Method of controlling air ratio in continuous heating furnace and method of heating steel material

Abstract

Provided are an excess-air-ratio control method for a continuous heating furnace and a method for heating a steel material, in which, in a continuous heating furnace, even when combustion control zones having different combustion loads and furnace temperatures are mixed, an excess air ratio can be controlled to an appropriate excess air ratio with high accuracy, environmental regulation values of NOx concentration and soot concentration are satisfied, energy saving properties and low NOx and soot discharge properties are more excellent than in the related art, and a reduction in iron loss due to oxidation of a steel material can be achieved at the same time.SOLUTION: A method for controlling an air ratio of a continuous heating furnace according to an aspect of the present invention includes a first measurement step of comparing a NOx concentration of an exhaust gas with a threshold value, a second measurement step of comparing a soot and dust concentration of the exhaust gas with a threshold value, and a third measurement step of comparing an O2 concentration with a threshold value for each control zone, wherein the air ratios of all the control zones are uniformly adjusted when the NOx concentration or the soot and dust concentration is equal to or higher than the respective threshold values, when the O2 concentration is equal to or less than the lower limit or equal to or more than the upper limit, the air ratio is adjusted for each control zone, and when the air ratio of the control zone is changed, the first measurement step is performed.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for controlling an air ratio in a continuous heating furnace and a method for heating steel materials. [Background technology]

[0002] In continuous reheating furnaces, air ratio control is carried out to achieve appropriate combustion conditions within the furnace, with the objectives of suppressing air pollutants, improving energy conservation, and controlling the oxidation of steel. The air ratio is the ratio of the actual amount of air to the theoretical amount of air required for fuel combustion.

[0003] Continuous heating furnaces are equipped with doors for loading and unloading steel materials, skids for transporting steel materials, etc., and air inevitably enters the furnace through these facilities. If a large amount of air enters the heating furnace, the actual air ratio in the furnace combustion field will be higher than the burner air ratio. Therefore, in order to achieve an appropriate air ratio, it is necessary to set the burner air ratio taking into account the air that has entered.

[0004] For example, Patent Document 1 describes an automatic NOx control method in which the NOx concentration in the combustion exhaust gas emitted from a heating furnace is measured, and when the NOx concentration exceeds a range between an upper limit based on the NOx emission concentration and the total amount regulation value and a lower limit set in consideration of black smoke, which has an inverse relationship with NOx, the furnace pressure, air ratio, combustion air temperature, etc. of the heating furnace are adjusted.

[0005] Furthermore, Patent Document 2 describes a combustion control method using oxygen concentration control in a fuel furnace, in which the NOx concentration and CO concentration in the exhaust gas in the chimney are continuously measured, and the optimum oxygen concentration setting value for the operating state of the furnace is determined from these values, the oxygen concentration in the exhaust gas in the combustion zone, flue, or chimney of the combustion furnace is continuously measured, proportional and integral calculations are performed on the deviation between the measured value and a predetermined setting value to calculate the air ratio, and the fuel flow rate or air flow rate is continuously controlled based on this.

[0006] Furthermore, Patent Document 3 describes a control method for adjusting the combustion air flow rate supplied to the burner by the steps of: measuring the fuel flow rate to the burner, the combustion air flow rate to the burner, the oxygen concentration in the furnace, and the furnace temperature for each heating zone; calculating the intrusion air flow rate for each heating zone based on a non-steady mass conservation equation that takes into account the fuel flow rate, combustion air flow rate, oxygen concentration in the furnace, and furnace temperature for each heating zone, as well as the advection and retention of exhaust gas within the furnace body; and calculating the combustion air flow rate to the burner that can maintain a desired air ratio within multiple heating zones based on the intrusion air flow rate for each heating zone. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-139966 [Patent Document 2] Japanese Patent Application Publication No. 5-79622 [Patent Document 3] Japanese Patent Application Publication No. 2019-60588 Summary of the Invention [Problem to be solved by the invention]

[0008] In controlling continuous heating furnaces, it is important to ensure that the NOx and particulate matter concentrations in the exhaust gas meet environmental regulations. Continuous heating furnaces are equipped with multiple combustion control zones, and the fuel gas flow rate supplied to each combustion control zone is generally different for each zone. NOx and particulate matter concentrations in the exhaust gas are measured in a chimney or other facility installed in the continuous heating furnace after the exhaust gases from each combustion control zone have merged. As a result, the influence of the air ratio in combustion control zones with relatively low fuel gas flow rates is overwhelmed by the conditions in combustion control zones with relatively high fuel gas volumes, making it difficult to adjust the air ratio appropriately for each combustion control zone.

