Accident Handling 43: What are the causes of flameout in circulating fluidized bed boilers?

Coal supply interruption is the main cause of combustion shutdown in circulating fluidized bed boilers

Coal supply interruption is the main cause of combustion shutdown in circulating fluidized bed boilers

(1) A fault in the 6kV or 400V plant power supply system causes the auxiliary equipment to lose power and stop operating. (2) The coal feeder failed and stopped operating, but the operators failed to detect this in time; as a result, there was a prolonged lack of coal supply, causing the bed temperature to drop below 650°C, without any burners either above or below the bed being activated to assist with combustion. (3) The return of material by the return conveyor is abnormal; when a large amount of bed material accumulates on top of the bed, the coal that is burning loses oxygen suddenly and goes out. (4) When the boiler is operating at low load, improper operation adjustments cause the bed temperature to drop to the point where the MFT mechanism activates. (5) Changes in coal quality (excessively high ash content, or reduced volatile matter and calorific value), for which the operators failed to make timely adjustments. (6) Failure of important plant auxiliary equipment leads to tripping, triggering the MFT protection. (7) Shutdown due to faults in the turbine and electrical systems; major interlock protection activates, and the boiler’s MFT activates. (8) A tube burst on the heating surface inside the furnace, resulting in a large amount of water and steam leaking out and causing a sharp drop in bed temperature. (9) The boiler’s MFT operates when other conditions for the MFT action are met.

①A failure in the coal feeder or an accumulation of coal that was not detected in time resulted in an excessively long period with no coal supply. ②The secondary return material is abnormal; there is excessive ash accumulation, which suddenly flows into the furnace. ③Poor coal quality or changes in coal quality, combined with untimely adjustments, lead to poor combustion and engine shutdown. ④The ratio of wind coal is improper.

1. The coal feeding system failure cannot be repaired. 2. The return feeder suddenly collapsed and spilled ash. 3. The fuel’s calorific value is too low or its moisture content is too high. 4. Leakage in the furnace’s water wall caused a sharp drop in the bed temperature in a certain area. 5. The protection action causes the boiler’s MFT to activate, thereby shutting off the main fuel supply.

Combustion shutdown in circulating fluidized bed boilers and its prevention: The shutdown of boilers is directly related to the safety and efficiency of power plants, and it requires great attention from every employee in such plants. After more than a decade of study and practice, the author has gained some insights, which are briefly summarized as follows: Fluidized bed combustion is a combustion method that lies between layered combustion and pulverized coal suspension combustion. Its risk of stalling is also somewhere in between the two. 1. Coal supply interruption is the main cause of combustion shutdown in circulating fluidized bed boilers. The shutdown of fluidized bed combustion is caused by a coal supply interruption. During fluidized bed combustion, there is a large amount of hot bed material in the bed; the bed temperature is typically between 850 and 1000°C. Over 95% of the bed material consists of hot ash, while about 5% is combustible material, mainly coke. The new fuel added to the combustion chamber per minute accounts for only about 1% of the bed material. A large amount of the hot bed material is an inert substance – ash – which does not compete with the newly added fuel for oxygen; rather, it provides abundant heat for heating and igniting the new fuel. Therefore, during the circulating fluidized bed combustion process, the ignition and combustion conditions for the newly added fuel are the best. When coal supply is about to be interrupted during combustion, 5% of the combustible material in the bed can sustain combustion for 3–5 minutes. Therefore, as long as a circulating fluidized bed can maintain continuous coal feeding and adjust the coal supply appropriately according to changes in load, it generally will not go out. 2. The main causes of coal supply disruptions and preventive measures. The main reason for coal supply disruptions is that the moisture content in the coal exceeds 8%, causing the coal to form bridges and blockages within the coal bin, preventing it from being discharged. Even if the operators detect this issue promptly, it may not be possible to resolve it in a timely manner. Burning coal with high moisture content leads to frequent coal clogging. Prevention of coal supply interruptions: Design larger dry coal storage areas to keep the moisture level below 8%; enhance monitoring of coal supply – installing alarms or audio alerts for coal supply interruptions is an effective measure to prevent such issues. 3. Preventing high-temperature slagging and low-temperature flameout: When the boiler load increases, it is necessary to increase the supply of air and coal ； Conversely, when the boiler load decreases, the air and coal supply need to be reduced. If the operator fails to do this, an increase in load will cause the temperature in the combustion chamber to keep dropping, eventually leading to the combustion chamber going out. When the load decreases, the temperature during combustion continues to rise, eventually leading to high-temperature slag formation and shutdown of the furnace. 4. Improper operation control of the material return led to a fire suppression incident. In small and medium-sized circulating fluidized bed boilers, improper feedback of material can also extinguish the flame in the combustion chamber, resulting in a shutdown. Some operators prefer to introduce return material after the boiler has been running for a while, but they fail to control the temperature of this return material; as a result, a large amount of it enters the combustion chamber and extinguishes the fire there. After the boiler is successfully ignited, when feeding back the material, release some of the ash from the backfed material before feeding it in. Since the return feeder is formed by a vulcanized seal, once the return air is turned on, a large amount of material flows into the combustion chamber, extinguishing the fire bed. 5. When the calorific value of coal changes significantly, adjust the coal supply to prevent shutdown due to low temperatures and slag formation at high temperatures. Generally, of course, when the calorific value of coal decreases, it is necessary to increase the coal supply. Of course, when the calorific value of coal increases, the amount of coal fed should be reduced. If adjustments are not made in a timely manner, as the calorific value of coal decreases, the temperature in the combustion chamber will keep dropping, eventually leading to flameout. As the calorific value of coal increases, the temperature in the combustion chamber rises as well, eventually leading to high-temperature slag formation and forcing the shutdown of the furnace. 6. Uncontrolled discharge of bed bottom slag caused the fluidized bed to shut down. Another cause of combustion shutdown during operation is the loss of control over the amount of material at the bottom of the discharge bed. A certain amount of bed material and a certain combustion temperature correspond to a certain boiler load. If the discharge of bottom slag gets out of control, and too much of the bed material is removed, resulting in too little bed material and a too thin layer of it, it becomes impossible to maintain a stable combustion chamber temperature, which can lead to the extinguishment of the fire in the combustion chamber. Conversely, if the slag cannot be removed smoothly, the amount of bed material increases and the layer grows taller; as a result, the pressure generated by the primary fan is insufficient to blow up the bed material, leading to a situation where combustion is suppressed. 7. Pulling the fuel gun out during ignition had already caused the engine to stall. When using a pre-heating chamber under the bed and an oil gun for ignition, once the temperature in the combustion chamber reaches 850–900°, the oil gun is gradually removed while the coal feed rate is increased gradually. Once it is confirmed that the coal being added has caught fire and the burning temperature is on the rise, withdraw the last oil gun. When the oil gun is withdrawn, the temperature of the fluidizing medium drops to a level slightly higher than the ambient temperature, which has a significant impact on combustion. If it is found that the combustion temperature drops rapidly and there is a risk of flameout after all the oil injectors have been removed, quickly reintroduce the oil injectors to assist with combustion. Improper operation of the oil removal gun is most likely to cause engine shutdown accidents.

The main cause of combustion shutdown in circulating fluidized bed boilers is water injection caused by tube bursts in the furnace.

It might be that the fluidized bed layer is too thin!