45 4 Production of Biofuels with Special Emphasis on Biodiesel Ayhan Demirbas ABSTRACT Biodiesel, an alternative biodegradable diesel fuel, is a renewable fuel that can be pro- duced from vegetable oils, animal fats, and used cooking oil, including triglycerides. It is derived from triglycerides by transesterication with methanol and ethanol. Con- cerns about the depletion of diesel fuel reserves and the pollution caused by the con- tinuously increasing energy demands make biodiesel an attractive alternative motor fuel for compression ignition engines. There are four different ways of modifying vegetable oils and fats to use them as diesel fuel, such as pyrolysis/cracking, dilution with hydrocarbons blending, emulsication, and transesterication. The most com- monly used process is the transesterication of vegetable oils and animal fats. 4.1 INTRODUCTION The term biofuel generally refers to liquid or gaseous fuels for the transport sec- tor that are predominantly produced from biomass. Biomass (all plants and living organisms) can be converted into liquid and gaseous fuels through thermochemical CONTENTS Abstract 45 4.1 Introduction 45 4.2 Vegetable Oils as Diesel Fuels 46 4.3 Biodiesel 47 4.4 Noncatalytic Transesterication with Supercritical Alcohol Transesterication 49 4.5 Transesterication Reaction Mechanism of Vegetable Oil 50 4.6 Variables Affecting Methyl Ester Yield 51 4.7 Comparison of Fuel Properties andCombustion Characteristics of Methyl and Ethyl Alcohols and Their Esters 52 4.8 Biodiesel Economy 53 4.9 Conclusions and Future Perspectives 53 References 53 © 2009 by Taylor & Francis Group, LLC 46 Handbook of Plant-Based Biofuels and biological routes. Biofuel is a nonpolluting, locally available, accessible, sustain- able, and reliable fuel obtained from renewable sources. The liquid biofuels being considered the world-over fall into the following categories: (1) vegetable oils and biodiesels, (2) alcohols, and (3) biocrude and synthetic oils (Demirbas 2006). Fig- ure 4.1 shows the resources of the main liquid biofuels for automotives. Figure 4.2 shows the main biomass conversion processes. 4.2 VEGETABLE OILS AS DIESEL FUELS The use of vegetable oils as alternative renewable fuel competing with petroleum was proposed at the beginning of the 1980s. The advantages of vegetable oils as die- sel fuel are their liquid nature, which implies portability, ready availability, renew- ability, higher heat content (about 88% of No. 2 petroleum diesel fuel), lower sulfur content, lower aromatic content, and biodegradability. A sustainable biofuel has two favorable properties, which are availability from renewable raw material and lower negative environmental impact than that of the fossil fuels. As an alternative fuel, vegetable oil is one of the renewable fuels. Vegetable and animal oils/fats, which Biofuels Biodiesel Bioethanol Rapeseed Soybean Palm Sunflower Wheat Maize Sugar Beet Potatoes FIGURE 4.1 Resources of main liquid biofuels for automotives. Biomass Conversion Feedstock ermochemical Conversion Biochemical Conversion Pyrolysis Gasification Liquefaction Bioethanol Biodiesel Syn-oil Syn-gas Bio-chemicals FIGURE 4.2 Main biomass conversion processes. © 2009 by Taylor & Francis Group, LLC Production of Biofuels with Special Emphasis on Biodiesel 47 mainly consist of the triglyceride of the straight chain fatty acid, are organic chem- icals made from carbon dioxide and water using solar energy. Vegetable oil is a potentially inexhaustible source of energy, with an energy content close to that of diesel fuel. Vegetable oils, such as palm, soybean, sunower, peanut, and olive oils, can be used as alternative fuels for diesel engines. Vegetable oils used as alternative engine fuels are all extremely viscous, with viscosities ranging from 10 to 20 times higher than petroleum diesel fuel. The major problem associated with the use of pure vegetable oils as fuels for diesel engines is caused by the high fuel viscosity in com- pression ignition engines. The vegetable oils pose many problems when used directly in diesel engines. These include (1) coking and trumpet formation on the injectors, to such an extent that fuel