Carbon dioxide separation plant using carbon dioxide as refrigerant in a refrigeration circuit, and method

Abstract

The system comprises a heat exchanger wherein flue gas containing carbon dioxide flows in heat exchange with streams of chilled carbon dioxide. The flue gas is cooled and carbon dioxide condensed by heat exchange is removed from the flue gas. The condensed carbon dioxide is partly processed in a refrigeration circuit and used as a refrigerant in the heat exchanger and partly removed as a CO2 product.

Description

CARBON DIOXIDE SEPARATION PLANT USING CARBON DIOXIDE AS REFRIGERANT IN A REFRIGERATION CIRCUIT, AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] The invention relates to a system or plant and to a method for separating carbon dioxide (CO2) from flue gas.BACKGROUND ART

[0002] 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 GG 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+O2G 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] Other CO2 separation systems and methods are disclosed in US2011 / 0271713 and FR2934170.

[0009] The separation of carbon dioxide from a CO2-containing flue gas is therefore a power-demanding process, which reduces the overall efficiency of the power generation process.

[0010] A method and a system capable of reducing the power required for carbon dioxide separation would therefore be welcomed in the art.SUMMARY

[0011] According to one aspect, disclosed herein is a system for producing CO2 out of a flue gas using a refrigerant circuit, wherein carbon dioxide is processed as a refrigerant. The system comprises a heat exchanger adapted to receive a compressed inlet flue gas stream and condense at least part of a carbon dioxide content thereof. The system further comprises at least a first separation drum adapted to receive a chilled flue gas stream containing at least partly liquefied CO2 from the heat exchanger, and to separate liquefied CO2 from the chilled flue gas stream. The system further comprises a collector line, adapted to collect liquefied carbon dioxide from the liquid outlet of the first separation drum. A liquefied CO2 collecting duct extends from the collector line through the heat exchanger and is adapted to deliver carbon dioxideto a refrigeration circuit. The liquefied CO2 collecting duct comprises a heat exchange section extending through the heat exchanger. Carbon dioxide flowing through the heat exchange section of the liquefied CO2 collecting duct vaporizes by absorbing heat from the flue gas and from hot a carbon dioxide flowing through the heat exchanger. The refrigeration circuit comprises: a CO2 compressor; a compressed CO2 duct having an inlet connected to a delivery side of the CO2 compressor, an outlet, and a heat exchange section between the inlet and the outlet and extending through the heat exchanger. The refrigeration circuit further comprises a CO2 return duct, which fluidly couples the outlet of the compressed CO2 duct to the collector line or to the liquefied CO2 collecting duct, upstream of the heat exchange section (71.1) of the liquefied CO2 collecting duct (71).

[0012] A CO2 outlet duct is connected to the outlet of the compressed CO2 duct to remove part of the carbon dioxide extracted from the flue gas by condensation.

[0013] According to another aspect, disclosed herein is a method for producing liquid CO2 out of a flue gas, the method comprising the following steps: a) delivering a stream of compressed flue gas containing CO2 through a heat exchanger in heat exchange with a flow of chilled CO2 circulating in a refrigeration circuit; b) at least partially condensing CO2 contained in the compressed flue gas by heat exchange with the chilled CO2 circulating in the refrigeration circuit; c) separating liquefied CO2 from the stream of compressed flue gas in at least a first separation drum; d) removing liquefied CO2 from the first separation drum; e) collecting liquefied CO2 from the first separation drum in a liquefied CO2 collecting duct, which comprises a heat exchange section extending through the heat exchanger; f) evaporating CO2 flowing through the heat exchange section of the liquefied CO2 collecting duct; g) compressing the CO2, evaporated in the heat exchange section of the liquefied CO2 collecting duct and exiting the heat exchanger, in a CO2 compressor of the refrigeration circuit; h) delivering a flow of compressed CO2 from the CO2 compressor through acompressed CO2 duct comprising a heat exchange section extending through the heat exchanger and cooling the flow of compressed CO2; i) dividing compressed and chilled CO2 at an outlet of the compressed CO2 duct into a first partial CO2 flow and a second partial CO2 flow; j) returning the first partial CO2 flow to the liquefied CO2 collecting duct upstream of the heat exchange section (71.1) thereof; k) removing the second partial CO2 flow from the refrigeration circuit as a CO2 product; l) removing CCh-lean flue gas from the heat exchanger.

