Reaction mechanisms and rate constants ofelementary steps in methane-oxygen flames

Abstract

The concentrations of all stable and unstable species have been measured in the reaction zone of lean and stoichiometric methane-oxygen flames burning at 40 torr. The rate constant of Reaction [1], CH4+OH→CH3+H2O, was found to be k1=3×1013×exp(−6000/RT) mole−1 cm3 sec−1. In stoichiometric flames, half of the fuel is consumed via Reaction [1′], CH4+H→CH3+H2 and at T=1600°K, k1′ is reported to be 3.2×1012. It is shown conclusively that the great majority of the methyl radicals is removed by Reaction [2], CH3+O→CH2O+H, with k2=1.3×1014 exp (−2000/RT). The rate-determining step is a reaction resulting in an increase of the number of particles, Reaction [13]: CH2O+M→CO+H2+M; the value k13=2.1×1016 exp(−35,000/RT) was derived. Another part of the formaldehyde disappears via Reaction [3]: CH2O+OH→CHO+H2O; the rate constant is k3≅2.3×1013 in the range T=1400°–1800°K. Most of the CHO radicals react with O2: CHO+O2→HO2+CO; k4≅3×1013 at T=1600°K. In lean flames, HO2 is removed mainly by reactions involving OH and O: HO2+OH(O)→O2+H2O(OH); and k5≅5×1013 at T=1600°K. Carbon dioxide is formed by Reaction [6]: CO+OH→CO2+H; the rate constant was found to be k6=2.8×1011 at T=1750°K, and the activation energy E6=5.5±2 kcal/mole in the range T=1500°–1900°K. In the burned gas of lean flames, the radical “pool” decays by Reaction [11]: H+O2+M→HO2+M,the rate constant (for M=O2) being k11=2.5×1015 at T=1900°K.