Kinetics of plasma assisted pyrolysis and oxidation of ethylene. Part 1: Plasma flow reactor experiments

Abstract

Pyrolysis and oxidation kinetics of ethylene were studied with and without a high-voltage plasma discharge at atmospheric pressure from 420K to 1250K. Experiments were performed in a nearly isothermal flow reactor using mixtures diluted in either argon, helium, or nitrogen to minimize the temperature changes from chemical reactions. At the end of the isothermal reaction zone, the gas temperature was rapidly lowered to quench the reaction. Gas composition was then determined using inline non-dispersive infrared analysis and sample extraction, where samples are stored in a multi-position valve for subsequent analysis with gas chromatography. Experiments were performed by fixing the flow rate or residence time in the reactor, and varying the temperature to achieve a reactivity map. The discharge region occupied approximately 11% of the total length of the isothermal reaction zone and could be placed anywhere along the length of the reactor. The discharge was found to enhance both the pyrolysis and oxidation reactions from 420K to the self-ignition temperature of the fuel. Ethylene pyrolysis was enhanced nearly as much as oxidation below 750K. For plasma-assisted pyrolysis, the results suggest that ethylene dissociation by electron-impact and collisional quenching with electronically excited states of argon, resulted in the direct formation of acetylene and the growth of larger hydrocarbons. During plasma-assisted oxidation, dissociation, and excitation of oxygen led to further fuel consumption, and enhanced the low temperature oxidative chemistry. Above 750K, thermal reactions began to couple to the plasma driven reactions providing further oxidation. At the highest temperatures, the radical production by thermal reactions became competitive and the effectiveness of the plasma discharge decreased.