Jet fuel starts as crude oil pulled from underground reservoirs and becomes usable aviation fuel through a series of refining steps, the most fundamental being distillation. A typical barrel of crude oil in the United States yields about 8% jet fuel by volume, with refineries able to shift some of that output toward diesel or back toward jet fuel depending on market demand. But crude oil is no longer the only feedstock. A growing share of jet fuel now comes from waste fats, plant oils, and even captured carbon dioxide, processed through entirely different chemical pathways to meet the same strict performance standards.
From Underground to the Refinery
Crude oil is a complex mixture of thousands of hydrocarbon molecules ranging from tiny, light gases to heavy, tar-like compounds. When a refinery receives a shipment of crude, the first job is to separate these molecules roughly by size, because size largely determines boiling point and, by extension, what kind of fuel or product each fraction becomes. Light molecules boil off at low temperatures and become gases or gasoline. Heavier molecules stay liquid longer and become diesel, lubricants, or asphalt. Jet fuel sits in the middle of this spectrum, in a range sometimes called kerosene.
The workhorse of this separation is atmospheric distillation. Crude oil is heated in a furnace and then fed into a tall column where temperature decreases from bottom to top. Molecules rise through the column until they reach a level cool enough to condense back into liquid, at which point they are drawn off. Optimization studies have found that the ideal crude oil inlet temperature for a distillation unit is around 382 °C, a value refined through simulation modeling to maximize the quality and yield of each fraction.1EUREKA: Physics and Engineering. Study and optimization of the factors affecting the crude oil distillation process using ASPEN HYSYS The kerosene cut, which becomes jet fuel, typically comes off at temperatures between roughly 150 °C and 275 °C. Laboratory comparisons of straight-run kerosene fractions with finished commercial kerosene show that the basic physical properties often match closely at this stage, though the commercial product still needs further treatment.2Al-Nahrain Journal of Science. Evaluation and Comparison between Crude Oil Straight Run Fractions and Its Commercial Fractions (Gasoline, Kerosene and Gas Oil) at Dura Oil Refinery
Cleaning and Cracking
The kerosene that drips off a distillation column is not ready for an aircraft engine. It contains sulfur compounds, nitrogen, and other contaminants that would corrode engine parts, create harmful exhaust, and degrade fuel stability. Two main refining processes clean it up: hydrotreating and, for heavier feedstocks, hydrocracking.
Hydrotreating pushes hydrogen gas through the kerosene at high temperature and pressure over a catalyst bed. The hydrogen reacts with sulfur and nitrogen atoms in the fuel, pulling them out as hydrogen sulfide and ammonia gases that are then captured and disposed of. Detailed chemical analysis of kerosene before and after hydrotreating confirms that the process dramatically changes the sulfur compound profile while also shifting the balance of hydrocarbon types in the fuel.3Elsevier / ScienceDirect. Comparison of gas and kerosene oils chemical composition before and after hydrotreating using comprehensive two-dimensional gas chromatography A related process called Merox sweetening handles a specific class of sulfur compounds, mercaptans, by converting them into less corrosive disulfides. This step does not actually remove sulfur from the fuel but changes it into a form that is less damaging to fuel system seals and metals.4Ijraset Journal For Research in Applied Science and Engineering Technology. Desulphurization of Jet Fuel using Merox Process: A Review
When refineries need to squeeze more jet fuel out of heavier crude oil fractions that would normally become heavy fuel oil or asphalt, they turn to hydrocracking. This process uses extreme pressures, high temperatures, and specialized catalysts to break large hydrocarbon molecules into smaller ones that fall within the kerosene range. Research on hydrocracking heavy vacuum gas oil, for example, has used pressures of 18 MPa and temperatures between 390 and 430 °C over nickel-tungsten catalysts to convert those heavy molecules into lighter, usable fuels.5Catalysts. Hydrocracking of Heavy Vacuum Gas Oil with Petroleum Wax Hydrocracking gives refineries flexibility: they can adjust conditions to favor jet fuel output over diesel or gasoline, depending on what the market needs.
