One of the major problems with biodiesel is its poor low-temperature flow proper­ties shown by its high cloud and pour points. At the cloud point, long-chain fatty

Fig. 7.15. The effect of ester chain length on cetane number. (Redrawn from Graboski and McCormick, 1998.)

Fig. 7.16. The effect of double bonds on cetane number. (From (▲) Graboski and McCormick, 1998; (■) Knothe et al., 2003.)

acid esters begin to form small wax crystals and when these reach 5 pm in size, the fuel begins to look cloudy. As the temperature decreases, the crystals grow and aggregate in a form which can plug filters and eventually it ceases to pour. Often low-temperature filterability (LTFT) is used as a low-temperature characteristic which is generally halfway between cloud and pour point. Several methods have been used to improve low-temperature characteristics including additives, branched chain esters and winterization.

Treatment with chemical additives, pour depressants, is one way that conven­tional diesel is modified to improve its low-temperature characteristics.

The molecular structure of pour depressants is polymeric hydrocarbon chains with polar groups which function by adsorption, co-crystallization, nucleation and improved wax solubility. Vegetable oils lack polar groups but ozonation can reduce the pour point (Soriano et al., 2005). Other cold flow improvers have been tested, and these reduced the pour point but had little effect on cloud points (Chiu et al., 2004).

Using branched alcohols, such as iso-propanol or iso-butyl alcohol in place of methanol and ethanol in the transesterification process, yields esters which have lower pour points. However, this appeared to increase the viscosity of the biodiesel and also resulted in incomplete esterification.

Winterization of biodiesel has been carried out which increases the unsaturated fatty acid esters. The saturated fatty acids are removed as they have higher melting points. The process reduces biodiesel yields and the high concentration of saturated fatty acids means a lower cetane value.