Production of low pressure liquid carbon dioxide from a power production system and method

Abstract

The present disclosure relates to systems and methods that provide a low pressure liquid CO.sub.2 stream. In particular, the present disclosure provides systems and methods wherein a high pressure CO.sub.2 stream, such as a recycle CO.sub.2 stream from a power production process using predominately CO.sub.2 as a working fluid, can be divided such that a portion thereof can be expanded and used as a cooling stream in a heat exchanger to cool the remaining portion of the high pressure CO.sub.2 stream, which can then be expanded to form a low pressure CO.sub.2 stream, which may be in a mixed form with CO.sub.2 vapor. The systems and methods can be utilized to provide net CO.sub.2 from combustion in a liquid form that is easily transportable.

Claims

1. A system configured for production of a low pressure liquid carbon dioxide (CO.sub.2) stream, the system comprising: a splitter configured for dividing a high pressure CO.sub.2 stream into a first portion and a second portion; a first expander configured for expanding and cooling the first portion of the high pressure CO.sub.2 stream; a heat exchanger for cooling the second portion of the high pressure CO.sub.2 stream against the cooled first portion of the high pressure CO.sub.2 stream exiting the expander; a second expander configured for expanding the cooled second portion of the high pressure CO.sub.2 stream so as to form the low pressure liquid CO.sub.2 stream; a combined stripping column and re-boiler, the stripping column being in line downstream from the second expander, and the re-boiler being in line downstream from the heat exchanger and upstream from the second expander; and a liquid/vapor separator positioned downstream from the second expander and upstream from the stripping column.

2. The system according to claim 1, wherein the first expander is configured for cooling the first portion of the high pressure CO.sub.2 stream to a temperature of −20° C. or less.

3. The system according to claim 1, wherein the heat exchanger is configured for cooling the second portion of the high pressure CO.sub.2 stream to a temperature of 5° C. or less.

4. The system according to claim 1, wherein the second expander is configured to expand the cooled second portion of the high pressure CO.sub.2 stream to a pressure that is 30 bar (3 MPa) or less but is greater than the triple point pressure of CO.sub.2.

5. The system according to claim 1, further comprising a compressor configured for receiving the first portion of the high pressure CO.sub.2 stream from the heat exchanger.

6. The system according to claim 1, further comprising: a combustor configured for combusting a carbonaceous or hydrocarbon fuel with oxygen in the presence of a recycle CO.sub.2 stream at a pressure of 100 bar (10 MPa) or greater and a temperature of 400° C. or greater to form a combustor exit stream comprising CO.sub.2; a turbine configured for expanding the combustor exit stream to generate power and form a turbine exit stream comprising CO.sub.2; a further heat exchanger configured for cooling the turbine exit stream; and a pump configured for pumping CO.sub.2 from the cooled turbine exit stream to form the high pressure CO.sub.2 stream.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

(2) FIG. 1 shows a flow diagram of a system according to embodiments of the present disclosure for formation of a low pressure liquid CO.sub.2 stream; and

(3) FIG. 2 shows a flow diagram of a system according to embodiments of the present disclosure for formation of a low pressure liquid CO.sub.2 stream utilizing a portion of a high pressure CO.sub.2 stream drawn from a power production process.

DETAILED DESCRIPTION

(4) 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.

(5) The present disclosure relates to systems and methods adapted for production of low pressure liquid carbon dioxide (CO.sub.2). The systems and methods particularly may be adapted to intake a stream comprising non-liquid CO.sub.2 (e.g., gaseous CO.sub.2 or supercritical CO.sub.2) and convert at least a portion of the non-liquid CO.sub.2 to liquid CO.sub.2. The intake stream may comprise a fraction of liquid CO.sub.2, however, the intake stream preferably comprises no more than about 25%, no more than about 10%, no more than about 5%, or no more than about 2% by weight liquid CO.sub.2.

