Supersonic Microwave Plasma: Potential and Limitations for Energy-Efficient CO2 Conversion

Supersonic flows provide a high thermodynamic nonequilibrium, which is crucial for energy-efficient conversion of CO2 in microwave plasmas and are therefore of great interest. However, the effect of the flow on the chemical reactions is poorly understood. In this work, we present a combined flow and plasma chemical kinetics model of a microwave CO2 plasma in a Laval nozzle setup. The effects of the flow field on the different dissociation and recombination mechanisms, the vibrational distribution, and the vibrational transfer mechanism are discussed. In addition, the effect of experimental parameters, like position of power deposition, outlet pressure, and specific energy input, on the CO2 conversion and energy efficiency is examined. The short residence time of the gas in the plasma region, the shockwave, and the maximum critical heat, and thus power, that can be added to the flow to avoid thermal choking are the main obstacles to reaching high energy efficiencies.