JP2015525846A - Triple expansion waste heat recovery system and method - Google Patents

Abstract

A waste heat recovery system is provided. The waste heat recovery system includes a Rankine cycle system for circulating a working fluid. The Rankine cycle system includes at least one first waste heat recovery boiler configured to transfer heat from a heat source to a working fluid. The Rankine cycle system also includes a first expander configured to receive heated working fluid from at least one first waste heat recovery boiler. The Rankine cycle further includes a second expander coupled to the at least one generator and a third expander. The waste heat recovery system also includes a condenser configured to receive working fluid at a low pressure from the first expander, the second expander, and the third expander for cooling, and cooling of the working fluid from the condenser. And a pump connected to the condenser for receiving the condensed stream. [Selection] Figure 1

Description

  This application relates generally to power generation, and more particularly to a system and method for recovering waste heat from a plurality of heat sources having different temperatures for the purpose of generating electricity.

  Many industrial power requirements come from power generation systems that provide electrical or mechanical power with minimal environmental impact and can be easily integrated into existing power grids or quickly installed as stand-alone units. You can make a profit. Combustion engines such as gas turbines or large reciprocating engines are suitable for power generation in industrial applications, but rely on increasingly costly fuels and generate exhaust and waste heat. One way to generate electricity from combustion engine waste heat without increasing waste heat emissions and without the need for additional fuel is to apply bottoming cycles. The bottoming cycle uses waste heat from a heat source such as an engine and converts the heat energy into electricity. The Rankine cycle is often applied as a bottoming cycle for a large combustion engine. The Rankine cycle is also utilized to generate power from geothermal or industrial heat sources. The basic Rankine cycle includes a turbo generator, a boiler, a condenser, and a feed pump.

  In one conventional system provided to generate electricity from waste heat, a Rankine cycle system using carbon dioxide as the working fluid is used in conjunction with a heat transfer heat exchanger. However, the amount of heat that can be recovered from the waste heat source is limited because the boiler inlet temperature of the working fluid rises after passing through the heat transfer heat exchanger. Boiler efficiency is reduced and heat input and power output are limited.

International Publication No. 2012/074940 Pamphlet

  Accordingly, there is a need for a Rankine cycle system that utilizes maximum waste heat and generates increased net power.

  An embodiment of the present invention provides a waste heat recovery system. The waste heat recovery system includes a Rankine cycle system for circulating a working fluid. The Rankine cycle system includes at least one first waste heat recovery boiler configured to transfer heat from a heat source to a working fluid. The Rankine cycle system also includes a first expander configured to receive heated working fluid from at least one first waste heat recovery boiler. The Rankine cycle system further includes a second expander coupled to the at least one generator and a third expander. The waste heat recovery system also includes a condenser configured to receive working fluid at a low pressure from the first expander, the second expander, and the third expander for cooling, and cooling of the working fluid from the condenser. A pump connected to a condenser for receiving a flow of condensed working fluid, the pump being a primary flow of working fluid flowing into the first waste heat recovery boiler, working fluid flowing into the second expander And the condensed working fluid is injected into the tertiary flow of the working fluid flowing into the third expander.

  An embodiment of the present invention provides a waste heat recovery system. The waste heat recovery system includes a Rankine cycle system for circulating a working fluid. The Rankine cycle system includes at least one first waste heat recovery boiler configured to transfer heat from a stream of hot gas or flue gas to a working fluid. The Rankine cycle system also includes a first expander configured to receive heated working fluid from at least one first waste heat recovery boiler. The Rankine cycle system further includes a second expander coupled to the first expander and a third expander coupled to the second expander, whereby the first expander, the second expander, and the second expander. The third expander and the third expander are directly or indirectly coupled in series with each other and further coupled to the generator. The waste heat recovery system also includes a condenser configured to receive working fluid at a low pressure from the first expander, the second expander, and the third expander for cooling. The waste heat recovery system further includes a pump connected to the condenser for receiving a cooled and condensed flow of working fluid from the condenser, the pump being a primary flow of working fluid that flows into the first waste heat recovery boiler. Secondary flow of working fluid flowing into the second expander via the first heat transfer heat exchanger, tertiary of working fluid flowing into the third expander via the second heat transfer heat exchanger It is configured to inject a condensed working fluid into the flow. The waste heat recovery system further includes at least one second waste heat recovery configured to heat the secondary flow of working fluid exiting the first heat transfer heat exchanger before entering the second expander. Including boiler.

