2514
D. B. Olson and
W.C.Gardiner
An Evaluation of Methane Combustion Mechanisms D. B. Olson” and W. C. Gardiner, Jr. Department of Chemistry, University of Texas, Austin, Texas 76712 (Received April 26, 1977) Publication costs assisted by the Robert A. Welch Foundation and the Petroleum Research Fund
Eight methane combustion mechanisms and their associated rate constant sets were studied by performing numerical simulations of a number of shock tube experiments. Seven of the mechanisms were taken from the recent literature, and one resulted from our own survey of the literature rate data for the combustion of H2, CO, CH4, C2H6, and CzH4. Two experimental parameters from each of seven different investigations were chosen to form an experimental data base with which to compare the mechanisms. The results of the calculations demonstrate the superiority of three mechanisms, which among themselves differ mainly in the methyl oxidation, formaldehyde decomposition, and Cz hydrocarbon reaction pathways.
Introduction The oxidation of methane has been the subject of numerous investigations in recent years due both to its practical importance and to the intrinsic chemical interest of a simple hydrocarbon combustion system. A considerable amount of experimental data is available in various forms, such as ignition delay times, or emission or absorption profiles of various species. Several mechanisms and rate constant sets have been published. In most instances, however, the mechanisms have been tested only on the data from one or two experiments and/or laboratories. Our aim was to test, by computer modeling, the more recently proposed mechanisms with a broader set of data than had been used before, with the goal of finding out if any mechanism would be capable of describing all the experimental data on methane combustion in the literature. In addition, it was anticipated that the computer simulations could be used to suggest which reactions or rate constants were most in need of experimental study for purposes of clarifying the reaction mechanism. As in any kinetic study, we did not expect to find the “correct” mechanism. What we hoped to find was a mechanism and rate constant set with which we could successfully model the CH4/02system over a wide range of temperature, pressure, and stoichiometry; a mechanism which reflected the whole current state of knowledge about H2/02/CO/ C02 reactions; and one which used the best available hydrocarbon pyrolysis mechanistic information and rate data. Method In order to facilitate comparisons between calculation and experiment for seven mechanisms and multiple data sets we had to choose suitable parameters from each experiment to model. This naturally restricted our data base to a small fraction of the total amount of experimental information available, but it was our expectation that a more meaningful evaluation could be made by comparing a few well chosen data points representing different types of experimental information rather than through a detailed study of only one or two types of experiments. The data chosen represent induction-period kinetics (induction times and early growth constants), reaction-zone kinetics (CO flame spectrum emission), and later-time kinetics (C02 emission and CH4 absorption). The conditions of the experimental data selected ranged from 1600 to 2500 K in temperature, 0.15 to 10.0 atm in pressure, and stoichiometric fuel-oxygen ratios, 4, from very rich, 4 = 17, t o very lean CO-containing mixtures seeded with trace amounts of CHI. The Journal of Physical Chemistry, Vol. 8 1, No. 25, 1977
The computer routine used a Gear-type integration of the set of differential equations describing the chemical kinetics and steady, variable-area, inviscid, compressible flow for incident shock waves, and constant density conditions for reflected shocks. Thermochemical and thermophysical properties were computed as functions of temperature using accurate polynomial representations of JANAF‘ data where possible. When JANAF data were not available, our own best estimates (except for CHBO data obtained from ref 2) were used, again in polynomial form. Computations were performed on a CDC 6400/6600 system. Comparisons of data and simulations were performed as follows. For each parameter selected and the corresponding set of conditions (temperature, mixture composition, pressure, and flow model), simulations were performed using each mechanism. The result in each case was expressed in logarithmic form by the match parameter pM = log (calculation/experiment).3
