A plant for producing carbon dioxide from a flue gas, and method

Abstract

The plant (1; 301; 401) comprises a heat exchanger (37; 337; 437) adapted to receive compressed flue gas containing carbon dioxide, and condense at least part of the carbon dioxide contained therein. A a carbon dioxide removal unit (39;39; 439) of the plant is adapted to remove liquefied carbon dioxide from the flue gas exiting the heat exchanger (37;337; 437). A carbon dioxide outlet duct (61; 391; 479) extends through the heat exchanger (37; 337; 437). The plant further includes a distillation section (6; 306; 406). The distillation section includes a distillation column (201; 360, 501), a reboiler (203; 363; 505) at the bottom of the distillation column (201; 360; 501) and an overhead condenser (205; 361; 503) at the top of the distillation column (201; 360; 501). Also disclosed herein is a method for removing carbon dioxide form a flue gas.

Description

A plant for producing carbon dioxide from a flue gas, and methodDESCRIPTIONTECHNICAL FIELD

[0001] Disclosed herein are plants and methods for removing carbon dioxide from a flue gas.BACKGROUND ART

[0002] Carbon dioxide (CO2) is generated by combustion of fuels, e.g. fossil fuels such as natural gas, in several power generation processes. CO2 is a greenhouse gas having a negative impact on the climate and is considered responsible for climate changes and in particular global warming. While continuous efforts are being made to reduce the amount of power generated by combustion of fossil fuels, these still remain one of the major sources of energy.

[0003] Fossil fuels are also used to supply fuel cells, where chemical energy is converted into electric energy through a chemical reaction not involving combustion. Typical fuel cells, such as Solid Oxide Fuel Cells (SOFC) use hydrogen as a fuel. Hydrogen can be produced from fossil fuels, such as methane (CH4) through reforming, according to the reactionCH4+H2O <--> CO + 3H2

[0004] Hydrogen reacts in the fuel cell with oxygen (O2) contained in ambient air, for instance, and generate electric power, according to the reaction2H2+O2^ 2H2O+electric energy

[0005] Flue gas from SOFC or other fuel cells contain a high concentration of carbon dioxide (CO2) and can contain residual H2 and CO.

[0006] Reducing the amount of CO2 released in the atmosphere is becoming an important aspect of the recent policies aimed at reducing the climate impact of anthropic activities. This also applies to flue gas from fuel cells.

[0007] It has been known for a quite long time to remove CO2 from flue gas by cryogenic separation. See e.g. EP2407741 and EP2365265. According to this known technology, compressed flue gas containing CO2 flows through one or more heat exchangers in sequence, also referred to as cold-boxes, where heat is removed from the flue gas. The temperature reduction causes separation of CO2 by liquefaction. The liquid CO2 is used as the refrigerant in the heat exchanger. To reduce the temperature of the liquefied CO2, the latter is expanded in an expander or through an expansion valve. The expanded and partly vaporized CO2 flows through the cold side of the heat exchangers), while the compressed flue gas flows through the hot side of the heat exchanger. The fully vaporized and heated carbon dioxide exiting the heat exchanger(s) is again compressed by an intercooled CO2 compressor train and finally collected in a storage or further transported in a pipeline.

[0008] Carbon dioxide removed from flue gas can contain a certain amount of impurities, often in the form of chemical species having a volatility higher than carbon dioxide. Known carbon dioxide separation methods and plants usually achieve a purity of 95% in the carbon dioxide product. A higher purity of the carbon dioxide from the separation systems would be preferred but is difficult to achieve with current technologies.

[0009] An improved plant and method for increasing the carbon dioxide purity would be welcomed in the art.SUMMARY

[0010] According to one aspect, disclosed herein is a plant for removing carbon dioxide from of a flue gas, including a distillation column for removing impurities, such as high-volatility impurities from carbon dioxide separated from the flue gas. As understood herein “high-volatility” is understood as a volatility higher than carbon dioxide. The resulting product carbon dioxide may have a purity of 99& or higher.

[0011] In embodiments disclosed herein, the plant comprises a heat exchanger adapted to receive compressed flue gas containing carbon dioxide, and condense at least part of the carbon dioxide contained therein. The plant further comprises a carbon dioxide removal unit, adapted to remove liquefied carbon dioxide from the flue gasexiting the heat exchanger. Carbon dioxide is liquefied in the heat exchanger by heat exchange with a refrigeration medium. As will become apparent from the following description of some embodiments, the refrigeration medium can be a refrigerant circulating in a closed refrigeration circuit. For instance, a mixed refrigerant can be used for such purpose. In other embodiments, to be described, liquefied, and expanded carbon dioxide, which has been separated from the flue gas, is used as a refrigerant in the heat exchanger.

[0012] This second kind of plants are referred to herein as auto-refrigerated plants, as the same chemical species (CO2) which is removed by liquefaction through chilling of the flue gas is used as the refrigerant medium. This is made possible by suitable pressures of the fluids circulating in heat exchange relationship in the heat exchanger.

[0013] The plant further includes a carbon dioxide outlet duct extending through the heat exchanger, wherethrough carbon dioxide is removed from the plant. The CO2- lean flue gas, from which carbon dioxide has been removed, is discharged through a gas discharge duct.

[0014] The plant also includes a distillation section, comprising: a distillation column, a reboiler at the bottom of the distillation column and an overhead condenser at the top of the distillation column. The reboiler and the overhead condenser can be arranged inside the distillation column, thus becoming an integral part thereof, or outside of the distillation column.

[0015] In embodiments disclosed herein, the carbon dioxide removal unit comprises an inlet fluidly coupled with the heat exchanger, a liquid outlet, and a gas outlet fluidly coupled with the gas discharge duct, to remove from the plant a gaseous phase collecting in the carbon dioxide removal unit.

[0016] An inlet of a cold side of the overhead condenser is fluidly coupled with a carbon dioxide feed line adapted to deliver carbon dioxide, which has been separated from the flue gas in the carbon dioxide removal unit, to an inlet of the cold side of the overhead condenser. An outlet of the cold side of the overhead condenser is adapted to route the carbon dioxide flowing through the cold side of the overhead condenser to the distillation column. A condensate outlet of the overhead condenser is configuredto return condensate carbon dioxide to the distillation column. A gas outlet of the overhead condenser is fluidly coupled with the gas discharge duct. By removing the gaseous phase from the carbo dioxide removal unit and discharging it from the plant, the duty required by the overhead condenser is reduced. Moreover, at least a major fraction of non-condensables is removed upstream of the distillation column, and a high purity of the carbon dioxide product can thus be reached more easily.

[0017] The above-summarized layout is adapted to use low-temperature carbon dioxide, which has been separated by liquefaction from the flue gas in the heat exchanger, as a cooling medium in the overhead condenser of the distillation column, to condense carbon dioxide collecting at the top of the distillation column.

[0018] The carbon dioxide entering the hot side of the overhead condenser is condensed by heat exchange with the lower-temperature carbon dioxide circulating in the cold side of the overhead condenser. The condensed carbon dioxide is routed to the distillation column, while the non-condensables, i.e. the impurities which have a higher volatility than carbon dioxide, such as H2, CO, CH4 or other impurities, depending upon the nature and origine of the treated flue gas, are collected at the gas outlet of the reboiler and removed from the plant along with the CO2-lean flue gas.

[0019] According to another aspect, disclosed herein is a method for removing carbon dioxide from a flue gas. The method comprises the following steps: delivering a stream of compressed flue gas containing carbon dioxide through a heat exchanger in heat exchange with a refrigerant; at least partially condensing carbon dioxide contained in the compressed flue gas by heat exchange against said refrigerant and removing the carbon dioxide from the flue gas; separating the condensed carbon dioxide from a gaseous phase and removing the gaseous phase; flowing the condensed carbon dioxide, removed from the flue gas, as a cooling medium through a cold side of an overhead condenser of a distillation column and condensing, through heat exchange therewith, gaseous carbon dioxide coming from the distillation column, thus separating carbon dioxide from non-condensablescontained therein; delivering the carbon dioxide exiting the cold side of the overhead condenser and carbon dioxide condensed in the overhead condenser into the distillation column; removing non-condensables from the overhead condenser; recycling liquid from the bottom of the distillation column in a reboiler and heating the recycling liquid in the reboiler through a heating medium; returning a gaseous flow from the reboiler into the distillation column and extracting liquid carbon dioxide from the reboiler.

[0020] Further features and embodiments of the plant and of the method according to the present disclosure are described below with reference to the attached drawings and set forth in the appended claims.

[0021] The flue gas delivered to the heat exchanger can be pre-treated in a flue gas pre-treatment section. The flue gas pre-treatment section can comprise one or more of the following: a dryer arrangement; a cooler arranged upstream of the dryer arrangement and adapted to pre-cool the flue gas; a water gas shift reactor unit; and a flue gas compressor.

[0022] The pre-treatment section can further include a liquid-vapor separator between the cooler and the dryer arrangement, adapted to remove condensate forming in the flue gas by pre-cooling the flue gas in the cooler.

