Title of Invention	method and apparatus for fuel gas moisturization and heating
Abstract	The Invention relates to a combined cycle system comprising. gas turbine (112, 212, 312), a steam turbine (118, 213, 313), aid a heat recovery shun generator (132, 232, 332), wherein gas turbine exhaust gas Is used in the heat recovery shun generator for generating steam for die steam turbine, said gas turbine exhaust gas flowing from am entry end to am ext end of the heat recovery steam generator mid wherein flu system additionally comprises: a fuel gas saturator (160, 260, 360) having an Inlet for hot saturator water, an Inlet for fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a saturator war heater (162, 262, 362); a flow path for flowing saturator water from said saturator war outlet to said saturator hear (162, 262, 362), said saturator heater being operatively coupled to a heat source In the heat recovery steam generator (132, 232, 332) for heating saturator war conducted thereto, using said heat notice, to produce hot saturator war; a flow path (266, 366) far flowing hat saturator water produced by the saturator heater to the hot saturator water inlet of the fuel gas saturator; a fuel superheater (164, 264, 364) for heating said saturated fuel gas; a flow path for flowing satiated fuel gas from said satiated fuel gas outlet to said fuel superheater (164, 264, 364) for heating said saturated fuel gas, to produce superheated, saturated fuel gas; and a flow — for flowing said superheated, satiated fuel gas to said gas turbine (112,212,312). (FIG. - 1)

Title of Invention

method and apparatus for fuel gas moisturization and heating

Abstract

The Invention relates to a combined cycle system comprising. gas turbine (112, 212, 312), a steam turbine (118, 213, 313), aid a heat recovery shun generator (132, 232, 332), wherein gas turbine exhaust gas Is used in the heat recovery shun generator for generating steam for die steam turbine, said gas turbine exhaust gas flowing from am entry end to am ext end of the heat recovery steam generator mid wherein flu system additionally comprises: a fuel gas saturator (160, 260, 360) having an Inlet for hot saturator water, an Inlet for fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a saturator war heater (162, 262, 362); a flow path for flowing saturator water from said saturator war outlet to said saturator hear (162, 262, 362), said saturator heater being operatively coupled to a heat source In the heat recovery steam generator (132, 232, 332) for heating saturator war conducted thereto, using said heat notice, to produce hot saturator war; a flow path (266, 366) far flowing hat saturator water produced by the saturator heater to the hot saturator water inlet of the fuel gas saturator; a fuel superheater (164, 264, 364) for heating said saturated fuel gas; a flow path for flowing satiated fuel gas from said satiated fuel gas outlet to said fuel superheater (164, 264, 364) for heating said saturated fuel gas, to produce superheated, saturated fuel gas; and a flow — for flowing said superheated, satiated fuel gas to said gas turbine (112,212,312). (FIG. - 1)

