METHOD FOR OPERATING A COMBUSTION SYSTEM Field of the Invention

The present invention relates to a method for operating a combustion system of a combustion section of a gas turbine engine comprising at least one main burner arrangement and at least one pilot burner arrangement, wherein the at least one main burner arrangement and the at least one pilot burner ar ^¬ rangement are operated with an air-fuel mixture and wherein the at least one main burner arrangement is operated with 0 ₂ ^_ lean air. The present invention further relates to a combus- tion system as well as to a combustion system embodied to perform the inventive method and to a gas turbine engine with such a combustion system.

Background to the Invention

In a gas turbine engine a working fluid, like air, being com ^¬ pressed by a compressor enters a burner section where combus ^¬ tion of a mixture of the working fluid and a fuel occurs. The resulting combustion gas drives a turbine through the expansion and deflection of the gas through the turbine of the gas turbine engine. The turbine work or a part thereof, is trans ^¬ ferred to the compressor through an interconnecting shaft. In order to achieve a reliable combustion a main flame in the combustion chamber had to be stable and robust. So that the combustion system regains its optimal performance the main flame has to be stabilized. Different methods are known to stabilise the main flame. One is to operate the main flame at a high temperature, like above 1200°C. Disadvantageously, this generates high und harmful amounts of combustion by ^¬ products . Thus, to provide a combustion system with low emissions, there is a need to achieve a lower overall temperature of the combustion. Lower temperatures can be achieved by burning lean air-fuel mixtures. However, such lean flames can be very unstable and only small changes in the equivalence ration can cause a disadvantage and troublesome flame out. From EP 1 110 034 Al it is known to endow the combustion system with a so called pilot burner, which provides a flame with a high tem ^¬ perature to ignite and stabilize the low temperature main flame. According to the high temperature of the pilot burner flame the overall performance of the combustion system, like temperature or waste emission, is affected negatively. This is especially beneficial for so called DLE-combustion systems (dry low emission) . An oxygen mole fraction is about 21% in air but in some cases the oxygen content in the air used for combustion may be decreased. Such applications could be for example a gas turbine equipped with flue gas recirculation, a gas turbine in a process that extracts oxygen from the air or some other process that reduces the oxygen content. In an en- vironment with low oxygen content the pilot flame loses its stabilizing effect. The adiabatic flame temperature decreases in low oxygen environment as more air is needed and thus more mass for the flame to heat. Consequently, the performance of the combustion system and hence the gas turbine engine is un- stable and unreliable.

It is a first objective of the present invention to provide a method for operating a combustion system of a gas turbine engine with a stabilized main flame and that results in a high efficient low emission combustion performance.

It is a further objective of the present invention to provide a combustion system as well as a gas turbine engine, which can be operated with a stabilized main flame and which can perform high efficient low emission combustion.

A still further objective of the present invention is to pro ^¬ vide a combustion system embodied to perform the inventive method to achieve an operation with a stabilised main flame as well as a combustion process with low emissions.

These objectives may be solved by a method, a combustion sys ^¬ tem and a gas turbine engine according to the subject-matter of the independent claims.

Summary of the Invention

Accordingly, the present invention provides a method for op ^¬ erating a combustion system of a combustion section of a gas turbine engine comprising at least one main burner arrange ^¬ ment and at least one pilot burner arrangement, wherein the at least one main burner arrangement and the at least one pi ^¬ lot burner arrangement are operated with an air-fuel mixture and wherein the at least one main burner arrangement is oper ^¬ ated with 02-lean air. It is proposed that the method comprises the step of: In ^¬ creasing the percentage of oxygen of the air-fuel mixture of the at least one pilot burner arrangement by feeding of air having a higher percentage of oxygen than the 02-lean air. Due to this, a stabilizing effect of the pilot flame in low oxygen content air combustion can be gained and increased in comparison with a method operating state of the art systems. The oxygen content of the combustion air for the main burner arrangement can be held lower than with a method used for state of the art systems as it is only the pilot flame that needs extra oxygen, what saves also costs. Moreover, the method allows the overall performance of the combustion sys ^¬ tem and hence the gas turbine engine to be reliable and ef ^¬ fective .

