Method and system for power production with improved efficiency

Abstract

The present disclosure relates to systems and methods that provide power generation using predominantly CO.sub.2 as a working fluid. In particular, the present disclosure provides for the use of a portion of the heat of compression from a CO.sub.2 compressor as the additive heating necessary to increase the overall efficiency of a power production system and method.

Claims

1. A power generating system comprising: a combustor configured to exhaust a combustion stream; a power production turbine configured to receive and expand the combustion stream and form a turbine exhaust stream; a first compressor configured to receive at least a portion of the turbine exhaust stream and form a compressed recycle CO.sub.2 stream; a recuperative heat exchanger configured to receive the turbine exhaust stream and the compressed recycle CO.sub.2 stream so that the compressed recycle CO.sub.2 stream is heated with heat withdrawn from the turbine exhaust stream; a first additive heating source arranged to provide heat for the compressed recycle CO.sub.2 stream in addition to the heat withdrawn from the turbine exhaust stream, the first additive heat source comprising a second compressor configured to receive and compress a portion of the turbine exhaust stream; and at least a second additive heating source arranged to provide heat for the compressed recycle CO.sub.2 stream in addition to the heat withdrawn from the turbine exhaust stream.

2. The power generating system of claim 1, further comprising a flow separator configured to separate the turbine exhaust stream into a first stream arranged for passage to the first compressor and a second stream arranged for passage to the second compressor.

3. The power generating system of claim 1, further comprising a pump configured to receive the first stream from the first compressor and the second stream from the second compressor and pressurize the first stream and the second stream in combination, the pump positioned downstream from the first compressor and the second compressor.

4. The power generating system of claim 1, wherein the recuperative heat exchanger is configured with a first flow path for passage of the turbine exhaust stream, a second flow path for passage of the compressed recycle CO.sub.2 stream, and at least a third flow path for passage of one or both of a stream from the first additive heat source and a stream from the at least a second additive heat source, wherein the first flow path and the at least a third flow path are configured for heating the second flow path.

5. The power generating system of claim 1, wherein the recuperative heat exchanger comprises a series of three or more heat exchangers or a series of three or more heating sections.

6. The power generating system of claim 1, further comprising one or more separators configured for separating at least water from the turbine exhaust stream.

7. The power generating system of claim 1, wherein the first compressor comprises a multi-stage, intercooled compressor.

8. The power generating system of claim 1, wherein the second compressor comprises an adiabatic, multi-stage compressor with no intercooling between compressor stages.

9. The power generating system of claim 1, wherein the at least a second additive heating source comprises an air separation plant configured for adiabatic compression.

10. The power generating system of claim 1, wherein the at least a second additive heating source comprises a gas turbine.

11. The power generating system of claim 1, wherein the at least a second additive heating source comprises a heated CO.sub.2 stream from a source that is external to the power generating system; preferably wherein the source external to the power generating system is a geological CO.sub.2 source or a CO.sub.2 pipeline.

12. A method of generating power, the method comprising: combusting a fuel with oxygen in the combustor in the presence of a recycle CO.sub.2 stream to produce a CO.sub.2 containing combustion stream; passing the CO.sub.2 containing combustion stream through a turbine to expand the CO.sub.2 containing combustion stream, generate power, and form a turbine exhaust stream; passing the turbine exhaust stream through a recuperative heat exchanger to withdraw heat from the turbine exhaust stream; compressing a portion of the turbine exhaust stream in a first compressor and form a compressed recycle CO.sub.2 stream; passing the compressed recycle CO.sub.2 stream through the recuperative heat exchanger so that the compressed recycle CO.sub.2 stream is heated with the heat withdrawn from the turbine exhaust stream; passing a stream from a first additive heat source through the recuperative heat exchanger so that the compressed recycle CO.sub.2 stream is heated by the stream from the first additive heat source, the first additive heat source comprising a second compressor configured to receive and compress a portion of the turbine exhaust stream; and passing a stream from a second additive heat source through the recuperative heat exchanger so that the compressed recycle CO.sub.2 stream is heated by the stream from the second additive heat source in combination with or as an alternative to the first additive heat source.

