Process Engineering Training Program MODULE 13 Kiln Volatiles Section
Content
1
CETIC “Volatiles” Group- Final Progress Report
2
CETIC Sub Commission “Behavior of Volatile Material in Kiln Systems
3
Investigation into Potential Low Temperature Volatilization
4
Factors Affecting Sulphate and Alkali Cycles in Rotary Kilns
5
Alkali Volatilization- A Review of Literature Available in 1977
6
A Study in the Volatile Cycles on HOPE # 2 kiln
7
Design and Experience with Bypasses for Chloride, Sulphate, and Alkalis
8
Kiln Gas Bleed Considerations
9
Ring Formations in Cement Kilns
10
Kiln Build-Up Meeting
11
Cement Seminar- Rings, Balls, and Build-Ups
12
Ring and Buildups in Cement Kilns
HBM Process Engineering Conference Minimization of Volatile Cycles
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 1
CETIC “Volatiles” Group- Final Progress Report
Blue Circle Indusuies PLC
Internal memo 3
SEE BELOW:-
‘rot?
C P KERTON Your re::
Date
21 December 1994
Copies
Stijec
CETIC “VOLAl-IUS” GROUP - FINAL PROGRESS REPORT
Herewith a copy of an English version of the final report from the CETIC GOUP which worked from 1990-1992, initially inspired by 3lue Circle’s initiatives. Copies of various French documents exchanged within the group are available from me or from Cb.ris Hoit. The text follows the order of the French original, intended for those who took part in the work. For those who have not been following progress so closely, the logical order of reading would be to stan with Appendix 2, which is a published paper based on the initial work of the group on cycles of chlorides, sulfates and alkalis driven from the burning zone. Next comes Appendix 3, which is Tom Lowe-s’ account of some practical applications of these concepts when burning petroleum coke. Returning then to the start of the main text, there is an account of further discussions and activity on this topic leading finally to Appendix 1 which looks at capture of SO:, in colder parts of a kiln system. (Those concerned soleIy with pollution abatemerit might well start here!) Proposals for possible further work are listed and those relating to sulfur behaviour will be pursued by joint exchanges of experiences from 1995. Suggestions for further distribution within Blue Circle will be welcomed. c I& ,/ f&L > c P Kerton Patent & Information Service Eric To:
;P L Rover (2 copies) C P B ‘Turner T M Lowes C J Holt L P Evans R M Mutter J M Lawton P A Longman
ca: \?%+!\2112CETIC
Blue Circle Technical Centre TC 94 049
CEl-lC SlJB-CO-ON
“BEHAVlOUR
O F
VOLATILE
M A - IN KILNS”
FINAL. PROGFZES REPORT MAY 1994
This report is strictly confidential within the Blue Circle Group. Additional copies should not be released outside Blue Circle without reference to the Technical Cenme.
Please apply to the hformation Servicesat
tie adcks give0 below.
September 1994
SUMMARY
Together with an earlier report (ISTN 92/5) this text sets down the major findings of a group which met from 1990 to 1993. For minor components volatilised in the burning zone (alkalis and sulfates), there are several successes on production kilns in reducing volatile cycles by attention to burning zone conditions, especially in relation to chemical decomposition of C&I,.
Correct diagnosis of conditions is assisted by
improvements to permanent on-line exit gas monitors and to suitable standardisation
of
sampling procedures for dry process kiln enuy material. The full effects of dust cycles in confusing results of sampling exercises remain to be established.
Sulfur volatilisation and absorption at lower temperatures have also been considered and the conditions of temperature and atmosphere which aid reactions for SO2 capture by various materials are outlined. Various permutations of actions are possible to abate emissions and some will not succeed at all points in a production line or may require moisture addition or increased residence time to improve their effectiveness.
Proposals for possible topics to continue this work are listed. (This is an English version of the official French text).
=
CONTEXTS
Paee No
1.
Introduction
2.
Task of the Sub-Commission
3.
Cycles of minor elements generated in the burning zone
4.
“Cold” cycles of minor elements
11
3.
Trace elements
14
6.
Future work
15
Figures
Appendix 1
SO? CAPTURE Chknical IMechanisms far Sulfur Dioxide Absorption in Cement Kilns and other Industrial Abatement Plant.
Appendix 2
TEXT FRO,M INTERNATIONAL SYMPOSIUM ON GAS CL&WING AT HIGH TEMPERATURES. Behaviour of Volatile Materials in Cement Kiln Systems.
Appendix 3
PAPER PRESENTED BY T M L0WE.S 100% Pet Coke - Problems and Solutions.
1.
INTRODUCTION
The group has the following memberx-
HOLDERBANK
P BORKI
3LUE
P KERTON (Animateur)/C HOLT
CIRCLE
CIMENTS LAFARGE
M DANDINE/K 30ULOT/M TOUSSSAINT/ X DUPONT-WAVRLN
CIMENTS
B POLGE/G Bw BERGERY/ G FLAMENT
FRANCAIS
ITALCEMENTI
R TACCHINI’
C3R
c MEYERS/J PARlSIS/P RENIER/ M BRUYERE
CIMENTS
D’OBOURG
ENCI
F LAMPROYEfR SPILLAERT W VAN LOO/F ERE?JS
The foIlowing meetings too& place:-
1.
1990 Autumn
Maastricht
ENCI
2.
1991 Spring
Greenhithe
BLUE
3.
1991 Autumn
St Antoing
CBWSCF
4.
1992 Spring
Frangey
LAFARGE
5.
1992 Autumn
OrbY
HOLDERBANK
6.
1993 Spring
Salerno
ITALCEMENTI
7.
1993 Autumn
Obourg
OBOURG
CIRCLE
This year there was one meeting at Obourg on the 4 and 5 November 1993.
TC94049
2.
2
Tt=chu.id
Cease, .!%@cabm 1994
TASK OF THE SUEKO~MMISSION
This is “to produce a state-of-the-art report concerning the behaviour of volatile material in kilns.”
This work originates from
1.
The increased use of ail sorts of secondary raw materials and fuels.
2.
The trend to produce more low alkali cements.
3.
Emission regulations which are becoming more and more rigorous.
Furthermore, during the work, important implications for kiln output and cement quality have been found in connection with control of cycles. Our principal recent activities are covered under the three following headings, 3-5.
(A copy of an associated paper by T M Lowes is included at the end of this report, having been presented at the same meeting in May 1994).
3.
CYCLES OF MINOR ELEMENTS GENERATED IN THE BURNING ZONE
We have already produced a first text on “classical” knowledge for this group of cycles
(ISTN 92/5): this was distributed in 1992. Supplementary work is described here.
Given a sample of material and its chemical analysis, one might think that all would become clear, but to calculate a chemical balance, it is also necessary to know the mass flows which are involved in the calculation, and there are a number of methods of deciding upon these. Each method has its own advantages and disadvantages, making them more (or less) appropriate at different points in the burning line. It is important to note that
1.
In the past this sophistication has not generally been adopted
2.
Various flow rates for hot raw meal entering the kiln can be calculated by different means, each one giving a different “burning zone volatility”.
We have continued exchanges on these topics to improve our understanding. One important parameter in the calculations is the flow rates of entrained solid particles between preheater stages, in particular between the kiln and the preheater tower. There are not many results available for this parameter (we note that there are not always the same sets of results available to explain observations!).
As far as these “classical” cycles of minor elements are concerned, there is much activity within member companies at present
There are two fields of particular
interest:
Firstly, the characterisation modified geometry
and the behaviour of hot raw meal (including the use Of
cyclones and special linings), and secondly the influence of
combustion and heat transfer on volatalisation
- especially in the burning zone but also
in precalciners. The influence of the local atmosphere close to clinker granules in the
rc94049
4
TecbniGai
Came. SenM I994
burning zone seems to play an even greater role than had been thought in the past: there are several results where volatilisation has been significantly reduced by more or less simple means (additional oxygen) and we trust that one day there will be a somewhat deeper understanding than indicated in our previous report.
This
understanding will also help us in applying results from the mathematical analysis of cycles from CBR which has shown great variations in alkali and sulfate volatilities
in
the burning zone in different kilns (even when taking account of the major volatilisation of chloride which explains a proportion of these differences).
This year we have tried to bring together knowledge on:
The most recent studies on control of cycles and characterising
hot raw meal
on the industrial scale;
Mathematical analysis of cycles and cyclone performance (from chemical analysis of samples);
The effects of the flame and kiln atmosphere ‘on volatility,
especially when
using petroleum coke.
Study
of
free energies indicates the compounds expected and which of them are the
most stable in the prevaiiing conditions of temperature, pressure and composition of the solid liquid and gaseous phases. Mr Berard-Bergery distributed a summary of the talk which he previously gave on this aspect of thermodynamics, He notes that without a certain knowledge of this subject it is difficult to make progress, given the need to explain the apparently contradictory results obtained from operating kilns. (We must certainly bear in mind the fact that we need to consider dynamic equilibria, not only static systems.) Thermodynamics allow us to determine the direction, intensity and speed of transformations of physical systems as a function of conditions. The possibility of a reaction is known from calculating the change in (Gibbs) free energy, if operating at constant temperature and volume or of free enthalpy at constant temperature and
pressure. Examining the trend of this value as a function of temperature, the reactions of formation or decomposition of a single chemical compound can be considered, and the order of stability in a family of compounds (chlorides, fluorides, sulfates etc) can be deduced. Hence, the reactions between one element and the compound of another element can be foreseen. There are quite a number of diagrams of free enthalpy as a function of temperature available (determined in the field of metallurgy) and also information on partial vapour pressures which
influence the equilibria.
Along a burning line it can be seen that for chlorides KCI is the most stable and easily vaporises producing a chloride cycle.
For fluorides CaF2 is the most stable at all
temperatures, and does not easily vaporise. For fluorides, phenomena are therefore very different for those encountered with chlorides. For sulfates K2SO4 is by far the most stable and CaSO4 the least. For C&30, stability also depends among other things on the partial pressure of oxygen, so that the sulfate cycle brought about by the decomposition and re-combination of &SO4 will be very different depending on the oxidation/reduction conditions at different points along a burning line. Na20 is more stable than f(20 and is found in clinker combined in the aluminate phase, whilst K20 tends to form &SO, if possible. It is interesting to see that sulfur, for example in the form of SO,, can be captured by carbonates because sulfates are more stable. At low temperatures sulfites can also be obtained.
CIMENTS FRANCAIS note that it is necessary to define the conditions in which a kiln must operate to give the desired results, in the case of high fluxes of sulfur or alkali. Results from CC3 confirm these ideas and they are being applied in other Works. Mr Flament has told us that the text which he presented at the Berlin Congress gives a good summary of his experiences at CCB.
At CCB, using 100% petroleum coke (ore-calcination with separate air at around 55-60% kcal) a good kiln output is found when there is a higher and more constant oxygen level at the kiln exit (1.5 - 2.0%), a greater fuel fineness (residue 3.6% compared with 5.0%), a more stable flame shape, and a less severe burning regime - obtained by means Of an
examination and adjustment of geometry in the burner region. The geometrical aspect helps to avoid too high a dust cycle (including cooler dust) and flushes, which can both cause alkali capture and blockages (contrary to the ideas of certain plant suppliers). &SO4 nodules are found in the interstitial clinker phase, and there is a need to consider quality aspects further. There is a sulfate: alkali ratio of 3.5, which is acceptable in the stable kiln regime: thus it is preferred to use only 50% petroleum coke for production purposes, so as to reduce the possibility of entering into a potentially difficult state. Coke brings 26% of the sulfur arriving in the kiln system.
The sulfate/alkali ratio is an important parameter to take into account, but it is not the only one. It is also necessary to have a good and continuous analysis of kiln exit gases to allow combustion conditions to be followed.
It is preferable that gas analysis is
automatically corrected for oxygen level so as to indicate other changes more clearly. It is equally necessary to continuously monitor precalciner
combustion conditions by
means of supplementary analysis of relevant compounds in the gas phase.
The “intensity of combustion” in the burning zone is an important factor to understand and use to control events. The decomposition of CaSO, in a reducing atmosphere is the key mechanism. Each kiln has its appropriate oxygen level in the kiln exit gases, which must be respected for a given sulfate input. The word “volatile” can lead some people into error:
whilst KCl
and NaCl are present in the form of gaseous molecules,
thermodynamics indicates that K20 and Na,O decompose in the flame and re-combine later. It can be useful to separately tabulate the calculated volatilities
of KCl NaCl,
K$S02, NaSO,, CaSO, instead of only Cl, K,O, NaZO, SO3.
We have discussed the design and operation of by-passes, certain of which take a significant dust burden at the kiln exit, which brings about difficulties in operation. In Ciments Francais kilns the range of dust burdens is from some ZOO-1,000 kg of dust per tonne of clinker. This dust burden reduces the performance of the lowest cyclone: if the value of the dust burden is not known it can be difficult to interpret results. (It
was
noted that Weber recorded low dust burdens in all the kilns which he analysed) Other
important parameters which must be known in order to diagnose a situation are the chemical analyses of good samples of coal and of the hot material coming into the kiln at the feed chute level.
Given the above-mentioned data, the measurement of CO? level at the kiln exit over a certain period allows separate calculation of decarbonisation in the kiln and the preheater/precalciner.
The mixture of dust and new raw meal which comes into the kiln
can then be estimated from the loss on ignition. It is suggested that methods using chemical tracers to estimate the material flow, for example K,O, can be falsified if the level of dust cycle is not known.
It seems probable that the geometry of a plant has a marked effect on dust entrainment. In the older generations of Dopol preheater, material fell a long distance from the bottom cyclone into the kiln.
The kiln entry material had a low loss of
ignition which could be falsely attributed to good decarbonisation.
The more recent
Dopols have a side entry for material: a bypass can then expect to encounter a lower dust burden. The older “lateral centrifuge” entry of F L Smidth also produces a poor bypass efficiency, due to dust entrainment.
High dust recycle levels also have the
inconvenience of increasing the probability of blockage (Mr DuPont-Wavrin noted that the Berthold Company is supplying an X-ray detector to monitor material flow rates ex-cyclone).
It is useful to calculate the effect of dust cycles on thermal performance. A heat and mass balance for each preheater stage allows the effect of dip tube geometry changes to be observed. Opinions vary as to the appropriate choice for different stages, not to mention the use or removal of cyclone exit flaps. A variety of experiences have been reported regarding Hasle Vortex Finders. At ENCI, excellent results have been recorded for over 3 years, whilst at CBR the tubes were lost in 3 months- Ciments Francais have observed the same range of lifetimes. There is consensus on the advantages of dense ceramic Hasle units in the kiln feed chute
(with
a minimum of exposed refractory cement?,” when they are set up with a good arrangement of air cannons. Some peopie have doubts as to their sensitivity to thermal shock in other regions of kilns during heating and cooling.
ITALCEMENTI
has described similar experience with a F L Smidth chloride bypass at
Picton, also used to assist the production of low alkali clinker. Here the high dust burden (some 1,000 kg per tonne) causes efficiency of a dust bypass.
the so-called “gas bypass” to have tbe
Tests are in hand at Nazareth with a purifier which
removes SO, by injection of raw meal and water (a Monsanto design). At Colaferro two geometrically identical preheater kilns produce respectively some 1,900 and 1,150 tonnes a day. The higher output kiln is fired with a mixture of coke and coal and produces
build-up
problems, whilst
the
other
operates
satisfactorily
with
100%
petroleum coke. The only difference that has been noted in combustion conditions is a higher secondary air temperature due to the use of IKN plates in the Fuller cooler.
Italcementi manages to use 100% petroleum coke on the Lepol process, even with 10%
over-grate firing, with emissions of SO, - except during build-up losses - having only pyritic material as origin. The residue is 10% as for coal. On the dry process it is necessary to drop the residue to 4% (or 5% for a coal/coke mixture). The precalciner gas does not contain any SO2 when coke is used in the burning zone. The sulfur leaves with the clinker, partly during occasional flushes.
In the past coke or anthracite was introduced into long granule-fed kilns. This helped formation of a good burning zone, but nowadays it is found that there is also a high SO2 emission.
This coke is now added to the main burner, a procedure which operates
satisfactorily if the burning zone is controlled via NO, monitoring to avoid the problems which can be caused by the sudden arrival in the burning zone of build-ups detached from internal cruciforms.
In the same way, LAFARGE has continued with a major programme of geometric "centralisation"
of burners, noting oxygen levels (typically some 3%) and SO, levels ex-
73.i~ re-sort Ls soicrlr confidemial w.rhio the Blue Ctie Group.
kiln as a function of flame momentum. Several Works
keep the centralising
mechanisms
on the kiln platform so that alignment can be corrected if there are changes after some weeks of operation. In such circumstances 100% coke can be used (3-5% S) on the dry process with a 5-10%
residue.
Automatic kiln entry material sampling systems are generally installed with a view to assuring safe operation (Pfaff) la Nouvelle.
and Lafarge gave an account of an in-depth study at Port
The company was particularly
interested in the impact of sampling
techniques on results. Here, there is a kiln fed at some 100 tonnes an hour (50% of the heat energy coming from petroleum coke) with 3% oxygen ex-kiln. The levels of all volatiles in the collected dust go down as a function of sample suction rate, reaching a plateau. The dust Is really a mixture of fine material (high in volatiles and easy to collect) and coarse material. Although the nominal isokinetic aspiration rate for the probe was some 30 litres per second, it was not aligned with the gas fIow direction and higher suction rates were therefore needed to obtain a representative sample. Lafarge express the hope that a standard method can be written up, suitable for use throughout the world. They have currently only two or three competent sampling teams in their French group. It was noted that it is a bad practice to make use of large probes and low capacity pumps to reduce blockages as far as possible. CBR reported on SO2 levels in the kiln system at Antoing. The level of some thousands ppm ex-preheater drops to 600 ppm at the stack. This loss is split
20% to the crusher,
20% to the mill and 60% to inleaking air. It was reported that at Rekingen the raw mill is run at a reduced throughput, in order to allow SO2 capture t o continue throughout the operating day. A few supplementary results from the simplified mathematical model have been distributed, with its application to the analyses of balance samples from various member companies, so that volatilisation,
entrainment and capture coefficients can be
calculated together with the performance of some cyclones.
ThLs remrr Ls snicdy codid~riaf w-if&in tie Blue CLv!e Grarw.
A significant range of values was noted, all calculated on the same basis. K20 is a good tracer to determine raw meal
entrainment by gases. For the calculated entrainment
values in the document distributed, one must consider the position and methods used to collect the kiln exit dust samples - with a probe in the kiln, in the riser duct or exbypass. Nevertheless we consider that it will be very useful to extend the tabIe of results already obtained by sending further analyses to CBR to gather a common table describing volatilities, volatilisation)
An example of a Blue Circle kiln (poor flame with high
is the only one to have been added this year, and the model remains to
be more widely used.
At BLUE CIRCLE some thermodynamic data lead us to think that in a typical kiln gas there is at llOO*C a sufficient reduction potential produced from 2,000 ppm of CO to reduce CaSO, with a consequently much higher voiatility.
A separate paper from Blue
Circle is appended, giving an account of UK experience with use of petroleum coke.
A paper from Blue Circle regarding the design of a bypass for a new Works with high chloride raw materials was discussed.
There was also an expected high content of
alkalis in the clinker, which could perhaps be reduced by the addition of even more chloride. The performance of existing by-pass systems indicates that despite the fact that (according to suppliers) there is a possible dust loss of some 200 to 250 g/Nm-’
in the gas extracted from the system it will be best to calculate with a
nominal level of 400
In such circumstances there will be a need for a raw meal
preparation system with a significantly higher throughput than normal.
Several remarks were made: there are examples of precalciners
blocked by sulfur; the
handling of by-pass dust rich in CaCl, is much more difficult; is the fuel penalty per percent of by-pass closer to 5% than the 1% used in this example? (This latter figure was supplied by the company which intends constructing the proposed Works.)
The animateur was invited at short notice to give a paper during an International Symposium of the Cleaning of Gases at High Temperatures in December 1993, which is
This remrr Ls sm’cr(;l cmfidemiai withiu tie Blue Cri-cle Group.
appended.
Several specialist workers in this field have encountered problems of
blockages and build-ups which provoke their interest. (Note that data on the equilibrium CO-CaSO~-CaS is given in "Sulphur Capture in Fluidised Bed Boilers:
the Effect of
Reductive Decomposition of CS04”, by A Lyngfelt and B Leckner, Chemical Engineering Journal, Volume 40. pages 59-69, 1989.)
4.
“COLD” CYCLES OF MINOR ELEMENTS LEMENTS
Information on the effect of internal cycles on emissions to the exterior has been exchanged, avoiding (if possible) examination of equipment for capture of such emissions which is left to the "Environment"
sub-commission, liaising with its animateur.
topic typically concerns cold cycles of SO2
This
formed at low temperatures in the
preheater, and capture of SO, emissions in long kilns and in the Lepol process involving cycles which originated in the burning zone.
For this topic it seems that most information ‘Environment”
sub-commission:
has already been treated by the
it remains to define more precisely the chemical
reactions which are involved and the domains in which these are the most (or the least) effective. We are interested in establishing information about chemical efficiency of absorption of SO2 as a function of conditions of atmosphere, humidity, temperature, residence time, particle size, chemical composition etc.
The animateur noted the classification
of absorption mechanisms given in two USA
papers: these concern tests carried out Davenport (Steuch) and at Lone-Star (Sheth)
The document sponsored by the British Pollution Inspectorate is also available. This reviews published information on removal of trace gases. It covers a range of both chemical species
and reactive materials, The sections relating to SO2 capture appear
to provide a useful framework within which the cement industry’s experience can be classified, see Appendix. Other industries
are interested in the possible future use Of
sorbents with increased reactivity (cement kiln dust?) and in “regenerable” agents
such
as calcium disilicate.
The reports of HOLDERBANK
to the Environment Sub-Commission have indicated that
emissions of some 1,000 to 1500 mg/Nm3 of SO2 were reduced to close to 350 mg/Nm3 during the operation of the raw mill.
The same effect could be obtained in direct
operation by the addition of Ca(OH)2 to the raw meal. This provides removal of 50%
SO2 at a stoichiometric level of 5; Polysius would suggest 80% removal at a ratio of 8. Not all users seem to have taken account of the need to use superstoichiometric quantities
of sorbents, which sometimes may react with only some 10% efficiency.
Work carried out in the UK by Lodge-Cottrell desulphurisation,
in the field of the electric power station
has shown an efficiency of 25% for dry lime injection, rising to 50%
in the presence of moisture, and also that sodium based reagents had a genuine action which was almost double that of calcium based sorbents (and that these could be introduced as solutions by means of simple nozzles).
It was mentioned that Rekingen Works had modified its raw mill throughput so that it operated 24 hours daily, thus capturing SO 3 to conform to emission regulations. The Santa Cruz Works of Lonestar
may do the same. In this class of activity, CIMENTS
FRANCAIS works on the basis of 50 to 75% absorption.
ENCI gave an account of experience with operation at different oxygen levels to reduce SO2 emissions from its two-stage preheater kiln. The degree of sulfation of clinker at Maastricht is 125%. This, along with other causes, produces an emission which must be reduced to comply with new regulations During 3 weeks the oxygen level at the kiln exit was altered from 1.5, 2.0, 2.5, 1.3, 2.0%. The results for SO2 level in the emitted gases and SO3 in the raw meal and clinker are in agreement, showing that emissions can be reduced. If the effects on quality and on kiln operation are acceptable, they intend to buy new fans to guarantee sufficient oxygen level at maximum kiln output (with fuzzy control). The re-installation of Magotteaux stirrers in the kiln has once again given positive results after the last stop.
At OBOURG
there is an emission problem similar to that at ENCI, which can be
resolved by a kiln exit oxygen level of 2 to 3 % but this could give a too high a chain temperature and an unacceptable reduction in output. Oxygen has been added (1,000 m3/h either by the primary air channel or beneath the flame) gaining 3 to 4 tonnes per hour of clinker at an acceptable chain temperature. The cost of oxygen is some 3 Belgian Francs per cubic metre for a permanent irstailation, but this could be
This reqorr is snict(y confidential within the Blue Ckfe Group.
offset if cheaper (higher sulfur
fuels can be used. It seems that at least a quarter, and
nore usually about a half of the SO2 disappears between the kiln and the stack, no doubt by capture on dust. Given the large volume of gas produced by this wet process kiln and its moist fuels, a new fan would be proportionately much more costly than for ENCI.
At Obourg it seems likely that the longer kiln can satisfy emission limits through control of excess air levels. For the other kiln, another method of reduction of peaks of SO, is being studied; NaHCOS injection in the exit gas duct at the upper end of the kiln. The trial installation from Solvay (about 400 kg/h of Na.I-IC03 powder) was leased for longer trials. Chemical efficiency is 100%.
As already noted ITALCEMENTI
is looking at a Monsanto system involving a water/meal
scrubber for SO,, with a cost expected to be only 20% for that of an “Untervaz” system-
LAFARGE has studied sulfur behaviour on a semi-wet Lepol grate. In the hot chamber there is an excellent capture of sulfur coming from the kiln, but there is also decomposition of pyritic sulfur on the grate. This starts at the transition from the cold to the hot chamber (500-600'C)
and is completed by the middle of the chamber.
TC9-4049
5.
15
Tecfioical Centre. Senrember 1994
TRACE ELEMENTS
lTALCEMENTl has presented a summary of results from 30 kilns, seeking to determine the amounts of 16 metals In stack dusts, and also of 5 inorganic micropollutants. Vibo works (precalciner),
At
the raw meal is dosed with CaF2 to influence the
decomposition of strontium sulfate and so limit the undesired effects brought about by SrO during alite formadon. On the Lepol process CaF2 also provides a less dusty clinker and a reduced need for kiln system cleaning. No changes were noted in the behaviour of other halogens or of alkalis.
OBOURG has provided, a list of balances over 2 years. BLUE CIRCLE has shown a table for retention of various elements in a number of kilns as a percentage of the quantity brought in by the kiln feed (including recycled dust). An examination of Blue Circle’s conclusions regarding behaviour of trace elements considered elements as being either non-volatile or partly volatile. 3-5 results were available for each element for the wet, semi-wet and dry processes. The percentage of the input found in stack dust was very low for the non-volatile elements (As, V and sometimes Cr - unless there was enrichment from refractories), thus reflecting tie good efficiency of de-dusting and the varied additional contributions
from fuels. The highest proportions escaping with stack
dust were for cadmium, lead and thallium in semi-wet kilns. The levels noted were influenced by the rate of removal of intemediate
dusts produced in the kiln processes.
For the dry process there was less enrichment of cadmium and lead.
We have specifically looked at German work previously published in ZKG, where the need to examine the chemical combination of elements is underlined. Mercury,
for
example, must always be oxidised in a kiln system, so that the vapour pressure of the uncombined element is not to be considered.
We have also received a supplement to the bibliographical list established in Autumn 1991, which concerns cold and trace element cycles. The published literature makes it quite evident that cement kiIns are reputed to be potential origins of emissions of SO,,
Tl, Pb - and perhaps Cd and Hg, if these latter are found in the region. There is also a certain interest in the upgrading of kiln dust,
TIC re,vrc is suictiv wnfide!zu*al’
witi tie BILE C’ric!e
G~OLT.
TC94049
17
Techical
Cam-e, ?&xBder i99d
FUTURE WORK
The Plenary session has asked for a new sub-commission to be set up in 1994/1995 to follow the topics left from the existing group, with the addition of a study of corrosion and the wider application of the mathematical model already developed. We must avoid repetition of work already done, and coverage of subjects which the Maintenance Group and the new Working Party on Flames are currently examining.
(For corrosion,
depending on the activity of the Maintenance Sub-Commission, we might envisage a search for methods of reducing its effects, as well as the identification of compounds which have a strongly corrosive action).
Our
group
is convinced that there remains much to be gained for the companies of
CETIC in bringing together the specialists who are concerned with Chemical phenomena within kiln systems
and avoiding the study of topics which are more or less “legal”.
All are invited to consider a new division of chemical and engineering work with regard to cycles, transport, clinkering, emissions, interaction with refractories, combustion, sampling (gas and solid), etc etc.
A review by the Sub-Committee has produced a list of ideas, (see following page), these points generally relate to factors which have an impact on quality, cycles, transport, clinkering of emissions, refractory attack, combustion, sampiing of gas and solids etc. Some of them are more relevant to groups in the Technical Commission. These ideas remain for immediate discussion.
(NOTE :
It was subsequently agreed to concentrate initially on topics related to the
behaviour of sulfur).
C P Kerton Technical Centre May 1994.
LIST OF TOPICS
Environment: Factors effecting organic emissions and their control.
Process: Water injection in planetary coolers. CO SO2, CaS04 balances.
Control of kiln build-ups and their chemical composition.
Effects of secondary firing on volatilisation/condensation
in Lepol grates and
their control. Methods of successful use of even higher levels of S in fuels.
IMethods
allowing the retention of SO3 in clinker.
Control of chloride volatilisation.
Applications of CO, analysers.
Determination of dust cycles in kilns and heat and mass balances for each preheater
stage.
Optimum fineness of petroleum coke for kiln and precalciner burners.
Effects of volatile materials on the long term and short term stability of kilns and their consequences for static and dynamic conuol strategies
(allowing early pro-active responses t o change).
Control strategies (anticipatory control) to restore operation of a disturbed kiln (effect on throughput, environment, quality etc).
Bypass control.
Effects of geometry on cyclone operation.
Effects of volatile cycles and dust cycles on cyclone dip tubes and on various refractories.
Control of sulfur cycles at low temperature.
Effects of heating and cooling on refractories and linings; success with rapid regimes.
Effects of parameters other than CO on sulfur volatilisation.
Correlations of SO2 signals with other process parameters.
Effects of injecting and additional fuel at different points in the process (for example solids at the kiln feed chute). Distribution of trace elements throughout kiln systems, and methods of control.
Interaction of volatiles and refractories.
Influence of SO3 on refractor+ life - both direct (chemical) and indirect (perhaps a lower BZT).
Correlations between SOS and free lime in operating kilns.
Effects of V and Ni (coming from coke) on refractories. Correct regulation for burners for different levels of coke fineness and coke mixtures.
Maintenance: Corrosion in colder zones of kilns (in relation to Cl and S)
Workplace hygiene and safety aspects from the point of view of volatiles
Product/Quality Treatment and use of dust rich in volatiles
Methods for the internal use and/or upgrading of dusts which cannot be dumped.
Effects of V and Ni (coming from coke).
Effects of SO3
and of sulfate/alkali ratio on clinker quality at different
levels of free-lime (is there an optimum level?)
Effects of marginally reducing conditions on quality.
Effects of halogens on cement behaviour (standards, etc).
Effect of clinker size grading on quality.
cix/tla 13.9.94 &4:x94.049
RT lo9 pot 1b.Col)
:A -.L---
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Tempercrure “t 0
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coo 6 0 0
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Free cncrg diagram for sulfates.
-20
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8
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fret energy diasram ior chiorid:s.
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Fret cncrgy diagram for fluorides.
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-
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t \ ‘,
450 400
350 3 0 0 zso 2 0 0 7.50 1 0 0 ‘s 0 0
L
20
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40
60
7 0 Dhbor
3-90
- -43.447
+
8 0
90
1 0 0
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110
1 2 0
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Km
so3
Cl
<125fr
13
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4.3
1.9
1.6
Ensemble
12.6
4.95
7.03
45op
3.6
4.4
2.3
>lSOp
3.1
Ensmble
5.3
4.2
2.1
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2.1
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0.7
0.8
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L
20
30
SO
40
60
7 0 Dhbor
3-90
- -43.447
+
8 0
90
1 0 0
R-t
- .a65
110
1 2 0
13*
4.042* ~&bor
Km
so3
Cl
<125fr
13
5.2
7.29
>125p
4.3
1.9
1.6
Ensemble
12.6
4.95
7.03
45op
3.6
4.4
2.3
>lSOp
3.1
Ensmble
5.3
4.2
2.1
Cl:
2.1
2.3
0.7
0.8
ENCI RELATION MG/M'.SO~
I
X
- %
02
1. General Information is presented here against the background of equipment/processes encountered outside the cement industry, where acid gas abatement may already be practised - and where ideas for transfer to our industry may originate. Our most frequent needs are to improve SC+ capture by calcium compounds in raw mills and/or compensate for the absence of this absorption when mills are not running. Whilst there is a 50 to 75% reduction in SC& levels in a number of cement Works when hydrated lime is suitably added, some sites need to understand why they record drops of only 20 to 40% and there also is an interest in better understanding the possibilities for using of alkaline sorbents. This note aims to provide suitable background information as an aid to better understanding when the complications and costs of the “Untervaz Solution” may have to be accepted. 2. Sorbent Injection 2.1 Basic Description of Technology and Principal Variations Sorbent injection is used primarily in pollution abatement as a means of reducing emissions of sulphur dioxide (SO-,> and other acid gases, such as HCI and HF. Material is usually injected into the gas stream as a fine powder, where it reacts with the acid gases, generating a dry product for collection in dust arresment equipment. Dry injection methods are particularly suitable for small boiler and incinerator plant or retrofit applications where the capital expenditure for other systems is prohibitively expensive. The efficiency of S&- removal is 40 - 80%, depending on the sorbenc used (most commonly calcium and sodium compounds). The sorbent can be injected at various points in the plant, according to the temperarure and conditions at which it is most reactive. The most common systems for boiler plant are: Furnace injection of calcium based compounds Heat exchanger injection of hydrated (slaked) lime (Ca(OH),, Post furnace injection of Ca(OHk at relatively high humidity Post furnace injection of sodium based compounds. In a cement process, we may see: “Classic” SG- capture in the lower stages of the preheater Hydrated lime injection at top of preheater or in conditioning tower (probably as slurry) Return of calcined meal to cooler parts of system (preferabiy with moisture) Wafer injection at suitable points to increase possibility of reactions with raw feed Hydrated lime addition to raw mill Injection of alkali compounds or solutions to exhaust gas duct SG- capture by limestone in raw mill in the presence of moisture. (Some different possibilities for injection are envisaged for Lepol systems.) 2.2 Principle of Operation The reaction between sulphur dioxide and dry sorbent is a heterogeneous one . (Reactions with moist sorbents are discussed later in Section 3.2). SC& molecules diffuse through the gas stream and are adsorbed on to the sorbent surface before diffusing into internal pores where chemical reaction occurs.
These mechanisms are exemplified in the classical kiln/preheater sulphate cycles. Dry calcium sorbents react with sulphur dioxide as follows: The first stage is calcination: CaC03 + CaO +CGCa(OH)z + CaO + Hz0 (These reactions occur at temperatures greater than 760 and 570°C respectively. Dolomitic limestone starts decomposition at a lower temperature.) The second stage is sulphation: CaO + SG_ + Y.Q + CaSO, With excess oxygen, complete oxidation to sulphate occurs at temperatures ahove 800°C. Below this temperature a mixture of sulphate, sulphite and sulphide is formed. (The optimum temperature range for direct reaction with hydrated lime is listed as 130 - 180 “C.) Sodium compounds react with SO, as follows: 2NaHCO,-Na&O,
+CG- + Hz0
Na&O, + sa* + ‘ha- -, lN&-so, +co, Sodium bicarbonate decomposes to sodium carbonate, which then reacts with S%- to form sodium sulphate. The these reactions are significant at temperatures above 130 - 180°C; at the lower temperatures in this range the favoured SO, reaction is directly with the carbonate, but at higher temperatures the thermal decomposition and sulphation reactions occur simultaneously. It is thought that the good performance of this sorbent may be explained by the fia that a shrinking core of bicarbonate is continuously decomposed, providing moisture at a fresh reaction interface for S%arriving through a permeable outer shell of sulphate A similar scheme is seen for potassium. 23 Selection and design considerations Dry sorbent injection is usually one of the cheapest abatement options for SG- removal, particularly for small or retrofit plant. as the capital cost is low. The choice of sorbent is a prime consideration and depends very much on availability. Calcium compounds used for dry injection are primarily naturally occurring limestone or dolomite or hydrated compounds derived from these raw materials. Reactivity is dependent on pre-treatment as well as natural properties. The sodium sorbents of interest are sodium bicarbonate (NaHCO,) and sodium sesquicarbonate (NaHCO,.Npl_C0,.2H,0). These occur naturally as nacholite and trona respectively. In making a choice, process economics are highly dependent on delivered price of reagent and rate of use, despite the initial lower cost and comparative simplicity of operation of the dry post-furnace injection processes. There are three temperature windows for calcium sorbent injection in boiler and incinerator systems, which also broadly apply to cement kilns. (Note that there is a temperature zone where neither group of reactions is very effective, especially in dry conditions.) Calcium sorbents can be injected directly into the furnace, where at temperatures of 1100 1250°C the calcination and sulphation reactions can occur, Calcium hydroxide Ca(OHX_ will react with SO, at about 550°C and hence can be injected before the heat exchanger.
At high levels of humidity calcium hydroxide will react with SO, at even lower temperatures (5 - 15°C above the saturation temperamre of the gas, as discussed in Section 3.2). water can be injected with the calcium sorbent into the duct between the heat exchanger and particulate abatement equipment or elsewhere Sodium sorbents are also injected in boiler plant to the duct between the heat exchanger and particulate abatement device, where the temperature is in or above the range 130 - 180°C. It is possible to inject sorbents at several points in the plant and to combine this technology with other abatement options. (No accounts of multiple injection systems are known for the cement industry). Handling problems may be encountered and not all sources of lime are equally effective in reacting (at a given fineness) in the time available at the point in the process where introduction is feasible. Reactivity increases as the surface area of the sorbent increases (particle size decreases), up to 40 m3/gm. The calcium:suiphur ratio is generally set at 2, but can be as high as 6, particularly for low sulphur coals where the mass of sorbent is still comparatively low. The reaction efficiency for limestone ranges from 40 - 50% at calcium:sulphur ratios of 2 - 4. Dolomites give greater conversion efficiencies, and this is attributed to the more open structure of the sorbent material which enables greater diffusion of gas into the pores of the sorbent. Conversion efficiencies of 70 - 80% are achieved with sorbents of hydrated lime Ca(OH), at 8OO”C, and sodium compounds also yield conversion efficiencies of 70 - 80%. (Use of a fabric Nter for particulate abatement is claimed to enhance SO, abatement efficiency by ca. 10% because the sorbent collected on the filter bags continues to react-with SO, during particle filtering. Dry sorbent injection to the filter bags after a cleaning cycle, is claims as an altemativelsupplementary method to enhance reaction.) Superstoichiometric quantities of reagent are usually needed because efficiency of reaction is low, and recycle may also be required. Efficiency may even be insufficient for elimination of either very high or very low SC& concenrrations at a realistic stoichiometric ratio. Use of “conditioned” (ca. 10% moist) hydrated-lime has been claimed to give improvements due to (a) breakage of particles when brought into contact with hot gases (so generating more surface area) and (b) cooling, which increases reaction efficiency. There are no known reports from the cement industry on this point, or on reactivity of different sources of limestone or dolomite. Limestone is the cheapest sorbent material currently in use. Lime (calcium hydroxide) is about 5 6 times more expensive than limestone and trona/nacholite are generally ten times more expensive than limestone. Some studies suggest N&HC03 becomes still more efficient at higher temperatures ( e.g., up to 815 deg C); it costs 2 to 4 times as much as hydrated lime. although consumption may be lower and there may be less residue to dispose of. (There is little or nothing known about the use. of alkali compounds at relatively high temperatures in the cement industry.) Combustion systems using high sulphur coals yield the most promising results where SO, levels are 2000 - 4000 ppm. At SC& concentrations < 1000 ppm the reaction is diffusion limited and it may be more difficult to achieve desired levels of efficiency. (In general, this technology is considered to be less efficient than the “wet” methods described later.) In boiler systems, the added sorbent and its interaction with the fly ash, can cause fouling of surfaces. Also, higher particulate loadings, decreased particle size and increased electrical resistiviry of the particles can impair the performance of collection devices. Handling and disposal of larger quantities of solid waste with properties different from fly ash or conventional scrubber sludge can be difficult and increase costs. For example, sodium salts are soluble in water and hence disposal requirements are more stringent.
3.1 Future Developments Research is continuing to enhance knowledge of-the appropriate mechanisms acting in these injection processes, with a view to developing alternative, moreeffective sorbents. For example, alkali metal additives in limestone enhance SO, abatement efficiency and early indications are that lime-containing waste materials, such as carbide mud and sugar mill mud, react faster and have a greater sorption capacity. The use of regenerable sorbents such as calcium silicates is another possibility. 3. Spray Dryers 3.1 Basic Description of Technology and Principal Variations
Spray drying is a standard chemical engineering operation used to produce dry powders of controlled particle size, density and moisture content Spray dryers are used in pollution abatement for the control of acidic species in a flue gas stream. Droplets of reagent are contacted with the flue gas in a reaction chamber - probably a modified conditioning tower in a cement Works. Liquid is continuously evaporating from the droplets in the chamber during the neutralisation reaction and the dry reaction product can be collected at the base of the chamber or in the dust abatement plant. A complete system consists of the spray dryer (atomiser and reaction chamber), associated slurry/ liquid handling equipment, a particulate collection devise and soiids recycling equipment. There are three types of atomiser in general use: rotary, two fluid or spray nozzles The reaction chamber can be a tower or dust, and the flow of the droplets and flue gas stream are usually co-current. Lime slurries are most often used, but sodium carbonate/bicarbonate solutions are also acceptable. 32 Principle of operation a) Lime spray driers
The atomiser generates dropiets of lime slurry which are injected into the flue gas stream in the reaction chamber. In the capture of sulphur dioxide, the chemical reactions which occur involve water and are believed to be: Liquid phase:
CaC03 + Sa- + %H,O -. CaS03.%Hz0 + Ca-
Gas/liquid phase:
Ca(OH)l
+.Sa- + Hz0 - CaSO,.‘/iHIO
+ lXH,O
Sa- is absorbed in the aqueous phase of freshly atomised droplets forming suiphurous acid, where the reaction of SO-, with lime or limestone proceeds rapidly, forming calcium sulphite which may later be oxidised and form gypsum in the presence of oxygen and water. As the droplets pass through the chamber, water evaporates to yield a porous particle which has a dry surface but a wet interior. Sa- diffuses into the wet sore of the particle and the reaction continues. The reaction of SO, with lime in the absence of any moisture is slow. Consequently, in order to extend the reactivity of the lime in the unit, the temperature near the exit is maintained just above the saturation point of the gas. As mentioned earlier (Section 2.2), these reactions can be involved in sorbent injection in cooler parts of a cement production line, for example when Ca(OH)z is injected to the preheater or Sa- reacts with limestone in the raw mill. In the absence of water, however, the reaction rate will be very slow - for example at the top of a preheater tower. Water injection to the preheater at Santa Cruz (without
adding any extra lime) was reported to allow 10-20% reduction in SO, levels. Failure to compare humidity levels and/or use fresh lime may account for several differences in experience of SGcapture in kiln systems. b) Spray driers using sodium salts
Dry S&- reacts with sodium carbonate/bicarbonate from low temperatures: and hence the requirement to enhance the reactivity by stringent control of temperature and humidity is not necessary. 3.3 Selection and Design Considerations The principal design parameters for spray dryers are droplet size and distribution, and inlet and outlet temperature. Multiple atomisers are used in order to achieve an even distribution of droplets in the reaction chamber, and the droplet size has to be such that the rate of evaporation is fast enough to prevent formation of scale in the reaction chamber as droplets/particles strike and stick to the walls, but slow enough to enable the reaction to occur. High inlet temperatures enable more water or lime to be injected, and low outlet temperatures (slightly above the saturation point of the gas) optimise the abatement efficiency of the spray dryer. Fine sprays and concentrated reagents have shorter drying and reaction times. Water evaporates from concentrated reagents rapidly and hence the neutralisation reaction occurs mainly between the acid gas and the porous particle. Fine droplets ( < 100 pm) are used with size tailored to the residence time of the flue gas and droplets in the chamber or duct. The residence time in a chamber is usually in the range 5 - 10 secs, with droplet size < 100 pm. For injection of slurry into a duct, reaction and drying times of 1 - 2 secs are typical. Residence times and evaporative heat available in an existing cement plant conditioning tower or gas duct system may limit the amount of SO, which can be scrubbed. The choice of sorbent will depend on its cost and availability: sodium salts give better “once through” efficiencies, but lime has a cost advantage over trone/nacholite and the calcium based reaaion product is insoluble in water which renders disposal easier, should this be necessary. Spray dryers have been successfully used in Europe for controlling emissions of acid gases, primarily for combustion plant and incinerators, using a lime sorbent which is recycled to improve its utilisation and achieving abatement efftciencies of > 99% and > 90% for HCl and SG- respectively. Efficiency can be enhanced by increasing the stoichiomeuic ratio for specific conditions of temperature and humidity, but the gains are limited by sorbent utiiisation, sorbent solubility and waste disposal costs. The Ca:S stoichiometric ratio is typicaiIy in the range 1 - 1.5 and liquid:gas ratios in the range 0.027 - 0.04 l/m3. Spray dryers offer several advantages over wet scrubbing, especially the fact that a dry product is formed which is easier to handle and dispose of than a liquid effluent. The capital cost, maintenance cost and energy requirements for the spray dryer system are lower than for wet scrubbing plant although reagent costs are higher. The particulate collection device can influence the operating conditions of the spray dryer. Acid gas removal can continue in a fabric filter but care has to be taken to prevent blinding of the bags. Electrostatic precipitators, however, can operate at temperatures nearer to the saturation point of the gas, hence the spray dryer outlet temperature can be lower which improves its abatement efficiency. The dry product from the spray dryer (hydrated &SO,) can be used for landfill, processed to yield anhydrite or pelletised to yield synthetic aggregate.
3.4 Scrubbers “Absorption” is a process which involves mass transfer between a soluble gas and a solvent in a contacting device; chemical reaction may or may not occur In process design, both the chemistry of the system and the physical structure of the equipment must be considered. Unfortunately, water alone is not effective at removing SO, from a gas stream because (unlike HCI) it is not very soiubie: an alkaline solution is needed The driving force for gas removal is the difference between the partial pressure of the soluble gas in the mixture and the vapour pressure of the solute gas in the liquid film in contact with the gas. Mass transfer occurs by molecular diffusion across the interface and the rate determining step can be in either the gas or the absorbent phase. When the gas is very soluble or reacts chemically with a reagent in the sorbent, the process is “gas phase controlled” Trace gas removal systems can be categorised by the solubility of the gas and by the reactivity of the system. SO-, is classed as “moderately soluble” in water (I-IF and HCI are very soluble), and sodium sulphite and-alkaline compounds are used with some success as additional reagents. Efficiencies of SC& removal of some 99% are attained in appropriate circumstances. There are no known accounts of the use of sodium sulphite solution for SO-, capture in the cement industry: usage in power stations generally appears to be associated with systems which treat the resultant chemical products to regenerate the solution of sulphite sorbent, or systems in which the sulphite is used alongside other reagents, providing an initial capture of sulphur as an alkaline compound for subsquent displacement reaction to form a more readily disposeable or saleableby-product. Gas absorbers which attain gas/liquid contact by bubbling dirty gas through a liquid are suitable for absorption processes which are “liquid phase controlled and those which involve spraying liquid through the gas stream are suitable for processes which are “gas phase controlled”. As absorption is a rate process, the concentration gradient (driving force for the reaction) and the (high) surface area of contact between the liquid and gaseous phase are crucial design parameters. The surface area is determined by the packing material or droplet size and this is usually achieved using packing materials which are coated with liquid or by droplet/bubble formation. The absorber design also has to provide a means for renewing the liquid absorbent so that a high driving force for mass transfer is maintainedGas and liquid flow rates and pressure drop across the absorber influence the driving force, the efficiency and in some cases the surface area (droplet formation). A good gas absorber design removes as much pollutant as possible in as small a space as possible. The choice of equipment depends on the abatement efficiency required, the energy and reagent requirements and the properties of the dirty gas stream.
c. P. KERTON, Blue Circle Te&nicai Centre, Greenhithe, &lay 1994 (Updated, July 1994).
- I r’
n
l-EMPERATURE C O N T .~R O L WATER
FLUE GAS BYPASS AROUND DRYER I- - - - - - - - - - - - - 1 I
I I
FLUE GAS ~no:.i POlLEn
+ C
J--J-
SPRAY DRYER WASTE RECYCLE
4-J
FABRIC FILTEH CATCH RECYCLE
L
E A N FLUE GAS
TO ASId DISPOSAL BIN
rBALL cc: M I L L _STLAKER
DETEN.TlON SLAKEFI NOT SHOWN a-l;.-,, FEED PUMP
APPENDIX 2
TEXT FROM INTERNATIONAL SYMPOSIUM ON GAS CLEANING AT HIGH TEMPERATURES Behaviour
of Volatile Materials in
Cement Kiln Systems
GAS CLEANING AT H I G H TEMPERATURES Edited
by
.
? q v
snlls o f riinny clicrnicnl (‘l’trc: usit ccnIc11I iidilslry convcnliun is loltuwrtt iii cxprcssinfi I’ atialyscs ill Icrrris of cIxitlcs. c.g.. CaO, S O , , hl,O,. clc., u s u a l l y 018 a ‘loss [ICC” b a s i s . i.e., allcr allowing, fnr ihc Ins5 ill wciglrl cvcnlually crpcricflcut rluc l o tlcslruclion o f clrbonalcs. clc.. during Iml lrcnlillcnl.)
“\‘01,A~1‘11.1~~S5”
ANI) hllXTIIANIShlS
01; VOI.A’1‘II,ISA-I’ION
‘1.11~ pritlcitd volnlil~ ~ICIIIC~IS arc K, N;I, Ct. S. I n Ihc cast of’ r a w nnlcrints, ccrlain sutrur cor~~t~~ur~d~ (rulfirtcs or ort:anics) c:ul rratlily clcco~~rt~sclvolnlilisc Indow Goo”C, bul IllOSl vdalitc co~ntmunds in raw inalcriids o n l y c v a p o r a l c pnrlially and a l higtlcr Icnitxralurcs as 111~ reed pa5scs low;rrtls llrc k i l n hrrnilig xonc. ‘I’hc rcsitluc rcmins i n lhc producl. cilllcr i n s o l i d ~cd~~iou ill IIIC plincitd phscs ol IIIC clinker o r as cliscrcic cocrqm~ncls. Whilst alnml all fcctl cllloritlr. w i l l cvatwmlc, lcs.scr aniounls 0r ollicr c o m p o u n d s do s o , wliilsl in corrlr:i’tl. liitd vcrlalilrs arc :ilii~o’;l ;ilw;iys ciilircly cvnpor;ilcd during cmiiIiIislion. lfv;rtKrrarrd volarilc~ rravcl hack up 11rc kiln wilt, 111c comlm~lion irmrt;;rnic comt~ountl’; (lilrcr;rlirrg I:~lwl Iical): i)
ii)
iii)
on llic Ucctl.
li)rmitlg lhc his
Or
n rccirculaliiig
eases and cordcrrsc a
inlcrnal votnlilc
s
load
n ;I lirrc rlrrsl r>r rcllrlc which i s fin;llty Irappcd i n lhc g a s clrxning s y s l c m o r r a w m i l l arrtl hx~mcs p a r t or an cxlcrilal vnlalilc c y c t c . a s 111151 i s parlly o r d$ly rl’lurncd ir1 tlrc sysic111 011 coltlcr strrl:rccs
ill lhc syslcrii. linming
llic Ixrsis
Or
hiltl~ups.
I’rcssurrs 10 cxploil cvcr more marginal rcscrvcs 0r r a w ni3lcrinls a n d fuels give rise l o irrcrcxirrg I;rmili;rrily willr llrc drm o f vohlilc spccics 011 process pcrlormarrcc. Wtrcri corrtlcrrsctt volalilcs rclurrr low;lrtls Ihc b u r n i n g zorrc, clctxriding o n Ihc overall clrcmicnl cr~nrliliorls and hrnint: ccrndiliorrs, llrry form a range or votalilc cnmpunds wlricll tlrcrr0clvcs cvapw;llc parli;rlly nri0 llrc cyctc only lids an utuilibriuni when ltx loiat quaillily Icarirrt: llrc syslcm (itI clirrkcr ;md mm-rclirrrrcd tlusl) equals ltral cnlcrirrc lhc s y s l c m . Alk;rlix
;md ~IIII~IICS ctrl~.rirrt! llrc trrrrrrirrp forrc irr trraclirc I:rrt:cly I’rlrrri :I \qr;lr;llc mr~llcr~ t’ll;lr<, iiumiscilrlu wills Ilrc prLicip;rl fcrrilc Ilux. ‘Tlrc lrvct or volalilcs irr rccircul;rliorr i s sit;rrilic;mlly grcalzr ll1911 lhcir lol;d ralc or irrlroducliori am1 lhc s~rbsln~~ccs it1 llrc vapour ptrasc’ cnri 115 irr v;rrious slalcs or dissoci:llion a n d recomlrinalion. III gcncml, Itrc prclcrrd clrloritlc corrrt~rmnd i s txrl:rsrium cllloritlc and only wlicrr lhcrc i s an cxccss I I I clrloriric lor cOcmic;rt crmrlrimrliori will) pol;lssium w i l l sodium ctrtoritlc lx hrmcd as : I rccircutalirrg vohlitc strxics. Alk;tli sulhlcs (Na2S0.,. K,SO,t cvatxxatc cony,rucnlly, tlisnprxzlring c n l i r c l y wticri Ilr;rlctl Ihr a loiig period. Cn.S04 ttcconiposcs arid hvcs rcsidu:d CnO (ill, trxitlising contlilior~st. s o lh:il CaO:CnS04 niclls c a n rorcll. T y p i c a l rccirculalirrg vrhtilr lrds rxtlrcssctl a3 XI ul’ llrc 1~1lal quimlily inlrotluccd a r c a s roilOws:
5llll;llc
(‘lrlrrridr(s) K-0
%
XXKt XX) . rLv1
Na20 SOJ
1.
IS0 - 200 2uo . R(X)
Ic!“u.utr,r = 0
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*(se3 ‘p!nbg
.~JlS!UJJqJ 3J!lS!JXJUeqJ [XI-J ‘So’U!llJS JJUlq/JUl2[~ %J!UJflq pJ[[OJlUO3 tunq 1~0s
+~JX![ZJJU+I iy S:X~U pg SNOil3V
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~cmpcraturc in a WCI prnccsr k i l n , a considcrablc dab-bank c l I t.-6urcrncnls 1115 been buill up. The USC o f lnwrr a s h rucl lowcrcd c l i n k e r K,O l c v c l b y O.lS%. dcspilc 111~ inIroduclior1 of n IiIIlc mnrc p&ssium IO lhc s y s t e m ( a n cxlra 0 . I 16 o n clinker). T h e dusl - rclurncd IO lhc kiln - had bccomc more rich in alkalis, so lhal Ihe proportion or K,O broughl in by solid fuel fell lrom 2 4 % I O IX% while ihai brough1 i n b y dust rcIIIrn rose from 18% I O 2 9 % . This s~rp.gcs~s 1ha1 KzO incotporaIion in clinker no1 only dcpcnds on 1hc quan1ily inlroduccd 11u1 also . a n d almvc all clsc _ o n IIIC type of malcrinl w h i c h b r i n g s i1 i n a n d perhaps o n 111~ pnsiIimr whcrc i1 is injccIctl. On screening 111~ clinker al 20 mm. chcmic~l analysis showed a K,O coriccnlralinr~ snmc IO% Irighcr i n Ihc cn;~rsc f r a c t i o n . The clinker a l k a l i COIIICIII has s~~cccssli~lly been rctlucccl i n lrials b y c a l c i u m chloride atldilinri nl l h c Ilamc.
Cnsc 6: A hil:h chlnridc coal ( - 0. I5 % Cl) can only bc ~isctl as a mix will1 anolhcr coal lo avoid Imild-ullc wilh Iyliically 2 % Cl a1 Ihc ImI1nm Or cyclone 4 (lhc lowcsl in lhc prchcalcr ~nwcr). hn;rlyTis of Inriltl~ul~s along IIIC kiln indica1ul chlnridc lc,vclr up IO 30% (al zero loss nn ignirinn) in 111~ coaIing Iron1 lhc base of cyclone 2 and 20% a1 SO 111 inlo Ihc kiln (tlcspilc iit hs’i ih:m 5 % lrvcl i n Imllr lmi :md rold p;nI’i 0r Ilic k i l n ) .
1m1 Inr some weeks will1 I h c objccIivc or reducing b u r n i n g ronc b y - p a s s . ICSIS WL volaIili.~~tinn b y p l a y i n g o n process paramcIcrs a n d prnducing a IighIly mincraliscd clinker will1 higher volalilc rclcnlion. D u r i n g changcovcr ~hcrc w a s some I c n d c n c y l o rnrrn sol1 build-ups in Ihc prchca~cr, bu1 wiIh ihc new regime csIablishcd ihcsc movctl lowards lhc kiln fcul chu~c wiIhou1 c a u s i n g m a j o r problems for k i l n opcniion. (EvidcnIly lhcrc a r c phcrIon1cna of boll1 shorI-Icrm a n d l o n g - I c r m sIabiliIy: o n c e a slablc b u r n i n g zone volaIili.saIion i s csIablishcd, in lakes Iimc f o r sIablc condiIions I O arrive higher u p the syslcm and in Ihc Iargc masses or malcrial in lhc build-ups and coalings already in cxislcncc.) The apparcn1 b u r n i n g ~nnc IcmpcraIurc w a s rcducul b y almu~ 1 2 0 ° C . while K,O volalilily droplwl IrIlm 70% III 60% i n 111~ b u r n i n g ~onc a n d IhaI ol S O , Iron1 RO% Iowartls lhc ranl:c SO% I O GO%, provitlcd lhnl k i l n cxi1 oxygen l c v c l w a s kcp1 a b o v e 2 % . Thcrc wcrc in~provcn~cnIs in kiln outpul raIc and rucl consumpIion and lhc cxpcrimcnlal Works adoplcd ccrlain Or Ihcsc changes during normal nl~ralinn rOr scvcral years. TCSIS wcrc c;Irricd 0111 invnlving variotrs N O , lcvcls ( t o Cnsc I I: IcmpcralrIrc) a s well as chcmic~lly rcrhlcing rh0C contliliruIs. so, :
C:1w 7:
Atldinl; a sccontl ln~~hc;l~cr c y c l n n c s~agc IO a l o n g d r y process k i l n ( a varinrIt of 1hc I:igutc I prnccss, will1 a higher k i l n IcngIh/dian~cIcr ralio a n d a single cyclone slagc nhnvc i1) yicldcd vnrimrs build-up problems. To resnlvc ~hcsc, IIIC kiln gas cxi1 0, lcvcl was iricrcA5rccl rrorll 0.5 In I .5 %, solid rd rcsiduc al 90 microns was rcduccd lo below 25 % and scvcral cmr~l~r~ss~d air “blaslcrs” wcrc inslallctl lo dislodge malcrial rrom lhe lower regions oC 111~ prchcnlcr. T h c s c acrions improved IIIC siIuaIior1 and sul~scqI~cnIly addiIional mcasurcs wcrc Iakcn: rcl~/l;iory slirrcrs wcrc addul l o I h c l i n i n g near the k i l n b a c k end, hurncr a i r vcltXiIy w a s incrcawd. a n d a “non-slick” lining was inslallcd in 1hc kiln cxi1 gas duel and cycl011c dip-lubes. ‘I’hcsc Crrm mntlc bcllcr ouIliu1 ralcs pnssihlc wilhoul b u i l d - u p s . IIc~cnIly. a Iiil;hcr snll’ur rd hlcntl 113s been nscd, :~ccomp;mictl b y s l a g (S - I X) :Imong lhc r;~w niix coml~~ncnIs. I’rchcaIcr blockage problems rccurrcd, bu1 b u i l d - u p s c;m be avoidctl if the SO, l c v c l i n snmplcs t a k e n Irnm the k i l n cnlry m a l c r i a l i s kcp1 b e l o w 2.5% b y lirnilirig lucl S conlcnl and sl;ig use i n lhc r a w m i x , provided lhal i n addilion l h c oxygen lcvcl at IIIC kiln back cm1 is kcp~ consis~cn~ly a1 or above 2%. Cnmpui~r c0nir01 or ihc kiln Iiclpr I n achicvc succc’i’i. rctlucinl: lhc vari:rhilily ol l h c 0, s i g n a l .
cxw Ii:
I ;~li~n:ih~ry tl:11:1 on minor clcnic111s cr;~n~plc, suITur vnl;rIiliIy i n ;I s~;~rrtlarrI rcginrc 0, OUI I~lls in ihc prcscncc Or 0,; r~~~~rihclcss a l I2W”C. The volalilily 0r m i n o r c l c m c n l s powdcrrtl m a l c r i a l lhan rOr grarIulcs.
conlirm clvccls crlrxcrvr.tl ill ln:tclicc. I:rlr (70% N,, 3 0 % C O ) i s close I O 100% a1 0% ihc CrrCd Or 0, is much ICSS al 1400°C lhan i n l h c laboralory i s a l s o m u c h grcalcr r0r
K,O a n d Na,O :
indicalc
llamc
‘I’hc r a l i o or S O , i n Slngc I V I n S O , i n r a w iri~l varied Iypically Iron1 I.R IO 2.7 lnr 111~ higher lcvcls ol NO., and was 3 . 0 [or a IOW 0, Icvcl. The clinker S O , c~nlcnl fell. In a parallel manner. for K,O 1hc ralin of l h c c o n l c n l i n S~ap,c I V IO Iha i n r a w meal varictl from 3.X IO 4 . 4 a n d rnr NazO rrom I .6 IO 2.0.
In gcncral, rultrcing cnndiIions a lower clinker SO,.
incrcascd SO, lcvcl a1 Slagc IV by a
hClOr 0r
2, also giving
CXSC 1 2 : S O , 11~s l~ccn tmmit~rcd ;II lhc kilt1 Iwck cd IO dclcrwiw hrc:~l rules for avoitlinl: hltKk:igc Icndcncics. ‘I’hc S O , sil:n:rl i s ncpisy :~nd dillicull l o inlcrltrcl willroul a krtowlAl:c or IhC hiSlWy Or IllC SySlCltl. C.G.. a rcccn~ brcak;rway of sulfalc b u i l d - u p m a l c r i a l a r r i v i n g i n IIIC lmrning ~nnc c a n give a h i g h S O , s i g n a l a1 111~ k i l n back end dcspilc tlrc prc~cncc ol Icr (,I k i l n >I,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rn ill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’ :rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)
l3TECl~S
OF
CONDENSATION
Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui 0 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul I . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC C O was tlc~cc~cd, Ihcrc w a s abnu1 2% S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc (2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or cliloridc-linsul tlclx,siIs.)
The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i l n ( g a s ) cxi1 dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion and h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc) ‘I’hc p l a n 1 p$omclry and IllC a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases. Cnr qrrcnching a n d scparalc dc-tluslirr~ l o par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.
