Gas-solid carbonation of Ca(OH)2 and CaO particles under non-isothermal and isothermal conditions by using a thermogravimetric analyzer: Implications for CO2 capture

The gas-solid carbonation of alkaline sorbents has been actively investigated as an alternative method to CO2 capture from industrial combustion sources and CO2 contained in the air. This study has a two-fold objective: firstly, quantify the gas-solid carbonation extent and the carbonation kinetics of Ca(OH)2 and CaO; and secondly, propose a reaction mechanism of gas-solid carbonation for CaO under dry conditions (relative humidity close to 0), i.e., when the action of water is negligible. The main results of our study have revealed that a high proportion of Ca(OH)2 nanoparticles were transformed into CaCO3 particles by gas-solid carbonation (carbonation extent, >0.94) under non-isothermal conditions. Moreover, this gas-solid reaction requires low activation energy (Ea≈6kJ/mol) at a constant heating rate of 5 or 10K/min. A similar carbonation extent was determined for gas-solid carbonation of in-situ synthesized CaO under non-isothermal conditions. However, the gas-solid carbonation of CaO takes place in a broader temperature range, implying a more complex thermokinetic behavior (overlapping of carbonation regimes or steps). Concerning the gas-solid carbonation of Ca(OH)2 and CaO under isothermal conditions, a high carbonation extent (>0.9) was determined for CaO at 600 (873K) and 800°C (1073K). Conversely, the gas-solid carbonation of Ca(OH)2 particles was relatively low (<0.56) at 400°C (673K) after 6h of reaction. This case is in agreement with the formation of a dense non-porous layer of carbonate mineral around the core of the reacting Ca(OH)2 particles, thereby limiting the transfer of CO2. Finally, an alternative reaction mechanism is proposed for the gas-solid carbonation of CaO, when the relative humidity is close to 0. This macroscopic control at high temperature avoids CO2 dissociation with molecular water at the CaO-CO2 interface. For these specific conditions, the mineralization of adsorbed CO2 on CaO particles implies a solid state transformation, i.e., CaCO3 formation from CaO-CO2 interactions. This could be explained by an atomic excitation than at high temperature allows the local migration of one oxygen atom from the solid towards the adsorbed CO2 leading to its mineralization into carbonate (porous or non-porous layer) around the reacting particles; chemically the mineralization of CO2 also implies the breaking of one covalent bond in the CO2 molecule.