Abstract:
Fuel gas is saturated with water heated with a heat recovery steam generator heat source. The heat source is preferably a water heating section downstream of the lower pressure evaporator to provide better temperature matching between the hot and cold heat exchange streams in that portion of the heat recovery steam generator. The increased gas mass flow due to the addition of moisture results in increased power output from the gas and steam turbines. Fuel gas saturation is followed by superheating the fuel, preferably with bottom cycle heat sources, resulting in a larger thermal efficiency gain compared to current fuel heating methods. There is a gain in power output compared to no fuel heating, even when heating the fuel to above the LP steam temperature.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to natural gas fired combined cycle power plants and, in particular, to a modified bottoming cycle for fuel gas saturation and heating to increase power output and thermodynamic efficiency.  
           [0002]    In conventional bottoming cycle Heat Recovery Steam Generators (HRSG) there is a large temperature difference between the hot gas and the cold water in the lower pressure economizer (LP-EC) resulting in thermodynamic exergy (thermodynamic potential) losses which limit the power output in the cycle. Heretofore there have been attempts to design bottoming cycles for better temperature matching in the HRSG, such as the Kalina cycle, which uses a multi-component fluid, e.g., ammonia and water, with non-isothermal boiling characteristics. Such multi-component fluid cycles provide better temperature matching in the entire HRSG and efficiency gains. However, significant practical difficulties exist in using multi-component fluids in bottoming cycles.  
           [0003]    Fuel heating is currently implemented in some combined cycle power plants for improving thermal efficiency. Although current fuel heating methods result in plant power output reduction, when heating the fuel above the LP steam temperature, the gain in thermal efficiency as a result of the decreased heat consumption makes fuel heating an economically attractive design option. However, there remains a need for a method and apparatus for achieving a better temperature matching in the HRSG while avoiding power plant output reduction.  
         BRIEF SUMMARY OF THE INVENTION  
         [0004]    The bottoming cycle design method according to a presently preferred embodiment of the present invention results in better temperature matching between the hot and cold heat exchange streams below the lowest pressure evaporator temperature by providing a water heating section for fuel gas saturation in parallel with the lower pressure economizer (LP-EC) in the heat recovery steam generator. Thus, the heat source for fuel gas saturation in the current invention is the gas turbine exhaust gases. The increased gas mass flow due to the addition of moisture results in increased power output from the gas and steam turbines. Fuel gas saturation is followed by superheating the fuel, preferably with bottom cycle heat sources, resulting in a larger thermal efficiency gain compared to current fuel heating methods. There is a gain in power output compared to no fuel heating, even when heating the fuel to above the LP steam temperature. As noted above, current fuel heating methods would result in a power output loss compared to no fuel heating. Thus, fuel gas saturation and subsequent super heating with the cycle of the invention results in increased power output and thermodynamic efficiency compared to a conventional combined cycle with fuel heating to the same temperature or a cycle with no fuel heating. This improved performance is a result of the reduced exergy losses in the HRSG with the modified bottoming cycle described.  
           [0005]    The invention is thus embodied in a combined cycle system including a gas turbine, a steam turbine, and a heat recovery steam generator, wherein gas turbine exhaust gas is used in the heat recovery steam generator for generating steam for the steam turbine, said gas turbine exhaust gas flowing from an entry end to an exit end of the heat recovery steam generator, and wherein the system further comprises a fuel gas saturator assembly for saturating fuel gas with water and heating the fuel gas, the heat recovery steam generator (HRSG) including a first water heater for heating water with heat from the exhaust gases, to define a heat source for the fuel gas saturator assembly; and a fuel gas superheater for superheating fuel gas that has been saturated and heated by the fuel gas saturator assembly for supply to the gas turbine.  