(6) Liquid CO.sub.2 produced according to the present disclosure can be produced at a low pressure in that the pressure of the produced liquid CO.sub.2 is less than 50 bar (5 MPa) but greater than the triple point pressure of CO.sub.2 so as to preferably avoid substantial formation of solid CO.sub.2. In some embodiments, the produced liquid CO.sub.2 can be at a pressure of down to about 6 bar (0.6 MPa), in particular about 30 bar (3 MPa) to about 6 bar 0.6 MPa), about 25 bar 2.5 MPa) to about 6 bar (0.6 MPa), or about 15 bar (1.5 MPa) to about 6 bar (0.6 MPa). The temperature of the produced liquid CO.sub.2 preferably is in the range of the saturation temperature at the given pressure. For example, the temperature can be in the range of about 5° C. to about −55° C., about −5° C. to about −55° C., or about −15° C. to about −55° C.

(7) Methods of producing liquid CO.sub.2 according to embodiments of the present disclosure generally can comprise cooling and expanding the CO.sub.2 from the intake stream. Depending upon the source of the intake stream, the methods may comprise one or more compression steps. In preferred embodiments, the intake CO.sub.2 can be at a pressure of about 60 bar (6 MPa) or greater, about 100 bar (10 MPa) or greater, or about 200 bar (20 MPa) or greater. In other embodiments, the pressure of the intake CO.sub.2 can be in the range of about 60 bar (6 MPa) to about 400 bar (40 MPa). The temperature of the intake CO.sub.2 may be greater than 10° C. or may be in the range of about 10° C. to about 40° C., about 12° C. to about 35° C., or about 15° C. to about 30° C. In some embodiments, the intake CO.sub.2 can be at about ambient temperature.

(8) An embodiment of a system and method according to the present disclosure useful in the production of liquid CO.sub.2 is shown in FIG. 1. As seen therein, a high pressure CO.sub.2 stream 24 may be cooled by passage through a water cooler 50 (which may be optional depending upon the actual temperature of the high pressure CO.sub.2 stream). The high pressure CO.sub.2 stream 24 is then divided into a first portion and a second portion using a splitter 68 (or other suitable system element configured for dividing a stream) to provide a high pressure CO.sub.2 side stream 57 that can be expanded, such as through a valve 58 or other suitable device, to form a cooling CO.sub.2 stream 56. The remaining high pressure CO.sub.2 stream 62 passes through a heat exchanger 10 where it is cooled by the cooling CO.sub.2 stream 56, which exits as CO.sub.2 stream 33. The cooled, high pressure CO.sub.2 stream 51 exiting the cold end of the heat exchanger 10 can be at a temperature of about 5° C. or less, about 0° C. or less, about −10° C. or less, or about −20° C. or less (for example, about 5° C. to about −40° C. or about 0° C. to about −35° C.). The cooled, high pressure CO.sub.2 stream 51 can be expanded to form the liquid CO.sub.2 stream. As illustrated in FIG. 1, the cooled, high pressure CO.sub.2 stream 51 first passes through a re-boiler 52, which is part of a stripping column 53 in FIG. 1, and thus supplies heating for the distillation therein, which is further described below. Passage through the re-boiler thus may be optional. The high pressure CO.sub.2 stream 55 leaving the re-boiler 52 is expanded to form the low pressure liquid CO.sub.2 stream 35 at a temperature and pressure in the ranges described above. In FIG. 1, stream 55 is expanded through a valve 48, but any device useful for expanding a compressed CO.sub.2 stream may be used. For example, the expansion device can be a work producing system, such as a turbine, which lowers the enthalpy of the CO.sub.2 between the inlet and the outlet and further lowers outlet temperature.

