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Chapter

175175

4

175

Combustion of Fuels

Combustion is the rapid chemical combination of oxygen with thecombustible elements of a fuel, resulting in the production of heat.

Combustion is accomplished by mixing fuel and air at elevated tem-peratures. The air supplies oxygen, which unites chemically withthe carbon, hydrogen, and a few minor elements in the fuel to pro-duce heat.

Steam has been generated from the burning of a variety of fuels. Inaddition to the common fossil fuels of coal, oil, and natural gas, todayan increased amount and varied supply of waste and by-product fuelsare used, such as municipal solid waste (MSW), coal mine tailings,and biomass wastes such as vine clippings and bagasse, a sugar cane

by-product. MSW also can have a large percentage of biomass becauseit contains yard waste. These fuels must be burned and their combus-tion products properly handled. They create a unique challenge intheir use because the fuel quality is significantly reduced due to the

fuels lower heating value and poor combustion characteristics. Inaddition, such fuels often present more restrictive emission limitations.The designs of the boilers are therefore unique to the combustion ofeach of these fuels.

4.1 The Combustion Process

The combustion process follows fundamental principles that must be

understood by the designers and operators of boilers and associated

equipment to ensure reliable service and high efficiency.

1. Control of air supply. The amount of air required depends on

the fuel, the equipment used for combustion, and the operating condi-

tions and is determined from manufacturers recommendations, which

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176 Chapter Four

are based on actual performance tests and operating experience. Too

much air results in an excessive release of hot gases from the stack

with a correspondingly high heat loss and reduction in efficiency. A

deficiency of air permits some of the fuel, unburned or only partially

burned, to pass through the furnace, which also results in a reductionin efficiency. It is therefore important that the best proportion of air

to fuel be determined and maintained in order to obtain the highest

efficiency possible.

2. Mixing of air and fuel. Air and fuel must be mixed thoroughly,

since each combustible particle must come into intimate contact with

the oxygen contained in the air before combustion can take place. If the

air distribution and mixing are poor, there will be an excess of air in

some portions of the fuel bed or combustion chamber and a deficiency

in others. Combustion equipment is designed with this principle inmind in an attempt to obtain the best possible mixing of fuel and air.

3. Temperature required for combustion. All around us we see

combustible material in intimate contact with air, and still it is not

burning. Actually, a chemical reaction is taking place, but it is so slow

that it is referred to not as combustion but as oxidation. The corrosion

(rusting) of steel when exposed to the atmosphere is an example of

this oxidation.

When the combustible material reaches its ignition temperature,

oxidation is accelerated and the process is called combustion. It is therapid chemical combination of oxygen with the combustible elements

of a fuel. Therefore, it is evident that it is important to maintain the

fuel and air mixture at a temperature sufficiently high to promote

combustion.

When the flame comes into contact with the relatively cool boiler

tubes, the carbon particles are deposited in the form of soot. When

boilers are operated at a very low capacity, the temperatures are

lower, which can result in incomplete combustion and excessive

smoke if combustion controls are not set properly.4. Time required for combustion. Air supply, mixing, and tempera-

ture determine the rate at which combustion progresses. In all cases an

appreciable amount of time is required to complete the process. When

the equipment is operated at an excessively high capacity, the time may

be insufficient to permit complete combustion. As a result, considerable

unburned fuel is discharged from the furnace. The rejected material

may be in the form of solid fuel or combustible gases. The resulting loss

may be appreciable and therefore must be checked and controlled.

These principles involving the process of burning (combustion) may

be understood by reference to Figs. 4.1 and 4.2. Here the principles of

combustion are applied to solid fuels burned on grates and to pulverized

coal, gas, and oil burned in suspension.



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Combustion of Fuels 177

Figure 4.1 The combustion process as applied to hand firing.

Figure 4.1 illustrates a hand-fired stationary grate installed under a

water-tube boiler. The coal is supplied by hand through the fire door. Airfor combustion enters through both the ashpit and the fire door. Primary

air comes through the stoker grate, and secondary air enters through

the fire door in this illustration. For the purpose of illustration, the fuel

bed may be considered as having four zones. Coal is added to the top or

distillation zone; next are the reduction and oxidation zones and, finally,

the layer of ash on the grates. The primary air that enters the ashpit

door flows up through the grates and ash into the oxidation zone, where

the oxygen comes into contact with the hot coal and is converted into

carbon monoxide (CO). [As noted later, carbon monoxide (CO) resultsfrom incomplete combustion.] As the gases continue to travel upward

through the hot-coal bed, this carbon monoxide (CO) changes to carbon

dioxide (CO2) as part of the combustion process. The exposure of the

coal in the upper zone to the high temperature results in distillation of

hydrocarbons (chemical compounds of hydrogen and carbon), which are

carried into the furnace by the upward flow of gases. Therefore, the

gases entering the furnace through the fuel bed contain combustible

materials in the form of carbon monoxide and hydrocarbons. The oxy-

gen in the secondary air that enters the furnace through the fire doormust combine with these combustibles to complete the combustion

process before they enter the boiler tube bank and become cooled.

From this discussion of the process it is evident that with hand fir-

ing a number of variables are involved in obtaining the required rate



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178 Chapter Four

Figure 4.2 The combustion process as applied to suspensionfiring.

of combustion and complete utilization of the fuel with a minimum

amount of excess air. The fuel must be supplied at the rate required

by the steam demand. Not only must the air be supplied in propor-

tion to the fuel, but the amount entering through the furnace doorsand the ashpit must be in correct proportions. The air that enters

through the ashpit door and passes up through the fuel bed deter-

mines the rate of combustion. The secondary air that enters directly

into the furnace is used to burn the combustible gases. Thorough

mixing of the combustible gases and air in the furnace is necessary

because of the short time required for these gases to travel from the

fuel bed to the boiler tubes. Steam or high-pressure air jets (overfire

air system) are used to assist in producing turbulence in the furnace

and mixing of the gases and air (see Sec. 5.11). A failure to distributethe coal evenly on the grates, variation in the size of the coal, and

the formation of clinkers result in unequal resistance of the fuel bed

to the flow of gases. (Aclinker is a hard, compact, congealed mass of

fuel matter that has fused in the furnace. It is often called slag.)



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Areas of low resistance in the fuel bed permit high velocity of gases

and accelerated rates of combustion, which deplete the fuel and fur-

ther reduce the resistance. These areas of low resistance have been

called holes in the fire.

As a result of these inherent shortcomings in hand firing and thephysical labor involved, mechanical methods of introducing solid fuel

into the furnace and automatically controlling the air supply have

been developed. These are explained in Chap. 5.Suspension firing of fuel in a water-cooled furnace is illustrated in

Fig. 4.2. This method may be utilized in the combustion of gaseousfuels without special preparations, of fuel oil by providing for atom-

ization, and of solid fuels by pulverization. The fuel particles and air inthe correct proportions are introduced into the furnace, which is at an

elevated temperature. The fine particles of fuel expose a large surface tothe oxygen present in the combustion air and to the high furnace tem-perature. The air and fuel particles are mixed either in the burner or

directly after they enter the furnace. When coal is burned by thismethod, the volatile matterhydrocarbons and carbon monoxideisdistilled off when the coal enters the furnace. These combustiblegases and the residual carbon particles burn during the short interval

of time required for them to pass through the furnace. The period oftime required to complete combustion of fuel particles in suspension

depends on the particle size of the fuel, control of the flow of combus-tion air, mixing of air and fuel, and furnace temperatures. Relatively

large furnace volumes are required to ensure complete combustion.The equipment used in the suspension burning of solid, liquid, andgaseous fuels is discussed in Chap. 5.

From these illustrations we note that the requirements for good

combustion are sufficient time of contact between the fuel and air,elevated temperature during this time, and turbulence to providethorough mixing of fuel and air. These are referred to as the three Ts

of combustiontime, temperature, and turbulence.

