WO2018181325A1

WO2018181325A1 - Air preheater

Info

Publication number: WO2018181325A1
Application number: PCT/JP2018/012452
Authority: WO
Inventors: 建聖渡邊
Original assignee: 住友重機械工業株式会社
Priority date: 2017-03-28
Filing date: 2018-03-27
Publication date: 2018-10-04
Also published as: MY195370A; PH12019502107A1; JPWO2018181325A1; JP6952108B2

Abstract

An air preheater that recovers heat from boiler exhaust gas and preheats air, said air preheater being equipped with a first tube provided inside an exhaust gas flow passage through which exhaust gas passes, and a second tube surrounding the first tube from the outer circumferential side thereof and extending along the first tube. Air flows in the first tube, and air at a higher temperature than the air flowing in the first tube flows in the second tube.

Description

Air preheater

The present invention relates to an air preheater.

As a conventional boiler system, one that heats a heat transfer medium by burning fuel is known (see, for example, Patent Document 1). The heated heat transfer medium circulates in a circulation system in the system. Further, the combustion gas generated by the combustion of the fuel is subjected to a heat exchange process or the like, and then discharged to the outside as an exhaust gas.

JP 2001-41415 A

In recent years, high moisture and high sulfur fuels tend to be used as boiler fuel. Exhaust gas generated by burning such fuel contains a lot of moisture. As moisture in the exhaust gas increases, the acid dew point (sulfuric acid dew point) increases. Here, the boiler system as described above may be provided with an air preheater that recovers heat of exhaust gas and preheats air. When the acid dew point of the exhaust gas is high, low temperature end corrosion (acid dew point corrosion) in which the tube disposed at the downstream end where the temperature of the exhaust gas becomes low may corrode may occur.

As a method for suppressing the occurrence of such low temperature end corrosion, there is a method of maintaining the exhaust gas temperature at a high level. However, increasing the temperature of the exhaust gas means increasing the sensible heat loss of the exhaust gas, resulting in a decrease in boiler efficiency of the boiler.

Accordingly, an object of the present invention is to provide an air preheater that can suppress the corrosion of the tube while suppressing a decrease in boiler efficiency.

An air preheater according to an aspect of the present invention is an air preheater that recovers heat of exhaust gas from a boiler and preheats air, and includes a first tube provided in an exhaust gas passage that allows exhaust gas to pass through, A second tube that surrounds the first tube from the outer peripheral side and extends along the first tube. Air flows through the first tube, and the first tube flows through the second tube. Air with a higher temperature than air flows.

An air preheater according to an embodiment of the present invention includes a first tube provided in an exhaust gas passage. The first tube can recover the heat of the exhaust gas flowing through the exhaust gas flow path by the air flowing through the first tube. Here, the air preheater includes a second tube that surrounds the first tube from the outer peripheral side and extends along the first tube. Thus, a double tube is comprised by the 1st tube and the 2nd tube. In addition, air having a higher temperature than the air flowing through the first tube flows through the second tube, which is a tube outside the double tube. Thereby, even if the exhaust gas becomes low temperature by removing heat, the first tube is surrounded by the second tube through which high-temperature air flows. Corrosion can be suppressed. As mentioned above, corrosion of a tube can be suppressed, suppressing the fall of boiler efficiency.

In the air preheater according to one embodiment, the air after passing through the first tube at least once may flow through the second tube. Since the air that has passed through the first tube at least once has recovered the heat of the exhaust gas, the temperature is higher than the air that is passing through the first tube on the most downstream side. Therefore, by flowing the air through the second tube, a structure for securing high-temperature air for flowing from the place other than the air preheater to the second tube becomes unnecessary.