(9) The expansion of the high pressure CO.sub.2 stream (e.g., from the range of about 60 bar (6 MPa) to about 400 bar (40 MPa)) to form the low pressure CO.sub.2 stream (e.g., at a pressure of about 30 bar (3 MPa) or less but greater than the triple point pressure of CO.sub.2) can result in a two phase product stream formed of a gas and liquid mixture having the same total enthalpy as the CO.sub.2 stream input to the valve (or other expansion device). The temperature of the two phase mixture leaving the valve (or a turbine per the exemplary, alternative embodiment noted above) particularly can be at the saturation temperature of the liquid at the reduced pressure. In FIG. 1, stream 56 exiting valve 58 and stream 35 exiting valve 48 may both be two phase streams. The two phase, low pressure CO.sub.2 stream 35 exiting valve 48 may be passed through a separator 9 to provide the CO.sub.2 vapor fraction stream 49 and the CO.sub.2 liquid fraction stream 36.

(10) In embodiments wherein the input high pressure CO.sub.2 stream is from an oxy-combustion power production system, the vapor fraction that can be separated from the low pressure liquid CO.sub.2 stream will contain the bulk of the inert gases (e.g., nitrogen, excess O.sub.2, and noble gases, such as argon) that are present in the oxygen source and the fuel source (e.g., natural gas). As a non-limiting example, an oxy-combustion power production process may be carried out with a 1% excess oxygen stream flow into a combustor, the oxygen stream being formed of approximately 99.5% oxygen and 0.5% argon. The resulting net CO.sub.2 product can include O.sub.2 at a 2% concentration and argon at a 1% concentration.

(11) According to the present disclosure, cooling of a CO.sub.2 product from a power system as exemplified above by indirect cooling means to a temperature which, on expansion through a valve to a pressure of, for example, 10 bar (1 MPa), results in a flash vapor fraction of approximately 4%. In various embodiments, the vapor fraction may be up to about 6%, up to about 5%, or up to about 4% by mass of the total liquid CO.sub.2 stream (e.g., stream 35 in FIG. 1). The vapor stream (e.g., stream 49 in FIG. 1) can comprise about 1% to about 75% by mass CO.sub.2 and about 25% to about 99% by mass of a combination of N.sub.2, O.sub.2, and argon (or other inert gases). In further embodiments, the vapor stream can comprise about 60% or greater, about 65% or greater, or about 70% or greater by mass of the combination of N.sub.2, O.sub.2, and argon (or other inert gases). The flash vapor fraction (e.g., stream 49 leaving the separator 9 in FIG. 1) may be vented to the atmosphere or captured. Production of the flash vapor stream is beneficial in embodiments where the input CO.sub.2 stream is derived from an oxy-combustion process as removal of the vapor fraction will prevent a build-up of inert argon and/or nitrogen (which may be present in natural gas and/or coal derived fuel gas that is combusted and which may be present in an oxygen stream derived from a cryogenic air separation plant). To form the flash vapor fraction, it can be useful to cool the high pressure CO.sub.2 stream (e.g., stream 62 in FIG. 1) to a temperature of about −30° C. or less or about −33° C. or less prior to expansion. In embodiments where the input high pressure CO.sub.2 stream is from a source that may be substantially or completely devoid of inert gases (and optionally oxygen), it may not be necessary to form the flash vapor fraction. In embodiments using natural gas fuel having a significant fraction of N.sub.2 in the oxy-fuel power production process, it can be useful to adjust the temperature to which the stream 51 is cooled so as to ensure the removal of the bulk of the N.sub.2 with the O.sub.2 and argon in stream 49 together with a minimum loss of CO.sub.2 in stream 49.

