Gas turbine assembly for power plant applications and method of operating the same

Abstract

The present invention relates to a gas turbine assembly for power plant applications and an operating method thereof. A gas turbine assembly for power plant applications fed by preheated fuel gas comprises: a compressor; a sequential burner assembly, which comprises a first combustion stage and a second combustion stage in series; a turbine, which is configured to be driven by hot gas to perform work on a rotor; a fuel source, which is configured to supply preheated fuel to a fuel gas distribution system, the fuel gas distribution system comprising a first pipeline for feeding the first combustion stage and a second pipeline for feeding the second combustion stage, at least the second pipeline is provided with at least a metal valve, between the metal valve and the second combustion stage there is a first conduit, and leaking fuel gas can accumulate in the first conduit; wherein the gas turbine also comprises a second conduit, the second conduit being configured to feed cooling air into the first conduit, wherein the cooling air has a temperature lower than the auto-ignition temperature of the fuel gas.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims priority to European patent application No. 19213681.0 filed on December 4, 2019, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of gas turbine assemblies (hereinafter simply denoted by GT) for power plant applications. As is well known, GT compresses air, mixes the compressed air with fuel, ignites the mixture in a combustor, and then expands the hot gases in a turbine to produce mechanical work on the shaft of the GT. In more detail, the GT of the present application is equipped with a sequential combustion technology (preferably the applicant's sequential combustion technology) to burn the fuel-air mixture in two sequential stages. In the first combustion stage, the compressed air is mixed with the fuel and burned to produce a hot gas flow. In the second stage, the fuel is injected into the hot gas, resulting in the self-ignition of the fuel due to the hot surrounding environment. With regard to the above-mentioned two-stage burner technology, the present invention solves the problem of shutting down the second stage and operating the GT assembly while the first stage is still in operation. Background Art

[0004] Generally speaking, a GT assembly includes the following components: a compressor, a combustor, and a turbine. An axial compressor pressurizes the air drawn into the compressor from the engine's environment. The compressed air leaves the compressor at its outlet into a fixed volume (called a "plenum"), where the main flow is distributed to the combustor assembly and a portion of the compressed air is used as cooling air. Typically, but not exclusively, the combustor assembly includes a plurality of combustor cans and burners, which mix the pressurized air with a fuel (gas or oil) and ignite this mixture. The resulting hot gases are then expanded through a turbine. The mechanical work done by the hot gas flow on the turbine blades is converted into rotational energy due to the circumferentially assembled blades of the GT rotor.

[0005] In order that both efficiency and emission targets are met, during full load as well as in partial and low load operation, a number of measures can be taken. One of those measures, but not limited to this, is the applicant's sequential combustion technology in its GT. This technology follows the principle of the reheat step in the thermodynamic cycle of the GT. Today, at least two different sequential GTs are known. According to a first embodiment, the first burner and the second burner are annularly formed and physically separated by a turbine blade stage called a high-pressure turbine. There is a second turbine unit (called a low-pressure turbine) downstream of the second burner. This kind of GT is produced by the applicant and is available on the market as "GT26". According to a second embodiment of the GT with sequential burners, the GT is not provided with a high-pressure turbine but the burner assembly is implemented in the form of a plurality of can burners arranged as a ring around the rotor. Each can burner includes a first stage and a second stage arranged in sequence. In addition, this second kind of GT is produced by the applicant and is available on the market as "GT36". Those two examples of GT assemblies (GT26 and GT36) are cited only as non-limiting examples in which the present invention can be applied. However, application of the present invention in a sequential can combustor GT may include higher (or better) advantages.

[0006] As is well known, the use of preheated fuel gas (hereinafter simply indicated by FG), for example FG preheated up to 300°C before feeding (or supplying) to the burner, in a sequential GT leads to beneficial effects on combustion efficiency and improves GT component performance. However, the use of this preheated fuel in the GT also involves some problems to be solved. Due to the high temperature of the fuel, conventional soft-sealed valves cannot be used along the fuel distribution line. Therefore, in the case of preheated fuel, the fuel distribution line must be provided with metal seat valves. Sometimes, the GT can be operated in the so-called "first stage operation only". In this condition, only the first combustion stage is operated, and the valves arranged along the fuel distribution line toward the second combustion stage are driven closed. However, the metal seat valves are not 100% tight but fuel leakage through these valves must be expected. This leakage can accumulate in the fuel distribution line directly upstream of the second combustion stage (i.e., in the pipes and fuel distribution system upstream of the second stage burner). The presence of leaking FG in the shut-down combustion stage and the potential accumulation of FG in the FG distribution system pose a risk to the safe operation of the GT components. Summary of the invention

[0007] Therefore, the main object of the present invention is to provide a new and inventive solution to overcome the above-mentioned problem of how to avoid undesirable accumulation of leaked FG in the fuel distribution line when the GT is used in first stage only operation. In particular, the main object of the present invention is to provide a GT assembly fed by preheated FG and a method for operating such a GT assembly.

