The Ideal Vegetable Oil-based Biodiesel Composition: A Review of

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The Ideal Vegetable Oil-based Biodiesel Composition: A Review of Social, Economical and Technical Implications S. Pinzi,† I. L. Garcia,† F. J. Lopez-Gimenez,‡ M. D. Luque de Castro,§ G. Dorado,| and M. P. Dorado*,† Department of Chemical Physics and Applied Thermodynamics, EPS, Edificio Leonardo da Vinci; Department of Agricultural Engineering, ETSIAM, Edificio Leonardo da Vinci; Department of Analytical Chemistry, Edificio Marie Curie; and Department Bioquı´mica y Biologıá Molecular, Campus Rabanales, C6-1-E17, UniVersidad de Co´rdoba, 14071 Co´rdoba, Spain ReceiVed December 15, 2008. ReVised Manuscript ReceiVed April 6, 2009

Though a considerable number of publications about biodiesel can be found in literature, several problems remain unsolved, encompassing economical, social, and technical issues. Thus, the biodiesel industry has come under attack by some environmental associations, and subsidies for biofuel production have been condemned by some governments. Yet, biodiesel may represent a truly competitive alternative to diesel fuel, for which fuel tax exemption and subsidies to energetic crops are needed. Biodiesel must increase its popularity among social movements and governments to constitute a valid alternative of energy source. In this sense, the use of nonedible oils to produce biodiesel is proposed in the present review. Moreover, the compromise of noninterference between land for energetic and food purposes must be addressed. Concerning technical issues, it is important to consider a transesterification optimization, which is missing or incomplete for too many vegetable oils already tested. In most cases, a common recipe to produce biodiesel from any raw material has been adopted, which may not represent the best approach. Such strategy may fit multifeedstock biodiesel plant needs but cannot be accepted for oils converted individually into biodiesel, because biodiesel yield will most likely fail, increasing costs. Transesterification optimization results depend on the chemical composition of vegetable oils and fats. Considering “sustainable” vegetable oils, biodiesel from Calophyllum inophyllum, Azadirachta indica, Terminalia catappa, Madhuca indica, Pongamia pinnata, and Jatropha curcas oils fits both current biodiesel standards: European EN 14214 and US ASTM D 6751 02. However, none of them can be considered to be the “ideal” alternative that matches all the main important fuel properties that ensure the best diesel engine behavior. In search of the ideal biodiesel composition, high presence of monounsaturated fatty acids (as oleic and palmitoleic acids), reduced presence of polyunsaturated acids, and controlled saturated acids content are recommended. In this sense, C18:1 and C16:1 are the best-fitting acids in terms of oxidative stability and cold weather behavior, among many other properties. Furthermore, genetic engineering is an invaluable tool to design oils presenting the most suitable fatty acid profile to provide high quality biodiesel. Finally, most published research related to engine performance and emissions fails in using a standard methodology, which should be implemented to allow the comparison between tests and biofuels from different origin. In conclusion, a compromise between social, economical, and technical agents must be reached.

1. Introduction Nowadays, the depletion of fossil fuel reserves and the necessity to reduce CO2 emissions in order to limit global warming are leading the research on alternative sources of energy. Among these alternatives are biofuels for internal combustion engines.1 In recent years, biofuel research has been directed mainly to explore plant-based fuels: that is, fatty acid methyl esters (FAME) of seed oils, and in some cases, fats.2 FAME, also known as biodiesel, is environmentally less contaminating, * Corresponding author. Phone: +34 957 218332; fax: +34 957 218417; e-mail: [email protected]. † Department of Chemical Physics and Applied Thermodynamics. ‡ Department of Agricultural Engineering. § Department of Analytical Chemistry. | Department Bioquı´mica y Biologıá Molecular. (1) Luque, R.; Herrero-Davila, L.; Campelo, J. M.; Clark, J. H.; Hidalgo, J. M.; Luna, D.; Marinas, J. M.; Romero, A. A. Energy EnViron. Sci. 2008, 1, 542–564. (2) Dorado, M. P. In Biofuels Refining and Performance; Nag, A., Ed.; McGraw Hill Professional: 2008; pp 107-148.

nontoxic, and biodegradable compared to diesel fuel.3 The usual raw materials being exploited commercially to produce biodiesel consist of edible fatty oils derived from rapeseed, soybean, palm, sunflower, and other plants. However, biodiesel from edible oils is controversial. Some nongovernmental organizations (NGO) and social movements (e.g., the Global Forest Coalition, among others), pinpoint the making of biofuels from edible raw materials as the main cause of increased global food market prices. Another claim against the use of biofuels is the depletion of ecological resources due to the intensive agricultural practices in the crop cultivation. Although international authorities such as the Food and Agriculture Organization of the United Nations (FAO) and the Austrian Biofuels Institute (ABI), among others, provide figures to demonstrate the small and nonsignificant aftereffect of biofuels in global economy,4 the focus must be (3) Azam, M.; Waris, A.; Nahar, N. M. Biomass Bioenergy 2005, 29, 293–302. (4) Bergsma, G.; Kampman, B.; Croezen, H.; Sevenster, M. Biofuels and their global influence on land aVailability for agriculture and nature; Delft: CE, 2006.

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Energy & Fuels, Vol. 23, 2009 Scheme 1. Transesterification Process

based biodiesel, together with some ethical, social, and economical considerations is presented. According to the conclusions of these sections, some nonedible low-cost vegetable oils with high potential to be used to produce biodiesel are recommended. Because of the limitations of the proposed vegetable oils, the influence of free fatty acids (FFA) content in biodiesel production are discussed, as well as potential modifications by genetic engineering. Finally, a comparative study concerning the best-fitting raw materials to produce biodiesel is presented. 2. The Ideal Chemical Structure of Biodiesel

put on nonedible oils instead of edible ones, to gain social acceptance of biodiesel. These arguments are not contradictory to the last annual report on the state of food and agriculture (2008), where FAO warns that policies concerning biofuels production should aim at the equal distribution of benefits between rich, developing, and poor countries.5 In this way, an increase in biofuels demand could help the rural development of less-favored countries. The FAO report concluded that both farm subsidies of biofuels and trade barriers that create artificial markets that benefit producers belonging to the Organization for Economic Co-operation and Development (OECD) countries at the expense of producers in developing countries should be removed. Another main concern in further usage of biodiesel is the economic viability. Several studies have identified that the price of feedstock oils is by far one of the most significant factors affecting the economic viability of biodiesel manufacturing.6-8 Approximately 70-95% of the total biodiesel production cost arises from the raw material.7 Therefore, to produce a competitive biodiesel, the feedstock price is a key factor that needs to be taken into consideration.2 It has been shown that biodiesel quality depends on fatty acid composition of raw materials (oils or fats). A biodiesel reaction is depicted in Scheme 1. Among the main fuel specifications related to chemical composition are cetane number; kinematic viscosity; oxidative stability; cold-flow properties in the form of cloud point (CP), pour point (PP), and cold-filter plugging point (CFPP); exhaust emissions; lubricity; and heat of combustion.9 Other parameters influenced by fatty acid composition are conversion rate of FAME and optimal amount of reagents involved in the transesterification reaction. Those parameters are also important in terms of economic viability of biodiesel production.2 According to these reasons, the use of nonedible, low-cost, and sustainable feedstocks compatible with a good quality of biodiesel should become a primary research target for the scientific community, thus facilitating the acceptance of biodiesel by both customers and vehicle manufactures. For this reason, the aim of this work is to provide a review about the main achievements concerning the influence of the chemical composition of biodiesel on several fuel topics (i.e., fuel properties, engine performance, etc.), focused on low-cost and nonedible oils. An approach to the biodiesel ideal chemical composition is also proposed. The ideal chemical structure of vegetable oils(5) FAO The state of food and agriculture (SOFA) 2008; Food and Agriculture Organization of the United Nations (FAO): Rome, 2008. (6) Dorado, M. P.; Cruz, F.; Palomar, J. M.; Lopez, F. J. Renewable Energy 2006, 31, 1231–1237. (7) Krawczyk, T. In International News on Fats, Oils and Related Materials; American Oil Chemists Society Press: Champaign, IL, 1996; Vol. 7, p 801. (8) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240. (9) Knothe, G. Energy Fuels 2008, 22, 1358–1364.

The influence of the chemical structure of fatty acids on biodiesel quality has been demonstrated.9-13 In this section, the optimal fatty acid profile of low-cost and nonedible vegetable oils, to be considered suitable and sustainable raw materials for biodiesel production, is discussed. According to this, main fuel properties are provided and analyzed, as shown in Table 1. 2.1. Iodine Value (IV). This parameter reflects total unsaturation regardless of the relative proportion of mono-, di-, tri-, and polyunsaturated compounds. In this sense, several authors have determined IV as a function to fatty acid profile.3,14 Results reveal that a high IV has been linked with low oxidation stability, causing the formation of various degradation products, which can negatively affect engine operability by forming deposits on engine nozzles, piston rings, and piston ring grooves. The effects of oxidative degradation represent a legitimate concern in terms of maintaining fuel quality of biodiesel.15 Biodiesel may oxidize more rapidly than conventional diesel fuel, particularly when the former is produced from highly unsaturated sources. The oxidation rate of biodiesel can be influenced by many factors, including temperature and chemical composition. The influence of fatty acid composition on biodiesel oxidation rate is higher than the influence of environmental conditions such as air, light, and the presence of metals.16 Monounsaturated fatty acid methyl esters (such as C18:1) are considered to be better than poly unsaturated ones (such as methyl linoleate (C18:2) and C18:3) in terms of oxidation stability, without any adverse effect on fuel cold properties.17 In particular, the number and position of double bonds in fatty acid esters affect the rate of oxidation.18 According to literature, the rates of oxidation have a relative value of 1 for oleates like methyl esters (ME) and ethyl esters (EE), 41 for linoleates, and 98 for linolenates. Small amounts of more highly unsaturated fatty compounds containing bis-allylic carbons have a significant strong effect on oxidative stability. In the case of lubricating oil dilution, highly unsaturated esters present in engine oil are suspected of forming highmolecular compounds, which may reduce the lubricating quality.19 The IV limit of 120 set by the European biodiesel standard (EN 14214) excludes several promising oil sources such as (10) Knothe, G. Fuel Process. Technol. 2005, 86, 1059–1070. (11) Canakci, M.; Sanli, H. J. Ind. Microbiol. Biotechnol. 2008, 35, 431– 441. (12) Harrington, K. J. Biomass 1986, 9, 1–17. (13) Ramos, M. J.; Fernańdez, C. M.; Casas, A.; Rodrı´guez, L.; Pe´rez, ´ . Bioresour. Technol. 2009, 100, 261–268. A (14) Schober, S.; Mittelbach, M. Lipid Technol. 2007, 19, 281–285. (15) Dunn, R. O. J. Am. Oil. Chem. Soc. 2002, 79, 915–920. (16) Knothe, G.; Dunn, R. O. J. Am. Oil. Chem. Soc. 2003, 72, 1155– 1160. (17) Imahara, H.; Minami, E.; Saka, S. Fuel 2006, 85, 1666–1670. (18) Durrett, T. P.; Benning, C.; Ohlrogge, J. Plant J. 2008, 54 (4), 593–607. (19) Knothe, G. Fuel Process. Technol. 2007, 88, 669–677.