In the air preheater according to one aspect, a plurality of first tubes may be provided in the exhaust gas flow path, and a second tube may be provided for a part of the plurality of first tubes. Among the plurality of first tubes, the second tube is provided only for those having a low exhaust gas temperature passing through and having a high possibility of low temperature end corrosion, and the passing exhaust gas temperature is high and there is a possibility of low temperature end corrosion. The second tube is not provided for low ones. Thereby, the manufacturing cost of an air preheater can be suppressed.

The air preheater which concerns on one form detects the temperature of the exhaust gas in the downstream from the area | region where the 1st tube is provided among the air temperature detection parts which detect the temperature of the air which flows through a 2nd tube, and an exhaust gas flow path An exhaust gas temperature detection unit that controls the flow rate of air that flows to the second tube based on detection results of the air temperature detection unit and the exhaust gas temperature detection unit. By performing such control, it is possible to flow air at an appropriate flow rate to the second tube.

According to the present invention, it is possible to provide an air preheater that can suppress the corrosion of the tube while suppressing a decrease in boiler efficiency.

It is a schematic structure figure of a boiler system provided with an air preheater concerning an embodiment of the present invention. It is a figure which shows schematic structure of the air preheater shown in FIG. It is a figure which shows schematic structure of the corrosion suppression structure of the air preheater shown in FIG.

Embodiments of the present invention will be described with reference to the drawings. However, the following embodiments are merely examples for explaining the present invention and are not intended to limit the present invention to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant description is omitted.

With reference to FIG. 1, the structure of the boiler system 100 provided with the air preheater 50 which concerns on this embodiment is demonstrated. The boiler system 100 is an external circulation type (circulating fluidized bed type) circulating fluidized bed boiler. The boiler system 100 includes a fluidized bed furnace 3 having a vertically long cylindrical shape. A fuel supply port 3a for supplying fuel is provided in the middle part of the furnace 3, and a gas outlet 3b for discharging combustion gas is provided in the upper part. The fuel supplied from the fuel supply device 5 to the furnace 3 is supplied into the furnace 3 through the fuel supply port 3a.

A cyclone 7 that functions as a solid-gas separator is connected to the gas outlet 3 b of the furnace 3. The discharge port 7a of the cyclone 7 is connected to a downstream gas processing system via a gas line. A return line 9 called a downcomer extends downward from the bottom outlet of the cyclone 7, and the lower end of the return line 9 is connected to the intermediate side surface of the furnace 3.

In the furnace 3, the solid material containing the fuel supplied from the fuel supply port 3a flows by the combustion / flowing air introduced from the lower air supply line 3c, and the fuel flows while the fuel flows, for example, about 800 to 900. Burn at ℃. A combustion gas generated in the furnace 3 is introduced into the cyclone 7 with accompanying solid particles. The cyclone 7 separates solid particles and gas by a centrifugal separation action, returns the solid particles separated via the return line 9 to the furnace 3, and removes the combustion gas from which the solid particles have been removed from the discharge port 7 a to the gas line. To the subsequent gas processing system.

In this furnace 3, a solid material called “in-furnace bed material” is generated and collected at the bottom, and the bed material is sintered and melted and solidified by the concentration of impurities (low melting point materials, etc.) in the in-furnace bed material, or It is necessary to suppress malfunctions caused by incombustible impurities. For this reason, in the furnace 3, the in-furnace bed material is discharged | emitted regularly or continuously outside from the discharge port 3d of the bottom part. The discharged bed material is supplied to the furnace 3 again after removing unsuitable materials such as metal and coarse particle diameter on a circulation line (not shown).

