mod 8-application of heat and mass balances

8/17/2019 Mod 8-Application of Heat and Mass Balances

1/218

Process Engineering

Training ProgramMODULE 8

Application of Heat and Mass BalancesSection Content

1 Application of H eat Balances to Process, Evaluation

2 H eat Balances –Im perial U nits

3 Paper 12 –Heat Balances

4 H eat Transfer

5 H eat Transm ission

Presentations

MASS, HEAT AND ENERGY BALANCES

HEAT TRANSFER


2/218

Blue Circle Cement

PROCESS ENGINEERING TRAINING

PROGRAM

Module 8

Section 1

Application of Heat Balances and Mass

Balances to Process, Evaluation


3/218

Application of Heat Balances

to Process Evaluation

P LayneJ. A. Stringer


4/218

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 possibleconsistent 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 thethird 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

h h h b l h b d i h ld i ld d il d f ll f h ili i


5/218

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.


6/218


7/218

3 1 PRELIMINARY CALCULATIONS


8/218

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

0.251 x( )

−

100

5100 = 0.238 kg./kg. of clinker

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 =

( )( )28.36100

1009636.0

−×

= 1.52 kg

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 =( )

( )28.36100

2.20100

−−

= 0.075 kg. of dry meal/kg of clinker

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

clinkerofwater/kgofkg1 02839.2x1.595

= 3 3 POTENTIAL HEAT IN COAL


9/218

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 x100

07.0= 0.0011 kg./kg. of clinker

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 1t to 2t then

Q = m S ( 2t - 1t )

where S is the mean specific heat of the material over the temperature range 1t to 1t 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


10/218

TABLE15.2 FUEL, FEED, CLINKER AND DUST DATA

COAL CLINKER

Moisture 5.0% Insoluble Residue 0.21Calorific Value of Dry

Coal

6750

kcal/kg

Si02 21.63

Analysis of Dry Coal A1203 6.57

Ash 15 3% Fe203 2 78 The coal is fed to the mill at 20oC i e the datum temperature to yield a 2t - 1t value and hence sensible heat


11/218

The coal is fed to the mill at 20 C, i.e. the datum temperature, to yield a 2t 1t 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 02 N2 Air Co C02 H20 S02vapor

20 0.218 0.248 0.240 0.249 0.198 0.435 0.143

100 0,220 0.248 0.240 0.249 0.211 0.447 0.147

200 0.223 0.249 0.242 0.250 0.221 0.452 0.150

300 0.227 0.250 0.243 0.252 0.230 0.457 0.154400 0.230 0.252 0.246 0.254 0.238 0.463 0.157

500 0.234 0.254 0.248 0.257 0.246 0.471 0.161

600 0.237 0.256 0.250 0.259 0.252 0.478 0,164

700 0.240 0.258 0.253 0.262 0.258 0.486 0.167

800 0.243 0.261 0.256 0.265 0.263 0.495 0.170

900 0.245 0.264 0.258 0.268 0.268 0.502 0.173

1000 0.247 0.266 0.260 0.270 0.271 0.512

1100 0.249 0.268 0.263 0.273 0.275 0.519

1200 0.251 0.271 0.265 0.275 0.278 0.527

1300 0.253 0.272 0.267 0.277 0.281 0.532

1400 0.255 0.275 0.269 0.278 0.284 0.542

1500 0.256 0.276 0.271 0.280 0.286 0.547

1600 0.258 0.278 0.272 0.282 0.289 0.5531700 0.258 0.280 0.273 0.283 0.291 0.561

1800 0.260 0.281 0.274 0,285 0.293 0.567

1900 0.262 0.282 0.276 0.286 0,294 0.573

2000 0.263 0.284 0.277 0.287 0.296 0.578


12/218

3.7 SENSIBLE HEAT IN RAW MATERIALS


13/218

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 + 7.646Cc - 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

Substituting in this formula data from Appendix 1, Table 15.6 gives


14/218

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

the dust losses, equal to =× 028.1595.1

075.00.048 kg./kg of clinker. Treating the dust loss moisture separately this

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

100

34.152.1 ×= 0.0204 kg./kg. of clinker. The heat required to evaporate this water at 20oC is 0.0204 x 586 =

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 x100

0.5 = 0.013 kg. of water/kg. of clinker.

