GB2446164A

GB2446164A - Gas Turbine Emissions Reduction with Premixed and Diffusion Combustion

Info

Publication number: GB2446164A
Application number: GB0702179A
Authority: GB
Inventors: Johan E Hustad
Original assignee: NTNU Technology Transfer AS
Current assignee: NTNU Technology Transfer AS
Priority date: 2007-02-05
Filing date: 2007-02-05
Publication date: 2008-08-06
Also published as: GB0702179D0

Abstract

A system for reducing emissions, particularly NOx emissions from a gas turbine system (1), provides air to a fuel gas line for partial premixing with the fuel gas flow. The gas turbine system has a first compressor (11), a combustion chamber (12) and a turbine (13). The fuel gas flow (21) is provided by one or more fuel gas lines (2). One or more inlets (3) provide oxygen to the compressor (11). Combustion is arranged to take place within the combustion chamber (12). One or more gas lines (4) are connected to the fuel gas lines (2), connected to the combustion chamber to provide a second gas flow (41) to be mixed to fuel gas supply. The second gas flow (41) includes oxygen and is premixed with the fuel gas before being injected into the combustion chamber (12). The premixed fuel reduces the combustion chamber temperature and flame length whereby emissions are reduced.

Description

GAS TURBINE

The present invention relates to a method and a system for reducing emissions from a gas turbine.

The protocol on reduction of acidification, overfertilisation and tropospheric ozone (The Gothenburg protocol) under the convention of long-range cross border air pollution was ratified in 1999. Under this protocol Norway is obliged to reduce the yearly emissions of various pollutants, one of them being nitrogen oxides NOR. The NO emissions should be reduced to 156.000 tons, equivalent to a reduction of 31 % compared to 2001 level or about 60.000 tons. The total emissions of NO in Norway per. 2001 are actually approximately at the same level as in 1990. Coastal activities such as fishing and ship traffic were responsible for about 39% of these emissions, road traffic for about 22%, land-based processes such as combustion and other processes for about 10 %, whereas offshore petroleum activities were responsible for about 20% of the emissions. The remainder of the emissions were due to other combustions, in particular due to motorized equipment, air-traffic and domestic heating.

The widespread use of gas turbines for off-shore power generation in Norway and the resulting emissions of CO2 and NO have incurred difficulties in fulfilling the obligations of the ratified treaties under the Kyoto agreement for CO2 and the Gothenburg protocol for NOR. In particular the reduction of NO emissions is proving to be a technological challenge. Although several methods are available at present for the reduction of the emissions of both, few of the available methods are practicable off-shore due to the high costs associated with them. The present invention seeks to resolve these issues and comprises a method for the reduction of NO by partial premixing of air or a diluent comprising some oxygen with the fuel gas flow.

The process of NO formation in gas turbines mainly comprises four different formation mechanisms which may be summarized as fuel NO formation, thermal NO formation, prompt NO formation and N20-intermediate mechanism.

Thermal NO is a result of the Zeldovich mechanism which is highly dependent upon combustion temperature. The reaction mechanism may be summarized as follows: Q+N2f-NO+N N + 02 NO + N N+ OH 4-'NO H In this reaction mechanism the reaction of 0 + N2 -NO + N is rate controlling.

This reaction is quite slow and is heavily temperature dependent.

Prompt NO is the result of the cracking of hydrocarbons and subsequent reaction with N2, according to the reaction mechanism: CH+N24-HCN+N This reaction mechanism is mainly limited by the amount of CH radicals present.

The N20-intermediate mechanism is important in fuel-lean, low temperature conditions. The three steps of this mechanism are: O+N2+M-N2O+M H+ N2O-NO NH Q+N2O4-NO+NO This mechanism becomes important in NO control strategies that involve lean premixed combustion.

Fuel NO is formed by the reaction of nitrous compounds in the fuel with excess oxygen in the combustion air or in the combustion chamber. Most gas turbines are fed with a feed mainly comprising methane and this reaction may thus in most circumstances be considered negligible for gas turbine systems.

