Boiler system, power generation plant, and boiler system operation method

Abstract

The purpose of the present invention is to achieve an operation state suitable for all load bands by supplying air at a flow rate sufficient to cool an unused burner and protecting a burner apparatus in a high-load zone, and setting an appropriate air flow rate in response to a decrease in heat load of a furnace in a low-load zone. This boiler system (2) comprises a boiler (10) having a plurality of burners (21), an air supply unit for supplying air to the burners (21), a cold gas damper (30d) for adjusting the amount of air supplied to the burners (21) by the air supply unit, and a control unit for controlling the cold gas damper (30d) so as to supply cooling air in an amount corresponding to the load of the boiler (10) when cooling air is supplied by the air supply unit to burners (21) that are not forming flames.

Description

Boiler system, power plant, and boiler system operation method

[0001] The present disclosure relates to a boiler system, a power plant, and a method of operating a boiler system.

[0002] Large boilers, such as power generation boilers, have a hollow furnace installed vertically, with multiple burners arranged circumferentially around the furnace wall. Large boilers also have a flue connected vertically above the furnace, with a heat exchanger for generating steam located in the flue. The burner injects a mixture of fuel and air (oxidizing gas) into the furnace, forming a flame, generating combustion gas that flows down the flue. A heat exchanger is installed in the area where the combustion gas flows, and water or steam flowing through heat transfer tubes constituting the heat exchanger is heated to generate superheated steam.

[0003] Such boilers are equipped with multiple burners (see, for example, Patent Document 1). The boiler also changes the number of burners that generate flames depending on the load. At this time, burners that are not generating flames may be subject to a shortened lifespan or damage due to radiant heat from the furnace. In particular, burners equipped with flame-holding mechanisms that are installed facing the interior of the furnace are susceptible to metal temperature increases due to radiant heat received by the flame-holding mechanism surface, potentially shortening their lifespan or causing damage. For this reason, cooling air is supplied to burners that are not generating flames to cool them, thereby preventing the shortened lifespan or damage of the burners.

[0004] JP 2023-76868 A

[0005] Unlike combustion air supplied to burners that are not generating a flame, cooling air supplied to burners that are not generating a flame is less likely to directly contribute to combustion in the boiler. Therefore, depending on the boiler load, an increase in the ratio of cooling air to the total amount of air supplied to the boiler (total air amount) can cause the air ratio near the flame to deviate from a specified range, which can affect boiler performance and operability, such as increasing the concentration of nitrogen oxides (NOx) in the combustion exhaust gas and increasing the exhaust gas temperature at the air preheater outlet due to an increase in the amount of air bypassing the air preheater.

[0006] The present disclosure has been made in consideration of the above circumstances, and aims to provide a boiler system, a power plant, and an operating method for a boiler system that can provide a sufficient air flow rate to cool unused burners and protect the burner equipment in the high load range, and that can maintain a suitable operating state in the full load range by adjusting the air flow rate in response to a decrease in the thermal load of the furnace in the low load range.

[0007] In order to solve the above problems, the boiler system, power plant, and method of operating a boiler system according to the present disclosure employ the following means: A boiler system according to one aspect of the present disclosure includes a boiler having a plurality of burners, an air supply unit that supplies air to the burners, an adjustment unit that adjusts the amount of air supplied to the burners by the air supply unit, and a control unit that controls the adjustment unit so that, when cooling air is supplied by the air supply unit to the burners that are not forming a flame, the amount of cooling air supplied corresponds to the load of the boiler.

[0008] A method for operating a boiler system according to one aspect of the present disclosure includes a boiler system comprising a boiler having a plurality of burners, an air supply unit that supplies air to the burners, and an adjustment unit that adjusts the amount of air supplied to the burners by the air supply unit, and includes a control step of controlling the adjustment unit so that when cooling air is supplied by the air supply unit to the burners that are not forming a flame, an amount of cooling air corresponding to the load of the boiler is supplied.

[0009] According to the present disclosure, in the high load range, sufficient air flow rate is supplied to cool unused burners to protect the burner equipment, and in the low load range, an appropriate air flow rate is set in response to the decrease in the thermal load of the furnace, thereby forming a flame with an appropriate combustion state at each load and optimizing the boiler as a whole.

[0010] Fig. 1 is a schematic configuration diagram showing a boiler system according to an embodiment of the present disclosure. Fig. 2 is a schematic configuration diagram showing a boiler system according to an embodiment of the present disclosure. Fig. 3 is a block diagram showing a control unit according to an embodiment of the present disclosure. Fig. 4 is a graph showing the relationship between the boiler load and the opening degree of the cold gas damper in the boiler system according to an embodiment of the present disclosure. Fig. 5 is a graph showing the relationship between the boiler load and the opening degree of the cold gas damper for each solid fuel in a boiler system according to a modified embodiment of the embodiment of the present disclosure.

[0011] Hereinafter, one embodiment of a boiler system, a power plant, and an operating method of a boiler system according to the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to this embodiment, and when there are multiple embodiments, it also includes configurations that combine the embodiments. In the following description, "up" and "above" refer to the upper side in the vertical direction, and "below" and "below" refer to the lower side in the vertical direction, and the vertical direction is not precise and may include errors.

[0012] The power plant 1 according to this embodiment includes a boiler system 2 that generates steam, a steam turbine (not shown) that is driven by the steam generated by the boiler system 2, and a power generation section (not shown) that generates electricity using the driving force of the steam turbine.

[0013] 1 is a schematic diagram showing the configuration of a boiler system 2 using solid fuel as the main fuel according to this embodiment. The boiler system 2 includes a boiler 10 and a mill 31. The boiler system 2 will be described in detail later.

