Cogeneration system

Abstract

To provide a cogeneration system capable of shortening a time required for start-up of a gas turbine and an exhaust heat recovery boiler and suppressing generation of nitrogen oxide at the time of start-up.SOLUTION: A system executes in this order: a preheating process of introducing steam from an external boiler 5 to nozzle steam piping 72 and preheating the nozzle steam piping 72, prior to starting a gas turbine 2; an external steam injection process of discharging the steam used for the preheating process from the nozzle steam piping 72 to the outside, then, supplying steam from the external boiler 5 to a fuel nozzle 25 via the nozzle steam piping 72 as nozzle steam immediately after starting the gas turbine 2, and increasing output of the gas turbine 2 while mixing the nozzle steam with fuel gas G1 and injecting the mixture; and a switching process of switching the nozzle steam from the steam from the external boiler 5 to the steam from an exhaust heat recovery boiler 4 when the gas turbine 2 and the exhaust heat recovery boiler 4 reach a steady operation state.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a cogeneration system, and more particularly to a cogeneration system provided with a gas turbine and an exhaust heat recovery boiler.

Background Art

[0002] As one type of cogeneration system that simultaneously performs power generation and steam generation, a system provided with a gas turbine and an exhaust heat recovery boiler is in use. In this type of cogeneration system, fuel gas is combusted in a combustor of the gas turbine, whereby power is generated by a generator coupled to a turbine shaft. Simultaneously, high-temperature exhaust gas discharged from the gas turbine is guided to the exhaust heat recovery boiler, and steam is generated by recovering exhaust heat. This type of cogeneration system is disclosed in, for example, Patent Document 1.

Prior Art Literature

Patent Literature

[0003]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0004] In the configuration disclosed in Patent Document 1, the steam generated in the waste heat recovery boiler is supplied to the outside, but a portion of the steam is injected into the combustor of the gas turbine. This is done to increase the mass flow rate of the gas guided from the combustor to the turbine by injecting steam into the combustor, thereby increasing the output of the turbine and the amount of power generated by the generator. In a cogeneration system, in addition to the configuration in which the steam generated in the waste heat recovery boiler is directly injected into the combustor as case steam to increase output, as described in Patent Document 1, there is also a configuration in which the steam is mixed with fuel gas and injected into the combustor from a fuel nozzle as nozzle steam. When fuel gas is burned, nitrogen oxides (hereinafter referred to as NOx) are generated in the high-temperature flame, but by mixing with nozzle steam, the combustion flame temperature can be lowered and the generation of NOx can be reduced.

[0005] In a cogeneration system equipped with a gas turbine and a heat recovery boiler, it takes time for the gas turbine and heat recovery boiler to reach a steady-state operating condition when starting them from a stopped state. During this start-up period, it is not possible to increase the turbine output, nor the amount of power generated or the power generation efficiency of the generator. This is because, during the start-up period, steam with a predetermined temperature and pressure cannot be obtained from the heat recovery boiler, and therefore, steam from the heat recovery boiler cannot be supplied as nozzle steam to be mixed with the fuel gas, and thus the reduction of NOx through nozzle steam mixing cannot be utilized. Thus, it is preferable to avoid a long start-up time required for the gas turbine and heat recovery boiler to reach a steady-state operating condition, and the generation of high concentrations of NOx during that start-up period, from the viewpoint of protecting the atmospheric environment.

[0006] The problem that this invention aims to solve is to provide a cogeneration system equipped with a gas turbine and a heat recovery boiler that can shorten the time required to start up the gas turbine and the heat recovery boiler, and suppress the generation of NOx during startup. [Means for solving the problem]

[0007] [1] In order to solve the above problems, the cogeneration system according to the present invention comprises a gas turbine that burns fuel gas injected from a fuel nozzle to rotate a turbine shaft, a generator that generates electricity by the rotation of the turbine shaft and supplies electricity to an external power demand unit, a waste heat recovery boiler that recovers waste heat from the gas turbine to generate steam and supplies steam to an external steam demand unit, a nozzle steam piping that supplies a portion of the steam generated in the waste heat recovery boiler to the fuel nozzle of the gas turbine as nozzle steam mixed with the fuel gas to suppress the generation of NOx, and an external boiler configured as a boiler device different from the waste heat recovery boiler and capable of introducing steam into the nozzle steam piping, and from a turbine stop state in which the operation of the gas turbine and the waste heat recovery boiler is stopped, the gas turbine and the waste heat recovery When the recovery boiler transitions to a turbine operating state in a steady-state operating condition, prior to starting the gas turbine, the following steps are performed in this order: a preheating step in which steam is introduced from the external boiler to the nozzle steam piping to preheat the nozzle steam piping, while steam from the heat recovery boiler has not yet been introduced into the nozzle steam piping; an external steam injection step in which, immediately after the gas turbine is started, steam from the external boiler is supplied to the fuel nozzle as nozzle steam via the nozzle steam piping, and the output of the gas turbine is increased while mixing and injecting the steam with the fuel gas; and a switching step in which, once the gas turbine and the heat recovery boiler reach a steady-state operating condition, the nozzle steam is switched from steam from the external boiler to steam from the heat recovery boiler.

