How sustainable aviation fuel is made
Jet fuel has to meet a strict standard, so a new fuel may only be used once its way of being made has been tested and approved. Eight ways are approved for fuel made on its own (in the standard's annexes, numbered A1 to A8), and a ninth lets an ordinary oil refinery feed a little fat, oil or synthetic liquid in with its crude. Each approved fuel is blended with ordinary jet fuel, at most half and for two of them a tenth, and the blend is then certified as ordinary jet fuel, so aircraft, pipelines and airports treat it like any other.
The approved ways with the plants and projects on this site's record that use each
| The way | Made from | At most, of the blend | Plants on record | Capacity a year |
|---|---|---|---|---|
| Fischer-Tropschannex A1, approved 2009 | Gas made from biomass, rubbish or carbon dioxide and hydrogen, built up into kerosene | half | 147 (25 running) | 14.5 million tonnes |
| HEFA (hydroprocessed fats and oils)annex A2, approved 2011 | Used cooking oil, animal fats, vegetable oils | half | 117 (27 running) | 34.1 million tonnes |
| Fermented sugars (SIP)annex A3, approved 2014 | Sugar from sugar cane or beet, turned into fuel by yeasts | a tenth | none yet | |
| Fischer-Tropsch with aromaticsannex A4, approved 2015 | As the first, with aromatics added so it behaves more like ordinary jet fuel | half | none yet | |
| Alcohol to jetannex A5, approved 2016 | Ethanol or isobutanol from sugar, starch, farm and forest waste, or captured carbon | half | 74 (8 running) | 8.3 million tonnes |
| Catalytic hydrothermolysisannex A6, approved 2020 | Fats and oils, cooked with water at high heat and pressure | half | none yet | |
| HEFA from algae oilannex A7, approved 2020 | Oil from one kind of algae | a tenth | none yet | |
| Alcohol to jet with aromaticsannex A8, approved 2023 | Alcohols from biomass, with aromatics made in the same process | half | none yet | |
| Co-processing in an oil refinerythe jet fuel standard itself | Fats and oils, or Fischer-Tropsch liquids, fed into an ordinary refinery with crude oil | 5 to 10 per cent of the refinery's feed | 32 (15 running) | 13.6 million tonnes |
Most of today's fuel is made the second way, from used cooking oil and animal fats, because the process is the cheapest and closest to an ordinary refinery's; the supply of those fats is the limit. The ways from alcohols, from gasified waste and from carbon dioxide and green hydrogen can use far more plentiful feedstock and are where most new plants are planned. Plants that make fuel from carbon dioxide and renewable power ("power to liquid" or e-fuels) are counted with the first way, which most of them use; making it through methanol is still being tested. Plants on record whose way is not one of these, or not yet stated: not stated 48, Alcohol-to-jet, Fischer-Tropsch, Power-t 1, Hydrothermal liquefaction 2, Power-to-liquid | Alcohol-to-jet | HEFA 1, Pyrolysis 8, other 84.
The three main families
From fats and oils. Waste cooking oil, animal fats and vegetable oils are treated with hydrogen to strip out the oxygen and break the molecules into the right size for jet fuel. It is the most mature way and makes most of the fuel today.
From sugars and alcohols. Sugar, starch or farm and forest waste is fermented into ethanol or a similar alcohol, which is then joined into longer molecules and refined into jet fuel. The alcohol can also be made from captured carbon.
From gas. Wood waste, household rubbish or other biomass is heated into a gas of carbon monoxide and hydrogen, or that gas is made from carbon dioxide and green hydrogen, and then built up into liquid fuel by the Fischer-Tropsch process, first used in the 1920s.
Towards fuel without blending
Ordinary jet fuel contains aromatics, which keep the seals in older fuel systems swollen and tight; most sustainable fuels have none, which is the main reason they are blended. Aircraft and engine makers have flown test aircraft on fuel with no fossil content at all, and the standards bodies are working on rules that would allow it, along with a larger share of fats and oils fed into ordinary refineries.
Every new way must pass a long series of tests (under the standard ASTM D4054) before it can be added. Several more are being assessed now.
Sources: ICAO, approved conversion processes (the annexes, their feedstocks, blend limits and the co-processing allowances); IATA, SAF technical certifications (the years approved); the eighth annex, alcohol to jet with aromatics, was approved in 2023 at up to half the blend (Hydrocarbon Processing, 8 August 2023). The plants and capacity are counted live from the site's record; capacity is as announced, not as built, and includes plants not yet running. To correct anything here, write to [email protected].