atomization does not occur properly or even is prevented as a result of plugged orices; (2) carbon deposits; (3) oil ring sticking; (4) thickening or gelling of the lubricating oil as a result of contamination by vegetable oils; and (5) lubricating problems. There are different ways of modifying vegetable oils and fats to use them as diesel fuel, and such methods as pyrolysis, dilution with hydrocarbons, and emulsi- cation have been considered. Figure 4.3 shows the use of vegetable oils as petroleum alternative fuels (Demirbas 2003). Emulsication with alcohols has been proposed to overcome the problem of high viscosity of vegetable oils (Madras, Kolluru, and Kumar 2004). 4.3 BIODIESEL The diesel made from oils/fats has been noted as an ecological fuel because the oils/ fats are a sustainable energy resource. Thus, alkyl esters of fatty acids that meet transportation fuel standards are called “biodiesel.” It has been also dened as the monoalkyl esters of the long chain fatty acids derived from renewable feedstocks, such as vegetable oils or animal fats, for use in compression ignition (diesel) engines. Biodiesel has become more attractive because of its environmental benets and the fact that it is made from renewable resources (Ma and Hanna 1999). One popular process for producing biodiesel from the fats/oils is trans-ester- ication of triglyceride by methanol (methanolysis) to make methyl esters of the straight chain fatty acid. Alcohols are primary or secondary monohydric aliphatic alcohols having one to eight carbon atoms. Among the alcohols that can be used in the transesterication reaction are methanol, ethanol, propanol, butanol, and amyl alcohol. Methanol and ethanol are used most frequently; ethanol is a preferred alco- hol in the transesterication process compared to methanol because it is derived Vegetable Oils Direct Use Conversion to Biodiesel by Alcoholysis Conversion to Hydrocarbons by Decarboxylation Dilution with Hydrocarbons FIGURE 4.3 Use of vegetable oils as petroleum alternative fuels. © 2009 by Taylor & Francis Group, LLC 48 Handbook of Plant-Based Biofuels from agricultural products and is renewable and biologically less objectionable in the environment. However, methanol is preferred because of its low cost and its physical and chemical advantages (polar and shortest chain alcohol). Methyl, ethyl, 2-propyl, and butyl esters have been prepared from canola and lin- seed oils through transesterication using KOH and/or sodium alkoxides as catalysts. In addition, (m)ethyl esters were prepared from rapeseed and sunower oils using the same catalysts. Today, biodiesel is made from a variety of natural oils. Important among these are soybean oil and rapeseed oil. Rapeseed oil, a close cousin of canola oil, dominates the growing biodiesel industry in Europe. In the United States, biod- iesel is being made from soybean oil because more soybean oil is produced than all other sources of fats and oil combined. There are many candidates for feedstocks, including recycled cooking oils, animal fats, and a variety of other oilseed crops. For the preparation of biodiesel using a catalytic method, the catalyst (KOH) is dissolved into methanol by vigorous stirring in a small reactor. The oil is transferred into the biodiesel reactor and then the catalyst/alcohol mixture is pumped into the oil. The nal mixture is stirred vigorously for 2 h at 340 K at ambient pressure. A successful transesterication reaction produces two liquid phases: ester and crude glycerin. The crude glycerin, the heavier liquid, settles at the bottom after a few hours of settling. The phase separation can be observed within 10 minutes and can be complete within 2 h of the settling. Complete settling can take as long as 20 h. After the settling is complete, water is added at the rate of 5.5%, v/v of the methyl ester of oil and then stirred for 5 minutes and the glycerin is allowed to settle again. Washing the ester is a two-step process, which is carried out with extreme care. A water wash solution at the rate of 28% by volume of oil and 1 g of tannic acid per liter of water is added to the ester and gently agitated. Air is carefully introduced into