Gas turbine unit

Abstract

Gas turbine unit comprising a compressor (2), a turbine (5) a combustor assembly (3) and a fuel supply assembly (4) configured to supply at least one first-stage fuel to the combustor assembly (3); the combustor assembly (3) comprising at least one combustor unit (7) provided with a liner (16) extending substantially along a combustor axis (B); the combustor unit (7) comprising a first-stage combustor (8) and a second-stage combustor (9), which is arranged downstream the first-stage combustor (8) along the gas flow direction (G); the first-stage combustor (8) comprising a first-stage burner (11), which is fed with first-stage air coming from the compressor (2) and first-stage fuel coming from the fuel supply assembly (4), and a first-stage combustion chamber (12), where the first-stage fuel is burned; the first-stage fuel being an ammonia-based fuel; wherein the second-stage combustor (9) comprises a second-stage burner (13) and a second-stage combustion chamber (14); the second-stage burner (13) being arranged downstream of the first-stage combustion chamber (12); the second-stage combustion chamber (14) being fed with hot gas leaving the first-stage combustion chamber (12) and passing through the second-stage burner (13) and with dilution air; the fuel supply assembly (4) being configured to supply the at least one first-stage fuel to the first-stage burner (11) so as the combustion stoichiometry of the first-stage burner (11) is unbalanced and an excess of first-stage fuel is present in the first stage burner (11) to obtain unburned first-stage fuel reaching the second-stage burner (13).

Description

TECHNICAL FIELD

[0001] The present invention relates to a gas turbine unit, in particular of a power plant. The invention also relates to a method for operating a gas turbine unit.DESCRIPTION OF PRIOR ART

[0002] As is known, a gas turbine unit for power plants comprises a compressor, a combustor assembly and a turbine.

[0003] In particular, the compressor comprises an inlet, supplied with air, and a plurality of blades compressing the passing air. The compressed air leaving the compressor flows into a plenum, i.e. a closed volume, and from there into the combustor assembly, where the compressed air is mixed with at least one fuel and combusted. The resulting hot gas leaves the combustor assembly and is expanded in the turbine, performing mechanical work.

[0004] Traditionally the fuel supplied to the combustor assembly of a gas turbine unit is natural gas or oil.

[0005] The market requires the gas turbine units to operate in the future with fuels different from natural gas or oil; in particular, gas turbine units should be able to correctly operate with high reactive fuels, such as for example hydrogen (H 2 ), mixtures containing hydrogen, or low reactive fuels such as Ammonia (NH 3 ), or mixtures of the above mentioned fuels or various types of liquid fuels.

[0006] Moreover, the combustor assemblies configured to operate with new fuels should be also able to respect the polluting emission limits.

[0007] In order to reduce these emissions and to increase operational flexibility, gas turbines have been developed which comprise a combustor assembly performing a sequential combustion cycle.

[0008] In general, a sequential combustor assembly comprises two combustors in series, wherein each combustor is provided with a respective burner and combustion chamber. Following the main gas flow direction, the upstream combustor is called "premix" combustor and is fed by the compressed air. The downstream combustor is called "sequential" or "reheat" combustor and is fed by the hot gas leaving the first combustion chamber. In addition both burners can be fed with fuel of the various types mentioned above.

[0009] The sequential combustor assemblies currently on the market are relatively cumbersome (in terms of axial length) and therefore expensive. The axial dimensions of these kinds of combustor assemblies can cause issues in terms of available space, for example, when retrofitting them into currently existing gas turbine units.

[0010] Furthermore, in general, a large structure has higher first costs and contains more parts, which need to be cooled, compared to a more compact structure.SUMMARY OF THE INVENTION

[0011] Therefore, it is a primary object of the present invention to provide a gas turbine unit which is efficient and cost effective and, at the same time, is able to operate also with different fuels, and in particular with ammonia-based fuels, without affecting the reliability of the combustor unit and guaranteeing polluting emissions under law limits

[0012] This object is attained, according to the present invention, by a gas turbine unit as claimed in claim 1.

[0013] A further aim of the present invention is to provide a method for operating a gas turbine unit with different fuels, and in particular with ammonia-based fuels. According to said object the present invention relates to a method for operating a gas turbine unit as claimed in claim 9.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described with reference to the accompanying drawings, which illustrate some non-limitative embodiment, in which: figure 1 is a schematic view, with parts removed for clarity, of a gas turbine unit provided with a combustor assembly according to the present invention; figure 2 is a schematic lateral section view, with parts removed for clarity, of a combustor assembly according to the invention; figure 3 is a perspective schematic view of a first detail of the combustor assembly of figure 2; figure 4 is a perspective schematic view of a second detail of the combustor assembly of figure 2; figure 5 is a schematic lateral section view, with parts removed for clarity, of a combustor assembly according to a variant of the present invention; figure 6 is a perspective schematic view of a first detail of the combustor assembly of figure 5; figure 7 is a perspective schematic view of the first detail of the combustor assembly according to a variant. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0015] Figure 1 is a schematic view of a gas turbine unit 1 for power plants according to the present invention.

[0016] Gas turbine unit 1 comprises a compressor 2, a combustor assembly 3, a fuel supply assembly 4 and a turbine 5. The compressor 2 and the turbine 5 extend along a main axis A.

[0017] In use, an airflow compressed in the compressor 2 is mixed with fuel and is burned in the combustor assembly 3. Fuel is supplied to the combustor assembly 3 by the fuel supply assembly 4 (only partially visible in figure 1 and 2). The burned mixture is then expanded in the turbine 5 and converted in mechanical power by a shaft 6, which is connected to a generator (not shown).

[0018] The combustor assembly 3 is a sequential combustor assembly and comprises a plurality of units 7 (only one being represented in figure 1). Each unit 7 comprises a first-stage combustor 8 and a second-stage combustor 9 sequentially arranged along the gas flow direction G. In other words, the second-stage combustor 9 is arranged downstream the first-stage combustor 8 along the gas flow direction G.

[0019] The terms downstream and upstream as used herein refer to the direction G of the main gas flow passing through the gas turbine.

[0020] The first-stage combustor 8 comprises: a first-stage burner 11, fed with first-stage fuel by the fuel supply assembly 4 and first-stage air, and a first-stage combustion chamber 12, where the first-stage fuel is burned.

[0021] The second-stage combustor 9 comprises: a second-stage burner 13 arranged downstream of the first-stage combustion chamber 12 and, preferably, at the outlet of the first-stage combustion chamber 12; a second-stage combustion chamber 14 fed with hot gas leaving the first-stage combustion chamber 12, with dilution air and, optionally, with a second-stage fuel coming from the fuel supply assembly 4.

[0022] In the example here disclosed and illustrated each unit 7 extends along a respective axis B (i.e. first-stage combustor 8 and second-stage combustor 9 extend along the same axis B). However, according to variant not shown the first-stage combustor and the second-stage combustor can be not aligned along a single axis. The second stage combustor 14 could be of can or can-annular type.

