Decarbonization has long been a priority for the aviation industry, but the journey to achieving net-zero has been far from linear. Since the first commercial flight took off with biofuels in 2008, the path towards sustainability has been full of obstacles.
Sustainable Aviation Fuel (SAF) has the potential to cut lifecycle greenhouse emissions by 80%, but to be implemented at scale, it must balance safety, accessibility, and costs.
Over the years, a range of technological pathways have emerged to crack this challenge. This article provides an overview of how SAF can be produced, highlighting the benefits and challenges with each approach.
Feedstocks: the SAF building block
Feedstocks are raw materials or inputs used in industrial processes to produce products. These can be natural or synthetic, and are typically unprocessed. Today, most SAFs are derived from one of these three kinds:
- First-generation crops are traditional food crops such as corn, sugarcane, soybean, and palm oil that can be converted into biofuels. While they are readily available, their use for SAF raises concerns about competition with food production and the impact on land use.
- Second-generation sources include non-food biomass such as agricultural residues (e.g., corn stover, wheat straw), forestry residues, and food waste. These feedstocks are more sustainable because they use parts of plants or waste materials that would otherwise be discarded. However, their supply is limited, which creates a bottleneck.
- Third-generation feedstocks primarily refer to algae and other microorganisms that can be cultivated on non-arable land using non-potable water. Algae offer high oil yields per acre and do not compete with food crops for land or freshwater resources. On the flip side, the technology is still in development, which makes it costly to produce and limits commercial scale.
Not all Sustainable Aviation Fuels (SAF) are produced from traditional biological feedstocks. Emerging technologies, such as Power-to-Liquid, use captured carbon and renewable hydrogen to overcome constraints. Below, we explain how feedstocks are treated to produce SAF.
Feedstock pre-treatment
A critical step in SAF production is ensuring that feedstocks are clean, stable, and suitable for conversion. Many feedstocks contain impurities such as metals, phosphorus, sulfur, or complex polymers that can damage catalysts and reduce fuel quality. Pre-treatment processes such as degumming, filtration, bleaching, and chemical hydrolysis help remove these contaminants. However, these can be energy-intensive, costly, and highly feedstock-specific. Pre-treatment systems, advanced catalysts, and energy-efficient technologies are driving innovation within this space. Additionally, SAF pathways that do not rely on biomass can bypass pre-treatment entirely, offering a promising alternative that can drastically reduce costs.
Established Production Pathways
Once treated, SAF can be produced through different methods. Each pathway comes with its own set of benefits and challenges, including differences in carbon reduction potential, production costs, and scalability. In addition to production considerations, SAF must meet strict safety and performance requirements. ASTM International, a global standards body, provides the certification framework that sets the guidelines for which SAF pathways are approved and under what blending limits.
The two most established pathways are:
- Hydroprocessed Esters and Fatty Acids (HEFA), which account for 80-90% of global SAF production today. HEFA runs lipids such as fats and oils through a refinery process with hydrogen to create synthetic paraffinic kerosene (SPK). SPK is a clean liquid fuel that burns nearly identically to conventional jet fuel, making it an attractive option. However, the limited availability and high cost of sustainable feedstocks are driving up prices.
- Co-processing lipids / Fischer–Tropsch liquids blends bio-based or synthetic feedstocks like used cooking oil, animal fats, or FT waxes, into existing refinery streams. This allows for incremental SAF production within the current infrastructure, which brings the benefit of low marginal cost. However, it requires strict traceability of renewable carbon and has a limited renewable share of just 5% blending under ASTM D1655, though efforts are underway to raise this to 30%.
Emerging Pathways
In recent years, several advanced technologies have emerged to overcome these barriers and expand production. Four pathways moving from pilot to commercialization include:
- Fischer-Tropsch Conventional (FT-SPK) can be divided into two main types based on whether the source is carbon and hydrogen: conventional FT and Power-to-Liquid (PtL). Both utilize abundant waste feedstocks such as municipal solid waste and agricultural residues. Feedstock supply chains for biomass and waste materials are still not well-established, and the complex gasification process means high upfront investment and lengthy construction processes.
- Alcohol-to-Jet (ATJ-SPK) converts alcohols like ethanol into synthetic paraffinic kerosene (SPK). Ethanol yield per acre is six times higher than oil, but it brings higher conversion costs compared to HEFA.
- Catalytic Hydrothermolysis Jet (CHJ) mimics the natural formation of fossil fuels by processing waste oils, free fatty acids (FFAs), and greases under high pressure and temperature. The technology is still in early pilot stages and requires specialized reactors.
- Synthesised Iso-Paraffins (SIP) convert sugar-based feedstocks, such as sugarcane juice, sugar beet, or cellulosic sugars, into farnesene through microbial fermentation. Fermentation costs are currently high, and SIP is limited to a 10% blend under current
ASTM approval. Production remains mostly at demonstration scale.
In addition to these, several pathways are currently in early research and ASTM evaluation phases. ReOil, for example, turns waste plastics into synthetic oils. Sign up for the Net0 Platform to view the full innovation landscape.
SAF End Products
Sustainable Aviation Fuel (SAF) end products are renewable or synthetic jet fuels that can be used directly in existing aircraft engines and fuel infrastructure. Typical SAF end products include synthetic paraffinic kerosene (SPK), iso-paraffins, and aromatic compounds, blended to meet stringent aviation fuel standards.
- Bio-fuels are the backbone of current SAF production. These are derived primarily from biological materials such as plant oils, animal fats, agricultural residues, and organic waste.
- E-SAF represents a revolutionary approach to SAF production that bypasses biological feedstocks entirely. These can be created by combining green hydrogen with captured carbon and then converted into synthetic hydrocarbons suitable for jet fuel.
These fuels deliver comparable energy density and combustion properties while significantly lowering lifecycle greenhouse gas emissions.
Conclusion
The extensive SAF landscape shows the complexity in decarbonizing the aviation industry. While innovation is taking place across the supply chain, a single solution replacing conventional fuels is still far off.
As the industry scales up from today’s 2% SAF usage to meeting ambitious climate targets, the continued development and optimization of these diverse production pathways will be essential for achieving sustainable aviation’s ultimate goal.
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