In most gas turbines the dominant NO formation mechanism is therefore the thermal NO mechanism, although for some very low emission gas turbines, the N20-intermediate mechanism may become dominant.

As is evident, several approaches to the reduction of NO emissions may be considered, either removing NO from the flue gas, or limiting the formation of NO during the combustion. Several well-known approaches to the removal of NO from flue gas flows are known, such as selective catalytic reduction (SCR) of NO usually performed by means of NH3 as the reducing agent. To enhance the process a catalyst should usually be present, an example of which may be V205/Ti02 SCR processes may reduce the NO emissions to a very low level, however there are several disadvantages to using SCR. The installation of the process equipment is costly, and the process should usually be conducted in a quite narrow temperature range. Typically a vertical down flow tower should be provided in which the required tower volume is quite large.

Furthermore the regeneration of NH3 is costly and the equipment requires a substantial amount of space. Undesirable side reactions may occur, and the addition of chemicals to a process stream should be avoided if possible.

Finally, catalyst poisoning may further limit the effectiveness of the process.

Thus the installation of SCR scrubbers or other SCR equipment is costly.

A separate approach to the problem is to reduce the amount of NO formed during the combustion itself, thus reducing the amount of NO emitted in the flue gas. The dominant NO formation mechanism is in most instances the thermal NO mechanism. As this reaction is mainly temperature driven, the main emphasis in combustion NO reduction efforts has been on the reduction of the temperature in the combustion chambers of the gas turbines. Amongst the various approaches to reducing the combustion temperature are, -the premixing of fuel in which a large volume of air is premixed with the fuel, -inserting steam or other diluents to the combustion chamber, -recycling of exhaust gas.

Premixing of the fuel is one of the most common approaches when designing a low NO or Ultra low NO burners (LNBs, ULNBs). The fuel is usually diluted using a large excess of air or other diluent such that the resulting combustion in the combustion chamber is no longer diffusion 0* controlled. Characteristics of the combustion flame are very long flame lengths and large diameter burners. By means of such processes, NO emissions may be reduced to about 25 ppm at 15 % per volume oxygen. There are however several particular problems associated with such burners, in particular that such systems may not easily be retrofitted to earlier systems already in use.

Thus if premixing of fuel to this degree is implemented, the entire combustion chamber comprising fuel lines, mixing zones etc must be replaced. The resulting combustion flame must furthermore be closely monitored in order for the flame not to be extinguished if a too high dilution ratio is used, and the instability of the flame may result in blow offs and system shutdowns. In order to reduce the possibility of such occurrences, a pilot burner is usually installed in the combustion chamber, in which the pilot flame is provided with a non-premixed fuel. In such systems the main NO contributing factor may be the pilot flame itself. The mixing of the fuel gas with the diluent must also be carefully performed such that eddies or zones having a high fuel ratio are not created, as these zones may cause backfire into the fuel line itself due to self ignition. Finally the instability of the flame may result in high noise levels detrimental to working conditions near the gas turbine. There are numerous examples in the art wherein such systems are discussed or implemented.

A different approach to the reduction of temperature in the combustion chamber is to increase the heat capacity of the mixture therein. Typically steam or other diluents may be introduced into the combustion chamber such that a portion of the heat resulting from the combustion is dispersed. By injecting water or other inert diluents into the combustion chamber, the mass flow through the turbine is furthermore increased, and also the resulting the power output. These systems are typically known as humidified gas turbines and are well-known in the art.

Other systems according to the art are described by amongst others Cheng in which the fuel gas is premixed with a desired amount of an inert gas such as steam. The premixing of the fuel gas flow with an inert gas such as steam increases the heat capacity of the mixture in the combustion chamber, thus reducing the temperature therein. The introduction of an inert gas into the fuel instead of inserting the inert into the combustion chamber also allows for a more homogenous mixture of fuel and inert in the combustion chamber and

I

thus reduces the amount of local hotspots were temperatures and NO formation rates are high. This is described in patents to Cheng, namely US6418724 and US6370862. However, one must still provide an inert such as steam to the system, and also provide means by which an inert is produced and inserted into the fuel gas flow.

