Microstructural Characterization of CaO-based Sorbents During CO2 Capture and Sorbent Regeneration

This work of thesis deals with the micro-textural properties characterization of calcium oxide as CO2 capture solid sorbent, that strongly influence the carbonation reaction step performances. The investigation of the sorbent micro-structure was carried out by means of both in-situ and ex-situ techniques, varying the precursor (CaCO3) activation conditions and sintering effects on the final sorbent pore network. X-ray diffraction and N2-adsorption techniques were used to identify correlations between the sorbent micro-textural properties and the average sorbent crystallite size, by considering completely calcined calcium carbonate samples under vacuum conditions and varying the high temperature (800-900°C) heating step period. With the aim to investigate the high temperature and time dependent phenomena, as pore generation, sintering processes and pore closure, in-situ X-ray small angle scattering techniques were carried out for the first time to investigate the sorbent micro-structure evolution during calcite decomposition and the CaO carbonation reaction. Firstly, preliminary ex-situ measurements were performed at the Advanced Photon Source facilities of the Argonne National Laboratory to test the X-ray small angle scattering technique capabilities to investigate high porous samples, such as completely calcined CaCO3 and partially carbonated CaO-based sorbents, and successful results were obtained, providing a detailed quantitative description of the sorbent micro-structure. Therefore, for the first time in-situ time-resolved X-ray small angle scattering tests were carried out to investigate the sorbent micro-textural properties generation during the CaCO3 calcination reaction and their evolution during the CaO carbonation reaction. Two different sets of experiments were considered, by performing CaCO3 calcination reactions below 800°C in pure nitrogen and at high temperatures (800°C and 900°C) in presence of CO2 (from 1% up to 50% in balance with N2) in the reaction atmosphere. Afterwards, CaO carbonation tests were performed at 550°C with 0.5% of CO2 in balance with N2. Because of the non-catalytic gas-solid nature of the CaO carbonation reaction, a modified random pore model was proposed in order to represent both the reaction kinetics and the sorbent micro-structure evolution over the reaction. In addition, a computation fluid dynamic study on a thermo-gravimetric analyzer was performed to quantify the external mass transfer effects on the kinetics of the CaO carbonation reaction.