Effective Transfer of CO2: From Flue Gas To Algal Cultivation

Abstract

Fossil fuel-fired thermal power plants are among the major contributors to the growing issue of global warming, owing to their substantial greenhouse gas emissions through the release of flue gas which contains 3 to 15% CO2. This escalation in the anthropogenic emissions drives severe ecological and economic damage, specifically accelerating global temperature increases, degrading ambient air quality, and an increasing decline in the volume and nutritional value of the agricultural yield around the globe. To mitigate these critical environmental impacts, Carbon Capture, Utilization, and Storage (CCUS) frameworks have emerged as a vital global strategy, with biological carbon utilization via microalgae cultivation representing a highly promising pathway. Traditional methods of CO2 delivery to grow microalgae heavily rely on conventional sparging which suffers from low residence times and mass-transfer efficiencies, leading to significant feed-gas loss, and high operational costs. This research is a part of a large project to demonstrate, characterize, and optimize a biorefinery process for converting a utility-source of CO2 into high-value products, specifically carbon nanofiber supercapacitors and ink, through algal cultivation. The primary objectives of this research are to maximize CO2 dissolution from the flue gas and to enhance the carbon transfer rate, thereby reducing effluent CO2 emissions. This work also provides a rigorous technical comparison between a hollow-fiber membrane (HFM) contactor system, orifice and tubing system, and simple sparging. The experimental data is used for preliminary design framework, which attempts to establish scalable operational criteria for industrial deployment. To overcome these issues, this research investigates two independent systems for improved carbon transfer: a selective absorption method utilizing a hollow-fiber membrane (HFM) contactor, and an enhanced sparging method employing an orifice and tubing configuration. To establish a theoretical maximum, the equilibrium concentration of CO2 was calculated by accounting for the algal growth medium’s salting-out effects and carbonate dissociation in the liquid medium. From the experiments using the hollow-fiber membrane contactor system, it was observed that HFM contactor systems transfer carbon more efficiently than traditional sparging, approaching the theoretical calculated maxima (~50-55%). However, the HFM contactor system eventually reaches a steady-state carbon dissolution over time. Additionally, system scale-up significantly drives up capital costs, as an upstream clarifier is required to protect the membranes from biofouling caused by the dense algal culture. Conversely, the orifice-and-tubing system eliminates the need for an upstream clarifier. Testing demonstrated that extending residence times through longer tubing and inducing turbulence via multiple orifices significantly enhances CO2 dissolution, reducing effluent gas concentrations. Because this equipment is straightforward to set up, and maintain, it incurs minimal capital and operational expenses. Despite its structural simplicity, the system achieves carbon transfer efficiencies remarkably close to the theoretical maximum (~100%). Furthermore, technical comparison revealed that the gas-liquid mass transfer equipment investigated in this research offer significantly lower operational costs (OPEX) in comparison to the traditional sparging methods. While both systems offer clear advantages over traditional sparging, further work is necessary to ensure fail-proof automated operation. Specifically, the setup must demonstrate the structural robustness required to meet peak CO2 demands during daytime growth spikes, consistently delivering an average of approximately 28 gCO2/day to support the cultivation of algae such as Nannochloropsis oceanica. Achieving this operational reliability is essential for its seamless integration into industrial facilities looking to reduce greenhouse gas emissions.