Microwave plasma reforming of CH4/CO2 mixtures: investigating the impact of high CO2:CH4 ratio and the fate of hydrogen sulfide

Microwave plasma dry methane reforming (DMR) is a promising route for converting biogas into synthesis gas while avoiding catalyst-related limitations. In this study, atmospheric-pressure microwave plasma reforming of CH 4 /CO 2 mixtures was investigated with a high CO 2 :CH 4 ratio of 3:1 to suppress soot formation. The effects of input power (1800-3000 W) and gas flow rate (20-40 SLM) on conversion, selectivity, product yields, and energy consumption were examined. Methane conversion ranged from approximately 30% to 90%, while CO 2 conversion varied between 25% and 67%. When expressed as converted moles, more than twice as many CO 2 molecules were converted compared to CH 4 , mainly due to reverse water-gas shift reaction. High CO 2 content effectively prevented soot formation but led to significant hydrogen consumption, resulting in low H 2 /CO ratios that are unfavorable for downstream synthesis processes. Energy consumption ranged from 80 to 130 kWh per kilogram of hydrogen. On the other hand, energy consumption for CO was in the range of 2.5–3 kWh/kg, making the process highly competitive when compared to other emerging technologies. For the first time, the fate of H 2 S under microwave plasma DMR conditions was examined. H 2 S was almost completely converted, predominantly forming COS and CS 2 , with possible trace amounts of SO 2 . These findings highlight the need for upstream H 2 S removal and careful balancing of CO 2 , steam, and CH to enable stable, soot-free operation with suitable syngas composition.