In offshore applications it is difficult to provide the required amount of steam for effective reduction of NO as this would require purification of salt water in order for the steam to be produced. Although this is technically feasible it is prohibitively costly. A large amount of equipment must furthermore be retrofitted to the system in order to allow the steam or di!uent to be injected.

Separate steam production units and steam lines must be provided necessitating major modifications of the process plant. There are also difficulties in stabilising the combustion flame at high diluent-to-fuel ratios.

A separate approach to the reduction of NO is to recycle some of the exhaust gas into the combustion chamber, thus providing a reduction in the amount of oxygen present in the chamber and consequently reducing the amount of oxygen available for NO formation. The recycling has the further effect of increasing the heat capacity in the combustion chamber. However soot problems as well as flame stability remain a problem.

The present invention seeks to reduce the problems identified above.

According to the present invention there is provided a system for reducing emissions from a gas turbine system, wherein said gas turbine system is arranged for energy production and / or propulsion by combustion of a combustible gas, wherein said gas turbine system comprises a first compressor, a combustion chamber and a turbine, wherein said gas turbine system is arranged for being provided with a fuel gas flow by one or more fuel gas lines, wherein said gas turbine system is arranged for being provided with a first gas flow from one or more inlets, wherein said first gas flow is arranged for providing oxygen for the combustion of fuel gas in said fuel gas flow, wherein said combustion is arranged for taking place within said combustion chamber, wherein said combustion in said combustion chamber is partially diffusion rate controlled, wherein one or more gas lines are arranged for being 1* connected to said fuel gas line or lines, wherein said gas line is arranged for providing a second gas flow for being mixed with said fuel gas flow, wherein said second gas flow at least partially comprises oxygen, whereupon said fuel gas flow is partially premixed with said second gas flow before injection into said combustion chamber.

The present invention also extends to a method for reducing emissions from a gas turbine system, wherein said gas turbine system produces energy by combustion of a combustible gas, wherein said gas turbine system comprises at least a first compressor, a combustion chamber and a turbine, wherein one or more fuel gas lines provde a fuel gas flow to said gas turbine system, wherein one or more inlets provide a first gas flow to said gas turbine system said first gas flow providing oxygen for the combustion of fuel gas flow, wherein said combustion takes place within said combustion chamber, wherein at least a portion of said combustion in said combustion chamber is diffusion rate controlled, wherein one or more gas lines are connected to said gas fuel line, wherein said gas line provides a second gas flow at least partially comprising oxygen, for mixing with said fuel gas flow, whereupon said fuel gas flow is partially premixed is partially premixed with said second gas flow before injection into said combustion chamber.

The invention further comprises the use of the system in a diffusion type gas turbine system as well as in the reduction of emissions from pilot burners in dry or wet low emission gas turbines.

Further advantageous features of the invention are described in the attached dependent claims.

Embodiments of the invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a highly schematic illustration of a gas turbine system (1) comprising a first compressor (11), a combustion chamber (12) and a turbine (13). A fuel gas line (2) furnishing a fuel gas flow (21) is shown providing fuel to the combustion chamber (12). A gas inlet (3) providing a first gas flow (31) to the gas turbine system (1) and an exhaust (9) arranged for removing an exhaust flow (91) is shown at respective ends of the system.

Figure 2a is a highly schematic illustration of gas turbine system (1) of an embodiment of the invention, in which a gas line (4) is arranged between the first compressor (11) and the fuel gas line (2) for providing a second gas flow (41) for partial premixing with said fuel gas flow (21). A second compressor (6) is arranged on the gas line (4) for further compression of the second gas flow (41).

Figure 2b illustrates the same situation as in Fig. 2a, but in which a heat exchanger (5) and a water injection system (7) is arranged on the gas line (4) for providing a water enriched flow (41') for partial premixing with said fuel gas flow (21).

Figure 3 is an illustration of an embodiment of the invention, in which a gas separation unit (8) is arranged on the gas line (4). Said gas separation unit (8) divides the second gas flow (41) into an oxygen enriched gas flow (42) and an oxygen depleted gas flow (43), wherein said oxygen depleted flow is provided for partial premixing with said fuel gas flow (21).

