Wondering how is biodiesel made? Fats or oils are reacted with methanol and a catalyst in a process called transesterification, which yields biodiesel and glycerin, then the fuel is separated and washed.
| Item | Typical value |
|---|---|
| Oil or fat reacted | About 100 pounds |
| Methanol added | About 10 pounds |
| Biodiesel produced | About 100 pounds |
| Glycerin co-produced | About 10 pounds |
| Feedstock per gallon of biodiesel | About 7.5 pounds |
These figures come from the U.S. Department of Energy’s Alternative Fuels Data Center and a University of Illinois farmdoc daily analysis. They are approximations and can change, so check the sources before you rely on them.
The short version: fats in, fuel and glycerin out
Biodiesel belongs to the wider family covered in our guide on what are biofuels. It is made from renewable lipids such as vegetable oils, animal fats and recycled grease.
The core reaction is transesterification. According to the Alternative Fuels Data Center, it is “a process that converts fats and oils into biodiesel and glycerin (a coproduct).”
The farmdoc daily analysis adds that the product is often called FAME, short for fatty acid methyl ester, because methanol is typically the reactant that creates the methyl esters.
What goes into a biodiesel plant?
Three kinds of inputs matter: a lipid feedstock, an alcohol and a catalyst.
- Feedstock: vegetable oils, animal fats or recycled grease, all of which contain free fatty acids.
- Alcohol: usually methanol, a short-chain alcohol.
- Catalyst: usually sodium hydroxide or potassium hydroxide. Enzymes are used rarely in the U.S., according to farmdoc daily.
The mix is close to a simple recipe. The Alternative Fuels Data Center says about 100 pounds of oil or fat react with 10 pounds of methanol and a catalyst, forming 100 pounds of biodiesel and 10 pounds of glycerin.
Read the proportions as a rule of thumb rather than a guarantee. Real plants adjust the mix to the feedstock they receive, but the balance shows that the alcohol is a small input and that glycerin is a steady, predictable output.
That predictability is useful when you picture a plant. Almost all of the mass in the oil ends up as fuel, and a smaller share leaves as glycerin.
Step by step: how the process runs
Plants differ in detail, but the sequence is broadly the same. The farmdoc daily description gives the following outline.
- Test and preheat the feedstock. While the feedstock preheats, its quality is tested for free fatty acid content.
- Load the reactor. With the temperature kept steady, feedstock goes into the reactor with methanol and the base catalyst.
- React. The oil molecules split and recombine with methanol to form methyl esters and glycerin.
- Settle and separate. Glycerin and biodiesel separate, with glycerin settling to the bottom and biodiesel rising to the top because of the difference in their density. Once settling time passes, the glycerol is drained from the bottom of the reactor.
- Wash. The methyl esters move to another tank where they are washed with water many times until the water runs clear.
The result is the finished fuel, which is then stored before it is blended or sold.
Two points are worth holding onto. First, the reaction needs the right conditions: the farmdoc daily account stresses keeping the temperature steady while the reactor is loaded. Second, separation is physical rather than chemical, because gravity does the work once the two liquids have different densities.
Washing then removes what the reaction leaves behind. Because the wash is repeated until the water runs clear, it works as a simple visual check on how clean the fuel is.
Why feedstock quality matters
Plants test the feedstock for free fatty acid content before it enters the reactor.
Feedstock also drives volume and cost. The farmdoc daily analysis estimates that about 7.5 pounds of feedstock are needed to make one gallon of biodiesel, along with 0.9 pounds of glycerin.
The Alternative Fuels Data Center also notes that research continues on algae as a potential feedstock. It is expected to give high yields from a smaller area of land than vegetable oils.
What happens to the glycerin?
Glycerin is not waste. The Alternative Fuels Data Center describes it as a coproduct commonly used in the manufacture of pharmaceuticals and cosmetics.
Farmdoc daily notes that crude and refined glycerol have several uses. Crude glycerin typically has a glycerol content of around 80 percent.
Because the mass balance above yields glycerin alongside the fuel, handling it is a routine part of plant operation rather than an afterthought.
How the finished fuel behaves
The chemistry that makes biodiesel also gives it distinct properties. Farmdoc daily lists four differences from petroleum diesel and renewable diesel.
- Energy content: FAME contains oxygen, giving it approximately seven percent less energy by volume.
- Storage life: the higher oxygen content can limit storage time because of oxidation that causes corrosion.
- Microbial fouling: poor storage tank management can let microbes corrode tanks and clog fuel lines.
- Cold behaviour: FAME biodiesel begins to freeze and form visible crystals at a relatively high temperature.
These traits explain why storage practice matters after the fuel leaves the plant.
Understanding the production route helps you read the rest of the biofuel picture. The reaction is simple on paper, but feedstock choice, reactor control and washing all shape the quality of what comes out.
When you compare any fuel, ask what the feedstock is, what reaction converts it, and what coproducts and limits come with the result. For biodiesel the answers are fats and oils, transesterification, glycerin, and the storage and cold-weather limits above.
Biodiesel versus raw oil and renewable diesel
Pouring raw oil in a tank is not the same as using biodiesel. The Alternative Fuels Data Center says fats and oils are much more viscous than biodiesel. It adds that low-level vegetable oil blends can cause long-term engine deposits, ring sticking and lube-oil gelling.
Biodiesel also differs from the fuel covered in our explainer on what is renewable diesel. Farmdoc daily says renewable diesel contains only hydrogen and carbon, so it is a hydrocarbon fuel like petroleum diesel. It is made by hydrotreating, in which a catalyst triggers reactions of hydrogen with the feedstock at high temperature and pressure.
Biodiesel is also a different product from ethanol, which is made from sugars and starches. Our page on what is bioethanol covers that route.
On the retail side, the Alternative Fuels Data Center says most distributors deliver blends of either B20 or B100, depending on the retailer’s preference.
Frequently asked questions
What is transesterification?
Show answer
It is the reaction that converts fats and oils into biodiesel and glycerin, using an alcohol and a catalyst.
What is biodiesel made from?
Show answer
Vegetable oils, animal fats and recycled grease are the usual feedstocks, according to farmdoc daily.
Why is biodiesel washed with water?
Show answer
The methyl esters are washed in a separate tank many times until the water runs clear, which cleans the fuel after the reaction and separation.
Does biodiesel have the same energy as petroleum diesel?
Show answer
Not quite. Farmdoc daily says FAME contains oxygen, which gives it approximately seven percent less energy by volume.
Is glycerin a by-product to throw away?
Show answer
No. It is a coproduct used in pharmaceuticals and cosmetics, among other uses.
