The nature of the feedstock is influencing a number of parameters such as the ester content, CN, CP/CFPP, viscosity and the oxidative stability. However, there is little influence on the heat of combustion, flash point, lubricity and emission. The ester content of the biodiesel is dependent on the feedstock due to the presence of FFAs which are not converted into esters, the amount of unsaponifiable fraction (1-2%) which is not removed during reaction and impurities such as dimers and polymers of TAG which are transformed into dimeric and polymeric alkyl esters.

The physical properties of biodiesel are mainly influenced by the fatty acid composition of the feedstock (see Chapter 4 for more details).

The melting point (MP) of FAME is mainly dependent upon the fatty acid alkyl chain:

• The longer the alkyl chain length the higher MP

• Presence of unsaturation determine lower MP

• Trans configuration and conjugation of FAME with identical C-atoms and unsaturation are leading to higher melting points

• Branching of alkyl chain is decreasing the MP

• The alcohol chain is influencing the MP: methyl>ethyl>iso-propyl.

In this way, the MP of the FAAE is highly influencing the physical properties (CP, CFPP and viscosity) of biodiesel. The CP is higher for longer alkyl chains,
unsaturation is leading to lower CP and trans and conjugated esters have higher CP. Similarly, the kinematic viscosity of biodiesel is also influenced. In addition, substituents in the FAAE are leading to substantial higher viscosity. Dimeric fatty acids also produce biodiesel with a higher viscosity.

Physical properties of individual FAAE and of biodiesel from various feedstocks are given in Table 5.5 and Table 5.6. The oxidative stability is mainly influenced by the degree of unsaturation, with allylic and crs-allylic position more easily oxidized.

Other factors are the presence of natural anti-oxidants. Purification of biodiesel by distillation is decreasing the oxidative stability as a part of tocopherols remain in the distillation residue. The presence of hydroperoxides, metals (FE and Cu) and pro-oxidants (e. g. chlorophylls) cause a lower oxidative stability. In relation to physical properties of biodiesel, there is a conflict between saturation and unsaturation. Biodiesel produced from more saturated feedstocks has a higher CN and a better stability. However, the cold properties are negatively influenced by a high degree of saturation. It has been observed that sedimentation of insoluble

contaminants in biodiesel prepared from soy and palm oil might occur well above CP (Van Hoed, 2010).

It is known that when microorganisms are cultivated on fat-type substrates (e. g. long-chain free-fatty acids, TAGs, fatty-esters, etc.) production of (intra­cellular, cell-bounded or extra-cellular) lipases is performed as a physiological response to the presence of fatty materials into the growth medium (Fickers et al., 2005). This secretion is obligatory in the case that TAGs or fatty-esters are used as substrates (Fickers et al., 2005; Papanikolaou and Aggelis, 2010). In contrast, a large variety of microorganisms are capable of utilizing soaps as well as free — fatty acids as sole carbon and energy source, regardless of the lipolytic capacities of the microorganisms used in order to break down fatty materials (Ratledge and Boulton, 1985; Papanikolaou and Aggelis, 2010). Specifically, for the case of the yeast Y. lipolytica, its culture on TAG-type substrates is accompanied by secretion of an extra-cellular lipase called Lip2p, encoded by the LIP2 gene (Pignede et al., 2000). This gene encoded for the biosynthesis of a precursor premature protein with Lys-Arg cleavage site. The secreted lipase was reported to be a 301-amino — acid glycosylated polypeptide which belongs to the TAGs hydrolase family (EC 3.1.1.3) (Pignede et al., 2000; Fickers et al., 2005). The Lip2p precursor protein was processed by the KEX2-like endoprotease encoded by the gene XPR6, whereas deletion of the above gene resulted in the secretion of an active but fewer stable pro-enzyme (Pignede et al., 2000). Simultaneously, other intra-cellular lipases (Lip7p, Lip8p) may also be produced and secreted into the culture medium, that present different fatty acid specificities, with maximum activity being displayed against D918:1 (oleic acid), 6:0 (capronic) and 10:0 (caprinic) fatty acids (Fickers et al., 2005).

