Abstract:
A method and system for operating at partial load a gas turbine system having a compressor, a combustor, and a turbine. The method and system may include the steps of lowering a flow of fuel to the combustor, extracting air from the compressor so as to lower a flow of air to the compressor, and returning the extracted air to the turbine or a component of the gas turbine system other than the combustor. Extracting air from the compressor raises a combustion temperature within the combustor. Raising the combustion temperature maintains a combustion exhaust below a predetermined level, maintains stable combustion, and extends turbine turndown values.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to gas turbines and more particularly relates to methods and systems to extend gas turbine turndown values during part load operations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gas turbines generally have high efficiency at peak and base load operations. This efficiency, however, generally decreases during part-load operations. Turbine operation and exhaust emissions compliance may become an issue at such lower loads. Specifically, reducing the load on the turbine or “turndown” generally may be accomplished by reducing the fuel flow to the combustor. This reduction in fuel flow, however, makes the air-fuel mixture leaner such that sustaining combustion becomes more problematic as combustion temperatures are reduced. Unstable combustion may lead to excessive gas emission levels as well as to mechanical instability. Such instability potentially may damage elements of the gas turbine system as a whole. A typical turndown value of about forty percent (40%) to about thirty percent (30%) of full load may be expected while maintaining emissions compliance. 
         [0003]    There is a desire, therefore, for improved methods and systems for gas turbine part-load operating conditions. Preferably, the improved methods and systems can extend the turndown value of a gas turbine within emissions compliance while maintaining or improving overall system efficiency. 
       SUMMARY OF THE INVENTION 
       [0004]    The present application thus provides a method of operating at partial load a gas turbine system having a compressor, a combustor, and a turbine. The method may include the steps of lowering a flow of fuel to the combustor, extracting air from the compressor so as to lower a flow of air to the combustor, and returning the extracted air to the turbine or a component of the gas turbine system other than the combustor. Extracting air from the compressor raises a combustion temperature within the combustor. Raising the combustion temperature maintains a combustion exhaust below a predetermined level such as a predetermined emissions compliance level. 
         [0005]    The present application further describes a gas turbine system. The gas turbine system may include a compressor with a compressor discharge, a combustor in communication with the compressor, and a turbine in communication with the combustor. A compressor discharge extraction may extend from the compressor discharge to the turbine such that air from the compressor discharge may be extracted and returned to the turbine during partial load operations. 
         [0006]    The present application further describes a gas turbine system. The gas turbine system may include a compressor and a combustor in communication with the compressor. The compressor may include a compressor discharge valve such that air from the compressor may be extracted during partial load operations. 
         [0007]    These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]      FIG. 1  is a schematic view of a gas turbine system as is described herein. 
           [0009]      FIG. 2  is a schematic view of an alternative embodiment of a gas turbine system as is described herein. 
       
    
    
     DETAILED DESCRIPTION  
       [0010]    Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  is a schematic view of an example of a gas turbine system  100 . Generally described, the gas turbine system  100  may include a compressor  110 , a combustor  120  with a number of cans  125 , and a turbine  130 . The gas turbine system  100  compresses ambient air in the compressor  110 . The ambient air is then delivered to the combustor  120  where it is used to combust a flow of fuel to produce a hot combustion gas. The hot combustion gas is delivered to the turbine  130  where it is expanded to mechanical energy via a number of blades within a hot gas path. The turbine  130  and the compressor  120  generally are connected to a common shaft  140  that may be connected to an electric generator or other type of load  150 . The load on the gas turbine system  100  may be determined by a load senor  155 . The load sensor  155  may be of conventional design. The gas turbine system  100  may be a Dry Low-NO x  (DLN) combustion system or any type of combustion system. The gas turbine system  100  may be part of combined cycle power plant or other types of generation equipment. 
         [0011]    Emissions compliance levels may vary according to location, type of generating equipment, operating conditions, and other variables. For the purposes herein, emissions compliance means a predetermined limit on gas turbine emissions that should not be exceeded. Emissions compliance generally focuses on NO x  and CO x  emissions and other types of byproducts. 
         [0012]    One known method of staying within emissions compliance during part-load operations is to reduce the angle of the inlet guide vanes about the compressor  110  and to activate an inlet bleed heat flow while considering a Fuel Stroke Reference. Such a control system is shown in commonly owned U.S. Pat. No. 7,219,040 entitled “Method and System for Model Based Control of Heavy Duty Gas Turbine.” 
         [0013]    In addition to the existing turbine designs, another emissions compliance method is to bleed off some of the compressed discharge air from the compressor  110  before it reaches the combustor  120 . Specifically, the fuel flow to the combustor  120  may be reduced during turndown. The reduction in fuel flow makes the air/fuel mixture leaner and reduces the temperature within the combustor  120 . Bleeding some of the compressor air also forces the temperature within the combustor  120  to increase so as to allow the gas turbine system  100  as a whole to operate at its intended fuel mixture. 
         [0014]    In addition to raising the temperature in the combustor  120 , this bleed air may be used to cool the parts of the turbine  130  within the hot gas path in a manner similar to existing compressor extractions. Specifically, in addition to existing extractions, the gas turbine system  100  also may have a number of cooling compressor stage extractions  160 . For example, a stage  9  compressor extraction  160  may be used to cool turbine stages  2  and  3  while compressor extractions  160  from stages  13 ,  17 , and  18 , may be used to cool stages  1 ,  2  and  3  of the turbine  130 . Other extraction locations and combinations may be used herein. 
