On the improvement of chemical conversion in a surface-wave microwave plasma reactor for CO₂ reduction with hydrogen (The Reverse Water-Gas Shift reaction)

Abstract

A novel surface-wave microwave discharge reactor configuration to generate syngas via gaseous CO2 reduction with H2 (non-catalytic Reverse Water-Gas Shift reaction) is studied in the context of power-to-chemicals concept. Improvement of CO2 conversion to maximize CO production is explored by adding an external cylindrical waveguide downstream of the plasma generation system. A 2D self-consistent argon model shows that power absorption and plasma uniformity are improved in the presence of the waveguide. We show experimentally that CO2 conversion is increased by 50% (from 40% to 60%) at the stoichiometric feed ratio H2:CO2 equal to 1 when using the waveguide. At higher H2:CO2 ratios, the effect of the waveguide on the reactor performance is nearly negligible. Optical emission spectroscopy reveals that the waveguide causes significant increase in the concentration of O atoms at a ratio H2:CO2 = 1. The effects of the operating pressure and cooling rate are also investigated. A minimum CO2 conversion is found at 75 mbar and ratio H2:CO2 = 1, which is in the transition zone where plasma evolves from diffusive to combined operation regime. The cooling rates have significant impact on CO2 conversion, which points out the importance of carefully designing the cooling system, among other components of the process, to optimize the plasma effectiveness.