asification of Fuels P A P E R S from the Symposia on Theoretical Aspects of Combustion and Gasification and Flame Studies on Interchangeability of Fuel Gases presented before the Divisions of Gas and Fuel Chemistry and Physical and Inorganic Chemistry at the Diamond Jubilee Meeting of the AMERICAN CHEMICAL SOCIETY. Another group of these papers was published in the May issue of INDUSTRIAL AND ENGINEERING CHEMISTRY. Interchangeability of Fuel Oases FLAME-STABILITY LIMITS OF METHANE, HYDROGEN, AND CARBON MONOXIDE MIXTURES Joseph Grumer, and Margaret E. Harris.
.I547
INTERCHANGEABILITY OF OIL GASES OR PROPANE-AIR FUELS WITH NATURAL GASES Joseph Grumer Margaret E. Harris, and Harold Schultz.
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Gasification of Solid Fuels COMBUSTION OF CARBON IN HIGH TEMPERATURE, HIGH VELOCITY AIR STREAMS J. M. Kuchta, A. Kant, and G. H. Damon..
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KINETICS OF STEAM-CARBON REACTION IN POROUS GRAPHITE TUBES H. F. Johnstone, C. Y. Chen, and Donald S. Scott.
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........................................................................ .................................................................. HYDROGEN-CARBON MONOXIDE REACTION IN A TUBULAR REACTOR WITH IRON-COPPER CATALYST W. M. Campbell and H. F. Johnstone.. ...........................................................................
.I570
-Stability Limits of Methane,
ogen, and Carbon Mono ixtures Joseph Grumer and Margaret E. Harris U. S. BUREAU OF MWES, PITTSBURGH, PA.
G a s companies frequently must interchange or supplement their send-out gases to meet consumer demand. To implement the method for predicting the interchangeability of fuel gases, the U. S. Bureau of Mines and the American Gas Association have cooperatively undertaken to study and systematize the flash-back and blowoff limits of primary gases (hydrogen, methane, carbon monoxide, etc.) and of multicomponent gases. Flame-stability data have been collected for binary fuels consisting of combinations of hydrogen, methane, and carbon monoxide. A new method of plotting these data was developed, so that three “composite flame-stability diagrams’’
for blowoff and three for flash back summarize all the limits for the three binary systems. Flame-stability measurements were also made for tertiary mixtures of these gases. Fairly good agreement was obtained between these experimental values and values calculated b y means of a simple mixture rule treating the tertiary mixture as combinations of binary fuels. The adequacy of such calculated flame-stability diagrams for industrial use remains to be determined, possibly by comparing predicted interchangeability of gases with past experience of gas utilities. These data may illustrate the noninterchangeability of a 490 B.t.u.coke oven gas with natural gas.
A
changed, without requiring preliminary appliance testing (I, 3). Only certain readily obtainable data on the fuel gases involved in the interchange are required: density, gas-line pressure, and flame-stability diagrams for both fuels. To implement this method and to obtain further basic information, the Bureau of Mines and the American Gas Association have cooperatively undertaken to study and systematize the combustion characteristics of primary gases (hydrogen, carbon monoxide, methane, ethylene, nitrogen, etc.) singly and in mixtures, with particular respect to blowoff and flash-back character-
BILITY to interchange or supplement gas supplies is becoming increasingly important in the gas industry. Because new send-out gases may not be suitable for maintaining stable flames on consumers’ appliances, the industry requires mean8 of predicting the performance of burners on company lines when fuel gases are interchanged. Recent studies by the Bureau of Mines have elucidated the fundamental principles of flame stabilization ( 4 , 6) and entrainment of air in gas burners (7‘). This information has been combined into relationships predicting the performance of burners in a community where fuel gases are July 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
154?
back and blowoff limits of binary mixtures graphically. I t was believed that such studies would reveal interactions between single gases that would persist in more complex mixtures. The following method was deaeloped. Measurements were madc of the flame-stability diagrams of a number of mixtures of two single gases covering the range of 0 to 100% of each. The fuels were made up in compressed-gas cylinders and analyzed on a mass spectrometer. Mixtures of these fuels with air were made up during runs by metering streams of fuel and dry compressed air from cylinders into mixing chambers upstream of the burner. A s only two flowmeters were required during a run, metering errors were less than when three or more streams are metered t o make up a m xture. Glass-moo1 flowmeters (9)having an accuracy of better than i l % were used. The data obtained in this manner were used to construct “composite flame-stability diagrams” for binary mixtures, such ah 1001
1
04
Figure 1.
