Fuel-Gas Analysis for Heating Value and Combustion Calculations K. M. WATSONAND N, H. CEAGLSKE, Department of Chemical Engineering, Uniuersity qf Wisconsin, Madison, Wis. of a hypothetical c o m p o u n d A TRIPLE-COMBUSTION scheme of fuelCnH2n+2 representing the average in calculating the efficiengas analysis to be carried out with the so-called composition of the saturated hycies of combustion in gasBureau of Mines apparabus is described. B y drocarbons. The indexnumber, fired equipment are the ratio of this method the average compositions and volumes n, may be evaluated from the flue gas to fuel gas, the ratio of air of both the saturated and unsaturated hydrodata of the usual combustion to fuel gas, and the amount of analysis. This type of data is water formed in c o m b u s t i o n . carbons of a gas sample are determined. The admirably suited for calculations The ease of determining thesefacresults of such a n analysis may be used as a basis of all kinds. From it, if the tors is greatly increased if the for combustion calculations. complete composition of the recomplete chemical analysis of Equations are developed whereby the heating mainder of the gas is known, may the fuel gas can be directly devalues of mixtures of either saturated or unbe c a l c u l a t e d the density and termined. h e a t i n g v a l u e as well as the I n dealing with the more coinsaturated hydrocarbon gases may be calculated stoichiometric relationships inplex fuel gases, it is difficult to from their average compositions. The healing volved in combustion. determine the data required for value of a complex fuel gas may be calculated Unfortunately no simple and such calculations by any of the from the results of a triple-combustion analysis sat i s f a c t o r y m e t ha d s have commonly used schemes of gas with an error not ordinarily exceeding 2 per cent. been devised for s e p a r a t i n g analysis, In analysis with the and identifying the unsaturated so-called Bureau of Mines apparatus (W),it is customary to determine separately the hydro- hydrocarbons of a gas. I n many fuel gases these unsaturated gen and carbon monoxide contents but to group the other com- compounds contribute more than 30 per cent of the heating bustible components as “illuminants” and as “methane” and value of the gas. Since this fraction may include com“ethane.” In a majority of fuel gases, paraffin hydrocarbons pounds of widely varying molecular weights and characterisother than methane and ethane are present of which this scheme tics, it is evident that a means of determining the compoeition takes no account. Furthermore, no information is obtained re- of the illuminants is necessary in any analytical scheme on garding the Composition of the mixture of unsaturated hydro- which combustion calculations are to be based. Calculation of the heating value of a fuel gas from its chemicarbons which is termed the “illurninants.” Such an analysis cal analysis is ordinarily not possible because of the unceris of little value as a basis for combustion calculations. Numerous schemes for the analysis of fuel gases by lique- tainty regarding the composition of the illuminants. If the faction and subsequent fractional distillation have been de- composition of the illuminants is determined by a liquefaction veloped. That of Podbielniak (5) is sufficiently convenient method, or if illuminants are absent, the heating value may be and simple to find considerable application in industrial labo- predicted with an accuracy sufficient for all ordinary requireratories. I n this scheme the components of the gas are identi- ments. Wojcicki (7) compared heating values calculated fied by their boiling points, and their quantities determined from chemical analysis to those experimentally determined for Polish natural gases in which no unsaturated compounds by plotting a distillation curve of the liquefied mixture. The method of Podbielniak yields excellent data when ap- were present. The heating values were calculated on the plied to natural or refinery gases. When volatile gases such basis of the index numbers of the paraffins present. It was as hydrogen or carbon monoxide are present, the fractional found that these results differed by less than 1.0 per cent from distillation must be supplemented by other methods in order those calorimetrically determined. Calculation of heating to determine completely the composition of the combustible value from chemical analysis is particularly valuable when components. If considerable quantities of the pentanes or only samples of limited size are available for laboratory exheavier hydrocarbons are present, differentiation between amination. the higher boiling components is difficult and the data become TRIPLE-COMBUSTION METHOD OF ANALYSIS uncertain as a basis for combustion calculations. The cost By extending the procedure ordinarily followed with the of the equipment and maintenance is also relatively high and the manipulation somewhat slower and more difficult than Bureau of Mines apparatus to include an additional total in the use of absorption or combustion methods of analysis. combustion with oxygen, data may be obtained from which For these reasons, liquefaction methods are not particularly the complete average compositions of both the paraffins and