The gas treatment system includes a gas heat exchange device 13 connected to the discharge port 7a of the cyclone 7 via a gas line, and a dust collector 15 connected to the discharge port 13a of the gas heat exchange device 13 via a gas line. And. The gas heat exchanger 13 is provided with a boiler tube 13b that superheats steam so as to cross the exhaust gas flow path. When the high-temperature exhaust gas sent from the cyclone 7 comes into contact with the boiler tube 13b, the heat of the exhaust gas is recovered into the steam in the tube, and the generated high-temperature steam is sent to the power generation turbine through the boiler tube 13b. An air preheater 50 that recovers the heat of the exhaust gas and preheats the air is provided at the discharge port 13a in the gas heat exchanger 13. The configuration of the air preheater 50 will be described later. The dust collector 15 removes fine particles such as fly ash that are still accompanying the combustible gas. As the dust collector 15, for example, a bag filter or an electric dust collector is adopted. The clean gas discharged from the discharge port 15 a of the dust collector 15 is discharged to the outside through the gas line and the pump 17 from the chimney 19.

Solid particles generated in the furnace 3 circulate in the circulation system 21 including the furnace 3, the cyclone 7, and the return line 9. In the following description, the fluid of solid particles is referred to as a heat transfer medium. In the circulation system 21, a heat exchange chamber 20 is formed between the return line 9 and the bottom of the furnace 3. A heat transfer medium is stored in the heat exchange chamber 20. A heat exchanger 22 is provided in the heat exchange chamber 20.

Next, a schematic configuration of the air preheater 50 will be described with reference to FIG. As shown in FIG. 2, the gas heat exchange device 13 includes an exhaust gas passage 101 through which the exhaust gas EG passes. The exhaust gas flow channel 101 includes

side wall portions

101a and 101b that face each other and side wall portions (not shown) that face each other in the front-rear direction of the paper surface. In the following description, the direction in which the exhaust gas EG flows in the exhaust gas channel 101 is referred to as the Z-axis direction. The upstream side with respect to the flow of the exhaust gas EG is the negative side in the Z-axis direction, and the downstream side is the positive side in the Z-axis direction. The direction in which the

side wall portions

101a and 101b face each other (the left and right direction in the drawing) is the X-axis direction. The direction in which the side wall 101a is arranged (the left side in the drawing) is the negative side in the X-axis direction, and the side in which the side wall 101b is arranged (the right side in the drawing) is the positive side in the X-axis direction. The front-rear direction on the paper is the Y-axis direction. The back side of the paper is the negative side in the Y-axis direction, and the front side of the paper is the positive side in the Y-axis direction.

The air preheater 50 recovers heat from the exhaust gas EG in the exhaust gas channel 101 and preheats the air. The air preheater 50 includes a plurality (three in this case) of

heat exchange units

51A, 51B, and 51C provided in the exhaust gas passage 101. The

heat exchange units

51A, 51B, and 51C are provided in this order from the upstream side to the downstream side with respect to the flow of the exhaust gas EG, that is, from the negative side to the positive side in the Z-axis direction.

The heat exchange units 51 A, 51 B, 51 C include a plurality of first tubes 52 provided in the exhaust gas passage 101, an inlet portion 53 communicating with each inlet of the plurality of first tubes 52, and a plurality of first tubes. And an outlet 54 that communicates with each outlet of the tube 52. The first tube 52 extends in the X-axis direction between the side wall 101a and the side wall 101b. In the first tube 52, air that exchanges heat with the exhaust gas EG flowing through the exhaust gas passage 101 flows. The first tube 52 extends at least over the entire length between the side wall 101a and the side wall 101b. The first tube 52 passes through the side wall 101a and opens outside the exhaust gas passage 101, and passes through the side wall 101b and opens outside the exhaust gas passage 101 (see, for example, FIG. 3). The plurality of first tubes 52 are arranged so as to be parallel to each other so as to be arranged in the Z-axis direction and the Y-axis direction. A gap is provided between the first tubes 52 adjacent to each other so that the exhaust gas EG can pass therethrough.

The inlet portion 53 is a box-shaped member having an internal space, is provided outside the exhaust gas flow channel 101, and is provided so that the inflow ports of the plurality of first tubes 52 communicate with the internal space. The inlet portion 53 expands the flow of air in the internal space so as to distribute the supplied air to each of the plurality of first tubes 52. The outlet portion 54 is a box-shaped member having an internal space, is provided outside the exhaust gas flow channel 101, and is provided so that the outlets of the plurality of first tubes 52 and the internal space communicate with each other. The outlet 54 collects the air that has flowed out from the plurality of first tubes 52.

