Environmental performance of sustainable aviation fuels
Kas Aghaei Nahri, Misagh (2025)
Diplomityö
2025
School of Energy Systems, Ympäristötekniikka
Kaikki oikeudet pidätetään.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe20251201113078
https://urn.fi/URN:NBN:fi-fe20251201113078
Tiivistelmä
Climate change is one of the most urgent global challenges, and aviation plays a disproportionately large role in warming despite contributing only 2-3% of global CO₂ emissions. This heightened impact stems from non-CO₂ effects, such as contrails, cirrus clouds, and nitrogen oxides, which amplify radiative forcing and make decarbonising aviation a priority for international climate policy. Sustainable Aviation Fuels (SAFs) have emerged as a central mitigation strategy, particularly electro-kerosene, hydrogen, and ammonia, which offer substantial long-term potential when produced using renewable electricity.
This thesis applies an attributional life cycle assessment (LCA) to compare these three emerging SAF pathways using a well-to-tank (WTT) system boundary and a functional unit of 1 MJ of fuel. Using GaBi and ISO 14040/44 standards, upstream processes including electrolysis, CO₂ capture, nitrogen separation, Fischer-Tropsch synthesis, and ammonia synthesis are modelled. Six configurations combining solar or wind electricity with AEL, PEM, or SOE electrolysis are assessed. Additional midpoint categories, freshwater eutrophication, marine eutrophication, and terrestrial acidification are also included.
Results show that solar-based systems consistently exceed the upstream emissions of conventional jet fuel (CJF = 17.13 g CO₂-eq/MJ). Solar + AEL produces 22.8 g CO₂-eq/MJ for e-kerosene, 15.4 for hydrogen, and 21.3 for ammonia, with both e-kerosene (+33%) and ammonia (+24%) exceeding CJF. Solar + PEM intensifies this trend, reaching 28.9, 20.1, and 26.2 g CO₂-eq/MJ, all of which are significantly higher than CJF (+69% to +52%). Only hydrogen under Solar + AEL and Solar + SOE (15.4-12.2 g CO₂-eq/MJ) achieves values slightly below CJF (-10% to -29%).
In contrast, wind-powered systems substantially outperform CJF across all fuels. Wind + AEL results in 7.5, 4.8, and 17.2 g CO₂-eq/MJ for e-kerosene, hydrogen, and ammonia, where e-kerosene (-56%) and hydrogen (-72%) show large reductions; ammonia is approximately equal to CJF. The best-performing pathway, Wind + SOE, achieves 6.8 g CO₂-eq/MJ for e-kerosene, 4.2 for hydrogen, and 10.1 for ammonia, representing reductions of 60%, 75%, and 41% relative to CJF.
Across all pathways, hydrogen consistently exhibits the lowest WTT climate impacts (4.2-20.1 g CO₂-eq/MJ), followed by ammonia and electro-kerosene. However, electro-kerosene remains the most feasible near-term option because it is fully compatible with existing aircraft engines and refuelling systems. Hydrogen and ammonia offer deeper long-term reductions but require advancements in storage, engines, and safety systems.
Overall, the findings demonstrate that electricity carbon intensity is the dominant determinant of SAF environmental performance. Wind-based electrolysis reduces WTT emissions by 65-85% compared to solar pathways and consistently outperforms CJF. Electrolyser choice further influences results, with SOE technologies offering the lowest impacts due to high electrical efficiency. These conclusions highlight that meaningful climate benefits from SAFs are achievable only when production is paired with low-carbon electricity systems and high-efficiency electrolysers.
This thesis applies an attributional life cycle assessment (LCA) to compare these three emerging SAF pathways using a well-to-tank (WTT) system boundary and a functional unit of 1 MJ of fuel. Using GaBi and ISO 14040/44 standards, upstream processes including electrolysis, CO₂ capture, nitrogen separation, Fischer-Tropsch synthesis, and ammonia synthesis are modelled. Six configurations combining solar or wind electricity with AEL, PEM, or SOE electrolysis are assessed. Additional midpoint categories, freshwater eutrophication, marine eutrophication, and terrestrial acidification are also included.
Results show that solar-based systems consistently exceed the upstream emissions of conventional jet fuel (CJF = 17.13 g CO₂-eq/MJ). Solar + AEL produces 22.8 g CO₂-eq/MJ for e-kerosene, 15.4 for hydrogen, and 21.3 for ammonia, with both e-kerosene (+33%) and ammonia (+24%) exceeding CJF. Solar + PEM intensifies this trend, reaching 28.9, 20.1, and 26.2 g CO₂-eq/MJ, all of which are significantly higher than CJF (+69% to +52%). Only hydrogen under Solar + AEL and Solar + SOE (15.4-12.2 g CO₂-eq/MJ) achieves values slightly below CJF (-10% to -29%).
In contrast, wind-powered systems substantially outperform CJF across all fuels. Wind + AEL results in 7.5, 4.8, and 17.2 g CO₂-eq/MJ for e-kerosene, hydrogen, and ammonia, where e-kerosene (-56%) and hydrogen (-72%) show large reductions; ammonia is approximately equal to CJF. The best-performing pathway, Wind + SOE, achieves 6.8 g CO₂-eq/MJ for e-kerosene, 4.2 for hydrogen, and 10.1 for ammonia, representing reductions of 60%, 75%, and 41% relative to CJF.
Across all pathways, hydrogen consistently exhibits the lowest WTT climate impacts (4.2-20.1 g CO₂-eq/MJ), followed by ammonia and electro-kerosene. However, electro-kerosene remains the most feasible near-term option because it is fully compatible with existing aircraft engines and refuelling systems. Hydrogen and ammonia offer deeper long-term reductions but require advancements in storage, engines, and safety systems.
Overall, the findings demonstrate that electricity carbon intensity is the dominant determinant of SAF environmental performance. Wind-based electrolysis reduces WTT emissions by 65-85% compared to solar pathways and consistently outperforms CJF. Electrolyser choice further influences results, with SOE technologies offering the lowest impacts due to high electrical efficiency. These conclusions highlight that meaningful climate benefits from SAFs are achievable only when production is paired with low-carbon electricity systems and high-efficiency electrolysers.