[0008] [2] In the embodiment of [1] above, the external boiler may generate steam at the same temperature and pressure as the waste heat recovery boiler in steady-state operation and supply the steam to the inside of the nozzle steam piping in the preheating step and the external steam injection step.

[0009] [3] In the embodiment of [1] or [2] above, the external boiler may be kept in operation for a period including the period from the turbine shutdown state to the external steam injection process, and may be stopped after the operation of the gas turbine and the waste heat recovery boiler has reached a steady-state operation state.

[0010] [4] In the embodiment of [3] above, the external boiler may continue to generate steam at the same temperature and pressure as the heat recovery boiler in steady-state operation, even during the period when the turbine is stopped, the heat recovery boiler may supply steam to the steam demand unit while the turbine is operating, and the external boiler may supply steam to the steam demand unit while the turbine is stopped.

[0011] [5] In any one embodiment of [1] to [4] above, the cogeneration system further comprises a compressor device that compresses air using electricity and steam as driving sources, the compressor device being driven by steam generated from the waste heat recovery boiler along with electricity as a driving source, and supplying the generated compressed air to an external compressed air demand unit, the steam generated from the waste heat recovery boiler assisting in the driving of the compressor device and containing components that have been reduced in pressure by the driving of the compressor device, and is supplied to the steam demand unit.

[0012] [6] In the embodiment of [5] above, the external boiler is kept running even when the turbine is stopped, and while the turbine is running, the steam generated from the waste heat recovery boiler assists in driving the compressor and is supplied to the steam demand unit, including components that have been depressurized by the operation of the compressor, and while the turbine is stopped, the steam generated from the external boiler assists in driving the compressor and is supplied to the steam demand unit, including components that have been depressurized by the operation of the compressor. [Effects of the Invention]

[0013] [1] In the cogeneration system according to the above invention, an external boiler is provided as a separate boiler device from the heat recovery boiler. Prior to starting the gas turbine, steam from the external boiler is introduced into the nozzle steam piping that supplies nozzle steam from the heat recovery boiler to the fuel nozzles of the gas turbine to preheat the nozzle steam piping. Furthermore, until the gas turbine and heat recovery boiler reach a steady-state operating condition, in the external steam injection process, steam from the external boiler is supplied to the fuel nozzles as nozzle steam through the preheated nozzle steam piping to operate the gas turbine. In this way, by generating steam with an external boiler independent of the heat recovery boiler and supplying it as nozzle steam through the preheated nozzle steam piping, the nozzle steam can be mixed with the fuel gas and the gas turbine can be operated even during startup when the steam generated from the heat recovery boiler has not yet reached the temperature and pressure of a steady-state operating condition. As a result, the output of the gas turbine can be increased earlier, and the time required for startup can be shortened. Furthermore, by mixing nozzle vapor with the fuel gas, it becomes possible to suppress NOx generation from the initial stages of startup.

[0014] [2] Here, if the external boiler generates steam at the same temperature and pressure as the heat recovery boiler in steady-state operation and supplies that steam to the inside of the nozzle steam piping in the preheating process and the external steam injection process, then, after the heat recovery boiler reaches a steady-state operation, the steam from the external boiler can be supplied as nozzle steam through the nozzle steam piping in the external steam injection process under the same conditions as when nozzle steam is supplied from the heat recovery boiler to the gas turbine via the nozzle steam piping, and injected into the gas turbine together with the fuel gas. For this reason, the operation of the gas turbine during the period required to reach a steady-state operation can be carried out while injecting gas from the fuel nozzle under the same conditions as in the steady-state operation. As a result, it is possible to effectively shorten the time to reach a steady-state operation and suppress NOx generation. Furthermore, in the switching process, when switching the nozzle steam from steam from the external boiler to steam from the heat recovery boiler, unnecessary fluctuations in conditions can be suppressed and the switching can be carried out smoothly.

[0015] [3] If the external boiler continues to operate, including the period from when the turbine is stopped until the external steam injection process, and is stopped after the gas turbine and heat recovery boiler have reached a steady-state operating condition, the preheating process and the external steam injection process can be carried out in a state in which the external boiler can stably supply steam at a predetermined temperature and pressure. As a result, the start-up period of the gas turbine and heat recovery boiler and the suppression of NOx generation can be effectively achieved by carrying out these processes. Once the heat recovery boiler reaches a steady-state operating condition, it is no longer necessary to use the external boiler to operate the gas turbine, so the energy required to operate the external boiler can be reduced by keeping it stopped.