Even if the terms "combustion section, main burner arrangement, pilot burner arrangement, air-fuel mixture, fuel, com ^¬ pressor section, valve, mixing device and feed line" are used in the singular or in a specific numeral form in the claims and the specification the scope of the patent (application) should not be restricted to the singular or the specific nu ^¬ meral form. It should also lie in the scope of the invention to have more than one or a plurality of the above mentioned structure ( s ) .

A gas turbine engine is preferably a whole plant, with at least a compressor section, a combustion section and a turbine section, further sections or devices may be provided. A main burner arrangement is intended to mean a functional ar ^¬ rangement and/or spatial assembly of pieces taking part of and/or assisting, building and/or participating in a combustion process, like an injector, a mixing device (swirler, premixer, mixer or parts thereof) or the like. A pilot burner arrangement is intended to means a functional arrangement and/or spatial assembly of pieces purposefully provided and/or constructed to assist the main burner arrangement and especially, to stabilise the main flame generated by the main burner arrangement. The pilot burner arrangement may be ar ^¬ ranged as and/or comprise at least one central pilot burner and/or at least one external pilot burner, which are both known in the art. An air-fuel mixture is intended to mean a mixture that is burnable in both burner arrangements. Preferably, it com ^¬ prises at least oxygen and at least one kind of fuel. The oxygen may be provided by air having an amount of oxygen as well as other gases. To provide several different fuels in the air-fuel mixture may be also possible. A mixing ratio of air and fuel in the air-fuel mixture may be any ratio suit ^¬ able for a person skilled in the art and may be selected for example in dependency of the fuel type, the burners used or other characteristics of the combustion system or the gas turbine engine, respectively, like a mass flow. Moreover, the fuel may be any fuel suitable for a person skilled in the art, like a gaseous fuel or a liquid fuel. Furthermore, it may be also feasible to provide a further compound in the air-fuel mixture, like a stabilizer. The air and/or the fuel of the air-fuel mixture and the mixture itself as well as a combustion gas resulting from the combustion of the air-fuel mixture can be also named as a working fluid. The air-fuel mixture is intended to be burned by the main burner arrange ^¬ ment or by the pilot burner arrangement. The air-fuel mixture can be the same for both burner arrangement but normally the air-fuel mixture of the main burner arrangement and the pilot burner arrangement differ from one another.

In this context, "02-lean" should be understood as having a an oxygen content that is lower than the oxygen contend or air and/or that is lower than an conventional compressor feed stream and/or as having an oxygen content of about 16% to 10% or even less. Preferably, the at least one main burner ar ^¬ rangement is operated with air having an oxygen content that is less than 13% or even less than 10%.

The feeding of the air having a higher percentage of oxygen than the 02-lean air may be performed at any location or time during the feeding process of the pilot burner arrangement. Thus, the air having a higher percentage of oxygen than the 02-lean air may be added to the air of the air-fuel mixture beforehand of mixing the air with the fuel or it may be added to the fuel beforehand of mixing the fuel with the air or it may be added to both beforehand of the mixing of both or it may be added after the mixing of the air and the fuel to the air-fuel mixture. In case the air having a higher percentage of oxygen than the 02-lean is added to the air beforehand of the mixing with the fuel the air is an 02-rich air or an 0 ₂ ^_ enhanced working fluid. "02-rich or -enhanced" should be un ^¬ derstood as with an oxygen content of about 18% to 30% and preferably as with an oxygen content close to atmospheric or of approximately 20% to 21%. The term "enrich" should be also understood as "supplement, add and/or dope". It might be also possible to mix only the fuel with 02-rich air without adding the 02-lean air beforehand or after the mixing of the fuel with the 02-rich air. In such a case a mass flow of the 0 ₂ - lean air provided for the pilot burner arrangement would be 0.

Hence, it is advantageous when the method further comprises the step of: Increasing the percentage of oxygen of an air of the air-fuel mixture of the pilot burner arrangement and/or the percentage of oxygen of a fuel of the air-fuel mixture of the pilot burner arrangement and/or the percentage of oxygen of the air-fuel mixture of the pilot burner arrangement by feeding of air having a higher percentage of oxygen than the 02-lean air.