In the present embodiment, the inlet 53 of the heat exchange unit 51A is provided on the positive side wall 101b in the X-axis direction, and the outlet 54 is provided on the negative side wall 101a in the X-axis direction. The inlet part 53 of the heat exchange unit 51B is provided on the negative side wall part 101a in the X-axis direction, and the outlet part 54 is provided on the positive side wall part 101b in the X-axis direction. The inlet 53 of the heat exchange unit 51C is provided on the positive side wall 101b in the X axis direction, and the outlet 54 is provided on the negative side wall 101a in the X axis direction.

In the present embodiment, heat exchange air is supplied in the order of the

heat exchange units

51C, 51B, and 51A. Specifically, a line L1 for supplying air from the outside of the air preheater 50 is connected to the inlet 53 of the heat exchange unit 51C. The outlet part 54 of the heat exchange unit 51C and the inlet part 53 of the heat exchange unit 51B are connected via a line L2. The outlet part 54 of the heat exchange unit 51B and the inlet part 53 of the heat exchange unit 51A are connected via a line L3. A line L4 is connected to the outlet portion 54 of the heat exchange unit 51A to allow preheated air to flow out to the outside in the air preheater 50. The air preheated by the air preheater 50 is supplied to various places where air is used in the boiler. For example, the air preheated by the air preheater 50 may be used as secondary combustion air for the furnace 3, or may be used as fluidizing air or combustion air at the bottom of the furnace 3.

The air preheater 50 configured as described above has a corrosion suppression structure 70 for suppressing low temperature end corrosion (acid dew point corrosion) of the first tube 52 at a position on the downstream end side in the flow of the exhaust gas EG. Is provided. Such corrosion at a low temperature is likely to occur when the moisture contained in the exhaust gas is large, when the sulfur content is high, or when the moisture and sulfur content is high. Examples of fuels that easily generate exhaust gas include coal having a high sulfur content, petroleum coke, coal having a high moisture content, and woody fuel such as forest land residue. In the present embodiment, the corrosion suppressing structure 70 is provided in the heat exchange unit 51C that is disposed on the most downstream side with respect to the flow of the exhaust gas EG. Moreover, the corrosion suppression structure 70 is provided with respect to the multiple 1st tube 52 arrange | positioned most downstream with respect to the flow of waste gas EG among the heat exchange units 51C. That is, the corrosion suppression structure 70 is not provided at a location where the temperature of the flowing exhaust gas EG is high and the possibility of low temperature end corrosion is low. Here, the corrosion inhibiting structure 70 is not provided for the first tubes 52 of the upstream side of the first tubes 52 of the

heat exchange units

51A and 51B and the heat exchange units 51C.

Referring to FIG. 3, the detailed configuration of the corrosion inhibiting structure 70 will be described. The corrosion suppression structure 70 includes a first tube 52, a second tube 71, an inlet portion 72 for the second tube 71, and an outlet portion 73 for the second tube 71. The end portion 52a on the outflow side of the first tube 52 in the corrosion suppressing structure 70 extends so as to protrude to the negative side in the X-axis direction from the side wall portion 101a in order to provide the inlet portion 72. The end portion 52b on the inflow side of the first tube 52 in the corrosion suppressing structure 70 extends so as to protrude to the positive side in the X-axis direction from the side wall portion 101b in order to provide the outlet portion 73.

