Chapter 2 LITERATURE REVIEW

Transcription

1 Chapter 2 LITERATURE REVIEW In this chapter, previous research related to vegetable oils, use of vegetable oils as fiiel, conversion of vegetable oils into biodiesel, performance, emissions and combustion characteristics of biodiesel as fuel, effect of biodiesel on engine wear and lubricating oil tribology etc. is summarized and discussed. The first section of this chapter presents information about use of vegetable oil as an alternative fuel. The next section summarizes conversion of vegetable oils into biodiesel using transesterification process. The next section focuses attention on performance and emission of biodiesel and ATF fuelled engines and effect of biodiesel and ATF on in-cylinder combustion parameters. The next section presents information on affect of biodiesel and ATF on wear of vital engine components. In the last section, effect of biodiesel and ATF on lubricating oil is discussed. 2.1 Vegetable oil as Fuel Recent events around the world have once again put energy security, and in particular petroleum import dependence, at the top of energy agenda of most of the countries. Significant interest in alternative fuel for transportation is the result of growing concern for the environmental impact of the fossil energy sources, which are limited. The emergence of global climate change as a critical energy and environmental policy issue has also heightened awareness that combustion of greenhouse gas emitting fossil fuels impose risks for the planet. The initial interest was mainly one of fuel supply security, but recently more attention has been focused on the use of renewable fuels in order to reduce the net production of CO2 from fossil fuel combustion sources. Biofuels may provide a partial solution to each of these problems, by displacing oil use in transport and by reducing greenhouse gas (GHG) emissions per liter of fuel consumed. Vegetable oils present a very promising and attractive alternative to mineral diesel since they are renewable and are produced easily in rural areas where there is an acute need for modern energy-sources. 35

2 Since the advent of the internal combustion engine vegetable oils have been tried as a fuel in a diesel engine, but later interest waned away due to cheap and plentiful supplies of petroleum fuels. However, periodic increase in petroleum prices due to higher demand, stringent emission norms, and feared shortages of petroleum fuels due to rapid depletion and net production of carbon dioxide (CO2) from combustion sources have rekindled interest in renewable vegetable oil fuels. It is only in recent years that systematic efforts have been made to utilize vegetable oil as fuels. Due to wide variation in soil, climatic conditions and competing uses of land etc. different nations are exploring different vegetable oils as potential fuel. For example, soybean oil in the United States and Brazil, rapeseed and sunflower oils in Europe, palm oil in Southeast Asia (mainly Malaysia and Indonesia), used frying oil in Japan and Austria, linseed and olive oil in Spain, cottonseed oil in Greece and coconut oil in Philippines are being considered as substitutes for diesel fuels [17, 24, 33, 34]. These oils are edible in nature and developing countries like India, which have shortage of edible oil supply, cannot afford edible oils as a substitute for mineral diesel. Increased pressure to augment production of edible oils has also put limitation on the use of these oils for production of biodiesel. To extend the use of biodiesel, the main concern is the economic viability of producing biodiesel. The price of feedstock (vegetable oil) is one of the most significant factors which constitute approximately 75-88% of the total biodiesel production cost [35]. A number of other oils like karanja oil, cotton seed oil, ricebran oil, rapeseed oil, maize oil, babasu oil and linseed oil etc. are being considered world-wide for use in engines [31]. Obviously, developing nations have to focus their attention on oils of non-edible nature, which are cheaper, as edible oils are in great demand for food purposes and are far too expensive as fuel at present. Vegetable oils have 90% of the heat content of diesel, and cetane number is also comparable to mineral diesel (table 1.8). High cetane number due to long chain hydrocarbon structure makes them suitable replacement for mineral diesel. The advantages of using vegetable oils as fuels are [36]: - Vegetable oils are liquid fuels from renewable sources. - They do not over-burden the environment with emissions. 36

3 - Vegetable oils have potential for making marginal land productive by their property of nitrogen fixation in the soil. - Vegetable oil's production requires lesser energy input in production. - Vegetable oils have higher energy content than other energy crops like alcohol. - Vegetable oil combustion has cleaner emission spectra. - Simpler processing technology. But the main property, which is substantially different from mineral diesel, is 'viscosity'. The viscosity of vegetable oil is several times higher than mineral diesel due to larger molecular mass and chemical structure [37]. Cloud point and pour point of vegetable oils is also higher. The high viscosity of vegetable oils ( cst) as compared to mineral diesel oil (3-4cSt) at 40 C leads to unfavorable pumping and spray characteristics and affects the fuel injection pressure, injection duration, injection pressure-time history, spray cone angle, and fuel atomization. The inefficient mixing of fuel with air contributes to incomplete combustion. The higher flash point and lower volatility of vegetable oils also affect its combustion and lubricating oil properties in long term usage. Several researchers reported that at least in short-term trials, vegetable oils deliver satisfactory engine performance and power output, often equal to or even slightly superior than mineral diesel fuel. However, on long term usage, vegetable oils cause engine problems. This was recognized in the early stages of renewed interest in vegetable oil-based alternative diesel fuels. Studies on the vegetable oil as a fuel reported that, long-term results with vegetable oil or blends with mineral diesel lead to severe engine deposits, sticking piston rings, crankcase oil contamination, injector coking and thickening of lube oil [38, 39, 40]. Vegetable oil had been tried in raw as well as processed (refined and filtered) form. Hawkins et al. [41] tested the long-term durability of such an engine with sunflower oil as fuel. The successful results of this test programme led to the manufacturer extending the warranty on this engine to cover this type of fuel. Engler et al. [42] found that engine performance tests using raw sunflower and cottonseed vegetable oils as alternative fuels gave poor results. While Engine tests with processed vegetable oils produced results slightly superior than similar tests for mineral diesel fuel. However, carbon deposits and lubricating oil 37

