Carbon Dioxide Capture System

Abstract

[Problem] To improve energy efficiency in a carbon dioxide recovery system. [Solution] A carbon dioxide recovery system 1 includes: an electrolytic reduction device 2 that produces a produced gas by the electrolytic reduction of carbon dioxide; and a carbon dioxide separation device 3 that separates the produced gas from the carbon dioxide. The carbon dioxide separation device has: a first gas diffusion electrode 51; a second gas diffusion electrode 52; a liquid chamber 53 that is formed between the first gas diffusion electrode and the second gas diffusion electrode and to which is supplied an electrolytic solution containing a compound for adsorbing / releasing protons in conjunction with oxidation-reduction; a first chamber 54 which is partitioned from the liquid chamber by the first gas diffusion electrode and to which the produced gas is supplied; and a second chamber 55 which is partitioned from the liquid chamber by the second gas diffusion electrode and in which the carbon dioxide separated from the produced gas flows. The electrolytic reduction device uses, as part of raw material, the carbon dioxide separated from the produced gas in the carbon dioxide separation device.

Description

[Technical field]

[0001] The present invention relates to a carbon dioxide capture system. [Background technology]

[0002] Patent Document 1 discloses a carbon dioxide electrolysis device that generates hydrocarbons with one or more carbon atoms, such as ethylene (C2H4), methane (CH4), methanol (CH3OH), and ethanol (C2H5OH), through an electrolytic reduction reaction of carbon dioxide (CO2). The carbon dioxide electrolysis device can be applied to a carbon dioxide capture system that separates and captures carbon dioxide from gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6622237 specification Summary of the Invention [Problem to be solved by the invention]

[0004] In a carbon dioxide electrolysis device, carbon dioxide is reduced at the cathode to produce hydrocarbons such as ethylene. The hydrocarbons produced at the cathode are discharged from the carbon dioxide electrolysis device together with unreacted carbon dioxide, so the carbon dioxide must be separated in a later process. One of the separation methods is cryogenic separation. However, when carbon dioxide is cooled at a pressure of several tens of atmospheres, similar to that of existing ethylene plants, it changes from a gas to a solid, which makes it more difficult to handle than a liquid, and reduces the efficiency of recovery and reuse. In addition, in order to recover carbon dioxide as a liquid, it is necessary to pressurize the carbon dioxide, which results in poor energy efficiency. Another separation method is to have amines absorb carbon dioxide. However, this method requires heating the amines when recovering carbon dioxide from the amines, which results in poor energy efficiency.

[0005] In view of the above background, an object of the present invention is to improve the energy efficiency in a carbon dioxide capture system. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, one aspect of the present invention is a carbon dioxide capture system (1), comprising: an electrolytic reduction device (2) which is supplied with a gas containing carbon dioxide and electrolytically reduces the carbon dioxide to produce a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen, and unreacted carbon dioxide; and a carbon dioxide separation device (3) which separates carbon dioxide from the mixed gas, wherein the carbon dioxide separation device comprises a first gas diffusion electrode (51) which is a cathode, a second gas diffusion electrode (52) which is an anode, a liquid chamber (53) which is formed between the first gas diffusion electrode and the second gas diffusion electrode and which is supplied with an electrolyte containing a compound which adsorbs and desorbs protons in association with oxidation and reduction, a first chamber (54) which is partitioned from the liquid chamber by the first gas diffusion electrode and to which the mixed gas is supplied, and a second chamber (55) which is partitioned from the liquid chamber by the second gas diffusion electrode and through which the carbon dioxide separated from the mixed gas flows, and the electrolytic reduction device uses the carbon dioxide separated from the mixed gas in the carbon dioxide separation device as part of a raw material.

[0007] According to this embodiment, since carbon dioxide is separated from the mixed gas in a gaseous state, the carbon dioxide separation device using electrochemistry can separate carbon dioxide from the mixed gas with less energy than a separation method using amines. This makes it possible to improve the energy efficiency of the carbon dioxide capture system.

[0008] In the above embodiment, the system may further include at least one moisture removal unit (4) that separates water from the product gas from which carbon dioxide has been separated in the carbon dioxide separation unit, and a product gas separation unit (5) that separates hydrocarbons from the product gas by cooling the product gas from which water has been separated in the moisture removal unit.

[0009] According to this embodiment, water and hydrocarbons can be efficiently separated from the generated gas. Furthermore, even if water is mixed into the generated gas in the electrolytic reduction device or the carbon dioxide separation device, the water is separated in the subsequent moisture removal device.

[0010] In the above aspect, the product gas separation apparatus may include a combustion furnace (6) that combusts at least a portion of the off-gas, which is the product gas from which hydrocarbons have been separated, and the moisture removal apparatus may execute an adsorption step of adsorbing water and releasing the product gas from which the water has been separated, and a desorption step of receiving heat generated in the combustion furnace and releasing the water adsorbed in the adsorption step.

[0011] According to this embodiment, the desorption step of the moisture removing device can be carried out by utilizing the combustion heat of combustible gases such as hydrogen and carbon monoxide contained in the generated gas, thereby improving energy efficiency.

[0012] In the above aspect, the system may include a first gas-liquid separation device (133) that separates liquefied water from the water-containing gas released in the desorption step of the moisture removal device, and a circulation passage (116) that returns the gas from which water has been separated in the first gas-liquid separation device to the moisture removal device, and the circulation passage may be arranged so as to be able to exchange heat with the combustion furnace.

[0013] According to this embodiment, the desorption step of the moisture removing device can be carried out by utilizing the heat generated in the combustion furnace.

[0014] In the above aspect, a part of the generated gas from which water has been separated in the water removing device may be supplied to the circulation passage.

[0015] According to this embodiment, a portion of the generated gas can be used to supply heat from the combustion furnace to the moisture removing device.

[0016] In the above aspect, the combustion apparatus may further include an exhaust gas passage (151) for supplying exhaust gas containing carbon dioxide and water, generated in the combustion furnace, to the electrolytic reduction.

[0017] According to this embodiment, the carbon dioxide and water generated in the combustion furnace can be used as part of the raw materials for producing hydrocarbons, carbon monoxide, and the like.

[0018] In the above aspect, the moisture removal devices (4A, 4B) may be provided in two units in parallel, and while one of the moisture removal devices is performing the adsorption process, the other moisture removal device may be performing the desorption process.

[0019] According to this embodiment, the electrolytic reduction device can be operated continuously.

[0020] In the above aspect, the carbon dioxide separation device may include a carbon dioxide circulation passage (71) connected to the inlet and outlet of the second chamber of the carbon dioxide separation device, a carbon dioxide return passage (75) connecting the carbon dioxide circulation passage and the inlet of the cathode chamber of the electrolytic reduction device, a mixed gas inlet passage (62) connecting the inlet of the first chamber of the carbon dioxide separation device and the outlet of the cathode chamber of the electrolytic reduction device, a product gas outlet passage (64, 66) connected to the outlet of the first chamber of the carbon dioxide separation device, and an electrolyte tank (82) connected to the inlet of the liquid chamber via an electrolyte supply passage (81) and connected to the outlet of the liquid chamber via a first electrolyte return passage (87).

[0021] According to this aspect, after the pressure of carbon dioxide is increased in the carbon dioxide circulation passage, carbon dioxide can be supplied to the electrolytic reduction device via the carbon dioxide return passage.

[0022] In the above aspect, a second gas-liquid separator (72) is provided in the carbon dioxide circulation passage, and the liquid separated in the second gas-liquid separator is fed to the electrolyte tank. returnThe electrolytic solution supply system may further include a second electrolytic solution return passage (95), a first pressure control valve (76) provided in the carbon dioxide return passage, a second pressure control valve (68) provided in the mixed gas inlet passage or the produced gas outlet passage, a third pressure control valve (flow rate control valve) (84) provided in the electrolytic solution supply passage or the first electrolytic solution return passage, a first pressure sensor (PS1) that detects the pressure in the first chamber as a first pressure, a second pressure sensor (PS2) that detects the pressure in the second chamber as a second pressure, a third pressure sensor (PS3) that detects the pressure in the liquid chamber as a third pressure, and a control device (7) that controls the first pressure control valve, the second pressure control valve, and the third pressure control valve so that the first pressure becomes equal to or greater than the third pressure and the third pressure becomes equal to or greater than the second pressure.

[0023] According to this embodiment, it is possible to make it difficult for the electrolyte to be mixed into the produced gas. Also, the electrolyte that has leaked into the carbon dioxide circulation passage can be returned to the electrolyte tank.

[0024] In the above aspect, the system may include a fourth pressure control valve (69) provided downstream of the first pressure control valve in the produced gas outlet passage, a gas return passage (97) connecting a top of the electrolyte tank and a portion of the produced gas outlet passage upstream of the fourth pressure control valve, and a fourth pressure sensor (PS4) that detects the pressure of the electrolyte tank as a fourth pressure, and the control device may control the first pressure control valve, the second pressure control valve, the third pressure control valve, and the fourth pressure control valve so that the first pressure is equal to or greater than the third pressure, the third pressure is equal to or greater than the fourth pressure, and the fourth pressure is equal to or greater than the second pressure.

[0025] According to this embodiment, the gas remaining at the top of the electrolyte tank can be returned to the product gas outlet passage via the gas return passage.

[0026] In the above embodiment, the hydrocarbon may include at least one of methane and ethylene.

[0027] According to this embodiment, at least one of methane, ethylene, and carbon monoxide can be produced from carbon dioxide.

[0028] In the above embodiment, the compound that adsorbs and desorbs protons with oxidation-reduction may be an organic compound having an oxidation-reduction potential of −1.0 V to 1.0 V based on the standard hydrogen electrode potential at pH 7. The compound that adsorbs and desorbs protons with oxidation-reduction may be a quinone compound.

[0029] According to these embodiments, carbon dioxide can be separated efficiently.

[0030] Another aspect of the present invention is a carbon dioxide capture system (1), comprising: an electrolytic reduction device (2) that is supplied with a gas containing carbon dioxide and electrolytically reduces the carbon dioxide to generate a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen, and unreacted carbon dioxide; and a carbon dioxide separation device (400) that separates carbon dioxide from the mixed gas. The carbon dioxide separation device comprises a carbon dioxide gas absorption section (401) that brings the mixed gas into contact with an electrolyte solution containing a compound that adsorbs and desorbs protons as a result of oxidation and reduction, and causes the electrolyte solution to absorb carbon dioxide in the mixed gas; an electrolyte membrane (416A); and a power supply (419) that is connected to the electrolyte membrane. the electrochemical cell (402) partitioned into a cathode chamber (417) and an anode chamber (418) by a membrane electrode assembly (416) including a cathode (416B) and an anode (416C) formed therein; a first passage (403) for flowing the electrolytic solution from the carbon dioxide gas absorption section to the anode chamber; a second passage (404) for flowing the electrolytic solution from the anode chamber to the cathode chamber; a third passage (405) for flowing the electrolytic solution from the cathode chamber to the carbon dioxide gas absorption section; and a gas-liquid separator (406) provided in the second passage for separating the electrolytic solution from the carbon dioxide gas, and the electrolytic reduction device uses carbon dioxide separated from the mixed gas in the carbon dioxide separator as part of a raw material.

