JPH07305833A - Heat-exchanger for radiant tube for heat treatment furnace, and preheating method for combustion air - Google Patents

Abstract

PURPOSE:To realize low NOX combustion and to save energy by a method wherein air preheated at the outer cylinder part of a heat-exchanger is recovered along an inner cylinder and guided to a burner part. CONSTITUTION:Exhaust gas (b) passing the outside of an outer cylinder 10 of a heat- exchanger 4 is heat-exchanged with air (a) for combustion, passing through a space between inner and outer cylinders 11 and 10, in relation of a counterflow. A number of fins 15 on the air side of the outer cylinder 10 are installed on the inner side of the outer cylinder 10 and fins 16 on the exhaust gas side of the outer cylinder 10 are also installed at an exhaust gas discharge part wherein the temperature of exhaust gas is decreased to a comparatively low value. Further, since the temperature of air (a) for combustion is increased to a maximum temperature at the tip part of the heat-exchanger 4, the inner cylinder 11 being a return pipe for the air (a) for combustion forms a double pipe of heat insulation structure, and a takeoff port for the air (a) for combustion is installed adjacently to an exhaust gas discharge port. The air (a) for combustion preheated by the heat-exchanger 4 is fed to a burner through a connection pipe 6 between the heat-exchanger 4 and the burner. This constitution increases the temperature of the air (a) for combustion to a high value and realizes low NOX combustion and saving of energy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion air preheating device for a radiant tube, which is compact and highly efficient in heat recovery and is suitable for energy saving.

[0002]

2. Description of the Related Art Conventionally, an air preheating device for combustion of a burner in a radiant tube of a heat treatment furnace such as a metal steel strip heating furnace is
As shown in FIG. 5, the heat exchanger 4 is attached to the radiant tube 1, the combustion air introduced from the inner cylinder 11 of the heat exchanger is folded back at the tip of the heat exchanger, and the outer cylinder 10 having the fins for promoting heat transfer is installed. When flowing, it was preheated by exchanging heat with the high temperature exhaust gas flowing outside the outer cylinder 10 in parallel flow, and supplied to the burner. (Unexamined-Japanese-Patent No. 63-15007) The counterflow type heat exchanger which introduces combustion air from an outer cylinder of a heat exchanger and supplies it to a combustion ignition burner through an inner cylinder also flows in a conventional heat exchanger. Some things have been realized only in the opposite direction. (Technical data of low NOx radiant tube burner with recuperator / from OTTO Co., Ltd.)

[0003]

The exhaust gas heat recovery portion of the conventional radiant tube is (a) heat recovery by a parallel flow system, and therefore there is a limit to recovery at the preheated air temperature. (B) The temperature of the combustion air cannot be raised too much from the regulated value of NOx. Due to this limitation, the combustion air has a temperature of about 400 ° C., and the exhaust gas temperature after exhaust heat recovery is 600 tens of degrees Celsius, and there is considerable room for improvement from the viewpoint of improving energy utilization efficiency. (FIG. 10) That is, in the conventional heat exchanger, the heat recovery rate of the heat exchanger was about 30% due to the limitation of the NOx value of the combustion burner, the parallel heat exchange method, and the like.

In such a radiant tube,
If it is attempted to enhance the heat recovery with combustion air, there is a limit that the temperature of the combustion air cannot exceed the exhaust gas temperature at the heat exchanger outlet because the flow inside the conventional heat exchanger is a parallel flow. (FIG. 10), the heat exchange area is increased by changing the method to the counter flow method, and the NOx is increased by self-circulation of the exhaust gas to the combustion air side in order to increase NOx due to the enhanced heat recovery.
Aim to reduce Ox. In addition, the combustion air outlet should be located closest to the burner in the middle of the heat exchanger, and the air return pipe should have a double-tube heat insulation structure to prevent the combustion air temperature from dropping and remove it from the air intake. A simple single tube is used between the parts to enhance heat exchange efficiency and enable exhaust heat recovery up to about 700 ° C. (Figure 11)

An object of the present invention is to improve the problems of a conventional air heat exchanger for combustion in a radiant tube, which has a low heat recovery rate and increases NOx when trying to increase it. A large heat transfer area is provided to increase the temperature of the supply combustion air and increase the amount of heat recovery, and low NOx
It is possible to realize energy saving by merely reworking the heat exchanger and its surroundings without significantly changing the existing heat exchanger installation part.