EN 14214; EN 14103

polyunsaturated FA

it increases (+++)

it increases (++)

low increase (+)

reactivity slightly decreases during esterification using supercritical methanol (-)

it increases less than the presence of monounsaturated FA do (++) it decreases less than the presence of monounsaturated FA do

Low CN (--) it increases more than with the presence of monounsaturated FA (+++)

Reactivity decreases using heterogeneous catalyst (--)

polynomial second order relationship1 µ ) 1.16E - 4M2 -0.0264M + 2.28

there is nonproportional relation it decreases (-)

it decreases (--)

OS decreases more Dramatically decrease than presence of stability (---) monounsaturated FA do(---) Most important factor: decrease CN. (---) It increases with the number of sequential CH2 (++)

OS (-) decreases with unsaturation

chain length

fatty acid with hydrogen groups increases µ

it decreases with C12:0 and C16:0

other factors

HHV ) 49.43 - 0.041SN - 0.015IV HHV ) 79.014 43.12d1

inversely proportional to CN

CN ) 46.3 + 5458/SN 0.225IV. It depends on density (+) in a second order polynomial function

interaction between biodiesel properties1

ref

36, 37

29, 143, 144

35, 142

10, 17

27

26, 40

141

17-19

3, 14

1 SN: saponification number; d: density; M: molecular weight (g/mol); +: low direct correlation between biodiesel property and fatty acid profile; ++: medium direct correlation; +++: significant direct correlation; -: low inverse correlation; --: medium inverse correlation; ---: significant inverse correlation.

methyl ester content

heat of combustion (HHV)

kinematic viscosity (µ)

low-temperature flow properties (CP and PP)

PM emissions

diunsaturated and triunsaturated FA

directly dependent (++) each double bond can add 253.8 g of iodine

monounsaturated fatty acids (FA)

EN 14214; nonlinear decrease with EN ISO 3104 unsaturation. First double bond presents higher effect

cetane number (CN)

NOx emissions

EN 14214; EN 5165

oxidation stability (OS)

standard

EN 14214; EN 14111 EN 14214; EN 14112

iodine value (IV)

property

Table 1. Influence of Oil Fatty Acid Profile on Biodiesel Properties degree of unsaturation

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sunflower and some nonedible low-cost oily crops. In fact, Schober and Mittelbach state that this limit cannot be argued as a suitable limit to describe or avoid the initial concerns about problems resulting from oxidative degradation.14 They state that IV itself is not the most suitable parameter to express biofuel stability, because it cannot weigh the significant difference in oxidative stability between mono-, di- and triunsaturated esters. As a matter of fact, it seems that parameters such as oxidation stability, linolenic acid ester content, and polyunsaturated esters content are better indicators of degradation tendencies.20 2.2. Cetane Number (CN). This parameter gives a measurement of the combustion quality during ignition. It provides information about the ignition delay (ID) time of a diesel fuel upon injection into the combustion chamber. Fuels with low CN tend to cause diesel knocking and show increased gaseous and particulate exhaust emissions (PM), due to incomplete combustion.21 Moreover, excessive engine deposits are reported. In general, biodiesel has higher CN values than fossil fuel, which is considered to be a significant advantage in terms of engine performance and emissions, allowing biodiesel-fuelled engines to run more smoothly and with less noise.22 Long ID times, with low CNsand subsequent poor combustionshave been associated to FAME with highly unsaturated components such as linoleic (C18:2) and linolenic (C18:3) acid esters. High CN values have been observed in saturated fatty acid esters, such as palmitic (C16:0) and stearic (C18:0) acid esters. Generally, the higher the chain length, the higher the CN value.12,23 Also, the more sequential CH2 groups in the fatty compound, the higher the CN. Knothe et al.24 studied the effect of the structure (branching, unsaturation, and length) of the fatty acid chain and the alcohol moiety on the CN of biodiesel. Such authors stated that the level of unsaturation of the fatty acid chains is the most significant factor causing lower CN. Also, results have shown that one saturated, long straight chain in a fatty ester suffices to provide a high CN. According to engine exhaust emissions, higher CN is correlated with reduced nitrogen oxides (NOx),25 although this may not always hold for all types of engine technologies.24 The connection between the structure of fatty acid esters and exhaust emissions was investigated by studying enriched fatty acid alkyl esters as fuel and using different vegetable oil esters with a wide range of iodine numbers.26 The NOx exhaust emissions reportedly increase with lower saturation and decreasing chain length, which can also lead to a connection with the CN of these compounds.27 Peterson et al.28 found that fatty acids with two double bonds had more effect on increasing NOx emissions than those with one double bond. On the other hand, PM was not influenced by chain length, but the higher reductions were found using methyl laurate and methyl palmitate.27 Changes in carbon monoxide (CO) and hydrocarbons (HC) could not be linearly correlated with unsaturation.28 (20) Mittelbach, M. Bioresour. Technol. 1996, 56, 7–11. (21) Mittelbach, M.; Remschmidt, C. Biodiesel: The ComprehensiVe Handbook; Martin Mittelbach: Graz, Austria, 2004. (22) Knothe, G.; Matheaus, A. C.; Ryan, T. W. Fuel 2003, 82, 971– 975. (23) Klopfenstein, W. J. Am. Oil. Chem. Soc. 1985, 62, 1029–1031. (24) Knothe, G.; Matheaus, A. C.; Ryan, T. W. Fuel 2003, 82, 971– 975. (25) Ladommatos, N.; Parsi, M.; Knowles, A. Fuel 1996, 75, 8–14. (26) Peterson, C. L.; Taberski, J. S.; Thompson, J. C.; Chase, C. L. Trans. ASAE 2000, 43, 1371–1381. (27) Knothe, G.; Sharp, C. A.; Ryan, T. W. Energy Fuels 2006, 20, 403–408. (28) Peterson, C.; Reece, D. Trans. ASAE 1996, 39, 805–816. (29) Freedman, B.; Bagby, M. O. J. Am. Oil. Chem. Soc. 1989, 66, 1601– 1605.

2.3. Gross Calorific Value (Higher Heating Value) and Net Calorific Value (Lower Heating Value). These fuel properties indicate the suitability of fatty compounds as diesel fuel. Due to higher oxygen content, FAME exhibit lower heating values than fossil diesel. So, to achieve adequate engine torque and power, an increasing of injection volumes is needed.21 However, this leads to higher specific fuel consumptions. Calorific value is not included in most fuel standards, but it is a limiting parameter within the European standard for FAME used as heating fuels (EN 14213). Freedman and Bagby29 developed a model to predict heating values from different fatty acids composition. Generally, the higher the chain length (number of carbons and hydrogens in FAME molecules), the higher the heating value.10 The increase in the ratio of these elements relative to oxygen also results in a heat content increase. A decrease in heat content is the result of fewer hydrogen atoms (i.e., higher unsaturation) in the fuel molecule. Therefore, from this point of view, oil sources with a high proportion of long-chain saturated compounds should be selected for transesterification.21 2.4. Brake-specific Fuel Consumption. BSFC is the ratio between mass fuel consumption and brake effective power, being inversely proportional to break thermal efficiency (BTE). Biodiesel-specific fuel consumption is expected to increase around 10-20% in relation to diesel fuel, since the loss of heating value of biodiesel must be compensated with higher fuel consumption. An indicator of the loss of heating value is the oxygen content in the fuel.30 Several researchers found a correlation between BSFC and oxygen content, concluding that the increase in BSFC is due to the oxygen enrichment from the fuel, but not from the air intake.31,32 2.5. Cold Weather Performance. One of the major problems associated with the use of biodiesel is poor flow properties at low temperatures.10 Partial solidification in cold weather may cause blockages of fuel lines and filters, leading to fuel starvation and problems during engine start-up.21 Provided that long-chain saturated fatty esters significantly increase CP and PP, reducing saturated fatty acid content of vegetable oils can improve cold temperature flow properties of biodiesel. To improve cold temperature flow characteristics of biodiesel, several proposals have been suggested, including winterization, additives, esterification with branched alcohols, and modification of oil chemical composition. Several authors have stated that the cheapest and more effective way to improve the lowtemperature flow properties of biodiesel is the optimization of fatty acid composition of the raw material.18,33,34 With this aim, Imahara et al.17 developed a prediction model to estimate CP of biodiesel from various oils/fats, providing a useful tool to determine optimal fatty acid methyl ester composition. They observed that CP depends mostly on saturated ester content, while the effect of unsaturated ester composition could be negligible. 2.6. Kinematic Viscosity. Fuel viscosity impacts on both injection and combustion efficiency. Higher viscosity leads to (30) Lapuerta, M.; Armas, O.; Rodrı´guez-Fernańdez, J. Prog. Energy Combust. Sci. 2008, 34, 198–223. (31) Rakopoulos, Ct. D.; Hountalas, D. T.; Zannis, T. C.; Levendis, Y. A. SAE Paper 2004-01-2924; 2004. (32) Graboski, M. S.; Ross, J. D.; McCormick, R. L. SAE Paper 961166; 1996. (33) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil. Chem. Soc. 1996, 73, 631–636. (34) Lee, I.; Johnson, L. A.; Hammond, E. G. J. Am. Oil. Chem. Soc. 1995, 72, 1155–1160. (35) Allen, C. A. W.; Watts, K. C.; Ackman, R. G.; Pegg, M. J. Fuel 1999, 78, 1319–1326.

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a higher drag in the injection pump, causing higher pressures and injection volumes, especially at low engine operating temperatures. As a direct consequence, the timing for fuel injection and ignition tends to be slightly advanced for biodiesel, which might in turn lead to increased NOx emissions due to higher maximum combustion temperatures.21 Allen et al.35 developed a method for predicting the kinematic viscosity of biodiesel from its fatty acid composition. Results showed that for saturated fatty acid esters, viscosity increased with carbon number in a curvilinear trend, rather than linear. Indeed, for unsaturated C18 esters, they observed a nonlinear decrease in viscosity, while increasing number of double bonds. The most significant effect was found for the first unsaturation. Contamination with small amounts of glycerides significantly increased biodiesel viscosity.35 2.7. Mono-, Di-, and Triglycerides (TG) or Triacylglycerols (TAG) Content. Fuel exceeding the limits of mono-, di-, and triglycerides defined in the EN 14214 standard may cause formation of deposits to injector nozzles, pistons, and valves. Indirect hints at high glycerides contents in biodiesel are correspondingly increased values for viscosity and carbon residue.21 There are only few studies about the influence of fatty acid composition of vegetable oils on transesterification yield. Abreu et al.36 studied the effect of heterogeneous catalysts and found that the activity of metal complexes increases for short chains. It is worthy to highlight that their results indicated that both the saturation degree and the alkyl-chain length are determinant factors in the catalytic activity. Stavarache et al.37 established the relationship between yield of FAME during ultrasound-assisted transesterification and the composition of fatty acids from different vegetable oils. In fact, they found that saturated fatty acids that have a natural preference for first and third positions in triglycerides were transesterified mostly at the beginning of the reaction, while the amount of unsaturated fatty acids esters increased as the reaction progressed.38 Warabi et al.39 studied the alkyl esterification in supercritical alcohol and observed that saturated fatty acids, including palmitic and stearic acids, had slightly lower reactivity than unsaturated fatty acids (i.e., oleic, linoleic, and linolenic acids). In conclusion, given the antagonistic requirements between low-temperature flow characteristics and the oxidative stability, NOx emissions, and CN, there is strictly no fatty acid profile providing a fuel for which all these parameters are optimal.18 However, various studies have suggested that biodiesel with high levels of methyl oleate may have excellent, if not optimal, characteristics with regard to ignition quality, NOx emissions, fuel stability, flow properties at low temperature, and iodine number according to the standard EN 14214.10,13 Furthermore, it is expected that biodiesel with an average of 1.5 double bonds per molecule will produce an equivalent amount of NOx to conventional diesel fuel.40 Lastly, given that polyunsaturated fatty acids have a disproportionably large effect on the auto(36) Abreu, F. R.; Lima, D. G.; Hamu´, E. H.; Wolf, C.; Suarez, P. A. Z. J. Mol. Catal A: Chem. 2004, 209, 29–33. (37) Stavarache, C.; Vinatoru, M.; Maeda, Y. Ultrason. Sonochem. 2007, 14, 380–386. (38) Richards, A.; Wijesundera, C.; Palmer, M.; Salisbury, P. AOCS Australian Workshop: Sydney, 2002; p 29. (39) Warabi, Y.; Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 283–287. (40) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M.; Tyson, K. S. EnViron. Sci. Technol. 2001, 35, 1742–1747.

