DE10325455A1 - Method for operating an annular burner arrangement in an intermediate heating stage of a multi-stage combustion device of a gas turbine - Google Patents

Abstract

In a method for operating an annular burner arrangement (10) in the intermediate heating stage of a multi-stage combustion device of a gas turbine, the burner arrangement comprising a plurality of similar burners (B1, ..., Bi, ..., Bn) arranged in a ring is improved Operation achieved in that at least a selected part of the burners (B1, ..., Bi, ..., Bn) taken in one in order to suppress high-frequency instabilities occurring during combustion in a predetermined unstable operating range of the burner arrangement (10) stable operating range is operated.

Description

TECHNICAL TERRITORY

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

STATE OF TECHNOLOGY

In a gas turbine with a reheater system becomes one second combustion zone generated by fuel in a High velocity flow polluted air is injected from a first turbine stage exit. Such high-speed flows into an exhaust-side combustion chamber occur, are often vulnerable for high frequencies instabilities which can generate high amplitude pulsations. The suppression of such Pressure pulsation is critical to maintaining the quality of the combustion process, and for avoiding deterioration of the burner structure. It is also It has been found that the excitation of such high frequency instabilities is sensitive is opposite minor changes the working point (such as changes in mass flow or temperature), so the type of combustion while of the operation of the machine quickly change from stable to unstable can.

In an older one not yet published application a procedure has been proposed to the applicant by which Walls in reheat burner assemblies can be interpreted that both the cooling effect as well as acoustic damping are maximized. As a result, the high frequencies for one wide range of operating parameters attenuated in the unstable area.

In DE-A1-199 39 235 describes a method for operating a combustion device described a gas turbine in which a plurality of in one common combustion chamber working burners of the same thermal Suppression performance so different from thermo-acoustic combustion instabilities are formed such that the flames or flame fronts generated by them are positioned distributed along the common axis. adversely is the different design of the burners, which leads to increased and maintenance costs.

The In contrast, the present invention shows another way of suppressing high-frequency vibrations in the combustion zone of reheat systems. she assumes an annular Burner arrangement with a plurality of similar ones arranged in a ring Burners, but operated under different conditions can be.

PRESENTATION THE INVENTION

It The object of the invention is a method for operating a burner arrangement in the reheating stage of a multi-stage incinerator to specify a gas turbine with which effective suppression of instabilities possible even if the burner arrangement is inherently unstable Operating area is operated.

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

A first preferred embodiment of the invention The process is characterized in that in the selected burners the heat release across from changed the other burners is that the changed Heat release in the selected one Distillers through a change the fuel supply is achieved, and that to change the fuel supply a controllable valve or a restriction is used in the fuel supply.

A second preferred embodiment is characterized in that the inlet temperature in the selected burners across from changed the other burners is and that the changed inlet temperature in the selected Burners through a targeted change the fuel supply in the burner arrangement of the first stage of the combustion device is achieved.

alternative For this purpose, the burner arrangement can be added in an operating range operated by water, and the changed inlet temperature in the selected one Burners through a targeted change water supply can be achieved.

A draws another preferred embodiment is characterized by the fact that the burners working in the unstable area are connected to the annular Burner arrangement can be operated in a suitable manner.

Further embodiments result from the dependent Claims.

SHORT EXPLANATION THE FIGURES

The In the following, the invention is to be described in connection with exemplary embodiments with the drawing closer explained become. Show it

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

3 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 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 TO EXECUTE THE INVENTION

The Stimulation of burn instabilities can have various causes have, such as a feedback mechanism between shear layer instabilities and fluctuations in heat release, or a feedback mechanism between the local heat release and fluctuations in the equivalence ratio due to a fluctuating column of air inside the burner element. Typically most describe these effects by a characteristic time delay, which is essentially on a convective time scale between the Fuel injection location (or: the origin of the shear layer instability) and the local one Location of the flame front based. As a result, these are mechanisms sensitive to the middle flame place.

The observed in a sequential combustion and high speed flow system Frequencies are typically in the 1-10 kHz range.

dargestellt werden, wobei L eine charakteristische Brennerlänge und U eine charakteristische Brennergeschwindigkeit sind. In diesem Bereich verursachen selbst kleine Variationen im Flammenort deutliche Wirkungen auf den Mechanismus der Schallerzeugung, d.h., Änderungen von stabilen zu instabilen Zuständen.The frequency f can also be dimensionless as a Strouhal number

are represented, where L is a characteristic burner length and U is a characteristic burner speed. In this area, even small variations in the flame location cause significant effects on the mechanism of sound generation, that is, changes from stable to unstable states.

