Abstract:
The invention refers to a sequential combustor arrangement including a first burner, a first combustion chamber, a dilution burner for admixing a dilution gas and a second fuel via a dilution-gas-fuel-admixer to the first combustor combustion products. The dilution-gas-fuel-admixer has at least one streamlined body which is arranged in the dilution burner for introducing the at least one second fuel into the dilution burner through at least one fuel nozzle, and which has a streamlined cross-sectional profile and which extends with a longitudinal direction perpendicularly or at an inclination to a main flow direction prevailing in the dilution burner. The streamlined body includes a dilution gas opening for admixing dilution gas into the first combustor combustion products upstream of the at least one fuel nozzle. The disclosure further refers to a method for operating a gas turbine with such a sequential combustor arrangement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 13180583.0 filed Aug. 15, 2013, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The invention refers to a sequential combustor arrangement for a gas turbine with admixing dilution gas into the sequential combustor arrangement. The invention additionally refers to a method for operating a gas turbine with admixing dilution gas into a sequential combustor arrangement. 
     BACKGROUND 
     Due to increased power generation by unsteady renewable sources like wind or solar existing gas turbine based power plants are increasingly used to balance power demand and to stabilize the grid. Thus improved operational flexibility is required. This implies that gas turbines are often operated at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures. 
     At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation and during transients, as these also count for cumulative emission limits. 
     State-of-the-art combustion systems are designed to cope with a certain variability in operating conditions, e.g. by adjusting the compressor inlet mass flow or controlling the fuel split among different burners, fuel stages or combustors. However, this is not sufficient to meet the new requirements. 
     To further reduce emissions and to increase operational flexibility sequential combustion has been suggested in DE 10312971 A1. Depending on the operating conditions, in particular on the hot gas temperature of a first combustion chamber, it can be necessary to cool the hot gases before they are admitted to a second burner (also called sequential burner). This cooling can be advantageous to allow fuel injection and premixing of the injected fuel with the hot flue gases of the first combustor in the second burner. 
     Good control of the inlet temperature to the second combustion chamber and good mixing of fuel injected in the second burner with the gases leaving the first combustor is a prerequisite for stable combustion with low emission values. However, the additional length required in the hot gas flow path for admixing dilution gas and fuel increases the cost of the gas turbine. In addition the stepwise cooling by admixing of dilution gas in a mixer followed by fuel injection can lead to an increase in the overall pressure drop of such a combustor arrangement. 
     SUMMARY 
     The object of the present disclosure is to propose a sequential combustor arrangement with a burner comprising means for admixing dilution gas and the second fuel between the first combustion chamber and the second combustion chamber. Such a “dilution burner” has to provide the proper inlet flow conditions for the second combustion chamber. In particular the hot gases are cooled to predetermined hot gas temperatures. Further, velocity distribution, oxygen and fuel content can be conditioned (e.g. controlled to a prescribed profile) for the second combustion chamber with proper admixing of dilution gas. 
     Deviations from prescribed inlet temperatures may result in high emissions (e.g. NOx, CO, and unburned hydrocarbons) and/or flashback in the dilution burner. Flashback and NOx are induced by the reduced self-ignition time for the injected fuel due to a high inlet gas temperature or high oxygen concentration, which causes earlier ignition (leading to flashback) or reduced time for fuel air mixing resulting in local hot spots during combustion and consequently increased NOx emission. Low temperature regions can cause CO emissions, due to the increased self-ignition time. This can reduce the time for CO to CO2 burnout, and a reduced local flame temperature, which can further slowdown the CO to CO2 burnout. Finally local hot spots may lead to overheating in certain regions downstream of the mixer. 
     Dilution gas can for example be compressed air or a mixture of air and flue gases of a gas turbine. Also compressed flue gases can be used as dilution gas. 
     According to an embodiment a sequential combustor arrangement comprises a first burner, a first combustion chamber, a dilution burner for admixing a dilution gas and a second fuel, and a second combustion chamber arranged sequentially in a fluid flow connection. The dilution burner comprises a dilution-gas-fuel-admixer for admixing a dilution gas and a second fuel to the first combustor combustion products leaving the first combustion chamber during operation. 
