Abstract:
A combined cycle power plant in which a gas turbine engine generates power, a heat recovery steam generator (HRSG) produces steam from high energy fluids produced from the generation of power in the gas turbine engine and a steam turbine engine generates additional power from the steam produced in the HRSG. The combined cycle power plant includes a heating element fluidly interposed between the steam turbine engine and the HRSG to heat fluid output from the steam turbine engine, which is to be fed to the HRSG and a control system to control an amount the fluid output from the steam turbine engine is heated by the heating element based on differences between HRSG and ambient temperatures.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to active stack temperature management via controlled steam extracted in a combined cycle power plant. 
     A combined cycle power plant utilizes a gas turbine engine and a steam turbine engine in combination to produce power. The power plant is arranged such that the gas turbine engine is thermally connected to the steam turbine engine through a heat recovery steam generator (“HRSG”). The HRSG is a non-contact heat exchanger that allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine engine exhaust gases. The HRSG is a large duct with tube bundles interposed therein such that water is heated to steam as the exhaust gases pass through the duct. 
     Modern combined cycles typically employ two (2) or three (3) steam generation pressures to recover energy from the gas turbine engine exhaust. These cycles also often include water supply temperature controls to maintain the tubes above the water dew point so as to avoid potential corrosion. Such cycles may set HRSG exit gas temperatures as low as about 150° Fahrenheit (about 66° Celsius) while, in some instances, a higher HRSG exit gas temperature is required. For example, a higher HRSG exit gas temperature may be required in accordance with local regulations to allow for a lower cost (shorter) stack but can result in an unavoidable performance reduction associated with recovery of less exhaust energy. 
     Higher HRSG exit gas temperatures have been previously achieved by reduction of low pressure surface area because the low pressure steam production thus sacrificed has the lowest potential to make work. By contrast, in a recent attempt to solve this problem, a low pressure steam extraction pipe and condensate feedwater heater achieve the required HRSG exhaust gas temperature across an ambient temperature range but, in this case, the hottest temperature in the ambient range tends to drive the design specifications for the extraction pipe and the condensate feedwater heater. Thus, at colder ambient temperatures, this recent arrangement would cause the HRSG exhaust gas temperature to tend to increase above the temperature requirement thus reducing thermal efficiency beyond what is required. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a combined cycle power plant is provided in which a gas turbine engine generates power, a heat recovery steam generator (HRSG) produces steam from high energy fluids produced from the generation of power in the gas turbine engine and a steam turbine engine generates additional power from the steam produced in the HRSG and includes a heating element fluidly interposed between the steam turbine engine and the HRSG to heat fluid output from the steam turbine engine, which is to be fed to the HRSG and a control system to control an amount the fluid output from the steam turbine engine is heated by the heating element based on differences between HRSG and ambient temperatures. 
     According to another aspect of the invention, a combined cycle power plant is provided and includes a gas turbine engine to generate power, a heat recovery steam generator (HRSG) to produce steam from high energy fluids produced from the generation of power in the gas turbine engine, a steam turbine engine to generate additional power from the steam produced in the HRSG, a heating element fluidly interposed between the steam turbine engine and the HRSG to heat fluid output from the steam turbine engine, which is to be fed to the HRSG and a control system to control an amount the fluid output from the steam turbine engine is heated by the heating element based on differences between an HRSG gas exit temperature and an inputted temperature setpoint. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the sole accompanying drawing, which is: 
       A schematic illustration of a combined cycle power plant. 
       The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the sole drawing, a combined cycle power plant  10  is provided. The power plant  10  includes a gas turbine engine system  12  with a combustion system  14  and a turbine section  16 . The power plant  10  further includes a steam turbine engine system  18 . The steam turbine engine system  18  includes a high pressure section  20 , an intermediate pressure section  22  and one or more low pressure sections  24  with multiple steam admission points at the different pressures. The low pressure section  24  exhausts into a condenser  26 . The steam turbine engine system  18  drives a generator  28  that produces electrical power. The gas turbine engine system  12 , the steam turbine engine system  18  and the generator  28  may be arranged on a single shaft  30 . Other configurations may be used. 
     The steam turbine engine system  18  is associated with a multi-pressure heat recovery steam generator (HRSG)  32 . The HRSG  32  is a counter flow heat exchanger in which feedwater that passes through the HRSG  32  is heated as exhaust gas output from the gas turbine engine system  12 , gives up heat and becomes cooler. The HRSG  32  has three (3) different operating pressures (high, intermediate, and low) with components to generate steam at the various pressures and temperatures. This steam is used as vapor feed to the corresponding stages of the steam turbine engine system  18 . The HRSG  32  may include, for example, a lower pressure section  34 , an intermediate pressure section  36  and a high pressure section  38 , each of which may generally include one or more economizers, evaporators and/or superheaters. 
     Condensate is fed from the condenser  26  to the HRSG  32  via one or more conduits  40  with the aid of a condensate pump  42 . A gland seal condenser  44  disposed downstream from the condensate pump  42  also may be used for secondary condensing operations. The condensate subsequently passes from the condensate pump  42  through the low pressure section  34  of the HRSG  32 . In a known manner, steam from the low pressure section  34  is fed to the low pressure section  24  of the steam turbine system  18  via a conduit  46 . Condensate and/or feedwater pass through the intermediate section  36  and are returned to the intermediate pressure section  22  of the steam turbine system  18  via a conduit  48 . Finally, condensate is passed through the high pressure section  38  of the HRSG  32  and is returned to the high pressure section  20  of the steam turbine system  18  via a conduit  50 . Hot water produced via the HRSG  32  also may be used for a fuel heating system  52 . 
