As the name suggests, pulse combustion control adopts an intermittent combustion method, using pulse width modulation technology to achieve temperature control of the kiln by adjusting the duty cycle (on-off ratio) of the combustion time. The fuel flow rate can be preset by pressure adjustment. Once the burner is working, it will be in full load state, ensuring that the gas outlet speed remains unchanged when the burner is burning. When the temperature needs to be raised, the burning time of the burner is lengthened and the intermittent time is reduced; when the temperature needs to be lowered, the burning time of the burner is reduced and the intermittent time is lengthened. The control chart is shown in Figure 1.
The main advantages of pulse combustion control are:
High heat transfer efficiency, greatly reducing energy consumption.
It can improve the uniformity of the temperature field in the furnace.
Precise control of the combustion atmosphere can be achieved without online adjustment.
It can improve the load regulation ratio of the burner.
The system is simple, reliable and low cost. Reduce the generation of NOx.
The adjustment ratio of ordinary burners is generally about 1:4. When the burner is working at full load, the gas flow rate, flame shape, and thermal efficiency can all reach the optimal state. However, when the burner flow rate is close to its At the minimum flow rate, the heat load is minimal, the gas flow rate is greatly reduced, the flame shape cannot meet the requirements, and the thermal efficiency drops sharply. When the high-speed burner operates below 50% of the full load flow rate, the above indicators are far away from the design requirements. gap. This is not the case with pulse combustion. No matter what the circumstances, the burner has only two working states, one is working at full load and the other is not working. The temperature is only adjusted by adjusting the time ratio of the two states, so pulse Combustion can make up for the defect of low burner regulation ratio, and can still ensure that the burner works in the best combustion state when low temperature control is required. When using a high-speed burner, the gas ejects quickly, forming a negative pressure around it, sucking a large amount of flue gas in the kiln into the main gas, and fully stirring and mixing, which prolongs the stagnation time of the flue gas in the kiln and increases the The contact time between the flue gas and the product improves the convective heat transfer efficiency. In addition, the flue gas and gas in the kiln are fully stirred and mixed, so that the gas temperature is close to the temperature of the flue gas in the kiln, improving the uniformity of the temperature field in the kiln and reducing high-temperature gas. Direct thermal shock to the heated object.
The adjustment of combustion atmosphere is an indispensable link to improve the performance of industrial kilns. However, traditional continuous combustion control can only measure the residual oxygen content of flue gas online and feed it back to the combustion atmosphere controller in real time. Only by adjusting and controlling the output of the combustion air flow actuator can the combustion atmosphere in the furnace be accurately controlled. Due to the reliability, lifespan and price of the zirconia sensor for detecting residual oxygen in flue gas, its use in industrial sites is often not ideal. Some kiln automatic control systems simply use a proportional follower to make the flow rate of combustion air and fuel flow into a fixed ratio. However, this method has to leave a large margin of combustion air and cannot achieve optimal results. Requirements for energy conservation and control of excess oxygen content (or excess air coefficient). Using the pulse combustion control method, the oil pressure and wind pressure can be adjusted to appropriate values ??at one time. After the system is put into operation, only these two pressures need to be kept stable. Measuring and controlling pressure is much simpler than flow. Fully automatic control can be adopted according to the actual situation of the system, or manual control can be adopted.
Compared with continuous combustion control, the number of instruments participating in the control in the pulse combustion control system is greatly reduced. There are only temperature sensors, controllers and actuators, omitting a large number of expensive flow and pressure detection and control mechanisms. Moreover, since only two-position switch control is required, the actuator has also changed from the original pneumatic (electric) valve to an electromagnetic valve, which increases the reliability of the system and greatly reduces the system cost. The industrial furnace control system uses an industrial PC as the control unit and adopts an advanced fieldbus architecture with powerful functions, rich graphics, and friendly user interface. All components are imported products, making the system more reliable.
The system has the following functions:
Real-time monitoring of temperature at various points in the furnace, flue gas residual oxygen, furnace pressure, oil (gas) pressure, combustion air pressure, fuel flow and Combustion air flow and other parameters.
It has upper and lower limit alarm functions and alarm printing function. The upper and lower alarm limits are set by the user, and the alarm record can be stored, and the user can query and print at will.
Each section in the furnace can be controlled to rise or fall according to the temperature value or temperature curve set by the user (see the figure below). The temperature rise uses pulse combustion control, and the temperature drop uses forced pulse air cooling control.
The residual oxygen in the flue gas in the furnace can be controlled according to the combustion atmosphere set by the user.
The furnace pressure can be controlled.
The incoming and outgoing materials of the kiln can be controlled.
It has historical data query function and can store, display and print historical data according to user needs.
It has report printing function and can print class reports, daily reports and monthly reports offline in real time.