Data Base The data base of 14 experimental parameters was chosen from seven recent shock tube investigations with the aim of selecting the widest possible variety of experiments and conditions. Dean and Kistiakowsky4 studied CO and COz thermal emission behind incident shock waves in C 0 / 0 2 / A r mixtures containing 180 or 500 ppm added CHI. From their data we chose to model (a) the C02 induction time, and (b) the COz exponential growth constant, both a t T = 2000 K in a CH4/02/CO/Ar = 0.05/2/4/94 mixture. The induction time was defined as the time until [CO,] = 2.7 X 1O-I’ mol ~ m - and ~ , the exponential growth constant was measured during the period when 2.7 X < [CO,] < 2.7 X mol ~ m - ~ . Jacobs and Gutman5i6 measured end-on exponential growth constants from CH* and C2* chemiluminescent emission behind reflected shock waves over the range 1700 < T < 2560 K, 0.3 < P < 2.0 atm, and 0.2 < 6 < 12. From the large amount of data available we chose to model a CH4/02/Ar = 1/1/98 mixture at (a) T = 1800 K, P = 1.5 atm, and (b) T = 2400 K, P = 1.9 atm. T o simulate the experimentally measured chemiluminescence growth constants we used twice the growth constant of the OH concentration. From the many possible sources of induction times we selected the incident shock wave experiments of Cooke and william^.^ A CH,/O2/Ar = 1.7/3.3/95 mixture a t (a) T =-1750 K, P = 0.28 atm, and (b) T = 2000 K, P = 0.33 atm were modeled. The induction time was defined in our
2515
Evaluation of Methane Combustion Mechanisms
calculations as the time at which the density gradient became negative; other possible definitions were confirmed to give very similar results. Jachimowski8 observed CO flame spectrum emission intensity behind incident shock waves over the range 1790 < T < 2580 K and 1.2 < P < 1.7 atm. A 2200 K experiment in a CH4/02/CO/Ar = 1/2/2/95 mixture was chosen for our data base. The parameters calculated were (a) the time until the maximum in the [CO] [O] product, and (b) the [CO] [O] magnitude at the time of maximum emission. Brabbs and Brokawg measured exponential growth constants of CO flame spectrum emission behind incident shock waves in C 0 / 0 2 / A r and CO/C02/02/Armixtures with various amounts of added CHI. We chose to model two different mixtures under the same conditions, T = 1600 K and P = 1.0 atm. The mixtures were CH4/O2/ CO/Ar = (a) 0.2/20/1/79 and (b) 0.20/20/1/79. TsuboilO and Tsuboi and Wagnerll reported a large amount of experimental induction time data and emission signals behind reflected shock waves over a wide range of composition and pressure. Tsuboi12additionally reported CH3 UV absorption data in the form of CH3 growth rates during the induction period. We chose to simulate these CH3 growth rates in a CH4/Oz/Ar = 1/2/97 mixture at P = 10 atm for (a) T = 1600 K and (b) T = 1820 K. The final data were taken from CH4 IR laser absorption experiments performed in our own laboratory in a very rich, CH4/02/Ar = 9/1/90 mixture at P = 0.25 atm. The details of this work are to be published e1~ewhere.l~ We modeled the change in absorption from 20 to 200 p s behind the incident shock waves a t (a) T = 2200 K and (b) 5” = 2500 K.
Mechanisms Eight mechanisms were chosen for study in this work. Skinner, Lifshitz, Scheller, and Burcat (SLSB)14 reported a 1 2 species, 23 reaction mechanism and rate constant set deduced from fitting CH4/OZ/Arand CH4/ 02/H2/Ar induction times and single pulse shock tube product distributions. Basically their mechanism was a H 2 / 0 2 / C 0 scheme with eight added hydrocarbon reactions. Methyl oxidation was treated by a global reaction and so CHzO and CHO were not considered. The methyl recombination reaction was the only consideration of Cz species. Cooke and Williams (CW),15in an extension of the work of Higgin and Williams,16 presented a 33 reaction mechanism considering 20 species. They performed comparisons with CH4/02/Ar and C2H6/02/Arinduction times. Good results were obtained when the main oxidation reaction of methyl, CH3 + O2 = CHzO + OH, was given a rate constant in the range from 1O’O to 10’l cm3 mol-ls-l. A limited consideration of C2 species was made. Jachimowski (J)8developed a kinetic scheme to model his CO flame spectrum data and ignition delays taken from the literature. The mechanism had 