Cnsr In: T o c x n m i n c IIIC rcnsibiliry oC p r o d u c i n g a sulraic-rich clinker
I n
wiIhou1 insInlling
a
lhc pasl. s c v c r a l empirical
limils have been l~roposcd
rOr
conccnlralions
d
volnlilcs
~cmpcraturc in a WCI prnccsr k i l n , a considcrablc dab-bank c l I t.-6urcrncnls 1115 been buill up. The USC o f lnwrr a s h rucl lowcrcd c l i n k e r K,O l c v c l b y O.lS%. dcspilc 111~ inIroduclior1 of n IiIIlc mnrc p&ssium IO lhc s y s t e m ( a n cxlra 0 . I 16 o n clinker). T h e dusl - rclurncd IO lhc kiln - had bccomc more rich in alkalis, so lhal Ihe proportion or K,O broughl in by solid fuel fell lrom 2 4 % I O IX% while ihai brough1 i n b y dust rcIIIrn rose from 18% I O 2 9 % . This s~rp.gcs~s 1ha1 KzO incotporaIion in clinker no1 only dcpcnds on 1hc quan1ily inlroduccd 11u1 also . a n d almvc all clsc _ o n IIIC type of malcrinl w h i c h b r i n g s i1 i n a n d perhaps o n 111~ pnsiIimr whcrc i1 is injccIctl. On screening 111~ clinker al 20 mm. chcmic~l analysis showed a K,O coriccnlralinr~ snmc IO% Irighcr i n Ihc cn;~rsc f r a c t i o n . The clinker a l k a l i COIIICIII has s~~cccssli~lly been rctlucccl i n lrials b y c a l c i u m chloride atldilinri nl l h c Ilamc.
Cnsc 6: A hil:h chlnridc coal ( - 0. I5 % Cl) can only bc ~isctl as a mix will1 anolhcr coal lo avoid Imild-ullc wilh Iyliically 2 % Cl a1 Ihc ImI1nm Or cyclone 4 (lhc lowcsl in lhc prchcalcr ~nwcr). hn;rlyTis of Inriltl~ul~s along IIIC kiln indica1ul chlnridc lc,vclr up IO 30% (al zero loss nn ignirinn) in 111~ coaIing Iron1 lhc base of cyclone 2 and 20% a1 SO 111 inlo Ihc kiln (tlcspilc iit hs’i ih:m 5 % lrvcl i n Imllr lmi :md rold p;nI’i 0r Ilic k i l n ) .
C:1w 7:
Atldinl; a sccontl ln~~hc;l~cr c y c l n n c s~agc IO a l o n g d r y process k i l n ( a varinrIt of 1hc I:igutc I prnccss, will1 a higher k i l n IcngIh/dian~cIcr ralio a n d a single cyclone slagc nhnvc i1) yicldcd vnrimrs build-up problems. To resnlvc ~hcsc, IIIC kiln gas cxi1 0, lcvcl was iricrcA5rccl rrorll 0.5 In I .5 %, solid rd rcsiduc al 90 microns was rcduccd lo below 25 % and scvcral cmr~l~r~ss~d air “blaslcrs” wcrc inslallctl lo dislodge malcrial rrom lhe lower regions oC 111~ prchcnlcr. T h c s c acrions improved IIIC siIuaIior1 and sul~scqI~cnIly addiIional mcasurcs wcrc Iakcn: rcl~/l;iory slirrcrs wcrc addul l o I h c l i n i n g near the k i l n b a c k end, hurncr a i r vcltXiIy w a s incrcawd. a n d a “non-slick” lining was inslallcd in 1hc kiln cxi1 gas duel and cycl011c dip-lubes. ‘I’hcsc Crrm mntlc bcllcr ouIliu1 ralcs pnssihlc wilhoul b u i l d - u p s . IIc~cnIly. a Iiil;hcr snll’ur rd hlcntl 113s been nscd, :~ccomp;mictl b y s l a g (S - I X) :Imong lhc r;~w niix coml~~ncnIs. I’rchcaIcr blockage problems rccurrcd, bu1 b u i l d - u p s c;m be avoidctl if the SO, l c v c l i n snmplcs t a k e n Irnm the k i l n cnlry m a l c r i a l i s kcp1 b e l o w 2.5% b y lirnilirig lucl S conlcnl and sl;ig use i n lhc r a w m i x , provided lhal i n addilion l h c oxygen lcvcl at IIIC kiln back cm1 is kcp~ consis~cn~ly a1 or above 2%. Cnmpui~r c0nir01 or ihc kiln Iiclpr I n achicvc succc’i’i. rctlucinl: lhc vari:rhilily ol l h c 0, s i g n a l .
cxw Ii:
I ;~li~n:ih~ry tl:11:1 on minor clcnic111s cr;~n~plc, suITur vnl;rIiliIy i n ;I s~;~rrtlarrI rcginrc 0, OUI I~lls in ihc prcscncc Or 0,; r~~~~rihclcss a l I2W”C. The volalilily 0r m i n o r c l c m c n l s powdcrrtl m a l c r i a l lhan rOr grarIulcs.
conlirm clvccls crlrxcrvr.tl ill ln:tclicc. I:rlr (70% N,, 3 0 % C O ) i s close I O 100% a1 0% ihc CrrCd Or 0, is much ICSS al 1400°C lhan i n l h c laboralory i s a l s o m u c h grcalcr r0r
Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui 0 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul I . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC C O was tlc~cc~cd, Ihcrc w a s abnu1 2% S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc (2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or cliloridc-linsul tlclx,siIs.)
1m1 Inr some weeks will1 I h c objccIivc or reducing b u r n i n g ronc b y - p a s s . ICSIS WL volaIili.~~tinn b y p l a y i n g o n process paramcIcrs a n d prnducing a IighIly mincraliscd clinker will1 higher volalilc rclcnlion. D u r i n g changcovcr ~hcrc w a s some I c n d c n c y l o rnrrn sol1 build-ups in Ihc prchca~cr, bu1 wiIh ihc new regime csIablishcd ihcsc movctl lowards lhc kiln fcul chu~c wiIhou1 c a u s i n g m a j o r problems for k i l n opcniion. (EvidcnIly lhcrc a r c phcrIon1cna of boll1 shorI-Icrm a n d l o n g - I c r m sIabiliIy: o n c e a slablc b u r n i n g zone volaIili.saIion i s csIablishcd, in lakes Iimc f o r sIablc condiIions I O arrive higher u p the syslcm and in Ihc Iargc masses or malcrial in lhc build-ups and coalings already in cxislcncc.) The apparcn1 b u r n i n g ~nnc IcmpcraIurc w a s rcducul b y almu~ 1 2 0 ° C . while K,O volalilily droplwl IrIlm 70% III 60% i n 111~ b u r n i n g ~onc a n d IhaI ol S O , Iron1 RO% Iowartls lhc ranl:c SO% I O GO%, provitlcd lhnl k i l n cxi1 oxygen l c v c l w a s kcp1 a b o v e 2 % . Thcrc wcrc in~provcn~cnIs in kiln outpul raIc and rucl consumpIion and lhc cxpcrimcnlal Works adoplcd ccrlain Or Ihcsc changes during normal nl~ralinn rOr scvcral years. TCSIS wcrc c;Irricd 0111 invnlving variotrs N O , lcvcls ( t o Cnsc I I: IcmpcralrIrc) a s well as chcmic~lly rcrhlcing rh0C contliliruIs. so, :
K,O a n d Na,O :
indicalc
llamc
‘I’hc r a l i o or S O , i n Slngc I V I n S O , i n r a w iri~l varied Iypically Iron1 I.R IO 2.7 lnr 111~ higher lcvcls ol NO., and was 3 . 0 [or a IOW 0, Icvcl. The clinker S O , c~nlcnl fell. In a parallel manner. for K,O 1hc ralin of l h c c o n l c n l i n S~ap,c I V IO Iha i n r a w meal varictl from 3.X IO 4 . 4 a n d rnr NazO rrom I .6 IO 2.0.
In gcncral, rultrcing cnndiIions a lower clinker SO,.
incrcascd SO, lcvcl a1 Slagc IV by a
hClOr 0r
2, also giving
CXSC 1 2 : S O , 11~s l~ccn tmmit~rcd ;II lhc kilt1 Iwck cd IO dclcrwiw hrc:~l rules for avoitlinl: hltKk:igc Icndcncics. ‘I’hc S O , sil:n:rl i s ncpisy :~nd dillicull l o inlcrltrcl willroul a krtowlAl:c or IhC hiSlWy Or IllC SySlCltl. C.G.. a rcccn~ brcak;rway of sulfalc b u i l d - u p m a l c r i a l a r r i v i n g i n IIIC lmrning ~nnc c a n give a h i g h S O , s i g n a l a1 111~ k i l n back end dcspilc tlrc prc~cncc ol Icr (,I k i l n >I,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rn ill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’ :rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)
l3TECl~S
OF
CONDENSATION
The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i l n ( g a s ) cxi1 dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion and h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc) ‘I’hc p l a n 1 p$omclry and IllC a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases. Cnr qrrcnching a n d scparalc dc-tluslirr~ l o par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.
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(fmm raw materials1
FIGURE 3 OXIDISED KILS SULFL.iR CYCLE ( . . . . . . . . . vapour phase: -solid phase)
FIGURE 3 OXIDISED KILS SULFL.iR CYCLE ( . . . . . . . . . vapour phase: -solid phase)
(fmm raw materials1
‘1‘115 cffccls o n cliukcr
iuay be sutntnariscd
as follows:
I’luxinl:
aclioii:
0 l 6 0
l o w e r lcmperalure o f firs1 l i q u i d phase f o r m a t i o n cbangc al’ l i q u i d viscosily allcraliun 0r sdxx Iciisioii OK l i q u i d inihlilic;il~o~i ul’ crystal ulorlhology.
c o m b i n e d williin Ihc silic; .md aluminalss o r as auliydrirc (CaSO,). which d~s~I~cs umrc slowly III~II a l k a l i sulf’ales, whilst llle fraclions of K,O aud NazO which are s o l u b l e ill w;ucr approach I .O ;mJ 0.S. rcspxiivsly. al a raliu OI’:IIJOUI I.S. Al sitll’alc:;ilk:ili raliiJ9 ahvc I .S Ircntls a r c wmewl~al crralic. I’ur IWSI uoru~l clinkers IIW prilicip4 rulli~~c phase w i l l bc alhil~italilc (Iwlassitlrri/roditr[rl s u l f a t e ) with a m a x i m u m WN;I r a t i o o f 3 . 0 . ‘I‘l~is IIllasc i s accompanied b y m i n o r quaniiiirs o f K,SO, a n d c a l c i u m Iangbsiniic. Na$O, bciug hmd o n l y Ibr u n u s u a l l y l o w K/Na raiios. A s w e l l a s lhe s o l i d solulion rffecls a n d the f o r m a t i o n o f compou~ids d e s c r i b e d almvr, v a r i o u s permulaliotis o f volalilcs ( e s p e c i a l l y in Itie p r e s e n c e o f f l u o r i n e ) c a n influcrux rhc s~ructurs arid belnviour o f aliic aml bcliir crysials ( M o i r & Classtx, l!J92).
I’l~ass rclaliurls:
I lydraulic l
aclivily:
lhe fL’;iclivtlics 01’ lhc cliukcr u~immls arc allcrcd b y sul~d mlulion ;uldlor by the cffccrs o f crystal syinrilctry (IQli leu~pcralure stabilization o f poly~norphs) arid/or cl’fucls occurritrg during liydraliotl (e.g., coaling of cc~uenl particles by insolubly salis).
( I I i s d i f f i c u l t lo isolale
Ihcsc Ihrcc classes of cffccl i i i praclics.)
I II gcuc~al. incorporalioo of a d d i t i o n a l clillkcr solC;~~e it1 a situatioll willi e~.ccss a l k a l i s y i e l d s a inore dilficuli “iiIlI>ilfCl\l grithhbilily” with adviiolagcs i n the Iua~kcl or hlmvctl early concrclc s1rc11g111 a n d workabilily. I’IIc cf~cc~r or utimJr ~WII~~II~IIIS WI the vircosily a n d shrlace lensioll of l i q u i d phascr c a n be cou~plrx. Luwcr viscosiliss e n c o u r a g e alilc ( c a l c i u m lrisilicale) forinalion. Calciuiu SUIT;IIL flux call. I~ow~!vsr. stabilise bclilc ( c a l c i u m disilicatc) antll~r CatIsc lhr prodtrcliori o f clinker alilc will1 lime i n c l u s i o n s . I:urlher, iu clinkers with a l o w alk;lli CWICIII, bclilc sI;tibilis;dicm due III cxccts S O , Icxls I~I tlilliuulr cl,lnl,iii;ll,ilily. S~rtm~ly c h e m i c a l l y fcducir$ comJilioiis I I I Ilic burriiug LOII(: cau Live a camcril will! par Ilow cliaiaclcrislics (tluc lo I’rsc K;,O a n d N+O), p o o r workhility ( d u e IO IIIC i n c r e a s e d conlcnl o f Iricalcium aluminalc arid ils rraclivily), poor slrcuglh (lower lricalcium silicalc conlenl) a n d Vilriill)lt colour. A l k a l i s rclainrd i i i clinker a r e prcscm cilhrr a s slablr sulhles o r iibSlllbd i n llld silicate arid alulnitialr siruc~urcs: ihcsc inlltrencc IIIC bchaviour o f fresh cor~crele a n d m o r t a r d u e l o Ilieir N;r20 hi\s a I~\WC matkcd ~ende~\cy than K,c) 10 ~OIIII wI~II~IIS ill various solubililier. I:or clinkers with (ruular) ratios of sulfiilc:lol;~l alkalis below 0.3, iIIItloSl c a l c i u m alurnimilr. a l l 111e sulh~e i s combimxl ill waltz soluble form, K$O,, beill& prcddomimmt. A pruporlion ol’ the a l k a l i s ale i n s o l i d soluiiou itI IIIC c l i n k e r aluciiinalr phase and lhis has ati advorle rlfccl ori the inirial cc~~~cnI rcaclivily illld Illus o n comxele a n d morlaf r h e o l o g y . For ralios belwceii 0 . 5 a n d I .O, a c e r t a i n quanlily of langbrinilc i s a l s o formed (and no1 a l l I:or r a t i o s alrove 1.0, si~nilicanl f r a c t i o n s o f lhr Sulfales a r e lltr a l k a l i s a r e wlul~lc).
II is gc~icf;~lly ~.up~~‘xd IhI (t~llicr Ihcl~~rs ljcitlg cqii;~lJ ll~c cxlcul 1,1’ vt8I:tlili*:tlit~ll ~~L,I~:I\c’I a s the Iliermal efficiency a l IIIC kiln iucrcascs. ‘I’liis i s prol~ably du1: Iu iltc lillliliug sllUci o f vapur wluralion b y a l k a l i c o m p o u n d s , a s c o n f i r m e d b y sludics of IIIC irca~msm o f k i l n dusl i n a I00 IIII~ diarnslcr hidiscd bed (‘l’clinnr rf ul, 1978) lo e x a m i n e ihc fcasibili~y o f produciril: cliriker ~~OIII WIII~III kilt1 flue dull will1 caplure of IIIC a l k a l i s distilled f r o m 111l: b e d for p o s s i b l e use ill 111e ferliliscr i n d u s t r y .
I I i s su~gerlcd IIuI
!II V’
v*
i;i
p*
=
J’ M,
= = =
M,
= -me..llilllM,1-.-. (1' - 1") WJ
salurhtd vapour coiicciilralioii wluralrd vapour p r e s s u r e OT au a l k a l i compouml g a s pcssur~ niolrcul;ir wcigld III’ valmiir m o l e c u l a r weigh1 o f g a s .
i i i Irai~sporl pscs ( k g / k g ) 1 ) ~:IIIIC imils 1
Give0 malluzmalical cxprcssiims f o r S;~lt~rillI2~l vapour prcssltrcs ilS ~m~ciicms of lc~llpcraluru a n d krmwlcdpz of kill) systru~ Icmpcralurc p r o f i l e s , the w~ura~cd val)uur conccmraiioll WI be calculaled f o r exh a l k a l i compout~d a n d IIIIIS he maximurn qu;miiiits cval~ra~cJ f r o m llle feed lxx unit IllilSS of g a s e s . ‘I’lieri, considcrirlg the amoums ol’ gas passirlg Il~rou~:h lhc kih al v a r i o u s temperatures, llx Iruc llllillllily ol volaiilcs Irar~sp~rlud per uuii m a s s o f c l i n k e r GUI be calc~tla~txl arld f r o m his krtowlcdp, ‘ideal* volaiilc cycles CarI lx tlsduccd. For example, saturated vapour KCI K,SO, Na$O,
pressures at IZOO’C
0. I8 am 0 . 8 x lo” illIll 0 . 1 3 x IO’] alIll
are (for IIIE
prrc
substances):
(0.6 x IO’j am with dccolnpition suI~Imssec1) (0.01 x IO” ~IIII with d e c o m p o s i t i o n s u p p r e s s e d )
c o m b i n e d williin Ihc silic; .md aluminalss o r as auliydrirc (CaSO,). which d~s~I~cs umrc slowly III~II a l k a l i sulf’ales, whilst llle fraclions of K,O aud NazO which are s o l u b l e ill w;ucr approach I .O ;mJ 0.S. rcspxiivsly. al a raliu OI’:IIJOUI I.S. Al sitll’alc:;ilk:ili raliiJ9 ahvc I .S Ircntls a r c wmewl~al crralic. I’ur IWSI uoru~l clinkers IIW prilicip4 rulli~~c phase w i l l bc alhil~italilc (Iwlassitlrri/roditr[rl s u l f a t e ) with a m a x i m u m WN;I r a t i o o f 3 . 0 . ‘I‘l~is IIllasc i s accompanied b y m i n o r quaniiiirs o f K,SO, a n d c a l c i u m Iangbsiniic. Na$O, bciug hmd o n l y Ibr u n u s u a l l y l o w K/Na raiios. A s w e l l a s lhe s o l i d solulion rffecls a n d the f o r m a t i o n o f compou~ids d e s c r i b e d almvr, v a r i o u s permulaliotis o f volalilcs ( e s p e c i a l l y in Itie p r e s e n c e o f f l u o r i n e ) c a n influcrux rhc s~ructurs arid belnviour o f aliic aml bcliir crysials ( M o i r & Classtx, l!J92).
;uldlor
r
iuay be sutntnariscd
as follows:
lcmperalure o f firs1 l i q u i d phase f o r m a t i o n al’ l i q u i d viscosily un 0r sdxx Iciisioii OK l i q u i d l~o~i ul’ crystal ulorlhology.
by
II is gc~icf;~lly ~.up~~‘xd IhI (t~llicr Ihcl~~rs ljcitlg cqii;~lJ ll~c cxlcul 1,1’ vt8I:tlili*:tlit~ll ~~L,I~:I\c’I a s the Iliermal efficiency a l IIIC kiln iucrcascs. ‘I’liis i s prol~ably du1: Iu iltc lillliliug sllUci o f vapur wluralion b y a l k a l i c o m p o u n d s , a s c o n f i r m e d b y sludics of IIIC irca~msm o f k i l n dusl i n a I00 IIII~ diarnslcr hidiscd bed (‘l’clinnr rf ul, 1978) lo e x a m i n e ihc fcasibili~y o f produciril: cliriker ~~OIII WIII~III kilt1 flue dull will1 caplure of IIIC a l k a l i s distilled f r o m 111l: b e d for p o s s i b l e use ill 111e ferliliscr i n d u s t r y .
I I i s su~gerlcd IIuI
!II V’
: Calciuiu
v*
i;i
p*
=
J’ M,
= = =
vily:
clivtlics 01’ lhc cliukcr u~immls arc allcrcd b y sul~d mlulion ccrs o f crystal syinrilctry (IQli leu~pcralure stabilization o f poly~norphs) cl’fucls occurritrg during liydraliotl (e.g., coaling of cc~uenl particles by y salis).
ale
M,
o
of a d d i t i o n a l clillkcr solC;~~e it1 a situatioll willi e~.ccss a l k a l i s y i e l d s grithhbilily” with adviiolagcs i n the Iua~kcl or hlmvctl early d workabilily.
>ilfCl\l
nker a r e prcscm cilhrr a s slablr sulhles o r iibSlllbd i n llld silicate arid ihcsc inlltrencc IIIC bchaviour o f fresh cor~crele a n d m o r t a r d u e l o Ilieir N;r20 hi\s a I~\WC matkcd ~ende~\cy than K,c) 10 ~OIIII wI~II~IIS ill I:or clinkers with (ruular) ratios of sulfiilc:lol;~l alkalis below 0.3, iIIItloSl combimxl ill waltz soluble form, K$O,, beill& prcddomimmt. A pruporlion n s o l i d soluiiou itI IIIC c l i n k e r aluciiinalr phase and lhis has ati advorle
salurhtd vapour coiicciilralioii wluralrd vapour p r e s s u r e OT au a l k a l i compouml g a s pcssur~ niolrcul;ir wcigld III’ valmiir m o l e c u l a r weigh1 o f g a s .
i i i Irai~sporl pscs ( k g / k g ) 1 ) ~:IIIIC imils 1
Give0 malluzmalical cxprcssiims f o r S;~lt~rillI2~l vapour prcssltrcs ilS ~m~ciicms of lc~llpcraluru a n d krmwlcdpz of kill) systru~ Icmpcralurc p r o f i l e s , the w~ura~cd val)uur conccmraiioll WI be calculaled f o r exh a l k a l i compout~d a n d IIIIIS he maximurn qu;miiiits cval~ra~cJ f r o m llle feed lxx unit IllilSS of g a s e s . ‘I’lieri, considcrirlg the amoums ol’ gas passirlg Il~rou~:h lhc kih al v a r i o u s temperatures, llx Iruc llllillllily ol volaiilcs Irar~sp~rlud per uuii m a s s o f c l i n k e r GUI be calc~tla~txl arld f r o m his krtowlcdp, ‘ideal* volaiilc cycles CarI lx tlsduccd.
Ihcsc Ihrcc classes of cffccl i i i praclics.)
~WII~~II~IIIS WI the vircosily a n d shrlace lensioll of l i q u i d phascr c a n iscosiliss e n c o u r a g e alilc ( c a l c i u m lrisilicale) forinalion. ~!vsr. stabilise bclilc ( c a l c i u m disilicatc) antll~r CatIsc lhr prodtrcliori o f i n c l u s i o n s . I:urlher, iu clinkers with a l o w alk;lli CWICIII, bclilc cxccts S O , Icxls I~I tlilliuulr cl,lnl,iii;ll,ilily. S~rtm~ly c h e m i c a l l y fcducir$ rriiug LOII(: cau Live a camcril will! par Ilow cliaiaclcrislics (tluc lo I’rsc r workhility ( d u e IO IIIC i n c r e a s e d conlcnl o f Iricalcium aluminalc arid lrcuglh (lower lricalcium silicalc conlenl) a n d Vilriill)lt colour.
= -me..llilllM,1-.-. (1' - 1") WJ
For example, saturated vapour
Sulfales
are
KCI K,SO, Na$O,
pressures at IZOO’C
0. I8 am 0 . 8 x lo” illIll 0 . 1 3 x IO’] alIll
are (for IIIE
prrc
substances):
(0.6 x IO’j am with dccolnpition suI~Imssec1) (0.01 x IO” ~IIII with d e c o m p o s i t i o n s u p p r e s s e d )
‘I’lrc Na
I~;III~IKJ~( capacity
14 air lo; vapours al lZ(XJ”C i cqj;lcily nl I?S(l”C i s ;IIJ~JIII
~SO,, < 0.S 1:/g. ‘I’hc
s
II cmi IljcrcInrc hc Iurcsccn Ilral (unless llic equilibrium vapour prcssurcs difCcr grally Irom urlur;jlrA values) lbcrc w i l l b c lilllc problem i n removing KCI from m a n y k i l n lluc dusls in a lluitliscd IJC~ witli a g a s llow ralc o f , s a y , 2 g per grammc o f dust, idll10ugb Ibe c a p a c i t y C0r sdhk removal m a y b c limilcd. The sa111c reasoning a p p l i e s 1 0 k i l n s , will1 wcl process kilns typically slmwinl: n ratio rjf a lilllc less 1ljan 2 g/g gas/solids in ~ljc burning z0rjc and ‘pcrljapr 2 . 7 5 l;Ig ;II IIW b a c k end. will1 corrcsl~nding values C0r lljc d r y prtxzcss (wilboul I)rcc;jlcitl;lliolj) Or I .4 ~$1; and I .91 g/g. I)c\pilc IIIC lacl IIWI qualilalivc diCCcrcr1ccs arc rrllcclctl in s:jmplc calculations, Suclj IimcT lnrjicr lljn~i tllosc cricounlcrcd i n
I~c~wcc~~ I W O k i l n s (OIJC d r y process a n d OIJC wet) “idcal” calcolalctl rccirculalinl; loads arc aboul IO praclicc. TIJC probable reasons include:
3)
In~~~w~~lclr ct~~~l:wl Iwlwccn g:iws :uul s o l i d s i n llir k i l n . wlicrc o n l y a sii1;1ll I’raclitui 01 lltc s o l i d surracc i s cxlxjsrd a l ; I given lime. ( I I i s cxpcclcd llial Ibcrc i% bcllcr coril;~cl i n llic cc~lrlcr dusly rqiojis Or lbc syslct~~.)
11)
Vul:llilis:llion c:b;lr;lclcrislics of IIIC alk:lli-ccrlJl;lirliIlg IniItcritIs ;,I a givcu IJI;IIJI. i.c., conrljinnliorl i n m0rc co~~iplcx silicalcs a n d aluminalcs. 1~01 o n l y sijihks.
0r co~~jp~rintls
wljicb arc condcnscd as/on solid dusl or
rtl0lc.
cl
I’raijspfjri
(1) Cl
I~~Ictluciion
0
I:orrnsliotj
b)
I~rjs~al~lc opcralion or p r o d u c t i o n k i l n s : praclical c o n d i t i o n s a r c 1101 cxaclly Il~osc cxl~clcd k)r very l o n g lcrm slabilily Or lcmpcralurc a n d malcrial Ilow.
nl vap0ur
prcssurc over scjlulions of a l k a l i c o m p o u n d s .
i~~lq~~c lrcnlmcnl d ibc Irnjjspjrl clicikcr nodules i n Ihc kill).
Or
or
ljcal and
‘I’IIANI
tbur: nil - 7(X1 g/1;: K,SO, - 1 g/1::
I W O limes bil;llcr.
Or
vnfxnjr
willrin
UJC bed
Or
Olbcr compounds. e.g., CaSO,, dcpcndin~ on alknli:sullaic ralio.
Tbc lurillcr tlcvclqjnrcn~ or a prctliclivc model w i l l Ijavc I O iakc accour~l o f such [actors, as well as illc crrccls Or coni~xlsilion Or kiln almospbcrc. In rcccril ycnrs llicrc has been mucll invcslig:llirjtj ol l;jclors /:nvcrninp, IIIC block;tgc of cyclones a n d lbcir pcc’k~rmancc i n biGI Icurpcralurc coal c~11iil~5lion proccsscs (itI Ilic Iiopc or prolccling Iurbinc blatlcs i n tlirrcl cycle clcclric power gcucr;tlicln syslcms). Wljcn linic (or limesi0nc) i s injcclcd lo nbsorb S O , . 111~ COIJI~HJ~JI~S a n d Il~crrnodynamic crilcria invoked a r c cxaclly 1110s~ cncounlcrcd in IIIC CCIIKIII intluslry - particularly wl~cn rclalivcly l1igb cbloridc coals arc oscd. II is probable Illal Iljcrc i s now sullicicnl n c a d c m i c knowlcdgc IO bclicr Irc31 o u r silualion a n d a l l o w improvctl mrxlcllinl; a n d undcrslanding. Anolbcr aspccl 10 consider i s knowlcdgc acquired rrnm sludy 0r ibc rcgciicraliori 0r CaO sorbcrils u.sul ror S O , s c r u b b i n g : a g a i n , dala p0lcnlially rclcvanf in k i l n sys~cms arc protluccd, l’or cxamplc, o n p r c s s u r c s o f S O , i n IIIC sy~cm CaSO.,I C;rSI CnO i n IIIC prescncc ol v a r i o u s conccnlrations ol C O a n d C O , . II i s Impcd Illal ibis paper promolcs cross-rcrliliulion bctwccn IIJCSC v a r i o u s fields or w o r k a n d lbc bcnly oC praclical knowlctlgc available i n ~ljc ccmcnl i n d u s t r y .
‘l’ltis Ical. ori~;in;llly I~lsul o n v a r i o u s irjlcrn~tl amI crlcrr~l rqK)rIs, Ita\: l~ccrj atl;~l~lcd 01t lllc b a s i s of urchjl di.uzussion will1 Icclmical slall rrom a mrmbcr ol ccmcnl companies. 7‘banks arc given lo all involved, as well as IO lbc Directors of WIIC Circle Industries PLC ror lbcir permission I O publish Ibis paper.
Clioi. G.-S. & Glaswr, 1:. I ’ . (19RR).‘I’bc Sulplrur Cycle i n Ccmcn~ K i l n s : Vap0ur I’rcssurcs and Solid~l’liasc Slabilily O r llrc Sulpbalc I’ljascs. Ccr~~ct~l rind Concrrlc Ilcsc:~rcl~, IR. 367.371. Kct~n. C . I ’ . & M u r r a y , It. 1. (1984). I’ortland CCIJIOJI I’rotluclicm. 111 Slntcljtrr nntl I’crlonnnnrr oC CWICII~S. c c l . I ’ . Ilarncs. Appliccl Scicncc I’ublisltcrs I.id. Ihrkirj~. ~IJ 705. 7 . x M o i r . (i. K . & Gksscr, I:. I ’ . (lYY2). hlincrnliscrs, h4otlificrs a n d Activators i n ~ljc Clinkcring I’rr~css. In 9111 It~lrrwtliowjl Corjgrcsi ~IJ lljc Cljcjrjislrg or CCIJJCII~. NCW Dribi, I n d i a , 1 9 9 2 : Congress Itcporls, Vnlumc I , National C o u n c i l r0r Ccmcnl a n d Iluildirj~ Malcri:tls, New Dcllji, p p . I2S- 154. Rilzmann, 338-343.
I l . (1971). C y c l i c I’bcrtomcna i n Rotary K i l n Systcrr~s. ~cjrlclll-l(nlk-Gip(,
I’cllmar, n.. Kljor, I . I I . . k Gregory, Glps, 3 I , 288-290.
S. (1979).
I’roccrsing or K i l n Dusi.
24.
APPExmX 3
PAPERPRESENTED BYT MLOWES 100% Pet Coke - Problems and Solutions
100% PET COKE - PROBLEMS AND SOLU-I-IONS “Good morning Lady and Gentlemen, The citie of my paper is 100% Petcoke - Problems and Solutions. It will give information on how Blue Circie Cement has moved from zero to 40% petcoke over a 3 year period, indicating the technological problems that need to be overcome for any works that seeks to fire 100% petcoke. Before beginning my presentation I would like to thank Gerard Flament of CCB and Jean Pierre Piliard of Ciments Lafarge for their help and information during this development phase within Blue Circle Cement. The paper mainiy deals with the dry process. LepoI and long dry processes.
However, the conclusions apply to wet,
Slide 1 This overhead indicates BCC’s approach to petcoke. Firstly, prior to 1991 there was zero use of petcoke because ic was coo much trouble and low ash coal was much “NICER”. However, in 1992 due to a significant recession, we moved up to 30% replacement and in 1993 40% overall on 10 Works. Some Wor:ks using none, 100% usage at times on a Lepol process, 80% on the large semi-wets at Northfleet and 65% on a wet process at our Masons Works. In 1993 the prices increased, consequently the financial benefit accrued to the project was only the same in 1993 as 1992. In 1994 there has been a slight market improvement, some Works can se11 everything they can make and consequently we are already failing to meet the 1994 plans for use co petcoke.
Slide 2 This slide gives some indications of why one should use petcoke. Firstly, its price can. be up to, and sometimes even more than, 50% less than the coal price per GJ. It can improve cement quality, if there is an excess of alkalies over sulphates. In addition, if you are coal mill limited on cement olant output it can, with the appropriate Hardgrove index, increase coal mill capacity. In addition, it can be claimed to help the environment as petcoke needs to be burnt and if it is burnt in conventional power stations the SO, emission {vi11 increase, whereas in most Of our processes it is significantly retained. However, the real reason for using it is that in a recession, the full kiln OUtpUt is not reouired and consequently operational costs are at a premium and petcoke can make a significant reduction in OperatiOnal costs.
Slide 3 This siide indicates reasons for not using petcoke. Firstly, it can increase SO? emissions, this even applies to a certain extent within a dry process as, for example, even in a precalciner betueen 6000 and 10000 ppm SO, is the maximum that can be absorbed before bypass into the preheater sys=m occurs. Due to the extra S02/S03 going into the system I don’t need to celi anybody there is significant increase in potential for ring formation and blockages. In addition the blending operations are increased unless one is reaily firing at 100% petroleum coke. .AIso there can be coal mill fineness problems having to meet the lower 90 micron residue and there is more variability in sulphur and Hardgrove index. in addition, some investment may be required.
Slide 4 This slide identifies the main problem of using petcoke which is controlling the S02/S03 cycles which leads to blockages and increased SO? emission. This depends on the total SO3 input, the Na30 equivalent, the molar ratio of SO3 to alkaiies in the clinker, the combinability temperature of the clinker and in addition to this, how well the kiln is controlled and the overall flame conditions that exist within the kiln.
Slide 5 This slide shows some facts concerning peccoke usage. Firstly, for price reasons the main interest lies in the higher sulphur petcoke which typically varies between 44% and S+% suiphur. For a dry process kiln with 800kcal/kg using 100% of a 5% sulphur peccoke, this is equivalent to around 1.33% SO3 on clinker. As most Works are limited to around 0.6 Nay0 equivalent, this corresponds to a molecular ratio of 2 to a clinker SO3 of 1.5%. .Above a molecular ratio of I, calcium iangbeinite, which is a salt of alkali and calcium sulphate with 2 molecules of calcium sulphate for 1 of potassium sulphate, forms in addition to calcium sulphate. Calcium lambonite decomposes at around 152O’C leaving alkali and calcium sulphate. Calcium sulphate decomposes at 145O’C. However, these decomposition temperatures are both in the presence of excess oxygen. If there is in excess of 2000 PP?J of CO in contact with caicium sulphate above a 1000°C it would break down to SO3 and calcium oxide and hence exacerbating any particular problems one has with SO2 emission and preheater/kiln blockages. For a molecular ratio greater than 2 in stage 4 raw meal, this generally is recognised as producing hard deposits.
Slide 6
This slide shows what conditions you need to avoid and what conditions you need to have to be able to use 10096 petcoke. Firstly, the conditions that need to be avoid& are chemical reduction in the burning zone, Over-burning and low Na,O equivalent. For 1009/o petcoke one needs to have a good flame, a readily combinable raw mix and a good kiln control of free lime and back end oxygen.
Slide 7 This slide shows what are the requirements of a good flame and uses the CEMFLAIME 1 information fairiy extensively. One needs to ensure that the burner has adequate momentum, e.g. around 7N/MW. This is to ensure that there is adequate air available to combust the voiatiles and carbon of the peuoleum coke on a micro mixed basis before the petcoke comes in contact wirh the burning zone at between 2 and 3 kiln diameters, depending on wheKher one has a high swirl or a zero swirl burner. It is importam Khar the ignition takes places very near the burner, a 60% bluff body will help to promote Khis by removing the jet establishmenr. region and creating an internal reverse flow zone. In addition, a secondary air temperature of greater than 800°C is required to achieve this early ignition. Petroleum coke is a by-product of a process which essentially means that all of the
lower temperature volaciles have been extracted and therefore peuoleum coke needs to reach around 900°C before its first volatiles can be released. T O cope with this lower volatile release rate and notentially a less porous stiumre of the carbon as a result of its formation process, a’90 micron residue of around 6-7% is required, following the general rule of +90 micron residue being 50% of the volatile matter. In addition, it is absolutely essential that a uniform enKrainmenK of secondary air takes place into the flame, particularly ensuring that the secondary air coming under the burner is not SKarved. To ensure the maximum possibilities a burner cenually lined u? the kiln is most important. Even if all of these criteria are met, one will still operate in reducing condition promoted by unburned carbon and CO if there is not adequate conuol of the back end oxygen. A recommended level is around a minimum of 3.5%.
Slide 8 This slide shows results from the CE:VFLAME 1 trials which indicates impact of back end oxygen on CO for both a medium volatile coal and flexicoke, with 90 micron residues of 1246 and 2%.
T’he measurements are made at the back end of the kiln simulator which is equivalent CO an L/D of 12. As you can see, for the flexicoke, co ensure r.har: rhere is a CO level less than 1000 PPM, one is essentiaiiy looking for 349’0 oxygen. A similar condition is i?Ot necessary for a medium voiatiie coal where ic does look char: in Che a-?;% region the conditions for less ihan 1000 ?Phl can be achieved.
Slide 9 This slide shows a typical situation on our dry process Works at Hope which indicates the impact of hard burning and CO/oxygen on stage 4 SO3. Hope Works has a clinker Molar Ratio of 2 with a British Coal. This was for a high volatile coal with a 90 micron residue of around 15% with the burner lined centrally up the axis and a momentum of at least 7NI1MW. It can be seen that hard burning increases the amount of SO3 in stage 4. However, it also increases the amount of NaZO equivalent and consequently not until one gets above around 1300 ppm of NOx is there a significant increase of excess SO3 over alkalies due CO the thermal breakdown of calcium sumhate in the burning zone. However, the most interesting point is the impact of back end oxygen OR the level of CO and its consequent impact on stage 4 SO3. It can be seen that once the back end oxygen drops down beiow 1.8%, i.e. to 1.4%, the CO increases from 500 porn to 2500 ppm and a corresponding increase of stage 4 SO3 from 3.2% to 4.5%. Under the conditions of the experiment, around 24 times the Na30 equivalent in the clinker was appearing the in stage 4 raw meal. Consequently, fo; a molar ratio Of 2, this corresponded to around 3.75% S03. It can be seen that under reducing conditions promoted solely by the lack of back end oxygen, the SO3 in stage 4 increased to 4.5% which is a totally unacceptable level and blockage problems occur on the Works at this ievei. The Works normally seek to run at around 3% stage 4 SO;.
Slide 10 While a good flame is important, alone it is not enough. Good kiln controi is required, i.e. using Linkman, to avoid continual SO3 recycling. For exampie, if one has a blaster operadon that breaks down deposits, if once it reaches the burning zone the conditions are SO hot that 90% of the SO3 is sent back into the preheater system, obviously the blaster operations are not being suitably supplemented.
In addition, it is very important to have a combinability temperature essentially less than 1500°C. Also, one operates at round about I-I?XJ free lime, or else a similar situation exists as to that I referred to previously under the need for good kiln conuol. Otherwise a continual recycling of the SOTin the syste.m wili occur leading to permanent problems associated with blockages and breakaway. In addition, it is important that one continually checks what is achieved for the stage 4 material. Preferably, this is supplemented by an SO2 probe that can be used to monitor what is happenin g in the back end of the kiln and even be linked into the high level control. For example, 3000 ppm of SO7 equates to around an extra 1.05% SO3 on stage 4 raw meal.
Slide 11 And finally, in conclusion, now we know how to do it, how can 100% petcoke be used with lower NOx emission, bearing in mind this normally mea&u running into on the verge of reducing conditions? The answer is, I don’t know at the moment. However, CEMFLAIME take place at the end of this year, will provide the answer. Thank you”
2 which is due to
--__-._---_-
---
100%PET COKE - PROBLEMS AND SOLbTIONs] _.. -_-_.. _--_-_.- ---------.---- .-. -__-
o BCC's approach to Pet Coke - Prior to 1991 zero use too much trouble, low ash coal much “NICER” - 1992 - 30% - 1993 - 40% 100% Lepol, 80% semi wet, 65% wet. - 1993 prices increased - 1994 market improvement - 1994 budget 35%
-
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1I-- 100% PET COKX - PROBLEMS AND SOLUTIONS / _.------_-_--.---__-- -___--. -- 1 -.-_-. ..-._.- . ..__._ _-. -. -_.-.._ ._.. - .__... _ _-. _- ,_.___ ,.__ -___ l
Why use Pet Coke? A Price - up to 50% < Coal/Gj A Improve cement quality A Increase coal mill capacity A Help the enviroment A Recession, fxlll kiln output not required
r
--- -_- _ -____. --__ ___.______ _.__ __,_____-_I__., _ -._____ _____ __._ _ __--
, I / 100% PET.-.-_ COKE PROBLEMS AND SOLUTIONS --.---- -..-- -_.-.- . .- . ..-.--- . . - . I ._ _._- ~----.-__.--.- -.----.-. _-----o Why not use Pet Coke? A Increase in SO2 emission A Increase in problems with rings and blockages A Blending operations increased A Coal mill fineness problems A S ,and Hardgrove variability A Investment may be required
I
.---__-__-.__-_- -.-__ --__ ..--
. . .._.- -_,.-..-. __.._______
-__-
_---
111000/o PET COKE__--..- PROBLEMS AND SOLUTIONS .__- - -__.__ ._. ._-_____. --____-__--_.--.-__-- ____.-_ --_.-. ___ .._....-... -.. o Main “PROBLEM” A Controlling SO2/SO3 cycles which leads to increased blockages and SO2 emission
0 Depends on:A Total SO3 input A Na20 equivalent A Clinker Molar Ratio A Combinability Temperature A Kiln Burning Control
A Hame Conditions
.-- -------- -_----
--
--
100% PET-COKE - PROBLEMS AND -------SOLUTIONS] .__ ---_.--- -___--.-.
A 5% S Pet Coke - 100% usage - 800 kcals/kg - 1.33% SO3 on clinker
A For 0.6 Na20 eq. MR = 2 for Clinker SO3 of 1.55 A Above a MR of 1, K2S04.2CaS04 - Calcium Lambinite and CaS04 form A Calcium Lambinite decomposes at 152OC, CaS04 -145OC in Excess 02 A 2OOOppm CO at above 1OOOC breaks down CaS04 A MR >2 in Stage w ray meal produces hard deposits. I,
-------.-... ----
-._-.-
---__-_-
FT Sx-j _--~ ---- _---_--- ____.---__._COKE ___ -_.. -_.__._. _ -,_ PROBLEMS __-.- -.- . ..-- - - --.-- AND l
FOR 100% Pet Coke need to AVOID: A Chemical Reduction in Burning Zone A Over-burning A Low Na20
o For 10096 Pet Coke need to IXAVE: A Good FLAME A Combinability < 145OC A Good Kiln Control of Free Lime and Back End oxygen
--.----------..-.. _ --...-_ ---..... --. ..-.
------
1 100% PET AND . . -SOLW=] .-. -. _---_.COKE - -..-_--..- -.__.._ -- PROBLEMS _. . . _ -._.._-_ . __ -_----_..----. ---l
Good FLAME as per CEMFLAME 1 A Burner Momentum - 7 N/MW A Bluff Body 60% A +9Ou residue 50% VM A Central Burner A Secondary Air > 800C A Back End Oxygen - 3.5%
I
I i
!
<
------.- ._-.- - .-.-. . .-. - _ _._ _._ . _ _ . . ._-------P-w-- .- .-. .-. .- - - - -
-
100%- PET PROBLEMS AND -----I ---_-- -_--.-.--SOLUTIONS -. . - c . -.- .--~-.--.--- - -COKE 0 Good FLAME alone not enough need: Ilr. Good Kiln Control - Linkman to avoid continual SO3 recycling A Raw meal residue low enough for good combinability A 14.5 % Free Lime. A Continual checking of performance A SO2 probe to monitor and control, eg 3000 ppm = 1.05% SO3 on Stage IV raw meal
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-.-- _..--___ ..__ .-- .__- --_----___-
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-COKE - PROBLEMS AND SOLUTIONS] I.-._- ~ .____.
__.--,. . ..: ._I
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---.
. -
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__.__
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..__.____
---...
o Now we know how to do it! 0 How can 100% pet coke be used with low Nbx emission
CEMFLAME 2 has the ANSWER
I_-_-
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 2
CETIC Sub Commission “Behavior of Volatile Material in Kiln Systems”
ISTN 9215 CETIC ~~-C~~~~IssIOh’
“BEHAVIOtiR OF VOLATILE lMATERL4~ J.N =N
SYSTEMS” REVIEW OF “CLASSICAL” ECNOWLEDGE ON &mOR ELE&fEiiS - JUNE 1992 Summary This report is based upon various documents originating within 3lue Circle and from external sources, supplemented by accounts of experience in Works operated by CETIC member companies. A review is given of factors governing the behaviour of minor feed constituents (Cl, K, Na, S) in cycles originating i!t the kiln burning zone and of the consequent effects on process plant performance. Recirculating loads of volatile species are formed and these are often implicated in the formation of build-ups, coatings and bloc.kages in cooler parts of a kiln system. Some relevant practical experience is listed and implications for product quality are summarised. In a given plant, burning zone temperature and atmosphere are the dominant driving forces for these cycles. A key area for action to gain control lies in the selection and preparation of fuels, burner settings and raw materials. Process control (including dust management) can also be important, especially as regards selection and maintenance of sensors which give direct or indirect information on burning zone atmosphere (i.e., chemically reducing or oxidising with respect to the volatiles). Several dry process Works have found useful results from study of the results of sampling and analysis of kiln entry meal on a regular basis. Differences between pIants may arise from design features or from characteristics of raw materials and fuels. Various modifications :o details of process design and operation/ control may be used to alleviate plant difficulties or modify clinker quality, depending on the local permutation of inputs and temperatures which is involved. For exampie, changes may be made to cyclone geometry or “non-stick” linings added, or benefits may be found from alterations to raw meal or fuel chemistry. These factors may also be considered when selecting new equipment. The prospects for further improvements in understanding and modelling of process phenomena associated with volatile cycles have improved in recent years with the completion of relevant thermodynamic studies in other areas of technology. The addition of secondary firing or precalcinarion can significantly alter the behaviour of some kiln systems, and appiication of the improved academic knowledge to some relevant situations may be worthy of encouragement.
IS-m 9215 CETIC SUB-COiMiullSSION
‘BEHAVIOUR OF VOLATDLE SYSTEMS”
REVIEW OF “CLASSICAL” JKNOWLEDGE
amTERI4L.S IN KILN
ON IMINOR ELEMENTS - JrJiuE 1992
CONTENTS Page No. 1
1.
INTRODUCTION
2.
VOLATILES: WHAT ARE THEY AND WHAT ARE THE MECHANISMS OF VOLATILISATION? 1
3.
CYCLIC NATURE OF VOLATILJZS 3.1 3.2 3.3 3.4
4.
3
EFFECTS OF VOLATILE CYCLES WET AND (LONG) SEMI-WET PROCESS LEPOL PROCESS DRY PROCESS
CHANGES N PROCESS CONDITIONS WHICH CAN INFLUENCE 7 BERAVIOUR OF VOLATILES 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9
5.
N KILN SYSTEMS
PARAMETERS NFLUENCNG VOLATTLE RECIRCULATION ITALCEIMENTI CBR LAFARGE OBOURG CMENTS FRANCAIS ENCI HOLDERBAlNK BLUE CIRCLE
EFFECTS ON CLINKER
13
Sulfate retention Reducing conditions Fundamental aspects 6.
TOWARDS A MODEL OF VOLATILE CYCLES
14
7.
EFFECTS OF CONDENSATION
17
CETlC SUB-COIMMISSION
“BEHAVTOUR OF VOLATILE IMATERIALS SYSTEMS”
IN KILN
REVIEW OF “CLASSICAL” KNOWLEDGE ON mOR ELEMENTS - JUNE 1992 1.
INTRODUCTION
This text, originally based on various reviews available within BCI and on external literature, has been adapted on the basis of discussion within the CETIC “Volatiles” Sub-Commission. During an examination of the available literature, some 190 apparentIy relevant articles were identified in “Chemicai Abstracts” for the period 1967-1991. It is interesting to note that a more detailed examination showed that 60 of these articles covered not only topics relevant to kiln operation, the environment and associated laboratory tests, but also: l
the production of mineral&d clinkers (despite efforts to exclude this topic from the list)
a
low temperature clinkering by chloride addition
l
the cement/sulfuric acid process
0
the use of waste materials in kiln feed.
65% of these articles came from. Eastern Europe, suggesting that there may be a knowledge base in the countries formerly behind the Iron Curtain. These subjects remind us that volatile components are nor merely bad news for cement producers, but may have useful effects on product performance and on economics in certain circumstances. 2.
VOLATI-LES: WHAT ARE THEY AND WHAT ARE THE MECHANISIMS OF VOLATILISATION?
The concern is with elements or compounds which partially or entirely evaporate in the kiln and which are transported towards cooler zones in kiln gases. Some can escape via the stack or are trapped in precipitator dust. Others condense as they leave the kiln and/or react with or condense on the feed in the preheater system. The principal volatile elements are : K, Na, Cl, S, Pb, TI, Cd, V, Hg, Zn and As. The elements F, V and As can be classed as “moderately volatile” and the others as “significantly”. They originate from inorganic and organic compounds in raw materials and fuel. In the case of raw materials, certain compounds can readily volatilise at a temperature below 600 deg. C, especially mercury, thallium and sulfur (if present as sulfide or in organic combination), but in generai volatile compounds in raw materials only evaporate pat-My as the feed passes along the kiln system and through the burning zone.. -The residue will be retained in the product, either in solid solution in the principal ‘phases of the clinker or as discrete
compounds - normally alkali chiorides and sulfates. T’ne typical proponions of volatiles in raw materials which evaporate while the feed passes through the !tiIn system are as foIIows (“primary volatiliries”, according to certain writers):
so3 K2O
NazO Cl, Hg F Pb, n C d , V, Zn
c/o 60 - 90 30 - 70 20 - 40 96 - 9 9 1 0 - 40 60 - 99 IO - 20
In contrast, fuei voiatiles are almost always entirely evaporated during combustion. Evaporated volatiles travel back up the kiln in the gas phase where: i>
they condense on the feed and form the basis of a recirculating volatile load in the kiln system
ii)
or they condense as a fine dust or fume which is finally trapped in the precipitator and which wiI1 then become part of an external volatile cycle if dust is partly or wholly returned to the kiln system
iii)
or they condense on a colder surface in the kiln system and form the basis of a build-up
iv)
or they escape through the stack either in the gas or vapour phase or as a fine fume.
The remainder of this text concentrates on the cycles of minor elements (Cl, K, Na, S) originating in the burning zone. The cycles of trace eiements and/or cy Iess hot parts of the system are not examined in detail. When volatiles condense on raw meal and are returned towards the burning zone, depending on the overall chemical conditions and burning conditions, they form a range of volatile compounds which themselves evaporate partially and are partly retained in the clinker - either as discrete compounds or in solid solution in the clinker phases. In general, chloride preferentially forms potassium chloride (KCl) and only when there is an excess of chloride over the needs for chemical combination with the available potassium will sodium chloride (NaCl) be formed as a recirculating volatile species. A large part of the alkalis and sulfate entering the burning zone is in practice present in the form of molten sulfates, forming a separate Iiquid phase immiscibIe with the principal ferrite flux.
2
The typical proportions of voiatiie compounds which evaporate in the burning zone (in the absence of reducing conditions) are as follows: % 40 - 60 40 - 60 40 - 100 60 - 90 20 - 40 97 - 99 96 - 99 80-100
K,SOq NaiSO? 2CaSO,.K$O, K,O in solid solution NaZO in solid solution KC1 NaCl CaSO,
In consequence, a cycle of recirculating volatiles develops, which only finds an equilibrium when the total quantity leaving the system (in clinker, dust and stack Iosses) equals that entering the system (in raw materiais and fuel). Various tables and figures are appended, showing melting and boiling points and vapour pressures. At this point the level of volatiles in recirculation can be significantly greater than the total mte of introduction of volatiles. The volatiles in the vapour phase can be in various states of dissociation and recombination. Alkali sulfates evaporate congruently, that is to say that they disappear entirely when heated for a long period, whilst &SO, decomposes and leaves residuaI CaO (in oxidising conditions). CaO:CaS03 melts can form. Typical recirculating volatile loads expressed as % of the total quantity introduced are as follows: %
Chloride
5ooiJ 200 - 650 150 - 200 200-800
K2O
Na,O so3
The effect which these recirculating volatiles exert on the formation of build-ups at the kiln (gas) exit depends on the composition (which governs the temperature of liquid formation and thus the position and hardness of the build-up) and on the quantity (which governs the rate of formation). The dependence on vapour pressure on temperature differs for various species and the “relative volatility” (or rank order of volatility) can vary in the cold and hot zones of the kiln system. In the past, several empirical limits have been proposed for the concentrations of volatiles admissible in a kiln, e.g., 0.03% Cl on clinker for preheater kilns. Today there is a tendency to prefer to specify concentrations tolerable at the kiln inlet (in hot kiln feed at zero loss on ignition). By way of indication, the concentrations of volatiles which can be tolerated in the lower stages of a preheater are typically given in the following ranges (exceptions being of special interest for study): 5% Cl 1.2 - 1.8 2.5 - 4.5 so3 2.5.3.5 Alkalis (eq. Na,O) 3
The FL.5 encrustation index R =
Total molar S inout Total K,O input + 0.5 (Total NazO input)
can be used as an indicator of the potential nature of any build-up, as follows: R31 0.7 < R < 0.9 R < 0.5
Hard build-ups based on SO, Relatively soft build-ups (easily removed) Carbonate-based build-ups in due course.
Most suIfates condense in the range 9000 - 1100 deg. C. The presence of fluorine can aggravate build-up problems due to the formation of fluoride compounds and. their aid to the formation of various silicates. Potassium chloride alone condenses between 800 and 900 deg. C (and sodium chloride at a slightly lower temperature). Build-ups can develop in the kiln feed chute or in the riser duct towards the bottom stage of the preheater. There is an optimum temperature for the capture of SO, by a freshly cakined raw meal (e.g., 880 deg. C in one study). Primary condensation is expected to be in the form of liquid alkali sulfates. It is often interesting to calculate the composition of the sulfate phase in kiln inlet material, with its addition of KCl. Fusion in the system Na,SO,/ K$O,/ CaSOJ KC1 begins at below 700 deg. C. Liquid films on dust particles are The origin of build-ups and as the thickness increases, internal temperature drops and new equilibria may be established and compounds formed. There is less literature on the phenomena governing condensation than on evaporation. The effects of atmosphere and the implications of precalciner operation are perhaps worthy of study, especially for sulfate compounds which can be present in various states of oxidation and which can react with water vapour to form bisultites and bisulfates. The characteristics are as follows:
Chemical State
Fuel and raw material
Clinker
Vapour
Oxidised s4+, 9’
Sulfares, e.g., gypsum, anhydrite, alum, etc.
Alkali sulfates and aBali/alkali metal sulfites
SO2 and (at low temp.)
Elemental S
Neutral, S
Reduced, S2-
so3
Pyrites, marcasite organic sulfides
Oldhamite, CaS, complex suiftdes of calcium, aluminium, etc
Non-volatiie
The oxidised sulfur cycle is illustrated among the appended figures. 3.
CYCLIC NATURE OF VOLATILES N JSXN SYSTE-MS
3.1
EFFECTS OF VOLATILE CYCLES
The volatilisation and condensation of volatiles in a kiln produce two undesirable effects on the kiln process: l
the formation of build-ups - possibly blockages and the possible emission of SO, from the system
a
transfer of heat away from the burning zone towards cooler parts of the system.
There are also effects on the clinker, which may be summarised as fofollows: Fluxing action: a lower temperature of first liquid phase formation l change of liquid viscosity l alteration of surface tension of liquid 0 modification of crystal morphology. 5
Phase re!arions: 0 the relative thermodynamic stabilities of the clinker minerals can be altered by solid solution effects. Hydraulic activity: 0 the reactivities of the clinker minerals are altered by solid solution and/or by the effects of crystal symmetry (high temperature stabilisation of polymorphs) and/or effects occurring during hydration (e.g., coating of cement particles by insolubie salts). (It is difficult to isolate these three classes of effect in practice.) 3.2
WET AND (LONG) SEMI-WET PROCESS
There is an obvious external cycie (dust return) as well as an internal cycle which has, perhaps, received less attention in the literature. In an example involving a coal with a sulfur content of 4.5% (not very differ&t from the level for a blend of coal and sulfur-rich petroleum coke), there was a high internal cycle and significant stack losses. In one BCI kiln these represented 42% of the quantity introduced to the system and were 5 times higher than the proposed BATNEEC concentration. Typically, FLS expect 30% or more of the sulfur entering a wet process kiln to escape via the stack. The external cycle can be influenced by the proportions of dust returned to the system by various routes. 3.3 LEPOL PROCESS The volatile cycle is more complicated on account of the additional external cycles due to dust from riddlings and cyclones, even though the basic situation seems similar to that for wet process kilns. However, there is a major opportunity for a volatile bleed (especially SO,) by disposing of riddlings and cyclone dust as well as that from precipitators. 3.4 DRY PROCESS As indicated above there are both internal and external volatiIe cycles. In general, sulfur escaping the preheater is found in the form of dust rather than SQ and forms part of the external cycle involving the raw mill, conditioning tower and precipitator. Further, it is expected that SO2 in the gas phase will be absorbed in the lower stages of the preheater, at least if there are no adverse conditions (e.g., chemically reducing conditions). When sulfide is present in the feed, a significant fraction will be lost from the preheater as SO, (e.g., 30 - 50%) and partly absorbed in the raw mill and the precipitator system. 6
4.
