21-06-2012, 03:33 PM
The Status of Biodiesel as an Alternative Fuel for Diesel Engine
11. The status of biodiesel pp. 71-75.pdf (Size: 115.03 KB / Downloads: 173)
Introduction
The large increase in number of automobiles in recent years has resulted in great demand for petroleum products. With crude oil reserves estimated to last only for few decades, there has been an active search for alternate fuels. The depletion of crude oil would cause a major impact on the transportation sector. Of the various alternate fuels under consideration, biodiesel, derived from vegetable oils, is the most promising alternative fuel to conventional diesel fuel (derived from fossil fuels; hereafter just “diesel”) due to the following reasons [1].
•
Biodiesel can be used in existing engines without any modifications.
•
Biodiesel is made entirely from vegetable sources; it does not contain any sulfur, aromatic hydrocarbons, metals or crude oil residues.
•
Biodiesel is an oxygenated fuel; emissions of carbon monoxide and soot tend to be reduced compared to conventional diesel fuel.
•
Unlike fossil fuels, the use of biodiesel does not contribute to global warming as CO2 emitted is once again absorbed by the plants grown for vegetable oil/biodiesel production. Thus CO2 balance is maintained.
•
The Occupational Safety and Health Administration classify biodiesel as a non-flammable liquid.
•
The use of biodiesel can extend the life of diesel engines because it is more lubricating than petroleum diesel fuel.
•
Biodiesel is produced from renewable vegetable oils/animal fats and hence improves fuel or energy security and economy independence.
Transesterification
The fatty acid triglycerides themselves are esters of fatty acids and the chemical splitting up of the heavy molecules, giving rise to simpler esters, is known as Transesterification. The triglycerides are reacted with a suitable alcohol (Methyl, Ethyl, or others) in the presence of a catalyst under a controlled temperature for a given length of time. The final products are Alkyl esters and Glycerin. The Alkyl esters, having favorable properties as fuels for use in CI engines, are the main product and the Glycerin, is a by-product. The chemical reaction of the Tri-glyceride with Methyl alcohol is shown below. With higher alcohols the chemical equation would change correspondingly.
Properties of biodiesel
The fuel properties of raw vegetable oil as listed in Table 1 indicate that the kinematic viscosity of vegetable oil varies in the range of 30–40 cSt at 38°C. The high viscosity of these oils is because of their large molecular mass in the range of 600–900 kg/m3. This is about 20 times higher than that of diesel fuel. The flash point of vegetable oil is very high (above 200°C). The heating values are in the range of 39–40 MJ/kg compared to 45 MJ/kg for diesel fuel. Heating values of various vegetable oils are nearly 90% of diesel fuel. The presence of chemically bound oxygen in vegetable oil lowers their heating values by about 10%. The cetane numbers are in the range of 35–50 [6] and is similar or close to that of diesel fuel. Long chain saturated, unbranched hydrocarbons are especially suitable for conventional diesel fuel. The long, unbranched hydrocarbon chains in the fatty acids meet this requirement. The above unique properties of vegetable oils help us to replace the conventional diesel fuel. Wang et al. [7] reported that the major disadvantage of vegetable oils is their inherent high viscosity.
Fuel consumption
Brake-specific fuel consumption (BSFC) is the ratio between mass of fuel consumption and brake effective power, and for a given fuel, it is inversely proportional to thermal efficiency. If the latter is unchanged for a fixed engine operation mode, the specific fuel consumption when using a biodiesel fuel is expected to increase by around 14% in relation to the consumption with diesel fuel, corresponding to the increase in heating value in mass basis. In other words, the loss of heating value of biodiesel must be compensated for with higher fuel consumption.
Conclusion
The problems with substituting vegetable oil for diesel fuels are mostly associated with their high viscosities, and low volatilities. The viscosity of vegetable oils can be reduced by transesterification. Transesterification is the most common method and leads to mono alkyl esters of vegetable oils and fats, known as bio-diesel. The production of biodiesel from vegetable oil is very simple. In the production of biodiesel it is observed that the base catalyst performs better than acid catalysts and enzymes. The biodiesel and their blends have similar fuel properties as that of diesel. It is also observed that biodiesel has similar combustion characteristics as diesel. Biodiesel engines offer acceptable engine performance compared to conventional diesel fueled engines.