Biodiesel is a replacement for diesel and is produced by reacting plant oils and animal fats with an alcohol to form a mixture of fatty acid esters in a reaction known as transesterification. Biodiesel is available commercially and should be regarded as a first-generation biofuel. The idea of splitting the triglycerides in fats and oils and using the resulting esters as a fuel has been around for a considerable time. Walton, in 1938, suggested the splitting of triglycerides (Graboski and McCormick, 1998), and there is a report of fatty acid esters being used as a fuel in the Congo in 1937 (Knothe, 2001). Subsequently, there have been a number of reports of using plant oil/diesel blends in engines where the problems of high viscosity of oil were encountered. One of the first reports of the use of esters was in 1980 using sunflower oil esters which appeared to remove many of the problems associated with untreated oils, in particular, viscosity. Since then, there has been a considerable number of reports on the production of fatty acid esters from a wide range of fats and oils. The European quality standards for fatty acid methyl esters, known as biodiesel, came into force in 2004 and are known as EN 14214 (biodiesel) and EN 14213 (heating fuel) (Schober et al., 2006).

Transesterification of plant oils is the conversion of the triglycerides which make up oils into fatty acid esters and glycerol. Triglycerides are the main component of fats and oils and consist of three long-chain fatty acids linked to a glycerol backbone. When the triglyceride reacts with an alcohol, the three fatty acids are released and combined with the alcohol to form alkyl esters. Transesterification of pure oils can be carried out rapidly with methanol and NaOH as the catalyst (Van Gerpen, 2005). Methanol is normally used as the alcohol, although ethanol, 2-propyl and 1-butyl will also suffice (Lang et al., 2001).

The reaction can be catalysed by alkalis, acids, lipase enzymes and inorganic hetero­geneous catalysts (Fukuda et al., 2001; Vincente et al., 2004). The conditions for catalysis are a temperature near to the boiling point of methanol (60°C), although room temperature will suffice with pure oil, a molar ratio of alcohol/oil of between 3:1 and 6:1, and NaOH as the catalyst. The stoichiometric molar ratio of methanol/ oil is 3:1 but in order to drive the reaction towards ester formation the ratio is increased to ratios of up to 9:1. The effect of the molar ratio of methanol/oil on the process of transesterification is shown in Fig. 7.9.

The transesterification reaction requires catalysis and apart from alkali catalysts others have been used including acids, enzymes and solid catalysts (Suppes et al., 2004; Vincente et al., 2004; Meher et al., 2006a). The alkali-catalysed transesterifica­tion is by far the fastest process (Fig. 7.10), but is sensitive to impurities in the raw materials.

The presence of water and free fatty acids in the oil consumes alkali, and forms soaps which in turn produce emulsions. Emulsions stop the separation of glycerol as the reaction proceeds, which reduces the yield of biodiesel (Fig. 7.11).

Fig. 7.9. The effect of the methanol/oil ratio on methyl ester production. MeOH, methanol; TAG, triacylglycerols; FAME, fatty acid methyl esters. (Redrawn from Freedman et al, 1986.)

Fig. 7.10. The production of methyl esters during NaOH-catalysed transesterification. (Redrawn from Freedman et al., 1986.)

Fig. 7.11. Effect of the presence of free fatty acids and water on the NaOH- catalysed transesterification of beef tallow. (Redrawn from Ma et al., 1998.)

R-COOH + KOH = R-COO-K+ + H2O (7.5)

fatty acid potassium soap

In extreme cases, the treated oil will set into a gel formed from a combination of glycerol and soap. An ester yield of less than 5% was obtained in the presence of 0.6% free fatty acids (Canakci and van Gerpen, 1999; Usta, 2005). Therefore, oils containing no water and less than 0.5% free fatty acids are required for successful alkali catalysis. These properties can be obtained with most plant oils, but waste cooking oils, rendered fats and some plant oils contain between 0.7 and 24% water and 0.01-75% free fatty acids (Zhang et al., 2003; Meher et al., 2006a; Canakci, 2007). Unfortunately, there are large amounts of unrefined plant oils, waste cooking oils and soapstocks available for biodiesel production. Acid catalysts, mainly sulfuric, hydrochloric and phosphoric acids, have not been used widely as the reaction is very much slower than the alkali catalysts (Fig. 7.12), but acid catalysis is not affected by free fatty acids.

Therefore, a two-stage process has been developed where in the first stage acid catalysis is used to esterify the free fatty acids, and the alkali-catalysed system is used in the second stage to transesterify the triglycerides (Zullaikah et al., 2005; Wang et al., 2006) (Fig. 7.13).

Fig. 7.13. The two-stage production of biodiesel from oil containing 50% free fatty acids. Stage one is catalysed by sulfuric acid and the second is alkali-catalysed. FFA, free fatty acids; FAME, fatty acid methyl esters; TAG, triacylglycerols. (From Zullaikah et a/., 2005.)