Production and analysis of bio-diesel from non-edible oils—A review
Introduction
Majority of the world's energy needs are supplied through petrochemical sources, coal and natural gases, with the exception of hydroelectricity and nuclear energy, all these sources are finite and at current usage rates will be consumed shortly [1]. Diesel fuel have an essential function in the industrial economy of a developing country and are used in transports, industrial and agricultural goods, etc. Compared to the rest of the world, India's demand for diesel fuels is six times that of gasoline. The consumption of diesel fuels in India in 1994–1995 was 28.30 million tonnes of diesel and 40.34 million tonnes of diesel in 2000–2001, which was 43.2% of the total consumption of petroleum products [2]. Economic growth is always accompanied by commensurate increase in the transport. This has stimulated the recent interest in alternative sources for petroleum-based fuels. An alternative fuel should be easily available, environment friendly and techno-economically competitive. One of such fuels is triglycerides and their derivatives. Vegetable oils, being renewable, are widely available from a variety of sources and have low sulphur contents close to zero and hence cause less environmental damage (lower green house effect) than diesel. Chemically the oils consist of triglyceride molecules of three long chain fatty acids that are ester bonded to a single glycerol molecule. These fatty acids differ by the length of carbon chain, orientation and position of double bonds in these chains. Thus bio-diesel refers to lower alkyl esters of long chain fatty acids, which are synthesized either by transesterification with lower alcohols or by esterification of fatty acids.
The use of vegetable oils, such as soya-bean, palm, sunflower, peanut and olive oil as alternative fuel has been around for 100 years when the inventor of the diesel engine Rudolph Diesel first tested peanut oil, in his compression ignition engine [3], [4]. Different countries are looking for different types of vegetable oils as substitutes for diesel fuels that depending upon the climate and soil conditions. For example, soya-bean oil in U.S., Rapeseed and Sunflower oils in Europe, palm oil in Southeast Asia and Coconut oil in Philippines are being used. The use of vegetable oils as sources of diesel would require more efforts to increase the production of oil seed and to develop new and more productive plant species with high yield of oil. Besides, some species of plants yielding non-edible oils. The planning commission of India has launched a bio-fuel project in 200 districts of the 18 states in India. It has recommended two plant species viz. Jatropha (Jatropha curcas) and Karanj (Pungamia pinnata) for bio-diesel production [5], [6], [7]. Both these plants may be grown on a massive scale on agricultural/degraded/waste lands, so that the chief resource may be available to produce bio-diesel on ‘farm scale’.
Vegetable oils occupy a prominent position in the development of alternative fuels although, there have been many problems associated with using it directly in diesel engine. These includes:
- 1.
High viscosity of vegetable oil interferes with the injection process and leads to poor fuel atomization.
- 2.
The inefficient mixing of oil with air contributes to incomplete combustion, leading to high smoke emission.
- 3.
The high flash point attributes to lower volatility characteristics.
- 4.
Lube oil dilution.
- 5.
High carbon deposits.
- 6.
Ring sticking.
- 7.
Scuffing of the engine liner.
- 8.
Injection nozzle failure.
- 9.
Types and grade of oil and local climatic conditions.
- 10.
Both cloud and pour points are significantly higher than that of diesel fuel. These high values may cause problems during cold weather.
These problems are associated with large triglycerides molecule and its higher molecular mass, which is avoided by chemically modified to vegetable oil in to bio-diesel that is similar in characteristics of diesel fuel [8], [9], [10].
The plant oils usually contain free fatty acids, phospholipids, sterols, water, odorants and other impurities. Because of these the oil cannot be used as fuel directly. To overcome these problem the oil requires slight chemical modification mainly transesterification, pyrolysis and emulsification. Among these, the transesterfication is an important process to produce the cleaner and environmentally safe fuel from vegetable oils.
Bio-diesel is defined as the mono alkyl esters of long chain fatty acids derived from renewable feed stock, such as vegetable oil or animal fats, for use in compression ignition engines [4]. Bio-diesel which is considered as a possible substitute of conventional diesel fuel is commonly composed of fatty acid methyl/ethyl esters, obtained from triglycerides by transesterification with methanol/ethanol respectively. Bio-diesel is compatible with convention diesel and both can be blended in any proportion.
