System and method for reducing nitrogen oxide in flue gas of steel rolling heating furnace

Abstract

The invention belongs to the technical field of steel rolling, and particularly discloses a system and method for reducing nitrogen oxides in flue gas of a steel rolling heating furnace, the system comprises a heating furnace, the heating furnace is communicated with a detection pipe, the detection pipe is sequentially provided with a dust remover, a desulfurization device, a cooler, a CO concentration sensor and an O2 concentration sensor, and the CO concentration sensor and the O2 concentration sensor are electrically connected with a PLC. And an air valve and a gas valve of the heating furnace are electrically connected with the PLC. The problems that in the prior art, due to the fact that an oxygen content real-time monitoring feedback mechanism is lacked, control over a valve deviates, dynamic matching of the atmosphere in a furnace cannot be achieved, and then NOX emission cannot be effectively controlled are solved. Real-time furnace atmosphere data are detected, the PLC is used for being in linkage with the air valve and the gas valve, closed-loop feedback control is formed, the air-fuel ratio can be dynamically adjusted, and therefore the emission amount of nitric oxide is reduced.

Description

Technical Field

[0001] This invention relates to the field of steel rolling technology, and more specifically to a system and method for reducing nitrogen oxides in flue gas from steel rolling heating furnaces. Background Technology

[0002] The billet heating furnace uses a mixture of blast furnace and coke oven gas as fuel to heat the billet. During the heating process, the high-temperature environment promotes the production of nitrogen oxides (NOx). X NO is generated, and the oxygen content in the flue gas inside the furnace is the determining factor. X Emissions are a key factor, directly affecting compliance with environmental emission standards and heating efficiency. The heating furnace is divided into five functional sections along the billet's travel direction: high-temperature section, soaking section, heating section, low-temperature section, and preheating section. The flue gas temperature in the high-temperature section can reach over 1000℃, while the flue gas temperature in the preheating section is over 700℃. The temperature gradients in each section are significantly different, which places stringent requirements on the precise control of the furnace atmosphere.

[0003] In terms of combustion control technology, current heating furnaces are controlled based on the calorific value of the gas, adjusting the air-gas ratio by setting a fixed proportion. However, due to the lack of a real-time oxygen content monitoring and feedback mechanism, there is a deviation between the controlled valve proportion and the actual valve proportion. This deviation directly affects the adjustment of the opening of the gas valves and air valves in each functional section, making it impossible to achieve dynamic matching of the furnace atmosphere, thus restricting NO control. X Effective control of emissions. Summary of the Invention

[0004] The purpose of this invention is to provide a system and method for reducing nitrogen oxides in the flue gas of steel rolling heating furnaces. This addresses the problem that existing control technologies lack a real-time oxygen content monitoring and feedback mechanism, leading to valve control deviations and hindering dynamic matching of the furnace atmosphere, thus preventing effective control of NO. X The issue of emissions.

[0005] To achieve the above objectives, the basic solution provided by this invention is as follows: a system for reducing nitrogen oxides in the flue gas of a steel rolling furnace, comprising a furnace, wherein the furnace is connected to a detection tube, and the detection tube is sequentially equipped with a dust collector, a desulfurization device, a cooler, a CO concentration sensor, and an O2 concentration sensor, wherein the CO concentration sensor and the O2 concentration sensor are electrically connected to a PLC controller, the air valve and the gas valve of the furnace are electrically connected to the PLC controller, a thermocouple is provided inside the furnace, the thermocouple is electrically connected to the PLC sensor, and the PLC sensor is electrically connected to a DCS system.

[0006] The beneficial effects of this invention are as follows: Flue gas is extracted from the furnace through a detection tube, and after pretreatment including dust removal, desulfurization, and cooling, the atmosphere data inside the furnace is captured in real time using a CO / O2 concentration sensor, providing data support for precise control. By leveraging a PLC controller to link the air valve, gas valve, and thermocouples, coordinated control of temperature and oxygen content is achieved, forming a closed-loop feedback control that reduces oxygen content and thus reduces nitrogen oxide emissions.

[0007] Option 2, which is a preferred option of the basic option, uses an infrared CO concentration sensor and an electrochemical O2 concentration sensor. Both sensors are suitable for industrial high-temperature flue gas environments with many impurities, have long service life and low maintenance costs, and ensure long-term stable operation of the system.

