1. Field of Endeavor
The present invention relates to the field of power plant technology. It refers to a method for operating a (stationary) gas turbine, and also to a gas turbine useful for carrying out the method.
2. Brief Description of the Related Art
A gas turbine with reheating (reheat gas turbine) is known (see, for example, U.S. Pat. No. 5,577,378, or “State-of-the-art gas turbines—a brief update”, ABB review 02/1997, FIG. 15, turbine type GT26), which combines a flexible operation with very low exhaust gas emission values.
The machine architecture of gas turbine of type GT26 is uniquely and suitably characterized for the realization of a concept which is the subject of the present invention, because:                in the compressor there is already a significant tapping off of compressor air at medium compressor pressures,        sequential combustion enables a high stability of combustion in the case of reduced values of oxygen surplus, and        a secondary air system is available which enables air to be tapped off from the compressor and cooled down, and the cooled-down air to be used for cooling the combustor and the turbine.        
The principle of the known gas turbine with reheating is reproduced in FIG. 1. The gas turbine 11, which is part of a combined-cycle power plant 10, includes two compressors, specifically a low-pressure compressor 13 and a high-pressure compressor 14, which are connected in series and arranged on a common shaft 15, and also two combustors, specifically a high-pressure combustor 18 and a reheat combustor 19, and associated turbines, specifically a high-pressure turbine 16 and a low-pressure turbine 17. The shaft 15 drives a generator 12.
The principle of operation of the plant is the following: air is inducted via an air intake 20 by the low-pressure compressor 13 and first compressed to an intermediate pressure level (about 20 bar). The high-pressure compressor 14 then further compresses the air to a high pressure level (about 32 bar). Cooling air is tapped off both at the intermediate pressure level and at the high pressure level and cooled in associated OTC coolers (OTC=Once-Through Cooler) 23 and 24, and via cooling air lines 25 and 26 is transmitted to the combustors 18, 19 and turbines 16, 17 for cooling. The remaining air from the high-pressure compressor 14 is guided to the high-pressure combustor 18 and heated there by combustion of a fuel which is fed via the fuel feed line 21. The resulting exhaust gas is then expanded in the subsequent high-pressure turbine 16 to a medium pressure level, performing work. After expansion, the exhaust gas is reheated in the reheat combustor 19 by combustion of a fuel which is fed via the fuel feed line 22, before it is expanded in the subsequent low-pressure turbine 17, performing further work.
The cooling air which flows through the cooling air lines 25, 26 is injected at suitable points of the combustors 18, 19 and turbines 16, 17 in order to limit the material temperatures to an acceptable degree. The exhaust gas which issues from the low-pressure turbine 17 is sent through a heat recovery steam generator 27 (HRSG=Heat Recovery Steam Generator) in order to produce steam which, within a water-steam cycle, flows through a steam turbine 29 and performs further work there. After the heat recovery steam generator 27 has been subjected to throughflow by the exhaust gas, the exhaust gas is finally discharged to the outside through an exhaust gas line 28. The OTC coolers 23, 24 are part of the water-steam cycle; superheated steam is produced at their outlets.
A high flexibility in operation is achieved as a result of the two consecutive combustions, which are independent of each other, in the combustors 18 and 19; the combustor temperatures can be set so that the maximum efficiency is achieved within the existing limits. The low exhaust gas values of the sequential combustion system are produced as a result of the inherently low emission values which can be achieved in the case of reheating (under certain conditions the second combustion even leads to a consumption of NOx).
On the other hand, combined-cycle power plants with single-stage combustion in the gas turbines are known (see, for example, U.S. Pat. No. 4,785,622 or U.S. Pat. No. 6,513,317), in which is integrated a coal gasification plant which is supplied with oxygen from an air separation unit (ASU) in order to provide the fuel, in the form of syngas which is produced from coal, which is required for the gas turbine. Such combined-cycle power plants are referred to as IGCC plants (IGCC=Integrated Gasification Combined Cycle).
The present invention is now based on the knowledge that, by the use of gas turbines with reheating according to FIG. 1 in an IGCC plant, the advantages of this gas turbine type can be utilized for the plant in a special way.
The highest flexibility and efficiency during the operation of an IGCC plant can be achieved if the air separation unit is not integrated and if undiluted fuels can be combusted. Using a gas turbine with reheating according to FIG. 1, this can be realized, while at the same time the emissions can be minimized on account of an alternative concept of the NOx controls. This type of process profits from the advantages of reheating.
The integration of a gas turbine in an IGCC plant typically influences both the compressor and the combustors, as is clear in FIG. 2. The combined-cycle power plant 30 of FIG. 2 includes a gas turbine 11 with a low-pressure compressor 13, a subsequent high-pressure compressor 14, a high-pressure combustor 18 with a subsequent high-pressure turbine 16, and a reheat combustor 19 with a subsequent low-pressure turbine 17. The compressors 13, 14 and the turbines 16, 17 are seated on a common shaft 15, by which a generator 12 is driven. The combustors 18 and 19 are supplied via a syngas feed line 31 with syngas as fuel which is produced by gasification of coal (coal feed line 33) in a coal gasification plant 34. A cooling device 35 for the syngas, a cleaning plant 36, and a CO2 separator 37 with a CO2 outlet 38 for discharge of the separated CO2, are connected downstream to the coal gasification plant 34.
For coal gasification in the coal gasification plant 34, oxygen (O2) is used, which is produced in an air separation unit 32 and fed via an oxygen line 32a. The air separation unit 32 obtains compressed air from the outlet of the low-pressure compressor 13. The nitrogen (N2) which also results during the separation is fed for example via a nitrogen line 32b to the low-pressure combustor 19 (and/or to the high-pressure combustor 18) for diluting the syngas.
For cooling the components of the combustors 18, 19 and turbines 16, 17, which are stressed by the hot gas, compressed cooling air is tapped off at the outlets of the two compressors 13 and 14, cooled in a downstream OTC cooler 23 or 24, and then via corresponding cooling lines 25 and 26 fed to places which are to be cooled.
At the outlet of the low-pressure turbine 17, a heat recovery steam generator 27 is arranged, which together with an associated steam turbine 29 is part of a water-steam cycle. The exhaust gas which issues from the heat recovery steam generator 27 is discharged to the outside via an exhaust gas line 28.
Such an integration of the gas turbine leads to                air being tapped from the compressor for the air separation unit in order to compensate for the fuel mass flow which is fed via the combustors; and        nitrogen (N2) being added for diluting the syngas fuels (CO-rich and H2-rich fuels) in order to control the production of NOx.        
When using syngas fuels, there are basically two possibilities for controlling the NOx:                It is general practice to control the NOx level in the case of CO-rich and subsequent H2-rich syngas fuels by the fuel being diluted with N2 from the air separation unit after the gasification (see FIG. 2).        An alternative to N2 dilution is the reduction of the flame temperature or, in the case of an afterburner or reheating, the reduction of the inlet temperature in the second stage of combustion.        
This possible alternative for a gas turbine with reheating offers the opportunity of controlling the production of NOx without noticeably forfeiting output.