May I ask for the normal shutdown procedures for a circulating fluidized bed boiler?

Circulating fluidized bed boilers eliminate the coal grinding system found in coal-fired boilers; instead, the coal is directly crushed into particles with a size of 0–8 mm, and these particles are then fed into the furnace’s bed (the dense phase zone) via a coal feeder. Hot primary air entering from below the bed ensures that the material in the bed and the coal particles are thoroughly fluidized, allowing for intense collisions and heat exchange between them, which facilitates rapid ignition and combustion. Large particles burn inside the bed ； The fine particles move with the flue gas to the upper part of the furnace (the dilute phase region), where they continue to burn and exchange heat. Some of these fine particles aggregate into larger particle clusters; unable to overcome the force of gravity, they move downward near the furnace walls, while the relatively less dense gas-solid mixture in the center of the furnace continues to move upward, thus creating a strong particle circulation within the furnace ； The relatively dilute gas-solid phase, carrying a large amount of unburned particles, exits the furnace and enters a cyclone separator, where most of the particles contained in the flue gas are separated. These particles are then sent back to the furnace bed via a J-valve feeding device to continue burning and being utilized, thus forming a large-scale external circulation of materials within the furnace. The flue gas thereafter also passes through the various heating surfaces of the boiler for heat exchange before leaving the furnace; after dust removal in an electrostatic precipitator, it is discharged into the atmosphere through a fan via the chimney. The representative equipment in the combustion system of a circulating fluidized bed boiler includes the forced draft fan, exhaust fan, J-valve fan, coal feeder, cyclone separator, J-valve return device, cold slag chamber, and ignition air duct under the bed. Other operations during shutdown: During the shutdown process, the operations on the steam and water system are the same for both types of boilers. In terms of the combustion system, fuel supply is gradually reduced and stopped according to respective procedures. After the fire is extinguished, the fans continue to run for about 5 minutes to remove any combustible materials from the furnace and flue ducts. Once the oxygen level rises above 15%, the fans are stopped and the dampers are closed. During the shutdown process, the operation that a circulating fluidized bed boiler needs to carry out in addition to what a coal powder boiler does is to pay attention to controlling the bed pressure in the furnace. This is because during the shutdown process, as the amount of fuel and air gradually decreases, the amount of material in circulation inside the furnace also decreases accordingly. A large amount of suspended material falls from the dilute phase region into the bed, causing a rapid increase in the bed pressure and differential pressure within the furnace. To maintain proper fluidization of the bed material and prevent coking due to poor local fluidization, as the amount of primary air is gradually reduced, it is necessary to remove a large quantity of bed material from the furnace through the slag cooler in order to maintain an appropriate bed pressure during shutdown. Therefore, during the shutdown process, the cold slagger generally remains in operation until the bed pressure reaches the required level. Especially in the event of a tube failure in the steam-water system inside the furnace, to prevent the material in the bed from caking due to contact with water, the igniter located in the air duct below the bed should be activated promptly. While maintaining the temperature of the furnace bed above 350°C, all of the material in the bed should be removed as much as possible. Shut-down cooling: The regulations regarding natural ventilation cooling and forced fan-driven cooling after shutting down are basically the same for circulating fluidized bed boilers and pulverized coal boilers. However, considering that after a normal shutdown, large amounts of material accumulate in the furnace bed of a circulating fluidized bed boiler as well as in the J-valve return device, its heat storage capacity is much greater than that of a pulverized coal boiler; therefore, it also requires a longer cooling time compared to a pulverized coal boiler. To create conditions that allow access to the furnace for maintenance, it is generally necessary to reduce the bed temperature below 50°C before stopping the operation of the fan ； For the J-valve return system, depending on the cooling requirements, the operating time of the J-valve fan is generally about 16 hours longer than that of the supply fan.

Are there no relatively specific standards?

When the bed temperature drops below 500, it can be assumed that all the coal in the bed has been burned completely. In case of an emergency, stop the coal feeder operation so that the material at the bottom can be discharged at any time.