A Life Cycle Assessment of Potential Pathways to Increase Sustainable Aviation Fuel Yields through CO<sub>2</sub> Upgrading Co-located with Corn Ethanol Production

<div>Alcohol-to-jet (ATJ) upcycling of ethanol to sustainable aviation fuel (SAF) is an attractive emerging pathway for SAF production, especially in the US Midwest with large-scale corn ethanol production. Only 39% of the corn carbon is converted to ethanol, 20% is emitted as CO<sub>2</sub>. Capturing the CO<sub>2</sub> to produce additional ethanol or SAF directly can increase the carbon yield. To guide technology selection, this work used life cycle assessment for several CO<sub>2</sub>-to-SAF production pathways. Additionally, improvements for corn ethanol production were explored by replacing natural gas burners with heat pumps for corn drying, which reduced the carbon intensity of corn ethanol by nearly 16%. But subsequent upgrading of the ethanol to SAF is only 4.5–20% better than conventional aviation fuel. By contrast, CO<sub>2</sub>-based alternative routes to SAF fared better, reducing carbon intensities between 83% and 90%. Gas fermentation of CO<sub>2</sub> to ethanol with subsequent ATJ upcycling to SAF was contrasted to Fischer–Tropsch conversion of CO<sub>2</sub> to SAF. Both streams require CO<sub>2</sub> conversion to CO, which can be produced using reverse water–gas shift or solid oxide electrolyzer cells. The Fischer–Tropsch synthesis shows a higher reduction in carbon intensity (up to 90%) compared to ATJ (up to 84.4%). For other impact categories, such as ozone depletion, ecotoxicity, and the like, the differences are of similar magnitude. Capturing CO<sub>2</sub> locally at the bioethanol factory and converting that CO<sub>2</sub> to ethanol might overall be preferable with a fermentation process that is quite like bioethanol production compared to Fischer–Tropsch synthesis for which products require a new transportation infrastructure. The aviation fuel yield from ATJ can reach 90%, higher than the 50–70% yield from Fischer–Tropsch synthesis, with gasoline and diesel fuel as major by-products for which markets will shrink in the future. Overall, ATJ appears to be the best choice for CO<sub>2</sub>-to-SAF using the synergy with corn ethanol factories for quick launch.</div>