Process Engineering Training Program MODULE 8 Application of Heat and Mass Balances Section
Content
1
Application of Heat Balances to Process, Evaluation
2
Heat Balances – Imperial Units
3
Paper 12 – Heat Balances
4
Heat Transfer
5
Heat Transmission
Presentations MASS, HEAT AND ENERGY BALANCES HEAT TRANSFER
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 8 Section 1
Application of Heat Balances and Mass Balances to Process, Evaluation
15
Application of Heat Balances to Process Evaluation
P Layne J. A. Stringer
1 INTRODUCTION The purpose of the rotary kiln is to increase the temperature of the raw material so that the chemical reactions leading to the formation of cement clinker can take place. The heat required to increase the temperature of the feed and for the chemical reactions is generated by burning fuel. It is clearly desirable on the grounds of cost to operate the kiln with as low a consumption of fuel as possible consistent with a high output of good quality clinker and to this end it is necessary to understand how the heat generated by burning fuel is utilized. Figure 15.1 shows the temperature of material and gas along a kiln and the five zones into which it is conventional to divide the kiln. In the first two zones, the temperatures are relatively low and the processes which the feed undergoes are mainly physical i.e. drying and preheating. In modern practice when the moisture content of the feed is relatively low, these processes are carried out in a separate preheater. In the third or calcining zone, chemical reactions start to take place, in particular the dissociation of calcium carbonate. In this zone it will be noted the material temperature rises only slightly despite a big change in gas temperature. In the fourth zone, the material is raised to around 1400 0 C at which the main clinker forming reactions can occur. The burning of fuel is arranged so that the gas temperature is a maximum in this zone. Finally ,in the fifth zone, the clinker is cooled by gas at a lower temperature. This process, of course, is largely performed in a separate cooler. The main quantities of heat involved in carrying out the processes in each of the five zones may be fairly readily determined and hence the overall heat requirement of the kiln system can be obtained. The relative length of each of the five zones is determined by several factors (e.g. difference in temperature of gas and material, gas velocity, volume loading) whose relationship is by no means fully understood.
2
HEAT AND MASS BALANCES
The economic aspects of any system involving the utilization of fuel can generally be tied to the thermal efficiency of the system. To determine this efficiency a heat balance has to be performed upon the system under equilibrium conditions. In this balance the heat supplied to, and lost from the system are equated. This balance takes no account of the internal modes of heat transfer but rather shows in the relative distribution of outgoing quantities what the input is required for. Figure 15.2 illustrates some of the parameters considered in making a heat balance for a wet process kiln with Fuller cooler. The dotted line encloses the system. Heat flows across the dotted line only are calculated; heat flows within this volume only are not considered in the overall heat balance. In making the heat balance chemical reactions (fuel combustion and clinker formation) and physical processes (for example, the evaporation of water) must be included. A prerequisite of making the heat balance is a knowledge of the various quantities of gases, and solids entering or leaving the system. A mass balance has therefore to be performed prior to calculation of the heat supplies or losses in the heat balance. The data required will consist of rates of feed of raw meal, fuel and air entering the system, which should equal the rates of removal of clinker, dust, and flue gases.
When the heat balance has been constructed it should yield a detailed account of all sources of heat utilization i.e. which functions use large amounts of the fuel, and which functions use a negligible amount of fuel. If greater thermal efficiency can be achieved in the system it will show which items are worthy of greater attention. In the wet process rotary kiln system a heat balance will show that virtually all the heat input is utilized between (a) the theoretical heat of reaction, (b) vaporization of the slurry moisture, (c) sensible heat of the exit gases, (d) shell losses
3
CONSTRUCTION OF THE HEAT BALANCE
A list of kiln data is shown in Table 15.1 to illustrate the measurements which are required to perform a heat balance, together with typical figures. The amount of data required depends somewhat upon the accuracy required of the resulting balance; the relative merits of the additional data are discussed in the appropriate sub-sections. The base lines upon which the heat balance is performed are selected mainly for simplicity of calculation. The quantities of heat involved are based upon lkg. of clinker and listed as kcal./kg. These may be converted to per cent standard coal/clinker by dividing the kcal./kg by 70 (standard coal is a theoretical coal with a gross calorific value of 7000 kcal./kg.). The quantities of sensible heat are calculated from a datum temperature which can be taken as ambient or some similar fixed value (e.g. 200 C). As an example, a heat balance on a wet process coal fired kiln will now be calculated. The relevant kiln data and analyses of raw meal, clinker, and fuel are set out in Tables 15.1 and 15.2. The data available in practice may be more or less than this amount, and it is thus not possible to completely standardise the procedure of heat balance determination.
3.1 PRELIMINARY CALCULATIONS From the data in the tables, the first requirement is to calculate the undetermined solid mass flows. The raw coal consumption is 0.251 kg./kg. of clinker. The coal moisture is 5%, thus the consumption of dry coal will be (100 − 5) 0.251 x = 0.238 kg./kg. of clinker 100 The ash content represents 15.3% of the dry coal, equivalent to 0.153 x 0.238 = 0.0364 kg./kg. of clinker. It is assumed that all the ash is absorbed in the clinker, therefore, the clinker derived from raw meal = 1 - 0.0364 = 0.9636 kg./kg. of clinker. The raw material also suffers a loss on ignition on passage through the kiln. Loss on ignition is determined by placing a small weighed sample of material in a cool furnace and raising the temperature to 900o C over 1 hour. After 3 - 4 hours at between 850o - 950o C the sample is removed, cooled in a desiccator and reweighed. The loss of weight determined represents mainly vapor from associated and combined water and carbon dioxide from carbonate dissociation and organic matter combustion. The raw meal suffers a loss on ignition of 35.28%, so that the quantity of raw meal required to produce this 0.9636 kg. of clinker =
(0.9636 × 100) = 1.52 kg (100 − 36.28) The raw meal further suffers a degree of dust loss, which is 0.06 kg./kg of clinker. The loss on ignition of the partly decarbonated dust is 20.2%, equivalent to 0.06 x =
(100 − 20.2) = 0.075 kg. of dry meal/kg of clinker (100 − 36.28)
The total raw meal required to produce lkg of clinker is therefore 1.52 + 0.75 = 1.595 kg. A slurry moisture of 39.2% will be equivalent therefore to 1.595 x 39.2 = 1.028 kg of water/kg. of clinker (100 - 39.2)
3.2 HEAT INPUT The total heat input is calculated by summing the various components containing both sensible and potential heat. In this example we must consider any sensible heat contained in the fuel, combustion air, and raw materials, plus the potential heats contained in the fuel and raw material.
3.3 POTENTIAL HEAT IN COAL Raw coal burnt per kg. of clinker is equivalent to 0. 238kg. of dry coal . Gross calorific value of dry coal = 6750 kcal./kg. Heat supplied by burning coal,0.238 x 6750 = 1607 kcal./kg of clinker. It will be noted that the gross calorific value has been used in which it is assumed that the water vapor from the combustion of the dry coal is condensed. In fact this water is carried out of the kiln as vapor and an allowance has to be made for this in calculating the sensible heat of the exit gases. 3.4 POTENTIAL HEAT IN RAW MATERIALS The dry raw meal contains 0.07% of combustible organic carbon equivalent to 1.595 x
0.07 = 0.0011 kg./kg. of clinker 100
Calorific value of carbon = 7828 Kcal./kg. Heat supplied by burning carbon is 0.0011 x 7828 = 8.6 Kcal./kg. of clinker 3.5 SENSIBLE HEAT IN COAL If the same quantity of heat is supplied to the same mass of different materials, and there are no chemical or physical changes of state, the resulting temperature rises are not the same, but depend on the specific heat of the material. Supposing that a quantity of heat Q is supplied to a given mass of material m, leading to a rise in temperature of the material from temperature t 1 to t 2 then Q = m S ( t 2 - t1 ) where S is the mean specific heat of the material over the temperature range t 1 to t 1 Some useful values of specific heats are shown in Tables 15.3 and 15.4. The sensible heat of a material is calculated in the above manner by calculating the heat contained in the material above a datum temperature (20oC in this case).
TABLE15.2 FUEL, FEED, CLINKER AND DUST DATA COAL Moisture Calorific Value of Dry Coal Analysis of Dry Coal Ash C H s N 0 Total Carbonate C02 (mineral) H20( > 110' i.e. Combined) Loss on Ignition (as measured) Organic Carbon
CLINKER Insoluble Residue Si02
5.0% 6750 kcal/kg 15.3% 76.0 4A 1.6 0.9 1.8
A1203 Fe203 CaO MgO S03 K20 Na20 Raw Meal 78.85% 34.87 1.34 36.28 0.07
0.21 21.63 6.57 2.78 66.31 1.09 0.44 0.68 0.29 Dust
20.2
The coal is fed to the mill at 20oC, i.e. the datum temperature, to yield a t 2 - t 1 value and hence sensible heat value of zero in this case. 3.6 SENSIBLE HEAT IN COMBUSTION AIR It is calculated later that the total air drawn into the system is 4,457 kg./kg. of clinker. Assuming this air is all at 22oC, its sensible heat is 4.457(22-20)0.24 = 2.14 kcal./kg. of clinker. TABLE 15.3 MEAN SPECIFfC HEATS OF UNDISSOCIATEO GASES BETWEEN 20'C AND t'C toC 20 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
02
N2
0.218 0,220 0.223 0.227 0.230 0.234 0.237 0.240 0.243 0.245 0.247 0.249 0.251 0.253 0.255 0.256 0.258 0.258 0.260 0.262 0.263
0.248 0.248 0.249 0.250 0.252 0.254 0.256 0.258 0.261 0.264 0.266 0.268 0.271 0.272 0.275 0.276 0.278 0.280 0.281 0.282 0.284
Air
Co
C02
0.240 0.240 0.242 0.243 0.246 0.248 0.250 0.253 0.256 0.258 0.260 0.263 0.265 0.267 0.269 0.271 0.272 0.273 0.274 0.276 0.277
0.249 0.249 0.250 0.252 0.254 0.257 0.259 0.262 0.265 0.268 0.270 0.273 0.275 0.277 0.278 0.280 0.282 0.283 0,285 0.286 0.287
0.198 0.211 0.221 0.230 0.238 0.246 0.252 0.258 0.263 0.268 0.271 0.275 0.278 0.281 0.284 0.286 0.289 0.291 0.293 0,294 0.296
H20 vapor 0.435 0.447 0.452 0.457 0.463 0.471 0.478 0.486 0.495 0.502 0.512 0.519 0.527 0.532 0.542 0.547 0.553 0.561 0.567 0.573 0.578
S02 0.143 0.147 0.150 0.154 0.157 0.161 0,164 0.167 0.170 0.173
Above 1500oC dissociation must be taken into account Data for 02, N2, Air, CO, C02, H20 vapor from Spiers: Technical Data on Fuel, 1962. Data for S02 from Perry : Chemical Engineers Handbook.
3.7 SENSIBLE HEAT IN RAW MATERIALS Slurry is fed to the kiln at 17oC, i.e. less than the datum temperature, therefore the sensible heat of the slurry will be a negative value on the input side of the heat balance. The specific heat of the dry raw material is taken as 02, (the specific heats of the main constituents are all approximately 0.2). Sensible heat in the dry raw material = 1.595(17 - 20)0.2 = -0.957 kcal./kg. of clinker. Sensible heat of slurry moisture = 1.028(17 - 20)1 = -3.84 kcal./kg. of clinker. Total sensible heat of feed = -4.041 kcal./kg. of clinker.
3.8 HEAT OUTPUT The heat output is also the sum of various components; but these are of a rather more complex nature than the input variables. Some basic knowledge of heats of reaction, Dalton's Law, and dewpoint are useful, and brief details on each of these can be found in Appendix 1.
3.9 THEORETICAL HEAT OF REACTION The heat required to convert raw meal into clinker can be calculated from first principles using basic heat of reaction data (Appendix 1, Table 15.6) : the composition of the raw meal is known. This method is illustrated and the effects of different lime saturation factors, silica ratios, alumina ratios, free lime, coal ash absorption and clay mineral type are discussed in Research Department Report SR-64/28/R-8. Various formulae have been developed by zur Strassen(1957) and Crichtoi (1938) to permit more rapid estimation of the theoretical heat. A formula of zur Strassen, which gives good agreement with calculations from first principles, is used here: Qt = 2.22A + 6.48Mc + 7.646C c - 5.116S - 0.59F where Qt is the theoretical heat in kcal./kg of clinker A is the weight in g of Al2O3 per 100g. of clinker Mc and Cc are the weights in g. of MgO and CaO per 100g. of clinker S is the % SiO2 in the loss free clinker analysis F is the % of Fe2 O3 and Mn 2O3 in the loss free clinker analysis
Substituting in this formula data from Appendix 1, Table 15.6 gives Qt = 2.2 x 6.57 + 6.48 x 1.09 + 7.646 x 66.31 -5.116 x 21.63 - 0.59 x 2.78 = 417 kcal./kg. of clinker In general theoretical heats of all clinker fall quite close to 420 kcal./kg, and this value can be adopted when insufficient data is available to apply the above formula.
3.10 HEAT TO EVAPORATE WATER The slurry moisture is equal to 1.028 kg./kg. of clinker. Incorporated in this figure is the moisture content of 0.075 the dust losses, equal to × 1.028 = 0.048 kg./kg of clinker. Treating the dust loss moisture separately this 1.595 leaves 1.028-0.048 = 0.98 kg./kg. of clinker of slurry moisture. It is assumed this water is evaporated at 20o C, at which temperature the latent heat is 586 kcal./kg. Therefore the heat required for the evaporation of slurry moisture is 0.98 x 586 = 574.3 kcal./kg. of clinker The raw material contains 1.34% of combined water equal (deducting the dust loss component) to 1.52 × 1.34 = 0.0204 kg./kg. of clinker. The heat required to evaporate this water at 20oC is 0.0204 x 586 = 100 11.9 kcal./kg. of clinker. (Note the heat of dissociation of combined water is included in the theoretical heat). The percentage of moisture in the coal is 5.0%, equal to 0.251 x
5.0 = 0.013 kg. of water/kg. of clinker. 100
The heat required to vaporize this moisture at 20oC = 0.013 x 586 = 7.59 kcal./kg. of clinker. In calculating the heat input to the kiln, the gross calorific value of the coal was used, thereby implying that the water vapor from the combustion of the hydrogen in the coal was condensed. In calculating the heat output of the kiln, therefore, the latent heat of vaporization of this water has to be included. The amount of water in the combustion products is .094 kg./kg. clinker (see section 3.12). Therefore heat to evaporate water in combustion products is 0.094 x 586 = 54.9 kcal./kg. of clinker.
