Kinetics of plasma assisted pyrolysis and oxidation of ethylene. Part 2: Kinetic modeling studies

Abstract

The kinetics of plasma-assisted pyrolysis and oxidation of ethylene have been numerically investigated. Combining plasma chemistry processes including electron-impact reactions, and reactions of electronically excited species with a comprehensive combustion mechanism, a plasma-assisted kinetic mechanism of ethylene pyrolysis and oxidation has been constructed. To test the accuracy of the constructed mechanism, numerical results were compared to experimental data obtained in a plasma flow reactor, performed under highly diluted conditions in argon at a pressure of 1atm for temperatures ranging from 520K to 1250K. Comparison of plasma-assisted pyrolysis results indicates little discrepancy between the model and experiments. Direct collisional quenching of electronically excited argon by ethylene is responsible for the low temperature enhancement of fuel consumption seen in the plasma-assisted pyrolysis experiments. Hydrocarbon radicals generally undergo addition and recombination reactions to yield several C3 and C4 hydrocarbon intermediates. As temperature increases, the plasma effects diminish and the reaction is overtaken by thermal pyrolysis. Comparison of experimental and modeling results for plasma-assisted oxidation of ethylene demonstrated relatively good agreement for most major and minor species. However, poor agreement was found for ethylene and acetaldehyde for T < 750K. In the oxidation system, collisional quenching of excited argon by O2 to generate the O-atom radical pool complemented the plasma-specific fuel dissociation reactions. The plasma was found to have different effects on the oxidation kinetics at different temperatures. At low temperatures, R+O2 type chemistry (R being a hydrocarbon radical) facilitates the formation of oxygenated species to enhance oxidation by way of formaldehyde. At intermediate temperatures, the formation of hydrocarbon and alcohol intermediates slows the oxidation process relative to the low temperatures. Finally, at high temperatures, plasma chemical reactions are unable to compete against the high temperature chain-branching reactions of the neutral chemistry that dominate and control the overall oxidation process.