[0014] The boiler 10 of this embodiment is a boiler that can generate superheated steam by burning pulverized fuel made by pulverizing solid fuel with a burner and exchanging the heat generated by this combustion with feedwater or steam. The solid fuel used can be biomass fuel, coal, or the like.

[0015] The boiler 10 has a furnace 11, a combustion device, and a combustion gas passage 12. The furnace 11 has a hollow rectangular cylindrical shape and is installed vertically. The furnace wall 101 that forms the inner wall surface of the furnace 11 is composed of a plurality of heat transfer tubes and fins that connect the heat transfer tubes together. The heat generated by the combustion of pulverized fuel is recovered by heat exchange with water and steam circulating inside the heat transfer tubes, and the temperature rise of the furnace wall 101 is suppressed.

[0016] The combustion device is installed in the lower region of the furnace 11. In this embodiment, the combustion device has a plurality of burners 21A, 21B, 21C, 21D, 21E, and 21F (hereinafter, sometimes collectively referred to as "burners 21") attached to the furnace wall 101. The burners 21 are arranged at equal intervals along the circumferential direction of the furnace 11 (for example, four burners installed at each corner of the rectangular furnace 11), and are arranged in multiple rows along the vertical direction. Note that, for convenience of illustration, only two burners of one set are shown in FIG. 1, and each set is denoted by the reference numerals 21A, 21B, 21C, 21D, 21E, and 21F. The shape of the furnace 11, the number of burner rows, the number of burners in one row, and the arrangement of the burners are not limited to this embodiment.

[0017] The burners 21A, 21B, 21C, 21D, 21E, and 21F are connected to a plurality of mills (pulverizers) 31A, 31B, 31C, 31D, 31E, and 31F (hereinafter, sometimes collectively referred to as "mills 31") via a plurality of pulverized fuel supply pipes 22A, 22B, 22C, 22D, 22E, and 22F (hereinafter, sometimes collectively referred to as "pulverized fuel supply pipes 22"). The mill 31 is, for example, a vertical roller mill having a rotatable grinding table (not shown) supported therein and a plurality of grinding rollers (not shown) supported above the grinding table so as to be rotatable in conjunction with the rotation of the grinding table. The solid fuel pulverized by the cooperation of the grinding rollers and the grinding table is transported to a classifier (not shown) provided in the mill 31 by primary air (carrier gas, oxidizing gas) supplied to the mill 31. The classifier separates the pulverized fuel into pulverized fuel having a particle size smaller than that suitable for combustion in the burner 21 and coarse pulverized fuel having a particle size larger than that. The pulverized fuel passes through the classifier and is supplied to the burner 21 together with primary air via the pulverized fuel supply pipe 22. The coarse pulverized fuel that does not pass through the classifier falls onto a grinding table inside the mill 31 due to its own weight and is re-ground. The burner 21 also has a flame-holding mechanism (not shown) at its tip that is installed so as to face the inside of the furnace 11.

[0018] An air register 23 is provided outside the furnace 11 at the installation position of the burner 21, and one end of an air duct 24 is connected to the air register 23. The other end of the air duct 24 is connected to a forced draft fan (FDF) 32. Air supplied from the forced draft fan 32 is heated by an air preheater 42 installed in the air duct 24 (details will be described later), and is supplied to the burner 21 via the air register 23 as secondary air (combustion air, oxidizing gas) and introduced into the furnace 11.

[0019] The combustion gas passage 12 is connected to the vertical upper part of the furnace 11. The combustion gas passage 12 is provided with superheaters 102A, 102B, and 102C (hereinafter sometimes collectively referred to as "superheaters 102"), reheaters 103A and 103B (hereinafter sometimes collectively referred to as "reheaters 103"), and an economizer 104 as heat exchangers for recovering heat from the combustion gas, and heat is exchanged between the combustion gas generated in the furnace 11 and feedwater or steam flowing through each heat exchanger. Note that the arrangement and shape of each heat exchanger are not limited to those shown in FIG. 1 .

[0020] A flue 13 is connected downstream of the combustion gas passage 12, and discharges the combustion gas whose heat has been recovered by the heat exchanger. An air preheater (air heater) 42 is provided between the flue 13 and the air duct 24, and heat is exchanged between the air flowing through the air duct 24 and the combustion gas flowing through the flue 13, thereby heating the primary air supplied to the mill 31 and the secondary air supplied to the burner 21, thereby recovering further heat from the combustion gas after heat exchange with water and steam.

[0021] A denitration device 43 may be provided in the flue 13 at a position upstream of the air preheater 42. The denitration device 43 supplies a reducing agent, such as ammonia or urea water, that has the ability to reduce nitrogen oxides to the combustion gas flowing through the flue 13. The denitration device 43 promotes a reaction between the nitrogen oxides (NOx) in the combustion gas to which the reducing agent has been supplied and the reducing agent through the catalytic action of a denitration catalyst provided in the denitration device 43, thereby removing and reducing the nitrogen oxides in the combustion gas. A gas duct 41 is connected to the flue 13 downstream of the air preheater 42. The gas duct 41 is provided with environmental devices, such as a dust collector 44, such as an electrostatic precipitator, that removes ash and the like from the combustion gas, and a desulfurization device 46 that removes sulfur oxides, as well as an induced draft fan (IDF) 45 that guides the exhaust gas to these environmental devices. The downstream end of the gas duct 41 is connected to a chimney 47, through which the combustion gas treated in the environmental device is discharged to the outside of the system as exhaust gas.

