EP3367001B1

EP3367001B1 - Second-stage combustor for a sequential combustor of a gas turbine

Info

Publication number: EP3367001B1
Application number: EP17158577.1A
Authority: EP
Inventors: Andrea Ciani
Original assignee: Ansaldo Energia Switzerland AG
Current assignee: Ansaldo Energia Switzerland AG
Priority date: 2017-02-28
Filing date: 2017-02-28
Publication date: 2020-12-23
Anticipated expiration: 2037-02-28
Also published as: EP3367001A1; CN108506962A; CN108506962B

Description

TECHNICAL FIELD

The present invention relates to a second-stage combustor for a sequential combustor of a gas turbine and to a method of controlling a sequential combustor of gas turbine with a first-stage combustor and a second-stage combustor.

BACKGROUND

As it is known, control of pollutant emissions is an objective of primary importance in design of any type of thermal machine and, in particular, of gas turbines for power plants. In fact, awareness of environmental risks drives towards regulations that set increasingly strict requirements. On the other hand, the organization of modern power market and the continuously variable demand do not allow to operate plants at constant load conditions. Instead, the need to meet demand fluctuations, including sudden rise or drop, and to contribute to control of grid frequency require quite flexible operation. The reduction of pollutant emissions, however, is made critical by such a flexible operation, because conflicting requirements must be balanced.
One of the problems to be addressed, for example, relates to emissions of nitrogen oxides (NOx) and carbon monoxide (CO). In fact, CO emissions are not normally an issue at full load, while attention is to be paid to the production of NOx, which tend to increase with temperature. Vice versa, lower temperature at part load helps keep NOx emissions low, but may prevent the complete oxidation of the carbon and favour the formation of carbon monoxide.
To solve this problem in gas turbines equipped with two-stage combustors, solutions have been proposed essentially based on control of the temperature of gas flow fed to the sequential combustors. Known solutions, however, are not satisfactory and a need is felt to further reduce emissions, in particular of NOx at full load and of CO at part load.
US 3 540 216 A discloses a combustor of a gas turbine, comprising a burner and a combustion chamber extending along a flow direction downstream of the burner. Downstream flame location and upstream flame location are defined within the combustion chamber in a flow channel formed by the burner and the combustion chamber. A downstream flame stabilizer is provided at the downstream flame location. A cross section of the flow channel changes gradually along the flow direction in a transition region between the burner and the combustion chamber, whereby the transition region prevents gas flowing through the combustion chamber from recirculating at the upstream flame location. Other examples of known combustors are disclosed in GB 2 287 312 A and in EP 3 015 772 A1 .

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a second-stage combustor for a sequential combustor of a gas turbine and a method of controlling a sequential combustor of gas turbine with a first-stage combustor and a second-stage combustor, which allow to overcome or at least to attenuate the limitations described.
According to the present invention, there is provided a second-stage combustor for a sequential combustor of a gas turbine, comprising:

a burner;
a combustion chamber extending along a flow direction downstream of the burner, wherein the burner and the combustion chamber form a flow channel and a downstream flame location and an upstream flame location are defined within the combustion chamber; and
a downstream flame stabilizer at the downstream flame location;
wherein a cross section of the flow channel changes gradually along the flow direction in a transition region between the burner and the combustion chamber, whereby the transition region prevents gas flowing through the combustion chamber from recirculating at the upstream flame location;
characterized by:
- an elongated streamlined body extending from the burner into the combustion chamber, wherein the burner is provided with a central body and the streamlined body extends from the burner substantially axially until an outlet of the combustion chamber and joins the central body in such a way to define a smooth transition without steps and possibly sharp edges, thereby being configured to prevent gas flowing through the combustion chamber from recirculating at the upstream flame location; and
- a selectively activatable upstream flame stabilizer at the upstream flame location, wherein the upstream flame stabilizer comprises at least one of:
  - an upstream electrode system, operable to provide ignition energy in the combustion chamber at the upstream flame location; and
  - an upstream pilot burner at the upstream flame location.

