Biodiesel is the renewable fuel derived from vegetable oils or animal fats. It is a diesel substitute made from biological feedstocks, including soybean or rapeseed oil, used cooking oil, tallow, lard, and other fats.
Manufacturers usually produce biodiesel through transesterification. In this chemical process, an oil or fat reacts with a short-chain alcohol—most commonly methanol—in the presence of a catalyst. The reaction creates fatty-acid esters, which form the fuel, and glycerol as a coproduct. This treatment lowers the raw oil’s viscosity so it can work more reliably in diesel fuel systems.
Biodiesel is different from straight vegetable oil, which can be too viscous and may cause deposits or injector problems. It is also different from petrodiesel, which is refined from petroleum. Fuel blends are labeled with a “B” number: B20 contains 20 percent biodiesel, while B100 is pure biodiesel. Its environmental benefits depend on the feedstock, farming practices, land use, and production method.