Full Text	METHOD AND APPARATUS FOR FUEL GAS MOISTURIZATION AND HEATING BACKGROUND OF THE INVENTION 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. 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 energy (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. 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 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 should be energy losses in the HRSG with the modified bottoming cycle described. 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. 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. 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. The herein described modified bottoming cycle and method is applicable in particular to natural gas fire combined cycle applications. BRIEF DESCRIPTION OF THE DRAWINGS 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: FIGURE 1 is a schematic representation of a conventional three pressure re- heat STAG cycle system; FIGURE 2 is a graph showing hot (gas) and cold (LP-EC) composite temperature with no fuel saturation for the system of FIGURE 1; FIGURE 3 is a schematic representation of a combined cycle power plant in accordance with the invention; FIGURE 4 is a more detailed schematic representation of a three pressure reheat STAG cycle with fuel gas saturation in accordance with one embodiment of the invention; FIGURE 5 is a graph of hot (gas) and cold (LP-EC-1&2 + SAT.HTR) composite temperature with fuel saturation in accordance with the first embodiment of the invention; FIGURE 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 FIGURE 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 A schematic of a conventional three pressure reheat combined cycle power plant with fuel heating 10 is shown in FIGURE 1. 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. 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). 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 tne 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. 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). 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. By way of illustration, and with reference to the cycle 10 shown in FIGURE 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%. FIGURE 2 shows a plot of the heat duty in millions of BTU"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 FIGURE 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. The basic concept of the invention can be understood with reference to the schematic representation of FIGURE 3. For convenience components that correspond to those identified above with reference to FIGURE 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. 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. 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. By way of example, a first preferred implementation of the foregoing concept for a three pressure reheat Combined Cycle Power Plant is shown in FIGURE 4. Again, for convenience, components that correspond to those identified above with reference to FIGURES 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. In the embodiment of FIGURE 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 should be connect energy 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. 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. 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 FIGURE 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. As an illustration, with the gas turbine and ambient conditions identical to those used for the example depicted in FIGURES 1 and 2, the proposed system design shown in FIGURE 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&2 as in the previous example. Fuel gas (100% Methane, CH4) enters the fuel gas saturator at a pressure of 400 psia and a temperature of 80oF. 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 CH4 and 14%v H2O. The saturated fuel gas is subsequently superheated to 365°F in the fuel superheating heat exchanger 264. FIGURE 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&2 and the saturator heater, for the system shown in FIGURE 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 FIGURE 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. 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 FIGURE 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. FIGURE 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 FIGURE 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. The apparatus and method of superheating the saturated fuel gas shown in FIGURE 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. The saturator bottoms pump 376 of FIGURE 6, and the like pumps illustrated in FIGURES 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. As described above with reference to e.g., FIGURE 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 FIGURE 3 could be replaced by the combination of a water input and a fuel/water heat exchanger, as shown in FIGURE 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 FIGURE 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 FIGURE 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. 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. 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. We Claim 1. A combined cycle system comprising a gas turbine (112, 212, 312), a steam turbine (118, 218, 318), and a heat recovery steam generator (132, 232, 332), 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 additionally comprises: a fuel gas saturator (160, 260, 360) having an inlet for hot saturator water, an inlet for fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a saturator water heater (162, 262, 362); a flow path for flowing saturator water from said saturator water outlet to said saturator heater (162, 262, 362), said saturator heater being operatively coupled to a heat source in the heat recovery steam generator (132, 232, 332) for heating saturator water conducted thereto, using said heat source, to produce hot saturator water; a flow path (266, 366) for flowing hot saturator water produced by the saturator heater to the hot saturator water inlet of the fuel gas saturator; a fuel superheater (164, 264, 364) for heating said saturated fuel gas; a flow path for flowing saturated fuel gas from said saturated fuel gas outlet to said fuel superheater (164, 264, 364) for heating said saturated fuel gas, to produce superheated, saturated fuel gas; and a flow path for flowing said superheated, saturated fuel gas to said gas turbine (112, 212, 312). 2. A combined cycle system as claimed in claim 1, wherein said fuel superheater (164, 264, 364) is operatively coupled to a heat source IP-EC in the heat recovery steam generator, for heating said saturated fuel gas using said heat source. 3. A combined cycle system as claimed in claim 2, wherein said saturator heater 262 is operatively coupled to a first portion LP-EC-1 of the heat recovery steam generator 232, said fuel superheater 264 is operatively coupled to a second portion IP-EC of the heat recovery steam generator 232, and wherein said second portion is upstream of said first portion with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator. 4. A combined cycle system as claimed in claim 1, wherein said heat recovery steam generator (232) comprises a low pressure evaporator LP-EV and wherein said heat source is downstream of said low pressure evaporator LP-EV with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator 232. 