CHAtYGES N P R O C E S S BEfIAVIOLJR OF VOL;ITILES
4.1
PAJiUMETERS
CO?iDITIONS rvHICH C&Y INFLUEYCE
NFLUENCNG VOLATILE RECIRCULATION
The principal parameters which influence volatile behaviour are as follows: temperature time type/composition concentration diffusion towards the solid surface (as controlled by clinker nodule size and flux content and composition) gas/solid contact other reactions kiln atmosphere (laboratory tests suggest that water vapour plays a role, as well as the “traditional” kiln gas components). (For summary, see table among appended figures.) Once a kiln is in operation, the main parameters available for the kiln operator to alter/control are th.e temperature and the atmosphere in the kiln. The rate of gas flow seems to be of secondary importance. Process design and local chemical conditions also play a part, determining the total quantities introduced to the system, chloride content, relative concentrations, combinability, fuel and to the firing system parameters and mixing within the clinker bed (with exposure of nodules to gas flow at the surface). It is often difficult to distinguish chicken from egg in the industrial operation of a kiln. For example, when CBR changed from gas (or indeed, to 92s) as a fuel in Canada in the past, there were effects on volatile cycles. This is to be expected due to the presence of a higher level of water vapour in the combustion products (e.g., coke < 5 % , lignite > 10%) and the higher vapour pressures of alkali hydroxides (see attached figures), but such changes are always accompanied by other alterations to the quantity and composition of volatiles introduced and/or the raw meal chemistry. The effect of nodule size on volatile behaviour seems equally. impossible to separate from that of the effects of liquid from alkali melts on clinker size grading (as discussed in past CETIC work). It is important to note that, in general, no-one measures “typical” volatile balances (save globally): available detailed balances are usually gained when there is a need to investigate a non-typical kiln system due to some problem or another. A number of illustrations follow, taken from the experiences of participants in the CETIC Group. 4 . 2 ITALCEIHENTI When CaC12 was introduced via the flame (0.3% on clinker) to volatilise K,O in a Gepol preheater kiln of 2000 t/d capacity (Stockertown, USA), it was noted that operation could continue for many _- hours with kiln entry levels of Cl of,around 3% with somewhat reduced 7
loss on ignition (heat pump effect). This indicates the capacity of this type of preheater to tolerate a little more Cl than cyclone preheaters (although long term results from Stockertown are still awaited). 4.3 CBR With chloride injection on a precalciner system (with bypass), there was a marked effect on Cl and alkali levels in kiIn inlet material but not on sulfate. (The same was true at Stockertown.) Full system equilibrium was not reached for three days. At another Works (about 60% heat energy input at the precalciner and no bypass), an upper sulfate limit on kiln inlet material of 3% was established in order to avoid any build-up problems (although perhaps not corrosion problems!). In this latter case the sulfate level was lower in the presence of chlorine and of potassium (see later data); at the same time there was also a certain reduction in SO, loss from pyritic material in the preheater. It is known that this precalciner rapidly forms build-ups above about 91.5 deg. C. In a wet process kiln, the formation of rings can be followed with the she&scanner and the flame setting altered to eliminate them (provided that action is taken within 2 to 3 hours). At Lixhe, various phenomena were noted when returning to coal firing after the use of gas, including a reduction in sulfate cycles and in the level of decarbonation at the preheater tower exit. This latter phenomenon results from a lower heat release within the preheater, notably from CaS04 recombination. The thermal effect is of the order of 75 kcal/kg with the inverse effect in the burning zone. Blue Circle have calculated the magnitude of this effect as 109 kg/kcal at Hope Works in the past. At Lixhe (dry process) the sulfate level in the kiln entry material can be reduced (and that of chloride raised) by increased burner momentum: wear of the burner tip allows the level to rise again in due course. A “non-stick” kiln feed chute lining from Hasle also gives good results. Also at Lixhe, a number of interesting relationships have been established from the results of three years’ operation (with analysis for volatiles in kiln inlet material twice per shift). For example: No. of kiln srops per monsh = -5.3 f 2.7 x %S03 (in hot meal) for cyclone blockages)
I?* = 0.88
%S03 (in hor meal). = 4.8 - 0.36 x % K,O (in hor meal)
R’ = 0.68
It was noted that the input of used tyres at the kiln back end gave good results in mechanical removal of build-ups. 4.4 LAFARGE A compilation of volatilisation levels was made in 1985 for all company kilns. The
company’s calculation of “volatility“ is somewhat different from that often encountered elsewhere. The following results (which have changed little since that time) were obtained: 8
%
so3
K2O
Na,O
I-w
56
34
14
Dry
80
69
26
Precalciner
55
49
55
(Retention levels are close to 100 % for preheater kilns and at lower levels for the other processes, being related to the level of dust disposal.) If is noted that Lepol (and wet) process kilns give Iower volatilisation levels than preheater kilns and that precalcinafion seems to influence these phenomena. Nevertheless, due to the small quantities of dust involved, the enrichment of volatiles in precipitator dust is highest for the Lepol process. (Italcementi note that for balances determined on 23 kilns the enrichment factor for dust compared with raw material is close to one for SO,, K,O and NazO in the dry process, but between 9 and 11 for the Lepol; in the case of chloride the comparison is between a factor of 8 for the dry process and 123 for the Lepol; the values for long, granule-fed semi-dry kilns are intermediate.) In examining the performance of a new low-NO, burner with a higher momentum than the previous one, it was noted that the CO signal could be made to disappear (for the same C+ level) with less decarbonation at the preheater exit and a higher level of kiln drive power (Amps). These effects were accompanied by an apparently higher BZT (with less clinker free-lime), less gaseous SO2 at the kiln exit, and kiln inlet material with less SO, and a higher loss on ignition at 1000 deg. C. 4.5 OBOURG Here the clinker KzO level is used as a control parameter for the wet process tiln and a considerable data-bank of measurements has beeri built up. At the end of 1990 the use of a lower ash fuel began and the clinker K,O level dropped from 0.69% (1990 mean) to 0.55% (mean for January/February 1991), despite the introduction of a little more potassium to the system (equivalent to. 1.43 % on clinker instead of 1.34%). Evidently the dust - returned to the kiln - had become more rich in alkalis. It can be shown that the proportion of $0 brought in by the solid fuel fell from 24% to 17.5% while at the same time that brought in by the dust return rose from 18.5% to 29 %. This observation leads to the conclusion that $0 incorporation in clinker does not only depend on the quantity introduced but also - and above ail else - on the type of material which brings it in and perhaps on the position where it is injected. On screening the clinker at 20 mm the same chemical analysis was found for each fraction with the exception of K,O, where the concentration was about 10% higher in the coarse fraction. Here the clinker alkali content has successfully been reduced by chloride addition at the flame. Ciments Franc$s have also demonstrated this effect with CaC12 addition to slurry (3.5% K,O in the dust in comparison with 2.7%). Blue Circle also once succeeded in demonstrating the efficacity of this method (accompanied by non-return of part of the dust) 9
in lowering clinker K,O ieveis by a half in a long kiln, despite the fact that some questions remained about the most appropriate place to introduce the chloride; it must be noted that there can be problems with the flow behaviour of precipitator dust above a certain chioride level. 4 . 6 CIivIENTS
FRANCAIS
On adding precalcination to a Lepol grate with (stoichiometrically) more sulfur than alkalis in the system, an increase was noted in sulfate and alkali levels at the grate and especially in the dust and nodules beneath the grate. More potassium sulfate was found in the clinker with precalcination; CaSO, was found in the cyclone dust and somewhat less in the precipitator dust, arising from kiln dust which had travelled across the layer of nodules on the grate. There was a little more SO2 emission with precalcination. On another Lepol kiln no such effects had been found on adding precalcination; at Frangey, Lafarge had noted a somewhat higher recycle of potassium and sulfates with Lepol precalcination. (At this latter Works, with use of chloride-rich substitute fuels, it is found necessary to carry out sampling over periods of at least a week to determine consistent volatile balances.) To use a high chloride coal (-0.15% Cl) on the dry process it has been found necessary to prepare a mix with another coal to avoid build-ups with typically 2% Cl at the bottom of cyclone 4. Analysis of build-ups along the kiln indicated that chloride levels reached 30% (at zero loss on ignition) in the coating from the base of cycione 2 and 20% at 50m into the kiln (despite its less than 5% level in both hot and cold parts of the kiln); run-out of material from a stopped kiln gave chloride levels rising up to 0.7%. During a past experimental campaign of burning chlorinated wastes, it was noted that the simultaneous presence of chloride and sulfate at high levels could give rise to emissions of HCI and SOz. Apart from this, the volatiles were all assimilated in the c!inker (burned at low temperature with free lime levels of up to some 13% and formation of CaCI,.C,S) or in the dust (and probably also trapped in refractories). 4.7 ENCI After the addition of a second stage to the preheater, various build-up problems were encountered. In 1985/6 the kiln exit 0, level was increased from 0.5 to 1.5%, the solid fuel residue at 90 microns was reduced to below 25 % and several “Cardox” units were installed. These actions improved the situation and in 1987 20m of “Magotteaux” stirrers were installed, the burner air velocity was increased to 100 m/s and a Hasle “non-stick” lining was installed in the duct and cyclone dip-tubes. These efforts made an output rate of some 110 t/h possible without build-up problems. More recently petroleum coke (3.5% S) has been fired (first at 4.5 t/h and afterwards 6.5 t/h), accompanied by oxygen addition at the flame and, finally, by the use of slag (S - 1%) as a raw mix component (5%, and then 10%) with yet more coke (8.0 to 8.5 t/h, i.e., - 60% of fossil fuel energy). At the start of 1991 the production rate was 120 t/h accompanied by preheater blockage problems. Duringr ,199l it was deduced that problems 10
with build-ups could be avoided if the SO, level in the kiln entry material was kept beiow 2.5%. This was possible with a coke input rate of about 8 t/h and a slag level of 10% provided the oxygen level at the kiln back end was kept consistently at 2%. If these conditions are not met, then kiln operation rapidly runs into problems. The “Fuzzy Logic” control system helps to achieve success, as the 0, signal standard deviation has fallen from 0.45% to 0.25% and that for the SO, in hot kiln-feed from 0.6% to 0.2%. 4 . 8 HOLDERBAhX In the past, Iaboratory data have been gathered on minor elements and these confirm the effects that are now more widely known. For example, suIfur volatility in a standard regime (70% Nz, 30% CO) is close to 100% at 0% 0, but falls in the presence of 0,; nevertheless the effect of O2 is much less at 1400 deg. C than at 1200 deg. C. The volatility of minor elements in the laboratory is also much greater for powdered material than for granules. (See appended illustrations, taken from external literature.) Currently there is interest in raw meal morphology and in the distribution of volatiles in the meal at the start of clinkering. A precalciner kiln system in Spain ran for many years with a Cl level in the kiln entry material between 3 and 4 % (that is to say about 0.5 % less than K20) and with no trace of CO. In this case there was about 1.1% SO, in the kiln entry meai and 4% K,O and no problems, but if CO was present there was about 2% SO, and 5% I(,0 in the hot kiln feed accompanied by Spur&e-based build-ups and cubic KC1 crystals. (It is also recognised in Ciments Franc@ that regular kiln operation helps to minimise the phenomenon of cementation by the freezing/thawing of chloride-based deposits.) While burning wastes at Clarkesville (wet process), it is found necessary to ensure that the clinker Cl content is always kept below 0.3 %, otherwise the kiln becomes unstable. At Origny there have been an enormous number of kiln stops caused by preheater blockages which, when sampled, do not contain many volatiles. This effect seems to have its origin in a liquid phase formed by calcite arising from a chalk with an extreme level of fineness which can decarbonate and recarbonate very rapidly. The problems have been much reduced by altered cyclone geometry and helped a little by the use of a mechanical cleaning device. 4.9 BLUE CIRCLE On one precalciner kiln it is difficult to find “typical” volatilities. For every determination there are almost always different values (40% for SO, and 50% for K,O changed to 25% and 4O%, for example). One can imagine that this is caused by variations in the nature of the raw materials and the content in the kiln feed of sulfate (and sulfite) captured in the meal after initial low temperature voiatilisation in the preheater. In order to examine the possibility of producing a sulfate-rich clinker (2% SC&) using certain available resemes of material and without installation of a by-pass on a new kiln, rests were carried out for about three weeks on a dry process kiln at another site (-35 t/h). The objective was to reduce the volatilisation in the burning zone by playing on process parameters and producing a lightly mineral&d clinker (-- 1% K,O, -0.15% NazO, and 11
- 0.15 % F, as usual at that Works, but with double the usual level of sulfate) and with a silica ratio a litT.ie lower than usual (-2.7 instead of -3.2), accompanied by a change in alumina ratio from 2.8 to 2.2. During the changeover there was some tendency to form soft build-ups in the preheater, but with the new regime established these moved towards the kiln feed chute without causing any major problems for kiln operation. The apparent burning zone temperature was reduced from about 1500 deg. C to 1380 deg. C, while K20 volatility dropped from 70% to 60% in the burning zone and that of Sq from 80% towards the range 50% to 60% provided that kiln exit oxygen level was kept above 2%. There were improvements in the output and fuel consumption of the kiln and, in fact, the experimental Works adopted certain of these changes during its normal operation for several years, until the asrival of demand for low alkali clinker. At Hope Works (dry process), tests were carried out involving various NO, levels as well as reducing conditions.
so, :
The ratio of SO3 in Stage IV to Scl, in raw meal varied typically from 1.8 to 2.7 for the higher levels of NO, and was 3.0 for a low 0, level. The clinker SO, content fell.
KzO and NazO :
In a parallel manner, for K,O the ratio of the content in Stage IV to that in raw meal varied from 3.8 to 4.4 and for NazO from 1.6 to 2.0.
In general, reducing conditions increase SO, level. at Stage IV by a factor of 2, giving a lower clinker SO,. As already described in a paper to the CETIC Technical Commission, at Cauldon Works (and later in other dry process Works) SO, has been monitored at the kiln back end to determine the local rules for avoiding blockage tendencies. The SO, signal is noisy and difficult to interpret without a knowledge of the history of the system, e.g., a reCent fall of sulfate build-up material arriving in the burning zone can give rise to a high SO, signal at the kiln back end despite the presence of a good flame and acceptable levels of volatiles in the kiln entry material. At Dunbar there has been success in reducing the number of kiln stops per year caused by preheater blockage from over 90 (1987) to less than 10 (1989), lost time hours having also fallen from around 450 per year to about a hundred. (There were also major gains in stops caused by rings and breakaways at the kiln entry seal.) There is no need to keep such monitoring equipment in permanent operation once the rules are established, but it expected that renewed investigations will be needed each time the conditions of operation change. In practice it is now found that with this know-how sulfurrich petroleum coke can be used (to a certain level) even on dry process kilns which in the past have given problems with just coal firing - but in several cases the build-ups seem to have moved from the preheater into the kiln (where they are destroyed). In a general manner it can be supposed that there are problems of both short-term and long-term and stability: once a stable burning zone volatilisation is established, one must wait for stable conditions to arrive higher up the system and in the large masses of material which form the build-ups and coatings already in existence.
12
During tests of SO, monitors at the exit of a long kiln with filter cake feed, it was noted that
the signal usually remained stable (below 100 vpm). But, when the oxygen level fell there was an inverse correlation between the 0, and SO, signals. In this case levels of around 1000 vpm SO, were reached, with considerable variations; it is supposed that cycles moved further up the kiln (nearer to the analyser) during low oxygen periods. At Plymstock Works (dry process), when changing the BZT from 1390 to 1500 deg. C the ratio of SO, in the fourth stage to that in the raw feed rose from 1.2 to over 4.0. Similar results were obtained ‘at Lichtenburg Works (South Africa). It is noted that a better understanding of volatile recirculation is useful for kiln operation because back end oxygen indicators can sometimes be misleading. At Mason’s Works (wet process), raising the 0, level gave higher levels of SO, and K20 retention in clinker (concentrations rising from 0.18 to 0.54% SO, and from 0.33 to 0.80% K20). Also for the wet process (Westbury Works): Kiln 1, “low momentum” flame, 2 % 0, - 2500 mg/Nm3 at kiln exit. Kiln 2, “good momentum” flame, 2 % 0, - 2 5 0 mg/Nm3 SO, at kiln exit 1 % 0 , - 1350 mg/Nm3 SO, at kiln exit. In practice alkalis are controlled on the wet process (by means of the LINKman system): Masons:
% alkali target
NO, set-point @pm at precip.) 550 4OO-500 300
< 0.55 0.55 - 0.7 > 0.7
Ravena (USA): - alkalis controlled by NO, set-point - sulfate/alkali ratio controlled by 0, set-point. 5.
EFFECTS ON CXANKER
Sulfate retention: In general, changin,0 from a situation with excess alkalis offers advantages. In general, a higher SO, content: improves early strength improves workability produces a more difficult “apparent grindability”. Increased ciinker alkali levels can also be associated with sulfate retention for Lepoi and wet
process kicilns, especially if there is already an excess alkali content. 13
The effects of minor components on the viscosity and surface tension of liquid phases can be complex. Lower viscosities encourage alire formation. Calcium sulfate flux can, however, stabilise belite and/or cause the production of clinker alite with lime inclusions. (Sulfate liquid systems are capable of influencing ionic transport and chemical combination despite the limited solubility of the principal clinker compounds.) In clinkers with a low al*ka.li content, there is the possibihty of belite stabilisation (difficuli combinability) due to excess SO,. Reducing conditions: Reducing conditions in the burning zone can give a cement with poor flow characteristics (due to free K,O and NazO), poor workability (due to the increased content of C3A and its reactivity), poor strength (lower C,S content) and variable colour. Fundamental aspects: At some future date, the production of lower LSF and/or mineral&d clinkers may be of interest. Alkalis retained in clinker are present either as stable sulfates or absorbed in the silicate and aluminate structures. NazO has a more marked tendency than K,O to form solutions in CIA. For ciinkers with (molar) ratios of sulfate:total alkalis below 0.5, almost all the sulfate is combined in water soluble form, K$O, being predominant. A proportion of the alkalis are in solid solution in the clinker C,A and this has an adverse effect on the initial cement reactivity and thus on concrete and mortar rheology. For ratios between 0.5 and 1.0, a certain quantity of langbeinite (2CaSO,.K$OJ is also formed (and not all the alkalis are soluble). For ratios above 1.0, significant fractions of the sulfates are combined within the sihcates and aluminates or as anhydrite (CaSOJ, which dissolves more slowly than alkali sulfates, whilst the fractions of KzO and Na,O which are soluble in water approach 1.0 and OS, respectively, at a ratio of about 1.5. At sulfate:alkaii ratios above 1.5 trends are somewhat erratic. For most normal clinkers the principal sulfate phase will be aphitalite with a maximum WNa ratio of 3.0. This phase is accompanied by minor quantities of K-$0, and calcium langbeinite, Na,S04 being found only for unusually low WNa ratios. As well as the solid soiution effects and the formation of compounds described above, various permutationsof volatiies (especially in the presence of fluorine) ‘can influence the stmcture of alite and belite crystals. (A comprehensive review of recent work forms part of the text of G K Moir and F P Glasser at the 1992 International Congress on Cement Chemistry in New Delhi.) 6.
TOWARDS A MODEL OF VOLATILE CYCLES
Various empirical volatility factors have been proposed and used with a certain measure of success. This section considers the possible approaches to a more fundamentally based model. It is generally supposed that (other factors being equal) the extent of voiatilisation decreases as the thermal efficiency of the kiln increases. An explanation may lie in the fact that this 14
is due to the limiting effect of vapour saturation by al,kaIi compounds. Studies by 3lue CircIe of the treatment of kiln dust in a 100 mm diameter fluidised bed tend to confirm this hypothesis. This study examined the feasibility of producing a low quality clinker from flue dust with capture of the alkalis distilled from the bed (for possible use in the fertiliser industry). Saturated vapour pressures at 1200 deg. C are (for the pure substances): KC1 0.18 atm 0.8 x 1O-3 am K2SO4 (0.6 x 10m3 atm with decomposition suppressed) Na,SO, 0.13 x lo3 atm (0.01 x 10s3 atm with decomposition suppressed) The transport capacity of air for vapcur at 1200 deg. C is thus KC1
700 g/g 4 g/g x0.5 g/g
K2S04
Na2S04
(The capacity at 1250 deg. C is about two times higher). It can therefore be foreseen that (unless the equilibrium vapour pressures differ greatly from saturated values) there will be little problem in removing KC1 from many kiln flue dusts in a fluidised bed with a gas flow rate of, say, 2 g per gramme of dust, although the capacity for sulfate removal may be limited. (A wet process kiln typically operates with a ratio of a little less than 2 g/g gas/soiids in the burning zone and perhaps 2.75 g/g at the back end; the corresponding values for the dry process are 1.4 g/g and 1.94 g/g). It is suggested that V” v* p*
= =
P M, M,
= = =
=
(I’*) (M,L_ (P - p*) c-q
saturated vapour concentration in transport gases (kg/kg) saturated vapour pressure ) ) same units of an alkali compound gas pressure > moIecuIar weight of vapour molecular weight of gas.
Given mathematical expressions for saturated vapour pressure as a function of temperature and knowledge of the temperature profile in the kiln system, the saturated vapour concentration can be calculated for each alkali compound and thus the maximum quantities evaporated from the feed per unit mass of gases. In considering the amounts of gas passing 15
through the kiln at vaiou~ temperatures, the true quantity of volatiles transported per unit mass of clinker can thus be calculated and from this knowledge, “ideal” volatile cycles can be deduced. (It is to be noted that, paradoxically, when alkali addition allows BZT to be reduced then blockage probIems can be lessened due to the dominance of temperature in the evaporation mechanism). Despite the fact that qualitative differences between two kilns (one dry process and one wet) are reflected in sample calculations, such “ideal” calculated recirculating loads are about 10 times larger than those encountered in practice. The probable reaSons are: Incomplete contact between gases and solids in the kiln, where only a small fraction of the solid surface is exposed at a given time. (It is expected that there is better contact in the colder dusty regions of the system.)
b)
Volatilisation characteristics of the alkali-containing minerals at a given plant. (Alkalis seem to be lost more e&y from silicates and aluminates than when present as sulfates).
cl
TEuISpOIt
d)
Reduction of vapour pressure over solutions of alkali compounds.
e>
Inadequate treatment of the transport of heat and of vapour within the bed of clinker nodules in the kiln.
0
Formation of other compounds, e.g., &SO,, depending on the alkaksulfate ratio.
Of compounds which are condensed ~/on solid dust or fume.
Unstable operation of production kilns, so that practical conditions are not exactly those expected for very long term stability of temperature and material flow. The further development of a predictive model will have to take account of such factors, as well as the effects of.composition of kiln atmosphere. In recent years there has been much investigation of factors governing the blockage of cyclones and their performance in high temperature coal combustion processes (in the hope of protecting turbine blades in direct cycle electric power generation systems). When lime (or limestone) is injected to absorb SO,, the compounds and thermodynamic criteria of interest in combustion systems are exactly those encountered in the cement industry - particularly when relatively high chloride coals are used. It is probable that there is now sufficient academic knowledge to better treat our situation and allow an improved modelIing and understanding. Another aspect to consider is knowledge acquired from study of the regeneration of CaO sorbents used for SO, scrubbing: again, data potentially relevant to kiln systems are produced, for example, on pressures of SO, in the system CaSO,/ CaS/ CaO in the presence of various concentrations of CO and CO, (see appended figures). 16
(It is interesting to note that volatile condensation has an effect on the kiln power signal used for process control. Any perturbation of the chemical composition of the kiln feed which raises volatile content will increase kiln Amps; a control strategy seeking a constant Amp&re signal would have the effect of reducing burning zone temperature, yielding under-burned clinker with a relatively high volatile content. A strategy using a constant fuel feed rate would be equally inadequate, due to the depression of burning zone temperature produced by the increased volatile load. The best control of product quality should result from a system based on observation of the peak clinker temperature, that is to say, indirectiy by kiln exit NO, control.)
7.
EFFECTS OF CONTXXSATION
The most probable primary condensation is in the form of liquid alkali sulfates, Melting in the ternary system Na$SO,/ K,SO,/ CaSO, starts at below 800 deg. C. Addition of KC1 increases the range of suIfate compositions which is liquid of this temperature and allows formation of liquid melts even below 700 deg. C. Deposits on the feed can provoke chemical reactions; they can equally cause adhesion and - as with deposits on surfaces initiate build-ups. (Direct condensation as calcium langbeinite is not expected on therrrtodynamic grounds.) While the literature tends to agree (although not totally) on vapour pressures of pure a&li compounds, information for the more complex species of interest in cement kilns is more rare. Studies at Aberdeen University have produced self-consistent results for sulfosilicate, sulfoaluminate and langbeinite. The order of volatility alters with temperature. As indicated earlier, liquid alkali sulfate systems have a poor dissolving power for most of the principal oxides of cement clinker. However, they have low viscosities and a low surface tension against silicates and thus cover and englobe these particles very effectively. It seems likely that the small quantity of silicate which is dissolved has a high mobility, so that the liquids are effective at producing a reaction (for preference towards C2S at 700 to 800 deg. C.). Stabiiisation of carbonates has been suggested (CaCO, can dissolve in the liquid phase in the presence of alkali sulfates in the range 880 - 900 deg. C., forming a liquid rich in CQ and the presence of fluoride can cause further complications - but equally (in combination with certain concentrations of other compounds) certain advantages as far as clinker quality is concerned.
C P KERTON June 1992 (English version, December 1992).
)jats~c;ld
w D, Trans. hwhy Svc.. 66 (81, I966 - 1973. (197W.
rcrry R 11 & Chil!on C II, Chemical Engineer’s tlandlmk, New York (S\h
cJi\ion
McGraw-Ililt,
1973).
/
Temperature (deg. C
~r3pt-1 to confirm that proportion volatilised
hating regime is dm-xteristic of raw mix for oiven 5
(i.e., (fiat volatilisation reacfion is first order with rap+% to alkali content).
ha from Palmer K & Bayik 0, pc~
R . epon M-117(1952] ud from
W&J
H. Rock ProdUCu 45 (2j.M - 68 (19413
/ I I i II f i
I350
-
Temperature (deg. Cl
MeI tinq
point
ea -c 8 0 0 ca 440-#SO edso-904 m9ou-954 CID
7 954
Meltinn &ae of (a) the CaSO ic SO -NazS04 System 4-w and (b) the Effect of 6 KC1
(“C)
(From
---c------ ,wpp- ------4~p-.
fue .air
Oxidized Portion of the S Cycle Relative to the Kiln. Dotted paths represent circulation in the vapour phase, solid l i n e s i n t h e s o l i d p h a s e ( s ) .
SLJAlAl,~l~Y 01: 1~AC’I’O1ci
\\‘IIICII INi~LUi:NC~
Il~l~VIOU1~O1~~lLNOI~VOLA'I'IL~COMI~UNDS1N 1WaN SYSTEMS 1.
Burning zone temperalure (alkali vapour pressures)
- level - variations
2.
Temperature profile of burning zone
3.
Comp&ition of alkali liquid systems in the burning zone
4.
Atmosphere in burning zone globally reducing locally reducing water vapour
‘IIll!I
MAJOR INFLUENCE
I’OSSll3LE CONTICOL ACTIONS
SECONDAItY INFLUENCE
0 0
Soft burning (including fluxes and mineralisers). Controlled burning.
:
Flame/burner settings. Fuel Characteristics.
0
Selection of raw feed chemistry (sulfate/alkali ratio).
0 0 0 0
Burner/flame settings. Fuel characteristics. Coal/coke fineness. Choice of fuel (solid, liquid, gas).
5.
Clinker size grading ’
0
Selection of raw feed chemistry (sulfate/alkali ratio) - including flux.
6.
Clinker llux content (density)
0
As above.
7.
Thermal efficiency (gas flow rates)
0
Process design/seleclion,
8.
Prehea’ter system design vertical cold areas/air inleaks anti-build-up lining geometry solid/gas loading
9.
Precalciner:
design operation
LO.
Composition of alkali phase in prehealer
11.
Dust return
12,
By-pass system
0 0 : 0 0
Process design/selection. Elimination of air inleaks / Insulation. Design. Type of precalciner. Throughput.
0
Selection of precalciner. Control of precalciner.
0 0
Selection of raw mix components. Selective quarryinglbcneficiation, 0
0
ConHol. Add. 0 = rrrrcertoin
X 0
Drnper
1, PCA Rcjwt MRS.68 (19.54).
Gtrr C B Kcil I:, ‘I’IZ (4). 7 _ 9 (I9Mj).
DUNBAR-HOURSLOST HOURS LOST
DUNBAR-KILNSTOPS NO. OF STOPS
!xo
100
400
80
._
300
60
200
40
100
20
0
0 PREHEATER BLOCKAGES 1987
RINGS/KIN INLET BUILD-UP 1988
1989
PREHEATER .BLOCKAGES
RINGSMILN INLET BUILD-UP
C2K53
lC
Log plot of the total pressure, in atmospheres, of rhe decomposition products of various sulphates occurring in the cement kiln. Abbreviations: C5S,S = 2Ca Si0&aS04 C A s= 3Cail 0 .CaSO &$33 = 2CaS&k2S044 Choi G-S & Ckser F P. Cement & Ccmxete Research 18. 367 - 374 (1988).
Phase diagram at atmospheric pressure for the system containing the solids CaSO,, CaO and CaS, as well us the gases SO,, CO and CO,. The reducing potenti equals p(CO)lp(COJ. Region A is where Cu# and CuSO, are the only solids; region B Fras &SO, repkced by CaS (see text). Contours of equd p(SOJ are c shown. ayhurst
A N & Tder R F, I Inst. Energy 64. 212 - 229 (fgg1).
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 3
Investigation into Potential Low Temperature Volatilization
INVSSTICATION IN'@3 PV!ZNTIAL LOW TFXPEXATUEE VOUTILISATIOH WITEIBT?BP~O&B3D
OxFo3DH!.3wOKgSKIL9,
PBSCALCINEB
AHDPREHUTERS-fSPERS
Objectives 1) To identify potential lov temperature volatile compounds in the raw materials of the'new Oxford Works, 2) To assess the quantities of volatile8 present ia the gas and Baterial streams at partiizular positions vitbin the system, in particular vithin the preheater and precalciner sreas.
3) To establish whether these calculated values will affect the conolusions on the size of bypass required for the nev Oxford Works.
The rav materials for the propose ,xford Works are fairly rich in alkalis, snd sulphates (up to 1.546 So3 and % 820). A bypass has been proposed l ti bleed off a proportion of the kiln'& to prevent these alkalis and sulphates causing problems in clinker quality and build up vithin the preheater and precalciner, The design of this bypass has been discussed extensively in the technical note STH 41/73. This note covered almost all possible condition8 which could affect the design of the bypass. flowever one area of uncertainty highlighted by SlY gl/lj vns the possibility of significant raw material volatility (;,3C%) at low temperatures (
higher tea eratures (*- 95OOC) a significant amount (.-low4 Xgs per clinker P of K2SO4 can be in the vapour phase. Tnns a high temperature in thi precalciner should be avoided as it vould have a detrimental effect on kiln operation.
Kg
3)
The vast Gjoritp of K2SO4 and rJa$Oq will be in the condensed phase on the suspended solids at the bypass position. This would mean that bleeding gas alone at the bypass position vi11 not reduce the quantity of alkalis in the kiln system. The most effective uay of reducing alkalis would be to bleed off the dust on which the alkalis condense.
4)
The quantity of KC1 in the precalciner for a precalciner operating temperature of 850 - 9ooOC is high (1-a on clinker) but this-is consistent with the values predicted in SE7 81/13 and; F&RlIfS of normal vorks operating experience. Thik is taken into acconnt in the bleed requiment.
S)
The effect of sulphur is summarised Fn Table 2. This table demonetratee that the source of sulphur, the kiln exhaust covsitlon and the presences of other minerals determines whether the sulphur will be bled off by the bypass, recycled within the preheater, or exhausted from the stack.
6)
If sulphur is present as calcium sulphate, or calcium sulphide tha t?o controlling parameters on the volatility are the gas temperature and gas composition. Houever the effect of gas composition reduces with decreasing temperatare and the evolution of SO2 will not be large (7% IBX) at temperatures experienced &the kiln back end, and will notie sicpificantly higher with reducing conditions than with oxidisiq conditions.
7)
Low temperature volatilisation due to the presence of certain mineral organic natter or iron pyrites will result in an increased SO2 emission from the stack but should not have any detrimental effect on the wlphur cycle in the precalciner/preheater system.
8)
Sulphur from the kiln fired fuel will tend to form a recirculating load betueen the kiln and preheater unless bled from the system via the bwss.
9)
Sulphur from the precalciner fuel will form a recirculating load in the same manner as sulphur from the kiln fuel. Houever this wxwt be bled from the system until the sulphnr has been recycled.
10)
Approximately 9546 of the So2 in kiln and preheater gas streams will be absorbed by the CaO in the kiln riser duct providing that the gas ad materials stream are in contact for sufficient time for an equilibrium to be reached. This time is very short due to large surface area of CaO.
Recommendationa 1) Further work should be carried out on the Oxford raw materials to establish the form in which the sulphur is present in the kiln feed. 2) As the wunt of lov temperature volatilisation is dependant on the form of sulphur in the raw meal a standard method for determining raw material volatility should be developed. Lf such a method were available it could be carried out as a normal laboratory test on the raw mater!-' The information could then be used either for design information for future works, or as inforznation to the kiln controller to enable mar efficient operation of the bypaas and kiln.
3) When the form of sulphw in the raw mterial has been established, a simulation of tjhe Oxford Works raw meal could be tried out at an existing XI W works. (possibly Plymstock). %upling throughout the system would then show where, and at vhat temperature, volatilisation and condensation of alkali and sulphur containing compwnds occuza.
CONTEXTS
Objective Conclusions Recomendations 1.
RiTRODUCTION
2.
THROREXCAL CONSIDEXATIONS 2.1 Volatiles Present in the Rack end Gas 2.2 Calculation of the Kaxidmii3
Amount of Katerial in
the Vapour Phase 3. SJLPZUR
COFi’AlfJING COXF’OUXDS
3.1 Reaction of CaO and SO2 3.2 Effect of Reducing Conditions on the CaS/CaS04 Equilibrium 3.3 Effect of Iron Sulphides
7
3.4 Effect of Xinerals 3.5 Effect of the presence of organic sulphur C' i contaminous compounds
7 8
3.6 Effect of sulphur in the fuel
8
4.
CONCLUSIONS
9
5.
RECOXNENDATIONS
10 11 12
7) Estimated Alkali Vapour in the Kiln/Preheater
13
Gas per Kg of Clinker (no bypass) 2) Effect of Sulphur on Raw Material
Volatility
74-15 16-23
a’. CRCULATION
LIST
24
INVESTI~TION
IF!$ POTENTIAL LOW TdI!XPERATURR VOIATILISATION
WITRIN T3Ei PROP$SXD OXFORD NEWWORXSKIE?, PFECALCIHER ANDPREHEmmsYsTEt?s
Objectives 1) To identify potential low temperature volatile compounds in the rav materials of the hew Oxford Worka. 2) To assess the qua&ities of volatile8 present in the ~8 and material streams at particular positions within the system, in particular VithiL the preheater and precalciner areas.
3) To establish whether these calculated values vi11 affect the ccnclnsions on the size of bypass required for the new Oxford Works.
1.
INTRODUCTION The Services Department of Research Division have produced a comprehensive assessment of the siie bypass that would be necessary tc prevent process problems due to volatile alkalis/sulphates within the pmposed Oxford new Works kiln syste (STN 81/13).
The report highlights problem sreaa micularly with reference to the effect of possible volatflisation vithin the prscalciner or preheater system.
The chemistry of the compounds concerned
is of a similar nature to tbat of materials studied by Eagineeriug Research and Development of the processinc of cement flue dusts. This note apples the theoretical predjctions of the previous flue dust work, together with other infomation awilable from published literature, to the potential problem of volatilisation in the Oxford preheater/precalciner system. The assumptions nsed'in SIN St/I3 acknovledge that a certain amount of lov temperature volatilisation may take place and allows for up to 3096 lov teaperature volatilisation with a bypass of up to 30% Rovever if low tenpersturc volatilisation vere to exceed 3cq6, then the following problems could occur.
il
The build up of a rec$rculating load within the preheater system causing the kiln feed to have a concentration of volatile cornpnents which could exceed the design capacity of the bypass.
ii)
The pssibility of material build up uithin the preheater system causing blockage problems.
iii) The release of vapour particularly So2 at a temperature below the temperature of the formation of free lime. This would mean that the bulk of the SO2 would not be recaptured by lime but released from the system in the stack exhaust gas, causing the bypass and preheater design to be oversized. 2.
THEORETICAL COEJDEFUTIONS 2.1 Volatiles Present in the Back end Gas . The first stage of the investigation was to identify all the volatile compounds present in the kiln exhaust gas; then from themdynamic IL -.J. c information on>heir ;zD ehavio.ar with temperature, compounds which could cause problems could be identified. The standard volatile com_r>onentT of X20, Na20, So
3
and CL were considered.
The form in which these
components will recondense is well documented, and is listed below. 1) Chloride will recondense as KCl, 2) K20 remaining and Na20 ,will recondense as K2sD4 and Na2S04.
3) If there'is excess sulphate, as in the Oxford case, Cam4 will be formed which preferentially forms the double salt (2 CaSO
4
g2s04) * 2.2 Calculation of the Maxfmum Amount of Material in the Tapour Phase The solid gas phase equilibria of El, K2S04 and Ba2sD4 has been / extensively studied at Barnstone by Khor?. Prom curves of saturation vapour pressure tempera-e for KCl, %s04 and Na SO (see Fig. 1) the saturation mass of vapour in the exhaust
2
4
gas can be calculated using the equation:
Where Ys equals the saturation mass of aUralA vamur in the exhaust e-9
Pa
is equal to the saturation vapour pressure (obtainable from Pig. 1)
P
is the total pressure (atzn)
Mv
i&-the molecular weight of the vapour component
M g
is the molecular weight of the gas.
Taking the total pressure to be one atmosphere and the molecular weight of the gas to be approximately 30 we obtain the curves shown in Pig. 2. If we assume that 4@ of the fuel is burnt in the kiln and 6% in the
precalciner, if there is no 'bleed, the mass of gas to clinker can be approximated to 0.55 Kg gas per Kg of clinker in the kiln and 1.7 X3 gas per Kg clinker in the precalciner.
Using this information we can
obtain the CuIves shown in figure 3. which give the narimum vapouz carrying capacity of the gases.
The czurve at a higfier level in the
precalciner indicates the higher gas quantities in the precalciner. By assuming the burning zone temperature to be 1450°C, temperature to be 1050°C,
the back errd
and the precalciner to have a maximum tezp-
erature of 950°C Table 1, can be constructed. This shows that we would expect all the available KC1 to be vapouris-A
hofnr- -a-h&
the kiln,? small amount of K.$04 to be'vapourised.&d quantity of Na2SC4. erature (85O'C
a negligible
If the precalciner is operated at a lower tv
is more ty-pioal.)
then the amount of K2SC4 vapourised
also becomes negligible. This analysis recognizes KCI. as low temperature volatile, hoverer this is also recognized in Sl!S 81/13, and the quantities of KC1 predieted in .the precalciner region by STB 81/13
(1.6% on clinker) com-
pares well the quantity predicted by this analysis over a temperature
-
range of 850 - 900°C (1-246 on clinker). The results of this fundamental approach show that the assumptions made in Sn 81113 concerning chloride recirculation are reasonable.
As the assumptions made in STM 81/l-J ~~ncernina alkali recimlatian are stx-mn to be reasonable
bv this inventiaatlon. anv wtemlal aaerstinz
procuemz voula nave w ari8e rrom an cute-rnatlve
source, prooablg
from sulphur contsining compounds. This is discussed in the following section. 3. SUUHUR
CONTldNISG
COMEWXES
.
Sulphur can be present in the kiln system from several different sources.
1)
Calcium sulphate and calcium sulphide in the rair materials.
ii)
Iron sulphides in the raw materials.
iii) Sulphur present in organic matter in the raw material. iv)
Sul?hur present in the fuel.
Ro similar partial pressure data to that used in the previous section wa available for CaSO
CaS and iron sulphide. In this case a fundamental 4' approach considering the thermodynamics of the reaction of SO2 and CaO MS considered. This investigation was complicated by the fact that the equilibria and rate of reaction are affected by the gas environment in which the reaction takes place e.g. reducing or oxidising, and the presence of certain minerals in the rav meal e.g. Si02, MgQ and FeO, 3.1 Reaction of CaO and SO2 Some of the many possible reactions of CaO and SO2 are listed belou3.
I)
caso4(s)
+
2’
CaS04(S)
+
yEI) 'O(g)
+ +
=2(s)
ca'o(8)
+ +
3
O2(g)
=2(g)
+
co2(S)
3)
c”s4(*)
4)
C=s4(s)
5)
+ @(*) + c”s(*) + 4 co(g) +
4co(g)
+
cas(8)
+
3 -4(9) + C"s(s) + 4 ys>
4c02(g)
+ 4 =2(g)
Beactions 2, 3 and 4 will only take place under reducing conditions and these will be discussed later. Of the two remaining reactions laboratory tests 4 on the adsorption of SO2 on cement raw meal have shorn that considerable adsorption of SO2 occurs in the temperatare range 600 to
goo’c according to the reverse of equstion (5).
6) 4 CaO(s)
+ 4
a2fg) 9 3 C=s4(s) + C=Scs)
From thermodynamic data we can obtain values of enthalpy, entmpy and Gibbs free energy for the reaction at different temperatures. Then using the Qan't Hoff isotherm the value of Kp (the equilbrium constant) can be found. DG
=
-2.303
%T Log
P
K -
(2)
F'rom the stoichiometry and assuming the actitivities equal to unity, it can be seen that Kp depends solely upon the partial pressure of &J ;,J/*L; 4.) & t h e From this knowledge we can obtain a curve (Figure =2* extent to which #e reactior6progreases to the right hand sic with _~._. --.---increasing temperature. Th2.s curve show that, at a kiln back end temperature of 1050°C all the SO2 would be in the gas phase vhere as over the precalciner temperature range (85O'C - 95O'C)
there would be
between 2% to 1446 dissociation to fern S02. This is within the 3096 low temperature volatillsatlon allowed for in the report STH 81/13. This equilibrium is for a static sitcation and does not consider the removal of SO2 from the system or the rate at which the reaction proceeds.
In the kiln/preheater system the compounds are constantly
being removed and replenished and so the equilibrium is also dependant on reaction rates and residence time of the compounds. This
analysis does shou however that the bulk of the SO2 will be generated from the decomposition of CaSo4 at a temperature greater than the typical temperature of the precalciner and so a bypaes at this Mgher temperature would reduce much of the So2 available for reforming CaSO
4
CaS in the precalciner. 3.2 Effect of Reducing Conditions on the CaS/CaSO, Equilibrium -t If sulphur containing limestone is roasted in air all the salphur is converted to C&O4 by the reverse of reaction (1)4. 7)
CaO
+
So2
+
8
02
+ Cas4
Reduction in the level of oxygen causes reaction (6) to be favoured which has bee,n found to be the major reaction in cement kiln exhaust gases. Further reduction of the level of oxygen causes reactions (2) and (4) to be favoured. The extreme case of reducing conditions is given by equation (3). If CaS is present a further reaction can occur with carbon dioxide5 according to the equation. 8) CaS
+ 3 co2
= cao
-+ so2
+
3co
!I'he temperature dependance of these reactions have been studied by Turkdogen and Olsson5 who obtained expressions for the equilibrium constants
10gpm2
(p~o/pco2)3
lo~W2
(pC02/pcO)
= - (20,000/T) + 9.27 =
-
(‘9617/T)
+
8.021
From these expressions the salphate/sulphide
(3) (4)
equilibrium diagrams can
be dravn vhich are shown in Figure 4, 5 and 6. These curves show that adjustment of the ol;ygen potential of the kiln atmosphere will adjust the level of sulphur retentio:n at higher temperatures.
and that this effect is more sensitive
At the relatively lov temperatures, 95O'C and
ii) the presence of potassium containing minerals very much enhances the decomposition of CaYO
4
when compared to the effect of analogz
non potassium co&ainiry~ minerals.
These obseroatione are valid
for temperatures below 12OO'C and at atmospheric pressure. The presence of minerals which enhance evolution of s02, by the same mechanism will obviously retard its recapture. This could mean that SO2 is lost from the system via the stack in greater amounts than that predicted by STN 61/13.
3*5
Effect of the presence of orwic sulphur contaminoua..compou.nds Another form in which sulphux can enter the kiln syetem is in the form of vegetable matter. !&is vi11 evolve SO2 at very much lover temperatures than mineral based
. If this was the mjor form of sulphnr in the rav % materials, this uould mean that almost all the raw material sulphnr would be lost via the stack causing the bypass tc be very much oversized. The effects illustrated in sections 3.4 and j.5 can be investigated by eqerimerit and would ahow up as enhanced volatility at lair temperatures. Experiments carried out on the Oxford raw material have not shown this fn be the case.
3.6 Effect of sulphur in the fuel: Sulphur in the fuel will be present in the kiln and precalciner gases a8 From the discussions of their reaction of CaO and SO2 (section j.1)
=2it was stated that considera't)le amonnts of sulphur vould be absorbed at
600 to 900'~.
In fact the peak rate of absorption for SO2 is at 850°C5,
Other factors affecting the amount of sulphur absorption are the concentration of CaO and S02, the surface area of the CaO and the . exposure time. In a kiln and preheater system the time for vhich CaO i8 exposed to So2 at a temperature where absorption can take place 18 very short. HoweV8r the fineness of the CaO particles means that an enormous surface area of CaO is presented to the gas stream. This is generally the overiding factor and the reaction vi11 tend to reach equilibrium very quickly.
Hence in the stage IV preheater cyclone8
there is almost 100% absorption of SO2 by the feed. With a precalciner the reaction is complicated by two streams of gas and material, at different temperatures combining. The precalciner gas stream will contain So2 from the fuel. The precalciner _.. material stream carries‘bffsolid of increasing CaO content (see ?ig. 8). Eouever there will be little recombination of CaO with SO2 in this stream, due to the gas temperature of approxinrately llOO°C. (see Figure 4). This will combine with gas and material streams from the kiln in the riser duct from the kiln.
(Por the RSP ONOQA system shorn in Figure
8, a mixing chamber is used),,
This material stream will also contain
CaO and So2 from the kiln fuel and rav materials. The gas temperatare is also around llOO°C and so there &ill also be very little combination CaO with S02.
As the gas and material progress up the riser duct the
endothermic production of CaO from CaCO streams.
3
coolstthe gas and material
This makes absorption of SC2 by the time more favourable.
The absorption should have reached a maximum at the point uhere the gas and materials streams separate. be close to the equilibrium
The absorption at this point till for this temperature (-. 900°C)
This is borne out by operating experience
with suspension preheater kilns. The presence of a bypass will mean that kiln produced SO2 will be bled off immediately whilst SO2 from the precalciner will be recycled before being bled from the system.
4.
CORCLUSIOBS 1) Of the potential problem causing compounds investigated &So,, Sa2s04, KC1 and sulphur containing compounds),Na2S04 could be effectively die+ cussed as a lov temperatie
volatile (see Table 1).
2) If the precalciner is operated at normal temperatures &850°C)
then
the amount of K2sD4 in the vaponr phase is insignificant. Sowever at higher temperatures (- 950°C) a significant amount (u10W4 Kgs ger Kg clinker) of K2S04 can be in the vapour phase. Thus a high temperature in the precalciner should be avoided as it would have a detrimental effect on kiln operation.
3) The vast majority of K SO and Na So will be in the condensed phase 2 4 2 4 on the suspended solids at the bypass position.
This would mean that bleeding gas alone at the bypass position will not reduce the quantity of alkalis in tha kil,~l system.
The most effective way of reducing
alkalis would be to bleed off the dust on which the alkalis condense.
4) The quantity of KC1 in the precalciner for a precalciner operating temperature of 850 - TCO'C is high (1-a on clinker) but this is -4% consistent with the values predicted in ST3 01/13 and&rem results of normal vorks operating experience.
This is taken into account in
the bleed requirement. 5) The effect of sulphur is wbrised in Table 2, This table demonstrates that the source of sulphur, the kiln exhaust composition and the presence of other minerals determines whether the sulphur vi11 be bled off by the bypass, recycled within the preheater, or exhausted from the stack. 6) If sulphur is present as calcium sulphate, or calcium sulphide the two control&g parameters on the volatility are the gas temperature and gas composition.
3ouever the effect of gas composition reduces with decreas-
ing temperature and the ev$ution of SO2 will not be large (1% mar) at temperatures experienced annthe kiln back end, and will not be significantly higher with reducing conditions than with oxidising conditions,
7) Low temperature volatilisation due to the presence of certain minerals, organic matter or iron Fyrites will result in an increased SO 2 emission from the stack but should not have any detrimental effect on the sulghur cycle in the precalciner/preheater 8)
system.
Sulphur from the kiln fired fuel will tend to form a recirculating load between the kiln and preheater unless bled from the system via the bmsa.
9) Sulphur from the precalciner fuel will form a recirculating load in the same manner as wlphur from the kiln fuel.
Houever this cannot be bled
from the system until the sulphur has been recycled. 10) Approximately
9546 of the SO2 in kiln and preheater gas streams will be
absorbed by tne CaO in the kiln riser duct providing that the gas and materials stream are in contact for '&fficient time for an equilibrium to be reached.
5.
This time is very short due to large 8urface area of CaO.
RECOI+2EDATIOBS 1) Further work should be carried out on the Oxford raw materials to establish the form in which the sulphur is present in the kiln feed. 2) As the amount of low temperature volatilisation is dependant on the
form of sulphur in the raw meal a standard method for determining rav material volatility should be developed.
If wch a method were available
it could be carried out as a normal laboratory test on the rau mterial. The information could then be used either for design information for future works, or as information to the kiln controller to enable zare efficient operation of the bypass and kiln.
3) When the form of sulphur in the raw material hae been established, a simulation of the Oxford Works raw meal could be tried out at an existing BCI dry works, (poselibly
Plymstock). Sampling throu&out
the system would then show where, and at what temperature, volatilisation and condensation of alkali and sulphur containing compounds occura*
1)
oxford Works - Modern Dry process with sulp;lur Rypass - an assessment of the technical risk.
STN M/13.
PA Loognarl,
D.S. Svift, March 1981.
2)
Processing of Cement Flue dust.. PhD Thesis, Jaw Huei Wor, December 1979.
3)
Sulphur pollution from coal combustion. Effect of mineral component on the thermal stabilities of sulphate ash dsd calcium sulphate.
Baker D.C. and Altar A. Ewironmental
Science and Technology. P!!ch 1981. 4)
Recirculation problems in Rotary Kiln Systems. H, Xitznann, Neubechun. 338-343.
5)
Translation of Zement-Kalk-Gips
(8)
1971.
Desulphurisation of hot reducing gases vith calcined Dolomite ET Turkdo6an & R.G, Olsson.
Irorrcuz&ing & Steel %king 1978 Ho. 4.
Estiloated Alkali Vawur in the Kiln/Freheater Gas per Kg of Clinker (no bypass
Position in System
WQ4 &3s/ Kg Clinker
Na2w4 %s/ Kg Clinker
4.8 x 1O-2
1 . 4 x 10 -2
Burning zone at 145OOC
0.55 Kgs gas/ Kg Clinker
IxL QdQT Clinker Complete Volatilisation Expected
I At 3ack End at 1050°C
0.55 &P @S/
8 x 1O-4
lo-5
-2 5 . 6 x 10
per Kg Clinker
Precalciner high temp. 95oOc 1.7 Kg3 es/
-5 5 I 10
Megligible
Negligible
Negligible
4 . 6 I 10 -2
Kg Clinker Precalciner
normal temp.850°C 1.7 &3s gas/ Kg Clinker
1 x 70
-2
2. TAB=
Effect of Sulphur on Rw Fkterial Volatility
Source of Sulphur
Conditions aff'ecting reactions
Likely effect Cam4 will be recycled being
CaS and CaSC
4
Oxidising
vapourised in the kiln and condensing to form CaSO
4’ in the precalciner unless tha SO2 level is reduced. CaS will tend to form CaSC
CaS and CaSO
4
Reducing
4
and act in
Similar to oxidising conditions except CaS will form in the precalciner and the So2 vi11 evolve at a sli&.tly lower temperature.
FeS2
The bulk of the sulphur will be converted to CaS but an increased quantity of .sulphur vi11 be lost from 'he system via the stack.
CaS and CaSO
4
CaS and CaS0
4
Presence of so* Fe203
Increased losses of So2 from
sodium and calcium
the stack when corn-red with
montmorillonite
pure CaS04
Presences of Potassium containing minerals
Y&e SO2 in the stack exhaust when compared vith the effect of SiO 2 etc.
Cod/.
l . . .
TABLE 2. Cont'd.
Source of Sulphur
Organic Sulphur
Conditions effecting reactions
LikeQ effect
A11 the sulphur vi11 be lost via the stack exhaust.
Sulphur the fuel
0
Temperature
("C)
Figure It Saturation Yapour Pressure of El, K SO ank Na SO 2 4 2 4
2FIGIJEE
.
Temperature @) Figure 2: Saturation Mass of KCl, K SO and NapS04.Yapom 2 4
in Air
-:18-
:
__- .- -_--
_‘.-
-.-
- --z
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 4
Factors Affecting Sulphate and Alkali Cycles in Rotary Kilns and the Implications
ANDTHE IMPLIC?YXONS OFTZESE EZFFECTS KITH RESPECT ?o P-S CDNTROL
SYNOPSIS
i)
To identify the fundmental
and empirical relationships governing
sulphate and alkali cycles in rotarykilns. ii) Using these relationships; to explain the kiln performance with reference to clinker sulphate ark3 alkali retentions at bth Northfleet and Hope M&s. iii) In the long term, to learn kx4 best to control the rotary kiln process with respect to sulphate and alkali levels, in order to qtimise the prciluction
of clinker that ineets current rmrket requirements.
Anexamii-la tion of alkali/sulphate cycles has 'keen carried out using
data frantrials
carriedcutatNxthf1ee-t andF@e b&As. The fundamental
an3 e-rpiricxl relationships qzvernirig these cycles are discussed along with deviations fran those relationships.
The magnitude of the suggested fat-
tors ki-iich cause deviations fran the fundamental relationship is estimated using data cbtained fran +mAs trials and ,&lished literature. is then used to mre closely mcdel the real situation.
This data
The resultant r&e1
when refinedmaybe used to estimate the recirculating load of
alkali/sulphate
(fran hereon described as wlatiles) within the rotary kiln
system for wxks where insufficient empirical data is available to calculate the recirculating load using the traditional average crass balance method.
Using the fundamental relationships discussed in this report, the
requirements for a control stratq for rreintaining volatiles in the kiln systxn is proposed.
a'constant level of
ll-Ls strategy is based a2 the
kiln X& signal.
i)
A simple idealised wlatiles cycle within a rotary kiln can be devloped frcmkncwledge
of the temperature profile and gas flows within
the kiln system. ii) The rregnitude
of the recirculating load of wlatiles in this simple
rwcdel is principally a functionof tb.e peak feed temperature. The quantity of gas pr unit of clinker is another impxtant factor since it determines the rraximum volatile recycling capacity of the kiln gases: i.e. the wet process, through its higher energy demand, results inrcrxe gas being available to carry a larger quantity of volatiles. Conversely, a precalciner with its smaller gas quantity within thekiln, nwinff
tm
90%
dec~rlnnatinn
&
theburning of 60% of the a& cutside
the kiln in the preheater, is unable tc suppxt such ahighlevelof volatiles as a suspension preheater !ciln or wet process kiln. iii) The idealised cycle does rzot fully explain the cbserved situation. However, through studying actual -rating data, the various effects that influence recirculation canbe estimated and amxe realistic model developed.
iv)
Tne four principal factors that alter volatiles recirculation b&aviour fran the ideal cycle are the gas/solid mixing, kiln atmsphere cqsition, dust insufflation and feed -ition. The level of gas/solid mixing in the burning zone is a function of the kilnvolume loading and the aznbined effect ofkiln speed and feed residence time.
The effect of pr mixingmuld
appear tobe to
decrease the actual quantity of mlatiles recirculation to around l/Sth to l/lOth that of the level predicted assuming an ideal cycle. The gas/solid mixing tith reference to the recapture of volatiles material is a function of the type of process e.g. suspension preheaters are highly efficient gas/solid mixers at the kiln back end whereas wet process kilns are rmch less efficient. VI
The effect of a reducing atrrosphere
is to reduce thequantityof SO3
retained in the clinker and increase the So3 lost as SO2 via the stack exhaust. vi)
Dust insufflation will typically cause a large increase in the volatile recirculating load.
vii) Volatile recirculation will tend to induce a cyclic pattern of behaviour for the quantity of mlatiles retained in the clinker *en clinkeris~ttoaconstant~~limemntent. The cyclic behaviour of the recirculating mlatiles causes changes in the apparent burning characteristics of the clinker bynodifying the quantity of flux in the pre-buming and burning mnes and also induces a~geinfhequantityof~trequiredto~~aconstant
-iv-
recirculating load, i.e. the heat requirement for mlatilisation the burning zc)ne will increase *xith increasing mlatile
in
content of the
feed.
This cyclic behaviour makes kiln axtroltc
a&t-ant
free lime
based on kiln amps exceedingly difficult. A amtrol strategy to mintain a am&ant
level of volatile recirculation till rerrove
cyclic behaviour.
this
Ihis stability can be achieved by maintaining a
constant burning mne teqerature
with a constant feed input. Mrk
carried out at I-b&pe Wrks has shown that the best relationship between an observed kiln pa.rar&er and the level of recycle tas found to be N&.
This wouldman
tain a ax&ant
thataaxtrolstratqybased
onK& shouldmin-
level of reci2miLation and allow the free lime wntent
to float inlinewith changes in the feed chemistry.
i)
F'urtherwrk
shouldbe
carried cut to identify the mlue of E, the gas/
solid mixing factor, particularly wi"Lh reference to the kiln ,param+ ters of mL.xne loading, kiln speed and kiln angle. This factor, along with thepeak
feed temperature, wuld allowamre
of the level of bxning
mne volatilisation to ba predicted.
ii) Ihe relationship between clinker m&hate studied to enanpss
accurate estimate
and q should be further
the effect of ,parameters
position, coal consmrpticn and kiln cutput.
such as B.E.02,
feed ax-
iii) The effect of oxidising or reducing atrmsphere cm mlatile cycles should 'ZE studied further.
Current theory and practical results indi-
cate an inverse relationship between the &antities of SO2 and 02 in the gas stream andhence a direct relationshipbatween
So3 in the
clinker and 02 in the gas stream. iv)
The effect of raw meal mineralogy shculd ke examined in order to attempt to qmntify its effect m the ideal cycle.
VI
The net effect of the thermal requirement for empczating and oHIdensing circulated mlatiles r&is to be determined mre specifically. This is of particular relevance where preheater cleaning causes sudden surges of mterial with a high mlatile content to enter the 'kiln.
vi)
Control strategies based on N& to mintain a azmstant mlatile recirculation load should be develo,Ded and evaluated in preference to those based mkiln amps and a amstant free Lime cmtent.
FIGURE t ------we
RECIRCULATING HOPE WORKS
Stack
VOLATILE
1977
BALANCE
Gas Stream
coal
0.123
K2°
2.624
K2°
0.019
Na20
0.033
Na20
0.180
Na20
0.005
S O3
0.381
so 3
4.930
so
0,480
C l
0,018
C l
1.120
Cl
F
0.000
P
0.019
F
K2°
Precipitator K2°
0.044
Na20
0.088
SO3
0.040
Cl
0.010
F
0.004
I Duet
-
-
-
1
1R e c i r c u l a t i n g K2° Na20 so C l
3
F
L
-Is
Preheater
n
82 jXj
2.457
K2°
2.605
O.l’t9
Na20
4.503
so 3
0.175 4.450
77.11
1.092
Cl
1,108
99.7
0.015
F
0.015
8.6
83.4 46.7
3.125 0.375
Feed
$-Retenlion -
K2°
0.624
K2°
0.668
Na20
0.218
Na20
0.226
Na20 0 . 2 0 0
so. 5
1.201
SO
1.2'11
so
Cl
0.009
Cl
0.019
C l
F
0.156
F
0.160
F
3
0.012 0.004
Volatilised i
K2° Nu20
Raw Feed
3
q6.6
3
1.300
53.3 %%,C
0.003
0.3
O.lGO
31.4
HOPE
WORKS
KILN : R E L A T I O N S H I P 6
YCH NX%
ZCGC
ICC%
0
21 I?102
Q
0
m/84/13
MD'IHE iMPLICATICNSOF'Z%SE EFFECTS WI'H RESPECT T0 PXCESS GXIWL
Page lb. SXNOPSIS OBJE~IVES
w
1.0 lzYmxum1m Figure1
Recirculating Volatile Balance t30
Figure2
raOpe Wrks No 2 Kiln Relationship v beween Kiln Exit W, and Clinker SO3 cbnt Sam Tine Base
ks 1977
2.0 FUNDAMENTAL RELATIONSHIPS ?G'FZTIG bQ LECYCLLES % Figure 3 Idealised K2SO4 cycle for a Northfleet Kiln Feed LSF 98% Figure 4 Idealised Cycle for a
pe Wrks Kiln Feed
3.0 DEXL..IONS FFXX¶ THE IDEAL CYCIE % 3.1 Gas-solid Mixing 3.2 The effect of cooler ,oart of
5 7 8 9 9
mixing and reaction rate in the
10 11 12
ical Trer& of So2 amxntratim in of Kiln Feed Ccqosition
13 14 15
Page PD. 4.0 IKPLIC~TICNS OF WUTILE OF KYI'ARY KIU?S
RECIRCUUVION
To 'IWZ pKCl5S.S
CCWjX)L
16
4.1 Burning Zone Temprature
17
4.2 Gas Qm-ktity
18
4.3 Xixing
18
Efficiency
Figure 7 S02, NO2 and 02 axxentrations Exhaust (dry basis)
in the Kiln
19
4.4 Gas solid mixing in the cooler ,mrt of the kiln rate 4.5 Gas Clarqpsitions
20
4.6 -Kiln Feed CDnpsition
21
4.7 Ixlst
21
Return
5 . 0 coNcus1ms
21
6.0
22
RECXMQXDATIONS
24
7.0 REFERENCES
APPENDIX 1 THEDRIES OF CORREcrED IDEAL CYCLES
Q3NSTRUC!XoNOF
IDEAL CYCE.S RM)
APPFxxx2 EEFECTOFREDJC APPEHDIx3 HEN PIPE
CD!JDITIONS
CN THE vOLU?ILIT'Y
OF ALKALI SULE'HATES
% F KWTILE
PECIRCUATIoN
AT WPE KXKS
ANDTHE
DIPLICATIONS OF'MESE
EFEECTS KtTH RESPECf'T3 PKCESS CrXTWL
OE!JEmIvE i)
To identify the fundamental and empirical relationships governing sulphate and alkali cycles in rotary kilns.
ii) Using these relationships; to explain the kiln
ce with
reference to clinker sulphate and alkali re entions at b&h No-fleet and I-Qpe Parks. iii) In the long term,
b the rotary kiln process
with respect to sulphate and plka prcduction of&clinker that
in order to optinise the nt mrket requiremnts.
1.0 1mmLUcr1oN Kiln based su &ate and alkali cycles ("volatile cycles") have almys
been of i.qx3rta.n \ to the cement raker, since the level of 4 ficant effect co the kiln characteristics, 'the recyclehasa % clinkerch ' try and the associated quality of the cement prcduced. Howaver, w1 e advent of dry process kilns these cycles have been 3 under r xtensive study due to build-up problems in the kiln ,preheater Q an relatively high alkali levels in the clinker. i%re r Q * enviromental to
pressures have added impetus to the study, due
increasing interest in the level of q and s4, &ssiom fram the
kiln exhaust stack.
ANDTHE IMPLIC?YXONS OFTZESE EZFFECTS KITH RESPECT ?o P-S CDNTROL
SYNOPSIS
i)
To identify the fundmental
and empirical relationships governing
sulphate and alkali cycles in rotarykilns. ii) Using these relationships; to explain the kiln performance with reference to clinker sulphate ark3 alkali retentions at bth Northfleet and Hope M&s. iii) In the long term, to learn kx4 best to control the rotary kiln process with respect to sulphate and alkali levels, in order to qtimise the prciluction
of clinker that ineets current rmrket requirements.
Anexamii-la tion of alkali/sulphate cycles has 'keen carried out using
data frantrials
carriedcutatNxthf1ee-t a&F&e b&As. The fundamental
an3 e-rpiricxl relationships qzvernirig these cycles are discussed along with deviations fran those relationships.
The magnitude of the suggested fat-
tors ki-iich cause deviations fran the fundamental relationship is estimated using data cbtained fran +mAs trials and ,&lished literature. is then used to mre closely mcdel the real situation.
This data
The resultant r&e1
when refinedmaybe used to estimate the recirculating load of
This re,mrt piqwses an alternative .meticd of estirwiting
t3e
recirculating volatiles based on the fundamental relationships governing the process of recirculation.
An ideal volatile cycle 'based
on these relationships is pro,oosed, along with ,mtulates for deviations frcax this ideal cycle.
Ran this rrcdel,
a new estimate for
the internal cycle is developed for mrious kiln amditions. These relationships are then used to support controlstrategybased onmintaining clinker
es levels, espe-
cially the sulphate level, at a constant va ue rather than to allcw them to vary over a large range of values. b 2.0 FUNDAEEVTAL,
fELATICNSHIPS AFFEcrrwG V3& C Y C L E S
The tm fmdamentalthe
'c ,parameters which affect the 4 volatilization of any rmteria e the temperature and its imle fraction in the gas stream (i.e. 3@al pressure). system these are iranife tity of gas atilabl
Inarotarykiln
the temperature of the feed and quan-
Q to carry the volatiles. The relationships
between temperature, \ m fraction and recirculation are discussed in more detail in ideal cycle.
ndix 1, along with details of 'mw to construct an e Ran these relationships we mn axstruct a diagram to
erial is wlatilised and recondensed. Fbr the ,p.xpcse 9 rt the recirculation of mly one wlatile - K2SO4 - is
ShGdhCWth
of thi
discuss9 , dlthough the same technique myba used for any mlatile. shows an ideal cycle diagram for Wrthfleet Mrks i%. 4 kiln. Fi e Q In studying this figure, cry rmst mncieve the zaw feed being fed to the kiln and rmving along the Xaxis until the feed temperature has
reached a pint at 5-6 'kiln diameters frm the nose ring where the K2SO4 in the kiln feed is able to volatilise. The K2SO4 then begins to vaporise into the gas stream where it attains the gas temperature and is trans~rtedback
along thekiln to apointtiere
perature falls to a level tiere the K2SO4 will reco
the gas teme (and rem
bine in the event of dissociation), at about 13 to 15 -zfiiiL ' ters fran the rose ring in the case of the first cycle.
les will con* tinue to build up the levels of volatile in the kiln bed until the maximm
quantity of K2SO4 tich can be
is reached. This.
will occur at the &peak feed temperatur
around three kiln diameters
frcm the nose ring.
any excess K2SO4 in with the clinker. Fbr
example, consider
approximately 25 cycles reached and K2SO4 culating load is 25% K2SO4 QI clinker. etical kiln feed history is illustrated in figure Theiqmrtantfhingtomtecnthiscycleisthe ity of K2SO4 mlatilised.
This is indicative of the
ility temperature of Hope tbrks kiln feed and the 1-r
1.92 kgs of gas per kg clinker in the burning zone and 2.75 kg of gas' per kg clinker at the back end.
E?qp Parks 'has 1.40 kg of gas par kg
clinker in tie burning zone and 1.94 kg of gas per kg clinker at the back end.
FiKurd 100
- Iticaliscd KSO!, c y c l e f o r n NorthI’leet
t
90
80
pdircction
70
4-edircctiorl
o f
gas
ol’ feed
80
50
b0
JO
20 solid gas
10
0
to
stream
t 0
K I L N DIflHETERS
F R O M HOOD
K i l n h’eed
LSF 985\:
L 0
C 0
&MN113 NO X O3SIlI1UlOA tOSZW SSW
3.0 DJZVZ~TIONS
FFCM TEE IDEAI;
CicLE
The volatile rrms balance in figure 1 gives details of the maan long tern recirculating ILoad at Wpe Wrks.
If ma consider K3SO4,
the equivalent average quantity of K2SO4 recirculating is 1.94% which is only l/lOth of that predicted by the thecretical
It is
these deviations fran the ideal cycle which are section, and, tiere pxsible, these deviations
in this
quantitative est
the ideal cycle axe given.
