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

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.