The main purpose of using wear-resistant refractory materials as lining for circulating fluidized bed boilers is to protect the steel shell of the furnace body from being eroded and worn by the materials being burned.
The low-quality coal burned in circulating fluidized bed boilers has a high ash content and large fly ash particle size. The separator separates the coal ash and circulates it for combustion, which makes the fly ash concentration in the flue gas very high. This leads to the collision heat transfer and serious erosion and wear problems of fly ash particles on the heating surface and lining in the furnace.
The circulating combustion path of the materials burned in the circulating fluidized bed boiler: coal powder (generally with a particle size of 0~10mm) and limestone powder are fed into the fluidized bed combustion chamber (dense phase zone) through the feeder, and the fuel boils and burns in this zone. The flue gas rises, passes through the dilute phase zone to the separator inlet and enters the cyclone separator, and the coarse particles sink and return to the combustion chamber through the return valve to burn again. The flue gas carries the fly ash and rises through the convection flue into the external heat exchanger. Refractory materials are mainly lined in the combustion chamber, ceiling, separator inlet, high-temperature cyclone separator and return valve.
Ⅰ. Working conditions of refractory materials in various parts
Taking a 220t circulating fluidized bed boiler as an example, the working conditions of refractory materials in various parts are as follows:
1. Dense phase zone
The depth of this zone is 220~1530mm and the length is 2100mm. Sandy media (coal powder and limestone powder) boil here. The pressure in this zone is 13818~20580pa, the flue gas velocity is about 1.2~1.8m/s, and the normal working temperature is 820~900℃.
2. Dilute phase zone
The area above the secondary air outlet is where the fine particles of fuel combustion are concentrated. The solid material concentration here is about 12~16kg/m3, the flue gas velocity is 1.2~3m/s, and the normal working temperature is 930-980℃.
3. Furnace top zone
The normal working temperature of the furnace top zone is 850~1100℃, and the flue gas velocity increases by 18m/s at the separator inlet.
4. Cyclone separator
The normal working temperature of this part is 850~930°C, and the flue gas velocity at the cyclone outlet can reach 29m/s.
5. Cyclone separator outlet main pipe
The smoke dust content here is 70~530g/m3, the normal working temperature is 850~930°C, and the maximum smoke flow rate is 18m/s.
Ⅱ. Factors determining mechanical wear
1. Flue gas velocity
Flue gas velocity is the most important factor affecting the wear of the furnace inner wall. Studies have shown that the amount of wear is proportional to the cubic power of the flue gas velocity. The size of the flue gas velocity directly affects the kinetic energy of the flowing fly ash and the amount of ash particles that impact the furnace inner wall per unit time.
2. Fly ash concentration
In CFB boilers, when the fly ash circulation rate is high, the combustion efficiency can be improved and the heat transfer effect can be enhanced. However, the high or low circulation rate also determines the concentration of solid particles in the flue gas in the furnace. Therefore, a higher circulation rate will cause the ash-containing flue gas flow to seriously wear the furnace inner wall. If the coal quality deteriorates, the ash content increases, and the amount of coal burned also increases, the fly ash concentration in the flue gas will increase sharply, which will increase the wear of the separator lining.
3. Fly ash impact probability coefficient
This is related to the particle characteristics of fly ash. The larger the particle, the greater the possibility of impact.
4. Ash particle wear characteristics
Ash particle wear characteristics refer to the influence of ash hardness, temperature, shape and particle size. If there are many hard substances in the ash, and the ash particles are coarse and angular, the wear characteristics of the ash particles will be enhanced.
5. Uneven distribution characteristics of fly ash concentration and flow rate
6. Materials of heating surface and lining
Under the same conditions, the better the wear resistance of the heating surface and lining materials, the smaller the wear amount; vice versa. According to the design requirements, the average working time of the boiler is 8600h, and the furnace is started and stopped 2 to 10 times a year. Each ignition takes 12 to 24 hours, and after ignition, the temperature is raised to 800°C at a rate of 65°C/h and kept warm for 8 hours. The service life of the furnace lining is required to be at least 2 years.
Ⅲ. Summary of the work and damage of lining refractory materials
1. Temperature
The working temperature of each part of the lining of the medium-temperature circulating fluidized bed boiler is between 800 and 1000℃. From the perspective of refractory materials, this belongs to the medium-temperature range. When selecting refractory materials, the medium-temperature physical and chemical properties of the materials should be considered first. For fired products, the sintering temperature is generally higher than this temperature range, so its physical and chemical properties at high temperatures can be directly used as the basis for selecting refractory materials. For amorphous refractory materials and unfired products, the medium-temperature physical and chemical properties must be examined. Organically bonded materials or hydration-bonded hydraulic materials often have the worst medium-temperature strength. These materials produce ceramic bonding at high temperatures, and the strength is greatly improved. Therefore, their physical and chemical indicators at high temperatures cannot be used as the basis for selecting refractory materials.
2. Thermal shock
Under normal operation, the circulating fluidized bed boiler starts and stops 2 to 10 times a year. In the early stage of commissioning, due to improper operation, the refractory lining was subjected to a strong thermal shock every year when it was started and stopped. If the refractory material has poor thermal shock resistance, the spalling of the refractory material caused by thermal shock will become a fatal factor for its damage.
3. Sand erosion wear
The flue gas velocity in the circulating fluidized bed boiler is large (up to 29m/s or more) and the solid material concentration is high. At medium temperature, high-speed flue gas carries a large amount of solid particles, which causes strong erosion and wear on the refractory lining, especially in the impact area of the cyclone, where erosion is most serious. For circulating fluidized bed boilers and cyclone separators with single-side return, the circulating flow of materials causes wear on the refractory lining.