[0009] The technology described in Patent Document 1 can comply with the upper limit of NOx emission concentration, but there is a risk that the regulation value for particulate matter cannot be met by only the lower limit of NOx concentration. Furthermore, the technology described in Patent Document 1 cannot set an air ratio appropriate for each combustion control zone, and there is room for improvement in the technology described in Patent Document 1.

[0010] In the technology described in Patent Document 2, the air ratio for multiple combustion control zones with significantly different fuel gas amounts is determined based on the exhaust gas oxygen concentration of the entire heating furnace, making it difficult to set an air ratio appropriate for each combustion control zone.In addition, no consideration is given to soot and dust.

[0011] The technology described in Patent Document 3 does not mention environmental conservation, and there is a risk that it will not be able to meet regulatory values ​​for NOx and particulate matter.

[0012] The present invention has been made in view of the above circumstances, and has an object to provide an air ratio control method for a continuous heating furnace, and a steel heating method, which can control the air ratio to an appropriate level with high accuracy even when combustion control zones with different combustion loads and furnace temperatures are present in a continuous heating furnace, satisfy environmental regulation values ​​for NOx concentration and soot concentration, and simultaneously achieve greater energy savings than before and a reduction in iron loss due to oxidation of steel. [Means for solving the problem]

[0013] The idea was to adjust the air ratio for the entire continuous heating furnace based on the measured values ​​of NOx and soot concentrations in the exhaust gas from the continuous heating furnace, and to adjust the air ratio for each combustion control zone based on the measured O2 concentration in each combustion control zone.Furthermore, in order to comply with the environmental standards for NOx, which is particularly important, it was decided that the NOx concentration would be measured after each change in the air ratio based on each measurement result.

[0014] The gist of the present invention, based on the above findings, is as follows. (1) An air ratio control method for a continuous heating furnace according to one aspect of the present invention is a method for controlling an air ratio for a continuous heating furnace having a plurality of combustion control zones, the method including: a first measurement step of measuring an NOx concentration in exhaust gas from the continuous heating furnace and comparing the NOx concentration with a predetermined threshold value; a second measurement step of measuring a soot concentration in the exhaust gas and comparing the soot concentration with a predetermined threshold value; and a third measurement step of measuring an O2 concentration in each combustion control zone and comparing the O2 concentration with a predetermined threshold value for each combustion control zone, wherein when the NOx concentration or the soot concentration is equal to or greater than the respective threshold value, the air ratios for all the combustion control zones are adjusted uniformly; when the O2 concentration is equal to or less than a lower threshold value or equal to or greater than an upper threshold value, the air ratio is adjusted for each combustion control zone; and when the air ratio for any of the combustion control zones is changed, the first measurement step is carried out. (2) In the air ratio control method for a continuous heating furnace described in (1) above, the third measurement step may further measure the CO concentration in each combustion control zone, compare the CO concentration with a preset threshold value for each combustion control zone, and adjust the air ratio for each combustion control zone if the CO concentration is equal to or greater than the threshold value.

[0015] (3) A method for heating steel according to another aspect of the present invention is a method for heating steel using a continuous heating furnace having a plurality of combustion control zones, comprising: a first measurement step of measuring a NOx concentration in exhaust gas from the continuous heating furnace and comparing the NOx concentration with a predetermined threshold; a second measurement step of measuring a soot concentration in the exhaust gas and comparing the soot concentration with a predetermined threshold; and a third measurement step of measuring an O2 concentration in each combustion control zone and comparing the O2 concentration with a predetermined threshold for each combustion control zone, wherein when the NOx concentration or the soot concentration is equal to or greater than the respective threshold, the air ratio for all of the combustion control zones is adjusted uniformly; when the O2 concentration is equal to or less than a lower threshold or equal to or greater than an upper threshold, the air ratio is adjusted for each of the combustion control zones; and when the air ratio for any of the combustion control zones is changed, the first measurement step is carried out. [Effects of the Invention]