Figure 4 is an alternative embodiment of the invention in which atmospheric air is provided by said gas line (4) for partial premixing with said fuel gas flow (21).

Figure 5 is a diagram showing the respective reduction in NO emissions by dilution ratio when N2 respectively air is mixed with the fuel gas flow (21).

This diagram is an empirical verification of the invention.

The invention will in the following be described referring to the attached figures and exemplary embodiments of the invention. Any combination of the below described embodiments should be considered to lie within the scope of the invention.

The invention comprises a system for reducing emissions, for example NO emissions from a gas turbine system (1), in which said system (1) comprises a compressor (11) a combustion chamber (12) and a turbine (13) as is known in the art. Said gas turbine system (1) is provided with a fuel gas flow (21) from one or more fuel gas lines (2) and a first gas flow (31) from one or more inlets (3), said gas flow being arranged for providing an oxygen containing mixture for the combustion of fuel gas in said fuel gas flow. Said fuel gas may be chosen from any suitable combustion gas such as methane, hydrogen, propane, butane etc. Said first gas flow (31) may be atmospheric air or any other gas flow comprising a suitable oxidant such as preferably oxygen for the combustion of the fuel gas.

The invention seeks to reduce the NO emissions from burners of so-called diffusion type. Most of the older gas turbines in use today are equipped with such diffusion type burners, in which the fuel gas flow (21) is injected into the combustion chamber (12) whereupon oxygen must diffuse to the flame surface for the combustion to take place. The diffusion of oxygen to the flame zone is rate limiting in such burners, and as the combustion or chemical reaction that takes place at the flame surface is stoichiometric, the flame temperature is very high, which enhances the production of thermal NON. A description of the concentration profiles in such diffusion burners may be found in textbooks, and is also described in US6418724.

These burners should not be confused with burners in which a prernixed fuel is injected into the combustion chamber for subsequent combustion, as the premixed fuel in such burners are diluted such that the fuel mixture is very close to the lower flammability limit of the fuel gas. However, as such burners are known to have quite unstable flames it is a common safety measure to install a pilot burner within the combustion chamber, wherein the pilot burner flame is a diffusion flame. The use of a pilot burner thus avoids situations where the premixed fuel flame is extinguished. However one of the main sources of NO emissions from such systems is the pilot burner. The pilot burner may furthermore be the sole energy source for such low emission systems for up to about 70-80 % of the total load of the gas turbine system, thus a further object of the invention is to reduce the emissions from such pilot burners.

Fig. 2a illustrates an embodiment which comprises a gas line (4) arranged for providing a second gas flow (41), for partial premixing with said fuel gas flow (21). Said second gas flow (41) should at least partially comprise oxygen, nd may in an ernbodfrnerit be atm.ospher!c a'r. The partia! prern!x!ng of said second gas flow (41) with said fuel gas flow (21) differs from the premixing of fuel as shown in the prior art in that the resulting mixture of the fuel gas flow (21) and said second gas flow (41) is near the upper flammability limit of the fuel gas instead of the lower flammability limit of the fuel gas. The combustion of said fuel gas within the combustion chamber (11) will thus be diffusion controlled.

The partial premixing of said gas flow (41) with said fuel gas flow (21) has several beneficial effects with respect to the reduction of NO emissions from said gas turbine systems (1). Firstly, the furnishing of oxygen to the fuel gas flow (21) will reduce the flame length within the combustion chamber (12) This reduction of flame length is of importance as it is on the flame surface that the majority of the NO formation occurs as previously described.

Secondly, partial premixing of said fuel gas flow (21) will result in a more homogenous flame in the combustion chamber (12), reducing the amount of fuel rich hotspots in the combustion chamber. It is known that such fuel rich hot spots are locations in which temperatures and thus NO formation rates are high.

Thirdly the partial premixing of said gas flow (41) with said fuel gas flow (21) results in a combustion chamber mixture having a higher heat capacity (Cr) than for an unmixed fuel gas flow. A further beneficial aspect of the premixing of the fuel gas flow (21) is to provide sufficient oxygen for the combustion so as for avoiding the emission of unburnt hydrocarbons (UHC) and the formation of CO gas due to the lack of oxygen in the process. The emission of both UHC's and CO gas should also be reduced as much as feasible duw to enviromental concerns.

