Abstract: This paper introduces the design of a PLC -based boiler flue gas desulfurization control system. The system is stable, safe, and reliable, improving the desulfurization efficiency and automation level of the desulfurization system.
Keywords: PLC, desulfurization, PID
Flue Gas Desulfurization Control System Design Based on PLC
Xiao Haijun, Sun Tao Shengxin
Abstract: This paper introduces design of control system of flue gas desulfurization. The automatic control system, which performed stably, safely and reliably, increases the efficiency and the automation level of the FGD system.
Key words: PLC Desulfurate PID
1. Introduction
China is a major coal-consuming country, and SO2 emissions from coal combustion are a major pollutant affecting urban air quality. Therefore, boiler flue gas desulfurization is an important means of reducing SO2 emissions.
Boiler flue gas desulfurization systems are highly complex, often placing stringent requirements on their control systems. PLC control has become an advanced, rapidly growing, and increasingly widely used control method. It boasts high reliability, resilience to harsh environments, comprehensive functionality, and cost-effectiveness, making it widely applicable in boiler flue gas desulfurization projects. The author participated in the design of a 2×75t/h boiler flue gas desulfurization control system for a heat source plant in Shandong Province. This system utilized a Siemens S7-200 programmable controller, with the host computer employing Beijing Yacon's KingView 6.53 industrial control software to monitor the desulfurization system.
2. Desulfurization system process flow
This heat source plant employs limestone-gypsum wet flue gas desulfurization technology. This technology uses limestone slurry as the desulfurizing agent, spraying and washing the flue gas within an absorption tower. This causes the sulfur dioxide in the flue gas to react and form calcium sulfite. Simultaneously, air is blown into the slurry in the absorption tower, forcing the calcium sulfite to convert into calcium sulfate. Gypsum is a byproduct of the desulfurization process. This method boasts high desulfurization efficiency (greater than 95%) and high operational reliability.
The system includes a flue gas heat exchange system, an absorption tower desulfurization system, a feeding system, a gypsum dewatering system, and a wastewater treatment system. This paper designs the PLC control system for the feeding system.
3. Introduction to Boiler Desulfurization System
(1) This system is a flue gas desulfurization project for 2x75t/h boilers. There are three pneumatic double slide valves in front of each boiler. They are opened when the system is running and closed when the system stops. It can be operated on-site or by the operator station in the control room. Each system has a 22kw Roots blower to blow limestone powder into the boiler for desulfurization. It is equipped with a local/remote control switch, which can be operated by the on-site control cabinet or the operator station in the control room. It can also be interlocked with the pneumatic valve in front of the boiler. If the pneumatic valve cannot be fully opened, the Roots blower cannot run.
(2) Each system has a 4kW feeder with variable frequency speed control. It has a control panel installed in a cabinet, which can be operated by the field control cabinet or the operator station in the control room to start and stop the feeder, reset the fault and control the feeder speed. It can also be interlocked with the Roots blower. If the Roots blower is not running, the feeder will not run either.
(3) The two systems share a limestone powder silo. The silo is equipped with three radio frequency admittance level switches: one high level and two low level. It has a local/remote control switch and can be operated by the field control cabinet, the field dust collector operation box, or the operator station in the control room. The field dust collector operation box is installed on the 0m floor, near the limestone powder silo inlet. It is used to operate the dust collector in the powder silo during feeding and can detect the height of limestone powder in the powder silo.
4. Hardware Components of the PLC Control System
(1) Central Processing Unit
The S7-200 uses the CPU226 CN as its central processing unit, featuring 24 digital inputs and 16 digital outputs, expandable to 248 digital inputs and 35 analog inputs. The S7-200 CPU boasts powerful instruction sets, enabling digital logic control, data processing, and closed-loop process control.
(2) Extension Module
The digital expansion modules include: EM223 CN, 8 inputs DC 24V/8 relay outputs; EM221 CN, 8 inputs DC 24V.
Analog expansion modules: 2 EM231 CN, 4 analog inputs, 12-bit resolution; EM232 CN, 2 analog outputs, 12-bit resolution.
5. PLC control system functions
The PLC control system performs functions such as data acquisition, analog quantity control, and sequential control.
(1) Data acquisition function
The host computer (industrial control computer) is equipped with KingView 6.53 monitoring software, which establishes a connection with the S7-200 via freeport communication to achieve data transmission.