The free-fatty acids (existed as initial substrate or produced after lipase hydrolysis of the TAGs/fatty-esters) will be incorporated, with the aid of active transport, inside the microbial cell. It is interesting to state that for the case of Y. lipolytica yeast, the various individual substrate fatty acids would be removed from the medium (and hence incorporated inside the microbial cell) with different rates (Papanikolaou et al. 2001, 2002a; Papanikolaou and Aggelis, 2003b). Specifically, regardless of the initial concentrations of the extra-cellular fatty acids, the incorporation rate of the lower aliphatic chain (lauric acid-12:0 and myristic acid-14:0) or unsaturated (A918:1 and linoleic acid-A9,1218:2) fatty acids is significantly higher than that of principally stearic (18:0) and to lesser extent palmitic (16:0) acid (Papanikolaou et al., 2001; Papanikolaou and Aggelis, 2003b). Moreover, the incorporated fatty acids will be either dissimilated for growth needs or become a substrate for endo-cellular bio-transformations (synthesis of ‘new’ fatty acid profiles which did not exist previously in the substrate) (Ratledge and Boulton, 1985; Koritala et al, 1987; Aggelis and Sourdis, 1997; Guo et al, 1999; Kinoshita and Ota, 2001; Papanikolaou et al, 2001, 2002a, 2007b; Papanikolaou and Aggelis, 2003a, 2003b, 2010).

The intra-cellular dissimilation of the various catabolized fatty acids is performed by reactions catalyzed by the various intra-cellular acyl-CoA oxidases (Aox). A significant amount of experimental work has been performed in relation with the elucidation of the above-mentioned reactions by using strains of the non­conventional yeast Y. lipolytica (Fickers et al, 2005). In fact, it has been revealed that the aforementioned biochemical process is a multi-step reaction requiring different enzymatic activities of five acyl-CoA oxidase isozymes (Aox1p through Aox5p), encoded by the POX1 through POX5 genes (Luo et al. 2002; Mlickova et al. 2004a, 2004b; Fickers et al. 2005). Aox3p is specific for short chain acyl-CoAs, Aox2p preferentially oxidizes long-chain acyl-CoAs while Aox1p, Aox4p and Aox5p do not appear to be sensitive in the chain length of the aliphatic acyl-CoA chain (Mauersberger et al. 2001; Luo et al. 2002; Fickers et al. 2005). It should also be noticed that genetically modified strains of Y. lipolytica namely JMY 798 (MTLY 36-2P) and JMY 794 (MTLY 40-2P) have been created from the wild-type W29 strain (Mlickova et al. 2004a, 2004b). These strains were subjected to disruptions of the genes implicated in the encoding of various intra­cellular Aox. The genetically engineered strains, hence, either under-expressed or did not at all express several of the enzymes implicated in the catabolism (^-oxidation) of aliphatic chains. When cultures were performed on oleic acid utilized as the sole substrate, although the genetically engineered strains showed almost equivalent microbial growth compared with the wild strain (W29) from which they derived, in contrast with W29 strain they presented significantly higher formation of lipid bodies and, hence, increased lipid accumulation (Mlickova et al. 2004a, 2004b). Therefore, the above-mentioned studies as well as various others reported in the literature (Aggelis and Sourdis, 1997; Papanikolaou et al, 2003; Szczesna-Antczak et al., 2006; Mantzouridou and Tsimidou, 2007) indicate that external addition of fat (ex novo lipid accumulation) can significantly enhance the bio-process of SCO production in various oleaginous microorganisms, but external utilization of fat mainly serves for the ‘improvement’ and ‘upgrade’ of a fatty material utilized as substrate [e. g. valorization of low-cost or waste fats so as to produce specialty lipids like cocoa-butter substitutes or substitutes of other high-added value lipids like illipe butter, shea butter, sal fat (Papanikolaou and Aggelis, 2010)] and not for the use of the SCO produced in the manufacture of bio-diesel.

Glycerol is one of the frequently produced bioalcohols, because glycerol is a waste product of biodiesel production. However, glycerol is rarely considered a biofuel, because it is not easy to burn and therefore is only useful for non­combustion-based fuel uses.

