Let's be clear about SAF fuel composition right from the start. It's not one single recipe. It's a family of jet fuels defined not by where they come from, but by their molecular structure and performance. That's the crucial shift in thinking. Whether it starts as used cooking oil, agricultural residues, or even captured carbon, the end product must meet the exacting specifications of Jet A or Jet A-1 fuel. This chemical parity is the non-negotiable foundation that allows SAF to be used in today's aircraft without modification. The magic—and the complexity—lies in the transformation process.
What You'll Find in This Guide
The Building Blocks: Feedstocks and Conversion Pathways
If you ask me what SAF is made of, I have to ask you back: at which stage? The feedstock or the final fuel? Most people get this confused. The feedstock is the raw material—the "what you start with." The composition of the final fuel is about the "what you end up with" after processing. This processing is called the conversion pathway, and it's the single biggest determinant of the fuel's cost, sustainability profile, and scalability.
Right now, the workhorse is the HEFA (Hydroprocessed Esters and Fatty Acids) pathway. It's mature, certified, and supplies over 90% of today's SAF. It uses fats, oils, and greases (FOGs). Think used cooking oil from restaurants, animal tallow from meat processing, or specific vegetable oils like camelina. The process strips out the oxygen and cracks the long-chain fatty acids into the shorter, branched hydrocarbons jet engines crave.
But here's a subtle error I see all the time: assuming all HEFA SAF is the same. The precise mix of feedstocks going into the biorefinery dramatically affects the yield and properties of the final fuel. Used cooking oil has different fatty acid profiles than, say, soybean oil. A refiner has to constantly tweak the hydroprocessing conditions to hit the exact Jet-A spec. This operational nuance is rarely discussed but is critical for consistent, high-volume production.
Comparing the Major Feedstock Categories
It's useful to break down the main contenders. The table below isn't just a list; it shows the trade-offs the industry is grappling with.
| Feedstock Category | Common Examples | Key Advantage | Primary Limitation / Concern |
|---|---|---|---|
| Lipids (Oils & Fats) | Used Cooking Oil (UCO), Animal Tallow, Camelina Oil | Technology-ready (HEFA pathway), high yield to jet fuel. | Limited, competed-for supply. UCO collection is a global logistics chain. |
| Lignocellulosic Biomass | Agricultural residues (corn stover), forestry waste, energy crops (switchgrass) | Massively scalable, avoids food competition, lower feedstock cost. | Complex, expensive conversion (FT, ATJ). Technology still scaling. |
| Sugar & Starch | Sugarcane, corn | Can use Alcohol-to-Jet (ATJ) pathway, established agri-infrastructure. | Direct food/fuel land-use debate. Sustainability depends heavily on farming practices. |
| Waste Streams | Municipal Solid Waste (MSW), industrial off-gases | High sustainability benefit (waste reduction), potentially very low carbon. | Feedstock inconsistency, pre-processing challenges, public perception ("trash to fuel"). |
| Power-to-Liquid (PtL) | Captured CO2 + Green Hydrogen (from electrolysis) | Ultra-low carbon potential, unlimited feedstock (CO2 from air/industry). | Extremely energy-intensive, requires vast cheap renewable electricity, very high cost today. |
Looking at this, the future of SAF composition is a multi-feedstock, multi-pathway story. Relying solely on used cooking oil won't cut it. The real game-changer, in my view, will be lignocellulosic and PtL routes, but they need time and massive investment to become cost-competitive.
The Rulebook: ASTM Standards and Blending Limits
This is where theory meets the tarmac. The SAF fuel composition is legally defined by the ASTM International standard D7566. This document is the bible. It doesn't just say "make sustainable fuel." It specifies, in intense chemical and physical detail, what the fuel from each approved pathway (Annexes A1 for HEFA, A2 for FT, etc.) must look like.
The most critical number for airlines and fuel suppliers is the blend limit. With very few, very recent exceptions, SAF must be blended with conventional Jet A/A-1. The current maximum for HEFA-SPK (Synthetic Paraffinic Kerosene) is 50%. For FT-SPK, it's also 50%. Why?
Safety and performance. Pure SAF, while chemically similar, can have different density, elastomer compatibility, or lubricity. Blending ensures the final mixture behaves identically to the fossil fuel the engine was certified on. It's a conservative, step-by-step approach. The industry is slowly testing and approving higher blends—United flew a 100% SAF flight on one engine in 2021—but for everyday operations, blending is the rule.
So, when you hear "SAF flight," it usually means a flight powered by a blend, say 30% SAF and 70% conventional jet fuel. The aviation fuel blending operation happens at specialized fuel depots, not at the airport ramp. It's a precise, regulated process.
A Molecular Breakdown: What's Actually in the Fuel?
Let's get molecular. Forget the feedstock for a moment. A drop of finished HEFA-SAF blended fuel is primarily a soup of paraffins (iso-alkanes and normal alkanes) and naphthenes (cycloalkanes). It has virtually no aromatics or sulfur. That last point is huge.
Conventional jet fuel contains aromatic compounds (like benzene derivatives). They're there for a reason: they swell the seals in older aircraft fuel systems to prevent leaks. Early 100% SAF tests revealed this issue—the seals shrank. Modern aircraft have compatible elastomers, but the global fleet is mixed. That's another reason for blending; the aromatics in the conventional portion keep the seals happy.
The near-zero sulfur content of SAF is a massive, unsung benefit. Sulfur in exhaust forms sulfate particles that become condensation nuclei for contrails. Contrails, especially persistent ones, contribute to aviation's warming effect. By removing sulfur, SAF composition directly leads to fewer and less impactful contrails. This is a non-CO2 benefit that doesn't get enough attention.
Here's a practical implication. Because SAF is more paraffinic, it often has a slightly higher energy density per kilogram than some conventional fuels. It's marginal, but in the weight-obsessed world of aviation, it can translate to a tiny efficiency gain. Not a selling point for the headlines, but a real operational note for flight planners.
The Real-World Hurdles and Future Ingredients
Talking about ideal SAF fuel composition is one thing. Delivering it at scale is another. The three biggest constraints are cost, feedstock availability, and the energy balance of production.
Cost is the elephant in the room. HEFA SAF is currently 2 to 4 times more expensive than fossil jet fuel. The feedstock itself is a major cost driver. Used cooking oil is no longer a waste product; it's a globally traded commodity. This highlights a painful truth: scaling SAF will create new supply chains and new competitions for resources.
The future composition will be shaped by economics and policy. Policies like the U.S. Inflation Reduction Act tax credits are explicitly tiered: fuels with a lower carbon intensity score get a higher credit. This incentivizes pathways beyond HEFA. It makes waste-based and PtL fuels more financially viable on paper.
I'm cautiously optimistic about PtL, or e-kerosene. Its composition can be almost identical to conventional fuel, including controlled aromatics, making 100% blends easier. But the "green hydrogen" part is the killer. You need enormous amounts of renewable electricity to split water. If that electricity isn't truly additional and cheap, the overall carbon benefit plummets. It's a energy vector problem, not just a chemistry problem.
The next decade will see new annexes added to ASTM D7566 for pathways like Alcohol-to-Jet (ATJ) from ethanol and methanol. Each new approved pathway expands the palette of acceptable SAF feedstocks and, by extension, the possible compositions of the fuel in your tank.