Feedstocks a major hurdle for sustainable aviation fuels adoption

Unabated, carbon dioxide emissions from global air traffic could quadruple by 2050, according to the German Aerospace Center (DLR). Carbon is only part of the story, as many experts believe the non-carbon effects of air transportation are even more impactful on the environment. Condensation trails (contrails) reduce heat dissipation from the earth and contribute to warming the atmosphere. Other climate-relevant emissions include nitrogen oxides, steam, soot and aerosols.

Aviation is a difficult industry to decarbonise. Nevertheless, air transport growth must be decoupled from environmental pollution with an immediate and scalable solution. The aviation industry has identified several potential options to make aviation more environmentally friendly including efficiency gains through operational and design improvements, combustion of low-emissions hydrogen or conversion to electricity via fuel cells, electric propulsion using zero emissions electricity, carbon offsets and sustainable aviation fuels (SAF). 

With technologies at different stages of readiness, an end to aviation emissions seems a distant dream. Hydrogen combustion, for example, offers a great opportunity to reduce climate impacts but requires completely different aircraft technologies, airport infrastructures and maintenance operations—and it will be many years before it makes a meaningful contribution.

In the near term, replacing conventional jet fuel with SAF is the best option we’ve got. “SAF has the potential to reduce nearly all impacts on climate and the environment,” says Professor Dr. Manfred Aigner from the DLR Institute of Combustion Technology in Stuttgart, Germany, Europe’s largest centre for aeronautics and space.  On September 12, 2022, Aigner provided a keynote presentation on “DLR Research for Sustainable Aviation – Sustainable Fuels & Clean Combustion” at the International Conference on Stability, Handling and Use of Liquid Fuels (IASH), in Dresden, Germany.

SAF technology is still nascent, with a limited number of viable pathways for emissions reduction and ASTM standards that allow a maximum 50% blend with conventional jet fuels—and in some cases only 10%.  However, the alternative fuels offer the potential to reduce carbon emissions by up to 99% compared to traditional jet fuel, depending on the technological pathway and feedstocks used, according to a December 2022 report by independent research provider Rhodium Group.  

SAF currently accounts for less than 0.1% of global jet fuel use. Aigner outlined a decarbonisation scenario from Mission Possible Partnership (MPP), an alliance of climate leaders focused on amplifying decarbonisation efforts. To achieve net zero by 2050, SAF’s share of final jet fuel demand needs to reach 13-15% by 2030, with 40-50 million tonnes (Mt) of supply from an estimated 300-400 SAF manufacturing plants. Currently, there are fewer than 10 plants. By 2050, 300-370 Mt of SAF is required from 1,600-3.400 manufacturing sites. 

Such a scenario requires massive investment in building SAF and renewable electricity capacity. MPP estimates USD40-50 billion in annual capital investments will be necessary for the 2020s expanding to USD270-290 billion in the 2040s.   

During his presentation, Aigner outlined several challenges in upscaling SAF production. At the current stage of the market, financing production is difficult. Aigner emphasised the importance of revenue certainty in encouraging investment. High feedstock costs and product price volatility are significant barriers. SAF can be two to six times more expensive than conventional jet fuels, which impacts the bankability of production facilities. 

First-of-a-kind plants are associated with high capital costs and a high degree of technology risk due to demand uncertainty. Aigner also mentioned the potential inclusion of a mechanism for non-carbon dioxide environmental impacts, although at this stage the mechanism remains unclear. 

Competition for feedstocks and fuels with other sectors, such as marine and road transportation, further complicates investments, particularly when road transportation offers greater opportunities and revenues.

Despite a laundry list of challenges, DLR believes the vision of zero-emission aviation is feasible. During his presentation, Aigner outlined several enablers of an effective transition including a need for more collaborative research on the climate impacts and effectiveness of both drop-in and non-drop-in fuels.

