From waste to asset: Turning ethanol production CO₂ into jet fuel

Manufacturing sustainable aviation fuel with CO₂ byproducts of ethanol production could reduce carbon intensity by more than 80% compared to fossil fuels.

The CO₂ released from corn during ethanol production could actually be a valuable, underutilized resource for producing aviation fuel rather than a waste byproduct, according to a study published in the SAE International Journal of Sustainable Transportation, Energy, Environment, & Policy.

Unlike the CO₂ from coal plants or cement kilns, which requires a lot of energy to capture, fermentation to produce ethanol releases very pure streams containing 85% CO₂ by volume or higher. As the corn plants sequestered CO₂ from the air, capturing the CO₂ released from fermentation and using it as fuel would reuse CO₂ without adding more to the atmosphere.

“It is exciting to explore whether this ‘waste’ stream can actually become a significant asset, turning inefficiency into advantage and accelerating the real-world application of emerging technologies,” said Stephen McCord, a research scientist in mechanical engineering at U-M and lead author of the study.

With aviation producing over a gigaton of fossil CO₂ emissions annually, sustainable aviation fuel produced from non-fossil carbon stocks can help reduce these emissions. Often made from biomass waste or cooking oils, small percentages of sustainable aviation fuels are already blended with conventional kerosene fuels, with the aviation industry and travelers pushing towards larger integration.

The United States produced 15.6 billion gallons of ethanol in 2023, releasing 48 megatons of CO₂, offering a route to produce sustainable aviation fuel at scale. With several different ways to make sustainable aviation fuel from bioethanol, the research team compared pathways to determine the ones with the lowest environmental impact.

“We hope to inform future development and policy by highlighting which routes are most promising for reducing aviation’s carbon footprint using existing waste resources,” said Volker Sick, former Director of the Global CO₂ Initiative and the DTE Energy Professor of Advanced Energy Research at U-M and senior author of the study.

The current corn-based sustainable aviation fuel production method chemically modifies ethanol to make aviation fuel through a process called Alcohol-to-Jet. Although it has a high fuel yield of 90%, this route only reduces carbon intensity by about 4.5% to 20% compared to kerosene jet fuel.

The research team compared this method to two CO₂-based routes. Both methods begin by converting captured CO₂ into syngas, a mixture of carbon monoxide (CO) and hydrogen gas (H₂). The gas fermentation route uses syngas to create ethanol as an intermediate step, then uses Alcohol-to-Jet to produce fuel. The Fischer-Tropsch Synthesis route instead feeds syngas into a reactor, synthesizing the long-chain liquid hydrocarbons that make up jet fuel.

A life cycle assessment found both approaches outperformed conventional methods, with Fischer-Tropsch Synthesis projected to reduce carbon intensity by up to 90% while gas fermentation was projected to reduce it by 84%.

When considering existing bioethanol facilities and workforce skills, gas fermentation followed by Alcohol-to-Jet came out on top as the path with the smoothest transition despite a slightly lower carbon intensity reduction.

“A variety of factors need to be considered when planning how to produce large quantities of sustainable aviation fuels from CO₂. Starting with CO₂ from corn ethanol fermentation promises the fastest path to scaling up this new industry,” said Sick.

As a follow-up, the research team assessed the economic competitiveness of these two pathways to understand which would operate best in real-world conditions and which could be deployed quickest in the US.

With electrification and hydrogen both facing significant technical and practical obstacles for long-distance air travel, hydrocarbon fuels will likely remain essential for aviation in the foreseeable future.

“These conversion routes provide a viable way to ‘defossilize’ aviation fuel and make meaningful progress towards reducing aviation’s carbon footprint—offering a realistic, near-term solution where alternatives are limited,” said McCord.