Research Team: Julie Witcover, Ph.D. (lead), Colin Murphy, Ph.D.
UC Campus(es): UC Davis
Problem Statement: In general, with just about any type of power (hydrogen, electric, etc), substantial volumes of low-carbon liquid fuels are necessary. However, aviation is widely considered to be the most difficult transportation mode to decarbonize. Substantial projected increases in long-distance air travel this century mean it remains a growing problem for California’s climate goals. Several lower carbon aviation fuels have received ASTM certification at specified blends in jet fuel and have been used in commercial use. Unusual among advanced biofuels, alternative aviation fuels have lined up investment, financing, and off-take agreements. In 2019, aviation fuels became eligible to earn Low Carbon Fuel Standard credits as an opt-in fuel. Interest from local governments and airports is surging. It is currently being discussed what the long-term carbon budget should be for aviation. Moreover, as California pushes to decarbonize transportation, how best to approach aviation fuels and harness the private sector interest without detracting from on-and off-road heavy duty sector decarbonization is paramount.
Project Description: This paper reviews literature on technological, market, and policy factors affecting the growth of alternative aviation fuels. At present, they represent a minimal fraction of global aviation fuel used but are a critical tool for lowering GHG emissions from aviation. Even with electric and hydrogen power, substantial volumes of low-carbon liquid fuels are likely needed; these will draw heavily on biomass. Beyond hydroprocessed esters and fatty acid (HEFA) fuels, technologies, including lower carbon e-fuels, remain pre-commercial. More jurisdictions are providing incentives for alternative aviation fuel, and some on-road biofuels may be redirected towards aviation in a favorable market, because production processes for these fuels overlap. Biomass feedstocks at different demand levels need to be sourced and evaluated for unintended impacts. Research suggests alternative aviation fuels improve air quality impacts compared to conventional jet fuel. Key uncertainties in scaling alternative jet fuel remain, including ongoing concerns about land use change from biofuels, how to right-size incentives with no technology clearly dominant, what the long-term carbon budget is for aviation, and how to build fuel delivery infrastructure.