[0006]

The present invention takes the following measures in order to solve such problems. (1) In a heat exchanger for introducing air preheating disposed inside an exhaust gas discharge pipe of a radiant tube for a heat treatment furnace, the heat exchanger is a double tube including an outer cylinder and an inner cylinder, and the outer cylinder and the inner cylinder are The radiant tube was connected to the inside of the furnace, the combustion air supply port was provided in the outer cylinder, and the preheated combustion air outlet in the inner cylinder was connected to the connecting pipe to the fuel ignition burner of the radiant tube. Is characterized by. (2) In the heat exchanger of the radiant tube for heat treatment furnace of (1), the combustion air supply port of the outer cylinder and the combustion air extraction port of the inner cylinder are arranged adjacent to the exhaust gas discharge port of the exhaust gas discharge pipe. It is characterized by

(3) In the heat exchanger of the radiant tube for the heat treatment furnace of (1), the combustion air supply port of the outer cylinder and the exhaust gas discharge port of the exhaust gas discharge pipe are arranged adjacent to each other, and An air outlet is provided between the tip of the outer cylinder and the exhaust gas outlet. (4) In (1) to (3), a plurality of fins are provided on the outer surface of the outer cylinder of the heat exchanger on the combustion air supply port side. (5) In (1) to (4), the inner tube of the heat exchanger is made of a double tube, the furnace inner tip of the double tube is closed, and the inner side of the other end of the double tube is released. It is characterized by a structure. (6) In (1) to (5), an ejector communicating with the inner cylinder is provided at the furnace inner end of the heat exchanger outer cylinder.

(7) When preheating the combustion air in the radiant tube by using the heat exchanger consisting of the double tube of the outer cylinder and the inner cylinder, the combustion air is introduced into the outer cylinder and passed through the wall surface of the outer cylinder. It is characterized in that after the heat is exchanged with the combustion exhaust gas in the radiant tube, the combustion air is sent to the fuel ignition burner through the inner cylinder. (8) Using a heat exchanger composed of a double tube of an outer cylinder and an inner cylinder,
When preheating the combustion air of the radiant tube, the combustion air was introduced into the outer cylinder, and after heat exchange with the combustion exhaust gas in the radiant tube through the wall surface of the outer cylinder, it was installed at the furnace inner tip of the heat exchanger outer cylinder. The feature is that the exhaust gas in the radiant tube is introduced from the ejector to the inner cylinder, and the mixed gas of the combustion air and the exhaust gas is sent to the fuel ignition burner.

[0009]

The present invention will be described in detail below. Effective utilization of energy closed in the process by recovering heat of radiant tube dispersed small capacity exhaust gas in heat treatment furnace with combustion air from high temperature to low temperature without significantly changing the conventional equipment structure. However, if the temperature of the combustion air rises, the temperature of the combustion flame rises accordingly, so that the amount of NOx generated increases.

