Basic Description of Boiler Post-Furnace Cooling and Slag Removal System
The boiler post-furnace cooling and slag removal system refers to the process where, after the coal in the boiler has been burned and discharged from the furnace, the slag temperature is still as high as 800-1100℃, thus requiring cooling. The system for cooling the slag is called the boiler post-furnace cooling system. For example, a slag cooler is a specialized cooling device. The system for transporting the cooled slag to a designated storage location is called the boiler post-furnace slag removal system. Examples include belt conveyors and scraper conveyors.
Introduction to Drum-type Cold Slag Machine:
Drum-type slag coolers are classified into spiral, louver, lifting plate, and honeycomb types. The honeycomb type (i.e., multi-tube type) further includes horizontal and inclined layouts. The working principle of a drum-type slag cooler is that spiral blades, louvers, and lifting plates are welded to the drum body. The rotation of the drum ensures full contact between the slag and the hot slag, transferring heat to the cooling water. This type of slag cooler is highly adaptable to different slag and coke sizes, eliminates the risk of coking, easily achieves continuous operation, and has relatively low operational requirements.
Introduction to belt conveyors:
A belt conveyor is a conveying device that uses a conveyor belt as a carrier and rotational torque as power. It consists of a frame, conveyor belt, motor, reducer, upper and lower idlers, etc. Belt conveyors are common conveying equipment. They can be further divided into flat belt conveyors and steeply inclined belt conveyors, etc.
PLC Overview
Programmable Controller (PLC) should ideally be abbreviated as PC, but to distinguish it from Personal Computer (PC), it is commonly referred to as PLC. It is a new type of industrial control device based on a microprocessor.
Features
1. The programming method is simple and easy to learn;
2. Powerful functionality and high performance-price ratio;
3. The hardware is complete, easy to use, and highly adaptable;
4. Contactless and wiring-free, with high reliability and strong anti-interference capability;
5. The system design, installation, and debugging workload is minimal;
6. Minimal maintenance workload and easy repair;
7. Small size and low energy consumption;
Description of electrical control for the slag cooler and belt conveyor in a traditional system.
Cold slag machine:
The control system is equipped with remote/local control. Remote control is DCS control, and local control is field control. The two control methods are interlocked.
When controlled by 2DCS, the speed of the slag cooler can be controlled according to the 4-20mA signal of the furnace pressure in the hearth, realizing automatic adjustment and automatic control, effectively reducing the amount of labor required to manually adjust the speed of the slag cooler;
3. The control cabinet is equipped with an emergency stop function, which can achieve emergency shutdown under any control mode.
4. The inlet and outlet of the slag cooler are equipped with PT100 temperature sensors, which transmit the signals directly to the DCS for real-time monitoring of inlet and outlet water temperature changes. When the temperature exceeds the set value, the DCS sends a control signal to promptly control the operation of the slag cooler.
5. A pressure transmitter is installed at the inlet of the cold slag machine. The pressure feedback signal of 4-20mA is transmitted to the DCS. When the pressure exceeds the upper and lower limits of the set value, the DCS gives a control signal to the control cabinet.
The slag cooler inlet is equipped with a differential pressure transmitter, which feeds back the water flow rate to the DCS as a 4-20mA signal. When low flow or flow interruption occurs, the DCS controls the slag cooler to alarm and shut down, effectively protecting the slag cooler itself.
7. The slag cooler has a speed sensor installed on the shaft end of the motor. A 4-20mA signal is transmitted to the DCS, which can monitor the equipment speed in real time.
8. The inlet and outlet valves of the slag cooler are intelligent integrated. They are equipped with stroke limit, over-torque, remote/local control, and valve position feedback output interface to send signals to the DCS or the main control cabinet, and can accept DCS 4-20mA regulating signals to control the inlet and outlet valve positions;
9. A proximity switch is installed on the base of the cold slag machine. When the cylinder experiences axial or radial displacement that exceeds the allowable value, the control cabinet will alarm and send the DPDT switch signal to the DCS to promptly notify maintenance personnel for adjustment.
Belt conveyor:
The control system is equipped with remote/local control. Remote control is DCS control, and local control is field control. The two control methods are interlocked.
2. Set up two-stage belt misalignment switches. When belt misalignment occurs, send a signal to the DCS to notify the central control personnel in a timely manner.
3. A pull-rope switch is installed so that when inspection personnel discover problems during on-site inspections, an emergency shutdown can be achieved under any control mode.
4. The control cabinet is equipped with an emergency stop function, which can achieve emergency shutdown under any control mode.
In traditional boiler systems, the post-furnace cooling system and the post-furnace slag removal system are two independent systems, operating independently and complementing each other. Furthermore, these two systems are typically supplied by different manufacturers, leading to problems with electrical interlocking and low compatibility, thus creating safety hazards during boiler operation. To address this issue, many construction companies require interlocking functions from various manufacturers during the bidding process. However, this results in fragmented control of different equipment, requiring numerous personnel to operate the equipment, leading to cost waste.
