BOILER AND METHOD FOR CONTROLLING BOILER

Abstract

An object of the present disclosure is to appropriately reduce NOx and CO. A boiler includes: a can body having water pipe; a burner for supplying primary fuel and air into the can body; a secondary fuel supply unit for supplying secondary fuel into the can body downstream of the burner in a flow direction of combustion gas; a cooling line for introducing a cooling fluid for reducing temperature of a predetermined space in the can body downstream of the burner in the flow direction of the combustion gas; a flow rate adjusting unit capable of adjusting a flow rate of the cooling fluid introduced into the can body from the cooling line; and a control unit for controlling the flow rate adjusting unit to control the flow rate of the cooling fluid such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

Description

BACKGROUND

1. Technical Field

The present disclosure relates to a boiler and a method for controlling the boiler.

2. Description of the Related Art

As the boiler for generating steam using heat obtained by burning a fuel gas, as disclosed in, for example, JP-A-2006-220373 and JP-A-2011-133180, there is a boiler which performs two-stage combustion by further including a fuel supply unit for supplying the fuel gas to a downstream side of a burner for supplying the fuel gas into a can body. JP-A-2006-220373 and JP-A-2011-133180 describe that NOx, CO, and oxygen concentrations contained in exhaust gas can be reduced with the above configuration. Further, JP-A-2011-133180 describes that the exhaust gas from the can body is drawn by an ejector and recirculated to the can body.

SUMMARY

For example, in JP-A-2011-133180, a position to which combustion gas is supplied is adjusted depending on an amount of combustion; however, there is room for further improvement in controlling temperature appropriately in the can body to reduce NOx and CO in the two-stage combustion described in JP-A-2006-220373 and JP-A-2011-133180.

An aspect of the present disclosure aims to provide the boiler for appropriately reducing NOx and CO and the method for controlling the boiler

According to an aspect of the present disclosure, there is provided a boiler including: a can body having water pipe; a burner connected to the can body and for supplying primary fuel and air into the can body; a secondary fuel supply unit for supplying secondary fuel into the can body downstream of the burner in a flow direction of combustion gas; a cooling line for introducing a cooling fluid for reducing temperature of a predetermined space in the can body downstream of the burner in the flow direction of the combustion gas; a flow rate adjusting unit provided in the cooling line and capable of adjusting a flow rate of the cooling fluid introduced into the can body from the cooling line; and a control unit for controlling the flow rate adjusting unit to control the flow rate of the cooling fluid introduced into the can body such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

According to an aspect of the present disclosure, there is provided a method for controlling a boiler including: a can body having water pipe; a burner connected to the can body and for supplying primary fuel and air into the can body; a secondary fuel supply unit for supplying secondary fuel into the can body downstream of the burner in a flow direction of combustion gas; a cooling line for introducing a cooling fluid for reducing temperature of a predetermined space in the can body downstream of the burner in the flow direction of the combustion gas; and a flow rate adjusting unit provided in the cooling line and capable of adjusting a flow rate of the cooling fluid introduced into the can body from the cooling line. The method controls the flow rate adjusting unit to control the flow rate of the cooling fluid introduced into the can body such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

According to an aspect of the present disclosure, there is provided the boiler for appropriately reducing NOx and CO and the method for controlling the boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a boiler according to a first embodiment;

FIG. 2 is a schematic partial cross-sectional view of the boiler according to the first embodiment;

FIG. 3 is a schematic block diagram of a control device according to the first embodiment;

FIG. 4 is a graph showing a relationship between an amount of primary fuel, an amount of secondary fuel, and an amount of cooling fluid;

FIG. 5 is a flowchart showing an example of a control method of the boiler according to the present embodiment;

FIG. 6 is a schematic cross-sectional view of the boiler according to a second embodiment; and

FIG. 7 is a schematic view of an ejector according to the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings, but the present disclosure is not limited thereto. Components of the embodiments described below can be combined as appropriate. In addition, some components may not be used.

In the following description, combustion gas is a concept including at least one of gas in which combustion reaction of fuel gas is completed and the fuel gas during the combustion reaction, and including all of a case where both the gas in which the combustion reaction of the fuel gas is completed and the fuel gas during the combustion reaction are included, a case where only the fuel gas during the combustion reaction is included, and a case where only the gas in which the combustion reaction of the fuel gas is completed is included.

First Embodiment

(Entire Structure of Boiler)

FIG. 1 is a schematic cross-sectional view of a boiler according to a first embodiment. FIG. 2 is a schematic partial cross-sectional view of the boiler according to the first embodiment. FIG. 2 is a schematic partial cross-sectional view when a boiler 1 according to the first embodiment is viewed from a Z direction described below, that is, when the boiler 1 is viewed from upward in a vertical direction. As shown in FIGS. 1 and 2, the boiler 1 according to the first embodiment has a can body 10, a blower 12, a duct 14, a burner 16, an exhaust stack 18, a fuel supply unit 20 (see FIG. 2), a primary fuel supply unit 22, a secondary fuel supply unit 24 (see FIG. 2), a cooling line 26 (see FIG. 2), a flow rate adjusting unit 28 (see FIG. 2), and a control device 30.

As shown in FIG. 1, the can body 10 has a main body 40, an upper header 42, a lower header 44, and a water tube group 50. In the present embodiment, an X direction is a flow direction of the combustion gas flowing in the main body 40. Further, a direction perpendicular to the X direction and in a vertical direction, which is upward in the vertical direction, is a Z direction. The Z direction is a direction being the same as the vertical direction in the present embodiment, but is not limited to the vertical direction. The main body 40 is a housing having a longitudinal direction in the X direction, and accommodates the water tube group 50 therein. The upper header 42 is a header connected to an end portion in the Z direction of the main body 40, here, the end portion in the vertical direction of the main body 40. The lower header 44 is a header connected to an end portion in a direction opposite to the Z direction of the main body 40, here, the end portion downward in the vertical direction of the main body 40.

