What are the pathways for producing drop-in biofuels and their technical challenges?

Drop-in biofuels are hydrocarbon fuels compatible with existing infrastructure, differing from oxygenated biofuels (ethanol, biodiesel). Production pathways: Hydrotreating/hydroprocessing: Vegetable oils, animal fats, or algal lipids hydrotreated to produce hydrocarbons; removes oxygen, saturates double bonds; renewable diesel (HVO), sustainable aviation fuel (SAF); commercial technology (Neste, Diamond Green). Gasification and Fischer-Tropsch: Biomass gasified to syngas, converted to liquid hydrocarbons; produces range of alkanes; capital intensive; Choren, Fulcrum BioEnergy. Pyrolysis and upgrading: Fast pyrolysis produces bio-oil; requires hydrotreating to upgrade to stable fuel; Ensyn, BTG. Fermentation to hydrocarbons: Engineered microbes produce isoprenoids, fatty alcohols, or alkanes directly; Amyris (farnesene), REG Life Sciences; lower yields than ethanol. Alcohol-to-jet (ATJ): Ethanol or isobutanol dehydrated, oligomerized, hydrogenated to jet fuel; Gevo, LanzaTech. Challenges: Feedstock cost and availability; hydrogen requirements for hydrotreating; process efficiency; meeting fuel specifications (ASTM D7566 for aviation); economics versus fossil fuels without policy support.