[0022] In the boiler 10, when the multiple mills 31 are driven, pulverized and classified pulverized fuel is supplied to the burner 21 together with primary air via the pulverized fuel supply pipe 22. Secondary air heated by the air preheater 42 is supplied to the burner 21 from the air duct 24 via the wind box 23. The burner 21 blows a pulverized fuel mixture, which is a mixture of pulverized fuel and primary air, into the furnace 11, and also blows secondary air into the furnace 11. The pulverized fuel mixture blown into the furnace 11 ignites and reacts with the secondary air to form a flame. A flame is formed in the lower region of the furnace 11, and high-temperature combustion gas rises within the furnace 11 and flows into the combustion gas passage 12. In this embodiment, air is used as the oxidizing gas (primary air, secondary air). However, oxidizing gases having a higher or lower oxygen content than air may also be used. Stable combustion in the furnace 11 can be achieved by adjusting the ratio of oxygen to the amount of fuel supplied within an appropriate range.

[0023] The combustion gas that has flowed into the combustion gas passage 12 exchanges heat with water and steam in a superheater 102, a reheater 103, and an economizer 104 arranged inside the combustion gas passage 12, and is then discharged into the flue 13, where nitrogen oxides are removed in a denitration device 43, and the combustion gas exchanges heat with primary air and secondary air in an air preheater 42, and is then discharged into the gas duct 41, where ash and the like are removed in a dust collector 44, and sulfur oxides are removed in a desulfurization device 46, and the combustion gas is then discharged to the outside of the system through a chimney 47. The arrangement of the heat exchangers in the combustion gas passage 12 and the arrangement of the devices from the flue 13 to the gas duct 41 with respect to the combustion gas flow do not necessarily have to be in the order described above.

[0024] Next, the boiler system 2 will be described in detail with reference to Figures 2 and 3. The boiler system 2 includes the boiler 10 described above and a plurality of mills 31 that pulverize the solid fuel to be supplied to the burner 21 of the boiler 10.

[0025] The mill 31 pulverizes the solid fuel supplied from the fuel supply unit 31 a inside, and discharges the pulverized solid fuel to the outside together with the primary air (carrier gas, oxidizing gas) supplied to the mill 31. The primary air and pulverized fuel discharged to the outside are led to the burner 21 of the boiler 10 via the pulverized fuel supply pipe 22.

[0026] The boiler system 2 includes a blower 30 that introduces primary air into the mill 31. The blower 30 is a device that blows primary air into the mill 31 to dry and transport the pulverized fuel. In this embodiment, the blower 30 includes a primary air fan (PAF) 33, a hot gas flow path 30a, a cold gas flow path (air piping) 30b, a hot gas damper 30c, and a cold gas damper (adjustment unit) 30d to appropriately adjust the flow rate and temperature of the primary air blown into the mill 31.

[0027] The hot gas flow path 30a branches off from the cold gas flow path 30b. The hot gas flow path 30a supplies a portion of the air delivered from the primary air fan 33 as hot gas that has been heated by passing through an air preheater 42. A hot gas gate valve 30e and a hot gas damper 30c are provided in the hot gas flow path 30a downstream of the air preheater 42. The hot gas gate valve 30e is an on-off valve that is switched between an open state and a closed state by a control unit 50. The opening degree of the hot gas damper 30c is controlled by the control unit 50. The flow rate of the hot gas supplied from the hot gas flow path 30a is determined by the opening degree of the hot gas damper 30c. The hot gas heated by the air preheater 42 is also guided to the other mills 31 via the hot gas flow path 30a.

[0028] The cold gas flow path 30b supplies a portion of the air sent out from the primary air ventilator 33 as cold gas at room temperature. A cold gas gate valve 30f and a cold gas damper 30d are provided in the cold gas flow path 30b. The cold gas gate valve 30f is an on-off valve that is switched between an open state and a closed state by the control unit 50. The opening degree of the cold gas damper 30d is controlled by the control unit 50. The flow rate of cold gas supplied from the cold gas flow path 30b is determined by the opening degree of the cold gas damper 30d. The cold gas sent out from the primary air ventilator 33 is also guided to other mills 31 via the cold gas flow path 30b.

[0029] The hot gas flow path 30a and the cold gas flow path 30b join at their downstream ends to form a primary air flow path 35. The flow rate of the primary air (i.e., the amount of air flowing through the primary air flow path 35) is the sum of the flow rate of the hot gas supplied from the hot gas flow path 30a and the flow rate of the cold gas supplied from the cold gas flow path 30b, and the temperature of the primary air is determined by the mixing ratio of the hot gas supplied from the hot gas flow path 30a and the cold gas supplied from the cold gas flow path 30b, and is controlled by the control unit 50. The primary air flow path 35 is provided with a flow meter (air amount detection unit) 35a that measures the flow rate of the primary air flowing through it.

[0030] In this embodiment, the blower section 30, the primary air flow path 35, the mill 31 and the pulverized fuel supply pipe 22 are included in the air supply section that supplies air (including primary air for combustion and cooling air) to the burner 21 of the boiler 10.

[0031] The boiler system 2 according to this embodiment does not always generate flames in all burners 21 (burners 21A, 21B, 21C, 21D, 21E, 21F), but changes the number of burners 21 that generate flames depending on the load on the boiler 10. Specifically, the higher the load on the boiler 10, the more burners 21 that generate flames. As a result, the boiler 10 includes burners 21 that generate flames and burners 21 that do not generate flames.

[0032] The burner 21 that forms a flame and the mill 31 that is connected via the pulverized fuel supply pipe 22 are in an operating state. On the other hand, the burner 21 that does not form a flame and the mill 31 that is connected via the pulverized fuel supply pipe 22 are in a stopped state.