           [0006]    In one embodiment, the fuel gas saturator assembly comprises a fuel gas saturator packed column, for saturating and heating fuel gas with heated water received from the first water heater of the HRSG. In another embodiment, the fuel gas saturator assembly comprises a water inlet for adding water to the fuel gas and a heat exchanger for heating fuel gas saturated with the water input at the water inlet. In this case, the heat exchanger receives and uses the heated water from the first water heater to heat the fuel gas. Whether a heat exchanger or a saturator column is used, in a preferred embodiment of the invention, the fuel superheater heats the saturated fuel gas using a heat recovery steam generator heat source.  
           [0007]    The invention is also embodied in a method for increasing power output and thermodynamic efficiency in a combined cycle system including a gas turbine, a steam turbine, and a heat recovery steam generator, wherein gas turbine exhaust gas is used in the heat recovery steam generator for generating steam for the steam turbine, said gas turbine exhaust gas flowing from an entry end to an exit end of the heat recovery steam generator, the method comprising the steps of adding water to and heating fuel gas to produce heated, saturated fuel gas, the heat being derived from the heat recovery steam generator, feeding the saturated fuel gas to a fuel superheater; further heating the saturated fuel gas in the fuel superheater to superheat the fuel gas; and feeding the superheated, saturated fuel gas to the gas turbine. In a preferred implementation, the saturated fuel gas is also heated with heat derived from a heat source in the heat recovery steam generator.  
           [0008]    The herein described modified bottoming cycle and method is applicable in particular to natural gas fire combined cycle applications.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of a presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a schematic representation of a conventional three pressure re-heat STAG cycle system;  
         [0011]    [0011]FIG. 2 is a graph showing hot (gas) and cold (LP-EC) composite temperature with no fuel saturation for the system of FIG. 1;  
         [0012]    [0012]FIG. 3 is a schematic representation of a combined cycle power plant in accordance with the invention;  
         [0013]    [0013]FIG. 4 is a more detailed schematic representation of a three pressure re-heat STAG cycle with fuel gas saturation in accordance with one embodiment of the invention;  
         [0014]    [0014]FIG. 5 is a graph of hot (gas) and cold (LP-EC- 1 &amp; 2 +SAT.HTR) composite temperature with fuel saturation in accordance with the first embodiment of the invention;  
         [0015]    [0015]FIG. 6 is a schematic representation of a combined cycle power plant with fuel gas saturation and integrated fuel superheater in accordance with another implementation of the invention; and  
         [0016]    [0016]FIG. 7 is a schematic representation of a combined cycle power plant with fuel gas saturation and integrated fuel superheater in accordance with yet another implementation of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    A schematic of a conventional three pressure reheat combined cycle power plant with fuel heating  10  is shown in FIG. 1.  
         [0018]    This example includes a gas turbine system  12  comprising a combustion system  14  and a gas turbine  16 , and a steam turbine system  18  including a high pressure section  20 , an intermediate pressure section  22 , and one or more low pressure sections  24  with multiple steam admission points at different pressures. The low pressure section  24  exhausts into a condenser  26 . The steam turbine  18  drives the generator  28  which produces electrical power. The gas turbine  12 , steam turbine system  18 , and generator  28  are arranged in tandem, on a single shaft  30 .  
         [0019]    The steam turbine system  18  is associated with a multi-pressure HRSG  32  which includes a low pressure economizer (LP-EC), a low pressure evaporator (LP-EV), a high pressure economizer (HP-EC- 2 ), an intermediate pressure economizer (IP-EC), an intermediate pressure evaporator (IP-EV), a low pressure superheater (LP-SH), a final high pressure economizer (HP-EC- 1 ), an intermediate pressure superheater (IP-SH), a high pressure evaporator (HP-EV), a high pressure superheater section (HP-SH- 2 ), a reheater (RH-SH), and a final high pressure superheater section (HP-SH- 1 ).  
