EP1205713B1 - Method of fuel injection in a burner - Google Patents

Description

TECHNICAL AREA

The present invention relates to a method of injecting fuel into a burner, which is designed as a double-cone burner, and to a burner for carrying out this method.

STATE OF THE ART

In burners, which supply liquid or gaseous fuel to a combustion chamber where the fuel burns at a flame front, so-called thermoacoustic fluctuations often occur. So also, for example, but not exclusively, in the very successfully used so-called double-cone burner, as in the EP 0 321 809 is described. In addition to the fluidic stability, mixture rupture fluctuations are a major reason for the occurrence of such thermoacoustic instabilities. Fluid-mechanical instability waves, which arise at the burner, lead to the formation of vortices (coherent structures), which influence the combustion and can lead to periodic heat release with the associated pressure fluctuations. The fluctuating air column in the burner leads to fluctuations in the mixing fraction with the associated fluctuations in the heat release.

These thermoacoustic vibrations pose a danger to any type of combustion application. They lead to high amplitude pressure oscillations, a limitation of the operating range and can increase pollutant emissions. This is especially true for low acoustic attenuation combustion systems. In order to allow high power conversion over a wide operating range in terms of pulsations and emissions, active control of the combustion vibrations may be necessary. Coherent structures play a crucial role in mixing processes between air and fuel. The dynamics of these structures consequently influence the combustion and thus the heat release. By influencing the shear layer between the fresh gas mixture and the recirculated exhaust gas, a control of the combustion instabilities is possible (eg. Paschereit et al., 1998, "Structure and Control of Thermoacoustic Instabilities in a Gas Turbine Bumer", Combustion, Science & Technology, Vol. 138, 213-232 ). One possibility is the acoustic stimulation ( EP 0 918 152 A1 ).

Through fuel staging, the flame position can be influenced and thus the influence of flow instabilities as well as time delay effects reduced.

Another mechanism that can lead to thermoacoustic oscillations are fluctuations in the mixture break between fuel and air

In DE 44 46 945 A1 discloses a double-cone burner, in which the delay time from the injection to the combustion of all fuel streams is the same size, so that disadvantageous thermoacoustic oscillations occur. At the in EP 0 592 717 A1 described premix burner, the fuel concentration at the outlet of the burner in the region of the burner axis is greater than the average fuel concentration in the exit plane of the burner. Thus, the combustion chamber can be operated near the lean extinction limit. Finally is off DE 198 09 364 A1 a burner is known in which via axially distributed fuel nozzles fuel is injected to improve the burner stability by damping the amplitude change. The burner is operated so that there is a decoupling of the fuel from the combustion and the dynamic pressure instability is reduced in the combustion chamber.

PRESENTATION OF THE INVENTION

The invention is therefore an object of the invention to provide a method and a Brenne the Doppelkegelbauart for performing such a method in which the occurrence of such thermoacoustic vibrations is reduced or even avoided.

It is a method for injecting fuel into a burner, which burner comprises an interior space enclosed by at least one shell, in which fuel is injected through fuel nozzles into a combustion air stream flowing in the interior, the resulting fuel / air mixture within a delay time τ flows to a flame front in a combustion chamber, and ignited there, wherein the burner (1) is a double-cone burner, wherein the burner (1) from at least two successive hollow cone bodies (8,9), which in the flow direction Have increasing conical inclination, and which part cone body (8,9) are arranged offset to each other, so that the combustion air (23) through a gap (7) between the part cone bodies (8,9) flows into the interior (22) is formed. In such a method, according to the invention, thermoacoustic oscillations are reduced or even completely avoided by injecting the fuel via fuel nozzles distributed over the entire burner length and arranged on the cone surfaces of the partial cone bodies (8, 9) on a specific area of the flame front in that the delay time τ between injection of the fuel and its combustion at the flame front for the different fuel nozzles corresponds to a distribution which varies systematically over the burner length and avoids combustion-induced vibrations.

In a conventional burner, experience has shown that the delay time τ between injection location and effective combustion at the flame front is essentially the same for all fuel nozzles distributed over the burner length. One finds an unsystematic slight variation around the mean value of the injection position. As a result, thermoacoustic vibrations can easily build up. The essence of the invention is thus to inject the fuel into the combustion air stream in such a way that no fuel nozzle that is essentially distributed over the entire burner length is set to the same delay time τ between injection site and effective combustion at the flame front, but that the delay time exceeds one Burner length systematically assumes varying distribution.

A first preferred embodiment of the invention is characterized in that the maximum time delay τ _max between Eindüsungsort and flame front in the range of τ _max = 5-50 ms, and that particularly preferably at a flow rate of the fuel / air mixture in the interior in the area of 20-50 m / s, the maximum time delay τ _{max is} in the range of τ _max = 5-15 ms, taking into account the displacement of the flame front position as a function of the flow velocity. If the method is used under such conditions, the thermoacoustic vibrations can be reduced particularly well.

