1 Introduction
In my country, the annual electricity consumption of fans accounts for more than 10% of the total consumption, making energy-saving research on fans of great practical significance. With the development of production, boilers are increasingly widely used in various fields of industrial production, becoming one of the important thermal equipment for developing the national economy. Therefore, improving the thermal efficiency of boilers is of paramount practical importance. The boiler fan frequency conversion control adopts a single-loop control system, which satisfies control requirements while ensuring high fan efficiency. Measuring the steam outlet temperature and its setpoint constitutes a differential PID industrial control system. The blower speed is adjusted by frequency conversion to change the air volume, thereby changing the furnace temperature and adjusting the steam temperature. Measuring the furnace negative pressure and its setpoint constitutes a differential PID control system to adjust the induced draft fan, changing the induced draft volume and maintaining the furnace in a slightly negative pressure state.
2. Process Principle Analysis
For pulverized coal boilers, the air supply is divided into primary air, secondary air, and tertiary air. Primary air primarily transports pulverized coal through the burner into the furnace and supplies the oxygen needed for the volatile matter in the pulverized coal to ignite and burn. When hot air is used for primary air supply, it also preheats the pulverized coal. Secondary air supplies the oxygen needed for complete combustion and ensures thorough mixing of air and fuel. The disturbance caused by secondary air promotes rapid, intense, and complete combustion. Tertiary air is the dry air discharged from the pulverizing system, commonly known as exhaust gas. It serves as the medium for transporting pulverized coal. It is called primary air during pulverization and only becomes tertiary air when supplied to the furnace through a separate nozzle. Tertiary air contains a small amount of fine pulverized coal, has a high velocity, and provides strong mixing during the combustion process. It also replenishes the oxygen needed for the burnout stage. Due to its low temperature and high water vapor content, it can lower the furnace temperature.
The total boiler air volume is controlled by adjusting the guide baffle at the inlet of the blower. Organizing boiler combustion requires coordinating the pressure and volume of primary and secondary air. The primary air volume should be adjusted to provide the oxygen required for the ignition of the volatile matter in the pulverized coal entering the furnace, while maintaining a certain air velocity to prevent blockage of the pulverized coal pipes and ensure the ejected jet has sufficient rigidity. Excessive or insufficient primary air volume, and excessively high or low air velocity, should be avoided. The secondary air volume, in addition to ensuring the oxygen required for coke combustion, must maintain a certain air velocity and control the mixing time with the primary air. It should have a strong mixing effect and the kinetic energy to penetrate the coke ash layer. This necessitates adjusting the air volume of each layer of secondary air according to specific conditions to achieve stable combustion and an appropriate excess oxygen content in the flue gas.
When the pressure inside the furnace becomes positive during operation, high-temperature flue gas and flames can escape from some openings and leaks, affecting environmental hygiene, endangering personal safety, and potentially causing coking in the furnace and fuel burners, overheating and deformation of the burners, rigid beams, and furnace walls, as well as unstable and unsafe combustion, reducing thermal efficiency. Therefore, the furnace should be maintained under negative pressure. However, excessive negative pressure will increase air leakage in the furnace and flue, lowering the furnace temperature, causing unstable combustion, increasing the amount of flue gas, exacerbating wear on the tail heating surfaces, increasing fan power consumption, and reducing boiler efficiency. Therefore, the furnace negative pressure value is generally best maintained between 30-50 Pa. During boiler operation, the pointer on the furnace negative pressure gauge often fluctuates slightly around the control value, sometimes even exhibiting large and violent fluctuations, indicating that the furnace negative pressure is always fluctuating.
Application of 3PLC and frequency converter in boiler fan frequency conversion
3.1 Process Control
Typically, when selecting a boiler's matching fan, short-term overload capacity must be considered, and an appropriate margin must be added to determine the model. When selecting a boiler, the boiler capacity must be determined based on the maximum process load and an appropriate margin. Given that the selection of these two stages is limited by product specification grading, the final fan capacity is often too large. Furthermore, the adjustment of the boiler's forced draft and induced draft fans is accomplished by regulating dampers. Therefore, when the airflow changes, from the perspective of the fan system, this is essentially artificially increasing resistance, wasting energy and money to meet the process and operating conditions' requirements for gas and liquid flow regulation. This outdated regulation method not only wastes valuable energy but also has poor regulation accuracy. To change this situation, the best approach is to adopt variable frequency speed control technology.
3.2 Control Method
The original induced draft fan operated under reduced-pressure start-up and power frequency mode, with airflow adjusted by regulating the damper. This control method had drawbacks: significant energy waste, poor real-time adjustment, high noise levels, and high labor intensity for workers. Therefore, a frequency converter upgrade was implemented. The closed-loop control principle of the induced draft fan is shown in Block Diagram 1. A pressure closed-loop circuit consisting of a differential transmitter, frequency converter, controller, and induced draft fan automatically controls the fan speed, maintaining a stable slight negative pressure in the furnace. This improves control accuracy, saves energy, and makes the induced draft fan control more rational.
Figure 2 shows the block diagram of the closed-loop control principle of the blower frequency conversion retrofit. The temperature closed-loop loop, composed of a temperature transmitter, frequency converter, controller, and blower, automatically controls the speed of the blower, so that the temperature of the boiler steam outlet remains at a stable value. This not only improves the control accuracy but also saves energy (electricity and fuel), making the blower control more reasonable.
3.3 Control Algorithm and PLC
The project uses Siemens S7-200 series PLCs to form an automation platform. The Siemens S7-200 series PLCs are capable of PID control, and their CPUs can support up to eight PID control loops.
PID control is implemented through the PID instruction function block. In S7-200, the PID loop instruction uses input and configuration information from the loop table to perform PID calculations and exchange data, making programming extremely simple. This instruction affects the special memory flag SM1.1 (overflow). PID calculations can only be performed when the top value of the logic stack is 1. This instruction has two operands: TBL and loop, where TBL is the starting address of the loop table. It is limited to the VB area and the data type is byte; loop is the loop number, which can be an integer from 0 to 7, so a maximum of 8 PID instructions can be used in the program. If two or more PID instructions use the same loop number, even if these instructions have different loop tables, unpredictable results will occur between these PID operations. Therefore, before directly using the PID instruction function block, all real numbers such as gain (KC), sampling time (TS), integration time (TI), and derivative time (TD) must be converted into real numbers between 0.0 and 1.0 so that the PID instruction function block can accept them. In other words, the actual physical quantities from the outside world are converted into data that the PID instruction can accept, i.e., input/output conversion and standardization processing.
4. Conclusion
This article provides a comprehensive description of the variable frequency control system for boiler fans. As a control device for boiler fans, its main task is to ensure the safe, stable, and economical operation of the boiler fans and reduce the workload of operators. The variable frequency control of the boiler fans utilizes PLC control, which can perform multiple functions such as automatic detection and automatic control during the frequency conversion process.