Biodiesel is typically produced by the transesterification of lipids (vegetable oil, soybean oil, waste oil, etc.) with an alcohol (typically methanol). If methanol is employed, the transesterification results in the production of methyl esters, which is used as the fuel, and glycerol which is the by-product as shown in Fig. 11.2. A total of 100 kg glycerol is produced for every ton of biodiesel manufactured.10 The main problem with bioglycerol is twofold: (1) it is a waste product so it is low concentration and impure and (2) there has not been a market for bioglycerol. Since glycerol is part of the waste stream, it is in low concentrations and in a highly basic aqueous environment, because the catalyst for transesterification is typically sodium or potassium hydroxide. This can be fixed by neutralization and distillation, but that is not cost effective. Secondly, glycerol is not commonly used as a fuel, because it does not burn well and cannot be easily electrochemically oxidized.1115 However, it may be useful for other chemical purposes, because it can be used to produce glyceric acid and dihydroxyacetone. It is important to note that the biodiesel process is not really producing a bioalcohol, but it is using a low energy density bioalcohol (methanol) to produce a higher energy density bioalcohol (bioglycerol), which is different than using carbohydrates to produce bioalcohols.

The increasing support for biofuels production over the last years in both developed and developing countries has been taking shape under a variety of policy tools aiming at several objectives: from increasing biomass, to land conversion, redistribution issues, fuel consumption, fuel and food prices, to cite a few. Subsidies, under various facets across countries, are the most commonly used measure in support of biofuels production. With a direct subsidy, for example, governments sustain farmers for every unit of biofuels/biomass produced. In European Union, United States, Brazil and now also in several developing countries (OECD, 2008), direct subsidies promote the use of set-aside lands for non-food crops cultivation and help in reducing various input costs such as fertilisers, feedstock and distribution.

Economic reasons advocate subsidies for biofuels production given that these cause reduction in GHG emissions. Therefore, to recognise biofuels for emission reduction and improving environmental quality, a GHG credit mechanism in the form of a subsidy is being considered as a viable instrument to incorporate (credit) that externality in the final price of biofuels commodities. Evidence of distortionary effects of subsidies is nonetheless common in economics such that caution should be used when implementing such tools (Koplow, 2006; Steenblik, 2007). The distortion would arise when using subsidies for unproductive investments with consequent market inefficiency (i. e. in production, consumption and prices) causing loss of well-being to the society and damaging the natural environment. Further debate considers the relationship between crude oil prices and food prices (Tyner, 2007). Over the last years, the rise in crude oil prices is putting considerable pressure on primary food prices (i. e. corn prices), and having a fixed subsidy on biofuels feedstock (e. g. ethanol) would certainly not help to keep food prices down. Contrarily, subsidising the biofuels industry is pushing higher investments in the sector causing food prices to increase with more damaging repercussions in the economies of the developing world. Tyner (2007) considers alternative policy mechanisms to a fixed per unit subsidy such as a variable rate linked to crude oil prices or higher subsidies to enhance third-generation biofuels (i. e. cellulose — based ethanol) to reduce agricultural prices and re-establish the balance between land for food cultivation and land for biofuels feedstock.

Other measures than subsidies can also be advocated for biofuels production. These are in the form of investment grants (from government and/or public institutions) to ensure that adequate start-up phases for agricultural feedstock conversion and efficient distribution at pumps take place. Furthermore, in the United States and European Union, forms of fuel excise tax credit are allowed for biofuels blenders. These can claim the tax credit for the blending content of renewable fuel used in a unit of (fossil) fuel sold. Also, carbon dioxide excise tax exemption is also practised in support of biofuels commodities consumption. Finally, an additional measure to support biofuels use aims at protecting domestic industries through the use of tariffs on imported biofuels goods. This instrument is currently used across a number of countries or block of countries and is more or less damaging on the competitiveness of international trade.

Various support policies are nonetheless being adopted across countries to promote biofuels use. The recent Commission Directive 2009/28/EC on the promotion of energy from renewables establishes Member States’ shares in renewables required by the Commission by 2020. Renewables shares as well as recent biofuels shares in 2007 (European Commission, 2009) are illustrated in Table 2.2.

