There are many different pathways to produce sustainable aviation fuel (SAF). There are different conversion technologies that can convert a wide range of biomass and waste feedstocks (including CO2) into jet fuel. At this stage, six different technology platforms are certified to produce SAF for use in commercial aviation. The basics:.
The FT process takes any carbon containing material and breaks it into individual building blocks in a gas form (synthesis gas). FT synthesis then combines these building blocks into SAF and other fuels. Two different FT processes have been certified by ASTM to date, one that produces a straight paraffinic jet fuel (SPK) and one that also produces additional aromatic compounds (SAK). Both processes can use any carbon containing starting material, according to the specification. Maximum blend ratio for both options is 50%. ASTM spec for SPK: D7566 – Annex 1 ; ASTM spec for SAK: D7566 – Annex 4
Hydrotreated Esters and Fatty Acids (HEFA)
HEFA refines vegetable oils, waste oils or fats into SAF through a process that uses hydrogen (hydrogenation). In the first step of the process the oxygen is removed by hydride-oxygenation. Next the straight paraffinic molecules are cracked and isomerised to jet fuel chain length. The process is similar to that used for Hydrotreated Renewable Diesel production, only with more severe cracking of the longer chain carbon molecules. Maximum blend ratio is 50%. ASTM spec: D7566 – Annex 2
Synthesized Iso-paraffins (SIP)
SIP is a biological platform, whereby microbes convert C6 sugars into farnesene, which after treatment with hydrogen can be used as SAF. Max blend ratio is 10%. ASTM spec: D7566 – Annex 3
Alcohol to Jet (AtJ)
AtJ converts alcohols into SAF by removing the oxygen and linking the molecules together to get the desired carbon chain length (i.e. oligomerisation). Currently there are two feedstocks approved for use in the AtJ technology: ethanol and iso-butanol. The source of the alcohol is not specified. Max blend ratio is 50%. ASTM spec: D7566-4
With co-processing, vegetable oils, waste oils and fats are processed along with conventional crude oil feedstocks in existing refining complexes. It is not per se a SAF focused production pathway, but more a result of the approval of feeding a small percentage vegetable oils into a refining complex. There is no annex to the D7566 specification for co-processing, but the use of biological oils is made possible by an amendment in the fossil jet fuel spec (D1655).
Note that the fact that a technology is certified does not mean that the fuel is also produced on a commercial scale. HEFA based biofuel is the only product that is commercially available today and powered over 95% of all SAF flights to date In addition to the six pathways described above, many more feedstock/technology combinations for SAF production are currently under development and in the process of getting ASTM certification.
If you want to learn more about technologies used to produce sustainable aviation fuels, please contact Oskar.