JPH0828810A - Radiant tube type heating device and its heating method - Google Patents

Abstract

PURPOSE:To contrive to elongate a life by unifying temperature distribution in a length direction of a radiant tube. CONSTITUTION:A combustion in shortage of air is executed in a burner 3 of a radiant tube, a secondary air is fed from a nozzle 7 set halfway moide the radiant tube along a flow of an exhaust gas, and unburnt fuel is burned. By this, an elongation of a life of the radiant tube and a unification of temperature of a material to be heated, and a decrease of NOx in the exhaust gas can be contrived.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiant tube type heating device and a heating method for lengthening the temperature distribution of a radiant tube in a radiant tube type heat treatment furnace so as to extend its life.

[0002]

2. Description of the Related Art Atmospheric heat treatment furnaces that produce continuous galvanized steel sheets, electromagnetic steel sheets, general cold rolled steel sheets, and heat treatment while adjusting the atmosphere used for the production of thick steel sheets and steel pipes are installed at the end of the radiant tube. A heating device using a radiant tube that supplies fuel and air to the burner to generate high-temperature combustion gas and transfers the heat of the high-temperature combustion gas to an object to be heated in a furnace is generally used.

As shown in FIG. 6, the general structure of the radiant tube is as shown in FIG.
There is a U-shape 62 in (b) and a W-shape 63 in (c), and in any radiant tube, a burner is attached to one end of the opening, and fuel and air are supplied to this burner. , The radiant tube burns to give heat to the object to be heated, and the exhaust gas is discharged from the other opening of the radiant tube.

As shown in FIG. 7, the tube surface temperature distribution in the longitudinal direction of the U-shaped radiant tube at three locations on one side and at six locations (a to f) in total is shown in FIG. 7 when the flame of the burner (hereinafter referred to as a frame) is b. In the vicinity of the point, the long frame exhibits a mountain-shaped temperature distribution having a peak temperature near the point e. Radiant tube life is highly dependent on this peak temperature and temperature distribution. The higher the peak temperature, the lower the creep rupture strength and the shorter the life of the radiant tube. Further, if the temperature distribution in the length direction becomes steep, the thermal stress increases and the life becomes short. For this reason, in order to extend the life of the radiant tube, a technique for eliminating the peak and flattening the temperature distribution in the length direction has been studied. To flatten the temperature distribution, there is a method to lower the peak temperature value by shifting the peak point backward by making the burner frame a long frame.
As shown in FIG. 7, the actual situation is that the decrease in peak temperature is small.

As another method, a plurality of nozzles are arranged so as to surround the base of the radiant tube, and atmospheric gas is sprayed from the plurality of nozzles to a locally high temperature portion of the radiant tube to cool the radiant tube. A method for uniformly heating the temperature distribution in the lengthwise direction of the tube (JP-A-2-4927), in which a plurality of nozzles or cylindrical slits are arranged so as to surround at least the base of a radiant tube having fins near the base. Then, the atmosphere gas is blown from the nozzle to the locally high temperature part of the radiant tube to cool it, and the temperature distribution in the longitudinal direction of the radiant tube is made uniform, thereby uniformly heating the steel strip (JP-A-3- No. 87321), excess air is burned by a burner at the inlet of the radiant cheve, From a fuel nozzle provided in, blow the fuel-free method (JP-A-5-248605) have been proposed in the tube along the flow of the exhaust gas flowing in the tube.

[0006]

[Patent Document 1] Japanese Patent Application Laid-Open No. 2-492
The methods disclosed in Japanese Patent Laid-Open No. 7-83321 and Japanese Patent Laid-Open No. 3-87321 all can make the temperature distribution of radiant cheves uniform, but use a large amount of atmospheric gas that was previously unnecessary for cooling the peak temperature portion. Since it is used, the energy consumption rate deteriorates, which is not only uneconomical, but also has the drawback that the device becomes complicated and expensive. Further, the method disclosed in JP-A-5-248605 is a two-stage combustion method in which a fuel injection nozzle is inserted into the radiant tube from outside the furnace, and an opening must be provided in the furnace wall and the radiant tube. The sealing property of the wall portion and the radiant tube opening becomes a big problem, and there is a concern about invasion of the atmosphere and exhaust gas into the furnace, and there is also a problem of escape of thermal expansion of the fuel injection nozzle.

