Abstract: This paper describes the air leakage problem in air preheaters of large thermal power units, focusing on the causes of leakage. A novel method for leakage control using a fuzzy controller is proposed, and a PLC-based gap control system is designed. Practical application shows that the system is reliable and can effectively reduce air leakage in the air preheater, providing a strong guarantee for the safe full-load operation of the unit.
Keywords: air preheater; air leakage; fuzzy logic; PLC
1 Introduction
Air preheaters are an important component of boiler equipment in thermal power plants. They are heat exchange devices that use the heat of flue gas at the tail end of the boiler to heat the air required for fuel combustion, thereby improving boiler efficiency. There are two common types of air preheaters: tubular and rotary. Rotary air preheaters are widely used in large and medium-sized power plants due to their advantages such as high heat transfer density, compact structure, corrosion resistance, long service life, and low operating costs[1].
However, in actual operation, various factors, such as ash accumulation, often cause air leakage of varying degrees[2]. The leakage rate of most air preheaters is around 10%, and the leakage rate of some air preheaters is over 20%. Air leakage in air preheaters greatly increases the output of blowers, primary air fans and induced draft fans, increasing energy consumption. In severe cases, it causes insufficient air volume to be sent into the furnace, resulting in low-load operation of the boiler and affecting the safe, economical and stable operation of the unit. Therefore, research on air leakage control is a very important topic. For a long time, engineers have been committed to solving this problem from different aspects such as structure, installation and operation protection[3,4]. Common air leakage control systems are generally divided into three types: mechanical, ultrasonic and eddy current. However, it is generally believed that mechanical control systems cannot guarantee the accuracy of control[5], and ultrasonic control systems are easily affected by soot blowing in the air preheater, resulting in relatively low sensitivity[6]. Therefore, most current air leakage control systems adopt eddy current, which has the characteristics of strong anti-interference ability and high sensitivity[7]. Meanwhile, PLC-based air leakage control systems have also achieved certain applications [8]. However, existing technical solutions still have some urgent problems to be solved, mainly in three aspects: (1) the application of advanced control strategies; (2) the enhancement of network communication functions; and (3) the improvement of human-machine interface. Therefore, this paper focuses on the radial sealing gap of the hot end of the air preheater rotor, selects an advanced high-performance high-temperature eddy current sensor, uses a PC, industrial control computer and PLC to form an industrial Ethernet, designs a gap control system based on Mitsubishi FX2N series PLC, and applies fuzzy control theory to tune the PID parameters of the collected signals. Actual operation shows that the system can effectively reduce air leakage of the preheater and provide a strong guarantee for the safe full-load operation of the unit.
2. Analysis of air leakage problem
Rotary air preheaters exchange heat by having hot and cold media (air and flue gas) flow alternately across the heating surface. There are two basic types of rotary air preheaters: the heating surface rotary type (also known as the Junker type) and the hood rotary type (also known as the Tommüller type). The hood rotary air preheater is less commonly used because of its high air leakage rate, which is difficult to eliminate effectively in actual operation [9]. At present, most of the large generator sets (mostly 300MW and 600MW) in my country use Junker air preheaters, the structure of which is shown in Figure 1.
Figure 1 Schematic diagram of air preheater principle
Fig.1 Schematic diagram of the principle of air preheater
Figure 2. Rotor "mushroom-shaped" deformation diagram
Fig.2 Mushroom distortion chart of the rotor
A Junkers air preheater is a type of preheater where the rotor rotates while the shroud remains stationary. Its main component is a relatively large (over 10 meters in diameter) low-speed rotor (rotor speed ≈ 1 r/min) that serves as the heat exchange element. High-speed flue gas flows upward through the left side of the rotor, heating the heat storage element (rotor sector compartment) within the rotor. When the heated heat storage element rotates to the right side (air side), air flows downward through the heat storage element, carrying away the heat and thus preheating the air.
Air leakage in air preheaters mainly includes two parts: carry-over leakage and direct leakage. Carry-over leakage is caused by air entering the flue gas side as the preheater rotates, resulting in leakage. This part of the leakage is determined by the structure of the rotary air preheater itself and cannot be eliminated. Existing studies have shown that its leakage rate is relatively small and is not the main source of leakage[10]. Direct leakage is caused by the pressure difference between the air side and the flue gas side and the gap between the moving and stationary parts of the air preheater. The larger the gap, the greater the pressure difference, and the greater the leakage. This is the main reason for air leakage in rotary air preheaters, and its leakage accounts for 80% to 90% of the total leakage[11]. Therefore, controlling the size of the gap is the key to controlling leakage.