         [0020]    Condensate is fed from condenser  26  to the HRSG  32  via conduit  34  with the aid of condensate pump  36 . The condensate subsequently passes through the LP-EC and into the LP-EV. In a known manner steam from the LP-EV is fed to the LP-SH and then returned to the low pressure section  24  of the steam turbine  18  via conduit  38  and appropriate LP admissions stop/control valve(s) schematically depicted at  40 . Feed water with the aid of feed water pump(s)  42  passes (1) through the IP-EC via conduit  44  and to the IP-EV via conduit  48 , and (2) through the HP-EC- 2  via conduit  46  and then on to the final HP-EC- 1  (conduit not shown). At the same time, steam from the IP-EV passes through the IP-SH and then flows through the reheater RH-SH via conduit  50 . The reheated steam is returned to the intermediate pressure section  22  of the steam turbine  18  via conduit  52 .  
         [0021]    Meanwhile, condensate in the final HP-EC- 1  is passed to the HP-EV. Steam exiting the HP-EV passes through the superheater sections HP-SH- 2  and HP-SH- 1  and is returned to the high pressure section  20  of the steam turbine  18  by way of conduit  54  and appropriate stop/control valves (if required, not shown).  
         [0022]    The source for fuel heater  56  in this example is an extraction  58  from the intermediate pressure economizer (IP-EC) outlet. Extraction from other sections of the HRSG or the steam turbine is also possible. Adding heat to the fuel from a bottom cycle energy source reduces the heat consumption by an amount equal to the heat added, with a corresponding reduction in the fuel gas consumption. Although there is a reduction in the plant net power output due to the use of a bottom cycle energy source for fuel heating, particularly when heating the fuel above the LP steam temperature, the reduction of the heat consumption would result in the increase in the thermodynamic efficiency if an appropriate heat source is selected. While the economical value of the increased thermodynamic efficiency is considerably higher than the cost of the lost power output in most instances, the benefit is nevertheless reduced due to the loss in the power plant output.  
         [0023]    By way of illustration, and with reference to the cycle  10  shown in FIG. 1, heating the fuel from 80° F. to 365° F. with the water leaving the exchanger  56  at a temperature of 130° F. results in an increase in combined cycle net efficiency by +0.6%, with a reduction in the net power output of −0.25%.  
         [0024]    [0024]FIG. 2 shows a plot of the heat duty in millions of BTU&#39;s per hour versus the corresponding temperature of the hot composite (gas) and the cold composite (boiler feed water), for the LP-EC section of the HRSG  32  in FIG. 1. Gases leaving the low pressure evaporator (LP-EV) and entering the LP-EC are typically between 290-330° F., and 313° F. is used for this example. In this example, a temperature differential of 25° F. exists at the gas inlet to the LP-EC, where the feed water is heated to 288° F., with this temperature mismatch increasing to approximately 60° F. at a gas temperature of 250° F., and further increasing to approximately 100° F. at the LP-EC exit where the gas enters the stack. This temperature mismatch is a source of exergy loss inherent in conventional Rankine bottoming cycles.  
         [0025]    The basic concept of the invention can be understood with reference to the schematic representation of FIG. 3. For convenience components that correspond to those identified above with reference to FIG. 1 are identified with similar reference numerals but are only discussed in particular as necessary or desirable to an understanding of the fuel saturation and heating components and process.  
         [0026]    Fuel gas is sent to a saturator  160 , where moisture is absorbed by direct contact with hot water in a packed or trayed column. The saturator bottoms water is heated with gas turbine exhaust gas in the saturator heater  162 . The saturator heater  162  is placed in an optimal location relative to other HRSG tube banks which heat the cycle working fluid. Makeup water is provided to the fuel gas saturator  160 , to replace the moisture absorbed by the gas. The saturated fuel gas leaving the saturator  160  is further heated in a fuel superheater  164  using, in the illustrated embodiment, a bottoming cycle heat source. The appropriate selection of bottoming cycle heat source(s) for the saturator heater and the fuel superheater results in a performance enhancement for the power cycle.  