The water tube group 50 has a plurality of water tubes 51, 52, 53 in which water or steam flows. The water pipes 51, 52, 53 are provided in the main body 40 and extend in the Z direction, and connect the upper header 42 and the lower header 44. As shown in FIG. 2, a plurality of water pipes 51 (outside water pipes) are located at both ends of the main body 40 and are aligned in a direction of the combustion gas flowing, that is, in the X direction. A plurality of water pipes 53 (central water pipes) are located inside the water pipes 51 and are aligned in the X direction. A plurality of water pipes 52 (intermediate water pipes) are located between the water pipes 51 and the water pipes 53, and are aligned in the X direction. Further, in the main body 40, a connecting wall 54 connecting the plurality of water tubes 51 is provided extending in the X direction. A space surrounded by the water pipes 51, the connecting wall 54, the upper header 42, and the lower header 44 forms a gas flow space. The water pipes 52 are arranged such that a distance between a part of the water pipes 52 in the X direction is longer than the distance between the other water pipes 52 in the X direction. That is, the water pipes 52 are partially removed. In the gas flow space, the space between the water pipes 52 having this long distance, that is, the space in which the pipe is removed is wider than the space between the other water pipes. Hereinafter, the space between the water pipes 52 having the long distance will be referred to as a combustion promoting space S. Since the combustion promoting space S is kept wide, combustion, that is, oxidation of CO in the combustion gas is promoted. The combustion promoting space S is not limited to the space between the water pipes 52, as long as it is a space kept wider than the other spaces surrounded by the water pipes, and may be a space surrounded by any water pipes. For example, in the main body portion 40, a diameter of the water pipe in the space upstream in the flow direction of the combustion gas is made smaller than that of the water pipes in the space downstream in the flow direction of the combustion gas, so that a pitch of the water pipes in the space upstream in the flow direction of the combustion gas may be larger than that of the water pipes in the space downstream in the flow direction of the combustion gas. In this case, the combustion promoting space S may be provided in the space upstream in the flow direction of the combustion gas. However, the combustion promoting space S may not necessarily be provided.

The blower 12 supplies air A for mixing with a primary fuel F1 described below. The duct 14 is a duct connected to the blower 12 and the air A supplied from the blower 12 flows therethrough. The primary fuel supply unit 22 is connected to the duct 14. Specifically, as shown in FIG. 2, the primary fuel supply unit 22 has a fuel supply line 60 and a primary fuel adjusting valve 62. One end of the fuel supply line 60 is connected to the fuel supply unit 20 for supplying the fuel F, and the other end thereof is connected to the duct 14. The fuel supply line 60 is provided with the primary fuel adjusting valve 62. The fuel F supplied from the fuel supply unit 20 flows through the fuel supply line 60, and at least a part of the flowing fuel F is supplied into the duct 14 as the primary fuel F1 through the primary fuel adjusting valve 62. The primary fuel F1 supplied from the fuel supply line 60 is mixed with the air A supplied from the blower 12 in the duct 14 to generate a mixed gas F1A. The fuel F (that is, the primary fuel F1 and a secondary fuel F2 described below) is a combustible fuel gas, such as natural gas and propane gas, but may be any fuel. Further, the primary fuel adjusting valve 62 is controlled to be opened and closed by the control device 30, to adjust a supply amount of primary fuel F1 to the duct 14.

Further, as shown in FIG. 1, the duct 14 has a pressure reducing member 12A upstream of a portion to which the fuel supply line 60 is connected, in flow of the air A. The pressure reducing member 12A is a member for reducing a pressure of the air A downstream thereof in the flow of the air A than the pressure of the air A upstream thereof in the flow of the air A, for example, by throttling a flow path. The pressure reducing member 12A is, for example, a punching metal in the present embodiment. The boiler 1 has an air differential pressure sensor 12B for detecting a differential pressure between the pressure downstream of the pressure reducing member 12A and the pressure upstream of the pressure reducing member 12A. The air differential pressure sensor 12B detects the differential pressure between the pressure of the air A downstream of the pressure reducing member 12A and the pressure of the air A upstream of the pressure reducing member 12A in the duct 14, and outputs information on the detected differential pressure to the control device 30.

As shown in FIG. 2, the primary fuel supply unit 22 further includes a pressure reducing member 64 and a fuel differential pressure sensor 66. The pressure reducing member 64 is a member provided in the fuel supply line 60 and for reducing a pressure of the primary fuel F1 downstream thereof in flow of the fuel F than the pressure of the primary fuel F1 upstream thereof in the flow of the fuel F, for example, by throttling a flow path. More specifically, the pressure reducing member 64 is provided downstream of the primary fuel adjusting valve 62 and upstream of a connection portion of the fuel supply line 60 with the duct 14. In the present embodiment, the pressure reducing member 64 is, for example, an orifice. Further, the fuel differential pressure sensor 66 detects a differential pressure between the pressure downstream of the pressure reducing member 64 and the pressure upstream of the pressure reducing member 64. The fuel differential pressure sensor 66 detects a differential pressure between the pressure of the primary fuel F1 downstream of the pressure reducing member 64 and the pressure of the primary fuel F1 upstream of the pressure reducing member 64 in the fuel supply line 60, and outputs information on the detected differential pressure to the control device 30.

The duct 14 is also connected to the main body 40 of the can body 10. The duct 14 is connected to a portion opposite to the X direction of the main body 40, that is, an upstream portion in the flow direction of the combustion gas. Further, a burner 16 is provided at a connection portion between the duct 14 and the main body 40. That is, it can be said that the burner 16 is connected to the can body 10 and is connected to a portion opposite to the X direction of the main body 40. The mixed gas F1A flowing through the duct 14 is supplied to the burner 16. The burner 16 supplies the mixed gas F1A, that is, the primary fuel F1 and the air A into the main body 40 of the can body 10.

The exhaust stack 18 is connected to the main body 40 of the can body 10 and, more specifically, connected to a portion in the X direction of the main body 40 (the most downstream portion in the flow direction of the combustion gas). The combustion gas in the main body 40 is discharged as exhaust gas from inside the main body 40 to the exhaust stack 18.

The secondary fuel supply unit 24 supplies the secondary fuel F2 into the can body 10 downstream of the burner 16 in the flow direction of the combustion gas. Specifically, as shown in FIG. 2, the secondary fuel supply unit 24 has secondary fuel supply lines 70, 74 and a secondary fuel adjusting valve 72. The secondary fuel supply line 70 is connected to the fuel supply line 60. More specifically, the secondary fuel supply line 70 is connected to the fuel supply line 60 at a portion between the connection point with the fuel supply unit 20 and the connection point with the primary fuel adjusting valve 62. Accordingly, in the fuel supply line 60, at least a part of the fuel F is supplied as the secondary fuel F2 from the fuel supply unit 20. The secondary fuel adjusting valve 72 is provided in the secondary fuel supply line 70. The secondary fuel adjusting valve 72 adjusts a supply amount of secondary fuel F2 to the secondary fuel supply line 70 by the control device 30 controlling opening and closing.