[0016] According to the present invention, in a continuous heating furnace, even if there are a mixture of combustion control zones with different combustion loads and furnace temperatures, it is possible to control the air ratio to an appropriate level with high precision, satisfy environmental regulation values ​​for NOx concentration and soot concentration, and simultaneously achieve greater energy savings than before and reduce iron loss due to oxidation of steel. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing an example of the configuration of a continuous heating furnace. [Figure 2] FIG. 1 is a flowchart showing the process flow of a control method for a continuous heating furnace according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] <Continuous heating furnace> Prior to describing a control method for a continuous heating furnace according to one embodiment of the present invention, an example of a continuous heating furnace to which the control method according to this embodiment can be applied will be described with reference to Fig. 1. Fig. 1 is a schematic diagram showing an example of the configuration of a continuous heating furnace.

[0019] A continuous heating furnace 1 is a device that heats continuously transported steel material S, for example, heating steel billets to a temperature required for hot rolling, which is performed after heating. The continuous heating furnace 1 has a furnace body 2 that extends in the furnace longitudinal direction, which is the direction in which the steel material S is transported. This furnace body 2 has a first heating zone 21, a second heating zone 22, a third heating zone 23, and a soaking zone 24 as combustion control zones, in this order, along the furnace longitudinal direction from the charging side to the unloading side of the steel material S. The combustion control zones are heated by a burner 3, which is a heating means. The burner 3 is supplied with fuel and air required to heat the steel material S, and heat is supplied to the steel material S from the combustion gas generated by the combustion of the fuel.

[0020] The continuous heating furnace 1 is also equipped with an exhaust system 4 that discharges exhaust gas from the furnace (each combustion control zone), and a chimney 5 that discharges the combustion exhaust gas from the exhaust system 4 into the atmosphere. The exhaust system 4 is also equipped with an induced draft fan as appropriate.

[0021] The continuous heating furnace 1 is equipped with a NOx concentration meter 6 that measures the NOx concentration in the exhaust gas discharged from the chimney 5, a soot meter 7 that measures the soot concentration, and an air ratio control unit 10 that controls the air ratio in the combustion control zones 21 to 24. The NOx concentration meter 6, the soot meter 7, and the air ratio control unit 10 constitute an automatic air ratio control device. The continuous heating furnace 1 is also equipped with an oxygen concentration meter 8 and a CO concentration meter 9 in each of the combustion control zones 21 to 24. Note that if the CO concentration is not used to control the air ratio, the CO concentration meter 9 may not be provided.

[0022] The steel material (steel billet) S is transported from the charging side to the extraction side of the continuous heating furnace 1 by a transport device 11. The transport device 11 may be, for example, a walking beam.

[0023] The air ratio control unit 10 controls the adjustment valve 12 to adjust the amount of air supplied from the blower 13. The air ratio control unit 10 also performs calculations such as the furnace atmosphere temperature, the temperature of the slab, the required fuel supply amount, and the fixed heat loss and effective fuel utilization rate. The air ratio control unit 10 is configured, for example, by a calculation device, and includes a calculation circuit configured from various processors and memories, etc. The air ratio control unit 10 also includes a communication interface for connecting to external devices, an input interface such as a keyboard, a mouse, a touchpad, and a touch panel, and a display device for displaying various information to a user (such as a designer).

[0024] <Air ratio control method> (First embodiment) A method for controlling an air ratio in a continuous heating furnace according to a first embodiment of the present invention will be described with reference to FIG. 2. FIG. 2 is a flow chart showing the flow of air ratio control in a continuous heating furnace according to this embodiment. The method for controlling an air ratio in a continuous heating furnace according to this embodiment is a method for controlling an air ratio in a continuous heating furnace having multiple combustion control zones, as described above. The method for controlling an air ratio in a continuous heating furnace according to this embodiment includes a first measurement step of measuring the NOx concentration in the exhaust gas from the continuous heating furnace and comparing the NOx concentration with a preset threshold value; a second measurement step of measuring the soot concentration in the exhaust gas and comparing the soot concentration with a preset threshold value; and a third measurement step of measuring the O2 concentration in each combustion control zone and comparing the O2 concentration with a preset threshold value for each combustion control zone. When the NOx concentration or the soot concentration is equal to or greater than the respective threshold value, the air ratio in all combustion control zones is adjusted uniformly. When the O2 concentration is equal to or greater than the threshold value, the air ratio is adjusted for each combustion control zone. When the air ratio in a combustion control zone is changed, the first measurement step is performed.