  An embodiment of the present invention provides a method for recovering waste heat for power generation using a working fluid in a Rankine cycle. The method includes injecting a primary stream of working fluid through at least one first waste heat recovery boiler to transfer heat from the working fluid hot gas or flue gas stream to the working fluid. The method also includes expanding the heated primary flow of working fluid through the first expander. The method further includes injecting a secondary flow of working fluid through the second expander and injecting a tertiary flow of working fluid through the third expander. Finally, the method combines the primary flow of the working fluid exiting the first expander, the second expander and the third expander, respectively, the secondary flow of the working fluid, and the tertiary flow of the working fluid. Passing through the auxiliary cooler and the condenser, respectively, to condense the combined working fluid and pass it to the pump.

  These and other features, aspects and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which: In the drawings, like features refer to like parts throughout the drawings.

1 is a diagram schematically illustrating a cycle of a recovery type waste heat recovery system according to an embodiment of the present invention. FIG. 2 is an exemplary diagram of the cycle shown in FIG. 1 as represented by a temperature-entropy diagram according to one embodiment of the present invention. FIG. 6 is a schematic diagram of a cycle of a recoverable waste heat recovery system according to another embodiment of the present invention. 6 is a flow chart illustrating example steps involved in a method for recovering waste heat for power generation using a working fluid in a Rankine cycle according to another embodiment of the present invention.

  In addressing elements of various embodiments of the present invention, the articles “a”, “an”, “the” and “said” are intended to mean that one or more elements are present. . The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than those described. Has been. Any examples of operating parameters do not exclude other parameters of the disclosed embodiments.

  FIG. 1 is a schematic diagram of a cycle of a recoverable waste heat recovery system 10 according to one embodiment of the present invention. The waste heat recovery system 10 includes a Rankine cycle system 12 for circulating the working fluid 14. In one embodiment, the working fluid is supercritical carbon dioxide. The Rankine cycle system 12 includes at least one first waste heat recovery boiler 16 configured to transfer heat from a heat source to the working fluid 14. Rankine cycle system 12 also includes a first expander 18 that is configured to receive heated working fluid 14 from at least one first waste heat recovery boiler 16. The Rankine cycle system 12 further includes a second expander 20 coupled to the first expander 18. The Rankine cycle system 12 further includes a third expander 22 coupled to the second expander 20, so that the first expander 18, the second expander 20, and the third expander 22 are in series. Are directly or indirectly coupled to each other and further coupled to the generator 24. Non-limiting examples of expanders 18, 20, 22 include gas turbines. In one embodiment, each of the first expander 18 or the second expander 20 or the third expander 22 may be independently coupled to a different generator. In another embodiment, the first expander 18, the second expander 20, and the third expander 22 may be coupled via a transmission. The waste heat recovery system 10 also includes a condenser 26 configured to receive the working fluid 14 in the low pressure stage 6 from the first expander 18, the second expander 20, and the third expander 22. In one embodiment, the condenser 26 utilizes a cryogenic fluid stream 27 to cool the working fluid 14. The waste heat recovery system further includes a pump 28 connected to the condenser 26 for receiving a cooled and condensed working fluid 14 stream from the condenser 26. The pump 28 has a primary flow (indicated by arrow 30) of the working fluid 14 flowing into the first waste heat recovery boiler 16 and a secondary flow (indicated by arrow 32) of the working fluid 14 flowing into the second expander 20. The condensed working fluid 14 is injected into the tertiary flow (indicated by arrow 34) of the working fluid 14 that flows into the third expander 22. Since the working fluid, carbon dioxide, has a fairly low critical temperature, condensation as in a normal Rankine cycle may not be achieved under warm ambient conditions. In this system, the condenser 26 should not be strictly limited to a device that fully condenses the working fluid to a liquid state, but may simply be a device that can cool the gas to a highly concentrated supercritical state. Need to understand. Similarly, the pump 28 can move and pressurize the gas exiting the condenser 26 as well as simply injecting liquid.