the aqueous layer while simultaneously stirring very gently. This process is continued until the ester layer becomes clear. After settling, the aqueous solution is drained and water alone is added at 28% by volume of the oil for the nal washing (Demirbas 2002). Figure 4.4 shows the catalytic biodiesel production diagram. The physical characteristics of the fatty acid (m)ethyl esters are very close to those of diesel fuel and the process is relatively simple. Furthermore, the (m)ethyl esters of the fatty acids can be burned directly in unmodied diesel engines, with very low deposit formation. The methyl and ethyl esters prepared from a particular vegetable oil have similar viscosities, cloud points, and pour points, whereas methyl, ethyl, 2-propyl, and butyl esters derived from a particular vegetable oil have similar gross heating values. However, their densities, which were 2 to 7% higher than those Biodiesel Methanol or Ethanol Catalyst Vegetable Oil Transesterification Reactor Glycerin FIGURE 4.4 Catalytic biodiesel production. © 2009 by Taylor & Francis Group, LLC Production of Biofuels with Special Emphasis on Biodiesel 49 of diesel fuels, statistically decreased in the order 2-propyl > ethyl > butyl esters. The higher heating values of the biodiesel fuels, on a mass basis, are 9 to 13% lower than No. 2 petroleum diesel. The viscosities of biodiesel fuels are twice that of No. 2 petro- leum diesel. The cloud and pour points of No. 2 petroleum diesel are signicantly lower than those of biodiesel fuels. The biodiesel fuels produced slightly lower power and torque and higher fuel consumption than No. 2 petroleum diesel. Biodiesel is an efcient, clean, 100% natural energy alternative to the petro- leum fuels. Among the many advantages of biodiesel fuel include the following: it is safe for use in all the conventional diesel engines; offers the same performance and engine durability as petroleum diesel fuel; is nonammable and nontoxic; and reduces tailpipe emissions, visible smoke, and noxious fumes and odors. It is better than diesel fuel in terms of the sulfur content, ash point, aromatic content, and biodegradability (Demirbas 2003). If biodiesel is valorized efciently for energy purposes, it would benet the environment and the local population through job creation, provision of modern energy carriers to rural communities, avoiding urban migration and reduction of CO 2 and sulfur levels in the atmosphere. 4.4 NONCATALYTIC TRANSESTERIFICATION WITH SUPERCRITICAL ALCOHOL TRANSESTERIFICATION In the conventional transesterication of animal fats and vegetable oils for biodiesel production, the free fatty acids and water always produce negative effects, since the presence of the free fatty acids and the water causes soap formation, consumes catalyst and reduces catalyst effectiveness, all of which result in a low conversion (Komers, Machek, and Stloukal 2001). The transesterication reaction may be car- ried out using either basic or acidic catalysts, but these processes are relatively time consuming and require the complicated separation of the product and the catalyst, which results in high production costs and energy consumption. In order to overcome these problems, Saka and Kusdiana (2001) and Demirbas (2002, 2003) proposed that biodiesel fuels may be prepared from vegetable oil via noncatalytic transesterica- tion with supercritical methanol (SCM). The SCM is believed to solve the problems associated with the two-phase nature of normal methanol/oil mixtures by forming a single phase as a result of the lower value of the dielectric constant of the methanol in the supercritical state. As a result, the reaction is completed in a very short time (Han, Cao, and Zhang 2005). Compared with catalytic processes under barometric pressure, the SCM process is noncatalytic, purication of the products is much sim- pler, requires lower reaction time and lower energy, and is more environmentally friendly. However, the reaction requires temperatures of 525 to 675 K and pressures of 35 to 60 MPa (Demirbas 2003; Kusdiana and Saka 2001). Supercritical transesterication is carried out in a high-pressure reactor (auto- clave). In a