Current projections (EurObserv’ER, 2009) also estimate that the European Union is near (5.3%) in reaching the target of 5.7% of renewable fuels under Commission Directive 2003/30/EC by 2010. In order to achieve the desired target, the European Union allows for certain tax measures to promote biofuels

use across Member States. Of particular interest are tariffs on ethanol imports. These correspond to 10.20/hl for denaturated ethanol and 19.20/hl for undenaturated ethanol. Although these measures are still seen as protectionist approaches to biofuels production (and therefore a threat to resource access) from developing countries’ perspective, biofuels industries in the European Union are relatively ‘new’ (compared to those already in place in Brazil or United States). Furthermore, the latest European Union enlargement and the restructuring of the energy market (and that of Eastern European economies) may be seen as arguments in favour of the use of tariffs on imported biofuels commodities to promote the development of a European biofuels market. Prevalent practices across the

European Union are also those incorporating tax rates into the selling of transport fuels which are comparable to 3.5% of total fuel use in the transport sector from 2010. On average, tax rates on biodiesel and ethanol are currently 50% lower than those on diesel and gasoline.

Likewise in the United States similar measures are used to support the biofuels chain (including consumption). These can be found in the form of tax incentives for fuel-switching engine cars or quality standards on fuels. Over the last years, though, the American public support has turned its attention to third-generation biofuels (e. g. biomass/cellulose-based biofuels), sustaining numerous projects. However, at present, excise tax credits (USD 0.135/l for ethanol and USD 0.264/l for biodiesel) and import tariffs are mainly used as instruments for biofuels support across states. The support policy for biofuels in the United States tends to apply low tariffs on imported biofuels commodities. Tariffs on ethanol are for example the equivalent of 1.2-2.5% of the tariffs in countries outside NAFTA. Blending practices are also notably applied to favour the re-export of biofuels goods in particular to the European Union.

In countries, such as Brazil, China, Japan and Canada, other specific, but analogous measures, are being implemented. Brazil has for long benefitted from tax exemptions, and also blending of ethanol to fossil fuels (ranging between 20-25% of ethanol content) is regulated according to government resolutions. Biodiesel blending to diesel mandates are in the figure of two and five per cent from 2013. On the international side, Brazil applies a high tariff (e. g. 20%) on imported biofuels commodities to protect the domestic market. China has only recently supported the production of biofuels though its promotion is still going through an experimental phase. The government, on the other hand, fully supports the distribution losses across the country. Blending with other fuels (enforced at ten per cent) is in force only in few cities (i. e. around 26 in 2006), and substantial subsidies are currently in place including forms of refund for value added.

Similar to China, the Japanese experience in biofuels production is also experimental, and most policies aim at setting targets for biofuels use in the transportation sector only. Canada, on the other hand, is a step forward compared to Asian countries. Compulsory mandates for blending ethanol and biodiesel in fossil fuels range between two and five per cent content by 2012. At federal level, Canadian government is heavily supporting (CAD 2.2 billion from 2008, OECD, 2008) biofuels production and consumption with additional tax exemption measures, subsidies and import tariffs (CAD 0.05/l) on imported biofuels commodities.

Ethanol is produced almost exclusively from corn in the USA. Corn is milled for extracting starch that is enzymatically treated for obtaining glucose syrup. Then, this syrup is fermented into ethanol. There are two types of corn milling in the industry: wet and dry. During wet milling process, corn grain is separated into its components.

Fermentation may be performed using S. cerevisiae at 30-32°C with the addition of ammonium sulfate or urea as nitrogen sources (Sanchez and Cardona, 2008). Proteases can be added to the mash to provide an additional nitrogen source (Bothast and Schlicher, 2005). Z. mobilis has also been researched for ethanol production from dry-milled corn starch (Krishnan et al., 2000). Other research efforts are oriented to the development of corn hybrids with higher extractable or fermentable starch content (Bothast and Schlicher, 2005).

Triticum spp.

Ethanol is produced from wheat (Fig. 4.10) by a process similar to that of corn. Some efforts have been done for optimising fermentation conditions (Thomas et al., 1996; Wang et al., 1999; Bayrock and Michael Ingledew, 2001; Barber et al., 2002; Soni et al., 2003). Cost is the main drawback of this alternative.