[0023] In some embodiments, the flue gas pre-treatment section comprises a filter section downstream of a dryer section of the dryer arrangement.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Reference is now made briefly to the accompanying drawings, in which:Fig. l illustrates a plant for removing carbon dioxide from a flue gas in an embodiment;Fig.2 illustrates a detail of the plant of Fig.l;Fig.3 illustrates a plant for removing carbon dioxide from flue gas in a further embodiment; andFig.4 illustrates a plant for removing carbon dioxide from flue gas in a yet further embodiment.DETAILED DESCRIPTION

[0025] In short, plants are disclosed herein, wherein compressed flue gas containing carbon dioxide is processed through a heat exchanger in heat exchange with a refrigerant. Carbon dioxide contained in the flue gas is condensed by heat exchange in the heat exchanger and the CO2-lean flue gas is discharged from the plant. The product carbon dioxide is further processed, e.g., stored, or transported. To enhance the purity of the product carbon dioxide, the plant includes a distillation section, wherein the carbon dioxide separated from the flue gas is purified and chemical species, having a higher volatility than carbon dioxide, are removed therefrom. The distillation section includes a distillation column which includes, or is combined with an overhead condenser and a reboiler. Low temperature carbon dioxide which has been separated from the flue gas is used as a refrigeration medium in the cold side of the overhead condenser of the distillation section. A gaseous blend of carbon dioxide and higher volatility species flows from the top of the distillation column through the hot side of the overhead condenser, in heat exchange with the carbon dioxide flowing through the cold side of the overhead condenser. By heat exchange, the gaseous carbon dioxide from the top of the distillation column is condensed and the non-condensable species are removed from the overhead condenser in gaseous form. At the bottom of the distillation column, liquid is circulated the reboiler. The high-volatility impurities and a portion of carbon dioxide vaporize in the reboiler and stream towards the top of the distillation column. The liquid collecting at the bottom of the reboiler, consisting of high- purity carbon dioxide, is removed as product carbon dioxide from the plant. Heat to the reboiler can be supplied by a fluid circulating in a closed refrigerant circuit. In other embodiments, the incoming flue gas can be used to supply heat to the reboiler.

[0026] Turning now to the drawings, a schematic diagram of a first embodiment of a plant according to the present disclosure is shown in Fig.l. The plant 1 comprises a pre-treatment section 3, and a carbon dioxide separation section 5, which will be referred herein as carbon dioxide liquefaction section 5. As will be explained in more detail below, however, the product carbon dioxide delivered by the carbon dioxideseparation section 5 can be in a liquid, gaseous, or supercritical state. Thus, even though reference is made herein to a liquefaction section 5, it shall be understood that carbon dioxide can be removed from the flue gas and delivered as a product of the plant in a non-liquefied condition.

[0027] The plant 1 further comprises a distillation section 6. The distillation section is aimed at removing residual components, in particular highly volatile gaseous components (non-condensables) from the carbon dioxide before delivering the carbon dioxide to a pipeline, a collector tank, or another plant for further processing.

[0028] The plant 1 can be used for instance to remove carbon dioxide from a flue gas produced by a fuel cell system, for instance a solid oxide fuel cell (SOFC) system, which uses natural gas as a fuel. Figs. 1 and 2 illustrate one embodiment of the liquefaction section 5. It shall be understood, that the system and method disclosed herein, aimed at improving the production of carbon dioxide by increasing the purity level thereof, can use a different carbon dioxide separation section, or liquefaction section. Some alternative embodiments of the carbon dioxide separation section are described later on.

[0029] The pre-treatment section 3 comprises a flue gas inlet 7 fluidly coupled to a source of flue gas, for instance a fuel cell arrangement, or another source of flue gas, such as a gas turbine engine, a boiler, or the like.

[0030] The pre-treatment section 3 can include a first liquid-vapor separator 9 having an inlet 9.1 fluidly coupled to the flue gas inlet 7, a liquid outlet 9.2 and a gas outlet 9.3.

[0031] The gas outlet 9.3 of the first liquid-vapor separator 9 is fluidly coupled with a suction side of a flue gas compressor unit 11, herein after referred to simply as compressor unit 11.

[0032] The compressor unit 11 is represented as a single compressor for the sake of simplicity. In some embodiments, the compressor unit 11 can include an intercooled multi-stage compressor or compressor train. In some embodiments, the delivery side of the compressor unit 11 is connected to a post-cooler 15 and a second liquid-vaporseparator 17, which comprises an inlet 17.1, a liquid outlet 17.2 and a gas outlet 17.3. Condensed water from the liquid outlet 9.2 of the first liquid-vapor separator 9 and from the liquid outlet 17.2 of the second liquid-vapor separator 17 is collected in a condensate removal line 21.

[0033] In some embodiments, the pre-treatment section 3 can include a water-gasshift reactor system 23 (shortly WGS reactor), adapted to convert carbon monoxide possibly contained in the flue gas and steam into carbon dioxide and hydrogen. This can be particularly useful when the flue gas processed in the system originates from a SOFC system.

[0034] The water gas shift reactor system 23 comprises a preheater 23.1, a heater 23.2, a reactor 23.3, a cooler 23.4, a water filter 23.5 and a water pump 23.6. The water filter 23.5 and the water pump 23.6 are located along a condensate recycling line 25 which feeds condensate from a third liquid-vapor separator to be described below.

[0035] As mentioned above, the compressor unit 11 usually includes a plurality of compressors or compressor stages with at least one interstage cooler, aka intercooler. In such case, a preferred location for the WGS reactor will be after a first stage of the compressor unit 11, directly before the interstage cooler. At this location preheater 23.1 and maybe even heater 23.2 may be avoided, when making use of the heat of compression. Depending of the water content of the incoming gas, that location may even allow to avoid pump 23.6.

[0036] The flue gas treated in the water-gas-shift reactor 23, if present, can be fed through further components of the pre-treatment section 3. In the embodiment of Fig.1 the pre-treatment section 3 includes, for instance, a mercury absorber 27 followed by a cooler 29 which is in turn fluidly coupled to the inlet 31.1 of a third liquid-vapor separator 31, including a liquid outlet 31.2 and a gas outlet 31.3. The third liquid-vapor separator 31 is adapted to separate water condensate from the flue gas. The condensate is delivered to the water-gas-shift reactor 23 through the condensate recycling line 25. If the water- gas-shift reactor 23 is not present, the liquid outlet 31.2 of the third liquidvapor separator 31 can be coupled to the condensate removal line 21, for instance.

[0037] The gas outlet 31.3 of the third liquid-vapor separator 31 is fluidly coupled toa flue gas dryer arrangement 33. The flue gas dryer arrangement 33 is schematically represented as an adsorption-type single flue gas dryer section 33.1 containing a desiccant bed 33.2. However, in order to allow continuous operation of the plant, the flue gas dryer arrangement 33 usually includes two flue gas dryer sections, such that while one flue gas dryer section is operating, the other flue gas dryer section undergoes a regeneration cycle to regenerate the desiccant, e.g., by a carbon dioxide stream or another water-lean gas stream, not shown. A filter 33.3 is arranged downstream of the flue gas dryer section, or of each flue gas dryer section to prevent adsorbent fines transport into the downstream section.

[0038] The dehydrated flue gas is delivered from the pre-treatment section 3 to the carbon dioxide liquefaction section 5 through a pre-treated flue gas inlet line 35.

[0039] In other embodiments, the water-gas shift reactor system 23 can be dispensed with. In such case, flue gas can be delivered from the flue gas inlet 7 directly to the flue gas cooler 29 and to the flue gas dryer arrangement 33, possibly after processing through the compressor unit 11 and liquid-vapor separator 17.

[0040] With continuing reference to Fig.1, the carbon dioxide separation section 5, referred herein also as carbon dioxide liquefaction section 5, is best shown in Fig.2. In this embodiment, the carbon dioxide liquefaction section 5 comprises a heat exchanger, aka cold box, 37, a liquefied carbon dioxide removal unit 39 and a refrigeration circuit 41. As will be described in more detail below, the refrigeration circuit 41 is a closed refrigeration circuit. A refrigerant, such as in particular a mixed refrigerant (MR), circulates and undergoes cyclic thermodynamic transformations to remove heat from the cold box or heat exchanger 37. The refrigeration circuit is separate from the flue gas and carbon dioxide circuit.

[0041] More in detail, the liquefied carbon dioxide removal unit 39 includes a first separation drum 43, an intermediate separation drum 45 and a further separation drum 47. The intermediate separation drum 45 will be referred herein also as the second separation drum, and the further separation drum 47 will be referred to herein also as the third separation drum. In other embodiments, not shown, the number of separation drums can be less than three or more than three. For instance, in one embodiment theliquefied carbon dioxide removal unit 39 can include only the first separation drum 43, or only the first and the third separation drums 43, 47, or more than three separation drums in sequence.

[0042] Generally speaking, each separation drum, except the last one, has a gas outlet fluidly coupled to an inlet of the next separation drum through a connection line which extends through the heat exchanger, or cold-box, 37.

[0043] When the CO2 terminal delivery conditions are higher than the liquefaction operating conditions, the liquefied carbon dioxide removal unit 39 further comprises a pumping unit which, in the schematic of Figs 1 and 2, is represented as a single pump 49. The pumping unit 49 has a suction side 49.1 and a delivery side 49.2. The suction side 49.1 of the pumping unit 49 is fluidly coupled to a reboiler of the distillation section, as will be described in more detail below.

[0044] Specifically, in the embodiment of Figs 1 and 2, the first separation drum 43 has an inlet 43.1, a liquid outlet 43.2 and a gas outlet 43.3. The second, or intermediate, separation drum 45 has an inlet 45.1, a liquid outlet 45.2 and a gas outlet 45.3. The third, or further, separation drum 47 has an inlet 47.1, a liquid outlet 47.2 and a gas outlet 43.3. The liquid outlets 43.2, 45.2 and 47.2 are connected to a collector line 51, which is fluidly coupled through a carbon dioxide feed line 52 to the distillation section 6 as will be described in more detail below.

[0045] The inlet 43.1 of the first separation drum 43 is connected to the pre-treated flue gas inlet line 35 through a first heat exchanging connection 53 extending through the heat exchanger 37. The gas outlet 43.3 of the first separation drum 43 is connected to the inlet of the second, or intermediate, separation drum 45 through a connection duct 55 comprising a second heat exchanging connection 55.1, extending through the heat exchanger 37. The gas outlet 45.3 of the second, or intermediate, separation drum 45 is connected to the inlet 47.1 of the third, or further, separation drum 47 through a connection duct 57, comprising a third heat exchanging connection 57.1 extending through the heat exchanger 37.

