Abstract: The quality of lime affects the final quality of steelmaking. This paper introduces the configuration, principle, and structural characteristics of the computer control system for No. 4 and No. 5 gas-fired lime kilns of Sansteel Group Mining Company, and details the gas-fired kiln control system. Keywords: Lime, gas-fired lime kiln, No. 4 and No. 5 kiln, computer control system I. Overview Lime is one of the most important raw materials in steelmaking. It has strong desulfurization and dephosphorization capabilities. The quality of lime directly affects the quality of steel and the service life of the furnace in steelmaking production. An important parameter for measuring the quality of lime is its activity. High activity results in rapid dissolution, rapid slag formation, and strong reactivity of lime during steelmaking. The earliest method used by Sansteel Mining Company to produce lime was by calcining coke and limestone together. The lime produced by this method had a high underburning rate and low activity because the coke and limestone were difficult to mix evenly. The gas-fired lime method produces lime by calcining limestone with a mixture of coal gas and air. This method produces lime with complete combustion, low underburning rate, and high activity. The gas-fired process has high requirements for the control of combustion temperature and gas flow rate. Computer control can meet the complex control requirements of the system. The computer control system for our newly commissioned No. 4 and No. 5 lime kilns uses Siemens S7-PLC and Advantech industrial PCs from Taiwan. II. System Composition and Network Communication Implementation To save investment, the original PLC systems for No. 1, No. 2, and No. 3 lime kilns remain unchanged; only the industrial PCs of the No. 1, No. 2, and No. 3 lime kiln computer operator stations are upgraded. The new No. 4 and No. 5 lime kiln PLCs use Siemens SIMATIC S7 PLCs, and the computer operator stations use Advantech P4 industrial PCs. The entire system uses MPI network, TCP/IP Ethernet, and RS232 network to enable resource sharing and redundancy among the No. 1, No. 2, No. 3, No. 4, and No. 5 lime kiln computer operator stations. The network configuration diagram is as follows: [align=center] Figure 1 Network Configuration of Lime Kiln Control System[/align] The hardware configuration has the following characteristics: 1. All digital and analog signals of the original No. 3 lime kiln, as well as all analog signals of No. 1 and No. 2, are controlled by a Siemens SIMATIC S7-300 series PLC. It mainly consists of one master rack with three slave racks. Each slave rack consists of an ET-200M and 7 input/output modules. The four racks form a PROFIBUS_DP network. 2. The digital signals of the original No. 1 and No. 2 lime kilns are still controlled by the OMRON C1000H PLC from Japan. The gas pressurization stations of No. 1 and No. 2 lime kilns are controlled by the remote PLC unit RT201 connected to the C1000H and the input/output modules of the C200H. Data from OMRON's C1000H PLC is connected via an RS232 serial port to the RS232 module on the PLCs of lime kilns #4 and #5, enabling hot backup of kilns #1 and #2 from the #3/#4/#5 operator stations. All digital and analog signals for the new #4 and #5 lime kilns are controlled by a Siemens SIMATIC S7-300 series S7-315DP PLC. The entire system utilizes a Siemens MPI network, employing MPI multi-point interface data communication technology to connect the SIMATIC S7-300 PLC to the industrial PC. Specifically, a CP 5611 card is inserted into each industrial PC. This card connects the industrial PC to the integrated MPI interface on the SIMATIC S7-300 PLC, forming an MPI network. The MPI communication baud rate reaches up to 187.5kbps. MPI communication is reliable, convenient, and fast. In addition, the industrial control computers of lime kilns #1, #2, and #3 form a network via TCP/IP Ethernet, achieving resource sharing and redundancy between the host computers. III. Control Process The production of lime by calcining limestone with a mixture of coal gas and air at a high temperature of approximately 1000 degrees Celsius is a complex process. Factors affecting lime activity are the temperatures of different sections of the gas-fired kiln, primarily the calcination zone temperature, which is generally between 800 and 1100 degrees Celsius. Here, the calcination zone temperature is used as the control target, and the inlet gas flow rate (the ratio of inlet air flow rate to coal gas flow rate) is used as the control factor. The output value of the coal gas flow rate is multiplied by the air/coal ratio to set the air flow rate, ensuring a constant air/coal ratio. The new system adds a variable frequency speed control system, allowing for variable frequency speed regulation of primary and secondary air to meet the complex control requirements of the system. IV. System Functions and Features The five lime kiln computer operation stations achieve mutual redundancy. The computer operation stations use the Microsoft Windows 2000 operating system. The programming language configuration software features a user-friendly interface and convenient operation. Each operator station can simultaneously collect real-time data from gas-fired kilns #1, #2, #3, #4, and #5. This involves collecting, analyzing, and storing various parameters from the underlying OMRON C1000H PLC and Siemens S7-300 PLC, with reports available for querying. The main system functions and features are as follows: 1. Automation of the limestone feeding process. Automatic control is achieved for timed lime discharge, the star-shaped lime discharger, the spiral lime discharger, the powder conveyor belt, and the multi-bucket elevator. 2. Remote control of the compressor and blower at the #3 gas pressurization station using the ET-200M's PROFIBUS_DP bus structure. 3. Remote control of the compressor and blower at the #1 and #2 gas pressurization stations using the OMRON RT201 remote communication module. 4. Three-dimensional dynamic simulation process display and control loop simulation display. The limestone feeding and quicklime discharge processes can be dynamically displayed. The system dynamically displays the operating conditions of kilns #1, #2, #3, #4, and #5, mainly showing the temperature, pressure, and flow rate of the gas-fired kilns, as well as the opening and closing status of fans, induced draft fans, and various important valves. The flow chart screen includes system flowcharts for gas-fired kilns #1, #2, #3, #4, and #5, as well as screens for pressurization stations #1 and #2, #3, and #4/#5. 5. Historical records for key parameters are available, with historical curves allowing users to easily view values ​​at any point within a year. Real-time curves are displayed one hour prior to the current time, with multiple curves displayed per page, showing the time and value for each point. Alarms are triggered and recorded for abnormal production conditions. 6. Records are added for each shift, including raw material consumption, gas consumption, and output. Accumulated flow rates and ash removal times are also recorded for operator reference. Various reports are available for viewing. V. Conclusion The computer operation stations of lime kilns #1, #2, #3, #4, and #5 achieve resource sharing and redundancy through MPI network and TCP/IP Ethernet. The system is reliable, has high control precision, and the produced lime meets the requirements of steelmaking production. Since its operation for over a year, the system has proven reliable and easy to operate.