4 contamination problems were reported with processed oil also. McDonnel et al. [43] investigated the use of a semi-refined rapeseed oil as a diesel fiael extender. Test results indicated that the rapeseed oil could serve as a fuel extender at inclusion rates up to 25%. As a result of using rapeseed oil as a fuel, injector life was shortened due to carbon buildup. However, no signs of engine wear or lubricating oil contamination were reported. The gum formation and carbon coking deteriorates the performance of vegetable oil for long term use. Korus et al. [44] reported that gum formation and carbon coking may occur immediately preceding and during combustion. Thermal polymerization may be the dominant gum forming reaction under combustion conditions since thermal polymerization has higher activation energy than oxidative polymerization and anaerobic conditions can occur within atomized fuel droplets. They suggested that carbon coking can be reduced with a lower degree of oil unsaturation and with better fuel atomization. Murayama et al. [45] investigated the feasibility of rapeseed oil and palm oil for diesel fuel substitution in a naturally aspirated D.l. diesel engine. It was found that both the vegetable oils delivered an acceptable engine performance and exhaust gas emissions for short term engine operation, but they caused carbon deposit build-ups and sticking of piston rings after extended engine operation. Practical solutions suggested to overcome the problems were: increasing the fuel temperature to over 200 C, blending 25% (v/v) mineral diesel in the vegetable oil, blending 25% (v/v) ethanol in the fuel, or converting the vegetable oils into methyl esters. Heating of the vegetable oil to reduce its viscosity have also been tried but the problem of deposits formation persists [31]. Almedia et al. [46] employed preheating of palm oil to reduce its viscosity, for use in a naturally aspirated direct injection four-stroke 70 kw diesel-generator. The high viscosity of palm oil resulted in poor atomization. carbon deposits, clogging of fuel lines and starting difficulties at low temperatures. When heated at 100 C palm oil gave lower viscosity, improved combustion and lower carbon deposits. On the other hand, Vellguth et al [\1\ reported that with preheated vegetable oils, carbon deposits were even more than the unheated vegetable oil. Bari et al. [47] observed higher CO and NOx emissions while using preheated crude palm oil fuelled diesel 38

5 engine. Kalam et al. [48] reported that preheated palm oil reduced emissions of CO, HC and PM but increased NOx as compared to mineral diesel. Kalam et al. [25] reported that blending of 30% coconut oil with mineral diesel in Isuzu 4FB1 IDI engine, produced higher brake power and net heat release rate with a net reduction in exhaust emissions (HC, NOx, CO, smoke and polycyclic aromatic hydrocarbon) and no significant carbon deposits were found on the injector nozzle tip. It was found that using vegetable oil as fuel, problems in DI engines and small IDI engines were more severe than IDI engine. IDI engines are known to be more tolerant of fuel quality. The causes of these problems were attributed to the polymerization of triglycerides which leads to formation of engine deposits as well as low fuel volatility and high fuel viscosity resulting in poor fuel atomization [12, 45, 50]. Basically, three different theories can be postulated to explain various durability problems that have been observed in vegetable oil fueled engines. These are: (i) high viscosity of the vegetable oils results in poor fuel atomization, which, in turn, lead to observed durability problems, (ii) the durability problems associated with the use of vegetable oil fuels, which result directly from molecular structure and the effect of molecular structure on the combustion chemistry, (iii) the durability problems due to incomplete combustion of the fuels (either spray or chemically induced) and the subsequent reaction of the fuels and partial combustion products with metal surfaces and lubricating oil [40]. An oxidative free-radical mechanism was suggested as governing triglyceride polymerization in lubricating oil contamination with sunflower oil as fuel [49]. It was reported that addition of antioxidants and dispersants is not sufficient to eliminate gum formation in vegetable oil fuelled engines [44]. Generally, use of vegetable oils reduces particulate emission and smoke opacity. But for other emissions, no definite trend is reported. Hemmerlein et al. [50] observed that energy consumption and engine performance were similar to operation with mineral diesel but higher exhaust emissions with rapeseed oil as a fuel. Emissions of CO and HC increased by up to 100 and 290%, respectively, compared to those for mineral diesel. but it is likely that these levels could be 39

