EP0576697B1 - Combustor chamber for a gas turbine - Google Patents

Description

Technical field

The invention relates to the combustion chamber of a gas turbine, in which catalytic burners are used in addition to classic burner types.

State of the art

The combination of diffusion burners and catalytic burners is known. It is used in a kind of mixed operation, whereby the combustion chamber is usually started up to a certain partial load in pure diffusion operation. Then more and more catalytic burners are switched on. The aim is to run the combustion chamber in full-load operation in purely catalytic operation. The catalytic burners are characterized by the fact that they remain operational even with a very lean fuel-air mixture. On the other hand, they also have typical disadvantages such as a lack of multi-fuel capability, slow controllability, problematic ignition and start-up.

On the other hand, combustion chambers for gas turbines based on premix burners are known, for example from EP-B1 29 619. Within a number of tubular elements, a pre-mixing / pre-evaporation process with a large excess of air takes place between the injected fuel and the compressor air before the actual combustion process downstream of a flame holder takes place. With this measure, the emission values of pollutants from combustion can be significantly reduced.

Combustion with the largest possible excess of air, given that the flame is still burning and then that not too much CO is produced, not only reduces the amount of NO _x pollutants, but also lowers other pollutants, namely how already mentioned about CO and unburned hydrocarbons. This optimization process can be driven in the direction of even lower NO _x values such that the space inside the combustion chamber for the combustion and after-reactions is dimensioned larger than would be necessary for the actual combustion. This allows a larger excess air number to be selected, with larger quantities of CO initially initially being produced, but these being able to react further to CO ₂ , so that ultimately the CO emissions remain low. On the other hand, due to the large excess of air, little additional NO is formed. Since several tubular elements take over the premixing in this known combustion chamber, only so many elements are operated with fuel in the load control that the optimum excess air number results for the respective operating phase (start, part load, full load).

Other types of premix burners in which flame holders can be dispensed with are known in the form of the double-cone burners according to EP-B1-0 321 809.

However, all combustion chambers with premix burners have the inadequacy that, at least in the operating states in which only some of the burners are operated with fuel, the flame stability is pushed close to the limit. In fact, due to the very lean mixture and the resulting low flame temperature under typical gas turbine conditions, the extinguishing limit is already reached with an excess air ratio of approximately 2.0.

A combustion chamber is known from US-A-4,040,252, in which catalytic afterburner is supplied with the combustion gases of a premix combustion and additional air.

Presentation of the invention

The invention tries to avoid all these disadvantages. It is based on the task of creating a measure by means of which the combustion chamber can be operated as close as possible to the lean extinguishing limit, ie in the area in which practically no NO _x is produced.

This is achieved according to the invention with the features of the claims.

The advantages of the invention can be seen, inter alia, in the pure support of the combustion chamber in critical phases, for example in the event of a temporary occurrence of vibrations in which the extinguishing limit for pure premix burners can be exceeded temporarily. The fact that the catalytic burners remain operational with a very lean mixture means that the control can be simplified to the extent that when loading and unloading the gas turbine and the combustion chamber, respectively, air ratio ranges can be traversed that cannot be passed through with pure premix combustion because of their lean extinguishing limit could.

This targeted use of the catalytic burner can avoid the disadvantages mentioned at the outset.

It is particularly expedient if the premix burner and the catalytic burner are designed to be interchangeable. This provides a simple means of adapting the burner configuration to the particular combustion chamber operation, for example with regard to fuel or pressure. In principle, the aim is to be able to operate the combustion chamber without a catalytic burner in order to take full advantage of the premix combustion. The interchangeability of the different types of burners can thus be regarded as a sensible option for switching on catalytic burners as required, whereby only as many catalytic burners are used as are required for stable operation of the combustion chamber.

It is also advantageous if the catalytic burners are provided with exhaust gas recirculation, the exhaust gas preferably being removed from the combustion chamber. This measure is based on the idea of giving the combustion mixture the minimum temperature necessary for the operation of the catalytic burner. This makes it possible to dispense with the pre-burners that were customary in catalytic burners.

It is particularly useful here if the entry of the combustion air for the catalytic burner is designed as a jet pump, the exhaust gas being drawn in from the combustion chamber via this jet pump.

