How to handle a circulating fluidized bed boiler after it shuts down?

1. If the coal feeder fails, shut down the furnace immediately and address the issue as soon as possible to restart ignition. 2. If the return material is abnormal, discharge ash urgently and increase the ash discharge rate; coal feeding must be prohibited. 3. Strictly control the steam temperature, turn off the desuperheating water, and activate the superheater drain valve. 4. If the fuel is natural gas, the air in the furnace needs to be replaced.

After the engine stops, the cause should be identified immediately: 1. If it is due to problems with the conveying or feeding system, these issues must be resolved right away. Once resolved, it is necessary to determine whether to restart the engine by igniting it directly or by using oil for ignition, depending on the bed temperature (if it is above 650 degrees, the engine can be started directly)； Otherwise, immediately shut down the fire and take measures to maintain heat and pressure; once everything is resolved, start up based on the bed temperature. 2. In the case of a return material issue (collapse of the return material device), the circulating ash should be discharged immediately, and the air flow and material ratios should be adjusted until normal operation is restored. 3. In the event of a power outage for plant use, the fire must be extinguished immediately, and measures should be taken to maintain heat and pressure. Subsequently, it depends on when power is restored to determine whether it is possible to start the boiler drum water level control system.

Fluidized bed combustion is a combustion method that lies between stratified 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; instead, 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 silos, preventing it from being discharged. Even if 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 blockages. 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 control of material return during operation led to a fire suppression incident. In small and medium-capacity circulating fluidized bed boilers, improper feeding of return material can also extinguish the flame in the combustion chamber, resulting in a shutdown. Some operators like 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, thereby 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 supplied 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, eventually leading to high-temperature slag formation and forcing a 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 lift the bed material, leading to a situation where combustion is suppressed. 7. Retreating the oil gun during ignition had already caused the engine to stop running. 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 supply is gradually increased. 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 re-introduce the oil injectors to assist with combustion. Improper operation of the oil removal gun is most likely to cause engine shutdown accidents.

After the circulating fluidized bed boiler shuts down, the following measures should be taken immediately: (1) Stop all coal feeders immediately and switch all automatic controls to manual mode. (2) Start the oil guns above and below the bed to rapidly increase the bed temperature, thereby facilitating combustion. (3) Appropriately reduce the air volume according to the situation, adjust the amount of circulating ash, and discharge some ash if necessary. (4) Reduce the water supply volume, control the drum water level at -30 mm, and closely monitor the water level. (5) Based on the downward trend in steam temperature, reduce or shut off the desuperheating water, and open the drain valve and vent steam to the atmosphere in order to cool the superheater. (6) If there is a fault in the coal feeding system, it should be repaired as soon as possible; if the system is blocked, it should be cleared promptly. (7) If the bed temperature meets the conditions for hot start, a hot start should be carried out immediately; once the steam temperature and pressure reach the rated values, the turbine can be restarted. (8) If the bed temperature does not meet the conditions for hot start, purging must be carried out followed by operation according to the cold start procedure. It is strictly prohibited to feed coal blindly, as this may lead to secondary combustion or furnace explosions, resulting in serious damage to the equipment. After the boiler shuts down, operators should promptly determine the cause; it is strictly prohibited to rely on luck or attempt to maintain the unit’s load by increasing the coal supply in order to raise the bed temperature. The above handling steps must be strictly followed to ensure the safe operation of the equipment.

If the eternal flame remains lit, it can be used to ignite the main burner and keep production going! If the permanent light goes out, the fuel gas valve must be shut off immediately; the dispatch team or the on-duty supervisor should be informed, the furnace should be purged, the cause identified, and ignition restarted! If the cause cannot be found, it is recommended to implement an emergency shutdown!