COMBINED CYCLE POWER GENERATING PLANT AND METHOD OF SUPPLYING COOLING STEAM FOR GAS TURBINE IN SAME

Abstract: In combined cycle power plant comprising combination of gas turbine plant and steam turbine plant, gas turbine cooling steam temperature is adjusted appropriately so that gas turbine can be made of easily obtainable material. Steam supplied from intermediate pressure evaporator is mixed into cooling steam supply passage for supplying therethrough exhaust steam from high pressure steam turbine into gas turbine high temperature portion as cooling steam, thereby cooling steam temperature is lowered without lowering of entire efficiency and material of less heat resistant ability becomes usable as material forming the gas turbine high temperature portion to be cooled so that design and manufacture of the gas turbine may be satisfied by less expensive and easily obtainable material.

Description: BACKGROUND OF THE INVENTION: Field of the Invention: The present invention relates to a combined cycle power plant in which a gas turbine plant and a steam turbine plant are combined and a cooling steam supply method for a gas turbine in operation thereof. Description of the Prior Art: A combined cycle power plant is a power system in which a gas turbine plant and a steam turbine plant are combined, wherein a gas turbine takes charge of a high temperature section of thermal energy and a steam turbine takes charge of a low temperature section thereof, so that the thermal energy is recovered for an effective use thereof and this power system is now being paid a high attention to. In the combined cycle power plant, research and development are being carried out aiming at a higher temperature gas turbine as one means for enhancing the efficiency. On the other hand, in order to attain such a higher temperature, improvement of cooling system which takes account of heat resisting ability of turbine structural elements must be pursued and as the result of various tries and errors, a steam cooled system which uses steam as cooling medium, instead of a prior art using compressed air, is in progress now. One example thereof is the Japanese laid-open patent application No. Hei 05(1993)-163960, wherein a cooling steam is obtained from intermediate pressure steam of a waste heat recovery boiler but there is obtainable no sufficient volume of the steam resulting in difficulty to effect a stable and secure cooling. Thus, as a further progress thereafter, putting eyes on a sufficient volume and a stable cooling, a cooling system using exhaust steam from a high pressure steam turbine as the cooling steam is now being developed. In the prior art steam cooled system as mentioned above, progress has been done from the system using intermediate pressure steam as the cooling medium to that using high pressure exhaust steam with result that the practicality has been enhanced further, but the high pressure exhaust steam is of a high temperature at same time, hence a gas turbine high temperature portion which is a portion to be cooled must be made of a selected material which can resist such a high temperature. A high temperature resistible material, being required to be of properties necessary for that material, becomes very expensive and in case of a turbine disc among others, for example, there is a large difficulty in obtaining an appropriate material therefore within limited conditions of price and the like and this leads to serious problem in the design and manufacture of the plant. EP-A-0 743 425 discloses a combined cycle power plant with the features of the preamble portion of claim 1. For lowering the temperature of the exhaust steam supplied from the high pressure turbine through the cooling steam supply passage as cooling steam for the high temperature portion of the gas turbine a passage is provided for mixing steam supplied from the intermediate pressure evaporator into the cooling steam supply passage. SUMMARY OF THE INVENTION: In view of the problem in the prior art, it is an object of the present invention to provide a combined cycle power plant and a method of operating a combined cycle power plant wherein cooling steam used as cooling medium is appropriately adjusted of its temperature with a functional stability of steam cooled system being by no means damaged, so that the gas turbine can be made of an easily obtainable material. In order to attain this object, the present invention provides a combined cycle power plant comprising the features of claim 1 and a method of operating such power plant comprising the steps of claim 5. Preferred embodiments of the power plant and of the method are defined in the dependent claims. That is, in the invention, the exhaust steam from the high pressure turbine is first selected as the cooling medium of the gas turbine high temperature portion. Then, said exhaust steam is not directly used as the cooling steam as it is to be supplied into the gas turbine high temperature portion but is heat-exchanged with the gas turbine fuel to be lowered of temperature and then supplied into the gas turbine high temperature