4.2 The Theory of Combustion

Combustion is a chemical process that takes place in accordance with

natural laws. By applying these laws, the theoretical quantity of air

required to burn a given fuel can be determined when the fuel analysis

is known. The air quantity used in a furnace, expressed as percentage

of excess above the theoretical requirements (excess air), can be

determined from the flue gas analysis.In the study of combustion we encounter matter in three forms: solid,

liquid, and gas. Melting is the change of phase from solid to liquid.

Heat must be added to cause melting. The change in the reversedirection, liquid to solid, is freezing or solidifying. The change of



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180 Chapter Four

phase from liquid to gas is called vaporization, and the liquid is saidto vaporize or boil. The change from the gaseous or vapor phase toliquid is condensation, and during this process the vapor is said to becondensing. Matter in the form of a solid has both volume and shape.

Aliquid has a definite volume, in that it is not readily compressible,but its shape conforms to that of the container. Agas has neither adefinite volume nor shape, since both conform to that of the container.

When liquids are heated, a temperature is reached at which vapor

will form above the surface. This vapor is only slightly above the liquidstate. When vapor is removed from the presence of the liquid andheated, a gas will be formed. There is no exact point at which a sub-stance changes from a gas to a vapor or from a vapor to a gas. It issimply a question of degree as to how nearly the vapor approaches a

gas. Steam produced by boiling water at atmospheric pressure isvapor because it is just above the liquid state. On the other hand, airmay be considered a gas because under normal conditions it is farremoved from the liquid state (liquid air). Gases follow definite laws

of behavior when subjected to changes in pressure, volume, and tem-perature. The more nearly a vapor approaches a gas, the more closelyit will follow the laws.

When considering gas laws and when making calculations in thermo-

dynamics (i.e., the relationship between heat and other forms of energy),

pressures and temperatures must be expressed in absolute units ratherthan ingauge values (read directly from gauges and thermometers).

Absolute pressures greater than atmospheric are found by adding the

atmospheric pressure to the gauge reading. Both pressures must beexpressed in the same units. The atmospheric pressure is accuratelydetermined by means of a barometer, but for many calculations theapproximate value of 14.7 psi is sufficiently accurate. For example, if apressure gauge reads 150 psi, the absolute pressure is 164.7 psia (read

as pounds per square inch absolute). Absolute pressures below zero

gauge are found by subtracting the gauge reading from the atmosphericpressure. When a gauge reads 5 psi, the absolute pressure would be14.75.0 9.7 psia. Pressures a few pounds above zero gauge and a few

pounds below (in the vacuum range) are frequently measured by a U-tube containing mercury and are expressed in inches of mercury. Manyof the pressures encountered in combustion work are nearly atmospher-ic (zero gauge) and can be measured by a U-tube containing water.

The zero on the Fahrenheit scale is arbitrarily chosen and has noscientific basis. Experiments have proved that the true or absolute zero

is 460 below zero on the Fahrenheit thermometer. The absolutetemperature on the Fahrenheit scale is found by adding 460F to the

thermometer reading. Absolute temperatures on the Fahrenheit scaleare called degrees Rankine (R). On the centigrade or Celsius scale, theabsolute temperature is determined by adding 273C to the centigrade



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thermometer reading. Absolute temperatures on the centigrade scaleare measured in kelvins (K).

Expressed in absolute units of pressure and temperature, thethree principal laws governing the behavior of gases may be stated

as follows (where V1 and V2 are, respectively, the initial and finalvolumes, P

1and P

2are, respectively, the initial and final absolute

pressures, and T1

and T2

are, respectively, the initial and finalabsolute temperatures):

Constant temperature. When the temperature of a given quantity

of gas is maintained constant, the volume will vary inversely as the

pressure. If the pressure is doubled, the volume will be reduced by

one-half:

V

V

1

2

P

P

2

1

Constant volume. When the volume of a gas is maintained con-

stant, the pressure will vary directly as the temperature. When the

temperature is doubled, the pressure also will be doubled:

P

P

1

2

T

T

1

2

Constant pressure. When a gas is maintained at constant pressure,

the volume will vary directly as the temperature. If the tempera-

ture of a given quantity of gas is doubled, the volume also will be

doubled:

V

V

1

2

T

T

1

2

In combustion work the gas temperature varies over a wide range.

Air enters the furnace or air heater, for example, at 70F, is heated in

some instances to over 3000F in the furnace, and is finally discharged

from the stack at between 300 and 400F. During these temperature

changes, the volume varies because the gases are maintained near

atmospheric pressure. This is most important because fans, flues and

ducts, boiler passes, etc., must be designed to accommodate these

variations in volume.

In addition to these physical aspects of matter, we also must consider

the chemical reactions that occur in the combustion process. All sub-

stances are composed of one or more of the chemical elements. The

smallest particle into which an element may be divided is termed an

atom. Atoms combine in various combinations to form molecules,

which are the smallest particles of a compound or substance. The



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182 Chapter Four

characteristics of a substance are determined by the atoms that make

up its molecules. Combustion is a chemical process involving the reac-

tion of carbon, hydrogen, and sulfur with oxygen.

The trading about and changing of atoms from one substance to

another constitute an exacting procedure. Substances always combinein the same definite proportions. The atoms of each of the elements

have a weight number referred to as the atomic weight. These weights

are relative and refer to oxygen, which has an atomic weight of 16.

Thus, for example, carbon, which is three-quarters as heavy as oxygen,

has an atomic weight of 12. The chemical and physical properties of

substances involved in the combustion process are given in Table 4.1.

When oxygen and the combustible elements or compounds are

mixed in definite proportions at an elevated temperature under ideal

conditions, they will combine completely. [The theoretical proportions(no deficiency and no excess) of elements or compounds in a chemical

reaction are referred to as the stoichiometric ratio.] This shows that a

given combustible element requires a definite amount of oxygen to

complete combustion. If additional oxygen is supplied (more than nec-

essary for complete combustion), the excess will not enter into the

reaction but will pass through the furnace unchanged. On the other

hand, if there is a deficiency of oxygen, the combustible material will

remain unburned. The law of combining weights states that the ele-

TABLE 4.1 Properties of Substances in the Combustion Process

Chemical Physical

Molec- Molec- Specific Specific Heating

ular Atomic ular weight, volume, value,

Name formula weight* weight* lb/ft3 ft3/lb Btu/lb State

Air 29 0.075 13.28 Gas

Carbon C 12 12 14,540 Solid

Carbon dioxide CO2

44 0.114 8.75 Gas

Carbon monoxide CO 28 0.073 13.75 4,355 Gas

Hydrogen H2

1 2 0.005 192.52 62,000 Gas

Nitrogen N2

14 28 0.073 13.75 Gas

Oxygen O2

16 32 0.083 12.03 Gas

Sulfur S2

32 64 4,050 Solid

Sulfur dioxide SO2

64 0.166 6.02 Gas

Water vapor H2O 18 0.037 26.80 Vapor

*Approximate.At 14.7 lb/in2 and 68 F.Air is a mixture of oxygen and nitrogen. This figure is the average accepted molecular

weight.At atmospheric pressure and 212F.



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ments and compounds combine in definite proportions that are in

simple ratio to their atomic or molecular weights.

Example The atomic and molecular weights of elements and compounds

are useful in determining the weights and volume of gases. It has beendetermined that at the same temperature and pressure a given volume of

all perfect gases will contain the same number of molecules. To test this

theory, calculate the volume in cubic feet of 32 lb of oxygen and 28 lb of

nitrogen (weights equivalent to their respective molecular weights).

Solution The specific volume in cubic feet per pound of oxygen and nitro-

gen as given in Table 4.1 are, respectively, 12.03 and 13.75 at a pressure of

14.7 lb/in2 and at 68F. Therefore,

32 lb

12.03 ft3

/lb

385 ft3

of oxygen28 lb 13.75 ft3/lb 385 ft3 of nitrogen

The weight of pounds of any substance, equal to its molecular weight, is

known as apound mole.

