WO2004109188A1

WO2004109188A1 - Method for the operation of an annular-shaped burner arrangement in an intermediate heating step of a multi-step combustion device of a gas turbine

Info

Publication number: WO2004109188A1
Application number: PCT/EP2004/051013
Authority: WO
Inventors: Peter Flohr; Alexander Ni; Christian Oliver Paschereit; Rajeshriben Patel; Majed Toqan
Original assignee: Alstom Technology Ltd
Priority date: 2003-06-05
Filing date: 2004-06-03
Publication date: 2004-12-16
Also published as: DE10325455A1

Abstract

The invention relates to a method for the operation of an annular-shaped burner arrangement (10) in the intermediate heating step of a multi-step combustion device of a gas turbine. Said burner arrangement comprises a plurality of similar burners (B1,..,Bi,..,Bn) which are arranged in a ring. The operation thereof is improved by virtue of the fact that at least one selected part of the burner (B1,..,Bi,..,Bn) used in a stable operational area is operated in a predetermined unstable operational area of the burner arrangement (10) for the absorption of high frequency instabilities occurring during combustion.

Description

DESCRIPTION

METHOD FOR OPERATING A RING-SHAPED BURNER ARRANGEMENT IN AN INTERMEDIATE STAGE OF A MULTI-STAGE COMBUSTION DEVICE OF A GAS TURBINE

TECHNICAL AREA

The present invention relates to the field of burner assemblies for gas turbines. It relates to a method for operating an annular Breπner arrangement according to the preamble of claim 1.

STATE OF THE ART

In a gas turbine with an reheater system, a second combustion zone is created by injecting fuel into a high speed flow of contaminated air exiting a first turbine stage. Such high-speed flows into a flue gas Combustion chambers are often susceptible to high-frequency instabilities that can generate high-amplitude pulsations. The suppression of such pressure pulsations is crucial for maintaining the quality of the combustion process and for avoiding deterioration of the burner structure. It has also been found that the excitation of such high-frequency instabilities is sensitive to slight changes in the operating point (such as changes in mass flow or temperature), so that the type of combustion can quickly change from stable to unstable during operation of the machine.

In an older, not yet published application by the applicant, a method has been proposed with which perforated walls in intermediate superheating burner arrangements can be designed in such a way that both the cooling effect and the acoustic damping are maximized. As a result, the high frequencies are attenuated for a wide range of operating parameters in the unstable range.

DE-A1-199 39235 describes a method for operating a combustion device of a gas turbine, in which a plurality of burners of the same thermal power working in a common combustion chamber are designed so differently to suppress thermo-acoustic combustion instabilities that those of the flames or flame fronts generated by them are positioned distributed along the common axis. The disadvantage here is the different design of the burners, which leads to increased production and maintenance costs.

In contrast, the present invention shows another way of suppressing high-frequency vibrations in the combustion zone of intermediate superheating systems. It is based on an annular burner arrangement with a plurality of similar ones arranged in a ring

Burners that can be operated under different conditions. PRESENTATION OF THE INVENTION

It is an object of the invention to provide a method for operating a burner arrangement in the intermediate heating stage of a multi-stage combustion device of a gas turbine, with which an effective suppression of instabilities is possible, even if the burner arrangement is operated in an inherently unstable operating range.

The object is achieved by the entirety of the features of claim 1. The essence of the invention is that a changed operating mode means that at least some of the individual burners are operated in a stable operating range. This detunes the entire system in such a way that high-frequency instabilities can no longer occur.

A first preferred embodiment of the method according to the invention is characterized in that the heat release in the selected burners is changed compared to the other burners, that the changed heat release in the selected burners is achieved by a change in the fuel supply, and in that for the change a controllable valve or a restriction in the fuel supply is used for the fuel supply.

A second preferred embodiment is characterized in that the inlet temperature in the selected burners is changed compared to the other burners, and in that the changed inlet temperature in the selected ones

Burners is achieved by a targeted change in the fuel supply in the burner arrangement of the first stage of the combustion device.

