(546b) Prospective Absolute Environmental Assessment of Sustainable Aviation Fuels

Aviation is a particularly challenging sector to decarbonize due to its high-energy-density fuel requirements and the lack of commercially available and scalable low-carbon technologies [1,2]. As such, it still heavily relies on fossil fuels, with a share of over 99% according to the International Air Transport Association (IATA) [3]. The sector is responsible for 2.5% of current global CO₂ emissions, the equivalent to around 950 Mt of CO₂ annually, and is predicted to surpass pre-pandemic levels by the end of 2025 [4].

Sustainable aviation fuels (SAFs) play a key role in the future aviation’s decarbonization strategy. In line with the sector’s commitment to achieving net-zero carbon emissions by 2050, SAFs are expected to provide over 60% of the required emissions reduction [3]. Furthermore, as drop-in replacements to fossil kerosene, they can make use of existing aircraft and fueling infrastructure, thus reducing the cost of decarbonization [2,5].

SAFs can be produced from different carbon feedstocks, such as biomass, organic waste, and captured CO₂, which, combined with the available array of synthesis technologies, result in several potential pathways. Nevertheless, the literature on existing alternatives is often fragmented, where some of the greener routes have never been investigated, while others were assessed applying a range of assumptions and data sources. Hence, a comprehensive harmonized assessment of the most prominent pathways is lacking. Moreover, existing studies provide limited insights into the environmental implications of their large-scale deployment beyond climate change mitigation and fail to contextualize the impact values using absolute thresholds. Lastly, such studies often assume fixed background data in the life cycle assessments (LCA), yet socio-economic systems are expected to evolve in the future, which may greatly impact the environmental footprint of green technologies [6].

Previous studies on synthetic fuels highlight the importance of transcending carbon footprint quantification in environmental assessments. More specifically, SAFs result in collateral damage to other impact metrics despite their excellent performance in reducing greenhouse gas emissions [7–9].

To this end, absolute environmental sustainability assessments (AESA) can provide insights into SAFs’ environmental performance relative to the planet’s ecological limits and carrying capacity [10]. These limits, known as planetary boundaries (PBs), define the safe operating space (SOS) for anthropogenic activities [11]. As such, this approach enables us to reach a more comprehensive understanding of the environmental impacts and implications of different SAF routes by quantifying their transgression levels to the SOS. Moreover, by coupling AESA with prospective data, we are able to account for the temporal evolution of impacts associated with expected changes in socio-economic systems and thus predict absolute environmental performance of both current and emerging strategies to produce SAF in the future [6,12].

Based on the above, in this work, we analyze current and prospective scenarios for SAF production and compare them to fossil jet fuel based on absolute sustainability criteria through the PB framework [11]. We developed detailed process simulations in Aspen Plus v12 for jet fuel production considering fossil and biogenic carbon sources, captured CO₂, renewable energy, and the relevant technologies for each feedstock type. The business-as-usual (BAU) fossil kerosene from crude oil was included as a benchmark for the analysis. The AESA was carried out in Brightway2 v2.4.6 [13] using the Ecoinvent v3.10 cut-off database [14] for the current scenarios and the premise v2.2.7 framework [12] to generate future background data. The impacts were calculated using the Environmental Footprint (EF) v3.1 method in combination with the IPCC 2021 method for climate change [6] and the LANCA v2.5 method for land use [10]. Absolute thresholds for interpreting the life cycle impact assessment (LCIA) values were taken from [10]. Furthermore, we consider both cradle-to-gate and cradle-to-grave scopes to include the emissions from the fuel use phase in the analysis.

The results indicate that the shift towards renewable feedstocks is necessary to keep jet fuel production within the SOS for the climate change impact category. Nevertheless, as expected, the climate change mitigation potential comes at the cost of burden shifting toward other categories, even in the prospective scenarios. In some cases, the renewable routes remain within the SOS despite the higher values compared to the BAU. However, they operate well beyond the planetary boundaries for many impact categories. This is true for both current and prospective scenarios; no scenario fully operates within the SOS for the downscaling principles applied in the study.

These results suggest that the adoption of one isolated pathway for SAF production, given the current state of technologies, would likely not be sustainable in absolute terms. Furthermore, though technology improvements can lead to overall reduced environmental impacts in the future, any solution is unlikely to fully prevent burden shifting from occurring. All in all, despite renewable feedstocks being necessary to reduce the global warming impact of the aviation sector, trade-offs are inevitable. Therefore, the combination of different strategies for jet fuel production should be investigated to ensure a better overall environmental performance beyond the context of climate change mitigation.

References

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