Therefore, a pound mole of oxygen equals 32 lb and a pound mole of

nitrogen equals 28 lb.

The following is an explanation of some of the chemical reactions

involved in combustion:

The volume of carbon dioxide (CO2) produced is equal to the volumeof oxygen (O

2) used. The carbon dioxide gas is, however, heavier than

the oxygen. The combining weights are 12 lb of carbon (1 12) and

32 lb of oxygen (2 16), uniting to form 44 lb of carbon dioxide, or 1

lb of carbon requires 2.67 lb of oxygen (32/12 2.67) and produces

3.67 lb of carbon dioxide (1 2.67 3.67). (See Table 4.2.) The com-

bustion of 1 lb of carbon produces 14,540 Btu.

When carbon is burned to carbon monoxide (CO) (Table 4.3), which is

incomplete combustion, the volume of oxygen used is only one-half of

that required for completely burning the carbon to carbon dioxide; thevolume of carbon monoxide produced is two times that of the oxygen

TABLE 4.2 Carbon Burned to Carbon Dioxide



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184 Chapter Four

TABLE 4.3 Carbon Burned to Carbon Monoxide

supplied. The heat released is only 4355 Btu/lb, but it is 14,540 Btu when

1 lb of carbon is completely burned. The net loss is, therefore, 10,185Btu/lb of carbon and shows the importance of completely burning the

combustible gases before they are allowed to escape from the furnace.

In the reaction shown in Table 4.4, the two molecules of carbon

monoxide (CO) previously produced, by the incomplete combustion of

two molecules of carbon, are combined with the necessary one mole-

cule of oxygen to produce two molecules of carbon dioxide. The 1 lb of

carbon produced 2.333 lb of carbon monoxide. Finally, however, the 1

lb of carbon produces 3.67 lb of carbon dioxide regardless of whether

the reaction is in one or two steps. The total amount of oxygenrequired, as well as the heat liberated per pound of carbon, is the

same for complete combustion in both cases.

Hydrogen is a very light gas with a high heat value. The combustion

of 1 lb of hydrogen gas liberates 62,000 Btu (Table 4.1). To develop this

heat, two molecules of hydrogen combine with one molecule of oxygen to

form two molecules of water (Table 4.5). One volume of oxygen is

required for two volumes of hydrogen. The weight relations are 1 lb of

hydrogen and 8 lb of oxygen, producing 9 lb of water. This water

appears as water vapor in the flue gases.

TABLE 4.4 Carbon Monoxide Burned to Carbon Dioxide



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The equations for these and some of the other reactions involved in

combustion are as follows:

TABLE 4.5 Combustion of Hydrogen

C O2 CO

2

2C O2 2CO

2CO O2 2CO

2

2H2 O

2 2H

2O

S O2 SO

2(sulfur dioxide)

2S 3O2 2SO

3(sulfur trioxide)

CH4

2O2 CO

2 2H

2O

Methane

2C2H

2 5O

2 4CO

2 2H

2O

Acetylene

C2H

4 3O

2 2CO

2 2H

2O

Ethylene

2C2H

6 7O

2 4CO

2 6H

2O

Ethane

Sulfur is an undesirable constituent in fuels. It has a heating value

of only 4050 Btu/lb (Table 4.1), contaminates the atmosphere with

sulfur dioxide unless controlled with air pollution control equipment,and causes corrosion in the flues, economizers, and air heaters. Some

forms of sulfur adversely affect pulverization.

In the combustion process, 1 lb of sulfur combines with 1 lb of oxy-

gen to form 2 lb of sulfur dioxide. (Actually, a portion of the sulfur is



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186 Chapter Four

converted to sulfur trioxide. The summation of all the sulfur oxides in

the flue gases is referred to as SOx.

.) The sulfur dioxide in the flue

gases can be approximated as follows:

SO2 lb/hK lb/h fuel burned 2 S/100

whereK the ratio of SO2

in flue gases to a theoretical amount result-

ing from the combustion of the sulfur in the fuel (frequently assumed to

be 0.95) andS the percentage of sulfur in the fuel. It is customary to

express sulfur oxide emission in pounds per million Btus of fuel burned.

SO2

lb/million Btu

Example Coal containing 1.5 percent sulfur with a Btu content of 11,500Btu/lb burns at the rate of 3 tons per hour. What is the sulfur emission in

pounds per million Btus input to furnace?

Solution

SO2

lb/h 0.95 3 2000 2 1.5/100 171

SO2

lb/million Btu 2.48

In practice, the oxygen supplied for combustion is obtained from theatmosphere. The atmosphere is a mixture of gases that for practical

purposes may be considered as being composed of the following:

Element Volume, % Weight, %

Oxygen 20.91 23.15

Nitrogen 79.09 76.85

Only the oxygen enters into chemical combination with the fuel.The nitrogen combines in small amounts with the oxygen to form

nitrogen oxides, commonly called NOx.

These are an atmospheric pol-

lutant. The amount of NOx

produced depends on the combustion

process. The higher the temperature in the furnace, the greater the

amount of nitrogen oxides. The remainder of the nitrogen passes

through the combustion chamber without chemical change. It does,

however, absorb heat and reduces the maximum temperature

attained by the products of combustion.

Since air contains 23.15 percent by weight of oxygen, in order tosupply 1 lb of oxygen to a furnace it is necessary to introduce

0.2

1

315 4.32 lb of air

171 1,000,000

3 2000 11,500

SO2

lb/h 1,000,000

lb fuel/h Btu/lb in fuel



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Since 1 lb of carbon requires 2.67 lb of oxygen (see Table 4.2), we

must supply

4.32 2.67 11.53 lb of air per pound of carbon

The 11.53 lb of air is composed of 2.67 lb of oxygen and 8.86 lb of

nitrogen.

11.53 2.67 8.86

lb air lb O2

lb N2

By referring to the equation for the chemical reaction of carbon and

oxygen (see Table 4.2), we find that 1 lb of carbon produces 3.67 lb of

carbon dioxide. Therefore, the total products of combustion formed byburning 1 lb of carbon with the theoretical amount of air are 8.86 lb of

nitrogen and 3.67 lb of carbon dioxide.

In a similar manner it can be shown that 1 lb of hydrogen requires

34.56 lb of air for complete combustion. The resulting products of

combustion are 9 lb of water and 26.56 lb of nitrogen.

Also, 1 lb of sulfur requires 4.32 lb of air, and therefore, the products

of combustion are 3.32 lb of nitrogen and 2 lb of sulfur dioxide.

The condition under which, or degree to which, combustion takes

place is expressed asperfect, complete, or incomplete. Perfect combus-tion, which we have been discussing, consists of burning all the fuel

and using only the calculated or theoretical amount of air. Complete

combustion also denotes the complete burning of the fuel but by supply-

ing more than the theoretical amount of air. The additional air does

not enter into the chemical reaction. Incomplete combustion occurs

when a portion of the fuel remains unburned because of insufficient

air, improper mixing, or other reasons. See Sec. 4.4 for a discussion of

coal analysis.

The values given in Table 4.6 will now be used in determining theamount of air required and the resulting products involved in the per-

fect combustion of 1 lb of coal having the following analysis:

Constituent Weight per lb

Carbon 0.75

Hydrogen 0.05

Nitrogen 0.02

Oxygen 0.09Sulfur 0.01

Ash 0.08

Total 1.00



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188 Chapter Four

TABLE 4.6 Theoretical Quantities Involved in Combustion of Fuel

Required Resulting quantities

Constituent O2

Air CO2

N2

H2O SO

2

Carbon (C) 2.67 11.53 3.67 8.86

Hydrogen (H) 8.00 34.56 26.56 9.00

Sulfur (S) 1.00 4.32 3.32 2.00

NOTE: All values are expressed in pounds per pound of fuel.