The secondary fuel supply line 74 is connected to a portion downstream of the connection portion of the secondary fuel supply line 70 with the secondary fuel adjusting valve 72 in flow of the secondary fuel F2. The secondary fuel supply line 74 is supplied with the secondary fuel F2 from the secondary fuel supply line 70 through the secondary fuel adjusting valve 72. The secondary fuel supply line 74 is also connected to the main body 40 of the can body 10. More specifically, the secondary fuel supply line 74 is connected to a portion in the X direction (downstream in the flow direction of the combustion gas) than a connection portion of the main body 40 with the burner 16. Therefore, the secondary fuel supply line 74 supplies the secondary fuel F2 from the secondary fuel supply line 70 into the can body 10 downstream of the burner 16 in the flow direction of the combustion gas. Furthermore, the secondary fuel supply line 74 is connected to a downstream side of a portion in which the combustion by the primary fuel F1 starts in the can body 10, that is, the downstream side of an ignition unit (not shown). The secondary fuel supply line 74 is preferably connected to a position in which temperature of the combustion gas is 800° C. or higher, that is, the position in which the secondary fuel F2 is appropriately self-combusted to a temperature capable of suppressing CO generation. Further, the secondary fuel supply line 74 is preferably connected to a portion opposite to the X direction (upstream in the flow direction of the combustion gas) than the combustion promoting space S. In the present embodiment, two secondary fuel supply lines 74 are provided branched from the secondary fuel supply line 70, and each of them is connected to the main body 40. However, the number of secondary fuel supply lines 74 is arbitrary, and may be one, for example.

As shown in FIG. 2, one end of the cooling line 26 is connected to the exhaust stack 18 and the other end thereof is connected to the secondary fuel supply line 70. The exhaust gas from the exhaust stack 18, that is, the combustion gas discharged from the can body 10 is supplied as a cooling fluid G0 to the cooling line 26. The cooling line 26 is connected to a portion downstream of the connection portion with the secondary fuel adjusting valve 72 and upstream of the connection portion with the secondary fuel supply line 74 of the secondary fuel supply line 70 in the flow of the secondary fuel F2. Therefore, the cooling fluid G0 flowing through the cooling line 26 is supplied to the secondary fuel supply line 74 through the secondary fuel supply line 70, and introduced into the can body 10 downstream of the burner 16 in the flow direction of the combustion gas from the secondary fuel supply line 74. Furthermore, the secondary fuel F2 is also supplied to the secondary fuel supply line 74. Therefore, in the secondary fuel supply line 74, the secondary fuel F2 and the cooling fluid G0 are mixed to generate a mixed gas F2A. The secondary fuel supply line 74 introduces the mixed gas F2A, that is, the secondary fuel F2 and the cooling fluid G0 into the can body 10.

The flow rate adjusting unit 28 is provided in the cooling line 26. The flow rate adjusting unit 28 adjusts a supply amount of cooling fluid G0 introduced into the can body 10 from the cooling line 26 through the secondary fuel supply line 74 by control of the control device 30. In the present embodiment, the flow rate adjusting unit 28 is a fan. The flow rate adjusting unit 28 draws the cooling fluid G0 (exhaust gas) from the exhaust stack 18 to supply it into the cooling line 26, and supplies the cooling fluid G0 supplied into the cooling line 26 into the can body 10 through the secondary fuel supply line 74. The flow rate adjusting unit 28 adjusts the supply amount of cooling fluid G0 to be supplied to the can body 10 by the control device 30 controlling rotational speed of built-in vane (not shown). However, the flow rate adjusting unit 28 is not limited to controlling the rotational speed, and may adjust the supply amount of cooling fluid G0 by any method, as long as it can adjust the supply amount of cooling fluid G0 to be supplied by the control of the control device 30. For example, the flow rate adjusting unit 28 may adjust the supply amount of cooling fluid G0 by controlling an opening degree of the built-in vane.

In the present embodiment, the cooling line 26, the secondary fuel supply line 70, and the secondary fuel supply line 74 are described as separate pipes. However, since the cooling line 26, the secondary fuel supply line 70, and the secondary fuel supply line 74 are connected to each other, the cooling line 26, the secondary fuel supply line 70, and the secondary fuel supply line 74 can be rephrased as one tube.

In the boiler 1 structured as described above, first, the primary fuel F1 introduced from the fuel supply line 60 and the air A supplied from the blower 12 are mixed in the duct 14 to generate the mixed gas F1A. The mixed gas F1A is supplied from the burner 16 into the main body 40 of the can body 10. The mixed gas F1A supplied into the main body 40 is ignited by the ignition unit (not shown), and the combustion gas with flame in the combustion reaction is formed by the burner 16. The combustion gas flows in the X direction while exchanging heat with the water pipes 51, 52, 53 in the main body 40. The cooling fluid G0 introduced from the cooling line 26 and the secondary fuel F2 introduced from the secondary fuel supply line 70 are mixed in the secondary fuel supply line 74, and introduced to a portion downstream of the burner 16 in the main body 40 in the flow of the combustion gas as mixed gas F2A. The mixed gas F2A is burned in contact with the combustion gas. Thus, the boiler 1 performs two-stage combustion by supplying the mixed gases F1A and F2A from the burner 16 and the secondary fuel supply line 74. The combustion gas supplied with the mixed gas F2A and burned in two stages further flows in the X direction while exchanging heat with the water pipes 51, 52, 53 in the main body 40, and is discharged from the exhaust stack 18 as the exhaust gas.

The boiler 1 according to the present embodiment can reduce an amount of NOx and an amount of CO contained in the combustion gas discharged from the can body 10 by performing the two-stage combustion in this manner. Here, in order to make the secondary fuel F2 self-burn to suppress the generation of CO, it is preferable to maintain temperature at a supply position of the secondary fuel F2 high to some extent. However, if the temperature at the supply position of the secondary fuel F2 is too high, combustion temperature (maximum temperature reached by the combustion) by the secondary fuel F2 may be too high, and the amount of NOx in the combustion gas may increase. Therefore, when performing the two-stage combustion in the boiler 1, it is preferable to maintain the temperature in the can body 10 within a predetermined range in order to suppress the amount of NOx and the amount of CO.

The boiler 1 according to the present embodiment maintains the temperature in the can body 10 within the predetermined range by introducing the cooling fluid G0 into the can body 10. The cooling fluid G0 is a fluid that is not burned by the combustion gas, and has a lower temperature than the combustion gas in the can body 10, so that the temperature in the can body 10 can be reduced. Here, the position at which the cooling fluid G0 is supplied in the can body 10, in other words, the position at which the secondary fuel supply line 74 is connected in the can body 10, is taken as the supply position. The cooling line 26 reduces the temperature of the supply position by supplying the cooling fluid G0 to the supply position in the can body 10. Here, a space in the can body 10 between the supply position and a position in the X direction by a predetermined distance from the supply position is taken as a predetermined space. The cooling line 26 can be said to reduce temperature of the predetermined space by supplying the cooling fluid G0 to the supply position. In the first embodiment, the predetermined space is a space from the supply position to which the secondary fuel F2 is supplied to a downstream side of the combustion gas. That is, it can be said that the cooling line 26 reduces the temperature of the space in which an unburned portion of the primary fuel F1, and the secondary fuel F2 or the secondary fuel F2 burn by supplying the cooling fluid G0 to the supply position. In other words, it can be said that the predetermined space is a space in which at least the secondary fuel F2 out of the primary fuel F1 and the secondary fuel F2 is burned. In the present embodiment, since the supply position is fixed, the predetermined space is a space whose position is fixed in the can body 10, and the position does not move in the can body 10.

