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Sustainable aviation fuels are a blanket term for 'drop-in' aviation fuels with lower carbon intensity than traditional fuels

Summary​

Sustainable aviation fuels are 'drop-in' aviation fuels with lower carbon intensity than traditional fuels. Regional definitions for sustainable aviation fuels vary, with the US requiring 50% lifecycle greenhouse gas emissions reduction and EU standards ranging from 50-70% depending on fuel type. To pass safety requirements, these fuels must meet strict technical standards like ASTM D7566. Three main production pathways have commercial significance due to the high allowable blending limit of 50%: HEFA-SPK from oil-based feedstocks, FT-SPK using syngas conversion, and ATJ-SPK from alcohol feedstocks. Each pathway faces unique challenges including feedstock sustainability, hydrogen requirements, and technical complexity. Sustainable aviation fuels are already available at major airports worldwide but their environmental benefits depend heavily on production methods and feedstock sources.

Sustainable aviation fuel is a broad term that is used to describe 'drop-in' aviation fuels that have a lower carbon intensity than traditional aviation fuels. Because classification as a sustainable aviation fuel often carries financial benefits such as tax credits and subsidies to boost production, there is some regional variation as to what is regulated as a sustainable aviation fuel. Additionally, to be considered a legally usable sustainable aviation fuel, the fuel also has to meet a number of strict technical standards.

Regional variation in definition​

In the United States, the definition of sustainable aviation fuels was expanded in the Inflation Reduction Act of 2022, which involves the following 1:

  1. The fuel meets ASTM D7566 or its successor standard.
  2. The fuel must achieve a lifecycle greenhouse gas emissions reduction of 50% compared to fossil fuels.
  3. The fuel must not be derived from fossil fuels.

In the European Union the eligibility criteria for sustainable aviation fuels is the following 2:

  1. The fuel is produced by an approved pathway detailed in ASTM D7566.
  2. The lifecycle greenhouse gas emissions reduction must be at least 50% for biofuels (65% for plants constructed after Jan 2021) and 70% for renewable fuels of non-biological origin such as e-jet fuels.
  3. The sourcing for the fuel feedstock is limited for crops that result in high levels of indirect land use change (ILUC)

Technical standards​

Aviation fuels are heavily regulated to meet technical standards that ensure that the fuel is safe for use. What is colloquially known as 'aviation fuel' or 'jet fuel' is really any fuel that meets the ASTM D1655 – Standard Specification for Aviation Turbine Fuels, which is universally accepted worldwide. The global standards for sustainable aviation fuel production are defined by the ASTM International standard ASTM D7566 (Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons). Contained within this standard are a number of approved pathways that have been rigorously tested to reach compliance, including the following three pathways that have been approved for 50% blending and have a high technological readiness level 3:

Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK) Hydroprocessed esters and fatty acids synthetic paraffinic kerosene is the only sustainable aviation fuel that has made an impact on a commercial scale in the aviation fuel market. Certified in 2011 with a 50% blending limit, HEFA-SPK is produced using a wide range of oil-based feedstocks including vegetable oils, animal fats, and waste cooking oils. The major drawback with HEFA-SPK is the same issue encountered with all oil-based biofuels: the sourcing of the feedstock is costly and tends to involve significant land use change that is often located in areas susceptible to deforestation such as Southeast Asia and South America. The sustainability of HEFA-SPK depends heavily on the feedstock used, showing significant benefits when utilizing used cooking oils, but performing worse than regular fossil fuels when palm oil is used 3. HEFA-SPK is already produced by a number of commercial facilities located throughout the world, and is sold in major airports such as San Francisco International Airport (SFO), Heathrow Airport (LHR), and Frankfurt Airport (FRA) 3.

The process of producing HEFA-SPK differs from the transesterification process used for the production of conventional biodiesel, and is a multi-step process that involves a number of processes including hydrotreating, which removes oxygen molecules to produce straight-chain hydrocarbons (alkanes) consisting of only carbon and hydrogen atoms. After this, the straight-chain hydrocarbons are hydrocracked (broken down into smaller molecules) and isomerized (rearranged) to produce the final product that meets the ASTM standard 4. One key consideration with HEFA-SPK is the large amount of hydrogen that is required for the hydrotreating process, which if sourced from fossil fuels as is usually the case, can result in a significant source of CO₂ emissions 3 4.

Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) The Fischer-Tropsch process utilizes syngas, a combination of carbon monoxide (CO), hydrogen (H₂), and sometimes carbon dioxide (CO₂), as its main feedstock, which then undergoes a catalytic conversion to produce synthetic kerosene. To supply syngas, a process called gasification is used, where a carbon-based feedstock such as biomass or municipal solid waste (MSW) is heated at high temperatures above 700°C in a controlled oxygen environment that does not allow full combustion of the material. Multiple Fischer-Tropsch plants have been built and operated in the United States to varying degrees of success. The FT-SPK process was the first sustainable aviation fuel to achieve technical certification back in 2009. It should be noted that the FT-SPK process is agnostic to the types of feedstocks used, meaning that it can utilize a wide range of feedstocks such as biomass, municipal solid waste, and renewable sources such as hydrogen and captured carbon dioxide which would classify the fuel as an e-jet fuel 3.

Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) The alcohol-to-jet process utilizes ethanol (or other alcohols such as isobutanol) as its main feedstock, preferably from a second generation source, but also via fermentation of industrial waste gases. The ethanol undergoes the following reactions: dehydration, oligomerization, hydrogenation, isomerization, and distillation, resulting in synthetic kerosene. One of the leaders in sustainable aviation fuel production is a company called LanzaJet, which utilizes an alcohol-to-jet pathway and is the first to produce sustainable aviation fuel on a commercial scale in the United States. The ATJ process was first approved for use using isobutanol as its feedstock in 2016 with technical approval granted to ethanol-based ATJ in 2018 3.

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Footnotes​

  1. Bracmort, K. (2024). Sustainable aviation fuel (SAF): An overview of current laws and legislation introduced in the 118th Congress (CRS Report No. IF12757). Congressional Research Service. https://www.congress.gov/crs_external_products/IF/PDF/IF12757/IF12757.4.pdf ↩

  2. European Commission, Joint Research Centre. (n.d.). Renewable energy – recast to 2030 (RED II). EU Science Hub. https://joint-research-centre.ec.europa.eu/welcome-jec-website/reference-regulatory-framework/renewable-energy-recast-2030-red-ii_en ↩

  3. Detsios, N., Theodoraki, S., Maragoudaki, L., Atsonios, K., Grammelis, P., & Orfanoudakis, N. G. (2023). Recent Advances on Alternative Aviation Fuels/Pathways: A Critical Review. Energies, 16(4), 1904. https://doi.org/10.3390/en16041904 ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  4. Han, J., Elgowainy, A., Cai, H., & Wang, M. Q. (2013). Life-cycle analysis of bio-based aviation fuels. Bioresource Technology, 150, 447–456. https://doi.org/10.1016/j.biortech.2013.07.153 ↩ ↩2