There are many different pathways to produce sustainable aviation fuel (SAF). These conversion technologies can transform a wide range of biomass and waste feedstocks (including CO2) into jet fuel. At this stage, eight different technology platforms are certified to produce SAF for use in commercial aviation.
Let’s look at 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 hydrodeoxygenation. Next, the straight paraffinic molecules are cracked and isomerized 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. The 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. oligomerization). Currently, there are two feedstocks approved for use in the AtJ technology: ethanol and iso-butanol. The source of the alcohol is not specified. The maximum blend ratio is 50%. ASTM spec: D7566-4 – Annex 5.
Catalytic Hydrothermolysis (CHJ)
CHJ converts fatty acid esters and free fatty acids into SAF via catalytic hydrothemolysis followed by any combination of hydrotreatment, hydrocracking, or hydroisomerization and fractionation. Max blend ratio is: ASTM spec: 50%. D7566 – Annex 6.
Hydroprocessed Hydrocarbons, Esters and Fatty Acids (HC-HEFA)
Bio-derived hydrocarbons and free fatty acids and fatty acid esters are upgraded in the HC-HEFA process similar to the HEFA process: they are hydroprocessed to saturate the hydrocarbon molecules and remove essentially all oxygen. A recognized bio source is the Botryococcus braunii species of algae. Max blend ratio is 10%. ASTM spec: D7566 – Annex 7.
With co-processing, vegetable oils, waste oils and fats, or FT-wax is, 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 co-feeding a small percentage of vegetable oils, or FT-wax into a refining complex. There is no annex to the D7566 specification for co-processing, but the use of biological oils, or FT-wax 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 eight pathways described above, many more feedstock/technology combinations for SAF production are currently under development and in the process of getting ASTM certification.