icting Interchangeability of Interchangeability of Oil Gases or Propane-Air Fuels with Natural Gases Joseph Grumer, Margaret E. Harris, and Harold Schultz U. 9. BUREAU OF MINES, PITTSBURGH, PA.
G a s companies using pipeline' natural gas generally provide emergency facilities which can replace or supplement their normal supply in the event of breakdown or abnormal demand. Oil gas and certain propane-air mixtures have been used for these purposes by the gas industry. A major limitation in the choice of a supplemental or substitute gas is the varying degree of interchangeability on appliances that had been adjusted on natural gas. A means of predicting interchangeability is therefore desirable. Recent studies have developed the fundamental theory underlying the stability of flames on burners and the entrainment of air by fuel jets. These principles were used to outline new concepts whereby burner performance with interchanged fuel gases may be predicted by caIcuIation. The body of data needed for
application of this method to practical problems of the gas industry is being collected under a cooperative agreement between the U. S. Bureau of Mines and the American Gas Association. Data at hand allow limited evaluation of interchanges of oil gases or propane-air fuels with natural gas. Calculations indicate that if natural gas is to be replaced by either fuel, the adjustment of appliances on the oil gas or propane-air gas is preferable to adjustment on natural gas. It is also indicated that on burners adjusted on natural gas, propane-air is more interchangeable than oil gas. Principles are outlined for guidance in determining additives to oil gas which can reduce flash-back or blowoff troubles on natural gas burners. T h e effects of burner conversion techniques on the interchangeability of these gases are also discussed.
T
predicting burner perforinance with interchanged gases have been explained ( 2 , 6). If a gas utility has had burners on its lines adjusted to natural gas, then in the units of Figure 1the air-entrainment performance points of all their burners are within the stable flame region. Thc exact location of each point or the number of such points is unknown. It is known only that no burner-performance points are in the blowoff or flash-back region. Moreover, very few burners will be operating with lean flames or near-stoichiometric flames or near the blowoff or flash-back curves, because they are commonly adjusted to give rich flames away from the flame-stability limits. The stable flame region of the adjustment fuel (fuel a ) can serve a dual purpose. It is the region in which burner flames of that fuel are stable, and it is also the region containing all the air-entrainment performance points of burners on a gas distribution system adjusted to that fuel. For the latter role approximate equations were derived in an earlier publication (Equations 4 and 7 of 6 ) for calculating the change in t,he region containing the airentrainment performance points of burners that results when one fuel gas is substituted for another. Using the units employed as coordinates in flame-stability diagrams, these two equstione may be -mitten as
HE interchangeability of fuel gases on consumer appliances has long been a major problem of thegasindustryandisbeconiing more important as pipeline natural gas is being made available over the country. Gas companies using pipeline natural gas generally provide emergency facilities, which can replace or supplement their normal supply in the event of breakdown or frequent above-normal demand. They require means of predicting the behavior of new fuels on their burners, so that they may provide for satisfactory emergencv send-out gases. A method of predicting burner performance with interchanged fuel gases was outlined in two previous publications ( 2 , 5 ) . This method is based on theoretical principles making use of the concept of critical boundary velocity gradients that define the flamestability region of a fuel and the principles of air entrainment by a fuel jet. These are combined into approximate relationships, which predict the change in the over-all performance of burners on gas-utilities lines when fuel gases are changed. Data required are the density, gas-line pressure, and flame-stability diagrams of the fuel gases involved in the exchange. Oil gases and propane-air fuels are being used or considered by many gas utilities as a replacement or supplement for natural gas. The suitability of thecie gases for such purposes cannot be evaluated accurately at this time, because pertinent factors, such as the effects of hot flame ports, shallow ports, noncircular ports, insufficient secondary air, and yellow tipping on the interchangeability of gases, are not considered. These remain to be studied in the future. Furthermore, natural gases, oil gases, and propaneair fuels vary in composition, and therefore only the method of reasoning is strictly applicable to each exchange. This paper is essentially limited to a prediction of the likelihood of flash back and blowoff on consumer appliances when oil gases or propane-air fuels are interchanged with natural gases.