5. A combined cycle system as claimed in claim 3, wherein said heat recovery steam generator 232 includes a low pressure evaporator LP-EV, said first portion LP-EC-1 of the heat recovery steam generator 232 is downstream of said low pressure evaporator LP-EV with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator 232, and said second portion LP-EC of the recovery steam generator is upstream of said low pressure evaporator LP-EV. 6. A combined cycle system as claimed in claim 1, comprising an input for adding make up water from a make up water source to said saturator water for replacing moisture absorbed by the fuel gas. 7. A combined cycle system as claimed in claim 6, wherein said input for adding make up water comprises a make up water inlet in said fuel gas saturator. 8. A combined cycle system as claimed in claim 6, wherein said heat recovery steam generator comprises at least one feed water transfer pump 242 for pumping feed water therethrough, and said input for adding make up water includes a flow path for directing at least a portion of a feed water output from said feed water transfer pump 242 to said fuel gas saturator 260. 9. A combined cycle system as claimed in claim 1, wherein at least a portion 368 of the water heated by the saturator heater 362 is diverted to the fuel superheater 364 for heating the saturated fuel. 10. A combined cycle system as claimed in claim 9, wherein the heat source for said saturator heater 362 is a different heat source than the heat source for said fuel superheater 364. 11.A combined cycle system as claimed in claim 1, wherein said saturator heater 262 comprises a heat exchanger for heating said saturator water with heat from said gas turbine exhaust, said heat exchanger being disposed in parallel to a low pressure economizer LP-EC-1 structure in said heat recovery steam generator. 12. A reheat cycle configuration for a steam turbine and gas turbine combined cycle system comprising: a steam turbine (118, 218, 318) connected to a load, a condenser (126, 226, 326) for receiving exhaust steam from the steam turbine and condensing said exhaust steam to water; a heat recovery steam generator (132, 212, 332) for receiving water from said condenser (126, 226, 326) and converting said water to steam for return to said steam turbine; at least one gas turbine (112, 212, 312) for supplying heat to said heat recovery steam generator in the form of exhaust gases; a fuel gas saturator assembly (160, 260, 360, 460) for saturating fuel gas with water and heating said fuel gas; said heat recovery steam generator (132, 232, 332) having a first water heater (162, 262, 362) for heating water with heat from said exhaust gases, to define a heat source for water for said fuel gas saturator assembly; and a fuel gas superheater (164, 264, 364, 464) for superheating fuel gas that has been saturated and heated by said fuel gas saturator assembly for supply to said gas turbine. 13. A reheat cycle configuration as claimed in claim 12, wherein said a fuel gas saturator (460) assembly comprises a water inlet for adding water to a fuel gas supply for said gas supply for said gas turbine and a heat exchanger for heating fuel gas saturated with water input at said water inlet; said heat exchanger receiving heated water from said first water heater. 14. A reheat cycle configuration as claimed in claim 12, wherein said fuel gas saturator assembly comprises a fuel gas saturator packed column (160, 260, 360) having an inlet for hot water from said first water heater, an inlet for fuel gas, an outlet for saturated fuel gas, and a water outlet. 15. A reheat cycle configuration as claimed in claim 12, wherein said fuel gas superheater (164, 264, 364, 464) heats said saturated fuel gas with heat derived from said heat recovery steam generator. 16. A reheat cycle configuration as claimed in claim 12, wherein at least a portion of the water heated by the first water heater 362 is diverted 368 to the fuel superheater 364 for superheating the saturated fuel and wherein saturator water output from said fuel superheater 364 is used to heat said fuel gas in said fuel gas saturator assembly 360. 17. A reheat cycle configuration as claimed in claim 16, wherein said heat recovery steam generator 332 comprises a second water heater 370 for heating said water diverted from first water heater 362 for input to said fuel superheater 364. 18. A reheat cycle configuration as claimed in claim 12, wherein said first water heater 262 is disposed in parallel to a low pressure economizer LP-EC-1 structure in said heat recovery steam generator 232. 19. A method for increasing power output and thermodynamic efficiency in a combined cycle system including a gas turbine (112, 212, 312), a steam turbine (118, 218, 318), and a heat recovery steam generator (132, 232, 332), 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: - supplying the fuel gas to a saturator (160, 260, 360, 460) to absorb moisture by direct contact with the hot water in a packed column of the saturator; - heating the demoistured fuel gas via heating of the bottom water at the saturator (160) with gas turbine exhaust gas in a saturator heater (162, 262, 362) disposed in an optimum location relative to the remaining tube-banks of the heat recovery steam generator (132, 232,332); - feeding the saturated fuel gas to a fuel superheater (164, 264, 364, 464); - further heating the saturated fuel gas in the fuel superheater (164) to superheat the fuel gas; - supplying make-up water to the fuel gas saturator (160) to replace the moisture absorbed by the fuel gas; and - feeding the superheated saturated fuel gas to the gas turbine. feeding the saturated fuel gas to a fuel superheater (164, 264, 364, 464); 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. 20. A method as claimed in claim 19, wherein said saturated fuel gas is superheated with heat derived from a heat source IP-EC; 370 in the heat recovery steam generator. 21.A method as claimed in claim 19, wherein said fuel gas is saturated and heated in a fuel gas saturator packed column (160, 260, 360) having an inlet for hot saturator water heated by the heat recovery steam generator, an inlet for fuel gas, an outlet for heated, saturated fuel gas, and a water outlet. 22.A method as claimed in claim 21, wherein at least a portion of said hot saturator water is diverted 368 to said fuel superheater 364 before being fed to said saturator 360. 23. A method as claimed in claim 21, wherein the saturator water is heated in a heat exchanger (162, 262, 362) disposed downstream of a low pressure evaporator LP-EV with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator. WE CLAIM 1. 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 having an inlet for hot saturator water, an inlet for fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a saturator water heater; a flow path for flowing saturator water from said saturator water outlet to said saturator heater, said saturator heater being operatively coupled to a heat source in the heat recovery steam generator for heating saturator water conducted thereto, using said heat source, to produce hot saturator water; a flow path for flowing hot saturator water produced by the saturator heater to the hot saturator water inlet of the fuel gas saturator; a fuel superheater for heating said saturated fuel gas; a flow path for flowing saturated fuel gas from said saturated fuel gas outlet to said fuel superheater for heating said saturated fuel gas, to produce superheated, saturated fuel gas; and a flow path for flowing said superheated, saturated fuel gas to said gas turbine. 