Automatic coal blending

Design of Automatic Coal Blending Control System for Coking Abstract: Based on the actual conditions of a certain coking plant, this paper designs an automatic coal blending control system for coking. It discusses the application of PLC software in the logical control of the coal blending system, and covers the main functions and components of the system, as well as the implementation aspects related to software and hardware. This includes the selection of weighing control instruments, the PLC electrical control system, the design of the IPC host monitoring software, system implementation, and anti-interference measures. Keywords: coking ； Coal blending ； Control system ； Introduction to the coking coal blending process of PLC 1 – A Certain Coal Coking Co., Ltd.: The 440,000 tons per year coking plant consists mainly of four workshops: coal preparation, coking, chemical product recovery, and power and maintenance. Coal preparation: clean coal yard, coal blending pit, crushing building, belt conveyors, etc ； Coking: coke ovens, quenching towers, coke screening buildings, belt conveyors, coke yard ； Chemical Production (Phase I): Comprises a primary cooler, a tar electrostatic precipitator, blowers, and tank areas, etc ； Power sources: fresh water, fire water, circulating water, substation, air compressor station, refrigeration station, etc. Coal blending process: Coal preparation – follows a process of first blending the coals and then crushing them. Electronic automatic coal blending scale for coal blending, cage-type crusher for crushing ； Coal blending – 4 coal receiving pits are installed on the coal preparation line; 4 belt batching scales are fitted on Coal Preparation Belt No. 1, and compressed air is provided for each coal bin. 2 System Design 2.1 System Architecture Weighing controller Weighing controller Weighing controller Weighing controller Industrial computer Printer Monitor RS232 RS485 Inverter 1# scale 4# scale Small conveyor Speed sensor Weight sensor Coal feeding 1# conveyor Figure 1 Schematic diagram of the composition of the automatic batching control system The hardware configuration of the automatic batching control system is shown in Figure 1. The instrument control system is composed of 4 sets of electronic belt scales for ingredient dosing, SA-201H type weighing control instruments, MBB weighing sensors, and SA-205 speed sensors. 1 set of **NAiSFP1-C72 type PIEs and 4 sets of **NAiSVF-7F small frequency conversion speed regulators serve as the electrical control devices for ingredient dosing. An Advantech IPC-610 industrial computer is used for real-time monitoring of the dosing process, with data communication between the upper-level computer and the weighing control instruments being achieved through RS-232 and RS-485 interfaces. 4 additional operators are added for use when single-unit control is required due to on-site operating conditions. 2.2 System Functions (1) Based on the set values specified for the ingredient requirements, the system automatically controls and adjusts the instantaneous feeding rate ； Through control and adjustment algorithm programs, the data in the workspace is processed mathematically and logically in accordance with the objectives of control and adjustment ； The system’s batching process can be monitored in real time through the dynamic display screen. (2) Query or automatically print and record the cumulative material dispensing amount, as well as any changes to the ingredient setting values ； Start/stop times and operation hours of the equipment in this unit ； Calculate and print the shift production volume and production reports. (3) A communication module can be used to establish a communication network between the batching scale and the host computer. (4) It emits an alarm signal when material discharge from the silo is hindered or when a malfunction occurs in the equipment, and it allows for the retrieval and printing of system alarm records. 2.3 Selection of weighing control instruments: The online continuous control of materials in this system is carried out by the SA-201H intelligent weighing control instrument. It is mainly based on the following considerations: 1. A single weighing control instrument is used for single-loop control; if one instrument fails, it only affects the control of one silo. 2. The instrument is directly connected to the weight sensor and speed sensor, eliminating the need for weight transmitters and speed transmitters. 3. The weighing control instrument uses PID control to regulate the speed of the small belt motor, providing automatic compensation in a timely manner to eliminate cumulative errors and ensure accurate and stable batching. 4. The instrument receives external 4mA–20mA current signals for dosage setting. 5. The instrument provides an instantaneous flow output: selectable from 4mA to 20mA, and can be sent to systems such as DCS. 6. The instrument provides digital output signals for conditions such as material shortage or excess, belt conveyor start/stop, and belt deviation. 