[0008] Option 3, a method for reducing nitrogen oxides in the flue gas of a steel rolling mill heating furnace, utilizes a combination of feedforward and feedback control to collaboratively reduce the concentration of nitrogen oxides in the flue gas. In the feedforward control: a target temperature is set at the initial stage of heating. The DCS system compares the actual temperature of the furnace during normal heating with the target temperature. When the actual temperature is lower than the target temperature, the PLC controller increases the opening of the gas valve until the actual temperature is 20°C lower than the target temperature, at which point the gas valve opening is reduced to maintain the temperature within a reasonable range. When the actual temperature is higher than the target temperature, the PLC controller adjusts the opening of the air valve to cool the gas, thereby controlling the fluctuation range of the gas calorific value within ±100 kcal / m³. 3 ; Feedback control: While the feedforward control raises and lowers the furnace temperature, some flue gas is discharged after passing through a dust collector, desulfurization unit, and cooler for dust removal, desulfurization, and dehydration. During this process, CO and O2 concentration sensors monitor the oxygen content in the detection tubes in real time. Based on the oxygen content, the opening of the air valve and gas valve of the furnace is controlled to further correct the fluctuation of the gas calorific value, thereby reducing the oxygen content in the furnace. Feedforward control takes temperature as the core objective and strictly controls the fluctuation of the gas calorific value within ±100 kcal / m³ by adjusting the gas valve opening in a gradient manner. 3 This avoids initial imbalances caused by fluctuations in fuel calorific value. Feedback control, based on real-time monitored oxygen content data, dynamically corrects the opening of air and gas valves, compensating for the shortcomings of feedforward control in handling sudden changes in fuel / air pressure and ensuring dynamic matching of the furnace atmosphere.

[0009] Option 4, which is the preferred option of Option 3, controls the initial opening degree of the air valve and gas valve of the heating furnace based on the calorific value of the gas.

[0010] Option 5, an optimal choice from Option 3, uses PID parameter settings to control the opening of the air and gas valves in the heating furnace within both feedforward and feedback control. This precise control of valve opening adjustment range and response speed reduces overshoot and settling time, ensuring stable furnace temperature and oxygen content within the target range. It avoids drastic oxygen content fluctuations caused by sudden valve adjustments, thereby stabilizing NO₂ levels. X Emission concentration.

[0011] Option 6, an optimized version of Option 3, utilizes feedforward control. When the difference between the target and actual temperatures exceeds 200℃, the opening of the air and gas valves increases by 5% each time, with adjustment time controlled within 5 seconds. When the difference is between 100-200℃, the opening increases by 2% each time, with adjustment time controlled within 5 seconds. When the difference is between 20-100℃, the opening increases by 1% each time, with adjustment time controlled within 5 seconds. When the difference is below 20℃, the opening decreases by 1% each time, with adjustment time controlled within 2 seconds. Adjustment rules are established based on temperature difference tiers, enabling rapid adjustment for large temperature differences and fine adjustment for small temperature differences. For temperature differences > 200℃, large and rapid adjustments are made to ensure heating / cooling efficiency; for temperature differences < 20℃, small and slow adjustments are made to avoid overshoot and oscillation.

[0012] Option 7, an optimal choice of Option 3, uses a 3% oxygen content as the setpoint in the feedback control. It compares this setpoint with the oxygen content of the flue gas in the detection tube. When the oxygen content in the detection tube is higher than 3%, the opening of the air valve is reduced to correct the oxygen content in the furnace; when the oxygen content is lower than 3%, the opening of the gas valve is reduced to correct the oxygen content in the furnace. It directly targets NO. X The key influencing factor for NO generation is to stably control the oxygen content in the furnace below 3%, thereby inhibiting NO at high temperatures from the core mechanism. X The generation of [the substance] meets the requirements for ultra-low emissions. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of a system for reducing nitrogen oxides in flue gas from a steel rolling furnace according to the present invention; Figure 2 This is a flowchart of a method for reducing nitrogen oxides in flue gas from a steel rolling furnace according to the present invention. Detailed Implementation

[0014] The present invention will be further described in detail below through specific embodiments: The reference numerals in the accompanying drawings of the instruction manual include: 1. heating furnace; 2. detection tube; 3. dust collector; 4. desulfurization device; 5. cooler; 6. CO concentration sensor; 7. O2 concentration sensor.