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oxidation of biodiesel,16 it is recommended to avoid their presence in TG to be used as raw material for biodiesel production. 3. Considerations about Biodiesel Produced from Vegetables Oils 3.1. Ethical Issues. The major obstacle for large-scale adoption of biodiesel from vegetable oils is the production of sufficient amounts of oilseed crops without significantly affecting food supply and cost. To reach this goal, researchers and industry have put a lot of effort proposing and developing alternative sources to produce biofuels. Although a large proportion of these efforts are focused on conversion of lignocellulosic feedstocks to ethanol (second-generation fuels), some strategies to design new crops to produce biodiesel are outlined below.18 It must be noted that there is currently a social controversy over biofuels produced from energy crops that are primarily used for feeding purposes. Yet, even in case the scientific community eventually finds an efficient technology to produce alcohol from agricultural pruning and forest residues that definitely will not compete with food, this will provide us with an insufficient quantity of biofuels. Plantations of trees to process biomass into alcohol may be needed, leading to the same social alarm caused by the use of energy crops (first-generation fuels) instead of crops for food. Increased demand for the production of edible oils for feeding purposes has put limitations on the use of these oils for biodiesel production. Some voices claim that edible oils are too important for human feeding to run vehicles.2 However, the FAO has calculated that 41.88 million km2 of land are available for agriculture, although just 15.06 million km2 are in use, and only 0.11 million km2 are used for biofuels production today, which is no more than 1% of that area. The FAO estimates that in 2030, 0.325 million km2 will be used for biofuels production, which is no more than 2% of total agricultural land use.41 Nevertheless, to avoid the use of edible crops to produce fuel, and the supposed subsequent increase of food price, nonedible crops developed in marginal lands could provide a sustainable option as biodiesel feedstocks. Biodiesel technology should not cause starvation in underdeveloped countries. In fact, it can have the opposite effect: it should be focused to help poor and developing countries to decrease their dependence to fossil oil imports, thus enhancing their Balance of Payments (BOP) and general welfare. However, it is important to mention again that either energy crops grown in marginal lands or biomass from forests to produce biofuels cannot nowadays provide the total amount of fuels required for the current high energy-dependent life. Therefore, to reduce the impact on climate change and other related problems including pollution, a change in the consumption habits is strongly recommended. 3.2. Economical Issues. One main concern in further usage of biodiesel is the economic viability of its production. A few years ago, biodiesel unit price was 1.5-3.0 times higher than that of petroleum-derived diesel fuel.11 But currently, due to the dramatic increase of crude mineral oil price, cost of biodiesel is not too far from diesel price (Table 2). Remarkably, in different countries, biodiesel price has always shown the same price as diesel fuel, even after the increase of fossil fuels price. The reason for this linearity is not clear, as other interests seem to control the market. (41) Konandreas, P.; Smithuber, J. Global Biofuel Production Trends and Possible Implication of Swaziland; Food and Agricultural Organization of United Nations: 2007.

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Manufacturing costs and raw feedstock prices are the main economic criteria to take into account for biodiesel production to compete with diesel fuel. Manufacturing costs include direct costs for oil extraction, reagents, operating supplies, and manpower, as well as indirect costs related to insurance and storage. For a complete analysis, fixed capital costs involved in the construction of processing plants and auxiliary facilities, distribution, and retailing must also be taken into consideration.6 In this sense, several studies have identified that the price of feedstocks is by far one of the most significant factors affecting the economic viability of biodiesel manufacture.6-8,42 In fact, approximately 70-95% of total biodiesel production cost arises from the cost of the raw material.42,43 Thus, to produce a competitive biodiesel, the feedstock price is a factor that needs to be taken into consideration. Several authors have found that a key factor to make biodiesel economically feasible is the application of tax credits.44,45 To promote biodiesel consumption, several countries have exempted biodiesel from their fuel excise tax. According to this, the European Union (EU) approved the biodiesel tax exemption program in May 2002 (Art. 21, Finance Law 2001).2 Despite this, some European countries, including Germany (considered one of the fathers of biodiesel), have started removing tax exemption. On Oct 22, 2008, the German federal government ¨ nderung der Fo¨rderung ratified the law entitled “Gesetz zur A von Biokraftstoffen” (Energy Tax Law).46 According to this, starting in January 2009, the German government will receive nine cents on the dollar more per liter of biodiesel (increasing from 0.18 to 0.21 Euros). Such a tax will increase to more than 65 cents on the dollar in 2012, putting obstacles to the development of German biofuel refineries, constituting a stimulus for the importation of cheaper biodiesel. Taxes remove the price advantage of biodiesel over conventional diesel fuel and should result in a massive decline in biodiesel output. A lower-cost biodiesel production can also be achieved by the optimization of the process. Because biodiesel chemical properties determine its feasibility as fuel, the optimization of reaction parameters can be exploited to maximize the yield of ester, thus achieving a low-cost chemical process and ensuring

appropriate chemical properties to guarantee adequate engine performance and appropriate exhaust emissions. In this sense, it is important to characterize the oil (i.e., fatty acid composition, water content, and other significant parameters) to determine the feasibility to convert the oil into biodiesel. 3.3. Ecological, Political, and Agronomical Issues. The renewed interest in the use of vegetable oils to produce biodiesel is due to its less polluting and renewable nature, compared to conventional petroleum-based diesel fuel. Biodiesel could be beneficial for environment, local population job creation, provision of modern energy carriers to rural communities, mitigation of human migration, and reduction of CO2 and sulfur levels in the atmosphere.47 Current biodiesel energy originates from the sun, through photosynthesis of biomass. However, to keep the main benefit of its use (i.e., to be an environmentally friendly energy), limiting factors such as the extensive use of land, irrigation, and labor practices (such as fertilizing, weed control, etc.) must be taken into consideration and reduced to minimal levels. Implementation of efficient farming practices to preserve soil fertility and to reduce the use of valuable inputs, such as fertilizers and water, gain special interest. The increasing development of biodiesel opens new challenges to the scientific community, including the production of renewable energy respecting natural ecosystems. Genetic engineering can nowadays be carried out in a clean, environmentally friendly, and cost-effective way, thus becoming an efficient approach to achieve such ecological, political, and agronomical goals, as discussed below. Another way to increase global vegetable oil production without harming ecosystems is to use marginal or nonarable wasteland.18 In addition, the set-aside rules of the EU Agricultural Policy specify a minimum area of obligatory set-aside (10% in 2001) of the total arable land, but also allow up to 50% of the total claimed area to be put into voluntary set-aside. Nevertheless, an exception has been introduced into the rules for managing set-aside land, allowing farmers to cultivate crops for nonfood purposes.48,49 It should be noted that increasing the set-aside area could lead to erosion problems, and may have an impact on arable land. Related to that, in response to the increasingly tight situation on the cereal market, the EU agriculture ministers recently approved a Commission proposal to remove the obligatory set-aside rate for autumn 2007 and spring 2008 sowings. Furthermore, the abolition of the compulsory set-aside from 2009 onward is part of the Common Agricultural Policy (CAP) Health Check proposal, which was adopted by the Commission on 20th May 2008 and it is currently under discussion in the Council, the European Parliament and other European Institutions.50 For all these reasons, another approach is the use of nonedible crops and trees, which have several advantages, such as growing in arid or less favored regions, requiring very little manpower and care, having high oil content, being resistant to plagues and drought, etc. The foliage could be used as manure, giving an added value to the crop. As an example, most trees and crops

(42) Connemann, J.; Fischer, J. The International Liquid Biofuels Congress. US National Biodiesel Foundation: Brazil, 1998, p. 15. (43) Haas, M. J.; McAloon, A. J.; Yee, W. C.; Foglia, T. A. Bioresour. Technol. 2006, 97, 671–678. (44) Bender, M. Bioresour. Technol. 1999, 70, 81–87. (45) Peterson, C. L. Transactions of the ASAE 1986, 29, 1413–1422. ¨ nderung der Förderung von (46) Vorblatt Entwurf eines Gesetzes zur A Biokraftstoffen. Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Oct. 20, 2008; pp 38. (http://www.bmu.de/files/pdfs/ allgemein/application/pdf/entw_foerderung_biokraftstoff.pdf).

(47) Demirbas, A. Energy ConVers. Manage. 2008, 49, 2106–2116. (48) Dorado, M. P.; Ballesteros, E.; Lopez, F. J.; Mittelbach, M. Energy Fuels 2004, 18, 77–83. (49) Graciani, A. L., Amores, A. G., Arnal Almenara, J. M., Chico Gaetan, J. M., Dorado, M. P. In 1st World Conference and Exhibition on Biomass for Energy and Industry; James & James (Science Publishers) Ltd.: London, 2001; pp 1560-1561. (50) Europa s Press Releases; EU: Brussels, 2008; Vol. IP/08/1069. Available at: http://europa.eu.

Table 2. Price of Biodiesel from Different Raw Materials and Diesel Fuel fuel diesel fuel RME (B100)a SME (B99)b PME (B99)c

price (USD/t), Sept 2007 price (USD/t), Sept 2008 ref 733 1020-1060 850-865 780-850

1017 1415 1185 990

145 145 145 145

a RME: rapeseed oil methyl ester, pure and without additives, matching EN 14214 standard and typically reaching -10 or -12 °C quoted in USD/MT on a free on board (FOB) Northwest Europe (North German ports, North France, Benelux and South-East UK) basis. b SME: soybean oil methyl ester, with a minimum of 99% biodiesel (B99), typically not matching the EN 14214 and reaching a 0/-5 °C CFPP, quoted in USD/MT on a cost, insurance, freight (CIF) ARA with the 6.5% duty paid included (T2). c PME: palm oil methyl ester, with a minimum of 99% biodiesel (B99), typically not matching the EN 14214 and reaching a +11/+15 °C CFPP, quoted in USD/MT on a cost, insurance, freight (CIF) ARA with the 6.5% duty paid included (T2).

