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What is MWVF?What is BiodieselThe differences between MWVF and RMETechnical summary of RME
Basic Chemistry of Organic FuelsGlossary of technical terms

The Basic Chemistry of Organic fuels.

Plants and animals absorb energy from the sun and store it in the form of fats, sugars or starches. It is for this reason that nearly all organic materials from the timber in trees to the carcasses of animals can be burned, and in burning they give off energy. Plants create special reserves of hydrocarbon in seeds in the form of oils, fats and starches, which provide a concentrated store of energy which is needed at the point of germination, when many very complicated chemical reactions are taking place. Wheat and grains generally store energy as starches, whereas rape and palm store energy as oils. Vegetable oils can be extracted from the mature seeds by compression, or by solvents, and some seeds provide prolific amounts of high energy oils, in particular sunflowers, rape, and palm. It is this source of oil that was first used in the development of compression ignition engines by Rudolph Diesel in 1895. His first engines were designed to run on a wide variety of vegetable oils including peanut oil. However, this was before more abundant supplies of mineral (fossil) fuels became available with the growth of the petrochemical industry, in which fossil hydrocarbons are extracted from within the earth’s crust, which were formed as a result of formerly living material.

It is now realized how much damage is being done to the environment by the burning of fossil fuels, and this is very much blamed for the effect of climatic perturbations, due to the increase in the levels of carbon in the atmosphere. All the time fossil sources of hydrocarbon are burned, carbon otherwise locked below the earth’s crust is expelled into the atmosphere through exhaust gasses. On a global basis, this amounts to millions of tons of carbon per year. To reverse this trend there is now an urgent need to reduce the burning of mineral hydrocarbons, and instead use renewable, lesser polluting hydrocarbon sources. For that reason, there is a need to reconsider the use of organic hydrocarbons, and also the processes that can be used to facilitate their economic combustion. A European life-cycle analysis showed that for every kilogram of biodiesel burned three kilograms of atmospheric carbon dioxide are consumed. This suggests that changing to bio-fuels could actually reduce levels of carbon in the atmosphere as well as preventing the increase of carbon levels.

Organic fats and oils exist in the form of triglycerides, consisting of three hydrocarbon chains connected together by glycerol. The bonding point is called an ester bond. From these many compounds can be derived various forms of fuel, often as alcohol, and carboxylic (fatty) acid methyl esters.

typical organic triglyceride with three hydrocarbon chains

Vegetable oil can be converted for use in a diesel engine by the process of transesterification. The three ester bonds within a triglyceride are first hydrolysed to form Free Fatty Acids, which are reacted with Methanol or Ethanol to from Methyl or Ethyl fatty acid esters,. This provides a thinner (less viscous) and more volatile fuel, and a by-product of glycerin. The process of transesterification separates the three bio-organic esters from the bond leaving each one as a free aliphatic hydrocarbon chain with a carboxylic acid group at one end. This feature allows living organisms to metabolize the otherwise un-reactive saturated chain, i.e. it makes the fuel bio-degradable. Hydrocarbon chains derived from inorganic materials (i.e. mineral or fossil sources) do not have this carboxylic acid end structure, and therefore they are not readily bio-degradable. For this reason, bio-diesel is a much safer and less toxic fuel than mineral diesel.

typical carboxylic acid group
terminating an organic hydrocarbon chain,
to form a fatty acid

Hydrocarbon chains comprise a series of carbon atoms linked together, each of which has two hydrogen atoms attached. These molecules may be of different chain lengths, and may also have double bonds in some places. They are generally stable but release considerable amounts of energy when burnt in the presence of oxygen. Combustion is activated by a small amount of energy but can release a great deal of energy in the right situation. In the complete combustion process, each pair of hydrogen atoms connects to one of oxygen making H₂O (water) and the carbon becomes CO₂ . A perfect reaction as in the case of burning bio-diesel would be as follows :-

Hydrocarbon chains derived from fossil fuels typified by the petrochemical industry do not have the carboxylic acid ester connection, and exist in many elaborate forms. For example petrol may be a mixture of the following molecules :-

a) b) and c) are alifatic compounds
d) an aromatic compound

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