6 reduced if the fuel injection timing is advanced. Emissions of NOx reduced, due to slower combustion and lower maximum combustion temperatures resulting from the use of rape-seed oil. Generally, soot and particulate emission are also reported to have reduced by up to 0.4 Bosch-number and 50%, respectively, although particulate emissions were up to 140% higher when combustion occurred in DI engines. The emissions of aromatics, aldehydes, ketones, and the SOF of particulate matter were higher when rape-seed oil was used [50]. Niemi et al. [51] reported more fine and ultra-fine particles when running a DI tractor engine on mustard oil, however smoke and HC emissions were lower. Jacobus et al. [52] conducted trials on sunflower oil. cottonseed oil. soybean oil, and peanut oil. It was reported that vegetable oils generally show slight improvements in thermal efficiency and indicated specific energy consumption; equal or higher gas-phase emissions; and significantly higher aldehyde emissions, including an increased percentage of formaldehyde. Th Several researchers tried to utilize used oils from the restaurants and animal fats as a fuel for diesel engine [53-56]. Yu et al. [55] conducted a study on waste cooking oil collected from the noodle industry. The oil was used as fuel in the engine without any fiarther treatment. The performance and emission characteristics were compared with mineral diesel. The experimental results indicated that combustion characteristics were generally similar to that of diesel. The energy released in the late combustion phase was higher, which was due to heavier molecular weight materials present in the waste cooking oil. The engine performance was similar to that of mineral diesel. The emissions of CO, NOx and SOx were higher for waste cooking oil compared to that of diesel. At high temperatures, tar like substance was found to have deposited in the combustion chamber [55]. Pugazhvadiv et al. [56] observed improved engine performance and reduced CO and smoke emissions with preheated waste frying oil. They reported that the waste frying oil preheated to 135 C could be used as a diesel fuel substitute for short-term engine operations. Preheated animal fat was reported be used in diesel engines with reduced smoke, hydrocarbon and carbon mono>cids^emissions with no major deterioration in engine performance [54] fy 40

7 Based on the literature, it was found that numerous studies involving use of straight vegetable oils and blends with mineral diesel had been carried out. Even though the physical and chemical properties of vegetable oils are close enough to diesel fuel to run diesel engines for short periods without any modifications, using vegetable oils in direct injection diesel engines results in severe engine deposits, dilution of the lubricating oil, injector choking, piston ring sticking etc. Long-term engine usage shows that engine durability with these fuels is questionable. Long term use of vegetable oils causes several problems in the engine, which may even lead to failure of the engine. Most of these problems are a consequence of high viscosity of the vegetable oils, which is a major contributing factor to the onset and severity of durability problems when using vegetable oils [40]. Since vegetable oils are not an ideal substitute fuel for diesel engines, they need to be modified in order to bring their properties further closer to mineral diesel. 2.2 Vegetable Oils: Fuel Formulation Techniques The fuel modifications are mainly aimed at reducing fuel viscosity, which would eliminate flow related problems. Considerable efforts have been put in to develop vegetable oil derivatives that approximate the properties and performance of the hydrocarbon fuels. The problems associated with vegetable oils can be overcome by following techniques. - Dilution - Microemulsion - Pyrolysis - Transesterification Dilution Dilution is one of the possible solutions to the viscosity problem of the vegetable oil. Dilution of vegetable oils can be accomplished with mineral diesel. a solvent or alcohol such as ethanol. Several researchers tried blending vegetable oils with mineral diesel as fuel. Pramanik [57] reported that 50% blend of jathropa oil can be used in diesel engine without any major operational difficulties but further study is suggested for the long term durability of the engine. Wang et al. [58] reported comparable engine performance with vegetable oils and lower CO, HC 41

8 and NOx emissions in case of vegetable oil fuelled engine. Several researchers investigated blends of vegetable oil and mineral diesel as fuel in engine. Most of them reported that direct use of vegetable oils and/or the use of blends of vegetable oils have generally performed unsatisfactory and considered to be impractical for both direct and indirect injection diesel engines. The high viscosity, acid composition, free fatty acid content, as well as gum formation due to oxidation and polymerization during storage and combustion, carbon deposits and lubricating oil thickening are obvious problems [14, 59]. Ramadhas et al. [60] reported that 50-80% of rubber-seed oil blends with mineral diesel gave the best performance but higher carbon deposits inside combustion chamber, frequent cleaning of fuel filter was required Microemulsions Another approach to solve the problem of the high viscosity of vegetable oils is to form microemulsions using short chain alcohols. Microemulsions with solvents such as methanol, ethanol and 1 -butanol have been investigated. A microemulsion is defined as a colloidal equilibrium dispersion of optically isotropic fluid microstructures with dimensions generally in the nm range formed spontaneously from two normally immiscible liquids. They can improve spray characteristics by explosive vaporization of the low boiling constituents in the micelles [14. 16]. Microemulsions because of their alcohol content have lower heating values than mineral diesel fuel, but the alcohols have high latent heat of vaporization and tend to cool the combustion chamber, which would reduce nozzle coking. Goering et al. [61] prepared microemulsion of soybean oil, mineral diesel, 190-proof ethanol and 1-butanol. The engine fuelled with this emulsion completed 200-hr EMA test without any operational difficulty. Lower engine wear, but higher deposits of carbon and lacquer on the injector tips, intake valves and top of the cylinder liners were reported. Short term performances of microemulsions of aqueous ethanol in soybean oil were nearly as good as that of mineral diesel, in spite of the lower cetane number and energy content. The durabilities were not determined [16]. Ziejewski et al. [62] prepared an emulsion of 53% (v/v) sunflower oil, 13.3% (v/v) 190-proof ethanol and 33.4% (v/v) 1-butanol. This non-ionic emulsion had viscosity of

