Power generation plant and method for operating power generation plant

Abstract

To improve energy efficiency of the entire power generation plant.SOLUTION: A power generation plant comprises: a high-pressure turbine 111A; a furnace 11; a boiler 10 that has a superheater 102 that superheats steam to be supplied to the high-pressure turbine 111A by exchanging heat with combustion gas, and a reheater 103 that superheats the steam discharged from the high-pressure turbine 111A by exchanging heat with the combustion gas generated in the furnace 11; a medium-low pressure turbine 111B to which the steam superheated in the reheater 103 is supplied, and that rotates with the supplied steam; a condenser 112 to which the steam discharged from the medium-low pressure turbine 111B is supplied, and that condenses the supplied steam; a steam header 54 that mixes the steam discharged from the high-pressure turbine 111A and the steam discharged from the medium-low pressure turbine 111B; and a mixed steam line L15 that connects the steam header 54 and the reheater 103, and leads the steam mixed in the steam header 54 to the reheater 103.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to power plants and methods of operating power plants. [Background technology]

[0002] A large boiler such as a power generation boiler has a hollow furnace that is installed vertically, and a plurality of burners are arranged on the furnace wall along the circumferential direction of the furnace. In addition, a flue is connected to the large boiler vertically above the furnace, and a heat exchanger for generating steam is arranged in the flue. Then, a flame is formed by the burner injecting a mixture of fuel and air (oxidizing gas) into the furnace, and combustion gas is generated and flows into the flue. A heat exchanger is installed in the area where the combustion gas flows, and water and steam flowing in a heat transfer tube that constitutes the heat exchanger are heated to generate superheated steam. For example, the facility of Patent Document 1 is known as a power generation plant equipped with such a boiler. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-106012 A Summary of the Invention [Problem to be solved by the invention]

[0004] In a power plant equipped with a boiler as described in Patent Document 1, superheated steam generated in the boiler is supplied to a steam turbine, which rotates and drives the steam turbine to generate electricity. After the steam has completed its work in the steam turbine, it is discharged from the steam turbine and guided to a condenser, where it is cooled and condensed. At this time, the exhaust heat of the steam is discarded in the condenser, which reduces the energy efficiency of the entire power plant.

[0005] The present disclosure has been made in consideration of the above circumstances, and has an object to provide a power generation plant and a method for operating the power generation plant that can improve the energy efficiency of the entire power generation plant. [Means for solving the problem]

[0006] In order to solve the above problems, the power plant and the power plant operating method of the present disclosure employ the following measures. a boiler having a first steam turbine rotated by steam supplied thereto, a furnace that generates combustion gas, a superheater that superheats steam supplied to the first steam turbine by heat exchange with the combustion gas generated in the furnace, and a reheater that superheats steam discharged from the first steam turbine by heat exchange with the combustion gas generated in the furnace; a second steam turbine that is supplied with steam superheated in the reheater and rotates by the supplied steam; a generator that generates electricity by the rotational driving forces of the first steam turbine and the second steam turbine; a condenser that is supplied with steam discharged from the second steam turbine and condenses the supplied steam; a mixing section that mixes the steam discharged from the first steam turbine and the steam discharged from the second steam turbine; and a mixed steam line that connects the mixing section to the reheater and leads the steam mixed in the mixing section to the reheater.

[0007] In addition, in a method of operating a power plant according to one embodiment of the present disclosure, the power plant includes a first steam turbine rotated by steam supplied thereto, a furnace that generates combustion gas, a boiler having a superheater that superheats steam to be supplied to the first steam turbine by heat exchange with the combustion gas generated in the furnace, and a reheater that superheats steam discharged from the first steam turbine by heat exchange with the combustion gas generated in the furnace, a second steam turbine to which the steam superheated in the reheater is supplied and which rotates by the supplied steam, a generator that generates electricity by the rotational driving forces of the first steam turbine and the second steam turbine, and a condenser to which steam discharged from the second steam turbine is supplied and which condenses the supplied steam, and the power plant also includes a mixing process for mixing the steam discharged from the first steam turbine and the steam discharged from the second steam turbine, and an introduction process for guiding the steam mixed in the mixing process to the reheater. Effect of the Invention

[0008] According to the present disclosure, the energy efficiency of the entire power plant can be improved. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram showing a power plant according to an embodiment of the present disclosure. [Diagram 2] 1 is a schematic configuration diagram showing a boiler system according to an embodiment of the present disclosure. [Diagram 3] 4 is a time chart showing a generator output and the like during normal operation and recirculation operation of the power plant according to the embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of a power plant and an operating method of a power plant 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 a plurality of embodiments, the present disclosure also includes a configuration in which the respective embodiments are combined. In the following description, up and above refer to the upper side in the vertical direction, and down and below refer to the lower side in the vertical direction, and the vertical direction is not precise and includes an error.

[0011] FIG. 1 is a schematic diagram showing a power generation plant using solid fuel as a main fuel according to this embodiment. The power plant 1 includes a boiler system 2 having a boiler 10 that generates steam, a steam turbine 111 that is driven by the steam generated in the boiler system 2, and a generator 113 that is connected to the steam turbine 111 and generates electricity by the rotational force of the steam turbine 111. Note that only the boiler 10 of the boiler system 2 is illustrated in FIG.

[0012] 2 is a schematic diagram showing the configuration of the boiler system 2. The boiler system 2 includes a boiler 10 that generates the above, a plurality of mills (pulverizers) 31 that pulverize the solid fuel to be supplied to the boiler 10, and a blower 30 that supplies air to the mills 31.

[0013] The boiler 10 generates steam by exchanging heat between the combustion gas generated in the furnace 11 and feed water in a heat exchanger. As shown in FIG. 1, the boiler 10 includes a furnace 11, a burner 21, and a combustion gas passage 12.

[0014] The furnace 11 is a hollow rectangular cylinder installed vertically. The furnace wall 101 that constitutes 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 to each other, and recovers the heat generated by the combustion of the pulverized fuel by heat exchange with water or steam flowing inside the heat transfer tubes, while suppressing the temperature rise of the furnace wall 101.