How Jet Fuel Relates to Diesel and Gasoline
Jet fuel, diesel, and gasoline all come from the same barrel of crude, but they occupy different boiling ranges and face very different performance requirements. Gasoline is the lightest of the three, boiling off first during distillation. Diesel overlaps with jet fuel’s boiling range but extends higher into heavier fractions. In the United States, roughly 46% of a barrel goes to gasoline and 31% to middle distillates like diesel and jet fuel, with jet fuel itself making up about 8%.6Elsevier. Alternative jet fuel feasibility
Because jet fuel and diesel share a similar distillation range, refineries can redirect the kerosene stream toward diesel production when diesel prices spike, and vice versa. This fungibility is a fundamental feature of refinery economics. It also means jet fuel supply is never entirely independent of diesel demand: a surge in trucking activity that drives up diesel consumption can tighten jet fuel supply even if air travel has not changed.
The critical difference lies in what aircraft engines demand. Jet fuel must remain liquid and pumpable at the extreme cold of high-altitude flight, where outside air temperatures can drop below −50 °C. It also must resist forming deposits at the high temperatures inside a jet engine’s fuel system. These twin requirements for cold-flow performance and thermal stability are far more stringent than anything diesel engines require, which is why jet fuel goes through additional testing and treatment that diesel does not.
Thermal Stability and Why It Matters
Inside a modern turbine engine, fuel does double duty. Before it burns, it circulates through the engine as a coolant, absorbing heat from hydraulic systems and engine oil. This means the fuel itself reaches high temperatures before combustion, and if it forms gummy deposits or breaks down chemically during that process, those deposits can clog fuel nozzles and degrade engine performance. This is why thermal oxidative stability is one of the most closely watched properties in jet fuel certification.
The standard test for this property heats fuel to 325 °C and measures how much deposit it leaves behind. Research into what drives deposit formation has found that the relationship between specific chemical compounds and fouling is not straightforward. In experiments adding various reactive compounds to synthetic kerosene, fouling actually passed through a minimum value before increasing, meaning a little bit of certain compounds can actually improve stability before too much starts causing problems. The concentration of any single reactive species turned out to be a poor predictor of thermal stability on its own.7Energy Science & Engineering. Sustainable aviation fuel: Impact of alkene concentration on jet fuel thermal oxidative test (JFTOT) This complexity is one reason jet fuel specifications are so detailed: you cannot simply look at one or two chemical properties and declare the fuel safe to fly.
Sustainable Aviation Fuels From Fats and Oils
The aviation industry is the hardest transportation sector to electrify. Batteries are far too heavy for long-haul flight, so the decarbonization strategy for aviation centers on sustainable aviation fuel, or SAF, which is chemically similar enough to conventional jet fuel that it can go into existing aircraft without modification. The most commercially mature pathway for making SAF starts not with crude oil but with waste cooking grease, animal fats, and plant-based oils.
This process, called hydroprocessed esters and fatty acids (HEFA), is currently the most established industrial method for SAF production.8Renewable and Sustainable Energy Reviews. Lipid feedstocks for sustainable aviation fuel (SAF) production via hydroprocessed esters and fatty acids (HEFA) pathway: Current status and challenges in quality, sustainability, availability and cost The chemistry involves stripping oxygen from the fat and oil molecules through a combination of reactions that produce water, carbon dioxide, and carbon monoxide as byproducts. What remains are long-chain hydrocarbons that then undergo cracking and isomerization, which reshapes them into branched molecules with the right size and cold-flow characteristics for jet fuel.9Current Opinion in Green and Sustainable Chemistry. Current outlook on sustainable feedstocks and processes for sustainable aviation fuel production The branching step is particularly important: straight-chain molecules would turn waxy at the low temperatures encountered during flight, so isomerization creates bends and branches that keep the fuel liquid.
HEFA fuel is already in commercial use at airports around the world, blended with conventional jet fuel. The main constraints are feedstock availability and cost. There is only so much used cooking oil in the world, and competition from biodiesel producers for the same feedstocks pushes prices up.
Fischer-Tropsch and Power-to-Liquid Fuels
A second major family of SAF production routes uses a process called Fischer-Tropsch synthesis, originally developed in the 1920s to make liquid fuels from coal. The modern version for SAF starts with a gas mixture of carbon monoxide and hydrogen, called synthesis gas. This gas can come from biomass, natural gas, or even captured COâ‚‚ combined with green hydrogen produced by electrolysis. The Fischer-Tropsch reaction builds up hydrocarbon chains from these simple gas molecules, and the resulting product can be further refined into kerosene that meets aviation standards.