     In addition, a heating element, such as a low pressure extraction feedwater heater  56 , is provided. The feedwater heater  56  is positioned about the conduit  40  and is operably disposed downstream from the condenser  26 . A further conduit  58 , such as a low pressure steam extraction pipe, may directly connect the feedwater heater  56  to the low pressure section  24  of the steam turbine engine system  18  whereby low pressure steam is extracted from the low pressure section  24  of the steam turbine engine system  18 , fed to the feedwater heater  56  and thus employed to raise the temperature of the condensate being fed to the HRSG  32 . The feedwater heater  56  thus tends to increase the supply temperature of the condensate to the HRSG  32 . 
     A valve  60 , such as a proportional valve or some other similar type of valve, is provided along the further conduit  58  to control an amount of the low pressure steam extracted from the low pressure section  24  of the steam turbine engine system  18 . The valve  60  is operated by a control system including a valve actuator  70 , a processing unit  80  and a temperature sensor  90 . The valve actuator  70  is operably coupled to the processing unit  80 . The processing unit  80  receives HRSG temperature measurements, such as HRSG exhaust gas temperature measurements, from the temperature sensor  90 , such as a thermocouple operably disposed within the HRSG  32 . 
     In cases where it is desirable to modulate the HRSG exhaust gas temperature to a setpoint that is higher than the naturally occurring temperature absent any mechanism for active temperature control, such as where a minimum HRSG exhaust gas temperature is defined by local regulations, the processing unit  80  compares the measured HRSG exhaust gas temperature to the an inputted temperature setpoint and/or a desired temperature setpoint and transmits a control signal, such as a proportional valve modulation signal, to the valve actuator  70  based on results on that comparison. The valve actuator  70  then opens or closes the valve  60  to control low pressure steam extraction accordingly. 
     While the feedwater heater  56  is designed such that a minimum HRSG exhaust gas temperature can be achieved across a given range of ambient temperatures, the minimum HRSG exhaust gas temperature would typically occur on the hottest day in the ambient range and, at every colder ambient temperature, the low pressure steam extraction would tend to be excessive without appropriate control of the valve  60  as provided by the control system described herein. A result of excessive low pressure steam extraction may be that the HRSG exhaust gas temperature would tend to increase above the required minimum thereby reducing thermal efficiency of the combined cycle power plant  10 . However, since the valve  60  as controlled by the processing unit  80  is able to modulate the low pressure steam extraction such that only the minimum required amount of low pressure steam is extracted to maintain the HRSG exhaust gas temperature above the required minimum for a given day having a given ambient temperature, such as the inputted temperature setpoint, the remaining flow will expand through the low pressure section  24  of the steam turbine system  18  thus increasing thermal efficiency of the combined cycle power plant  10 . 
     In accordance with alternative embodiments, the steam for use in the feedwater heater  56  may be extracted from the high or intermediate pressure sections  20 ,  22  of the steam turbine engine system  18 . Another alternative would be to have the processing unit  80  infer the HRSG exhaust gas temperature based on a low pressure economizer inlet temperature with a relationship between the low pressure economizer inlet temperature and the HRSG gas exhaust temperature understood. The processing unit  80  would then transmit the flow modulation signal based on the low pressure economizer inlet temperature. 
     Still other alternative embodiments include the addition of valves and conduits or the modification of existing valves and conduits to bypass the steam turbine engine system  18 . In these embodiments, extracted steam for use in the feedwater heater  56  is drawn from conduits  46 ,  48  and  50  directly. For example,  FIG. 1  illustrates a conduit  47  that extends from the conduit  46  to the feedwater heater  56 , thereby bypassing the steam turbine engine system  18 . Also, after steam exits the high pressure section  20  of the steam turbine engine system  18 , the steam proceeds through a cold reheat pipe  471  and merges with intermediate pressure superheated steam from conduit  472 . Extracted steam from either cold reheat pipe  471  or conduit  472  could also be used to supply the feedwater heater  56 . 
     The feedwater heater  56  can be provided in any heat exchanging configuration and may incorporate additional or alternative heating elements. 
     In accordance with further aspects of the invention, a method of operating a combined cycle power plant  10  is provided. The method includes directing condensed low pressure steam, from the steam turbine engine system  18  and the condenser  26  toward the HRSG  32 , heating the condensed low pressure steam prior to the admission thereof into the HRSG  32  and controlling an amount of the heating of the condensed low pressure steam. The heating is achieved by exposing in a feedwater heater  56  the condensed turbine exhaust to an amount of low pressure steam, which is extracted from the steam turbine engine system  18 . The controlling, as described above, is achieved by modulating the amount of the low pressure steam to which the condensed turbine exhaust is exposed based on a comparison of a temperature measurement of the HRSG  32 , an ambient temperature measurement and a predefined temperature setting, which may be defined in accordance with the ambient temperature measurement. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.