EARLY STAGES OF PYROLYSIS AND OXIDATION OF METHANE

Measurement of post shock-front density-gradients generated by shock waves propagating through CH4 Ar and CH4/O2/Ar mixtures provided data for estimating the rate constant for the primary dissociation step [CH4 + Ar → CH3 + H + Ar] in the bimolecular regime. A least-squares analysis of 24 runs covering the temperature range 1950-2770°K and total densities (2.37-8.91) × 10-6 mol cm-3 gave for k1(T) = 0.6 Z CH ( E0 RT)4.2 exp ( (-E0 RT)/4.2! cm3 mol-1 s-1, with E0 = 103.2 kcal/mole. The measured activation energy was 85.8 ± 1 kcal/mole. Within our experimental scatter the rate constant for CD4 was equal to that for CH4 but (possibly) with a slightly higher activation energy (+1.7 kcal). To fit the computed profiles of the density gradients to those observed, over particle times 0.5-50 μs, a 12-step mechanism was postulated and a reasonable set of rate constants derived. Our data combined with room temperature values for the termolecular association of CH3 with H atoms suggest that the association rate constant varies as ( 1 T)1.2. At CH4 O2 ratios 3.3 and 6.7, combustion is initiated when molecular oxygen is attacked by the primary products of pyrolysis. Ten additional steps are required to account for the recorded density gradient profiles. The sensitivity of the psfdg technique for imposing sharp criteria on proposed reactions that occur during the induction period of a combustion process is demonstrated in these studies. Comparisons were made between the 22-step mechanism derived in this investigation and a variety of mechanisms proposed in the literature. © 1979.