[0033] The burners 21 that are forming a flame are supplied with a mixture of pulverized fuel and primary air for combustion by an air supply unit, while the burners 21 that are not forming a flame are supplied with cooling air by an air supply unit.

[0034] 3, the boiler system 2 includes a control unit 50. The control unit 50 controls the hot gas gate valve 30e, the hot gas damper 30c, the cold gas gate valve 30f, the cold gas damper 30d, etc. The control unit 50 also receives information detected by the flow meter 35a.

[0035] When supplying cooling air to burners 21 that are not forming a flame, control unit 50 controls the opening degree of cold gas damper 30d so that the amount of cooling air supplied corresponds to the load on boiler 10. Specifically, control unit 50 controls the opening degree of cold gas damper 30d so that the amount of cooling air supplied to burners 21 that are not forming a flame decreases as the load on boiler 10 decreases.

[0036] The control unit 50 may determine the amount of cooling air based on a pre-stored map or the like that defines the relationship between the load of the boiler 10 and the amount of cooling air, and control the opening degree of the cold gas damper 30d so that the amount of cooling air is achieved. In this case, the method for controlling the opening degree of the cold gas damper 30d so that the amount of cooling air is achieved is not particularly limited. For example, the control unit 50 may control the opening degree of the cold gas damper 30d based on a pre-stored map or the like that defines the relationship between the amount of cooling air and the opening degree of the cold gas damper 30d. The control unit 50 may also control the opening degree of the cold gas damper 30d based on the amount of cooling air detected by the flow meter 35a.

[0037] Furthermore, the method by which the control unit 50 acquires the load of the boiler 10 is not limited. For example, the load command value of the boiler 10 may be used as the load of the boiler 10. Furthermore, the control unit 50 may derive the load of the boiler 10 based on the amount of solid fuel supplied to all mills 31, i.e., the total amount of fuel supplied to the boiler 10.

[0038] The control unit 50 (Controller) includes, for example, a CPU (Central Processing Unit: Processor), a main memory, and a secondary storage. The control unit 50 may also include a communication unit for transmitting and receiving information to and from other devices. The main memory is composed of, for example, a writable memory such as a cache memory or a RAM (Random Access Memory), and is used as a working area for reading the CPU's execution program and writing data processed by the execution program. The secondary storage is a non-transitory computer-readable storage medium. The secondary storage is, for example, a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, or a semiconductor memory. For example, a series of processes for realizing various functions is stored in the form of a program in a secondary storage device, and the CPU reads this program into the main storage device and executes information processing and arithmetic operations to realize various functions. Note that the program may be pre-installed in the secondary storage device, provided in a state stored in a computer-readable storage medium, or distributed via wired or wireless communication means. Examples of computer-readable storage media include magnetic disks, magneto-optical disks, CD-ROMs, DVD-ROMs, and semiconductor memories.

[0039] Next, the flow of air supplied to the burners 21 that are forming a flame and the burners 21 that are not forming a flame will be described.

[0040] [Burner (Ignition Burner) Forming the Flame] The mill 31 corresponding to the burner 21 forming the flame is in an operating state as described above. Primary air, a mixture of hot gas flowing through the hot gas flow path 30a and cold gas flowing through the cold gas flow path 30b, is supplied from the blower 30 via the primary air flow path 35 to the operating mill 31. At this time, the openings of the hot gas damper 30c and the cold gas damper 30d are controlled by the control unit 50 so that the primary air has a predetermined temperature (the temperature at the outlet or inlet of the mill 31) and flow rate. The hot gas gate valve 30e and the cold gas gate valve 30f are also opened by the control unit 50.

[0041] The operating mill 31 receives solid fuel from the fuel supply unit 31a and pulverizes the solid fuel inside. The operating mill 31 discharges the pulverized solid fuel (pulverized fuel) together with the supplied primary air to the outside. The mixture of the pulverized fuel and primary air discharged to the outside of the mill 31 is led to the burner 21 of the boiler 10 via the pulverized fuel supply pipe 22.

[0042] Secondary air preheated by an air preheater 42 is supplied to the wind box 23 of the boiler 10 via an air duct 24. A wind box damper (not shown) that adjusts the flow rate of secondary air supplied to each burner 21 is provided in the wind box 23 for each burner 21. The opening degree of the wind box damper is adjusted by the control unit 50 in accordance with the operating state of each burner 21 (ignition / extinguishing, load on the boiler 10, amount of fuel supply, etc.).

[0043] The burner 21 that forms the flame uses the supplied primary air and secondary air as combustion air to burn the supplied pulverized fuel, and forms the flame. At this time, the amount of secondary air is adjusted by the opening of the wind box damper so that the air ratio near the flame falls within a predetermined range.

[0044] [Burners Not Forming Flames (Extinguishing Burners)] As described above, the mills 31 corresponding to the burners 21 not forming flames are in a stopped state. Only cold gas flowing through the cold gas flow path 30b from the blower 30 is supplied as cooling air to the stopped mills 31 via the primary air flow path 35. The controller 50 controls the opening of the cold gas damper 30d so that the amount of cooling air supplied decreases as the load on the boiler 10 decreases. An example of the control performed by the controller 50 will be described with reference to FIG. 4 . As shown in FIG. 4 , the controller 50 gradually reduces the opening of the cold gas damper 30d when the load on the boiler 10 is low. Specifically, the controller 50 reduces the opening of the cold gas damper 30d when the load on the boiler 10 is low, compared to when the opening of the cold gas damper 30d is kept constant in accordance with the load on the boiler 10 (see the dashed line). Note that the control performed by the controller 50 is not limited to this. For example, the control unit 50 may control the cold gas damper 30d so that the opening degree gradually decreases as the load on the boiler 10 decreases.