13. The method of claim 12, further comprising dividing the turbine exhaust stream to form a first turbine exhaust portion for compression in the first compressor and second turbine exhaust portion for compression in the second compressor.

14. The method of claim 13, where a mass ratio of the first turbine exhaust portion to the second turbine exhaust portion based on the total mass of the turbine exhaust stream is 50:50 to 99:1.

15. The method according to claim 12, wherein one or more of the following conditions apply: the CO.sub.2 containing combustion stream has a temperature of 500° C. to 1,700° C. and a pressure of 100 bar (10 MPa) to 500 bar (50 MPa); a pressure ratio across the turbine is 5 to 12; the heat is withdrawn from the turbine exhaust stream in a recuperative heat exchanger comprising three or more sections or comprising three or more individual heat exchangers.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Having thus described the disclosure in the foregoing general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

(2) FIG. 1 is a flow diagram of a prior art power production cycle;

(3) FIG. 2 is a flow diagram of a further prior art power production cycle; and

(4) FIG. 3 is a flow diagram of a power production system and method according to an exemplary embodiment of the present disclosure including a plurality of compressors for compressing a recycle CO.sub.2 stream and deriving heat therefrom for input to a recuperator heat exchanger.

DETAILED DESCRIPTION

(5) The present subject matter will now be described more fully hereinafter with reference to exemplary embodiments thereof. These exemplary embodiments are described so that this disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art. Indeed, the subject matter can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms “a”, “an”, “the”, include plural referents unless the context clearly dictates otherwise.

(6) The present disclosure relates to systems and methods that provide power generation using predominantly CO.sub.2 as a working fluid. In particular, the process uses a high pressure/low pressure ratio turbine that expands a mixture of a high pressure recycle CO.sub.2 stream and combustion products arising from combustion of the fuel. Any fossil fuel, particularly carbonaceous fuels, may be used. Non-limiting examples include natural gas, compressed gases, fuel gases (e.g., comprising one or more of H.sub.2, CO, CH.sub.4, H.sub.2S, and NH.sub.3) and like combustible gases. Solid fuels—e.g., coal, lignite, petroleum coke, bitumen, biomass, and the like, or viscous liquid fuels may be used as well with incorporation of necessary system elements. For example, a partial oxidation combustor can be used to convert the solid or viscous liquid fuel to a fuel gas that is substantially free of solid particles. All fuel and combustion derived impurities in an oxidized state, such as sulfur compounds, NO, NO.sub.2, CO.sub.2, H.sub.2O, Hg, and the like can be separated from the power cycle for disposal with substantially or completely no emissions to the atmosphere. As noted previously, other fuels likewise may be utilized. Pure oxygen can be used as the oxidant in the combustion process. In some embodiments, combustion temperature may be regulated by diluting the oxygen with CO.sub.2 in ratios as otherwise noted herein.

(7) The hot turbine exhaust is used to partially preheat the high pressure recycle CO.sub.2 stream. In combination with this heating, the recycle CO.sub.2 stream can be further heated using additive heating that can be derived from the compression energy of a CO.sub.2 compressor. The operating conditions for the CO.sub.2 compressor can vary as further described herein. For example, in some embodiments, it can be useful to utilize a CO.sub.2 compressor inlet temperature that is higher than normal approach to ambient cooling means. The minimum inlet temperature of the stream entering the CO.sub.2 compressor, for example, can be approximately the dew point of water at the operating conditions. In some embodiments, the CO.sub.2 compressor can have an inlet temperature of about 50° C. to about 250° C. Optionally other heating means providing heat at a temperature level below about 400° C. can be used in addition to the heating available from the CO.sub.2 compression. Such means can include heat transferred from the air compressors of a cryogenic air separation plant operating partially or completely in the adiabatic mode without intercooling. When such heat is utilized, the air compressors preferably can be operated with pressure ratios above 2.5 in the adiabatic stages.