[0023] With reference to figure 2, the second-stage burner 13 comprises a mixing device 15, which is preferably a swirler (in figure 2 the swirler 15 is sectioned along an axial plane). The dilution air and the second-stage fuel are injected at the mixing device 15 of the second-stage burner 13.

[0024] In this way, the second-stage combustion chamber 14 is supplied with dilution air, second-stage fuel and hot gas leaving the first-stage combustion chamber 12 and passing through the mixing device 15.

[0025] The mixing device 15 is positioned downstream of the outlet of the first-stage combustion chamber 12 and there is no injection of air and / or fuel between the first-stage combustion chamber 12 and the second-stage burner 13. The expression "no air injection" means that air with a combustion role is not injected. Air with a cooling role could possibly be injected between the first stage combustion chamber 12 and the second stage burner 13.

[0026] In other words, there is no mixer or mixing stage between the first-stage combustor 8 and the second-stage combustor 9. The combustor assembly 3 comprises a liner 16 which extends along the longitudinal axis B and comprises a basically rotational symmetric or even cylindrical portion 17a, substantially defining the first-stage combustion chamber 12, a converging portion 17b defining the second-stage burner 13 and an inlet portion of a mixing zone 18 and a diverging portion 17c an end portion of the mixing zone which is followed by the second-stage combustion chamber 14. The mixing zone 18 is therefore preferably defined by a narrowing which is followed by an expansion facing into the second combustion chamber 14.

[0027] The narrowing has the effect of accelerating the passage of the burned flow in order to enhance mixing and to avoid flashback, while the expansion has the effect of reducing pressure loss.

[0028] Advantageously, the second-stage burner 13 is arranged in a large cross-sectional area. This allows to keep the pressure losses of the hot gas leaving the first-stage combustion chamber 12 low. Moreover, as will be detailed in the following, the positioning of the second-stage burner 13 in the largest area of the converging portion 17b also gives enough design space for the structure of the mixing device 15 and for the injection of dilution air and second-stage fuel, which are injected at the mixing device 15.

[0029] In this way the dilution air, the second-stage fuel and the hot gas leaving the first-stage combustion chamber are mixed simultaneously in the mixing zone 18 downstream of the second-stage combustor 13.

[0030] The combustor assembly 3 also preferably comprises an axial symmetric or even cylindrical central body 19 and an outer casing 20.

[0031] The central body 19 is preferably arranged along the axis B and extends along the combustor assembly 3 across the first-stage combustor 8 and at least a portion of the second-stage combustor 9.

[0032] In particular, the central body 19 preferably extends along the second-stage burner 13 and along the mixing zone 18 until reaching the second-stage combustion chamber 14.

[0033] The central body 19 is supplied at least with the second-stage fuel.

[0034] Preferably, the central body 19 comprises also a cooling system. The cooling system could be realized by means of an annular conduit 23a surrounding a central second stage fuel conduit 23b. The cooling air circulating in the annular conduit 23a may be discharged, for example as film cooling, at the outer surface and / or at the terminal face of the central body 19 facing into the second-stage combustion chamber 14.

[0035] According to a variant not shown, the downstream part of the central body 19 can house a Helmholtz damper, in order to counteract thermoacoustic instabilities.

[0036] According to a variant not shown, the central body is not cylindrical, but is shaped in a way that the cross section of the mixing zone 18 is reducing along the axis B. Preferably, also in this configuration the central body is opened so as to the discharge air in the second-stage combustion chamber 14.

[0037] The outer casing 20 surrounds at least a portion of the liner 16 for defining an annular chamber 22 around the liner 16. Said annular chamber 22 being supplied with second-stage air (from now on called "dilution air"). The first-stage air and the dilution air are preferably coming from a plenum (not shown in the attached drawings). As is known, ambient air enters compressor 2 and is compressed. Compressed air leaves compressor 2 and enters the plenum, which is a volume defined by a plenum casing (not shown).

[0038] In particular, the outer casing 20 surrounds the portion 17a of the liner 16, substantially defining the first-stage combustion chamber 12, and the converging portion 17b of the liner 16. Cooling channels (not shown) could be implemented in the liner portion 17c and fed from the said plenum.

[0039] With reference to figure 2, the first-stage burner 11 of the first-stage combustor 8 is represented schematically by a box.

[0040] As anticipated, the second-stage burner 13 comprises a mixing device 15, which comprises a swirler.

[0041] Dilution air and second-stage fuel are fed at the swirler 15 and preferably through the swirler 15.

[0042] With reference to figures 2-4, the swirler 15 comprises a plurality of hollow struts 24, which are circumferentially spaced one another around the axis B. Each strut 24 extends radially from the central cylindrical body 19 to the outer casing 20. The radial direction r along which each strut extends is defined starting from the combustor axis B.

[0043] As will be detailed in the following, the mixing device 15 is configured in such a way that the second-stage fuel flow is substantially embedded in the dilution air flow and thereby initially separated from the hot gas coming from the first-stage combustion chamber 12.

[0044] Each strut 24 comprises a wall 25 shaped to define a leading edge 26 (better visible in figure 2) which faces the first-stage combustion chamber 12 and a trailing edge 27 which is arranged opposite to the leading edge 26 along the combustor axis B and faces the second-stage combustion chamber 14.

[0045] The trailing edge 27 is arranged downstream of the leading edge 26 considering the gas flow direction G.

[0046] The outer wall 25 is also shaped to define two sides 28a 28b which are respectively comprised between the leading edge 26 and the trailing edge 27.

[0047] The trailing edge 27 comprises an outlet opening 29. Preferably, the outlet opening 29 extends for the entire radial height of the trailing edge 27 and, more preferably, for the entire circumferential length of the trailing edge 27.

[0048] Inside each strut 24 a cavity 30 is defined.

[0049] The sides 28a and 28b are respectively shaped so as to define an outer curved surface.

[0050] Preferably, each side 28a 28b is shaped so as to defines an outer curved surface defining substantially one lobe.

[0051] The side 28a has an outer convex shape, while the side 29a has an outer concave shape, or vice versa. In the example here illustrated sides 28a and 28b are shaped so as to define one concavity. According to a variant, the sides 28a and 28b can be shaped so as to define more than one concavity.

[0052] Preferably, the sides 28a and 28b are shaped so as to be curved also internally.

[0053] Preferably the sides 28a and 28b extend substantially radially so as the distance between the sides 28a and 28b increases radially. In this way, more cooling air is ejected at larger radii compared to smaller radii. In this way the larger amount of hot gas coming at larger radii is compensated to obtain a homogenous mixing between all three streams (hot gas - dilution air - second stage fuel).

[0054] With reference to figure 2, showing a portion of the swirler 15 along an axial sectional view, the sides 28a and 28b are shaped to preferably form a section of a circle, when looking in lateral direction on the trailing edge 27 to compensate for the converging liner 16 and ensure a correct mixing of the dilution air with the hot gas coming from the first-stage combustor 8.

