Currently, boiler fans in my country's power plants, especially primary air fans, generally suffer from high energy consumption and noise levels during operation. Several power plants in the North China Power Grid have already applied variable frequency drives (VFDs) to centrifugal primary air fans in both existing and newly built units. After speed regulation, fan power consumption is reduced, operating efficiency is improved, and plant power consumption is lowered, resulting in significant energy savings. However, some retrofit projects have encountered new problems: such as "air grabbing" under heavy load conditions; overheating and damage to the front bearing of the primary air fan motor; and overload protection tripping of the VFD during primary air fan RB operation, leading to unit MFT (Main Fuses Trip) activation, seriously affecting the safe and economical operation of the fans and boilers.
Thoughts on primary air fan RB (Reverse Pressure Regulation): Why did the same primary air fan successfully achieve RB when using inlet damper regulation, but fail after variable frequency speed control modification? How to prevent a significant drop in primary air pressure after RB? How to control a sharp drop in steam temperature after RB? Is a constant pressure method or a sliding pressure method better for the unit during RB?
I. The meaning of rapid load shedding by the unit
The meaning of rapid load shedding (RB or RunBack): When one of the main auxiliary units of the unit, such as the primary air fan, forced draft fan, induced draft fan, air preheater, boiler steam-driven feedwater pump, and boiler water circulation pump, fails, the Coordinated Control System (CCS) quickly issues a command to reduce the actual load of the unit by a certain margin. The boiler and turbine main controllers adjust the combustion, feedwater, steam temperature, and turbine DEH control systems to ensure that the unit's load and related parameters ultimately reach the capacity operating conditions of a single auxiliary unit, thus guaranteeing safe operation.
RB is one of the unit's safety functions. To realize the RB function, the CCS and BMS control systems must coordinate their actions. Except for the RB command for the primary air turbine, which is issued by the BMS itself, all other RB commands are issued by the CCS. The logic diagram of RB is shown in Figure 1.
The RB module operates according to its internally set load reduction rate and target load command. The boiler load decreases at a predetermined rate. The reduction in fuel quantity is regulated by the fuel regulator and coordinated by the BMS system, which, according to certain logic, stops the corresponding coal (pulverized coal) feeders or activates the corresponding oil guns. During the RB process, the turbine inlet pressure is automatically controlled by the steam turbine. When the BMS receives the RB command, it first issues an alarm signal and sends a data recording (DL) signal to the data acquisition system (DAS). Simultaneously, it shuts down the top-level coal mill. Next, the CCS reduces the speed of the coal feeders on each operating level. If the RB command persists 10 seconds after the coal pulverizer on level F stops, the BMS stops the coal mill on level E, while the CCS continues to reduce the coal feeder speed. After 10 seconds, if the RB command still exists, the BMS stops the coal mill on level D, finally keeping the coal mills on levels A, B, and C running.
After a single forced draft or induced draft fan trips due to an accident, the forced draft and induced draft fans on the same side will automatically trip and stop operating through interlocking.
If the RB command still exists after the three coal mills (D, E, and F) are stopped, it indicates that another auxiliary machine with the same function has also malfunctioned, resulting in an MFT (Maintenance Time Trip) action.
II. Implementing the RB function during primary air fan speed regulation
Whether the primary wind turbine's RB function can be implemented depends on the following two factors.
1. Characteristics of primary air fans and their systems
(1) Parameters and margins of a single primary air fan
For large-scale units, each primary air fan is typically designed for 50% of the unit load. Larger design capacity is more beneficial for achieving the primary air fan's RB (regenerative braking) function, but detrimental to energy conservation. Excessive fan design margin leads to excessive energy consumption per fan, especially when damper regulation is used, resulting in significant energy wastage at the fan's throttling losses. Even with variable frequency speed control, selecting excessively large head and flow margins can cause the fan to operate to the left of its performance curve's peak at low loads, making parallel operation difficult and leading to "air competition" between fans. Improving the load-carrying capacity of a single primary air fan requires measures such as reducing air preheater leakage, improving primary air system piping and dampers, and refining thermal control interlocking protection logic.
(2) System air leakage
Power plants using positive pressure direct-fired pulverizing systems generally report low success rates for primary air fan RB (reverse rotation) operations. The load-carrying capacity of a single primary air fan is insufficient, often leading to tripping of all coal mills or MFT (Maintenance Force Trip) activation. The root cause is often not inadequate fan selection, but rather severe system air leakage. During primary air fan RB operations, as a single primary air fan operates, the load gradually decreases, causing air leakage in the air preheater (hereinafter referred to as the air preheater) to continuously increase. During the system switching of the number of operating coal mills, the resistance of the primary air system pipeline changes, causing primary air to take a shortcut, bypassing the two air preheaters and the primary air connecting valve with a large volume of air. This results in a significant amount of leaking air flowing back out of the inlet of the tripped fan.
① Air leakage in primary air duct
Check for leaks in manholes, flanges, and other areas within the primary air ducts, eliminate leaks, and reduce air leakage. If necessary, pressure test the pulverizing system and use smoke bombs to check for leaks.
② Air preheater leakage
Factors affecting air preheater leakage include primary air pressure, flue gas temperature, and manufacturing process.
Air preheater leakage rate and primary air leakage rate are different concepts. The former refers to the total leakage of primary and secondary air. For a three-compartment rotary air preheater, the design leakage rate is generally 6% to 10%. Among them, primary air leakage accounts for the vast majority of the total leakage, reaching over 80%. At low loads, the primary air leakage rate accounts for 30% to 40% of the total primary air volume, or even higher.
The air preheater leakage rate is a key performance indicator for achieving standard commissioning of the unit, and it is generally achievable in the initial stage of operation. However, during long-term operation, the leakage rate often exceeds the standard. The increased air preheater sealing gap is closely related to factors such as low-temperature corrosion of the air preheater, rotor deformation, and wear of the sealing plates.
As the unit load continues to decrease, the leakage rate of the primary air system shows an increasing trend; under the same load, the primary air leakage rate is related to the operating mode, such as the operating primary air pressure and the number of coal mills in operation.
Ash blockage in the air preheater increases the resistance of the primary air system duct network, limiting the output of the blower.
(3) Coal mills not in operation
The RB logic does not consider the ventilation situation of the non-operational coal mills, only tripping the upper-level operating coal mills and keeping only 2-3 operating coal mills in the middle and lower levels. When the ventilation of the non-operational coal mills is not in operation, it will also lead to a decrease in system air pressure, affecting the normal combustion of primary air.
1) Primary air fan inlet and outlet doors
The inlet and outlet doors of the fans have poor airtightness; one fan is running while the other is shut down for emergency repairs.
