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As one of the three main equipment in thermal power plants, the boilers in such plants suffer from failures such as pipe leaks due to high-temperature oxidation and corrosion in their high-temperature heating surfaces, including the water wall, superheater, reheater, and economizer tubes of coal-fired boiler systems. 1 General mechanism of high-temperature corrosion The process of high-temperature corrosion on the flue gas side of water wall tubes is relatively complex. It is generally believed that the occurrence of high-temperature corrosion is related to the following factors: a, high sulfur content in coal ； b. The coal dust flame, which contains combustible materials, directly impacts the wall surface ； c. The water wall is often in a reducing atmosphere. d. When the sulfur content in coal is high, the chemical composition of the deposits on the outside of the water wall tends to promote the occurrence of high-temperature corrosion. If the outer surface of the water wall tubes is frequently exposed to flames containing a large amount of unburned coal dust, ferrous sulfide (FeS2) adheres to the tube walls along with the coal dust particles and ash; the atomic sulfur and SO3 generated through catalysis within the furnace cause high-temperature corrosion of the water wall ； Under oxygen-deficient conditions, high levels of reducing gases such as H2S and CO near the water wall can also cause high-temperature corrosion of the water wall. Studies have shown that in a reducing atmosphere, when the concentration of H2S in the flue gas exceeds 0.01%, it causes severe corrosion to steel, with the corrosive effect being strongest in the range of 300°C to 500°C. 2 Measures to prevent high-temperature corrosion: Given the characteristic of coal with a high sulfur content being used as fuel, specific measures can be taken to prevent high-temperature corrosion in the boiler, as well as to avoid flame sticking to the furnace walls and the formation of a reducing atmosphere in the high-temperature areas of the furnace: 1) Measures to prevent high-temperature corrosion of the water wall: a. Select appropriate thermal parameters and furnace structure parameters, ensuring that the temperature at the furnace outlet is suitable. By selecting an appropriate distance between the upper row of burners and the side water wall, as well as between the lower row of burners and the corner of the cold ash hopper, it is possible to prevent the flame from directly striking the water wall, thus avoiding slag formation on the furnace’s water wall and high-temperature corrosion. Select an appropriate distance between the upper burner and the bottom of the screen to keep the smoke temperature at the bottom of the screen at a low level, thereby preventing high-temperature corrosion of the tube screen. b. By selecting the parameters of the optimized internal threaded tube appropriately, heat transfer on the working fluid side can be enhanced, the surface temperature of the water wall tubes can be reduced, and high-temperature corrosion can be prevented. c. The burnout air utilizes an optimized dual-air-flow structure and arrangement; the burnout air nozzles consist of two streams of air: the airflow in the central area is non-rotating and penetrates directly into the center of the furnace to supply the air required for burning out ； Rotating airflow is used at the edge air inlets to create an oxidizing atmosphere on the water wall, effectively preventing coal powder particles from eroding the water wall. At the same time, the optimized arrangement of the burnout air nozzles ensures that the burnout air covers the entire primary air flow across the width of the furnace, preventing coal dust particles from escaping. This helps to effectively avoid high-temperature corrosion in the areas of the burner near the side walls. d. Optimize the angle of the burner expansion cone to prevent premature flame spread from scouring the water wall. e. Arrange the burners properly, ensuring they are at a sufficient distance from the side walls as well as the cold ash hoppers, to prevent the flames from striking the water-cooled walls. f. Optimize the arrangement of burnout air; in addition to the main burnout air, side burnout air is arranged near the side walls to create a low-temperature wind shield that protects the side walls from being eroded by the flame. g. The above measures are effective in reducing high-temperature corrosion of the water wall, but they cannot completely eliminate it. Based on design experience with the combustion of high-sulfur anthracite, it is recommended to apply corrosion-resistant coatings to the water wall and high-temperature heating surfaces. Although the initial investment for applying such coatings is high, it prevents