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

3.12 COMBUSTION PRODUCTS


15/218

The carbon in the fuel and raw material are burnt thus

C 02 CO2

+ →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 2H2O

+ →4kg. 32kg. 36kg.

The sulphur in the fuel-is burnt this

S O2 SO2

+ →32kg. 32kg. 64kg.

On the basis of I kg. of clinker the fuel combustion should yield

0.238 x100

76 x

12

44 = 0.663kg. of carbon dioxide

0.238 x4

36

100

4.4× = 0.094kg of water vapor

44 0.7× = 0. 0257 kg. of carbon dioxide/kg. of raw meal


16/218

1210× 0. 0257 kg. of carbon dioxide/kg. of raw meal

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 x100

8.1= 0. 0043kg

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 x100

0.9= 0.0021 kg./kg. of clinker

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 28.1% by volume

CO 0.1% by volumeO2 0.85% by volume

N2 (by difference) 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 28.2% by volume

O2 0.8% by volume

N 71 00% b l

The O2 content of 0.8% represents the excess air. The ratio by volume of nitrogen to oxygen in air is 3.76,

h f h i h i i h i i b i h


17/218

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) =+

×0.00211.971

1.97167.8%

The percentage of excess air is, therefore

%6.410067.8

3.1=×

The weight of nitrogen in the excess air is

×100

4.61.971 = 0.0907 kg./kg. of clinker

and the weight of complimentary oxygen is

=31.3

0.09070.0274 kg./kg. of clinker

The total weight of air entering the kiln (i.e. combustion air plus excess air), per kg. of clinker is

Combustion air .595 + 1.971 + 2.566

Excess air 2.566 x 4.6/100 =0.118

Total 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.

h l i ff b h f d i k /k f li k

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


18/218

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

%6.55100

3.36100

3.36

2.201002.20

3.361003.36

=×

−

−− −

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

582.0209.0100

6100

2.5734.870.075).-(1.595 =×+

+ kg./kg clinker

(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 1.049

from combustion of coal 0.094

from coal moisture 0.013from water vapour in air 0.0133

Total 1.1693

SO2 from combustion of coal 0.0076


19/218

O2 from excess air 0.0274

N2 from coal 0.0021

from combustion air 1.9710

from excess air 0.0907Σ

2 N= 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 115o

C. I kg. of clinker is made every 0.0019 min. Theweight 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 2.566

Excess Air 0.118

Cooler Exhaust Air 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 determination of the shell losses from kilns, coolers etc. is a difficult problem.


20/218

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

4

3

4

2r TTAq −αε=

where 2T and 3T are the absolute temperatures of the shell and the surroundings respectively, ε is theemissivity of the surface and γ is a constant. Convection takes place according to an equation of the form

( )32c tthAq −=

where 2t and 3t are the temperature of the shell and the surroundings respectively and h is a coefficient whosevalue 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 anycoating. 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 isequivalent to 1.11 x

510 x 0.0019 = 210.9 kcal./kg. of clinker.

3.20 HEAT LOST IN MAKING DUST

Th d l i kil i id bl i i d i i d i i h f diffi l k

The percentage decarbonation has been estimated as 55.6% equivalent to 0.209 kg. of carbon dioxide per kg.

f d t A i thi b di id t f th di i ti f l i b t th i ht f


21/218

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 x44

100 = 0.029 kg./kg. of clinker

At 20oC, 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.

The heat required for the evaporation of this moisture at 20oC (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 x100

34.1 = 0.001 kg./kg of clinker.

The heat required to evaporate this water at 20oC 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


22/218

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

CO2 28.1 28.2

CO 0.1

O2 0.85 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 x8.0

0.8)-(0.85= 0.0017 kg./lg. of clinker

Carbon monoxide reacts with oxygen thus

2CO + O2 → 2CO2

(56kg.) (32kg.) (88kg.)