3.11 SENSIBLE HEAT OF EXIT GASES To calculate the sensible heat lost by the exit gases, the total masses of the constituent gases have first to be calculated, via appropriate mass balances.
3.12 COMBUSTION PRODUCTS The carbon in the fuel and raw material are burnt thus C
CO2
02 →
+ 12kg.
32kg.
44kg.
(A small fraction of the carbon is burnt to CO and not CO2 this is allowed for later). The hydrogen in the fuel is burnt thus 2H2
O2 →
+ 4kg.
2H2O
32kg.
36kg.
The sulphur in the fuel-is burnt this S
O2 →
+ 32kg.
SO2
32kg.
64kg.
On the basis of I kg. of clinker the fuel combustion should yield 0.238 x
76 44 x = 0.663kg. of carbon dioxide 100 12
0.238 x
4.4 36 × = 0.094kg of water vapor 100 4
0.238 x
1.6 64 × = 0.0076kg of sulfur dioxide 100 32
The oxygen required for combustion, per kg. of clinker is 0.238 x
76 32 4.4 32 1.6 32 × + 0. 238 x × + 0.238 x × = 0.57kg. 100 12 100 4 100 32
The 0.7% organic carbon in the raw meal is also burnt, consuming 0.7 32 × = 0.0186 kg. of oxygen/kg. of raw meal 100 12
equivalent to 0.0186 x 1.595 = 0.0297 kg. of oxygen/kg of clinker, to give
0.7 44 × = 0. 0257 kg. of carbon dioxide/kg. of raw meal 10 12 equivalent to .0257 x 1.595 = 0.0409 kg. of carbon dioxide/kg. of clinker. A small part of this oxygen for combustion comes from the coal; per kg. of clinker this is 0. 238 x
1.8 = 0. 0043kg 100
The weight ratio of nitrogen to oxygen in air is 3.31 (assuming the nitrogen includes all the inert gas). Therefore the weight of nitrogen in the air required for combustion is (0.57 + 0.0297 - 0.0043) x 3.31 = 1.971 kg./kg. of clinker. There is also some nitrogen in the coal equal to 0.238 x
0.9 = 0.0021 kg./kg. of clinker 100
3.13 EXCESS AIR IN THE EXIT GASES We must now consider the excess air in the exit gas (i.e. air in excess of the combustion requirements) as shown in the exit gas analysis. CO2 CO O2 N2 (by difference)
28.1% by volume 0.1% by volume 0.85% by volume 70.95% by volume Σ = 100.0
If the combustion had been complete the volume of CO would have burnt to an equal volume of CO2 by combining with half its volume of O2 The gas analysis would have then been CO2 O2 N2
28.2% by volume 0.8% by volume 71.00% by volume Σ = 100.0
(The slight contraction in volume and the resulting correction which should be made to bring the analysis back to 100% basis has been neglected the error is insignificant at low CO contents).
The O2 content of 0.8% represents the excess air. The ratio by volume of nitrogen to oxygen in air is 3.76, therefore the N2 content representing the excess air is 0.8 x 3.76 = 3.1%, the remaining N2 being the combustion-air and the coal. The N2 content being due to combustion air is (71.0 - 3.1) ×
1.971 = 67.8% 1.971 + 0.0021
The percentage of excess air is, therefore 3.1 × 100 = 4.6% 67.8 The weight of nitrogen in the excess air is 4.6 × 1.971 = 0.0907 kg./kg. of clinker 100 and the weight of complimentary oxygen is 0.0907 = 0.0274 kg./kg. of clinker 3.31 The total weight of air entering the kiln (i.e. combustion air plus excess air), per kg. of clinker is Combustion air Excess air Total
.595 + 1.971
+ 2.566
2.566 x 4.6/100 =0.118 2.684
3.14 OTHER SOURCES OF WATER VAPOUR The combustion of the fuel provides one source of water vapor, the other sources consisting of the feed, coal moisture, and included water vapor in the combustion air. The total water vapor given off by the feed is 1.028 + 0.021 = 1.049 kg./kg. of clinker. The air entering the kiln will contain some water vapor. In this country the average weight of water per kg. of dry air is of the order of 0.005 kg. On this basis the quantity of water vapor per kg. of clinker is 2.6591 x 0.005 = 0.0133kg
3.15 OTHER SOURCES OF CARBON DIOXIDE
Some. of the feed leaves the kiln as dust which is only partially decarbonated. Loss on ignition of the dust is 20.2% compared with 36.3% of the feed. Assuming the losses on ignition represent the degree of decarbonation, the percentage decarbonation of the dust on a loss free basis is 36.3 20.2 − 100 − 36.3 100 − 20.2 × 100 = 55.6% 36.3 100 − 36.3 Therefore the carbon dioxide evolved by the dust is 55.6 x (0.3487 + 0.0257) = 0.209 kg./kg. of dust 100 The dust loss of 6% on clinker is equivalent to 0.075 kg. of dry raw meal/kg. of clinker. Therefore the carbon dioxide derived from the feed is 34.87 + 2.57 6 (1.595 - 0.075). × 0.209 = 0.582 kg./kg clinker + 100 100 (It has been assumed that the dust has been completely dried, i.e. slurry moisture and combined water have been removed).
3.16 HEAT CONTENT Summation of the constituent gas weights per kg. of clinker results in the following (in kg.). H2O
from feed from combustion of coal from coal moisture from water vapour in air
Total
CO2
Total
1.049 0.094 0.013 0.0133 1.1693
from feed from combustion of coal
0.582 0.663 1.245
SO2
from combustion of coal
0.0076
O2
from excess air
0.0274
N2
from coal from combustion air from excess air Σ N2 =
0.0021 1.9710 0.0907 2.0638
The heat required to raise these gases from 20oC to 212oC, a temperature difference of 192oC, is (1.1693 x 0.452 x 192)+ (1.245 x 0.222 x 192)+(0.0076 x 0.15 x 192)+ (0.0274 x 0.223 x 192)+(2.0638 x 0.249 x 192) = 254.5 kcal./kg. of clinker. Sensible heat of exit gases = 254.5 kcal./kg. clinker.
3.17 COOLER EXAUST AIR 931 kg./min of air is exhausted from the cooler at 115oC. I kg. of clinker is made every 0.0019 min. The weight of air/kg. of clinker is therefore 931 x 0.0019 = 1.773 kg./kg. of clinker. The sensible heat contained in this air is 1.773 x (115 - 20) x 0.241 = 40.6 kcal./kg. of clinker. With a rotary or planetary cooler, this item would not occur. The total air drawn into the kiln and cooler per kg. of clinker, (in kg.) is Combustion Air Excess Air Cooler Exhaust Air
2.566 0.118 1.773 4.457 kg.
This figure has been used in section 3.6 to calculate the sensible heat of air entering the system.
3.18 SENSIBLE HEAT OF CLINKER The clinker leaves the cooler at 124oC. The sensible heat in the clinker/kg of clinker is 1 x 0.188 x (124 - 20) = 19.6 kcal./kg. of clinker
3.19 SHELL LOSS
The determination of the shell losses from kilns, coolers etc. is a difficult problem. From the outer surface of the kiln shell heat is transferred to the surroundings by two means. Radiation takes place according to an equation of the form
(
q r = Aε α T2 − T3 4
4
)
where T2 and T3 are the absolute temperatures of the shell and the surroundings respectively, ε is the emissivity of the surface and γ is a constant. Convection takes place according to an equation of the form q c = hA(t 2 − t 3 ) where t 2 and t 3 are the temperature of the shell and the surroundings respectively and h is a coefficient whose value depends on a number of factors including the dimensions of the kiln and the air velocity over the kiln. By measuring the temperature and emissivity along a kiln shell the heat loss can be estimated using formulae of the form of equations noted above. Numerous measurements have to be made as there is a very large variation in the temperatures at various points on the shell. The temperature at any particular point depends on the corresponding temperatures in the kiln, the type and thickness of the brickwork and the thickness of any coating. The shell loss from a modern kiln is of the order of 4070 kcal./hr.m 2 of surface, though a very wide variation is to be expected from this value. Measurements have also to be made of cooler, preheater, coal mill and connecting ducting temperatures as there is a substantial, though relatively smaller, shell loss from these as well. It will be evident from the above equation that the shell loss depends on the temperature conditions in the kiln and the kiln geometry. On the whole the temperature conditions in a kiln do not vary much with output. (There is, however, a tendency for temperature to rise with output), In consequence the shell loss remains substantially constant whatever the output. In this way the shell loss differs from for example, the exit gas loss and the clinker loss which increase with output. For the purpose of the heat balance, the total shell loss of the system is taken as 1.11 x 10 5 kcal./min. This is equivalent to 1.11 x 10 5 x 0.0019 = 210.9 kcal./kg. of clinker. 3.20 HEAT LOST IN MAKING DUST The dust leaving a kiln varies considerably in quantity and composition and it is therefore difficult to make an accurate estimate of the heat loss associated with the dust. The usual method is to assume the dust is partially decarbonated dry raw meal. In this example the degree of decarbonation is estimated on the basis of the loss on ignition of the dust. A fuller analysis would permit a more accurate estimate. o
The dust loss is 0.06 kg./kg. of clinker. The dust leaves the system at the exit gas temperature of 212 C. Assuming a specific heat of 0.21 (i.e. as for CaCO 3 ) the sensible heat loss is 0.06 x 0.21 x (212 - 20) = 2.42 kcal./kg of clinker.
The percentage decarbonation has been estimated as 55.6% equivalent to 0.209 kg. of carbon dioxide per kg. of dust. Assuming this carbon dioxide to come from the dissociation of calcium carbonate, the weight of calcium carbonate dissociated is 0.06 x 0.209 x
100 = 0.029 kg./kg. of clinker 44
o
At 20 C, the heat of dissociation of calcium carbonate is 422 kcal./kg. hence the heat required to partially decarbonate the dust is .029 x 422 = 12.24 kcal./kg. of clinker. The heat loss associated with the dust is, therefore, 2.42 + 12.24 =14.66 kcal./kg. of clinker. Also associated with the dust is the heat required to dry its slurry moisture and combined water. The slurry moisture as shown in section 3.10 is equal to 0.048 kg./kg. of clinker. o
The heat required for the evaporation of this moisture at 20 C (again using a latent heat of 586 kcal./kg.) is 0.048 x 586 = 28.13 kcal./kg. of clinker. The combined water (1.34%) amounts to 0.075 x
1.34 = 0.001 kg./kg of clinker. 100
o
The heat required to evaporate this water at 20 C is 0.001 x 586 = 0.59 kcal./ kg. of clinker. The total heat required for evaporation of water associated with the dust is therefore 28.13 + 0.58 = 28.72 kcal./kg. of clinker. The total heat loss associated with the dust is therefore 14.66 + 28.72 = 43.38 kcal./kg. of clinker. It will be noted that the heat required to vaporize and heat up to the exit gas temperature the water in that part of the feed lost as dust and also the sensible heat of the carbon dioxide evolved by the dust have been estimated earlier. It is, of course, possible to consider those heat quantities under the heading of heat lost in making dust. On some works dust is returned to the kiln. Where this happens calculations should be based on the net dust loss (i.e. total dust loss. minus that returned), the returned dust being considered as part of the feed.
3.21 HEAT LOST BY INCOMPLETE COMBUSTION The presence of carbon monoxide in the exit gas indicates that combustion of the carbon in the fuel (or raw meal) has not been complete and this represents a loss of heat. The weight of carbon monoxide is calculated from the exit gas analysis % by volume
% by volume after elimination of CO 28.2
28.1 0.1 0.85
CO2 CO O2
0.8
This 0.8% was shown in section 3.13 to represent 0.0274 kg. of oxygen/kg. of clinker. Therefore the oxygen required to combine with the carbon monoxide present is 0.0274 x
(0.85 - 0.8) = 0.0017 kg./lg. of clinker 0.8
Carbon monoxide reacts with oxygen thus 2CO
+
(56kg.)
O2 (32kg.)
→
2CO2 (88kg.)
Therefore the weight of carbon monoxide combining with 0.0017kg. of oxygen is 0.0017 x
56 = 0.003kg 32
The heat lost in burning carbon to carbon monoxide instead of carbon dioxide is 2417 kcal./kg of carbon monoxide. The heat lost by incomplete combustion is therefore 0.003 x 2417 = 7.25 kcal./kg of clinker.
TABLE 15.5 HEAT BALANCE HEAT INPUT Coal Combustion Sensible Heat
kcal./kg. of Clinker
% of heat input
1607.0 0.0
99.59 0
8.60 - 4.04
0.53 - 0.25
2.13
0.13
1613.69
100.00
Theoretical Heat
417.00
25.84
Evaporation of Water Heat to vaporize slurry moisture Heat to vaporize combined water in feed Heat to vaporize coal moisture Heat to vaporize water from combustion
574.30 11.90 7.59 54.9
35.59 0.74 0.47 3.41
Sensible Heat of Exit Gases Sensible Heat of Exhaust Air from Cooler
254.5 40.60
15.77 2.52
Sensible Heat of Clinker
19.60
1.21
Shelf Loss
210.90
13.07
Dust Loss
43.38
2.69
7.25
0.45
1642.02
101.76
28.33
1.76
Feed Combustion of Organic Matter Sensible Heat Air, Sensible Heat TOTAL HEAT OUTPUT
Sensible Heat in Exhaust Gases
Loss due to Incomplete Combustion TOTAL Heat Unaccounted for
3.22 HEAT UNACCOUNTED FOR All the various items in the heat balance have now been calculated, and are summarized in Table 15.5. In this particular case the heat unaccounted for is 28.8 kcal./kg. of clinker, only 1.8% of the total heat input. In making any heat balance, there is likely to be some heat unaccounted for. The relative size of this factor gives some measure of the accuracy of the balance and the data on which it is based. However, it has to be remembered that an error in one item may be cancelled out by errors in other items, resulting in a misleadingly small out of balance. Although inaccurate data or the failure to consider certain factors are the most likely causes of a large heat unaccounted for, the possibility of unsteady conditions at the time of making measurements should also be taken into account, For example, it will be readily appreciated that after lighting up a cold kiln the heat input will exceed the heat output until steady conditions are reached. In certain instances, the determination of all the items (and in particular the shell loss) in the heat balance may not be possible and these items are then included as heat unaccounted for.
