Kinetic Modeling of CO2 Biofixation by Microalgae and Optimization of Carbon Supply in Various Photobioreactor Technologies

Photosynthetic microorganisms like microalgae or cyanobacteria can be used to fix CO2 from industrial effluents in a sustainable way. However, the gaseous CO2 must be first transferred into the liquid phase in the form of dissolved inorganic carbon (DIC) to then be assimilated and thus biofixed by microalgae. This article introduces and validates a model able to relate effects of those parameters on relevant quantities, such as CO2 biofixation rates and CO2 use efficiency as characterized by CO2 removal from the gas phase. The ability to predict carbon fluxes in the process as a function of operating parameters is first illustrated for lab-scale photobioreactors, emphasizing the difficulty to optimize both CO2 biofixation rates (which implies maximizing biomass growth) and CO2 removal from the gas phase (which implies working at low DIC concentrations). As two technologies presenting different gas- liquid mass transfer performances, mechanically stirred versus airlift systems are then discussed. Covered raceways are revealed to be of interest, reaching up to 80% in CO2 use efficiency, while the large flow rate needed for sufficient mixing in airlift systems facilitates the CO2 supply to the culture to the detriment of CO2 use efficiency, typically only a few percent in usual operating conditions. Finally, the potential of a multistage strategy is investigated for a typical CO2-enriched flue gas. The relevance of biological fixation as a carbon sink and of system arrangement (i.e., series, parallel, or in combination) will be discussed in terms of biomass production, surface requirement, and carbon removal efficiency.