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.