[0014] In a system and method according to the present disclosure, therefore, the liquefied CO2 collected at the collector line and the liquefied CO2 from the liquefied CO2 collecting duct are combined in a single flow of liquid CO2 which is fed through the heat exchange section of the liquefied CO2 collecting duct and vaporized. The vaporized CO2 stream at the outlet of the heat exchange section of the liquefied CO2 of the collecting duct is then delivered to the compressor of the refrigeration circuit. In contrast to other systems and methods of the current art, a single pressure level and a single compressor train for managing the CO2 is thus required, which results in a substantially simplified arrangement.

[0015] Further features and embodiments of the system and of the method disclosed herein are described below, reference being made to the attached drawings, and are set forth in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Reference is now made briefly to the accompanying drawings, in which:Fig. l is a diagram of a system according to the present disclosure in one embodiment;Fig.2 is an enlarged detail of the diagram of Fig.1, showing the heat exchanger; andFig.3 is an enlarged detail of the diagram of Fig.l in a further embodiment.DETAILED DESCRIPTION

[0017] Fig. l shows a schematic diagram of a system 1 according to the presentdisclosure in one embodiment. The system 1 comprises a flue gas pre-treatment section 3 and a carbon dioxide separation section 5 (shortly CO2 separation section). The product CO2 separated from the flue gas in the CO2 separations section can be in gaseous, liquid or supercritical 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 CO2 is in liquid form.

[0018] The system 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. 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 CO2 concentration in the flue gas, use of the system and method disclosed herein to treat flue gas of different origin is not excluded.

[0019] The flue gas pre-treatment section 3 comprises a flue gas inlet 7, which is 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 or the like.

[0020] The flue gas pre-treatment section 3 can include a first liquid-vapor, or liquidgas 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.

[0021] 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.

[0022] 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. The delivery side of the compressor unit 11 is connected to a post-cooler 15 and a second liquid-vapor, or liquid-gas separator 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.

[0023] In some embodiments, the flue gas pre-treatment section 3 can include a water-gas-shift reactor system 23 (shortly WGS reactor), adapted to convert carbon monoxide, possibly contained in the flue gas, and steam into carbon dioxide and hydrogen.

[0024] In some embodiments, 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 water condensing at the bottom of a third liquidvapor, or liquid-gas separator to be described below.

[0025] As mentioned above, the compressor unit 11 usually includes a plurality of compressors or compressor stages with at least one interstage cooler or 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 preheater23.1 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.

[0026] The flue gas treated in the water-gas-shift reactor system 23, if present, can be fed through one or more additional components of the flue gas pre-treatment section 3.

[0027] In the embodiment of Fig.1 the flue gas pre-treatment section 3 includes, for instance, a mercury absorber 27 followed by a cooler 29. The flue gas can flow through the mercury absorber 27 for mercury removal and thereafter through the hot side of the cooler 29, in heat exchange with a cooling medium flowing through the cold side of the cooler 29. The outlet of the hot side of the cooler 29 is fluidly coupled to the inlet 31.1 of a third liquid-vapor, or liquid-gas separator 31, including a liquid outlet31.2 and a gas outlet 31.3.

[0028] The third liquid-vapor separator 31 is adapted to separate water which has been condensed following flue gas cooling in the cooler 29. The condensed water collected at the bottom of the third liquid-vapor separator 31 can be delivered to the water- gas-shift reactor system 23 through the condensate recycling line 25. If the water- gasshift reactor system 23 is not present, the liquid outlet 31.2 of the third liquid-vaporseparator 31 can be fluidly coupled to the condensate removal line 21, for instance.

[0029] The gas outlet 31.3 of the third liquid-vapor separator 31 can be fluidly coupled to a flue gas dryer arrangement 33 adapted to remove moisture from the cooled flue gas. The flue gas dryer arrangement 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 system, the flue gas dryer 33 usually includes two flue gas dryer units, such that while one flue gas dryer unit is operating, the other flue gas dryer unit 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 section 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.

[0030] The dehydrated flue gas is delivered from the flue gas pre-treatment section 3 to the carbon dioxide liquefaction section 5 through a pre-treated flue gas inlet line 35.

[0031] 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 gas dryer arrangement 33, possibly after processing through the compressor unit 11 and liquid-vapor separator 17.

[0032] The carbon dioxide liquefaction section 5 is shown in more detail in Fig.2.

