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·day exceeding that obtained using 1% CO₂/N₂ and urea as the N-source (0.099 ± 0.28 g/L·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.