[0016] [4] In this case, the external boiler continues to generate steam at the same temperature and pressure as the heat recovery boiler in steady-state operation, even when the turbine is stopped. The heat recovery boiler supplies steam to the steam demand section while the turbine is operating, and the external boiler supplies steam to the steam demand section while the turbine is stopped. This configuration allows the external boiler to continue generating steam and supplying it to the external steam demand section in place of the heat recovery boiler, even when the gas turbine and heat recovery boiler are stopped. When starting up the gas turbine and heat recovery boiler from a turbine-stopped state, the preheating process and the external steam injection process can be effectively carried out by receiving steam that has already reached a predetermined temperature and pressure from the external boiler, which is still in operation. This configuration can be suitably applied to situations such as when a factory or other facility uses a cogeneration system to generate both electricity and steam during the daytime when the price of electricity sold from power generators is high, but uses the cogeneration system only to generate steam at night when it is cheaper to purchase electricity from power generators than to generate electricity with the cogeneration system.

[0017] [5] The cogeneration system further includes a compressor that compresses air using electricity and steam as power sources, and the compressor is driven by steam generated from a heat recovery boiler along with electricity as power sources, and the generated compressed air is supplied to an external compressed air demand unit, and the steam generated from the heat recovery boiler assists in driving the compressor and is supplied to the steam demand unit containing components that have been depressurized by the operation of the compressor, in which case even if the amount and pressure of the steam generated from the heat recovery boiler exceeds the amount and pressure required by the steam demand unit, the amount and pressure of the steam can be used to assist in driving the compressor, thereby reducing the power required to drive the compressor and making effective use of the steam energy.

[0018] [6] In this case, the external boiler continues to operate even during the period when the turbine is in a stopped state. While the turbine is in an operating state, the steam generated from the waste heat recovery boiler assists in driving the compressor device, contains components depressurized by the driving of the compressor device, and is supplied to the steam demand section. While the turbine is in a stopped state, when the steam generated from the external boiler assists in driving the compressor device, contains components depressurized by the driving of the compressor device, and is supplied to the steam demand section, even during periods when the gas turbine and waste heat recovery boiler are stopped, such as at night, the steam generated from the external boiler can continue to supply steam to the steam demand section and assist in driving the compressor device. Then, when starting up the gas turbine and the waste heat recovery boiler, by performing the preheating step and the external steam injection step using steam from the external boiler, as described above, shortening of the startup period and reduction of NOx generation can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] [Figure 1] It is a schematic diagram showing the configuration of a cogeneration system according to an embodiment of the present invention. [Figure 2] It is a diagram for explaining temporal changes in the state of main components of the cogeneration system. MODE FOR CARRYING OUT THE INVENTION

[0020] Hereinafter, a cogeneration system according to an embodiment of the present invention will be described with reference to the drawings.

[0021] [Configuration of Cogeneration System] FIG. 1 shows the configuration of a cogeneration system 1 according to an embodiment of the present invention. The cogeneration system 1 includes a gas turbine 2, a generator 3, a waste heat recovery boiler 4, and an external boiler 5.

[0022] The gas turbine 2 includes a combustor 21, a compressor 22, and a turbine 23. Fuel gas G1 is injected into the combustor 21 via a fuel nozzle 25, and the fuel gas G1 is combusted in the combustor 21 by air compressed by the compressor 22. At this time, steam is mixed into the fuel gas G1 introduced into the combustor 21 as nozzle steam described later. Combustion gas generated by combustion of the fuel gas G1 is guided from the combustor 21 to the turbine 23, and drives the turbine 23 to rotate a turbine shaft 24.

[0023] The turbine shaft 24 of the gas turbine 2 is coupled to the generator 3, and the generator 3 generates power through rotation of the turbine shaft 24. Electric power obtained by power generation is supplied to an external power demand unit D1. The compressor 22 is also coaxially coupled to the turbine shaft 24, and rotation of the turbine shaft 24 is used for driving the compressor 22 along with power generation in the generator 3.

[0024] The waste heat recovery boiler 4 recovers waste heat from the gas turbine 2 to generate steam. Specifically, in the gas turbine 2, high-temperature exhaust gas discharged after driving the turbine 23 is introduced into the waste heat recovery boiler 4 via an exhaust gas path 71. The waste heat recovery boiler 4 generates steam from water by the heat of the exhaust gas. Part of the steam generated by the waste heat recovery boiler 4 is appropriately depressurized by a pressure reducing valve 76 via a steam supply pipe 75 connected to a steam generation unit 41, and then supplied to an external steam demand unit D2.