The downstream flame stabilizer allows control of the second-stage combustor such that, in certain conditions, the flame is anchored and stabilized at the downstream flame location, which may be set as close to the combustion chamber outlet as desired. In particular, the flame may be stabilized at the downstream flame location at high-load operating conditions (e.g. full load or for current load above a high load threshold). As a result, a very low post-flame residence time is achieved for the mixture burned in the combustion chamber. Near full load conditions, flame temperature is very high and allows complete oxidation of carbon, thus avoiding or at least reducing production of CO. High flame temperature would be instead detrimental for NOx emissions, but it is effectively compensated by the low post-flame residence time. Accordingly, extremely high flame temperature can be achieved, within acceptable levels of NOx emissions. Moreover, only a small portion of the combustion chamber liner is exposed to such extremely high temperature, to the benefit of component lifetime. Due to the downstream stabilizer, the downstream flame location can be accurately set and does not change remarkably.
On the other hand, the second-stage combustor can be controlled to set the flame at the upstream flame location at part load, in particular for current load below a low load threshold (which may be lower than or equal to the high load threshold). Flame temperature is lower at part load and production of NOx is normally not an issue. Instead, post flame residence time is high and complete oxidation of carbon takes place, thus keeping CO emissions low.
Flow recirculation or backflow may be created by a sudden increase in the cross section of the flow channel along the flow direction and is usually exploited in known combustors to create stagnation regions at the inlet of the combustion chamber, because flame anchoring is favoured by stagnation regions. However, flame anchoring may be difficult to release in the presence of recirculating flow, so stabilization of the flame at the downstream flame location might be prevented or made critical once the flame has been anchored at the upstream flame location. A smooth transition from the burner to the combustion chamber, instead, helps release the flame from the upstream flame location as desired. Stabilization of the flame at the upstream flame location may be achieved through control of the inlet temperature of the mixture to be burned in the combustion chamber.
The downstream flame stabilizer may be implemented by any measure suitable to ensure ignition of the mixture at the downstream flame location, whether by increasing reactivity of the mixture, or by providing a local heat source or by favouring stagnation of the mixture locally.
The streamlined body helps maintain a smooth transition in the flow channel between the burner and the combustion chamber. For example, the streamlined body may smoothly join a central body of the burner to prevent backflow.
Firm flame stabilization at the upstream flame location is achieved by the upstream flame stabilizer. Fluctuations of the inlet gas temperature reflect on the ignition time of the mixture and flame location. The upstream flame stabilizer allow to compensate for changes of flame location which could otherwise affect combustion in case of control purely based on inlet temperature. The fact that the upstream flame stabilizer may be selectively activated allows to easily release the flame from the upstream flame location e.g. when gas turbine load increases and there is an advantage to stabilize the flame at the downstream flame location.
The upstream flame stabilizer may be implemented e.g. by increasing reactivity of the mixture or by providing a local heat source.
A combination of upstream electrode system and upstream pilot burner may be exploited as desired.
According to an aspect of the invention, thus, the downstream flame stabilizer comprises at least one of:

a downstream electrode system, operable to provide ignition energy in the combustion chamber at the downstream flame location;
a downstream pilot burner at the downstream flame location; and
a change in cross section of the combustion chamber along the flow direction at the downstream flame location, the change in cross section being configured to cause gas flowing through the combustion chamber to recirculate at the downstream flame location.

