Analysis of Operating Defects and Proper Installation of Circulating Fluidized Bed Boilers

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Abstract: Circulating fluidized bed boilers are environmentally friendly and energy-efficient boilers that align with the future development direction of thermal power plants. Currently, they have entered the stage of large-scale and commercial production internationally, and domestic power plants are also beginning to adopt them one after another. However, it is after all a new type of application technology; its boiler structure is special, and its combustion method differs fundamentally from that of coal powder boilers. There is still relatively little experience available in China in this area. To this end, several prominent issues identified during installation and use are discussed, along with some insights and experiences. }Pj Keywords: Circulating fluidized bed boiler ； Proper installation ； Reasonable operation ； Wear ； air leakage ； The quality of the wind cap F enables circulating fluidized bed boilers to operate with high efficiency, low pollution, and good adaptability to different types of coal. It can be burned with almost any type of fuel, such as peat, bituminous coal (including high-sulfur coal), anthracite, gangue, coke, industrial waste, municipal garbage, and so on. By directly adding desulfurization agents such as limestone into the bed, the investment is low and the desulfurization efficiency is high (when Ca/S = 1.5–2.0, the desulfurization efficiency can reach 85%–90%). This type of furnace represents the development direction for environmentally friendly and energy-efficient power plants today. Internationally, circulating fluidized bed boilers have entered the stage of large-scale and commercial production, and an increasing number of manufacturers in China have also begun to develop and produce such boilers. Power plants equipped with circulating fluidized bed boilers can be found throughout the country. Circulating fluidized bed combustion technology is a new technique; the boiler structure is special, and the combustion method differs fundamentally from that of coal powder boilers. In terms of installation technology, there is still a certain gap in China compared to coal powder boilers. This article only presents my views on a few prominent issues; problems similar to those in coal-fired boilers are not discussed further. p?o 1 Wear issue 3Om: Circulating fluidized bed boilers use coal particles with a particle size of less than 13 mm, and the fluidization air velocity is very high (usually 5–10 m/s). Wear is the biggest problem with this boiler. 7;4 /q 1.1 Wear of the combustion zone u7^ The area around the furnace’s air distribution plates is surrounded by water-cooled walls on all four sides. Within a height range of 3.5 m above the air distribution plates, pins are welded to both the inner and outer sides of these water-cooled walls, and a \"high-temperature and wear-resistant casting material\" is poured over this structure; metal guards are installed on the outer side. The thickness of the refractory layer on the fire-facing side is usually 20 mm. This area is called the “combustion zone”. -@The space enclosed by the X(mO combustion zone is called the \"combustion well\"; it serves as the core of combustion in a circulating fluidized bed boiler, where the entire process of coal particle fluidization, combustion, and burnout takes place. All wear in the combustion zone is borne by high-temperature and wear-resistant castables; the material composition of these castables and the quality of their installation are key factors in reducing wear. Regarding nywZH, the first issue is material selection. There are many manufacturers in China that produce high-temperature and wear-resistant castables, and there is a wide variety of such materials; the wear resistance of these castables varies significantly depending on the temperature. Some materials exhibit the highest wear resistance in the range of 1,400–1,600°C, but their wear resistance is very low in the temperature range of 800–1,000°C; therefore, the selection of materials is crucial. The combustion temperature of circulating fluidized bed boilers is 800–1,000°C; therefore, materials with the highest wear resistance in this temperature range should be selected. The construction quality of 8N is also an important issue; the following points should be taken into consideration during construction: GHgc (1) The water-cement ratio must be well controlled – generally, 7% to 8% water should be added to the casting material – and it is necessary to follow the instructions provided in the material’s user manual strictly. An increase of 1% in the water content results in a decrease of about 20% in the strength of the castable (no empirical evidence available). The water used for construction must be clean, with a pH level that meets the requirements. For yE0/B (2), mixing must be