A guide to approved and developed SAF production pathways
Sustainable Aviation Fuel (SAF) is being developed from a growing range of feedstocks and conversion technologies.
As these technologies progress from research and development through technical evaluation and approval, the aviation fuels industry uses a wide range of acronyms to describe different production routes and synthetic fuel components.
Terms such as HEFA-SPK, FT-SPK, ATJ-SPK, SIP, SATF and SBC are therefore increasingly common within SAF development, production and testing. This guide explains the terminology behind key SAF pathways, identifies the pathways currently incorporated into ASTM D7566, and provides an overview of additional technologies that remain under development.
What is a SAF pathway?
A Sustainable Aviation Fuel pathway describes the combination of feedstock and conversion technology used to produce a synthetic aviation fuel component.
Potential feedstocks range from oils and fats to alcohols, sugars, biomass and other renewable or waste-derived materials.
The conversion technology determines how those feedstocks are transformed into hydrocarbons suitable for aviation use.
Because aviation fuels operate in an extremely safety-critical environment, a new pathway cannot simply be introduced into commercial service. It must undergo extensive technical evaluation to demonstrate that the resulting fuel has suitable properties and performance.
Understanding SAF pathway status
Approved ASTM D7566 pathway
A synthetic aviation fuel pathway incorporated into an ASTM D7566 Annex has completed the required evaluation process for inclusion within the specification.
The resulting Synthetic Blending Component must still be manufactured, blended, tested and certified in accordance with the requirements of the applicable Annex and ASTM D7566.
Pathway in development
A pathway that has not yet been incorporated into an ASTM D7566 Annex remains under development or evaluation.
These technologies may use different feedstocks or conversion processes and can ultimately progress towards ASTM approval following the required technical evaluation.
This distinction is important: not every SAF technology currently being researched is approved for commercial aviation use.
What is ASTM D7566?
ASTM D7566 – Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons defines requirements for aviation turbine fuels containing approved synthetic blending components.
Each approved production pathway is incorporated into a dedicated Annex within the specification. The current specification contains eight synthetic blending component Annexes, A1 through A8.
Once an ASTM D7566 fuel has been correctly produced, blended, tested, certified and released, it is regarded as ASTM D1655 aviation turbine fuel.
Approved SAF pathways in ASTM D7566
ASTM D7566 Annexes at a glance:
| Annex | SAF Pathway | Acronym |
| A1 | Fischer-Tropsch Synthetic Paraffinic Kerosene | FT-SPK |
| A2 | Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene | HEFA-SPK |
| A3 | Synthesised Iso-Paraffins | SIP |
| A4 | Fischer-Tropsch Synthetic Kerosene with Aromatics | FT-SKA / SPK/A |
| A5 | Alcohol-to-Jet Synthetic Paraffinic Kerosene | ATJ-SPK |
| A6 | Catalytic Hydrothermolysis Jet | CHJ |
| A7 | Hydrocarbon-Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene | HC-HEFA-SPK |
| A8 | Alcohol-to-Jet Synthetic Kerosene with Aromatics | ATJ-SKA |
What is FT-SPK?
FT-SPK stands for Fischer-Tropsch Synthetic Paraffinic Kerosene.
The Fischer-Tropsch process converts suitable carbon-containing feedstocks into synthetic hydrocarbons. Potential feedstocks can include biomass as well as other carbon sources.
FT-SPK was the first synthetic blending component pathway incorporated into ASTM D7566.
ASTM D7566: Annex A1
What is HEFA-SPK?
HEFA-SPK stands for Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene.
The HEFA process converts oils and fats through hydroprocessing to produce synthetic paraffinic kerosene.
Potential sustainable feedstocks include used cooking oils, vegetable oils and animal fats.
ASTM D7566: Annex A2
What is SIP?
SIP stands for Synthesised Iso-Paraffins.
This pathway produces synthetic iso-paraffinic hydrocarbons from hydroprocessed fermented sugars.
ASTM D7566: Annex A3
What is FT-SKA?
FT-SKA refers to Fischer-Tropsch Synthetic Kerosene with Aromatics.
The pathway builds on Fischer-Tropsch technology while incorporating aromatic hydrocarbons into the resulting synthetic kerosene.
You may also encounter terminology such as SPK/A in relation to this pathway.
