Is purging required for both cold start and hot start of a circulating fluidized bed? Is the procedure the same? Why is purging necessary?

1. Purging before boiler ignition (mandatory before cold start). Before purging the boiler and igniting it with an oil igniter, it is very important to conduct an internal inspection of the air ducts and air chambers downstream of the oil igniter, in order to check for any leakage of the bed material. All bed material that has fallen into the air ducts and air chambers should be removed to prevent the air nozzles from becoming clogged during purging. Before each start-up and before starting up again after the main fuel is cut off, the furnace, cyclone separator, and tail heating surface areas must be purged (except during hot start-ups). Before starting each fan, it is necessary to ensure that the air pathway from the blower inlet to the chimney is unobstructed, in order to prevent damage to the furnace and flue ducts caused by positive or negative pressure. 2. Hot-starting does not require purging. 3. Purging procedure: The general sequence is to first start the induced draft fan, followed by the high-pressure fluidization fan, the primary and secondary fans, and one coal feeding pressure booster fan; the air flow rate is adjusted according to the purging requirements, and all fans are set to automatic mode. After the purging is complete, all these fans will remain in their operational state for subsequent startups. 1) Start the induced draft fan. Close the electric baffle door at the inlet of the exhaust fan, open the electric baffle door at the outlet of the exhaust fan, close the short-circuit switch of the exhaust fan, and start the motor of the exhaust fan. Once the motor is running and has reached the specified speed, adjust the negative pressure in the furnace to (-125Pa～-245Pa). 2) Start the two high-pressure fluidized air fans via sequential control. 3) Start the secondary fans and primary fans, close the air inlet control dampers of the fans, and close the short-circuit switch of the blower to operate the motor. After the fan motor is operating properly, adjust the air inlet damper of the fan to set the primary air flow at the minimum level required for fluidization, namely 173,300 Nm3/h. Keep the total secondary air flow at its lowest value, while ensuring that the secondary air flow to the upper and lower layers of the front and rear walls is at an appropriate level. 4) Start a coal feeding booster fan. Close the inlet control damper of the coal feeding booster fan, and close the short-circuit switch to start the motor. Once the motor is operating properly, open the baffle to enable the fan to run normally; at the same time, open the outlet baffle of the fan (open the coal feeding air bypass valve when the coal feeding pressure booster fan is not in use). 5) Once the above steps are completed, the control system automatically adjusts the unit to the “ready for purging” state. 6) Purge the unit for at least 5 minutes at at least 30% of the full airflow rate (not exceeding 40%). 7) After the boiler has been purged, restore the Main Fuel Cutout (MFT) status and carry out the following steps: (A) All secondary air control dampers should be set to their minimum opening position. (B) Maintain an air volume equivalent to a fluidization wind speed of 1.2 m/s. (C) Position the following equipment at the specified locations: 1) Place the oil igniter under the bed and the adjusting dampers for the furnace fluidization air (leading to the air chamber) in place, so as to achieve an appropriate amount of combustion air. 2) Adjust the inlet damper of the blower to establish a minimum fluidization air volume of 173,300 Nm3/h. 3) Ensure a proper supply of fuel to the oil igniter under the bed, as well as unobstructed purge lines.