Algae can also be used to produce energy in a number of ways. One of the most competent ways is through exploitation of the algal oils to produce biodiesel. Algal biomass contains three major components: carbohy­drates, proteins, and lipids/natural oils (Dunahay et al., 1996). Because the natural oil made by microalgae is in the form of triacylglycerol molecule, which is the right kind of oil for producing biodiesel, microalgae are the exclusive focus in the algae to biofuel arena. Actual biodiesel yield per hectare is about 80% of the yield of the parent crop oil given in Table 1.5.

In view of Table 1.5, microalgae emerged to be the only source of biodiesel that has the potential to completely replace petroleum diesel. Unlike other oil crops, microalgae grow extremely rapidly and many are exceedingly rich in oil. Microalgae commonly dou­ble their biomass within 24 h. Biomass doubling times during exponential growth are commonly as short as

3— 4 h. Oil content in microalgae can exceed 70% by weight of dry biomass (Metting, 1996; Spolaore et al.,

2006) . Oil levels up to 50% are quite common. Oil pro­ductivity, the mass of oil produced per unit volume of the microalgal broth per day, depends on the algal growth rate and the oil content of the biomass. Microal­gae with high oil productivities are desired for produc­ing biodiesel.

CHEMICAL TRANSESTERIFICATION PROCESS FOR BIODIESEL PRODUCTION

The source oil used in making biodiesel consists of tri­glycerides (Figure 1.7), in which three fatty acid

molecules are esterified with a molecule of glycerol. In biodiesel production, triglycerides are reacted with methanol in a reaction known as transesterification or alcoholysis. Transesterification produces methyl esters of fatty acids that are biodiesel and glycerol (Figure 1.7). The reaction occurs stepwise: triglycerides are first con­verted to diglycerides, then to monoglycerides and finally to glycerol.

At equilibrium, transesterification needs 3 mol of alcohol for every mole of triglyceride to produce 1 mol of glycerol and 3 mol of methyl esters (Figure 1.8). In­dustrial processes use 6 mol of methanol for each mole of triglyceride (Fukuda et al., 2001). This large excess of methanol ensures that the reaction is driven in the di­rection of methyl esters, i. e. toward biodiesel. Yield of methyl esters exceeds 98% on a weight basis.

Transesterification is catalyzed by acids and alkalis (Fukuda et al., 2001). Alkali-catalyzed transesterification is about 4000 times quicker than the acid-catalyzed reac­tion. Thus, alkalis such as sodium and potassium hy­droxide are frequently used as commercial catalysts at a concentration of about 1% by weight of oil. Alkoxides such as sodium methoxide (CH3ONa) act like better cat­alysts than sodium hydroxide and are being increas­ingly used. Use of lipases offers significant advantages, but it is currently not feasible because of the relatively high cost of the catalyst (Chisti, 2007). Alkali-catalyzed transesterification is carried out at about 60 °C under one atmospheric pressure, as methanol boils off at 65 °C at atmospheric pressure. Under these conditions, reaction takes about 90 min to complete (Meher et al.,

CH2-COO-R CH-COO-R’ + ЗСН3ОН CH2-COO-R" Methanol |Triglyceridi|

FIGURE 1.8 Transesterification of oil to biodiesel.

2006) . A higher temperature can be used in combination with higher pressure, but the process becomes expen­sive. During reaction, methanol and oil do not mix; hence, the reaction mixture shows two liquid phases. Other alcohols can be used, but methanol is the least expensive. To stop yield loss due to saponification reac­tions (soap formation), the oil and alcohol must be dry and the oil should have a least of FFAs. Biodiesel is recovered by repeated washing with water to remove glycerol and methanol.