The possibility of using vegetable oils as fuel has been recognized since the advent of diesel engines. Speaking to the Engineering Society of St. Louis, Missouri, in 1912, Rudolph Diesel, said, “The use of vegetable oils for engine fuels may seem insignificant today, but such oils may become in course of time as important as petroleum and the coal tar products of the present times.”
Vegetable oils have higher viscosity than commercial diesel fuel, which means they are thicker and flow less easily. The high viscosity of raw vegetable oil reduces fuel atomization and increases penetration. These features contribute to the formation of engine deposits, piston ring sticking, injector coking, and thickening of oil.
To reduce the viscosity of vegetable oils, methods have been developed such as dilution (blending), micro-emulsification, pyrolysis (thermal cracking), and transesterification.
1. Dilution (Blending)
Crude vegetable oils can be blended directly or diluted with the diesel fuel to improve viscosity. Dilution reduces the viscosity, engine performance problems such as injector coking, and more carbon deposits. However, dilution is not suitable for long term use in a direct injection engine.
Pure biodiesel or 100% biodiesel is referred to as B100. A biodiesel blend is pure biodiesel blended with petrodiesel. Biodiesel blends are referred to as BXX. The XX indicates the amount of biodiesel in the blend (i.e., a B90 blend is 90% biodiesel and 10% petrodiesel).
Microemulsion is another approach to reduce the viscosity of vegetable oils. Let us discuss how it happens. Emulsion is a dispersion of oil in water or water in oil and this dispersion is also known as macroemulsion. On the other hand, microemulsion is a thermodynamically stable dispersion of oil in water and water in oil. Both used surfactant as the principal agent to enable water and oil to mix. Let us further understand the meaning of thermodynamically stable with the following example:
When a spoonful of salt is added to a cup of water, one can observe that within a very short span of time the salt will dissolve inside the water, becoming the product of salt + water. As there was no need of input energy in any form, means that this reaction was thermodynamically stable, and spontaneous. The two reactants (in this case, salt and water) prefer to react and maintain stability as products.
Microemulsion is not used for synthesis of biodiesel. It is just a technique to reduce the viscosity of oil.
3. Pyrolysis (Thermal Cracking)
Pyrolysis is a method of conversion of one substance into another through heating or heating with the aid of the catalyst in the absence of air or oxygen. It involves heating in the absence of air or oxygen and cleavage of chemical bonds to yield small molecules. The material used for pyrolysis can be vegetable oils, animal fats, natural fatty acids and methyl esters of fatty acids. The liquid fuel produced from this process has almost identical chemical components to conventional diesel fuel.
Transesterification, also called alcoholysis, is a chemical reaction of an oil or fat with an alcohol in the presence of a catalyst to form esters and glycerol. It involves a sequence of three consecutive reversible reactions where triglycerides (TG) are converted to diglycerides (DG) and then DG are converted to monoglycerides (MG) followed by the conversion of MG to glycerol. In each step an ester is produced and thus three ester molecules are produced from one molecule of TG.
Among the alcohols that can be used in the transesterification reaction are methanol, ethanol, propanol, butanol, and amyl alcohol. Methanol and ethanol are used most frequently. However methanol is preferred because of its low cost. A catalyst is usually used to improve the reaction rate and yield. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side. It also gives glycerol as a byproduct which has a commercial value.
Transesterification is the most viable process adopted known so far for the lowering of viscosity and for the production of biodiesel. Thus biodiesel is the alkyl ester of fatty acids, made by the transesterification of oils or fats, from plants or animals, with short chain alcohols such as methanol and ethanol in the presence of catalyst and glycerin is consequently a by-product from biodiesel production.
5. Use of Catalysts in Transesterification
The catalysts used in the transesterification are broadly divided into two types: a) Base catalyst and b) Acid catalyst
The base catalysts are preferred over acid catalysts, due to their capability of completion of reaction at higher speed, requirement of lower reaction temperature, and their higher conversion efficiency as compared to acid catalysts.
6. Advantages of Biodiesel
Biodiesel is renewable, energy efficient, and can be used in most diesel engines with no or only minor modifications. It is made from either agricultural or recycled resources. Biodiesel is environmental friendly because of lower HC emissions, smoke and soot reductions, lower CO emissions, reduction of greenhouse gases. Biodiesel contains no sulphur. Biodiesel does not produce greenhouse effects, because the balance between the amount of CO2 emissions and the amount of CO2 absorbed by the plants producing vegetable oil is equal.
Biodiesel is biodegradable and renewable and can help reduce dependency on foreign oil. It helps to lubricate the engine itself, decreasing engine wear. Biodiesel can be used directly in compression ignition engines with no substantial modifications of the engine. B20 can be used without engine modifications.
P. B. Weisz, W. O. Haag and P. G. Rodeweld, Science, 1979, 206(4414), 57.