[0025] At least a portion of the steam generated by the heat recovery boiler 4 that is not supplied to the steam demand unit D2 is supplied to the gas turbine 2 through a nozzle steam pipe 72 connecting the heat recovery boiler 4 and the gas turbine 2. Specifically, the nozzle steam pipe 72 is configured as a pipe that is connected to the steam generation unit 41 of the heat recovery boiler 4 at one end, branching off from the steam supply pipe 75, and connected to the fuel nozzle 25 of the gas turbine 2 at the other end. A portion of the steam generated in the heat recovery boiler 4 is supplied to the fuel nozzle 25 as nozzle steam, which is mixed with the fuel gas G1, through the steam supply pipe 75. By mixing steam as nozzle steam with the fuel gas G1 injected from the fuel nozzle 25 into the combustor 21 of the gas turbine 2, the temperature of the combustion flame when the fuel gas G1 burns can be lowered. The reduction in the combustion flame temperature can reduce the generation of NOx associated with combustion.

[0026] The external boiler 5 is configured as a separate boiler system from the heat recovery boiler 4. In other words, the external boiler 5 does not use exhaust gas from the gas turbine 2 as a heat source like the heat recovery boiler 4, but rather heats water to produce steam using a heat source independent of the gas turbine 2, such as the combustion of city gas G2, and operates independently of the heat recovery boiler 4 and the gas turbine 2.

[0027] Steam generated in the external boiler 5 can be introduced into the nozzle steam piping 72 via the external steam introduction piping 74. In other words, the steam generation section 51 of the external boiler 5 and the nozzle steam piping 72 are connected by the external steam introduction piping 74, and the inside of the nozzle steam piping 72 and the inside of the external steam introduction piping 74 are mutually connected. The steam from the external boiler 5 introduced into the nozzle steam piping 72 via the external steam introduction piping 74 can be used to preheat the nozzle steam piping 72 in the preheating process, which will be described later. In addition, the steam from the external boiler 5 introduced into the nozzle steam piping 72 via the external steam introduction piping 74 can also be supplied to the fuel nozzle 25 of the gas turbine 2 and used as nozzle steam in the external steam injection process, which will be described later. Furthermore, the steam generated in the external boiler 5 can also be supplied to the steam demand section D2 via the external steam introduction piping 74 and the steam supply piping 75. In other words, by switching a valve (not shown), it is possible to switch between supplying nozzle steam to the fuel nozzle 25 via nozzle steam piping 72 and steam to the steam demand section D2 via steam supply piping 75, using either steam from the waste heat recovery boiler 4 or steam from the external boiler 5.

[0028] Although the configuration is arbitrary, in this cogeneration system 1, the steam generated in the heat recovery boiler 4 can be used not only as nozzle steam, but also as case steam injected into the combustor 21 in the form of superheated steam. In other words, the nozzle steam piping 72 can be branched midway to provide case steam piping 73, and the steam supplied from the heat recovery boiler 4 via this case steam piping 73 can be mixed with compressed air generated by the compressor 22 of the gas turbine 2 in the mixer 26, and the system can be configured to inject it as superheated steam. Then, the gas turbine 2 can be operated while injecting this superheated steam into the combustor 21. By injecting superheated steam as case steam at a position corresponding to the outside of the flame inside the combustor 21, it is possible to increase the mass flow rate of gas introduced into the gas turbine 2 while suppressing excessive temperature rise at the inlet of the turbine 23, thereby increasing the output of the gas turbine 2. Steam from the external boiler 5, introduced into the nozzle steam piping 72 via the external steam introduction piping 74, can also be introduced into the case steam piping 73.

[0029] Furthermore, although optional, this cogeneration system 1 can have a compressor unit 6 independently of the gas turbine 2 and the heat recovery boiler 4. The compressor unit 6 is a device that compresses air, driven by both electricity and steam. In the compressor unit 6, the built-in air compressor 62 generates compressed air from externally taken-in air G3, using the rotation of an electrically driven motor 63 as well as the rotation of a steam turbine 61 powered by steam as its driving source. A compressor pipe 77 is provided, branching off from the middle of the steam supply pipe 75 that supplies steam from the steam generation section 41 of the heat recovery boiler 4 to the steam demand section D2. A portion of the steam supplied to the steam supply pipe 75 is introduced into the steam turbine 61 of the compressor unit 6 via the compressor pipe 77. The steam introduced into the steam turbine 61 is depressurized to assist in driving the air compressor 62. This depressurized steam component then passes through the depressurized steam piping 78, merges with the steam that was not used to assist in driving the compressor unit 6, and is then supplied to the steam demand unit D2. The compressed air generated by the compressor unit 6 is supplied to the external compressed air demand unit D3. Steam from the waste heat recovery boiler 4 and steam from the external boiler 5 can also be switched and supplied to the compressor piping 77 via the steam supply piping 75.

[0030] [Operation of the cogeneration system] Next, the operation of the cogeneration system 1 described above will be explained. In this cogeneration system 1, there are periods in which the gas turbine 2 and the heat recovery boiler 4 are operated to generate electricity with the generator 3 and produce steam with the heat recovery boiler 4, and periods in which the gas turbine 2 and the heat recovery boiler 4 are shut down. For example, the gas turbine 2 and the heat recovery boiler 4 are operated during the day and shut down at night.