21 reactions for 13 species. No C2 species were considered. The methyl oxidation by 02 was similar to that considered by CW, with the rate constant estimated to be l o l l cm3 mol-l s-l. Bowman (BY7 presented a 30 reaction mechanism for 16 species deduced from his experiments on CH4/OZ/Ar mixtures. Rate constants for five reactions were varied to obtain agreement with the experimental times of occurrence and magnitudes of 0 and OH radical concentration overshoots. The rate of methyl reaction with molecular oxygen was inferred to be 1O1O cm3 mol-1 s-l at 2000 K, somewhat smaller than the rate determined by CW or by J. The rate of H + 0 2 = OH + O2 required was five times
the value of Schott.18 Methyl recombination and 0-atom attack on ethane were included although no reaction pathways for the resulting ethyl radical were provided. Brabbs and Brokaw (BB)9reported rate constants for methyl oxidation which, when combined with the mechanism and rate constants taken from their earlier work,lg give a 12 reaction mechanism for 14 species. They concluded that the analysis of their CO flame spectrum exponential growth data was compatible only with the reaction CH3 0’ = CH30 0 followed by either CH30 M = CH20 H M or CH30 CO = CH3 + COS. Rate constants for these reactions were reported. We used the CH30 decomposition as the second step in our calculations. BB made no consideration of Cz species. In an EPA technical report, Engleman (E)’ presented the results of an extensive survey and evaluation of the literature data on 322 methane-air reactions for 25 species. Primary consideration was given to reactions of importance in stoichiometric combustion from 1500 to 2500 K at 1atm. For the E mechanism, we selected the reactions flagged in the report as “probably” or “possible” important, excluding the reactions of nitrogen containing species and of CH. This reduced the mechanism to one of 54 reactions and 16 species. Four additional reactions (94, 231, 254, and 322 in the Engleman report numbering system) were removed in order to use our kinetics program as dimensioned for 50 reactions. Careful tests were performed to ensure that these four reactions were unimportant for the present calculations. The mechanism contained seven CH30 reactions for which rate data were obtained from estimates by Benson, Golden, and Shaw.’O No C2 species were considered. Tsuboi (T)l2reported a 39 reaction mechanism considering 17 species. Comparison of induction times for several mixtures over a wide density range resulted in rate constants for methyl oxidation and for formyl decomposition. The rate constant for the CH3 + O2 reaction was deduced to be 1010.6cm3 mol-’ s-l at 2000 K, in good agreement with that of CW and of J. Since the calculations of the present work were for densities considerably less than those considered by Tsuboi, we replaced his reported pressure dependent unimolecular CH4 decomposition reaction with the recent bimolecular rate constant reported by Roth and Just.21 More detailed consideration of the Cz hydrocarbon reactions was made by Tsuboi than in any of the previous mechanistic studies. For our own work, the OG mechanism, we performed a literature survey and assembled a mechanism and rate constant set reflecting what we feel are the best available rate data. Starting with a H2/02/CO/C02/Ar kinetic scheme2’ (16 reactions), we added the CH4 decomposition and oxidation steps (4 reactions), CH3 oxidation (3 reactions), and CH20 and CHO reaction pathways (9 reactions). At this point the mechanism could be considered to be an extension of that of Bowman.’ To this set we added reactions for the decomposition and combustion of C2H6, C2Hs, and CzH4 (12 reactions), and an abbreviated set of C2H3 and CzH2reactions ( 5 reactions), giving the mechanism a total of 49 reactions considering 20 species. A listing of the OG mechanism and rate constant set is available from this journal as supplementary material (see paragraph at end of text regarding supplementary material). Unimolecular decomposition reactions pose a special problem in formulating any mechanism to be used over a variety of conditions. Care must always be exercised to make sure that the rate expressions for these reactions are applicable for the densities and temperatures to be
+
+ + +
+
+
The Journal of Physical Chemistry, Vol. 87, No. 25, 7977
2518
D. B. Olson and W. C. Gardiner PM
MECHANISM
-1
SLSB
cw J
B BE
E T
PM
r
0
+1
?