As described above, the boiler 1 reduces the temperature of the predetermined space in the can body 10 by introducing the cooling fluid G0 into the can body 10. Further, the boiler 1 maintains the temperature in the can body 10 within the predetermined range by adjusting the amount of cooling fluid G0 to be supplied by the control device 30. The control device 30 will be described below.

(Structure of Control Device)

FIG. 3 is a schematic block diagram of the control device according to the first embodiment. As shown in FIG. 3, the control device 30 has a control unit 80 and a storage unit 82. The control device 30 is a computer for controlling the boiler 1. The storage unit 82 is a memory for storing operation contents of the control unit 80, information on program, and the like. The storage unit 82 includes, for example, at least one external storage device such as a RAM (Random Access Memory), a ROM (Read Only Memory), or a flash memory.

The control unit 80 is a computing device, that is, a CPU (Central Processing Unit). The control unit 80 has an air controller 84, a primary fuel controller 86, a secondary fuel controller 88, and a fluid controller 90. The air controller 84, the primary fuel controller 86, the secondary fuel controller 88, and the fluid controller 90 perform processing to be described below by reading software (program) stored in the storage unit 82. However, the air controller 84, the primary fuel controller 86, the secondary fuel controller 88, and the fluid controller 90 may be respectively constituted by dedicated hardware circuits.

The air controller 84 calculates the supply amount of air A supplied to the can body 10 and controls the supply amount of air A supplied to the can body 10 so as to be the calculated supply amount. Specifically, the air controller 84 calculates the supply amount of air A depending on, for example, a combustion stage instructed to the boiler 1, that is, an instruction of what kind of combustion stage the boiler 1 is operated in. For example, in the present embodiment, information indicating a relationship between the combustion stage and the supply amount of air A is stored in the storage unit 82. The air controller 84 reads this information from the storage unit 82 and substitutes the instructed combustion stage into the relationship to calculate the supply amount of air A. For example, the supply amount of air A is set to be larger as the instructed combustion stage is larger, that is, as the combustion is higher. A method for calculating the supply amount of air A is not limited to this, and may be set arbitrarily. Further, in the boiler 1 in the present embodiment, a plurality of combustion stages is set for each amount of combustion, and for example, four of stop, low combustion, medium combustion, and high combustion are set.

The air controller 84 controls the blower 12 so that the amount of air A thus calculated is supplied. For example, the air controller 84 supplies the calculated amount of air A to the duct 14 by adjusting an opening degree of a damper (not shown) provided upstream of the pressure reducing member 12A shown in FIG. 1. For example, the air controller 84 obtains information on the differential pressure between the upstream side and the downstream side of the pressure reducing member 12A from the air differential pressure sensor 12B shown in FIG. 1. The air controller 84 obtains the amount of air A actually supplied to the duct 14 from the information on the differential pressure detected by the air differential pressure sensor 12B, and adjusts the opening degree of the damper to actually supply the calculated amount of air A based on the obtained actual supply amount of air A. The air controller 84 is not limited to adjusting the opening degree of the damper when controlling the supply amount of air A. For example, the air controller 84 may control the supply amount of air A by controlling rotational speed of the blower 12 by the inverter, or control the supply amount of air A using both the inverter and the damper.

The primary fuel controller 86 calculates the supply amount of primary fuel F1 to be supplied to the can body 10 and controls the supply amount of primary fuel F1 to be supplied to the can body 10 so as to be the calculated supply amount. Specifically, the primary fuel controller 86 calculates a target supply amount of primary fuel F1 based on the supply amount of air A to the duct 14. The primary fuel controller 86 obtains information on the differential pressure between the upstream side and the downstream side of the pressure reducing member 12A from the air differential pressure sensor 12B shown in FIG. 1, and obtains the supply amount of air A to the duct 14 from the obtained information on the differential pressure. The primary fuel controller 86 may obtain information on the supply amount of air A from the air controller 84. Then, in the present embodiment, information indicating a relationship between the supply amount of air A and the supply amount of primary fuel F1 is stored in the storage unit 82. The primary fuel controller 86 reads the information indicating the relationship between the supply amount of air A and the supply amount of primary fuel F1 from the storage unit 82, and substitutes the obtained supply amount of air A into the relationship to calculate the target supply amount of primary fuel F1. As described above, in the boiler 1, the primary fuel controller 86 controls to supply the primary fuel F1 of the target supply amount calculated based on the information indicating the relationship between the supply amount of air A and the supply amount of primary fuel F1; however, a method for supplying the primary fuel F1 of target supply amount is not restricted to this. For example, the boiler 1 may be provided with a mechanical governor in the fuel supply line 60, and may be controlled to supply the primary fuel F1 of the target supply amount by changing an operation amount of governor depending on the differential pressure between the upstream side and the downstream side of the pressure reducing member 12A (that is, the differential pressure detected by the air differential pressure sensor 12B). That is, the operation amount of governor may be set in association with the supply amount (differential pressure) of air A, and the governor may be operated depending on the supply amount of air A to supply the primary fuel F1 of the target supply amount. Note that the target supply amount of primary fuel F1 is set such that ratio of oxygen contained in the combustion gas after the combustion of the primary fuel F1 is, for example, 6% or more and 10% or less, preferably about 8%. The ratio of oxygen contained in the combustion gas refers to the ratio of the amount of oxygen contained in the combustion gas to a total amount of combustion gas. The combustion gas after the combustion of the primary fuel F1 refers to the combustion gas in a state in which the combustion reaction of the primary fuel F1 is completed, and it can be said that it refers to the combustion gas in the state in which the combustion reaction of the primary fuel F1 is completed and the secondary fuel F2 is not supplied. Since the combustion reaction may continue in a very small amount even in the above-described “combustion gas in the state in which the combustion reaction is completed”, “completion of the combustion reaction” may not mean 100% completion of the combustion reaction, that is, complete combustion. The target supply amount of primary fuel F1 can also be rephrased as being set such that an air ratio in the mixed gas F1A is a predetermined value. In this case, the air ratio is preferably, for example, 1.4 or more and 2.0 or less. The target supply amount of primary fuel F1 is not limited to the above description and may be calculated by any method.