The factors which are amsidered discussed in mre detail belcw. ?he
f the effect of "
\ be of major iiqm2ance
are
ors are:-
i)
The effect of gas/solid ' the burning zone. +
ii)
The effect of gas/s&i
g and reaction rate in the oc;oler
,"rts of the kiln. iii)
The effect of kiln atmsphere.
zone of a rotary kiln, vqmrisation
till take
volatilised is in mntactwith the unsaturated
hut
this wntact,
gas..
The rotary kiln is very inefficient at ~rcducing
as the bulk of therraterial is contained in the feed bed
which presents cxiLy a szmllsurface
to the gas strfxrnatany
time.
For this reason a mixing factor (E) with dimensionless values between 0 andlneeds
to be intrcxduced.
The mlue of E will approach 1 tien
the gas/solid mixing is very efficient and allows all the feed +a acne into cc&act
with the gas.
Thus the value of E represents the frac-
tion of the total solid surface ewsed to the gas stream. Using this interpretation be can say that E will be a function of the volume loading of the kiln: (a small~l.umeloading.presents
w ger surface per unitmss of feed than ahigh volume loading) and the ccmbjned u function of kiln speed and kiln angle. A high Q . factor wuld be expected with alowkiln
slope and ahighki
Work is currently being carried cut has
m aza-quter
and vice versa. ncdelling as to
the precise value of E; the 'nest value ZL r hasedcnpresent information is 0.1 to 0.2. + .l to 0.2 implies a decrease in the
The use of values of E maximum arw3unt
of volatilisation % the Iwrning zone by a factor of 5
to 10.
0 0
3.2 TIE EFFECT OF GAS SOL
"
ANDKEACI'ICNRATEIN'-!XEC0LERPARl'OF
THEKIlxsi!Ls~ % This e ect is very mch a function of the type of process. For example, a dry process kiln &i&has intimate mixing of gas + solids at temperatures tiere reactions 6311 cccur very ally all the volatiles till be captured. The mixing of this part of the process can therefore be said to apprwch
unity.
For a wt or semi-wet process, the tilk of feed
material is retained within the feed bed and therefore less intimate mixing is likely resulting in a much 1-r level of Mlatiles captured (circa 50% efficiency).
being
This later case is illustrated in da'~a obtained fran an HES bum carried out at Pamstone ibrksc21 see figure 3 here the quantities of K20 and SO3 retained in porthole samples 1 taken from the chain section outlet, and porthole 3 taken fra a point just prior to the burning zone, show v-q little difference (i.e. change in K20 and only 0.5% change in SO3 content).. The stirring ction in the chain section of a long wet 'kiln does increase the mixin efficiency, 9 hence there is a sharp decrease in So3 content
rtholelto
the
stack of around 70% and in K20 mntent of around v 906. It is possible that the actual mixing efficiency within a lon %t kiln is likely to be prirrarily a function of the chain
This section refers i
'tally to the oxidising potential of
the kiln atrrosphere and its -ifi? eff upon the volatilisation of sO3. The fact that a reducing
‘In atxrxphere as indicatedby alowbck Q end oxygen content (generally < 1.0%) gives rise to lu48er clinker SO3 andhigher exhaustSO2 \n thehigher oxidistig conditionhas
well documented
myle and Ferd41,
been
B.-KG&~] and ~la~urne~~~.
e Eetails of th possible rre&ani'sins mncemedwith this process stay be found in values
' 2 tile the effect is illustrated in figure 6. The & in figure 6 will be dependent upon the type of process
(e.g. wet Q), feed and fuel -ition and burner design. The feature is the shape of the curve i.e. a very rapid change in SO2 content in the exhaust gas (andhence an equivalent change in the SO3 in the clinker) for a sznall&ange
in oxidisingpotential
FIQURE 5
K2°
0.13
SO 3
0.32
BARNSTONE
HES
TRIAL
NO. 6 AUGUST 1980 -
VOLATILE RECIRCULATION
Feed K2° s o3
I.77 5.‘+3
I I I 1 1 A
End of Kiln Back
Temp. 2-300 C ’
Temp.
800~
Temu
Ti?ICAL
2500
TRZXDS
OF SO2 CO?;CE?XX4TION
IY KIL!J EXYAUST
-
NZARLY ALL SULFUR ?XTTED AS SO2 Y7 v 2000
1 I
1500
.?ROBABLE TREYD WL"r! LOU ALKALI FEED 1000
----__
500
I.0
2.0
02 CO?lCEXTMTION
Extracted
A.0
3.0
(I)
from :
Doyle and r"er& A?$icatiocs of Flue Gas halysis to Cement i
5.0
of the 'kiln gases from reducing to oxidisirg cmditions. pattern has teen observed at LXorthfleet
A similar
Wr'ks during tests cm the So2
monitor where SO2 is practically undetectable at hack end oxygen levels of qeater than 1.0% tit very quickly rises to levels of &me 500 ppn when back end oxygen falls below 1.0&[71.
A change in SO2
concentration in the exlmust gas at Sbrthfleet of 0 the equimlent of approximtely
500 ppn So2 is 0.4% SO3 a2 clinker. b b v
3.4 TE-E EFFFCI OF KILN FEED (XXKSITION This can be categorised into tin0
i) ii)
The effect of the rawrreal chemis & ‘The effect of the rawrieal
try after being rrcdified &
by
recirculation. @Y mly differ in the absolute quantities
cementrawrEa1s
of volatiles, but also tt*o Tit meals -tith the same mlatiles
amtent
may have very differe ' eralcgy. Mrk by Baker and r~tmarrC81' for %Y exarqle, has that certain minerals have a greater propensity to
inlpartaninde encourage the
ationof
SO3 in particular.
These minerals
' ualcfiaracter to the particular rawmaal. and there is
no way as yet tern ex
estimating their effect upon the recirculation pat-
empirical 4%
techniques.
irculation of the mlatiles themselves causes the content of @ volatiles entering the burning zone to increase. This has tm affects upn the burning characteristics of the raw meal:-
i)
Both the armuntof flux within the burr&g zone and the apparent length of the burning zone will tend 'io T. in increase in kiln amps due to increased volatile load could, for example, be mistaken for an indication of overburning and any resultant reduction inburning zone terrrperature axld lead to an increase in clinker free lime.
ii)
An increase in fines within
the systemwillalso tend to encourage clinker ctions w which in turn should decrease the free Ii, axtent. T7 77 The evapration or disocciation of the wlatlles in the burning zone will de-r-and a significant quantity XT high grade heat fran the flame.
ver, is recovered further ?%is high grade heat k dcxn the kiln as lw grade h t when the wlatiles recondense. ' effect" is shown in appendix + tiat the recirculating wlatiles make
Pa estimation of this 'heat 3.
The overall effect *
a significant demand (up %100 kcals/kg clinker) upon the heat atilable in the
zone andhence they tend to increase
the totalheatinputreqirementto
the kiln.
\ f&s could aggrevate each other and with the net system, raw mealch
'stry, and the oontrolstrategy
+
smployed.
3.5 THE EFFECT OF RETURNED lxiZ?I A rmjor differenc:e
between the idealised cycle and the real
situation is, that although much of the recirculated mterial condenses cm solids derived frcan the kiln feed, these solids are generally held in suspension and therefore swept frcm the kiln in the gas stream.
If the dust is returned, especially by
lation, then
the observed cycles should bqin to approach th mixing factor (E) will begin to approach unity. The greater the armmtof dust return , the greater the Y recirculating load and hence the clos
clinker mlatiles till to the system (except for at t-ratures
burning mne is considerably
within the
ter than cne at-sphere and hence, ir
theory, should have an k-finite %pacity for volatilisation).
4.C
0 e IMPLICATIONS OFKKx4T&E FECIRCUIATION~ PIE Pm C?N'I'floLOF mARY IiIL~S This s discusses the implications that mlatile 4 recirculatio has onkiln mntrol. ijherever possible the individual factors dis ed in the previous section are related to kiln cofkrol 42 parame s uch as burning zone temperature and kiln feed rate. SOme fo of the factors such as recapture of mlatiles will tend to be related to
c&-
ss design rather than any &aarameter
part of a control strategy.
tNhich can be altered as
These should be rmted, as it is highly.
probable that any control strategy considering mlatiles recirculation muldbe dependent to a certain extents the type of,orocess.
The quations given in table lcgl shm the iqxtance of temperature as a factor amtrolling
mlatile recirculation, because
the amunt of recirculation has a logarithmic relationship to temperature.
In a dxy process kiln, the usual strategy is to control
the kiln to produce clinker to a selected range of fr by mnitoring the kiln amps.
' e cmntent w This signal is a function of, amngst
Y7 other things, the amount of flux present in the )u . However, in v section 3.4 it was pstulated that volatile the armuntof flux in thekilnwith litt inputratetothekiln.
Thekilnmps
sane degree cn the rragnitude of the strategybasedmthe
irculation will mxlify T or m change in the fuel
' therefore be dependent to & tile cycle.
Akiln control
ining a steady level of kiln amceptofiii? '
amps is likely to disturb the
'les cycle when recovering fran a % process disturbance khich ' in turn affect the ammtofkrning zone flux and hence kiln -Q For example, a perturbation in kiln feedchenistrythat' Zip.
amps set pint
eases mlatiles till tend to,increase 'kiln % react by reducing the BZT to maintain the the clinker produced would be both underburnt in mlatiles.
Q
makltain
tant coal feed strategy is similarly insufficient to le kiln control,
as any increase in recirculating
Q will tend to depress the temperatures in the burning zone vo1ti andhence theheat inwtto the kiln will heed tobe wied in order to control the solids temperature andhence the clinker free lime content.
In order to mintain a specific temperature in the burning zone, the best ,Darameter
to consider is therefore the peak feed teqerature.
Unfortunately present day instrumentation is unable to supply a reliable direct indication of ,oeak feed temperature. An alternative method of establishing a peak feed temperature is to measure the parameter qon which the peak feed temperature depe gas teqxxature.
fhe W
This maybe carried out indirectlyby&asuring
a
parameter dependent upon the peak gas temperat This will then be a function of the peak that the feed rate is n-aintained
at a
constant level. The
relationship between NO, and clinker So as alreadybeen discussed 4 (see figure 2). Further evidence fo the relationship between wlatiles content and N& is show in fi concentration is seen to foil
. Here the kiln exit So2 & exy step change in Q, highlighting
the particularly strong correla % ' between these tko ,oarameters.Clll 4.2 GAS CXJANTI'IY
0 0
This, as mention %? previously, is principally a process design variable. Any
on the mlatiles cycle due to gas quantity
changes assoc' ted with fuel changes are liable to be masked by the greater s * ty to changes in the burning imne temperature. 4 The appr te strategy txxards gas quantity control should therefore Qmintdin constant gas flew through the kiln together with be-try a
t feed rate.
4.3 MEQNG EFFICIENCY This factor is essentially determined by the process design. As discussed in section 3.1, the efficiency (E) is determined by a Ccnr
bined function of the kiln speed and 'kiln slo,pe which determines the amount of effective surface area of feed within the burning zone. As a consequence of the gecmetry of the circle, a significant change in volume loading will only have a small change in the effective surface area of the feed within the typical limits of kiln volume loading. As thekiln
angle is fixed, thekiln
speed is the cnly
meter tich can have any influence ctl the wlatile postulated that this influence should not be lar
cess' le paraFITIt is cycle. rasmallrange
F of kiln speeds as there are tw opposing effects. An increase in kiln speed would increase the rate at which effecti 32 surface of feed is exposed for volatilisation
to take pla but decrease the time +L available for evapration. I t is p tulated that the likely effect is for them to cancel each other unlikely to be critical in
'In speed is therefore a a constant level of
volatile recirculation.
\a function of process design. Changes due to This is similarly this effect mul design but
causetl in bet process kiln by altering the chain
e ges during operation of the kiln are unlikely to affect
(Gas wition tich affects this will be volatiles r ure. a discuss t h e next section). a 4-5
=fFF= As was stated in section 3.3 a &ange in oxidising~tentialof the kiln gases can have a large effect cn the recirculation of SO3. If a strategy of maintaining a constant wlatiles achieved, a constant back end oqgen
cycle is to be
level will be necessary.
4.6 K!ZN FEED CMFOSITION The effect a2 kiln mntrol of the kiln feed rrcdified by mlatiles has already keen discussed with reference to temperature in section 4.1.
If%wever, even if a constant level of recycle is rmintained,
there stillmyke changes in the feed chemistry entering the'azming zone due to changes in feed &en&try of the raw f qUate blending.
cau edby inade-
Pius, as with the strategyofhrning -9 a target
free line ken &anges in LSF of the feed muld
instability, a
v control strategy based cm mlatiles also r*es mw feed of consistent chmistry. 4.7 DUST KEI'UEN A dust return system tends
rate 01 a discrete level (ie.
either on or off).
either turning cn or off a dust
return systan till dis
cycles as the change in iciln wla-
tiles input will affect culating load is
Ps the size of the recirdependent, the mjor factor
thatwuldbe
affected
- the mixing factor. 'Ihis muld
take cm anew
and a mew volatiles recirculation equilibrium % bymaintaining a steady Wrning zone tmperature.
lised volatiles cycle within a rotary kiln can be devekncwledge of the temperature profile and gas flows within the kiln system.
ii) The magnitude of the recirculating load of mlatiles in this simple rmdel is principally a function of the peak feed temperature. Another factor, the quantity of gas per unit of clinker, is iqxxtant since it determines tie mxirmm mlatile recycling capacity of the kiln gases: i.e. the wet process through its higher in more gas being available to otrry a larger Conversely, a precalciner with its smaller gas
y within the kiln,
owing to 90% decarbonation and the burning of
-1 outside
the kiln in the preheater, is unable to supp%kuch a high level of volatiles as a suspension preheater ki iii) The idealised cycle does not
wet L;rocess kiln. the observed situation.
Ffowaver, through studying actual cpe ing data the various effects n that influence recirculation estimated and a mre realistic model developed. iv) The four principal fa(;tors tit alter mlatiles recirculation bahavia= fra the ideal \ cyc .e are the gas/solid mixing, kiln atmqhere fflation and feed amposition. The level of
solidmixing
in the bnxning zone is a function of the
kiln mime4!ti 1 ' g and the ambined effect of kiln speed and feed residen
') e. Q
l3e effect ofpr mixing wuld appear tobe to
the actual quantity of wlatiles recirculation, to around l/lOth that of the level predicted assuming an ideal cycle. The gas/solid mixing with reference to the recapture of volatiles material is a function of the type of process e.g. suspension pr*
heaters are highly efficient gas/solid mixers at the kiln 'back end whereas wet process kilns are mch less efficient.
VI
The effect of a reducing atmosphere is to reduce the quantity of so3 contained in the clinker and increase the So3 lost as SO-2 via the
w
stack exhaust. vi)
IXlst insufflation till typically cause a large
in the
volatile recirculating load. vii) Volatile recirculation will tend to indu
a cyclic pattern of beha-
viour for the quantity of volatiles retain & in the clinker hhen clinker is burnt to a amstantfree The cyclic behaviour of the re
wntent.
'
e? ating volatiles muses changes in
the apparent burning characteris @%zs of the clinker byrzdifying quartity of flux in the
the
uming zones and also induces
a change in-the
tomintain
a constant
recirculating load.
basedcxlki e
exceedingly difficult.
cyclic 43OUT.
This stability can be achieved by titaining
This cyclic b&ml Q mkeskilnmntroltoconstantfreelime Aamtrolstrategy
to n-&n-
tain a 03 tan level of volatile recirculation will rezmve this
burning zonetmperaturewith
aoonstantfeed
a
input. Wrk
out at Hope Works has st-hown that the best relationship betm an observed kiln parameter and the level of recycle was found to be PI%.
This muldman that a aontrolstrategybesed QIQ shouldmin-
tain a asnstant level of recirculation and allow the free lime content
6.
FBXMGBDATIONS
i)
E'urther
mrk should be carried cut to identify the mlue of E, the gas/
solid mixing factor, particularly with reference tc the kiln parameters of volume loading, kiln speed and kiln angie. This factor, along with the peak feed temperature, kould allowarmre
acwe estimate of the level of burning zone tolatilisation to redicted. Y7 T? ii) The relationship bst*en clinker sulphate an F.Dx should be further studied to envss the effect of ,mrameters\ s as 3.E.02, position, ~1 consmption
and kiln ou
iii) The effect of oxidising or reducin should be studied further.
feed am-
. 6 sphere cc mlatile
cycles
Current%zry and practical results indi-
cate an inverse relationship the gas stream and hence
the quantities of SO2 and 02 in 4 ect relationship between SO3 in the
clinker and 02 in the gasQ s . iv)
The effort nf raw
ek , 1
ergiccv &c&d be examined~~im-order
to
f; atterqt to quan VI
The net eff
its effect m the ideal cycle. % the thermal requirement for evaprating
and cm-
densing c' e cul ed vclatiles reeds to ke detemiined mre specifically. This is & icular relemnce where preheater cleaning causes sudden SIX f mterial with a high volatile cmntent to enter the kiln. vi)
c3 Control strategies based on Mx to maintain a constant mlatile recirculation load should be developed and emluated
in preference to
those based on kiln amps and a constant free lime ontent.
7.
F33ERExcEs
1)
Weber P.
Heat Transfer in Fbtary Kilns with due Regard to Cyclic
Processes and Phase Formation. ZKG tiglish Special Edition 1963. 2)
Longman P.A. Swift LG. Oxford Mrks -~twem Dry Process with sulphur By-Pass - An asseswnent of the technical Risk.‘ SI'N 81/13.
3)
Rogers A.R.
Assessment of the potential to q-tin&sew performance by
application of improved prczess sensors : tinit Kiln September 20-24th 1982. TN 82/28. 4)
Coyle B.W. and Fenk F.W. Applications of works operation.
5)
Brmm A.W.
FwkPrcducts,
Nov19
Chadbume J.F. Kilns.
. FkSXCh
, SR-65/7/M&3.
Continuous i% 'tar' 9 g of Gaseous Emissions an Cement
Paper presented to
Control Association, Jun 7)
gas analysis to wnent %
Retention of Alkali and SuLph 4.2 in Clinker.
Division Repx~tis 1 and 2 SP-65/33
6)
* of I-bps wxks Y7 v
d Meeting of the fir EMlution 4 9.
Lorimer A.D.J.
cklline 802 Qnitoring : Assessment of LXXJV and
Electrcchemical
cell
: May to October1984. %terns at Northfleet
m/84/21. EBkr D.C. and k A. Effect
of '
Ashand
8ul.phu.r mllution &an coal ambustion.
Cxnponents cn the Them&l. Stabilities of sulphated
cium S&hate.
EZlvironmental
Sciences and Technology,
March @@ .
9)
.
The processing of Cement Flue Ibst, PhD Thesis, ?he
m University of Aston in Birmingham, December 1979.
10)
Haspel D.W.
Le@ Grate Mt Transfer bechanisns. TM. IxJH.005,
Weardale bbrks,
1973.
11)
Analyser 2.3nk at the ?A &3vre lbrks. CETIC mviromt
Gq2pelle M. Sub-m-mission
12)
Turkdqan
September 1984.
E.T. and Olsson R.G.
with Calcined l%lmite. 13)
Myers J.C.
Sul&ur
Desulphurisation of Wt Reducing Gases
Iron Making and Steel ?&king No. 4, 1978.
Balances in @m%t Kilns and
Thesis, University of Texas,
Austin, 1977.
& D.
stenson
De
r 1984
@Y
ilers.
!4Sc
(i) APPENDLyl THM)RIES OF FECIFCULATICN, 3-E CTINSI'X~ION OF IDEAL CYCLES
The volatile aanponents of aarnent raw mals and fuels are vaporised ken the feed temperature is raised to a level tvhere the vapour pressure of the volatiles bscunes significant.
The wlatile
caqments will then impart a partial pressure to the gas stream. Cn cooling, the vqmur pressure of the arnponents will decrease causing condensation.
E?y measuring the kapour pressure of certain cmpments
at different temperatures, enpirical relationships for the saturation vapour pressure and temperature have been established.
Equations
relating saturation vap0u.r pressure to temperature are given in table 1, equations 2 to 7 and shown graphically in figures (i) to (iii) C91. The saturation repour pressure can bs used to calculate the saturation vapcur mncentratiun vf
= P* PT
by applyirlg Raoults Law:
Mv Gj--
(1)
where vf is the saturation concentration of bap3ur in the carry%
gas
(kg/kg
9as)
l
P* is the saturation vapour pressure (any pressure unit) P is the pressure of the gas (same units) Mv and m are rm1ecula.r lHeights of the vapour and gas reqpectively.
An example for the saturation axcentrations
of I@1 and K2S04
in air is shown in figure iv. Examples of bw to construct a volatile cycle using equations ltc 7 is shcmlbs!low.
(ii) The first stage in oonstnxting
an ideal cycle is to obtain a
feed and gas temperature profile. For this, kxwledge
of the ox-
binability of the particular feed for a particular targetted free lime must be available.
This can be cbtained fran empirical data. This
will give the pfaak feed temperature.
In the exarrples
shown below the
peak feed temperature for the wet process is lSOO°C and 1490°C for the dry process.
Azonedheatbalance
rrcdel for the kiln can then be used
to derive a temperature profile.
Fbr cur examples the temperature
profiles are sho+m in figures v and vi (the oscillations in figure (v) are due to graph interpretation; thecurveshouldbemth).
By
use of equations 2-7, the partial pressure of the volatile QsmFonents maybe calculated. Equation lwill concentration. ccqosition.
The mlecular
then give the saturation vapour
weight of the gas is estimated frcm its
The calculations are sumnar ised in tables 2 and 3.
By mking
assumptions for the gas quantities at various &mints
in the kiln, the actual amount of mlatile &per !kilc.grarn of clinker can be found.
2-ii.s is s-ised in table 4.
Table 4 can be used to amstruct
an idealised cycle as seen in
figures (vii) and (viii). Ekplamtion
of the idealised q&e
To explain the idealised cycle, an example of akiln
to&ich
is fed the equivalent of 2.0% K2SO4 in the feed and fuel is 0311sidered.
Cm the first cycle the feed will be tranqmrted
by the
turning action of kiln until, in the case of the dry process in figure (vii), a pk.nt4.5
diameters frm the rrxe ring is reached.
feed reaches a teqerature
where significant empration
of
Here the K2SO4 Qn
(iii)
occur as K20 and sO3.
The m:.atile afnpments are taken up 'by the gas
stream &ere they rapidly achieve gas tmperature
and are tranqmrted
away fran the burning zone to the back end by the kiln draught. At twelve 'kiln diameters the gas tenperature has fallen to a level tiere significant condensation as K2SO4 on cccur.
This is then inmx-
porated txck into the feed and transported fomds along with the fresh K2SO4 fran the kiln feed. On the second ideal cycle there is an quimlent of 4.0% ~2~04 in the kiln feed approaching the burning zone.
The cycles will con-
tinue until a +.ntattw kiln diameters tiere the riexi~um allckable evaporation determined by feed temperature is reached. Ebr the dry process curve this is 16% m clinker after eight idealised cycles. Cn the ninth cycle the K2SO4is raroved fran the kiln in the clinker as the equivalent
of
2.0% K2SO4.
The corrected cycle The idealised cycles shown in figures (vii ) and (viii ) are rx3t followzd in practice. The factors of 'mixing efficiency in the burning zone and backerd, dust loss and reducing conditions all account for deviations fran the idealised cycle.
Tables 5 and 6 give results of calculations
based m assumptions of suitable Mlues for these factors. Fran fhese tables, figures (ix) and (x) are cbtained. 'Ihe solid lines cn figures (ix) and (x) indicate the 1evels of mlatile corrected for the mixing efficiency.
The broken lines show further axmctions
to decrease the
amount of material recaptured in the cooler parts of the kiln due to dust loss and reducing conditions.
Saturated Vapour Pressure of KCl,
TABLE - - ^ - - I -I-
K SO and Na SO
3--4- Z-4
Reference
LoglO
(p,/atm)
= - '47oo
5 9oo
Decomposition Suppreoscd Loglo
( ps/atm)
Loglo
(po/ntm)
Hart and Laxton, 19671 llalntead, 1970 (cf. Koaugi, 1967; Duboia et al, 19613)
P '- ' "O" 5 'O"
6.23
Cubicdfotti Kuneahea,
,‘ DccomnPnit.lPn I
Loglo
(pa/Atm)
=
-y
-t
7*oe1
I
1400 - 1625
and 1972
g t:u h
0.30
5 g“,
0.25-
‘0 Lla 5 2
0.20 -
0.15 -
0.10 -
1200
1250
Temperature (“Cl
Saturated Vauour 6 omce:
JANAZ',
Pressure of SC1
1960 or J.snz, 1969)
TJ,zzz ( ii) ______---s-.-
A-
Decomposition
unsupprrsscd
DecomposXon
suppfms*d
3-
.5-
.o-
S-
10 -
OS-
1150
I
1200
I
12so
1 1350
I
13M)
Temperature ["Cl
Saturated Vawur and Decomuosition ISource:
?ressures of K,sO.
Eart and Lax-ton, 1967; Salstead,
19701
1.c ?ICc?z (iii) ------------
0.'
0.f
z
0.1
ri) T z ; 3 :: v
0.6
g
0.5
&
P v 1 1 ",
0.4
;;:
0.3
0.2
0.1
0.0 Temperature
Saturated
Vaoour and Deconuosition
(Source:
Cubicciotti
("Cl
Pressures of Ha SO
and Xeneshea,
1972)
2-4
.C u
1V
C 0 .-
Temper abre ( C ) l
Saturation
Capacity of XC1 ad IT 2SO - - j -Vau0u.r ------in Air from llOo-l’jOo°C
FXCURL (v) _..--------
:wL. t PI~oce:is F e e d uh _..--__ ps tempeniturc ------..__ --^----_ p-oSi1.e -----..------------.--
6 - F E E D TENP 6 - GM TEW
E’IGUNL (vi) ..--------c2500
0
proTilL? :I)ry ‘rOCCS9 - - - -temperature - - ^^-CI________------L.------- -Feeh - L - - dand - - gas
- FEELI TEHP
2000
SD0
- “.-. .-.~-‘.-7*‘-i’ --i+---- “--T-
K I L N OIFlMETERS
.__ -,-,-__-_ .-+-- -+..-- -. 14 -2 o *-.. 10 11 +s-.-;‘s-12
FROM NOSE RING
-.. -I-. _. _ _-, 15
IE
Gas t
i
Sabrated Concentration i.3 qas (!&kg p.5.) Kcl
WC
K2So4
K2SO4
-2504
?ia2SO4
0
500
1250
.2.$6
3.07x10-3
7.02~10-3
1
600
1350
infinity
0.031
0.032
*,
0.099
0.123
0.169
0.232
0.031
0.032
2.93
3.94x10-3
3.24x10-3
0.474
1.45xlO'3
3.64xlo-3
2
1150
1450
0.601
3
1500
1500
knit*
4
1870
1350
5
2om
1260
.
1.99x10- 3
0.169 9.95
.25x10-3 0.232 I.
infinity 0
I,
6
zcoo
1130
7
1950
10(X
8
1870
950
9.95
9
1710
910
1.45
4.29
0.023
10
1550
850
0.281
0:429
9.65X10-3
11
14.60
810
0.111
0.140
4.71x10-3
12
1370
axI
0.040
0.042
3.9SXlo-3
13
1330
730
45.27
0.0246
0.0239
9.59x1&4
14
1290
690
5.62
0.0148
0.0131
15
1250
650
2.46
3.67x10-3
.02~10-3
16
1203
600
1.16
~27~10-3
-06x10-3
0.091 0.046
T?%aLz 3.
DE ?5cGss
Satc.zit+d so Of
Feed. tcqY
Ciln
WC
Concemraticn
ia 9 (kq/kg 9s) KC1
K2So4
-2So4
XL
K2SO4
.*2SO4
liam eers
0 1
605 aao
1370
0.0400
0.0423
1417
I.
0.0688
O.oaol
4.
0.111
0.140
*a
0.0681
0.079
6,
3.78
0.0118
0.0101
950
84
0.046
4,
0.031
L-lfinhy
2
1594
1460
0.434
3
2110
1416
infhity
4
2274
1273
,.
5
2227
0.737 infinity
6
2106
925
7
1940
8%
430.0
a*
0.0204
a
1799
a74
3.465
159.4
0.0144
9
1678
a56
0.983
2.208
0.0107
10
157s
a42
0.360
0.583
a.4xo-3
11
14a5
a30
0.144
0.192
6.a2x10-3
12
1407
a21
o.ffi14
0.0700
5.81x10-3
13
1338
al4
0.0272
0.026a
S.llxLO-3
14
1277
ma
0.0125
15
1224
803
16
1177
798
0.0108
4.57x10-3
;.03x10-3
t.59x10'3
4.17x10-3
3.02x10-3
2.05x10-3
3.79x10-3
(xiii)
TAaLz 4.
Sal 33LriJ Tl,ss
KC1
.%2SO4
K2g4
0
in5.nity
5.72
6.25
1
..
9.04
11.45
2
LS.87
20.02
3
9.74
lo.a7
1.69
1.44
61s -. 1n:sr+y I.
2.17
1.7S
Dry mess
7.75
a.00
124 scc/x;$ :Ks ‘a up w
24.7
30.1
5 diamzter
$2.23
33.0
1.43 kg/k; clizkz
7.75
a.0
Aker 3
732
2.4.a
2.06
Diemeters
6.1
14s
0.44
0.264
1.99
7
4.65
27.9
a
2.07
14.1
9
2.12
7.65
10
1.67
2.9s
wet prccess 213 sx/lZi :s ?d Up to S
11
1.37
1.G
Oiaetezs
12
1.16
1.21
2.499 icg/kq di-k
13
1.02
0.29
After 5
14
0.96
&meters
1s
0.03
3.059
1 6
0.75
4
514.0
5
6.6
6
Sas c3 solid 11 12
infinity 1.
13
28.4 12.2
0.382 13.9
infizity
33.9
42.3
,a
12.2
12.3
5.45
5.33
13800
7.32
7.31
14
817
2.49
2.14
1719
4.53
4.01
15
324
1.20
0.913
752
2.65
2.1s
16
170
0.61
0.408
354
1.31
0.94
20
0.44
kg/k;/cli*e
R
.
t:
t
c
+.
h
FIGURE (viii) --1----1-----
cycle for -----_----------^---__^___ Idcnliscd
NA2SOlt
so 6 - D R Y PROCES f.!l - Y E T PROCES
50
40
so
20
10
0
KILN DIRMETERS FROH NOSE RING
(xvi) Table 5
Corrected Cycle for Mixing Efficiency No. of kiln diameter
0
!
Concentration (% &per Kg Clinker Dry E2m !SS
T
Concentration (S pr Kg Clinker IGet EYOI ss
KC1
K2SO4
Na2SO4
KC1
K2SOq
Na2SO4
Infinit
1.14
1.21
123
0.434
0.35
1.97
2.29
Infinit:
1.55
1.6
3.17
4.04
‘I
4.94
6.02
1
8,
2 3
I,
1.95
2.17
‘I
4
0.8
0.34
0.29
1,
5
.32
6
1.6
146
0.496
0.412
.22
29
0.09
0.053
7
.8
5.50
8
0.57
2.82
9
0.42
1.53
10
0.33
0.59
11
0.27
0.29
12
0.23
0.24
13
0.20
0.06
14
0.18
15
0.17
16
0.16
11
lilfinit
I,
12 13
tions
%2 = 0.2
-
30.3
40.1
Infinity
39.0
49.2
EBE m
12.89
14.6
I,
14.0
14.7
= 0.95
5.72
5.60
15870
9.02
8.41
14
856
2.61
2.25
1975
5.21
4.61
15
340
1.26
0.96
865
3.65
2.47
16
1 179
1
USlimp
11.60 1.55
-
T
0.64
[
0,43
= 0.85
hii)
bxclusicn (1 per kg Clinker) cry prccess
T
CmclL;sim (3 per kg Clhkr) Xet Praess KU
11 12
infinity 4.
13
36.4
48.1
15.4
17.5
infinity I.
6.86
6.72
19KXl
?ss.ui&ons
QSO4
Xa2SO4
46.8
1 59.0
!x5; lass
16.8
17.6
= 20% cn
10.8
10.1
Clinker
14
1027
3.13
2.70
2370
6.25
5.53
15
408
1.51
1.15
l@iO
4.38
2.46
16
215
0.77
0.52
4aa *
1.81
1.30
11
121
160
156
197
Pfhdng
12
51.2
58.4
56
58.8
Cmditioixi
13
22.9
22.4
36.1
33.6
SQ3
14
10.4
9
20.a
18.4
RecoroirationS
14.6
15
S.04
3.a4
9.88
decxe.asedby
16
i.56
1.72
6.04
4.32
75%
11
146
192
la7
236
ca7biii
12
61.6
70
67.2
70.4
13
27.4
26.9
43.2
40.4
14
12.5
10.8
25
22.1
17.5
11.8
15
6.04
4.60
16
3.08
2.08
7.24
5.2
ZffeG
~‘IcilJlikb ( ix ..---------.w
)
Corrc?cLcd Cplc i-'or KL'CO'I _____---.-----^------.--.---
20
<
@ - D R Y HIXINC FRCTOR CJ- YET HfXfNG FACTOR A - DRY DUST LOSS 0 - YET DUST 1 x - D R Y REDUCI x - Y E T REDUCI + - D R Y CBH8II y - Y E T CbHBIt
16
16
i.i;oLlt,
3.
&if
X
'ITJ GA 'I'0 SOLIIJ
V
--w-e --I--- l I I \ ----I I 6
\x , \
14
\
1
\
1
\ \
-I-
\
12
‘Q
x
L
4
2
0
f 0
KILN DIRMETERS
FROM NOSE RING
’
,
’
y\ ’ \
6
8
’
J
\ \ -\
10
\ >( \I I
\1
\ ’ \ ’ \G \
FIGUHC (x) ------.e---
C o r r e c t e d Cplc l*‘or IJA2L;ch _-__..-------^I-------c----
6 - D R Y tlfXINC FRCTCJR 2 - Y E T tiIX1Nf.i FRCTOR - D R Y D U S T LOSS @ - YET DUST LOSS x - DRY REDUCING x - YET REDUCING -t- - D R Y COMElINED ‘f - YET COMBINED
<
K I L N DIRMETERS
SULID
‘I’0 cxs
cA!i TO SOLID
FROM NOSE RING
Effect of Reducing Conditions cm the Volatility of Alkali Sulphates The precise mechanisins
by which reducing conditions within the kiln
remove SO2 frm alkali sulphates is not fully understood. real effect observed and documented by my other mxkers
Eiowever,
it is a
in this field, e.g.
Chatiume ,4 and by Brcx& fran within Blue Circle; A similar chernicalprccess to the removal of SO2 is the desul@rising of steel using lime.
This has been extensively studied by Turkdogan and
01ss0n. 021 The reactions described by Tuxkdogan and Olsson involve only calcium sulphate.
However, for cur plrFoses we riay irake the reasonable assumption
that the alkali sulphates till undergo similar reactions, as is suggested by their relative psitions
in the periodic table of elfments. I
Calcium sulphate will deccqcse reaction
under the effect of heat alone by the
:
CaSOq(s)
= CSO(~)
+ SO2(g)
4 h$2(g)
Wwever under reducing amditions
&Gs = 408 W ml-l the follwing
(1)
reactions will be
preferred: Ea.504
+
cqg) =
cao(s)
e(s) =
+ S02(g)
+ ~203~)
AGs
=(s) + “co(g) 4@
+
15
Id
ml-l
=04(s)
+
C&04(S)
-+ 4CO(g) = CELLS +
where&?
is the c%ange in Gibbs F'ree E&qy at 25°C and L atmosphere
4C02(g)
Reabsorption of SO2 till corer bythe $CaOo)
AGS
-
+ 297 kJ m31-1 183
kJ
ml-l
reactions:
+ 4SO2(g) = 3CaSO4(,) f c~s(~)
;lGs - 787 W x11-l
(2)
(3) (4)
or in the presence of oxygen by the reaction:
-o(s)
+ SZ(g)
+ $02(g)
= -Q4(s)
.GS
(6)
- 409 kiJ id-1
The wlue:GS is an indicator of the pssibility of reactions occurring under standard conditions (289K and 1 amsphere).
If the mlue
ofSGs is negative, then therrm$mmically the reaction should proceed qmntaneous1y.
Generally the mre Ixksitive,UY, the rmre heat is required in
order to m&e the reaction take place.
Thus cne would expect reaction 4 to
be favoured rather than reaction 1. This analysis takes m amount of the kinetics of the reaction. Howaver, sane idea of the reaction kinetics under oxidising and reducing conditions my be c&An& by mnsidering the relative reactivities of SO2 ami so3.
?hermlecule SCl3is ilDre reactive than sO2, both in terms ofkine-
tics and therrr&ynamics, so the preferential formation of So3 rather than SO2 wuld lead tomre sulphurbeing
in a reactive formtiere it muld
react with tie available GO, KS, t&20 etc. 'Ihe equilibriabetween and SO3 have been studied by LMyers.C13]
So2
Fran the eqirical equations
obtained byMyers, the graph in figure 2 (i) can be axstru~ed.
This
shms that the higher the percentage 02 present, the more likely is the formationof SO3 andhence the formtionof sul+ates whichtill. be mptured bythemcuningfeed.
The lcwoqqen curves shas thatsulphurwill be
present as SO2 which can escape in the'kiln exhaust. Ektreme reducing conditions lead to the formation of H2S reaqnisable
by its “bad egg"
odour: inthis case therewuldbe virtuallymrecapture on-feed.
of sulphurbythe
0 d
0
a
0
%
St
0
-
0
n
0
(xx-ii)
ua
NLlIS1J3hN83
0
20s
PD EQS’QI
::
x
‘.
0 4
0
(xxiii) APPENDIX3 Heat pipe effect of volatile circulation at F!op? ibrks
i)
Latent Heats of Vapourisation/Disscciation KC1
+
670
Kcals/Kg
N.B. V = Vqxmrisation D = Dissociation
(v)
K2S04
+ 563 Kcals/Kg + 1805 Kcal.s/Kg
Na2SO4
+ 733 Kcals/Kg + 2096 Kcd.s/Kg
(A?
t 675 Kc&/Kg
(D)
Obtained fran Khor
caso4
c91
Recirculating volatiles K20
2.457 % on clinker
Na20
0.149 % on clinker
SO3
4.509 % on clinker
Cl
1.092 % on clinker
This is quivaledto KC1
2.29 % on clinker
K2SO4 1.94 % on clinker Na2SO4 0.34 % on clinker CaS04 5.46 % on clinker Therefore Heat pipe effect, assuming all mqxmznts KC1
16.46 Kcals/Kg
disscciate fully, is:
clinker
K2SO4 10.92 Kcds/Kg clinker 35.02 Kcals/Kg
clinker
Na2SO4 2.49 Kcds/Kg clinker 7.13 Kcals/Kg
clinker
CaSO4 36.85 Kcals/Kg clinker Total 108.87 Kcals/Kg clinker Therefore rraxinnrm
total heat pipe effect for I-bps is 109 Kc&s/Kg clinker.
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 5
Alkali Volatilization- A Review of Literature Available in 1977
ALKALI A iU3VIEFw
VOLATILISATION
OP LITERATURE AVAILBBcg IN 1977
Information from 119 published references has been collated and critically reviewed. The effect of alkalis on clinker properties and kiln operation are briefly discussed, followed by an outline of the origins of alkalis in raw materials and fuels and of the published physical properties of relevant pure compounds. Known physical and chemical factors affecting volatilisation are assessed - a key objective of this review. Finally, information on the distribution of alkalis in clinker is presented, together with an outline of initial attempts at theoretically estimating recirculating loads and of practically reducing their magnitude. From the review it is concluded that there is general agreement on the factors affecting the volatilisation of alkalis. Volatilisation is increased by smaller nodule size, lower flux content in the clinker, higher levels of chloride, lower levels of sulphur, higher temperatures, longer heating periods and by higher contents of water vapour in the kiln gases. It Is not clear what the effects of other factors are (over the ranges studied in the available literature), and it may be inferred that these are relatively small: gas flow rate and composition, alkali content, presence of fluoride, LSF and S/R and oxidising conditions in the kiln.. Several of these factors appear worthy of further study. There is, nevertheless, insufficient quantitative evidence to predict the degree of volatilisation of alkalis from a given raw mix under particular conditions. The interplay of phase equilibrium and transport phenomena is probably too complicated for it to be worth while attempting to simply calculate volatilisations from known thermochemical data: more complex modelling is needed. In practical terms, the circulation of alkalis is affected as much by their condensation and deposition as by their volatilisation. It would be useful to devote some attention to this hitherto somewhat neglected aspect of the phenomenon.
ALKALI
VOLATILISATION
ISTN S7/4
A REVIEW OF LITERATURE AVAILABLE IN 1977
C O N T E N T S Page
Nr.
1.
INTRODUCTION
1
2.
EFFECTS OF ALKALIS
3
2.1
Effects of high alkali levels in clinker 2.1.1 Alkali-aggregate reaction 2.1.2 Air-setting of cement 2.1.3 Effect on strength
2.2
Effects of high alkali levels on the manufacturing process 2.2.1 Effect on kiln fuel consumption 2.2.2 Corrosion of refractories 2.2.3 Build-up and preheater blockages.
3.
a
SOURCES OF ALKALIS 3.1
Alkalis in limestones
3.2
Alkalis in clays and shales
3.3
Alkalis in fuels
3.4
Methods of reducing alkali ,zontents
of materials
4.
PHYSICAL PROPERTIES OF ALKALI METAL COMPOUNDS
11
5.
FACTORS
12
5.1
AFFECTING
VOLATILISATION
Physical factors 5.1.1 Nodule and particle size 5.1.2 Effect of the liquid content of the clinker 5.1.3 Gas flow rates 5.1.4 Duration and temperature of heating Cont./...
-iiN Page r
5.2 - Chemical factors 5.2.1 K20 and Na20 contents 5.2.2 Effect of chloride and fluoride 5.2.3 Effect of SO2 and SO3 5.2.4 Effect of water vapour 5.2.5 Alkali containing minerals 5.2.6 Effect of LSF and SR 5.2.7 Effect of kiln atmosphere
6.
DISTRIBUTION OF ALKALIS IN CLINKER
28
7.
ALKALI CIRCDLATION IN THE KILN SYSTEM
31
7.1
Estimation of alkali circulation
7.2
Reducing alkali circulation 7.2.1 Removal and treatment of flue dust 7.2.2 Bypasses on preheater kilns 7.2.3 Other processes for alkali reduction
8.
CONCLUSIONS
9.
REFERENCES
10.
NAME INDEX TO REFERENCES
11.
TABLE 1
40
.
ISTN 87/4
ALKALI VOLATILXSATION 1.
- A REVIEW OF LITERATURE AVAILABLE IN 1977
INTRODUCTION The following report summarises published information available in 1977 on the subject of the behaviour of volatile alkalis in cement clinker manufacture.
.These
materials are distinguished from other clinker-forming
materials by being solid at the lower temperatures encountered in the kiln
system
but vaporising at higher temperature - for instance in the
kiln burning zone.
The effect of this is that they circulate within
the kiln and preheater system and can, under.certain attain high concentrations.
circumstances,
The quantitive effect on alkali levels in
the clinker on removing some of this recirculating material by a by-pass may not be immediately predictable.
Together with the compounds of the alkali metals sodium and potassium, it is also convenient to consider other compounds involving sulphur, chlorine and (less commonly) fluorine, which have boiling points within the same range and which can exhibit similar behaviour.
Interest in the behaviour of alkalis in cement manufacture first arose in connection with the possibility of obtaining potash for use as a fertiliser from clays, feldspars and other unconventional materials
(I).
A considerable amount of work was done on this topic
in the United States during the First World War, as at that time most of that country's supply of potash was imported from Germany. application,
In this
it was necessary to ensure that as high a proportion of
the potash in the raw feed as possible was volatilised and recovered
in the flue dust, and that this potash was water-soluble: this latter feature could be achieved by heating in an oxidising atmsphere (2). ! This m&hod of producing @ash was uneconmic when ample supplies of potashwereavailable
fromother sources andsomrkwas discontinued
after the early 1920's. The problem of alkali levels in CempJls was once mre brought into pmninencewheh
it was suggested (3) thatcemants
withhigh
alkali levels reactedwith certainaggregates, suchas
scxw chertsr
shales and limestones, to produce destructive -ion-
This led
various national specifying authorities to limit the alkali content of. cementS, ark3 so it was necessaryto levels present.
investigatein5ms
of r&uciqg
In the 1950's, the use of the suspension preheater
for dry process kilns intr&ucedfurther
problems as alkalis are
recirculated within the kiln/prehsater system quite efficiently, so t&&clinker
alkali levels cxnnotba
flue dust (which rarely &ibits rich in alkalis).
reduced simplybydiscardingthe
a separately-collectable
FurLher, the high levels of recirculating alkali
salts tend to condense at critical pints build-ups.
fraction
in the system and create
This high level of recirculation can be reduced (with soma
increase in fuel qmsuqtion)
by bleeding off sune of the gas from the
kilnbackendanddiscar~ngtheassociatedltustburden. of bleedrequiredto
Thedegree
produce given results cannot always be predicted
accurately. Research has been undertak~~atvaxious
establishuents, with the
aim of constructing a theoretical model for alkali behaviour in
kiln/preheater system and devising laboratory tests which can be applied to raw materials to predict their behaviour in large scale kiln systems under various circumstances. Possible areas for such research are identified in the course of this review. 2.
EE7?Ems OF ALKALIS
2.1 Effects of high alkali levels in clinker 2.1.1 AUcdli-aqqreqate reaction It was found by Stanton in 1940 (3) that cracking in concrete structures
couldbeattributedto
reaction
betweenalkalisinthe
cemant and certain minerals in the aggregate, This phexxxmmn
wds
investigated by a number of organisiations, notably the US Wrreau of Reclamation (41, frcm which it became apparent that the problem had occurred with aggregates containing amorphous or microcrystalline silica.
Examples of such aggregates are opaline cherts, siliceous
shales and limestones and scma types of andesite, rhyolite and obsidian (5). theUSAto
As a result of this work it becama standard practice in
specify a rfaximm value of 0.6% by weight for equivalent
Na20 content (Na20 aggregates.
+ 0.658 K-20) for the cement to be used with such
This limit was intrcxluced
as a tentative revision to the
ASIM standard for cemant in June 1959 and in subsequent versions (e.g.
6) it is included as a characteristic which can be specified by
the purchaser if he considers it desirable. The mst serious alkali-aggregate reaction problem were found in the USA- mainly on the West Coast and in Nebraska and Kansas.
Reactive aggregates were also found in North Germany (Schleswig-Holstein) and Denmark (71, but flint gravels found in Germany and the UK have not generally been considered to be reactive. Alkali-aggregate reaction is also known to occur occasionally in other countries, e.g. Portugal and New Zealand (8). Alkali limits are included in the standard specifications of some countries.
For example Brazil (91, Peru (lO).and Venezuela (11)
followed the ASTM in setting an optional limit of 0.6% for (Na20 + 0.658 K20) where a reactive alkali is expected, and Mexico (12) has set limits of 1.2, 0.9, 1.2, 0.8 and 0.9% for Types I, II, III, IV and V cements reapectivsly, with an optional lower limit of 0.6% for use in concrete with reactive aggregate. It has, however, been suggested (13) that alkali contents below 0.6X, perhaps as low as 0.35X,
can still lead to reaction in some
circumstances. Potash and soda generally have an equivalent effect in causing expansion (7) but under some circumstances (14) it appears that more potash than soda can be tolerated without expansion. The form of the alkali does not seem to make a great difference (13) in the long term, although water-soluble alkali is liberated more rapidly from crystalline than from glassy alkali-containing phases in early stages of hydration.
It is suggested that the significant
factor appears to be the concentration of hydroxide ions in the pore solution (15).
2.1.2 Air setting of cement Rapid air-setting of c-t in storage is due $ the' formation of syngenite (X2SO4.CaS04.H20) (16, 17) and thus it is generally desirable to keep the K20 level in the clinker as low as possible where other factors indicate that air-setting-may be a problem. 2.1.3 Effect on strenqth It appears to be generally accepted in the literature that, at levels typically found in cm-e&s , increasing the aUcali.contentof the clinker tends to increaseearlystrengths strengths.
anddepress ultimate
Forexample,AS?Mtestson2in.~~cubesgavethe
following results for cemnts with different soluble K20 contents (and similar levels of potential C3S, C3A and fineness) (18):
Cartpressive strength at
Cexnt with 0.03% soluble K20 (MPa)
Carwtwith 0.61% soluble K$ @Pa)
1 &Y
8.41
9.79
3days
17.58
18.34
7day-5
27.72
24.41
28 days
45.09
32.41
The sama investigation found, however, that clinker fran which the alkali (0.57% Na20 equivalent, minly as potassium salts) had.been remved
by reburning with ammniumchloride
had lowx strengths atall
tinkas _.thahthe addition.
samcemantreburned
DatxlreportedbyMussgnug
ina similarmannerwithoutNH4Cl (21) ix-ldicatedthestrengthof
high-alkali clinker as being 100% greater than that of low-alksli clinker atlday, subsequently.
2O%greater
at3 days and similar at7days
These findings were basedon
clinkers withalkalisulphatecontents
and
tests co alargentznber
of
beW Oand4.0%
2.2 Effects of high alkali levels on the manufacturing process 2.2.1 Effect on kiln fuel consm@ion The evaporation of alkalis in the burning zone consums high mature, Mature.
heat at
which is subsequently liberated at a lmer
Additionalfuelisrequiredtomaintainthesamburning
zone temperature under these conditions, and Weber (22) estimates thad this can be up to 31 kcal/kg of clinker for a specific suspension preheater kiln,12 kc&/kg
for aIqolkiln
and 5 kcal/kg for awt
process kiln, the difference being explained by the opportunity for effective use of the low-grade heat liberated in each mse. EIowever,
this fuel penalty my not be realised in practice,
because the presence of alkalis will tend to pramte clinker minerals atalowar
the fornation of
temperature, thus permitting the burning
zone temperature to be reduced, other things being equal (19, 20).
2.2.2 Corrosion of refractories Thamstserious said not to be cWcal
adverse effect of alkalis on kiln linings is but machanical (23, 24, 25). AlJ6i.i
salts -
notably
K2SC4 and Kc1 - condense on the brick at teqeratures
of
700-10bO"C and fill the pores, thus increasing the risk of cracking and spalling with temperature changes.
In a kiln used part of the
time for the nanufacture of white cemaht under reducing conditions, it was found (261 that sulphides, e.g. KE'eS2, brick.
me deposited on the
Although the sulphides did not damage the brick and possibly
even tended to strengthen it, under oxiding conditions theywere converted to sulphates with highly deleterious results.
2.2.3 E?uild-up
and preheater blockaqes
The volatile ca-qounds evaporated in the burning zone tend to recondense on dust particles and in the cooler parts of the kiln system.
There my ba a considerable teqerature
range in tiich these
canpounds are in the liquid state (see Section 2) and in parts of the kiln system in this temperature range, severe build-up problems can OCCUT.
Places where this happens axe the inletendof the kiln and
the preheater cyclones (271, in which coatings can lead to much impaired flow of materials and ultimately complete blockage (28). "Rule of thumb" limits have been quoted (29) of 0.015% for chloride and 1.0 for the mlecular
ratio of sulphate to alkalis in the raw mix.
Although clogging problems are widespread with suspension preheater kilns, it has boer~ them (30).
claimed thatshaftpreheaters are notsubjectto
3.
souw=Es OF ALKALIS 3.1
Alkalis in limestones Innormal cases,publisheddata
suggests thatthelimastone
the raw mix is only a minor source of volatile carpounds.
in
Average
values for 345 American limastones are (31):
K20
0.33%
Na$
0.05%
=3
0.05%
Cl
0.22%
Anumber
of Lkrbyshirelimestones
had higher SO3 contents~ in
the range 0.1 to 4.8% (32). One~uld~alkaliproblems calcareousmai%rials mud.
Suchmaterials
(33) for ewmple,
tobecanali.kelywhenusing
of marine origin, such as coral andaragonite are used in Hawaii and the southern UnitedStates but as far as is known no particular problems due to
high sodiumchloride contents are encountered. Inecpdments
carried outatawet-prccess xorks where sea _ water was used to-h oyster shell rawmaterial and tom&e uq the raw slurry (341, it was found that substitution of fresh water decreased the Na$ levels in the clinker to scene &xx-k
but also
increased the K20 content, so that only a sr&l reduction of total alkalicontentcould be obtained in this way.
3.2 Alkalis in clays and shales These mterials raw mix.
normally provide the bulk of the alkalis in the
Minerals with high alkali contents include feldspars, for
example orthcclasewhich can contain up to12%
can contain I+ to17% X2Oand Na2O,andclayminerals,
albitewhich
such as micas and
illite (35).
Analysis of 33 illites (36) showed a range of Na2C
contents.frcxn
0.05 to 1.05% with a maan of 0.27% and X2C contents fran
4.6 to 11.0% with a mean of 6.7%. 3.3 Alkalis in fuels Coals contain alkalis both in themineralmtter,~~andX~ together forming 1 to 6% of a typical British coal ash, and a.s sodium and potassium chlorides, either in the free stateor coal stitances.
adsorbed on the
Chlorine levels can be up to 1% ;37).
are typicallymlow in sulphur, as arecoals
British coals
franthewastem
United
States, and Ruhr coal contains about 2.8% So3 (35). Eastern LE coals are higher in sulphur as the following analysis (%'by coal) confirms (38): Illinois coal w
0.15%
Nazo
0.12%
=3
10.48%
Cl
0.22%
(Ash
9.58%)
weight of dry
It has been suggested (39) that coal my contain iodine which is found in flue dust, but this muld not ha expected to have a significant effect. Heavy fuel oils, for ewrrple No. 6 Fuel Oil Bunker
C) include
about 1% of water containing dissolved salts,‘mainly sodium chloride, so that atypical ashmycontain of up to about 0.1% of the oil.
32%Na20,
ckesponding
tocontents
Sulphur levels can be high, ranging
frcxn about 2% (SO31 for a low sulphur oil with 0.04% ash to 10% GO31 for a high sulphur oil with 0.02% ash (40). Natural gas contains no alkalis.
Sulphurlevels
(mainly in tbe
form of H2.S) can ba quite high in the raw gas, but pipeline specifications generally limit H2S contents to 4 ppxn by voluma to avoid corrosion, so that rmst of this is removed (41).
(4~ by
volume of H2S corresponds to about 18.6 ppn by weight of S03, or about 0.0015 g SC3 per 1000 kcal). Wastemterials
usedas
fuelcouldcontain
appreciable
volatiles, depending 'on the sour&. . Addition of chlorinated hydrccarbon waste to fuel has keen proposed (42) as amans of reduciiq clinker alkali levels. 3.4
Methods of reducinq alkali contents of materials It does not appear to be possible to reduce the contents of
alkalis in clays and shales by any practicable nmns. experiments by the Portland Cement Association (43)
A series of investigated
leaching, flotation and heating methods and showed that the only
mtl-&.5 to reduce the alkalicontentof
the mterialappreciably
ware
(a1 heating at a temperature and LSF approaching those used in cemant manufacture and (b) heating with concentrated mineral. 4.
Pl-lYSICALPROPElRTIES
acid.
OFALSAJLI MILTALCOMPOUNDS
Melting and boil& teqeratures
(at 1 amphere)
for scma
potassium; sodium and calcium cq&nds a.re given in‘&ble 1.
SOUrCeS
for these are (40) and (441, except where otherwise indicated.
It is
not'clear how significant these repours are under practical kiln co&iitioti, since various eutectics form; K2SO4/aSOq/I@1systemthelmest
for example in the
fusion teaiperatures
are in the range
650-700°C (26). VaqoU.pressures
have been investigated fran an early date (47).
Fig. 1 shows saturated repour pressures of KCH, andNaF
&OH, KCl, &Cl, KF
(40) together with approximate values for K2S04 (48) and
Na2sOq (49);
these latter should be treatedwith scma caution as they
are ex%rapolated beyond the range of validity claimad for the equations but should indicate general trends. For further details of sadim chloride see (51, for potassium fluoride SW [SlJ and for potassium and sodium sulphates (52). A considerable amunt
nf wxk on
the behaviour of potassium sulphate at high temperatures has baen carried out in connection with its possible use as a seed in magnetohydrodynamic power generation (53, 54). the partial presssures of the VazTious
&an calculation of
species present at teqeratures
in the range 1500-2000'K and 1 amphere pressure when
sulphur-ccntaining ctxnpcunds
fuel oil is burnt, it appears that the potassium
present in significant quantities muld be K-$04,, K$O3
andKOH. Althouqn me vapour pressures \Flg.ll
agree v1 general. terms
with the order of volatility observed in practice, one muld anticipate considerable difficulty in using them to predict absolute volatilisation levels at given temperatures. Plbst of the species involved do not cccur as pure canpounds in the solid state and pressures andteqeratures
inside nodules of clinker, for instance,
are not wall known. 5.
FxxxxsAFFE13TINGvoLATILIs~IoN By "volatilisation" we m3n the departure fran heat-treated arterial of a proportion of a stitance llliX.
This heattreatmtzntmayba
case volatilised stitances
originally present in a raw
under labratoryccnditions,
are generally r-v&l fran the system, or
in
tendto
recycled,'giving a lowar overall %xJ%e obtain~under
in which
condense and ba
of vclatilisation".
Values
different circumstances must therefore be carpared with
care. 5.1 Physical factors 5.1.1 Nodule and particle size It muld be expected that under certain conditions the rate'of loss of volatile suketances from nodules could be limited by their
diffusion frcm the interior of the nodule to the surface. Experiments by the-FCA with various sizes of nodules of raw mix and with rebmning clinker nodules of various sizes tend to confirm this' idea (551, althouqh the author suggests elsewhere (56) that in practice the effect of nodule size my be small in cmparison temperature and residence~tima
with factors such as
at-mature. This
view is supported
bv a series of statistically-controlled experiments at NIIITsenrent (57).
Results of Goes Ad Keil (58) and Draper (55) have been
replotted on a lag/log scale (Fig.21 and agree (with the exception of one value for lmn pellet diameter), indicating a relationship of the form: volatilisationd
1
.3'd JAnalysis of pieces of clinker of different s&as
(59) showA
K20
levels to be higher & the larger pieces. Although it appears that the effect of nodule size on volatilization is not very great in practice (for example, a variation in pill size of f 25% would alter volatilisation by C 7.5% - lo%),
it
could nevertheless be useful to investiuate it more thoroughly, both to verify the relationship, which is based on very limited data, and as a guide to the factors limiting alkali volatilisation under various ccxnbinations
of burning tirre and temperature.
Scma mrk has been &ne with beds of poxdered raw mterials
of
various depths (55, 591, the results of which broadly parallel those for nodulised raw mterials.
No publications dealing'with the effect
of the size of individual particles on volatilisation have been found; it seems reasonable to propose that below a certain size this muld have little effect but it muld be useful for stamlardising -imental
results to put this ptulate
to the test.
5.1.2 Effect of the liquid content of the clinker Since the effect of increasing the liquid content is to reduce the porosity of the clinker, this factor would be expected to lower the rate of alkali volatilisation (58, 60, 61). Mussgzmg, howaver, found no correlation baMaen
total alkali (KS + Na$) and Fe203
contents of a number of clinkers (211 anda separate series of experim31.ts
showed that the A/F r‘t' a 10 was the least significant -of the
five factors studied (571, being overshadoWed by temperature, time, sulphate content and pellet size.
Itwmld'te
of interest to
investigate the magnitude of this effect in store detail, as there appears to be scrne disagreeman tas to its iqortance. 5.1.3 Gas flow rates The effect of gas flow rates on the volatilisation of alkalis has been studied to a limited extent only, for example by Jackson and Morgan (621
who detected ho significant variation. I&oratory
vimants
on volatilisation rates scmatimas incorporate means for
passing a gas canposition approximating to kiln gas over the raw mix (551, for instance 77% N2, 20% CO2 and 3% 02 (56).
So2 (see Section
4.2.3.) and water vapour (Section 4.2.4.) have been added to this.gas in varying quantities, but no results of altering its flow rate have
been found. It is possible that this could be significant in saz .cases, as significant variations in volatilisation with stirrins a bed of powder have been reported (59). 5.1.4 Euration and temperature of heating There is general agreemant that the degree of volatilisation increa.%es
with the tim andwith
the Mrperature
of heat treatment,
although it has been noted (60) that after heating fran 700°C to 14OO"C,
little additional volatilisation takes place on subsequent
heating at 1400°C for a further 30 minutes. The effects of duration and teqerature in rnagni.tGde
are thought to be highly significant and about equal
(57).
The rate of volatilisation frcxn a particular raw mix under gives temperature andother investigated, buts-
definedexternal
&hditions
has not been
interesting conclusions can be drawn fran
additional analysis of reported data. An assumption which is widely made, albeit usually implicitly, is that the proportion of a species volatilised on heating for a given time and at a particular temperature is characteristic of the raw mix, i.e.
that the volatilisation rate is proportional to the amunt
present (a). Thus: -da=ka dt:
- a0 r 1h = orkdf atJ
a-
tI
In (at) (a,>
=
-kt
where at is the amount of alkali present at.time t and a0 the amount present at the start of heating. is, if the volatilisation
If this assumption is correct, that
reaction is first order with respect to K20
or Na20 content, plotting the log of the amount of alkali remaining against time should give a straight line.
Data from Palmer and
Baylees (34) and Woods (63) were analysed in this way and results for selected mixes are shown in Fig. 3.
It will be noted that although
the fit is very good in most cases, the line of best fit does not always pass through the point corresponding to 100% original alkali content at time t=O.
To explain this, it is necessary to consider t
heating conditions which were applied.
In (34)
the samples were
heated for 20 minutes at 1800°F (980°C) to decarbonate kthem and immediately transferred into a high-temperature furnace maintained at 2600°F (1427"C), time.
in which they were keept for the appropriate length oi
The heating programme in (63) is not stated.
predicted initial alkali level substantially below
In the case of a 100X,
a possible
explanation is that part of the alkali is driven out while the sample is being heated up to the experimental temperature.
This appears to
be reasonable, although it is surprising that, if this is
SO,
no
correlation was found between the rate constant k and the initial alkali level ao.
This could be due to the presence in the mix of
chloride levels appreciably less than those needed to react with all the K20 (the addition of chloride is known to increase volatilisatic
significantly - see Section 4.2.2.). (34)
The line shown for Mix 12 of
with added CaC12 shows a very low level of K20’ remaining at the
start of heating at 1427"C,
and a rate constant which, although the
highest of those observed, is not much higher than that of the same mix without CaC12 (not shown in Fig. 3), - 0.0820 and - 0.0771 respectively.
In the latter case, however, no volatilisation appeared
to have taken place at lover temperatures.
Another problem is presented by results such as those for Mix 5 of (34)
and Mix 3564, 125O"C,
of (63).
In these cases, it appears
that volatilisation does not begin until some time after the start of heating at the final temperature. K20 volatilisation takes plz&e
after 60 minutes, at 125O'C
In the case of Mix 3565 of (631,
at llOO"C,
no
at 12OO'C it begins only
it begins after 30 minutes and at 1300°C
and higher temperatures it starts immediately.