9 cst at 40 C, cetane number of 25 and an ash content of less than 0.01%. In a 200 hour laboratory screening engine endurance test, no significant deterioration in engine performance (lower power output, exhaust temperature, intake manifold pressure and lesser smoke) were observed but difficulty in starting the engine at room temperature, irregular injector needle sticking, heavy carbon deposits, incomplete combustion and increase in lubricating oil viscosity were reported Pyrolysis Pyrolysis is the conversion of one substance into another by means of heat with or without the aid of a catalyst. The pyrolyzed material can be vegetable oil, animal fat. natural fatty acid and methyl esters of fatty acids. The pyrolysis of fats has been investigated for more than 100 years, especially in those areas of the world which lack petroleum deposits. Many investigators have studied the pyrolysis of triglycerides to obtain products suitable for diesel engines. Thermal decomposition of triglycerides produces the compounds of various classes' like alkanes, alkenes, alkadines, aromatics and carboxylic acids [16]. Zaher et al. [63] chemically modified used vegetable oil by thermal cracking in the presence of 2% calcium oxide as a catalyst and tested its 50% blend with diesel as a fuel in a single cylinder diesel engine. They reported that the product obtained by thermal cracking of frying oils had viscosity of 6.0 cst, flash point of 55 C and 40.5 kj/kg heating value. Also, the performance of a diesel engine in terms of output power, brake thermal efficiency, and brake specific fuel consumption was not markedly changed by blending mineral diesel with the cracked product (50%) blend). Milne et al. [64] reported that conversion of various biomass materials (including triglycerides) through cracking over zeolite catalyst produced products containing significant aromatic fractions and were much closer to gasoline than diesel fuel. It needs to be noted that the equipment for thermal cracking and pyrolysis is expensive for modest throughputs. In addition, while the products are chemically similar to petroleum-derived gasoline and diesel. the removal of oxygen during thermal processing also removes environmental benefits of using an oxygenated fuel. It produced some low value materials and, sometimes, more gasoline than diesel [14, 65]. 43

10 2.2.4 Transesterification Transesterification is a well-known reaction in organic chemistry. The industrialscale processes for transesterification of vegetable oils were initially developed in the early 1940's to improve the separation of glycerol during soap production. At that time, this process was used for the separation of glycerol, which was needed for explosive production [66]. Transesterifcation is a method to lower the viscosity of vegetable oils by breaking up the triglyceride molecules and separating the fatty acid molecules from the glycerin molecule. The transesterification process is the reaction of a triglyceride (fat/oil) with alcohol to form esters and glycerol. This makes the properties of vegetable oils and animal fats closer to mineral diesel. There is an energy advantage in methyl ester production from vegetable oils. Ratio of energy outputs to energy inputs is approximately 4.5:1 in case of sunflower oil [67,68]. Details of the process of transesterification and biodiesel production are given in following section. 2.3 Biodiesel as Diesel Engine Fuel The best way to use vegetable oil as a fuel is to convert it to biodiesel. The term "biodiesel" generally refers to methyl esters (sometimes called "fatty acid methyl ester", or FAME) made by transesterification, a chemical process that reacts a feedstock oil or fat with methanol in the presence of a catalyst. The feedstock can be vegetable oil, such as that derived from oilseed crops (e.g. soy, sunflower, rapeseed, etc.), used frying oil (e.g. yellow grease from restaurants) or animal fat (beef tallow, poultry fat or pork lard). The biodiesel is quiet similar to conventional mineral diesel in its main characteristics. However, biodiesel has a number of advantages (table 2.1). These include better lubricity (i.e. lower engine friction), virtually no aromatic compounds or sulphur, and a higher cetane number compared to mineral diesel. Therefore, biodiesel is a strong candidate to replace mineral diesel if the need arises. The advent of ultra low sulfur diesel fuels and their reduced lubricity may motivate use of low concentration biodiesel blends to ensure adequate lubricity. The conversion of triglycerides into methyl or ethyl esters through the transesterification process reduces the molecular weight to onethird that of the triglyceride, reduces the viscosity by a factor of eight or higher 44

11 and increases the volatility. Biodiesel does not contain any petroleum products, but it is compatible with conventional (mineral) diesel and can be blended at any level with mineral diesel to create a stable biodiesel blend. The level of blending with petroleum diesel is referred as Bxx, where xx indicates the amount of biodiesel in the blend (i.e. B40 blend is 40% biodiesel and 60% mineral diesel). It can be used in CI engine without any major modification in the engine hardware. In addition to biodiesel, the transesterification process typically yields co-products such as rushed bean "cake", an animal feed, and glycerin. Glycerin is a valuable chemical used for making many types of cosmetics, medicines and foods, and its co-production improves the economics of making biodiesel. Table 2.1: Fuel properties of biodiesel and mineral diesel [29] Property Cetane number Lubricity Biodegradability Toxicity Oxygen Aromatics Sulphur Cloud point Flash point Effect on natural, butyl rubber Biodiesel 51 to up to 11% F can degrade Low-sulphur diesel 44 to 49 very low - - very low 18-22% ppm F no impact Transesterincation Process Transesterification is the most effective and widely used technique for formulating the vegetable oil for use in diesel engine. All vegetable oils and animal fats primarily consist of triglycerides. Transesterification is a well established chemical reaction, in which alcohol reacts with triglycerides of fatty acids in presence of a catalyst. A catalyst is usually used to improve the reaction rate and yield. It is a reversible reaction of fat or oil with a primary alcohol, in which, alcohol combines with the triglycerides to form one mole of glycerol and three moles of mono alkyl esters (biodiesel). These esters (biodiesel) contain 10 to 11% oxygen by weight, which may improve combustion compared to hydrocarbonbased diesel fuels in an engine. The cetane number of biodiesel is around 50. The stoichiometry for the transesterification reaction requires 3:1 molar ratio of 45