The primary fuel controller 86 controls the primary fuel supply unit 22 so that the primary fuel F1 of the target supply amount thus calculated is supplied. Specifically, the primary fuel controller 86 controls the opening degree of the primary fuel adjusting valve 62 to supply the primary fuel F1 for the calculated supply amount to the can body 10. In the present embodiment, the primary fuel controller 86 may obtain information on the differential pressure of the primary fuel F1 between the upstream side and the downstream side of the pressure reducing member 64 from the fuel differential pressure sensor 66, and may obtain the amount of primary fuel F1 actually supplied to the can body 10 from the information on the differential pressure. In this case, the primary fuel controller 86 adjusts the opening degree of the primary fuel adjusting valve 62 so that the amount of primary fuel F1 actually supplied is the target supply amount. The primary fuel controller 86 is not limited to adjusting the opening degree of the primary fuel adjusting valve 62 when controlling the supply amount of primary fuel F1. For example, the primary fuel controller 86 may control the supply amount of primary fuel F1 by means of the mechanical governor provided in the fuel supply line 60, or may control the supply amount of primary fuel F1 using both the governor and the primary fuel adjusting valve 62.

The secondary fuel controller 88 calculates the supply amount of secondary fuel F2 to be supplied to the can body 10, and controls the supply amount of secondary fuel F2 to be supplied to the can body 10 so as to be the calculated supply amount. Specifically, the secondary fuel controller 88 obtains the information on the differential pressure of the primary fuel F1 between the upstream side and the downstream side of the pressure reducing member 64 from the fuel differential pressure sensor 66, and obtains the supply amount of primary fuel F1 to the can body 10 from the information on the obtained differential pressure. The secondary fuel controller 88 may obtain information on the supply amount of primary fuel F1 from the primary fuel controller 86. Then, in the present embodiment, information indicating the relationship between the supply amount of primary fuel F1 and the supply amount of secondary fuel F2 is stored in the storage unit 82. The secondary fuel controller 88 reads the information indicating the relationship between the supply amount of primary fuel F1 and the supply amount of secondary fuel F2 from the storage unit 82, and substitutes the obtained supply amount of primary fuel F1 in the relationship, to calculate the target supply amount of secondary fuel F2. The target supply amount of secondary fuel F2 is set such that the ratio of oxygen contained in the combustion gas after the combustion of the secondary fuel F2 is, for example, 2% or more and 6% or less, preferably about 4%. The combustion gas after the combustion of the secondary fuel F2 refers to the combustion gas in a state in which the combustion reaction of the secondary fuel F2 is completed. Furthermore, it may be rephrased that the combustion gas after the combustion of the secondary fuel F2 is the combustion gas in a state in which the combustion reaction of both the primary fuel F1 and the secondary fuel F2 is completed, and may be rephrased that it is the combustion gas (exhaust gas) discharged from the can body 10. Further, it can be rephrased that the target supply amount of secondary fuel F2 is set such that the air ratio in the case of summing the mixed gas F1A and the mixed gas F2A is a predetermined value. In this case, the air ratio is preferably, for example, 1.1 or more and 1.4 or less. The target supply amount of secondary fuel F2 is not limited to the above description and may be calculated by any method.

Thus, the target supply amount of secondary fuel F2 is set depending on the supply amount of primary fuel F1. When the supply amount of primary fuel F1 is adjusted by the information on the differential pressure from the fuel differential pressure sensor 66, the supply of the secondary fuel F2 is also appropriately performed by responding to the supply amount of primary fuel F1. However, a method for calculating the target supply amount of secondary fuel F2 is not limited to this. For example, an oxygen concentration sensor may be provided in the exhaust stack 18, and the secondary fuel controller 88 may set the target supply amount of secondary fuel F2 such that the ratio of oxygen contained in the combustion gas after combustion of the secondary fuel F2 is a predetermined ratio in response to a detection result of oxygen concentration contained in the combustion gas in the exhaust stack 18 by the oxygen concentration sensor.

The secondary fuel controller 88 controls the secondary fuel supply unit 24 so that the secondary fuel F2 of the target supply amount thus calculated is supplied. Specifically, the secondary fuel controller 88 controls the opening degree of the secondary fuel adjusting valve 72, to supply the can body 10 with the secondary fuel F2 for the calculated supply amount. The secondary fuel controller 88 is not limited to adjusting the opening degree of the secondary fuel adjusting valve 72 when controlling the supply amount of secondary fuel F2. For example, the secondary fuel controller 88 may control the supply amount of secondary fuel F2 by means of the mechanical governor provided in the secondary fuel supply line 70, or may control the supply amount of secondary fuel F2 using both the governor and the secondary fuel adjusting valve 72.

Note that it can be said that the supply amount of secondary fuel F2 is an amount capable of consuming about 8% of oxygen contained in the combustion gas to about 4% by combustion, and it is proportional to the supply amount of primary fuel F1. That is, the secondary fuel controller 88 increases the supply amount of secondary fuel F2 in proportion to an increase of the supply amount of primary fuel F1.

The fluid controller 90 controls the flow rate adjusting unit 28 to change the amount of cooling fluid G0 supplied into the can body 10 depending on the supply amount of secondary fuel F2. More specifically, the fluid controller 90 calculates the supply amount of cooling fluid G0 to be supplied to the can body 10 and controls the supply amount of cooling fluid G0 to be supplied to the can body 10 so as to be the calculated supply amount. The fluid controller 90 controls the flow rate adjusting unit 28 to supply the can body 10 with the cooling fluid G0 for the calculated supply amount.

The fluid controller 90 calculates the amount of cooling fluid G0 to be supplied into the can body 10 based on the supply amount of secondary fuel F2. Specifically, the fluid controller 90 calculates the supply amount of cooling fluid G0 such that the temperature of the predetermined space in the can body 10 is within a predetermined temperature range. The predetermined temperature range is 800° C. or more and 1200° C. or less, but more preferably 1000° C. or more and 1200° C. or less. Further, as described above, since the predetermined space is a space in which the unburned portion of the primary fuel F1 and the secondary fuel F2, or the secondary fuel F2 is burned, it can be said that the fluid controller 90 controls the flow rate adjusting unit 28 to adjust a flow rate of the cooling fluid G0 such that the combustion temperature by the primary fuel F1 and the secondary fuel F2 is within the predetermined temperature range. In other words, it can be said that the fluid controller 90 supplies the cooling fluid G0 for the flow rate capable of setting the combustion temperature in the predetermined space within the predetermined temperature range. For example, in the present embodiment, information indicating the relationship between the supply amount of secondary fuel F2 and the supply amount of cooling fluid G0 such that the temperature of the predetermined space in the can body 10 is within the predetermined temperature range is stored in the storage unit 82. The air controller 84 reads the information from the storage unit 82, and substitutes the supply amount of secondary fuel F2 into the relationship to calculate the supply amount of cooling fluid G0. Since the supply position of the secondary fuel F2 (connection position of the secondary fuel supply line 74) is provided at a position in which the temperature of the combustion gas is 800° C. or higher, it can also be said that the fluid controller 90 controls the supply amount of cooling fluid G0 so that a temperature at a predetermined position is 1200° C. or less.