This delay is perhaps
due to the alkali-containing materials initially requiring to react with other components before the alkalis are released.
The correlation coefficients were recalculated in selected cases including the point a = 100X;, t = 0.
As would be expected, the
correlation was usually slightly improved where the original line of best fit passed near this point, but where significant changes in k and/or a0 resulted, the effect on the correlation was unpredictable. It vould therefore be desirable to carry out experiments on the rate of volatilisation, in which the actual alkali contents at t = 0 are measured.
If the relation suggested by the data already examined was ,. confirmed, this would mean that the proportion of alkali volatilised
under given conditions was independent of the absolute amunt
present
(until; of course, all the alkali was lost). An alternative way of indicating the propensity of a raw mix to lose alkali on heating has been proposed by Palmar and Rayless (34). This is the "resistance factor“ which is the sum of the percentages OI alkali remaining after heating for 10, 30, 50 and 70 minutes at 2600'F (1470°C).
This empirical mathod appears to be suitable for cuqar'i3-g
raw mixes with each other, but does not permit the prediction of volatilisations
under other conditions.
Its use has not been taken up
by other mrkers. Maintaining clinker atan elevated temperature has been used as a n&hod of reducing its alkali content (64, 65). A tE Bureau of Reclamation specification for low heat, low alkali cement was ntet by reheating the clinker frcm the kiln to 1427°C in a converted rotary cooler-
Na$ equivalents decreased frcm 0.889 to 1.19% in the
(loss-free) raw mix to 0.411 to 0.890% in the clinker and to 0.315 to 0.716% in the treated clinker. was abxt550
The heat input to the clinker treater
kcal/lq clinker,which
renders themathodunattractive
in nomal circmnstances. Data obtained by the Portland Cement Association (56) for a number of kilns show that total alkali volatilisation tends to decrease as the residence time of the mterials
in the burning zone
decreases, but the effect of other factors is tcogreatto quantitative analysis.
permit a
The effect of mrature magnitude to that of tine (51).
on volatilisation is canparable in It has been stated that "the burning
of cement clinker takes place within a temperature range where a canparatively small rise in the temperature will cause a noticeable increase in the amount of alkalis volatilised" (66).
It is therefore
said to be necessary to control the burning zone temperature accuratfAy.
Itcertainlyseems
to be the case that there is a big
increase bet- ll.50 and 13OO'C (particularly bat- 1200 and 1250°C) in the percentage of the original K20 volatilised in one hour (591, although in normal conditions one would expect rmst clinker to reach these teqeratures
in a kiln. Kate constants have been
calculated fran the data of Wads (63) at a range of matures
for
K20 and Na20 volatilisation frcxn one raw mix and these are shown in Fig. 4.
The rate constant increases roughlylinearlywith
temperature, the effect of tgmperature being much more significant for K2Othan
for Na20.
Itmightbeexpe&ed
fromtheorythatlqeK~uld
be proportional to l/T ("K), but the limited data available do not appear to confirm this and further exp~imznts
muld be desirable.
The increase in K$ volatilisation in a given tima with temperature .follows a logistic curve (591, since the absolute rate of volatilisation after a given tima initially increases with temperature ;% (as the effect of the increase in kwith temperature wthe daninant factor) and then declines again (as the amount of alkali remaining becomes the dxninant factor). 'i>- ature at which volatilisation begins can be as low as.. 700-800°C for El (21) but is rrore typically in the region of 1100°C.
The volatilities of the various species are discussed in mre detail below. 5 . 2 Chemical factors 5.2.1 KS and Nag contents It has already been suggested (in Section 4.1.4.
above) that the
kinetics of alkali volatilisation are first order with respect to RIO and Na$, and if this is the case the percentage volatilisation is Hcmever,dataobtainedbytheFa
independent of the absolute levels.
(67) suggest that reduction in alkalis in fullsize with alkalicontentof
the rawmix.
kilns increases
These findings are not
necessarilycontradictory,as
clays andshales withhigher K$and
Na$ my, for ample ,contain
thein inaless firmlyminbined
form,
lost at loam temperatures, but this is certainly an area which xmld repay investigation. As far as the relative volatilisation of Na$) and K20 is concerned, it appears (56) that volatilisation of Na20 is negligible where Kg volatilisation is less than 30%, and is at about half the level of additional K20 volatilisation above the 30% "threshold". 5.2.2 Effect of chloride and fluoride The early studies on the recovery of potash fran cement flue dust and other minerals (62) showad that the amount of KF volatilised under given conditions could be increased by addition of calcium or sodiumchloride.
Itwas found to be~necessary to add just over the
quantity needed to combine with the K20 to give KCl. more effective in promoting volatilisation than
NaCl
CaCi2.
was rather
With the
interest in reducing total alkali in the clinker, the effects of NaCl and CaCl2 -(63)
on Na20 volatilisation also were investigated, and Woods
found that although NaCl
K20 than was CaC12, increased by NaCl
was indeed more errective in liberating
the Na20 content of the clinker was noticeably addition.
The use of calcium chloride was studied ~ in full size kilns by Holden (68), who found that the removal of alkali by addition of calcium chloride could be estimated for CaC12 additions up to 1.5% by the empirical equation:
Ar f
0.559 K, [CaClZI 100 - L
where A, * total alkali removal expressed as Na20, efficiency of utilisation of CaCl2, material and [CaClz]
K, = apparent
L = loss on ignition of raw
= % of CaC12 added (dry basis). The efficiencies
of CaCl2 utilisation observed were 27% to 34% (average 32%) for wet process kilns and 45% to 56% (average 48%) for long dry process kilns. Addition of 35% commercial grade hydrocholoric acid has been used as a cheaper alternative to CaC12 where this was not readily available locally (69).
No particular problems were encountered in adding the
acid to the slurry tanks and control of the quantity added was better than when flake calcium chloride was used.
Problems have arisen with low volatilisation and build-up problems when CaC12 is used in long dry process kilns(70). Although no published data are to hand, it would be expected that levels of
chloride addition would cause severe problems in preheater kilns and muld probably have little effect on alkali reduction. Contrary to the above results, laboratory tests on smrrles
man
six works in Central Asia (71) showed that addition of NaCl did not appreciably increase and inmstcases
reduced the Na2C1contentof
the
clinker, but that CaC12 was in rmst cases more effective than NaCl. Although it is known that addition of chlorides decreases alkal contents, further mrk needs to be Qne before the precise effect at various temperatures can be predicted. Eata on the effect of fluorides on alkali volatilisation are
spar=.
woods (63) states that where calcium fluoride was added to a
raw mix to produce a high early strength cerrwt, the clinker mntaine less potash than usual butahoutthe
same amount of soda. He suggest
that addition of CaF2 &uld probably only be of value in pramting volatilisation where the raw mix was high in KS but relatively ion i. Na20.
According to Sprung and van Seebach (721, theremdymmic
considerations favour the formation of calcium fluoride and alkali matal
sulphates
The amunts cases.
rather
thanalkali
fluorides
of fluoride in rawmaterials
andcalciumsulphate.
are, however, small in mst
Cost and possible toxicity muld probably make fluoride
addition unattractive as a means of increasing alkali volatilisatior even if it were mre effective, 5.2.3 Effect of So2 and So3 The water-soluble alkali salts remaining in clinker are almst
entirely sulphates (16), chlorides having been lost through volatilisation.
The effect of sulphur compounds 0; the alkali content
of the clinker is therefore considerable, the sulphur deriving both from the fuel and from sulphur compounds naturally occurring in (or added to) the raw materials.
The sulphur content of the clinker is
typically 50 to 65% of the total sulphur input from all these sources, so that it is possible for the sulphur content of the clinker to be higher than that of the raw feed (22).
As the vapour pressure of
alkali sulphates is considerably lower than that of chlorides (Fig.1) it would be expected that higher levels of sulphur in the clinker would tend to prevent alkali volatilisation and this is in fact observed (54, 56). greater
than
on
The effect on K20 volatilisation is appreciably
Na20
volatilisation
(62).
K20,
Na20
and
chloride
contents of flue dust are reduced even in the presence of chloride in the feed (73) - which can be advantageous in improving the efficiency of electrostatic precipitation - so, it would seem, demonstrating that alkali
sulphates
are
formed
or
volatile chlorides and oxides.
retained
in
Statistical
preference
to
examination
the shows
more that
SO3 content is second in importance only to time and temperature in affecting alkali volatilisation (57). Operating data (56) from ITS kilns
show
a
relation
between
volatilisation
of
SO3
and K20,
although
it is not clear what is the significance of this and there is some evidence (56) that the effect of SO3 is greater at higher temperatures.
The equilibrium between calcium sulphate and CaO, SO2
and
been
SO3
lOOO”C,
has
extensively
investigated.
At temperatures above
combustion of sulphur compounds in the fuel results
predanhantly
in SC2 (271, which can react with lime in the
temperature range 600-900°C. 3
4 CaO + 4 SO2. . 3 CaSO4 + CaS The xraximum
absorption of SC2 by raw meal was obtained at 880°C;
this is significant since a high proportion of the sulphur in the kiln exit gas can be absorbed by raw meal in the preheater (74). Snaller amounts of sulphur can also be absorbed byrawmterials,
where these
are dried by kiln exit gases in the rawmill. The decaqosition
of pure C&O4 begins at about 900-lOOO'C, but
is initially very slow.
Sax5CaSC4remainsunconvertedatabout
1385“C, which is the melting point of the CaO-CaSO4 entectic (75). The decaqosition
rate is much increased by the addition of other
materials, eg silica, alumina, kaolin and iron odde, and by the presence of water vapour.
-sition
atmosphere, when CaS is formed. for mmfacturing
cen?ent
is also faster in a reducing
This fact has been used in processes
and sulphuric acid, by heating a mix of
anhydrite, coke and argillaceous materials in a rotary kiln (76). -
4
3 - 4
+x
+m2+cas
+c&-+4c.&+4=2
The lime reacts with the argillaceous ccqxxents minerals and the SC2 is separated fran the exit gas.
to give clinker It is necessary
to maintain oxidising conditions in the exit gas to avoid the formation of elemental sulphur, for instance by reaction of the H2S andSO2.
It has been claimed that low-alkali ceinentcanbeproduced
in this prccess by adding calcium chloride in the usual way (77). This 'is perhaps unexpected in view of the effect of even a small percentage of added calcium sulphate on alkali volatilisation, and no doubt results from dissociation of the CaC12, allowing alkalis to evolve as chlorides. Further mrk on this aspect should investigate in greater detail the effect of varying levels of sulphate addition at various temperatures and in the presence of other materials; also, the relative effects of So2 in the kiln atxmsphere and calcium sulphate in thefeed. 5.2.4 Effect of water vapour Fran the relatively high vqxmr pressures of KOH and NaOH it might be exqected that water vqour
+muld pramte
volatilisation and
this is confirmed by laboratory expariments by Goes and Keil (581, which gave the following results
H2OiIlkilngas % byvol.
K20 volatilisation with 30 min at 1330°c %
14OOY %
0
64
83
5
76
100
10
100
n.d.
Wcods (70) reports that satka works have claimad that spraying small Zn-ounts
of water (approx.
2ml/kg clinker) into the burning zone
directly above the flame my reduce the clinker alk$i -0.1 to 0.2% (on clinke?z).
content by
This effect might have sme relevance to
the observed tendency for gas (andperhaps oil-fired) kilns to have higher alJcali
volatilisations than coal-fired kilns (18, 54, 65) since
more water vapour muld ba produced by ccanbustion of hydrocarbons. (Spiers
reports moisture levels in canbustion products as: 2 - 5% coke 10 - 15% Lignite 7 - 9% coking coass 5 - 7% Anthracite 1 0 % - H.F.O. 1
5.2.5 Alkali-containinq minerals Scdiumandpotassiumin
rawmterials
can occur ina number of
different minerals (see Section 2) and these can vary in their tendency to lose alkalis (57).
Elpertits
with rawmaals
containing
mica, illite and orthaclase feldspar (58) showad that alkali w lost mre readily fran mica and illite than from feldspar at temperatures of 900-1250°C.
It is suggested that this my be due to difference in
bond energies for the various minerals. Another factor may explain differences between minerals; for example in gr eensand (glauconite) the silicate is hydrated, (62) leading to formation of K20 in the presence of H$ when heated, allowing the formation of Koti, tiich has a higher vapour pressure than
KS.
(see 4.2.4.)
It muld be useful to investigate the response to heat treatmant of various potash and scda containing minerals in mre detail, and this muld require sama a-cans of allowing for the effects of other factors and also devising a quantitative msasure of the "availability" of K2OandNa20
in theminerals.
5.2.6 Effect of LSF and SR Although little reliable infomtion
on this subject has been
found, such as there is indicates thatitwxld further Czq&nmts.
be useful tocarryout
Ekperirrwts on remving alkalis frcmraw
materials by heating (43) indicated that there was no mlatilisation on heating mixtures of clay or shale with 10% or less of limastone to fusion and that alkalis wsre not released in significant quantity until the amuntof
lime approached that of a normal cenwt raw mix.
The data are shown in Fig. 5 where percentage loss of alkali is shown in relation to the proportion of CaC03 in the mix, and in Fig. 6, which shows alkali loss in relation to the TLSF - which has been estirrated
since the caqxxsition of the shale was not reported. It
will be seen fran these figures that the proportions investigaM
do
not permit us to say whether ease of volatilisation varies with LSF within the range normlly
found, or whether aminimumlima
required, above which there is little variation.
content is
Earlier experiments
(62) had shown that KS could be volatilised frm greensand (glauconite)/l~stone/cdlcium
chloride mixes containing as little as
one third limestone, if these were heated to temperatures just below that of fusion.
The effect of silica ratio on alkali volatilisation has not been investigated. Tbare is scma evidence (68) of a higher, calculated c3S content leading to lowar alkali levels in clinker, and it ms found (61) that clinker alkali levels could ba reduced by replacing part of the argillaceous mterial
with sand.
Howevex, in this case the effect
of increased S.R. was not distinguished fran the effects of lmer alkali levels in the sand reducing &-se in the raw feed and the necessary higher burning matures, which one wuld
expect to ba
more significant. 5.2.7 Effect of kiln a-here In normal centant cment/sulphuric
cperations, as distinct fran the
acid process (see 5.2.3 above), the limited
information available frm laboratory tests (59) suggests that volatilisation is not greatly affected by the kiln amphere. results indicate that a reducing amphere pramtes
The
volatilisation at
1200°C and 1250°C and perhaps slightly depresses it at 13OO"C, but the data do notreallyperm.ituseful
conclusions to be drawn.
FundamantaJ.ly,a reducingatmxiphere
is expectedtoprcm&e
volatilisation of alkali sulphates in the burning zone. 6.
DISTRIEUTIOJJ7OF~SINCLIN?GZR The distribution of alkalis is not revie++ed
in detail, as the
literature on this subject is very extensive. E'rcm the efforts to utilise flue dust during the First World War, it had been noted (78) that potash in flue dust existed in
several different forms with different degrees of solubility in water or acid, but no substantial progress k~s made until the 1930's and 1940's, when ah extensive programe (US) Natioml
of mrk GELS carried out at the
Bureau of Standards, in cooperation with the Portland
Cement Association (79 - 84).
Newkirk (85, 86) concluded that the
alkali-containing compounds M&A3 andKC2jSl2ere presence of SC3 andCaSO4 which xas imiscible
and reacted to give an alkalisulphate
with other phases. For R$/SO3
the SC3 reactedpreferehtiallywith the KS.
phase is imzdiately
unstable in the
mlecular
phase
ratios
The alkalisulphati
soluble in mter (87) although alkali fran other
phases is not. Segnit (881 suggested that at least some, and possibly a considerable part, of the soda in Portland cemnt
occurs in the C$.
On the other hand, investigations at the Bulgarian Building Research Institute appeared to suggest that both KS and Na-$ are predminantly in the form of aluminates (89).
Toropov and Dobrovolksky (60) support
Newkirk's opinion that most of the KS is in belit&;
they excluded
clinkers with free alkali sulphate from consideration. Pollitt and Brown (16) generally mnfirm
Newkirk's findings on
alkali sulphates, noting that equilibrium conditions are not attained in a kiln, so that works clinkers tend to contain mre calcium sulphate thah do latxlratory
clihkers.
They also suggestthatscme
potash could cxcu~ in the C3A and that t&h potash and soda could exist in alite in small quantities.
Azelitskaya and co-workers (90, 91)
found that addition of
gypsum to raw mixes containing alkalis diminished t?ne effect of alkalis, which (at a raw mix SO3 content of 0.6% or less) was to decrease C3S
content.
With the addition of.5 to 10% gypsum to the raw
mix (which also raised the LSF and hence the C3S content), alkalis were in the form of their sulphates.
When investigating dust return,
Luginina and Shaposhnikova (92) found that the presence of high levels of alkali sulphate tended to reduce alite formation and to give alite crystals-with inclusions of belite and alkali glass and eroded edges. Other work (93) confirmed that the addition of gypsum to alkali-containing mixes tends to convert the alkalis to soluble sulphates;
in this work the optimum SO3 level in the clinker appears
to be about 4%.
Subsequent work by Abbassi (94) confirms the general trend of previous
investigations.
Alkalis can exist in clinker separately, as
sulphates or carbonates, or in other phases: preferentially.
C3A dissolves soda
However, all the phases can contain at least some
potash and soda, the amounts in the flux phase being below 0.2%.
The
ratio of the K20 content of the C2S to that of the C3S is usually about IO, the maximum K20 content found for C2S 1.9%.
in this work being
The Na20 ratio varies, but is about 10 for max. 0.6% for Na20
recorded in the C~S.
More Na20 was found (relatively) in the C3A at a
max. of 1.7% (cp, 3.2% max. for K20).
7.
ALK?GICIRCULATION
INTKEKIINSYSTEM
7.1 Estimation of alkali circulation Equations to give the concentration of a volatile cqnent i at various points in the system have been derived by Ritmann
for
Polysius AG (28, 95). According to these, in a system without a bypass, the attmntof
the ith caqmentrecycled
to the incaning mterials
frcxnthe kiln gases
(referred to as the adsorbed phase, since
this is believed to consist rtkainly of alkali carpounds condensed on raw material particles) is given by (Eli + Bri*) .ai.Ci Xi = l- c2i.ai ci
represents the amount of the.cmponent
Bri* =Bri/Ci
in the raw maal
where Bri is the amount of the cmponent
in the fuel
ffl i
represents the volatilisation of i from the raw ma1
52i
represents the volatilisation of i from the recycled naterial, and
ai
represents theextenttowhich the component is reabsorbed from the gases in the preheater. Note that Xi, Ci and Bri can be in any consistent units of ItaSS,
such as (but not necessarily) g/kg of clinker or per cent of clinker. $li, f2i ad ai are fractions not percentages. It is ~instructive this equation.
to consider the assurrptions
mde in deriving
If the various value& are shown in a diagram Ri is the
content of the ccmponent i in the clinker:
Xi T ar
Then if Xin and Xi(n-1) are the values of Xi after n and (n-l) :ycles
respectively:
Xin = Bri + Ci Eli + Xi(n-1) 2i
But in equilibrium conditions the amount of each component circulating is constant,so that
xi -
= Bri + Ci Eli + Xi E2i
ai
Xi s ai (Bri + cilli) (1-a c2i > or Xi 31 ai Ci (Eli
+ Bri*)
(1-a C2i)
.This
assumes that the whole of the volatile component in the
fuel is volatilised and that no raw feed is lost to the stack (or dumped) from the kiln/preheater system.
Using the same symbols and assumptions, it is clear that Ri (1 content of component i in the clinker) will be the amount not volatilised, i.e.
Ri = (IL-&li)
Ci + Cl-f2i)
Xi
. i
hence Ri = (l-
Ci + (l-[*i)
= Ci (l-aiE2i )
E(l-Eli)
+ ai Bri" (1-{2;)
II
f ai (
If ws can neglect the volatile content of the fuel, for exaqle for K20 and Nag if oil or gas is used, Bri* = 0 and [ (l-Eli)
Ri = ci
+ ai (Eli -czi)
1
(l-aic2i) = Ci (l-Eli (l-ail 1 ( 7l-aiE2il ) If a fraction V of the kiln gases is by-passed, then the volatile ccqonent
i passing on to the preheater will be reduced to a
level Xi Cl-V)/ai,
and the axrount
recirculating will be Xi (1-V).
We
can therefore represent the effect of a by-pass in the above equation by replacing ai by a'i where a'i = ai (1-V) These assm@ions
are cleiuly open to criticism. The concept of
differing volatilisations for raw mterial
and recycled mterial
is
swrted by soma qrtitalevidence (28) which suggests that alkalis are rtore readily volatilised frcm recycled mterial raw mterials,
than from
although other mrk (55, 59) indicates that alkalis in
flue dust are less readily volatilised than those in raw materials. However, the values for any single ccqqent
do not autamtically
take
into account the amount of other components present. For example, the volatilisation of KS will be. increased by the presence of chloride and&creased by the presence of SO3.
As these ccaqxments are lost
fran the system to different extents, the proportions in the recirculating material willvaryas the system tends towards equilibrium.
Although the volatile content of coal ash is relatively
small, it is not necessarily all lost (96) and sma alkalis my reach the clinker from this source.
It is also possible that SO3 fran oil
fuel could be re-absorbed at relatively high ixqeratures.
Ibere
seems no reason to assume that dust lost frcm the prekzker
will
contain alkalifrcmnthekilngases
only and thathoalkaliwill
be
lost frcm the raw material (55). It appears that in practice the factorsE1
a.udc2 are estimatedempirically
in a particular case, so
that while these equations may be useful for predicting the effect of varying the level of ah existing by-pass, for example, they muld probably ba of little value in predicting the bahaviour of a raw mterial
in aproposed
hew system.
Teoreanu and Puri (94) adopt different ratios which seem to be less gene-rally applicable than those of Ritzmann. They &, hcmver, intrcduce the possibility of non-equilibrium conditions in the kiln, for instance where alkali-containing coatings build up over a period and then break free.
It is clear that in addition to the factors affecting volatilisation which have been discussed at scnte length abve, the condensation and recirculation of alkalis are also important. One mans which has been adopted for following alkali circulation is the use of radioisotopes such as K42.
Inanexperimantonawetprccess
kiln (981 it was found that the man number of cycles was about five, the time for each being 110-120 minutes. This value is on the high side, caqaredwith (99, 100).
those derivedbycalculaticm
frmalkalibalances
The efficiency of capture of dust by the preheatar system
varies according to the type. Ritmann
quctes typical values for dust
collection efficiency for cyclone preheabars of 60-90% for stages 3 and 4 (1011.
The total capture of alkali-ccntaining dust will
therefore be very high, and hence the alkali levels in'the flue dust are very low (102) and the circulating load very high. kilns a proportim
& wet process
of the alkali, say 40% of the KS (221, is
precipitated, and alkali levels in the flue dust are mderate
(102).
The typical Le@ kiln (22) emits little dust and apparently traps aln-ostno alkali in the grate:
very high values have been reported
for the alkali content of the flue dust (22). 7.2 Reducinq alkali circulation 7.2.1 Rezmval and treatnwt
of flue dust
Over a significant period of tima, the kiln systmmustbe
zero.
the net gain of alkalis in
Therefore, apart frm the small amunt
of alkali in the stack gas which is not precipitated in the dust collectors, all the alkali put into the systemmustleave either in
the clinker or in the flue dust. Thus, it is only possible to reduce the alkali level in the clinker to a level below that arriving in the rawmeal
if the flue dust is either dmped
or treated to reduce its
alkali content before being returned to the process (55).
In a wet
process kiln, discarding the dust reduced the equivalentNa20 clinker fran 0.68% to 0.54% (68). As dust rermml amour&of
circulating alkali, however, the rduction
in the
will reduce the in clinker alkali
that can be achieved by discarding dust will be less than the alkali content of the reamed Anumber
dust.
of ways of treating the dust to reduce its alkali
contenthavebeenproposed.
As the finer fraction contains a higher
proportion of alkalis (103,104) it should be possible to reduce alkali circulation by returning only the coarser fraction of the dust, although it is doubtful whether the differential muld often be large enough to mke this procedure mrthwhile. For reasons discussed above, there is a considerable variation in water solubility of K20 between flue dusts, ranging in one study (103) fran 2 to 96%.
Because of this the efficiency of leaching
vaxies, but it has frequently baen used or proposed (105 - 108). is obviously best suited for use with uet process kilns.
Problerrs
It may
arise in disposal of the salt solution produced, as it is rarely econanic to recover these salts as solids. Another mans of reducing the alkali content of flue dust is to heat it to, say, 1200°C. bed reactor (109).
This can be done conveniently in a fluidised
7.2.2- Bypasses on preheater kilns In preheater kilns - especially 4stage
cyclone preheater kilns
-virtually all the alkalis in the gas franthe
kiln are absorbed by
raw feed in the preheater.
in the flue dust is
Thealkalicontent
therefore very low and little reduction in the alkali content of the clinker or in the circulatingloadcan treating the dust.
be obtainedbyremving
or
One way in which this could can be achieved is by
removing part of the gas and/or dust before it enters the preheater, cooling it, collecting the dust and discharging the gas to the a?mosphere (21). of alkali.
Problems tend to arise in practice with the txlild-w
Although it muld be preferable to cool the gases by uater
spray, as the gas volumes needing cleaning are nmch less in this case, there is so far little practical experience of this, ccoling by mixing with cold air having been used in rmst cases (ll.0).
It appea.rs
that
the effect of a bypass is greatest up to about 20% bleed (991, above which the increased fuel consumption, which is about 40 kcal/kg of clinker at 10% bleed (211, becomes excessive in relation to the alkali reduction obtained.
Suggested bypass levels (ill) are 2-4% to reduce
operational problem, eg blockages, and S-15% to reduce alkali levels intheclinke.r.
A possible modification proposed to the bypass (112)
is to pass the bled-off gas through an initial cyclone and return the coarse dust (with a low alkali content) direct to the kiln. Although this means thatmre of alkaliremval,
gas has to be bled off tomintain
the same level
the quantity of dust to be collected is much less.
Incorporation of a bypass enables the maximum permissible chloride content of the raw fe& (see Section 1.2.3.) to be increased frcxn 0.015% to about 0.075% at 10% bypass (29). As wxld be -ted the effect of a bypass in reducing K2Ois Na$.
It has baen sugges&
itnaybecaae
greater thau the effect on
that if high levels of bypass are needed
desirable to use a precalciner system (ll3,
volIxrrs of gas passing through the kiln is thenmuch increase in fuel consuqtion
ll4)
as the
reducedandthe
at high bypass levels (eg 50 to 100%) is
less important. Although alkali problems are n-c&often
eucounteredwith
cyclone
preheaterkilns,bypassescanbeusedwithgratepreheaterkilnsdlso (IS, IX). In this case, the cleaned bypass gas can be used for
7.2.3 Other processes for alkali reduction Same alternative n&hods beenproposed.
for reducing alkali circulation have
One of these (ll7,
118) involves using excess hot air
fran the cooler for drying pellets on a grate preheater. The kiln gases~~dbepassedthro~htfiecdlciningpartof passed toa dust-collecting systemtogether hadbeen
thegrateandthen
with the cooler air which
passed through the other part of the grate.
An alternative mathod of lowaring alkali circulation in suspension preheater kilns which has been proposed (although not yet used in practice) is to pass some or all of the kiln exit gases over a cooled surface (ll9)
to produce a deposit of condensed alkalis and
high-i
dust.
Possible modifications to this process include the
use of~water-cooledtubes
or of bdies
system and a cooling/alkali remval
exchanged
plant.
betwzn
the kiln
8. CONCLUSIONS From the review it is concluded that there is general agreement on the factors affecting the volatilisation of alkalis. Volatilisation is increased by-smaller nodule size, lower flux content in the clinker, higher levels of chloride, lower levels of sulphur, higher temperatures, longer heating periods and by higher contents of water vapqur in'the kiln gases. It is not clear what the effects of other factors are (over the ranges studied in the available literature), and it may be inferred that these are relatively small: gas flow rate and composition, alkali content, presence of fluoride, LSF and S/R and oxidising conditions in the kiln. Several of these factors appear worthy of further study. There is, nevertheless, insufficient quantitative evidence to predict the degree of volatilisation of alkalis from a given raw mix under particular conditions. The interplay of phase equilibrium and transport phenomena is probably too complicated for it to be worth while attempting to simply calculate volatilisations from knoun thermochemical data: more complex modelling is needed. In practical terms, the circulation of alkalis is affected as much by their condensation and deposition as by their volatilisation. It would be useful to devote some attention to this hitherto somewhat neglected aspect of the phenomenon.
T. G. BURNHAM.
RRC/TGB/JLMC/D31:CPK/LEX32 11.03.87:12.02.R8.
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2.
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3.
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9.
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10.
INSTITU'IO DE INVESTIGACION TECNCIKXA INDUSIXAL TECNICAS: C&auto Portland Tipo I, Normal. IIT= 334.009 &@ember 1971)
Specification for
of alkali-
TECNICAS: Cimntomrtland:
Y DE NORMAS
11. COMISION VENEZOLANA NOEUQS INDUSIRIALES: Especificaciones para cwanto Portland, CCCA: Ce 100 (1972) 12.
DIRECCIONGENERAL lx% Cl-1955
DENORM?lS:
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low
43.
PORTER, E.S. and ToI;ER, H.J.: Remving alkalies fran raw materials. Portland Carwt Association report MP-99, 1961
44.
KAYE, G.W.C. and LABY, T.H.: Tables of physical and chemical constants. Imdon, Ion-, 14th edition, 1973
45.
SEE, J-B.: Fluorspar and fluorine cmpounds in high-terqerature smelting and refining of matals, Minerals Science and Ezqineering, 1976, 2, (41, 217-241
46.
IWAKYID, N., SUITQ, M., HAM%%TSU, S. and SA'IOH, of inorganic fluorides. Transactions of the JWRI, 1973, 2, (21, 204-207
47.
J?CKSON, D.D. and M3RGAN, J.J.: Measurmsntof Mporpressures of certainpotassiumcunpounds. Journal of Industrial andEngineering Chexnistry,192l,~, (21, llO-ll8
48.
HALSTEAD, W.D.: Saturatedvapour pressure of @assiumsulphate. Transactions of the Faraday Scciety, 1970, 66, (81, 1966-1973
49.
CUBICCICYITI, D. and KECJESHEA, F.J.: Thermdynamics sodium sulfate. High Tmparature Science, 1972, 4, 32:40
50.
BA-, R-J., IEFEVER, R.A. and WILCOX, W.R.: Evaporation of sodium chloridemalts. Journal of Crystal Growth, 1971, 2, 317-323
51.
TmoR, L.: The vaporization tiermdynamics of fused lithium and potassium fluorides. Journal of Chemical vcs, 1976, 5, (81, 777-783
52.
FICAILIRA, P.J., UY, O.M., MUENW, D.W. and MARGRAVE, J.L.: Mass spectramatric studies at high teqeratures: XXIX, Thend. decaqmsition and sublimation of alkali rmatal sulfates. Journal of the Amarican Ceramic Society, 1968, 51, (101, 574-577
53.
HART, A-B., QGLNER, G.C., HAISTERD, W-D., IFXTON, J.W. and TIDY, D.: Scme factors in seed recovery. ' International SyqmsiumonMagnetchydrodynamic Electrical Power Generation, Paris, 1964, Session 7, Paper 89.
I.: Malting points
of vaporization of
54.
chemistryof HART, A.B. and LAxroN, J.W.: Samaspectsofthe m.h.d. seed. Philosophical Transactions of the Royal Scciety of London, A, 1967, 261, 541-557. .
55.
DRAPER, J.: Iaboratorystudyof alkalimmmlin the cement burning prOCSs - a progress report. Portland Cement Association report MRS-68, 1954.
56.
DRA?TR, J.: Alkaliremval in the cement burning process -a study of operating data. Portland Cemant Association report MRs-69, 1954.
57.
EWTIN, Z.B., FRIDMAN, I.A. and mKBICV, V.K.: Study of the volatilisation of K20 using mthmatical vimantal design Tsemnt, 1966, (41, 7-9.
58.
GOFS,C andKEn, F.: The behaviour of alkalis in cmentburning Tonindustrie-Zeitmg, 1960, 84, (81, 125-133.
59.
ANDERSON, E and NESTELL, R.J.: The volatilisation of potash fran cenentmterials. Journal of Industrial and Engineering Chemistry, 1917, 2, (31, 253-261. _
60.
T0ROK)V, NA. and cOBROVOL'SKIY, K.A.: Volatilisation of sodium and potassiumoxides during clinker burning andtheir distribution in the minerals. Tsement, 1965, (31, 6-7.
61.
GRAZHDANSKIY, S.A., KINSTLW, reducing the alkalicontentof Tsemnt, 1965, (31, 18-19.
62.
J?CKSCN, D.D. and MlRGAN, J.J.: An application of the vapor pressures of potassium cmpounds to the study of the recovery of potash by volatilisation. Journal of Industrial and E&nearing Chemistry, 1921, 2; (41, 292-295.
63.
KOIX, H.: Remving alkalis by heating with admixtures. Rock Products, 1942, 5, (21, 66-68. (a reprint of FCA report M-74, 1941.1
64.
LMWUR, H. M&.: ~ethodof reducing alkali content in Portland cerrPntclinker. Portland Cement Association report M-74, 1941.
65.
TXWXJR, H. McC., MCMASTER, E.T. andJACQUES, alkali Portlandcemntclinker. Rook Products, 1943, 46, (71, 59, 67, 68, 74.
and ABUIA, S.Ya.: m of clinker.
K.M.
W.: Manufacturing low
66.
CARLSEN, H.: The behaviour of alkalis in cement raw materials during the burning process. Rock Products Annual Cement Industry Operations Sem,inar;1965. (reprinted in Rock Products, 1966, 2, (51, 87, 88,' 157)
67.
LOVELAND, R.A.: Reduction of alkalies in cement kilns (study by questionnaire) Portland Cement Association report MRS-61, 1948.
68.
HOLDEN, E.R.: Reduction of alkalies in Portland cement - use of calcium chloride Industrial and Engineering Chemistry, 1950, 42, (21, 337-341.
69.
GILLILAND, J.L.: Removal of alkalies by use of hydrochloric acid. Symposium on alkali removal and problems, Milwaukee, Wis., 1959. Portland Cement Association report M-158, 1960.
70.
WOODS, II.: Reduction of alkalies in cement manufacture. Portland Cement Association report M-149, 1956.
71.
NDDEL'MAN, V.I. and UVAROVA, 1.T.: Method of reducing alkali contents of clinkers from cement works in Central Asia. Tsement, 1968, (41, 12-13.
72.
SPRUNG, S. and SEEBACH, H.M. von: Fluorine balance and fluorine emission of cement kilns. Zement-Kalk-Gips, 1968, z, (11, l-8.
73.
BANIT, F.G. and VASILIK, A.V.: on exit gas dust removal. Tsement, 1972, (61, 17-18.
74.
HATANO,H.: The behaviour of sulphur in the suspension preheater kiln system. Zement-Kalk-Gips, 1972, 25, (11, 18-19
75.
COLUSSI, I and LONGU, V.: 11 Cemento, 1974, 71, (21,
76.
Gypsum finds new role in easing sulfur shortage REMIREZ, R.: Chemical Engineering, 1968, 75, (241, 112-114.
77.
Production of cement and sulphuric acid from gypsum. Cement and Lime Manufacture, 1968, 41, (41, 53-60.
78.
MERZ, A.R. and ROSS, W.H.: The nature of recombined potash in cement mill dust. Journal of Industrial and Engineering Chemistry, 1919, 2, (11, 39-45.
79.
BROWNMILLER, L.T. and EOGUE, R.H.: The sys tern CaO-NaZO-AlZO3. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 25, 1932.
Influence of gypsum-bearing additives
Thermal decomposition of calcium sulphate. 75-98.
80.
BROWNMILLER, L.T.: A study of the system lime-potash-alumina. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 30, March 1933.
81.
TAYLOR, W.C.: Phase equilibria studies on mixtures of the compounds 4CaO.Al2O3~Fe203-2CaO.Fe203-K2O.A12O3. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 37, September 1938.
82.
TAYLOR, W.C.: The system 2.CaO.SiO2,K 20.CaO.Si02 and other phase .equilibrium studies involving potash. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 40, September 1941.
83.
TAYLOR, W.C.: Further phase-equilibrium studies involving the potash compounds of Portland cement. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 43, December 1942.
84.
EUBANK, W.R. and BOGDE, R.H.: Preliminary study on portions of the systems Na20;CaO-A1203-Fe203 and Na20-CaO-Fe203-Si02. Portland Cement Association Fellowship at the National Bureau of Standards, Paper No. 50, March 1948.
85.
NEWKIRK, T.F.: Effect of SO3 on the alkali compounds of Portland cement clinker. Journal of Research of the National Bureau of Standards, 1951, 47, (51, 349-356.
86.
NEWKIRK, T.F.: The alkali phases in Portland cement clinker. Proceedings of the 3rd International Symposium on the Chemistry of Cement, London, 1952, pp.151-171.
87.
ELLINGSON, O.A., GILL-AM, J.L. and KOPANDA, J.E.: "water-soluble alkali" in cements. ASTM Bulletin, 1956, (Feb), 63-66.
88.
SEGNIT, E.R.: Further data on the system Na20-CaO-Si02. American Journal of Science, 1953, 251, 586-601.
89.
BA3ATSCHEV, G.N. and RADEVA, K.K.: Alkalis in Portland cement clinker. Silikattechnik, 1961, 12, (11, 33-35.
90.
BUDNIKOV, P.P., AZELITSKAYA, R.D. and LOKOT', A.A.: Effect of additions of gypsum on mineral formation in alkali-containing cement clinker. Zhurnal Prikladnoi Khimii, 1968, (5), 953-957.
91.
AZELITSKAYA, R.D., PONOMAREV, I.F., BLONSKAYA, V.M., KARBYSHEV, M.G., LOKOT', A.A., and STEPANOV, V.M.: .The effect of gypsum on the phase composition of alkali-containing clinker. Tsement, 1969, (2).
Determination of
92,
LIx;ININA, I.G. and SHAPCSHNIKUVA, M.A.: Characteristics of clinker mineral formation in thepresenceof alkalisulphates. Tsemant, 1970, (81, 18-20.
93.
EUIT, Yu.M., KADS~SKII, V-E., TURETSKII, A.M. and iANN, N.S.: Effect of gypsum on the properties of an alkali-containing cmarit. Tsemant, 1971, (41, 14-16.
94.
AHHASSI, G.: A contribution to the study of the influence of alkali metals an the canposition and hydration of clinker phases. Revue des Materiaux de Construction, 1974, (6911, 315-322.
95.
RITZMANN, H.: Hmtokeepalkalies frcxnstealingprebeater efficiency. Rock Products, 1974, 77, (21, 6669. (originally presented as a paper entitled "Raw maal preheater and alkali problems" attheRcxkProducts 8th International Cemant Industry Sminar, 1972).
96.
PCYITIZ, N.S. and CHEESMAN, R-D.: Effect of coal ash on the liberation andnature of cmtantmi1lpotash. Journal of Industrial and Engineering Chmistry 1918, 10, (21, 109-Ill. (See also correspondence from E. Anderson and R.J. Nestell and fran E.O. Rhodes and J.J. Porter in (121, 1030-1033).
97.
-, I. and PURI, A.: Cycle of volatile substances in rotary kilns for Portlahdcemntclinker. Z-t-Kalk-Gips, 1975, 25, (91, 377-379.
98.
IEHMANN, W.S. and PIASSMANN, E.: Determination of the alkali circulation with the aid of the radio-isotope K42 in a long wat process rotary kiln. Zmant-Kalksips, 1957, 10, (31, 89-93.
99. WEBER, P.: Alkali problms and alkali elimination in heat ecoimnisiq dry process kilns. Zearant-Kalk-Gips, 1964, 17, (81, 335-344. 100. VlRlloRENKcrv, V.I. and MLKONSKY, preheaterkilns. Tsemxit, 1965, (61, 12-14.
B.V.:
Alkali circulation in cyclone
101. RITZMANN, H.: The effect of dust cycles on the heat consuqtion rotary kiln plants with raw maal preheaters. Zemant-Kalk-Gips, 1971, 24, (121, 576-580. 102. SPRLm, s.: The chemical andmineralogical cmposition dust. Tanindustrie-Zeitung, 1966, 90, (lo), 441-449,
of cemnt
of
kiln
103. LITYNSKI, T. and GODEK, J.: K2C and CaO detennination in foreign and Polish cemant dusts. Zmant-Kalk-Gips, 1965, g, (101, 534-535. 104. HEIIbIANN, T.: Treatmantof dust frcmcemntkilns. British patent 1,145,827, published 19th March 1969, assigned to -F.L. Smidth & Co. A/S. 105. KESTER, BE.: Alkali reduction by kiln dust leaching. Symposium on alkali remval and problems, Milwaukee, Wis., 1959. Portland Cement Association report M-158, 1960. 106. STEVENS, H.A.: Wetrecoveryof kiln dustandsubsquentalkali reduction. Symposium on alkali removal. and problems, Milwaukee, Wis., 1959. Portland &meat Association report M-158, 1960. 107. BADE, E.: Process for reducing the alkali cycle in clinker burning. Zanent-Kalk-Gips, 1962, 15, (91, 403-408. 108. YURGANUV, N.N., SAFONOV, N.A. and BRODKINA, E-R,: Method for reducing alkali circulation when returning dust to rotary kilns. Tseiwnt, 1966, (11, 10-n. 109. WATSCN, D and ERW, A.W.: Treatment of waste products frcm Portland cement manufacture. US patent 4,001,030 (4th January 19771, assigned to Associated PortlandCerrwt Manufacturers Limited. experience andconsiderations 110. EGNN, Wand-HARD, U.: Cementwxks relating to the design of bypass system. Zerrwt-Kalk-Gips, 1972, 25, (61, 281-282. 111. HAEMms, P.: Alkali behaviour in a suspension preheater using a gas by-pass - practical experience at the Rillito installation. IEEE Cement Industry Technical Conference, Tucson, Ariz. 1976. ll.2. SCHLUJ!ER, H.: Process for reducing the alkaliandchlorine suspension preheater kilns. Zerwt-Kalk-Gips, 1972, 25, (11, 20-22.
cycles in
The development and application of 113. CHRISTIANSEN, S. and MADSEN, G-M.: precalciners for cement kilns. Reek Products, 1975, 78, (51, 85-88, 126. 114. SMIMH, F.L. & CO. A/S: ~ethodof burning cement. British patent 1,023,910, published 30th March, 1966. 115. lIED&DT, H.: Measures for reducing the alkali cycle in the Le@ kiln. VDZ Congress on Process Technology of Cmznt Manufacture, Dusseldorf, 1971, 164-167.
116. -T, E.A. and HEIAN, G.A.: Alkali remval SyStem, Rmk Products, 1973, 7fi, (51, 60-64, 179.
via grate - kiln
117. CLAUSEN, C.F.: U+mlkali cemmt fran high-efficiency kilns? Rock Products, 1960, 63, (11, 148, 149, 152, 154, 164. 118. POLYSIUS GMEi: Process andapparatus for producing -toflow alkalicontent. British patent 874,818, published 10th August 1961. 119. VEIGEZ, J.F.: Problems of alkali reduction in the Htildt system. Sympsosium on alkali remval and problem, Milwaukee, Wis., 1959. Portland Cetrent Asscciation report M-158, 1960,
RRC/TGB/JMLC/b31 11.03.87.
TABLE 1 Melting andimiling c-unds.
pints of selected calcium, potassiumand
Camp&d
CaCO3 --cam2
Mslting point ("Cl
Eoiling Point ("Cl
C!ecoqases 899 (calcite) 772 - 782
> 1600 - 2000
c*2
1386 - 1423 (45)
Ca(OHI2
deccmpses 580
Cd0
2570 - 2600
-
sodium
2500
2850 - 3000
4
1450
KP3
891
KC1
776 -
790
1407 - 1500
ET
846 -
858
1505
IUJH
360 -
380
1320 - 1322
KiQ K2Si03
ckcqoses
ckarposes
350
976
K2Si$5
1015 * 10
K2s04
1069
*a2C03
851
dL3XiVpSeS
N&l
801
1413
NaF
988 - 997 (46)
1695
NaOH
318
1390
*aZo
sublines 1275
Na2=2o5
874
Na2SiO3
1088
Na4SiO4
1018
*a2=4 RRC/+IGB/JMLC/b31 11.03.87.
1689
890
.
:
;.’
fi ,...I
t:‘.
!.: L .
--.-
. . . .:..
: -‘-..--+ :__: i_
.: .. ! ---CL... .: 8.
-:I
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I-
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,
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) I._!
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:\.
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:..[:I.: I : -..I:
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:-
~-.~:--::j..-~:~:
_ ,-.I ‘:
7: :I20 vol2tilisation (Log -scel.
- .- -/__ -.. :.---/ : F&pure 3 ---_,-.-i i : ..- i--‘--_-e i--q. -_. , I 1-. _-!------i --.-.._. . .._ ! .-- LLJ--L .q ..____ T:. ..i -Lp:; j _ j ic;. .c ;
t I . TW\‘\. .
3 7
c
p-1 ( .bh
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;
--y-y--.,
i-i I 4 i-. . . I --.,-.I ‘-.I ______ &- -i- _---t’-~--f .-..---i .-.1 .I .-1.... j __.. i ______ I-. _.__ /r---- ;- i- - --- --.---L ! : ____ i l..- i : I ---.\p.I 1 t ! j. I I j I ./ mien ] .\
: i /_i\i:-+-i-i-; - _- ..i.- -.*-:.&-- - c---. -.I.. . . i. ._ 1..--i _._. .___ i .____. i’___._ i. . I T-77
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Figure 4 3
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:-...--,.: !
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: : i
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F&me 5 . .._~ _.,_ -. __._- j___..--_ _ .__-T.--.-. - .3 t7-------, 5; : _-_1 : ._’ _ .; r9/ .____ _ ..____.. - -- ._-- -- --_. . 1 / 2 ‘f- -7 3; ..i .--.. --... ;. : :__ _ _ *- I ;-- : ;--,-. c-,..-“--.‘ -_-_i --.. -- _ _ - _-.. _._- _. .__._. ._.. ____ ! ‘--[i -.-i *-j -. -.- : i ;.. . :... i . ..-) .._. : j --. / I _ __-_ &.‘__ __-----_ --. ---_-___---:-.+--;--;. I-- 7 . i yy-y1 I ; __.. ;T j . j _ ; . 8 . ’ A------.:-.-- .-_---.. --,-I--.- :_-__-___ 1-m ._- -.---+--i : T: 1 I t I
.--._--_--- -__/ ._ __ - ..-- I
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50;
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Figure 6
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Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 6
A Study in the Volatile Cycles on HOPE #2 Kiln
A series of seven tests have been conducted at Hope Works during which the No. 2 kiln was operated at selected, controlled levels of kiln back end NOx (equivalent to burning zone temperature) and oxygen in order to study the effects on volatile cycles of these different regimes.
Each detailed
study period lasted at least twelve hours and samples were collected from various points around the kiln and preheater system for analysis.
CONCLUSIONS
The test results indicate that the chemical proportions of the minor constituent volatile components at Hope lead to the following cycle levels in stage IV meal.
The typical fluoride cycle level is 1.2 to 1.4 times the input to the system and does not change substantially as the kiln temperature or atmosphere is modified, over the range studied.
The chloride cycle is 20 to 30 times the total chloride input and increases slightly with increasing temperature, although this may be due to improved kiln stability at higher temperatures. The chloride cycle is not modified by vide changes to kiln atmosphere.
3.
The chloride cycle is a low temperature cycle and cannot be significantly modified by variation of kiln conditions.
4.
Chloride in the cycle combines with potassium where available. However,
at low temperatures - equivalent to NOx levels of below
1,000 ppm - some sodium is also involved.
5.
The total potassium cycle is approximately 3 times the input level, however about two thirds of this is present in Stage
IV in conjunction
with chloride and so cannot be controlled except by incorporation of a bleed sys tern.
6.
About one third of the potassium in Stage IV is either feed based on its first passage through the preheater or derived from a potassium sulphate
based
cycle.
This
cycle ratio varies from 1.05 (minimal
recycle) at 900 ppm kiln NOx level to 1 .25 a t 1 , 4 0 0 p p m
NOx (25%
This portion can be controlled by operation at low
r e c y c l e 1.
temperatures.
7.
The sodium cycle level varies between 1
.2 and 2.0 times that in the
rarnneal over the temperature range examined.
8.
At low temperatures some sodium becomes involved in the low temperature chloride cycle and this boosts the sodium cycle by about
9.
10%.
The majority of the sodium in the system is involved in a sodium sulphate
based
cycle.
This is strongly temperature dependent, and
the rate of increase also appears
to be increasing with temperature.
Over the temperature range investigated the cycle rose from 1.35 times t h e l e v e l i n t h e feed at low temperatures to 2.0 times at high X0x levels.
10. The total sulphur cycle is temperature dependent and rises from 1
.5
to 2.0 times that of the input over the temperature range investigated.
11. The alkali sulphate cycle has already been summarised in Points 6 and 9 and makes up about 35% of the total sulphate in Stage IV material. These cyclic levels can be seen to be lower than the total sulphur cycle
levels.
12. The calcium sulphate recycle is strongly temperature dependent and rises from about 2.5 times the level in the feed at lov NOx levels to 4 times at high (1,450 ppn)
NOx levels.
13. The calcium sulphate cycle is also increased by a move into a reducing kiln atmosphere.
At about 1,200 ppm NOx the cycle
increased from 3.1 under oxidising conditions to 5.5 times the feed level under reducing conditions.
14. For potassium and sodium there are indications of a slov but steady increase in losses of these components from the preheater system as temperatures increase.
This may become part of an external
cycle or may be lost to atmosphere.
This could be established by
a longer term study of the levels of these components in the precipitator and stack dusts.
15. Sulphur is in overall balance within the system belov NOX levels of 1,200 to 1,300 ppm, but above this level the loss increases sharply with rising temperature.
Again this may become part of
an external cycle or may be lost from the systen,
hovever in this
exercise no precipitator or stock dust samples were collected so this cannot be confirmed.
16. When the kiln a:mosphere moves into reducing conditions the losses of alkalies and sulphur from the kiln system increases sharply.
In viev of these conclusions it is recommended that, at
1.
Hope:
Kiln back-end oxygen level is maintained at 2.0 to 2.52 (as measured).
2.
Kiln back-end NOx levels are maintained below 1200
ppm,
although
the most appropriate NOX level will depend on detailed raw mix chemistry.
3.
Communication between kiln burner and personnel cleaning the preheater be improved.
4.
During cleaning operations short term reductions in raw meal feed vhich
avoid
the
inception
of
reducing
conditions
will
be
beneficial in the longer term.
NOTE :
Cycle =
quan:ity i n S:age I V - o n a l o s s f r e e b a s i s quantity in feed on a loss free basis
F u e l i n p u t l e v e l s f o r a l k a l i e s , fluorides and sulphates have not been taken into account as the levels are relatively
lou and constant, as
indicated from the small number of fuel samples that were analysed. The chloride input from coal has been taken into account.
1.
INTRODIJCTIOfi Since the development of the suspension preheater based dry process for cement manufacture the study of the inherent cycling effects of the potentially
volatile
components,
which
are present as minor constituents
of the raw materials and fuels, has become increasingly important.
In
the less thermally efficient plants a natural loss of a portion of these volatile components occurred in the waste gases so automatically controlling
the
recirculation
levels
level in the clinker product.
within
the
burning
system
and
the
Previously this loss of volatile
components has also been higher than would be found today due to the capabilities volatilised
of
the
available
components
zones of the kiln.
pollution
condense
on
control
fine
I n the suspension
dust
equipment, particles
preheater,
with
as
the
in
the
cooler
its
greatly
increased surface contact between gas and particle and repeated separation
of
gas
and
particle, the recovery and retention of
volatilised components will be almost complete.
This leads to
increased proportions of these components in clinker and to the development of operational problems in the pyro-processing stage due to the
quantities
through
of the potentially sticky components that build-up
cyclic
processes.
These effects limited the immediate impact
of the suspension preheater technology in the industry as a whole for a number of years.
Nevertheless increasing fuel prices and the
requirement for even larger process units provided the incentive for identifying the causes of and solving the problems associated with volatile targetted
component at
cycles 1~3.
controlling
alkali
the recirculation levels wi:hin kiln
In general this work has been levels
such
in
clinker
boundaries
and/or
that
maintaining
allow
reasonable
operation, whilst the solutions have been limited to indirect
methods
of
control;
modification of the raw mix, change of fuel,
installation of a bleed system, manual or automatic cleaning of areas where excessive bui Id-up occurs .
During the developaent phase of the high level control project at Hope Works it became evident
volatile
cycle
were produced as the combustion chamber conditions were modified.
The
more
consistent
control
that and
considerable greater
changes
understanding
in of
the
process
conditions
vithin the kiln that vas developed through the reliable measurement and interpretation of kiln back end conventional
instrumentation
NOx level in association vith the more
offers
the
potential
for
deliberate
modi-
fication and control of the volatile cycles by variation of process conditions of
on
a
troublesome
plant
scale.
volatile
cycles
This would then permit the minimisation and
hence
lead
to
improved
kiln
stability
and operating times.
Consequently,
it was decided to investigate the exient of variations in
the volatile cycles vhich could develop by operating a kiln under a number of temperature and gas compositional (oxidising/reducing) Over the course of the last tvo years, as
operational
restraints
regimes. and
manpower availability have permitted, a series of seven studies have taken place on No. 2 kiln system at Hope Works.
In these studies three
different kiln back end NOx levels (800 to 1,400 ppm) atmosphere
conditions
as shovn in Table 1 . relationship
betveen
(reducing
to
high
excess
oxygen)
and three kiln were
targetted
Previous vork 4j5y6 has shovn the general kiln
h’Ox level and burning zone temperature.
Although it is not possible to
equate a particular NOx level vith an
actual definitive material or flame temperature at the fron: end of
the
k i l n , t h e t y p i c a l r e l a t i o n s h i p f o r N o . 2 k i l n a t H o p e i s shovn i n F i g . 1 .
For control purposes the advantages of NOx m e a s u r e m e n t a r e t h a t a b o v e a base
level NOx generation is proportional to the temperature of the
burning zone - v i t h i n s p e c i f i c l i m i t a t i o n s , - t h e r e i s a v e r y f a s t reaciion
time, the signal suffers relatively little process noise, and
the true reading
is
unaffected
by
front
end
dust
cycles.
2.
VOLATILZ
2.1
CYCLES
General
Considerations
7,8,9,10
The minor components that are generally considered to be involved in
major
volatile
cycles
are
the
chloride,
alkali,
and
sulphur
s p e c i e s , with some attention also being paid to fluorides.
Prime
control of the volatile component cycles is performed in the design stage of a Works project through the selection of rav’ materials and fuels in order to optimise the relative and absolute levels
of
the
potentially
volatile
components
the inclusion of a bleed system. defined
the
volatiles
during
the
pyro-processing
and,
if
necessary,
Once these factors have been
will develop internal and external cycles an internal cycle being
stages;
totally within the kiln and preheater system, vhilst an external cycle will leave the system but be returned after a time lag (for instance
with
the
precipitator
are kept steady, the cycles equilibria
are
reached,
dust).
will
Where processing condition2
continue
to
develop uniil
at which time the total amounts of
volatile entering the system will be balanced by the quantities leaving the system.
The degree of volatilisation
and the rates at which the
equilibrium are established will depend on:
(i) (ii) (iii) (iv) (VI
(vi) (vii) (viii) (ix) (xl
The
species, their
chemical
forms,
and
concentrations.
The volume of gases. The intimacy of contact between gas and solid. The vapour pressures of the salts. Possibility of dissociation or further reaction. Rate
of
diffusion
to
and
from
solid/gas
interfaces.
Degree of saturation of gas. K i l n a:mosphere. Kiln
temperatures.
Time/temperature profile of material within the kiln.
lost
of these factors are to some degree inter-related, and so in
norma 1 operation the only methods available to control the degree of volatilisation
will be the kiln internal atmosphere and
temperature. The major cycles will be internal cycles between the burning zone and preheater where an individual cycle time of twenty to thirty minutes would be expected.
Smaller
quantities
of
volatile
components may be involved in the longer term external cycle which develops raw
via
the
precipitator
dusts
and
volatile
collection
in
the
These would be expected to have much longer cycle
mill.
times of up to twenty four hou:s.
2.2
Fluorides
l1
Fluorides can be found naturally in raw materials or be deliberately added in small quantities to the raw mix. proportions burning
its
mineralising
process.
action
has
a
beneficial
In low
effect
on
the
Generally it has a low volatility and causes
few operational problems, although a level of above 0.25Z
in
clinker may lead to setting problems particularly in winter. However,
cases have been cited vhere a hard dense build-up has
developed
in
preheaters,
where the build-up contains high (over II)
proportions of fluoride.
2.3
Chlorides
l2
Chlorides are derived from the raw ma:erials
and the kiln fuel.
Either source will only have very small quantities of inherent chloride, with
the
but high
the
high
volatilities
collection
efficiency
of of
these the
compounds cyclone
together
preheater
systems will lead to the development of a greatly enhanced cycle. The chlorides have a high affinity for the alkalies in general and potassium
in
particular.
This property - together with the high
volatility K20
- has been used in wet process kilns to control clinker
levels by addition of CaC12 to the rav mix o: fuel, which
leads to loss of KC1 with the exhaust gas from the kiln. In the suspension
preheater,
however,
the
volatilised
material
is
recaptured within the system unless a bleed is utilised between the
kiln
and
riser duct.
It is generally considered that no more
than 3X of the chloride passing from the preheater to the kiln vi11 l e a v e the system with the clinker.
Although
considerably
higher levels have been noted in individual samples of clinker this is probably due to a ‘push’ of kiln feed, or a semi flush situation as thermodynamic consideration indicate that no chloride should pass through the burning zone.
On many SP kiln systems
some degree of preheater cleaning is necessary on a regular basis and this may help to control the chloride cycle by forcing the kiln conditions into a situation which permits a brief increase in clinker chloride level (i .e s reduced material temperature, increased
material
loading
and
flux
level).
Small amounts of chlorides will also leave the preheater system with the waste gas stream.
Results of detailed balances at Rope
and Plymstock suggest that the preheater loss can be of a similar order to that in clinker.
Taking the total loss of chloride from
the system as between 2 and 5% of the feed to the burning zone it would then be expected that a circulating load of 2 0 to 5 0 times
the rota1
2 . 4 A l k a l i e s 13p
chloride input vould develop.
l4
The major source of alkalies will be the raw
mix;
notably the
clay component although minor quantities can arise from :he fuels. The initial free alkalies will behave in one of three ways:
1.
Remain in the material being processed and become incorporated formed .
2.
3e
in
the
clinker
constituents
This happens to Na20
converted
carbonates,
into
different
hydroxides - by
that
are
being
to a greater degree than K20.
compounds
-
chlorides,
reaction
with
other
sulphates,
constituents
of the raw mix.
3.
Diffuse to the surface of the process material and volatilise.
I n i t s ini:ial s t a t e K20 b e g i n s t o v o l a t i l i s e o v e r a w i d e r a n g e o f temperature, depending on the form of clay in which it was incorporated,
but irrespective of source it would be expected to
have volatilised almost completely at burning zone temperatures although some may have been at least partially stabilised by conversion to the less volatile bed.
Once
volatilised
it
will
sulphate form within the material react
sulphates at the rear of the kiln. dust particles.
to
form
chlorides
and
These will then deposit on
I n i t i a l l y , Na20 i s l e s s v o l a t i l e t h a n K20 d u e t o
its higher bond energy and so a greater proportion of the kiln feed Na20
would be expected to pass through the burning zone in
clinker without participating in the volatile cycles. Na20
will react with SO2 and SO3
to form sulphates towards the
r e a r o f t h e k i l n , and with chloride vhere i n e x c e s s o f K20. and sulphate,
Volatilised
this species is present
Where alkali is present in excess of chloride
alkali carbonates will be formed.
Each of these
alkali compounds will deposit in a liquid state on the surface of dust particles in the cooler zones of the kiln and lower preheater stages and will enter the volatile cycles as the dust is separated out in the cyclones.
Direct contact onto kiln or preheater The compounds
surfaces may lead to the development of build-up. will
then
re-enter
the
kiln
where
the
degree of
volatilisation
will depend on the species and the kiln conditions.
Volatility
decreases
from
chloride to carbonate to sulphate, and hence
sulphates are more likely. to pass through the burning zone. Nevertheless the likely range o f burning zone temperatures cover the
thermal
increase
area
in
significantly
which
alkali
volatilities
are
with
rising
temperature.
likely
to
2 . 5 S u l p h u r 15s16~17
Sulphur can e n t e r the system in a number of forms from either fuels or raw materials.
A limited amount may evaporate in the upper
preheater stages and escape from the system in the exhaust gases. In general SO2 and SO3 can form in the high temperature areas and be transferred to the gas phase. In the cooler areas of the kiln back end and preheater system, sulphates will form and re-enter the material stream.
Preferentially
alkali
sulphates
will
be
produced with excess sulphate combining with free lime or calcium carbonate that is available in these areas.
k’here r e d u c i n g
conditions exist the equilibrium will favour the existence of SO2 a n d SO3,
so
restricting
the
formation
of
sulphates.
In this
situation loss of sulphur oxides by way of the stack may increase, but where the gas stream passes through the raw mill the majority of
the sulphur oxides would be expected to react with the high
active
surface calcium compounds which are produced in the milling This will :hen return to the kiln in the raw mix as part
process.
of an external cycle.
The high boiling points of the alkali sulphates would indicate that
relatively
Bowever,
low
dissociation
levels may
of
volatilisation
occur, particularly
would under
be
expected.
the
reducing
conditions which can exist to some degree within the burning zone. Calcium sulphate also has a high boiling point, but is even more susceptible
to
dissociation,
so
a
higher
from this compound would be expected.
recirculation
A s CaSC&
of
sulphate
cannot recycle as
a compound the lime from this compound remains in clinker as free
lime
- making burning more difficult - whilst the SO3 is carried
in the gas stream CO
the kiln back end where it reacts to form
a l k a l i o r c a l c i u m sulphate.
3.
PROCEDURZS
The intention was to operate the kiln under controlled back end oxygen and kiln temperature regimes, w i t h k i l n backend NOx l e v e l b e i n g u s e d a s an indication of change in temperature. were 800, 1,200 and 1,400 ppm.
The
target
&OX
levels
For each of these temperature conditions
two back end oxygen levels were targetted; (norma 1 oxygen 1 .
selected
1.8% (moderate) and 2.5%
At Hope Works there is a positive offset of 0.6 to
0.8% for the measured value of kiln back end oxygen as compared to the real value due to the type of sampling system.
In addition to these
s i x t e s t s , a further run was conducted at 1,200 ppn~ NOx with CO evident at the kiln back end - reducing conditions in the burning zone (Table 1). Each test run lasted at least twelve hours, over which period samples of preheater feed, kiln feed (ex Stage IV cyclone) and clinker were taken at hourly intervals.
