Abstract:
A system and method for supercharging a combined cycle system includes a forced draft fan providing a variable air flow. At least a first portion of the air flow is directed to a compressor and a second portion of the airflow is diverted to a heat recovery steam generator. A control system controls the airflows provided to the compressor and the heat recovery steam generator. The system allows a combined cycle system to be operated at a desired operating state by controlling the flow of air from the forced draft fan to the compressor and the heat recovery steam generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to concurrently filed application Ser. No. ______, titled GAS TURBINE COMPRESSOR INLET PRESSURIZATION AND FLOW CONTROL SYSTEM, filed jointly in the names of John Anthony Conchieri, Robert Thomas Thatcher, and Andrew Mitchell Rodwell and application Ser. No. ______, titled GAS TURBINE COMPRESSOR INLET PRESSURIZATION HAVING A TORQUE CONVERTER SYSTEM, filed jointly in the names of Sanji Ekanayake and Alston I. Scipio, each assigned to General Electric Company, the assignee of the present invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The subject matter disclosed herein relates to combined cycle power systems and more particularly to supercharged combined cycle systems with air flow bypass. 
       BACKGROUND 
       [0003]    Combined cycle power systems and cogeneration facilities utilize gas turbines to generate power. These gas turbines typically generate high temperature exhaust gases that are conveyed into a heat recovery steam generator (HRSG) that produces steam. The steam may be used to drive a steam turbine to generate more power and/or to provide steam for use in other processes. 
         [0004]    Operating power systems at maximum efficiency is a high priority for any generation facility. Factors including load conditions, equipment degradation, and ambient conditions may cause the generation unit to operate under less than optimal conditions. Supercharging (causing the inlet pressure to exceed the exhaust pressure) turbine systems as a way to increase the capacity of gas-turbine is known. Supercharged turbine systems typically include a variable speed supercharging fan located at the gas turbine inlet that is driven by steam energy derived from converting exhaust waste heat into steam. The supercharging fan is used to increase the air mass flow rate into the gas turbine so that the gas turbine shaft horsepower can be augmented. 
         [0005]    A problem with conventional supercharged combined cycle systems is that they are uneconomical due primarily to the prevailing “spark spread.” Spark spread is the gross margin of a gas-fired power plant from selling a given amount of electricity minus the cost of fuel required to produce that given amount of electricity. Operational, maintenance, capital and other financial costs must be covered from the spark spread. Another problem with conventional supercharged systems is that controlling the inlet fan is difficult. In many cases, the return on investment of such systems is not attractive. Conventional supercharged combined cycle systems do not provide customers with sufficient system flexibility, output and efficiency over the system life cycle. Additionally, those systems require significant modifications and are sometimes not compatible with legacy systems. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In accordance with one exemplary non-limiting embodiment, the invention relates to a combined cycle system including a gas turbine subsystem having a compressor and an output side that provides an exhaust, and a heat recovery steam generation subsystem having an inlet. An exhaust duct is coupled to the gas turbine system and the inlet for transporting the exhaust to the heat recovery steam generation system. The system also includes a controllable air stream source that produces an air flow and a ducting assembly coupled to the controllable air stream source that conveys at least a portion of the air flow to the compressor. A bypass coupled to the controllable air stream source and the exhaust duct adapted to selectively convey at least a portion of the air flow to the inlet is also provided. 
         [0007]    In another embodiment, a supercharging system is provided, the system including a forced draft fan providing a variable air flow. A duct that directs at least a portion of the air flow to a compressor and a bypass subsystem that diverts at least a portion of the air flow to a heat recovery steam generator are also provided. The system includes a control system coupled to the bypass subsystem and the forced draft fan. 
         [0008]    In another embodiment, a method of operating a combined cycle system includes determining a first operating state and determining a desired operating state. The method includes determining a first mass flow quantity of air to be provided to a compressor and a second mass flow quantity of air to be provided to a heat recovery steam generator to achieve the desired operating state. The method includes providing source of controllable air flow, selectively conveying the first mass flow quantity of air into the compressor; and selectively conveying the second mass flow quantity of air to the heat recovery steam generator. 
         [0009]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of certain aspects of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic illustration of an embodiment of a supercharged combined cycle system with air bypass. 
           [0011]      FIG. 2  is a schematic illustration of another embodiment of a supercharged combined cycle system with air bypass. 
           [0012]      FIG. 3  is a flow chart of an embodiment of a method implemented by a supercharged combined cycle system with air bypass. 
           [0013]      FIG. 4  is a chart illustrating a result accomplished by a supercharged combined cycle system with air bypass. 