It has a dynamic process diagram that can display the process flow diagram of the entire kiln, dynamically display the parameters of each point in the furnace in real time, and dynamically display the flame combustion status in the furnace in real time.
In actual application, when the ordinary pulse width modulation method is used to adjust the combustion duty cycle, when the duty cycle is close to 0% or 100%, the interruption or combustion time is too short, and the on-site The operating effect was not ideal, so we introduced the concept of minimum time and set the minimum time of interruption and combustion to 3 seconds. When the duty cycle is close to 0% or 100%, the solution can be solved by extending the corresponding combustion and interruption time. This question. As a new technology, pulse combustion has broad application prospects and can be widely used in ceramics, metallurgy, petrochemical and other industries. It will play a major role in improving product quality, reducing fuel consumption, and reducing pollution. It is an innovation in automatic control in the industrial furnace industry. , will become the development direction of industrial furnace combustion technology in the future.
1. The equipment uses various combustion gases as the medium and is heated by various burners. The maximum temperature is 1200°C.
2. The furnace frame is made of various large and medium-sized steel sections and is welded on-site. The shell sealing plate is a color steel plate, and the high-aluminum full-fiber refractory wire blanket module is the furnace lining. The sealing and energy-saving effect is obvious.
3. The trolley frame is welded by a combination of various large I-beams, channel steels, angle steels and thick steel plates.
4. The trolley transmission uses all wheels as driving wheels, which ensures reliable driving. The transmission system adopts a "three-in-one" motor-reducer installation method is shaft-mounted, with compact structure, firm assembly, and flexible entry and exit. , simple operation and convenient maintenance.
5. The refractory masonry of the trolley adopts high-aluminum shaped brick structure, which has good sealing effect with the furnace body and high compressive strength. Pads are placed on the trolley surface for stacking workpieces. The trolley side panels are all made of cast parts to ensure that the trolley body does not deform and is durable. The sealing between the furnace car and the furnace lining adopts a refractory fiber sealing block electric push rod automatic compression structure. The opening and closing of the side seal are interlocked with the entrance and exit of the furnace car.
6. The furnace door adopts a high-aluminum all-fiber refractory thrown wire blanket and a steel combined frame structure, with an electric hoist for lifting and lowering. The furnace door sealing mechanism adopts a long and short lever spring-type automatic compression cam mechanism and a soft edge sealing device. Ensure no friction up and down, ease, safety and reliability.
7. The chimney is equipped with automatic furnace pressure control, butterfly valve, etc., which can adjust the cooling speed.
8. High-speed burners are used for heating, evenly distributed on both sides. Continuous proportional regulation of combustion. The actuator adjusts the air volume, and adjusts the gas volume through the proportional valve to achieve air-fuel ratio combustion. The gas and air volume are set with a lower limit limiter. The gas pipe of each burner is equipped with a control solenoid valve. Each burner Equipped with an independent and complete combustion controller, with automatic ignition, flame detection, fire extinguishing alarm and automatic gas shutoff. This fully ensures the stability and safety of the combustion temperature control system.
9. Characteristics of the burner
The high-speed burner basically achieves complete combustion of fuel and combustion air in the combustion chamber. The burned high-temperature gas is sprayed at a speed of 100m-150m/s. out, thereby strengthening the convection heat transfer, promoting air circulation in the furnace, and achieving the purpose of uniform furnace temperature, so that the heat preservation is uniform at ≤±10℃.
The burner
a. Small combustion chamber volume
b. High combustion gas outlet velocity
c. Adjustment ratio of the burner Large, 1:10
d, automatic ignition and flame monitoring
e, each burner failure alarm function
f, each combustion air low pressure protection
g. Interlocking safety control of large and small fires for each burner
h. Combustion status display, fault alarm display
i. Temperature curve design and modification, save and print
j, operation prompts, fault prompts
k, combustion fan control (switch), furnace door control (switch), air-fuel ratio control, process safety chain control
p>10. Preheater
Use GC type tube type heat exchanger with inserted spoiler to increase the preheating temperature of the air. When the furnace temperature is 1000°C, the air temperature will be preheated to 300°C. -350℃.
GC type high-efficiency insert heat exchanger, under the same heat transfer coefficient, the air side pressure loss is lower than that of ordinary insert heat exchangers. Its value is around 1500Pa, thus reducing power consumption.
When the flue gas temperature is 600°C, the comprehensive heat transfer coefficient is above 45 W/M2°C. When the flue gas temperature is ≥900°C, the comprehensive heat transfer coefficient is above 55 W/M2°C.
The heat exchanger is designed to use heat-resistant steel and stainless steel according to different temperatures, and the layout adopts temperature uniformity and thermal stress relief measures.
