Automatic control characteristics and application

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Automatic control characteristics and application of regenerative heating furnace

I. Introduction

regenerative heating furnace is a new type of heating furnace used in steel rolling plant, which has the advantages of efficient combustion, recycling flue gas and low carbon dioxide emission. It is widely used in industrial enterprises and plays an important role in promoting energy conservation and emission reduction

II. Working principle and characteristics of regenerative heating furnace

the efficient regenerative combustion system of regenerative heating furnace is mainly composed of regenerative burner and reversing system. It is divided into three main parts: preheating section, heating section and soaking section. The principle is to use the regenerator to pre store heat, so as to recover the damp heat of the temperature regulating flue gas to the greatest extent and improve the combustion air temperature

the regenerator of the new regenerative heating furnace now generally uses ceramic balls or honeycomb as the regenerator, which has a large surface area, greatly improves the heat transfer system, and greatly reduces the volume of the regenerator. Coupled with the new and reliable automatic control technology and the high preheating temperature of the preheating medium, the exhaust gas preheating can be recycled close to the limit. It is a new type of heating furnace with high efficiency and energy saving

the main field equipment involved in the control are: thermocouples for furnace temperature measurement in each section; Thermocouple for measuring flue gas temperature before and after gas preheater; Thermocouple for flue gas temperature measurement in each section and in front of the smoke extractor; Gas, air and flue gas flow measurement orifice and differential pressure transmitter in each section; Gas, air and flue gas flow regulating valves of each section; Gas shut-off valve and air/flue gas three-way directional valve in front of burners on both sides of each section; Micro differential pressure transmitter for furnace pressure measurement and flue gate for furnace pressure regulation; Fan inlet air inlet valve for air pressure regulation; Gas header shut-off valve and pressure regulating valve; Other safety protection interlocking equipment, etc

III. reversing principle

the reversing device is an important part of the heating furnace, and the whole combustion process is completed by the abstract device. It can be said that it is the heart of the whole heating furnace. Its reversing principle is: in the initial state, when the reversing device is in a fixed state, it transmits gas and air to the burner on one side of the furnace, realizes mixed combustion in the furnace, and discharges flue gas from the burner on the other side of the furnace. After a cycle (120s-180s), it changes direction to realize periodic reversing. The reversing device generally adopts a two cylinder, two position four-way reversing valve, which has four channels. Each action opens two channels and closes two channels at the same time to realize the periodic reversing of gas supply and drainage

IV. automatic control system

the control system of regenerative heating furnace generally includes:

⑴ reversing control system; ⑵ furnace temperature control system; ⑶ pressure control system in the furnace; (4) safety protection control system; (5) smoke air ratio control; (6) functions of HMI human-machine dialogue interface

1. Selection of reversing control system equipment

reversing control is the top priority of the whole heating furnace combustion and control system, and it is the key control system of combustion control. In other words, the normal operation of the reversing control system determines the normal combustion and temperature control of the whole heating furnace. Therefore, the computer control system is adopted in the control system. The sensors collect various variables from the PLC, and then the PLC drives the corresponding reversing device and the corresponding actuator respectively according to the set control mode and target value to adjust the process variables and realize the adjustment and control of temperature, pressure and flow

the operator can set various thermal parameters of the furnace through the HMI interface of the industrial computer through the keyboard and mouse, and the computer will send them to the industrial computer for processing according to the set parameters and display them on the HMI At the same time, you can view various historical parameters and print various production reports at any time. The audible and visual alarm system can alarm the fault immediately and prompt the operator. The treatment method is an advanced and practical computer control system at present

2. Commutation control

the commutation control system has two parts: automatic and manual control. Automatic control is adopted during normal operation, and manual control is adopted only under maintenance and fault conditions

automatic control: the commutation control system is equipped with automatic and manual change-over switches. When the system is under automatic control, the commutation will realize periodic automatic commutation according to the pre programmed automatic control program and the set commutation time. No personnel are required to operate in this commutation process

manual control: manual control is mainly used in maintenance and when the system is in fault state In the control program, the change-over valve and gas quick cut valve are manually controlled, which can make the change-over valve act in the manual state and make the quick cut valve open and close

smoke exhaust temperature control: the system has certain requirements for the smoke exhaust temperature, which generally cannot be higher than 180 degrees. When it is higher than 180 degrees, it must be forced to reverse. Therefore, there are two methods to control the smoke exhaust temperature in the reversing control system: one is to reduce the amount of smoke passing through the regenerator, and the other is to change the cycle time of reversing to control the temperature