The bran fraction, which would normally be a waste product of the wheat milling industry, can be used as the sole medium to produce enzyme complexes (Dorado et al., 2009). The proposed process could be potentially integrated into a wheat milling process to upgrade the wheat flour milling by-products into platform chemicals of a sustainable chemical industry (Du, Lin, et al., 2008). If the production of co-products is optimised and residues are integrated into the process, ethanol from wheat may become a serious competitor of gasoline as a fuel.

The practical use of free lipase in reaction systems suffers from technological difficulties such as contamination of the products with residual enzymatic activity and economic difficulties such as the use of enzyme for a single reactor pass.

Hence, part of the overall potential enzymatic activity is lost. If the lipase is immobilized, it becomes an independent phase within the reaction system, which may easily be retained in the reactor with concomitant advantages of preventing contamination of the products and extending its useful active life. Furthermore, as mentioned in Section 6.7, immobilization provides a more rigid external backbone for lipase molecule, allowing it to maintain its activity at higher temperatures than if it is in free-form. Therefore, the reaction optimum temperature is expected to increase, which results in faster rate of reaction. In addition, immobilization of lipases has been proposed as a countermeasure to the high water content usually present in WO (Fukuda et al., 2001). Furthermore, by immobilization, the enzyme is dispersed over a large surface, which results in an enhanced catalytic performance, especially in organic media, which is the case in biodiesel production. It was shown that lipase from C. antarctica performed better when immobilized on ceramic beads than in free-form (Al-Zuhair et al., 2008). Similar results were also found using lipase from P. cepacia (Shah and Gupta, 2006).

The main advantage of immobilization of lipase, however, is the ability of repeated use. The ability to use the immobilized enzyme repeatedly is actually the factor that determines its effectiveness. Due to the negative effect caused by by-product glycerol adsorption on the surface of the immobilized lipase, a loss in activity is inevitable with repeated uses. However, the immobilized lipase retained more than 70% of its initial activity even after more than ten cycles. This was found when using different lipases immobilized on different solid surfaces, such as Novozym 435 (Wei et al., 2004) and P. fluorescens lipase immobilized on toyonite (Iso et al, 2001). Organic solvents are usually used to dissolve the by-product glycerol, which clogs the active sites of the immobilized lipase. By using t-butanol as solvent, Wang et al. (2006) showed that there is no obvious loss in biodiesel yield even after immobilized lipase from T. Lanuginosa was used for 120 cycles. Even better results were found by Li et al (2006) using immobilized lipase from T. Lanuginosa and Novozyme 435; with the number of cycles reaching 200. However, as mentioned earlier, the addition of organic solvent has inherent problems, such as, diluting substrates and requiring additional solvent recovery unit.

From an economical point of view, a continuous reaction process without the use of any organic solvent is needed for the industrial production of biodiesel. It has been shown that the activity of immobilized lipase could be significantly increased and deactivated enzyme could be regenerated when t-butanol was used for an immersion pretreatment of the enzyme (Chen and Wu, 2003). It was shown that the activity of pretreated Novozyme 435 increased about tenfold in comparison to the enzyme not subjected to pretreatment. In addition, following complete deactivation by methanol, washing the enzyme with t-butanol successfully regenerated the enzyme and restored up to 75% of its original activity level. Recently, it was found that activity, methanol tolerance and operational stability of immobilized lipase from Candida sp. 99-125 can be significantly enhanced by pretreatment with 1 mM salts solutions of CaCl2 and MgCl2 (Lu et al, 2010). The reason might be that these
salts incorporate with the protein to form a more stable molecule that resists conformational change induced by high methanol concentration.

Lignocellulosic feedstocks are often more difficult to break down into their constituent parts in comparison with first generation feedstocks. Therefore, the conversion technologies are also more costly. A schematic diagram of a potential ethanol production process from cellulosic feedstock is shown in Fig. 9.6.

The lignocellulosic feedstock after collection is too bulky and needs to be converted to an optimal size by mechanical steps like chipping, grinding and milling. These size reduction steps are necessary to achieve an optimal size of the feedstock.

