Combined-cycle power plant

Abstract

The combined-cycle power plant (1) comprises a plurality of gas turbines (2a, 2b), a single heat recovery steam generator (3) fed by the exhaust gases produced by the gas turbines (2a, 2b), and a single steam turbine (4) fed by the steam produced by the heat recovery steam generator (3).

Description

[0001] COMBINED-CYCLE POWER PLANT

[0002] DESCRIPTION

[0003] The present invention relates to a combined-cycle power plant.

[0004] A combined-cycle power plant produces electrical energy by combining two cycles, one gas cycle and one steam cycle.

[0005] In the first cycle, energy production takes place through the combustion of natural gas (so-called “open” cycle).

[0006] In the second cycle, the heat contained in the combustion exhaust gases of the natural gas is recovered to produce steam.

[0007] The traditional architecture of a combined-cycle power plant comprises, in cascade, a gas turbine, a heat recovery steam generator, and a steam turbine.

[0008] In the plant, the gas turbine drives an electric generator, and the heat of the exhaust gases from the gas turbine generates steam, which is used to produce additional electrical energy by means of a steam turbine.

[0009] Plants that must deliver a high nominal electrical power may include multiple lines operating in parallel; in such cases, when opting for a combined cycle, existing technology provides that each line includes a gas turbine and an associated heat recovery steam generator.

[0010] Combined-cycle power plants have rapidly spread because they have high efficiency, in terms of electrical energy production yield, together with a consequent reduced environmental impact in terms of pollutants released into the atmosphere per unit of power generated, compared to traditional so-called open-cycle plants.

[0011] However, such plants — especially those designed to provide a high nominal electrical power — sometimes suffer from excessive overall dimensions that make installation in confined spaces impossible.Similarly, existing open-cycle plants are often confined in limited spaces that do not allow their conversion to combined cycle according to traditional technology.

[0012] The technical task addressed by the present invention is therefore to provide a combined-cycle power plant that overcomes the technical drawbacks of the known art, by compacting the overall dimensions of the heat recovery steam generator, optimizing the structure, components, and instrumentation, while maintaining high reliability and safety and a high nominal electrical power, and enabling the above-described minimization of the related environmental impact.

[0013] This technical task, as well as these and other objects, according to the present invention, are achieved by providing a combined-cycle power plant characterized in that it comprises a plurality of gas turbines, a single heat recovery steam generator fed by the exhaust gases produced by said plurality of gas turbines, and a single steam turbine fed by the steam produced by said heat recovery steam generator.

[0014] In a preferred embodiment of the invention, the plant comprises two gas turbines feeding said heat recovery steam generator.

[0015] In a preferred embodiment of the invention, the plant comprises a manifold connecting said plurality of gas turbines and said heat recovery steam generator, said manifold having a mixing chamber for the exhaust gas flows coming from said plurality of gas turbines, a plurality of exhaust gas inlet paths to the mixing chamber, and a single exhaust gas outlet path from the mixing chamber, wherein each exhaust gas inlet path is connected to an exhaust gas discharge path of a corresponding gas turbine, and said single exhaust gas outlet path is connected to a single exhaust gas supply path to said heat recovery steam generator.

[0016] In a preferred embodiment of the invention, said inlet paths and said single outlet path have a respective horizontal longitudinal axis.

[0017] In a preferred embodiment of the invention, the plant with two gas turbines has said manifold shaped as a “T”.In a preferred embodiment of the invention, said plurality of gas turbines is sized to provide each a nominal electrical power between 20 MW and 800 MW.

[0018] In a preferred embodiment of the invention, each gas turbine is sized to provide an exhaust gas flow in a range between 25 and 1100 kg / s and an exhaust gas flow temperature in a range between 400°C and 700°C.

[0019] In a preferred embodiment of the invention, at least one gas turbine operates from a minimum of 50% of its nominal power up to 100% of its nominal power.

[0020] Further features and advantages of the invention will become more apparent from the description of a preferred but non-exclusive embodiment of the combined-cycle power plant according to the invention, illustrated by way of non-limiting example in the accompanying drawings, in which:

[0021] • Figure 1 shows a simplified diagram of the combined-cycle power plant;

[0022] • Figure 2a schematically shows a plan view of the exhaust gas manifold sectioned along a horizontal mid-plane;

[0023] • Figure 2b shows a side elevation view of the exhaust gas manifold of Figure 2a;

[0024] • Figure 3 schematically shows the heat recovery steam generator 3 of the combined-cycle power plant.