[0008] As a preface, it can be provided that the GT assembly of the present invention is a GT assembly for a power plant, comprising:

[0009] a compressor configured to compress incoming air;

[0010] a sequential combustor assembly configured for mixing a fuel (at least FG) in compressed air and for producing a hot gas flow, wherein the combustor assembly comprises in series a first combustion stage and a second combustion stage;

[0011] a turbine configured to be driven by the hot gases to perform work on a rotor;

[0012] A fuel source is configured for preheating fuel and for supplying the preheated fuel to a fuel distribution system including a first line for feeding the first combustion stage and a second line for feeding the second combustion stage.

[0013] Referring to the burner assembly, preferably, the burner assembly comprises a plurality of can burners, wherein each can burner comprises a first combustion stage and a second combustion stage in series. A first line of the fuel distribution system is configured to feed all first combustion stages, and a second line of the fuel distribution system is configured to feed all second combustion stages.

[0014] Referring to the fuel source, this source is configured for feeding a plurality of different kinds of fuel; according to the present invention, the claimed preheated fuel is FG fed to a single conduit and split into two lines downstream.

[0015] As is known, sometimes the GT can be operated in a so-called "first stage only operating mode", in which the second stage of the burner is shut down and the valves of the second stage fuel distribution system are driven closed. Since the fuel is preheated (between 250°C and 320°C, preferably at 300°C), the control valves of the fuel distribution system are metal seat valves, and therefore during the first stage only operating mode, the fuel upstream of those valves can leak through the metal seat valves. As a result, some preheated fuel can accumulate upstream of the shut down second combustion stage, specifically in the pipe (called the first pipe) used as a collector for feeding the second burner.

[0016] In this situation, there is a possibility of ignition of the fuel in the first duct (with associated damage) when in contact with the hot gases on the burner side. In order to avoid this problem, according to the main definition of the invention, the GT also includes a duct (the claimed second duct) configured to feed cooling air, and wherein this second duct is connected on one side to the first duct (where the leaked fuel accumulates), producing a lean air-fuel mixture. Since the cooled air reveals a temperature (between 320°C and 360°C, preferably at a temperature of 340°C) below the auto-ignition temperature of the fuel, any improper ignition of the mixed medium is avoided.

[0017] As is known, the GT further comprises a control unit configured for selectively operating the GT in the first stage only mode and (in this case) for closing the valve of the second stage fuel line. Preferably, the feed of cooling air to the second duct is activated by the control unit only during the first stage only operating mode (preferably continuously during the first stage only operating mode).

[0018] Please note that with the expression "first and second conduits" we do not mean necessarily a single conduit, but a distribution line comprising more than one conduit.

[0019] Preferably, the GT also comprises a heat exchanger (possibly a so-called through cooler or "OCT") configured for cooling the compressed air drawn off from the compressor and for feeding this cooled air to the turbine for cooling purposes. In the case of the present invention, the second duct is a duct connected on the other side to the outlet of the heat exchanger, so that the cooling air fed in the second duct is part of the cooling air drawn off from the heat exchanger. In this case, for the purposes of the present invention, an additional purge gas source (e.g. nitrogen) is not required.

[0020] In addition, the cooling air fed in the second duct air also serves as purge air when the second stage of the combustion chamber is switched.

[0021] Of course, the present invention can also be defined as a method for operating a GT assembly for a power plant fed by preheated fuel (ie, FG). The method may include the following steps:

[0022] a) providing a GT assembly having a sequential burner as described above;

[0023] b) preheating the fuel (preferably between 250°C and 320°C, more preferably at 300°C) before feeding the fuel into the fuel distribution system;

[0024] c) controlling the GT to operate in a first stage only operating mode, wherein a valve driving the second stage fuel line is closed;

[0025] d) feeding (preferably continuously during the first stage only operating mode) cooled air in the second duct at a temperature below the auto-ignition temperature of the preheated fuel leaking in the first duct (preferably between 320°C and 360°C, more preferably at 340°C).

[0026] Preferably, step d) is performed so that when the second stage is switched off, the cooling air also serves as purge air.

[0027] It should be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Other advantages and features of the present invention will be apparent from the following description, drawings and claims.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention which are believed to be novel are set forth with particularity in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0029]

[0013] Further benefits and advantages of the present invention will become apparent after careful reading of the detailed description, with appropriate reference to the accompanying drawings.

