Kinetics of plasma-assisted oxidation of highly diluted hydrocarbon mixtures excited by a repetitive nanosecond pulse discharge

Plasma-assisted oxidation of atmospheric pressure hydrogen- and hydrocarbon-oxygen mixtures diluted in argon is analyzed by kinetic modeling, over a wide range of temperatures. In the experiments, preheated reactant mixtures are excited by a repetitively pulsed, double dielectric barrier ns discharge in plane-to-plane geometry, at near-isothermal conditions. Plasma images and temperature distributions in the discharge indicate that the reactant flow is nearly uniform, justifying the use of quasi-0D approximation in kinetic modeling. The kinetic model is based on a plasma chemistry mechanism combined with a conventional combustion reaction mechanism. The model does not contain adjustable parameters such as reduced electric field in the plasma, used in a number of previous 0D modeling studies. Comparison of the modeling predictions with the experimental data for hydrogen and methane oxidation exhibits good agreement between measured and predicted fuel concentrations over a wide range of temperatures, showing that the yield of primary radicals generated in the plasma is predicted accurately. Concentrations of intermediate hydrocarbon species predicted by the model are also in good agreement with the experiments, with the exception of acetylene below the hot ignition point. For ethylene and propane oxidation, the model overpredicts fuel consumption at low temperatures, also overpredicting concentration of CO, the dominant oxidation product, and underpredicting acetaldehyde concentration. This indicates that low-temperature pathways of formaldehyde formation, a major precursor for CO, as well as low-temperature reactions of several radicals which are precursors for formaldehyde and acetaldehyde, are not represented accurately in both conventional reaction mechanisms used. Although concentrations of intermediate hydrocarbon species in ethylene and propane are predicted relatively well, kinetics of formation and decay of acetylene remains not understood. This may be due to inaccurate branching ratio for dissociative quenching of metastable argon by heavy hydrocarbon species, as well as deficiencies of the conventional reaction mechanisms. (paper)