Table 1 Main Controlled Equipment of the Slag Cooler
Optimized application of four programmable logic controllers in cooling and slag removal systems
In the overall boiler system, the two systems are interconnected at different levels, so they can be combined into one. This integration facilitates equipment linkage and compatibility. The following analysis uses the boiler slag removal system upgrade at Leshan Keer Alkali Industry Co., Ltd. as an example. This project is equipped with 2 x 25T boilers, 2 slag coolers, one trough belt conveyor, and 2 flat belt conveyors.
1) Control method setting:
1. There are two startup methods: independent start/stop and one-key start/stop.
2. Delayed sequential start-up is only used when using one-button start-up. The start-up sequence is: No. 3 flat belt conveyor → No. 2 flat belt conveyor → No. 1 trough belt conveyor → slag cooler, with a delay time of 10 seconds. Delayed sequential stop-up is also used when using one-button stop-up. The stop-up sequence is: slag cooler → No. 1 trough belt conveyor → No. 2 flat belt conveyor → No. 3 flat belt conveyor, with a delay time of 20 seconds to ensure that there is no conveying material on the equipment.
3. It has interlocking protection function. When the last stage equipment fails and stops, all the upstream equipment will stop immediately; when the intermediate equipment fails and stops, all the upstream equipment of the failed machine will stop immediately to prevent material accumulation.
4. The slag cooler uses frequency conversion control to adjust its speed.
II) Programmable Logic Controller Selection
The cost of restarting the boiler ranges from 8,000 to 20,000 RMB. To ensure the boiler's safe and smooth operation, the stability of the programmable logic controller (PLC) is of paramount importance; therefore, international brands are given priority when selecting a PLC.
Well-known international brands include Siemens (Germany), Rockwell Automation (AB), Mitsubishi, Koyo, OMRON, Modicon, LS Electric, GEEFanuc, and Panasonic Electric Works (NAIS). Among these, Siemens and Mitsubishi are particularly representative international brands.
Siemens was the preferred brand for this project. Siemens PLCs include the S7200, S7300, and S7400 series. Based on a comprehensive analysis of the project's functions, the S7224 CPU from the S7200 series was deemed to offer the best price-performance ratio.
S7224 CPU Performance Overview: Main Performance: Integrated 14 inputs/10 outputs for a total of 24 digital I/O points, 2 inputs/1 outputs for a total of 3 analog I/O points, can be connected to 7 expansion modules, and can be expanded to a maximum of 168 digital I/O points or 38 analog I/O points;
Other features: 20KB program and data storage space, 6 independent high-speed counters (100KHz), 2 high-speed pulse outputs of 100KHz, 2 RS485 communication/programming ports, with PPI communication protocol, MPI communication protocol and free mode communication capability;
Features: This unit also features several new functions, such as built-in analog I/O, position control characteristics, self-tuning PID function, linear ramp pulse command, diagnostic LEDs, data logging, and recipe functions. It is a new type of CPU with analog I/O and powerful control capabilities.
(III) Selection of Expansion Modules
The number of digital and analog expansion modules will be determined based on the number of points required by the device.
2. The EM277 network module will be used for communication with the host computer.
(iv) CPU Communication
1. Communication with the touchscreen uses RS485 serial communication.
2. Communication with the host computer is via industrial Ethernet through the M277 network module. Alternatively, it can be connected to a fiber optic transceiver via the EM277 and then communicate with the host computer via fiber optic cable.
Five Summary
I) Technical Summary
1. By adopting PLC control, the problems of incompatibility and unsatisfactory interlocking effects between the two systems were solved, greatly ensuring the safe operation of the boiler.
The use of RS485 serial communication between the 2PLC and the frequency converter provides a better communication method for the frequency converter when using PID control, and realizes the automatic adjustment function of the frequency converter.
The 3PLC's communication with the host computer via industrial Ethernet greatly improves the system's reliability and stability. Furthermore, the touchscreen operation reduces the number of switch cables, avoiding operational problems caused by switches and wiring issues, and also improves the aesthetics of the control cabinet, making operation more intuitive.
II) Economic Summary
Before the project was upgraded, this position was staffed by six operators working in three shifts. According to data provided by Leshan Keer Alkali Industry Co., Ltd., the monthly salary for each employee in this position was 1480 yuan. Therefore, the total annual cost for this position was 1480 × 6 × 12 = 106560 yuan.
PLC supplier quotations are as follows: Table 2
Therefore, it is evident that the cost can be recovered within one year after adopting PLC. For the user, this not only improves the safety of boiler operation but also reduces operating costs and saves human resources. This demonstrates that the application of PLC in boiler post-furnace cooling and slag removal systems is effective and feasible. Surrounding the boiler body, there are many auxiliary devices serving the main body, and these are provided by multiple equipment manufacturers. Based on the analysis of this case, it is clear that applying PLC to boiler auxiliary equipment to improve various auxiliary systems is effective and feasible.
