## 3 Results and Findings from the Existing Literature
### Life Cycle Analysis (LCA)
GHG emissions were quantified, resulting in values of 7 and \(-\)25 gCO\({}_{2}\)e/MJ, showing a potential WTW GHG emissions reduction of at least 92%. This highlights the potential of such methods to significantly reduce GHG emissions and enhance energy security. It was reported that by enhancing biogas content through purification, the cogeneration plant efficiency increased [56]. Additionally, the enrichment of biogas led to reduced emissions, thus ensuring a more sustainable energy production route. The life cycle implications of the methanol production process revealed significant influences from electricity prices, electrolyser capital costs, and plant capacity factors [57]. Furthermore, the study suggested that the methanol production process's competitiveness is expected to increase in the mid-term future, driven primarily by new European policy directions. [PERSON] [58] highlighted the environmental superiority of green hydrogen over blue hydrogen, as the former is carbon-free and does not require CO\({}_{2}\) storage. Further, the life cycle analysis pinpointed the need for advancements in essential processes for synthetic fuel production and stressed the potential synergy of local production combined with imported hydrogen or fuels. From a life cycle viewpoint, the innovative process of combining the power-to-methanol process with MSW gasification presents a forward-thinking solution, valorising carbon within the MSW and decreasing dependency on fossil fuels [59]. The life cycle analysis also highlighted the potential of the hybrid scheme, which, considering future renewable energy trends and decreasing power costs, could emerge as an attractive solution in the near future. [PERSON], [PERSON] [69] undertook a comprehensive life cycle assessment focusing on fleet emissions and the overall climate impact. The methodology utilised involved producing hydrogen via water electrolysis followed by Fischer-Tropsch synthesis, aiming at power-to-liquid production. The study considered the environmental impacts and potential of PtL, particularly for the aviation sector. One striking result showed that PtL derived from wind energy could reduce environmental impact by up to 42%. Another noteworthy finding was the potential reduction in warming impact due to the phenomenon of contrail-induced cloudiness. In 2018, [PERSON], [PERSON] [63] focused on a life cycle analysis (LCA) of OME3-5 e-fuels produced through the anhydrous trioxane and OME1 pathways, considering inputs such as flue gas, water, electricity, and steam. OME3-5 e-fuels have been identified as a promising option for decarbonising heavy-duty transportation in the future. [PERSON], [PERSON] [64] reported that where a substantial portion of electricity generation relies on fossil fuels, e-fuel production using renewable power demonstrates significant advantages in terms of GHG intensity. However, even with 47% renewable power in the mix, the life cycle emissions remain only 25% lower than those of traditional grey methanol. The findings indicate that the GHG footprint of e-fuels remains higher than that of grey fuels, particularly in regions with a larger share of fossil-based grid electricity. This review highlights the significance of transitioning to cleaner energy sources to optimise the sustainability of e-ammonia production. [PERSON], [PERSON] [66] highlighted that abatement costs for e-fuels were considerably higher, ranging from 544 to 6424 V/tCO2-eq, when compared to competing vehicle fuel technology options. Furthermore, the analysis revealed a significant improvement in e-fuel production costs and life cycle emissions by 2030, with estimates indicating 3.24 V/leq diesel and 6.63 gCO2-eq/MJ, underscoring the potential for cost reduction and emissions mitigation in 2030. The global quest for sustainable energy sources represents a critical challenge, and the United Nations Sustainable Development Goals (UNSDGs) offer a comprehensive framework to address this challenge.