Abstract: This paper introduces the control scheme and specific hardware and software design of the computer control system for a steam injection boiler. It realizes the automatic control tasks of the boiler water supply system, negative pressure regulation system, and boiler combustion system. Furthermore, it automatically adjusts the air-coal ratio using a self-optimizing method for thermal efficiency, achieving the goal of economical combustion. System operation shows that this system greatly improves the stability and economy of boiler operation, reduces the labor intensity of operators, and achieves satisfactory control results. Keywords: Steam injection boiler; Thermal efficiency; Combustion system 1 Introduction The newly renovated boiler at the steam injection station of Gaosheng Oil Production Plant in Liaohe Oilfield adopts a forced circulation single-tube direct current operating mode. Industrial soft water is pressurized by a plunger pump and enters the economizer. After absorbing waste heat and increasing in temperature, the soft water enters the heating tube. In the heating tube, it absorbs the radiant heat of the high-temperature flame and flue gas in the combustion chamber and vaporizes into high-temperature steam, which is then output to various gas and oil wells through steam pipelines. Air is introduced into the blower through the duct, pressurized by the blower, and then enters the air preheater. In the preheater, it is heated into hot air before entering the furnace. Pulverized coal in the feed silo is fed into the burner via the coal conveying pipeline by the primary fan, where it mixes thoroughly with the preheated air and is injected into the combustion chamber for combustion. The high-temperature flame and flue gas generated by combustion first heat the water in the water-cooled wall tubes in the combustion chamber. Then, the high-temperature flue gas passes sequentially through the economizer and the air preheater, heating the working fluids (water and air) of these heated surfaces. During the heat transfer process, the temperature of the flue gas gradually decreases. It then passes sequentially through a multi-tube dust collector and a bag filter to remove most of the fly ash carried in the flue gas. An ash storage tank is located below each of the multi-tube and bag filters to collect flue gas ash. After secondary dust removal, the flue gas finally enters the chimney and is discharged into the atmosphere. The ash produced after fuel combustion falls into the ash hopper at the bottom of the combustion chamber. An auger driven by a motor transports it to the outlet at one end of the ash hopper, where it enters the bottom ash storage tank. Each storage tank is equipped with a level sensor. When the ash level reaches a certain height, the sensor generates a high-level signal. This signal is sent to the PLC, which then issues a soot blowing control signal to open the compressed air solenoid valve. Compressed air then blows the ash through the ash conveying pipe into the main ash bin. This heat pipe boiler system differs from traditional liquid level boilers. The control tasks for this system are: 1. Maintaining the boiler furnace pressure between -10 mmH2O and 0 mmH2O; 2. Maintaining a constant boiler feedwater flow rate while keeping steam consumption constant; 3. Maintaining a constant output steam pressure to achieve economical combustion; 4. Achieving automatic ignition control and automatic control of processes such as soot blowing. 2. Control Scheme Design According to the control requirements, to maintain the furnace pressure within the required negative pressure range, a closed-loop control is constructed with furnace pressure as the controlled variable and steam flow rate as the feedforward variable (control block diagram omitted). For feedwater control, the output value of the feedwater flow rate measuring instrument is used as feedback. The deviation between the feedwater flow rate setpoint and the feedback value is used for PID calculation. The result of the calculation, plus the steam flow rate feedforward variable, is used as the input control variable for the plunger pump frequency converter. The output of the frequency converter is used to control the speed of the plunger pump motor, thereby controlling the feedwater flow rate of the feedwater pump (specific control principle diagram omitted). Automatic control of boiler combustion is the most important part of this system. The effectiveness of the combustion system control directly affects the economic efficiency of the system operation. The main function of this combustion control system is to automatically adjust the coal feed rate, automatically search for the optimal blast rate, and automatically search for new optimal operating conditions when environmental conditions change (e.g., changes in the calorific value of pulverized coal), i.e., it has adaptive capability. In this control system, boiler steam pressure is the main controlled variable. PID calculations are performed using the deviation between the given steam pressure and the actual measured value, and