Combined cycle plant

CA 02437442 2003-08-08

TITLE OF THE INVENTION

Combined cycle plant

BACKGROUND OF THE INVENTION

The present invention relates to a combined cycle plant that combines a conventional thermal power plant and a gas turbine plant.

In a well-known combined cycle plant combining a 10 conventional thermal power plant comprising a boiler, steam turbine, condenser, etc. and a gas turbine plant comprising a gas turbine and heat recovery steam generator, the steam generated by the boiler of the conventional thermal power plant and the steam generated by the heat recovery steam generator of the gas turbine plant are put together to drive the steam turbine .

Prior art relating to this type of power plant is disclosed, for example, in the Japanese laid-open

Patent Publication No. 2000-220412.

SUMMARY OF THE INVENTION

A power plant is equipped with a system for dumping steam'generated by a boiler into a condenser

CA 02437442 2007-01-11 during start-up, shut-down and load rejection. The prior art mentioned above relates to a power plant that combines a conventional thermal power plant and a gas turbine plant and so is equipped with two steam generators, i.e. a boiler and a heat recovery steam generator, therefore two dump systems are required for dumping the generated steam into the condenser. Consequently, the system layout is complicated and multiple inlet ports are required on the condenser.

The present invention provides for constructing a dump system for dumping generated steam into a condenser without increasing the number of inlet ports on the condenser of an existing plant.

A combined cycle plant according to the present invention is equipped with a dump system that connects a boiler pipe with the condenser and dumps the steam generated by a boiler into the condenser, bypassing a steam turbine, and a pipe that connects a heat recovery steam generator pipe with the dump system into the condenser.

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- 2a Certain exemplary embodiments may provide a combined cycle plant comprising: a conventional thermal power plant having a boiler, a boiler pipe including a main steam pipe, a cold reheat pipe and a hot reheat pipe, a steam turbine, a condenser, and a condensate water/feedwater system; a gas turbine plant having a gas turbine, a heat recovery steam generator recovering heat from the gas turbine exhaust, and a heat recovery steam generator pipe supplying the generated steam from the heat recovery steam generator to the steam turbine, a dump system connecting the boiler pipe with the condenser and dumps the steam generated by the boiler into the condenser, bypassing the steam turbine; and a pipe connecting the heat recovery steam generator pipe with the dump system into the condenser, wherein the dump system into the condenser is a turbine bypass system equipped with a flash tank, and the point of connection between the heat recovery steam generator and the dump system into the condenser is located on the flash tank of the turbine bypass system or on a pipe near the flash tank.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic diagram of the combined cycle plant according to the embodiment 1 of the

CA 02437442 2003-08-08 invention;

Fig. 2 is a schematic diagram of the combined cycle plant according to the embodiment 2 of the invention;

Fig. 3 is a schematic diagram of the combined cycle plant according to the embodiment 3 of the invention;

Fig. 4 is a schematic diagram of the combined cycle plant according to the embodiment 4 of the invention;

Fig. 5 is a schematic diagram of the combined cycle plant according to the embodiment 5 of the invention; and

Fig. 6 is a schematic diagram of the combined cycle plant according to the embodiment 6 of the invention .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described hereunder with reference to Fig. 1 to Fig. 6.

<Embodiment 1>

A preferred embodiment of a combined cycle plant according to the present invention is described

CA 02437442 2003-08-08 hereunder with reference to Fig. 1.

The combined cycle plant is comprised of two power plants: a conventional thermal power plant comprising a boiler 10, a boiler pipe including a main steam pipe 60, cold reheat pipe 61 and hot reheat pipe 62, high-pressure (HP), intermediate-pressure (IP) and low-pressure (LP) turbines 21,22 and 23, respectively, a condenser 25, and,a condensate water/feedwater system; and a gas turbine plant comprising a gas turbine 40, a heat recovery steam generator 50 that recovers heat from the exhaust of the gas turbine 40, and a heat recovery steam generator pipe including a HP pipe 70 and a LP pipe 71.

In Fig. 1, feedwater supplied to the boiler 10 is heated by an economizer 12 and steam is generated by an evaporator 13. The generated steam, while being superheated through a primary superheater 14 and a secondary superheater 15, is directed as superheated steam into the HP turbine 21 through the main steam pipe 60 connecting the evaporator 13 with the HP turbine 21.

Steam generated by the heat recovery steam generator 50 is merged into the main steam pipe 60

CA 02437442 2003-08-08 through the HP pipe 70 connecting a HP drum 51 with the main steam pipe 60 and is also directed as superheated steam into the HP turbine 21.

