Sustainable Aviation Fuel Technology: Production Pathways and Feedstocks

SAF Production Pathways: An Overview

Sustainable Aviation Fuel (SAF) is jet fuel produced from non-petroleum feedstocks that, when assessed on a lifecycle basis, generates significantly fewer greenhouse gas emissions than conventional jet fuel from crude oil. The defining characteristic of SAF is not its chemical composition — which must be virtually identical to conventional Jet-A to be certified as a drop-in fuel — but its production pathway and feedstock. ASTM International's D7566 standard, which governs the certification of alternative jet fuels, currently approves seven distinct production pathways for SAF, each using different feedstocks and production chemistry.

The lifecycle greenhouse gas reduction of SAF relative to conventional jet fuel ranges from approximately 30% to over 90%, depending on the pathway and feedstock. The range reflects genuine variation in the carbon content and land use implications of different feedstocks, the energy intensity of different production processes, and the geographic source of feedstock. Understanding these differences is essential for evaluating competing claims about SAF's climate benefit and for making procurement decisions that genuinely deliver emissions reductions rather than superficially claiming them.

The seven ASTM-approved pathways as of 2024 are: HEFA (Hydroprocessed Esters and Fatty Acids), FT (Fischer-Tropsch synthesis from biomass gasification), ATJ (Alcohol-to-Jet), CHJ (Catalytic Hydrothermolysis of fats and oils), DSHC (Direct Sugar to Hydrocarbon from engineered yeast), HC-HEFA (Hydroprocessed Hydrocarbons from Botryococcus braunii algae), and co-processing of bio-feedstocks in conventional oil refineries. Of these, HEFA accounts for the overwhelming majority of current SAF production — approximately 80–90% of all SAF produced commercially as of 2024 — with FT and ATJ together representing most of the remainder.

HEFA: The Dominant Current Pathway

Hydroprocessed Esters and Fatty Acids (HEFA) is the most commercially mature SAF production pathway and the source of nearly all SAF produced at commercial scale today. HEFA converts fats, oils, and greases (FOGs) — animal tallow, used cooking oil (UCO), palm oil, camelina oil, and other lipid feedstocks — into a hydrocarbon fuel through hydroprocessing, a chemical process that removes oxygen from the fatty acid molecules and cleaves the ester bonds using hydrogen under elevated temperature and pressure.

The HEFA process begins with a hydroprocessing step (also called hydrotreating) that converts the triglycerides in the feedstock into normal paraffins (straight-chain hydrocarbons) plus propane and water. The normal paraffins are then hydroisomerized — rearranged into branched-chain isomers — to improve the cold-flow properties of the resulting fuel, which would otherwise solidify at temperatures encountered at cruise altitude. Finally, the hydroisomerate is hydrocracked (broken into smaller carbon-chain fragments) and distilled to produce a blend of naphtha, jet fuel, and diesel fractions. The jet fuel fraction typically represents 40–60% of the total product, with naphtha and diesel as co-products.

The largest commercial HEFA producers include Neste (Finland), which operates the world's largest SAF production capacity at its Rotterdam and Singapore refineries with a combined renewable products capacity of approximately 3.3 million tonnes per year (though not all renewable output is SAF); World Energy (US), which operates the sole dedicated SAF refinery in North America at Paramount, California; and TotalEnergies, which has converted conventional refining capacity at its La Mède facility in France to co-process bio-feedstocks. Airlines including Delta, American, United, Lufthansa, and British Airways have signed long-term HEFA SAF offtake agreements with these producers, in most cases securing supply commitments for 5–10 years.

The primary limitation of HEFA is feedstock availability and sustainability. The most sustainable HEFA feedstocks — used cooking oil and animal tallow — are available in limited quantities globally. ICAO estimates total sustainable UCO availability at approximately 10–20 million tonnes per year globally; at a conversion efficiency of roughly 0.7 tonnes of SAF per tonne of feedstock, this represents 7–14 million tonnes of potential HEFA SAF — compared to global jet fuel demand of approximately 300 million tonnes per year. Even converting 100% of available UCO to SAF would supply only 4–5% of global jet fuel demand, illustrating the structural limitation of HEFA alone as a decarbonization pathway.