ReViews

mentioned below grow well on wasteland and therefore can tolerate long periods of drought and dry conditions.2 4. Nonedible and Low-cost Vegetable Oils with High Potential to Be Used As Raw Materials to Produce Biodiesel In terms of sustainability, provided the previous technical, ethical, economical, and social considerations, potentially suitable vegetable raw materials for biodiesel production are described below. The selection has been done considering low input crops and the most promising ones, according to their properties (Tables 3 and 4).2 In the following sections, potentially suitable low-cost vegetable oils for biodiesel production are analyzed separately, in terms of fuel properties and their suitability to be used as alternative diesel fuel. Finally, a comparison between them facilitates finding out the best fitting low-cost vegetable oil for biodiesel production. 4.1. Jatropha curcas. J. curcas is a perennial plant, native and widely spread throughout many tropical countries. It grows readily in poor and stony soil. It is drought- and disease-resistant, and its oil yields high-quality biodiesel.51,52 Biodiesel produced from J. curcas oil meets all the requirements stipulated by the EU standard EN 14214.53 However, as J. curcas is still a wild plant, the initiation of systematic selection and breeding programs is a prerequisite for sustainable utilization of this plant for biodiesel production.54 The FFA content of J. curcas seed oil (JCSO) varies depending on the quality of the feedstock. Although Sivaprakasam and Saravanan55 reached 91% yield on jatropha oil methyl ester (JOME) using alkaline transesterification and jatropha oil poor on FFA, Berchman and Hirata56 developed a technique to produce biodiesel from crude JCSO containing high levels (15%) of FFA. Some researchers have proposed the use of immobilized enzymes, such as those from Chromobacterium Viscosum, Candida rugosa, and Sus scrofa porcine pancreas as catalyst.57,58 In this sense, Modi et al.59 proposed the use of propan-2-ol as an acyl acceptor for immobilized Candida antarctica lipase B. Additionally, Zhu et al.60 proposed the use of a heterogeneous solid superbase catalyst (catalyst dosage of 1.5%) and calcium oxide, at 70 °C for 2.5 h, with a 9:1 methanol:oil molar ratio to produce biodiesel. Kumar et al.61 obtained values of brake thermal efficiency of jatropha oil methyl ester comparable to diesel fuel values, higher values of CH and CO emissions, but lower values of NOx exhaust emissions. 4.2. Pongamia pinnata (Karanja Seed Oil). This nonedible oil tree is drought-resistant, tolerant to salinity, moderately frost hardy, and is commonly found in East Indies, Philippines, and (51) Becker, K.; Makkar, H. P. S. Lipid Technol. 2008, 20, 104–108. (52) Banapurmath, N. R.; Tewari, P. G.; Hosmath, R. S. Renewable Energy 2008, 33, 1982–1988. (53) Kumar, A.; Sharma, S. Ind. Crops Prod. 2008, 28, 1–10. (54) Foidl, N.; Foidl, G.; Sanchez, M.; Mittelbach, M.; Hackel, S. Bioresour. Technol. 1996, 58, 77–82. (55) Sivaprakasam, S.; Saravanan, C. G. Energy Fuels 2007, 21, 2998– 3003. (56) Berchmans, H. J.; Hirata, S. Bioresour. Technol. 2008, 99, 1716– 1721. (57) Shah, S.; Sharma, S.; Gupta, M. N. Energy Fuels 2004, 18, 154– 158. (58) Shah, S.; Sharma, S.; Gupta, M. N. Indian J. Biochem. Biophys. 2003, 40, 392–399. (59) Modi, M. K.; Reddy, J. R. C.; Rao, B. V. S. K.; Prasad, R. B. N. Bioresour. Technol. 2007, 98, 1260–1264. (60) Zhu, H.; Wu, Z. B.; Chen, Y. X.; Zhang, P.; Duan, S. J.; Liu, X. H.; Mao, Z. Q. Chin. J. Catal. 2006, 27, 391–396. (61) Senthil Kumar, M.; Ramesh, A.; Nagalingam, B. Biomass Bioenergy 2003, 25, 309–318.


India.2 Several scientists have investigated and proposed karanja oil as a potential source of biodiesel.3,62-66 Most researchers have conducted the transesterification of P. pinnata oil by using methanol and potassium hydroxide.64,66,67 Due to its high FFA content, some researchers have proposed the esterification of the FFA with H2SO4, prior to transesterification with NaOH.68 In all cases, karanja oil has shown promising properties to be used as a raw material to produce biodiesel, saving large quantities of edible vegetable oils. Diesel engine performance tests have been carried out with karanja oil methyl ester (KOME) and its blend with diesel fuel from 20 to 80% by volume (v/v).65 Results have revealed a reduction in exhaust emissions together with an increase in torque, brake power, thermal efficiency, and reduction in brake-specific fuel consumption compared to diesel fuel. Prakash et al.69 optimized transesterification of karanja oil using the Taguchi optimization methodology70 and carried out performance and emission tests using diesel fuel and biodiesel blends. Among the blends, 20% KME showed better performance characteristics compared to other blends. They observed better BTE, BSFC, and indicated thermal efficiency (ITE). With regard to exhaust emissions, 20% blend slightly increased the NOx due to the higher specific gravity of the fuel. Both the PM emission and smoke density were low.69 4.3. Madhuca indica (Mahua Oil). This is a deciduous tree that belongs to the family Sapotaceae. It can reach up to 21 m high. Several approaches to produce biodiesel from this crop can be found in literature.2 In this sense, Ghadge and Raheman have proposed a two-step pretreatment to reduce high FFA levels. Transesterification was carried out adding 0.25 (v/v) methanol and 0.7% KOH. Fuel properties were found to be comparable to those of diesel fuel.71 Other authors have proposed different successful alternatives to produce biodiesel from this species: ethanol and sulfuric acid, and methanol and sodium hydroxide.72-74 Excepting water content, the fuel properties of mahua biodiesel are within the limits specified by the ASTM D 6751-02 and EN 14214 standards.71 Besides calorific value, all other fuel properties of mahua biodiesel were found to be higher than high-speed diesel fuel.75 Raheman and Ghadge75 measured engine performance and emissions of biodiesel obtained from mahua oil and its blends with diesel fuel. They observed that BSFC increased while BTE decreased by increasing the proportion of biodiesel in the blends. Smoke level and CO were reduced, whereas NOx increased with (62) Naik, M.; Meher, L. C.; Naik, S. N.; Das, L. M. Biomass Bioenergy, 2008, 32 (4), 354-357. (63) Meher, L. C.; Dharmagadda, V. S. S.; Naik, S. N. Bioresour. Technol. 2006, 97, 1392–1397. (64) Meher, L. C.; Naik, S. N.; Das, L. M. J. Sci. Ind. Res. 2004, 63, 913–918. (65) Raheman, H.; Phadatare, A. G. Biomass Bioenergy 2004, 27, 393– 397. (66) Karmee, S. K.; Chadha, A. Bioresour. Technol. 2005, 96, 1425– 1429. (67) Vivek, G. A. K. J. Sci. Ind. Res. 2004, 63, 39–47. (68) De, B. K.; Bhattacharyya, D. K. Lipid Fett 1999, 101, 404–406. (69) Prakash, N.; Arul Jose, A.; Devanesan, M. G.; Viruthagiri, T. Indian J. Chem. Technol. 2006, 13. (70) Rao, R. S.; Kumar, G.; Prakasham, R.; Hobbs, S. P. J. Biotechnol. J. 2008, 3, 510–523. (71) Ghadge, S. V.; Raheman, H. Biomass Bioenergy 2005, 28, 601– 605. (72) Raheman, H.; Ghadge, S. V. Fuel 2007, 86, 2568–2573. (73) Ghadge, S. V.; Raheman, H. Bioresour. Technol. 2006, 97, 379– 384. (74) Puhan, S.; Vedaraman, N.; Sankaranarayanan, G.; Ram, B. V. B. Renewable Energy 2005, 30, 1269–1278. (75) Raheman, H.; Ghadge, S. V. Fuel 2008, 87, 2659–2666. (76) Nabi, M. N.; Akhter, M. S.; Zaglul Shahadat, M. M. Bioresour. Technol. 2006, 97, 372–378.

2.3

18.5

4

10.2

4-6

5.5

5-6

35

5.9

HeVea brasiliensis

Brassica carinata

Brassica carinata (low erucic) Camelina satiVa

Terminalia catappa

Asclepias syriaca

Calophyllum inophyllum 17.9

Cynara cardunculus

5

1.3

8.7

20

25

42.7

34.8

32

14-16

42-44

10-17

24.6

42

20-25.1 41-51

C 18:3

60

13.7

48.7

28

15-16

35-37

17-25

39.6

15

8.9-13.7 C20:0 (1.3)

C14:0 (1) C20:0 (3-3.3)

2.1

C16:1 (2.5) C24:0 (2.6)

C16:1 (6.8)

36-37 C20:1 (15-16) C22:0 (1-2) C22:1 (3) 1.2

15

FFA (%)

98

401

22

0.019

0.5

95

85 32.47

>99 652

93

20-25

49

97.44

0.054-6.1 29.9-38.3

98.27

84.46

91.9

40

83

90.4

27-392

40-50

90-91

581

oil content yield FAME (w/w %) (w/w %)

2.2

10.8

17

C24:0 1 (1-3.5) 8.3

C16:1 (0.8)

other acids

10-17 Erucic: 45.4; Gadoleic: 10.3 15-16

16.3

42.1-35.3 0.3

C 18:2

44.5-71.3 10.8-18.3

36.5-41

C 18:1

MeOH 6:1 (M), CH3CH2ONa 0.2:1 (M) MeOH 6:1 (M), CH3ONa-KOH 1.1% (w/w), 60 °C, 60 min Pretreatment acid catalyzed (H2SO4), MeOH 6:1 (M), KOH 1.5%, 65 °C, 4 h

MeOH 4.6:1 (M), KOH 1.4%, 45 °C, 30 min MeOH 6:1 (M), NaOH 1.5% (w/w), 25 °C, 60 min

acid pretreatment catalyzed with H2SO4, MeOH 9:1 (M), KOH, MeOCH3 0.8%, 45 °C, 120 min one-step alkali catalyzed: MeOH 18%, NaOH 1.6%, 70 °C, 60 min MeOH 6:1 (M), NaOH 1-1.5% (w/w), 65 °C, 40-180 min Pretreatment acid catalyzed (H2SO4), MeOH 6:1 (M), NaOH 0.7% (w/w), 60 °C, 180 min MeOH 6:1 (M), NaOH 0.6% (w/w), 65 °C, 60 min MeOH 6:1 (M), NaOH 1% (w/w), 60 °C 60 min + ether petroleum

transesterification conditions

From seed. 2 From kernel. 3 Surface response pretreatment. 4 Refined oil. Transesterification yield from refined camelina oil reached 89% (acid value of 6.0).