As described above, the fluid controller 90 calculates the supply amount of cooling fluid G0 to the can body 10 based on the supply amount of secondary fuel F2. However, a method for calculating the supply amount of cooling fluid G0 is not limited to this. For example, the fluid controller 90 may calculate the supply amount of cooling fluid G0 to the can body 10 based on the supply amount of primary fuel F1, may calculate the supply amount of cooling fluid G0 to the can body 10 based on the supply amount of air A to the duct 14, or may calculate the supply amount of cooling fluid G0 to the can body 10 based on the combustion stage instructed to the boiler 1. In this case, for example, the fluid controller 90 stores in the storage unit 82, information indicating a relationship between the supply amount of primary fuel F1 and the supply amount of cooling fluid G0, information indicating a relationship between the supply amount of air A and the supply amount of cooling fluid G0, or information indicating a relationship between the combustion stage and the supply amount of cooling fluid G0. The fluid controller 90 reads the information from the storage unit 82 and substitutes the supply amount of primary fuel F1, the supply amount of air A, or the combustion stage into the relationship to calculate the supply amount of cooling fluid G0. The boiler 1 may be provided with a nitrogen oxide concentration sensor in the exhaust stack 18, and the supply amount of cooling fluid G0 to the can body 10 may be calculated based on a detection result of the nitrogen oxide concentration sensor. In this case, the fluid controller 90 obtains information on nitrogen oxide concentration in the cooling fluid G0 in the exhaust stack 18 detected by the nitrogen oxide concentration sensor. Then, the fluid controller 90 calculates the flow rate of the cooling fluid G0 from the obtained nitrogen oxide concentration such that the content of the nitrogen oxide is within a predetermined range. The fluid controller 90 controls the flow rate adjusting unit 28 to supply the can body 10 with the cooling fluid G0 for the calculated flow rate.

As described above, the cooling fluid G0 can reduce the temperature of the combustion gas, and the temperature can be further reduced as the supply amount of cooling fluid G0 increases. Here, the combustion temperature in the can body 10 depends on the combustion amount, that is, the supply amounts of the primary fuel F1, the secondary fuel F2, and the air A; however, the supply amount of secondary fuel F2 is calculated based on the supply amount of primary fuel F1, and the supply amount of primary fuel F1 is calculated based on the supply amount of air A. That is, the supply amounts of the primary fuel F1, the secondary fuel F2, and the air A are related to each other. Therefore, it can be said that the combustion temperature in the can body 10 depends on the supply amount of primary fuel F1. Therefore, it can be said that the fluid controller 90 changes the supply amount of cooling fluid G0 depending on the supply amount of primary fuel F1 in order to maintain the temperature in the predetermined space in the predetermined temperature range. That is, the fluid controller 90 increases the amount of cooling fluid G0 to be supplied as the supply amount of primary fuel F1 increases, and reduces the amount of cooling fluid G0 to be supplied as the supply amount of primary fuel F1 is reduced.

The supply amount of cooling fluid G0 will be further described. FIG. 4 is a graph showing a relationship between the amount of primary fuel, the amount of secondary fuel, and the amount of cooling fluid. In FIG. 4, a horizontal axis is the supply amount of primary fuel F1. A line segment L1 indicates the relationship between the supply amount of primary fuel F1 and the supply amount of secondary fuel F2. As indicated by the line segment L1, the secondary fuel controller 88 linearly increases the supply amount of secondary fuel F2 in response to a linear increase of the supply amount of primary fuel F1. That is, a rate at which the flow rate of the secondary fuel F2 increases when the supply amount of primary fuel F1 increases is substantially constant. A line segment L2 indicates a relationship between the supply amount of primary fuel F1 and the supply amount of cooling fluid G0. As indicated by the line segment L2, when the supply amount of primary fuel F1 linearly increases, the fluid controller 90 increases the supply amount of secondary fuel F2 in the form of a quadratic curve convex downward. That is, the fluid controller 90 controls the supply amount of cooling fluid G0 such that the rate increases at which the supply amount of cooling fluid G0 increases when the supply amount of primary fuel F1 increases, as the supply amount of primary fuel F1 increases. In the case of a high combustion state in which the supply amount of primary fuel F1 is large, a rate of increase in temperature of the can body 10 may be high. The fluid controller 90 can preferably suppress temperature rise in the high combustion state by increasing the supply amount of cooling fluid G0 in the high combustion state as in the line segment L2.

The control device 30 is structured as described above. Next, a flow of method of supplying fuel and the like by the control device 30 will be described based on a flowchart. FIG. 5 is a flowchart illustrating a control flow of the control unit according to the first embodiment. As shown in FIG. 5, first, the control unit 80 calculates the supply amount of air A by the air controller 84, and controls an air blowing amount so that the calculated amount of air A is supplied (Step S10). Then, the control unit 80 calculates the supply amount of primary fuel F1 based on the supply amount of air A by the primary fuel controller 86, and controls the primary fuel adjusting valve 62 so that the calculated amount of primary fuel F1 is supplied (Step S12). Then, the control unit 80 calculates the supply amount of secondary fuel F2 based on the supply amount of primary fuel F1 by the secondary fuel controller 88, and controls the secondary fuel adjusting valve 72 so that the calculated amount of secondary fuel F2 is supplied (Step S14). Then, the control unit 80 calculates the supply amount of cooling fluid G0 such that the predetermined space in the can body 10 has the predetermined temperature range (for example, 800° C. or more and 1200° C. or less) based on the supply amount of secondary fuel F2 by the fluid controller 90, and controls the flow rate adjusting unit 28 so that the calculated amount of cooling fluid G0 is supplied (Step S16). Thus, the present process is completed by supplying the primary fuel F1, the air A, the secondary fuel F2, and the cooling fluid G0.

