What are the requirements of CFB boiler for coal particle size?
The coal particles are too coarse, which will inevitably lead to the small circulating flow rate of the particles in the fluidized bed boiler, and the evaporation capacity can not reach the design value. The lower part of the combustion chamber has high temperature and the upper part of the boiler is lower. In order to solve this problem, as one of the means of operation, often used to increase air flow operation, so that the larger particles can be brought to the upper part of the combustion chamber combustion, improve the upper part of the combustion chamber temperature, reduce the lower part of the combustion chamber temperature, slagging, improve coal particle combustion effect, improve evaporation. The wear rate of the heating surface and anti wear refractory lining is proportional to the 3 side of the air velocity, and the result of large air flow accelerates the wear of the boiler.
The major heat loss in boiler combustion is solid incomplete combustion loss q4. The amount of carbon containing less than 2% of CFB circulating fluidized bed boiler general bed bottom, lower than that of pulverized coal combustion boiler. However, the carbon content of fly ash is higher than 10%, higher than that of pulverized coal furnace, especially for the fine particles in coal. When the calorific value of coal is higher and the volatile content is lower, the carbon content of fly ash is up to 20%~30%. Seriously affect the boiler combustion efficiency.
The average particle size of coal is too large. At the designed fluidization rate, the fine particles are blown out of the dense phase bed, and a large amount of coarse particles are burned in the dense phase bed (the increase of the combustion fraction), which releases a large amount of heat. Because the arrangement of the heating surface at the lower part of the combustion chamber is definite, the excessive heat can not be absorbed, and the lower part of the bed is increased. The result is that the coal can not be added on the one hand, and on the other hand, it is easy to produce slag at high temperature.In actual operation, in order to control the combustion conditions, with the load operation, the operators can control the bed temperature by increasing the primary air volume or reducing the amount of coal supply, so as to ensure the safe and stable operation of the boiler. This brings two results: first, reduce the amount of coal into the boiler, it will inevitably reduce the boiler load, which can reduce the operating pressure of the auxiliary system equipment. Two, add a wind to suppress bed temperature rise, meet the normal fluidization of large particles, at the same time can increase the amount of coal with large load, which led to a large number of fine particles blown to the tail, as fly ash carbon content increased.
Circulating fluidized bed boiler in order to call its fluidized state, the coal particles in the furnace has strict requirements, generally required into the furnace powder particle size shall not exceed 13mm. And the proportion of coal particle size in each range should meet the requirements of boiler design