14.4.1 Heterogeneous catalysts for biodiesel production

A major concern with using a homogeneous alkali catalyst, such as sodium methylate, for biodiesel production is the large quantity of water used for washing the residual catalyst and soap from the biodiesel. The ratio of water used is typically 1:1, i.e., for every gallon of biodiesel produced, one gallon of washing water is needed. This ratio can be reduced to 10% of the volume of biodiesel produced if the soap is split by acidulation prior to or during washing (Van Gerpen, 2005). The consumption of wash water can be virtually eliminated if facilities are in place to recycle and reuse the wash water.

Another drawback of using a homogeneous base catalyst is soap formation due to the high FFA content (e.g., greater than 5 wt%) of the feedstocks, such as waste vegetable oils or microalgal oil. Soap formation in biodiesel production not only adversely affects the biodiesel yield but also makes biodiesel separation from the byproduct glycerin very difficult. When high concentrations of FFAs are present, a two-step process, i.e., a strong acid-catalyzed esterification followed by a base-catalyzed process, is needed to avoid the problem of significant soap formation (Canakci and Van Gerpen, 1999, 2001).

The advantages of using heterogeneous catalysts in biodiesel production include the elimination of water-washing or dry-washing of the post-reaction mixture for catalyst and soap removal. Other advantages include decreased production of waste water, reuse of the heterogeneous catalysts, high productivity per unit of reactor capacity, and easier scale-up for continuous-flow processes. It is also claimed that heterogeneous catalysts can catalyze both the esterification and transesterification reactions so that the two-step process is not required to deal with high FFA feedstocks (Furuta et al., 2004). A major disadvantage of using heterogeneous catalysts, however, is the high catalyst cost and higher operating cost due to the elevated temperatures usually required (Kiss et al., 2010; Sakai et al., 2009). To date, no cost-effective and highly efficient heterogeneous catalyst has been identified for practical commercial use.

Heterogeneous catalysts for biodiesel production are mainly metal hydroxides and oxides and other non-metal-based compounds (Borges and Diaz, 2012; Semwal et al., 2011; Chouhan and Sarma, 2011). The catalyst supports are typically aluminum, activated carbon, and organic resin. Table 14.1 provides an overview of the current efforts in exploring suitable heterogeneous catalysts for biodiesel production. Generally, all of the reported catalysts lack activity at low operating temperatures (65°C or lower). Even with extended periods of reaction time, the overall biodiesel yields or vegetable oil conversion rates catalyzed by heterogeneous catalysts are low (less than 98%) and are not comparable with those by homogeneous catalysts. This incomplete transesterification usually translates to poor quality biodiesel and it is likely that the fuel will not meet the standards specified by the ASTM.

There are various methods to increase the catalyst activity, one of which is to raise the operating temperature to 120°C, or even to 200°C and higher, so that the metal catalysts start to show their advantageous catalytic activity (Jitputti et al., 2006; Garcia et al., 2008; Carmo et al., 2009: Peng et al., 2008; Park et al., 2010). However, this will obviously increase operating costs. Other cost considerations must be made when using heterogeneous catalysts in biodiesel production. This process requires a much higher methanol-to-vegetable oil molar ratio, as high as 60:1, in order to achieve a reasonable reactivity (Benjapornkulaphong et al., 2009; Garcia et al., 2008; Carmo et al., 2009; Nakagaki et al., 2008). Such a high methanol-to-oil molar ratio leads to additional effort in methanol recovery, re-purification, and reuse, and thus adds to the operating cost (Sakai et al., 2009; Kiss et al., 2010). Some catalyst producers recommend that the fuel be vacuum distilled to remove the products of incomplete reaction, which is an indication of low catalyst activity.

Most of the heterogeneous catalysts described in the literature were tested in a powder form or a slurry suspended in a vegetable oil and methanol mixture with high mixing intensity (up to 1500 rpm) in a batch mode. Keeping these catalysts suspended in large-scale, continuous-flow reactors would be a challenge. Pelletized or other shaped heterogeneous catalysts are typically not developed unless a catalyst has proved to be effective and worth the effort. At that stage, some engineering challenges must be overcome, such as the strength and pore sizes of the structure and ensuring adequate capability for mass transfer. Catalyst poisoning is also a serious consideration for commercialization. Another problematic phenomenon is the reusability of the heterogeneous catalysts. Some catalysts appear to lose their reactivity quickly due to leaching after being used for only a few batches (Liu et al., 2007, 2008; Faria et al., 2008; Di Serio et al., 2010; Chen et al., 2012).