Therefore the weight of carbon monoxide combining with 0.0017kg. of oxygen is

0.0017 x32

56 = 0.003kg

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


23/218

HEAT BALANCE

HEAT INPUT kcal./kg. of % of heat input

Clinker Coal

Combustion 1607.0 99.59

Sensible Heat 0.0 0

Feed

Combustion of Organic Matter 8.60 0.53Sensible Heat - 4.04 - 0.25

Air,

Sensible Heat 2.13 0.13

TOTAL 1613.69 100.00

HEAT OUTPUT

Theoretical Heat 417.00 25.84

Evaporation of Water

Heat to vaporize slurry moisture 574.30 35.59

Heat to vaporize combined water in feed 11.90 0.74

Heat to vaporize coal moisture 7.59 0.47

Heat to vaporize water from combustion 54.9 3.41

Sensible Heat in Exhaust Gases

Sensible Heat of Exit Gases 254.5 15.77

Sensible Heat of Exhaust Air from Cooler 40 60 2 52 3.22 HEAT UNACCOUNTED FOR


24/218

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 USES OF THE HEAT BALANCE

4.1 SIMPLIFICATION OF THE HEAT BALANCE

Calculation of a heat balance along the lines described above is lengthy and tedious, and may require data

which 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 relevantcontributions 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

If suitable data is available, either first hand or by reasonable approximation then this type of calculation can


25/218

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

Back end temperature reduction also produces a significant effect upon heat requirement. A reduction of 10oC

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 theorder 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).


26/218

4.24 SHELL LOSSES

Sh ll l t f i l l ti f th h t l i b t 13% i thi R d ti i h ll


27/218

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.

(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. 0oC, 20

oC). Table 15.6 contains the heats of reaction at 20

oC of the main

ti i i th t i l i th kil It h ld b i t d th t th ti d t il


28/218

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 20oC.

The heat of reaction at some other temperature, t, can be found from the data in Table 15.6 by assuming the

reactants are brought from t to 20o

C then react and the products are taken from 20o

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 → CaCO3

3 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 was 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.


29/218


30/218


31/218

Blue Circle Cement

PROCESS ENGINEERING TRAINING

PROGRAM

Module 8

Section 2

Heat Balances – Imper ial Units

PAPER NO.5

HEAT BALANCES


32/218

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

HEAT BALANCES


33/218

(BTU/lb UNITS)

1. INTRODUCTION

The purpose of the rotary kiln is to increase the temperature of the raw material so that the chemical reactionsleading 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

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


34/218

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 above1400°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.

5. UNITS

5 1 C ti l U it


35/218

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 solidsentering 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 atequilibrium.

When equilibrium exists, the mass flow into the system in unit time will equal the mass flow from the system

(Fig.8).

7. REFERENCE TEMPERATURE

For determining the heat balance it is necessary to define a reference or datum temperature on which all


36/218

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. SENSIBLE HEAT

8.1 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 - 1t )( 1t - t ref )

= 4-x 0.242 (390-68)

Q = 311.7 BTU/lb clinker

8.2 Specific Heat

Intermediate values can be interpolated, most easily by plotting a SH/temperature curve over the appropriate

range.


37/218

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 = ( )23

12211331

tt

)t-(t t-tSH-)t-(t t-tSH

−

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 = 0.223100-200

20)-(1000.220-20)-(200

= 0.2254

All heat quantities (Q) associated with sensible heat can be calculated knowing mass (m), temp (t xo

) andmean SH ( x

oref

o tSt − ).

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 oi

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


38/218

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 - LHV100

H9M 2+ BTU/lb

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

For a coal fired wet process kiln the heat supplied by the coal is calculated as follows (similarly for other kiln

processes):

l /lb


39/218

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 x100

5-100= 23.8%

(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 ( 2O ) to give carbon dioxide (C 2O ) is as follows:

C + 2O → C 2O

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 2O from coal combustion = 0.23812

44

100

76××

Referring back to Section 10.2, it must be understood that in the absence of a suitable excess of oxygen, someof 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.


40/218

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 ref t 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 ref

ot 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. ref ot = 68°F

LHV68°F = 1052.8 BTU/Ib H O2

SH68°F-414°F of WV = 0.4526

BET = 414°F

0.98 lb slurry H O2

/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)

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


41/218

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 32 O ex clay per 1001b clinker

M c C c = lb mgO and CaO ex MgC 3O and CaC 3O per 1001b clinker respectively

S, (F+Mn) = % Si 2O and % (FFe 2 3O + Mn 2 3O ) 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 32 O , MgO, CaO, Si 2O and Fe 2 3O in the clinker.

i.e. clinker: A1 32 O 6.57%, MgO 1.09%, CaO 66.31%, S 2O 21.63%, Fe 32 O 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

b) montmorillonite 725.8 BTU/1b

c ) illite 723 1 BTU/lb


42/218

c ) illite 723.1 BTU/lb

It can be seen that kaolinite minerals give theoretical heat approaching 755 BTU/1b, but montmorillonite an

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.