[0045] 2, the hot gas gate valve 30e is closed by the control unit 50, and no air flows through the hot gas flow path 30a. On the other hand, the cold gas gate valve 30f is opened by the control unit 50.

[0046] Furthermore, the fuel supply unit 31 a does not supply solid fuel to the mill 31 that is in a stopped state. The cooling air supplied to the stopped mill 31 passes through the mill 31 and is discharged to the outside. The cooling air discharged to the outside of the mill 31 is led to the burner 21 of the boiler 10 via the pulverized fuel supply pipe 22.

[0047] Secondary air preheated by an air preheater 42 is supplied to the wind box 23 of the boiler 10 via an air duct 24 .

[0048] The burner 21 that is not generating a flame uses the supplied cooling air to cool the burner nozzle, etc., thereby suppressing damage caused by radiant heat. The cooling air that has cooled the burner 21 is discharged into the furnace 11 of the boiler 10.

[0049] This embodiment provides the following advantageous effects. In the boiler system 2, combustion air is supplied to the burners 21 that are generating flames via an air supply unit, and cooling air is supplied to the burners 21 that are not generating flames via an air supply unit, thereby suppressing damage to the burners 21 due to radiation. Therefore, the total amount of air supplied to the boiler 10 (total air amount) includes the combustion air supplied to the burners 21 that are generating flames and the cooling air supplied to the burners 21 that are not generating flames. This embodiment includes a control unit 50 that controls the cold gas damper 30d to supply cooling air to the burners 21 that are not generating flames in an amount corresponding to the load of the boiler 10. This allows the amount of cooling air supplied to the burners 21 that are not generating flames to be adjusted to an amount corresponding to the load of the boiler 10. Therefore, the ratio of the amount of cooling air to the total air amount can be adjusted to an amount corresponding to the load of the boiler 10. In other words, the ratio of the amount of combustion air to the total amount of air can be set to a ratio according to the load of the boiler 10, thereby creating a favorable combustion state in the boiler 10 and ensuring a stable state for the boiler system as a whole. This improves the performance and operability of the boiler 10. Furthermore, since the burner 21 can be cooled with an appropriate amount of cooling air, damage to the burner 21 due to heat can be suppressed, and the life of the burner 21 can be extended.

[0050] Furthermore, the lower the load on the boiler 10, the fewer the number of burners 21 that form flames, and therefore the more burners 21 to which cooling air is supplied. Therefore, when the amount of cooling air supplied to the burners 21 is constant, the lower the load on the boiler 10, the greater the ratio of the amount of cooling air to the total amount of air. On the other hand, in this embodiment, the control unit 50 controls the cold gas damper 30d so that the amount of air supplied to the burners 21 that are not forming flames decreases as the load on the boiler 10 decreases. This makes it possible to suppress an increase in the ratio of the amount of cooling air to the total amount of air, even when the boiler 10 is operating in a low load range.

[0051] Specifically, an increase in the ratio of the amount of cooling air to the total amount of air raises concerns about the following points (1) to (3).

[0052] (1) Because cooling air is supplied to the burner 21 via the same system as primary air, an increase in the ratio of cooling air volume to the total air volume is equivalent to an increase in the primary air volume. As the ratio of primary air volume to the total air volume increases, the ratio of secondary air volume to the total air volume decreases. This reduces the amount of secondary air supplied to the wind box 23, causing a decrease in pressure within the wind box 23. Consequently, the pressure difference between the furnace 11 of the boiler 10 and the wind box 23 decreases, potentially preventing the secondary air from being properly introduced from the wind box 23 to the furnace 11 of the boiler 10, even when the wind box damper is adjusted. This could result in the air ratio near the flame deviating from the specified range, potentially affecting the performance of the boiler 10, such as increasing the concentration of nitrogen oxides (NOx) in the combustion exhaust gas. On the other hand, in this embodiment, even when the boiler 10 is operating in a low-load range, the increase in the ratio of cooling air volume to the total air volume can be suppressed, allowing secondary air to be properly introduced from the wind box 23 to the furnace 11 of the boiler 10. Therefore, a flame can be suitably formed in the boiler 10 .

[0053] (2) Furthermore, when the amount of cooling air increases, the amount of air bypassing the air preheater 42 (i.e., flowing through the cold gas flow path 30b) also increases. This means that the exhaust gas is not sufficiently cooled in the air preheater 42, and the temperature of the exhaust gas discharged from the air preheater 42 to the gas duct 41 increases. As a result, high-temperature exhaust gas is introduced into equipment (such as the dust collector 44, induced draft fan 45, and desulfurization device 46) located downstream of the exhaust gas flow from the air preheater 42, which could potentially exceed the upper gas temperature limits of the equipment or reduce the efficiency of the boiler 10. On the other hand, in this embodiment, when the boiler 10 operates in the low-load range, the amount of cooling air is reduced, thereby suppressing an increase in the amount of air bypassing the air preheater 42. This suppresses an increase in the temperature of the exhaust gas discharged from the air preheater 42 to the gas duct 41.