14.4.2 Ultrasonic processing

Application of ultrasonication in chemical reaction systems has been extensively researched due to its effective production of radicals for initiating chemical reactions and its localized micro-scale cavitation and mixing (Shol, 1988; Flint and Suslick, 1991). The study of chemical changes induced by ultrasonication is now recognized as the science of sonochemistry and the chemical reactors that incorporate ultrasound are referred to as sonochemical reactors (Mason, 2000; Thompson and Doraiswamy, 1999).

The insolubility of methanol in vegetable oils is a limiting factor for the transesterification of triacylglycerides for biodiesel production (Van Gerpen, 2005). To overcome this problem, mechanical mixing is typically applied in conventional processes to improve the reaction rate. Recognizing its effectiveness in creating micro-scale cavitation and intensified local mixing, researchers have attempted to apply ultrasonication in transesterification of vegetable oils and animal fats for biodiesel production (e.g., Ji et al., 2006; Wu et al., 2007; Hanh et al., 2009).

The level of positive effect of ultrasonication on transesterification varies largely among reports. One consistent advantage is the shorter reaction time, which ranges from one minute (Teixeira et al., 2009) to a few minutes (Stavarache et al., 2005; Singh et al., 2007; Kumar et al., 2010a). Although most work has been done with conventional homogeneous catalysts, similar phenomena were observed in systems where heterogeneous catalysts were used (Kumar et al., 2010b; Yu et al., 2010).

Currently, most reports published on ultrasound-assisted transesterification are laboratory evaluations. The exact mechanism behind the enhancement by ultrasonication is not yet clearly understood. Based on experience with other ultrasound-assisted chemical systems, the ultrasonic cavitation combined with the radial motion of the ultrasonic cavitation bubbles are proposed to be the reason for the accelerated reaction rates (Mootabadi et al., 2010). The localized high intensity energy generates microbubbles that create increased interfacial areas to overcome the mass transfer limit. Such high energy density may also overcome the activation energy to initiate the transesterification (Singh et al., 2007). However, such a phenomenon is significant only in the close proximity to the ultrasound transducer (Monnier et al., 1999a, 1999b; Cintas et al., 2010). Another important finding under specific experimental conditions is that chemical radicals are not the reason for the enhanced transesterification under ultrasonication as claimed in other sonochemical systems (Kalva et al., 2009). This may be due to the fact that the moderate intensity of ultrasonication used for biodiesel production does not generate enough energy to produce radical formation.

Some use of ultrasound-assisted systems in smaller biodiesel production plants has been reported but no technical details are provided. According to the nature of ultrasonication and the experience obtained from other sonochemical systems, chemically effective and economically viable sonochemical reactors for biodiesel production still require additional research and development, especially regarding the engineering aspects of the sonochemical reactors (Thompson and Doraiswamy, 1999). The widespread commercial application of ultrasonication biodiesel production may take another decade. A robust yet highly productive transesterification system may be created through a combination of sonochemical reactors with heterogeneous catalysts for biodiesel production, which may also be more suitable for small producers (He and Van Gerpen, 2012a).

14.4.3 Supercritical processing

Conventional processes for biodiesel production use homogeneous catalysts, such as sodium methoxide or hydroxide which require water washing or ‘dry washing’ with absorbent materials to remove soaps from the crude biodiesel. This washing step requires extra equipment and consumes the homogeneous catalyst, which increases the overall operating cost. One non-catalyzed approach for biodiesel production is the processing of vegetable oils in supercritical methanol. Saka and his team first reported non-catalytic supercritical biodiesel preparation starting in the late 1990s and have published extensively in this field (e.g., Saka and Kusdiana, 1999, 2001a, 2001b; Kusdiana and Saka, 2001a, 2004a; Warabi et al., 2004a, 2004b; Saka et al., 2006).