11

2-3

20

Azadirachta indica

1

0.5

16-28.2

Madhuca indica

2.4-8.9

3.7-7.9

Pongamia pinnata

C 18:0

13.4-15.3 6.4-6.6

C 16:0

Jatropha curcas

raw material (oil)

Table 3. Chemical Properties of Nonedible Vegetable Oils and Transesterification Conditions fatty acid composition (%) ref

catalyst

done

2, 104, 149

3, 78

90, 92

93

catalyst catalyst

86, 89, 148

48

82

52, 79-81

3, 76, 147

3, 71, 73

64, 65, 67, 69

51, 55, 56, 146

2 tests

done

done3

done

done

transesterification optimization

2332 Energy & Fuels, Vol. 23, 2009

1500 100 31

+2

100 100 +6.4 -104

+12 +13 24

0

-35

-50 -356

-27

-4 -6 4.83 -8 6.425 4.3 -0.95 -6 4.6-5.2 13.2 4.3 4 -4 -10 5.1 -9 4 1.90 39550 422004 36970 27727 39250 33000-37200 Camelina satiVa Terminalia catappa Asclepias syriaca Calophyllum inophyllum Cynara cardunculus

56.9 464 57.1 50 57.3 59

8.7

37800-38450 36100 36800 40100 37500-38650 36000

1 At 25 °C. 2 Percentage of brake specific fuel consumption (relative to petrodiesel based fuel). 3 Test using 15% biodiesel/diesel fuel blend. 4 Camelina oil without transesterification. 5 At 20 °C. 6 60% blend with diesel fuel. 7 Units: kg cal/mol.

48 86, 89, 148 93 92 78 104, 107 138 155 83.2 152-157 71.5 117 1500 1500

1500 1500 1500 1500 1500 1500 100 (80 for BSFC) 100 100 100 100 100 +14.5 +8 +26 25.63-29 29.51 21 +8 -73 -81 -43 +10 -20 -5 -4 -3-7 -8.3 6 5

speed (rpm) IV (gl2/100 g) load (%)

>120 95-107 86.5-90 74.2 70-74 121-145 92-128 >3.5 and 6 3.23 2.35

>51 50-58.5 54.53 56.61 51-57.87 43-44.81 52 EN 14214 standard Jatropha curcas Pongamia pinnata Madhuca indica Azadirachta indica HeVea brasiliensis Brassica carinata low erucic)

CN raw material (oil)

heat of combustion kJ/kg)

kinematic oxidation CP PP viscosity stability (h) (°C) (°C) (40 °C; mm1/s)

NOx

PM

HC

CO

maximum BTE (%) BSFC (%)2

running conditions exhaust emissions relative to petrodiesel-based fuel (%)

Table 4. Fuel Properties of FAME from Nonedible Vegetable Oils

51, 52, 61, 150, 151 52, 65, 69, 138 3, 72 3, 76 52, 79, 81 82, 83


ref

ReViews

an increase of biodiesel percentage from mahua oil in the blends. They considered that mahua oil methyl esters were safely blended with diesel fuel up to 20%, without significantly affecting engine performance (BSFC, BTE) and emissions (smoke, CO, and NOx), and thus it was recommended as a suitable alternative fuel for diesel engines. 4.4. Azadirachta indica (Neem Oil). This large tree grows in almost all types of soils. It thrives well in arid and semiarid climate with maximum shade temperature as high as 49 °C, bearing rainfalls as low as 250 mm/year.3 Nabi et al. have produced biodiesel from neem oil by using 20% methyl alcohol and 0.6% anhydrous NaOH catalyst. Reaction temperature was kept at 55-60 °C. Compared with conventional diesel fuel, exhaust emissions including smoke and CO were reduced, whereas NOx emissions were increased with diesel fuel-biodiesel blends, except when the exhaust gas recirculation (EGR) was applied. According to their results, they recommended it as an environment-friendly alternative fuel for diesel engines.76 4.5. Calophyllum inophyllum (Nagchampa/Polanga Oil). This tree thrives in xerophytic habitats. It grows best in deep soil or exposed sea sands. The rainfall requirement is just 750-5000 mm/year.3 Polanga oil contains 24.96% saturated and 72.65% unsaturated acids.77 Saturated fatty acid alkyl esters increase cloud point, cetane number, and stability. The free fatty acid content of unrefined filtered nagchampa oil was found to be 22%, with an acid value of 44 mg KOH/g.78 Sahoo et al.78 obtained a comparatively higher flash point than petroleum diesel fuel for nagchampa oil methyl ester (NOME), indicating better safety conditions during storage. All characterization tests of biodiesel demonstrated that the most important properties are very close to those of diesel fuel. The performance of diesel engines was slightly better in terms of BTE, BSFC, smoke opacity, and exhaust emissions including NOx for the entire range of operations.78 4.6. HeWea brasiliensis (Rubber Seed Oil). This rubber tree originates from the Amazon rain forest (Brazil). Although there are variations in the oil content from different countries, the average oil yield is 40%2 and contains 17-20% saturated and 77-82% unsaturated fatty acids.79 To check its feasibility as a source to produce biodiesel, several studies have been undertaken. Ikwuagwu et al.79 prepared methyl esters of rubber seed oil using excess of methanol (6 M), containing 1% NaOH as a catalyst. The biofuel properties showed similar values compared to those of diesel fuel, with the exception of the oxidative stability. Ramadhas et al.80 performed a prior acid-catalyzed esterification to reduce the high FFA content, followed by an alkaline esterification. The lower blends of biodiesel with diesel fuel increased BTE and reduced both BSFC and exhaust emissions.80,81 4.7. Brassica carinata (Ethiopian Mustard Oil). This is an adequate oil-bearing crop that is well-adapted to marginal regions. It is drought-resistant and grows in arid regions. Ethiopian mustard presents up to 6% saturated hydrocarbon chains. B. carinata oil from wild species presents high erucic acid content (which is toxic), although cultivars with low erucic acid are used as food by Ethiopians. B. carinata adapts better and is more productive in adverse conditions than B. napus., offering the possibility of exploiting the Mediterranean marginal areas for energetic purposes.82 (77) Hemavathy, J.; Prabhakar, J. V. J. Am. Oil. Chem. Soc. 1990, 67, 955–957. (78) Sahoo, P. K.; Das, L. M.; Babu, M. K. G.; Naik, S. N. Fuel 2006, 86, 448–454.

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Dorado et al.48 described a low-cost transesterification process of B. carinata oil and found negative effects of singular fatty acids (e.g., erucic acid) in the alkali-catalyzed transesterification. Cardone et al.83 found that B. carinata biodiesel produces lower levels of particulate matter but higher levels of NOx concentrations with respect to diesel fuel. The soluble organic fraction of biodiesel particulate suggested that the carcinogenic potential is lower than that of petroleum diesel.83 4.8. Camelina satiWa (Gold-of-pleasure Oil/False Flax). Budin et al.84 studied the composition of C. satiVa oil and concluded that this low-input crop presents food and nonfood exploitation potential. The oil yield from this species is similar to that of the spring rapeseed; however, lower fertilizer and pesticide requirements lead to a substantial cost reduction,85 being a more environmentally friendly crop. Since production cost of C. satiVa is relatively low compared to many other oil crops, including rapeseed, corn, and soybean, it is an attractive potential crop for biofuels. The free fatty acid content of raw C. satiVa oil is 3.1% (acid value equals to 6.0), whereas refined oil presents a value around 0.05% (acid value is 0.1). The iodine value (152-157), far exceeds the limits of all biodiesel standards, due to very high level of polyunsaturated fatty acids in the C. satiVa oil (Table 3). However, the high iodine value of the C. satiVa oil methyl esters does not seem to lead to a rapid deterioration of lubricating oil.86 Fro¨hlich and Rice86 have investigated the production of methyl esters from C. satiVa oil. Biodiesel was prepared by means of a single-stage esterification using methanol and KOH. They compared two methods of transesterification developed by Freedman et al.87 and by Maurer.88 Steinke et al.89 developed both alkali and lipase-catalyzed alcoholeysis of C. satiVa oil. Fro¨hlich and Rice86 tested biodiesel from this species in two light transport vehicles. Fuel consumption and general vehicle operation resulted to be similar to those observed using rapeseed oil methyl esters. Fuel-specific properties of Camelina satiVa oil methyl esters are largely within specification, though low-temperature behavior could be a problem under certain weather conditions. 4.9. Asclepias syriaca (Milkweed Oil). The common milkweed is native from the North East and North Central of the United States of America, where it grows on roadsides and undisturbed habitat.90 The seed contains 20-25% (dry weight) of triglycerides, composed of over 90% unsaturated fatty acids with nearly 50% of linoleic acid and less than 2% of linolenic acid (Table 3).91 On the basis of the fatty acid profile, the oil is expected to provide an alternative source to biodiesel production. (79) Ikwuagwu, O. E.; Ononogbu, I. C.; Njoku, O. U. Ind. Crops Prod. 2000, 12, 57–62. (80) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Fuel 2005, 84, 335–340. (81) Ramadhas, A. S.; Muraleedharan, C.; Jayaraj, S. Renewable Energy 2005, 30, 1789–1800. (82) Cardone, M.; Mazzoncini, M.; Menini, S.; Rocco, V.; Senatore, A.; Seggiani, M.; Vitolo, S. Biomass Bioenergy 2003, 25, 623–636. (83) Cardone, M.; Prati, M. V.; Rocco, V.; Seggiani, M.; Senatore, A.; Vitolo, S. EnViron. Sci. Technol. 2002, 36, 4656–4662. (84) Budin, J. T.; Breene, W. M.; Putnam, D. H. J. Am. Oil. Chem. Soc. 1995, 72, 309–315. (85) Downey, R. K. J. Am. Oil. Chem. Soc. 1971, 48, 718–722. (86) Fro¨hlich, A.; Rice, B. Ind. Crops Prod. 2005, 21, 25–31. (87) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil. Chem. Soc. 1984, 61, 1638–1643. (88) Maurer, K. Landtechnik 1991, 46, 604–608. (89) Steinke, G.; Schonwiese, S.; Mukherjee, K. D. JAOCS 2000, 77, 367–371. (90) Holser, R. A. Ind. Crops Prod. 2003, 18, 133–138. (91) Adams, R. P.; Balandrin, M. F.; Martineau, J. R. Biomass 1984, 4, 81–104. (92) Holser, R. A.; Harry-O’Kuru, R. Fuel 2006, 85, 2106–2110.