12 alcohol to oil, however, since the reaction is reversible, in practice excess alcohol (6:1) is required to shift the equilibrium to the products side in order to raise product yield. Methanol and ethanol are used most frequently, especially methanol because of its low cost and its physical and chemical advantages (polar and shortest chain alcohol). It can quickly react with triglycerides and catalyst easily gets dissolved in it [ ]. The transesterification reaction can be catalyzed by both homogeneous and heterogeneous catalyst. Homogeneous catalyst includes alkalis and acids. The most commonly used alkali catalysts are sodium hydroxide, potassium hydroxide, carbonates and corresponding sodium and potassium alkoxides such as sodium methoxide. sodium ethoxide, sodium propoxide and sodium butoxide, etc. Sulphuric acid, sulfonic acids and hydrochloric acid are usually used as acid catalysts. Lipases can also be used as bio-catalyst in an enzyme catalyzed transesterification process. Several researchers have reported the kinetics for both acid and alkali catalyzed transesterification reactions. Freedaman et al. [70] reported the transesterification reaction of soybean oil and other vegetable oils with alcohols, examining the effects of type of alcohol, molar ratio, type and amount of catalyst, and reaction temperature on rate constants and reaction order. The S-shaped curves of the effects of time and temperature on ester formation for a 30:1 ratio of butanol and soybean oil (SBO), 1% H2SO4 and 77-ll7 C at 10 C intervals indicated that the reaction began at a slow rate, proceeded at a faster rate and then slowed again as the reaction neared completion. Ester formation essentially completed in 3 hours at 117 C compared to 20 hr at 77 C with acid catalyst (H2SO4). Alkali-catalyzed transesterification required 6:1 molar ratio of alcohol to oil and takes nearly one hour at 60 C for reaction to complete [71]. Alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is often used commercially [14, 71]. Alkaline catalysts have the advantage of being less corrosive to industrial equipment than acid catalysts. The reaction rate constants for the alkali-catalyzed reaction were reported to be much higher than those for the acid-catalyzed reactions. 46

13 The most important variables affecting the yield of biodiesel from transesterification are, - Reaction temperature - Molar ratio of alcohol to oil - Catalyst - Reaction time - Presence of moisture and free fatty acids. Reaction temperature The rate of reaction is strongly influenced by the reaction temperature. However, given enough time, the reaction will proceed to near completion even at room temperature. Generally, the reaction is conducted close to the boiling point of methanol (60 C to 70 C) at atmospheric pressure. The maximum yield of esters occurs at temperatures ranging from 60 C to 80 C at a molar ratio (alcohol to oil) of 6:1 [16. 51, 71, 72]. Further increase in temperature is reported to have a negative effect on the conversion [16], Several researchers have studied the effect of temperature on conversion of oils and fats into biodiesel. Darnoko andcheryan [73] studied the effect of temperature on palm oil transesterification was studied at a catalyst (KOH) concentration of 1% and a methanol/oil molar ratio of 6:1. The rate of transesterification in a batch reactor increased with temperature up to 60 C. Higher temperatures didn't reduce the time to reach maximum conversion. Freedman et al. [71] studied the transesterification of refined soybean oil with methanol at a molar ratio of 6:1, 1% NaOH catalyst, at three different temperatures 60 C, 45 C and 32 C. They reported that after 0.1 hr, ester yields were 94, 87 and 64% for 60 C, 45 C and 32 C temperature respectively. After 1 hr, ester formation was identical for 60 C and 45 C and only slightly lower at the 32 C reaction temperature. It showed that temperature clearly influenced the reaction rate and ester yield and transesterification can proceed satisfactorily at ambient temperatures, if given enough time, in the case of alkaline catalyst [71]. In addition it has also been reported that methyl ester yields are lower when higher temperatures were used, presumably because the saponification reaction is more temperature sensitive [74]. 47

14 Molar ratio One of the most important variables affecting the yield of ester is the molar ratio of alcohol to vegetable oil. The stoichiometry of the transesterification reaction requires 3 moles of alcohol per mole of triglyceride to yield 3 moles of fatty esters and 1 mole of glycerol. To shift the transesterification reaction to the right, it is necessary to use either a large excess of alcohol or to remove one of the products from the reaction mixture. The second option is preferred wherever feasible, since in this way, the reaction can be driven to completion. When 100% excess methanol is used, the reaction rate is at its highest. An acid catalyzed reaction needs a 30:1 ratio of BuOH to soybean oil, while an alkali-catalyzed reaction requires only a 6:1 ratio to achieve the same ester yield for a given reaction time [70]. Freedman et al. [71] studied the effect of molar ratio (from 1:1 to 6:1) on ester conversion with vegetable oils. Soybean, sunflower, peanut and cotton seed oils behaved similarly and achieved highest conversions (93-98%) at a 6:1 molar ratio. Significant amounts of partially reacted mono- and diglycerides will be present when the alcohol to oil ratio is too low. On the other hand, an excessive amount of alcohol makes the recovery of the glycerol difficult. Hence ideal alcohol/ oil ratio has to be established empirically, considering each individual process. Molar ratios greater than 6:1 do not increase yield much (already 98-99%)), however interferes with separation of glycerol. Thus, a molar ratio of 6:1 is normally used in industrial processes to obtain methyl ester yields higher than 98%) (w/w) [16, 65]. Catalyst Catalysts for transesterification process are classified as alkali, acid, or enzyme. Alkali-catalyzed transesterification is much faster than acid-catalyzed process [65, 72]. However if a vegetable oil contains higher free fatty acid and more water, acid-catalyzed transesterification is suitable. Sodium alkoxides are among the most efficient catalysts [85]. Sodium hydroxide and potassium hydroxide, due to their low cost are widely used in industrial biodiesel production because of their 48

