Benchmarking microwave-induced CO$_2$ plasma splitting against electrochemical CO$_2$ reduction for a comparison of promising technologies

Plasma conversion technology is an emerging technique under development to activate, convert or valorize gas molecules such as CO$_2$, N$_2$, CH$_4$, NH$_3$ and others. A large-scale application beyond the lab-scale demonstrator unit requires assessment of the efficiency of this new technology. The straightforward approach for assessment of the efficiency is benchmarking with the other well-established technologies of similar technology readiness level (TRL). In this paper we present a benchmarking of the atmospheric pressure microwave-induced CO$_2$ plasma splitting with electrochemical CO$_2$ conversion, via both low-temperature and high-temperature electrolysis. An additional step of oxygen removal in case of the plasma reactor is implemented due to the difference in the output stream of the plasma (gas mixture containing CO$_2$, CO, and O$_2$) and the electrochemical reactor (typical gas mixture on cathode containing CO$_2$ and CO). For the benchmarking, a comprehensive set of comparison parameters that are applicable for both the plasma and the electrochemical route is identified and grouped in three comparison categories: performance, interfaces, and economics. ... mehrThe comparison of these parameters demonstrates that in terms of the electric power consumption (EPC; power required for production of one Nm$^3_{CO}$) plasma conversion technology (∼20 kWh/Nm$^3_{CO}$) is in the ballpark with the other two electrochemical technologies (∼4–20 kWh/Nm$^3_{CO}$). The key features of the plasma conversion technology are relatively large conversion (up to 56%) and moderate energy efficiencies (up to 27%). Also, CO$_2$ gas of reduced purity of only 98% can be used without decrease of the performance, and CO output values are currently at 3.5 slm (standard litre per minute). Fast on/off response time of order of minutes, and no need for the hot standby indicate that the plasma conversion is particularly suitable for use of intermittent renewable energy sources. The aspects that require further development include optimization of the process towards lower EPC$_{total}$ values, improved oxygen gas separation, and reliable ignition of the plasma.