Gas turbines with sequential combustion comprise two combustors as shown for example in FIG. 1. The gas turbine 10 of FIG. 1 has a rotor 11 which is concentrically surrounded by a casing 12. A compressor 13 compresses air which feeds into a first combustor comprising first combustion chamber 14 and first burners 17. The first burners 17 are supplied with a fuel by means of a first fuel supply 18. The hot gas generated in the first combustor 14, 17 flows through hot gas channel 21 into a second combustor comprising a second combustion chamber 15 and second burners 20. The second burners 20 are supplied with a fuel by means of a second fuel supply. The hot gas from the second combustor 15, 19 enters the turbine 16 to do work.
Usually, a high-pressure turbine is positioned between the first combustor 14, 17 and the second combustor 15, 19. However, in this case the high-pressure turbine has been omitted. Instead, dilution air may be injected into the hot gas channel 21 by means of a dilution air supply 23.
In part load operation of gas turbines, the combustor hot gas temperature typically decreases. At a certain hot gas temperature limit, CO emissions rise and set a limit to the emission guarantee range. The CO emissions typically rise when the gas turbine load is reduced from 100% to part-load is shown in FIG. 2, where the CO emission is shown as a function of the gas turbine relative load RLGT, with the peak at 12 characterising the ignition of a second combustor. At a certain load limit LTL a predetermined CO limit LTCO is exceeded. At the same time the second combustor is characterised by a certain pressure drop which has a negative impact on performance.
To solve this problem, two different approaches have been proposed in the prior art:                1. Starting the second combustor at the highest possible load. However, this is not very effective because the hot gas temperature for the first combustor needs to be maintained within limits imposed by the high pressure turbine lifetime (when a high pressure turbine is provided between first and second combustor).        2. Switch off some of the second burners at low part-load, so that the remaining ones are fired at higher exit temperatures at a lower load. Such solution has the drawback of high stress to the low pressure turbine (16 in FIG. 1) and is limited by the temperature limits of the boiler (in a combined cycle power plant).        
Further related prior art has been identified as follows:
Document DE 103 12 971 A1 discloses a gas turbine assembly, especially for a power unit, comprising a compressor and two downstream combustion chambers in series followed by a turbine. There is a cooling unit arranged between the two combustion chambers. This solution is based on sequential combustion without a high pressure turbine. Due to the missing high pressure turbine between the two combustors comparatively high gas temperatures result at the exit of the first combustor, which would result in a spontaneous ignition when fuel is injected at the second combustor, leading to excessive component thermal stresses near the fuel injection and an insufficient mixing of fuel and air resulting in high emission values.
To avoid this disadvantage it is proposed to cool down the hot combustion gas from the first combustion chamber before injecting fuel into the hot combustion gas to form the fuel-oxidant-mixture for the second combustion chamber. By cooling the hot combustion gas from the first combustion chamber or first combustion step the exhaust gas temperature can be reduced enough that the ignition of the injected fuel is delayed long enough to allow a detached flame and a sufficient mixture in order to have a homogenous and lean fuel-oxidant-mixture.
Document WO 03/038253 relates to the conversion of a sequentially-fired gas turbine unit, essentially comprising at least one compressor, a high pressure combustion chamber, a high pressure turbine, a low pressure combustion chamber and a low pressure turbine. The rotating parts of the compressor, high pressure turbine and low pressure turbine are arranged on a common rotor and on conversion all claimed considerations associated with said gas turbine unit can be advantageously simplified. The converted gas turbine unit comprises a reduced compressor, the original high pressure combustion chamber (operating at lower pressure), the original low pressure combustion chamber and the original low pressure turbine, whereby a reduction of the compressor is achieved by means of removing, without replacement, a number of the final high pressure stages, in relation to the direction of flow or by removing the same from drive and the original high pressure turbine is removed from function without replacement. The high pressure turbine is just a transfer channel between the high pressure combustion chamber and the low pressure combustion chamber. The hot gases flow from said high pressure combustion chamber through said transfer channel directly into the low pressure combustion chamber.
Document EP 2 206 959 A2 discloses a gas turbine system including a fuel reformer system comprising a fuel inlet configured to receive a fuel slipstream, an oxygen inlet configured to introduce an oxygen slipstream, a preconditioning zone configured to pre-treat the fuel slipstream, a mixing zone comprising a premixing device configured to facilitate mixing of the fuel slipstream and the oxygen slipstream to form a gaseous premix, a reaction zone configured to generate a syngas from the gaseous premix, a quench zone configured to mix a fuel stream into the syngas to form a hydrogen-enriched fuel mixture, and a gas turbine configured to receive the fuel mixture.
Document WO 2010/112318 A1 relates to a method for the low CO-emission partial load operation of a gas turbine having sequential combustion, wherein the air ratio of the operative burners of the second combustion chamber at partial load is kept below a maximum air ratio, and to a gas turbine for carrying out said method. In order to reduce the maximum air ratio, a number of modifications are performed in the operating concept of the gas turbine, individually or in combination. One modification is an opening of the adjustable compressor guide vane row before turning on the second combustion chamber. In order to turn on the second combustion chamber, the adjustable compressor guide vane row is quickly closed and fuel is conducted into the burners of the second combustion chamber in a synchronized way. A further modification is the turning off of individual burners at partial load.
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