Abstract:
An airflow control system for a combined cycle power generation system according to an embodiment includes: airflow control system for a combined cycle power generation system, comprising: an airflow generation system for attachment to a rotatable shaft of a gas turbine system, the airflow generation system drawing in an excess flow of air through an air intake section; a mixing area for receiving an exhaust gas stream produced by the gas turbine system; and an air extraction system for extracting at least a portion of an excess flow of air generated by the airflow generation system to provide bypass air, and for diverting the bypass air into the mixing area to reduce a temperature of the exhaust gas stream; wherein the reduced temperature exhaust gas stream is provided to a heat recovery steam generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. application Ser. Nos. ______, GE docket numbers 280650-1, 280685-1, 280687-1, 280688-1, 280692-1, 280707-1, 280714-1, 280730-1, 280731-1, 281003-1, 281004-1, 281005-1 and 281007-1 all filed on ______. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The disclosure relates generally to power generation systems, and more particularly, to systems and methods for cooling the exhaust gas of power generation systems. 
         [0003]    Utility power producers use combined cycle (CC) power generation systems because of their inherent high efficiencies and installed cost advantage. CC power generation systems typically include a gas turbine, a heat recovery steam generator (HRSG), and a steam turbine. The heat recovery steam generator uses the hot exhaust gas from the gas turbine to create steam, which drives the steam turbine. The combination of a gas turbine and a steam turbine achieves greater efficiency than would be possible independently. 
         [0004]    Operational flexibility to meet varying power grid demands at different times of the day is an important consideration in CC power generation systems. The issue becomes more important as intermittent energy sources such as solar and wind are integrated into the power grid. To this extent, CC power generation systems powered by fossil fuels must be capable of increasing/decreasing power output as required to accommodate such intermittent energy sources. 
         [0005]    Non-steady state emissions from a CC power generation system (e.g., during start-up) are generally closely scrutinized by regulatory authorities. During start-up, emission control devices employing selective catalytic reduction (SCR) and carbon monoxide (CO) catalysts are not active. To avoid thermal stresses in the steam turbine, the gas turbine has to be held at a lower load to control the HRSG inlet temperature to around 700° F. Since emission are higher at lower gas turbine loads and the emission control devices are not yet active, emissions during start-up can be an order of magnitude higher than those at steady state operation. Further, operating gas turbines at lower loads for a considerable amount of time also reduces the power provided to the power grid during the crucial start-up period. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    A first aspect of the disclosure provides an airflow control system for a combined cycle power generation system, including: an airflow generation system for attachment to a rotatable shaft of a gas turbine system, the airflow generation system drawing in an excess flow of air through an air intake section; a mixing area for receiving an exhaust gas stream produced by the gas turbine system; and an air extraction system for extracting at least a portion of an excess flow of air generated by the airflow generation system to provide bypass air, and for diverting the bypass air into the mixing area to reduce a temperature of the exhaust gas stream; wherein the reduced temperature exhaust gas stream is provided to a heat recovery steam generator. 
         [0007]    A second aspect of the disclosure provides a turbomachine system, including: a gas turbine system including a compressor component, a combustor component, and a turbine component; a shaft driven by the turbine component; a fan coupled to the shaft upstream of the gas turbine system, the fan drawing in an excess flow of air through an air intake section; a mixing area for receiving an exhaust gas stream produced by the gas turbine system; an air extraction system for extracting at least a portion of an excess flow of air generated by the fan to provide bypass air, and for diverting the bypass air into the mixing area to reduce a temperature of the exhaust gas stream; a heat recovery steam generator for receiving the reduced temperature exhaust gas stream and for generating steam; and a steam turbine system for receiving the steam generated by the heat recovery steam generator. 
         [0008]    A third aspect of the disclosure provides a combined cycle power generation system, including: a gas turbine system including a compressor component, a combustor component, and a turbine component; a shaft driven by the turbine component; an electrical generator coupled to the shaft for generating electricity; a fan coupled to the shaft upstream of the gas turbine system, the fan drawing in an excess flow of air through an air intake section; a mixing area for receiving an exhaust gas stream produced by the gas turbine system; an air extraction system for extracting at least a portion of an excess flow of air generated by the fan to provide bypass air, and for diverting the bypass air into the mixing area to reduce a temperature of the exhaust gas stream; a heat recovery steam generator for receiving the reduced temperature exhaust gas stream and for generating steam; and a steam turbine system for receiving the steam generated by the heat recovery steam generator. 