The data acquisition function will collect, process, calculate, retrieve and store various information quantities (analog quantities and switch quantities) in the production process according to the required sampling speed, analog-to-digital conversion accuracy and scanning cycle, and configure them into various screens in the form of text, charts, curves, etc., and display them on the LCD screen of the host computer to provide the operator with timely information on the unit's operating status.
Specific functions of data acquisition:
• Display: Includes simulation display, operation display, group display, alarm display, etc.
• Tabular records: including periodic records, real-time records, and incident sequence records, etc.
• Historical data storage and retrieval.
(2) Analog control
The desulfurization system's operating parameters are input to the PLC, which uses PLC and PID software to control the analog output, achieving automatic adjustment. This analog control function regulates the frequency of the feeder's frequency converter. The amount of limestone desulfurizing agent is controlled by the feeder's frequency converter. Since the feed rate directly affects desulfurization efficiency, and to achieve both economical operation and environmental standards, the feed rate needs to be controlled based on the SO2 content. The SO2 content setpoint can be modified on the host computer. The monitored SO2 value after desulfurization is compared with the setpoint, and the frequency of the feeder's frequency converter is adjusted after PID calculations to control the feed rate.
(3) Sequential Control System (SCS)
The SCS controls the unit's motors, pneumatic valves, solenoid valves, etc. The SCS system in this project will be designed with selectable functional groups, subgroups, and individual control modes. Each subgroup's control function can be started and stopped independently. Operators can select automatic or manual operation modes via the CRT keyboard. During automatic program execution, any fault will trigger an interrupt signal, halting the running program and returning it to a safe state. The cause of the interruption will be displayed on the screen. When manual operation is selected, the SCS system's permission settings prevent operator error. Interlocking and protection commands have the highest priority, while manual commands take precedence over automatic commands. The "start," "stop," "open," and "close" commands of the controlled equipment are interlocked, ensuring the controlled equipment moves in a safe direction. The SCS's protection and interlocking functions are always active and cannot be manually deactivated by operators.
6. Design concept of control software
The software component is a key part of achieving desulfurization control. The software is designed with a modular structure to ensure strong portability and future functional expansion.
(1) Program flow
The desulfurization system procedure is shown in Figure 1:
Figure 1. Flowchart of the desulfurization system
(2) Closed-loop control
To achieve both economical operation and compliance with environmental standards, the feed rate needs to be controlled based on the SO2 content. Figure 2 shows the PID function diagram of the PLC analog closed-loop control in this desulfurization system. An SO2 concentration sensor detects the SO2 concentration in the reactor. The transmitter converts the sensor's output current signal into a standard-range current signal, which is then sent to the analog input module. After A/D conversion, a digital quantity proportional to the SO2 concentration is obtained. The CPU compares this digital quantity with the SO2 setpoint, calculates the error value according to the PID control algorithm, and sends the result (digital quantity) to the analog output module. After D/A conversion, it becomes a current signal used to control the feeder's delivery rate, thereby controlling the SO2 concentration in the boiler and achieving closed-loop control.
Figure 2 PID Functional Block Diagram
The S7-200 PLC provides built-in PID control instructions. For analog closed-loop control, analog expansion modules (EM231, EM232) are used to implement PID control. The PID function instructions designed in this paper are shown in the figure.
As shown in Figure 3, by simply setting the PID parameters and running the PID control instructions, the output control value can be obtained, achieving analog closed-loop control. This system adopts the on-site experimental tuning method, directly tuning the parameters in the process control system. The SO2 concentration change is observed in real-time via the SO2 curve on the host computer interface, and the PID parameters are modified to achieve the ideal control value.
Figure 3 PID control instructions
7. Conclusion
By applying a Siemens S7-200 PLC, reliable and efficient real-time control and monitoring of the desulfurization system were achieved, improving the system's reliability, safety, and automation level. Practical application at a heat source plant in Shandong Province has proven that this system is reliable, has a low failure rate, and offers excellent environmental protection and economic benefits.
References:
[1] Liao Changchu. PLC Programming and Application [M]. 3rd ed. Beijing: China Machine Press, 2008.
[2] Huang Jing. Electrical Control and Programmable Logic Controllers [M]. Beijing: China Machine Press, 2003.