[0015] A plurality of burners 21 are provided. In detail, the burners 21 are arranged in a plurality of stages along the vertical direction, with each set consisting of burners arranged at equal intervals along the circumferential direction of the furnace 11. For convenience of illustration, only one burner 21 is shown in Fig. 1 and Fig. 2. The shape of the furnace, the number of burner stages, the number of burners in one stage, the arrangement of the burners, and the like are not limited to this embodiment.

[0016] As shown in Fig. 2, the burner 21 is connected to the mill 31 via a pulverized fuel supply pipe 22. The burner 21 burns the solid fuel pulverized by the mill 31 as fuel. The burner 21 also has a flame holding mechanism (not shown) at its tip, which is installed so as to face the inside of the furnace 11.

[0017] An air box 23 is provided outside the furnace 11 at the mounting position of the burner 21. One end of an air duct 24 is connected to the air box 23. The air box 23 supplies air to all of the burners 21. A forced draft fan (FDF: Forced Draft Fan) 32 is connected to the other end of the air duct 24. The 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 burners 21 via the air box 23 as secondary air (combustion air, oxidizing gas), and is introduced into the furnace 11. The amount of secondary air supplied to the air box 23 is determined according to the amount of air used by all of the burners 21.

[0018] As shown in Fig. 1, the combustion gas passage 12 is connected to the vertical upper part of the furnace 11. In the combustion gas passage 12, a superheater 102, a first reheater 103A, a second reheater 103B (hereinafter, these may be collectively referred to as "reheaters"), and a coal economizer (not shown) are provided as heat exchangers for recovering heat from the combustion gas, and heat is exchanged between the combustion gas generated in the furnace 11 and feed water or steam flowing inside each heat exchanger. The arrangement and shape of each heat exchanger are not limited to the form shown in Fig. 1.

[0019] As shown in Fig. 2, the boiler 10 is connected to a flue 13 through which the combustion gas (hereinafter, may be referred to as "exhaust gas") whose heat has been recovered by the heat exchanger is discharged. An air preheater 42 is provided in the flue 13. The air preheater 42 exchanges heat between the air flowing through the hot gas passage 30a and the air duct 24 and the combustion gas (exhaust gas) flowing through the flue 13. By performing this heat exchange, the air preheater 42 heats 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.

[0020] The air preheater 42 has a first heat exchange section 42a to which air (primary air) flowing through the hot gas flow passage 30a is introduced, and a second heat exchange section 42b to which air (secondary air) flowing through the air passage 24 is introduced. The first heat exchange section 42a has, for example, a plurality of heat transfer tubes (not shown) and performs heat exchange between the primary air flowing through the heat transfer tubes and the exhaust gas flowing outside the heat transfer tubes. The second heat exchange section 42b has, for example, a plurality of heat transfer tubes (not shown) and performs heat exchange between the secondary air flowing through the heat transfer tubes and the exhaust gas flowing outside the heat transfer tubes.

[0021] The first heat exchanger 42a and the second heat exchanger 42b are each defined to have an upper limit of heat quantity that can be exchanged. The upper limit of heat quantity is defined based on the heat transfer area of ​​the heat transfer tube. In the boiler system 2 according to the present embodiment, the upper limit of heat quantity of each heat exchanger is defined according to the amount of primary air and the amount of secondary air required when only solid fuel is burned in the boiler 10. That is, in the case of the amount of primary air and the amount of secondary air, the first heat exchanger 42a and the second heat exchanger 42b are configured so that the total amount of heat exchanged in the first heat exchanger 42a and the second heat exchanger 42b is maximized. The amount of primary air that becomes the upper limit of heat quantity in the first heat exchanger 42a is, for example, 40% of the total amount of air (the sum of the amount of primary air and the amount of secondary air) supplied to the boiler 10. In addition, the amount of secondary air that becomes the upper limit of heat quantity in the second heat exchanger 42b is 60% of the total amount of air (the sum of the amount of primary air and the amount of secondary air) supplied to the boiler 10.

[0022] A gas duct 41 is connected to the flue 13 downstream of the air preheater 42. The gas duct 41 is provided with an exhaust gas thermometer 14 for measuring the temperature of the exhaust gas flowing inside. The downstream end of the gas duct 41 is connected to a chimney (not shown), and the exhaust gas treated by the environmental equipment is discharged to the outside of the system.

[0023] The mill 31 is, for example, a vertical roller mill in which a grinding table (not shown) is supported inside so as to be driven and rotatable, and a plurality of grinding rollers (not shown) are supported above the grinding table so as to be rotatable in conjunction with the rotation of the grinding table. The mill 31 grinds the solid fuel supplied from the fuel supply unit 31a by the cooperation of the grinding rollers and the grinding table. The solid fuel ground 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 classifies the fuel into pulverized fuel having a particle size equal to or smaller than that suitable for combustion in the burner 21 and coarse pulverized fuel having a particle size larger than the particle size. The pulverized fuel passes through the classifier and is supplied to the burner 21 together with the primary air via the pulverized fuel supply pipe 22. The coarse pulverized fuel that does not pass through the classifier falls onto the grinding table by its own weight inside the mill 31 and is ground again.

[0024] The blower 30 is a device that blows primary air for drying and transporting the pulverized fuel into the inside of the mill 31. The amount of primary air supplied to the boiler 10 is determined according to the amount of air used by the burner 21. In order to appropriately adjust the flow rate and temperature of the primary air blown into the inside of the mill 31, in this embodiment, the blower section 30 is equipped with a primary air fan (PAF) 33, a hot gas flow path (air supply line) 30a, a cold gas flow path 30b, a hot gas damper 30c, and a cold gas damper 30d.

[0025] The hot gas passage 30a branches off from the cold gas passage 30b. The hot gas passage 30a supplies a part of the air sent out from the primary air ventilator 33 as hot gas heated by passing through the air preheater 42. The hot gas passage 30a is provided with a hot gas damper 30c downstream of the air preheater 42. The opening degree of the hot gas damper 30c is controlled by a control device (not shown). The flow rate of the hot gas supplied from the hot gas passage 30a is determined by the opening degree of the hot gas damper 30c. The hot gas heated by the air preheater 42 is also led to other mills 31 via the hot gas passage 30a.