Finally, the catalytic burners are advantageously arranged in the primary zone of the combustion chamber in highly stressed wall parts, where they perform a kind of heat shield function. This measure makes it possible to dispense with the usual wall cooling at the relevant points, which meets the requirement for the surface to be cooled as little as possible.

Brief description of the drawing

Fig.1

einen Teillängsschnitt einer Brennkammer;

Fig.2

einen Querschnitt durch einen Vormischbrenner;

Fig.3

eine Brenneranordnung im Querschnitt;

Fig.4

eine Brennstoffregelkurve zum Belasten der Brennkammer im Gasbetrieb.

In the drawing, an embodiment of the invention is shown schematically.
Show it:

Fig. 1

a partial longitudinal section of a combustion chamber;

Fig. 2

a cross section through a premix burner;

Fig. 3

a burner assembly in cross section;

Fig. 4

a fuel control curve for loading the combustion chamber in gas operation.

Only the elements essential for understanding the invention are shown. The system does not show, for example, the arrangement and arrangement of the combustion chamber on the rotating machines, the provision of fuel, the control devices and the like. The direction of flow of the working means is indicated by arrows.

Way of carrying out the invention

In FIG. 1, several premix burners 10 and catalytic burners 20 are arranged in the dome-shaped end of a combustion chamber in the combustion chamber wall 1. The latter are located locally at locations that would normally need to be cooled heavily. They essentially consist of the actual catalyst 21, which is surrounded by a bell-shaped housing 22. A fuel feed 23 penetrates the housing wall, gas preferably being used as the fuel. The combustion air is conducted into the interior of the housing via an annular air inlet 24. The combustion air at the outlet of the gas turbine compressor (not shown) generally has a temperature of approximately 350 ° C. This is not enough to keep the catalytic combustion going.

The air inlet 24 is therefore designed as a jet pump. During operation, hot fuel gas is sucked out of the combustion chamber 25 into the interior of the housing via this jet pump. This is done via exhaust nozzles 26, which are distributed around the circumference of the catalytic converter and are cooled by the combustion air. The jet pump and the exhaust gas nozzles are dimensioned in such a way that the amount of exhaust gas sucked in is large enough to safely reach the critical temperature of, for example, 550 ° C. required for the catalyzer. As an example, it should be mentioned that about 3 parts of exhaust gas at 1200 ° C are sucked into 10 parts of combustion air at a temperature of 350 ° C.

The schematically shown premix burner 10 according to FIGS. 1 and 2 is a so-called double-cone burner, as is known for example from EP-B1-0 321 809. It essentially consists of two hollow, conical partial bodies 11, 12 which are nested one inside the other in the direction of flow. The respective central axes 13, 14 of the two partial bodies are offset from one another. The adjacent walls of the two partial bodies in their longitudinal extent form tangential slots 15 for the combustion air, which in this way reaches the interior of the burner. A first fuel nozzle 16 for liquid fuel is arranged there. The fuel is injected into the hollow cone at an acute angle. The resulting conical liquid fuel profile is enclosed by the combustion air flowing in tangentially. The concentration of the fuel is continuously reduced in the axial direction due to the mixing with the combustion air. The burner can also be operated with gaseous fuel. For this purpose, gas inflow openings 17 distributed in the longitudinal direction are provided in the region of the tangential slots in the walls of the two partial bodies. In gas operation, the mixture formation begins with the combustion air thus already in the zone of the inlet slots 15. It goes without saying that mixed operation with both types of fuel is also possible in this way.

At the burner outlet, a fuel concentration that is as homogeneous as possible is established over the loaded cross-section. A defined dome-shaped return flow zone is created at the burner outlet, at the tip of which the ignition takes place.

The mode of operation of the invention is now explained on the basis of the fuel control curve in FIG. 4. The burner arrangement shown in FIG. 3 is used as the basis for this and the assumption is made that the burners are only switched on or off in groups. In this case, it proves to be expedient to first ignite the internal burners and then gradually start up further external fuel elements. For this purpose, the burners are divided into six groups with the following members: group u = 9 elements, group v = 6 elements, group w = 3 elements, group x = 6 elements, group y = 6 elements, group z = 6 elements. The burners of groups u, v, w, x and y are premix burners, those of group z are catalytic burners. The groups are designated as such in Fig. 3.