4
4.1
USES OF THE HEAT BALANCE
SIMPLIFICATION OF THE HEAT BALANCE
Calculation of a heat balance along the lines described above is lengthy and tedious, and may require data which is not always available. Certain simplifications may be justified, however, without too much approximation. On the heat input side of the balance it is reasonable to treat the burning of the fuel as the sole source, as the other inputs rarely exceed 1% of the total. On the heat output side of the balance the quantities are more evenly divided. As indicated in section 3.9 it is reasonable to assume a value of 420 kcal./kg. of clinker for the theoretical heat. The heat required to vaporize the slurry moisture represents the major constituent on the output side, but can fairly readily be calculated. The heat to vaporize the combined water in the feed and coal moisture can only be ignored if the relevant contributions in the raw meal and coal compositions are also small, i.e. < 2%, and < 20% respectively. The sensible heat in the exit gases represents the most tedious part of the calculation, but is also one of the major constituents in the balance. The sensible heat of clinker can be calculated very readily. The shell loss cannot be calculated with any accuracy without numerous surface temperature measurements. In general the shell loss can be taken, as shown in section 3.19 as about 4070 kcal./hr.m 2 of surface, which may decrease with increasing kiln size. Small wet process kilns may, however, have shell losses as high as 20% of the heat input, and this value probably gives rise to the greatest inaccuracies. If the dust loss is not known, the assumption of 5% on clinker and 30% decarbonisation is probably a fair approximation, from which the heat loss can be calculated. Finally, the heat loss due to incomplete combustion may be ignored if the %CO in the exit gas is suitably small, i.e. < 0.5%.
4,2 PROGRAMMED HEAT BALANCES
If suitable data is available, either first hand or by reasonable approximation then this type of calculation can be programmed. By using a computer it is possible to reproduce the results of the above heat balance within a matter of seconds. This method also has much greater flexibility as it is also possible to vary some of the input data and predict their likely effects upon fuel economy. A computer program has been developed by Eng. R & D, Barnstone on similar lines to the above method to construct a kiln and cooler heat balance. By introducing a suitable loop into the program it is possible without introducing large errors to investigate the effects of selected input variables upon the heat requirements of the kiln. The variables investigated using this technique are slurry moisture, back end temperature, back end oxygen, dust losses, and shell losses. The results of these variations are plotted graphically in Fig. 15.3 (a) - (e), and discussed below based on the sample calculation.
4.2.1 SLURRY MOISTURE Variation in slurry moisture content has a marked effect upon heat input in that a reduction of 0.5% in slurry moisture yielded a saving of about 1% in coal consumption. By the use of suitable additives greater reductions may be obtained in slurry moisture, and hence coal consumption. It is therefore important to run at the lowest practicable slurry moisture content in the interests of fuel economy.
4.2.2 BACK END TEMPERATURE o
Back end temperature reduction also produces a significant effect upon heat requirement. A reduction of 10 C in back end temperature resulted in a fuel saving of about 1%. Some limitations may be encountered with for example dewpoint (leading to corrosion), but again small reductions can produce appreciable savings.
4. 2. 3 DUST LOSSES A large source of fuel wastage is seen in the effects of dust loss. If in the sample heat balance the dust loss were doubled, the fuel requirement would rise by a factor of about 6 ½ %. On some kilns dust losses of the order of 24%, four times the sample value, are obtained representing an enormous waste of fuel. The dust removed also provides a large disposal problem, as it is rarely in the case/of a wet process returned to the kiln (by e.g. insufflation at the kiln hood).
4.24 SHELL LOSSES Shell losses represent a fairly large proportion of the heat losses, i,e. about 13% in this case. Reduction in shell losses could only be gained by improving the insulation of the kiln lining which is rarely possible. If however improved insulation were possible, then a reduction of say 25% in shell loss could yield a saving of heat input of about 3%. 4.2.5 BACK END OXYGEN Back end oxygen content, as shown on Fig, 15.3 (e), has a significant but less drastic effect upon heat requirements than the above variables. Accurate control of the back end oxygen is still a very effective method of saving fuel, as a reduction from say 2.5% to 0.5% can save about 1% of fuel consumption. It is therefore important to meter the back end oxygen content as accurately as possible, and work at the lowest practicable value. It is not possible to apply this type of treatment with sufficient accuracy to a practical system as it considers the effects of one variable isolation. In practice changes in one input variable would affect others, e.g. changes in slurry moisture would result in changed values of back end temperature, dust loss, etc. The heat balance does however highlight the order of savings which may be achieved by small improvements in the more significant variables, i.e. slurry moisture, back end temperature, and dust loss. As fuel represents a major proportion of production cost even small improvements in fuel efficiency can be very worthwhile.
5
REFERENCES
Crichton, D.C., 1938, Rotary Kiln Heat Balance by Equations. A.p.C.M. Ltd, Research Dept. 6 pp. zur Strassen, H., 1957. The Theoretical Heat Requirements for Cement Burning. Zement - Kalk - Gips, 10.1., p 1-12.
APPENDIX I - HEAT OF REACTION, DALTON'S LAW, AND DEWPOINT 1
HEAT OF REACTION
In order to carry out certain chemical reactions it is necessary to supply heat. These reactions are said to be endothermic. An important example in this context is the dissociation of calcium carbonate. CaCO3
→ CaO + CO2
In other reactions, however, heat is evolved and these are said to be exothermic. The combustion of coal or oil are obvious examples. The combination of the oxides CaO, SiO2, Al2O3 and Fe2O3 in the burning zone is another example. A specific quantity of heat is associated with any one reaction. This heat of reaction is found to vary with temperature in most reactions, It is normal to present heat of reaction data in terms of an isothermal reaction
(i.e. the products of the reaction are assumed to be brought to the initial temperature of the reactants) at some o o o arbitrary reference temperature (e.g. 0 C, 20 C). Table 15.6 contains the heats of reaction at 20 C of the main reactions occurring in the material in the kiln, It should be appreciated that these reactions do not necessarily o take place at the reference temperature of 20 C. The heat of reaction at some other temperature, t, can be found from the data in Table 15.6 by assuming the o o reactants are brought from t to 20 C then react and the products are taken from 20 C to t.
2
DALTON'S LAW-OF PARTIAL PRESSURE
This law states that the pressure exerted by a mixture of non-reaction gas is equal to the sum of pressures which each gas would exert if it alone occupied the total volume of the mixture at the same temperature i.e. PV = V (P1 + P2 + P3 . . . .) etc. where P and V are the pressures of the mixture and P1, P2, P3, etc. are the partial pressures of the individual gases, In this context the law finds an important application in connection with the dewpoint of gases; this is dealt with in the next section. The partial pressure of carbon dioxide in kiln gases determines the conditions at which the decarbonation reactions occur (see Figure 15.4). For any particular temperature there is a partial pressure of carbon dioxide below which dissociation occurs.. CaCO3
→
CaO + CO2
and above which the reverse reaction takes place: CaO + CO2
3
→
CaCO3
DEWPOINT OF A GAS
Consider a gas in which the partial pressure of water vapor is P w . On cooling the gas temperature is reached where the water vapor begins to condense. This temperature is the dewpoint of the gas and it varies with P w as shown in Figure 15.5. P w is equal to 1 atmosphere (760mm. Hg) at the boiling point of water. The dewpoint temperature of a gas can be calculated from the weight composition as illustrated. The temperature of slurry in the back end of a kiln (or nodules in a semi dry process preheater) has to be raised to the dewpoint of the exit gas before drying commences. As long as the surface of the material remains damp, the temperature of the slurry is maintained at about the dewpoint of the gas. Once the surface moisture is removed, the slurry temperature starts to rise and may exceed the boiling point of water before the slurry is completely dried.
From the above it is evident that the dewpoint represents the lowest possible exit gas temperature, Since some cooling takes place between the kiln exit and the stack, it is obviously desirable to work with a somewhat higher temperature to prevent condensation in ducts, precipitators etc. Additionally, any sulphur trioxide in the flue gases will result. in an increased dewpoint. It is suggested that the kiln exit temperature be o maintained about 50 C above the dewpoint.
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 8 Section 2
Heat Balances – Imperial Units
PAPER NO.5 HEAT BALANCES
1.
Introduction
2.
Objectives of the Heat Balance
3.
Internal Heat Exchange
4.
Control Volume
5.
Units
6.
Mass Balance
7.
Reference Temperature
8.
Sensible Heat
9.
Heat of Reaction
10.
Combustion of Coal
11.
Latent Heat
12.
Heat of Clinker Formation (Theoretical Heat)
13.
Shell Losses
14.
Heat Unaccounted For
15.
Uses of Heat Balance
16.
Conclusion
Appendix I Fundamentals of Heat Balances
HEAT BALANCES (BTU/lb UNITS) 1.
INTRODUCTION
The purpose of the rotary kiln is to increase the temperature of the raw material so that the chemical reactions leading to the formation of cement clinker can take place. The heat required to increase the temperature of the feed and for the chemical reactions is generated by burning fuel. The plant operator is interested in the economic aspects of kiln fuel utilization. For a given kiln system this is governed by the thermal efficiency at which the system is being operated. Clearly it is desirable on the grounds of cost to operate the kiln at the lowest possible fuel consumption. But this must be consistent with the highest practicable output of acceptable quality clinker. 2.
OBJECTIVES OF THE HEAT BALANCE
The heat balance equates the heat supplied to - consumed in – and lost from the kiln system under equilibrium (steady normal operating) conditions, as shown schematically in Fig.l. Consideration of the heat balance enables the following objectives to be met: a)
To account for the energy actually used
b)
To monitor plant performance regularly
c)
To evaluate the effects of changes in materials, plant and process operations on fuel consumption
d)
To decide where to give priority in the works improvement plan to reduce fuel consumption
e)
To provide data for improved plant design, i.e: refurbishment, modification, new plant
f)
To achieve the basic objectives of kiln operation, i.e. maximum output of acceptable clinker at lowest possible fuel consumption.
3.
INTERNAL HEAT EXCHANGE
The heat balance ignores the internal modes of heat transfer, but rather illustrates in the relative distribution of outgoing quantities what the heat input is required for. Nevertheless it is worth looking at the internal heat transfer to see how the heat is usefully used in the kiln system.
Fig.2 shows the temperature of material and gas along a kiln and the five zones into which it is conventional to divide the kiln. In the first two zones, the temperatures are relatively low and the processes which the feed undergoes are mainly physical, i.e. drying and preheating. In modern practice these processes are carried out in a separate preheater. In the third zone, chemical reactions start to take place, in particular the dissociation of calcium carbonate (calcining or decarbonation). In preheater kilns about 30% and in precalciner kilns up to 95% of this process occurs in the preheater system. In this zone, it will be noted that the material temperature rises only slightly despite a big change in gas temperature. In the fourth zone, the material is raised to above 1400°C at which temperature the main clinker-forming reactions occur. The burning of fuel is arranged so that the gas temperature is a maximum in this zone. Finally, in the fifth zone, the clinker is cooled by gas at a lower temperature. This process, of course, is largely completed in a separate cooler. The main quantities of heat involved in carrying out the processes in each of the five zones can be readily determined, and hence the overall heat requirement of the kiln system can be obtained. The relative length of each of the five zones is determined by several factors (e.g. difference in temperature of gas and material, gas velocity, volume loading), and there is considerable overlapping of the processes. It is the external heat exchange factors with which we are primarily concerned in the heat balance, and the principles to be discussed in this paper are listed in Fig.3.
4.
CONTROL VOLUME
4.1
Concept of Control Volume
The control volume is the system enclosed by external boundaries across which the heat flows occur. The heat balance is concerned with these cross boundary flows which must be measured/calculated. If is these heat flows that the operator aims to control as much as possible. Fig.4 shows the control volume for a wet process kiln with grate cooler. Feed and fuel and primary and secondary air enter the system. Clinker and flue dust, kiln exhaust gases and cooler exhaust air leave the system. Some heat is also lost from the kiln and cooler shells. Where a preheater is installed, inleaking air and preheater shell losses must also be considered (Fig.5). If there is a precalciner, coal will also enter the system at the preheater. With rotary and planetary coolers there will be no cooler exhaust air to consider.
4.2
Steady State Conditions
It is essential that the system within the control volume is operating under steady equilibrium conditions for the heat balance to be valid. Under steady state conditions the total rate of heat flowing into the control volume will equal the total rate of heat leaving the control volume (Fig.6). If equilibrium has not been achieved, e.g. kiln heating up; feed or fuel rate changes occurring, feed, fuel or clinker quality changing etc, then heat entering and leaving will not be equal and there will be errors in the heat balance. Lack of steady state conditions is the most common cause of serious errors in heat balances.
5.
UNITS
5.1
Conventional Units
The normal convention is to use to the Système International d'Unitès (SI) units for the heat flow - kilocalorie (kcal) to produce one unit - kilogramme (kg) of clinker. i.e. kcal/kg clinker It is important to understand the difference between net and gross bases of expressing these units. The term kcal/kg clinker implies net basis, which is normally used for comparisons. The significance of net and gross units will be discussed later. 5.2
USA Units
In the USA the units used are based on a mix of the British thermal unit (Btu) and the American short ton (T) systems. i.e. mBtu/T(m = 1 million) where 1 kcal/kg = 3.6 x 10 −3 mBtu/T or 1 mBtu/T = 277.78 kcal/kg 6.
MASS BALANCE
A prerequisite for making the heat balance is a knowledge of the various quantities of gases and solids entering or leaving the system (control volume). A mass balance has, therefore, to be performed prior to calculation of the heat supplies or losses in the heat balance. The data required will consist of rates of raw meal, fuel and air entering the system, which should equal the rates of clinker, dust and flue and exhaust gases leaving the system. (Fig.7). Again, it is essential that the mass balance is made under steady state conditions. The mass balance will actually be partly measured and partly calculated, and it is the measured parameters that must be at equilibrium. When equilibrium exists, the mass flow into the system in unit time will equal the mass flow from the system (Fig.8). i.e. gas flow W 1 = gas flow W 2 (=W) But with heat Q added, the temperature of the masses W 1 and W 2 will not be equal. i.e. gas temp T 1 gas temp T 2
7.
REFERENCE TEMPERATURE
For determining the heat balance, it is necessary to define a reference or datum temperature on which all quantities of sensible heat are based. An obvious reference temperature could be 32°F, but more commonly a temperature nearer to ambient is used for convenience. In the UK a reference temperature (t ref ) of 20°C (68°F) is used. In tropical areas a higher temperature, 25 (77°F) or 30°C (86°F) may be defined.
