NC I PLES O F CO MBU S T I O NI N

T HE S T EA M BO I LER

FU RNA CE

By

ART HU R D PRAT T

P‘

uélz’

sfied by

T HE BA BCO CK 8: W I LCO"CO .

NEW YORK

Copyright, 19mg, byT he Babcock Wilcox CO‘

.

PR INC IPLES O F COMBUSTI ON IN

T HE STEAM BOI LER

FURNACE

SEC T I ON PAG E

INTRODUCTION

T HE CHEM ISTRY OF COMBUSTION

DENS ITY,WE IGHT AND VOLUME OF G ASES

HEAT OF COMBUST ION

SPECIF IC HEAT

T EMPERATURES DEVELOPED IN COMBUSTION

AIR AND COMBUST ION

COMBUSTION FORMULAE

COMBUSTION LOSSES

SMOK E .

G ENERAL CONCLUS IONS

T HE COMPUTAT ION OF COMBUSTION DATA

COAL

WOOD

O IL

NATURAL G AS

BY-PRODUCT COK E OVEN G AS

BLAST FURNACE G A S

"III HEAT BALANCE

SOLID OR LIQU ID FUELS

G ASEOUS FUELS

I N T RO DU C T I O N

HE function of a boiler furnace is the generation of themaximum amount of heat from a given quantity of a

specific fuel,and if such function is to be properly fulfilled

,

it is essential that the furnace Operator understand the broaderprinciples involved in combustion . Unfortunately

,from the

standpoint of effi cient steam generation,the statement is too

frequently accepted as true that theoretical generalizations andmathematical formulae are of but little value to the operatingengineer . To an extent

,such statements may be true

,but on

the other hand it is to be remembered that combustion is purelya chemical phenomenon and as such can be properly investigatedand controlled only by chemical means . Experience resultingfrom the “cut and try methods of ordinary actual practise inthe burning of individual fuels is unquestionably an importantfactor in the bringing about of efficient furnace results

,but it is

obvious that such results will be most readily secured when thisexperience is combined with a full knowledge of the theory ofcombustion and the properapplication of the available methodsused in obtaining combustion data. Further

,the importance of

such knowledge is today greater than it has ever been . Mostapparatus for the generation of power has reached a state of

development where there is but little likelihood of any great increase in economy . The generally accepted types of steam boilerused in present day power plant practise have the inherent abilityto absorb heat efficiently and from this standpoint may be includedwith the apparatus from which much more cannot be expected .

I f we accept this statement as true,the effi cient generation

of“ steam in the boiler proper becomes in reality a question of

efficient combustion,and it is this phase of boiler practise

the efficient generation of heat in the boiler furnace—in whichthere is the greatest and in fact the only field for appreciableimprovement .Power plant owners are coming more and more to appreciate

the necessity for intelligence in the boiler room— the reinforcingof experience in firing by a full knowledge of the theory of combustion—and in the growing number of plants where this needis realized suitable apparatus is installed for the determination

and checking of combustion results . In plants not so equipped,the possible savings due to the intelligent use of such apparatusand the proper appl icat ion of the data so obtained In

preventable losses,are In aggrega te enormous .

T HE CHEMISTRY O F COMBUSTION

E chemistry of combustion as applied to boiler fum ace

practise is elementary but for a proper comprehension ofthe subj ect it seems advisable to include a brief con

sideration of the general principles involved,together with data

covering the combining qualities of the constituents of the fuelsordinarily encountered in steam generation .

The smallest quantity of an element or a com pound thatis capable of separate existence is taken as the physical unitof matter and is called a molecule . Molecules are composed ofatoms of elements which may be defined as the smallest unitof an element which can enter into or be expelled from a compound . Atoms never exist singly but in combination with one

or more atoms to form a molecule . Molecules of the elementarygases such as oxygen

,nitrogen and hydrogen

,are supposed to

consist of two atoms .

A chemical reaction between elements or compounds is arearrangement of the atom s of the constituent elements intoa new combination of molecules . Such reactions always occurin acco rdance with fixed and invariable weight relations which arecharacteristic of the elements involved, and definite volumetricChanges based on the number of gaseous molecules reacting andproduced .

El ements are designated by symbols,and compounds by

combinations of the symbols of their constituent elements .

Subscripts are affixed to the symbols to designate the numberof t imes the combining or atomic weight of the element occurs .It follows that from the symbol of a compound so expressed andthe atomic weight of the elements involved

,the proportionate

parts by weight of the various constituents entering into the compound may be readily determined .

The elementary substances encountered in combustion workare oxygen

,nitrogen

,hydrogen

,carbon . and sulphur . The

symbols of these elements together with their atomic weightsare given in Table I .

COMBUST ION

Combustion is the phenomenon resulting from any Chemicalcombination evolving heat . Oxygen is the sole supporter of

9

combustion,and a combustible therefore may be defined as a

substance capable of combining with oxygen to produce heat .The speed of combustion depends upon the affinity of the combustible element for oxygen

,and to a lesser extent upon the

'

conditions under which combustion takes place . This Speedmay vary from the very slow,

as in the case of rust formation,to

the instantaneous,as in the explosion of confined powder .

From the standpoint of heat production for steam generatingpurposes

,combustion may be defined as the rapid combination

of the combustible elements of fuel with oxygen, while in thissense the term combustible implies the capacity of an elementfor combin ing rapidly with oxygen to produce heat .

Combustion is said to be complete when the combustibleelements and compounds have unitedwith all of the oxygen withwhich they are capable of entering into combination .

TABLE 1ELEMENTS AND COMPOUNDS ENCO UNTERED "

I N COMBU STION

A utom ic Weight Molecular Weight

Substance FormAccurate A pproxi Accura te

So lidHydrog enOxyg enSulphur So lidNitrog en iCa rbon MonoxideCa rbon D ioxideMethaneAcetyleneEthyleneEthaneS ulphur DioxideHydrog en S ulphideWa ter Va porAir

'*Atom ic sym bo l.T T he m olecu la r weight of C has not been definitely determ ined . Ca rbon exists in a

num ber of form s each of wh ich probably ha s its own m olecu la r weight . T he la test investig a tions indica te that a m olecu le of ca rbon in a ny form consists of at least 12 a tom s.

IA tm ospheric nitrogen a s dist inguished from chem ica lly pure nitrogen wh ich ha s a n

a tom ic weight slightly less than

For the commercial production of heat it is essential thatthe combustible elements have a strong and ready affinity foroxygen . Carbon and hydrogen which are by far the most important of combustible elements encountered in the commonfuels meet this requirement admirably . These occur either ina free or combined state in all fuels

,liquid

,solid and gaseous .

The combustible elements and the compounds in which theyappear in any of the fuels used for commercial heat generationare given in Table I . This ‘ table gives the symbols of theelements and their compounds which occur in combustion worktogether with their molecular weights . It also includes the noncombustible elements and compounds

,a knowledge of which is

necessary in the obtaining and application of combustion data"A IR

As we find in nature the combustible matter for the generationof heat

,so from the same source we obtain

,in the oxygen of the

air,the necessary supporter of combustion.

Atmospheric air is a mechanical mixture— as distinguishedfrom a chemical compound— of oxygen

,nitrogen, and slight

amounts of carbon dioxide, water vapor, argon and otherinert gases . For engineering purposes , the carbon monoxideand the inert gases are ordinarily included with the nitrogen andof the Slightly varying prOportiorfs of oxygen and nitrogen givenby different authorities the generally accepted values are

By Volum e ByWeightP er Cent P er Cent

I 2 3 I 5

The oxygen with its strong affinity for the combustible con

stituents of the fuel,under the proper conditions of temperature

which will be discussed hereafter,separates itself from its

mechanical union with nitrogen and enters into chemical com binat ion with the available combustible, thus fulfilling its functionin the promotion of combustion . The nitrogen serves no purposein combustion and in fact is a source of direct loss in that itabsorbs heat in its passage through the furnace and carries offa portion “of such heat in leaving the boiler ; further, as a uselessconstituent it necessitates in the design of the furnace, boiler

and flue,space for its ' accommodation

,

‘

which,were it possible

or practicable to supply oxygen alone to the fuel,would not

be required .

The combination of oxygen with the combustible elements andcompounds is

,as stated

,in accordance with fixed laws . Considered

as a chemical reaction the manner of such combination is simpleand may be readily computed from the molecular weights givenin Table I . Assuming complete combustion and that the exactamount of oxygen required is supplied and utilized in combination

,

these reactions and the resulting combinations are as given inT able 2 .

TABLE 2CHEMICAL REACTIONS O F COMBUSTION

Reaction

Ca rbon (to CO )Ca rbon (to C0 2)Ca rbon MonoxideHydrog enS u lphur (to 8 0 2)S ulphur (to SO 3 )MethaneAcetyleneEthyleneEthaneHydrog en Sulphide

It is important to note from this table that carbon may enterinto combination with oxygen to form two compounds

,carbon

monoxide and carbon dioxide . In burning to carbon monoxide,carbon has not combined with all of the oxygen with which it iscapable of entering into combination and is not therefore com

pletely oxidized . In burning to carbon dioxide it has combinedwith allof the oxygen possible and oxidization is complete . Carbonmonoxide may unite with an additional amount of oxygen to formcarbon dioxide and in this way the carbon of the original combination may become completely oxidized . The fact that carbonmay enter into these two combinations with oxygen is of thegreatest importance in furnace efficiency and will be discussedhereafter at greater length in the consideration of the heat ofcombustion and air supply .

12

2C+ 02= 2CO

aC—i—z o z

'

z z CO2

2CO+ 02= 2CO 2

2H2+ O

Z= 2H

ZO

S+ 02=SO

2

ZS+ 30 2= 25 0

3

CH4—i—20 2

=CO2 +2H

ZO

2C 2H 2+ 50 2=4C0 2+ 2H

20

C 2H 4+ 30 2=2C0

2+ 2H20

2C2H

6+ 70 2=4C0 2+6H 20

s S —30 2- 2H

20 -2SO

2

T EMPERATURE

Before discussing in detail the effects of supplying oxygenfor combustion in excess of the requisite amount or of supplyingless than the amount required

,the other important factor of

combustion,viz .

,temperature

,should be considered .

The speed of combustion is, as stated, dependent upon theaffinity of the combustible matter for oxygen and the conditionsunder which combustion takes place . The chief of these conditions is that of temperature. The mere fact that oxygen isbrought into the presence of a combustible substance does not ofnecessity mean that combustion will follow .

Every combustible substance has a temperature called itsignition temperature to which it must be brought before it willuni te in chemical combination with oxygen and below which suchcombination will not take place ; and this ignition temperaturemust exist -with oxygen present or there will be no combustion .

The ignition temperature of different combustible substancesvaries greatly . These temperatures for various fuels and for thecombustible constituents of the fuels used in boiler practise aregiven in Table 3 .

TABLE 3IG NITION TEMPERATURES

Com bustible Substance

SulphurFixed Ca rbon—Bitum inous Coa lFixed Ca rbon—Sem i-bitum inous Coa lFixed Ca rbon—Anthra cite Coa lAcetyleneEthaneEthyleneHydrogenMethaneCa rbon Monoxide 12 10

It is of interest to note that the temperature of ignition ofthe gases of a coal vary from each other (see flame) and areconsiderably higher than the ignition temperature of the fixedCarbon of the coal . The ignition temperature of coal is theignition temperature of it s fixed carbon content

,since the gaseous

I 3

constituents are ordinarily distilled off,though not igni ted

,before

such temperature is attained .

When combustion has started,the heat evolved in the

oxidization o f the combustible matter will maintain under properconditions sufli ciently high temperatures for further ignition .

I 4

DENS ITY W E IG HT A ND VOLUME O F

GASES

N the computation of combustion data it is frequently necessary to know the density

,weight and volume of air and of

the various gases encountered in commercial practise .

The density of a ga s (commonly expressed by the symbol A )which is ordinarily referred to that of air as standard

,is the weight

of unit volume of the gas divided by the weight of an equalvolume of pure dry air

,the conditions of temperature and

pressure being the same .

The weight per cubic foot of a gas,ordinarily designated by

S,is,under standard conditions

,called the specific weight . With

the weight of air at atmosphere pressure and varying temperatureconditions known

,the weight of any gas at the same temperature

m ay be computed from the relations of density and specific weightas indicated by

(a ir)"A ( I )

the subscripts 1 simply indicating that the air and the gas,the

weight ofwhich is required,are at the same temperature .

The Specific volume of a gas,usually designated by the symbol

V,or the cubic feet per pound

,will obviously be the reciprocal of

its specific weight,or

I

5.

While it is perhaps easier and more convenient to computeweight and volumetric"data of gases from their relative densitiesand a table of weights and volumes of air

,such values may be

computed from the characteristic equation of a perfect gas,viz

PV: RT

where P= absolute pressure in pounds per q are foot,V=volum e per pound in cubic foot

,

T = absolute temperature,

R= a constant varying with the gas and derived from therelations existing between the pressure

,volume and

temperature of the gas in question .

I S

This pressure—volume-temperature relation for any gas,as

indicated by the constant R,represents the expression

_ Povo

T0

where the subscripts0represent a set of standard conditions .

Since the volume (and hence the specific weight) of a gas is afunction of both temperature and pressure

,it is necessary

,in

order that there may be a suitable basis for comparison,that

all volumes be reduced to some such standard set Of conditions .These conditions

,as ordinarily accepted

,are a pressure of

poundsper square inch (2 I pounds per square foot)and a temperature of 3 2 degrees Fahrenheit .Table 4 gives the weights and volumes of a ir at atmospheric

pressure and different temperatures .

TABLE 4VOLUME AND WEIG HT OF A IR

A T ATMO SPHERIC PRESSU RE

I 6

With the values of P0and T

Othus fixed (see Absolute

Temperature below) the value of the constant R for any gas asgiven in formula (3 41) may be expressed as

V459

thu's offering a means of determining the value of R directlyfrom the specific volume of the gas . Since the specific volumeof a gas is the reciprocal of the weight per cubic foot

,and for

any two gases the weights per cubic foot vary inversely as theirm olecular weights

,where the value of R for any gas is known

,

the value for any other gas m aythus be determined from therelations of the molecular weights of the two gases

,viz

-3045 V,

N2—Molecular Weight= 28 R= 5

02—Molecular Weight : 32 R= x

x z: 32 28

R

From the value of R as given in formula (3 -6) it is possibleto express the characteristic equation of a perfect gas in what isperhaps a more convenient form for general use

,as

PV Pov

0

T T (W)0

From the characteristic equation of a perfect gas,it is

obvious that the volume of a gas will vary inversely as the absolute pressure and directly as the absolute temperature . Incombustion work the variation in the pressure

“

Of the gasesencountered is small . The temperature range covered

,however,

is large,and because of the effect of temperature Change on

volume,it is perhaps well to define here “absolute temperature .

”

Experiment shows that if the temperature of a perfect gas at

32 degrees Fahrenheit is increased one degree, the pressure beingkept constant

,the gas expands part of its volume . If such a

rate of expansion per one degree increase in temperature held goodat all temperatures

,and experiment shows that such is the case

above 32 degrees, if its pressure is kept constant, the'

g as woulddouble in volume with an increase in temperature above 32 degreesof degrees Fahrenheit . Under a reduction of temperature of seconds below 32 degrees (corresponding to an

I 7

ultimate temperature of 49 1 .64—32=459 .64

i

degrees Fahrenheitbelow zero) the gas would disappear . While undoubtedly somechange in the law would occur before the lower temperature could

'

be reached,there is no reason why the law may not be used over

the temperature range in which it is known to hold .

