I
BERNARD GREIFER and RAYMOND FRIEDMAN Atlantic Research Corp., Alexandria, Va.
How Automobile Exhaust Gases Burn An Approach to Air Pollution Control Direct-flame afterburners for automobiles could convert smog-producing hydrocarbons into harmless carbon dioxide and water. But it isn’t as simple as it sounds before translating theoretical possibility into commercial product the range of operating conditions must be known. Investigations with simulated engine exhausts reveal that afterburner inlet temperature is of prime importance
-
installation of noncatalytic afterburners in the exhaust lines of automobiles is a possible means of lowering the concentration of hydrocarbons in the effluent gases, thereby reducing air pollution. However, with the low fuel content of typical engine exhaust gases, maintenance of an afterburner flame under all conditions of engine operation is questionable. This investigation was designed to determine whether mixtures of gases reported in automobile engine exhausts are capable of being burned under the most favorable laboratory conditions, with inlet temperature control and optimum addition of secondary air. Flammability limits of mixtures of fuels can occasionally be predicted by Le Chatelier’s principle ( Z ) , but they are no longer so correlated when carbon monoxide and hydrogen are prominent constituents, as in the case for automobile exhaust gases. I n this investigation, mixtures of methane, isobutane (representing all the paraffin hydrocarbons except methane), ethylene (representing all the olefins), acetylene (representing the alkynes), hydrogen, carbon monoxide, nitrogen, air, carbon dioxide, and water vapor were chosen to simulate reported exhaust-gas analyses (7, 6, 7, 11-16, 20-22) with regard to calorific content, carbon-hydrogen ratio, carbonoxygen ratio, oxygen-nitrogen ratio, carbon monoxide-carbon dioxide ratio, and paraffin-olefin ratio. The simulated engine exhausts were preheated, and their flammability was determined with a flat-flame burner such as has been described by Powling (5, 78) and by Friedman (8).
T H E
Experimental The apparatus and experimental technique were designed to supply a uniformly mixed, heated gas mixture of known composition to the burner. I n the experimental arrangement the composition of the blends was adjusted by metering each gas through a sapphireorifice flowmeter. Flow rates were controlled by diverting a portion of the mixture from the burner. Water vapor was added by bubbling the gas mixture through a flask of heated water; the percentage of water was determined by the ratio of the partial pressure of water vapor (equal to the vapor pressure of the water in the flask) to the total pressure in the flask. The gas blend was then preheated and passed through a thermally insulated line to the burner, which was also heavily wrapped with insulation. The flat-flame burner was particularly suited to the study of very slow flames near the limits of flammability because the geometry and low throughput velocity minimized convective disturbances which might make a flame unstable, and the inner diameter (3.75 inches) was large enough to minimize wall quenching of the flame. The burner had a hollow wall through which silicone oil was circulated from the thermostatically controlled bath which also constituted the preheater. The circulating oil also heated both the body of the burner and the gas-flow line and cooled the top rim of the burner, thus maintaining the gas at the desired temperature. Five disks of 100-mesh stainless steel wire cloth spaced throughout the 18-
inch length of the burner assured laminar flow with a flat velocity profile. A 96% silica glass cylinder and a perforated copper plate provided a zone of uniform back pressure above the flame and minimized convective disturbances from the surrounding atmosphere. The plate was cooled by circulating 180’ F. water. Without cooling, the plate would become heated by the flame and radiate heat to the burner, causing uncontrolled preheating. On the other hand, overcooling would cause water vapor from the combustion products to condense and run back into the burner. The gas mixture was ignited manually a t the top of the burner, the copper plate being raised momentarily. The flame stabilized in the form of a thin, motionless, horizontal disk in the space between the burner rim and the top plate. The gas composition and flow rate were varied slowly and corresponding changes in the flame were noted ; the composition at which the flame could no longer be stabilized on the burner, a t any gas flow rate, was taken as the flammability limit.