[0033] In some embodiments, the carbon dioxide liquefaction section 5 comprises a heat exchanger, aka cold box, 37, a carbon dioxide removal unit 39 and a refrigeration circuit 41. The refrigeration circuit 41 is a refrigeration circuit, which uses carbon dioxide, separated from the flue gas, as a refrigerant fluid and wherefrom part of the processed carbon dioxide is removed as CO2 product. The CO2 removed from the refrigeration circuit is stored in a storage, to be described, or transported in a pipeline, for instance. Differently from cryogenic carbon dioxide separator systems of the prior art, the system disclosed herein comprises a refrigeration circuit wherein carbon dioxide separated from the flue gas undergoes cyclic thermodynamic transformations, including compression, cooling, expansion, and heating by heat removed from the flue gas. Thus, carbon dioxide separated from the flue gas by chilling against expandedcarbon dioxide is partly used as a refrigerant in the closed refrigeration circuit and partly removed from the system as product CO2.

[0034] More in detail, the carbon dioxide removal unit 39 includes a first separation drum 43, a further, or intermediate, separation drum 45 and a yet further, or final, separation drum 47. The intermediate separation drum 45 will be referred herein also as the second separation drum and the final separation drum 47 will be referred to herein also as the third separation drum. Similarly, components or devices belonging to or combined with the further, or intermediate separation drum 45 and wit the yet further, or final, separation drum 47, will be referred to as “second” and “third” component or device.

[0035] While in the embodiment illustrated in Fig.2 the system 1 comprises three separation drums 43, 45, 47, 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 39 can include only the first separation drum 43, or only the first separation drum 43 and the second separation drum 45, or only the first separation drum 43 and the third separation drum 47, or an additional separation drum in sequence with the first, second and third separation drums 43, 45, 47.

[0036] In general, the carbon dioxide removal unit 39 includes at least a first separation drum and a further separation drum.

[0037] 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.

[0038] Specifically, in the embodiment of Fig.1, 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 first, second and third liquid outlets 43.2, 45.2 and 47.2 are connected to a collector line 51 through respective first, second and third control and de-pressurizing valves 43.4, 45.4 and 47.4

[0039] The inlet 43.1 of the first separation drum 43 is connected to the pre-treatedflue gas inlet line 35 through a first heat exchanging connection 53 extending through the heat exchanger or cold box 37, along which the flue gas is chilled in heat exchange with evaporating carbon dioxide flowing in the cold side of the cold box or 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.

[0040] The connection 57 and the intermediate or second separation drum 45 form a fluid connection between the gas outlet 43.3 of the first separation drum 43 and the inlet 47.1 of the further, or third, separation drum 47.

[0041] 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. l the expansion device includes an expander 59.3. In some embodiments the expander 59.3 can be drivingly coupled to an electric generator 60 to convert mechanical power generated by the expander 59.3 into electric power. In other embodiments the expander 59.3 can be connected to either a CO2 compressor, to be described, or to the flue gas compressor unit 11, or both, such as to minimize power demand of the respective machines. In simpler embodiments, the expander 59.3 is replaced by a lamination or expansion valve.

[0042] The gas discharge duct 59 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.

[0043] The refrigeration circuit 41 comprises a CO2 compressor 61. In some embodiments, the CO2 compressor 61 can include one or more stages, or can comprise a trainincluding two or more compressors in sequence. In the schematic of Figs 1 and 2 the CO2 compressor 61 comprises two compressors 61.1 and 61.2 in series. In some embodiments, the CO2 compressor 61 can be an inter-cooled compressor. In the schematic of Figs. 1 and 2, the CO2 compressor 61 comprises an intercooler 61.3. To further reduce the temperature of the carbon dioxide processed by the CO2 compressor 61, the CO2 compressor 61 can include, or can be combined with, an after-cooler 63. Reference number 65 indicates a driver, for instance an electric motor, which drives the CO2 compressor or compressor train 61 in rotation. Upstream of a suction side of the CO2 compressor 61, a suction drum 67 can be arranged, which removes any liquid fraction possibly contained in the CO2 entering the CO2 compressor 61.

[0044] In the embodiment shown in Figs. 1 and 2, the refrigeration circuit 41 is an open circuit, which processes liquefied carbon dioxide from the separation drums 43, 45, 47 and only partially recirculates the CO2 in the refrigeration circuit, while a fraction of the CO2 is removed from the refrigeration circuit as CO2 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 CO2.

[0045] Specifically, a liquefied CO2 collecting duct 71 extends from the collector line 51 to the suction drum 67 upstream of the CO2 compressor 61. The liquefied CO2 collecting duct 71 comprises a heat exchange section 71.1 which extends through the heat exchanger or cold box 37.