The second tube 71 surrounds the first tube 52 from the outer peripheral side and extends along the first tube 52. The central axis of the second tube 71 and the central axis of the first tube 52 may extend so as to coincide with each other. However, the central axis of the first tube 52 is deviated as long as the air flow is not affected. Also good. The end portion 71a on the inflow side of the second tube 71 extends so as to protrude to the negative side in the X-axis direction from the side wall portion 101a. However, the end portion 71a has a smaller protruding amount than the end portion 52a of the first tube 52, and is disposed on the positive side in the X-axis direction with respect to the end portion 52a. The end portion 71b on the outflow side of the second tube 71 extends from the side wall portion 101b so as to protrude to the positive side in the X-axis direction. However, the end portion 71b has a smaller protruding amount than the end portion 52b of the first tube 52, and is disposed on the negative side in the X-axis direction than the end portion 52b. Note that the

end portions

71a and 71b of the second tube 71 do not have to protrude from the

side wall portions

101a and 101b, and need only extend at least over the entire length between the side wall portion 101a and the side wall portion 101b. In addition, although the 2nd tube 71 is provided with respect to all the 1st tubes 52 shown by FIG. 3, the 2nd tube 71 is with respect to a part of several 1st tube 52. It is provided. That is, the second tube 71 is not provided in the first tube 52 on the upstream side with respect to the flow of the exhaust gas EG, and is exposed to the exhaust gas EG.

The material of the first tube 52 and the second tube 71 is not particularly limited, but a general material such as carbon steel may be applied in order to reduce the material cost. That is, by adopting the corrosion inhibiting structure 70, it is not necessary to employ an expensive material resistant to corrosion as the tube material itself. However, the present invention does not exclude the use of these expensive materials as the material of the first tube 52 and the second tube 71.

The inlet portion 72 is a box-shaped member having an internal space, is provided outside the exhaust gas flow channel 101, and is provided so that the inflow ports of the plurality of second tubes 71 communicate with the internal space. The inlet part 72 expands the flow of air in the internal space so as to distribute the supplied air to each of the plurality of second tubes 71. The inlet portion 72 for the second tube 71 is provided inside the outlet portion 54 of the heat exchange unit 51C, and airtightness is ensured so as not to communicate with the internal space of the outlet portion 54. The end portion 52 a of the first tube 52 extends so as to penetrate the inlet portion 72 and is disposed so as to open in the internal space of the outlet portion 54.

The outlet part 73 is a box-shaped member having an internal space, is provided outside the exhaust gas flow channel 101, and is provided so that each outlet of the plurality of second tubes 71 and the internal space communicate with each other. The outlet portion 73 collects the air that has flowed out from the plurality of second tubes 71. The outlet portion 73 for the second tube 71 is provided inside the inlet portion 53 of the heat exchange unit 51C, and airtightness is ensured so as not to communicate with the inner space of the inlet portion 53. The end portion 52 b of the first tube 52 extends so as to penetrate the outlet portion 73 and is disposed so as to open in the internal space of the inlet portion 53.

In the second tube 71, air A2 having a higher temperature than the air A1 flowing through the first tube 52 flows. As the air A2 flowing through the second tube 71, air supplied from anywhere may be adopted as long as the temperature is higher than that of the air A1. The temperature of the air A1 is not particularly limited, but may be about 30 to 90 ° C. However, in the present embodiment, the air after passing through the first tube 52 flows through the second tube 71 at least once. In the present embodiment, air discharged from the heat exchange unit 51A is employed as the air A2. The temperature of such air A2 may be approximately 200 ° C. or higher. Accordingly, the inlet 72 is connected to a line L6 branched from the line L4 extending from the outlet 54 of the heat exchange unit 51A (see also FIG. 2). Thereby, the air supplied from the line L 6 flows to the second tube 71 through the inlet portion 72. A line L7 is connected to the outlet portion 73. Although the connection destination of the line L7 is not particularly limited, it may be connected to various places where air is used in the boiler, similarly to the line L4. However, since the air A2 passes through the second tube 71 outside the double pipe, the air pressure decreases. Therefore, the line L7 can reuse the recovered heat by supplying air to a position where high pressure is not required (for example, the upper part of the furnace or the seal air).

