Study of apparent effective ionization coefficient in CH4:N2 and CH4:O2 gas mixtures

Methane (CH4) has a high heating value, is relatively inexpensive, and is a widely available gas. For the conversion of methane into liquid products, a novel approach proposes to use non-thermal plasma processes. Efficient utilization of plasma requires precise plasma chemical models that rely on accurate input data such as the reduced effective ionization coefficient alpha(ea)/N. Such data are available for pure CH4, but N-2 or O-2 are typically also present in plasma reforming of CH4, and no experimental data about reduced effective ionization coefficient exist for these mixtures. The present study determined the alpha(ea)/N in CH4:N-2 and CH4:O-2 gas mixtures in the pressure range of 10-800 Torr and reduced electric field strength E/N range of 40-1200 Td utilizing a steady-state non-self-sustaining Townsend discharge. Experimental results were compared with calculations of the Boltzmann equation solver BOLSIG+ and a more accurate model that accounted for detachment from O-2, as well as O-3(-) formation and charge transfer reactions. In CH4:N-2 mixtures, alpha(ea)/N decreased with N-2 content, and BOLSIG+ calculations agreed reasonably well with measurements. In CH4:O-2 mixtures, alpha(ea)/N did not change with the mixture composition, and at low E/N values, the data calculated by the refined model fitted the experimental data markedly better compared to fitting with BOLSIG+ calculations. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/).