3. Furnace temperature control

during the operation of the heating furnace, it is extremely important to ensure the air-fuel ratio of combustion. Too high air-fuel ratio will oxidize the surface of the billet and increase the heat loss. The air-fuel ratio is too low, so that the fuel can not be completely burned, causing the outflow of gas, wasting fuel and polluting the environment. To ensure the rationality of the air-fuel ratio and the controllability of the temperature in the heating section. Now the double fork limiting control mode is adopted, that is, the furnace temperature control circuit with the temperature in the furnace as the main ring and the air gas as the auxiliary ring. In this way, when the furnace temperature is low, the air volume can be increased first and then the gas volume can be increased. When the furnace temperature is too high, reduce the amount of gas first and then the amount of air to realize the cross control of air and gas. The specific control process is: input the set heating curve into the computer according to the process requirements, and the process detection is controlled according to the preset curve. If there is deviation, change the flow of gas and automatically change the air flow in proportion, so as to control the temperature of the three sections of the furnace, so that the temperature of the three sections of the furnace is controlled within the required range

4. Pressure control in the furnace

in order to reduce the interference of reversing on the pressure regulation in the furnace, remote control of the regulating valves of the gas exhaust manifold and the air exhaust manifold is adopted to ensure the stability of the furnace pressure. The specific control process is: set the furnace pressure control value on the computer, and the computer automatically jumps the set value by controlling the opening of the gas exhaust and air exhaust regulating valve to ensure that the furnace pressure is within the set value range

5. Smoke air ratio control

the purpose of air-fuel ratio control is to control the air/gas flow ratio of each section within the range of the set air-fuel ratio, so that the burner can be fully and completely burned. The purpose of smoke air ratio control is to control the smoke/air flow ratio of each section within the range of the set smoke air ratio, so as to make the heat accumulator work in the best state, achieve the best heat utilization and energy saving effect, and maintain the stability of furnace temperature. The control of smoke air ratio is very critical to the control effect of regenerative combustion. After repeated debugging and actual production, we believe that the smoke air ratio between 1.0 and 1.3 is more appropriate, which varies according to the heating section, heating time and steel grade

6. Safety protection system

when the gas header is low, slowing indicates that this kind of technology can also be used in other parties. Jinan new era Gold Testing Instrument Co., Ltd. is absolutely a high-end enterprise in terms of the working efficiency of the experimental machine. When the pressure of the air header is lower than 3kpa, and the blower and induced draft fan are powered off, the control system will automatically cut off the quick cut-off valve of the gas header. The reversing valve is equipped with upper and lower proximity switches. The signal sent by the proximity switch is connected to the indicator light and the alarm system. When the solenoid valve controlling the reversing valve is energized for 10 seconds at a time and the proximity switch does not send a signal, the alarm system will send an alarm signal and delay the closing of the quick cut-off valve of the gas header. The power supply system adopts Santak UPS for power supply, so as to ensure that the safety protection system of the system operates and achieve the purpose of safe operation in the case of power loss

7. Functions of HMI interface

⑴ popular WinCC interface, convenient man-machine dialogue mode. Operators can perform various operations and set various process parameters through keyboard and mouse. If there are problems in operation and control, HMI will directly prompt errors

⑵ main screen of HMI

① process monitoring diagram: it can conveniently and intuitively monitor the thermal parameters of each section of the heating furnace, such as furnace temperature, furnace pressure, air volume, gas and air pressure, and dynamically display the reversing action status and the switching degree of the regulating valve

② historical temperature curve: it can draw the temperature history curve of each point in the production process, so that the operator can intuitively understand the temperature trend from the personnel, so as to make timely adjustment

③ alarm diagram: give audible and visual alarms for furnace temperature, exhaust gas overtemperature, furnace pressure too high, gas and air header pressure too low, and prompt fault points and suggested treatment methods at the same time

④ database: the industrial computer of this control system can store all kinds of process parameters that need to be saved in the database to improve the commercial value of food packaging. Various historical data and curves can be retrieved and queried conveniently and quickly through the query menu. You can also print the report operation steps required by different users: table

v. conclusion

since the 3 # walking beam heating furnace of Jigang was put into operation in early 2004, the automatic control system has operated stably, the exhaust gas temperature is controlled below 180 ℃, the billet in the furnace is heated evenly, and the heating capacity reaches the design output of 120t/h. The regenerative combustion control adopted by it plays a very important and positive role in improving the output and quality of billets, prolonging the service life of heating furnace, reducing oxidation and burning loss, and saving energy and consumption. (end)

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