Electroreduction of CO₂/CO to C₂ Products: Process Modeling, Downstream Separation, System Integration, and Economic...

Direct electrochemical reduction of CO₂ to C₂ products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO₂ is first electrochemically reduced to CO and subsequently converted to desired C₂ products has the potential to overcome the limitations posed by direct CO₂ electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO₂ conversion routes to C₂ products. For the indirect route, CO₂ to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO₂ conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO₂ conversion pathways are presented, which includes CO₂ capture, CO₂ (and CO) conversion, CO₂ (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m², and an electricity price of <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO₂/CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO₂ electrolyzers and possible solutions are discussed as well.