In a further aspect of the invention the method comprises the step of: Enriching the air of the air-fuel mixture of the at least one pilot burner arrangement and/or the fuel of the air-fuel mixture of the at least one pilot burner arrangement and/or the air-fuel mixture of the at least one pilot burner arrangement with air so that a percentage of oxygen of the air-fuel mixture burned by the at least one pilot burner ar- rangement is at least 5% higher than an a percentage of oxy ^¬ gen of the air feed to the at least one pilot burner arrange ^¬ ment. This ensures a proper ignition of the pilot flame and a stabilised combustion process. In this context, the air feed to the pilot burner arrangement is the air being provided to the pilot burner arrangement beforehand of the mixing process (fuel) . The described and beneficial effects can be even more enhanced, when the oxygen content is at least 8% or prefera ^¬ bly at least 10% higher. As stated above the at least one main burner arrangement is operated the with 02-lean air. Thus, advantageously only the air/fuel/air-fuel mixture provided for the pilot burner ar ^¬ rangement has to be substituted with oxygen, reducing the overall costs since high percent and especially pure oxygen is expensive.

In a further realisation of the invention the air of the air- fuel mixture of the at least one pilot burner arrangement be- forehand of the mixing with the air having a higher percent ^¬ age of oxygen than the 0 ₂ -lean air is 0 ₂ -lean air. In other words, the air feed to the at least one pilot burner arrange ^¬ ment has an oxygen content that is less than 13%. Thus, the addition of air with higher oxygen contend than 0 ₂ -lean air is especially effective, resulting in a stable combustion.

In an especially beneficial embodiment of the invention it is provided, that the method further comprises the step of: Pro ^¬ viding the 0 ₂ -lean air from an oxygen-extraction process. By using this air as a working fluid of the combustion process the working fluid can be used for several processes, namely, as a working fluid of a compressor section, for an oxygen extraction or separation and for the combustion. Thus, the working fluid can advantageously be cycled back (from the compressor section via the extractor to the combustion section) to be used in the further process steps of the turbine engine. This reduces a need to freshly supply an equal amount of working fluid as that being withdrawn from the compressor section for the oxygen extraction.

As stated above, the resulting working fluid is 0 ₂ -lean with an oxygen content of about 16% to 10% or even less. Hence, in case of an 0 ₂ -rich fluid with up to 30% oxygen, the oxygen extraction process can extract up to 20% 0 ₂ -content and nor ^¬ mally at least about 10% 0 ₂ -content. Generally, the extracted amount depends on the operating mode and what will be possi ^¬ ble due to the process and the specifications of a used com ^¬ bustion system. A pressure level in the system especially be- fore and after the oxygen extraction depends strongly on load, ambient conditions and the oxygen extraction process. An example for 100% load and 15°C ambient air temperature would be 20.2 bar (a) pressure before O ₂ separation and 19.95 bar (a) after O ₂ separation.

Thus, the extraction process is realised by using a cycling path/arrangement that is adapted to supply a compressed 0 ₂ - rich working fluid exiting the compressor section to an ex- ternal unit for an oxygen extraction process and back feeding the 02-lean working fluid to the turbine engine or the com ^¬ bustion chamber of the combustion section, respectively, to be burnt therein. The advantage of this concept is to use all gas turbine engine compressor working fluid for the oxygen process enabling high oxygen production. Furthermore, leading the processed working fluid back into the gas turbine engine advantageously enables the full performance of the gas tur ^¬ bine engine cycle.

In a further step of the invention the 02-lean working fluid or the resulting air-fluid mixture is burnt by the main burner arrangement and the resulting combustion gas is ex ^¬ panded through a turbine of the turbine section for producing electricity. Hence, the height of the use of the oxygen ex ^¬ traction whit this gas turbine engine type is to gain a power production instead of power consumption while producing oxygen . According to a further realisation of the invention the method comprises the step of: Providing the air having a higher percentage of oxygen than 02-lean air from a compressor discharge of a compressor section of the gas turbine en ^¬ gine. Hence, air being present in the gas turbine engine may be used. Thus, advantageously, no further air supply is needed. As stated above, the lion's share of the compressor discharge is applied to the oxygen extraction process. For supplementing the air of the air-fuel mixture of the at least one pilot burner arrangement with air having a higher per- centage of oxygen than 02-lean air only a small fraction of the compressor discharge is needed and used. Therefore, a feed line for the air of the compressor discharge is provided that leads from or connects the compressor section to/with the at least one pilot burner arrangement.