• – die Einlasstemperatur an der Zwischenüberhitzungs-Brenneranordnung mittels eines abgestuften Betriebs von Brennergruppen in der ersten Verbrennungsstufe; und
• – die Wärmefreisetzung in der Zwischenüberhitzungs-Brenneranordnung mittels individueller Regelventile oder -düsen in den Brennstoff-Versorgungsleitungen der Zwischenüberhitzungsbrenner.

The acoustic interaction in an annular burner arrangement for reheating with any number of burners can be described as follows using a model:
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 the aid of which one can avoid the occurrence of high-frequency vibrations, namely

• - The inlet temperature at the reheater burner arrangement by means of a staged 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 inhomogeneities, by the two above Process (inlet temperature, heat release) introduced be on the stability of the Burner arrangement has been investigated using numerical methods.

• – Wenn Imω > 0 ist, verschwinden die Pulsationen mit der Zeit und der zugehörige Pulsationsmode ist stabil.
• – Wenn Imω < 0 ist, wachsen die Störungen exponentiell mit der Zeit und der zugehörige Pulsationsmode ist instabil.
• – Der dazwischenliegende Fall Imω = 0 entspricht dem neutralen Mode. Der Imaginärteil Imω der Frequenz charakterisiert die Wachstumsrate der Störungen und gibt ein Mass für die Instabilität der Brenneranordnung.

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 f (r →, t) = F (r →) exp (iωt), (1) the interference amplitude F is exclusively a function of the spatial coordinates. Mathematically, the problem of stability is reduced to a problem of eigenfunctions. The solution to the problem yields 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 with 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.

For stability analysis the following simplifying approximations are used: The annular burner arrangement becomes infinitely thin assumed (see: S.R. Stow, A. Dowling, Thermoacoustic oscillations in an annular combustor, ASME Turbo Expo 2001, Paper 2001-GT-0038, (2001)).

The radial dependence of the disturbances is neglected within this approximation. 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. In 1 is a schematic illustrated development of an annular burner assembly 10 with n burners B1, .., Bi, .., Bn reproduced in a common combustion chamber 11 with a combustion chamber outlet 12 work. 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) caused by the shape of the burner arrangement 10 is set. The flames F1, .., Fi, .., Fn emanating from the burners B1, .., Bi, .., Bn are in 1 also drawn. The distance between the outlet of the i-th burner Bi and the associated flame Fi is designated as x _{f, i} .

The Brenner model that 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 determined the acoustic behavior of the individual burner in the stability analysis. The input sizes of the model are: geometry of the burner, pressure drop in the fuel supply line, fuel mass flow, mean hydrodynamic and thermodynamic parameters (speed, Pressure, density, temperature) of the flow in the burner, location of the flame front and fuel mix length. The mechanism of pulsation excitation used is that of Rayleigh (J.W.S. Rayleigh, The theory of sound, 1945, New York Dover Publications). Acoustic waves that spread in the burner at the burner inlet, at its outlet and on the flame reflected. Pressure fluctuations result in fluctuations in the equivalence ratio and consequently in the heat release, which then stimulate pressure vibrations. The mechanism described above represents a closed control loop with feedback, which is then unstable if the Rayleigh criterion is met (see e.g. the DE-A1-199 39 235). The aim of the investigation was to examine the Sensitivity of the control loop to the different above Control parameters. In the above Article by A. Ni et al. is the case of an individual Brenners has been considered. In the present case, the collective Effects due to the acoustic interaction between the burners over the in the annular burner arrangement propagating acoustic waves are also taken into account.

eines einzelnen Brenners in Abhängigkeit von der zugehörigen dimensionslosen Frequenz, Strouhalzahl Str, wiedergegeben. In dieser Berechnung ist die Position der Flammenfront X_f/L = 0,53. Man sieht im Ergebnis eine ausgeprägte (negative) Spitze bei einer Frequenz von ungefähr Str = 2, die auch in Experimenten festgestellt worden ist. In 3 ist die Abhängigkeit der Wachstumsrate G.R. bei dieser Strouhalzahl von der Position der Flammenfront (X_f/L) dargestellt. Man erkennt eine starke Abhängigkeit der Wachstumsrate von X_f/L. Eine Verschiebung der Flammenfront von weniger als X_f/L = 0,53 bringt den (Str = 2)-Mode vom instabilen in den stabilen Zustand und wieder zurück. Übertragen auf die dimensionsbehafteten Grössen heisst das, dass einer Verschiebung der Flammenfront um einige wenige Zentimeter vom instabilen in den stabilen Zustand führt und zurück. Dieses Ergebnis stimmt mit Experimenten überein, die in einer Testanordnung mit einem einzelnen Brenner durchgeführt worden sind.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. In 2 is the behavior of the pulsation growth rate

of a single burner as a function of the associated dimensionless frequency, Strouhal number Str. In this calculation, the position of the flame front is X _f / L = 0.53. The result is a pronounced (negative) peak at a frequency of approximately Str = 2, which has also been found in experiments. In 3 the dependence of the growth rate GR at this Strouhal number on the position of the flame front (X _f / L) is shown. 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 dimensional sizes, this means that a shift of the flame front by a few centimeters leads 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 annular burner arrangement, the system is unstable if the flame fronts of all 24 burners have the coordinate X _f / L = 0.67. The growth rate of instability is shown in the table 4 played. If the flame fronts of all 24 burners have the coordinate X _f / L = 0.4, the system is stable.

