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Deflagration in circulating fluidized bed boilers and its prevention. Circulating fluidized bed boilers offer advantages such as wide fuel adaptability, high combustion efficiency, effective desulfurization, low nitrogen oxide emissions, high combustion intensity, a small furnace cross-sectional area, simple fuel preprocessing and coal feeding systems, as well as flexible load regulation, making it easy to achieve comprehensive utilization of ash and slag. Since the late 1950s, it has been widely adopted in China as an environmentally friendly and energy-saving boiler. However, the operation technology of fluidized bed boilers is a new type of applied technology in China, and it is not yet fully mature. During operation, especially during the commissioning of new boilers, some unexpected accidents are inevitable. Although boiler deflagration accidents occur infrequently, once they happen, they can be highly destructive and dangerous, even causing significant losses to the company’s operations and the lives of its employees. 1. Several situations in which deflagration occurs: Boiler deflagration takes place when the concentration of combustible substances in the furnace is within the deflagration limit; upon exposure to an open flame or when the temperature reaches the ignition point, a violent explosion occurs, with the combustion products rapidly spreading outwards into the surrounding space. The following describes several situations in which deflagration can occur in fluidized bed boilers. 1.1 Intense combustion: If too much fuel is added or the process of stopping the combustion is delayed, this results in an excessive amount of fuel remaining in the bed material after combustion has stopped. Under conditions of low oxygen, the carbon in this fuel does not burn completely, producing large amounts of CO. At the same time, the fuel heats up to high temperatures inside the furnace, causing flammable gases such as methane and hydrogen to be released. Due to the decrease in the surface temperature of the bed material after the fire is extinguished, these flammable gases fail to come into contact with an open flame and accumulate within the boiler furnace. When the fire is ignited, as the fan starts operating, the bed material begins to fluidize; the hot material rises from below, and at that point the flammable gases come into contact with the open flame, causing immediate combustion. If the concentration of the flammable substances is within the deflagration limit, a deflagration will occur. Some stokers, worried that the bed temperature might drop too quickly and cause the fire to go out when starting the fire, add a small amount of fuel before turning on the blower. The fuel that enters the furnace in this way not only releases flammable gases but also a large amount of coal dust that participates in the combustion process; this not only increases the likelihood of a detonation but also intensifies its severity. 1.2 Sudden influx of large amounts of return material leads to deflagration; circulating fluidized bed boilers all have a material circulation system. During the operation of a fluidized bed > circulating fluidized bed boiler, a large amount of solid particles circulate within a circulation loop that includes the combustion chamber, separator, and return device. The circulation ratio of such boilers is typically between 5 and 20; in other words, 5 to 20 times as much ash as the amount of coal fed enters the combustion chamber again through the return device for further combustion. The material involved in this circulation is fine ash with a diameter of around 0.1 mm, which has good fluidity and continuously enters the furnace driven by the return air. If the return air flow is too low during operation, the material inside the returner will cease to be fluidized, leading to blockage of the returner. Fine ash will accumulate within it; once this ash has accumulated to a certain amount, it will start to move due to its own weight, or the material will become fluidized again when the air flow is increased as part of operational adjustments. At that point, tons of fine ash can enter the furnace in a short period of time. Due to the large surface area of the fine ash, the return air mixes rapidly with the air and fills the furnace; moreover, fine ash typically contains about 20% carbon, which makes it highly prone to deflagration in the high-temperature environment of the furnace. 1.3 Oil and gas deflagration: Fluidized bed boilers generally use diesel for ignition, and during this process, the oil injector may stop functioning due to factors such as impurities in the oil, improper adjustment of the ignition air, or too low oil pressure. After the fire is extinguished, if the oil valve is not detected and closed in time, the atomized fuel will continue to be sprayed into the furnace, resulting in oil mist filling the entire area from the furnace to the tail flue and even up to the chimney outlet. If ignition occurs again at this time or if it comes into contact with other open flames, deflagration of the entire system will take place. At 5:14 on August 19, 2000, the 35t/h fluidized bed/circulating fluidized bed boiler at one of Pingmei Group Company’s own power plants was started up. With an oil pressure of 1.2–1.6 MPa, two ignition nozzles sprayed fuel in a misted form into the furnace; the bottom layer of material, 450–500 mm thick, began to flow and heat up. At 5:24, 10 minutes after ignition, it was observed that the temperature of the bed was starting to drop. The boiler operator checked and found that both ignition nozzles had stopped functioning; he immediately tried to restart