Biodiesel

Green Chemistry Jemma Vickery jv2237@bris.ac.uk

Introduction

One of key aims of Green Chemistry involves the development of chemical processes, which use renewable energy resources. Diesel is a common fuel used to power large vehicles and heavy machinery, however its derivation from crude non renewable energy resources leaves an irreversible impact on the environment. Biodiesel provides a key alternative to diesel using renewable crude products in place of non renewable fossil fuels and is just as efficient as diesel in powering diesel engines.

Picture of a Blue Sun Biodiesel pump.

Biodiesel consists of methyl esters of fatty acids and can be derived from farm crops including soybeans, cottonseed, sunflower seeds and rape seeds.

The concept of using vegetable oils to develop engine fuels in place of diesel dates back to 1895 when Rudolf Diesel developed the first engine to run on peanut oil, unfortunately Diesel died before his idea was fully recognised. However fuel shortages in the 1970s sparked further interest in diversifying fuel resources and biodiesel was developed as an alternative to diesel.

Picture of Rudolf Diesel, see reference (5).

Biodiesel is easily synthesised by chemically changing particular oils and fats in one of three different routes, with the majority of biodiesel produced via the first route:

1. Base catalysed transesterification of the oil with alcohol

2. Direct acid catalysed esterification of the oil with methanol

3. Conversion of the oil to fatty acids, and then to alkyl esters with acid catalysis

The general synthetic procedure involves reacting an alcohol with useless fats and oils in the presence of a catalyst, usually sodium or potassium hydroxide. The glycerine is allowed to settle and the supernatant is biodiesel which contains a mixture of methylated fatty acids and methanol. The catalyst remains dissolved in the glycerol fraction and the esters are purified. The alcohol used is in excess, promoting an efficient rate of reaction and can be recovered at the end of the procedure and reused.

Biodiesel has numerous advantages over diesel. It is non-flammable, non- explosive and biodegradable. It creates fewer sulphur, carbon monoxide and carbon dioxide emissions, which results in less acid rain and a reduced impact on global warming. Biodiesel has a higher centane rating than petrodiesel, which means combustion of biodiesel is more efficient. The only real downside to biodiesels is that they produce more NOx emissions but these can be reduced through the use of catalytic converters.

See references (4,5)