[0054] (3) Furthermore, when the forced draft fan 32 and the primary air fan 33 are arranged in parallel, the proportion of the discharge air volume of the primary air fan 33 increases and the proportion of the discharge air volume of the forced draft fan 32 decreases, which could cause the rotor blade opening of the forced draft fan 32 to approach full closure, resulting in a deterioration in controllability. On the other hand, in this embodiment, when the boiler 10 operates in the low load range, the amount of cooling air is reduced, which makes it difficult for the proportion of the discharge air volume of the primary air fan 33 to increase. Therefore, the rotor blade opening of the forced draft fan 32 is unlikely to approach full closure, which can prevent the controllability of the forced draft fan 32 from deteriorating.

[0055] In this embodiment, the control unit 50 controls the opening of the cold gas damper 30d so that the amount of cooling air supplied to the burners 21 that are not generating a flame corresponds to the load of the boiler 10. In this way, the amount of cooling air supplied can be adjusted simply by controlling the opening of the cold gas damper 30d. Therefore, the amount of cooling air can be easily adjusted.

[0056] Furthermore, when the control unit 50 controls the cold gas damper 30d based on the amount of air detected by the flow meter 35a, the amount of cooling air supplied to the burners 21 that are not forming a flame can be more accurately set to an amount that corresponds to the load of the boiler 10. Therefore, the ratio of the amount of cooling air to the total amount of air can be more accurately set to a ratio that corresponds to the load of the boiler 10.

[0057] Furthermore, in this embodiment, the control unit 50 stops the supply of solid fuel from the mill 31 to the burners 21 that are not forming a flame, and controls the cold gas damper 30d so that the mill 31 supplies air in an amount corresponding to the load of the boiler 10. This allows an appropriate amount of cooling air to be supplied to the burners 21 that are not forming a flame in the boiler 10 to which solid fuel is supplied from the mill 31, thereby suppressing damage to the burners 21 due to radiant heat. Furthermore, the amount of cooling air supplied to the burners 21 that are not forming a flame can be adjusted to an amount corresponding to the load of the boiler 10. Therefore, the ratio of the amount of cooling air to the total amount of air can be more accurately adjusted to a ratio corresponding to the load of the boiler 10.

[0058] [Modification] Next, a modification of this embodiment will be described with reference to Fig. 5. In this modification, the control performed by the control unit 50 is different. In the above embodiment, an example was described in which the control unit 50 controls to adjust the opening of the cold gas damper 30d so that an amount of cooling air corresponding to the load on the boiler 10 is supplied when cooling air is supplied to the burners 21 that are not forming a flame. In this modification, in addition to the above control, the control unit 50 controls to adjust the opening of the cold gas damper 30d so that an amount of cooling air corresponding to the type of solid fuel supplied to the mill 31 is supplied when cooling air is supplied to the burners 21 that are not forming a flame.

[0059] An example of control performed by the control unit 50 of this modified example will be described. First, the control unit 50 determines the type of solid fuel used in the boiler 10 based on a value calculated from a heat absorption index (HAI), which is the ratio of the heat exchange rate in the superheater 102, reheater 103, and economizer 104, which are convective heat transfer units, to the heat exchange rate in the furnace 11, which is a radiative heat transfer unit. Specifically, if the calculated value is 0.9, the solid fuel is determined to be a low fuel ratio solid fuel. If the calculated value is 1.0, the solid fuel is determined to be a medium fuel ratio solid fuel. If the calculated value is 1.1, the solid fuel is determined to be a high fuel ratio solid fuel. The fuel ratio is calculated as fixed carbon / volatile matter. The lower the fuel ratio, the more volatile matter is contained, which means higher combustibility and earlier completion of combustion. Therefore, the amount of radiative heat transfer in the furnace 11 tends to be relatively large, and the HAI tends to be small.

[0060] Next, the control unit 50 controls the opening of the cold gas damper 30d to supply cooling air in an amount corresponding to the determined fuel ratio of the solid fuel. Specifically, as shown in Fig. 5, when the solid fuel is determined to be a low fuel ratio, the temperature in the furnace 11 becomes higher, so the opening of the cold gas damper 30d is made larger than in the case of solid fuels with other fuel ratios. Furthermore, when the solid fuel is determined to be a medium fuel ratio, the opening of the cold gas damper 30d is made larger than in the case of solid fuels with a high fuel ratio. In this way, the control unit 50 controls the cold gas damper 30d so that the opening becomes larger the lower the fuel ratio of the solid fuel.

[0061] For solid fuels where the calculated value is greater than 0.9 and less than 1.0, or greater than 1.0 and less than 1.1, a parameter obtained by proportionally dividing the set value for each calculated value is used. Specifically, for example, if the calculated value is 0.95, an intermediate value between the parameter when the calculated value is 0.9 and the parameter when the calculated value is 1.0 is reflected as the parameter.

[0062] 4, regardless of the fuel ratio of the solid fuel, the control unit 50 may control the cold gas damper 30d so that the opening degree gradually decreases as the load on the boiler 10 decreases. In the example of FIG. 4, the opening degree of the cold gas damper 30d is set to not be smaller than the minimum opening degree m.

[0063] This modified example provides the following advantageous effects. Because the fuel ratio (ease of combustion) varies depending on the type of solid fuel, the temperature of the furnace 11 of the boiler 10 changes depending on the type of solid fuel supplied to the boiler 10. Therefore, the radiant heat to the burner 21 also changes depending on the type of solid fuel. Therefore, the amount of cooling air required to cool the burner 21 that is not generating a flame also changes depending on the type of solid fuel. In this embodiment, the control unit 50 controls the aperture of the cold gas damper 30d to supply an amount of cooling air that corresponds to the type of solid fuel supplied to the mill 31. This allows the amount of cooling air supplied to the burner 21 that is not generating a flame to be adjusted to an amount that corresponds to the type of solid fuel. Therefore, the ratio of the amount of cooling air to the total amount of air can be adjusted to a ratio that corresponds to the type of solid fuel. This makes it possible to create a favorable combustion state in the boiler 10 while ensuring an amount of cooling air that corresponds to the type of solid fuel.

