Carbon dioxide conversion by solar-enhanced microwave plasma: Effect of specific power and argon/nitrogen carrier gases

Abstract Plasma-chemical processes are ideally suited for the conversion of low-value gaseous feedstock such as carbon dioxide (CO2) into fuels and chemicals. However, the reliance on low-pressure operation of traditional plasma methods to achieve high efficiency can limit their economic viability and net sustainability benefits. Solar-Enhanced Microwave Plasma (SEMP) CO2 conversion exploits the interaction between solar radiation and a CO2-Ar or CO2-N2 microwave discharge to achieve increased energy efficiency while operating at atmospheric pressure conditions. Moreover, by incorporating solar energy, SEMP processes exhibit the same specific energy input (SEI) as conventional plasma methods. The effects of electric power, solar power, flow rate, and gas composition on solar absorption, conversion, and energy efficiencies in SEMP CO2 conversion are investigated. Experimental results show up to ∼21% absorption of solar power obtained under CO2-Ar operation. In all the studied conditions, the absorbed solar radiation leads to increased CO2 conversion efficiency; e.g. an increase from ∼8% to ∼9% is obtained at 900 W electrical power and 75 W absorbed solar power. The use of nitrogen as auxiliary gas leads to higher conversion and energy efficiencies than those using argon. The highest conversion efficiency obtained with CO2-N2 is ∼15.5%, whereas the highest conversion efficiency using CO2-Ar is ∼9.5% – both observed at the same SEI of 1.8 eV/molecule. The results suggest the potential of the SEMP processing of CO2-N2 mixtures, as found in flue gas from fossil-fuel power plants, as a viable approach for CO2 conversion and utilization.