Predicting Interchangeability of Fuel Gases on Burners Figure I may be considered to be the flame-stability diagram for natural gases ( 3 ) . It was measured with the natural gas described in Table I. Diagrams such as Figure l and their use in
1554
P:
+6
p, -4 = 0
[(l - d")S]
(1)
where 4
[F2Szdo]a
~ _ _ _ [SZdO] [l - (1 - do)FS]a
and
F = fuel-gas concentration, fraction of stoichionietric S = stoichiometric fuel-gas concentration do = specific gravity of fuel gas p o = fuel gas line pressure Subscript a designates fuel gas used for adjusting hurriers, and subscript z designates new fuel gas.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. I
Fuel Gasification lM).ocO 80,w
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1.6
2.0
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3.2
loo
OAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
Figure 1. Flame-Stability Diagram for Natural Gas 91.5% CHI, 5.2% C&B, 1.3% CaHa, 0.9% coz, 0.6% Nz, 0.2% 0.2% CiHm 0.1% C4Hs
0.4
0.8
12
1.6
20
24
28
32
GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
Figure 2. Flame-Stability Diagram for Oil Gas 1 33.4% C a n , 37.4% CHd, 14.0% Nt, 15.2% He.
100.000 80,000 60,000 40,000
20 000
GAS CONCENTRATION~-F~ACTIONOF'STOICHIOMETRC
Figure 3.
Predicted Burner Performance in Community Changing from Fuel Gas a to n
To calculate the loci of air-entrainment performance points of burners in a community adjusted to fuel a but using fuel x, the 'coordinates of the two curves on the flame-stability diagram of fuel a are substituted in Equations 1 and 2, and the equations are solved for coordinates for fuel z. Two new curves are obtained, which are called curves lz and 22 in the following discussion. Although flame-stability curves of fuel a are assumed to be known, Equations 1 and 2 do not give flame-stability curves of fuel .c. The flame-stability curves of any fuel must be determined in the laboratory or obtained by some other manner. The new curves are now the new limits of the region containing the airentrainment performance points of all burners in the community under consideration. The flame-stability diagram of fuel must I now be measured experimentally, just as was done for fuel a. If the limits of the new air-entrainment performance region for fuel July 1952
Figure 4. Predicted Burner Performance in Community Changing from Fuel Gas a to x 2 are within the stable flame region of fuel 2, then fuel 2 can be used in place of fuel a. If the new borders are outside the stable flame region of fuel x,it appears possible to determine whether the fuels arenot exchangeable or whether the situation can be remedied by minor changes in burner adjustments, composition of fuel 2, or gas-line pressure. Furthermore, as fuel z may be a mixture of fuel a with a supplemental fuel, the method can be used to predict burner performance with supplemental fuel gases used to meet peak-load demands.
Interchanges between Natural Gas and Oil Gas Interest is growing in the gas industry in the use of oil gas to supplement supplies of natural gas or t o replace it in the event of a pipeline breakdown. Recently, Stookey ( 6 ) commented on the
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
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Table I. Analysis
91.5 5.2 1.3
CaHe GHio
0.2 0.2 0.1
C4Hs
con
0.9 0.6
S I
......
Hz 0 2
Specific gravity Stoichiometric fuel gas, % Gas-line pressure Heating value, B.t.u./cu. foot
8 800
600 400
200
C
I
/INObdrners
h dI 1 1 Flash-back curve for a
!
100 04
Figure 5.
08
12 16 20 GAS CONCENTRATION FRACTION of
24
STOICHIOMETRIC
2R . .
I
_9 7_
Effect on Flash-Back Trouble Zones of Adding Propane or Nitrogen to Oil Gas Natural gas-oil gas interchanges
“paradox” that natural gas is interchangeable with oil gas but oil gas does not appear to be completely interchangeable with natural gas. Others have made a point of the one-way interchangeability of natural gas with respect to oil gas. Once completely understood, the factors behind this seeming paradox may be useful to the industry, because utilities preparing for conversion to natural gas may benefit by converting first to oil gas instead of waiting and converting directly to natural gas. Gas burners on a utility’s lines are adjusted to a particular fuel, so that their air-entrainment performance points fall within the flame-stability diagram of the fuel. When an exchange is considered, the adjustment gas is the one that the burners in the community are adjusted to, not the fuel that the gas company is using predominantly.
Natural Gas
Oil Gas 1
...........
Air CH4 CnHs CzH4 CaHs
i5 100
Composition of Fuel Gases
37.4
.....
33.4
..... . . t . .
..... ..... n.I..
14.0
15.2
............ 0,613 9.07
0.680 10.3
Same for both 1061
966
Canadian Natural Gas Propane-Air ...... 56.7 86.7 ..... 10.4 .....
...... ...... ...... ......
.....
0.1
..... ..... .....
...... 1.9
...... 0.9
0.635 9.09
43.3
..... ..... .
I
.
.
j
.....