Fischer-Tropsch kerosene is notable for being extremely clean: it is almost entirely made up of straight-chain and branched paraffins, with virtually no aromatic compounds or sulfur. Process evaluations have shown that both simple and more complex Fischer-Tropsch configurations achieve very high hydrocarbon yields, around 99%, and meet the ASTM D7566 specifications that govern SAF blendstock quality.10Energy Conversion and Management. Sustainable power-to-liquids aviation fuels: Modelling and comparison of two Fischer-Tropsch upgrading process concepts A further advancement allows the addition of aromatics to the Fischer-Tropsch kerosene, producing what is called FT-SPK/A, a fully formulated fuel that can in principle qualify as Jet A-1 without needing to be blended with conventional fuel at all.11Energy Science & Engineering. Sustainable aviation fuel: Pathways to fully formulated synthetic jet fuel via Fischer–Tropsch synthesis
When the starting feedstock is captured COâ‚‚ and renewable hydrogen, the resulting fuel is sometimes called e-kerosene or power-to-liquid fuel. A comparative economic assessment of several COâ‚‚-based SAF pathways, including Fischer-Tropsch, methanol-to-jet, and ethanol-to-jet routes, has examined how costs vary across the EU, the United States, and the Middle East.12Fuel. Comparative techno-economic assessment of CO2-based SAF pathways: Fischer-Tropsch, methanol-to-jet and ethanol-to-jet across three global regions: EU, USA and Middle East The economics are still challenging. These fuels require large amounts of green electricity for hydrogen production, and current costs are several times higher than conventional jet fuel. But they represent the only pathway to jet fuel that does not depend on any biological feedstock at all.
Alcohol-to-Jet and Other Emerging Pathways
A third SAF pathway converts alcohols, primarily ethanol and isobutanol, into jet fuel through a sequence of catalytic steps borrowed from traditional petroleum refining and petrochemical processing. The alcohol-to-jet process dehydrates the alcohol into an alkene, oligomerizes those small molecules into longer chains in the kerosene range, and then hydrogenates the result into a stable paraffinic fuel.13ChemSusChem. The Alcohol-to-Jet Conversion Pathway for Drop-In Biofuels: Techno-Economic Evaluation This pathway is attractive because ethanol production is already a massive global industry, meaning there is an existing supply chain to draw on. The catch is that converting ethanol into jet fuel adds significant cost and complexity compared to simply burning the ethanol in a car.
Researchers are also exploring more exotic feedstocks. Lignin, the rigid structural polymer in wood and crop residues, can be broken down into small aromatic molecules and then built back up into jet-fuel-range hydrocarbons. Laboratory work has demonstrated controllable transformation of lignin into both aromatic and cycloparaffinic compounds in the right carbon number range for jet fuel, with selectivity for the desired products reaching above 94%.14PubMed Central. From lignin to cycloparaffins and aromatics: directional synthesis of jet and diesel fuel range biofuels using biomass Lignin is abundant and currently treated as a low-value byproduct of the paper and bioethanol industries, so finding a way to convert it into aviation fuel at scale would be a significant win.
The Blend Wall and Why 100% SAF Is Hard
Even when a sustainable aviation fuel meets every chemical specification, it still faces a practical compatibility challenge. Current ASTM standards limit most SAF types to a maximum of 50% by volume when blended with conventional jet fuel. The reason is partly about engine seals. Conventional jet fuel contains aromatic compounds that cause the rubber o-rings in aircraft fuel systems to swell slightly. That swelling is actually by design: it creates a tight seal. Pure SAF, especially Fischer-Tropsch and HEFA fuels, contains very few aromatics, so the seals might not swell enough, raising the risk of leaks.
Research using optical measurements of rubber swelling has found that the relationship between aromatic content and seal performance is more nuanced than the simple 8% aromatics minimum currently required by specifications. Some fuel blends with less than 8% aromatics still produced swelling in the conventional fuel range, suggesting that the current rules may be more conservative than necessary.15Energy & Fuels. Measurements of Nitrile Rubber Absorption of Hydrocarbons: Trends for Sustainable Aviation Fuel Compatibility The industry is actively working to revise these standards, and some SAF formulations, like FT-SPK/A with added aromatics, are designed specifically to work around this limitation.