Any combination of downstream electrode system, downstream pilot burner and change in cross section may be exploited as desired.
According to an aspect of the invention, the downstream electrode system comprises a set of downstream electrodes on the streamlined body at the downstream flame location and a downstream voltage supply line running inside the streamlined body.
According to an aspect of the invention, the downstream pilot burner comprises a set of downstream fuel nozzles on the streamlined body at the downstream flame location and a downstream fuel supply line running inside the streamlined body.
The streamlined body, besides providing its typical aerodynamic function, is exploited also to accommodate voltage and/or fuel supply lines. Thus, downstream flame stabilizer can be obtained with virtually no impact on the gas flow through the combustion chamber, which is not appreciably affected.
According to an aspect of the invention, the change in cross section of the combustion chamber is defined by at least one of a truncated downstream end of the streamlined body and a circumferential step in a combustor liner delimiting the combustion chamber around the downstream flame location.
According to an aspect of the invention, at least one intermediate flame stabilizer is provided at a respective intermediate flame location between the upstream flame location and the downstream flame location.
Additional stabilized flame locations may be provided along the combustion chamber as desired to further reduce pollutant emissions.
According to an aspect of the invention, there is provided a gas turbine comprising a sequential burner, the sequential burner including a first-stage combustor and a second-stage combustor as defined above.
According to an aspect of the invention, the gas turbine comprises a controller configured to set a current flame location at the downstream flame location in a first operating condition, corresponding to load values above a first load threshold, and to set the current flame location at the upstream flame location in a second operating condition, corresponding to load values below a second load threshold, not exceeding the first load threshold.
According to an aspect of the invention, the controller is configured to control a hot gas temperature of hot gas flowing from the first-stage combustor to the second-stage combustor.
Controlling the hot gas temperature, besides being generally beneficial to combustion, helps stabilizing flame position. In some conditions, hot gas temperature control may be sufficient for stabilization at the upstream flame location.
According to an aspect of the invention, the controller is configured to activate the downstream flame stabilizer selectively in the first operating condition.
According to an aspect of the invention, the controller is configured to activate the upstream flame stabilizer selectively in the second operating condition.
According to an aspect of the invention, there is provided a method of controlling a sequential combustor of gas turbine with a first-stage combustor and a second-stage combustor including:

a burner;
a combustion chamber extending along a flow direction downstream of the burner, wherein the burner and the combustion chamber form a flow channel and a downstream flame location and an upstream flame location are defined within the combustion chamber;
an elongated streamlined body extending from the burner into the combustion chamber, wherein the burner is provided with a central body and the streamlined body extends from the burner substantially axially until an outlet of the combustion chamber and joins the central body in such a way to define a smooth transition without steps and possibly sharp edges, thereby being configured to prevent gas flowing through the combustion chamber from recirculating at the upstream flame location; and
a selectively activatable upstream flame stabilizer at the upstream flame location, wherein the upstream flame stabilizer comprises at least one of:
- an upstream electrode system, operable to provide ignition energy in the combustion chamber at the upstream flame location; and
- an upstream pilot burner at the upstream flame location;
- the method comprising stabilizing a current flame location at the downstream flame location in a first operating condition, corresponding to load values above a first load threshold, and stabilizing the current flame location at the upstream flame location in a second operating condition, corresponding to load values below a second load threshold, not exceeding the first load threshold.

According to an aspect of the invention, stabilizing the current flame location comprises controlling a hot gas temperature of hot gas flowing from the first-stage combustor to the second-stage combustor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, which show some non-limitative embodiment thereof, in which:

