The use of microalgae to sequester carbon dioxide was proposed in the past by a number of authors (Benemann, 1997; Sheehan et al., 1998; Chisti, 2007; Skjanes et al., 2007).

Microalgae have been proposed as systems for the sequestration of carbon diox­ide (Sawayama et al., 1995; Zeiler et al., 1995; Ono and Cuello, 2006; Cheng et al., 2006; de Morais and Costa, 2007a, b) and the production of biofuels (Chisti, 2007). The biofuels include biogas (methane) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Minowa et al., 1995; Sawayama et al., 1995; Miao and Wu, 2006; Xu et al., 2006; Chisti, 2007), and biohydrogen (Fedorov et al., 2005), and the direct use of algae in emulsion fuels (Scragg et al., 2003).

Microalgae should be considered to have the following features:

• Higher photosynthetic efficiency than terrestrial plants.

• Rapid growth rate, doubling times of 8-24 h.

• High lipid content 20-70%.

• Direct capture of carbon dioxide, 100 t algae fix ~183 t carbon dioxide.

• Can be grown on a large scale.

• Will not compete with terrestrial plants in food production.

• Produce valuable products.

• Freshwater and marine species.

• Have a much better yield of oil per hectare: oil palm 5000 t/ha, algae 58,700 t/ha (Chisti, 2007).

Figure 3.12 shows a possible system for carbon dioxide sequestration and biofuel production using carbon dioxide from a stationary carbon dioxide source such as a power station. Microalgae can fix large quantities of carbon dioxide but it is likely that only a proportion of the carbon dioxide in the flue gases will be removed. Also the flue gases from power stations contain other gases which may affect the growth of microalgae. A number of studies have been carried out on the effect of flue gases on microalgae. Nannochloris sp. was shown to grow in the presence of 100 ppm

Fig. 3.12. Possible sequestration of carbon dioxide from a power station and use of Biodiesel algal biomass to produce biodiesel.

nitric oxide (NO) (Yoshihara et al., 1996). Tetraselmis sp. could grow in flue gas containing 185 ppm sulfur oxides, 125 ppm nitrogen oxides and 14.1% carbon dioxide (Matsumoto et al., 1995). A Chlorella sp. was also found to grow in the pres­ence of various combinations of sulfur and nitrogen oxides (Maeda et al., 1995).