Abstract:
An airflow control system for a gas turbine system according to an embodiment includes: an airflow generation system for attachment to a rotatable shaft of a gas turbine system for 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 the excess flow of air generated by the airflow generation system to provide bypass air; an enclosure surrounding the gas turbine system and forming an air passage, the bypass air flowing through the air passage and around the gas turbine system into the mixing area to reduce a temperature of the exhaust gas stream; and an exhaust processing system for processing the reduced temperature exhaust gas stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. application Ser No. ______, GE docket numbers 280650-1, 280685-1, 280687-1, 280692-1, 280707-1, 280714-1, 280730-1, 280731-1, 280815-1, 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]    Exhaust gas from power generation systems, for example a simple cycle gas turbine power generation system, often must meet stringent regulatory requirements for the composition of the exhaust gas released into the atmosphere. One of the components typically found in the exhaust gas of a gas turbine power generation system and subject to regulation is nitrogen oxide (i.e., NO x ), which includes, for example, nitric oxide and nitrogen dioxide. To remove NO x  from the exhaust gas stream, technology such as selective catalytic reduction (SCR) is often utilized. In an SCR process, ammonia (NH 3 ) or the like reacts with the NO x  and produces nitrogen (N 2 ) and water (H 2 O). 
         [0004]    The effectiveness of the SCR process depends in part on the temperature of the exhaust gas that is processed. The temperature of the exhaust gas from a gas turbine power generation system is often higher than about 1100° F. However, SCR catalysts need to operate at less than about 900° F. to maintain effectiveness over a reasonable catalyst lifespan. To this extent, the exhaust gas from a simple cycle gas turbine power generation system is typically cooled prior to SCR. 
         [0005]    Large external blower systems have been used to reduce the exhaust gas temperature of a gas turbine power generation system below 900° F. by mixing a cooling gas, such as ambient air, with the exhaust gas. Because of the possibility of catalyst damage due to a failure of an external blower system, a redundant external blower system is typically utilized. These external blower systems include many components, such as blowers, motors, filters, air intake structures, and large ducts, which are expensive, bulky, and add to the operating cost of a gas turbine power generation system. Additionally, the external blower systems and the operation of the gas turbine power generation system are not inherently coupled, thus increasing the probability of SCR catalyst damage due to excess temperature during various modes of gas turbine operation. To prevent SCR catalyst damage due to excess temperature (e.g., if the external blower system(s) fail or cannot sufficiently cool the exhaust gas), the gas turbine may need to be shut down until the temperature issue can be rectified. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    A first aspect of the disclosure provides an airflow control system for a gas turbine system, including: an airflow generation system for attachment to a rotatable shaft of a gas turbine system for 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 the excess flow of air generated by the airflow generation system to provide bypass air; and an enclosure surrounding the gas turbine system and forming an air passage, the bypass air flowing through the air passage and around the gas turbine system into the mixing area to reduce a temperature of the exhaust gas stream. 
         [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 for 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 the excess flow of air generated by the fan to provide bypass air; an enclosure surrounding the gas turbine system and forming an air passage, the bypass air flowing through the air passage and around the gas turbine system into the mixing area to reduce a temperature of the exhaust gas stream; an airflow controller for controlling the air extraction system to maintain the reduced temperature exhaust gas stream at a temperature of less than about 900° F.; 
         [0000]    and a selective catalytic reduction (SCR) system for processing the reduced temperature exhaust gas stream. 
         [0008]    A third aspect of the disclosure provides a 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 for 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 the excess flow of air generated by the fan to provide bypass air; an enclosure surrounding the gas turbine system and forming an air passage, the bypass air flowing through the air passage and around the gas turbine system into the mixing area to reduce a temperature of the exhaust gas stream; an airflow controller for controlling the air extraction system to maintain the reduced temperature exhaust gas stream at a temperature of less than about 900° F.; and a selective catalytic reduction (SCR) system for processing the reduced temperature exhaust gas stream. 
         [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 simple cycle gas turbine power generation system according to embodiments. 
           [0012]      FIG. 2  depicts an airflow generation system according to embodiments. 
