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 αea/N. Such data are available for pure CH4, but N2 or O2 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 αea/N in CH4:N2 and CH4:O2 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 O2, as well as O3− formation and charge transfer reactions. In CH4:N2 mixtures, αea/N decreased with N2 content, and BOLSIG+ calculations agreed reasonably well with measurements. In CH4:O2 mixtures, αea/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.