## 3 Results and Findings from the Existing Literature
### Charting Data: Exploring Optimal Methods Both Economically and Environmentally  
A variety of e-fuel production methods have been explored globally. Some examples follow. Direct fermentation and gasification plus fermentation: used in China, this method utilises corn, cassava, and corn cob feedstocks to produce ethanol-to-jet fuel (ETJ) [67]; integrated catalytic deoxygenation and hydrothermal gasification: employed in the US, the process used esters and fatty acids as feedstocks, aiming for hydrogen gas production [70]; hydrogenation processes: various methods of hydrogenation have been utilised across multiple countries including Abu Dhabi, Italy, and Turkey, using different sources of CO2 and H2 to produce synthetic methane, methanol, and other e-fuels [71]; power-to-liquid (PtL) processes: both the UK and Germany have used methods involving direct air capture (DAC) units, water electrolysis, and refinery plants, targeting power-to-liquid production for jet fuel hydrocarbons [51]; and Fischer-Tropsch (FT) synthesis: utilised in several studies, spanning the US and Europe, this process has been used with various feedstocks like H2, waste CO2, e-hydrogen, and biomass to produce diverse e-fuels [54]. The economic viability of e-fuel production processes varies. For example, the hydrogenation of CO2 from DAC in Abu Dhabi and Benevento indicated the electrolyser as the most expensive component, with the least cost of methane production being between 3.1  
Figure 7: Techno-economic (TEA) and life cycle analyses (LCA) on e-fuels supporting UNSDGs. and 3.9 USD kgCH\({}_{4}\)\({}^{-1}\) when using point-source capture [71]; the integrated catalytic deoxygenation and hydrothermal gasification in the US showcased that scenario 2 (0.39 USD/L) was more cost-effective than scenario 1 (0.62 USD/L) [68]; a study in Italy using hydrogenation, electrolysis, and aqueous-phase reforming showed that the advanced scenario could produce fuels at a minimum selling price of 1.84 USD/kg [53]. However, it is essential to consider that while some methods might currently seem less economically viable, such as the e-methanol production in Italy at 960 USD/t, they have the potential to become competitive in the future due to evolving market conditions and policy frameworks [57]. Life cycle assessment (LCA) results provide insights into the environmental sustainability of the different methods. The ETJ production in China using direct fermentation and gasification indicated the lowest GHG emission at 31.66 gCO\({}_{2}\)eq/M] jet fuel [67]; PL production in the UK revealed a greenhouse gas potential of 21.43 gCO\({}_{2}\)eq/M]SAF, which is below the UK mandate of a 50% emission reduction compared to fossil jet fuel [51]; a unique method in the US integrating Fischer-Tropsch fuel production with corn ethanol by product CO\({}_{2}\) showcased an impressive WTW GHG emissions reduction by 90-108% against petroleum fuels [68]; and another study in the US involving nuclear energy demonstrated the potential to achieve a WTW GHG emissions reduction of at least 92% [55]. Conclusively, while economic viability is a crucial factor in the widespread adoption of e-fuel production methods, the combined techno-economic and life cycle assessments highlight the importance of balancing costs with environmental sustainability. As the global community continues to prioritise low-carbon solutions, these integrated assessments become pivotal in charting the path forward for sustainable aviation fuels and other e-fuels [64]. These studies employed an integrated approach that combines life cycle assessment (LCA) and techno-economic analysis (TEA) to assess the baseline climate change impact and cost of a standard system. In this analysis, both the LCA model and TEA model (Figure 8) share common scenarios, system boundaries, and input parameters. Figure 8: Integrated LCA and TEA framework reviewed in this study.