Methane coupling to C₂H_x hydrocarbons by injection into the afterglow of a hydrogen plasma

Ethylene is mainly produced via naphtha cracking, which causes 1% of global CO₂ emissions. Due to the electrification of naphtha crackers and the circularization of polymer industry, methane will become a major unused byproduct. We investigate the direct conversion of methane to ethylene in a microwave plasma reactor. A challenge in methane coupling is optimizing hydrocarbon selectivity and preventing soot formation. According to the Kassel mechanism, the selectivity is determined by a delicate balance between high temperatures (> 2000K), fast heating and quenching rates (10⁶ – 10⁷ K/s) and short residence times.

To enter the residence time regime required for ethylene selectivity, we feed methane downstream of H₂ plasma, mimicking the two-step Huels process in a microwave discharge. We use gas chromatography to investigate product distribution and methane conversion. We find increased ethane and ethylene yield at low pressures and powers, shifting towards acetylene at higher powers. Additionally, we use in-situ Raman scattering from H₂ to measure rovibrational temperature at the mixing point and in the plasma. Gas temperatures were around ~700 – 800 K in the methane injection region for all conditions tested. Assuming negligible methane recirculation to the upstream plasma, we conclude that H-radical induced chemistry was the dominant reaction pathway. We will compare these results with ongoing experiments, in which thermal conversion is expected to increase with methane injected in hotter regions.