[0031] Figure 2 schematically shows the state of the main components of the cogeneration system 1 over time. For each device, the operating state is shown along the vertical axis, between the stopped state and the steady-state operation state. The higher up the vertical axis, the more the device is starting up and approaching the steady-state operation state. Figure 2 mainly shows the start-up phase, which is the transition from the turbine stopped state, where the gas turbine 2 and the heat recovery boiler 4 are stopped, to the operating state, where the gas turbine 2 and the heat recovery boiler 4 are operating in a steady-state operation state.

[0032] In order to transition from a nighttime turbine shutdown state to a daytime turbine operation state, during the morning startup, prior to starting up the gas turbine 2 and the heat recovery boiler 4, the external boiler 5 is operated to generate steam. In this initial state, no steam, including steam from the heat recovery boiler 4, is introduced into the nozzle steam piping 72.

[0033] At time T1, the preheating process is performed by introducing steam from the external boiler 5 into the nozzle steam piping 72. By passing the steam introduced from the external boiler 5 through the nozzle steam piping 72, the walls of the nozzle steam piping 72 are heated by the steam, and preheating occurs from a low temperature state. During preheating, the steam introduced from the external boiler 5 into the nozzle steam piping 72 and passed through is discharged after passing through the nozzle steam piping 72. If the cogeneration system 1 has case steam piping 73, steam from the external boiler 5 is also introduced into the case steam piping 73, and the case steam piping 73 is also preheated. Note that as the supply of steam to the nozzle steam piping 72 begins at time T1, the steam flow rate of the external boiler increases.

[0034] During the preheating process, the nozzle steam piping 72 is sufficiently preheated by steam from the external boiler 5, and when the inside of the nozzle steam piping 72 reaches a predetermined temperature, the preheating process is completed at time T2. In other words, the introduction and ventilation of steam from the external boiler 5 to the nozzle steam piping 72, which had been continuing since time T1, is stopped. Note that during the preheating process from time T1 to time T2, the steam used for preheating is not introduced into the gas turbine 2 via the fuel nozzle 25 or mixer 26, but is discharged to the outside of the cogeneration system 1.

[0035] Subsequently, at time T3, the gas turbine 2 is started and operation begins. Immediately after the gas turbine 2 is started, at time T4, when the rotational speed of the gas turbine reaches the level of steady operation, the external steam injection process is started. That is, from time T4 onwards, steam from the external boiler 5 is reintroduced into the nozzle steam piping 72, and this steam from the external boiler 5 is supplied to the fuel nozzle 25 as nozzle steam. It is then mixed with fuel gas G1 and injected into the combustor 21. In this way, the output of the gas turbine 2 is gradually increased while fuel injection is performed using the steam generated in the external boiler 5 as nozzle steam. At time T4, almost simultaneously with, or immediately after, the injection of nozzle steam and fuel gas G1 begins, the generator 3 and the waste heat recovery boiler 4 also begin operation.

[0036] After injecting nozzle steam and fuel gas G1 at time T4, the output of the gas turbine 2 increases, and as a result, the power output of the generator 3 increases. A little later, the temperature and pressure of the steam generated in the heat recovery boiler 4 also increase.

[0037] Subsequently, at time T5, when the operating state of the generator 3 and the heat recovery boiler 4 reaches a steady-state operation, a switching process is executed at time T6. That is, the steam supplied to the fuel nozzle 25 is switched from steam from the external boiler 5 to steam from the heat recovery boiler 4. From time T6 onward, with the switching of the steam source as nozzle steam in the switching process in between, the operation of the gas turbine 2 and the operation of the heat recovery boiler 4 continue in a steady-state operation. As shown in Figure 2 as "Heat Recovery Boiler Steam Flow Rate", the flow rate of steam supplied from the heat recovery boiler 4 to the fuel nozzle 25 and the steam demand unit D2 increases from time T6 to a steady state set to the same flow rate as the external boiler 5. Then, it is sufficient to start supplying steam from the heat recovery boiler 4 to the steam demand unit D2. If the steam from the heat recovery boiler 4 is also used for case steam injection, the steam flow rate is further increased thereafter, as shown in Figure 2.

[0038] As described above, in the cogeneration system according to this embodiment, an external boiler 5 is provided separately from the waste heat recovery boiler 4, and when starting up the gas turbine 2 and the waste heat recovery boiler 4, the steam from the external boiler 5 is used to perform the preheating process and the external steam injection process. This shortens the time required to start up the gas turbine 2 and the waste heat recovery boiler 4 compared to conventional cogeneration systems, and also reduces the NOx generated during startup. In conventional cogeneration systems, an external boiler is not provided, and the preheating process and external steam injection process using steam from an external boiler are not performed. In conventional cogeneration systems, after starting up the gas turbine and the waste heat recovery boiler, the nozzle steam piping is waited to become sufficiently hot due to the steam generated from the waste heat recovery boiler, and then, after the waste heat recovery boiler has started up to a steady-state operating state and can supply nozzle steam at a predetermined temperature and pressure, nozzle steam is supplied to the fuel nozzle via the nozzle steam piping and injected together with the fuel gas. Consequently, during those waiting periods, the fuel gas is injected without mixing it with nozzle steam, and the gas turbine is operated, resulting in the generation of high concentrations of NOx during that time.

