EP2848865A1

EP2848865A1 - Thermoacoustic stabilization method

Info

Publication number: EP2848865A1
Application number: EP13184151.2A
Authority: EP
Inventors: Franklin Marie Genin; Naresh Aluri; Bruno Schuermans
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2013-09-12
Filing date: 2013-09-12
Publication date: 2015-03-18

Abstract

The invention relates to a method for thermoacoustic stabilization of gas turbine combustors (1) with multiple burners (2), which are arranged in at least one burner group (3) and each of the burners (2) is supplied with fuel (5) and oxidant (6). The method is characterized in operating neighboring burners (2) within that burner group (3) at different nominal velocities of oxidant through controlling of the pressure drop of the oxidant flow across individual burners (2) (velocity staging). In a preferred embodiment identical burners (2) are used wherein at least one burner (2) of the burner group (3) is both operated with a different amount of oxidant (6) and with a different amount of fuel (5) than the other burners (2) in that burner group (3). With such a velocity and fuel staging the flame temperatures are equal which means on one hand that there is no NOx penalty and on the other hand that a good mitigation for pulsation is achieved.

Description

BACKGROUND OF THE INVENTION

The present invention relates to gas turbine combustion systems. It refers to a thermoacoustic stabilization method, which keeps engine pulsations under control for the lifetime of the engine.

PRIOR ART

Modern gas turbines for power generation rely on the lean operation to fulfill the legal requirements of pollutant emissions. Therefore, premix burners, for example EV burners (EV stands for environmental) as described in general in EP 0 321 809 B1 or US 4,932,861 are used. The flame stabilization that is necessary by using lean fuel relying mostly on free standing recirculation regions. These flames are typically very sensitive to flow perturbations and easily couple with the dynamics of the combustion chamber to lead to thermoacoustic instabilities. These flow dynamics have a strong impact on the combustion quality, components lifetime, etc., and are thus undesirable.
Known methods for thermoacoustic stabilization currently used in gas turbines rely on passive control, and use either fuel staging in burner groups (operation concepts where groups of burners are defined to operate at different power through fuel staging), burner staging (burners with multiple fuel nozzles to combine the stability brought by rich zones to leaner flame regions) or Helmholtz resonators/dampers.
In the first two cases the basis for stabilization is the creation of hot regions within the flow which deviate from the overall lean conditions (staging in flame temperature). The drawback of such approaches is the promoted NOx creation in the hotter parts of the combustor.
Alternatively, it is known state of the art to use Helmholtz resonators/dampers, see for example EP 1158247 B1 , WO 2010/115980 A2 , US 8205714 B2 for reducing thermoacoustic instabilities in combustion chambers. Using these devices causes additional costs and they are, however, not always implementable due to space constraints and/or retrofit ability constraints. Furthermore, inherent to the operation of a Helmholtz damper and to avoid backflow of hot gases into the dampers, purge air is required to flow within the dampers, hence removing air from the main burners and making the flames hotter, with the same consequences as noted above. The same considerations apply to other known damping features, for example soft walls.
Current state of the art in mitigation of combustor pulsation is the implementation of burner temperature staging at engine level. The response to the individual burners to acoustic perturbations is changing with flame temperature. Staging the burner temperatures allows then to detune consecutive burners, such that they cancel out their individual responses.
As a disadvantage, all above mentioned known stabilization methods have NOx penalties due to the spared air to the damper or to the temperature spread within/across the burners.

SUMMARY OF THE INVENTION

It is one object of the present application to provide a method of thermoacoustic stabilization for gas turbine combustion systems composed of multiple burners which overcomes the disadvantages of the prior art methods.
This object is obtained by a method according to claim 1.
The present method for thermoacoustic stabilization of gas turbine combustors with multiple burners, wherein the burners are arranged in at least one burner group and each of them is supplied with fuel and oxidant, is characterized in operating neighboring burners in that burner group at different nominal velocities of the oxidant by an oxidant pressure drop across the individual burners. This is called in the following "velocity staging".
The nominal burner velocity is proportional to the mass flow of oxidant through the burner. The burner velocity U_burner is defined by the following equation: $U_{burner} = {mdot}_{burner} / (ρ \cdot A)$