Alternatively and/or additionally, the air having a higher percentage of oxygen than 02-lean air may be providing from an external oxygen source. In this context, "external" is in- tended to mean a source that is no integral part of the gas turbine engine.

In a preferred embodiment of the invention the method further comprises the step of: Mixing the air of the - resulting - air-fuel mixture of the at least one pilot burner arrangement with the air having a higher percentage of oxygen than 0 ₂ ^_ lean air and the fuel resulting in the air-fuel mixture for operating the at least one pilot burner arrangement. Hence, all three components - the air ( 02-lean) , the fuel and the air having a higher percentage of oxygen than 02-lean air - are mixed simultaneously. Thus, an air-fuel mixture that is homogenously mixed can be combined used. Alternatively and/or additionally, the mixing sequence may be performed in steps. Either, mixing the air ( 02-lean) of the - resulting - air-fuel mixture of the at least one pilot burner arrangement with the air having a higher percentage of oxygen than 02-lean air first and subsequently adding the fuel. Or mixing the fuel with the air having a higher percentage of oxygen than 02-lean air first and subsequently adding the air ( 02-lean) of the - resulting - air-fuel mixture of the at least one pilot burner arrangement. It would also be possible to mix both the air ( 02-lean) of the air-fuel mixture of the at least one pilot burner arrangement and the fuel with the air having a higher percentage of oxygen than 02-lean air and subsequently mixing the resulting mixtures to get the air- fuel mixture to be burned. The mixing may be facilitated with any means feasible for a person skilled in the art, like an active - driven - component or a passive component, like a selected construction of a part or piece that influences a flow path of the air/fuel/oxygen. This may be a specifically constructed wing or a spatial arrangement of such wings. A mass flow of the air ( 02-lean) is not specifically con ^¬ trolled; it is dependent on the pressure drop over the burner arrangement ( s ) which depends on the engine load. However, the air (having a higher percentage of oxygen than 02-lean air) and fuel mass flow can be controlled by each feeding pres ^¬ sure. Hence, the method further comprises the step of: Deter ^¬ mining a feeding pressure of the air having a higher percentage of oxygen than 02-lean air and/or determining a feeding pressure the fuel and, controlling a flame stability of a pi ^¬ lot flame of the at least one pilot burner arrangement due to the determined feeding pressure of the air having a higher percentage of oxygen than 02-lean air and/or the feeding pressure the fuel. Thus, two independent parameters can be used to influence the combustion process and the flame sta ^¬ bility.

This can be easily done when the method comprises the step of: Controlling a feeding of - a flow of - the air having a higher percentage of oxygen than 02-lean air by at least one valve and/or controlling a feeding of a fuel flow by at least a further valve.

The present invention further relates to a combustion system of a combustion section of a gas turbine engine comprising at least one main burner arrangement and at least one pilot burner arrangement, wherein the at least one main burner arrangement and the at least one pilot burner arrangement are operateable with an air-fuel mixture and wherein the at least one main burner arrangement is operateable with 02-lean air.

It is proposed that the combustion system is characterised by at least one mixing device that is able to increasing the percentage of oxygen of the air-fuel mixture of the at least one pilot burner arrangement by feeding of air having a higher percentage of oxygen than the 02-lean air.

Due to this, a stabilizing effect of the pilot flame in low oxygen content air combustion can be gained and increase in comparison with state of the art systems. The oxygen content of the combustion air for the main burner arrangement can be held lower than in state of the art systems as it is only the pilot flame that needs extra oxygen, what saves also costs. Moreover, the overall performance of the combustion system and hence the gas turbine engine is reliable and effective. Hence, the enrichment of the fluids can be achieved construc ^¬ tively easy. Moreover, by using a mixing device the mixing process can be realised homogeneously.

The supply of the different working fluids may be provided easily when the combustion system comprises at least one feed line for the air having a higher percentage of oxygen than 02-lean air, at least one feed line for the 02-lean air and/or at least one fuel feed line.

