(571g) Reducing Carbon Loss in Bioethanol Plants: Co-Producing Formic Acid from CO?

In 2023, the United States produced 15,620 million gallons of ethanol, with 14,249 million gallons consumed domestically. For cellulosic feedstocks such as corn stover, the theoretical maximum ethanol yield is approximately 80 gallons per ton of biomass—equivalent to 0.237 tons of ethanol per ton of feed. This represents a 76.3% mass loss, predominantly residing in CO₂ and byproduct streams. In a representative cellulosic bioethanol facility modelled by the National Renewable Energy Laboratory (NREL), only about one-third of the carbon input is converted into ethanol, while the remaining two-thirds is lost as CO₂ during fermentation and combustion processes. Thus, there potentially is significant room to improve the carbon conversion efficiency in bioethanol processes.

This paper investigates strategies to enhance carbon efficiency in bioethanol production by valorizing CO₂ emissions into valuable chemical products. We identify formic acid as a particularly promising co-product due to its low theoretical energy requirement for production—270 kJ/mol of CO₂—making it the most energetically favorable option among potential reduction products such as lactic acid, ethylene glycol, and propanol. Additionally, formic acid exhibits a favorable economic profile, where its market price exceeds the minimum theoretical cost of electricity required for its synthesis from CO₂.

We examine several process scenarios, including utilizing surplus electricity generated on-site at the NREL plant to electrolytically reduce a portion of CO₂ into formic acid, as well as importing electricity to reduce all available CO₂. Sensitivity analyses are conducted to evaluate the economic viability of each scenario under varying electricity, CO₂, and formic acid prices. The results of these studies provide insights into the conditions under which carbon utilization becomes economically attractive as well as assist in identifying optimal deployment strategies for future bioethanol facilities.