1.23
9.28
Same for both
1040
1120
Consider the exchange of “oil gas” No. 1 in place of the natural gas described in Table I. Figures 1 and 2 are the flame-stability diagrams for these two fuels. Figure 3 predicts burner performance in a community changing from natural gas to oil gas 1 without readjusting any burners. Curves 1s and 2s were obtained by substituting in Equations 1 and 2 the coordinates of the flash-back and blowoff curves of natural gas and solving for values of F , and g. for oil gas 1. The loci of air-entrainment performance points of burners in the community for oil gas are between curves 1x and 22. The flamestability diagram for oil gas (Figure 2) is superimposed in Figure 3 on the diagram of performance points. It is seen that the air-entrainment performance points of burners in a community undergoing this exchange will shift to mixtures that are leaner relative to the flame-stability diagram of oil gas than was the case initially for the air-entrainment performance region of these burners in relation to the flame-stability diagram of natural gas. I n addition to the shift to leaner flames, the flame-stability dia ram for oil gas has a much deeper and wider flash-back region &an natural gas, as may be seen by comparing Figures 1 and 2. Because of these two effects, the air-entrainment performance points of many burners fall within the flashback region of oil gas when that gas replaces natural gas on burners adjusted to the latter. Now consider the exchange of natural gas to replace oil gas. Here, oil gas is the base gas-that is, the gas for which the burners in the community were last adjusted. When natural gas is supplied to these burners, the air-entrainment performance points will be richer relative to the flame-stability diagram of natural gas than they were for oil gas. This exchange is illustrated in
60000 40.000
20,000
12
16
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32
GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
Figure 6.
Flame-Stability Diagram for Oil Gas 2
29.1% CH,, 26.2% C2H4, 11.8% Hz, 10.6% N2,0.2% C3Hs, 22.1% C a s
1556
Figure 7.
Predicted Burner Performance in Community Changing from Fuel Gas a to x
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 7
Fuel Gasification Figure 4. No burners will flash back with natural gas, but burners that were close to blowoff with ail gas will blow off with natural gas. The relative weights of the region in Figure 4 showing burners blowing off and the region in Figure 3 showing burners flashing back need evaluation. To do this accurately requires information that is still lacking on the statistical distribution of burners in the flame-stability diagram of fuel a. However, it is known that burners must be adjusted to fuel a, so that they can tolerate chance-increased gas-line pressure without blowing’ off and be turned down without flashing back and that most burners are adjusted for rich flames. On this basis, it would be expected that considerably fewer burners would fall in blowoff in Figure 4 than would fall in flash back in Figure 3.
A community adjusted to oil gas can use natural gas most of the year and still be better fitted to use oil gas a t any time as a supplementing gas or as a replacement than a community adjusted to natural gas. If a gas-distribution system is to be adjusted to natural gas, provision may be made during conversion to improve its tolerance for oil gas. The preferred band of performance points for burners using oil gas and natural gas interchangeably is above line g (the maximum critical boundary velocity gradient for flash back for oil gas). If this condition is met, no burners will flash back with either oil gas or natural gas. The current procedure for gas conversion is to enlarge the diameter of flame ports to some value determined by trial with natural gas. Because flash back with natural gas is so difficult, the flame ports may be enlarged more than necessary for natural gas and to a point where flash back can occur with oil gas. This may explain why one city will find oil gas not so readily exchangeable with natural gas than elsewhere. Flash back can be avoided, as the recommended input rate, B.t.u. per hour, and the number of flame ports are generally known for each burner. The maximum diameter to which the flame may be enlarged for natural gas without causing flash back with oil gas may then be calculated by Equation 3
where A , = fraction of primary air required stoichiometrically in burner stream B, = volume of primary air required stoichiometrically per unit volume of fuel C = recommended maximum input rate or a fraction thereof corresponding to a turned-down flame, B.t.u. per hour
100
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D, = maximum diameter for flame ports, if flash back is to be avoided with oil gas, inch gc = maximum critical boundary velocity gradient for flash back of oil gas, sec.-l Ha = heating value of natural gas, B.t.u. per cu. foot n = number of flame ports p = gas-line pressure, cm. p = density of burner stream, grams per cc. Subscript a = natural gas (adjustment gas) Subscript x = oil gas (substitute gas) Equation 3 is obtained by combining equations 1 and 7 of (2). There are indications that Equation 3 can be used advantageously in designing new natural-gas burners. An alternative, when it can be done without yellow tipping, is to adjust on natural gas for extra-soft flames and thereby provide richer flames on oil gas with fewer burners flashing back. Such adjustments have been found desirable in practice ( 4 ) . In addition to burner adjustment as indicated above, the exchangeability of oil gases and natural gases may be improved by changing the composition of the oil gas with additives. The performance points of natural-gas burners that flash back with oil gas can be located in the flame-stability diagram of nahural gas, as follows (blowoff is not considered important in this exchange): The coordinates of the flash-back curve of the oil gas are substituted in Equations 1 and 2. The equations are solved for the coordinates for natural gas. The resulting curve 2a is plotted with the flash-back curve of natural gas (Figure 5 , A ) . The area between the flash-back curve for natural gas and curve 2a contains the performance points for natural-gas burners that will flash back with oil gas. It is defined as a “trouble zone.” This particular trouble zone contains burners adjusted slightly rich on natural gas. Considering gas-appliance adjustment practice, there may be a large number of these burners, most of them located at the higher g values in the zone. Figure 5 , A , also indicates the type of additives that should be used with oil gas to improve its utilization as a substitute for natural gas. The additive should lower the flash-back curve, have a lower density, and/or require more air stoichiometrically than oil gas. These two effects, particularly the latter, tend to eliminate the trouble zone. Propane, for example, meets two of these requirements. The exact amount of propane that is to be added will vary with the natural gas, the oil gas, and the burner
l Figure 9.