[0046] The liquefied CO2 collecting duct 71 collects condensed carbon dioxide from the liquid outlet 43.2, 45.2 and 47.2 of the three separation drums 43, 45, 47 through the control and de-pressurizing valves 43.4, 45.4 and 47.4, as will be described in more detail later on.

[0047] The delivery side of the CO2 compressor 61 is fluidly coupled to an inlet of a compressed CO2 duct 73. The compressed CO2 duct 73 extends from the inlet to an outlet 85 and includes a heat exchange section 73.1 between which extends through the cold box or heat exchanger 37. The outlet 85 of the compressed CO2 duct is fluidly coupled to a CO2 return duct 75 which connects the outlet 85 of the compressed CO2 duct 73 to the collector line 51. An expansion valve 77 is positioned along the CO2 return duct 75, such that the compressed and chilled CO2 delivered from thecompressed CO2 duct 73 into the CO2 return duct 75 is at least partly expanded and the temperature thereof is reduced.

[0048] The compressed CO2 duct 73 is further fluidly coupled with a CO2 outlet duct 79, which comprises a heat exchange section 79.1 extending through the heat exchanger, or cold box 37, and which is configured to deliver carbon dioxide to a carbon dioxide storage 81. A pressure adjusting unit 83 is positioned along the CO2 outlet duct 79, between the heat exchange section 71.1 thereof, which extends through the heat exchanger 37, and the outlet 85 of the compressed CO2 duct 73. In the embodiment of Figs 1 and 2 the pressure adjusting unit 83 comprises a pressure-reduction valve.

[0049] The system 1 operates as follows.

[0050] A CCh-rich flue gas is fed to the system 1 through the flue gas inlet 7. CO2- rich flue gas can be generated by a fuel cell arrangement, such as a SOFC arrangement, and can contain carbon dioxide (CO2) in combination with other species, such as steam (H2O), carbon monoxide (CO), hydrogen (H2), and possible contaminants. The flue gas is compressed in the flue gas compressor unit 11 and cooled in the post-cooler 15 to remove water therefrom. Condensed water is removed from the compressed flue gas in the first liquid-vapor separator 17 and collected in the condensate removal line 21.

[0051] 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.

[0052] After water-gas-shift conversion (if provided) the flue gas flows through optional mercury absorber 27 and through 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 33.1 to remove residual moisture and is finally fed from the flue gas pre-treatment section 3 to the carbo dioxide liquefaction section 5.

[0053] 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 in heat exchange with the CO2 from the refrigeration circuit 41 and from the collector line 51, such thata fraction of carbon dioxide contained in the flue gas is liquefied and separated from the flue gas stream in the first separation drum 43.

[0054] 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 exchanging connection 55.1, extending through the heat exchanger or cold box 37, where the flue gas is further chilled in heat exchange with the CO2 from the refrigeration circuit 41 and from the collector line 51. A second fraction of carbon dioxide is liquefied and separated from the gaseous stream in the intermediate separation drum 45.

[0055] 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 CO2 circulating in the refrigeration circuit 41 and from the collector line 51. The carbon dioxide liquefied in the heat exchanging connection 57.1 is separated from the gaseous stream in the further separation drum 47.

[0056] 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 expanded in the expander 59.3, which generates mechanical power therewith. The mechanical power can be converted into electric power by generator 60. 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 the gas discharge duct 59. 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.

[0057] The liquid carbon dioxide collected at the first separation drum 43, intermediate separation drum 45, and further separation drum 47 is used as refrigerant in the refrigeration circuit 41. Specifically, liquid carbon dioxide is de-pressurized through the control and de-pressurizing valves 43.4, 45.4 and 47.4 and delivered through the liquefied CO2 collecting duct 71 which extends through the cold box or heat exchanger 37. Carbon dioxide entering the liquefied CO2 collecting duct 71 can be a two-phasecarbon dioxide and can evaporate by removing heat from the flue gas flowing in the first, second and third heat exchanging connections 55.1, 57.1 and 59.1.

[0058] Vaporized carbon dioxide flowing in duct 71 downstream of the heat exchanger 37 enters the suction drum 67, and after removal of possible residual liquid carbon dioxide, the carbon dioxide in vapor or gaseous form is pressurized again in the CO2 compressor 61 and cooled in the intercooler 61.3 and in the after-cooler 63.

