Transport Blog

What is Biodiesel?

Published by Sam Page on 30 September 2011

Biodiesel is a carbon neutral fuel which can be used to replace convenional diesel. It is derived from a process known as transesterification whereby the oils produced by oliferous plants (typically in the UK we are talking about rapeseed or sunflower oil as the major sustainable sources) are combined under the correct conditions with a methoxide catalyst to cause separation of the oil into usable fuel oil and glycerol by-product. 

In layman's terms, transesterification can be thought of as the process of converting one ester into another ester. An ester is a chemical combination of fatty acids attached to alcohol. Animal and vegetable fats, oils and biodiesel are examples of esters. 

If both vegetable oil and biodiesel are esters, why is it not practical to use vegetable oil in a diesel engine instead of going through the process of creating biodiesel? In other words, why is there a need for transesterification? 

The answer lies in the difference in viscosity, that is the thickness or resistance to flow, between the two esters. Vegetable oil has too high a viscosity for diesel engines, designed for fossil diesel, to cope with. This is because the constituent alcohol molecule of the vegetable oil ester, glycerol, is very large. Hence we need to reduce the thickness of the vegetable oil by replacing the glycerol with an alcohol that is smaller in molecular size, methanol, and thus create a different ester.




This is what the process of transesterification allows us to do. By converting the vegetable oil ester into the biodiesel ester, it separates the larger glycerol molecules from the fatty acids within the vegetable oil. The methanol combines with the fatty acids producing smaller methyl esters thus creating the more free flowing biodiesel.

Given that transesterification is the process of converting one ester into another, it has to be noted that the process is reversible. This point is highlighted in the section on How to make Biodiesel.


Benefits of using biodiesel:

  • Better lubricity resulting in longer life for diesel engines
  • Sustainable production
  • Significantly kinder to the environment
  • Improved fuel economy - up 8%
  • Much lower fuel duty - around 20 pence per litre cheaper than fossil diesel

N.B. Biodiesel that is made from used vegetable oil releases land for food crops.