[0025] [First measurement step (steps S1 and S2)] In the first measurement step, the NOx concentration of the exhaust gas from the continuous heating furnace is measured, and the measured NOx concentration is compared with a preset threshold value. The measurement location may be any location where all exhaust gas from the continuous heating furnace 1 is collected, such as the location where all exhaust gas from the chimney 5 or the exhaust system 4 is collected. Therefore, the exhaust gas from the continuous heating furnace 1 refers to a mixture of exhaust gases discharged from the combustion control zones 21-24. While there are no particular limitations on the method for measuring the NOx concentration, chemiluminescence or ultraviolet absorption methods are preferred for rapid measurement results. The NOx concentration is preferably measured using an automatic measurement system or automatic measuring instrument conforming to JIS B 7982:2002. The measured NOx concentration is sent to the air ratio control unit 10.

[0026] The air ratio control unit 10 compares the NOx concentration with a preset threshold. If the NOx concentration is less than the threshold (step S2 / YES), a second measurement step is performed. If the measured NOx concentration is equal to or greater than the threshold (step S2 / NO), a first air ratio adjustment step is performed. The threshold may be set arbitrarily, for example, so long as it is set so that even if the NOx concentration exceeds the threshold, it does not exceed the environmental regulation value set by the local government where the continuous heating furnace is installed. As an example, the threshold may be set to a value that is 60% of the environmental regulation value. In this case, since the environmental regulation value is usually set based on a uniform exhaust gas standard in terms of O2 conversion (a conversion for displaying the concentration of substances in exhaust gas under air concentration conditions of standard oxygen concentration), the measured NOx concentration is also converted to O2 using the following formula, and the converted NOx concentration is compared with the threshold. C = Cs × (21-On) / (21-Os) In the above formula, C: Converted NOx concentration (ppm) Cs: Measured NOx concentration (ppm) On: Standard oxygen concentration (%) (standard oxygen concentration when environmental regulation value is converted to O2; for example, 12%) Os: Oxygen concentration in exhaust gas (%) is.

[0027] [First air ratio adjustment step (step S3)] In the first air ratio adjustment step, the air ratio control unit 10 uniformly reduces the air ratio in each of the combustion control zones 21-24 of the continuous heating furnace 1 by a predetermined value. Most of the NOx generated in the continuous heating furnace 1 is produced by the high-temperature oxidation of N2 in the exhaust gas. Therefore, the generation of NOx can be suppressed by reducing the O2 concentration in the continuous heating furnace 1. Therefore, the amount of NOx generated can be reduced by reducing the air ratio.

[0028] The amount of air supplied to each of the combustion control zones 21 to 24 is reduced to lower the air ratio. Here, if the air ratio is changed significantly in a short period of time, the furnace environment may change suddenly, and dust may be generated suddenly. Therefore, the amount of decrease in the air ratio (predetermined value) is set to a value that does not cause a sudden change in the furnace environment (does not cause a sudden generation of dust) based on past performance (such as the furnace condition when the air ratio is changed). The amount of decrease in the air ratio may be, for example, 0.03.

[0029] By uniformly controlling the air ratio in each of the combustion control zones 21 to 24, it is possible to quickly suppress an increase in the NOx concentration before the NOx concentration exceeds the environmental regulation value. After the first air ratio adjustment step, the first measurement step is carried out.

[0030] [Second measurement step (steps S4 and S5)] In the second measurement step, the dust concentration of the exhaust gas from the continuous heating furnace is measured, and the measured dust concentration is compared with a preset threshold value. The measurement position may be any position where all of the exhaust gas from the continuous heating furnace 1 is collected, such as the position where all of the exhaust gas from the chimney 5 or the exhaust system 4 is collected. The method for measuring the dust concentration is not particularly limited, but an electrodynamic measurement method is preferable because it allows for highly sensitive analysis. The measured dust concentration value is sent to the air ratio control unit 10.