Alternatively, the burner arrangement can be operated in an operating range with the addition of water, and the changed inlet temperature in the selected burners can be achieved by deliberately changing the water supply. Another preferred embodiment is characterized in that the burners operating in the unstable region are operated in a suitably distributed manner via the annular burner arrangement.

Further embodiments result from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in more detail below using exemplary embodiments in connection with the drawing. Show it

1 shows an exemplary (schematic) annular burner arrangement with n burners in a reduced coordinate system (x-circumferential coordinate; z-axial coordinate) for the implementation of the invention;

2 shows the growth rate of the pulsations of a single burner as a function of the Strouhal number;

3 shows the dependence of the growth rate at the Strouhal number 2 on the distance X _{f of} the flame front from the outlet of the associated burner; and

4 shows a table of the growth rate of a burner arrangement with 24 identical burners, which are operated in different ways in a stable and unstable mode. WAYS OF CARRYING OUT THE INVENTION

The excitation of combustion instabilities can have various causes, e.g. a feedback mechanism between shear layer instabilities and fluctuations in heat release, or a feedback mechanism between local heat release and fluctuations in the equivalence ratio, due to a fluctuating air column within the burner element. Typically, most of these effects can be described by a characteristic time delay, which is essentially based on a convective time scale between the location of the fuel injection (or: the origin of the shear layer instability) and the local position of the flame front. As a result, these mechanisms are sensitive to the central flame location.

The frequencies observed in a sequential combustion and high speed flow system are typically in the 1-10 kHz range. f -L The frequency f can also be dimensionless as a Strouhal number Str = - -

are represented, where L is a characteristic burner length and U is a characteristic burner speed. In this area, even small variations in the location of the flame cause significant effects on the

Mechanism of sound generation, i.e. changes from stable to unstable states.

The acoustic interaction in an annular burner arrangement for reheating with any number of burners can be described with a model as follows:

The model assumes that there are, in particular, two parameters or methods for introducing inhomogeneities into the reheat burner arrangement with only minor structural changes to the burners, with their Help you can avoid the occurrence of high frequency vibrations, namely

- The inlet temperature at the reheater burner arrangement by means of a graduated operation of burner groups in the first combustion stage; and

- The heat release in the reheat burner arrangement by means of individual control valves or nozzles in the fuel supply lines of the reheat burner.

The influence of the inhomogeneities caused by the two above Procedures (inlet temperature, heat release) are introduced, the stability of the burner arrangement has been examined using numerical methods.

The conventional linear stability approach forms the basis of the investigation. According to this approach, the flow equations are linearized and the development of flow disturbances is checked. The flow disturbances (pressure, speed, etc.) are assumed to have an exponential time dependency

(1) f (r, t) = F (7) exp (iωt), the interference amplitude F being exclusively a function of the spatial coordinates. Mathematically, the problem of stability is reduced to a problem of eigenfunctions. The solution to the problem results in the eigenfunctions F (r) and natural frequencies ω of possible pulsation modes of the burner arrangement. The natural frequencies are generally complex. Taking into account the exponential dependence of the flow disturbances on time according to equation (1), the following stability criteria can be formulated:

- If Imω> 0, the pulsations disappear over time and the associated pulsation mode is stable. - If Imω <0, the disturbances grow exponentially with time and the associated pulsation mode is unstable. - The intermediate case Imω = 0 corresponds to the neutral mode. The imaginary part Imω of the frequency characterizes the growth rate of the disturbances and gives a measure of the instability of the burner arrangement.

The following simplifying approximations are used for the stability analysis: The annular burner arrangement is assumed to be infinitely thin (see: SR Stow, A. Dowling, Thermoacoustic oscillations in an annular combustor, ASME Turbo Expo 2001, Paper 2001 -GT-0038, (2001)). Within this approximation the radial dependence of the disturbances is neglected. This is true if the wavelength is much greater than the radial thickness of the toroidal toroidal arrangement. This approximation reduces the number of independent spatial variables from 3 to 2, namely to a circumferential coordinate and an axial coordinate. FIG. 1 shows a schematic development of an annular burner arrangement 10 with n burners B1, .., Bi, .., Bn, which work in a common combustion chamber 11 with a combustion chamber outlet 12. The coordinate z denotes the circumferential direction and x the axial extent in the flow direction. For a given value of the axial coordinate x, the flow pattern is periodic with the period L (x), which is determined by the shape of the burner arrangement 10. The flames F1, .., Fi, .., Fn emanating from the burners B1, .., Bi, .., Bn are also shown in FIG. 1. The distance between the outlet of the i-th burner Bi and the associated flame Fi is designated as X _f .

