Design, Modeling and Optimization of Modified MEA Scrubbing Process for Post Combustion CO2 Capture

Abstract

Post-combustion CO2 capture with aqueous monoethanolamine (MEA) scrubbing is a promising and well-proven technique for reducing atmospheric CO2 emissions. The MEA scrubbing process is suitable for treating flue gas from coal-fired power plants because of its high CO2 capture capacity and its ability to be retrofitted into existing power plant facilities. However, the MEA scrubbing process is not cost effective in terms of CO2 capture, in particular for the energy required for solvent regeneration. To overcome this issue, studies have been conducted to reduce the solvent regeneration energy through modifying the process configuration. However, the majority of these modified processes call for additional capital costs due to the requirement for additional equipment. The objective of this study is therefore to determine the optimal configuration for reducing the cost of CO2 capture. The operating expenditure (OPEX) and capital expenditure (CAPEX) were considered together to reduce the total CO2 capture cost. Firstly, analysis of the conventional MEA scrubbing process energy system was carried out to determine the key variables for reducing the solvent regeneration energy. These key variables were the temperature at the stripper top, and the temperature approach at the cross heat exchanger. Analysis of the existing modified MEA scrubbing process was then carried out. The modified MEA scrubbing processes can be classified into three groups based on their energy reduction mechanism. The energy reduction mechanism of group I involves an increasing in the lean loading at the stripper bottom, while that of group II involves decreasing the solvent inlet temperature at the stripper top, and that of group III involves increasing the heat recovery. Combination of the multiple modified MEA scrubbing processes exhibiting positive interactions was then investigated. Absorber intercooling, cold solvent split, and rich vapor compression were selected as the optimal combination based on quantitative studies. For the combined configuration, the equivalent energy decreased 5.7%, from 1.22 GJe/ton CO2 to 1.15 GJe/ton CO2. Following energy consumption minimization, the additional CAPEX was calculated as a penalty term. Subsequently, the superstructure model of the modified configurations was prepared, involving six different modified configurations and various split flow configurations. As the cost model was built into the superstructure, the superstructure model calculated the OPEX and CAPEX terms simultaneously. Finally, optimization of the superstructure model of the modified MEA scrubbing configurations was carried out, simultaneously solving the process variables for six different modified configurations. The objective of scenario I was to minimize the equivalent energy without considering the CAPEX term. As a result, the equivalent energy for CO2 capture and compression decreased 22.1%, from 1.30 GJe/ton CO2 to 1.02 GJe/ton CO2. The objective of scenario II was to minimize the total cost, i.e., the sum of the OPEX and CAPEX terms. As a result, the total cost of CO2 capture and compression decreased 10.2%, from €54.7/ton CO2 to €51.0/ton CO2. The annualized cost reduction was therefore €25.7 M/yr for a 630 MWe power plant.