[0055] In other words, the sides 28a and 28b are shaped so as to form, at the trailing edge 27, preferably a right angle α with the liner 16 and preferably a right angle β with the central cylindrical body 19.

[0056] With reference to figures 3 and 4, the cavity 30 of each strut 24 houses a respective second-stage fuel injection unit 32 which is fed with the second-stage fuel. Preferably the second-stage fuel is fed to the second-stage fuel injection unit 32 through the central cylindrical body 19. In other words, the second-stage fuel injection unit 32 is fluidly connected to central cylindrical body 19 and specifically to the central second stage fuel conduit 23b.

[0057] According to a variant, only some of the plurality of struts are housing respective second-stage fuel injection units. Referring to figure 4, the second-stage fuel injection unit 32 comprises a finger 35 and a plurality of nozzles 36 extending from the finger 35.

[0058] The finger 35 is hollow and is fluidly connected to the central body 19 (specifically to the central second stage fuel conduit 23b). Preferably, the finger 35 is radially arranged and is located in the cavity 30 substantially equidistant from the sides 28a and 28b.

[0059] The nozzles 36 extends transversally from the finger 35 and are supplied with the second-stage fuel through the finger 35.

[0060] Each nozzle 36 has an inlet 37 at the finger 35 and an outlet 38. The axial length (intended as the length measured along the combustor axis B) of each nozzle 36 is defined so as the outlet 38 of each nozzle 36 is arranged at the outlet opening 29 of the respective strut 24.

[0061] According to a variant not shown the outlet 38 of each nozzle 36 is arranged upstream of the outlet opening 29.

[0062] Preferably, looking in lateral direction, the nozzles 36 are arranged so as the second-stage fuel ejects in a direction perpendicular to the trailing edge 27. In this way the second-stage fuel would not impinge against the inner wall of the liner 16 in the mixing zone 18.

[0063] The inlets 37 of the nozzles 36 are aligned along a substantially radial direction on a face of the finger facing the outlet opening 29.

[0064] The nozzles 36 can have the same diameter or they can have different diameters as illustrated in the example of figure 4.

[0065] The diameters of the nozzles 36 can be differentiated in order to generate a desired fuel mixture fraction distribution at the outlet opening 29.

[0066] In the non-limiting example here illustrated, the nozzles 36 arranged in the middle area have the minimum diameter. Furthermore, these nozzles 36 could be equidistant along the trailing edge of the fuel injector 39 or with varying distance.

[0067] The nozzles 36 are preferably connected one another to create substantially a "nozzle wall 39" which has a wavy shape.

[0068] Preferably, the nozzle wall 39 is shaped so as to be substantially radially arranged and having a wavy shape similar to the wavy shape of the sides 28a and 28b.

[0069] The fuel nozzles 36 of the nozzle wall 39 extend preferably along a rectilinear path which is inclined in all three directions to follow the shape of the sides 28a 28b.

[0070] The wavy shape of the inner faces and the outer faces of the sides 28a and 28b and the wavy shape of the nozzle wall 39 comprising the nozzles 36 has the effect of improving the mixing of the dilution air, the second-stage fuel and the hot gas coming from the first-stage combustion chamber 12.

[0071] The lobed profiles of the sides 28a and 28b and of nozzle wall 39, in fact, has the effect of creating the vortexes required to mix the different flows.

[0072] The cavity 30 of each strut 24 is fed with dilution air coming from the annular chamber 22 around the liner 16.

[0073] With reference to figure 3, the liner 16 is preferably provided with a plurality of openings 40 each of which connects the respective cavity 30 with the annular chamber 22.

[0074] Preferably, each opening 40 is associated to at least one baffle 42 which is configured, as will be detailed better in the following, to guide the flow of air inside the cavity 30. The aim of the baffle 42 is mainly to ensure the cooling of the leading edge 26.

[0075] In particular, the baffle 42 is configured to guide the flow of air behind the second-stage fuel injection unit 32 (see also figure 2). In this way the dilution air exits the outlet opening 29 together with the second-stage fuel exiting the second-stage fuel injection unit 32, and, in particular, the dilution air flow surrounds the second-stage fuel flow coming from the injection unit 32. In this way, the fuel rich regions at the exit of the second-stage burner 13 have an average amount of dilution air.

[0076] According to a variant not shown, each opening can be associated to more than one baffle.

[0077] To avoid the direct contact of undiluted first-stage hot gas with the second stage fuel, which likely would cause flashback, the dilution air is supplied such that the dilution air flow is placed between the hot gas stream and the second-stage fuel stream.

[0078] Consequently, the second-stage flame temperature peaks, which would occur due to imperfect mixing of the fuel, are at least partly compensated by additional dilution air in such locations. Thereby, in the second-stage combustion chamber 14, even in relatively fuel rich zones the local flame temperature can be relatively low, and therefore the amount of NO x emissions is low.

[0079] The dilution air is therefore injected in axial direction as it exits from the outlet opening 29 at the trailing edge 27. In this way the dynamic part of the total pressure is not dissipated as it normally occurs in classical prior art mixing stages arranged upstream of the second-stage burner. Therefore, the pressure loss of the combustor assembly according to the present invention is reduced compared to a classical prior art combustor assembly.

[0080] Contrary to the most common dilution air injection systems, which are generally designed to inject the dilution air into the hot gas in a "jet-in-crossflow" manner between the first stage combustor and the second stage burner, the solution according to the present invention is designed to inject the dilution air through the struts 24, which allow a parallel injection of the dilution air into the hot gas path. Mainly, the angle between the main flow direction of the hot air coming from the first stage combustor 8 and the dilution air flow at the outlet opening 29 of the struts 24 is zero. Only some local mixing due to the swirl shape of the struts 24 can create some vortices leading to mixing in the downstream mixing zone 18.

[0081] In this way the dilution air, which is a considerable quantity of air compared to the air coming from the first-stage combustor 8, keeps the highly reactive fuel in the core of the dilution air cold for a longer time and allows it to mix with the dilution air until it sees the hot air from the first stage combustor 8, which leads to the chemical reaction. Advantageously the shape of the swirler 15, and in particular the shape of each strut 24, allows a correct mixing of the hot gas flow coming from the first-stage combustor 8 with the dilution air and second-stage fuel fed through the swirler 15.

[0082] In addition, a correct dimensioning of the circumferential distance between each strut 24 can ensure that the vortexes created by each of the strut 24 can combine better with those from the neighbouring lobed strut 24. This allows to apply the claimed solution also in small burners

[0083] Furthermore, the features of second-stage combustor 9 improve the shielding of the second-stage fuel flow. In this way the amount of second-stage fuel nozzles 36 can be increased, providing a superior mixing of the fuel with the other flows.