(8) It has been discovered according to the present disclosure that power production efficiency can be improved through provision of additive heating as defined herein, such additive heating particularly being provided at a temperature level below about 400° C. (e.g., in the range of about 100° C. to about 400° C.). The provision of the additive heating can overcome the large difference in the specific heat of CO.sub.2 at a typical high pressure turbine inlet of about 300 bar (30 MPa) and the specific heat of CO.sub.2 at a typical low pressure turbine exhaust pressure of about 30 bar (3 MPa). This difference is evident in the table provided below.

(9) TABLE-US-00001 CO.sub.2 specific heat CO.sub.2 specific heat Temperature K (kJ/kg) at (kJ/kg) at (° C.) 30 bar (3 MPa) 300 bar (30 MPa) 300  (26.85) 1.18 1.95 350  (76.85) 1.05 2.00 400 (126.85) 1.02 1.90 450 (176.85) 1.03 1.63 500 (226.85) 1.06 1.47 600 (326.85) 1.10 1.31 750 (476.85) 1.17 1.23 1000 (726.85) 1.24 1.28

(10) A power production method according to the present disclosure particularly can comprise a series of steps that can provide for improved efficiency. The method can comprise passing a compressed, heated recycle CO.sub.2 stream into a combustor. The compressed, heated recycle CO.sub.2 stream can be formed as further described below. In the combustor, a fuel can be combusted with the oxidant (e.g., oxygen of at least 98% or at least 99% purity, optionally diluted with CO.sub.2) in the presence of the recycle CO.sub.2 stream to produce a CO.sub.2 containing stream. The CO.sub.2 containing stream from the combustor can have a temperature of about 500° C. or greater (e.g., about 500° C. to about 1,700° C. or about 800° C. to about 1,600° C.) and a pressure of about 100 bar (10 MPa) or greater (e.g., about 100 bar (10 MPa) to about 500 bar (50 MPa)). The CO.sub.2 containing stream can be passed through a turbine to expand the CO.sub.2 containing stream, generate power, and form a turbine exhaust stream comprising CO.sub.2. The CO.sub.2 containing stream can be expanded across the turbine at a pressure ratio of less than 12 or less than 10 (e.g., about 5 to about 12). In alternate embodiments, high pressure ratios as noted herein may be used, such as in the case of utilizing a plurality of turbines, as described in U.S. Pat. Pub. No. 2013/0213049, the disclosure of which is incorporated herein by reference in its entirety.

(11) The turbine exhaust stream can be processed to remove combustion products and any net CO.sub.2 produced by combustion of the fuel. To this end, the turbine exhaust stream can be cooled by passage through a heat exchanger. Any heat exchanger suitable for use under the temperature and pressure conditions described herein can be utilized. In some embodiments, the heat exchanger can comprise a series of at least two, at least three, or even more economizer heat exchangers. A single heat exchanger with at least two sections, at least three sections (or even more sections) can be used. For example, the heat exchanger may be described has having at least three heat exchange sections operating across different temperature ranges. Withdrawn heat from the turbine exhaust stream can be utilized for heating the recycle CO.sub.2 stream as described below.

(12) The turbine exhaust stream can be divided into two or more portions. The first portion can comprise 50% or greater, 70% or greater, or 90% or greater (but less than 100%) of the total mass flow of the turbine exhaust stream. The first turbine exhaust portion is cooled preferably at a temperature that is less than the water dew point after leaving the heat exchanger. The first turbine exhaust portion can be passed through a separator to remove water and can be further treated to remove other combustion products or impurities. The resulting stream can be described as a main recycle CO.sub.2 stream, and this stream can be compressed such as in a multi-stage compressor with intercooling between the stages. Preferably, the main recycle CO.sub.2 stream is compressed to a pressure of about 40 bar (4 MPa) to about 100 bar (10 MPa). In some embodiments, the main recycle CO.sub.2 stream is compressed to a pressure of about 60 bar (6 MPa) to about 100 bar (10 MPa) or about 67 bar (6.7 MPa) to about 80 bat (8 MPa).