[0031] According to this embodiment, since carbon dioxide is separated from the mixed gas in a gaseous state, the carbon dioxide separation device using electrochemistry can separate carbon dioxide from the mixed gas with less energy than a separation method using amines. This makes it possible to improve the energy efficiency of the carbon dioxide capture system.

[0032] In the above aspect, the carbon dioxide gas absorption unit includes a housing (431) that extends vertically and stores the electrolyte at a bottom thereof, a gas inlet (411) that is provided at a lower part of the housing and through which the mixed gas is supplied, a gas outlet (412) that is provided at an upper end of the housing and through which the mixed gas is discharged, a nozzle (432) that is provided at an upper part of the housing and sprays the electrolyte, a fourth passage (433) that supplies the electrolyte stored at the bottom of the housing to the nozzle, a tray (434) that is provided in the housing between the gas inlet and the nozzle and temporarily stores the electrolyte supplied from the nozzle, and an electrolyte outlet (414) provided on the tray, and the electrolyte outlet is connected to the first passage and the third passage (434) is connected to the second passage (434). the passage is connected to the fourth passage, a first flow control valve (436A) is provided in the third passage, and a second flow control valve (436B) is provided in the fourth passage closer to the bottom of the housing than the third passage, the power source, the first flow control valve, and the second flow control valve are controlled by a control device (7), and the pH of the electrolytic solution stored in the bottom of the housing is pH1, the pH of the electrolytic solution flowing in a portion of the fourth passage closer to the nozzle than the third passage is pH2, the pH of the electrolytic solution flowing through the third passage is pH3, and the pH of the electrolytic solution flowing through the first passage is pH4, the control device may control the power source, the first flow control valve, and the second flow control valve so as to satisfy pH3≧pH2≧pH1≧pH4≧5.5.

[0033] According to this embodiment, it is possible to efficiently absorb carbon dioxide gas into the electrolytic solution and release the carbon dioxide gas from the electrolytic solution. Effect of the Invention

[0034] According to the above aspect, it is possible to improve the energy efficiency in the carbon dioxide capture system. [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 is an explanatory diagram showing a carbon dioxide capture system according to a first embodiment; [Diagram 2] FIG. 1 is an explanatory diagram showing an electrolytic reduction device according to a first embodiment. [Diagram 3] FIG. 1 is an explanatory diagram showing a carbon dioxide separation device according to a first embodiment. [Figure 4] An explanatory diagram showing the carbon dioxide separation mechanism in a carbon dioxide separator. [Diagram 5] FIG. 13 is an explanatory diagram showing a carbon dioxide capture system according to a second embodiment. [Figure 6] FIG. 11 is an explanatory diagram showing a carbon dioxide separator according to a third embodiment. [Figure 7] FIG. 11 is a schematic diagram showing a carbon dioxide separator according to a fourth embodiment. [Figure 8] FIG. 13 is a detailed explanatory diagram showing a carbon dioxide separator according to a fourth embodiment. [Figure 9] Graph showing recovery rate of each component in Example 1 [Figure 10] Graph showing recovery rate of each component in Example 2 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Hereinafter, an embodiment of a carbon dioxide capture system according to the present invention will be described. As shown in Fig. 1, a carbon dioxide capture system 1 according to the embodiment has, as its main components, an electrolytic reduction device 2 and a carbon dioxide separation device 3. The carbon dioxide capture system 1 further has a moisture removal device 4, a product gas separation device 5, and a combustion furnace 6. The carbon dioxide capture system 1 also has a control device 7 that controls each device and a flow control valve, which will be described later. The control device 7 has a processor, a memory, and a storage device that stores a program, and controls each device by executing the program.

[0037] The electrolytic reduction device 2 supplies a gas containing carbon dioxide and electrolytically reduces the carbon dioxide to generate a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen, and unreacted carbon dioxide. The mixed gas is discharged from the cathode 16 side of the electrolytic reduction device 2. At the cathode of the electrolytic reduction device 2, as represented by the following chemical formulas (1) to (4), carbon dioxide is reduced depending on the catalyst species added to the electrode and the operating conditions to obtain a product containing ethylene as a main product and by-products such as carbon monoxide, methane, and hydrogen. 2CO2+12H + +12e - →C2H4+4H2O ...(1) CO2+8H + +8e - →CH4+2H2O ...(2) CO2+2H + +2e - →CO+H2O ...(3) 2H + +2e - →H2...(4) At the anode of the electrolytic reduction device 2, water is oxidized to generate oxygen, as represented by the following chemical formula (5). 2H2O→O2+4H + +4e - ...(5) The electrolytic reduction device 2 may be a three-chamber electrolytic reduction device having a cathode gas chamber and a cathode liquid chamber separated by a cathode, which is a gas diffusion electrode, and an anode liquid chamber separated from the cathode liquid chamber by a separator and in which an anode is disposed, or an electrolytic reduction device using a membrane electrode assembly (MEA) in which an electrolyte membrane is sandwiched between a cathode and an anode.

[0038] An example of the electrolytic reduction device 2 is shown in Fig. 2. The electrolytic reduction device 2 has an electrolytic cell 14 having a cathode chamber 11 and an anode chamber 12 separated from each other by a membrane electrode assembly 10. The membrane electrode assembly 10 has an electrolyte membrane 15, a cathode 16 provided on one side of the electrolyte membrane 15, and an anode 17 provided on the other side of the electrolyte membrane 15. Gaseous carbon dioxide is supplied to the cathode chamber 11. An anolyte is supplied to the anode chamber 12. The cathode 16 and the anode 17 are connected to a DC power source 19.

[0039] The anolyte is an aqueous solution in which an electrolyte is dissolved. The electrolyte includes potassium, sodium, lithium, or at least one of these compounds. The electrolyte may include, for example, at least one selected from the group consisting of LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3, LiHCO3, NaHCO3, and KHCO3.

[0040] The electrolyte membrane 15 is formed of a proton conductor that allows protons to pass therethrough. The electrolyte membrane 15 is, for example, a solid polymer electrolyte membrane, and is preferably a fluorine resin-based cation exchange resin membrane having sulfonic acid groups.

[0041] The cathode 16 is a gas diffusion electrode. The cathode 16 is permeable to gases including carbon dioxide. The cathode 16 may be a porous conductive substrate such as carbon paper, carbon felt, or carbon cloth on which a water-repellent coating such as polytetrafluoroethylene is formed. The conductive substrate is connected to the negative electrode of a DC power source 19 and receives a supply of electrons. A catalyst is supported on the cathode 16. The catalyst may be a known carbon dioxide reduction catalyst, and includes at least one of, for example, a Group 11 element such as copper, a Group 12 element such as zinc, a Group 13 element such as gallium, a Group 14 element such as germanium, or a metal compound thereof. The metal compound includes at least one of an oxide, a sulfide, and a phosphide. The catalyst is preferably one suitable for reducing carbon dioxide to produce ethylene, and is preferably a material obtained by combining, for example, copper or a copper compound with a metal of a Group 11 element, a Group 12 element, a Group 13 element, or a Group 14 element, and a metal compound thereof.

[0042] The anode 17 is made of, for example, a metal material such as titanium, nickel, molybdenum, platinum, gold, silver, copper, iron, lead, or an alloy material of these metals, a carbon-based material such as carbon, or a conductive ceramic. The shape of the anode 17 may be a flat plate, a mesh, or a porous body having a plurality of openings. The anode 17 supports an oxygen generating catalyst such as platinum or iridium.

[0043] The DC power supply 19 converts power obtained by thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, or the like into DC as necessary and supplies the power to the cathode 16 and the anode 17. From the viewpoint of reducing carbon dioxide emissions, it is preferable to use power obtained by solar power generation, wind power generation, hydroelectric power generation, or the like that utilizes natural energy (renewable energy) as the DC power supply 19. The DC power supply 19 applies a voltage so that the cathode 16 has a negative potential with respect to the anode 17. It is preferable that the DC power supply 19 obtains the potential of the cathode 16 using a reference electrode and controls the voltage applied so that the potential of the cathode 16 is within a predetermined range.

[0044] The cathode chamber 11 has an inlet 24 and an outlet 25. As shown in Figs. 1 and 2, the inlet 24 of the cathode chamber 11 is connected to a carbon dioxide supply source 27 via a first supply passage 26. The carbon dioxide supply source 27 is not particularly limited as long as it is a facility capable of supplying carbon dioxide gas, and is preferably a storage tank or the like. The outlet 25 of the cathode chamber 11 is connected to the first supply passage 26 via a gas circulation passage 28. A flow control valve 29 is provided between the carbon dioxide supply source 27 and the first supply passage 26. The flow control valve 29 controls the flow rate of the carbon dioxide gas supplied from the carbon dioxide supply source 27 to the first supply passage 26.

[0045] The anode chamber 12 has an inlet 31 and an outlet 32. The inlet 31 of the anode chamber 12 is connected to a water supply source 34 via a second supply passage 33. The water supply source 34 supplies liquid water to the second supply passage 33. The outlet 32 ​​of the anode chamber 12 is connected to the second supply passage 33 via an anode liquid circulation passage 35. A flow control valve 36 is provided between the water supply source 34 and the second supply passage 33. The flow control valve 36 controls the flow rate of liquid water supplied from the water supply source 34 to the second supply passage 33. The second supply passage 33 is provided with a flow sensor S12 that measures the flow rate of water flowing therethrough.

[0046] A gas circulation flow rate control device 38 that discharges a portion of the gas circulating inside the gas circulation passage 28 is provided in the gas circulation passage 28. The outlet of the gas circulation flow rate control device 38 is connected to the cathode outlet passage 39. The gas circulation flow rate control device 38 controls the flow rate and pressure of the gas circulating through the gas circulation passage 28 and the cathode chamber 11 by discharging the gas to the cathode outlet passage 39.

[0047] The anode fluid circulation passage 35 is provided with a gas-liquid separator 41. The gas-liquid separator 41 separates gas from the anode fluid and delivers the gas to an anode outlet passage 42. The anode fluid circulation passage 35 may also be provided with an electrolyte concentration controller 43 for adjusting the electrolyte concentration of the anode fluid within a predetermined range. The electrolyte concentration controller 43 may include a sensor for detecting the electrolyte concentration of the anode fluid, an electrolyte fluid supplier for supplying new anode fluid of a predetermined concentration, and a drainage device for discharging a portion of the circulating anode fluid.