4 41
'
MECHANISM
-1 SLSB
cw J
B
BB E T
0G
OG Expt Expt
Flgure 1. Comparison of computed and experimental parameters for the data of Dean and Kistiak~wsky,~ in the form of the match parameter, p M =I log (calculated parameter/experimental parameter). The eight mechanisms are denoted by their author abbreviations as noted In the text. The bottom entry, Expt, represents the base 10 logarithm of the estimated error bound of the data. Two bars are presented for each mechanism representing the pMresuits for the two parameters selected from this experiment to model. Only one bar is given corresponding to Expt since the estimated errors for the two measurements were found to be similar. The data parameters modeled are (a) the COPinduction time, and (b) the COPexponential growth constant, both at 2000 K in a CH4/02/CO/Ar = 0.05/2/4/94 mixture. The length of the bar represents the degree to which the mechanism fails to simulate the data parameter, with p M ranging from -1 to +1 for errors in the calculatlons of 1/10 and 10; a value of p M = 0 represents an exact match.3
modeled. Most shock tube experiments are performed at densities and temperatures such that unimolecular reaction rate constants may be in a pressure-dependent falloff region and neither the high pressure nor the low pressure limiting rate expression applies. RRK or RRKM calculations may be used to obtain rate expressions for the conditions of interest, but these calculations would be too cumbersome to include in routine modeling. In addition, shock tube studies often cover wide ranges of temperature, but at almost constant density. The degree of falloff from the high pressure rate constant for this situation increases with increasing temperature, requiring a nonlinear rate expression with significantly lower activation energy than that of the high pressure rate constant. The high pressure rate constant, in any case, is almost never applicable and its use can result in an overestimate of the rate by orders of magnitude, The unimolecular decomposition rate constants for the OG mechanism were treated as follows. For CHI we used the bimolecular, low-pressure limit decomposition rate constant reported by Roth and Just." The pressure dependence of this reaction obtained by Hartig, Troe, and Wagnerz3indicates that the bimolecular rate expression is valid for pressures less than a few atmospheres. Only in the simulations of the Tsuboi12 experiments would we expect this rate constant to be an overestimate, and even for these conditions we estimate the effect to be small. The decompositions of C2H6 and CzHS are at neither the high nor the low pressure limit for the conditions of the present calculations. We used for the CzH6 decomposition the bimolecular rate expression recently measured in our l a b o r a t ~ r ywhich , ~ ~ shows the increasing falloff from the high pressure limit with increasing temperature. This experiment covered the range of conditions 1330 < T < 2500 K, and 0.10 < P < 0.50 atm. From the same investigation we also used a CzH6decomposition rate constant obtained by performing RRK calculations of the The Journal of Physical Chemistry, Vol. 81, No. 25, 1977
t
t 0
tl
Flgure 2. Same as Figure 1 for the data of Jacobs and G ~ t m a n . ~ ~ ~ The data parameters are the exponential growth constants for a CH4/02/Ar = 1/1/98 mixture at (a) T = 1800 K, P = 1.5 atm, and (b) T = 2400 K, P = 1.9 atm. MECHANISM
PM
I
-1
tl
SLSB
cw J
BEB
1
T
OG Exp t
!
t+l
Figure 3. Same as Figure 1 for the data of Cooke and Williams.' The data parameters are the induction times for (a) T = 1750 K, P = 0.28 atm, and (b) T = 2000 K, P = 0.33 atm in a CH4/OP/Ar= 1.7/3.3/95 mixture.
amount of falloff from the high pressure limit for the same temperature and pressure range as given above. We used bimolecular rate constants for the decomposition of CH20, CHO, CzH4,and C2H3.
Results The logarithmic error parameters pM from calculations of each experimental parameter for each of the eight mechanisms are shown in Figures 1-7. For each mechanism there are two bars representing the pM for the two calculations for each experiment. Examination of Figures 1-7 shows the performance of each mechanism for the experiments simulated in this work. Some experiments are seen to be fairly well modeled by most mechanisms (cf. Figure 4) while others are only very poorly described (cf. Figures 6 and 7). In order to provide an overall measurement of the performance of each mechanism, we calculated the average deviation for the 14 simulations, Zlpn/il/l4, as shown in Figure 8, a measure of the absolute value of the errors in the calculations. OG is seen to be the only mechanism to simulate the data within the experimental error, although T and B also give good results. In order to investigate the direction of the errors in the simulated results, we calculated the mean, pM = ZpM/14, and standard deviation of the pM errors as shown in Figure
2517
Evaluation of Methane Combustion Mechanisms PM
PM MECHANISM
7
HECHANISN
1
-1
-1
tl
SLSB
SLSB
cw
cw
J
J
B
B
BB
BB
E
E
T
T
OG
OG
Expt
Expt
Figure 4. Same as Figure 1 for the data of Jachimowski.' The data parameters are (a) the time until the maximum in the [CO] [O] product and (b) the magnitude of the [CO] [O]product at maximum, both for a T = 2200 K, P = 1.4 atm, CH4/O2/CO/Ar = 1/2/2/95 mixture.