     The first fuel and second fuel can be the same type of fuel, e.g. both gaseous or both liquid fuel and come from the same source or can be different fuel types provided from different fuel sources. 
     The dilution-gas-fuel-admixer has at least one streamlined body which is arranged in the dilution burner with at least one fuel nozzle for introducing the at least one fuel into the dilution burner. The streamlined body has a streamlined cross-sectional profile which extends with a longitudinal direction perpendicularly or at an inclination to a main flow direction prevailing in the dilution burner. Upstream of the at least one fuel nozzle the at least one streamlined body of the dilution-gas-fuel-admixer comprises a dilution gas opening for admixing dilution gas into the first combustor combustion products. 
     According to another embodiment the dilution gas opening is dimensioned such that during operation the mass flow ratio of the first combustor combustion products to the mass flow of dilution gas admixed through the dilution gas opening is in a range of 0.5 to 2. Preferably the dilution gas opening is dimensioned such that during operation the mass flow ratio in a range of 0.7 to 1.5, and most preferably in the range of 0.9 to 1.1. 
     Specifically the dilution gas opening is directed parallel to the main flow direction of the first combustor combustion products leaving the first combustor during operation. In particular the dilution gas opening is directed to release dilution gas parallel to the main flow at the location of the dilution gas opening. 
     According to another embodiment the ratio of the flow area for first combustor combustion products at the location of the dilution gas openings to the flow area of dilution gas opening is in the range of 1 to 10. Preferably the ratio of flow areas is in the range of 2 to 7 and most preferably in the range of 4 to 6. 
     According to a further embodiment of the sequential combustor arrangement the dilution-gas-fuel-admixer has at least two dilution gas openings for admixing dilution gas into the first combustor combustion products. The at least two dilution gas openings for admixing dilution gas into the first combustor combustion products are formed as slots, which extend from lateral surfaces of the streamlined body of the dilution-gas-fuel-admixer towards the flow area of the first combustor combustion products, i.e. normal to the longitudinal extension of the stream lined body. 
     Specifically the height of the slots can be determined as a function of the temperature distribution and/or velocity distribution of the first combustor combustion products entering the dilution burner. In particular the slot height can be basically proportional to the temperature difference between the hot gas at the corresponding position of the slot and the temperature of the dilution gas. Since the dilution gas admixed through the slot is proportional to the slot height the cooling effect of the dilution gas is proportional to the slot height and an inhomogeneous temperature profile at the exit of the first combustion chamber can be equalized with a matched height distribution; e.g. a slot with a large height in regions of high temperature and a slot a small height in a region of low temperature. 
     Alternatively the height of the slots can determined as a function of the position in the longitudinal direction of the streamlined body. 
     The height distribution can be a simple linear function with the height increasing from one end of the slot to the other. It can be any curve or have a reduced height towards the end of the slot, close the side walls of the dilution burner because the temperature might already be reduced due to wall cooling in the dilution burner or the combustion chamber. 
     According to another embodiment a vortex generator is arranged on at least one lateral surfaces of the streamlined body. 
     The vortex generators can for example be of triangular shape with a triangular lateral surface converging with the lateral surface upstream of the vortex generator, and two side surfaces essentially perpendicular to a central plane of the streamlined body. The two side&#39;s surfaces converge at a trailing edge of the vortex generator. This trailing edge is typically just upstream of the corresponding fuel nozzle. 
     According to yet another embodiment the trailing edges of the streamlined body are provided with at least two lobes in opposite transverse directions with reference to a central plane of the streamlined body. At the position of the dilution gas openings the width of cross section of the streamlined body (normal to the flow direction of the main flow at the location of the dilution gas opening) is reduced to increase the flow area in the dilution burner. The ratio of the sum of cross section of the dilution gas openings to the area of the largest cross section of the streamlined burner downstream of the dilution gas openings can for example be in the order of 0.1 to 2 or even larger. According to one embodiment this ratio is in the range of 0.5 to 1.5. 
