What is biodiesel?

Biodiesel is a type of diesel. It is made from animal fat or vegetable oil. It is also possible to convert vegetable oils and animal fats into synthetic diesel, but that's another story.

Imagine a comb with three teeth. Vegetable oil molecules and animal fat molecules are shaped like that. The teeth are the biodiesel. The backbone is glycerine. To make vegetable oil or animal fat into diesel, we have to break off the teeth and then bandage the broken ends. We do this by mixing the oil or fat with an alcohol and a catalyst, and letting the brew undergo a chemical reaction which converts the oil or fat into fatty-acid alkyl esters (biodiesel) and glycerine. Each oil or fat molecule yields three biodiesel molecules and one glycerine molecule.

There's nothing exotic about the ingredients. The alcohol can be methanol (wood alcohol) or ethanol (grain alcohol, gin, vodka, etc.). The catalyst can be lye (caustic soda, also known as sodium hydroxide) or caustic potash (potassium hydroxide).

If we use methanol, we'll get fatty-acid methyl esters (FAME): If we use ethanol, we'll get fatty-acid ethyl esters (FAEE): Either way, we'll have long chain molecules that look almost like hydrocarbons, except that there's a methyl or ethyl group where the chain (a tooth) was connected to the glycerine backbone.

Using ethanol instead of methanol adds a couple of complications: the catalyst must be caustic potash because caustic soda won't dissolve properly in ethanol, and the ethanol must be totally free of water, or anhydrous, which is tough to achieve.

Making biodiesel from waste veggie oils is also more complex, because they contain free fatty acids - the heating process breaks up the comb-shaped molecule, but not the way we want it. This means the biodiesel brewer must add an extra, carefully calculated amount of catalyst to neutralise those free fatty acids so they don't wear out the engine's fuel injection system prematurely.

Warning

Some biodiesel ingredients are toxic and flammable. Make sure you know the appropriate safety precautions before you try to make your own. Journey to Forever's "Make your own biodiesel" page gives an introduction to the basic safety issues. I will not be held responsible for any problems you might have - you have been warned.

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