I
i
Figure 7. Same as Figure 1 for the data of Olson and GardIner.l3 The data parameters are the change in IR absorbance (mostly due to CH4 disappearance) from 20 to 200 ps in a P = 0.25 atm, CH,/O2/Ar = 9/1/90 mixture at (a) T = 2200 K and (b) T = 2500 K.
1 PM 1 / 1 4
PM HECHANISH
I
b
-1
&
SLSB
cw J B BE
E T
OG Expt
tl
Figure 5. Same as Figure 1 for the data of Brabbs and Brokaw.' The data parameters are the CO flame spectrum emission exponential growth constants at T = 1600 K, P = 1.0 atm for (a) CH,/O,/CO/Ar = 0.02/20/1/79 and (b) CH,/O2/CO/Ar = 0.20/20/1/79. KCHANISEI
+l
MECHANISM
7
t
0
0.2
0.4
0.6
SLSB
cw J B 88
E T OG
Expt
Figure 8. The average deviation of the pMerrors for the 14 simulations of Figures 1-7. The mechanisms denoted as in Figure 1. The Expt bar represents the average deviation of the estimated uncertainty of the experimental data.
PM I
I
-1 SLSB
cw J B BB E
T OG Expt
Figure 6. Same as Figure 1 for the data of Tsuboi.'' The data parameters are the CH3 growth rates during the induction period for a P = 10 atm, CH,/02/Ar = 1/2/97 mixture at (a) T = 1600 K and (b) T = 1820 K.
9. Here the signs of the errors are kept to see whether the mechanisms are too fast or too slow. Positive error is defined as fast (too short induction time, too large growth constant or rate, or too large change in absorption). We
observe that all of the mechanisms predict too rapid reaction. The relative ranking is the same as in Figure 8 except for BB, which shows both slow and fast errors almost equally, and so considerably reduced mean error. Again OG is the only mechanism whose calculations fall within the experimental error limits.
Discussion The data chosen for simulation cover a wide span of composition, temperature, pressure, and experimental methods. Probably the fraction of data from CO flame emission studies is too large, but the usefulness of this type of data seems to be well founded. Extensions of the analysis introduced here could include some of the detailed composition or pressure dependencies which have been mea~ured.~~'J~ For our purposes, however, the present data base is satisfactory. The selection of a relatively small number of experimental parameters for comparison reduces the amount of computer output such that we could understand the results of a simulation more readily. Our results confirm that this method of intercomparing mechanisms is highly useful. Given the method of intercomparison, what are the main results? Essentially we find one mechanism that gives better results than the rest, with two others also achieving The Journal of Physlcal Chem/sfty, Vol. 81, No. 25, 1977
2518
D. B. Olson and W. C. Gardiner
PM t
a
T r
MECHANISM
-o!5
0
0.5
I
1 .o
J
B
BB E
Expt OG
Figure 9. The mean of the pMerrors for the 14 simulations of Figures 1-7, +/- one standard deviation. The mechanisms are denoted as in Figure 1. The Expt bar represents +/- one standard deviation of the estimated errors for the experimentaldata. Positive pMbeing defined as more rapid reaction, all mechanisms are seen to calculate too rapid reaction.
