Investigation of Energy-Saving Designs for an Aqueous Ammonia-Based Carbon Capture Process

Abstract

In an aqueous ammonia-based postcombustion carbon capture (PCC) process, the regeneration energy of the CO2 lean solvent dominates the overall energy consumption. The energy reduction in the CO2 stripper can be achieved by either formulating new solvents or optimizing the process configurations. The ammonia concentration in the lean solvent is an important design parameter. A high concentration of ammonia results in a lesser lean solvent required at a constant CO2 removal efficiency, e.g., 90%; however, it encourages the formation of ammonium bicarbonate in the reflux, which brings the desorbed CO2 back into the stripper. To achieve constant CO2 removal, extra solvent in the stripper needs to be vaporized, which results in higher energy consumption than that required in applying a lean solvent with a low ammonia concentration. In this study, an index, which is the vaporization ratio of the solvent to the captured CO2, is used to evaluate the energy required for regenerating the lean solvent. A higher value of the index indicates that extra solvent needs to be vaporized, as compared to the captured amount of CO2. This index unifies the energy-saving concepts of the pressurized stripper and advanced stripper configurations. The former increases the stripper temperature, which favors CO2 desorbed from the rich solvent, while the latter lowers the top temperature of the stripper, which enhances the CO2 purity at the top. Both methods, which are the pressurized stripper and advanced stripper configuration, can achieve the energy-saving purpose by reducing the vaporization ratio, because a higher CO2 purity at the top leads to lesser solvent required to be vaporized for constant CO2 removal. In addition, the energy reduction achieved by stripper modifications, which include the rich-split process, the interheating process, and the integration of both configurations, is investigated. The results indicate that the energy-saving effect of the rich-split process integrated with interheaters (IHs) is not as promising as the literature claims. Once the design parameters of the rich-split process are selected properly, the rich-split process without IHs can achieve the same energy-saving effect as that achieved by process of integration with IHs.