         [0015]    In this example, a compressor discharge extraction  170  from a compressor discharge  175  of the compressor  110  also may be used to cool an early stage of the turbine  130  in a manner similar to the compressor stage extractions  160  described above. The compressor stage extraction  170  may extend from the compressor discharge  175  to the first or second stage of the turbine  130 . Other positions may be used herein. 
         [0016]    Alternatively, the energy of the compressor discharge extraction  170  may be used for any desired operation with respect to the gas turbine system  100  or the power plant as a whole via a heat exchanger  180  or other type of heat transfer device. The heat exchanger  180  may be of conventional design. For example, the heat exchanger  180  may be in communication with the compressor discharge  175  and other elements of the combined cycle power plant as described above. 
         [0017]    Operation of the extractions  160 ,  170  may be performed with the use of an exhaust temperature sensor  190 . The exhaust temperature sensor  190  may be in communication with the exhaust flow from the turbine  130  so as to sense the output temperature therein. The exhaust temperature sensor  190  may be of conventional design. The exhaust temperature sensor  190  may be in communication with an extraction flow control valve  200 . The extraction flow control valve  200  may be a conventional three-way valve that forwards the air of the compressor discharge extraction  170  either towards the turbine  130  for cooling therein or towards the heat exchanger  180  for use with the combined cycle power plant or otherwise. A further turbine temperature sensor  195  may be used with respect to the parts within the hot gas path of the turbine  130 . Other sensors may be used herein. 
         [0018]    A similar flow control valve  165  may be positioned about the compressor stage extractions  160  such that the compressor stage extractions also may be used to control the temperature of the combustor  120  or for other purposes. For example, the compressor extraction  160  may be used to cool the various stages of the turbine  130  as described above as well as for the stability of the combustor  120  during part-load operations. Specifically, the compressor stage extractions  160  may be used during part load operations to limit the air sent to the combustor  120  while cooling the turbine  130  or otherwise. The extraction flow control valve  165  may be a three-way valve as described above and may be in communication with the heat exchanger  180  or a similar type of device such that the heat and energy of the compressor stage extractions  160  also may be in communication with other elements of the combined cycle power plant as described above. 
         [0019]    The amount, location, and temperature of the extractions  160 ,  170  may be determined by the temperature sensors  190 ,  195  in association with a controller  210 . The controller  210  may be any type of programmable microprocessor. More than one controller  210  may be used. The controller  210  may store performance parameters, curves, equations, look up tables, other data structures as well as immediate feedback from the temperature sensors  190 ,  195 , from the load sensor  155 , and from other types of input. Specifically, the controller  210  may adjust selectively the location and volume of the source and the destination of the extractions  160 ,  170  based upon the exhaust temperature, the temperature of the parts in the hot gas path of the turbine  130 , and/or the load on the gas turbine system  100  as a whole. The controller  210  also may completely shutdown certain cans  125  within the combustor  120 . Shutting the combustor cans  125  down may further extend turndown values. The controller  210  may provide for shutdown of one or more of the cans  125  and vary the extractions  160 ,  170  so as to maintain a predetermined exhaust temperature and maintain the gas turbine system  100  within emissions compliance. 
         [0020]    As is shown in  FIG. 1 , an exhaust gas recirculation  220  to the turbine  130  generally may be used to reduce certain emissions at full-load operations.  FIG. 2  shows the use of an exhaust gas recirculation  220  for part-load operations. Specifically, the exhaust gas recirculation may be fed to the compressor  110  and/or the combustor  120 . The exhaust gas recirculation  220  may be used to control the amount of oxygen in the air sent to the combustor  110  so as to increase the temperature of the combustor  120  by utilizing the heat and energy of the exhaust gas. Alternatively, the exhaust gas recirculation  220  may be delivered to the turbine  130  on a selective basis depending upon operations within the early stages of the turbine  130 . The exhaust gas recirculation  220  may be delivered to the inlet, the discharge, or to any stage of the compressor  110  or the turbine  130  or to any combustor location. The exhaust gas recirculation  220  may be selectively delivered based upon operating conditions. 
         [0021]    In use, the combination of these various techniques may reduce the turndown value of the gas turbine  100  as a whole to about 14.3% or less of full-load with a fuel consumption decrease of about nine percent (9%) or more. These turndown values may be achieved by maintaining the temperature of the combustor  120  above the minimum operating limits by controlling the amount of intake air. Air for part-load operations may be controlled by the selected extractions  160 ,  170  from the compressor discharge  175  and the compressor stages, by decreasing the number of compressor cans  125  in operation, and/or by returning exhaust gases selectively to the combustor  120 , the compressor  110 , and/or the turbine  130 . Various combinations of these techniques also may be used. Likewise, the use of the compressor extractions  160 ,  170  reduces the temperature of the parts in the hot gas path of the turbine  130  so as to extend part life. The heat and energy of the extractions  160 ,  170  further may be delivered to the heat exchanger  180  so as to increase overall plant thermal efficiency or for other purposes. 
         [0022]    It should be apparent that the forgoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.