Table 5 gives the densities, weights and volumes understandard conditions

,of the commercial gases encountered in

combustion problems,as well as the values of the constant R .

.TABLE 5

DENS ITY ,WEIG HT AND VOLUME OF G ASES

A T ATMO SPHERIC PRESSU RE A ND 3 2 DEG REES FAHRENHEIT

Relative Density Weight Cubic Feet Va lue ofper per ConstantRCubic Foot Pound in

Air: 1 Hydrog en Pounds Cubic FOOt PV=RT

Ca rbon MonoxideCa rbon DioxideMethane .

AcetyleneEthyleneEthaneSulphur DixoideCa rbonl i

S ulphur

*Based on a pproxim ate m olecu la r weights.

TSoHd.

flf ca rbon can be conceived to exist a s a g as under standa rd condit ions its relativedensity wou ld be its weight per cubic foot .0668 pounds

,and its volum e cubic

feet per pound.

From the foregoing it is evident that under a constant pressure

,the volume of a gas will vary directly as the number of

degrees between its temperature and the temperaturedegrees Fahrenheit . To simplify the application of the law

,a

new thermometric scale is constructed,the point corresponding

to —460 degrees Fahrenheit being taken as the zero point, andthe degrees being of the same magnitude as those on the Fabrenheit scale . Temperatures referred to this new scale are called

18

https://www.forgottenbooks.com/join

HEAT O F COMBUSTI ON

nter into a direc t combination toa definite amount of heat is eithered . Such amount of heat is called

the heat of combination and from its very definition may beeither posit ive or negative . When a compound is decomposedinto its constituent elements the amount of heat absorbed or

evolved is exactly the same a sthat which was evolved or absorbedin the original formation of the com pofm d. When both com bination and decomposition. are involved in a complex chemical changethe heat produced or absorbed is the net result of the two reactions .

HEAT OF COMBUSTIONSince the term combustion

,as used in furnace practise

,is

limited to the rapid chemical com bination of the combustibleconstituents of a fuel and oxygen

,with a resulting production

of heat, the heat of combustion of a fuel is obviously the heatevolved in the complete oxidization of such combustible elementsthrough union with oxygen . The heat of combustion is thusthe heat of combination of a specific set of elements andcompounds

,the combinat ion of whi ch with oxygen always

results in the production of heat . It follows that the heat of

combination of a compound . which results from the unionof a single combustible element with oxygen to produce heat isthe same as the heat of com bustion of that element .The principles controlling the development of heat by

combustion as generally accepted as authoritative are thosepropounded by Berthelot . His “second law ” is of particularinterest in combustion as limited to furnace practise

,and as

applied to such practise may be stated as followsThe heat energy evolved in any chemical change in the

boiler furnace,where no mechanical work is done

,i . a

,evolved

through the union of combustible elements with oxygen, is dependent upon the final products of combustion and in no wayupon any intermediate combination or combinations that mayhave occurred in reaching the final result .The application of this law may be readily shown by exampleA coal fire from which all of the volatile constituents have

been driven and which consists of incandescent coke may for

20

the present purpose be considered as consisting entirely of,

carbon . If air is introduced under the fire the oxygen im m edi

ately breaks its mechanical union with nitrogen and enters intochemical combination with carbon to form carbon dioxide

(C+ 2 O= CO2 ) . Each unit of carbon has combined with the

maximum amount of oxygen with which it can exist as a compound . The oxygen on the other hand is capable of uniting with

,

additional carbon and as the unit of carbon dioxide passes upwardthrough the fuel bed under the influence of draft it encountersother free carbon with which it unites to form carbon monoxide

(CO 2

—C 2CO ) , thus “satisfying the affinity of oxygen forcarbon .

” If no additional oxygen is encountered in the furtherpassage through the fuel bed

,these particular molecules

,as

representative of the products of combustion,will issue from the

fuel bed as carbon monoxide . If no additional oxygen is encountered in the furnace the total heat available for later absorptionby the boiler is that due to the combustion of carbon to carbonmonoxide regardless of the fact that at one stage of the processthe carbon had been completely oxidized and carbon dioxidehad been produced . If

,on the other hand

,additional oxygen is

encountered in the furnace,the temperature is above the ignition

point of carbon monoxide, and this temperature is maintained asufficient length of time for further combustion

,z'

. a,if the gases

are not cooled below the ignition temperature by the boilerheating surface before further combustion can be completed

,

the carbon of the carbon monoxide will unite with additionaloxygen to form carbon dioxide (2CO+ 2 O 2CO

Q ) . The totalheat evolved and available for absorption in such cases will bethat

,due to the burning of carbon to carbon dioxide regardless of

the two intermediate steps .

That combustible substances exist is,under the laws of

Chemical c ombination,an absolute indication that at some time

there was expended an amount of energy in some transformableshape equivalent to the heat of combustion of the individualsubstance considered . While it is not within the provinceof the present article to discuss the reactions which broughtabout the state of existence of the combustible substancesas used for commercial heat“ generation

,the above statement

may be accepted as true and the principles involved S imply

as being of the general laws covering the conservation of energy .

The heat of combustion of a fuel,or as it is sometimes called

,

the calorific value,as used in boiler practise

,is the amount of

heat expressed in B . t . u . generated by the complete combustionor oxidization of one pound of the fuel in question . The amountof heat so generated is a constant for any given combination of

combustible elements and compounds,and in accordance with

Berthelot’s second law is irrespective of the manner in whichcombustion takes place

,so long as it is complete .

The unit of measure of qua ntity of heat is, as stated above,the B . t . u . Until recently this has ordinarily been defined asthe amount of heat necessary to raise the temperature of onepound of water at a definite temperature

,one degree Fahrenheit .

The value as now generally accepted is 1 73mm of the amount ofheat necessary to raise the temperature of one pound of waterfrom 32 deg rees to 2 12 degrees Fahrenheit .Table 6 gives the heat of combustion of what may be termed

the “pure .fuels”whether elements or compounds . These are found

in various combinations in the fuels encountered in boiler practise .

TABLEHEAT O F COMBUSTION

BY CALORIMETRIC DETERM INATION

H eatVa lue—B . t. u. per Pound

H igher Lower or Nett H igher

Hydrogen . 5 2920

Ca rbon (to CO )Ca rbon (to CO Z )C a rbon MonoxideCa rbon in CO §MethaneAcetyleneEthyleneEthaneS u lphur (to 80 2)

Sulphur (to 80 3 )

1There is a considerable discrepancy between lower heat va lues a s g iven by differentauthorities, the va ria tion being due to m ethods of com putat ion and assum ptions. (See text .)T he va lues gi ven a re those of G . A . Goodenough .

i At 32 degrees Fahrenheit and a tm ospheric pressure.

§ Per pound of ca rbon in ca rbon m onoxide,z'

. e.

,pounds of CO .

Hea ting Va lue by Ca lorim etry, see Discussion, page 23 .

It appears from Table 6 that when one pound of carbon isburned to carbon monoxide the heat produced is B . t . u .

less than if the carbon were completely oxidized or burned tocarbon dioxide . That such a difference exists in the amount ofheat evolved in the burning of a fuel in two different ways offersthe possible source of one of the most prolific of furnace losses .

This will be discussed at greater leng th in connection withair supply and combustion .

The heat Of combustion of a fuel is the basis upon which theeffi ciency of a steam boiler is computed and is therefore Of thegreatest importance .

MEASUREMENT OF HEAT OF COMBUSTIONThe most satisfactory method of determining the heat value

Of any fuel is by the direct measurement of the heat evolvedduring combustion in a calorimeter . Descriptions of fuel calorimeters and the methods of their Operat ion are given by numerousauthorities and need no discussion here .

For solid fuels and most liquid fuels,calorimeters of the

“bomb ” type in which combustible Substances are burned in aconstant volume of oxygen

,give the most satisfactory results .

With such calorimeters, properly Operated, combustion will becomplete

,all of the heat generated will be absorbed and measured

,

and heat from external sources can either be excluded or haveproper correction made for its presence .

For gaseous fuels calorimeters of the continuous or constantflow type are ordinarily used

,the"unker calorimeter being a c

cepted as standard for this class Of work .

The accuracy of the determination of the heat value of a fuelby calorimetry is largely a question of the personal equation ; themore careful the manipulation of the instrument the more accuratewill be the results . With carefulmanipulation

,the results should

be accurate to within a fraction of one per cent .For solid and liquid fuels separate determinations are necessary

for the heat value of each specific fuel . For elements and combustible compounds entering into gaseous fuels the heats of

combustion have been determined by so many authorities thatdefinite values may be accepted as correct without determination .

In View Of the difficulties of computing the heat values Of suchcombustibles this fact is fortunate .

23

COMPUTATION OF HEAT OF COMBUST ION

While the heat value of a fuel may, as stated, be most satisfactorily determined by actual experiment in a calorimeter, itfrequently happens that such apparatus is not available . Undersuch conditions approximate heat values may be determined forcertain fuels by computation from the ultimate chemical analys isof the fuel . The formula for such computation in most generaluse and which for most coals gives reasonably accurate results isthat of Dulong . This formula

,using approximate figures

,is

B . t . u . per pound : C (H—gH—zio 50 S (4)the symbols representing the proportionate parts by weight of

carbon,hydrogen

,oxygen and sulphur in the fuel

,while the co

efficients represent the approximate heating values of theconstituents with which they appear in the formula . The term

0(H—g )

ls assumed to contain a correction for the hydrogen In

the fuel which is combined wi th oxygen and exists as moisture .

Dulong’

s formula will g ive as stated, very close approximationsfor the heat value of most coals—probably within 2 or 3 percent . There are

,however

,certain sources of possible error

in the use of the formula even for the fuels with which itgives the most accurate results

,and since these sources of

error offer the explanation of'

why the formula is not applicableto all fuels

,and particularly to gaseous fuels

,their discussion

seems warranted .

(a ) Carbon and sulphur are the only elements in coal in afree state

,and but a portion Of these constituents may occur in

elementary form . The carbon may be present as graphite or asamorphous carbon

,the heating values of which are entirely

different . The sulphur may exist as FeS2 (pyrites) . Further

,

the sulphur may be burned to SO2or 5 0

3 ,in the production of

which the amount of heat evolved is widely different . (SeeTable

(5) If a portion of the carbon and hydrogen are combined ashydrocarbons

,the heating value of such combinations are far

different than if the elements existed separately,since in such

case the heat of combination or of dissociation would have to be

24

considered . This factor makes questionable the heat value of aportion of the carbon and probably of all of the hydrogen .

(c) The term (PI—g) which is assumed to be correct for thatportion of hydrogen contained in the moisture is not a properassumption since a portion of the oxygen unquestionably existsin a free state in all fuels .

(d) An additional portion of the oxygen is in all probabilitycombined with nitrogen in certain organic nitrates and some maypossibly exist in combination as carbonates in mineral matterforeign to the coal .All of these factors tend toward error . While with most coals

the error is small,it is unfortunately, with the generally accepted

co-efficients,one of excess . In the case Of gaseous fuels

,however

,

in view pa rticularlv of items (é) and (5 ) above, the chance Of erroris great . The magnitude of error will depend in such cases uponthe individual set Of hydrocarbons present in the fuel . If we hadfor instance a fuel com posed of C

SH

GO

Q ,the constituents m ight

be united in such a number Of different combinations as to giveresults varying with the manner of combination

,from per cent

less to per cent greater than the result which would be Ob

ta ined from the application Of Dulong’

s formula,which assumes

that all of the oxygen is combined with hydrogen as water .

Numerous other formulae of an empirical nature,

for thedetermination of the heat value “Of fuels have been offered byvarious authorities . Most of these are’ based upon a series ofchemical analyses

,and while they give reasonably accurate results

in the case of individual classes of coal,they fail when an attempt

made to apply them not only to other Classes Of fuel,but even

to other Classes Of coal .The only accurate and reliable heating value of a fuel is that

determined experimentally with a calorimeter,and such determ i

nation Should correctly be reported as a part Of the ultimate orproximate chemical analys is of the fuel .In the case of commercial gases where the proportionate parts

by weight may be readily determined,the heating value may be

accurately computed from a table of the heat values of individualconstituents

,which values have been definitely fixed by numerous

calorimetric experiments .

HIGHER AND LOWER HEAT VALUES

The heat value of a fuel as defined is known as the “higherheat value and is ordinarily accepted as the standard in thiscountry . In the case of fuel containing hydrogen, and thisincludes practically all fuels in commercial use, there 18 anothervalue known as the “lower ”

,

“net ” or“available ” heat value

,

in the determination of which an attempt is made to allow for thelatent heat recovered in the condensation of the water vaporformed in the combustion Of hydrogen . For example : In thecalorimetric determination of the heat value of a fuel containinghydrogen

,the products of combustion are cooled to approximately

the temperature of the original mixture,say 62 degrees Fahren

heit . In cooling the products to this temperature the water vaporformed by the combustion of hydrogen is condensed

,and the

result,expressed in B . t . u .

,after being corrected for sulphur and

like factors,z’

. e .,the higher heat value

,includes the latent heat

of water vapor given up in such condensation .

If the lower value be represented by H; and the weight ofwater produced per pound of fuel by w

,the lower heat value may

be determined from —wr (5 )

where Hpequals the higher heat value and r is a factor which

varies with the percentage of hydrogen in the fuel,the amount

of air or oxygen used in'

combustion,the moisture in the air and

the temperature to which the products of combustion are cooledin the calorimeter. T OO frequently r is simply taken as the latentheat of steam either at 32 degrees or 2 12 degrees, though in calorimetric work neither of these temperatures are apt to occur .With the lower heat value so defined

,the difference between

the higher and the net value will obviously be the total heat ofthe steam or water vapor as it escapes less the sensible heat ofan equivalent weight Of water at the temperature of the fuel andof the oxygen before combustion take s place .

The lower heat value is in common use in Great Britain andin most foreign countries . In this country the higher value isalmost universally accepted

,and this is the standard recommended

by the American Society of Mechanical Engineers .

Any attempt to make use of the lower heat value introducesa source of possible error in the proper temperature for use in

26

https://www.forgottenbooks.com/join

SPEC IF IC HEAT

heat Of combustion of any substance from its verynature must have an. important bearing on the temperaturewhich will result from the burning of Such substance .

Before discussing the temperatures so developed,a knowledge of

the specific heats is necessary . This subj ect is important in thecomputation of much combustion data

,and for this reason is

considered at length .

The specific heat of a substance is the amount of heatexpressed in thermal units required to raise unit weight of thesubstance through one degree of temperature

,the units in this

country being one pound and one degree Fahrenheit .The specific heat of all substances varies with the temperature .

Since all substances vary in volume or in pressure with changesin temperature

,it is necessary to distinguish between the specific

heats at constant volume and at constant pressure,expressed

ordinarily as C,and C

9 ,respectively .