In the case of carbon, the values given in Table 4.7 are found as follows:

0.75 2.67 2.00lb C/lb coal lb O2 reqd/lb C lb O2 reqd/lb coal

2.00 4.32 8.64lb O2/ lb coal lb air reqd/lb O2 lb air reqd/lb coal

0.75 3.67 2.75lb C/lb coal lb CO

2/lb C lb CO

2/lb coal

8.64 2.00 6.64lb air reqd/lb coal lb O

2reqd/lb coal lb N

2/lb coal

The values for hydrogen and sulfur are found in a similar manner.

Note in Table 4.7 that the weight of fuel and air supplied is equal tothe weight of the resulting quantities.

0.92 lb 10.022 lb 10.942 lbfuel less ash air reqd total input

2.75 lb 7.722 lb 0.45 lb 0.02 lb 10.942 lbCO

2N

2water vapor SO

2total output

Therefore, when analyzing Table 4.7, it takes approximately 10 lb

of air to burn 1 lb of coal, and this combustion results in approximate-

ly 11 lb of flue gas (10.942) on a theoretical basis.In practice, it is necessary and economical to supply more air than

the theoretical amount in order to obtain complete combustion. The

air supplied to a combustion process in an amount above that theoret-

ically required is known asexcess air.

The flue gas analysis is effective in determining the amount of air

supplied for combustion, as indicated by Fig. 4.3. This graph shows how

the amount of excess air used in the combustion process can be

calculated by the percentage of either carbon dioxide or oxygen in the

flue gases. When a single fuel is burned, the carbon dioxide content ofthe flue gases provides a satisfactory index of the amount of excess air

being used. This can be explained by the fact that with the complete

combustion of 1 lb of carbon, 3.67 lb of carbon dioxide is produced. (See

Table 4.2.)



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TABLE 4.7 Theoretical Quantities Involved in Combustion of Coal

Required Resulting Quantities

WeightConstituent per lb O

2Air CO

2N

2H

2O SO

2

Carbon 0.75 2.00 8.64 2.75 6.64

Hydrogen 0.05 0.40 1.728 1.328 0.45

Sulfur 0.01 0.01 0.043 0.033 0.02

Total 0.81 2.41 10.411 2.75 8.001 0.45 0.02

Correction for

N2

in coal 0.02 0.02

O2

in coal 0.09 0.09 0.389 0.299

Corrected total 2.32 10.022 2.75 7.722 0.45 0.02

NOTE: All values are expressed in pounds per pound of coal.

Therefore, the amount of carbon dioxide formed depends on the

amount of carbon burned. When a relatively large amount of air is

used, the fixed amount of carbon dioxide gas will be diluted and the

percentage correspondingly lowered. Conversely, if only a small amount

of excess air is used, there will be less dilution, and the percentage ofcarbon dioxide will be relatively high. For a given percentage of excess

air, fuels with higher carbon-hydrogen ratio will have a higher percent-

age of carbon dioxide in the flue gases than fuels with lower carbon-

hydrogen ratio. For a given percentage of excess air, the flue gases from

a coal-fired furnace will have a higher percentage of carbon dioxide than

when fuel oil is burned. For example, flue gas will contain 12 percent

carbon dioxide when 54 percent excess air is used with bituminous coal

and only 27 percent excess air with fuel oil (see Fig. 4.3).

The percentage of oxygen in the flue gases provides an adequatemeasurement of excess air when either single or multiple fuels are

being used. The oxygen in the flue gases represents that portion

which entered but did not combine with the combustible elements in

the fuel. This oxygen in the flue gases and the nitrogen with which it

was mixed are the excess air. The theoretical oxygen and therefore air

requirement is approximately proportional to the heat content of the

fuel even with variations in the carbon-hydrogen ratio. For a given

percentage of oxygen, the excess air is approximately the same for

either coal or fuel oil. For example, a flue gas will contain 6 percentoxygen when 40 percent excess air is used with bituminous coal and

38.8 percent excess air with fuel oil.

The flue gas analysis is obtained by use of the Orsat, as explained in

Sec. 4.8, which is an apparatus where gaseous constituents are



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190 Chapter Four

measured by absorption in separate chemical solutions. Modern facili-

ties use continuous emission-monitoring (CEM) equipment; however,

the Orsat remains an important piece of test equipment to verify theresults from electronic equipment. The analysis includes carbon diox-

ide (CO2), oxygen (O

2), and carbon monoxide (CO). When the sum of

these three constituents is subtracted from 100, the remainder is

assumed to be nitrogen (N2).

The excess air in percentage of the theoretical requirements can be

calculated by the following formula:

Percentage of excess air 100

However, when there is no carbon monoxide present, the formula

becomes

Percentage of excess air 100

The use of these formulas will be shown by application to the follow-

ing flue gas analyses.

Analysis CO2

O2

CO N2

A 13.7 3.5 1.8 81.0

B 13.5 5.5 0.0 81.0

O2

0.263N2O

2

O2

12CO

0.263N2

1

2CO

O2

Figure 4.3 Carbon dioxide (CO2) and oxygen (O

2) in percentage by volume com-

pared with the excess air used when various types of fuels are burned.



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Analysis A:

Percentage of excess air 100 13.9

Analysis B:

Percentage of excess air 100 34.8

For analysis A, the excess air is 13.9 percent; for B, it is 34.8 percent.

Free oxygen is present in both analyses, but the presence of carbon

monoxide in A indicates incomplete combustion.

When 1 lb of carbon is burned to carbon dioxide, 14,540 Btus are

released, but when it is burned to carbon monoxide only 4355 Btusare released, a loss of 10,185 Btu/lb (see Table 4.1). Therefore, it is

important that the carbon monoxide content of the products of com-

bustion be at a minimum. It also means that the carbon monoxide

content of the products of combustion can be used to control the

amount of excess air. When there is a trace of carbon monoxide in

the flue gases, the air supply is increased.

When the percentages of carbon, hydrogen, and sulfur in solid fuels

are known, the heating value can be approximated by using Dulongs

formula:

Heating value, Btu/lb of fuel 14,540C 62,000(H 18O) 4050S

Theoretical air required, lb/lb fuel 11.53C 34.56(H 18O) 4.32S

It is preferable to determine the heating value of a fuel by actuallydeveloping and measuring the heat. This is accomplished by com-

pletely burning a carefully weighed sample of the fuel in a calorime-ter. The heat produced causes a temperature rise in a known quantity

of water. The temperature rise is indicative of the heating value ofthe fuel.

Physical characteristics of fuel are, in many cases, more importantin practical application than chemical constituents. Means have beendevised for determining some of these characteristics by simple controltests that can be made by the plant operators.

As noted previously, fuel and air mixing to ensure complete com-

bustion is not perfect, and therefore excess air is required. However,because the excess air that is not used for combustion leaves the unit

as part of the flue gas at stack exit temperatures, the amount ofexcess air should be minimized. The energy required to heat the air

from ambient to stack temperature is lost heat. Each design of com-bustion equipment has its excess air requirements, and Table 4.8shows typical ranges for various fuels and methods of firing.

5.5

0.263 81.05.5

3.512 1.8

0.263 81.0 12 1.8 3.5



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192 Chapter Four

Excess air also can be considered as providing a safety factor above

the actual air that is required for combustion. A boiler that is operat-

ing without sufficient air has the potential for a dangerous condition

because the fuel-air mixture is fuel-rich, and the possibility of an

explosion exists.

4.3 The Air Supply

Supplying oxygen, as contained in air, is an important consideration

in the combustion process. In a typical case in which 15 lb of air is

required per pound of coal (assuming approximately 50 percent excess

air), it is necessary to deliver 15 tons of air to the furnace and to pass

almost 16 tons of gases through the boiler for each ton of coal burned.

(See Table 4.7, where 10 lb of air is required per pound of coal and, with

50 percent excess air, 15 lb of air is required.) In many installations

the ability to supply air is the limiting factor in the rate of combustion.