[0084] The combustor assembly of the present invention allows moreover to inject at the first-stage combustor 8 most of the kind of fuels, even ammonia or hydrogen-based fuels. While in the classic combustor assemblies this kind of fuels would create a major issue due to the high temperature reached at the second-stage burner inlet, the present invention circumvents this issue, since the dilution air is injected at the second-stage burner 13, thus inducing a temperature increase further downstream, mostly in the second stage combustion chamber 14.

[0085] Finally, the combustor assembly 3 of the present invention has a reduced axial length.

[0086] Figures 5-7 shows a variant off the present invention wherein the same reference numbers used in figures 1-4 are used for indicating identical or similar parts.

[0087] According to the variant shown in figures 5-7, when the fuel supply assembly 4 supplies ammonia-based fuel as first-stage fuel, the second-stage fuel is not supplied to the second-stage burner 13 and only dilution air is supplied to the second stage burner 13.

[0088] In figures 5-7, in fact, the second stage fuel supply assembly (comprising the second stage fuel injection units 32) is not present.

[0089] In particular, in this case, the fuel supply assembly 4 supplies ammonia-based fuel in excess with respect to first-stage air coming from the compressor 2. In this way, in the first-stage combustion chamber 12 burns a mixture of fuel / air which is unbalanced (i.e. rich in fuel). In this way unburned ammonia-based fuel reaches the inlet of the second-stage burner 13.

[0090] In other words, the quantity ammonia-based fuel supplied by the fuel supply assembly 4 to the first stage burner 11 is regulated so as the combustion stoichiometry of the first-stage burner 11 is unbalanced on fuel.

[0091] Since there will be more ammonia-based fuel compared to the available amount of oxygen (rich combustion), the ammonia-based fuel cannot burn completely within the first stage combustion chamber 12. In this case, NH 3 of the ammonia-based fuel will be either burned (4NH 3 + 3O 2 -> 6H 2 O + 2N 2 ) or cracked (2NH 3 -> N 2 + 3H 2 ).

[0092] The dilution air injected at the second-stage burner 13 will allow to burn the rest of the ammonia-based fuel (now in the form of hydrogen H 2 , which was generated within the first-stage combustion chamber 12).

[0093] By doing so the amount of NOx can be drastically reduced, compared to a simple burning of ammonia-based fuels.

[0094] The regulation of the amount of first-stage air fed to the first-stage burner 11 and of the amount of dilution air fed to the second-stage burner 13 is preferably defined by the geometrical features defined in the design step and, therefore, basically constant over the load and not actively controlled.

[0095] According to a variant not shown, also the amount of air supplied to the first-stage burner 11 and the second stage burner 13 can be actively controlled, for example taking into account the relative load of the engine, the fuel composition, the ambient conditions (ambient temperature and humidity).

[0096] As already disclosed, also when the fuel supply assembly 4 supplies ammonia-based fuel as first-stage fuel, dilution air is supplied through the mixing device 15 already detailed.

[0097] In particular, the mixing device 15 represented in figures 5 and 6 has the same structure detailed in the embodiment of figures 1-4, but it does not comprise the injection units for the supply of the second-stage fuel.

[0098] In figure 7 it is illustrated a variant of the mixing device 15 of figures 5 and 6, wherein a mixing element 132 is arranged in the cavity 30 of at least one strut 24.

[0099] The mixing element 132 is shaped so as to define a proper velocity profile of the dilution air at the outlet opening 29.

[0100] In the non-limiting example here illustrated, the mixing element has a shape substantially similar to the one of the injection unit 32 of figure 3 and 4.

[0101] The alternative solution disclosed with reference to figure 5-7 are therefore particularly focused on ammonia-based fuels.

[0102] However, the solution disclosed in figure 1-4 could be also used in case of ammonia-based fuels, with the precaution of setting the fuel supply assembly 4 to supplies ammonia-based fuel as first-stage fuel and not to supply second-stage fuel. Only dilution air is supplied to the second stage burner 13.

[0103] The solution disclosed in figures 1-4 could be considered more flexible in the operation of the combustor assembly 3, as fuels different from the ammonia-based ones can be supplied to the combustor unit 7 without any required hardware modification.

[0104] Finally, modifications and variants can be made to the assembly described herein without departing from the scope of the present invention, as defined in the appended claims.

Examples

Embodiment Construction

[0015]Figure 1 is a schematic view of a gas turbine unit 1 for power plants according to the present invention.

[0016]Gas turbine unit 1 comprises a compressor 2, a combustor assembly 3, a fuel supply assembly 4 and a turbine 5. The compressor 2 and the turbine 5 extend along a main axis A.

[0017]In use, an airflow compressed in the compressor 2 is mixed with fuel and is burned in the combustor assembly 3. Fuel is supplied to the combustor assembly 3 by the fuel supply assembly 4 (only partially visible in figure 1 and 2). The burned mixture is then expanded in the turbine 5 and converted in mechanical power by a shaft 6, which is connected to a generator (not shown).

[0018]The combustor assembly 3 is a sequential combustor assembly and comprises a plurality of units 7 (only one being represented in figure 1). Each unit 7 comprises a first-stage combustor 8 and a second-stage combustor 9 sequentially arranged along the gas flow direction G. In other words, the second-stage combustor 9 ...

TECHNICAL FIELD

[0001] The present invention relates to a gas turbine unit, in particular of a power plant. The invention also relates to a method for operating a gas turbine unit.DESCRIPTION OF PRIOR ART

[0002] As is known, a gas turbine unit for power plants comprises a compressor, a combustor assembly and a turbine.

[0003] In particular, the compressor comprises an inlet, supplied with air, and a plurality of blades compressing the passing air. The compressed air leaving the compressor flows into a plenum, i.e. a closed volume, and from there into the combustor assembly, where the compressed air is mixed with at least one fuel and combusted. The resulting hot gas leaves the combustor assembly and is expanded in the turbine, performing mechanical work.

[0004] Traditionally the fuel supplied to the combustor assembly of a gas turbine unit is natural gas or oil.

[0005] The market requires the gas turbine units to operate in the future with fuels different from natural gas or oil; in particular, gas turbine units should be able to correctly operate with high reactive fuels, such as for example hydrogen (H 2 ), mixtures containing hydrogen, or low reactive fuels such as Ammonia (NH 3 ), or mixtures of the above mentioned fuels or various types of liquid fuels.

[0006] Moreover, the combustor assemblies configured to operate with new fuels should be also able to respect the polluting emission limits.

[0007] In order to reduce these emissions and to increase operational flexibility, gas turbines have been developed which comprise a combustor assembly performing a sequential combustion cycle.

[0008] In general, a sequential combustor assembly comprises two combustors in series, wherein each combustor is provided with a respective burner and combustion chamber. Following the main gas flow direction, the upstream combustor is called "premix" combustor and is fed by the compressed air. The downstream combustor is called "sequential" or "reheat" combustor and is fed by the hot gas leaving the first combustion chamber. In addition both burners can be fed with fuel of the various types mentioned above.