According to a preferred refinement, the at least one mixing device that is positioned to be feed with air having a higher percentage of oxygen than the 02-lean air by the at least one feed line for the air having a higher percentage of oxygen than the 02-lean air and with the 02-lean air by the at least one feed line for 02-lean air and with fuel by the at least one fuel feed line. As a result, the mixture has a uniform composition which provides a continuous combustion. The mixing operation of the at least one mixing device results in the air-fuel mixture that operates the at least one pilot burner arrangement or in the air-fuel mixture being burned by it .

In case the mixing is done in subsequently performed steps an at least second mixing device may be provided.

According to a further aspect of the invention the at least one feed line for the air having a higher percentage of oxy ^¬ gen than 02-lean air connects a compressor section of the gas turbine engine with the at least one pilot burner arrange ^¬ ment. Hence, the air having a higher percentage of oxygen than 02-lean air can be supplied to the at least one pilot burner arrangement constructively easy.

Alternatively and/or additionally, the at least one feed line for the air having a higher percentage of oxygen than the 0 ₂ ^_ lean air connects a high pressure air source, especially an external high pressure air source, with the at least one pi ^¬ lot burner arrangement. Thus, the feed line may be con ^¬ structed independently from constructional limitations of the compressor section.

A feed of the air having a higher percentage of oxygen than the 02-lean air to the at least one feed line for the air having a higher percentage of oxygen than the 02-lean air can be adjusted and thus controlled constructively easy when the combustion system comprises least one valve to control a feed of air having a higher percentage of oxygen than the 02-lean air to the at least one feed line for the air having a higher percentage of oxygen than the 02-lean air. Moreover, a feed of a fuel flow to the at least one fuel feed line can be in ^¬ fluenced when the combustion system comprises least one fur ^¬ ther valve to control a feed of fuel to the at least one feed line for the fuel. The invention further relates to an inventive combustion system for operating the inventive method or in other words, to an inventive combustion system being operated with the inventive method. Further, the invention relates to a gas tur ^¬ bine engine with at least one inventive combustion system.

The above-described characteristics, features and advantages of this invention and the manner in which they are achieved are clear and clearly understood in connection with the following description of exemplary embodiments which are ex- plained in connection with the drawings.

Brief Description of the Drawings The present invention will be described with reference to drawings in which: shows a schematically and sectional view of a part of a gas turbine engine unit with an upper half of a gas turbine engine and a unit for an oxygen ex ^¬ traction process, shows in a perspective view a burner head of a com bustion system of the gas turbine engine from

FIG 1, shows a sectional view through a part of a combus ^¬ tion chamber and the burner head of FIG 2, shoes an enhanced view of a pilot burner arrange ^¬ ment of the burner head of FIG 2, shows a schematically and perspective view an exem plarily mixing device of the pilot burner arrange ^¬ ment of FIG 4 and shows a sectional view through a part of a combus ^¬ tion chamber and an alternatively embodied burner head of an alternative combustion system.

Detailed Description of the Illustrated Embodiments

The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the gas turbine engine 14 unless otherwise stated. If used and not oth ^¬ erwise specified, the terms axial, radial and circumferential are made with reference to a rotational axis 58 of the gas turbine engine 14.

FIG 1 shows an example of a gas turbine engine unit 44 with a gas turbine engine 14 and a unit 46 for an oxygen extraction process in a sectional view. The gas turbine engine 14 com- prises, in flow series of a working fluid, like air 22, 24 or a combustion gas 48, an air inlet 50 and a compressor section 28 with a compressor 52, a combustion section 12 with a combustion system 10 and a turbine section 54 with a turbine 56, which are generally arranged in flow series in the direction of a longitudinal or rotational axis 58. The gas turbine en ^¬ gine 14 further comprises a shaft 60 which is rotatable about the rotational axis 58 and which extends longitudinally through the gas turbine engine 14. The shaft 60 drivingly connects the turbine section 54 to the compressor section 28.

