JP7739792B2 - Radiant tube heating device - Google Patents

Description

This invention relates to a radiant tube heating device, specifically characterized by a structure that reduces nitrogen oxides produced in combustion gases.

A radiant tube heating device is suitable for use, for example, when heating a heat treatment furnace or the like while keeping the atmosphere clean.It is equipped with a cylindrical radiant tube and a combustion burner attached to one end of the radiant tube, and the fuel gas and combustion air ejected from the combustion burner are combusted in the combustion area within the radiant tube.
Radiant tube heating devices are required to efficiently raise the ambient temperature in heat treatment furnaces, etc., and also to reduce the amount of nitrogen oxides (NOx), such as nitric oxide and nitrogen dioxide, which are generated during combustion and are harmful to the environment.

Japanese Unexamined Patent Publication No. 64-46516 Japanese Patent Application Laid-Open No. 2016-205644

NOx (thermal NOx) is produced when nitrogen and oxygen contained in combustion air react at high temperatures. Suppressing local temperature increases during combustion and reducing the oxygen concentration in the combustion area are considered effective ways to reduce NOx. From this perspective, related technologies for reducing NOx include those described in the above-mentioned patent documents.
Patent Document 1 discloses a two-stage combustion burner in which a small cylindrical secondary air nozzle is provided in the center of the burner tip (burner head) that is inserted into a radiant tube, and multiple gas nozzles and primary air nozzles are further provided on the same circumference concentric with this secondary air nozzle.

Patent Document 2 also discloses that a cylindrical temperature rise suppressor is placed around the tip opening of the combustion burner inserted into the radiant tube, and a flow path formed between the inner wall surface of the radiant tube and the outer surface of the temperature rise suppressor is used to recirculate a portion of the burned gas, thereby reducing the oxygen concentration in the combustion area.

The present invention was developed in light of the above circumstances, and aims to provide a radiant tube heating device with a novel structure that can suppress the generation of NOx, thereby enriching the technology related to NOx suppression.

Thus, the present invention provides:
A cylindrical radiant tube,
a combustion burner attached to one end side of the radiant tube;
a cylindrical temperature rise suppressor disposed in a combustion region within the radiant tube, the temperature rise suppressor defining an inner flow passage located in a central portion of the combustion region and an outer flow passage located in a peripheral portion of the combustion region;
Equipped with
The burner head of the combustion burner inserted into the radiant tube has:
a first nozzle portion that ejects fuel gas and combustion air toward the combustion region at a peripheral edge of the head in a plan view seen from the axial direction;
a second nozzle portion that protrudes in the axial direction more than the first nozzle portion at a head central portion in a plan view seen from the axial direction and ejects the combustion air from a tip thereof toward the combustion region;
is formed,
The temperature rise suppressor is disposed downstream in the direction of combustion gas flow, away from the tip of the second nozzle portion so as not to overlap with the tip of the second nozzle portion.

According to the radiant tube heating device of the present invention, combustion air is ejected in two stages, from the first nozzle section and the second nozzle section. Therefore, in the combustion zone directly below the first nozzle section in the first stage, combustion occurs in a gas-rich state with a high fuel gas ratio. The first-stage combustion gas generated directly below the first nozzle section passes through the second nozzle section and is then mixed with the combustion air from the second nozzle section.
Here, in the present invention, a cylindrical temperature rise suppressor is arranged downstream of the combustion burner in the direction of combustion gas flow, and most of the first-stage combustion gas flows into the outer flow path formed between the temperature rise suppressor and the radiant tube before being sufficiently mixed with the combustion air from the second nozzle section, and combustion continues in a gas-rich state.
On the other hand, the combustion air ejected from the second nozzle portion flows into the inner flow passage of the temperature rise suppressor in a state where it is slightly mixed with the combustion gas from the first stage, and combustion occurs in an air-rich state.
This state of separation into gas-rich and air-rich gases is maintained while the gas moves along the longitudinal direction of the temperature rise suppression body, and after passing through the temperature rise suppression body, the entire combustion gas gradually mixes, and slow combustion continues.

