Installation of anti-wear walls in fluidized bed boilers

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Comparative Study on Anti-wear Diaphragm Technology and Anti-wear Beam Technology for Circulating Fluidized Bed Boilers. Research Background on CFB Power Generation: Circulating Fluidized Bed (CFB) boilers represent a relatively mature technology for utilizing clean coal. They offer advantages such as wide fuel adaptability, good operational performance, and strong environmental benefits. They are widely used in industries in China such as waste rock power generation, non-ferrous metallurgy, chlor-alkali chemistry, papermaking, and textiles, serving as an important solution for power and steam supply. With the rapid development of circulating fluidized bed boilers, some problems have also emerged. In particular, boiler manufacturers do not give sufficient consideration to domestic coal types and operating conditions during design, resulting in severe wear inside some of these boilers and a short operational lifespan. 2 Mechanism of wear The wear of the water wall in circulating fluidized bed boilers is caused by their operating mechanism; under the influence of the \"ring-core\" flow pattern within the furnace, the air flow at the center moves upward, while the flow along the walls moves downward. As the flow moves in a wall-attached manner, the velocity increases continuously (as shown in Figure 1), causing the water-cooled walls to wear out severely due to prolonged erosion. It is generally believed that there is a relationship between the wear rate of ash on the pipe wall and its velocity, ash concentration, and ash particle size, as given by Equation (1): where E represents the wear rate, in μm/100h ； W──gray velocity, m/s ； D──Gray particle size, mm ； U──Ash concentration, kg/(m2·s). Figure 1 Schematic diagram of the wear mechanism inside the furnace. In a circulating fluidized bed boiler, the furnace chamber is in a state of rapid fluidization; in the regions near the walls of the furnace, particle clusters flow downward along the water-cooled walls at an increasing speed, with their concentration rising exponentially. According to equation (1), the wear rate of the water wall tubes in a circulating fluidized bed boiler due to ash is proportional to the cube of the wall-adjacent flow velocity and to the wall-adjacent flow concentration. In circulating fluidized bed boilers, high wall-adjacent flow velocities and high concentrations are the main reasons for severe wear of the water wall; if low-quality coal with high ash content is used, the problem of water wall wear becomes even more pronounced. Figure 2 Schematic diagram of the main wear areas inside the furnace. The key area for anti-wear measures in a circulating fluidized bed boiler is the interface between the refractory and wear-resistant materials in the dense phase region and the smooth tube water wall in the dilute phase region. Refractory and wear-resistant materials in the dense phase zone are installed on the water wall tubes, and they do not cause wear to those tubes ； In the dilute phase region, the material concentration is low, and the flow direction of the material is consistent with that of the water wall tubes; therefore, significant wear generally does not occur. In the transition zone between the two, the particle concentration is relatively high, and the direction of the tubes does not match the flow direction of the material, which can lead to the formation of vortices that cause wear. 3. Research objects: Eight 240 t/h CFB boilers in a power plant. These boilers consist mainly of a furnace, a high-temperature insulated separator, a self-balancing “U”-shaped return valve, and a rear convection flue. They feature a single-furnace membrane-type water wall; the separation of circulating material is achieved through a high-temperature insulated cyclone separator. The superheaters are arranged in three stages, with two spray desuperheaters in between. The rear flue is equipped with three economizers as well as primary and secondary air preheaters. Figure 3 Schematic diagram of the boiler structure. Two high-temperature insulated separators are arranged between the combustion chamber and the tail convection flue in the boiler. A non-mechanical return valve is located beneath the return leg of each high-temperature insulated separator; the material return process is self-balancing. The fluidization seal air is supplied separately by a high-pressure fan. The housings of the separators and the return valve are made of steel plates, with insulating materials as well as wear-resistant and fire-resistant materials lining them. For these 8 boilers, the heating surface wear is severe, and their continuous operation time is generally only 3 to 4 months, which is below the national average. The application of the anti-wear partition technology aims to reduce boiler wear; therefore, the power plant first adopted this technology to renovate a demonstration boiler. Five