UKy-CAER CO_2 Capture System Secondary Stripping: Effectiveness for Reducing the Solvent Regeneration Energy and Capital Cost Reduction

摘要

The U.S. Department of Energy (DOE) has set a cost goal of <$40/tonne of CO_2 captured to be achieved in 2020 by demonstration processes and by 2025 for commercialization of 2nd generation capture technologies. To achieve this goal, UKy-CAER has focused on research in cooperation with industrial, non-profit, and government partners to develop and scale-up CO_2 capture technologies for power generation sector utilization. In a project primarily funded by U.S. Department of Energy, National Energy Technology Laboratory (US DOE NETL), a 0.7 MWe heat-integrated CO_2 capture system (CCS) developed by UKy-CAER has been in operation since the spring of 2015, accruing more than 3500 hours of operation, at Kentucky Utilities E.W. Brown Generating Station, a 750 MWe commercial coal-fired power generation plant, located in Harrodsburg, KY. There are two novel concepts included in the UKy-CAER CCS. First, a two-stage stripping process for solvent regeneration increases the solvent working capacity, reduces the energy required for solvent regeneration, and reduces capital costs. A primary, conventional steam-driven stripping column is followed by a secondary air-stripping column to recover additional CO_2. At the commercial scale, the gas stream exiting the secondary air-stripper is recycled to the power plant boiler combustion air enriching the flue gas with CO_2, and increasing the gas phase CO_2 partial pressure in the absorbing column. Further reduction of the carbon loading in the lean solvent solution results in a higher free amine concentration, and a lower liquid phase CO_2 concentration at the top of the absorber. Lower liquid phase CO_2 concentration and higher gas phase CO_2 concentration will increase the driving force for CO_2 diffusion through the liquid/gas reaction film and result in higher mass transfer. Second, a heat integrated liquid desiccant loop recovers waste energy from the CCS and improves power plant efficiency. A two-stage cooling tower has been designed where the bottom section utilizes the liquid desiccant to remove moisture from the cooling air that is fed to the conventional cooling tower operation in the top section, increasing the cooling of the water, lowering the feed cooling water temperature. In a commercial scale version of the UKy-CAER CCS, the cooling tower would be fully integrated with the power plant and reducing the relative humidity of the cooling air will lower the turbine condenser cooling water temperature, reducing the steam turbine back pressure and improving overall efficiency. The focus of this paper is on the effectiveness of two-stage stripping. First, experimental data is presented showing that the amount of CO_2 released from the secondary stripper is 20% or greater, of that captured in the absorber and that lean solvent feed to the absorber, as low as 0.9 mol C/kg solvent, using the heat typically rejected to environment, resulting in a higher solvent working capacity and a reduction in the energy required for solvent regeneration. Second, a capital cost comparison will be presented with estimates from a technical and economic analysis (TEA) performed by Electric Power Research Institute (EPRI) and WorleyParsons where the costs associated with the CO_2 Removal & Compression are found to be lower for the UKy-CAER CCS, with a 12% lower cost associated with the CO_2 Removal System and a 15% lower cost associated with CO_2 Compression & Drying.