Outdoor cultivation of microalgae in a coal-fired power plant for conversion of flue gas CO₂ into microalgal direct combustion fuels

Microalgae have piqued renewed interest as a sustainable biofuel feedstock owing to their high CO₂ conversion efficiency. However, the major limitation of microalga-based biofuel production is low productivity. In this study, CO₂ in flue gas emitted from the coal-fired power plants was fixed through mass microalgal cultivation using only sunlight as an energy source. To minimize the cost and energy required to supply the flue gas and efficiently utilize the microalgal biomass, a polycarbonate (PC) greenhouse and polymeric photobioreactors were installed near the power plant stack. Four different microalgal strains (Chlamydomonas reinhardtii, Chlorella sorokiniana, Neochloris oleoabundans, and Neochloris oleoabundans #13) were subjected to semi-continuous culturing for 1 month. The maximum biomass productivity was achieved by the N. oleoabundans #13 strain (0.703 g L⁻¹ day⁻¹). Additionally, polymerase chain reaction analysis revealed that the individual microalgal culture was not cross-contaminated with other microalgal cultures in this cultivation system, owing to the structural properties of photobioreactor comprising individual modules. The lipid content and calorific productivity of N. oleoabundans #13 biomass were 45.70% and 3.553 kJ L⁻¹ day⁻¹, respectively, which indicate satisfactory performance of biomass as a direct combustion fuel. The CO₂ fixation rate, which was calculated based on the carbon content in the biomass, was 0.309 g CO₂ L⁻¹ day⁻¹. Therefore, large amounts of CO₂ can be reduced using the large-scale microalgal cultivation system, which enables efficient biological CO₂ conversion and maximizes microalgal biomass utilization.