           [0013]      FIG. 3  depicts an enlarged view of a portion of the gas turbine power generation system of  FIG. 1  according to embodiments. 
           [0014]      FIG. 4  is an illustrative cross-sectional view of the bypass enclosure and the compressor component of the gas turbine system taken along line A-A of  FIG. 1 . 
           [0015]      FIG. 5  is an illustrative cross-sectional view of the bypass enclosure and the compressor component of the gas turbine system taken along line B-B of  FIG. 3 . 
           [0016]      FIG. 6  is a chart showing an illustrative relationship between the flow of bypass air into a mixing area and the temperature of the exhaust gas stream at different load percentages of a gas turbine system, according to embodiments. 
           [0017]      FIG. 7  shows a schematic diagram of a simple cycle gas turbine power generation system according to embodiments. 
       
    
    
       [0018]    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 
       [0019]    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. 
         [0020]      FIG. 1  is a block diagram of a turbomachine (e.g., a simple cycle gas turbine power generation system  10 ) that includes a gas turbine system  12  and an exhaust processing 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 . 
         [0021]    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 . 
         [0022]    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 . 
         [0023]    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. 
         [0024]    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  may continue to flow in a downstream direction  34  towards the exhaust processing system  14 . The downstream end  30  of the turbine component  22  may be fluidly coupled via a mixing area  33  to a CO removal system (including, e.g., a CO catalyst  36 ) and an SCR system (including, e.g., an SCR catalyst  38 ) of the exhaust processing system  14 . As discussed above, as a result of the combustion process, the exhaust gas stream  32  may include certain byproducts, such as nitrogen oxides (NO x ), sulfur oxides (SO x ), carbon oxides (CO x ), and unburned hydrocarbons. Due to certain regulatory requirements, an exhaust processing system  14  may be employed to reduce or substantially minimize the concentration of such byproducts prior to atmospheric release. 
         [0025]    One technique for removing or reducing the amount of NO x  in the exhaust gas stream  32  is by using a selective catalytic reduction (SCR) process. For example, in an SCR process for removing NO x  from the exhaust gas stream  32 , ammonia (NH 3 ) or other suitable reductant may be injected into the exhaust gas stream  32 . The ammonia reacts with the NO x  to produce nitrogen (N 2 ) and water (H 2 O). 
         [0026]    As shown in  FIG. 1 , an ammonia evaporator system  40  and an ammonia injection grid  42  may be used to vaporize and inject an ammonia solution (e.g., stored in a tank  46 ) into the exhaust gas stream  32  upstream of the SCR catalyst  38 . The ammonia injection grid  42  may include, for example, a network of pipes with openings/nozzles for injecting vaporized ammonia into the exhaust gas stream  32 . As will be appreciated, the ammonia and NO x  in the exhaust gas stream  32  react as they pass through the SCR catalyst  38  to produce nitrogen (N 2 ) and water (H 2 O), thus removing NO x  from the exhaust gas stream  32 . The resulting emissions may be released into the atmosphere through a stack  44  of the gas turbine system  12 . 
         [0027]    The ammonia evaporator system  40  may further include, for example, a blower system  48 , one or more heaters  50  (e.g., electric heaters), and an ammonia vaporizer  52 , for providing vaporized ammonia that is injected into the exhaust gas stream  32  via the ammonia injection grid  42 . The ammonia may be pumped from the tank  46  to the ammonia vaporizer  52  using a pump system  54 . The blower system  48  may include redundant blowers, while the pump system  54  may include redundant pumps to ensure continued operation of the ammonia evaporator system  40  in case of individual blower/pump failure. 
         [0028]    The effectiveness of the SCR process depends in part on the temperature of the exhaust gas stream  32  that is processed. The temperature of the exhaust gas stream  32  generated by the gas turbine system  12  is often higher than about 1100° F. However, the SCR catalyst  38  typically needs to operate at temperatures less than about 900° F. 
