Corrosion mechanism of heating surface of pulverized coal boiler

The corrosion parts of pulverized coal boiler are usually water wall tubes, superheater tubes, reheater tubes and economizer tubes, which are called "Four Tubes" In the process of pulverized coal combustion, these parts contact with flame and combustion products directly, and suffer from high temperature, corrosion and stress In China, the total installed capacity of the existing units is close to 300 million kilowatts, but more than 90% of them are 200-300 MW units with sub critical capacity Among them, the overdue service units have exceeded 25 million KW The superheaters and Reheaters of these units are mostly made of low alloy steel pipes (such as g10212crmo t11t12, etc.) Material factors and bad conditions in operation lead to more early tube explosion Local statistics show that the emergency repair time caused by boiler tube explosion accounts for more than 40% of the unplanned outage time of the whole unit and more than 70% of the unplanned outage time Boiler tube explosion not only seriously affects the normal operation of the generator, but also causes great economic losses to the power plant The cause of boiler tube explosion is very complex Tube manufacturing, metallurgical quality, installation, welding and other original quality problems There are also high temperature and high pressure on the inner wall, high temperature gas corrosion on the outer wall, particle erosion, operation system, management and maintenance, etc there are no less than a dozen of these factors, but about 90% of the essential reason for tube explosion is the joint action of high temperature, corrosion and stress, resulting in material damage and crack growth Among the various failure factors, stress damage, corrosion in steam pipe and fatigue are related to the reasons of taking materials, manufacturing, processing, maintenance and operation management, while corrosion (high temperature oxidation, corrosion of molten salt and sulfur, stress corrosion) and related wear are the objects that surface engineering technology can pay attention to and try to solve High temperature oxidation refers to the reaction between the metal elements (mainly FE) that make up the furnace tube and oxygen in the flame above and below 1200 ℃ in the furnace to generate oxide The oxide is incompatible with the original material in phase composition, structure and other aspects, resulting in stress, and finally delamination and thinning of the furnace tube Sulfur corrosion is caused by the oxidation of s in coal to SO2 or SO3 When the air coefficient is low or the content of flue gas is high, it will be converted into H2S, which will form FES with the Fe element in the furnace tube, resulting in the furnace tube delamination Hot corrosion is the formation of S-containing molten salt by sulfide at its dew point temperature The molten salt is liquid and contacts with the pipe wall, so it is a more serious corrosion Erosion is the erosion of the surface of the furnace tube by the ash entrained in the smoke flow It is a physical process Under the joint action of the above factors, the furnace tube is seriously damaged