Effect of Cooling Performance of Quenching Systems on CO ₂ Dissociation in a Microwave Plasma

Plasma-assisted CO2 conversion is attracting growing attention. The post-discharge gas quenching is one of the promising methods to enhance CO2 splitting. In this study, we experimentally investigate the effects of effluent cooling on CO2 conversion in microwave plasma at 100 Torr (13.3 kPa). A novel interchangeable heat exchanger system, capable of operating in quenching rod (QR) mode and quenching tube (QT) mode with an adjustable distance from the plasma, was developed to investigate CO2 conversion under various cooling conditions. Significant differences in conversion and energy efficiency between the two cooling modes were observed at specific energy input (SEI) above 4 eV/molecule. At low SEI, there was no significant difference in energy efficiency (η) and dissociation degree (β) between the modes; however, as energy density increased, the difference became more pronounced. At 5.5 eV/molecule, the QT mode exhibited η = 39% and β=73% while the QR mode showed η = 32% and β=60%. A qualitative comparison of the calculated cooling and reaction characteristic times showed that the observed difference in CO2 conversion between the QT and QR modes is primarily attributed to the insufficient suppression of CO and O2 re-oxidation reactions in the QR mode at high SEI conditions. Therefore, the suppression of CO combustion through adequate cooling could be the key consideration in the design of heat exchangers for high-SEI operations, in order to achieve both high CO2 conversion and high energy efficiency.