Using waste CO2 to increase ethanol production from corn ethanol biorefineries: Techno-economic analysis

Sustainable conversion of carbon dioxide (CO2) to value-added chemicals or fuels shifts a linear “cradle to grave chemicals or fuels manufacturing model” to a circular carbon economy. A key consideration for CO2 utilization is the economic viability of converting specific CO2 sources based on scale, purity, etc. Among different CO2 sources, the corn dry mill ethanol plant with onsite waste CO2 upgrading is considered as a low hanging fruit strategy for implementing CO2 utilization with minimum requirements for gas purification. This study investigates the detailed techno-economic analysis of using a hybrid bio-electrochemical process to convert waste CO2 streams from corn dry mill facilities into ethanol and demonstrates the impact of CO2 utilization on the biorefinery economics. The bio-electrochemical CO2 conversion process combines water electrolysis to hydrogen (H2), electrolysis of CO2-to-carbon monoxide (CO), and gas fermentation to ethanol. With onsite CO2 conversion, total ethanol yield can be potentially improved by 45%. In this study, variations of H2:CO ratio ranging from 0 to 5 are explored to understand the impact of gas mixture composition on economic viability. The techno-economic analysis results show single-pass carbon yield in the biological conversion step can be potentially improved up to 100% when using H2 as an alternative energy source. Upgrading waste CO2 generated from a corn dry mill facility via biological CO2 to ethanol conversion is likely to be cost effective under the following conditions: high CO2 electrolysis energy efficiency (≥70% theoretical), low electricity cost (≤$0.02/kWh), and high CO2 electrolysis conversion efficiency (≥50% CO2 input).