Cement Chemistry Handbook - Fuller

FULLER - CEMENT CHEMISTRY - HANDBOOK

CEMENT CHEMISTRY Table of Contents 1. INTRODUCTION 1.1 INTRODUCTION 1.2 DEFINITIONS - cement, concrete, cement types, raw materials etc. 2. COMPOSITION 2.1 COMPOSITION - basic calculation/formulas - chemical shorthand etc. 2.2 MODULES 2.3 MINERAL COMPOSITION 3. TYPES OF CEMENT 3.1 TYPES OF CEMENT 3.2 CEMENT STANDARDS 3.3 CEMENT QUALITY - MAIN FACTORS 4. MANUFACTURE 4.1 MANUFACTURE OF CEMENT - grey, mixed and white cement - wet, dry and semi-dry process 4.2 RAW MIX 4.3 RAW MATERIALS 4.4 CHEMICAL COMPOSITION AND CONTROL OF RAW MIX 4.5 PHYSICAL CONTROL AND COMPOSITION OF RAW MIX 4.6 BURNABILITY OF RAW MIX 4.7 CLINKERISATION 4.8 INFLUENCE OF THE RAW MIX ON CLINKER FORMATION AND BURNABILITY 5.PROCESS AND KILNS 5.1 TYPES OF KILNS - wet, dry & semi-dry 5.2 WET KILN - main features - process 5.3 DRY KILN 5.3.1 LONG KILN 5.3.2 SP KILN 5.3.3 ILC-E KILN 5.3.4 ILC KILN 5.3.5 SLC KILN 5.3.6 SLC-S KILN 5.3.7 SLC-I KILN 5.4 ASH ABSORPTION 5.5 VOLATILE MATTER

Chemistry Bible Rev.0; 7 Dec 00

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FULLER - CEMENT CHEMISTRY - HANDBOOK 5.6

MAIN FEATURES DURING BURNING

6. HEAT OF REACTION & HEAT TRANSFER 7.FUEL 7.1

TYPES OF FUEL 7.1.2 COAL 7.1.3 FUEL OIL 7.1.4 GAS 7.1.5 WASTE FUELS

8.COMBUSTION 9. COAL & OIL 9.1 9.2 9.3 9.4 9.5 9.6

FINENESS OF COAL DRYING OF COAL ASH CONTENT GAS CONTENT MINOR COMPONENTS REQUIREMENT FOR AIR

10. PROCESS GAS


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1.1

INTRODUCTION

Cement is a material that binds together solid bodies (aggregate) by hardening from a plastic state. Many materials act as adhesives or cement according to this definition. The cement referred to above, which is used for civil engineering and the construction industry, is portland cement. Portland cement is hydraulic and develops strength primarily by the hydration of the di- and tri-calcium silicates it contains. Hydraulic means that the paste of cement and water will harden under water. Lime, on the other hand, will harden due to the reaction with carbon dioxide from the air. In 1793, an Englishman by the name of Smeaton found that a form of hydraulic cement was produced when a mixture of limestone and clay were burned together. He was the first producer of portland cement. Another Englishman, Joseph Aspdin obtained the first patent for portland cement in 1824. He burned a mixture of chalk and clay in a lime kiln to form a clinker and ground the clinker to cement. Aspdin is regarded as the inventor of portland cement. The early product did not have the same quality of cement currently produced today. Two main developments have improved the quality of cement: •

Addition of gypsum to regulate the setting time

•

Higher burning temperature to obtain higher lime containing silicates

Today, portland cement (portland cement clinker with a small addition of gypsum) is used worldwide and is the primary construction material for roads, bridges, runways, tunnels, and buildings. In addition, blended cements are produced using portland cement clinker, a small amount of gypsum and another additive such as blast furnace slag, fly ash, or other pozzolans. Some blended cement has improved performance over ordinary portland cement with greater resistance to alkali and sulfate attack. 1.2

DEFINITIONS - cement, concrete, cement types, raw materials


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Cement:

cement is a mix of cement clinker and gypsum ground together into a powder.

Gypsum:

calcium sulfate is used to regulate setting time.

Concrete:

concrete is a mix of cement (13%), water (7%), fine aggregate –6mm (32%), and coarse aggregate +6mm (48%) which hardens to solid mass

Cement types:

there are numerous types of cement for many different applications. ASTM defines the chemical and physical requirements

Ordinary

general-purpose concrete

Rapid hardening

precast concrete

Alkali resistant

used with reactive aggregates

Sulfate resistant

used in applications requiring resistance against sulfate attack

White cement

special architectural concrete

Low heat

massive concrete like dams

Masonry cement Blended cements

mortar bonding brick or block

use an addition of slag or other pozzolans to achieve some of the properties of alkali or sulfate resistance

Raw Materials:

The common raw materials for manufacture of Portland cement are: Limestone / Marble / Marl Clay / Shale Sand Iron ore or pyrite ash

Limestone and clay are primarily found near the plant, where as, sand and iron ore are usually brought in because of their preferred chemical composition.


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FULLER - CEMENT CHEMISTRY - HANDBOOK Today, there is considerable interest by our society for the reuse of waste materials. Some of these can be utilized in the raw mix feed to the kiln. Other materials such as waste oils and solvents can be safely disposed of in the burning zone of the rotary kiln because of the high temperatures. 2.1 COMPOSITION - basic calculation/formulations - chemical shorthand The most common compounds in cement chemistry are: Formula

Name

Source

SiO2

silicium dioxide

quartz, sand

Al2O3

aluminium oxide

Fe2O3

iron oxide

CaO

calcium oxide

lime

CaCO3

calcium carbonate

limestone, marble, marl

MgO

magnesium oxide

MgCO3

magnesium carbonate

K2O

potassium oxide

Na2O

sodium oxide

H2O

water

N2

nitrogen

79 volume percent in air

O2

oxygen

21 volume percent in air

CO

carbon monoxide

CO2

carbon dioxide

SO2

sulfur dioxide

A typical analysis of clinker is given in the table below:


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FULLER - CEMENT CHEMISTRY - HANDBOOK Table 1 CLINKER COMPOSITION SiO2 Al2O3 Fe2O3 CaO MgO K2O+Na2O SO3 Insoluble residue Ignition loss Total Free CaO

Clinker % 22.51 5.16 2.55 64.97 2.61 1.7 0.22 0.1 0.06 99.88 0.8

Min limit % 17 4 0.1 62 0 0.3 0 0 0

Max limit % 26 10 5 69 4 2 1.5 1 3

0

2.5

* In the table, the term ignition loss is defined as the loss in weight by heating the sample to a temperature of ~900°C.

2.2 MODULES Modules determine the proportioning of the raw mix. The three commonly used modules are: SILICA MODULUS

MS

ALUMINA MODULUS

MA

LIME SATURATION FACTOR

LSF

The silica modulus is defined as the ratio of silica to the sum of alumina and iron oxide:

MS=

SiO2 Al2 O3 + Fe2 O3

The silica modulus for clinker is normally between 2.4 and 2.7. The amount of melt phase in the burning zone is a function of MS, when MS is high; the amount of melt is low. Therefore, when the MS is too high, the formation of nodules might be too slow, resulting in less nodulization. The material becomes dusty and impedes the kiln operation. Generally, a higher MS relates to a harder burning mix.


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FULLER - CEMENT CHEMISTRY - HANDBOOK The alumina modulus is defined as:

MA =

Al2 O3 Fe2 O3

The alumina modulus for clinker varies between 1.6 and 2.0. The temperature by which the melt forms depends on the MA. The lowest temperature is obtained when the MA is 1.6. The MA also affects the color of clinker and cement. The higher the MA, the lighter the cement. The lime saturation factor, LSF, is defined as the ratio of available lime to the theoretical lime required by the other major oxides in the raw mix to form clinker and is determined from the following equation:

CaO LSF= 2.8* SiO2 + 118 . * Al2 O3 + 0.65* Fe2O3 The LSF is usually expressed as a percentage. The LSF for clinker is in the range of 88 and 98%. The theoretical maximum amount of lime, CaO, that can be combined with the acidic oxides has been derived from phase diagrams and can be calculated as follows: CaOmax = 2.80*SiO2 + 1.18*Al2O3 + 0.65*Fe2O3 For cement, a variation of the previous equations has to be used to account for the addition of gypsum to cement. The amount of CaO has to be reduced by the amount bound in gypsum CaSO4. The LSF formula for cement is then:


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CaO − 0.7 * SO3 2 .8* SiO2 +1.18* Al2 O3 + 0.65* Fe2 O3

LSF =

2.3 MINERAL COMPOSITION During burning in the cement kiln, the lime is combined with the acidic oxides to form cementitious minerals. The main minerals are: Table 2 CLINKER MINERALS Formula

2

Alite Belite

2CaO.SiO

Tricalcium aluminate

3CaO.Al O

Calcium alumino ferrite

3

formula

weight %

weight %

70

35

45

20

18

3

15

1

2

0.5

2

CS 3

3

CA 2

3

4

4CaO.Al O .Fe O

Free lime

Min

CS 2

2

Max

3

3CaO.SiO

2

Short

C AF

CaO

Note: The short formula designation is used throughout the cement industry.

