In-furnace desulfurization has been widely used in circulating fluidized bed boilers. SO2 is removed by reacting with limestone during the process of sulfation after calcination in air combustion. Although CaCO3 is the main component of limestone, there are also other impurities such as CaMg(CO3)(2) and SiO2 which can influence the desulfurization. The porous CaO produced by calcination plays an important role in sulfation, and water vapor in the furnace influences the calcination. This work aims to understand the impacts of impurities and water vapor on limestone calcination. Two kinds of China limestone were used to investigate the issues in a rotatable fluidized bed reactor. Mercury injection apparatus (MIP), scanning electron microscopeenergy dispersive spectrometer (SEM-EDS), and X-ray diffraction (XRD) techniques were employed to analyze the pore structure, micromorphology, and crystal structure of the Ca-O calcined, respectively. The results show that the water vapor improves the calcination rate and shortens the reaction time, and those influences are stronger for higher impurity limestone possibly because of more defects in the crystal structure. Water vapor can directly influence the chemical reaction of calcination without affecting the diffusion property of CO2. Higher water vapor content results in slightly lower ultimate degree of conversion of limestone, but for different kinds of limestone the difference is not obvious. The results of SEM and MIP also mean that the existence of water vapor improves sintering and growth of grains. The results of XRD give further evidence to the previous conclusion. These tests and analysis give rise to the mechanisms behind the impacts of water vapor on limestone calcination: the binding ability of H2O to active site O* in Ca-O is stronger than that of CO2 center dot H2O tends to replace CO2 on the active site to increase the release of CO2 in calcination. Water vapor also accelerates sintering, most possibly in the initial stage when the sintering neck is formed. There exist two possibilities: H2O molecules are absorbed on the active site of Ca-O* to promote the formation of the sintering neck of Ca-O by interaction between H2O molecules (such as hydrogen bond). Water vapor can also act as a solvent to improve the solid state diffusion from surface to sintering neck which also benefits the fusion and growth of minicrystals.