Towards an Integrated Process for CO2 Capture and Utilization: Cultivation of Scenedesmus acutus Using Gaseous CO2 and NH3

Integrating CO2 scrubbing from flue gas with its utilization in algae cultivation represents a potential means of lowering the cost of CO2 capture. Towards this goal, this study sought to assess the feasibility of using gaseous a CO2/NH3 stream, derived from CO2 capture using aqueous ammonia, as a C- and N-source for algae cultivation. Scenedesmus acutus was cultured in 800 mL photobioreactors using gaseous CO2/NH3 in mole ratios varying from 7 to 18. Excellent growth of Scenedesmus acutus was observed, the average growth rate for CO2/NH3 = 10 of 0.171 +/- 0.015 g/L center dot day exceeding that obtained using 1% CO2/N-2 and urea as the N-source (0.099 +/- 0.28 g/L center dot day). Under optimal growth conditions (CO2/NH3 mole ratio of 10), CO2 utilization ranged from 57 to 72%, while the NH3 utilization was >90%. The CO2/NH3 feed rate was also found to exert a significant effect on algae productivity, with excessive feed rates leading to the accumulation of NH3 in the culture at concentrations that were toxic to the algae. Consequently, to avoid the toxic effects of high NH3 concentrations (>2.0 mM), it proved necessary to balance the NH3 supply with the algae growth rate so that excessive NH3 accumulation was prevented. This indicates that for practical applications, a CO2/NH3 feed control strategy would be required that takes into account the ammonium ion concentration in solution and the pH so as to avoid significant concentrations of free NH3. Analysis of the harvested biomass revealed a high protein (>= 47 wt%) and a low ash content (< 3.6 wt%), suggesting it would be well suited for use as animal feed or as a feedstock for the production of bioplastics.

Automated summary

Integrating CO2 scrubbing from flue gas with its utilization in algae cultivation is a cost-effective means of CO2 capture. This study assessed the feasibility of using gaseous CO2/NH3 stream derived from CO2 capture as a C- and N-source for algae cultivation. The results showed that Scenedesmus acutus had excellent growth using CO2/NH3, with a higher growth rate compared to conventional N-sources. A balanced supply of NH3 was necessary to avoid toxic effects, and the harvested biomass had high protein content and low ash content, making it suitable for animal feed and bioplastics production.