[0026] The control device (Controller) includes, for example, a CPU (Central Processing Unit: Processor), a main memory, a secondary storage, etc. Furthermore, the control device may include a communication unit for transmitting and receiving information to and from other devices. The main storage device is composed of writable memory such as cache memory and RAM (Random Access Memory), and is used as a working area for reading out programs executed by the CPU and writing data processed by the programs. The secondary storage device is a non-transitory computer readable storage medium, such as a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, or a semiconductor memory. A series of processes for realizing various functions is stored in a secondary storage device in the form of a program, for example, and various functions are realized by the CPU reading the program into the main storage device and executing information processing and arithmetic processing. The program may be installed in the secondary storage device in advance, 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.

[0027] The cold gas passage 30b supplies a portion of the air sent out from the primary air ventilator 33 as cold gas at room temperature. A cold gas damper 30d is provided in the cold gas passage 30b. The opening degree of the cold gas damper 30d is controlled by a control device. The flow rate of the cold gas supplied from the cold gas passage 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 passage 30b.

[0028] The hot gas flow path 30a and the cold gas flow path 30b join at the downstream end 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 device.

[0029] The combustion gas flowing into the combustion gas passage 12 exchanges heat with water and steam in the superheater 102, reheaters (first reheater 103A, second reheater 103B) and economizer arranged inside the combustion gas passage 12, and is then discharged into the flue 13, where nitrogen oxides are removed in a denitration device (not shown), and the combustion gas exchanges heat with primary air and secondary air in an air preheater 42, and is then discharged into a gas duct 41, where ash and the like are removed in a dust collector (not shown), where sulfur oxides are removed in a desulfurization device (not shown), and the combustion gas is then discharged to the outside of the system from a chimney (not shown). Note that the arrangement of the heat exchangers in the combustion gas passage 12 and the devices from the flue 13 to the gas duct 41 with respect to the combustion gas flow does not necessarily have to be in the order described above.

[0030] Next, the superheater 102, the reheaters (first reheater 103A, second reheater 103B), and the economizer (not shown) provided in the combustion gas passage 12 as heat exchangers will be described in detail. Note that Fig. 1 does not accurately show the positions of the heat exchangers (superheater 102, first reheater 103A, second reheater 103B) in the combustion gas passage 12, and the arrangement order of the heat exchangers with respect to the combustion gas flow is not limited to that shown in Fig. 1. In addition, in the following description, the intermediate / low pressure turbine 111B will be described as one steam turbine, but the intermediate / low pressure turbine 111B may be divided into two steam turbines, an intermediate pressure turbine and a low pressure turbine.

[0031] The steam turbine 111 is composed of, for example, a high-pressure turbine 111A and an intermediate / low-pressure turbine 111B. The steam heated by the superheater 102 of the boiler 10 is guided to the high-pressure turbine 111A via a steam line L3. A high-pressure steam flow control valve 57 is provided on the steam line L3. The steam introduced into the high-pressure turbine 111A drives the high-pressure turbine 111A to rotate, and is then guided to the second reheater 103B of the boiler 10 via a steam line L4. The steam reheated in the second reheater 103B is introduced into the first reheater 103A and resuperheated. The steam discharged from the first reheater 103A is guided to the intermediate / low-pressure turbine 111B via a steam line L5, and drives the intermediate / low-pressure turbine 111B to rotate. A condenser 112 is connected to the intermediate / low-pressure turbine 111B via a steam line L8. The steam that has driven the medium-low pressure turbine 111B is guided to the condenser 112 via a steam line L8, where it is condensed into water by heat exchange with cooling water (e.g., seawater or river water) in the condenser 112. The condenser 112 is connected to the economizer via a water supply line (not shown).

[0032] As shown in FIG. 1, the power plant 1 further includes a booster turbine 51, a first heat exchanger 52, a second heat exchanger 53, and a steam header .

[0033] The boost turbine 51 has a turbine 51a that rotates in response to the flow of steam circulating through the first high-pressure steam line L11, a compressor 51b that boosts the pressure of steam circulating through the first recirculation line L13, a rotating shaft 51c that connects the turbine 51a and the compressor 51b, and a transmission 51d provided on the rotating shaft 51c. In the boost turbine 51, the turbine 51a rotates upon receiving the flow of steam circulating through the first high-pressure steam line L11. Accordingly, the rotary shaft 51c rotates and the compressor 51b rotates. At this time, the rotation speed of the compressor 51b is changed to a desired rotation speed by the transmission 51d. As the compressor 51b rotates, the steam introduced into the compressor 51b through the first recirculation line L13 is compressed. As a result, the steam circulating through the first recirculation line L13 is boosted. The steam boosted by the compressor 51b is discharged to the second recirculation line L14. In addition, the steam that drives the turbine 51a is discharged to the second high-pressure steam line L12.

[0034] Further, a first high-pressure steam line L11 branches off from the steam line L4. The first high-pressure steam line L11 connects a midpoint of the steam line L4 to the booster turbine 51. The first high-pressure steam line L11 guides a portion of the steam flowing through the steam line L4 to the booster turbine 51. A second electric valve 59, a first heat exchanger 52, and a second heat exchanger 53 are provided in this order from the upstream side at a midpoint of the first high-pressure steam line L11. The second electric valve 59 is an on-off valve that can be switched between an open state and a closed state to switch between a state in which steam flows through the first high-pressure steam line L11 and a state in which steam does not flow through the first high-pressure steam line L11. The second electric valve 59 is controlled by a control unit to be switched between the open state and the closed state.

[0035] Further, a first electric valve 58 is provided in the steam line L4 downstream of the branch position of the first high-pressure steam line L11. The first electric valve 58 is an on-off valve that can be switched between an open state and a closed state to switch between a state in which steam flows to the steam line L4 downstream of the first electric valve 58 and a state in which steam does not flow therethrough. The first electric valve 58 is controlled by a control unit to be switched between the open state and the closed state.

[0036] The upstream end of a second high-pressure steam line L12 is connected to the booster turbine 51. Steam that drives the booster turbine 51 is discharged to the second high-pressure steam line L12. The second high-pressure steam line L12 connects the booster turbine 51 and a steam header 54. The second high-pressure steam line L12 guides the steam that drives the booster turbine 51 to the steam header 54. The second high-pressure steam line L12 is provided with a pressure regulating valve 55. The pressure regulating valve 55 is a valve whose opening can be adjusted. The pressure regulating valve 55 can adjust the pressure of the high-pressure steam flowing through the second high-pressure steam line L12 by adjusting its opening. The opening of the pressure regulating valve 55 is controlled by a control unit. The control unit may control the opening of the pressure regulating valve 55 so that the pressure of the steam flowing from the second high-pressure steam line L12 into the steam header 54 and the pressure of the steam flowing from the second recirculation line L14 into the steam header 54 are the same pressure.