15 cheapness [16, ]. Partly due to faster esterification and partly because alkaline catalysts are less corrosive to industrial equipment than acidic catalysts, most commercial transesterifications are conducted with alkaline catalysts. The alkaline catalyst concentration in the range of 0.5 to 1% (w/w) yields 94 to 99% conversion of vegetable oils into esters. Further, increase in catalyst concentration does not increase the conversion and it adds to extra costs because it is necessary to remove it from the reaction medium at the end [14. 16]. Reaction time The conversion rate increases with reaction time. Freedman et al. [71] transesterified peanut, cottonseed, sunflower and soybean oils under the condition of methanol to oil molar ratio of 6:1, 0.5% sodium methoxide catalyst and 60 C. An approximate yield of 80% was observed after 1 minute for soybean and sunflower oils. After 1 hour, the conversions were almost same for all four oils (93-98%). Ma et al. [14] studied the effect of reaction time on transesterification of beef tallow with methanol. The reaction was very slow during the first minute due to the mixing and dispersion of methanol into beef tallow. From one to five minutes, the reaction proceeded very fast. The apparent yield of beef tallow methyl esters surged from 1 to 38%. It was reported that acid catalyzed transesterifcation take longer time for the reaction to complete. IVIoisture and free fatty acids Starting materials used for alkali-catalyzed transesterification of triglycerides must meet certain specifications. The triglyceride should have an acid value less than 1 and all materials should be substantially anhydrous. If the acid value is greater than 1, more NaOH is required to neutralize the free fatty acids. Water also caused soap formation, which consumed the catalyst and reduced catalyst efficiency. The resulting soap causes an increase in viscosity, formation of gels and makes the separation of glycerol difficult. Freedman et al. [71] stated that ester yield reduced significantly if the reactants did not meet these requirements. Sodium hydroxide or sodium methoxide reacted with moisture and carbon dioxide in the air, which diminished their effectiveness. They compared both crude and refined vegetable oils as feedstock and found that the yield of methyl esters reduced from 93-98% 49

16 for the refined oil to 67-86% for the crude oil. This was attributed mostly to the presence of up to 6.66% free fatty acids in the crude oil. Ma et al. [14] investigated the effects of free fatty acids and water on transesterification of beef tallow with methanol. The results showed that the water content of beef tallow should be kept below 0.06% (w/w) and free fatty acid content of beef tallow should be kept below 0.5% (w/w), in order to get the best conversion. Water content was a more critical variable in the transesterification process than free fatty acids [14]. Alcantara et al. [73] transformed three fatty materials - bean oil, used frying oil. and tallow - with sodium methoxide into two different types of products by transesterification and amidation reactions with methanol and diethylamine. respectively. Amides enhance the ignition properties of petrochemical diesel fuel. They also reported that stirring speed plays important role and very low conversion was reported at low speed (360 rpm) Recent Developments in Transesterification Several researchers have studied the transesterification process with different vegetable oils and different catalyst. Vicente et al. [74] found higher conversion with methoxide catalyst but these catalysts are very expensive and are hygroscopic in nature. They transesterified sunflower oil with different catalysts and found that the purity of biodiesel with methoxide, KOH and NaOH was similar (near 100%) but yield with KOH and NaOH was and 85.9t% (w/w) respectively which could be further improved by optimizing reaction parameters. For an alkalicatalyzed transesterification process, triglycerides should have low value of free fatty acids (FFA) and alcohol must be substantially anhydrous. Soap formation lowers the yield of esters and renders the separation of ester and glycerol and water washing of ester difficult [14, 65, 71]. Van Gerpan [66] reported that up to about 5% FFA, the reaction can be catalyzed with an alkali catalyst with additional catalyst to compensate for the catalyst loss in soap formation. The soap that is created during the reaction is either removed with the glycerol or is washed out during the water wash. When the FFA level is above 5%, the soap inhibits separation of the methyl esters and glycerol and contributes to emulsion formation during the water wash. For these cases, an acid catalyst, such as sulfuric acid, can 50

