Developing an off-site bicarbonation absorber system to promote microalgal fixation of CO2 in exhaust gas from biogas upgrading

Abstract

In order to address the risk of explosion due to CH₄ from exhaust gas produced during biogas upgrading in closed carbon fixation systems employing photosynthetic microalgae, an off-site bicarbonation absorber system was developed to promote microalgal CO₂ fixation under atmospheric pressure. The abundant CO₂ in the biogas upgrading exhaust gas (≥90 vol% CO₂, ≤10 vol% CH₄) reacted with a Na₂CO₃ solution in the off-site bicarbonation absorber to produce NaHCO₃, which was used as carbon source for microalgal growth in enclosed column photobioreactors. After the reaction, CH₄ was discharged outside the bicarbonation absorber because it did not react with the Na₂CO₃ solution and was extremely difficult to dissolve in water, thereby avoiding the explosion risk due to accumulated CH₄ in the enclosed column photobioreactors. The experimental results showed that the Spirulina growth rate first increased 1.7 times, peaking at 0.6 g/L/d, and then decreased when the bicarbonation reaction time (optimal 50 min), absorber diameter (optimal 10 cm), initial Na₂CO₃ concentration (optimal 173 mM), and exhaust gas aeration rate (optimal 100 sccm) increased. The optimal molar ratio of NaHCO₃ to total inorganic carbon in the bicarbonation absorber solution reached 79 %. The sufficient HCO₃^- supply and suitable pH of the microalgal solution improved the synthesis of photosynthetic pigments in the microalgal cells and enhanced their photochemical efficiency and carbon sequestration rates.