(12) Preferably, the majority of the concentration of O.sub.2 and argon (and other inert gases) from the input CO.sub.2 stream is removed in the flash vapor fraction such that the CO.sub.2 liquid fraction stream (e.g., stream 36 in FIG. 1) has only a minor concentration of N.sub.2, O.sub.2, and argon—e.g., about 1% or less, about 0.5% or less, or about 0.2% or less by mass. This minor concentration of N.sub.2, O.sub.2, and argon can be stripped from the CO.sub.2 liquid fraction stream, such as by using a distillation apparatus (e.g., the stripping column 53 in FIG. 1). Alternatively to the illustration of FIG. 1, a stripping section may be fitted in the lower part of the flash separator. In embodiments utilizing the stripping column, a re-boiler (component 52 in FIG. 1 as discussed above) can be included to withdraw remaining available heat from part or all of the high pressure CO.sub.2 stream (e.g., stream 51 in FIG. 1). Such heating can be varied to provide the necessary liquid to vapor ratio to reduce the oxygen concentration in the net liquid CO.sub.2 product (stream 54 in FIG. 1). The oxygen concentration in the net liquid CO.sub.2 stream can be no more than about 25 ppm, no more than about 20 ppm, or no more than about 10 ppm.

(13) In further embodiments, the product liquid CO.sub.2 stream 54 can be pumped to a high pressure and heated in heat exchanger 10 (or in a further heat exchanger or by further means) for delivery into a CO.sub.2 pipeline. The product liquid CO.sub.2 stream particularly may be pumped to a pressure of about 100 bar (10 MPa) to about 250 bar (25 MPa).

(14) Returning to FIG. 1, the top product 63 leaving the stripping column 53 may be further reduced in pressure if desired, such as in valve 64 and then combined with CO.sub.2 stream 33. The combined streams may be compressed in compressor 34 to provide a return high pressure CO.sub.2 stream 21, which may be, for example, combined with the input high pressure CO.sub.2 stream 24 or added to a further CO.sub.2 containing stream (see FIG. 2).

(15) The foregoing embodiments for forming a low pressure liquid CO.sub.2 stream can be economically desirable in that about 95% or greater, about 96% or greater, or about 97% or greater by mass of the CO.sub.2 in the net low pressure CO.sub.2 stream (e.g., stream 35 in FIG. 1) can be removed as the low pressure liquid CO.sub.2 stream. In the embodiments described above, about 1.5% to about 2.5% by mass of the net CO.sub.2 product may be vented to the atmosphere with the combined N.sub.2, O.sub.2, and argon stream (e.g., stream 49 in FIG. 1), thus providing a CO.sub.2 removal efficiency of about 97.5% to about 98.5%. In embodiments wherein the above-described method is carried out in connection with a closed cycle power system using CO.sub.2 as the working fluid, the stream 49 preferably is vented to the atmosphere because removal of the inert components is desirable to keep their partial pressure and concentration as low as possible. Optionally, the stream 59, following pressure reduction in valve 60, can be routed through a set of passages in the heat exchanger 10 to provide extra refrigeration for cooling the stream 62 before the stream 59 is vented.

(16) The utilization of an input high pressure CO.sub.2 stream 24 provides a unique ability to provide indirect cooling to the high pressure CO.sub.2 stream. As described in relation to the embodiments above, the indirect cooling can be provided by dividing out a portion of the high pressure CO.sub.2 stream at near ambient temperature and then expanding this divided portion of the high pressure CO.sub.2 stream to a temperature of about −20° C. or less, about −30° C. or less, or about −40° C. or less (e.g., approximately −40° C. to about −55° C.). This can be achieved by reducing the pressure of the high pressure CO.sub.2 stream 24 down to less than about 20 bar (2 MPa), less than about 10 bar (1 MPa), or less than about 8 bar (0.8 MPa) (e.g., about 20 bar (2 MPa) to about 5 bar (0.5 MPa) or about 12 bar (1.2 MPa) to about 5 bar (0.5 MPa), particularly about 5.55 bar (0.555 MPa)). The resulting liquid plus vapor stream (e.g., stream 56 in FIG. 1) in then used to cool the bulk high pressure CO.sub.2 stream indirectly in a heat exchanger.

