I. Introduction Circulating fluidized bed combustion (CFB) is widely recognized in China as a clean and efficient combustion technology. CFB combustion is a combustion technology between stoker combustion and chamber combustion, employing fluidized bed combustion. It features wide adaptability, high combustion efficiency, high combustion intensity, uniform temperature distribution, efficient desulfurization due to low-temperature staged combustion, low nitrogen oxide emissions, wide load adjustment range, low pollutant emissions, and good ash and slag utilization performance. It is an environmentally friendly boiler and a new type of boiler strongly promoted by the state. Its combustion process is as follows: Fuel is fed into the furnace through the pre-furnace coal feeding system. The air supply system consists of primary air (blast) and secondary air. Primary air is fed into the furnace from the lower part of the furnace bed, mainly ensuring the fluidization of the fuel bed; secondary air is fed into the combustion chamber in stages at multiple points, mainly increasing the oxygen content in the furnace to aid combustion. After combustion, the material is broken down into smaller particles and enters the separator along with the flue gas. After solid-gas separation, most of the particles are returned to the furnace by the return feeder at the bottom of the separator, ensuring a sufficiently high ash concentration in the furnace to guarantee fluidization. The flue gas is then extracted by the induced draft fan after passing through a dust collector. The boiler control system mainly includes automatic control of the steam drum water level, main steam pressure, furnace pressure, coal feeding, and boiler interlock control. The steam drum water level, main steam pressure, and furnace pressure primarily control the fans and pumps. Traditional control methods use dampers or valves to adjust the air volume and flow rate. While simple, this method essentially achieves regulation by artificially increasing resistance, wasting significant electrical energy. Variable frequency speed control, on the other hand, offers high efficiency, eliminates additional slip losses associated with speed regulation, provides a wide speed range, high precision, and stepless speed regulation. It easily achieves coordinated control and closed-loop control. Because it can utilize the original squirrel-cage motor, it is particularly suitable for the technical transformation of old equipment. It retains the advantages of the original motor's simple structure, reliability, durability, and convenient maintenance, while achieving significant energy savings, making it an ideal method for energy conservation in boiler fans and pumps. II. System Introduction The control system adopts advanced control equipment such as PLC, touch screen, AI regulator, and frequency converter to implement effective linkage control of the steam boiler. The system has interlocking/manual switching functions. The PLC provides reliable interlocking control and alarm prompts for the boiler, ensuring safe operation. The use of frequency converters for motor start-stop control saves a significant amount of energy, achieving economical combustion in the boiler. The following is an example of a 10-ton fluidized bed control system: 1. System control overall block diagram is as follows: 2. Main monitored parameters: Steam drum water level: The steam drum water level is one of the most important parameters in the boiler control system. By employing a single-chamber balance vessel and a differential pressure transmitter, a 4-20mA current signal proportional to the steam drum water level can be obtained. The measuring instrument converts this current signal into a water level signal, directly displaying the steam drum water level. This water level signal also serves as the automatic adjustment signal for the steam drum water level, participating in the system's automatic regulation. Main steam pressure: A pressure transmitter converts the main steam pressure into a 4-20mA current signal, which is then displayed by the regulating instrument. Furnace pressure: A micro-pressure transmitter converts the furnace pressure into a 4-20mA current signal, which is then displayed by the display instrument. Simultaneously, the display instrument assesses the furnace pressure, triggering an alarm and shutting down the furnace when the pressure exceeds the limit. Temperature System Measurement: Temperature monitoring points include furnace temperature, feedwater temperature, flue gas temperature, air preheater after temperature, cold air temperature, dust collector after temperature, and return material temperature. Signals measured by the temperature sensing elements are sent to the temperature display instrument for display, and anomaly detection is performed. In case of anomalies, an alarm signal is sent to the PLC. Flow Measurement: Steam flow measurement uses a vortex flow meter, converting the steam flow into a frequency signal, which is then displayed on the flow recorder. Other parameters include primary air pressure, secondary air pressure, and induced draft/blower motor speed. These parameters can be connected to a digital display instrument or a PLC analog input module and displayed on a touchscreen. 