(359b) Application of Membrane CO2 Absorber in NH4oh Looping Process for Enhanced Algae Growth
AIChE Annual Meeting
Friday, November 19, 2021 - 12:51pm to 1:12pm
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 (NOx, SOx 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.
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