portion for cooling thereof. There is available as the cooling steam a sufficient amount of the exhaust steam of the high pressure turbine, but this exhaust steam is of a high temperature at same time, hence it is heat-exchanged with the gas turbine fuel to be lowered of temperature, thereby the gas turbine high temperature portion to be cooled can be made of a material which is of a less heat resistant ability and yet a stable cooling can be effected without the entire efficiency being lowered. In a preferred embodiment reheater is provided as a means for reheating steam which has cooled a high temperature portion to be cooled of said gas turbine. That is, the construction is such that the steam which has cooled the gas turbine high temperature portion can be led into the reheater so as to be reheated there, thereby even if the cooling steam after used for cooling of the gas turbine is not high enough for a predetermined temperature for reason that the gas turbine is not of a high temperature enough or the cooling has not been effected sufficiently or the like, then the cooling steam is reheated by the reheater and a necessary heat for the downstream steam turbines such as the intermediate steam turbine can be secured resulting in enhancement of the thermal efficiency. According to the method of operating such power plant the inlet pressure of the high pressure turbine is adjusted to 16.1809 to 17.1616 MPa (165 to 175 ata), thereby the high pressure exhaust steam property is lowered to 330 to 250°C and 3.4323 to 2.9419 MPa (35 to 30 ata) at the outlet of the high pressure turbine, thus the cooling steam can be lowered of the temperature without the efficiency of the downstream equipments, the intermediate pressure turbine for example, being lowered and the downstream gas turbine high temperature portion to be cooled can be made of a material of less heat resistant ability and less expensive and yet a stable cooling can be effected without the entire efficiency being lowered. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a diagrammatic view showing a combined cycle power plant of a first embodiment example serving to explain aspects of the present invention. Fig. 2 is a diagrammatic view showing a combined cycle power plant of an embodiment according to the present invention. Fig. 3 is a diagrammatic view showing a combined cycle power plant of a further example serving to explain aspects of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS: A first example serving to explain certain aspects of the present invention will be described with reference to Fig. 1. All the embodiments according to the present invention are made on the basis of a combined cycle power plant consisting basically of three portions of a gas turbine, a steam turbine and a waste heat recovery boiler and the whole aspect of this combined cycle power plant will be described first. In Fig. 1, numeral 10 designates a gas turbine plant, which comprises as main equipments a gas turbine 11, an air compressor 12 driven by the gas turbine 11, and a combustor 13 for burning a fuel together with a compressed air supplied from the air compressor 12. There are provided within the gas turbine 11 moving blades, stationary blades and the like (not shown), which correspond to a high temperature portion 11a which requires a cooling thereof. The combustor 13 comprises therein a tail tube cooled portion 13a which extends from a combustion chamber rear portion to a combustion gas outlet, and this tail tube cooled portion 13a is also a portion requiring a cooling thereof, like said high temperature portion 11a. Also provided are a cooling steam supply passage 14 for supplying therethrough a cooling steam to the high temperature portion 11a of the gas turbine 11 and a cooling steam bifurcation passage 15, bifurcated from the cooling steam supply passage 14, for supplying therethrough the cooling steam to the tail tube cooled portion 13a of the combustor 13. Numeral 20 designates a waste heat recovery boiler, which uses an exhaust gas of the gas turbine 11 as a heat source and comprises therein portions of a low pressure steam generating portion, an intermediate pressure steam generating portion and a high pressure steam generating portion. If equipments included in said respective steam generating portions are shown approximately in the order of steam flow therein, then the low pressure steam generating portion is composed of a low pressure economizer 21, a low pressure evaporator 22, a low pressure superheater 23, etc. and the intermediate pressure steam generating portion is composed of an intermediate pressure economizer 24, an intermediate pressure feed water pump 25, an intermediate pressure evaporator 26, an intermediate pressure first reheater 27, an intermediate pressure second reheater 28. etc. Also, the high pressure steam generating portion is composed of a high pressure first economizer 29, a high pressure feed water pump 30, a high pressure second economizer 31, a high pressure evaporator 32, a high pressure first superheater 33, a high pressure second superheater 