8. 8.1
SENSIBLE HEAT Heat v Temperature
If the same quantity of heat is supplied to the same mass of different materials and there are no chemical or physical changes of state, the resulting temperature rises are not the same, but depend on the specific heats of 'he materials. The heat contained by the material giving rise to its temperature is its sensible heat. Suppose 'that a quantity of heat Q is supplied to a given mass of material m, leading to a rise in temperature of the material from Lemperature t° 1 to t° 2 then: Q = mS (t° 2 - t° 1 ) where S is the mean specific heat of the material over the temperature range t° 1 to t° 2 . The sensible heat of each material is calculated in the above manner by calculating the heat contained in the material above a datum temperature. e.g. Consider a grate cooler exhaust of 4 lb air at 390°F per lb of clinker produced, reference temperature 68°F: Q = ms (t ref - t 1 )( t 1 - t ref ) = 4-x 0.242 (390-68) Q = 311.7 BTU/lb clinker
8.2
Specific Heat
8.2.1
Tables
Some useful values of mean specific heats are given in Tables 1 and 2. It can be observed that mean specific heats usually vary with temperature. The tables give mean specific heats between a reference temperature (68°F) and t x . 8.2.2
Interpolated Values
Intermediate values can be interpolated, most easily by plotting a SH/temperature curve over the appropriate range. e.g. mean SH of clinker between 68°F and 660°F from Fig.9 is 0.210 by interpolation. 8.2.3
Calculated Values
It is possible to calculate the mean SH between any two temperatures using the data in the tables as follows: SH t 2 -t 3 =
SH t 1 - t 3 (t 3 - t 1 ) - SH t 1 - t 2 (t 2 - t 1 ) (t 3 − t 2 )
e.g. Oxygen SH 68°F to 212°F = 0.220 SH 68°F to 392°F = 0.223 SH 212°F to 392°F = 0.223
(200 - 20) - 0.220 (100 - 20) 200 - 100
= 0.2254 All heat quantities (Q) associated with sensible heat can be calculated knowing mass (m), temp (t o x ) and mean SH ( St o ref − t o x ). 9.
HEAT OF REACTION
In order to carry out certain chemical reactions, it is necessary to supply heat. These reactions are said to be endothermic. An important example in this context is the dissociation of calcium carbonate. CaCO 3 → CaO + CO 2 (886.7 BTU/lb clinker) In other reactions, however, heat is evolved and these are said to be exothermic. The combustion of coal or oil are obvious examples. The combination of the oxides CaO, SiO 2 , A1 2 O 3 and Fe 2 O 3 in the burning zone is another example. A specific quantity of heat is associated with any one reaction. This heat of reaction is found to vary with temperature in most reactions. It is normal to present heat of reaction data in terms of an isothermal reaction (i.e. the products of the reaction are assumed to be brought to the initial temperature of the reactants) at some
arbitrary reference temperature (e.g. 32°F, 68°F). Table 3 contains the heats of reaction at 68°F of the main reactions occurring in the material in the kiln. It should be appreciated that these reactions do not necessarily take place at the reference temperature of 68°F.
The heat of reaction at some other temperature, to, can be found from the data in Table 3 by assuming the reactants are brought from t°C to 68°F, then react and the products are taken from 68°F to t°
10.
COMBUSTION OF COAL
10.1
Calorific Value
When coal is combusted in air the combustion products include water vapor from the hydrogen in the coal. The calorific value (CV) of coal as determined is the gross value. The water vapor from the dry coal combustion is condensed in the test apparatus, giving up ILS latent heat which 'is included in the water bath measurement of the heat evolved (Fig.10). Hence the CV test gives the "higher heating value" or gross CV of the coal. In the kiln system, the water vapor from coal combustion is discharged to atmosphere via the stack. Condensation occurs in the atmosphere and the latent heat is then given up outside the control volume. Hence only the "lower heating value" or net CV of the coal is utilized. The net CV of the coal can be calculated from the measured gross CV if the hydrogen content is known, i.e:
CV net = CV gross - LHV
M + 9H 2 BTU/lb 100
where M = % moisture in coal (wet basis) H = % hydrogen in coal (dry basis) LHV = latent heat 1= 1056 BTU/lb For typical UK kiln coals the gross to net factor is approximately 0.96. (For heavy fuel oil it is 0.94 and for natural gas 0.90.) Calculation of kiln fuel consumption using the net CV of the fuel gives a true value for the heat actually consumed. It must be used when making comparisons between kilns fired by different quality coals or different fuels.
10.2
Heat Supplied by Coal
For a coal fired wet process kiln the heat supplied by the coal is calculated as follows (similarly for other kiln processes): Dry coal CV = 12150 BTU/lb gross Coal moisture = 5% As-fired coal consumption = 25.1% Net CV = 12150 x 0.96 = 11664 BTU/lb Dry coal cons. = 25.1 x
100 - 5 = 23.8% 100
(0.238 lb coal/lb clinker) Heat input = 0.238 x 11664 = 2776 BTU/lb The term “2776 BTU/lb" implies the net or actual fuel consumption.
10.3
Combustion Products from Coal
The combustibles in coal are carbon, hydrogen and sulfur. The reaction for the combustion of carbon (C) in oxygen ( O 2 ) to give carbon dioxide (C O 2 ) is as follows: C
+ O2 →
C O2
12 lb + 32 1 b → 44 lb where the weights represent the relative proportions of the reactants and product from their atomic weights.
For the wet process kiln with 0.238 lb coal per lb clinker and 76% C in the coal: C O 2 from coal combustion = 0.238 ×
76 44 × 100 12
= 0.663 lb C O 2 /lb clinker Using this value and the mean SH Of C O 2 between the reference and kiln exit temperatures, the sensible heat in the C O 2 from the coal can be calculated. Similar calculations are carried out for all other products in the kiln exit gas.
Referring back to Section 10.2, it must be understood that in the absence of a suitable excess of oxygen, some of the carbon will burn to carbon monoxide (CO) only, in which case some of the potential heat input will be lost. This CO loss must be calculated in the heat balance.
10.4
Heat From Organic Carbon in Raw Meal
Some raw meals can have a significant amount of organic carbon present, which contributes to the heat input. Although normally relatively small at up to about 10 kcal/kg, in the case of an oil shale for example, the heat input may be large (Rawang Works oil shale gives about 270 BTU/lb of total heat input of 1440 BTU/lb clinker).
11.
LATENT HEAT
When water is heated, its temperature rises to 212°F, this involves the sensible heat between t ref and 212°F. Further heating does not cause further temperature rise, but converts the water to steam (water vapor) without increasing the temperature. This is the latent (not sensible) heat of vaporization (LHV) of water, i.e. the heat required to accomplish the physical change of state from liquid to gas. The calculation can be made considering the LHV of water at the reference temperature and the mean SH of water vapor from t o ref and the exit gas temperature. Alternatively the calculation can use the mean SH of water between the reference temperature and 212°F, the LIHIV o-F water at 212°F and the mean SH of water vapour between 212°F and the exit gas temperature. For convenience we use the former calculation. e.g. t o ref = 68°F LHV68°F = 1052.8 BTU/Ib H 2 O SH68°F-414°F of WV = 0.4526 BET = 414°F 0.98 lb slurry H 2 O /lb clinker Q LHV68°F = 0.98 x 1052.8 = 1031.7 BTU/lb clinker Q SH/WV68-414°F = 0.98 x 0.4526 (414-68) = 153.5 BTU/lb clinker Total heat required to dry slurry and heat water vapor to exit gas temperature. Q tot = 1031.7 + 153.5 = 1185.2 BTU/lb clinker
12.
HEAT OF CLINKER FORMATION
12.1
Calculation of Theoretical Heat
The heat required to convert the raw meal to clinker is termed the theoretical heat. Regardless of the relative efficiency of the kiln system, this heat must be supplied to produce the Bogue clinker compounds. It can be calculated from first principles by using basic heat of reaction data (Table 3) if the composition of the raw meal is known. However, the required data is seldom available, and the calculation is tedious. Various formulae have been developed to permit more rapid estimation of the theoretical heat. A formula by zur Strassen (1957) which gives good agreement with basic calculations is: Q th = 4.002A t + 11.683M c + 13.786C c - 9.224S - 1.054 (F + Mn) BTU/lb clinker where Q th = theoretical heat of clinker formation A t = lb Al 2 O 3 ex clay per 1001b clinker M c C c = lb mgO and CaO ex MgC O 3 and CaC O 3 per 1001b clinker respectively S, (F+Mn) = % Si O 2 and % (FFe 2 O 3 + Mn 2 O 3 ) in loss free clinker
As an approximation, zur Strassen's formula can be simplified for application to typical high grade limestone and shale raw mixes as follows: Q th = 4.002A + 11.683M + 13.786C - 9.224S - 1.064F BTU/lb clinker where A, M, C, S and F are the % A1 2 O 3 , MgO, CaO, Si O 2 and Fe 2 O 3 in the clinker. i.e. clinker: A1 2 O 3 6.57%, MgO 1.09%, CaO 66.31%, S O 2 21.63%, Fe 2 O 3 2.78% Q th = 4.002 x 6.57 + 11.683 x 1.09 + 13.786 x 66.31 - 9.224 x 21.63 - 1.064 x 2.78 ∴Theoretical heat = 751 BTU/lb clinker Generally, theoretical heat values for OPC (Type I) clinker range from about 720 to 755 BTU/lb clinker. The latter value is usually taken in the absence of full data. However, the theoretical heat depends on raw meal mineralogy and clinker composition, and is affected by coal ash absorption.
12.2
Effect of Clay Mineral Type LSF 94% SR 2.3 AR 2.4heavy fuel oil (HFO) firing a)
kaolinite
754.7 BTU/lb
b)
montmorillonite
725.8 BTU/1b
c)
illite
723.1 BTU/lb
It can be seen that kaolinite minerals give theoretical heat approaching 755 BTU/1b, but montmorillonite and illite minerals are much easier to combine.
12.3
Effect of LSF Kaolinite, SR 2.3 AR 2.4 HFO firing a)
LSF 89%
732.8 BTU/lb
b)
LSF 94%
754.7 BTU/Ib
It can be seen that reducing the LSF significantly reduces the theoretical heat.
112.1
Effect of Coal-Ash Absorption Kaolinite, LSF 94%, SR 2.3, AR 2.4 Coal firing (15% ash) a)
Ash absorbed 3.75%
745.7 BTU/1b
b)
Ash absorbed 7.5%
736.9 BTU/Ib
It can be seen that coal ash acts as a mineraliser, making combination easier.
12.5 a)
Effect of Cement Type HSR (Type V) clinker, coal firing LSF 90% SR 2.3 AR 0.7
706.9 BTU/lb
Although the LSF is lower, the theoretical heat is well below what would be expected due to LSF and coal firing. b)
LH (Type IV) clinker, coal firing
LSF 84% SR 2.0 AR 0.7
676.6 BTU/Ib
As would be expected the theoretical heat of Type IV clinker is very low.
12.6
Burnability
Theoretical heat should not be confused with burnability. The latter includes raw meal fineness and homogeneity, mineralisers, free lime level etc.
13.
SHELL LOSSES
Heat is transferred from the outer surfaces of the kiln (and cooler etc) shell to the surroundings by two means. Most of the heat is lost by radiation (Fig.11a). Radiation takes place according to the equation:
(
Q r = Aεσ T2 − T3 4
4
)
Where Q r = heat lost in BTU/h by radiation A = area of shell in ft² T 2 T 3 = absolute temperature (t°F + 460) of shell and surroundings respectively ε = surface emissivity - rough steel equals 98% of black body at 930°F ( ε can be measured) σ = Stefan's constant 0.173 x 10 −8 BTU/ft²h(°R 4 ) Some heat is lost by convection (Fig 11a). Convection takes place according to the equation: Q c = 0.13 CA (t 2 - t 3 ) 1.25 where Qc = heat lost in BTU/h by convection t 2 t 3 = temperatures of shell and surroundings respectively C = constant (unity for large bodies with all round convection in still air) a correction has to be applied for wind velocity. By measuring the temperature along the kiln shell, the heat loss can be estimated using the above equations. Numerous measurements have to be made as there is a very large variation in the temperatures at various points on the shell. Measurements have also to be made of cooler, preheater, coal mill and connecting ducting temperatures as there is a substantial, though relatively smaller,. shell loss from these as well. Usually a nomogram is used to read off the shell losses (see Figs.11c and d).
The determination of the shell losses from kilns, coolers, etc is a difficult problem. The shell loss from a modern BCI operated kiln is of the order of 4070 kcal/h× m² of surface, although a very wide variation is to be expected from this value. For a moderate to large wet process kiln this would give a shell loss value of the order of 210 kcal/kg clinker. For a conventional suspension preheater kiln the shell loss would be about 90 kcal/kg clinker. It is evident from above that the shell loss depends on the temperature conditions in the kiln and the kiln geometry. On the whole, the temperature conditions in a kiln do not vary much with output. In consequence, the shell loss remains substantially constant whatever the output. However, as shown in Fig.11b, radiation losses increase exponentially with shell temperature. Hence the importance of good refractory, coating and firing conditions for fuel economy.
14.
HEAT UNACCOUNTED FOR
In making any heat balance, there is usually some heat unaccounted for. The relative size of this factor gives some measure of the accuracy of the balance and the data on which it is based. However, it has to be remembered that an error in one item may be cancelled out by errors in other items, resulting in a misleadingly small heat unaccounted for value. Although inaccurate data or the failure to consider certain factors are the most' likely causes of a large heat unaccounted for, the possibility of unsteady conditions at the time of making measurements should also be considered. For example, it will be readily appreciated that, after lighting up a cold kiln, the heat input will exceed the heat output until steady conditions are reached as the system absorbs heat. In certain instances, the determination of all the items (in particular the shell loss) in the heat balance may not be possible and these items are then included as heat unaccounted for.
15.
USES OF THE HEAT BALANCE
15.1
Simplification of the Heat Balance
Calculation of a heat balance along the lines described above is lengthy and tedious, and may require data which are not always available. Certain simplifications may be justified, however, without too much approximation. On the heat input side of the balance, it is reasonable to treat the burning of the fuel as the sole heat source, as the other inputs rarely exceed 1% of the total. On the heat output side of the balance, the quantities are more evenly divided. It is reasonable to assume a value of 755 BTU/lb of clinker for the theoretical heat for Type I clinker. The heat required to vaporize the slurry moisture represents the major constituent on the output side, but can fairly readily be calculated. The heat to vaporize the combined water in the feed and coal moisture can be ignored if the relevant contributions in the raw meal and coal compositions are small, i.e. less than 2% and 20% respectively. The sensible heat in the exit gases represents the most tedious part of the calculation, but is also one of the major constituents in the balance. The sensible heat of clinker can be calculated very readily. The shell loss cannot be calculated with any accuracy without numerous surface temperature measurements. In general, the shell loss can be taken
as about. 1500 BTU/hrft² of surface, which may decrease with increasing kiln size. Small wet process kilns may, however, have shell losses as high as 20% of the heat input and this value probably gives rise to the greatest inaccuracies. If the dust loss is not known, the assumption of 5% on clinker and 30% decarbonisation is possibly a fair approximation, from which the heat loss can be calculated. Finally, the heat loss due to incomplete combustion may be ignored if the % CO in the exit gas is small, i.e. less than 0.2%.
