A Physics Constrained Methodology for the Life Cycle Assessment of Sustainable Aviation Fuel Production

Feedstock-to-fuel conversion, or “Fuel Production”, is a major contributor to greenhouse gas (GHG) emissions in life cycle assessment (LCA) of sustainable aviation fuels (SAF) from wastes. Here we construct and demonstrate an original mass and energy conserved chemically rigorous LCA methodology for the production of Hydroprocessed Esters and Fatty Acids-Synthetic Paraffinic Kerosene (HEFA-SPK) from Used Cooking Oil (UCO). This study proposes and demonstrates the use of; (i) the chemical composition of the UCO, (ii) the ASTM properties of HEFA-SPK, and (iii) the elemental mass and energy conserved reaction mechanism which converts one to the other, as physical constraints for the specific LCA of any UCO derived HEFA-SPK. With application of these constraints, the emissions embodied in UCO HEFA-SPK Fuel Production is found to range from 4.2 to 15.7 gCO2e/MJSAF depending on the renewability of the energy and hydrogen utilized. Imposition of (i)-(iii) as modelling constraints, derives a HEFA-SPK yield of 49 mass% a priori. This finding aligns with experimental literature, but brings attention to the higher yield estimations of 70–81% observed in current LCA tools. We show that this impacts the end LCA significantly as it adjusts allocation of emissions. A replication study of CORSIA’s (10.5 gCO2e/MJSAF) default core LCA value for Fuel Production, quantifies the increase at +5.3 gCO2e/MJSAF, or 15.8 gCO2e/MJSAF as total for Fuel Production. As the embodied emissions are significantly dependent on the specifics of the scenario assessed, we highlight reporting a definitive GHG intensity for any UCO derived HEFA-SPK as generic will be inaccurate to an extent.