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.


43/218

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 = ( )4

3

4

2 TTA −εσ

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:

Qc= 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

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 beexpected from this value. For a moderate to large wet process kiln this would give a shell loss value of the


44/218

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 kilngeometry. 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 datawhich 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

as about. 1500 BTU/hrft² of surface, which may decrease with increasing kiln size. Small wet process kilnsmay, 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


45/218

p y pp y

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 reductionsmay 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 i b k d t t lt i f l i f b t 1% S li it ti b t d ith

the kiln (by insufflation at the kiln hood) or by dust scoops on the kiln shell, thereby reducing fuelconsumption.


46/218

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 savin

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 managementteam to follow the progress being made and to participate in the optimization of fuel consumption.

References

TABLE I

MEAN SPECIFIC HEATS OF UNDISSOCIATED GASES BETWEEN 68-F AND t-


47/218

t°F O2 N2 Air CO CO2 H2O

Vapor

SO2

68 0.218 0.248 0.240 0.249 0.198 0.435 0.143

212 0.220 0.248 0.240 0.249 0.211 0.447 0.147

392 0.223 0.249 0.242 0.250 0.221 0.452 0.150

572 0.227 0.250 0.243 0.252 0.230 0.457 0.154

752 0.230 0.252 0.246 0.254 0.238 0.463 0.157

932 0.234 0.254 0.248 0.257 0.246 0.471 0.1611112 0.237 0.256 0.250 0.259 0.252 0.478 0.164

1292 0.240 0.258 0.253 0.262 0.258 0.486 0.167

1472 0.243 0.261 0.256 0.265 0.263 0.495 0.170

1652 0.245 0.264 0.258 0.268 0.268 0.502 0.173

1832 0.247 0.266 0.260 0.270 0.271 0.512

2012 0.249 0.268 0.263 0.273 0.275 0.519

2192 0.251 0.271 0.265 0.275 0.278 0.527

2372 0.253 0.272 0.267 0.277 0.281 0.532

2552 0.255 0.275 0.269 0.278 0.284 0.542

2732 0.256 0.276 0.271 0.280 0.286 0.547

2912 0.258 0.278 0.272 0.282 0.289 0.5533092 0.258 0.280 0.273 0.283 0.291 0.561

3272 0.260 0.281 0.274 0.285 0.293 0.567

3452 0.262 0.282 0.276 0.286 0.294 0.573

3632 0.263 0.284 0.277 0.287 0.296 0.578

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


48/218

TABLE 3 - HEATS OF REACTION AT 68°F


49/218


50/218


51/218


52/218


53/218


54/218


55/218

FIG.8 GAS FLOWING THROUGH A SYSTEM


56/218


57/218


58/218


59/218

Heat Loss vs Surface Temp


60/218

0

2000

4000

6000

8000

10000

12000

0 200 400

H e a t L o s s ( k c a l / m 2 / h )

Strong Wind (10 C)

Strong Wind (20 C)

Strong Wind (30 C)

Med Wind (10 C)

Med Wind (20 C)

Med Wind (30 C)

Still Wind (10 C)

Still Wind (20 C)

Still Wind (30 C)


61/218

APPENDIX I

FUNDAMENTALS OF HEAT BALANCES

1. Preliminary Considerations


62/218

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 car-

bon, CaO) in the material streams entering the kiln will equal the weight of the same component in thematerial 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 beshown 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

vi) The air used for combustion contains a small quantity of water vapor which is heated to the exit gastemperature (in addition water may be sprayed into the cooler).

vii) Excess hot air is exhausted from a Fuller (grate) type cooler.


63/218

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 combustionrepresents 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 unitsused 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.

b) Vaporization of the slurry moisture

c) Sensible heat of the exit gases

d) Shell losses


64/218

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 heatare 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 x100

5)-(100= 0.238 lb/lb of clinker

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 ra material also s ffers a loss on ignition on passage thro gh the kiln Loss on ignition is determined b

36.28-100

100x0.9636= 1.512 lb

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:


65/218

p y , q

( )( )

075.028.361002.2010006.0 =

−−× lb 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:

39.2)-(100

39.2x1.587= 1.023 lb of water/lb of clinker

3.2 Heat input

The total heat input is calculated by summing the various components containing both sensible and potentialheat. 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.