As a result, the present invention suppresses local increases in combustion temperature and reduces the generation of NOx compared to a hypothetical case in which no temperature rise suppression body is installed.

In the present invention, the burner head of the combustion burner is provided with:
a first nozzle portion that ejects combustion air toward the combustion region at a peripheral edge of the head in a plan view seen from the axial direction;
a second nozzle portion that protrudes in the axial direction more than the first nozzle portion at a head central portion in a plan view seen from the axial direction and ejects the combustion air and fuel gas from a tip end thereof toward the combustion region;
It may be one that forms.

In this case, too, the combustion air is ejected in two stages, from the first nozzle section and then from the second nozzle section. In the combustion zone directly below the second nozzle section, combustion takes place in a gas-rich state with a high ratio of fuel gas, and then the fuel gas is mixed with the combustion air from the first nozzle section.
Here, a cylindrical temperature rise suppressor is arranged downstream of the combustion burner in the direction of combustion gas flow, and most of the combustion gas directly below the second nozzle section flows into the inner flow path of the temperature rise suppressor before being sufficiently mixed with the combustion air from the first nozzle section, and combustion continues in a gas-rich state.
On the other hand, the combustion air ejected from the first nozzle section flows into the outer flow passage formed between the temperature rise suppressor and the radiant tube in a state slightly mixed with the combustion gas directly below the second nozzle section, and burns in an air-rich state.
Therefore, compared to a case where a hypothetical temperature rise suppressor is not provided, a local increase in combustion temperature can be suppressed, and NOx generation can be suppressed.

In the present invention, a heat exchanger can be attached to the other end of the radiant tube to heat the combustion air to be supplied to the combustion burner in advance by heat exchange with the high-temperature gas inside the radiant tube.
As mentioned above, the present invention can suppress the proportion of NOx contained in the combustion gas, thereby raising the preheating limit (the upper limit of the heating temperature of the combustion air) regulated by the Air Pollution Control Act. Therefore, by attaching a heat exchanger to the other end of the radiant tube and heating the combustion air, the thermal efficiency of the entire radiant tube heating device can be improved.

1 is a diagram showing a radiant tube heating device according to an embodiment of the present invention attached to a heat treatment furnace. FIG. 2A and 2B are enlarged views of the burner head in the combustion burner of FIG. 1, in which (A) is a front view and (B) is a cross-sectional view taken along the line B-B of (A). FIG. 2 is a side view of the temperature rise suppressor shown in FIG. 1 . FIG. 2 is a vertical cross-sectional view of the temperature rise suppressor of FIG. 1 . 2A is a front view of the temperature rise suppressor of FIG. 1, FIG. 2B is a front view of a temperature rise suppressor of a different configuration, and FIG. 2C is a front view of a temperature rise suppressor of a further different configuration. 4A and 4B are diagrams for explaining the operation of the radiant tube heating device of the same embodiment. 10A and 10B are enlarged views of a burner head in a radiant tube heating device according to another embodiment of the present invention, in which (A) is a front view and (B) is a cross-sectional view taken along the line BB of (A). 4A and 4B are diagrams for explaining the operation of the radiant tube heating device of the same embodiment.

The present invention will be described in detail below with reference to the accompanying drawings, in which: Fig. 1 shows a radiant tube heating device 10 according to an embodiment of the present invention, and more specifically shows the device attached to the wall 1a of a heat treatment furnace 1.
Radiant tube heating device 10 includes a cylindrical radiant tube 12 that penetrates furnace wall 1a from the inside to the outside, a combustion burner 20 attached to one end 12a of radiant tube 12, a temperature rise suppressor 40 disposed in the combustion region inside radiant tube 12, and a heat exchanger 60 disposed inside the other end 12b of radiant tube 12. Radiant tube heating device 10 combusts fuel gas and combustion air ejected from combustion burner 20 in the combustion region inside radiant tube 12. The combustion gas generated at this time flows through radiant tube 12 toward the other end 12b, as shown by the arrows in the figure.