[0048] The carbon dioxide in the cathode chamber 11 diffuses into the cathode 16 and is reduced (see chemical formula (1)). This produces a product gas containing ethylene as the main product and by-products such as methane, hydrogen, carbon monoxide, and formic acid. The product gas mixes with the unreacted carbon dioxide in the cathode chamber 11 to produce a mixed gas. The mixed gas may contain water and hydrocarbons such as propane and butane. The main components of the mixed gas are ethylene and carbon dioxide.

[0049] The product gas containing the product produced by the reduction reaction of carbon dioxide and unreacted carbon dioxide circulates through the gas circulation passage 28 and is sent to the cathode outlet passage 39 from the gas circulation flow rate regulator 38 .

[0050] In the anode 17, water and hydroxide ions in the anolyte are oxidized to generate gaseous oxygen (see chemical formula (5)). The gaseous oxygen is separated from the anolyte by the gas-liquid separator 41 in the anolyte circulation passage 35 and is sent to the anode outlet passage 42.

[0051] In the electrolytic reduction device 2, the catalyst supported on the cathode 16 and the potential of the cathode 16 are preferably set so that the faradaic efficiency for ethylene production at the cathode 16 is 30% or more, preferably 50% or more. Here, the faradaic efficiency is defined as the ratio of the current that contributes to the production of each product to the total current flowing through the electrolytic cell 14. In the electrolytic reduction device 2, the catalyst supported on the cathode 16 is preferably selected so that the selectivity for ethylene production at the cathode 16 is 30% or more.

[0052] The gas flowing through the anode outlet passage 42 is mainly oxygen. Note that the gas flowing through the anode outlet passage 42 may also contain carbon dioxide.

[0053] The mixed gas discharged from the cathode outlet passage 39 of the electrolytic reduction device 2 is supplied to the carbon dioxide separation device 3. The carbon dioxide separation device 3 separates carbon dioxide from the mixed gas. As shown in FIG. 3, the carbon dioxide separation device 3 has a first gas diffusion electrode 51 which is a cathode, a second gas diffusion electrode 52 which is an anode, a liquid chamber 53 which is formed between the first gas diffusion electrode 51 and the second gas diffusion electrode 52 and to which an electrolytic solution containing a compound which adsorbs and desorbs protons in association with oxidation-reduction is supplied, a first chamber 54 which is partitioned from the liquid chamber 53 by the first gas diffusion electrode 51 and to which the generated gas is supplied, and a second chamber 55 which is partitioned from the liquid chamber 53 by the second gas diffusion electrode 52 and to which the carbon dioxide separated from the generated gas flows. In this embodiment, the carbon dioxide separation device 3 has a stack 59 in which a first unit, in which a second gas diffusion electrode 52, a liquid chamber 53, and a first gas diffusion electrode 51 are arranged in this order, and a second unit, in which the first gas diffusion electrode 51, the liquid chamber 53, and a second gas diffusion electrode 52 are arranged in this order, are alternately stacked at intervals from each other. A first chamber 54 is formed between two adjacent first gas diffusion electrodes 51, and a second chamber 55 is formed between two adjacent second gas diffusion electrodes 52.

[0054] Each of the first gas diffusion electrode 51 and the second gas diffusion electrode 52 has a porous conductor. The porous conductor preferably has a large specific surface area in order to increase the reaction area. The specific surface area of ​​the porous conductor is preferably 1 m2 or more in BET adsorption measurement. 2 / g or more, more preferably 100m 2 / g or more, more preferably 500m 2 The surface resistance of the porous conductor is preferably as low as possible, and is preferably 1 kΩ / □ or less, and more preferably 200 Ω / □ or less. The porous conductor may be, for example, a carbon sheet, a carbon cloth, or a carbon paper.

[0055] In order to minimize the voltage drop (IR drop) due to solution resistance, the first gas diffusion electrode 51 and the second gas diffusion electrode 52 are preferably placed as close as possible to each other without contacting each other. A separator may be inserted between the first gas diffusion electrode 51 and the second gas diffusion electrode 52. The separator has insulating properties and allows the electrolyte to pass through. The separator may be selected from, for example, a porous membrane of polyolefin represented by polyethylene and polypropylene, a porous membrane of polyester, aliphatic polyamide, or aromatic polyamide, a nonwoven fabric, or the like.

[0056] The first gas diffusion electrodes 51 are connected to the negative electrode of a DC power source 61 , and the second gas diffusion electrodes 52 are connected to the positive electrode of the DC power source 61 .

[0057] The cathode outlet passage 39 of the electrolytic reduction device 2 is connected to the inlets of each of the first chambers 54 via a mixed gas inlet passage 62. A downstream portion of the mixed gas inlet passage 62 branches out corresponding to each of the first chambers 54. A blower 63 that sends out the generated gas toward each of the first chambers 54 is provided upstream of the mixed gas inlet passage 62.

[0058] The outlets of the first chambers 54 are connected to a first vessel 65 via a plurality of first product gas outlet passages 64. The first vessel 65 is connected to a second product gas outlet passage 66. The product gas discharged from each first chamber 54 passes through one of the plurality of first product gas outlet passages 64, the first vessel 65, and the second product gas outlet passage 66, and is discharged from the carbon dioxide separation apparatus 3. A pressure control valve 68 and a pressure control valve 69 are provided in the second product gas outlet passage 66, in this order from the first vessel 65 side. It is preferable that the outlets of the first chambers 54 are located above the inlets of the first chambers 54.

[0059] The inlet and outlet of each second chamber 55 are connected by a carbon dioxide circulation passage 71. It is preferable that each outlet of each second chamber 55 is disposed lower than each inlet of each second chamber 55. A second vessel 72, a blower 73, and a third vessel 74 are provided in this order in the carbon dioxide circulation passage 71 from the outlet side to the inlet side of the second chamber 55. Carbon dioxide gas mainly flows through each second chamber 55 and the carbon dioxide circulation passage 71.

[0060] The second vessel 72 functions as a gas-liquid separator. The bottom of the second vessel 72 is disposed below the outlets of the second chambers 55, and the carbon dioxide circulation passage 71 extends downward from the outlets of the second chambers 55 toward the second vessel 72. In this way, when the electrolyte liquid leaks from each liquid chamber 53 to each second chamber 55, the leaked liquid accumulates at the bottom of the second vessel 72.

[0061] The gas components in the second vessel 72 flow from the top of the second vessel 72 to the blower 73. The blower 73 sends out the gas in the carbon dioxide circulation passage 71 toward the third vessel 74. A carbon dioxide return passage 75 for returning the circulating carbon dioxide gas to the inlet of the cathode chamber 11 of the electrolytic reduction device 2 is connected to the carbon dioxide circulation passage 71. The carbon dioxide return passage 75 is preferably connected to the third vessel 74 and the first supply passage 26. A pressure control valve 76 is provided in the carbon dioxide return passage 75.

[0062] Each of the inlets of the liquid chambers 53 is connected to the electrolyte tank 82 via an electrolyte supply passage 81. The electrolyte supply passage 81 branches out corresponding to each of the liquid chambers 53. The electrolyte supply passage 81 is provided with a pump 83, a flow control valve (pressure control valve) 84, and a temperature regulator 85 in this order from the electrolyte tank 82 side. The pump 83 sends the electrolyte from the electrolyte tank 82 to each of the liquid chambers 53. The temperature regulator 85 regulates the temperature of the electrolyte. The temperature regulator 85 regulates the electrolyte to, for example, room temperature to 80° C. or less. Each of the outlets of the liquid chambers 53 is connected to the electrolyte tank 82 via a first electrolyte return passage 87. As a result, the electrolyte circulates through the electrolyte tank 82, the electrolyte supply passage 81, each of the liquid chambers 53, and the first electrolyte return passage 87.

[0063] A portion of the electrolyte supply passage 81 between the pump 83 and the flow rate control valve 84 is connected to the electrolyte tank 82 via a circulation passage 88. The circulation passage 88 is provided with an electrolyte sensor CS1 that detects information including at least one of the composition and concentration of the electrolyte passing through the circulation passage 88. The electrolyte sensor CS1 may be, for example, an absorptiometer or a conductivity sensor. The absorptiometer is preferably an ultraviolet-visible spectrophotometer.

[0064] The electrolyte tank 82 is connected to a high-concentration electrolyte tank 92 via an electrolyte supply passage 91. A high-concentration electrolyte is stored in the high-concentration electrolyte tank 92. The high-concentration electrolyte has a higher concentration of electrolytes and a compound that adsorbs and desorbs protons in association with oxidation-reduction, which will be described later, than the electrolyte stored in the electrolyte tank 82. The electrolyte supply passage 91 is provided with a pump 93 that sends the high-concentration electrolyte from the high-concentration electrolyte tank 92 to the electrolyte tank 82.

[0065] The bottom of the second vessel 72 is connected to the electrolyte tank 82 via a second electrolyte return passage 95. A pump 96 is provided in the second electrolyte return passage 95 to pump the electrolyte from the second vessel 72 toward the electrolyte tank 82. As a result, the electrolyte that has been separated from the carbon dioxide gas and accumulated at the bottom of the second vessel 72 is returned to the electrolyte tank 82 via the second electrolyte return passage 95.

[0066] The top of the electrolyte tank 82 is connected to a portion of the second produced gas outlet passage 66 between the pressure control valve 68 and the pressure control valve 69 via a gas return passage 97. A temperature regulator 98 is provided in the gas return passage 97. The pressure of the gas phase in the electrolyte tank 82 is controlled by the pressure control valve 69, and the gas in the electrolyte tank 82 flows to the second produced gas outlet passage 66.

[0067] The first vessel 65 and the third vessel 74 are provided with pressure sensors PS1 and PS2 for measuring the internal pressure of the gas phase portion. A pressure sensor PS3 for measuring the internal pressure is provided in a portion between the flow control valve 84 and the end of the liquid chamber 53 side of the electrolyte supply passage 81. A pressure sensor PS4 for measuring the internal pressure of the gas phase portion is provided in the electrolyte tank 82. A flow sensor FS1 is provided in a portion of the mixed gas inlet passage 62 on the first chamber 54 side from the blower 63. A flow sensor FS2 is provided in a portion of the carbon dioxide circulation passage 71 between the blower 73 and the third vessel 74. A flow sensor FS3 is provided in a portion of the electrolyte supply passage 81 between the pump 83 and the flow control valve 84. The electrolyte tank 82 is provided with a temperature sensor TS1 for measuring the temperature of the charged liquid therein. The pressure sensors PS1 to PS4, the flow sensors FS1 to FS3, and the temperature sensor TS1 are connected to the control device 7. In another embodiment, the pressure sensor PS1 may be provided in a portion of the mixed gas inlet passage 62 between the blower 63 and the first chamber 54. The pressure sensor PS3 may be provided in the first electrolyte return passage 87.