The partial premixing ratio of said second gas flow (41) to said fuel gas flow (21) may be in the range 0,5 to 5 by volume.

As is shown in fig. 5, an increase in the dilution-fuel-ratio (DFR) increases the reduction in N0 emissions. The stoichiometr,c reaction mechanism of air and methane is given below: CH4 +2(02 +79/21 N2) -> CO2 +2H2Q +2 (79/21) N2 This reaction mechanism does not take into account CO formation nor NO formation.

The influence of the DFR has also been studied in the prior art, however it has been believed that an increase in the supply of oxygen to the fuel gas flow (21) would greatly increase the emissions of N0. This is a misconception in the prior art. It has empirically been proven to be false as is illustrated in fig. in which is shown the relative reduction of NO emissions when premixing the fuel gas flow (21) with an inert compared to the reduction in NO emissions when using air.

The increase in N0 emissions due to the increased oxygen present in the fuel is compensated by the increase in DFR, an in crease in DFR that is rendered possible by the oxygen present. This somewhat counterintuitive result is due to the fact that when using inert gases the possible DFR is limited. If DFR is increased above a certain limit, blowoff conditions may occur. A blowoff is initiated when an insufficient amount of oxygen is present for the combustion and the flame lifts off. When providing for instance air for the premixing of the fuel gas flow (21), it is possible to increase the DFR to higher levels than previously imagined to be feasible as the oxygen comprised in the air will reduce the flame length as well as stabilising the flame, and thus allow a higher DFR before blow off. The higher DFR results in a lowering of N0 emissions comparable to or better than is achieved by pre-mixing an inert into the fuel gas flow(21).

The maximum DFR is mostly limited by safety concerns, as it is desirable to avoid backfire into the fuel alimentation line (2). As the upper flammability limit of natural gas in air is about 15% per volume, care should be taken to be above said limit. It should be noted that the upper flammability limit is temperature and pressure dependent. The theoretical limit to the premixing of the fuel gas flow is thus 85/15 or 15 % fuel to 85% air at volumetric ratios, giving a DFR equal to 5,67 by mass. If the DFR is increased above this threshold, there is a potential risk of backfire into the fuel gas line (2). As there might be eddies or other local spots in the fuel gas line (2) after the premixing, it is prudent to avoid approaching the theoretical DFR limit.

The one or more gas lines (4) are arranged for providing for providing compressor as said gas flow (41) air for partial premixing with said fuel gas flow (21). This is a very simple solution, solely comprising taking a portion of the compressor air and using said portion of the compressor air for said second gas flow (41). The use of compressor air has several advantages as it requires a small amount of retrofitting onto the system before use. A gas flow line (4) and a mixing zone in said fuel gas line (2) is sufficient to achieve a system of the invention. No further pretreatment should be required, rendering this solution much simpler than the solutions known in the prior art wherein is shown the pie-mixing of the fuel gas flow (21) with steam or other inert compounds. The compressor air is furthermore partially compressed thus reducing the need for possible recompression before partial premixing with the fuel gas flow (21). A first system is shown in fig. 2a, in which is also shown a compressor (6) as described below.

The one or more gas lines (4) may be arranged for providing atmospheric air for the premixing of said fuel gas flow (21), see fig. 4. This has the advantage of avoiding the installation of said gas line (4) from said first compressor (11). This embodiment would necessitate the compression of the air before mixing with said fuel gas flow (21). The fuel gas flow (21) should in most instances be provided to the combustion chamber at a pressure higher than the pressure in the combustion chamber. This implies that the atmospheric air should be compressed before mixing with the fuel gas flow (21). The compression may take place in several stages with intermediate cooling if necessary. Compression of said gas flow (41) might also be necessary when said gas flow (41) comprises compressor air or exhaust air (91). Examples of configurations according to this embodiment are shown in figs. 2a-4.

