The oxide skin of supercritical circulating fluidized bed boiler refers to the oxide skin of a certain thickness formed after the oxidation film gradually eases after the boiler operates for a long time, and there is often a large difference in the expansion coefficient between the oxide skin and the steel pipe substrate. After the boiler is shut down for cooling, the oxide skin will fall off, which will lead to the blockage of the heating surface pipe. Moreover, a large amount of oxide scale will accumulate after falling off, which will lead to the reduction or interruption of the steam volume on the tube wall of the heating surface and the deterioration of the steam cooling effect in the tube, which will directly lead to the tube wall overheating or tube explosion. In general, in order to prevent oxide skin from falling off, positive measures shall be taken in design, manufacturing, installation and operation. Specifically:
1. During the overall design of the boiler, the wall temperature measuring points of thermal deviation must be reduced as much as possible, and the measuring points shall be checked regularly to ensure the accuracy of wall temperature measurement. Due to the thermal deviation of the heating surface, the steam temperature of the heating surface shall be controlled according to the allowable temperature of the metal. The metal temperature shall be strictly controlled during operation to ensure that no overheating occurs.
2. The selection of high temperature heating surface pipes shall be reasonably considered according to the high temperature oxidation resistance margin. For platen superheater, primary superheater and final reheater heating surfaces, SA213-TP347HFG and SUPER304H can be used as materials.
3. The enthalpy increase, resistance drop and inlet and outlet forms of superheaters at all levels shall be designed reasonably to control and reduce the flow deviation.
4. The hanger shall be designed reasonably to ensure sufficient margin, and the single hanger shall be rejected. The displacement of the hanging device in cold state is considered as 40%~60% of that in hot state, so that the platen heating surface can be pretensioned in cold state to increase the deformation resistance.
5. Ensure the free expansion of the platen heating surface. At the place where the platen heating surface passes through the wall, a metal expansion joint with reasonable structure shall be used. At the same time, the optimized elbow structure is adopted at the outlet of the platen heating surface to increase the flexibility of the platen and eliminate the deformation caused by the blocked expansion.
6. During operation, desuperheating water and soot blowing shall be used steadily in strict accordance with the designed startup and shutdown modes, load change and temperature change rate, so as to reduce thermal deviation, avoid overheating and sudden temperature change, and strengthen steam and water supervision; Forced ventilation cooling is strictly prohibited for boilers with scale problems after shutdown.
7. In the process of startup, shutdown and load change, try to restrain the periodic temperature fluctuation and temperature change rate of the heating surface, and slow down the peeling of oxide skin.
8. During the maintenance, the oxide skin detector shall be used to detect the oxide skin of the superheater and reheater, and the service life of the pipes shall be evaluated and the pipes with severe oxidation shall be replaced in time.
9. Strengthen the inspection of the heating surface and header, and ensure that the inside of the heating surface is clean and free of debris. From the actual use of the supercritical circulating fluidized bed boiler at present, its scale problem is far less serious than that of the pulverized coal boiler, which is also a major advantage of the supercritical circulating fluidized bed boiler.
There are two main reasons for the scale falling off in the high-temperature heating surface of supercritical circulating fluidized bed boiler. One is that the scale reaches a certain thickness; The other is the frequent, large and high rate of temperature change. At ordinary times, we should timely check the preparation for boiler operation and regularly check whether there are hidden dangers in the boiler.
Post time: Nov-11-2022