[0039] In contrast, in the cogeneration system 1 according to this embodiment described above, a preheating process is performed during startup, when transitioning from a turbine-stopped state to a turbine-operating state, and the nozzle steam piping 72 is preheated using steam from an already operating external boiler 5. This prevents condensation of nozzle steam due to cooling on the walls of the nozzle steam piping 72. Therefore, even if nozzle steam is supplied to the fuel nozzle 25 through the nozzle steam piping 72 immediately after the gas turbine 2 starts up, problems such as difficulty in measuring and controlling the mass flow rate and excessive cooling of the combustion flame due to the mixing of water droplets produced by condensation into the steam are less likely to occur. There is no need to wait for the nozzle steam piping 72 to be heated by the steam generated in the waste heat recovery boiler 4 in order to avoid steam condensation. Furthermore, by implementing an external steam injection process, after the nozzle steam piping 72 is preheated by the preheating process, steam can be introduced from the external boiler 5 into the preheated nozzle steam piping 72 without having to wait for the waste heat recovery boiler 4 to fully start up to a steady-state operating state. This steam can then be used as nozzle steam to lower the combustion flame temperature of the fuel gas G1.

[0040] In this way, by performing the preheating process and the external steam injection process, it is possible to mix nozzle steam with fuel gas G1 and inject it into the combustor 21 immediately after starting up the gas turbine 2, without waiting for the nozzle steam piping 72 to be heated by the steam from the heat recovery boiler 4, and without waiting for the heat recovery boiler 4 to start up until it reaches a steady-state operating condition in which it can generate steam at the predetermined temperature and pressure required for the nozzle steam. This allows for an early increase in the output of the gas turbine 2 and the generator 3, and shortens the start-up time required to reach a steady-state operating condition. It also suppresses fluctuations in start-up time due to ambient temperature, etc. Furthermore, because nozzle steam can be mixed with fuel gas G1 and injected immediately after starting up the gas turbine 2, the reduction of NOx generation by lowering the temperature of the combustion flame can be implemented immediately after starting up the gas turbine 2. The combined effects of shortening the start-up time and suppressing the NOx generation concentration effectively reduce the total amount of NOx generated during the start-up time.

[0041] Furthermore, in the cogeneration system 1, if a case steam piping 73 is provided, preheating the case steam piping 73 during the preheating process can suppress the condensation of steam due to cooling on the pipe walls of the case steam. The case steam needs to be introduced into the gas turbine 2 in the form of superheated steam, and if steam condensation occurs, it could lead to damage to the turbine 23. By preheating the case steam piping 73 with steam from the external boiler 5, the effects of steam condensation can be suppressed, making it possible to inject the case steam under stable conditions and under controlled conditions.

[0042] In this embodiment, the external boiler 5 is already started up at time T1 when the preheating process begins, and its performance and start-up timing are not particularly limited as long as it can generate steam with sufficient temperature and pressure. However, it is preferable that the external boiler 5, when performing the preheating process and the external steam injection process, generates steam at the same temperature and pressure as the waste heat recovery boiler 4 when it reaches a steady-state operating condition after time T5, and supplies that steam into the nozzle steam piping 72. Then, in the preheating process, the nozzle steam piping 72 is heated to the same state as after reaching a steady-state operating condition, and in the external steam injection process, the output of the gas turbine 2 can be increased while supplying nozzle steam under the same conditions as after reaching a steady-state operating condition. As a result, the start-up time is shortened and NOx is reduced by the preheating process and the external steam injection process, and this can be achieved efficiently. In addition, in the switching process, unnecessary fluctuations in conditions can be avoided, and the steam source can be switched smoothly.

[0043] If the external boiler 5 continues to operate from before time T1, when the turbine is stopped, until time T5, when the external steam injection process is completed, it can supply steam to perform the preheating process and the external steam injection process, effectively shortening the start-up time of the gas turbine 2 and the waste heat recovery boiler 4 and reducing NOx emissions. After the waste heat recovery boiler 4 has started up to a steady-state operating state, the waste heat recovery boiler 4 will independently supply nozzle steam and steam to the steam demand section D2, and there will be no need to use the external boiler 5. Therefore, the operation of the external boiler 5 can be stopped thereafter (time T6).

[0044] During the period before time T1 when the turbine is stopped, i.e., at night, the external boiler 5 may be stopped. However, if the cogeneration system 1 is installed in a factory that operates 24 hours a day, and there is a demand for steam at the steam demand section D2 even at night, the external boiler 5 should continue to operate even while the turbine is stopped. In Figure 2, at time T7, which corresponds to evening, the operation of the gas turbine 2, the waste heat recovery boiler 4, and the generator 3 is stopped, and the turbine is in a stopped state. The external boiler 5 is restarted a little before this time T7. Then, throughout the period when the turbine is stopped after time T7, i.e., until the next morning (until time T6 of the next day), the operation of the external boiler 5 continues.