An object of the present invention is to easily prevent local high temperature portions from being generated in the radiant tube, to make the temperature distribution in the longitudinal direction of the radiant tube uniform, to prolong the life of the radiant tube, and to heat the object to be heated. It is an object of the present invention to provide a radiant tube type heating device and a heating method for making the temperature uniform and reducing NOx in exhaust gas.

[0008]

[Means for Solving the Problems] The present inventors have conducted various test studies in order to achieve the above object. As a result, the burners of the radiant cheves burned due to lack of air, and the secondary combustion air was supplied from the nozzles that were provided along the flow of the exhaust gas up to the middle of the radiant cheves, and the unburned components due to the lack of air in the burners were radiant cheves. By burning up to the outlet of, it is possible to make the temperature distribution in the length direction of the radiant cheve uniform without lowering the combustion efficiency, it is confirmed that the peak temperature can be lowered and the NOx of the exhaust gas can be reduced, This invention was reached.

That is, the first invention of the present application is characterized in that an air blowing nozzle is provided along a flow of exhaust gas from a burner portion provided at an inlet of the radiant tube to a midpoint of the radiant tube. Is.

In the second invention of the present application, the burner of the radiant tube burns due to lack of air, and the secondary combustion air is blown from the nozzle provided along the flow of the exhaust gas up to the middle of the radiant tube, and the burner is not burned. A heating method in a radiant tube type heat treatment furnace, which is characterized in that a fuel of a fuel content is burned.

Further, according to the third invention of the present application, the radiant tube is burned at an air ratio of 0.5 to 1.0, and secondary combustion is performed from a nozzle provided along the flow of the exhaust gas up to the middle of the radiant tube. It is a heating method in a radiant tube heat treatment furnace, characterized in that the working air is blown into the burner so that the total air ratio becomes 1.05 to 1.3 to burn the unburned fuel.

[0012]

In the first invention of the present application, the air blowing nozzle is provided along the flow of the exhaust gas from the burner portion provided at the inlet of the radiant tube to the middle of the radiant tube.
Secondary combustion air can be blown in, and the burner can burn due to lack of air, the local high temperature part in front of the burner is eliminated, and unburned components in the exhaust gas can be burned to the outlet of the radiant tube, which improves combustion efficiency. The temperature distribution in the length direction of the radiant tube can be made uniform without decreasing the temperature.

In the second invention of the present application, the burner of the radiant tube burns due to lack of air, the secondary combustion air is blown from the nozzle provided along the flow of the exhaust gas up to the middle of the radiant tube, and unburned in the burner. By burning a portion of the fuel, the local high temperature part in front of the burner is eliminated, and the temperature distribution in the length direction of the radiant tube can be made uniform without lowering the combustion efficiency, and the peak temperature is lowered. Therefore, low NOx is also possible. Further, by suppressing the peak temperature, the creep rupture strength is increased and the life of the radiant tube can be extended.

In the third invention of the present application, the radiant tube is burned at an air ratio in the burner of 0.5 to less than 1.0, and secondary combustion is performed from a nozzle provided along the flow of the exhaust gas up to the middle of the radiant tube. By blowing the working air so that the total air ratio becomes 1.05 to 1.3 and burning the unburned fuel in the burner, the local high temperature part in front of the burner is eliminated and the combustion efficiency is reduced. Without doing so, the temperature distribution in the length direction of the radiant tube can be made uniform, the peak temperature can be lowered, and low NOx can be achieved. Further, by suppressing the peak temperature, the creep rupture strength is increased and the life of the radiant tube can be extended.