feasible where only a generally applicable method of deter- the illuminants may Abe calculated, No additional equipment is needed and the time required for the complete analysis is mining combustion characteristics is required. A complete scheme of analysis combining liquefaction, ab- increased by only about 25 per cent. The sample is first submitted to the complete procedure of sorption, and combustion methods has been described by Wood (8). The data obtained by this method are complete the Bureau of Mines method (2). The carbon dioxide is absorbed in potassium hydroxide solution, the illuminants in but the procedure is b n g and difficult. It has been pointed out by Parr (4) and recently empha- fuming sulfuric acid, and the oxygen by phosphorus or alkasized by Baxter (1) that the results of the Bureau of Mines line pyrogallate. The carbon monoxide and hydrogen are type of analysis have more significance if the average mo- determined by fractional combustion with cupric oxide a t lecular composition of the paraffin hydrocarbons is reported 300” C., followed by the determination of the carbon dioxide rather than assuming that only methane and ethane are pres- formed, The carbon monoxide content of the original gas ent. The paraffin content of the gas is reported in terms is equal to the carbon dioxide formed, whereas the hydrogen
T
HE immediate objectives
70
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 15, 1932
content is equal to the contraction in volume accompanying thia first combustion. The residual gas is assumed to consist of paraffin hydrocarbons and nitrogen. This gas is completely burned by introducing i t into a pipet filled with pure oxygen in contact with a spiral of incandescent platinum wire. The contraction in volume and the carbon dioxide formed in the combustion are determined. From these data the volume and average composition of the paraffins may be calculated (4). The combustion reaction is as follows: C,,Hp,+2
+ (1.5n + 0.5)O~= nCOz + (n f l)HzO
(1)
If V is the volume of the paraffins in the sample, the equations for the contraction and carbon dioxide formed in the combustion are represented, respectively, by Av = 1.5V COZ = Vn
+ 0.5Vn
where Av is the contraction in volume accompanying the second combustion. Combining Equations 2 and 3, J7
= AV
- 0.5 COO 1.5
coz n=T-
(4)
Au"
- [ AV + 1 / 2 c 0
+ 3/2H2]
Since V', the volume of the illuminants, is determined by absorption in fuming sulfuric acid, combination of Equations 7 and 9 permits direct solution for b. The volume of carbon dioxide formed from the combustion of the illuminants is the difference between the carbon dioxide formed in the third combustion and the sum of the volumes of carbon dioxide formed from the paraffins and the carbon monoxide. That result'ing from the paraffins is measured in the second combustion and is equal to Vn, from Equation 3. Thus, COS' = C02' - (Vn CO) (10) where COzu= C02 formed in third combustion per 100 cc. of sample
+
Combination of Equations 8 and 10 permits direct solution for a. The application of this method is illustrated by the following data from the analysis of a typical carbureted water gas: B U R E A U OF MINES ANALYSIS
VOLUME
The contraction, AD', and the carbon dioxide resulting from the combustion of the illuminants contained in 100 cc. of the original gas are, respectively,
+ :)
COS' = V'a
The contraction resulting from the combustion of the illuminants may be obtained as the difference between the contraction in volume accompanying the third combustion of the entire gas and the sum of the contractions resulting from the combustion of the carbon monoxide, hydrogen, and the paraffins. The contraction from the combustion of the paraffins is measured in the second combustion as Au, based on a sample of 100 cc. From the combustion of the carbon monoxide and hydrogen there will be contractions equal to respectively one half and three halves of the volumes of these components in the sample. Thus,
(9)
where Au" = contraction in third aombustion per 100 cc. of original gas sample CO = percentage of CO in Sam le He = percentage of Hz in smn$e
(5)
The nitrogen content of the original sample is determined by difference, as usual. For determining the composition of the illuminants, a second sample is taken into the apparatus, treated with potassium hydroxide solution to remove the carbon dioxide, and completely burned in the slow-combustion pipet with an excess of pure oxygen. The size of the sample to be taken for this third combustion depends on the oxygen demand of the gas. It is desirable to use as large a sample as possible without the volume of the products of combustion and excess oxygen exceeding the capacity of the buret. I n working with the usual types of manufactured city gas, a sample of about 35 cc., burned with 90 cc. of pure oxygen, will be satisfactory. The contraction in volume and the carbon dioxide formed in the combustion are determined and calculated to the basis of 100 cc. of the original gas. By means of these data the average composition of the illuminants may be calculated as follows: I n 100 cc. of the original gas there are V' cc. of unsaturated compounds having an average composition CaHb. When burned with oxygen, the following reaction takes place:
Au' = V'( 1
Av'
71
CONTRACTION
cc.