It is possible to calculate the amount of the four main minerals by using the BOGUE formulas, however, the calculation is only approximate. The actual amount present of any mineral can be determined by microscopy or faster by Xray diffraction. Using % wt., the BOGUE formulas for clinker are:

3

CS=

2

2

3

2

4.07*CaO –7.60* SiO – 1.43* Fe O – 6.72* Al O


8

3

FULLER - CEMENT CHEMISTRY - HANDBOOK 2

CS=

2

2

3

2

3

8.60* SiO + 1.08* Fe O + 5.07* Al O – 3.07*CaO 2

2

3

(or C S= 2.867*SiO -0.7544* C S ) 3

CA =

2

2

3

2.65*Al O - 1.69* Fe O

4

C AF =

3

2

3

3.04*Fe O

As mentioned above, other minerals are present and have to be taken into 4

account. These are free lime (CaO) , calcium sulfate (CaSO ) and periclase (MgO). The free lime should be under 2.0%. A higher amount indicates poor burning or faulty composition of the raw mix. Too high a free lime will result in volume instability in the cement mortar or concrete. Calcium sulfate in cement 3

comes from the addition of gypsum and from clinker. In clinker, SO comes from 2

the sulfur in the fuel used and from sulfur in the raw materials (pyrites FeS ). MgO can cause late expansion in concrete. A maximum of 6% MgO is allowed in ordinary portland cement. Cements with higher MgO contents 4-5%, rapid cooling of the clinker is of more value for offsetting the adverse effects of high magnesia contents. For other types of cement, the maximum limit varies.

The minerals in clinker are mainly found as crystals. A smaller amount is present in the so-called glass phase. During burning, some liquid phase is formed at the high temperature in the burning zone (BZ). The liquid phase ranges from 20 to 27% in normal clinker, however, some of the liquid does not have time to form crystals during cooling. The liquid phase @ 1450°C is determined by the following equation:


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2

2

3

% Liquid Phase = 3.00A + 2.25F + (MgO + K O + Na O + SO )

3. TYPES OF CEMENT

The clinker from the kiln system is ground to cement with a small addition of gypsum in order to control the time of setting. If no gypsum is present, the cement will set rapidly when water is added. The amount of gypsum added is 3 to 5% by weight. The formula for gypsum is: 4

2

CaSO ,2H O 4

- which is the di-hydrate form

2

CaSO ,0.5H O

- which is the hemi-hydrate form (plaster of paris)

4

CaSO

- which is the anhydrite form

The gypsum molecule is crystallized with two molecules of water. Gypsum occurs in nature as gypsum rock. Other sources of gypsum are waste gypsum from exhaust gas desulfurisation at power plants, surplus gypsum from fertilizer factories or cement manufacturing exhaust gas scrubbers. Other materials can be added to clinker and gypsum to make cement. If the addition rate is small, the product can still be called portland cement.

The most common types of cement can be divided into three main groups:

a) PORTLAND CEMENTS:


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ORDINARY PORTLAND CEMENT RAPID HARDENING CEMENT SULFATE RESISTANT CEMENT LOW HEAT CEMENT WHITE CEMENT

b) COMPOSITE CEMENTS BLASTFURNACE CEMENT FLY ASH CEMENT POZZOLAN CEMENT OTHER BLENDED CEMENTS

c) OIL WELL CEMENTS

a) PORTLAND CEMENTS are by far the most common type of cements produced around the world. The most widely used type is ORDINARY PORTLAND CEMENT. RAPID HARDENING CEMENT is a portland cement that develops strength faster than ordinary portland cement. It is manufactured by grinding the clinker and gypsum finer in the cement mill. Good clinker quality is needed and the addition of gypsum is a little higher to maintain setting time and increase strength development. SULFATE RESISTANT CEMENT is used where a higher resistance is required


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to sulfate-bearing waters. The cement composition has a lower content of C A, less than 8%. LOW HEAT CEMENT is used where low heat development during hardening is required, i.e. in large concrete structures like dams. The rate of strength development is lower than that of ordinary portland cement. Although, the final strength may be higher. WHITE CEMENT is used where a white color is wanted for a facade of buildings. It is manufactured from raw materials with low content of iron chromium and manganese.

b) COMPOSITE CEMENTS also known as blended cements start with Portland cement clinker and gypsum but also have the addition of another material. Some additions become hydraulically activated as they react with the Portland cement clinker. To mention a few: Blastfurnace slag cement is a cement made by grinding together portland cement clinker and granulated blastfurnace slag in the proper proportions. Masonry cement is a combination of portland cement clinker, limestone, and a small addition of an air-entraining agent to produce a more workable, rapid hardening mortar than ordinary portland cement. It can also be made by intergrinding mixtures of portland cement with hydrated lime, granulated slag, or other waterproofing agents with inert fillers. Pozzolanic cements are produced by mixing together portland cement and a pozzolana. A pozzolana is a material which is capable of reacting with lime in


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the presence of water at ordinary temperatures to produce cementitious compounds.

c) OIL WELL CEMENTS Oil well cements are used for cementing the steel casing of gas & oil wells to the walls of the bore-hole and to seal porous formations. Usually, portland cements more coarsely ground than normal, with the addition of special retarders to allow for slow-setting conditions are used.

Cement standards define the various types of cement. Today, there are two main standards: • •

The US standard defined in ASTM C 150 The European standard EN 197-1

ASTM uses 5 main classes for Portland cement. There are also a number of composite or blended cements defined by ASTM. The European standard has three main strength classes each divided into two subgroups or 6 classes in all.

Tables 3 and 4 show some of the physical requirements defined by ASTM. Table 3

ASTM CEMENT TYPES Compressive Strengths – for ASTM C109 Cubes Number

Type of Cement


3 days

7 days

28 days

compressive

compressive

compressive

strength

strength

strength

13


MPa

MPa

I

Ordinary Portland cement

12

19

IA

OPC air entraining

10

16

II

Moderate sulfate resistant

10

17

14

MPa

Moderate heat of hydration II A

II + air entraining

8

III

High early strength

24

III A

III + air entraining

19

IV

Low heat of hydration

V

High sulfate resistance

8

7

17

15

21

Note: SI units are the standard .To convert to psi : SI * 142.23.

Table 4 ASTM CEMENT TYPES Air Content, Fineness, Soundness and Setting Time


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ASTM

Air content

TYPE

Fineness

Autoclave

Setting

Blaine

expansion

minutes

2

vol% I IA II II A III III A

m /kg

%

Initial ,min

max

Max 12

>280

<0.80

45

375

Min=16,max= 22

>280

<0.80

45

375

Max 12

>280

<0.80

45

375

Min=16,max= 22

>280

<0.80

45

375

Max 12

<0.80

45

375

Min=16,max= 22

<0.80

45

375

IV

Max 12

>280

<0.80

45

375

V

Max 12

>280

<0.80

45

375


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Table 5 European Standard EN 197-1 Types of Cement Cement type

Designation

Notation

Clinker , K ,%

Additive

I

Portland cement

I

95-100

Gypsum Filler

II

Portland slag

II/A-S

80-94

S=Granulated

cement

II/B-S

65-79

blastfurnace slag

Portland silica fume

II/A-D

90-94

D=Silica fume

Portland pozzolana

II/A-P

80-94

P=Natural pozzolana

cement

II/B-P

65-79

II/A-Q

80-94

II/B-Q

65-79

Portland fly ash

II/A-V

80-94

cement

II/B-V

65-79

II/A-W

80-94

W=calcareous

II/B-W

65-79

fly ash

Portland burnt shale

II/A-T

80-94

T=burnt shale

cement

II/B-T

65-79

Portland l/st cement

II/A-L

80-94

II/B-L

65-79

Portland composite

II/A-M

80-94

cement

II/B-M

65.79

Blastfurnace cement

III/A

35-64

III/B

20-34

III/C

5-19

IV/A

65-89

IV/B

45-64

V/A

40-64

V/B

20-39

cement

III

IV

V

Pozzolanic cement

Composite cement


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Q=industrial pozzolana

V=siliciceous fly ash

L=limestone

M=different materials


All of the above European types of cement are the result of many years of work on a unification for the different standards. In European standards, the three main strength classes are divided into two subgroups or 6 classes in all.

Table 6 European Standards for Strengths Type

Class

2 days

7 days

28 days

compressive

compressive

compressive

strength

strength

strength

MPa

MPa

MPa

>=16

32.5-52.5

I,II,III,IV, & V

32.5

I,II,III,IV, & V

32.5 R

>=10

32.5-52.5

I,II,III,IV, & V

42.5

>=10

42.5-62.5

I,II,III,IV, & V

42.5 R

>=20

42.5-62.5

I,II,III,IV, & V

52.5

>=20

>=52.5

I,II,III,IV, & V

52.5R

>=30

>=52.5

For both ASTM and EN, there are a number of other physical requirements. For these it is necessary to reference the two standards. The standards also specify some chemical requirements. For ASTM they are:


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Table 7 ASTM Chemical Specifications Cement type

I and I A

II and II A

III and III A

IV

V

2

SiO , min.% 2

20.0

3

Al O , max % 2

6.0

3

Fe O , max% MgO, max%

6.0 6.0

6.0

6.0

6.0

6.0

3.0

3.0

3.5

2.3

2.3

3.5

Not applicable

4.5

Not applicable

Not applicable

Ign.loss, max%

3.0

3.0

3.0

2.5

3.0

Ins.res.max%

0.75

0.75

0.75

0.75

0.75

3

SO , max% 3

C A <= 8% 3

SO , max% 3

C A > 8%

3

C S ,max%

35

2

C S,min%

40

3

C A, max% 4

8

15

7

5

3

C AF+2(C A),max%

25

Optional requirements: 3

C A max%,for

8

moderate sulfate res.


18


C A max%,for high

5

sulfate rest. 3

3

C S+C A, max

58

%,mod. heat of hydration 2

2

Na O+0.658K O,

0.60

0.60

0.60

0.60

0.60

max%, low alkali

3.3 CEMENT QUALITY - MAIN FACTORS

Cement quality is just as important for the manufacturer of the product as it is for the producers of concrete and concrete products, contractors and the private consumer.

The cement manufacturers have to meet the standards, the clients requirements and at the same time be competitive. The plants also have a role in fulfilling environmental requirements and assisting in the disposal of various waste products.

The consumer has a range of requirements for the cement depending on the type of concrete product, i.e. all purpose ready mix, precast, or pumped concrete to mention a few.