     Besides the sequential combustor arrangement a gas turbine with such a sequential combustor arrangement and a method for operating a gas turbine with such a sequential combustor arrangement are the subject of the present disclosure. 
     Such a gas turbine comprises at least a compressor, a sequential combustor arrangement and a turbine, wherein the sequential combustor arrangement has a first burner, a first combustion chamber, a dilution burner with a dilution-gas-fuel-admixer, and a second combustion chamber arranged sequentially in a fluid flow connection. 
     According to one embodiment the method for operating such a gas turbine comprises the following steps:
         compressing the inlet gas in the compressor, admixing a first fuel in the first burner,   burning the resulting mixture of compressed inlet gas and fuel in the first combustor to obtain first combustor combustion products,   admixing a dilution gas and a second fuel to the first combustor combustion products with the dilution-gas-fuel-admixer,   burning the mixture of first combustor combustion products, second fuel, and dilution gas in the second combustion chamber to obtain second combustor combustion products, and   expanding the second combustion chamber combustion products also called hot gas in the turbine.       

     Before combustion the mixture of first combustor combustion products, second fuel, and dilution gas in the second combustion chamber to obtain second combustor combustion products in the second combustion chamber the first combustor combustion products, second fuel, and dilution gas are at least partly mixed in the dilution-gas-fuel-admixer. 
     For admixing, the second fuel is introduced into the dilution burner with a dilution-gas-fuel-admixer having at least one streamlined body which is arranged in the dilution burner. The second fuel can be injected through at least one fuel nozzle from the streamlined body. The at least one streamlined has a streamlined cross-sectional profile which extends with a longitudinal direction perpendicularly or at an inclination to a main flow direction prevailing in the dilution burner. From the at least one streamlined body dilution gas is admixed via at least one dilution gas opening into the first combustor combustion products upstream of the at least one fuel nozzle. 
     Specifically the mass flow ratio of the mass flow of first combustor combustion products entering the dilution burner to the mass flow of dilution gas admixed through the dilution gas opening is in the range of 0.5 to 2. Preferably the ratio is in the range of 0.7 to 1.5, and most preferably the ratio is in a range of 0.9 to 1.1. 
     According to a further embodiment the dilution gas is injected into the dilution gas burner through the dilution gas opening in a direction within a maximum deviation of 20° to the flow direction of the main flow direction of the first combustor combustion products at the point of injection. Ideally the dilution gas is injected into the dilution gas burner through the dilution gas opening parallel to the flow direction of the main flow of the first combustor combustion products. 
     According to another embodiment of the method the dilution gas is injected into the dilution gas burner through the dilution gas opening with the same average velocity as the average flow velocity of the first combustor combustion products at the point of injection or within a maximum deviation of 30% to the average flow velocity of the first combustor combustion products at the point of injection. 
     Injection of dilution gas with the same flow direction and/or same velocity as the flow direction, respectively the same flow velocity as the flow of the combustion products of the first combustor reduces the pressure loss upon injection of the dilution gas. 
     In such a configuration dilution gas and combustion products of the first combustor can flow parallel to each other along the streamlined body. Downstream of the openings for admixing dilution gas the fuel can be injected practically into the dilution gas, and all three fluids: combustion products of the first combustor, dilution gas and fuel can be mixed in one step. For good mixing of these three fluids vortex generators can be arranged on at least one lateral surfaces of the streamlined body. Alternatively the trailing edges of the streamlined body can be provided with at least two lobes in opposite transverse directions relative to a reference to a central plane of the streamlined body. 
     According to a further embodiment of the method the dilution gas is admixed into the first combustor combustion products through at least two dilution gases opening, which are formed as slots. Each of these slots extends from a lateral surface of the streamlined body of the dilution-gas-fuel-admixer towards the flow area of the first combustor combustion products (i.e. normal to the lateral surface of the streamlined body). 
     According to yet another embodiment the mass flow distribution of injected dilution gas in longitudinal direction of the streamlined body is determined as a function of the temperature distribution and/or velocity distribution of the first combustor combustion products entering the dilution burner. Typically more dilution gas is injected into regions with higher temperature than into a region with a lower temperature. 