In description of FIG. 5, detection of the combustion stage, supply control of the air A based on the combustion stage, supply control of the primary fuel F1 based on the supply amount of air A, supply control of the secondary fuel F2 based on the supply amount of primary fuel F1, and supply control of the cooling fluid G0 based on the supply amount of secondary fuel F2 are performed in this order. However, each control is not limited to being performed in this order. For example, the control device 30 may store in the storage unit 82 in advance, the information (table) indicating the relationship between the combustion stage and the supply amount of air A, information indicating a relationship between the combustion stage and the supply amount of primary fuel F1, information indicating a relationship between the combustion stage and the supply amount of secondary fuel F2, and information indicating a relationship between the combustion stage and the supply amount of cooling fluid G0. In this case, the controller 30 reads the information, calculates the supply amount of air A, the supply amount of primary fuel F1, the supply amount of secondary fuel F2, and the supply amount of cooling fluid G0 from the combustion stage instructed to the boiler 1, and supplies the air A, the primary fuel F1, the secondary fuel F2, and the cooling fluid G0 so that they are the calculated supply amounts. After performing the supply control in this manner, the control device 30 may finely adjust the supply amounts by performing the supply control of the primary fuel F1 based on the supply amount of air A, the supply control of the secondary fuel F2 based on the supply amount of primary fuel F1, and the supply control of the cooling fluid G0 based on the supply amount of secondary fuel F2 as described above.

As described above, the boiler 1 according to the present embodiment has the can body 10 having the water pipes 51, 52, 53, the burner 16, the secondary fuel supply unit 24, the cooling line 26, and the flow rate adjusting unit 28, and the control unit 80. The burner 16 is connected to the can body 10 and supplies the primary fuel F1 and the air A into the can body 10. The secondary fuel supply unit 24 supplies the secondary fuel F2 into the can body 10 downstream of the burner 16 in the flow direction of the combustion gas. The cooling line 26 introduces the cooling fluid G0 for reducing the temperature in the predetermined space in the can body 10 downstream of the burner 16 in the flow direction of the combustion gas. The flow rate adjusting unit 28 is provided in the cooling line 26 and can adjust the flow rate of the cooling fluid G0 to be introduced into the can body 10 from the cooling line 26. The control unit 80 controls the flow rate adjusting unit 28 and supplies the flow rate of the cooling fluid G0 to be introduced into the can body 10 such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

The boiler 1 according to the present embodiment suppresses the generation of CO by setting the temperature of the predetermined space in the can body 10 to 800° C. or higher. Further, the boiler 1 reduces NOx by setting the temperature of the predetermined space in the can body 10 to 1200° C. or less by the cooling fluid G0. Here, for example, when the boiler 1 having a plurality of combustion stages performs low combustion as described above, if the combustion stage is shifted to increase the amount of combustion from the required load, since the temperature rise is large, it may be difficult to maintain the temperature in the predetermined space at 800° C. to 1200° C. On the other hand, the boiler 1 according to the present embodiment can control the temperature in the predetermined space to 800° C. to 1200° C. by cooling with the cooling fluid G0. Therefore, according to the boiler 1, even if the combustion stage is changed, NOx and CO can be appropriately reduced by controlling the flow rate adjusting unit 28 to adjust the flow rate of the cooling fluid G0. Furthermore, the boiler 1 according to the present embodiment may control the supply amount of the cooling fluid G0 depending on the supply amount of primary fuel F1, the secondary fuel F2, or the air A. In this case, for example, the boiler 1 can suppress that the supply amount of the cooling fluid G0 is excessive and the temperature is too low when the supply amount of primary fuel F1, the secondary fuel F2 or the air A is small, or suppress that the supply amount of the cooling fluid G0 is insufficient and the temperature is too high when the supply amount of primary fuel F1, the secondary fuel F2 or the air A is large.

The control unit 80 controls the flow rate adjusting unit 28 such that the rate increases at which the flow rate of the cooling fluid G0 increases when the supply amount of primary fuel F1 increases, as the supply amount of primary fuel F1 increases. The boiler 1 can appropriately suppress the temperature rise in the high combustion state by thus increasing the supply amount of the cooling fluid G0 in the high combustion state.

The secondary fuel supply unit 24 (secondary fuel supply line 70) is connected to the cooling line 26, and supplies the secondary fuel F2 into the can body 10 in a state of being mixed with the cooling fluid G0. Since the boiler 1 supplies the secondary fuel F2 into the can body 10 in a state of being mixed with the cooling fluid G0, it is possible to restrain the temperature from being excessively raised by the cooling fluid G0 while being suitably burned in two stages by the secondary fuel F2.

However, the cooling line 26 may not be connected to the secondary fuel supply unit 24, and the secondary fuel F2 and the cooling fluid G0 may be separately supplied into the can body 10 without being mixed. In this case, the cooling line 26 is preferably connected to the can body 10 upstream of the secondary fuel supply unit 24 in the flow of the combustion gas. That is, in this case, the cooling line 26 is connected to a position between the ignition unit (not shown) and the secondary fuel supply unit 24 (secondary fuel supply line 74). Therefore, in this case, it can be said that the secondary fuel supply unit 24 supplies the secondary fuel F2 into the can body 10 downstream of the cooling line 26 in the flow direction of the combustion gas. Thus, by supplying the secondary fuel F2 to the downstream side of the cooling fluid G0, the combustion gas can be cooled, and the reaction with the secondary fuel F2 can be slowed to suppress the temperature rise. Further, by supplying the secondary fuel F2 to the downstream side of the cooling fluid G0, that is, by supplying the cooling fluid G0 to the upstream side of the secondary fuel F2, it is possible to block the flame of the combustion gas by the primary fuel F1 from going downstream by the cooling fluid G0. Thus, it is possible to restrain the flame of the combustion gas from coming into contact with the secondary fuel F2, and to suppress the temperature rise due to the secondary fuel F2 burning with the flame.

The flow rate adjusting unit 28 is a fan for supplying the exhaust gas discharged from inside the can body 10 into the cooling line 26. The cooling line 26 introduces the exhaust gas supplied from the flow rate adjusting unit 28 into the can body 10 as the cooling fluid G0. The boiler 1 can suitably cool the inside of the can body 10 by using the exhaust gas as the cooling fluid G0. Further, by introducing the exhaust gas into the can body 10, it also functions as EGR (Exhaust Gas Recirculation), and NOx can be suitably reduced. Furthermore, by setting the flow rate adjusting unit 28 as a fan, it can suitably take in the exhaust gas as the cooling fluid G0, and suitably control an intake amount of the exhaust gas.