         [0009]    The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawing that depicts various embodiments of the disclosure. 
           [0011]      FIG. 1  shows a schematic diagram of a combined cycle (CC) power generation system according to embodiments. 
           [0012]      FIG. 2  depicts an enlarged view of a portion of the CC power generation system of  FIG. 1  according to embodiments. 
           [0013]      FIG. 3  shows a schematic diagram of a CC power generation system according to embodiments. 
           [0014]      FIG. 4  depicts an enlarged view of a portion of the CC power generation system of  FIG. 3  according to embodiments. 
           [0015]      FIG. 5  is an illustrative cross-sectional view of the bypass enclosure and the compressor component of the CC power generation system taken along line A-A of  FIG. 3 . 
           [0016]      FIG. 6  is an illustrative cross-sectional view of the bypass enclosure and the compressor component of the CC power generation system taken along line B-B of  FIG. 4 . 
           [0017]      FIG. 7  depicts a schematic diagram of a CC power generation system according to embodiments. 
           [0018]      FIG. 8  is an illustrative chart depicting various operating conditions during a typical start-up process according to embodiments. 
       
    
    
       [0019]    It is noted that the drawing of the disclosure is not to scale. The drawing is intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawing, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    As indicated above, the disclosure relates generally to power generation systems, and more particularly, to systems and methods for cooling the exhaust gas of power generation systems. 
         [0021]      FIGS. 1 and 3  depict block diagrams of turbomachine systems (e.g., combined cycle (CC) power generation systems  10 ). According to embodiments, each CC power generation system  10  includes a gas turbine system  12  and a heat recovery steam generator (HRSG system  14 ). The gas turbine system  12  may combust liquid or gas fuel, such as natural gas and/or a hydrogen-rich synthetic gas, to generate hot combustion gases to drive the gas turbine system  12 . 
         [0022]    The gas turbine system  12  includes an air intake section  16 , a compressor component  18 , a combustor component  20 , and a turbine component  22 . The turbine component  22  is drivingly coupled to the compressor component  18  via a shaft  24 . In operation, air (e.g., ambient air) enters the gas turbine system  12  through the air intake section  16  (indicated by arrow  26 ) and is pressurized in the compressor component  18 . The compressor component  18  includes at least one stage including a plurality of compressor blades coupled to the shaft  24 . Rotation of the shaft  24  causes a corresponding rotation of the compressor blades, thereby drawing air into the compressor component  18  via the air intake section  16  and compressing the air prior to entry into the combustor component  20 . 
         [0023]    The combustor component  20  may include one or more combustors. In embodiments, a plurality of combustors are disposed in the combustor component  20  at multiple circumferential positions in a generally circular or annular configuration about the shaft  24 . As compressed air exits the compressor component  18  and enters the combustor component  20 , the compressed air is mixed with fuel for combustion within the combustor(s). For example, the combustor(s) may include one or more fuel nozzles that are configured to inject a fuel-air mixture into the combustor(s) in a suitable ratio for combustion, emissions control, fuel consumption, power output, and so forth. Combustion of the fuel-air mixture generates hot pressurized exhaust gases, which may then be utilized to drive one or more turbine stages (each having a plurality of turbine blades) within the turbine component  22 . 
         [0024]    In operation, the combustion gases flowing into and through the turbine component  22  flow against and between the turbine blades, thereby driving the turbine blades and, thus, the shaft  24  into rotation. In the turbine component  22 , the energy of the combustion gases is converted into work, some of which is used to drive the compressor component  18  through the rotating shaft  24 , with the remainder available for useful work to drive a load such as, but not limited to, an electrical generator  28  for producing electricity, and/or another turbine(s). 
         [0025]    The combustion gases that flow through the turbine component  22  exit the downstream end  30  of the turbine component  22  as a stream of exhaust gas  32 . The exhaust gas stream  32  flows in a downstream direction  34  into a mixing area  36  and toward/into the HRSG system  14 . 
