Aviation is one of the most challenging sectors to decarbonise—and one of the most urgent. With global air travel consuming nearly 300 million metric tonnes of fuel annually and contributing roughly 2.5% of global CO₂ emissions, the need for climate-conscious alternatives is pressing. Unlike road or rail transport, where electrification is increasingly viable, jetliners cannot simply be plugged into the grid. The physics of flight demand high energy density, and for now, that means liquid fuels.
This is where Sustainable Aviation Fuel (SAF) enters the picture. SAF is a ready-to-use, drop-in alternative to conventional jet fuel that integrates seamlessly with existing aircraft, engines, and airport infrastructure. SAF represents the most promising near-term solution for reducing aviation’s climate impact without requiring radical changes to the aviation system.
The technologies behind SAF production reveal how diverse feedstocks—including captured carbon dioxide—are transformed into jet fuel. From established methods to frontier innovations in biological, chemical, and electrochemical conversion, we examine the future pathways that could redefine aviation’s relationship with the climate.
1. Transition to SAF
Aviation fuel—commonly known as jet fuel—is traditionally derived from petroleum, much like other transportation fuels. In contrast, SAF is produced from renewable and low-carbon feedstocks, including waste oils, agricultural residues, and even captured carbon dioxide. Depending on the feedstock and production pathway, SAF can achieve net lifecycle greenhouse gas emission reductions of 80% or more compared to conventional jet fuel. Its compatibility with existing infrastructure, combined with its climate benefits, has positioned SAF as a cornerstone of global aviation decarbonisation strategies.
International bodies like the International Civil Aviation Organisation (ICAO) and the International Air Transport Association (IATA) have set ambitious SAF adoption targets for 2050. Meanwhile, regional initiatives are driving deployment:
- The EU’s ReFuelEU Aviation initiative mandates a 2% SAF blend by 2025, scaling to 70% by 2050.The U.S.
- SAF Grand Challenge aims for 3 billion gallons of SAF annually by 2030 and 35 billion by 2050, supported by tax credits under the Inflation Reduction Act.
2. How SAF Is Made: A Two-Step Process
All SAF production pathways—whether commercial or experimental—follow a core two-step framework—each involving multiple substeps tailored to the feedstock and technology.
2.1 Step 1: Feedstock Conversion
Renewable and low-carbon feedstocks are first transformed into intermediate compounds. These intermediates typically include bio-oils, alcohols, or biocrude, depending on the input material and conversion method.
2.2 Step 2: Fuel Upgrading
The intermediates are then refined through advanced upgrading techniques such as hydroprocessing, catalytic reforming, or hydrogenation. These processes ensure the final product meets stringent aviation fuel specifications for energy density, combustion performance, and safety.
3. Feedstock for SAF
The choice of feedstock plays a critical role in determining the sustainability, scalability, and carbon reduction potential of SAF. Feedstocks typically fall into four main categories:
- Lipid-based: Includes plant oils, used cooking oil, animal fats, and other waste greases. These are commonly used in HEFA pathways.
- Sugar-based: Derived from carbohydrate-rich biomass such as corn, wheat, and bagasse. These feedstocks are typically fermented into alcohols for Alcohol-to-Jet production.
- Lignin-rich: Comprises crop residues, forestry waste, and woody biomass. These are suitable for thermochemical conversion methods like pyrolysis and hydrothermal liquefaction.
- Carbon dioxide: Captured either from industrial point sources or directly from the atmosphere. This feedstock enables novel biological, chemical, and electrochemical pathways that transform CO₂ into jet fuel.
4. Commercial SAF Technologies
To date, two SAF technologies have reached commercial scale, each offering distinct pathways for converting renewable feedstocks into jet-grade fuel.
4.1 Hydroprocessed Esters and Fatty Acids (HEFA)
HEFA is the most mature SAF technology, converting lipid-based feedstocks—such as plant oils, waste fats, and greases—into jet fuel through hydroprocessing. The process involves catalytic deoxygenation and hydroisomerisation, yielding hydrocarbons that meet stringent aviation specifications.