[0015] like Figure 1 As shown: A system for reducing nitrogen oxides in flue gas from a steel rolling furnace includes a furnace 1, which is connected to a detection tube 2. The detection tube 2 is sequentially equipped with a dust collector 3, a desulfurization device 4, a cooler 5, a CO concentration sensor 6, and an O2 concentration sensor 7. The CO concentration sensor 6 is an infrared CO concentration sensor, and the O2 concentration sensor 7 is an electrochemical O2 concentration sensor. All of the above devices are existing technologies. The CO concentration sensor 6 and the O2 concentration sensor 7 are electrically connected to a PLC controller. The air valve and gas valve of the furnace 1 are electrically connected to the PLC controller. A thermocouple is installed inside the furnace 1, and the thermocouple is electrically connected to the PLC sensor. The PLC sensor is electrically connected to a DCS system.

[0016] Example 2 like Figure 2 The method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace involves a combination of feedforward and feedback control to reduce the concentration of nitrogen oxides in the flue gas. The feedforward control involves controlling the initial opening of the air and gas valves of the heating furnace 1 based on the calorific value of the gas (different calorific values ​​result in different air and gas combustion ratios; the valve opening is controlled by the air and gas flow rates corresponding to the combustion ratio). At the initial heating stage, a target temperature is set for the heating furnace 1. The DCS system compares the actual temperature of the heating furnace 1 during normal heating with the target temperature, adjusting the cycle every 10 seconds. When the actual temperature is lower than the target temperature, the PLC controller increases the gas valve opening until the actual temperature is 20°C lower than the target temperature, then decreases the gas valve opening to maintain the temperature within a reasonable range. When the actual temperature is higher than the target temperature, the PLC controller adjusts the air valve opening to cool the furnace, thereby controlling the fluctuation range of the gas calorific value within ±100 kcal / m³. 3 Specifically, when the difference between the target temperature and the actual temperature is above 200℃, the opening of the air valve and the gas valve increases by 5% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is between 100-200℃, the opening of the air valve and the gas valve increases by 2% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is between 20-100℃, the opening of the air valve and the gas valve increases by 1% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is below 20℃, the opening of the air valve and the gas valve decreases by 1% each time, with the adjustment time controlled within 2 seconds. Feedback control: While the temperature of the heating furnace 1 is raised and lowered via feedforward control, some flue gas is discharged after dust removal, desulfurization, and dehydration through the dust collector 3, desulfurization device 4, and cooler 5. During this process, the CO concentration sensor 6 and O2 concentration sensor 7 monitor the oxygen content in the detection tube 2 in real time. Based on the oxygen content, the opening of the air valve and gas valve of the heating furnace 1 is controlled to further correct the fluctuation of the gas calorific value, thereby reducing the oxygen content in the furnace. Specifically, with an oxygen content of 3% as the set value, it is compared with the oxygen content of the flue gas in the detection tube 2 for pre-control. The pre-control value of oxygen content is 2%. Each time the adjustment is made, when it approaches the target value (when it is 20°C away from the target temperature), pre-control is performed to reduce the overshoot and avoid oscillation. When the oxygen content in the detection tube 2 is higher than 3%, the opening of the air valve is reduced to correct the oxygen content in the furnace; when the oxygen content is lower than 3%, the opening of the gas valve is reduced to correct the oxygen content in the furnace. In both feedforward and feedback control, the opening of the air valve and gas valve of the heating furnace 1 is controlled by setting PID parameters.

[0017] The valve has a minimum opening degree. At this minimum, the actual flow rate and the detected flow rate will deviate significantly, causing control instability. This is called the blind zone, or insensitive dead zone in control terminology. When the deviation between the detected and setpoint values ​​is less than the insensitive dead zone, no adjustment is performed. When the deviation exceeds the insensitive dead zone, normal PID control is applied. The insensitive dead zone is the set minimum opening degree for both the air valve and the gas valve, which is 10%. When the valve opening reaches this value, no adjustment is made.

[0018] Figure 2 In the diagram, + indicates positive feedback or a given value; - indicates negative feedback; × indicates signal coupling adjustment, used for compensating for temperature parameters and gas control quantities; ÷ indicates the air-fuel ratio in combustion control, which is the ratio of air volume to gas flow rate; when the oxygen content is higher than 3%, it means the oxygen content in the flue gas is high, and the air valve opening is adjusted, which is called high selection; when the oxygen content is lower than 3%, it means the oxygen content in the flue gas is low, and the gas valve opening is adjusted, which is called low selection.