[0045] It is preferable that the external boiler 5 maintains an operating state that generates steam at the same temperature and pressure as the heat recovery boiler 4 when it is in steady operation, even during the period when the turbine is stopped after time T7. Then, while the turbine is operating (daytime), the heat recovery boiler 4 supplies steam to the steam demand unit D2, and while the turbine is stopped (nighttime), the external boiler 5 supplies steam to the steam demand unit D2. This ensures that steam is supplied to the steam demand unit D2 under the same conditions throughout the entire period. Furthermore, since the external boiler 5 is already in a steady operating state at night, when the gas turbine 2 and heat recovery boiler 4 are started up in the morning (after time T1 the next day), the steam from the external boiler 5 already has sufficient temperature and pressure and can be used directly in the preheating process and external steam injection process from time T1. Once the waste heat recovery boiler 4 reaches a state where it can generate steam at a temperature and pressure sufficient to carry out those processes, the supply of steam to the steam demand unit D2 is taken over from the external boiler 5 to the waste heat recovery boiler 4, and the external boiler 5 can be shut down as described above. In addition, in facilities such as factories that operate 24 hours a day, where the supply of steam to the steam demand unit D2 is required even at night, the supply of electricity to the power demand unit D1 is often required throughout the night. However, since the selling price of electricity from power generators is low at night, the demand at the power demand unit D1 can be met by purchasing electricity from power generators rather than generating electricity with the generator 3 of the cogeneration system 1.

[0046] Furthermore, if the cogeneration system 1 is equipped with a compressor device 6 and uses a portion of the steam generated from the heat recovery boiler 4 to assist in driving the compressor device 6, the energy of the surplus steam can be effectively utilized to produce compressed air. For example, if the steam pressure required by the steam demand unit D2 is lower than the steam pressure generated by the heat recovery boiler 4, the steam will be reduced in pressure by the pressure reducing valve 76 before being supplied to the steam demand unit D2. The energy from the pressure reduction can then be effectively utilized to assist in driving the compressor device 6.

[0047] The compressor unit 6 can be assisted in driving not only by steam from the heat recovery boiler 4 but also by steam from the external boiler 5. Therefore, if the external boiler 5 continues to operate even when the turbine is stopped, such as at night, the system should be configured as described above for switching the supply of steam to the steam demand unit D2 at time T7. During the daytime when the turbine is operating, steam generated from the heat recovery boiler 4 assists in driving the compressor unit 6, while at night when the turbine is stopped, steam generated from the external boiler 5 assists in driving the compressor unit 6. In either period, the steam generated from the boiler should be used to assist in driving the compressor unit 6 and should be supplied to the steam demand unit D2 containing components that have been depressurized by the operation of the compressor unit 6. By doing so, in addition to supplying steam to the steam demand unit D2, the supply of compressed air by the compressor unit 6, which uses steam as part of its driving source, can be stably continued even at night when the turbine is stopped.

[0048] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the invention. For example, in the above configuration, by using the external boiler 5 to perform both the preheating process and the external steam injection process, it is possible to significantly shorten the start-up time of the gas turbine 2 and the waste heat recovery boiler 4 and suppress the generation of NOx during start-up. However, even if only one of the preheating process or the external steam injection process is performed, some effect can be obtained in shortening the start-up time of the gas turbine 2 and the waste heat recovery boiler 4 and suppressing the generation of NOx during start-up. Furthermore, when only the preheating process is performed, the heat source for preheating is not limited to the external boiler 5, but any external heat source can be used. [Explanation of Symbols]

[0049] 1. Cogeneration System 2 Gas Turbines 21 Combustor 22 Compressor 23 Turbines 24 Turbine shaft 25 Fuel Nozzle 26 Mixer 3 Generators 4. Waste heat recovery boiler 5. External boiler 6. Compressor unit 71 Exhaust gas path 72 Nozzle steam piping 73 Case Steam Piping 74 External steam introduction piping 75 Steam supply piping D1 Electricity demand department D2 Steam demand section D3 Compressed Air Demand Section G1 Fuel Gas G2 City Gas G3 Air