3. Main Control Loop Automatic Drum Water Level Control: The drum water level is a crucial parameter affecting the safe operation of the boiler. Excessively high water levels can disrupt the normal operation of the steam-water separation device, leading to increased water carryover in the steam, increased scaling on the pipe walls, and reduced steam quality. Insufficiently low water levels can disrupt water circulation, causing water-cooled wall tube rupture, and in severe cases, dry-boil and damage to the drum. Therefore, both excessively high and low water levels can cause major accidents. Its controlled variable is the steam drum water level, while the regulating variable is the feedwater flow rate. By adjusting the feedwater flow rate, the material inside the steam drum achieves dynamic equilibrium, keeping the changes within the allowable range. The AI ​​controller, based on the water level signal measured by the differential pressure transmitter, directly controls the frequency of the frequency converter after calculation. When the water level is high, the speed decreases; when the water level is low, the speed increases, ensuring the steam drum water level remains constant at the set level. A switch is used to select whether pump #1 or #2 is in operation or on standby. The block diagram is as follows: Automatic control of furnace negative pressure: The magnitude of the furnace negative pressure is affected by the induced draft and forced draft volumes. If the furnace negative pressure is too low, flames will erupt from the furnace and blast furnace gas will leak outwards, endangering the safety of equipment and personnel. If the negative pressure is too high, the furnace air leakage will increase, resulting in increased flue gas losses and increased induced draft fan power consumption. The block diagram is as follows: AI industrial controllers are used for regulation in these control loops. The AI ​​instrument has advanced control algorithms with fuzzy logic PID regulation and parameter self-tuning functions. When the error is large, fuzzy logic is used for adjustment to eliminate the PID saturation phenomenon. When the error decreases, an improved PID algorithm is used for adjustment, and it can automatically learn and remember some characteristics of the controlled object during adjustment to optimize the effect. It has the characteristics of no overshoot, high precision, simple parameter determination, and good control effect even for complex objects. Its overall adjustment effect is more obvious than that of general PID algorithm. The model of the instrument is AI-808EI4X3L2L2. The main setting parameters of the instrument are: HIAL: upper limit alarm; LOAL: lower limit alarm; Df: hysteresis (dead zone, hysteresis), used to avoid frequent alarm actions caused by fluctuations in the measured input value. Ctrl: control mode, using AI artificial intelligence adjustment/PID adjustment, Ctrl=1. M5: hold parameter, mainly determines the integral action in the adjustment algorithm. Similar to the PID integral time, the smaller M5 is, the stronger the integral action of the system. When M5=0, integral and AI artificial intelligence adjustment are canceled, and it becomes a PD controller. P: Rate parameter, proportional to the change in measured value when the instrument output changes by 100% per second. P = 1000 / unit increase of measured value per second (the system defines one unit as 0.1). T: Lag time. The smaller t is, the stronger the proportional and integral actions, while the relatively weaker the derivative action, but the stronger the overall feedback effect; conversely, the larger t is, the weaker the proportional and integral actions, while the relatively stronger the derivative action. Ctl: Output cycle, reflecting the speed of instrument operation and adjustment. Sn: Input feedback signal type, Sn = 15, signal is 4-20mA. Opt: Output mode, Opt = 4, output is 4-20mA. CF: System function selection, allows selection of the system's adjustment direction. The steam drum water level and main steam pressure adopt a reverse action adjustment mode, while the furnace negative pressure adopts a positive action adjustment mode. 4. Debugging of control loop instruments. The AI ​​instrument is the core control device of this system, and its control effect directly affects the normal operation of the entire system. During commissioning, the boiler is best operated under pressure, as the load on the pumps and fans is significantly higher than normal, resulting in more ideal commissioning results. When lacking empirical control parameters, PID self-tuning is necessary to achieve optimal control. However, typical control instruments operate on-off control during self-tuning, with the output either at its maximum or minimum. Significant output fluctuations are unacceptable in a boiler system. The AI-808 controller features a manual self-tuning mode, effectively addressing this issue. The procedure is as follows: switch the instrument to manual output mode, adjust the output using the △ and ▽ keys to closely match the setpoint, and then initiate self-tuning. This limits the instrument's output to ±10% of the current manual output, preventing large fluctuations. In most cases, a single self-tuning session is sufficient for satisfactory control. If control deviations occur, they can be corrected by fine-tuning the M5, P, and T parameters. 