What Is Biodiesel?

Biodiesel is a liquid biofuel composed of mono-alkyl esters of long-chain fatty acids, produced by reacting vegetable oil, animal fat, or waste cooking oil with a short-chain alcohol. It is chemically distinct from the raw oils it comes from and from the petroleum diesel it substitutes for, and that distinction is the point: the esterification step lowers viscosity by roughly an order of magnitude, which is what makes the fuel usable in an unmodified compression-ignition engine. In commercial practice the alcohol is almost always methanol, so the product is a fatty acid methyl ester, commonly abbreviated FAME.

Biodiesel should be separated from renewable diesel, which is made from similar feedstocks but by hydrotreating rather than esterification. Hydrotreating strips the oxygen out entirely and yields a hydrocarbon that is chemically indistinguishable from petroleum diesel, whereas biodiesel retains its ester oxygen and therefore has different cold-flow, solvency, and storage behavior. The US Department of Energy maintains a technical comparison of renewable diesel and its relationship to biodiesel that sets out where the two fuels can and cannot be used interchangeably.

Production Chemistry

The dominant route is base-catalyzed transesterification. Triglyceride, a glycerol backbone carrying three fatty acid chains, reacts with methanol in the presence of sodium or potassium methoxide, exchanging the glycerol for three methyl groups and releasing glycerol as a co-product. The reaction is equilibrium-limited, so methanol is supplied in excess, typically six moles per mole of triglyceride against a stoichiometric requirement of three, and runs at around 60 degrees Celsius near atmospheric pressure. Glycerol separates by gravity and is drawn off, and the ester phase is washed to remove residual catalyst, soap, and alcohol before drying. Free fatty acids in low-cost feedstocks such as trap grease consume the base catalyst and form soap, so those streams are pre-treated by acid-catalyzed esterification. About 100 kilograms of glycerol accompanies every metric ton of biodiesel, and finding markets for that stream has been a persistent factor in plant economics.

Fuel Properties and Specifications

Biodiesel carries roughly 8 to 9 percent less energy per gallon than petroleum diesel, but its higher cetane number and its intrinsic lubricity partially offset that in engine performance, and the lubricity benefit is significant enough that low blends are used to restore film strength lost when sulfur was removed from petroleum diesel. Combustion produces less particulate matter, carbon monoxide, and unburned hydrocarbon, with a modest increase in oxides of nitrogen for many engine and calibration combinations. The governing specification, ASTM D6751 for B100 as a blendstock, controls cetane number, flash point, water and sediment, sulfated ash, total and free glycerin, oxidation stability, and cold soak filtration; the ASTM biodiesel specification tables published by the Alternative Fuels Data Center list the limits alongside the parallel standards for finished blends. Two properties dominate field problems: cold flow, since saturated esters from animal fat and palm oil begin to gel at higher temperatures than soy-derived esters, and oxidation stability, since unsaturated chains form peroxides and sediment during storage.

Blending and Supply

Biodiesel is normally sold as a blend designated by its volume fraction, so B5 is 5 percent biodiesel and B20 is 20 percent. Blends up to B5 fall within the ordinary diesel specification and require no special handling, B20 is approved by many engine manufacturers subject to fuel quality conditions, and B100 sees limited use in controlled fleets. The fuel is a mild solvent, so it loosens deposits in tanks and lines that were laid down by petroleum diesel, and filters are typically changed early after conversion. US biodiesel production capacity stood at about 2.1 billion gallons a year in January 2024 and had been essentially flat, while capacity for renewable diesel expanded sharply over the same period, with soybean oil, corn oil, canola, used cooking oil, and rendered animal fats as the principal feedstocks.

Applications

Biodiesel has applications in a range of sectors, including:

• Heavy-duty road freight and long-haul trucking fleets
• Transit buses and municipal vehicle fleets
• Agricultural and construction equipment
• Rail locomotives and marine vessels
• Stationary generators and backup power units
• Home and commercial heating oil blends, sold as bioheat