Figure 1 is simplified block diagram of a gas turbine assembly;
Figure 2 is a longitudinal section through a sequential combustor including a second-stage combustor in accordance to an embodiment of the present invention;
Figure 3 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention;
Figure 4 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention;
Figure 5 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention;
Figure 6 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention;
Figure 7 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention; and
Figure 8 is a longitudinal section through second-stage combustor in accordance to another embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 shows a simplified view of a gas turbine assembly, designated as whole with numeral 1. The gas turbine assembly 1 comprises a compressor section 2, a combustor assembly 3 and a turbine section 5. The compressor section 2 and the turbine section 3 extend along a main axis A. An airflow compressed in the compressor section 2 is mixed with fuel and is burned in the combustor assembly 3, possibly added with dilution air. The burned mixture is then expanded to the turbine section 5 and converted in mechanical power.
A controller 7, which is configured to define a setpoint for the gas turbine, receives state signals from sensors 8 and operates the gas turbine through actuators 9 to provide a controlled power output.
The combustor assembly 3 is a two-stage sequential combustor and comprises a plurality of can combustors 10 arranged around the main axis A. Each of the can combustors 10, one of which is shown in Figure 2, comprises a first-stage combustor 12 and a second-stage combustor 13 sequentially arranged and defining a flow channel 15.
More specifically, the first-stage combustor 12 comprises a burner 16 and a combustion chamber 17. A fuel lance 18 for feeding fuel to the second-stage combustor 13 extends axially through the combustion chamber 17.
The second-stage combustor 13, which is illustrated in greater detail in Figure 3, comprises a burner 20, a combustion chamber 21 and a transition element 22 for coupling to the turbine section 5, here not shown.
The burner 20 is configured to admix a hot gas flow received from the first-stage combustor 12 and fuel received through the fuel lance 18. The burner 20 comprises an outer wall 24 and a central body 25 which extends along a burner axis. Within the burner 20, the flow channel 15 is delimited by the wall 24 and by the central body 25. Fuel is injected in the flow channel 15 through injectors (not shown).
The combustion chamber 21 extends along a flow direction downstream of the burner 20. In one embodiment, the combustion chamber 21 comprises an outer liner 27, an inner liner 28 and an elongated streamlined body 30. The outer liner 27 surrounds the inner liner 28 at a distance therefrom, so that a cooling channel 31 is defined between the outer liner 27 and the inner liner 28. The inner liner 28 delimits the flow channel 15 outwards in the combustion chamber 21 and joins the wall 24 of the burner 20 in such a way to define a smooth transition without steps and possibly sharp edges. In one embodiment, an edge of the wall 24 matches an adjacent edge of the inner liner 27.
The streamlined body 30 extends from the burner 20 into the combustion chamber 21 substantially axially until an outlet thereof and joins the central body 25 of the burner 20 also in such a way to define a smooth transition without steps and possibly sharp edges.
On account of the smooth connection between the inner liner 27 and the wall 24 on the one side and between the streamlined body 30 and the central body 25 on the other side, in a transition region 32 between the burner 20 and the combustion chamber 21 a cross section of the flow channel changes gradually along the flow direction. Therefore, the transition region is configured to prevent gas flowing through the combustion chamber 21 from recirculating and creating stagnations regions.
A downstream flame location 33 and an upstream flame location 34 are defined within the combustion chamber 21 at a distance from one another. In one embodiment, the downstream flame location 33 is defined at an outlet of the combustion chamber 21 and the upstream flame location 34 is defined in the transition region 32 between the burner 20 and the combustion chamber 21, e.g. at an inlet of the combustion chamber 21. In this manner, the whole space available in the combustion chamber 21 can be exploited to minimize the post flame residence time at full load and high temperature and improve air and fuel mixing, thus reducing NOx emissions, and to maximize the post flame residence time at part load and low temperature, thus reducing CO emissions. However, the distance between the upstream flame location and the downstream flame location could be lower than in the example of Figures 2 and 3.
A downstream flame stabilizer 35 and an upstream flame stabilizer 36 are provided at the downstream flame location 33 and at the upstream flame location 34, respectively.
In one embodiment, the downstream flame stabilizer 35 comprises a downstream electrode system, operable to provide ignition energy in the combustion chamber 21 at the downstream flame location 33. The downstream electrode system comprises a set of downstream electrodes 37 on the streamlined body 30 at the downstream flame location 33 and a downstream voltage supply line 38 running inside the streamlined body 30 and the fuel lance 18. Through the downstream electrodes 37, the downstream flame stabilizer 35 produces sparks across the combustion chamber 21 and causes ignition of the mixture flowing through the combustion chamber 21 irrespective of temperature conditions and of the self-ignition time of the mixture. In this respect, the self-ignition time of the mixture may be even so long that the mixture would not self-ignite within the combustion chamber 21, but the downstream flame stabilizer 35 is in any case capable of stabilizing the flame at the downstream flame location 33.