The heat exchanger portion is not a conventional parallel flow type in which air is introduced from the inner cylinder and is preheated in the outer cylinder portion, but the air preheated in the outer cylinder portion is recovered through the inner cylinder and is transferred to the burner portion. The counter flow type that guides shall be adopted. (Fig. 2) When the exhaust gas is high in temperature, many fins for heat transfer promotion are installed on the inner surface of the outer cylinder to promote heat transfer on the low temperature side of the combustion air. The heat exchanger capacity was improved by extending the exchanger part outside the furnace to increase the heat transfer area and installing fins in the low temperature part on the exhaust gas side of the outer cylinder. (Figure 3)

The total heat transfer rate K is approximately 1 / K≈
1 / α + 1 / β (α: exhaust gas side heat transfer coefficient, β: combustion air side heat transfer coefficient), and α = α _r + α _c (α _r : heat transfer coefficient by radiation, α _c : heat transfer coefficient by convection) ), In FIG.
As shown in (4), if the heat recovery is enhanced, the exhaust gas outlet temperature will decrease and α _r will decrease significantly. Therefore, on the outside of the outer cylinder (exhaust gas side), the heat transfer coefficient α _r due to radiation is greatly reduced and the heat transfer coefficient K is lowered, so the heat transfer coefficient α _c due to convection.
It is effective to determine the number, length, and height of the fins so that the number of fins increases.

Here, the combustion air outlet is arranged adjacent to the exhaust gas outlet, and the thermal expansion of the outer cylinder and the inner cylinder is released to the inside of the furnace in the radiant tube (FIG. 1) and the combustion air outlet. Is the part closest to the fuel ignition burner in the middle part between the tip of the heat exchanger and the air intake port, and the heat expansion of the outer cylinder between the combustion air intake port and the extraction port is released to the combustion air intake port side ( 5). Moreover, by making the inner cylinder a double tube, heat exchange between the combustion air is prevented, and a decrease in the air temperature that has reached the maximum temperature at the tip of the heat exchanger is prevented. (FIG. 8) That is, an air heat-insulating layer is generated on the wall surface of the inner cylinder to prevent convective heat transfer, and by using a double tube, radiant heat transfer is also prevented and the combustion air temperature is maintained.

Further, a suction port of about 15 mmφ is provided at the tip portion of the heat exchanger, and a high temperature of about 20% (about 1000 ° C.) with respect to the amount of combustion air introduced by the ejector effect.
The exhaust gas self-circulates to the combustion air side. (FIG. 2) That is, in the ejector 12, the flow velocity on the combustion air side is increased and the static pressure at that portion is decreased to draw in the exhaust gas. On the combustion air side, total pressure (Pa) = dynamic pressure (Pa)
d) + static pressure (Pas), and the exhaust gas side similarly has total pressure (Pg) = dynamic pressure (Pgd) + static pressure (Pgs). Normally, the static pressure on the combustion air side is higher than the static pressure on the exhaust gas side (P
as> Pgs), Pas is converted into dynamic pressure by increasing the flow velocity V of the ejector section combustion air, and Pas = Pa
-(Ρ _a V ² / 2G) <Pgs [ρ _a : combustion air density, G: gravitational acceleration], and the exhaust gas dynamic pressure ρ _g v corresponding to this static pressure difference (Pgs-Pas) ²
/ 2G [ρ _g : exhaust gas density, v: ejector part exhaust gas flow velocity] is generated in the ejector part, and exhaust gas circulation of approximately v × (exhaust gas introduction nozzle cross-sectional area) is generated.
Further, by installing an ejector at the most advanced portion of the heat exchanger, the exhaust gas dynamic pressure (Pgd) in the radiant tube contributes to the circulation though it is small. (Figure 4)

Therefore, the exhaust gas introducing nozzle having a hole diameter for narrowing the flow passage so as to increase the flow velocity at the folded portion of the tip end portion of the heat exchanger on the side of the combustion air and ensuring a necessary exhaust gas circulation amount therein. By providing, the exhaust gas circulates to the combustion air side in the ejector section 12 in accordance with the difference between the static pressure on the air side and the total pressure on the exhaust gas side. By doing so, the dynamic pressure on the exhaust gas side can be effectively utilized, and it is not necessary to install a fan for circulating the exhaust gas, and O ₂ is about 18 when the exhaust gas circulation amount is 20%.
%, NOx is about 3 compared to the case where exhaust gas circulation is not performed.
It can be reduced by 0% and the allowable generation value becomes 120 ppm or less. The greater the circulation amount, the greater the NOx suppressing effect, but if it is too large, combustion becomes unstable, so 20 to 30% is the limit for the combustion air amount. [Firefield Association Lecture Boiler / Thermal Nuclear Power Generation Technology Association]