[0059] Cooled carbon dioxide is then delivered through the compressed CO2 duct 73 and liquefied by heat exchange while flowing through the heat exchange section 73.1 of the compressed CO2 duct 73 extending through the heat exchanger 37. Carbon dioxide cooling is achieved by heat exchange with the evaporating carbon dioxide flowing through the liquefied CO2 collecting duct 71 and relevant heat exchange section 71.1 thereof.

[0060] The flow of liquefied carbon dioxide is then split in: a first partial flow which is returned through the CO2 return duct 75 and the expansion valve 77 towards the liquefied CO2 collecting duct 71; and a second partial flow which represents the CO2 product that is delivered through the CO2 outlet duct 79 to the carbon dioxide storage 81. Before reaching the carbon dioxide storage 81, the partial flow of carbon dioxide delivered from the outlet 85 of the compressed CO2 duct 73 to the carbon dioxide storage 81 can be expanded in valve 83 to the required pressure and heated through the heat exchange section 79.1 of the CO2 outlet duct 79 by removing further heat from the heat exchanger 37.

[0061] If the pressure in the carbon dioxide storage 81 is higher than the carbon dioxide pressure in point 85, the pressure adjusting device 83 can comprise a pressureboosting unit, such as a pump instead of an expansion valve. An embodiment using a pressure-boosting unit 83 instead of an expansion valve is shown in Fig.3. The remaining components shown in Fig.3 and labeled with the same reference numbers of Fig.2 are the same as described above in connection with Figs. 1 and 2 and will not be described again.

[0062] The following Table 1 summarizes exemplary and not limiting values of pressure and temperature in several points of the system shown in Fig.2. Each point islabeled with a letter from A to O. The letters are reported in the first column of Table 1 and shown in Fig.2:Table 1

[0063] 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 system (1) for producing CO2 out of a flue gas, the system comprising:- a heat exchanger (37) adapted to receive a compressed inlet flue gas stream and condense at least part of a carbon dioxide content thereof;- a first separation drum (43) adapted to receive a chilled flue gas stream containing at least partly liquefied CO2 from the heat exchanger (37), and to separate liquefied CO2 from the chilled flue gas stream; wherein the first separation drum (43) comprises an inlet (43.1), a liquid outlet (43.2), and a gas outlet (43.3);- a collector line (51), adapted to collect liquefied carbon dioxide from the first separation drum (43);- a refrigeration circuit (41) and a liquefied CO2 collecting duct (71) extending from the collector line (51) through the heat exchanger (37) and adapted to deliver carbon dioxide to the refrigeration circuit (41); wherein the liquefied CO2 colleting duct (71) comprises a heat exchange section (71.1) extending through the heat exchanger (37); and wherein the refrigeration circuit comprises: a CO2 compressor (61); a compressed CO2 duct (73) having an inlet connected to a delivery side of the CO2 compressor (61), an outlet (85), and a heat exchange section (73.1) between the inlet and the outlet and extending through the heat exchanger (37); and a CO2 return duct (75), which fluidly couples the outlet (85) of the compressed CO2 duct (73) to the collector line (51) or to the liquefied CO2 collecting duct (71), upstream of the heat exchange section (71.1) of the liquefied CO2 collecting duct (71);- and a CO2 outlet duct (79) connected to the outlet (85) of the compressed CO2 duct (73).

2. The system (1) of claim 1, further comprising a first de-pressurizing valve (43.4) between the liquid outlet (43.2) of the first separation drum (43) and the collector line (51).

3. The system (1) of claim 1 or 2, further comprising:at least a further separation drum (45; 47) adapted to receive a chilled flue gas stream containing at least partly liquefied CO2 from the heat exchanger (37), and to separate liquid CO2 from the chilled flue gas stream; wherein the further separation drum (45; 47) comprises an inlet (45.1; 47.1), a liquid outlet (45.2; 47.2), and a gas outlet (45.3; 47.3), the liquid outlet (45.2; 47.2) being fluidly coupled with the collector line (51); and a fluid connection between the gas outlet (43.1) of the first separation drum (43) and the inlet (45.1; 47.1) of the further separation drum (45; 47), the fluid connection extending through the heat exchanger (37) and comprising a heat exchanging connection (55.1; 57.1).

4. The system (1) of claim 3, further comprising a further de-pressur- izing valve (45.

4. 47.4) between the liquid outlet (45.2; 47.2) of the further separation drum (45; 47) and the collector line (51).

5. The system (1) of any one of the preceding claims, further comprising an expansion valve (77) positioned in the CO2 return duct (75).

6. The system (1) of any one of the preceding claims, further comprising a pressure adjusting unit (83) along the CO2 outlet duct (79).