[0068] The electrolyte is composed of a solute and a solvent in which the solute is dissolved. The solute generates carbonic acid when dissolved in the solvent, or generates bicarbonate ions or carbonate ions by ionization. The solute may include at least one selected from the group consisting of bicarbonates and carbonates of alkali metals, and bicarbonates and carbonates of alkaline earth metals. Specifically, the solute may be NaHCO3, KHCO3, LiHCO3, Na2CO3, K2CO3, or Li2CO3. The solvent may be water.

[0069] The electrolyte preferably contains a compound dissolved therein that adsorbs and desorbs protons in association with oxidation and reduction. The compound is preferably an organic compound having an oxidation-reduction potential of -1.0 V to 1.0 V based on the standard hydrogen electrode potential at pH 7. The compound is, for example, a quinone-based compound, an indophenol-based compound, an indigo-based compound, or the like, and is preferably a quinone-based compound. The compounds may be, for example, chloramine-T, o-tolidine, 2,5-dihydroxy-1,4-benzoquinone, p-aminodimethylaniline, o-quinone, 1,2-diphenol, p-aminophenol, 1,4-benzoquinone, 2,6,2′-trichloroindophenol, indophenol, phenol blue, 2,6-dichlorophenol indophenol (DCPIP), 2,6-dibromo-2′-methoxyindophenol, 1,2-naphthoquinone, 1-naphthol-2-sulfonic acid indophenol, toluylene blue, dehydroascorbic acid / ascorbic acid, N-methylphenazinium methosulfate (PMS), thionine, phenazine ethosulfate, 1,4-naphthoquinone, toluidine blue, thioindigo disulfonate, methylene blue, 2-methyl-1,4-naphthoquinone ( It is preferable that the composition contains at least one selected from the group consisting of vitamin K3), indigo tetrasulfonate, methyl capri blue, indigo trisulfonate, indigo disulfonate, 2-hydroxy-1,4-naphthoquinone, 2-amino-N-methylphenazine methosulfate, indigo monosulfonate, brilliant alizarin blue, 2-methyl-3-hydroxy-1,4-naphthoquinone, 9-methyl-isoalloxazine, anthraquinone-2,6-disulfate, neutral blue, riboflavin, anthraquinone-1-sulfate, phenosafranin, safranine T, lipoic acid, acridine, neutral red, cystine / cysteine, benzyl viologen, 1-aminoacridine, methyl viologen, 2-aminoacridine, 2,8-diaminoacridine, and 5-aminoacridine.

[0070] The quinone-based compounds include hydroquinone-based compounds containing hydroxyl groups and benzoquinone-based compounds obtained by oxidation of hydroquinone-based compounds and containing carbonyl groups. The quinone-based compounds contained in the electrolyte have functional groups with relatively high electron-withdrawing properties compared to oxygen. As a result, when the raw material gas provided in the electrolyte contains not only carbon dioxide but also oxygen, the quinone-based compounds are more likely to receive electrons generated by the electrode reaction on the first gas diffusion electrode 51 side than oxygen due to the presence of the relatively large electron-withdrawing functional groups of the quinone-based compounds. That is, the quinone-based compounds are more easily reduced than oxygen, and the benzoquinone-based compounds containing carbonyl groups are reduced to hydroquinone-based compounds containing hydroxyl groups, while oxygen is less likely to be reduced. This prevents the hydroquinone-based compounds after the reduction reaction from being immediately oxidized by oxygen. That is, the hydroquinone-based compounds with electron-withdrawing groups are resistant to oxidation by oxygen.

[0071] The functional group of the quinone compound may be, for example, a sulfonate. The sulfonate has a sulfo group (-SO3) and an alkali metal element, and may be, for example, sodium sulfonate or potassium sulfonate. In addition, the number of functional groups may be one or more from the viewpoint of increasing electron-withdrawing ability. In addition, the number of functional groups may be one or more, and is preferably two or more and four or less from the viewpoint of increasing electron-withdrawing ability.

[0072] The quinone compound is a hydroquinone compound that contains a hydroxyl group in addition to the functional group after reduction reaction. The number of hydroxyl groups is preferably 2 to 4 in order to ensure aromaticity. The hydroquinone compound is preferably, for example, disodium 4,5-dihydroxy-1,3-benzenedisulfonate (tiron) or potassium hydroquinonesulfonate.

[0073] Moreover, the quinone-based compound is a benzoquinone-based compound that contains a carbonyl group in addition to the functional group after the oxidation reaction. The benzoquinone-based compound becomes a compound in which a hydroxy group is replaced with a carbonyl group by the oxidation reaction at the second gas diffusion electrode 52. The chemical formula (6) of the benzoquinone-based compound is shown below. Here, R1 to R4 are H (hydrogen), functional groups, etc. [ka] As a specific example of a benzoquinone-based compound, the chemical formula (7) of 4,5-dihydroxy-1,3-benzenedisulfonic acid is shown. [ka]

[0074] Next, the operation of the carbon dioxide separation device 3 will be described with reference to Fig. 4. In the first gas diffusion electrode 51 serving as the cathode of each liquid chamber 53, the benzoquinone-based compound (Q) in the electrolyte is reduced to a hydroquinone-based compound (QH2) as shown in the following chemical formula (8). Q+2H + +2e - →QH2...(8)

[0075] At this time, protons (H + ) is absorbed into the hydroquinone-based compound, so that the pH of the electrolyte in the vicinity of the first gas diffusion electrode 51 becomes relatively higher than that before the reduction reaction. When the pH becomes relatively higher, carbon dioxide, due to its nature, becomes more soluble in water, which is the solvent for the electrolyte. As a result, carbon dioxide in the generated gas in each first chamber 54 dissolves into the electrolyte in each liquid chamber 53 via the first gas diffusion electrode 51. Then, carbon dioxide becomes hydrogen carbonate ions in the electrolyte through the reactions of the following chemical formulas (9) to (11). Chemical formulas (9) to (11) are equilibrium reactions. CO2+H2O⇔H2CO3...(9) H2CO3⇔H + +HCO3 - ...(10) HCO3 - ⇔H+ +CO3 2- ...(11)

[0076] In the second gas diffusion electrode 52 serving as the anode of each liquid chamber 53, the hydroquinone-based compound (QH2) in the electrolyte is oxidized to a benzoquinone-based compound (Q) as shown in the following chemical formula (12). QH2→Q+2H++2e - ...(12)

[0077] At this time, protons (H + ) is released, so that the pH of the electrolyte in the vicinity of the second gas diffusion electrode 52 becomes relatively lower than that before the oxidation reaction. When the pH becomes relatively lower, the equilibrium state of bicarbonate ions and carbonic acid in the chemical formulas (9) to (11) shifts toward the carbonic acid side. This generates carbon dioxide. As a result, the carbon dioxide in the electrolyte is released into each of the second chambers 55 via each of the second gas diffusion electrodes 52. In this way, the carbon dioxide gas in the mixed gas in each of the first chambers 54 is separated in a gaseous state into each of the second chambers 55. At this time, the hydrocarbons, oxygen, hydrogen, and the like in the mixed gas are not dissolved in the electrolyte and are maintained in the first chamber 54. This allows the carbon dioxide separator 3 to separate carbon dioxide from the mixed gas.

[0078] Due to the oxidation-reduction reaction of the quinone compound, the pH on the first gas diffusion electrode 51 side becomes relatively high, and the pH on the second gas diffusion electrode 52 side becomes relatively low. This creates a pH gradient in the electrolyte, and allows bicarbonate ions, carbonic acid, and carbonate ions to migrate to the second gas diffusion electrode 52 side. This allows the flow rate of carbon dioxide gas that can be separated into the second chamber 55 to be increased.

[0079] The mixed gas (produced gas) from which carbon dioxide has been separated flows from each first chamber 54 through the first vessel 65 to the second produced gas outlet passage 66. The carbon dioxide gas separated in the second chamber 55 circulates through the carbon dioxide circulation passage 71 and each second chamber 55. At this time, the electrolytic solution is separated from the carbon dioxide gas in the second vessel 72 and returned to the electrolytic solution tank 82 through the second electrolytic solution return passage 95. The carbon dioxide gas flowing through the carbon dioxide circulation passage 71 is returned to the electrolytic reduction device 2 through the carbon dioxide return passage 75 and the first supply passage 26 by opening the pressure control valve 76. The electrolytic reduction device 2 uses the carbon dioxide separated from the produced gas in the carbon dioxide separation device 3 as part of a raw material.

[0080] The control device 7 detects the pressure PL1 of the first chamber 54, the pressure PL2 of the second chamber 55, the pressure PL3 of the liquid chamber 53, and the pressure PL4 of the electrolyte tank 82 based on signals from the pressure sensors PS1, PS2, PS3, and PS4. The control device 7 may set the pressure of the first vessel 65 detected by the pressure sensor PS1 to the pressure PL1 of the first chamber 54, the pressure of the third vessel 74 detected by the pressure sensor PS2 to the pressure PL2 of the second chamber 55, and the pressure of the electrolyte supply passage 81 detected by the pressure sensor PS3 to the pressure PL3 of the liquid chamber 53. The control device 7 controls the pressure control valves 68, 76, and the flow control valve 84 so that the pressure PL1 of each first chamber 54 is equal to or greater than the pressure PL3 of each liquid chamber 53, and the pressure PL3 of each liquid chamber 53 is equal to or greater than the pressure PL2 of each second chamber 55 (PL1≧PL3≧PL2). The control device 7 may control the pressure control valves 68, 69, 76 and the flow control valve 84 so that the pressure PL1 of each first chamber 54 is equal to or greater than the pressure PL3 of each liquid chamber 53, the pressure PL3 of each liquid chamber 53 is equal to or greater than the pressure PL4 of the electrolyte tank 82, and the pressure PL4 of the electrolyte tank 82 is equal to or greater than the pressure PL2 of each second chamber 55 (PL1≧PL3≧PL4≧PL2). By making the pressure PL1 of each first chamber 54 equal to or greater than the pressure PL3 of each liquid chamber 53, the electrolyte in the liquid chamber 53 is prevented from leaking through the first gas diffusion electrode 51 to the first chamber 54. By making the pressure PL3 of each liquid chamber 53 equal to or greater than the pressure PL4 of the electrolyte tank 82, even if gas is mixed into the liquid chamber 53 from the first chamber 54, separation of the gas and the electrolyte in the electrolyte tank 82 is promoted. In addition, when the pressure PL3 in each liquid chamber 53 becomes equal to or higher than the pressure PL2 in each second chamber 55, carbon dioxide in the electrolyte is easily released into the second chamber 55 through the second gas diffusion electrode 52. The electrolyte leaked into each second chamber 55 is separated by the second vessel 72 and returned to the electrolyte tank 82.