The upstream flame stabilizer 36 comprises an upstream electrode system, operable to provide ignition energy in the combustion chamber 21 at the upstream flame location 34. The upstream electrode system comprises a set of upstream electrodes 39 on the streamlined body 30 at the upstream flame location 34 and an upstream voltage supply line 40. In one embodiment (not shown), the upstream electrodes 39 may be arranged on the central body 25 of the burner 20 at the interface between the burner 20 and the combustion chamber 21.
The downstream flame stabilizer 35 and the upstream flame stabilizer 36 are selectively activated by the controller 7 on the basis of the load determined for the gas turbine assembly 1. When the load exceeds a high load threshold, the controller 7 activates the downstream flame stabilizer 35 and deactivates the upstream flame stabilizer 36, thus setting a current flame location at the downstream flame location 33. Instead, when the load is below a low load threshold, the controller 7 activates the upstream flame stabilizer 36 and deactivates the downstream flame stabilizer 35. Accordingly, the current flame location is set at the upstream flame location 34. The low load threshold does not exceed the high load threshold.
In one embodiment, illustrated in figure 4, a second-stage combustor 113 comprises a burner 120 and a combustion chamber 121. The combustion chamber 121 comprises a downstream flame stabilizer 135 at a downstream flame location 133, substantially as described with reference to Figure 2 and 3, and an upstream flame stabilizer 136 at an upstream flame location 134 at an inlet of the combustion chamber 121. The burner 120 and the combustion chamber 121 are configured to prevent recirculation of hot gas flow at the upstream flame location 134.
The upstream flame stabilizer 136 is defined by an upstream pilot burner that comprises upstream fuel nozzles 139 and an upstream fuel supply line 140 coupled to the upstream fuel nozzles 139 and running through the fuel lance 18. The upstream fuel nozzles 139 are uniformly distributed in a circumferential direction on the streamlined body, here designated by 130, and may be provided in the form of radially extending injection conduits which end at an intermediate radial distance between the streamlined body 130 and the inner liner, here 128, of the combustor chamber 121. As an alternative embodiment (not shown), the upstream fuel nozzles may be provided in the form of openings in a lateral surface of the streamlined body 130.
Figure 5 shows another embodiment of the invention. In this case, a second-stage combustor 213 comprises a burner 220 and a combustion chamber 221. The combustion chamber 221 in turn comprises a downstream flame stabilizer 235 at a downstream flame location 233 and an upstream flame stabilizer 236 at an upstream flame location 234 at an inlet of the combustion chamber 221. The burner 220 and the combustion chamber 221 are configured to prevent recirculation of hot gas flow at the upstream flame location 234.
The downstream flame stabilizer 235 is defined by a downstream pilot burner having a set of downstream fuel nozzles 241 on the streamlined body, here designated by 230, at the downstream flame location 233 and a downstream fuel supply line 242 coupled to the downstream fuel nozzles 241 and running inside the streamlined body 230. The downstream fuel nozzles 241 are uniformly distributed in a circumferential direction on the streamlined body 230, and may be provided in the form of radially extending injection conduits which end at an intermediate radial distance between the streamlined body 230 and the inner liner, here designated by 228, of the combustor chamber 221. The downstream fuel nozzles may be provided also in the form of openings in a lateral surface of the streamlined body 230, as an alternative.
Likewise, the upstream flame stabilizer 236 is defined by an upstream pilot burner (as already described with reference to Figure 4) that comprises upstream fuel nozzles 239 and an upstream fuel supply line 240 coupled to the upstream fuel nozzles 239 and running through the fuel lance 18.
Fuel supply to the downstream flame stabilizer 235 and to the upstream flame stabilizer 236 is determined by the controller 7 based on current load of the gas turbine 1. Specifically, the controller activates the downstream flame stabilizer 235 and deactivates the upstream flame stabilizer 236 when the current load exceeds a high load threshold; and activates the upstream flame stabilizer 236 and deactivate the downstream flame stabilizer 235 when the current load is below a low load threshold.
In one embodiment, illustrated in Figure 6, a second-stage combustor 313 comprises a burner 320 and a combustion chamber 321. The combustion chamber 321 comprises an outer liner 327, an inner liner 328 and an elongated streamlined body 330. The inner liner 328 delimits a flow channel 315 outwards in the combustion chamber 321 and joins a wall 324 of the burner 320 in such a way to define a smooth transition without steps and possibly sharp edges, substantially as already described with reference to Figures 2 and 3.
