Patent Publication Number: US-2009235634-A1

Title: System for extending the turndown range of a turbomachine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the operation of a turbomachine, and more particularly to a system for extending the turndown range by heating the inlet-air. 
     Turbomachines, such as gas turbines, aero-derivatives, or the like, commonly operate in a combined-cycle and/or cogeneration mode. In combined-cycle operation, a heat recovery steam generator, which generates steam, receives the exhaust-gas from the turbomachine; the steam then flows to a steam turbine that generates additional electricity. In a co-generation operation, a portion of the steam generated by the heat recovery steam generator is sent to a separate process requiring the steam. 
     Combined-cycle and cogeneration plants are rated to generate the maximum amount of energy (mechanical, electrical, etc) while operating at baseload. However, baseload operation, though desired by operators, is not always feasible. There may not be a demand in the energy market (electrical grid, or the like) for all of the energy generated at baseload. Here, the powerplant must either shutdown or operate at partload, where less than the maximum amount of energy is generated. 
     Turbomachines are typically required to maintain emissions compliance while generating power. A turbomachine operating at partload, may not maintain emissions compliance over the entire partload range, (from spinning reserve to near baseload). Turndown range may be considered the loading range where the turbomachine maintains emissions compliance. A broad turndown range allows operators to maintain emissions compliance, minimize fuel consumption, and avoid the thermal transients associated with shutting down the powerplant. 
     For the foregoing reasons, there is a need for a system for extending the turndown range. The system should reduce the fuel consumed by the turbomachine while operating at the partload range. The system should not require significant changes to the turbomachine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with an embodiment of the present invention, a system for extending a turndown range of a turbomachine operating at partload, the system comprising: a turbomachine comprising a compressor, which receives an inlet-air; a combustion system; and a turbine section; wherein the turbomachine produces an exhaust-gas; a heat recovery steam generator (HRSG), wherein the HRSG receives a portion of the exhaust-gas and produces steam; and at least one air preheater comprising at least one heat exchanging section, wherein the at least one air preheater heats the inlet-air before the inlet-air flows to the compressor; wherein a portion of the at least one heat exchanging section receives a fluid at a temperature allowing for heating of the inlet-air; and wherein the fluid flows from a source external to the turbomachine; and wherein heating the inlet-air reduces an output of the turbomachine and extends the turndown range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustrating an example of a system for extending the turndown range of a turbomachine in accordance with a first embodiment of the present invention. 
         FIG. 2  is a schematic illustrating an example of a system for extending the turndown range of a turbomachine in accordance with a second embodiment of the present invention. 
         FIG. 3  is a schematic illustrating an example of a system for extending the turndown range of a turbomachine in accordance with a third embodiment of the present invention. 
         FIG. 4  is a schematic illustrating an example of a system for extending the turndown range of a turbomachine in accordance with a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of preferred embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention. 
     The present invention may be applied to a wide variety of turbomachines including, but not limiting of, gas turbines, aero-derivative combustion turbines, and the like. An embodiment of the present invention takes the form of an application and process that may heat the air entering a turbomachine (hereinafter “gas turbine”) to increase the turndown range. 
     An embodiment of the present invention has the technical effect of extending the turndown range by heating the air (hereinafter “inlet-air”) entering the compressor of the gas turbine. As described below, the inlet-air is heated by an energy source external to the gas turbine. 
     Referring now to the Figures, where the various numbers represent like elements throughout the several views,  FIG. 1  is a schematic illustrating an example of a system  100  for extending the turndown range of a gas turbine  105  in accordance with a first embodiment of the present invention. 
       FIG. 1  illustrates a site comprising a gas turbine  105 : a heat recovery steam generator (HRSG)  110 ; a stack  115 ; and an air preheater  155 . Generally, the gas turbine  105  comprises an axial flow compressor  120  having a shaft  125 . Inlet-air  130  enters the compressor  120 , is compressed and then discharged to a combustion system  135 , where a fuel  140 , Such as natural gas, is burned to provide high-energy combustion gases which drives the turbine section  145 . In the turbine section  145 , the energy of the hot gases is converted into work, some of which is used to drive the compressor  120  through the shaft  125 , with the remainder available for useful work to drive a load such as the generator, mechanical drive, or the like (none of which are illustrated). The exhaust-gas  150  from the turbine section  145  may then flow to the HRSG  110 , which may transfer a portion of the exhaust-gas  150  energy into steam (not illustrated). 
     During baseload operation, the combustion system  135  may ensure that the exhaust-gas  150  flowing out of the stack  115  meets the site emissions requirements. Depending on the turndown range of the gas turbine  105 , certain partload operations may violate the site emissions requirements, which may require the shutdown of the gas turbine  105 . An increase in the turndown range may avoid the need to shutdown the gas turbine  105 . Also, an extended turndown range allows for operating the gas turbine  105  at lower loads, while maintaining emissions compliance and consuming less fuel  140 . 