Supercritical processing of vegetable oils can achieve the necessary transesterification without the need for a catalyst. Once in the supercritical stage, methanol has a much stronger solvent effect that dissolves the vegetable oils and thus overcomes the insolubility between the vegetable oils and methanol, allowing the reaction to proceed quickly in a homogeneous phase at a high reaction temperature. The enhanced solvent capability of supercritical methanol is attributed to its much weakened hydrogen bonds and highly reduced polarity (Yamaguchi et al., 2000). Most reported research uses supercritical methanol as the solvent. The operating conditions are close to or above the supercritical properties of methanol (i.e., 240°C and 8.1 MPa) and in the range of 270–430°C. Although a higher temperature provides a more effective supercritical fluid, considering the thermal stability of the unsaturated fatty acids and the increased operating cost due to high temperature and high pressures, a temperature range of 270–300°C is recommended (He et al., 2007b; Imahara et al., 2008).

In addition to the elimination of the catalyst, another advantage of supercritical methanol processing is its tolerance of a high FFA concentration in the feedstock. This is due to the fact that FFA can be esterified directly to methyl esters (Kusdiana and Saka, 2001b).

Generally water in the vegetable oil adversely affects the process (e.g., soap formation which requires extra catalyst) and the biodiesel quality (e.g., hydrolyte formation) in the conventional alkali-catalyzed transesterification process (He and Van Gerpen, 2012b). However, in supercritical methanol processing, the presence of water was found to have no negative effect on biodiesel (i.e., methyl esters) production (Kusdiana and Saka, 2004a), because hydrolysis of triacylglycerides caused by the presence of water, if it occurs, leads to the formation of free fatty acids, which are then esterified into methyl esters. Therefore, a small amount of water in the vegetable oil or animal fat is not a major concern in supercritical biodiesel production.

Short chain, primary alcohols are the logical choice for transesterifying vegetable oils and animal fats (Warabi et al., 2004a). However, other solvents can also be used in supercritical processing for biodiesel production, such as dimethyl carbonate (Ilham and Saka, 2010), methyl acetate and other carboxylate esters (Saka and Isayama, 2009; Goembira et al., 2012; Niza et al., 2012).

Recognizing the potential advantage of triacylglyceride hydrolysis in supercritical methanol to free fatty acids and subsequent esterification to methyl esters, a two-step process has also been studied by researchers. In this two-step process, vegetable oil was hydrolyzed in sub-critical water for 20 min at 270°C into free fatty acids and glycerin byproduct. After the mixture of glycerin and excess water was separated out, the free fatty acids were fed into a supercritical methanol reactor and esterified into methyl esters at a comparable operating temperature and for a comparable reaction time (Kusdiana and Saka, 2004b; Minami and Saka, 2006). Using acetic acid to replace water for triacylglyceride hydrolysis under subcritical conditions was also studied by the same group and claimed to have better effectiveness (Saka et al., 2010).

14.4.4 Purification by adsorbents and resins

Magnesium silicate is widely used to remove free fatty acids and other polar compounds from used cooking oils to extend their life. It can also be used to purify biodiesel by adsorbing free glycerin, soaps, methanol as well as monoglycerides and sterol glucosides. Approximately 1% magnesium silicate powder is added to the biodiesel at 60–65°C. However, the exact treatment level depends on the amount of contaminants to be removed. The mixture is agitated for 20 minutes and then the adsorbent is removed by filtration. Typically, the residual methanol is removed from the crude biodiesel before the adsorbent is added so the active sites on the adsorbent particles are not overwhelmed by the alcohol (Berrios and Skelton, 2008).

An alternative process uses ion exchange resins to remove free glycerin and soaps. The resins usually consist of small (∼0.5 mm) styrene beads with treated surfaces. The beads are placed in fixed beds and the biodiesel is pumped through the beads. When soap molecules in the biodiesel contact the resin beads, hydrogen ions from the beads are exchanged with the sodium (or potassium) ions from the soap (Wall et al., 2011). Thus, removal of the soap causes an increase in the acid value of the biodiesel. Soap levels above about 2500 ppm may cause the resulting fuel to exceed the ASTM specified acid value. Glycerin is removed by adsorption to the resin bead surface, a connection that can be overcome by washing the beads with methanol, this allows the resin beads to regenerate. It is also claimed that magnesium silicate can also be utilized in fixed beds with regeneration by methanol wash.

