01. Frontier overview
Circulating fluidized bed (CFB) boiler technology is a relatively mature clean combustion technology. CFB boilers have excellent fuel adaptability and can burn almost any fossil fuel. Limestone can be directly added to the furnace to remove 90% of SO2, and the emission concentration of NOx is low, only 1/4 of that of pulverized coal furnaces. Boiler ash has good activity and can be used as building filling materials. Among them, thermal insulation refractory materials have become an important part of boilers with their fire resistance, wear resistance and thermal insulation effects. Due to the different functions of various positions in the circulating fluidized bed boiler, the thermal insulation refractory materials required at various positions of the boiler are also different.
02. Causes of damage to boiler refractory materials
The damage of refractory materials can be divided into two situations: ① wear of refractory materials; ② damage of refractory materials.
The working conditions in the furnace that cause the wear of the refractory materials are: ① The working temperature in the boiler furnace is 900~1050℃; ② The redox atmosphere in the furnace; ③ Flue gas scouring and impact: The furnace is generally 3~6m/s, and the separator is 20~30m/s.
Flue gas scouring and impact refers to the wear caused by the impact of fluid or solid particles on the material surface at a certain speed and angle. Scouring wear: The impact angle between the particles and the solid surface is small and close to parallel. Under the combined force of the vertical component velocity and the tangent component velocity, the particles form a planing effect on the solid surface, thereby gradually destroying the refractory materials. Impact wear: The impact angle is large and close to vertical. The particles hit the solid surface at a certain speed to cause cracks and deformation. Long-term impact damages the solid surface and the deformed layer will fall off.
There are several types of refractory material damage: ① Thermal spalling; ② Structural spalling; ③ Mechanical stress spalling. Thermal spalling is caused by the rapid temperature change and uneven heating during the start-up and shutdown of the boiler, which causes a temperature difference inside the refractory material, thereby generating stress, causing the refractory material to crack and spall. Structural spalling is the change in material composition (qualitative change) during the long-term use of the boiler, and the surface material spalling. The reason why mechanical stress causes refractory spalling is due to the different thermal expansion coefficients of the refractory material and the metal structure (temperature and pressure measuring elements, refractory material grab nails, etc.) that passes through the refractory material.
03. Working conditions at various positions of the boiler
The company's 75t/h boiler is ignited under the bed. When the under-bed ignition method is used, the temperature rises quickly in this part. The maximum temperature during ignition can reach 1200~1400℃, the temperature changes quickly, has a high thermal shock stability, and is not easy to fall off. Due to the small number of particles and low wear resistance requirements, high-temperature resistant castables can be selected.
The working temperature of the fluidized bed surface is between 800~1100℃, and wear-resistant refractory materials are laid between the hoods of the fluidized bed.
The working temperature of the circulating fluidized bed boiler furnace is between 900~1000℃. The material and ash concentration in the dense phase layer and semi-dense phase layer is very high, and the airflow direction changes frequently during constant boiling and circulation. The requirements for the refractory materials on the four walls of the fluidized bed are very high. They must have both high fire resistance and adhesion, and must have high wear resistance. Its structural type adopts the method of welding pins on the water-cooled wall tubes and coating refractory plastic.
The working temperature at the flue gas turning point on the furnace top is 850~1100℃. The refractory and wear-resistant layer of the furnace roof adopts the following types, namely, refractory castable casting or special-shaped brick laying or refractory wear-resistant plastic (add pins when using membrane wall) straight cylinder and cone circulating fluidized bed boiler cyclone separator is designed to separate carbon particles and ash particles in flue gas. The particles in the separator have a high speed, the working conditions are bad, the working temperature is 800~950℃, and there is a possibility of secondary combustion. The lining material must have high wear resistance. High-aluminum or corundum castables are mostly used.
The working temperature is 800-950℃, the particle concentration is high, but the particle size is fine, the heat capacity of the ash is large, and the thermal shock to the lining is large. The working conditions are not bad, and high-aluminum materials are generally used. However, the construction conditions of these parts are poor, and special attention should be paid to the construction process to ensure the construction quality.
The temperature of the tail flue is low, the wear of the lining is small, and ordinary refractory bricks can be used for construction.
04. Matters to be noted when using refractory materials
①Clean water must be used, and the amount of water added is 6%~8%;
②Use a forced mixer, and all mixing tools must be clean. Mix until the material is uniform;
③When mixing, the amount of material added should not be less than the whole bag, and dry mix for 15 minutes before adding water to achieve the purpose of uniform mixing;
④The castable is reinforced with the metal surface of the boiler through a large number of pins. The pin is a metal material, and the thermal expansion coefficient is much greater than that of the refractory material, so the pin needs to be preheated before installation;
⑤The casting surface of all molds should be coated with a layer of engine oil;
⑥Each batch of materials must be poured within 10-30 minutes after mixing. It is advisable to pour to the specified thickness at one time and vibrate until it is completely sealed;
⑦Demolding 24 hours after pouring, and the total curing time is 3 days.
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