Methane coupling in a microwave plasma reactor governed by temperature

Scaling the plasma process of methane (CH₄) coupling to acetylene (C₂H₂) to industrial scale requires knowledge of the dominant chemical and transport processes. This study investigates the influence of hydrogen (H₂) recycling and pressure in a CH₄ microwave plasma reactor by combining simple models with experimental in-situ diagnostics.

Our experiments are conducted at 700 W and 10 slm, with the CH₄ feed diluted with 40-60% H₂. We used in-situ Raman scattering spectroscopy of H₂ to determine the gas temperature. From 50 mbar to 125 mbar, the peak temperature increases from 2100 K to 3000 K. This trend is attributed to radial plasma contraction, which concentrates power and reduces the plasma volume. In the same pressure range, conversion efficiency increased from 25% to 50%, with approximately 100% C₂H₂ selectivity and negligible soot production. Our minimum measured energy requirement is 9.4 kWh/kg-C₂H₂.

To study the mechanisms responsible for the observed results, we developed a simple 2D core-shell model. It matches the experimental results well and will be used to demonstrate the underlying mechanisms. Our results show that industrial performance, such as the Hüls process at 10.3 kWh/kg-C₂H₂, can be achieved and even surpassed by lab-scale microwave plasma reactors. This allows for optical diagnostics and process optimization in a relevant parameter space.