[0025] • Figure 4 shows an excerpt from a finite element distribution study intended to demonstrate the correct distribution of the exhaust gases at the inlet of the heat recovery steam generator, in a side view, where the values of the linear velocity, indicated by “v”, are represented by a grayscale scale;

[0026] • Figure 5 shows an excerpt from a finite element distribution study intended to demonstrate the correct distribution of the exhaust gases at the inlet of the heat recovery steam generator, in a top view, where the values of the linear velocity, indicated by “v”, are represented by a grayscale scale.With reference to the above-mentioned figures, a combined-cycle power plant is shown, indicated overall by reference numeral 1.

[0027] The plant 1 comprises a plurality of gas turbines 2, a single heat recovery steam generator 3 fed by the exhaust gases produced by the gas turbines 2, and a single steam turbine 4 fed by the steam produced by the heat recovery steam generator 3.

[0028] In the preferred embodiment illustrated and described herein, two gas turbines 2a, 2b are provided to feed the heat recovery steam generator 3.

[0029] In other solutions not illustrated or described herein, more than two gas turbines feeding the heat recovery steam generator 3 may also be provided.

[0030] Advantageously, the plant 1 comprises a manifold 5 connecting the gas turbines 2a, 2b and the heat recovery steam generator 3.

[0031] The manifold 5 comprises a mixing chamber 6 for mixing the exhaust gas flows coming from the gas turbines 2a, 2b, inlet paths 7a, 7b for the exhaust gases into the mixing chamber 6, and a single outlet path 8 for the exhaust gases from the mixing chamber 6.

[0032] Obviously, each exhaust gas inlet path 7a, 7b is connected to an exhaust discharge path 9a, 9b of a corresponding gas turbine 2a, 2b, while the single exhaust gas outlet path 8 is connected to a single exhaust gas supply path 10 to the inlet duct of the heat recovery steam generator 3.

[0033] In Figure 2a, the exhaust gas flow coming from the first gas turbine 2a is indicated by arrow Fl, the exhaust gas flow coming from the second gas turbine 2b is indicated by arrow F2, and the mixed exhaust gas flow exiting from the outlet path 8 is indicated by arrow F3.

[0034] The configuration of the manifold 5 is important to ensure correct distribution of the mixed exhaust gas flow F3 exiting from the outlet path 8.

[0035] In this regard, the exhaust gas inlet paths 7a, 7b have horizontal longitudinal axes La, Lb, and the outlet path 8 has a horizontal longitudinal axis Lc.In practice, the exhaust gas manifold 5, in the case of a plant 1 with two gas turbines 2a, 2b, has a special “T”-shaped configuration, preferably arranged horizontally.

[0036] A junction angle a between the two exhaust gas flows Fl and F2 is set to an optimal value dictated by an adequate distribution of the gases at the inlet of the heat exchange surfaces of the steam generator.

[0037] In the preferred embodiment illustrated and described herein, the heat recovery steam generator 3 is of the natural circulation type, with horizontal exhaust gas flow and vertical heat exchange tubing 11 in which water circulates and is transformed into steam.

[0038] The heat recovery steam generator 3 comprises, in sequence, one or more economizers 3a, one or more evaporators 3b, and one or more superheaters 3 c that complete the evaporation of the water and the superheating of the steam.

[0039] The exhaust gas flow F3 exiting from the manifold 5 enters a diverging diffuser 13 of the heat recovery steam generator 3 through the supply path 10, and passes sequentially through the superheater 3c, the evaporator 3b, and the economizer 3a before being discharged into the atmosphere through a stack 12.

[0040] The exhaust gas flow exiting the stack 12 is indicated by F4.

[0041] In the vertical tubes 11, the flow of liquid water entering the economizer 3 a of the heat recovery steam generator 3 is indicated by F5, and the flow of superheated steam exiting the superheater 3c of the heat recovery steam generator 3 is indicated by F6.

[0042] The duct system 5, 6, and 10 ensures that the exhaust gas flow F3 is distributed uniformly over the entire vertical extent of the tubing 11.

[0043] An advantageous plant size 1 has been identified in which the gas turbines are each sized to provide a nominal electrical power between 20 MW and 800 MW.In particular, an advantageous plant size 1 has been identified in which each gas turbine is sized to supply an exhaust gas flow in a range between 25 kg / s and 1100 kg / s and an exhaust gas temperature in a range between 400°C and 700°C.

[0044] The plant 1 provides that at least one gas turbine operates from a minimum of 50% of its nominal electrical power up to 100% of its nominal electrical power.