The streamlined body 330 extends from the burner 320 into the combustion chamber 321 substantially axially until an outlet thereof and joins a central body 325 of the burner 320 also in such a way to define a smooth transition without steps and possibly sharp edges.
A downstream flame location 333 and an upstream flame location 334 are defined in the combustion chamber 321 at a distance from one another and a downstream flame stabilizer 335 and an upstream flame stabilizer 336 are provided at the downstream flame location 333 and at the upstream flame location 334, respectively.
The downstream flame stabilizer 335 comprises a change in cross section of the combustion chamber 321 along the flow direction at the downstream flame location 333. The change in cross section is configured to cause gas flowing through the combustion chamber 321 to recirculate at the downstream flame location 333. In the embodiment of Figure 6, the change in cross section of the combustion chamber 321 is defined by a truncated downstream end 330a of the streamlined body 330 and by a circumferential step 328a in the inner liner 327 delimiting the combustion chamber 321 around the downstream flame location 333. In other embodiments not shown, however, the change in cross section maybe defined only by the truncated downstream end of the streamlined body or only by the circumferential step of the inner liner.
The upstream flame stabilizer 336 may comprise an upstream electrode system as disclosed with reference to Figures 2 and 3. As an alternative, the upstream flame stabilizer may comprise an upstream pilot burner.
Figure 7 shows another embodiment, in which a second-stage combustor 413 comprises a burner 420 and a combustion chamber 421. A downstream flame location 433 and an upstream flame location 434 are defined in the combustion chamber 421 at a distance from one another and a downstream flame stabilizer 445 is provided at the downstream flame location 433. The downstream flame stabilizer 435 may be of any kind previously described, e.g. including an electrode system, a pilot burner or a change in cross section of the flow channel. In the example Figure 7, electrodes 437 are provided on a streamlined body 430 of the combustion chamber 421, with a voltage supply line 438 in the streamlined body 430.
A transition region 432 between the burner 420 and the combustion region 421 is configured to prevent gas flowing through the combustion chamber form recirculating at the upstream flame location 434, as described with reference to Figures 2 and 3.
The controller 7 is configured to control an inlet gas temperature of hot gas from the first-stage combustor 12 to the second-stage combustor 413 and to selectively activate the downstream flame stabilizer 435 as desired based on the current load of the gas turbine 1. For the purpose of controlling the inlet gas temperature, the controller 7 may act e.g. on a power split or power ratio of power delivered by the first-stage combustor 12 to power delivered by the second-stage combustor 413, and/or on a flow of dilution air admixed to the hot gas from the first-stage combustor 12 before entering the second-stage combustor 413. The controller 7 uses temperature control to set a current flame location at the upstream flame location 434, in this case without the aid of a upstream flame stabilizer. In one embodiment (not shown), the controller may set the current flame location also at the downstream flame location by controlling the inlet gas temperature instead of or in addition to using a stabilizer in the combustion chamber. In this case, the current flame location may be set at the upstream flame location by temperature control or with the aid of a selectively activatable upstream flame stabilizer.
In one embodiment, shown in Figure 8, a second-stage combustor 513 comprises a burner 520 and a combustion chamber 521. A downstream flame stabilizer 535 and an upstream flame stabilizer 536 are respectively provided on a streamlined body 530 of the combustion chamber 521 at a downstream flame location 533 and at an upstream flame location 534, which are defined in the combustion chamber 521. In addition, one or more intermediate flame locations 550 are defined in the combustion chamber between the upstream flame location 234 and the downstream flame location 233 and a respective intermediate flame stabilizer 551 is provided at each intermediate flame location 550. The burner 520 and the combustion chamber 521 are configured to prevent recirculation of hot gas flow at the upstream flame location 534.
The downstream flame stabilizer 535 and the upstream flame location 536 may comprise an electrode system, having a set of downstream electrodes 537 and a downstream voltage supply line 538 and a set of upstream electrodes 539 and an upstream voltage supply line 540, respectively. Each intermediate flame stabilizer 551 comprises a respective set of intermediate electrodes 552 at the respective intermediate flame location 550 and a respective voltage supply line 553, running in the streamlined body 530 and in the fuel lance 18.
As the downstream flame stabilizer 535, the upstream flame location 536, each of the intermediate flame stabilizer 551 is selectively activatable by the controller 7 to set a current flame location at the respective intermediate flame location 550 based on a current load of the gas turbine 1 as desired.
Finally, it is evident that the described second-stage combustor and method of controlling a sequential combustor may be subject to modifications and variations, without departing from the scope of the present invention, as defined in the appended claims.

