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Biofuels are made by breaking biomass into useful intermediates—such as sugars, oils, bio-oil, or synthesis gas—then converting and upgrading those intermediates into fuel. There is no single “chemical way”: some routes begin with fermentation, others with heat or pressure, and many combine biological conversion with chemical reactions and catalysts.
What happens between biomass and finished fuel?
A typical conversion pathway can be understood as feedstock → pretreatment or deconstruction → intermediate → conversion and upgrading → finished fuel. Not every facility uses every step. The feedstock and desired product determine the process.
Biomass may be separated into component chemicals before being converted into fuels, chemical intermediates, or other products. For example, a plant might turn carbohydrates into sugars, oils into fatty-acid-derived intermediates, or woody material into bio-oil or synthesis gas. Those intermediates are then processed into a fuel with the required properties. The U.S. Department of Energy describes these conversion technologies as producing liquid transportation fuels, including diesel and jet fuel, as well as chemical intermediates, co-products, and biopower (DOE conversion technologies).
Three broad ways to convert biomass
Biochemical conversion
Biochemical routes use biological processes, often to make sugars or other carbohydrate intermediates. Fermentation can turn those sugars into alcohols such as ethanol; chemical processing can then convert an alcohol into a different fuel. Thus, a biofuel route can be biological at the front end and chemical at the back end.
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Thermochemical conversion
Thermochemical routes use heat, sometimes with pressure and catalysts, to make intermediates such as bio-oil or synthesis gas. Those intermediates generally need further processing before they become finished fuels. Gasification, for example, makes synthesis gas that can be converted into hydrocarbons.
Chemical upgrading
Upgrading adjusts an intermediate’s composition and properties to produce a usable fuel. Depending on the route, it can include reactions such as dehydration, hydrogenation, hydrocracking, hydroisomerization, or synthesis from gas. The term “chemical” therefore describes a range of operations, not one universal reactor or recipe.
How biomass becomes sustainable aviation fuel
Sustainable aviation fuel (SAF) is a useful example because different biomass feedstocks can be transformed into drop-in hydrocarbon fuel through distinct routes. In the United States, the Alternative Fuels Data Center (AFDC) describes several approved pathways, each with its own feedstocks, process and ASTM pathway designation. Approval and blend limits are pathway-specific and can change; check the current ASTM D7566 annex and applicable requirements for a particular fuel.
| Pathway | Feedstock and intermediate | Main conversion steps | AFDC pathway and blending information |
|---|---|---|---|
| HEFA | Triglyceride oils, including plant or animal oils and waste fats, oils and greases. | Hydroprocessing breaks down fatty-acid chains; hydroisomerization and hydrocracking further process the material. | HEFA-SPK, ASTM D7566 Annex A2; AFDC lists a 50% blending limit. |
| Alcohol-to-jet (ATJ) | Ethanol or isobutanol, which can be produced from starchy or cellulosic feedstocks through fermentation or gasification. | Dehydration, oligomerization, hydrogenation and hydrotreatment convert alcohol into jet-range fuel. | ATJ-SPK, ASTM D7566 Annex A5. AFDC’s table and accompanying text differ on the blending limit; consult the current standard and the approved pathway rather than relying on a single general figure. |
| Gasification and Fischer–Tropsch (FT) | Woody biomass and other eligible materials, including municipal solid waste, agricultural and forest wastes, and energy crops; the intermediate is synthesis gas. | Gasification produces synthesis gas, which Fischer–Tropsch synthesis converts into hydrocarbons. | FT-SPK; AFDC lists the pathway among approved options. The cited AFDC pathway description does not give a blend limit for this row. |
| Catalytic hydrothermolysis | Clean free-fatty-acid oil combined with preheated water. | The mixture passes through a catalytic hydrothermolysis reactor and is processed into jet fuel. | ASTM D7566 Annex A6; AFDC lists a 50% blending limit. |
These pathway descriptions and blend figures are from the U.S. DOE AFDC’s SAF overview. The ATJ discrepancy is a reason to verify the exact approved case and current standard rather than treating a table entry as a universal allowance.
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Other familiar examples: ethanol and biodiesel
Ethanol
Ethanol is commonly produced by fermenting carbohydrate-rich biomass. Cellulose and hemicellulose are also being explored as sources of fermentable sugars, but converting them requires processing the plant material into usable intermediates. NREL’s indexed Biofuels Basics page describes these approaches; the page’s detailed content was not available for direct verification here, so no finer process claims are made.
Biodiesel
Biodiesel can be made by reacting an alcohol—usually methanol—with vegetable oil, animal fat, or recycled cooking grease. This is distinct from the SAF pathways above: similar oil-based feedstocks can serve different processes and produce fuels with different specifications.
Which routes are ready to scale?
Commercial maturity differs by pathway. The International Energy Agency (IEA) identifies established biofuel pathways, while describing biomass-to-Fischer–Tropsch as at demonstration stage and hydrothermal liquefaction (HTL) and fast-pyrolysis upgrading as less mature. Its status descriptions are not a guarantee that a particular project is commercially viable; project economics, feedstock access and fuel qualification still matter. See the IEA’s biofuels overview.
HEFA can use existing processing approaches and waste oils, but waste fats and oils are limited resources. The IEA identifies feedstock diversification, reducing the cost gap, and clear policy and regulatory frameworks as important to expanding biojet production. A pathway that works chemically does not automatically work at the scale or cost needed for broad deployment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What determines whether a biofuel is sustainable?
“Bio-based” does not by itself establish that a fuel is low-carbon or sustainable. The result depends on the feedstock, how it is produced or collected, the energy used in conversion, and the boundaries of the lifecycle assessment. Land and water use also matter. A waste-derived fuel and a fuel made from a dedicated crop can have different impacts even if their final molecules are similar.
- Feedstock availability: waste oils, residues and other non-food feedstocks can support production, but their supply is finite.
- Process energy and yield: conversion inputs and the amount of usable fuel produced affect both cost and lifecycle performance.
- Land and water: crop-based supply must be assessed for its land and water demands.
- Certification and standards: a fuel must meet the relevant pathway and product requirements for its intended use.
The scale of the challenge is substantial. The IEA reports that biofuels supplied more than 3.5% of global transport energy demand in 2022, while biojet kerosene accounted for less than 0.1% of aviation fuel demand that year. In the IEA’s Net Zero Emissions by 2050 Scenario, biojet kerosene reaches around 10% of aviation fuel demand in 2030; that is a scenario requirement, not a forecast. In the same scenario, about 40% of biofuels consumed in 2030 come from wastes, residues and dedicated non-food-competing crops, compared with an estimated 9% in 2021. These figures describe different measures and scenario assumptions, not assured outcomes.
Why there is no one chemical recipe
Biomass is a broad category, not a uniform raw material. Sugars, oils, woody matter and organic wastes have different compositions, so they produce different intermediates and call for different reaction sequences. The route also depends on the target fuel: producing ethanol is not the same process as producing jet-range hydrocarbons.
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The practical question is therefore not simply which reaction turns biomass into fuel. It is whether a particular feedstock can be supplied sustainably, converted economically, upgraded to the required specification, and certified for its intended use.
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