Mitigating NH3 Vaporization from an Aqueous Ammonia Process for CO2 Capture

An aqueous ammonia process (AAP) offers several advantageous technological features over other existing reactive absorption-based CO2 capture processes such as increased CO2 absorption loading capacity, no oxidative solvent degradation, no corrosion problems, high CO2 absorption fluxes and low energy input needed for solvent regeneration. It has also the potential of capturing multiple flue gas components (SO2, NOX, and CO2) and producing value added chemicals, such as ammonium sulfate, ammonium nitrate and ammonium bicarbonate, which are commonly used as fertilizers. Unfortunately, a major drawback of the AAP is NH3 volatility resulting in NH3 vaporization to the flue gas. Therefore, the current article presents the results of experimental and numerical investigations directed at in-depth understanding of the AAP and at developing of new methods for mitigating the unwanted NH3 vaporization. For this purpose three types of reactor configurations are studied: (i) packed bed, (ii) falling film and (iii) membrane. The bench-scale experiments realized in the counter-current packed bed reactor reveal, that NH3 vaporization can be minimized under the conditions of low temperature, pH, and flow rate of flue gas and under the conditions of high pressure and flow rate of aqueous ammonia. Further, from the detailed 2D modeling of the AAP realized in the falling film reactor it is found, that NH3 vaporization can be mitigated by making use of the mechanisms of negative enhancement of mass transfer and of migrative mass transport. Finally, the potential benefits of using membrane facilitated AAP reactors are discussed.