May-2024

Technology pathways for sustainable aviation fuel (SAF)

To meet CO2 emissions reduction targets by 2050, accelerating the development and deployment of SAF is the best option to decarbonise the aviation industry.

Svetlana van Bavel and Chippla Vandu
Shell Catalysts & Technologies

Article Summary

Refiners have successfully utilised easier-to-process used cooking oils (UCO) and animal fats to produce bio-SAF for some time. Given concerns about the limited availability of these feedstocks, fuel suppliers will need to process more challenging feedstocks, such as biomass residues and renewable (green) hydrogen and carbon dioxide (CO₂). In particular, synthetic aviation fuels (eSAF) made using power-to-liquids (PTL) technology have high potential owing to the virtually limitless supply of feedstocks, namely solar and wind energy or nuclear power, water, and CO₂. Combining the production of eSAF using PTL and bio-SAF made using biomass-to-liquids (BTL) technology can be an enabler for early projects. Crucially, the technologies for eSAF and bio-SAF have high technology readiness levels.

Aviation is one of the fastest-growing sources of greenhouse gas (GHG) emissions and one of the most challenging sectors to decarbonise. At present, aviation is responsible for a relatively small proportion of global GHG emissions, viz. 3% in 2019, though the environmental impact of air traffic goes beyond CO₂ emissions, as the formation of contrails and clouds amplifies its overall climate effects.

Although air travel fell significantly during the COVID‐19 pandemic, it rebounded strongly in 2023, coming very close to the pre-COVID peak level, and is expected to continue to grow. While advances in aircraft design have enhanced operational efficiency, with newer models consuming up to 20% less fuel, the surge in air traffic can be expected to negate these gains. As a result, international aviation could contribute up to 22% of global carbon emissions by 2050.

Difficulties in decarbonising aviation can be attributed to several factors. Aircraft have long lifespans so it would take decades to replace the existing fleet. Safety requirements mean high scrutiny and long lead times for the adoption of new technologies, such as battery-electric or hydrogen-powered aircraft, especially for long-haul flights.

Consequently, the industry will continue to rely on high-energy-density fuels such as kerosene for decades. In addition, there are significant costs associated with many decarbonisation solutions, especially solutions that require modifications to aircraft and fuel supply infrastructure.

The aviation industry will, therefore, need to use all available solutions and measures to decarbonise – no single solution will be enough on its own – and SAF is the only scalable in-sector option to help materially reduce emissions in the period to 2050.

Specifications for SAF ensure it can be used as a drop-in fuel, allowing it to be blended with conventional kerosene-based jet fuel and used in the world’s existing aircraft fleet without the need for redesign or upgrade.

When used unblended, currently available SAF, made from hydroprocessed esters and fatty acids (HEFA), has the potential to cut life-cycle carbon emissions by up to 80% compared to conventional jet fuel. SAF made of bio-residues (bio-SAF) and synthetic aviation fuels made from renewable power, water and CO₂ (eSAF) can achieve even higher life-cycle emissions reductions.

Whereas 100% SAF is being developed and tested, SAF is currently limited to blends of up to 50% with conventional jet fuel. There are indications that the highly paraffinic nature of fuels such as SAF produced through Fischer–Tropsch-based BTL or PTL pathways can have benefits by reducing particulate matter emissions and contrail formation.

Drivers of SAF adoption
Passenger attitudes and regulatory frameworks are the main drivers for SAF adoption. Passengers’ awareness of environmental sustainability is rising, compelling the aviation industry to explore and adopt low-emission alternatives. Although high-quality offset schemes continue to play a role, there is a notable rise in interest in SAF. Perhaps more relevant, however, are the regulatory frameworks and incentives in place in the EU and the US.

In the EU, legislation sets targets and obligations for overall SAF supply, with sub-targets for eSAF supply (see box opposite for definitions). The Renewable Energy Directive II (RED II) is the legal framework for the development of clean energy across all sectors of the EU economy. In addition, the recently adopted ReFuelEU Aviation regulation requires aircraft operators, EU airports, and aviation fuel suppliers to reduce the EU’s GHG emissions from aviation with mandated levels for SAF supplied in EU airports.

These mandated levels, summarised in Table 1, are relatively modest for 2025 but will escalate rapidly over time to 70% SAF incorporation in 2050, half of which should be eSAF.

Table 2 shows that a minimum production volume of 8,900 ktpa of SAF will be required by 2035, of which 2,200 ktpa should be eSAF (these figures are likely to be conservative as they are based on the 2019 EU jet fuel market and exclude potential market growth). It should be noted that imports are allowed, so the SAF does not have to be produced in the EU, just supplied in the EU.

In the US, the adoption of SAF is encouraged through tax credits, specifically under the Inflation Reduction Act at the federal level, in addition to specific state-level Low Carbon Fuel Standard credits, stimulating domestic production. Although the outlook for SAF-specific tax credits remains uncertain beyond 2027, alternative incentives, such as the 45Q tax credit for carbon capture and storage and the 45V credit for clean hydrogen, are in place until at least 2032. These provisions can contribute to the viability of specific SAF-related projects in the US.

SAF feedstocks and technology pathways
Regulatory frameworks, such as the EU’s RED II and ReFuelEU Aviation, define SAF incorporation targets and specify feedstock types. In the EU, UCO-based diesel comprises a substantial 19% of total biodiesel consumption, and this is projected to double by 2030. Despite the relative ease of processing through hydrotreating, this feedstock faces supply limitations, particularly as the industry seeks to scale up production to make a more substantial impact on reducing carbon emissions.

Although more challenging to process, organic feedstocks such as agricultural residues and biowaste can be processed using a variety of methods. An example is Shell Fiber Conversion Technology, which converts lignocellulosic biomass such as wet distillers grains from corn ethanol plants into enhanced-protein animal feeds, distillers corn oil, and cellulosic ethanol.