     The present invention extends the turndown range by heating the inlet-air  130 . Generally, the output (electrical, mechanical, or the like) of a gas turbine  105  is governed by the amount of mass-flow entering the compressor  120 . The mass-flow may be considered the product of the density and the volume-flow of the inlet-air  130  entering the compressor  120 . The amount of volume-flow entering the compressor  120  may vary on the ambient temperature conditions and the angle of Variable Inlet Guide Vanes (IGVs), if present on the gas turbine  105 . The IGV angle may determine the flow area at the inlet of the compressor  120 . The IGV angle may be reduced to a minimum angle, limiting the amount of turndown. At the minimum IGV angle, a corresponding minimum volume-flow is drawn into the compressor  120 . 
     In the present invention, the heating of the inlet-air  130  decreases the density, allowing less dense inlet-air  130  to enter the compressor  120 . Here, at a given load point the volume-flow entering the compressor  120  may remain constant, however the mass-flow decreases due to the decrease in density of the inlet-air  130 . As discussed, the output of the gas turbine  105  may be determined by the mass-flow entering the gas turbine  105 ; therefore less output is produced due to the heating of the inlet-air  130 , compared to not heating of the inlet-air  130 . 
     The heating of the inlet-air  130  also increases the temperature (hereinafter “compressor discharge temperature”) of the air  130  exiting the compressor  120 . This heated inlet-air  130  then enters the combustion system  135 . The heated air  130  aids in reaching the overall universal reference temperature (“firing temperature”) of the gas turbine  105 . The heated inlet-air  130  allows the gas turbine  105  to consume less fuel  140  to obtain the firing temperature. Here, more fuel  140  would be consumed if unheated inlet-air  130  entered the compressor  120 . 
     Overall, the present invention incorporates at least one air preheater  155 , which may be installed upstream of the compressor  120 . The air preheater  155  may be a heat exchanger, or the like. The air preheater  155  may be sized to adequately heat the inlet-air  130  to a temperature that increases the turndown range. 
     Generally, the temperature of the unheated inlet-air  130  may be determined by the ambient conditions or the outlet temperature of any air conditioning system (not illustrated) located upstream of the air preheater  155 . An embodiment of the present invention may increase the temperature of the inlet-air  130  to any temperature allowed for by the air preheater  155 . However, the increase in temperature of the inlet-air  130  may be limited by at least one of several factors, such as but not limiting of, the geometrical limitations of the air preheater  155 ; a temperature that may violate a thermal, operational, or mechanical limitation; or the like. For example, but not limiting of, the system  100  may increase the temperature of the inlet-air  130  from approximately 59 degrees Fahrenheit to approximately 120 degrees Fahrenheit. Here, the inlet-air  130  may have an inlet flowrate of 3,000,000 pounds/hour. 
     The system  100 , illustrated in  FIG. 1 , includes at least one air preheater  155 , a preheater supply line  160 ; and a preheater discharge line  165 . The preheater supply line  160  allows a portion of the exhaust-gas  150 , or other fluid, such as, but not limiting of, water, steam, or the like, to flow from the HRSG  110  to the air preheater  155 . In this first embodiment of the present invention, an end of the preheater supply line  160  is connected to a portion of the HRSG  110 , where the exhaust-gas  150  may be extracted. The preheater supply line  160  receives a portion of the exhaust-gas  150  from the HRSG  110 . The exhaust-gas  150  may flow through the preheater supply line  160 , which may have an opposite end connected to a portion of the air preheater  155 . 
     This first embodiment of the present invention allows a user to determine where the exhaust-gas  150  is extracted from on the HRSG  110 . The present invention may allow a user to optimize the location on the HRSG  110  where the exhaust-gas  150  is extracted and sent to the air preheater  155 . A user may consider a variety of factors when determining the optimized location on the HRSG  110 . These factors may include, for example, but not limiting of, the following. Temperature: the temperature of the fluid used to increase the temperature of the inlet-air  130  (exhaust-gas  150 , water, steam, or the like), should be higher than the maximum desired temperature that the inlet-air  130  may be raised to by the air preheater  155 . The maximum desired temperature might be used for sizing the air preheater  155 . Flow: flow of the fluid should be sufficient to supply the air preheater  155 , while maintaining sufficient flow for other demands from the HRSG  110 , or the like. Fluid type: the use of water, if available, as the fluid for increasing the temperature of the inlet-air  130  may be optimum, possibly requiring less mass-flow and a relatively smaller sized air preheater  155 . Energy Source: the fluid may derive from an energy source that may be utilized without negatively impacting the overall benefits of heating the inlet-air  130 . The energy source may include, for example, but not limiting of, outlet from a condenser or fuel heater  175 ; packing flows, or the like; exhaust-gas  150 : discharge from the stack  115 ; any other energy source external to the bottoming cycle. 
     For example, but not limiting of, an operator of the site may use a portion of the exhaust-gas  150  flowing towards the condenser (not illustrated). Here, this energy may be considered ‘low value’ because the energy needed to create steam may have been already extracted. However, another site, may extract the exhaust-gas  150  from another area of the HRSG  110 . Here, for example, but not limiting of, an operator may decide that instead of restricting the flow of the exhaust-gas  150  entering a section of the HRSG  110 , divert a portion of the exhaust-gas  150  to the air preheater  155 . 