Radiant tube 12 is a one-piece metal pipe made of, for example, cast steel, with a circular cross section, and has a horizontal U-shape in side view. Radiant tube 12 has one end 12a and the other end 12b that run parallel to each other through furnace wall 1a along the furnace interior and exterior, a semicircular turn portion 12c that protrudes toward heating chamber 2 (inside the furnace), and a hollow portion 13 that runs continuously through the inside of these.
The shape of the radiant tube 12 may be other than U-shaped, such as W-shaped, as needed.

The combustion burner 20 mixes and burns fuel gas and combustion air, emitting a long, thin flame in the combustion region within the radiant tube 12. The combustion burner 20 comprises a mounting member 21 attached to the radiant tube 12, a cylindrical body 22 connected to the mounting member 21, and a burner head 23 attached to the tip of the cylindrical body 22. The combustion burner 20 is attached to the radiant tube 12 so that the cylindrical body 22 and burner head 23 are inserted into the hollow portion 13 at one end 12a of the radiant tube 12 and are coaxial with the hollow portion 13.

A fuel branch pipe 26 and an air intake pipe 27, which form part of the fuel supply pipe 25, are connected to the mounting member 21. Fuel gas is supplied via the fuel branch pipe 26, and combustion air is supplied via the air intake pipe 27. The supplied fuel gas and combustion air flow through the interior of the cylindrical body 22 and are supplied to the burner head 23.

Figure 2 is an enlarged view of the burner head 23 of the combustion burner 20. The burner head 23 includes a blocking member 29 that blocks the axial end of the cylindrical body 22, and as shown in Figure 2(A), has a second nozzle portion 32 formed in the center of the head in a plan view from the axial direction, and a first nozzle portion 31 formed on the periphery of the head around the second nozzle portion 32.

As shown in Figure 2 (B), the second nozzle section 32 is a cylindrical section that protrudes axially further than the first nozzle section 31, and has a through-hole 33 formed therein that penetrates axially. A portion of the combustion air that flows through the interior of the cylinder 22 and is sent to the burner head 23 further flows through the through-hole 33 and is sent to the opening 33a at the tip. The combustion air is then ejected from the tip opening 33a of the second nozzle section 32 toward the combustion region.

On the other hand, the first nozzle section 31 is an annular section that extends radially outward from the base end side of the second nozzle section 32 and is connected to the peripheral wall of the cylindrical body 22. Four burner tips 35 are attached to the first nozzle section 31 at 90° intervals in the circumferential direction so as to be concentric with the second nozzle section 32. A fuel gas supply pipe 36 is connected to the burner tips 35, and fuel gas supplied from the fuel branch pipe 26 is sent to the burner tips 35 through the supply pipe 36 arranged inside the cylindrical body 22 and is sprayed from the nozzle holes 35a of the burner tips 35 toward the combustion area.

Furthermore, the first nozzle section 31 has four through-hole air nozzle holes 37 formed at 90° intervals circumferentially so as to be concentric with the second nozzle section 32. A portion of the combustion air that flows through the interior of the cylinder 22 and is sent to the burner head 23 is ejected from the air nozzle holes 37 toward the combustion area.

As described above, the combustion burner 20 of this embodiment is a two-stage combustion burner that ejects combustion air in two stages. In the combustion burner 20, first, the fuel gas ejected from the first nozzle portion 31 and the combustion air are mixed together, causing combustion in the first stage. Note that the mixed gas of the fuel gas and the combustion air is ignited by a pilot burner (not shown).
Here, the combustion air supplied to the first stage combustion is only a portion of the theoretically required amount of air, so combustion in the fuel region directly below the first nozzle section 31 is gas-rich combustion with a high ratio of combustion gas. Then, after passing through the second nozzle section 32, a portion of the first stage combustion gas is mixed with the combustion air from the second nozzle section 32. Therefore, the combustion gas around the area passing through the second nozzle section 32 is in an air-rich state in the center and a gas-rich state in the peripheral portion.