[0030] The invention itself, however, may best be understood by reference to the following detailed description of the invention, which, taken in conjunction with the accompanying drawings, describes exemplary embodiments of the invention, in which:

[0031] Figure 1 is a schematic view of an example of a GT assembly for a power plant suitable for being provided with the present invention;

[0032] Figure 2 is used for Figure 1 A schematic view of an example of a combustor section of a GT assembly;

[0033] Figure 3 The present invention is provided with an embodiment of Figure 2 Schematic view of an example of a fuel feed line of a combustor section. DETAILED DESCRIPTION

[0034] In conjunction with the accompanying drawings, the technical content and specific implementation of the present invention are described below according to the preferred embodiments, which are not intended to limit the scope of practice of the present invention. Any equivalent variations and modifications made according to the attached claims are covered by the claims claimed by the present invention.

[0035] The present invention will now be described in detail with reference to the accompanying drawings.

[0036] View now Figure 1 , which is a schematic view of an embodiment of a GT assembly for a power plant with sequential combustion suitable for being provided with the present invention. Figure 1A GT 1 is disclosed having a rotor 6 defining an axis 2 and comprising in series along a main flow M:

[0037] Compressor section 3,

[0038] a combustor section 4, in which the compressed air is mixed with fuel and this mixture is burned to generate a hot gas flow,

[0039] The turbine section 5 , in which the hot gas flow expands, performs rotational work on the rotor 6 .

[0040] according to Figure 1 , the rotating part of the rotor 6 driving the generator 7 downstream of the turbine 5. In addition, Figure 1 The GT 1 is a GT in which the burner section comprises in series a first combustion stage 9 and a second combustion stage 10, which are fed with fuel by a first fuel distribution line 11 and a second fuel distribution line 12, respectively. The first and second fuel distribution lines 11 and 12 are fed by a common fuel supply source 13. According to the invention, the fuel supply source 13 is a fuel source configured for conveying at least one fuel, namely FG, wherein this fuel is a preheated fuel, i.e. a fuel preheated to at least 300°C before being fed into the first and second fuel distribution lines 11 and 12. Figure 1 In FIG. 8 , reference numeral 8 refers to a heat exchanger, preferably an OTC, which is fed by the compressed air flowing out of the compressor C and is configured to cool this air and to supply this cooled air to the turbine 5 for cooling purposes. Figure 1 The GT 1 of the type disclosed in is particularly useful for implementation in conjunction with the present invention, i.e., minimal new components are added to the assembly.

[0041] View now Figure 2 , which is Figure 1 An enlarged view of an embodiment of a burner section 4 of FIG. Figure 2 A can burner 4 among a plurality of can burners 4 is disclosed, which are arranged in a ring around the turbine axis and form a burner section 4. In each can burner 4, two combustion stages are enclosed in series. Each can burner 4 is housed in a relative inlet opening of an outer casing 14, which defines a plenum 15, in which compressed air is delivered by a compressor 3. Each can burner 4 has an axis 16 and comprises a first burner 9 and a second burner 10 in series along a main flow M. In particular, the first burner 9 comprises a plurality of first burners 17, followed by a first combustion chamber 18. The second burner 10 comprises a second burner assembly 19, followed by a second combustion chamber 20. According to Figure 2In the embodiment of the present invention, the fuel is fed to the second burner 19 by a second fuel distribution line 12, which in this case extends outside the combustor casing 21. Downstream of the second combustion chamber 20, there is a transition duct 22 for directing the hot gases towards the turbine 5. Figure 2 The second fuel distribution line 12 may be implemented in the form of a circular collector configured to feed all of the can burner second stages 10. Figure 1 As disclosed in , the second fuel distribution line 12 is connected to a fuel source 13 which also feeds the first fuel distribution line 11 .

[0042] View now Figure 3 , which is Figure 1 and Figure 2 Schematic view of an example of a fuel feed system for a burner section of GT 1. As mentioned before, the fuel source 13 is configured for conveying preheated fuel (preferably from 230°C to 300°C) in a first fuel distribution line 11 and in a second fuel distribution line 12, which are configured for feeding this fuel to the first combustion stage 9 and the second combustion stage 10 of GT 1, respectively. Due to the high temperature of the fuel, each line 11, 12 of the fuel feed system is provided with at least one valve, preferably a metal seat valve. Figure 3 In the example disclosed in , the first fuel distribution line 11 and the second fuel distribution line 12 originating from a common pipeline 23 leading from a preheated fuel source 13 are provided with two shut-off valves 24 and one control valve 25 downstream of the shut-off valve 24 and upstream of the combustion stages 9 and 10. According to the above, there is a pipeline 26 between the control valve 25 and the second combustion stage 10, preferably in the form of a collector. In the case of the first stage only operating mode, the valves 24, 25 along the second fuel distribution line 12 are driven closed in order to avoid fuel feeding to the shut-down second combustion stage 10. However, since those valves 24, 25 are metal seat valves (soft sealing valves cannot be used due to the high fuel temperature), fuel leakage occurs and therefore some of the leaked fuel can accumulate in the pipeline 26 between the control valve 25 and the second combustion stage 10. In order to avoid any undesired ignition of this accumulated fuel in the pipeline 26 (in the case of hot gases being sucked from the combustion chamber, ignition can occur directly). According to a general definition of the invention, cooled air is fed to this duct 26 (i.e. air having a temperature below the auto-ignition temperature of the fuel leak, for example not exceeding 340° C.). As a result, two effects are produced. On the one hand, a lean mixture is produced, having a temperature below the flammable limit of the fuel, and on the other hand, a purge flow of the leaking fuel from the duct 26 is produced. According to Figure 3In the embodiment of the invention, the cooling air fed in the duct 26 is part of the cooling air drawn from the heat exchanger 8, which in turn is fed by compressed air and is configured to produce the cooling air to be supplied to the turbine 5. Since this heat exchanger 8 is already present in the GT assembly, it is only necessary to provide a duct 27 (which comprises at least a valve 28) connecting the outlet of the heat exchanger 8 to the duct 26 for the purpose of carrying out the invention. Preferably, this feeding of cooling air to the duct 27 can be carried out continuously during the first stage operating mode only.

