The aviation industry is responsible for 2% of the total GHG emissions and 10% of the fuel consumption worldwide, with a predicted market growth of 3.7% a year. It is expected that a large portion of the current global fleet will continue to be operational until 2040-2050 calling for the development of drop-in alternatives based on sustainable resources [1].

Significant research efforts have been devoted to identifying routes to produce liquid fuels from CO₂ and H₂ leveraging the Fisher-Tropsch (FT) synthesis. One option is to feed CO₂ and H₂ in a reverse water gas shift reactor (RWGS) and use the resulting syngas in a traditional FT process [2]. The limitations of this approach are: (i) the RWGS reactor operates at high temperatures (600-1000 °C); and (ii) the FT reactor produces a significant amount of wax, a mixture of heavy hydrocarbons that requires additional upgrading units, such as a hydrocracking (HC) reactor to improve the yield of liquid fuels. The direct hydrogenation of CO₂ to liquid fuels in a single step is more appealing, but usually the formation of short chain hydrocarbons is favoured [3]. Yao et al. (2020) [4] synthesized a novel Mn-Fe-K based catalyst capable of converting CO₂ and H₂ with excellent yield and selectivity towards liquid hydrocarbons in the jet fuel range (C8–C16) and minimal wax production.

In this work, we carry out a techno-economic analysis and life-cycle assessment (LCA) of two SAF production processes: a one-step process (1sFT) based on the above-mentioned catalyst; and a two-step process (2sFT) based on the work of [2]. Both processes are fed with CO2 from direct air capture (DAC) and green H2. 1sFT and 2sFT are simulated at scale using Aspen HYSYS to evaluate their economic performances and obtain inventories for the LCA. The environmental assessment is conducted in SIMAPRO 9.3, using Ecoinvent 3.8 Cut-Off database for the background process inventories. The fuel obtained from 1sFT and 2sFT is compared against the fossil-based alternative considering different environmental KPIs, as for SAF to be truly sustainable the reduction is GHG emission should not be associated with high collateral ecological damage, a problem often referred to as burden shifting. ReCiPe2016 is used as life-cycle impact assessment method [5] and GWP100 is evaluated alongside the monetized endpoint impacts to human health, ecosystem quality and resource scarcity. 

Our results show that the one-step process is superior both in economic and environmental terms to the two steps process, due to a lower capital cost, higher selectivity towards liquid hydrocarbons and lower energy requirements. On a well-to-wake basis the 1sFT process is predicted to reduce GHG emissions by 75% and the 2sFT process by 58%. The analysis of the endpoint environmental impacts confirms a better environmental performance of the synthetic fuels compared to the fossil counterpart, with the 1sFT outperforming the 2sFT by a larger extent, with 1sFT and 2sFT having a total externality cost 44% and 22% lower that the fossil fuel, respectively. At current CO2 and H2 prices the productions cost of low-carbon synthetic fuels is predicted to be 6-8 times higher than fossil-based aviation fuels, which is in agreement with the literature, and a combination of feedstock cost reduction and policy intervention (e.g. carbon taxation) is necessary for synthetic fuels to become cost competitive.