typical run, the autoclave is charged with a given amount of the vegetable oil and liquid methanol with changed molar ratios. The autoclave is supplied with heat from an external heater, and the power is adjusted to give an approximate heat- ing time of 15 min. The temperature of the reaction vessel can be measured with an iron-constantan thermocouple and controlled at ±5 K for 30 min. The transesteri- cation occurs during the heating period. After each run, the gas is vented, and the © 2009 by Taylor & Francis Group, LLC 50 Handbook of Plant-Based Biofuels autoclave is poured into a collecting vessel. All the contents are removed from the autoclave by washing with methanol. Table 4.1 shows critical temperatures and criti- cal pressures of various alcohols. Table 4.2 shows comparisons between the catalytic methanol method and supercritical methanol method for the production of biodiesel from vegetable oils by transesterication. 4.5 TRANSESTERIFICATION REACTION MECHANISM OF VEGETABLE OIL Transesterication consists of a number of consecutive, reversible reactions. The triglyceride is converted stepwise to diglyceride, monoglyceride, and nally glyc- erol. The formation of alkyl esters from monoglycerides is believed to be a step that determines the reaction rate, because monoglycerides are the most stable inter- mediate compound (Ma and Hanna 1999). Several aspects, including the type of catalyst (alkaline, acid, or enzyme), alcohol to vegetable oil molar ratio, temperature, water content, and free fatty acid content, have an inuence on the course of the transesterication. In the transesterication of vegetable oils and fats for biodiesel TABLE 4.2 Comparisons between the Catalytic Methanol (MeOH) method and Supercritical Methanol (SCM) Method for the Production of Biodiesel from Vegetable Oils by Transesterification Method Catalytic MeOH Process SCM Method Methylating agent Methanol Methanol Catalyst Alkali (NaOH or KOH) None Reaction temperature (K) 303–338 523–573 Reaction pressure (MPa) 0.1 10–25 Reaction time (min) 60–360 7–15 Methyl ester yield (wt%) 96 98 Removal for purication Methanol, catalyst, glycerol, soaps Methanol Free fatty acids Saponied products Methyl esters, water Smelling from exhaust Soap smelling Sweet smelling TABLE 4.1 Critical Temperatures and Critical Pressures of Various Alcohols Alcohol Critical Temperature (K) Critical Pressure (MPa) Methanol 512.2 8.1 Ethanol 516.2 6.4 1-Propanol 537.2 5.1 1-Butanol 560.2 4.9 © 2009 by Taylor & Francis Group, LLC Production of Biofuels with Special Emphasis on Biodiesel 51 production, free fatty acids and water always produce negative effects, as the pres- ence of free fatty acids and water causes soap formation, consumes catalyst, and reduces catalyst effectiveness, all of which result in a low conversion (Ali, Hanna, and Cuppett 1995). The transesterication is an equilibrium reaction and the trans- formation occurs essentially by mixing the reactants. In the transesterication of the vegetable oils, a triglyceride reacts with an alcohol in the presence of a strong acid or base, producing a mixture of fatty acids, alkyl esters, and glycerol. The stoichio- metric reaction requires 1 mol of a triglyceride and 3 mol of the alcohol. However, an excess of the alcohol is used to increase the yields of the alkyl esters and to allow its phase separation from the glycerol formed (Bala 2005). A reaction mechanism of the vegetable oil in the SCM was proposed based on the mechanism developed by Krammer and Vogel (2000) for the hydrolysis of the esters in sub- or supercritical water. The basic idea of supercritical treatment is the effect of the pressure and temperature on the thermophysical properties of the sol- vent, such as dielectric constant, viscosity, specic weight, and polarity (Kusdiana and Saka, 2001). 