July 1952
Predicted Burner Performance in Community Changing from Fuel Gas Q to x
INDUSTRIAL AND ENGINEERING CHEMISTRY
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natural gas and should be compared with Figures 3 and 7 . No attempt has been made here to evaluate yellow tipping, as its effect on the interchangeability of gases has not been adequately investigated to date. Such studies are contemplated for the future.
Interchanges between Natural Gas and Propane-Air
Figure 10. Flame-Stability Diagram for Propane
adjustments in the city. The limits probably are from 10 to 25% propane. The flame-stability diagram is given in Figure 6 for oil gas 2, which is a mixture containing 22.1% propane, 0.2% propylene, and 77.7% oil gas 1; 7.60% stoichiometric, 0.874 specific gravity. The effect of adding 22% propane to oil gas 1 is shown in Figures 5, B , and 7. The former shows that the trouble zone has been reduced in height, and, what is more important, in width. Figure 7 predicts burner performance for this exchange and should be compared with Figure 3. Nitrogen is another additive to be considered. It has the advantage that it lowers the flash-back curve. The flame-stability diagram is shown in Figure 8 for oil gas 3, a mixture containing 13.0% nitrogen and 87.0% oil gas 1; 11.8% stoichiometric, 0.725 specific gravity. However, oil gases with added nitrogen require less air stoichiometrically and are heavier than originally. This means that the trouble zone for oil gas Kith added nitrogen may be widened to include more burners than the original oil gas, as can be seen by comparing Figure 5, A and C. Figure 9 predicts burner performance for the exchange of oil gas 1plus nitrogen for
Predicting the interchangeability of these two gases can be illustrated by the following example, which permits comparison with the actual experience of a gas company. Howell (4), of the Dominion Natural Gas Co., Ltd., Brantford, Ontario, Canada, has reported that his company found a 43.2% propane-in-air fuel to be exchangeable for natural gas after adjustment of some customer appliances. Lower propane percentages gave excessive flash-back difficulties. The two gases are described in Table I. Figure 10, which has been drawn from data in the literature (1, 3))is the flame-&ability diagram for all propane gases. The stoichiometry, specific gravity, and heating value are different for each propane-air fuel. Figure 10 differs perhaps insignificantly from Figure 1 for natural gases. The latter is virtually identical with the diagram for methane. Accordingly, one average diagram may bc used for all saturated hydrocarbon fuels ( 1 ) or, perhaps better, one for methane and natural gases (Figure 1) and a second for propane and higher hydrocarbon fuels (Figurp 10). This is a minor question which experience will answer. Figure 11 predicts burner performance in a community such as that serviced by the Dominion Natural Gas Co., Ltd. (J), where burners were adjusted to the Canadian natural gas and then supplied with propane-air (Table I). As with oil gas, the performance points of burners undergoing this exchange shift to leaner flames relative to the propane-air flame-stability diagram than was the case initially with reference to natural gas. This effect would worsen as the propane percentage in the fuel is lowered, as can be seen from Equations 1 and 2 and the stoichiometry involved; this explains the trend observed by the above gas utility. The flash-back region for propane-air is more comparable in depth and width to that of natural gas than to the flash-back region for oil gas. Therefore, far fewer burners will flash back in Figure 11 than in Figure 3. Noreover, those that do flash back or blow off can be made to operate stably by reducing the air entrained by the burner. This adjustment is generally feasible and simple and is accomplished by throttling air ports. Figure 11 indicates that most burners in the communitv v d l
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Figure 11. Predicted Burner Performance in Community Changing from Fuel Gas a to x
1558
Figure 12. Predicted Burner Performance in Community Changing from Fuel Gas a to x
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 7
Fuel Gasification perform satisfactorily and that relatively few burners will flash back or blow off. A more exact estimate could be made, if the statistical distribution of burners over a flame-stability diagram were known. The reverse exchange, from propane-air to natural gas on burners adjusted to propane-air, is illustrated in Figure 12. Virtually complete satisfactory performance of burners is indicated, showing the advantage of adjusting a community to propane-air, if it and natural gas are to be used interchangeably. The comments made above for oil gas about advantageous burner adjustment on natural gas also apply t o propane-air. Only flash-back and blowoff limitations on interchangeability of gases have been considered above. The U. S. Bureau of Mines, in cooperation with the American Gas Association, plans to apply fundamental concepts as fast as possible to the remaining limitations, with the hope of making available to the gas industry adequate means of predicting which gases may be supplied satisfactorily to each community.