where mdot_burner is the mass flow of oxidant passing through a burner, ρ is the oxidant density upstream of the burner, and A is a characteristic cross section of the burner, for simplicity, "burner velocity" will be used in the following to describe the equivalent mass flow of oxidant going through the burner.
The core of the invention is to operate neighboring burners of gas turbine combustors at different nominal mass flows of oxidant by an oxidant pressure drop across the individual burners. The oxidant is for example air or air with water addition etc.
This leads to thermoacoustic stabilization at minimal implementation costs. Furthermore, it can be retrofitted.
According to a preferred embodiment of the invention an added value is to modulate the specific powers of the individual burners in relation with their nominal velocities (velocity and fuel staging), keeping the temperature spread across burners to a low value and hence reducing NOx penalty. The stabilization is as effective as the state of the art approaches but without the undesired increasing of NOx that is associated with the known prior art methods.
Preferred embodiments of the invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1: shows a view of an annular combustor, schematic of the front segments 360°, with premix burners of the EV type (double-cone type) arranged in groups of four;
Fig. 2: shows a schematic view of a burner in Fig.1 in longitudinal direction of the burner for explanation of the nomenclature;
Fig. 3: shows a schematic view of a burner group with four burners according to Fig.1 in homogeneous operation with stability issues;
Fig. 4: shows a schematic view of a burner group according to Fig. 1 with unequally distributed fuel and same mass flow of oxidant (prior art);
Fig. 5 and 6: show two embodiments of the present invention in a schematic view with two burner groups according to Fig. 1, both with equally distributed fuel to each burner, but with differently staged amount of air to the burners; and
Fig. 7: shows a preferred embodiment of the present invention in a schematic view of a burner group according to Fig. 1 with velocity and fuel staging.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

