Method for operating a combined-cycle power plant with cogeneration, and a combined-cycle power plant for carrying out the method

Abstract

The invention relates to a method for operation of a combined-cycle power plant with cogeneration, in which method combustion air is inducted in at least one gas turbine, and in which method the exhaust gas emerging from the at least one turbine is passed through a heat recovery steam generator (HRSG) in order to generate steam. The electricity production can be decoupled from the steam production in order to restrict the electricity production while the heat provided by steam extraction remains at a constant level. A portion of the inducted combustion air can be passed through at least one turbine to the HRSG without being involved in the combustion of the fuel in the gas turbine. This portion of the combustion air can be used to operate at least one supplementary firing in the heat recovery steam generator.

Claims

1. A method for operation of a combined-cycle power plant with cogeneration, in which method combustion air is inducted in at least one gas turbine, is compressed and is supplied to at least one combustion chamber for combustion of a fuel, and a resultant exhaust gas is expanded in at least one turbine, producing work, and in which method an exhaust gas which emerges from the at least one turbine is passed through a heat recovery steam generator in order to generate steam, wherein the heat recovery steam generator is part of a water-steam circuit, wherein heat is provided by extracting steam from at least one steam turbine, the method comprising: passing a portion of the inducted combustion air through the at least one turbine to the heat recovery steam generator without being involved in the combustion of the fuel in the at least one gas turbine to restrict electricity production while heat provided by the extracting of steam from the at least one steam turbine remains constant; and using the portion of the combustion air that is passed through the at least one turbine without being involved in the combustion of the fuel in the at least one gas turbine to operate at least one supplementary firing in the heat recovery steam generator; and increasing a mass flow of the portion of the inducted combustion air that is passed through the at least one turbine to the heat recovery steam generator without being involved in the combustion of the fuel in the at least one gas turbine at low load operation relative to high load operation, wherein the steam extracted is used in a desalination plant selectively operated with low-pressure steam or intermediate-pressure steam, and in order to restrict the electricity production, the operation of the desalination plant is additionally converted from intermediate-pressure steam to low-pressure steam.

2. The method as claimed in claim 1, wherein the at least one combustion chamber has only one combustion chamber and the at least one turbine has only one turbine.

3. A method for operation of a combined-cycle power plant with cogeneration, in which method combustion air is inducted in at least one gas turbine, is compressed and is supplied to at least one combustion chamber for combustion of a fuel, and a resultant exhaust gas is expanded in at least one turbine, producing work, and in which method an exhaust gas which emerges from the at least one turbine is passed through a heat recovery steam generator in order to generate steam, wherein the heat recovery steam generator is part of a water-steam circuit, wherein heat is provided by extracting steam from at least one steam turbine, the method comprising: passing a portion of the inducted combustion air through the at least one turbine to the heat recovery steam generator without being involved in the combustion of the fuel in the at least one gas turbine to restrict electricity production while heat provided by the extracting of steam from the at least one steam turbine remains constant; and using the portion of the inducted combustion air to operate at least one supplementary firing in the heat recovery steam generator, wherein the at least one combustion chamber is comprised of a first combustion chamber and a second combustion chamber that are sequentially arranged and wherein the at least one gas turbine is designed for sequential combustion and has two sequentially arranged combustion chambers comprising the first and second combustion chambers and wherein the at least one turbine in which the resultant exhaust gas is expanded is comprised of two turbines for expansion of the resultant exhaust gases, wherein the portion of the inducted combustion air which is not used for combustion of the fuel is provided for operation of the at least one supplementary firing by switching off the second combustion chamber.

4. The method as claimed in claim 3, wherein the at least one gas turbine is provided with variable inlet guide vanes which are set at the same time to a maximum open position when the second combustion chamber is switched off.

5. The method as claimed in claim 1, wherein the at least one supplementary firing is operated at an input to the heat recovery steam generator.

6. The method as claimed in claim 5, wherein the heat recovery steam generator contains a first superheater, and the at least one supplementary firing is operated downstream from the first superheater.

7. The method as claimed in claim 1, wherein the desalination plant comprises units for multiple-effect distillation that operate with low-pressure steam, each of said units comprising an apparatus for thermal vapor compression which operates with intermediate-pressure steam and is switched on in order to restrict the electricity production.