The end user wants a durable concrete which will stand up to heavy usage on infrastructure, be frost resistant, withstand alkali aggregate reaction and at the same time have a good appearance in the finished structure. Chemistry Bible Rev.0; 7 Dec 00

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The cement quality depends on the clinker manufacturing process, the cement milling, the fineness, and any changes to the cement after milling.

Composition can also vary within a single type of cement. Even ordinary portland cement has subtle differences. For example, the gypsum addition rate and with limestone where up to 5% might have been added during grinding.

The cement fineness can be varied during grinding with a finer product reacting faster. Fineness is expressed as: specific surface area (Blaine), residue and particle size distribution.

The chemistry of the clinker is important. The clinker mineral composition has to be considered. The main minerals in the clinker are: •

3

2

3

4

C S, C S, C A,C AF

The main reactions at hydration in an idealized form are:

3

2

x

y

2

1) C S + (3-x+y) H O = C SH + (3-x) Ca(OH) 2

2

x

y

2

2) C S + (2-x+y) H O = C SH + (2-x) Ca(OH) 3

2

2

8

4

19

3) C A + 13.5 H O = ½ (C AH + C AH ) 3

2

3

6

4) C A + 6 H O = C AH 3

2

2

3

19

5) C A + Ca(OH) + 18 H O = C AH


20


4

2

3

4

31

6) C A +3 CaSO + 31H O = C A.3 CaSO .H 3

4

2

3

4

12

7) C A + CaSO + 12H O = C A. CaSO .H 4

3

8) C AF – reactions similar to C A 2

2

9) CaO + H O = Ca(OH) 2

2

10) MgO + H O = Mg(OH) 4

2

4

2

11) CaSO + 2 H O = CaSO ,2 H O 4

1

2

2

1

2

2

4

2

12) CaSO , / H O + 1 / H O = CaSO ,2 H O

In all of the above reactions, water reacts with the hydration product minerals increasing the volume of the solid phase. Each reaction differs in velocity, volume change, and in the nature of the hydration products. These reactions are the background for the setting time and strength development to the solid state from the plastic phase when water is first added to the cement.

Of prime importance is the state of the gypsum, as di-, hemi-, or anhydrite, in the cement as it first reacts with water. Depending upon that state, the gypsum and 3

C A reactions with water can result in normal, false or flash set.

False set is the premature stiffening of the cement paste due to most of the gypsum being either hemi-hydrate or soluble anhydrite due to overheating. Upon mixing with water, crystallization of reformed gypsum causes stiffening. This stiffening can be broken upon remixing without additional water. False set can happen either by fast set of gypsum hemi-hydrate or because of a slow reaction


21


3

between C A with water. The slow reacting C A can be caused by prehydration 2

or by carbonation with CO . False set can be prevented by lowering the mill exit temperature, thereby, reducing the degree of gypsum dehydration, the amount of gypsum added to the mix, or replacing part of the gypsum with a natural insoluble anhydrite.

3

On the other hand, flash set occurs if the C A is more reactive than gypsum with water. The setting is characterized by a high evolution of heat and short setting time. Flash set can be prevented by adding more gypsum to the cement or by dehydrating the gypsum to a more reactive form, i.e. hemi-hydrate or anhydrite.

Therefore, as the cement is being ground, the mill material temperature must carefully be controlled. Between 90-130°C, the gypsum changes into calcium sulfate hemihydrate (plaster of paris) by releasing 1.5 molecule water: 4

2

CaSO ,2H O 4

4

2

2

CaSO ,½ H O + 1½ H O

2

CaSO ,½H O

4

(a)

2

CaSO + ½ H O

(b)

The cement mill material temperature is controlled primarily by cooling the mill with an internal water spray in the second compartment. Additional cooling is accomplished with air in the separator. The cement mill exit temperature should not reach 130°C and is usually targeted at 110°C.


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Finally, if the cement material temperature has not been controlled in the mill system, the cement might enter the storage silo at too high a temperature causing dehydration of the gypsum. The rate of the transformation increases with temperature and with falling dew point. The change after equation (a) is rapid at a temperature of 90-130°C. The water molecule released can give rise to formation of lumps in the cement and to scaling on the storage silo wall by 2

4

.

. 4

2

formation of Syngenite K SO CaS0 H O.

4. MANUFACTURE 4.1

MANUFACTURE OF CEMENT - grey, mixed and white cement - wet, dry and semidry process

The manufacture of Portland cement is divided into the three main parts: a) b) c)

Preparation of the kiln feed Burning in the kiln Grinding of the clinker with gypsum and other additions

The description here will concentrate on the process for ordinary grey cement with some comments on the other types of cement. The dry process is used to make the majority of the cement produced in the world. The wet process, however, is still used where fuel cost has allowed it. The wet process can furthermore be justified where the raw materials are very wet such as chalk, a soft limestone, and clay. An intermediate solution is the semi-dry process where the raw mix is prepared as slurry. The slurry can then be filtered to remove a portion of the water before the burning or the slurry may be pumped directly into a dryer crusher working in unison with the kiln. 4.2

RAW MIX


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FULLER - CEMENT CHEMISTRY - HANDBOOK The raw mix must have a composition that will produce a clinker of the proper analysis. The difference in the composition of the raw mix and the clinker is threefold. First, is the change in each of the materials as they are heated up in the kiln. The changes are due to a loss in weight mainly from the release of carbon dioxide and water. Second, is a change due to absorption of ash from coal used as fuel. There is also a change due to absorption of sulfur in the fuel. Finally, there is a change due to the small dust loss in the exhaust gas. Some of this dust is returned to the process but some might be wasted as in a bypass. In a wet process, the dust may be discarded in order to reduce the alkali content in the clinker. The raw mix must therefore compensate for these changes and losses; otherwise, the clinker will not have the correct chemical and mineralogical composition. The way in which this is done will be explained below. 4.3

RAW MATERIALS

Many raw materials are suitable for the production of cement. In principle, as long as they can be mixed to give the right composition of the clinker, they can be used. There are some restrictions naturally. They must be available in large quantities and be economically feasible. In addition, there might also be restrictions on use due to minor components in the raw materials. Limestone is the largest component used in producing cement. It is available as CaCO3 in marble, limestone, chalk and marl. Limestone is sometimes found together with magnesium carbonate. Only small amounts of MgO can be tolerated in cement due to the risk of the expansion reaction in the concrete. Limestone containing a large amount of magnesium carbonate is called dolomite. In nature, limestone is found in many places mixed with clay and/or shale. Clay and shale contain SiO2, Al2O3 and Fe2O3. In some cases a type of limestone is found that is quite close in chemical composition to the cement composition. When the limestone is of a higher purity than the requirement for the raw mix,


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FULLER - CEMENT CHEMISTRY - HANDBOOK then other raw materials must be added to the mix. The amount of limestone is calculated using the formula below:

CaO LSF= 2.8* SiO2 + 118 . * Al2 O3 + 0.65* Fe2O3 Sand is a mineral very rich in silica, SiO2. It is a very hard and abrasive mineral. It is used when the mix is insufficient in silica. It will increase the MS or the silica ratio:

MS =

SiO2 Al 2 O3 + Fe2 O3

Iron can be used in the form of iron ore, usually an iron oxide, or as a waste product from the fertilizer industry, such as pyrite ash. It is used to regulate or reduce the alumina modulus, the ratio of alumina to iron oxide:

MA =

Al2 O3 Fe2 O3

Bauxite an alumina mineral rich in Al2O3 and can be used to increase the MA. Fly ash, one of the waste materials from the power generation industry is also used as a raw material. This is known as pulverized fly ash, PFA. Typically, this is higher in SiO2 content. The number of components used in the raw mix is typically 4-5 raw materials depending upon the need for correction of the three main modules: LSF, MS, MA. The physical nature of a raw material is also important. Very wet materials can be the reason for choosing the wet or semi-dry process. Very abrasive materials like sand and basalt are costly to crush and grind to the fine state needed in the raw mix. The chemical variation in the raw material is also important. If there are great variations, more homogenization will be required.


25

FULLER - CEMENT CHEMISTRY - HANDBOOK 4.4

CHEMICAL COMPOSITION AND CONTROL OF RAW MIX

The chemical composition of the raw mix has to be prepared correctly to yield a good clinker. Also, the variation in the raw mix going to the kiln has to be small to obtain good burning conditions for the kiln and preheater. The first step in the mix design is the determination of the chemical composition of the raw materials. It is common to have an approximate analysis of each raw material and use this for the calculation of the mix ratios. A sample after the raw mill is easily obtained for analysis. An analysis can be performed quickly using X-ray fluorescent analysis, XRF. Timely adjustments can then be made to the raw mill weighfeeders. A typical calculation of a raw mix and the corresponding clinker can be made using a simple spreadsheet like EXCEL with its SOLVER function, i.e. as shown in table no.8 below. The calculations made in the table show that 5 raw materials have been available at this plant. This has made it possible to satisfy 4 conditions, one less than the number of raw materials. The four conditions here are: LSF

=

94

MS

=

2.75

MA

=

1.90

Na2O+ 0.658*K2O

=

0.64

The alkalies often have to be restricted to satisfy a requirement for low alkali cement. Low alkali cement is needed when the aggregate contains reactive silica, which can give an expansion in concrete.