(13) The second portion of the turbine exhaust stream can be compressed to form a heated, compressed second turbine exhaust portion. The second turbine exhaust portion can comprise the balance of the turbine exhaust not present in the first portion (e.g., 50% or less, 30% or less, or 10% or less (but greater than 0%) of the total mass flow of the turbine exhaust stream). Preferably, the second turbine exhaust portion can be withdrawn from the turbine exhaust between the second and third heat exchange sections (e.g., the second and third heat exchangers in the series moving from hot to cold—in other words, the heat exchangers working between the lowest temperature and an intermediate temperature). The second turbine exhaust portion is preferably compressed so as to achieve a temperature in the range of about 100° C. to about 400° C. and a pressure of about 40 bar (4 MPa) to about 100 bar (10 MPa). In some embodiments, the pressure can be about 60 bar (6 MPa) to about 100 bar (10 MPa) or about 67 bat (6.7 MPa) to about 80 bar (8 MPa). The second turbine exhaust portion can be re-introduced to the heat exchanger, preferably passing from the hot end of the intermediate temperature heat exchanger to the cold end of the low temperature heat exchanger. The cooled second turbine exhaust portion can be at a temperature that is below the water dew point, and the cooled stream can be passed through one or more separators to remove water and any other impurities. The remaining stream is a secondary recycle CO.sub.2 stream, and it can be combined with the main recycle CO.sub.2 stream. Such combining can be at a variety of points. For example, the main recycle CO.sub.2 stream can be added to the cooled second portion of the turbine exhaust after passage through the low temperature heat exchanger and before passage through the separator. Alternatively, the main recycle CO.sub.2 stream and the secondary recycle CO.sub.2 stream can be combined after water separation or at another point of the cycle. Net CO.sub.2 produced from combustion can be withdrawn at this point, such as for use in enhanced oil recovery, for sequestration, or the like.

(14) In some embodiments, the second turbine exhaust portion can be compressed using multi-stage compression wherein there is no inter-cooling between stages followed by inter-cooling between later stages. Compressed and heated gas of the second turbine exhaust portion exiting the non-cooled stages can be introduced to the heat exchanger as otherwise described above, and the so-cooled stream can be subjected to the inter-cooled compression before combining with the first turbine exhaust portion. The number of non-cooled stages (x) and inter-cooled stages (y) can independently be 1 or more, 2 or more, or 3 or more (e.g., 1 to 5 or 2 to 4).

(15) The total recycle CO.sub.2 stream (formed of the main recycle CO.sub.2 stream and the secondary recycle CO.sub.2 stream) can be pumped to a pressure suitable for passage into the combustor. Preferably, the total recycle CO.sub.2 stream is pumped to a pressure of at 100 bar (10 MPa) or greater or about 200 bar (20 MPa) or greater, such as about 100 bar (10 MPa) to about 500 bar (50 MPa). The compressed recycle CO.sub.2 stream is then passed back through the heat exchangers to be heated. The compressed recycle CO.sub.2 stream is heated using the heat withdrawn from the turbine exhaust stream (which can be characterized as the heat of combustion that remains in the turbine exhaust stream). The heat in the turbine exhaust stream, however, is insufficient to achieve a close temperature approach between the turbine exhaust stream and the heated, compressed recycle CO.sub.2 stream at the hot end of the heat exchanger. According to the present disclosure, the heat from the compressed, second turbine exhaust portion can be used as additive heating to reduce the temperature differential between the turbine exhaust stream and the heated, compressed recycle CO.sub.2 stream leaving the heat exchanger and entering the combustor. The additive heating can be characterized as the heat of recompression and is separate from the heat of combustion that is present in the turbine exhaust. The use of the additive heating can be beneficial to reduce temperature differential between the turbine exhaust stream and the heated, compressed recycle CO.sub.2 stream leaving the heat exchanger and entering the combustor to about 50° C. or less, about 40° C. or less, or about 30° C. or less, such as about 10° C. to about 50° C., or about 20° C. to about 40° C.

