## 4 Discussion  
### Techno-Economic Analysis Research on e-Fuel Production  
The techno-economic analysis of e-fuel production and utilisation presents a rapidly evolving landscape of opportunities, challenges, and implications. Various production technologies have been assessed across multiple studies, revealing contrasting efficiencies and intricacies based on geographical, technological, and economic contexts. 1. Production technologies and their efficiencies. This is a recurrent theme observed across the studies and includes, for example, the integration of catalytic deoxygenation, hydrothermal gasification, direct air capture (DAC), and methanation [70; 71]. These methods, combined with other processes such as Fischer-Tropsch and electrolysis, are central to producing synthetic fuels, including sustainable aviation fuel (SAF), methane, and e-diesel. The overall efficiency of these methods, however, varies. In the UK, for instance, the system efficiency for power-to-liquids stood at 25.6%, while in the US, a carbon conversion rate of 99% was achieved in a modelled FT process [51; 55]. Such variations emphasise the need for continuous optimisation and the incorporation of experimental and kinetics data into model improvements. 2. Capital and operational expenditures. Major contributors to costs have been identified in different scenarios. The significant expenses were associated with the electrolyser, the PV field, and H\({}_{2}\) storage [71]. The research by [53] further underscored the capital implications of processes like aqueous-phase reforming (APR), which led to a 6.6% increase in fixed capital investment. Operational expenses are equally impactful. Direct manufacturing costs, for instance, saw a decrease of 22% when APR was employed in Italy. 3. Break-even points and return-on-investment calculations. Break-even points, often denoted by metrics such as the minimum fuel selling price (MFSP), provide crucial insights into the economic feasibility of e-fuel production across different contexts. In the US, MFSP values varied based on nuclear power plant capacities, highlighting the significance of the energy source in determining production costs [55]. Similarly, the levelised cost of methane (LCOM) in Abu Dhabi and Benevento and the levelised cost of electricity (LCOe) in Saudi Arabia provide crucial benchmarks for assessing the economic implications of e-fuel production [52; 71]. While some studies suggest favourable return-on-investment scenarios--like the feasible methanol production costs observed in Turkey in 2022--others hint at the need for further economic incentives or technological improvements [56]. The LCOM values in Abu Dhabi and Benevento, for instance, exceeded prevailing market prices for methane [71]. This divergence accentuates the importance of contextualising techno-economic analyses and recognising the fluidity of market dynamics, technological advancements, and policy implications. In China the techno-economic analysis considers the costs associated with gasifying six types of crop straws, the Fischer-Tropsch (FT) synthesis process, and the required infrastructure and operations, evaluating these costs in terms of capital expenditure (CapEx) and operational expenditure (OpEx). The study demonstrates the economic viability of producing sustainable aviation fuel (SAF) from crop straws through the gasification-FT pathway, especially given China's abundant agricultural biomass resources in 2024. Corn straw, in particular, shows significant promise, supporting the potential for large-scale SAF applications in the future [73]. The analysis considers the costs associated with electrolysis for hydrogen production, direct air capture (DAC) for CO\({}_{2}\), and the processes of condensation and TEG absorption for producing synthetic natural gas (SNG). The study reveals that SNG costs can be significantly reduced with US tax credits for low-carbon hydrogen production.