[0037] The upstream end of the first recirculation line L13 is connected to the medium / low pressure turbine 111B. A part or all of the steam that has completed its work in the medium / low pressure turbine 111B is discharged to the first recirculation line L13. The amount of low-pressure steam discharged to the first recirculation line L13 (in other words, the flow rate of low-pressure steam flowing through the first recirculation line L13) is determined by the aperture of a low-pressure steam flow control valve 56 provided in the steam line L8. Note that a flow control valve may be provided in the first recirculation line L13, and the amount of low-pressure steam discharged to the first recirculation line L13 may be adjusted by adjusting the aperture of the flow control valve. The first recirculation line L13 connects the intermediate / low pressure turbine 111B and the booster turbine 51. The first recirculation line L13 guides the low pressure steam to the booster turbine 51.

[0038] The upstream end of a second recirculation line L14 is connected to the booster turbine 51. Steam boosted by the booster turbine 51 is discharged to the second recirculation line L14. The second recirculation line L14 connects the booster turbine 51 and a steam header 54. The second recirculation line L14 guides the steam boosted by the booster turbine 51 to the steam header 54. A second heat exchanger 53 and a first heat exchanger 52 are provided in this order from the upstream side at a midpoint of the second recirculation line L14.

[0039] The first heat exchanger 52 exchanges heat between the steam flowing through the first high-pressure steam line L11 and the steam flowing through the second recirculation line L14. The first heat exchanger 52 heats the steam flowing through the second recirculation line L14 by performing heat exchange.

[0040] The second heat exchanger 53 performs heat exchange between the steam flowing through the first high-pressure steam line L11 and the steam flowing through the second recirculation line L14. The first heat exchanger 52 performs heat exchange to heat the steam flowing through the second recirculation line L14.

[0041] The downstream ends of the second high-pressure steam line L12 and the second recirculation line L14 are connected to the steam header 54. Steam that has flowed through the second high-pressure steam line L12 and the second recirculation line L14 is guided to the steam header 54. The steam header 54 mixes the steam flowing in from the second high-pressure steam line L12 and the steam flowing in from the second recirculation line L14.

[0042] The upstream end of a mixed steam line L15 is connected to the steam header 54. Steam mixed in the steam header 54 is discharged to the mixed steam line L15. The mixed steam line L15 connects the steam header 54 to a midpoint of the steam line L4. In detail, the downstream end of the mixed steam line L15 is connected to a portion of the steam line L4 downstream of the first electric valve 58. The mixed steam line guides the steam mixed in the steam header 54 to the second reheater 103B via the steam line L4.

[0043] Next, the behavior of the power plant 1 according to this embodiment will be described. Air adjusted to a predetermined amount and temperature is introduced as primary air (transport gas) into the mill 31 by the blower 30. The blower 30 generates primary air of a predetermined amount and temperature by mixing air flowing through the hot gas flow passage 30a (air heated by heat exchange with exhaust gas in the air preheater 42) with air flowing through the cold gas flow passage 30b. The amount and temperature of the primary air are adjusted according to the amount and type of solid fuel supplied to the mill 31 so as to be suitable for drying in the mill 31, transportation in the pulverized fuel supply pipe 22, combustion in the burner 21, and the like.

[0044] When the mill 31 is driven, the pulverized fuel that has been dried, pulverized, and classified is supplied together with the primary air to the burner 21 via the pulverized fuel supply pipe 22. In addition, secondary air heated by an air preheater 42 is supplied to the burner 21 from an air duct 24 through an air box 23. The burner 21 blows a pulverized fuel mixture, which is a mixture of the pulverized fuel and the 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.

[0045] When a flame is formed in the lower region of the furnace 11, high-temperature combustion gas rises inside the furnace 11 and flows into the combustion gas passage 12 inside the boiler 10. In this embodiment, air is used as the oxidizing gas (primary air, secondary air), but the oxidizing gas may have a higher or lower oxygen content than air, and stable combustion in the furnace 11 can be achieved by adjusting the ratio of the amount of oxygen to the amount of fuel supplied within an appropriate range.

[0046] The combustion gas flowing into the combustion gas passage 12 exchanges heat with water or steam in heat exchangers (superheater 102, first reheater 103A, second reheater 103B, economizer, etc.) arranged inside the combustion gas passage 12, and is then discharged into the flue 13, exchanges heat with primary air and secondary air in the air preheater 42, and is then discharged into the gas duct 41 and exhausted to the outside of the system from the chimney.

[0047] As described above, when the combustion gas flows through the combustion gas passage 12, the heat of this combustion gas is recovered by the superheater 102, the reheater (first reheater 103A, second reheater 103B), and the economizer. On the other hand, the feed water supplied to the boiler 10 is preheated by the economizer, and then heated to become steam when passing through the heat transfer tubes constituting the furnace wall 101, and is guided to a steam separator (not shown). The steam separated by the steam separator is introduced into the superheater 102 and superheated by the combustion gas. The superheated steam generated by the superheater 102 is supplied to the high-pressure turbine 111A via the steam line L3, and drives the high-pressure turbine 111A to rotate. The steam that drives the high-pressure turbine 111A to rotate is discharged to the steam line L4.

[0048] The flow of steam discharged from the high-pressure turbine 111A will be described separately for a case where the steam discharged from the intermediate / low-pressure turbine 111B is not guided to the second reheater 103B (in other words, recirculated) (hereinafter referred to as "recirculation operation"), and a case where the recirculation operation is performed. In addition, hereinafter, the operation where the recirculation operation is not performed will be referred to as "normal operation".

[0049] [Normal operation] When performing normal operation, the low pressure steam flow control valve 56 and the first electric valve 58 are fully opened, and the second electric valve 59 is fully closed. The steam discharged from the high-pressure turbine 111A is introduced through a steam line L4 to the second reheater 103B and the first reheater 103A in that order and is superheated again. The resuperheated steam is supplied to the medium-low pressure turbine 111B through a steam line L5 and drives the medium-low pressure turbine 111B to rotate. The rotating shaft of the steam turbine 111 drives the generator 113 to generate electricity. The steam discharged from the medium-low pressure turbine 111B is cooled in the condenser 112 to become condensed water, and is sent again to the economizer through the feedwater line. In the case of normal operation, the steam discharged from the medium-low pressure turbine 111B does not flow into the first recirculation line L13.