[0031] The air ratio control unit 10 compares the dust concentration with a preset threshold value. If the dust concentration is less than the threshold value (step S5 / YES), a third measurement step is performed. If the dust concentration is equal to or greater than the threshold value (step S5 / NO), a second air ratio adjustment step is performed. The threshold value may be set arbitrarily, for example, so long as it is set so that even if the threshold value is exceeded, it does not exceed the environmental regulation value set by the local government where the continuous heating furnace is installed. As an example, a value that is 60% of the environmental regulation value may be set as the threshold value. In this case, the O2 converted dust concentration is compared with the threshold value, similar to the NOx concentration.

[0032] [Second air ratio adjustment step (step S6)] In the second air ratio adjustment step, the air ratio control unit 10 uniformly increases the air ratio in each of the combustion control zones 21-24 of the continuous heating furnace 1 by a predetermined value. Soot, which accounts for a large proportion of the dust generated in the continuous heating furnace 1, tends to be generated easily in an unburned environment. Therefore, increasing the O2 concentration in the continuous heating furnace 1 can suppress the generation of dust. Therefore, increasing the air ratio can reduce the amount of dust generated.

[0033] The amount of air supplied to each of the combustion control zones 21 to 24 is increased to raise the air ratio. Here, if the air ratio is changed significantly in a short period of time, the furnace environment may change suddenly, which may result in the sudden generation of NOx. Therefore, the increase in the air ratio (predetermined value) is set based on past performance (furnace conditions when the air ratio was changed, etc.) to a value that does not cause a sudden change in the furnace environment (does not cause the sudden generation of NOx). The increase in the air ratio may be, for example, 0.03.

[0034] By uniformly controlling the air ratio in each of the combustion control zones 21 to 24, it is possible to quickly suppress an increase in the dust concentration before the dust concentration exceeds the environmental regulation value. After the second air ratio adjustment step, the steps are sequentially performed from the first measurement step. As described above, steps S1 to S6 control all of the combustion control zones in the continuous heating furnace 1 uniformly.

[0035] [Third measurement step (steps S7 to S9)] In the third measurement step, the O2 concentration in each combustion control zone 21-24 is measured and compared with a preset threshold. While the O2 concentration measurement method is not particularly limited, it is preferable to employ laser absorption spectroscopy to measure the O2 concentration across the furnace width, as this method allows for easy measurement of the average O2 concentration within the combustion control zone. Laser absorption spectroscopy is a measurement technique that measures the concentration of each component based on the amount of laser light absorbed as it passes through the gas being measured. Laser absorption spectroscopy offers superior responsiveness compared to typical suction-type O2 concentration measurements. Furthermore, while suction-type measurements reflect the O2 concentration of the gas near the furnace wall, the O2 concentration within the combustion control zones 21-24 is not uniform across the furnace width. Therefore, by employing laser absorption spectroscopy to measure across the furnace width, the average O2 concentration within each combustion control zone can be measured. The measured values ​​of the O 2 concentration in each of the combustion control zones 21 to 24 are sent to the air ratio control unit 10 .

[0036] The air ratio control unit 10 compares the O2 concentration measured for each combustion control zone 21-24 with a preset threshold value. The O2 concentration value at which heat loss is minimized is set as the target value, with a lower threshold value (lower limit threshold) and a higher threshold value (upper limit threshold) set as the target value. Here, heat loss refers to the total loss due to exhaust gas discharged from the combustion control zone and unburned gas loss. The minimum heat loss can be determined from the correlation between heat loss and O2 concentration calculated using the temperature and flow rate of exhaust gas discharged from the combustion control zone and the unburned gas concentration.

[0037] If the measured O2 concentration is equal to or lower than the lower threshold (step S8 / NO), the air ratio of the combustion control zone where the O2 concentration is equal to or lower than the lower threshold is increased (step S9). If the measured O2 concentration is higher than the lower threshold (step S8 / YES), it is determined whether the measured O2 concentration is lower than the upper threshold (step S8'). If the measured O2 concentration is equal to or higher than the upper threshold (step S8' / NO), the air ratio of the combustion control zone where the O2 concentration is equal to or higher than the upper threshold is decreased (step S10). After steps S9 and S10, the process is repeated sequentially from step S1. Furthermore, if the measured O2 concentration is less than the upper limit threshold (step S8' / YES), the series of determinations ends, but the process may be repeated sequentially from step S1.