Coal-Based Jet Fuel
Crude oil and biological feedstocks are not the only starting points. South Africa has been making jet fuel from coal for decades, a legacy of the apartheid-era need for fuel self-sufficiency. Coal can be converted to jet fuel through two routes: direct liquefaction, which dissolves coal under heat and pressure with hydrogen to create liquid hydrocarbons, and indirect liquefaction, which first gasifies the coal into synthesis gas and then uses Fischer-Tropsch chemistry to build fuel molecules from scratch.
These two routes produce fuels with very different chemical profiles. Direct coal liquefaction yields a kerosene with high aromatic content, while the indirect Fischer-Tropsch route produces fuel that is almost entirely paraffinic, with nearly no aromatics at all.16Applied Mechanics and Materials. Component Characteristics of Coal-Based Jet Fuel and Petroleum-Based Jet Fuel The high aromatic content from direct liquefaction is actually a concern for combustion quality and emissions, while the near-zero aromatics from the indirect route create the same seal-swelling compatibility issues discussed above. Neither matches conventional petroleum-based jet fuel’s balanced composition without additional blending or processing. Coal-based jet fuel also carries a very high carbon footprint compared to petroleum-derived fuel, since the gasification and liquefaction steps are energy-intensive. It remains relevant primarily in countries with abundant coal and limited domestic oil production.
From Refinery to Wing
Making jet fuel is only half the story. Getting it from the refinery to the aircraft without contamination is an elaborate logistical operation. Jet fuel travels through dedicated pipelines, is stored in purpose-built tanks, and is tested repeatedly at each transfer point. Water is one of the most persistent threats: even small amounts dissolved in the fuel can freeze at altitude and block fuel filters. Microbial growth in fuel-water interfaces is another headache, because bacteria and fungi can form mats that clog filters and corrode tank linings.
Reviews of the jet fuel supply chain have documented how multiple layers of safeguards are deployed from the refinery gate all the way to the aircraft wing, including filtration, sampling protocols, and quality management systems at each handoff point between pipeline operators, storage terminals, and fueling trucks.17Process Safety Progress. Safeguards: A key process safety tool in jet fuel management from refinery to aircraft wings At major airports, fuel arrives through a hydrant system of underground pipes connected directly to gate positions, so the fuel never touches open air between the storage tank and the aircraft. Smaller airports rely on fuel trucks, which adds another transfer step and another opportunity for contamination, making quality testing at the point of delivery even more critical.
Cleaner Fuels and the Contrail Problem
The environmental conversation around jet fuel has historically focused on COâ‚‚ emissions, but researchers increasingly recognize that contrails, the white lines aircraft leave across the sky, may contribute as much or more to aviation’s total warming effect as the COâ‚‚ itself. Contrails form when soot particles in engine exhaust act as seeds for ice crystal formation in cold, humid air. The more soot, the more ice crystals, and the greater the warming effect of the resulting artificial clouds.
Flight tests comparing conventional Jet A-1 with SAF blends have found dramatic reductions in soot when burning the cleaner fuels. HEFA-based blends produced roughly 45–53% less soot than conventional fuel, and a Fischer-Tropsch-based semi-synthetic fuel showed similar reductions of about 50%. Those soot reductions translated into 45–74% fewer ice crystals forming in the engine exhaust plumes.18Communications Earth & Environment. Cleaner burning aviation fuels can reduce contrail cloudiness The biggest reductions came from fuels specifically designed to minimize naphthalene, a type of two-ringed aromatic compound that turned out to be a particularly efficient soot precursor.
Modeling work on how soot particles activate as ice crystal nuclei has added further detail to this picture. The size of the primary soot particles matters, not just their total number. As biofuel content increases and aromatic concentrations drop, both the number and the size of soot particles shrink, changing how many of them are able to trigger ice formation.19Environmental Science & Technology. Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles This finding suggests that even partial SAF blends, well below 50%, could meaningfully cut aviation’s contrail-related warming, offering a climate benefit that starts paying off long before the industry achieves 100% SAF adoption.