In the present invention, the air ratio in the burner is preferably from 0.5 at which combustion in the burner is stable to less than 1.0 at which unburned materials remain. Further, it is preferable that the air ratio is 1.05 to 1.3 inclusive of the amount of secondary combustion air from the secondary combustion air blowing nozzle and the amount of air from the burner. When the air ratio exceeds 1.3, the gas temperature inside the radiant tube becomes low, and the amount of heat generated from the surface of the radiant tube becomes small. As the fuel used in the present invention, coke oven gas, blast furnace gas or a mixed gas of coke oven gas and blast furnace gas, propane gas, liquefied natural gas, heavy oil and the like can be used. The present invention is also applicable to the case where the input heat amount changes with time, and in this case also, the air ratio in the burner is set to 0.5 to less than 1.0, and the secondary combustion air blowing nozzle is used. The secondary combustion air is blown in, and the total air ratio is set to 1.05 to 1.3 by the sum of the air amount in the burner and the blown air amount. Further, the air injection position for secondary combustion is in the range from the front and rear of the middle part in the straight pipe type radiant tube, and from the first straight pipe part to the rear end of the first curved pipe part in the U-shaped radiant tube and the W-shaped radiant tube. If it is good.

The inner diameter of the air blowing nozzle is preferably 5% or more of the inner diameter of the radiant tube. If it is smaller than this, the required amount of air does not flow. The outer diameter of the air blowing nozzle is preferably 30% or less of the inner diameter of the radiant tube. If it is larger than this, combustion in the burner is hindered and the surface temperature of the radiant tube does not reach the target temperature. The material of the secondary combustion air blowing nozzle may be SCH24, heat resistant cast steel, refractory, silicon carbide, silicon nitride, or the like.

[0017]

【Example】

Embodiment 1 Details of the present invention will be described below with reference to FIGS. 1 and 2 showing an embodiment. FIG. 1 shows a horizontal cross-sectional view of a U-shaped radiant tube, FIG. 2 shows a case of another U-shaped radiant tube, and FIG. 1 (a) shows a W-shaped radiant tube. The horizontal sectional view and (b) are horizontal sectional views in the case of being applied to a straight type radiant tube. In FIG. 1, 1 is a U-shaped radiant tube installed through a furnace wall 2 of the heating furnace, 3 is a burner of the U-shaped radiant tube 1, and a combustion air supply pipe 4 is provided on a side portion of the burner 3. A fuel blow-in pipe 5 is provided in front of the burner 3, and the fuel blown from the fuel blow-in pipe 5 is burned by the combustion air supplied from the combustion air supply pipe 4. Reference numeral 6 denotes a secondary combustion air supply pipe provided in the vicinity of the fuel injection pipe 5 of the burner unit 3. The secondary combustion air supply pipe 6 penetrates the burner 3 and extends to the middle of the U-shaped radiant tube 1 along the flow of the exhaust gas. The secondary combustion air is blown from a nozzle 7 opened in the middle of the radiant tube 1.

In FIG. 2A, 11 is a W-shaped radiant tube installed through the furnace wall 12 of the heating furnace, 1
Reference numeral 3 denotes a burner of the W-shaped radiant tube 11, the combustion air supply pipe 14 is provided on the side of the burner 13, and the fuel blowing pipe 15 is provided in front of the burner portion 13 to burn the fuel blown from the fuel blowing pipe 15. It is configured to be burned by the combustion air supplied from the commercial air supply pipe 14. Reference numeral 16 denotes a secondary combustion air supply pipe provided near the fuel injection pipe 15 of the burner 13. The secondary combustion air supply pipe 16 penetrates the burner 13 and extends along the flow of the exhaust gas to the middle of the W-shaped radiant tube 11. The secondary combustion air is blown from a nozzle 17 opened in the middle of the tube 11.

In FIG. 2B, 21 is a straight radiant tube installed through the furnace wall 22 of the heating furnace, and 23 is a burner of the straight radiant tube 21, which supplies combustion air to the side of the burner 23. The pipe 24 is provided with a fuel blowing pipe 25 in front of the burner 23, and is configured to burn the fuel blown from the fuel blowing pipe 25 with the combustion air supplied from the combustion air supply pipe 24. 26 is an air supply pipe for secondary combustion provided near the fuel injection pipe 25 of the burner 23.
Is extended to the middle of the straight radiant tube 21 along the flow of the exhaust gas, and the secondary combustion air is blown from the nozzle 27 opened in the middle of the straight radiant tube 21.