The number of pounds of fuel that can be burned per square foot of

stoker grate area depends on the amount of air that can be circulated

through the fuel bed. A means must be provided to supply the

required amount of air to the furnace. The products of combustion must

be removed from the furnace and circulated over the heat-absorbing

TABLE 4.8 Ranges of Excess Air Requirements for Various Fuelsand Methods of Firing

Fuel Excess air, % by weight

Pulverized coal 1520Coal

Fluidized bed combustion 1520

Spreader stoker 2535

Water-cooled vibrating grate stoker 2535

Chain and traveling grate stoker 2535

Underfeed stoker 2540

Fuel oil 315

Natural gas 315

Coke oven gas 315

Blast furnace gas 1530

Wood/bark 2025

Refuse-derived fuel (RDF) 4060

Municipal solid waste (MSW) 80100

SOURCE: Babcock & Wilcox, a McDermott company.



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surfaces. Finally, the pollutants must be removed from the resulting

flue gases before they are discharged through the stack to the atmos-

phere.

This circulation of gases is caused by a difference in pressure,

referred to in boiler practice as draft. Draft is the differential in pres-sure between two points of measurement, usually the atmosphere

and the inside of the boiler setting.

A differential in draft is required to cause the gases to flow through

a boiler setting. This required differential varies directly as the square

of the rate of flow of gases. For example, when the flow is doubled, the

difference in draft between two points in the setting will increase four

times. For a given amount of fuel burned, the quantity of gases pass-

ing through the boiler depends on the amount of excess air being used.

The draft differential across the boiler tube bank and the differential

created by an orifice in the steam line are used to actuate a flowmeter.

Recording flowmeters are calibrated under actual operating conditions

by use of the flue gas analysis so that when the steam flow output and

the gas flow coincide, the optimal amount of air is being supplied.

A draft gauge in the form of a U-tube is partly filled with water. A

sampling tube inserted in the boiler setting is then connected to one end

of the U-tube, while the other end remains open to the atmosphere. The

difference in the height of the two columns of water is a measure of

the draft.

The scales on these gauges are calibrated in inches, and the differ-

ence in height of the columns is read in inches of water. The draft

gauges in common use are mechanical or dry-type gauges. However,

the scales are calibrated in inches of water.

A pressure in the furnace slightly lower than that of the atmos-

phere (draft) causes the air to enter, thus supplying the oxygen

required for combustion. A draft at the boiler outlet greater than that

in the furnace causes the products of combustion to circulate through

the unit. The rate of flow or quantity of air supplied can be regulated

by varying the draft differential.

The principle of draft and air regulation is explained by reference

to the hand-fired boiler, using a natural draft, shown in Fig. 4.4 and

the graph of Fig. 4.5. The stack produces a draft of 1.0 in of water,

which is regulated by the stack damper to give the required furnace

draft. As the capacity increases, more air is required to burn the addi-

tional fuel, and the stack damper must be opened to compensate for

the draft loss caused by the increased flow of gases. Draft loss occursacross the fuel bed, the boiler, the damper, and the breeching (flue).

Finally, at 100 percent capacity, the stack damper is wide open and no

more air can be supplied, thus limiting the ability of the furnace to

burn additional coal efficiently.



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194 Chapter Four

Figure 4.4 Flow of air and flue gas through a hand-fired boiler.

Figure 4.5 Draft in a hand-fired boiler.

The type of fuel determines the amount of draft differential

required to produce a given airflow through the fuel bed. This is one

of the reasons that more load can be carried with some fuels than

with others. Figure 4.6 shows the draft normally required to burn

several different types of fuels at varying rates.



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Early boiler designs met their total draft requirements with natural

draft that was supplied by the height of the stack. As units became

larger and included additional heat traps, such as superheaters, econ-

omizers, and air heaters (and thereby having a higher draft loss), itwas not practical to draft the entire unit from the stack. These units

required fans in addition to the stack, using a forced-draft (FD) fan

alone or in combination with an induced-draft (ID) fan.

Most combustion equipment uses forced-draft fans for supplying

air to the furnace either through the burners or through the grates.

An enclosure, called a wind box, provides the air distribution for

proper combustion. The wind box is a reinforced, metal-cased enclo-

sure that attaches to the furnace wall and incorporates the burners

and distributes the combustion air. It also can refer to the air distri-bution ductwork under a stoker. It can be located on a furnace wall

or on all the furnace walls depending on the burner configuration.

The attachments to the furnace walls must be gas-tight and must

permit thermal expansion between the casing and the tubes.

Figure 4.6 Fuel bed draft differential required for various fuels.



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196 Chapter Four

Figure 4.7 Flow of air and flue gas through an underfeed stoker-firedboiler.

Depending on the size of the boiler, the wind box can be arranged in

compartments on the front and rear furnace walls to optimize the

control of combustion.

The forced-draft (FD) fan supplies the air at a pressure above that

of the atmosphere and forces it into the furnace. Figures 4.7 and 4.8show the application of forced draft to a stoker and the draft and

wind-box pressure at various boiler capacities. This is the same unit

as that shown in Fig. 4.4 except that an underfeed stoker has been

substituted for the hand-fired grates. The forced-draft fan produces

a pressure under the stoker and causes the necessary air to flow up

through the fuel bed. An increase in capacity requires an increase in

pressure under the stoker to cause the additional air to flow through

the fuel bed. The stack produces the draft necessary to circulate the

gases through the boiler and breeching (flue). By automatic regula-tion of the stack damper, the furnace draft is maintained constant at

0.05 in of water. Operating a furnace at a constant draft slightly

below atmospheric pressure is referred to as balanced draft. When

the combustion rate increases, more air is added to the furnace, and

a corresponding increased amount of flue gas is removed. This

results in a greater flow but maintains a constant pressure in the

furnace. Since the total effect of the stack is now available in over-

coming the resistance through the boiler, a higher capacity can be



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obtained than with the same unit operating without a forced-draft

fan and with hand-fired grates.

The stack provided an adequate means of circulating the air and

gases through hand-fired grates or burners and boilers. However, the

application of stokers necessitated the use of forced-draft fans, and

induced-draft fans are required when economizers, air heaters, or flue

gas cleaning equipment is applied to balanced-draft boilers. Modern

boilers that burn solid fuels are nearly all balanced-draft units incor-porating a forced-draft (FD) fan and an induced-draft (ID) fan.

The pressurized furnaces used in connection with package oil- and

gas-fired boilers require only forced-draft fans. Figure 4.9 shows how

the pressure developed by the forced-draft fan is used to produce the

flow of air and flue gases through the entire unit. Combustion air

flows from the forced-draft fan through the supply duct to the wind

box, into the furnace, and through the boiler, economizer, and inter-

connecting flues to the stack. All the energy required is supplied by

the forced-draft fan. At the maximum capacity of 200,000 lb of steamper hour, the static pressure at the forced-draft fan outlet must be

19.6 in of water (gauge). The pressure drop across the wind box is

maintained nearly constant at all steam loads by adjustable louvers.

This creates a high velocity at the burners and promotes thorough

Figure 4.8 Draft and wind-box pressure in an underfeed stoker-fired boiler.



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198 Chapter Four

Figure 4.9 Air and flue gas pressure in a 200,000 lb/h pressurized fuel oil and naturalgas-fired package boiler.

mixing of air and fuel, thereby maintaining good combustion efficiency

at lower steam loads. Note the draft loss differential for the boiler and

economizer and how it increases as capacity increases.

The pressure-furnace principle had been applied to large pulverized-

coal-fired boilers. The necessary air and gas flow was developed with

the forced-draft fan, but a number of design and operating problems

were introduced such as flue gas leaks from the furnace into operating

areas of the plant. In addition, the requirement for flue gas cleaning

equipment, such as SO2 scrubbers and fabric filters, increased thedraft loss and made the pressurized unit unattractive. Therefore,

these types of units are designed today as balanced-draft boilers

incorporating both FD and ID fans.

