Abstract:
In method for operating a gas turbine ( 11 ), the compressed air is fed to a combustor ( 18, 19 ) for the combustion of a coal syngas, and the resulting hot gases are expanded in a subsequent turbine ( 16, 17 ). Some of the compressed air is separated into oxygen and nitrogen, and the oxygen is used for producing the syngas. In a first combustor, ( 18 ) syngas is combusted and the resulting hot gases are expanded in a first turbine ( 16 ), and in a second combustor syngas is combusted, using the gases which issue from the first turbine ( 16 ), and the resulting hot gases are expanded in the second turbine ( 17 ). The two combustors ( 18, 19 ) are operated with undiluted syngas, and the first combustor flame temperature (T F ) is lowered compared with the operation with natural gas (T NG ), while the second combustor ( 19 ) is operated in the normal operation (T NG ) for natural gas.

Description:
This application is a Continuation of, and claims priority under 35 U.S.C. §120 to, International Application No. PCT/EP2007/062986, filed 29 Nov. 2007, and claims priority therethrough under 35 U.S.C. §§119, 365 to Swiss application no. 01956/06, filed 1 Dec. 2006, the entireties of both of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     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 CO 2  separator  37  with a CO 2  outlet  38  for discharge of the separated CO 2 , are connected downstream to the coal gasification plant  34 . 
     For coal gasification in the coal gasification plant  34 , oxygen (O 2 ) is used, which is produced in an air separation unit  32  and fed via an oxygen line  32   a . The air separation unit  32  obtains compressed air from the outlet of the low-pressure compressor  13 . The nitrogen (N 2 ) which also results during the separation is fed for example via a nitrogen line  32   b  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 (N 2 ) being added for diluting the syngas fuels (CO-rich and H 2 -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 H 2 -rich syngas fuels by the fuel being diluted with N 2  from the air separation unit after the gasification (see  FIG. 2 ).   An alternative to N 2  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. 
     SUMMARY 
     One of numerous aspects of the present invention includes a method for operating a gas turbine which operates together with a coal gasification plant and which can be characterized by a reduction of NOx emissions without noticeable loss of output and flexibility of operation. 
     Another aspect include a gas turbine with reheating which comprises two combustors and two turbines, wherein in the first combustor syngas is combusted, using the compressed air, and the resulting hot gases are expanded in the first turbine, and wherein in the second combustor syngas is combusted, using the gases which issue from the first turbine, and the resulting hot gases are expanded in the second turbine, the two combustors are operated with undiluted syngas, and the flame temperature in the first combustor of the gas turbine is lowered compared with the operation with natural gas, while the second combustor is operated in the normal mode which is designed for natural gas. 
     One aspect of the method includes that CO-rich syngas is used as fuel, and in that the flame temperature in the first combustor of the gas turbine is lowered by 50-100 K compared with the operation with natural gas. 
     Another aspect of the method includes that H 2 -rich syngas is used as fuel, and the flame temperature in the first combustor of the gas turbine is lowered by 100-150 K compared with the operation with natural gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be subsequently explained in more detail based on exemplary embodiments in conjunction with the drawings. In the drawings: 
         FIG. 1  shows the simplified schematic diagram of a combined-cycle power plant with a gas turbine with reheating or sequential combustion according to the prior art; 
         FIG. 2  shows the simplified schematic diagram of an IGCC plant with a gas turbine with reheating or sequential combustion, wherein the nitrogen which is produced during air separation is used for diluting the syngas; and 
