Revisiting the chemical kinetics of CH3 + O2 and its impact on methane ignition

Abstract

The reaction between the methyl radical and molecular oxygen is one of the most important combustion reactions—it plays a controlling role in methane ignition. Although the reaction kinetics of CH3 + O2 has been extensively studied (at least experimentally) in the literature, high-level theoretical studies are still desired to understand the chemistry of this “small but complicated” reaction system and provide rate constants over wide range of temperature and pressure. In the present study, the rate coefficients of three major reaction channels of CH3 + O2, i.e., CH3OO stabilization (R1), CH3O + O channel (R2), and CH2O + OH channel (R3), were computed with temperature ranging from 300 to 2500 K and pressure from 0.01 to 100 atm by combining high-level quantum chemical calculations (CCSD(T)-F12/cc-pVQZ-F12//QCISD/6-311++G(2df,2p)) and RRKM/master equation simulations (with variable reaction coordinate transition state theory simulation for the barrierless channels). The computational results were compared with previously reported rate constants based on a thorough literature review. They show satisfactory agreement with those most recent measurements over the limited experimental temperature ranges—the discrepancies are within a factor of two for the three dominant reactions. The rate constants of R2 and R3 have very high sensitivity coefficients to the ignition delay time of methane. The impact of this reaction kinetics on methane ignition was evaluated by putting our currently computed rate coefficients into two recently reported kinetic models for methane combustion. There was a significant difference on the simulated ignition delay for an RCM experiment by replacing the rate constants of R2 and R3 with the newly computed results. It indicates the risk of estimating rate constants by simple extrapolation from experimental measurements over limited temperature ranges. The modified Arrhenius representations of the reaction channels of both CH3 + O2 and CH3OO dissociation were provided for utility in combustion modeling.