         [0026]    The HRSG system  14  generally comprises a heat exchanger  40  that recovers heat from the exhaust gas stream  32  of the gas turbine system  12  to produce steam  42 . The steam  42  may be used to drive one or more steam turbine systems  44 . The combination of the gas turbine system  12  and the steam turbine system  44  generally produces electricity more efficiently than either the gas turbine system  12  or steam turbine system  44  alone. The steam  42  generated by the HRSG system  14  may also be used in other processes, such as district heating or other process heating. In embodiments, the HRSG system  14  may further include a duct burner system  46  that is configured to burn fuel  48  (e.g., natural gas) in a combustion chamber  50  in order to increase the quantity and/or temperature of the steam  42  generated in the HRSG system  14 . 
         [0027]    As depicted in  FIG. 1 , an air generation system including, for example, a fan  60 , may be coupled to the shaft  24  of the gas turbine system  12  upstream of the gas turbine system  12 . The fan  60  may be used to draw in a supply of cooling air (e.g., ambient air) through the air intake section  16 . At least a portion of the air drawn in by the fan  60  may be used to lower the temperature of the exhaust gas stream  32 . The fan  60  may be fixedly mounted (e.g. bolted, welded, etc.) to the shaft  24  of the gas turbine system  12 . To this extent, the fan  60  is configured to rotate at the same rotational speed as the shaft  24 . 
         [0028]    The compressor component  18  has a flow rate capacity and is configured to draw in a flow of air (e.g., ambient air) via the air intake section  16  based on its flow rate capacity. In operation, the fan  60  is designed to draw in an additional flow of air through the air intake section  16  that is about  10 % to about  40 % of the flow rate capacity of the compressor component  18 . In general, the percentage increase in the flow of air may be varied and selectively controlled based on several factors including the load on the gas turbine system  12 , the temperature of the air being drawn into the gas turbine system  12 , the temperature of the exhaust gas stream  32  at the SCR catalyst  38 , etc. 
         [0029]    As depicted in  FIG. 2 , an inlet guide vane assembly  62  including a plurality of inlet guide vanes  64  may be used to control the amount of air available to the fan  60  and the compressor component  18 . Each inlet guide vane  64  may be selectively controlled (e.g., rotated) by an independent actuator  66 . Actuators  66  according to various embodiments are shown schematically in  FIG. 2 , but any known actuator may be utilized. For example, the actuators  66  may comprise an electro-mechanical motor, or any other type of suitable actuator. 
         [0030]    The actuators  66  may be independently and/or collectively controlled in response to commands from an airflow controller  100  to selectively vary the positioning of the inlet guide vanes  64 . That is, the inlet guide vanes  64  may be selectively rotated about a pivot axis by the actuators  66 . In embodiments, each inlet guide vane  64  may be individually pivoted independently of any other inlet guide vane  64 . In other embodiments, groups of inlet guide vanes  64  may be pivoted independently of other groups of inlet guide vanes  64  (i.e., pivoted in groups of two or more such that every inlet guide vane  64  in a group rotates together the same amount). Position information (e.g., as sensed by electro-mechanical sensors or the like) for each of the inlet guide vanes  64  may be provided to the airflow controller  100 . 
         [0031]    The increased flow of air provided by the fan  60  may increase the air pressure at the compressor component  18 . For example, in the case where the flow of air is increased from about 10% to about 40% by the operation of the fan  60 , a corresponding pressure increase of about 5 to about 15 inches of water may be achieved. This pressure increase may be used to overcome pressure drop and facilitate proper mixing (described below) of cooler air with the exhaust gas stream  32  in the mixing area  36 . The pressure increase may also be used to supercharge the gas turbine system  12 . 
         [0032]    Referring to  FIGS. 1 and 2 , an air extraction system  70  may be employed to extract at least some of the additional flow of air provided by the fan  60  (e.g., any airflow greater than flow rate capacity of the gas turbine system  12 ). A flow of air  72  may be extracted using, for example, one or more extraction ducts  74  ( FIG. 2 ). The extracted air, or “bypass air” (BA) does not enter the gas turbine system  12 , but is instead directed to the mixing area  36  through bypass ducts  76  as indicated by arrows BA, where the bypass air may be used to cool the exhaust gas stream  32 . The remaining air (i.e., any portion of the additional flow of air generated by the fan  60  not extracted via the extraction ducts  74 ) enters the compressor component  18  of the gas turbine system  12  and flows through the gas turbine system  12  in a normal fashion. If the flow of remaining air is greater than the nominal airflow of the gas turbine system  12 , a supercharging of the gas turbine system  12  may occur, increasing the efficiency and power output of the gas turbine system  12 . 