0riginal sample A fter KOH A fter HI SO^ After KOH After P Atter CuO_(lst oombustion) AIter KOH On added for opmbustion After oombustion (2nd oombustion) After KOH
100.0 97.4 90.3 89.0 88.3 48.4 15.6 81.1 75.7 64.2
2.6 = %
con
-
20.9 Av 11.5 = con
TOTAL COMBUBTION (THIRD COMBUSTION)
Volume of aample After KOH On added for opmbustion After oombustion After KOH
33.0 32.2 91.8 85.9 63.7
38.1 22.2
From Equations 4 and 5 and the data of the second combus tion, J' = 20.9
- 5.75
= 1o
1.5
11 5 n = - = 1.14 10.1
In the total combustion, 38 1 Contraction per 100 cc. of gas = = 115.4 = Av" 0.330 COZformed per 100 cc. of gas = __ 22'o = 67.3 = 0.330
GO%'
From Equations 9 and 10, Au' = 115.4
Cot'
=
67.3
-
20.9 11.5
++ 16.4 + 59.81 32.91 = 22.9
From Equations 7 and 8 b
- (g-
1)4 = 4.72
22 9 a = - = 2.73 8.4
COMPLETEANALYSIS
con
CmH4.72
0: Hi
(illurninants)
co C1.14Hd.or(paraffins) N1
% .2.6 8.4 0.7 39.9 32.9 10.1 6.4
1oo.o
18.3
ANALYTICAL EDITION
72
TABLE I. TOTAL HEATING VALUESOF UNSATURATED
CALCULATION OF HEATING VALUE The heating values of the lower paraffin hydrocarbon gases are represented by the following equations with errors of less than 0.5 per cent. These equations were derived from the data of the International Critical Tables and the recent experimental values of Rossini (6). Total Heating Value: Gram calories er gram mole = 156,70011 56,100 11) B. t. u. per cu, at 60" F., 80 in., satd. H20= 73211 262 112) Net Heating Value: Gram calories er gram mole = 146,20011 45,500 (13) B. t, u.per cu. at 60" F., 30 in., satd. H20 = 683n 212 (14)
+
8. 8.
+
Vol. 4, No. 1
+ +
HYDROCARBON GASES
CA#
(Unit: gram calories per gram mole) EXPERIMENTAL H.V. CALCD. H. V. FROM Ea. 16
ERROR %
Ethylene Propylene Isobutylene Amylene Benzene Toluene Acetylene
0.00
4-0.40 0.00
-0.86 0.00 -0.3 -0.7
ACCURACY OF METHOD
It must be recognized that the methods described above are subject to the limitations of the grade of commercial gasanalysis equipment usually employed. The average composition of the unsaturated hydrocarbons is determined from differences between relatively large experimentally determined values. This result is particularly uncertain when the Total Heating Value: unsaturated hydrocarbons constitute only a small fraction of Gram calories per gram mole = 98,200a 28,200b 28,800 the total combustible components. However, in such cases (15) accurate knowledge of the composition of these compounds is B. t. u. per cu. ft. at 60" F., 30 in., aatd. HzO = 459a 132b + 135 (16) of correspondingly lesser importance. If reasonable care is Net Heating Value: used, the results of the analysis should be satisfactory as a Gram calories per gram mole = 98,200a 22,900b 28,800 (171 basis for all ordinary combustion calculations. I n carrying out the analysis particular care must be exerB. t. u. per cu. ft. at 60" F., 30 in., satd. HcO = 459a 107b 136 (18) cised that the two samples used are of exactly the same composition. The samples should be stored over mercury, out The errors in the use of these equations are shown in Table I, of contact with rubber tubing or stoppers which might absorb in which values calculated from Equation 15 are compared to hydrocarbons. Mercury must be used in the buret and comthose taken from a critical compilation by Kharasch (3). bustion pipet of the analytical apparatus. When the heavier It will be noted that these equations break down entirely hydrocarbons are present, care must be taken that each comwhen applied to acetylene. However, for the lower olefins bustion with oxygen is complete. The mixture of excess and aromatics, the