26


Table 8 Calculation of Raw Mix RAWMIX NO:

1

HEAT CONSUMPTION COAL HEAT VALUE Hi COAL ASH

750 7420 9.4

Limeston e 980257

Shale

Fly ash

Sand

Bottom ash

980576

980578

980575

980577

SiO2 Al2O3 Fe2O3 CaO MgO Mn2O3

5.52 0.58 0.23 51.81 0.36 0.02

54.84 15.12 7.33 0.81 1.33 0.07

54.58 26.72 9.42 1.49 0.71 0.08

78.41 3.76 1.70 5.84 1.43 0.02

39.39 18.30 28.69 3.34 0.76 0.14

14.47 3.40 1.64 43.49 0.47 0.03

TiO2 P2O5 K2O Na2O SO3 LOI @900 oC TOTAL ClC Free CaO LSF(SO3) LSF Na2O+0.658*K2O MS MA

0.03 0.02 0.14 0.06 0.19 40.74 99.70 0.006 0.03

0.83 0.04 2.76 0.34 8.71 7.95 100.13 0.012 0.07

1.56 0.17 2.05 0.35 0.45 2.15 99.73 0.009 0.68

0.17 0.02 1.07 0.88 0.08 6.53 99.91 0.006 0.17

0.83 0.12 1.50 0.31 4.14 2.44 99.96 0.030 0.17

X-PLANT

Raw meal

Coal ash

(LOI free)

970988

22.14 5.19 2.51 66.53 0.72 0.04

36.28 17.09 31.98 8.48 0.79 0.13

22.28 5.31 2.79 65.98 0.73 0.04

0.19 0.03 0.44 0.13 0.61 34.64 99.53 0.007 0.085

0.29 0.05 0.67 0.20 0.93 0.00 99.29 0.010 0.130

0.78 0.11 1.09 0.32 2.48

0.29 0.05 0.68 0.20 0.95 0.00 99.29 0.010

94 95

94 95

2.87 2.07

2.87 2.07

Raw meal

317 318

-3 0

1 1

3 3

0 2

6.81 2.52

2.44 2.06

1.51 2.84

14.36 2.21

0.84 0.64

Clinker

99.53 0.000

0.00 93 93.7 0.64 2.75 1.90

CaO(SO3) C3S C2S C3A C4AF Min. weight% Weight % (X) Max. weight %

65.32 56.87 21.28 9.34 8.49 80.00 83.11 90.0

4.00 4.09 10.0

2.50 7.00 7.0

1.00 4.14 5.0

0.00 1.47 5.0

99.80 99.80 100.2

99.05

0.95

100.00

The chloride content is also shown. Chloride is a volatile component and can form coatings together with alkalies in the preheater. The chloride content has to be restricted in preheater kiln systems to 0.015% by weight in the kiln feed. . 4.5

PHYSICAL CONTROL AND COMPOSITION OF RAW MIX

The raw mix must contain the proper fineness and be homogenized before Chemistry Bible Rev.0; 7 Dec 00

27

Setpoint

94 0.67 2.75 1.90

FULLER - CEMENT CHEMISTRY - HANDBOOK entering the kiln. For the old fashioned wet process and for the semi-dry process, the kiln feed is slurry with water content of 33 to 40%. In the dry process, the kiln feed is a dry powder with a typical moisture content of 0.5 to 1.5%. The fineness of the raw mix is measured on sieves .The normal sieves with respective residues used are: 90µ or 0.09 mm

10-20% retained

(equivalent to ASTM 170)

200µ or 0.20 mm

0.5-1.5% retained

(~ equivalent to ASTM 70)

• •

Also, the composition of the residue is important. Free silica (quartz) will for instance result in poor reactivity or burnability of the material in the burning zone. When the coarse particles have a similar composition to the kiln feed (less quartz) then a greater amount of residue can be tolerated. In the wet process, the slurry moisture should be as low as possible but still be transportable via slurry pumps. The amount of water that is evaporated from slurry with a moisture content of 30% is 0.66 kg/kg clinker, while a moisture content of 35% requires evaporation of 0.83 kg water/kg clinker. 4.6

BURNABILITY OF RAW MIX

The reactivity of the kiln feed for slurry or raw meal is checked by the burnability test in the laboratory. The procedures can vary from different kiln suppliers but in principle a few small nodules are made of the raw mix ground to a fixed sieve residue. Usually, three sets of nodules each of different sieve residues (5, 10 and 15% residue on 0.09-mm sieve) are burned for a given time and at a given temperature. The clinker formed is then crushed and ground determining the amount of free CaO. The free CaO is compared to the free CaO content expected or found on a standard raw mix and classified as hard, normal or easy burnability. If the raw mix is hard to burn, then the raw mix may have to be ground finer or the composition might have to be altered. The first changes would normally be made to the MS or LSF. The Chemistry Bible Rev.0; 7 Dec 00

28

FULLER - CEMENT CHEMISTRY - HANDBOOK burnability can be estimated from the below formula: CaO (1400oC) = 0.35*(LSF-96)+1.58*(MS-1.6)+0.55*(A44)+0.12*(R125) where: A44= acid insoluble residue> 44µ in % R125 = total residue > 125µ in %

4.7

CLINKERISATION

The reaction zones that occur as the raw mix is fed to the pyro system are: 1)

Drying Zone: < 100°C, evaporation of free water

2)

Preheating Zone: 100-750°C, loss of bound water in clays

3)

Calcining Zone: 750-1000°C, decomposition of carbonates CaCO3, MgCO3 and others in the calcination zone. The CO2 leaves the kiln with the exhaust gas. CaO and MgO are formed.

4)

Burning Zone: 1000-1450°C, some liquid is formed and the fusion forms clinker minerals C2S and then C3S.

5)

Cooling Zone: 1450-1300°C, the melt solidifies and the material crystallizes, cooling zone.

For dry preheater kilns without a precalciner, the material entering the rotary kiln is 40 to 50% calcined. When a calciner is installed, the material is 80 to 95% calcined when entering the kiln. A calciner temperature of ~ 875°C will usually result in a calcination of approximately 9095%. The advantages of a modern dry kiln with a preheater compared to a long wet kiln are: - Smaller kiln - Lower fuel consumption - Less replacement of refractory due to longer lining life in the burning zone - Better process control - Larger production capacity The advantages of a precalciner dry kiln compared to a preheater kiln are: - Smaller kiln


29

FULLER - CEMENT CHEMISTRY - HANDBOOK - Better and easier process control - Longer refractory life in the burning zone - Two step firing with approx. 60% firing in the calciner and 40% firing in the kiln. - Larger production capacity 4.8

INFLUENCE OF THE RAW MIX ON CLINKER FORMATION AND BURNABILITY

The clinker formation is very important for plant operation and for cement quality. Fine dusty clinker will be difficult to handle in the grate cooler and a large dust circulation may start between the cooler and the burning zone. The coating in the burning zone can become quite porous and unstable. Grinding of fine clinker calls for a higher power consumption in the cement mill. The two factors determining the clinker formation and the clinker size are: a)

Agglomeration and nodulization in the burning zone due to liquid formation 3

b)

Formation and growth of C S crystals working against nodulization

The nodulization depends on the liquid to bind the fine particles together. The formation is a function of particle size and the amount of liquid. In the rotary kiln, o

the liquid phase will start forming around 1340 C and amounts to 20-25 %. An


30


increase in temperature does not increase the amount of melt substantially. 3

Formation of C S starts, the rate increases and the size of the crystals increase. The formation and growth of crystals in the burning zone eventually stops the agglomeration. Four main characteristics of clinker formation are:

•

Alite size: measure of kiln burning zone temperature rise 1200-1450°C where belite is combining w/ CaO to form alite crystals. Rapid heating is desirable w/ alite size ranging from 15-20µ , whereas, slow heating results in sizes of 60µ or greater.

•

Belite size: reflects retention time in burning zone above 1400°C. Maximum retention time is desired w/ average crystal length of 25-40µ . Shorter retention time = 5-10µ .

•

Belite color: rate of initial cooling to below 1000°C. Rapid cooling is desired resulting in clear crystals. Slower cooling gives yellow to amber colored crystals.

•

Alite Birefringence: difference between refractive indices of blue/red light


31


related to maximum kiln temperature which is desired, birefringence of 0.0080.010. A cooler burning zone yields ~ 0.002. Figure 1

Melt Formation as a Function of Temperature

Figure 2

% Liquid as a Function of MS, MA and LSF


32


The raw mix chemistry has a major influence on the process. A high MS will result in less liquid phase formation and require a higher burning temperature. Lowering the MS will give better burning and nodulzation. A lower MA results in a 3

higher liquid phase at a lower temperature. A higher LSF will give more C S formation. Depending upon the level of LSF, a higher LSF will result in a higher burning zone temperature and above a certain level the nodulization is impeded and the clinker gets more dusty.

5. PROCESS AND KILNS 5.1

TYPES OF KILNS - wet, dry & semi-dry

The dry process is used predominantly today because of the lower heat consumption and the better process control compared to the wet process. The wet process is only used when fuel is very cheap or the raw materials are very wet not making it economically feasible to replace it. 5.2

WET KILN

The wet kiln was for many years the standard equipment in the industry. Fuel was cheap and the process of slurry preparation was easy. Homogenization in silos and large slurry basins blended the slurry perfectly. The wet kiln had to perform drying, preheating, calcination, burning and often clinker cooling in one piece of equipment. However, the wet kiln has some limitations:


33

FULLER - CEMENT CHEMISTRY - HANDBOOK -Heavy fuel consumption -Process control difficult -Production over 1500 tpd clinker difficult -High refractory cost 5.3

DRY KILN

A brief review of the FLS/FULLER dry kiln types is found in table no.9 below. The table is included because there is a connection between cement chemistry and the choice of kiln type. The layout of the different kiln types is shown on the figure on enclosure 2. 5.3.1 LONG DRY KILN The long dry kiln with a cross section for heat exchange is not installed any more. It has been superceded by the more efficient preheater kiln systems 5.3.2 SP KILN The Suspension Preheater type, SP, has preheater cyclones but no calciner. The number of cyclones is 4 to 6. The material going into the kiln after the preheater has a degree of calcination of 40 to 50 %. The last half of the calcination takes place in the kiln. This means that the necessary amount of heat exchange in the kiln is larger than in the kiln types with a separate calciner. The kiln has to be larger for a given production. Due to the calcination in the kiln, the charge is fluidized by CO2 giving the material a chance to flow freely. 5.3.3 ILC-E KILN The In-Line-Calciner using Excess Air type, ILC-E, has no tertiary air duct as all the air passes through the kiln. A small calciner is built into the riser duct and the air for combustion is drawn through the kiln. 5.3.4 ILC KILN The In-Line-Calciner type, ILC, has a calciner in line with the kiln. The preheater is a single string with the calciner built into the riser duct. Combustion air is drawn from the grate cooler through a separate tertiary air duct. A damper in the tertiary air duct allows