[0009] The sequential combustor assemblies currently on the market are relatively cumbersome (in terms of axial length) and therefore expensive. The axial dimensions of these kinds of combustor assemblies can cause issues in terms of available space, for example, when retrofitting them into currently existing gas turbine units.

[0010] Furthermore, in general, a large structure has higher first costs and contains more parts, which need to be cooled, compared to a more compact structure.SUMMARY OF THE INVENTION

[0011] Therefore, it is a primary object of the present invention to provide a gas turbine unit which is efficient and cost effective and, at the same time, is able to operate also with different fuels, and in particular with ammonia-based fuels, without affecting the reliability of the combustor unit and guaranteeing polluting emissions under law limits

[0012] This object is attained, according to the present invention, by a gas turbine unit as claimed in claim 1.

[0013] A further aim of the present invention is to provide a method for operating a gas turbine unit with different fuels, and in particular with ammonia-based fuels. According to said object the present invention relates to a method for operating a gas turbine unit as claimed in claim 9.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described with reference to the accompanying drawings, which illustrate some non-limitative embodiment, in which: figure 1 is a schematic view, with parts removed for clarity, of a gas turbine unit provided with a combustor assembly according to the present invention; figure 2 is a schematic lateral section view, with parts removed for clarity, of a combustor assembly according to the invention; figure 3 is a perspective schematic view of a first detail of the combustor assembly of figure 2; figure 4 is a perspective schematic view of a second detail of the combustor assembly of figure 2; figure 5 is a schematic lateral section view, with parts removed for clarity, of a combustor assembly according to a variant of the present invention; figure 6 is a perspective schematic view of a first detail of the combustor assembly of figure 5; figure 7 is a perspective schematic view of the first detail of the combustor assembly according to a variant. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0015] Figure 1 is a schematic view of a gas turbine unit 1 for power plants according to the present invention.

[0016] Gas turbine unit 1 comprises a compressor 2, a combustor assembly 3, a fuel supply assembly 4 and a turbine 5. The compressor 2 and the turbine 5 extend along a main axis A.

[0017] In use, an airflow compressed in the compressor 2 is mixed with fuel and is burned in the combustor assembly 3. Fuel is supplied to the combustor assembly 3 by the fuel supply assembly 4 (only partially visible in figure 1 and 2). The burned mixture is then expanded in the turbine 5 and converted in mechanical power by a shaft 6, which is connected to a generator (not shown).

[0018] The combustor assembly 3 is a sequential combustor assembly and comprises a plurality of units 7 (only one being represented in figure 1). Each unit 7 comprises a first-stage combustor 8 and a second-stage combustor 9 sequentially arranged along the gas flow direction G. In other words, the second-stage combustor 9 is arranged downstream the first-stage combustor 8 along the gas flow direction G.

[0019] The terms downstream and upstream as used herein refer to the direction G of the main gas flow passing through the gas turbine.

[0020] The first-stage combustor 8 comprises: a first-stage burner 11, fed with first-stage fuel by the fuel supply assembly 4 and first-stage air, and a first-stage combustion chamber 12, where the first-stage fuel is burned.

[0021] The second-stage combustor 9 comprises: a second-stage burner 13 arranged downstream of the first-stage combustion chamber 12 and, preferably, at the outlet of the first-stage combustion chamber 12; a second-stage combustion chamber 14 fed with hot gas leaving the first-stage combustion chamber 12, with dilution air and, optionally, with a second-stage fuel coming from the fuel supply assembly 4.

[0022] In the example here disclosed and illustrated each unit 7 extends along a respective axis B (i.e. first-stage combustor 8 and second-stage combustor 9 extend along the same axis B). However, according to variant not shown the first-stage combustor and the second-stage combustor can be not aligned along a single axis. The second stage combustor 14 could be of can or can-annular type.

[0023] With reference to figure 2, the second-stage burner 13 comprises a mixing device 15, which is preferably a swirler (in figure 2 the swirler 15 is sectioned along an axial plane). The dilution air and the second-stage fuel are injected at the mixing device 15 of the second-stage burner 13.

[0024] In this way, the second-stage combustion chamber 14 is supplied with dilution air, second-stage fuel and hot gas leaving the first-stage combustion chamber 12 and passing through the mixing device 15.

[0025] The mixing device 15 is positioned downstream of the outlet of the first-stage combustion chamber 12 and there is no injection of air and / or fuel between the first-stage combustion chamber 12 and the second-stage burner 13. The expression "no air injection" means that air with a combustion role is not injected. Air with a cooling role could possibly be injected between the first stage combustion chamber 12 and the second stage burner 13.

[0026] In other words, there is no mixer or mixing stage between the first-stage combustor 8 and the second-stage combustor 9. The combustor assembly 3 comprises a liner 16 which extends along the longitudinal axis B and comprises a basically rotational symmetric or even cylindrical portion 17a, substantially defining the first-stage combustion chamber 12, a converging portion 17b defining the second-stage burner 13 and an inlet portion of a mixing zone 18 and a diverging portion 17c an end portion of the mixing zone which is followed by the second-stage combustion chamber 14. The mixing zone 18 is therefore preferably defined by a narrowing which is followed by an expansion facing into the second combustion chamber 14.

[0027] The narrowing has the effect of accelerating the passage of the burned flow in order to enhance mixing and to avoid flashback, while the expansion has the effect of reducing pressure loss.

[0028] Advantageously, the second-stage burner 13 is arranged in a large cross-sectional area. This allows to keep the pressure losses of the hot gas leaving the first-stage combustion chamber 12 low. Moreover, as will be detailed in the following, the positioning of the second-stage burner 13 in the largest area of the converging portion 17b also gives enough design space for the structure of the mixing device 15 and for the injection of dilution air and second-stage fuel, which are injected at the mixing device 15.

[0029] In this way the dilution air, the second-stage fuel and the hot gas leaving the first-stage combustion chamber are mixed simultaneously in the mixing zone 18 downstream of the second-stage combustor 13.

[0030] The combustor assembly 3 also preferably comprises an axial symmetric or even cylindrical central body 19 and an outer casing 20.

[0031] The central body 19 is preferably arranged along the axis B and extends along the combustor assembly 3 across the first-stage combustor 8 and at least a portion of the second-stage combustor 9.

[0032] In particular, the central body 19 preferably extends along the second-stage burner 13 and along the mixing zone 18 until reaching the second-stage combustion chamber 14.

[0033] The central body 19 is supplied at least with the second-stage fuel.

[0034] Preferably, the central body 19 comprises also a cooling system. The cooling system could be realized by means of an annular conduit 23a surrounding a central second stage fuel conduit 23b. The cooling air circulating in the annular conduit 23a may be discharged, for example as film cooling, at the outer surface and / or at the terminal face of the central body 19 facing into the second-stage combustion chamber 14.

[0035] According to a variant not shown, the downstream part of the central body 19 can house a Helmholtz damper, in order to counteract thermoacoustic instabilities.

