The fatty acid profile of biodiesel will reflect the fatty acid profile of the oil used in its production. Table 7.14 gives the fatty acid profiles of biodiesel produced from a variety of oils. The most striking differences are perhaps the biodiesel produced from palm, coconut and linseed oil. Both palm and coconut oil contain high levels of lauric acid and linseed a high concentration of linolenic acid. This will have a profound effect on the characteristics of the biodiesel. The high level of saturated fatty acids in palm biodiesel will mean poor cold temperature characteristics with a cloud point of 8°C and a pour point of 6°C. Linseed biodiesel has a high proportion of unsaturated fatty acid esters and should exhibit rapid oxidation and polymer formation. The properties of biodiesel is affected by the proportion of long — and short-chain fatty acids and the presence of one or more double bonds (saturated and unsaturated) (Fig. 7.14).

Fig. 7.14. The effect of chain length and degree of saturation on cetane number, oxidative stability, lubricity and cold flow.

Biodiesel has a similar energy content and viscosity as diesel in addition to a number of positive attributes such as increased flash point, non-toxic, rapidly bio­degradable, lower emissions and increased lubricity (Table 7.15). Negative aspects are the poor low-temperature characteristics and oxidative stability. Many of these properties are affected by the fatty acid ester content depending on chain length and the presence of double bonds.

Other parameters which are included in the table are acid value and oxidative stability. Acid value is the measure of the unsaturation of the fatty acids in the mixture

Table 7.15. Properties of various biodiesel. (From Graboski and McCormick, 1998; Sheehan et al., 1998; Williamson and Badr, 1998; Srinvastava and Prasad, 2000; Mittlebach and Gangl, 2001; Al-Widyan et al., 2002; Antolin et al., 2002; Haas, 2005; Sarin et al., 2007.) Density Cetane Energy Viscosity Cloud Pour Flash Acid value Oxidation Oil source (mg/ml) number content (kJ/kg) (cPs) point (°C) point (°C) point (°C) (gig/100 g) stability (h) Soybean 0.88 52.5 40.0 4.2 2 -1 169 0.15 3.8 Rapeseed 0.88 52.2 40.5 5.8 -1 -6 162 0.16 5.6 0.32 Sunflower 0.88 48.0 40.0 4.22 0 -4 1 83 0.179 0.2 1.73 Cotton 0.88 51.2 40.2 — — 3 110 — — Palm oil 0.87 53.0 39.5 4.5 8 6 174 0.24 13.37 Tallow 0.88 58.8 40.0 4.45 12 9 106 — Linseed 0.88 — 40 3.32 0 — — 0.335 — Jatropha — 57.1 — 4.4 4 — 163 0.48 3.23 Pongamia — 55.1 — 4.16 4 — 141 0.1 2.35 Cooking oil 0.88 — 39.3 15.1 0 0 109 Soybean 0.88 51.3 — 4.3 6 — 169 soapstock

measured as gI2/100 g sample. Oxidative stability is the time required to induce the production of volatile breakdown products when incubated at elevated temperatures, in hours.