Unsustainable HEFA feedstocks — particularly virgin palm oil, which is associated with deforestation and consequent land-use-change (LUC) emissions — can actually generate higher lifecycle GHG emissions than conventional jet fuel when LUC effects are included in the accounting. This concern has led European and US regulatory frameworks to exclude or significantly penalize palm-based HEFA in SAF accounting, and it underscores why feedstock certification and traceability are critical to the SAF value proposition.

Power-to-Liquid: SAF from CO₂ and Renewable Electricity

Power-to-Liquid (PtL) SAF, also called e-fuel or synthetic kerosene, is produced by combining green hydrogen (hydrogen produced by electrolysis of water using renewable electricity) with CO₂ (captured from the atmosphere or from concentrated industrial sources) through the Fischer-Tropsch or methanol synthesis process to produce hydrocarbons that can be refined into jet fuel. PtL represents the only truly scalable long-term SAF pathway: unlike biomass-based pathways that are constrained by land availability and sustainable feedstock limits, PtL's theoretical production capacity is limited only by the availability of renewable electricity and CO₂ capture infrastructure.

The PtL production chain begins with electrolysis. A proton exchange membrane (PEM) or alkaline electrolyzer splits water molecules into hydrogen and oxygen using electrical energy. For the hydrogen to be "green," the electricity must be generated from renewable sources — solar, wind, hydropower — with no fossil fuel combustion. The hydrogen is then combined with CO₂ in a reverse water-gas shift (RWGS) reaction to produce synthesis gas (syngas — a mixture of hydrogen and carbon monoxide). The syngas feeds a Fischer-Tropsch synthesis reactor, which polymerizes the syngas molecules into long-chain hydrocarbons (synthetic crude, or syncrude). The syncrude is then hydrocracked and refined into jet fuel and other hydrocarbon products.

The lifecycle GHG emissions of PtL SAF depend critically on the source of the CO₂ feedstock and the carbon intensity of the electricity used for electrolysis. Using direct air capture (DAC) CO₂ — CO₂ extracted directly from the atmosphere — combined with electricity from solar or wind generation with near-zero carbon intensity produces SAF with lifecycle emissions of less than 5% of conventional jet fuel: the carbon released when the SAF is burned was originally taken from the atmosphere, completing a near-closed carbon cycle. Using CO₂ from industrial flue gas capture reduces the feedstock challenge but raises questions about whether the CO₂ would have been emitted regardless, and the lifecycle accounting is correspondingly more complex.

Current PtL production is at demonstration and early commercial scale. Atmosfair in Germany operates a PtL plant producing a few hundred tonnes of SAF per year. Hifuel (a Finnish-German startup), Norsk e-Fuel (Norway), and Infinium (US) are constructing larger facilities targeting 5,000–50,000 tonnes per year capacity. The scale required to impact global jet fuel supply is many orders of magnitude larger — a single commercially meaningful PtL facility at the scale of a conventional refinery would produce approximately 100,000–500,000 tonnes of SAF per year, requiring gigawatts of dedicated renewable electricity. The capital cost of such a facility is in the billions of dollars, and the current cost of PtL SAF — approximately $5–15 per liter — is 4–12 times the cost of conventional jet fuel.

Cost reduction trajectories for PtL depend primarily on the cost of renewable electricity (which has declined dramatically over the past decade and is expected to continue declining), the cost of electrolysis (where significant learning curve reductions are projected as scale increases), and the cost of CO₂ capture. Optimistic projections suggest PtL SAF could reach price parity with conventional jet fuel by 2040 in regions with excellent renewable electricity resources and CO₂ capture infrastructure; more conservative analyses suggest 2050 or later. The range of uncertainty is large, and policy support — carbon pricing, SAF mandates, offtake guarantees — will significantly affect the pace of cost reduction.

