An insight into the reaction kinetics of CH3 + O2(a1Δg) and its enhancement effect on methane ignition

The reaction between CH3 and O2(a1∆g) is crucial to understand the effects of electronically excited oxygen in plasma-assisted combustion of methane and other hydrocarbons. In the present work, multireference quantum chemical methods were used to investigate the potential energy surface of CH3 + O2(a1∆g). The RRKM/master equation simulation was employed to compute the rate coefficients of various pathways to this reaction over the temperature range of 300–2000 K and a pressure range of 0.1–100 atm. Special attention has been paid to the nonadiabatic transition between the excited state and ground state, which directly leads to a quenching channel from CH3 + O2(a1∆g) to CH3 + O2(X3∑g−). This quenching reaction has been overlooked by previous theoretical and kinetic modeling studies. We also conducted kinetic modeling to examine the effect of this reaction on the ignition enhancement of methane oxidation. Although the channel of CH3 + O2(a1∆g) quenching to CH3 + O2(X3∑g−) has nonnegligible rate constants comparing with other reaction channels, modeling result with the inclusion of 5% O2(a1∆g) in molecular oxygen shows that the titled reactions shorten the ignition delay time of methane by more than twenty times at 900 K, 1 atm. The ignition enhancement is mainly from the chain branching channels to CH2O + OH and CH3O + O which has been greatly promoted by excess energy from O2(a1∆g). The present study uncovers the kinetic mechanism of this nonadiabatic reaction and provides reasonable rate coefficients for further kinetic modeling of plasma-assisted combustion of methane and other hydrocarbons.