14.5.1 Traditional feedstocks

Currently, soybean oil is the dominant feedstock in the US while canola and corn oils, animal fats, and yellow greases, are the supplemental feedstocks. Soybean oil is a co-product of the soybean meal industry. Conventional soybean seeds contain about 18–20% oil. Despite the low oil content, soybean oil produced in the US is still available in large quantities due to the vast amount of soybean production. For example, 2.97 billion bushels or approximately 80 million metric tons of soybeans were produced in 2012 (USDA, 2012a), which is approximately equivalent to 17.6 × 10⁶ m³ (or approx. 4.6 billion gallons) of oil. In 2012, 1.12 million metric tons (2.48 billion pounds) of canola seed was produced, which is equivalent to approximately 146 million gallons of oil (USDA, 2012b). However, most of the traditional feedstocks have established markets and cannot easily migrate to the biodiesel industry.

The ‘double-zero’ rapeseed or canola oil is the major feedstock for biodiesel in Europe. Rapeseed adapts well to a wide variety of climate and soil conditions and is cultivated widely around the world. The Canadian modified cultivar of rapeseed or canola is very low in the undesirable erucic acid and glucosinolates, and is now the main cultivar planted for its oil in Europe and North America. Total biodiesel production by the EU-27 was 9.57 million tons (approx. 2.31 billion gallons) in 2010 (European Biodiesel Board, 2012). Although there were imported feedstocks such as palm oils, this production was mainly from rapeseed/canola oils.

It is predicted that rapeseed/canola production in 2012 will be 20 million metric tons in the EU-27 and 14.5–15.5 million metric tons in Canada (Mielke, 2012). If 45% oil content (CGC, 2012) is assumed, the canola oil produced by the EU-27 and Canada in 2012 will be approximately 9.0 million tons (or 2.6 billion gallons) and 6.75 million tons (or 1.95 billion gallons), respectively. It is expected that the biodiesel production by the EU-27 will increase in 2012. Therefore, in order to keep growing, biodiesel producers in Europe must explore other feedstock opportunities while competing with the food market for canola oil.

Vegetable oils are composed mainly of triacylglycerides, which are the glycerin esters of different fatty acids. When processed for food use, these oils contain very low free fatty acids, and are very low in sulfur and other heterogeneous chemicals. This makes them high quality feedstocks for biodiesel production, especially when using homogeneous alkali-catalyzed conversion processes. When using food-grade vegetable oil as the feedstock, soap formation is typically not a concern in biodiesel production.

Different oils and fats are characterized by the fatty acid chains that are present in the triacylglycerides. The nomenclature for identifying fatty acids is commonly C_X:Y, where X is the number of carbon atoms in the fatty acid chain and Y is the number of double bonds. The fatty acid profiles for common vegetable oils and animal fats are shown in Table 14.2.

Chemically, animal fats are composed of the same compounds as those in seed oils but with fatty acid profiles that contain larger fractions of saturated fatty acids (C_16:0 and C_18:0). Animal fats are the byproducts of the livestock and poultry industries, and are relatively inexpensive feedstocks. Although large quantities of animal fats are produced annually, these fats have established markets as food and food additives (such as butter, lard, and shortenings), animal feed additives, and industrial products such as fatty acids, soap, paints, and lubricants.

Greases are generally used vegetable oils and/or animal fats. Yellow grease, or waste vegetable oil (WVO), consists mostly of used cooking oils, and brown grease is typically recovered from grease traps of restaurants and food processing plants. Greases usually contain large amounts of free fatty acids and other polymerized and/or hydrolyzed compounds after repeated uses at elevated temperatures. Due to the manner of its collection, greases are commonly high in water content which complicates the conversion process. Brown grease is even more problematic due to its contamination by cleaning agents. Therefore, greases are low quality feedstocks for biodiesel production, and special processing and/or pretreatment are required to avoid problems caused by impurities in order to have a quality biodiesel product.

14.5.2 Emerging feedstocks

Emerging feedstocks, including camelina, jatropha, pennycress, and microalgae, have also been researched and utilized for biodiesel production. Camelina, a member of the Brassica family, has many species but the most important for biodiesel is Camelina sativa (Vollmann et al., 1996). It has recently attracted the attention of farmers in the northern US (McVay and Lamb, 2008). Camelina adapts well to marginal lands having dry and cool climate conditions with a short growing season that, with adequate rainfall, allows for double cropping. The yield of camelina in North America is about 890–1,350 kg/ha (800–1,200 lb/acre) and the oil content of camelina seeds varies from 30 to 40% (McVay and Lamb, 2008). Camelina oil consists of up to 88% unsaturated fatty acids (Putnam et al., 1993), which contribute to the desirable cold flow properties of the biodiesel produced from it. However, camelina seeds are very small (only up to 2 g per 1,000 seeds) and the plant is very strong when ripe, which makes harvesting camelina very difficult.