15.2
Programmed Heat Balance
If suitable data are available, either first hand or by reasonable approximation, then this type of calculation can be programmed. By using a computer, it is possible to reproduce the results of the heat balance within a matter of seconds. This method also has much greater flexibility as it is possible to vary some of the input data and predict their likely effects upon fuel economy.
15.3
Significance of Variables
By introducing a suitable loop into the program, it is possible to investigate the effects of selected input variables upon the heat requirements of the kiln without introducing large errors. For example, variables investigated using this technique were slurry moisture, back end temperature, back end oxygen, dust losses and shell losses. The results of these variations were plotted graphically in Fig.12 (a) - (e) and are discussed below, based on the example heat balance for a wet process kiln.
15.3.1 Slurry Moisture Variation in slurry moisture content has a significant effect on heat input. A reduction of 0.5% in slurry moisture yields a saving of about 1% in coal consumption. By the use of suitable additives, greater reductions may be obtained in slurry moisture and hence, coal consumption. It is therefore important to run at the lowest practicable slurry moisture content in the interests of fuel economy.
15.3.2
Back End Temperature
Reduction in back end temperature also produces a significant effect upon heat requirement. A reduction of 50°F in back end temperature results in a fuel saving of about 1%. Some limitation may be encountered with dew-point (leading to corrosion) but even moderate reductions can produce appreciable fuel savings.
15.3.3 Dust Losses A large source of fuel wastage occurs due to dust loss. If in the example heat balance the dust loss were doubled, the fuel requirement would rise by a factor of about 6½%. On some wet process kilns, dust losses of the order of 20% have been reported, representing a large waste of fuel. Some or all the dust can be returned to
the kiln (by insufflation at the kiln hood) or by dust scoops on the kiln shell, thereby reducing fuel consumption.
15.3.4
Shell Losses
Shell losses represent a fairly large proportion of the heat loss, i.e. about 13% in the example. Reduction in shell losses can only be gained by improving the insulation of the kiln lining which is often not possible. However, if improved insulation is possible, then a reduction of say 25% in shell loss could yield a fuel saving oil about 3%.
15.3.5 Back-end Oxygen Back-end oxygen content has a less significant effect on heat requirements than the above variables. Accurate control of the back-end oxygen is still an effective method of saving fuel however, as a reduction from say 3.5% to 1.5% can save about 1% of fuel consumption. It is therefore important to monitor and control the back-end oxygen content as accurately as possible at the optimum practicable value (1.5 - 2.0%). 15.3.6 Interaction of Variables It is not possible to apply this type of study to a practical system with sufficient accuracy as it considers the effects of one variable in isolation. In practice, changes in one input variable affect others, e.g. changes in slurry moisture will result in changed values of back-end temperature, dust loss, etc. The isolated variable approach does, however, highlight the order of savings which may be achieved by small improvements in the more significant variables, i.e. slurry moisture, back-end temperature and dust loss etc. As fuel represents a major proportion of production cost, even small improvements in fuel efficiency are worthwhile. 16.
CONCLUSION
The essence of the heat balance is that the high fuel consuming items become apparent. Changes in fuel consumption can be ascribed to particular changes in the process and remedial/improvement action decided. Priorities for improvements can be established. As a works improvement plan progresses, the heat balance will show the real savings being achieved. The simple presentation enables all members of the management team to follow the progress being made and to participate in the optimization of fuel consumption. References Crichton, D.C., 1938. Rotary Kiln Heat Balances by Equations. APCM Ltd, Research Department. zur Strassen, H., 1957. The Theoretical Heat Requirements for Cement Burning. Zement-Kalp-Gips, 10.1, p 1-12. APCM Ltd, Research Department, 1964. The Effect of Chemical Composition on the Heat Requirements of Raw Mixes, SR/28, 1964.
TABLE I MEAN SPECIFIC HEATS OF UNDISSOCIATED GASES BETWEEN 68-F AND tt°F
O2
N2
Air
CO
CO2
68 212 392 572 752 932 1112 1292 1472 1652 1832 2012 2192 2372 2552 2732 2912 3092 3272 3452 3632
0.218 0.220 0.223 0.227 0.230 0.234 0.237 0.240 0.243 0.245 0.247 0.249 0.251 0.253 0.255 0.256 0.258 0.258 0.260 0.262 0.263
0.248 0.248 0.249 0.250 0.252 0.254 0.256 0.258 0.261 0.264 0.266 0.268 0.271 0.272 0.275 0.276 0.278 0.280 0.281 0.282 0.284
0.240 0.240 0.242 0.243 0.246 0.248 0.250 0.253 0.256 0.258 0.260 0.263 0.265 0.267 0.269 0.271 0.272 0.273 0.274 0.276 0.277
0.249 0.249 0.250 0.252 0.254 0.257 0.259 0.262 0.265 0.268 0.270 0.273 0.275 0.277 0.278 0.280 0.282 0.283 0.285 0.286 0.287
0.198 0.211 0.221 0.230 0.238 0.246 0.252 0.258 0.263 0.268 0.271 0.275 0.278 0.281 0.284 0.286 0.289 0.291 0.293 0.294 0.296
H2O Vapor 0.435 0.447 0.452 0.457 0.463 0.471 0.478 0.486 0.495 0.502 0.512 0.519 0.527 0.532 0.542 0.547 0.553 0.561 0.567 0.573 0.578
SO2 0.143 0.147 0.150 0.154 0.157 0.161 0.164 0.167 0.170 0.173
Above 2732°F dissociation must be taken into account Data for O2, N2, Air, CO, CO2, H2O Vapor from Spiers : Technical Data on Fuel, 1962 Data for SO2 from Perry : Chemical Engineers' Handbook
TABLE 3 - HEATS OF REACTION AT 68°F
The heat quantity refers to the compound underlined The + and - signs indicate endothermic and exothermic reactions respectively Data from Report SR-64/28/R-8
FIG.8 GAS FLOWING THROUGH A SYSTEM
Heat Loss vs Surface Temp Strong Wind (10 C)
Heat Loss (kcal/m2/h)
12000
Strong Wind (20 C)
10000
Strong Wind (30 C)
8000
Med Wind (10 C)
6000
Med Wind (20 C)
4000
Med Wind (30 C)
2000
Still Wind (10 C) Still Wind (20 C)
0 0
200
400 Still Wind (30 C)
Surface Temperature (C)
APPENDIX I FUNDAMENTALS OF HEAT BALANCES 1.
Preliminary Considerations
In making a heat balance, the total heat supplied to the system is equated to the total heat leaving the system under equilibrium conditions. This makes no particular allowance for the internal heat exchanges occurring, but shows how the heat used may be divided from a consideration of the heat input and output quantities. A prerequisite of making the heat balance is the calculation of the various quantities of gases, liquid and solids entering or leaving the system. The total weight of feed, fuel and air entering the system will equal the total weight of the clinker, dust, air and gases leaving the system. Likewise the weight of any component (e.g carbon, CaO) in the material streams entering the kiln will equal the weight of the same component in the material streams leaving the kiln. 2.
Heat Supplied to the Kiln
The heat supplied to the kiln may be considered to come almost entirely from the fuel, although the raw materials may contain a small percentage of organic material which contributes some heat to the system when it burns. If the material feeds are above the datum temperature a small quantity of sensible heat will also be shown on this side of the heat balance. 3.
Heat Expenditure
The ways in which heat is used in the kiln and the various heat losses may be divided as follows: a)
Theoretical Heat The net total of heat required for the various chemical reactions, i.e dissociation of carbonates, formation of silicates and aluminates in the burning zone and the removal of combined water from clay minerals. It is assumed that the reactions take place at the datum temperature.
b)
Heat Lost in Exhaust Gases i)
The water in the feed is evaporated and heated to the exit gas temperature.
ii) The CO2 from the dissociation of carbonates is heated to the exit gas temperature. iii) The gases from the combustion of fuel and organic matter in the feed are discharged at the exit gas temperature. iv) Any excess air used above that required for combustion is heated to the exit gas temperature. v)
Any moisture in the fuel as fired is evaporated and is heated to the exit gas temperature.
vi) The air used for combustion contains a small quantity of water vapor which is heated to the exit gas temperature (in addition water may be sprayed into the cooler). vii) Excess hot air is exhausted from a Fuller (grate) type cooler. c)
Heat Lost in Clinker Shell Loss There are losses through the kiln shell and hood and the walls of the kiln, cooler, preheater and coal mill by radiation and convection.
d)
Dust Loss The dust carried out of the kiln is heated to the exit gas temperature and may have partially reacted.
e)
Heat Lost by Incomplete Combustion Any carbon monoxide present due to imperfect combustion represents a loss of heat.
4.
Basis and Datum Temperature
In a heat balance on a kiln it is simplest to make calculations on the basis of a given weight of clinker, usually 1 kg or 11b. Heat quantities are expressed as kilocalories or British thermal units respectively. Hence the units used will be kcal/kg and Btu/lb respectively. The heat quantities are calculated from a datum temperature. This can be taken as the atmospheric temperature or some similar fixed temperature (e.g. 60°F, 20°C).
HEAT BALANCE WET CHAINED KILN WITH GRATE COOLER (BTU/lb UNITS)
1.
INTRODUCTION
It is clearly desirable on the grounds of cost to operate the kiln with as low a consumption of fuel as possible consistent with a high output of good quality clinker and, to this end, it is necessary to understand how the heat generated by burning fuel is utilized. This requires the construction of a heat balance. 2.
THE HEAT BALANCE
When the heat balance has been constructed, it should yield a detailed account of all sources of heat utilization, i.e. which functions use large amounts of fuel and which functions use a negligible amount of fuel. If greater thermal efficiency can be achieved in the system, it will show which items are worthy of greater attention. in the wet process rotary kiln system, a heat balance will show that virtually all the heat input is utilized between: a) The theoretical heat of reaction
b) Vaporization of the slurry moisture c) Sensible heat of the exit gases d) Shell losses The base lines upon which the heat balance is performed are selected mainly for simplicity of calculation. The quantities of heat involved are based upon 1 lb of clinker and listed as BTU/lb. The quantities of sensible heat are calculated from the datum temperature (68°F).
CONSTRUCTION OF THE HEAT BALANCE As an example, a heat balance on a wet process coal fired kiln will now be calculated. The relevant kiln data and analyses of raw meal, clinker and fuel are set out in Tables 1 and 2. (The data available in practice may be less than this amount, and it is thus not possible to completely standardize the procedure of heat balance determination.)
3.1
Preliminary Calculations
From the data in the tables, the first requirement is to calculate the undetermined solid mass flows. The clinker output is 34.72T/h. The raw coal consumption is 0.251 lb/lb of clinker. The coal moisture is 5%, thus the consumption of dry coal will be: 0.251 x
(100 - 5) = 0.238 lb/lb of clinker 100
Hence, coal moisture = 0.013 lb/lb of clinker The ash content represents 15.3% of the dry coal, equivalent to 0.153 x .238 = 0.0364 lb/lb of clinker. It is assumed that all the ash is absorbed in the clinker. Therefore, the clinker derived from raw meal is: 1 -0.0364.= 0.9636 lb/lb of clinker The raw material also suffers a loss on ignition on passage through the kiln. Loss on ignition is determined by placing a small weighed sample of material in a cool furnace and raising the temperature to 1652°F over one hour. After 3-4 hours at between 1562°-1742°F, the sample is removed, cooled in a desiccator and reweighed. The loss of weight determined represents mainly vapour from associated and combined water and carbon dioxide from carbonate dissociation and organic matter combustion. The raw meal suffers a loss on ignition of 36.28% so that the quantity of raw meal required to produce this 0.9636 lb of clinker is:
0.9636 x 100 = 1.512 lb 100 - 36.28 The raw meal further suffers a degree of dust loss, which is 0.06 lb/lb of clinker. The loss on ignition of the partly decarbonated dust is 20.2%, equivalent to: 0.06 ×
(100 − 20.2) = 0.075 lb (100 − 36.28)
lb dry meal/lb of clinker
the total raw meal required to produce 1 1b of clinker is, therefore, 1.512 + 0.075 = 1.587 lb. A slurry moisture of 39.2% will be equivaliant, therefore, to: 1.587 x 39.2 = 1.023 lb of water/lb of clinker (100 - 39.2)
3.2
Heat input
The total heat input is calculated by summing the various components containing both sensible and potential heat. In this example, we must consider any sensible heat contained in the fuel, combustion air and raw materials, plus the potential heats contained in the fuel and raw material.
3.3
Potential Heat in Coal
Raw coal burnt per lb of clinker is equivalent to 0.238 lb of dry coal. Gross calorific value of dry coal = 12170 BTU/lb. Heat supplied by burning coal: 0.238 x 12170 = 2896 BTU/lb of clinker (gross) It will be noted that the gross calorific value has been used in which it is assumed that the water vapor from the combustion of the dry coal is condensed. In fact, this water is carried out of the kiln as vapor and an allowance has to be made for this in calculating the sensible heat of the exit gases.
3.4
Potential Heat in Raw Materials (Organic Carbon)
The dry raw meal contains 0.07% of combustible organic carbon equivalent to: 1.587 x
0.07 = 0.0011 lb/lb of clinker 100
Calorific value of carbon = 14114 BTU/lb Heat supplied by burning carbon is 0.0011 x 14114 = 15.5 BTU/lb of clinker.
3.5
Sensible Heat in Coal
The coal is fed to the mill at 68°F (the datum temperature) to yield a nil t 2 − t 1 value and hence a sensible heat value of zero in this case: 0.238 x (68 – 68) 0.23 = nil
3.6
Sensible Heat in Combustion Air
It is calculated later (Section 3.17) that the total air drawn into the system is 4.438 lb/lb of clinker. Assuming this air is all at 72°F, its sensible heat is: 4.438 (72-68) 0.24 = 4.26 BTU/lb of clinker
3.7
Sensible Heat in Raw Materials
Slurry is fed to the kiln at 63°F, i.e. less than the datum temperature. Therefore, the sensible heat of the slurry will be a negative value on the input side of the heat balance. The specific heat of the dry raw material is taken as 0.2 (the specific heats of the main constituents are all approximately 0.2). Sensible heat in the dry raw material = 1.587 (63-68) 0.2 = -1.587 BTU/lb of clinker Sensible heat of slurry moisture 1.023 (63-68) 1 = -5.115 BTU/lb of clinker Total sensible heat of feed = -6.702 BTU/lb of clinker
3.8
Heat Output
The heat output is also the sum of various components, but these are of a rather more complex nature than the input variables.