[0081] The control device 7 obtains the concentration of the quinone-based compound in the electrolyte based on a signal from the electrolyte sensor CS1, and when the concentration of the quinone-based compound is equal to or lower than a predetermined judgment value, the control device 7 drives the pump 93 to supply the high-concentration electrolyte from the high-concentration electrolyte tank 92 to the electrolyte tank 82. The high-concentration electrolyte has a higher concentration of the quinone-based compound and electrolyte than the electrolyte supplied to each liquid chamber 53. This increases the concentration of the quinone-based compound in the electrolyte, and the concentration of the quinone-based compound in the charged liquid is maintained within a predetermined range.

[0082] 1, the product gas from which carbon dioxide has been separated in the carbon dioxide separation device 3 (hereinafter referred to as the first treated gas) is sent from the second product gas outlet passage 66 through a passage 101 to the moisture removal device 4. A compressor 102 that pressure-transmits the first treated gas toward the moisture removal device 4 is provided in the passage 101. The moisture removal device 4 separates water from the first treated gas.

[0083] In this embodiment, the moisture removal device 4 includes a first moisture removal device 4A and a second moisture removal device 4B arranged in parallel with each other. The first and second moisture removal devices 4A and 4B may be adsorption towers filled with a moisture adsorbent. The moisture adsorbent releases (desorbs) moisture by being subjected to a heat treatment. The moisture adsorbent may be a desiccant such as crystalline zeolite (molecular sieve).

[0084] An inlet of the first moisture removal device 4A is connected to a passage 101 via a passage 103. An outlet of the first moisture removal device 4A is connected to a passage 104. An inlet of the second moisture removal device 4B is connected to the passage 101 via a passage 105. An outlet of the second moisture removal device 4B is connected to a passage 106. The passages 104 and 106 are connected to the inlet of the product gas separation device 5 via a passage 107. On-off valves 111-114 are provided in the passages 103-106, respectively.

[0085] The first and second moisture removal devices 4A and 4B are connected to a purge gas circulation passage 116. The purge gas circulation passage 116 includes a passage 121 extending from a portion of the passage 103 between the on-off valve 111 and the first moisture removal device 4A, a passage 122 extending from a portion of the passage 105 between the on-off valve 113 and the second moisture removal device 4B, a circulation passage 123 connected to the passages 121 and 122, a passage 124 connecting the circulation passage 123 to a portion of the passage 104 between the first moisture removal device 4A and the on-off valve 112, and a passage 125 connecting the circulation passage 123 to a portion of the passage 106 between the second moisture removal device 4B and the on-off valve 114.

[0086] The passages 121, 122, 124, and 125 are provided with on-off valves 126, 127, 128, and 129, respectively. The circulation passage 123 is provided with a first heat exchanger 131, a cooler 132, a gas-liquid separator 133, a blower 134, a flow control valve 135, the first heat exchanger 131, and a combustion furnace 6, in this order from the passage 121 side. The portion of the circulation passage 123 between the flow control valve 135 and the first heat exchanger 131 is connected to the passage 107 via a passage 138. The passage 107 is provided with a flow control valve 139. The portion of the circulation passage 123 between the blower 134 and the flow control valve 135 is connected to the passage 101 via a return passage 141.

[0087] The combustion chamber of the combustion furnace 6 is connected to the anode outlet passage 42 of the electrolytic reduction device 2 via a passage 142. A second heat exchanger 143 is provided in the passage 142. A passage 144 through which hydrogen and carbon monoxide separated in the product gas separation device 5 flow is connected to a portion of the passage 142 between the electrolytic reduction device 2 and the second heat exchanger 143. An oxygen supply source 147 is connected to a portion of the passage 142 between the electrolytic reduction device 2 and the second heat exchanger 143 via a flow control valve 146. A fuel supply source 149 is connected to the passage 144 via a flow control valve 148. The fuel supply source 149 may be a tank or a pipeline that supplies gas fuel such as natural gas or hydrogen. The gas fuel supplied from the fuel supply source 149 is mixed with the hydrogen and carbon monoxide flowing through the passage 144. Oxygen and the fuel containing hydrogen and carbon monoxide are mixed in the passage 142, heated in the second heat exchanger 143, and then supplied to the combustion furnace 6.

[0088] The combustion furnace 6 may be an oxygen combustion furnace. In the combustion chamber of the combustion furnace 6, oxygen supplied from the passage 142 and fuel containing hydrogen and carbon monoxide are combusted. Exhaust gas generated by the combustion mainly contains carbon dioxide and water. The exhaust gas is supplied from the combustion furnace 6 to the first supply passage 26 through an exhaust gas passage 151. The exhaust gas passage 151 may be provided with a blower 150 for sending the exhaust gas to the first supply passage 26. As a result, carbon dioxide contained in the exhaust gas is reused as part of the raw material in the electrolytic reduction device 2. The exhaust gas passage 151 passes through the second heat exchanger 143. As a result, the exhaust gas and the gas containing fuel and oxygen flowing through the passage 142 exchange heat in the second heat exchanger 143, and the gas flowing through the passage 142 is heated.

[0089] The first and second moisture removal devices 4A, 4B perform an adsorption process for adsorbing water from the generated gas, and a desorption process for releasing the water adsorbed in the adsorption process by receiving heat generated in the combustion furnace 6. When one of the first and second moisture removal devices 4A, 4B is performing the adsorption process, the other of the first and second moisture removal devices 4A, 4B performs the desorption process. The first and second moisture removal devices 4A, 4B may switch between the adsorption process and the desorption process at a predetermined time interval.

[0090] By opening the on-off valves 111, 112, 127, and 129 and closing the on-off valves 113, 114, 126, and 128, the first moisture removal device 4A performs the adsorption process, and the second moisture removal device 4B performs the desorption process. As a result, the first process gas flowing through the passage 101 passes through the first moisture removal device 4A, and moisture is removed. The first process gas from which moisture has been removed by the first moisture removal device 4A or the second moisture removal device 4B is called the second process gas. Most of the second process gas is sent to the product gas separation device 5 through the passage 107. At this time, by opening the flow control valve 139, a part of the second process gas passes through the passage 138 and is supplied to the circulation passage 123. A part of the second process gas circulates through the purge gas circulation passage 116 as a purge gas.

[0091] The purge gas flowing through the circulation passage 123 is heated in the combustion furnace 6. At this time, the purge gas exchanges heat with the fuel, oxygen, and exhaust gas without mixing with them. The purge gas heated in the combustion furnace 6 is supplied to the second moisture removal device 4B via the passage 125. As a result, the moisture adsorbent in the second moisture removal device 4B is heated by the purge gas, and moisture is desorbed from the moisture adsorbent. As a result, the moisture adsorbent in the second moisture removal device 4B is regenerated.

[0092] The moisture desorbed from the moisture adsorbent passes through the first heat exchanger 131 and the cooler 132 together with the purge gas and is cooled. The purge gas containing moisture is cooled by heat exchange with the purge gas that has passed through the flow control valve 135 in the first heat exchanger 131. The purge gas containing moisture is further cooled in the cooler 132, and the moisture is liquefied.

[0093] The moisture-containing purge gas that has passed through the cooler 132 is separated into liquid water in the gas-liquid separator 133. The liquid water separated in the gas-liquid separator 133 is supplied to the second supply passage 33 via the passage 152 and sent to the anode chamber 12 of the electrolytic reduction device 2.

[0094] By opening the on-off valves 113, 114, 126, and 128 and closing the on-off valves 111, 112, 127, and 129, the second moisture removal device 4B performs the adsorption step and the first moisture removal device 4A performs the desorption step. As a result, moisture is removed from the first process gas flowing through the passage 101 in the second moisture removal device 4B, and the first process gas is then sent to the product gas separation device 5. In addition, heated purge gas is supplied to the first moisture removal device 4A, and the moisture adsorbent in the first moisture removal device 4A is regenerated.

[0095] The product gas separation device 5 separates hydrocarbons from the second process gas. The product gas separation device 5 may be a cryogenic separation device. In this embodiment, the product gas separation device 5 separates methane and ethylene from the second process gas independently of each other. The off-gas from which methane and ethylene are separated from the second process gas mainly contains carbon monoxide and hydrogen. The off-gas is sent to the combustion furnace 6 via the passage 144 and the passage 142 and used as fuel. When carbon monoxide is the target substance, it is also possible to recover hydrogen and carbon monoxide.

[0096] A flow sensor FS4 is provided at the end of the first supply passage 26 on the electrolytic reduction device 2 side to measure the flow rate of the gas supplied to the cathode chamber 11 of the electrolytic reduction device 2. A flow sensor FS5 is provided at the cathode outlet passage 39 to measure the flow rate of the generated gas passing through the inside, and a gas composition sensor CS2 is provided to detect the composition of the generated gas. A flow sensor FS6 is provided at the carbon dioxide return passage 75 to measure the flow rate of the carbon dioxide gas passing through the inside. A flow sensor FS7 is provided at a portion of the passage 138 between the flow control valve 139 and the passage 107 to measure the flow rate of the second process gas passing through the inside. A flow sensor FS8 is provided at a portion of the circulation passage 123 between the blower 134 and the flow control valve 135 to measure the flow rate of the purge gas passing through the inside. A flow sensor FS9 is provided at a portion of the passage 144 between the fuel supply source 149 and the generated gas separation device 5 to measure the flow rate of the off-gas passing through the inside. A temperature sensor TS2 is provided at a portion of the circulation passage 123 between the combustion furnace 6 and the passages 124 and 125 to measure the temperature of the purge gas passing through the inside. An oxygen concentration sensor OS1 for measuring the oxygen concentration in the gas containing fuel and oxygen supplied to the combustion furnace 6 is provided at the end of the passage 142 on the second heat exchanger 143 side.

[0097] The control device 7 may obtain the flow rate and composition of the produced gas flowing through the cathode outlet passage 39 based on signals from the flow sensor FS5 and the gas composition sensor CS2, and control the current supplied by the DC power supply 19 based on the flow rate and composition of the produced gas. The control device 7 may increase the current supplied by the DC power supply 19 as the flow rate of the produced gas decreases or the ethylene concentration decreases.

[0098] The control device 7 may obtain the flow rate of carbon dioxide flowing through the carbon dioxide return passage 75 based on a signal from the flow sensor FS6, and control the aperture of the flow control valve 29 based on the flow rate of carbon dioxide flowing through the carbon dioxide return passage 75. The control device 7 may increase the aperture of the flow control valve 29 as the flow rate of carbon dioxide flowing through the carbon dioxide return passage 75 decreases.

[0099] The control device 7 may obtain the flow rate of the purge gas flowing through the circulation passage 123 based on a signal from the flow sensor FS8, and control the aperture of the flow control valve 139 based on the flow rate of the purge gas flowing through the circulation passage 123. The control device 7 may increase the aperture of the flow control valve 139 as the flow rate of the purge gas flowing through the circulation passage 123 decreases.