         [0033]    The bypass air may be routed toward the mixing area  36  downstream of the turbine component  22  through one or more bypass ducts  76 . The bypass air exits the bypass ducts  76  and enters the mixing area  36  through a bypass air injection grid  110  ( FIG. 1 ), where the bypass air (e.g., ambient air) mixes with and cools the exhaust gas stream  32 . In embodiments, the temperature of the exhaust gas stream  32  generated by the gas turbine system  12  is cooled by the bypass air from about 1100° F. to about 600° F.-1000° F. in the mixing area  36 . The bypass air injection grid  110  may comprise, for example, a plurality of nozzles  112  or the like for directing (e.g., injecting) the bypass air into the mixing area  36 . The nozzles  112  of the bypass air injection grid  110  may be distributed about the mixing area  36  in such a way as to maximize mixing of the bypass air and the exhaust gas stream  32  in the mixing area  36 . The nozzles  112  of the bypass air injection grid  110  may be fixed in position and/or may be movable to selectively adjust the injection direction of bypass air into the mixing area  36 . 
         [0034]    A supplemental mixing system  38  ( FIG. 1 ) may be positioned within the mixing area  36  to enhance the mixing process. The supplemental mixing system  38  may comprise, for example, a static mixer, baffles, and/or the like. 
         [0035]    As depicted in  FIG. 2 , the air flow  72  into each extraction duct  74  may be selectively and/or independently controlled using a flow restriction system  80  comprising, for example, a damper  82 , guide vane, or other device capable of selectively restricting airflow. Each damper  82  may be selectively controlled (e.g., rotated) by an independent actuator  84 . The actuators  84  may comprise electro-mechanical motors, or any other type of suitable actuator. The dampers  82  may be independently and/or collectively controlled in response to commands from the airflow controller  100  to selectively vary the positioning of the dampers  82  such that a desired amount of bypass air is directed into the mixing area  36  via the bypass ducts  76 . Position information (e.g., as sensed by electro-mechanical sensors or the like) for each of the dampers  82  may be provided to the airflow controller  100 . 
         [0036]    Bypass air may be selectively released from one or more of the bypass ducts  76  using an air release system  86  comprising, for example, one or more dampers  88  (or other devices capable of selectively restricting airflow, e.g. guide vanes) located in one or more air outlets  90 . The position of a damper  88  within an air outlet  90  may be selectively controlled (e.g., rotated) by an independent actuator  92 . The actuator  92  may comprise an electro-mechanical motor, or any other type of suitable actuator. Each damper  88  may be controlled in response to commands from the airflow controller  100  to selectively vary the positioning of the damper  88  such that a desired amount of bypass air may be released from a bypass duct  76 . Position information (e.g., as sensed by electro-mechanical sensors or the like) for each damper  88  may be provided to the airflow controller  100 . Further airflow control may be provided by releasing bypass air from one or more of the bypass ducts  76  through one or more metering valves  94  controlled via commands from the airflow controller  100 . 
         [0037]    The airflow controller  100  may be used to regulate the amount of air generated by the fan  60  that is diverted as bypass air through the bypass ducts  76  and into the mixing area  36  relative to the amount of air that enters the gas turbine system  12  (and exits as the exhaust gas stream  32 ) in order to regulate the temperature at the HRSG system  14 . The amount of bypass air flowing through the bypass ducts  76  into the mixing area  36  may be varied (e.g., under control of the airflow controller  100 ) as the temperature of the exhaust gas stream  32  changes, in order to regulate the temperature at the HRSG system  14 . 
         [0038]    The airflow controller  100  may receive data  102  associated with the operation of the CC power generation system  10 . Such data may include, for example, the temperature of the exhaust gas stream  32  as it enters the mixing area  36 , the temperature of the exhaust gas stream  32  at the HRSG system  14  after mixing/cooling has occurred in the mixing area  36 , the temperature of the air drawn into the air intake section  16  by the fan  60 , and other temperature data obtained at various locations within the CC power generation system  10 . The data  102  may further include airflow and pressure data obtained, for example, within the air intake section  16 , at the inlet guide vanes  64 , at the fan  60 , at the entrance of the compressor component  18 , within the extraction ducts  74 , within the bypass ducts  76 , at the downstream end  30  of the turbine component  22 , and at various other locations within the CC power generation system  10 . Load data, fuel consumption data, and other information associated with the operation of the gas turbine system  12  may also be provided to the airflow controller  100 . The airflow controller  100  may further receive positional information associated with the inlet guide vanes  64 , dampers  82 ,  88 , valve  94 , etc. It should be readily apparent to those skilled in the art how such data may be obtained (e.g., using appropriate sensors, feedback data, etc.), and further details regarding the obtaining of such data will not be provided herein. 
