Biodiesel and bioethanol

Biodiesel and bioethanol are liquid biofuels made from biological material rather than from crude oil. Because the plants used to make them absorb carbon dioxide as they grow, these fuels are described as renewable, and burning them returns carbon that was recently taken from the atmosphere. They are most often used as partial or complete substitutes for conventional diesel and petrol in road transport.

Biodiesel is produced from vegetable oils or animal fats through a chemical reaction called transesterification, in which the oil is reacted with an alcohol, usually methanol, in the presence of a catalyst. This converts the oil into fatty-acid alkyl esters, which are the biodiesel itself, and produces glycerol as a by-product that can be purified and used elsewhere. The finished fuel can power ordinary diesel engines, often blended with petroleum diesel; the proportion is commonly written as B20 or B100, where the number gives the percentage of biodiesel.

Bioethanol is an alcohol made by fermenting sugars from crops such as sugar cane, maize or sugar beet, in the same way that fermentation produces drinking alcohol, followed by distillation. It is blended with petrol in proportions such as E10 or E85. Biofuels can reduce dependence on imported oil and lower some emissions, but they also raise concerns about contamination, fuel quality, and competition between fuel crops and food production for farmland. Their overall environmental benefit depends heavily on how the feedstock is grown and processed.

Frequently asked questions

How is biodiesel made?

Vegetable oil or animal fat is reacted with an alcohol such as methanol and a catalyst, a process called transesterification, which yields biodiesel esters and glycerol as a by-product.

What is the difference between biodiesel and bioethanol?

Biodiesel is made from oils and fats and replaces diesel; bioethanol is an alcohol made by fermenting plant sugars and is blended with petrol.

Why is glycerol mentioned with biodiesel?

Glycerol is the main by-product of the transesterification reaction. It is separated from the fuel and can be refined for use in other products.