ASTM D7566: Annex A4
What is ATJ-SPK?
ATJ-SPK stands for Alcohol-to-Jet Synthetic Paraffinic Kerosene.
Alcohol-to-Jet technology converts suitable alcohol feedstocks into synthetic hydrocarbons for aviation fuel.
Approved inputs within the current pathway include specific alcohol-derived feedstocks defined by ASTM D7566.
ASTM D7566: Annex A5
What is CHJ?
CHJ stands for Catalytic Hydrothermolysis Jet.
This pathway uses catalytic hydrothermolysis to convert suitable lipid-based feedstocks into synthetic aviation fuel components.
Potential feedstocks include oils and fats.
ASTM D7566: Annex A6
What is HC-HEFA-SPK?
HC-HEFA-SPK refers to Hydrocarbon-Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene.
The pathway extends HEFA-related processing to hydrocarbon-containing biological feedstocks.
ASTM D7566: Annex A7
What is ATJ-SKA?
ATJ-SKA stands for Alcohol-to-Jet Synthetic Kerosene with Aromatics.
The pathway uses Alcohol-to-Jet technology to produce synthetic aviation kerosene containing aromatic hydrocarbons.
It is the most recently added of the eight pathways currently included within ASTM D7566.
ASTM D7566: Annex A8
SAF pathways in development
Development of new Sustainable Aviation Fuel production technologies continues.
A number of additional pathways are being investigated using alternative feedstocks and conversion technologies. These pathways do not currently have their own ASTM D7566 Annex and should therefore be distinguished from the approved pathways above.
Terminology encountered within current SAF development includes:
CPK-0
Cycloparaffinic Kerosene
A synthetic kerosene pathway incorporating cycloparaffinic hydrocarbons.
HBtJ
Hybrid Bio-Thermocatalytic Jet
A developing pathway combining biological and thermocatalytic processes for the production of aviation fuel.
HEFA-SKA
Hydroprocessed Esters and Fatty Acids Synthetic Kerosene with Aromatics
A developing HEFA-derived route intended to produce synthetic kerosene containing aromatic hydrocarbons.
ItJ-SCP
Isoprene-to-Jet Synthesised Cycloparaffins
A developing pathway using isoprene as a feedstock for aviation fuel production.
KtJ-SPK
Ketones-to-Jet Synthetic Paraffinic Kerosene
A developing route for converting ketone intermediates into synthetic paraffinic kerosene.
LP-HEFA-SKC
Low Pressure HEFA Synthetic Paraffinic Kerosene Containing Cycloparaffins
A developing HEFA-related pathway producing synthetic kerosene containing cycloparaffinic hydrocarbons.
MtJ-CKA
Methanol-to-Jet Cycloparaffinic Kerosene with Aromatics
A developing Methanol-to-Jet pathway intended to produce aviation kerosene containing cycloparaffinic and aromatic hydrocarbons.
MtJ-SPK
Methanol-to-Jet Synthetic Paraffinic Kerosene
A developing pathway using methanol as the starting material for the production of synthetic paraffinic kerosene.
POtK-SKA
Pyrolysis Oil to Jet Synthetic Kerosene with Aromatics
A developing route using pyrolysis oil as a feedstock for synthetic aviation kerosene containing aromatic hydrocarbons.
PtJ-SKA
Plastics-to-Jet Synthetic Kerosene with Aromatics
A developing conversion route investigating the production of synthetic aviation kerosene from suitable plastic-derived feedstocks.
SBtJ
Sewage Biosolids to Jet
A developing pathway investigating the conversion of sewage biosolids into aviation fuel.
TOJ
Tall Oil Jet
A developing pathway using tall oil-derived feedstock for aviation fuel production.
Essential Sustainable Aviation Fuel terminology
Understanding the pathway acronyms is only part of SAF terminology. Several broader terms are frequently used throughout development, testing and certification.
What does SAF mean?
SAF stands for Sustainable Aviation Fuel.
SAF is aviation turbine fuel containing hydrocarbons produced from feedstocks and processes that satisfy applicable sustainability requirements. SAF is intended to reduce lifecycle greenhouse gas emissions compared with conventional fossil-derived aviation fuel while maintaining the stringent safety and performance requirements of aviation turbine fuel.
