Method to integrate regenerative rankine cycle into combined cycle applications using an integrated heat recovery steam generator

Abstract

A system is disclosed that incorporates a regenerative Rankine cycle integrated with a conventional combined cycle. This novelty requires minimal changes to a conventionally designed Heat Recovery Steam Generator and uses an added duct firing array(s) to boost the enthalpy of combustion turbine exhaust. The higher enthalpy in said exhaust is then extracted with the co-shared heating elements of the conventionally designed combined cycle to produce high pressure main and reheat steam. In practice, the condensate stream from the condenser is bifurcated such that a separate and dedicated feedwater flow, used for regeneration, is directed to feedwater heaters and then converted to steam with the provided additional enthalpy at the same pressure and temperature as the main steam in the conventional combined cycle. The fractional amount of condensate that is not sent through the feedwater heaters is directed to the HRSG to be heated in conventional fashion.

Claims

1. A method for generating electric power that incorporates the use of a regenerative Rankine cycle with a combined cycle, the method comprising the steps of: Bifurcating the condensate from a condenser into two or more separate condensate feed streams whereby the condensate in at least one condensate feed stream is pressurized to feedwater and sent directly to a heat recovery steam generator and the condensate in at least one condensate feed stream is pressurized to feedwater and sent to one or more common heating elements that is co-shared with the first stream first being preheated by a one or more feedwater heaters utilizing extraction steam from an extraction turbine; generating steam in a parallel cycle using a regenerative Rankine cycle and co-mixing said steam with the steam produced in a traditional non-regenerative combined cycle and transferring the steam to an extraction steam turbine having one or more extraction ports; converting the steam into electricity through the use of an extraction steam turbine and generator and extracting some of the steam for heating feedwater.

2. The method of claim 1, wherein additional heat enthalpy is supplied to the common heating elements and used to boost the temperature and enthalpy of the combustion turbine exhaust flow such that there is additional enthalpy in said combustion turbine exhaust flow to generate steam for use in a regenerative Rankine cycle.

3. The method of claim 1, wherein the separately fired duct burner is placed inside the combustion turbine exhaust ducting and before the heat recovery steam generator.

4. The method of claim 1, wherein the commonly fired heating elements of the heat recovery steam generator may be configured in a once through or drum design.

5. The method of claim 1, wherein the method may be utilized in conjunction with a single pressure or multiple pressure heat recovery steam generator.

6. The method of claim 2, wherein some or all of the additional heat enthalpy supplied to the co-shared heating element is generated from combusting fuel in at least one duct burner.

7. The method of claim 2, wherein the additional heat enthalpy supplied to the co-shared heating element may be generated from fossil fuel or non-fossil fuel or a combination of both.

8. The method of claim 2, wherein substantially all of the additional heat enthalpy supplied to the co-shared heating element is utilized to generate steam.

9. The method of claim 2, wherein some or all of the additional heat enthalpy supplied to the co-shared heating element is supplied through the use of one or more duct burners placed in the combustion turbine exhaust ducting and before the commonly fired heating element.

10. The method of claim 2, wherein the low temperature enthalpy supplied by the duct burner is used for steam dearation or low pressure steam for power production.

11. The method of claim 2 where additional enthalpy is added by conventional duct firing and there is a proportional increase in the low temperature feedwater flow to provide additional capacity and energy through a straight through non-regenerative Rankine cycle in the conventional manner of duct firing.

12. The method of claim 2 where a combination of increase of the low temperature feedwater flow in conjunction with an increase of the high temperature regenerative feedwater is used in parallel to optimize both capacity and energy output.

13. A method to generate reheated steam utilizing a co-shared heating element in a regenerative Rankine cycle used in conjunction with a combined cycle, the method comprising of: Partially expanded steam from the high pressure turbine exhaust is sent to a co-shared heating element to boost said steam to a temperature that is compatible with the hot reheat steam produced by the heat recovery steam generator for mixing with total mix directed to the intermediate pressure turbine inlet; a duct burner to provide for the necessary enthalpy into the separately fired heating element to reheat the steam from the high pressure turbine exhaust is located upstream of the heat recovery steam generator.

14. A method for generating electric power that incorporates the use of a regenerative Rankine cycle with a combined cycle, the method comprising the steps of: Bifurcating the condensate from a condenser into two or more separate condensate feed streams whereby the condensate in at least one condensate feed stream is pressurized to feedwater and sent directly to a heat recovery steam generator and the condensate in at least one condensate feed stream is pressurized to feedwater and sent to at least one co-shared heating element first being preheated by a one or more feedwater heaters utilizing cold reheat steam from a non-extraction turbine; generating steam in at least one co-shared heating element and transferring the steam to a non-extraction steam turbine; converting the steam into electricity through the use of a non-extraction steam turbine and generator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Drawing 1 is a sketch that diagrammatically shows the proposed concept. The drawing shows an inner feedwater loop, shown in dotted lines, employing feedwater heaters supplying additional feedwater flow in a designated flow path such that common heating elements and added duct firing results in a separate regenerative Rankine cycle. The duct firing shown is an added array and is not to be confused with conventional duct firing used to increase the steaming capacity of the HRSG. The novelty's proposed additional duct firing does not increase the feedwater flow rate from the condenser directly to the HRSG to produce more steam; this novelty proposes a separate loop method allows the feedwater to be preheated in a separate loop using extraction flows from the steam turbine with additional enthalpy added for steam production using a dedicated duct burner array. The novelty's added duct firing precludes the installation of a conventional duct firing array but does not impact or impede the operation of using the new array for conventional duct firing and can be used in tandem with the proposed novelty.

