11.08.2015   BIOFUELS

Strain selection

Not all microalgae accumulate high concentration of oil, but there are a number of freshwater and marine species that do. Some examples of the oil levels accumulated

Table 7.5. Comparison of an open raceway and closed photobioreactor. (Adapted from Chisti, 2007.)

Parameter

Raceway

Photobioreactor

Biomass production per year (kg)

100,000

100,000

Volumetric productivity (kg/m3/day)

0.117

1.535

Areal productivity (kg/m2/day)

0.035

0.072

Biomass concentration (kg/m3)

0.14

4.00

Dilution rate (day-1)

0.25

0.384

Area needed (m2)

7,828

5,681

Oil yield (m3/ha)a

42.6 (37.5)b

58.7 (51.6)

Carbon dioxide consumption per year (kg)

183,333

183,333

aBased on 30% oil in biomass.

b( ) values in tonnes per hectare; compare this with rapeseed at -1 t/ha.

image128

Fig. 7.7. Various ways of utilizing microalgae for the production of energy including biodiesel.

are given in Chisti (2007) and Scragg (2005). However, not all microalgal oils are suitable for biodiesel production, as some contain high levels of unsaturated fatty acids which reduce the oxidative stability of the biodiesel. In many cases, high oil accumulation is only found under some form of stress such as nitrogen limitation (Illman et al., 2002), and so growth may have to be in two stages in order to obtain high levels of oil. In contrast, heterotrophic growth in glucose stimulated oil accu­mulation in Chlorella protothecoides (Miao and Wu, 2006; Xu et al., 2006). Strain selection will also be important depending on the type of cultivation system used.