In operation of the gas turbine engine 14, air 24, which is taken in through the air inlet 50 is compressed by the com ^¬ pressor section 28 and delivered to the combustion section 12 or burner section. The compressed air 24 passing through the compressor section 28 is an 0 ₂ -rich working fluid with about 21% O ₂ content. The combustion section 12 comprises a burner plenum 62, one or more combustion chambers 64 with a wall encasing the combustion chamber 64 and exemplarily one burner 66 fixed to the combustion chambers 64 defined by a double wall can 68 and at least one burner 66 fixed to each combus ^¬ tion chamber 64. The combustion chamber 64 and the burner 66 are located inside the burner plenum 62. After traveling the compressor section 28 the 0 ₂ -rich flow medium or air 24 enters a diffuser 70 of the compressor section 28 and is discharged from the diffuser 70 into a channel 72 extending through the combustion section 12 and connecting the gas turbine engine 14 or the combustion section 12 with the unit 46 for the oxygen extraction process. The unit 46 for the oxygen extraction process is embodied in such a way to extract O ₂ from the heated 0 ₂ -rich air 24. Therefore, the extraction unit 46 comprises for example an ion transport membrane to perform the oxygen extraction process (not shown in detail) .

After the oxygen is extracted the now 0 ₂ -lean flow medium or air 22 has an O ₂ content of approximately 13% to 14%, depend- ing on the efficiency of the extraction process. The gas tur ^¬ bine engine unit 44 comprises a return channel 74 connecting the extraction unit 46 and the gas turbine engine 12. Hence, after the extraction process the 02-lean flow medium or air 22 flows back to the gas turbine engine 14 travelling the re ^¬ turn channel 74 to enter the combustion section 12. The channels 72 and 74 are sealed against one another to prevent a mixing of the 02-rich air 24 and the 02-lean air 22 (not shown in detail) .

The 02-lean air 22 entering the combustion chamber 64 or the burners 66, respectively, is mixed with a gaseous or liquid fuel 26 (details see below) . The resulting air-fuel mixture 20 is then burned and a resulting combustion gas 48 or work- ing gas from the combustion is channelled via a transition duct 76 to the downstream turbine section 54.

The turbine section 54 comprises a number of blade carrying discs 78 attached to the shaft 60. In the present example, the turbine section 54 comprises two discs 78 each carry an annular array of turbine blades 80. However, the number of blade carrying discs 78 could be different, i.e. only one disc 78 or more than two discs 78. In addition, guiding vanes 82 of a turbine cascade, which are fixed to a stator 84 of the gas turbine engine 14, are disposed between the turbine blades 80. Between the exit of the combustion chamber 64 and the leading turbine blades 80 inlet guiding vanes 88 are pro ^¬ vided . The combustion gas 48 from the combustion chamber 64 enters the turbine section 54 and drives the turbine blades 80 which in turn rotate the shaft 60. The guiding vanes 82, 86 serve to optimise the angle of the combustion or working gas 48 on to the turbine blades 80. The compressor section 28 comprises an axial series of guide vane stages 88 and rotor blade stages 90. FIG 2 shows a burner head 92 of the combustion system 10 of the combustion section 12. The burner head 92 comprises a main burner arrangement 16 (see FIG 3) and a pilot burner ar ^¬ rangement 18 which are operateable with the air-fuel mixture 20 (see FIG 4) . The pilot burner arrangement 18 comprises several pilot burners 94 each having an injection nozzle 96 to inject the air-fuel mixture 20 in to the downstream com ^¬ bustion chamber 64 for ignition in the pilot flame 30 (see FIG 3) . 02-lean air 22 is injected through holes 98 or simi- lar from the burner plenum 62 into the pilot burner arrangement 18 (not shown in FIG 3) .