Coal samples were also taken periodically. Each
sample vas analysed for major oxides, fluoride, chloride, tr’a20, SO3 using an XRF analyser.
K20 and
Ultimate and calorific value analyses were
also performed on :he c o a l s a m p l e s . Throughout each test period and, in most cases, for a period before the test began, kiln
operating
conditions
were
monitored.
Aver age kiln
conditions, and the range of conditions for each test are detailed in T a b l e 2 . Tne results of chemical analyses of Stage IV material are presented graphically for each test to show the variation with time together with selec:ed
process data in Appendix I.
During each test a number of plant measurements and material temperatures
- air
flovs, shell, gas
- vere completed, and a kiln operation log
maintained in order to build up sufficient information for heat and mass balances. of
the
This raw data is summarised
alkali
cycle
studies
this
in Appendix 2, but in the context
information
is
relatively
meaningless
as the dominant factor in the overall heat balance vas the raw meal feed rate
vhich varied significantly from test to test, notably as a function
of whether the raw mill was in operation. period
Additionaliy,
over the time
covered, modifications were made to the top stage cyclones which
changed the external dust cycle for the system.
4.
EX?ERI.%ESTAL 4.1
F!XSULTS
Fluorides
Average fluoride levels on a loss free basis for each test are given in Table 3 together with the ratios of Stage 4 material and clinker fluoride levels to the fluoride level of the preheater feed
material.
The level of fluoride in Stage
is generally
IV
1.15 to 1.45 times that in the preheater feed indicating low levels of recirculation.
At such low levels of recirculation and
initial input it is difficult to be definite about trends, but plots of this data - Figure 2 - i n d i c a t e i n i t i a l l y a s l i g h t decrease
in
average
recirculation
with
increasing
temperature,
folloved by a climb back to the original level. This pattern has been seen before in experimental burning of mineralised on the dry process. volatilisation
is
Experience
more
here
dependent
on
suggested the
that
potential
the mix than on material temperatures (Table 4).
clinkers
fluoride
flux
level
in
It is known that
an increase in flux level vi11 reduce component volatilities due to changes in diffusion rate and reduced surface area, and so it is
possible that in the longer term over the lower temperature
range the effect of increasing flux level vith temperature has a greater
negative
temperature
effect
than
considerations
the
for
straight
this
forvard
volatility/
component.
At higher temperatures the recirculation rises back up to its previous
level, as
temperature
effects
take
precedence.
These
trends are confirmed by the way in vhich fluoride levels in clinker and preheater feed are similar at low temperatures, but clinker levels lower than those of the preheater feed at higher temperatures. X0x
Plots of Stage IV material fluoride level against
level for each individual test (Figure 3) also suggest a
slight
increase
time s c a l e .
in
recirculation
with
temperature
over
this
short
There
is
no
indication
of
any
change
in
fluoride
recirculation
level being brought about by variation of the kiln back end oxygen level.
These observations cannot cover the situation where there is an excess
of
alkalies
over
sulphate.
In
this
instance
alkali
fluorides can form from CaF2 and so a different trend would be anticipated.
4.2 Chlorides
Initial comparison of the proportions of chloride in the Stage IV material
with
X0x
level for individual tests suggests a stable or
slightly falling level of chloride in Stage IV with increasing temperature (Figure 4) .
However,
higher
X0x
levels
were
generally
associated with higher raw meal input levels and a lower coal to clinker ratio over these test periods.
This gives a lower total
chloride input to the system and so the lower Stage IV levels would seem to reflect the lower input quantity.
AS
chloride is volatile
at relatively low temperatures it can evaporate before the burning tone,
and so has a short cycle time.
Figur? 5 compares the test
average da:a for the ratio of Stage IV to total input chloride quantities and shows a slowly increasing chloride cycle vith temperature,
but
this
cycle
is
basically
ratio falling between 20 and 30. at the higher NOx
a
large
cycle
with
this
There are also indications that
levels the chloride cycle had not reached
equilibrium as Stage IV chloride levels were still rising with t i m e a t r e l a t i v e l y s t e a d y NOx l e v e l s .
During the time period
in
which these tests took place the Works
were still maintaining a regular preheater cleaning campaign to control
preheater
build
up, and
the
regular
seni-flush
situations
vhich this incorporates may have helped to control the S:age chloride
levels
in
the
lower
half
of
the
predicted
range.
IV
Certainly it has been noted at Elope t h a t o c c a s i o n a l l y c l i n k e r s a m p l e s c o n t a i n h i g h (+ 0.1X) l e v e l s o f c h l o r i d e .
Furthermore,
t h e h i g h e s t t e s t a v e r a g e c l i n k e r c h l o r i d e l e v e l (0.162) vas n o t e d i n T e s t 1 , during vhich period the kiln was unstable. This is no evidence that modification of the kiln atmosphere by moving into reducing conditions causes any change in chloride cycle as the Stage IV to total input chloride ratio for Test 5 is very similar to those from Tests 1 and 2 which had similar NOX levels.
4 -3 Potassium
The potassium (K20) cycle measured in the Stage IV cyclone during these tests vas betueen 3.5 and 6.5 times the input level from the rav meal.
A S the proportion in coal is relatively small and vi11
not vary very much it has been ignored in these studies in order to simplify the exercise.
Figure 6a presents average data for each
test for the ratio of K20 in Stage IV material to that in the preheater oxygen
feed,
plotted
against
average
NOx
level.
For
the
normal
tests, this K20 ratio can be seen to be increasing (from
3.85 to 4.4) as NOx rises from 900 to 1,400 ppm.
For the moderate
o x y g e n t e s t s t h e p i c t u r e i s l e s s c l e a r v i r h t h e r a t i o s f o r t h e lov (Test 3) and high NOx (Test 6) :ests
being similar vith a sharp dip
i n T e s t 1 , at the medium NQx l e v e l .
It has previously been
suggested that K20 vi11 pre ferentially
react
a
lov
temperature
cycle.
vith
chloride
to
form
It is also noticeable that the test
which does not fit in with the general picture - T e s t 3
- also has
a very high chloride level in Stage IV.
the
preferential reaction of
K20
Assuming
that
in Stage IV is vith all the available
chloride it can be seen that 65 to 752 of the potassium in Stage IV will be bound to chloride.
This cycle is a low temperature cycle
and can only be controlled by use of a bleed system.
K20 vhich has not reacted with chloride will nov react to form sulphates.
Table 6 also includes the ratios of K20 a s s u l p h a t e i n
Stage IV to K20 in the preheater feed.
In this case the level of
K20 as sulphate in Stage IV rises from 1.05 times the K20
level in
the preheater feed at low #Ox levels to 1.24 times the preheater feed level at high NOx levels.
At both moderate and normal oxygen
levels this ratio increases as kiln NOX level rises. At the lower NOx levels (850
to 900 ppm)
the indications are that potassium
sulphate passes through the kiln without becoming involved in an internal cycle.
In fact at these temperatures, it is likely that
some Na20 becomes involved in the chloride cycle, as insufficient potassium was measured in Stage IV to totally account for the measured chloride level. and the trend shovs
These ratios are plotted in Figure 6b,
up clearly.
conditions test (tes:
The K2S04 ratio for the reducing
5) is slightly higher than those of the other
tests at 1,200 X0x level, but this increase is too slight to be convincing proof of an increased cycle under reducing conditions.
Figure 6c plots the average ratios for each test between the K20 levels in clinker and preheater feed.
This clearly shows that at
low NOx levels the system is in equilibrium in terms of K20, however as the NOx level rises the output level of K20 falls belo= that of the input. is approaching 202.
By an EiOx level of 1,400 ppm this inbalance Bowever, with the onset of reducing
conditions this inbalance approaches 50f.
There art two possible
explanations for such an offset betveen output and input levels. Firstly,
it is possible that a build up of concentrated K2SO4
KCL is developing;
and
this material sets on the surfaces vi:hin the
preheater and so prevents the material re-entering the cycle. Secondly, there is a loss of K20 from the system.
This may pass
to atmosphere through the stack, or be collec:ed within the raw mill and precipitators and be returned to system as part of a long term external cycle.
4.4
Sodium
Test average data for Xa20
levels
- on a loss free basis - around
the kiln system are presented in Table 7, showing that the Na20 level in Stage IV is between 1.6 and 2 times that in the preheater feed.
Figure 7a plots the ratio of Stage IV Na20 to preheater
feed Na20 against NOx level.
This
suggests
that
initially
r a t i o f a l l s s l i g h t l y a s NOx r i s e s b e f o r e c l i m b i n g a g a i n . previous
the In the
section it was suggested that at low NOx levels some Na20
reacts with the chlorides so becoming involved in the low temperature Ka20
cycle.
Whilst only low proportions of the available
- p o s s i b l e 1 0 % - become involved in this way, it can still
s i g n i f i c a n t l y i n c r e a s e t h e t o t a l Na20 r a t i o o f Na2S04
cycle.
I n F i g u r e 7b t h e
i n S t a g ? I V - as calculated from a chloride,
potassium and sulphate balance - to preheater feed Na20 is plotted. as
NOx
This shows an increase in ihe cycle derived from increases.
The data suggests that the Stage IV Na20
Na2SO4 level
- as SO4 - rises from about 1.35 times the level in the preheater f e e d a t l o w NOx t o a b o u t 2 . 0 t i m e s a t t h e h i g h NOx l e v e l . T h e results of moderate and normal oxygen level tests suggest that the recycle can be reduced slightly by operation at higher oxygen leve 1s , although this is not born out by the low oxygen test. AS
with K20, clinker output and preheater input Na20
Salanced
a t l o w NOx l e v e l s , b u t a s t h e NOx l e v e l r i s e s a b o v e 1 , 3 0 0
the level in clinker falls quickly;
being betveen 20 and 30% lower
than the input level at 1,400 ppm NOx. certain
temperature
l e v e l Na2SO4
temperature dependant. Ira2SO4,
levels are
As Na20
This suggests that above a
recirculation levels are strongly is almost completely present as
t h i s m a k e s i t p o s s i b l e to control and in some cases signifi-
c a n t l y r e d u c e t h e fia20 r e c y c l e b y conirol
of kiln conditions.
4.5 Sulphur
The level of sulphur - calculated as SO3 - measured in Stage IV material and in clinker was very variable during the course of each test as the levels react strongly to changes in both kiln temperature and
atmosphere.
However,
average values for the
samples taken around the kiln system for each test T a b l e 8 - show a more consistent picture.
- given in
The ratio between
sulphur as SO3 in Stage IV and in raw meal is plotted against kiln NOx level in Figure 8a and shows that over the temperature range investigated the total sulphur cycle rises from 1.6 to 2.6 tonnes the sulphur input from the raw meal, rising as the NOx level rises.
There is also a substantial increase in cycle once
reducing conditions are encountered - from 2.0 to 3.0 at 1,200 ppm NOx.
In the previous two sections the alkali sulphate cycles have been discussed.
A S these cycles increase only slowly with temperature,
these effects have been removed from the sulphate cycle by calculating the amount of sulphur likely to be present as alkali in raw meal and Stage IV and subtracting these from the total quantities.
The remaining figure can be considered to be the
sulphur which takes part in a calcium sulphate derived cycle; at ilope this makes up approximately two thirds of the sulphur in Stage IV.
In this case the ratio rises from about 2.5 at low NOx
levels to around 4 at an NOx level of 1,450 as decomposition of CaSO4 moves
increases. into
This cycle also increases quickly as the kiln
reducing
conditions; from about 3.1 to 5.5 under the
test conditions.
As
in the previous two sections at low NOx levels the sulphur
output
in
clinker is approximately in equilibrium with that in the
raw meal, whiist a s NOx rises above 1 ,300 ppm clinker sulphur levels are lower than the inputs.
Study of the data from
individual tests shows that after a sharp rise in temperature the sulphur
cycle
initially
hours to stabilise. at
higher
responds
very
quickly
but
takes
several
This suggests that if the kiln is held stable
temperatures
the
recycle
ratio
is
likely
to
rise
even
above the levels measured to date, and an external cycle will develop. In reducing conditions the clinker to raw meal sulphur ratio
also
drops
substantially,
from equilibrium to 0.55 when
operating at a 1,200 ppm NOx level at the kiln back end.
5.
GZERAL
DISCUSSION
The results indicate that with improved control of the kiln and operation at lower temperatures and under steady oxidising conditions it would be possible to reduce the alkali levels in Stage IV by a moderate amount, and the sulphate level significantly.
Over the
temperature range investigated the K20 level in Stage IV material was reduced by about 10% at the lowest temperatures, whilst Na20 and SO3 levels in Stage IV material both fell by over 302. It must also be recalled that the highest kiln back end NOx level that was targetted was 1,400 ppm.
Shortly before this test work began the kiln was
commonly run at an NOx level of about 1,800 ppm, and when an NOx monitor was first installed
NOx levels of about 2,500 ppm
were common.
At that latter NOx value SO3 levels of up to 7.0% were recorded as compared to the figures of 2.4 to 2.7 obtained during the low temperature test work.
A similar effect on Stage IV SO3 level was
observed during the new Oxford Works Simulation Trials at Plymstock in 1980 lg,
as shown in Figure 9, although in this case extra iron was
added to the raw mix to permit the large reduction in burning zone temperature. Overall the general trends and size of cycle for each component fit in with theoretical studies for the vapour pressure v. temperature relationships (Figure 10).
During the early days of high level control test work it was noted that raising the measured kiln back end oxygen level from its previous normal level of about 1.5% to around 2.3 to 2.5% resulted in considerably more stable kiln operation.
This was presumably a two tier effect, as a
normal level of 1.5% gave the potential for the kiln to dip into reducing conditions when any disturbance occurred.
The first effect
of this would be to modify the flame and hence temperatures around the system.
Secondly the reducing conditions vould strip volatiles -
notably sulphate - from the system, so upsetting the equilibrium and modifying the quantities of low temperature flux and increasing the likelihood of high levels of build up around the kiln system. This
e f f e c t o f o x y g e n o n s u l p h u r l e v e l s in clinker has also been noted at Northfleet Works 2o d u r i n g t e s t i n g o f an SO2 continuous measurement probe,
and at Westbury.
deliberately the
raised
consistency
in
of
In the first case oxygen levels were
order
cement
to
stabilise
clinker
SO3
levels
to
improve
quality, whilst in the latter the oxygen
level was reduced in order to increase the sulphate and alkali cycles, so
allowing
the
Stabilization
control
and
of
reduction
alkali of
levels
the
on
volatile
these
wet
process
cycles
will
lead
kilns.
to
more
c o n s i s t e n t c l i n k e r q u a l i t y , and there will be a lower and more consistent heat sink on the front end of the kiln resulting from the e v a p o r a t i v e l o a d 21 3 22.
this may be partly offset by the
However,
reduction in low temperature flux level at the rear of the kiln. Further study would be necessary
In
the
past
most
methods
for
to
the
investigate
prediction
of
this.
volatile
recirculation
have been based on the oxide input levels, without taking real account of their inter-dependancy and the forms in which the recirculation develops.
The results of this exercise clearly show the need for a
more detailed breakdown into the components of the volatile cycles. kiiilst The
early studies by Research Division did take this approach, 23. installation
increased cycles .
kiln
of
‘Linkman’ high
stability
level
control
systems
will
give
and permit improved studies of volatile
A further exercise on a dry process site with excess alkali
would be of benefit in expa riding t h i s s t u d y t o c o v e r t h e compl?te of
possible
volatile
components.
range
6.
PR..C?IC.AL
I.QLICATIOWS FOR ROPE WORKS
Tne da:a shows that the major significant cycle at 'dope is calcium sulphate based.
This cycle increases markedly with increasing
temperature and also increases greatly with the on-set of reducing conditions.
Consequently it can be minimised by operation at lov kiln
temperatures and in an oxidising environment.
It is not, however,
simply enough to avoid CO at the kiln back-end, as significant reducing conditions can occur within the burning zone before an obvious increase in CO is noted at the kiln back-end.
Previous work has shovn that a
water wash sampling system tends to give an offset in oxygen analysis and that this offset will depend on the water flow rate through the sampling system.
In general during the test periods this offset fell
betveen 0.6 and 0.8;
a kiln back-end oxygen reading of 2.5% vas
equivalent to a true reading of 1.9% (offset 0.6).
In order to
minimise recycle it is advisable to maintain the kiln back-end oxygen reading - from the existing kiln instrumentation - between 2.0 and 2.5%.
Kiln front end temperatures should be at the lowest level
compatible with steady kiln operation and will depend on the rav mix chemistry and condition of the firing system, but is likely to generally fall in the back-end
NOx range
of 800 to 1000ppm.
It is significant that the periods when kiln operation is most likely to fall outside these ranges are during the routine cleaning operations, vhich is also the time when the concentrations of volatile components within the kiln are likely to be the highest. Extra attention to kiln operation during these periods should result in minimisation of volatile retention, vhilst lack of attention is likely to cause the majority of the potentially volatile components cleaned from the preheater to be driven back and redeposited within the system. In order to fully control kiln operation at these times it is necessary that the kiln burner be notified before cleaning commences.
7.
COSCLCSIONS
6 KECOM?fENDATIONS
The test results indicate that the chemical proportions of the minor constituent volatile components at Hope lead to the following cycle levels in stage IV meal.
1.
The typical fluoride cycle level is 1 .2 t o 1 .4 times the input to the system and does not change substantially as the kiln temperature or atmosphere is modified, over the range studied.
2.
The chloride cycle is 20 to 30 times the to:al
chloride input and
increases slightly with increasing temperature, although this may be due to improved kiln stability at higher temperatures.
The ch lor ide
cycle is not modified by wide changes to kiln atmosphere.
3.
Chloride in the cycle combines with potassium where available. Eowever , at low temperatures - equivalent
to
NOx levels of below
1,000 ppm - some sodium is also involved.
4.
The chloride cycle is a low temperature cycle and cannot be significantly
5.
modified
by
variation
of
kiln
conditions.
Tne total potassium cycle is approximat ely 3 times the input level, however about two thirds of this is present in Stage IV in conjunction with chloride and so cannot be controlled except by incorporation of a bleed system.
r 0 .
About one third of the potassium in Stage IV is either feed based on its first passage through the preheater or derived from a potassium sulphate
based
cycle.
recycle) at 900 ppm kiln recycle 1.
This cycle ratio varies from 1.05 (minimal NOx level to 1.25 at 1,400 ppm NOx (25%
This portion can be controlled by operation at low
temperatures,
7.
The sodium cycle level
varies between 1 .2 and 2.0 times that in the
rawmeal over the temperature range examined.
8.
A t lou temperatures some sodium becomes involved in the low temperature chloride cycle and this boosts the sodium cycle by about 10%.
9.
The majority of the sodium in the system is involved in a sodium sulphate
based
cycle.
This is strongly temperature dependent, and
the rate of increase also appears to be increasing with temperature. Over the temperature range investigated the recycle rose from 1.35 times the level in the feed at low temperatures to 2.0 times at h i g h NOx l e v e l s .
10.
T'ne total sulphur cycle is temperature dependent and rises from 1.6 to 2.0 times that of the input over the temperature range investigated
11.
.
The alkali sulphate cycle has already been summarised in Points 6 and 9 and makes up about 35% of the total sulphate in Stage IV material.
These cyclic levels can be seen to be lover than the
total sulphur cycle levels.
12
Tne
calcium
sulphate
recycle
is strongly temperature dependent and
rises from about 2.5 times the level in the feed at low NOx levels t o 4 t i m e s a t h i g h ( 1 , 4 5 0 ppm)
13
NOx l e v e l s .
The calcium sulphate cycle is also increased by a move into a reducing
kiln
atmosphere.
At about 1,200 ppm NOx the cycle
increased from 3.1 under oxidising conditions to 5.5 times the feed
14.
level
under
reducing
conditions.
For potassium and sodium there are indications of a slow
but steady
increase in losses of these components from the preheater system
as
temperatures
increase.
This may become part of an external
cycle or may be lost to atmosphere.
This could be established by
a longer term study of the levels of these components in the precipitator
15.
and
stack
dusts.
Sulphur is in overall balance within the system below NOx levels of 1,200 to 1,300 ppm, but above this level the loss increases sharply
with
rising
temperature.
Again this may become part of
an external cycle or may be lost from the system, however in this exercise no precipitator or stack dust samples were collected so
16.
this
cannot
be
confirmed.
When
the kiln atmosphere moves into reducing conditions the losses
of alkalies and sulphur from the kiln system increases sharply.
In vieu of these conclusions it is recommended that, at hope:
1.
Kiln back-end oxygen level is maintained at 2.0 to 2.52 (as measured 1.
2.
Kiln back-end NOx levels are mainta ined b e low 1200 ppm, although the most appropriate NOx level will depend
on detailed raw mix
chemistry.
3.
Communication between kiln burner and personnel cleaning the preheater be improved.
4.
During cleaning operations short term reductions in raw meal feed which avoid the inception of reducing conditions will be beneficial
in
the
longer
term.
REFERENCES
1.
G. Mussnung 'Contribution to the Alkali Problem in Suspension Preheater Kilns (Bumboldt Energy
Committee of
8 December
2 .
the
1961,
German
Kiln)‘.
Cement
Annual Meeting of the Heat and Makers
Association,
Dusseldorf.
'Coal and the US Cement Industry, Parts 1 and 2.'
World Cement
Technology, Jan/Feb. 1982 and March 1982. 3 .
Weber 'Heat Transfer in Rotary Cement Kilns' Section 4.5.
4 .
3aspe1, Lorimer, Southern, Taylor 'Blue Circle High Level Kiln Control' IEEE 1987.
5 .
A. Lorimer 'Fuel Savings Arising from the Operation of High Level Control at Elope Works', Research Division, Process Development Dept., F N
6 .
86/68.
Haspel, Taylor, Kerton 'High Level Kiln Control based on NOx Monitoring'. Paper to Cetic
7 .
Chemical Commission Mee:ing,
Locker, Spring and Opitz
'Reactions
Liege, June 1985 (ETN 85/21).
associated uith
kiln gases.
Cyclic Processes of Volatile Substances, Coatings, Removal of Rings'. VDZ Congression on Process Technology of Cement Manufac:ure, 1971. a .
Davis and Longman 'Design and Experiences with Bypasses for Chloride, Sulphate and Alkalies'.
9.
Bra-, A.W. 'Chemical Consequences of High Efficiency Systems' Group Technical Conference 1976.
10. Danoe and Steuck 'Behaviour of Volatile Matter in Cement Kiln Systems' FLS
Publication.
11. Hawkins and Wilson 'Fluorine Containing Phase causing Blockages Suspension
Preheaters'.
in
American Ceramic Society, Pacific Coast
Regional Meeting, Los Angeles, Oct. 31, 1977.
12. Polysius Review NO. 58 'Chlorine and its Behaviour in Preheater Kilns'.
13. Goes and Keil 'The Behaviour of Alkalies in Cement Burning'. Tonindustrie-Zeitung
84(8)
125-133, 1960.
14. H. Carlson 'The Behaviour of Alkalies in Cement Raw Materials during the Burning Process'.
Rock Products Cement Industry Operations
Seminar 1965.
15. Goldman, Kreft and Schutte 'Cyclic Phenomena of Sulphur in Cement Kilns' World Cement Technology, November 1981.
16. Hatano 'The Behaviour of Sulphur in Suspension Preheater Kiln Systems' Zement Kalk Gips, l/1972,
p. 18-19.
17. Etoc P 'The Cement Kiln - An efficient Trap for Sulphur' Ciment, Beton,
Platzer, Chaux (701) 210-213, 1976.
18. Ritzemann 'Recirculation Problems in Rotary Kiln Systems' ZKG(8), 338-343, 1971.
19. ‘-Lxperimental Prodquction
of a Minor HESC
at Plymstock Works, in order to
Investigate The Potential froblems Associated with The Production of a Similar Material at a New Oxford Works' Eng.RD TN 81/13.
20. A Lorimer 'On-line SO2
Monitoring:
Cell Systems at Northflee::
Assessmen: of RDUV and Electrochemical
May to October 1984'. Research Dept.
Em 84/21.
21. D W Haspel 'Cement Kiln Alkali/Sulphate Cycles under Equilibrium Conditions',
Research Division, Process Development Dept., Fh'/'84/93.
22. D G Stenson 'Factors affecting Sulphate and Alkali Cycles in Rotary Kilns and the implications of these Effects with Respect to Process Control' Research Dept. ETN/84/13.
TABLE 1
- EST TARGET CONDITIONS
co -
NOx level
BE02 level -
1
1,200
1.8
Nil
2
1,200
2.5
Nil
3
800
1.8
Nil
4
800
2.5
Nil
5
1,200
AS required
Positive
6
1,400
1.8
Nil
7
1,400
2.5
Nil
Test No.
TASL.E
No.
Test
2
-
TEST AVERGE
Average
Range of
NOx l e v e l
NOx values
CONDITIONS
Average
BE02
Range
of
level
-BE02 value-
1
1,244
573
t0
1,938
1.67
1.20 to 2.05
2
1,214
840
to 1,587
2.25
1.94 to 2.67
3
995
5 2 7 to 1,472
2.11
I .50 to 2.80
4
a68
543 to 1,330
2.47
I
.37 to 3.08
5
1,223
9 0 9 to 1,418
1.57
0.9 to 1.76
6
1,462
1,008 to 1,780
1.37
0 . 9 to 2 . 2 8
7
1,365
1,100 to 1,608
2.72
1.88 to 3.14
Ave CO
level
TABLE 3 -
Test No. I, Fluoride in
AtERAGE FLUORIDE LEVELS FOR EACH TFiST (LOSS FREE BASIS)
-1
2
3
4
5
6
7
0.109
0.109
0.109
0.109
0.155
0.186
0.185
0.105
0.105
0.13
0.11
0.16
0.16
0.16
0.126
0.128
0.154
0.142
0.179
0.235
0.218
1.16
1.17
1.41
1.30
1.15
1.26
1.18
0.963
0 -963
1.193
1.009
1.032
0.870
0.845
raw meal
Z fluoride in clinker
2 fluoride in Stage IV
Ratio o f fluoride in Stage IV to raw meal
Ratio o f fiuoride in clinker to raw meal
TABLE 5 -
Test
TEST AVE.RXE CHLORIDE LEVELS (LOSS FREE BASIS)
No.
1
2
2
4
5
6
7
1.
% Chloride input
0.060
0.060
0.063
0.064
0 -059
0.059
o.oc+
2.
X Chloride clinker
0.16
0.14
0.009
0.017
0.008
0.008
0.010
3.
% Chloride in Stage I V
1.295
1.405
1.702
1.419
1.346
1.519
1.627
4.
Ratio 3:l
21.6
23.4
27.0
22.2
22.8
25.7
25.4
TABLE 6 -
Test
EST AVER;IGE KpO LEVELS (LOSS FREE BASIS)
1
2
3
4
5
6
7
1.
% K20 in raw meal
0.65
0.622
0.623
0.636
0.651
0.646
0.678
2.
% K20 in c linker
0.565
0.551
0.64
0 -615
0 -465
0.53
0.56
3.
% K20 in Stage IV
2.505
2.618
2 -749
2.452
2.387
2.811
2 -976
4.
% K20 in SCage IV
1.752
1.851
2.089
1.781
1.585
2,028
2 -133
0.753
0.761
0.66
0.671
0 -802
0 -783
0.843
Ratio 3:1
3.85
4.21
4.41
3 -85 '5
3 -667
4.351
4.389
Ratio 5:l
1.158
1.223
1.059
1.054
1.232
1.212
1.243
Ratio 2:l
0.86
0 -89
1.03
0.97
0.68
0 -82
0.83
as chloride c % K20 in Srage IV 2. as sulphate
TAELE 7
Test
LEVELS (LOSS FREE BASIS)
1
2
3
4
5
6
7
0.218
0.20
0.202
0.202
0.233
0.231
0 -277
1.
% Na20
2.
2; Na20 in clinker
0.211
0.207
0.221
0.227
0.172
0.195
0.196
3.
% fia20 S t a g e I V
0.329
0.294
0.327
0.323
0 -330
0.471
0.499
4.
% Na20
IV
0.005
0.006
0.033
0.052
0.00
0.011
0.00
IV
0 -324
0.288
0 -294
0.271
0.330
0.460
OTFFJ
R a t i o o f 3:1
1.509
1.470
1.619
1.599
1.416
2.039
1.801
R a t i o o f 5:1
1.488
1 .44
1.455
1.342
1.416
1.99
1.801
R a t i o o f 2:1
0.97
1.03
1.09
1.12
0.74
0 -84
0.71
as 5.
in raw meal
- TEST AVERAGE NA20
i n Stage
chloride
% N a 2 0 i n Stage a s sulphate
TABE
Test
8
-
TEST AVEUGE
1 -
SO3 LEVELS (LOSS FREE BASIS)
2
3
4
5
6
7
1.419
1.473
1.668
1.696
1.
2, SO3 in raw meal
1.53
1.369
1.417
2.
% SO3 in rm (CaO)
0.696
0 -582
0.627
3.
% SO3 in clinker
1.507
1.417
1.449
1.485
0.838
1.400
1.425
4.
% SO3 in Stage IV
3.176
3.193
2.356
2.710
4.433
4.426
4.005
5.
% SO3 as CaS04 as
2.118
1.862
1.515
1.812
3 -326
3.166
2.644
R a t i o 4:l
2.076
2.142
1.663
1.910
3 .OlO
2 -653
2 -361
R a t i o 5:2
3.043
3.199
2.416
2.932
5.373
3 -856
3 -470
R a t i o 3:l
0 -985
1.035
1.023
1.047
0.569
0.84
0.84
in Stage IV
% a s
CaSO4
66.7
58 .3
64.3
66.9
75 .o
71.5
66 .o
X
RELATIONSHIP -BE;TwEEN NOx AND LX&T
HOPE WORKS No 2 KILN:
x
x
hourly ATerAg data from 3/5/85
x 0
daily August
I
r
730 1350 Burning Zone Temperature ( "cl
I
1400
(av.Worka pyru rending)
I
1450
AVel%gt
7985
dAtA
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634
PLY?WJ!OCK
WORKS.. VARIATION OF SDLPHATE CYCLE WITH BZT
I
2c.b so3
300
ex ntnge IV a8
of total SO, in feede
I 400 (meal
I 500
+ coal)
X?PEE;DIS
!
GUPHICAL REPRESESTATIOTS OF VARiATI04‘S STAGE IV MATERIAL CklEYISiKV
IK
0% A?i IYDIVIDOAL TEST EASIS
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APPENDIX 2
PLANT .?EASUREiHEATS
.9/?/37 3
3-27 --
l220
lGl3
..-
23-o
I-P?
~0,7 .-
lG7 20.3 .-
q I-?
Kc1
t
I so
-
3
-+a-
133
QO
536
33t
I BLC,
I
S-11 ---I822 I
I 2
3 4 s 6 ‘31
APPENDIX1 EOPE h7XK.S ALKALI CYW BuRNI%
ZONE c.kimIcAL
TADER 4UILIBRIUd
Material
%Feed
% B.E.
LOAD
COhDITIONS
% Prcduct 1982 clinker Average
Absolute % Recycled
Na20
O-2
0.4 (x2)
0.2
0.25
0.2% (xl)
K20
0.6
6.0 (x10)
0.6
0.55
5.4% (x9)
SO3
1.3
6.5 (x5)
1.3
1.3
5.2% (x4)
cao (caSO4 If
0.37
0.74 W)
0.37
0.37%
0.37% (xl
Cl
MJOI
2.0%
0.19
F
13.17% * Eetemined
fran sulphate balance.
Assuming the average heat requirement to vacourise - deccxqmse alkalis averages 800 kcals/kg (600 to 1000 kcals/Ig[l] 1 then average aBali vapourisation load on th,n burning zone is 100 kcah/kg.
[ll
Vapourisation only - No decmpos ition of alkali salts.
m 84/93
Over a period of years, various extimtes drive off the alkali/sulphates
of the heat required to
in the burning zone have been made. The
majority are in the order of 100 kcals/kg.
However ETIU 83/13 has sho+.n that
this can easily tax-y on a short term basis by i 50 kcals/lcg disturbance in-the Burning Zone Tarcjerature.
due to a minor
This note references scxne of
the various source& on this subject. 1.
"Heat Transfer in Rotary Kilns" Weber. P. ZIGS Spcial
Publication X0 9.
Alkali cycle equivalent to 100 Kcals/kg clinker. 2.
Lepol Grate Heat Transfer Mechanism D-W.
Haspel June, 1973
ikxaxA 005 At least 70 kcals/kg clinker of heat are recovered as a resu+lUor the condensation of alkalis/recrmbination 3.
Hope Works Alkali Cycles - See
of SO3 on the Lepl grate.
Appendix
I.
The mean heat required to vapourise the alkali/sul?hates at Eope is in the order of 100 kcals/kg.
A further 10 kcals/kg is involved in the
melting/fusion of the salts. The exothermic clinkering rcuctions cccuring in the burning zone are equivalent to -97 kcals/kg (typical 1982).
APPENDIX II HOPE 5XRKS CLINERING ~thermic
REACTIONS
Reactions:Heat of Form&ion
Hope % (1982 Av) Clinker Canposition
k&b/kg clinker
C3S
-126.2 kcal/kg
62.7
-79.1
C2S
-174 kcal/kg
9.2
-16.0
C3A
- 3.7 kcal/kg
10.4
- 0.38
C4M
-20.1 kcal/kg
7.8
- 1.57
Total
-97.05 km&/kg clinker
90.1%
Note heat required to prcduce C3S fran C2S is +2.8 kcals/kg reactant Endothermic Reactions:tlO0 kcals/kg
Alkali Recycling Latent heat of Z&ion
(circa 10% of vapourising load) + 10 kcals/kg
ll0 kcals/kg Net (Ekothermic/~doth~c~
+ 13 kcals/kg
Useful Heat in Gases and Clinker Dust Useful Heat in Gases (Sp Heat xA T) circa 0.17 x
T = 17 kcals/kg/lOO°C
Useful Heat in Clinker (SP Heat x A T) "
T=
0.30x
30 kcals/kg/lOO"C
APPENDIX Iv Z-IBM!
DISmmCE
Area to b= cledned.
?!SSCC3ATED
WItTYI KILB CLEYNFG
(HOPE)
8m (circumference) x 5m (length) = 4Om*
AssumS thickness circa l/4 - l/3 m Total rr&erial to be cleaned - circa 10m3 Assuming S.g of 0.8 - 1.0, Material hocked
down 8 to 10
totes,
which circa 20% - 30% are alkalis. With additional heat requirenent to m&e clinker 300 kcaI.s/kg This material is equivalent to 3 'Lonnes of clinker (distrilxlt& over two hours?) or5
tOMeS
of feed
D. W. Has@ November 1984
of
APPENDIX III PHYSICALC0NsmNE
ASSCCIATEDWITE! ALKALI CYCLES
Latent Heat of Vapxrisation/Eemqmsition Kcl
+670 kcaldkg
W)
K2s04
+563 kcals/kg
W)
+1805 kcals/kg (D) Na2SO4
t-733 kcals/kg W) +2096 kcals/kg (D)
CasOi
+675 kcaldkg
(D)
v = Vapxr D = Dissociate Later&He&of
Fusion
KC1
+96.6 kcals/kg (M-P.
K2SO4
+ 57.6 kcals/'Kg
Na2SO4
+ 42-S kcals/kg (M.P.
722°C)
(M-P. 1069°C) 884°C)
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 7
Design and Experience with Bypasses for Chloride, Sulphate, and Alkalis
DESIGB AND EXPERIENCE WITB BYPASSES FOR CBLORIDE, SULFATE .tvl, ALKALIS By Xessrs P.. Davis and P-A. Longman Blue Circle Industries, United Kingdom 1.
INTRODUCTION During the production of Portland cement clinker, some of the minor components $n the raw materials and fuel, notably the alkalis, sulphur and chloride compounds are volatilised
in the kiln. These com-
ponents are then swept with the kiln gases to the cooler parts of the kiln system uhere they condense.
In the less thermally efficient pro-
cesses, such as the wet and long dry process, a significant proportion of these volatiles can *escape' from the system either in the prccipitator/5ag house dust or up the stack.
In the Lepol or XL process,
which we have successfully operated for more than 20 years, fairly high levels of volatiles can also be tolerated. This is partly because the nodule grate is not a particularly effective scrubber and partly.because
precipitator and
intermediate cyclone dust can be
discarded thereby lowering the volatile recirculating load. Similarly, in two stage preheater processes moderate volatile inputa can be tolerated. However, in the more thermally efficient processes, such as the four stage suspension preheater process, greater amounts are "trapped" in the feed as It passes down the preheater and are carried back into the kiln, where a proportion again evaporates.
This repeated evapora-
tion and condensation leads to the gradual formation of an internal
volatile recirculating load until a steady state is reached(l).
-2When the first suspens'ioti.preheater
processes were:-built
1950's the importance of recirculating‘volatlles~was
In-tt.
m.t fully appr
ciated and- a- number bf plants simply could- not run.&ecausc.of build and blotikages at" thi Wla inlet and in. the--louer, stages: of the prcheater.
I,n some instances-the clinker produced uaa of unacceotable qualitv because af its hinher alkali content in comoarlson ulth that produced in the simpler, but leas thermally efficient processes. Over the yeears, as a result ef studies made by both the plant svooliers and the cement manufacturers (see references) an understand$ng has emerged of the factors affecting the formation of the_ volatile recirculatinn load in a four stage ausuension oreheater process, and of the effect such a recircnlatlon has on plant ~4 mance and cement quality. As a reaul.t, It is possible, when employing the four stage suspension preheater process for cement manufacture to predict the maxImum levels of chloride. alkalis and sdohate that can be tolerate< in the raw materials and fuel.
Where these levels are just exceeded
the situation can often be recovered, ad kiln operation and cement qiiaLitp,ptob1en!s.
avoided, by extr$cti-ng
'befbre. tA#! preheatar
a proportion of the kiln gases
such-.tI& a .sufficient
proportion of the unwanted
volatile! components- are "bled" qut the system flowever, operation of such a."bJeed" or Wla bypass,also depletes. the preheater of some of Its he,- uith,the.reault. that -on-a susoension preheater process the kiln bypass has to be llmited$to_about 25% otherwise there is insufficient heat for effective preheating(2).
-3Bith the introduction of precalciners in the mid 1970's it vas generally recognised that this limitation could now be overcome and that by burning a substantial proportion of the total fuel in the precalciner it would-be possible to bleed all of the kiln gases, if necessary, and still achieve sufficient decarbonation and calcinatioa
in the preheater(3r4,S)*
Moreover, whereas in a suspension preheater
process a fuel penalty of about 4 kcal kg'1 of clinker is incurred for each 1% bypass, in the precalciner process this is roughly half at around 2 kcal kg-l. At that time, therefore, the precalciner pt'ocesi looked particularly attractive in this respect and partly for this reason'gained videspread
adoption.
Since then operational'experience and results
have been obtained vhich have cast doubt on the effectiveness of the kiln bypass on this type of process. In Blue Circle Industries, in addition to operating a number of conventioaal
suspension preheater processes with no bypasses, UC have
experience in running one suspension preheater process fitted ulth a small bypass to remove cnlorlde and two precalciner processes fitted vith bypasses to remove sulphur and alkalis respectively. In this paper, we outline the chemical factors which determine. the volatile recirculating load in a four stage preheater process and the effects this can have on kiln operation and cement quality. Our experience in operating both four stage preheater and precalclner kilns with and vithout by$asses and how the bypass performance compares vith design predictions is-also discussed.
-4CFEMICZLL, 2.1
PACTORS
Volatile
Components
The two main factors which vi11 dictate uhether a kiln bppasz necessary, and if so hov large, are:-
i)
The total quantities of alkalis (sodiumGiAd potassium), color and eulphur present in the rav materials aad fuel which vi11 determine the overall composition of the clinker.
ii)..,The
volatility of these components in the kiln system which WI
determine-the nature of the volatile recirculating load. The main source of alkalis is the clay and shale phases in the secondary rau materials and in general the presence of al'alis in t1 primary calcarcous raw material or the fuel arises from coataf"~
-'ic
wtth argillaceous materials. Chloride is normally present as alkali chloride in the raw materials and fuel but the presence of organic chloride compounds in
coal or other fuels is possible. Sulphur can occur as a variety of organfc and inorganic compounti in both fuels and raw materials and in a variety of oxidation states. The volatility of these components in the kiln system Is complex and is dependent on the follouing interrelating factors:-
i>
Whether the volatile components occur in the fuel or the raw materials.
-
5
-
ii) The relative amounts of the total alkalis, sulphur and chloride entering the system. iii) The burnability of the mix, uhich can be influenced by the overall chemistry of the clinker., the fineness of the raw feea 'and whether mineralisers such as fluorine are present. iv) The burning environment in the kiln system, which vi11 be influenced by (iii) above, the level of free lime required, the nature of the fuel and firing system employed and whether carbon and/or sulphlde is present in the raw materials. 2.2
Volatile
Cycle
In practice, the volatile recirculating load in a kiln system vi.11 gradually build up to equilibrium and it is customary for researchers and workers in this field to estimate the final balance of volatlles recirculating by regarding the build up in cycles.
A number
of papers have been published on this subject (6-9) and this is an approach which we have been using for some 20 years, uith the assistance of computer programmes for the past 10 years. In our current computer model the first cycle is regarded as where fuel and feed first enter the kiln system.
In general the vola-
tiles In the fuel are assumed to evaporate completely as the fuel comSusts, while of the volatiles in the raw materials, only a proportion are assumed to evaporate - the remainder being retained in the clinker.
-6 Of the volatiles that have been evaporated a certain propor: (generally most) are assumed to condense on the feed in the prehe such that In the secood cycle the material entering the burning zt contains both recirculating volatile compounds and volatile compel present in the raw materials. For the second cycle a proportion of the volatlles in the raw materials will again evaporate in the burning zone and the rentaind stay in the clinker.
Similarly, a proportion of the recirculating
compounds will also be evaporated uhile the remainder are retained ihe clinker.
With each ensuing cycle, therefore, the quantity of
volatile compounds recirculating and the quantity retained in the clinker gradually builda up until the total quantity entering the system via the feed and fuel equals the total quantity leaving
c
system 'via the clinker and stack. A simple illustration of this is shoun in Figure 1. Rouever,
tsoastruct
such a balance it is necessary to have a knowledge of the
recirculating compounds being formed and the volatllities
of both
these compounds and those present in the raw materials.
Below, ue
.summarisf
the information we have acquired from both laboratory
investigations and studies of full scale plant of the behaviour of chlorine, sulphur and alkalis in suspension preheater processes. It was on these data that we based the design of our bypasses for our precalciner plants, and uhich in the light of our more recent experiences now require modifying.
-7
-
2.3 Chloride Virtually all the chloride input to a kiln system vi11 be volatilised at burning zone temperatures and in general less than 3% of that entering the burning zone will leave via the clinkerc2). During the process of evaporation and being swept down the kiln the chloride will preferentially react with potassfrrm
to form
potassium chloride and in general sodium chloride vi11 only form if there is an excess of chlorine over potassium. A very small percentage of this alkali chloride may escape up the stack as fume or fine particles of dust but the bulk will condense out on the feed. By itself, potassium chloride till largely condense out in the temperature range of 800 - 900°C and as a result it can often cause blockages and form hard deposits in the back end chute or in the riser pipe to the lower preheater stage.
If sodium chloride is also present
the temperature range over which the chlorides vi11 condense till be extended generally dounwards i.e. 700 - 900°C and in terms of kiln operation this may present fever problems.
A similar effect occurs in
the presence of sulphates. 2.4 Sulphur The behaoiour of sulphur in the kiln is more complex than that of chloride and is strongly influenced by the form of the sulphur and the burning conditions.
-aIn general, sulphides present in the raw materials wfll‘read~ oxidise and evqorate such.that most if not all are driven off by time the feed reaches the burning zone.
Calcium sulphate, whether
present in the raw materials or. fotmed in the kiln cycle, although not particularly'oolatil,e, readily dissociates at burning zone tur peratures particularly under:prevailing
reducing conditions, and y
only a small proportion of this sulphate- is retained in the clinke: Alkali sulphatea similarly dissociqte
id's reducing cvirooment.
However, in an oxidising enviromaent they are'nore stable with the result that up to 40%. of the total alLd.1 sdphates entering the burning zone map be retained In the clinker, It follows that the bulk of the sulphate retained in the clinks is alkali based - either as alkali sulphate - @i?la)2S04 or as, alkali calcium sulphates such as calcium langbeinite This is illustrated
It
2CaSOf,.K2suq-
19 Figure 2. . In contrast the sulphate evaporate
may be alkali sulphates, dissociated sulphates or oxidised sulphide. This mixture vi11 condense and/or react over a tide range of texperatures as it passes to the cooler parts of the kiln. SO2 gas vfl readily react uith calcium oxide in the decarbonated feed at the bat end of the kiln but any uhich is not trapped in this ny say well pa: through all of the preheater and escape up the stack(lO*ll). Most sulphate compounds will condense at temperatures of 900-1100°C.
In our expeience if the alkalis and sulphates are more
or less in balance this condensation forms a light loosely bound depa sit in the fourth stage and kiln inlet which can be scoured away by
-9passing feed or if necessary easily removed. Aowever, if there is an excess of sulphate over alkalis the deposit can be more sticky and harder to remove.
Densified layers of calcium sulphate and sulphosi-
licates 2(2CaO.S102)CaS04
(Figure 3) may form which vi11 adversely
affect kiln performance and. require special cleaning facilities(12r13).
If fluorfne is present In the rav materials the
situation may be exacerbated by it promoting the formation of intermediary compounds such as the fluorinated (CllA7.CaF2),
calcium dumim:e
spurrite 2(2. CaO .SiO2 )CaC03 and various calcium sulpho-
silicates. 2.5
Alkalis
In general a substantial proportion of the recirculating alkalis will be associated uith either the chloride or sulphate as discussed above. Houever, there ulll always be a tendency for some alkalis, par ticularlp Na20, to be retained in solid solution in the main clinker phases(14).
The proportion of alkalis retained in this uap in the
clinker will increase if there is an excess of alkalis over sulphate entering the system or if reducing conditions prevail in the burning zone.
Reductioa
will lead to dissociation and preferential loss of
SO2 although with severe reductioa, alkali iron sulphide compounds such as ?ZeS2 may form in the clinker which results in a greater retention of alkali and sulphur in the clinker than is otherwise expected. In general up to 50% of allalis entering the burning zone as alkalis oxides may be retained in the clinker.
- 10 If there is an excess of al.kali over sulphate uhere condensation occurs at the back end of the kiln hard alkali carbonate based deposits may form. PRACTICAL IWLICATIONS 3.1 Kiln Operatioa Published data, coupled vith our OM experience, suggests that the formation of blockages and hard'deposits in the fourth stage and kiln chute are likely to start presenting problems when the level of chloride or sulphate in the feed material leaving stage 4 exceeds 1.5% or 3.5% respectively.
In addition such levels of sulphate may also
enhance ring formation in the kiln itself thereby restricting output. The quantity and nature of the volatile recirculating load will obviously vary from Works to Works, but in general practical terms in order to avoid exceeding the above limits it will be necessary to restrict the total volatile input to the system, when &pressed as a percentage
of
the clinker output, to 0.025 - 0.030% chloride and
around 1.5% sulphate. Normally,
the alkalis vi11 be bound up ulth the chloride and
sulphate and the levels that can be tolerated in the material leaving the fourth stage till be dictated by these components.
If alkali is
predominantly present as alkali carbonate, however, we vould expect to be able to tolerate up to at least 3% alkalis in material leaving stage 4.
Thus in terms of kiln operation it may be possible to
- 11 -
tolerate a total alkali
iIIQUt
to the SysteZI expressed as a percentage
of the clinker output of 1.2% equivalent xa20 without running into any serious problems. '3.2 Cement Quality In terms of cement quality the obvious factor which may dictate limiting the quantity of alkalis retained in the,clinkar vi11 be the necessity to manufacture a low alkali cement in order to avoid expansive alkali aggregate reactions occurring in the concrete.
In this
respect the ASTM limit of'O.6% equivalent Xa20 is generally recognised worldtide.
Aowever,
even if the productioa of a low alkali cement is
not required, it may still be necessary to limit the level of al!calis retained in the clinker.
For example alkali sulphates vi11 shorten
setting times, enhance the early strength of the cement at the expense of its late strength, and will render the cement more prone to air setting during etorage.
These effects vi11 normally become 'noticeable
-Jhen around 1% alkali sulphate is present in the clinker and will be significant at the 2% level. If there is an excess of alkalis over sulphate then the alkalis will enter into solid solution in the main clinker phases and in general late strength will be depressed. In this situation If the clinker is also subjected to reducing conditions then the resultant cement will be prone to adsorbing moisture from the atmosphere and its flowability
properties uill noticeably deteriorate. This effect is
illustrated in Figure 4.
- 12 If there is an excess of sulphate over alkalis then the formation of calcium langbeinite
(2CaSQ.K2S04)
will enhance strengths at all
ages.
If high levels of sulphate are retained in the clinker this till limit the level of gypsum addition that can be made at the c&en& mill.
This in turn will lead to an increase in the milling energy
required to grind to a given surface area or cement strength. Although chloride is not usually retained in the clinker in appreciable
quantities, there is always a risk where the chloride
input is high that unstable kiln operating conditions will lead to periods of production where high levels are retained in the clinker. This could present problems if reinforced and/or prestressed concrete is prepared from this cement since the chloride may migrate through the concrete and lead to corrosion of the reinforcement. Iu the U.K. it is recommended that prestressed concrete contains less than 0.06% chloride when expressed as a percentage of the cement component.
In practice, after allowing for the contribution from the
aggregates and water, this means limiting the amount of chloride in the cement to less than 0.03%. 3.3. hyuass Design Should a kiln bypass be required to control the quantities of volatiles recirculating in the kiln or retained in the clinker, then this Is nornally located at the kiln inlet.
The objective is to bleed
off the kiln gases containing volatiles without removing excessive quantities of dust.
- 13 -
Some plant suppliers have found the best location for such a take off is from the riser immediately above the kiln and along the line of its axis. Houever, promising results have also been obtaihed with bypasses located on the side of the riser and fitted with a deflector which keeps the hot gases from the kiln apart from the incoming feed. The hot gases bled from the system ulll, of course, need to be quenched to freeze the volatilised material, and then cooled and filtered.
As close to the take-off point as possible, the..gas should
either be quenched tith cold air to a temperature of 250°C and dedusted in a glass bag filter, or air quenched to around 4OO"C, water cooled to 150°C in a conditioning tower and dedusted in an electrostatic precipitator.
Variable speed fans are preferred for the quench
air and filter fans to give a reduced electrical power consumptioa should it be found possible to operate at less than the design bypass percentage.
Tfius the capital kost of the bypass and its ancillary
equipment, whilst only a small proportion of the total cost of the plant, is nevertheless significant. In sizing the bypass, the procedure we have adopted is to construct a similar volatile recirculating load to that illustrated in Figure 1 but with a proportion of the volatiles now bled from the system.
This is illustrated for the suspension preheater and pre-
calciner process in Figures 5 and 6 .respectlvely.
For these calcula-
tions it is assumed that the bypass is 100% efficient i.e. a 50% bypass vill bleed off 50% of all of the volatiles approaching it in the gas stream.
By running our computer programme for different
- 14 bypass levels it is then possible to plot the stage 4 and- clinker volatile contents against X bypass and from these plots read off the level of. bleed required. 3.4
Bypass
Operation
In addition to the capital cost, a bypass facility vi11 also incur running and maintenance coats.
As mentioned previously there
vi11 be a fuel penalty and this is illustrated for both the suspension preheater akd the precalciner processes in Figure 7.
Bypass dust will
need to be handled and disposed of and inevitably there vi11 be a depletion of rav material reserves. At this stage, therefore, it is uorth considering some of the alternative
eolutions.
In terms of kiln operation a slight excess of
volatiles above the limits discussed in section 3.1 can often be dealt with by installing poking facilities to remove hard deposits in the riser pipe and in the lower stages of the preheater.
-An imbalance
between sulphate and alkalis can be restored by the deliberate addition of an appropriate source of volatile compound to the feed.
Uh
chloride inputs can often be avoided, for example, by purchasing a special lov chloride coal which although generally more expensive than the normal fuel employed we have found that in some cases its use is cheaper overall than operating a bypass. In tens of cement quality modifications to the setting and strength characteristics of the cement caused by the presence of alkali compounds can be countered by adjusting the main chemical parameters.
?or example it say be possible to offset the effects of
alkali sulphate by raising the silica ratio.
- 15 If
a
Works is only required to manufacture low alkali cement for
some of the time then It is conceivable that alternative raw materials sufficiently low in alkalis could be acquired for this purpose or it may be more economic to use selective quarrying. For example, Blue Circle Industries are presently converting an old Lepol process plant to a precalciner.
The level of sulphur in the kiln feed uas such as
to require a sulphur bypass in the updated process.
Aowever,
detailed
geological investigation showed that a large proportion of the sulphur was contained vithin a well defined, approximately lm thick, band in the shale quarry.
In terms of both capital and operating cost it was
shown that it would be-more economic to discard this high sulphur material by selective quarrying:
therefore, no bypass has been
installed. In some instances rather than operating a bypass to remove volatiles it may be more convenient to make further additions to the feed in order to produce a fully mineralised propertiee.
clinker with enhanced cement
This is a technology which ue have developed and suc-
cessfully applied in Blue Circle Industries (15-17). 4.
PRACTICAL EWERIENCE 4.1 Suspension Preheater Plants without Bypass Facilities Blue Circle Industries currently operate a number of suspension preheater plants without bypass facilities. Samples of kiln feed, fuel and clinker are regularly analysed on the Works and on several occasions samples have been taken from the stages of the preheaters ia order to construct a volatile balance.
For the purposes of this paper
-de have selected four plants to illustrate the range of our experience.
- 16 The total volatile input at these plants derived from both the raw materials and fuels and expressed on a clinker basis is as follows:-
Plant
lA
1B
2
3
4A
4B
Fuel
Oil
coal
coal
coal
Gas
Oil
Total S as SO3
,0.90
0.95
1.70
0.45
0.1s
1.05
K20
1.30
1.30
0.65
0.35
0.60
0.60
NaZO
0.20
0.15
0.20
0.10
0.50
0.50
Cl
0.02
0,ozs
0.02
0.04
0.00s
0.00s
Further details of these plants, the typical levels of volatiles found in the stage 4 feed and the typical equivalent ??a20 content of the clinker are shown in Figure 8. Plant 1 was originally oil fired but in 1981 it was converted to coal firing.
By necessity a low chloride coal is used 1:; order to
avoid having to install and operate a chloride bypass.
With both
fuels the alkali and sulphate input is high but more or less in balance uith the result that although build ups do form in the riser and fourth stage they are relatively easy to remove. The resultant clinker, however, has a high equivalent Na20 coutent, the bulk of which is present as alkali sulphate.
Although the
productioo of a low alkali cement is aot necessary to counter the adverse effect the alkali sulphates have on the late strength of the cement the Works have traditionally incorporated some sand in their mix to lover the total alkali input and raise the silica ratio.
Hore
recently, mineralisers have been successfully used at this Works to
- 17 achieve both a further Improvement in late strengths and a reduction in the level of alkali retained in the clinker. Plant 2 is also fired uith a specially purchased low chloride coal in order to avoid a high chloride recirculating load. The raw materials "are relatively low in alkalis with the result that a low alkali clinket,uith an equivalent Na20 of less than 0.60% can be easily produced.
Unfortunately, these rau materials also con-
tain fluorine and appreciable quantities of sulphur and uhile both these components can be limited by selective quarrylng their presence leads to the formatioa of hard back end deposits uhich require regular removal.
!4'hile such removals do create a certain amount of kiln
instability this is not excessive.
Overall
this
plant
demonstrates
that vith comprehensive cleaning facilities high volatile recir culating loads can be tolerated, and the formation of hard back end deposits can be successfully controlled, vlthout having to resort to the operation of a kiln bypass. A further consequence of the presence of fluorine, hovever, is that the early activity of the oement is depressed with the result that, in viatet in particular, setting times are extended and any placed concrete has a tendency to "bleed". Various solutions to this problem have been tried but have either been impractical or uneconomic.
More recently, however, this problem has been overcome by the
use of mineralisers. Plant 3 is fired ulth a locally available coal.
As a consequence
it3 chloride Input is high and would normally require the operation of
- 18 a small chloride bypass.
Houever, the sulphate and alkali inputs are
both relatively low, and more or less in balance, vith the result that by operating facilities to regularly clear away deposit? which form in the riser and stage 4 the need to bleed any of the kilns' gases has been avoided.
The resultant cement has a very low alkali coatent well
belov 0.60% equivalent Xa20. Plant 4 contains bath an oil fired kiln and a gas fired kiln. 30th kilns have a lov chloride input but a high alkali input which results in the equivalent Xa20 in the clinker exceeding 0.60%. However, as extended cements are mainly produced at this Works this does not present any problem. . In term of kiln operation, the gas fired kiln has an excess
of alkalis over sulphate uhile the oil fired
kiln has an excess of sulphate over alkalis. Both kilns form deposits in the riser and fourth stage which are easily removed. Construction of the volatile recirculating load for each of the above 'kilns has indicated that the proportion of volatiles evaporated in the burning zone of each is similar.
In all cases (Figure 9) vlr
tually all the chloride, approximately 85% of any excess sulphate over alkalis, around 65% of alkali sdphate and about 60% of excess alkalis over sulphate was evaporated. 4.2 Chloride Bypass - Plant 5 In addltioo to the above suspension preheater processes, a further one is operated in the Blue Circle group *with to remove chloride.
a small bypass
Up to 1975 this plant uas oil fired, thereafter
it has been fired with natural gas.
- 19 The raw materials employed on this 53orks comprise a high grade limestone, a marl and a sand.
Both the limestone and marl contain
chloride with the result that the total chloride input to the kiln system, as a percentage of the clinker output, is around 0.10 - 0.15%. In designing the bypass for this plant it uas assumed that both retention of chloride in the clinker and loss up the stack uould be negligible.
As a consequence it was predicted (Figure 10) that a kiln
bypass of some 8 - 10% would be required to lower the concentration of chloride in the stage 4 feed to an acceptable level.
In practice,
however, up to 0.01% chloride can be retained in the clinker vhile a somewhat smaller amount leaks up the stack uith the result that the actual bypass operated is in the range 6-8X. Volatile and mass balances have been constructed for this plant with both fuels.
i>
These have indicated that:-
the proportions of volatlles evaporated in the buhling zoue are similar to the other suspension preheater plants operated in the Blue Circle Group
ii) the bypass is also effective at bleeding sulphur and al!calis from the system iii) changing from oil to natural gas did not affect the performance of the bypass although some change in clinker chemistry and
cement quality and grindability as a result of the lover sulphur input was noted. 4.3
Alkali Sypass - Plant 6 This is a 2500 tonnes per day oil fired precalciner process uhich
- 20 -
came on stream in December 1981.
The tav materials consist of a high
grade limestone, a siliceous limestone, a clay and iron oxide. The total volatile input, as expressed as a percentage of the clinker output, i s : -
% Total S as SO3
0.75
K20
0.75
Nat0 Cl
.0.3s 0.01
Use of these raw materials produces a mix with a relatively high silica ratio of around 3.5 and a difficult combinability. Consequently it was assumed when designing this plant in the late 1970's that the degree of evaporatioo which would be achieved in the burning zone uould be at least the same as that found in our suspensiou preheater processes. Against this background and assuming losses up the stack would be negligible it was predicted that the bypass required to produce a lov alkali cement would be 30% (Figure 11). At the time all the main plant suppliers agreed with thi's prediction. However,
since the plant has been in operation difficulties have
been encountered in producing a low al'kali cement. Close monitoring of the process, during a series of trials carried out In conjunction with the plant supplier, has established that while the bypass is currently limited to only about 25% the main reason for this failure
- 21 -
to produce a low al'kali ceaent is that the degree of evaporation a'chleved in the burning tone is lower than *was expected.
Further
investigation has indicated that the main reason for this is the very much higher solid to gas ratio achieved ia. a precalciner'kiln
which
restricts the sueeping action of the gase&*). On this basis (Figure 11) it is estimated that the bypass would have to be increased to around 60% in order to produce a suitably low al!=11 clinker. .AI.temative
solutfoas have been considered.. Attempts to increase
the volatility in the burning zone by raising the lime saturation and silica ratio of the feed, or by burning more fuel in the rotary kiln rather than in the precalciner were only partially successful. Adding chloride to the system to enhance the volatility of the alkalis or selective quarrying of the raw materials to lower the alkali input have been considered but are expensive. At present the Works have found the least expensive solution is to buy In an alternative low alkali clay and produce low al'kali cement on a campaign basis pending modification to the bypass. 4.4 Sulphur Bypass - Plant 7 This is a 1200 tonnes per day coal fired precalciner process which was first lit up in September 1982. The raw materials consist of a high grade and a low grade limestone.
Both these and the fuel
employed contain appreciable quantities of sulphur *with
the result
that there Is a gross excess of total sulphate over al:kalis entering
- 22 the system.
The total volatile input, as expressed as a percentage of
clinker output is:I Total S as SO3
2.30
K2O
1.05
Na20
0.65
Cl
0.01
These raw materials produce a mix uith a relatively lou silica ratio and an easy combinability and it was anticipated that volatility in the burning zone could be low.
In addition, from the form of the
sulphur in the rau materials it vas recognised during the design stage that this might be volatillsed in the upper stages of the preheater and escape up the stack,
Comercial decfsioas dictated that the plant
had to be ordered before the rav materials could be comprehensively tested and consequently when sizing the bypass we decided to take the possible conservative view that only a limited quantity of SO2 may escape up the stack and that the volatility of the excess sulphate over alkalis entering the burning zone as calcium sulphate could well be dissociated to the same degree in a precalciner process as in the suspension preheater process.
Accordingly, ue estimated (Figure 12)
that the bypass would have to be at least 30X and that cleaning facilities similar to those employed at our plant 2 may also be required. At the same time our calculations showed that while the clinker sulphate content would be sufficiently lowered (Figure 13) to pemit an acceptable
gypsum addition, the allcall content of the clinker
- 23 (Figure 14) uould remain high.
Fortunately, the production of a low
al'kali cement is not required at this Works'and, although the alkalis are mainly present as alkali sulphatcs, their effect on the cement strength is also not critical (indeed it is beneficial) as the majority of the cement produced at this Works is extended with an addition of natural pozzolan.
However, the presence of alkali sulpha-
tes uas expected to lead to air setting problems necessitating appropriate counter measures. In general the plant supplier agreed ulth these conclusions. Their calculations shoved that while a lover bypass would probably suffice, a 25% bypass should be installed to cater for all eventualities and therefore they raised no objection to our proposal to install a 30% bypass. Ho-vet, uhen the plant was fully commissioned in November 1982 it became clear that the Win could be successfully operated wlthout having to bleed any of the kiln's gases. Since at that time the fuel cousumption guaranteed by the plant supplier still needed to be checked opportunity was taken while performing this exe&se to take a range of balance samples at different bypass settings.