           [0014]      FIG. 5  is a flow chart of an embodiment of a method implemented by a supercharged combined cycle system with air bypass. 
           [0015]      FIG. 6  is a chart illustrating a result accomplished by a supercharged combined cycle system with air bypass. 
           [0016]      FIG. 7  is a chart illustrating a result accomplished by a supercharged combined cycle system with air bypass. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Illustrated in  FIG. 1  is a schematic illustration of a supercharged combined cycle system with air bypass (SCCAB system  11 ) in accordance with one embodiment of the present invention. The SCCAB system  11  includes a gas turbine subsystem  13  that in turn includes a compressor  15 , having a compressor inlet  16 , a combustor  17  and a turbine  19 . An exhaust duct  21  may be coupled to the turbine  19  and a heat recovery steam generator subsystem (HRSG  23 ). The HRSG  23  recovers heat from exhaust gases from the turbine  19  that are conveyed through HRSG inlet  24  to generate steam. The HRSG  23  may also include a secondary burner  25  to provide additional energy to the HRSG  23 . Some of the steam and exhaust from the HRSG  23  may be vented to stack  27  or used to drive a steam turbine  27  and provide additional power. Some of the steam from the HRSG  23  may be transported through process steam outlet header  28  to be used for other processes. The SCCAB system  11  may also include an inlet house and cooling system  29 . The inlet house and cooling system  29  is used to cool and filter the air entering the compressor inlet  16  to increase power and avoid damage to the compressor  15 . 
         [0018]    The SCCAB system  11  also includes a forced draft fan  30  used to create a positive pressure forcing air into the compressor  15 . Forced draft fan  30  may have a fixed or variable blade fan (not shown) and an electric motor (not shown) to drive the blades. Forced draft fan  30  may be driven by a variable frequency drive (VFD  31 ) that controls the rotational speed of the electric motor by controlling the frequency of the electrical power supplied to the motor. VFD  31  provides a number of advantages, including energy savings from operating at lower than nominal speeds. Another advantage is that VFD  31  may be gradually ramped up to speed lessening the stress on the equipment. The forced draft fan  30  provides a controllable air stream source though a duct assembly  32  and may be used to increase the mass flow rate of air into the compressor  15 . The quantity of air going into the compressor is controlled by the VFD  31 . The compressor inlet  16  may be configured to accommodate slight positive pressure as compared to the slight negative pressure conventional design. 
         [0019]    The SCCAB system  11  may also include a bypass  33  (which may include external ducting) that diverts a portion of the air flow from forced draft fan  30  into the exhaust duct  21 . This increased air flow provides additional oxygen to the secondary burner  25  to avoid flame out or less than optimal combustion. Bypass  33  may be provided with a flow sensor  35  and a damper valve  37  to control the airflow through the bypass  33 . A control system  39  may be provided to receive data from flow sensor  35  and to control the damper valve  37  and the VFD  31 . Control system  39  may be integrated into the larger control system used for operation control of SCCAB system  11 . The airflow from the bypass is conveyed to the exhaust duct  21  where the temperature of the combined air and exhaust entering the HRSG  23  may be modulated. 
         [0020]    Illustrated in  FIG. 2  is another embodiment of a SCCAB system  11  that includes a pair of gas turbine subsystems  13 . In this embodiment, the exhaust of the pair of gas turbine subsystems  13  is used to drive a single steam turbine  27 . In this embodiment, an inlet house  41  is positioned upstream of the forced draft fan  30 , and a cooling system  43 , where the airflow from the fan may be cooled, is positioned downstream of the forced draft fan  30 . The bypass  33  is coupled to the cooling system  43 . One of ordinary skill in the art will recognize that although in this embodiment two gas turbine systems  13  are described, any number of gas turbine systems  13  in combination with any number of steam turbine(s)  27  may be used. 