Air preheaters are designed with specialized sealing devices to reduce gaps and minimize air leakage. The sealing device generally consists of seven parts: hot-end radial seal, cold-end radial seal, axial seal, cold-end bypass seal, hot-end bypass seal, sector plate seal, and central cylinder seal. Its total air leakage Q can be expressed by equation (1):
Q_total = (Q1 + Q2 + Q3 + ... + Q7) + Q8 (1)
Where: Q1 - radial leakage at the hot end; Q2 - radial leakage at the cold end; Q3 - axial leakage.
Q4 - Hot-end bypass seal leakage; Q5 - Cold-end bypass seal leakage; Q6 - Sector-shaped plate seal leakage.
Q7 - Air leakage in the center cylinder seal; Q8 - Air leakage due to carryover.
Existing research results indicate that among the various factors causing air leakage, radial sealing leakage at the hot end is the most important factor for air preheater leakage, accounting for 30% to 50% of the total [12]. This is due to the "mushroom-shaped" deformation of the preheater rotor during hot operation, which leads to an increase in the radial sealing gap, as shown in Figure 2. The rotor expands due to heat during operation. Due to the temperature difference between the hot and cold ends of the rotor, which is generally around 250°C, the expansion amounts of the hot and cold ends are different, resulting in mushroom-shaped deformation of the rotor. The amount of deformation can be expressed by the following formula [11]:
(2)
In the formula: is the deformation, ε=12×10 -6 /℃ (thermal expansion coefficient of steel), H is the rotor height, D is the rotor diameter, R is the rotor radius, and is the temperature difference.
Therefore, the air leakage control system is mainly designed to control the radial sealing gap at the hot end. This is also the fundamental focus of this study: to design a control system that minimizes the radial sealing gap at the hot end to reduce air leakage.
3 Clearance Control System
3.1 System Working Principle
The clearance control system mainly consists of five parts: clearance measuring mechanism, operator station (host computer), master control station (slave computer, i.e., PLC system), actuator, and safety interlock device. (See Figure 3.)
A remote monitoring method is employed, configuring two medium-sized PLCs to monitor each section of the radial sealing gap at the hot end. An industrial control computer acts as the host, managing the lower-level PLCs using a cyclical query-response mechanism. It also serves as the system's communication processor, sending collected field signals and operational status data to the local area network for further processing and use. Furthermore, it can control the operation of lower-level PLCs and field devices based on commands from the upper level. Additionally, the system includes a hub interface for connecting to an industrial Ethernet network, enabling communication with a higher-level control station or server. A 50-ohm terminator is installed at each line endpoint to enhance communication's anti-interference capability.
This approach has the following advantages:
(1) It realizes decentralized control and on-site segmented control.
(2) Using PLC to control the underlying data acquisition and necessary data processing of the entire system is beneficial to giving full play to the role of PLC.
(3) The industrial control computer and the PLC use point-to-point communication, which is fast and reliable.
Figure 3. Block diagram of the control system
Fig. 3 Frame diagram of control system
3.2. System Composition
3.2.1 PLC System
The PLC control system is shown in Figure 4. The host computer is the Yanxiang IPC-810 industrial computer, and the slave computer is the Mitsubishi FX2N series PLC. The I/O module adopts the area control module, which has the advantage of being able to be installed on site and communicate with the PLC host through a twisted pair cable, greatly reducing the number of cables and saving costs. At the same time, it can avoid the introduction of some interference signals as much as possible. In this way, a communication local area network is established between the IPC industrial computer and the two PLCs, and the communication protocol adopts the TCP/IP protocol [13]. It is worth mentioning that since the communication layer in this system is not complicated, the two PLCs in the air leakage control system do not need to operate at the same time. The backup PLC-B automatically switches to use when the main PLC-A fails, and only one PLC is running at any time.