34, etc. Numeral 40 designates a steam turbine plant, which comprises a high pressure turbine 41 supplied with a high pressure steam from the high pressure second superheater 34 of the waste heat recovery boiler 20, an intermediate pressure turbine 42 supplied with an intermediate pressure steam from the intermediate pressure second reheater 28 and a low pressure turbine 43 supplied with an exhaust steam from the intermediate pressure turbine 42 and a low pressure steam from the low pressure superheater 23. It is to be noted that the construction is made such that an exhaust steam from the low pressure turbine 43 is condensed into water at a condenser 44 provided downstream thereof and this water is circulated to the waste heat recovery boiler 20 via a boiler feed water pump 45 and a gland steam condenser 46 sequentially. Numeral 50 designates a stack, numeral 51 designates a generator driven by the gas turbine plant 10 and numeral 52 designates a generator driven by the steam turbine plant 40. While various equipments are named and described schematically, the present example relates to a combined cycle power plant consisting of said gas turbine plant 10, said waste heat recovery boiler 20 and said steam turbine plant 40 and includes further the following construction. That is, an intermediate pressure steam supply passage 35 connects at its one end to the intermediate pressure evaporator 26 of the waste heat recovery boiler 20 so as to take out an intermediate pressure steam therefrom and this intermediate pressure steam supply passage 35 connects at its the other end to the cooling steam supply passage 14 at a position downstream of a bifurcation position of the cooling steam supply passage 14 and the cooling steam bifurcation passage 15 so as to make the intermediate pressure steam mixed into the cooling steam supply passage 14. Also, a back-up spray water supply passage 36 connects at its one end to the cooling steam supply passage 14 at a position further downstream of a connection position of the cooling steam supply passage 14 and the intermediate pressure steam supply passage 35 and at its the other end to the intermediate pressure feed water pump 25 upstream thereof. Numeral 37 designates a gas turbine cooling steam recovery passage, which connects to an inlet side of the intermediate pressure second reheater 28 and numeral 38 designates a tail tube cooling steam recovery passage, which connects to an outlet side of the intermediate pressure second reheater 28. In the present example constructed as above, cooling of the high temperature portion 11a of the gas turbine 11 and the tail tube cooled portion 13a of the combustor 13, while in a steady operation, is done as follows. That is, a high pressure exhaust steam from the high pressure turbine 41 is supplied into the cooling steam supply passage 14 to be bifurcated on the way, wherein one portion thereof flows to the combustor 13 via the cooling steam bifurcation passage 15 for cooling the tail tube cooled portion 13a and the other flows to the gas turbine 11 via the cooling steam supply passage 14 for cooling the high temperature portion 11a thereof. At this time, the cooling steam flowing to the gas turbine 11 via the cooling steam supply passage 14 is mixed with the intermediate pressure steam supplied from the intermediate pressure evaporator 26 via the intermediate pressure steam supply passage 35, so that temperature of the cooling steam in the cooling steam supply passage 14 is lowered. If one example of such temperature lowering is to be shown, the temperature of the high pressure exhaust steam coming out of the high pressure turbine 41 is approximately 370°C and if a turbine portion, especially a movable portion such as a moving blade and a disc, is exposed to the steam of that temperature, it will be put in a severe heat resisting condition and an expensive material will be needed for overcoming same, which results in aggravation of the economical conditions. But the intermediate pressure steam coming from the intermediate pressure evaporator 26 is of a temperature of approximately 250°C and if this intermediate pressure steam is mixed into said high pressure exhaust steam of approximately 370°C, then the temperature of this mixed steam lowers to approximately 330°C and the material of said turbine movable portion such as the moving blade and the disc can be satisfied by a less expensive and easily obtainable material, which results in a favorable turn in the economical conditions. The intermediate pressure steam supplied from the intermediate pressure evaporator 26 to be mixed into the high pressure exhaust steam is approximately same in the pressure as the high pressure exhaust steam coming out of the high pressure turbine 41, hence there occurs no lowering of the entire efficiency due to such mixing and a predetermined temperature lowering can be effected. The tail tube cooled portion 13a of the combustor 13, being a fixed structural element, is resistible with a less expensive material even to the temperature condition of same 370°C, hence the intermediate