In a further embodiment of the invention, the fuel is injected in such a way that the time delay distribution over the burner length towards the burner end is designed to be substantially linearly decreasing from the maximum value τ _max by a maximum delay difference Δτ to a minimum value at the burner end of τ _max -Δτ. This simple distribution can be realized with relatively little effort and shows an efficient effect. It can be seen that preferably the delay difference Δτ is set in the range of 10-90% of the maximum value τ _max , in particular in the range of more than 50% of the maximum value τ _max

The method is a double-cone burner in which the burner of at least two successive hollow cone bodies, which have an increasing conical inclination in the flow direction, and which part cone bodies are arranged offset to each other, so that the combustion air through a gap between the part cone bodies in the Interior flows, is formed. Especially in this, already mentioned above, premix-type double-cone burner, the method can be applied particularly favorable.

Furthermore, the invention relates to a double-cone burner for carrying out the above method, wherein the fuel nozzles are arranged on the cone surfaces of the Teikegelkörper on lines for a region of the flame front wherein the fuel nozzles are divided into groups, and wherein each of a group of Fuel nozzles is arranged in a line such that all the fuel nozzles of a group are responsible for the supply of the same area in the flame front. Particularly preferred in such a burner, the fuel nozzles are distributed such that the number of lines is greater than the average number of fuel nozzles of a group. It turns out that a division of the total of 32 nozzles of a double-cone burner in 8 groups on 8 lines with 4 nozzles is advantageous.

BRIEF EXPLANATION OF THE FIGURES

Fig. 1a)

einen konventionellen Doppelkegelbrenner mit typischer Brennstoffeindüsung;

Fig. 1b)

die bei einem Brenner nach Fig. 1a) auftretende schematisierte Verzugszeitverteilung über die Brennerlänge;

Fig. 2

eine lineare Verzugszeitverteilung;

Fig. 3

eine zweidimensionale Stabilitätsanalyse von Verzugszeitverteilungen;

Fig. 4a)

einen Doppelkegelbrenner mit verteilter Brennstoffdüsenanordnung; und

Fig. 4b)

mögliche Verzugszeitenverteilungen bei einem Brenner nach Fig. 4a).

The invention will be explained in more detail with reference to embodiments in conjunction with the drawings. Show it:

Fig. 1a)

a conventional double-cone burner with typical fuel injection;

Fig. 1b)

the schematic delay time distribution occurring over the torch length in the case of a burner according to FIG. 1a);

Fig. 2

a linear delay time distribution;

Fig. 3

a two-dimensional stability analysis of delay time distributions;

Fig. 4a)

a dual cone burner with a distributed fuel nozzle assembly; and

Fig. 4b)

possible delay time distributions in a burner according to Fig. 4a).

WAYS FOR CARRYING OUT THE INVENTION

nicht mehr erfüllt ist, d.h. Wärmefreisetzung und Druckmaximum nicht mehr in Phase sind. Damit ist der treibende Mechanismus für das Auftreten von thermoakustischen Schwingungen unterbunden. Die Darstellung des Rayleigh-Kriteriums nach Fouriertransformation im Frequenzbereich zeigt diesen Zusammenhang noch deutlicher: $$G\left(x\right)=2\int \left|S_{\mathit{pq}}\left(x,,f\right)\right|\cos \left({\phi }_{\mathit{pq}}\right)df$$

wobei S_pq das Kreuzspektrum zwischen Druckfluktuationen p' und Fluktuationen der Wärmefreisetzung q' darstellt und φ_pq die Phasendifferenz. Durch Wahl der korrekten Phasendifferenz zwischen Wärmefreisetzung (beeinflussbar durch den Zeitverzug) und dem Drucksignal kann der Rayleigh-Index auf G(x) < 0 eingestellt werden und damit ist das System gedämpft.If one influences the time lag between the fuel injection and the periodic heat release, ie the flame front, one can control the combustion instabilities. The basic idea of the invention is to disrupt the time delay τ between the periodic heat release at the flame front and the pressure fluctuation during the injection, so that the Rayleigh criterion $$G\left(x\right)=\frac{1}{T}\underset{0}{\overset{T}{\int }}p\text{'}\left(x,,t\right)q\text{'}\left(x,,t\right)dt<0$$

is no longer satisfied, ie heat release and maximum pressure are no longer in phase. This prevents the driving mechanism for the occurrence of thermoacoustic oscillations. The representation of the Rayleigh criterion after Fourier transformation in the frequency domain shows this relationship even more clearly: $$G\left(x\right)=2\int \left|S_{\mathit{pq}}\left(x,,f\right)\right|\cos \left({\phi }_{\mathit{pq}}\right)df$$

where S _{pq represents} the cross spectrum between pressure fluctuations p 'and fluctuations of the heat release q' and φ _pq the phase difference. By choosing the correct phase difference between heat release (influenced by the time delay) and the pressure signal, the Rayleigh index can be set to G (x) <0 and thus the system is muted.