(16) In some embodiments, additive heating can be provided by other means in combination with or as an alternative to the heat of recompression. For example, heated CO.sub.2 from an external source can be utilized. Such external source can be, for example, CO.sub.2 withdrawn from a geological source, CO.sub.2 taken from a pipeline, or the like. In such embodiments, splitting of the turbine exhaust stream can be unnecessary, and the heated CO.sub.2 can be input to the system in the same manner as the heat of recompression described above. The additional CO.sub.2 can be withdrawn from the system with the net CO.sub.2 product and can be returned to the heat source. In such manner, a recycled CO.sub.2 from an external source completely outside of the power production system can be utilized as additive heating. Alternatively, part or all of the additive heating can be from a gas turbine exhaust or from a condensing stream.

(17) An exemplary embodiment of a system according to the present disclosure is shown in FIG. 3. The embodiment is described in relation to an exemplary embodiment of a combustion method utilizing defined parameters. Specific temperatures and pressures thus can vary based upon the specific operation conditions.

(18) In the embodiment of FIG. 3, a turbine exhaust stream 55 at 728° C. and 30 bar (3 MPa) passes through three economizer heat exchangers in series 29, 27, and 26 leaving as stream 46 at 46° C. and 29 bar (2.9 MPa). Heat exchanger 29 may be characterized as a high temperature heat exchanger, heat exchanger 27 may be characterized as an intermediate temperature heat exchanger, and heat exchanger 26 may be characterized as a low temperature heat exchanger. It is understood that the terms “high temperature,” “intermediate temperature,” and “low temperature,” are intended to only describe the operating temperature ranges of the three heat exchangers relative to one another. The stream 46 is cooled in a water cooled heat exchanger 58 to 17.2° C., and a condensed water stream 56 is separated in the phase separator vessel 53. An overhead CO.sub.2 gas stream 61 leaves the phase separator vessel 53 and enters a two stage centrifugal CO.sub.2 recycle compressor 21 (stage 1) and 22 (stage 2), wherein a discharge stream 44 from the first stage compressor 21 is cooled in an intercooler 23 to 17.2° C., exits as stream 45, and is then compressed in the second stage compressor 22 to form stream 48 at 80 bar (8 MPa). This main recycle compressor discharge stream 48 joins with stream 47, and the combined stream 69 is cooled in a water cooled heat exchanger 24 to a temperature of 22.7° C. In other embodiments, this temperature can be in the range of 10° C. to about 30° C. Condensed water 68 is separated in a phase separator 67 producing the total recycle CO.sub.2 stream 49, which is in the supercritical state and has a high density of 850 Kg/m.sup.3. A net product CO.sub.2 stream 62, equivalent to the carbon in the fuel gas converted to CO.sub.2 in the combustor, is removed from the system (after cooling, as illustrated, or before cooling) for sequestration, use in enhanced oil recovery, or the like.

(19) The total recycle CO.sub.2 stream 49 is cooled in heat exchanger 70 to a temperature of 17.2° C. then enters a multi-stage centrifugal pump 25 with a discharge pressure of 305 bar (30.5 MPa) to from a high pressure CO.sub.2 recycle stream 50, which is heated in the three economizer heat exchangers in series 26, 27 and 29 leaving as stream 54 at a temperature of 725° C. and 302 bar (30.2 MPa). The stream 54 is heated to 1154° C. in combustor 30 by the direct combustion of a natural gas stream 40 with a 99.5% O.sub.2 stream 41, both at 320 bar (32 MPa). In the exemplified embodiment, modeling was done with pure CH.sub.4 as the fuel gas. The mixed stream of recycle CO.sub.2 and combustion products 57 enters a power turbine 31 with a discharge pressure of 30 bar (3 MPa) and exits as turbine exhaust stream 55.

(20) As seen in the table above, the difference in the specific heat of CO.sub.2 at 300 bar (30 MPa) and 30 bar (3 MPa) increases as the temperature drops from 1000 K (727° C.). In light of this difference, additive heating is required to achieve a very close temperature approach between the turbine exhaust stream 55 and the recycle CO.sub.2 stream 54, and such additive heating can be supplied, for example, in the “low temperature” economizer heat exchanger 26 and/or the “intermediate temperature” economizer heat exchanger 27. According to the present disclosure, the additive heating can be provided by utilizing the adiabatic heat of compression of part of the recycle CO.sub.2 stream which, in the exemplary embodiment, is compressed to a pressure of about 29 bar (2.9 MPa) to about 80 bar (8 MPa).