  In one embodiment, the first waste heat recovery boiler 16 is the primary stream of working fluid 14 (arrow 30) that enters the first expander 18 from the first stream 17 of hot gas or the first stream 17 of flue gas. Including a heat exchanger section configured to transfer heat to. As shown in FIG. 1, the Rankine cycle system 12 also heats from the primary flow 30 of the working fluid 14 exiting the first expander 18 to the secondary stream 32 of the working fluid 14 prior to entering the second expander 20. Includes a first heat transfer heat exchanger 36 configured to move the air. In one embodiment, the first heat transfer heat exchanger 36 is an intermediate temperature heat transfer heat exchanger. Further, Rankine cycle system 12 is configured to transfer heat from a secondary flow 32 of working fluid exiting second expander 20 to a tertiary flow 34 of working fluid 14 prior to entering third expander 22. A second heat transfer heat exchanger 38. In one embodiment, the second heat transfer heat exchanger 38 is a low temperature heat transfer heat exchanger.

  Further, in one embodiment, the Rankine cycle system 12 is the primary flow of the working fluid 14 after exiting the first expander 18, the second expander 20, and the third expander 22, respectively, before entering the capacitor 26. 30 and an auxiliary cooler 40 for pre-cooling the combined flow of the secondary flow 32 of the working fluid 14 and the tertiary flow 34 of the working fluid 14. In a combined thermoelectric (CHP) system, the heat obtained in the auxiliary cooler 40 from the precooling action may be used for external processes. In one embodiment, the auxiliary cooler 40 is derived from a precooling action in the Rankine cycle system 12 by transferring heat to the primary stream 30 of the working fluid 14 to preheat before entering the waste heat recovery boiler 16. Use the heat generated.

  As shown in FIG. 1, the cycle of the waste heat recovery system 10 includes one main loop cycle 42 indicated by stages 1, 2, 3H, 4H, 5H and 6. The waste heat recovery system 10 also includes a second loop cycle 44 and a third loop cycle 46 that are parallel to the main loop cycle 42. Such a cascade connection of the second and third loop cycles 44 and 46 uses the first heat transfer heat exchanger and the second heat transfer heat exchanger to make the first and second extracts. The additional residual superheat from the expanded carbon dioxide (working fluid 14) after expansion in the pandas 18, 20 is effectively utilized. As shown in FIG. 1, the second loop cycle 44 is shown by stages 1, 2, 3I, 4I, 5I and 6, and the third loop cycle 46 is made up of stages 1, 2, 3L, 4L, This is indicated by 6.

  FIG. 2 is an exemplary diagram of a cycle of the waste heat recovery system 10 shown in FIG. 1 as represented by a temperature-entropy diagram according to one embodiment of the present invention. Temperature (Celsius temperature) is shown on the vertical Y-axis, and entropy (kilojoules per Kelvin) is shown on the horizontal X-axis. Temperature-entropy diagram 50 shows main loop cycle 42 (indicated by stage 1-2-3H-4H-5H-6-1), second loop cycle 44 (stage 1-2-3I-4I-5I-6). ) And the third loop cycle 46 (indicated by stages 1-2-3L-4L-6-1). In the main loop cycle 42, the liquid working fluid 14 (shown in FIG. 1) coming from the condenser 26 is injected to a very high pressure (eg 300 bar) in stage 2 and then heated in the waste heat recovery boiler 16. After being heated to a temperature close to that of the waste heat source, the working fluid 14 generates electrical power in the first expander 18 (shown in FIG. 1). The working fluid 14 undergoes an expansion process, during which the temperature and pressure of the working fluid 14 drops from stage 3H to 4H. Further, the low pressure working fluid 14 exiting the first expander 18 is cooled in a first heat transfer heat exchanger 36 (shown in FIG. 1), in which case the working fluid is fed to the working fluid 14 after the pump. Heat is transferred to a secondary stream 32 (as shown in FIG. 1) of working fluid 14 that deviates from primary stream 30. This secondary stream 32 also expands in the second expander 20 (stages 3I to 4I), this second expander operates at a lower temperature and is similar in the second heat transfer heat exchanger 38. The method heats the working fluid tertiary stream 34 (shown in FIG. 1), where the temperature further drops from stage 4I to 5I. In one embodiment, the secondary stream 32 is optionally further heated in an additional heat exchanger section in the waste heat recovery boiler to a higher temperature, i.e. as high as possible in the first stream. it can. The tertiary stream 34 (shown in FIG. 1) of the working fluid 14 is also heated after the pump and by the secondary stream 32 in the second heat transfer heat exchanger 38 (shown in FIG. 1). It deviates from the high-pressure line (primary stream 30) and expands in the third expander 22 from stage 3L to 4L and is then combined with the primary stream 30 and the secondary stream 32 at low pressure in stage 6. In one embodiment, the combined stream of working fluid 14 is cooled or condensed by heating one of the other streams 30, 32, or 34 of the working fluid before being cooled and condensed. Further cooling can be done in the exchanger. With respect to condensation, the carbon dioxide working fluid 14 is cooled below a critical temperature of 30 ° C., otherwise a cooled and highly concentrated gas is formed in the condenser 26 to be fed to the feed pump.