Claims

A second-stage combustor for a sequential combustor of a gas turbine, comprising:
a burner (20; 120; 220; 320; 420; 520);

a combustion chamber (21; 121; 221; 321; 421; 521) extending along a flow direction downstream of the burner (20; 120; 220; 320; 420; 520), wherein the burner (20; 120; 220; 320; 420; 520) and the combustion chamber (21; 121; 221; 321; 421; 521) form a flow channel (15) and a downstream flame location (33; 133; 233; 333; 433; 533) and an upstream flame location (34; 134; 234; 334; 434; 534) are defined within the combustion chamber (21; 121; 221; 321; 421; 521); and

a downstream flame stabilizer (35; 135; 235; 335; 435; 535) at the downstream flame location (33; 133; 233; 333; 433; 533);

wherein a cross section of the flow channel (15) changes gradually along the flow direction in a transition region (32; 432) between the burner (20; 120; 220; 320; 420; 520) and the combustion chamber (21; 121; 221; 321; 421; 521), whereby the transition region (32; 432) prevents gas flowing through the combustion chamber (21; 121; 221; 321; 421; 521) from recirculating at the upstream flame location (34; 134; 234; 334; 434; 534);

characterized by:
an elongated streamlined body (30; 130; 230; 330; 430; 530) extending from the burner (20; 120; 220; 320; 420; 520) into the combustion chamber (21; 121; 221; 321; 421; 521), wherein the burner (20; 120; 220; 320; 420; 520) is provided with a central body (25) and the streamlined body (30; 130; 230; 330; 430; 530) extends from the burner (20; 120; 220; 320; 420; 520) substantially axially until an outlet of the combustion chamber (21; 121; 221; 321; 421; 521) and joins the central body (25) in such a way to define a smooth transition without steps and possibly sharp edges, thereby being configured to prevent gas flowing through the combustion chamber (21; 121; 221; 321; 421; 521) from recirculating at the upstream flame location (34; 134; 234; 334; 434; 534); and

a selectively activatable upstream flame stabilizer (36; 136; 236; 336; 536) at the upstream flame location (34; 134; 234; 334; 434; 534), wherein the upstream flame stabilizer (36; 136; 236; 336; 536) comprises at least one of:
an upstream electrode system (39, 40; 539, 540), operable to provide ignition energy in the combustion chamber (21; 121; 221; 321; 421; 521) at the upstream flame location (34; 134; 234; 334; 434; 534); and