Figure 4 PLC control system
Fig. 4 PLC control system
3.2.2 Measuring Mechanism
The measuring mechanism is used to detect the minimum gap between the bottom surface of the sector plate and the radial sealing plate. A high-temperature eddy current sensor is installed inside the air preheater, rigidly connected to the adjusting sector plate. As the sector plate moves up and down, the sensor moves accordingly. The probe signal processing unit converts the detected gap value into a standard electrical signal and sends it to the main control unit. It compares the signal with a given value and determines the direction of the actuator (upward, downward, stationary) based on the sign of the deviation. The magnitude of the deviation determines the action time, or the deviation is compared with a set deviation band to determine how to execute the action, thus forming an automatic gap control system. Its mechanism is shown in Figure 5 below.
Figure 5. Gap measuring mechanism
Fig. 5 Measurement unit of gap
3.2.3 Implementing Agency
The actuator is the device that drives the sector plates to track the rotor's hot deformation. It includes a limit switch box, crossbeam, gearbox, lifting mechanism, transmission box, frame, tie rods connecting the preheater sector plates, and scales indicating the sector plate position. Each air preheater has two sector plates, resulting in four actuators. Rigidly adjustable sector plates are used, which can move up and down under the drive of the lifting mechanism. Simultaneously, the radial sealing plate is folded into a zigzag line during installation and becomes approximately straight when hot, thus helping to control the gap between them within design specifications during hot operation. Each actuator uses a dual-motor drive structure with one motor operating and one motor as a backup. A mechanical transmission system converts the motor rotation into the up-and-down movement of the sector plates, with a moving speed of approximately 1.5–2 mm/min.
3.2.4 Security Interlock Device
Safety interlocking devices mainly include four types: rotor stop detection switch, gap upper and lower limit travel switches, sector plate limit deformation switch, and motor thermal overload relay. Their main function is to issue an alarm signal when a system fault occurs, such as the preheater spindle stopping or rotating at too low a speed.
4 Control Algorithm
Because the mushroom-shaped deformation of the rotor after heating during air preheater operation is complex and its precise mathematical model is difficult to obtain, a parametric fuzzy self-tuning PID control method is proposed to systematically control the radial air leakage gap, based on the traditional PID control strategy. Combining empirical knowledge of PID parameter tuning, the parameter adjustment rules are determined as follows:
Rule 1: When the system output exceeds a given value, reduce the integral coefficient KI .
Rule 2: When the system rise time is greater than the required rise time, increase KI ;
Rule 3: When the system output fluctuates in steady state, the differential coefficient KD should be increased appropriately.
Rule 4: The system output should be sensitive to interference signals; KD should be appropriately reduced.
Rule 5: If the rise time is too long, increase the proportional gain coefficient KP ;
Rule 6: If the system output oscillates, reduce KP ;
Rule 7: Rule 2 has higher priority than Rule 5. That is, when the rise time is too long, adjust KI first, then adjust KP .
Based on the above rules, a fuzzy control strategy based on linguistic control rules is adopted, and a fuzzy control table for modifying KP , KI , and KD is designed. The fuzzy subsets of the linguistic values of the input and output variables are {NB++,NB+,NB,NM,NS,NO,PO,PS,PM,PB,PB+,PB++}. The input variable is the difference E between the air leakage gap and the setpoint. Let the universe of discourse for the error E be X, and the universe of discourse for the control variable u be Y. Quantizing their magnitudes into 12 levels, we have X = Y = {-5,-4,-3,-2,-1,-0,+0,+1,+2,+3,+4,+5}. The element with the highest membership degree in the fuzzy subset is selected as the control variable using the maximum membership degree method.
To meet the requirements of PID parameter self-tuning under different deviations E and deviation change rate ΔE, the fuzzy relationship between the three PID parameters and the deviations E and ΔE is found by continuously detecting E and ΔE during operation. Then, the fuzzy control rules are used to modify the PID parameters online, and a parameter fuzzy self-tuning PID controller is constructed to finally achieve real-time control of the radial air leakage gap.
5. Conclusion
Practical results show that after the device was put into use, the system performance was significantly improved, the air leakage rate was reduced to below 5%, the unit operated stably, the boiler efficiency was improved, energy consumption was reduced, and operating costs were reduced. It is a very considerable energy-saving project.
The innovation of this paper lies in the following: based on the analysis of the causes of air leakage in the air preheater of large thermal power units, a new method for air leakage control using a fuzzy controller is proposed, and a PLC-based gap control system is designed.
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