(17) The systems and methods of the present disclosure are particularly beneficial when used in combination with a power production method utilizing a CO.sub.2 working fluid, such as the systems disclosed in U.S. Pat. No. 8,596,075, the disclosure of which is incorporated herein by reference in its entirety. In particular, such process can use 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. Preferably, the fuel is a gaseous fuel; however, non-gaseous fuels are not necessarily excluded. Non-limiting examples include natural gas, compressed gases, fuel gases (e.g., comprising one or ore 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, and the like, may be used as well with incorporation of necessary system elements (such as with the use of a partial oxidation combustor or a gasifier to convert the solid or heavy liquid fuels to a gaseous form). Liquid hydrocarbon fuels may also be used. Pure oxygen can be used as the oxidant in the combustion process. The hot turbine exhaust is used to partially preheat the high pressure recycle CO.sub.2 stream. The recycle CO.sub.2 stream is also heated using heat derived from the compression energy of a CO.sub.2 compressor, as further discussed herein. All fuel and combustion derived impurities such as sulfur compounds, NO, NO.sub.2, CO.sub.2, H.sub.2O, Hg and the like can be separated for disposal with no emissions to the atmosphere. A CO.sub.2 compression train is included and comprises high efficiency units that ensure minimum incremental power consumption. The CO.sub.2 compression train can particularly provide a recycle CO.sub.2 fuel compressor flow that can be recycled in part to the combustor and directed in part to the liquid CO.sub.2 production components as the input high pressure CO.sub.2 stream.

(18) FIG. 2, for example illustrates a power production system combined with elements as described herein to produce the net CO.sub.2 product derived from carbon in the primary fuel in the form of a low pressure liquid with an oxygen content in a minimal range as described herein. An embodiment of such system is described in the Example below in connection to FIG. 2.

(19) The magnitude of the total CO.sub.2 net product flow can be vary depending upon the nature of the fuel used. In embodiments utilizing a natural gas fuel, the total CO.sub.2 net product flow can be about 2.5% to about 4.5% (e.g., about 3.5%) of the total recycle CO.sub.2 fuel compressor flow. In embodiments utilizing a typical bituminous coal (e.g., Illinois No. 6), the total CO.sub.2 net product flow can be about 5% to about 7% (e.g., about 6%) of the total recycle CO.sub.2 fuel compressor flow. The quantity of recycled CO.sub.2 used for refrigeration can be in the range of about 15% to about 35% or about 20% to about 30% (e.g., about 25%) by mass of the net CO.sub.2 product flow.

(20) In some embodiments, liquid natural gas (LNG) can be used as a refrigeration source in a manner such as described in U.S. Pat. Pub. No. 2013/0104525, the disclosure of which is incorporated herein by reference in its entirety. In particular embodiments, the LNG can be heated to a temperature approach to the condensing temperature of the CO.sub.2 turbine exhaust (e.g., at a pressure of about 20 bar (2 MPa) to about 40 bar (4 MPa)). The turbine exhaust flow leaving the water separator can be dried in a desiccant drier to a dew point below about −50° C. before being liquefied using refrigeration derived from the high pressure LNG, which is in turn heated. The liquid CO.sub.2 can now be pumped to a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa) using a multi-stage centrifugal pump. The high pressure natural gas will be at a temperature typically in the range of about −23° C. (for turbine exhaust leaving the economizer heat exchanger at about 20 bar (2 MPa)) to about 0° C. (for turbine exhaust leaving the economizer heat exchanger at about 40 bar (4 MPa)) using a 5° C. approach to the saturation temperature of CO.sub.2 at these pressures. This cold, high pressure natural gas can be used to pre-cool the high pressure CO.sub.2 at about 60 bar (6 MPa) to about 400 bar (40 MPa) prior to expansion to produce liquid CO.sub.2 in the pressure range of about 6 bar (0.6 MPa) to about 30 bar (3 MPa). This refrigeration can be supplemented by additional refrigeration derived from expansion of high pressure CO.sub.2 as described above to give a temperature of the cooled net CO.sub.2 product which on expansion to the required pressure of the liquid CO.sub.2 product results in a gas fraction containing about 50% to about 80% by mass of (O.sub.2+N.sub.2+Ar). The effect is to significantly reduce the quantity of additional CO.sub.2 which must be recycled for refrigeration.

