1 Introduction
Coal-fired power plants play a significant role in the development of my country's power industry, accounting for over 80% of the country's total electricity generation. However, while providing ample electricity, they also pollute and damage the environment. During power generation, power plants produce large amounts of industrial waste (fly ash or fly ash). To ensure the safe operation of boiler systems and protect the environment, this fly ash must be removed and transported away promptly, and the waste must be comprehensively utilized. Currently, pneumatic ash removal systems are widely used, and coal-fired power plants are required to improve dust removal efficiency and promote the comprehensive utilization of fly ash. However, in actual operation, the stability and reliability of the ash conveying system are unsatisfactory, and the causes and locations of operational failures are varied, leading to decreased dust removal efficiency, shutdown of the pneumatic ash conveying system, excessive emissions of flue gas, and environmental pollution from ash water, thus affecting the normal production of the power plant.
2. Development of pneumatic conveying technology in power plants
Pneumatic conveying is a method of transporting granular materials from one location to another within a closed pipeline using compressed air (or other gases) as a carrier and in a specific mixing ratio. The main task of a pneumatic ash removal system is to use a silo pump as a transmitter and compressed air as power to dry-transport the fly ash collected by the electrostatic precipitator to the ash silo along the ash removal pipeline. The dry ash in the ash silo is then transported by truck, or mixed into wet ash and transported away by truck.
In the 1920s, pneumatic conveying technology began to be applied in coal-fired power plants, mainly for conveying fly ash from the bottom of dust collectors, using steam extractors as the air source. In the mid-1950s, a few power plants in China also began to adopt steam-extraction negative pressure pneumatic conveying systems. The disadvantages of this system were low output, short conveying distance, severe equipment wear, high steam consumption, and poor system safety and economy, generally limiting its use to small and medium-sized power plants. After the 1960s, positive pressure conveying technology began to be applied in China. Entering the 1980s, many power plants successively introduced various types of advanced ash removal equipment and related technologies from developed countries, further promoting the development of fly ash pneumatic conveying technology in domestic power plants. Suspension conveying technology has evolved from single suction conveying to pressure conveying and combined suction-pressure conveying, and plug conveying technology has also been successfully applied in domestic coal-fired power plants. The theoretical research on gas-solid two-phase flow, which serves as the theoretical basis of pneumatic conveying technology, and the design calculation methods for conveying systems have also been continuously improved. Meanwhile, due to the rapid development of manufacturing technology and materials engineering, as well as the significant progress in control and sensing technologies, the conveying distance, conveying concentration, system output, equipment manufacturing processes, and automation management level of pneumatic conveying systems have been greatly improved, thereby enhancing system reliability and engineering economy.
Electrostatic precipitators were widely used in coal-fired power plants in China starting in the 1970s. They have three irreplaceable advantages for the comprehensive utilization of fly ash: dry dust collection, which allows fly ash to maintain its original good activity; high dust collection efficiency, which can collect the finest dust particles with the highest utilization value to the greatest extent; and its own multi-field dust collection structure, which has the characteristic of particle size classification of dry ash, can realize the separation, storage and utilization of coarse, medium and fine ash.
With the implementation of my country's sustainable development strategy and the development of environmental protection and comprehensive utilization of fly ash, the application prospects of pneumatic ash removal technology in coal-fired power plants will become increasingly promising.
3. Automated Design Principles of Ash Removal Process
The pneumatic ash removal system uses a silo pump as the transmitter and compressed air as the power source to dry-transport the fly ash collected by the electrostatic precipitator to the ash silo along the ash removal pipeline. The entire process is carried out through sealed pipelines. The system is equipped with a dedicated air compressor as the power source for dry ash transportation and also serves as the control air source. At the end of the system, there is a dry ash silo for storing coarse and fine ash. The ash from the first and second electrostatic precipitators is coarse ash, while the ash from the third and fourth electrostatic precipitators is fine ash. Under normal circumstances, the fly ash from the third and fourth electrostatic precipitators can only be sent to the fine ash silo, and the coarse ash from the first and second electrostatic precipitators can only be sent to the coarse ash silo. When the fine ash silo malfunctions, the fine ash can be sent to the coarse ash silo.
The ash removal system is designed to include an ash hopper, ash silo, and auxiliary equipment under the dust collector, an air compressor, an ash storage tank, conveying equipment, pipelines, a silo pump, valves, etc. The ash conveying pipeline connects to an electrostatic precipitator and leads to the dust collector. After filtration by the dust collector, the fine ash powder enters the dry ash silo. An electric three-way valve at the ash hopper allows control of whether dry or wet ash removal is used. A level gauge monitors the amount of dry ash powder stored in the dry ash silo. When the dry ash powder in the ash hopper reaches different positions, the intensity of the natural beta rays emitted by the ash powder hitting the detector varies. The position of the dry ash powder is determined by the difference in ray intensity detected by the detector. After being discharged from the ash hopper, the dry ash enters the silo pump through the inlet valve and is discharged from the outlet valve, ultimately being discharged to the ash storage tank for collection and reuse.