[0036] According to a variant not shown, the central body is not cylindrical, but is shaped in a way that the cross section of the mixing zone 18 is reducing along the axis B. Preferably, also in this configuration the central body is opened so as to the discharge air in the second-stage combustion chamber 14.

[0037] The outer casing 20 surrounds at least a portion of the liner 16 for defining an annular chamber 22 around the liner 16. Said annular chamber 22 being supplied with second-stage air (from now on called "dilution air"). The first-stage air and the dilution air are preferably coming from a plenum (not shown in the attached drawings). As is known, ambient air enters compressor 2 and is compressed. Compressed air leaves compressor 2 and enters the plenum, which is a volume defined by a plenum casing (not shown).

[0038] In particular, the outer casing 20 surrounds the portion 17a of the liner 16, substantially defining the first-stage combustion chamber 12, and the converging portion 17b of the liner 16. Cooling channels (not shown) could be implemented in the liner portion 17c and fed from the said plenum.

[0039] With reference to figure 2, the first-stage burner 11 of the first-stage combustor 8 is represented schematically by a box.

[0040] As anticipated, the second-stage burner 13 comprises a mixing device 15, which comprises a swirler.

[0041] Dilution air and second-stage fuel are fed at the swirler 15 and preferably through the swirler 15.

[0042] With reference to figures 2-4, the swirler 15 comprises a plurality of hollow struts 24, which are circumferentially spaced one another around the axis B. Each strut 24 extends radially from the central cylindrical body 19 to the outer casing 20. The radial direction r along which each strut extends is defined starting from the combustor axis B.

[0043] As will be detailed in the following, the mixing device 15 is configured in such a way that the second-stage fuel flow is substantially embedded in the dilution air flow and thereby initially separated from the hot gas coming from the first-stage combustion chamber 12.

[0044] Each strut 24 comprises a wall 25 shaped to define a leading edge 26 (better visible in figure 2) which faces the first-stage combustion chamber 12 and a trailing edge 27 which is arranged opposite to the leading edge 26 along the combustor axis B and faces the second-stage combustion chamber 14.

[0045] The trailing edge 27 is arranged downstream of the leading edge 26 considering the gas flow direction G.

[0046] The outer wall 25 is also shaped to define two sides 28a 28b which are respectively comprised between the leading edge 26 and the trailing edge 27.

[0047] The trailing edge 27 comprises an outlet opening 29. Preferably, the outlet opening 29 extends for the entire radial height of the trailing edge 27 and, more preferably, for the entire circumferential length of the trailing edge 27.

[0048] Inside each strut 24 a cavity 30 is defined.

[0049] The sides 28a and 28b are respectively shaped so as to define an outer curved surface.

[0050] Preferably, each side 28a 28b is shaped so as to defines an outer curved surface defining substantially one lobe.

[0051] The side 28a has an outer convex shape, while the side 29a has an outer concave shape, or vice versa. In the example here illustrated sides 28a and 28b are shaped so as to define one concavity. According to a variant, the sides 28a and 28b can be shaped so as to define more than one concavity.

[0052] Preferably, the sides 28a and 28b are shaped so as to be curved also internally.

[0053] Preferably the sides 28a and 28b extend substantially radially so as the distance between the sides 28a and 28b increases radially. In this way, more cooling air is ejected at larger radii compared to smaller radii. In this way the larger amount of hot gas coming at larger radii is compensated to obtain a homogenous mixing between all three streams (hot gas - dilution air - second stage fuel).

[0054] With reference to figure 2, showing a portion of the swirler 15 along an axial sectional view, the sides 28a and 28b are shaped to preferably form a section of a circle, when looking in lateral direction on the trailing edge 27 to compensate for the converging liner 16 and ensure a correct mixing of the dilution air with the hot gas coming from the first-stage combustor 8.

[0055] In other words, the sides 28a and 28b are shaped so as to form, at the trailing edge 27, preferably a right angle α with the liner 16 and preferably a right angle β with the central cylindrical body 19.

[0056] With reference to figures 3 and 4, the cavity 30 of each strut 24 houses a respective second-stage fuel injection unit 32 which is fed with the second-stage fuel. Preferably the second-stage fuel is fed to the second-stage fuel injection unit 32 through the central cylindrical body 19. In other words, the second-stage fuel injection unit 32 is fluidly connected to central cylindrical body 19 and specifically to the central second stage fuel conduit 23b.

[0057] According to a variant, only some of the plurality of struts are housing respective second-stage fuel injection units. Referring to figure 4, the second-stage fuel injection unit 32 comprises a finger 35 and a plurality of nozzles 36 extending from the finger 35.

[0058] The finger 35 is hollow and is fluidly connected to the central body 19 (specifically to the central second stage fuel conduit 23b). Preferably, the finger 35 is radially arranged and is located in the cavity 30 substantially equidistant from the sides 28a and 28b.

[0059] The nozzles 36 extends transversally from the finger 35 and are supplied with the second-stage fuel through the finger 35.

[0060] Each nozzle 36 has an inlet 37 at the finger 35 and an outlet 38. The axial length (intended as the length measured along the combustor axis B) of each nozzle 36 is defined so as the outlet 38 of each nozzle 36 is arranged at the outlet opening 29 of the respective strut 24.

[0061] According to a variant not shown the outlet 38 of each nozzle 36 is arranged upstream of the outlet opening 29.

[0062] Preferably, looking in lateral direction, the nozzles 36 are arranged so as the second-stage fuel ejects in a direction perpendicular to the trailing edge 27. In this way the second-stage fuel would not impinge against the inner wall of the liner 16 in the mixing zone 18.

[0063] The inlets 37 of the nozzles 36 are aligned along a substantially radial direction on a face of the finger facing the outlet opening 29.

[0064] The nozzles 36 can have the same diameter or they can have different diameters as illustrated in the example of figure 4.

[0065] The diameters of the nozzles 36 can be differentiated in order to generate a desired fuel mixture fraction distribution at the outlet opening 29.

[0066] In the non-limiting example here illustrated, the nozzles 36 arranged in the middle area have the minimum diameter. Furthermore, these nozzles 36 could be equidistant along the trailing edge of the fuel injector 39 or with varying distance.

[0067] The nozzles 36 are preferably connected one another to create substantially a "nozzle wall 39" which has a wavy shape.

[0068] Preferably, the nozzle wall 39 is shaped so as to be substantially radially arranged and having a wavy shape similar to the wavy shape of the sides 28a and 28b.

[0069] The fuel nozzles 36 of the nozzle wall 39 extend preferably along a rectilinear path which is inclined in all three directions to follow the shape of the sides 28a 28b.

[0070] The wavy shape of the inner faces and the outer faces of the sides 28a and 28b and the wavy shape of the nozzle wall 39 comprising the nozzles 36 has the effect of improving the mixing of the dilution air, the second-stage fuel and the hot gas coming from the first-stage combustion chamber 12.