     Alternatively or in combination the mass flow distribution of injected dilution gas in longitudinal direction of the streamlined body is determined as a function of the position along the longitudinal direction of the streamlined body. 
     Cooling of the streamlined body can be required. In particular upstream of the location of the dilution gas openings the streamlined body is exposed to the hot combustion products of the first combustor. According to one embodiment a cooling gas is used to cool external surface of the streamlined body. Typically, most of the fluid added from the streamlined body is dilution gas. Specifically the ratio of dilution gas to cooling gas is greater than 2; preferably the ratio is greater than 5, or even up to 20 or 50. Cooling and dilution gas can have the same composition and can come from the same source. 
     A local high oxygen concentration can have a similar effect as a local high temperature, e.g. leading to fast reaction, consequently to reduce the available time for mixing before self-ignition occurs, high combustion temperatures, increased NOx emissions and possibly flash back. A local low oxygen concentration can have a similar effect as a local low temperature, e.g. leading to slow reaction, consequently to increased CO and UHC (unburned hydrocarbon) emissions. Therefore the admixing of dilution gas can be distributed to adjust the oxygen concentration in the gas leaving the dilution burner. 
     The dilution-gas-fuel-admixer can also be combined with dampers or as connecters to damping volumes as described in the European patent application EP12189685, which is incooperated by reference. 
     The gas turbine can include a flue gas recirculation system, in which a part of the flue gas leaving the turbine is admixed to the compressor inlet gas of the gas turbine. 
     Different cooling technologies might be used in the cooling zones. For example effusion cooling, impingement cooling or convective cooling or a combination of cooling methods can be used. 
     Referring to a sequential combustion the combination of combustors can be disposed as follows:
         Both, the first and second combustors are configured as sequential can-can architecture.   The first combustor is configured as an annular combustion chamber and the second combustor is configured as a can configuration.   The first combustor is configured as a can-architecture and the second combustor is configured as an annular combustion chamber.   Both, the first and second combustor are configured as annular combustion chambers.       

     Different burner types can be used. For the first combustor so called EV burner as known for example from the EP 0 321 809 or AEV burners as known for example from the DE195 47 913 can for example be used. Also a BEV burner comprising a swirl chamber as described in the European Patent application EP12189388.7, which is incorporated by reference, can be used. In a can architecture a single or a multiple burner arrangement per can combustor can be used. Further, a flamesheet combustor as described in US2004/0211186, which is incorporated by reference, can be used as first combustor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure, its nature as well as its advantages, shall be described in more detail below with the aid of the accompanying schematic drawings. 
       Referring to the drawings: 
         FIG. 1  shows a gas turbine with a sequential combustion arrangement with a first burner, first combustion chamber, a mixer for admixing dilution gas, a second burner, and a second combustion chamber; 
         FIG. 2  shows a gas turbine with a sequential combustion arrangement in annular architecture with a first burner, first combustion chamber, a dilution burner, and a second combustion chamber; 
         FIG. 3  shows a gas turbine with a sequential combustion arrangement in annular architecture with a first burner, counter flow cooling of the first combustion chamber, a first combustion chamber, a dilution burner, and a second combustion chamber; 
         FIG. 4  shows a gas turbine with a sequential combustion arrangement in can architecture with a first burner, first combustion chamber, a dilution burner, and a second combustion chamber; 
         FIG. 5  shows a dilution burner with a dilution-gas-fuel-admixer with vortex generators and carrier gas injection; 
         FIG. 6  shows a dilution burner with a dilution-gas-fuel-admixer and a thin trailing edge; 
         FIG. 7  shows a dilution-gas-fuel-admixer with slots for admixing dilution gas on both sides of the streamlined body; 
         FIG. 8  shows a dilution burner with a dilution-gas-fuel-admixer with vortex generators, two axial staged gas openings on each side of the streamlined body and carrier gas injection; 
         FIG. 9  shows a perspective view of dilution-gas-fuel-admixer with vortex generators, cooling gas injection from the trailing edge and carrier gas injection; 
         FIG. 10  shows a perspective view of dilution-gas-fuel-admixer with lobes for vortex generation and carrier gas injection. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a gas turbine  1  with a sequential combustor arrangement  4 . It comprises a compressor  3 , a sequential combustor arrangement  4 , and a turbine  5 . 