4.4 Coal

Coal was formed by the decomposition of vegetation that grew in pre-

historic forests. At that time the climate was favorable for very rapid

growth. Layer upon layer of fallen trees was covered with sediment,and after long periods of aging, the chemical and physical properties

of the now ancient vegetation deposits were changed, through various

intermediate processes, into coal. The process of coal formation can be

observed in the various stages on the earth today. However, present-day



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formations are insignificant when compared with the magnitude of

the great coal deposits.It is estimated that 100 years is required to deposit 1 ft of vegetation

in the form known aspeat, and 4 ft of peat is necessary for the forma-

tion of 1 ft of coal. Therefore, it requires 400 years to accumulateenough vegetable matter for a 1-ft layer of coal. The conversion frompeat to coal requires ages of time. In some areas where other fuel isscarce, the peat is collected, dried, and burned.

The characteristics of coal depend on the type of vegetation fromwhich it was formed, the impurities that became intermixed with thevegetable matter at the time the peat bog was forming, and the aging,time, temperature, and pressure. It is apparent that the characteristics

of coal vary widely.

For example, peat often contains partially decomposed stems, twigs,and bark. Peat is progressively transformed to lignite, which eventuallycan become anthracite when provided with the proper progression ofgeologic changes. However, this transformation takes hundreds of years

to complete.Coal is a heterogeneous material that varies in chemical composition

according to location. In addition to the major organic ingredients of C,H

2, and O

2, coal also contains impurities. The impurities that are of

major concern are ash and sulfur.

The ash results from mineral or inorganic material that was intro-duced during formation of the coal. Ash sources include inorganic sub-stances, such as silica, which are part of the chemical structure of the

plants. Dissolved mineral grains that are found in swamp water arealso captured by the organic matter during the formation of coal. Mud,shale, and pyrite are deposited in pores and cracks of the coal seams.

Sulfur occurs in coal in three forms:

1. Organic sulfur, which is part of the coals molecular structure

2. Pyritic sulfur, which occurs as the mineral pyrite

3. Sulfate sulfur, which is primarily from iron sulfate

The highest sulfur source is sulfate iron, which is found in water.Fresh water has a low sulfate concentration, whereas salt water is

high in sulfate. Bituminous coal is found deposited in the interior ofthe United States, where oceans once covered the region, and thiscoal has a high sulfur content.

Coal can be classified as follows, and typical analyses are shown in

Table 4.9.

Peat. Peat is the first product in the formation of coal and consists of

partially decomposed plant and mineral matter. Peat has a moisture

content of up to 70 percent and a heating value as low as 3000 Btu/lb.



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200

TABLE4.9

RepresentativeAnalysesofWood,

Peat,andCoalonan

AsReceivedBasis

Proximateanalysis

Ultimat

eanalysis

Calorific

Volatile

Fixed

va

lue,

Kindoffuel

Moisture

matter

carbon

Ash

Sul

fur

Hydrogen

Carbon

Nitrogen

Oxygen

Btu/lb

Wood

6.25

49.50

1.10

43.15

5800

Peat

56.70

26.14

11.17

5.99

0.64

8.33

21.03

1.10

62.91

3586

Lignite

34.55

35.34

22.91

7.20

1.10

6.60

42.40

0.57

42.13

7090

Subbituminous

24.28

27.63

44.84

3.25

0.36

6.14

55.28

1.07

33.90

9376

Bituminous

3.24

27.13

62.52

7.11

0.95

5.24

78.00

1.23

7.47

1

3,919

Semibituminous

2.03

14.47

75.31

8.19

2.26

4.14

79.97

1.26

4.18

1

4,081

Semianthracite

3.38

8.47

76.65

11.50

0.63

3.58

78.43

1.00

4.86

1

3,156

Anthracite

2.80

1.16

88.21

7.83

0.89

1.89

84.36

0.63

4.40

1

3,298

SOURCE:Ad

aptedfromE.S.Moore,

Coal,W

iley,NewYork.



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Although it is not an official coal classification, it is used as a fuel in

some parts of the world.

Lignite. Lignite is the lowest ranking of coal with a heating value of

less than 8300 Btu/lb and a moisture content as high as 35 percent.The volatile content is also high, and therefore lignite ignites easily.

Subbituminous. These coals are noncoking; i.e., they have minimal

swelling on heating and have a relatively high moisture content of 15 to30 percent. They are high in volatile matter and thus ignite easily. Theyalso have less ash and burn cleaner than lignite. They have a low sulfurcontent, generally less than 1 percent, and a heating value between 8300

and 11,500 Btu/lb. Because of the low sulfur content, many power plants

have changed to subbituminous coal in order to limit SO2 emissions.

Bituminous. This coal is the one most commonly burned in electric

utility boilers, and it has a heating value between 10,500 and 14,000Btu/lb. As compared with lignite and subbituminous coals, the heat-ing value is higher and the moisture content and volatile matter arelower. The high heating value and its relatively high volatile matter

enable these coals to burn easily when fired as pulverized coal. Sometypes of bituminous coal, when heated in the absence of air, soften

and release volatiles and then form coke, a porous, hard, black product.Coke is used as fuel in blast furnaces to make iron.

Anthracite. This is the highest ranked coal. It has the highest contentof fixed carbon, ranging from 86 to 98 percent. It has a low volatilecontent, which makes it a slow-burning fuel. Its moisture content islow at about 3 percent, and its heating value can be as high as 15,000

Btu/lb. Anthracite is low in sulfur and volatiles and burns with a hot,clean flame. It is used mostly for domestic heating as well as some

metallurgical processes. However, utility-type boilers are designed toburn this low-volatile coal in pulverized coal-fired boilers. Refer to

Fig. 2.42.

Preparation plants are capable of upgrading coal quality. In thisprocess, foreign materials, including slate and pyrites, are separated

from the coal. The coal may be washed, sized, and blended to meetthe most exacting power plant demands. However, this processingincreases the cost of coal, and an economic evaluation is required to

determine whether the cost can be justified. If the raw coal availablein the area is unsatisfactory, the minimum required upgrading mustbe determined. Utility plants obtain the lowest steam cost by select-ing their combustion equipment to use the raw coal available in the



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202 Chapter Four

area. However, in order to meet sulfur dioxide emission limits, manyutilities use low-sulfur coals that often have to be transported signifi-cant distances. For example, many plants located in the midwest ofthe United States use coals shipped from Montana and Wyoming.

For over 20 years, Powder River Basin (PRB) coals, where minesare located across eastern Wyoming and Montana, have led in the

supply of these low-sulfur coals. The annual production of this coal

has increased significantly, from less than 10 million tons per year in

the mid-1970s to over 300 million tons per year today. Nearly all this

coal is used in electric power plants.

Most of this coal produces less than 1 lb of SO2

per million Btus,

thus making it a popular choice, even though it has a relatively low

heat content, ranging from 8400 to 9300 Btu/lb. The effect on the SO2

removal requirements is simplified, as discussed in Chap. l2. Sincethis coal can be mined efficiently and economically because its seams

lie close to the surface, it also has a price advantage over coals from

other locations. However, transportation costs are higher than those

for coals from eastern regions.

Many utilities, independent power producers (IPPs), and large

industrial plants have switched from eastern bituminous coals to

PRB coal for reasons of cost alone. Others are using the fuel because it

reduces emissions of SO2

without the need to install costly scrubbers.

However, the use of PRB coals has some negative effects that must be

handled. The fuel has a higher moisture content, which results in a

lower heating value. It also has a lower ash softening temperature

and a higher ash content.

These characteristics often result in greater fouling and slagging of

boiler heating surfaces, and dust control in and around the power

plant is more diffcult. Fugitive dust can ignite and even explode

under certain conditions, and this puts plant personnel and equip-

ment at risk, making it even more important to have good housekeep-

ing procedures. The higher ash content places additional burden on

the ash handling system and on the needed landfill to handle the

waste material.

However, these potential operating difficulties can be managed, and

PRB coals will continue to be used because of their lower cost and

lower sulfur content.

All the many factors involved in obtaining the lowest-cost steam

production must be taken into consideration. For a new facility, this

would involve the selection of combustion equipment for the coal thatis available in the region or for the coal that may be imported to the

plant. For an existing facility, a selection of the type of coal to best

match existing combustion equipment may be necessary.