5. Boiler Interlocking: Boiler interlocking plays a crucial role in ensuring the safe operation of the boiler. In this example, consider the following interlocking conditions: Other interlocking conditions include: (1) When the blower is running, if the induced draft fan stops, an alarm will be issued and the blower will stop at the same time. (2) When the blower is running and the water level is extremely low, an alarm will be issued and the blower and coal feed will stop at the same time. (3) When the secondary fan frequency converter fails, an alarm will be issued, coal feed will stop, and the blower and induced draft fan will stop. III. Operation and Management 1. Induced Draft Fan Control Start-up Conditions: When the induced draft fan starts, the following conditions must be met: the "Induced Draft Fan Stop" button is not pressed, the "Emergency Shutdown" button is not pressed, the induced draft fan frequency converter has no fault signal, and the induced draft fan is in a stopped state. At this time, press the "Induced Draft Fan Start" button, and the induced draft fan will start running, and the "Induced Draft Fan Running Indicator" will light up. Adjust the instrument to control the speed of the induced draft fan. Stop Conditions: When any of the following conditions are met, the induced draft fan will stop running: the "Emergency Shutdown" button is pressed, the induced draft fan frequency converter fails, and the "Induced Draft Fan Stop" button is pressed. Speed ​​Adjustment: Ⅰ. Manual Adjustment: When the AI-808 intelligent regulating instrument for "furnace negative pressure" displays the output status, press the menu key; the "MAN" indicator light will illuminate. At this time, press the △ and ▽ keys to adjust the instrument's output current, which will change the induced draft fan speed via the frequency converter. Ⅱ. Automatic Adjustment: The instrument automatically adjusts the output current, changing the induced draft fan speed via the frequency converter, and automatically controls the furnace negative pressure. During automatic adjustment, when the instrument displays the measured value or set value status, press the △ and ▽ keys to adjust the instrument's set pressure, thereby changing the magnitude of the furnace negative pressure. 2. Blower Control Start-up Conditions: The blower should meet the following conditions to start: The "Blower Stop" button and the "Emergency Shutdown" button have not been pressed; the blower frequency converter does not issue a fault signal; the blower is in a stopped state. There is no "High Steam Pressure" signal; the induced draft fan and feedwater pump are running. At this time, press the "Blower Start" button; the blower will start running, and the "Blower Running Indicator Light" will illuminate. Adjust the instrument to control the blower speed. Stop Conditions: The blower will stop running when any of the following conditions are met: the "Emergency Shutdown" button is pressed; the blower frequency converter malfunctions; the induced draft fan stops; the feedwater pump stops; a high steam pressure alarm occurs; a high furnace negative pressure alarm occurs; a low steam drum water level alarm occurs; and the "Blower Stop" button is pressed. Speed ​​Adjustment: I. Manual Adjustment: When the AI-808 displays the output status, press the menu key; the "MAN" indicator light will illuminate. At this time, press the △▽ keys to adjust the instrument's output current, which will change the blower speed through the frequency converter. II. Automatic Adjustment: The instrument automatically adjusts the output current, changes the blower speed through the frequency converter, and automatically controls the steam pressure. During automatic adjustment, when the instrument displays the measured value or set value status, press the △▽ keys to adjust the instrument's set pressure, thereby changing the steam pressure. 3. Coal Feeder Control Start-up Conditions: The following conditions must be met when the coal feeder starts: the "Coal Feeder Stop" button is not pressed, the "Emergency Shutdown" button is not pressed, the coal feeder frequency converter does not issue a fault signal, and the coal feeder is in a stopped state. When the blower is running, press the "Coal Feeder Start" button to start the coal feeder. The "Coal Feeder Operation Indicator" will illuminate. Adjust the speed setting device to change the coal feeder speed. Stop conditions: The coal feeder will stop running when any of the following conditions are met: the "Emergency Shutdown" button is pressed, the coal feeder frequency converter malfunctions, the blower stops, and the "Coal Feeder Stop" button is pressed. Speed ​​adjustment: After the coal feeder starts running, rotate the coal feeder speed regulator to adjust the coal feeder speed. Clockwise adjustment increases the speed, and counterclockwise adjustment decreases the speed. Simultaneously, if the blower speed is lower than the set speed, the coal feeder speed will be 0; if the blower speed is higher than the set speed, the coal feeder speed will increase to the set speed. 