(685b) NH4oh Looping with Membrane CO2 Absorber and Distributed Stripper for Enhanced Algae Growth

The University of Kentucky Center for Applied Energy (UK CAER) has devised a unique, integrated CO₂ capture and utilization technology. CO₂ from coal-fired power generation flue gas is first captured at half the operating cost of a typical aqueous CO₂ capture system (CCS), distributed in an aqueous stream and then fixed by algae in bioreactors where the algae production is increased by 50% over that with a typical intermittent nutrient feeding system. Lower CCS operating cost is achieved by eliminating the flue gas pretreatment step for cooling and SO₂ removal, eliminating steam extraction from the power generation steam cycle for solvent regeneration, and eliminating CO₂ compression. Higher algae production is achieved by continuous, just-in-time nutrient feed to the bioreactors directly from a distributed solvent regenerator, which maintains the bioreactor pH for optimum growth.

The process starts with a uniquely configured membrane absorber, where the flue gas is indirectly contacted with an ammonium hydroxide (NH₄OH) solvent. Dissolved NH₃ is attractive for both CO₂ capture and as an algae nutrient. For CO₂ capture it is inexpensive, has a low regeneration energy, is thermally- and oxidatively-stable and has a viscosity near that of water, which makes is easy to transport. Numerous 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^{[1, 2, 3]}. NH₃ is attractive as an algae nutrient due to its low cost.

The rich NH₄OH solvent is pumped to a set of distributed regenerators which are co-located with the algae bioreactors. Solvent pumping, transport and distribution reduces the balance of plant (BOP) cost compared to a typical aqueous CCS related to the flue gas duct and boost fan required to transport the flue gas. The energy required for the distributed solvent regeneration is supplied by solar-thermal panels eliminating the need for steam extraction from the power generation steam cycle. After solvent regeneration, the product stream contains both the CO₂ captured from the flue gas and volatized NH₃ from the solvent. This product stream is fed directly to the bioreactors, eliminating the need for compression of the CO₂ stream. The relative amounts of CO₂ and NH₃ in the product stream are adjusted and controlled by a controlling the regeneration conditions (pressure and temperature). The continuous feed of the right ratio of nutrients overcomes the typical inhibition of algae growth resulting from frequent pH swings in the bioreactor due to unbalanced (intermittent) feeding systems for CO₂ and N. Also, because the regenerators will operate at pressure and be located in close proximity to the bioreactors, there is no worry about pressure drop when sparging the gas into the algae. Sparging produces small bubbles which is beneficial for mass transfer efficiency.

One known challenge when using an NH₄OH solvent is high NH₃ emission. Hydrophobic membranes are used for CO₂ capture using an aqueous NH₃ solution^{[4, 5]} without the direct contact between flue gas and aqueous solution. Additionally, UK CAER CO₂ capture and utilization process manages NH₃ slip in three extra measures. First, NH₃ slip is minimized by working with minimal species partial pressure, which is proportional to the concentration in the liquid. Hence, lowering the capture solvent concentration will lower the NH₃ partial pressure. Second, UK CAER’s previous work has demonstrated that the addition of Zn²⁺ into NH₃ solutions to chelate the NH₃ can reduce NH₃ volatility. Third, the configuration of the membrane CO₂ absorber utilizes condensed water from the flue gas to continually wash the gas-side of the membrane to reduce fouling and recapture NH₃ slip.

Additional details about the UK CAER unique, integrated CO₂ capture and utilization technology will be presented along with technology development plans.

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