Milkweed oil contains more than 6% of palmitoleic acid, which is usually found in smaller amounts in vegetable oils. This is a very interesting fact, because methyl palmitoleate is a strong candidate to enhance fuel properties, besides methyl oleate.9 Holser and O’Kuru92 analyzed fuel properties of both methyl and ethyl esters of milkweed seed oil. Milkweed biodiesel exhibits pour and cloud point values that may suggest an improved cold weather performance. Highly unsaturated ester structures (e.g., linolenate) oxidize more rapidly than saturated ester structures, leading to fuel degradation, reducing its quality. 4.10. Terminalia catappa. This tree is popularly known in Brazil as “castanhola”. It has been studied by Dos Santos et al.93 The tree is tolerant to strong winds, salt spray, and moderately high salinity in the root zone. It grows principally in freely drained, well-aerated, sandy soils. The oil can be obtained from the kernels of the fruit, with yields around 49% w/w.94 Castanhola oil fatty acid composition is similar to that of the conventional edible oils. Dos Santos et al.93 compared basic and acid catalysis and observed that basic catalysts are more efficient than acid ones. In the presence of basic catalysts, an average yield of FAME of ca. 93% was obtained. Although the fruit is edible, the kernel is nonedible and is considered a waste. However, it might also be used to produce biodiesel, giving added value to this crop. 4.11. Ricinus communis (Castor Oil). This plant is native from Central Africa, being cultivated in many hot climates. The oil contains up to 90% of ricinoleic acid, which is not suitable for nutritional purposes due to its laxative effect. The hydroxycarboxylic acid is responsible for the extremely high viscosity of castor oilsalmost a hundred times the value observed in other fatty materials.21 Transesterification reactions from this oil have been carried out mainly by using both ethanol and NaOH, as well as through enzymatic methanolysis.95,96 Several authors have studied the influence of catalyst on biodiesel yield from castor oil. Results showed that the most efficient transesterification of castor oil was achieved in the presence of sodium methoxide and acid catalysts.97 The viscosity of castor oil-based biodiesel is extremely high at low temperatures, and the melting point of methyl ricinoleate is close to 0 °C. Furthermore, the cetane number of methyl ricinoleate, and therefore neat castor oil biodiesel, do not meet the minimum requirements for biodiesel standard specifications. The oxidative stability of methyl ricinoleate is significantly lower than that of its nonhydroxylated counterpart (methyl oleate), and is even lower compared to methyl linoleate.9 To recommend this biodiesel as an alternative to diesel fuel, more research is needed. 4.12. Cuphea ssp. (Cuphea). This genus includes species such as C. carthagenensis, C. painteri, C. ignea, C. Viscosissima, and C. llaVea. Cuphea grows in temperate and subtropical (93) Dos Santos, I. C. F.; de Carvalho, S. H. V.; Solleti, J. I.; Ferreira de La Salles, W.; Teixeira da Silva de La Salles, K.; Meneghetti, S. M. P. Bioresour. Technol. 2008, 99, 6545–6549. (94) Abdullah, A. H.; Anelli, G. RiVista di Agricoltura Subtropicale e Tropicale 1980, 74, 245–247. (95) De Oliveira, D.; Di Luccio, M.; Faccio, M.; Dalla Rosa, C.; Bender, J. P.; Lipke, N.; Amroginski, C.; Dariva, C.; Oliveira, J. V. Appl. Biochem. Biotechnol. 2005, 122. (96) Fagundes, F. F.; Garcia, R. B.; Costa, M.; Borges, M. R. J. Biotechnol. 2005, 118, 166–169. (97) Meneghetti, S. M. P.; Meneghetti, M. R.; Wolf, C. R.; Silva, E. C.; Lima, G. E. S.; Silva, L. L.; Serra, T. M.; Cauduro, F.; de Oliveira, L. G. Energy Fuels 2006, 20, 2262–2265.

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climates. The seeds of these plants contain around 30-36% oil.98 Fatty acid composition of the oil comprises major quantities of caprylic acid (73% in C. painter and 3% in C. ignea), capric acid (18% in C. carthagenensis, 24% in C. painteri, 87% in C. ignea, and 83-86% in C. llaVea), and lauric acid (57% in C. carthagenensis).99 The correlation analysis between fatty acid composition of cuphea oil and environmental crop factors as latitude, elevation, and temperature have been studied by Ghebretinsae et al.100 They observed that environmental factors contribute significantly, and in particular, with respect to the ratio of lauric/capric and lauric/myristic acids. Genetically modified oil of C. Viscosissima presents relatively low viscosity, enhancing its performance as an alternative to diesel fuel.101 Also, the atomization properties suggest better fuel performance, owing to the presence of short-chain triglycerides, compared to traditional vegetable oils comprising predominantly long-chain triglycerides.102 4.13. Cynara spp. This genus includes species such as C. humilis (thistle), C. cardunculus (cardoon), and C. scolymus (artichokes). Although the leaf stalks of these species can be eaten, they are considered weed in many countries. The flower buds and stems of this genus can also be used for food purposes. The oil to produce biodiesel is extracted from the nonedible seeds, therefore not competing with food markets and increasing the value and profitability of the plant. Moreover, this genus presents a high degree of rusticity, is resistant to plagues, dry conditions, and frost, is highly efficient in the use of water and nutrients, and has reduced agrochemical needs. The low inputs management required, the advantages of increasing biodiversity by including C. cardunculus in agroecological systems, and its adaptability to native Mediterranean regions make this crop a potentially optimum alternative for a sustainable agriculture in those regions.103 The lignocellulosic biomass of cardoon can be used as a solid biofuel, and seed-oil can be derived to biodiesel production, making its cost lower compared to that of sunflower oil.104 Encinar et al. transesterified C. cardunculus oil by using methanol and several catalysts (sodium hydroxide, potassium hydroxide, and sodium methoxide) to produce biodiesel.105,106 C. cardunculus methyl esters provide a significant reduction in particulate emissions, mainly due to reduced soot and sulfate formation.107


Several nonedible feedstocks exhibit high level of FFA, as can be seen in Table 3. This represents a key problem during common alkaline transesterification. Alkaline catalyst reacts with FFA and produces soap (saponification reaction), reducing the

biodiesel yield, and preventing the separation of esters, glycerol, and washing water. Soap formation also increases the viscosity and leads to gel formation.87,108-111 In general, the use of alkaline catalysts in transesterification reactions is not recommended in feedstock with FFA contents above 0.5%.108,112,113 Homogeneous mineral acids (i.e., H2SO4) have been used as catalysts for raw materials with high FFA content. During the esterification step (usually called “pretreatment”), the acid catalyst converts the FFA into esters. Triglycerides are then converted into FAME via transesterification with alkaline catalyst. During pretreatment, the main factor to monitor is water formation, due to its inhibiting effects in the transesterification.11 Di Serio et al.114 showed the possibility to perform a simultaneous esterification and transesterification, using low concentrations of homogeneous Lewis acid catalysts (e.g., carboxylic acids of given metals). However, this process has also some associated problems related to the need of separating catalysts from products by downstream purification. Demirbas115 proposed a supercritical transesterification process as an alternative method to the previous two-steps catalyzed process. The advantages of the supercritical process are that catalysts are not required, both esterification and transesterification reactions happen simultaneously, and the methodology is neither sensitive to FFA nor water. Nevertheless, the supercritical process requires a high molar alcohol/feedstock ratio (around 40-42:1), involving high-energy consumption (with high reaction pressures, around 35-40 MPa, and reaction temperatures generally higher than 300 °C). Side reactions including thermal decomposition and dehydrogenation of unsaturated fatty acid methyl esters may occur in case reaction parameters values exceed the optimal levels.110,116 Recent studies using heterogeneous catalysts (e.g., acidic and basic solid resins immobilized lipases) have been reported. These catalysts allow the use of different feedstocks, requiring lower investment costs and less downstream process equipment, as compared to supercritical processes. Marchetti et al.117 carried out a conceptual design of these alternative production plants with a techno-economical analysis and concluded that the supercritical approach is not an economically feasible alternative, due to its high operating costs. Heterogeneous catalysts have some advantages, since they can be easily separated from the reaction products, and reaction conditions can be less intensive than those required under supercritical conditions.118 Most research on the use of heterogeneous catalysts has been focused on solid base catalysts.118,119 Solid acid catalysts have been largely ignored for biodiesel synthesis, due to lower reaction rates and undesired side reactions found for homogeneous mineral acids.119 Nevertheless, since acid catalysts can

(98) Kaliangilee, I.; Grabe, D. F. J. Seed Technol. 1988, 12, 107–113. (99) Graham, S. A. CRC Crit. ReV. Food Sci. Nutr. 1989, 28, 139–173. (100) Ghebretinsae, A. G.; Graham, S. A.; Camilo, G. R.; Barber, J. C. Ind. Crops Prod. 2008, 27, 279–287. (101) Geller, D. P.; Goodrum, J. W.; Knapp, S. J. Ind. Crops Prod. 1999, 9, 85–91. (102) Geller, D. P.; Goodrum, J. W.; Siesel, E. A. Trans. ASAE 2003, 46, 955–958. (103) Raccuia, S. A.; Melilli, M. G. Field Crop Res. 2007, 101, 187– 197. (104) Fernańdez, J.; Curt, M. D.; Aguado, P. L. Ind. Crops Prod. 2006, 24, 222–229. (105) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J.; Tejedor, A. Energy Fuels 2002, 16, 443–450. (106) Encinar, J. M.; Gonzalez, J. F.; Sabio, E.; Ramiro, M. J. Ind. Eng. Chem. Res. 1999, 38, 2927–2931. (107) Lapuerta, M.; Armas, O.; Ballesteros, R.; Fernańdez, J. Fuel 2005, 84, 773–780.

(108) Canakci, M.; Gerpen, J. V. Trans. ASAE 2001, 44, 1429–1436. (109) Haas, M. J. Fuel Process. Technol. 2005, 86, 1087–1096. (110) Ma, F. R.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (111) Dorado, M. P.; de Almeida, J. A.; Schellert, C.; Ballesteros, E.; Lo¨hrlein, H. P.; Krause, R. Trans. ASAE 2002, 45, 525–529. (112) Ma, F.; Clements, L. D.; Hanna, M. A. Trans. ASAE 1998, 41. (113) Dorado, M. P.; Ballesteros, E.; Arnal, J. M.; Gomez, J.; Gimenez, F. J. L. Energy Fuels 2003, 17, 1560–1565. (114) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nastasi, M.; Santacesaria, E. J. Mol. Catal A: Chem. 2005, 239, 111–115. (115) Demirbas, A. J. Sci. Ind. Res. 2005, 64, 858–865. (116) He, H.; Wang, T.; Zhu, S. Fuel 2007, 86. (117) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Fuel Process. Technol. 2008, 89, 740–748. (118) Di Serio, M.; Tesser, R.; Pengmei, L.; E., S. Energy Fuels 2008, 22, 207–217. (119) Lotero, E.; Goodwin, J. G. J.; Bruce, D. A.; Suwannakarn, K.; Liu, Y.; Lopez, D. E. Catalysis 2006, 19, 41–84.