Liquids and solids,because of their low co-efficients of

expansion,vary but little in volume under a temperature change

Of one degree and for these substances therefore there is but littledifference in the specific heat at constant volume and that atconstant pressure . With gases

,on the other“ hand

,there is a

decided distinction . When any heat is added to a gaseoussubstance

,its volume may be kept constant

,in which case no

external work is done,or the gas may be allowed to expand during

the addition Of heat,the pressure being kept constant . The

specific heat at constant volume therefore will always be less thanthat at constant pressure by the amount of heat required to dothe work of expansion against external pressure .

Under both specific heat at constant pressure and that atconstant volume it is necessary to distinguish still further betweenz’

m z‘

a flm neom and m a m Specific heat .The instantaneous Specific heat of a substance is the amount

of heat that must be added to a unit weight of such substance atsome definite temperature to increase its temperature one degree,under given conditions of pressure or volume .

The mean specific heat of a substance,over a given tempera

ture range,is the value by which such range must be multiplied

28

to determine the quantity of heat necessary to raise unit weightof the substance through the range under the conditions ofpressure or volume which exist .In the computation of combustion data the mean specific heat

should be used .

From the definition of a B . t . u . as hitherto accepted (seepage when the specific heat Of water is given as unity

,such

value would express the instantaneous specific heat at constantpressure

,at the standard temperature (usually 62 degrees Fahren

heit) . From the definition now accepted—namely, I vgvth of theheat required to raise one pound. of water from 32 degrees to2 12 degrees Fahrenheit—where the specific heat is given as one,such value is the mean specific heat between 32 and 2 12 degrees .Except in the case of water vapor

,the variation with pressure

in the specific heat of the gases ordinari lyencountered in combustion work is negligible . In the case of water vapor

,where it

is_necessary to deal with any considerable range of pressures, this

variation would be an appreciable factor,but in the commercial

gases involved in combustion,the partial pressure exerted by

water vapor,either in gases before combustion or in the exhaust

gases is rarely over one pound absolute . With such a limitedpressure range and in view of the fact that the water vaporcontent of the gases is sm all

,the effect of such variation in

pressure on the specific heat of the gas as a whole may beneglected .

The range of pressure in the gases encountered in boiler workis so limited— varying from that at which the commercial gasesare introduced into the furnace for combustion to the suctionunder which they are drawn over the boiler heating surfaces—thatin the computation of combustion data the gases may be safelyassumed to be at a constant pressure . The specific heat atconstant pressure is the specific heat which should be used

,and

any results based on the assumption of a constant pressure of thegases as a whole

,and in which the variation in the specific heat of

the water vapor content with Change of pressure is neglected,will be well within the limits of accuracy of practically all combustion data computation .

While the variation in specific heat with pressure can beneglected

,the variation with temperature is a very appreciable

29

factor and must be given proper consideration where accuracyis desired .

The results of the great amount of experimental work thathas been done in the determ inationof the specific heat of gasesare unfortunately not in complete agreement . From the workof Holbom and Henning

,Langen , Pier and Austin, however, the

specific heats of the diatomic gases (H 2 ,0

2 ,N

2and CO) and of

carbon dioxide and water vapor are pretty definitely determined .

The values for these gases which follow are apparently the mostauthoritative of those that-have been Offered .

The general formula for the specific heat of a gas at constantpressure may be expressed by the function

a+bt+cfi +dt3

(6 )

The mean specific heat of a gas between the temperaturest,and $

2will be then

or by integration5 —t z 1+

[02+ £1) t

, )2—22

‘

Z

CARBON DIO"IDEThe value of the instantaneous specific heat at constant

pressure of CO2 ,as given by Holborn and Henning

,in terms of

the Fahrenheit scale is

O . 1‘

9 83+ 8 3 5 x 10‘ 7 t ' -I 6 .7 x 10 9 12 (8)

Values as determined by this formula decrease rapidly attemperatures above 2400 degrees Fahrenheit . That such adecrease occurs appears questionable

,and for this reason it

seems advisable to modify the formula in such a manner as tocontinue the increase in specific heat with temperature in alogical way . Mathias Pier investigated the specific heat Of CO

2

at high temperatures and the values as determined by him areabove those of Holborn and Henning . A modification of Holborn

30

and Henning’s formula (8) for use in the case of temperaturesabove 2 200 degrees D . which appears to give logical results is

x 10 ' 7 t -2 3 .4 x 10' 9 z2 + o .22 x 10

"“ t3

This formula gives values for the specific heat of CO,above

2200 degrees Fahrenheit greater than those of Holborn andHenning and somewhat less than those Of Pier .

Formula which should be used for temperatures up to2200 degrees Fahrenheit

,in terms of mean specific heat at

constant pressure for a temperature range o—t,in accordance

with the relation between instantaneous andmean specific heatsas indicated by formulae (6) and (7 ) will become

x I O“ 7 Z x10 9 22 (t o)

For a range of definite temperatures,23—1

2 ,the constants will be

the same as in (I O), the values of t , and £2 being substituted asindicated in

For temperatures above 2200 degrees,the mean specific heat

at constant pressure between 0 and t degrees Fahrenheit becomesfrom formula (9 )

x ro w x 10- 9 z2+ 5 . 5 x I o“ l a t3 (1 1 )

and for a temperature range t1—t

2 ,the proper value may be com

puted in accordance with values of and £2indicated by using

the constants as given in

CARBON MONO"IDE AND N ITROGENHolbom and Henning give the instantaneous specific heat of

nitrogen at constant pressure as

t ( 12 )

Their investigations extended to a tem perature of 2456

degrees Fahrenheit and appear to Offer the most authoritativevalues . In the absence of data at higher temperatures it isnecessary to accept this formula for all temperaturesThe mean specific heat between 0 and 1 becomes then

opr , z‘

( 13 )

and f‘

Or a range tl

— z‘

2as indicated in the case of carbon dioxide .

Formulae (12 ) and (13 ) will also give the specific heat ofcarbon monoxide .

* See “Experim ents on the Ra te of Heat Transfer from a Hot G as to a Coo lerMetallic Surfa ce .

” T he Babcock Wi lcox Co .

,19 16 .

3 1

Mean Specific Hea t—.Wa ter Va por. 5 5 . 54 .53 .5 2 . 5 1 . 50 .49 .46 .45

Mean Specific Hea t—Hydrog en4-0 3-9 318 3-7 3-6 3-5

.29 .28 .27 .26 .25 .24 .23 .22 .21 .20 .19

Mean Specific Hea t, Ca rbon Monoxide, Ca rbon Dioxide, Oxyg en, Nitrogen, Air

FIGURE 1

O"YGENThe data on the specific heat of oxygen is meagre . Holbom

and Austin experimented with oxygen mixed with 9 per centnitrogen up to temperatures of 1160 degrees Fahrenheit

,while

Langen and Pier investigated at higher temperatures . The bestformula Offered is apparently one which gives values somewhathigher than those of Langen and Pier, but which agrees morenearly in values with that proposed by Holborn and Henning .

This formula for the instantaneous specific heat or oxygen atconstant pressure is

154+ 19 t

and for the mean specific heat over the range o— t

57 0 4-2 02 154 .000009 5 z

HYDROGEN

Holborn and Henning give as the mean molecular specificheat of hydrogen

m apr ,=6 . 5 8+ 32 z

‘

This in terms of mean specific heat becomes

Q M: 32 9 + 66 1

WATER VAPOR

The formula for the specific heat of water vapor is based onthe values given In Marks and Davis’ Steam Tables . Thisformula for the instantaneous specific heat at a constant pressureof one pound absolute (which may be accepted as correct for thepartial pressure of the water vapor in the gases of combustiondata work over the range of draft pressure or suction found) is

x I O ‘ 7 Z+ 2 82 5 x I t"

The mean specific heat for the range o—z‘ will be

x 104 1+ 942 x 112 (1 9)

For a range Of temperature l‘

l—t

2this means specific heat

will beCp, 1 _ t2

—Z t (20)

it See “Experim ents on the Rate of Hea t Transfer from a Hot G as to a Cooler Meta llicSurfa ce .

” T he Babcock 81. Wi lcox Co .

,19 16.

33

The specific heat of a gaseous mixture is found by multiplyingthe percentage by weight of each of the constituent gases by thespecific heat of that gas and dividing the sum of the productsby 100 .

Investigations of the specific heats of other important gasesencountered in combustion work

,over any considerable tempera

ture range are lacking,thorigh it is possible in one or two

instances,to give formulae from which approximate values m ay

be computed . In the computation of combustion work such gases

(CH C2H

4 ,etc .) are ordinarily dealt with at atmospheric or at

least at low temperatures, under which conditions reasonablyaccurate values are available . Further

,the percentages of such

gases in the ordinary gaseous fuels is not such as to cause any

great error in the determination of the specific heat of the gas asa whole through the use of inaccurate or questionable specific heatsfor these individual constituents . What appear to be the mostauthoritative values for the specific heats of these gases at 60and 600 degrees are given in Table 7 .

TABLE 7MEAN SPEC IFIC HEATS AT CONSTANT PRESSURE

AND ORD INARY TEMPERATURES

Methane t

fEthylene 33 5+o .0002 1 t

The mean specific heats between 0 and t the gas temperatureof the ordinary gases encountered in combustion work

,and of

water vapor are Shown graphically in Figure 1.

34

https://www.forgottenbooks.com/join

the formula for the determination of the theoretical temperaturedeveloped may be expressed

Mapz‘

l-f Heat 1t

2

where tlfi tem perature of fuel mixture,

t2= tem perature evolved in combustion,

£1,and ckz m ean specific heats of fuel mixture and products

of combustion,respectively .

S ince t,is unknown

,is also unknown

,and

,as stated

,the

method Of trial and error must be used . This method is bestillustrated by example

,and isperhaps most fully indicated by the

consideration ‘

of a gaseous fuel . Assume then,blast furnace gas

having an analysis as follows

aresCO

H2

2 3

CH 5 3 30

CO2

I

N2 5 8-9 5 5 7 -42

If this gas is burned with 20 per cent excess air the productsof combustion from Table 8 will be

*I t is to be noted in the case of fuels conta ining hydrogenous constituents,since no

condensation of water vapor occurs,the lower or a va ilab le hea t va lue of such constituents

is the proper va lue for use in the com puta tions. These va lues m ay be taken from Table 6

36

If the temperature of the fuel mixture before combustion is2 50 degrees Fahrenheit, the computations involved in the use offormula (22) under the assumed conditions, expressed in tabularform are

*From 20 per cent excess a ir. TIncludes N 2from excess a ir.

The heat energy of the fuel mixture above 0 degrees willthen

M . ch , “ .tl= .277245 x B . t . u .

Since z,is unknown it is necessary to assume a trial value in

order to compute With computed for such tria lvalue

,formula (22) may be solved for t, and the value of I , so

determined used for a second trial .If then we assume t the theoretical temperature evolved

under the conditions of combustion given as 3000 degreesFahrenheit

,we have

Substituting in formula (22)

5288 3 5 t,

t2=2 7 10 degrees

37

U sing as a second trial value 1 degrees,we have

‘Products—M 1

5 807

0364

02 7 5

L 299 8

Substituting again in formula (22)5 23 124 t

,

122 272 1 degrees

The theoretical temperature evolved under the assumed conditions will thus be approximately 27 3 5 degrees Fahrenheit .The above method may be continued if more accurate resultsare desired .

In the consideration of the theoretical temperature it isevident that the time element

,z'

. e .

,the length of time necessary

to complete combustion,does not enter

,though in actual practise

this is an appreciable factor .In practise

,the temperature which

,for a given fuel

,is theo

retically possible, is never obtained . The main factor in theburning of ordinary fuels which results in a temperature belowthat theoretically possible

,is the dilution of the products of

combustion through the introduction Of a greater amount of airthan is required for complete oxidization

,z'

. a,the presence of

excess air . Under such conditions there are present in theproducts of com bustion am ounts Of oxygen and nitrogen inexcess of the amounts required for combustion

,which excess

weights must be heated from the temperature at which they a re

introduced to the ultimate temperature of the gases . In using aportion of the definite amount of heat that a given fuel willgenerate for so increasing the temperature of these excess weightsof oxygen and nitrogen

,the temperature of the ultimate mixture

will be reduced to below that which would exist were there noexcess gases to be heated .

Temperatures below the theoretical will also result from aninsuffi cient air supply . Under such conditions there is a loss inthe heat generated due to incomplete combustion of carbon

(burning to CO instead of CO 2 ) .

38

A further reduction below the theoretical temperature occursthrough loss in radiation . While the tim e element does not enterinto any computation involving form ula in practise

,since the

quantity of heat radiated from a given mass of fuel is a functionof the time during which combustion takes place

,it is obvious that

a portion of the heat generated will be lost through radiation,

such loss increasing as combustion is slower .

The two important reactions of combustion

2H20

2= 2H

2O and 2CO+ O 2

: 2C02

are reversible and if such dissociation occurs it would have adecided effect on the temperature developed in combustion . Theamount of dissociation which takes place under the temperaturesdeveloped in boiler furnace practise is not definitely known butis probably inappreciable . For commercial combustion this factormay be considered as neglig ible .

From the factors involved it is evident that the better the combustion

,z'

. a,the more complete with the minimum of excess air

,the

higher the temperature developed,and it follows that the better

the combustion and the higher the temperature,again assuming

the ability of the boiler to efficiently absorb heat,the better the

efficiency . It is very difficult with the means available to determine accurately the actual temperature developed in commercialcombustion

,and hence to make use of such temperature as a

measure of the efficiency of combustion . Fortunately there areother methods by which such efficiency may be determined witha considerable degree of accuracy .

FLAME

The appearan ce of combustion,z’

. a,the “look of the mass of

fuel and of the products of combustion,offer to the experienced

eye a measure of the temperatures developed . While the useof such a method can lead only to the most approximate results

,

and at best serve simply as a check of more accurate determ inations

,it is perhaps worth while to consider it .

The physical evidence by which the temperature and thedegree and the extent of combustion in a boiler furnace maybe j udged

,is the appearance of the flame

,the fuel itself being

visible but rarely . Flame may be defined as a mass of intensely

39

heated gas in a state‘

of combustion,though it -is possible for

flame to exist as gas not actually in such state . The luminosityof flame

,or the characteristic which gives its visibility

,is due to

the heating to incandescence of the unconsumed part icles ofcombustible matter present in the gases

,and the variation in the

colors of flame is due to the difference in the degree of heat comm unica ted to these particles . The higher the temperature “

of

these particles the whiter -the flame . The length and volumeof the flame will vary with the com bustible elements present

,

and the “ thoroughness with which the air and combustible elements. are mingled

,and since such number will decrease with an

increase inthe completeness of combustion,the shorter the flame

,

in the absence of any outside cooling medium,the more rapid and

complete the combustion .

If it were possible for the combustion of a ny fuel to be com

plete and instantaneous there would be no visible flame,since

both carbon dioxide and water vapor are Invi sible . Visible flame,then, is evidence of incomplete or non-combustion

,but such

evidence in the commercial furnace means simply that the combustion has not taken place with sufficient rapidity to evolve heatinstantaneously .

It follows from the above that for a given amount of fuelburned

,a short flame will ordinarily mean rapid and complete

combustion,a longer flame delayed combustion

,and a very long

flame imperfect or non-combustion .

