Boiler heating surface wear has always been a serious problem in thermal power plants. If no measures are taken or the measures are taken improperly, frequent boiler tube bursting and shutdown accidents often begin to occur after the boiler is put into operation for a few months. In particular, the boilers burning inferior coal (the proportion of coal grinding stone mixed with high ash content) are more serious, and the water wall wear of circulating fluidized bed (CFB) boilers is more prominent, the wear and tear of the water wall leakage is 5 to 6 times that of the pulverized coal furnace. According to statistics, the accidental shutdown of the circulating fluidized bed boiler due to wear and tear accounts for about 50% of the total number of accidental shutdowns. The direct harm of wear is mainly manifested in the following two aspects: ① Thinning the pipe wall. Every 2~3 months of operation of the CFB boiler, the water wall wears 1~2mm, the most seriously worn parts are 3~4mm, and even leakage occurs, which seriously affects the safe operation of the unit, increases the workload of temporary maintenance and overhaul of the power plant, and causes great losses to the power plant. ② A sudden burst accident occurred, resulting in an emergency shutdown of the furnace for repairs, not only disrupts the normal production order of the power plant and reduces the power generation, but also increases the labor intensity and maintenance costs of workers, which directly affects the economic benefits and operational safety of thermal power plants.
In order to improve the safety and economy of CFB boiler operation, reduce the number of unplanned outages and the occurrence of leakage accidents, reduce operation and maintenance costs, and improve the utilization rate of the unit,it is necessary to investigate and analyze the causes, and take scientific and effective anti-wear measures for the worn water wall.
(1) The main reason for the wear of the water wall
The wear of CFB boiler water wall tubes is mainly concentrated in three areas: wear at the junction of water wall and refractory material; wear of tube walls in the corners around the furnace; wear of irregular tube walls.
①Coal particles cause wall wear of the water-cooled wall. The CFB boiler has a large amount of bed material and circulating material in the combustion chamber, which prolongs the residence time of coal particles in the furnace. Under the action of the air flow with sufficient speed, strength and rigidity from the air distribution plate of the furnace bottom, these particles move upward in the inner core area, and flow back down along the water wall of the furnace in the outer annular area, as shown in Figure 1. These solid particles flowing downward will directly impact the water wall tube, causing rapid erosion and wear of the water wall tube. In addition, a large amount of fly ash particles are contained in a large amount of ash powder and flue gas produced when coal is burned. In the complex dynamic process of combustion inside the furnace, violent impact and high-speed scouring of the water-cooled wall tube causes the water-cooled wall wall surface to be worn into small platforms of different shapes, resulting in the thinning of the water-cooled wall wall thickness.
②Due to the existence of depressions or protrusions in the tube bundle, the particle flow changes abruptly here, which in turn causes rapid wear on the part. In the process of installation and maintenance, there are folds, staggered joints or the surface of the welding joint is not polished and smooth; the ribs and temporary welding parts during installation are not cut clean; the water-cooled wall fins have depressions or protrusions that are not welded, repaired and polished; there are depressions or protrusions at the refractory joints; the upper and lower gaps of the pipes are uneven, and it is easy to block large particles of ash to form steps, and so on. These factors will cause the ash to change its direction when it slides down, and eddy currents will occur locally, which not only cause rapid wear on the welded joints and ribs, but also cause serious erosion and wear on the nearby water wall. Poor castable quality or construction quality will also cause the castable to fall off, leaving the water wall tubes exposed and subject to wear. Zhejiang University and other units have found through cold tests that the bosses in the turning area change the flow direction of the sliding particles, so that the wear rate here is 5-10 times higher than that of the smooth wall surface, and the larger the turning angle, the more serious the wear.
③ The particle size of the incoming pulverized coal is large, and the particle size distribution far exceeds the design value. When adding the starting bed material to the boiler, the control of the particle size of the bed material is ignored, or the boiler slag is directly used as the starting bed material. These factors will cause the difficulty of fluidization of the furnace material. In order to alleviate the problem of poor fluidization, the general measure is to increase the primary air volume. However, increasing the primary air volume on the one hand makes the particle size of the upper part of the furnace too large, resulting in increased wear on the water wall (The amount of wear is proportional to the square of the particle diameter. When the speed of the particles and the impact angle with the water wall are constant, the larger the particle size, the greater the amount of wear). On the other hand, when the primary air volume increases, the flue gas velocity will increase. For flue gas with a certain fly ash concentration, the flue gas velocity will increase, and the impact force and scouring force of the fly ash particles on the water wall tube wall will also increase, so that the wear is aggravated [generally the relationship between the wear amount and the speed of the flue gas is n (n ≥ 3) power.
④ Stones in coal are the source of accelerated wear. The hardness of the stones contained in the material is 30 to 40 times that of the ash content. Although the proportion of the stones is less than 1/10 of the ash content, the harmful effects formed by the stones are beyond the reach of the ash content. After the investigation of many thermal power plants, the same conclusion was reached, that is, removing the “three pieces” (stone, iron, and wood) in the raw coal, especially the stone, is the most effective way to solve the problem of wear and tear.
In addition, the loose sealing of the furnace causes flue gas leakage, the primary wind speed and air volume are too large during operation, the coal sowing air and the cooling slag air are too large, and the air volume and air pressure of the shut-off valve are unreasonable, which are also the reasons for the wear of the water wall.
