CO2 sorption reactions at 20 bar and two different temperatures (i.e., 180 and 220 degrees C) using a K-Mg-based CO2 sorbent were carried out in a custom-designed high-pressure thermogravimetric analyzer (pressurized bubbling fluidized bed reactor on a scale) coupled with a gas chromatograph. The experimental apparatus, including the thermogravimetric analyzer, was custom-designed to measure weight changes caused by either CO2 sorption or water sorption or both. Analysis of the CO2 sorption reaction revealed that water sorption takes place rapidly with a moderate CO2 sorption rate at the early stage of the reaction. Then, the reaction migrates to CO2 sorption with simultaneous water desorption. Therefore, the mechanism of the CO2 sorption reaction is assumed to consist of fast hydration of K2CO3 and MgO, formation and decomposition of KHCO3, and finally carbonation of Mg(OH)(2) resulting in MgCO3 as the main product. K2CO3 is assumed to provide an efficient pathway for CO2 and water to travel into the core region of the sorbent via a reversible reaction between K2CO3 and KHCO3. (C) 2015 Elsevier Ltd. All rights reserved.