an upstream pilot burner (139, 140; 239, 240) at the upstream flame location (34; 134; 234; 334; 434; 534).
The second-stage combustor of claim 1, wherein the downstream flame stabilizer (35; 135; 235; 335; 435; 535) comprises at least one of:
a downstream electrode system (37, 38; 437, 438; 537; 538), operable to provide ignition energy in the combustion chamber (21; 121; 221; 321; 421; 521) at the downstream flame location (33; 133; 233; 333; 433; 533);

a downstream pilot burner (241, 242) at the downstream flame location (33; 133; 233; 333; 433; 533); and

a change in cross section (328a, 330a) of the combustion chamber (21; 121; 221; 321; 421; 521) along the flow direction at the downstream flame location (33; 133; 233; 333; 433; 533), the change in cross section (328a, 330a) being configured to cause gas flowing through the combustion chamber (21; 121; 221; 321; 421; 521) to recirculate at the downstream flame location (33; 133; 233; 333; 433; 533), wherein the change in cross section (328a, 330a) of the combustion chamber (321) is defined by at least one of a truncated downstream end (330a) of the streamlined body (330) and a circumferential step (328a) in a combustor liner (328) delimiting the combustion chamber (321) around the downstream flame location (233).
The second-stage combustor according to claim 2, wherein the downstream electrode system (37, 38; 437, 438; 537; 538) comprises a set of downstream electrodes (37; 437; 537) on the streamlined body (30; 430; 530) at the downstream flame location (33; 433; 533) and a downstream voltage supply line (38; 437; 538) running inside the streamlined body (30; 430; 530).
The second-stage combustor according to claim 2 or 3, wherein the downstream pilot burner (241, 242) comprises a set of downstream fuel nozzles (241) on the streamlined body (230) at the downstream flame location (233) and a downstream fuel supply line (242) running inside the streamlined body (230).
The second-stage combustor according to any one of the preceding claims, comprising at least one intermediate flame stabilizer (551) at a respective intermediate flame location (551) between the upstream flame location (34; 134; 234; 334; 434; 534) and the downstream flame location (33; 133; 233; 333; 433; 533).
A gas turbine comprising a sequential burner, the sequential burner including a first-stage combustor (12) and a second-stage combustor (13; 113; 213; 313; 413; 513) according to any one of the preceding claims.
The gas turbine according to claim 6, comprising a controller (7) configured to set a current flame location at the downstream flame location (33; 133; 233; 333; 433; 533) in a first operating condition, corresponding to load values above a first load threshold, and to set the current flame location at the upstream flame location (34; 134; 234; 334; 434; 534) in a second operating condition, corresponding to load values below a second load threshold, not exceeding the first load threshold.
The gas turbine according to claim 7, wherein the controller (7) is configured to control a hot gas temperature of hot gas flowing from the first-stage combustor to the second-stage combustor.
The gas turbine according to claim 7 or 8, wherein the controller (7) is configured to activate the downstream flame stabilizer (35; 135; 235; 435; 535) selectively in the first operating condition.
The gas turbine according to claim 9 as appended to claim 6, wherein the controller (7) is configured to activate the upstream flame stabilizer (36; 136; 236; 336; 536) selectively in the second operating condition.
A method of controlling a sequential combustor of a gas turbine with a first-stage combustor (12) and a second-stage combustor (13; 113; 213; 313; 413; 513) according to claim 1; the method comprising stabilizing a current flame location at the downstream flame location (33; 133; 233; 333; 433; 533) in a first operating condition, corresponding to load values above a first load threshold, and stabilizing the current flame location at the upstream flame location (34; 134; 234; 334; 434; 534) in a second operating condition, corresponding to load values below a second load threshold, not exceeding the first load threshold.
The method according to claim 11, wherein stabilizing the current flame location comprises controlling a hot gas temperature of hot gas flowing from the first-stage combustor to the second-stage combustor.