Next, we will explain the temperature-rise suppressor 40. The temperature-rise suppressor 40 is made of SiC (ceramic), which has both high thermal conductivity and high impact resistance. As shown in Figures 1, 3, 4, and 5(A), it is a cylindrical member overall, with a through-hole 41 that runs axially through the interior. The outer surface of the temperature-rise suppressor 40 is formed with multiple (here, five) spiral grooves 43, and spiral ridges 44 that protrude along the boundaries between these grooves 43. When placed within the radiant tube 12, these multiple ridges 44 contact or are close to the inner wall surface of the radiant tube 12. In this example, these multiple ridges 44 make it easy to accurately position the temperature-rise suppressor 40 at a predetermined position in the hollow portion 13 of the radiant tube 12.

When the temperature rise suppression body 40 is placed in the combustion region of the radiant tube 12, the through holes 41 function as an inner flow path for circulating gas present in the central portion of the combustion region, and the grooves 43 formed on the outer surface of the temperature rise suppression body 40 function as an outer flow path for circulating gas present in the peripheral portion of the combustion region.

1, the temperature-rise suppression body 40 is disposed downstream in the combustion gas flow direction so as not to overlap with the tip of the second nozzle portion 32, and a separation region 47 is formed between the combustion burner 20 (more specifically, the tip of the second nozzle portion 32) and the temperature-rise suppression body 40. Here, the separation distance L1 (see FIG. 6) from the combustion burner 20 can be, for example, 200 to 600 mm.
This separation area 47 mixes the combustion air ejected from the second nozzle portion 32 with a portion of the first stage combustion gas, thereby achieving combustion in an air-rich state in the inner flow path 41 of the temperature rise suppression body 40.

The shape of the temperature-rise suppressor 40 is not limited to the above and can be modified as appropriate. For example, as shown in Fig. 5(B), the temperature-rise suppressor 40 may have six spiral grooves 43 and an equal number of ridges 44 protruding between the grooves 43 and the ridges 44. The number of grooves 43 and ridges 44 may be any number as long as they are plural and equal in number.
5(C), a temperature-rise suppressor 40C may be used in the same manner as described above, in which six (plural) ridges 44 are symmetrically provided on the outer peripheral surface of a cylindrical body and linearly protrude along the axial direction, and an equal number of grooves 43a are formed linearly along the axial direction between the ridges 44. Note that although the grooves 43a have a substantially fan-shaped cross section along the width direction, they may also have an arc-shaped cross section similar to the above.
The ceramic constituting the temperature rise suppressor may include, in addition to SiC, for example, WC, B ₄ C, alumina (Al ₂ O ₃ ), aluminum nitride, TiN, mullite, and the like.

Next, the heat exchanger 60 will be described. As shown in FIG. 1, the heat exchanger 60 comprises a substantially cylindrical main body 62 and a hemispherical tip 63. The main body 62 has a spiral groove formed on its peripheral wall. An air intake pipe 27 is disposed within the main body 62, directing combustion air toward the combustion burner 20. An air vent 62a formed in the rear end wall of the main body 62 is connected to an air branch pipe 78, which constitutes part of the air supply pipe 76. The combustion air flows into the main body 62 via the air branch pipe 78 and the air vent 62a. The combustion air flowing into the main body 62 is preheated by the heat of the high-temperature gas and then supplied to the combustion burner 20 via the air intake pipe 27, as described above. Meanwhile, the exhaust gas (combustion gas) after heat exchange with the combustion air is discharged from the exhaust port 16 formed on the other end 12b of the radiant tube 12, through the flue 65a in the exhaust pipe 65, and to the outside.

Next, the operation of the radiant tube heating device 10 of this example will be explained using Figures 1 and 6. In this example, the combustion burner 20 attached to one end 12a of the radiant tube 12 is configured to eject combustion air in two stages, from the first nozzle section 31 and the second nozzle section 32. Therefore, in the combustion region directly below the first nozzle section 31 of the first stage, combustion occurs in a gas-rich state with a high fuel gas ratio, as shown in Figure 6. The first-stage combustion gas generated directly below the first nozzle section 31 then passes through the second nozzle section 32 and is mixed with the combustion air from the second nozzle section 32.

