Thread Content
Improvement of Boiler Wall Structural Design 1. Introduction Since my country introduced units above 300MW (including 300MW) in the 1980s, it has become the main unit for thermal power generation in my country. Through the joint efforts and digestion and absorption of power plants, design institutes, manufacturing plants, installation companies and other units, the overall unit has reached localization and serialization. Insulation furnace wall materials with low thermal conductivity, light bulk density, good earthquake resistance and no pollution (or low pollution) are also widely used in boiler furnace wall insulation. Continuously improve and perfect the structure. This article will describe these two aspects in conjunction with the company's boiler wall materials and structures. 2. Comparison of the boiler wall structure before and after improvement. Our company adopted the furnace wall structure and material selection as shown in Figure 1 in the 1980s and 1990s. Figure 2 shows our company’s newly improved furnace wall structure and material selection. 1. Drawbacks of the furnace wall structure in Figure 1: a. It can be seen from the structure that the aluminum silicate refractory fiber felt used for the inner layer cannot be closely attached to the water-cooled wall or wall-clad tube, thus forming an internal air corridor. In order to eliminate the internal air corridor, a 100mm high and 50mm thick annular wall sealing belt is cast with lightweight insulation castable on the upper part of the rigid beam (as shown in Figure 1). However, with the long-term operation of the boiler, boiler vibration, different linear expansion coefficients of the two materials, and poor and difficult-to-control construction conditions, after many years of operation, the lightweight insulation castable will crack until the whole piece falls off, eventually causing the inner layer to form an entire hot air corridor (channel) that runs through (from the furnace bottom to the furnace top), making the thermal insulation effect * * Decrease, the furnace top temperature increases, and the overall thermal efficiency of the boiler decreases. b. The main thermal insulation material is foam asbestos board (or rock wool) with heavy bulk density, poor thermal shock performance, unstable thermal conductivity and pollution caused by construction and operation. After the boiler has been operated for a long time (3 to 4 years), the fibers of the insulation material age and break, forming a cavity on the top of the boiler, thus affecting the insulation effect. This kind of material is basically no longer used in large boilers (unless a very small number of individual power plants are forced to overuse it due to other reasons). c. The outer layer of thermal insulation has exposed wire mesh and fixed plates on the fixed insulation (furnace wall). The heat flow is transferred to the wire mesh and other places through the fixed steel (insulation pin) plate for external heat transfer. The temperature of the outer surface of the boiler insulation increases and the insulation effect is reduced. In short, after the boiler has been running for a period of time (about 2 to 3 years), the thermal insulation effect of the boiler body in the original structural form can no longer meet the requirements of the boiler for its thermal insulation workpiece. 2. The structure of Figure 2 makes up for the shortcomings of the structure of Figure 1. a. The inner layer is made of conductive hard ceramic fiber board, and its outer surface is engaged with the surface of the heating surface tube. This material is semi-rigid and can be closely combined with the shape of the outer surface of the heating surface for a long time. Even if the boiler operates for a long time, it will not form an internal air corridor (channel). To ensure the thermal insulation effect of the boiler, this material is easy to install, and the insulation pins can be easily penetrated and fixed to the heating surface tube. Physical and chemical indicators of this material: Al2D3SiO2Fe2O3 Bulk weight Slag ball content Elastic shrinkage rate 1000℃ Thermal conductivity coefficient 1000℃ 50～54% 40～48%＜1%200Kg/m3＜5%＞80%＞4%＜ 0.156w/m·kb. The main thermal insulation materials should be those with light bulk density, good seismic resistance and low pollution (or no pollution) during construction and operation, such as composite silicate boards or high-temperature centrifugal glass plates (cotton). c. Add 20mm thick high-temperature glass wool (felt) with heat-reflective film to the outer layer of the wire mesh that fixes the insulation wall. Use Ф4 round steel clips to insert into the main insulation body with a pitch of 500mm × 500mm to fix this material. Block and reflect the heat from the wire mesh, making the outer surface temperature of the boiler furnace wall * * reduce. 3. The thermal insulation calculation structure is shown in Figure 3. The calculation summary is as follows: Temperature between fins (°C) 420 400 380 360 80mm layer away from fins 349 333 316 300 200mm layer away from fins 108 105 100 95 225mm layer away from fins 40 39 38 36 Note: 1. The calculation assumes environmental conditions, the ambient temperature is 25°C, the wind speed is zero, and the calculated temperature is at the protective plate of the outer insulation layer of the insulation structure. 2. Different insulation layer surfaces have different heat exchange capabilities with the surrounding environment. The above table shows the surface temperature value of a new bright galvanized sheet as the insulation layer. When the pot has been running for a period of time and the galvanized surface loses its luster, the surface temperature can drop by 2 to 3°C compared with the original value. Through structural improvement and the selection of boiler furnace wall insulation for each insulation layer, the requirements of the boiler technical conditions for coal-fired power station boilers (DS268-88) are met. That is, under normal operating conditions of the boiler equipment, when the ambient temperature is 25°C, the surface temperature of the furnace wall of the indoor boiler shall not exceed 50°C, and the outdoor boiler shall not exceed 45°C, and the heat dissipation generally shall not exceed 290 watts/meter2 (w/m2). 4. Conclusion 1. Since the temperature of the fin part of the furnace wall is generally about 40 to 60°C higher than the surface temperature of the tube wall, only by insulating the fin part can the accuracy of the insulation design calculation be achieved. 2. Since there is still a large amount of heat in the furnace wall part that is transmitted to the wire mesh through the insulation pins to achieve good results, the complete insulation effect must eliminate the heat transferred in this part. High-temperature glass wool (felt) with a reflective film is used outside the wire mesh for external insulation to eliminate the above phenomenon. 3. The main insulation layer uses thermal insulation materials with low thermal conductivity, good thermal shock performance, good waterproof performance, easy installation and guaranteed product quality when used in large areas, so that the boiler wall can work normally for a long time.