[0045] In the illustrated case of two gas turbines, since it may be provided that one gas turbine operates at 50% load of its nominal electrical power (full load) while the other gas turbine is not operating, the two gas turbines can supply a minimum electrical power equal to 25% load of the total nominal electrical power.

[0046] Gas turbine load control in the range between 50% load and full load for each gas turbine is advantageously carried out either by controlling gas turbine load through direct variation of the combustion heat rate or through operation of an exhaust gas diverter damper located between the gas turbines and the heat recovery steam generator for exhaust gas flow partialization.

[0047] Advantageously the exhaust gas composition is mainly referred to clean natural gas; the referenced solution considers then a N2 content in the range 65-85 % of the total exhaust gas released by the gas turbines.

[0048] The plant 1 is particularly reliable and safe compared to traditional plants that provide as many heat recovery steam generators as gas turbines, since, thanks to the use of a single heat recovery steam generator, it drastically reduces overall dimensions, components, and instrumentation, and consequently the risk of malfunctions.

[0049] The provision of a single heat recovery steam generator consequently reduces plant costs and, thanks to its compact dimensions, makes it possible to optimize the plant layout, also enabling the conversion of an existing open-cycle plant into a combined-cycle plant, thereby reducing the associated environmental impact relative to the generated power.The originality of the invention lies in having provided a plant concept focused on connecting multiple gas turbines to a single heat recovery steam generator.

[0050] This new plant concept overcomes a technical limitation related to the well-known difficulty of achieving proper distribution of exhaust gases in the inlet duct of the heat recovery steam generator, which has so far always discouraged the adoption of a plant configuration different from that involving the connection of a single gas turbine to a single heat recovery steam generator. The invention provides a particular construction of the exhaust gas ducts that makes it possible to broaden plant design options by adopting a single heat recovery steam generator to which multiple gas turbines are connected, while at the same time maintaining adequate distribution of the gases over the heat exchange surfaces.

Claims

CLAIMS1. Combined cycle power plant (1), characterized by comprising a plurality of gas turbines (2a, 2b), a single heat recovery steam generator (3) fed by the exhaust gases produced by said plurality of gas turbines (2a, 2b), and a single steam turbine (4) fed by the steam produced by said heat recovery steam generator (3).

2. Combined cycle power plant (1) according to claim 1, characterized by comprising two gas turbines (2a, 2b) feeding said heat recovery steam generator (3).

3. Combined cycle power plant (1) according to any of the preceding claims, characterized by comprising a manifold (5) connecting said plurality of gas turbines (2a, 2b) to said heat recovery steam generator (3), said manifold (5) featuring a mixing chamber (6) for the exhaust gas flows coming from said plurality of gas turbines (2a, 2b), a plurality of inlet paths (7a, 7b) for the exhaust gases into the mixing chamber (6), and a single outlet path (8) for the exhaust gases from the mixing chamber (6), where each exhaust gas inlet path (7a, 7b) is connected to an exhaust gas outlet path (9a, 9b) of a corresponding gas turbine (2a, 2b), and said single exhaust gas outlet path (8) is connected to a single exhaust gas feeding path (10) leading to said heat recovery steam generator (3).

4. Combined cycle power plant (1) according to claims 2 and 3, characterized by the fact that said manifold (5) has a "T" configuration.

5. Combined cycle power plant (1) according to any of the preceding claims, characterized by the fact that said plurality of gas turbines (2a, 2b) is sized to individually provide a nominal electric power ranging between 20 MW and 800 MW.

6. Combined cycle power plant (1) according to the previous claim, characterized by the fact that each gas turbine (2a, 2b) is sized to provide an exhaust gas flow (Fl, F2) in a range between825 and 1100 kg / s and an exhaust gas flow temperature (Fl, F2) in a range between 400°C and 700°C.

7. Combined cycle power plant (1) according to any of the preceding claims, where at least one gas turbine (2a, 2b) operates at a minimum load of 50% of its nominal power up to 100% of its nominal power.

8. Combined cycle power plant (1) according to the preceding claim, where gas turbine load control in the range between 50% load and full load for each gas turbine is carried out by controlling gas turbine load through direct variation of the combustion heat rate.

9. Combined cycle power plant (1) according to claim 7, where gas turbine load control in the range between 50% load and full load for each gas turbine is carried out through operation of an exhaust gas diverter damper located between the gas turbine and the heat recovery steam generator for exhaust gas flow partialization.

10. Combined cycle power plant (1) according to any of the preceding claims, where the exhaust gas composition is referred to clean natural gas with a N2 content in the range 65-85 % of the total exhaust gas.9

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