TABLE 7ATEMPERATURE AND APPEARANCE OF FLAME*

Tem peratureAppearance Of F lam e Deg rees Fahrenheit

Da rk Red

Dull RedDu ll Cherry RedFu ll Cherry Red

Clea r Cher ry Red

Deep O rang eWhiteB right WhiteDa z z ling White

Hos. W . Ha ys.

40

The temperature evolved in combustion may be approx imated“from the appearance of the fuel mass or the flame in accordancewith the preceding table . Such figures are of necessity but theroughest approxim ations

,but

,in connection with the flam e length

,

are of some value where apparatus for more accurate determinaextent degree of combustion 18 not available .

A IR A ND COMBUSTION

US far,in the abstract . consideration of combustion

,

the presence of sufficient oxygen for combination with~

oxidizable substances,and of a temperature sufficient to

bring about the chemical combinations of combustion,have

simply been assumed . As a matter of fact,given proper tem

perature conditions, it is the physical introduction of oxygen intothe presence of combustible substances in such manner as toassure complete oxidizat ion

,and at the sam e time to assure the

utilization of all or of ‘ the maximum proportion so supplied,

that is the most important and difficult problem in the burningof all fuels .

The source of supply of the oxygen necessary for combustionis

,as stated

,the air . From the proport ionate parts by weight

of oxygen and nitrogen as given,namely

,O

2=2 per cent

and N per cent,it is Obvious that to supply one pound

of oxygen for combustion it will be necessary to supply

1

pounds of air,and that in this weight of air there will be

pounds of nitrogen which serves no useful function incombustion .

We have seen in Table 2,the chemical combinations Occurring

in the union of oxygen with the combustible elements and compounds found in the fuels used for the commercial generation of

heat . From the manner of such combinations and dissociations,and a consideration of the atomic weights of the elements involved

,the proportionate part by weight of the elements entering

into the resulting compounds may be readily computed as well asthe weights of the products of combustion . With the amount ofoxygen required for combustion thus known the amount of airrequired will be indicated from the oxygen—nitrogen ratio existing in air .The methods of such computations are clearly indicated by

example,and since the relation of the products of combustion

to the combustible elements of the fuel is the most importantfactor in the determination of the efficiency of combustion , itappears advisable to illustrate such computations fully .

42

https://www.forgottenbooks.com/join

weight of oxygen to form eighteen parts by weight of water vapor .

Hence in one pound of water vapor we have

1 pound H2O 2

. 1 I 1 pounds H2+ .889 pounds O 2

Since the ratio of hydrogen to oxygen in water vapor is thus1 to 8 it will require 8 pounds of oxygen for the complete combustion of one pound of

'

hydrogen,which m eans

,as for the

combustion of carbon,

pounds of air required to burn one pound of hydrogen .

The nitrogen present in this weight of air will be

8 x pounds N2

and the products of combustion of one pound ofhydrogen will be

1 pound H2+ 8 pounds 0

22 9 pounds H2

O

8 x pounds N2

As typical of the combustible compounds consider ethylene

(the CH series are all computed in a similar manner)C2H

42 2C+4H

or from atomic weights

Thus one pound of ethylene is composed of

8 5 7 pounds C+ . I 43 pounds H

T o burn .8 5 7 pounds of carbon will require

8 5 7 x pounds 02

To burn . 143 pounds of hydrogen will require

143 x pounds O2

The total oxygen required then will be

22 86+ poundsand the total air

x 43 2 2 148 2 pounds

The products of combustion will beC02 H,OPounds Pounds Pounds

8 5 7 pounds C+ 2 .286 pounds 02

. 143 pounds H2+ pounds 0

2

pounds air x .768 5 (per cent N in air)

44

The methods of computation are simple but,as stated

,are

considered at length because of their importance,particularly in

the case of gaseous fuels . Table 8 gives the results of suchcomputations

,in terms of weight, for all of the combustible

elements and compounds encountered in commercial fuels .

Table 9 gives such values in terms of volume .

TABLE 8COMBUSTION DATA

I N TERM S O F PO U NDS PER PO U ND O F FU EL

Products of Com bustionPounds

Ca rbon (to CO Z )Ca rbon (to CO )Ca rbon MonoxideSu lphurHydrog enMethaneAcetyleneEthyleneEthane .

Hydrogen Su lphide

TABLE 9COMBUSTI O N DATA

I N TERM S OF CU B IC FEET P ER CU B IC . FO OT O F FU EL

Products of Com bustionCubic Feet

Ca rbon MonoxideHydrog en

AcetyleneEthyleneEthaneHydrogen Sulphide

Considered from a chemical standpoint,the supplying of j ust

theproper amount of oxygen or of air for perfect combustion, as

indicated by Table 8,appears Simple . I t is

,however

,the physical

diffi culty encountered in the introduction of j ust the properamount of oxygen that is the main source of the losses occurringin the burning of any fuel .I t may be well to distinguish here between perfect and com

plete combustion . Perfect combustion,as shown in Table 8

,is

the result of supplying the requisite amount of oxygen forunion with all of the oxidizable constituents of the fuel andutilizing in combustion all of the oxygen so supplied . Completecombustion on the other hand

,results from the oxidization of

all the combustible constituents of the fuel but does not ofnecessity imply the utilizat ion of all of the oxygen supplied .

If perfect combustion could be accomplished in a boiler furnacethere would be no unavoidable combustion losses . While combustion is complete but not perfect, there are, as will be shown,losses due to the supplying of too great an amount of oxygen

,

and hence air,and it follows that the more nearly complete

combustion can be made to approach perfect combustion,the less

the loss that will occur in the burning of any fuel . It is in factthis problem—the seeking after perfect combustion— that is theproblem of furnace design .

I t is obvious from the foregoing that the real measure of theefficiency of combustion is to be found in the relations existingbetween the amount of air theoretically required for the burningof any fuel and the amount of air actually supplied for suchcombustion and before considering the possible furnace lossesresulting either from incomplete combustion or from thesupplying of too great an amount of oxygen it is necessary tounderstand the method of determining these relations .

The calculations involved in the determination of the weightof air required for the perfect combustion of a pound of a givenfuel have been indicated in the computations of Table 8 . For

such determination an analysis of the fuel is necessary,this

analysis in the case of solid and liquid fuels being given in termsof weight

,and in the case of gaseous fuels either in terms of

weight or of volume . While the analysis of gaseous fuels areordinarily given in terms of volume

,it is perhaps best to trans

form such analysis to a weight basis,since the results are usually

desired in terms of weight .

46

With the data of Table 8 available,the development of

formulae to give directly the theoretical amount of air necessaryfor the perfect combustion of any fuel is simple . Such formulaeare given hereafter . There are

,however

,no suitable or reliable

means of measuring or weighing the air actually admitted to aboiler furnace

,and the only means of determining the amount

of such air is from the analysis of the products of combustionordinarily called flue gases . In making use of such analysiscertain assumptions , discussed hereafter, are necessary , butthese assumptions are such that the results obtained from theproper consideration of a properly made analysis are well withinthe error of a boiler test as a whole .

The apparatus used in the determination of the constituentsof flue gases and the methods of Operating such apparatus havebeen too often described to need discussion here . In the ordinary commercial analysis the proportionate parts by volume ofcarbon dioxide

,carbon monoxide and oxygen are determined

,

the difference between the sum of these constituents and 100

per cent being assumed as nitrogen .

Where combustion is complete,regardless of the amount of

excess air, the only products of combustion that can result fromthe burning of any fuel are CO

2 ,SO

2 (or 5 03 ) H20 and N

2.

The ordinary commercial analysis then is in reality simply am easure of the completeness of combustion of the carbon contentof a fuel . Properly used

,however

,such analysis may be made

to give combustion data from which furnace losses may be computed within the required limits of accuracy .

It seems proper to emphasize here the necessity,where

accurate results are desired,of considering flue gas analyses only

in conjunction with analyses of the fuel burned . As an exampleof the errors that may arise where the two analyses are notconsidered together we may take the tables of preventable losses“corresponding to varying percentages of carbon dioxide presentin the flue gases

,which are given in numerous publications .

Such tables give an arbitrary percentage Of carbon dioxide which,

if it could be obtained would represent no preventable furnace loss,

with increasing losses for lesser percentages of carbon dioxide .

For any fuel there will be a definite percentage of carbondioxide that must correspond to perfect combustion and hence

47

to zero preventable loss,but such percentage will vary not only

for different classes of fuels but even widely with different fuelsof the same cla ss . How wide this variation in carbon dioxidemay be for perfect combustion with different fuels is indicatedby the computations of combustion data given later, the rangein the examples of fuel taken being from per cent in thecase Of by-product coke oven ga s to per cent in the case ofblast furnace gas . From these figures it is obvious that CO

,

tables are not to be accepted as a measure of preventable furnaceloss

,regardless of the class of fuel burned

,and that for the

intelligent use of a flue g as analysis, analysis of the fuelburned is also essential .

COMBUSTION FORMULAE

A IR REQ U IRED FO R COMB U S T IO N

T H carbon,hydrogen

,and sulphur the only com

bustible elements found in the fuels used forcommercial steam generation

,it is

,as stated

,a simple

matter from the data of Table 8,to construct a formula for the

amount of air theoretically required for the complete combustionof a pound of any fuel . This may be expressed as follows :

Pounds air required per pound fuel 2

0C+ 34 . 56 (H S

where C,H

,O and S represent the percentages by weight of

carbon,hydrogen

,oxygen and sulphur .* As in the case of for

Omula the term (II—g )

assumes that all of the oxygen

constituent is free to uni te with the hydrogen to form watervapor

,such an assumption in the computation of the amount of

air required leading to a negligible error . This formula,reduced

to the Simpler form in which it is ordinarily used becomes

Pounds air required per pound fuel

C O S34 56 (3

+ IH— ‘

S‘ l g )

In the case of gaseous fuels,it would be necessary

,in order

to make use of formula to break the hydro-carbons intotheir constituent elements

,and it is simpler to make use of a

formula based directly upon the data of Table 8 . For this classof fuels the formula may be expressed as follows

Pounds air required per poundfuel

CO+ 34. 56 H 2+ CH4

—l C2H

2+ C2H

4+

C2H

6+ 6 . 10 H

2S O

2 (25 )

With gaseous fuels,where the analysis is commonly given on

a volumetric basis,it is sometimes desirable to express the amount

*Wh ile the constants a re thosedeterm ined in the ca lcu lations for Table 8 .

49

of air required in terms of cubic feet . On the basis of the data ofTable 9 , formula (2 becomes then

Cubic feet air required per cubic foot gas :

2 .39 (CO—l

’

CH4+ I C

2H

2+ C2H

4+ I 6 74

C2H

6O

2 (26)

PRODUCTS OF COMBUSTION

The data of Table 8,also makes it possible to determine

directly what the products of theoretically perfect combustionwill be .

Products of combustion per pound fuel

CO22 3 .667 C

O

S 022 2 . S

N22 8 .8 5 C+ 26 . 56 H2+ 3 32 S

—l—N 2

With the actual weights of the products of combustion thusknown

,they may be expressed in terms of percentage by weight

,

and if”desired these latter values mayy

adily be transformed intovalues giving percentages by volume .

As in the case of air required for combustion it is perhapsS impler to express the products of combustion of a gaseous fueldirectly in terms of the data Of Table 8 .

Products of combustion,one pound fuel

”

CO2= L 5 7 CO + 2 .7 5 CH4

+ 3 . 39 C2H

2+ 3 . I 4 C

2H

4+ 29 3

C2H

6+ CO

2

OH

202 9 (H CH

4+ 0 .69 C 2

H2+ C

2H

4+

C2H

6+ 0 . 5 3 H ,

S+H,O

SO2= L 88 H

QS

N2= I .89 CO + 26 . 56 H ,

+ 13 .28 CH4+ C

2H

2+ 12 38

C2H

4+ H

,S+N

2

50

https://www.forgottenbooks.com/join

in the fuel tim es the weight of dry products of combustion perpound of carbon must equal the total weight of dry products .

This, expressed as a formula,is,in terms of weights per pound

of fuel,

Weight of C burned x dry gas per pound C 2 total dry gas

Total dry ‘gas per pound fuelDry gas per pound C “

Weight C burned per pound fuela

The actual weight of carbon in the fuel must reappear in theflue gases in the same amount either as carbon dioxide or carbonmonoxide and (a ) may be written as

Total dry productsDry gas per pound C=

Weight C in flue gases

This relation must hold whether expressed in terms of actualweights or in terms of percentage by weights and (6) thus maybe written

Dry gas per pound C

r3r CO 2

+ 79; CO

where the symbols represent percentages by weight .Formula (5 ) may be transferred to volumetric form by multi

plyIng each term by its relative density (see page 19 ) and becomes

1 1 co,+ s O

,+ 7

(2 )3

7

the symbols representing the volumetric percentages of the constituent gases as given by a flue gas analysis .The principal assumption of formula (2 7) is that the analys is

as used is of dry gas .

All fuels in common use contain a greater or lesser amountof moisture . The loss due to such moisture is computed wherea heat balance is given but the weight of thi s moisture is sometimes overlooked in computing total gas weight . All air suppliedfor combustion also contains a certain amount of moisture, andthough this weight may be computed and the loss resultingtherefrom determined

,the weight is ordinarily inappreciable

and the loss commonly included with the unaccounted losses .

Dry gas per pound C

Aside from the moisture in the fuel and in the air suppliedfor combustion, which moisture will appear as water vapor in theflue gases

,there will also be an appreciable weight of water vapor

due to the burning of the hydrogen content of the fuel . Thisweight

,with perfect combustion

,may be as high as 15 per cent

of the total for certain gaseous fuels . (See by-product coke

fin gas .)We have then present in the flue gases but not measured in

the ordinary analysis,a considerable amount of moisture in the

form of water vapor . Water is commonly used as the displacement medium in the collection of the sample of gas for analysis

,

and further,during the analysis itself the gas sample comes into

contact with water . The effect of these various factors tendstoward a saturation of the gas being

'

analyz ed and from theresults obtained with all classes of fuel the assumption seemswarranted that such gases are actually saturated . Under theseconditions proportionate parts of the water vapor content of

the gas will be absorbed with the different constituents “of suchgas and the res

nyting analysis may be safely assumed to be

that of a dry gas . How nearly correct such an assumption ismay be seen from the various examples of the computations ofcombustion data which follow .

A further source of error in formula (27 ) is one resultingfrom the presence of sulphur innumerous fuels . Such sulphur

,

as shown in Table 2,ordinarily burns to 5 0

2 ,which will be

absorbed in the flue gas analjisis a s c arbon dioxide . With fuelslow in sulphur the error arising from this source is small and

/

can

be safely neglected . With fuels high in sulphur and low incarbon

,however

,as in the case of certain middle western coals

,

the error may be ' of sufficient amount to warrant consideration .

In an example given later for a coal containing S andper cent C . the error is shown to be as great a s 4

per cent .

It is entirely possible in determining the weight of dryproducts of combustion per pound of fuel from formula (27 ) tomodify the actual carbon weight as given by the ultimate analysisto correct for the sulphur content of the fuel

,and where accuracy

is desired,and the sulphur content is appreciable

,such a cor

rection Should be made .