  FIG. 3 is a schematic diagram of a cycle of a recoverable waste heat recovery system 70 according to another embodiment of the present invention. The waste heat recovery system 70 is the same as the waste heat recovery system 10 shown in FIG. 1 except that the waste heat recovery system 70 includes the second waste heat recovery boiler 21. In this embodiment, the second loop cycle 44 acts on the secondary stream 32 of working fluid 14 in the first waste heat recovery boiler 16 after it is first heated in the first heat transfer heat exchanger 36. A second waste heat recovery boiler 21 is utilized that utilizes hot flue gas or fluid 19 to further heat to a temperature equal to the fluid primary stream 30. The heating action of the secondary stream 32 of the working fluid 14 in the second waste heat recovery boiler 21 may lead to a thermodynamic advantage that allows higher efficiency at the lower peak temperature of the waste heat recovery system 70. is there.

  FIG. 4 is a flowchart illustrating the steps involved in the method 100 for recovering waste heat for power generation using a working fluid in a Rankine cycle. In step 102, the method includes injecting a primary stream of the working fluid through at least one first waste heat recovery boiler to transfer heat from the hot gas or flue gas stream to the working fluid. In step 104, the method includes expanding the heated primary flow of working fluid through the first expander. Further in step 106, the method includes diverting the secondary flow of working fluid from the primary flow via the second expander. In step 108, the method includes diverting the tertiary flow of working fluid from the primary flow via the third expander. Finally, in step 110, the method includes a primary flow of working fluid exiting the first expander, the second expander, and the third expander, respectively, a secondary flow of working fluid, and a tertiary flow of working fluid. Condensing the combined working fluid by passing the combined through an auxiliary precooler and a condenser, and directing the condensed working fluid to the pump.

  Advantageously, the present invention leads to high efficiency of the waste heat recovery system by utilizing carbon dioxide, which can be heated to a very high temperature as the working fluid. Carbon dioxide is a non-toxic and thermally stable working fluid. The system and method, which uses three expanders and utilizes a triple expansion process with cascaded heat transfer heat exchangers, maximizes the available waste heat induced in the system. Pull out power. Furthermore, the heating effect of the secondary flow of the working fluid in the second waste heat recovery boiler can lead to a thermodynamic advantage that allows higher efficiency at the lower peak temperature of the waste heat recovery system.

  Moreover, those skilled in the art will recognize the interchangeability of various features from the various embodiments. Similarly, the various method steps and features described and other known equivalents relating to such methods and features may be combined and adapted by those skilled in the art to build additional systems and techniques in accordance with the principles of the present disclosure. Can do. Of course, it is to be understood that all such objects and advantages described above may be achieved by a particular embodiment. Thus, for example, one of ordinary skill in the art will appreciate that the systems and techniques described herein do not necessarily achieve the other objects and advantages that may be taught or presented herein, as taught herein. It will be understood that the invention may be embodied or implemented in such a way as to achieve or optimize one advantage or group of benefits.

  While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. The appended claims are intended to cover all such modifications and variations as fall within the true spirit of the invention.

1, 2, 3H, 3I, 3L, 4H, 4I, 4L, 5H, 5I, 6th stage 10, 70 Waste heat recovery system 12 Rankine cycle system 14 Working fluid 16 Waste heat recovery boiler 17 First flow 18 of hot gas First expander 19 High temperature flue gas or fluid 20 Second expander 21 Second waste heat recovery boiler 22 Third expander 24 Generator 26 Condenser 27 Low temperature fluid flow 28 Pump 30 Primary working fluid Stream 32 Working Fluid Secondary Stream 34 Working Fluid Tertiary Flow 36 First Heat Transfer Heat Exchanger 38 Second Heat Transfer Heat Exchanger 40 Auxiliary Precooler 42 Main Loop Cycle 44 Second Loop Cycle 46 Third Loop Cycle 50 Temperature-Entropy Diagram 100 Recovering Waste Heat for Power Generation Using Working Fluid in Rankine Cycle Method 102 Injecting a primary stream of working fluid through at least one waste heat recovery boiler to transfer heat from the stream of hot gas or flue gas to the working fluid Step 104 Working fluid through the first expander Step 106 diverting the heated primary flow of the first flow through the second expander diverting the secondary flow of the working fluid from the primary flow 108 diverting the third flow of the working fluid from the primary flow through the third expander Step 110 A combination of the primary flow of the working fluid, the secondary flow of the working fluid, and the tertiary flow of the working fluid that respectively exits the first expander, the second expander, and the third expander Condensate the combined working fluid by passing it through the precooler and condenser, and the condensed working flow Step further induced pump