-12 -One or more second compressors (6) arranged for compressing said second gas flow (41) may be arranged on said gas line (4) such that a desired pressure is achieved before the partial premixing. In conjunction with or independently of said compressor train, heat exchangers (5) may be arranged on said gas line (4), wherein said heat exchangers are arranged for cooling said second gas flow (41), see fig. 2b. The compression of gas is more effective when the gas is at a low temperature. A system according to this embodiment is shown in fig. 2b, in which is shown the gas line (41) being arranged from the compressor (11) such that compressor air is provided for the partial premixing. Heat exchangers (5) might also be provided if said gas flow (41) comprises exhaust gas (91) or atmospheric air.

The temperature of the second gas flow (41) is reduced by injecting water into said second gas flow (41), see fig. 2b. The injected water both serves as a cooling medium and has a very high heat capacity. The injected water will cool the flow and the high heat capacity of water will further increase the heat capacity of the mixture in the combustion chamber. This will reduce the combustion temperature, reducing the formation rate of NOR. The water may be injected as droplets or steam according to any suitable procedure.

A control unit is arranged on said gas line (4) in order for controlling the flow of said second gas flow (41). Said control unit may comprise eiements for measuring the flow of fuel gas (21), the combustion temperature, the NO emissions, and other pertinent parameters so as for controlling said second gas flow (41) in an efficient manner. Any suitable control system may be adapted to this purpose.

A gas separation unit (8) may be arranged on said gas line (4), in which said gas separation unit is arranged for separating said second gas flow (41) in an oxygen enriched flow (42) and an oxygen depleted flow (43). The oxygen depleted flow (43) may advantageously be used as said second gas flow (41).

The gas separation unit (8) may be of any type suitable for gas separation, such as amongst others a membrane system, cryogenic separation and I or pressure swing adsorption. An example of a possible configuration according to the invention is shown in fig. 3. A gas separation unit (8) might also be provided -13-if needed where said gas flow (41) comprises exhaust gas (91) or atmospheric air.

The oxygen depleted flow may comprise between 1-21 % oxygen by volume according to system parameters. The system performance may thus be improved if an excess of oxygen is detected in the combustion chamber, or if system parameters indicate that a less rich fuel mixture should be provided.

The gas line (4) may be arranged for providing flue gas (91) as said second gas flow (41). In this embodiment there is no need for reducing the oxygen levels of said second gas flow (41), as the flue gas flow (91) has a previously reduced oxygen content due to the combustion. A flue gas flow may comprise about 15% 02. The flue gas flow (91) further comprises CO2 and H20 as per the reaction mechanism shown above and will thus have a rather high IS heat capacity. It may thus prove unnecessary to provide water to said second gas flow (41).

Embodiments of the invention may be used in any gas turbine system in which the combustion is mostly diffusion controlled such as conventional burners comprising diffusion or swirl burners or for pilot burners typical of dry low emission (OLE) or wet low emission (WLE) systems. The system and method allows for simple retrofitting of existing gas turbine systems at low cost and high efficiency. For conventional burners the emissions of NO may be expected to be reduced to about 20-30 ppm or lower, whereas for DLE and / or WLE systems NO emissions may be expected to be reduced to about 5 ppm or even lower. The increase in mass throughput in the system has the further benefit of increasing the turbine effectiveness, thus allowing the system to function at approximately the same efficiency levels as gas turbines having higher NO emissions. The use of air has the major advantage of reducing the amount of retrofitting needed for NO reduction to be achieved.

It will be appreciated that variations in, and modifications to, the embodiments as illustrated and as described above may be made in accordance with the invention as defined by the accompanying claims.

Claims

-14 -Claims.