The feasibility of anaerobic digestion application comes through enhancing the efficiency of anaerobic digesters. In the case of solid feedstocks, this task is challenging, since the rate limiting step has been recognised to be the disintegration and the hydrolysis of the particulate organic matter. Some of the studied methods for enhancing the biogas production are:

• Pretreatment methods: They are applicable mainly when high solid feedstock is involved. In general, pretreatment methods can be divided into three main types according to the means used for altering its structural features: mechanical, physicochemical and biological. Mechanical pretreatment is almost always applied before any other kind of pretreatment, and actually refers to milling, through which reduction of particle size of solids is achieved. The reduction in particle size leads to an increase of available specific surface. Both physicochemical and biological pretreatment methods may enhance biodegradability, but physicochemical methods yield in general higher efficiencies. During physicochemical pretreatment, the feedstock is exposed to acid, alkaline or oxidative conditions, at ambient or high temperature. The use of high temperatures without the addition of some chemical agent, called thermal pretreatment, can also be used. Combinations of two or more physical and chemical pretreatment methods are also possible, such as acid-catalysed steam explosion, ammonia fiber explosion (AFEX) and CO2 explosion.

For lignocellulosic feedstocks, steam pretreatment, lime pretreatment, liquid hot water and ammonia based pretreatments seem to have high potential (Hendriks and Zeeman, 2009). The main effect of these methods is to dissolve the hemicellulose and alter the lignin structure, improving the accessibility of the cellulose to hydrolytic enzymes. In the case of municipal activated sludge, the goal of pretreatment is to rupture the cell wall and to facilitate the release of intracellular matter in the aqueous phase for subsequent degradation and enhance dewaterability. Various pretreatment methods have also been studied (Weemaes and Verstraete, 1998). Ultrasonic pretreatment seems to be promising, since full-scale studies have showed an improvement in sludge dewaterability (Khanal et al, 2007).

• Use of additives (Yadvika et al., 2004):

— The addition of powdered leaves, crop residues, etc. seem to increase the biogas production; the additives create a more favourable environment for the microorganisms and offer sites for the substrate local concentration through adsorption which seem to have a positive impact on biogas production (Chandra and Gupta, 1997; Dar and Tandon, 1987; Somayaji and Khanna, 1994; Babu et al., 1994).

— The addition of microbial strains (such as cellulolytic bacteria and fungi or cell lysate) increases the substrate digestibility (Tirumale and Nand, 1994; Attar et al., 1998; Geeta et al., 1994; Dohanyos et al., 1997).

— The addition of inorganic elements, adsorbents or chelating agents seems to help through various ways, by: (1) increasing the density of bacterial flocs (Shimizu, 1992), (2) contributing to the formation of vital metal — containing enzymes (Geeta et al., 1990), (3) solubilising trace elements via combining a chelating agent with a metal (Gaddy, 1994), and (4) increasing stability via adsorption (Patel et al., 1992; Patel and Madamwar, 1994).

While studies on GHG emissions from biofuel systems abound, economic assessments are still rare and difficult to compare due to different assumptions for feedstocks and conversion technologies (Bridgwater, 2009). Economic data is not

available in the public domain due to confidentiality as the conversion technologies are still under development. Nevertheless, several sources provide estimates of the economic viability of biofuel systems as discussed below:

On a life cycle basis, the costs of biofuels are mainly contributed by:

• the costs of feedstock cultivation, preparation and delivery;

• the capital costs for manufacturing plants for conversion into biofuels;

• other costs such as labour, utilities, maintenance, insurance, etc.

The following sections give an overview of the feedstock and capital costs and discuss how they influence final biofuel prices.

Biodiesel is a mixture of fatty acid alkyl esters (FAAE) (mainly methyl esters) produced from lipids via transesterification (Fig. 5.2) of the acylglycerides or esterification (Fig. 5.3) of fatty acids.

Theoretically 1 mol triglycerides is reacting with 3 moles alcohol producing 3 moles esters and 1 mol glycerol.

Methanol is the major alcohol used because of the lower price but other alcohols can also be used such as ethanol, isopropanol and butanol. Although the latter alcohols can give better fuel properties, they are not used on an industrial scale due to their higher price and processing problems.