         FIG. 3  shows the simplified schematic diagram of an IGCC plant with a gas turbine with reheating or sequential combustion, wherein according to an exemplary embodiment of the invention, undiluted syngas is used as fuel and the flame temperature in the first combustor is reduced for reducing the NOx emission. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In  FIG. 3 , an IGCC plant with a gas turbine with reheating or sequential combustion, which is operated according to principles the invention, is shown in a much simplified schematic diagram. For like plant components, the same designations are used in this case, as in  FIG. 2 . The nitrogen (N 2 ) which accumulates in the air separation unit  32  is no longer used here for dilution of the syngas which is used as fuel, but the syngas is injected into the combustors  18 ,  19  in an undiluted state. For this, in the first combustor  18  the flame temperature T F  is lowered compared with the temperature T NG  which prevails during normal natural gas operation, whereas in the second combustor  19  the nominal flame temperature T NG  which is provided for natural gas operation is maintained. As a result of lowering the flame temperature in the first combustor  18 , the production of NOx can be controlled without a crucial loss of output occurring. Thus, in the case of a gas turbine without reheating or sequential combustion, a reduction of output by 10% and a reduction of efficiency by 1% results by lowering the flame temperature in the (single) combustor by 100° C., whereas lowering the flame temperature in the first combustor of a gas turbine with sequential combustion brings about a reduction of output by only about 1% and a reduction of efficiency by only 0.1%. 
     The extent of lowering the flame temperature T F  in the first combustor  18  in this case depends upon the type of syngas which is used: 
     In the case of an IGCC plant with CO-rich syngas, the following situation ensues:
         The flame temperature T F  in the first combustor  18  is lowered by 50-100 K compared with the natural gas operation.   The second combustor  19  is operated with the nominal flame temperature, but with a lower inlet temperature which results on account of the reduced flame temperature in the first combustor.   Consequently, an operation of the two combustors  18 ,  19  without dilution of the syngas with nitrogen is possible; the low heating value (LHV) of the undiluted syngas lies within the order of magnitude of 12-14 (MJ/kg); diluted syngas fuels have low heating values in the order of magnitude of 5-7 (MJ/kg).   On account of the fact that only the first combustor  18  is operated with reduced flame temperature or reduced turbine inlet temperature, the second combustor operates in an unaltered state with the flame temperature which is provided for natural gas operation so that the turbine exhaust temperature (of the second turbine  17 ) is the same as in the case of natural gas operation. Therefore, no loss of output and efficiency results for the combined-cycle process of the IGCC plant. This is a special characteristic of the gas turbine with reheating, in which the flame is controlled by the inlet conditions instead of by the exhaust conditions.   In a gas turbine without reheating, however, a reduced flame temperature or turbine inlet temperature results in a significant reduction of the turbine exhaust temperature. This causes an appreciable loss of output and efficiency for the combined-cycle process of the IGCC plant in comparison with the natural gas operation.
           The plant can therefore be operated with the following advantages:
               The highest possible flexibility and the highest possible efficiency result during the operation of the IGCC plant; the air separation unit  32  does not have to be integrated and the fuel does not have to be diluted with nitrogen.   A small reduction of the flame temperature is achieved during the combustion of the undiluted CO-rich fuel in order to control the production of NOx.   No change is necessary in the mechanical layout of compressor and turbine of the gas turbine which is designed for natural gas. The absent syngas dilution leads to a smaller difference in the heating value between natural gas and undiluted syngas in comparison to diluted syngas.   The operating concept for the gas turbine is simple and flexible.   The operation of the entire plant is flexible on account of the absent integration of the air separation unit  32 .   The unaltered exhaust temperature of the gas turbine leads to high output and high efficiency for the IGCC plant.   
               