Jatropha is a perennial shrub that grows in various harsh climates and poor soil conditions. The jatropha plant and its fruits and seeds contain poisonous toxins so it is considered to be a non-food crop (Dias et al., 2012). Jatropha seeds contain up to 35% non-edible oil (Kumar and Sharma, 2008) and its oil yield can be as high as 1,900 kg/ha (1,690 lb/acre), making it a good candidate for biodiesel production. Jatropha fruits ripen year round and harvesting is very labor-intensive. Unless automated harvesting machines are invented, jatropha fruit harvesting is unlikely to be cost-effective. Jatropha is being actively explored by countries that have limited agricultural land but are still interested in seeking oil sources for biofuels such as India, China, and other developing countries (Kumar et al., 2012; Yang et al., 2012; Mofijur et al., 2012). However, the suitability of jatropha as a large-scale domesticated crop has been questioned and discussions are underway to find sustainable ways to produce it (Contran et al., 2013; Kumar et al., 2012).

Pennycress is a collective name applied to a group of species belonging to the Brassicaceae family. Field pennycress, Thlaspi arvense, is a variety that has been identified as having industrial potential, especially for use as a feedstock for biodiesel. The seeds of pennycress contain up to 36% oil and are high in erucic and linoleic acids (Moser et al., 2009b). Pennycress seeds are small. Seeds harvested in Peoria, Illinois, have average dimensions of 1.87 × 1.35 × 0.74 mm and 1000 seeds weighed only 0.97 g at 9.5% moisture (Evangelista et al., 2012). However, the seed yield can approach 1,420 kg/ha (1,265 pounds/acre). Field pennycress is basically considered to be an annual weed, growing in open spaces and roadsides (Mitich, 1996). This species is tough and its seeds can have a long life, lasting approximately 20–30 years (Koundinya and Hansen, 2012). Evaluation of biodiesel produced from pennycress oil has reported desirable properties due to the high contents of erucic (32.8 wt%) and linoleic (22.4 wt%) acids. The biodiesel produced from pennycress oil has a cetane number close to 60 and exhibits favorable properties at low temperatures (Moser et al., 2009a). To be established as a feasible feedstock for biodiesel production, more studies are needed on pennycress to establish its agronomic, harvesting, and processing requirements as well as its environmental and social impacts.

One of the most promising feedstocks for biodiesel production is believed to be microalgae oil (Wu et al., 2012; Ahmad et al., 2011; Stephens et al., 2010; Chisti, 2008). As a potential source for biofuel production, microalgae have been exploited and researched extensively (e.g., Sheehan et al., 1998). Microalgae can be cultivated in open ponds or in bioreactors (Chen et al., 2011). If strains of microalgae contain 30% lipids, the productivity of algal oil can be as high as 58,700 L/ha (6,270 gal/acre); as a comparison to traditional oil seeds, canola productivity is only 1,190 L/ha (127 gal/acre) (Chisti, 2007). Practical production of microalgal lipids for biodiesel production still faces many challenges. Among them, cost effectiveness is the most critical (Stephens et al., 2010; Harun et al., 2011). Difficulties exist not only in producing oil efficiently from high lipid strains but also in cost-effective cultivation and harvesting systems. Of particular significance is the high energy requirement for the de-watering and drying processes. Currently, research and development on producing biodiesel from microalgal oils is very active, and operations at pilot and demonstration scales are reported, but no reports are available yet on commercial microalgal biodiesel production. The fatty acid profiles of these emerging plant oils are provided in Table 14.3.