     In use, the system  100  operates while the gas turbine  105  is not at baseload. As the gas turbine  105  unloads, the present invention may divert a portion of the exhaust-gas  150  to the air preheater  155  via the preheater supply  160 . The exhaust-gas  150  may flow through an inlet portion of the air preheater  155 . As the inlet-air  130  flows through the air preheater  155 , the heat from the exhaust-gas  150  is transferred to, and increases the temperature of, the inlet-air  130 . After flowing through the air preheater  155 , the exhaust-gas  150  may flow through the preheater discharge line  165  to the stack  115  and/or the HRSG  110 . 
       FIGS. 2 through 4  illustrate alternate embodiments of the present invention. A key difference between all embodiments of the present invention is the source of energy used to increase the temperature of the inlet-air  130 . The discussions of  FIG. 2 through 4  focus on the differences between each alternate embodiment and the embodiment illustrated in  FIG. 1 . 
       FIG. 2  is a schematic illustrating an example of a system  200  for extending the turndown range of a gas turbine  105  in accordance with a second embodiment of the present invention. Here, the primary difference between this second embodiment and the first embodiment is the addition of at least one external energy source (EES)  170 , which provides the energy for increasing the temperature of the inlet-air  130 . 
     The EES  170  may provide sufficient energy to heat the inlet-air  130  to the temperature that allows for extending the turndown range. As illustrated in  FIG. 2 , the EES  170  may eliminate the need for extracting the exhaust-gas  150  from the HRSG  110 . In this second embodiment, the exhaust-gas  150  may be used for other purposes and/or may flow through the stack  115 . Alternatively, the EES  170  may operate in conjunction with the embodiment of illustrated in  FIG. 1 . Here, the EES  170  may operate as the primary energy system for increasing the temperature of the inlet-air  130  and the extraction from the HRSG  110 , may serve as a secondary energy system (and vice-versa). 
     The EES  170  may include at least one of the following energy systems: a wind turbine, a boiler, an engine, an additional combustion turbine, an additional HRSG, a power plant, a solar energy source, geothermal energy source, fuel cell/chemical reaction, external process, and combinations thereof; none of which are illustrated in  FIG. 2 . Each of the aforementioned energy system may indirectly or directly increase the temperature of the inlet-air  130 . 
     For example, but not limiting of, a wind turbine may indirectly increase the temperature of the inlet fluid  130 . Here, the energy generated by the wind turbine may heat water within a tank (not illustrated) integrated with the preheater supply line  160 . The heated water may flow through the preheater supply line  160  to the air preheater  155 . After flowing through the air preheater  155 , the heated water may flow through the preheater discharge line  165 , which may be integrated with the EES  170 . Alternatively, for example, but not limiting of, a boiler may directly increase the temperature of the inlet fluid  130 . Here, the preheater supply line  160  may be integrated with a portion of the boiler. The steam or hot water generated by the boiler may flow through the preheater supply line  160  and the air preheater  155 . After flowing through the air preheater  155 , the steam or hot water may flow through the preheater discharge line  165 , which may be integrated with the EES  170 . 
       FIG. 3  is a schematic illustrating an example of a system  300  for extending the turndown range of a gas turbine  105  in accordance with a third embodiment of the present invention. Here, the primary difference between this third embodiment and the first embodiment is the addition of the fuel heater  175 . Some gas turbines  105  use heated fuel  140  as a way to increase performance. The fuel heater  175  commonly heats the fuel  140  on the site where the gas turbine  105  is located. The fuel heater  175  may have the form of a heat exchanger, or the like. 
     As illustrated in  FIG. 3 , the exhaust-gas  150  may exit the HRSG  110  via the preheater supply line  160 . In an embodiment of the present invention, the air preheater  155  may include multiple portions allowing for a plurality of inlet flows. As illustrated in  FIG. 3 , the air preheater  155  may include a first inlet portion integrated with the fuel heater discharge line  185 , and a second inlet portion integrated with the preheater supply line  160 . 
     In this third embodiment, the preheater supply line  160  may be integrated with a fuel heater supply line  180 . Here, a portion of the exhaust-gas  150  may flow into the fuel heater  175 . Another portion of the exhaust-gas  150  may flow into the air preheater  155 . After flowing through the fuel heater  175 , the exhaust-gas  150  may flow through the fuel heater discharge line  185  to the air preheater  155 . After flowing to the air preheater  155 , the exhaust-gas  150  may then flow through the preheater discharge line  165  to the stack  115  and/or the HRSG  110 , as previously described. 
       FIG. 4  is a schematic illustrating an example of a system  400  for extending the turndown range of a gas turbine  105  in accordance with a fourth embodiment of the present invention. Here, the primary difference between this fourth embodiment and the first embodiment is that the exhaust-gas  150  is extracted from the stack  115 , as opposed to the HRSG  110 , as illustrated in  FIG. 1 . 
     In this fourth embodiment of the present invention, an end of the preheater supply line  160  is connected to a portion of the stack  115 , where the exhaust-gas  150  is extracted. The exhaust-gas  150  may flow through the preheater supply line  160 , which may have an opposite end connected to a portion of the air preheater  155 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. 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. 
     Although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.