a satisfactory overall performance. We find that it is possible to simulate this set of data with an average deviation less than that of the experiments, although not every individual calculated parameter was within the estimated error bounds of the data. This indicates that many parts of the more successful mechanisms are probably correct. We shall see below, however, that our data base is not sufficient to determine details of some reaction pathways. Only the effective rates are defined. An examination of the individual mechanisms is necessary in order to relate the various reaction rates and pathways to the results of our calculations. We took mechanisms developed for specific limited applications and applied them to conditions far from what was intended by the original workers. Our aim here, however, was to find a general mechanism and so it is of interest to discover where limited mechanisms fail, since this provides information about what reactions or rate constants are lacking. Before making a detailed examination of some of the mechanisms, a general overview will be useful. Several significant features were observed upon comparing reaction profiles from the eight mechanisms. First we noted very different post-reaction-zone species concentrations. The profiles from SLSB, CW, and BB did not show a rapid decrease of hydrocarbon concentrations during the reaction zone to very low levels. Instead, there was an equilibrium of these species a t levels larger than expected. This behavior we interpret as the result of incomplete pathways for the combustion of CH3, CHzO, CHO, and higher hydrocarbons, allowing these species to continue to exist in an oxidizing environment. Second, we observed a direct relation between overall performance and the extent of consideration of the reactions of C2 species. Indeed, the mechanisms rank in Figure 9 in the same sequence as in order of increasing Cz hydrocarbon reactions, except for CW, which seems to be a special case. A closer examination of the CW mechanism leads us to conclude that while it does contain a large set of C2reactions, the thermal decomposition of most hydrocarbon species was omitted, reaction with molecular oxygen being the major destruction pathway for these species. The C2 reactions would have been expected to be of importance in rich mixtures, but the present results lead us to conclude that they are also important under conTha Journal of Physical Chemistw, Vol. 81, No. 25, 1977
ditions of excess oxygen. Examination of the simulations of the Tsuboi12 CH3 production rates provides insight about the importance of these reactions. We find smaller CH3 concentrations, in addition to smaller growth rates, predicted by the OG, T, and B mechanisms during the induction period. This probably results from the methyl-methyl reactions which give C2 species as products which may then be oxidized, thus providing an alternate route to the direct oxidation of the methyl radical itself. The importance of these pathways seems to have been largely ignored previously. Upon comparing the three more successful mechanisms, OG, T, and B, we find that the reaction pathways for CH4, CH3, CHZO, and CO oxidation are very similar. As previously discussed, Bowman's mechanism considers fewer Cz reaction pathways. It also set the important H O2 reaction rate to be much faster than currently accepted.'* Among the reactions for the primary radical attack on CH4, we find the largest variation among the three mechanisms to be Tsuboi's CH4 + OH rate constant, which was derived from an evaluation of the literature rate data. The rate which was chosen is a factor of 4 lower than that used by OG or B at 2000 K; the high temperature measurements scatter over this range. An important reaction sequence where large ambiguity exists is
+
CH, t 0, = CH,O t OH CH,O t M = C M O t H t M
followed by CHO t M = CO
+Ht
M
Although CH30 has also been suggested as the product of the reaction of CH3 with Oz (see, for example, ref 9), OG, T, and B use the above CHzO sequence instead, with some variations in the rate constants. The OG and B schemes are very similar using rate constants for the first two reactions in cm3 mol-l s-l units of 1O'O and about 1010.3 at 2000 K, assigning the CHzO decomposition an activation energy of about 36 kcal mol-', much less than the bond dissociation energy. We consider the rate constants that we used for this sequence of reactions to be only tentative choices with large uncertainty values, useful only until better data are available. Tsuboi accepts the CHzO + M rate constant measured by Schecker and JostZ5which has an activation energy of 7 2 kcal mol-'. For the T mechanism at 2000 K the first two reaction rate constants are 1010.6and in the same units as before. Since these reactions are very important in the methane combustion mechanism, we performed some additional calculations. First we took the OG mechanism and replaced the rate constants for this sequence with those used by Tsuboi (k(CH3+02)X 4, and CHzO + M replaced by the ref 25 value). Simulations of our data base resulted in an average deviation of the error parameter, pM, of 0.10, compared with 0.07 and 0.12 for OG and T, respectively. Alternatively we replaced the first reaction of this sequence with the two reactions and rate constants from the BB mechanism CH, t 0, = CH,O t 0 CH,O - M = CH,O + H t M
The average deviation of the error parameter was found for this mechanism to be 0.11. We are left with the conclusion that our data base is insufficient to determine details of these reactions, or even whether the methoxy pathways exist. Further experimental data are needed which are really sensitive to the rates of these reactions,
Evaluation of Methane Combustion Mechanisms
such as independent CHzO decomposition rate constants or other intermediate species profiles. The calculated CHpO concentrations, for example, for an experiment with 4 = 1, T = 2000 K, and P = 1 atm, differ during the induction period by more than a factor of 10 between the OG and the T mechanisms. No experimental data, however, are available with which to compare such predictions. There do exist other types of experimental data which we have not considered in our data base: product distributions,26 temperature dependence of ignition del a y ~ , ~ Jmixture ~@ dependence of exponential growth constant^,^^^ and others. Extending the comparative analysis introduced here to a wider data base including essentially different experimental parameters would promise to provide important further information about the mechanism of methane oxidation.
