A rate equation theory for the pore size distribution of calcined CaCO3 in calcium looping
CaCO3 calcination is an important step in calcium looping, and the formed pore structure of porous CaO is critical for subsequent carbonation towards carbon dioxide. Therefore, it is necessary to investigate the evolution of the pore structure of the sorbent in the calcination step. A mathematical model describing the pore size distribution during the calcination of the CaCO3 particle was developed. CaCO3 calcination is calculated following a shrinking core model at the CaO–CaCO3 interface, and CO2 diffuses through the porous CaO layers. During the decomposition of CaCO3, after the departure of the CO2 molecule from its original lattice, a vacancy will be formed that will diffuse inside the solid, and the collision and coagulation of the vacancy results in pore formation. A rate equation theory was proposed to describe the vacancy coagulation and pore evolution inside the solid, with rate expressions derived for the pore size distribution function with time evolution. To validate the developed model, the evolution of the pore size distribution during CaCO3 calcination was experimentally measured in a high-temperature furnace combined with the nitrogen adsorption method. It was found that there is a characteristic bimodal distribution for the pore structure of calcined CaCO3, with average pore sizes of ∼2.8 nm and ∼50 nm. The calculated results agree well with the experimental data, and the relative importance of growth and coagulation was discussed.