High-rate CO2 sequestration using a novel venturi integrated photobioreactor and subsequent valorization to microalgal lipids†
Recovery of waste CO2 from flue gas involves two phase gas–liquid interactions for application in enhanced microalgal growth, CO2 bio-fixation and increased production of microalgal lipids for 4th generation biofuels. Mineralized CO2 as bicarbonate between pH 10 and 7 constitutes the major inorganic carbon for phototrophic microalgae. Consolidating Carbon Capture and Usage (CCU) technology with microalgal lipid synthesis, the present work exhibits design and optimization of a microbubble assisted CO2 sequestration reactor to recover CO2 from low concentration sources (5% v/v) and maximize microalgal lipid productivity without any nutrient stress. A venturi microbubble generator was amalgamated with a bubble column and the Reynolds number was used to define the hydrodynamic flow regime. At a steady volumetric liquid flow rate QL, corresponding to laminar flow conditions, the gas flow rate QG was varied in the range 0.1–1 L min−1 to determine the volumetric mass transfer coefficient KLa and the CO2 mass transfer efficiency. KLa and QG values were found to be linearly dependent in the QG range 0.1–0.7 L min−1, while CO2 capture efficiencies in the range 86% to 98% were obtained. The optimized medium recirculation sufficed for culture agitation and further aeration for mixing was not required. A correlation coefficient was derived, relating the change in gas–liquid interfacial area with the change in QG. The resultant system achieved a biomass yield of 0.454 g L−1 and a carbon content of 38.9% volatile suspended solids (VSS). Furthermore, a lipid accumulation of 35.9% on a dry cell weight basis and a lipid productivity of 11.64 mg L−1 d−1 exhibited feasible valorisation of sequestered carbon.