Claims (23)

At least one first waste heat recovery boiler (16) configured to transfer heat from a heat source to the working fluid (14);
A first expander (18) configured to receive the heated working fluid (14) from the at least one first waste heat recovery boiler (16);
A Rankine cycle system (12) for circulating working fluid (14), comprising a second expander (20) and a third expander (22) coupled to at least one generator (24); ,
A condenser configured to receive the working fluid (14) at low pressure from the first expander (18), the second expander (20), and the third expander (22) for cooling. (26) and
A pump (28) connected to the condenser (26) for receiving a cooled and condensed flow of the working fluid (14) from the condenser (26), wherein the pump comprises the first waste heat; A primary flow (30) of the working fluid (14) flowing into the recovery boiler (16), a secondary flow (32) of the working fluid (14) flowing into the second expander (20), and the third extract. A waste heat recovery system (10) comprising a pump (28) configured to inject the condensed working fluid (14) into a tertiary flow (34) of the working fluid (14) flowing into a panda (22).   The waste heat recovery system (10) of claim 1, wherein the working fluid (14) is carbon dioxide.   The working fluid (14), wherein the first waste heat recovery boiler (16) enters the first expander (18) from a first stream of hot gas or a first stream of flue gas (17). The waste heat recovery system (10) of any preceding claim, comprising a heat exchanger section configured to transfer heat to the primary stream (30).   The working fluid before the Rankine cycle system (12) enters the second expander (20) from the primary flow (30) of the working fluid (14) exiting the first expander (18) The waste heat recovery system (10) of claim 1, comprising a first heat transfer heat exchanger (36) configured to transfer heat to the secondary flow (32) of (14).   The waste heat recovery system (10) according to claim 4, wherein the first heat transfer heat exchanger (36) is an intermediate temperature heat transfer heat exchanger.   The secondary flow (32) of the working fluid (14) leaving the first heat transfer heat exchanger (36) before the Rankine cycle system (12) enters the second expander (20). 2. The waste heat recovery system (10) according to claim 1, comprising a second waste heat recovery boiler (21) configured to heat a).   The second waste heat recovery boiler (21) before entering the second expander (20) from a second stream of hot gas or a second stream of flue gas (19). 7. A heat exchanger section configured to transfer heat to the secondary stream (32) of the working fluid (14) exiting one heat transfer heat exchanger (36). Waste heat recovery system (10).   The working fluid before the Rankine cycle system (12) enters the third expander (22) from the secondary flow (32) of the working fluid (14) exiting the second expander (20) The waste heat recovery system (10) of claim 1, comprising a second heat transfer heat exchanger (38) configured to transfer heat to the tertiary flow (34) of (14).   The waste heat recovery system (10) according to claim 5, wherein the second heat transfer heat exchanger (38) is a low temperature heat transfer heat exchanger.   The Rankine cycle system (12) exits the first expander (18), the second expander (20) and the third expander (22) before entering the condenser (26). A combined flow of the primary flow (30) of the working fluid (14) after, the secondary flow (32) of the working fluid (14), and the tertiary flow (34) of the working fluid (14). The waste heat recovery system (10) according to claim 1, comprising an auxiliary cooler (40) for pre-cooling the water.   A primary flow of the working fluid (14) exiting the Rankine cycle system (12) from the first expander (18), the second expander (20) and the third expander (22), respectively. (30), the secondary flow (32) of the working fluid (14), and the tertiary flow (34) of the working fluid (14) are combined to generate heat for the external process. The waste heat recovery system (10) of claim 1, comprising a combined thermoelectric (CHP) system for providing.   The combined thermoelectric (CHP) system transfers heat from precooling action to the primary stream (30) of working fluid (14) to preheat before entering the waste heat recovery boiler (16). The waste heat recovery system (10) of claim 11, wherein the waste heat recovery system (10) is configured to.   The waste heat recovery system (10) of claim 1, wherein the condenser (26) cools the working fluid (14) and the pump (28) compresses the cooled gas rather than injecting liquid. At least one first waste heat recovery boiler (16) configured to transfer heat from the hot gas or flue gas stream to the working fluid (14);
A first expander (18) configured to receive the heated working fluid (14) from the at least one first waste heat recovery boiler (16);