(21) Returning to the exemplary embodiment of FIG. 3, a portion of the cooling turbine exhaust stream 51 between the two economizer heat exchanger sections 27 and 26 at a temperature of 138° C. can be withdrawn and compressed in a single stage or multi-stage adiabatic compressor 28 producing stream 52 at 246° C. and 80 bar (8 MPa). The compressed and heated stream 52 re-enters the hot end of economizer heat exchanger 27, and the stream is passed through heat exchanger 27 and heat exchanger 26 where it cools and leaves as stream 47 at 54° C. The entire heat of compression in compressor 28 supplied by work stream 34 is thusly transferred to the high pressure recycle CO.sub.2 stream, and this heat input is equivalent to heat of combustion delivered in the combustor 30 since it reduces the hot end temperature difference. The flow-rate of stream 51 is maximized to achieve a significantly small temperature difference between streams 65 and 66 at the inlet to the high temperature economizer heat exchanger 29. This temperature difference between streams 65 and 66 preferably is about 50° C. or less, about 40° C. or less, about 30° C. or less, about 20° C. or less, particularly about 10° C. to about 50° C., or about 20° C. to about 40° C. As discussed above, stream 47 joins with the main recycle compressor discharge stream 48 for cooling in heat exchanger 24 to 22.7° C. The additive heating provide by CO.sub.2 compression as described above provides for increased efficiency in the power production system.

(22) Note that other sources of low temperature level heating (e.g., gas turbine exhaust or condensing stream) can be utilized as the additive heating. The exemplary embodiment of FIG. 3 includes the cryogenic air separation plant 81 main air flow 42a which has been adiabatically compressed to 5.7 bar (0.57 MPa) and 223° C. entering the hot end of economizer heat exchanger 27 as stream 42 and leaving heat exchanger 26 as stream 43 at 54° C. In some embodiments, stream 42 may arise from stream 42b, which is illustrated as heat derived from a gas turbine 83. Although not illustrated in FIG. 3, in some embodiments, the O.sub.2 stream can be supplied from the air separation plant at 80 bar (8 MPa) and ambient temperature and can be mixed with CO.sub.2 from stream 49 to give 25 mol % O.sub.2 that can be compressed to 320 bar (32 MPa) before being heated to 725° C. in the economizer heat exchangers 27, 26 and 29. In practice, this CO.sub.2+O.sub.2 compressor can also feature a hot gas compressor section as has been shown for the CO.sub.2 recycle compressor. In FIG. 3, cooling water inlet streams are represented as streams 38, 59, 72, and 36, while the respective outlet streams are represented as streams 39, 60, 74, and 37. The compressor power inputs are illustrated in FIG. 3 as elements 32 and 34, and such power inputs may be electric or may be turbine driven. The CO.sub.2 pump electric power input is illustrated as element 33. The turbine shaft power output is illustrated as element 64 from the generator 63.

(23) The exemplary embodiment described was evaluated with ASPEN modeling software using actual machine efficiencies, heat exchanger temperature differences, and system pressure drops giving a net efficiency of 58.5% (LHV basis). The calculation was based on a thermal input of 500 MW to the combustor 30.

(24) Although the disclosed systems and methods may be particularly applicable to combustion systems and methods for power production, a broader application to efficient heating of a gas stream is also encompassed. As such, in some embodiments, the present disclosure can relate to a method for heating a gas stream, and particularly for heating a recirculating gas stream. The recirculating gas stream may be any gas stream that is continuously cycle through stages of heating and cooling, optionally including stages of compression and expansion.