However, the cooling fluid G0 is not limited to the exhaust gas, and may be any fluid as long as it is a fluid capable of reducing the temperature in the can body 10 raised by the combustion gas. More specifically, it is preferable that the cooling fluid G0 be a fluid that is not burned by the combustion gas, and be a fluid that is cooler than the combustion gas in the can body 10. The cooling fluid G0 is preferably a gas, but may be a liquid. Examples of the cooling fluid G0 other than the exhaust gas include steam, water, and an inert gas. That is, the cooling line 26 may introduce at least one or more of the exhaust gas discharged from inside the can body 10, the steam, the water, and the inert gas into the can body 10 as the cooling fluid G0. By using such a cooling fluid G0, the temperature rise in the can body 10 can be suitably suppressed. Note that examples of the inert gas include nitrogen, carbon dioxide, and argon.

Second Embodiment

Next, a second embodiment will be described. A boiler 1a according to the second embodiment shows an example in which the steam is used as the cooling fluid. Descriptions of portions of the second embodiment having the same structure as that in the first embodiment will be omitted.

FIG. 6 is a schematic cross-sectional view of the boiler according to the second embodiment. As shown in FIG. 6, the boiler 1a according to the second embodiment has a cooling line 26a and a flow rate adjusting unit 28a. One end of the cooling line 26a is connected to the upper header 42 or a steam header (not shown), and the other end thereof is connected to the secondary fuel supply line 70 through an ejector 76a. The cooling line 26a is supplied with the steam from the upper header 42 as a cooling fluid G0a. The cooling fluid G0a flowing through the cooling line 26a is supplied to the secondary fuel supply line 74 through the secondary fuel supply line 70 and introduced into the can body 10 in a state of being mixed with the secondary fuel F2. The flow rate adjusting unit 28a is an on-off valve which is controlled to be opened or closed by the fluid controller 90 of the control device 30. The flow rate adjusting unit 28a adjusts the supply amount of the cooling fluid G0a into the can body 10 by being controlled to be opened and closed. Since the cooling fluid G0a is the steam, the cooling line 26a may not be provided with the fan for taking in the cooling fluid G0a.

FIG. 7 is a schematic view of the ejector according to the second embodiment. As shown in FIG. 7, the ejector 76a has an inner cylinder 76a1 and an outer cylinder 76a2. The inner cylinder 76a1 is provided inside the outer cylinder 76a2. One end of the inner cylinder 76a1 is connected to the cooling line 26a, and the other end thereof is tapered. One end of the outer cylinder 76a2 is connected to the secondary fuel supply line 70, and the other end thereof is connected to the secondary fuel supply line 74. The cooling fluid G0a supplied from the cooling line 26a to the inner cylinder 76a1 is injected from the inner cylinder 76a1 into the outer cylinder 76a2, and flows from the outer cylinder 76a2 to the secondary fuel supply line 74. On the other hand, the secondary fuel F2 is drawn into the outer cylinder 76a2 from inside the secondary fuel supply line 70 by injection of the cooling fluid G0a into the outer cylinder 76a2. The secondary fuel F2 drawn into the outer cylinder 76a2 flows from the outer cylinder 76a2 to the secondary fuel supply line 74, and is mixed with the cooling fluid G0a.

As in the boiler 1a according to the second embodiment, even if the steam is used as the cooling fluid G0a, NOx and CO can be appropriately reduced as in the first embodiment. The boiler 1a according to the second embodiment has the ejector 76a connected to the cooling line 26a through which the cooling fluid G0a flows and the secondary fuel supply line 70 through which the secondary fuel F2 flows. The ejector 76a injects the cooling fluid G0a from the cooling line 26a into an inside thereof to draw the secondary fuel F2 from the secondary fuel supply line 70, and supplies the secondary fuel F2 and the cooling fluid G0a to the secondary fuel supply line 74 connected thereto. Thus, the boiler 1a has the ejector 76a for taking in the secondary fuel F2 using vapor pressure of the cooling fluid G0a, so that the secondary fuel F2 can be appropriately taken therein to be mixed with the cooling fluid G0a even when a supply pressure of the secondary fuel F2 is low. In addition, it is also possible to perform secondary combustion, for example, using a low dryness steam.

Claims

1. A boiler comprising:

a can body having water pipe;

a burner connected to the can body and for supplying primary fuel and air into the can body;

a secondary fuel supply unit for supplying secondary fuel into the can body downstream of the burner in a flow direction of combustion gas;

a cooling line for introducing a cooling fluid for reducing temperature of a predetermined space in the can body downstream of the burner in the flow direction of the combustion gas;

a flow rate adjusting unit provided in the cooling line and capable of adjusting a flow rate of the cooling fluid introduced into the can body from the cooling line; and

a control unit for controlling the flow rate adjusting unit to control the flow rate of the cooling fluid introduced into the can body such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

2. The boiler according to claim 1, wherein the control unit controls the flow rate adjusting unit such that a rate increases at which the flow rate of the cooling fluid increases when a supply amount of the primary fuel increases, as the supply amount of the primary fuel increases.

3. The boiler according to claim 1, wherein the secondary fuel supply unit is connected to the cooling line, and supplies the secondary fuel into the can body in a state of being mixed with the cooling fluid.

4. The boiler according to claim 1, wherein the secondary fuel supply unit supplies the secondary fuel into the can body downstream of the cooling line in the flow direction of the combustion gas.

5. The boiler according to claim 1, wherein the cooling line introduces at least one or more of exhaust gas discharged from the can body, steam, water, and inert gas into the can body as the cooling fluid.

6. The boiler according to claim 3, wherein

the flow rate adjusting unit is a fan for supplying the exhaust gas discharged from the can body into the cooling line, and

the cooling line introduces the exhaust gas supplied from the flow rate adjusting unit into the can body as the cooling fluid.

7. (canceled)

8. The boiler according to claim 2, wherein the secondary fuel supply unit is connected to the cooling line, and supplies the secondary fuel into the can body in a state of being mixed with the cooling fluid.

9. The boiler according to claim 2, wherein the secondary fuel supply unit supplies the secondary fuel into the can body downstream of the cooling line in the flow direction of the combustion gas.

10. The boiler according to claim 3, wherein the secondary fuel supply unit supplies the secondary fuel into the can body downstream of the cooling line in the flow direction of the combustion gas.

11. The boiler according to claim 2, wherein the cooling line introduces at least one or more of exhaust gas discharged from the can body, steam, water, and inert gas into the can body as the cooling fluid.

12. The boiler according to claim 3, wherein the cooling line introduces at least one or more of exhaust gas discharged from the can body, steam, water, and inert gas into the can body as the cooling fluid.

13. The boiler according to claim 4, wherein the cooling line introduces at least one or more of exhaust gas discharged from the can body, steam, water, and inert gas into the can body as the cooling fluid.