2. A combined cycle system according to claim 1, wherein said fuel superheater is operatively coupled to a heat source in the heat recovery steam generator, for heating said saturated fuel gas using said heat source. 3. A combined cycle system according to claim 2, wherein said saturator heater is operatively coupled to a first portion of the heat recovery steam generator, said fuel superheater is operatively coupled to a second portion of the heat recovery steam generator, and wherein said second portion is upstream of said first portion with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator. 4. A combined cycle system according to claim 1, wherein said heat recovery steam generator includes a low pressure evaporator and wherein said heat source is downstream of said low pressure evaporator with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator. 5. A combined cycle systern according to claim 3, wherein said heat recovery steam generator includes a low pressure evaporator, said first portion of the heat recovery steam generator is downstream of said low pressure evaporator with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator, and said second portion of the heat recovery steam generator is upstream of said low pressure evaporator. 6. A combined cycle system according to claim 1, further comprising an input for adding make up water from a make up water source to said saturator water for replacing moisture absorbed by the fuel gas. 7. A combined cycle system according to claim 6, wherein said input for adding make up water comprises a make up water inlet in said fuel gas saturator. 8. A combined cycle system according to claim 6, wherein said heat recovery steam generator includes at least one feed water transfer pump for pumping feed water therethrough, and said input for adding make up water includes a flow path for directing at least a portion of a feed water output from said feed water transfer pump to said fuel gas saturator. 9. A combined cycle system according to claim 1, wherein at least a portion of the water heated by the saturator heater is diverted to the fuel superheater for heating the saturated fuel. 10. A combined cycle system according to claim 9, wherein the heat source for said saturator heater is a different heat source than the heat source for said fuel superheater. 11. A combined cycle system according to claim 1, wherein said saturator heater comprises a heat exchanger for heating said saturator water with heat from said gas turbine exhaust, said heat exchanger being disposed in parallel to a low pressure economizer structure in said heat recovery steam generator. 12. A reheat cycle configuration for a steam turbine and gas turbine combined cycle system comprising: a steam turbine connected to a load, a condenser for receiving exhaust steam from the steam turbine and condensing said exhaust steam to water; a heat recovery steam generator for receiving water from said condenser and converting said water to steam for return to said steam turbine; at least one gas turbine for supplying heat to said heat recovery steam generator in the form of exhaust gases; a fuel gas saturator assembly for saturating fuel gas with water and heating said fuel gas; said heat recovery steam generator including a first water heater for heating water with heat from said exhaust gases, to define a heat source for said fuel gas saturator assembly; and a fuel gas superheater for superheating fuel gas that has been saturated and heated by said fuel gas saturator assembly for supply to said gas turbine. 13. A reheat cycle configuration according to claim 12, wherein said a fuel gas saturator assembly comprises a water inlet for adding water to a fuel gas supply for said gas turbine and a heat exchanger for heating fuel gas saturated with water input at said water inlet; said heat exchanger receiving heated water from said first water heater. 14. A reheat cycle configuration according to claim 12, wherein said fuel gas saturator assembly comprises a fuel gas saturator packed column having an inlet for hot water from said first water heater, an inlet for fuel gas, an outlet for saturated fuel gas, and a water outlet. 15. A reheat cycle configuration according to claim 12, wherein said fuel gas superheater heats said saturated fuel gas with heat derived from said heat recovery steam generator. 16. A reheat cycle configuration according to claim 12, wherein at least a portion of the water heated by the first water heater is diverted to the fuel superheater for superheating the saturated fuel and wherein saturator water output from said fuel superheater is used to heat said fuel gas in said fuel gas saturator assembly. 17. A reheat cycle configuration according to claim 16, wherein said heat recovery steam generator includes a second water heater for heating said water diverted from said first water heater for input to said fuel superheater. 18. A reheat cycle configuration according to claim 12, wherein said first water heater is disposed in parallel to a low pressure economizer structure in said heat recovery steam generator. 19. 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, said fuel gas being heated with heat 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. 20. A method as in claim 19, wherein said saturated fuel gas is superheated with heat derived from a heat source in the heat recovery steam generator. 21. A method as in claim 19, wherein said fuel gas is saturated and heated in a fuel gas saturator packed column having an inlet for hot saturator water heated by the heat recovery steam generator, an inlet for fuel gas, an outlet for heated, saturated fuel gas, and a water outlet. 22. A method as in claim 21, wherein at least a portion of said hot saturator water is diverted to said fuel superheater before being fed to said saturator. 23. A method as in claim 21, wherein the saturator water is heated in a heat exchanger disposed downstream of a low pressure evaporator with respect to a flow direction of said gas turbine exhaust through the heat recovery steam generator. The invention relates to a combined cycle system comprising a gas turbine (112, 212, 312), a steam turbine (118, 218, 318), and a heat recovery steam generator (132, 232, 332), 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 additionally comprises: a fuel gas saturator (160, 260, 360) having an inlet for hot saturator water, an inlet for fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a saturator water heater (162, 262, 362); a flow path for flowing saturator water from said saturator water outlet to said saturator heater (162, 262, 362), said saturator heater being operattvely coupled to a heat source in the heat recovery steam generator (132, 232, 332) for heating saturator water conducted thereto, using said heat source, to produce hot saturator water; a flow path (266, 366) for flowing hot saturator water produced by the saturator heater to the hot saturator water inlet of the fuel gas saturator; a fuel superheater (164, 264, 364) for heating said saturated fuel gas; a flow path for flowing saturated fuel gas from said saturated fuel gas outlet to said fuel superheater (164, 264, 364) for heating said saturated fuel gas, to produce superheated, saturated fuel gas; and a flow path for flowing said superheated, saturated fuel gas to said gas turbine (112, 212, 312).