[0046] The connection 57 and the intermediate, or second, separation drum 45 forms a fluid connection between the gas outlet of the first separation drum and the inlet ofthe further, or third, separation drum 47.

[0047] The gas outlet 47.3 of the third, or further, separation drum 47 is fluidly coupled with a gas discharge duct 59. The gas discharge duct 59 has a first heat exchange section 59.1 and a second heat exchange section 59.2 arranged in sequence and extending through the heat exchanger 37. An expansion device, such as an expansion valve or an expander, is located along the gas discharge duct 59 between the first heat exchange section 59.1 and the second heat exchange section 59.2. In the schematic of Fig.2 the expansion device includes an expander 59.3, wherethrough CCh-lean flue gas is expanded.

[0048] The delivery side of the pumping unit 49.2 is connected to a pressurized carbon dioxide outlet duct 61, wherethrough the pressurized liquid carbon dioxide is removed. The carbon dioxide outlet duct 61 includes a heat exchange section 61.1 extending through the heat exchanger 37.

[0049] The refrigeration circuit 41 comprises a compression section, a refrigerant cooling and condensing section, a refrigerant expander section, a heat rejection section, and a heat absorption section. A refrigerant flows through the refrigeration circuit 41 and undergoes cyclic thermodynamic transformations to pump heat out of the heat exchanger 37 using mechanical power generated by a driver, for instance an electric motor or a turbine.

[0050] In the schematic of Figs 1 and 2 the driver is shown at 63 and the compression section is represented as a single compressor 65. It shall be understood that the compression section can include more than one compressor, or a multistage compressor, possibly an intercooled multistage compressor or an intercooled compressor train including a plurality of compressors in sequence and at least one intercooler between the delivery side of an upstream compressor and the suction side of a downstream compressor.

[0051] In the embodiment of Figs. 1 and 2, the refrigerant cooling and condensing section comprises refrigerant cooler 67.1, a refrigerant condenser 67.2, and a further cooling section, which is positioned in the distillation section 6, as will be described in more detail below, where compression heat of the refrigerant is used in a distillationreboiler. The refrigeration circuit further comprises a heat rejection section including a heat rejection flow path 69, and a heat absorption section including a heat absorption flow path 73 both extending through the heat exchanger 37. A refrigerant expansion device, for instance an expansion valve 71, or an expander, is placed between the heat rejection flow path 69 and the heat absorption flow path 73.

[0052] The distillation section 6 comprises a distillation column 201, a reboiler 203 at the bottom of the distillation column 201 and an overhead condenser 205 at the top of the distillation column. The purpose of the distillation section is to remove impurities, i.e., contaminants, having a volatility higher than carbon dioxide, from the liquefied carbon dioxide delivered through the collector line 51 and the carbon dioxide feed line 52, before delivering the carbon dioxide, removed from the flue gas, to the pressurized carbon dioxide outlet duct 61. A purity of as high as 99.99% of the carbon dioxide product delivered at 61 can be obtained with the distillation section 6 described herein.

[0053] Impurities or contaminants having a volatility higher than carbon dioxide may include different chemical species. For instance, if flue gas is generated by a SOFC system, such contaminants may include hydrogen, carbon monoxide and methane.

[0054] The reboiler 203 has a cold side which is connected to the bottom of the distillation column 201 through a recirculation line 207 wherethrough CCh-rich liquid is withdrawn from the bottom of the distillation column 201 and fed to the reboiler 203. A return line 209 fluidly couples the cold side of the reboiler 203 to the distillation column 201 and returns a gaseous flow containing contaminants and vaporized carbon dioxide to the distillation column 201.

[0055] When referred to a heat exchanger, a “cold section” as used herein is the section of the heat exchanger where a fluid flows, which receives heat by heat exchange from a “hot side” of the heat exchanger. Thus, as used herein, a “hot side” of a heat exchanger is the side where a fluid flows, which transfers heat to the fluid which flows in the cold side of the heat exchanger.

[0056] While in the schematic of Fig.2 the reboiler 203 is shown as a component separate from the distillation column 201, in some embodiments the reboiler 203 mayalso be incorporated directly into the bottom section of the distillation column 201.

[0057] Liquid carbon dioxide is removed from the reboiler 203 through a carbon dioxide removal line 210, featuring a liquid outlet of the reboiler 203. A pressure control valve 221, or an expansion valve, can be positioned along the carbon dioxide removal line 210, the latter being fluidly coupled with the carbon dioxide outlet duct 61 through pump 49.

[0058] Heat is delivered to the reboiler by the compressed refrigerant, for instance fed from the refrigerant condenser 67.2. The refrigerant flows through a hot side of the reboiler and is used herein as a heating medium in the hot side of the reboiler 203. Thus, the hot side of the reboiler 203 features the further cooling section of the refrigeration circuit 41, provided in combination with the refrigerant cooler 67.1 and the refrigerant condenser 67.2, as mentioned above.

[0059] Thus the thermal energy required to partly vaporize the liquid coming from the bottom of the distillation column 201 is provided by compression heat. Using an additional source of heat is not excluded, for instance if more heat is needed than that available from the refrigerant.

[0060] A gaseous stream containing vaporized carbon dioxide and contaminants in the form of more volatile species in gaseous form flows through a hot side of the overhead condenser 205, which is fluidly coupled to the top of the distillation column 201 through an inlet connection 211. A return connection 213 fluidly couples a condensate outlet 212 of the overhead condenser to the distillation column 201 and is adapted to return carbon dioxide, which condenses in the overhead condenser 205, to the distillation column 201. The inlet of the cold side of the overhead condenser 205 is fluidly coupled with the collector line 51 through the carbon dioxide feed line 52, and receives liquid carbon dioxide therefrom. The outlet of the cold side of the overhead condenser 205 is fluidly coupled at 215 to the distillation column 201, such that the carbon dioxide delivered through the collector line 51 and the carbon dioxide feed line 51 to the overhead condenser 205, and used therein as a cooling medium, is fed to distillation column 201.

[0061] While in the schematic of Fig.2 the condenser 205 is shown as a componentseparate from the distillation column 201, in some embodiments the overhead condenser 205 may also be incorporated directly into the distillation column 201 top section.

[0062] The operation of the plant 1 is clear from the description above, and is summarized as follows.

[0063] A CCh-rich flue gas is fed to the plant 1 through the flue gas inlet 7 (Fig.l). CCh-rich flue gas can be generated by a fuel cell arrangement, such as a SOFC arrangement, and contains carbon dioxide in combination with other species, such as water, carbon monoxide, hydrogen, and possible contaminants, in addition to carbon dioxide. The flue gas is compressed in the flue gas compressor unit 11 and cooled in the post-cooler 15 to remove water therefrom. Condensate (condensed water) is removed from the compressed flue gas in the first liquid-vapor separator 17 and collected in the condensate removal line 21.

[0064] The compressed flue gas from the first liquid-vapor separator 17 is then processed in the water-gas-shift reactor system 23, if required, e.g., to convert carbon monoxide and water into carbon dioxide and hydrogen.

[0065] After water-gas-shift conversion (if provided) the flue gas flows through optional mercury absorber 27 and through the cooler 29. The flue gas is pre-cooled in the cooler 29. Condensate (water) separating from the flue gas by cooling in cooler 29 is removed from the flue gas in the second liquid-vapor separator 31. After condensate removal, the flue gas is processed through the dryer section 33.1 to remove residual moisture and is finally fed from the pre-treatment section 3 to the carbo dioxide liquefaction section 5.

[0066] In the carbon dioxide liquefaction section 5 the flue gas flows firstly through the first heat exchanging connection 53, where the flue gas is chilled by heat exchange against the refrigerant circulating in the refrigeration circuit 41, such that a fraction of carbon dioxide is liquefied and separated from the flue gas stream in the first separation drum 43.

[0067] The gaseous fraction of the flue gas exits the first separation drum 43 and flows through the connection duct 55 and through the second heat exchangingconnection 55.1, extending through the heat exchanger 37, where the flue gas is further chilled by heat exchange against the refrigerant. A second fraction of carbon dioxide is liquefied and separated from the gaseous stream in the intermediate separation drum 45.

[0068] The gaseous stream from the intermediate separation drum 45 flows through the connection duct 57 and the third heat exchanging connection 57.1 extending through the heat exchanger 37 in heat exchange with the refrigerant. The carbon dioxide liquefied in the heat exchanging connection 57.1 is separated from the gaseous stream in the further separation drum 47.

[0069] The stream of the resulting CCh-lean flue gas flows from the further separation drum 47 through the heat exchange section 59.1 of the gas discharge duct 59 and expands in the expander 59.3. The temperature of the expanded CCh-lean flue gas is thus reduced and the chilled CCh-lean flue gas flows through the second heat exchange section 59.2 to remove heat from the heat exchanger 37. The expanded CCh-lean flue gas is finally discharged through a cold side of the refrigerant cooler 67.1 and used therein to remove compression heat from the refrigerant.

[0070] The resulting, CCh-lean flue gas can be processed in different ways depending upon the composition thereof. For instance, if the flue gas is generated by a SOFC arrangement, the CCh-lean flue gas can contain hydrogen and can be fully or partly recycled towards the fuel cell arrangement.

