Improving microalgae growth and carbon capture through micro-size bubbles generation in flat-panel photobioreactors: Impacts of different gas sparger designs on mixing performance

Anthropogenic emissions like carbon dioxide (CO2) necessitate the need for immediate mitigation of excessive CO2 concentrations, so as to ease the impacts of climate change. Mass carbon capture and storage (CCS) via microalgae cultivations have been proven prominent for their CO2 capturing abilities and prospects of converting CO2 into commercially valuable bioproducts. However, current microalgae cultivation systems have yet to achieve the satisfactory production rates required for mass CCS. To improve microalgae growth, CO2 gas bubbles are often fed to sustain microalgae cultures in photobioreactors (PBRs). Studies have shown significant increases in microalgae growth and CO2 capturing rates with the reduction of gas bubble sizes to micro-size. In this work, 3 distinct gas spargers were designed, and their mixing performances were investigated in a 10 L flat-panel PBR. Detailed analysis of micro-size bubbles generation via gas spargers, along with the cost analysis and carbon tax estimations for the sparger designs and operations, were discussed. The results of this work showed that spargers with micro-size pores were not able to produce micro-size bubbles. The customised sintered metal spargers with novel double sparger orientation resulted in higher mixing performances, compared to perforated spargers and commercial spargers. Findings advance the understanding of mixing and CO2 mass transfer, important milestones for photobioreactor scale-up studies and partly fulfil the circular bioeconomy concept where microalgae cultivations address both environmental and economic aspects.