pressure steam may not be mixed into the cooling steam bifurcation passage 15 actively, unless needed specifically. Also, as a back-up measure for the case where the intermediate pressure steam cannot be obtained as expected, a hot water supplied from the intermediate pressure feed water pump 25 via the back-up spray water supply passage 36 is sprayed into the cooling steam supply passage 14 so that the temperature of the cooling steam in the cooling steam supply passage 14 is lowered securely. The cooling steam which has cooled the high temperature portion 11a of the gas turbine 11 is recovered into the intermediate pressure second reheater 28 via the gas turbine cooling steam recovery passage 37, and the cooling steam which has cooled the tail tube cooled portion 13a of the combustor 13 comes to the outlet side of the intermediate pressure second reheater 28 via the tail tube cooling steam recovery passage 38 to join with another steam which has been heated through the intermediate pressure second reheater 28 to be further supplied to the intermediate pressure turbine 42 for recovery. According to the present example constructed as mentioned above, the cooling steam for cooling the high temperature portion 11a of the gas turbine 11 is obtained by the high pressure exhaust steam from the high pressure turbine 41 being mixed with the intermediate pressure steam from the intermediate pressure evaporator 26 to be lowered of temperature, thereby the high temperature portion 11a of the gas turbine 11 is required less severely to be heat resistant and a manufacturing cost of the turbine can be saved greatly. Next, an embodiment according to the present invention will be described with reference to Fig. 2. It is to be noted that same components or parts as those in the mentioned first example are given same reference numerals in the figure with repeated description being omitted as much as possible and a featured construction of the present embodiment will be described mainly. In the present embodiment, there is provided a fuel heater 16 in the cooling steam supply passage 14 for supplying therethrough the high pressure exhaust steam from the high pressure turbine 41 to the gas turbine 11, said fuel heater 16 being positioned upstream of a bifurcation position of the cooling steam supply passage 14 and the cooling steam bifurcation passage 15 for supplying therethrough the cooling steam to the combustor 13, so that a heat exchange may be done between a fuel to be supplied into the combustor 13 and the high pressure exhaust steam from the high pressure turbine 41. That is, the high pressure exhaust steam supplied from the high pressure turbine 41 via the cooling steam supply passage 14 is first cooled at the fuel heater 16 by the fuel to be supplied into the combustor 13 to be then bifurcated, wherein one portion thereof flows to the combustor 13 via the cooling steam bifurcation passage 15 for cooling the tail tube cooled portion 13a and the other flows to the gas turbine 11 via the cooling steam supply passage 14 for cooling the high temperature portion 11a thereof. If one example of a temperature lowering of the high pressure exhaust steam cooled at the fuel heater 16 is to be shown, supposing that the temperature of the high pressure exhaust steam coming out of the high pressure turbine 41 is approximately 375°C, then a turbine portion, especially a movable portion such as a moving blade and a disc, is exposed to the steam of that temperature to be put in a severe heat resisting condition and an expensive material will be needed for overcoming same, which results in aggravation of the economical conditions. But the temperature of the fuel of gas turbine coming into the fuel heater 16 is as low as 300°C or less and if the high pressure exhaust steam is cooled by that fuel, then the temperature of the high pressure exhaust steam is lowered to approximately 330°C and the material of said turbine movable portion such as the moving blade and the disc can be satisfied by a less expensive and easily obtainable material, which results in a favorable turn in the economical conditions. According to the present embodiment constructed as mentioned above, the cooling steam for cooling the high temperature portion 11a of the gas turbine 11 and the tail tube cooled portion 13a of the combustor 13 is obtained by the high pressure exhaust steam from the high pressure turbine 41 being cooled by the gas turbine fuel to be lowered of temperature, thereby said high temperature portions 11a, 13a are required less severely to be heat resistant so that employment of an easily obtainable material becomes possible and the manufacturing cost can be saved greatly. In the present embodiment also, like in the first example, the cooling steam which has cooled the high temperature portion 11a of the gas turbine 11 is recovered into the intermediate pressure second reheater 28 via the gas turbine cooling steam recovery passage 37. That is, because said cooling steam can be reheated by the intermediate pressure second reheater 28, even if the cooling steam after used for the