[0071] The liquid carbon dioxide exiting the first separation drum 43, the second separation drum 45 and the third separation drum 47 at the respective liquid outlets 43.2, 45.2 and 47.2 is collected in the collector 51 at a suitable pressure to be delivered through the carbon dioxide feed line 52 to the overhead condenser 205. Valves 51.1, 51.2 and 51.3 can be arranged between each liquid outlet 43.2, 45.2, and 47.2 and the collector 51, such that carbon dioxide flows through the collector 51 and the carbon dioxide feed line 52 towards the overhead condenser 205 at the desired pressure and temperature conditions.

[0072] The carbon dioxide flowing through the cold side of the overhead condenser 205 can partly evaporate by removing heat from the gaseous flow which is routedthrough the hot side of the overhead condenser 205.

[0073] After flowing through the cold side of the overhead condenser 205, the carbon dioxide, which usually contains contaminants and / or a fraction of more volatile components, is delivered to the distillation column 201, where carbon dioxide collects at the bottom of the distillation column and the more volatile species are removed in gaseous form at the top of the distillation column 201. Part of the carbon dioxide in gaseous form flows upstream and exits the top of the distillation column and enters, as a blend of non-condensable, more volatile gaseous species, and carbon dioxide, in the hot side of the overhead condenser 205.

[0074] The heat exchange between the blend of gaseous species flowing through the hot side of the overhead condenser 205 and the chilled carbon dioxide flowing through the cold side of the overhead condenser 205 causes carbon dioxide streaming through the hot side of the overhead condenser 205 to condense. The more volatile, non-con- densables escape from the overhead condenser 205 at a gas outlet 218 of the overhead condenser 205 and are routed through line 219 to the gaseous flow leaving the gaseous outlet 47.3 of the most downstream separation drum, i.e. the third separation drum 47, through the gas discharge duct 59. Condensed carbon dioxide in the hot side of the overhead condenser is removed from a condensate outlet 212 of the overhead condenser and returned through the return connection 213, to the distillation column 206.

[0075] A pressure control valve 217 can be positioned along the line 219 which fluidly couples the gas outlet of the overhead condenser 205 with the gas discharge duct 59.

[0076] Carbon dioxide escaping in gaseous state from the top of the distillation column 201, and which condenses in the overhead condenser 205, is returned to the distillation column 201.

[0077] Liquid collected at the bottom of the distillation column 201 is recirculated in the reboiler 203, where the liquid is heated by heat exchange against the refrigerant from the refrigerant condenser 67.2, which circulates in the hot side of the reboiler 203. Heat transferred from the refrigerant flowing through the hot side of the reboiler 203 evaporates the more volatile species contained in the liquid blend flowing throughthe cold side of the reboiler 203.

[0078] Highly pure carbon dioxide can thus be removed in liquid form from the bottom of the reboiler 203, and is delivered through the carbon dioxide removal line 210, and the suction line 49.1, to the pumping unit 49.

[0079] The liquid carbon dioxide delivered by pump 49 is heated in the heat exchange section 61.1 extending through the heat exchanger 37 to the required final temperature. The pressure at which the pumping unit 49 pressurizes the liquid carbon dioxide is such that the carbon dioxide remains in the liquid state and does not change state from liquid to vapor. This avoids the need to compress the carbon dioxide product, which is discharged from the heat exchanger 37.

[0080] The refrigerant circulating in the closed refrigeration circuit 41 can be a mixed refrigerant, the composition whereof can be selected such that the required outlet temperature of the carbon dioxide in the carbon dioxide outlet duct is achieved.

[0081] In some embodiments the mixed refrigerant contains at least two components, one component being carbon dioxide and the other being a component having a boiling point temperature higher than carbon dioxide.

[0082] In some embodiments, the mixed refrigerant contains a blend of carbon dioxide, and one or more hydrocarbons CxHycontaining from 1 to 5 carbon atoms, preferably from 1 to 5 carbon atoms and possibly nitrogen. For instance, the one or more hydrocarbons can be selected from the group consisting of propane, propylene, isobutane, iso-pentane.

[0083] In some embodiments, the mixed refrigerant may contain carbon dioxide from 10 to 85% and nitrogen from 0 to 5% and at least one hydrocarbon as outlined above. In some embodiment, the mixed refrigerant can contain, in addition to carbon dioxide and possibly nitrogen, from 10 to 70% of one or more hydrocarbons with 3 carbon atoms, from 1 to 60% of one or more hydrocarbons containing 4 carbon atoms and from 1 to 30% of one or more hydrocarbons containing 5 carbon atoms.

[0084] Percentages are expressed in volume.

[0085] In the system described above streams of liquefied carbon dioxide (containing impurities having a higher volatility than carbon dioxide) from the first separation drum 43, the second separation drum 45, and third separation drum 47 are collected at collector 51 and purified in the distillation column 201, wherewith more volatile species, such as CO, H2 and possible contaminants, are removed from the carbon dioxide, before the latter is fed through line 61 to a pipeline, a collection tank, or a further processing plant. Carbon dioxide with a purity level up to 99.99% can be obtained with the use of the distillation column 201, in combination with the overhead condenser 205.

[0086] In the embodiment shown in Figs 1 and 2, a further advantage is achieved, by heat integration, in that compression heat from the closed refrigeration circuit 41 is used to drive the reboiler 203 of the distillation column 201, thus reducing or eliminating the need for additional thermal energy to drive the reboiler 203 of the distillation section 6.

[0087] The carbon dioxide flowing through the heat exchange section 61.1 removes heat from the heat exchanger and the temperature and pressure thereof may increase up to the final pressure and temperature values which are required in an outlet pipeline or storage unit. The carbon dioxide at the outlet of the heat exchanger 37 can be in a liquid or in a supercritical condition.

[0088] While a pumping unit 49 is usually desirable, in some embodiments the pressure of the liquefied carbon dioxide at the outlet of the cold side of the reboiler 203 can be sufficiently high to maintain the liquid or supercritical state at the outlet of the heat exchanger 37. In such case, the pumping unit 49 can be dispensed with and the final pressure of the separated and purified carbon dioxide collected in the pressurized carbon dioxide outlet duct 61 is achieved by compression of the flue gas in the compressor unit 11.

[0089] A further embodiment of a carbon dioxide separation plant according o the present disclosure is shown in Fig.3.

[0090] While in Figs 1 and 2 the carbon dioxide separation plant includes a closed refrigeration circuit 41, in which a dedicated refrigerant fluid flows and undergoescyclic thermodynamic transformations, the embodiment of Fig.3 is a so-called autorefrigerated separation plant, wherein liquefied carbon dioxide is used as refrigerant for the incoming flue gas, to condense carbon dioxide contained therein. The plant 301 of Fig.3 can include a pre-treatment section, not shown in Fig.3. The pre-treatment section can be the same or similar to the pre-treatment section 3 shown in Fig.l. Pretreated, CCh-rich flue gas enters the carbon dioxide removal section, which is labeled here 305, and which will be referred to as carbon dioxide liquefaction section, even though it shall be understood that in some embodiments, the carbon dioxide removed from the flue gas treated in the carbon dioxide removal section 305 is in gaseous condition.

[0091] The plant 301 further includes a distillations section 306, aimed at removing high-volatility contaminants from the carbon dioxide separated from the flue gas before the carbon dioxide leaves the plant 301, as will be explained below.

[0092] A pre-treated flue gas inlet line 335 delivers pre-treated flue gas to a heat exchanger 337 of the carbon-dioxide liquefaction section 305. This latter comprises a heat exchanger 337, wherethrough compressed flue gas from the pre-treatment section (not shown) is fed in heat exchange with chilled carbon dioxide and chilled vent gas. Heat is thus removed from the compressed flue gas to cause condensation of carbon dioxide contained therein.

[0093] The carbon dioxide is condensed in a carbon dioxide removal unit 339, wherefrom condensed carbon dioxide is removed delivered as a cooling medium to an overhead condenser of a distillation column, to be described.

[0094] The embodiment of Fig.3 is a simplified embodiment of an auto-refrigerated plant, with a minimum of components, which will be described in detail below. Those skilled in the art will understand from the following description that a more complex layout can be foreseen, for instance with a larger number of heat exchanging sections in the heat exchanger 337 and a larger number of gas / liquid separators.

[0095] Turning now to the simplified embodiment of Fig.3, the incoming compressed flue gas flows through a first heat exchanging connection 341 and a second heat exchanging connection 343. The outlet end of the first heat exchangingconnection 341 is fluidly coupled to a hot side of a reboiler (to be described) of the distillation section 306, before entering the second heat exchanging connection 343. The first heat exchanging connection 341 features a flue gas pre-cooling duct, adapted to partly cool the flue gas upstream of a reboiler for the purposes which will be described below, The second heat exchanging section 343 features a flue gas cooling duct downstream of the reboiler.

[0096] The outlet end of the second heat exchanging connection 343 is fluidly coupled with a separator drum 345. The separator drum 345 comprises an inlet 345.1, a liquid outlet 345.2 and a gas outlet 345.3. Carbon dioxide which has been liquefied by heat exchange against the chilled carbon dioxide circulating though the cold side of the heat exchanger 337, collects at the bottom of the liquid / gas separator. The noncondensables, are removed at the top of the separator drum 345 through the gas outlet 345.3. and vented trough a gas discharge duct 351 after flowing through a heat exchange connection 353, where the non-condensables are heated by heat removed from the incoming flue gas (heat exchanging connections 341, 343).

[0097] The distillation section 306 comprises a distillation column 360, combined with an overhead condenser 361, which is adapted to condense carbon dioxide exiting at the top of the distillation column 360. The distillation section 306 further comprises a reboiler 363, aimed at removing non-condensables or highly volatile species from the carbon dioxide delivered to the distillation section 306 from the separator drum 345.

[0098] As in the previous embodiment, also in the schematic of Fig.3 the reboiler 363 and the overhead condenser 361 are depicted as components arranged outside the distillation column 360. In other embodiments, one or both the reboiler 363 and the overhead condenser 361 can be arranged inside the distillation column 360.