After having driven the HP turbine 21, the steam 5 flows through the cold reheat pipe 61 and is directed to a reheater 11 of the boiler 10. The steam superheated by the reheater 11 is supplied to the IP turbine 22 through the hot reheat pipe 62. Steam generated by the heat recovery steam generator 50 is merged into the hot reheat pipe 62 through the LP pipe 71 connecting the LP drum 52 with the hot reheat pipe 62 and is also directed as superheated steam into the IP turbine 22.

After having driven the IP turbine 22, the steam is directed to the LP turbine 23 through a crossover pipe 63. The steam, after having driven the LP turbine 23, is then directed to the condenser 25 where it is turned into condensate water.

The condensate water condensed by the condenser

25 is then pressurized by a condensate pump 30, heated by a gland steam condenser 31, and split into condensate for the conventional thermal power plant and feedwater for the heat recovery steam generator 50 of the gas turbine plant.

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A condensate pipe 65 connecting the condenser 25 with a deaerator 34 is installed in the condensate water system of the conventional thermal power plant.

The condensate water in the conventional thermal 5 power plant is heated by the LP heater 32, deaerated by the deaerator 34, and then turned into feedwater.

A feedwater pipe 66 connecting the deaerator 34 with the boiler 10 is installed in the feedwater system of the conventional thermal power plant. The feedwater in the conventional thermal power plant is pressurized by the feedwater pump 36, heated by the HP heater 37, and then returned to the boiler 10.

The feedwater in the gas turbine plant is led through a feedwater pipe 72 into the heat recovery steam generator 50, which recovers heat from the exhaust of the gas turbine 40, and is turned into steam by heat exchange with the gas turbine exhaust. The steam is merged into the main steam pipe 60 through the HP pipe 70 connecting the HP drum 51 with the main steam pipe 60, and serves as superheated steam to drive the HP turbine 21.

In addition, a portion of the feedwater, having been separated inside the heat recovery steam generator 50 and directed to the LP drum 52, is

CA 02437442 2003-08-08 turned to steam by heat exchange with the gas turbine exhaust. The steam is merged into the hot reheat pipe 62 through the LP pipe 71 connecting the LP drum 52 with the hot reheat pipe 62, and serves as superheated steam to drive the IP turbine 22.

In normal operation, a power generation cycle is repeated as described above and electric energy is generated by the steam turbine generator 24, which is driven by the directly-connected HP, IP and LP steam turbines 21, 22 and 23, and also by the gas turbine generator 44, which is driven by the directlyconnected gas turbine.

The conventional thermal power plant is equipped with a system for dumping the generated steam into the condenser in case of start-up, shut-down and load rejection where the generated steam cannot be directed into the steam turbine. In an embodiment according to the present invention, there is provided a turbine bypass system 1 that is branched from the main steam pipe 60 and connected to the condenser 25 through the turbine bypass valve 8 and corresponding pipe. With this turbine bypass system 1, the steam generated by the boiler 10 can be dumped into the condenser 25.

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There is provided a turbine bypass system for the steam generated by the heat recovery steam generator 50: a HRSG HP turbine bypass system 2 that is branched from the HP pipe 70 and connected to the turbine bypass system 1 and a HRSG LP turbine bypass system 3 that is branched from the LP pipe 71 and connected to the turbine bypass system 1.

In an embodiment according to the present invention, the HRSG high pressure turbine bypass system 2 and HRSG low pressure turbine bypass system 3 which are the second and third turbine bypass systems are connected to the first turbine bypass system 1. Furthermore, the turbine bypass system 1 from the connecting point of the HRSG high pressure turbine bypass system 2 and of the HRSG low pressure turbine bypass system 3 to the steam condenser 25 is shared as a bypass system for steam generated by the combustion boiler 10 and by the exhaust heat recovery boiler 50.

In construction of the parallel power generation system comprising the steam power generation plant, the gas turbine, and the exhaust heat recovery boiler, sharing of a part of turbine bypass system 1 with the turbine bypass system in the exhaust heat

CA 02437442 2003-08-08 recovery boiler 50 such as described above, allows reduced size and complexity of the steam condenser 25 and also reduced cost of the steam condenser and of the turbine bypass system.

Also, in the case where a gas turbine plant is added to an existing steam power generation plant, large reconstruction of a steam condenser may be required and this may not be possible to do depending on the particular circumstances. However, it may be possible to solve these problems by utilizing a turbine bypass system as described above.

To start-up this power plant, the conventional thermal power plant is started first, and then the gas turbine plant is started. Accordingly, the boiler

10 is started first and the steam generated by the boiler 10 is discharged into the condenser 25 through the turbine bypass system 1 in the beginning, and then, when the pressure and temperature of the steam generated by the boiler 10 become high enough to be supplied to the steam turbine, the steam is fed into the steam turbine.