17 be used to esterify the free fatty acids to methyl esters [66]. Acid catalyst (HCl, H2SO4) are advantageous for oils having high FFA content, as acid catalyzes the FFA esterification to produce fatty acid methyl ester, increasing the biodiesel yield, but the reaction time and alcohol requirement increases substantially [75]. Lang et al [76] prepared esters of different oils using Methyl, ethyl, 2-propyl and butyl alcohols. They reported almost same heating value for all esters but pour point decreased with longer chain alcohols. Butyl esters showed improved cold flow properties. So higher chain alcohols should be preferred to improve the cold flow properties. During thermogravimetric (TGA) analysis, esters were found to be considerably less volatile than the mineral diesel. Boocock et al. [74] and Zhou et al. [77] proposed the addition of a co-solvent (tetra-hydro-furan and methyl tertiary butyl ether) to create a single phase, and this accelerates the reaction so that it reaches substantial completion in few minutes. The primary concerns with this method are the additional complexity of recovering and recycling the co-solvent. Additional concerns have been raised about the hazard level associated with the co-solvents most commonly proposed, tetra-hydro-furan and methyl tertiary butyl ether [66]. Haas et al. [78] have shown that acid-catalyzed esterification can be used to produce biodiesel from low-grade by-products of the oil refining industry, such as soapstock. Soapstock, a mixture of water, soaps, and oil, is dried, saponified, and then esterified with methanol or some other simple alcohol using an inorganic acid as a catalyst. For soybean acid oil containing 59.3t% (w/w) FFA, maximum esterification occurred at 65 C and 26 hours reaction time at a molar ratio of total FFA/methanol/sulfuric acid of 1:15:1.5. The procedure relies on large excess of alcohol, and the cost of recovering this alcohol determines the feasibility of the process. Transesterification without catalyst Several researchers [79,80] tried transesterification without using any catalyst in supercritical methanol, which eliminates the need for the water washing. Diasakou et al. [81] investigate the non-catalytic transesterifcation of soybean oil with methanol. They used high temperature in the range of 220 and 235 C at 55 and 62 bar initial pressure for varying molar ratio of methanol to alcohol (6:1 to 27:1). 51

18 They reported that methyl ester content was 85% after 10 hour reaction time at 235 C and 27:1 methanol to oil molar ratio. Saka & Kusdiana [80, 82] found that in supercritical methanol, reaction requires only four minutes and also presence of water did not affect ester yield. It was found that this new supercritical methanol process requires shorter reaction time and simpler purification procedure, but very high pressure (45-65 bar), temperature (350 C) and very high molar ratio of alcohol to oil (42:1) was required. Enzymatic transesterification Fukuda et al [65] reported that both extracellular and intracellular lipases are also able to effectively catalyze the transesterification of triglycerides in either aqueous or nonaqueous systems. It is reported that the by-product, glycerol, can be easily recovered without any complex process in enzymatic transesterification, and also that free fatty acids contained in waste oils and fats can be completely converted to methyl esters. Mittelhetch [83] reported the lipase catalyzed transesterification of sunflower oil. Watanabe et al. [84] conducted three-step methanolysis of waste oil by using three columns packed with 3 g of immobilized Candida antarctica lipase. Lipase catalyzed esterification is reported to be a viable method for the production of alkyl esters from tallow, vegetable oil, and greases [85]. Enzymes like lipases can also be used as biocatalysts for transesterification but cost is the main hurdle to commercialize enzymes as catalyst [14, 65, 86]. Several researchers transesterified the waste vegetable oils and found different optimum quantities of reactants and process variables [75, 88]. Transesterification using heterogeneous catalyst More recently, there has been an increase in research activities for development of heterogeneous catalyst (enzymes and solid catalyst), because their utilization in the transesterification reaction greatly simplifies and economizes the posttreatment (separation and purification) of the products. The use of heterogeneous catalyst does not produce soaps through fatty acid neutralization and triglyceride saponification. However, heterogeneous catalyzed reaction also requires extreme reaction conditions, while the methyl ester yield and the reaction time are still 52

19 unfavourable compared to the alkali catalysts [74, 89]. Marchetti et al. [86] compared different technologies (table 2.2). Table 2.2: Comparison of different technologies to produce biodiesel [105 Variable catalysis Reaction temperature ( C] Alkali catalysis Lipase Catalysis Supercritical alcohol Acid FFA in raw Material Saponified products Methyl esters esters Esters Water in raw with Material Interference with reaction No influence _ Interference reaction Yield of methyl Esters Normal Higher Good Normal Recovery of Glycerols Difficult Easy. Diflficuh Purification Repeated Washing None - Repeated washing Production cost of catalyst Cheap Expensive Medium Cheap Four different continuous process flow sheets for biodiesel production from vegetable oil or waste cooking oil under alkaline or acidic conditions on a commercial scale were developed by Zhang et al. [90].Detailed operating conditions and equipment designs for each process were obtained. A technological assessment of these four processes was carried out to evaluate their technical benefits and limitations. Analysis showed that the alkali-catalyzed process using virgin vegetable oil as the raw material required the fewest and smallest process equipment units [90]. In summary, base catalysts are most common, since the process is faster and reaction conditions are moderate. In the base-catalyzed process, homogenous catalysts dissolve fully in the glycerol layer and partially in the methyl esters layer. Washing of biodiesel for separation of excess methanol, catalyst and glycerin present in the biodiesel layer is essential. 53