[0099] Specifically, the liquid outlet 345.2 of the separator drum 345 is fluidly coupled, through a carbon dioxide feed line 371, with an inlet of a cold side of the overhead condenser 361. A pressure control valve 373 is positioned along the carbon dioxide feed line 371.

[0100] The cold side of the overhead condenser is in turn fluidly coupled with thedistillation column 360, to return carbon dioxide which vaporizes in the cold side of the overhead condenser 361 to the distillation column 360. Reference 375 designates the fluid coupling between the outlet of the cold side of the overhead condenser 361 and the distillation column 360. Reference 377 designates the fluid coupling between the top of the distillation column 360 and the inlet of the hot side of the overhead condenser 361, while reference 379 designates the fluid coupling between the condensate outlet 378 of the overhead condenser 361, and the distillation column 360, wherethrough condensed carbon dioxide from the overhead condenser 361 is returned to the distillation column 360.

[0101] Specifically, condensed carbon dioxide is thus returned from the condensate outlet 379 of the overhead condenser 361, through line 379, to the upper part of the distillation column 360 and heated carbon dioxide exiting the cold side of the overhead condenser 361 is fed through line 375 to the distillation column 360.

[0102] Non-condensables, i.e. gaseous species having a higher volatility than carbon dioxide are removed through a gas outlet 376 of the overhead condenser 361 through a line 380, which is fluidly coupled with the gas discharge duct 351.

[0103] The reboiler 363 comprises a hot side, wherethrough compressed flue gas flows. The flue gas is cooled partly in the first heat exchanging connection 341, before being delivered, through a line 381, to the inlet of the hot side of the reboiler 363. The outlet of the hot side of the reboiler 363 is fluidly coupled through a line 383 with the second heat exchanging connection 343.

[0104] The flue gas flowing through the hot side of the reboiler 363 heats CCh-rich liquid drawn from the bottom of the distillation column 360 and delivered through a line 385 to the cold side of the reboiler 363. The outlet of the cold side of the reboiler 363 is fluidly coupled through a line 387 with the distillation column 360.

[0105] Condensed and purified carbon dioxide is removed from the liquid outlet of the reboiler 363 through a carbon dioxide removal line 390. The carbon dioxide removal line 390 is fluidly coupled with a carbon dioxide outlet duct 391, which comprises a heat exchanging section 391.1 extending through the heat exchanger 337, where the liquefied carbon dioxide removed from the bottom of the reboiler 363removes heat from the compressed flue gas flowing through the first heat exchanging connection 341 and the second heat exchanging connection 343.

[0106] An expansion valve 393 is positioned along the carbon dioxide removal line 390, between the reboiler 363 and the heat exchanging section 391.1 of the carbon dioxide outlet duct 391.

[0107] A further valve 352 is provided along the gas discharge duct 351, the further valve 352 featuring an expansion device for the CCh-lean flue gas.

[0108] The operation of the plant 301 is clear from the above description and is summarized as follows.

[0109] Compressed, and possibly pre-treated flue gas, enters the carbon dioxide liquefaction section 305 through the pre-treated flue gas inlet line 335, and is partly cooled in the first heat exchanging connection 341, before entering the hot side of the reboiler 363, where the compressed flue gas exchanges heat against CCh-rich liquid drawn from the bottom of the distillation column 306 through line 385. The liquid contains mainly carbon dioxide and fractions of more volatile components, such as H2, CO or the like. The compressed flue gas enters the reboiler 363 after a first partial cooling in the flue gas pre-cooling duct 341, to reduce the temperature difference between the flue gas flowing through the hot side of the reboiler 363 and the liquid flowing through the cold side of the reboiler 363, thus preventing film boiling of the liquid in the reboiler, which would reduce the efficiency of the heat exchange therein.

[0110] After cooling in the reboiler 363 through heat exchange against the liquid drawn from the bottom of the distillation column 360, the flue gas is further cooled in the second heat exchanging connection 343, to cause condensation of carbon dioxide therein.

[0111] The flue gas flow, which contains condensed carbon dioxide, flows through the separator drum 345 to separate condensed carbon dioxide from the flue gas. The condensed carbon dioxide, containing contaminants having a higher volatility than carbon dioxide, flows through the liquid outlet 345.2 of the separator drum 345, through valve 373, through the carbon dioxide feed line 371, and through the cold sideof the overhead condenser 361, to remove condensation heat from the vapor flow entering the hot side of the overhead condenser 361 from the top of the distillation column 360, causing condensation of carbon dioxide in the hot side of the overhead condenser 361.

[0112] Carbon dioxide condensed in the overhead condenser 361 and removed therefrom from the outlet of the hot side of the overhead condenser 361, i.e. from the condensate outlet 378 of the overhead condenser 361, and from the outlet of the cold side of the overhead condenser 361 is returned to the distillation column 360, while noncondensables, i.e. gases having a volatility higher than carbon dioxide are removed from the overhead condenser 361 at the gas outlet 376 thereof and collected through line 380 in the gas discharge duct 351, which also collects gas from the separator drum 345.

[0113] Gas is vented through the gas discharge duct 351 from the liquefaction section 5.

[0114] Liquid carbon dioxide containing impurities with a higher volatility than carbon dioxide circulates from the bottom of the distillation column 360 through the reboiler 363, where heat from the incoming flue gas is used to evaporate the more volatile components, while purified liquid carbon dioxide, with a purity level which may achieve 99.99%, is extracted from the liquid outlet of the the reboiler and removed from the liquefaction section 5 through a carbon dioxide removal line 390 and the carbon dioxide outlet duct 391 which extends through the heat exchanging section 391.1 where heat is removed from the heat exchanger 337 by the outflowing carbon dioxide.

[0115] A further embodiment of an autorefrigerated plant is illustrated in Fig.4.

[0116] In this embodiment, the plant, labeled 401, can include a flue gas pre-treat- ment section like the one shown in Fig. 1, and not shown in Fig.4, and a carbon dioxide separation section 5 (shortly carbon dioxide separation section). The product carbon dioxide separated from the flue gas in the carbon dioxide separation section can be in gaseous or liquid conditions. In the following description the carbon dioxide separation section 5 will be referred to also as carbon dioxide liquefaction section 5.However, it shall be understood that this does not limit the scope of the present disclosure to a system where the product carbon dioxide is in liquid form.

[0117] The system 401 can be used for instance to remove carbon dioxide from a flue gas produced by a fuel cell system, for instance a solid oxide fuel cell (SOFC) system, which uses natural gas as a fuel. While treatment of flue gas from fuel cells with a system according to the present disclosure is particularly advantageous, in view of the relatively high carbon dioxide concentration in the flue gas, use of the system and method disclosed herein to treat flue gas of different origin is not excluded.

[0118] Pre-treated, e.g., dehydrated flue gas, is delivered from the flue gas pre-treat- ment section (not shown) to the carbon dioxide liquefaction section 5 through a pretreated flue gas inlet line 435.

[0119] In some embodiments, the carbon dioxide liquefaction section 5 comprises a heat exchanger, aka cold box, 437, a carbon dioxide removal unit 439 and a refrigeration circuit 441. As the plant of Fig.4 is an auto-refrigerated plant, the refrigeration circuit 441 uses carbon dioxide, separated from the flue gas, as a refrigerant fluid. In this embodiment, part of the processed carbon dioxide is removed as carbon dioxide product. The carbon dioxide removed from the refrigeration circuit is stored in a storage, to be described, or transported in a pipeline, for instance.

[0120] The plant 401 also comprises a distillation section 406, which is used to remove impurities, e.g. high-volatility components which are contained in the carbon dioxide removed from the flue gas, to obtain a high purity level in the carbon dioxide product delivered by the plant 401.

[0121] In the embodiment of Fig.4, the carbon dioxide removal unit 439 includes a plurality of separation drums, i.e. liquid / gas separators, wherein liquefied carbon dioxide is separated from the flue gas. In the embodiment of Fig.4, the plant carbon dioxide removal section 405 comprises a first separation drum 443, a further, or intermediate, separation drum 445 and a yet further, or final, separation drum 447. The intermediate separation drum 445 will be referred herein also as the second separation drum and the final separation drum 447 will be referred to herein also as the third separation drum. Similarly, components or devices belonging to or combined with thefurther, or intermediate separation drum 445 and wit the yet further, or final, separation drum 47, will be referred to as “second” and “third” component or device.

[0122] While in the embodiment illustrated in Fig.4 the system 401 comprises three separation drums 443, 445, 447, in other embodiments, not shown, the number of separation drums can be less than three, or more than three. For instance, in one embodiment the liquefied carbon dioxide removal unit 439 can include only the first separation drum 443, or only the first separation drum 443 and the second separation drum 445, or only the first separation drum 443 and the third separation drum 447, or an additional separation drum in sequence with the first, second and third separation drums 443, 445, 447.

[0123] Each separation drum, except the last one, has a gas outlet fluidly coupled to an inlet of the next separation drum through a connection line which extends through the heat exchanger, or cold-box, 437.

[0124] Specifically, in the embodiment of Fig.1, the first separation drum 443 has an inlet 443.1, a liquid outlet 443.2 and a gas outlet 443.3. The second or intermediate separation drum 445 has an inlet 445.1, a liquid outlet 445.2 and a gas outlet 445.3. The third or further separation drum 447 has an inlet 447.1, a liquid outlet 447.2 and a gas outlet 443.3. The first, second and third liquid outlets 443.2, 445.2 and 47.2 are connected to a collector line 451 through respective first, second and third control and de-pressurizing valves 443.4, 445.4 and 447.4

[0125] The inlet 443.1 of the first separation drum 443 is connected to the pre-treated flue gas inlet line 435 through a first heat exchanging connection extending through the heat exchanger or cold box 437, along which the flue gas is chilled by heat exchange against evaporating carbon dioxide flowing in the cold side of the cold box or heat exchanger 437.