That is, the gist of the present invention is that in a radiant tube exhaust gas heat exchanger, the flow directions of the high-temperature fluid (exhaust gas) and the low-temperature fluid (combustion air) are counter-current, and the logarithmic mean temperature difference is used. To increase the amount of heat recovery, increase the length of the heat exchanger, strengthen the fins in the low temperature part outside the outer cylinder, and increase the exhaust heat recovery rate to about 50% to save energy. . To increase NOx due to enhanced heat recovery, the O ₂ concentration of the combustion air is reduced by the self-circulation of exhaust gas to the combustion air side at the tip of the heat exchanger, and the combustion speed of the flame is delayed to generate NOx. This is an exhaust heat recovery enhanced heat exchange combustion method characterized in that the amount is reduced.

[0016]

【Example】

Embodiment 1 FIG. 1 is an embodiment of the present invention, in which a radiant tube 1 includes a fuel supply pipe 3, an air supply pipe 5, a fuel ignition burner 2 and an exhaust gas discharge pipe 13. Here, the combustion air is the heat exchanger 4 installed in the exhaust gas discharge part of the radiant tube.
Is preheated by the heat exchanger and is guided to the fuel ignition burner 2 through the connecting pipe 6 between the heat exchanger portion and the burner portion. For the radiant tube 1, coke oven gas (COG) or natural gas (LNG), which is an iron-made by-product gas, is used, which is burned with preheated combustion air in the burner section, and outside the tube while passing through the radiant tube. Radiation from the surface gives heat to steel materials. Steel to be heated (cold-rolled steel plate, etc.) is continuously carried into the heat treatment furnace 14 from the steel plate charging port, heated to about 800 ° C. in the furnace, and taken out from the steel product carrying port. The temperature in the furnace is about 900 ° C, and the temperature of the exhaust gas b on the heat exchanger entrance side also rises to about 1000 ° C.
The exhaust gas b is made to face the combustion air a, heat-exchanged by the heat exchanger 4, guided to the fuel ignition burner 2, and burned with the fuel c. At this time, the combustion air a is 70% as shown in FIG.
Energy is saved by the amount preheated to 0 ° C., but NOx accompanying combustion increases.

Therefore, the combustion air a is heat-exchanged with the exhaust gas b by the heat exchanger 4 to be preheated, and a part of the exhaust gas b is circulated to the combustion air side through the ejector 12 at the tip of the heat exchanger. In other words, if a high-efficiency ejector that also uses the dynamic pressure of exhaust gas is used, the amount of exhaust gas mixed can be efficiently increased, the O ₂ concentration of combustion air can be reduced to 18%, and the combustion speed in the fuel ignition burner 2 can be reduced. Low NO
x burning becomes possible. By optimally designing the hole diameter of the ejector part as shown in Fig. 4 (exhaust gas introduction nozzle diameter: about 15φ, inner cylinder throttle part diameter: about 35φ), the exhaust gas circulation amount is about 20%, and exhaust gas circulation is performed. A NOx reduction of about 30% was achieved compared to when it was not.