4. Feedwater Pump Control #1 Feedwater Pump Start-up Conditions: The following conditions must be met for #1 feedwater pump to start: the "Feedwater Pump Stop" button and the "Emergency Shutdown" button must not be pressed; the feedwater pump inverter must not issue a fault signal; the feedwater pump must be in a stopped state; and the #1/#2 pump switch must be switched to #1. At this time, pressing the "Feedwater Pump Start" button will start the feedwater pump. Adjustment Instrument Control #1 Feedwater Pump Speed ​​Stop Conditions: The #1 feedwater pump will stop running when any of the following conditions are met: the "Emergency Shutdown" button is pressed; the feedwater pump inverter malfunctions; the #1/#2 pump switch is switched to #2; and the "Feedwater Pump Stop" button is pressed. Speed ​​Adjustment: I. Manual Adjustment: When the "Drum Water Level" adjustment instrument displays output status, press the menu key; the "MAN" indicator light will illuminate. At this time, pressing the △▽ keys will adjust the instrument's output current, which will change the feedwater pump speed via the inverter. II. Automatic Adjustment: The instrument automatically adjusts the output current, changes the feedwater pump speed via the frequency converter, and automatically controls the steam drum water level. During automatic adjustment, when the instrument displays the measured value or set value, pressing the △▽ keys adjusts the set liquid level, thereby changing the steam drum water level. #2 Feedwater Pump Start-up Conditions: The #2 feedwater pump must meet the following conditions to start: the "Feedwater Pump Stop" button has not been pressed, the "Emergency Shutdown" button has not been pressed, the feedwater pump frequency converter has no fault signal, the feedwater pump is in a stopped state, and the pump switch has not been switched to #1 pump. At this time, pressing the "Feedwater Pump Start" button will start the feedwater pump. Adjust the instrument to control the #2 feedwater pump speed. Stop Conditions: The #2 feedwater pump will stop running when any of the following conditions are met: the "Emergency Shutdown" button is pressed, the feedwater pump frequency converter malfunctions, the pump switch is switched to #1, and the "Feedwater Pump Stop" button is pressed. Speed ​​Adjustment: Same as the #1 pump adjustment method. Once all equipment has started and no faults have occurred, the system can be switched to interlock control mode. 5. The interlock alarm control system should have a reliable alarm device. When the following conditions occur, the buzzer will sound an alarm, and the corresponding alarm indicator light will flash: high furnace negative pressure, high steam pressure, high steam drum water level, low steam drum water level, high furnace temperature, high flue gas temperature, high air preheater after-temperature, low feedwater temperature, induced draft fan frequency converter failure, blower frequency converter failure, coal feeder frequency converter failure, feedwater pump frequency converter failure, etc. Upon the occurrence of any of these abnormalities, the PLC will decide to stop the corresponding equipment based on the interlock conditions. Pressing the "Mute" button will stop the buzzer sounding, and the alarm indicator light will remain on until the alarm signal is cleared. IV. Conclusion The system uses intelligent instruments for automatic adjustment and PLC interlock protection, greatly improving control accuracy and stability. It is a relatively ideal control scheme, and the boiler frequency conversion retrofit has the following advantages: 1. Frequency conversion starting has minimal impact on the power grid. Due to frequency conversion regulation, the fan can achieve soft starting, starting at low frequency with a very small starting current and extended starting time. This avoids the impact on the power grid and mechanical equipment caused by several times the rated starting current under large inertial loads, effectively extending the motor's lifespan. Furthermore, frequency conversion allows for on-demand starting and stopping; 2. On-demand airflow adjustment avoids waste. Since the frequency converter controls the fan's airflow by adjusting the fan speed, it eliminates the need for damper adjustment, with an adjustment range from 0% to 100%. Therefore, the airflow can be adjusted freely according to production process requirements, reducing unnecessary waste; 3. Energy-saving operation saves significant energy. With frequency conversion, the fan is no longer constantly operating at full load, and the fan valves are fully open, resulting in significant energy savings; 4. Reduced fan workload extends service life. After adopting frequency conversion, the fan operates at a lower speed for most of its working time, thus greatly reducing the mechanical stress and electrical shock of the fan, significantly extending its service life and reducing maintenance intensity; 5. With frequency conversion, the start and stop times can be set, reducing impact corrosion on flues, ducts, and dampers, thereby extending the service life of many parts, effectively improving the maintenance cycle of the corresponding equipment, and saving a lot of maintenance costs; 6. It allows the motor to be directly connected to the fan, reducing transmission costs; 7. The reduced operating speed of the motor and fan improves lubrication conditions, reducing transmission device failures; 8. The reduced system pressure alleviates pressure and sealing conditions on pipelines, extending their service life, etc.