[0038] The air ratio of each combustion control zone is adjusted by adjusting the amount of air supplied. Here, if the air ratio is changed significantly in a short period of time, the furnace environment may change suddenly, and NOx or dust may be generated suddenly. Therefore, the change amount (predetermined value) of the air ratio is set based on past performance to a value that does not cause a sudden change in the furnace environment (does not cause a sudden generation of NOx and dust). The change amount of the air ratio may be, for example, 0.02.

[0039] By adjusting the air ratio for each combustion control zone, it is possible to achieve a combustion state suitable for each combustion control zone, which has different combustion loads and furnace temperatures, that is, high thermal efficiency control. After adjusting the air ratio, the first measurement step is carried out.

[0040] The control time interval is set to a time interval long enough to allow changes in NOx concentration, dust concentration, and O2 concentration due to changes in the air ratio to become apparent. This allows heat loss to be suppressed while stably satisfying environmental regulations. If the control time interval is too short, each measurement process will be performed before the effects of the air ratio change become apparent, which may result in unstable control. Also, if the control time interval is too long, control may not be able to keep up with increases in NOx concentration or dust concentration, and the NOx concentration, dust concentration, and O2 concentration may not be able to be within the desired range. The control interval depends on the size of the continuous heating furnace, etc., but can be, for example, 6 minutes.

[0041] Second Embodiment Furthermore, in the third measurement step, it is preferable to measure the CO concentration within each combustion control zone in addition to the O2 concentration and compare it with a predetermined threshold. The CO concentration can directly determine the unburned state of fuel gas within each combustion control zone, thereby improving control accuracy compared to measuring the O2 concentration alone. There are no particular restrictions on the method for measuring the CO concentration, but it is preferable to use laser absorption spectroscopy to measure the CO concentration across the furnace width, as with measuring the O2 concentration.

[0042] If the measured CO concentration exceeds the threshold, the air ratio is increased by a predetermined value for each combustion control zone. The reason for increasing the air ratio when the CO concentration exceeds the threshold is to completely combust the CO and suppress unburned loss. The threshold can be set to the CO concentration value at the O2 concentration that minimizes heat loss, based on the correlation between the O2 concentration and CO concentration of the exhaust gas emitted from the combustion control zone.

[0043] Here, if the air ratio is increased significantly in a short period of time, the furnace environment may change suddenly, and NOx may be generated suddenly. Therefore, the increase in the air ratio (predetermined value) is set based on past performance (furnace conditions when the air ratio was changed, etc.) to a value that does not cause a sudden change in the furnace environment (does not cause a sudden generation of NOx). The increase in the air ratio may be, for example, 0.02. After adjusting the air ratio, the first measurement step is carried out.

[0044] The present invention has been described above using the present embodiment. However, the technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0045] For example, the continuous heating furnace described with reference to FIG. 1 is merely an example, and may be modified within the scope of the present technology.

[0046] Furthermore, in the above embodiment, for ease of explanation, it has been described that step S8 and step S8' are performed in this order, but this order may be reversed. [Example]

[0047] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0048] Steel materials were heated in a continuous heating furnace while controlling the air ratio. In Example 1 of the present invention, the air ratio was controlled using the method of the first embodiment described above, and in Example 2 of the present invention, the air ratio was controlled using the method of the second embodiment. In Comparative Example 1, the air ratio was controlled based solely on the NOx concentration in the exhaust system, and in Comparative Example 2, the air ratio was controlled solely on the O2 concentration in each combustion control zone. The control time in each example was 3 hours. The NOx and soot concentrations were measured in the stack exhaust system, and the measured NOx and soot concentrations were converted to O2 for comparison with threshold values ​​based on environmental regulations. The O2 and CO concentrations in each combustion control zone were measured using laser absorption spectroscopy.