[0126] The first heat exchanging connection is actually split into two sections, labeled 453.1 and 453.2, respectively. The flue gas flows through the first section 453.1, and then delivered to a reboiler, to be described, of the distillation section 406, before flowing through the second section 453.2.

[0127] The gas outlet 443.3 of the first separation drum 443 is connected to the inlet of the second, or intermediate, separation drum 445 through a connection duct 455 comprising a second heat exchanging connection 455.1, extending through the heat exchanger 437. The gas outlet 445.3 of the second, or intermediate, separation drum 445 is connected to the inlet 447.1 of the third, or further, separation drum 447 through a connection duct 457, comprising a third heat exchanging connection 457.1 extending through the heat exchanger 437.

[0128] The connection 457 and the intermediate or second separation drum 445 forms a fluid connection between the gas outlet 443.3 of the first separation drum 443 and the inlet 447.1 of the further, or third, separation drum 447.

[0129] The gas outlet 447.3 of the third, or further, separation drum 447 is fluidly coupled with a gas discharge duct 459. The gas discharge duct 459 has a first heat exchange section 459.1 and a second heat exchange section 459.2 arranged in sequence and extending through the heat exchanger 437. An expansion device, such as an expansion valve or an expander, is located along the gas discharge duct 459 between the first heat exchange section 459.1 and the second heat exchange section 459.2. In the schematic of Fig.4 the expansion device includes an expansion device for the CO2- lean flue gas. In the embodiment of Fig.4, the expansion device comprises an expander 459.3. In some embodiments the expander 459.3 can be drivingly coupled to an electric generator 460 to convert mechanical power generated by the expander 459.3 into electric power. In other embodiments the expander 459.3 can be connected to either a carbon dioxide compressor 461, to be described, or to the flue gas compressor unit of the pre-treatment section (not shown), such as to minimize power demand of the respective machines. In simpler embodiments, the expander 459.3 is replaced by a lamination or expansion valve.

[0130] The gas discharge duct 459 can be fluidly coupled selectively to an incinerator or to a recycle line adapted to recycle the gas flowing therethrough to a flue gas source, e.g. towards a fuel cell system, not shown.

[0131] The refrigeration circuit 441 comprises a carbon dioxide compressor 461. In some embodiments, the carbon dioxide compressor 461 can include one or morestages, or can comprise a train including two or more compressors in sequence. In the schematic of Fig.4 the carbon dioxide compressor 461 comprises two compression stages 461.1 and 461.2 in series. In some embodiments, the carbon dioxide compressor 461 can be an inter-cooled compressor. In the schematic of Fig. 4, the carbon dioxide compressor 461 comprises an intercooler 461.3. To further reduce the temperature of the carbon dioxide processed by the carbon dioxide compressor 461, the carbon dioxide compressor 461 can include, or can be combined with, an after-cooler 463. Reference number 465 indicates a driver, for instance an electric motor, which drives the carbon dioxide compressor or compressor train 461 in rotation. Upstream of a suction side of the carbon dioxide compressor 461, a suction drum 467 can be arranged, which removes any liquid fraction possibly contained in the carbon dioxide entering the carbon dioxide compressor 461.

[0132] In the embodiment shown in Fig.4, the refrigeration circuit 441 is an open circuit, which processes liquefied carbon dioxide from the separation drums 443, 445, 447 and only partially recirculates the carbon dioxide in the refrigeration circuit, while a fraction of the carbon dioxide is removed from the refrigeration circuit as carbon dioxide product, which is collected in a storage, for instance, as will be described in more detail below. The carbon dioxide removed from the system can be liquid, gaseous or supercritical carbon dioxide. Before removal from the plant 401, the carbon dioxide is purified through the distillation section 406, as will be described in more detail below.

[0133] Specifically, a carbon dioxide collecting duct 471 extends from the collector line 451 to the suction drum 467 upstream of the carbon dioxide compressor 461. The carbon dioxide collecting duct 471 comprises a heat absorption section 471.1 which extends through the heat exchanger or cold box 437 and which features a heat absorption section, adapted to absorb heat from the flue gas which flows through the heat exchanger 437.

[0134] The carbon dioxide collecting duct 471 collects condensed carbon dioxide from the liquid outlet 443.2, 445.2 and 447.2 of the three separation drums 443, 445, 447 through the control and de-pressurizing valves 443.4, 445.4 and 447.4, as will be described in more detail below. The carbon dioxide collected in the carbon dioxidecollecting duct 471 can be partly evaporated through expansion in the de-pressurizing valves 443.4, 445.4 and 447.4.

[0135] The delivery side of the carbon dioxide compressor 461 is fluidly coupled to an inlet of a compressed carbon dioxide duct 473. The compressed carbon dioxide duct 473 extends from the inlet through a heat exchange section 473.1, featuring a heat rejection section, which extends through the cold box or heat exchanger 437. The compressed carbon dioxide duct 473 is further fluidly coupled to the collector line 451 through a carbon dioxide expansion device 477 arranged downstream of the heat exchange section 473.1. The carbon dioxide expansion device 477 can be an expansion valve or an expander. The compressed and chilled carbon dioxide from the heat exchange section 473.1 is at least partly expanded and the temperature thereof is reduced.

[0136] The flow of expanded carbon dioxide streaming from the carbon dioxide expansion valve 477 is split into two partial streams. A first partial stream is returned to the collector line 451 through a pressure control valve 521. A second partial stream is fed to the distillation section 406 through a carbon dioxide feed line 517, which includes a pressure control valve 519. Thus, part of the expanded carbon dioxide is used as a refrigerant in the autorefrigerated carbon dioxide liquefaction system, and another part of the expanded carbon dioxide is purified as described below in the distillation section 406 and removed from the plant 401 as carbon dioxide product.

[0137] The distillation section 406 comprises a distillation column 501, an overhead condenser 503 fluidly coupled with the top of the distillation column 501 and a reboiler 505 fluidly coupled with the bottom of the distillation column 501.

[0138] As mentioned with regard to the previously described embodiments, the overhead condenser 503 and / or the reboiler 505 can be arranged inside the distillation column 501.

[0139] Heat delivered by the flue gas to the reboiler 505 is transferred to liquid extracted from the bottom of the distillation column 501 and used to evaporate the high- volatility components contained in the carbon dioxide collecting at the bottom of the distillation column. As described with reference to the embodiment of Fig.3, flue gas is delivered as a heating medium to the reboiler 505 after a first pre-cooling stepthrough the first section 453.1 of the heat exchanging connection, to reduce the temperature difference between the flue gas and the liquid delivered to the reboiler, thus preventing film boiling in the pipes of the reboiler. The first section 453.1 of the heat exchanging connection features, therefore, a flue gas pre-cooling duct. The subsequent second section 453.2 features a flue gas cooling duct for further cooling the flue gas and condense carbon dioxide contained therein, upstream of the first separation drum 443.

[0140] The flue gas is delivered from the first cooling section 453.1 to the reboiler 505 through a first line 509 and returned towards the heat exchanger 437, and specifically to the second cooling section 453.2, through a second line 511.

[0141] The hot side of the reboiler 505, wherethrough flue gas flows, transfers heat to the cold side of the reboiler 505, which receives CCh-rich liquid from the bottom of the distillation column 501. The inlet of the cold side of the reboiler 505 is fluidly coupled with the bottom of the distillation column 501 through a line 513, such that CCh-rich liquid is delivered from the bottom of the distillation column 501 to the inlet of the cold side of the reboiler 505.

[0142] The outlet of the cold side of the reboiler 505 is fluidly coupled to the distillation column 501 through a line 515, which returns evaporated, high-volatility components as well as evaporated carbon dioxide to the distillation column 501. Since high-volatility components are removed by evaporation from the liquid carbon dioxide entering the cold side of the reboiler, high-purity, liquid carbon dioxide collects at the bottom of the reboiler and can be removed therefrom as described below.

[0143] A gaseous flow, consisting of evaporated carbon dioxide and components having a volatility higher than carbon dioxide, is removed from the top of the distillation column 501 through a line 516 and enters the overhead condenser 503. Carbon dioxide is condensed in the overhead condenser 503 by heat exchange against chilled carbon dioxide from the compressed carbon dioxide duct 473, downstream of the heat exchanger 437, i.e. downstream of the heat exchange section 473.1.

[0144] For circulating chilled carbon dioxide from the heat exchange section 473.1 to the cold side of the overhead condenser 503, a carbon dioxide feed line 517 extendsfrom the carbon dioxide expansion valve 477 to the inlet of the cold side of the overhead condenser 503, through the further pressure control valve 519. The carbon dioxide feed line 371517 is further fluidly coupled to collector 451 through the pressure control valve 521. As mentioned above, a portion of the chilled carbon dioxide which expands in the carbon dioxide expansion valve 477 is routed to the overhead condenser 503 and removed from the plant 401 after purification.

[0145] The outlet of the cold side of the overhead condenser 503 is fluidly coupled with the distillation column 501 through a line 518, which delivers vaporized or partly vaporized carbon dioxide to the distillation column 501.

[0146] Condensed, high-purity carbon dioxide is removed from the liquid outlet of the reboiler 505 through a liquid carbon dioxide removal line 531, which is fluidly coupled to a carbon dioxide outlet duct 479, with the interposition of a pressure adjusting unit 533 In the embodiment of Fig. 4 the pressure adjusting unit 533 comprises a pressure-reduction valve, or an expansion valve. In other embodiments (not shown) the pressure adjusting unit 533 can include a pump.