11. Control system
The system mainly adjusts and controls the flow rate of the gas pipeline of each furnace, the flow rate of flue gas and the dilution air volume by detecting the temperature and pressure of the furnace. It is also equipped with a natural gas main pipe quick cut-off device.
The level of furnace pressure has a great influence on the use effect of the heating furnace. When the furnace pressure is high, the furnace gas will rush out of the sealing gaps of the furnace body to form airflow flushing, which will affect the furnace door and furnace sealed with fiber materials. The bottom compression has a greater impact. At the same time, the high-temperature air flow will also have an impact on the environment around the furnace and the control components. When the furnace pressure is low, cold air is sucked in from the sealing gap. In addition to increasing the oxidation of the workpiece, the high temperature in the furnace will be quickly drawn out by the negative pressure, causing a waste of fuel. To this end, a furnace pressure measuring point is installed on the exhaust duct to control the electric flue gas valve to keep the furnace pressure at a slightly positive pressure state
The furnace adopts zone furnace temperature control, and each zone is equipped with a thermocouple to measure The temperature enters the multi-point recorder to centrally track and record the temperature in the furnace.
12. Safety interlock system
The safety interlock between the trolley and the furnace door. When the furnace door is not opened to a certain position, the trolley will be locked in and out. When the trolley seal is not opened, The trolley will be locked in and out.
When the air, gas pressure, and compressed air pressure do not meet the specified requirements, the combustion of the burner will not be started. If it is burning, it will be shut down safely.
13. Main features of the equipment
1. Good energy-saving effect: The furnace lining of this equipment is all made of high-aluminum refractory fiber. Compared with refractory bricks, it has smaller thermal conductivity and smaller heat capacity. , so the thickness of the refractory layer is small and the heat absorption is greatly reduced.
This equipment adopts a high-speed temperature-adjusting burner system, with a large ejection speed of 100m/s, which can effectively stir the furnace gas, make the furnace temperature uniform, and the burner system burns completely, so that the fuel can be fully utilized. . Using furnace pressure zero control and full sealing technology, the maximum joint surface (the sealing surface between the furnace car and the furnace body) is at zero furnace pressure, the furnace gas does not leak, and the cold air does not penetrate, so that the heat energy generated by combustion can be effectively Land utilization.
2. High degree of automation: the furnace doors and furnace cars are all electric, with operating consoles, so operators can easily control the operation of the furnace doors and furnace cars. The furnace door and furnace car have stroke control and can automatically stop running when they reach a limited limit position to ensure safety. The combustion system has a complete set of ignition, large and small fire operation, detection, flameout alarm, flameout cut-off and re-ignition functions, and each set of burners has an independent control box, which can be controlled individually. Each control box interface can be connected to the temperature controller in the instrument room, so that the entire system can be automatically controlled.
Pipeline parameters are automatically controlled. The pipeline pressure of combustion air and fuel can be set and automatically adjusted to control the amount of combustion air and fuel at the optimal ratio to ensure high combustion efficiency and eliminate black smoke.
The furnace pressure is automatically controlled by comparing the furnace pressure signal with the set value through the pressure transmitter, transmitting the signal to the chimney actuator, and automatically controlling the pressure in the furnace by changing the opening of the chimney.
The temperature in the furnace is controlled by an advanced intelligent digital display temperature controller, which forms a closed-loop control with the temperature measuring element and the automatic control burner. It has high precision, high flexibility, anti-interference and high reliability. The temperature control system can automatically calculate, operate, display and store the heat treatment production process curve to achieve full process control.
The instrument cabinet is equipped with operating value displays for temperature, furnace pressure, each burner, and each pipeline parameter, as well as abnormal situation alarms and emergency protection measures to ensure operational safety.
Hot standby sequence and production transfer sequence
1. If the water jacket leaks, the stirring rod leaks, or the stirring water seal leaks, it is not suitable to convert to hot standby furnace. If the air branch pipe valve is seriously not closed tightly or the saturated steam valve is seriously leaking and cannot be repaired and replaced during hot standby, it is not suitable to be used as hot standby.
2. When planning hot standby, you should choose a furnace with better furnace conditions for hot standby. If you choose a hot standby furnace with a lowered fire layer and severe slagging in the furnace, the furnace condition will inevitably worsen. .
3. For generators that are planned to be hot-standby, the furnace should be inspected in advance, and the parts with a small amount of slag should be drilled to break the slag, and drilled at various points of the furnace section to make all points of the furnace section Basically the same tightness.
4. Reduce the ash layer appropriately.
5. Appropriately thicken the coal seam.
6. Check whether the maximum relief valve bell cover is flexible in rising and falling.
7. Reduce the air volume before hot standby.
8. Seal the double vertical pipe water seal and pull up the maximum relief valve in time according to the furnace outlet pressure.
9. Close the air valve and saturated steam valve of the branch pipe strictly.