Experimental Analysis and Model-Based Optimization of Microalgae Growth in Photo-Bioreactors Using Flue Gas

Three strains of microalgae, Chlorella sp., Tetraselmis suecica and Synechocystis PCC 6803, were grown with the flue gas generated by natural gas combustion. Continuous exposure of cultures to flue gas significantly reduced all algal growth, while Chlorella sp. showed the highest tolerance to high concentration CO₂. Two methods can be used to overcome flue gas inhibitory effect on biomass growth: reduce in-flow CO₂ concentration or apply on-off mode for flue gas treatment. To design efficient strategies for flue gas utilization in algal photo-bioreactors, we employed a Monod-based kinetic model to describe phototrophic biomass growth in response to CO₂ control dynamics. Via static optimization, the model designed the control dynamics of in-flow CO₂ at low concentration to support the maximal growth rate by gradually increasing CO₂ concentration. On the other hand, we also tested a much simpler, but industrial feasible, on-off control mode for direct flue gas treatment. Based on the reported algal kinetic parameters (e.g., maximal specific growth rate and inhibition constant), the model showed that gas-on (~10 seconds) and gas-off (5~9 minutes) had more than 90% of theoretically optimal biomass growth rate (as achieved by optimal control of dynamic in-flow CO₂ profiles). The models developed in this study could be useful for rational design of algal bioprocess using high concentration flue gases.