[0100] The control device 7 may obtain the temperature of the purge gas supplied to the first or second moisture removal device 4A, 4B based on a signal from the temperature sensor TS2, and control the aperture of the flow control valves 146, 148 based on the temperature of the purge gas. The control device 7 may increase the aperture of the flow control valves 146, 148 when the temperature of the purge gas is equal to or lower than a predetermined lower limit.

[0101] The control device 7 may obtain the oxygen concentration of the gas flowing through the passage 142 based on a signal from the oxygen concentration sensor OS1, and may control the aperture of the flow control valve 146 based on the oxygen concentration. The control device 7 may increase the aperture of the flow control valve 146 when the oxygen concentration of the gas flowing through the passage 142 is equal to or lower than a predetermined lower limit.

[0102] The control device 7 may obtain the flow rate of the off-gas flowing through the passage 144 based on a signal from the flow sensor FS9, and control the aperture of the flow control valve 148 based on the flow rate of the off-gas. The control device 7 may increase the aperture of the flow control valve 148 as the flow rate of the off-gas decreases.

[0103] The effects of the carbon dioxide capture system 1 are described below. Carbon dioxide in the generated gas discharged from the electrolytic reduction device 2 is separated in a gaseous state from the generated gas by the carbon dioxide separation device 3, making it easy to capture and reuse the carbon dioxide. The carbon dioxide separation device 3 using electrochemistry can separate carbon dioxide from hydrocarbons with less energy than a separation method using amines. This makes it possible to improve the energy efficiency of the carbon dioxide capture system 1.

[0104] The moisture adsorbent in the moisture removal device 4 is regenerated by utilizing the heat of combustion of the off-gas, which contains carbon monoxide and hydrogen separated from the generated gas, and oxygen generated at the anode 17 of the electrolytic reduction device 2. This can improve the energy efficiency of the carbon dioxide capture system 1. In addition, since the carbon dioxide and water generated by the combustion of the off-gas are reused as raw materials for the electrolytic reduction device 2, the amount of carbon dioxide emitted into the external environment can be reduced.

[0105] The moisture remover 4 has first and second moisture removers 4A and 4B arranged in parallel. Therefore, one of the first and second moisture removers 4A and 4B can perform the adsorption step, and the other can perform the desorption step. This enables the electrolytic reduction device 2 to be operated continuously.

[0106] The carbon dioxide separation device 3 has a stack 59 in which a first unit, in which a first gas diffusion electrode 51, a liquid chamber 53, and a second gas diffusion electrode 52 are arranged in this order, and a second unit, in which a second gas diffusion electrode 52, a liquid chamber 53, and a first gas diffusion electrode 51 are arranged in this order, are alternately arranged at intervals. This allows the carbon dioxide separation device 3 to be formed compactly. Each liquid chamber 53 is connected to an electrolyte tank 82, and the electrolyte circulates. This makes it easy to replenish and replace the electrolyte, and the quality of the electrolyte can be maintained. This allows the carbon dioxide separation capacity to be maintained constant.

[0107] Second embodiment Next, a carbon dioxide capture system 200 according to a second embodiment will be described. As shown in Fig. 5, the carbon dioxide capture system 200 according to the second embodiment differs from the carbon dioxide capture system 1 according to the first embodiment in that it has a carbon dioxide separator 201 in the passage 142. The other configuration of the carbon dioxide capture system 200 according to the second embodiment is similar to that of the carbon dioxide capture system 1 according to the first embodiment.

[0108] The carbon dioxide separator 201 has the same configuration as the carbon dioxide separator 3 shown in FIG. 3. The carbon dioxide separator 201 separates carbon dioxide from the gas discharged from the anode outlet passage 42 of the electrolytic reduction device 2. The gas discharged from the anode outlet passage 42 of the electrolytic reduction device 2 contains oxygen as a main component and may contain a small amount of carbon dioxide. The gas discharged from the anode outlet passage 42 of the electrolytic reduction device 2 is supplied to each first chamber 54 through the mixed gas inlet passage 62 of the carbon dioxide separator 201, and carbon dioxide is separated. The gas discharged from each first chamber 54 of the carbon dioxide separator 201 is called a third treated gas. As shown in FIG. 5, the main component of the third treated gas is oxygen. The third treated gas is passed through the second heat exchanger 143 by the passage 142 and supplied to the combustion furnace 6.

[0109] The carbon dioxide return passage 75 of the carbon dioxide separation device 201 is connected to the first supply passage 26 by the carbon dioxide return passage 202. As a result, the carbon dioxide separated from the gas discharged from the anode outlet passage 42 of the electrolytic reduction device 2 is returned to the electrolytic reduction device 2 via the carbon dioxide return passage 202 and the first supply passage 26, and is used as part of the raw material.

[0110] A flow sensor FS10 for measuring the flow rate of gas flowing inside and a gas composition sensor CS3 for detecting the composition of gas flowing inside are provided in a portion of the passage 142 between the electrolytic reduction device 2 and the carbon dioxide separation device 201. A flow sensor FS11 for measuring the flow rate of carbon dioxide gas flowing inside is provided in the carbon dioxide return passage 202.

[0111] The control device 7 may obtain the flow rate and composition of the gas flowing through the passage 142 based on signals from the flow sensor FS10 and the gas composition sensor CS3, and control the current supplied by the DC power supply 19 based on the gas flow rate and composition. The control device 7 may increase the current supplied by the DC power supply 19 as the gas flow rate decreases or the oxygen concentration decreases.

[0112] The control device 7 may obtain the flow rate of carbon dioxide flowing through the carbon dioxide return passage 202 based on a signal from the flow sensor FS11, and control the aperture of the flow control valve 29 based on the flow rate of carbon dioxide flowing through the carbon dioxide return passage 202. The control device 7 may increase the aperture of the flow control valve 29 as the flow rate of carbon dioxide flowing through the carbon dioxide return passage 202 decreases.

[0113] According to the carbon dioxide capture system 200 of the second embodiment, even if carbon dioxide leaks from the cathode chamber 11 to the anode chamber 12 through the membrane electrode assembly 10 of the electrolytic reduction device 2, the leaked carbon dioxide can be captured in the carbon dioxide separation device 201 and reused as a raw material for the electrolytic reduction device 2.

[0114] Third embodiment In the third embodiment, the carbon dioxide separator 3 of the first and second embodiments is replaced with a carbon dioxide separator 300. The carbon dioxide separator 300 has a partially modified configuration compared to the carbon dioxide separator 3. In the carbon dioxide separator 300, the same configuration as that of the carbon dioxide separator 3 is given the same reference numerals and description thereof is omitted.

[0115] As shown in Fig. 6, in the carbon dioxide separation device 300, the inlets of the first chambers 54 are disposed above the outlets. The first vessel 65 functions as a gas-liquid separation device. The bottom of the first vessel 65 is disposed below the outlets of the first chambers 54, and each of the first generated gas outlet passages 64 extends downward from the outlets of the first chambers 54 toward the first vessel 65. In this way, if the electrolyte liquid leaks from each liquid chamber 53 to each first chamber 54, the leaked liquid will accumulate at the bottom of the first vessel 65.

[0116] The gas components in the first vessel 65 flow from the top of the first vessel 65 to the second produced gas outlet passage 66. The bottom of the first vessel 65 is connected to the electrolyte tank 82 via a second electrolyte return passage 95. A pump 96 is provided in the second electrolyte return passage 95 to send the electrolyte from the first vessel 65 toward the electrolyte tank 82. As a result, the electrolyte separated from the produced gas and accumulated at the bottom of the first vessel 65 is returned to the electrolyte tank 82 via the second electrolyte return passage 95.

[0117] The control device 7 controls the pressure control valves 68, V5, and V6 so that the pressure PL2 in each second chamber 55 becomes equal to or greater than the pressure PL3 in each liquid chamber 53 and the pressure PL3 in each liquid chamber 53 becomes equal to or greater than the pressure PL1 in each first chamber 54 (PL2≧PL3≧PL1). By making the pressure PL2 in each second chamber 55 equal to or greater than the pressure PL3 in each liquid chamber 53, the electrolyte in the liquid chamber 53 is prevented from leaking through the second gas diffusion electrode 52 into the second chamber 55. The electrolyte leaked into each first chamber 54 is separated by the first vessel 65 and returned to the electrolyte tank 82. In addition, the control device 7 may control the pressure control valves 68, 69, 76 and the flow control valve 84 so that the pressure PL2 of each second chamber 55 is greater than or equal to the pressure PL3 of each liquid chamber 53, and the pressure PL3 of each liquid chamber 53 is greater than or equal to the pressure PL4 of the electrolyte tank 82, and the pressure PL4 of the electrolyte tank 82 is greater than or equal to the pressure PL1 of each first chamber 54 (PL2≧PL3≧PL4≧PL1).

[0118] (Fourth embodiment) In the fourth embodiment, the carbon dioxide separation apparatus 3 of the first and second embodiments is replaced with a carbon dioxide separation apparatus 400. As shown in Fig. 7, the carbon dioxide separation apparatus 400 has a carbon dioxide gas absorption section 401, an electrochemical cell 402, a first passage 403, a second passage 404, a third passage 405, and a gas-liquid separation device 406.

[0119] The carbon dioxide gas absorbing unit 401 brings the mixed gas into contact with an electrolyte solution containing a compound that adsorbs and desorbs protons in association with oxidation and reduction, and causes the electrolyte solution to absorb carbon dioxide in the mixed gas. The electrolyte solution may be the same as the electrolyte solution flowing through the carbon dioxide separator 3 of the first embodiment. The electrolyte solution may be, for example, potassium hydroquinone sulfonate. The carbon dioxide gas absorbing unit 401 may bring the mixed gas into contact with the electrolyte solution by, for example, countercurrent contact, parallel contact, or bubbling. The carbon dioxide gas absorbing unit 401 may spray the electrolyte solution toward the mixed gas, or may bring the mixed gas into contact with the electrolyte solution using a hollow fiber membrane.

[0120] Carbon dioxide gas absorbing section 401 has a gas inlet 411 to which the mixed gas is supplied, a gas outlet 412 from which the mixed gas is discharged, an electrolyte inlet 413 to which an electrolyte is supplied, and an electrolyte outlet 414 from which the electrolyte is discharged.

[0121] The electrochemical cell 402 is divided into a cathode chamber 417 and an anode chamber 418 by a membrane electrode assembly 416. The membrane electrode assembly 416 includes an electrolyte membrane 416A, and a cathode 416B and an anode 416C provided on both sides of the electrolyte membrane 416A and connected to a power source 419. The cathode 416B is disposed in the cathode chamber 417, and the anode 416C is disposed in the anode chamber 418. The cathode 416B is connected to the negative electrode of the power source 419, and the anode 416C is connected to the positive electrode of the power source 419. The electrolyte membrane 416A, the cathode 416B, and the anode 416C may have the same configuration as the electrolyte membrane 15, the cathode 16, and the anode 17 of the first embodiment. The cathode 416B and the anode 416C may have the same configuration as the first gas diffusion electrode 51 and the second gas diffusion electrode 52 of the first embodiment.