Full Text

METHOD AND APPARATUS FOR FUEL GAS
MOISTURIZATION AND HEATING
BACKGROUND OF THE INVENTION
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.
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 energy
(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.
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
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 should be energy losses in the HRSG with the modified bottoming
cycle described.
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.
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.
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.
The herein described modified bottoming cycle and method is applicable in
particular to natural gas fire combined cycle applications.
BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIGURE 1 is a schematic representation of a conventional three pressure re-
heat STAG cycle system;
FIGURE 2 is a graph showing hot (gas) and cold (LP-EC) composite
temperature with no fuel saturation for the system of FIGURE 1;
FIGURE 3 is a schematic representation of a combined cycle power plant in
accordance with the invention;
FIGURE 4 is a more detailed schematic representation of a three pressure reheat
STAG cycle with fuel gas saturation in accordance with one embodiment
of the invention;
FIGURE 5 is a graph of hot (gas) and cold (LP-EC-1&2 + SAT.HTR) composite
temperature with fuel saturation in accordance with the first embodiment of
the invention;
FIGURE 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
FIGURE 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
A schematic of a conventional three pressure reheat combined cycle power
plant with fuel heating 10 is shown in FIGURE 1.
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.
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).
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 tne 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.
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).
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.
By way of illustration, and with reference to the cycle 10 shown in FIGURE 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%.
FIGURE 2 shows a plot of the heat duty in millions of BTU"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
FIGURE 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.
The basic concept of the invention can be understood with reference to the
schematic representation of FIGURE 3. For convenience components that
correspond to those identified above with reference to FIGURE 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.
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.
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.
By way of example, a first preferred implementation of the foregoing concept
for a three pressure reheat Combined Cycle Power Plant is shown in FIGURE
4. Again, for convenience, components that correspond to those identified
above with reference to FIGURES 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.
In the embodiment of FIGURE 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 should be connect energy
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.
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.
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
FIGURE 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.
As an illustration, with the gas turbine and ambient conditions identical to
those used for the example depicted in FIGURES 1 and 2, the proposed
system design shown in FIGURE 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&2 as in the previous example.
Fuel gas (100% Methane, CH4) enters the fuel gas saturator at a pressure of
400 psia and a temperature of 80oF. 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 CH4 and 14%v H2O. The saturated
fuel gas is subsequently superheated to 365°F in the fuel superheating heat
exchanger 264.
FIGURE 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&2 and the saturator heater, for the system shown in FIGURE
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 FIGURE 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.
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 FIGURE 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.
FIGURE 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
FIGURE 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.
The apparatus and method of superheating the saturated fuel gas shown in
FIGURE 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.
The saturator bottoms pump 376 of FIGURE 6, and the like pumps illustrated
in FIGURES 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.
As described above with reference to e.g., FIGURE 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
FIGURE 3 could be replaced by the combination of a water input and a
fuel/water heat exchanger, as shown in FIGURE 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 FIGURE 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 FIGURE 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.
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.
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.
We Claim
1. A combined cycle system comprising a gas turbine (112, 212, 312), a
steam turbine (118, 218, 318), and a heat recovery steam generator
(132, 232, 332), 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 additionally
comprises:
a fuel gas saturator (160, 260, 360) having an inlet for hot
saturator water, an inlet for fuel gas, an outlet for saturated fuel gas, and
a saturator water outlet;
a saturator water heater (162, 262, 362);
a flow path for flowing saturator water from said saturator water
outlet to said saturator heater (162, 262, 362), said saturator heater being
operatively coupled to a heat source in the heat recovery steam generator
(132, 232, 332) for heating saturator water conducted thereto, using said
heat source, to produce hot saturator water;
a flow path (266, 366) for flowing hot saturator water produced by
the saturator heater to the hot saturator water inlet of the fuel gas
saturator;
a fuel superheater (164, 264, 364) for heating said saturated fuel
gas;
a flow path for flowing saturated fuel gas from said saturated fuel
gas outlet to said fuel superheater (164, 264, 364) for heating said
saturated fuel gas, to produce superheated, saturated fuel gas; and a flow
path for flowing said superheated, saturated fuel gas to said gas turbine
(112, 212, 312).
2. A combined cycle system as claimed in claim 1, wherein said fuel
superheater (164, 264, 364) is operatively coupled to a heat source IP-EC
in the heat recovery steam generator, for heating said saturated fuel gas
using said heat source.
3. A combined cycle system as claimed in claim 2, wherein said saturator
heater 262 is operatively coupled to a first portion LP-EC-1 of the heat
recovery steam generator 232, said fuel superheater 264 is operatively
coupled to a second portion IP-EC of the heat recovery steam generator
232, and wherein said second portion is upstream of said first portion with
respect to a flow direction of said gas turbine exhaust through the heat
recovery steam generator.
4. A combined cycle system as claimed in claim 1, wherein said heat
recovery steam generator (232) comprises a low pressure evaporator
LP-EV and wherein said heat source is downstream of said low pressure
evaporator LP-EV with respect to a flow direction of said gas turbine
exhaust through the heat recovery steam generator 232.
5. A combined cycle system as claimed in claim 3, wherein said heat