3.5 Sensible Heat in Coal

The coal is fed to the mill at 68°F (the datum temperature) to yield a nil 12 tt − value and hence a sensibleheat value of zero in this case:


66/218

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

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:


67/218

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:

1.587

0.075x 1.023 = 0.048 lb/lb of clinker

Treating the dust loss moisture separately, this leaves 1.023 -0.048 = 0.98 lb/lb of clinker of slurry moisture. Itis 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:

100

1.34x1.512= 0.0203 lb/lb of clinker

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:

In calculating the heat input to the kiln, the gross calorific value of the coal was used, thereby implying thatthe 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:


68/218


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 C O2

+ →12 lb 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 2H2O+ →

4 lb 32 lb 36 lb

The sulfur in the fuel is burnt thus:

S O2 S O2

+ →32 lb 32 lb 64 lb

On the basis of 1 lb of clinker the fuel combustion should yield:

0.238 x32

64

100

6.1 × = 0.0076 lb of sulfur dioxide

The oxygen required for combustion per lb of clinker is:

326132443276


69/218

lb57.032

32

100

6.1238.0

4

32

100

4.4238.0

12

32

100

76238.0

=××+××+××

The 0.7% organic carbon in the raw meal is also burnt, consuming:

0187.012

32

100

7.0=× lb of oxygen/lb of raw meal, equal to:

0.0187 x 1.587 = 0.0297 lb of oxygen/lb of clinker, to give:

0257.012

44

100

7.0=× lb of carbon dioxide/lb of raw meal, i.e:

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:

lb0043.0100

8.1283.0 =×

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 0021.0100

9.0=× lb/lb of clinker

CO2 29.1% by volume

CO 0.1% by volume

O2 0.85% by volume

N2 (by difference) 70.95% by volume


70/218

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) %9.67

0021.0971.1

971.1=

+

×

The percentage of excess air is therefore:

and the weight of complimentary oxygen is:

3.31

0.0867= 0.0262 lb/lb of clinker

The total weight of air entering the kiln (i.e. combustion air plus excess air) per lb of clinker is:


71/218

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 is20.2% compared with 36.3% of the feed.

Assuming the losses on ignition represent the degree of decarbonation, the percentage decarbonation of the

d t l f b i i

Therefore, the carbon dioxide evolved by the dust is:

100

55.6x (0.3487 + 0.0257) = 0.208 lb/lb of dust

The dust loss of 6% on clinker is equivalent to 0 075 lb of dry raw meal/lb of clinker Therefore the carbon


72/218

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) 578.0208.0100

6

100

2.57)(34.87=×+

+ lb/lb clinker

(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) 1.043 )

)


73/218

)

from combustion of coal 0.094 ) ) 1.1634

from coal moisture 0.013 )

)

from water vapor in air 0.0134 )

CO2 from feed 0.582 )

) 1.24

from combustion of coal 0.663 )

SO2 from combustion of coal 0.0076

O2 from excess air 0.0262

N2 from coal 0.0021 )

)

from combustion air 1.9710 ) 2.0598

)from excess air 0.0867 )

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

Combustion Air 2.566

Excess Air 0.113

Cooler Exhaust Air 1.769

4 438 lb


74/218

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 = ( )4

3

4

2 TTA −εσ

where A is the area, T2 and T3 are the absolute temperatures of the shell and the surroundings respectively, εis the measured emissivity of the surface and σ is the Stefan Boltzmann constant (0.173 – 10-8 BTU/hrft²R 4)

Convection takes place according to an equation of the form:

q c = hA ( )32 tt − 1.25

where t2 and t3 are the temperature of the shell and the surroundings respectively and In is a coefficient whosevalue 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

Measurements have also to be made of cooler, preheater, coal mill and connecting ducting temperatures asthere 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


75/218

4.41 x 10 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 x44

100= 0.029 lb/lb of clinker

At 68°F, the heat of dissociation of calcium carbonate is 760 BTU/lb. Hence, the heat required to partiallydecarbonate the dust is:


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 combined water (1.34%) amounts to:

0.075 x100

1.34= 0.001 lb/lb of clinker

The heat required to evaporate this water at 68°F is:


76/218


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,

mod 8-application of heat and mass balances

Documents