[0050] [Recirculation operation] When performing the recirculation operation, the first electric valve 58 is fully closed, and the second electric valve 59 is fully open. Also, the low-pressure steam flow control valve 56 is opened. The opening degree of the low-pressure steam flow control valve 56 is appropriately set.

[0051] Steam discharged from the high-pressure turbine 111A is introduced into the first high-pressure steam line L11 via the steam line L4. The steam flowing into the first high-pressure steam line L11 is cooled by heat exchange with steam circulating through the second recirculation line L14 in the first heat exchanger 52 and the second heat exchanger 53. The steam that has completed heat exchange in the second heat exchanger 53 drives and rotates the turbine 51a of the booster turbine 51. The steam that has driven and rotated the turbine 51a is discharged from the booster turbine 51 and introduced into the second high-pressure steam line L12. The steam that has flowed into the second high-pressure steam line L12 is reduced in pressure by the pressure control valve 55 and introduced into the steam header 54.

[0052] A part or all of the steam discharged from the intermediate / low pressure turbine 111B is introduced into the compressor 51b of the booster turbine 51 via the first recirculation line L13, and is pressurized by the compressor 51b. The steam pressurized by the compressor 51b is introduced into the second recirculation line L14. The steam that flows into the second recirculation line L14 is heated by heat exchange with the steam circulating in the first high pressure steam line L11 in the second heat exchanger 53 and the first heat exchanger 52. The steam that has completed the heat exchange in the first heat exchanger 52 is introduced into the steam header 54.

[0053] The steam header 54 mixes the steam introduced into the steam header 54 via the second high-pressure steam line L12 and the steam introduced into the steam header 54 via the second recirculation line L14. The steam mixed in the steam header 54 is introduced into the boiler 10 via the mixed steam line L15 and a part of the steam line L4. Specifically, it is introduced into the second reheater 103B. The steam introduced into the boiler 10 flows in the order of the second reheater 103B and the first reheater 103A, and is superheated in each reheater. The resuperheated steam is supplied to the intermediate / low pressure turbine 111B via the steam line L5.

[0054] In this way, in the recirculation operation, a part or all of the steam discharged from the intermediate / low pressure turbine 111B is guided again to the intermediate / low pressure turbine 111B without passing through the condenser 112.

[0055] During the transition from normal operation to recirculation operation, both the steam flow during normal operation and the steam flow during recirculation operation occur.

[0056] Next, the relationship between the output of the generator 113 in the power plant 1 according to this embodiment, the steam recirculation amount, the steam intake amount of the medium-low pressure turbine 111B, the operating state of the boost turbine 51, the opening degree of the low-pressure steam flow control valve 56, and the opening and closing states of the first electric valve 58 and the second electric valve 59 will be explained using the time chart of Figure 3.

[0057] In the section from t0 to t1, the power plant 1 is operating normally. That is, the low pressure steam flow control valve 56 and the first electric valve 58 are fully open, and the second electric valve 59 is fully closed. As a result, no steam is guided to the booster turbine 51, and the booster turbine 51 is in a stopped state. At this time, the flow rate of steam guided to the medium / low pressure turbine 111B via the steam line L5 (medium / low pressure turbine steam intake rate) is set to 1000 t / h. In addition, the steam recirculation rate (the flow rate of steam guided to the reheater 103 (first reheater 103A, second reheater 103B) via the first recirculation line L13 and the second recirculation line L14) is set to 0 t / h. In addition, the output of the generator 113 is set to 50% of the maximum output.

[0058] In the section from t1 to t4, the power plant 1 transitions from normal operation to recirculation operation. First, in the section from t1 to t2, the second electric valve 59 is gradually switched from a fully open state to a fully closed state. At this time, the low-pressure steam flow control valve 56 and the first electric valve 58 are maintained in a fully open state. As the second electric valve 59 opens, steam is guided to the booster turbine 51, so the booster turbine 51 starts up and gradually increases its rotation speed. In addition, the low / medium pressure turbine steam intake amount, steam recirculation amount, and generator output are kept constant at the same values ​​as in the section from t0 to t1.

[0059] In the section from t2 to t3, the aperture of the low-pressure steam flow control valve 56, the open / closed states of the first electric valve 58 and the second electric valve 59, and the operating state of the booster turbine 51 are maintained. As a result, the medium / low pressure turbine steam intake amount, the recirculation amount, and the generator output are kept constant at the same values ​​as in the section from t1 to t2.

[0060] In the section from t3 to t4, the first electric valve 58 is gradually switched from the fully open state to the fully closed state. In addition, the aperture of the low-pressure steam flow control valve 56 is reduced. At this time, the second electric valve 59 is maintained in the fully open state. At this time, the speed of the booster turbine 51 is gradually increased. In addition, according to the aperture of the low-pressure steam flow control valve 56, the medium-low pressure turbine steam intake amount, steam recirculation amount, and generator output increase.

[0061] In the section after t4, the power plant 1 performs recirculation operation. The first electric valve 58 is fully closed, and the second electric valve 59 is fully open. In addition, the low-pressure steam flow control valve 56 is open. In detail, the low-pressure steam flow control valve 56 is opened close to fully closed at the time of t4, and the opening degree is gradually increased from there over time, and after being opened fully, the fully open state is maintained. At this time, the booster turbine 51 maintains its rotation speed. Furthermore, the steam recirculation amount is maintained near a predetermined flow rate (in this embodiment, αt / h). Furthermore, as the steam recirculation amount increases, the intermediate-pressure turbine steam intake amount also increases. Specifically, the intermediate-pressure turbine steam intake amount is maintained near a predetermined flow rate (in this embodiment, 1000t+αt / h). Accordingly, the generator output is also maintained near a predetermined value (in this embodiment, 50+β% of maximum output).