8. A combined-cycle power plant comprising: at least one gas turbine with a compressor for compression of inducted combustion air, a first combustion chamber for combustion of a fuel using the compressed combustion air, and at least one turbine for expansion of exhaust gases created during the combustion; a water-steam circuit with at least one steam turbine and a heat recovery steam generator through which exhaust gases which emerge from the at least one gas turbine flow, wherein a capability to extract steam is provided for the at least one steam turbine; and a controllable bypass provided in the at least one gas turbine, via which a portion of the compressed combustion air is introduceable into the at least one turbine by bypassing the first combustion chamber; and a supplementary firing is provided in the heat recovery steam generator, in which fuel can be burnt in order to heat the exhaust gases introduced, using the combustion air which is passed via the bypass, wherein the controllable bypass is controlled such that a mass flow of the portion of the combustion air that is introduced into the at least one turbine by bypassing the first combustion chamber via the controllable bypass is greater during low load operations as compared to high load operations, wherein the combined-cycle power plant has an associated desalination plant comprising multiple-effect distillation devices supplied with low-pressure steam from the at least one steam turbine, each of the multiple-effect distillation devices comprising an associated thermal vapor compression device operable with intermediate-pressure steam from the at least one steam turbine.

9. The combined-cycle power plant as claimed in claim 8, wherein a valve is arranged in the bypass.

10. The combined-cycle power plant as claimed in claim 8, wherein the at least one gas turbine is designed for sequential combustion, and has a second combustion chamber, such that the first and second combustion chambers are two sequentially arranged combustion chambers and the at least one turbine for expansion of the exhaust gases created during the combustion is comprised of two turbines for expansion of the exhaust gases.

11. The combined-cycle power plant as claimed in claim 8, wherein the controllable bypass comprises a valve that is controllable to adjust an amount of the portion of the compressed combustion air that is passed through the bypass to bypass the first combustion chamber.

12. The combined-cycle power plant as claimed in claim 11, wherein the controllable bypass is configured such that each and every combustion chamber of the gas turbine is bypassed by the portion of the compressed combustion air that is passed through the bypass.

13. The combined-cycle power plant as claimed in claim 8, wherein the controllable bypass is configured to restrict electricity production of the at least one gas turbine while heat provided by extraction of steam by the at least one steam turbine remains constant.

14. The method as claimed in claim 1, wherein the passing the portion of the inducted combustion air through the at least one turbine to the heat recovery steam generator without being involved in the combustion of the fuel in the at least one gas turbine to restrict electricity production while heat provided by the extracting of steam from the at least one steam turbine remains constant comprises: passing the portion of the inducted combustion air through at least one controllable bypass such that the portion of the inducted combustion air bypasses the at least one combustion chamber and is not involved in the combustion of the fuel in the at least one gas turbine, the at least one controllable bypass having a valve to control an amount of the portion of the inducted combustion air that passes through the at least one controllable bypass.

15. The method as claimed in claim 3, wherein the portion of the inducted combustion air additionally bypasses the first combustion chamber.

16. The combined-cycle power plant as claimed in claim 8, wherein the supplementary firing comprises a first supplementary firing arranged in the heat recovery steam generator at an input of the heat recovery steam generator, and a second supplementary firing arranged downstream from a first superheater.

17. The combined-cycle power plant as claimed in claim 8, wherein the thermal vapor compression devices are designed such that they can be switched off.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be explained in more detail in the following text with reference to exemplary embodiments in conjunction with the drawing, in which:

(2) FIG. 1 shows a simplified schematic of a combined-cycle power plant with a connected desalination plant according to one exemplary embodiment of the invention;

(3) FIG. 2 shows a schematic of a combined-cycle power plant comparable to that in FIG. 1, in particular with sequential combustion in the gas turbine and a connected desalination plant, as is suitable for carrying out the method according to the invention; and

(4) FIG. 3 shows a schematic of a desalination plant, which can be operated within the scope of the invention selectively only with low-pressure steam or with low-pressure steam and intermediate-pressure steam.