Acknowledgment The authors wish to thank Bernard Lewis and Guenther von Elbe for suggestions received during the course of this work. This research is supported in part by the American Gas Association (Project PDC-BGU).
Nomenclature d = specific gravity g = boundary velocity gradient, sec. -1
n = number of flame ports p = static pressure head, cm. A = fraction of primary air required stoichiometrically, in burner stream B = volume of primary air required stoichiometrically per unit volume of fuel C = input rate, B.t.u. per hour D = diameter, inches F = fuel-gas concentration, fraction of stoichiometric H = heating value, B.t.u. per cu. foot S = stoichiometric fuel-gas concentration p = density, grams per cc,
Subscripts
a = fuel gas used for adjusting burners c = maximum None = plane of burner ports o = a t plane of fuel-gas orifice x = substitute fuel gas
Literature Cited (1) Bollinger, L. M., and Williams, D. T., Natl. Advisory Comm. Aeronaut., Tech. Note 1234 (1947). (2) Grumer, J., IND. ENG.CHEM..41,2756 (1949).
(3) Harris, M. E., Grumer, J., von Elbe, G., and Lewis, B., “Third
Symposiumon Combustion, Flame and Explosion Phenomena,” p. 80, Baltimore, Williams & Wilgins Co., 1949. (4) Howell, F. D., American Gas Association, Production and Chemical Conference, May 1950. (5) Lewis, B.,and Grumer, J., Gas Age, 105,25 (1950). (6) Stookey, K.W., Am. G a s J . , 173,18 (1950). R ~ C E I V Efor D review October 15, 1961.
ACCEPTED April 7, 1952
Combustion of Carbon in High Temperature, High Velocity Air
Streams
J. M. Kuchta, A. Kant, and G. H. Damon CENTRAL EXPERIMENT STATION, U. S. BUREAU OF MINES, PITTSBURGH, PA.
T h e kinetics of the combustion of spectroscopic grade electrode carbon has been studied for the reactions that occur when cylinders of this material are placed in high temperature, high velocity air streams. Data collected for air velocities between 28 and 540 feet per second and for air temperatures between 900’ and 1200O C. indicate that the absolute reaction rate is proportional to the 0.47 power of the velocity (YO.&) and independent of air temperature. This velocity effect is in good agreement with theoretical calculations in which diffusion is assumed to be the controlling factor. Good agreement is also
obtained between experimental data and calculations based on the film resistance concept. The surface temperature of the carbon increases with increasing air velocity and a constant air temperature. The data indicate that the reaction rate should change from diffusion controlled to chemically controlled as the air velocity is increased to some limiting value. It is concluded that the reaction of carbon and oxygen (air), under the given conditions, is diffusion controlled but that at high air velocities the rate is approaching that to be predicted on the basis of the Arrhenius equation.
THE
the surface, the reaction on the surface, and the transport of the gaseous products from the surface. Thus, the controlling factors of the reaction would be diffusion or chemical reactivity, depending on which is the slower process. At relatively low temperatures the reaction tends t o follow an equation of the Arrhenius type. Thus, for a fist-order reaction
4 study of the combustion rate of carbon a t high temperatures and air velocities has become of interest because of the increasing use of solid fuels for rapid combustion purposes. Most of the work in this field has been limited t o investigations at relatively low temperatures and velocities; considerable data have been obtained a t high temperatures, but at rather low velocities. The primary object of this investigation was to observe the effect of temperature and air velocity in the region of rapid combustion, where the transport of oxygen molecules t o the carbon surface seemingly plays the most vital role. The over-all heterogeneous reaction of oxygen and carbon can be pictured as involving the transport of the gaseous reactants to
July 1952
Essentially, K Orepresents the surface reaction where it is assumed that the primary reaction is the formation of carbon dioxide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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