34

FULLER - CEMENT CHEMISTRY - HANDBOOK for balancing the air between the calciner and the kiln. 5.3.5 SLC KILN The Separate-Line-Calciner type, SLC, is a double string preheater with the calciner in one string. The calciner is placed in parallel to the kiln riser duct. The combustion air for the calciner is atmospheric air heated in the grate cooler and transported through a tertiary air duct. The gas from the calciner and the gas from the kiln are not mixed and pass through two separate strings of preheater. . 5.3.6 SLC-S KILN The Separate-Line-Calciner –Special type, SLC-S, is a single string system with the calciner placed at the side of the riser duct from the kiln. The gas stream from the calciner is mixed with the gas from the kiln riser duct and pass through one string of preheater cyclones. There is only one main ID fan. An adjustable restrictor at the top of the riser duct makes the distribution of air between the calciner/tertiary air duct/cooler and the kiln/riser duct. 5.3.7 SLC-I KILN Separate-Line-Calciner –with In-line-calciner in kiln string type, SLC-I. The system is a development of the SLC system. It has the two strings as the SLC system but with a calciner also in the kiln string. The SLC-I system can be used for upgrading of a SLC system by the installation of a small calciner in the kiln string. The firing in the two calciners will be: -SLC-calciner:

40-50% of total firing

-ILC-calciner:

10-15% of total firing

Table 9 Comparison of Kiln Systems Type

SLC

SLC-I

SLC-S

ILC

ILC-E

SP

tertiary air

Yes

Yes

yes

Yes

no

no

11000

9000

4500

7500

4000

3500

production, mtpd


35

FULLER - CEMENT CHEMISTRY - HANDBOOK calciner, % fuel

55/60

bypass, max% of kiln gas

100

55 SLC:40/50 ILC:10/15 100

inferior fuel in calciner

Yes

Yes

55/60

55/60

15/25

5

30

60

25

30

yes

medium

no

UNAX, KWh/t FOLAX, KWh/t

12/14

11/13

22/24

22/24

22/24

22/24

18/20

17/19

40

40

70

70

70

70

min.production

%

max.content

Na2Oeq

1.0-1.5

1.0-1.5

1.0-1.5

1.0-1.5

1.0-1.5

1.0-1.5

in %

SO3

0.8-1.2

0.8-1.2

0.8-1.2

0.8-1.2

1.0-1.6

1.0-1.6

(clinker basis)

Cl

0.015

0.015

0.015

0.015

0.023

0.023

.

5.4

ASH ABSORPTION

The ash from the coal used for combustion will be absorbed in the clinker. This is shown on the calculation sheet above (table 8). In the example, a coal with a heat value of 7420 kcal/kg and an ash content of 9.4% is used. The heat consumption in the kiln is 750 kcal/kg. The amount of ash absorbed is then: Ash absorption = (750/7420) * 9.4 = 0.95 % of clinker Therefore, it is necessary to analyse the coal ash to allow for this addition to the raw mix. 5.5

VOLATILE MATTER

Some of the chemicals in the materials going into the burning zone evaporate. The components can come from the raw materials, fuel and waste products. The four most important are: potassium, sodium, sulfur and chloride. There are others but they are normally of minor importance. These minor materials, heavy metals or certain organic compounds can have important implications on the environment for a given plant. The plant should be aware of the different minor constituents to prevent any problems. The four volatile elements mentioned above evaporate in the burning zone and condense again in the colder parts of the kiln system. The colder parts are the


36

FULLER - CEMENT CHEMISTRY - HANDBOOK outer walls causing coating and build-up. Volatiles will also condense on the raw meal particles, as they are colder than the gas carrying the volatiles. Some of the volatiles may escape from the kiln system partly being caught in the filter or escaping into the atmosphere. The volatiles that do not leave the system with the exhaust gas either remain in circulation in the kiln system or leave with the clinker. The volatiles can accumulate in the kiln and preheater causing problems with in build-up in the cyclones and riser duct. It is important to be able to foresee any problems that may occur before the start up of a new plant or a conversion of an existing plant to prevent the possibilities of plugging. A certain portion of the volatiles in the material flowing into the burning zone of the kiln will evaporate at the high temperature. The portion that evaporates is defined as the evaporation factor called ε

(epsilon). A portion of the volatile

material leaves the system with the exhaust gas. This is referred to as a valve V. An internal circulation of volatiles takes place and the circulation factor is called K. The part of the volatile leaving the kiln with the clinker is the residual part called R. A simple layout of a kiln system is shown below:

Figure 2 Circulation of volatile in simple kiln system

ε


37


Evaporation factor

ε = d/b= (b-c)/b=1- c/b

Valve

V = e/d= (a-c)/(b-c)

Circulation factor

K = b/a

Residual component

R = c/a

For one unit of feed: a = 1 the material balance is: K*(1-ε ) + Kε V = 1;

K = 1/(1-ε (1-V)) ;

R = K*(1-ε ) The circulation factor K is the amount of compound going to the burning zone when feeding a unit of 1 (one) to the system. R is the amount going into the clinker. It is possible to calculate the circulation K and the residual in clinker R when the evaporation factor ε and the valve V are known. The enclosed table and figure give evaporation factors and valves for typical cases.

Table 10 Melting and Boiling Points of Alkali Salts COMPOUND


K

Na

38


Melting

Boiling

Melting

Boiling

temp.

temp.

temp.

temp.

°

°

C

°

C

°

C

C

-oxide

Decompose

350

Sublime

1275

-carbonate

894

Decompose

850

Decompose

-sulfate

1074

1689

884

Decompose

-chloride

768

1411

801

1440

-hydroxide

360

1320

328

1390


39


Table 11

Evaporation factor Kiln valve,wet kiln,nodule operating Kiln valve,wet kiln, dust

Typical values for ε and V Symbol K2O Na2O 0.20-0.40 0.10-0.25 ε 0.50 0.70 Vo

Cl

SO3

0.990-0.996

0.35-0.80

0.70

0.60

Vo

0.40

0.60

0.60

0.40

0.20

0.50

0.60

0.40

0.55

0.80

0.60

0.35

0.70

0.85

0.80

0.60

~1

~1

~1

~1

~1

~1

~1

~1

Cyclone preheater valve 1-

Vo Vo Vo Vo Vo Vc

0.35

0.50

0.35

0.45

stage Cyclone preheater valve 2-

Vc

0.20

0.45

0.20

0.30

stage Cyclone preheater valve 1-

Vc

0.15

0.40

0.05

0.15-0.50

0.60

0.70

0.50

0.55

0.60

0.80

0.70

0.30

Cooling tower valve

Vc Vm Vkt

~1

~1

~1

~1

Electrostatic precipitator

Vf

0.40

0.70

0.30

0.50-0.80

operating Long dry kiln Kiln valve,1-stage kiln Kiln valve,2-stage kiln Kiln valve,4-stage kiln Kiln valve,precalciner kiln

stage Dedusting cyclone valve Raw mill valve

valve

The evaporation of alkalies is larger when chloride is high. This is at times used to increase the evaporation in the burning zone by adding CaCl2. Sulfur is difficult to evaluate. Some sulfur in the raw mix is present free in various organic compounds or in pyrites. Approximately, 50% of the sulphur burns off in the top stages of the preheater tower. CaCO3 assisted by moisture catches some of it in the rawmill. SO2 in the preheater also reacts with calcium carbonate with a maximum around 800°C. Sulfur in combination with alkalies behaves differently than SO2 from fuel. Excess sulfur, sulfur not bound as alkali sulfates, can be calculated as:


40

FULLER - CEMENT CHEMISTRY - HANDBOOK Excess S = 1000*%SO3- 850*%K2O-650*%Na2O < 250 g/100 kg clinker where: All percentages are calculated on clinker basis SO3 is total from rawmix + fuel

To ensure trouble free operation of a preheater kiln the following limits apply: Table 12 Limits for Volatiles Raw mix burnability Na2O + K2O (% clinker basis) Cl (% on clinker basis) SO3 (% on clinker basis) from raw mix + fuel ;

Easy

Hard

Max 1.5 %

Max 1.0 %

Max 0.023 % Max 0.023 % Max 1.6 % Max 1.0%

Or excess sulfur under 250 g/100 kg clinker If the natural valves are insufficient, then a kiln bypass can be installed. The bypass will take part of the kiln gas before the preheater and transport it to a separate cooling and dedusting system. The bypass gas has to be cooled immediately to 350oC to avoid clogging. The cooling takes place in a swirling chamber with atmospheric air. Some dust will be removed with the bypass gas (2-3% with 10% bypass.) 5.6

MAIN FEATURES DURING BURNING

Chemical control during operation of the kiln system is divided into the following: -Feed composition -Product quality of clinker -Emission control -Fuel -Preheater The raw meal must have the correct quality with little variation. The standard deviation for LSF should be less than 1% and corresponding less than 0.1 for MS and MA. Large variations will result in irregular kiln operation resulting in problems with ring formation and coating in the preheater, as well as, requiring Chemistry Bible Rev.0; 7 Dec 00

41

FULLER - CEMENT CHEMISTRY - HANDBOOK higher fuel consumption. Performing a free lime analysis on an hourly basis monitors the product quality of the clinker. The analysis can be made either on an average hourly sample or on an hourly spot sample. Environmental authorities stipulate emission control in many countries. The plants have to control and continuously register plant emission of dust, SO2 and other constituents in the exhaust gas. The results have to be reported back to the authorities. The type of fuel used in cement production is either pulverised coal, fuel oil, natural gas, or waste products. Pulverised coal is usually produced at the site in a coal mill that dries and grinds the raw coal to a fineness of approximately 15% retained on the 0.09-mm sieve and moisture content of 1 to 2 %. The residue and the moisture content vary according to the type of coal. Some types of coal with high gas content have a high tendency toward self-ignition, which has to be taken into account. Coal with low content of volatiles like semi-anthracite has to be ground very fine to promote ignition. An important component in heavy fuel oil and coal is the sulfur content. The sulfur has to be taken into account together with the alkalies. Sulfur content in heavy fuel oil above 5% will usually cause build up problems. Fuel analysis should be made regularly by either the supplier or the plant laboratory. The preheater has to be kept free from coating that can clog the cyclone outlet or increase the pressure drop in the riser duct. This can be followed by regular sampling of the material going into the kiln and analyse for chloride, sulfur, and alkalies.