         [0027]    The addition of moisture to the fuel gas at the fuel gas saturator  160  increases the mass flow of the fuel gas. This increased mass flow increases power output of both the gas and steam turbines. Moreover, the use of low grade energy, which would not be useful for steam production, to introduce moisture and thus increase mass flow to the fuel, results in the gain in thermodynamic efficiency. This is also reflected as a reduction of the temperature mismatch in the HRSG below the LP-EV gas exit temperature and a corresponding decrease in thermodynamic exergy losses in the HRSG section.  
         [0028]    By way of example, a first preferred implementation of the foregoing concept for a three pressure reheat Combined Cycle Power Plant is shown in FIG. 4. Again, for convenience, components that correspond to those identified above with reference to FIGS.  1  and/or  2  are identified with similar reference numerals but are only discussed in particular as necessary or desirable to an understanding of the fuel saturation and heating components and process.  
         [0029]    In the embodiment of FIG. 4, a section of the low pressure economizer (LP-EC) has been modified by placing a fuel saturator water heating coil section (SAT.HTR)  262  in parallel with an economizer section (LP-EC- 1 ). This modification results in the reduction of the temperature mismatch and exergy loss in the HRSG  232  below the LP-EV gas exit temperature, and a corresponding efficiency enhancement with fuel saturation. While in the illustrated embodiment of saturator heater is shown in parallel to the LP-EC- 1 , it could, for example, be arranged in an intertwined arrangement with the LP-EC- 1 , or placed at other locations in the HRSG.  
         [0030]    The heated saturator water is sent to saturator  260  via conduit  266 , where moisture is absorbed by the fuel gas by direct contact with the hot saturator water. The saturator bottoms water is returned to the saturator water heater  262 , e.g., with the aid of a saturator bottoms pump  268 . Makeup water is provided, for example, from the feed water pump  242  output as shown at F, to the fuel gas saturator  260 , to replace the moisture absorbed by the gas. Although makeup water for fuel saturation is shown as taken from the feed water transfer pump  242  discharge and/or from the fuel superheater  264 , the saturator water (saturator makeup) could be taken from any other location in the cycle, or from an outside source. Thus, the illustrated source(s) are not to be limiting in this regard.  
         [0031]    The saturated fuel gas leaving the saturator  260  is further heated in fuel superheater  264 , preferably using a bottoming cycle heat source. The heating source for the fuel superheating in this example is IP-EC discharge water, via conduit  258 , but other heat sources could be used. In the embodiment of FIG. 4, the IP-EC discharge water is returned to the IP-EC as shown at G and/or is used as makeup water for fuel saturation, as mentioned above.  
         [0032]    As an illustration, with the gas turbine and ambient conditions identical to those used for the example depicted in FIGS. 1 and 2, the proposed system design shown in FIG. 4 results in a +1.0% gain in combined cycle net efficiency, and a +0.9% gain in combined cycle net output. In this example, the gas leaves the LP-EV at 313° F. and the saturator bottoms water is heated to 298° F. in the saturator heater (SAT.HTR) which is placed, as noted above, in parallel with LP-EC- 1 . The boiler feed water is heated to 288° F. in LP-EC- 1 &amp; 2  as in the previous example.  
         [0033]    Fuel gas (100% Methane, CH 4 ) enters the fuel gas saturator at a pressure of 400 psia and a temperature of 80° F.. The fuel gas leaves the saturator at 284° F. saturated with water vapor. The saturated fuel gas leaving the gas saturator has a composition of approximately 86%v CH 4  and 14%v H 2 O. The saturated fuel gas is subsequently superheated to 365° F. in the fuel superheating heat exchanger  264 .  
         [0034]    [0034]FIG. 5 shows the plot of heat duty in millions of BTUs per hour versus the corresponding temperature of the hot composite (gas) and the cold composite (boiler feed water heating, and saturator bottoms water heating) for the HRSG section LP-EC- 1 &amp; 2  and the saturator heater, for the system shown in FIG. 4. In this example, a temperature differential of 15° F. exists at the gas inlet to the HRSG sections after the LP-EV, with the temperature differential increasing to approximately 35° F. at a gas temperature of 250° F. and further increasing to 80° F. at the HRSG exit. The proposed cycle design of FIG. 4 has thus resulted in a substantial reduction of the temperature mismatch (and exergy losses) in this example for gas temperatures between 313° F. and approximately 240° F., and a smaller reduction in the temperature mismatch at lower gas temperatures.  
