Biofuels are not made one way. Different fuels come from different feedstocks by different chemistry, and the four main routes are worth understanding because they explain almost everything else — why some fuels are cheap and others costly, why some drop straight into the pipeline and others do not.
Fermentation & distillation — ethanol
The oldest route. Yeast ferments sugar into alcohol and carbon dioxide; the dilute alcohol is then distilled and dehydrated to anhydrous fuel ethanol. Sugar crops like sugarcane ferment directly; starch crops like corn first need milling and enzymes to convert starch into fermentable sugar. The leftover solids become animal feed (distillers’ grains), and at sugarcane mills the fibre (bagasse) is burned for energy. See ethanol.
Transesterification — biodiesel
Oils and fats are reacted with an alcohol (methanol) and a catalyst, converting triglycerides into fatty-acid methyl esters (FAME) — biodiesel — with glycerin as a by-product. It is a relatively simple, established process, but because FAME differs chemically from petroleum diesel it is used in blends. See biodiesel.
Hydrotreating — renewable diesel & SAF
The same fats and oils can instead be reacted with hydrogen at high temperature and pressure. Hydrotreating strips out oxygen and reshapes the molecules into hydrocarbons chemically near-identical to petroleum diesel or jet fuel — yielding renewable diesel and sustainable aviation fuel that drop into existing fuel without blend limits.
Thermochemical conversion — advanced fuels
For tough, fibrous biomass that cannot simply be fermented, heat does the work. Gasification turns biomass into a synthesis gas that can be built up into liquid fuels; pyrolysis heats biomass without oxygen into a bio-oil that is then upgraded. The parallel biochemical route uses enzymes to break plant fibre into sugars for fermentation. These are the routes behind advanced and cellulosic biofuels, and the reason they are harder and costlier than first-generation production.
Ethanol production, step by step
The corn dry-mill route, the most common in the United States, shows the full sequence. Grain is milled to a flour and slurried with water; enzymes then break its starch into fermentable sugars. Yeast ferments those sugars over a couple of days into a “beer” of roughly 10–15% alcohol, releasing carbon dioxide that can be captured. Distillation concentrates the alcohol, and a final dehydration step (typically molecular sieves) removes the last water to yield anhydrous fuel ethanol. The solids left behind are dried into distillers’ grains for animal feed, and corn oil is separated for use as a biodiesel feedstock. Sugarcane skips the enzyme step entirely, fermenting the cane’s sucrose directly — which is why it is simpler and less energy-intensive.
Refining, finishing and quality
Whichever route is used, the crude product is not yet a saleable fuel. Biodiesel is washed to remove residual catalyst, soap and glycerin, then dried; renewable diesel and SAF are fractionated and finished like petroleum streams. Every finished fuel must then meet a published specification — ASTM or EN standards governing properties such as water content, cetane or octane, cold-flow behaviour and oxidation stability — before it can enter the distribution system. This finishing-and-testing stage is what separates fuel-grade product from a laboratory reaction, and it is a significant part of a plant’s cost and complexity.
Choosing a route
Which route a producer chooses depends on the feedstock and the target fuel. Sugar and starch favour fermentation; oils and fats favour transesterification or hydrotreating; woody and fibrous biomass needs the thermochemical or enzymatic routes. The economics, the carbon intensity and the finished fuel’s compatibility with existing engines all follow from that first choice of process and feedstock. As a rule, the simpler and more abundant the sugar or oil, the cheaper the fuel — which is why first-generation routes dominate today and why the advanced routes that unlock tougher biomass remain the harder, costlier frontier.