6. HEAT OF REACTION & HEAT TRANSFER Chemistry Bible Rev.0; 7 Dec 00

42


The chemical change from raw material to clinker requires heat for two reasons. The first is due to the heat of reaction for the transformation to clinker. Secondly, because the clinker process is not an ideal or 100 % effective process, heat is lost from the kiln system as: •

Radiation loss from all outer surfaces

•

Heat loss with the gasses from the kiln

•

Excess hot air from the clinker cooler

•

Heat loss with hot clinker

Heat effects the chemical reactions, the formation of solutions and changes in the state of aggregation such as melting or vaporization. The heat effects are called exothermic, when a reaction is accompanied by heat evolution. When heat is absorbed, then the reaction is endothermic. The dissociation of CaCO3, calcium carbonate, is a typical endothermic reaction: CaCO3

CaO + CO2 – 422 kcal/g

The double arrow signifies that the reaction can be reversed. In the preheater, this is called recarbonation. The order of magnitude of the heat of recarbonation is normally evaluated from the temperature profile and the temperature difference between the lowermost and second lowermost cyclone in the preheater tower. When planning a new plant or when making a kiln conversion it is important to know the heat of reaction for the process. The analysis is made in the laboratory of the equipment supplier. Basically, there are the following heat changes: Table 13 Reactions During Heating Temp °C <100 100 - 400 400 - 750

Reaction Evaporation of free water Absorbed water evaporates Decomposition of clay minerals, Kaolinite

metakaolinite


43

Heat change Endothermic Endothermic Endothermic

FULLER - CEMENT CHEMISTRY - HANDBOOK 600 - 900 600 - 1000 800 - 1300

Decomposition of metakaolinite to free reactive oxides Endothermic Decomposition of carbonates to free reactive oxides Endothermic Reactive oxides form intermediate or final clinker Exothermic

1300 - 1380 1250 - 1500

minerals Formation of clinker melt from aluminates and ferrites formation of aliteC3S the principal clinker mineral

Endothermic Endothermic

The reactions within the kiln system take place at a slightly negative pressure and in an oxidizing atmosphere. Reduction does not normally take place in the kiln system apart from a reducing zone in the riser duct of the preheater to reduce the emission of NOx. The first five reactions in the table take place rapidly with a velocity determined by the transfer of heat from the gas to the solid material. The last two reactions are determined first by the contact rate of the reactive components present in the solid phases and later in the burning zone by diffusion of the reactive components in the clinker liquid phase. The total of the heat reactions during clinker formation is endothermic. An example of the heat of reaction is:

Table 14 Heat of reaction Evaporation of combined water Decomposition of clay minerals Dissociation of carbonates Formation of clinker minerals

Kcal/kg 20 35 475 -130

Combustibles in the raw mix Total heat of reaction

-15 385

The heat of reaction is the theoretical heat consumption for the kiln system. Since the process is not ideal, heat losses exist in the system. The losses of


44

FULLER - CEMENT CHEMISTRY - HANDBOOK heat come from the following: ♦ Hot exhaust gas ♦ Heat loss from surfaces of the kiln system, i.e. radiation loss ♦ Excess air from the clinker cooler ♦ Heat lost with clinker after the cooler Some of the loss can be utilized for drying of the raw materials and of raw coal. Table 15 Typical Heat Consumption for Different Systems

Specific heat consumption for kiln systems Wet kiln Long dry kiln 1-stage preheater kiln 2-stage preheater kiln 4-stage preheater kiln Semi-dry kiln w/ preheater & calciner 5-stage kiln w/ preheater & calciner 5-stage preheater kiln w/calciner and latest cooler type

kcal/kg clinker 1400 1100 1000 900 800 950 725 690

The dry process is always chosen unless the raw materials have moisture contents above 20-30%. The efficiency of the heat exchange in a cyclone is the same as the separation efficiency due to the rapid heat transfer between material and gas. Usually, there is only a temperature difference of 5°C between the exit gas and material leaving the cyclone. There is, however, a variation in efficiency between the cyclones as we go lower down in the preheater. This is due to the change in the design of the cyclones. At the high temperature in the lower cyclones these cyclones do not usually have an internal vortex. The vortex or central pipe is difficult to construct in a material that will last at the high temperatures.

7.FUEL 7.1

TYPES OF FUEL


45


The most common types of fuel are: coal, fuel oil and natural gas. The most common fuel is coal with heavy fuel oil being second. Natural gas is used where available and is an excellent fuel. Many waste products from a variety of industrial sources are also used as a fuel source. 7.1.2 COAL Coal is found all over the globe. Coal originates from plants, that over many millenniums have been transformed into coal. The age of coal results in different composition and quality. Anthracite and hard coal are old types, while lignite and peat are younger types. Raw coal is a combination of coal, ash and water. The carbon is the main constituent in the coal, but there are also hydrocarbons, oxygen, nitrogen and sulfur often as pyrites FeS2. Heating the coal in a non-oxidising atmosphere drives out some of these constituents as gas also referred to as volatiles and tar. The coal is then changed into coke. Younger coal has a higher gas content than older coal. They are easier to ignite than the older coal. They are also prone to self-ignition during storage. Table 16 CLASSIFICATION OF COALS Lignite

Bituminous coal

Anthracite

% % % %

40-50 40 – 50 10-25 5-25

5-10 10 – 40 1-3 10-20

0–3 5 1 5-10

C H S N+O Hs Hi

% % % % Kcal/kg Kcal/kg

56 4 1 19 5120 4820

70 3 1 3 6625 6310

78 2 1 2 7100 6900

weight volume

Kg/kg Nm3/kg

7.1 5.5

9.2 7.1

9.9 7.6

Total

Nm3/kg

6.0

7.4

7.8

CO2 + SO2 H2 O

vol% vol%

17.8 10

17.6 6.5

18.9 4.5

Total moisture Volatiles Hygroscopic water Ash

Examples of commercial grades of coals Chemical composition: Carbon Hydrogen Sulphur Nitrogen + oxygen Heat value Gross Net

Combustion air and gas Combustion air required Composition

Wet , 0%

of combustion

oxygen

gases


46

FULLER - CEMENT CHEMISTRY - HANDBOOK N2 Dew point

vol% °C

72.2 46

75.9 38

76.6 31

Table 17 Typical Petroleum Cokes Type

%H2O

%Volatiles

%Fixed C

%Ash

%Sulfur

Gross Heat

Hardgrove

Green delayed

8

11

82

0.5

4

Value (kcal/kg) 8000

60-100

Fluid

0

5

86

1.0

4

8000

25-30

Proximate analysis of coal: The proximate analysis of a coal sample gives values for moisture, volatile matter, ash and fixed carbon. It is performed under detailed laboratory procedures, which can be found in other reference material. The volatile matter is °

the portion of the coal when heated to 900 C without air is driven off as gases. Fixed carbon is the residue remaining after the volatile matter is driven off. The °

ash content is found by heating the sample to 800 C. The moisture in coal is divided into free moisture and hygroscopic moisture, where the free moisture is the moisture lost by air-drying. The volatile matter, fixed carbon, ash, and moisture add up to 100%.

The amount and composition of ash varies from one coal to another. The amount of ash and its composition has to be known, as it will be a part of the clinker. Some coals have a very abrasive ash and a high wear index, which is of value especially for vertical coal mills. Chemistry Bible Rev.0; 7 Dec 00

47


Coal dust can be dangerous and cause explosions. It can also self-ignite at room temperature. For that reason a safety index for a particular coal is assigned according to the characteristics of the coal.