EXAMPLE

(21) Embodiments of the present disclosure are further illustrated by the following example, which is set forth to illustrate the presently disclosed subject matter and is not to be construed as limiting. The following describes an embodiment of a combined power production system and method and system and method for production of low pressure liquid CO.sub.2, as illustrated in FIG. 2.

(22) As seen in FIG. 2, a natural gas fuel stream 42 (which in this Example is pure methane) at about 40 bar (4 MPa) is compressed to about 320 bar (32 MPa) in a compressor 44 to provide a compressed natural gas fuel stream 43, which in turn enters a combustion chamber 1 where it combusts in a preheated oxidant stream 38, which comprises about 23% by mass of oxygen mixed with about 77% by mass of diluent CO.sub.2. In the illustrated embodiment, the total oxygen quantity contains approximately 1% by mass more oxygen than is required for stoichiometric combustion. The combustion products are diluted in the combustor 1 by a heated recycle CO.sub.2 stream 37 at about 304 bar (30.4 MPa) and about 707° C. A combustor exit stream 39 at a temperature of about 1153° C. is passed to a turbine 2 inlet, the turbine being coupled to an electric generator 3 and a main CO.sub.2 recycle compressor 4.

(23) The combustor exit stream 39 is expanded in the turbine 2 to provide a turbine exit stream 45 at about 30 bar (3 MPa) and about 747° C., which in turn is passed through an economizer heat exchanger 15 and is cooled to about 56° C. leaving as cooled turbine exit stream 16. The cooled turbine exit stream 16 is further cooled against cooling water in a water cooler 7 to near ambient temperature (stream 17 in FIG. 2). The cooled turbine exit stream 17 is passed through a separator 6 where a liquid water stream 18 is separated from a gaseous CO.sub.2 overhead stream 19, which itself is divided into separate flows (streams 22 and 20 in FIG. 2).

(24) The gaseous CO.sub.2 overhead bulk stream 22 enters the CO.sub.2 recycle compressor 4, which operates with an intercooler 5 and compresses the ambient temperature gaseous CO.sub.2 overhead bulk stream 22 (derived from the turbine exit stream 45) from a pressure of about 28.2 bar (2.82 MPa) to about 63.5 bar (6.35 MPa)—i.e., compressed CO.sub.2 stream 23.

(25) The gaseous CO.sub.2 overhead fraction stream 20 is used to dilute the 99.5% O.sub.2 stream 28 (which is at a pressure of about 28 bar (2.8 MPa)) that is produced by the cryogenic air separation plant 14. Combined streams 20 and 28 form the low pressure oxidant stream 26, which is compressed to about 320 bar (32 MPa) (stream 27) in a compressor 11 with inter-coolers 12. The high pressure oxidant stream 27 is heated in the economizer heat exchanger leaving as the preheated oxidant stream 38 at about 304 bar (30.4 MPa) about 707° C.

(26) A first side-stream 32 at about 110° C. is taken from the heating high pressure recycle CO.sub.2 flow and heated to about 154° C. (stream 31 in FIG. 2) in side heat exchanger 13 against a heat transfer fluid (entering the side heat exchanger as stream 30 and exiting as stream 29) which removes heat of compression from the air compressors in the cryogenic air separation plant 14. The ASU has an atmospheric air feed 40 and a waste nitrogen exit stream 41 which is vented to the atmosphere.

(27) A second side-stream 61 at a temperature of about 400° C. is taken from the heating high pressure recycle CO.sub.2 stream and used in the turbine 2 for internal cooling.

(28) The compressed CO.sub.2 stream 23 at about 63.5 bar (6.35 MPa) and about 51° C. is cooled in a heat exchanger 46 against cooling water to provide stream 47 at about 17.5° C. with a density of about 820 kg/m.sup.3, which is pumped in a multi-stage centrifugal pump 8 to a pressure of about 305 bar (30.5 MPa). The pump discharge flow is divided into two parts.