53

The first term of formula viz . ( I 1

represents not only the weight of CO2resulting from the com

bustion of carbon,but includes as well the SO

2resulting from

the combination of sulphur . If the weight of CO2and SO

2result

ing from the combustion of one pound of carbon and one poundof sulphur

,respectively

,were the same

,the necessary correction

,

for the proper determination of the weight of dry products ofcombustion per pound of fuel from formula (27 ) could be madesimply adding the sulphur content to the carbon content of thefuel . The CO

2resulting from the combustion of one pound of

carbon, however, is, from Table 8, pounds,while the weight

of SO2from one pound of sulphur is pounds . The corrective

factor must be in the ratio of these weights,and the correct value

S

products of combustion per pound of fuel then instead of being

11 CO,+ 8 0

,+ 7 (N ,

+ CO )'

3 (C0 2+CO )

should be,where accuracy is desired

,

1 CO2+ 8 O

2+ 7 (N 2

+ CO)3 (CO 2

+CO)Formula (27 ) then, may be accepted as correct for any

fuel,for the computation of the data which it is presumed

to give, namely, the weight of dry gas per pound of carbon,

or by multiplying the weight so determined by the weight ofcarbon in the fuel properly corrected for the sulphur equivalent

,

the weight of dry gas per pound of fuel . It is not to be acceptedhowever

,without additional data in the way of fuel analysis

,in

the computation of total gas weights or in the computation of theamount of air supplied for combustion . The chief reason for thisstatement lies in the fact that practically all fuels contain acertain amount of hydrogen . The oxygen supplied for the combustion of this hydrogen does not appear in the dry flue gasesand is not accounted for by formula while the nitrogenwhich accompanied the oxygen so utilized does appear in the drygases and in the analysis . It is not always made clear why, inSpite of this fact

,formula (27 ) can be safely used for the com

putation Of the dry gas per pound of carbon or per pound of

instead of being (C-l S) will be (C+ The weight Of dry

54

fuel,and a word of explanation on this feature seems advisable .

The carbon content of the fuel must all appear in the dry gasesin the exact amount as in the fuel

,either as carbon dioxide

or as carbon monoxide . The basis of formula (27 ) is, as has beenshown

,simply the weight relation between a known quantity of

one constituent of the dry gases (carbon) and the total weight ofsuch gases

,regardless of the composition of such total weight

or the sources of its constituents, and with the weight and thepercentage weight of a single constituent known

,the total weight

is Obvious .

A IR SUPPLIED FOR COMBUSTION

A number of formulae based upon a volumetric flue gasanalysis have been offered for the computation Of the weight ofair supplied per pound of fuel burned . While certain of theseformulae give reasonably accurate results for specific classes offuels

,none is applicable to all fuels .

Unquestionably the best method of determining the weightof air supplied

,and in fact the only method that may be safely

used for all fuels,is through the use

,

of formula (27 ) or (27-5)giving the dry products of combustion per pound of carbon or offuel

,and . in conj unction with this formula

,certain data of perfect

combustion which may be obtained from Table 8 .

It is customary and proper to report a fuel analysis on a dryor moisture free basis. On such a basis, where

,

total gas weightsare desired

,the water vapor in

i

the flue gases resulting from thepresence of moisture in the fuel should be computed separately

,

and in the proposed method of determining the air supplied forcombustion

,neglecting the moisture content of such air

,the

results Obtained are in terms of dry fuel .Assuming complete combustion of the hydrogen present in

any fuel,the water vapor content of the flue gases from this

source must be a constant weight regardless of the amount of airsupplied for combustion . This weight may be readily determinedfrom the percentage of hydrogen in the fuel (total weight perpound) and the data of Table 8 . Obviously then, the total weightof the products of combustion per pound of dry fuel for anyamount of excess air must equal the dry products of combustionper pound as given by formula (27-6) plus the constant weight of

Less unconsum ed C in a sh.

5 5

the water vapor formed in the burning of the hydrogen content .Further

,the total weight Of .theproducts of combustion of the dry

fuel must equal the weight of air supplied plus the"

weight of thefuel which is burned

,and appears in the flue gases . Hence

,

Dry products per pound fuel+H-

2,O from H

2

Dry air supplied per pound (Weight fuel in g a ses)’le

Dry air supplied per pound2 Dry products per poundH

2O from H

2

— (Weight fuel in gases)

From the weight of air supplied as so determined,and the

weight theoretically required as computed from Table 8 or byformula the amount of excess air may be readily found

,as

may be the ratio of air supplied to that theoretically necessary,

which value,assuming complete combustion

,is in the last

'

analysis,

the true measure of the efficiency of combustion .

This method,a s stated, necessitates an analysis of the fuelas

well as of the flue gases . There is one of the formulae offered forthe direct computation of the amount of air supplied for com bustion

,based on a volumetric flue gas analysis alone

,which while it

is not applicable to all fuels,will give reasonably accurate results

for most solid and liquid fuels, and for this reason should bediscussed . This formula as ordinarily given is

N2

CO2CO

where the symbols represent the volumetric percentages of carbonmonoxide

,carbon dioxide and nitrogen .

This formula with the constant is derived as followsThe last term of formula (27 )

7 N ,

3 (CO 2+ CO)

must represent the weight of nitrogen supplied by the air,plus the

weight of nitrogen in the fuel itself . For the particular fuel

(coal containing one per cent N 2) and combustion conditions (20per cent excess air) from which the constant in formula

(28) was determined, the nitrogen content of the fuel was approximately fi

lm Of the total weight Of nitrogen in the dry products

of combustion .

*Ex . in the case of coa l ( I -A sh) .

Dry air supplied per pound C _ (28 )

Since the nitrogen is per cent by weight of the airsupplied for combustion

,the weight of air supplied per pound of

carbon for the conditions assumed would be thenN

27 (512—

535)

30 32 N 2

.768 5 x 3 (CO 2—j—CO ) (CO 2

—l—CO )Since the correction to the term 7 N 2

will vary not onlywith the nitrogen content of the fuel but also with the amountof excess

'

a ir supplied,and for this reason the formula must be

only approximate at best,it would perhaps be best to make no

attempt to correct for the nitrogen content of the fuel,in which

case the constant instead of being would becomeand the weight of air supplied per pound of carbon will be

7 N 2N

o

.768 5 x 3For the determination of the weight of dry a1r per pound of

fuel from this formula,where the sulphur content of the fuel is

low,this value may be obtained by m ultiplying

l

form ula (284 ) bythe pecentag e by weight of carbon in the fuel . With fuels highin sulphur a correction may be made to modify the carboncontent as in the case of the determination of the dry productsper pound of fuel

,though in view of the approximate nature of

the formula,thisrefinement is probably not warranted . If such

modification is desired,the cornective factor instead of being

based,as in the previous case

,upon the ratio of 5 0

2to CO 2,

should be based on the weight ratio of N2

. in the products ofcombustion of one pound of carbon and one pound of sulphurrespectively

,or from Table 8

, 3 . 32 to With such correc

tion the weight of dry a ir supplied per pound of fuel would be

(C

The error of this formula will depend as stated,not only

upon the nitrogen content of the fuel but also upon the amount ofexcess air supplied . While this error is practically negligible forsolid and liquid . fuels

, in gaseous fuels it is sufficiently large tomake the formula useless . The reason for this is clear if weconsider blast furnace gas

,where with ordinarily good comb

tion,the weight of nitrogen In the fuel itself may be alm ost

57

great as that in the air introduced for combustion . (See examplefollowing .)Numerous other formulae are offered for the determination

of the ratio of air to that required . Such formulae,however

,are

based on the relations of nitrogen and oxygen existing in theflue gases

,and are incorrect in that they do not take into con

sideration the fact that while,with most fuels

,practically all of the

nitrogen shown was introduced with the air supplied,this nitrogen

is composed of that which accompanied the oxygen used in thecombustion of carbon and appearing as carbon dioxide or carbonmonoxide

,and that which accompanied the oxygen used in the

combustion of hydrogen,this latter amount of oxygen not

appearing in the flue gas analysis . Hence the relation ofnitrogen and oxygen in the dry flue gases cannot be used as

indicative of similar relat ions existing in the air supplied .

This criticism does not apply to formula (28) since this is anexpression of carbon-nitrogen relations

,and does not involve

oxygen . The criticism of formula (28) as to nitrogen contentof the fuel is applicable to the air ratio formulae usuallyoffered . These air ratio formulae are ordinarily

"

so subj ectto error and are so narrowly applicable that they are notIncluded here .

The errors resulting from the proper use of flue gas analysisin the computation of combustion data are well within the errorof boiler testing as a whole . There is

,however

,a real source of

possible error in the making of the analyses,and in practise

there are several features that should be carefully watched whereaccuracy in the fuel results is desired . These are of suffi cientimportance to warrant discussion and

,assuming a proper design

of analysis apparatus,the errors to be guarded against may be

listed as followsFirst . Care should be taken that the sample of gas for

analysis is an average sample . This is the feature which shouldbe most carefully watched and is perhaps the most difficult ofachievement . No hard and fast rules can be la id down for themethods of Obtaining such average sample and it is largely aquestion of commonsense . The sample should be drawn fromthe main body of the gases and in a location where the possibilityof dilution through air infiltration is a minimum .

58

https://www.forgottenbooks.com/join

COMBUSTION LOSSES

ITH the methods of computing combustion dataavailable

,it is now possible to consider the losses

which occur in the burning of fuel under a steamboiler . Certain of such losses are not

,strictly speaking

,com

bustion losses,but “it is customary to consider all losses together .

The results of the computations of these losses constitute the“heat balance ” of a boiler test which indicates the distribution oflosses . Where a test is not accompanied by such a heat balance

,

or at least by sufficient data from which it may be computed,the

results should not ordinarily be accepted as absolutely reliable .

These losses,together with the methods of their computa

tions areFz

'

rsl . Loss due to fine m oisture conta ined in Inefnel.

All of the moisture . in the fuel must be heated from atmos

pheric temperature (or from the temperature of the fuel wherethis is above that at atmosphere) to 2 12 degrees

,the temperature

at which steam is formed,assuming atmospheric pressure

,and

the steam so formed must be heated to the temperature of thefurnace gases . Since in passing over the boiler heating surfacethe temperature will ultimately be reduced to that of the escapinggases

,the first and last temperatures are those that need be con

sidered.

The B . t . 11. loss from this source per pound of fuel may beexpressedPer cent Moisture x I 2 .48 (T (29 )

where z= tem pera ture of atmosphere or fuel,T = tem perature of escaping flue gases

,

9 70 .4= latent heat of evaporation at atmospheric pressure,

48=mean specific heat of superheated steam at atmos

pheric pressure . (In reality this value will varyslightly with different values of T

,but the varia

tion is small and .48 may be taken as representingthe value for ordinary exit gas temperatures .)

In the case of gaseous fuels introduced into the furnace themoisture content already exists as vapor . The temperature ofthis vapor is the same as that of the gas with which it is “mixed

,

60

but its partial pressure is below that corresponding to suchtemperature

,except where the gas is saturated

,a condition which

rarely occurs . Such water vapor then,existing at a temperature

above saturation,or above the temperature corresponding to its

partial pressure,is in reality superheated steam

,and in increasing

its temperature to that of the escaping gases the question ofthe expenditure of heat in changing its condition

,i . e .

,latent

heat expenditure,is not involved .

The loss due to the moisture content of gaseous fuels will beexpressed then

Per cent moisture .48 (T -t) (3 0 )

Where the gaseous fuel is introduced into the furnace ator near atmospheric temperatures the specific heat of the watervapor content will be considerably lower than The use ofthis value

,however

,as the mean specific heat over the range

t—T will lead to a negligible error only .

Second. Loss due to m oisz’nre form ed in Me ba nning of

From Table 8,each pound of hydrogen burnedw ill result in

the form ation of 9 pounds of water vapor . Thi s moisture mustbe heated as in the case of the moisture in the fuel and the loss ‘

may be expressed

Per cent H2x 9 12— t)

—l .48 (T (3 1 )

In the case of hydrogen,since water is an actual product of

combustion the latent heat must be taken into consideration,

regardless of the fact that the moisture appears in the productsof combustion as water vapor

,and whether the fuel is solid

,liquid

or gaseous .

Ynim ’. Loss due to m oi sture in Me a ir.

The weight Of water vapor per pound of dry air may be determ ined from readings of the wet and dry bulb thermometersand a set of psychrometric tables .

This weight times the weight of dry air supplied per poundof fuel

,as determined by the methods which have been indicated

,

will give the total moisture in the air supplied per pound of fuel

(W) . S ince this moisture is already in the form of water vapor,

as in the case of the moisture content of gaseous fuels,the question

61

of the expenditure of heat in changing its condition is not involvedand the loss from this source will be

W x .48 x (T —t) (3 2 )

Foa rz‘n. Loss due to dea l ca rried away in the dry c/zim ney

g a ses .

The weight of gas per pound of fuel burned (W) may becomputed by the methods indicated . In the case of solid fuelswhen the weight of dry gas per pound of carbon as given byformula (27 ) is multiplied by the carbon content of the fuel, theproper value of the carbon for use is the percentage of carbonactually burned and appearing in the flue gases, i . e .

,the carbon

content corrected for any unconsumed carbon in the ash andrefuse .

The heat lost in the dry Chimney gases then,

-is measuredby this weight of gas (W) and the difference between thetemperature of the escaping gases and that of the atmosphere .

I t may be expressedW (T—t) .24 (33 )

where .24 is taken as the mean specific heat of the gas betweenthese temperature limits . Since this specific heat will vary withthe temperature of the escaping gases and with their composition

,

it would be well to compute its value where the most accurateresults are desired . The value .24 though probably somewhatlow

,is,however

,ordinarily accepted .

P ifik. Loss due to Me incomplete com onstion of ca rbon .

This loss may be expressed

COC

CO 2 COx x 10 160

in which C is the weight of carbon which is burned and appears inthe flue gases

,i . e .

,corrected for solid fuels

,as in the case of the

proceeding loss,for such unconsumed carbon as appears in the

ash . The constant 10160 represents the number of heat unitsgenerated in burning one pound of carbon in ca roon m onox ide

to carbon dioxide . The term CO in which the symbolsCO 2 CO

’

(N . B .—T he loss due to m oisture in the a ir is frequently not com puted and

is included with the una ccounted losses. )

62

represent the volum etric percentages of the constituents asshown by analysis

,is an expression denoting the m a g/ct of the

carbon present in the carbon monoxide constituents,and perhaps

needs explanation .

From formula the weight of carbon monoxide in theflue gas is given by

7 CO

3 (C0 2 CO )

If this expresses the weight of carbon monoxide in terms of

volumetric percentages of the constituents, obviously the weightof carbon in the carbon monoxide must be é"of this amount or

CO

CO 2 CO

S ix tn. Loss due to ca roon appea ring in unconsum ed refuse

This loss may ordinarily be determined only in the case ofsolid fuels . It is expressed

c x C x 14600100

where c2 weight of ash per pound of fuel,C2

per cent of unconsumed combustible matter inthe ash

,

c 2 weight of unconsumed carbon in terms of totalcarbon per pound of fuel .

The unconsumed combustible inatter in the refuse is assumedto be entirely carbon for which 14600 B . t . u . per pound is takenas the approximate heat value . This assumption will give ri se toan error which is negligible .

Seventn. Radia tion a nd una ccounted losses .