[0122] The first passage 403 connects the electrolyte outlet 414 of the carbon dioxide gas absorbing unit 401 and the anode chamber 418 of the electrochemical cell 402. The first passage 403 allows the electrolyte to flow from the carbon dioxide gas absorbing unit 401 to the anode chamber 418. The first passage 403 may be provided with a pump for transporting the electrolyte.

[0123] The second passage 404 connects the anode chamber 418 and the cathode chamber 417, and allows the electrolytic solution to flow from the anode chamber 418 to the cathode chamber 417. A gas-liquid separator 406 is provided in the second passage 404. The gas-liquid separator 406 separates the electrolytic solution from gas components in the electrolytic solution.

[0124] The third passage 405 connects the cathode chamber 417 of the electrochemical cell 402 and the electrolyte inlet 413 of the carbon dioxide gas absorbing unit 401. The third passage 405 allows the electrolyte to flow from the cathode chamber 417 to the carbon dioxide gas absorbing unit 401. The third passage 405 may be provided with a pump for transporting the electrolyte.

[0125] In the carbon dioxide separation device 400, the electrolyte circulates in the order of carbon dioxide gas absorption section 401, first passage 403, anode chamber 418, second passage 404, gas-liquid separator 406, cathode chamber 417, and third passage 405. The electrolyte is reduced in cathode chamber 417, and the pH increases.

[0126] In carbon dioxide gas absorption section 401, the mixed gas comes into contact with the electrolyte having a relatively high pH, ​​and carbon dioxide gas in the mixed gas dissolves in the electrolyte. The chemical reactions at this time are similar to those of chemical formulas (9) to (11) above. In the electrolyte, carbon dioxide takes the form of hydrogen carbonate ions. This removes carbon dioxide gas from the mixed gas. The mixed gas from which carbon dioxide gas has been removed is discharged from gas outlet 412 and sent to moisture remover 4 via passage 101.

[0127] The electrolyte with carbon dioxide dissolved therein is sent to the anode chamber 418 via the first passage 403. In the anode chamber 418, the electrolyte is oxidized, and the pH of the electrolyte decreases. The pH of the electrolyte flowing from the anode chamber 418 to the second passage 404 becomes lower than the pH of the electrolyte flowing from the cathode chamber 417 to the third passage 405. When the pH of the electrolyte decreases, the bicarbonate ions in the electrolyte receive protons and change to carbon dioxide. The chemical reactions at this time are the same as those of the above chemical formulas (9) to (11). As a result, the carbon dioxide becomes gaseous and is desorbed from the electrolyte.

[0128] The carbon dioxide gas separated from the electrolytic solution in the anode chamber 418 and the electrolytic solution are sent to the gas-liquid separator 406 via the second passage 404. In the gas-liquid separator 406, the carbon dioxide gas and the electrolytic solution are separated. The electrolytic solution flows from the gas-liquid separator 406 to the cathode chamber 417 via the second passage 404. The carbon dioxide gas flows from the gas-liquid separator 406 to the electrolytic reduction device 2 via the carbon dioxide return passage 75. In the cathode chamber 417, the electrolytic solution is reduced and the pH increases, making it capable of absorbing carbon dioxide gas again.

[0129] According to the carbon dioxide separator 400 described above, the potential difference between the cathode 416B and the anode 416C can be made smaller than in the carbon dioxide separator 3 according to the first embodiment, and energy efficiency can be improved.

[0130] FIG. 8 shows a more detailed configuration of the carbon dioxide separation device 400 described above. The carbon dioxide gas absorption unit 401 has a housing 431 that extends vertically. The electrolyte is stored in the bottom of the housing 431. A gas inlet 411 to which the mixed gas is supplied is provided in the lower part of the housing 431. A gas outlet 412 from which the mixed gas is discharged is provided in the upper end of the housing 431. The carbon dioxide gas absorption unit 401 also has a nozzle 432 that is provided in the upper part of the housing 431 and sprays the electrolyte, a fourth passage 433 that supplies the electrolyte stored in the bottom part of the housing 431 to the nozzle 432, and a tray 434 that is provided between the gas inlet 411 and the nozzle 432 in the housing 431 and temporarily stores the electrolyte sprayed from the nozzle 432. A plurality of trays 434 may be provided. The electrolyte outlet 414 may be provided in the tray 434.

[0131] The third passage 405 is connected to the fourth passage 433. The connection part of the fourth passage 433 with the third passage 405 becomes the electrolyte inlet 413. A first flow control valve 436A is provided in the third passage 405. A second flow control valve 436B is provided in the fourth passage 433 closer to the bottom of the housing 431 than the third passage 405. A first pump 438A is provided between the third passage 405 and the nozzle 432 in the fourth passage 433. The electrolyte stored in the bottom of the housing 431 is supplied to the nozzle 432 through the fourth passage 433 by the first pump 438A, and the electrolyte is sprayed from the nozzle 432. The electrolyte sprayed from the nozzle 432 flows down inside the housing 431, passes through the tray 434, and is stored in the bottom of the housing 431.

[0132] A water supply valve 439 for supplying water may be provided at the bottom of the housing 431. The electrolyte is diluted by supplying water to the bottom of the housing 431 via the water supply valve 439. A mist eliminator 441 is provided at the gas outlet 412. The mist eliminator 441 collects mist of the electrolyte suspended in the discharged mixed gas.

[0133] The electrochemical cell 402 is partitioned into a plurality of cathode chambers 417 and a plurality of anode chambers 418 by a plurality of membrane electrode assemblies 416. The plurality of membrane electrode assemblies 416 are arranged in parallel with a gap between them. The cathodes 416B or the anodes 416C of adjacent membrane electrode assemblies 416 face each other. As a result, the plurality of anode chambers 418 and the plurality of cathode chambers 417 are arranged alternately in the electrochemical cell 402.

[0134] An end of the first passage 403 on the anode chamber 418 side branches and connects to each of the multiple anode chambers 418. An end of the second passage 404 on the anode chamber 418 side branches and connects to each of the multiple anode chambers 418. An end of the second passage 404 on the cathode chamber 417 side branches and connects to each of the multiple cathode chambers 417. An end of the third passage 405 on the cathode chamber 417 side branches and connects to each of the multiple cathode chambers 417.

[0135] A first tank 405A and a second pump 438B are provided in the third passage 405 in this order from the electrochemical cell 402 side. The first flow control valve 436A is provided between the second pump 438B and the electrolyte inlet 413. The first tank 405A temporarily stores the electrolyte discharged from each cathode chamber 417.

[0136] In the first passage 403, a second tank 403A, a third pump 438C, and a third flow control valve 436C are provided in this order from the electrolyte outlet 414 side. The second tank 403A temporarily stores the electrolyte discharged from the electrolyte outlet 414. In addition, the electrolyte collected in the mist eliminator 441 flows into the second tank 403A.

[0137] The second passage 404 is provided with a gas-liquid separator 406, a fourth pump 438D, and a fourth flow control valve 436D, in this order from the anode chamber 418 side. The gas outlet 412 of the gas-liquid separator 406 is connected to the carbon dioxide return passage 75 via a fifth flow control valve 436E.

[0138] A flow meter FS21 for measuring the flow rate of the electrolyte flowing therethrough is provided in a portion of the third passage 405 between the first flow control valve 436A and the fourth passage 433. A flow meter FS22 for measuring the flow rate of the electrolyte flowing therethrough is provided in a portion of the fourth passage 433 between the first pump 438A and the nozzle 432. A flow meter FS23 for measuring the flow rate of the electrolyte flowing therethrough is provided in a portion of the first passage 403 between the third flow control valve 436C and the anode chamber 418. A flow meter FS24 for measuring the flow rate of the electrolyte flowing therethrough is provided in a portion of the second passage 404 between the fourth flow control valve 436D and the cathode chamber 417.

[0139] The housing 431 is provided with a pH sensor pHS1 for measuring the pH of the electrolyte stored in the bottom of the housing 431. A pH sensor pHS2 for measuring the pH of the electrolyte flowing through the fourth passage 433 is provided in a portion between the first pump 438A and the nozzle 432. The third passage 405 is provided with a pH sensor pHS3 for measuring the pH of the electrolyte flowing through the third passage 405. The pH sensor pHS3 may be provided in the first tank 405A, for example. The first passage 403 is provided with a pH sensor pHS4 for measuring the pH of the electrolyte flowing through the first passage 403. The pH sensor pHS4 may be provided in the second tank 403A, for example.

[0140] The housing 431 is provided with a level gauge LS1 for measuring the level of the electrolyte stored in the bottom thereof. The first tank 405A is provided with a level gauge LS2 for measuring the level of the electrolyte stored therein. The second tank 403A is provided with a level gauge LS3 for measuring the level of the electrolyte stored therein. The gas-liquid separator 406 is provided with a level gauge LS4 for measuring the level of the electrolyte stored therein. The gas-liquid separator 406 is also provided with a pressure sensor PS11 for measuring the pressure of the gas phase portion therein.

[0141] The control device 7 is connected to the power source 419, the first to fourth pumps 438A to 438D, the first to fifth flow rate control valves 436A to 436E, the water supply valve 439, the first to fourth flow meters 446A to 446D, the first to fourth pH sensors 447A to 447D, the first to fourth level gauges 448A to 448D, and the pressure sensor PS11. The control device 7 controls the power source 419, the first to fourth pumps 438A to 438D, and the first to fifth flow rate control valves 436A to 436E.

[0142] The pH of the electrolyte stored at the bottom of the housing 431, i.e., the pH detected by pH sensor pHS1, is defined as pH1. The pH of the electrolyte flowing in the portion of the fourth passage 433 closer to the nozzle 432 than the third passage 405, i.e., the pH detected by pH sensor pHS2, is defined as pH2. The pH of the electrolyte flowing through the third passage 405, i.e., the pH detected by pH sensor pHS3, is defined as pH3. The pH of the electrolyte flowing through the first passage 403, i.e., the pH detected by pH sensor pHS4, is defined as pH4.

[0143] The control device 7 controls the power source 419, the first flow control valve 436A, and the second flow control valve 436B so as to satisfy pH3≧pH2≧pH1≧pH4≧5.5.