The burner model described in a previous article (A. Ni, W. Polifke and F. Joos, Ignition delay time as a contribution to thermo-acoustic instability in sequential combustion, ASME Turbo Expo 2000, Paper 2000-GT-0103 ( 2000)) has been proposed, determines the acoustic behavior of the individual burner in the stability analysis. The input variables of the model are: geometry of the burner, pressure drop in the fuel supply line, fuel mass flow, average hydrodynamic and thermodynamic parameters (speed, pressure, density, temperature) of the flow in the burner, location of the flame front and Fuel mixing length. The mechanism of pulsation excitation used is that given by Rayleigh (JWS Rayleigh, The theory of sound, 1945, New York Dover Publications). Acoustic waves that propagate in the burner are reflected at the burner inlet, at its outlet and at the flame. Pressure fluctuations result in fluctuations in the equivalence ratio and consequently in the heat release, which then excite pressure fluctuations. The mechanism described above represents a closed control loop with feedback, which becomes unstable when the Rayleigh criterion is met (see, for example, DE-A1-199 39 235). The aim of the investigation was to test the sensitivity of the control loop to the various above

Rule parametric. In the above Article by A. Ni et al. the case of a single burner has been considered. In the present case, the collective effects due to the acoustic interaction between the burners via the acoustic waves propagating in the annular burner arrangement are also taken into account.

The results of the calculations which have been carried out in order to clarify the influence of the unevenness due to different burner characteristics on the growth rate of the pulsations are considered below. The calculations have been carried out for a real geometry of the burner arrangement. FIG. 2 shows the behavior of the pulsation

G.R. of a single burner depending on the

2π associated dimensionless frequency, Strouhal number Str, reproduced. In this calculation, the position of the flame front is Xf / L = 0.53. The result is a pronounced (negative) peak at a frequency of approximately Str = 2, which has also been found in experiments. FIG. 3 shows the dependence of the growth rate GR at this Strouhal number on the position of the flame front (X _f / L). One can see a strong dependence of the growth rate on X _f / L. A shift in the flame front of less than X _f / L = 0.53 brings the (Str = 2) mode from the unstable to the stable state and back again. Transferred to the dimensioned sizes, this means that a shift in the flame front by a few centimeters from the unstable to the stable state and back. This result is consistent with experiments conducted in a single burner test setup.

The calculations for a single burner are consistent with those for an annular burner arrangement with n = 24 identical burners. According to the calculations for the ring-shaped burner arrangement, the system is unstable if the flame fronts of all 24 burners have the coordinate Xf / L = 0.67. The growth rate of instability is shown in the table in FIG. 4. If the flame fronts of all 24 burners have the coordinate Xf / L = 0.4, the system is stable.

The results of the calculations for the mixed cases in which some burners are stable and some burners are unstable are shown in the table in FIG. 4. The burners are numbered consecutively in the azimuthal direction. The first column of the table shows the consecutive order numbers of the unstable burners with the dimensionless flame front position Xf / L = 0.67. All other burners are operated stably and have the coordinate X _f / L = 0.4. Both

The influence of the burners, which have been throttled to stability, has also been investigated in calculations. The throttling has been simulated by reducing the fuel supply to the respective burner. The third column of the table contains the numbers of the burners that have been throttled by 50%. The fourth column shows the calculated growth rate for the respective burner configuration. The data in the 10th row of the table mean, for example, that the three burners 1, 2 and 3 have the dimensionless flame front position X _f / L = 0.67 and the burners 1 and 2 are throttled.