Results The gas mixtures investigated were based to a large extent on the mass spectrometer analyses of automobile exhausts from the literature (73, 20). Initial mixtures representing actual exhaust-gas analyses were modified in various ways by the addition of oxygen and diluent gases so as to approach fuel-deficient flammability limits. For those compositions which would give stable combustion, although close VOL. 51, NO. 8
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AUGUST 1959
953
CRITICAL
minimum chemical reaction rate in order to be self-propagating, and this rate is a function of the flame temperature, among other things ( 3 , 4 ) . The adiabatic flame temperature of any mixture in which all the fuel is burned completely may be calculated by the following thermodynamic relation (valid for flame temperatures below 2000" K.) :
FLOW
NON-CRITICAL
FLOW
z: ni h,i
BURNER COPPER T O
COPPER TUBING CIRCULATING 180' F WATER
FLAT FLAME&
WCOR
100-MESH S. S. SCREEN
CARBON DIOXIDE
DISCARD
CIRCULATING OIL-
v
GLASS
ELlEF VALVE CHECK
VALVE
PUMP
WATER-VAPOR
I
INJECTOR
PREHEATER
Blending, preheating, and combustion of 10-component gas mixtures were carefully designed and controlled
to limit mixtures, burning velocities were determined. These were of the order of 6 to 15 cm. per second. h'o direct measurment of the completeness of combustion was attempted, as reactions of simple mixtures burning as stable flames in this range of burning velocities and stoichiometry have repeatedly been shown to go to completion, insofar as oxidation of carbon and hydrogen is concerned. Nitrogen compounds constitute an exception. Ninety-four compositions within the ranges indicated in Table I were considered to represent valid limit mixtures. Flammability data were taken over a range of burrier inlet temperatures from a lower limit of 86" F. (303' K.) to an upper limit of 440' F. (500' K.), determined by the desire to avoid preflame reactions upstream of the flame (77). The detailed operations and specific limit mixtures obtained have been described elsewhere ( 9 ) . The resulting data were correlated (Figure 1) in terms of the calculated adiabatic flame temperature and a correlation parameter designated F. The
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Table I.
Range of Gas Concentrations in Limit Mixtures Concentration,
Gas
02 Nz COP
co
Hz CH4 CzH. Isobutane CzHz HzO vapor
Mole % 5.8P19.9 29.09-81.21 0 -40.61 0 - 8.09 0 - 3.50 0 - 1.91 0.27- 1.43 0 - 1.18 0 - 0.35 1.92-12.33
calculated adiabatic flame temperature is defined as the temperature to which the product gases would be heated during constant-pressure adiabatic combustion, if the mixture could be burned. Heat losses are ignored in this calculation, making the calculated temperature an upper limit which is approached, but can never be exceeded, in practice. The choice of flame temperature as a correlating parameter is based on the concept that the flame requires a certain
INDUSTRIAL AND. ENGINEERING CHEMISTRY
=
zyj ( H i
-
ETO)]
(1)
Here, nC is the number of moles of the z'th fuel component in the mixture, h,, is the molar heat of combustion at constant pressure and 298" K. of the z'th component to the products gaseous carbon dioxide and gaseous water, j 3 is the number of moles of the I'th combustion product ( y ? values being related to n, values by stoichiometric balances), and ( H , - H o ) ?is the difference in enthalpy of thefrh combustion product between the value at flame temperature (T,)and the initial temperature of the mixture ( T O ) . As the (Hf)2 term depends upon the flame temperature. Equation 1 may be solved for this temperature by an iterative process. The summation is taken over all fuel components, including ethylene. Values of h, vary with temperature less than 1% over the range of burner inlet temperatures considered ; hence 298 " K values may be used with satisfactory accuracy. Tabulations of h, for the common gases are readily available (70, 79). All the adiabatic flame temperatures were between 1200" and 1620 ' K.. increasing with increasing paraffin hydrocarbon content (relative to the other fuels) and decreasing with increasing hydrogen content (relative to the other fuels). The effect of gas composition is included in the empirically developed correlation parameter, F, which depends upon the relative contributions to the total enthalpy release of the different species in the fuel mixture. By definition :
The parameter F may be understood by the following explanation. Mixtures of the fuels hydrogen and acetylene with air in stoichiometric proportions have very high flame speeds, implying rapid combustion kinetics; stoichiometric mixtures of air with methane. isobutane. or carbon monoxide have low flame speeds. and presumably slower combustion kinetics; ethylene-air mixtures have intermediate flame speeds. The parameter F increases with increasing pro-
A U T O M O B I L E E X H A U S T GASES portions of slower-burning fuels and decreasing proportions of faster-burning fuels. Figure 1 shows that flame temperatures increase with increasing values of F. This is to be expected, as a mixture with intrinsically slower kinetics, on being diluted, will cease to burn a t a higher temperature than a mixture with faster kinetics, the flammability limit being viewed as the condition for which the rate of heat loss from the flame becomes too large relative to the rate of heat generation. In defining F, the mixture of six fuel gases has been divided into three types of gases: slow-burning (carbon monoxide, methane, and isobutane), fastburning (hydrogen and acetylene), and the remaining ethylene, which burns a t a rate intermediate between the two. The six fuel gases employed in this investigation are not the only ones found in automobile exhausts, but they do represent the classes of compounds present. The F value of genuine automobile exhausts may be calculated by considering alkanes to be in the slow-burning group (and their normalized heats of combustion, or 2 values, added with carbon monoxide, methane, and isobutane); alkynes to be in the fastburning group (and their normalized heats of combustion, or 2 values, subtracted from the above total, along with