Current state of the art in mitigation of combustor pulsation is the implementation of burner temperature staging at engine level. The response of individual burners to acoustic perturbations is changing with flame temperature. Staging the burner temperatures allows then to detune consecutive burners, such that they cancel out their individual responses.
A quantification of the response of individual burners to acoustic perturbations can be made through measurements of flame transfer functions (FTF hereafter). In particular, comparing the phase of the FTFs of two burners will show whether the burners are detuned (out of phase) or not.
Typically the oxidant mass flow through a burner is assumed to be very similar in all burners.
It was found out that a phase shift similar to that due to temperature staging can be obtained by changing individual burners' operating velocities. Furthermore, increasing the burner velocity together with the fuel mass flow can be achieved to keep the flame temperature constant. Practically, this is equivalent to estimating the impact of a burner specific power onto the FTF. The shift in phase during burner velocity variations is found to be comparable to that obtained during temperature variations. Consequently, the possibility to detune the burners' responses is shown while still allowing for homogeneous operation.
Any gas turbine set where individual combustion chambers are connected to two or more burners can benefit from this invention. The practicality of the present invention resides in the fact that identical burners are used.
It is common art to protect burners in gas turbines with hardware device that reduce the risks of loose objects from damaging the burners, and further entering the combustion chamber / turbine. Such devices take, for instance, the form of grids, perforated plates, etc. Without loss of generality, such devices are hereafter referred to as "sieves".
In a cheap and efficient approach, the pressure drop right upstream of the burner can be controlled on a burner by burner basis, by implementing varying sieves upstream of the burners. Thereby, the oxidant mass flow will be redistributed across burners, providing more oxidant flow to the burner with low pressure drop (higher burner velocity) and less oxidant flow to the burners with an additional pressure drop induced by the sieve. This arrangement leads to a burner velocity staging which provides additional stability to the system.
In a preferred embodiment, orifices are implemented in the fuel distribution of the burner groups to control the fuel mass flows according to the respective burner oxidant mass flow and approach homogeneous flame temperature operation of the different groups.
The present velocity staging concept is illustrated here for a specific annular combustor with a predefined burner grouping. It is clear, however, that a similar velocity staging can be achieved in all other gas turbines types where multiple burners are used, in annular, cannular or silo combustors.
Fig. 1 shows a schematic view of an annular combustor 1 of the front segments 360° (front plate 4), with 24 premix burners 2 of the EV type (double-cone type). The burners 2 are arranged in 8 groups 3, each of four burners 2. One group 3 is circled with a dotted line in Fig. 1. The following figures (Fig. 3 to Fig. 7) focus on such a group of burners.
Fig. 2 shows in a schematic view such an EV burner 2 from Fig.1 in the longitudinal direction of the burner for explanation of the nomenclature and should always be discussed in connection with the following figures. The burner 2 opens in the front plate 4. Fuel 5 and oxidant 6 are supplied to the burner 2. A longer fuel line 5 means more fuel mass flow and a thicker oxidant line 6 means more oxidant flow. The burner is surrounded with a sieve 7, the thickness of the dashed line indicates the blockage strength. Reference number 8 indicates the flame front.
Fig. 3 shows a schematic view of a burner group 3 with four burners 2 according to Fig. 1 (Prior Art). An average mass flow flue (M_fuel=m_fuel_avg) is supplied to each of the burners 2 (all burners 2 get the same amount of fuel 5) and the same amount of oxidant 6 is supplied to each of the burners 2 (equal mass flow). This leads to a homogeneous operation with an average flame temperature (Tflame-average) in the flame front 8 which has the lowest NOx, but suffers from pulsation (instability).
Fig. 4 shows a second schematic view of a burner group 3 with four burners 4 according to Fig. 1 (Prior Art). A pulsation mitigation according to the known prior art is here achieved wherein the fuel only is staged (unequally fuel distribution-the burner 2 below in Fig. 4 has a lower fuel mass flow (M_fuel<m_fuel_avg) than the other three burners (M_fuel>m_fuel_avg)), and all burners 2 get same amount of oxidant 6 as indicated by the arrows 6 with the same thickness. This leads to flame staging, the flame temperature of the lower burner 2 in Fig. 4 is lower than the average flame temperature (Tflame<Tflame_avg), while the flame temperature of the other three burners 2 in Fig. 4 is higher than the average flame temperature (Tflame>Tflame avg). This has the disadvantage of increasing NOx emissions.
Fig. 5 and Fig. 6 show two embodiments of the present invention in a schematic view with two burner groups 3 according to Fig. 1, both with equally distributed fuel 5 to each burner 2 (M_fuel = m_fuel_avg), but with differently staged amount of oxidant 6 (see different thickness of arrows 6) to the burners 2 (velocity staging).
In Fig. 5 a higher amount of oxidant 6 (higher mass flow) is supplied to one burner 2 (see burner 2 below in Fig. 5) while the other three burners 2 are each supplied with a lower amount of oxidant 6 as can be seen by the thinner lines in Fig. 5. This is achieved by using different sieves 7, the sieve 7 for the lower burner 2 has a lower blockage strength (see the thinner line in Fig. 5) comparing to the other three burners 2. Therefore, the flame temperature of the lower burner 2 is lower than the average flame temperature (Tflame<Tflame_avg), while the flame temperature of each of the other three burners 2 is higher than the average flame temperature (Tflame > Tflame_avg).
In Fig. 6 a lower amount of oxidant 6 is supplied to one burner 2 (see burner 2 below in Fig. 6 with a lower mass flow of oxidant 6) while the other three burners 2 are each supplied with a higher amount of oxidant 6 as can be seen by the thicker lines in Fig. 6. This is achieved by using different sieves 7, the sieve 7 for the lower burner 2 has a higher blockage strength (see the thicker line in Fig. 6) comparing to the other three burners. Therefore, the flame temperature of the lower burner 2 is here higher than the average flame temperature (Tflame>Tflame_avg), while the flame temperature of each of the other three burners 2 is lower than the average flame temperature (Tflame<Tflame_avg).
The application in velocity staging in combination with uniform fuel injection is a thermoacoustic stabilization method, but leads also to flame temperature staging which should be avoided because of the NOx penalty.
Fig. 7 shows the preferred embodiment of the present invention in a schematic view of a burner group 3 according to Fig. 1 with both velocity and fuel staging. In Fig. 7 a lower amount of oxidant 6 is supplied to one burner 2 (see burner 2 below in Fig. 7) while the other three burners 2 are each supplied with a higher amount of oxidant 6 as can be seen by the thicker lines in Fig. 7. This is achieved by using different sieves 7, the sieve 7 for the lower burner 2 has a higher blockage strength (see the thicker line in Fig. 7) comparing to the other three burners. In addition, the burner 2 below in Fig. 7 which operates with lower amount of oxidant 6 has also a lower fuel mass flow than the other three burners 2 (see shorter fuel supply line) line. This leads to a uniform flame temperature (Tflame_avg) across the burners 2 (no NOx penalty) and stabilization of the GT operation.
Of course, these embodiments do not limit the scope of protection, for example such a staging concept could be done such that staging is applied within a group of 4 burners, keeping two burners at one condition and the other two at another, or equivalently to groups of less/more than 4 burners. The number of variations is high and relatively straightforward, hence not explicitly included.
To summarize the disclosure: An approach to thermoacoustic pulsations mitigations is proposed whereby a velocity staging between burners is applied. Such an approach permits neighboring burners to be detuned hence increasing the stability. Combining this to a fuel distribution that matches the oxidant distribution, the combustor can be operated near homogeneous conditions, so that the penalty in pollutant emissions is reduced to its minimum.
This approach can be implemented in a number of different ways, for example installation of different burner sizes, of burners with different pressure drop characteristics, etc. In a preferred arrangement, all burners are identical, and the individual burners pressure drops are controlled by the implementation of different sieves (already implemented in the engines, however currently with same characteristics for all burners) upstream of the burners. This approach leads to velocity staging with minimal cost because such sieves are inexpensive.
Homogeneous operation of the burners lead to a more uniform flow distribution at the combustor-turbine interface, hence allowing a hotter operation which cancels out the performance penalty due to the potentially increased combustor pressure drop. The proposed staging concept is applicable to any gas turbine system composed of multiple burners (annular, cannular, silo).