By configuring as described above, each burner 3, 13, of each radiant tube 1, 11, 21
In No. 23, a rated amount of fuel is injected from each of the fuel injection pipes 5, 15, 25, and the air ratio in each of the burners 3, 13, 23 from each of the combustion air supply pipes 4, 14, 24 is 0.5 to 1.0. Nozzle 7, in which the secondary combustion air is supplied to the secondary combustion air supply pipes 6, 16, 26 from the respective secondary combustion air supply pipes 6, 16, 26 in the middle of each radiant tube 1, 11, 21 From 17, 27, the total air ratio is 1.05
It is possible to supply 1.3 so that the unburned fuel is burned by each burner 3, 13, 23. By performing two-stage combustion in each radiant tube 1, 11, 21 as described above, the surface temperature distribution in the longitudinal direction of each radiant tube 1, 11, 21 is made uniform, and the peak temperature of the local high temperature is also compared to the conventional one. It can be lowered by about 30 ° C. Further, the NOx value in the exhaust gas at the outlets of the radiant tubes 1, 11 and 21 was also reduced by about 20%, and unburned substances were not confirmed.

Example 2 The outer diameter shown in FIG. 1 is 173 mm, the wall thickness is 8 mm, and the total length is 3,8.
Using a 60 mm U-shaped radiant tube, a coke oven gas (hereinafter referred to as COG) having a calorific value of 4,600 Kcal / Nm ³ was used as fuel, and the theoretical air amount was 4.71 Nm ³ / N.
When burned at m ³ , burner rating is 55,000
It is 0 Kcal / H (COG: about 12 Nm ³ / H).
In the burner, a rated amount of fuel is injected, and the air amount is supplied at an air ratio of 0.9 Nm ³ / H, which is 0.9 Nm ³ / H. Combustion due to insufficient air suppresses the generation of a local high temperature part in front of the burner.
To the residual fuel in the exhaust gas burned due to the lack of air, an air blowing nozzle having an inner diameter of 21.6 mm provided along the exhaust gas flow immediately before the first curved pipe portion of the U-shaped radiant tube was used to generate 11.3 Nm ³ / H. The secondary combustion air was blown in to burn. At this time, the air ratio of the air in the burner and the air from the air blowing nozzle provided in the middle of the radiant tube was set to 1.1. The surface temperature distribution in the length direction of the radiant tube in this case was measured. The result is shown in FIG. In addition, a to f of the radiant tube length direction position correspond to a to f positions of FIG. As shown in FIG. 3, the surface temperature distribution in the length direction of the radiant tube was made uniform, and the local high temperature value could be reduced by about 30 ° C. from the conventional value. Further, the NOx value at the radiant tube outlet was also reduced by about 20%, and no unburned matter was confirmed. Furthermore, when the present invention is applied to a radiant tube of a continuous annealing furnace, it has become possible to extend the life of the radiant tube from 3 years to 4 years. Further, although the air blowing nozzle for secondary combustion is exposed to high-temperature exhaust gas, there is no problem such as damage because the relatively low temperature secondary combustion air flows inside and is cooled.

Example 3 Further, using the above U-shaped radiant tube, the burner was charged with a rated amount of fuel, and the air ratio in the burner was 0.4.
0.5, 0.9, 1.0, and secondary combustion air was blown from the secondary combustion air blowing nozzle so that the total air ratio was 1.1, and burned, and the furnace temperature was set to 900 ° C. In this case, the surface temperature distribution in the length direction of the radiant tube was measured. The result is shown in FIG. 4 in comparison with the conventional example in which the air ratio in the burner was 1.1. In FIG. 4, a is an air ratio of 0.4 in the burner, b is an air ratio of 0.5 in the burner,
c is an air ratio of 0.9 in the burner, d is an air ratio of 1.0 in the burner, and x is a conventional example. Further, a to f in the radiant tube length direction position correspond to a to f positions in FIG. As shown in FIG. 4, by setting the air ratio in the burner to 0.5 to less than 1.0 and the total air ratio to 1.1, the peak temperature can be lowered as compared with the conventional example, and the length of the radiant tube can be reduced. The surface temperature distribution in the depth direction can be made uniform. However, the air ratio at the burner is 0.
In the case of 4a, the peak temperature is higher than that in the conventional example, and the difference in the surface temperature distribution in the length direction of the radiant tube is large.