         [0035]    As noted above, the heat source for fuel superheating after saturation could be an extraction from other points in the HRSG or the Steam Turbine. Further, while in the example of FIG. 4, water leaving the fuel superheater is returned to the IP-EC, that water could be admitted to any other appropriate location in the bottoming cycle, or to the fuel saturator as makeup water.  
         [0036]    [0036]FIG. 6 is a further illustrative embodiment of the invention, in which the saturated fuel gas leaving the saturator  360  is superheated using the saturator bottoms liquid rather than cycle working fluid as in the embodiment of FIG. 4. As illustrated, the saturator bottoms liquid is initially heated in the heat exchanger  362  with heat from the HRSG exhaust gases. An extraction  368  from the outlet of heat exchanger  362  is sent to heat exchanger  370  for further heating. As illustrated, heat exchanger  370  is placed upstream of heat exchanger  362  in the HRSG  332 . Both saturator heaters  362  and  370  are placed in an optimal location relative to other HRSG tube banks which heat the cycle working fluid. The outlet  372  from heater  370  is used as the heat source for superheating the saturated fuel gas in heat exchanger  364 . The outlet liquid stream  374  from heater  364  is admitted to the fuel gas saturator  360  after being rejoined with the other flow  376  from the outlet of heat exchanger  362 , for direct contact heat and mass transfer with the fuel gas.  
         [0037]    The apparatus and method of superheating the saturated fuel gas shown in FIG. 6 provides a performance benefit due to additional moisture absorption by the gas, and increased safety in the system. The increased safety of this system is due to the elimination of the potential of fuel gas mixing with the cycle working fluid, which is a potential safety hazard when using cycle working fluid as the heat source for heat exchange with the fuel gas.  
         [0038]    The saturator bottoms pump  376  of FIG. 6, and the like pumps illustrated in FIGS. 3 and 4, may be located at other positions in the system and additional pumps may be added to the system depicted. Heater  370  may be eliminated from the system for some cycle designs with the extraction from the outlet of heater  362  sent directly to heater  364 . Furthermore, although not illustrated in particular, the saturation water (saturator make-up, or saturator bottoms) in any of the illustrated embodiments could be heated with low grade heat sources available in the cycle, such as lube oil heat, which are normally rejected to cooling water. This would result in a further performance enhancement to the proposed cycle.  
         [0039]    As described above with reference to e.g., FIG. 3, the fuel gas saturator assembly for adding water to and heating the fuel gas may be a saturator packed column. As an alternative, the saturator packed column shown in FIG. 3 could be replaced by the combination of a water input and a fuel/water heat exchanger, as shown in FIG. 7, while obtaining similar thermodynamic benefits of moisturizing the fuel. The choice of device (packed column or heat exchanger) would be determined by the heat and mass transfer effectiveness of the device, and the overall power plant economics. In FIG. 7, the makeup water is sprayed into the fuel gas at the inlet to the heat exchanger (water atomization for spraying would be either using a pressure atomized nozzle, air atomized nozzle, or steam atomized. If steam or air atomized configurations are used it would be extracted from the cycle.). The two phase fuel/water mixture is heated in heat exchanger  460  using heat extracted from an optimum HRSG location as shown in FIG. 7, with a closed loop system. The saturated fuel gas leaving heat exchanger  460  is further superheated in heat exchanger  464  prior to entering the gas turbine combustor. The system otherwise generally corresponds to the other embodiments described hereinabove.  
         [0040]    As will be appreciated, the invention can be applied to a single pressure or multi-pressure combined cycle power generation system with or without reheat.  
         [0041]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.