These losses which are either impossible or impracticable tomeasure

,include

(a ) Radia tion loss, wh ich in term s of percentage will vary withthe siz e of the unit

,the condition of the setting and like

fa ctors.

(5 ) Losses due to unburned vola tile hydrocarbons, and

(c) Loss due to the com b ina tion of carbon and m oisture,with theconsequent form a tion of hydrog en (C H

20=CO 20H) ,

wh ich m ay or m ay not be burned . T h is a ction m ay occur

when m oist fuel is thrown on an incandescent fuel bed.

(d) O ther losses not a ccounted above.

63

The total of the losses under item seven is taken as the difference between 100 per cent and the boiler effi ciency plus the sumof the six losses as computed .

The actual computations of two typical heat balances are givenin examples hereafter .

Of the losses listed which can be computed the first,second

and third items are only to an extent controllable . Sincethe moisture content of all fuels and of air

,and the hydrogen

content of most fuels must be accepted as found,the only manner

is which these losses may be kept at a minimum for a givenfuel is by the reduction of the exit gas temperature to the lowestpossible or practicable figure . Assuming proper combustion

,

the exit gas temperature is a function of the heat absorbingability of the boiler

,and is thus rather a question of boiler design

than of combustion proper . If,on the other hand

,the effi cient

absorbing power of the boiler is assumed,these three losses are

controllable to the extent that exit gas temperatures are dependentupon combustion .

The fourth loss is more truly a combustion loss though sinceit is affected by exit gas temperatures this too is dependent onboiler design . Obviously with a given fuel

,and for a given exit

gas temperature,the greater the gas weight

,i . e .

,the greater the

excess air,the greater the loss of heat in the chimney gases .

This loss is kept at a minimum when complete com bustion ismade to approach perfect combustion .

The fifth loss is entirely a combustion loss and is to beprevented only by the admission of sufficient air for completecombustion and in a manner that such complete combustion isassured . In endeavoring to bring about such conditions the tendency is toward the introduction of too great an amount of air, inwhich case the carbon monoxide loss will be reduced or preventedat the expense of a loss resulting from the fourth source . It isto be remembered that while the absence Of carbon monoxide inthe flue gases indicates complete combustion

,it does not of neces

sity indicate efficient combustion .

The sixth loss,which can only be determined with solid fuels

,

is not properly speaking a combustion loss and is the result ofthe physical factors entering into the design of furnaces

,stokers

or grates,and in the operation of the apparatus . Assuming

64

the best of design this loss is minimiz ed through properOperation .

I t will be noted from the foregoing that the two main factorsupon which the extent of all combustion losses depend are theamount of air supplied for combustion and the temperature of

the gases leaving the boiler heating surfaces . T he factor of airsupply can

,within limits

,be controlled

,but if we assume the

ability of a boiler to absorb heat efli ciently, the factor Of exitgas temperature can only be controlled to the extent that it isdependent upon air supply . In View of this fact the effect ofair supply on exit g as temperature must be considered .

On first thought it would appear that s ince large quantitiesof excess air introduced into the furnace would reduce thetemperature of the products of combustion before the boilerheating surfaces are encountered

,such dilution would result in

lower exit gas temperatures and it is of course entirely possible tocarry this dilution to the products of combustion in the furnaceto a point where such a decrease in ultimate temperature wouldresult . In practise

,however

,even where the amounts of excess air

correspond to the most inefficient combustion,this excess instead

of decreasing,tends to increase the exit gas temperature .

The common explanation of this apparent phenomenon isthat the excess air in passing through and mingling with theactual products of combustion absorbs heat from such productsmore readily than will the boiler heating surfaces

,and a con

siderable port ion of the heat so absorbed is carried off in theescaping gases . Such a statement Offers by far the simplestexplanation

,and one which accounts for a part at least of the

increase of exit gas temperature with an increase of excess ai r .

The other factor l ea ding to such a result is dependent uponheat transfer rates

,difference in temperature between the gases

and the absorbing surface,the percentage of total heat absorbed

through radiation and the percentage of total absorption throughconvection . Any attempt to explain the high exit temperaturesaccompanying large amounts of excess air on such a basis lea dsto a complication of theories that are not within the scope of

the present article .

If we accept the foregoing as correct,it is obvious that the

stack loss due to excess air will increase with such excess,not

65

only because additional amounts of air must be heated fromatmospheric temperature to that of the escaping gases

,but also

because the ultimate temperature will, within ordinary limits, behigher as the amount of excess is increased

,the two factors thus

combining to increase the possible loss under item four,as listed

previously .

The effect of incomplete combustion in the furnace may beeither to reduce or increase exit gas temperatures .

If the combustion of a given fuel is not completed in thefurnace before the combustible gases come into contact with theboiler heating surfaces

,the temperature evolved in the furnace

,

and hence the temperature of the products of combustion,will

be less than if such combustion were complete . If such un

consumed or partially consumed gases pass from the boiler andup the stack without encountering somewhere in the settingsuffi cient additional oxygen for the completion of combustion

,or

temperatures under which combination resulting in furthercombustion will take place

,the result on the ultimate flue gas

temperature would be to reduce it below what it would be ifcombustion had been complete in the furnace . If

,on the Other

hand,these part ially consumed gases encounter at some point in

their passage over the boiler heating surface sufficient oxygenfor continued combustion with a temperature above the ignitionpoint

,such combustion will occur . In boiler practise this is

known as delayed or secondary combustion,and ordinarily will

take place at such a point within the boiler setting as toa ppreciably increase the temperature of the exit gases abovethat which would result from complete combustion in the furnace .

https://www.forgottenbooks.com/join

constituents,and it is the wholly or partially incomplete com

bustion of these constituents that causes smoke from all fuelssolid

,liquid or gaseous .If the volatile hydrocarbo‘ns are not consumed in the furnace

,

and there is no secondary combustion, there will of course be adirect loss resulting from the non-com bustion of these constituents. While Certain of these unconsumed gases may appearas visible smoke, the loss from this source cannot be measuredwith the ordinary flue gas analysis apparatus

,and must of

necessity be included with the unaccounted losses .

Where the combustion of the hydrocarbon constituents is incomplete a portion of the carbon component ordinarily appearsas soot particles in the smoke . In the burning of hydrocarbonsthe hydrogen constituent unites with oxygen before the carbonfor example

,in the case of ethylene (C 2

H4 )

G2H4+ 2 O 2 2 H

2O+ 2 C

If after the hydrogen is “satisfied ” there is sufficient Oxygenpresent with which that carbon component may uni te, andtemperature conditions are right

,such combination will take

place and combustion will be complete . If on the other handsufficient oxygen is not present

,or if the temperature is reduced

below the combining temperature of carbon and oxygen,regard

less oi the amount of oxygen present,the carbon will pass off

unconsumed as soot .The direct loss from unconsumed carbon passing off in thi s

manner is probably rarely in excess of one per cent of the totalfuel burned even in the case of the densest smoke . The lossdue to unconsumed or partially consumed volatile hydrocarbons

,

on the other hand,though not indicated by the appearance of

the gases issuing from a stack,may represent a very appreciable

percentage of the total fuel fired .

While the loss represented by the visible constituents ofsmoke leaving a chimney may ordinarily be considered negligible

,

there is a loss due to the presence of unconsumed carbon andtarry hydrocarbons in the products Of combustion which

,while

not a direct combustion loss, may result in a much greater lossin efficiency than that due to visible smoke . These constituentsadhere to the boiler heating surfaces

,and acting as an insulating

68

layer greatly reduce the heat absorbing ability of such surfaces .

From the foregoing it is evident that the stack losses indica ted by smoke

,whether visible or invisible

,result almost

entirely from improper com bustion . Assuming a furnace of

prOper design and fuel burning apparatus of the best, there willbe no obj ectionable smoke where there is good combustion . Onthe other hand a smokeless chimney is not necessarily indicativeof proper or even of good combustion . Large quantities of

excess air in diluting the products of combustion naturally tendtoward a smokeless stack

,but the possible combustion losses

corresponding such excess supply have been Shown .

G ENERAL CONCLUS IONS

N View of the great number of factors involved in the combust ion of any fuel

,and the great variation in the charac

teristics not only of different classes of fuel, but of differentfuels of the same class

,it is

'

obvious that the specific requirements for the proper combustion of an individual fuel must beconsidered as a distinct problem . It is possible

,however

,from

the foregoing,to draw certain general conclusions as to the com

bustion requirements of a ny fuel, whether solid, liquid or gaseous,and since such conclusions form the basis of the design of allcombustion apparatus

,they are' worthy of careful note .

These general requirements of proper combustion m ay besummarized as followsFirst . The adm issiOn of an air supply such as will assure

sufficient oxygen for complete combustion .

Second . Since complete combustion is not of necessityefficient combustion

,it must be secured wi thout permitting the

dilution of the products of combustion with excess air . Itfollows then

,that

Third . The air supply should be admitted at the prOpertime and in such a manner that the oxygen of the air comes intofree and intimate contact with the combustible substances ofthe fuel . In the case of solid fuels this means not only intocontact with the solid particles of the oxidizable substances

,

but also with the combustible gases as they are distilled fromthe fuel .Fourth . The gases must be maintained at a temperature at

or above their ignition point until combustion is complete .

Theore tically,as has been indicated

,the most efficient com

bustion is that resulting in the maximum temperature possible .

In practice,there are frequently factors which

,from the stand

point of Operating commercial efli ciency, makes it advisable tokeep furnace temperatures somewhat below those which couldbe obtained were this the sole factor involved .

Fifth . An additional requirement wh ich has to do with thephysical rather than the chemical aspect of combustion is thatproper provision must be made for the expansion of gases duringthe period of their combustion .

70

In considering combustion it is necessary,though perhaps

difficult for the average boiler user,to distinguish between the

purely chemical changes that accompany oxidization and thepurely physical aspect of the later transformation of heat energyin the passage of the products of combustion through the boiler

,

i . e .

,the absorption of heat by the boiler from such gases . The

efficiency of combustion is thus independent of the ability of theboiler under which combustion takes place to absorb heat

,and

in the requirements of proper combustion j ust summarized suchability is either assumed or neglected .

From the general conclusions drawn it would seem perhapsa Simple matter to meet the requirements of proper combustion .

Unfortunately,however

,such is not the case and it is

,as stated

heretofore,the physical and mechanical details encountered in

attempting to fulfill such requ irements that render the problemof proper combustion difficult . Assuming proper furnace formand adequate combustion temperatures

,the problem is solely

one of air admission and admixture . T he factors entering intothe problem and the methods used to bring about the desiredresults are so widelyvaried for different fuels, that it is necessary

,as stated

,to consider each class of fuel specifically for any

but the m ost general statements .

7 1

T HE COMPUTATION O F COMBUSTION

DATA

HE methods of computing combustion data as discussedin the foregoing

,and the very widely .

differing data resulting from the combustion of different classes of fuel

,i . e.

,

the wide variation in possible or probable flue gas analyses,products

of combustion and air supplied per pound of fuel for differentcombustion conditions are

,to the writer’s mind

,best illustrated

by example ,For this reason typical examples of the different classes of

fuel used for the commercial production of heat under steamboilers are considered in the following . Except in the case ofcoal where the analyses vary over a wide range

,the analyses

of the fuels taker-i are sufficiently near an average to allow theresults to be plotted in such manner that for a given flue gasanalysis (i . e .

,per cent CO 2 ) , the weight of the products of com

bustion and the amount of excess air corresponding to suchanalysis

,m ay be determined directly for the specific class of fuel

considered with a degree of accuracy sufficient for approximatework . Such graphic representations are therefore included .

Given a coal having ultimate

Moisture per cent

B . t . u .

,per pound 143 5 1

7 2

With perfect combustion the oxygen and air required,and

the products of combustion per pound of coal will be as follows :

Weight Required—Poundsper PoundCoa lPounds

C .7986

H2

.0502 .402

O 2N

2.0186

S .0 118 .012 .05 1

A sh .O 7SI

2. 544.043 u 186*

.000

SO2a s CO Z

.000

*A ir and N2equiva lents of O

2incoa l.

The weight of a ir theoretically required for the combustionof one pound of coal is then pounds . For each 20 percent in excess of this amount (i . e .

,each pounds above

there will appear in the products of combustion

x .2 3 15 2 . 500 pounds O 2

x 7 68 5 2 16 60 pounds N2

and the weights of the products of combustion per pound of coalfor varying amounts of excess air will be :

TABLE A

WeightProductsPerfectCom bustion 20 PerCent 40 P er Cent 60 P erCent 80 PerCent

Expressed in terms of percentage weight,these values are

TABLE B

Expressed in terms of percentage weight of dry products ofcombustion these values are

TABLE C

20 PerCent 40 Per Cent 60 Per Cent 80 Per Cent

If we convert these percentages by weight of the dryproducts of combustion into terms of percentage by volume afterthe method given on page 19 , the values as given in Table Cbecome

TABLE D

20 Per Cent 40 Per Cent 60 P er Cent 80 P er Cent

I I I I 000 00 I

For the purpose of comparing the results as computed asabove with those obtained through the use of the combustion

74

https://www.forgottenbooks.com/join

The weight of dry air supplied per pound of coal,using the

actual carbon weight,will be

x pounds

while the weight using the carbon weight corrected for thesulphur equivalent (in this Case the ratio of the nitrogen in theair supplied for the combustion of carbon and sulphur to CO

2and

SO2respectively) will be

x pounds

The actual weight of air Supplied per pound of dry fuel willbe the total products of combustion per pound of dry coal

,less

the weight per pound which is burned and appears in suchproducts

,or

,from Table A and the weight of ash as given by

the analysisI pounds

For this particular coal then,the errors above

,using the uncor

rected and the corrected values Of carbon as applied to formulaare per cent and per cent respectively .

Formulae (28, 28a and 288 ) will, as stated, give results withinreasonably accurate limits

,with fuels having a low nitrogen con

tent,the error varying with the percentage O i nitrogen and with

the amount of air used for combustion . With fuels of highnitrogen content

,the error may be as great as 80—90 per cent

(see Blast Furnace Gas) and for such fuels these formulae arenot to be relied upon .

The heat of combustion per pound of dry coal computedfrom formula (4) will be

.0427

8

1449 2 B . t . u .

14600 x 7 9 86+ 62000 ( 0502 ) + 40 5O x 0 118

as against the calorimetrically determined value 143 5 1 B . t . u .

,

an error of per cent .To indicate the amount of possible error with high sulphur

fuels,in the determination of the dry products of combustion

per pound of fuel from formula (27) where the carbon content isnot corrected for the sulphur equivalent

,let us assume a coal

having the analysis given below . The weight of oxygen and air

76

theoretically required,and the weight of the products of perfect

combustion per pound of dry coal will be as follows :

Weight Required—Pounds Products of Com bustion—Pounds per Pound Coa lper PoundCoa lPounds

.6125

.0448 .3 584 .4032

.1062

.0100

S .0442 .0442

A sh . 1823

02in Coa l .1062 .4588

.0000 .4032

.0884

.0000

The air required per pound of dry coal for perfect combustionis thus pounds . If we assume the coal to be burned with20 per cent excess air, there will appear in the products of combustion,

in addition to the weights j ust given,

x .2 3 15 2 . 3860 pounds O 2

x .768 5 2 12 8 13 pounds N 2

With 20 per cent excess air then,the weight of the pro

ducts of combustion per pound of dry coal,these weights

expressed in terms of percentage weight,expressed in terms

of percentage weight of dry products, and expressed in terms ofpercentage volume of dry products

,are as follows

77

Under the assumed conditions,the weight of drygas per pound

of carbon will be from formula (27 )

1 1 x I 5 .6O 3+ S x x 80 .8 50

3 x_

I 6-363 5 POunds

Multiplying this value by .6 12 5 , the weight of carbon per poundOf

'

dry coal, we have as the dry products per pound

x pounds

as against the computed value above

13 2 4 032 2 8 pounds

an error Of approximately per cent . If,on the other hand

,

the weight of carbon is corrected for the sulphur equivalent,we

have

x pounds

which value checks with the computed weight .