14.6.1 Feedstocks with high content of free fatty acids

Most biodiesel is currently produced using high quality soybean oil catalyzed by homogeneous alkali catalysts. However, other feedstocks such as waste vegetable oils and/or microalgal lipids can be high in FFAs requiring a different approach. The FFA content ranges between 2 and 7% for used vegetable oils, 5 and 30% for animals fats, and 0 to 100% for some low quality trap grease (Van Gerpen et al., 2004). If a feedstock contains high levels of FFA, typically 5% or higher, alkaline catalysts are not suitable for use in triacylglyceride transesterification. This is because the alkaline catalysts, such as sodium methoxide, will react with the FFA directly to produce fatty acid salts or ‘soaps’. Soap formation not only consumes the catalysts but also causes the reacting mixture to emulsify. This can lead to difficulties in processing and to a low quality biodiesel product. Early research by Freedman et al. (1984) demonstrated the significant FFA effect on the transesterification process catalyzed by alkaline catalysts. At the same operating conditions, the product yield of methyl esters was reduced from 93–98% to 67–86% due to the presence of a significant amount (approx. 6.7%) of free fatty acids in the oil. Soap generally partitions into the byproduct glycerin and is considered waste if not recovered with additional processing. Generally speaking, if a feedstock contains no more than 1–2% FFA in alkaline-catalyzed transesterification, the FFA can be ignored; if 2–5% FFA, additional alkaline catalyst is needed to account for the consumption of catalyst due to soap formation and special attention needs to be paid to the procedures. An acid-catalyzed pretreatment, or distillation to remove the FFAs, is required if the oil or fat contains more than 5% FFAs.

It has been shown that strong acid-catalyzed transesterification is too slow and requires much higher alcohol levels; however, acid catalysts are very effective in catalyzing the esterification of FFA to methyl esters (Canakci and Van Gerpen, 1999). Therefore, a two-step process was proposed and is now considered to be the standard process for converting high FFA feedstocks, e.g., waste vegetable oils, into biodiesel (Canakci and Van Gerpen, 2001; Van Gerpen et al., 2004). In this two-step process, a strong acid is used to first convert the FFA into alkyl esters through an esterification process, and then the triacylglycerides are converted to alkyl esters via alkaline-catalyzed transesterification. The byproduct of the FFA esterification reaction is water. It has to be removed before the mixture proceeds to the transesterification process, which follows the same steps as the transesterification of high quality oil described previously.

14.6.2 Impurities that affect product quality

14.7.1 Feedstock availability

Despite its advantages, biodiesel production is constrained by the availability of feedstock. Biodiesel production in the US was approximately 1.05 billion gallons in 2011 (US EPA, 2012a), and approximately 830 million gallons in the first nine months of 2012 (US EPA, 2012b). The National Biodiesel Board expects the total biodiesel production to be up to 2 billion gallons (or 7.3 × 10⁶ m³) in the next few years (Jobe, 2012). The estimate requires a considerable increase in feedstock supply, and most likely will require major new sources beyond soybean oil.

As the world’s largest biodiesel producers, the EU-27 and North America are facing a challenge of limited feedstock supply. Biodiesel production in the US will grow as the Renewable Fuel Standard mandates expand and world demand for diesel fuel increases. Biodiesel production in the US will be limited to approximately 2 billion gallons/year because of feedstock availability, as mentioned above. Before economically viable microalgal and jatropha oils are available for biodiesel production, biodiesel producers will find no easy solution to the issue of feedstock shortage. Feedstock suppliers will balance their profitability between selling to existing markets, especially the food market, and to the biodiesel industry.

Another limitation preventing the biodiesel industry from expanding is the cost relative to petroleum diesel fuel. It is generally agreed that biodiesel is more expensive than petroleum diesel in the market, and the major cause of this is the feedstock cost. An estimate for a plant of 17,400 tons/year (5 million gallons/year) has revealed that oil is the dominant contributor, at 80.6%, of the total production cost (Van Gerpen et al., 2006). This observation is consistent with an early study by Haas et al. (2006) on a 37,850 m³/year (or 10 million gallon/year) plant, in which the feedstock (soybean oil) cost was 88% of the total cost. Although the byproduct, glycerin, can be recovered and marketed to offset some of the cost, the dominance of the oil cost will not change much.

Varying production costs are expected when different business models are adopted; however, the production cost is still dependent on the cost of the feedstock. It is logical, therefore, to use low cost feedstocks, such as animal fats and waste vegetable oils or greases. In today’s market, low cost feedstock usually means low quality feedstock. Additional cost is needed to upgrade and handle the low quality feedstocks.