In this example of the radiant tube heating device 10, the cylindrical temperature rise suppression body 40 is positioned downstream of the combustion burner 20 in the direction of combustion gas flow, and the first-stage combustion gas flows into the outer flow path 43 formed between the temperature rise suppression body 40 and the radiant tube 12 before being sufficiently mixed with the combustion air from the second nozzle section 32, and combustion continues in a gas-rich state.
On the other hand, the combustion air ejected from the second nozzle portion 32 mixes slightly with the first-stage combustion gas in the separation region 47, and then flows into the inner flow passage 41 of the temperature rise suppression body 40, where it burns in an air-rich state.
This state of separation into gas-rich and air-rich mixtures is maintained while the combustion gas moves (during the length L2) along the longitudinal direction of the temperature-rise suppression body 40. After passing through the temperature-rise suppression body 40 (after reaching the region indicated by L3 in the figure), the entire combustion gas gradually becomes mixed, and slow combustion continues.

As a result, in the radiant tube heating device 10 of this embodiment, local increases in combustion temperature are suppressed and NOx generation is reduced compared to when a hypothetical temperature rise suppression body is not installed.

The high-temperature combustion gas thus generated passes through the radiant tube turn 12c and is sent to the other end 12b of the radiant tube 12, as shown in Figure 1. During this time, the high-temperature combustion gas radiates heat into the heating chamber 2 through the wall of the radiant tube 12.

The combustion gas that reaches the other end 12b of the radiant tube 12 flows from the tip 63 of the heat exchanger 60 shown in Figure 1 along each of the outer spiral grooves formed in the main body 62 of the heat exchanger 60. This allows for efficient preheating of new combustion air flowing within the inner spiral groove located inside the outer spiral groove. Furthermore, because the temperature rise suppression body 40 suppresses the proportion of nitrogen oxides in the combustion gas, the upper limit of the heating temperature of the combustion air due to the above preheating can be increased, significantly improving thermal efficiency.

[Example]
As described above, a radiant tube heating device equipped with a combustion burner that ejects combustion air in two stages was used to measure the amount of NOx contained in the combustion gas with and without a temperature rise suppressor, and the NOx reduction effect was confirmed. The temperature rise suppressor used had an inner diameter of 107.5 mm, an axial length of 615 mm, and a spiral groove on its outer circumferential surface. Here, the temperature rise suppressor was placed at a distance L1 (see Figure 6) of 440 mm from the combustion burner.
According to this evaluation, it was confirmed that the amount of NOx generated when the furnace atmosphere temperature was controlled to 900°C could be reduced by 45% by providing the temperature rise suppressor.

7 is a diagram showing the schematic configuration of a radiant tube heating apparatus 10B according to another embodiment of the present invention. Like the radiant tube heating apparatus 10, the radiant tube heating apparatus 10B of this embodiment also ejects combustion air in two stages, but in this radiant tube heating apparatus 10B, fuel gas is ejected from the second nozzle 32B.
7, in the radiant tube heating apparatus 10B of this example, the burner head 23B of the combustion burner 20B inserted into the radiant tube 12 is provided with a first nozzle section 31B having an air nozzle hole 37 for ejecting combustion air, and a second nozzle section 32B having a burner tip 35 disposed inside a through-hole 33 penetrating in the axial direction for ejecting fuel gas and combustion air. Of the components of this example, those common to the radiant tube heating apparatus 10 described above are designated by the same reference numerals and will not be described again.