7. It features a standard RS485 communication interface with a data rate of 2400–19200 bits, allowing 4 SA-201H instruments to be connected to the same computer system. The external interface of the instrument is shown in Figure 2. Weight sensor, speed sensor, fixed-value input, switch inputs 1-4, AC220 power supply (input terminal), PID speed control output, RS485 communication, instantaneous flow rate output, switch output 1: electric vibrator, switch output 2: operation status (output terminal), 888888888888889. Figure 2: Schematic diagram of the principle of the SA201H intelligent weighing controller. 2.4 PLC electrical control system. 2.4.1 Characteristics of the PLC application software. The control software for the coal preparation PLC system consists of a control program and a screen monitoring program. The control program runs within the CPU of the PLC controller; it is a program written in the \"ladder diagram language\" or \"function block language\" using Concept2.6. Select by process, process execution, start prediction, alarms, information transmission, and other sections. Starting the reverse material flow sequence in the program allows several belt-driven devices to be started at intervals, thereby reducing the \"pressure drop\" that occurs when multiple devices are started simultaneously” ； Sequential stopping of the forward material flow enables several belt-driven devices to achieve a \"delayed shutdown,\" reducing the time during which the equipment operates idly and saving electrical energy. The video monitoring program was developed using Intellution IFIX3.0. The full coal preparation system includes one guidance screen, one main screen, 3 operation screens, and an alarm summary screen, among others. It is easy to switch between different screens. The guide screen is used for switching between different task screens. The main screen is used to display the entire coal preparation process. On this screen, the status of various conveyor belts and mechanical equipment is indicated by different colors, and alarm messages are conveyed through flashing, etc. In addition, information such as the level in the silos, the current consumption of motors, the ash content of the coal, and the amount of coal is also displayed. The operation panel is used by operators to start and stop different material flows. It displays the operating status of the equipment, as well as information on automatic/manual mode, standalone/connected mode, level deviation, and wire pull fault. 2.4.2 Logical Control Devices The PLC in this system serves only as a logical control device, responsible for controlling the sequential/immediate start and stop of the 4 weighing belt motors in the system, as well as implementing electrical interlocks. According to the requirements of the batching process, the digital input (DI) signals of the entire batching automatic control system include: large belt operation status, running status, debugging status, full start, sequential start, start command, stop command, and start signal after a fault in the coal feeding system is resolved. The digital output (DO) signals include: start command for Motor 1 of the small belt, start command for Motor 2 of the small belt, start command for Motor 3 of the small belt, start command for Motor 4 of the small belt, and stop signal in case of a fault with the electronic scale. During normal operation, 10 seconds after the main belt starts, the smaller belts #1 to #4 are started in sequence. The interval between successive starts is determined by the distance between the discharge openings of the smaller belts and the speed of the main belt, in order to ensure that the hoppers are aligned. If the main belt stops suddenly due to a fault, after the smaller belts have begun operating, the PLC controls them to stop immediately and sends out a fault alarm signal. During normal shutdown, an automatic shutdown signal is sent from the outside, causing the smaller belts to stop sequentially. Once all of the smaller belts have stopped, a contact signal is sent to the external control cabinet for the larger belts in order to halt them as well. 2.5 Design of the IPC host computer monitoring software: The host computer monitoring software utilizes VC language and ACCESS database. (1) System functional structure: System locking, Administrator login, System settings, Administrator password, Locking password, System clock, Communication management, Material timer settings, Saving time, First shift start time, Ratio settings, Reports, Exit, Production log, Hourly records, Cumulative material volume, Cumulative silo volume. Figure 3: Software module diagram of the batching control system. (2) Database structure: The upper-level computer system uses an ACCESS database; a total of 9 data tables have been designed, including those for instrument parameters. The field definitions are as follows: Instrument parameters: Machine number, Name, Material type, Cumulative amount, Set