[0147] The carbon dioxide outlet duct 479 comprises a heat exchange section 479.1 extending through the heat exchanger, or cold box 437, and is configured to deliver carbon dioxide to a carbon dioxide storage 481.

[0148] The operation of the plant shown in Fig.4 is clear from the above description and is summarized below.

[0149] A CCh-rich flue gas is fed to the carbon dioxide separation section 5 of the system 401 through the pre-treated and compressed flue gas inlet line 435. The flue gas flows firstly through the first section 453.1 of the heat exchanging connection 53, where the flue gas is cooled by heat exchange against chilled carbon dioxide.

[0150] Partially cooled flue gas flows then through line 509 as heating medium through the reboiler 505 and returned to the heat exchanger 437 through line 511. Next, the flue gas flows through the second section 453.2 of the heat exchanging connection 453 and is further chilled by heat exchange against carbon dioxide, streaming through the heat absorption section 471.1 and the heat exchange section 479.1. At least part ofthe carbon dioxide contained in the flue gas condenses by heat exchange in the heat exchanger 437.

[0151] From the heat exchanging connection 453 the chilled flue gas containing condensed carbon dioxide flows into the first separation drum 443.

[0152] The gaseous fraction of the flue gas exits the first separation drum 443 and flows through the connection duct 455 and through the second heat exchanging connection 455.1, extending through the heat exchanger or cold box 437, where the flue gas is further chilled by heat exchange against the carbon dioxide from the refrigeration circuit 441 and from the reboiler 505. A second fraction of carbon dioxide is liquefied and separated from the gaseous stream in the intermediate separation drum 445.

[0153] The gaseous stream from the intermediate separation drum 445 flows through the connection duct 457 and the third heat exchanging connection 457.1 extending through the heat exchanger 437 in heat exchange with carbon dioxide circulating in the refrigeration circuit 441 and from the collector line 451. The carbon dioxide liquefied in the heat exchanging connection 457.1 is separated from the gaseous stream in the further separation drum 447.

[0154] The stream of the resulting CCh-lean flue gas flows from the further separation drum 447 through the heat exchange section 459.1 of the gas discharge duct 459 and expanded in the expander 459.3, which generates mechanical power therewith. The mechanical power can be converted into electric power by generator 460. The temperature of the expanded CCh-lean flue gas is thus reduced and the chilled CO2- lean flue gas flows through the second heat exchange section 459.2 to remove heat from the heat exchanger 437. The expanded CCh-lean flue gas is finally discharged through the gas discharge duct 459. The resulting, CCh-lean flue gas can be processed in different ways depending upon the composition thereof. For instance, if the flue gas is generated by a SOFC arrangement, the CCh-lean flue gas can contain hydrogen and can be fully or partly recycled towards the fuel cell arrangement.

[0155] Part of the liquid carbon dioxide collected at the liquid outlets 443.2, 445.2 and 447.2 of the first separation drum 443, second separation drum 445, and third separation drum 447 is used as refrigerant in the refrigeration circuit 441. Specifically,liquid carbon dioxide is de-pressurized through the control and de-pressurizing valves 443.4, 445.4 and 447.4 and delivered through the carbon dioxide collecting duct 471 which extends through the cold box or heat exchanger 437. Carbon dioxide entering the carbon dioxide collecting duct 471 can be a two-phase carbon dioxide and can evaporate by removing heat from the flue gas flowing in the first, second and third heat exchanging connections 455.1, 457.1 and 459.1.

[0156] Vaporized carbon dioxide flowing in duct 471 downstream of the heat exchanger 437 enters the suction drum 467, and after removal of possible residual liquid carbon dioxide, the carbon dioxide in vapor or gaseous form is pressurized again in the carbon dioxide compressor 461 and cooled in the intercooler 461.3 and in the aftercooler 463.

[0157] Cooled carbon dioxide is then delivered through the compressed carbon dioxide duct 473 and liquefied by heat exchange while flowing through the heat exchange section 473.1 of the compressed carbon dioxide duct 473 extending through the heat exchanger 437. Carbon dioxide cooling is achieved by heat exchange against the evaporating carbon dioxide flowing through the liquefied carbon dioxide collecting duct 471 and relevant heat absorption section 471.1 thereof.

[0158] The flow of liquefied carbon dioxide is expanded in the carbon dioxide expansion valve 477 and split into two partial streams. A first partial stream recirculates in the refrigeration circuit 441. Specifically, the first partial flow is returned to the collector 451 through the valve 521 and reused as refrigerant in the refrigeration circuit 441.

[0159] A second partial stream is delivered through the carbon dioxide feed line 517 to the overhead condenser 503 of the distillation section 406 and flows through the cold side of the overhead condenser 503 to remove condensation heat from the carbon dioxide flowing from the head of the distillation column 501 through the hot side of the overhead condenser 503.

[0160] Carbon dioxide from the overhead condenser 503 (both from the cold side as well as from the hot side) is delivered to the distillation column 501 through lines 518 and 520. Specifically, line 518 fluidly couples the outlet of the cold side of theoverhead condenser with the distillation column 501 and line 520 fluidly couples the condensate outlet 522 of the overhead condenser 503 with the distillation column 501.

[0161] Carbon dioxide product at high purity level, e.g. as high as 99.99%, is extracted in liquid form from the bottom of the reboiler 505 through carbon dioxide removal line 531 and carbon dioxide outlet duct 479. The vapor or gas phase from the reboiler 505 is delivered through line 515 back to the distillation column 501.

[0162] Non-condensables, including mainly species having a volatility higher than carbon dioxide are vented through a gas outlet 502 of the overhead condenser 503 though a line 504, which is fluidly coupled to the gas discharge duct 459, wherefrom the gas can be recycled, vented, or delivered to an incinerator, for instance, depending upon the gaseous species contained therein.

[0163] The CCh-rich liquid collecting at the bottom of the distillation column 501 is delivered to the hot side of the reboiler 505. Heat from the incoming, and partly cooled flue gas drives the reboiler 505, causes vaporization of highly volatile species, and possibly a fraction of the carbon dioxide contained in the incoming liquid from the bottom of the distillation column 501. The gaseous species forming in the reboiler 505 are routed back through line 515 and reach top of the distillation column 501. The liquid collecting in the reboiler 505 is highly pure carbon dioxide representing the product of the plant, and is removed through the carbon dioxide removal line 531 and the carbon dioxide outlet duct 479.

[0164] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A plant (1; 301; 401) for removing carbon dioxide from of a flue gas, the plant comprising: a heat exchanger (37; 337; 437) adapted to receive compressed flue gas containing carbon dioxide, and condense at least part of the carbon dioxide contained therein; a carbon dioxide removal unit (39;39; 439), adapted to remove liquefied carbon dioxide from the flue gas exiting the heat exchanger (37;337; 437); a carbon dioxide outlet duct (61; 391; 479) extending through the heat exchanger (37; 337; 437), wherethrough carbon dioxide is removed from the plant; a gas discharge duct (59; 351; 459); and a distillation section (6; 306; 406), comprising: a distillation column (201; 360, 501), a reboiler (203; 363; 505) at the bottom of the distillation column (201; 360; 501) and an overhead condenser (205; 361; 503) at the top of the distillation column (201; 360; 501); wherein the carbon dioxide removal unit (39; 339; 439) comprises an inlet (43.1; 45.1, 47.2; 345.1; 443.1, 445.1, 447.1) fluidly coupled with the heat exchanger (37; 337; 437), a liquid outlet (51; 345.2; 451), and a gas outlet (47.3; 345.3; 447.3) fluidly coupled with the gas discharge duct (59; 351; 459), to remove from the plant a gaseous phase collecting in the carbon dioxide removal unit (39; 339; 439); wherein an inlet of a cold side of the overhead condenser (205; 361; 503) is fluidly coupled with a carbon dioxide feed line (52; 371; 517) adapted to deliver carbon dioxide, which has been separated from the flue gas in the carbon dioxide removal unit (39; 339; 439), to the cold side of the overhead condenser (205; 361; 503); wherein an outlet of the cold side of the overhead condenser (205; 361; 503) is adapted to route the carbon dioxide flowing through the cold side of the overhead condenser to the distillation column (201; 360; 501); wherein a condensate outlet (212; 378; 522) of the overhead condenser (205; 361; 503) is configured to return condensate carbon dioxide to the distillation column (201; 360; 501); and wherein a gas outlet (218; 376; 502) of the overhead condenser (205; 361; 503) is fluidly coupled with the gas discharge duct (59; 351; 459).

2. The plant (1; 301; 401) of claim 1, wherein the reboiler (203; 363; 505) has a hot side adapted to circulate a heating medium therethrough, and a cold side adapted to receive a CCh-rich liquid collecting at the bottom of the distillation column (201, 360; 501), and to recirculate a gaseous flow, generated by heating the CCh-rich liquid with the heating medium, through the distillation column (201; 360; 501).

3. The plant (1; 301; 401) of claim 2, wherein a liquid outlet of the reboiler (203; 363; 505) is fluidly coupled with the carbon dioxide outlet duct (61; 391; 479).

4. The plant (1; 301; 401) of any one of the preceding claims, wherein the carbon dioxide removal unit (39; 339; 439) comprises a first separation drum (43; 343; 443), adapted to receive a chilled flue gas stream containing at least partly liquefied carbon dioxide from the heat exchanger (37; 337 437), and to separate liquid carbon dioxide from the chilled flue gas stream; wherein the first separation drum (43; 343; 443) comprises an inlet (43.1; 443.1; 345.1) fluidly coupled with the heat exchanger (37; 337; 437), a liquid outlet (43.2; 345.; 443.2), and a gas outlet (43.3; 345.3; 443.3).