The flow rate of the air A2 with respect to the second tube 71 may be controlled according to the situation. Specifically, the corrosion suppression structure 70 may include an air temperature detection unit 81, an exhaust gas temperature detection unit 82, a flow rate adjustment unit 83, and a control unit 84. The air temperature detector 81 detects the temperature of the air A 2 flowing through the second tube 71. Note that the air temperature detection unit 81 may directly measure the air A2 in the second tube 71. However, the air temperature detection unit 81 may be a position immediately before flowing into the second tube 71 (for example, the line L6 and the inlet 72) or immediately after flowing out. (For example, the line L7 and the outlet 73) may be measured. The exhaust gas temperature detector 82 detects the temperature of the exhaust gas EG downstream of the region of the exhaust gas flow channel 101 where the first tube 52 is provided. That is, the exhaust gas temperature detection unit 82 detects the temperature of the exhaust gas EG in the region of the exhaust gas flow channel 101 on the downstream side of the heat exchange unit 51C of the air preheater 50. The flow rate adjusting unit 83 adjusts the flow rate of the air A 2 with respect to the second tube 71. The flow rate adjusting unit 83 may be configured by, for example, a valve provided in the line L6. The control unit 84 controls the flow rate of air that flows to the second tube 71 based on the detection results of the air temperature detection unit 81 and the exhaust gas temperature detection unit 82. The control unit 84 adjusts the flow rate of the air A 2 by sending a control signal corresponding to the flow rate to the flow rate adjustment unit 83.

Next, functions and effects of the air preheater 50 according to the present embodiment will be described.

First, an air preheater according to a comparative example will be described. The air preheater according to the comparative example is not provided with the corrosion suppressing structure 70 as in the present embodiment, and the second tube 71 is not provided in the first tube 52 on the most downstream side, and the exhaust gas EG is not provided. It is exposed to. Here, when high moisture and high sulfur fuel are used as the fuel for the boiler, the exhaust gas EG generated by burning the fuel contains a large amount of moisture. As the moisture in the exhaust gas increases, the acid dew point (sulfuric acid dew point) increases (although it differs depending on the fuel used, it is generally 110 to 130 ° C). When the acid dew point of the exhaust gas EG is high, in the air preheater according to the comparative example, the low temperature end corrosion (acid dew point corrosion) in which the first tube 52 disposed at the downstream end where the temperature of the exhaust gas EG becomes low is corroded. ) May occur.

As a method for suppressing the occurrence of such low temperature end corrosion, there is a method of maintaining the temperature of the exhaust gas EG high. However, increasing the temperature of the exhaust gas EG means that the sensible heat loss of the exhaust gas EG increases, and as a result, the boiler efficiency of the boiler decreases. Further, when the first tube 52 is made of a material that is resistant to corrosion, a high-grade material must be used, which increases the manufacturing cost.

On the other hand, the air preheater 50 according to this embodiment includes a first tube 52 provided in the exhaust gas passage 101. The first tube 52 can recover the heat of the exhaust gas EG flowing through the exhaust gas flow channel 101 by the air flowing through the first tube 52. Here, the air preheater 50 includes a second tube 71 that surrounds the first tube 52 from the outer peripheral side and extends along the first tube 52. Thus, the first tube 52 and the second tube 71 constitute a double tube. In addition, air having a higher temperature than the air flowing through the first tube 52 flows through the second tube 71 that is the outer tube of the double tube. Thereby, even if the exhaust gas EG becomes low temperature by removing heat, the first tube 52 is surrounded by the second tube 71 through which high-temperature air flows. Corrosion by low temperature exhaust gas EG can be suppressed. As mentioned above, corrosion of a tube can be suppressed, suppressing the fall of boiler efficiency.

