Abstract
The carbonation reaction of CaO with CO~2~ mainly consists of two stages: the fast surface chemical reaction stage and the ensuing product layer diffusion control stage. The determination of the former stage is very important for an efficient capture of CO~2~ using multiple cyclic calcination and carbonation reactions (CCRs) method. The CaO carbonation with CO~2~ and the product layer thickness of CaCO~3~ produced were numerically simulated using Bhatia's unified random pore model (URPM) and four equations developed for estimating product layer thickness. Analysis of the numerical results indicated large discrepancies of the product layer thickness computed from the four methods, showing that it might not be suitable to determine the fast chemical reaction stage based only on the product layer thickness. The concentration profile of the reaction reactant (CO~2~) (vs) reaction time should be a direct indicator of the reaction extent. It was also found that the changing tendencies of the overall CO~2~ dimensionless concentration inside the CaO particle and the overall carbonation reaction rate (vs) reaction time coincided at the same time, both decreasing towards zero. It shows that the latter (reaction rate) can also be considered to determine the completion of the fast chemical reaction stage, the same as the former (CO~2~ concentration in CaO particles). Furthermore, the reaction rate can be experimentally determined separately for the two indicators (product layer thickness and CO~2~ concentration). Therefore, the overall reaction rate was suggested as a new and feasible indicator to determine the fast chemical reaction stage of CaO with CO~2~. Copyright ยฉ 2007 Curtin University of Technology and John Wiley & Sons, Ltd.