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Because of the varied and complex nature of coal, a number of

methods for evaluating and specifying have been developed. One of

the simplest methods of controlling coal quality is the proximate

analysis, which includes moisture, volatile matter, fixed carbon, ash,

heating value, and sometimes sulfur.The procedure for obtaining a proximate analysis must be in

accordance with ASTM1 laboratory procedures, which are generally

as follows:

Moisture. A 1-g sample of coal is placed in an oven where the tem-

perature is maintained at 220F for 1 h. The difference between the

weight before and after drying is the amount of moisture removed.

Volatile matter. The sample is next placed in a furnace in a covered

crucible, where the temperature is maintained at 1700F for a period

of 7 min. The gaseous substance driven off is called volatile matter.

Fixed carbon. The lid is now removed from the crucible, the fur-

nace temperature is increased, and the crucible is allowed to

remain in the furnace until the combustible has been completely

burned. The loss in weight as a result of this burning is the amount

of fixed carbon.

Ash. The residual material in the crucible is ash.

Heating value. The heating value of coal may be determined by

use of a bomb calorimeter. The bomb calorimeter provides a means

of burning a small sample of coal under controlled conditions and

measuring the resulting temperature rise in a given quantity of

water. A 1-g sample (a gram is 1/453.6 lb) of pulverized and dried

coal is placed in a tray. The tray is then placed in a steel bomb with

a fuse wire arranged to extend into the tray of coal. The bomb is

then closed, connected to an oxygen tank, and pressurized. A mea-

sured amount of water is poured into the calorimeter bucket. The

bucket is placed in the calorimeter, and the bomb is carefully sub-

merged in the water. A stirring device agitates the water to maintain

uniform temperature. A calibrated thermometer permits the operator

to observe the temperature. The coal is now ignited by means of the

fuse wire and the temperature rise is noted. The heating value of

the coal in Btu/lb is found by multiplying the temperature rise by a

constant for the specific calorimeter.

Coal analyses are expressed in three different ways depending on the

constituents included. The designation as received or as fired refers to


1American Society for Testing and Materials (ASTM).



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204 Chapter Four

an analysis in which the actual moisture is included. When the expres-sion moisture-free or dry coal is used, the analysis considers the mois-ture as having been removed. Since neither moisture nor ash adds tothe heating value of coal, analyses are calculated to exclude these con-

stituents and in this way to give a true indication of the nature of thecombustible material. When the moisture and ash are not included, theanalysis is referred to as moisture- and ash-free or combustible. Sulfurhas some heating value but is nevertheless an objectionable constituent

of coal because of its corrosiveness and because of its contribution to airpollution, which is commonly referred to as acid rain.

The ultimate analysis of coal includes carbon (C), hydrogen (H),sulfur (S), oxygen (O), and nitrogen (N). This analysis is performed ina chemical laboratory. It is required for heat-balance calculations and

the determination of the theoretical air requirements. Table 4.9 givesa typical representation of the proximate and ultimate analysis offuels on an as-received basis, classified by rank according to the pro-gressive changes from wood to anthracite. Analyses of coal types vary

based on their location throughout the world.In this ultimate analysis, the moisture content is included in the

hydrogen and oxygen values. The calculations in Table 4.10 show howthe ultimate analysis for bituminous coal can be expressed with the

moisture as a separate item. The calculations also show how the analy-

sis can be converted to a dry and ash-free basis.Ash is an inert material, but its characteristics frequently determine

the desirability of a coal for a given installation. Because of the impor-

tance of the fusion or melting temperature of the ash, tests have beendevised to determine this property.2 The ash to be tested is molded intoa small pyramid, placed in a test furnace, and exposed to a steadilyincreasing temperature. The atmosphere surrounding the sample iscontrolled, and the temperature is measured while the pyramids are

observed through a peephole in the side of the furnace (Fig. 4.10). The

temperature of the pyramids is noted and recorded at three stages ofmelting: initial deformation (Fig. 4.10a), when the tip of the pyramidfirst shows a change; ash-softening temperature (Fig. 4.10b), when the

pyramid forms in a sphere; and melting point (Fig. 4.10c), when the ashbecomes fluid and the sphere flattens. These three temperatures arereported in reference to ash fusion. Fusion temperatures provide ashmelting characteristics and are used to determine the potential for slag-

ging under normal operating conditions. The degree of slagging candetermine the firing method for the boiler.

When coal is heated in the absence of air or with a large deficiencyof air, the lighter constituents are volatilized and the heavier hydro-

carbons crack, liberating hydrogen and leaving a residue of carbon.

2 Such tests are used by the American Society for Testing and Materials (ASTM).



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This carbon residue that contains the ash and a part of the sulfur of

the original coal is called coke. The principal uses for coke are in theproduction of pig iron in blast furnaces and in iron foundries. Because

it is smokeless in combustion, it has been used for space heating.

The coking tendency of coal is expressed by the free-swelling index

(FSI). This test is made by grinding a sample of coal to pass a no. 60

sieve and then heating 1 g under specified conditions that also have

been described by ASTM. The profile obtained by heating the sample

is compared with a standard set of profiles to determine the FSI of the

sample. The standard profiles are expressed in one-half units from 1

to 9. Coals having an FSI below 5 are referred to asfree-burning, sinceparticles do not tend to stick together and form large lumps of coke

when heat is applied but remain separate during the combustion

process. Coals having an FSI above 5 are referred to as caking or cok-

ing, since the particles swell and tend to stick together when heated.

Figure 4.10 Ash fusion temperature determination

TABLE 4.10 Bituminous Coal Ultimate Analysis: Calculation of Moisture and Ash-FreeBasis

From tables Separate moisture Dry Combustible

S 0.95 0.95 0.98 1.06H 5.243.24

19* 4.88 5.04 5.44

C 78.00 78.00 80.62 87.01

N 1.23 1.23 (10.0324) 1.27 (10.0735) 1.37

O 7.473.2489* 4.59 4.74 5.12

Ash 7.11 7.11 7.35

Moist 3.24

100.00 100.00 100.00 100.00

*1 lb H2 8 lb O

2 9 lb H

2O.



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206 Chapter Four

The caking, or coking, characteristics of coal affect its behavior

when it is burned on hand-fired grates or stokers. When a coal cakes,

the smaller particles adhere to one another, and large masses of fuel

are formed on the grates. This action reduces the surface area that is

exposed to oxygen and therefore retards the burning. Since theselarge pieces of coke do not burn, a portion is discharged to the ashpit

as unburned carbon. For efficient combustion, a coking coal requires

some agitation of the fuel bed to break up the coke masses in order to

maintain uniform air distribution. Free-burning coals, on the other

hand, may be burned successfully without fuel bed agitation. These

characteristics (coking and free-burning) need not be considered when

coal is burned in the pulverized form.

Coal that has a relatively high percentage of volatile matter is termed

soft; that which has a lower percentage of volatile matter is termed hard.Many bituminous coals are considered soft coals, whereas anthracite

coal is considered a hard coal. When coal is heated, the volatile matter

has a tendency to be distilled off in the form of combustible gases known

as hydrocarbons. These volatile gases liberated from coal must be

burned in the combustion space above the fuel bed. A large combustion

space must be provided to burn these gases and thereby eliminate fuel

loss and smoke. Because hard coal has a relatively lower percentage

of volatile matter, it burns with a short flame, and most of the com-

bustion takes place in the fuel bed.When soft coal is burned in pulverized form, the volatile material is

distilled off and burns as a gas. This makes it relatively easy to main-

tain ignition and complete combustion with a minimum flame travel.

Hard coal is also burned in the pulverized form, and in this case, each

particle is a small portion of carbon that must be burned by contact

with oxygen. The combustion of these carbon particles requires an

appreciable amount of time, resulting in a long flame travel and a

tendency for the fire to puff out at low loads and when starting up.