20 2.3.3 Biodiesel Washing Currently, biodiesel is washed with processes such as bubble washing, spray (mist) washing, counter current washing and agitation. The basic principle in bubble washing is washing the biodiesel by bubbles. Air-bubbles rise through the water into biodiesel and carry a thin water film around them. This bubble washes the biodiesel around it and reaches the surface of biodiesel. Once it reaches the top, the bubble explodes sending the water downwards. The water going down also cleans the biodiesel and hence biodiesel will be washed during upward motion of air bubbles and downward motion of water. In spray washing, a superfine spray above wash-tank sends a mist of water droplets down onto the surface creating zero agitation. It washes biodiesel slowly. Since this method involves production of superfine spray of water to avoid agitation in the biodiesel, it is very tedious and water consuming process. The water used in this process cannot be reused. After some time, the water gets settled and biodiesel can be separated. In counter current washing, biodiesel and water will be flowing in opposite directions for effective cleaning of impurities in biodiesel. This method uses least amount of water among all processes. In this method, long columns are used for the cleaning of biodiesel. Biodiesel is introduced into the column at the top and hot water (around 70 C) at the bottom. After the cleaning process, the mixture is collected in tanks and the process is repeated three to four times. For washing the biodiesel at quicker rates, agitation or stir washing is used. This method is not effective when the transesterification biodiesel yield is low. In this method, biodiesel and water are mixed using a mechanical stirrer for 5 minutes until the mixture appears homogeneous. Then the mixture is allowed to settle for 2-3 hours. The top biodiesel layer is siphoned off and this method is repeated 3-4 times. Esters are named according to the alcohol that is used, such as methanol, ethanol, or butanol, and the source of the triglyceride. If methanol or ethanol is 54

21 reacted with soybean oil. it is called methyl ester of soybean oil or ethyl ester of soybean. 2.4 Performance, Emission and Combustion Characteristics of Biodiesel Several researchers have published data for biodiesel emissions obtained with different engines and vehicles, and with different test procedures. In this section a brief overview of performance, emissions and combustion data of biodiesel produced with different feed-stocks during steady-state as well as transient engine test cycles is presented Performance Characteristics Biodiesel has lower heating value. (8-10% lower than mineral diesel on weight basis) because of presence of substantial amount of oxygen in the fuel molecule but at the same time biodiesel has a higher specific gravity (0.88) as compared to mineral diesel (0.85) hence overall impact is approximately 4-5% lower energy content per unit volume. Several experimental investigations have been carried out by researchers around the world to evaluate the engine performance of different biodiesel blends. Altin et al. [91] studied the effect of sunflower oil, cottonseed oil, soybean oil and their methyl esters in a single cylinder, four stroke direct injection diesel engine. They observed slight reduction in torque and power produced and increased bsfc in case of biodiesel fuelled engines. Similar results were reported by Kaufman and Ziejewski [92] and Antolin et al. [93] for sunflower methyl ester; Clark et al. [98], Mcdonald et al. [94] for soybean esters; Petreson et al. [95] for rapeseed oil biodiesel and Phuhan et al. [96] for mahua biodiesel, Cetkinya et al. [68] for used cooking oil biodiesel, and similar results for other biodiesel [95, 97]. Carraretto et al. [99] carried out investigations using different blends of biodiesel and mineral diesel (i.e. 100%, 80%. 70%, 50%, 30%, 20% and 0% (v/v) of biodiesel, respectively) in a six cylinders direct injection diesel engine. The increase in biodiesel percentage in the blend leads to slight decrease in both power and torque over the entire speed range. In particular, with pure biodiesel, there was 55

22 a reduction by about 3% in the maximum power and about 5% in the maximum torque. Moreover, with pure biodiesel. the maximum torque was found to be delivered at relatively higher engine speed. However, Al-widyan et al. [100] reported slightly increased power and lower bsfc for waste oil biodiesel fuelled engines. Agarwal and Das [101] also reported lower bsfc for linseed oil biodiesel blends. Moreno et al. [102] investigated sunflower methyl ester and its blends in an IDl engine and found that blends of 25 to 50% were suitable to be used as fuel. Torque and power were maintained at the same level as mineral diesel where as specific fuel consumption increased. Slight decrease in torque at higher SME contents but reversed trend at lower SME contents was reported by Silva et al. [103]. Raheman and Phadatre [104] reported average 6% increased brake power output for a karanja oil biodiesel up to 40% blend (B40) and with a further increase in the biodiesel percentage in the blend engine power reduced slightly. Bsfc was observed higher in case of biodiesel fuelled engines but brake specific energy consumption (bsec) and the thermal efficiency was reported to be either comparable or higher in comparison to mineral diesel fuelled engine. Kelvin et al. [104] for different fatty acid esters, Canakci [105] for blend of mineral diesel with soybean ester and Dorado et al. [88] for waste olive oil ester reported increased brake specific fuel consumption but no significant change in thermal efficiency. Usta et al. [106] reported slightly increased power and thermal efficiency with addition of tobacco seed oil methyl ester to mineral diesel in turbocharged IDl engine. Al-widyan et al. [100] reported improved efficiency for blends of ethyl ester of used vegetable oil. Lafrogia et al. [97] reported 10 % increase in thermal efficiency with biodiesel. Agarwal and Das [101] for linseed oil biodiesel and Raheman and Phadatre [107] for karanja oil biodiesel reported improved efficiency for all the blends of biodiesel and particularly B20 blend showed the maximum efficiency in a constant speed stationary diesel engine. 2.5% improvement in efficiency with 20% blend of linseed oil with mineral diesel was reported [101]. Senatore et al. [108] analyzed the performance and emission of a turbocharged direct injection diesel engine fueled with rapeseed oil methyl ester and diesel fuel. It was reported that performance is substantially unaffected if the comparison is made in terms of equivalence ratio. Fuel consumption was also reported unaffected in terms of brake specific energy consumption. No definite 56