The common safety index is the ratio of % fixed carbon to % volatiles. A high safety index means a lower chance for a coal dust explosion. The safety index for coal varies from 1 for high volatile lignite up to 15-16 for petcoke and anthracite. Ultimate analysis of coal: In the ultimate analysis of coal, carbon, hydrogen, sulphur, nitrogen and oxygen are determined. Chemical analysis: In the chemical analysis of coal, the inorganic composition is determined on the coal ash. The values are used for the calculation of the raw mix and clinker composition. Heat value: The heat value or calorific value is important for the evaluation of the coal and for the heat economy of the kiln. The difference between the gross and net heat value is the heat of evaporation of the water from combustion and the evaporation of the water. The approximate calculation is: Hnet = Hgross – 5.85(9*%H + % Water in sample)

[kcal/kg]

7.1.3 FUEL OIL Fuel oil is used for cement kilns at many plants. The fuel oil most commonly used


48

FULLER - CEMENT CHEMISTRY - HANDBOOK is a heavy fuel oil. The lighter fuel oil types are often used for lighting up kilns. Typical analysis for oil is: Table 18 TYPICAL FUEL OILS Gas oil 86.3

86.2

86.1

H%

12.8

12.4

11.8

S%

0.9

1.4

2.1

o

0C

0.880

0.905

0.960

15oC

0.870

0.895

0.960

At 2° Engler Temperature in

0.880

0.865

0.880

0

60

120

°C for 2° Engler Kcal/kg/oC

0.485

0.480

0.465

Gross, Hs

Kcal/kg

10875

10550

10375

Net, HI

Kcal/kg

10200

9900

9750

Kg/kg

14.4

14.2

14.0

3

Nm /kg

11.1

11.0

10.8

Wet, 0% O2

Nm3/kg

11.80

11.68

11.51

CO2+SO2

%

13.7

13.9

14.1

H2O

%

12.0

11.8

11.4

N2

%

74.3

74.3

74.5

Specific heat Calorific value

Air for combustion

Combustion gases

Heavy fuel oil

C% Composition

Specific gravity, kg/litre

Light fuel oil

Dew point Theoretical flame temperature

o

C

50

50

49

o

C

2160

2120

2120

The composition of fuel oils does not vary as much as coals. There are, however, changes from place to place. The different organic components may vary. An important value is the specific gravity because the calorific value is related to it. Specific gravity changes with temperature. This is important to take into account when a heat balance has to be made. The volume consumption of the oil has to be converted into a weight consumption in order to calculate the heat input. Another important value is the viscosity of the oil. For the burners at Fuller and FLS, the viscosity has to be 2° Engler (or 18 centiStoke) for proper atomisation of the oil. Some oils require a temperature of 120°C others have a higher requirement up to 170°C. The sulfur content could be quite high and should be limited to a maximum of 5%


49

FULLER - CEMENT CHEMISTRY - HANDBOOK S. 7.1.4 GAS Natural gas is an excellent fuel source for cement kilns because it is neat and clean as well as being easy to use. The installation is simple and the composition of the gas has very little variation. Although, there is a risk for explosions and special safeguards have to be installed and maintained. The table below has typical values for natural gas:

Table 19 Examples of Typical Natural Gases Overseas


50


Compositio n

Dutch

Calorific

CH4 C2H6 C3H8 C4H10 < C5 Gross Net

Total CO2 H2O N2 Dew point

Kg/Nm3 Nm3/Nm3 Nm3/Nm3 Vol% Vol% Vol% O C

10.91 8.44 9.20 9.80 18.60 71.60 59

wet , 0% oxygen free)

Combustion gases

value Air for combustion

Vol% Vol% Vol% Vol% Vol% Kcal/Nm3 Kcal/Nm3

(Groningen) 81.76 2.73 0.38 0.13 0.16 8500 7580

* The natural gas contains varying amounts of nitrogen.


51

Sahara 86.50 9.42 2.63 1.06 0.09 10780 9750 13.96 10.80 11.52 10.60 17.70 71.70 58

North Sea (typical)

91.80 3.50 0.80 0.30 0.33 9700 8760 12.60 9.75 10.60 9.80 18.50 71.70 59


Table 20 Domestic Natural Gas Properties City

Gross

Specific

Heating

Gravity

CH4

C2H6

C3H8

Components of Gas (% volume) C4H10 C5H12 C6H12 CO2

N2

Misc.

+

Value 3

Baltimore, MD Birmingham, AL Boston, MA Columbus, OH Dallas, TX Houston, TX Kansas City, MO Los Angeles, CA Milwaukee, WI New York, NY Phoenix, AZ Salt Lake City, UT San Francisco, CA Washington, D.C.

(Btu/ft ) 1051 1024 1057 1028 1093 1031 945 1084 1051 1049 1071 1082 1086 1042

0.590 0.599 0.604 0.597 0.641 0.623 0.695 0.638 0.627 0.595 0.633 0.614 0.624 0.586

94.40 93.14 93.51 93.54 86.30 92.50 72.79 86.50 89.01 94.52 87.37 91.17 88.69 95.15

3.40 2.50 3.82 3.58 7.25 4.80 6.42 8.00 5.19 3.29 8.11 5.29 7.01 2.84

0.60 0.67 0.93 0.66 2.78 2.00 2.91 1.90 1.89 0.73 2.26 1.69 1.93 0.63

0.50 0.32 0.28 0.22 0.48 0.30 0.50 0.30 0.66 0.26 0.13 0.55 0.28 0.24

0.00 0.12 0.07 0.06 0.07 … 0.06 0.10 0.44 0.10 0.00 0.16 0.03 0.05

0.00 0.05 0.06 0.03 0.02 … … 0.10 0.02 0.09 0.00 0.03 0.00 0.05

0.60 1.06 0.94 0.85 0.63 0.27 0.22 0.50 0.00 0.70 0.61 0.29 0.62 0.62

0.50 2.14 0.39 1.11 2.47 0.13 17.1 2.60 2.73 0.31 1.37 0.82 1.43 0.42

0.06 He

0.01 He

Gas is difficult to ignite with an ignition temperature of 600°C. Therefore, the kiln lining has to be hot prior to starting the gas burner without a pilot. It is an advantage that the content of sulfur is low, the volume of the combustion products is high and has a high dew point. The heat loss with the exit gas is consequently high, but this is compensated by a low requirement for primary air. 7.1.5 WASTE FUELS A variety of waste fuels are burned at cement plants. They consist of old tires, used lubrication oil, and used solvents, i.e. waste plastic. Proper precautions


52


have to be in effect to protect not only the personnel but the environment as well. Cement plants have accepted the responsibility for disposing of these many different wastes and in some cases are financially rewarded. Table 21 Typical Waste Fuel Analysis Type of

Heating value

Volatiles

Moisture

Ash

Sulfur

Dry Combustible

Density

Waste Paper Wood Rags Garbage Coated fabric:

(Btu/lb) 7572 8613 7652 8484 10996

(%wt.) 84.6 84.9 93.6 53.3 81.2

(%wt.) 10.2 20.0 10.0 72.0 1.04

(%wt.) 6.0 1.0 2.5 16.0 21.2

(%wt.) 0.20 0.05 0.13 0.52 0.79

(%wt.)

(lb/ft3)

78.8

23.9

rubber Coated felt:

11054

80.87

1.5

11.39

0.80

88.61

10.7

vinyl Coated fabric:

8899

81.06

1.48

6.33

0.02

93.67

10.1

vinyl Polyethylene

19161

99.02

0.15

1.49

0

98.51

5.7

film Foam: scrap Tape: resin-

12283 7907

75.73 15.08

9.72 0.51

25.3 56.73

1.41 0.02

74.7 43.27

9.1 9.5

13202 11428

100.0 75.06

1.72 0.56

0.13 4.56

0 0.02

99.87 95.44

6.4 23.4

covered glass Fabric: nylon Vinyl scrap

8.COMBUSTION Combustion is the chemical reaction between oxygen and fuel. The main component of coal and fuel oil is carbon, which is oxidised, or burns to carbon dioxide CO2 with the evolution of heat. The oxidisation of coal can start at room temperature with a coal of high reactivity or high volatile content. Ignition of the raw coal can take place in the storage area if the material is not stacked properly. If the temperature surpasses a certain limit, the ignition temperature, a fire can easily start if oxygen is available. The ignition temperature for coals varies in relation to the gas or volatile content.


53

FULLER - CEMENT CHEMISTRY - HANDBOOK Lignites and bituminous coals with volatile content over 30% have low ignition temperatures whereas anthracite, petcoke and semi-anthracite require a high ignition temperature. In the burner, fuel is mixed with primary air as the two streams exit the burner nozzle. The mixing depends on the direction of the stream and of the mixing energy or velocity. The temperature of the primary and secondary air is also important. The temperature of the flame can be calculated under ideal conditions. The maximum theoretical flame temperature is found by fuel ignition with the stoichiometric air requirement. The practical value is approximately 80 to 85% of the calculated one. For coal, the water will evaporate first followed by gas evolution from the volatile portion of coal. The rest of the fine and porous coal particles burn with a visible flame due to the emission from the fine particles in the flame. In contrast, natural gas can burn with hardly any visible flame. The fine coal particles will oxidise to CO and later to CO2 as more oxygen becomes available. The flame front travels with a certain velocity increasing with temperature and is also dependent on the injected air/fuel stream for a steady combustion to be maintained. Coal with a low content of gas like anthracite and semi-anthracite requires a high ignition temperature, and usually a small amount of primary air is used. The conditions for fuel oil are similar to coal. The oil droplets evaporate to gas that ignites and burns. The flame is luminous because carbon or soot particles are formed during the combustion. The ignition of oil is more difficult than with coal due to the slower mixing of air and oil and to the evaporation of the oil droplets. The heat is transferred from the flame to the lining and the charge in the kiln by radiation. The radiation follows: R= e*k*( T14- T24)

[kW/m2] e = coefficient of emission, max=1.


54

FULLER - CEMENT CHEMISTRY - HANDBOOK K is a constant T1 and T2 are surface temperatures in °Kelvin The coefficient of emission for coal e is close to ~1 at the coal flame temperature. This is due to the content of particles in the coal flame. For oil, the value of e is 0.7 to 0.9 and for gas only 0.2 to 0.6. Radiant heat is transferred quickly to the lining and to the charge surface. But the transfer of heat by conduction into the charge from the charge surface and from the lining is a lot slower. This is one of the determining factors for the length of the burning zone. The heat transfer is better when the charge is turning over rather than sliding on the lining.