Other conditions being equal, it is necessary to resort to finer pulver-ization when hard coal is burned than when soft coal is burned. Thus

consideration must be given to the volatile content of the coal both in

designing equipment and in operation.

Coal as it is removed from the mine contains some moisture, and

the amount may be increased by exposure to the weather before it

reaches the plant. Moisture represents an impurity in that it adds to

the weight but not to the heating value of coal. It enters the furnace

in the form of water and leaves as steam in the flue gas. Heat gener-

ated by the fuel actually must be expended to accomplish this con-version. Normally it is to the operators advantage to burn coal with

a low moisture content to prevent the loss of heat that results from

converting the water into vapor or steam. However, when coal is



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burned on grates or stokers, there are conditions that make it advan-

tageous to have a small percentage of moisture present. This moisture

tends to accelerate the combustion process, keep the fuel bed even, and

promote uniform burning. The advantages gained by the presence of

the moisture may then balance the loss resulting from the heatrequired for its evaporation. Coals having 7 to 12 percent moisture

content are recommended for use on chain- and traveling-grate stokers.

The addition of moisture to promote combustion of coal is referred to

as tempering.

Coal transportation costs are a significant part of the overall fuel

cost. Nearly 60 percent of all coal deliveries is made by rail, and

slightly less than 20 percent is transported by barge. The remaining

coal delivery is equally divided between truck shipment and a contin-

uous belt system when the power plant is located at the coal mine.

Coal with a high moisture content presents some difficult handling

problems. During the winter season, this moisture freezes while the coal

is in transit, making it very difficult to remove the coal from the rail-

road cars.

The methods used to thaw coal cars are gas burners under the cars,

thawing sheds equipped with radiant electric heaters, and thawing

sheds with steam coils for heating air, which is circulated around the

cars. Car shakers are helpful, but they will not remove frozen coal.

Coal removed from the storage pile during snowy or rainy weather

also may contain a high percentage of moisture. The wet coal adheres

to the chutes, causing problems in the flow of the coal to the stoker or

pulverizer mill. In many plants this has become a serious operating

problem. Improvised methods of rapping the pipe, using an air lance,

etc., have proved marginally effective in getting the coal to flow. In

the design of the plant, the coal feed pipe from the bunkers to the

stoker or mill should be as nearly perpendicular as possible with no

bends or offsets. Access openings should be provided so that when

stoppages do occur they can be relieved quickly.

Electric vibrators have proved beneficial when the pitch of the

chute is insufficient to promote flow. In some installations, hot air is

passed through the coal pipe with the coal. The larger the coal size,

the less water it will retain. Therefore, some relief from freezing in

cars and the stoppage of chutes can be obtained by using a coarser

grade of coal. This, however, usually means an increase in cost.

The practice of applying antifreeze solutions to the coal when it is

loaded at the mines has proved effective in reducing moisture pickupin transit. The added coal cost can be justified by the reduced han-

dling costs at the plant. In fact, for hot, dry weather, where winds

could create coal dust clouds, the coal is often sprayed with oil or an

antifreeze solution to settle the fines.



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208 Chapter Four

In general, coal-fired power plants are located in remote areas

where the outdoor storage of coal has not been a major problem. Coal

dust from open coal piles and storm water runoff from these coal piles

had to be dealt with, but the remoteness of the sites made these prob-

lems rather minimal.However, with the worldwide implementation of various health and

environmental regulations, the covering of coal piles is being consid-ered as a requirement at many power plant sites. Figures1.6 and 1.7

show an example of a plant site in Florida where the environmentallysensitive location made it necessary to install coal storage domes.These domes protect the coal piles from rain, wind, and moisturebuildup. Dry coal burns more efficiently, and the coal pile is protected

from erosion and runoff. A dry stockpile reduces the risk of moisture

buildup in pockets, which can cause spontaneous combustion, a com-mon problem associated with piles of coal.

The coal domes shown in Fig. 4.11 provide coal storage for a power

plant located in Taiwan. Each of the domes is designed to store over150,000 tons of coal, and they are approximately 475 ft in diameterand 150 ft in height. Because of the location of the site adjacent to theocean, these domes were built with aluminum. Domes located in

areas where the environment is not corrosive can be constructedusing galvanized steel.

Internal cladding prevents dust accumulation on the internal struc-ture. This cladding provides a sealed, gap-free surface that resistsdust accumulation and minimizes potential explosion hazards due to

dust buildup.

Figure 4.11 Coal storage domes for power plant in Taiwan. (Geometrica, Inc.)



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Figure 4.11 also shows the coal conveyors to and from the coal stor-age domes. Each dome is designed to house the coal feed conveyor, thestacking and reclaiming equipment, and the coal transfer system tothe exit conveyor. Ventilation and filtering systems are also built into

the domes to provide normal ventilation as well as ensuring againstany methane or smoke buildup.

A schematic of the coal conveying system and the stacking andreclaiming equipment is shown in Fig. 4.12. A photograph of the system

in operation is shown in Fig. 4.13.With the use of coal so important to the production of low-cost elec-

trical energy, and with protection of the environment being such avital requirement, innovative methods of handling and burning coal,such as these coal storage domes, are now being used.

Particle size is an important consideration in the selection of coal. Thesize requirement for different equipment varies widely. Coal burned ongrates must have a certain size composition to regulate the passage ofair through the fuel bed, while for a pulverized-coal burner the coal

must be reduced in size to small, powderlike particles to promoterapid and complete combustion.

There is a tendency for coal particles to fracture when mechanicalforce is applied. Dense, hard coals resist fracture and retain their size

during handling. Soft coals shatter easily, break up into small particles

when handled, and therefore are said to be friable. Size degradationduring shipment depends on the coal friability, which identifies itsease of crumbling, and the techniques, methods, and number of trans-

fers that should be used. This size degradation is not a concern whenthe coal is to be pulverized.

When coal containing a range of sizes is dropped through a chute orother coal-conveying equipment, the fine and coarse particles segregate.If this condition occurs in the coal supply to a stoker, the fine is

admitted to one section and the coarse to another. The resistance of

the fuel bed to airflow varies, resulting in different rates of airflowthrough the grates and subsequent variations in the rates of combus-tion. Coal-conveying systems for supplying stokers should be

designed to prevent this segregation. Refer to Fig. 5.9 for a typicalcoal distributor across a stoker inlet.

Fine coal particles have a greater tendency to retain moisture, andwet fine coal will not flow readily through chutes and spouts. The

clogging of coal-conveying equipment by fine wet coal becomes sotroublesome that coal with coarser particles sometimes must be

selected. Once the fine wet coal is in the furnace, there is a tendencyfor the combustion gases to carry the small particles of coal. This

results in a loss of heat due to the unburned carbon.

The type of combustion equipment dictates the size of coal that is

required. Therefore, it is necessary to designate coal size by stating



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Figure4.1

2Aschematicdiagramshowinginternalcoalconveyorsystemwithincoalstorage

domes(Geome

trica,

Inc.)

210



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the largest and smallest pieces and the percentage of the various-

sized particles, such as 114 in by14 in with not more than 15 percent

minus 14. For stoker-fired units, coal sizing is specified to meet the

stoker requirements, and therefore no additional sizing equipment

is required at the plant. For pulverized-coal-fired boilers, a maximumdelivered top size is specified with no limitation on the percentage of

fines.

A characteristic known asgrindability is considered when selecting

coal for pulverizer plants. Some coals are harder and therefore more

Figure 4.13 Coal conveyor system in operation (Geometrica, Inc.)



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212 Chapter Four

difficult to pulverize than others. The grindability of coal is tested, and

the results are reported in accordance with the Hardgrove standard,

which has an index of 100. A weighed, screened sample is placed in a

laboratory test mill, and a given preestablished amount of energy is

applied. The ratio of the fineness produced in the test sample to thatproduced when the same amount of energy was expended on a sample

of standard coal is the Hardgrove value for the coal tested. The index is

relative, since large values, such as 100, represent coals that are

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