         [0039]    Based on the received data  102 , the airflow controller  100  is configured to vary as needed the amount of bypass air flowing through the bypass ducts  76  into the mixing area  36  to maintain the temperature at the HRSG system  14  at a suitable level. This may be achieved, for example, by varying at least one of: the flow of air drawn into the air intake section  16  by the fan  60  (this flow may be controlled, for example, by adjusting the position of one or more of the inlet guide vanes  64  and/or by increasing the rotational speed of the shaft  24 ); the flow of air  72  into the extraction ducts  74  (this flow may be controlled, for example, by adjusting the position of one or more of the dampers  82 ); and the flow of bypass air passing from the extraction ducts  74 , through the bypass ducts  76 , into the mixing area  36  (this flow may be controlled, for example, by adjusting the position of one or more of the dampers  88  and/or the operational status of the metering valves  94 ). 
         [0040]    The airflow controller  100  may include a computer system having at least one processor that executes program code configured to control the amount of bypass air flowing through the bypass ducts  76  into the mixing area  36  using, for example, data  102  and/or instructions from human operators. The commands generated by the airflow controller  100  may be used to control the operation of various components (e.g., such as actuators  66 ,  84 ,  92 , valve  94 , and/or the like) in the CC power generation system  10 . For example, the commands generated by the airflow controller  100  may be used to control the operation of the actuators  66 ,  84 , and  92  to control the rotational position of the inlet guide vanes  64 , dampers  82 , and dampers  88 , respectively. Commands generated by the airflow controller  100  may also be used to activate other control settings in the CC power generation system  10 . 
         [0041]    As depicted in  FIGS. 3 and 4 , instead of using external bypass ducts  76 , the gas turbine system  12  may be surrounded by a bypass enclosure  111 . The bypass enclosure  111  may extend from, and fluidly couple, the air intake section  16  to the mixing area  36 . The bypass enclosure  111  may have any suitable configuration. For instance, the bypass enclosure  111  may have an annular configuration as depicted in  FIG. 5 , which is a cross-section taken along line A-A in  FIG. 3 . The bypass enclosure  111  forms an air passage  113  around the gas turbine system  12  through which a supply of cooling bypass air (BA) may be provided for cooling the exhaust gas stream  32  of the gas turbine system  12 . 
         [0042]    An air extraction system  114  may be provided to extract at least some of the additional flow of air provided by the fan  60  and to direct the extracted air into the air passage  113  formed between the bypass enclosure  111  and the gas turbine system  12 . The air extraction system  114  may comprise, for example, inlet guide vanes, a stator, or any other suitable system for selectively directing a flow of air into the air passage  113 . In the following description, the air extraction system  114  comprises, but is not limited to, inlet guide vanes. As shown in  FIG. 6 , which is a cross-section taken along line B-B in  FIG. 4 , the air extraction system  114  may extend completely around the entrance to the air passage  113  formed between the bypass enclosure  111  and the compressor component  18  of the gas turbine system  12 . 
         [0043]    As depicted in  FIG. 4 , the air extraction system  114  may include a plurality of inlet guide vanes  116  for controlling the amount of air directed into the air passage  113  formed between the bypass enclosure  111  and the gas turbine system  12 . Each inlet guide vane  116  may be selectively and independently controlled (e.g., rotated) by an independent actuator  118 . The actuators  118  are shown schematically in  FIG. 4 , but any known actuator may be utilized. For example, the actuators  118  may comprise an electro-mechanical motor, or any other type of suitable actuator. 
