Conclusion

The authors reviewed large-scale algal culture systems. Open pond systems have lower productivity of algal biomass, require larger land areas, and involve larger land costs. However, their operation cost is lower. On the other hand, closed systems can achieve high-density culture, and the volume of algal culture can be reduced. The reduction of culture volume decreases the land costs. However, a certain land area is still required for the collection of solar energy, and the operating cost is larger than that of open systems. Moreover, solar radiation, temperature, and other factors regulating algal productivity are significantly affected by location. Suitable culture systems have to be

(^Seawater")

Drying system

Figure 4. Conceptional system of CO2 fixation by microalgae.

(^Seawater")

Drying system

Figure 4. Conceptional system of CO2 fixation by microalgae.

Micro Algae System Design

Figure 5. Design of CO2 removal system in cement industry.

Atmosphere

Figure 5. Design of CO2 removal system in cement industry.

adopted according to the target products and available environmental conditions.

Extensive studies of biological CO2 fixation using mi-croalgal cultures have been pursued for the past 10 years. A primary goal is the complete removal of CO2 in discharged gas emitted by such an on-site system. Because of the land-area requirements and the current CO2 mitigation cost of $264/ton as carbon, it is to be difficult at pres ent to apply microalgal cultures to CO2 removal. Most nations are seriously considering the increase of atmospheric CO2 concentration, and intensive efforts to reduce the anthropogenic CO2 emission will be made. Microalgae culture may be one of the important processes used for such efforts (55).

Regarding technological use of microalgal biomass, several examples, including historical findings, have been pre sented. Microalgal production of useful chemicals and energy resources have been extensively investigated. Processes that utilize the majority of the resulting microal-gal biomass as energy sources would be desired. Such processes may allow the recycling of evolved CO2 from human energy consumption rather than a one-way emission, as is the case with fossil fuels. The following six products can be produced from microalgal biomass for use as fuels: hydrogen (through biophotolysis), methane (through anaerobic digestion), ethanol (through yeast or other alcohol fermentation), triglycerides (through extraction of lipids), methyl ester fuels (through transesterification of lipids), and liquid hydrocarbon (from Botryococcus braunii).

Increasing attention has been paid to resource sustain-ability in all industries. It must be considered whether resources used for manufacturing products can be sustainable. Developing technologies of microalgal culture will be expected to provide sustainable resources.

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