CaCO₃ Crystallite Evolution during CaO Carbonation: Critical Crystallite Size and Rate Constant Measurement by In-Situ Synchrotron Radiation X‑ray Powder Diffraction

In this work, the evolution of the CaCO₃ crystalline phase during the CaO–CO₂ reaction was investigated by means of in situ synchrotron radiation X-ray powder diffraction performed at the Advanced Photon Source (APS) facilities of the Argonne National Laboratory. CO₂ absorption experiments were carried out in a high temperature reaction capillary with a controlled atmosphere of pure carbon dioxide (CO₂ partial pressure of 1 bar) and in the temperature range between 450 and 750 °C, using CaO-based sorbents obtained by calcination of commercial calcium carbonate. The Rietveld refinement method was applied to estimate the average size of the CaCO₃ crystallites formed during a carbonation time of 20 min, as a function of the carbonation temperature and of the initial calcination conditions. Local maxima were observed in the CaCO₃ crystallite size versus time curves and were identified as the critical CaCO₃ crystallite sizes, marking the transition between the first fast carbonation stage and the second reaction stage controlled by product-layer diffusion. A relationship between this parameter and the reaction temperature, as well as with the initial (at the beginning of carbonation) CaO crystallite size, were found. The CaCO₃ critical crystallite sizes were used to estimate the active surface areas of the CaO sorbent particles where CaCO₃ crystals form and grow. The computed active surface areas were utilized to calculate the kinetic parameters of the surface carbonation reaction: a reaction rate constant of 4.41 × 10^–4 mol/m² s, with zero-activation energy, was obtained