Abstract: Desulfurization technology is one of the most effective and optimal way to achieve clean coal combustion in circulating fluidized bed boilers,which is low-cost,no wastewater produced and simple to operate.In addition,there are many influencing factors and the reported data is significantly scattered,which make it too difficult to derive a generalized approach to predict the desulfurization performance,and there is not a reasonable explanation for this seemly abnormal declination in desulfurization efficiency with increasing Ca/S ratio in the process of practical application,and the simple Ca/S ratio cannot be fully represent the amount of limestone actually involved in desulfurization,it is difficult to fully describe the desulfurization law of limestone in the furnace. Based on the movement and distribution law of materials in the boiler,the new concept of"effective limestone inventory"was proposed in order to explore the essence of determining the desulfurization efficiency of limestone in the furnace,and through comprehensive analysis on desulfurization in circulating fluidized bed boilers.A mathematical model was established according to the principle of ash mass balance.The validity of model was validated by comparing with the experimental results of industrial circulating fluidized bed boilers with different scale burning coal particles and coal slime.The results reveal that the key factor that actually determines the desulfurization efficiency in CFB boilers is"the effective limestone inventory".This theory and model provide significantly useful guideline for effectively improving the desulfurization efficiency in the furnace.Based on the theory,it concludes that the key technologies to achieve efficient desulfurization in the furnace include:( 1) optimizing the limestone particle size distribution to effectively increase the limestone quantity in the external circulation and the residence time in the limestone furnace;( 2) smoothing the way to discharge the bottom ash so the loss of the effective limestone can be minimized;( 3) improving the gas-solid contact efficiency so the activity of the effective limestone can be fully utilized.