In the radiant tube heating device 10B of this example, combustion in a gas-rich state with a high ratio of fuel gas occurs in the combustion region immediately below the second nozzle section 32B, as shown in Fig. 8. The combustion gas immediately below the second nozzle section 32B flows into the inner flow passage 41 of the temperature-rise suppressor 40 before being sufficiently mixed with the combustion air from the first nozzle section 31B, and combustion in a gas-rich state continues.
On the other hand, the combustion air ejected from the first nozzle section 31B mixes slightly with the combustion gas directly below the second nozzle section 32B in the separation region 47 and flows into the outer flow path 43 formed between the temperature rise suppression body 40 and the radiant tube 12, resulting in combustion in an air-rich state with a high ratio of combustion air.
This state of separation into gas-rich and air-rich gases is maintained while the combustion gas moves along the longitudinal direction of the temperature-rise suppression body 40. After passing through the temperature-rise suppression body 40, the entire combustion gas gradually becomes mixed, and slow combustion continues.

As a result, in the radiant tube heating device 10B of this example, local increases in combustion temperature are suppressed and NOx generation is reduced compared to when a hypothetical temperature rise suppression body is not installed.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the spirit of the present invention.

10, 10B Radiant tube type heating device 12 Radiant tube 12a One end 12b Other end 20, 20B Combustion burner 23, 23B Burner head 31, 31B First nozzle portion 32, 32B Second nozzle portion 40, 40B, 40C Temperature rise suppressor 41 Through hole (inner flow path)
43, 43a Concave groove (outer channel)
60 Heat exchanger

Claims (3)

A cylindrical radiant tube,
a combustion burner attached to one end side of the radiant tube;
a cylindrical temperature rise suppressor disposed in a combustion region within the radiant tube, the temperature rise suppressor defining an inner flow passage located in a central portion of the combustion region and an outer flow passage located in a peripheral portion of the combustion region;
Equipped with
The burner head of the combustion burner inserted into the radiant tube has:
a first nozzle portion that ejects fuel gas and combustion air toward the combustion region at a peripheral edge of the head in a plan view seen from the axial direction;
a second nozzle portion that protrudes in the axial direction more than the first nozzle portion at a head central portion in a plan view seen from the axial direction and ejects the combustion air from a tip thereof toward the combustion region;
is formed,
the temperature-rise suppressor is disposed downstream in a combustion gas flow direction and away from the tip of the second nozzle portion so as not to overlap with the tip of the second nozzle portion, and is disposed at a position where fuel gas can be combusted in the inner flow path and the outer flow path,
During combustion of the combustion burner,
In the inner flow passage, combustion is carried out in an air-rich state in which the ratio of combustion air is higher than in the outer flow passage,
A radiant tube heating device characterized in that combustion in the outer flow passage is carried out in a gas-rich state in which the ratio of fuel gas is higher than in combustion in the inner flow passage . A cylindrical radiant tube,
a combustion burner attached to one end side of the radiant tube;
a cylindrical temperature rise suppressor disposed in a combustion region within the radiant tube, the temperature rise suppressor defining an inner flow passage located in a central portion of the combustion region and an outer flow passage located in a peripheral portion of the combustion region;
Equipped with
The burner head of the combustion burner inserted into the radiant tube has:
a first nozzle portion that ejects combustion air toward the combustion region at a peripheral edge of the head in a plan view seen from the axial direction;
a second nozzle portion that protrudes in the axial direction more than the first nozzle portion at a head central portion in a plan view seen from the axial direction and ejects the combustion air and fuel gas from a tip end thereof toward the combustion region;
is formed,
the temperature-rise suppressor is disposed downstream in a combustion gas flow direction and away from the tip of the second nozzle portion so as not to overlap with the tip of the second nozzle portion, and is disposed at a position where fuel gas can be combusted in the inner flow path and the outer flow path,
During combustion of the combustion burner,
In the inner flow passage, combustion is carried out in a gas-rich state in which the ratio of fuel gas is higher than in the outer flow passage,
A radiant tube heating device characterized in that combustion in the outer flow passage is carried out in an air-rich state in which the ratio of combustion air is higher than in combustion in the inner flow passage . A radiant tube type heating device as described in either claim 1 or 2, characterized in that a heat exchanger is attached to the other end of the radiant tube, which preheats the combustion air to be supplied to the combustion burner by heat exchange with the high-temperature gas in the radiant tube.