1
LL 0 8
12 16 20 24 GAS CONCENTRATION FRACTION OF STOICHIOMETRIC
Flame-Stability Diagram for iOO%
28
32
I
Methane
H2, percent
0 12.6 260 I ! I
46.4 CHd.
Dercent
70.7 84.6 100 ‘ I
istics, and to determine the possible application of such generalizations to the problems of interchangeability of fuel gases and improvement of burner design. Initial attention was given to the gases methane, hydrogen, and carbon monoxide, because these are usually present in fuels supplied by utilities to the public. This paper presents a graphical method of concisely recording experimentally determined flame-stability limits of binary mixtures of these three gases and a semiempirical method of estimating flame-stability limits for tertiary mixtures from experimental data for simpler fuels. The methods proposed should be generally applicable to other gases.
Single-Component Fuels The flash-back and blowoff limits of a fuel may each be represented by a single curve of “critical boundary velocity gradients,” g p or ge, versus fuel gas-air composition in the burner, expressed as the fuel-gas “fraction of stoichiometric,” F (1,3 ) . These two curves comprise the flame-stability diagram of a fuel. Figure 1 is the flame-stabilitp diagram of methane (99.6% CHI). The two curves of a flame-stability diagram separate three regions of flame behavior on burners-one in which flames flash back, one in which flames are stable, and another in which flames blow off. Such a diagram is characteristic of the fuel gas and is not conditioned by any test burner or group of burners. Its use in predicting burner performance with interchanged fuel gases has been described (1, 3 ) . Flame-stability diagrams for methane, hydrogen, and carbon monoxide have been included as part of the following figures. Data for pure hydrogen were obtained from the literature (8), while data for pure carbon monoxide mere obtained by extrapolating to 0% hydrogen values given by Walker and Wright ( 9 ) for mixtures of carbon monoxide with 1.0 and 1.8% hydrogen. Diagrams such as Figure 1 can be obtained for any gas or combination of gases by measurement. However, it would be most timeconsuming to do so for all fuels.
Binary Mixtures Previous studies of propane-hydrogen (6), propane-carbon monoxide, and methane-carbon monoxide mixtures ( 9 ) have demonstrated that the flash-back and blowoff limits of the single gases cannot be combined linearly t o give the limits of binary mixtures. This conclusion was also reached in the Bureau of Mines laboratory for other gases. These observations led to a decision to systematize the flash1548
o
0.2 0.4
o 6 o a 1.0 os 0.6
0.4
0.2
o
Figure 2. Critical Boundary Velocity Gradients for Flash Back of Methane-Hydrogen Fuels Composite diagram
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 7
Fuel Gasification Figures 2 and 3, which summarize the flash-back and blowoff limits, respectively, for all mixtures of hydrogen and methane. Each of these two graphs consists of a family of curves along which the fuel-air composition, F , expressed as a fraction of the stoichiometric fuel percentage, is cohstant. Each curve is a plot of critical boundary velocity gradients for either flash-back or blowoff versus ratios of hydrogen to methane. From 0 to 50% hydrogen the ratio is H,/CHa, and from 50 to 100% hydrogen it is CH4/H2. This is done to avoid a value of infinity. Figures 2 and 3 can be used to draw the flash-back and blowoff curves of a particular hydrogen-methane fuel by reading off the ordinates on each F curve corresponding to the desired H I / C H ~ratio and plotting these ordinates (gradients for flash-back or for blowoff) against the F values. Similarly, Figures 4 and 5 are for the binary system of hydrogen and carbon monoxide, while Figures 6 and 7 are for the system of methane and carbon monoxide. The data in Figures 6 and 7 parallel the trends reported by Walker and Wright
I
0 12.6 26.0
HZ,percent
46 4
70.7
(9) for these gases. The graphical method outlined is applicable to any binary system of gases. These three sets of composite flame-stability diagrams are different from one another. The methane-hydrogen system of fuels shows that the addition of a little methane to hydrogen drastically lowers the flash-back curves but does not severely affect the blowoff limits. The addition of a little hydrogen to methane has no pronounced effect. The addition of a little methane or hydrogen to carbon monoxide strongly affects the flash-back and blowoff curves, but the addition of a little carbon monoxide to hydrogen or methane has only a mild effect. The flash-back and blowoff limits for these three systems increase strongly in the order methane-carbon monoxide, methane-hydrogen, and carbon monoxide-hydrogen. The flash-back region of hydrogen-carbon monoxide mixtures is also much wider than those of the other two systems, extending on the average from about F = 0.6 to about F = 2.2. I n all three systems the slopes of the blowoff curves are different from the flash-back curves.