In the automatic control system for dry ash removal, the PLC can take corresponding actions based on the ash level signal (high, normal, low) transmitted from the level gauge and the pressure signal indicated by the pressure gauge above the silo pump. When the level gauge indicates a high level, the system opens the feed valve and closes the discharge valve, air inlet valve, auxiliary blowing valve, and filter membrane solenoid valve. Dry ash is discharged from the ash hopper into the silo pump for transfer. As the silo pump continuously stores ash, the internal pressure gradually increases. When the pressure exceeds a set value i, the electrical contacts of the pressure gauge above it are closed, and the pressure gauge icon on the host computer display screen turns red. At this time, the control system closes the feed valve, the electric three-way valve closes the dry ash removal inlet, and the dry ash removal stops. Afterward, the system opens the discharge valve, opens the filter membrane solenoid valve, auxiliary blowing valve, and air inlet valve, and starts the air compressor. Under air pressure, the dry ash inside the silo pump is transported to the ash storage tank through the ash pipe. During this period, the internal pressure of the silo pump continues to rise. When the pressure exceeds the set value ii, the system will shut down the air compressor and then automatically close the inlet valve, the auxiliary blowing valve, and the solenoid valve with filter membrane. After a period of time, the dry ash inside the silo pump gradually decreases, and the pressure drops. When the pressure is lower than the set value i, the electrical contact pressure gauge disconnects, and the system display screen turns green. At this time, the system will automatically close the discharge valve, open the inlet valve, and open the electric three-way valve to re-enter the dry ash removal process. The dry ash removal system block diagram is shown in Figure 1.
4. PLC control design of the ash removal system
Based on the system's functional requirements and considering its reliability, a Siemens S7-200 series PLC was selected to control the ash conveying system. The S7-200 PLC offers a variety of functions, making programmable control more flexible and convenient; it has expansion modules for easy system expansion; and its integrated PPI interface provides powerful communication capabilities, enabling communication between the host PC and the PLC. The host PC can perform programming and monitor program execution.
4.1 Modular Software Design
Traditional programming methods execute each instruction linearly or sequentially, resulting in programs that are difficult to read and verify. This system design methodology emphasizes the modular structure and hierarchical nature of software design. Before designing the program, the overall composition and modular structure of the software are analyzed and designed. The program is then refined from top to bottom during the design process, making it easier to implement control for systems with complex control structures and functions. The control system structure is shown in Figure 2.
The system has both manual and automatic control functions, and the host computer can monitor the ash conveying system.
4.2 User Software Function Design
(1) Main functions of the programmable control section:
Timed programmable ash removal: The PLC takes corresponding measures based on the ash powder position signal (high, normal, low) transmitted from the level gauge and the pressure value signal indicated by the pressure gauge on the electric contact above the silo pump.
High ash level priority ash discharge: When the ash level gauge indicates that the ash level is high, the PLC will prioritize ash discharge according to the interruption request.
Program-controlled automatic ash conveying: When the system is in program-controlled execution mode, the PLC automatically conveys ash to the dry ash removal system;
Remote operation: When the system is in remote operation mode, operators can perform remote manual control from the control room.
2) Main functions of the upper-level monitoring section:
Process flow diagram and trend chart display: The host computer can display the system process flow diagram and track the changes in silo pump pressure for different time periods such as 1 hour, 8 hours, 12 hours, and 24 hours;
Parameter display and alarm screen display: When the pressure gauge on the electrical contact above the silo pump indicates a high value, the system automatically issues a warning sound and displays a red warning signal; when the level gauge monitors the material level in the ash hopper for high/low values, the system issues an alarm signal, etc.
Statistical management functions and various report display and printing: The system can summarize and print reports on historical data (operation records of on-duty personnel, number of ash removals, ash removal time, pressure curves, etc.);
Production process event and alarm recording: The system can track and record alarms for abnormal events during the production process;
The programmable control system is connected to the main plant's computer: the system can be connected to the main plant's MIS system, which facilitates the registration, data transmission, and reporting of work record information.
5. Conclusion
This paper selects a Siemens S7-200 series PLC to control the ash conveying. In the dry ash removal system, an electrical contact pressure gauge is used to control the feeding/discharging process of the silo pump. By setting the I and II values of the electrical contact pressure, the dry ash removal operation can be automatically controlled, effectively avoiding problems such as pipeline blockage caused by unclear control of the feeding/discharging process in the original system. Real-time monitoring and fault display and recording improve the system's automation level, increase work efficiency, and enhance the comprehensive utilization rate of fly ash.
