Note: Solar conversion efficiencies are for visible light. The credit for biomass fuel is $25/barrel of oil.

Note: Solar conversion efficiencies are for visible light. The credit for biomass fuel is $25/barrel of oil.

9%, based on total light. The lower productivity reflects what could be expected based on present experience. The higher productivity is the theoretical limit. CO2 mitigation costs greatly depend on the productivity of algae and solar radiation. Microalgal strains that can achieve higher pho-tosynthetic efficiency at higher solar radiation are necessary. A photobioreactor, in which microalgae can convert from CO2 to biomass at higher photosynthetic efficiency, is also required.

Further research into microalgae, photobioreactors, the downstream process of microalgal culture, and the burning process of microalgal biomass is still needed. If such research is successful and the economic projections shown in Table 4 are verified, this system will represent one of the most promising technologies for future CO2 mitigation processes.

A Designed System for the Reduction of CO2 Emission: Case II—Cement Plants

Coccolithophorids are unicellular planktonic marine algae that produce elaborate structures called coccoliths, comprising scales or plates of CaCO3. In the oceans, huge blooms of coccolithophorid algae occur. These blooms have been recognized as contributing to ocean floor sediment formation, and algae play an important role in the global carbon cycle by recycling CaCO3.

We have chosen coccolithophorid algae as model organism to investigate biomineralization and have focused on the ecological significance of CaCO3 recycling. We have proposed CO2 fixation by artificial weathering of waste concrete and coccolithophorid algae cultures (Fig. 4). During artificial weathering of waste concrete suspended in seawater, atmospheric CO2 can be absorbed and dissolved as bicarbonate ions, which are a major source of coccolith particles. Coccolithophorid algae can use bicarbonate ions to form CaCO3 particles. As a result, CO2 absorbed by artificial weathering can be mineralized and fixed permanently. Artificial weathering ofwaste concrete is also a useful method to supply bicarbonate ions to cells of the coccolithophorid alga Emiliania huxleyi.

The CO2 fixation method by artificial weathering of waste concrete and coccolithophorid algae cultures can be applied to the reduction of CO2 emission in cement plants. A designed system for the reduction of CO2 emission in cement plant is shown in Fig. 5. Coccoliths can be used as an alternative to limestone, which is a carbonate source for cement production. In the cement industry, CO2 is mainly produced by the decomposition of limestone during the burning of cement clinker. If CaCO3 recycling can be achieved by artificial weathering of waste concrete and coc-colithophorid culture, CO2 emission by the cement industry might be reduced (Fig. 5).

Moreover, it is proposed that microalgal biomass with fixed CO2 products may be stored in concrete. If microalgal biomass can be stored in concrete without the decomposition of the biomass back to CO2, removal of anthropogenic CO2 may be achieved.

CO2 absorption by artificial weathering of concrete has been determined. When concrete (calcium content was 15% wt/wt) pieces were weathered by CO2 and seawater, over 310 mg CO2/g concrete could be absorbed and dissolved in the form of HCO;T (54). The amount of CO2 absorbed is directly related to calcium content in concrete, 310 mg CO2/g concrete is equal to almost 1 g CO2/g Portland cement (calcium content is almost 46%).

On the other hand, amount of emitted CO2 during a cement production is 708 kg CO2/ton cement. The amount of absorbed CO2 by the weathering of waste concrete is greater than that of emitted CO2 during a cement production. Moreover, CO2 is absorbed by following coccolitho-phorid algae cultures.

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