Simulating Study On Ash Deposits And Ash Blowing In Superheater Of SP Boiler In Cement Kiln

Air Quenching Cooler(AQC)and Suspension Preheater(SP)waste heat boilers are are widely used in Chinese cement industry to recover waste heat for power generation,where the recovery efficiency is lower and the cost of power generation is higher compared with the advanced international level.Besides,the problem of ash deposition is serious in SP waste heat boiler.It is an important way to improve the efficiency of waste heat recovery and improve the efficiency of power generation as well as reduce the cost of power generation by blowing ash and reducing the erosion of boiler tube bundles caused by soot blowing.In this thesis,the superheater of SP waste heat boiler in waste heat recovery power generation system of a cement kiln was taken as the research object,and the numerical simulation method was adopted to study the problems of ash deposition,ash blowing and tubes erosion induced by ash blowing inside the superheater.The influences of flue gas velocity,concentration of flue gas and arrangement angle of elliptical heat exchanger tube(the angle between the length of elliptical tube and the sharp angle of airflow direction)on ash deposition were investigated by establishing a comprehensive growth model.Afterwards,we fitted the simulation results based on asymptotic dust growth model to determine the effects of flue gas velocity,dust concentration and elliptical tube arrangement angle on the time characteristics of ash deposition.In addition,Tabakoff erosion model was advocated to study the flow field and erosion of superheater of SP boiler under direct air blowing.The effects of elliptical tube arrangement angle,soot blowing pressure,soot blowing distance,nozzle diameter,soot blowing position and nozzle shape on soot blowing and soot blowing erosion were obtained.Based on the minimum cost method and the analysis of the above research results,the optimum period of soot blowing for circular and elliptical tubes was obtained.The specific conclusions are listed as follows:(1)The ash deposition rate of tube bundles increases with the increase of flue gas velocity and dust concentration.However,the time for ash deposition to reach a stable value decreases with the increase of flue gas velocity and concentration.The steady deposition amount of ash decreases with the increase of flue gas velocity,but has nothing to do with the change of dust concentration.When the arrangement angle of elliptical tube changes in the order of 45 °,0 °,25 °,65 ° and 90 °,the amount and time of ash deposition increase in turn.Compared with elliptical tubes with different arrangement angles,the time for ash deposition to reach stability is the shortest,but the amount of ash deposition is the largest.Based on the above conclusions,it is safe to conclude that the effect of anti-ashing for elliptical tube is better than that of circular tube.The resistance loss and heat transfer coefficient of the elliptical tube increase with the increase of the arrangement angle.The resistance of the elliptical tube is equal to that of the circular tube when the arrangement angle is 65 °,and the heat transfer coefficient is equal to that of the circular tube when the arrangement angle is 45 °.(2)When the arrangement angels(?)are chosen as 0 ° and 25 °,the soot blowing effects of the elliptical tube are better than those of the circular tube.Compared with the circular tube with ? of 45 °,the soot blowing effects of the elliptical tube with ? of 65 ° and 90 ° are weaker than those of the circular tube.The maximum erosion rate of elliptical tube soot blowing decreases with the increase of the arrangement angle.When the arrangement angle is 45 °,the maximum wear rate of elliptical tube soot blowing is slightly larger than that of circular tube soot blowing.The effects of soot blowing,the maximum wear of superheater tube bundle and the average wear of tube bundle increase with the increase of soot blowing pressure and the diameter of soot blowing tube but the decrease of soot blowing distance.For triangular,rectangular,circular and elliptical nozzles(the ratio of length diameter to d short diameter is 2:1,3:1 and 4:1),the longer the lateral distance of the nozzle,the better the soot blowing effect is.The soot blowing effect of rectangular nozzle is better than that of circular nozzle,and the maximum wear rate of rectangular nozzle is slightly higher than that of circular nozzle.The overall wear of triangular nozzle is the lowest,while the maximum wear and average wear of elliptical nozzle are the highest.