     The sequential combustor arrangement  4  comprises a first burner  10 , a first combustion chamber  11 , and a mixer  12  for admixing a dilution gas  32  to the hot gases leaving the first combustion chamber  11  during operation. Downstream of the mixer  12  the sequential combustor arrangement  4  further comprises a second burner  13 , and a second combustion chamber  14 . The first burner  10 , first combustion chamber  11 , mixer  12 , second burner  13  and second combustion chamber  14  are arranged sequentially in a fluid flow connection. The sequential combustor arrangement  4  is housed in a combustor casing  31 . The compressed gas  8  leaving the compressor  3  passes through a diffusor  30  for at least partly recovering the dynamic pressure of the gas leaving the compressor. 
     The sequential combustor arrangement  4  further comprises a first combustion chamber cooling zone with a first cooling channel  15  which is delimited by the first combustion chamber wall  24  and a first jacket  20 , which is enclosing the first combustion chamber wall  24 . It comprises a mixer cooling zone with a second cooling channel  16  which is delimited by a mixer wall  25  and a second jacket  21 , which is enclosing the mixer wall  25 . It comprises a second burner cooling zone with a third cooling channel which is delimited by a second burner wall  26  and a third jacket  22 , which is enclosing the second burner wall  26 . It also comprises a second combustion chamber cooling zone with a fourth cooling channel  18 , which is delimited by a second combustion chamber wall  27 , and a fourth jacket  23 , which is enclosing the second combustion chamber wall  27 . 
     Compressed gas  8  is fed into the first cooling channel  15  as cooling gas  33  at an upstream end (relative to the hot gas flow direction) and flows through the first cooling channel  15  parallel to the main flow direction of the hot gas flow in the first combustion chamber  11 . After passing through the first cooling channel  15  the cooling gas  33  enters the second cooling channel for cooling the mixer. After at least partly cooling the mixer the cooling gas  33  is fed into the dilution gas inlet  19  and admixed to the hot gas as dilution gas  32  in the mixer  12 . 
     Compressed gas  8  is also fed into the fourth cooling channel  18  as cooling gas  33  at a downstream end (relative to the hot gas flow direction) and flows in counter flow to the main flow direction of the hot gas flow in the second combustion chamber  14 . After passing through the fourth cooling channel  18  the cooling gas  33  enters the third cooling channel  17  at a downstream end (relative to the hot gas flow direction) and flows in counter flow to the main flow direction of the hot gas flow in the second burner  13 . After cooling the second combustion chamber wall  27  and the second burner wall  26  the cooling gas  33  is fed to the second burner  13 . The cooling gas  33  can for example be fed to the second burner  13  as cooling gas, e.g. as film cooling gas or diffusion cooling. Part of the cooling gas  33  can already be fed to the hot gas  9  in the second combustion chamber  14  during cooling of the second combustion chamber wall  27  (not shown). 
     A first fuel  28  can be introduced into the first burner  10  via a first fuel injection, mixed with compressed gas  8  which is compressed in the compressor  3 , and burned in the first combustion chamber  11 . Dilution gas  32  is admixed in the subsequent mixer  12 . A second fuel  29  can be introduced into the second burner  13  via a second fuel injector  51 , mixed with hot gas leaving the mixer  12 , and burned in the second combustion chamber  14 . The hot gas leaving the second combustion chamber  14  is expanded in the subsequent turbine  5 , performing work. The turbine  5  and compressor  3  are arranged on a shaft  2 . 
     The remaining heat of the exhaust gas  7  leaving the turbine  5  can be further used in a heat recovery steam generator or boiler (not shown) for steam generation. 
