SAF (Sustainable Aviation Fuel)

Sustainable Aviation Fuel (SAF) is a drop-in jet fuel produced from non-petroleum feedstocks through several conversion pathways, with lifecycle greenhouse-gas (GHG) emissions substantially below conventional Jet A and Jet A-1. SAF is approved for use in commercial aviation under ASTM D7566 (and is then re-designated as ASTM D1655 finished jet fuel after blending with petroleum jet) and is the principal near-term decarbonization pathway for the aviation sector.

Approved SAF pathways

ASTM D7566 currently approves the following SAF conversion pathways, each with a specific blending limit:

• HEFA-SPK (Hydroprocessed Esters and Fatty Acids — Synthetic Paraffinic Kerosene) — up to 50% blend; produced from oils and fats via hydrotreating, similar process chemistry to renewable diesel
• FT-SPK (Fischer-Tropsch SPK) — up to 50% blend; produced from gasification of biomass or municipal solid waste, then F-T synthesis
• ATJ-SPK (Alcohol-to-Jet SPK) — up to 50% blend; produced by oligomerization of bio-derived ethanol or isobutanol
• CHJ (Catalytic Hydrothermolysis Jet) — up to 50% blend; produced from triglycerides via catalytic hydrothermolysis
• HC-HEFA-SPK — up to 10% blend; produced from algal oils

Each pathway is approved through a multi-year ASTM committee process following demonstration testing.

HEFA dominance

In current global SAF production, the HEFA-SPK pathway dominates by a wide margin. HEFA shares process chemistry with renewable diesel: both are produced by hydrotreating oils and fats over a hydroprocessing catalyst. A renewable-diesel facility can be retooled to produce HEFA SAF with limited additional capital expenditure, although the kerosene fraction's yield is typically lower than renewable diesel's distillate fraction.

Lifecycle GHG savings

Public peer-reviewed studies and ICAO CORSIA's default lifecycle factors document SAF lifecycle GHG savings, relative to conventional jet fuel, in these typical ranges:

• HEFA from used cooking oil: 70–85% reduction
• HEFA from soybean oil: 35–55% reduction (varies with ILUC handling)
• FT-SPK from forestry residues: 75–90% reduction
• ATJ from corn ethanol: 30–55% reduction
• ATJ from sugarcane: 65–80% reduction

Specific values depend on the feedstock supply chain, the conversion facility's energy mix, and the program's ILUC accounting methodology.

Federal incentives

SAF receives an enhanced incentive under the U.S. Section 45Z Clean Fuel Production Credit at 26 USC § 45Z. The maximum SAF credit is $1.75 per gallon at the lowest CI, compared with $1.00 per gallon for non-SAF transportation fuels. Treasury's implementing notices (IRS Notice 2025-10 series) set out the SAF-specific pathway and CI calculation requirements. The OBBBA July 2025 law extended Section 45Z through 31 December 2029.

State and EU drivers

State-level SAF demand is reinforced through California's Low Carbon Fuel Standard (LCFS), which awards LCFS credits to SAF supplied into California, and through analogous programs in Oregon (CFP) and Washington (CFS). The European Union's ReFuelEU Aviation regulation (Regulation (EU) 2023/2405) imposes a SAF blending mandate at EU airports starting at 2% in 2025 and rising to 70% by 2050. Together, these federal, state, and supranational drivers shape SAF's near-term demand profile.

ICAO CORSIA

The International Civil Aviation Organization's Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) recognises SAF as a CORSIA-eligible fuel under specific sustainability criteria defined in the ICAO Sustainability Criteria document. SAF used on covered international flights reduces the operator's CORSIA offsetting obligation; the framework has been operational since 2021.

Drop-in compatibility

The "drop-in" property of approved SAF means that the blended jet fuel behaves identically to conventional Jet A or Jet A-1 in turbofan and turboprop engines, requires no modification to the aircraft's fuel system, and meets all the same airworthiness specifications. This property is what allows SAF to be introduced into the existing distribution network — pipelines, refuelling trucks, airport hydrants — without segregated infrastructure. The 50% blending limit on most ASTM D7566 SAF pathways reflects the engine-certification testing that has been performed to date; the 50% cap is expected to rise in future ASTM revisions as additional engine-OEM testing data become available.

Sources

Standards: ASTM D7566 (alternative jet fuels), ASTM D1655 (finished jet fuel). U.S. statute: 26 USC § 45Z; OBBBA July 2025. EU regulation: Regulation (EU) 2023/2405 (ReFuelEU Aviation). ICAO CORSIA SARPs documented in ICAO Annex 16 Volume IV. CARB pathway library for LCFS-certified SAF pathways.