The oxidation of methane at elevated pressures: Experiments and modeling

A detailed chemical kinetic model has been developed for methane oxidation which is applicable over a wide range of operating conditions. A reaction mechanism, originally developed for high-temperature methane oxidation, was expanded and extended to include reactions pertinent to the lower temperature, elevated pressure conditions encountered in the flow reactor experiments performed in the study. The resulting 207-reaction, 40-species mechanism is capable of reproducing the experimental species concentrations for each of the cases studied. The concentration profiles of reactant, intermediate, and product species, including CH₄, CH₂O, CH₃OH, H₂, C₂H₆, C₂H₄, CO, and CO₂, were obtained in the High Pressure Optically Accessible Flow Reactor (HiPOAcFR) facility for temperatures ranging from 930 to 1000 K and pressures of 6 and 10 atm. Based on the model, no appreciable change in reaction pathway was observed over the pressure and temperature range studied, with HO₂ providing the major route for CH₃ oxidation. CH₂O was found to be a vital intermediate for all of the CH₄ oxidation paths. In addition, inclusion of trace amounts of CH₂O measured at the initial sampling location into the model initial conditions greatly reduced the predicted time to onset of fuel disappearance and enhanced the model agreement. This result is consistent with past engine studies which have found that CH₂O is a significant pro-knock additive when added to a methane base fuel. The expanded reaction mechanism was also tested against shock-tube ignition delay and laminar flame speed data and was found to be in good agreement with the relevant experimental data.