5. The Shortcomings of High FFA Content in Oils to Biodiesel Production

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simultaneously carry out the esterification and transesterification, they could help in the processing low-cost feedstocks involving nonedible oils with high content of FFA, thereby lowering overall production costs.43 Currently, there are extensive reports about enzyme-mediated alcoholysis for biodiesel production. These processes are classified into immobilized lipase,120 whole cell catalyst,121 and liquid lipase-mediated alcoholeysis for biodiesel production.122 Lipase enzymes from microorganisms such as Mucor miehei, Rhizopus oryzae, Candida antarctica, and Pseudomonas cepacia have shown suitability to be used for biodiesel production. They catalyze both transesterification of TG and esterification of FFA in one step. Enzymatic reactions are carried out at moderate temperatures, providing high ester yields. This method cannot be currently used in industry owing to high enzyme costs and the problems related to its deactivation caused by feed impurities.118 According to literature, lipase cost reduction constitutes the major issue for lipase-mediated alcoholysis for biodiesel industrialization. Generally speaking, there are two strategies to reduce lipase cost. One method involves the development of new lipases, fermentation optimization, and downstream processing improvements. Another way is to improve/extend the operational life of the lipase, which can be achieved through enzyme immobilization.122 Lipase-catalyzed transesterification of three nonedible oils (jatropha, karanja, and putranjiva) has been carried out by Haldar.123 This author obtained the maximum yield using 3:1 methanol-to-oil molar ratio, at 40 °C during 8 h. Shah et al.57 used three different lipases (from Chromobacterium Viscosum, Candida rugosa, and Sus scrofa porcine pancreas) in the solventfree transesterification of jatropha oil. Only the lipase immobilized from C. Viscosum on Celite-545 provided an appreciable yield (71%). 6. Genetic Modifications to Optimize Biodiesel Fuel Properties Living beings can be improved by traditional Mendelian approaches (e.g., classical agriculture and farming). Yet, there are currently powerful molecular tools that can assist breeders to improve species, breeds and cultivars in a quicker and more efficient way. Besides, the use of molecular biology methodologies in general, and genetic engineering in particular, allows genetic manipulations to produce transgenics that are not possible with the Mendelian crosses, due to species-specific sexual barriers. Genetic engineering techniques include the use of site-directed mutagenesis, gene promoters, transcription factors, antisense RNA, RNA interference (RNAi), etc. Durrett et al.18 discussed the ways in which molecular biology could address some of the major issues related to biodiesel. The main concerns focus on the increase of oil content in oilseed plants and the improvement of the fatty acid profile to enhance the fuel properties of biodiesel. Both targets are discussed in this section. 6.1. Increasing Oil Content in Oilseed Plants. Viable strategies to increase oil content in seeds have been developed, although additional work is needed. In addition, research at an early stage has also suggested several paths to produce oil in (120) Caballero, V.; Bautista, F. M.; Campelo, J. M.; Luna, D.; Marinas, J. M.; Romero, A. A.; Hidalgo, J. M.; Luque, R.; Macario, A.; Giordano, G. Process Biochem. 2009, 44, 334–342. (121) Li, W.; Du, W.; Liu, D. H. J. Mol. Catal., B 2007, 45, 122–127. (122) Du, W.; Li, W.; Sun, T.; Chen, X.; Liu, D. Appl. Microbiol. Biotechnol. 2008, 79, 331–337. (123) Haldar, S. K.; Nag, A. Open Chem. Eng. J. 2008, 2, 79–83.

vegetative tissue rather than in seeds. Durrett et al.18 proposed to combine these approaches to develop high-yield energy crops. Moreover, genetic engineering allows the dwarfing of the stalks for easier harvesting, as well as to increase the harvest index (seed yield divided by biomass). Although the genes controlling dwarfism have an unknown function, many genes that control height are known.124 Genetic engineering has been used to produce cuphea cultivars with reduced seed shattering and increased seed oil.125,126 6.2. Improving Fatty Acid Profile. Genetic modification of fatty acid composition offers a method to address most fuel property issues simultaneously. For example, the presence of some metabolites (e.g., methyl palmitoleate and esters of decanoic acid) could be increased. Both are strong candidates to improve fuel properties, besides methyl oleate.9 Provided that oils for both food and biodiesel markets need raw material showing similar properties, that is, high monounsaturated fatty acids content, plant breeding to improve biodiesel quality has been based until recently on goals and techniques developed by oilseed breeders targeting edible oils markets. They produced oils primarily composed of monounsaturated oleic acid, together with some saturated acids, and as little of the oxidation-prone linolenic acid as possible. Breeders have been adopting both transgenic (including antisense RNA and RNAi) and nontransgenic approaches for the generation of higholeate germplasm.127 6.2.1. Transgenic Approaches. The high percentage of polyunsaturated fatty acids makes Camelina satiVa and HeVea brasiliensis, among many other nonedible oils, more susceptible to oxidation, thus being undesirable for fuel production and other industrial applications. To improve the oxidative stability of biodiesel some efforts have been carried out. Although a traditional approach recommends the addition of antioxidants to the fuel, a transgenic strategy consists on reducing the content of unsaturated (particularly polyunsaturated) fatty acids present in the input oil. An increased understanding of the pathways involved in the synthesis of tocopherols and tocotrienols (natural antioxidants present in plants) has provided transgenic strategies to manipulate the levels of these antioxidants in soybeans and other crops.128,129 Such alterations could therefore be incorporated when developing oil crops for biodiesel. Lu and Kang130 developed an in planta method to generate transgenic C. satiVa plants with less content of polyunsaturated fatty acids. They demonstrated that C. satiVa is very susceptible to Agrobacterium-mediated transformation by floral dipping along with vacuum infiltration. As visual selection marker, a fluorescent protein (DsRed) was used. A genetic improvement of agronomic characters of C. satiVa oil, including the oilseed fatty acid profile, has been carried out. New clones present less content of polyunsaturated fatty acids and increased content of monounsaturated acids. In contrast to the dominating unsaturated C18 fatty acid, medium-chain fatty acids (MCFA) are nearly absent in the oil of traditional raw materials to produce biodiesel (soybean seeds, sunflower seeds, canola seeds, etc.). Modification of the chemical composition in nonedible oils to higher contents of C8-C14 fatty acids would provide new possibilities for oleo(124) Gressel, J. Plant Sci. 2008, 174, 246–263. (125) Knapp, S. J.; Crane, J. M. Crop Sci. 2000, 40, 299–300. (126) Knapp, S. J.; Crane, J. M. Crop Sci. 2000, 40, 301–301. (127) Murphy, D. J. Eur. J. Lipid Sci. Technol. 2007, 109, 296–306. (128) Cahoon, E. B.; Hall, S. E.; Ripp, K. G.; Ganzke, T. S.; Hitz, W. D.; Coughlan, S. J. Nat. Biotechnol. 2003, 21, 1082–1087. (129) Karunanandaa, B.; Qi, Q. G.; Hao, M. Metab. Eng. 2005, 7, 384– 400. (130) Lu, C.; Kang, J. Plant Cell Rep. 2007, 27, 273–278.

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Figure 1. Influence of fatty acid profile of vegetable oils to produce biofuels on cetane number and oxidative stability.

Figure 2. Iodine value of methyl esters from different vegetable oils.

chemical usages. The esters of decanoic acid appear to be the most suitable saturated shorter-chain esters to produce biodiesel from fatty acids (esters of lauric acid present too high melting point, whereas esters of octanoic acid exhibit too low cetane

number).9 For this purpose, the genes responsible for the accumulation of MCFA in cuphea oil have been transferred to a target oil crop. The best result showed up to 7.9% of C10.131 Unfortunately, the low yield of novel fatty acids in transgenic

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Figure 3. Kinematic viscosity of methyl esters from different vegetable oils.

Figure 4. Cetane number of methyl esters from different vegetable oils.

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Figure 5. Gross heat of combustion of methyl esters from different vegetable oils and its comparison to diesel fuel.

plants is a common problem and is one of the main reasons that limit the widespread production of unusual fatty acids using such approach.18 It is clear that more basic and applied research is needed to decipher the molecular basis underlying the plant metabolism and physiology. The direct use of cuphea oil as fuel was studied considering that viscosity increases with the number of acyl carbons, and decreases with the presence of double bonds.35 Thus, low molecular-weight triglycerides have lower viscosities than those typically found in conventional plant oils. Low-molecular-weight TAG, from tributyrin (4:0) to tricaprin (10:0), are predicted to have better fuel atomization characteristics than conventional TAG.102 Seed oil with higher levels of tricaproin (6:0) and tricaprylin from a mutant of Cuphea Viscosissima had a coking index comparable to that of No. 2 diesel fuel.101 However, one of the main problems derived from the direct use of lowmolecular-weight triacylglycerols is the poor cold-temperature flow properties. Gressel124 indicated that several nonedible oilseed plants could be converted to efficient biofuel crops by rendering them less toxic, using transgenic approaches. This author emphasized the importance of both a control of toxic wild crops and genetic engineering in the release of toxic compounds in nonedible oilseed crops. To use the byproduct cake generated in the process for animal feeding, the omission of curcin132 and phorbol ester from Jatropha curcas seeds,133 the release of ricin from castor oil, and the exclusion of cytotoxic compounds from Pongamia pinnata and Calophyllum inophyllum are some goals of transgenic modification for biodiesel production. 6.2.2. Mixed Approaches. Velasco et al.134 studied the inheritance of increased oleic acid concentration in a high-erucic acid Ethiopian mustard mutant oil developed through mutagenesis. They found that the monogenic inheritance of increasingly higher oleic acid levels in the high-erucic acid line could (131) Stoll, C.; Lu¨hs, W.; Zarhloul, M. K.; Brummel, M.; Spener, F.; W., F. Eur. J. Lipid Sci. Technol. 2008, 108, 277–286. (132) Lin, J.; Li, Y.-X.; Zhou, X.-W.; Tang, K.-X.; Chen, F. Mitochondrial DNA 2003, 14, 311–317.

facilitate the transfer of this trait to zero-erucic acid Ethiopian mustard germplasm. Recent isolation of a natural mutant of castor bean with high oleic acid and low ricinoleic acid concentration diversifies the potential uses of castor oil.135 Nevertheless, genetic engineering of castor through silencing the fatty hydroxylase gene (responsible for the conversion of oleic to ricinoleic acid) leads to accumulation of high levels of oleic acid that would be of interest to the biodiesel market.135,136 Fatty acid composition of nonedible oils could be altered to some extent through interspecific hybridization. However, a more targeted approach would be to silence ∆-9 or -12 desaturase genes to increase the accumulation of stearic or oleic acids, respectively.137 In summary, genetic engineering techniques can be used to modulate (blocking, reducing, or enhancing) the gene expression of selected genes and thus their downstream processing (translation to peptides/proteins). Nevertheless, it should be taken into account that once the gene expression of a particular gene is altered by genetic engineering (which may block some metabolic pathways), the cell may show altered regulatory behaviors, trying to compensate the physiological alteration. This may represent the reduced or increased synthesis of other metabolites in the metabolomic (133) Makkar, H. P. S.; Aderibigbe, A. O.; Becker, K. Food Chem. 1998, 62, 207–215. (134) Velasco, L.; Fernandez-Martinez, J. M.; De Haro, A. Crop Sci. 2003, 43, 106–109. (135) Rojas-Barros, P.; de Haro, A.; Munoz, J.; Fernandez-Martinez, J. Crop Sci. 2004, 44, 76–80. (136) Sujatha, M.; Reddy, T. P.; Mahasi, M. J. Biotechnol. AdVances 2008, 26, 424–435. (137) Liu, Q.; Singh, S. P.; Green, A. G. Plant Physiol. 2002, 129, 1732– 1743. (138) Banapurmath, N. R.; Tewari, P. G.; Hosmath, R. S. Renewable Energy 2008, 33, 2007–2018. (139) Vicente, G.; Martinez, M.; Aracil, J. Energy Fuels 2006, 20, 394– 398. (140) In Proceedings of the VI World renewable energy congress. Korbitz, W., Ed., Brighton, UK, July 1, 2000; Permagon: Amsterdam, Netherlands, pp 1258-1261.

