The coefficient of heat conductivity of ash is small, the ash accumulating on the boiler heating surface will greatly affect the heat transfer of boiler heating surface, thus reducing boiler efficiency. If ash accumulates on the convective heating surface of cross-sectional area of flue, which will reduce the cross-sectional area of the duct, increase the flow resistance, make the output of suction fan insufficient and reduce the operation load, in severe condition, block the tail flue and force the boiler to stop for maintenance. Due to ash accumulation, the flue gas temperature rises and the operating safety of the heating surface in the back will be affected.     （lsh）