Accident Handling 10: What are the main causes of coking in the bed material of circulating fluidized bed boilers?

1. The impact of fuel. If the ash melting point of coal is low, when the coal particles melt into a liquid state or soften at the high temperatures in the furnace, they stick together. Moreover, the heat generated by their own combustion cannot be dissipated in time, resulting in coking. Secondly, an excessive amount of coal is supplied during operation, resulting in a high coal content in the bed; this raises the temperature of the bed and makes the combustion atmosphere more reducing. As a result, the ash particles in the coal easily reach a molten or softened state, leading to coking. Furthermore, excessive variations in coal type, an inappropriate selection of the fuel preparation system, and large coal particle sizes with a high proportion of coarse particles can also severely affect the fluidization of the bed, leading to overheating in the dense phase region and coking. 2. Influence of operating parameters: If the amount of primary air during operation is too low, or if it is reduced below the level necessary for fluidization, this will result in poor fluidization of the material layer, leading to localized overheating. Once coking occurs in certain areas, it will cause the surrounding particles to stick together, thereby expanding the area affected by coking. This situation mainly occurs during startup, because the material layer is too low at that time and the air flow rate is low, preventing the entire material layer from reaching a uniform state of good fluidization. Furthermore, the bed pressure difference is a parameter that reflects the thickness of the fuel bed in the combustion chamber. During boiler operation, the thickness of this bed directly affects the quality of fluidization; if the bed is too thick, it may lead to poor fluidization, resulting in coking in the furnace or even flameout. 3. Effects of return material: If the wind flow for returning material is too low, or if the return material device becomes blocked due to the collapse of refractory materials, or if there is an uncontrolled leakage of recycled ash caused by high differential pressure in the material layer, the return material cannot be properly sent back into the furnace, which can lead to excessively high bed temperatures and coking. If coal is added at this time to maintain pressure and steam temperature, the bed temperature will rise sharply due to the combined effect of the return material not returning to the furnace and the addition of coal, resulting in coking on the bed. If the return material temperature is too high during operation, coking may occur inside the return material device. 4. Structural impacts: Poor design of the air distribution plate, unreasonable placement of the air nozzles, or damaged nozzles can lead to uneven air distribution, resulting in certain layers of material not being fluidized and thus coking forming. Furthermore, the return valve is poorly designed; the return air may cause combustion of combustible materials within the valve body, thereby increasing the return temperature and leading to coking inside the returner. 5. Issues with operating personnel

Too little bed material was added during ignition start-up, and too much oil was supplied when the oil injection gun was used. During the pressure increase process, if coal is added too quickly or in excessive amounts, or if coal is added without adding air, a large number of unburned coal particles accumulate together and suddenly explode. Improper operation during fire suppression led to the shutdown of all fans before the oxygen level had started to rise, and before the volatiles in the fuel in the furnace bed had been completely burned out. The primary air flow rate is too low, below the critical fluidization air flow rate ； The combustion load is too high, resulting in excessively high combustion temperatures ； The coal particles are too large or the deformation temperature of the ash is low ； Excessive slag discharge, improper handling procedures ； Abnormal material return or blockage in the return conveyor ； Coal supply to the burner is interrupted due to improper handling ； The load was increased too quickly, and the operation was improper ； The air cap is damaged or deformed, allowing ash to fall into the air chamber and resulting in uneven air distribution ； The bed temperature gauge is inaccurate or not functioning properly, leading to misjudgments by the operators ； The bed layer is too thick, and slag is not removed in a timely manner ； The magnet separator malfunctions, allowing clumps of iron or insulation material to enter the furnace and disrupt the normal fluidization state. The ash fusion point of coal types is too low, while the combustion temperature is higher than the ash fusion point.