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In this work, the reaction mechanism in non-thermal plasma enabled methane conversion driven by different sources, i.e. microwave discharge, nanosecond pulsed dielectric barrier discharge, and spark discharge, is investigated. The behaviour of reactive species is monitored using the optical emission spectroscopy (OES), while the final gaseous products are analysed with gas-chromatograph. A zero-dimensional (0D) reaction kinetics model is established to reveal the underlying reaction processes. By changing pressure, different plasma modes (diffuse and filamentary) are achieved and the relative emission intensity of different species and distribution of gaseous productions are manipulated. It is found that there is a strong correlation between the intensity-ratio of C2A → X and CH A → X, gas temperature, and the relative concentration of C2 products in different discharge forms, i.e. higher intensity ratio of C2/CH is associated with higher gas temperature and more unsaturated hydrocarbon products. Assisted with the 0D reaction kinetics model, the relationship between the intermediate species and final products is illustrated. Based on the investigation in this work, it is proposed that intensity ratio of OES can be widely adapted as a non-intrusive and real-time process monitoring in the plasma-enabled methane conversion. [ABSTRACT FROM AUTHOR]

In order to comprehensively investigate the discharge characteristics of an atmospheric-pressure radio-frequency (RF) pulsed discharge in Ar/CH4/CO2, a phase-resolved measurement is given in this paper. Firstly, the discharge characteristics of RF plasma in a gas mixture of Ar, CO2 and CH4 are investigated under different parameters. It is found peak power is more efficient than duty cycle in increasing the discharge area. Besides, a phase-resolved morphology of RF plasma is given. The discharge in the positive cycle is longer in length and stronger in emission intensity than in the negative cycle. Secondly, different activation paths of species have been obtained by using phase-resolved optical emission spectroscopy. Ar species is mainly activated by electron collision effect, while CH and C2 are mainly activated by Ar metastable species. Finally, detailed electron kinetics is analyzed. The results show that most energy is transferred to the vibrational excitation compared to elastic activation, electronic excitation, and ionization under different ratios of CO2 and CH4 to Ar. Besides, under a higher ratio of CH4 and CO2 to Ar, the generating rate of species and power loss rate are higher. It proves that higher fraction of CO2 and CH4 is better for the conversion rate and energy efficiency under RF discharge in theory, which guides the reforming of CH4 and CO2 in RF discharge. [ABSTRACT FROM AUTHOR]

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]