         [0029]    According to embodiments, an airflow generation system  56  may be used to provide cooling air for lowering the temperature of the exhaust gas stream  32  to a level suitable for the SCR catalyst  38 . As depicted in  FIG. 1 , the airflow generation system  56  may include a fan  58 . The fan  58  may be coupled to the shaft  24  of the gas turbine system  12  upstream of the gas turbine system  12  to provide cooling air (e.g., ambient air) drawn in through the air intake section  16  that may be used to lower the temperature of the exhaust gas stream  32 . The fan  58  may be fixedly mounted (e.g. bolted, welded, etc.) to the shaft  24  of the gas turbine system  12  using a mounting system  60 . To this extent, the fan  58  is configured to rotate at the same rotational speed as the shaft  24 . In other embodiments, as shown in  FIG. 2 , a clutch mechanism  62  may used to releasably couple the fan  58  to the shaft  24  of the gas turbine system  12 . This allows the fan  58  to be selectively decoupled from the shaft  24  if not needed. When the clutch mechanism  62  is engaged, the fan  58  is coupled to the shaft  24  and is configured to rotate at the same rotational speed as the shaft  24 . Clutch coupling/decoupling commands may be provided to the clutch mechanism  62  via an airflow controller  100  ( FIG. 1 ). An adjustable speed drive system may also be used to couple the fan  58  to the shaft  24  to allow the fan  58  to be rotated at a different speed than the shaft  24 . 
         [0030]    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  58  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. 
         [0031]    As depicted in  FIGS. 1 and 3 , the gas turbine system  12  may be surrounded by a bypass enclosure  70 . The bypass enclosure  70  extends from, and fluidly couples, the air intake section  16  to the mixing area  33 . The bypass enclosure  70  may have any suitable configuration. For instance, the bypass enclosure  70  may have an annular configuration as depicted in  FIG. 4 , which is a cross-section taken along line A-A in  FIG. 1 . The bypass enclosure  70  forms an air passage  72  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 . 
         [0032]    An air extraction system  74  may be provided to extract at least some of the additional flow of air provided by the fan  58  (e.g., any airflow greater than the flow rate capacity of the compressor component  18  of the gas turbine system  12 ) and to direct the extracted air into the air passage  72  formed between the bypass enclosure  70  and the gas turbine system  12 . The air extraction system  74  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  72 . In the following description, the air extraction system  74  comprises, but is not limited to, inlet guide vanes. As shown in  FIG. 5 , which is a cross-section taken along line B-B in  FIG. 3 , the air extraction system  74  may extend completely around the entrance to the air passage  72  formed between the bypass enclosure  70  and the compressor component  18  of the gas turbine system  12 . 
         [0033]    As depicted in  FIG. 3 , the air extraction system  74  may include a plurality of inlet guide vanes  78  for controlling the amount of air directed into the air passage  72  formed between the bypass enclosure  70  and the gas turbine system  12 . Each inlet guide vane  78  may be selectively controlled (e.g., rotated) by an independent actuator  80 . Actuators  80  according to various embodiments are shown schematically in  FIG. 3 , but any known actuator may be utilized. For example, the actuators  80  may comprise an electro-mechanical motor, or any other type of suitable actuator. 
         [0034]    The bypass air does not enter the gas turbine system  12 , but is instead directed to the mixing area  33  through the air passage  72  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 not extracted via the air extraction system  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 flow rate capacity of the compressor component  18  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 . 
         [0035]    The actuators  80  of the air extraction system  74  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  78 . That is, the inlet guide vanes  78  may be selectively rotated about a pivot axis by the actuators  80 . In embodiments, each inlet guide vane  78  may be individually pivoted independently of any other inlet guide vane  78 . In other embodiments, groups of inlet guide vanes  78  may be pivoted independently of other groups of inlet guide vanes  78  (i.e., pivoted in groups of two or more such that every inlet guide vane  78  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  78  may be provided to the airflow controller  100 . 
         [0036]    The increased flow of air provided by the fan  58  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  58 , 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 downstream exhaust processing system  14 . The pressure increase may also be used to supercharge the gas turbine system  12 . 
         [0037]    The bypass air flows toward and into the mixing area  33  downstream of the turbine component  22  through the air passage  72 . In embodiments, the bypass air exits the air passage  72  and is directed at an angle toward and into the exhaust gas stream  32  in the mixing area  33  to enhance mixing. In the mixing area  33 , the bypass air (e.g., ambient air) mixes with and cools the exhaust gas stream  32  to a temperature suitable for use with the SCR catalyst  38 . 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 less than about 900° F. in the mixing area  33 . 