What does SATF mean?
SATF stands for Synthetic Aviation Turbine Fuel.
It is the broader term for aviation turbine fuel containing synthesised hydrocarbons. A synthetic aviation turbine fuel is not necessarily sustainable; sustainability depends on factors including the feedstock, production process and applicable sustainability criteria.
What does SBC mean?
SBC stands for Synthetic Blending Component.
An SBC consists of synthesised hydrocarbons produced in accordance with the requirements of an applicable ASTM D7566 Annex.
The SBC is blended with conventional aviation fuel within the permitted limits for that pathway before the finished fuel is tested and certified to the requirements of ASTM D7566.
How are new SAF pathways approved?
Bringing a new aviation fuel pathway into commercial use requires extensive evaluation.
ASTM D4054 – Standard Practice for Evaluation of New Aviation Turbine Fuels and Fuel Additives provides a framework for developing the technical data required to evaluate new aviation fuels and synthetic blending components.
ASTM D4054 is therefore an important bridge between the development of a new fuel technology and its potential inclusion as an approved synthetic blending component pathway within ASTM D7566.
Why does SAF pathway development require extensive testing?
Different feedstocks and conversion technologies can produce fuels with different chemical compositions and physical properties.
Testing is therefore essential throughout SAF development and qualification to demonstrate that candidate fuels are compatible with aircraft, engines and aviation fuel infrastructure.
Depending on the stage of development and fuel pathway, properties requiring evaluation can include:
- Flash point
- Dielectric constant
- Ethanol content
- Particle contamination and cleanliness
- Water separation characteristics
- FAME contamination
- Cetane/ignition quality
Reliable laboratory data helps fuel developers, manufacturers and industry stakeholders assess new pathways while maintaining the safety and performance requirements demanded by commercial aviation.
Sustainable Aviation Fuel testing and quality control
Whether you are developing a new SAF pathway, evaluating Synthetic Blending Components or carrying out routine aviation fuel quality control, reliable testing is critical to ensuring fuel safety, performance and compliance.
Stanhope-Seta provides laboratory instruments and reference materials supporting the development, evaluation and routine quality control of Sustainable Aviation Fuel, Synthetic Aviation Turbine Fuel and Synthetic Blending Components.
Sustainable Aviation Fuel Pathways Frequently Asked Questions
How do you know if a SAF pathway is approved?
A synthetic aviation fuel pathway included within an ASTM D7566 Annex has completed the evaluation and specification-development process required for incorporation into D7566. The resulting fuel must still be produced, blended, tested and certified in accordance with the requirements of the relevant Annex and specification.
Are all SAF production pathways approved for aviation use?
No. New feedstocks and conversion technologies continue to be developed. Pathways that have not yet been incorporated into an ASTM D7566 Annex remain under development or evaluation and should not be considered equivalent to approved ASTM D7566 pathways.
How many SAF pathways are currently included in ASTM D7566?
ASTM D7566 currently contains eight synthetic blending component pathways within Annexes A1 through A8.
These include FT-SPK, HEFA-SPK, SIP, FT-SKA/SPK-A, ATJ-SPK, CHJ, HC-HEFA-SPK and ATJ-SKA.
What is the difference between ASTM D4054 and ASTM D7566?
ASTM D4054 provides the evaluation framework used to generate technical data for new aviation turbine fuels and fuel additives. ASTM D7566 is the fuel specification containing the synthetic blending component pathways that have successfully progressed through the approval process and been incorporated into individual Annexes.
What is the difference between SAF and a Synthetic Blending Component?
A Synthetic Blending Component (SBC) is the synthetic hydrocarbon component manufactured according to the requirements of an ASTM D7566 Annex.
Sustainable Aviation Fuel (SAF) is the aviation fuel resulting from an eligible sustainable production route and satisfying the applicable technical and sustainability requirements. The synthetic blending component is therefore part of the process used to produce the finished aviation fuel.
What happens after a Synthetic Blending Component is produced?
The component is blended with conventional aviation turbine fuel in accordance with the limits and requirements of its ASTM D7566 Annex. The finished batch must then satisfy the applicable ASTM D7566 requirements before certification and release. Once released, the fuel is regarded as ASTM D1655 aviation turbine fuel.