WO OD

Given a wood (pine) having the following ultimate analysis

Moisture per cent

Heat value per pound dry wood,B . t . u . 9 15 3

78

With perfect combustion the Oxygen and air required perpound of dry wood and the products of combustion per poundwill be as follows :

*A ir and N 2 equiva lents of O z in wood.

The weight of air theoretically required for the combustionone pound of dry wood is thus pounds . For each 20

per cent in excess of this weight (i . e .

,each pounds of air

a bove there will appear in the products of combustion .

x 2 3 15 2 2 8 14 pounds 0 2

x .768 5 pounds N 2

a nd the weight of the products of combustion per pound of dry“wood for varying amounts of excess air will be

TABLE A

Weight Weight Products—Va rying Am ounts Excess Air—PoundsProductsPerfect

Com bustion 20 Per Cent 40 P er Cent 60 P er Cent 80 Per Cent 100 Per Cent

I I -933

79

Expressed in terms of percentage weight,these values are

T A BLE 'B

Expressed in terms of percentage weight of dry products ofcombustion these values are :

TABLE C

Per Cent Weight Dry Products—Va rying Am ounts Excess Air

20 Per Cent 40 P er Cent 60 Per Cent 80 P er Cent 100 P er~Cent

I I00.000 I

Converting the percentages by weight into terms of percentage

,

by volume of dry products of combustion,the values of

Table C become :

TABLE D

20 Per Cent 40 P er Cent 60 P er Cent 80 Per Cent

I 00.000

80

In order to compare these results with those Obtained throughthe use of the combustion formulae

,assume that the wood is

burned with 60 per cent excess air and that the flue gas analysisshows per cent CO per cent 0

2,and per

cent N2

The dry gas per pound of carbon from formula (27) will be

1 1 x x x

3 x 12 . 5 14

Since the wood contains no sulphur,no correction for this

constituent is necessary to the carbon weight,and the weight of

dry products of combustion per pound of dry wood is

pounds

x . 503 12 pounds

which checks with the value of Table A,viz

pounds

Since for each pound of dry wood burned there are 46 10pounds of contained moisture

,and S ince from the hydrogen con

tent there will appear in the flue gase’s 5 5 8 pounds of watervapor, the total weight of products per pound of wood will be

2 1 78 pounds

The weight of dry air supplied per pound of ca rbon fromformula (28) 15

x

and the dry air supplied per pound of wood

x 5 03 12 9 7 18 pounds

Since the nitrogen content of the wood is so small as not toappear in the computations of the products of combustion, thisvalue will check with the weight of air determined from Table Aand the ash weight

,viz .

"

17 72 1 pounds

or with the value from the theoretical amount of air required and60 per cent excess

,viz

x pounds

If we accept the analysis taken as typical of all woods,the

approximate weights of the products of combustion per pound

81

C I“.

28 26 4 22 20 18 16

Products of Com bustion per Pound of O il—PoundsFIGURE 2

O I L . CO Z—Products per Pound O il . CO Z—Per Cent of Excess Air

https://www.forgottenbooks.com/join

pounds above there will appear in the products ofcombustion

x .2 3 15 2 .649 6 pounds of 02

x pounds of N2

and for varying amounts of excess air the weights of the productsof combustion per pound of Oil will be

TABLE A

Expressed in terms of percentage weight,these values are

TABLE B

Expressed in terms of percentage weight of dry products ofcombustion

,these values are :

Per CentWeight Dry Products—Varying Am ounts Excess Air

20 Per Cent 40 P er Cent 6o Per Cent 80 Per Cent 100 Per Cent

I I

84

Converting these percentages by weight of the dry productsof combustion into terms of percentage by volume, the values ofTable C become

TABLE D

20 PerCent 40 Per Cent 60 Per Cent 80 PerCent

100 000

Assume,for the purpose of comparing the data thus com

puted with the results Obtained from the use Of the combustionformulae

,that the oil is burned with 20 per cent excess air and

that the flue gas analysis shows per cent COper cent 0

2,and per cent N

2.

The weight of dry products of combustion per pound of

carbon is,from formula (27 )

11 x 8 x 7 x

3 x

Multiplying this value by the weight of Carbon per pound ofoil we have as the weight of dry gas per pound of oil.

x pounds

as against the value from Table A

— I . I 43 2 I 6 .69 5 pounds

If the carbon weight is corrected for the sulphur equivalent,

the two values may be made to check exactly,and we have as

the weight of dry gas per pound of Oil

x pounds

The total gas weight per pound of oil will be the dry gasweight plus the weight of moisture resulting from the burningof . 127 pounds of hydrogen, or

x pounds

85

Per Cent Excess Air —Per Cent20 40 60 80

13 12 I I 10 9 8 7P roducts Of Com busti on per Pound Dry Wood —Pounds

FIGURE 3WOOD. CO

z—Products per Pound Dry Wood. CO Per Cent of Excess Air

86

and since all of the fuel will appear in the products of combustion,

the weight of air supplied per pound of oil will be

12 pounds

This value checks with the computed value of the theoreticalrequirement plus 20 per cent excess

,or

x pounds

The weight of air supplied per pound of carbon from formula

(28 ) isx pounds

and the weight per pound of Oil,using the corrected carbon weight

,

x pounds

the slight difference between this value and the actual weightbeing due to the nitrogen content of the Oil .If the analysis of oil taken be accepted as typical for this

class of fuel,the weight of the products of combustion per pound

of Oil for different percentages of COQ ,and the per cent of excess

air corresponding to such CO2may be determined directly from

Figure 3 .

NA T U RA L G A S

Given a natural gas (Ohio) having an analysis by volumeas follows

P er Cent

Carbon Monoxide

Hydrogen

Methane

Ethylene

Hydrogen Sulphide

Oxygen

Carbon Dioxide

N itrogen

I

Converting this analysis by volume to one by weight we haveVolum e per Weight per Weight PerCentbyCubic Foot Cubic Foot Pounds Weight

CO .0045 x .07806 .0003 5 1

H2 .0 182 x .00562 000 102

CH,

.9 3 3 3 x .04500 .041999 2

CQH,.002 5 x .07808 .00019 5

H , S .00 18 x .096002

.00017 3

0 2003 5 x .0892 1 0003 12 0 677

C0 2.0022 x . 12341 .000272

N 2.0340 x .07807 .002654

.04605 8

The weight of the gas , is thus, under standard conditions,046058 pounds per cubic foot .With perfect combustion the oxygen and air required per

pound of gas, and the products of combustion per pound willbe as follows :

Products of Com bustion—Pounds Per Pound G as

.0068

02in G as .0294

*.0068 .0226*

.0000

SO Z as C02

.0000 1 .0000

*Air and N , equiva lents of 0 , present in g as.

The weight of air theoretically required for the combustionof one pound of gas is thus pounds . For each 20 per

88

cent in excess of this weight (i . e .,each pounds) , there

will appear in the products of combustionx .2 3 15 2

.7 36 56 pounds O 2

x .768 5 2 pounds N,

and for varying amounts of excess air the weights of theproducts of combustion per pound of gas will be

Expressed in terms of percentage weight these values are

T A BLE IB

20 P er Cent 40 Per Cent 60 Per Cent 80 Per Cent

Expressed in terms of percentage weight of dry products ofcombustion these values are

TABLE C

Convert ing these percentages by weight of the dry productsof combustion into terms of percentage by volume

,the values of

Table C becomesTABLE D

Per Cent Volum e Dry Products—Varying Am ounts Excess Air

20 Per Cent 40 Per Cent 60 P er Cent 80 Per Cent 100 Per Cent

100 000

For the purpose of comparison between the results so com

puted with those obtained from the combustion formulae, assumethat the ga s is burned with 40 per cent excess air and the flue

ga s analysis shows per cent CO, ,

per cent O, ,

and

per cent N,

.

The dry gas per pound of carbon from formula (27) will be

1 I x x x

3 . x

The weight of carbon per pound of gas will be

pounds

From CO .00762 x é;From CH

,.9 1188 x

From C,H

4.0042 3 x $5.

From CO,

.00 59 1 x 1312 0 0 16 12

Total carbon .69 2414 pounds

Multiplying the weight of dry gas per pound of carbon by thisvalue

,we have as the dry gas per pound of gas burned

x pounds

while the value from Table A for 40 per cent excess air is

pounds

While the difference in these values is negligible, theymay be made to check still more closely if the carbon weightis corrected for the sulphur equivalent . The weight of sulphurper pound of g a s is

00376 x pounds

https://www.forgottenbooks.com/join

Per Cent Excess A ir—Per Cent20 40 60 80

30 28 26 24 22 20 18

P roducts of Com bustion per Pound of G as -P0unds

FIGU RE 4N A T U RAL G A S . CO

z—Productsl

per Pound G asCo

z—Per Cent Excess Air

9 2

Since,as was shown

,the gas under standard conditions

weighs .0460 5 8 pounds per cubic foot, the heat value per cubicfoot will be

“

22037 x B . t . u .

To illustrate the methods of computation where volumetricresults are desired

,assume the same natural gas analysis as given

above . The volumes of oxygen and air required for combustionand the volumetric products per cubic foot of gas will be asfollows

Products of Com bustion—Cubic FeetRequiredCubic Foot per Cubic Foot G a s

.0045 .0085.0344 .0182

5

.0284 .0050

.0102 .0018 .0018

.9449 .0035 .0018

.003 50 .0 1674*

.003 5 .0132'

ale

.9449 .0000

.0018

.9467 .0000

*A ir and N2equiva lents of 0

2present in g as.

N . B .—I t is of interest to note tha t because of the volum etric relations of CO

,0

2and

C02,H

2 ,0

2andH

20

,andH

25,02,H

20 and 5 0

2 ,the tota l volum e of products is not

equa l to the volum e of the g as plus the volum e of the a ir supplied .

One cubic foot of gas will thus require cubic feet ofair for perfect combustion . If we assume

,as in the computations

on a weight basis, that the gas is burned with 40 per cent excessair

,there will appear in the products of combustion in addition to

the volumes given above

x .40 x .209 1= .7 5 38 cubic feet 0 2

x .40 x .7909 cubic feet N2

93

For 40 per cent excess air then, the volumes of the productsof combustion

,these volumes expressed in terms of percentage

volume,and expressed in terms of percentage volum e of dry

products Will be

Erasing"P”

9467

7 5 38

1 1

The dry gas analysis asthus computed on a direct volumetricbasis may be considered to check the analysis computed on

the basis of weight,the maximum difference being per cent .

T he slight d ifference is due to'

i

the fact t hat the weights ofoxygen required per pound of the various combustible Substancesas given in Table 8 do not exactly check with the corresponding volumes '

of oxygen required as given in Table 9 . Thevariation between these Sets of corresponding values resultsfrom the use of the approximate instead of the accurate atomicand molecular weights in the computation of the proportionateparts byWeight of the constituents of the combustible substancesin Table

.

8 ' Any error arising from this source may be neglected .

The heat value per cubic foot of this natural gas may becomputed from the analysis by volume and Table 6 as follows '

Volum e per B . t . 11. perI Cubic Foot Cubic Foot I B t ' 11.

CO .0045 xH

2.0 182 x

CH .9 33 3 xC2H

4.002 5 x

H28 .00 18 x

B . t . u . per cubic foot

This value checks with the value already com puted from theanalysis by weight .If we accept the analysis taken as typical of natural gas, the

approximate weights of the products of combustion per pound ofgas burned

,and the percentage of excess air, corresponding to

different percentages of COQ ,for this class of fuel, may be

determ ined directly from Figure 4 .

94

BY-PRO DU CT CO K E OVEN

Given a by-product coke oven gas havingvolume as follows :

Carbon Dioxide

Carbon Monoxide

Methane

Hydrogen

Nitrogen

I

Converting the analysis by volume to one by weight,we have

Volum e per Weight per Weight P er CentCubic Foot Cubic Foot Pounds Weight.007 5 x . 12 341 0009 3

.0600 x .07806 00468 1

.28 15 x .04500 0 1267

5 300 x .00 562 .0029 8

. 12 10 x .07807. 00945

.0307 1 1

The weight of the gas is thus,under standard conditions

,

.0307 1 pounds per cubic foot .

With perfect combustion,the oxygen and air required per

pound of gas,and the products of combustion per pound will be

as follows

Required per Pound G asPounds

1.0000

The weight of . air theoretically required for the combustionof one pound of gas is thus pounds . For each 20 per

9 5

cent in excess of this amount (i . e .

,each pounds above

59) there will appear in the products of combustionx .23 15 : . 503 pounds 0 2

x pounds N2

and for varying amounts of excess air the weights of the productsof combustion per pound of gas will be :

TABLE A

Expressed in terms of percentage weight these values areTABLE B

Expressed in terms of percentage weight of dry products ofcombustion these values are :

TABLE C

20 Per Cent 40 Per Cent 60 Per Cent 80 Per Cent 100 PerCent

96

Converting these percentages by weight of the dry productsof combustion into terms of percentage by volume, the values ofTable C become

TABLE D

Com bustion 20 Per Cent 40 Per Cent 60 PerCent 80 Per Cent 100 Per Cent

For the purpose of comparing the results so computed withthose obtained from the combustion formulae

,assume that the

gas is burned with 40 per cent excess air and that the flue gasanalysis shows per cent CO per cent O and

per cent N2

.

The weight of dry gas per pound of carbon from formula

(27 ) will be

The weight of carbon per pound of gas burned is

From C02

.0303 x fr .0083

From CO . 1524 x .065 3

From CH4

.4126 x g . 3094

Total carbon . 38 30 pounds

and the weight of dry products per pound of gas

x . 3830 pounds

The weight of hydrogen per pound of gas burned is

From H2

From CH4

.4126 xTotal H

2.2002 pounds

and the weight of water vapor formed in the burning of thishydrogen will be

2002 x pounds

97

Per Cent Excess A ir—Per Cent20 40 60 80

22 20 18 16 14 12

Products of Co m bustion per Pound G a s —Pounds

FIGU RE 5BY-PRODU CT COK E OVEN G A S. Co z

—P roducts per Pound G asCO Z—Per Cent Excess Air

98

https://www.forgottenbooks.com/join

BLA S T FU RNA CE G AS

Given a blast furnace gas having an analysis by volume,

a s followsP er Cent

Carbon Dioxide 12 . 50

Carbon MonoxideHydrogenNitrogen 5

Converting the analysis by volume to one by weight,we have

Kitchiopii fi

e

tflfit. 12 50 x . 12 341 .0 1543

2 540 x .07806 .019 8 3

03 50 x .00562 00020

5 860 x .07807 .045 7 5

.08 12 1 1

The weight of the gas is thus,under standard conditions

,

08 12 1 pounds per cubic foot .With perfect combustion the oxygen and air required per

poundof gas,and the products of combustion per pound

,will be

as follows

The weight of air theoretically required for the combustionof one pound of gas is thus .68 5 7 pounds . For each 20 per centin excess of this amount ( z

'

. e .,each . 137 14 pounds above .68 5 7 )

there will appear in the products of combustion

. 137 14 x 2 3 15 2 0 3 17 5 pounds 0 2

. l 37 i 4 x pounds N2

*Whi le blast furnace g as conta ins a considerable a m ount of m oisture,va rying with the

wa ter in the cha rge and the am ount used for dam pening , it is custom a ry to g ive the ana lysison a dry ba sis, reporting the m oisture sepa rately in term s of g ra ins per cubic foot of g as .