         [0044]    The actuators  118  of the air extraction system  114  may be independently and/or collectively controlled in response to commands from the airflow controller  100  to selectively vary the positioning of the inlet guide vanes  116 . That is, the inlet guide vanes  116  may be selectively rotated about a pivot axis by the actuators  118 . In embodiments, each inlet guide vane  116  may be individually pivoted independently of any other inlet guide vane  116 . In other embodiments, groups of inlet guide vanes  116  may be pivoted independently of other groups of inlet guide vanes  116  (i.e., pivoted in groups of two or more such that every inlet guide vane  116  in a group rotates together the same amount). Position information (e.g., as sensed by electro-mechanical sensors or the like) for each of the inlet guide vanes  116  may be provided to the airflow controller  100 . 
         [0045]    The bypass air does not enter the gas turbine system  12 , but is instead directed to the mixing area  36  through the air passage  113  as indicated by arrows BA, where the bypass air may be used to cool the exhaust gas stream  32 . The remaining air (i.e., any portion of the additional flow of air generated by the fan  60  not extracted via the air extraction system  114 ) enters the compressor component  18  of the gas turbine system  12  and flows through the gas turbine system  12  in a normal fashion. If the flow of remaining air is greater than the nominal airflow of the gas turbine system  12 , a supercharging of the gas turbine system  12  may occur, increasing the efficiency and power output of the gas turbine system  12 . 
         [0046]    The bypass air flows toward and into the mixing area  36  downstream of the turbine component  22  through the air passage  113 . In embodiments, the bypass air exits the air passage  113  and is directed at an angle toward and into the exhaust gas stream  32  in the mixing area  36  to enhance mixing. In the mixing area  36 , the bypass air (e.g., ambient air) mixes with and cools the exhaust gas stream  32  to a temperature suitable for use in the HRSG system  14 . In embodiments, the temperature of the exhaust gas stream  32  generated by the gas turbine system  12  is cooled by the bypass air from about 1100° F. to about 600° F.-1000° F. in the mixing area  36 . 
         [0047]    As depicted in  FIGS. 3 and 4 , the distal end  120  of the bypass enclosure  111  may curve inwardly toward the mixing area  36  to direct the bypass air at an angle toward and into the exhaust gas stream  32  in the mixing area  36 . The intersecting flows of the bypass air and the exhaust gas stream  32  may facilitate mixing, thereby enhancing the cooling of the exhaust gas stream  32 . A flow directing system  122  may also be provided to direct the bypass air at an angle toward and into the exhaust gas stream  32 . Such a flow directing system  122  may include, for example, outlet guide vanes, stators, nozzles, or any other suitable system for selectively directing the flow of bypass air into the mixing area  36 . 
         [0048]    An illustrative flow directing system  122  is shown in  FIG. 4 . In this example, the flow directing system  122  includes a plurality of outlet guide vanes  124 . Each outlet guide vane  124  may be selectively controlled (e.g., rotated) by an independent actuator  126 . The actuators  126  are shown schematically in  FIG. 4 , but any known actuator may be utilized. For example, the actuators  126  may comprise an electro-mechanical motor, or any other type of suitable actuator. In embodiments, the flow directing system  122  may extend completely around the exit of the air passage  113  formed between the bypass enclosure  111  and the turbine component  22  of the gas turbine system  12 . 
         [0049]    A supplemental mixing system  38  ( FIG. 1 ) may be positioned within the mixing area  36  to enhance the mixing process. The supplemental mixing system  38  may comprise, for example, a static mixer, baffles, and/or the like. 
         [0050]    As shown in  FIG. 4 , bypass air may be selectively released from the bypass enclosure  111  using an air release system  130  comprising, for example, one or more dampers  132  (or other devices capable of selectively restricting airflow, e.g. guide vanes) located in one or more air outlets  134 . The position of a damper  132  within an air outlet  134  may be selectively controlled (e.g., rotated) by an independent actuator  136 . The actuator  136  may comprise an electro-mechanical motor, or any other type of suitable actuator. Each damper  132  may be controlled in response to commands from the airflow controller  100  to selectively vary the positioning of the damper  132  such that a desired amount of bypass air may be released from the bypass enclosure  111 . Position information (e.g., as sensed by electro-mechanical sensors or the like) for each damper  132  may be provided to the airflow controller  100 . Further airflow control may be provided by releasing bypass air from the bypass enclosure  111  through one or more metering valves  140  ( FIG. 4 ) controlled via commands from the airflow controller  100 . 
