Thermodynamics of Producer Gas Combustion J
Application to Internal Combustion Engines A. P. OLESON AND RICHARD WIEBE Northern Regional Research Laboratory,
U. S . Department of Agriculture, Peoriu, Ill.
T
The composition and heats of combustion of producer gas derived from various raw materials are given. A nomograph is presented for calculating the heats of combustion of producer gas mixtures containing hydrogen, carbon monoxide, and methane. The theoretical air-fuel ratios of producer gas are compared with those of other fuels used in internal combustion engines, and attention is called to their importance in connection with carburetor design. The heat of combustion of a mixture of producer gas with theoretical air is considerably lower than that of gasoline or alcohol; and lower power output must be expected from such a mixture run under identical conditions with the liquid fuels mentioned above. The thermodynamic properties of two typical producer gas mixtures are calculated, and the results shown on charts. A sample calculation gives temperatures and pressures a t various points of the Otto cycle, as well as work, mean effective pressure, and efficiency. The effects of compression ratio and intake-manifold pressure on mean effective pressure and thermal efficiency are shown by charts. It is pointed out that extreme compression ratios are not so practicable for a n Otto cycle engine as an increase in manifold pressure, since a relatively low supercharge pressure gives a power output equivalent to that of gasoline. I t would be possible to use such a fuel in a Diesel gas engine.
HIS laboratory has been interested in the possible utilization of agricultural r k d u e s as sources of producer gas in this country. A comprehensive laboratory investigation was made with two gas generators, from which certain results are drawn for use in this paper. A detailed account of these investigations will be published later. As a supplement to the experimental work, thermodynamic calculations on two representative gaseous mixtures are presented here in order fully to evaluate all the possibilities in the use of such a fuel. These calculations enable one to predict temperature, maximum pressure, mean effective pressure, work, and thermal efficiency of these mixtures when used as fuels in internal combustion engines, as well as to make comparisons with similar data obtained for octane (13, 14, 98) and for alcohol (27). A large amount of work has been done on various fuels that may be used in portable gas generators, as evidenced by a voluminous literature. An extensive bibliography on "Theory, Design, Fuels, Performance, Utilization, and Economics of Gas Producers" is available on application to thie laboratory. Woods (2.9) made two series of calculations of the maximum temperature of combustion of a producer gas mixture having the following composition: carbon dioxide, 2%; carbon monoxide, 31%; methane, 0.5%; hydrogen, 9.5%; nitrogen, 57%, This gas was mixed with various proportions of air a t a fixed compression ratio of 6 to 3 and a t an initial temperature of 212' F. I n his first series Woods took into account only the dissociation of water into hydrogen and oxygen, and of carbon dioxide into carbon monoxide and oxygen; in the second set of calculations the formation of nitric oxide was also included. For a theoretical mixture the maximum temperature of combustion was 4627'
Rankine by the first method, and 4597' R. by the second, a lowering of 30' R. due to the nitric oxide reaction. Making the same assumptions, a temperature of 4595" R., almost identical with the second of the two figures, was obtained when the data TABLE I. EQUILIBRIUM CONCENTRATION OF DISSOCIATION PROD- for producer gas mixture 2 (Table 11) were used, although in this work the formation of atomic hydrogen and oxygen, as well of UCTS,IN MOLEPER CENT, AS A FUNCTION OF PRESSURE AND ABSOLUTETEMPERATURE OH, was also considered. This should tend to reduce the tem3960' R-. 4 6 8 0 " R .perature still further; however, the slight difference in comCompo312.16 954.7 1947.7 316.76 964.4 1963.4 position may, a t least in part, account for the discrepancy. The nent lb./sq.in. Ib. Ib. lb. lb. Ib. importance of dissociation a t high temperatures and pressures is NO 0.094 0.560 0.445 0.391 0.128 0.101 74.310 74.646 74.822 75.335 75.416 76.462 NI illustrated in Table I. Even though the concentrations of some OH 0.086 0.070 0.824 0.426 0.336 0.129 0.139 e. 104 0.084 HP 0.038 0.028 0.021 components may seem insignificant, these values, when multiH 0.001 0.001 0.016 0.032 0.003 0.010 plied by their heats of dissociation, make an appreciable contri6.408 6.427 Hs0 6.104 6.370 5.937 6.184 2.243 co 1.688 1.377 0.397 0.300 0.539 bution to the internal energy of the system. 17.264 17.422 17.528 15.989 15.368 16.336 CO: 0 0.048 0.022 0.003 0.001 0.001 0.013 Table I1 gives the average composition on a dry basis and the On 0.096 0.739 0.560 0.447 0.191 0.140 heat of combustion at constant volume (water as vapor) of proc 5400' R-. ducer gas made from various fuels in portable gas generators as 988.4 2003.9 Compo326.6 nent Ib. Ib. lb. well as in a unit designed in this laboratory. NO 1.246 1.115 1.484 It should be understood that there are considerable variations 71.977 72.859 73.309 N¶ of gas composition, depending on type of generator, moisture 1.250 1.016 1.707 OH 0.222 0.373 0.270 Composition at 520° H.: Hi content of the fuel or charge when water is added separately, and H 0.066 0.210 0.102 &On 2 CO 30%. HI 5.362 Ha0 4.909 5.579 10%; Xk,id: N ~b7%j , changing conditions within the gas generator during combustion, 5.918 4.602 3.769 co aa well as variations due to starting, acceleration, and speed of 11.246 12.848 13.672 0.169 0.106 0.342 the engine used in combination with the unit. Spiers and Giffen 1.147 1.884 1.392 oa (23) made extensive testa on the performance of a converted gas+
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
which are tolerated in the petrol engine, where the effect on power is small, would lead to serious power lo& in the gas engine”. Schniirle (21),too, found that maximum power output was obtained with a definite mixture containing 90% of theoretical fuel. The engines used in the two experiments wcre very different. The thermodynamic calculations were made similarly to those previously mentioned (13,27), The perfect gas law has been assumed to hold throughout the calcylations. The equation for the combustion of mixture 1 with air reads as follows (proportions by volume) :
TABLE 11. AVERAGECOMPOSlTIONS AND HEATS O F COMBUSTION OF PRODUCER GASMADEFROM VARIOUSFUELS Source of Producer Gas Charcoal (68) T ,,w-tamn
I-..
r n ~ fin) r \_”,
--Gas CO, 2.0
CO 31.0
Com oeitionCH4 Na 9.6 0.6 57.0
&
H, E B.t.u./ B.t.;./ Cu. Ft.* Cu. Ft.* 130.1 129.5
“_..I
Water added 0.5 30.6 5.3 63.8 No water added 6.25 23.15 17.95 0.70 61.95 126:s l29:3 4.5 26.0 15.0 1.4 53.1 Anthracite (68) 137.3 136.7 Wood (66) 13.8 17.2 18.0 1.9 49.1 121.8 121.4 Corncobsa Choppedinsmaupiecee 10.7 14.7 21.9 3.2 49.5 138.3 135.8 Whole 15.5 15.4 11.8 4.1 53.2 119.1 118.7 Producer gas mixt., No. 1 14.0 16.0 18.0 2.0 50.0 118.9 118.4 No. 2 2.0 30.0 10.0 1.0 57.0 132.8 132.2 * Values used for low heats of combustion (water as vapor) at 60’ F. and 14.7 pounds per square inch are as follows: H heat of combustion at constant pressure; E heat of combustion at constant volume For CO 1/zOz : no,. H = 121,L393 B.t.