recovery steam generator 232 includes a low pressure evaporator LP-EV,
said first portion LP-EC-1 of the heat recovery steam generator 232 is
downstream of said low pressure evaporator LP-EV with respect to a flow
direction of said gas turbine exhaust through the heat recovery steam
generator 232, and said second portion LP-EC of the recovery steam
generator is upstream of said low pressure evaporator LP-EV.
6. A combined cycle system as claimed in claim 1, comprising an input for
adding make up water from a make up water source to said saturator
water for replacing moisture absorbed by the fuel gas.
7. A combined cycle system as claimed in claim 6, wherein said input for
adding make up water comprises a make up water inlet in said fuel gas
saturator.
8. A combined cycle system as claimed in claim 6, wherein said heat
recovery steam generator comprises at least one feed water transfer
pump 242 for pumping feed water therethrough, and said input for adding
make up water includes a flow path for directing at least a portion of a
feed water output from said feed water transfer pump 242 to said fuel gas
saturator 260.
9. A combined cycle system as claimed in claim 1, wherein at least a portion
368 of the water heated by the saturator heater 362 is diverted to the fuel
superheater 364 for heating the saturated fuel.
10. A combined cycle system as claimed in claim 9, wherein the heat source
for said saturator heater 362 is a different heat source than the heat
source for said fuel superheater 364.
11.A combined cycle system as claimed in claim 1, wherein said saturator
heater 262 comprises a heat exchanger for heating said saturator water
with heat from said gas turbine exhaust, said heat exchanger being
disposed in parallel to a low pressure economizer LP-EC-1 structure in said
heat recovery steam generator.
12. A reheat cycle configuration for a steam turbine and gas turbine combined
cycle system comprising:
a steam turbine (118, 218, 318) connected to a load,
a condenser (126, 226, 326) for receiving exhaust steam from the
steam turbine and condensing said exhaust steam to water;
a heat recovery steam generator (132, 212, 332) for receiving
water from said condenser (126, 226, 326) and converting said water to
steam for return to said steam turbine;
at least one gas turbine (112, 212, 312) for supplying heat to said
heat recovery steam generator in the form of exhaust gases;
a fuel gas saturator assembly (160, 260, 360, 460) for saturating
fuel gas with water and heating said fuel gas;
said heat recovery steam generator (132, 232, 332) having a first
water heater (162, 262, 362) for heating water with heat from said
exhaust gases, to define a heat source for water for said fuel gas
saturator assembly; and
a fuel gas superheater (164, 264, 364, 464) for superheating fuel
gas that has been saturated and heated by said fuel gas saturator
assembly for supply to said gas turbine.
13. A reheat cycle configuration as claimed in claim 12, wherein said a fuel
gas saturator (460) assembly comprises a water inlet for adding water to
a fuel gas supply for said gas supply for said gas turbine and a heat
exchanger for heating fuel gas saturated with water input at said water
inlet; said heat exchanger receiving heated water from said first water
heater.
14. A reheat cycle configuration as claimed in claim 12, wherein said fuel gas
saturator assembly comprises a fuel gas saturator packed column (160,
260, 360) having an inlet for hot water from said first water heater, an
inlet for fuel gas, an outlet for saturated fuel gas, and a water outlet.
15. A reheat cycle configuration as claimed in claim 12, wherein said fuel gas
superheater (164, 264, 364, 464) heats said saturated fuel gas with heat
derived from said heat recovery steam generator.
16. A reheat cycle configuration as claimed in claim 12, wherein at least a
portion of the water heated by the first water heater 362 is diverted 368
to the fuel superheater 364 for superheating the saturated fuel and
wherein saturator water output from said fuel superheater 364 is used to
heat said fuel gas in said fuel gas saturator assembly 360.
17. A reheat cycle configuration as claimed in claim 16, wherein said heat
recovery steam generator 332 comprises a second water heater 370 for
heating said water diverted from first water heater 362 for input to said
fuel superheater 364.
18. A reheat cycle configuration as claimed in claim 12, wherein said first
water heater 262 is disposed in parallel to a low pressure economizer
LP-EC-1 structure in said heat recovery steam generator 232.
19. A method for increasing power output and thermodynamic efficiency in a
combined cycle system including a gas turbine (112, 212, 312), a steam
turbine (118, 218, 318), and a heat recovery steam generator (132, 232,
332), 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:
- supplying the fuel gas to a saturator (160, 260, 360, 460) to absorb
moisture by direct contact with the hot water in a packed column of
the saturator;
- heating the demoistured fuel gas via heating of the bottom water
at the saturator (160) with gas turbine exhaust gas in a saturator
heater (162, 262, 362) disposed in an optimum location relative to
the remaining tube-banks of the heat recovery steam generator
(132, 232,332);
- feeding the saturated fuel gas to a fuel superheater (164, 264, 364,
464);
- further heating the saturated fuel gas in the fuel superheater (164)
to superheat the fuel gas;
- supplying make-up water to the fuel gas saturator (160) to replace
the moisture absorbed by the fuel gas; and
- feeding the superheated saturated fuel gas to the gas turbine.
feeding the saturated fuel gas to a fuel superheater (164, 264, 364,
464);
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.
20. A method as claimed in claim 19, wherein said saturated fuel gas is
superheated with heat derived from a heat source IP-EC; 370 in the heat
recovery steam generator.
21.A method as claimed in claim 19, wherein said fuel gas is saturated and
heated in a fuel gas saturator packed column (160, 260, 360) having an
inlet for hot saturator water heated by the heat recovery steam generator,
an inlet for fuel gas, an outlet for heated, saturated fuel gas, and a water
outlet.
22.A method as claimed in claim 21, wherein at least a portion of said hot
saturator water is diverted 368 to said fuel superheater 364 before being
fed to said saturator 360.
23. A method as claimed in claim 21, wherein the saturator water is heated in
a heat exchanger (162, 262, 362) disposed downstream of a low pressure
evaporator LP-EV with respect to a flow direction of said gas turbine
exhaust through the heat recovery steam generator.
WE CLAIM
1. 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 having an inlet for hot saturator water, an inlet for fuel
gas, an outlet for saturated fuel gas, and a saturator water outlet;
a saturator water heater;
a flow path for flowing saturator water from said saturator water outlet to
said saturator heater, said saturator heater being operatively coupled to a heat
source in the heat recovery steam generator for heating saturator water
conducted thereto, using said heat source, to produce hot saturator water;
a flow path for flowing hot saturator water produced by the saturator heater
to the hot saturator water inlet of the fuel gas saturator;
a fuel superheater for heating said saturated fuel gas;
a flow path for flowing saturated fuel gas from said saturated fuel gas outlet
to said fuel superheater for heating said saturated fuel gas, to produce
superheated, saturated fuel gas; and
