Abstract: Ultra-supercritical coal-fired power generation technology is an important way of coal clean utilization.Among the ultra-supercritical boilers,the furnace arch at the joint between the furnace outlet of the П type boiler and the horizontal flue is easy to form ash deposition due to the existence of the recirculation zone,which seriously affects the safe operation of the boiler.Through the mechanism analysis of the ash deposition of the furnace arch in the ultra-supercritical coal-fired boiler,including the adhesion force between the ash particle and the sediment and the force of destructing ash deposition the mathematical model of ash deposition of the furnace arch and the pre-judging model of ash collapse were established to give a prediction result of whether the ash collapse would occur,which could provide a guidance plan for the safe operation and maintenance of the power station.The ash deposition and ash collapse pre-judging model obtained the moving speed of the coal ash particles in the flue gas,the depth of the sediment layer and the surface tension based on adopting the coal quality parameters,boiler design parameters and operating data,so as to quantitatively calculate the adhesion force of ash particles in the per unit area of inclined plane of the furnace arch and the impact of the ash particles on sediments.The theoretical maximum ash deposition height and the actual maximum ash deposition height at the furnace arch position were obtained according to the equilibrium relationship among the adhesion force,gravity and frictional force in per unit area.When the theoretical maximum ash deposition height is greater than the actual maximum ash deposition height,the friction and adhesion force of the sediment layer are greater than the component force of gravity along the inclined plane of the furnace arch,and the ash deposition of the furnace arch does not collapse,otherwise,when the maximum ash deposition height is less than the actual maximum ash deposition height,ash collapse will occur.For the 1 000 MW ultra-supercritical П-type coal-fired boiler of Second Electric Power Generation Co.,Ltd. Taizhou Zhejiang province,the characteristics of the ash deposition and ash collapse at the furnace arch were calculated and analyzed.The results show that,the when the furnace outlet temperature is 1 035 ℃ and the design coal is burned,the actual ash height at furnace arch is 2.48 m,which is less than the gravity balance height of 6.69 m.The furnace arch of the boiler does not occur ash collapse,and the design and operation of the furnace arch is reasonable and feasible.The pre-judgment model of ash deposition and ash collapse at the furnace arch of the ultra-supercritical boiler has certain reference value for boiler design and optimization operation.