It now turns out that the time delay from the point of injection at the fuel nozzles to the flame front in existing premix burners over the entire injection length of the

Premixed gas is constant at certain operating points. For example, in a double-cone burner according to the prior art as shown in Fig. 1a).

In this example to be understood longitudinal section through a double-cone burner 1, as for example from the EP 0 321 809 is known, the upper gap 7 between the two conical burner shells 8 and 9 can be seen. The combustion air 23 passes through this gap 7 to the distributed over the burner length fuel nozzles 6 over into the interior 22, wherein the fuel is detected and enclosed by the passing air 23. In the interior 22 of the burner 1, the combustion air stream flows to form a in

Flow direction propagating conical fuel column along the stomata. 5

The fuel / air mixture then passes into the combustion chamber 2, where it ignites at a flame front 3.

In such a double-cone burner, the delay time τ, which elapses between the injection at the fuel nozzles 6 to the ignition at the flame front 3, almost constant for all positions of the fuel nozzles, as shown schematically in Fig. 1b) (the coordinate x extends from the outlet 10 of the burner 1 to its rear end, ie in the figure 1a) from right to left). In other words, no systematic variation of the delay times τ as a function of the fuel nozzle position along the burner 1 can be observed (eg shorter delay times for nozzles 6 near the burner exit 10), but rather a more or less random distribution which fluctuates only a little as a function of injection location x.

As shown in FIG. 2, it is proposed according to the invention to set a distribution of the delay time over the burner length instead of the hitherto substantially constant time delay from the fuel injection 6 to the flame front 3. The distribution is set in a first choice so that the delay times τ linearly varies by a delay time difference Δτ, linearly increasing from a minimum τ _max -Δτ to the maximum in the rear burner region of τ _max .

In a two-dimensional representation, in FIG. 3 the burner stability as a function of the parameters Δτ (x-axis) and τ _max (y-axis) for a delay time distribution is indicated as in FIG. 2. As an example, three values for the behavior at different flow velocities in the burner are given as individual measured values: for low flow velocity 17, for average flow velocity 18 and for high flow velocity 19. In general, it turns out that two basically unstable, here hatched areas form. On the one hand, an unstable region 16 short delay times.

Almost regardless of the choice of Δτ, the burner is not acoustically stable here for such high flow velocities. A second, island-like region 13 of unstable behavior is found for low velocities, ie high values of τ _max , and for small values of Δτ.

It can easily be seen that the stability of a burner, which operates with its typical operating values mostly close to the island 13, can be stabilized both by an increase in the flow velocity according to arrow 15, and by an increase in the delay time difference Δτ, ie by a Shift of the operating point in the graph according to arrow 14 to the right. Because of practical reasons, the value of τ _{max is} not always in the stable low range gem. 15 is a shift by setting increased Verzugseitendalterenzen Δτ, ie over more spread delay times, often an efficient and feasible alternative.

Typically, the delay times for burners are in the range of τ = 5-50 ms, for double-cone burners normally in the range of 5-15 ms at flow rates of 10-50 m / s. Δτ can be varied over a wide range now, but typically find variations of Δτ = 0.5 τ _max application in double cone burners, a variation of Δτ proves ≥ 0.5 τ _max be particularly advantageous.

Such a distribution can be implemented technically on a double-cone burner serving as an exemplary embodiment, as already illustrated in FIG. 1, by way of a simple modification of the fuel injection into the combustion air stream 23. The fuel nozzles 6 become no longer arranged directly on the column 7 between the two shells 8 and 9 here, but are resp. embedded in the cone surfaces of the elements 8 and 9, and thereby systematically set the delay times. For this purpose, the fuel nozzles 6 can be divided into groups, and in each case the fuel nozzles of a group are arranged on lines 20 which follow the flow lines along the burner contour. Nozzles of a group feed a certain area of the flame front, but with a different delay time τ between moment of injection and arrival at the flame front 3. It is advantageous to form as many groups as possible to form a uniformly submerged flame for additional scattering of the time delay. In the case of the number of 32 nozzles typical for double-cone burners due to the pressure drop, for example, a division into 8 groups, each of which 4 nozzles (each two per cone 8 or 9) being arranged on 8 lines with the same time delay, is suitable for the thermoacoustic oscillations prevent.

