Time-resolved characteristics and chemical kinetics of non-oxidative methane conversion in repetitively pulsed dielectric barrier discharge plasmas

Abstract

Repetitively pulsed discharge is an emerging non-thermal plasma technology in the chemistry field given its advantages of high electron energy and moderate gas temperature. The focus of this paper is on the temporal evolution of physical and chemical processes in the plasma-assisted non-oxidative conversion of methane (CH4). Positive and negative streamer discharge plasmas were generated by a microsecond-pulse power source (pulse rising time ~500 ns and full width at half maximum ~8 μs) in a well-designed dielectric barrier discharge reactor. The performance of CH4 conversion (conversion rate, product yield, energy conversion efficiency, etc) was investigated in the first instance. A CH4 conversion rate of 6.2%–9.6% was acquired in the negative streamer discharges, which is on average 6% higher than that in the positive streamer discharges. To explain this phenomenon, the temporal evolution of voltage–current waveforms and ultrafast ICCD photographs and optical emission spectra were recorded. As expected, significant differences between the two different forms of discharges were found. Compared with those of the positive streamer discharges, characteristics of only one, big breakdown, higher initial voltage and current peak, a more stable and nearly uniform luminescent process, and higher CH (A2Δ − X2Π) emission intensity and rotational temperature were observed in the negative streamer discharges, which coincide with the promotion of CH4 conversion performance. A simplified chemical kinetics analysis of CH4 discharge was also implemented to demonstrate the influence of characteristic differences on CH4 conversion performance.