         [0021]    In operation, the SCCAB system  11  provides increased air flow into the HRSG  23  resulting in a number of benefits. The SCCAB system  11  may provide an operator with the ability to optimize combined cycle plant flexibility, efficiency and lifecycle economics. For example, boosting the inlet pressure of the gas turbine subsystem  13  improves output and heat rate performance. The output performance of the SCCAB system  11  may be maintained flat (zero degradation) throughout the life cycle of SCCAB system  11  by increasing the level of supercharging (and parasitic load to drive the forced draft fan  30 ) over time commensurate with the degradation of SCCAB system  11 . The use of the VFD  31  to power the forced draft fan  30  enables and substantially improves system efficiencies under partial-supercharge conditions. Another benefit that may be derived from the SCCAB system  11  is the expansion of the power generation to steam production ratio envelope. This may be accomplished by modulating the exhaust gas temperature at HRSG inlet  24  with air from the forced draft fan  30 . Another benefit that may be derived from the SCCAB system  11  is an improved start up rate as a result of the reduction in the purge cycle (removal of built up gas). The SCCAB system  11  may also provide an improved load ramp rate resulting from the modulation of the exhaust temperature at the exhaust duct  21  with air from the forced draft fan  30  provided through the bypass  33 . The forced draft fan  30  of the SCCAB system  11  also provides an effective means to force-cool the gas turbine subsystem  13  and HRSG  23 , reducing maintenance outage time and improves system availability. The forced draft fan  30  provides comparable benefit for simple cycle and combined-cycle configurations for all heavy-duty gas turbine systems  13  delivering in the range of 20% output improvement under hot ambient conditions with modest capital cost. 
         [0022]    The SCCAB system  11  may implement a method of maintaining the output of a combined cycle plant over time (method  50 ) as illustrated with reference to  FIGS. 3-4 . In  FIG. 3  the method  50  may determine the current state (method element  51 ), and may determine a desired state (method element  53 ). The desired state may be to maintain a nominal output over time to compensate for performance losses. Performance losses typically arise as a result of wear of components in the gas turbine over time. These losses may be measured or calculated. The method  50  may determine the required increased air mass flow to maintain the desired output (method element  55 ). Based on that determination, the method  50  may adjust the air mass flow into the compressor inlet  16  (method element  57 ). The method  50  may adjust the combined air and exhaust mass flow into the HRSG inlet  24  (method element  59 ). 
         [0023]      FIG. 4  illustrates the loss of output and heat rate over time (expressed in percentages) of a conventional combined cycle system and a SCCAB system  11 . Gas turbines suffer a loss in output over time, as a result of wear of components in the gas turbine. This loss is due in part to increased turbine and compressor clearances and changes in surface finish and airfoil contour. Typically maintenance or compressor cleaning cannot recover this loss, rather the solution is the replacement of affected parts at recommended inspection intervals. However, by increasing the level of supercharging using forced draft fan  30  output performance may be maintained, although at a cost due to the parasitic load to drive the forced draft fan  30 . The top curve (unbroken double line) illustrates the typical output loss of a conventional combined cycle system. The second curve (broken double lines) illustrates the expected output loss with periodic inspections and routine maintenance. The lower curve (broken triple line) shows that the output loss of an SCCAB system  11  may be maintained at near 0%. Similarly, the heat rate degradation of a conventional combined cycle system (single solid curve) may be significantly improved with an SCCAB system  11 . 
         [0024]      FIG. 5  illustrates a method of controlling the steam output of a SCCAB system  11  (method  60 ). Method  60  may initially determine the current state (method element  61 ). The method  60  may also determine the desired output and steam flow (method element  63 ). The method  60  may determine the required increased air flow (method element  65 ) to the compressor inlet  16  and the HRSG inlet  24 . Method  60  may then adjust the air flow into the compressor inlet  16  (method element  67 ) and the combined exhaust and air flow into the HRSG inlet  24  (method element  69 ), to provide the desired steam output. 
         [0025]      FIG. 6  illustrates expanded operating envelope to maintain constant steam flow. The vertical axis measures output in MW and horizontal axes measures steam mass flow. The interior area (light vertical cross hatch) shows the envelope of a conventional combined cycle system. The envelope of an SCCA  11  is shown in diagonal cross hatching, and a larger area illustrates the performance of an SCCA  11  combined with secondary firing in the HRSG  23 . 
         [0026]      FIG. 7  is a chart that illustrates the improved operational performance of an SCCAB system  11  at a specific ambient temperature in comparison with conventional combined cycle systems at minimum and base loads. The horizontal axis measures output in MW and the vertical axis measures heat rate (the thermal energy (BTU&#39;s) from fuel required to produce one kWh of electricity). The chart illustrates the improved efficiency delivered by the SCCAB system  11 . 
         [0027]    The foregoing detailed description has set forth various embodiments of the systems and/or methods via the use of block diagrams, flowcharts, and/or examples. Insofar as such block diagrams, flowcharts, and/or examples contain one or more functions and/or operations, it will be understood by those within the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware. It will further be understood that method steps may be presented in a particular order in flowcharts, and/or examples herein, but are not necessarily limited to being performed in the presented order. For example, steps may be performed simultaneously, or in a different order than presented herein, and such variations will be apparent to one of skill in the art in light of this disclosure. 
         [0028]    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.