Managing carbon dioxide mass transfer in photobioreactors for enhancing microalgal biomass productivity

Mitigating greenhouse-gas emissions, of which fossil-derived carbon dioxide (CO2) is the dominant component, is becoming increasingly imperative. One of the tools for lowering the demand for fossil carbon is cultivation of microalgae, which are fast-growing photosynthetic microorganisms that utilize sunlight for energy and CO2 as their carbon source. In addition, microalgae can provide feedstock to replace fossil sources, particularly for transportation fuels. In open and closed microalgal cultivating systems (also called open ponds and photobioreactors, respectively), CO2 can be sparged into the culture medium through a gas distributor; CO2 molecules diffuse through the gas-liquid interface and dissolve into the culture medium, from which they can be taken up for the biosynthesis of microalgal cells. Due to the modest solubility of CO2 in water, optimal design and operating variables (e.g., inlet gas flow rate, sparger characteristics, CO2 concentration in the inlet gas, and the height of a PBR or sump) are required to increase the CO2 mass transfer rate into the medium and, consequently, CO2 uptake and biomass productivity. The concepts and phenomena discussed in this work apply to photobioreactors and open ponds that are sparged with CO2. This review systematically evaluates how the key design and operating variables affect bubble behavior and the rate of CO2 delivery into the medium. The review also addresses advanced strategies that are being employed to increase the rate of CO2 transfer, but with lower costs than with sparging.