7. The system (1) of claim 6, wherein the pressure adjusting unit (83) is selected from the group consisting of: a pressure-reduction valve; a pressure-boosting unit.

8. The system (1) of any one of the preceding claims, wherein the CO2 compressor (61) is an intercooled compressor.

9. The system (1) of any one of the preceding claims, wherein the CO2 compressor (61) is fluidly coupled to an after-cooler (63).

10. The system of any one of the preceding claims, further comprising a flue gas pre-treatment section (3).

11. The system of claim 10, wherein the flue gas pre-treatment section (3) comprises at least one of the following:- a dryer section (33.1);- a cooler (29) arranged upstream of the dryer section (33.1) and adapted to pre-cool the flue gas- a liquid-vapor separator (31) between the cooler (29) and the dryer section (33.1), adapted to remove condensate which forms in the flue gas by pre-cooling the flue gas in the cooler (29).- a filter section (33.3) downstream of the dryer section (33.1).- a water gas shift reactor system (23).- a flue gas compressor (11);- a combination thereof.

12. A method for producing liquid CO2 out of a flue gas, the method comprising the following steps: delivering a stream of compressed flue gas containing CO2 through a heat exchanger (37) in heat exchange with a flow of chilled CO2 circulating in a refrigeration circuit (41); at least partially condensing CO2 contained in the compressed flue gas by heat exchange with the chilled CO2 circulating in the refrigeration circuit (41); separating liquefied CO2 from the stream of compressed flue gas in at least a first separation drum (43); removing liquefied CO2 from the first separation drum (43); collecting liquefied CO2 from the first separation drum (43) in a liquefied CO2 collecting duct (71), which comprises a heat exchange section (71.1) extending through the heat exchanger (37); evaporating CO2 flowing through the heat exchange section (71.1) of the liquefied CO2 collecting duct (71); compressing the CO2, evaporated in the heat exchange section (71.1) of the liquefied CO2 collecting duct (71) and exiting the heat exchanger (37), in a CO2 compressor (61) of the refrigeration circuit (41); delivering a flow of compressed CO2 from the CO2 compressor (61) through a compressed CO2 duct (73) comprising a heat exchange section (73.1) extending through the heat exchanger (37) and cooling the flow of compresseddividing compressed and chilled CO2 at an outlet (85) of the compressed CO2 duct (73) into a first partial CO2 flow and a second partial CO2 flow; returning the first partial CO2 flow to the liquefied CO2 collecting duct (71) upstream of the heat exchange section (71.1) of the liquefied CO2 collecting duct (71); removing the second partial CO2 flow from the refrigeration circuit as a CO2 product; removing CCh-lean flue gas from the heat exchanger (37).

13. The method of claim 12, further comprising the step of de-pressur- izing liquefied CO2 removed from the first separation drum (43) upstream of the liquefied CO2 collecting duct (71).

14. The method of claim 12, further comprising the following steps: separating further liquefied CO2 from the stream of compressed flue gas in a further separation drum (45; 47) positioned downstream of the first separation drum; wherein the first separation drum (43) and the further separation drum (47) are fluidly connected by a fluid connection between a gas outlet (43.1) of the first separation drum (43) and an inlet (45.1; 47.1) of the further separation drum (45; 47), the fluid connection extending through the heat exchanger (37) and comprising a heat exchanging connection (55.1; 57.1); removing liquefied CO2 from the further separation drum (45; 47); and collecting liquefied CO2 from the further separation drum (45; 47) in said CO2 collecting duct (71).

15. The method of claim 14, further comprising the step of de-pressur- izing liquefied CO2 removed from the further separation drum (45; 47) upstream of the liquefied CO2 collecting duct (71).

16. The method of any one of claims 12 to 15, further comprising the step of expanding the first partial CO2 flow between the outlet (85) of the compressed CO2 duct (73) and the liquefied CO2 collecting duct (71).

17. The method of any one of claims 12 to 16, further comprising the step of adjusting a pressure of the second partial CO2 flow removed from therefrigeration circuit as a CO2 product; the step of adjusting said pressure comprising: expanding the second partial CO2 flow; or boosting the pressure of said second partial CO2 flow.

18. The method of any one of claims 12 to 17, further comprising the step of expanding the CCh-lean flue gas removed from the heat exchanger (37).

19. The method of claim 18, wherein the step of expanding the CCh-lean flue gas comprises the step of expanding the CO2 lean flue gas in an expander (59.3) and generating mechanical power therewith.

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