When the turbine bypass system 1 is not carrying steam or the amount of steam from the boiler 10 into the turbine bypass system 1 becomes less than a

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- 10 specified amount (threshold), for example after the steam generated by the boiler 10 is all directed into the steam turbine, the gas turbine plant is started. The steam generated by the heat recovery steam generator 50 is directed, through the HRSG HP turbine bypass system 2 and HRSG LP turbine bypass system 3, into the turbine bypass system 1 and then discharged into the condenser 25.

When the pressure and temperature of the high10 pressure steam from the heat recovery steam generator become high enough to be mixed into the main steam, and when the pressure and temperature of the low-pressure steam from the heat recovery steam generator 50 become high enough to be mixed into the hot reheat steam, each steam from the heat recovery steam generator 50 is mixed into the main steam pipe 60 and hot reheat pipe 62, respectively, and the turbine bypass system 1 does not carry steam.

As the turbine bypass system 1 is put into operation for each of the boiler 10 and heat recovery steam generator 50 in turn as described above, the turbine bypass systems 2 and 3 from the heat recovery steam generator 50 can each be connected to the turbine bypass system 1.

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- 11 The above operation is .maintained by a control unit 80 that monitors the opening state of the turbine bypass valve 8 and permits the start-up of the gas turbine 40 only when the above described operating condition is met.

In an event of load rejection or steam turbine tripping, generated steam from either the conventional thermal power plant or from the gas turbine plant can no longer be directed into the steam turbine, therefore the generated steam cannot be handled by the turbine bypass system 1 alone. Each boiler pipe and HRSG pipe is equipped with a release valve 64 for releasing the steam generated in the conventional thermal power plant and gas turbine plant into the air.

In an embodiment of the combined cycle plant according to the present invention, because it is not necessary to install a system for dumping the generated steam into the condenser in each of the conventional thermal power plant and the gas turbine plant, the generated steam can be dumped into the condenser without increasing the number of inlet ports on the condenser of an existing plant.

As the operating states of the gas turbine plant

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- 12 and the conventional thermal power plant are monitored and each plant is started in turn accordingly, the steam generated in the gas turbine plant and in the conventional thermal power plant are not dumped into the condenser at the same time. Thus, the capacity of the turbine bypass system 1 can be decreased.

A typical exemplary embodiment of the present invention is described above. However, there are different constructions available for the system for dumping the generated steam into the condenser, alternative embodiments are described hereunder.

<Embodiment 2>

Fig. 2 shows a schematic diagram of a further 15 preferred embodiment of a combined cycle plant according to the present invention. The devices and components which are the same as in Fig. 1 are denoted the same and no further explanation is given of them. The description hereunder addresses only the differences. In Fig. 2, the control unit 80 is not shown.

The combined plant cycle is equipped with the turbine bypass system 1 as a system for dumping the generated steam into the condenser. The turbine

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- 13 bypass system of this embodiment is branched from the main steam pipe 60 at the outlet of the primary superheater 14, directed through the pipe and a flash tank 4, and then piped and connected to the condenser

25. With this turbine bypass system 1, the steam generated at start-up of the boiler 10 is dumped into the condenser 25.

The combined cycle plant is also equipped with another turbine bypass system for dumping the steam generated by the heat recovery steam generator 50:

the HRSG HP turbine bypass system 2 that is branched from the HP pipe 70 and connected to the flash tank 4, and the HRSG LP turbine bypass system 3 that is branched from the LP pipe 71 and connected to the flash tank 4. The point of connection of each turbine bypass system from the heat recovery steam generator 50 can be located directly on the flash tank 4 or alternatively on the pipe near the flash tank 4.

<Embodiment 3>

Fig. 3 shows a schematic representation of a further preferred embodiment of a combined cycle plant according to the present invention. The devices and components which are the same as in Fig. 1 and Fig. 2 are denoted the same and no further

CA 02437442 2003-08-08

- 14 explanation is given of them. The description hereunder addresses only the differences. In Fig. 3, the control unit 80 is not shown.

As the turbine bypass system for the boiler 10, the combined cycle plant is equipped with the HP turbine bypass system la that is branched from the main steam pipe 60 and connected to the cold reheat pipe 61 and the LP turbine bypass system lb that is branched from the hot reheat pipe 62 and connected to the condenser 25. A main steam pipe drain system 5 connecting the main steam pipe 60 with the condenser 25 is also installed. The main steam pipe drain system 5 and LP turbine bypass system lb serve as a system for dumping the generated steam into the condenser.

The HRSG HP turbine bypass system 2 is connected to the main steam pipe drain system 5 and the HRSG LP turbine bypass system 3 is connected to the LP turbine bypass system lb so as to serve as the turbine bypass system for the steam generated by the heat recovery steam generator.