9. COAL & OIL 9.1

FINENESS OF COAL

The coal has to be dried and ground in a coal mill. The coal mill can be an airswept ball mill with separator or a vertical mill. The coal fineness depends on the type of coal as seen from table 22: Table 22 Coal Properties Type

Volatiles %

Ash %

Hygroscopic water, %

Anthracite Semi-

<5 5 – 15

5 3-5

<2 2–6

Anthracite Low volatile bituminous coal Bituminous coal High volatile bituminous coal Lignite

15 – 20 20-30 30 – 40 40 – 50

5–8 8 – 10 10 – 20 15 – 30

2–6 2–6 2–6 10 – 25

Sieve residues of coal meal 5-7% +0.09 mm 1 % + 0.2 mm residues increasing to coarser values 30% + 0.09 mm 3 – 4 % + 0.2 mm

The grindability is usually given as a hardgrove index number. The relationships between the hardgrove index and the grindability in kWh/t for different types of grinding applications are given below. The Hardgrove index for coals can vary


55

FULLER - CEMENT CHEMISTRY - HANDBOOK from 40 to over 100 for very soft lignite or petcoke. An important aspect for the choice between a ball mill and a vertical mill is the wear index. Some coals have a high ash content with an abrasive constituent resulting in a high rate of wear. In such cases, a ball mill might be chosen or a vertical mill with stronger wear resistant parts.

Figure 3 HARDGROVE-GRINDABILITIES 40

spec.grindability- KWh/t

35 30 25 20 15 10 5 0 40

50

60

70

80

90

Hardgrove index ballmill, 5%+ 0.09 mm ballmill, 10% + 0.09 mm ballmill, 20% + 0.09 mm vertical mill, 5%+0.09mm vertical mill, 10% +0.09 mm Vertical mill, 20% + 0.09 mm

H

RDG A ROV E -G R IN DA

I L B IT IE S

50

0 ex

40

s pe c.g rin da bi lity -K W h/t

35 30 25 20 15 10 5 0 4 0

bal lm i l,5 % + bal lm i l,2 0% ver tica lm il,

9.2

60 7 H ra dg rov e ind 0 .09 m + 0. 9 10 % +0.

m b alm m v ert 09 V m ert

8 0

90

il ,10 % +0 ical m il,5 % ica lm il, 20%

.09 m m +0. 09 m + 0. 09

m

DRYING OF COAL

Raw coal contains varying amounts of water. The raw coal has to be dried to facilitate grinding and handling but the drying must not go beyond the limit of safety. The coal is dried in the coal mill only to the recommended minimum


56

FULLER - CEMENT CHEMISTRY - HANDBOOK residual moisture content to reduce the risk for fires and explosions. This residual moisture content is found by using the graph in figure 4 below according to the actual hygroscopic moisture of the coal. The plant laboratory determines the hygroscopic moisture and residual moistures at various drying temperatures. A detailed procedure to determine surface and hygroscopic moisture of coal meal is found in another reference. Presented here is an overview of the program. Free moisture, or surface moisture, is the water lost by air drying a prepared coal sample at an ambient temperature of 30°C. Further drying of the coal sample at various temperatures between 30 and 105°C drives off the remaining water. The ratio of the change in sample weight between 30 and 105°C divided by the weight at 105°C, represents the hygroscopic moisture. A graph of the residual moistures at various drying temperatures shows the correct operating temperature for the outlet of the coal mill. Usually, a coal mill outlet temperature of between 60 and 80°C is correct. The inlet temperature to the coal mill can be as high as 300 to 350°C. The inlet temperature depends on the quantity of air through the mill, and requires the dew point of the outlet air after the coal mill to be 20°C higher than the outlet air temperature to avoid condensation and clogging in the transport system following the mill. For example, to dry one kg of coal with 10% moisture 1.2 kg drying air at 300°C is required.

12 10 8 6 4 2 0 1

2

3

4

Hygroscopic w ater,%

9.3

ASH CONTENT


57

5

6

Residual moisture,%

Figure 4 Recommended moisture in coal dust


The ash content in various coals differs in quantity as well as composition. It is necessary to know the exact composition of the ash because it combines with the raw mix in the kiln system. The ash analysis is performed by the plant laboratory or by the coal supplier. The ash content of coal will consume some heat during combustion due to chemical changes of the minerals in the coal. This reduces the flame temperature. In coals with a high ash content, it can be difficult to obtain a sufficiently high flame temperature for the process. 9.4

GAS CONTENT

The gas content is important for the ignition of the coal. Anthracite and some petcoke have low gas contents and consequently are very difficult to ignite. Only very fine grinding can compensate for the lack of gas. Sometimes a very high gas content can result in problems with the mixing air. This can impede ignition and proper burning which can result in a low flame temperature. 9.5

MINOR COMPONENTS

Coal also contains alkalies, sulfur and chloride. These constituents have to be taken into consideration for the calculation of the raw mix and of the clinker composition. These volatile components will participate in the internal circulation of volatile components in the kiln, calciner and preheater. 9.6

REQUIREMENT FOR AIR

The combustion of fuel requires air to oxidise the carbon, hydrogen and sulfur in the flame into the combustion products of carbon dioxide, sulfur dioxide and water. In addition, a certain amount of excess air is used to obtain complete combustion without formation of carbon monoxide. The reactions are: C+ O2 CO2 + heat


58

FULLER - CEMENT CHEMISTRY - HANDBOOK 4H + O2

2H2O + heat

S + O2

SO2 + heat

The water content in the coal is evaporated. The minimum amount of oxygen required for combustion, Omin, can be calculated by: Omin = (0.01)*(32/12*C+16/2*H +32/16*S+32/14*N-O) [kg O/kg fuel] Therefore the required amount of air, Lmin, is: Lmin = (Omin / 1.429)

[Nm3O/kg fuel]

Lmin = [Omin / (0.21*1.429)]

[Nm3air/kg fuel]

The total volume of the combustion gas is: Vmin= 0.0889*C +0.2231*H – 0.0263*O + 0.0333*S +0.0124*H2O [Nm3gas/kg fuel] Where: C, H, S, N, O, H2O are in %wt. of the fuel

Combustion takes place with surplus air to ensure complete combustion. The surplus of air is referred to as excess air and is symbolised by the Greek letter lambda, λ . The excess air in the combustion air is calculated from the formula:

λ = 1/(1-79/21*O2/(100-CO2-O2)) For example, the amount of excess air is 1.20% when O2= 3.6 vol% and CO2=15.4 vol%. If there is incomplete combustion CO is present and the formula cannot be used. Incomplete combustion increases the heat consumption of the kiln appreciably and it is dangerous for the precipitator. The kiln exhaust will also contain CO2 from the decarbonation of carbonates in the raw mix. For a normal raw mix, the amount of CO2 is 0.55 kg or 0.28 Nm3 per one kg of clinker.


59


10. PROCESS GAS The process gas contains combustion products from the fuel with some excess air, as well as, products from heating the raw mix. The various components are described further. The amount of combustion gas can be calculated from the equation: min

2

3

V = 0.0889*C +0.2231*H – 0.0263*O + 0.0333*S +0.0124*H O [Nm gas/kg fuel]

This amount has to be increased by the excess air:

λ = 1/(1-79/21*O2/(100-CO2-O2)) The CO2 released by the heating of carbonates in the preheater, calciner and kiln must also be added. The amount for a normal clinker is approximately 0.55 kg per kg of clinker or 0.28 Nm3. Finally, there is the additional volume from evaporation of water in the raw mix. In most systems, the preheater gases are taken to a raw mill and/or coal mill and used for drying the respective material. SO2 is present in the exhaust gases from oxidation of the fuel S and decomposition of sulfates. Most of the SO2 is totally scrubbed in the preheater by K2O, Na2O, and CaO. Due to the lower volatility, the alkali sulfates will most likely leave the system with the clinker. However, the calcium sulfate ends up evaporating in the burning zone leading to an internal cycle condensing at the backend of the kiln. The exhaust gas also contains SO2 from the sulfides and pyritic sulfur present in the raw materials. They are oxidised in the preheater and a portion escapes with the exhaust gas. By having the exhaust gas go to a raw mill and/or conditioning tower, scrubbing of the SO2 takes place reducing the emissions. NOx is formed by combustion in the kiln and calciner. Fuel NOx is formed when combustion occurs below 1200°C. The amount of fuel NOx depends on: 

Amount of N in fuel


60

FULLER - CEMENT CHEMISTRY - HANDBOOK 

Excess oxygen



Flame temperature

When combustion temperatures are higher than 1200°C, the NOx formed is thermal NOx. This happens in the kiln-burning zone. The NOx formed is predominantly NO, a small part (5%) of which in the colder parts of the preheater is oxidised to NO2. Thermal NO increases with flame temperature above 1200°C, with retention time, and with increasing free oxygen. Thermal NO produced in the kiln-burning zone is related to: 

Flame temperature



Flame shape



Type of fuel



Excess oxygen



Gas retention time in the burning zone



Load temperature



Load retention time in the burning zone

A short, hot flame, excess combustion air and high secondary air temperature will all increase NO formation. While fuel moisture, dust insufflation, or a high dust entrainment in secondary air will reduce the NO. A harder burning mix will always lead towards a higher NO, however, overburning should be avoided. It is x

2

common to express NO as NO for regulatory purposes.

The NOx formed can be reduced in different ways. Ammonia can be injected in the top cyclone. Another way is to have a reduction zone in the calciner, which is


61

FULLER - CEMENT CHEMISTRY - HANDBOOK the feature of the low-NOx calciner. In the reduction zone, the following reaction takes place between 800-1100°C reducing the NOx emissions:

[N] + NO

N2 + [O]

Another component sometimes found in the exhaust gas is CO. CO is usually formed from incomplete combustion of the fuel. At temperatures around 680°C, oxidation of CO to CO2 takes place in the presence of excess oxygen. CO emissions at the stack may be contributed from poor mixing of the fuel/combustion air or not enough combustion air in the burning zone. Organic C found in the raw materials tend to oxidise at temperatures below 680°C with roughly 20% of the oxidation forms CO regardless of the amount of oxygen present. The above holds true when the level of organic C in the raw material is 1.4 g/kg clinker or more. For example, 2 g C/kg clinker with a 15% conversion rate corresponds to about 250 ppm CO at 5% oxygen.


62

Cement Chemistry Handbook - Fuller

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