The results of
this investigation are shovn by the dotted lines on Figures 12-140 The main points to emerge from this evaluation were:i>
around 50% of the total sulphide present in the rav materials evaporated in the upper stages of the preheater and escaped up the stack
MS
- 24 ii) the proportions of volatiles evaporated in the burning zone were substantially lower than in any of our other plants. As a consequence, acceptable lev.$ls of total sulphate in both the stage 4 material and the clinker are obtained without having to resort to bleeding any of the kiln's gases* Hovever, the level of alkali sulphates in the clinker Is high and as expected the Works have been
forced to counter airsetting problems. Follouing
these observations we have developed a laboratory test
method for assessing the potential for sulphur to burn off over the temperature range likely to be encountered in the upper stages of a preheater. 'By subjecting raw materials to this test irom a number of sites ue have shown that this cannot be readily predicted from the chemistry alone but is dependent on a number of factors including the form the sulphur occurs in the raw naterials, the temperature at which the calcareous component in the feed starts decarbonating and the atmosphere in the preheater, which may itself be influenced by the composition of the feed. DISCUSSION XND CONCLUSIONS 1.
In modern, high thermal efficiency dry process kiln systems, continuity of kiln operatioa and clinker cement quality are more susceptible to adverse effects of volatile components in the kiln feed and fuel.
Wet, long dry and Lepol or ACL process plants are
not so sensitive. 2.
In some circumstances it has been found that such effects can be more economically countered by selective quarrying, or by the use
- 25 of alternative raw materials or fuel, or by adjusting or mineraUsing the chemistry of the clinker, or by adopting intensive and automated techniques for clearing build ups from the riser pipe, rather than by resorting to a bypass. 3.
The adoption of precalciner processes has made it practicable to employ bypasses of up to 100% of the rotary kiln exhaust gas, compared tith a practicable limit of about 25% vith a simple suspension preheater system.
4.
However, with a precalciner process only about 40% of the fuel is burned in the rotary kiln itself and recent experience shovs that the volatilisation
of alkalis and sulphates in the kiln are
lower than in normal preheater kilns. This reduces the effectiveness of the bypass where the objective is to limit the al'kali or the sulphate content of the, clinkers. 5.
While siting a bypass to remove chloride is relatively easy, that required to remove alkalis and sulphur Is more complicated and is dependent on the interaction of these compounds and their volatility in both the preheater and the rotary kiln and is therefore more difficult to size.
6.
-Although
plant data provides a number of useful reference points
from which future predictions can be based, there is also a need for the properties of the raw materials and fuel to be e!UplOyed
to be thoroughly evaluated in the laboratory.
- 26 -
For our part in Blue Circle we have both a comprehensive modern laboratory which can carry out such investigations and coaslderable background experience and data from the operatioa of plants. 6.
ACKXOWLEDGEHENTS The authors wish to thank Blue Circle Industries PLC for their permission to publish this paper and to colleagues and Associate cornpanics vithin the Blue Circle Group for their contributions.
7.
REFERENCES 1.
Reactions associated vith the kiln gases: Volatile cycles, buildups, ring removal. F.W. Lecher, S. Spmng C D. Opitz. Zement-Kalk-Gips,
2.
1972, 25, (l), l-12.
Bypass systems for preheater and flash calciner kilnsN.U. Biege h L.J, Parsons .
'
Pit and Quarry, 1978, 2 (L), 91-97. 3.
Second generation precalclnlng
with bypass alternatives for
alkali control. F.I. Kohanowski h J.L. Shy. Proceedings of the 13th International Cement Seminar, Chicago, 1977, 98-108. 4.
Eeductioo of alkali and sulfur content of clinker by kiln bypass in flash calciner systei. J. Tjarshawsky
& E.S. Porter
In Process technology of cement sanufacturing, VDZ Kongress L977, 652-659.
- 27 5.
New cement plant in Abu-Dhabi uith precalciner 'kiln process with 100% kiln gas bypass system. N. Xakamura
& 2. Tojo.
Clments, BiGtons, Platres, Cbaux, 1982, (7381, 268-274. 6.
Redustioo of alkali and chlor%e cycles in the suspensioa preheater
kiln.
3. Schliiter
Zement-Kalk-Gips, 7.
1972, 25, (l), 20-22
Raw meal preheater and alkali problans. B. RItzmann. Proceedings of the Eighth International Cement Industry Seminar, Chicago, 1972, 39-47.
a.
Cyclic behaviour of volatile components in dry process plants for burning cement clinker. W. Danowski h U. Strobel. Silikattechnik, 1977, 28, (2), 40-43.
9.
Hethod for predicting cyclic behaviour of deterious substances in cement
'kilns.
W. Kreft. Zement-Kalk-Gips, 10.
1982, 35, (9), 456-459.
The behaviour of sulphur in cement clinker burning. S. Sprung. Tonindustrle-Zeitung,
1965, 89, (5/6),
124-130.
- 23 11.
The behaviour of sulphur in the suspension preheater kiln. H. Eatano. Zement-Kallc-Cips, 1972, 2, (l), 18-19.
12.
Investigatioos
of the formation of rings in rotary cement kilns.
3.93. Sylla, Zement-Kalk-Gips, 13.
1974, 27, (LO), 499-508.
Xing Formation in Rotary Cement Kilns. D. Opitz. Schriftenreche der Zementundustrie
14.
No. 41, 1974.
The distribution of al'kalts in Portland cenent clinker. H.W.U. ?ollitt h A.W. Broun. Proceedings of the Fifth Internatioaal
Symposium on the Chemistry
of Cement, Tokyo, 1968, Vol.1, Part I, 322-333. 15.
U.K. Patent No. 1498057.
16.
Use of mineralisers
to produce high strength cement.
G.R. Long. Proceedings of the Fifth International Conference on cement rnicroscapy. 17.
Nashville, 1983, 86-98.
Improvements in the early properties of Portland cement. G.K. Yoir. Proceedings of the Royal Society discussion meeting, London, 1983, 127-138.
- 29 -
3.8.
Processing of 'kiln dust B. Tettmar, S.R. Khor and S. Gregory Process Technology of Cement Manufacture WZ Koogress 1977, 658-663.
FIG 2. Calcium Langbeinite (arrowed blue) with associated alkali sulphate (arroued red) in production clinker.
FT.G 3. Laths of calcium sulphosilicate (arrowed yellow) within dense cal clum sulphate rich deposit from sintering zone in kiln.
FIG 4. Dark chain like droplets (arroued white) of atmospheric moisture occurring on the surface of a polished section of a clinker manufactured in a reducing environment.
W
,*i-b - !4T.i =i !I G !-a. ‘1, cr, +L ‘2 :..: iy LLiL
c
3
u . ,y ‘t
z
.Ti -
1
-
:i 27 a
ail .._.... c
.; & x.-
I-
I.
L
i--.-l.-.: ,.! ., . . . . . _If II-_. . .L. ~ I l-l. . . . . . . . . . .._.__..
!Z -
-....... . . . . . . . . .
‘I-‘ z.- :ri &.,_
Figure 7-f&l Consumption vs % Bypass I_1100 73 s 3
* 1000 iii 2
/ /
.Ig
/
;/
/Suspension
P 51 g00
0 0 0
8 -43 A.2 3 .g 8 0 0
Precalciner Kiln Air separate) High Dust Loss - - - Low Dust Loss
% cd i-L 10
20
30
4 0 50 60 % Bypass
70
80
90
100
000000 LnLnOu3LnLo a3a3cnm-cn 4 - 4
Figure Kiln Volatilities of E3CI Suspension Preheater Process (No Bypass) -$-Typical -.. 100 90
Chloride Sulphate (other than Alkali Sulphate)
80 n
70
QS-
Alkali SMphate
6s 3,
A‘lkali S (other than Alkali Sulphate co .
Llr
(u kI
20 10 0
Fiqure IO-Chloride Level 4 Stage 4 vs % Bypass in PLd 5 11 10 9 8 7 Predicted Level
6
- A - - - Actual Level
5 4 3 ommende Bypass
2 1
4
6 ‘10 E&pass
0
10
12
Figurell, -E%quivaleht NazO in Clinker (Ye) vs ‘lo Bypass in Plant 6.
---I-
Predicted levels for suspensionprahealer kiln volati(ities Revised prediction using Bypass trial data
Actual levels from Bypass trial mendat ion
40 50 .60 z!!LBBy~
80
Predicted lev& for different kiln volatibties w - - L --- Actual iwels from Bypass trial
&commended
1
0
5
I
10
I
15
1
20
I
,
30 25 -‘lo Bypass -
,
1
1
1
J
35
40
45
50
55
FJgure14--E4uivaient
NgzO in Clinker(%) vs % Bypass in Plant 7
2.0 f-#+zdicted IQVQ~ for different kiln volatilitibs _c-m-- “Actual IQVQIS. from Bypass trial.
1.8 I.6 -
ecommended Bypass
0.6 o-4 o-2 0’ 0
I 5
I 10
I 15
I 20
I 25
1 30
B ‘lo y p a s s
I 35
1 40
I 45
1 50
1 55
I I I 1 I
:
- ’
I. _’L.. .F
In ,C i
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 8
Kiln Gas Bleed Considerations
KILN GAS SLEED CONSIDERATION!fj
The process under consideration wiil need a chloride bleed in order that regular kiln operation can be maintained. With the present raw materials, the required bleed level will be 30 to 50% of kiln gases. At this bleed level, clinker equivalent soda levels will be very high, at 0.9 to 0.95%, but could be reduced by 0.1% by use of a bleed of up to 75%. Other options for clinker alkali reduction are the use of alternative raw materials, which is also likely to reduce the bleed requirement for kiln operation, or addition of CaCl, which will have a major effect on alkali level but will require a larger gas bleed. Dust loss with the bleed is likely to be 200 to 250 gnns/Nm’ of gas extracted from the system but design should assume a level of 400 grms/Nma.
2.
INTRODUJCTION Since the development of the suspension preheater based dry process for cement manufacture, the study of the inherent cycling effects of the potentially volatile components - which are present as minor constituents of the raw materials and fuels - have become increasingly Important. In the older, less thermally efficient processeS, a natural loss of a portion of these volatile components occurred in the waste gases, so automatically controlling the recirculation levels within the burning system and the level in the clinker product. In the suspension preheater, with its greatly increased surface contact between gas and particle and repeated separation of gas and particle, the recovery and retention of volatilised components will be almost complete. This leads to Increased proportions of these components in clinker and within the kiln system; which can lead to the development of operational problems in the pyre-processing stage due to the quantities of potentially Where sticky components that build-up through cyclic processes. necessary, a portion of the kiln gas is bled off in order to remove a portion of the volatile components and so control the levels of these components either in clinker because of cement quality requirements or in the kiln system because of the potential to cause blockages within the preheater. The minor components that are generally considered to be involved in major volatile cycles are the fluoride, chloride, alkali and sulphur species - although other elements do also become involved in cycles to a much lesser degree (V, As, Pb, TL, Cd, Hg, Zn), these are not important for this study. 1
Prime control of the volatile component cycles is performed in the design stage of a works project through the selection of raw materials and fuels in order to optlmfse the relative and absolute levels of the potentially volatile components and, where necessary, the inclusion of a bleed system. Once these factors have been defined, the voiatiles will develop internal and external cycles during the pyre-processing stages; an internal cycle being totally within the kiln and preheater system, whilst an external cycle will leave the system but be returned after a time lag (for instance with the precipitator dust). Where processing conditions are kept steady, the cycles will continue to develop until equilibria are reached, at which time the total amounts of volatile entering the system will be balanced by the quantities leaving the system. The degree of volatilisation and the ratez established will depend on:
at which the equilibrium are
(a)
The species, their chemical forms and concentrations.
b)
The volume of gases.
(cl
The intimacy of contact between gas and solid.
(4
The vapour pressures of the salts.
(e)
Possibility of dissociation or further reaction.
(0
Rate of diffusion to and from solid/gas interfaces.
kc)
Degree of saturation of gas.
(h)
Kiln atmosphere.
(i)
Kiln
(j)
Time/temperature profile of material within the kiln.
temperatures.
Most of these Factors are to some degree inter-related and so in normal operation the only methods available to control the degree of volatilisation and eventual concentration in clinker and kiln system will be the kiln internal atmosphere and temperature, and the proportion of gas bleed from the kiln exit. The major cycles will be internal cycles between the burning zone and preheater where an individual cycle time of twenty to thirty minutes would be expected. Smaller quantities of volatile components may be involved in the longer term external cycle which develops via the precipitator dusts and volatile colIection in the raw mill.. These would be expected to have much longer cycle times of up to 24 hours. Any kiln gas bleed from a suspension preheater system obviously has a 2
fuel penalty associated with it. For a standard preheater, it is generally considered that a 30% bleed approximately represents the maximum economically justifiable level, whilst in a precalciner the equivalent kiln exit gas volume can be between 30 and 45% of that of a simple suspension preheater which makes bleeds of up to 100% justifiable under certain circumstances. Because of this factor, control of the cycles for improved process operation is easier on a precalciner than on a suspension preheater. Conversely control of the proportions of the volatiles in clinker is more difficult due to the reduced volatilisation that occurs in precalciner kilns - a consequence of factors (b) (c) (f) (i) and (j).
3.
GENERAL FUZVIEW OF PROPERTIES OF VOLATILE COMKNENTS
3.1
General
Comments
In practice, the proportion of each potentially volatile compound which evaporates within the kiln can vary significantly depending on the species and the type of process. Typical ranges that have been reported in the literature are set out in Table 1, whilst Table 2 details melting and boiling point data for the major components. In the past, empirical limits have been proposed for the total concentrations of volatile input to a kiln system but modern practice is to specify the concentrations that can be - tolerated in the lower stages of a preheater. The maximum reported ranges that are generally accepted at this point as being unlikely to cause any operating problems are: chloride so, alkalies
1.0 to 1.5% 2.5 to 4.5% 2.5 to 3.5%
It is possible to operate successfully with significantly higher levels of individual components, however, the overall effect on kiln operation would depend on the relative proportions of individual compounds and the effort put to cleaning the preheater interior. The individual components are discussed in the following sub-sections. 3.2
Fluorides Fluorides can be found naturally in raw materials or be deliberately added in small quantities to the raw mix. In low proportions its mineralising action has a beneficial effect on the burning process. Generally, it has low volatility and causeS few operational problems, although a level of above 0.25% in clinker may lead to setting problems, particularly in winter. Cases have been cited, however, where a hard dense build-up has developed in preheaters where the build-up contains a high (over 1%) proportion of fluoride.
3.3
Chlorldq Chlorides are derived from the raw materials and the kiln fuel. The high volatilities of these compounds, together with the high collection efficiency of the cyclone preheater systems, will lead to the development of a greatly enhanced cycle. The chlorides have a high affinity for the alkalies in general and potassium in particular. This property, together with the high volatility, has been used in kilns (commonly on the wet process, occasionally on the SP process) to control clinker K,O levels by addition of CaCl, to the raw mix or fuel, which leads to loss of KC1 with the kiln bleed or the exhaust gas from the kiln system. In the suspensfon preheater, the volatilised material is recaptured within the system unless a bleed is utilised between the kiln and riser duct. It is generally considered that no more than 3% of the chloride passing from the preheater to the kiln will leave the system with the clinker. Al*though considerably higher levels have been noted in individual samples of clinker, this is probably due to a ‘push’ of kiln feed, or a semi-flush situation as thermodynamic considerations indicate that no chlorideshould pass through the burning zone. On many SP kiln systems some degree of preheater cleaning is necessary on a regular basis and this may help to control the chloride cycle by forcing the kiln conditions into a situation which permits a brief increase in clinker chloride level (i.e. reduced material temperature, increased material loading and flux level). Small amounts of chlorides will also leave the preheater system with the waste gas stream. Taking the total loss of chloride from the system as between 2 and 5% of the feed to the burning zone, it would then be expected that a circulating load of 20 to 50 times the total chloride input could develop in a system without a kiln gas bleed. No reports of low temperature chloride volatilisation within the preheater
have been identified. 3.4
Alkalies The major source of alkalies will be the raw mix; notably the clay component, although minor quantities can arise from the fuels. The initial free alkalies will behave in one of three ways:
(1)
Remain in the material being processed and become incorporated in the clinker constituents that are being formed. This happens to Na,O to a greater degree than K,O.
(2)
Be converted into different compounds - chlorides, sulphates, carbonates, hydroxides - by reaction with other constituents of the raw mix.
(3)
Diffuse to the surface of the process material and volatilise.
4
In its initial state, K,O begins to volatilise over a wide range of temperature, depending on the form of clay In which It was incorporated but irrespective of source, it would be expected to have volatilised almost completely at burning zone temperatures, although some may have been at least partially stabilised by conversion to the less volatile sulphate form within the material bed. Once volatilised it will react to form chlorides and suiphates - chlorides preferentially - at the rear of the kiln. These will then deposit on dust particles. Initially Na,O is less volatile than K,O due to its higher bond energy and so a greater proportion of the kiln feed Na,O would be expected to pass through the burning zone in clinker without participating in the volatile cycles. Volatflised Na,O will react with SO, and SO, to form sulphates towards the rear of the kiln and with chloride where this species is present in excess of K,O. Where alkali is present in excess of chloride and sulphate, alkali carbonates will be formed. Each of these alkali compounds will deposit in a liquid state on the surface of dust particles in the cooler zone of the kiln and lower preheater stages and will enter the volatile cycles as the dust is separated out in the cyclones. Direct contact onto kiln or preheater surfaces may lead to the development of build-up. The compounds will then re-enter the kiln where the degree of volatllfsation will depend on the species and the kiln conditions. Volatility decreases from chloride to carbonate to sulphate and, hence, sulphates are more likely to pass through the burning zone. Nevertheless, the likely range of burning zone temperatures cover the thermal area in which alkali voiatilities are likely to increase significantly with rising temperature. In general, precalciner kilns have significantly lower burning zone temperature than are common in other processes and, hence, alkali sulphate volatilisation in particular is lower in precalciners than in other processes. 3.5
SulDhur Sulphur can enter the system in a number of forms from either fuels or raw materials. A limited amount may evaporate in the upper preheater stages and escape from the system in the exhaust gases. In general, SO1 and SO, can form in the high temperature areas and be transferred to the gas phase. In the cooler areas of the kiln back-end and preheater system, sulphates will form and re-enter the material stream. Preferentially alkali sulphates will be produced with excess sulphate combining with free lime or calcium carbonate and hydroxide that Is available in these areas. Where reducing conditions exist, the equilibrium will favour the existence In this of SO, and SO,, so restricting the formation of sulphates. situation loss of suiphur oxides by way of the stack may increase but where the gas stream passes through the raw mill, the mafority of the sulphur oxides would be expected to react with the high active surface calcium compounds which are produced in the milling process. This will then return to the kiln in the raw mix as part of an external cycle. The high boIIing points of the alkali sulphates would Indicate that relatively low levels of volatilisation would be expected. However, dissocfation may occur, particularly under the reducing conditions which 5
can exist to some degree within the burning zone. Calcium sulphate also has a high boiling point but is even more susceptible to dissociation, so a higher recirculation of sulphate from this compound would be expected. As CaSO, cannot recycle as a compound, the lime from this compound remains in clinker as free lime - making burning more difficult - whilst SOS is carried in the gas stream to the kiln back-end where it reacts to form alkali or calcium sulphate.
4.
BLEED REOUIREMENT The proposed new line is based on a precalciner process. The raw mix contains such high levels of chloride and alkali that the system would be inoperable without a kiln gas bleed. In addition, the alkali level in the clinker could potentially restrict the markets open to this material. A series of raw mix designs have been considered and associated with this was a series of calculated material analyses based on the i3CI volatilisation model, showing the effect of gas bleed levels between zero to 100%. The results of these calculations are presented agafn in Table 3. ,Mixes 6 and 8 represent the proposed range of clinker chemistry and from the material analyses these would have a chloride content in the raw meal of approximately 0.22%. Figure 1 plots the effect of kiln gas bypass on the chloride level in the material passing from preheater to kiln and indicates that a bypass requirement of 33% is anticipated in order to restrict the chloride level at this point to 1%. Figure 2 indicates the effect of increased chloride content in the raw meal, with chloride levels of 0.25% and 0.30% requiring 38% and 48% bypasses respectively. The levels of chloride bleed would also control the levels of alkali and sulphate in the kiln inlet material to acceptable levels for kiln operation. With the level of bleed necessary for the control of chloride in order to maintain satisfactory process operation and with the proposed raw mix, alkali levels in clinker are likely to be about 0.95% Na, equivalent, as indicated in Figure 3. Such an alkali level is likely to be acceptable in many areas but may not be acceptable in parts of the world where ASR is a significant consideration. Lower clinker alkali levels can be achieved in one of three ways:
(1)
Increased bypass level. Increasing the bypass level to 75% would be expected to reduced the clinker equivalent soda level to between 0.8 and 0.85% depending on the mfx (Figure 3). In this case the majority of the reduction would be in the K,O concentration due to its higher volatility.
(2)
Use of alternative raw materials. The use of an alternative raw material to clay could significantly reduce the input of alkalies and chloride to the process. As an example, mix 11 in Table 3 considers the effect of using 2% of a European bauxite in the mix. 6
This would reduce the cIay usage by about WY% and increase the sandstone requirement but the overall effect on the volatile components is to reduce the concentrations by between 25 and 40%. The immediate result of this is to reduce the kiln gas bleed requirement for chloride control from 33% to 22%. At this lower bleed level a clinker equivalent soda level of about 0.7% would be anticipated, whilst high bleed levels would permit further reduction in a similar manner to case 1 (e.g. 50% bleed 0.6% equivalent soda - the exact effects would obviously depend on the full chemical analysis of the material used). (3)
Addition of CaCl when lower alkali clinker is required. This could be used to make a separate quality clinker as needed. The chloride would preferentially react with alkali and thus increase the volati$ty of this species. However, this route would require that the bleed system has the potential for operation at higher levels possibly up to 100% - during such periods of manufacture.
OTHER EFFECTS OF KILN GAS BLEEDS As the kiln gas bleed removes gas and material at high temperature, this obviously has a significant heat penalty on the process. A first order estimate of the fuel penalty can be obtained from Fig. 4. The dust that is extracted from the process has a high volatile content and so cannot be re-used within the kiln system. Consequently any dust drawn out with the gas bleed will effectively increase the raw meal to clinker factor and so require increased capacity in all the stone processing and meal preparation stages through to the kiln feed point. Recently the major manufacturers have all been making significant efforts to reduce the amount of dust extracted for a given gas bleed percenta e. Currently, Q most will design for a dust loss of 150 to 250 grms/Nm of gas bleed, whilst users are commonly reporting figures of 150 to 400 gnns/Nm’ with extremes of up to 600 grms/Nm’. The effect of this range on dust loss from the system and consequent extra raw meal requirement for the two proposed plant sizes is shown in Table 4. It is assumed that in normal operation the dust loss will be approximately 200 to 250 grrns/Nma but the design parameters for raw meal preparation should allow for up to 400 grms/Nm’.
AD JL/JAS
7
TABLE 1: VOLATWY FLANGES (a) Primary Volatilitks
01) Specific Volatflkies
%
‘56
SO,
60-90
KS4
40-60
K,O
30-70
NaS4
40-60
Na,O
20-40
2CaSO,.Ks,
40-100
Cl, l-b
96-99
K,O In solid solution
60-90
F
IO-40
Nai in solid solution
20-40
I
!
i
Pb, Tl I
Cd, V, Zn
SO-99
KC1
I O-20
NaCI
96-99
CASO,
80-100
I
97-99
Primary volatilities indicate reported range of volatilities irrespective of full chemical form. Specific volatilities indicate the reported range of volatllities more common compounds.
for the
TABtE 2 : MELTING 6 l3OLlNG POWI- DATA T Melt&g Point Boiling Point D&XX&l OC Oc OC SoDiUM Chloride
800
Sulphate
884
over 1200
1440 - 1465
Carbonate
850 - 854
dissociates
Hydroxide
319 - 328
1390 I
fOTA9XJ-M Chloride
700 - 768
1407 - 1411
Sulphate
1075
1689
Zarbonate
894 - 897
bIy&oxide
320
Calcium Sulphate
1297
over 1200
disszlates
1320
1350
bJ.9r 1113 1.93
LO1
14.49 3.34 2.23 42.w 3.73
13.94 3.49 2.X 4.92 3.35
0.13 0. $1
0.14 0.5.3 0.56 0.227
22: 93.0 2.6 1
::: 43.21 35.05 0.09 0.33 cz9 96.0 2.4 1.5
w.0 0
0 0 0.49 0.7a 0.81 0.331
20 2 0.51 0.80 0.33 0.138
54 5 0.57 0.82 0.86 0.W
0.41
0.42
0.81 15.16 1.27 11.036
0.66 2.4a 1.16 1.91
m 10
0.59 0.87 0.90 0.368
0 0 0.9 O.Bc 0.36 0.34
0.43
0.45
0.39
0.40
0.47 1.33 1.W 0.67
0.43 0.87
O.Br 6.00 1.34 1.63
::2
z
22.2 5.12 3.42 65.60
1.23 1.38
m
0.41
0.42
0.Y
0.49 1.43 1.10 0.71
0.44 0.9 O.% 0.387
3.71 3.4 3. w 7.64
10 0.60
20
Y, 5
2 0.4s
0.Y
0.41 0.234
65 13 8 11
E
0.63
0.54
0.54
0.57
0-n
0.33
0.37 0.57 0.67 0.29
1.67 0.86 1.034
96.0
23.6
0.47 0.9
0:67 O.ZS!
f-43 L3l j-61 L 076 Lrn 2.4
co 10
0.241
21.63 3 .
21.39 s. 35 3.56
0.25 0.59 0.U 0.75 33.0 2.6 _ 1.5
z o:a7
0 0 0.43 0.51 0.60 0.229
9 5 0.55 0.39 0.91 0.367
0.26
54 19 8 10
14.05
1-S 23.8
% 13 8 I2
8.: 0147
2
3
_--. -7 TABLE 4 : WI%CT OF DLJZED LJZVEL AND DUST LOADING Plant output Bleed % I
Dust Load grm/Nm3 .
c
=
5500 tpd Actual Dust Los
I
-
7500 tpd
Raw Meal Equivalent
Actual Dust Loss
Raw Men1 Iquivolent
Av tpd
Max tpd
Av tpd
Max tpd
Av tpd
Max tpd
Av W
Max tpd
30
100 200 300 400 500 600
78.4 156.8 235.1 313.5 391.9 470.3
82.5 165 247.5 330 412.5 495
117.4 234.8 352.2 469.7 587.1 704.5
123.6 247.2 370.8 494.4 618 741.6
106.9 213.8 320.6 427.5 534.4 641.3
112.5 225 337.5 450 562.5 675
160.1 320.2 480.3 640.4 800.6 960.7
168.5 337.1 505.G 674.2 842.7 1011.2
50
100 200 300 400 500 600
130.6 261.3 391.9 522.5 653.1 783.8
137.5 275 412.5 550 687.5 825
195.7 391.4 587.1 782.8 978.5 1174.2
206 412 618 824 1030 1236
178.1 356.3 534.4 712.5 890.6 1068.8
187.5 375 562.5 750 937.5 1125
266.9 533.7 800.6 1067.4 1334.3 1601.1
280.9 561.8 842.7 1123.6 1404.5 1685.4
70
100 200 300 400 500 600
182.9 365.8 548.6 731.5 914.4 1097.3
192.5 385 577.5 770 962.5 1155
274 547.9 821.9 1095.9 587. I 1643.8
288.4 576.8 865.2 1153.6 618 1730.3
249.4 498.8 748.1 997.5 1246.9 1496.3
262.5 525 787.5 1050 1312.5 1575
373.6 747.2 1120.8 1494.4 1868 2241.6 J
-
393.3 786.5 1179.8 1573 1966.3 2359.6 -__-_
Three potential raw mixes were selected for the bypm calculation (on the basis of kiln operation and quality).
(1)
For the situation where a bypass was in operation, the fuel consumption wa5 assumed to rise by PKcallKg per 1% of kiln gzs bleed from the system and 1% dust was assumed to be lost by 10% bypass in operation (precalciner process). The fuel consurnptions and dust losses on clinker basis were estimated as follows:
(2)
The estimated clinker analysis has been calculated for the situation where a bleed of 0, 10, 20 50% and 100% of kfln gasa fs in operation. When a bleed is In operation, the levels of volatiles leaving the system in the clinker was calculated on the basis of the following assurnptiom
0)
All the recirculating voIatiIes are potentially available to leave the system via a bleed.
(ii)
The following volatillsation NaCl KC1 Kfl.4 KG04
(3)
99 99 40 40
rata were assumed:
ExcessK,O E x c e s sNa,O Ca!Xl, tCasOq.K&04
35 35 40 40
The total volatile input is the sum of the volatlles from the raw feed and from the fuel expraed on a clinker basis.
WV, aa rr$ OA 1 .a 1.. 0 .:. . :. . .._’ . .. .:
__’
cy
H k
;
. . .. ._’
. . . . . .. . . . .. . . . . . . . . .. . . . .. . . .. .
0 04
0 -
to
e-4
-
to d
+x 6
I’
I / i i i
’
I
,
, I I
I
I
I i
I
, 4. .I’
.I’ ;I1
I
’ :
i
I
i
, I
!
I ! i
i
I
I
i
:
1 1
I , ’ I i : ; * !
i
i L
I
j
/ ; j / i . i ;ii i i i i ; i i i
1 r
i
t
i .III
t I
I
I
I
I
I
I
I
, ! ; I :! t i i I; i-i 1;. ::
I
:
, 1: ’ 11-i a
t i i ‘I i i i !I;
0 co
0 -3
Fig. rt.
E S T I M A T E D H E A T C O N S U M P T I O N v s *lo G A S B L E E D O F F AT THE KILN BACK END
HEAT CONSLJt.4P1ON ()I cd/kg NETT) 950
925
-
9 0 0
- - -
;i;.
r 875
--
.“__..
,‘ib l/
850
-.. -- -- ..-
---
--_--.-- -.-. --
..--. --
-.--
DUST LOSS
/=’ .0 /’
;./ 64
.
SUSPENSION PREHEATF~~ K I L N (100% F U E L IN)ECTION I N T O T H E Kl
r
825
-
.--.e
..--
-. - P R E C A L C I N E R K I L N --._ (LO’/. FUEL INJECTION INrO THE
.fl
/ - .’
IUST
*. 8 0 0 liiiclc 0 10
To
30 ‘1.
LO G A S BLEED
50 OFF
A T ItIE
LOSS EX KILN
*I. O N
CLINHtmH
I
1
I
I
GO
70
no
90
KILN
DACK EN0
KILN]
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 9
Ring Formations in Cement Kilns
Ring formations in cement kilns Greg Palmer Plant
Superintendant,
Queensland
Cement
Ltd,
Introduction Ring formations in cement rotary kilns have been known to induce operational instability and restrict gas flow to the point where production must be stopped. Rings that can cause severe operational problems in long wet kilns have been found in the following areas:
Australia process instability are caused by mechanisms 2.3 and 4.
0 electrostatic attraction
Freeze-thaw mechanisms The freeze/thaw mechanism is used to advantage in the burning zone of cement rotary kilns, in this zone coating protects the refractory lining from aggressive chemical attack and reduced the thermal energy loss from the kiln. In fact, in the burning zone, coating will build-up to a steady state level of some 20 to 30cm in thickness. This is achieved as the rotary action of a cement kiln is an ideal environment where temperature fluctations and tumbling feed will expose a cooler surface to freeze any liquid. The freeze/thawing mechanism is totally dependent on the composition present and the temperature in the vicinity of the solid. If the temperature increases or the composition changes, for example by chemical reaction, then it is possible that the mineral system may reach the eutectic point or congruent melting point at a different position along the kiln axis. The composition of the mineral system provides valuable information as historical interpretation of the concentration levels can be linked to various types of rings. At a fixed temperature, the mineral concentration determines whether or not the eutectic or peritectic points will be reached. In an industrial environment, it is not possible to quantify the exact mineral system that may exist at any one moment in a dynamic- kiln environment. Mainly because the concentrations of fluxing and mineralising elements such as alkalies, sulphates and chlorides are changing due to volatilisation or condensation. Thus, as the concentrations vary, the position of liquid formation will also vary along the kiln axis. An important source of alkali, sulphate and chloride is from the electrofilter dust being returned to the kiln. This dust can contain concentration levels many times that of kiln feed which, and if not monitored, can cause rapid concentration buildup.
The source of material can also be categorised as there is a number of streams available, as shown in Figure 1, that can effect the composition. Thus, it can be seen that ring formation is a dynamic process with a number of streams affecting the composition at any point and a number of different mechanisms which can bind the material together. This makes any investigation extremely difficult, as it is not possible to hold all but one parameter constant so that its effect can be evaluated. To overcome this problem investigations rely upon an empirical approach, past history and industrial observations. In rotary kilns most ring formations that cause
The role of alkali salts and alkali aluminates Many investigations into the role of alkalies, sulphate and chlorides in ring formations by freeze thawing have been undertaken and are well documented(4). (5). (6). (71. Choi and Glassern discuss the role of sulphur in clinker production by synthesising, calcium langbeinite (CaL), calcium a l u m i n o s u l p h a t e (&A&) a n d c a l c i u m silicosulphat’e (C&,S) from chemical reagents. The presence of sulphur has both beneficial and adverse affects: it improves burnability by acting as a fluxing agent. Choi and Glasse@) found that at low temperatures, below l,lOO’C, the evaporation process is
Transition zone Chain area Kiln discbarge area To prevent rings forming it is necessary to understand the mechanisms that assist in formation. There are a number of mechanisms, detailed by Rumpfc2), that can lead to ring formations, however, there are many factors. that contribute or aid a mechanism. Due to this complexity it is not possible to study each factor in isolation. To overcome this problem investigations rely upon an empirical approach, past history and industrial observations. Using a combination of both XRF chemical analysis and petrographic analysis, it is possible to determine the mechanism most likely to cause a build-up. This paper discusses transition zone and kiln discharge rings that have caused operational problems over the last few years. The investigation found that the rings were formed by a combination of combustion thermodynamics and freeze/thaw mechanisms. There was no evidence of any alkali sulphate or sulphate spurrite induced rings. Mechanism of ring formation The main mechanisms for rotary cement kiln rings were categorised by Rumpfm into the following groups: 0 melting or softening of the surface due to friction 0 melting or freezing from the loss or input of heat 0 interlocking of particles that have been built-up by particles held together by surface forces 0 interlockirg of fine needle like particles
WORLD CEMENT DECEMBER
1590
dominated by alkali sulphates. However, this is in direct contradiction to Holderbank investigations where the low temperature evaporation process is dominated by alkali chlorides. However, the latter process is more in line with what is observed in an industrial process. Alkali sulphates and chlorides have possibly the lowest eutectic point in the clinkering system: K2S04-Na2S04 binary system has a eutectic point of less than 900%. The source of alkali and sulphates is either from volatilisation from the feed entering the burning zone or a sulphur rich fuel. As previously mentioned, from 1 ,lOO% upwards, the volatilisation process is dominated by alkali sulphates and chlorides. Since the only mechanism of ‘bleeding’ the system in a long wet kiln of alkali sulphate or chlorides is by dust loss or substition in the clinker phases, it is easy to see how concentration can quickly reach critical levels. As the alkali salts are transported, via the gas stream towards the feed end of the kiln the dew point temperature will be reached allowing condensation of alkali sulphates. In the zone where alkali sulphates can remain liquid its low surface tension will ensure that it wets the clinker crystals present. The identification of alkali sulphates can be done both chemically and microscopically. If sulphate is a major mechanism, then it would be expected that the percentage SOS, Na,O or K20 found in a rin,g would be several times the amount found in clinker. Also, if a sample is prepared for microscopical observation, then alkali sulphates can be high-lighted by potassium hydroxide etching, which makes the identification
reasonably straightforward. The ferrite and aluminate phases, C4AF and &A, and alkali aluminates must be considered in ring formation as they are a significant component in the clinker matrix, accounting for around 17-20 per cent of the total composition. With an alumina modulus of 1.38 in a pure &A-C4AF mix, the eutectic point will be reached at 1338OC. However, substitution of Na+l and K+l ions can occur in the ferrite and aluminate phases, but this does not necessarily mean that the eutectic point will decrease. In fact Bogue(16j reports for the ternary system K20-CaO-A1203 the eutectic point is above 1300%. The KA-CdAF-C2F system was similar showing a eutectic point, again above 1300°C. A study of the different systems in which Na+l could be susbtituted into &A or C4AF similarly showed that the eutectic point was about the same level as if K-7 w a s s u b s t i t u t e d . S o e v e n t h o u g h t h e alumina and the ferrite components will determine the potential liquid phase, it is the presence of sulphate and chlorides which will determine the eutectic point or congruent melting point at the lower temperature range in a cement kiln. Spurrite formation Spurrite formation should also be considered at this stage as it has long been associated with ring formations. Spurrite or sulphate spurrite has been recognized as a contributor in ring formation by the intergrowth or matting of the distinct prism shaped crystals, The formation of spurrite Ca; (SiO& C03, has been extensively studied;(l). (lo). (l’) and recently Bolio-Arced and Glasser@b investigated the role of mineralisers H20, F a n d C l i n t h e
Table I The Chemical analysis of the different rings
Kiln No
6
Location (m) Sample Date
6
6
6
5
21m
Om
30m
Ring 2611189
Ring 717189
Nose Ring 1 o/5/89
3Om-36m Clinker 2116l89
Ring 27111185
Na,O
0.7
0.70
1.5
0.7
0.53
K20
0.11
0.14
0.5
0.11
0.10
so3
0.53
0.55
2.60
0.57
0.77
p205
0.06
0.11
0.10
0.12
0.12
LOI
0.32
0.71
3.22
0.56
0.32
SR
2.67
2.63
2.29
3.52
2.56
AR
1.65
0.86
1.21
1.46
1.41
LSF
94.4
83.3
% Liquid (Lea) 1338°C AR> 1.38
21.3%
98.4
r.
95.7
81.2
18.1%
26.4%
19.1
27.1
% Liquid (Lea) 1338=X AR < 1.38 14cvc
24.3
I+;
1::
Fly Ash
Flaw Meal
Coal Ash lnsutflated
oust
Alng Clinker Oust
e Combustion Gas VolatIlizeed Elements
Figure 7. The source of material can also be catagorised as there is a number of streams available that can effect the composition.
formation of spurrite. A number of reaction mechanisms have been proposed for the formation of spurrite with the reaction frequently beginning with either C&S or C$S. It is generally accepted that spurrite is not - stable above 900%. Thus, if the alite mechanism is responsible for spurrite formation, then the alite must be transported to the cooler region in the gas stream. For alite crystals transported this way, it is only a matter of time before the C3S will react with CO2 or SO3 to form spurrite. Bolio-Arceo a n d Glassero) also studied the mechanism of spurrite f o r m a t i o n f r o m C2S with CaF2 a n d CaClz as mineralisers in the reaction species. It was found that at least 0.2% CaC12 or CaFl was required. The role of spurrite in ring formation can easily be misunderstood, particularly if CaFz is available, for with these compounds present, a liquid phase would be produced, raising the question, ‘Which comes first - the interlocking of spurrite crystals
or the adhesion by freeze/thawing? Investigations conducted at Holderbank suggest that ring formation is a combination of solidification binding the spurrite crystals which further assists with crystals growth. Regardless if spurrite is suspected of being the binding mechanism, then microscopic examination will easily identify these crystals by their long prismatic, needle-like shape. However, it must be pointed out that investigations(‘l) have shown that. the concentration of spurrite can vary radially through a ring and, at or near the hot surface the concentration of spurrite can be very small. It would therefore be recommended that a radical cross-section of a ring be sampled when examining for spurrite. Ash rings Ash rings are worth mentioning briefly as they have been historically associated with ring formations. In coal fired kilns ‘ash’ rings have been reported to form in the transition zone and cooler inlet areas. These rings are formed as liquified ash, from the flame, settles on the surface of clinker crystals c r e a t i n g a r e a s w i t h h i g h SiO;, a n d l o w CaO concentrations. In these areas only belite can be formed. Holderbank has reported that the chemical composition of these rings is similar to ordinary clinker. Microscopically, these rings are very porous with both ash and clinker particles aggerating to form particles in the range of 100pm to 350pm. Cooling by rotation of the kiln or from colder secondary air will ensure solidification of the liquid phase. The presence of belite streaks caused by the welding together of the crystals by ash drops is commonly associated with these rings.
SF7 Clinker.
c.5 CIinker
Figure 2. SiO,-Al,O,-Fe,O,
Ternary diagram.
industrial observations and interpretations
Rings forming in Kiln 6 have, on occasions, caused severe operating problems to the point where it has been necessary to stop production. The rings were forming in the upper transition zone and at the kiln discharge (nose ring). It was initially felt that rings forming in the transition zone were sulphate rings caused by excessive SOs being returned in the electrofilter dust. While the ‘nose ring’ formations were caused by dusting, as the clinker cooled by secondary air at approximately 500°C, it would transform belite from the beta to gamma type. The dilatation of the belite crystal as it undergoes this transformation will cause disintegration of the crystal. However, the gamma belite transformation is uncommon as a high LSF will cause a slight excess of lime to remain in belite which acts as stabiliser. Extensive investigations “‘sing both XRF analysis and petrographic examination, were carried out on the rings and the following discussion shows that a completely different mechanism was responsible. The chemical analysis of the different rings is shown in Table 1, while Figure 3 and Figure 10 form part of the petrographic examination.
Figure 3. K6 Ring 3Q36m.
Transition zone rings
The chemical analysis of the K6 ring at the 30-36m mark on 26 January, 1989, shows no abnormal levels of alakali, SO3 or Cl, in fact the concentrations are similar to normal clinker. The silica and alumina ratio are 2.63 and 0.86, respectively. While the silica modulus is normal for ordinary clinker the alumina modulus is very low. Plotting the alumina and silica modulii on the ternary diagram, Figure 2, shows that the composition has a tendency to form rings; however, this tendency is only slightly more than normal SR clinker. While SR clinker is known to form a thicker coating in the burning zone, it could not be said that unwanted ring formation was associated with this type of clinker production. The percentage liquid phase present in the sample, as determined by Lea(ln* is low and calculated at 12.4 per cent. Even though the sample has a low percentage liquid the high iron content means that the matrix viscosity will be low and effectively wet any surrounding particles. Petrographic examination is shown in Figures 3 and 4; two different sections of the ring at different magnifications. In this section the belite has been etched blue, alite is etched brown and the closed pores can be seen as black areas. The very dense nature of the ring is evident by the close packing structure of the different phases and a very low open porosity. Figure 4 shows that the matrix is ferrite rich with the ferrite phase being identified by its unetched light grey appearance. There is no sign of any alkali sulphate and as the ferrite phase has a low viscosity it is the most likely binding agent in this case. The freeze/thawing of the kiln feed by the tumbling motion is further enhanced in this area by the increases or decreases in the heat
Figure 4. K6 Ring 3036m.
Figure 5. K6 Ring 27m.
flux profile due to combustion thermodynamics. Mathematical modelling of the heat flux profile along the axial length of the kiln shows an area from 20m to 35m where, given the right chemical composition, the eutectic point could easily be reached as a result of a changing temperature profile. The heat flux profile is that of a long lazy flame with the flame temperature dropping from 1460°C, at 20m to 129OOC at 26m then increasing again-to 13OOOC at 29m. Thus, the most likely mechanism for ring formation is an increasing flame temperature combined with a higher percentage of liquid ferrite phase. The liquid phase would be solidified by the tumbling kiln charge’.
Figure 6. K6 Ring
Figure 7. K5
21m.
Ring 30m.
Figure 8. K5 Ring 30m.
The chemical composition of the K6 ring at the 21m mark on 7 July, 1989, has a silica and alumina ratio of 2.29 and 1.21 respectively. Plotting these modulii on the ternary diagram, figure 2, shows that its composition has the greatest tendency to form a ring. The percentage liquid phase, approximately 23.4 per cent, is higher than the ring formed at the 30-36m mark, but only marginally higher than normal clinker at 1,338OC. The alkali content, KzO(0.5%), NazO(1.5%) and S03(2.60%) are higher than typically found in clinker. liowever, even at these concentrations it would be unlikely that alkali sulphates could be the binding mechanism. Petrographic examination shows that the ring is
very dense with almost no porosity. Also, as would be expected for material in this section there was almost no alite with belite being the major phase present. The belite crystals varied in size from 10 to 50 pm; the large crystals would suggest that a temperature around 1500°C was reached. The belite structure was internally disorganised, with what appears to be alkali sulphate on or near the surface. Figure 5 shows a section of the ring with a band of idiomorphic periclase, round greyish crystals, which is indicative of the ring being in contact with a dolomite lining. Associated with the periclase is a large amount of free lime. The matrix is predominately ferrite with a small amount of coarse aluminate. Figure 6 is a higher magnification view of the sample further away from the periclase band. The shape and internal disorganisation of the belite make it impossible to interpret; however, it is again most likely that the binding phase is the ferrite rich matrix. The heat flux profile reduces significantly from 20m to 25m, thus any small changes in the axial position of the feed would expose the feed to large changes in the surface temperature. As mentioned earlier the flame temperature changes from 1460°C at 20m to 1290°C at 26m, but it must be remembered that these predictations are based on a mathematical model and in the real environment the combustion thermodynamics could shift the heat flux profile in any direction along the kiln axis. Again, it would appear that a varying heat flux profile in the region of 20m to 27m is inducing ring formation by a freeze/thaw mechanism of a ferrite rich matrix. The chemical analysis of the K5 ring gives an alumina and silica modulii of 1.41 and 2.56, respectively. Plotting these values on the ternary diagram, Figure 2, shows that the ring formation tendency is not all that dissimilar to ordinary clinker. Petrographic examination of the ring, as shown in Figures 7 and 8, reveal a very dense structure with some closed pores (black areas) and even less open pores (mat grey in colour). The most evident feature in Figure 7 is the nesting of belite, blue etched crystals, which could be attributed to an inhomogeneous feed. Figure 8 shows a higher magnified view of the ring. The alite crystals, etched brown, have a maximum size of approximately 50pm but many small alite crystals are also present. The matrix is predominately ferrite rich, particulariy in areas of belite clusters. Some aluminate can be seen as a slightly darker mat grey colour. Similarly, the mathematical modelling of the heat flux profile in this kiln is very similar to Kiln 6 with an increase in the flame temperature around the 30m mark. Thus, it would appear that the same mechanism is responsible for the rings. That is varying flame temperature assisting a high liquid content which is being freezed by a tumbling kiln ‘charge’. Nose ringsample The sample labelled ‘nose ring’, 10 May 89, has a chemical composition which is very similar to ordinary clinker. The percentage sulphate is the only minor constitutent that could be considered slightly high. The silica and alumina modulii are 3.52 and 1.46, respectively and plotting these
proposed mechanism in this case would be a ver) hot burning zone combined with a hot cooling zone so that as the clinker leaves the kiln, any clinker dust re-entering the kiln will adhere to a mobile matrix phase and solidify on contact or as it comes into contact with coler secondary air.
Figure 9. K6 ‘Nose’ Ring.
Conclusions It is concluded that ring formation, in Kiln 6, is caused by freeze/thawing brought on by a combination of combustion thermodynamics and a low alumina ratio slurry. Mathematical modelling of the kiln heat flux has shown that the flame appears to be long and lazy with an increase in the flame temperature around the 30m mark. The chemical composition has also shown that the alumina modulus is consistently low allowing for a greater percentage of liquid phase to be present at low temperatures and that it is very near the eutectic point of the &A-C4AF system. It is also possible that an inhomogeneous feed may further exacerbate the problem. There was no evidence to suggest that alkali sulphates or chlorides formed part of the binding mechanism. References
figure 10. K6 ‘Nose’ Ring.
values on the ternary diagram, Figure 2, shows that the ring composition has the least tendency of all the samples to form a ring. Petrographic examination of this sample, is perhaps the most interesting, and typical views are shown in Figures 9 and 10. The sample has a very open porous framework of crystals cemented together by an interstitial matrix of fine aluminate and ferrite phases, as shown in Figure 9. The major phase present is an idiomorphic alite with a size ranging from 240pm to 100pm. The larger aiite shows signs of decomposition with cracking and secondary belite formation, see Figure 10. The belite crystals on the other hand appear mainly as clusters with crystal size ranging from 10 to 50pm. The etching of belite shows areas of different reactivity which is believed to be caused by alkali sulphate on or near the crystal surface(15,. Both the size of the alite and belite crystals would suggest that a sintering temperature of at least 1500% was reached. In addition, the alite decomposition would suggest that the cooling rate, between the burning zone and the kiln discharge, was too slow. Thus, if the kiln cooling zone was relatively long and hot then hot clinker with a mobile matrix will crystallize as it meets the cooler secondary air as it leaves the kiln. The fact that solidification take:s ulace as the material discharges from the kiln is evident by the rapid cooling of the matrix. Petrographic examination of the matrix shows it is composed of evenly distributed fine ferrite and aluminate crystals. The WORLD CEMENT DECEMBER 1990
(1) OPITZ, D - Ring and Coating Formation in Cement Kilns S c h r i f f e n r e i h e d e r Z e m e n t i n d u s t r i e , H:41/1974. V e r e i n d Deutscher Zementwerks EV Dusseldorf. (2) RUMPF. H - Prooerties. Bindino. Mechanisms and Strenoth of Agglomerates Aufberei?ungs-Tec’knik. No 3. 1970. ” (3) LONG, G R - Deleterious Raw Materials and Their Effects on Clinker Burnability. Proceedings of the fifth fnternationaf Conference on Cement Microscopy March 14.17,1983. Nashville Tennessee USA. (4) SPRUNG, S - influence of Process Technology on Cement Properties. Translation ZKG No 10 ~577 1986. (5) CHOI GANG-SOON and GLASSER. F P -The Sulphur Cycle in Cement Kilns: Vapour Pressures and Solid-Phase Stability of the Suiphur Phases. Cement and Concrete Research V78 ~367 1988. (6) SYLLA, H M - Untersuchungen zur Bildung von Ansatzringer in Zementdrehofen. Zemenf-Kalk-Gips No 10 ~499 1974. (7) STRUNG, J. KNOFEL. 0. DREIZLER. S, DREIZLER. I, and BERGISCHGLADBACK. Influence of Alkalies and Sulphur on the Properties of Cement Parts I. II and Ill. Zemenl-Kafk-Gips No 5 ~130 1985. (8) MACGREGOR MILLER. F - Dustv Clinker and Grindabilitv Problems. Rock Products ~152 April i98O. (9) BOLIO-ARCEO. H and GLASSER. F P - Formation of Spurrite Cae(SiO&COJ. Cemenf and Concrete Research V20(2) p301 1990. (10) SYLtA, H M - Investigations on the Formation of Rings in Rotary Cement Kilns. Zement-Kalk-Gips (10) p499 1974. (11) BECKER. F and SCHRAMLI. W - Build-up of Rings Caused by Spurrite Formation. Cement and lime Manufacrure V42(9) p91 1969. (12) HOFMANNER, F - Microstructure of Portland Cement C l i n k e r . H o l d e r b a n k Managemenf a n d C o n s u l t i n g L t d . Holderbank Switzerland 1973. (13) IMLACH. J A - Determination of the Cause of Ring Formation in Kiln No 5. Darra. Holderbank Management and Consultina Ltd. Reoort No MA8613355iE. (14) IMLACH. J A 1 Analysis of Cause of Ring formation in Kiln 6 at 98-121 Feet from Kiln Outlet. Holderbank Management and Consultino Ltd. Reoort No MA8913613lE. (15) IMLACH: J A and MISTELI. 8 - Texture Evaluation of Ring and Slab Samples Taken from Kilns 5 and 6 at Darra. #Holderbank M a n a g e m e n t a n d C o n s u l t i n g Lfd. Report No MA89l758lE. (16) BOGUE. R H - The Chemistry of Portland Cement. Reihhold Publishing Corp. 1965. (17) LEA, F M - The Chemistry of Cement and Concrete 3rd edition. Edward Arnold (Publishiers) Ltd. 1983.
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 10
Kiln Build-Up Meeting
KILN BUILD-UP MEETING
5 May 1998 - Fairfield House, Stanhope Present:
P.Greeno J. Collinson D.Belemy A.Bainbridge K.Atkinson
M.Mutter L.Evans A.Edwards
J.Wilson S.Dodd
A meeting was organised to review the causes of kiln build-ups at Weardale, Cookstown and Dunbar works. Each works has specific build-ups and the aim of the meeting was to review any commonality in the build-ups and methods for reducing the frequency of stops. A review of each works past and present problems was the basis of the meeting.
Weardale Works
Three areas of build-up appear at Weardale
(iii)
Lepol roof and kiln inlet chute, with material often falling off the roof and into the kiln. Kiln back-end, often a long build-up of around 20 feet long and up to 2 foot thick. Feed can often be held up in this area causing problems with overloading of the riddlings system. Back of the burning zone - this. material can often be reached and removed by water jetting.
The works are currently using open cast coal with a sulphur level of 0.9 - 1 %, and around 30 % petcoke with a sulphur level of around 5 %. Recently water has been injected into the flame at around 5 litres per minute, which has had the benefit of tightening the flame; The water injection has also had the effect of softening and reducing some of the build-up in the burning zone. It is proposed to increase the water addition rate progressively to attempt to remove build-up further back in the burning zone. The back-end build-up is thought to be alkali-sulphate based. Sulphate to alkali molar ratio is running around 1.5 - 1.7 on OPC and 1.7 to 2 on SRC. Slag is also being used in the raw mix, which is having a number of effects:
(i)
(ii)
Kiln outputs have increased as the level of slag used has increased. The slag tends not to create as much build-up.
The slag addition is reducing the overall sulphate input as it is replacing higher sulphate shale. The works is also attempting to select the best shale to minimise sulphate input. Additional problems are experienced with the production of SRC, where nodule breakdown on the grate is an issue leading to a dusty kiln and increased bed blinding. On occasions the kiln can build up in 48 hours on an SRC run.
Cookstown Works
Cookstown reported that whilst in the past many similar areas of build-up occurred, steps have been taken to reduce their formation. Since 1992 the cyclone dust has been removed from the system and mixed with the CKD. This material is then transferred to the cement mills and incorporated into the final product. It was commented that if the cyclone dust is returned to the process then the build-ups reoccur very rapidly. The benefit of this is two-fold, the first effect is to reduce bed blinding and the second is to reduce the sticky alkali-sulphate material entering the kiln and forming a build-up. Grate/chute build-ups still occur at Cookstown, although their effect has been minimised by the installation of silicon carbide refractory on the roof (around half of the length of the above calciner)
and sides of the Lepol grate, and also in the chute. There are no reports of any difficulties in keeping the silicon carbide in these areas. The grate and chute have around 14 air cannons installed although their effectiveness has not been quantified. The chute and hearth were also modified around the same period to allow an increased area for gas flow. 1997 saw a significant number of stops due to long, tapered build-ups through the burning zone. These were often hard build-ups, and due to hardness and the shape of the taper, were difficult to remove by any other method apart from stopping the kiln and digging out the build-up. These build-ups were associated with the burner itself (a new FCT pipe) and the position of the burner. Since the reinstallation of the burner and its alignment, only two burning zone build-ups have occurred, both being rings as opposed to tapers. The result of this is that the rings can be shot away and a kiln stop avoided.
Dunbar Works
Dunbar works manages to achieve a good burning zone coating (although sometimes a little deficient at the nose ring). The build-up problem would appear to be around the 35 and 45-metre length from the burner, with distinct rings being formed at these points. There is no real build-up in the riser, and there is no riser-cleaning or air cannons. The effect of the rings is to create a dam in the kiln causing raw meal to be held back and spill out of the back end seal. Amongst recent solutions are the speeding of the kiln to 4 rpm to move the material away from the seal, modification of the seal to reduce spillage and a rubbing ring to be installed later in the year. The works have been trying to identify the cause of the rings. The’ stage 2 inlet oxygen is controlled at 4 % in an attempt to reduce sulphate volatilisation. Observations are that there is a link between NOx and spillage, and that less build-up is present if the fuel split between kiln and precalciner is kept around the 40/60 % (worse spillage at 45/55 % split). The build-up is sometimes claimed to be clinkered. Discussion
Having identified the types and areas of build-ups in each of the kilns, it is important to classify the types of build-up being formed. It would appear that the experiences of Dunbar could be a different problem to those at the back of the kiln at Weardale (and those previously experienced at Cookstown). Dunbar need to evaluate the composition of the build-up to identify whether it is due to volatile recirculation or whether the rings are due to clinker being carried to the back of the kiln and depositing on the sticky melt in the kiln. Such build-ups can be characterised as a dense yellow/brown material, made up of material with fine particles. The principle of analysing the “building blocks” - the size of particles in the build-ups - will be pursued by Technical Centre on samples of build-up received in the future. On a kiln where cooler control is poor, and fine, dusty clinker is periodically produced due to overburning, such build-ups can be expected. In terms of sulphate control for Dunbar, much of the raw material sulphate is lost in the preheater tower as it is present in the raw meal as sulphite. This acts as a bleed for sulphur in the system, although some sulphate is returned to the system via the GCT dust. If the build-ups turn out to be sulphate based then further examination of the volatile cycles is required. Cookstown build-up is currently in the burning zone, and as previously stated, tend to be rings as opposed to tapers. It is likely that the rings are originating from the return of the dust from the cooler when the air cannons are fired on the IKN. The solution here would be to attempt a redesign of the inlet around the IKN to reduce the use of the cannons and therefore the amount of dust returned to the burning zone. This leaves a comparison of the three Lepol kilns to review how to reduce the back-end build-ups at Weardale. The obvious difference is that the cyclone dust at Cookstown is completely removed from the system whereas this is not carried out at Weardale. When this method was tialled at Weardale, the
cyclone dust was removed from one kiln and returned to the other kiln. The kiln with cyclone dust, not suprisingly, performed poorly. The kiln without cyclone dust resulted in a reduction in output with a hotter kiln. Whilst the trial was possibly not conclusive, the removal of dust from the cyclones and addition to the cement mill at Weardale will potentially increase the cement alkali level above the 0.6 soda equivalent level. All of the riddlings are returned to all three kilns. Riddlings are often screened on other Lepol plants so that the smaller fractions, which have the higher volatile contents, can be removed from the kiln system and further reduce volatile cycles. It is proposed to trial this at Cookstown with the results being fed on to Weardale. Cookstown comment that removal of the CKD and cyclone dust will reduce the soda equivalent in the cement by around 0.02. It is also commented that the amount of cyclone dust and the level of alkalis in the clinker could reduce by bleeding off this material. In the trial carried out at Wear-dale in removing the cyclone dust, around 2 tph material was collected, with around two thirds of the material having to be dumped in the quarry. The quantities and qualities of the materials around the system need to be reviewed and compared with Cookstown, to examine whether the dust could be returned to the cement mills. This will also allow a sulphate balance to be carried out to examine the extent of circulation around the system. The recirculation of the volatile materials - alkalis and sulphates - is related to effects in the burning zone such as flame impingement and reducing conditions in the flame, as well as the high sulphate to alkali ratio. Calcium sulphate will decompose in the burning zone due to contact of the flame with the material bed in the presence of CO, and also excessive temperatures in the burning zone. Therefore it is important to ensure that a tight flame can be produced to avoid the flame touching the bed. This, in principle, can be achieved by the use of a bluff body in the burner pipe. It was agreed to trial such a device when an appropriate design is provided by the Technical Centre. The position and angle of the burner will also be review. Both burners are in different positions at present but this has not shown any significant differences in operation. If the burner is pushed into the kiln the secondary air will be better entrained and produce a narrower flame. Coal mill and oxygen control at Weardale will also be reviewed so a more stable operation in the burning zone can be achieved. NOx control is currently used as a control parameter and variations in this parameter, along with the SO2 at the back of the kiln can also give indicators as to the extent of the recirculation in the system. An action plan of areas to review has been compiled as a result of these discussions.
Process Parameters
02, NO,, S02, CO, (stack and back end).
2.
Kiln feed, temperature profile and suction profile.
3.
Secondary air temperature, coal feedrate.
4.
Cement quality - sulphate - max 3.5% - alkalis - max 0.6% - slump - early strength/reactivity - 28 day properties
5.
Cement quality
6.
Effect on product properties of any considered change.
7.
Alkali bleed via auxiliary stack.
8.
Kiln/cooler exhaust/dust to burning zone.
9.
Lepol best practice for dust return/build-up.
10. Secondary firing.
1.
Cookstown max speed 1.8 Weardale max speed ? Does this affect build-up?
2.
Silicon carbide on grate areas/kiln areas bricks.
3.
Grate chute design at CKN - compare with Weardale and hearth design.
4.
Grate
SC
Operation - bed depth and variation (fixed speed) - doors open/closed - pan angles
5.
Recirculation fan - use/benefits/effects.
6.
Microphone on grate - information on bed blinding.
Raw Meal (either for SR or OPC) 1.
Fluorspar addition - benefits? - where in process?
2.
Secondary r.m.‘s - Shale ‘Z’ usage - Slag - PFA (up to 3% shale replacement nodule problems) . + nodule strengthener to increase use of pfa - increased low stone usage
3.
Raw meal residue - now - with secondary r.m.‘s
4.
Chemistry of particle range in raw meal.
5.
Raw meal variability effect - improve -
1.
Coal/petcoke mix - impact of fuel sulphur - water Injection
2.
Flame
3.
Coal Residue
4.
Coal ash composition and % in coal.
5.
Fuel blending in coal store.
6.
Fuel composition - frequency of sampling.
7.
PF moisture < 1%.
8.
Coal mill control.
9.
bluff body firing pipe position in relation to nose ring firing pipe angle secondary air temperature
NOx/kiln burning strategy (N.B. control of kiln with low NOx burner?)
10.
Kiln camera - Quadtech?
11.
Shell temperature for build-up.
12.
Sulphate balance/SO2 sample.
13.
Inleaking air through system.
14.
Temperature profile through grate - comparison between works.
15.
Removal of coal mill cyclone off cooler.
recirc/grate material SO4
1.
Quantity of riddlings.
2.
Quality and psd of riddlings.
3.
Screening - possible without breaking up nodules?
4.
Position of riddlings return - back onto grate vs kiln chute.
5.
1 Kiln no riddlings return, other riddlings from both kilns - high alkali clinker.
6.
Nodule strength improvement.
Cyclone Dust 1.
Quantity of dust - OPC and SR.
2.
Quality of dust - chemistry - particle size - hot/cold cyclones - efficiency of cyclones between kilns compare with CKN. Vortex finders in cyclones.
3.
Return of dust to cement mills - how much
4.
Cost of tipping dust? Can it be sold?
5.
Return to other kiln.
6.
Return all/proportion to noddy pans/elsewhere.
7.
Predict effects of all/partial removal.
8.
Pelletizing of dust.
9.
Intermediate fan efficiency.
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 11
Cement Seminar- Rings, Balls, and Build-Ups
Table of Contents Page 1.
INTRODUCTION
1
2.
LOCATION OF RINGS
2
2.1
Classification
2
3.
THEORETICAL ASPECTS OF RING AND DEPOSIT FORMATION
4
CHARACTERISTICS OF VARIOUS RING AND DEPOSIT TYPES
5
4. 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.0 4.9
5.
