Numerical Simulation On Platen Superheater Tube Overtemperature For1820t/h Supercritical Boiler

Supercritical coal-burning technology has become the most advanced technology at present because of its higher efficiency, lower emission, easier regulation for load, and steadier operation. In actual operation process, the overtemperature problem sometimes appears on the superheater wall in supercritical boiler because of high-temperature and high-pressure steam, limited wall materials and boiler furring on the superheater wall due to long-time operation.The supercritical boiler of1820t/h in capacity is taken as the research object, and the platen superheater area in the boiler is studied by numerical simulation. The temperature distribution simulated in different working conditions is contrastively analysed. The realizable k-ε model is used for gas flow simulation. The equations of the mixture fraction and the scalar probability density function(PDF) method are employed to calculate the volatile combustion. In order to trace the coal particle, the stochastic tracking model was employed for it. The two competing rates model was used for volatilization of pulverized coal particle. Diffusion dynamic combustion model was employed for char combustion. P-1radiation model was employed for radiation.After reasonable mathematical-physical models are chosen, the velocity distribution and the temperature distribution in furnace and platen superheater area are simulated in the original condition. The temperature distribution in different positions of the furnace and platen superheater area is simulated in conditions of different excess air coefficients and different ratio of secondary air to primary air.In the original condition namely BMCR condition, the temperature variation tendency in platen superheater area rises first, then declines and rises at last along the direction of furnace height, and the highest temperature point appears in the lower end of the tube panels (around the tube elbow). For a single tube panel, the average temperature around the tube is gradually reduced from outer tube to inner tube. Along the direction of tube panel height, the temperature of the tube area declines because of the weaker role of flue gas on tube panels when the flue gas temperature gradually declines.For the excess air coefficient, increasing excess air coefficient to1.15is conducive to complete combustion of pulverized coal. The whole furnace temperature level which rises results in a wide range of high-temperature area in the platen superheater area. When the excess air coefficient increases to1.25, the high-temperature area around the platen superheater gradually decreases with the decreasing furnace temperature. Therefore, the combustion efficiency and moderate temperature of the platen superheater should be comprehensively considered when the excess air coefficients are chosen.For the ratio of primary air to secondary air, when the total air volume keeps constant and the ratio of the secondary air gradually increases, namely the ratio of the primary air gradually decreases, the flame center in furnace gradually lower. Consequently, the overtemperature of the platen superheater can be effectively avoided and the service life of the platen superheater is extended.