(29) High pressure recycle CO.sub.2 stream 25 from the pump discharge flow is passed through the economizer heat exchanger 15 and functions as the flow from which the first side-stream and the second side-stream are taken (as discussed above).

(30) The stream 24 from the pump discharge flow comprises the net CO.sub.2 product stream derived from carbon in the natural gas. Stream 24 preferably can include an additional content of CO.sub.2 for use in refrigeration. The additional CO.sub.2 content can be up to about 50% by mass, up to about 40% by mass, or up to about 30% by mass of the recycle CO.sub.2. In some embodiments, the additional CO.sub.2 content can be about 5% to about 45% by mass, about 10% to about 40% by mass, or about 15% to about 35% by mass of the recycle CO.sub.2.

(31) The high pressure CO.sub.2 stream 24 is cooled to near ambient temperature in a water cooler 50 and divided into two parts. High pressure CO.sub.2 fraction stream 57 is reduced in pressure to about 8.2 bar (0.82 MPa) in valve 58 to form a cooling CO.sub.2 stream 56, which is a two phase mixture at a temperature of about −45° C. The cooling CO.sub.2 stream 56 is passed through heat exchanger 10 where it evaporates and heats to near ambient temperature leaving as CO.sub.2 stream 33.

(32) High pressure net CO.sub.2 product stream 62 is passed directly into the heat exchanger 10 where it is cooled against the cooling CO.sub.2 stream 56 to a temperature of about −38° C. leaving as cooled high pressure net CO.sub.2 product stream 51. This stream is then passed through a small re-boiler 52 in the base of a stripping column 53 leaving as stream 55. This stream is reduced in pressure to about 10 bar (1 MPa) in valve 48 to form a two phase net CO.sub.2 product stream 35, which is then passed through a separator 9.

(33) The overhead vapor stream 49 exiting the top of the separator 9 encompasses about 4% by mass of the flow of two phase net CO.sub.2 product stream 35 and is formed of about 30% by mass CO.sub.2 and about 70% by mass of a combination of O.sub.2 and argon. The overhead vapor stream 49 is reduced in pressure in valve 60 and then vented to the atmosphere (stream 59 in FIG. 2). Optionally, stream 59 can be heated in heat exchanger 10 to near ambient temperature providing extra refrigeration and then further heated to above ambient temperature to make the vent stream buoyant.

(34) The liquid CO.sub.2 stream 36 exiting the separator 9 at a pressure of about 10 bar (1 MPa) comprises about 96% by mass of the flow of two phase net CO.sub.2 product stream 35. Stream 36 is fed to the top of the stripping column 53.

(35) Exiting the bottom of the stripping column 53 is the low pressure liquid CO.sub.2 product stream 54, which comprises the net CO.sub.2 produced from carbon in the primary fuel feed to the power system. In the illustrated embodiment, stream 54 has an oxygen content below 10 ppm.

(36) The top product stream 63 exiting the stripping column 53 is reduced in pressure to about 8 bar (0.8 MPa) in valve 64 and added to CO.sub.2 stream 33. Combined streams 33 and 63 are compressed in compressor 34 to about 28.5 bar (2.85 MPa). The discharge stream 21 compressed in the CO.sub.2 compressor 34 is mixed with gaseous CO.sub.2 overhead bulk stream 22 and compressed back up to about 305 bar (30.5 MPa) in the CO.sub.2 compressor 4 and the pump 8.

(37) In the above example, specific values (e.g., temperature, pressure, and relative ratios) are provided to illustrate working conditions of an exemplary embodiment of the present disclosure. Such values are not meant to be limiting of the disclosure, and it is understood that such values may be varied within the ranges as otherwise disclosed herein to arrive at further working embodiments in light of the overall description provided herein.

(38) 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.