Section snippets
Transesterification reaction
Transesterification [11] also called alcoholysis is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis except that an alcohol is used instead of water [1]. This has been widely used to reduce the viscosity of the triglycerides. The transesterification is represented as:
If methanol is used in this process then it is called methanolysis. Methanolysis of triglycerides represented in Eq. (2).
Transesterification is one of the reversible reactions and
Effect of free fatty acids and moisture
The free fatty acid and moisture content are key parameters for determining the viability of the vegetable oil transesterification process. To carry the base catalyzed reaction to complete; the three free fatty acid (FFA) value lower than 3% is needed. The higher the acidity of the oil, smaller is the conversion efficiency. Both, excess as well as insufficient amount of catalyst may cause soap formation [14]. The starting materials used for base catalyzed alcoholysis should meet certain
Quantification of methyl esters
Various analytical methods were developed for analyzing mixtures containing fatty acids esters and mono-, di-, and tri-glycerides obtained by the transesterification of vegetable oils. Analysis were performed by a thin layer chromatography/flame ionization detector (TLC/FID). Steryl alcohol used as an internal standard. From plots of areas and weight ratios of methyl linoleate and mono-, di-, and tri-linolein, linear equations were developed from which response factors were calculated.
Fuel properties and specification of bio-diesel
Since bio-diesel is produced in quite differently scaled plants from vegetables oils of varying origin and quality, it was necessary to install a standardization of fuel quality to guarantee engine performance without any difficulties. The parameters, which define the quality of bio-diesel, can be divided into two groups. One group contains general parameters, which are also used for mineral oil based fuel and the other group especially describes the chemical composition and purity of fatty
Performance of bio-diesel in diesel engine
Conventional Internal Combustion Engines can be operated with bio-diesel without major modification [36]. In comparison to diesel, the higher cetane number of bio-diesel results in shorter ignition delay and longer combustion duration and hence results in low particulate emissions and minimum carbon deposits on injector nozzles. It is reported that if an engine is operated on bio-diesel for a long time, the injection timing may be required to be readjusted for achieving better thermal
Teardown analysis
Fraer et al. [40] reported that 1996, Mack MR 688p model vehicle having six cylinders its compression ratio of 16.5:1 and producing the power 300 hp at 1950 rpm used in postal purposes. The engine and fuel system components were disassembled, inspected and evaluated to compare wear characteristics after 4 years of operation and more than 6,00,000 miles accumulation on B 20 no difference in wear or other issues were noted during the engine teardown. The cylinder heads of B20 engines contained a
Recommendation for development of bio fuels [41]
- •
As the stock of fossil fuel is getting depleted, emphasis should be given to renewable sources of fuel such as Bio-Fuel crops and tree brone oilseeds.
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DST funding is required for land resources management water resources, mineral, fossil fuels.
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Design, develop and popularize appliances and equipments specifically for rural application.
- •
Prima facie, bio-diesel seems to have significant potential to contribute to Indian's energy security, the need of the hour is to under take R&D on sustainable
Environmental considerations
In view of environmental considerations, bio-diesel is considered Carbon neutral because all the CO2 released during consumption had been sequestered from the atmosphere for the growth of vegetable oil crops. The combustion of bio-diesel has reported to emit lesser pollutants compared to diesel [39]. This indicates that the engine exhaust contains no SO2, and shows decreasing emissions of PAH, CO, HC, soot and aromatics. The NOx emission is reported to be in the range between ±10% as compared
Economic feasibility of bio-diesel
India has rich and abundant resources of edible and non-edible oil seeds, the production of which can be stepped up manifolds if the government provides incentives to farmers for production of bio-diesel. The economic feasibility of bio-diesel depends on the price of crude petroleum and the cost of transporting diesel over long distances to remote areas. It is a fact that the cost of diesel will increase in future owing to the increase in its demand and limited supply. Further, the strict
Conclusion
Alternate fuels for diesel engines have become increasingly important due to decreasing petroleum resource and environmental consequences of exhaust-gases from petroleum-fuelled engines. A number of studies have shown that triglycerides hold promise as alternative fuels for diesel engines. However, the high viscosity, low volatility and poor cold flow properties of triglycerides, which result in severe engine deposits, injector choking and piston ring sticking have prevented triglycerides from
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