hydrogen and acetylene) ; and alkenes and alkadienes to be in an intermediate category, being treated like ethylene. The effect of initial mixture temperature merits discussion. Data points from both preheated and nonpreheated gas mixtures group together in Figure 1. This is a consequence of the nature of the flame-temperature parameter, which is a function of the initial mixture temperature as well as the mixture composition. The correlation is therefore known to be valid for inlet temperatures from 86' to 440' F. and is believed to be usable for higher inlet temperatures. This result does not mean that burner inlet temperature has a negligible effect on flammability; on the contrary, the data show that it has a strong effect, as preheating a given mixture substantially influences its flame temperature. The correlation exists because those limit mixtures at the lower inlet temperatures have higher chemical heating values, and about the same flame temperatures, as the mixtures a t higher inlet temperatures. The inlet temperature necessary to bring a given mixture just to the limit correlation line may be predicted by calculating F for the mixture (Equation 2 ) , obtaining the corresponding flame temperature from Figure 1, and calculating the inlet temperature consistent with this flame temperature (Equation 1).
For rapid estimates, the latter step may be facilitated by the approximate rule that a 100' F. rise in inlet temperature corresponds to an 80' F. rise in flame temperature. Further details of the procedure are available elsewhere (9). Table I1 includes calculated adiabatic flame temperatures and values of F for a number of typical exhaust-gas analyses taken from the literature. These analyses were considere secondary air added to make a stoichiometric mixture, unless sufficient air was already present. The nature of the flammability correlation shows that the optimum proportion of secondary air to be added is that amount which provides just the stoichiometrically necessary quantity of oxygen. Further air additions would reduce the flame temperature and decrease flammability. All mixtures were assumed to be at an inlet temperature of 500' K. (440' F.). Table I1 and Figure 1 show that, for the assumed inlet temperatures, deceleration and idling exhausts should be flammable. Cruising and acceleration apparently produce exhaust gases which do not burn under these conditions. The flame temperature of any gas mixture may be brought up to the correlation line, and even above it, by assuming a higher gas inlet temperature, because the system cannot distinguish between thermal energy supplied from the chemical reaction or from an external source. It is calculated that the acceleration exhaust may be made to burn if it is first heated to 890' F., while
it would be necessary to heat the cruising exhaust to 1240' F. to achieve this result. Slow chemical reactions would be expected to occur in fuel-containing gases as hot as 1240' F., so that thecomposition and probably the temperature would be continuously changing with time (77). A complete analysis of this situation would require a detailed condideration of the pertinent heat-transfer conditions and chemical kinetics. However, a reasonable approximation of flammability may be made without taking these complex factors into account. I n extrapolating the results of this investigation to actual automobile exhaust gases, it is important to realize that the test mixtures have been burned in the laboratory under the most ideal conditions of fuel-air mixing, laminar gas flow with flat velocity profiles, and absence of turbulent disturbances a t the burner mouth. Although engineering designs of noncatalytic afterburners may approach these ideal conditions to a greater or lesser degree, it is extremely unlikely that they could exceed the efficiency of the flat-flame burner in maintaining a self-propagating flame. Therefore, the results of this investigation may be considered to be a boundary condition: if a gas mixture does not burn on the flat-flame burner, it cannot be burned in any noncatalytic afterburner; while if it does burn on the flatflame burner, this does not mean necessarily that it can be made to burn inside a particular afterburner. Such infor-
Figure 1. Correlation of limit mixtures b y means of adiabatic flame temperature and correlation parameter F. Average mixture which falls below the curve will not burn VOL. 51, NO. 8
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corresponding to conditions of deceleration and idling may be burned in an ideal afterburner by introducing the gas mixtures at 440’ F.; for acceleration and cruising exhausts, inlet temperatures should be 890’ and 1240’ F., respectively. Acknowledgment
This research was supported by the Air Pollution Foundation, San Marino, Calif., and the guidance provided by W. L. Faith of that organization is appreciated. The flame of this burner floats motionless as a thin horizontal disk in the space between the burner rim and the top plate
literature Cited
(1) Chandler, J. M., Cannon, W. A., Neerman, J. C., Rudolph, A., J . Air Pollution Control Assoc. 5 , 65 (1955). (2) Coward, H. F., Jones, G. W., “Limits of Flammability of Gases and Vapors,” U. S. Bur. Mines Bull. 503, 5-8 (1952). (3) Egerton, A. C., Powlin , J., Proc. Roy. Sac. (London)A-193, 172 fi948). (4) Zbid.. D. 190. (Sj Egerton, A. C., Thabet, S., Zbid., A-211, 445 (1952). (6) Elliot, M. A,, Nebel, G. J., Rounds, F. G., J. Air Pollution Control Assoc. 5 , 103 (1955). (7) Fitton, A,, Roy. SOC. Health (Gr. Brit.), June 13, 1956. (8) Friedman, R., “Fou rth Symposium (International) on Combustion,” p. 259, Williams & Wilkins, Baltimore, I
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1951
mation must come from an evaluation of each afterburner design. Conclusions O n the basis of correlated combustion tests predictions of automobile exhaustgas flammability may be made from a knowledge of the composition and afterburner inlet temperature. If the calculated adiabatic flame temperature of the exhaust gas, taking into account preheat temperature, is less than a critical value, it cannot burn. This value, ranging from 1200’ to 1620’ K.,
Table I I .