[0071] The lobed profiles of the sides 28a and 28b and of nozzle wall 39, in fact, has the effect of creating the vortexes required to mix the different flows.

[0072] The cavity 30 of each strut 24 is fed with dilution air coming from the annular chamber 22 around the liner 16.

[0073] With reference to figure 3, the liner 16 is preferably provided with a plurality of openings 40 each of which connects the respective cavity 30 with the annular chamber 22.

[0074] Preferably, each opening 40 is associated to at least one baffle 42 which is configured, as will be detailed better in the following, to guide the flow of air inside the cavity 30. The aim of the baffle 42 is mainly to ensure the cooling of the leading edge 26.

[0075] In particular, the baffle 42 is configured to guide the flow of air behind the second-stage fuel injection unit 32 (see also figure 2). In this way the dilution air exits the outlet opening 29 together with the second-stage fuel exiting the second-stage fuel injection unit 32, and, in particular, the dilution air flow surrounds the second-stage fuel flow coming from the injection unit 32. In this way, the fuel rich regions at the exit of the second-stage burner 13 have an average amount of dilution air.

[0076] According to a variant not shown, each opening can be associated to more than one baffle.

[0077] To avoid the direct contact of undiluted first-stage hot gas with the second stage fuel, which likely would cause flashback, the dilution air is supplied such that the dilution air flow is placed between the hot gas stream and the second-stage fuel stream.

[0078] Consequently, the second-stage flame temperature peaks, which would occur due to imperfect mixing of the fuel, are at least partly compensated by additional dilution air in such locations. Thereby, in the second-stage combustion chamber 14, even in relatively fuel rich zones the local flame temperature can be relatively low, and therefore the amount of NO x emissions is low.

[0079] The dilution air is therefore injected in axial direction as it exits from the outlet opening 29 at the trailing edge 27. In this way the dynamic part of the total pressure is not dissipated as it normally occurs in classical prior art mixing stages arranged upstream of the second-stage burner. Therefore, the pressure loss of the combustor assembly according to the present invention is reduced compared to a classical prior art combustor assembly.

[0080] Contrary to the most common dilution air injection systems, which are generally designed to inject the dilution air into the hot gas in a "jet-in-crossflow" manner between the first stage combustor and the second stage burner, the solution according to the present invention is designed to inject the dilution air through the struts 24, which allow a parallel injection of the dilution air into the hot gas path. Mainly, the angle between the main flow direction of the hot air coming from the first stage combustor 8 and the dilution air flow at the outlet opening 29 of the struts 24 is zero. Only some local mixing due to the swirl shape of the struts 24 can create some vortices leading to mixing in the downstream mixing zone 18.

[0081] In this way the dilution air, which is a considerable quantity of air compared to the air coming from the first-stage combustor 8, keeps the highly reactive fuel in the core of the dilution air cold for a longer time and allows it to mix with the dilution air until it sees the hot air from the first stage combustor 8, which leads to the chemical reaction. Advantageously the shape of the swirler 15, and in particular the shape of each strut 24, allows a correct mixing of the hot gas flow coming from the first-stage combustor 8 with the dilution air and second-stage fuel fed through the swirler 15.

[0082] In addition, a correct dimensioning of the circumferential distance between each strut 24 can ensure that the vortexes created by each of the strut 24 can combine better with those from the neighbouring lobed strut 24. This allows to apply the claimed solution also in small burners

[0083] Furthermore, the features of second-stage combustor 9 improve the shielding of the second-stage fuel flow. In this way the amount of second-stage fuel nozzles 36 can be increased, providing a superior mixing of the fuel with the other flows.

[0084] The combustor assembly of the present invention allows moreover to inject at the first-stage combustor 8 most of the kind of fuels, even ammonia or hydrogen-based fuels. While in the classic combustor assemblies this kind of fuels would create a major issue due to the high temperature reached at the second-stage burner inlet, the present invention circumvents this issue, since the dilution air is injected at the second-stage burner 13, thus inducing a temperature increase further downstream, mostly in the second stage combustion chamber 14.

[0085] Finally, the combustor assembly 3 of the present invention has a reduced axial length.

[0086] Figures 5-7 shows a variant off the present invention wherein the same reference numbers used in figures 1-4 are used for indicating identical or similar parts.

[0087] According to the variant shown in figures 5-7, when the fuel supply assembly 4 supplies ammonia-based fuel as first-stage fuel, the second-stage fuel is not supplied to the second-stage burner 13 and only dilution air is supplied to the second stage burner 13.

[0088] In figures 5-7, in fact, the second stage fuel supply assembly (comprising the second stage fuel injection units 32) is not present.

[0089] In particular, in this case, the fuel supply assembly 4 supplies ammonia-based fuel in excess with respect to first-stage air coming from the compressor 2. In this way, in the first-stage combustion chamber 12 burns a mixture of fuel / air which is unbalanced (i.e. rich in fuel). In this way unburned ammonia-based fuel reaches the inlet of the second-stage burner 13.

[0090] In other words, the quantity ammonia-based fuel supplied by the fuel supply assembly 4 to the first stage burner 11 is regulated so as the combustion stoichiometry of the first-stage burner 11 is unbalanced on fuel.

[0091] Since there will be more ammonia-based fuel compared to the available amount of oxygen (rich combustion), the ammonia-based fuel cannot burn completely within the first stage combustion chamber 12. In this case, NH 3 of the ammonia-based fuel will be either burned (4NH 3 + 3O 2 -> 6H 2 O + 2N 2 ) or cracked (2NH 3 -> N 2 + 3H 2 ).

[0092] The dilution air injected at the second-stage burner 13 will allow to burn the rest of the ammonia-based fuel (now in the form of hydrogen H 2 , which was generated within the first-stage combustion chamber 12).

[0093] By doing so the amount of NOx can be drastically reduced, compared to a simple burning of ammonia-based fuels.

[0094] The regulation of the amount of first-stage air fed to the first-stage burner 11 and of the amount of dilution air fed to the second-stage burner 13 is preferably defined by the geometrical features defined in the design step and, therefore, basically constant over the load and not actively controlled.

[0095] According to a variant not shown, also the amount of air supplied to the first-stage burner 11 and the second stage burner 13 can be actively controlled, for example taking into account the relative load of the engine, the fuel composition, the ambient conditions (ambient temperature and humidity).

[0096] As already disclosed, also when the fuel supply assembly 4 supplies ammonia-based fuel as first-stage fuel, dilution air is supplied through the mixing device 15 already detailed.

[0097] In particular, the mixing device 15 represented in figures 5 and 6 has the same structure detailed in the embodiment of figures 1-4, but it does not comprise the injection units for the supply of the second-stage fuel.

[0098] In figure 7 it is illustrated a variant of the mixing device 15 of figures 5 and 6, wherein a mixing element 132 is arranged in the cavity 30 of at least one strut 24.

