The table below shows the seven different internationally approved processes through which sustainable aviation fuel (SAF) can currently be made. Approval for additional production processes is expected in the coming years.
Producing sustainable aviation fuel
| Pathways and processes | Feedstock options | Producers using the pathway | Date of approval | Current blending limit |
|---|---|---|---|---|
| Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) | biomass (forestry residues, grasses, municipal solid waste) | 2009 | up to 50% | |
| Hydroprocessed Esters and Fatty Acids (HEFA-SPK) | algae, jatropha, camelina | Alt Air | 2011 | up to 50% |
| Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP) | microbial conversion of sugars to hydrocarbon | Amyris | 2014 | up to 10% |
| FT-SPK with aromatics (FT-SPK/A) | renewable biomass such as municipal solid waste, agricultural wastes and forestry residues, wood and energy crops | 2015 | up to 50% | |
| Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) [isobutanol] | agricultural waste products (stover, grasses, forestry slash, crop straws) | Gevo | 2016 | up to 30% |
| Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) [ethanol] | agricultural waste products (stover, grasses, forestry slash, crop straws) | LanzaTech | 2018 | up to 50% |
| Catalytic hydrothermolysis synthetic jet fuel (CHJ) | Triglyceride-based feedstocks (plant oils, waste oils, algal oils, soybean oil, jatropha oil, camelina oil, carinata oil and tung oil) | ARA and Euglena | 2020 | up to 50% |
| High Hydrogen Content Synthetic Paraffinic Kerosene (HHC-SPK) | biologically derived hydrocarbons such as algae | IHI World | 2020 | up to 10% |
Find out more about the sustainability background of SAF and view the Beginner's Guide to Sustainable Aviation Fuel.
Testing and certifying a new energy source
SAF must have the same chemical qualities and characteristics as conventional jet fuel. This is important to ensure that manufacturers do not have to redesign engines or aircraft, and that fuel suppliers and airports do not have to build new fuel delivery systems. To ensure technical and safety compliance, SAF must undergo strict laboratory, ground, and flight tests under an internationally-recognised standard.
Why does a blend limit exist?
At present, SAF must be blended with conventional jet fuel (up to 50%). Some less environmentally favourable components of conventional jet fuel (e.g. sulphur) allow seals to swell in engines and prevent fuel leaks. Newer engines do not have this concern, and SAF has been performance tested at 100% in military aircraft. While SAF production volumes remain low, a blend limit does not hamper the use of SAF, however, it is expected that the blend limit will eventually increase to 100%.
Testing
Safety is the aviation industry’s top priority, therefore the process for testing potential new fuels is extremely rigorous. Through testing in laboratories, in equipment on the ground, and under the extreme conditions of in-flight operations, an exhaustive process determines the suitability of SAF.
Researchers develop SAF that has similar properties to conventional jet fuel, Jet A-1. Tests look at specific fuel consumption at several power settings, from ground idle to take-off speed, which is then compared to performance with conventional jet fuel. The amount of time it takes for the engine to start, how well the fuel stays ignited in the engine, and how the fuel performs in acceleration and deceleration, are all tested thoroughly. Tests are also completed to ensure fuels don’t have a negative impact on the materials used in building aircraft and components.
Once the lab and ground testing have been completed, the fuel is tested on aircraft under normal operating conditions. During the test flight, pilots perform a number of standard tests, as well as simulating exceptional circumstances, to ensure the fuel can withstand use under any operating conditions.