ignition using a torch. Subsequently, an explosion occurred inside the furnace and in the combustion system, and flames erupted from the furnace door, causing injuries to people. Post-inspection revealed that a small portion of the boiler’s insulation had come loose, and there were leaks in the sealing areas and expansion joints. Due to the high positive pressure, the marble blocks in the rear flue were dislodged, resulting in a short circuit in the smoke and air system; therefore, it was necessary to shut down the boiler. Post-analysis identified the following reasons for the deflagration: (1) The boiler operator lacked a strong sense of responsibility. Failing to thoroughly test the atomization performance of the igniter before ignition, and attempting to ignite again without conducting a proper inspection after an ignition failure, were the direct causes of the explosion accident, which resulted from excessive oil fume concentrations in the furnace coming into contact with an open flame. (2) Inappropriate adjustment of the ignition air and too low oil pressure led to the interruption of fuel injection and thus extinguishment of the fire; after the fire was out, it was not detected in time, or no measures were taken to restart ignition, which resulted in a deflagration. (3) Poor atomization of the oil gun, clogged nozzle, incomplete combustion of the oil, and a large amount of oil vapor in the furnace. Therefore, this deflagration occurred as the fuel supply from the fuel gun was interrupted, causing a large amount of oil and gas to fill the combustion system; the temperature of these combustible gases reached their ignition point, leading to a violent deflagration when they came into contact with an open flame. 1.4 Re-ignition of combustibles in the flue During the operation of a circulating fluidized bed boiler, an incident of re-ignition of combustibles in the flue may occur. In such cases, the following phenomena occur: the exhaust gas temperature rises sharply, the temperatures at the outlets of the primary and secondary air also increase, and the negative pressure in the flue and combustion chamber changes drastically, even turning into positive pressure ； Black smoke emerges from the chimney; smoke or sparks may also leak out from areas where the exhaust fan housing is not airtight. The main reasons for this problem are improper combustion adjustment and unreasonable air supply, which result in combustible materials entering the flue ； Excessive negative pressure in the furnace draws unburned combustible materials into the flue ； Dust blockage in the return material device reduces the efficiency of the separator, causing unburned particles to fill and enter the flue. 2 Prevention of boiler deflagration: In view of the above common causes of deflagration, the following measures should be taken during the operation of fluidized bed > circulating fluidized bed boilers to prevent deflagration. (1) When lighting the fire, it is necessary to first start the exhaust fan to ventilate for 5 minutes before turning on the supply fan, in order to ensure that any flammable gases accumulated inside the furnace are expelled and to prevent contact with an open flame. (2) When shutting down the boiler, coal feeding must be stopped first. When the bed temperature approaches stability or shows a slight downward trend, stop supplying air to the fan, in order to prevent an excessive amount of coal remaining in the bed after the fire is extinguished, which could lead to the generation of large quantities of flammable gases and dry coal dust. (3) After extinguishing the fire and before restarting it, try to prevent any fuel from entering the furnace; do not add fuel first and then start the blower when restarting the fire. (4) When backflow causes blockage and a large amount of ash accumulates during operation, the ash is removed through the ash discharge system. (5) If the oil gun fails to extinguish the fire during the ignition process, the oil valve should be closed first; after allowing the fan to operate for 5 minutes to ensure ventilation, ignition should be attempted again. (6) During the ignition process, if the nozzle of the oil gun becomes clogged and poor atomization occurs, it will be difficult to raise the bed temperature to the required level for coal addition; in such cases, ignition should be stopped, the nozzle of the oil gun should be cleaned or replaced before attempting to ignite again. (7) During the ignition process, it is essential to control the amount of coal added; generally, the total amount of coal added should not exceed 20% of the amount of bed material. (8) If an abnormal increase in flame temperature is detected, combustion adjustment should be intensified to bring the air-coal ratio within an appropriate range ； If it is caused by ash blockage in the feedback device, the ash blockage inside the device must be removed immediately ； If the re-ignition of combustible materials in the flue causes the exhaust gas temperature to exceed 300°C, fire suppression measures must be taken immediately: all access panels and shutters should be tightly closed, ventilation must be stopped, and either a fire extinguishing device should be used inside the flue or steam applied to put out the fire. Once the exhaust gas temperature returns to normal, the system can remain stable for a while before the access panels are opened again to check that there are no sources of fire in the flue; only after about 15 minutes of ventilation using the exhaust fan can the boiler be restarted. By implementing the above measures, explosions in fluidized bed > circulating fluidized bed boilers can be prevented, ensuring the safe and efficient operation of the units.