Aigner highlighted DLR’s proposal for a Power-to-Liquids (PtL) Research, Technology & Demonstration Platform. PtL is synthetically produced liquid hydrocarbon involving the conversion of syngas into SAF via a Fischer–Tropsch reaction using green hydrogen. The project will be the largest research facility for electricity-based fuel production and has received EUR12.7 million (USD13.7 million) from the German Federal Ministry for Digital and Transport for the planning phase. Additional funding for implementation will be determined at the end of 2023.

The demonstration plant is a semi-industrial scale and will produce up to 10,000 tonnes per year (t/a) and includes a research module (100 t/a PtL) for the optimisation and evaluation of PtL. Aigner emphasised the importance of systematic technical fuel assessments in developing SAF including the use of data-assisted tools to optimise fuel design, including the use of probabilistic machine learning models.

DLR considers its role in supporting the energy transition one of an architect. The organisation is involved in fundamental research through to applications and maintains close ties with the aviation industry and wider economy. The German company is focused on developing new aircraft and engine concepts in combination with SAF developments. 

A breakthrough in clean aviation is possible with the co-optimisation of SAF and combustor system design, says Aigner.  New properties of fuels will enable innovative optimisation of the combustor system to maximise performance and minimise emissions. Co-optimisation will offer enhanced pre-vaporisation, premixing and homogenous burning and provide opportunities to significantly reduce nitrous oxide (NOx), soot and ultra-fine particle emissions, he says.

Demand for SAF is on the rise. However, significantly more is needed, and feedstock availability is a major concern. The International Energy Agency’s (IEA) Renewables 2022 report forewarned of a feedstock supply crunch for biodiesel, renewable diesel and bio-jet fuel producers from 2022 to 2027. Wide-ranging cooperation and the alignment of interests are needed to enable scaling. It is time for aviation to claim the feedstocks that it requires.

The United States has set a target to reduce aviation emissions by 20% by 2030.  U.S. President Joseph Biden’s Inflation Reduction Act (IRA), passed on August 16, 2022, introduced tax incentives for low-carbon jet fuel to help the government achieve its stated goal of at least three billion gallons of SAF per year by 2030 and 35 billion in 2050. The current supply is 4.5 million gallons annually.

Europe’s ReFuelEU Aviation initiative is targeting 5% SAF in aviation fuel in 2030 and 65% in 2050, whereas the United Kingdom has committed to 10% by 2030 and up to 75% by 2050.  Government mandates and incentives are beginning to shift the needle in the jet fuel market.

However, not all sources of SAF are equal. Current levels of sustainable feedstocks are inadequate and there is a lack of funding for research into the development of new feedstocks. Greater investment in production facilities is necessary to unlock future availability of feedstocks. Doubts have also been raised around the feedstock sources that will be permitted by the European Union in the future.

The technical unreadiness of some SAF technologies is a major hurdle. Seven ASTM-certified pathways exist for producing SAF with hydro-processed esters and fatty acids (HEFA), Fischer-Tropsch (FT-SPK), and Alcohol-to-Jet (AtJ)— broadly considered the most important in the current decade.

HEFA is commercially viable and the least expensive to produce. However, feedstocks, such as cooking oil, are not indefinitely scalable and high feedstock costs mean they are likely only a near-term decarbonisation option. HEFA accounts for more than two-thirds of new capacity by 2026 in some projections, potentially more if feedstock issues can be resolved.

The Rhodium Group report believes AtJ and gas FT are potential medium-term solutions, although they are currently only at laboratory or pilot scale and require better investment. Rhodium Group imagines the costs of PtL will prevent it as a medium-term SAF application, with a need for superior clean electricity sources and declines in carbon capture and electrolyser costs. 

Municipal solid waste offers great potential for the FT-SPK and AtJ pathways, although it suffers from mismanagement with much of the waste currently disposed of via dumping or burning. In the United States, waste-to-energy plants already account for a small but steady source of electric power. According to the U.S Department of Energy’s Energy Information Administration, 14,000 gigawatt hours (GWh) of electricity is produced each year from around 60 WTE plants with a total generating capacity of 2,051 megawatts (MW). None of the existing plants have a generating capacity greater than 100 megawatts (MW).