I
84.6 100
0
0.2 04 0.6 0.8 1.0 0.8 0.6 0.4 0.2
L - - - - - g A Hz% d
0
Figure 3, Critical Boundary Velocity Gradients for Blowoff of Methane-Hydrogen Fuels
Figure 4. Critical Boundary Velocity Gradients for Flash Back of Carbon MonoxideHydrogen Fuels
Composite diagram
Composite diagram
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I N D U S T R I A L A N D E N G I N E E R ING C H E M I S T R Y
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~
I
CO. oercent
Figure 6. Critical Boundary Velocity Gradients for Flash Back of Carbon MonoxideMethane Fuels Composite diagram
*
o
L
o
0.2 0.4 0.6 0.8
h
co
A
o
u
1.0 0.8 0.6 0 4
C
O
-
0.2
u o
4
Hz
Figure 5. Critical Boundary Velocity Gradients for Blowoff of Carbon MonoxideHydrogen Fuels Composite diagram
Tertiary Mixtures Measuring the critical boundary velocity gradients of all possible tertiary mixtures would be time-consuming and would result in a complex group of graphs or tables. The prospects grow less feasible for quaternaq- mixtures and so on. Although the simple linear mixture rule could not correctly average the flash-back and blowoff gradients of single gases to obtain those of binary mixtures, i t seemed likely that the gradients of binary fuels and some single fuels could be thus combined to give the gradients of multicomponent fuels. The “coke oven” gases listed in Table I were used to test this possibility. These test fuels were selected as extremes in composition. Gas 1 was chosen to simulate a real coke oven gas and to test the above proposition for a high hydrogen, medium methane, and low carbon monoxide fuel; ?Yo. 2, for 1550
( F k G a i c o n c h r a t ) o n , f;actio;
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 7
-Fuel
Gasification
-
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08 12 1s 2.0 2.4 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
GAS CONCENTRATION. FRACTION OF STOICHIOMETRIC
28
Figure 9. Calculated Flame-Stability Diagrams and Experimental Points for Coke Oven Gas 2
Figure 8. Calculated Flame-Stabilit Diagrams and Experimental Points for Coke &en Gas 1 26.3% C H ~ 10.6% ,
co,4
29.4% CH4, 31.7% CO, 38.7% H P
. 6N ~~ ~ 5,a . 4 H* ~~
600,000
a mixture of equal percentages of the three; No. 3, for a high hydrogen, medium carbon monoxide, and low methane fuel; and No. 4,for a high carbon monoxide and low hydrogen and methane. The linear mixture rule is the normal way of obtaining a weighted sum of anything and is used here as Equation 1: g, +
1
+
... = n,gy
400.000
200.000
100,000
+ nzg. + . . .
(1)
> 80,M)O z"
60,000
u
where g = flash-back or blowoff gradient and n = fractional concentrations of each component. Values of g,, gs, etc., can be read from a flame-stability diagram, such as Figure 1, or from composite flame-stability diagrams, such as Figures 2 and 3. If the fuel contains three single gases, a , p , and y, which are to be considered as a combination of two binary fuels ( a @)and (a y ) , then QI may be divided between p and y in the ratio of @/y. It remains t o be determined by experiment which single gases are to be a , p , or y and which binary combinations are to be y, x , etc. Experiments with the four gases in Table I led t o the following conclusions :
+
+
% 40.000 iY
0
d
20,000
Tube diameter. cm.