An arrangement of the fuel nozzles 6 on such lines 20 now allows the setting of delay time distributions in an entire region 21, as shown in Fig. 4b). Of course, other arrangements of the fuel nozzles to resp. possible in a burner, which lead to a targeted, thermoacoustic vibrations preventing systematic distribution of delay times, and the presented embodiment and the specified, substantially linear distributions are to be understood as exemplary only.

NAME LIST

1

Double-cone burner

2

combustion chamber

3

flame front

4

Wall of the combustion chamber

5

Streamlines of the fuel / air mixture

6

fuel nozzles

7

Gap between the conical burner shells

8th

Inner conical burner bowl at 7

9

Outer conical burner bowl at 7

10

Front end of the double cone burner

11

Constant time delay

12

Delay distribution

13

Unstable range of high delay times

14

Stabilizing shift after large distribution widths

15

Stabilizing shift after short delay times

16

Unstable range of short delay times

17

Behavior at low flow velocity

18

Behavior at medium flow rate

19

Behavior at high flow velocity

20

Lines for same area of the flame front

21

Adjustable time delay range

22

inner space

23

Combustion air flow

Claims (9)

1. Method for injecting fuel into a burner (1), which burner (1) comprises an interior space (22) which is enclosed by at least one shell (8, 9), in which fuel is injected through fuel nozzles (6) into a combustion air flow (23) which flows in the interior space (22), the resulting fuel/air mixture, within a delay time (τ), flows towards a flame front (3) into a combustion chamber (2), and ignites there, wherein the burner (1) is a double-cone burner, in which the burner (1) is formed from at least two hollow partial cone bodies (8, 9), which are positioned one upon the other and which in the flow direction have an increasing cone inclination, and which partial cone bodies (8, 9) are arranged in an offset manner in relation to each other, so that the combustion air (23) flows into the interior space (22) through a gap (7) between the partial cone bodies (8, 9), wherein
   the fuel is injected by means of fuel nozzles (6), which are distributed over the entire length of the burner and arranged on the cone surfaces of the partial cone bodies (8, 9) on lines (20) which feed a defined region of the flame front, in such a way that the delay time (τ) between injection of the fuel and its combustion in the flame front (3) for the different fuel nozzles corresponds to a distribution (12) which systematically varies over the length of the burner, which avoids combustion-driven oscillations.
2. Method according to Claim 1, characterized in that the maximum time delay (τ_max) between injection location (6) and flame front (3) is in the region of τ_max= 5 - 50 ms.
3. Method according to Claim 2, characterized in that at a flow velocity of the fuel/air mixture in the interior space in the region of 20 - 50 m/s, the maximum time delay (τ_max) is in the region of τ_max = 5 - 15 ms.
4. Method according to one of the above claims, characterized in that the fuel is injected in such a way that the time delay distribution (12) over the length of the burner is designed in a way in which it basically linearly decreases towards the burner end (10) from the maximum value τ_max by a maximum delay difference (Δτ) to a minimum value at the burner end (10) of T_max - Δτ.
5. Method according to Claim 4, characterized in that the delay difference (Δτ) is in the region of 10 - 90% of the maximum value (τ_max), especially in the region of more than 50% of the maximum value (τ_max).
6. Burner (1) for implementing a method according to one of Claims 1 to 5, wherein the burner (1) comprises an interior space (22) which is enclosed by at least one shell (8, 9), in which fuel is injected by means of fuel nozzles (6), which are arranged along the entire length of the burner, into a combustion air flow (23) which flows in the interior space (22), the resulting fuel/air mixture within a time delay (τ) flows towards a flame front (3) into a combustion chamber (2), and ignites there, wherein the burner (1) is a double-cone burner, in which the burner (1) is formed from at least two hollow partial cone bodies (8, 9) which are positioned one upon the other and which are arranged in an offset manner in relation to each other, so that the combustion air (23) flows into the interior space (22) through a gap (7) between the partial cone bodies (8, 9), wherein the fuel nozzles are arranged on the cone surfaces of the partial cone bodies (8, 9) on lines (20) which feed a defined region of the flame front, characterized in that the partial cone bodies (8, 9) have an increasing cone inclination in the flow direction.
7. Burner (1) according to Claim 6, characterized in that the fuel nozzles (6) are divided into groups, wherein one group of fuel nozzles (6) is arranged on a line (20) in each case in such a way that all the nozzles (6) of the group feed a defined region of the flame front (3) with a different time delay (τ).
8. Burner (1) according to Claim 7, characterized in that the number of lines (20) is greater than the average number of fuel nozzles (6) of a group.
9. Burner (1) according to Claim 8, characterized in that the burner has altogether 32 nozzles, which are divided into 8 groups on 8 lines (20) with 4 nozzles each.

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