CO2 recovery from CPU vent by Ca-looping process

One key unit operation in oxyfuel power plants is the CO2 purification unit (CPU), expected to be needed in most of oxyfuel installations to reduce the content of non-condensable gases (mainly O2, N2 and Ar), originated from the oxidant excess required for the combustion, air in-leakages and the non-100% purity of the O2 produced in the cryogenic ASU. Auto-refrigerated cryogenic processes are typically proposed for the CPU, where high purity CO2 is condensed from the impure CO2 stream at pressures of 15-35 bar and temperatures close to the triple point one. In cryogenic purification processes, a relevant amount of the CO2 originated by the fuel combustion and contained in the initial impure CO2-rich stream is lost with the vent stream rich of the non-condensable gases. As a result, by venting the CO2 in the non-condensable gases, which can be composed by about 30-40% of CO2 on a molar basis, the overall carbon capture rate of the plant may reduce to less than 90%. In order to reduce this loss, some processes have been proposed to recover the CO2 contained in the vent stream originated in the CPU. The first process is proposed by Praxair and is based on a vacuum pressure swing adsorption (VPSA) process. The second one is proposed by Air Products and is based on the PRISM ® membrane separation system. In addition to CO2, such a system is also capable of recovering the O2 in the non-condensable gas stream, to be recycled to the furnace, also reducing in this way the consumption for O2 production in the ASU. In this work, a CO2 recovery system based on the Calcium looping (CaL) process is assessed. Such a process has been widely assessed in recent years for application on flue gases from coal-fired power plants [1]. CaL is a high temperature regenerative sorption process operated in a dual fluidized bed system which utilizes CaO as CO2 sorbent. CO2 contained in a stream at low concentration (combustion gases or, in this case, CPU vent stream) is contacted with CaO in a fluidized bed reactor (carbonator) operating at around 650°C, where CaCO3 is generated according to the exothermic carbonation reaction: → The limestone formed in the carbonator is decomposed back into CaO and high concentration CO2 in another fluidized bed reactor (calciner) according to the reverse calcination reaction. Since this reaction is highly