Slagging Diagnosis Of Furnace Based On Temperature Difference Of Water Wall Back Fire Side

Coal-fired units of 600MW and above have gradually become the main power generating units in China.Due to the complexity and variability of coal,low NO_x combustion technology,especially the implementation of ultra-low emission technology,boiler slagging has been a hot issue troubling the safe and economic operation of power plants.It is difficult to directly observe and diagnose furnace slagging in real time.It is of great significance to adopt effective technical means to monitor furnace slagging and its trend in real time.Based on the structural characteristics of the water wall,it is believed that the surface shape of the slag layer will be different with different slag thicknesses.Therefore,a new slag layer growth model is defined.Taking a 600 MW supercritical once-through boiler as the research object,according to the slag layer growth model,while considering the influence of the welding foot structure on the heat conduction of the water wall,a more realistic physical model of the slagging water wall is established.FLUENT is used to simulate the temperature distribution of the slagging water wall,and the feasibility of the numerical model is verified.According to the actual possible operating parameter range of the boiler,624 operating conditions have been formulated for numerical simulation of the temperature field of the slagging water wall,including:furnace smoke temperature 1300～1800 K(6 points),the convective heat transfer coefficient of the working fluid 3000～100000 W/(m~2·K)(8 points),working fluid temperature 520～700 K(5 points)and slag layer thickness 0～65 mm(12 points).The simulation results:(1)As the slagging thickens,the surface temperature of the slag layer gradually approaches the furnace smoke temperature,and the temperature of the backfire side of the water wall gradually approaches the temperature of the working fluid;(2)From the clean water wall to the slagging thickness of 65 mm,The temperature difference on the backfire side(the temperature difference between the midpoint of the fin and the midpoint of the pipe wall on the backfire side)varies from 1.02 to 128.18 K;(3)The temperature difference on the backfire side has a good dynamic range and measurement resolution,and comprehensively considering the convenience of installation and maintenance,the temperature difference on the back side of the fire is selected as the characteristic parameter for judging the degree of slagging.The temperature difference on the backfire side:(1)It has a close linear positive correlation with the furnace smoke temperature;(2)When the slagging thickness reaches 35 mm,it is almost no longer affected by the furnace smoke temperature;(3)It decreases with the increase of convective heat transfer coefficient and tends to a constant temperature;(4)The working fluid temperature has little effect on it and can be ignored.Using GA-SVR,the relationship between the slagging thickness and the temperature difference on the backfire side,the furnace smoke temperature and the convective heat transfer coefficient of the working fluid is used to perform regression fitting.The corresponding slagging thickness could be obtained by inputting the last three variables,so as to realize the diagnosis of the slagging condition of the water wall.