Interplay of Transport, Plasma Concentration, and Chemistry in Microwave Discharges

Microwave plasma is investigated as gas conversion technology for its compatibility with intermittent power sources and its promise in efficiency and selectivity, e.g. in dissociation of CO₂ or carbon coupling in CH₄. Understanding of transport and power density and their relation to plasma concentration are key for optimization of performance and scale up of the technology. In this work, we demonstrate the dominant phenomena at play in a comparison of CO₂ and CH₄ discharges in Forward Vortex (FV) and Reverse Vortex (RV) flow, involving maps of composition (CO₂, CO, O₂ and O) and temperature measured with Raman scattering.

Large differences in temperature and composition are observed. In CO₂, we measure temperatures below 4500 K in RV versus 6000 K in FV and consequently lower dissociation degrees in the plasma core for RV versus FV. In CH₄, the macroscopic production of soot is entirely suppressed. It will be shown how these observations correlate with measured global gas conversion performance.

In effect, a decoupling of core temperature and composition with power density is achieved, enabling concentrated plasma conditions at different gas temperature. Furthermore, the work shows that plasma concentration results from a balance between ambipolar diffusion and recombination. Such aspects underline the importance of gas flow geometry for reactor parameters and it will be sketched how these can play an important role in future reactor engineering and design, in particular in relation with ongoing R&D for electrification of process industry.