BEST MODES FOR CARRYING OUT THE INVENTION

(5) An operating concept for the gas turbine in a combined-cycle power plant with cogeneration, which results in a large exhaust gas flow from the gas turbine with a low exhaust gas temperature at the same time opens up a wide range for supplementary firing in the heat recovery steam generator, thus making it possible to ensure high steam production for a connected desalination plant, which is operated using the steam, even when the demand for electricity from the grid system is low at the same time. The supplementary firing can in this case be restricted to the input of the heat recovery steam generator as a result of which, however, the steam production is restricted. If, in contrast, further supplementary firing is additionally provided between the superheaters which are arranged in the heat recovery steam generator, the steam production can be increased considerably, but the steam turbine will probably have to be switched off because the temperature level of the steam produced at the output of the heat recovery steam generator is too low.

(6) FIG. 1 shows a simplified installation layout of a combined-cycle power plant with a connected desalination plant according to one exemplary embodiment of the invention, by means of which a corresponding operating concept can be implemented. The illustrated combined-cycle power plant 10 comprises a gas turbine 11, a water-steam circuit 12 and a desalination plant 15. The gas turbine 11, which drives a first generator G1, comprises a compressor 18, a combustion chamber 19 and a turbine 20. The compressor 18 inducts combustion air via an air inlet 16, compresses it and then emits it to the combustion chamber 19, where together with a fuel that is introduced, it feeds a combustion process which produces hot exhaust gases, which are expanded in the downstream turbine 20, producing work. The amount of inducted combustion air can be controlled via variable inlet guide vanes 17.

(7) The hot exhaust gas from the gas turbine 11 flows through a heat recovery steam generator HRSG 13, which is arranged in the water-steam circuit 12, in order there to convert feedwater, from a feedwater tank 28, to superheated steam via appropriate economizers 26, 27 and superheaters 22, 24. Appropriate pumps P2 and P3 are provided in order to direct the feedwater to the HRSG 13. Furthermore, a high-pressure drum 25 is provided in a manner known per se, as well as a valve V5, by means of which the inlet flow to the high-pressure drum 25 can be controlled. The high-pressure steam, which is produced in the heat recovery steam generator 13 is fed via a valve V3 of the high-pressure turbine 29 to a steam turbine 14, which is arranged in the water-steam circuit and drives a further generator G2, where it is expanded to an intermediate pressure, before entering an intermediate-pressure turbine 30 of the steam turbine 14. The low-pressure steam exhausted by the intermediate-pressure turbine 30 is then passed through a corresponding low-pressure turbine 31, in order finally to be condensed in a condenser 32 and pumped back to the feedwater tank 28 by means of a condensate pump P1. A cooling medium, which is conveyed by means of a further pump P4, flows through the condenser 32.

(8) Steam is extracted from the steam turbine 14 via a valve V4 between the intermediate-pressure turbine 30 and the low-pressure turbine 31, and is fed to a desalination plant 10, which, for example, may be designed as shown in FIG. 3. The condensate created in the desalination plant is fed back into the circuit via a dotted line in FIG. 1.

(9) In the installation shown in FIG. 1, two supplementary firings 21 and 23 are provided in the heat recovery steam generator 13, one (21) of which is arranged directly at the input of the heat recovery steam generator 13 (so-called duct firing), while the second (23) is arranged between the two superheaters 22 and 24 (so-called inter-bank firing). Both supplementary firings 21 and 23 are supplied with suitable fuel via appropriate valves V1 and V2.

(10) The special feature of the installation shown in FIG. 1 is now that a variable proportion of the combustion air that has been compressed in the compressor 18 bypasses the combustion chamber 19 via a bypass 33, which is arranged in the gas turbine 11 and can be controlled by means of a valve V6, and this proportion of the combustion air is therefore not involved in the combustion in the gas turbine 11. This ensures a constantly high exhaust gas mass flow even when the load on the gas turbine 11 is relatively low. Such operation leads to a lower exhaust gas temperature, which allows independent control of the steam generation by means of the supplementary firings 21 and 23 in the heat recovery steam generator 13.