value, Time per cycle, Number of cycles operated, Tare weight, Rated flow rate, Speed of the main belt, Speed of the secondary belt, Flow regulation coefficient, Calibration coefficient, Sensor range, Speed coefficient, Minimum control current, Maximum control current, Pneumatic control current, Start-up delay, Alarm time, Operating mode ； System parameters: system password, lock password, instrument communication port, baud rate, startup delay, window width, window height ； Alarm record: Silo, start time, end time, alarm type: Set value change: Time, silo, material type, original set value, new set value ； Ratio changes: time, material type, ratio ； Instantaneous recording: time, set value, instant ； Material type summary: Material type, date, Shift 1, Shift 2, Shift 3 ； Cumulative record: Machine number, date, set value, name, material type, Shift A cumulative total, Shift B cumulative total, Shift C cumulative total, daily cumulative total ； Hourly record: warehouse code, date, material type, period 1, …, period 24. 3 System Implementation 3.1 Control of Material Ratios The feeding tasks in this system are controlled based on the ratio inputs and calculations provided by the upper-level computer; that is, the components of various raw materials are entered into the computer, the calculation methods and parameters for formulating the mixtures are specified, and the computer carries out the calculations directly, setting and coordinating the ratios for each type of material. 3.2 Control of material quantity: Once the required weight for a certain amount of material is determined, the host computer transmits this set value to the weighing control instrument via the communication interface. The material falls from the silo onto the weighing belt, and its weight is detected by the weighing sensor. A feedback voltage signal representing the material flow rate (ranging from 0 mV to 30 mV) is sent to the weighing control instrument, where it is processed to determine the current PV value. After PID processing, a control signal ranging from 4 mA to 20 mA is generated and sent to the frequency converter, thereby changing its U/F output value and thus adjusting the speed of the conveyor belt in order to control the amount of material delivered. 3.3 Operation mode ① Manual: The silo and the weighing belt are started and stopped using manual buttons; a speed control potentiometer on-site is used to regulate the speed of the small belt motor via its frequency converter, while the weighing instrument directly monitors the flow rate and load. ②Semi-automatic: The start and stop of the small belt motor are manually controlled via the control panel in the operation room, while the PID output from the weighing control instrument is used to regulate the frequency converter so as to achieve the desired speed for instant flow control in the silo. ③Fully automatic: The amounts of each ingredient are uniformly set by the upper-level computer based on the predetermined formulation. It is capable of collecting and processing the weighing signals as well as the on/off status signals from the control circuits related to each ingredient, thereby enabling automatic control over various processes such as material feeding from the silos, weighing and detection, adjustment of material quantities, and selection and switching between different silos. 3.4 Anti-interference measures ① At the input of the system’s main power supply, isolation voltage stabilizers and UPS units are installed to reduce interference from other power systems on the system. ②Install grounding devices to avoid interference caused by chaotic grounding. ③The large belt operation status signal (the normally open contacts of the AC contactor for the large belt motor) is subject to AC interference signals of over 100 volts; an intermediate relay is added to isolate this interference. 4 Application Effects From the hardware components and working principle of this system as described above, it can be seen that by using PLCs with stored logic in place of wiring logic, the external wiring required for control devices is reduced, which significantly shortens the design and construction time of control systems. Moreover, such systems offer excellent functional capabilities. At the same time, since PLCs utilize modern large-scale integrated circuit technology and are manufactured using strict production processes, their internal circuits feature anti-interference technologies, which ensures that the entire electrical control system has high reliability and stability. This system has been in operation since March 2003, operating smoothly and reliably; its control functions and performance meet the design requirements. The accuracy of individual measurements is ±0.5%, while the precision of ingredient control reaches ±1%. The accuracy error of quantitative material feeding is better than ±1.5%. The batching process proceeded smoothly, the coke oven operation remained stable, and the production conditions were improved.