[0064] The present disclosure is not limited to the above-described embodiments and may be modified as appropriate without departing from the spirit and scope of the present disclosure. For example, in the above-described embodiments, an example in which the forced draft fan 32 and the primary air fan 33 are arranged in parallel is described, but the present disclosure is not limited to this. For example, the forced draft fan 32 and the primary air fan 33 may be arranged so that a portion of the air pressurized by the forced draft fan 32 is guided to the primary air fan 33.

[0065] The boiler system, the power plant, and the method of operating the boiler system described in the above-described embodiments can be understood, for example, as follows: A boiler system according to a first aspect of the present disclosure includes a boiler (10) having a plurality of burners (21), an air supply unit that supplies air to the burners (21), an adjustment unit (30d) that adjusts the amount of air supplied to the burners (21) by the air supply unit, and a control unit (50) that controls the adjustment unit (30d) so as to supply an amount of cooling air corresponding to the load of the boiler (10) when cooling air is supplied by the air supply unit to the burners (21) that are not forming a flame.

[0066] In a boiler system, combustion air is supplied to burners generating flames via an air supply unit, and cooling air is supplied to burners not generating flames via an air supply unit to prevent burner damage due to radiation. Therefore, the total amount of air supplied to the boiler (total air amount) includes combustion air supplied to burners generating flames and cooling air supplied to burners not generating flames. The above configuration includes a control unit that controls the adjustment unit to supply cooling air to burners not generating flames in an amount corresponding to the boiler load. This allows the amount of cooling air supplied to burners not generating flames to be adjusted to correspond to the boiler load. Therefore, the ratio of the amount of cooling air to the total air amount can be adjusted to correspond to the boiler load. In other words, the ratio of the amount of combustion air to the total air amount can be adjusted to correspond to the boiler load, thereby achieving a favorable combustion state in the boiler and optimizing the boiler system as a whole.

[0067] In addition, in the boiler system according to the second aspect of the present disclosure, in the first aspect, the control unit (50) controls the adjustment unit (30d) to reduce the amount of cooling air supplied to the burner (21) that is not forming a flame as the load of the boiler (10) decreases.

[0068] The lower the boiler load, the fewer the number of burners that form flames, and therefore the more burners to which cooling air is supplied. Therefore, if the amount of cooling air supplied to the burners is constant, the lower the boiler load, the greater the ratio of the amount of cooling air to the total amount of air. On the other hand, with the above configuration, the control unit controls the adjustment unit so that the amount of air supplied to burners that do not form flames decreases as the boiler load decreases. This makes it possible to suppress an increase in the ratio of the amount of cooling air to the total amount of air, even when the boiler is operating in a low load range.

[0069] Furthermore, in a boiler system according to a third aspect of the present disclosure, in the first or second aspect, the air supply unit has an air pipe (30b) that conveys air to the burner (21), the adjustment unit (30d) has a damper (30d) that is provided in the air pipe (30b) and adjusts the amount of air flowing through the air pipe (30b) by changing the opening degree, and the control unit (50) controls the opening degree of the damper (30d) so that cooling air is supplied to the burner (21) that is not forming a flame in an amount corresponding to the load of the boiler (10).

[0070] In the above configuration, the control unit controls the opening of the damper so that the amount of cooling air supplied to the burners that are not generating flames corresponds to the boiler load. In this way, the amount of cooling air supplied can be adjusted simply by controlling the opening of the damper. Therefore, the amount of cooling air can be easily adjusted.

[0071] Furthermore, the boiler system according to a fourth aspect of the present disclosure is any one of the first to third aspects, and includes an air amount detection unit (35a) that detects the amount of air supplied to the burner (21), and the control unit (50) controls the adjustment unit (30d) based on the amount of air detected by the air amount detection unit (35a).

[0072] In the above configuration, the control unit controls the adjustment unit based on the amount of air detected by the air amount detection unit. This makes it possible to more accurately adjust the amount of cooling air supplied to burners that are not forming flames to an amount that corresponds to the boiler load. Therefore, it is possible to more accurately adjust the ratio of the amount of cooling air to the total amount of air to a ratio that corresponds to the boiler load.

[0073] In addition, a boiler system according to a fifth aspect of the present disclosure is any one of the first to fourth aspects, and includes a plurality of mills (31) capable of supplying air or a mixture of pulverized solid fuel and air to specific burners (21) via the air supply unit, and the control unit (50) controls the adjustment unit (30d) to stop the supply of solid fuel from the mills (31) to the burners (21) that do not form a flame, and to supply cooling air from the mills (31) in an amount corresponding to the load of the boiler (10).

[0074] In the above configuration, the control unit stops the supply of solid fuel from the mill to the burners that do not form a flame and controls the adjustment unit to supply air from the mill in an amount corresponding to the boiler load. This allows cooling air to be supplied to the burners that do not form a flame in the boiler to which solid fuel is supplied from the mill, thereby suppressing damage to the burners due to radiant heat. Furthermore, the amount of cooling air supplied to the burners that do not form a flame can be adjusted to an amount corresponding to the boiler load. Therefore, the ratio of the amount of cooling air to the total amount of air can be more accurately adjusted to a ratio corresponding to the boiler load.

[0075] Furthermore, in a boiler system according to a sixth aspect of the present disclosure, in the fifth aspect, the load of the boiler (10) is the amount of solid fuel supplied to the boiler (10).

