Life Cycle Assessment and Environmental Optimization of Alternative Aviation Fuels

Abstract

The aviation sector is a significant and growing source of global greenhouse gas emissions (GHGs), accounting for about 2.5% of global CO₂ and up to 4% of total climate impact when non-CO₂ effects are included. In response to the urgency of decarbonizing this sector, this thesis conducts a comprehensive Life Cycle Assessment (LCA) of fourteen alternative aviation fuel pathways, using the GREET 2024 model to evaluate their emissions, energy use, and environmental trade-offs. The study covers a wide range of fuels, including Alcohol-to-Jet (ATJ), Sugar-to-Jet, Hydroprocessed Esters and Fatty Acids (HEFA) from various feedstocks (canola, corn, soy, palm), and hydrogen-based systems, including hydrogen for fuel cell applications. Emissions are assessed across Well-to-Pump (WTP), Pump-to-Wake (PTW) and Well-to-Wake (WTW) stages, for two aircraft classes: Single Aisle (SA) and Large Quad (LQ). In the Well-to-Pump (WTP) analysis, Alcohol-to-Jet fuel from cellulosic biomass emerged as the most sustainable option, with the lowest total CO₂ emissions (14.11 g), no land use change emissions, and the highest energy efficiency (3258 kJ), supported by renewable energy credits in the GREET 2024 model. In contrast, corn-based standalone SAF showed the highest WTP emissions (57.84 g CO₂), along with significant land use change impacts (9.01 g CO₂e) and elevated CH₄ and NOₓ emissions. Sugar-to-Jet (STJ) fuels offered intermediate performance, with the biological route showing lower total CO₂ (21.29 g) but higher total energy use (4537 kJ). The lignocellulosic forest residue pathway showed the lowest GHG-100 emissions (1.93 g CO₂e) and total energy input (1026 kJ), confirming that waste-derived fuels are environmentally preferable. These findings highlight that feedstock type and conversion method significantly affect upstream emissions, positioning cellulosic and residue-based pathways as leading candidates for decarbonizing aviation fuel production. The normalized results show that HEFA from corn oil has the lowest WTW emissions, with emissions (0.0914 gCO_2e/kg·km) for SA and emissions (0.1050 gCO_2e/kg·km) for LQ, reducing GHG emissions by more than 85% compared to conventional jet fuel. ATJ from cellulosic biomass also performs exceptionally well, reaching emission (0.1400 gCO_2e/kg·km) for SA and (0.1440 gCO_2e/kg·km) for LQ, reflecting over 81–85% emission reductions. Hydrogen as a fuel for fuel cells produce the lowest use-phase emissions, achieving near-zero emissions (0.015 gCO_2e/kg·km) for SA, though their total WTW emissions (0.2350–0.2520 gCO_2e/kg·km) depend on the hydrogen production method. Fuels based on coal and natural gas using Fischer-Tropsch synthesis show the highest normalized emissions, with WTW values of emission (0.9635 gCO_2e/kg·km) ` for SA and emission (1.0000 gCO_2e/kg·km) for LQ, making them the least sustainable options. Energy and water use were also analyzed. For instance, ATJ-Cellulosic had the lowest total energy demand (3258 kJ), mostly sourced from renewables, and the highest energy delivery efficiency, exceeding 99.6%. In contrast, STJ pathways consumed more energy (up to 4537 kJ), and soy-based HEFA required the most water (2358 cm³/MJ), due to irrigation. In conclusion, the study identifies waste-based and renewable fuels—especially ATJ-cellulosic, HEFA-corn oil, and hydrogen fuel cells—as the most promising solutions for reducing aviation emissions. While each fuel type has limitations in cost, infrastructure, or feedstock availability, these options offer the strongest potential for decarbonizing air travel and achieving long-term climate goals.