[0019] Traditional control methods only target temperature, without real-time monitoring of oxygen content within the area or providing feedback on oxygen content. This fails to reflect the combustion status and achieve precise oxygen control. Uncontrolled oxygen content in the flue gas leads to high oxygen levels during emissions, increasing the equivalent concentration of nitrogen oxides. The method described above controls the oxygen content inside the heating furnace to below 3%, the oxygen content at the chimney collection point to below 8%, and nitrogen oxide emissions to below 150 mg / m³. 3 The heating furnace has been running continuously for 6 months without any exceedance of nitrogen oxides, meeting the requirements for ultra-low emissions.

[0020] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A system for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace, characterized in that, The device includes a heating furnace (1), which is connected to a detection tube (2). The detection tube (2) is sequentially equipped with a dust collector (3), a desulfurization device (4), a cooler (5), a CO concentration sensor (6), and an O2 concentration sensor (7). The CO concentration sensor (6) and the O2 concentration sensor (7) are electrically connected to a PLC controller. The air valve and the gas valve of the heating furnace (1) are electrically connected to the PLC controller. The heating furnace (1) is equipped with a thermocouple, which is electrically connected to the PLC sensor. The PLC sensor is electrically connected to the DCS system.

2. The system for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace according to claim 1, characterized in that, The CO concentration sensor (6) is an infrared CO concentration sensor, and the O2 concentration sensor (7) is an electrochemical O2 concentration sensor.

3. A method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace, characterized in that, The concentration of nitrogen oxides in flue gas is reduced by the combined action of feedforward control and feedback control. In the feedforward control, the target temperature is set in the initial stage of heating of the furnace (1). The DCS system compares the actual temperature of the furnace (1) during normal heating with the target temperature. When the actual temperature is lower than the target temperature, the PLC controller increases the opening of the gas valve until the actual temperature is 20°C lower than the target temperature. Then, the gas valve opening is reduced to keep the temperature within a reasonable range. When the actual temperature is higher than the target temperature, the PLC controller adjusts the opening of the air valve to cool down the gas, thereby controlling the fluctuation range of the gas calorific value within ±100 kcal / m³. 3 ; Feedback control: While the temperature of the heating furnace (1) is raised and lowered by the feedforward control, some flue gas is discharged after being cleaned, desulfurized and dehydrated by the dust collector (3), desulfurization device (4) and cooler (5). During this process, the CO concentration sensor (6) and O2 concentration sensor (7) monitor the oxygen content in the detection tube (2) in real time. The opening degree of the air valve and gas valve of the heating furnace (1) is controlled according to the oxygen content to further correct the fluctuation of the gas calorific value, thereby reducing the oxygen content in the furnace.

4. The method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace according to claim 3, characterized in that, The initial opening degree of the air valve and gas valve of the heating furnace (1) is controlled by the calorific value of the gas.

5. A method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace according to claim 3, characterized in that, In both feedforward and feedback control, the opening degree of the air valve and gas valve of the heating furnace (1) is controlled by setting PID parameters.

6. The method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace according to claim 3, characterized in that, In feedforward control, when the difference between the target temperature and the actual temperature is above 200℃, the opening of the air valve and the gas valve increases by 5% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is between 100-200℃, the opening of the air valve and the gas valve increases by 2% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is between 20-100℃, the opening of the air valve and the gas valve increases by 1% each time, with the adjustment time controlled within 5 seconds; when the difference between the target temperature and the actual temperature is below 20℃, the opening of the air valve and the gas valve decreases by 1% each time, with the adjustment time controlled within 2 seconds.

7. The method for reducing nitrogen oxides in flue gas from a steel rolling mill heating furnace according to claim 3, characterized in that, In the feedback control, the oxygen content is set at 3%, and compared with the oxygen content in the flue gas in the detection tube (2). When the oxygen content in the detection tube (2) is higher than 3%, the opening of the air valve is reduced to correct the oxygen content in the furnace; when the oxygen content is lower than 3%, the opening of the gas valve is reduced to correct the oxygen content in the furnace.

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