Integration of KOH-based CO2 absorption and Ca(OH)2-triggered mineralization: Process tracking and kinetic analysis

Abstract

KOH-based CO₂ absorption was integrated with Ca(OH)₂-triggered mineralization under ambient conditions. CO₂ is chemically absorbed into the aqueous KOH solution in two consecutive absorption stages, reacting rapidly with OH^- to produce CO₃^2- and slowly with CO₃^2- to produce HCO₃^- in the 1st and 2nd stage, respectively. The total CO₂ loading reaches 0.95 mol CO₂/mol KOH. CO₂ absorption rate in the 1st stage is determined by the diffusion of CO₂ and is thus independent of the OH^- concentration and enhanced by increasing inlet CO₂ concentration. In the 2nd stage, CO₂ absorption rate is determined by the absorption reaction and linearly decreased with decreasing CO₃^2- concentration. The prepared K₂CO₃ and KHCO₃ solutions exhibited similar performance toward mineralization by Ca(OH)₂. After 20 min of reaction under a Ca/C molar ratio of 1.0, KOH regeneration efficiency reached 75.9 % from K₂CO₃ and 76.1 % from KHCO₃. Mineralization of the CO₂-rich absorption solution occurred rapidly. Under a Ca/C molar ratio of 1.1, KOH regeneration efficiency reached 73.3 % after 5 min and 83.9 % at steady state after 20 min of reaction. Dissolution of Ca(OH)₂ is likely the rate-controlling step and XRD and SEM analysis confirmed the selective conversion of Ca(OH)₂ (portlandite) to CaCO₃ (calcite) during the mineralization process.