a flow path for flowing said superheated, saturated fuel gas to said gas
turbine.
2. A combined cycle system according to claim 1, wherein
said fuel superheater is operatively coupled to a heat source in the heat
recovery steam generator, for heating said saturated fuel gas using said heat
source.
3. A combined cycle system according to claim 2, wherein said saturator
heater is operatively coupled to a first portion of the heat recovery steam
generator, said fuel superheater is operatively coupled to a second portion of
the heat recovery steam generator, and wherein said second portion is
upstream of said first portion with respect to a flow direction of said gas
turbine exhaust through the heat recovery steam generator.
4. A combined cycle system according to claim 1, wherein said heat
recovery steam generator includes a low pressure evaporator and wherein
said heat source is downstream of said low pressure evaporator with respect
to a flow direction of said gas turbine exhaust through the heat recovery
steam generator.
5. A combined cycle systern according to claim 3, wherein said heat
recovery steam generator includes a low pressure evaporator, said first
portion of the heat recovery steam generator is downstream of said low
pressure evaporator with respect to a flow direction of said gas turbine
exhaust through the heat recovery steam generator, and said second portion
of the heat recovery steam generator is upstream of said low pressure
evaporator.
6. A combined cycle system according to claim 1, further comprising an
input for adding make up water from a make up water source to said
saturator water for replacing moisture absorbed by the fuel gas.
7. A combined cycle system according to claim 6, wherein said input for
adding make up water comprises a make up water inlet in said fuel gas
saturator.
8. A combined cycle system according to claim 6, wherein said heat
recovery steam generator includes at least one feed water transfer pump for
pumping feed water therethrough, and said input for adding make up water
includes a flow path for directing at least a portion of a feed water output
from said feed water transfer pump to said fuel gas saturator.
9. A combined cycle system according to claim 1, wherein at least a
portion of the water heated by the saturator heater is diverted to the fuel
superheater for heating the saturated fuel.
10. A combined cycle system according to claim 9, wherein the heat source
for said saturator heater is a different heat source than the heat source for said
fuel superheater.
11. A combined cycle system according to claim 1, wherein said saturator
heater comprises a heat exchanger for heating said saturator water with heat
from said gas turbine exhaust, said heat exchanger being disposed in parallel
to a low pressure economizer structure in said heat recovery steam generator.
12. A reheat cycle configuration for a steam turbine and gas turbine
combined cycle system comprising:
a steam turbine connected to a load,
a condenser for receiving exhaust steam from the steam turbine and
condensing said exhaust steam to water;
a heat recovery steam generator for receiving water from said condenser and
converting said water to steam for return to said steam turbine;
at least one gas turbine for supplying heat to said heat recovery steam
generator in the form of exhaust gases;
a fuel gas saturator assembly for saturating fuel gas with water and heating
said fuel gas;
said heat recovery steam generator including a first water heater for heating
water with heat from said exhaust gases, to define a heat source for said fuel
gas saturator assembly; and
a fuel gas superheater for superheating fuel gas that has been saturated and
heated by said fuel gas saturator assembly for supply to said gas turbine.
13. A reheat cycle configuration according to claim 12, wherein said a fuel
gas saturator assembly comprises a water inlet for adding water to a fuel gas
supply for said gas turbine and a heat exchanger for heating fuel gas
saturated with water input at said water inlet; said heat exchanger receiving
heated water from said first water heater.
14. A reheat cycle configuration according to claim 12, wherein said fuel
gas saturator assembly comprises a fuel gas saturator packed column having
an inlet for hot water from said first water heater, an inlet for fuel gas, an
outlet for saturated fuel gas, and a water outlet.
15. A reheat cycle configuration according to claim 12, wherein said fuel
gas superheater heats said saturated fuel gas with heat derived from said heat
recovery steam generator.
16. A reheat cycle configuration according to claim 12, wherein at least a
portion of the water heated by the first water heater is diverted to the fuel
superheater for superheating the saturated fuel and wherein saturator water
output from said fuel superheater is used to heat said fuel gas in said fuel gas
saturator assembly.
17. A reheat cycle configuration according to claim 16, wherein said heat
recovery steam generator includes a second water heater for heating said
water diverted from said first water heater for input to said fuel superheater.
18. A reheat cycle configuration according to claim 12, wherein said first
water heater is disposed in parallel to a low pressure economizer structure in
said heat recovery steam generator.
19. 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,
said fuel gas being heated with heat 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.
20. A method as in claim 19, wherein said saturated fuel gas is
superheated with heat derived from a heat source in the heat recovery steam
generator.
21. A method as in claim 19, wherein said fuel gas is saturated and heated
in a fuel gas saturator packed column having an inlet for hot saturator water
heated by the heat recovery steam generator, an inlet for fuel gas, an outlet for
heated, saturated fuel gas, and a water outlet.
22. A method as in claim 21, wherein at least a portion of said hot saturator
water is diverted to said fuel superheater before being fed to said saturator.
23. A method as in claim 21, wherein the saturator water is heated in a
heat exchanger disposed downstream of a low pressure evaporator with
respect to a flow direction of said gas turbine exhaust through the heat
recovery steam generator.
The invention relates to a combined cycle system comprising a gas turbine (112,
212, 312), a steam turbine (118, 218, 318), and a heat recovery steam
generator (132, 232, 332), 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 additionally comprises: a fuel
gas saturator (160, 260, 360) having an inlet for hot saturator water, an inlet for
fuel gas, an outlet for saturated fuel gas, and a saturator water outlet; a
saturator water heater (162, 262, 362); a flow path for flowing saturator water
from said saturator water outlet to said saturator heater (162, 262, 362), said
saturator heater being operattvely coupled to a heat source in the heat recovery
steam generator (132, 232, 332) for heating saturator water conducted thereto,
using said heat source, to produce hot saturator water; a flow path (266, 366)
for flowing hot saturator water produced by the saturator heater to the hot
saturator water inlet of the fuel gas saturator; a fuel superheater (164, 264, 364)
for heating said saturated fuel gas; a flow path for flowing saturated fuel gas
from said saturated fuel gas outlet to said fuel superheater (164, 264, 364) for
heating said saturated fuel gas, to produce superheated, saturated fuel gas; and
a flow path for flowing said superheated, saturated fuel gas to said gas turbine
(112, 212, 312).

