Research is underway to produce synthetic fuels such as hydrocarbon fuels by reacting hydrogen derived from renewable energy with carbon dioxide and other substances, and their use towards a decarbonized society is being considered.¹ In this study, we focus on high-temperature co-electrolysis of CO₂ and H₂O using a solid oxide electrolysis cell (SOEC), which is one of the promising CO₂ conversion technologies. The objective of this study is to evaluate the influence of the electrolysis process and operating conditions on the fuel synthesis process based on heat and mass balance analysis and to propose designs for a new fuel production system.

Experimental

A commercial chemical process simulation software Aspen Plus was used to simulate a fuel synthesis process using SOEC high-temperature co-electrolysis. In this simulation, H₂O and CO₂ were considered to be supplied to the SOEC for co-electrolysis, and a fuel synthesis process such as a CH₄ production reactor was connected to the outlet of the SOEC to calculate the overall system efficiency. Figure 1 describes an SOEC and a fuel synthesis reactor of the SOEC high-temperature co-electrolysis process created by Aspen Plus.

Results and discussion

Figure 2 shows one of the simulation results calculated using the process model shown in Figure 1, where the operating temperature of the SOEC is 800 ℃ and the reaction pressure is 1 bar. Figure 2 shows the change in gas composition after the co-electrolysis process in the SOEC by varying H₂O flow rate from 1 to 5 mol min^-1 relative to CO₂ flow rate of 1 mol min^-1. As shown in Figure 2, the conversion rate of CO₂ to CO increases with increasing H₂O flow rate due to the reverse water-gas shift reaction, and the ratio of H₂O to CO peaked at H₂O/CO₂=2, and then gradually decreased. Since the ratio of these gases affects the efficiency of fuel synthesis to CH₄ and other downstream gases, we have studied the appropriate fuel synthesis ratio based on the simulation results. In this presentation, we report on the results of the performance evaluation of co-electrolysis and fuel synthesis process based on the obtained simulation results, by conducting electrolysis tests using SOEC cells and evaluating cell performance and downstream gas composition under different operating conditions, based on actual experimental results.

Acknowledgements

This work was partially supported by JSPS KAKENHI Grant Number JP21K04981.