Process design of absorption-membrane hybrid CO₂ capture systems for coal-fired power plant

Abstract

Global warming is now a global issue and about 42% of CO2 emissions are associated with power plants in 2013 (IEA, 2015). Significant efforts have been made to the development of CO2 capture technologies for power plants. Technical progress being made in the area of CO2 capture has mainly focused on single separation technology, namely, absorption, adsorption, cryogenic systems, membrane, etc (Freeman et al., 2012). In order to complement shortcomings of each technology and to maximize synergetic benefits, the attention of hybrid process using more than one CO2 capture technology simultaneously has been recently drawn. This study focuses on process design of CO2 capture systems in coal-fired power plant by combining absorption and membrane separation methods together. Two absorption-membrane hybrid options for separating CO2 from flue gas are investigated. Process modeling was carried out with Unisim® and Matlab® in an integrated manner. The hybrid process considered in this study is categorized to series and parallel schemes, according to the linking structure between absorption and membrane process. For the hybrid series process, the flue gas emitted from the boiler is subjected to the absorption process to remove CO2, while non-treated flue gas is fed into the membrane process in which a portion of the combustion air for the boiler is used as sweep gas to generate pressure gradient. CO2 is then passed from the flue gas into the sweep air stream which is fed to the boiler. Accordingly, CO2 concentration of the flue gas is increased, with which energy required for desorbing CO2 in the stripper can be reduced. In the hybrid parallel process, the flue gas discharged from the boiler is divided into two streams. One is fed to the absorption process, and the other is used as air sweep gas in membrane process. As CO2 concentration of the flue gas increases and the feed flowrate to the absorption process decreases, there is a possibility to reduce capital cost of absorption process. Case study was performed such that key design variables, including absorption CO2 removal rate (only to parallel process), MEA solvent flowrate, the ratio of air entering the membrane, and membrane area were varied and their techno-economic impacts were evaluated in a holistic manner. With the case study, it was found that hybrid process based on series arrangement can reduce reboiler duty for a stripper column, while hybridized parallel process can reduce the absorption column cost. In order to fully appreciate hybrid process between absorption and membrane, further detailed verification for potential negative impacts on power generation would be required if air sweeping were utilized in hybrid arrangement.