In FIG 3 the burner head 92 and a part of the combustion chamber 64 are shown in a simplified view. The main burner arrangement 16 is embodied as a central burner (not shown in detail) that is circumferentially surrounded by the pilot burners 94, which in turn are exemplarily embodied as external pilot burners 94. The main burner arrangement 16 and the pilot burner arrangement 18 are feed with the 02-lean air 22 from the oxygen extraction process. To stabilise the main flame 100 resulting after ignition of an air fuel mixture 20 of the main burner arrangement 16 the pilot burner arrange ^¬ ment 18 is embodied selectively for this purpose. As stated above, the pilot burner arrangement 18 is feed with 02-lean air 22. This may cause an unstable pilot flame 30 and the pi ^¬ lot burner 94 may lose his stabilising effect. By mixing the air 22 used for the pilot flame 30 with air 24 having a higher percentage of oxygen than the 02-lean air 22, a normal, 21% or higher oxygen mole fraction can be reached. Thus, the air 22 of the air-fuel mixture 20 of the pilot burner ar ^¬ rangement 18 beforehand of the mixing with the air 24 having a higher percentage of oxygen than the 02-lean air 22 is 02- lean air 22. As can be seen in FIG 4, which shows an enhanced view of one pilot burner 94, the pilot burner arrangement 18 comprises a source for the air 24 having a higher percentage of oxygen than the 02-lean air 22 and additional pieces to provide the air 24 having a higher percentage of oxygen than the 02-lean air 22. The air 24 having a higher percentage of oxygen than the 02-lean air 22 is provided by a feed line 36 shared by all pilot burners 94 and that connects the compressor section 28 of the gas turbine engine 14 with the pilot burner ar ^¬ rangement 18.

For mixing of the air 24 having a higher percentage of oxygen than the 02-lean air 22 with the 02-lean air 22 and the fuel 26 the combustion system 10 comprises a mixing device 34 that is able to increase the percentage of oxygen of the air-fuel mixture 20 of the pilot burner arrangement 18 by feeding of air 24 having a higher percentage of oxygen than the 02-lean air 22. To provide the air 24 having a higher percentage of oxygen than the 02-lean air 22, the air 22 and the fuel 26 for the mixing device 34 the combustion system 10 comprises a feed line 36 for the air 24 having a higher percentage of oxygen than the 02-lean air 22, a feed line 38 for the 02 - lean air 22 (not shown in detail in FIG 3 and 4, see FIG 2) and an fuel feed line 40 for the fuel 26, wherein the mixing device 34 is positioned to be feed with air 24 having a higher percentage of oxygen than the 02-lean air 22, with the 02-lean air 22 and with the fuel 26 by the respective feed line 36, 38, 40. The fuel 26 is provided from a separate manifold 102 shared by all pilot burners 94. The resulting air-fuel mixture 20 is than discharged from the mixing device 34 to the nozzle 96 to be injected into the combustion cham ^¬ ber 64 where the oxidation process occurs in the pilot flame 30.

The lion's share of the air 24 having a higher percentage of oxygen than the 02-lean air 22, in other words, the 02-rich air 24 with about 21% 0 ₂ , is applied to the oxygen extraction process in unit 46. Only a small fraction is led from the compressor section 28 or from its diffuser 70 via feed line 36 to the pilot burner arrangement 18. The main flame 100 is feed by an air-fuel mixture 20 that contains the same air 22 with reduced oxygen content (0 ₂ ^_ lean) as the air 22 feed to the pilot burner arrangement 18 but this mixture 20 is not doped with air 24 having a higher percentage of oxygen than the 02-lean air 22.

The mixing device 34 can be embodied as a pre-mixer or as a swirler. In FIG 5 one embodiment of one useable mixing device is exemplarily shown. A mixing cavity 104 that receives the flow mediums (air 22, air 24 having a higher percentage of oxygen than the 02-lean air 22, fuel 26) has two swirler wings 106 that are arranged in close proximity to the respec ^¬ tive nozzle 96 for injection in the combustion chamber 64 (see FIG 4) .

The method for operating the combustion system 10 comprises the following steps: Increasing the percentage of oxygen of the air-fuel mixture 20 of the pilot burner arrangement 18 by feeding of air 24 having a higher percentage of oxygen than the 02-lean air 22, and specifically, an air 22 of the air- fuel mixture 20 of the pilot burner arrangement 18 by feeding of air 24 having a higher percentage of oxygen than the 02- lean air 22 and further specified by enriching the air 22 of the air-fuel mixture 20 of the pilot burner arrangement 18 with air (24) 24 so that an oxygen content of the air-fuel mixture 20 burned by the pilot burner arrangement 18 is at least 5% higher or about 8% higher than an oxygen content of the air 22 feed to the pilot burner arrangement 18 beforehand of the mixing with the air 24. Preferably, the air 22 is en- riched by the air 24 so that the oxygen content of the re ^¬ sulting air-fuel mixture 20 is about 21%.