         [0038]    As depicted in  FIGS. 1 and 3 , the distal end  82  of the bypass enclosure  70  may curve inwardly toward the mixing area  33  to direct the bypass air at an angle toward and into the exhaust gas stream  32  in the mixing area  33 . 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  84  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  84  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  33 . 
         [0039]    An illustrative flow directing system  84  is shown in  FIG. 3 . In this example, the flow directing system  84  includes a plurality of outlet guide vanes  86 . Each outlet guide vane  86  may be selectively controlled (e.g., rotated) by an independent actuator  88 . The actuators  88  are shown schematically in  FIG. 3 , but any known actuator may be utilized. For example, the actuators  88  may comprise an electro-mechanical motor, or any other type of suitable actuator. In embodiments, the flow directing system  84  may extend completely around the exit of the air passage  72  formed between the bypass enclosure  70  and the turbine component  18  of the gas turbine system  12 . 
         [0040]    A supplemental mixing system  90  ( FIG. 1 ) may be positioned within the mixing area  33  to enhance the mixing process. The supplemental mixing system  90  may comprise, for example, a static mixer, baffles, and/or the like. The CO catalyst  36  may also help to improve the mixing process by adding back pressure (e.g., directed back toward the turbine component  22 ). 
         [0041]    As shown in  FIG. 3 , bypass air may be selectively released from the bypass enclosure  70  using an air release system  92  comprising, for example, one or more dampers  94  (or other devices capable of selectively restricting airflow, e.g. guide vanes) located in one or more air outlets  96 . The position of a damper  94  within an air outlet  96  may be selectively controlled (e.g., rotated) by an independent actuator  98 . The actuator  98  may comprise an electro-mechanical motor, or any other type of suitable actuator. Each damper  94  may be controlled in response to commands from the airflow controller  100  to selectively vary the positioning of the damper  94  such that a desired amount of bypass air may be released from the bypass enclosure  70 . Position information (e.g., as sensed by electro-mechanical sensors or the like) for each damper  94  may be provided to the airflow controller  100 . Further airflow control may be provided by releasing bypass air from the bypass enclosure  70  through one or more metering valves  110  ( FIG. 3 ) controlled via commands from the airflow controller  100 . 
         [0042]    The airflow controller  100  may be used to regulate the amount of air generated by the fan  58  that is diverted as bypass air into the mixing area  33  through the air passage  72  relative to the amount of air that enters the gas turbine system  12  (and exits as the exhaust gas stream  32 ) in order to maintain a suitable temperature at the SCR catalyst  38  under varying operating conditions. A chart showing an illustrative relationship between the flow of bypass air into the mixing area  33  and the temperature of the exhaust gas stream  32  at different load percentages of the gas turbine system  12  is provided in  FIG. 6 . In this example, the chart in  FIG. 6  depicts: 1) temperature variation of an exhaust gas stream  32  of a gas turbine system  12  at different load percentages of the gas turbine system  12 ; and 2) corresponding variation in the flow of bypass air as a percentage of the exhaust gas stream  32  (bypass ratio) needed to maintain the temperature at the SCR catalyst  38  at a suitable level (e.g., 900° F.) at different load percentages of the gas turbine system  12 . As represented in the chart in  FIG. 6 , the amount of bypass air flowing through the air passage  72  into the mixing area  33  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 SCR catalyst  38 . 
         [0043]    The airflow controller  100  ( FIG. 1 ) may receive data  102  associated with the operation of the gas turbine power generation system  10 . Such data may include, for example, the temperature of the exhaust gas stream  32  as it enters the mixing area  33 , the temperature of the exhaust gas stream  32  at the SCR catalyst  38  after mixing/cooling has occurred in the mixing area  33 , the temperature of the air drawn into the air intake section  16  by the combined action of the fan  58  and the compressor component  18  of the gas turbine system  12 , and other temperature data obtained at various locations within the gas turbine power system  10 . The data  102  may further include airflow and pressure data obtained, for example, within the air intake section  16 , at the air extraction system  74 , at the fan  58 , at the entrance of the compressor component  18 , within the air passage  72 , at the downstream end  30  of the turbine component  22 , at the flow directing system  84 , and at various other locations within the gas turbine 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  78 , outlet guide vanes  86 , damper  94 , valve  110 , 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. 