The calculations can be summarized as follows:

1. The stability depends on the number of unstable burners. The more unstable burners there are, the greater the growth rate. 2. The stability depends on the mutual positioning of the unstable burners. Comparing the data in rows 5 and 7, it can be concluded that there are 4 unstable burners in both cases; however, the second case is more stable (the growth rate is smaller). There are three unstable burners in rows 3 and 4; however, they are both more stable than the case in row 7.

3. The stability is also strongly determined by the number and position of the throttled burners.

The following conclusions can be drawn from the analysis:

- Small changes in the location of the flame front for each of the individual burners push the system into or out of a stable operating range.

- The controlling parameters of the system are given the given geometry and acoustic boundary conditions

- inlet temperature,

- heat release in the burner arrangement,

- pressure drop in the fuel nozzle, and

- Mixing and ignition times. - In a ring-shaped system with a plurality of burners, the respective combination of unstable and stable burners has a decisive influence on the resulting growth rate of the instability.

- The stability of the overall system depends on the number of unstable burners. The more burners are unstable, the greater the growth rate. The stability of the burner assembly depends on the relative position of the unstable burner within the system. A coherent group of unstable burners makes the entire system unstable. Statistically distributed unstable burners do not necessarily lead to an unstable combustion system.

From the analysis, an operating concept according to the invention can now be developed, in which the introduction of small but controlled inhomogeneities can be used in the reheating burner assembly to effectively prevent the occurrence of high frequency vibrations. These inhomogeneities can either be introduced into the reheat burner assembly itself, or they can be introduced upstream in the first stage burner assembly such that an azimuthally inhomogeneous temperature field extends into the reheat burner assembly.

In order to suppress high-frequency instabilities occurring during combustion, at least a selected part of the burners B1, .., Bi, .., Bn are operated in a stable operating range in a predetermined unstable operating range of the burner arrangement 10.

On the one hand, this can be done by changing the heat release in the selected burners compared to the other burners, in particular the changed heat release being achieved in the selected burners by changing the fuel supply. A controllable valve or a restriction in the fuel supply is preferably used to change the fuel supply.

Alternatively, the inlet temperature in the selected burners can be changed compared to the other burners, in particular the changed inlet temperature in the selected burners being achieved by a targeted change in the fuel supply in the burner arrangement of the first stage of the combustion device.

If the burner arrangement is operated in an operating range with the addition of water, the changed inlet temperature in the selected burners can also be achieved by a targeted change in the water supply.

LIST OF REFERENCE NUMBERS

10 burner arrangement (ring-shaped) 11 combustion chamber

12 combustion chamber outlet

B1, B2, Bi, Bn burner

F1, F2 ₎ Fi, Fn flame x axial coordinate z circumferential coordinate

Claims

1. A method for operating an annular burner arrangement (10) in the intermediate heating stage of a multi-stage combustion device of a gas turbine, which burner arrangement comprises a plurality of similar burners (B1, .., Bi, .., Bn) arranged in a ring, characterized in that to suppress high-frequency instabilities occurring during combustion in a predetermined unstable operating range of the burner arrangement (10), at least a selected part of the burners (B1, .., Bi, .., Bn) is operated in a stable operating range.

2. The method according to claim 1, characterized in that in the selected burners the heat release is changed compared to the other burners.

3. The method according to claim 2, characterized in that the changed heat release in the selected burners is achieved by changing the fuel supply.

4. The method according to claim 3, characterized in that a controllable valve or a restriction in the fuel supply is used to change the fuel supply.

5. The method according to claim 1, characterized in that in the selected burners, the inlet temperature is changed compared to the other burners.

6. The method according to claim 5, characterized in that the changed inlet temperature in the selected burners is achieved by a targeted change in the fuel supply in the burner arrangement of the first stage of the combustion device.

7. The method according to claim 5, characterized in that the burner arrangement is operated in an operating range with the addition of water, and that the changed inlet temperature in the selected burners is achieved by a targeted change in the water supply.

8. The method according to claim 1, characterized in that the remaining burners operating in the unstable region are operated in a suitable manner distributed over the annular burner arrangement (10).