(336c) Sustainable Aviation Fuel Production: An Enviro-Economic Assessment of Direct CO2 Hydrogenation

Reducing the greenhouse gases emissions from human activities is a key challenge towards net-zero and a sustainable future (IEA 2020). Transportation plays an important role in everyday life and the aviation industry emits 2% of the total GHG emissions, with a consumption of 10% of the fuel worldwide, and is recognised as a particularly hard to decarbonise sector. Several challenges reduce the number of alternatives to fossil fuels for the aviation sector, including the high energy content per unit mass and volume required and the long service life of the airplanes. 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 (Royal Society 2023). According to IEA (2022) low-carbon hydrogen-derived liquid fuels will gain significant market share in the sector, as batteries and hydrogen are not feasible alternatives, especially for long-rage flights, due to the low energy content per unit of mass and volume respectively (Karadotcheva et al. 2021). Moreover, hydrogen-fuelled aircrafts would require a complete redesign and advanced materials to avoid hydrogen embrittlement and leakage (Airbus 2020). Lastly, drop-in fuels can leverage existing infrastructure for fuel distribution and storage, which is an important enabler for a fast transition.

Sustainable aviation fuel (SAF) is a broad term that includes biofuels and synthetically derived hydrocarbons, or e-fuels. Significant research efforts have been devoted to identifying routes to produce liquid fuels from CO₂ and H₂ leveraging the Fisher-Tropsch (FT) synthesis, a well-established thermochemical process to produce hydrocarbons of different chain length distribution from syngas, a mixture of CO and H₂. However, syngas is nowadays produced from steam methane reforming or coal gasification and carries a high environmental burden. To reach a significant reduction in the greenhouse gas emissions, CO₂ (biogenic of from direct air capture, DAC) must be used as the carbon source in the FT process along with sustainable H₂. One option is to convert CO₂ and H₂ in a reverse water gas shift (RWGS) reactor and use the resulting syngas in a traditional FT process (Zang et al. 2021). The main limitations of this two-step approach are: (i) RWGS reaction requires high temperatures to achieve a good conversion (600-1000 °C); and (ii) the production of wax, a mixture of heavy hydrocarbons, 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 the selectivity towards liquid fuels is usually low, and the formation of short chain hydrocarbons is favoured (Gao et al. 2017). Recently, Yao et al. (2020) 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 Zang et al. (2021). As feedstock, we consider CO₂ from DAC as carbon source, available at 25 °C and 1 bar (Keith et al. 2016) and H₂ from water electrolysis using wind energy, available at 20 bar and 25 °C (Al Qahtani et al. 2021). Both feed streams are assumed to have no impurities. 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 (Huijbregts et al. 2017) and GWP100 is evaluated alongside the monetized endpoint impacts to human health, ecosystem quality and resource scarcity. The monetisation factors to translate the endpoint impacts to a common monetary basis are obtained from Dong et al. (2019) converted to $₂₀₂₁.

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-tank basis both 1sFT- and 2sFT-derived fuels have a negative GWP potential and outperform the fossil-fuel counterpart, while on a well-to-wake basis the 1sFT process is predicted to reduce GHG emissions by 75% and the 2sFT process by 60% (Figure 1a). The analysis of the endpoint environmental impacts confirms a better environmental performances 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 externalities cost 80% and 30% lower that the fossil fuel, respectively (Figure 1b). The largest share of externalities cost in both the one-step and two-step processes is attributed to the damage to resource scarcity due to the high energy requirements of DAC and therefore alternative DAC processes should be considered to move further away from the fossil-dependency of the aviation sector. At current CO₂ and H₂ prices the productions cost of low-carbon synthetic fuels is predicted to be 4-6 times higher than fossil-based aviation fuels, which is in agreement with the literature. And even if the CO₂ and H₂ prices were to decrease significantly, policy interventions such as carbon taxation would likely remain necessary for synthetic fuels to become cost competitive. Future work will focus on include additional options for H₂ and CO₂ procurement, including blue hydrogen, alternative DAC processes, and biogenic carbon sources.

Acknowledgement: Funding from the Engineering and Physical Sciences Research Council (EPSRC) for the research under the UKRI Interdisciplinary Centre for Circular Chemical Economy programme (EP/V011863/1) is gratefully acknowledged.

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