Current Production Scale vs. Demand

The gap between current SAF production and the volume required to meaningfully decarbonize aviation is enormous and represents the central challenge for SAF policy and investment. Global SAF production in 2023 reached approximately 600,000 tonnes — less than 0.2% of global jet fuel demand of approximately 300 million tonnes. Even the most optimistic near-term projections, which assume full utilization of announced production capacity and successful commissioning of all projects currently under construction, suggest global SAF supply reaching 3–5 million tonnes by 2027 — still less than 2% of demand.

IATA's 2050 net-zero carbon commitment requires SAF to supply approximately 65% of aviation's fuel needs in 2050 — approximately 150–200 million tonnes per year at projected traffic levels. Bridging from 0.6 million tonnes in 2023 to 150+ million tonnes in 2050 requires average annual growth of approximately 35% for 27 consecutive years — unprecedented in any energy industry. The International Energy Agency's Sustainable Development Scenario requires SAF production to reach 30 million tonnes by 2030, 130 million tonnes by 2040, and over 400 million tonnes by 2050 to keep aviation on a 1.5°C-compatible pathway.

The investment required to achieve these scale targets is correspondingly large. BloombergNEF estimates cumulative investment in SAF production infrastructure of $3.5–4.5 trillion through 2050 to reach the IEA's ambitious scenario. For context, total global oil and gas investment in 2022 was approximately $500 billion. Redirecting even a fraction of hydrocarbon investment toward SAF production would dramatically accelerate the industry's scaling trajectory, but this requires policy frameworks that make SAF economically competitive with conventional jet fuel — either through carbon pricing that raises the cost of fossil jet fuel, mandates that require airlines to blend a minimum percentage of SAF regardless of cost, or subsidies that reduce SAF production costs.

Mandates, Policy, and the SAF Market

The primary policy instruments for driving SAF adoption are blending mandates (which require airlines or fuel suppliers to incorporate a minimum percentage of SAF in jet fuel sold), carbon pricing (which makes conventional jet fuel more expensive relative to SAF), and production incentives (which reduce SAF production costs through tax credits or subsidies). The United States, European Union, United Kingdom, and several other jurisdictions have implemented various combinations of these instruments, with significant differences in ambition and implementation timeline.

The European Union's ReFuelEU Aviation regulation, which entered into force in 2024, establishes mandatory SAF blending targets for all fuels uplifted at EU airports: 2% by 2025, 6% by 2030, 20% by 2035, 34% by 2040, 42% by 2045, and 70% by 2050. Significantly, the regulation includes a sub-mandate for "synthetic fuels" (PtL): 1.2% of total fuel by 2030, rising to 35% by 2050. This PtL sub-mandate is intended to drive investment in power-to-liquid technology at a scale that would not occur through market forces alone, given PtL's currently high cost. The regulation applies to all aircraft with MTOW above 5.7 tonnes departing EU airports, capturing virtually all commercial aviation.

The US Inflation Reduction Act (IRA) of 2022 created a Sustainable Aviation Fuel Tax Credit of $1.25–$1.75 per gallon for SAF meeting a 50%+ lifecycle GHG reduction threshold, with the credit increasing to $1.75 per gallon for SAF meeting a 100% reduction threshold. This credit significantly improves the economics of US SAF production relative to conventional jet fuel and has driven a wave of investment announcements in US SAF production capacity. Airlines including United, American, Delta, and Southwest have signed multiyear SAF offtake agreements partially on the strength of the IRA credit's economic impact.

Book-and-claim accounting, which allows airlines to purchase SAF credits separate from the physical fuel that flows through their aircraft, has been essential for making SAF commercially accessible to airlines whose supply chains cannot physically co-mingle SAF with their fuel uplift. Under book-and-claim, an airline purchases a certificate representing a defined quantity of SAF produced and blended somewhere in the global jet fuel supply chain; the airline claims the emissions reduction associated with that SAF volume for its corporate sustainability reporting, even though the physical SAF may have been burned by a different airline at a different airport. IATA's CORSIA methodology and the Roundtable on Sustainable Biomaterials (RSB) SAF certification program both support book-and-claim accounting, enabling multinational corporate travel programs and smaller airlines without direct SAF supply access to participate in the SAF market.