Exhaust Fan Deposits Slurry Rings Cyclone and Grate Preheater Deposits * Meal Ring (Calcininq Ring) in Long Kilns Middle Rings in Large Preheater Kilns Sinter Rings (excl. coal-ash rings) Coal Ash Sinter Rings Clinker Rings / Cooler Inlet Deposit Kiln Charge Balls
METHODS
OF
REMOVAL/ELIMINATION
TABLES 1 - 7 APPENDICES I - III
5 6 7 8 9 10 10 12 12
13
RINGS, BALLS AND BUILD-UPS
1.
INTRODUCTION
Rings and deposits are accumulations of solid materials (from the powdery kiln charge) in the rotary or static sections of clinker production lines. They have been encountered since the earliest days of rotary kiln production, with each development in process technology, e.g. grate and cyclone preheaters, grate cooler, bringing with them their own specific type of deposit. Rather than being of academic interest, ring and deposit formation has an appreciable influence on personnel plant operations, frustrating-operations by their impairing or even impeding production, and annoying the company management by lowering production (and sales) and increasing production costs. As a direct consequence of rings and deposits, the gas and material flow through the kiln is restricted, resulting in a reduced kiln output. Especially in the sinter zone, the presence of rings can interfere with combustion of the fuel and can result in improper combustion. From time to time unstable rings and deposits can break away leading to blockage or mechanical damage in the cooler, or in cyclone blockages. The partial shedding of coating from the exhaust fan blades results in severe vibration which mostly requires a short shutdown for complete removal. The breaking of a ring almost always causes a flush of material into the burning zone and a temporary loss of stable kiln operations. The formation of deposits in cyclones results in extra costs for the labour needed to remove the deposits by poking. The introduction of air canons (big blasters) provides a method for their regular automatic removal and has been installed in Group plants with persistant preheater blockages. High pressure water jets may also be employed. In the worst cases, a complete shutdown is necessary allow entrance to the affected area and mechanical removal of the blockage with compressed air drills. This shutdown invariably weakens the sinter zone refractories, and accelerates the next shutdown for rebricking.
to
2. LOCATION OF RINGS
2.1 Classification Unwanted build-ups may be classified with regard to the type of material from which they are formed, either sintered or unsintered. Within these two groups the various types can be classified as follows: unsintered: ---------. exhaust fan deposits . cyclone and grate preheater deposits . slurry or mud rings . meal rings sintered: -------. middle ring . sinter ring . clinker ring . "snowman" in grate cooler . kiln charge ball Process technological characteristics of such build-ups e.g. kiln type, location, temperature of gas and kiln charge can be seen in Table 1. Material technological characteristics e.g. state of kiln charge, enrichment in various elements, and type of texture are summarized in Table 2. The location of the various types of the above rings and deposits can be seen in Fig. 1.
RINGS AND BUILD- UPS IN DIFFERENT KILN SYSTEMS
CYCLONE PREHEATER KILN
.
ASH RINGS L/D - 5-7 /
I GRATE PREHEATER
\
- KILN -
WET KILN
COATING ON INLET TO COOLER
/
c
MIDDLE RINGS LARGE DRY KILNS MEAL RINGS (LONG KILNS) L/D - 7-13 SINTER RINGS L/D - 2-7 CLINKER RINGS L/D- O-2
MUD BALLS SLURRY RING
/
/
3. THEORETICAL ASPECTS OF RING AND DEPOSIT FORMATION Although of much practical significance, little quantitatively based, fundamental knowledge is available on the formation of deposits from solids suspended in gas streams. In a qualitative way, however, the more important features of such processes are known. The formation of a deposit is always a dynamic process in which the factors responsible for formation outweigh the forces of degradation. In general, the stronger the forces of destruction, the more unlikely the chance of deposit formation, but when this does occur, a strong, hard to remove agglomeration is the result. After the transport of material to the area of deposition, a definite force is required to make it adhere to the wall. This can range in magnitude from that caused by turbulence within the gas stream, increasing to centrifugal forces when the stream changes direction, to that due to mechanical pressure. Whereas preheater deposits involve the first two, mechanical pressure certainly plays a part in ring formation within the rotating kiln. The forces according to Rumpf considered to cause deposit formation can be grouped as follows: A
-
melting or softening of surface due to friction or collision
B
-
melting or freezing due to addition or removal of heat
c
-
interlocking of aggregates built up of finer particles held together by surface forces
D
-
interlocking of long fibrous particles
E
-
electrostatic
attraction
The mechanisms B, C and D are the ones encountered in kiln operations. In general, the finer the powder, the greater the tendency towards agglomeration, and in many cases the absence of particles under a critical size (e.g. 5 urn) ensures freedom from deposition.
4.
CHARACTERISTICS OF VARIOUS RING AND DEPOSIT TYPES
Tables 3 and 4 contain a list of typical properties of rings and build-ups encountered within the "Holderbank" Group plants (with full chemical analysis being provided in Appendices I, II and III). Included are such factors as volatile element concentrations and moduli of the deposited materials. As an indication of the texture, the size of the pores and the particles or aggregates of particles, from which the materials were built up, is given. In many cases the mineralogical composition is also given.
4.1 Exhaust Fan Deposits In the case of kilns with pressure filter systems, in which unfiltered dust-laden gas passes through the exhaust gas fan, deposit formation causes problems. These arise when the deposit falls off one blade, and brings the rotating fan out of balance. Deposits of up to 3.5 kg/blade are known to occur.
Characteristic
properties:
Exhaust fan deposits, composed of the finest raw meal particles are usually red-brown, hard and quite brittle. They exhibit a compact layered 8%. structure and have a very low porosity of Their chemical and mineralogical composition is basically that of the raw meal but often the plate-shaped clay particles are preferentially deposited parallel to the blade surfaces. Due to their long stay in the system, fan deposits are enriched in the volatile components K20, Na20, so3. Typical values include the following: LSF SR K20 + Na20 SO3
20 1.0 2.1 4.7
-
100 1.5 3.0 % 6.0 %
The SO3 is usually present as anhydrite (CaS04).
Bindinq mechanism: In this case, the temperature is such that liquid phase involvement - aqueous or molten salts - can be ruled out. The dust particles, because of the fan rotation, strike the blade surfaces with a high velocity and are so compacted. As the texture of the surface, after even a short time in operation, possesses undulations in the order 0.5 - 20 urn, the smaller dust particles can be mechanically "locked-on". Subsequent development of the deposit follows by an identical mechanism.
4.2 Slurry Rings (including mud balls)
Characteristic
properties:
These occur in long wet kilns and are composed of the partially dried kiln charge somewhat enriched in alkalis and S03. They are soft and can usually be broken - and hence prevented - by heavier chains. The H20 content lies between 20 and 30%, a range in which clay materials exhibit a sticky, plastic consistency. The content of the alkalis which greatly increases the tendency to adhesion (influence on rheological properties) can be up to 10% K20 + Na20, and about the same level of S03. In many cases, balls form (in addition) on the chain links by the same mechanism. A typical example of a mud ball is plant I in Table 4. Bindinq mechanism: The binding mechanism is the well-known ability of clays to form a sticky, plastic mass when containing the correct quantity of H20, and to harden on the further water loss. To this mechanism must also be added the crystallization of K2SO4 solution and the further strengthening of the structure by formation of CaS04. Photo la gives an example of such a ring.
4.3 Cyclone and Grate Preheater Deposits
Characteristics: These deposits form on the roofs, walls, outlet and riser pipes of cyclone preheaters, in the hot chamber of grate preheaters, and vary considerably in appearance and homogeneity. In general, they have a light colour varying from cream to brown to pink, indicating that the component particles had not been heated higher than 1200°C. In some cases, darker zones of harder burnt material can be observed. Depending on their place of deposition, they range from a dense, compact, definitely layered structure, hard to break to a porous (30%) material with only moderate strength with less obvious layering. The former type is typical of cyclone cones and discharge pipes while the latter is to be found in the transition and swirl chambers. Soft deposits can, however, also be found in the cyclones. From a chemical viewpoint, this deposit type usually is characterized by a concentration of the volatile elements in the following range: K20 = 1 - 30% Na20 = 0 - 2%
so3 = 1 - 35% Cl = 1 - 25%
In some cases, therefore, deposits can occur with no appreciable increase in concentration. Typical analyses found for deposits are given in Table 3 and Appendix I. The mineralogical composition of preheater deposits differs as would be expected from that of the raw meal in that the clays are essentially decomposed, and a reaction to form intermediate minerals has taken place. Minerals containing only the volatile elements can also be found. Amongst the minerals found in preheater deposits are the following: raw meal:
calcite quartz
CaC03 SiO2
normal intermediate phases: free lime periclase mayenite
CaOf MgO C12A7
typical deposit phases with low melting point: sylvite halite langbeinite arcanite
KC1 NaCl 2CaS04 . K2S04 K2S04
typical deposit phases without melt involvement: carbonate spurrite sulfate spurrite anhydrite
Formation
2C2S . CaC03 2C2S . CaS04 CaS04
mechanism:
The binding substance in this deposit type is the low melting point Na20, K20, SO3, Cl based compounds. These are molten in the kiln gas and are deposited on the cyclone walls and pipes, or first on dust particles which then themselves are deposited out of the gas stream in these areas. Cooling on contact or with increasing thickness results in an appreciable strengthening of the originally sticky deposit. Because of the extensive duration of stay in the kiln system, a reaction takes place with gaseous CO2 and SO3, resulting in the formation of lath shaped spurrite and sulfate spurrite which additionally strengthen the texture. Typical textures for unsintered, preheater and kiln inlet deposits can be seen in photos lb - 1d.
4.4 Meal Ring (Calcining Ring) in Long Kilns
Characteristics and formation: The meal rings, often called "calcining rings" in long kilns, are in their properties and mechanism of formation very similar to those of preheater deposits in heat exchanger kilns. This is perhaps not surprising in that both build-ups occur in the same temperature zone. Meal rings are mostly less troublesome than preheater deposits because often, due to their relatively poor strength, thermal fluctuations, kiln deformation and the action Of the material stream, they fall off periodically under their own weiqht. A typical example of a
4.5 Middle Rings in Large Preheater Kilns
Characteristics: Unlike meal rings, middle "rings" are dense (fine grained) of low porosity, very hard and seldom fall off during operations. Although termed as a ring they are rather more elongated, like a band, being often some 15 - 20 m long extending from 7 to 11 diameters from the outlet, e.g. 35 - 55 m for a 5 m Qj kiln. Unlike previous types, this deposit is clinker-like in colour indicating it being composed of well burnt kiln charge. Perpendicular to the direction of deposition, the fine layered structure can be seen showing the curvature of the kiln shell. The chemical composition of middle rings is very similar to that of clinker. This is surprising because considering the long duration of the stay in the kiln, no increase in concentration of the alkalis or SO3 takes place, and often the ring shows lower volatile element values than for clinker. Typical analyses of a middle ring are given in Table 5. The minerals found in middle rings are the clinker minerals alite, belite, aluminate, ferrite and free CaO, the alite having often decomposed into microscopically mixed belite and free CaO, resulting from the temperature at the site of the ring being under the lower stability temperature of alite (i.e. 1260°C).
Formation
Mechanism:
The mechanism of bonding is the freezing of the clinker alumino-ferrite melt. Due to a long cool flame, the clinker has a tendency to be fine, and the smallest clinker particles of 150 - 450 urn are carried back by the flame and deposited onto the kiln wall in a zone where temperatures of below 1250°C exist. The particles immediately freeze in place, and because the kiln charge is still fine, it does not possess sufficient abrasive action to remove the growing ring. The typical compact structure of a middle ring can be seen in photo le.
4.6 Sinter Rings (excluding coal-ash rings) Characteristics: These occur at the beginning of the sinter zone some 4 - 5 D from the kiln outlet. They are greyish-black in colour, strong and are (usually) agglomerations of small clinker pellets and clinker dust. No layer structure is obvious because of the presence of large pores and voids. In general, the chemical composition is that of the clinker with no appreciable concentration of volatile elements. From a mineralogical viewpoint, the normal clinker minerals alite, belite, aluminate, ferrite and to form free CaO are observed, with reactions belite and CaOfree, spurrite and belite being found with increasing depth in the ring, i.e. decreasing temperature, similar to the case of middle rings.
Bonding mechanism: The bonding is created by the freezing of the alumino ferrite clinker liquid in the case of pure sinter rings. This phenomenon occurs especially at the beginning of the sinter zone, where the liquid phase is just starting to form (approx. 1280°C). Due to the rotation of the kiln, the charge in this zone freezes with each kiln revolution: a new wet layer sticks on, and with time a thick deposit builds up consisting of particles of less than 1 mm.
4.7 Coal Ash Sinter Rings Characteristics: In kilns fired with a high ash content coal, sinter/coal ash rings can form at 7 - 8.5 D from the kiln outlet. They are dense, often layered and sometimes glassy in appearance and built up from particles some 150 - 250 urn in size. They are rather less dense and have larger pores and voids than middle rings. Photo 1f gives an example of the microstructure of such material, showing the coal ash layers.
From the viewpoint of their chemical and mineralogical composition they are essentially similar to clinker, exhibiting the minerals alite, belite, aluminate, ferrite and free CaO. With decreasing temperature (increasing ring depth) reactions to form spurrite and calcite take place, and also the transformation of alite -> belite + CaOf and /3beliteerbelite. Details of the chemistry and mineralogy are given in Table 6 . No enrichment of the volatile phases can be observed. Because of the enrichment in coal ash, the belite content is higher than that of the clinker, and tends to be found in layers. Formation
mechanism:
The bonding medium here is the sticky molten coal ash particles and perhaps to a slight extent, the alumino ferrite clinker liquid phase occurring by a mechanism such as in Fig. 2 showing the typical build up during kiln rotation. Fig. 2: ash layer (sticky)
kiln charge a)
Mechanism for ring formation ash layer + sticking kiln charge
kiln
charge
b)
ash layer / kiln charge / ash layer
kiln charge
c)
The molten coal ash droplets adhere to the exposed kiln lining at a point and temperature at which they are still partially fluid and sticky. When this sticky layer passes under the kiln charge on each rotation, it is assumed that a single layer of the still very fine kiln charge adheres to it. Because of the presence of fine crystalline alite and the overall occurrence of liquid phase, it must be assumed that the material temperature at the position of the ring lies above 126O'C.
The alite crystals are very small and certainly much smaller than those of the clinker. Because of this, it can definitely be said that the ring is not formed from clinker dust blown back down the kiln.
4.8 Clinker Rings / Cooler Inlet Deposit (snowman) Such rings and deposits are formed from normal size clinker granules and have a high porosity containing many voids. They are usually not troublesome to kiln operations as they can easily be removed. Their composition and mineralogy is identical to clinker, but in some cases, rings of up to 3.5% K20 and 3.0% SO.3 have been observed. The mechanism of bonding is the freezing of the clinker liquid phase as the clinker passes through the cooling zone (ring) or on falling down the chute into a grate cooler, grate kilns being usually operated so as to have no cooling zone within the kiln itself.
4.9 Kiln Charge Balls Kiln balls occur in cases where a tendency to meal or sinter ring already exists and can be up to 1 m in diameter. The chemical composition is, thus, an important factor. They are usually found upstream of meal or sinter rings. They are usually made up of already calcined material and can have a porosity of up to 558, consisting of many fine pores. Often they consist of a hard dense porous core, surrounded by the majority of porous material. The core usually is a piece of coating from say the lower heat exchangers or the transition chamber, and often has a composition different from the kiln charge in the area of formation. Differences in composition can be seen in Table 7.
The mechanism of meal ball formation can be due to either, or a combination of the following factors: - stripping and subsequent "balling" of old, excess coating - agglomeration enhanced by available clinker and/or salt melt - ring section acts as a dam, retaining "pieces" of material for long periods. Radial growth of the pieces occurs by compaction and adherence of fresh surface due to continual rolling action of the pieces/balls over the charge. In most cases, no liquid phase participation in sufficient quantities is possible so that the balls behave like a snowball and by their own pressure material sticks to the surface. This mechanism is similar to that of deposition on the exhaust fan blades.
5.
METHODS
OF
REMOVAL/ELIMINATION
An important prerequisite for minimizing the tendency to form objectionable coatings and rings, is stable kiln operation. This applies to the composition, fineness and feed rate of the raw material and fuel, and burning zone heat control. The tendency to form coatings in the kiln is reduced by lowering the dust load of the kiln gas. Objectionable coatings and rings which are formed as a consequence of high concentrations of various circulating elements can be obviated by appropriate reduction of the cycles in question. This can be achieved by: - Employing different raw materials and/or fuel with lower concentration of the offending element. This is generally not practicable. - Control of the raw meal milling so as to reduce the concentration of the very fine particles of sizes under 20 urn.
- Intervention into cyclic discarding dust in which have become concentrated, bypass installation which of the kiln gas.
process by either the circulating elements or by means of a extracts a portion
The penetration of false air into the preheater and kiln inlet chambers should be avoided, as such cold areas will act as sites for preferential build-up. In order to reduce the tendency to form sinter rings, it is in the first place necessary to reduce the proportion of fusible matter in the clinker, i.e. the lime standard and silica modulus should be increased. "Coating-inactive" bricks have also proved successful in certain cases, in reducing the tendency toward sinter ring formation. In coal-fired kilns a coal with a normal ash content should be employed as coals having ash contents of 40% are characterized by a very high tendency to ring formation. No general approach can be given to the effectiveness of other measures, e.g. alteration to firing conditions, as these represent variables which are peculiar to the particular installation. Clinker rings can be avoided by shifting the flame further back, thus increasing the clinker temperature at the kiln outlet. As a result of this, however, the "stickiness" range of the clinker is shifted towards the cooler inlet. Coatings can then be formed on the cooler inlet chute. This is particularly problematic with satellite coolers. In instances where this occurs with grate coolers, these coatings can be eliminated by the use of water cooled plates on the inlet chute.
Table 1:
7
r' AND LOCATION OF RINGS AND COATING (according to Opftr, 1974)
kiln
type
coatfng slurry
type(s)
all rlng
long
wet
location
temperature ('C) charge gas
exhaust blades
fan
128/180
-
drying
zone
150/300
< 100
700/1100
700/800
coatfng
dry (preheater)
stage 3 & 4
coating
Lepol
walls/roof of hot chamber
grate
coating
long dry
Inlet
meal ring middle ring
preheater long dry wet
calcining
kiln ball
charge
slnter
chamber zone II #I
calclnfng zone
100
lOOO/llOO
1100/1400 41400 41400 <1400
---
"("%8"" <1200 <1200
all
beglnnlng of sinter zone
1400/1600
1250/1350
slnter zone coatfng
all
slnter
1600/1800
1350/l450
clinker
all
end of slnter zone
coating
ring
all
1)
1000/l
rfng
.
grate cooler entrance
zone
800/1600
600/800
1200/1400
1200
Table
Type
2:
GKNPRAL
MATERIAL CHARACTERISTICS
Location
K
Chbractar Base Material
OF VARIOUS RING TYPES
Enriched in
Texture
-----
: n
Raw Heal
Coating
all
EXh8U8t Blade8
fan
Slurry ring
NL
Drying
Lone
Int. Med.
Clinker
t
Ne,K
t
Packing
'P8rtlcle' 8ire
+
den8e
fine
t
denee
fine
(du8t
Coating
-
DS
S t a g e Ji4
t
w/ P
Halls/roof of
t
Coating
DL
Inlet
chamber
t
Heal ring Hiddle ring
WL DS
Calclnfng .
Coating
hot chamber
DL
zone
”
l
t
t
t t
t
pOr0ll*
fin8
t
poKOU8
fine
t -
t
porou8
fine
denee I .
flne
+
porou8
fine
t +
*
4’
l .
-Kiln charge ball
ALL
Calcining zone
Sinter ring
ALL
Beginning of 8inter xone
t
poroun
large
Sinter xone coating
ALL
Sinter
t
I pcwOU8
large
Cl lnker
ALL
End of sinter
prwOU8
large
PorOU8
large
t*
tone
ring
Lone
-
Coeting
Grat8
COOl cr
t l
coating center
lrblt 3 :
alrnl
Volrtllt Eltatnts
.ocrtla
Alktll Sulphul RJtio
K20
PROPERTIES
ModlIlI SR
An
ff
BUILD-UPS
EXAMINED
Gtnerrl tppttrrnct LSF
Strtngtl
Colour
Ttrturt
Colour
Htntrtlogy
,f strtrks
Ltytr
SwpltI
sire (jIltI
Pore
Bld.lJnlts fpd
I - 10
.I - 10
0.16
1.76
2.71
62
stron9
Yellw
rtdlbrwn
0.36
2.05
2.77
66
strong
ytllw
rtd/brwn
0.85
2.25
3.44
124
WJk
yellow
none
0.02
2.8
2.5
149
strong
allw/ irwn
dtrL brown
0.67
2.4
1.69
339
strong
yellow
none
0.19
0.38
3.33
1.70
180
strong
*td/brom
none
3.6
0.07
1.46
1.45
196
E
28.0
1164.91
3.01
2.26
103
mtdlum
brown
blttlgrttn
ports fllltd
KCI-200 chrvgt 1-5
B
23.6
2.31
3.2
2.6
104
wrk
brown
none
ports fllltd
KC1 cont. chrrgt 2-3
4.66
2.62
2.36
69
tllwl
none
ports filled
KC1 cont. chrrgt O-5
0.08
1.61
1.59
157
A
1st cyc
7.6
A
Ind cyc
11.4
0.16
A
lrd cyc " .
4.6
0.09
1.8
1.3
0.65
1.09
c
Ith c c 'Pjpt r
0.8
C
' (Cone .
6
1.6 24.0 2.56 36.0
dtnse porous
dtnst porous
-
0
.
F
User 'lpt
0.61
31.5
0.16
3.6
dense portus
strong
i rwn
VI- 5 I - 10
Cl- 3 2- 6
10 - 20
10 - 20
1: : 2:
2: : 5:
ma or: crlcltt, -i-K su p at spurrite, KC1 minor: s urTltc,Cr Ii t
"
4.0
:fln nltt .
6.4
0.19
1.6
1.1
10.2
0.27
2.5
1.3
196
strong
none
1.3
0.69
3.2
2.13
114
WtJk
none
4.6
1.65
2.09
5.69
62
weak
none
porars
6tnsr
porous .
6.9
0.34
17.6
1.48
0.68
75
strong
,rd/broua
blrck
dtnst porous
IO - 1s
2: : 3: >l 100
5 - 10
20 - 30 Iu.KCl cant 20 - 30
4 I E0
Table 4 :
Plant
(*let, (bitt)
Ring TYPL
K20
ma20
MUd
110 a
4.1
0.40
Heal
100 a
2.18
0.16
Ball
hlkrll Sulphur Ratio
Volrtlla Elements
.ocrtlon
SO3
0.14
K
Middle
45
l
0.16
0.06
0.01
0.13
1.56
F
Middle
37 a
0.12
0.07
0.05
0.71
0.84
1.01
0.24
1.01
1.14
0.34
0.03
0.21
1.21
Ash/ Slnter Ash/ Slnter N
Ash/ Slnter
32.5 8
18.5 a
0.95
0.20
nodull SR
0.44 14.2
0.20
1.56
0.56
PROPERTIES ff RIMiS EXAMINED
2.36
2.13
Generrl appearance
AH
LSF
Strength
1.40
110
strong
1.46
10
strong
Colour
1.6
92
strong
yellow/ brown
2.32
00
rtrong
yellow/ brown I
01
2.4
2.88
79
a0
strong
strong
Hlnerrlogy
brow/ 9w
Texture Pore She (pm)
Bld.Unlts fpd
2- 3
2- 5
He or - Sulphrte d-purrlte
100
50 - 100
#(nor - C2S, C4AF, mite. vrrlous sulphrto
100
50 - 100
g;; c25, C3A. C4AF
150 - 250
150 - 450
50
50 - 100
Et;; CZS, C3A* C4AF
200
100 - 200
C3S. C S, C3A, C&Of spurrl t e
100 - 150
200
Et&
200 - 500
150 - 300
$5, C3A, C4Af
I EP
TABLE 5 :
ring internal
sample
Loss
Typical analyses of at various depths
on iqn.
0.47
ring middle 1.82
a
middle
ring external 0.94
ring
clinker
0.37
Si02
23.0
17.8
21.9
21.9
Al203
4.9
5.1
5.5
5.6
Fe203
3.0
2.9 *-.
3.1
3.2
66.8
66.1
66.4
70.7
1.1
1.0
0.95
0.95
so3
0.13
0.07
0.27
0.48
K20
0.16
0.09
0.30
0.35
Na20
0.06
0.06
0.12
0.14
TiO2
0.25
0.23
0.25
0.25
m203
0.02
0.02
0.02
0.02
p205
0.24
0.21
0.20
0.20
Cl'
0.01
0.01
0.01
0.01
Total
99.74
100.36
100.01
99.57
LS
92
122
CaO M90
#
96
93
SR
2.91
2.23
2.55
2.49
AR
1.63
1.76
1.77
1.75
TABLE
Chemistry and.Mineraloqy
6:
ciln point depth in ring
29.5
of a (coal ash) ring
32.5
under hot surf ace
middle
near shell f I
chemical analysis: si02 u203 -203 CaO M90 K20 Na20
so3 Loss on ign.
24.65 5.68 4.23 61.27 0.49 0.58 0.36 0.15 2.61
24.61 5.70 4.29 61.92 --0.79 0.34 0.03 0.27 1.63
21.77 5.48 3.53 65.31 0.64 0.17 0.20 0.34 2.48
-
Total
100.0
LS SR AR
78.1 2.49 1.34 1.0
free lime
ninerals {wt. %) alite p<
99.68
100.0
79.0 2.46 1.33 4.0
93.7 2.42 1.55 2.7
det.
and pbelit
nJ
30
ry35
d -
40
N
15
55
-
60
d
40
rJ
30
liquid phase (aluminate + ferrite)
ni 15
N
15
d
15
carbonate spurrite+CaOf
Id15
h/ 10
&
15
< 5
45
E\belite texture: average pore size (estimated) p
400
-
800
400
-
800
I I
100 - 300 1
TABLE
Chemical composition of core and rim of meal ball and the kiln charge composition at the correspondrnq zone
7:
k i l n
b a l l
rim
Loss on iqn. I
20.9
A1203
I
5.6
Fe203
'
Ca0 s03
I I
kiln
core
charge
calcininq zone I
3.9
Si02
I
4.3
I
I
'-17.2 8.0
1 I I
1.9
I
21.2 5.7
2.8
4.4
2.7
63.0
62.5
62.1
1.7
2.2
1.9
LSF
94
104
I
91
SR
2.4
1.4
1 I
2.5
Cl'
0.1
I I
0.1 I I
K20
2.1
1.5
1 I
0.1
2.3'
APPENDIX I
APPENDIX II: CHEMICAL ANALVSIS
MA-Mat.
Nr.
Plant Locatlon Loss on Ign. sio2
A1203 Fe 03 Ca 6 MgO SO3 K2D Na20 TiO Mn2 6 3
I I I
50'393 D
53'550 G
I Rlser Plpe
11.8 9.2
:*2" 49:3 1.2
15.9
3.6 0.98 0.09 0.07
;:-05
0.11 2.7
Total
100.65
:"R LSF
I I
OF RISER PIPE AND KILN INLET DEPOSITS
1.61 1.59 157
54’155 1 54’118
A
I3
I
Kiln Inlet (OS)
10.1
7.4 f*$
1;*:
46:l
5:::
2:::
E 8.4
z5 0:15
0.01 0.09
252:; '10.6
3:4
0.19 0.15
:
4:*; 0:83 4.6 7.6
it*:1
53’956 H Kiln Inlet (DG) 0.57 12.4 2: 39:8 2.3 27.6 :*:r
0:os 0.38 0.27
0:19 0.16 0.08 0.09
5.0
0.03 0.05 2.1
99.7
102.82
101.23
98.83
3.12 2.13 114
2.09 5.89 52
1.48 0.68 95
2.47 1.31
196
APPENDIX
MA-Mat.
Nr.
Plant Location Loss on ign. SfO2 Al203 ;;a03 I? t.3 so3 K2D Na20 ii02 Mn2D3 p
Total CaOf SR . EF
III:
53’952 I
CHEMICAL ANALYSIS OF RINGS FROM,ROTARY
52'040
52'037
K
F
100-l 10m
45m
37m
10.7 14.7
t:: 0.40 0.18 0.06 0.23 0.23
0.47 23.0 4.9 3.0 66.4 1.1 0.13 0.16 0.06 0.25 0.02 0.24 0.01
98.89
2.19 1.48 110
ii:; 52.3 0.79
52’178 M
SECTION OF KILN
49’643 1
50'524
N
J
35m
18.3m
1.63 24.6
0.12 22.84
FL! 63:l 2.1 0.77 0.72 0.07 0.31 0.04 0.08 0.05
45:: 61.9 0.79 0.27 0.34 0.03
35:; 63.9 0.92 1.07 1.07 ' 0.24 0.36 0.15 0.24
8.11 21.3 4.5
99.74
100.44
99.68
100.25
27-h 1:63 92
2.73 2.32 88
2.46 1.33 79
2:::
32.5m
L
45’061
El 1:84 88
1OOm
2:::
:.:6 0:95 0.28 0.28 0.02 0.08 0.28
:*: 50:8 1.13 14.2 2.18 0.16 0.22 0.04 0.24 0.01
100.56
99.38
2.88 1.55 88
0.62 2.36 1.46 78
5;::
PHOTOTABLE
I
1 a)
l0J.u
I
1:
REM 84/108
Mud ball in chains
IOU/ u
lc) KC1
1 el
REM 841545
S.E.M. MICROGRAPHS OF DEPOSITS
1 b)
REM 84/506
Compact cyclone 2 deposit
ld)
crystals
REM 84/79
eY
1 f)
,xm
,
Spurrite
crystals
REM 84/69
REM 84/72
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 13 Section 12
Rings and Buildups in Cement Kilns
BURNING
ISSUES
address those most frequently encountered. inches w.g., -0.01, -0.1, -0.15, etc. Quite posThere are problems associated with the sibly the most serious effect on hood presburning of waste fuels which can be attributed to flame position, alkalies, chlorides and sulphur. Figure 2 shows a basic understanding of air volume changes attributable to changes in temperature. Our experience indicates that many people tend to forget this relation. They comments “I didn’t increase the air flow,” “the flame looks like it is on the load. It wasn’t yesterday; someone must have moved the burner.” Figure 2 shows the increase in volume caused by temperature changes. One cubic foot of air at 100°F weighs about 0.071 pounds. The weight of one MAJOR CAUSES fig.1 cubic foot of air seems insignifi- sure sampling over the years has been our 1 . Overheating cant, but at each of these plotting attempt to “bum on the nose.“ All changes 2. Slow clinker quench the weight of air is the same in fuel ignition are immediately detected by 3. Fuel impingement on the burning zone (0.071 pounds), only the volume hood pressure changes-there is no dampload has changed. 4.4 cubic feet of air ening effect as there is when the flame is 4 . Long flame at 2000°F still weigh 0.071 away from the nose. 5 . Chlorides pounds. Figure 3 shows the different pressure 6 . Suiphur We often hear the question, conditions found in the kiln discharge hood. 7 . Potassium “Where is the best place to sam- This is why more than one sample point is 8 . Mechanical restrictions ple the kiln discharge hood pres- needed, with all manifolded together to sure?” But the real question is serve as one sample source. The result is the EVALUATION PROGRAMME. “where on the hood does the measurement of an average pressure. 1. Sampling sample point (or points) give a It is interesting to note the effect higher 2. Care of sample (temperature, air and pressure reading that permits rel- secondary air temperatures have on the kiln moisture considerations) ative control?” That is, where are discharge hood pressure. The increase of 3. Samples which are consistent and the conditions today similar to secondary air temperature increases the volrepresentative what they were yesterday? The ume of air as well as the velocity. This 4. Documentation of conditions before and next question is “what is the cor- increase of velocity tends to drive the secduring time of the problem rect kiln discharge hood pres- ondary air and dust toward the top of the sure?” The kiln discharge hood hood. This condition always creates a dustSOLUTIONS should be at a slightly negative ing and puffing at the top of the hood over 1. Alter raw materials and fuel pressure to permit observation the kiln, whereas the bottom side of the kiln 2. Control internal alkali, sulphur and chloride by instruments or persons with- may be at slightly negative pressure. cycle out undue overheating and dusty A change in secondary air temperature a. Install a kiln gas by-pass for preheater conditions. From the standpoints can move the flame position up or down. and calciner kilns of good housekeeping and mainCertainly, a change of secondary air temperb. Do not return as much total dust, tenance, the hood pressure ature wilt alter the fuel ignition rate, but the especially where precipitator fields discharge should be slightly negative. This concern in this example is the positioning of to individual conveyors value should be c. Determine time cycle for build-up determined by trial r Adjust kiln burner (permit clinker quench, and error for each fig.3 shorten burning zone length, eliminate fuel POSITIVE system. It is always impingement on the load, locate burner on advisable periodikiln center line and slope) POSITIVE PRESSUR tally to review the 4. Adjust kiln material and gas temperature selected set point to profile determine if condi5. increase kiln rotational speed NEGATIVE tions have changed. 6. Install kiln internal restrictions such as Once the desired dams or orifice rings kiln discharge hood 7. Maintain the secondary air temperature pressure is selected, consistently that is the target, whether it is 0.05 he problems caused by rings and build-ups in a kiln system always create turmoil and frequently a loss of production. Most of these problems can be controlled if not eliminated. Figure 1 indicates the major causes of rings and build-ups. There must be a good evaluation programme, which includes a review of the literature. When this is accomplished, there are definitely solutions to the problems of rings and build-ups in the kiln. Frequently the solution requires forgetting some preconceived ideas. This section will not cover all of the rings and build-ups that can occur, but will
T
BURNING
ISSUES
secondary air temperature is increased. Velocity through the cooler throat increases to 1275 feet per minute. This increase of velocity tends to raise the flame path, which usually causes the burning zone to cool off and the calcined material to flush into the burning zone. These examples show why it is more important to maintain a constant secondary air temperature than to attempt to reach the highest possible temperature.
clinker and past a thermocouple sensor. Methods for aspirating air from the clinker cooler have proved to be impractical primarily because of wear created by the clinker dust. Quite possibly the calciner kiln system permits the most accurate measurement of combustion air temperature. The calciner kiln system aspirates combustion air from the clinker cooler as tertiary air for the calciner. In spite of its inaccuracy, the thermocouple placed in the clinker cooler throat has been accepted as indicating a usable relative secondary air temperature for day-to-day kiln operation. This method of detecting secondary air temperature is fine if we remember that m It is a relative temperature and may read much higher because of radiated heat from the clinker. n It may increase or decrease, depending upon changes in the clinker cooler bed, without much real change in air temperature. Fluctuation of secondary air temperature is one of the major causes of rings and build-ups. The kiln flame and location must be controlled to maintain a stable operation. A stable kiln operation should create the pattern of coating and a ring formation shown in figure 7. This drawing shows only a small amount or no coating from the burning zone to the kiln discharge end. The ring
the flame path. Figure 4 shows the burner positioned on the kiln centre line and slope. This position has been adjusted during fig.6 operation to compensate for the secr - - - - - x ondary air’s tendency to lift the flame path. In this example the intent is to direct the flame tip on the kiln centre line and slope. Figure 4 indicates that the average secondary air temperature is 1000°F. The volume of secondary air passing though the fixed throat area has a velocity of 900 feet per minute. The system in figure 5 is identical to that in figure 4 except that the secondary air temperature has been reduced to 700°F. The velocity of the secNormally, attempts to achieve the maxiondary air through the clinker cooler throat mum secondary air temperature produce has now been reduced to 715 feet per cyclical operation of the kiln. This promotes minute. The flame path has been lowered the production of clinker and the tip is no longer on the kiln centre burned in a reducing atmosline and slope, resulting in fuel impinging phere, slow quench of the on the load. The problem of fuel impingeclinker minerals, dusting in the C ,I ’ ment on the load is definitely more prokiln discharge hood and kiln ., ‘, ., nounced when the burner is adjusted to turn ring formation. the flame toward the load. Microscopic It is also important to recog: analyses often indicate that the clinker was nise that the secondary air temproduced in a reducing atmosphere on this perature recorded by most date, whereas the day before this was not plants is a relative temperature. the case. The secondary air temperature Figure 6 shows what happens when the is usually detected by placing a thermocouple the that forms 80-115 feet from the kiln dissomewhere near clinker cooler throat area. charge end is in the area where calcination is The value indicated by this complete and the liquids begin to form. The method of sensing not only location of this ring depends upon the burnmeasures the air temperaing zone length. It is formed because of the ture, but it also detects coexistence of calcined material, a small radiated heat from the amount of liquid, and material still in the clinker and the flame. A solid phase. This creates prime conditions true secondary air temper- for build-up. The ring does not adhere to the ature is measured by aspirefractory, is not dense and is, very fragile. It rating a portion of the set- breaks up and falls out when the kiln temondary air away from the perature is changed by alterations to the cal-
B U R N I N G I S S U E S cining zone and material preparation. It may fall out when flame length and location change. This ring is regarded as an asset because it serves as an orifice that increases the gas
such as a decrease in secSLOW ondary air temperature, may QUENCH create the condition where the flame tip is projected through the load (fig 9). This lengthening of the flame causes fuel I I impingement on LONG FLAME the load, but also causes the conical long flame ring build-up shown in figure 8. When this ring is detected, it can be broken up and dropped out by shortening the flame. _ _ ----.-IThis type of ring can also be prevented with a short flame velocity at its location. This tends to hold with its tip directed on the kiln centre line back and mix aerated material. While the and slope. ring is present kiln operation tends to be staAnother example of a long flame is ble, with less material flushing into the burning zone. If all conditions remain stable, the ring remains and assists operation. It does not grow substantially as the stable operating time increases. For several days often the ring falls out, kiln operation may be cyclic and it is difficult to keep the raw load out of the burning zone. We have often reviewed kiln operators’ logs and found this NOSE RING KILN to be a common scenario. DISCHARGE SLOW QUENCH --I_--Figure 8 shows the ring caused by a long flame. This ring may also be formed when the flame path is directed into the load. In shown in fig 10. In this case the flame tip is the latter case, the kiln may have experi- at least directed on the kiln slope and paralenced stable operation with the flame lel to the kiln centre line. There are apparently sufficient liquids available to produce a sticky environment which promotes the development of a material ball. Balls which are 6 to 12 inches in diameter have been found in the middle of the calcining zone. A few of these balls grow to diameters of 6-8 feet. The larger balls look alarming directed toward the load, and this location when they are first seen passing through the may be satisfactory as long as the flame tip burning zone. Burning with a shorter flame is not on the load. However, when the flame length prevents additional balls forming tip is directed into the load, any change, unless they are caused by a high concentration of alkalies, sulphur, and chlorine. The nose ring (fig 11) has fig.10 been described as an “ash ring.” Some kilns operate with a nose ring most of the time. This tends to restrict clinker i P---discharge from the kiln. KILN DISCHARGE Microscopic evaluations of clinker produced during the
\
I- t i e . 1 2
presence of a nose ring indicate the presence of slow quench. The nose ring permits a very slow quench of the clinker because the material is pooled when it passes out of the burning zone. Quick quench of the clinker minerals must be completed within the kiln or it will not be achieved. Slowly quenched clinker causes the C?S to revert back to C,S and free lime. Further slow cooling causes the C,S fin the beta state or high-temperature form) to change to the gamma state of C,S (a lowtemperature form). The gamma form of C2S is a dust and no longer forms a nodule. This dust is picked up by the flow of air and carried back into the kiln where it enters the burning cycle again. The slow quench cycle continues as long as the nose ring persists to act as a dam. The suspended particles returning with combustion air are easily preheated because the surface area is maximised. The liquid available at the kiln nose permits adherence of the dust particles, and the building of the nose ring continues. Figure 12 shows an example of a snowman on the clinker cooler back wall. Some snowmen grow tall enough to reach the burner pipe. Generally, the larger the kiln, the larger the snowman. Depending upon the installation procedure of refractory over dead grates, some kiln systems form snowmen on the clinker cooler side wall near the throat. The snowman build-up is caused by the same problem that promotes the nose ring build-up - that is, slow quench of the clinker. Microscopic evaluation of clinker shows whether the material was slowly or quickly quenched and whether C,S changed from the beta to the gamma state. Both the nose ring problem and the snowman build-up can be eliminated by adjusting the kiln burning operation so that the clinker is quickly quenched within the kiln.
B U R N I N G I S S U E S We have learned to live with a dusty kiln temperature increased from 2600°F to discharge hood, especially in larger kilns. 2750°F and NOx fell from 750 ppm to 350 The old small wet-process kilns were sel- ppm. The clinker went from slowly dom dusty because the fuel consumption quenched to quickly quenched. The clinker cooler snowmen were eliminated, the kiln discharge HIGH SULPHUR RING hood cleared and THICKNESS CAN BE 1 we could see the flame and burning zone. In addition the 28-day compressive s t r e n g t h s increased by 600 psi over a 90 day KICN period without DISCHARGE any increase of fineness. Figure 13 displays a ring formation was high and we could not gain quick which occurs in the calcining zone or the enough ignition to burn on the nose. This area where the promoted the quick quench of clinker gas temperature is within the kiln. We also found that the old sufficiently low to wet kiln produced the most reactive clinker, permit condensawhich permitted a lower fineness for similar tion of sulphur compressive strength levels. and chloride comOur most recent experience of putting pounds. This ring KILN GAS this flame technology into practice was with is a part of the a large wet kiln. It was necessary to remove alkali, sulphur, BYPASS large snowmen from the clinker back wall. a n d chloride These snowmen were giants, lo-12 feet high cycle. All kilns and 6-8 feet in diameter at the bottom. The have a variety of kiln discharge hood was so dusty that we rings in this area: could not see the nose of the kiln. The nose some consist of a refractory had to br replaced every six small amount of months and the nose castings every 12 PU”kY coating. months. The kiln burner was adjusted to With larger rings shorten the flame: this reduced the burning (fig 13) the kiln has to be shutdown to physzone by about 45 per cent. The burning zone ically remove the build-up. The elimination of the cause normally requires a fuel change, such fig.14 as a lower sulphur fuel, a n d the return of less kiln VELOCITY dust. If a microscopic evaluation of the clinker indicates production in a reducing HATES ANi atmosphere, the TEMPERATURE burner should be BYPASS QUENCH Al R adjusted to elimiIN RISER nated fuel impingement on the load. This will permit a higher clinker sulphur
L
level which removes a similar amount from the cycle. If the long wet and dry kilns use an electrostatic precipitator, the dust collected in the final fields can be wasted as high alkali, sulphur, and chloride material. The electrostatic precipitator works well as a kiln gas bypass system for the long wet and dry kiln systems. Since the solidified alkali, sulphur, and chloride particles are very small, they are concentrated in the final field of the precipitator, and are easily separated and removed from the system. Figure 14 shows some areas in the suspension preheater where problem build-ups often occur. As we proceed up the preheater in the direction of the kiln gas flow, the first problem area is at the kiln feed shelf. This problem on a preheater kiln is either caused by leakage of ambient air into the system or by operating with a high level of carbon monoxide in the exit gas. Ambient air leak-
fig.15
19OO’F T O 21OO’F
age causes a localised condensation of alkali, sulphur and chloride compounds. These chemicals are vaporised in the burning zone and exit as a kiln gas until temperature conditions are sufficiently low (about 1800°F) to cause condensation to the liquid state. Normally, the preheater kiln exit gas temperature is above the condensation point. When ambient air leaks into the kiln feed end housing there is a localised cooling of the kiln gas at the leakage source that results in build-up at that point. A different type of calciner kiln system build-up at the feed shelf and feed end housing walls can also be caused by leakage air. This build-up is caused when the kiln feed is nearly calcined and there are C,AF liquids present. However, if the gas dust concentration is sufficiently high, the liquid will adhere to the dust particle rather than to the surface of the wall, thereby preventing a build-up.
B U R N I N G I S S U E S also ensure that the
quench air exits to bypass the kiln AP = 2.0' KI LN induced draught fan. 75O*F T O ilOO°F I i - - + - Alkali, sulphur GAS e and chloride comBYPASS -0.5. TO -1.5” pounds create no -2.5" TO -3.5” o build-up problems if they exist in either QUENCH CHAMBER m ! 7 em -1900’F T O 2100’F the gaseous state or I ‘. 7-J the solid state. QUENCH AIR ’ However, if they / exist in the liquid state, they behave like water on dust. fig.16 The secret to efficient i kiln gas bypass sysThis situation can be artificially duplitern operation is taking a portion of the kiln cated by the introduction of dust from the exit gas at plus 1900°F and instantaneously Stage III cyclone material discharge and/or quenching it to about 750°F. This permits creating a rough feed shelf surface which the alkali, sulphur and chloride compounds causes the a splashing of the feed out into the gas stream. Dust re-entrained in the kiln exit gas by a rough feed shelf surface will increase the dust lost through a kiln gas bypass system, so the feed shelf must have a smooth surface when running a kiln gas bypass system. Figure 15 shows a build-up above the kiln gas bypass take-off and within the quench chamber. The build-up in the kiln riser above the kiln gas bypass take-off is caused by the leakage of quench air from GAS AND DUST the quench chamber. Proper sizing of the bypass quench chamber inlet can ensure that quench air does not enter the riser duct. DJST The example in fig 16 shows the parameGAS ---c ters used for design and adjustment of the quench chamber inlet. A two-inch pressure loss through the quench chamber inlet will
-I
to pass from the gaseous state directly to the solid state without passing through the liquid state. Some designers and operators quench to higher temperature levels, i.e. 900°F to 1100°F. Our experience has found more potential for build-ups in the quench chamber at these higher temperatures. The kiln gas bypass system appears to work best when the quench chamber and kiln riser duct take-off are placed above the kiln. As the gas and dust exit the kiln, the dust is thrown against the feed shelf while the gas is turned upward. This separates dust particles from the kiln exit gas stream. The cleaned gas tends to pass on the kiln side of the riser duct for a short time. Figure 17 shows the desired quench chamber position and fig 18 indicates the desired operating parameters for a kiln gas bypass quench chamber. In our experience a quench chamber operated with these parame ters will not product any build-ups, and will
NCH
CHAMBER
operate with no dust in the bottom of the chamber. Kiln gas bypass dust collector material contains 0.520 per cent of clinker, 20-25 per cent So,, and 4.5-5.0 per cent K20. If the percentage of sulphur as SO3 is less, for example 16 per cent, the bypass system is taking too much dust from the kiln riser, etc. There are always answers to problems with rings and build-ups. The solution is usually found when the attitude of the operator is that “we cannot continue to live with this problem.” JI This paper was first presented by the author, Floyd C Hamilton, of Hamilton Technical Services lnc, Roanoke, Virginia, for the National Lime Association meeting, St Louis, Missouri, United States, October 1997.
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
HBM PROCESS ENGINEERS CONFERENCE
• Minimization of Volatile Cycles
MINIMISATION OF VOLATILE CYCLES
1.
SUMMARY
Concentration of minor components within kiln systems due to volatile cycles can lead to significant operational problems on all types of process, with consequent loss of output. A portion of some of these volatile materials is also lost to the atmosphere, and minimisation of this “leakage” is also increasingly becoming an environmental concern. understanding of the factors that effect the magnitude of these cycles is an essential part of improved kiln control leading to effective control of the cycles. Successful implementation requires co-operation between chemist, process engineer, mechanical engineer and operators. 2.
INTRODUCTION
A limited number of minor components in the raw mix and/or fuel can become highly concentrated within the cement kiln system and then create operational problems. The minor components that are generally considered to be involved in major volatile cycles are the chloride. alkali (Sodium and Potassium) and sulphur species - although other elements do also become involved in cycles to a much lesser degree (such as Fl V, As, Pb, Tl Cd, Hg and Zn) they have not been identified as causing operational problems and so will not be considered at this time. These substances are present in the raw materials in low proportions in a variety of forms, but are likely to evaporate or decompose under the temperature regimes found in the burning zone. Once this happens they become associated with the gas stream and cool as this losses its heat to the material bed, until they either condense or react to form compounds that will condense. At this point they are present in the gas stream as potentially sticky liquids and will adhere to any surface with which contact is made; this can be a particle surface or a vessel side wall. Once the liquid has condensed on to a particle this can still stick to any wail with which contact is made until the temperature drops to a sufficiently low level for the liquid to solidify. In the suspension preheater and Lepoi processes the temperatures at which the potential compounds are liquid coincide with those found close to the kiln hearth and lower preheater stages (SP kiln) or above calciner (Lepol kiln), and this leads to material building up on the walls in these areas, or to ring formation at the very back of the kiln. In the older long chained kilns, excessive volatile cycles can contribute to ring formation. In either situation, at best this reduces the duct or kiln dimensions and so causes increased pressure drop and - probably - extra dust generation, whilst at worst it reduces output, increases kiln instability, and causes blockages to develop in the preheater system: the end result being significant kiln down time. In general, in the more thermally efficient processes (precalciner, suspension preheater and Lepol) most of the volatiles will condense in the preheater and only a small fraction will condense on the precipitator dust or escape up the stack. In contrast in the less thermally efficient processes (wet and long dry) a higher proportion of the volatiles will pass through the kiln system and condense on the precipitator - or bag house - dust or escape up the stack. Commonly where volatiles condense onto a dust the finer dust fraction will develop a higher concentration of the volatile component, due to the higher surface area of the finer dust.
Where a component is partially volatilised in the burning zone and then partially recombines into the material stream within the kiln/preheater system it is possibie for a large amount of the component to continuously recycle around the kiln system. This is called an internal cycle. Where some of the component is collected with a dust stream externally to the kiln system and then returned to the kiln, this is referred to as an external cycle. 3.
GENERAL REVIEW OF THE PROPERTIES OF THE MAJOR VOLATILE COMPONENTS
3.1 Chlorides
Chlorides are derived from the raw materials and the kiln fuel. The high voiatilities of these compounds, together with the high collection efficiency of the cyclone preheater systems, will lead to the development of a greatly enhanced cycle. The chlorides have a high affinity for the alkalies in general and potassium in particular. This property together with the high volatility has been used in kilns (commonly on rhe wet process, occasionally on the SP process) to control clinker K2O levels by addition of CaCl2 to the raw mix or fuel, which leads to loss of KCl with the kiln bleed or the exhaust gas from the kiln system. In the suspension preheater, the volatilised material is recaptured within the system unless a bleed is utilised between the kiln and riser duct. It is generally considered that no more than 3% of the chloride passing- from the preheater to the kiln will leave the system with the clinker. Although considerably higher levels have been noted in individual samples of clinker, this is probably due to a “push” of kiln feed, or a semi-flush situation as thermodynamic considerations indicate that no chloride should pass through the burning zone. On many SP kiln systems some degree of preheater cleaning is necessary on a regular basis and this may help to control the chloride cycle by forcing the kiln conditions into a situation which permits a brief increase in clinker chloride level (i.e. reduced material temperature, increased material loading and flux level). Small amounts of chlorides will also leave the preheater system with the waste gas stream. Taking the total loss of chloride from the system as between 2 and 5 % of the feed to the burning zone, it would then be expected that a circulating load of 20 to 50 times the total chloride input could develop in a system without a kiln gas bleed. No reports of low temperature chloride volatilisation within the preheater have been identified. 3.2
Alkalies
The major source of alkalies will be the raw mix; notably the clay component, although minor quantities can arise from the fuels The initial free alkalies will behave in one of three ways:
1)
Remain in the material being processed and become incorporated in the clinker constituents that are being formed. This happens to Na2O to a greater degree than K2O
2)
Be converted into different compounds - chlorides, sulphates, by reaction with ofner constituents of the raw mix.
3)
Diffuse to the surface of the process material and volatilise.
carbonates, hydroxides
In its initial state, K2O begins to volatilise over a wide range of temperature, depending on the form of clay in which it was incorporated but irrespective of source, it would be expected to have volatilised almost completely at burning zone temperatures, although some may have been at least partially stabilised by conversion to the less volatile sulphate form within the material bed. Once volatilised it will react to form chlorides and sulphate - chlorides preferentially - at the rear of the kiln. These will then deposit on dust particles. Initiallv Na2O is less volatile than K2O due to its higher bond energy and so a greater proportion of the kiln feed Na2O would be expected to pass through the burning zone in clinker without participating in the volatile cycles. Volatilised Na2O will react with SO2 and SO, to form sulphates towards the rear of the kiln and with chloride where this species is present in excess of K2O Where alkali is present in excess of chloride and sulphate alkali carbonates will be formed Each of these al “-=smm the surface of dust particles in the cooler zone of the kiln ad lower preheater stages and will enter the volatile cycles as the dust is separated out in the cyclones. Direct contact onto kiln or preheater surfaces may lead to the development of build-up, The compounds will then re-enter the kiln where the degree of volatilisation will depend on the species and the kiln conditions. Volatility decreases from chloride to carbonate to sulphate and, hence, sulphates are more likely to pass through the burning zone. Nevertheless, the likely range of burning zone temperatures cover the thermal area in which alkali volatilities are likely to increase significantly with rising temperature. In general, precalciner kilns have significantly lower burning zone temperature than are common in other processes and, hence, alkali sulphate volatilisation in particular is lower in precalciners than in other processes. 3 . 3
Sulphur
Sulphur can enter the system in a number of forms from either fuels or raw materials. A limited amount may evaporate in the upper preheater stages and escape from the system in the exhaust gases. In general, SO, and SO, can form in the high temperature areas and be transferred to the gas phase. In the cooler areas of the kiln back-end and preheater system sulphates will form and re-enter the material stream. preferentially alkali sulphates will be produced with excess sulphate combining with free lime or calcium carbonate an$li@ru&&5li~-i5Xailablein~e~~ .--~~~r-‘l-----‘t’:----~~a~~~~~~~~~~~~~, so restricting the formation of sulphates. In this situation loss of sulphur oxides by way of the stack may increase but where the gas stream passes through the raw mill the majority of the sulphur oxides would be expected to react with the high active surface calcium compounds which are produced in the milling process. This will then return to the kiln in the raw mix as part of an external cycle. The high boiling points of the alkali sulphates would indicate that relatively low levels of volatilisation would be expected. However, dissociation may occur, particularly under the reducing conditions which can exist to some degree within the burning zone. Calcium sulphate also has a high boiling point but is even more susceptible to dissociation, so a higher recirculation of sulphate from this compound would be expected. As CaSO4 cannot recycle as a compound, the lime from this compound remains in clinker as free lime - making burning more difficult whilst SO, is carried in the gas stream towards the kiln back-end where it reacts to form alkali or calcium sulphate.
4.
CONTROL OF VOLATILE CYCLES
As indicated earlier the major species involved in volatile cycles are chlorides, alkali and sulphur based components. As the development of a volatile cycle depends on the evaporation or dissociation and condensation of a range of compounds the meiting boiling and dissociation temperatures of these compounds will be of major significance, whilst volatility characteristics will also be of importance. Boiling and melting point data is presented in figure 1, and volatility data in figure 2. As chemical analysis gives details of the volatile components in the feed materials of in material from within the system in terms of the primary oxides or elemental forms - IWO, K20, SO3 and Cl - the volatilities are normally expressed in the same terms. and the ranges of volatility for each that are commonly found within the cement kiln are given in figure 3. These can be considered to be the primary volatilities for these components, and in each case the tabulated figures show a wide range of volatilities. This doesn’t help to extend our potential understanding of what is happening within the kiln. The volatiles however will be present in the material bed in the kiln as compounds for each of which the volatility will depend on physical characteristics such as those set out in figures 1 and 2. As indicated in section 3 there will be an order of priority (ease) with which these compounds will form. Therefore it is possible to study the development of a volatile cycle in terms of the physical properties of these compounds. The specific volatilities that have been found for each of the compounds that are likely to form are set out in figure 4. This shows for instance, that alkali in combination with chloride is more likely to develop a cycle than that combined with sulphate. In the same way, sulphate combined with calcium is more likely to volatilise than that present in an alkali form. Where volatiles are present in the raw materials some degree of cycle is bound to develop. However, the overall cycles can be governed to a degree by optimising the relative proportions of each volatile component to maximise the potential for the formation of those compounds with the lower specific volatilities. The prime control of the volatile component cycle is performed in the design stage of a works project through the selection of raw materials and fuels in order to optimise the relative and absolute levels of the potentially volatiie components and, where necessary, the inclusion of a bleed system. However, whenever changes are made to a sourcing of raw materials (including fuels) the potential affection in the volatile balance needs to be considered. Where processing conditions are kept steady, the cycles will continue to develop until equilibria are reached, at which time the total amounts of volatile entering the system will be balances by the quantities leaving the system. The degree of volatiisation and the rates at which the equilibrium are established will depend on:
(a)
The species, their chemical forms and concentrations.
(b)
The volume of gases.
(c)
The intimacy of contact between gas and solid.
(d)
The vapour pressures of the salts.
(e)
Possibility of dissociation or further reaction.
(f)
Rate of diffusion to and from solid/gas interfaces.
(g)
Degree of saturation of gas.
(h)
Kiln atmosphere.
(i)
Kiln temperatures.
(j)
Time/temperature profile of material within the kiln.
Most of these factors are to some degree inter-related and so in normal operation the only methods available to control the degree of volatilisation and eventual concentrations in clinker and within the kiln system will be the kiln internal atmosphere and temperature, and the proportion of gas bleed from the kiln exit or material bleed from the system. As indicated above, it should be possible to modify the volatile cycle by variation of the kiln internal atmosphere (oxygen level) and burning zone temperature. In the late 1980’s a series of kiln trials were conducted at Hope Works to investigate the effect of these kiln conditions (BZT and BEO,) on the volatile cycles. This study covers the situations of significant chloride, alkali, and sulphate inputs. The major conclusions were:1.
The chloride cycle is 20 to 30 times the total chloride input and increases slightly with increasing temperature, although this may be due to improved kiln stability at higher temperatures. The chloride cycle is not modified by wide changes to kiln atmosphere (Figure 5).
2.
Chloride in the cycle combines with aetassiu~availahle, However, at low temperatures - equivalent to NOx Eve1.s‘ m o p$ii - some sodium is also involved.
3.
The chloride cycle is a low temperature cycle and cannot be significantly modified by vat-ration or rum conanions.
4.
The total potassium cycle is approximately 3 times the input level (Figure 6), however about two thirds of this is present in Stage IV in conjunction with chloride and so cannot be controlled except by incorporation of a bleed system.
5.
About one third of the potassium in Stage IV is either f& based on its first passage through the preheater or derived from a potassium sulphate based cycle. This cycle ratio varies from 1.05 (minimal recycle) at 800 ppm kiln NOx level to 1.25 at 1,400 ppm NOx (25% recycle). This portion can be controlled by operation at low temperatures (Figure 7).
6.
The sodium cycle level varies between 1.2 and 2.0 times that in the raw meal over the temperature range examined.
7.
At low temperatures some sodium becomes involved in the low temperature chloride cycle and this boosts the sodium cycle by about 10%.
8.
The majority of the sodium in the system is involved in a sodium sulphate based cycle. This is strongiy temperature dependent, and the rate of increase also appears to be increasmg with temperature. Over the temperature range investigated the recycle rose from 1.35 times the level in the feed at low temperatures to 2.0 times at high NOx levels (Figure 8).
9.
The total sulphur cycle is temperature dependent and rises from 1.6 to 2.6 times that of the input over the temperature range investigated (Figure 9). Low oxygen levels increase the suiphur cycle.
10.
The alkali sulphate cycle has already been summarised in points 6 and 9 and makes up about 35% of the total sulphate in Stage IV material. These cyclic levels can be seen to be lower than the total suiphur cycle levels (Figures 7, 8 and 9).
11.
The calcium sulphate recycle is strongiv temoerature dependent. The quantity of SO, as %&(ii$L~V rises from^-“aoout 2.5 times tne level in the feed at low NOx levels to 4 times at high (1,450 ppm) NOx levels (Figure 10).
12.
The calcium suiphate cycle is also increased by a move into a reducing kiln atmosphere. At about 1,200 ppm NOx the level of SO, as CaSO4 increased from 3.1 under oxidising conditions to 5.5 times the feed levei under reducing conditions (Figure 10).
13.
For potassium and sodium there are indications of a slow but steady increase in losses of these components from the preheater system as temperatures increase. This may become part of an external cycle or may be lost to atmosphere. This could be established by a longer term study of the levels of these components in the precipitator and stack dusts.
14.
Sulphur is in overall balance within the system below NOx levels of 1,200 to 1,300 ppm, but above this level the loss increases sharply with firing temperature. Again this may become part of an external cycle or may be lost from the system to the atmosphere, however in this exercise no precipitator or stack dust samples were collected or SO2 emission measurements made so this cannot be confirmed.
15.
When the kiln atmosphere moves into reducing conditions the losses of alkalies and sulphur from the kiln system to the atmosphere increase sharply.
Overall the results show that the chloride cycle and its associated alkali cycle cannot be controlled by process conditions. Cycles of alkali present in the form of alkali sulphate can be controlled and minimised by careful control of the burning zone to the lowest practical temperature. Calcium sulphate based cycles can also be minimised by burning to low temperatures, but in this case careful control of the kiln atmosphere {back end 02 level) is also necessary to ensure that the on-set off reducing conditions is not possible. While these tests were conducted on a suspension preheater kiln, the critical area is the burning zone and so the general results will be equally applicable to other processes. The susceptibility of calcium sulphate to increased volatilisation under even borderline reducing conditions also emphasises the potential for the sulphate cycle to be affected by the condition of the firing system, by the type of fuel, and by cooler operation. Low secondary air temperature, a low volatile fuel, poor coal drying, or low momentum in the firing pipe are likely to promote slow initial combustion, in which case significant combustion is likely to continue to occur once the jet has fully expanded. This may give reducing conditions immediately above the material bed and encourage sulphate volatilisation. Where this is possible it is recommended that the firing pipe is aiigned along the axis of the kiln in order to ensure the maximum kiln length is available for jet expansion. A “cool” flame is also likely to produce a long burning zone which will hold the potentially volatile materials for a longer time period within the temperature range at which volatilisation could occur; this will also promote higher degrees of volatilisation. 5.
CONCLUSIONS
From a Process Engineer’s point of view, the easiest way to minimise kiln volatile cycles is to push the problem to the Works Chemist and expect the raw materials to be selected to give the minimum practical inputs of potentially volatile materials. This also requires the levels that do exist in the raw materials to be balanced to give the maximum opportunity for the formation of compounds with relatively low specific volatilities. The quantities of material recirculating can also be controlled by the selective removal of a dust fraction from the process. Examples of this are the selective dumping of the finest precipitator dust fraction on the wet process at Ravena, and the dumping of Lepoi cyclone dust at Cookstown. The magnitude of the volatile cycles can also be reduced by the careful control of kiln conditions. In general volatile cycles will increase slowly with increasing burning zone temperature or length. There will also be a major increase in sulphate cycle if reducing conditions develop close to the material bed. This increase will start to occur well before a significant increase in kiln backend CO level becomes apparent. This emphasises the need to possess optimised fuel preparation and firing systems and an efficient clinker cooler.