Example 4 Furthermore, using the above U-shaped radiant tube, a rated amount of fuel was injected into the burner, the air ratio in the burner was kept constant at 0.9, and the total air ratio was supplied from the secondary combustion air blowing nozzle. The secondary combustion air was blown into the furnace so that the values were 1.1, 1.3, and 1.4 to perform combustion, and the surface temperature distribution in the length direction of the radiant tube when the furnace temperature was 900 ° C. was measured. The result is shown in FIG. 5 in comparison with the conventional example in which the air ratio in the burner was 1.1. In FIG. 5, e is a total air ratio of 1.1, f is a total air ratio of 1.3, g is a total air ratio of 1.4, and x is a conventional example. Further, a to f in the radiant tube length direction position correspond to a to f positions in FIG. As shown in FIG. 5, in the case of e and f in which the total air ratio is 1.1 and 1.3, the peak temperature can be lowered as compared with the conventional example, and the surface temperature distribution in the length direction of the radiant tube is It has been made uniform.
However, when the total air ratio is 1.4 g, the surface temperature distribution in the length direction of the radiant tube can be made uniform, but the radiant tube surface temperature is low as a whole.

[0024]

As described above, according to the present invention,
It is possible to prevent the local high temperature part of the radiant tube from occurring and to make the temperature distribution in the length direction uniform, extend the life of the radiant tube, make the temperature of the heated object uniform and reduce NOx in the exhaust gas. it can.

[Brief description of drawings]

FIG. 1 is a horizontal cross-sectional view of a U-shaped radiant tube.

2A and 2B show a case where the present invention is applied to another type of radiant tube, in which FIG. 2A is a horizontal sectional view when applied to a W-shaped radiant tube, and FIG. 2B is a straight tube type radiant tube. It is a horizontal sectional view at the time of doing.

FIG. 3 is a graph showing the relationship between the position in the U-shaped radiant tube length direction and the tube surface temperature in Example 2.

FIG. 4 is a graph showing the relationship between burner air ratio, U-shaped radiant tube lengthwise position, and tube surface temperature in Example 3;

FIG. 5 is a graph showing the relationship between the total air ratio, the U-shaped radiant tube lengthwise position, and the tube surface temperature in Example 4.

FIG. 6 shows a model of a radiant tube,
(A) is a straight pipe type, (b) is a U-shaped, and (c) is a W-shaped plan.

FIG. 7 is a graph showing a relationship between a U-shaped radiant tube lengthwise position and a tube surface temperature in the case of a short frame and a long frame according to a conventional combustion method.

[Explanation of symbols]

 1,62 U-shaped radiant tube 2,12,22 Furnace wall 3,13,23 Burner 4,14,24 Combustion air supply pipe 5,15,25 Fuel injection pipe 6,16,26 Secondary combustion air supply Tubes 7, 17, 27 Nozzles 11, 63 W-shaped radiant tubes 21, 61 Straight radiant tubes

Claims (3)

[Claims] 1. A radiant tube type heating device comprising an air blowing nozzle provided along a flow of exhaust gas from a burner portion provided at an inlet of the radiant tube to a midpoint of the radiant tube. 2. A burner of a radiant tube burns due to lack of air, and secondary combustion air is blown from a nozzle provided along the flow of exhaust gas up to the middle of the radiant tube to burn unburned fuel in the burner. A heating method in a radiant tube type heat treatment furnace characterized by the above. 3. A burner of a radiant tube is burned at an air ratio of 0.5 to less than 1.0, and secondary combustion air is totally discharged from a nozzle provided along the flow of exhaust gas up to the middle of the radiant tube. The air ratio is 1.05-1.
A method for heating in a radiant tube type heat treatment furnace, characterized in that the unburned fuel in the burner is burned by being blown into the burner so that the amount becomes 3.