WEAR ANALYSIS AND REFRACTORY MATERIAL SELECTION OF CASTABLES AT EACH POSITION OF CIRCULATING FLUIDIZED BED BOILER

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 put into the furnace to remove 90% of SO2, and the emission concentration of NOx is low, only 1/4 of that of pulverized coal furnace. Boiler ash has good activity and can be used as building filling material. Among them, thermal insulation refractory materials have become an important part of boilers due to their effects of fire resistance, wear resistance and thermal insulation. Due to the different functions of each position of the circulating fluidized bed boiler, the thermal insulation and refractory materials required by each position of the boiler are also different.refractory castables.
Boiler structure
The design fuel is bituminous coal, which can be mixed with a certain proportion of gas. The amount of gas fuel mixed with the boiler is 30% of the total fuel amount (calculated by heat). The boiler is suitable for semi-open air arrangement. The boiler adopts a combination of front hanging and rear support, and the elevation of the boiler operation floor is 7m. The boiler adopts simple single boiler, natural circulation, dispersed descending pipe, balanced ventilation, adiabatic cyclone gas-solid separator, circulating fluidized bed combustion method, and convection heating surface is arranged in the flue of the convection shaft.
Reasons for the destruction of boiler refractories
The damage of the refractory material is divided into two situations: ① the wear of the refractory material; ② the damage of the refractory material.
2.1 Wear of refractory materials
The working conditions in the furnace that cause the wear of the fire-attached materials are: ① the working temperature in the boiler furnace is 900~1050 ℃; ② the redox atmosphere in the furnace; The device is 20~30m/s.
Flue gas erosion and impact refer to the wear caused by the impact of fluid or solid particles on the material surface at a certain speed and angle. Erosion wear: The impact angle of the particles with the solid surface is small and close to parallel. Under the combined force of the vertical velocity and the tangential velocity, the particles form a planing effect on the solid surface, thereby gradually destroying the refractory material. Impact wear: The impact angle is large, close to vertical. The particles hit the solid surface at a certain speed, causing them to crack and deform. Long-term impact damages the solid surface and the deformed layer falls off.
2.2 Destruction (peeling) of refractory materials
The damage of refractory materials is as follows: ① thermal spalling; ② structural spalling; ③ mechanical stress spalling. Thermal spalling is due to the rapid temperature change and uneven heating during the start and stop of the boiler, resulting in a temperature difference inside the refractory material, resulting in stress, resulting in cracking and peeling of the refractory material. Structural spalling is the change of material composition (qualitative change) during the long-term use of the boiler, and the surface material peels off. The reason for the spalling of the refractory due to mechanical stress is the difference in the thermal expansion coefficients of the refractory and the metal structures (temperature measuring, load-measuring elements, refractory gripping pins, etc.) passing through the refractory.
The working conditions of each position of the boiler
3.1 Ignition air duct
The company’s 75t/h boiler is ignited under the bed. When the under-bed ignition method is used, this part heats up quickly, and the maximum temperature during ignition can reach 1200~1400 ℃. The temperature changes quickly, and it has high thermal shock resistance and stability, and is not easy to fall off. Due to the small number of particles and low requirements for wear resistance, high temperature resistant castables can be selected.
3.2 Fluidized bed surface
The working temperature of the bed surface of the fluidized bed is between 800 and 1100 °C, and the wear-resistant refractory material is laid between the air caps of the fluidized bed.
3.3 Boiler furnace
The working temperature of the circulating fluidized bed boiler furnace is between 900 and 1000 °C, and the dense and semi-dense layer materials and ash concentrations are very large. The requirements for the refractory material of the four walls of the chemical bed are very high, which must have both high refractoriness and adhesion, and high wear resistance. Its structural type adopts the method of coating fire-resistant plastics on the welding pins on the water-cooled wall pipes.
3.4 Furnace top wear layer
At the turning point of the flue gas on the top of the furnace, the working temperature is 850~1100℃. The refractory and anti-wear layer of the furnace top adopts the following types, namely, refractory castable pouring or special-shaped brick laying or refractory wear-resistant plastic (add pins when membrane wall is used)
3.5 Cyclone separator
Cyclone separators of straight and conical circulating fluidized bed boilers are designed to separate carbon particles and ash particles in flue gas. There is the possibility of secondary combustion, and the lining material must have high wear resistance. High alumina or corundum castables are mostly used.
3.6 Riser and return feeder
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 technology to ensure the construction quality.
3.7 Tail flue
The temperature of the tail flue is low, and the wear of the lining is small, so it can be completely built with ordinary refractory bricks.
Material selection for each part
The main refractory materials used in the 75t/h CFB boiler are: corundum wear-resistant plastic, diatomite thermal insulation castable, high-strength wear-resistant castable, phosphate refractory concrete, lightweight thermal insulation brick, diatomite thermal insulation brick , refractory bricks, etc.
Among them, the wear-resistant layer at the lower part of the furnace and the furnace outlet are mainly made of corundum wear-resistant plastic; the inlet flue and target area of ​​the separator are made of high-strength wear-resistant castables; Strength and wear-resistant castables; phosphate refractory concrete is used for air distribution device, ignition air duct and furnace door; diatomite insulation bricks and refractory bricks are mainly used for each heating surface at the tail
Pay attention to these issues 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 cleaned. Mix until the material is uniform;
③ When stirring, 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 mixing evenly;
④ The castable is reinforced with a large number of pins and the metal surface of the boiler. The pin is a metal material, and the thermal expansion coefficient is much larger than that of the refractory material, so the pin needs to be preheated before installation;
⑤ The pouring surface of all molds should be coated with a layer of oil;
⑥ Each batch of materials must be poured within 10-30 minutes after stirring, and should be poured to the specified thickness at one time, and vibrated until it is completely sealed;
⑦ Release the mold 24 hours after pouring, and the total curing time is 3 days.
concluding remarks
More than 50% of the problems of CFB boilers are caused by castables. The selection and construction of castables have a decisive impact on the service life of CFB boilers. Only by fully understanding the working principle, wear characteristics and characteristics of various castables of CFB boilers can we ensure the construction quality and increase the service life of CFB boilers.