[0062] According to this embodiment, the following advantageous effects are obtained. In this embodiment, the steam header 54 mixes the steam discharged from the high pressure turbine 111A and the steam discharged from the medium / low pressure turbine 111B, and the mixed steam line L15 guides the steam mixed in the steam header 54 to the reheaters (first reheater 103A, second reheater 103B). That is, a part or all of the steam that has completed work in the medium / low pressure turbine 111B is guided to the second reheater 103B via the steam header 54 and the mixed steam line L15. In this way, since a part of the steam is circulated between the reheater and the medium / low pressure turbine 111B, the amount of steam introduced into the reheater can be increased compared to the case where the steam that has completed work in the medium / low pressure turbine 111B is not circulated to the reheater. Therefore, the heat transfer efficiency in the reheater is improved, and the amount of recovered heat can be increased. Therefore, the amount of work of the medium / low pressure turbine 111B can be increased. Therefore, the energy efficiency of the entire power plant 1 can be improved.

[0063] In this embodiment, the steam discharged from the high pressure turbine 111A and the steam discharged from the medium / low pressure turbine 111B are mixed in the steam header 54, and the mixed steam in the steam header 54 is guided to the reheater. As a result, compared with the case where the steam discharged from the high pressure turbine 111A and the steam discharged from the medium / low pressure turbine 111B are separately guided to the reheater, the temperature and pressure of the steam introduced into the reheater can be made uniform, and the difference between the reheater metal temperature and the steam temperature guided to the reheater can be made uniform, so that heat can be efficiently recovered and the amount of heat recovered in the reheater can be more suitably increased. Therefore, the work load of the medium / low pressure turbine 111B can be increased. Therefore, the energy efficiency of the entire power plant 1 can be improved.

[0064] In addition, in this embodiment, the first heat exchanger 52 and the second heat exchanger 53 are provided to exchange heat between the steam discharged from the high-pressure turbine 111A and guided to the steam header 54 (hereinafter referred to as "high-pressure steam") and the steam discharged from the intermediate / low-pressure turbine 111B and guided to the steam header 54 (hereinafter referred to as "low-pressure steam"). This makes it possible to reduce the temperature difference between the high-pressure steam and the low-pressure steam guided to the steam header 54. Therefore, the temperature of the steam introduced into the reheater via the steam header 54 can be more suitably uniformized, and the difference between the reheater metal temperature and the steam temperature guided to the reheater can be uniformized, so that heat can be efficiently recovered and the amount of recovered heat in the reheater can be more suitably increased. Therefore, the workload of the intermediate / low-pressure turbine 111B can be increased. Therefore, the energy efficiency of the entire power plant 1 can be improved.

[0065] In addition, this embodiment includes a booster turbine 51 that boosts the pressure of low-pressure steam with high-pressure steam. This makes it possible to reduce the pressure difference between the high-pressure steam and the low-pressure steam guided to the steam header 54. Therefore, the pressure of the steam introduced into the reheater via the steam header 54 can be more suitably uniformed, making it possible to efficiently supply the high-pressure steam turbine exhaust steam and the intermediate / low-pressure steam turbine exhaust steam to the reheater, thereby more suitably increasing the amount of heat recovered in the reheater. This makes it possible to increase the workload of the intermediate / low-pressure turbine 111B. This makes it possible to improve the energy efficiency of the entire power plant 1.

[0066] In this embodiment, the booster turbine 51 has a transmission 51d that changes the rotation speed of the second turbine. This makes it possible to boost the low-pressure steam to a desired pressure. Therefore, the pressure difference between the high-pressure steam and the low-pressure steam guided to the steam header 54 can be more suitably reduced.

[0067] In this embodiment, the steam line L8 is provided with a low-pressure steam flow control valve 56 that adjusts the flow rate of steam flowing through the steam line L8. This makes it possible to adjust the flow rate of steam guided from the second turbine to the condenser 112, and also to adjust the flow rate of steam guided from the second turbine to the steam header 54.

[0068] Also, for example, in a boiler that is intended to use a specific solid fuel (coal as an example), there are cases where another solid fuel (biomass fuel such as biomass pellets as an example) that has a lower moisture content than the specific solid fuel is used. In this case, the amount of heat required to dry the fuel is less than when the specific solid fuel is used, so the temperature of the primary air (carrier gas) introduced into the mill 31 is lowered. In addition, when using biomass fuel as a solid fuel with a low moisture content, it is necessary to lower the temperature of the primary air (carrier gas) introduced into the mill 31 from the viewpoint of preventing ignition within the mill 31, since biomass fuel is more flammable (easier to ignite) than coal. The temperature of the primary air introduced into the mill 31 (the temperature at the inlet of the mill 31) is adjusted by adjusting the flow rate distribution of hot air heated by heat exchange with the exhaust gas in the air preheater 42, and cold air bypassing the air preheater 42. For this reason, if the temperature of the primary air introduced into the mill 31 is lowered, the flow rate of hot air passing through the air preheater 42 decreases, which reduces the amount of heat recovered from the combustion gas in the air preheater 42, and there is a possibility that the exhaust gas temperature at the outlet of the air preheater 42 will increase.

[0069] In addition, compared to combustion ash (coal ash) when coal is used, the combustion ash (biomass combustion ash) produced when biomass fuel is used tends to have more ash adhering to heat exchangers such as the furnace wall 101, superheater 102, reheater (first reheater 103A, second reheater 103B), and economizer (not shown) of the boiler 10, and the amount of heat recovered by each heat exchanger tends to be lower. As a result, the amount of heat recovered from the combustion gas in each heat exchanger of the boiler 10 decreased, and there was a possibility that the temperature of the combustion gas introduced into the air preheater 42 and the temperature of the exhaust gas at the outlet of the air preheater 42 would increase.

[0070] If the exhaust gas temperature at the outlet of the air preheater 42 rises, the heat loss due to the exhaust gas discharged to the outside of the system increases, which may lead to a decrease in the energy efficiency of the boiler 10. In addition, if the exhaust gas temperature at the outlet of the air preheater 42 becomes higher than the heat resistance temperature of various devices downstream of the air preheater 42, the various devices downstream of the air preheater 42 may be damaged by the heat of the exhaust gas.

