One of the major challenges of using plasmas for CO 2 conversion is the removal of oxygen in the plasma effluent. In this work an efficient removal of oxygen ( > 99 % removal) from the product gas stream in a 2.45 GHz microwave plasma reactor at atmospheric pressure is reported. This is achieved by addition of H 2 gas enabling the reverse water gas shift reaction. The addition of H 2 takes place either immediately after the microwave resonator, or inside the resonator, affecting the plasma size and optical emission in the latter case. The performance is comparable for both positions of H 2 addition. Already 2 vol.% of H 2 admixture results in O 2 removal below 0.1 vol.% from the product gas stream, however at the cost of lower CO 2 conversion. An increase of H 2 flow leads to a rise of the CO 2 conversion in accordance to the thermodynamics of the reverse water gas shift reaction. CO 2 conversions of up to 65%, and CO energy yields of up to 0.27 kg CO kWh −1 are achieved with less than 0.1 vol.% O 2 in the product gas. The H 2 /CO ratio in the product gas lies in the range between 1.8–2.2 which makes it suitable for synthetic fuel processes such as Fischer–Tropsch. The performance comparison demonstrates that the presented method of O 2 removal by H 2 admixing to the microwave CO 2 plasma is a very efficient process compared to other O 2 removal methods yielding the highest energy yields. • Reverse water gas shift in the microwave plasma at atmospheric pressure. • Increase of the CO2 conversion with a simultaneous O2 removal. • Highest energy yield compared to other plasmas used for the CO2 conversion with O2 removal step. • H2/CO ratio about 2 realised in the product gas.
Hecimovic et al. (Tue,) studied this question.