[0036] Drawing 2 is similar to Drawing 1 but shows the additional embodiment of reheating that would be available, if deployed, under this novelty. In this scheme, the cold reheat steam is bifurcated with the majority of steam flowing to the common and co-shared heating elements and the remaining steam flow used for regenerative heating in the first point heater.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The numbers and data shown are general approximations only in order to more fully delineate the principles of the proposed novelty and the overall flow schematic should not be construed as a final thermodynamic analysis. Referring to Drawing 1, if we assume a closed operating Rankine cycle, condenser 1 condenses the steam flow 18 from the low pressure steam turbine 12. This novelty separates that amount of condensate into two streams 2 and 3 where stream 2 is the additional mass flow rate used for regeneration and absorbs the heat from steam extractions from appropriate ports in the extraction turbine. In practice, the fraction dedicated to the regenerative portion of the condensate flow 2 from the condenser is, typically, about 40-45% of total condensate flow. However, these values can be adjusted for cycle optimization. The pre-heated feedwater 7 is shown in Drawing 1 as a dedicated feed to the co-shared heating elements 8. The amount of condensate 3 used for non-regenerative cycle operation is fed directly to the HRSG 9 for feedwater heating, evaporating and superheating and then directed to the High Pressure (HP) steam turbine 11. Condensate 2 flows through the regenerative heater #3 4, then through heater #2 5 and then completes its pre-heating through heater #1 6. Typically, in traditional Rankine regenerative reheat cycles that are non-critical, the first point heater (heater #1) 6 receives steam extraction from the cold reheat line; this embodiment of reheat is described further in Drawing 2. The herein embodiment description assumes that the first point heater 6 receives its extraction flow from the cold reheat line from the HP turbine 11. For simplicity, boiler feed pumps and other associated flow lines, such as feedwater drip lines, have not been shown.

[0038] The amount of reheating, and the number of feedwater heaters, is an economic evaluation whereby the cost of preheating is evaluated against the gain in efficiency; typically large coal plants use 7 or 8 heaters; if a new facility is used, an economic evaluation will determine the number of feedwater heaters used. Drawing 1 shows only three for simplicity. While this novelty permits heating close to the saturation point, it is assumed here for illustrative purposes that the pre-heated feedwater 7 is heated to approximately 500 F. Heating elements 8 provide sensible heating, evaporation and superheating required for production of main steam.

[0039] While the exhaust of the combustion turbine 13 is shown as 1160 F, the additional duct firing 14 adds heat such that the overall gas temperature is now 1540 F. The amount of heat required to evaporate and superheat the main steam and to reheat the steam from the feedwater 7 would then bring down the combustion turbine's exhaust gas temperature as the gas flow travels from the high temperature heating elements to the lower heating elements (feedwater heating and economizers). Since there would be excess heat in the lower temperature end of the HRSG due to the duct firing and heating the 500 F preheated feedwater, excess enthalpy is used for lower steam pressure generation and to preheat the steam used for dearation. In this manner, any increase in the stack temperature, as compared to the stack temperature when no regenerative steam is being produced and there is no duct firing, can be held to a minimum

[0040] Referring again to Drawing 1, the feedwater 7 is heated in the co-shared heating elements used for production of steam and reheat steam in the non-regenerative combined cycle, the feedwater stream which is now superheated steam 10 is directed to the inlet of the HP steam turbine 11 where it is mixed with the main steam produced by the CT exhaust flow in the HRSG 9 at the same pressure and enthalpy for expansion in the HP turbine 11. A separate line 10, as shown in Drawing 1 may be necessary depending on the design of the existing turbine; otherwise, the steam is fed to the turbine in a common header. It is noted that this example depicts a three pressure combined cycle and that the low pressure steam 15, and the intermediate pressure steam 16 are directed to the IP/LP steam turbine 12 as appropriate. The main steam 17, the intermediate pressure steam 16 and the low pressure steam 15 have all been generated with minimal changes to the HRSG 9. The primary design parameter proposed in this novelty is that the heating of the separated and designated regenerative feedwater 7 is performed by integrating with the heating elements required for the combined cycle although larger carrying capacity is required. These co-shared heating elements 8 and the added duct firing 14 in the duct upstream of the conventionally designed HRSG 9 where the said HRSG design is, essentially, unaltered and the stack temperature 23 remains, essentially, unchanged.

[0041] Referring to Drawing 2, the addition of a reheat section is shown in conjunction with the production of main steam produced by the previously described regenerative Rankine cycle in Drawing 1. The cold reheat working fluid 24 is a separate loop used to reheat that portion of the main steam that has been generated through a regenerative Rankine cycle. It is noted that the main steam produced by the HRSG using solely the waste heat of the CT 13 is reheated through the HRSG operation only. Drawing 2 is the same as Drawing 1 except for the addition of the specific equipment and lines required for reheating of the main steam. In Drawing 2, we follow the assumption that most non-critical Rankine cycles take the first point heater steam extraction 22 from the cold reheat line 19. The remaining fraction of the cold reheat 24 is then directed to a co-shared reheater 21 used by the traditionally designed combined cycle. The reheated steam 20 is directed to the intermediate steam line 16 and mixed with the combined cycle's production of intermediate steam and directed to the IP/LP steam turbine 12. Although a separate line is shown, the delivery of the hot reheat may also use a co-shared header, drum and other heating elements. The reheating process does not impact the stack temperature 23.