[0076] In the above configuration, the boiler load is the amount of solid fuel supplied to the mill, and thus, an amount of cooling air corresponding to the amount of solid fuel supplied to the mill can be supplied to the burners that are not forming a flame.

[0077] In addition, in the boiler system according to the seventh aspect of the present disclosure, in the fifth or sixth aspect described above, the control unit (50) controls the adjustment unit (30d) to supply an amount of cooling air corresponding to the type of solid fuel supplied to the boiler (10) when supplying cooling air to the burner (21) that is not forming a flame.

[0078] Since the fuel ratio (ease of combustion) varies depending on the type of solid fuel, the temperature of the boiler furnace changes depending on the type of solid fuel supplied to the boiler. Therefore, the radiant heat to the burner also changes depending on the type of solid fuel. Therefore, the amount of cooling air required to cool the burner that is not forming a flame also changes depending on the type of solid fuel. In the above configuration, the control unit controls the adjustment unit to supply an amount of cooling air depending on the type of solid fuel supplied to the boiler. This makes it possible to adjust the amount of cooling air supplied to the burner that is not forming a flame to an amount depending on the type of solid fuel. Therefore, the ratio of the amount of cooling air to the total amount of air can be adjusted to a ratio depending on the type of solid fuel.

[0079] A power plant according to a first aspect of the present disclosure includes a steam boiler system (2) according to any one of the first to seventh aspects, and a power generation unit that generates power using steam generated by the boiler system (2).

[0080] In a method for operating a boiler system according to a first aspect of the present disclosure, the boiler system (2) comprises a boiler (10) having a plurality of burners (21), an air supply unit that supplies air to the burners (21), and an adjustment unit (30d) that adjusts the amount of air supplied to the burners (21) by the air supply unit, and comprises a control step of controlling the adjustment unit (30d) to supply an amount of cooling air corresponding to a load on the boiler (10) when cooling air is supplied by the air supply unit to the burners (21) that are not forming a flame.

[0081] REFERENCE SIGNS LIST 1: Power plant 2: Boiler system 10: Boiler 11: Furnace 12: Combustion gas passage 13: Flue 21: Burner 21A: Burner 21B: Burner 21C: Burner 21D: Burner 21E: Burner 21F: Burner 22: Pulverized fuel supply pipe 22A: Pulverized fuel supply pipe 22B: Pulverized fuel supply pipe 22C: Pulverized fuel supply pipe 22D: Pulverized fuel supply pipe 22E: Pulverized fuel supply pipe 22F: Pulverized fuel supply pipe 23: Wind box 24: Wind duct 30: Blower section 30a: Hot gas flow path 30b: Cold gas flow path (air piping) 30c: Hot gas damper 30d: Cold gas damper (adjustment section) 30e : Hot gas gate valve 30f : Cold gas gate valve 31 : Mill 31a : Fuel supply section 32 : Forced draft fan 33 : Primary air fan 35 : Primary air flow path 35a : Flow meter (air amount detection section) 41 : Gas duct 42 : Air preheater 43 : Denitrification device 44 : Device 45 : Induced draft fan 46 : Desulfurization device 47 : Chimney 50 : Control section 101 : Furnace wall 102 : Superheater 102A : Superheater 102B : Superheater 102C : Superheater 103 : Reheater 103A : Reheater 103B : Reheater 104 : Economizer

Claims

1. A boiler system comprising: a boiler having a plurality of burners; an air supply unit that supplies air to the burners; an adjustment unit that adjusts the amount of air supplied to the burners by the air supply unit; and a control unit that controls the adjustment unit so that, when cooling air is supplied by the air supply unit to the burners that are not forming a flame, an amount of cooling air corresponding to the load of the boiler is supplied.

2. A boiler system as described in claim 1, wherein the control unit controls the adjustment unit so that the amount of cooling air supplied to the burners that are not forming a flame decreases as the load on the boiler decreases.

3. A boiler system as described in claim 1, wherein the air supply unit has an air pipe that transports air to the burner, the adjustment unit has a damper provided in the air pipe that adjusts the amount of air circulating through the air pipe by changing its opening degree, and the control unit controls the opening degree of the damper so as to supply an amount of cooling air corresponding to the load of the boiler to the burner that is not forming a flame.

4. A boiler system as described in claim 1, further comprising an air volume detection unit that detects the volume of air supplied to the burner, and the control unit controls the adjustment unit based on the volume of air detected by the air volume detection unit.

5. A boiler system as described in claim 1, comprising a plurality of mills capable of supplying air or a mixture of pulverized solid fuel and air to specific burners via the air supply unit, wherein the control unit controls the adjustment unit to stop the supply of solid fuel from the mills to burners that do not form a flame, and to supply cooling air from the mills in an amount corresponding to the load of the boiler.

6. The boiler system according to claim 5, wherein the load of the boiler is the amount of solid fuel supplied to the boiler.

7. A boiler system as described in claim 5, wherein the control unit controls the adjustment unit so that, when supplying cooling air to the burner that is not forming a flame, an amount of cooling air corresponding to the type of solid fuel supplied to the boiler is supplied.

8. A power generation plant comprising: a boiler system according to any one of claims 1 to 7; and a power generation unit that generates power using steam generated by the boiler system.

9. A method for operating a boiler system comprising: a boiler having a plurality of burners; an air supply unit for supplying air to the burners; and an adjustment unit for adjusting the amount of air supplied to the burners by the air supply unit, the method comprising a control step of controlling the adjustment unit so as to supply an amount of cooling air corresponding to the load of the boiler when cooling air is supplied by the air supply unit to the burners that are not forming a flame.

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