         [0051]    The airflow controller  100  may be used to regulate the amount of air generated by the fan  60  that is diverted as bypass air into the mixing area  36  through the air passage  113  relative to the amount of air that enters the gas turbine system  12  (and exits as the exhaust gas stream  32 ) in order to control the temperature at the HRSG system  14  under varying operating conditions. The amount of bypass air flowing through the air passage  113  into the mixing area  36  may be varied (e.g., under control of the airflow controller  100 ) as the temperature of the exhaust gas stream  32  changes, in order to regulate the temperature at the HRSG system  14 . 
         [0052]    As shown schematically in  FIG. 4 , the bypass enclosure  111  may be provided with one or more access doors  150 . The access doors  150  provide access through the bypass enclosure  111  to the various components of the gas turbine system  12  (e.g., for servicing, repair, etc.). 
         [0053]    In other embodiments, as depicted in  FIG. 7 , the gas turbine casing  160  itself can be used in lieu of the enclosure  111 . This configuration operates similarly to the system depicted in  FIGS. 3 and 4 , except that the air extraction system  114  and flow directing system  122  are disposed within the gas turbine casing  160 . The fuel/combustor inlets  162  of the combustor component  20  of the gas turbine system  12  may extend through the gas turbine casing  160  (e.g., for easier access). In this configuration, bypass air (BA) passes between the gas turbine casing  160  and the exterior of the compressor component  18 , combustor component  20 , and turbine component  22 . Other components depicted in  FIGS. 3 and 4 , such as the air intake section, HRSG system, airflow controller, etc. are not shown for sake of clarity in  FIG. 7 . 
         [0054]    In operation, a portion of the air drawn in by an air generation system (e.g., the fan  60 ) is directed by the air extraction system  114  as bypass air into an air passage  164  formed between the gas turbine casing  160  and the exterior of the compressor component  18 , combustor component  20 , and turbine component  22 . The bypass air exits the air passage  164  and is directed by at an angle by the flow directing system  122  toward and into the exhaust gas stream  32  in the mixing area  36 . In the mixing area  36 , the bypass air (e.g., ambient air) mixes with and cools the exhaust gas stream  32 . The temperature of the exhaust gas stream  32  generated by the gas turbine system  12  may be cooled by the bypass air from about 1100° F. to about 600° F.-1000° F. in the mixing area  36 . 
         [0055]    As detailed above, the airflow controller  100  may receive a wide variety of data  102  associated with the operation of the CC power generation system  10  and the components thereof. Based on the received data  102 , the airflow controller  100  is configured to vary as needed the amount of bypass air flowing through the air passage  113 ,  164  into the mixing area  36  to regulate the temperature at the HRSG system  14 . This may be achieved, for example, by varying at least one of: the flow of air drawn into the air intake section  16  by the fan  60  and compressor component  18  of the gas turbine system  12 ; the flow of air directed into the air passage  113 ,  164  via the air extraction system  114  (this flow may be controlled, for example, by adjusting the position of one or more of the inlet guide vanes  116 ); and the flow of bypass air passing through the air passage  113 ,  164  into the mixing area  36  (this flow may be controlled, for example, by adjusting the position of one or more of the dampers  132  and/or the operational status of the metering valves  110 ). 
         [0056]    Examples of the start-up operation and the normal steady state operation of the CC power generation system  10  will now be provided with reference to  FIGS. 1, 3, and 8 . 
         [0057]    Start-Up Operation 
         [0058]    During a start-up process in a conventional CC power generation system, the gas turbine needs to be parked at a lower load (e.g., compared to the minimum emissions compliance load (MECL)), which results in higher NO x  and CO emissions. This is done, for example, to maintain the temperature of the steam entering the steam turbine to around 700° F. to avoid thermal stresses in the steam turbine. This lower load is indicated by point “A” in  FIG. 8 . 
         [0059]    In contrast, according to embodiments, in a CC power generation system  10  including a fan  60  for generating bypass air for cooling an exhaust gas stream  32  of a gas turbine system  12 , the gas turbine system  12  can be parked at a higher load (as indicated by point “B” in  FIG. 8 ) with a higher exhaust temperature. At the higher exhaust temperature, the NO x  and CO emissions in the exhaust gas stream  32  are lower. The temperature of the exhaust gas stream  32  of the gas turbine system  12  can be controlled using the bypass air (BA) to provide an inlet temperature of about 700° F. at the HRSG system  14 . This results in lower start-up NO x  and CO emissions and also helps to increase the power output of the gas turbine system  12  during start-up. Comparing point A and point B in  FIG. 8 , for example, it can easily be seen that the gas turbine system  12  can be operated (point B) at a higher temperature and higher load than a conventional gas turbine system (point A), while still providing an inlet temperature of about 700° F. at the HRSG system  14 . 