The results of the calculations for the mixed cases where some burners are stable and some burners are unstable are in the table of 4 compiled. 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 X _f / L = 0.67. All other burners are operated stably and have the coordinate X _f / L = 0.4. The influence of the burners, which have been throttled to stability, has also been examined in the 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.

• 1. Die Stabilität hängt von der Anzahl der instabilen Brenner ab. Je mehr instabile Brenner vorhanden sind, desto grösser ist die Wachstumsrate.
• 2. Die Stabilität hängt von der wechselseitigen Positionierung der instabilen Brenner ab. Vergleicht man die Daten in den Reihen 5 und 7, kann man zu dem Schluss kommen, dass in beiden Fällen 4 instabile Brenner vorhanden sind; jedoch ist der zweite Fall stabiler (die Wachstumsrate ist kleiner). In den Reihen 3 und 4 gibt es drei instabile Brenner; sie sind jedoch beide stabiler als der Fall in Reihe 7.
• 3. Die Stabilität wird auch stark durch die Anzahl und Position der gedrosselten Brenner bestimmt.

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.

• – Kleine Änderungen im Ort der Flammenfront bei jedem der einzelnen Brenner schieben das System in einen stabilen Betriebsbereich hinein bzw. aus dem Bereich heraus.
• – Die steuernden Parameter des Systems sind bei vorgegebener Geometrie und akustischen Randbedingungen – Einlasstemperatur, – Wärmefreisetzung in der Brenneranordnung, – Druckabfall in der Brennstoffdüse, und – Mischungs- und Zündzeit.
• – In einem ringförmigen System mit einer Mehrzahl von Brennern hat die jeweilige Kombination von instabilen und stabilen Brennern einen entscheidenden Einfluss auf die resultierende Wachstumsrate der Instabilität.
• – Die Stabilität des Gesamtsystems hängt von der Anzahl der instabilen Brenner ab. Je mehr Brenner instabil sind, umso grösser ist die Wachstumsrate.
• – Die Stabilität der Brenneranordnung hängt von der relativen Position der instabilen Brenner innerhalb des Systems ab. Eine zusammenhängende Gruppe von instabilen Brennern macht das gesamte System instabil. Statistisch verteilte instabile Brenner führen nicht notwendigerweise zu einem instabilen Verbrennungssystem.

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 time.
• - 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 arrangement 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.

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

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

This can happen on the one hand in that in the selected burners the heat release across from the other burners have changed the modified one in particular heat release in the selected Distillers through a change the fuel supply is reached. To change the fuel supply is preferably a controllable valve or a restriction used in the fuel supply.

alternative can in the selected Burner the inlet temperature has changed compared to the other burners, the modified one in particular Inlet temperature in the selected burners through a targeted change the fuel supply in the first stage burner assembly Combustion device is reached.

If the burner arrangement in an operating area with the addition of water is operated, but can also change the inlet temperature in the chosen Burners through a targeted change water supply can be achieved.

10

burner arrangement (Annular)

11

combustion chamber

12

combustor

B1, B2, Bi, Bn

burner

F1, F2, Fi, Fn

flame

x

axial coordinate

Z

Circumferential coordinate

Claims (8)

Method of 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 arranged in a ring (B1, .., Bi, .., Bn), characterized in that in order 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 by itself in a stable operating range. A method according to claim 1, characterized in that in the selected Burners heat release across from changed the other burners is. A method according to claim 2, characterized in that the changed heat release in the selected Burners by changing the Fuel supply is reached. A method according to claim 3, characterized in that change the fuel supply a controllable valve or a restriction is used in the fuel supply. A method according to claim 1, characterized in that in the selected Burner the inlet temperature has changed compared to the other burners. A method according to claim 5, characterized in that the changed Inlet temperature in the selected Burners through a targeted change the fuel supply in the first stage burner assembly Combustion device is reached. A method according to claim 5, characterized in that the burner assembly in an operating area with the addition of Water is operated, and that the changed inlet temperature in the selected one Burners through a targeted change the water supply is reached. A method according to claim 1, characterized in that the remaining burners operating in the unstable area via the annular burner arrangement ( 10 ) are distributed in a suitable manner.