     In the example shown here compressed gas  8  is admixed as dilution gas  32 . Typically compressor gas  8  is compressed ambient air. For gas turbines with flue gas recirculation (not shown) the compressor gas is a mixture of ambient air and recirculated flue gas. 
     Typically, the gas turbine system includes a generator (not shown) which is coupled to a shaft  2  of the gas turbine  1 . The gas turbine  1  further comprises a cooling system for the turbine  5 , which is also not shown as it is not subject of the invention. 
     Different exemplary embodiments of the sequential combustor arrangement with a dilution burner are shown in  FIGS. 2 to 4 . Details of different exemplary embodiments of the dilution burner are shown in  FIGS. 5 to 10 . 
     The embodiment of  FIG. 2  differs from the combustor arrangement of  FIG. 1  in that the mixer  12  and second burner  13  are replaced by a dilution burner  35 . In particular the separate mixer  12  and second fuel injector  51  can be replaced by one component. The dilution burner  35  comprises dilution burner walls  36  delimiting the hot gas path, and a dilution-gas-fuel-admixer  34  arranged in the hot gas path of the dilution burner  35 . The hot first combustor combustion products  37  enter directly into the dilution burner  35  without any prior cooling. The Second fuel  29  and dilution gas  32  is supplied via the dilution-gas-fuel-admixer  34 . In the example shown the dilution gas  32  is feed into the dilution-gas-fuel-admixer  34  via a dilution gas feed  46  from the third cooling channel  17  enclosing the dilution burner  35 . 
     The dilution gas  32  can also be supplied from other sources as for example directly from the compressor diffusor  30  or from the first cooling channel  15 . 
     Like in burner arrangement of  FIG. 1  compressed gas  8  is fed into the fourth cooling channel  18  as cooling gas  33  at a downstream end (relative to the hot gas flow direction) and flows in counter flow to the main flow direction of the hot gas flow in the second combustion chamber  14 . After passing through the fourth cooling channel  18  the cooling gas  33  enters the third cooling channel  17  at a downstream end (relative to the hot gas flow direction) and flows in counter flow to the main flow direction of the hot gas flow in the dilution burner  35 . After cooling the second combustion chamber wall  27  the cooling gas  33  is fed to the dilution burner  35 . Finally, after cooling the dilution burner wall  36  the cooling gas is used as dilution gas  32 . 
     The embodiment of  FIG. 3  is based on  FIG. 2 . In this example the dilution burner and cooling scheme of the first burner  10  is changed. The cooling gas  33  cools the first combustion chamber  11  in a counter flow arrangement. After cooling the first combustion chamber  11  the cooling gas  33  enters a burner hood which guides the cooling gas  33  into the first burner  10 . 
     The embodiment of  FIG. 4  is based on  FIG. 2 . In this example the dilution burner and cooling scheme is unchanged but a sequential combustion arrangement in a can architecture and with a flame sheet burner as first burner  10  is shown. A plurality sequential combustion arrangement in a can architecture is arranged circumferentially spaced on a radius around the axis of the gas turbine (not shown). 
       FIG. 5  shows a cross section of a dilution burner  35  with a dilution-gas-fuel-admixer  34  interposed between two side walls  36  of the dilution burner  35 . In this example the dilution-gas-fuel-admixer  34  comprises a streamlined body  42  with a leading edge section  47  and a trailing edge section  48 , as well as vortex generators  41  attached to the lateral walls of the streamlined body  42 . Between the leading edge section  47  and the trailing edge section  48  dilution gas openings  44  are arranged, facing in a downstream direction of the flow of the first combustor combustion products  37  for injecting the dilution gas  32  in the same direction as the main flow. At the location of the dilution gas openings  44  the width of cross section of the streamlined body is reduced to increase the flow area in the dilution burner by the flow area A d  of dilution gas opening  44 . Upstream of the dilution gas opening  44  the flow area in the dilution burner was equal to the flow area for first combustion products A c . 
     For cooling of the leading edge section  47  cooling gas  45  can be injected through cooling holes. Typically a film cooling can be applied for the leading edge section  47 . 