Biodiesel can be produced from a variety of feedstocks including edible vegetable oils (soybean, rapeseed, palm, sunflower, palm kernel and coconut), animal fats, non­edible oils (jatropha, camelina, rice bran, pongomia, thelvetia, etc.) and side-streams from refining (soapstock, acidulated soapstock and deodorizer distillates). A future feedstock will be algae growing either in open fields or closed reactors. The yield of oil/ha is estimated to be at least ten times higher and can be produced at any place.

The cost for the production of biodiesel consists of 85% of the feedstocks. A process model to estimate biodiesel production cost has been developed. This flexible model can be modified to calculate the effects on capital and production costs of changes in feedstocks costs, changes in the type of feedstocks employed, in the value of the glycerol co-product and change in process chemistry and technology (Haas et al., 2006).

According to the feedstock and technology used, a distinction is made between biodiesel from first and second generation (Table 5.1). Biodiesel of the first generation is considered as FAAE produced by the traditional alkaline catalyzed transesterification reaction from refined edible vegetable oils and animal fats. Biodiesel from the second generation is the production of fatty acid methyl esters (FAME) or other esters from resources other than edible oils and in most cases using alternative technologies. It is obvious that these resources are not in competition with food/feed production and can be considered as more sustainable and more ethical.

Biofuels from the third generation produced from lipid resources are oils and fats generating power and heat (CHP, couple heat and power) in stationary diesel engines and green diesel produced by hydrotreating of oils and fats producing linear alkanes, propane, CO, CO2 and water.

Why are FAMEs suitable as diesel fuel? Conventional diesel fuel for transportation (DF2) is a product obtained by cracking of petroleum and consist mainly of long chain unbranched alkanes (C14-C24) with a boiling range of 180— 240°C, cetane number (CN) of 40-50 and heat of combustion of 45 000 kJ/kg. Biodiesel has a similar chemical structure (except for the presence of the ester function) of long chain (C12-C22) with a higher boiling range (250-450°C), CN between 40 and 80 and heat of combustion of 40 000 kJ/kg. Due to the similarity in structure, CN and energy value fatty acid esters are readily replacing diesel.

Biodiesel is miscible with petrodiesel in all concentrations, namely blends B5, B20, etc. which corresponds to the percentage of biodiesel in diesel. Blend up to 20% can be used without modification of the engines. Higher blending will require modifications due to the solvent properties of the esters which are affecting the rubber tubings and fittings.

In a comparison between biodiesel and diesel the following observations can be made:

1. The CN for the biodiesel from soybean and rapeseed oil is slightly lower. The CN of esters correlates well with the boiling points. The CN from palm oil and animal fats are higher.

2. The heat of combustion is 13% lower than for DF2. However, due to the higher density there is eight per cent difference expressed in volume.

3. The viscosity of biodiesel is two times higher.

4. Biodiesel has higher cloud point (CP) and cold filter plugging point (CFPP).

5. Biodiesel is an oxygenated fuel which results in a cleaner burning.

6. Biodiesel has a higher lubricity which is advantageous in low sulfur content.

7. Biodiesel has a higher flash point.

8. Biodiesel does not contain sulfur.

9. Biodiesel has lower fine particulate matter, lower polyaromatic hydrocarbons and SO2 emissions.

10. Biodiesel has higher NOx emissions.

Biodiesel can only be commercialized and sold as biodiesel on condition when it complies with biodiesel standards EN14214:2009 (EN) or ASTM D 6751 (USA). The European specifications are summarized in Table 5.2.

The most important parameters concern the ester content (minimum 96.5%) and the acid value (maximum 0.5 mg KOH/g). The ester content is influenced by the quality of the technology and processing but also by the composition of the used feedstock. The unsaponifiable fraction (sterols, tocopherols, hydrocarbons, etc.) present in the vegetable oils in a range of one to two per cent stays in the biodiesel will decrease the theoretical ester content already to 98-99%. Other important parameters are sulfur, phosphorous, alkali metals, total contamination and non-reacted acylglycerols. The CP is region dependent while there is a difference in the standard for oxidative stability between the EU and the USA (rapeseed biodiesel has a higher oxidative stability than soy biodiesel). Some of these parameters are more difficult to achieve when alternative feedstocks are used. Usage of additional pre-refining and/or post-treatment is required to guarantee the compliance with the biodiesel standards.