Documents:

331-CAL-2000-FORM-27.pdf

331-cal-2000-granted-abstract.pdf

331-cal-2000-granted-assignment.pdf

331-cal-2000-granted-claims.pdf

331-cal-2000-granted-correspondence.pdf

331-cal-2000-granted-description (complete).pdf

331-cal-2000-granted-drawings.pdf

331-cal-2000-granted-form 1.pdf

331-cal-2000-granted-form 18.pdf

331-cal-2000-granted-form 2.pdf

331-cal-2000-granted-form 3.pdf

331-cal-2000-granted-form 5.pdf

331-cal-2000-granted-letter patent.pdf

331-cal-2000-granted-others.pdf

331-cal-2000-granted-reply to examination report.pdf

331-cal-2000-granted-specification.pdf

331-cal-2000-granted-translated copy of priority document.pdf

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Patent Number

212594

Indian Patent Application Number

331/CAL/2000

PG Journal Number

49/2007

Publication Date

07-Dec-2007

Grant Date

04-Dec-2007

Date of Filing

07-Jun-2000

Name of Patentee

GENERAL ELECTRIC COMPANY.

Applicant Address

ONE RIVER ROAD SCHENECTADY, NEW YORK M12345 USA.

Inventors:

#	Inventor's Name	Inventor's Address
1	RANASINGHE JATILA (NMN)	841 RED OAK DRIVE NISKAYUNA, NEW YORK 12309 USA.
2	SMITH RAUB WARFIELD	12 HUCKLEBBERY LANE BALLSTON LAAKE NEW YOURK 12309. USA.

PCT International Classification Number

F 0 2C 6/18

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	09/340,510	1999-07-01	U.S.A.