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).
Table 7.10. A comparison of the properties of diesel and plant oil.
Property Diesel Rapeseed oil
Density (kg/l) 0.84 0.778–0.91 Viscosity (cSt) 2.8–3.5 37–47 Flash point (°C) 64–80 246–273
Cetane numbera 48–51 38–50
Calorific value (MJ/kg) 38.5–45.6 36.9–40.2
aCetane number is an indicator of the ignition quality of the fuel and is linked to ignition delay. Standards have been set for cetane number measured against hexadecane (cetane) assigned a value of 100.
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).
+ = +
1
2 2 3
NaOH catalyst 2
2 3 3
3
2 2 3
triglyceride methanol glycerol methyl esters
CH CH OH R COOCH
CH 3 CH OH CHOH R COOCH
CH CH OH R COOCH
(7.4)
The reaction can be catalysed by alkalis, acids, lipase enzymes and inorganic hetero- geneous 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 transesterifica- tion 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).
0 20 40 60 80 100 120
0 1 2 3 4 5 6
Molar ratio (MeOH/oil)
Product(wt %)
TAG Fame
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.)
R-COOH KOH = R-COO K H O2
fatty acid potassium soap
− +
+ + (7.5)
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).
0 5 10 15 20 25 30 35 40 45
0 0.2 0.4 0.6
Free fatty acid (%)
Ester(%)
0 H2O 0.9% H2O
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.)
Fig. 7.10. The production of methyl esters during NaOH-catalysed transesterification.
(Redrawn from Freedman et al., 1986.) 0
20 40 60 80 100 120
0 2.5 5 7.5 10 20 30
Time (min)
Conversion(%)
Alternative catalysts
However, alkalis and acids are not the only catalysts which can be used in the trans- esterification reaction and these include enzymes and solid catalysts. Some of the solid catalysts are listed in Table 7.11.
Transesterification using heterogeneous catalysts has been investigated using basic zeolites and alkaline metal compounds. Metal oxides, hydroxides and alkoxides have been used to transesterify rapeseed oil (Gryglewicz, 1999) where calcium oxide was the most effective. Metal oxides and those loaded with Al2O3, SiO2 and MgO were also used to treat rapeseed oil (Peterson and Scarrach, 1984).
Oil extracted from Pongamia pinnata has been transesterified using a solid Li/
CaO catalyst even in the presence of 0.48–5.75% free fatty acids (Meher et al.,
0 20 40 60 80 100 120
0 2 4 6 8 10 12
Time (h)
Esters(%)
Fig. 7.12. Acid-catalysed esterification of rice bran oil containing 75.8% free fatty acids. (From Zullaikah et al., 2005.)
0 20 40 60 80 100 120
0 1 2 3 4 5 6 7 8 9 10
Time (h)
Components(%)
FFA FAME TAG
First stage
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 al., 2005.)
2006b) and Jatropha curcas oil using CaO (Zhu et al., 2006). A number of modified zeolites have been used successfully to transesterify soybean oil (Suppes et al., 2004).
Much of the research has been with solid base catalysts but solid acid catalysts have also been used. Tungstated zirconia, a solid super acid catalyst, has been used to transesterify soybean oil at 200–300°C, and has given a conversion of over 90%
(Furutaet al., 2004). More recently, amorphous zirconia combined with titanium and aluminium has been shown to give over 95% conversion of soybean oil at 250°C (Furutaet al., 2006).
Microbial lipases have the ability to transesterify oils in the presence of metha- nol. These enzymes function in the presence of water and the catalyst and salts do not need removing at the end of the reaction (Table 7.12). However, the enzymes are more expensive than the simple inorganic catalysts. Some of the expense of using enzymes can be reduced by enzyme immobilization which allows a continu- ous process and increases the working life of the enzyme (Ban et al., 2001; Fukuda et al., 2001).
Table 7.11. Solid catalysts used to produce biodiesel.
Oil Catalyst Reference
Soybean Zeolite Suppes et al. (2004) Metals (Ti, Si)
Jatropha curcas Calcium oxide Zhu et al. (2006) Pongamia pinnata Calcium oxide Meher et al. (2006b) Glyceryl tributyrate Li-calcium oxide Watkins et al. (2004) Soybean Lewis acid Di Serio et al. (2005)
Rapeseed Metal oxides Peterson and Scarrach (1984) Rapeseed Metal oxides, hydroxides, Gryglewicz (1999)
Mixture of oils Fe-Zn cyanide complex Sreeparasanth et al. (2006) Soybean oil Solid super acid
(sulfated Zi and Sn)
Furuta et al. (2004)
Table 7.12. Enzymatic transesterification.
Oil Lipase Conversion (%) Reference
Rapeseed Alcaligenes sp. immobilized on activated bleaching earth
80 Du et al. (2006) Rapeseed Candida rugosa 97 Linko et al. (1998)
Sunflower Mucor meihei 83 Selmi and
Thomas (1998) Waste cooking
grease
Pseudomonas cepacia and Candida antarctica
85.4 Wu et al. (1999) Sunflower Pseudomonas fluorescens 82 Mittelbach (1990) Palm kernel P. cepacia 15–72 Abigor et al. (2000) Soybean Rhizopus oryzae immobilized 90 Ban et al. (2001) Cotton seed oil C. antartica 100 Royon et al. (2007) Soybean oil C. antartica >90 Watanabe et al. (2002)
Transesterification has also been carried out using supercritical methanol, ethanol, propanol and butanol. The process does not require a catalyst but high tem- peratures (∼300°C) and pressures (8 MPa) (Cao et al., 2005; Demirbas, 2006a,b).