LIST OF REFERENCE NUMERALS

1: combustor
2: burner
3: group of burners
4: front plate
5: fuel
6: oxidant
7: sieve
8: flame front

Claims

Method for thermoacoustic stabilization of a gas turbine combustor (1) with multiple burners (2), arranged in at least one burner group (3) and each of the burners (2) is supplied with fuel (5) and oxidant (6),
characterized in that operating neighboring burners (2) in that burner group (3) at different nominal velocities by pressure drop of oxidant (6) across the individual burners (2).
Method as claimed in claim 1, characterized in that for velocity staging burners (2) with different burner sizes are used.
Method as claimed in claim 1, characterized in that for velocity staging burners (2) with different pressure drop characteristics are used.
Method as claimed in claim 1, characterized in that for velocity staging identical burners (2) are used, wherein at least one burner (2) of the burner group (3) is supplied with a different nominal mass flow of oxidant (6) than the other burners (2) in that burner group (3) by an oxidant pressure drop across the individual burners (2).
Method as claimed in claim 4, characterized in that the at least one burner (2) of the burner group (3) is supplied with a lower mass flow of oxidant (6) than the other burners (2) of that burner group (3).
Method as claimed in claim 4, characterized in that the at least one burner (2) of the burner group (3) is supplied with a higher mass flow of oxidant (6) than the other burners (2) of that burner group (3).
Method as claimed in one of claims 1 to 6, characterized in that all burners (2) of the burner group (3) are supplied with the same mass flow of fuel (5).
Method as claimed in one of claims 1 to 6, characterized in that for fuel staging said burners (2) are operated with a different mass flow of fuel (5).
Method as claimed in claim 4, characterized in that the at least one burner (2) of the burner group (3) operated with a different mass flow of oxidant (6) than the other burners (3) is supplied with a different mass flow of fuel (5) than the other burners (2) in that burner group (3).
Method as claimed in claim 5, characterized in that the at least one burner (2) of the burner group (3) operated with a lower mass flow of oxidant (6) than the other burners (3) is supplied with a lower mass flow of fuel (5) than the other burners (2) in that burner group (3).
Method as claimed in claim 6, characterized in that the at least one burner (2) of the burner group (3) operated with a higher mass flow of oxidant (6) than the other burners (3) is supplied with a higher mass flow of fuel (5) than the other burners (2) in that burner group (3).
Method as claimed in one of claims 4 to 11, characterized in that the mass flow of oxidant (6) supplied to the burners (2) and the pressure drops are controlled by implementation of different perforated sieves (7).