c" 9
2z g
dE
10000 8000
6000 4000
2000
1,000
800
For flash back the experimental points in Figures 8 to 11 for the four test gases are unlike those for binary mixtures of hydrogen a n d carbon monoxide but resemble the data for mixtures of (CO CH,) or (Ha CH,). Accordingly, for flash back, y and z in Equation 1 are taken to be (CO CH4) and (Hz CH,). The flash-back curves in Figures 8 to 11 were calculated with Equation 1 from Figures 2 and 6, the composite flame-stability diagrams for flash back for the binary systems of (CO CH,) and (Hz CHI). The agreement between calculated and experimental flash-back gradients seems satisfactory. For blowoff, the experimental points in Figures 8 to 11 are in the range of mixtures of (Hz CO) and (HZ CHa) but greatly above the range of mixtures of (CO CH,). Accordingly, for blowoff, y and B in Equation 1 appear to be the binary systems (CO Hz) and (Hz CHI). The best fit of the experimental blowoff limits in Figures 8 to 11is obtained if the hydrogen is proportioned between methane and carbon monoxide so that the ratio of Hz/CO N 0.2. Examination of the curves in Figure 5 shows a sharp change of slope near H,/CO ,- 0.2, which indicates that the reactivity of carbon monoxide is strongly accelerated by
+
+
+ +
+
+
+
+
July 1952
+
+
+
600
loo+1
'
0 1 8
12 " 1" 6 2 I0 24 GAS CONCENTRATION. FRACTION OF STOICHIOMETRIC
" 28
Figure 10. Calculated Flame-Stability Diagrams and Experimental Points for Coke Oven Gas 3 10.3% CHc, 34.0% CO, 55.7% Hn
the addition of hydrogen up to this point. When more hydrogen is added there appears to be an averaging effect between it and the species "activated" carbon monoxide. The blowoff curves in Figures 8 to 11 have been calculated on the basis that these fuels are mixtures of the binary system (H, (2%) (Figure 3)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
+
1551
200
ow
100 000
80 000 60,000
Figure
11. Calculated Flame-Stability Diagrams and Experimental Points for Coke Oven Gas 4 16.3yo CHa, 66.27~ CO, 17.5% H:
and activated carbon monoxide (identical t o H2/C0 = 0.2 in Figure 5 ) . The agreement between calculated and experimental gradients seems satisfactory.
Interchanges of Coke-Oven Gas 1 and Natural Gas It may be both unnecessary and estremely difficult to devise means of estimating flame-stability diagrams of multicomponent
Figure 12. Predicted Burner Performance in Community Changing from Fuel Gas a to x
fuels more exactly than above. The gas industry seems best fitted to determine whether estimated flame-stability diagrams are as good as measured ones in meeting its needs. Its experience should be compared with conclusions reached from both types of diagrams. Examination of the interchangeability of coke-oven gas and natural gas is such a test. Examples have been given, using measured flamestability diagrams of predicting burner performance Tith interchanged fuel gases ( 1 , 3). The method requires that the gas-line pressure, the specific gravity, and the flame-stability diagrams of the fuels involved be known. With these data, burner-performance diagrams such as Figures 12 and 13 are constructed, predicting burner performance in a community changing from its adjustment gas (fuel a ) to another 40 000
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r‘
5
4,000
t5 E
2000
9 3
, 2
$
1000 800 600 400
tr
5 200
100
08
Figure 14. Figure 13.
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Predicted Burner Performance in Community Changing from Fuel Gas a to x
12 16 20 24 28 GAS CONCENTRATION. FRACTlOh O F STOlCHlOMETR1C
32
Flame-Stability Diagram for Natural Gas
91.5% CHI, 5.2% CzHs 1.3% CaHs, 0.9% COS, 0.6% Nz, 0.2% CaHo,’O.Z%CiHlo, 0.1% C&
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 7
Fuel Gasification (fuel z). The space between curves 1s and 2s represents, in effect, the port loading and gas-air ratio on burners in a community adjusted to fuel a but using fuel x . Superimposed on this is the flame-stability diagram of fuel s. Thus, regions are marked out in which burners in the community wlll burn stably,
Table I.
Composition of Test “Coke Oven” Gases and Natural Gas No. 1
No. 2
No. 3 55.7 10.3 34.0
No. 4
Natural Gas
Hydrogen % 58.4 38.7 17.5 ...... Methane ’% 26.3 29.4 16.3 91.5 Carbon &onoxide, % 10.6 31.7 66.2 ...... Nitrogen, % ’ 4.6 0.1 .......... 0.6 Ethane, % .................... 5.2 Pro ane % 1.3 Carton hioxide, % 0.1 0.1 0.9 Propylene, % .................... 0.2 Butane, % .................... 0.2 Butylene % .................... 0.1 Stoichiordetric fuel gas, 19.35 18.2 24.2 21.9 9.07 %. Specific gravity 0.336 0.497 0.425 0.743 0.613 Heating value, B.t.u./ cu. foot 490 526 394 433 1067
.................... ..........
gas utilities making change-overs similar to that of Figure 12 have found i t necessary to convert every burner to natural gas because of the objections deducible from Figure 12. Now consider the reverse exchange, from natural gas (fuel a ) to coke-oven g a s (fuel z). Figure 13 predicts burner performance in a community undergoing this exchange. It is seen that many burners will fall in the flash-back range but very few in the blowoff range. The airentrainment performance region is shifted considerably toward leaner and sharper flames than existed with natural gas. This exchange would also require that many burners be converted to use coke-oven gas. Figures 12 and 13 illustrate the use of estimated flame-stability diagrams of multicomponent fuels in predicting burner performance with interchanged fuel gases.