FIG. 2 shows a heat exchanger with an exhaust gas self-circulation function which enhances the heat recovery. The exhaust gas b passing outside the outer cylinder 10 of the heat exchanger 4 exchanges heat with the combustion air a passing between the inner cylinder 11 and the outer cylinder 10 in a counterflow relationship.
A large number of air-side fins 15 of the outer cylinder are installed inside the outer cylinder as usual. Since the heat exchanger 4 is extended to the outside of the heat treatment furnace 14 this time, the exhaust gas side fins 16 of the outer cylinder are also installed in the exhaust gas discharge portion where the exhaust gas has a relatively low temperature. (FIGS. 2 and 3) Further, since the combustion air a has the highest temperature at the tip of the heat exchanger 4, the inner cylinder 11 serving as the combustion air return pipe is replaced with a double pipe having a heat insulating structure as shown in FIG. The combustion air outlet is installed adjacent to the exhaust gas outlet, and is supplied to the fuel ignition burner 2 through the connecting pipe 6. (FIG. 1) As a circulation amount of exhaust gas as an ejector effect, a circulation amount of about 20% was stably obtained even if the combustion capacity was changed (1/4 to 4/4 load). The circulation amount of the exhaust gas was calculated by measuring the O ₂ concentration of the combustion air heat-exchanged by the heat exchanger 4.

Example 2 Under the exhaust gas conditions of Example 1, as shown in FIG. 5, heated combustion air was supplied to the fuel ignition burner 2 in the shortest time to reduce the heat radiation loss in the heat exchanger and the connecting pipe 6. Can be minimized. Further, when this high efficiency exchanger is applied to the existing equipment, there is an advantage that the existing connecting pipe 6 can be used as it is. However, the thermal expansion of the outer cylinder between the combustion air intake port and the discharge port will be escaped by having the outer cylinder seal part by the O-ring in FIG. 6, but the heat recovery effect is improved, so that the heat exchanger When the lengths are the same, the temperature drop due to heat radiation can be prevented by about 50 ° C., and the heat exchanger for obtaining the combustion air at 700 ° C. becomes more compact than that of the first embodiment. At this time, a heat insulating structure with a double pipe is adopted for the inner cylinder 11 which is also a return pipe for the combustion air which has been shortened. Further, in order to reduce NO _X , if an ejector is provided as shown in FIG. 2 and exhaust gas is circulated, NO _X can be reduced.

[0020]

As described above, according to the present invention, exhaust gas self-circulation is performed in the heat exchanger portion and the O ₂ concentration can be controlled, so that low NOx combustion in the combustion burner can be realized. Therefore, a means for suppressing the generation of NOx can be provided, so that the heat of the high temperature exhaust gas can be recovered to the low temperature and the temperature of the combustion air can be increased, and effective energy saving can be achieved. Further, if the energy saving is enhanced in this way, the amount of fuel and combustion air is reduced, and further, the air dilution currently being performed to lower the exhaust gas temperature is eliminated, and the exhaust gas temperature and flow rate of the exhaust system are reduced. Can be realized, exhaust gas blower 9
It is possible to reduce the load and reduce the size of the heat treatment furnace piping system. By self-circulating the exhaust gas, the combustion in the burner section becomes slow, the radiant tube surface temperature distribution is averaged, and the radiant tube life is extended.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of the present invention (combustion air outlet and exhaust gas outlet are at the same position)

2 is a side view of the heat recovery enhanced heat exchanger of FIG.

FIG. 3 is a cross-sectional view of a heat recovery enhanced heat exchanger,

FIG. 4 is an enlarged view of the ejector section,

FIG. 5 is an overall configuration diagram showing an embodiment of the present invention (combustion air outlet is located directly below a fuel ignition burner),

FIG. 6 is a side view of a heat exchanger that takes out combustion air from a heat exchange intermediate portion,

FIG. 7 is a cross-sectional view of the heat exchanger of FIG.

FIG. 8 is a diagram showing a double pipe structure of an inner cylinder;

FIG. 9 is a side view of a conventional heat exchanger,

FIG. 10 shows an example of a temperature pattern of a conventional heat exchanger,

FIG. 11 is an example of a temperature pattern of a heat recovery enhanced heat exchanger.