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network. Thus, caution should be exercised when predicting the phenotypic effects of genetic engineering approaches. 7. The Ideal Raw Materials to Produce Biodiesel: A Comparative Study Although studies about nonedible oils to produce biodiesel can be found in the literature, comparative studies between them are missing. Tables 3 and 4 show the most important fuel quality parameters considering biodiesel from different raw materials. Several authors have used different methodologies to analyze properties, as well as different transesterification conditions, thus making difficult the comparisons between results. Banapurmath et al.138 carried out comparative analysis of BTE and exhaust emissions (HC, CO, and NOx) of a diesel engine fueled with sesame, karanja, and jatropha oil methyl esters. Results showed that BTE decreased with the use of biodiesel from jatropha oil. Azam et al.3 carried out an exhaustive study about the fatty acid composition of 75 vegetable oils, proposing mathematical calculation of CN, IV, and saponification value of each oil. Results showed that oil methyl esters of 26 out of the 75 (including Azadirachta indica, Calophyllum inophyllum, Jatropha curcas and Pongamia pinnata) were suitable to be used as biodiesel. Also, these biofuels meet the major specifications of biodiesel standards on USA, Germany, and Europe. However, a comparative study concerning biodiesel optimization and engine performance of those oils was missing. Vicente et al.139 carried out a comparative study between different raw materials (i.e., sunflower, rapeseed, high- and lowerucic Ethiopian mustard, and waste olive oils), all of them suitable for biodiesel production in Spain. The FFA content of these vegetable oils varied from 0.02 to 6.47% and were transesterified with methanol using potassium hydroxide as catalyst. Viscosity, peroxide value, and acid value were within the EU biodiesel specifications. The optimization of most potential feedstock is either missing or incomplete. In most cases, only some parameters have been taken into consideration and optimized, not taking into account all involved parameters.90,92,93 Without a complete optimization under methodologically homogeneous optimization process, a comparison between different FAME yields depicted in Table 3 is not enough to provide as much useful information as it could otherwise. Moreover, following an appropriate optimization process, higher values of FAME yield could be expected. As showed in Table 3, 12 vegetable oils have been proposed as raw materials to produce biodiesel. However, the transesterification optimization has been carried out only for five of them, and in two cases only the catalyst amount was optimized. To determine the best working conditions and to save resources, optimization must be carried out for each raw material, separately. In fact, many authors produce and test biodiesel considering the optimization procedures developed by other works, as a generic recipe. This could be acceptable in a multifeedstock plant, where the target is to produce biodiesel from different raw materials, without involving dramatic changes in the working conditions. However, a previous customized optimization, considering each oil individually, is strongly recommended. This could help to both increase the knowledge about the process and find out the most suitable conditions in case a multifeedstock is selected.140 (141) (142) (143) (144)

Krisnangkura, K. J. Am. Oil. Chem. Soc. 1986, 73, 471–474. Knothe, G.; Steidley, K. R. Fuel 2005, 84, 1059–1065. Demirbas, A. Energy Sources 2003, 25, 721–728. Demirbas, A. Energy ConVers. Manage. 2000, 41, 1609–1614.

The optimization of the combination of several variables can be accomplished in a reasonable time using an effective design of experiments. The use of a screening process followed by a multifactorial design or a surface response represents a powerful solution that involves the following advantages: (1) more information per experiment than unplanned approaches; (2) reduced number and cost of experiments; (3) calculation of the interactions among experimental factors within the range studied, with better process understanding; and (4) easier determination of the operational conditions for scale-up processes. The screening process is one of the most powerful techniques of design of experiments. With a small number of experimental runs it is possible to select the most important factors that have a significant effect on response variable (in case of biodiesel optimization should be the yield of the transesterification reaction). The most common screening experiment techniques are Taguchi and Plackett-Burman.73 The influence of fatty acid composition of different nonedible oils, in terms of CN and oxidative stability, is shown in Figure 1. The most suitable fatty acid composition should contain high percentage of monounsaturated fatty acids (as oleic and palmitoleic acids)9,13 and minimum amounts of polyunsaturated acids (such as linolenic acid). Likewise, cautious balanced ratios between saturated and C18:2 are highly recommended. Camelina satiVa oil, low erucic Brassica carinata oil and HeVea brasiliensis seed oils exhibit less suitable fatty acid profiles for biodiesel production when compared to others, due to their high percentage of polyunsaturated fatty acids (Figure 1 and Table 3). Also, it can be seen from Table 3 that they exceed the IV value and the linolenic acid limit established by the European biodiesel standard EN 14214. Figure 2 shows the comparison of IV from different vegetable oils, according to EN 14214. Indeed, methyl esters from Asclepias syriaca oil shows the highest IV, due to its high content of linoleic acid. Despite this, such fatty acid content is low. Biodiesel from soybean oil also exceeds the IV limit established by the EN 14214, as depicted in Figure 2. Due to the crucial importance of viscosity in engine performance, Table 4 and Figure 3 show viscosity values from different FAME samples. Only a few low-cost oils exceed maximum values of kinematic viscosity established in the EN 14214, but none of them exceed the ASTM D 6751 limit. Surprisingly, methyl esters from HeVea brasiliensis oil, which presents a high unsaturated fatty acid content, also shows a high viscosity value (see Tables 3 and 4).81 This could be due to the formation of high molecular-weight compounds from polyunsaturated fatty acids.14 However, Brassica carinata (high and low erucic acid) does not show the same trend, providing similar average of mono- and polyunsaturated compounds.83,113 As mentioned for transesterification optimization studies, there is also a lack of information related to engine performance and (145) Biodesel Report. Kingsman S. A.; Maillard, X., Ed.; Lausanne: Switzerland Kingsman: Paris, 2007. (http://www.kingsman.com/images/ SampleRpts/Biodiesel/070919WeeklyBiodieselReport.pdf). (146) Om Tapanes, N. C.; Gomes Aranda, D. A.; de Mesquita Carneiro, J. W.; Ceva Antunes, O. A. Fuel 2008, 87, 2286–2295. (147) Eevera, T.; Rajendran, K.; Saradha, S. Renewable Energy 2009, 34, 762–765. (148) Bernardo, A.; Howard-Hildige, R.; O’Connell, A.; Nichol, R.; Ryan, J.; Rice, B.; Roche, E.; Leahy, J. J. Ind. Crops Prod. 2003, 17, 191– 197. (149) Curt, M. D.; Sańchez, G.; Fernańdez, J. Biomass Bioenergy 2002, 23, 33–46. (150) Agarwal, D.; Agarwal, A. K. Appl. Therm. Eng. 2007, 27, 2314– 2323. (151) Sarin, R.; Sharma, M.; Sinharay, S.; Malhotra, R. K. Fuel 2007, 86, 1365–1371.

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exhaust emissions using different biofuels and their blends with diesel fuel. Moreover, there is a remarkable difference in the technical procedures. Some reports do not present their results referred to standard conditions, thus preventing the comparison with different engines and places (atmospheric conditions, latitude, altitude, fuels, etc.). Table 4 summarizes engine performance and emissions using 100% biodiesel from different raw materials. Due to the correlation between BSFC and the heating values,21,30 it should be expected that FAME with similar heating values depict similar BSFC and BTE values. According to Table 4 and Figure 5, a discrepancy between BSFC obtained for Pongamia pinnata, Madhuca indica, and Brassica carinata methyl esters (that exhibit an increased BSFC compared to diesel fuel in the range of 8%, 26% and 13%, respectively) and their gross heat of combustion (around 36000 kJ/kg in every case) can be found. This incongruity confirms the difficulty of the comparisons of the results achieved from different engine tests carried out using a wide range of diesel engines of different sizes and types, under different working conditions, and remarks the necessity to establish standard condition for engine tests. On the basis of these partial results, it is only possible to state that Calophyllum inophyllum, Brassica carinata (low erucic) and Azadirachta indica methyl esters may show better engine performances in terms of BTE and BSFC, due to their high heating values (Figure 5), as compared to the other sustainable FAME. However, engine tests for Azadirachta indica are missing. Figures 2-Figures 5 show the comparison between FAME from vegetable oils (with a great potential for a sustainable production of biodiesel) and other conventional biodiesel (e.g., rapeseed oil methyl ester and soybean oil methyl ester). It should be noticed that mostly all of these raw materials satisfy the biodiesel ASTM standard. However, only six biodiesel from “sustainable” vegetable oils (Calophyllum inophyllum, Terminalia catappa, Azadirachta indica, Madhuca indica, Pongamia pinnata and Jatropha curcas) are under European standard limits. According to Figure 5, they have a high heat of combustion (even higher than the outspread soybean oil methyl ester, and above 36 000 kJ/kg). C. inophyllum and A. indica oil methyl esters provide the highest calorific values (Figure 5). The fatty acid profile of the previously mentioned six vegetable oils (Figure 1) show similar composition for C. inophyllum, T. catappa, A. indica, and M. indica oils (40% of unsaturated fatty acids and 40% of monounsaturated fatty acids), whereas P. pinnata and J. curcas oils have about 20% of saturation and between 40 and 70% of monounsaturated acids. According to Table 4, these two vegetable oils exhibit better low-temperature flow properties than other oils with higher level of saturation. Among them, J. curcas oil shows lower oxidation stability than P. pinnata oil, due to its higher content of di- and polyunsaturated fatty acids (Figure 1). C. inophyllum, P. pinnata, and J. curcas oils show high FFA content (22, 8.3, and 15%, respectively), whereas the latter exhibits different values, depending on the quality of the feedstock. High FFA content represents the most important drawback for an economically sustainable implementation of biodiesel. 8. Conclusions Since Rudolf Diesel ran his engine on peanut oil in 1900, research about biofuels for diesel engines has experienced

significant growth, mainly during the last decades. Different oils and fats have been transesterified and tested in diesel engines. However, some basic and crucial issues related to this topic remain to be addressed: (1) There is a correlation between fatty acids composition and transesterification optimization results. However, a considerable number of papers have used a common recipe, most likely decreasing transesterification yield and increasing costs, depending on the raw material. In too many cases, when optimization has been carried out, not all involved parameters have been taken into consideration. (2) When biodiesel performance and emissions have been tested, a lack in the use of a standard methodology has been observed. This makes the comparison between different biofuels difficult. (3) Among the studied low-cost vegetable oils, biodiesel from Calophyllum inophyllum, Azadirachta indica, Terminalia catappa, Madhuca indica, Pongamia pinnata, and Jatropha curcas oils fit both the EN 14214 and the US ASTM D 6751-02 standards. Despite that none of them is considered to be the ideal biodiesel that ensures a perfect diesel engine behavior, biodiesel from P. pinnata and J. curcas oils are more suitable to be used under cold climates when compared to the others. (4) In search of the ideal biodiesel composition, high presence of monounsaturated fatty acids (as oleic and palmitoleic acids), reduced presence of polyunsaturated acids, and controlled saturated acids content are strongly recommended features. C18:1 and C16:1 are the best-fitting acids in terms of oxidative stability and cold weather behavior. Genetic engineering and transgenesis should focus on the exclusion of undesirable fatty acids, through reducing or blocking the gene expression of the corresponding coding genes. That can be coupled with the enhancement of the expression of desirable target genes, leading to the generation of the appropriate fatty acids. (5) Biodiesel production has been questioned lately by some social organizations and political actions, which may represent some serious misrepresentations. Even if most of such antibiodiesel arguments are invalid, some warranties must be provided. Among the conditions that could increase biodiesel popularity are the use of nonedible raw materials and the compromise of noninterference with lands for food crops. Only by providing a socially accepted alternative energy could biodiesel eventually be considered a transitional energy, until more appropriate renewable energies such as nuclear fusion are developed. Acknowledgment. Authors gratefully acknowledge support for this research from the Spanish Ministry of Education and Science (ENE2007-65490/ALT) and from “Junta de Andalucıá,” Spain (Grupo PAI TEP 169, BIOSAHE). Special thanks are given to Mr. Christian Schellert (Fachgebiet Agrartechnik, University of Kassel, Germany) for excellent assistance. EF801098A

The Ideal Vegetable Oil-based Biodiesel Composition: A Review of

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