Acknowledgment The authors wish to thank Bernard Lewis and Guenther von Elbe for suggestions received during the course of this work. The measurements for Figures 11 and 14 were made b y Harold Schultz, Explosives and Physical Sciences Division, Bureau of Mines. This research is supported by the American Gas Association (Project PDC-3-GU).
Nomenclature flash back, or blow off. If most burners are within the stable flame region of the flame-stability diagram of fuel x, fuel x may be substituted for fuel a in the community. If many burners are outside the stable-flame region, it appears possible t o determine whether the exchange is not feasible or whether the situation can be remedied by charges such as minor burner adjustments, minor adjustments in composition of fuel x, or minor changes in line pressure. Existing data indicate the feasibility of the method. Consider a community adjusted to coke-oven gas 1 (fuel a ) and changing to natural gas (Table I) (fuel 2) without adjusting burners. Figure 14 is the flame-stability diagram for natural gas. The estimated flash-back curve and blowoff curve in Figure 8 is taken as the flame-stability diagram of gas 1. Figure 12 shows what burner performance may be anticipated for this exchange. No flash-back trouble is indicated, but many burners are predicted to blow off. Moreoverl the air-entrainment performance region has been displaced considerably toward richer flames with natural gas than existed with coke-oven gas. (If there was no displacement, curve 22 would center about the abscissas of unity.) This indicates that large, soft, or yellow-tipped flames may be expected on most burners. A more detailed’analysis is possible in principle (1, 3) but cannot be given before completion of studies of such factors as yellow tipping and the statistical distribution of burners over a flame-stability diagram. I n practice,
g = boundary velocity gradient, sec.-* n = fractional concentration of a component in a mixture
F = fuel-gas concentration, fraction of stoichiometric
Subscripts a = fuel gas used for adjusting burners x = substitute fuel gas y = component in a mixture z = component in a mixture
Literature Cited (1) Grumer, J., IND.ENQ.CHEM.,41,2756 (1949). (2) Iberall, A. S., J . Research Natl. Bur. Standards, 45, 398 (1950). (3) Lewis, B., and Grumer, J., Gas Age, 105,25 (1950). (4) Lewis, B., and von Elbe, G., J . Chem. Phys., 11, 76 (1943). (5) Lewis, B., and von Elbe, G., Trans.Am. SOC.Mech. Engrs., 1948, 307.
(6) Reiter, 8. H., and Wright, C. C., IND.ENG. CHEM., 42, 691 (1950). (7) von Elbe, G., and Grumer, J., Ibid., 40, 1123 (1948). (8) von Elbe, G., and Mentser, M., J. Chem. Phys., 13,89 (1945). (9) Walker, P. L., Jr., and Wright, C. C., Division of Gas and Fuel
Chemistry, 117th Meeting, AMERICANCHEMICAL SOCIETY, Houston, Tex., 1950. RBOEIXVED for review October 15, 1951. ACCEPTEDApril 7, 1952.
* * * * * Studies designed to solve the peak load problems that occur i n the gas industry are reported in a paper entitled “Production of Oil Gases as Substitutes for Natural Gases,” by H R. Linden, J. J. Guyer, C. A. M. Hall, and C.H. Riesz, which was presented a t the Production and Chemical Conference, American Gas Association, i n May. Three methods for producing high B.t.u. oil gases t h a t are completely interchangeable with natural gases were studied. Results of the tests follow: Production of low-gravity, low true oil gas heating value oil gases requires high severities of cracking, and unless tars of high free carbon and pitch content are not objectionable, this method would be restricted to distillate oil peak load operations where oil cost differential is of lesser importance than high set capacity. Best results are obtained with the catalytic autohydrogenation process if the hydrogen to ethylene ratio of the feed oil gas is high (above one). This requires moderately high severities of cracking or production of the oil gas under moderate pressure. The reaction is exothermic and heat exchange equipment is required in addition to a reactor. However, catalyst replacement is the only major processing cost anticipated. Except for a small reduction of oil efficiencies, pressure gasification does not introduce new operating problems, and i t should be equally applicable to distillate and residual oil operation. Greatest utility would be in new continuous gasification equipment, now being developed on pilot plant scale, as modification of existing high B.t.u. oil gas sets would be necessary to permit pressure buildup during make-oil admission. July 1952
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