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Biofuels are made by converting biomass into fuel-ready intermediates—such as sugars, oils, bio-oil or synthesis gas—and then processing those intermediates into usable fuels. There is no single chemical recipe: the route depends on the feedstock and the desired fuel. Some pathways rely on biological conversion, such as fermentation, while others use heat, catalysts or both. Many commercial processes combine these approaches.
How biomass becomes fuel
A useful way to understand biofuel production is to follow the material through four stages: feedstock, preparation, an intermediate, and conversion or upgrading. A plant may combine or omit steps depending on its feedstock and process. The U.S. Department of Energy describes biomass conversion as producing transportation fuels, chemical intermediates, co-products and biopower through deconstruction or fractionation followed by synthesis and upgrading.
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- Choose a feedstock: Examples include carbohydrate-rich crops, woody biomass, plant oils, animal fats, used cooking oil and other wastes.
- Prepare and separate it: Pretreatment or deconstruction makes useful components accessible, such as sugars, oils or fatty acids.
- Make an intermediate: Depending on the route, this may be an alcohol, bio-oil or synthesis gas.
- Convert and upgrade: Biological processing, heat, catalysts or combinations of these turn the intermediate into a fuel with the required properties.
The distinction between “biological” and “chemical” is not absolute. Fermentation uses microorganisms to make an alcohol, but chemical upgrading can then turn that alcohol into jet fuel. Thermochemical routes use heat to make bio-oil or gas intermediates, which may also need catalytic upgrading. DOE’s conversion technology overview and its SAF strategy presentation transcript describe these broad routes.
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Routes that can produce sustainable aviation fuel
Sustainable aviation fuel (SAF) provides a clear example of how different feedstocks can reach a similar finished product by different process sequences. The U.S. Department of Energy Alternative Fuels Data Center (AFDC) describes several approved pathways, each tied to a specific ASTM pathway and blend limit. Those limits are pathway-specific, and the applicable standard should be checked for the exact fuel and approval in question.
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| Pathway | Feedstock and intermediate | Conversion sequence | AFDC-listed ASTM pathway and blend limit |
|---|---|---|---|
| HEFA-SPK | Plant or animal oils, including waste fats, oils and greases; triglyceride feedstock | Hydroprocessing breaks fatty-acid chains, followed by hydroisomerization and hydrocracking. | ASTM D7566 Annex A2; 50% blend limit |
| Alcohol-to-jet (ATJ-SPK) | Ethanol or isobutanol, which can be made from cellulosic or starchy feedstock through fermentation or gasification | Dehydration, oligomerization, hydrogenation and hydrotreatment convert the alcohol into jet fuel. | ASTM D7566 Annex A5; AFDC’s table lists 50%, while its accompanying text describes 30% for approved ethanol and isobutanol cases |
| Fischer–Tropsch (FT-SPK) | Woody biomass and other listed sources, including municipal solid waste, agricultural and forest wastes, and energy crops; synthesis gas | Gasification makes synthesis gas, which is converted to hydrocarbons by Fischer–Tropsch synthesis. | AFDC describes the pathway; its cited pathway details do not establish a blend limit here. |
| Catalytic hydrothermolysis | Clean free-fatty-acid oil and preheated water | The mixture passes through a catalytic hydrothermolysis reactor to produce a fuel intermediate. | ASTM D7566 Annex A6; 50% blend limit |
These pathway descriptions and limits come from the AFDC sustainable aviation fuel overview. Its ATJ table and explanatory text give different blend limits, so the figure should not be treated as a universal limit; consult the current ASTM annex and the approval for the specific pathway. ASTM approvals and limits are pathway-specific and can change.
HEFA: oils and fats to hydrocarbons
HEFA uses oils and fats rather than sugars. Hydroprocessing removes oxygen-containing components and breaks fatty-acid chains; subsequent hydroisomerization and hydrocracking help produce hydrocarbons suitable for aviation fuel. Because waste oils and animal fats can be used, HEFA illustrates how an existing feedstock stream can support a fuel pathway—but the available quantities are not unlimited.