[0049] The test conditions were as follows: each threshold value and the amount of increase or decrease in the air ratio. Threshold: NOx concentration: 70ppm Dust concentration: 5mg / Nm 3 O2 concentration: Lower threshold 2%, upper threshold 3% CO concentration: 2000ppm -Predetermined amount of increase or decrease in air ratio NOx concentration standard: -0.03 Dust concentration standard: +0.03 O2 concentration standard: +0.02 or -0.02 CO concentration standard: +0.02

[0050] To comprehensively evaluate the three aspects of environmental friendliness, energy efficiency, and scale loss, we measured the time that either the NOx concentration or the soot concentration exceeded the standard value (environmental regulation value x 0.6), the fuel consumption rate (relative difference from the average value), and scale loss.Scale loss was calculated based on the oxygen concentration inside the furnace.

[0051] The test was judged to be good if either the NOx concentration or the particulate matter concentration exceeded the standard value for 5 minutes or less, the reduction rate in fuel consumption was 2.0% or more, and the scale loss was 0.92% or less. The test was judged to be bad if any of the above conditions were not met. The results are shown in Table 1.

[0052] [Table 1]

[0053] It was confirmed that in Inventive Example 1, the time over the standard value was maintained at a low value of 2 minutes, while the fuel consumption rate reduction ratio was 2.0% or more, and scale loss was also low. Furthermore, Inventive Example 2 was able to further suppress soot and unburned CO compared to Inventive Example 1, so the time over the standard value was shorter and the fuel consumption rate reduction ratio was able to be further increased.

[0054] In Comparative Example 1, control was based only on NOx concentration, so the time the soot concentration exceeded the standard value was 10 minutes, the reduction rate in fuel consumption rate was small, and the scale loss was also high, resulting in a poor evaluation.

[0055] In Comparative Example 2, control was based solely on the O2 concentration in each combustion control zone, which resulted in a high reduction rate in fuel consumption and a low amount of scale loss. However, the time that the NOx concentration and soot concentration exceeded the standard values ​​was long at 40 minutes, resulting in a poor evaluation. [Explanation of symbols]

[0056] 1. Continuous heating furnace 2 Furnace body 3 burners 4. Exhaust system 5. Chimney 6 NOx concentration meter 7 Soot meter 8. Oxygen concentration meter 9 CO concentration meter 10 Air ratio control unit 21 First heating zone (combustion control zone) 22 Second heating zone (combustion control zone) 23 Third heating zone (combustion control zone) 24. Burning control zone

Claims

1. 1. A method for controlling an air ratio in a continuous heating furnace having a plurality of combustion control zones, comprising: a first measuring step of measuring a NOx concentration in exhaust gas from the continuous heating furnace and comparing the NOx concentration with a predetermined threshold value; a second measuring step of measuring a dust concentration in the exhaust gas and comparing the dust concentration with a predetermined threshold value; O within each combustion control zone ２ The concentration is measured, and the O ２ a third measuring step of comparing the concentration with a preset threshold value; When the NOx concentration or the soot concentration is equal to or greater than the respective threshold values, the air ratios of all the combustion control zones are adjusted uniformly; The above O ２ When the concentration is equal to or less than the lower threshold or equal to or more than the upper threshold, the air ratio is adjusted for each combustion control zone; The air ratio control method for a continuous heating furnace, wherein the first measuring step is carried out when the air ratio in the combustion control zone is changed.

2. In the third measurement step, a CO concentration within each combustion control zone is further measured, and the CO concentration is compared with a predetermined threshold value for each combustion control zone; 2. The method for controlling an air ratio in a continuous heating furnace according to claim 1, wherein the air ratio is adjusted for each of the combustion control zones when the CO concentration is equal to or higher than the threshold value.

3. A method for heating steel materials using a continuous heating furnace having a plurality of combustion control zones, comprising: a first measuring step of measuring a NOx concentration in exhaust gas from the continuous heating furnace and comparing the NOx concentration with a predetermined threshold value; a second measuring step of measuring a dust concentration in the exhaust gas and comparing the dust concentration with a predetermined threshold value; O within each combustion control zone ２ The concentration is measured, and the O ２ a third measuring step of comparing the concentration with a preset threshold value; When the NOx concentration or the soot concentration is equal to or greater than the respective threshold values, the air ratios of all the combustion control zones are adjusted uniformly; The above O ２ When the concentration is equal to or less than the lower threshold or equal to or more than the upper threshold, the air ratio is adjusted for each combustion control zone; A steel heating method, wherein the first measuring step is carried out when the air ratio in the combustion control zone is changed.

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