thorough; use a forced mixer to achieve a paste-like consistency. The mixed casting material should not be stored for too long. ^For Om3 (3), use a small-diameter vibrator or a plate vibrator to compact the pouring material. czZv6 (4): The temperature rise curve of the furnace is determined strictly in accordance with the data provided by the material supplier. ~M(;D 1.2: Wear of the horizontal flue – The horizontal flue is the passage through which flue gas moves from the furnace chamber to the cyclone separator; its structure typically consists of high-temperature and wear-resistant bricks for the bottom surface and side walls, while the top is covered with high-temperature and wear-resistant cast materials. On the water wall surrounding the inlet of the horizontal flue, pins are welded to the fire-facing side, and high-temperature and wear-resistant castables are applied. The high-speed flue gas carries unburned coal particles and fly ash in amounts ten times that of the coal fed in, and it enters the horizontal flue after rotating 90°. This causes impact on the water wall at the inlet,; therefore, a 400 mm wide castable must be applied to the fire-facing side of the water wall at the inlet. The flow area of the horizontal flue is small, so the velocity of the flue gas increases after it enters the horizontal flue from the furnace. At the same time, the cross-section of the horizontal flue is in the shape of a converging nozzle, causing the flow velocity of the smoke gas to increase gradually within the horizontal flue. The wind velocity at the inlet of the cyclone separator reaches 12–18 m/s, which causes significant wear on the four walls and may also lead to vibration of those walls. The material selection and construction requirements for the high-temperature and wear-resistant castable here are the same as those in the combustion zone. There are also material selection issues regarding U-type high-temperature and wear-resistant bricks, as well as the refractory mortar used for bricklaying; it is necessary to use materials with the highest wear resistance in the temperature range of 800–1,000°C. Errors in material selection can lead to permanent problems, severely affecting the safe and economical operation of the power plant. ~The side wall of the horizontal flue cannot be constructed as a single wall; adding additional bricks will not resolve the underlying issue, as prolonged exposure to washout and vibration can cause the wall to collapse. The side walls and guard plates should be connected using heat-resistant steel bars, with at least one connection every 500 mm in the vertical direction. For walls without guard plates, the adjacent walls should be connected to form a single unit; similarly, connections should be made at intervals of at least 500 mm in height. w621 1.3 Wear of cyclones and downcomers (>Wv). The function of a cyclone is to use centrifugal force to separate ash particles from flue gas; after separation, the flue gas enters the rear vertical shaft ； The ash particles descend to the material leg and enter the combustion shaft through the return conveyor to be burned again. The upper dome and lower cone of the cyclone separator are made of high-temperature and wear-resistant cast materials, while the middle cylindrical section is constructed using high-temperature and wear-resistant bricks; protective plates are installed on the outside of the cyclone separator. The leg is cylindrical in shape, with casting material filled inside; it has a water cooling jacket in the middle, and a protective plate is installed on the outside. The flue gas carries ash particles into the cyclone separator, creating high-speed rotating air currents that cause wear on the inner walls of the separator; particularly, the section of the cylinder wall opposite the horizontal flue experiences the most severe wear due to the direct impact of these air currents. Several points should be noted during the installation of the AHbz cyclone separator: (1) The outer protective plates of the separator must remain concentric, and the brick supports and hooks must be installed firmly while ensuring the correct dimensions. >o (2) In addition to ensuring their internal quality, high-temperature and wear-resistant bricks as well as high-temperature and wear-resistant castables must also have proper installation dimensions. The cylinder, cone, and any cross-section at the top must all maintain concentricity, with a smooth surface to prevent severe localized wear. The connection area with the leg should have a smooth transition to prevent coking at the throat, which could lead to blockages in the cyclone separator. (hz (3) Expansion joints are neat, of correct size, and have appropriate filler. The expansion joint is too small; as a result of poor expansion during unit operation, the furnace wall or castable material falls off ； If the expansion joint is too large, it can cause localized wear due to the expansion joint. H