[0071] Furthermore, when biomass fuel is burned in the boiler 10, the temperature of the steam guided to the reheaters (first reheater 103A, second reheater 103B) may decrease, causing the output of the generator 113 to fall below the rated output, resulting in a decrease in the amount of power generated. This is because, as described above, biomass combustion ash tends to adhere more easily to the heat transfer surfaces of the heat exchangers (superheater 102, reheater, etc.) provided in the combustion gas passage 12 than coal ash, resulting in a decrease in the heat absorption efficiency of the heat exchanger. In addition, as described above, the biomass fuel requires a decrease in the temperature of the primary air introduced into the mill 31 from the viewpoint of preventing ignition in the mill 31, and the increase in the temperature of the exhaust gas discharged from the air preheater 42 (i.e., the amount of heat exchange in the air preheater 42 decreases) is also a factor in the decrease in the amount of power generated by the generator 113.

[0072] On the other hand, in this embodiment, the amount of steam introduced into the reheaters (first reheater 103A, second reheater 103B) can be increased. Therefore, the heat transfer efficiency in the reheaters (first reheater 103A, second reheater 103B) is improved, and the amount of recovered heat can be increased. Therefore, the temperature of the exhaust gas discharged from the boiler 10 can be reduced. Therefore, a decrease in the energy efficiency in the boiler 10 can be suppressed. Also, a situation in which various devices downstream of the air preheater 42 are damaged by the heat of the exhaust gas can be suppressed.

[0073] The present disclosure is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit and scope of the present disclosure. For example, in the above embodiment, an example has been described in which the steam discharged from the intermediate / low pressure turbine 111B is boosted using the boost turbine 51 that utilizes the energy of the steam discharged from the high pressure turbine 111A, but the present disclosure is not limited to this. For example, the steam discharged from the intermediate / low pressure turbine 111B may be boosted by a device that utilizes external power.

[0074] In the above embodiment, the steam discharged from the medium-low pressure turbine 111B is heated using the first heat exchanger 52 and the second heat exchanger 53 that utilize the heat of the steam discharged from the high pressure turbine 111A, but the present disclosure is not limited to this. For example, the steam discharged from the medium-low pressure turbine 111B may be heated by a device that utilizes heat supplied from the outside.

[0075] The power plant and the method of operating the power plant described in the above-described embodiment can be understood, for example, as follows. A power plant according to a first aspect of the present disclosure includes a boiler (10) having a first steam turbine (111A) rotated by supplied steam, a furnace (11) generating a combustion gas, a superheater (102) superheating steam to be supplied to the first steam turbine (111A) by heat exchange with the combustion gas generated in the furnace (11), and a reheater (103A, 103B) superheating steam discharged from the first steam turbine (111A) by heat exchange with the combustion gas generated in the furnace (11), and a second steam turbine (103A, 103B) to which the steam superheated in the reheaters (103A, 103B) is supplied and which is rotated by the supplied steam. the first steam turbine (111A) and the second steam turbine (111B), a generator (113) that generates electricity by the rotational driving force of the first steam turbine (111A) and the second steam turbine (111B), a condenser (112) that is supplied with steam discharged from the second steam turbine (111B) and condenses the supplied steam, a mixing section (54) that mixes the steam discharged from the first steam turbine (111A) and the steam discharged from the second steam turbine (111B), and a mixed steam line (L15) that connects the mixing section (54) to the reheaters (103A, 103B) and guides the steam mixed in the mixing section (54) to the reheaters (103A, 103B).

[0076] The above-mentioned configuration includes a mixing section that mixes the steam discharged from the first steam turbine and the steam discharged from the second steam turbine, and a mixed steam line that guides the steam mixed in the mixing section to the reheater. That is, a part or all of the steam that has completed work in the second steam turbine is guided to the reheater via the mixing section and the mixed steam line. Since a part of the steam is circulated between the reheater and the second steam turbine in this way, the amount of steam introduced into the reheater can be increased compared to a case in which the steam that has completed work in the second steam turbine is not circulated to the reheater. Therefore, the amount of heat recovered in the reheater can be increased. Therefore, the amount of work of the second steam turbine can be increased. Therefore, the energy efficiency of the entire power plant can be improved.

[0077] In the above configuration, the steam discharged from the first steam turbine and the steam discharged from the second steam turbine are mixed in a mixing section, and the steam mixed in the mixing section is guided to the reheater. This makes it possible to equalize the temperature and pressure of the steam introduced into the reheater, compared to a case in which the steam discharged from the first steam turbine and the steam discharged from the second steam turbine are separately guided to the reheater. This makes it possible to increase the amount of heat recovered in the reheater. This makes it possible to increase the workload of the second steam turbine. This makes it possible to improve the energy efficiency of the entire power plant.

[0078] A power plant according to a second aspect of the present disclosure, in the above-mentioned first aspect, is provided with heat exchange sections (52, 53) that exchange heat between steam discharged from the first steam turbine (111A) and guided to the mixing section (54) and steam discharged from the second steam turbine (111B) and guided to the mixing section (54).

[0079] The above configuration includes a heat exchange section that exchanges heat between steam discharged from the first steam turbine and guided to the mixing section (hereinafter referred to as "high pressure steam") and steam discharged from the second steam turbine and guided to the mixing section (hereinafter referred to as "low pressure steam"). This makes it possible to reduce the temperature difference between the high pressure steam and the low pressure steam guided to the mixing section. Therefore, the temperature of the steam introduced into the reheater via the mixing section can be more suitably uniformed. Therefore, the amount of heat recovered in the reheater can be more suitably increased. Therefore, the workload of the second steam turbine can be increased. Therefore, the energy efficiency of the entire power plant can be improved.

[0080] A power plant according to a third aspect of the present disclosure, in the first or second aspect described above, includes a booster section (51) that boosts the pressure of steam discharged from the second steam turbine (111B) and led to the mixing section (54) with steam discharged from the first steam turbine (111A) and led to the mixing section (54).

[0081] The above-mentioned configuration includes a boosting section that boosts the low-pressure steam with high-pressure steam. This makes it possible to reduce the pressure difference between the high-pressure steam and the low-pressure steam introduced to the mixing section. This makes it possible to more suitably equalize the pressure of the steam introduced to the reheater via the mixing section. This makes it possible to more suitably increase the amount of heat recovered in the reheater. This makes it possible to increase the workload of the second steam turbine. This makes it possible to improve the energy efficiency of the entire power plant.

