CO₂, NO_x and SO_x removal from flue gas via microalgae cultivation: A critical review

Flue gas refers to the gas emitting from the combustion processes, and it contains CO₂, NO_x, SO_x and other potentially hazardous compounds. Due to the increasing concerns of CO₂ emissions and environmental pollution, the cleaning process of flue gas has attracted much attention. Using microalgae to clean up flue gas via photosynthesis is considered a promising CO₂ mitigation process for flue gas. However, the impurities in the flue gas may inhibit microalgal growth, leading to a lower microalgae-based CO₂ fixation rate. The inhibition effects of SO_x that contribute to the low pH could be alleviated by maintaining a stable pH level, while NO_x can be utilized as a nitrogen source to promote microalgae growth when it dissolves and is oxidized in the culture medium. The yielded microalgal biomass from fixing flue gas CO₂ and utilizing NO_x and SO_x as nutrients would become suitable feedstock to produce biofuels and bio-based chemicals. In addition to the removal of SO_x, NO_x and CO₂, using microalgae to remove heavy metals from flue gas is also quite attractive. In conclusion, the use of microalgae for simultaneous removal of CO₂, SO_x and NO_x from flue gas is an environmentally benign process and represents an ideal platform for CO₂ reutilization.