3.9
Theoretical Heat of Reaction
A derivation of zur Strassen's formula can give a good approximation for the theoretical heat using the clinker oxide values: Q th = 4.002A.+ 11.683M + 13.786C - 9.2245S - 1.064F where A, M, C, S and F are the weight % of the clinker oxides, i.e: Q th = 4.002 x 6.57 + 11.683 x 1.09 + 13.786 x 66.31 - 9.224 x 21.63 - 1.064 x 2.73 = 752.0 BTU/lb of clinker
3.10
Heat to Evaporate Water
The slurry moisture is equal to 1.023 lb/lb of clinker. Incorporated in this figure is the moisture content of the dust losses, equal to: 0.075 x 1.023 = 0.048 lb/lb of clinker 1.587 Treating the dust loss moisture separately, this leaves 1.023 -0.048 = 0.98 lb/lb of clinker of slurry moisture. It is assumed that this water is evaporated at 68°F, at which temperature the latent heat is 1056 BTU/lb. Therefore, the heat required for the evaporation of slurry moisture is: 0.98 x 1056 = 1035 BTU/lb of clinker The raw material contains 1.34% of combined water equal (deducting the dust loss component) equal to: 1.512 x 1.34 = 0.0203 lb/lb of clinker 100 The heat required to evaporate this water at 68°F is: 0.0203 x 1056 = 21.4 BTU/lb of clinker (Note the heat of dissociation of combined water is included in the theoretical heat). The percentage of moisture in the coal is 5.0%, equal to: 0.251 x
5.0 = 0.013 lb of water/lb of clinker 100
The heat required to vaporize this moisture at 68°F = 0.013 x 1056 = 13.73 BTU/lb of clinker
In calculating the heat input to the kiln, the gross calorific value of the coal was used, thereby implying that the water vapor from the combustion of the hydrogen in the coal was condensed. In calculating the heat output of the kiln, therefore, the latent heat of Vaporization of this water has to be included. The amount of water in the combustion products is 0.094 lb/lb clinker (see Section 3.12). therefore, heat to evaporate water in combustion products is: 0.094 x 1056 = 99.3 BTU/lb of clinker
3.11
Sensible Heat of Exit Gases
To calculate the sensible heat lost by the exit gases, the total masses of the constituent gases have first to be calculated, via appropriate mass balances.
3.12
Combustion Products
The carbon in the fuel and raw material are burnt thus: C
O2 →
+ 12 lb
C O2
32 lb
44 lb
(A small fraction of the carbon is burnt to CO and not C O2. This is allowed for later). The hydrogen in the fuel is burnt thus: 2H2
O2 →
+ 4 lb
2H2O
32 lb
36 lb
The sulfur in the fuel is burnt thus: S
O2 →
+ 32 lb
S O2
32 lb
64 lb
On the basis of 1 lb of clinker, the fuel combustion should yield: 0.238 x
76 44 × = 0.663 lb of carbon dioxide 100 12
0.238 x
4.4 36 × = 0.094 lb of water vapor 100 4
1.6 64 × = 0.0076 lb of sulfur dioxide 100 32
0.238 x
The oxygen required for combustion per lb of clinker is: 0.238 ×
76 32 4.4 32 1.6 32 × + 0.238 × × + 0.238 × × = 0.57 lb 100 12 100 4 100 32
The 0.7% organic carbon in the raw meal is also burnt, consuming: 0.7 32 × = 0.0187 lb of oxygen/lb of raw meal, equal to: 100 12 0.0187 x 1.587 = 0.0297 lb of oxygen/lb of clinker, to give: 0.7 44 × = 0.0257 lb of carbon dioxide/lb of raw meal, i.e: 100 12
0.0257 x 1.587 = 0.0408 lb of carbon dioxide/lb of clinker A small part of this oxygen for combustion comes from the coal; per lb of clinker, this is: 0.283 ×
1.8 = 0.0043 lb 100
The weight ratio of nitrogen to oxygen in air is 3.31 (assuming the nitrogen includes all the inert gas). Therefore, the weight of nitrogen in the air required for combustion is: (0.57 + 0.0297 - 0.0043) x 3.31 = 1.971 lb/lb of clinker. There is also some nitrogen in the coal equal to: 0.238 ×
3.13
0.9 = 0.0021 lb/lb of clinker 100
Excess Air in the Exit Gases
We must now consider the excess air in the exit gas (i.e. air in excess of the combustion requirements) as shown in the exit gas analysis:
CO2 CO O2 N2 (by difference)
29.1% by volume 0.1% by volume 0.85% by volume 70.95% by volume 100.0
If the combustion had been complete, the volume of CO would have burnt to an equal volume of CO2 by combining with half its volume of O2. The gas analysis would have then been: CO2
23.2% by volume
O2
0.9% by volume
N2
71.00% by volume 100.0
(The slight contraction in volume and the resulting correction which should be made to bring the analysis back to 100% basis has been neglected - the error is insignificant at low CO contents). The O2 content of 0.8% represents the excess air. The ratio by volume of nitrogen to oxygen in air is 3.76. Therefore the N2 content representing the excess air is: 0.8 x 3.76 = 3.01% the remaining N2 being the combustion air and the coal. The N2 content being due to combustion air is: (71.0 - 3.01) ×
1.971 = 67.9% 1.971 + 0.0021
The percentage of excess air is therefore: 3.01 × 100 = 4.4% 67.9
The weight of nitrogen in the excess air is: 4.4 x 1.971 = 0.0867 lb/lb of clinker 100
and the weight of complimentary oxygen is: 0.0867 = 0.0262 lb/lb of clinker 3.31 The total weight of air entering the kiln (i.e. combustion air plus excess air) per lb of clinker is: Combustion Air
0.595 + 1.971 = 2.566
Excess Air
2.566 x 4.4/100 = 0.113
Total Air
2.566 + 0.113 = 2.679
(Also cooler exhaust air, Section 3.17)
3.14
Other Sources of Water Vapor
The combustion of the fuel provides one source of water vapor, the other sources consisting of the feed, coal moisture and included water vapor in the combustion air. The total water vapor given off by the feed is: lb/lb of clinker 0.020 = 1.043 The air entering the kiln will contain some water vapor. In this country, the average weight of water per lb of dry air is of the order of 0.005 lb. On this basis, the quantity of water vapor per lb of clinker is: 2.679 x 0.005 = 0.0134 lb
3.15
Other Sources of Carbon Dioxide
Some of the feed leaves the kiln as dust which is only partially decarbonated. Loss on ignition of the dust is 20.2% compared with 36.3% of the feed. Assuming the losses on ignition represent the degree of decarbonation, the percentage decarbonation of the dust on a loss free basis is:
36.3 20.2 − 100 − 36.3 100 − 20.2 × 100 = 55.6% 36.3 100 − 36.3
Therefore, the carbon dioxide evolved by the dust is: 55.6 x (0.3487 + 0.0257) = 0.208 lb/lb of dust 100 The dust loss of 6% on clinker is equivalent to 0 .075 lb of dry raw meal/lb of clinker. Therefore, the carbon dioxide derived from the feed is: (1.587 - 0.075)
(34.87 + 2.57) 6 + × 0.208 = 0.578 lb/lb clinker 100 100
(It has been assumed that the dust has been completely dried, i.e. slurry moisture and combined water have been removed).
3.16
Heat Content
Summation of the constituent gas weights per lb of clinker results in the following (in lb): H2O
from feed (free + combined) from combustion of coal from coal moisture from water vapor in air
CO2
from feed
1.043 ) ) 0.094 ) ) 0.013 ) ) 0.0134 )
from combustion of coal
0.582 ) ) 0.663 )
SO2
from combustion of coal
0.0076
O2
from excess air
0.0262
N2
from coal
0.0021 ) ) 1.9710 ) ) 0.0867 )
from combustion air from excess air
1.1634
1.24
2.0598
The heat required to raise these gases from 68°F to 414°F, a temperature difference of 346°F, is: (1.1634 x 0.452 x 346)+(1.245 x 0.222 x 346)+(0.0076 x 0.15 x 346)+ (0.0262 x 0.223 x 346)+(2.0598 x 0.249 x 346) = 457.4 BTU/lb of clinker, i.e: Sensible heat of exit gases = 457.4 BTU/lb clinker
3.17
Cooler Exhaust Air
2052 lb/min of air is exhausted from the cooler at 240°F. 1 lb of clinker is made every 0.00086 min. The weight of air/lb of clinker is, therefore: 2052 x 0.00086 = 1.765 lb/lb of clinker The sensible heat contained in this air is: 1.765 x (240 - 68) x 0.241 = 73.1 BTU/lb of clinker
The total air drawn into the kiln and cooler per lb of clinker (in lb) is:
Combustion Air
2.566
Excess Air
0.113
Cooler Exhaust Air
1.769 4.438 lb
This figure has been used in Section 3.6 to calculate the sensible heat of air entering the system.
3.18
Sensible Heat of Clinker
The clinker leaves the cooler at 255°F. The sensible heat in the clinker/lb of clinker is: 1 x (255 - 68) 0.188 = 35.2 BTU/lb of clinker.
3.19
Shell Loss
Heat is transferred from the outer surface of the kiln shell, to the surroundings by two means. Radiation takes place according to an equation of the form:
(
q r = Aεσ T2 − T3 4
4
)
where A is the area, T2 and T3 are the absolute temperatures of the shell and the surroundings respectively, ε is the measured emissivity of the surface and σ is the Stefan Boltzmann constant (0.173 – 10-8 BTU/hrft²R4) Convection takes place according to an equation of the form: q c = hA (t 2 − t 3 ) 1.25 where t2 and t3 are the temperature of the shell and the surroundings respectively and In is a coefficient whose value depends on a number of factors including the dimensions of the kiln and the air velocity over the kiln. By measuring the temperature and emissivity along a kiln shell, the heat loss can be estimated using formulae of the form of equations noted above. Numerous measurements have to be made as there is a very large variation in the temperatures at various points on the shell. The temperature at any particular point depends on the corresponding temperatures in the kiln, the type and thickness of the brickwork and the thickness of any coating. The shell loss from a modern kiln is of the order of 1500 BTU/hrft2 of surface, though a very wide variation is to be expected from this value.
Measurements have also to be made of cooler, preheater, coal mill and connecting ducting temperatures as there is a substantial, though relatively smaller, shell loss from these as well. For the purpose of this heat 5 balance, the total shell loss of the system is taken as 4.41 x 105 BTU/min. This is equivalent to: 4.41 x 105 x 0.00086 = 379.3 BTU/lb of clinker
3.20
Heat Loss in Making Dust
It is difficult to make an accurate estimate of the heat loss associated with the dust. The usual method is to assume the dust is partially decarbonated dry raw meal. In this example, the degree of decarbonation is estimated on the basis of the loss on ignition of the dust. The dust loss is 0.06 lb/lb of clinker. The dust leaves the system at the exit gas temperature of 414°F. Assuming a specific heat of 0.21 (i.e. as for CaCO3), the sensible heat loss is: 0.06 x (414 - 68) 0.21 = 4.36 BTU/lb of clinker The percentage decarbonation has been estimated as 55.6% equivalent to 0.209 lb of carbon dioxide per lb of dust. Assuming this carbon dioxide to come from the dissociation of calcium carbonate, the weight of calcium carbonate dissociated is: 0.06 x 0.209 x
100 = 0.029 lb/lb of clinker 44
At 68°F, the heat of dissociation of calcium carbonate is 760 BTU/lb. Hence, the heat required to partially decarbonate the dust is: 0.029 x 760 = 22.04 BTU/lb of clinker The heat loss associated with the dust is, therefore: 4.36 + 22.04 = 26.4 BTU/lb of clinker Also associated with the dust is the heat required to dry its slurry moisture and combined water. The free slurry moisture associated with the dust loss as shown in Section 3.10 is equal to 0.048 lb/lb of clinker. U4 The heat required for the evaporation of this moisture at (again using a latent heat of 1056 BTU/lb) is: 0.048 x 1056 = 50.69 BTU/lb of clinker
The combined water (1.34%) amounts to: 0.075 x
1.34 = 0.001 lb/lb of clinker 100
The heat required to evaporate this water at 68°F is: 0.001 x 1056 = 1.056 BTU/lb of clinker The total heat required for evaporation of water associated with the dust is, therefore: 50.69 + 0.59 = 51.28 BTU/lb of clinker The total heat loss associated with the dust is, therefore: 26.40 + 51.28 = 77.68 BTU/lb of clinker It will be noted that the heat required to vaporize and heat up to the exit gas temperature the water in that part of the feed lost as dust and also the sensible heat of the carbon dioxide evolved by the dust have been estimated earlier. It is, of course, possible to consider those heat quantities under the heading of heat lost in making dust. On some works, dust is returned to the kiln. Where this happens, calculations should be based on the net dust loss (i.e. total dust loss minus that returned), the returned dust being considered as part of the feed.
3.21
Heat Lost by Incomplete Combustion
The presence of carbon monoxide in the exit gas indicates that combustion of the carbon in the fuel (or raw meal) has not been complete and this represents a loss of heat. The weight of carbon monoxide is calculated from the exit gas analysis: % by volume
CO2
28.1
CO
0.1
O2
0. 35
% by volume after elimination of CO 28.2
0.8
This 0.8% was shown in Section 3.13 to represent 0.0262 lb of oxygen/lb of clinker. Therefore, the oxygen required to combine with the carbon monoxide present is:
0.85 − 0.8 0.0262 × = 0.0016 lb/lb of clinker 0.8 Carbon monoxide reacts with oxygen thus: 2CO
+
(56 1 b)
O2 (32 lb)
→
2CO2 (88 1 b)
Therefore, the weight of carbon monoxide combining with 0.0016 kg of oxygen is: 0.0016 ×
56 0.003 kg 32 =
The heat lost in burning carbon to carbon monoxide instead of carbon dioxide is 4358 BTU/lb of carbon monoxide. The heat lost by incomplete combustion is, therefore: 0.003 x 4358 = 13.07 BTU/lb of clinker
3.22
Heat Unaccounted For
All the various items in the heat balance have now been calculated and are summarized in Table 3. In this particular case, the heat unaccounted for is -48.7 BTU/lb of clinker, only 1.7% in excess of the total heat input. When the determination of all the items (in particular the shell loss) in the heat balance are not possible, these items are then included as heat unaccounted for.