         [0044]    Based on the received data  102 , the airflow controller  100  ( FIG. 1 ) is configured to vary as needed the amount of bypass air flowing through the air passage  72  into the mixing area  33  to maintain the temperature at the SCR catalyst  38  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 combined action of the fan  58  and the compressor component  18  of the gas turbine system  12 ; the flow of air  72  allowed into the air passage  72  via the air extraction system  74  (this flow may be controlled, for example, by adjusting the position of one or more of the inlet guide vanes  78 ); and the flow of bypass air passing through the air passage  72  into the mixing area  33  (this flow may be controlled, for example, by adjusting the position of one or more of the dampers  94  and/or the operational status of the metering valves  110 ). 
         [0045]    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 air passage  72  into the mixing area  33  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  80 ,  88 ,  98 , clutch  62 , valve  110 , and/or the like) in the gas turbine power generation system  10 . For example, the commands generated by the airflow controller  100  may be used to control the operation of the actuators  80 ,  88 , and  98  to control the rotational position of the inlet guide vanes  78 , outlet guide vanes  86 , and dampers  94 , respectively. Commands generated by the airflow controller  100  may also be used to activate other control settings in the gas turbine power generation system  10 . 
         [0046]    As shown schematically in  FIG. 3 , the bypass enclosure  70  may be provided with one or more access doors  112 . The access doors  112  provide access through the bypass enclosure  70  to the various components of the gas turbine system  12  (e.g., for servicing, repair, etc.). 
         [0047]    In other embodiments, as depicted in  FIG. 7 , the gas turbine casing  120  itself can be used in lieu of the enclosure  70 . This configuration operates similarly to the system depicted in  FIG. 1 , except that the air extraction system  74  and flow directing system  84  are disposed within the gas turbine casing  120 . The fuel/combustor inlets  122  of the combustor component  20  of the gas turbine system  10  may extend through the gas turbine casing  120  (e.g., for easier access). In this configuration, bypass air (BA) passes between the gas turbine casing  120  and the exterior of the compressor component  18 , combustor component  20 , and turbine component  22 . Other components depicted in  FIGS. 1 and 3 , such as the air intake section, exhaust processing system, airflow controller, etc. are not shown for sake of clarity in  FIG. 7 . 
         [0048]    In operation, with continuing references to  FIG. 7 , a portion of the air drawn in by the fan  58  and the compressor component  18  is directed as bypass air into an air passage  124  formed between the gas turbine casing  120  and the exterior of the compressor component  18 , combustor component  20 , and turbine component  22  by the air extraction system  74 . The bypass air exits the air passage  124  and is directed by at an angle by the flow directing system  84  toward and into the exhaust gas stream  32  in the mixing area  33 . In the mixing area  33 , the bypass air (e.g., ambient air) mixes with and cools the exhaust gas stream  32  to a temperature suitable for use with the SCR catalyst  38  ( FIG. 1 ). 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 less than about 900° F. in the mixing area  33 . 
         [0049]    The use of an airflow generation system  56  including a fan  58  in lieu of conventional large external blower systems and/or other conventional cooling structures provides many 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.) is eliminated. This reduces manufacturing and operating costs, as well as the overall footprint, of the gas turbine power generation system  10 . The footprint is further reduced as the fan  58  draws in air through an existing air intake section  16 , rather than through separate, dedicated intake structures often used with external blower systems. 
         [0050]    Use of the fan  58  provides a more reliable and efficient gas turbine power generation system  10 . For example, since the bypass air used for cooling in the mixing area  33  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 additional flow of air generated by the fan  58  may be used to supercharge the gas turbine system  12 . 
         [0051]    Power requirements of the gas turbine power generation system  10  are reduced because the fan  58  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. 
         [0052]    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). 
         [0053]    When an element 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 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. 
         [0054]    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. 
         [0055]    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.