4.2 Alcohol-to-Jet (AtJ)
AtJ upgrades sugar-derived alcohols like ethanol or butanol into SAF via dehydration, oligomerisation, and hydrogenation. Most current AtJ facilities rely on food crops such as corn or wheat to produce the necessary alcohol intermediates.
4.3 The Food-versus-Fuel Dilemma
While both HEFA and AtJ are technically capable of using waste-based feedstocks—such as used cooking oil or cellulosic ethanol—the commercial reality is more complex. Present-day HEFA production still depends heavily on food-grade oils, and AtJ facilities continue to use edible grains. Although these fuels are classified as sustainable due to their non-fossil origins, they raise serious ethical concerns about diverting food resources for fuel—especially in a world where nearly 30% of the global population faces moderate or severe food insecurity.
5. Emerging SAF Technologies: Beyond Food-Based Feedstocks
To overcome the food-versus-fuel dilemma, researchers and developers are advancing next-generation SAF technologies that rely on non-edible biomass and captured carbon dioxide. These innovations aim to unlock more equitable, scalable, and climate-resilient pathways for aviation decarbonisation. Two promising biomass-based technologies are gaining traction for their ability to convert lignin-rich and waste-derived feedstocks into jet fuel.
5.1 Hydroprocessed Catalytic Fast Pyrolysis Oil (HCFPO)
HCFPO employs catalytic fast pyrolysis—a rapid thermal decomposition process—to convert lignin-rich biomass into bio-oils. These oils are then upgraded through hydroprocessing to meet aviation fuel standards. This pathway offers a viable route for utilising forestry residues, crop waste, and other non-food biomass.
5.2 Catalytic Hydrothermolysis Jet (CHJ)
CHJ mimics the geological formation of crude oil by applying heat and pressure to biomass in a water-rich environment. The resulting biocrude is subsequently refined into SAF using conventional upgrading techniques. CHJ is particularly suited to wet biomass and organic waste streams, expanding the range of usable feedstocks.
6. The Carbon Dioxide Frontier
Among the most transformative innovations in SAF development is the use of carbon dioxide itself as a feedstock. This approach not only helps reduce atmospheric CO₂ levels but also enables a circular carbon economy—turning waste emissions into valuable fuel.
Researchers are exploring three primary pathways for converting CO₂ into SAF:
- Biological: Microorganisms metabolise carbon dioxide into oils or alcohols, which can be upgraded into jet fuel.
- Chemical: Advanced catalytic systems directly synthesise hydrocarbons from CO₂ through thermochemical reactions.
- Electrical: Electrochemical reactors powered by renewable energy reduce CO₂ into fuel precursors such as syngas or alcohols.
Europe is leading in electrochemical conversion technologies, while Australia is pioneering biological approaches—particularly in integrating biowaste and carbon capture.
6.1 Carboxylic Acid to Jet-Fuel (Catoj) Technology
A standout example is Catoj—short for Carboxylic Acid to Jet-Fuel—which combines biowaste and captured CO₂ to produce microbial carboxylic acids. These acids are then converted into hydrocarbons in a single-step process suitable for SAF, gasoline, and diesel. By streamlining the conversion pathway, Catoj improves overall efficiency and reduces production complexity—potentially lowering costs and accelerating commercial viability.
7. Looking Ahead
While many of these technologies are still in early development, they offer compelling alternatives to food-based SAF. Their success will depend on:
- Continued R&D investment
- Supportive policy frameworks
- Strategic partnerships across industry and government
As aviation grapples with its climate footprint, SAF offers a bridge between today’s infrastructure and tomorrow’s sustainability. The challenge now is to scale these solutions—ethically, efficiently, and equitably.
Also See
CO2 to Jet Fuel: Our SAF Moment
Sustainable Aviation Fuel: Bio Jet Fuel & CO₂ Use Innovations Solving the Food vs Fuel Dilemma
Technological Evolution of Bioethanol from First to Third Generation
Adeerus Ghayan 