     In the streamlined body  42  ducts are provided to feed gaseous fuel  38  and liquid fuel  39  to the fuel injection nozzles  43  for injecting the fuel  38 ,  39 . In the example shown the nozzles  43  are arranged at the trailing edge of the streamlined body  42 . In addition, carrier gas  40  can be injected from opening adjacent to the fuel nozzles  43 . 
     To enhance mixing vortex generators  41  are extending from the lateral sides of the streamlined body  42 . 
     The example of  FIG. 6  is based on  FIG. 5 . It differs from the example of  FIG. 4  in that it has no cooling or carrier air is injected at the trailing edge. Without the air injection the trailing edge section  48  and in particular the trailing edge can be designed thinner thereby reducing losses. 
     In addition the second combustion chamber  14  which is arranged downstream of the dilution burner  35  is indicated in  FIG. 6 . In this example the cross section of the flow path is increasing towards the dilution burner  35  for flame stabilization. 
       FIG. 7  shows an embodiment of a dilution burner  35  with a rectangular flow cross section. Such dilution burners can be arranged circumferentially around the axis of a gas turbine with a radial direction r pointing away from the axis. Typically the longitudinal direction of the streamlined body  42  is parallel to the radial direction r when installed in the gas turbine. The cross section is shown from the downstream end of the dilution burner  35 . The dilution-gas-fuel-admixer  34  of this example has only one fuel nozzle  43  for injecting either gaseous or liquid fuel  38 ,  39 . The trailing edge section  48  is extending on both sides of the nozzle  43 . A dilution gas opening  44  is arranged on both sides of the trailing edge section  48 . The dilution gas opening  44  has the form of a slot extending in radial direction r. The height h of the slot is linearly increasing from an inner slot height h i  to an outer slot height h o . The slot height is determined by the downstream end of the leading edge section  47 . 
       FIG. 8  shows a cross section of a dilution burner  35  with a dilution-gas-fuel-admixer  34  interposed between two side walls  34  of the dilution burner  35  similar to  FIG. 5 . In this example the dilution-gas-fuel-admixer  34  comprises gas openings  44 a and  44 b in an axially staged arrangement, facing in a downstream direction of the flow of the first combustor combustion products  37  for injecting the dilution gas  32  in the same direction as the main flow. 
       FIG. 9  shows a perspective view of dilution-gas-fuel-admixer  34  with vortex generators. It is similar to the embodiment shown in  FIG. 5 . However, in this example fuel  38 ,  39  is injected from circular, respectively annular fuel nozzles  43  arranged at the trailing edge  48 . In addition cooling gas  45  is injected from a slot which is extending along the trailing edge  48  of the streamlined body  42 . Carrier gas  40  can be injected via an annular opening, which is arranged coaxially around the fuel nozzles  43 . To enhance mixing of fuel  38 ,  39  with the dilution gas  32  and first combustor combustion products  37  vortex generators  41  are arranged upstream of the fuel nozzles  43  on the lateral walls  49  of the streamlined body  42 . In the example show a section of the dilution burner wall  36  is indicated. Typically the streamlined body  42  of the dilution-gas-fuel-admixer  34  extends normal to the dilution burner wall  36  into the flow path of the dilution burner. 
       FIG. 10  shows a perspective view of another embodiment of a dilution-gas-fuel-admixer  34 . In this embodiment no vortex generators extend from the streamlined body. Instead the streamlined body comprises lobes  50  in opposite transverse directions with reference to a central plane  52  of the streamlined body  42  for vortex generation. 
     For all shown arrangements can or annular architectures or any combination of the two is possible. EV, AEV or BEV burners can be used for can as well as for annular architectures. 
     The mixing quality of the mixer  12  is crucial for a stable clean combustion since the burner system of the second combustion chamber  14  requires a prescribed inlet conditions. 
     All the explained advantages are not limited to the specified combinations but can also be used in other combinations or alone without departing from the scope of the disclosure. Other possibilities are optionally conceivable, for example, for deactivating individual burners or groups of burners at part load operation. Further, the cooling gas and the dilution gas can be re-cooled in a cooling gas cooler before use as cooling gas, respectively as dilution gas.