4.6 VARIABLES AFFECTING METHYL ESTER YIELD The most important variables affecting the methyl ester yield during the transesteri- cation reaction are molar ratio of the alcohol to vegetable oil and the reaction tem- perature. The viscosities of the methyl esters from the vegetable oils were slightly higher than that of No. 2 diesel fuel. Figure 4.5 shows a typical example of the rela- tionship between the reaction time and the temperature (Demirbas 2002). A hazelnut kernel oil sample was used; the critical temperature and the critical pressure of meth- anol were 512.4 K and 8.0 MPa, respectively. The variables affecting the ester yield during the transesterication reaction were molar ratio of the alcohol to the vegetable 0 20 40 60 80 100 0 100 200 300400 Reaction Time (sec) Yield of Methyl Ester, wt% 450 K 493 K 503 K 513 K 523 K FIGURE 4.5 Changes in yield percentage of methyl esters as treated with subcritical and supercritical methanol at different temperatures as a function of reaction time. Molar ratio of vegetable oil to methyl alcohol: 1:41. Sample: hazelnut kernel oil. © 2009 by Taylor & Francis Group, LLC 52 Handbook of Plant-Based Biofuels oil, reaction temperature, reaction time, water content, and catalyst. It was observed that increase in the reaction temperature, especially to supercritical temperatures, had a favorable inuence on the ester conversion (Demirbas 2002). The stoichiomet- ric ratio for the transesterication reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of fatty acid ester and one mole of glycerol. Higher molar ratios result in higher ester production in a shorter time. As has been mentioned above, in the catalyzed methods, the presence of water has negative effects on yields of methyl esters. However, the presence of water posi- tively affected the formation of methyl esters in our supercritical methanol method. Figure 4.6 shows the plots for yields of methyl esters as a function of water content in the transesterication of triglycerides. Most diesel engines are designed to use highly lubricating, high sulfur content fuel. Recent environmental legislature has forced diesel fuel to contain only a mini- mum amount of the sulfur for lubricating purposes. Thus, the slightly higher viscos- ity of biodiesel is helpful and lubricating to most diesel motors. Compared to No. 2 diesel fuel, all of the vegetable oils are much more viscous, much more reactive to oxygen, and possess higher cloud point and pour point temperature. The ash point of all vegetable oils is far above that of diesel fuel, reecting the nonvolatile nature of the vegetable oils. Vegetable oils are not directly volatile, but crack during distil- lation into a series of hydrocarbons or can be converted by transesterication into more volatile methyl esters. 4.7 COMPARISON OF FUEL PROPERTIES AND COMBUSTION CHARACTERISTICS OF METHYL AND ETHYL ALCOHOLS AND THEIR ESTERS In general, the physical and chemical properties and the performance of the ethyl esters are comparable to those of the methyl esters. The methyl and ethyl esters have almost the same heat content. The viscosities of the ethyl esters are slightly higher and the cloud and pour points are slightly lower than those of the methyl esters. 0 012345 20 40 60 80 100 Water Content, % Methyl Ester, % Supercritical methanol Alkaline catalyst Acid catalyst FIGURE 4.6 Plots for yields of methyl esters as a function of water content in transesteri- cation of triglycerides. © 2009 by Taylor & Francis Group, LLC Production of Biofuels with Special Emphasis on Biodiesel 53 Engine tests demonstrate that methyl esters produce slightly higher power and torque than the ethyl esters (Bala 2005). Some desirable attributes of the ethyl esters over methyl esters are: they have signicantly lower smoke opacity, lower exhaust tem- peratures, and lower pour point. The ethyl esters tend to have more injector coking than the methyl esters. 