[Explanation of symbols]

 1 radiant tube 2 fuel ignition burner 3 fuel supply pipe 4 heat exchanger 5 air supply pipe 6 connecting pipe 7 furnace wall 8 combustion air blower 9 exhaust gas blower 10 outer cylinder 11 inner cylinder 12 ejector 13 exhaust gas discharge pipe 14 heat treatment furnace 15 outside Cylinder air side fin 16 Outer cylinder exhaust gas side fin 17 Inner cylinder throttling part 18 Exhaust gas introduction nozzle 19 Outer cylinder seal part 20 Inner cylinder heat insulation cylindrical pipe a Combustion air b Exhaust gas c Fuel gas Pa Combustion air Total pressure Pad Combustion air dynamic pressure Pas Combustion air static pressure Pg Exhaust gas total pressure Pgd Exhaust gas dynamic pressure Pgs Exhaust gas static pressure V Ejector combustion air velocity v Ejector exhaust gas velocity

Claims (8)

[Claims] 1. A heat exchanger for preheating introduced air, which is arranged inside an exhaust gas discharge pipe of a radiant tube for a heat treatment furnace, wherein the heat exchanger is a double pipe consisting of an outer cylinder and an inner cylinder, and the outer cylinder and the inner cylinder. Is connected to the inside of the furnace of the radiant tube, the combustion air supply port is provided in the outer cylinder, and the preheated combustion air outlet in the inner cylinder is connected to the connecting pipe to the fuel ignition burner of the radiant tube. A heat exchanger in a radiant tube for a heat treatment furnace, characterized in that 2. The heat exchanger for a radiant tube for a heat treatment furnace according to claim 1, wherein the combustion air supply port of the outer cylinder and the combustion air extraction port of the inner cylinder are adjacent to the exhaust gas discharge port of the exhaust gas discharge pipe. A heat exchanger in a radiant tube for a heat treatment furnace, characterized in that 3. The heat exchanger for a radiant tube for a heat treatment furnace according to claim 1, wherein the combustion air supply port of the outer cylinder and the exhaust gas discharge port of the exhaust gas discharge pipe are arranged adjacent to each other, and the combustion of the inner cylinder is performed. A heat exchanger in a radiant tube for a heat treatment furnace, wherein an air outlet is provided between the tip of the outer cylinder and the exhaust gas outlet. 4. The radiant tube for a heat treatment furnace according to claim 1, wherein a plurality of fins are provided on the outer surface of the outer cylinder of the heat exchanger on the combustion air supply port side. Heat exchanger. 5. The inner tube of the heat exchanger comprises a double tube,
The heat exchange in the radiant tube for a heat treatment furnace according to any one of claims 1 to 4, wherein the furnace inner end of the heavy pipe is closed and the inner end of the other end of the double pipe is opened. vessel. 6. An ejector communicating with the inner cylinder is provided at the furnace inner end of the outer cylinder of the heat exchanger.
A heat exchanger in a radiant tube for a heat treatment furnace according to any one of 1. 7. When preheating the combustion air in the radiant tube by using a heat exchanger composed of a double tube of an outer cylinder and an inner cylinder, the combustion air is introduced into the outer cylinder and the radiant is passed through the wall surface of the outer cylinder. A method for preheating combustion air in a radiant tube for a heat treatment furnace, characterized by sending combustion air to a fuel ignition burner through an inner cylinder after heat exchange with combustion exhaust gas in the tube. 8. When preheating the combustion air in the radiant tube by using a heat exchanger composed of a double tube of an outer cylinder and an inner cylinder, the combustion air is introduced into the outer cylinder and the radiant is passed through the wall surface of the outer cylinder. After exchanging heat with the combustion exhaust gas in the tube, the exhaust gas in the radiant tube is introduced into the inner cylinder from the ejector provided at the furnace inner tip of the heat exchanger outer cylinder, and the mixed gas of combustion air and exhaust gas is used as the fuel ignition burner. A method for preheating air for combustion in a radiant tube for a heat treatment furnace, characterized by feeding.