5. The plant (1; 401) of claim 4, further comprising: at least a further separation drum (47; 447) adapted to receive a chilled flue gas stream containing at least partly liquefied carbon dioxide from the heat exchanger (37; 437), and to separate liquid carbon dioxide from the chilled flue gas stream; and wherein the further separation drum (47; 447) comprises an inlet (47.1 ; 447.1), a liquid outlet (47.2; 447.2), and a gas outlet (47.3; 447.3); and a fluid connection between the gas outlet (43.3; 443.3) of the first separation drum (43; 443) and the inlet (47.1; 447.1) of the further separation drum (47; 447), the fluid connection extending through the heat exchanger (37; 437).

6. The plant (1; 401) of claim 5, wherein the gas outlet (47.3; 447.3) of the further separation drum (47; 447) is fluidly coupled with the gas discharge duct (59; 459).

7. The plant (1; 401) of claim 5 or 6, wherein the fluid connection between the gas outlet (43.3; 443.3) of the first separation drum (43; 443) and the inlet(47.1; 447.1) of the further separation drum (47; 447) comprises: at least an intermediate separation drum (45; 445) adapted to receive a chilled flue gas stream containing at least partly liquefied carbon dioxide from the heat exchanger, and to separate liquid carbon dioxide from chilled flue gas stream; wherein the intermediate separation drum (45; 445) comprises an inlet (45.1; 445.1), a liquid outlet (45.2; 445.2), and a gas outlet (45.3; 445.3); and a heat exchanging connection fluidly coupling the gas outlet of the first separation drum with the inlet of the intermediate separation drum, the heat exchanging connection (55.1; 455.1) extending through the heat exchanger (37; 437).

8. The plant (1) of any one of the preceding claims, further comprising a closed refrigeration circuit (41) adapted to circulate a refrigerant through the heat exchanger (37) and chill the flue gas therewith.

9. The plant (1) of claim 8, wherein the closed refrigeration circuit (41) comprises a refrigerant compression section, a refrigerant cooling and condensing section, a refrigerant expansion section, a heat rejection section, and a heat absorption section.

10. The plant (1) of claim 9, wherein the closed refrigeration circuit is adapted to deliver heat to the reboiler (203).

11. The plant (1) of claim 9 or 10, wherein the refrigerant cooling and condensing section extends through a hot side of the reboiler (203) and is adapted to deliver heat from the refrigerant to the reboiler.

12. The plant (1) of any one of claims 9 to 11, wherein: the closed refrigeration circuit (41) further comprises: a refrigerant compressor (65) in the compressions section; and a refrigerant expansion device (71) in the refrigerant expansion section; wherein the refrigerant cooling and condensing section is positioned between the refrigerant compressor (65) and the refrigerant expansion device (71) ; the heat rejection section comprises a heat rejection flow path (69) extending through the heat exchanger (37) between the refrigerant cooling and condensing section and the refrigerant expansion device (71), and adapted to remove heat from therefrigerant; and the heat absorption section comprises a heat absorption flow (73) path extending through the heat exchanger (37) between the refrigerant expansion device (71) and the refrigerant compressor (65), adapted to absorb heat from the flue gas flowing through the heat exchanger (37).

13. The plant (1) of any one of claims 8 to 12, further comprising a pumping unit (49) having a suction side fluidly coupled with the liquid outlet of the reboiler (203) and a delivery side fluidly coupled with the carbon dioxide outlet duct (61); wherein the pumping unit (49) is adapted to: remove carbon dioxide from the liquid outlet of the reboiler (203), pressurize the carbon dioxide removed from the reboiler (203), and circulate the pressurized carbon dioxide to the carbon dioxide outlet duct (61) through the heat exchanger (37).

14. The plant (1) of any one of claims 8 to 13, wherein the closed refrigeration circuit contains a mixed refrigerant.

15. The plant (1) of claim 14, wherein the mixed refrigerant comprises: a blend comprising carbon dioxide and at least one or more hydrocarbons, in particular containing from 1 to 5 carbon atoms, preferably from 3 to 5 carbon atoms; or a blend comprising at least two hydrocarbons, in particular containing from 1 to 5 carbon atoms, preferably from 3 to 5 carbon atoms.

16. The plant (1; 301; 401) of any one of the preceding claims, wherein an expansion device (59.3; 352; 459.3) is arranged along the gas discharge duct (59; 351; 459), and is adapted to expand gas flowing through the gas discharge duct; and wherein the gas discharge duct extends downstream of the expansion device through the heat exchanger.

17. The plant (1; 301; 401) of any one of the preceding claims, further comprising a carbon dioxide expansion valve (221; 393; 533) arranged between the liquid outlet of the reboiler (203; 363; 505) and the carbon dioxide outlet duct (61; 391; 479).

18. The plant (1; 301; 401) of claim 17, wherein the carbon dioxideoutlet duct (61; 391; 479) comprises a heat exchange section (61.1; 391.1; 479.1) downstream of the expansion valve (221; 393; 533), and extending through the heat exchanger (37; 337; 437) and adapted to remove heat therefrom.

19. The plant (301; 401) of any one of claims 1 to 9 and 12 to 18, wherein the hot side of the reboiler (393; 505) is adapted to receive compressed flue gas as heating medium.

20. The plant (301; 401) of claim 19, wherein an inlet of the hot side of the reboiler is fluidly coupled to a flue gas pre-cooling duct (341; 453.1) extending through the heat exchanger (337; 437), such as to receive a flow of pre-cooled flue gas.

21. The plant (301; 401) of claim 20, wherein an outlet of the hot side of the reboiler (363; 505) is fluidly coupled with the inlet (345.1; 443.1) of the carbon dioxide removal unit (339; 439) through a flue gas cooling duct (343; 453.2) extending through the heat exchanger (337; 437).

22. The plant (401) of any one of claims 4 to 7: further comprising a carbon dioxide collecting duct (471) fluidly coupled with the first separation drum (443) and extending through the heat exchanger (437); wherein the carbon dioxide collecting duct (471) is fluidly coupled with a carbon dioxide compressor (461) and comprises a heat absorption section (471.1) extending through the heat exchanger (437).

23. The plant (401) of claim 22, wherein a carbon dioxide expansion device (477) is fluidly coupled with the carbon dioxide collecting duct (451) and with the cold side of the overhead condenser (503), such that expanded carbon dioxide from the carbon dioxide expansion device (477) is split into a first partial stream recycled to the carbon dioxide collecting duct (451) and a second partial stream delivered to the cold side of the overhead condenser (503).

24. The plant (301) of any one of claims 4 to 7, wherein the liquid outlet (345.2) of the first separation drum (345) is fluidly coupled with the cold side of the overhead condenser (361), such that liquefied carbon dioxide collecting in the firstseparation drum (345) is used to condense carbon dioxide in the hot side of the overhead condenser (376).

25. A method for removing carbon dioxide from a flue gas, the method comprising the following steps: delivering a stream of compressed flue gas containing carbon dioxide through a heat exchanger (37; 337; 437) in heat exchange with a refrigerant; at least partially condensing carbon dioxide contained in the compressed flue gas by heat exchange against said refrigerant and removing the carbon dioxide from the flue gas; separating the condensed carbon dioxide from a gaseous phase and removing the gaseous phase; flowing condensed carbon dioxide removed from the flue gas as a cooling medium through a cold side of an overhead condenser (205; 361; 503) of a distillation column (201; 360; 501) and condensing, through heat exchange therewith, gaseous carbon dioxide coming from the distillation column (201; 360; 501), thus separating carbon dioxide from non-condensables contained therein; delivering the carbon dioxide exiting the cold side of the overhead condenser (205; 361; 503) and carbon dioxide condensed in the overhead condenser into the distillation column (201; 360; 501); removing non-condensables from the overhead condenser (205; 361; 503); recycling liquid from the bottom of the distillation column (201; 360; 501) in a reboiler (203; 363; 505) and heating the recycling liquid in the reboiler through a heating medium; returning a gaseous flow from the reboiler (203; 363; 505) into the distillation column (201; 360; 501) and extracting liquid carbon dioxide from the reboiler.

26. The method of claim 25, further comprising the steps of: flowing the liquid carbon dioxide extracted from the reboiler (203; 363; 505) through the heat exchanger (37; 337; 437); and removing chilled carbon dioxide from the heat exchanger (37; 337; 437).

27. The method of claim 25 or 26, wherein the refrigerant is a refrigerant flowing in a closed refrigeration circuit (41), in which the refrigerant is cyclicallycompressed, cooled and condensed, expanded, and heated by heat exchange against flue gas in the heat exchanger.

28. The method of claim 27, wherein the refrigerant is a mixed refrigerant.

29. The method of claim 28, wherein the mixed refrigerant comprises: a blend comprising carbon dioxide and at least one or more hydrocarbons, in particular containing from 1 to 5 carbon atoms, preferably from 3 to 5 carbon atoms; or a blend comprising at least two hydrocarbons, in particular containing from 1 to 5 carbon atoms, preferably from 3 to 5 carbon atoms.

30. The method of claim 27, 28 or 29, comprising the step of using compressed refrigerant as the heating medium in the reboiler (203).

31. The method of claim 25 or 26, wherein the refrigerant is a flow of expanded carbon dioxide separated from the compressed flue gas.

32. The method of claim 31, comprising the step of using compressed flue gas as heating medium in the reboiler (363; 505).

33. The method of claim 32, comprising the steps of: pre-cooling the flue gas in the heat exchanger (337; 437); flowing the pre-cooled flue gas through the reboiler (363; 505) and transfer heat from the pre-cooled flue gas to the liquid recycled from the bottom of the distillation column (360; 501) through the reboiler (363; 505).

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