CO2 Conversion in Nonuniform Discharges: Disentangling Dissociation and Recombination Mechanisms

Motivated by environmental applications such as synthetic fuel synthesis, plasma-driven conversion shows promise for efficient and scalable gas conversion of CO2 to CO. Both discharge contraction and turbulent transport have a significant impact on the plasma processing conditions, but are, nevertheless, poorly understood. This work combines experiments and modeling to investigate how these aspects influence the CO production and destruction mechanisms in the vortex-stabilized CO2 microwave plasma reactor. For this, a two-dimensional axisymmetric tubular chemical kinetics model of the reactor is developed, with careful consideration of the nonuniform nature of the plasma and the vortex-induced radial turbulent transport. Energy efficiency and conversion of the dissociation process show a good agreement with the numerical results over a broad pressure range from 80 to 600 mbar. The occurrence of an energy efficiency peak between 100 and 200 mbar is associated with a discharge mode transition. The net CO production rate is inhibited at low pressure by the plasma temperature, whereas recombination of CO to CO2 dominates at high pressure. Turbulence-induced cooling and dilution of plasma products limit the extent of the latter. The maxima in energy efficiency observed experimentally around 40% are related to limits imposed by production and recombination processes. Based on these insights, feasible approaches for optimization of the plasma dissociation process are discussed.