cooling of the gas turbine 11 is of a temperature which is less than a predetermined level for reason that the high temperature portion 11a of the gas turbine 11 is not of a sufficiently high temperature or that the cooling thereof has not been done sufficiently, that cooling steam can be reheated by said reheater 28, thereby necessary heat for the downstream steam turbines such as the intermediate pressure turbine can be secured and this results in enhancement of the thermal efficiency. Next, a second example serving to explain certain aspects of the present invention will be described with reference to Fig. 3. It is to be noted that same components or parts as those in the mentioned first example are given same reference numerals in the figure with repeated description being omitted. In the present example, the high pressure exhaust steam from the high pressure turbine 41 is supplied into the cooling steam supply passage 14 to be then bifurcated, wherein one portion thereof flows to the combustor 13 via the cooling steam bifurcation passage 15 for cooling the tail tube cooled portion 13a and the other flows to the gas turbine 11 via the cooling steam supply passage 14 for cooling the high temperature portion 11a thereof. The cooling steam is theretofore made up at the high pres sure turbine 41 as follows, wherein the high pressure steam supplied into the high pressure turbine 41 is set to pressure of 16.1809 to 17.1616 MPa (absolute) (165 to 175 ata) and temperature of approximately 566°C by an appropriate control means (not shown). Then, the steam supplied into the high pressure turbine 41 in said pressure and temperature changes to pressure of 2.9419 to 3.4323 MPa (absolute) (30 to 35 ata) and temperature of 330 to 250°C at an outlet of the high pressure turbine 41. That is, in order to obtain a high pressure steam property of said 2.9419 to 3.4323 MPa (absolute) (30 to 35 ata), 330 to 250°C at the high pressure turbine 41 outlet, taking account of an ordinary turbine expansion, the pressure at an inlet of the high pressure turbine 41 is required to be 16.1809 to 17.1616 MPa (absolute) (165 to 175 ata) when the inlet temperature is selected to approximately 566°C which is an ordinary value. The cooling steam coming out of the high pressure turbine 41 is supplied to the high temperature portion 11a of the gas turbine 11 and the tail tube cooled portion 13a of the combustor 13 via the cooling steam supply passage 14 and the cooling steam bifurcation passage 15 and the temperature of this cooling steam is made up to 330 to 250°C as mentioned above. Because this temperature range is not of a specifically severe requirement as a heat resistant performance of a material composing said high temperature portion 11a and the tail tube cooled portion 13a, hence the material of the turbine movable portion such as the moving blade and the disc can be satisfied by a less expensive and easily obtainable material, which results in a favorable turn in the economical conditions. It is to be noted that in the present example, the steam property is specified at the high pressure turbine 41 inlet and the cooling steam of predetermined property is obtained at the high pressure turbine 41 outlet and the heat resisting temperature condition at said high temperature portion 11a and tail tube cooled portion 13a is satisfied without the cooling steam being made up otherwise, but if there occurs an unexpected variation in the steam making up function before the high pressure turbine 41 or in the operation of the high pressure turbine 41 or the like, an intermediate pressure steam can be supplied into the cooling steam supply passage 14 for hack-up via the intermediate pressure steam supply passage 35. Further, said cooling steam is set to the pressure of 2.9419 to 3.4323 MPa (absolute) (30 to 35 ata) at the high pressure turbine 41 outlet and while this pressure range is maintained, there occurs no performance lowering of the downstream intermediate pressure turbine 42, which results in no fear of efficiency lowering of the entire system. Also, the steam supplied into the intermediate pressure turbine 42 returns to 566°C by heating at the intermediate pressure second reheater 28, recovery heat at the gas turbine plant 10 and the like, and a low pressure steam at an inlet portion of the low pressure turbine is made up to a pressure of 303.975 to 607.95 kPa (absolute) (3 to 6 ata). According to the present example and embodiment as mentioned above, the cooling steam for cooling the high temperature portion 11a of the gas turbine 11 and the tail tube cooled portion 13a of the combustor 13 is made up to a favorable property at the high pressure turbine 41 outlet, thereby there occurs no fear of efficiency lowering of the downstream intermediate pressure turbine and the structural material of the high temperature portion 11a of the gas turbine 11 and the tail tube cooled portion 13a of the combustor 13 is required of a less severe heat resistant ability without a specific equipment being provided otherwise and the turbine manufacturing cost can be saved greatly.

