Low temperature oxidation of methane in a nanosecond pulsed plasma discharge

Abstract

A study of the low temperature chemistry of methane oxidation in a nanosecond repetitively pulsed (NRP) discharge is undertaken. Both laser absorption spectroscopy measurements of temperature and formaldehyde, as well as gas chromatograph sampling of major products are used to evaluate the kinetic mechanism of a stoichiometric methane/oxygen mixture with 75% helium dilution. In addition, a predictive tool for calculating electron collision reactions, excited and ionized species reactions, and combustion reactions has been built. A model for methane, oxygen, and helium plasma has been assembled for use with this new tool, and is incorporated with a low temperature combustion mechanism. Comparisons between the model and the measured species are in agreement for CH4 and O2 consumption, as well as production of H2O, CO, CO2, and H2. However, CH2O, CH3OH, C2H6, C2H4, and C2H2 are not accurately predicted. Path flux analysis reveals that methane oxidation proceeds through one of two intermediates: CH3 or CH2, which are created primarily by electron collision reactions and H-abstraction reactions by plasma-generated radicals. The major species resulting from CH2 oxidation are generally well predicted, while those resulting from methyl oxidation are poorly predicted. Thus, further investigation of low temperature (400-500 K) methyl reactions, particularly the consumption pathways of CH3O2, are needed to bring the model into agreement with the present measurements.