Abstract : Based on the analysis of the original boiler control system, this paper proposes a transformation scheme using programmable logic controller (PLC) control, and presents the system hardware composition. Various control and regulation functions of the boiler are realized through PLC. Keywords : Boiler control; Programmable logic controller; Frequency converter; Energy saving. Two 15t/h boilers in a rubber factory were originally monitored by instruments, which was difficult to achieve satisfactory results. The boiler thermal efficiency was lower than the design target, resulting in high labor intensity for workers and serious environmental pollution. The original boiler operation control method involved adjusting the switches or valves of the blower, grate, and water pump, which could not achieve precise and continuous regulation, resulting in low control accuracy. I. Control Scheme The basic task of automatic control of the boiler combustion process is to adapt the combustion heat to the load requirements while ensuring the safe and economical operation of the boiler. Given a specific outlet water temperature, it is necessary to adjust the ratio of blast air to coal feed to ensure the boiler operates in the optimal combustion state. Simultaneously, a certain negative pressure should be maintained in the furnace to maintain boiler thermal efficiency, prevent overheating and flame ejection, and ensure personnel safety and environmental hygiene. 1. Boiler Drum Water Level Control System The drum water level is an important parameter affecting the safe operation of the boiler. Excessive water level can disrupt the normal operation of the steam-water separator, leading to increased water carryover in the steam, increased scaling on the pipe walls, and negatively impacting steam quality. Insufficient water level can disrupt water circulation, causing water-cooled wall tube rupture, and in severe cases, dry-boil and damage to the steam drum. The steam drum water level control system essentially maintains the balance of water inlet and outlet in the boiler. By adjusting the amount of water inlet, it achieves balance, maintaining the steam drum water level near the center line to improve boiler evaporation efficiency and ensure production safety. Since the boiler water level system is a self-balancing controlled object, the phenomenon of false water levels during operation—where a sudden increase in load causes a drop in steam drum pressure, leading to a sudden increase in boiling and a rise in water level—can be correctly judged in practical applications using a three-impulse control system of water level, steam flow, and feedwater flow (see Figure 1). 2. Application of Variable Frequency Technology to Control Airflow and Water Flow: In traditional boiler control methods, equipment such as feedwater pumps, blowers, and induced draft fans operate at maximum speed. The adjustment of blower and induced draft volume is achieved by adjusting the opening of the induced draft baffle and blower baffle. This control method wastes a lot of energy on the baffles. Replacing the control scheme of dampers, baffles, and valves with a frequency converter drive, using the frequency converter to regulate the motor speed, achieves regulation of water pump flow, induced draft, and blower air volume. This not only has a wide adjustment range but also a smooth adjustment characteristic curve, enabling continuous and stable regulation and energy saving. In the boiler control system, using a frequency converter to regulate the water pump speed, thereby regulating the water flow, can significantly save energy. Using a frequency converter to regulate the motor speed also provides motor protection functions, such as timely alarms and shutdowns in case of overcurrent, phase loss, or overheating, greatly extending the motor's service life. The frequency converter also has a soft-start function, which reduces the impact on the power grid and improves the quality of power consumption for the enterprise. II. New System Structure The system uses two Dell microcomputers as host computers, one as the main unit and the other as the auxiliary unit, forming a dual-machine redundant system. The system communicates with the lower-level PLC via an MPI multi-point interface, enabling centralized monitoring and unified scheduling of the boiler's operation and achieving remote control. Operators can also monitor the boiler's operating status at any time via computer and control and set parameters for fans and pumps. Furthermore, various data from the boiler's operation and network management system can be stored in the computer's database and displayed or printed. III. New System Configuration 1. Software: Microsoft Windows 2000 operating system, Siemens WinCE V5.1 configuration software, Siemens Softnet-S7 software, Siemens S7-300 programming software SETUP7 v5.2+spl. 2. Hardware configuration: P4 1.8G CPU, 256M DDR memory, 32M graphics card; 40G hard drive; 4 USB 2.0 ports; 17-inch Dell CRT monitor, touch-screen industrial keyboard, and mouse set. 3. Programmable Logic Controller (PLC). The system utilizes the Siemens S7-300 series, with an S7-315-DP programmable controller as the CPU. It features a built-in PID module, 48k of memory, and I/O expansion to 2048 points, along with a Siemens CP5611 communication card. IV. Safety Measures for the Control System 1. Communication Network Redundancy: To ensure reliable system operation, the computer control communication network employs a dual-redundant fiber optic ring network. A failure in one network line will not affect normal system operation. 2. PLC Redundancy: Two sets of Siemens S7-300 PLCs provide redundant control. Each set consists of identical modules, with one working and one as a backup. 3. Power Supply Redundancy: The Siemens S7-300 series PLC modules use DC 24V power. Two independent AC 220V power supplies are provided on-site, supplying power to the input terminals of two Siemens SITOP power supplies (20A). This ensures continuous power supply even if any AC power supply fails or any SITOP power supply is damaged, achieving system power redundancy. Due to the adoption of computer control, the boiler is always in an economical combustion state, which greatly reduces pollution such as flue gas and dust. At the same time, the oxygen content of flue gas and the carbon content of slag are significantly reduced, resulting in significant energy saving. After adopting frequency conversion technology, the impact on the power grid during equipment startup is reduced, the service life of the motor is extended, and maintenance costs are reduced. References: [1] Zhu Chuanbiao. Fundamentals of Industrial Boiler Technology [M]. Shanghai Far East Press, 1996, 12. [2] SIMATIC WinCC Configuration Manual [K]. [3] SIMATIC S7-300 Programmable Controller Hardware and Installation Manual [K].