(25) A gas stream G that may be subject to heating according to the present disclosure may be any gas; however, it can be particularly advantageous for the gas stream G to comprise CO.sub.2, such as being at least about 10%, at least about 25%, at least about 50%, at least about 75%, or at least about 90% by mass CO.sub.2. A recirculating gas stream G particularly may be at increased temperature T.sub.1 (e.g., about 500° C. to about 1700° C.) and a pressure P.sub.1 that enables forming a desired amount of heat of compression—e.g., a pressure of less than about 40 bar (4 MPa). The gas stream G at pressure P.sub.1 and temperature T.sub.1 can be cooled, such as by passage through a recuperative heat exchanger. Preferably, cooling is such that the gas stream G is cooled to a temperature T.sub.2 that is less than T.sub.1. In some embodiments, cooling can be carried out using a series of multiple heat exchangers (e.g., 2, 3, or more heat exchangers) or using a heat exchanger that includes a plurality of heat exchange sections or using a combination thereof. The individual heat exchangers (or heat exchange sections) can exchange heat at different temperature ranges, which ranges may overlap. Use of multiple heat exchangers and/or heat exchange sections enables streams to be added or withdrawn at different temperature ranges.

(26) The gas stream G can be separated into a first fraction G.sub.1 and a second fraction G.sub.2. Such separation can occur after the gas stream G has been cooled to the temperature T.sub.2 or to an intermediate temperature T.sub.int that is between T.sub.1 and T.sub.2. The temperature T.sub.2, for example, can be the temperature at the cold end of the recuperative heat exchanger (or the heat exchanger or heat exchange section working over the lowest temperature range), and the temperature T.sub.int, for example, can be a temperature at the cold end of a second heat exchanger (or second heat exchange section) in a series of three or more heat exchangers (or heat exchange sections). Preferably, the second gas fraction G.sub.2 can be withdrawn at an intermediate temperature prior to further cooling of the first gas fraction G.sub.1. After the gas stream fraction G.sub.1 has been cooled, it can then be compressed to a greater pressure P.sub.2 that preferably can be greater than P.sub.1. Such compression, for example, can be carried out with a multi-stage compressor that is intercooled. The pressure P.sub.3 can be, for example, about 40 bar (4 MPa) to about 100 bar (10 MPa), about 60 bar (6 MPa) to about 100 bar (10 MPa) or about 67 bar (6.7 MPa) to about 80 bat (8 MPa).

(27) The withdrawn gas stream fraction G.sub.2 can be separately compressed to a pressure P.sub.3 that also preferably is greater than P.sub.1. The pressure P.sub.3 can be in the same range of pressure P.sub.2; however, P.sub.2 and P.sub.3 do not necessarily need to be identical. In some embodiments, the gas stream fraction G.sub.2 can be compressed using adiabatic compression with no intercooling so as to heat the gas stream fraction G.sub.2 to a temperature T.sub.3 that is greater than T.sub.2. In embodiments wherein the gas stream fraction G.sub.2 can be withdrawn at the intermediate temperature T.sub.int, T.sub.3 preferably is greater than T.sub.int. The heat from the compressed gas stream fraction G.sub.2 can be withdrawn and used as additive heating to the recirculating gas stream as further described below.

(28) After the compression heat has been withdrawn from gas stream fraction G.sub.2, the gas stream fraction G.sub.1 and the gas stream fraction G.sub.2 can be combined to form a combined recirculating gas stream G.sub.C. The recirculating gas stream G.sub.C will have a pressure that is substantially similar to the pressure P.sub.2 and/or P.sub.3 and can be pumped to a greater pressure P.sub.4 that is greater than P.sub.2 and greater than P.sub.3. Such pumping is desirable is the recirculating gas stream G.sub.C is being utilized in a high pressure application. In some embodiments, however, the pressure P.sub.2 and/or P.sub.3 may be suitable and no further compression may be required.