Moreover, in the air preheater 50, since the high temperature air flows through the second tube 71 exposed to the exhaust gas EG, the temperature of the tube wall of the second tube 71 is also maintained at a temperature at which low temperature end corrosion does not occur. it can. Therefore, it is not necessary to use a high-grade material that is resistant to corrosion as the material of the first tube 52 and the second tube 71, and thus the manufacturing cost can be suppressed.

In the air preheater 50, the air after passing through the first tube 52 flows through the second tube 71 at least once. Since the air that has passed through the first tube 52 at least once has recovered the heat of the exhaust gas EG, it is more than the air A1 that is passing through the first tube 52 of the most downstream side corrosion suppression structure 70. The temperature is high. Therefore, by flowing the air through the second tube 71, a structure for securing high-temperature air for flowing into the second tube 71 from a place other than the air preheater 50 becomes unnecessary.

In the air preheater 50, a plurality of first tubes 52 are provided in the exhaust gas flow path 101, and a second tube 71 is provided for a part of the plurality of first tubes 52. Among the plurality of first tubes 52, the second tube 71 is provided only for those having a low exhaust gas temperature passing through and having a high possibility of low temperature end corrosion, and passing through the exhaust gas temperature is high, allowing low temperature end corrosion. The second tube 71 is not provided for those having low properties. Thereby, the manufacturing cost of the air preheater 50 can be suppressed.

The air preheater 50 includes an air temperature detector 81 that detects the temperature of the air flowing through the second tube 71, and the temperature of the exhaust gas EG downstream of the region of the exhaust gas channel 101 where the first tube 52 is provided. And a control unit 84 that controls the flow rate of the air flowing to the second tube 71 based on the detection results of the air temperature detection unit 81 and the exhaust gas temperature detection unit 82. By performing such control, air having an appropriate flow rate can be flowed to the second tube 71.

The present invention is not limited to the embodiment described above.

For example, in the above-described embodiment, air flows in the order from the heat exchange unit 51C on the downstream side to the heat exchange unit 51B and the heat exchange unit 51A for each heat exchange unit. Instead of this, for example, the order of introduction of air into the heat exchange unit may be changed, and air may be simultaneously supplied in parallel to each of the

heat exchange units

51A, 51B, 51C.

In the above-described embodiment, the air A2 is drawn from the line L4, but the drawing position of the air A2 is not particularly limited. For example, the air A2 may be drawn from the lines L2, L3, etc. In addition, when the temperature of the air drawn out from the lines L2 and L3 is not sufficient, auxiliary heating may be performed.

In the above-described embodiment, air A2 and air A1 flow so as to face each other, but they may flow in the same direction.

DESCRIPTION OF SYMBOLS 50 ... Air preheater, 52 ... 1st tube, 71 ... 2nd tube, 81 ... Air temperature detection part, 82 ... Exhaust gas temperature detection part, 84 ... Control part, 101 ... Exhaust gas flow path.

Claims

An air preheater that recovers heat of boiler exhaust gas and preheats air,
A first tube provided in an exhaust gas passage for allowing the exhaust gas to pass through;
A second tube surrounding the first tube from the outer peripheral side and extending along the first tube;
Air flows through the first tube,
An air preheater in which air having a higher temperature flows through the second tube than air flowing through the first tube.
The air preheater according to claim 1, wherein the air after passing through the first tube flows through the second tube at least once.
A plurality of the first tubes are provided in the exhaust gas flow path,
The air preheater according to claim 1 or 2, wherein the second tube is provided for a part of the plurality of first tubes.
An air temperature detector for detecting the temperature of the air flowing through the second tube;
An exhaust gas temperature detector for detecting the temperature of the exhaust gas downstream of the region where the first tube is provided in the exhaust gas flow path;
The control unit according to any one of claims 1 to 3, further comprising: a control unit that controls a flow rate of air flowing to the second tube based on detection results of the air temperature detection unit and the exhaust gas temperature detection unit. Air preheater.