Insights into the Role of H2O in the Carbonation of CaO Nanoparticle with CO2

The enhancement of H2O on the carbonation reaction of CaO with CO2 is now widely recognized in the calcium-looping systems. However, the microscopic origins of steam-enhanced reactions remain unclear. A new insight into this issue from the atomic level is provided. We performed molecular dynamics (MD) simulations using a recently developed ReaxFF reactive force field to study the role of H2O on the carbonation reaction of CaO for enhancing CO2 capture. First, the effects of H2O on the carbonation reaction of CaO with CO2 were investigated by MD simulations combined with thermogravimetric analysis (TGA) experiments. Our calculation results well-supported by the TGA experiments showed that H2O just enhances CaO carbonation at the diffusion-controlled stage, whereas there is little influence on the kinetic stage. Then, we analyzed the properties of ion/gas diffusion and the solid product layer to deeply understand the role of H2O in the diffusion-controlled stage. It was found that the ion/gas diffusion could be significantly improved by H2O. In addition, H2O promotes the formation and growth of the solid product layer. We believed that it is attributed to the following facts. The existence of OH derived from H2O dissociation promotes CO32– formation and increases the thickness of the CaCO3 product layer. The protons can penetrate toward the center of the CaO particle by forming new hydroxyl groups with the solid oxygen, thereby destroying the reactant structure and altering the reaction activity, which leads to improved carbonation.