4.
SIMPLIFICIATION OF THE HEAT BALANCE
Calculation of a heat balance along the lines described above is lengthy and tedious, and may require data which are not always available. Certain simplifications may be justified however, without too much approximation. On the heat input side of the balance, it is reasonable to treat the burning of the fuel as the sole source, as the other inputs rarely exceed 1% of the total. On the heat output side of the balance, the quantities are more evenly divided. As indicated in Section 3.9, it is reasonable to assume a value of 757 BTU/lb of clinker for the theoretical heat. The heat required to vaporize the slurry moisture represents the major constituent on the output side, but can fairly readily be calculated. The heat to vaporize the combined water in the feed and coal moisture can only be ignored if 'he relevant contributions in the raw meal and coal compositions are also small, i.e. less than 2% and 20% respectively. The sensible heat in the exit gases represents the most tedious part of the calculation, but is also one of the major constituents in the balance. The sensible heat of clinker can be calculated very readily. The shell loss cannot be calculated with any accuracy without numerous surface temperature measurements. In general, the
shell loss can be taken, as shown in Section 3.19, as about 1500 BTU/hrft² of surface, which may decrease with increasing kiln size. Small wet process kilns may, however, have shell losses as high as 20% of the heat input and this this value probably gives rise to the greatest inaccuracies. If the dust loss is not known, the assumption of 5% on clinker and 30% decarbonisation is probably a fair approximation, from which the heat loss can be calculated. Finally the heat loss due to incomplete combustion may be ignored if the % CO in the exit gas is suitably small, i.e. less than 0.2%.
TABLE 1 - KILN DATA
TABLE 2 - FUEL, FEED, CLINKER AND DUST DATA
TABLE 3 - HEAT BALANCE
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 8 Section 3
Paper 12 - Heat Balances
PAPER NO. 12 HEAT BALANCES
CONTENTS
1.
INTRODUCTION
2.
HEAT AND MASS BALANCES 2.1
3.
4.
CONSTRUCTION OF THE MASS BALANCE 3.1
Moisture Content
3.2
Data
3.3
Potential Heat in Coal
3.4
Sensible Heat
3.5
Specific Heats
3.6
Theoretical Heat of Reaction
3.7
Combustion Products
3.8
Calculate Excess Air
3.9
Sensible Heat of Clinker
3.10
Shell Loss
COMPLETED BALANCE 4.1
5.
Control Volume
Unaccounted Heat
USES OF THE HEAT BALANCE 5.1
Simplification of the Heat Balance
5.2
Programmed Heat Balance
5.3
Effect of Variables
HEAT BALANCES 1.
INTRODUCTION
The purpose of the rotary kiln is to increase the temperature of the raw material so that the chemical reactions leading to the formation of cement clinker can take place. The heat required to increase the temperature of the feed, for the chemical reactions to take place, is generated by burning fuel. It is clearly desirable on the grounds of cost to operate the kiln with as low a consumption of fuel as possible consistent with a high output of good quality clinker and, to this end, it is necessary to understand how the heat generated by burning fuel is utilized.
2.
HEAT AND MASS BALANCES
The economic aspects of any system involving the utilization of fuel can generally be tied to the thermal efficiency of the system. Overall efficiency is known from the dry fuel consumption and is easily converted into any preferred set of units. To understand what drives the fuel consumption , the inefficiencies must be investigated. To determine the inefficiencies, a heat balance has to be performed upon the system and this must be done under equilibrium conditions. In this balance, the heat supplied to, and lost from, the system are equated. This balance takes no account of the internal modes of heat transfer but rather illustrates the relative distribution of energy usage quantities and their location. A pre-requisite of making the heat balance is a knowledge of the various mass flows of gases and solids entering or leaving the system. A mass balance has, therefore, to be performed prior to calculation of the heat supplies or losses in the heat balance. By convention masses are specific masses. Individual mass flows are thus divided by the clinker production rate and are thus referred to 1 kg.
2.1
Control Volume
A prerequisite to undertaking a beat balance is the decision regarding what is to be included and what is not. An imaginary envelope known as the control volume needs to be set around the system or component of system in question. Such a control volume is illustrated in fig 1. Only mass and heat flows which cross the boundary need to be quantified.
3.
CONSTRUCTION OF THE MASS BALANCE
3.1 Moisture Content To avoid complexities the following calculations consider only dry materials hence the first step is to calculate all masses to dry basis:e.g.
Component A has mass
10 tph
and moisture of for example
5% 10x(100 - 5) = 9.5 100
The dry mass will be The moisture is (10-9.5) = 0.5 tph
All the individual moisture contents can thus be collected and accounted separately, if required. 4.2 Data A list of main mass flows is shown below. INPUT Feed Coal Air
OUTPUT Unknown tph 5 tph
Feed Temp LOI Feed LOI Dust Coal Temp Coal Ash Coal CV Exit Gas Temp Clinker ex cooler Considering the Flows of solids only.
Clinker Dust Combustion Gas CO2
60 deg. C. 35% 30% 20 deg. C 10% 6900 kcal/kg coal (net) 360 deg. C 180 deg. C
INPUT Feed tph x (100-LOI)/100 = solid from feed Coal tph x (100-ash)/100 = ash from coal
OUTPUT
40 tph 2 tph
Clinker= 40 tph. Dust = Measured Value as % Clinker= 2.0 tph At steady state INPUT = OUTPUT i.e.
Feed tph x (100-35000 + 5 x 10/100 = 40 + 2 Feed tph x 0.65
= 42 -.5
Feed tph
= 41.5 / 0.65 = 63.8462 tph
Since we always refer this back to a unit mass of clinker this is: = 63.8462 / 40 tph meal/tph clinker This is numerically the same as kg meal per hr/kg clinker per hr = 1.596 kg meal per kg clinker. Thus we know that we need 1.596 kgs of meal to make 1 kg clinker. This figure is used (by the laboratory) to facilitate stock control and to maintain production records. As this figure is affected by dust loss and coal ash, and since these may be variable, the so called meal to clinker factor is generally adjusted in line with stock and sales movements. Although greatly simplified the above forms the basis of the mass balance. The CO2 generated is (63.8462 - 2) x 35/100
= 21.646 tph = 21.646/40 kg C02/kg clinker = 0.541 kg/kg
Note: In this case it is assumed that CO2 derives only from the net meal input. The 2 tph of dust loss reduces the meal quantity which could generate the CO2. The base lines upon which the heat balance is performed are selected mainly for simplicity of calculation. In the same way as specific mass the quantities of heat involved are also based upon 1 kg of clinker and listed as kcal/kg. The quantities of sensible heat are calculated from a datum temperature which can be taken as ambient or some similar fixed value (e.g. 20°C).
3.3.
Potential Heat in Coal
Dry coal burnt is 5 tph this is equivalent to 5 / 40 kg of dry coal/kg clinker.(0.125 kg /kg) Net calorific value of dry coal = 6900 kcal/kg. Heat supplied by burning coal: tph dry coal/tph clinker x 6900 kcal/kg of clinker (net).(862.5 kcal/kg in this case) (6900 kcal/kg is a typical value for Wankie Colliery Coal)
3.4
Sensible Heat
If the same quantity of heat is supplied to the same mass of different materials and there are no chemical or physical changes of state, the resulting temperature rises are not the same, but depend on the specific heats of the materials. Supposing that a quantity of heat Q is supplied to a given mass of material m, eading to a rise in temperature of the material from t 1 to t 2 then: Q = mS (t2 – t1) where S is the mean specific heat of the material over the temperature range t1 to t2 Some useful values of specific heats are shown in section 6 below. The sensible heat of a material is calculated in the above manner by calculating the heat contained in the material above a datum temperature (20°C in this case). Each of the masses above has a temperature this temperature relates to a specific heat.
3.5
Specific Heats
For simplicity the specific heats are considered as fixed values and not dependant upon temperature. Values (kcal/kg °C) as follows:
3.6
Clinker
0.19
Flue gas
0.26
Limestone
0.20
Air
0.24
Coal
0.20
CO2
0.24
Theoretical Heat of Reaction
Various formulae have been developed to permit more rapid estimation of the theoretical heat. A formula of zur Strassen (1957), which gives good agreement with calculations from first principles, is: Qt = 2.22At + 6.48Mc + 7.646Cc - 5.116S - 0.59F where Qt is the theoretical heat in kcal/kg of clinker, At is the g Al2O3 from clay per 100g clinker, Mc and Cc are the g MgO and CaO from MgCO3 and CaCO3 respectively per 100g clinker, S is the % SiO2 in the loss free clinker analysis and F is the % Fe2O3 + Mn2O3 in the loss free clinker. In general, the theoretical heat of many clinkers falls in the range 400-420 kcal/kg, and the latter value can be used when insufficient data are available to apply the above formula.
3.7
Combustion Products
Fuels generally consist of various proportions of carbon and hydrogen. In addition fuels may contain sulfur, nitrogen and combined oxygen. If a fuel burns perfectly in air to produce carbon dioxide and water only i.e. it burns out all the oxygen present, this is said to be stoichiometric combustion. When residual oxygen is present in the combustion gas, excess air is said to have been used. By convention excess air is expressed as a percentage of stoichiometric mass. In the calculation of combustion the following will be used: One kg of a notional coal combines stoichiometrically with 10 kg of air and produces 11 kg of combustion products.
3.8
Calculate Excess Air
Excess air is determined from details of the combustion. The graph fig 2 facilitates the determination of excess air from residual oxygen content. Example of calculation of air used and combustion product generated. If Fuel Burnt is 5 tph Clinker produced is 40 tph Fuel per unit Clinker is .125 kg/kg If excess air is 10% Then combustion air used is (.125 x 10)
= 1.25 kg/kg
Excess air is (.125 x 10)xl0/100) Total
= 0. 125 kg/kg = 1.375 kg/kg
Combustion gas is (.125 x 10+0.125
= 1.5 kg/kg
3.9
Sensible Heat of Clinker
The clinker leaves the cooler at 180°C. The sensible heat in the clinker/kg of clinker is 1 x 0.19 x (180 - 20) = 30.4 kcal/kg of clinker. 3.10
Shell Loss
The determination of the shell losses from kilns, coolers, etc is a difficult problem. From the outer surface of the kiln shell, heat is transferred to the surroundings by two means. Radiation takes place according to an equation of the form: qr = A (T24 x T3 4 ) where A is the area, T2 and T3 are the absolute temperatures of the shell and the surroundings respectively, is the emissivity of the surface and is a constant. Convection takes place according to an equation of the form: qc = hA (t2- t3) where t2 and t3 are the temperature of the shell and the surroundings respectively and h is a coefficient whose value depends on a number of factors including the dimensions of the kiln and the air velocity over the kiln. By measuring the temperature along a kiln shell, the heat loss can be estimated using complex formulae or the graph fig 3. Measurements have also to be made of cooler, preheater, coal mill and connecting ducting temperatures as there is a substantial, though relatively smaller, shell loss from these as well.
4.
Completed Balance
Having determined the mass and heat flows we can now construct the complete balance. INPUT Feed Coal Total Air TOTAL IN OUTPUT Dust CO2 Combustion Gas Clinker Shell Loss Heat of reaction Heat unaccounted for TOTAL OUT
tph 63.8 5.0 55.0 123.8
kg/kg 1.596 0.125 1.375 3.096
kcal/kg 1.596x(60-20)x.2 .125 x 6900 1.375x(20-20)x24
2.0 21.6 60.0 40.0
0.05 0.541 1.5 1
0.05x(360-200x.2 0.541x(360-20)x.24 1.5 x (360-20)x.26 1 x (180-20) x .19
123.6
12.8 862.5 875.3
3.091
Error in mass balance= .2/l23.846 x 100 = 0.16%
3.4 44.2 132.6 30.4 220.0 420.0 24.7 875.3
Error in Heat Balance= 24.7/875.3 x 100 = 2.8 %
4.1
Unaccounted Heat
All the various items in the heat balance have now been calculated. In making any heat balance, there is likely to be some "heat unaccounted for." This can occur for a number of reasons : i)
Instantaneous fuel consumptions give a better picture of performance than taking values from production records. These latter figures inevitably contain fuel consumed in warm up after stops and if stops are frequent the difference between instantaneous and average fuel may be large.
ii)
The accuracy or repeatability of gas analysis can give an optimistic view of inleak and lead to conclusions of a lower fuel consumption than reported average.
iii) Shell losses are frequently understated generally because of the scale of measurement required to provide better accuracy. The relative size of this factor gives some measure of the accuracy of the balance and the data on which it is based. However, it has to be remembered that an error in one item may be cancelled out by errors in other items, resulting in a misleadingly small out of balance. Although inaccurate data or the failure to consider certain factors are the most likely causes of a large unaccounted heat flow, the possibility of unsteady conditions at the time of making measurements should also be taken into account. For example, it will be readily appreciated that, after lighting up a cold kiln, the heat input will exceed the heat output until steady conditions are reached. In certain instances, the determination of all the items (in particular the shell loss) in the heat balance may not be possible and these items are then included as heat unaccounted. 5.
USES OF THE HEAT BALANCE
5.1
Simplification of the Heat Balance
Calculation of a heat balance along the precise lines is lengthy and tedious, and may require data which are not always available. The above example illustrates a number of short circuits that can be made yet still allow significant insight into the way heat is being spent. On the heat input side of the balance, it is reasonable to treat the burning of the fuel as the sole source, as the other inputs rarely exceed 1% of the total. On the heat output side of the balance, the quantities are more evenly divided. As indicated above, it is reasonable to assume a value of 420 kcal/kg of clinker for the theoretical heat.