Summary A systematic intercomparison of several methane combustion mechanisms has been carried out by computer modeling. Simulations of selected data parameters from a variety of shock tube experiments were performed and the results compared. This data base was adequately modeled by several mechanisms. Consideration of methyl-methyl and Cz hydrocarbon reactions was found to be a necessary component of a successful mechanism. Areas where the available experiments do not allow selections between different reaction pathways or between different rates were pointed out. Acknowledgment. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This research was also supported by the Robert A. Welch Foundation and the US. Army Research Office. Supplementary Material Available: A listing of the OG mechanism and rate constant set ( 5 pages). Ordering information is available on any current masthead page. References and Notes (1) D. R. Stull and H. Prophet, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 37 (1971). (2) V. S. Engleman, EPA Technical Report, EPA-600/2-76-003, 1976. (3) W. C. Gardlner, Jr., J. fhys. Chem., paper presented at this symposium.
2519 A. M. Dean and G. B. Kistiakowsky, J. Chem. phys., 54, 1718 (1971). N. F. Jacobs, Ph.D. Thesis, Illinois Institute of Technology, 1969. N. F. Jacobs and D. Gutman, Paper presented at the 16lst National Meeting of the American Chemical Society, Los Angeles, Calif., 1971. D. F. Cooke and A. Williams, Combust. Flame, 24, 245 (1975). C. J. Jachimowski, Combust. Flame, 23, 233 (1974). T. A. Brabbs and R. S. Brokaw, Symp. (Inf.)Combust.[froc.], 75th, 893 (1975). T. Tsuboi, Jpn. J . Appl. fhys., 14, 1371 (1975). T. Tsuboi and H. Gg. Wagner, Symp. (mt.) Combust. [ f r o c . ] , 75th, 883 (1975). T. Tsuboi, Jpn. J . Appl. fhys., 15, 159 (1976). D. B. Olson and W. C. Gardiner. Jr.. Combust. Flame. in Dress. G. B. Skinner, A. Lifshitz, K. Scheller, and A. Burcat, J. Chem. ‘fhys., 56, 3853 (1972). D. F. Cooke and A. Williams, Symp. (Int.)Combust. [ f r o c . ] , 73th, 757 (1971). R. M.”. Higgin and A. Williams, Symp. (Int.) Combust. [ R o c . ] , 72h, 579 (1969). C. T. Bowman, Symp. (Int.)Combust. [ f r o c . ] , 75th, 869 (1975). G. L. Schott, Combust. Flame, 21, 357 (1973). T. A. Brabbs, F. E. Belles, and R. S.Brokaw, Symp. (Int.) Combust. [ f r o c . ] , 73th, 129 (1971). S.W. Benson, D. M. Golden, and R. Shaw, EPA Technical Report EPA 600/2-75-019, 1975. P. Roth and Th. Just, Ber. Bunsenges. fhys. Chem.,79, 683 (1975). See, for examDle. W. C. Gardnlner, Jr., W. G. Mallard, M. McFarland. K. Morinaga, J. H. Owen, W. T. Rawlins, T. Takeyama, and B. F. Walker, Symp. (Int.) Combust. [ R o c . ] , 74th, 61 (1974). G. L. Schott and R. W. Getzinger, “Physical Chemistry of Fast Reactions”, Vol. I, B. P. Levitt, Ed., Plenum Press, London, 1973, Chapter 2. R. Hartig, J. Troe, and H. Gg.Wagner, Symp. (Int.) Combust. [froc.], 73th, 147 (1971). D. 6.Olson, T. Tanzawa, and W. C. Gardiner, Jr., Int. J. Chem. Kinet., submitted for publication. H. G. Schecker and W. Jost, Ber. Bungsenges. fhys. Chem., 73, 521 (1969). A. Lifshitz, K. Scheller, A. Burcat, and G. B. Skinner, Combust. Fbme, 16, 311 (1971).
Discussion D. B. OLSON. I think the compilers of rate constants should do us all a great service and delete old and no good literature values. DORENINDRITZ (Frick Chemistry Lab). The compilers of rate constants do a tremendous service by scouring the literature for all previous determinations. Total omission of a value that they have determined to be in error does not tell the researcher whether or not they have considered the value at all. It is important that they do not totally omit “bad” or “old” values.
The Journal of Physical Chemistry, Vol. 81, No. 25, 1977