the result is used to adjust the coal feed rate. After the coal feed rate is determined, the optimal air feed rate is found online through a step-search optimization algorithm. The control principle diagram of the boiler combustion control system is shown in Figure 1. [align=center] Figure 1 Boiler Combustion Control System Block Diagram[/align] 3 Control System Design 3.1 System Hardware Configuration Based on the above process flow and control requirements, we designed a computer automatic control system based on an upper and lower computer structure. The lower computer of this system consists of a Siemens S7-300 series PLC, responsible for the direct control of field equipment and the acquisition and processing of field parameters. The upper computer uses a Taiwan Advantech industrial computer for real-time display of field data, report printing, trend curve plotting, and other functions. The specific structure of the upper and lower computer system is shown in Figure 2: [align=center] Figure 2 Upper and Lower Computer System Structure Diagram[/align] PS307: Power module, providing 24V DC power supply for the PLC system. CPU315: A type of CPU module in the S7-300 series. It has data processing and communication functions with the host computer. IM365: An interface module that integrates two PLCs mounted on different racks. SM331: An analog input module that connects to the outputs of various temperature, pressure, and other sensors, converting analog signals into digital signals for use by the PLC and the host computer. SM332: An analog output module whose outputs are used to control the frequency converters of the blower, induced draft fan, powder feeder, and plunger pump. SM321: A digital input module used to receive equipment status signals and monitored signals (such as high bottom ash level). SM322: A digital output module used to output various control commands and alarm signals issued by the PLC (such as low inlet water pressure, boiler shutdown, etc.). The host computer is equipped with a CP5611 communication interface card, which uses the MPI communication protocol for data communication with the PLC. 3.2 System Software Design The host computer system software was developed on the Windows 98 platform. The monitoring software used was KingSCADA version 6.01, with a total of 256 points. The system included a main monitoring screen, equipment control screen, trend curve screen, alarm screen, report screen, equipment debugging screen, frequency converter control screen, system screen, and low-voltage power distribution diagram screen. It performed various display, control, and alarm functions on the field data transmitted from the PLC. The slave computer software used the Siemens Step7 software package and was programmed using ladder logic to complete data acquisition, data processing, PID calculation, and alarm functions. A flowchart of the entire slave computer software workflow is available from the author. The functions of each program are as follows: Initialization program: Resets some data storage devices and timers. Accumulation subroutine: Accumulates and calculates total power consumption, total water supply, and total pulverized coal consumption. Ignition subroutine: Performs boiler ignition. Negative pressure regulation subroutine: Adjusts the boiler furnace negative pressure within the allowable range. Combustion regulation subroutine: Maintains stable steam pressure and performs automatic air-coal ratio optimization calculation to ensure economical combustion. Flow regulation subroutine: Regulate the water flow rate. Soot blowing subroutine: When the material level of the multi-pipe ash bin, bag ash bin, and bottom ash bin is high, call this subroutine to realize the soot blowing function. Alarm subroutine: When an abnormal situation occurs during system operation, it can automatically alarm, the alarm bell rings, and the corresponding indicator light flashes. Shutdown subroutine: Includes normal shutdown and emergency shutdown. When the normal shutdown button is activated, the normal shutdown subroutine is called. When an emergency occurs in the system, the emergency shutdown subroutine is called to immediately stop the equipment operation. 4 Conclusion The system operates normally on site and the operation effect is good. It has achieved the initial design requirements. The boiler combustion control scheme with self-optimization of air-coal ratio parameter is adopted, which realizes the function of dynamically finding the best air volume during system operation, thereby ensuring the economic efficiency of system combustion. Compared with the operation of the boiler before the modification, it saves energy and reduces the labor intensity of workers. It is expected that the modification cost can be recovered within one year. References [1] Ren Guoliang. Industrial Boiler Self-Optimizing Combustion Control System, Modern Energy Saving, 1994, 6: 11-14 [2] Siemens (China) Co., Ltd. SIMATIC S7-300 Programmable Controller User Manual, 2001 [3] Siemens (China) Co., Ltd. SIMATIC S7-300 Programmable Controller Template Specification, 2001