Comparative life cycle assessment of bio-based sustainable aviation fuel pathways

The aviation industry has started to look towards sustainable aviation fuel (SAF) as a potential alternative that can reduce the overall emissions of aircraft transportation. As such, this study aims to assess and compare the potential environmental impacts of two SAF production pathways: hydrothermal liquefaction (HTL) and gasification followed by Fischer-Tropsch (G+FT). Using a life cycle assessment (LCA) framework, the analysis focuses on a gate-to-gate approach, considering the stages from biomass receipt to biofuel separation with 20 kg of microalgae biomass as the common feedstock for both routes. SimaPro software v.9.6.0.1 is utilized to model the system while the ReCiPe 2016 method is employed at the midpoint and endpoint levels. Results show that the G+FT route exhibits higher potential impacts across all 18 midpoint categories evaluated. Specifically, human carcinogenic toxicity in G+FT (8.39 kg 1,4-DCB) is 25 times greater than in HTL (0.334 kg 1,4-DCB), while freshwater ecotoxicity (1.06 kg 1,4-DCB) and marine ecotoxicity (0.86 kg 1,4-DCB) in G+FT exceed HTL values (0.235 kg 1,4-DCB and 0.171 kg 1,4-DCB) by factors of 4.5 and 5.0, respectively. Contribution analysis reveals that the Fischer-Tropsch stage in the G+FT pathway and the hydrotreatment phase in HTL contribute the most emissions to these categories. At the endpoint level, the G+FT route causes greater damage to human health (3.82E-02 DALY), ecosystems (8.91E-04 species.yr), and resources (1.04E-03 USD2013) compared to HTL by factors of 18.0, 18.1, and 19.0, respectively. Potential improvements to the technical and environmental performance of the two pathways may be further explored to pave the way for commercial microalgae based SAF.