Rate-Equation-Based Grain Model for the Carbonation of CaO with CO2

A rate-equation-based grain model was developed to describe and predict the complex behaviors of the carbonation reaction of CaO with CO2 in calcium looping. In the model, the assumption of a uniform CaCO3 film at the grain scale was replaced with the product island morphology, and the rate equation theory was used to calculate the growth of product islands; this modified grain model was integrated into a particle scale model in which gas diffusion inside a CaO particle and the pore plugging effect were considered. The macroscopic kinetics of the carbonation reaction—including the initial fast stage and the later product layer diffusion stage—could be predicted successfully using the developed theory and was validated by comparison with experimental data. The effects of structural parameters on the carbonation kinetics were discussed. Furthermore, a nanometer-scale grain design criterion for CaO sorbent was proposed to optimize particle structure and achieve maximum CaO conversion in the initial fast stage; this concept was validated and supported by the developed rate-equation-based grain model. This developed model provides a link between microscopic mechanisms at the grain level and the gas diffusion behavior inside a sorbent particle, and it can be used to describe the macroscopic kinetics of a gas–solid reaction.