Alcohol-to-jet: alcohol to jet-range hydrocarbons
ATJ starts with an alcohol such as ethanol or isobutanol. Dehydration removes water to form smaller molecules, oligomerization joins them into longer chains, and hydrogenation and hydrotreatment help produce the desired fuel. The alcohol itself may come from biological fermentation or, in some routes, gasification-derived intermediates. This is a good example of why calling a process simply “chemical” can hide biological steps earlier in the chain.
Gasification and Fischer–Tropsch: biomass through synthesis gas
Gasification converts woody biomass into synthesis gas, a mixture used as a building block for fuels. Fischer–Tropsch synthesis then converts that gas into hydrocarbons. The route can accommodate a range of biomass and waste feedstocks described by AFDC, but feedstock flexibility does not by itself establish commercial readiness or economic viability.
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Catalytic hydrothermolysis: oil processed with water
In the AFDC-described route, clean free-fatty-acid oil is combined with preheated water and passed through a catalytic reactor. It is distinct from HEFA in its process sequence and pathway approval, even though both draw on lipid feedstocks.
Other familiar examples: ethanol and biodiesel
Not every biofuel is a drop-in hydrocarbon. Ethanol is commonly made by fermenting carbohydrate-rich biomass; research has also examined using cellulose and hemicellulose, which require breaking down plant material to make fermentable sugars. Biodiesel is made by reacting an alcohol—usually methanol—with vegetable oil, animal fat or recycled cooking grease. These descriptions appear on NREL’s Biofuels Basics page; its indexed content supports this general context, not a detailed process specification.
Why a working reaction does not guarantee a scalable fuel
A pathway’s prospects depend on more than whether its chemistry works. Feedstock supply, conversion cost, sustainability, process energy and fuel certification all shape whether it can contribute at scale. The International Energy Agency (IEA) biofuels overview notes that waste oils and animal fats support current routes but are limited resources; broader deployment depends on expanding both technologies and sustainable feedstock sources.
- Feedstock availability: Competing uses and limited supplies can constrain routes built around waste oils, fats or other specific materials.
- Technology maturity: The IEA reports that established pathways coexist with less mature options. Biomass-to-Fischer–Tropsch is at demonstration stage, while upgrading routes involving hydrothermal liquefaction (HTL) or fast pyrolysis remain less mature.
- Cost and policy: The IEA identifies the cost gap and clear policy and regulatory frameworks as factors affecting biojet deployment.
- Environmental performance: A bio-based origin does not automatically make a fuel low-carbon or sustainable. Feedstock choice, land and water use, process energy and the lifecycle boundary used for assessment matter; DOE’s SAF strategy presentation explicitly flags appropriate land and water use.
- Certification: Aviation fuel must meet pathway-specific technical requirements and blend limits; a promising conversion route is not the same thing as an approved fuel.
What current figures and scenarios do—and do not—say
The IEA reports that biofuels supplied over 3.5% of global transport energy demand in 2022, while biojet kerosene accounted for less than 0.1% of aviation fuel demand that year. These figures describe observed shares, not the potential of any single process.
In the IEA’s Net Zero Emissions Scenario, biojet kerosene reaches around 10% of aviation fuel demand in 2030. In the same scenario, roughly 40% of biofuels consumed in 2030 come from wastes, residues and dedicated non-food-competing crops, compared with an estimated 9% in 2021. These are scenario values, not certain forecasts or observed outcomes.
In a DOE presentation transcript, Senior Analyst Zia Haq described HEFA as commercially viable and commercially available at the time of that presentation, while saying other listed pathways needed substantial research and development to reduce costs and improve sustainability. That statement is useful historical context, not a current status update; the IEA’s more recent pathway discussion also distinguishes established routes from those still at demonstration or less mature stages.
How to judge a biofuel pathway
When evaluating a claim about a new or established biofuel, ask what material enters the plant, what intermediate is produced, what transformations follow, and what evidence supports the final fuel’s certification and environmental performance. A pathway is meaningful only when its feedstock can be sourced sustainably and its finished fuel can meet the relevant standard at a viable cost.
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