Claims

1. A gas turbine rotates the turbine shaft by burning fuel gas injected from a fuel nozzle, A generator that generates electricity by the rotation of the turbine shaft and supplies power to an external power demand unit, A waste heat recovery boiler that recovers waste heat from the aforementioned gas turbine to generate steam and supplies the steam to an external steam demand unit, A nozzle steam pipe supplies a portion of the steam generated in the heat recovery boiler to the fuel nozzle of the gas turbine as nozzle steam, which is mixed with the fuel gas to suppress the generation of nitrogen oxides. It comprises an external boiler configured as a separate boiler system from the aforementioned heat recovery boiler, and capable of introducing steam into the nozzle steam piping, During the startup phase, when transitioning from a turbine-stopped state, where the gas turbine and the heat recovery boiler are stopped, to a turbine-operated state, where the gas turbine and the heat recovery boiler are operated in a steady-state operation, Prior to starting the gas turbine, a preheating step is performed in which steam is introduced from the external boiler into the nozzle steam piping while steam from the heat recovery boiler is not yet introduced into the nozzle steam piping, After the steam used in the preheating process is discharged to the outside through the nozzle steam piping, and immediately after starting the gas turbine, an external steam injection process is performed in which steam from the external boiler is supplied to the fuel nozzle as nozzle steam via the nozzle steam piping, and the output of the gas turbine is increased while mixing it with the fuel gas and injecting it. When the gas turbine and the heat recovery boiler reach a steady-state operation, a switching process is performed to switch the nozzle steam from steam from the external boiler to steam from the heat recovery boiler. Perform these steps in this order: A cogeneration system in which the external boiler continues to operate, including the period from the turbine shutdown state to the external steam injection process, and is shut down after the operation of the gas turbine and the waste heat recovery boiler reaches a steady-state operating state.

2. The cogeneration system according to claim 1, wherein the external boiler generates steam at the same temperature and pressure as the waste heat recovery boiler in steady-state operation, and supplies the steam to the inside of the nozzle steam piping in the preheating step and the external steam injection step.

3. The external boiler continues to generate steam at the same temperature and pressure as the heat recovery boiler during steady-state operation, even while the turbine is stopped. While the turbine is operating, the heat recovery boiler supplies steam to the steam demand section. The cogeneration system according to claim 1, wherein the external boiler supplies steam to the steam demand section while the turbine is stopped.

4. The aforementioned cogeneration system further comprises a compressor device that compresses air using electricity and steam as power sources. The compressor device is driven by steam generated from the waste heat recovery boiler, along with electricity, and supplies the generated compressed air to an external compressed air demand unit. The cogeneration system according to claim 1 or 2, wherein the steam generated from the waste heat recovery boiler assists in driving the compressor device and is supplied to the steam demand unit, containing components that have been depressurized by the operation of the compressor device.

5. The external boiler continues to operate even during the period when the turbine is shut down. While the turbine is operating, the steam generated from the heat recovery boiler assists in driving the compressor and is supplied to the steam demand section, containing components that have been depressurized by the operation of the compressor. The cogeneration system according to claim 4, wherein, while the turbine is stopped, steam generated from the external boiler assists in driving the compressor device and is supplied to the steam demand section containing components that have been depressurized by the operation of the compressor device.

6. A gas turbine rotates the turbine shaft by burning fuel gas injected from a fuel nozzle, A generator that generates electricity by the rotation of the turbine shaft and supplies power to an external power demand unit, A waste heat recovery boiler that recovers waste heat from the aforementioned gas turbine to generate steam and supplies the steam to an external steam demand unit, A nozzle steam pipe supplies a portion of the steam generated in the heat recovery boiler to the fuel nozzle of the gas turbine as nozzle steam, which is mixed with the fuel gas to suppress the generation of nitrogen oxides. An external boiler, configured as a boiler system different from the aforementioned heat recovery boiler, capable of introducing steam into the nozzle steam piping, It has a compressor device that compresses air using electricity and steam as driving sources, During the startup phase, when transitioning from a turbine-stopped state, where the gas turbine and the heat recovery boiler are stopped, to a turbine-operated state, where the gas turbine and the heat recovery boiler are operated in a steady-state operation, Prior to starting the gas turbine, a preheating step is performed in which steam is introduced from the external boiler into the nozzle steam piping while steam from the heat recovery boiler is not yet introduced into the nozzle steam piping, After the steam used in the preheating process is discharged to the outside through the nozzle steam piping, and immediately after starting the gas turbine, an external steam injection process is performed in which steam from the external boiler is supplied to the fuel nozzle as nozzle steam via the nozzle steam piping, and the output of the gas turbine is increased while mixing it with the fuel gas and injecting it. When the gas turbine and the heat recovery boiler reach a steady-state operation, a switching process is performed to switch the nozzle steam from steam from the external boiler to steam from the heat recovery boiler. Perform these steps in this order: The compressor device is driven by steam generated from the waste heat recovery boiler, along with electricity, and supplies the generated compressed air to an external compressed air demand unit. The external boiler continues to operate even during the period when the turbine is shut down. While the turbine is operating, the steam generated from the heat recovery boiler assists in driving the compressor and is supplied to the steam demand section, containing components that have been depressurized by the operation of the compressor. A cogeneration system in which, while the turbine is stopped, steam generated from the external boiler assists in driving the compressor device and is supplied to the steam demand section, containing components that have been depressurized by the operation of the compressor device.

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