The diagram does not show the actual starting process of the gas turbine, which begins at approx. 20% engine speed with the initial ignition via the centrally arranged pilot burner 5 and is completed when the engine reaches the nominal engine speed and the synchronization.

In the circuit diagram in FIG. 4, only the loading process from idle is explained. The load P in [%] is plotted on the abscissa and the excess air number (lamda) is plotted on the ordinate. The parameters K ₃₆ , K ₃₀ , K ₂₇ , K ₂₄ , K ₁₈ , K ₁₅ , K ₁₂ , K ₉ stand for a number of 36, 30, 27 .... 9 burners in operation. It is the optimal switching curve when loading the combustion chamber in gas operation.

The stability limits for pure premix combustion are plotted with S _V. For comparison, the stability limit for the pure diffusion combustion mentioned at the beginning is also mentioned with S _D. It can be seen here that this limit S _{D is} at a very high excess air ratio. However, the required low NO _x values could not be achieved with such a driving style. As a guideline, it can be stated that diffusion combustion would result in approx. 300-500 ppm NO _x emissions for modern gas turbines alone.

With pure premix combustion, on the other hand, the required NO _x limit values can easily be fallen below. However, the stability limit is low due to the low flame temperature. The range between ignitability and extinguishing is relatively narrow for the safe operation of the combustion chamber over the full load range.

According to the diagram, the combustion chamber is started up with 12 burners according to the thickly drawn switching curve from idling to 15% load. The groups u and w are in operation. Due to the increase in gas supply, the excess air figure at 15% load has become so low that burner group v is now switched on while group w is switched off at the same time. There are therefore 15 premix burners in operation. The further control curve when the load is increased is then determined in such a way that the excess air figure is constantly in the same range. For this purpose, in the example shown, the burner groups x, y and w are switched on at loads P = 27%, 44%, 64% and 86% respectively. switched off for fine grading.

According to the invention, group z with the catalytic auxiliary burners is additionally put into operation at 86% load. This results in a driving style directly on the stability limit. It goes without saying that the new measure can be applied not only at full load, but also, if necessary, at partial load. Basically, the catalytic burner can be used to reach operating points that are not possible with pure premix combustion, since the latter must always maintain a certain safety distance from the extinguishing limit.

To explain this, it should be mentioned that, when stochastic noise is quite common, vibrations of the order of 10 to 20 mb already prevail. This leads to very considerable fluctuations in the excess air figure, which only has the values indicated in FIG. 4 in the mean value, but actually fluctuates in a range around this mean value. And this fact leads, depending on the amplitude of the vibration, on the one hand to significantly increased NO _x values and on the other hand to a dangerous approach to the extinguishing limit. The difference of 10-15 mb pressure fluctuation leads to an additional 5-8 ppm NO _x .

The new way of driving on the extinguishing limit means that the NO _x values of 20 ppm that can be achieved today can certainly be significantly undercut.

Reference list

1

Combustion chamber wall

5

Pilot burner

10th

Premix burner

11

Partial body

12th

Partial body

13

Central axis

14

Central axis

15

tangential slots

16

Fuel nozzle

17th

Gas inflow opening

20th

catalytic burner

21

Catalytic

22

casing

23

Fuel supply

24th

Air intake

25th

Combustion chamber

26

Exhaust nozzle

Claims (5)

1. Combustion chamber, in particular for gas turbines, in which there are arranged on a common combustion chamber wall (1) premixing burners (10) with a incident flow of combustion air, and catalytically supported burners (20), preferably gas-operated, the main combustion being carried out by the premixing burners.
2. Combustion chamber according to Claim 1, characterized in that the premixing burners (10) and the catalytic burners (20) are of exchangeable configuration.
3. Combustion chamber according to Claim 1, characterized in that the catalytic burners (20) are provided with a waste-gas feedback (24, 26), the waste gas preferably being extracted from the combustion chamber (25).
4. Combustion chamber according to Claim 3, characterized in that the inlet of the combustion air for the catalytic burners is constructed as a jet pump (24), the waste gas being sucked from the combustion chamber (25) via this jet pump.
5. Combustion chamber according to Claim 1, characterized in that the catalytic burners (20) are arranged in the primary zone of the combustion chamber in highly loaded wall parts.

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