Insight into the Mechanism and Effect of H2O on CaO Sulfation by Density Functional Theory

The CaO sulfation is the main reaction to capture SO2 by limestone in circulating fluidized boilers. The mechanism and effects of H2O on CaO sulfation were investigated by density functional theory and experiments. The reaction paths and energy barriers for SO4 2– forming on CaO and CaSO4 surfaces as well as the outward diffusion of Ca2+/O2– in CaO and CaSO4 crystals were examined. Under conditions without H2O, the outward diffusion of Ca2+ on the CaSO4 surface is the rate-determining step for CaO sulfation because it has the largest energy barrier (3.63 eV) of all steps in the sulfation reaction. Under conditions with H2O, X-ray diffraction analysis indicates that H2O cannot directly accelerate the diffusion of Ca2+ in the CaSO4 layer. However, with the assistance of H2O, the interactions between Ca2+ and SO4 2– on the CaSO4 surface become weaker, the energy barrier for the diffusion of Ca2+ decreases from 3.63 to 1.62 eV, and thus the outward diffusion of Ca2+ on the CaSO4 surface is enhanced. The rate-determining step of the sulfation reaction under conditions with H2O became O2– diffusion in the CaSO4 layer with an energy barrier of 3.42 eV, which means that H2O decreases the largest energy barrier for sulfation by 0.21 eV. In addition, H2O can also enhance the adsorption of SO2 on a CaSO4 surface.