Several technologies have been developed to produce hydrocarbon biofuels – renewable diesel (RD) and sustainable aviation fuel (SAF) – from fats, oils, and greases (FOG), with the hydroprocessing of esters and fatty acids (HEFA) representing one of the most mature pathways. In its current form, HEFA is mainly reliant on the hydrodeoxygenation (HDO) reaction, which has several drawbacks since HDO requires large amounts and pressures of hydrogen, feedstocks of high purity and cost, as well as problematic sulfided catalysts that risk contaminating the biofuel product with sulfur. A process based on decarboxylation/decarbonylation (deCOx) offers an attractive alternative to HDO, since it requires lower amounts and pressures of hydrogen, feedstocks of low purity and cost, and simple supported metal catalysts. Herein, several geographically distributed oleaginous feedstocks – ranging from municipal waste feeds (brown grease) to pine chemicals (tall oil and rosin) – were upgraded to RD and SAF via deCOx. Powdered and engineered Ni-based catalysts were used for FOG-to-RD conversion via deCOx, evaluating deoxygenation over reducible and non-reducible oxides.
Engineered alumina-based catalyst showed superior deoxygenation activity and stability for up to 300 hours on stream. Similarly, quantitative conversion of FOG to SAF was achieved over bifunctional Ni-Cu-based catalysts with zeolitic supports, with deCOx and isomerization occurring in a single step. Initial screening studies performed in a semi-batch reactor revealed that upgrading distilled tall oil (DTO) over a Ni-Cu-based catalyst afforded all types of hydrocarbons comprising SAF, namely n-alkanes, iso-alkanes, cycloalkanes, and aromatics. The same combination of feed, catalyst, and reaction conditions were applied in a fixed-bed reactor for a continuous experiment, consisting of two 72-hour cycles with catalyst regeneration in between. DTO conversion remained quantitative (~100%), with aromatic yields ≥80% regardless of time-on-stream. Most liquid products fell within the carbon number and boiling point range of jet fuel across all samples. Notably, the reaction produced all hydrocarbon classes found in SAF, with particular abundance of aromatic hydrocarbons. Since ~20% aromatics are required to swell elastomeric seals and prevent leaks in aircraft fuel systems, seal compatibility testing confirmed that the aromatics-rich SAF blendstock exhibited a volume swell percentage comparable to qualified SAF blends. Catalysts used for deoxygenation reactions were characterized using various techniques – including N2 physisorption, X-ray diffraction, X-ray photoelectron spectroscopy, microscopy, and temperature-programmed methods – to rationalize trends, propose reaction pathways, and elucidate structure-activity relationships. Finally, to evaluate the economic and environmental feasibility of this technology, techno-economic and lifecycle analyses were conducted on an integrated plant combining catalytic deoxygenation and hydrothermal gasification, producing hydrogen for converting tall oil fatty acid to SAF. The analyses revealed a minimum fuel-selling price of USD$0.39/L – lower than that of existing SAF pathways (USD$1.4/L) – with greenhouse gas emissions of 5.1g CO2-eq/MJ, which is 94% lower than fossil jet fuel (85g CO2-eq/MJ).