What is biodiesel?
Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. These products are also known as fatty-acid methyl esters.
In this representation, is the long hydrocarbon part of the fatty-acid residue.
Relationship to triglycerides
Vegetable oils contain triglycerides: esters formed from glycerol and long-chain carboxylic acids. The long-chain acid residues in these triglycerides are retained when biodiesel is produced.
Vegetable oil
Contains triglyceride molecules in which three fatty-acid residues are joined to a glycerol-derived backbone by ester links.
Biodiesel
Contains the long-chain acid residues as separate methyl esters rather than as parts of a triglyceride.
Vegetable oils are renewable starting materials. Converting their triglycerides into methyl esters produces a less viscous liquid that is more suitable for use as a fuel.
| Material | Key structural feature | Role in biodiesel production |
|---|---|---|
| Vegetable oil | Triglycerides containing three ester links | Supplies the long-chain fatty-acid residues |
| Biodiesel | Methyl esters, | Forms the fuel mixture |
Carbon-neutral claim
Biodiesel made from plant oils may be described as approximately carbon neutral because the carbon dioxide released when the fuel burns can be balanced against carbon dioxide absorbed from the atmosphere as the plants grew by photosynthesis.
This description does not account for every source of emissions. Energy may be required to grow and irrigate crops, extract the vegetable oil, heat the reaction mixture and process the finished fuel. If that energy comes from fossil fuels, additional carbon dioxide is released.
Growing crops for fuel can also use land that might otherwise be available for food production, so the wider environmental and ethical effects need to be considered.