               

     In the case of an IGCC plant with H 2 -rich syngas with a CO 2  displacement reaction for the CO 2  separation, the following situation ensues:
         The flame temperature T F  in the first combustor  18  during operation with H 2  is lowered by 100-150 K compared with the natural gas operation.   The second combustor  19  is operated with the nominal flame temperature for natural gas operation.   Consequently, an operation of the two combustors  18 ,  19  without dilution of the syngas with nitrogen is possible; the low heating value of H 2  lies in the order of magnitude of 30-35 (MJ/kg); the low heating value of the supplementary fuel and of undiluted H 2  is in the same order of magnitude, which leads to a less serious mismatch of turbine and compressor.   Diluted H 2 -fuels have a low heating value in the order of magnitude of 6-9 (MJ/kg).   On account of the fact that only the first combustor  18  is operated with reduced flame temperature or turbine inlet temperature, the turbine exhaust temperature of the gas turbine is the same as in the case of natural gas operation. Therefore, no loss of output and efficiency for the combined-cycle process of the IGCC plant results.   In a gas turbine without reheating, however, a reduced flame temperature or turbine inlet temperature results in a significant reduction of the turbine exhaust temperature. This causes an appreciable loss of output and efficiency for the combined-cycle process of the IGCC plant in comparison to the natural gas operation.   The plant can therefore be operated with the following advantages:
           The highest possible flexibility and the highest possible efficiency result during the operation of the IGCC plant; the air separation unit  32  does not have to be integrated and the fuel does not have to be diluted with nitrogen.   The costs for installation and also for operation and maintenance of the plant are reduced. No equipment for compression of N 2  is necessary.   A small reduction of the flame temperature is achieved during the combustion of the undiluted H 2 -fuel in order to control the production of NOx.   No change is necessary in the mechanical layout of compressor and turbine of the gas turbine which is designed for natural gas. The absent syngas dilution leads to a smaller difference in the heating value between natural gas and undiluted syngas in comparison to diluted syngas.   The operating concept for the gas turbine is simple and flexible.   The operation of the entire plant is flexible on account of the absent integration of the air separation unit  32 .   The unaltered exhaust temperature of the gas turbine leads to high output and high efficiency for the IGCC plant.   On account of the absent integration of the air separation unit the operation of the plant is flexible and can be started quicker.   
               

     The precondition for the concept of the not fully integrated air separation unit, which is described above, is that undiluted coal gas can be used in the two combustors of the gas turbine. 
     The main technical challenges for the combustion of such undiluted coal gas in the combustors of the gas turbine are:
         the achievement of low emission values,   sufficient distances to the limits for flashbacks and pulsations,   maintaining operating flexibility during fluctuations in the quality of the coal gas, and the possibility of the use of supplementary fuel (natural gas or oil),   the tapping and returning of cooling air in the hot gas regions of combustor and turbine.       

     The gas turbine with reheating meets these challenges particularly well in the case of syngas applications for the following reasons: 
     1. An advantage in the case of NOx emission, which via the optimum choice of the combustion temperatures in the two combustors can be transferred to syngas applications, is specific to the gas turbine with reheating. 
     2. The combustion stability and the operational flexibility of the gas turbine with reheating are greater than in the case of a comparable machine with single-stage combustion. Operating limits, at a given flame temperature, are typically provided by flame extinction and flashback and/or emission levels, which leads to a permissible range of fuel qualities and fuel reactivities. In the case of a turbine with reheating, these operating limits are appreciably broadened because the two combustion systems enable the operation with two different or independent flame temperatures, i.e., a lower temperature in the first stage and a higher temperature in the second stage, with only slight disadvantages with regard to NOx emission. 
     3. The requirements for the gas pressure can be minimized if no diluted N 2  is injected into the first and second combustion system, which typically operates at a pressure of &gt;30 bar or in the range of 15-20 bar. 
     LIST OF DESIGNATIONS 
     
         
         
           
               10 ,  30  Combined-cycle power plant 
               11  Gas turbine 
               12  Generator 
               13  Low-pressure compressor 
               14  High-pressure compressor 
               15  Shaft (gas turbine) 
               16  High-pressure turbine 
               17  Low-pressure turbine 
               18  Combustor (high-pressure combustor) 
               19  Combustor (reheat combustor) 
               20  Air intake 
               21 ,  22  Fuel feed line 
               23 ,  24  OTC cooler 
               25 ,  26  Cooling line 
               27  Heat recovery steam generator 
               28  Exhaust gas line 
               29  Steam turbine (steam cycle) 
               31  Syngas feed line 
               32  Air separation unit 
               32   a  Oxygen line 
               32   b  Nitrogen line 
               33  Coal feed line 
               34  Coal gasification plant 
               35  Cooling device 
               36  Cleaning plant 
               37  CO 2  separator 
               38  CO 2  outlet 
           
         
       
    
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.