In-furnace desulfurization technology is usually used in circulating fluidized bed (CFB) boilers, and natural limestone is the most common calcium-based sorbent for-sulfur removal. Limestone desulfurization could be influenced by water vapor in the flue gas and also impurities such as SiO2, Al2O3, and Fe2O3 contained in the natural limestone. The limestone desulfurization process can be divided into two steps: calcination to produce CaO and sulfurization of CaO formed. This paper studies the impacts of water vapor and impurities on the limestone calcination and sulfinization. of CaO calcined in a microfluidized bed reactor analyzer (MFBRA). The apparent activation energy under different operating conditions was first Calculated, and then the influence of water vapor and impurities on the activation energy was preliminarily discussed. The results show that reaction temperature, water vapor,,and impurities all have influences on the indirect desulfurization of limestone under the condition of standard pressure and air atmosphere. For calcination, the-apparent activation energy decreases Slightly in the presence of water vapor, and it does not change with the impurities. It is speculated that the existence of water vapor may produce an intermediate product Ca(OH)(2), which has-a lower activation energy for decomposition than that for CaCO3. The impurities in the crystal structure of CaCO3 may affect the connection of CaO*-CO2 and improve the porosity of product layer which in turn accelerates the release of the gas product CO2 without affecting the activation energy. For the sulfurfization. of CaO Calcined; both water vapor and impurities decrease the apparent activation, energy. This may result:from the improvement in solid-state diffusion of Ca2+ ions in the presence of water vapor or impurities. Further investigations are needed in order to confirm these speculations.