4.8 BIODIESEL ECONOMY The cost of the biodiesel fuels varies depending on the base stock, geographic area, variability in the crop production from season to season, the price of the crude petro- leum, and other factors. Biodiesel has over double the price of diesel. The high price of biodiesel is largely due to the high price of the feedstock. However, biodiesel can be made from other feedstocks, including beef tallow, pork lard, and yellow grease, which are relative cheaper raw materials. Cooking oils can also be used as cheaper raw materials. The problem with processing these low-cost oils and fats is that they often contain large amounts of free fatty acids (FFA) that cannot be converted to biodiesel using an alkaline catalyst. A review of twelve economic feasibility studies shows that the projected costs for biodiesel from oilseed or animal fats have a range of US$0.30 to $0.69/l, includ- ing the meal and glycerin credits and the assumption of reduced capital investment costs by having the crushing and/or esterication facility added onto an existing grain or tallow facility. Rough projections of the cost of biodiesel from vegetable oil and waste grease are US$0.54 to $0.62/l and US$0.34 to $0.42/l, respectively. With pretax diesel priced at US$0.18/l in the United States and US$0.20 to $0.24/l in some European countries, biodiesel is thus currently not economically feasible, and more research and technological development are needed. 4.9 CONCLUSIONS AND FUTURE PERSPECTIVES The biodiesels are the mono-alkyl esters of long chain fatty acids derived from renewable feedstocks, such as vegetable oils or animal fats, for use in diesel engines. The purpose of the transesterication of vegetable oils to their methyl esters is to lower the viscosity of the oil. The main factors affecting transesterication are molar ratio of glycerides to alcohol, catalyst, reaction temperature and pressure, reaction time, and the contents of free fatty acids and water in oils. Vegetable oils are a renewable and potentially inexhaustible source of energy, with an energy content close to that of diesel fuel. The vegetable oil fuels are not feasible because they are more expensive than petroleum fuels. However, with recent increases in petroleum prices and the uncertainties concerning petroleum availabil- ity, there is renewed interest in vegetable oil fuels for diesel engines. REFERENCES Ali, Y., M. A. Hanna, and S. L. Cuppett. 1995. Fuel properties of tallow and soybean oil esters. JAOCS 72:1557–1564. © 2009 by Taylor & Francis Group, LLC 54 Handbook of Plant-Based Biofuels Bala, B. K. 2005. Studies on biodiesels from transformation of vegetable oils for diesel engines. Energy Edu. Sci. Technol. 15:1–43. Demirbas, A. 2002. Biodiesel from vegetable oils via transesterication in supercritical methanol. Energy Convers. Mgmt. 43:2349–2356. Demirbas, A. 2003. Biodiesel fuels from vegetable oils via catalytic and non-catalytic super- critical alcohol transesterications and other methods: A survey. Energy Convers. Mgmt. 44:2093–2109. Demirbas, A. 2005. Biodiesel production from vegetable oils by supercritical methanol. J. Sci. Ind. Res. 64:858–865. Demirbas, A. 2006. Global biofuel strategies. Energy Edu. Sci. Technol.17:32-63. Han, H., W. Cao, and J. Zhang. 2005. Preparation of biodiesel from soybean oil using super- critical methanol and CO 2 as co-solvent. Process Biochemistry 40:3148–3151. Komers, K., J. Machek, and R. Stloukal. 2001. Biodiesel from rapeseed oil and KOH. II. Composition of solution of KOH in methanol as reaction partner of oil. Eur. J. Lipid Sci. Technol. 103:359–362. Krammer, P., and H. Vogel. 2000. Hydrolysis of esters in subcritical and supercritical water. Supercrit. Fluids 16:189–206. Kusdiana, D., and S. Saka. 2001. Kinetics of transesterication in rapeseed oil to biodiesel fuels as treated in supercritical methanol. Fuel 80:693–698. Ma, F., and M. A. Hanna. 1999. Biodiesel production: A review. Biores. Technol. 70:1–15. Madras, G., C. Kolluru, and R. Kumar. 2004. Synthesis of biodiesel in supercritical uids. Fuel 83:2029–2033. Saka, S., and D. Kusdiana. 2001. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 80:225–231. © 2009 by Taylor & Francis Group, LLC . by Taylor & Francis Group, LLC 46 Handbook of Plant- Based Biofuels and biological routes. Biofuel is a nonpolluting, locally available, accessible, sustain- able, and reliable fuel obtained. Dilution with Hydrocarbons FIGURE 4. 3 Use of vegetable oils as petroleum alternative fuels. © 2009 by Taylor & Francis Group, LLC 48 Handbook of Plant- Based Biofuels from agricultural products. function of reaction time. Molar ratio of vegetable oil to methyl alcohol: 1 :41 . Sample: hazelnut kernel oil. © 2009 by Taylor & Francis Group, LLC 52 Handbook of Plant- Based Biofuels oil,