A Rankine cycle system (12) for circulating working fluid (14), comprising a second expander (20) and a third expander (22) coupled to at least one generator (24); ,
A condenser configured to receive the working fluid (14) at low pressure from the first expander (18), the second expander (20), and the third expander (22) for cooling. (26) and
A pump (28) connected to said condenser (26) for receiving a cooled flow of said working fluid (14) from said condenser (26), said pump being said first waste heat recovery boiler; The working fluid (14) flowing into the second expander (20) through the primary flow (30) of the working fluid (14) flowing into (16) and the first heat transfer heat exchanger (36). And the third flow (34) of the working fluid (14) flowing into the third expander (22) via the second heat transfer heat exchanger (38). A pump (28) configured to inject fluid (14);
Configured to heat the secondary stream (32) of the working fluid (14) exiting the first heat transfer heat exchanger (36) before entering the second expander (20). A waste heat recovery system (10) comprising at least one second waste heat recovery boiler (21).   The waste heat recovery system (10) of claim 14, wherein the working fluid (14) is carbon dioxide.   The at least one first waste heat recovery boiler (16) or the at least one second waste heat recovery boiler (21) from a stream of hot gas or flue gas to the first expander (18). Configured to transfer heat to the primary flow (30) of the working fluid (14) entering or the secondary flow (32) of the working fluid (14) entering the second expander (20). The waste heat recovery system (10) according to claim 14.   The first heat transfer heat exchanger (36) moves from the primary flow (30) of the working fluid (14) exiting the first expander (18) to the second expander (20). The waste heat recovery system (10) of claim 14, wherein the heat recovery system (10) is an intermediate temperature heat transfer heat exchanger configured to transfer heat to the secondary stream (34) of the working fluid (14) prior to entering. ).   The second heat transfer heat exchanger (38) moves from the secondary flow (32) of the working fluid (14) exiting the second expander (20) to the third expander (22). The waste heat recovery system (10) of claim 14, wherein the waste heat recovery system (10) is a low temperature heat transfer heat exchanger configured to transfer heat to the tertiary stream (34) of the working fluid (14) prior to entering.   The Rankine cycle system (12) exits the first expander (18), the second expander (20) and the third expander (22) before entering the condenser (26). Then, the combined flow of the primary flow (30) of the working fluid (14), the secondary flow (32) of the working fluid (14), and the tertiary flow (34) of the working fluid (14). The waste heat recovery system (10) of claim 14, comprising an auxiliary cooling or combined thermoelectric (CHP) system (40) for pre-cooling. A method for recovering waste heat to generate electricity using a working fluid (14) in a Rankine cycle (12),
A primary flow (30) of the working fluid (14) through at least one first waste heat recovery boiler (16) to transfer heat from a stream of hot gas or flue gas to the working fluid (14). Injecting, and
Expanding the heated primary flow (30) of the working fluid (14) via a first expander (18);
Diverting the secondary flow (32) of the working fluid (14) from the primary flow (30) via a second expander (20);
Diverting the tertiary flow (34) of the working fluid (14) from the primary flow (30) via a third expander (22); and the first expander (18) and the second expander. (20) and the primary flow (30) of the working fluid (14) exiting the third expander (22), the secondary flow (32) of the working fluid (14), and the working fluid, respectively. Condensing the combination of the working fluid (14) by passing the combination of the tertiary flow (34) of (14) through an auxiliary cooler (40) and a condenser (26); Directing the working fluid (14) further to the pump (28).   The secondary stream (32) of the working fluid (14) is passed through a first intermediate temperature heat transfer heat exchanger (36) for preheating prior to delivery to the second expander (20). 21. The method of claim 20, further comprising the step of passing.   Prior to delivery to the second expander (20), the secondary stream (32) exiting the first heat transfer heat exchanger (36) is routed to a second waste heat recovery boiler (21). 21. The method of claim 20, further comprising the step of passing.   In order to preheat the tertiary flow (34) of the working fluid (14) before delivery to the third expander (22), the tertiary flow (34) of the working fluid (14) The method of claim 21, further comprising the step of passing through two cold heat transfer heat exchangers (38).