<Embodiment 4>

Fig. 4 shows a schematic representation of a further preferred embodiment of a combined cycle

CA 02437442 2003-08-08

- 15 plant according to the present invention. The devices and components which are the same as in Figs.

to 3 are denoted the same and no further explanation is given of them. The description hereunder addresses only the differences. In Figure 4, the control unit 80 is not shown.

The combined cycle plant of this embodiment is equipped with the secondary superheater bypass system 6 connecting the main steam pipe 60 with the condenser 25. The secondary superheater bypass system 6 and LP turbine bypass system lb are installed to serve as the system for dumping the generated steam into the condenser.

The HRSG HP turbine bypass system 2, branched from the HP pipe 70 and connected to the secondary superheater bypass system 6, and HRSG LP turbine bypass system 3, branched from the LP pipe 71 and connected to the LP turbine bypass system lb, are installed to serve as. the turbine bypass system for the steam,generated by the heat recovery steam generator 50.

<Embodiment 5>

Fig. 5 shows a schematic representation of a further preferred embodiment of a combined cycle

CA 02437442 2003-08-08

- 16 plant according to the present invention. The devices and components which are the same as in Figs. 1 to 4 are denoted the same and no further explanation is given of them. The description hereunder addresses only the differences. In Figure 5, the control unit 80 is not shown.

The combined cycle plant is equipped with a superheater inlet dump system 7 connecting the main steam pipe 60 with the condenser 25. The superheater inlet dump system 7 and turbine bypass system 1 serve as the system for dumping the generated steam into the condenser.

The HRSG HP turbine bypass system 2, branched from the HP pipe 70 and connected to the turbine bypass system 1, and HRSG LP turbine bypass system 3, branched from the LP pipe 71 and connected to the superheater inlet dump system 7, are installed to serve as the turbine bypass system for the steam generated by the heat recovery steam generator 50.

Embodiment 6>

Fig. 6 shows a schematic representation of a further preferred embodiment of a combined cycle plant according to the present invention. The devices and components which are the same as in Figs. 1 to 5

CA 02437442 2003-08-08

- 17 are denoted the same and no further explanation is given of them. The description hereunder addresses only the differences. In Figure 6, the control unit 80 is not shown.

In the combined cycle plant, an attemperator 9 is installed on the turbine bypass system 1, and both HRSG HP turbine bypass system 2 and HRSG LP turbine bypass system 3 are connected to the turbine bypass system 1. The point of connection is located on a pipe connecting the turbine bypass valve 8 to the attemperator 9.

A combined cycle plant can come into existence in several different ways such as, for example, where a new combined cycle plant is constructed from the scratch, where a gas turbine plant is added to an existing conventional thermal power plant or where a new boiler and a new gas turbine plant are installed but the existing steam turbine is still used.

When a new power plant according to the present invention is to be constructed, installing respective dump systems for dumping the steam generated in the conventional thermal power plant and gas turbine plant into the condenser is not necessary. Even in the case where a gas turbine plant is to be added to

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- 18 an existing conventional thermal power plant, no additional inlet port needs to be installed on the condenser to receive a dump system for the steam generated by the heat recovery steam generator, and so large-scale remodeling of the condenser is not needed.

The meaning of reference numbers in the figures are as follows:

1···Turbine bypass system, 2---High pressure turbine-bypass system of HRSG, 3···Low pressure turbine bypass system of HRSG, 4··Flash tank,

5··-Drain system of main steam pipe, 6---Bypass system of secondary superheater, 7··-Dump system at superheater inlet, 8---Turbine bypass valve,

9··Attemperator, 10---Boiler, 11·Reheater,

12···Economizer, 13···Evaporator, 14··-Primary superheater, 15···Secondary superheater, 21---HP (high-pressure) turbine, 22··-IP (.intermediatepressure) turbine, 23---LP (low-pressure) turbine, 24---Steam turbine generator, 25···Condenser,

30···Condensate pump, 31··-Gland steam condenser, 32---LP (low-pressure) heater, 34···Deaerator,

36···Feedwater pump, 37---HP (high-pressure), heater, 40---Gas turbine, 41···Compressor, 42···Turbine,

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- 19 43···Combustor, 44··-Gas turbine generator, 50··’Heat recovery steam generator, 51··-HP drum, 52·--LP drum, 60··-Main steam pipe, 61··-Cold reheat pipe, 62··’Hot reheat pipe, 63··Crossover pipe, 64··-Release valve,

70··-HP pipe, 71---LP pipe, 72···Feedwater pipe,

80···Control unit.

According to the present invention, installing respective dump systems for dumping the steam generated in the conventional thermal power plant and the gas turbine plant into the condenser is not necessary, therefore a system for dumping the generated steam into the condenser can be constructed without increasing the number of inlet ports on the condenser of an existing plant.