(359b) Application of Membrane CO2 Absorber in NH4oh Looping Process for Enhanced Algae Growth

An energy-efficient and durable way to capture and store CO₂ for utilization is absorbing CO₂ into an aqueous solution for use as an in-situ carbon feedstock and using solar energy to produce to chemicals or biomass, such as via biofixation using algae. This is a sustainable process for CO₂ capture and utilization ^{[1, 2]}. Currently, most algae utilization systems utilize the CO₂ product stream after it has been separated from the CO₂-containing gas (flue gas). Unfortunately, this approach is not cost-effective on a large scale.

Dissolved NH₃ is attractive for both CO₂ capture and as an algae nutrient. For CO₂ capture, the ammonium hydroxide (NH₄OH) solvent has several advantages including low cost, no degradation in the presence of O₂, SO₂ or heat, high capture capacity, low regeneration energy, as well as the potential for capturing multiple acid gases (NO_x, SO_x and CO₂) in the flue gas ^[3,4]. Studies have shown that the scrubbing capacity of NH₃ is approximately 0.9-1.2 kg of CO₂/kg of NH₃, with a CO₂ removal efficiency of ~99% and half the solvent regeneration energy than that of 30 wt% MEA ^[5-7]. Its low cost makes NH₃ attractive as an algae nutrient. A practical, cost-effective, integrated CO₂ capture and utilization technology has been developed, constructed and tested at the University of Kentucky Center for Applied Energy Research (UK CAER) for algae production with three unique features. First, an aqueous NH₃ solution functions as both CO₂ capture absorbent and as an algae nutrient. Second, a downward flow, gas-liquid, indirect contact membrane CO₂ absorber is used, where the capture and biofixation is decoupled and NH₃ slip is minimized by including <1 wt% chelating compound in the solvent. Condensate from the saturated flue gas continually washes the gas side of the absorber membrane to ensure long-term operation. Third, solar energy-powered, distributed regenerators are installed near the algae bioreactor modules to provide local, just-in-time distribution of CO₂ and NH₃ at the appropriate ratio.

The UK CAER integrated capture and utilization system uses 10 cfm flue gas produced from either coal or natural gas combustion to simulate an industrial CO₂ source. The bench scale process consists of the membrane CO₂ absorber, solar powered flash stripper, algae bioreactors, and balance of plant, including NH₃ and water make-up systems. Commercially available polymeric membranes, whose surfaces are rendered hydrophobic, are used in the membrane absorber without the direct contact between flue gas and aqueous capture solution ^{[8, 9]}. During operation, flue gas is fed into the membrane tube-side followed by a water-wash column to remove any NH₃ slip prior to stack emission. The NH₄OH solvent flows through the shell side of the absorber and the exiting CO₂-richsolution is pumped to a pressurized solar energy heater and a solvent regenerator. Pressure is controlled in the regenerator to produce a product stream with a CO₂:NH₃ ratio of ~7, which is most appropriate as feed to the algae bioreactors. A modularized bioreactor, ~10 m², is used for performance evaluation.

Experimental results will be presented from operation of the UK CAER bench scale process detailing CO₂ capture and mass transfer enhancement, NH₃ slip and operation stability due to NH₃ salt formation and algae production. Control of NH₃ slip by solvent formulation optimization will be presented. A 2 M NH₄OH solvent is employed to keep the NH₃ vapor pressure low and a chelating agent (Zn²⁺, amine and ionic liquid) is added to further reduce NH₃ volatility. Development of the membrane absorber will also be presented. Configuration of the membrane absorber utilizes condensed water from the flue gas to continually wash the gas-side to reduce fouling and recapture NH₃ slip. Hydrophobic coatings, such as with Teflon AF2400^®, of the membrane are also considered. Preliminary bench scale studies will be detailed showing that NH₃ slip is manageable and controllable. Finally, Scenedesmus acutus (UTEX B72) algae productivity results will be presented when continuously fed with the appropriate CO₂:NH₃ ratio, optimized based on pH. Algae productivity is determined based on regular gravimetric culture density measurements and the algae yield at harvest. Culturing and harvesting follow standard practices. Measured productivity is compared with modeled productivity for the same period/climatic conditions, using an existing model.

References

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[2] Shah, M., Mahfuzur, R., Liang, Y., Cheng, J. J., & Daroch, M., Frontiers in plant science, 2016, 7, 531.

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[4] Li, K., Yu, H., Qi, G., Feron, P., Tade, M., Yu, J., & Wang, S., Applied Energy, 2015, 148, 66-77.

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[8] Villeneuve, K., D. Roizard, J.C. Remigy, M. Iacono, and S. Rode., Separation and Purification Technology, 2018, 199, 189-197.

[9] Molina, C. T., & Bouallou, C., Clean Techn Environ Policy, 2016, 18, 2133-2146.