Biodiesel consists of fatty acid methyl esters (FAME) created as the result of a reaction between an alcohol and oils/fats of vegetable or animal origin. Methanol (wood alcohol) is usually the alcohol of choice, but ethanol may also be used. The conversion process releases glycerine, a by-product of the biodiesel production process.
Biodiesel may be used both as a carbon neutral fuel in diesel engines, and as a biofuel oil. The European diesel standard, EN 590, generally permits adding up to 5% biodiesel, while a number of passenger cars, lorries and buses may run on 100% biodiesel with a few minor adjustments.
In principle, the structure of all oils/fats is the same, regardless of whether they are of vegetable or animal origin. They consist largely of triglycerides, and only the proportion of individual fatty acids differs. It is especially the content of unsaturated fatty acids in relation to the saturated fatty acids that is important for the product. As the following table shows, palm oil and animal fat contain almost the same level of unsaturated fatty acids, while the level is far higher in rapeseed oil and soy bean oil.
| || ||Animal fat||Rapeseed oil||Palmoil|| |
| ||Palmitic acid (16:0)||24||4||42|| |
| ||Palmitoleic acid (16:1)||4||0||0|| |
| ||Stearic acid (18:0)||16||2||4|| |
| ||Oleic acid (18:1)||43||60||43|| |
| ||Linoleic acid (18:2)||11||20||10|| |
| ||Lenolenic acid (18:3)||1||10||0|| |
| ||Other||1||4||1|| |
A high level of unsaturated fatty acids lowers the congealing point, not just for the oil/fat, but also for the resulting FAME. This is particularly significant in connection with the use of pure biodiesel, while it is less important for admixtures.