[0099] The mixing element 132 is shaped so as to define a proper velocity profile of the dilution air at the outlet opening 29.

[0100] In the non-limiting example here illustrated, the mixing element has a shape substantially similar to the one of the injection unit 32 of figure 3 and 4.

[0101] The alternative solution disclosed with reference to figure 5-7 are therefore particularly focused on ammonia-based fuels.

[0102] However, the solution disclosed in figure 1-4 could be also used in case of ammonia-based fuels, with the precaution of setting the fuel supply assembly 4 to supplies ammonia-based fuel as first-stage fuel and not to supply second-stage fuel. Only dilution air is supplied to the second stage burner 13.

[0103] The solution disclosed in figures 1-4 could be considered more flexible in the operation of the combustor assembly 3, as fuels different from the ammonia-based ones can be supplied to the combustor unit 7 without any required hardware modification.

[0104] Finally, modifications and variants can be made to the assembly described herein without departing from the scope of the present invention, as defined in the appended claims.

Examples

Embodiment Construction

[0015]Figure 1 is a schematic view of a gas turbine unit 1 for power plants according to the present invention.

[0016]Gas turbine unit 1 comprises a compressor 2, a combustor assembly 3, a fuel supply assembly 4 and a turbine 5. The compressor 2 and the turbine 5 extend along a main axis A.

[0017]In use, an airflow compressed in the compressor 2 is mixed with fuel and is burned in the combustor assembly 3. Fuel is supplied to the combustor assembly 3 by the fuel supply assembly 4 (only partially visible in figure 1 and 2). The burned mixture is then expanded in the turbine 5 and converted in mechanical power by a shaft 6, which is connected to a generator (not shown).

[0018]The combustor assembly 3 is a sequential combustor assembly and comprises a plurality of units 7 (only one being represented in figure 1). Each unit 7 comprises a first-stage combustor 8 and a second-stage combustor 9 sequentially arranged along the gas flow direction G. In other words, the second-stage combustor 9 ...

Claims

1. Gas turbine unit comprising a compressor (2), a turbine (5), a combustor assembly (3) and a fuel supply assembly (4), configured to supply at least one first-stage fuel to the combustor assembly (3); the combustor assembly (3) comprising at least one combustor unit (7) provided with a liner (16) extending substantially along a combustor axis (B); the combustor unit (7) comprising a first-stage combustor (8) and a second-stage combustor (9), which is arranged downstream the first-stage combustor (8) along the gas flow direction (G); the first-stage combustor (8) comprising a first-stage burner (11), which is fed with first-stage air coming from the compressor (2) and first-stage fuel coming from the fuel supply assembly (4), and a first-stage combustion chamber (12), where the first-stage fuel is burned; the first-stage fuel being an ammonia-based fuel; wherein the second-stage combustor (9) comprises a second-stage burner (13) and a second-stage combustion chamber (14); the second-stage burner (13) being arranged downstream of the first-stage combustion chamber (12); the second-stage combustion chamber (14) being fed with hot gas leaving the first-stage combustion chamber (12) and passing through the second-stage burner (13) and with dilution air; the fuel supply assembly (4) being configured to supply the at least one first-stage fuel to the first-stage burner (11) so as the combustion stoichiometry of the first-stage burner (11) is unbalanced and an excess of first-stage fuel is present in the first stage burner (11) to obtain unburned first-stage fuel reaching the second-stage burner (13).

2. Gas turbine unit according to claim 1, wherein the second stage burner (13) comprises a mixing device (15); wherein the dilution air is injected at the mixing device (15).

3. Gas turbine unit according to claim 2, wherein the mixing of the dilution air, the unburned first-stage fuel and the hot gas leaving the first-stage combustion chamber (12) is done at the trailing edge of the mixing device (15) within the second-stage burner (13).

4. Gas turbine unit according to claim 2 or 3, wherein the mixing device (15) comprises a swirler provided with a plurality of struts (24), radially arranged about the combustor axis (B); dilution air being fed through at least one strut (24) of the plurality of struts (24).

5. Gas turbine unit according to claim 4, wherein each strut (24) comprises a wall (25) shaped to define: • an inner cavity (30); • a leading edge (26), which faces the first-stage combustion chamber (12); • a trailing edge (27), which is arranged opposite to the leading edge (26) along the combustor axis B and comprises an outlet opening (29) which faces the second-stage combustion chamber (14); • two sides (28a; 28b), which are respectively comprised between the leading edge (26) and the trailing edge (27).

6. Gas turbine unit according to claim 5, wherein the liner (16) is provided with at least two openings (40), each of which connects a cavity (30) of a respective strut (24) with a dilution air source (22); wherein each opening (40) is preferably associated to at least one baffle (42) which is configured to guide the flow of air inside the cavity (30) .

7. Gas turbine unit according to any of claims 5-6, wherein the sides (28a, 28b) are respectively shaped so as to define an outer curved surface; wherein each side (28a, 28b) is shaped so as to define an outer curved surface defining at least one lobe.

8. Gas turbine unit according to any of claims 5-7, wherein the sides (28a, 28b) are shaped so as to form, at the trailing edge (27), a right angle (α) with the liner (16).

9. Method for operating a gas turbine unit (1); the gas turbine unit (1) comprising a compressor (2), a turbine (5), a combustor assembly (3) and a fuel supply assembly (4), configured to supply at least one first-stage fuel to the combustor assembly (3); wherein the combustor assembly (3) comprises at least one combustor unit (7) provided with a liner (16) extending substantially along a combustor axis (B); the combustor unit (7) comprising a first-stage combustor (8) and a second-stage combustor (9), which is arranged downstream the first-stage combustor (8) along the gas flow direction (G); the first-stage combustor (8) comprising a first-stage burner (11), and a first-stage combustion chamber (12),; wherein the second-stage combustor (9) comprises a second-stage burner (13) and a second-stage combustion chamber (14); the second-stage burner (13) being arranged downstream of the first-stage combustion chamber (12); the method comprising: • supplying a first stage air to the first-stage burner (11) ; • supplying a first-stage fuel to the first-stage burner (11) so as the combustion stoichiometry of the first-stage burner (11) is unbalanced and an excess of first-stage fuel is present in the first stage burner (11); the first-stage fuel being an ammonia-based fuel; • supplying dilution air to the second-stage burner (13).

10. Method according to claim 9, wherein the step of supplying dilution air to the second-stage burner (13) comprises injecting dilution air at a mixing device (15) arranged in the second-stage burner (13).

11. Method according to claim 10, wherein the mixing of the dilution air, the unburned first-stage fuel and the hot gas leaving the first-stage combustion chamber (12) is done at the trailing edge of the mixing device (15) within the second-stage burner (13).

12. Method according to claim 10 or 11, wherein the mixing device (15) comprises a swirler provided with a plurality of struts (24), radially arranged about the combustor axis (B); dilution air being fed through at least one strut (24) of the plurality of struts (24).

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