In the above the vaporization of all water is ignored, for a dry process plant such a simplification is justified but not so for a wet process plant. Coal moisture can only be ignored if the relevant contributions in the raw meal and coal compositions are also small, i.e. less than 2% and 20% respectively. The sensible heat in the exit gases represents the most tedious part of the calculation, but is also one of the major constituents in the balance. The sensible heat of clinker can be calculated very readily. The shell loss cannot be calculated with any accuracy without numerous surface temperature measurements. In general, the shell loss can be taken, as shown , as about 4070 kcal/hr.m² of surface, which may decrease with increasing kiln size. (For a kiln shell the loss is usually close to 10% of the heat input to the kiln). Small wet process kilns may, however, have shell losses as high as 20% of the heat input and this value probably gives rise to the greatest inaccuracies. If the dust loss is not known, the assumption of 5% on clinker and 30% decarbonisation is probably a fair approximation, from which the heat loss can be calculated. 5.2
Programmed Heat Balance
If suitable data is available, either first hand or by reasonable approximation, then this type of calculation is easily adapted for Lotus or similar spreadsheet application. The further extension of the development of the heat balance (almost into the PC games market) has produced a range of kiln simulators. These give operators hands on experience without any adverse consequences. As a further development it is possible to break the heat balance into a number of control volumes calculate the balances sequentially and thereby produce a powerful diagnostic toolkit. Although any variable in the heat balance may affect another, it is possible to chance a variable without introducing large errors and thus to investigate the effects of selected input variables upon the heat requirements of the kiln. The variables investigated using this technique are slurry moisture, back end temperature, back end oxygen, dust losses and shell losses.
5.3 5.3.1
Effect of Variables Back-End Oxyzen
Back-end oxygen content, as shown in Fig 4 (a), has a significant but less drastic effect upon heat requirements than the above variables. Accurate control of the back-end oxygen is still a very effective method of saving fuel as a reduction from say 3.5% to 1.5% can save about 1% of fuel consumption. It is therefore important to meter the back-end oxygen content as accurately as possible and work at the lowest practicable value (i.e. 1.5-2.0%).
5.3.2
Exit Gas Temperature
Back end temperature reduction also produces a significant effect upon heat requirement. A reduction of 10°C in exit temperature resulted in a fuel saving of about 1%. Some limitations may be encountered with, for example, dew-point (leading to corrosion) but again, small reductions can produce appreciable savings.
5.3.3
Dust Losses
A large source of fuel wastage is seen in, the effects of dust loss. If in the sample heat balance the dust loss were doubled, the fuel requirement would rise by a factor of about 6½%. On some kilns, dust losses of the order of 24%, four times the sample value, are obtained representing an enormous waste of fuel. The dust removed also provides a large disposal problem, as it is rarely (in the case of a wet process) returned to the kiln (by e.g. insufflation at the kiln hood).
5.3.4
Shell Losses
Shell losses represent a fairly large proportion of the heat losses, i.e. about 25% in the worked example. Reduction in shell losses could only be gained by improving the insulation of the kiln lining by addressing the issues on the cooler efficiency and by increasing the output (similar total heat loss for a higher output). if however improved insulation were possible, then a reduction of say 25% in shell loss could yield a saving of heat input of about 3%. it is not possible to apply this type of treatment with sufficient accuracy to a practical system as it considers the effects of one variable in isolation. In practice changes in one input variable would affect others, e.g. changes in meal or coal moisture would result in changed values of exit temperature, dust loss, etc. The heat balance does, however, highlight the order of savings which may be achieved by small improvements in the more significant variables, i.e. moisture, exit temperature and dust loss. As fuel represents a major proportion of production cost, even small improvements in fuel efficiency can be worthwhile.
Figure 2
EXCESS AIR Excess Air% vs Oxygen% Excess Air %
100 80 Excess Air %
60 40 20 0 0
2
4 6 Oxygen %
8
10
Figure 3 Heat Loss VS Shell Temperature
Heat Loss (kcal/m2/min)
Datum Ambient 21 deg C
300
Wind Velocity (0 m/s)
200 100
Wind Velocity (1.5 m/s)
0 0
50 100 150 200 250 300 350 400 Shell Temperature (deg C)
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 8 Section 4
Heat Transfer
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
Module 8 Section 5
Heat Transmission
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
PRESENTATIONS MASS, HEAT AND ENERGY BALANCES
HEAT TRANSFER
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
PRESENTATION
Mass, Heat and Energy Balance
Mass, Heat and Energy Balances
I don’t need to do this any more
Its on my computer !!
Introduction • Mass Balance – Boundary – Input/output
• Types of heat – – – –
Sensible/latent heat Heat generated by fuels Heat of reaction Heat losses
• Practical application
Mass Balance Works Example
What Crosses the boundaries
SENSIBLE AND LATENT HEAT
Sensible & Latent Heat • Sensible Heat – Heat required to raise 1kg of substance by 1 deg C
• Latent Heat – Heat required to change the phase of 1 kg without change of temperature eg. Vaporization of water to steam
Fuels
How much heat comes from the same quantity of different types of solid and liquid fuels
Calorimeter
Gross CV and net CV • Test fuel is burned inside a steel vessel • Heat released raises temperature of water and vessel • For optimum combustion, the vessel is pressurised with oxygen. • Hydrogen reacts to water which condenses ⇒latent heat is released ⇒gross CV
–Thermometer –Coal +O2
–Water bath
How much heat do I get when I Burn a fuel? Calorific value (as fired) x mass = kcals/kg of the fuel x mass of fuel
Main Combustion Reactions SIMPLIFIED SCHEME
CnHm + (n + m) O2 4 C + O2
nCO2 + m H2O 2 2 CO
CO + OH
CO2 + H
H + OH
H2O
CO + 1/2 O2 + M
CO2 + M
COMBUSTION PRODUCTS Combustibles in Coal are : Carbon, Hydrogen, Sulphur Taking Carbon : C + O2 12 kg + 32 kg
C O2 44 kg
In example balance: 0.238 kg coal with 76% C Hence : CO2 from coal combustion C O2 = 0.238 x 0.76 x 44/12 = 0.663kg/kg clinker
Heat supplied by Coal Example: Dry coal CV = 6750 Kcal/kg gross Nett CV = 0.96 Gross CV Therefore: Nett CV = 6480kcal/kg As fired coal consumption = 25.1% at 5% H20 Therefore: Dry Coal consumption = 23.8% = 0.238kg coal/kg clinker Therefore: Heat input = 0.238 x 6480 = 1542 kcal/kg ______________________________________________________ This implies nett value - all comparisons should be nett basis
Table 4 Ultimate Analys is o f Typical Fuels Fue l Type s Gas
HFo il
Gas Oil
AS H C H N S O To tal
0.0 73.3 23.8 2.5 0.0 0.4 100.0
0.0 85.4 11.4 0.0 2.8 0.4 100.0
0.0 86.1 13.2 0.0 0.7 0.0 100.0
c v(g ro s s ) c v(ne t) rho (kg /m3)
12734 11477 0.757
10248 9675 0.96
10893 10225 0.83
P'c o ke
0.0 90.0 3.0 2.0 5.0 0.0 100.0 8380 8268
S tandard c o al 11.0 73.5 4.7 1.4 1.9 7.5 100.0 7000 6756
COMBUSTION OF COAL ANALYSIS (%by WT) C H O N S
REQUIRED O2 STOIC'METRIC 10%EXCESS
69.00 4.50 7.60 1.40 0.50 TOTAL N2 ASSOCIATED TOTAL AIR
1.840 0.360 (0.076) 0.005 2.129 7.004 9.133
2.342 7.705 10.047
COMBUSTION PRODS STOIC'METRIC 10%EXCESS 2.530 0.405 0.014 0.010 2.959 7.004 9.963
N.B. QUANTITIES IN KG/KG COAL
2.530 0.405 0.213 0.014 0.010 3.172 7.705 10.877
Chemical Reaction • Endothermic reaction - heat is consumed – Calcium Carbonate breaks down to CaO (lime) and CO2 when heated – it takes heat in ⇒ the reaction is endothermic.
• Exothermic reaction - heat is released – CaO (lime) reacts with Silica and the cement minerals are formed – the process gives out heat ⇒ the chemical reactions is exothermic.
Heat of Reaction • Heat of reaction is the difference between the heat absorbed in decarbonating the limestone and the heat released in forming the clinker minerals. • It should be noted that raw meal chemistry affects the reaction heat, the heat absorbed by the process gets bigger as the LSF of the materials rises.
Reaction Heat. Variation w ith LSF 460
kcal/kg
440
420
LSF
400
380 80
85
90
95
LSF
100
105
HEAT OF REACTION EXOTHERMIC REACTIONS Clinkerization
kcal/kg cl
C2S + C3S C3A C4AF Alkali Sulphates TOTAL
+ 104.5 + 1.5 + 1.4 + 18.1 + 125.5
Combustibles Clinker
kcal/kg
Carbón
+34.9
Q reaction = -550.1 + 125.5 + 34.9 = -389.7
Q reaction=
~ - 390 kcal/kg Clinker
HEAT OF REACTION ENDOTHERMIC REACTIONS: Calcination and Decomposition CaCO3 MgCO3 Crystal Water Other Silicates CaSO4 Combined Alkalis TOTAL
kcal/kg cl - 491.8 - 22.5 - 27.2 + 7.5 - 6.4 - 9.4 -550.1
–Heat losses
Convection
Radiation Losses
Losses
Shell Heat Losses Convection Radiation
Air
Shell Losses vs Shell Temperatures 250 225
Kcal/(m2.min)
200 175 150
Wind Velocity 1.5 m/S
125 100 75
Wind Velocity 0 m/s 50 25 0 50
100
150
200
250
300
SHELL TEMPERATURE ºC
350
400
Reference or Datum Temperature 20 deg C Mass
1 Kg Clinker
Waste Gas
Heat Losses
Feed
P/H Fuel
Kiln
Air Exhaust Air
Clinker
Cooler Air
Inleak
–By pass
TEMPERATURE OF GAS AND MATERIAL IN IN ROTARY KILN
GA S
TE
MA TE
COOLING ZONE
COOLER
CLINKERING ZONE
M PE
GAS
RA TU
RE
MATERIAL RIA L
TE
MP
ER AT U
CALCINING ZONE
KILN
RE
PREHEATING ZONE
PREHEATER PRECALCINER
DRYING ZONE
HEAT CONS UMPTION
LONG WET kca l/kg
THEORETICAL HEAT EVAP ORATION EXIT GAS LOS S to DUS T COOLER EXHAUS T CLINKER RADIATION TOTAL % THERMAL EFF
MBtu/s t
%
420 529 264
1.51 1.90 0.95
29.0% 36.5% 18.2%
72 20 144
0.26 0.07 0.52
5.0% 1.4% 9.9%
1449
5.22
100.0% 29.0%
NB.1 H2O Eva p =1/3 of TOTAL HIGH EXIT GAS LOS S HIGH RADIATION LOS S
HEAT CONS UMPTION
LONG DRY kca l/kg
THEORETICAL HEAT EVAP ORATION EXIT GAS LOS S to DUS T COOLER EXHAUS T CLINKER RADIATION
420
TOTAL % THERMAL EFF
MBtu/s t
%
312
1.51 0.00 1.12
44.1% 0.0% 32.7%
79 20 122
0.28 0.07 0.44
8.3% 2.1% 12.8%
953
3.43
100.0% 44.1%
NB.2 HIGH EXIT GAS LOS S HIGH RADIATION LOS S
HEAT CONS UMPTION kca l/kg
S EMI DRY LEPOL MBtu/s t
%
THEORETICAL HEAT EVAP ORATION EXIT GAS LOS S to DUS T COOLER EXHAUS T CLINKER RADIATION
420 109 97
1.51 0.39 0.35
52.1% 13.5% 12.0%
81 20 79
0.29 0.07 0.28
10.0% 2.5% 9.8%
TOTAL % THERMAL EFF
806
2.90
100.0% 52.1%
NB.3 12-14% H2O LOW EXIT GAS LOS S
HEAT CONS UMPTION kca l/kg THEORETICAL HEAT EVAP ORATION EXIT GAS LOS S to DUS T COOLER EXHAUS T CLINKER RADIATION TOTAL % THERMAL EFF
420
DRY PROCES S 4 S TAGE MBtu/s t %
184
1.51 0.00 0.66
53.6% 0.0% 23.5%
83 20 77
0.30 0.07 0.28
10.6% 2.6% 9.8%
784
2.82
100.0% 53.6%
NB.4 NIL H2O HIGH EXIT GAS LOS S HEAT RECOVERY?
HEAT CONS UMPTION kca l/kg THEORETICAL HEAT EVAP ORATION EXIT GAS LOS S to DUS T COOLER EXHAUS T CLINKER RADIATION TOTAL % THERMAL EFF
420
PRECALCINER 4 S TAGE MBtu/s t %
185
1.51 0.00 0.67
54.5% 0.0% 24.0%
83 20 62
0.30 0.07 0.22
10.8% 2.6% 8.1%
770
2.77
100.0% 54.5%
NB.5 NIL H2O HIGH EXIT GAS LOS S S P EC HEAT LOS S LOWER
Kiln Heat Consumption 1730
1000
1720
950 900
1710
850 1700
800
1690
750
1680
700 0
2
4
6
Exit Oxygen %
8
kcals/kg (dry)
kcal/kg w et
Effect of Kiln Exit Oxygen
WET DRY
Summary:
Heat Balance
•Consistent boundary •Mass balance first •Identify opportunities to improve energy efficiency •Allows means of comparison of energy strategies •Process simulation •Gives steady state answer actual usage may be higher due to warm ups etc following shutdowns
Watt = 1 joule / sec 4.186joules = 1 calorie 2400watts = 2400 joule/sec Time
=
140 secs
Heat delivered = 2400 x 140 /(4.186 x1000) = 80 kcal
Blue Circle Cement
PROCESS ENGINEERING TRAINING PROGRAM
PRESENTATION
Heat Transfer
Heat Transfer • 3 Modes of heat transfer – Conduction – Convection – Radiation
Convection
Radiation Losses
Conduction
Losses
Conduction • Transfer of heat from one part of a body to another part of the same body, or from one body in physical contact with another body, without T1 appreciable particle movement within the body • Q = U * A * ∆T
Brick Shell
T3 T2
Convection • Transfer of heat from one point to another point within a fluid by mixing one part of the fluid with another • Natural convection - fluid motion by temperature related density differences • Forced convection - Motion caused by mechanical means (fans - windspeed)
Convection Natural Convection 1.25
Conv = h .( T avg - T amb )
Forced Convection
Wind Conv = ( 7 + 7
3.6
) . s . ( T avg - T amb )
Shell Heat Losses Convection Radiation
wind
Air
Shell Losses vs Shell Temperatures 250 225
Kcal/(m2.min)
200 175 150
Wind Velocity 1.5 m/S
125 100 75
Wind Velocity 0 m/s 50 25 0 50
100
150
200
250
300
SHELL TEMPERATURE ºC
350
400
Radiation • Transfer of heat from one body to another, not in contact with it, by means of wave motion through space
Rad = E . SB . (T1 − T2 ) 4
T1
4
T2
Reference or Datum Temperature 20 deg C
Mass
1 Kg Clinker