(11) At the same time, because of the reduced combustion air flow, suitable combustion parameters can be maintained in the combustion chamber 19, with the consequence that the hazardous-substance emission can be kept low even when the load on the gas turbine is comparatively low. Since the oxygen content in the exhaust gas from the gas turbine is increased considerably in comparison to conventional operation as a result of bypass operation, the supplementary firing in the heat recovery steam generator 13 can be operated on a large scale without additional external air.

(12) Another possible way to implement a comparable method is shown in FIG. 2 for a combined-cycle power plant 40, which comprises a gas turbine 34 with sequential combustion. In the case of sequential combustion such as this, as is known by way of example from the document EP 1 914 407 A2, first and second combustion chambers 19 and 36, respectively, in the gas turbine 34 are respectively connected one after the other to a downstream expansion turbine 35 and 37, respectively. In this case, when the electricity production is restricted, the second combustion chamber 36 is switched off, and the inlet guide vanes 17 are opened completely at the same time. This maintains the full exhaust gas mass flow even when the load on the gas turbine 34 is comparatively low, with the first combustion chamber 19 being operated close to its nominal operating point and with the hazardous-substance emissions remaining low.

(13) The resultant exhaust gas temperature is low, and, even in this situation, allows independent control of the steam generation by means of the supplementary firings in the heat recovery steam generator 13. The oxygen content in the exhaust gas of the gas turbine is also considerably increased in comparison to conventional operation in this type of operation, allowing the supplementary firing in the heat recovery steam generator 13 to be operated on a large scale without additional external air.

(14) However, in addition to switching off the second combustion chamber 36, it is also possible to provide a bypass, as shown in FIG. 1, for the first combustion chamber 19, in order to supply a proportion of the compressed air, bypassing the first combustion chamber 19. This makes it possible to ensure that the first combustion chamber 19 can be operated with a flame temperature close to the nominal operating point, and to ensure that this results in a low exhaust gas temperature with a high mass flow at the same time, as a result of which, as already described above, independent control of the steam generation by the supplementary firings 21 and 23 is possible.

(15) The two supplementary firings 21 and 23 illustrated in FIG. 1 and FIG. 2 have the advantage that the first additional firing 21 essentially makes it possible to ensure a minimal steam temperature at the outlet of the heat recovery steam generator 13, in order to keep the steam turbine 14 within its load limits, while the purpose of the second supplementary firing 23 is essentially to generate and to control the desired amount of steam.

(16) A further influencing option consists in designing the desalination plant 15, as shown in FIG. 3, such that it can be operated via separate extraction lines selectively with intermediate-pressure steam from the intermediate-pressure turbine 30 or with low-pressure steam from the low-pressure turbine 31. This can be done by using desalination units 15a-15d in the desalination plant 15, with so-called multiple-effect distillation devices (MED) 38 in combination with thermal vapor compression devices (TCV) 39. The multiple-effect distillation devices 38 require low-pressure steam at a pressure of about 0.5 bar, while the thermal vapor compression devices 39 require steam at a pressure of about 3 bar.

(17) In the exemplary embodiment shown in FIG. 3, a steam turbine 14 is shown having a common high-pressure turbine 29 and intermediate-pressure and low-pressure turbines 30 and 31 arranged in two parallel paths, which respectively drive a generator G3 and G4 and receive intermediate-pressure steam from the high-pressure turbine 29 via valves V7 and V8, while the high-pressure turbine 29 receives high-pressure steam 41 via the valve V3. The desalination plant 15 is connected to the upper path of the steam turbine 14, with the operation of the thermal vapor compression devices 39 being controlled via appropriate valves V9. The lower path interacts directly with the condenser 32.

(18) A desalination plant 15 of this kind can be operated alternatively in two different operating modes: in one operating mode, the desalination units 15a-15d are operated without the thermal vapor compression devices 39 (valve V9 closed), in order to achieve maximum electricity production. In the other operating mode, the thermal vapor compression devices 38 are likewise operated, in order to maintain the production of drinking water, while the electricity requirement is low at times or seasonally.

(19) One advantage of this configuration is that the supplementary firing in the heat recovery steam generator, which normally has to be designed for partial load operation of the gas turbine, can be reduced in its size, because less steam is required for the desalination of the same amount of water as a result of the combined operation of the multiple-effect distillation devices 38 and the thermal vapor compression devices 39.