## 3 Results and Findings from the Existing Literature
### Techno-Economic Analysis (TEA)  
The techno-economic analysis (TEA) is a vital component in evaluating the viability of e-fuels, providing insights into their economic feasibility, cost-effectiveness, and environmental impacts. This section systematically reviews various TEA assessments for e-fuels, exploring innovative production pathways, cost reduction strategies, and sustainability metrics. By examining methods such as catalytic deoxygenation, hydrothermal gasification, direct air capture, and power-to-liquid processes, this review highlights the economic and environmental potentials of e-fuels. The findings aim to inform policy decisions, guide industry practices, and identify areas for further research in the development of sustainable fuel alternatives. In the quest to develop e-fuels, the techno-economic analysis (TEA) serves as a crucial tool in assessing the feasibility, cost-effectiveness, and environmental impact of various production pathways. This section reviews relevant TEA assessments regarding the development of e-fuels. It includes various methods such as the integration of catalytic deoxygenation and hydrothermal gasification methods for producing hydrogen gas from esters and fatty acids, emphasising tall oil fatty acid (TOFA) [51]; direct air capture (DAC) combined with hydrogenation of CO2 to produce synthetic methane, as investigated by [PERSON] et al. [52]; and the integration of DAC, offshore wind farms, and electrolysis for producing power-to-liquid (PLL) SAF by [PERSON] and [PERSON] [51] in the UK. In a recent comprehensive study conducted in 2023, [PERSON] et al. [51] developed a detailed TEA concentrating on integrated catalytic deoxygenation and hydrothermal gasification methods. The primary aim was to produce H2 gas from esters and fatty acids, with a specific emphasis on tall oil fatty acid (TOFA). Notably, the study revealed that the minimum fuel selling price (MFSP) for scenario 2 was USD 0.39/L, a figure lower than the USD 0.62/L observed in scenario 1. A pivotal recommendation stemming from the study was the imperative need for concentrating efforts on process optimisation and model improvement [51]. This optimisation would be particularly advantageous, especially if guided by the inclusion of both experimental data and kinetics data. The overarching theme of the analysis centered on the TEA and LCA impacts associated with sustainable aviation fuel (SAF) production, utilising integrated catalytic deoxygenation and hydrothermal gasification methods. The findings indicate that optimising these processes could significantly reduce the minimum fuel selling price (MFSP), underscoring the importance of incorporating experimental and kinetics data for further improvements. Direct air capture (DAC) is widely investigated to capture CO2 emissions from decentralised sources [52]. As an alternative to geological storage, CO2 from DAC can be reacted with H2 from water electrolysis driven by renewable energy to produce synthetic CH4, increasing the penetration of renewable energies and leading to a circular carbon economy [52]. In DAC methanation, [PERSON] et al. [52] estimated the total project cost (TPC), levelised cost of CO2 removal (LCCR), and levelised cost of methane (LCOM). The process involved the hydrogenation of CO2 sourced from DAC, H2 generated from water electrolysis, and renewable PV sources, culminating in the production of synthetic methane (CH4). The results indicated that cities with varying solar energy distribution might have to invest heavily in H2 storage tanks. The major cost contributors were found to be the electroly, followed by the PV field, H2 storage, and reactors [52]. The LCOM was estimated at 4.9-8.2 USD-kgCH4\({}^{-1}\) using integration with the DAC plant investigated, while it was 3.1-3.9 USD-kgCH4\({}^{-1}\) when utilising integration with point-source carbon capture.