The mass flow of the air 22 is not specifically controlled. It is dependent on the pressure drop over the burner (s) which depends on the engine load. A mass flow of the fuel 26 is controlled by a feeding pressure which gives a parameter to control the stability of the pilot flame 30 and thus the main flame 100. Hence, the method comprises the steps of: Deter- mining a feeding pressure of the fuel 26 and controlling a flame stability of the pilot flame 30 of the pilot burner ar ^¬ rangement 18 due to the determined feeding pressure of the fuel 26. Hence, the combustion system comprises a sensor to detect the feeding pressure of the fuel 26 (not shown) and a valve (not shown), located in or at the fuel feed line 40, to control a feed of fuel 26 to the feed line 40 for the fuel 26. Alternatively, it may be possible to add the 02-rich air in two steps to air 22 and fuel 26 and mix the resulting mix ^¬ tures thereafter. Or it might be also possible to mix only the fuel 26 with 02-rich air 24 without adding the 02~lean air 22 beforehand or after the mixing of the fuel 26 with the 02-rich air 24. In such a case a mass flow of the 02~lean air 22 provided for the pilot burner arrangement would be 0 (not shown) .

In FIG 6 an alternative exemplary embodiment of the combus- tion system 10 is shown. Identical components, features and functions are denoted by the same reference numerals. How ^¬ ever, to distinguish the exemplary embodiment of FIG 6 over that of FIG 1 to 5 the letter 'a' has been added to the ref ^¬ erence numerals of the components that are designed differ- ently in the exemplary embodiments of FIG 6. The description below is substantially limited to these differences compared to the exemplary embodiment of FIG 1 to 5, wherein reference is made to the description of the exemplary embodiment in FIG 1 to 5 with respect to identical components, features, and functions.

FIG 6 shows an alternative embodiment of the combustion sys ^¬ tem 10 or of its pilot burner arrangement 18, respectively. The combustion system 10a of FIG 6 differs from the combus- tion system 10 of FIG 1 to 5 in that the feed line 36a is em ^¬ bodied differently. The air 24 having a higher percentage of oxygen than the 02-lean air 22 or, in other words, the fresh 02-rich air 24 is feed from an external source 108 (connec- tion line not shown for better presentability) to a separate air manifold 110. The feed line 36a connects the manifold 110 with the mixing device 34. The external source 108 can be compressor discharge air that is lead out from the gas engine 14 and in to the burner head 92 via a control valve or it can be some other external high pressure air source 108.

The mass flow of the air 22 is not specifically controlled. It is dependent on the pressure drop over the burner (s) which depends on the engine load. According to this embodiment a mass flow of the air 24 having a higher percentage of oxygen than the 02-lean air 22 and a mass flow of the fuel 26 may be controlled by each feeding pressure which gives two parame ^¬ ters to control the stability of the pilot flame 30 and thus the main flame 100. Hence, the method comprises the steps of: Determining a feeding pressure of the air 24 having a higher percentage of oxygen than the 02-lean air 22 and controlling a flame stability of the pilot flame 30 of the pilot burner arrangement 18 due to the determined feeding pressure of the air 24 having a higher percentage of oxygen than the 02-lean air 22. Thus, the combustion system comprises a sensor to detect the feeding pressure of the air 24 having a higher percentage of oxygen than the 02-lean air 22 (not shown) and a valve 32, located in or at the feed line 36a for the air 24 having a higher percentage of oxygen than the 02-lean air 2, to control a feed of the air 24 to the feed line 36a.

It may be also possible to determine feeding pressure of the fuel 26 by a sensor and to control the flame stability of the pilot flame of the pilot burner arrangement due to the deter ^¬ mined feeding pressure the fuel 26. Therefore, a further valve may be located in or at the fuel feed line (not shown) .

It should be noted that the term "comprising" does not ex- elude other elements or steps and "a" or "an" does not ex ^¬ clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con ^¬ strued as limiting the scope of the claims.

Although the invention is illustrated and described in detail by the preferred embodiments, the invention is not limited by the examples disclosed, and other variations can be derived therefrom by a person skilled in the art without departing from the scope of the invention.