         [0060]    Normal Operation 
         [0061]    During normal operation, a portion of the flow of air generated by the fan  60  may be used to supercharge the compressor component  18  of the gas turbine system  12 , thereby boosting the power output of the gas turbine system  12 . Further, the bypass air mixed back into the exhaust gas stream  32  of the gas turbine system  12  increases the flow into the HRSG system  14  and reduces the temperature of the flow. This allows increased firing in the duct burner system  46  without reaching the tube temperature limit of the HRSG system  14  (e.g., around 1600° F.). This allows increased power output from the bottoming cycle of the CC power generation system  10 . In embodiments, the power output of the CC power generation system  10  can be increased, for example, by 10 to 15% compared to the power output of a conventional CC power generation system (i.e., no fan). 
         [0062]    In embodiments, several parameters can be regulated depending, for example, on power grid demand, to control the power output of the CC power generation system  10 , including:
   1) the amount of supercharging of the compressor component  18  of the gas turbine system  12 ;   2) the amount of bypass flow provided to the mixing area  36  to cool the exhaust gas stream  32 ;   3) the ratio (i.e., “bypass ratio”) of bypass flow versus flow into the gas turbine system  12  provided by the air extraction system  70 ;   4) the amount of firing of the duct burner system  46  (e.g., to move temperature of the exhaust gas flow to a target level after the bypass air has been injected into the exhaust gas stream  32 ); and   5) the amount of overfiring or underfiring of the gas turbine system  12  (to provide as much energy as feasible in the topping cycle).   
 
         [0068]    Many advantages may be provided by the disclosed CC power generation system  10 . For example, the power output of the gas turbine system  12  may be increased due to the supercharging of the inlet air to the compressor component  18  by the fan  60 . Further, high duct burner firing is possible without reaching the HRSG tube temperature limit, resulting in higher bottoming cycle power output. In addition, the gas turbine system  12  may be run during start-up at a higher load point. This results in lower emissions and an exhaust gas stream  32  having a temperature higher than that needed for the steam turbine system  44 . 
         [0069]    The use of an air generation system (such as fan  60 ) in lieu of conventional large external blower systems and/or other conventional cooling structures provides many other advantages. For example, the need for redundant external blower systems and associated components (e.g., blowers, motors and associated air intake structures, filters, ducts, etc.) required by conventional exhaust stream cooling systems is eliminated. This reduces manufacturing and operating costs, as well as the overall footprint, of the CC power generation system  10 . The footprint is further reduced as the fan  60  draw in air through an existing air intake section  16 , rather than through separate, dedicated intake structures often used with external blower systems. 
         [0070]    Use of the fan  60  provides a more reliable and efficient CC power generation system  10 . For example, since the bypass air used for cooling in the mixing area  36  is driven by the shaft  24  of the gas turbine system  12  itself, large external blower systems are no longer required. Further, at least a portion of the higher than nominal flow of air generated by the fan  60  may be used to supercharge the gas turbine system  12 . 
         [0071]    Power requirements of the CC power generation system  10  are reduced because the fan  60  is coupled to, and driven by, the shaft  24  of the gas turbine system  12 . This configuration eliminates the need for large blower motors commonly used in conventional external blower cooling systems. 
         [0072]    In various embodiments, components described as being “coupled” to one another can be joined along one or more interfaces. In some embodiments, these interfaces can include junctions between distinct components, and in other cases, these interfaces can include a solidly and/or integrally formed interconnection. That is, in some cases, components that are “coupled” to one another can be simultaneously formed to define a single continuous member. However, in other embodiments, these coupled components can be formed as separate members and be subsequently joined through known processes (e.g., fastening, ultrasonic welding, bonding). 
         [0073]    When an element or layer is referred to as being “on”, “engaged to”, “connected to” or “coupled to” another element, it may be directly on, engaged, connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to”, “directly connected to” or “directly coupled to” another element, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0074]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0075]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.