anti-wear partitions were installed around the furnace at various heights, at an appropriate upward angle (as shown in Figure 4). These partitions are welded to the fins located between the tubes of the membrane water wall, with a certain expansion gap left between them and the water wall tubes. Based on the technical design principles of anti-wear baffles, the power plant aims to use this technology to gradually reduce the flow rate and concentration of ash adhering to the wall surfaces, thereby extending the service life of the water wall. After the adoption of the anti-wear partition technology, the operating time between successive startups of the demonstration boiler increased to 5 months; however, it declined thereafter, with a record period of downtime of less than 60 days at one point. The number of water-cooling tube replacements required during each shutdown for maintenance also increased significantly, severely affecting production operations on site. Observations made inside the furnace during the shutdown period revealed irregular deformation as well as loosening and detachment of the anti-wear partitions. In the areas where these partitions had detached, wear was significantly greater compared to other areas. The thickness measurement records show that after 90 days of operation at the location where the anti-wear partition had fallen off, the local wear amount had already exceeded 1.5 mm; moreover, wear marks were also found about 1 meter above the fifth anti-wear partition, indicating that the wear was moving upward. Figure 4 shows the structure diagram of the anti-wear partition. The addition of this anti-wear partition caused certain changes in the boiler’s operating parameters; by comparing the historical data before and after the modification, it was found that the bed temperature of the boiler increased by 15°C. Obviously, due to unreasonable structure and design, the flow rate and concentration of the ash adhering to the wall decreased excessively, which reduced the heat transfer coefficient and ultimately affected the boiler’s load-carrying capacity. Additionally, because the anti-wear protection period was short, the anti-wear partitions failed to achieve the desired effect, resulting in losses that outweighed any benefits. The application effect of the anti-wear beam technology differs from that of anti-wear baffles. The anti-wear beam technology consists of claws and fire-resistant, wear-resistant plastic materials to form anti-wear protrusions (anti-wear beams); as the fluid flows along the wall, these protrusions slow down its velocity and concentration, thereby reducing wear. Since the main component of the anti-wear beam is a fire-resistant and wear-resistant plastic, and the entire beam is formed in one piece, its structure is strong and reliable. Moreover, since the welding work is mainly concentrated on the pins, construction is easier. To ensure project quality and anti-wear performance, the design and construction were entrusted to the Clean Energy Technology Research Institute of China Huaneng Group. Special attention was paid to determining the installation height and structural dimensions of the anti-wear beams, in order to minimize the impact on the boiler’s heat transfer while still ensuring effective anti-wear protection. Figure 5 Schematic diagram of the anti-wear beam structure. When applying the anti-wear beam technology in boilers, the existing anti-wear partitions were removed simultaneously. In terms of actual application, the continuous operation time of the 8 240 t/h circulating fluidized bed boilers equipped with this technology increased to 8–10 months on average, and the number of water wall elements that needed to be replaced during shutdown for maintenance decreased from 60–80 to less than 10. By comparing the operating data before and after the modification, the bed temperature increased by 5°C under the same operating conditions, and the boiler’s operation was not affected. Figure 6: Distribution diagram of the anti-wear beams inside the furnace. 6 Analysis and conclusions: 1) In terms of improving the flow characteristics at the side walls, the principles behind anti-wear beam technology and anti-wear partition technology are similar; however, due to their durability, anti-wear beams provide a better protective effect than anti-wear partitions ； 2) The anti-wear partitions will limit the operating capacity of the boiler; after detailed calculations, the arrangement of the anti-wear beams has no impact on the boiler’s operation, ensuring stable boiler performance ； 3) After adopting the anti-wear beam technology, the continuous operation cycle of 240t/h circulating fluidized bed boilers can be extended from 3–4 months to 8–10 months, while the workload for each shutdown for maintenance is significantly reduced ； 4) The anti-wear beam technology offers excellent protective effects and should be the preferred technique for anti-wear treatment in circulating fluidized bed boilers.