14.7.2 Cold flow properties and oxidative stability

Biodiesel is prone to the issue of poor low temperature operability due to its content of long-chain, saturated fatty acids. The cold flow properties of biodiesel, as characterized by its cloud point (CP), pour point (PP), and cold filter plugging point (CFPP), are less satisfactory than petroleum diesel. CP is the temperature at which a fuel starts to show observable crystals upon cooling under defined conditions (ASTM D2500, 2005). PP is the lowest temperature at which a fuel can maintain its flowability or ability to be pumped. The CFPP for biodiesel is defined as the highest temperature at which a given volume of fuel fails to pass through a standardized filtration device under specified testing conditions.

All cold flow properties are related to the melting points (m.p.) of the fuel constituents and their solubility in the fuel. A high m.p. component will crystallize and precipitate out when its concentration in the fuel is beyond its solubility. The components of biodiesel generally have higher melting temperatures than those of petroleum diesel, especially the long-chain, saturated fatty acid esters. The longer the fatty acid chain, the higher the melting point of the component. When unsaturated bonds are present in the alkyl chain, the m.p. of the component decreases considerably. For example, the m.p. of methyl oleate (C_18:1) is −20°C, while the m.p. of linoleate (C_18:2) is −35°C. Another factor affecting the cold flow properties is the solubility of a component in the fuel. A high m.p. component will not crystallize unless its concentration is higher than the quantity the fuel can dissolve. The final CP or PP of a biodiesel is the collective outcome of the properties of the individual methyl esters and their liquid–solid equilibria in the fuel. The CP and PP of biodiesel largely depend on the fatty acid profiles of the feedstock, as shown in Table 14.4. Traceable impurities, such as plant sterol glucosides, can also affect the biodiesel cold flow properties. Plant sterol glucosides have higher melting points and limited solubility in biodiesel. Once in biodiesel, they may crystallize and serve as the nuclei for other high m.p. components to agglomerate.

Biodiesel is relatively unstable compared to petroleum diesel. This is mainly due to the chemical characteristics of the biodiesel constituents. The fatty acid chains contain variable numbers of unsaturated carbon to carbon (C–C) bonds, depending on the feedstock, with the carboxyl groups linked with alkyl groups at one end. These unsaturated C–C double bonds are non-conjugated in structure and are thermochemically vulnerable to oxidation. The mechanism is that alkyl radicals are formed first at the positions of unsaturation following an attack by oxidants; then hydroperoxides are formed, followed by a series of degradation and polymerization reactions. The presence of free mineral acids catalyzes the formation of alkyl free radicals (Shahidi, 2005; Frankel, 2005). The relative rate of oxidation depends on the level of unsaturation. The more double-bonds an oil or a fat has, the more it is prone to oxidation. The relative vulnerability of the oleic (C_18:1), linoleic (C_18:2), and linolenic (C_18:3) chains is 1:12:26 after 30 days of storage (Chapman et al., 2009). Other literature reports even higher oxidation rates of 1:40:80 for the same fatty acids (Witting, 1965). This difference is largely attributed to the environment to which the oil or fat is exposed. A large number of chemicals can be produced from such oxidation reactions, including aliphatic alcohols, aldehydes, short-chain organic acids, and polymers (Loury, 1972; Neff et al., 1993; Andersson and Lingnert, 1998; Bondioli et al., 2002). The adverse consequences of such chemicals in biodiesel are reduced flash point, rancidity, increased acidity, increased viscosity, and accelerated fuel degradation. Biodiesel oxidative instability may be accelerated due to the impurities present in the fuel. Minerals such as rust from the containers serve as catalysts for decomposition reactions. Incompletely reacted products, indicated by the presence of excess monoglycerides and free glycerin, likely provide a reactive environment. The potential for high moisture retention in biodiesel (15–25 times more than that in petroleum diesel; He et al., 2007a) also provides favorable conditions for biodiesel oxidation.

To ensure its quality in long-term storage, biodiesel needs to strictly meet the ASTM D6751 or EN21414 specifications. Rusty tanks need to be thoroughly cleaned before being filled with biodiesel. Direct sunlight and high temperatures should be avoided. Nitrogen blanketing should be used to reduce air contact and moisture absorption in the biodiesel. For the long-term storage of biodiesel, a biocide application is recommended to prevent biological contamination.