has been empirically related to the exhaust-gas composition. If the flame temperature exceeds this value, the gas mixture will burn in the laboratory on a flat-flame burner; whether it will burn in a particular practical afterburner must be determined by trial. The optimum proportion of secondary air to add to an exhaust mixture entering an ideal afterburner is the stoichiometrically required quantity, as excess air lowers the flame temperature. O n the basis of certain reported analyses for automobile exhausts (73, ZO), the present results imply that exhausts
Flame Temperatures and Correlation Parameter F for Typical ExhaustGas Analyses” Addition of secondary air was assumed
Reportcd Compositions Operating condition Ref. Deceleration [ZO, Lab. A ] Deceleration 120, Lab. B] Deceleration ( 73)
a
Idling Cruising
( 73)
Acceleration
(73)
(73)
For 440’ F. (500’ K.) gas inlet temperature.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Calcd. Tr, K.
Fb
1744 2087 1350 1431 974 1159
42 43 39 35 48 49
Dimensionless.
(9) Greifer, B., Friedman, R., “Combustiblity of Simulated Automobile Exhaust Gases,” Air Pollution Foundation (Los Angeles), Rept. 2 5 (1958). (10) Hilsenrath, J., ed., “Tables of ‘Thermal Properties of Gases,” Natl. Bur. Standards, Circ. 564 (1955). (11) Hutchison, D. H., Holden, F. R., J . Air Pollution Control Assoc. 5 , 71 (1955). (12) Larson, G. P., Chipman, J. C., Kauper, E. K., Ibid.,5 , 84 (1955). (13) Magill, P. L., Hutchison, D. H., Stormes, J. iM.,Proc. 2nd Natl. Air Pollution Svmaosium. Pasadena. Calif.. 1952, p. 71.‘ (14) Neerman, J. C., Parsons, J. L., Bryan, F. R.. J.Air Pollution Control Assoc. 6. 38 (19 5 6 ) . (15) Patton, H. W., Lewis, J. S., Proc. 3rd Natl. Air Pollution Symposium, Pasadena, Calif., 1955, p. 74. (16) Payne, J. Q., Sigworth, H. W., Proc. 2nd Natl. Air Pollution Symposium, Pasadena, Calif., 1952, p. 62. (17) Pease, R. N., “Equilibrium and Kinetics of Gas Reactions,” pp. 209-11, Princeton Univ. Press, Princeton, N. J., 1942. (18) Powling, J., Fuel 2 8 , 25 (1949). (19) Rossini, F. D., ed., “Selected Values of Physical and Thermodynamic Properties ofHydrocarbons and Related Compoundq,” Am. Petrol. Inst. Research .Project 44, pp. 444-59, Carnegie Press, Pittsburgh, Pa., 1953. (20) Rounds, F. G., Bennett, P. A,, Nebel, G. J., J. Air Pollution Control Assoc. 5 , 109 (1953). (21) Twiss, S. B., Teague, D. M., Bozek, J. W., Sink, M. V., Zbid.,5 , 75 (1955). (22) Walker, J. K., O’Hara, C. L., Anal. Chem. 27, 825 (1955). A
RECEIVED for review October 9, 1958 ACCEPTED March 30, 1959 A.1.Ch.E. Meeting, Salt Lake City, Utah, September 1958.