(29) The recirculating gas stream G.sub.C (optionally at the pressure P.sub.4) can be passed to the recuperative heat exchanger such that the gas stream G.sub.C is heated by the cooling gas stream G. The heat withdrawn from the compressed gas stream fraction G.sub.2 can be added to the recirculating gas stream G.sub.C. Such additive heating can be carried out after pumping to pressure P.sub.4. In some embodiments, the additive heating can be carried out in the recuperative heat exchanger. For example, if a single recuperative heat exchanger is used, the heat of compressed gas stream fraction G.sub.2 can be input to the heat exchanger at a suitable point to provide the additive heating to the recirculating gas stream G.sub.C in the desired temperature range. In embodiments wherein a plurality of heat exchanger (or heat exchange sections) are used, the heat of compressed gas stream fraction G.sub.2 can be added to one or more of the lower temperature heat exchangers (or heat exchange sections). For example, during compression, gas stream fraction G.sub.2 can be heated to a temperature in the range of about 100° C. to about 400° C., and the heat from the compressed gas stream fraction G.sub.2 can be added to one or more heat exchangers (or heat exchange sections) working in this temperature range. In FIG. 3, for example, compressed gas stream fraction G.sub.2 would equate to stream 52, which is passed through heat exchangers 26 and 27, which are working at a lower temperature range than heat exchanger 29. Generally, a series of heat exchangers such as illustrated in FIG. 3, comprises three heat exchangers that each transfer in separate temperature ranges (which ranges may overlap). In the example of FIG. 3, heat exchanger 29 can be characterized as operating in a temperature range R.sub.1, heat exchanger 27 can be characterized as operating in a temperature range R.sub.2, and heat exchanger 26 can be characterized as operating in a temperature range R.sub.3. As illustrated, since heat exchanger 29 is at the hot end of the series and heat exchanger 26 is at the cold end of the series, the temperature relationship of the series of heat exchangers would be R.sub.1>R.sub.2>R.sub.3.

(30) The use of the additive heating provided by the compression heat in compressed gas stream fraction G.sub.2 can be beneficial to bring the temperature of the combined recirculating gas stream G.sub.C significantly close to the temperature of gas stream G prior to cooling. For example, the recirculating gas stream G.sub.C after passing through the recuperative heat exchanger and receiving the heat from the compressed gas fraction G.sub.2 can have a temperature T.sub.4 that is within 50° C. of T.sub.1. Typically, the temperature T.sub.4 of recirculating gas stream G.sub.C after passing through the recuperative heat exchanger will remain below T.sub.1. In such embodiments, recirculating gas stream G.sub.C after passing through the recuperative heat exchanger and receiving the heat from the compressed gas fraction G.sub.2 can have a temperature T.sub.4 that is less than T.sub.1 by no more than 50° C.

(31) The approach of T4 to T1 can be further improved through addition of heat from one or more additional sources. Such additional heat source can comprise any device or combination of devices configured to impart heating to a stream that is sufficient to heat a gas stream as described herein so that the gas stream achieves the desired quality and quantity of heat. As non-limiting examples, the additional heat source can be one or more of a combustion heat source, a solar heat source, a nuclear heat source, a geothermal heat source, and an industrial waste heat source. The additional heat source may include a heat exchanger, a heat pump, a power producing device, and any further combination of elements (e.g., piping and the like) suitable to form, provide, or deliver the necessary heat.

(32) The method for heating a recirculating gas stream can further comprise one or more steps. For example, the gas stream G may be a stream exiting a turbine. As such, the pressure P.sub.1 of gas stream G can be less than an earlier pressure P.sub.0 of the gas stream before passage through the turbine. In some embodiments, P.sub.0 can be substantially similar to P.sub.4 (e.g., within 10%, within 5%, or within 2% thereof). In some embodiments, recirculating gas stream G.sub.C can be subjected to a superheating step after exiting the hot end of the heat exchanger (i.e., after being re-heated in the heat exchanger and receiving the additive heat of compression from G.sub.2). For example, recirculating gas stream G.sub.C can be heated with heat of combustion, with solar heating, with nuclear heating, with geothermal heating, with industrial waste heating, or with any combination thereof. In some embodiments, recirculating gas stream G.sub.C can be so-heated and then passed through a turbine for expansion and power production. The stream leaving the turbine may then be characterized again as gas stream G.

(33) Many modifications and other embodiments of the presently disclosed subject matter will come to mind to one skilled in the art to which this subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the present disclosure is not to be limited to the specific embodiments described herein and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.