T he m oisture content is ordina rily about 30 or 3 5 g ra ins per cubic foot .

and for varying am ounts of excess air the weights of the productsof combustion per pound of gas will be

TABLE A

WeightProduc tsPerfect

Com bustion 20 Per Cent 40 Per Cent 60 Per Cent 80 Per Cent

Expressed in terms of percentage weight,these values are

TABLE B

P er CentWeightProductsPerfectCom bustion 20 P er Cent 40 P er Cent 60 Per Cent 80 P er Cent

I00 .000 I I I I

Expressed in terms of percentage weight of dry products ofcombustion

,these values are :

TABLE C

Per C ent Excess Air—Per Cent20 40 60 80

L 9

P roducts of Com bustion per Pound Dry G as—PoundsFIGU RE 6

BLAST FU RNACE G A S. Co .,—Products per;P0und G as

COz—Per Cent Excess Air

102

https://www.forgottenbooks.com/join

Since all of the gas appears in the products of combustion,

the weight of air supplied per pound of dry gas burned must be

poundswhich checks with the value computed from the weight of a irtheoretically required and 40per cent excess air viz . :

.68 5 7 x pounds

Blast furnace gas offers the best example of the unsuitab ilityof formula (28) for application in the case of all fuels

,for not

only is the nitrogen content high (over 50 pe r cent) , but it islarge in ' proportion to the total nitrogen in the products of

combustion, even with great amounts of excess air .

If,for the present example

,we apply this formula

,the weigh t

of air supplied per pound of carbon will be

and the weight of air supplied per pound of gas

x pounds

As compared with the correct weight pounds) formula (28)results in an error of per cent

,and the error would be still

greater were the gas burned with less than 40 per cent excess air .

The heat value per pound of the blast furnace gas,from the

analysis by weight and Table 6,is

CO .24418 x 4380 2 1069 5

H,

.00246 x 620002

B . t . u .

Since the gas,under standard conditions

,weighs as shown

,

08 12 1 pounds per cubic foot,the heat value per cubic foot is

08 12 1 x 1222 2 99 2 B . t . 11.

Or,the heat value per cubic foot

,from the volumetric analysis

and Table 6,is CO .2 54 x 342 2 869

H .03 5 x 348 2 12 2

B . t . u .

2

If we accept the analysis of blast furnace gas taken as typicalof this fuel as a class

,the approximate weights of the products of

combustion per pound of dry gas burned,and the percentages

of excess air,corresponding to various percentages of carbon

dioxide,may be determined directly from Figure 6 .

104

HEA T BA LANCE

in the case of the combustion data just discussed,the

computations involved in the determination of the distribution of losses in a boiler test

,z'

. e.

,the “heat balance

,

”

are best illustrated by example .

SO L ID O R LIQ U ID FU ELS

Where the Weight of fuel burned can be actually weighed,

(3 . g ,coal

,oil

,orwood) or accurately measured (e. g ,

natural gas) ,the computations are direct . The radiation loss and the smalllosses which cannot be computed from ordinary test data are

,as

stated,grouped

,and are taken as the difference between 100 per

cent and the sum of the known and distributable losses .

As an example of this class of heat balance,let us consider

one of the tests * at the plant of the Detroit Edison Company,in which the test data and calculated results necessary for thecomputation of the heat balance were as follows

Wet Bulb Thermometer,Degrees Fahrenheit

Temperature Boiler Room,Degrees Fahrenheit

T emperature Exit Gases,Degrees Fahrenheit

C,Per Cent

H Per CentO Per CentN Per CentS,Per Cent

1

A sh,Per Cent/

Moisture ln Coal,Per Cent

B . t . 11. per Pound Dry CoalA sh and Refuse (Per Cent Dry Coal)Unconsumed Carbon in Ash, Per Cent

CO Per CentFlue Gas O Per CentAnalysis CO

,Per Cent

N, ,Per Cent

Evaporation from and at 2 12 Degrees PoundDry Coal

,Pounds

elf“Tests of La rge Boi lers a t the Detroit Edison Com pany.”—D. S ."a cobus, Trans.

A . S . M . E .

‘ —Volum e 33 .

105

The heat absorbed by the boiler per pound of dry coal is

x B . t . u .

and the efficiency of the boiler

1079 1 per cent

Loss Due to M oisture e

’

u Coa l

The moisture in the coal,

per cent,becomes in terms

dry coalper cent

and the loss due to this moisture content is

.0 19 5—7 3) B . t . u .

2 5 140002 0 18 per cent

Loss Due 50 Me B uru z’

ug of Hydrog eu

This loss per pound of dry coal is

0 5 56 x 9 ( 5 7 5 B . t . u .

642 14000 2 4. 5 8 per cent

L oss Due 10 Hea t e’

u Dry Cfiz’

m uey G a s

The weight of dry gas per pound of carbon from formula (27) is

11 x r4.oo+ 8 x —l3

The weight of carbon per pound of dry fuel is 7842 pounds .

Certain of this carbon,however

,is not burned

,as evidenced by

the unconsumed carbon in the ash . Expressed in terms of totalcarbon

,the unburned weight is

1 pounds

.0703 x .3 15 2 22 1 per cent

and the weight of carbon burned per pound of dry coal,and

passing off with the chimney gases is

7842 1 1 pounds

106

https://www.forgottenbooks.com/join

L oss Due to Ga roou iu tfi e A sk

This loss from formula (3 5 ) is

0 703 x6 0 2

100

x 14 0 32 3 3 u

32 3 3 + 140002 2 3 1 per cent

Radia tion a ua’U u a eeouu tea

’L osses

The radiat ion and unaccounted losses will be

14000 16 16+ 39+ 227+ B . t . u .

or3 37 14000 2 per cent

The complete heat balance is thenB . t . 11. Per Cent

Heat absorbed by boiler 1079 1

Loss due to moisture in coal 2 5

Loss due to moisture formed in burning H,

642

Loss in dry chimney gases 16 16 1 1. 54

Loss due to moisture in air 39

Loss due to incomplete combustion of C 227

Loss due to unconsumed C in ash 32 3

Radiation and unaccounted losses 3 37

I 4000 I

I t is of interest to note that the radiation and unaccountedlosses for the test considered are as low as per cent .Generally speaking

,these losses are one of the best indications

of the accuracy of a boiler test,and where a heat balance shows

an excessive unaccounted loss it is well to scrutin ize the testdata most carefully before accepting the results without question .

G A SEO U S FU ELS

With certain gaseous fuels it is impossible accurately tomeasure the amount of fuel burned without resorting to methodsof metering which are not available in most tests . In suchcases the heat absorbed by the boiler per unit of fuel burned

,

and therefore the efficiency of the boiler,cannot be directly

determined . Since,however

,all of the combustion losses

,except

,

108

of course,the radiation and unaccounted loss

,can be computed

directly,a heat balance not only indicates the distribution of

losses,but offers a means of indirectly determining the boiler

efficiency . For such determination it is necessary to assumethe radiation and unaccounted loss,but experience has fixed theamount of such loss within reasonably accurate limits .

Blast furnace gas is the fuel in most common use that cannotreadily be measured

,and we will consider a test with this gas in

which the data necessary for the computation of a heat balanceis as follows

Temperature Boiler Room,Degrees

Temperature Exit Gases,Degrees

CO Per CentAndy“ Dry

co,Per Cent

Gas by Volumeat 60 Degrees

H2’Per Cent

N Per CentMoisture in Gas (Grains per Cubic Foot) , GrainsTemperature Gas“ Entering Burner Degrees

CO Per CentFlue Gas O

, ,Per Cent

A na1y3 1s CO,Per Cent

N, ,Per Cent

We will assume that the radiation and unaccounted loss,

including the loss due to the moisture in the air supplied forcombustion is per cent .It would

,of course

,be possible to compute the heat balance

either on a volumetric or on a weight basis,but since the common

combustion formulae are in terms of weight the latter basisappears preferable .

Converting the analysis of the gas by volume to one byeight

,we have

Volum e per Weight perCubic Foot Cubic Foot

CO,

. 130 x . 12341

CO 2 56 x 0 7806

H , 0 32 x 0 0562d

N . 5 82 x 0 7807

.08 164

The weight .of the gas then at 60 degrees is .08 164 poundsper cubic foot .The heat value of the gas will be

2447 3 x 4380 2

0022 1 x 62000

B . t . 11. per pound

or at 60 degrees

x 0 8 1642 9 8 7 B . t . u . per cubic foot .

Since the gas enters the boiler at a temperature above thatof the atmosphere

,there is available in the gas for absorption by

the boiler a definite amount of sensible heat aside from the heatdeveloped by the combustion of the gas

,and the heat balance

therefore must be computed on the basis of above atmospherictemperature .

This sensible heat per pound of gas wi ll bee

where 5 2 mean specific heat of gas,

T 2 tem pera ture entering gas,t2 tem perature atmosphere .

The mean specific heat of the gas between 60 degrees and

300 degrees is

CO,

0 0008 3x 180—0 000000 17x 37200) 2 .039 8

CO x 180) 0 5 78

H,2 0 022 (3 .29 + 0 00266 x 180) 007 3

x 180) 13 15

Mean "

Specific heat 2 2 364

and the total heat value per pound of gas above 60 degrees is

(300 B . t . u .

The computation of the heat balance proper is as follows

Loss Due to M oisture iu G a s

The gas contains 3 1 grains of moisture per cubicterms of weight per pound of gas this value is

x 3 12 3 79 grains per pound

or 0 541 pounds of moisture per pound of g as .

https://www.forgottenbooks.com/join

L oss Due to [ ueoueplete Com oustiou of Ga roou

This loss from formula (3 4) is

6x . 15 8 5 x 10 1602 443 0 B . t . u .

—z 1266 2 35 0 per cent

The heat absorbed by the boiler per pound of gas burned,by

difference,assuming the radiation and unaccounted loss as

per cent“or B . t . u .

,is

1266 13 48+ 24 43+ 23

or

1266 2 700 6 per cent

The complete heat balance is thenB . t . 11. Per Cent

Loss due to moisture in gasLoss due to moisture formed in burning H

,

Loss in dry chimney gasesLoss due to incomplete combustion of CRadiation and unaccounted loss (assumed)

Absorbed by boiler (by difference) 5

The above method may be followed for any fuel where anactual weight or measurement of fuel is not possible

,but where

such weight or volume can be determined the method used inthe case of coal

,preceding

,preferable .

I N DE"Abso lute tem pera tureAbsolute z ero

A irand com bustionCom position of

Effect ofexcesson exit g as tem perature

Efiect of insufficient on exit g astem perature

ExcessInsufii cient

Loss due to m oisture inRequired for com bustionSupplied for com bustionWeight and volum e of

B last furnace g as, com putation of combustion da ta

British therm a l unitBy-product coke oven gas, com putat ion

of com bustion data

Ca rbon dioxide,specific heat of

Ca rbon,loss due'

to unconsum ed

Ca rbon m onoxide,loss due to pres

ence of

Ca rbon m onoxide,specific heat of

Cha ra cterist ic equation of gases

Chem istry of com bustionChem ica l reactions of com bustion

Coa l,com putation of com bustion da ta

Com binat ion, heat ofCom bust ionA ir required forChem ica l reactions ofChem istry ofCom pleteCom putation of data

,blast furna ce

gas

Com putationofdata,by—product g as

Com putation of da ta , coa lCom putation of data

,natura l g as

72

20

95

72

PA G E

20 22

Com bustion 9

Com putation of data,oil 83

Com puta tion of data,wood 78

Genera l requirem entsof proper 70

Hea t of 20,22

I ncom plete 62

Losses 60

Perfect 46

Products of,weight 45 , 50

Products of, vo lum e 45 , 5 1

Speed of 13

Tem peratures developed in 3 5

Com plete com bustion 10, 46

Density of gases 15 , 18

D issociat ion 39

Dulong’sform u la . 24

Excess a ir 56, 65 , 69

Effect of on exit gas tem peratures 65

Flam e 39

Flam e as a m easure of tem perature 40

G as ana lysis 47 , 58

Assum ptions of 5 7

Conversion of 19

Effect of presence of m oisture 5 7

Errors of 58

Gases 15

Cha ra cteristic equation of 15

Density of 15 , 18

Specific heat of 32 , 34

Vo lum e of 15 , 18

Weight of 15 , 18

Ga seous fuels,heat balance 108

Genera l conclusions 70

Heat ba lance 60,105

Ga seous fuels 108

Solid and liquid fuels 105

Heat of com bination 20

Hea t of com bust ionCom putation of

Mea surem ent of

I NDE"Coutiuuea’

Heat, specificHeat va lue

,high andlow

H ydrogen, loss due to burningHydrogen, specific hea t ofI gnit ion, speed ofI gnit ion tem peraturesI ncom plete com bustionof C , lossduetoInstantaneous specific heatI nsufficient a ir

,effect of on exit tem

perature

I ntroduction

Mean specific heatMoisture in fuel, com bustion lossdue toMoisture in a ir

,com bust ion loss due to

Moisture form ed in burning H lossdue to

Natura l gas,com putati0n ofcom bustionda ta 87

N itrogen, specific heat of 3 1

O i l,com putation of com bustion data 83

Oxygen, specific heat of 33

Perfect com bustion 46

Products of com bustion,weight 45 , 50

Products of com bustion,volum e 45 , 5 1

Radiat ion loss 63

Rea ctions,chem ica l

,of com bustion 12

Sm oke

Solid a nd liquid fuels,heat ba lance

Specific heatA t constant pressure

Specific heatA t constant vo lum e

Genera l form u laI nstantaneousMeanOf C0

,

O f co

orH,

orN,

O f 0 ,

O f water vaporSpeed of com bustionSulphur

,effect on g as ana lysis

Su lphur,correction for

Tem peratureAbsoluteDeveloped in com bust ionEffect of excess a ir on exitEffect of insuffi cient a ir on exitFla m e a s a m ea sure ofI gnit ion

U na ccounted losses in hea t ba lanceU nconsum ed C in fuel, loss due toU nit

,Brit ish therm a l

Vo lum e,of a ir

Volum e,of ga ses

Wa ter va por,specific heat of

Wa ter vapor,in a ir

,loss due to

Weight of a irWeight of ga sesWood

,com putation ofcom bustion da ta

Zero , absolute

16

18

33

61

16

18

I 9