frequent shutdowns for tube replacement due to corrosion-induced thinning, making it cost-effective in the long term. 2) Measures to prevent high-temperature corrosion of the convective heating surfaces: a. Arrange the positions of the heating surfaces appropriately, so that those where the working fluid temperature is high are located in areas with relatively lower flue gas temperatures. The flue gas flows from the furnace outlet sequentially to the screen superheater, the high-temperature superheater, and the high-temperature reheater; the temperature of the working fluid in these heat-exchanging surfaces increases from low to high ； Additionally, for the heating surfaces of the panel superheater, high-temperature superheater, and high-temperature reheater in areas where the flue gas temperature is relatively high, a co-current arrangement is adopted; the flue gas temperature at the inlet is relatively high. This ensures that the temperature on the surface of the tubes in the heat-exposed area remains at a low level; a lower wall temperature helps to prevent high-temperature corrosion effectively. b. Throttling rings are used to reduce the deviation between tubes and to control the wall temperature of the heated surfaces, keeping it below the temperature at which high-temperature corrosion occurs, thereby effectively preventing such corrosion. c. In the selection of materials for the heating surface tubes, steel with good corrosion resistance was used. A large amount of SA-213TP347H austenitic stainless steel is used for the tubes of screen superheaters, high-temperature superheaters, and high-temperature reheaters. 3 Examples of Preventing High-Temperature Corrosion (I) The 2×300MW boilers in the first and second phases of a certain plant: The sulfur content on an as-received basis for the coal used in the engineering design was 2.9%, while the sulfur content of Coal 1 used for verification was 4.5%, and that of Coal 2 was 2.86%. To prevent high-temperature corrosion of the water wall, the following series of measures were adopted: (1) In the high-temperature areas of the water wall, thermal spraying of aluminum was used on the fire-facing side of the water wall. By this method, a thin layer of aluminum is formed on the surface of the tube wall, and the dense Al2O3 layer formed on the surface of this aluminum layer serves to prevent corrosion. (2) A horizontal concentration burner is used, with rich pulverized coal primary air on the fire-facing side and lean pulverized coal primary air on the backside, to maintain an oxidizing atmosphere near the water wall. (3) A small cutting circle is adopted, with each corner burner divided into two groups of horizontal rich-lean burners, along with higher primary air ratios, peripheral air ratios, as well as primary air speeds and peripheral air speeds, in order to prevent the flame from scouring the water wall. (4) The large furnace and burners are divided into two groups, etc., to reduce the heat load on the furnace wall. (5) During operation, pay attention to even air distribution to prevent the flame from leaning against the wall. The two units in Phase 1 were put into operation in 1999 and 2000 respectively, while the two units in Phase 2 were commissioned in 2004; the water wall is performing well. (II) 2×300MW flame boilers in Phase I and Phase II of a certain plant: The sulfur content on an ash-free basis for the coal specified in the engineering design is 2.29%. The actual sulfur content in the coal used in operation varies to some extent, generally ranging from 2.5% to 2.7%, and at times it can reach 4%. The commissioning dates of the four reactors were: December 1998, August 1999, March 2003, and August 2003. At the time of initial operation, the water wall was not sprayed; after operating for some time, corrosion and thinning occurred throughout the straight tube section of the upper furnace chamber, mainly in the areas above the furnace arch that were not covered by the pre-combustion zone. The situation was even more severe on the side walls, with a corrosion rate of 1.5 mm per year. Later, the power plant carried out supersonic spraying on the side walls of the water wall, using CT45 as the spraying material (a nickel-based chromium-nickel alloy), and the operation has been satisfactory. High-temperature corrosion also occurred on the lower part of the divider screen. In 1999, the power plant applied a coating to the corroded areas of the divider screen in Boiler #2. However, this coating provided only corrosion protection and no wear resistance; it wore off after some time of operation. In 2004, the power plant hired another domestic manufacturer to apply another coating. This time, the coating material offered both corrosion and wear resistance, resulting in satisfactory operational performance.