Design and characterization of a solar-enhanced microwave plasma reactor for atmospheric pressure carbon dioxide decomposition

The decomposition of carbon dioxide (CO₂) is a primary step in carbon re-utilization approaches aimed to fulfill fuels and chemicals demands and mitigate environmental emissions. Plasmachemical CO₂ decomposition processes can be highly efficient; however, their reliance on electrical energy can limit their economic viability and sustainability advantage. In contrast, solar thermochemical CO₂ decomposition approaches can have limited efficiency, but their direct use of the most abundant form of renewable energy affords them the greatest sustainability potential. Solar-enhanced microwave plasma (SEMP) chemical synthesis, based on the direct interaction between microwave plasma and concentrated solar radiation, is investigated as a novel approach to combine the advantages of plasmachemical and solar thermochemical processes. SEMP is motivated by the potential for synergistic effects between solar photons and plasma species, implied by the markedly greater spectral absorption of nonequilibrium CO₂ plasma compared to that of equilibrium CO₂, to lead to enhanced chemical decomposition. The design, development, and characterization of a SEMP reactor for atmospheric pressure CO₂ decomposition is presented. The reactor is powered by a 1.25 kW magnetron and by up to 525 W of incident radiative power from a high-flux solar simulator. Experimental results reveal that the microwave plasma absorbs up to 20% of concentrated solar radiation at a solar-to-electrical power ratio of 0.5, and that relative absorption decreases with increasing solar input power. However, conversion efficiency and plasma energy efficiency increase with increasing solar power, up to 9% and 25% respectively, for a solar-to-electrical power ratio of 0.75. The enhanced process performance appears to be a consequence of the greater power density in the plasma caused by the direct absorption of solar radiation.