Non‐oxidative methane conversion in diffuse, filamentary, and spark regimes of nanosecond repetitively pulsed discharge with negative polarity.

Methane conversion for higher hydrocarbons was carried out in the diffuse, filamentary, and spark regimes of nanosecond repetitively pulsed discharge with negative polarity in a pin‐to‐plate reactor. The diffuse discharge was characterized as a regime with low‐plasma energy, and only trace hydrogen and C2H6 were found in this regime. In filamentary regime, the conversion rate reached 2.2–7.6% with energy conversion efficiency (ECE) of 8.8–12.7%. Meanwhile the selectivity of C2H6, C2H4, C2H2, and C3 were 24.3–18.6%, 9.3–7%, 5.6–7.3%, and 0–5%, respectively. As for the spark regime, C2H2 became the main hydrocarbon product with the selectivity of 12.8%, conversion rate of 44.7–74.4% and ECE of 12.3%. Electrical measurements and ICCD photographs showed that the short pulse could prevent the streamer turning into spark discharge, and thus realize smooth transitions among each regime. Optical emission spectroscopy was used to investigate the plasma chemistry in each regime. It found that the highest spectral line, gas temperature, and electron density varied in these regimes. These results suggested that, comparing with electron impact reactions, the thermal chemistry became more and more important as the plasma energy increased. Highlights: The discharge regimes of nanosecond repetitively pulsed discharge with negative polarity for methane conversion has been investigated in a pin‐to‐plate reactor. Smooth transition among the diffuse, filamentary, and spark regimes are realized by changing voltage and pulse width of the negative nanosecond repetitively plasma (NRP).The short pulse can inhibit the growth of the unstable bright spots around the electrodes, and thus prevents the streamer turning into spark discharge.The main final products can be controlled from C2H6 to C2H2 with different discharge regimes.The detected maximum CH4 conversion was 7.6% in filamentary regime and 74.4% in spark regime, respectively.The plasma chemistry in each regime was analyzed with the optical emission spectroscopy (OES). [ABSTRACT FROM AUTHOR]