[0082] A power plant according to a fourth aspect of the present disclosure is the same as that of the third aspect, wherein the boost section (51) includes a first turbine (51a) that rotates upon receiving the flow of steam discharged from the first steam turbine (111A), a second turbine (51b) that boosts the pressure of steam discharged from the second steam turbine (111B) by rotating, a rotating shaft (51c) that connects the first turbine and the second turbine, and a transmission (51d) that changes the rotation speed of the second turbine provided on the rotating shaft.

[0083] In the above configuration, the boosting section has a transmission that changes the rotation speed of the second turbine. This allows the low-pressure steam to be boosted to a desired pressure. Therefore, the pressure difference between the high-pressure steam and the low-pressure steam introduced to the mixing section can be more suitably reduced.

[0084] A power plant according to a fifth aspect of the present disclosure is, in any one of the first to fourth aspects described above, further comprising a steam line (L8) connecting the second steam turbine (111B) and the condenser (112), and the steam line (L8) is provided with a flow control valve (56) for adjusting the flow rate of steam flowing through the steam line (L8).

[0085] In the above configuration, the steam line is provided with a flow rate control valve for controlling the flow rate of steam flowing through the steam line, thereby making it possible to control the flow rate of steam guided from the second turbine to the condenser and to control the flow rate of steam guided from the second turbine to the mixing section.

[0086] A method for operating a power plant according to a first aspect of the present disclosure is a method for operating a power plant (1), in which the power plant (1) includes a boiler (10) having a first steam turbine (111A) rotated by supplied steam, a furnace (11) generating a combustion gas, a superheater (102) superheating steam to be supplied to the first steam turbine (111A) by heat exchange with the combustion gas generated in the furnace (11), and reheaters (103A, 103B) superheating steam discharged from the first steam turbine (111A) by heat exchange with the combustion gas generated in the furnace (11), and a second steam turbine (111B) which is supplied with steam superheated by the first steam turbine (111A) and the second steam turbine (111B) and rotates by the supplied steam, a generator (113) which generates electricity by the rotational driving force of the first steam turbine (111A) and the second steam turbine (111B), and a condenser (112) which is supplied with steam discharged from the second steam turbine (111B) and condenses the supplied steam, and the steam turbine comprises a mixing step of mixing the steam discharged from the first steam turbine (111A) and the steam discharged from the second steam turbine (111B), and an introduction step of guiding the steam mixed in the mixing step to the reheaters (103A, 103B). [Explanation of symbols]

[0087] 1: Power plant 2: Boiler system 10: Boiler 11: Furnace 12: Combustion gas passage 13: Flue 14: Exhaust gas thermometer 21: Burna 22:Powdered fuel supply pipe 23: Wind box 24: Wind road 30: Blower section 30a: Hot gas flow path 30b: Cold gas passage 30c: Thermal gas damper 30d: Cold gas damper 31: Mill 32: Forced ventilation fan 33: Primary air ventilator 35: Primary air passage 41: Gas duct 42: Air preheater 42a: 1st heat exchange section 42b: Second heat exchange section 51: Booster turbine 51a: Turbine 51b: Compressor 51c: Rotating shaft 51d: Transmission 52: 1st heat exchanger 53:Second heat exchanger 54: Steam header 55: Pressure control valve 56: Low pressure steam flow control valve 57: High pressure steam flow control valve 58: First motor-operated valve 59: Second motor valve 101: Furnace wall 102:Superheater 103A: 1st reheater 103B:Second reheater 111: Steam turbine 111A: High pressure turbine 111B: Medium and low pressure turbine 112: Condenser 113: Generator L11: First high pressure steam line L12: Second high pressure steam line L13: First recirculation line L14: Second recirculation line L15: Mixed steam line L3: Steam line L4: Steam line L5: Steam line L8: Steam line

Claims

1. a first steam turbine rotated by the supplied steam; a boiler including a furnace that generates a combustion gas, a superheater that superheats steam to be supplied to the first steam turbine by heat exchange with the combustion gas generated in the furnace, and a reheater that superheats steam discharged from the first steam turbine by heat exchange with the combustion gas generated in the furnace; a second steam turbine that is supplied with the steam superheated in the reheater and is rotated by the supplied steam; a generator that generates electricity using a rotational driving force of the first steam turbine and the second steam turbine; a condenser to which the steam discharged from the second steam turbine is supplied and which condenses the supplied steam; a mixing section that mixes the steam discharged from the first steam turbine and the steam discharged from the second steam turbine; a mixed steam line connecting the mixing section and the reheater and guiding the steam mixed in the mixing section to the reheater.

2. 2. The power plant according to claim 1, further comprising a heat exchange section that exchanges heat between the steam discharged from the first steam turbine and guided to the mixing section and the steam discharged from the second steam turbine and guided to the mixing section.

3. 2. The power plant according to claim 1, further comprising a booster section that boosts the pressure of the steam discharged from the second steam turbine and led to the mixing section, with the steam discharged from the first steam turbine and led to the mixing section.

4. 4. The power plant according to claim 3, wherein the boosting section includes a first turbine that rotates by receiving a flow of steam discharged from the first steam turbine, a second turbine that boosts the pressure of the steam discharged from the second steam turbine by rotating, a rotating shaft that connects the first turbine and the second turbine, and a transmission that changes the rotation speed of the second turbine provided on the rotating shaft.

5. a steam line connecting the second steam turbine and the condenser; 2. The power plant according to claim 1, wherein the steam line is provided with a flow rate control valve for controlling a flow rate of the steam flowing through the steam line.

6. 1. A method of operating a power generation plant, comprising: The power generation plant comprises: a first steam turbine rotated by the supplied steam; a boiler including a furnace that generates a combustion gas, a superheater that superheats steam to be supplied to the first steam turbine by heat exchange with the combustion gas generated in the furnace, and a reheater that superheats steam discharged from the first steam turbine by heat exchange with the combustion gas generated in the furnace; a second steam turbine that is supplied with the steam superheated in the reheater and is rotated by the supplied steam; a generator that generates electricity using a rotational driving force of the first steam turbine and the second steam turbine; a condenser to which the steam discharged from the second steam turbine is supplied and which condenses the supplied steam, a mixing step of mixing steam discharged from the first steam turbine and steam discharged from the second steam turbine; and an introducing step of introducing the steam mixed in the mixing step into the reheater.

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