1. A system for reducing emissions from a gas turbine system, wherein said gas turbine system is arranged for energy production and / or propulsion by combustion of a combustble gas, vhere:n sd gas turbine system comprises a first compressor, a combustion chamber and a turbine, wherein said gas turbine system is arranged for being provided with a fuel gas flow by one or more fuel gas lines, wherein said gas turbine system is arranged for being provided with a first gas flow from one or more inlets, wherein said first gas flow is arranged for providing oxygen for the combustion of fuel gas in said fuel gas flow, wherein said combustion is arranged for taking place within said combustion chamber, wherein said combustion in said combustion chamber is partially diffusion rate controlled, wherein one or more gas lines are arranged for being connected to said fuel gas line or lines, wherein said gas line is arranged for providing a second gas flow for being mixed with said fuel gas flow, wherein said second gas flow at least partially comprises oxygen, whereupon said fuel gas flow is partially premixed with said second gas flow before injection into said combustion chamber.

2. A system as claimed in Claim 1, wherein the partial premixing ratio of said second gas flow to said fuel gas flow is in the range from 0,5 to 5 by weight.

3. A system as claimed in Claim 1 or Claim 2, wherein said one or more gas lines are arranged running from said first compressor for providing compressor as said second gas flow.

4. A system as claimed in Claim 1 or Claim 2, wherein said one or more gas lines are arranged for providing atmospheric air as said second gas flow.

5. A system as claimed in Claim 1 or Claim 2, wherein said one or more gas lines are arranged for providing exhaust gas as said second gas flow. -15-

6. A system as claimed in any preceding claim, wherein said one or more gas lines are provided with one or more heat exchangers arranged for cooling said second gas flow before mixing with said fuel gas flow.

7. A system s ciirrid in any preceding ciaim, wherein one or more second compressors are arranged for compressing said second gas flow before mixing with said fuel gas flow.

8. A system as claimed in any preceding claim, wherein a water injection system is arranged for providing water to said second gas flow before mixing the resulting flow with said fuel gas flow.

9. A system as claimed in any preceding claim, wherein a control unit is arranged on said gas line, in which said control unit is arranged for controlling the flow of said second gas flow.

10. A system as claimed in any preceding claim, wherein a gas separation unit is arranged on said gas line, in which said gas separation unit is arranged for separating said second gas flow into an oxygen enriched flow and an oxygen depleted flow.

11. A system as claimed in Claim 10, wherein said oxygen depleted gas flow is mixed with said fuel gas flow.

12. A system as claimed in Claim 11, wherein said oxygen depleted gas flow comprises between 1-21 % oxygen by volume.

13. A system as claimed in Claim 12, wherein said oxygen depleted gas flow comprises about 15 % oxygen.

14. A method for reducing emissions from a gas turbine system, wherein said gas turbine system produces energy by combustion of a combustible gas, wherein said gas turbine system comprises at least a first compressor, a combustion chamber and a turbine, wherein one or more fuel gas lines provide a fuel gas flow to said gas turbine system, -16 -wherein one or more inlets provide a first gas flow to said gas turbine system said first gas flow providing oxygen for the combustion of fuel gas flow, wherein said combustion takes place within said combustion chamber, wherein at least a portion of said combustion in said combustion chamber is diffuson rate ccntro!!ed, wherein one or more gas lines are connected to said gas fuel line, wherein said gas line provides a second gas flow at least partially comprising oxygen, for mixing with said fuel gas flow, whereupon said fuel gas flow is partially premixed is partially premixed with said second gas flow before injection into said combustion chamber.

15. A method as claimed in Claim 14, wherein the partial premixing ratio of said second gas flow to said fuel gas flow is in the range 0,5 to 5 by weight.

16. A method as claimed in Claim 14 or Claim 15, wherein said one or more gas lines are arranged running from said first compressor such that said second gas flow mainly comprises compressor air.

17. A method as claimed in Claim 14 or Claim 15, wherein said one or more gas lines are connected to the atmosphere such that said second gas flow mainly comprises atmospheric air.

18. A method as claimed in Claim 14 or Claim 15, wherein said one or more gas lines are connected to an exhaust such that said second gas flow mainly comprises exhaust gas.

19. A method as claimed in Claim 14 or Claim 15, wherein said second gas flow is cooled before mixing with said fuel gas flow.

20. A method as claimed in any of Claims 14 to 19, wherein said second gas flow is compressed before mixing with said fuel gas flow.