equations represent the heating values oxygen and combustion products should be repeatedly passed within the probable accuracy of the experimental data. into the combustion pipet over the incandescent platinum The heating values of hydrogen and carbon monoxide from spiral and returned to the buret for measurement until a the data of Rossini (6) are as follows: constant volume is attained. By heating the spiral only when gas is being passed into the pipet, this can be accomHYDROQEN plished with minimum opportunity for oxidation of nitroTotal Heating Value: gen. Gram calories per gram mole = 68,310 The accuracy of the heating value calculated from the B. t. u. per cu. ft. at 60" F., 30 in., satd. HzO = 319 analysis is dependent largely on the accuracy of the analysis Net Heating Value: and on the accuracy of the determinations of the heating Gram calories per gram mole = 57,750 B. t. u. per cu. ft. at 60" F., 30 in., satd. HzO = 270 values of the pure components on which Equations 11 through 18 are based. There is considerable uncertainty as to the heatCARBON MONOXIDE ing value of pure ethylene. The value given in Table I is Gram calories per gram mole = 67,620 an average of two values given by Kharasch which differ from B. t. u. per cu. ft. at 60" F., 30 in., satd. HzO = 316 each other by over 4 per cent. If reasonable Care is used in the analysis, it is believed that By means of the above data and Equations 11 through 18, it is possible to calculate the heating value of a complex fuel gas the calculated heating value will rarely deviate from the corfrom the results of the triple-combustion method of analysis rect one by more than 2 per cent unless a large amount of previously described, This method is illustrated by the acetylene is present. In several tests of the method on carfollowing calculation of the total heating value of the car- bureted water gases, this limit of error was not exceeded. I n working with heavy gases which burn with difficulty in the bureted water gas whose analysis is given above: combustion pipet, somewhat larger errors may be encountered. Basis of calculation = 1.0 gram mole of fuel gas H. V. of hydrogen = 0.399 X 68,310 = 27,250 H. V. of carbon monoxide = 0.329 X 67,620 = 22,250 LITERATURE CITED H. V. of unsaturated CZ.73H4.72 = 0.084 (2.73 X The heating values of the lower unsaturated hydrocarbon gases of average formula C.Ha are satisfactorily represented by the following formulas, which were derived from the data of Kharasch (3'):
98.200)
+
+
+
+
+ (4.72 X 28,200) + 28,800
H. V. of paraffins (C1.14H4.2!3) = 0.101 (1.14 X 156,700)
+ 56,100
Total H. V., gram calories per gram mole
+
+ +
.
I
= 36,400
= 23,750 = 109,650
This calculated result corresponds to 512 B. t. u. per cubic foot measured at 60" F., 30 inches of mercury, and saturated with water vapor. The actual heating value of the gas indicated by a calibrated recording gas calorimeter was 518 B. t.u. per cubic foot. The error in the heatingvalue calculated from the chemical analysis was -1.2 per cent.
(1) Baxter, Coolo. School of M i n e s Mag., Jan., 1930. (2) Fieldner, Jones, and Holbrook, Bur. Mines, Tech. P5pET 320 (1925). (3) Kharasch, Bur. Standards J . Remarch, 2, 359 (1929). (4) Parr, "Fuel, Gas, Water, and Lubricants," p. 99, McGrtaw-Hill 1922. (5) Podbielniak, IND.ENQ.CHEM.,Anal. Ed., 3, 177-88 (1931). (6) Rousini, Bur. Standards J. Research, 6 , 1-35 (1931).
(7) Wojcicki, "Transactions of the Fuel Conference, World Pow Conferenoe, London, 1928," Vol. I, p. 829. (8) Wood, Fuels Science Practice, 9, 288-90 (1930). RECEIVED June 29, 1931.