Claims: A combined cycle power plant comprising a combination of a gas turbine plant (10), a waste heat recovery boiler (20) and a steam turbine plant (40); said gas turbine plant (10) being constructed of a gas turbine (11), an air compressor (12) driven by said gas turbine (11) and a combustor (13) for burning fuel together with compressed air supplied from said air compressor (12); said waste heat recovery boiler (20) being for generating steam by exhaust heat of said gas turbine (11) as heat source and being constructed of portions, mutually connected by pipings, of a high pressure steam generating portion, an intermediate pressure steam generating portion and a low pressure steam generating portion; said high pressure steam generating portion comprising a high pressure economizer (29,31), a high pressure feed water pump (30), a high pressure evaporator (32) and a high pressure superheater (33,34); said intermediate pressure steam generating portion comprising an intermediate pressure economizer (24), an intermediate pressure feed water pump (25), an intermediate pressure evaporator (26) and an intermediate pressure reheater (27,28); said low pressure steam generating portion comprising a low pressure economizer (21), a low pressure feed water pump, a low pressure evaporator (22) and a low pressure superheater (23); and said steam turbine plant (40) being constructed of steam turbines, mutually connected by pipings, of a high pressure turbine (41), an intermediate pressure turbine (42) and a low pressure turbine (43), said high pressure turbine (41) being supplied with high pressure steam from said high pressure steam generating portion, said intermediate pressure turbine (42) being supplied with intermediate pressure steam from said intermediate pressure steam generating portion, and said low pressure turbine (43) being supplied with low pressure steam from said low pressure steam generating portion; said combined cycle power plant further comprising means for lowering a temperature of exhaust steam supplied from said high pressure turbine (41) through a cooling steam supply passage (14) as cooling steam into a high temperature portion (11a,13a) of said gas turbine plant (10) to be cooled; characterized in that said means for lowering a temperature of exhaust steam supplied from said high pressure turbine (41) comprises a fuel heater (16) provided to said cooling steam supply passage (14) to cool the cooling steam in a heat exchanging relation with fuel for said gas turbine (11). The combined cycle power plant according to claim 1, characterized in that a reheater (28) is provided as a means for reheating steam which has cooled the high temperature portion (11a,13a) to be cooled of said gas turbine (11). The combined cycle power plant according to claim 1 or 2, characterized in that said means for lowering a temperature of exhaust steam supplied from said high pressure turbine (41) further comprises a supply passage (35) for mixing steam supplied from said intermediate pressure evaporator (26) into said cooling steam supply passage (14). The combined cycle power plant according to claim 1, 2 or 3, characterized in that said fuel heater (16) is positioned upstream of a bifurcation position between a bifurcation passage (15) for supplying cooling steam to a tail tube cooled portion (13a) of the combustor (13) and the cooling steam supply passage (14) for supplying cooling steam to a high temperature portion (11a) of the gas turbine (11). A method of operating a combined cycle power plant according to any one of claims 1 to 4, comprising the steps of: supplying cooling steam from the high pressure turbine (41) as cooling steam into a high temperature portion (11a,13a) of said gas turbine plant (10) to cool the same, wherein an inlet temperature of said high pressure turbine (41) is set to approximately 566°C, an inlet pressure of said high pressure turbine (41) is adjusted to 16.1809 MPa (absolute) to 17.1616 MPa (absolute) (165 to 175 ata) and exhaust steam of said high pressure turbine (41) is maintained to a temperature of 330 to 250°C and pressure of 3.4323 MPa (absolute) to 2.9419 MPa (absolute) (35 to 30 ata), and lowering the temperature of the cooling steam from said high pressure turbine (41) in a heat exchanging relation with fuel for the gas turbine plant (10). The method according to claim 5, comprising the step of additionally supplying an intermediate pressure steam into the cooling steam from the high pressure turbine (41) to compensate an unexpected variation in the generation of steam before or in the high pressure turbine (41).

Inventors: HIROKAWA KAZUHARU, MORI HIDETAKA, SUGISHITA HIDEAKI, UEMATSU KAZUO, HIROKAWA, KAZUHARU, MORI, HIDETAKA, SUGISHITA, HIDEAKI, UEMATSU, KAZUO, HIROKAWA, Kazuharu, Takasago Mach. Works, MORI, Hidetaka, Takasago Res. &amp; Dev. Center, SUGISHITA, Hideaki, Takasago Res.&amp; Dev.Center, UEMATSU, Kazuo, Takasago Mach. Works
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1998
Main classification: F02C6
IPC codes: F02C7, F01K23, F02C6