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

Abstract

Photosynthetic microorganisms like microalgae or cyanobacteria can be used to fix CO 2 from industrial effluents in a sustainable way. However, the gaseous CO 2 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 CO 2 biofixation rates and CO 2 use efficiency as characterized by CO 2 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 CO 2 biofixation rates (which implies maximizing biomass growth) and CO 2 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 CO 2 use efficiency, while the large flow rate needed for sufficient mixing in airlift systems facilitates the CO 2 supply to the culture to the detriment of CO 2 use efficiency, typically only a few percent in usual operating conditions. Finally, the potential of a multistage strategy is investigated for a typical CO 2 -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.

In situ, High-Resolution Quantification of CO2Uptake Rates via Automated Off-Gas Analysis Illuminates Carbon Uptake Dynamics in Cyanobacterial Cultures

“…PCC 7942 (Omata et al, 1999), Scenedesmus obliquus (Azov, 1982), Chlamydomonas reinhardtii (Palmqvist, Sjoberg, & Samuelsson, 1988), as well as species of red algae (Johnston, Maberly, & Raven, 1992). Meanwhile, however, as also noted by others (Pruvost, 2022), despite supporting greater maximum volumetric CO 2 uptake rates, increasing also unfortunately led to a decrease in CO 2 utilization efficiency by the culture, which occurs as a result of mass transfer limitations and disparate rates of supply vs. fixation. Specifically, for example, when PCC 7002 was grown under continuous illumination at either 75 or 150 E, the maximal CO 2 utilization efficiency decreased from about 25% at a of 1,000 ppm to just about 8% at 7,500 ppm (Supplemental Figure 5).…”