[0043] Although the present invention has been explained with respect to its preferred embodiment(s) as mentioned above, it should be understood that many other possible modifications and variations may be made without departing from the scope of the present invention. Therefore, it is contemplated that the appended claim(s) will cover such modifications and variations that fall within the true scope of the present invention.

Claims

1. A gas turbine fed by preheated fuel gas for power plant applications, the gas turbine comprising: a compressor configured to compress incoming air; a sequential combustor assembly configured for feeding at least a fuel in the compressed air and producing a hot gas flow, wherein the combustor assembly includes a first combustion stage and a second combustion stage in series; a turbine configured to be driven by the hot gas flow to perform work on a rotor; a fuel source configured for supplying preheated fuel to a fuel gas distribution system, said fuel gas distribution system comprising a first line for feeding said first combustion stage and a second line for feeding said second combustion stage, at least said second line being provided with at least a metal valve, between said metal valve and said second combustion stage there being a first conduit in which leaking fuel gas can accumulate; Characterized in that the gas turbine further comprises a second duct configured to feed cooling air into the first duct, wherein the cooling air has a temperature lower than the auto-ignition temperature of the fuel gas; Wherein, the second conduit comprises at least a valve and the gas turbine further comprises: a control unit configured for operating the gas turbine in a first stage only mode, wherein the valve of the second line is driven to close, the control unit being configured for feeding the cooling air in the second duct by opening the valve of the second duct during operation in the first stage only mode; and Therein, the second line for feeding the second combustion stage comprises at least an upstream shut-off valve and at least a downstream control valve.

2. The gas turbine according to claim 1, characterized in that The control unit is configured to continuously feed the cooled air in the second duct during operation in the first stage-only mode.

3. The gas turbine according to claim 1, characterized in that The gas turbine further comprises: a heat exchanger configured for cooling compressed air flowing out of the compressor and for feeding this cooled air to the turbine for cooling purposes, the second duct being connected downstream to the outlet of the heat exchanger so that the cooling air fed in the second duct is a portion of the cooling air led out of the heat exchanger.

4. The gas turbine according to claim 3, characterized in that The heat exchanger is configured to cool the compressed air at a temperature between 320°C and 360°C.

5. The gas turbine according to claim 3, characterized in that The heat exchanger is configured to convey the cooled air at a pressure higher than a pressure of the leaking fuel accumulated in the first conduit so that the cooled air also serves as purge air.

6. The gas turbine according to claim 1, characterized in that The combustor assembly includes a plurality of can combustors, each can combustor including the first combustion stage and the second combustion stage in series.

7. The gas turbine according to claim 1, characterized in that The fuel source is configured to preheat the fuel to a temperature of up to 320°C.

8. A method for operating a gas turbine for a power plant having a sequential burner, the method comprising the steps of: a) providing a gas turbine with a sequential burner according to claim 1; b) preheating the fuel gas before feeding the fuel gas distribution system; c) controlling the gas turbine for operation in a first stage only mode, wherein a valve of the second line is driven closed; d) Feeding cooled air in said second conduit at a temperature below the auto-ignition temperature of preheated fuel leaking into said first conduit.

9. The method according to claim 8, characterized in that: Said step b) is carried out so that said fuel is preheated to a temperature of at most 320°C.

10. The method according to claim 8, characterized in that: Said step d) is performed so that said cooled air is compressed air flowing out of said compressor and cooled by a heat exchanger at a temperature between 320°C and 360°C.

11. The method according to claim 8, characterized in that Said step d) is performed continuously during operation in said first stage only mode.

12. The method according to claim 8, characterized in that Said step d) is performed so that said cooled air also serves as purge air, said purge air having a pressure higher than the pressure of the leaked fuel accumulated in said first conduit.

Images