21. A method as claimed in any of Claims 14 to 20, wherein water is mixed with said the second gas flow before the resulting flow is mixed with said fuel gas flow.

22. A method as claimed in any of Claims 14 to 21, wherein said second gas flow is separated into an oxygen enriched flow and an oxygen depleted flow.

23. A method as claimed in Claim 22, wherein said oxygen depleted gas flow is mixed with said fuel gas flow.

24. A method as c!aimed in Clm 22 or Clam 23, wherein said oxyQeri depleted gas flow comprises between 1-21 % oxygen by volume.

25. A method as claimed in Claim 24, wherein said oxygen depleted gas flow comprises about 15 % oxygen.

26. Use of a system as claimed in any of Claims 1 to 13 for reducing NO emissions from a diffusion type gas turbine system.

27. Use of a system as claimed in any of Claims 1 to 13 for reducing NO emissions from a pilot burner in Dry or Wet Low Emission gas turbine systems.

28. A gas turbine system with reduced emissions substantially as hereinbefore described with reference to the accompanying drawings.

29. A method of reducing emissions from a gas turbine system substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows: 1. A system for reducing emissions from a gas turbine system, wherein said gas turbine system is arranged for energy production and/or propulsion by combustion of a combustible gas, wherein said gas turbine system comprises a first compressor, a combustion chamber and a turbine, wherein said gas turbine system is arranged for being provided with a fuel gas flow by one or more fuel gas lines, wherein said gas turbine system is arranged for being provided with a first gas flow from one or more inlets, wherein said first gas flow is arranged for providing oxygen for the combustion of fuel gas in said fuel gas flow, wherein said combustion is arranged for taking place within said combustion chamber, wherein said combustion in said combustion chamber is partially diffusion rate controlled, wherein one or more gas lines are arranged for being connected to said fuel gas line or lines, said fuel gas line or lines providing fuel gas to the diffusion rate limited combustion, wherein said gas line is arranged for providing a second gas flow for being mixed with said fuel gas flow, wherein said second gas flow at least partially comprises oxygen, whereupon said fuel gas flow for the diffusion rate controlled combustion is partially premixed with said second gas flow before injection into said combustion chamber.

3. A system as claimed in Claim I or Claim 2, wherein said one or more gas lines are arranged running from said first compressor for providing compressor as said second gas flow.

4. A system as claimed in Claim I or Claim 2, wherein said one or more gas lines are arranged for providing atmospheric air as said second gas flow.

5. A system as claimed in Claim I or Claim 2, wherein said one or more gas lines are arranged for providing exhaust gas as said second gas flow.

7. A system as claimed in any preceding claim, wherein one or more second compressors are arranged for compressing said second gas flow before mixing with said fuel gas flow.

13. A system as claimed in Claiml2, wherein said oxygen depleted gas flow comprises about 15 % oxygen.

14. A method for reducing emissions from a gas turbine system, wherein said gas turbine system produces energy by combustion of a combustible gas, wherein said gas turbine system comprises at least a first compressor, a combustion chamber and a turbine, 2O wherein one or more fuel gas lines provide a fuel gas flow to said gas turbine system, wherein one or more inlets provide a first gas flow to said gas turbine system said first gas flow providing oxygen for the combustion of fuel gas flow, wherein said combustion takes place within said combustion chamber, wherein at least a portion of said combustion in said combustion chamber is diffusion rate controlled, wherein one or more gas lines are connected to said gas fuel line, said fuel gas line providing fuel gas to the diffusion controlled combustion, wherein said gas line provides a second gas flow at least partially compnsing oxygen, for mixing with said fuel gas flow, whereupon said fuel gas flow for the diffusion controlled combustion is partially premixed with said second gas flow before injection into said combustion chamber.

24. A method as claimed in Claim 22 or Claim 23, wherein said oxygen depleted gas flow comprises between 1-21 % oxygen by volume.

26. Use of a system as claimed in any of Claims I to 13 for reducing NOx emissions from a diffusion type gas turbine system.

27. Use of a system as claimed in any of Claims I to 13 for reducing NOx emissions from a pilot burner in Dry or Wet Low Emission gas turbine systems.