Chapter 14 - Chlorella-Based Composites

Abstract

Reducing the discharge of CO2 into atmosphere is one of the crucial goals to be achieved in this century, as the increasing CO2 concentration is causing global warming. Biological fixation of CO2 by microalgae has attracted a lot of attention as the most sustainable method. Such biological CO2 fixation systems produce a large amount of microalgae instead of discharging CO2 into the atmosphere. Consequently, the resultant microalgae will be required to be effectively used as fodder, building materials, as well as in other applications. In this chapter, first the feasibility of fixation of CO2 from flue gas is described on the basis of a life-cycle assessment study of a hypothetical system incorporated in a 1000-MW liquefied natural gas power station, indicating that such CO2 fixation system could be practical with an efficient utilization of resultant microalgae as fodder. Compounding of chlorella with versatile plastics is a new strategy for retaining a large amount of biologically fixed CO2 in resultant composites for long period with a potential application to building materials. Heat-pressuring chlorella with slightly polar polyvinylchloride (PVC) produces a novel composite that satisfies the requirements of rigid or plasticized PVC, depending on the chlorella content. Compounding chlorella with hydrophobic polyethylene (PE) or poly(propene) (PP), however, necessitates slight grafting of PE or PP with maleic anhydride (MA) for acquisition of compatibility between the components. Melt-mixing PE with 0.25wt% of MA, followed by further melt-mixing the resultant MA-modified PE (MPE) with chlorella, yields a novel MPE–chlorella composite with tight interfaces between chlorella grains and the MPE matrix, because of the formation of ester bonds between chlorella and MPE. On the basis of this finding, compounding chlorella with another hydrophobic PP is successfully attained through melt-mixing them with maleic anhydride–grafted PP as a compatibilizer to yield a novel, compatibilized PP–chlorella composite. These chlorella composites can be readily shaped into required moldings. Therefore, the compounding of chlorella with these versatile plastics will be an essential base for realizing a practical system of biological CO2 fixation.