[0144] The control device 7 controls the fifth flow control valve 436E based on the pressure of the gas phase portion in the gas-liquid separation device 406 acquired by the pressure sensor PS11. EXAMPLES

[0145] Example 1 The carbon dioxide separator 3 according to the first embodiment was prepared as follows, and its effect was confirmed. The carbon dioxide separator was a three-chamber electrolysis cell having a first chamber (cathode chamber), a second chamber (anode chamber), and a liquid chamber, which were partitioned by a first gas diffusion electrode (negative electrode) and a second gas diffusion electrode (positive electrode). The liquid chamber was formed between the first gas diffusion electrode and the second gas diffusion electrode. The first and second chambers were set in size so that the gas passing through each chamber would efficiently contact the first and second gas diffusion electrodes. In addition, the liquid chamber was formed thin so that the liquid junction resistance was small. The first and second gas diffusion electrodes were formed by applying a solution in which polytetrafluoroethylene (PTFE) was dispersed at 30 wt% to the surface of carbon paper (porosity 70%, thickness 0.4 mm) and baking it at a temperature of 350°C for 20 minutes in a nitrogen atmosphere electric furnace.

[0146] The electrolyte was prepared as follows. First, an aqueous solution containing 0.7M disodium 4,5-dihydroxy-1,3-benzenedisulfonate and 1M potassium hydrogen carbonate was prepared. 30mL of this aqueous solution was added to a single-chamber electrolysis cell using carbon paper as the negative and positive electrodes, and constant-current electrolysis was performed while blowing in humidified carbon dioxide gas at a flow rate of 100mL / min. During this time, the amount of hydrogen contained in the gas discharged from the outlet of the electrolysis cell was monitored by gas chromatography, and electrolysis was terminated when the amount of hydrogen produced reached the amount required to oxidize half of the 4,5-dihydroxy-1,3-benzenedisulfonic acid in terms of the current flow.

[0147] A DC voltage was applied to the first gas diffusion electrode and the second gas diffusion electrode, and 20 mL of electrolyte was circulated in the liquid chamber at 5 mL / min. A generated gas containing humidified 2.01 vol% CO2, 1.00 vol% C2H4, 0.10 vol% CH4, 0.10 vol% CO, and N2 as a balance gas was circulated in the first chamber at 100 mL / min, and N2 gas was circulated in the second chamber at 100 mL / min. The composition of the outlet gas of the second chamber was analyzed by gas chromatography. The results are shown in FIG. 9. From FIG. 9, about 80% of CO2 was recovered at the current efficiency, and C2H4, CH4, and CO were below the detection limit. From this result, it was confirmed that CO2 moved from the generated gas in the first chamber to the second chamber and was separated from the generated gas. It was also confirmed that C2H4, CH4, and CO did not move to the second chamber.

[0148] Example 2 The carbon dioxide separation device and electrolyte used were the same as those used in Example 1. A DC voltage was applied to the first gas diffusion electrode and the second gas diffusion electrode, and 20 mL of electrolyte was circulated in the liquid chamber at 5 mL / min. A generated gas containing humidified 0.201 vol% CO2, 1.00 vol% C2H4, 0.10 vol% O2, and N2 as a balance gas was passed through the first chamber at 100 mL / min, and N2 gas was passed through the second chamber at 100 mL / min. The composition of the outlet gas of the second chamber was then analyzed by gas chromatography. The results are shown in FIG. 10. From FIG. 10, about 60% of CO2 was recovered in terms of current efficiency, and C2H4 and CO2 were below the detection limit. From these results, it was confirmed that CO2 moved from the generated gas in the first chamber to the second chamber and was separated from the generated gas. It was also confirmed that C2H4 and CO2 did not move to the second chamber.

[0149] Although the specific embodiment has been described above, the present invention is not limited to the above embodiment and can be modified in a wide range of ways. [Explanation of symbols]

[0150] 1: Carbon dioxide capture system 2: Electrolytic reduction device 3,400: Carbon dioxide separator 4: Moisture removal device 4A: 1st moisture removal device 4B:Second moisture removal device 5: Produced gas separation device 6: Combustion furnace 7: Control device 51: First gas diffusion electrode 52: Second gas diffusion electrode 53:Liquid chamber 54: 1st room 55: 2nd room 62: Produced gas inlet passage 64: First generated gas outlet passage 65: First vessel 66: Second product gas outlet passage 68: Pressure control valve 69: Pressure control valve 71: Carbon dioxide circulation passage 72: Second vessel (gas-liquid separator) 74: 3rd Vessel 75: Carbon dioxide return passage 76: Pressure control valve 81: Electrolyte supply passage 82: Electrolyte tank 84: Pressure control valve 87: First electrolyte return passage 95: Second electrolyte return passage 116: Purge gas circulation passage 133: Gas-liquid separation equipment 401: Carbon dioxide gas absorption section 402: Electrochemical cell 403: 1st aisle 404:Second aisle 405: 3rd aisle 406: Gas-liquid separation equipment 411: Gas inlet 412: Gas outlet 413: Electrolyte inlet 414: Electrolyte outlet 416: Membrane electrode complex 416A: Electrolyte membrane 416B : Cathode 416C : Anode 417: Cathode chamber 418: Anode chamber 419 :Power supply 431: Housing 432: Nozzle 433: 4th aisle 434: Tray 436A: First flow control valve 436B: Second flow control valve PS1: Pressure sensor PS2: Pressure sensor PS3: Pressure sensor PS4: Pressure sensor

Claims

1. 1. A carbon dioxide capture system comprising: an electrolytic reduction device that is supplied with a gas containing carbon dioxide and electrolytically reduces the carbon dioxide to generate a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen and unreacted carbon dioxide; and a carbon dioxide separation device for separating carbon dioxide from the mixed gas, the carbon dioxide separation device includes a first gas diffusion electrode which is a cathode, a second gas diffusion electrode which is an anode, a liquid chamber which is formed between the first gas diffusion electrode and the second gas diffusion electrode and to which an electrolytic solution containing a compound which adsorbs and desorbs protons in association with oxidation-reduction is supplied, a first chamber which is partitioned from the liquid chamber by the first gas diffusion electrode and to which the mixed gas is supplied, and a second chamber which is partitioned from the liquid chamber by the second gas diffusion electrode and to which carbon dioxide separated from the mixed gas flows, The electrolytic reduction device is a carbon dioxide recovery system that uses the carbon dioxide separated from the mixed gas in the carbon dioxide separation device as part of a raw material.

2. At least one moisture remover that separates water from the product gas from which carbon dioxide has been separated in the carbon dioxide separation device; 2. The carbon dioxide capture system according to claim 1, further comprising a product gas separation device that separates hydrocarbons from the product gas by cooling the product gas from which water has been separated in the moisture removal device.

3. a combustion furnace for combusting at least a portion of the off-gas, which is the product gas from which hydrocarbons have been separated in the product gas separation device; 3. The carbon dioxide capture system according to claim 2, wherein the moisture removal device performs an adsorption process in which water is adsorbed and the product gas from which the water has been separated is released, and a desorption process in which heat generated in the combustion furnace is received and the water adsorbed in the adsorption process is released.

4. a first gas-liquid separator for separating liquefied water from the water-containing gas released in the desorption step of the moisture remover; a circulation passage for returning the gas from which water has been separated in the first gas-liquid separation device to the moisture removal device, The carbon dioxide recovery system according to claim 3 , wherein the circulation passage is arranged so as to be capable of exchanging heat with the combustion furnace.

5. The carbon dioxide capture system according to claim 4 , wherein a portion of the generated gas from which water has been separated in the water removal device is supplied to the circulation passage.

6. 4. The carbon dioxide recovery system according to claim 3, further comprising an exhaust gas passage for supplying exhaust gas containing carbon dioxide and water, generated in the combustion furnace, to the electrolytic reduction.

7. The carbon dioxide capture system according to claim 3 , wherein the moisture removal devices are provided in two units in parallel, and while one of the moisture removal devices is performing the adsorption process, the other moisture removal device performs the desorption process.

8. The carbon dioxide separation device is a carbon dioxide circulation passage connected to an inlet and an outlet of the second chamber of the carbon dioxide separation device; a carbon dioxide return passage connecting the carbon dioxide circulation passage and an inlet of a cathode chamber of the electrolytic reduction device; a mixed gas inlet passage connecting an inlet of the first chamber of the carbon dioxide separation device and an outlet of the cathode chamber of the electrolytic reduction device; a product gas outlet passage connected to an outlet of the first chamber of the carbon dioxide separation device; The carbon dioxide capture system according to any one of claims 1 to 7, further comprising: an electrolyte tank connected to an inlet of the liquid chamber via an electrolyte supply passage and connected to an outlet of the liquid chamber via a first electrolyte return passage.

9. A second gas-liquid separator provided in the carbon dioxide circulation passage; a second electrolyte return passage for returning the liquid separated in the second gas-liquid separator to the electrolyte tank; a first pressure control valve provided in the carbon dioxide return passage; a second pressure control valve provided in the mixed gas inlet passage or the product gas outlet passage; a third pressure control valve provided in the electrolytic solution supply passage or the first electrolytic solution return passage; a first pressure sensor that detects the pressure in the first chamber as a first pressure; a second pressure sensor that detects the pressure in the second chamber as a second pressure; a third pressure sensor that detects the pressure of the liquid chamber as a third pressure; 9. The carbon dioxide capture system according to claim 8, further comprising a control device that controls the first pressure control valve, the second pressure control valve, and the third pressure control valve so that the first pressure is equal to or greater than the third pressure and the third pressure is equal to or greater than the second pressure.

10. a fourth pressure control valve provided in the product gas outlet passage downstream of the first pressure control valve; a gas return passage connecting a top of the electrolyte tank and a portion of the produced gas outlet passage upstream of the fourth pressure control valve; a fourth pressure sensor that detects the pressure of the electrolyte tank as a fourth pressure, 10. The carbon dioxide capture system of claim 9, wherein the control device controls the first pressure control valve, the second pressure control valve, the third pressure control valve, and the fourth pressure control valve so that the first pressure is equal to or greater than the third pressure, the third pressure is equal to or greater than the fourth pressure, and the fourth pressure is equal to or greater than the second pressure.

11. 2. The carbon dioxide capture system of claim 1, wherein the hydrocarbons include at least one of methane and ethylene.

12. 2. The carbon dioxide recovery system according to claim 1, wherein the compound that adsorbs and desorbs protons with oxidation-reduction is an organic compound having an oxidation-reduction potential of −1.0 V to 1.0 V based on the standard hydrogen electrode potential at pH 7.

13. 2. The carbon dioxide recovery system according to claim 1, wherein the compound that adsorbs and desorbs protons in association with oxidation-reduction is a quinone compound.

14. 1. A carbon dioxide capture system comprising: an electrolytic reduction device that is supplied with a gas containing carbon dioxide and electrolytically reduces the carbon dioxide to generate a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen and unreacted carbon dioxide; and a carbon dioxide separation device for separating carbon dioxide from the mixed gas, The carbon dioxide separation device is a carbon dioxide gas absorption unit that brings the mixed gas into contact with an electrolyte solution containing a compound that adsorbs and desorbs protons as a result of oxidation and reduction, and causes the electrolyte solution to absorb carbon dioxide in the mixed gas; an electrochemical cell partitioned into a cathode chamber and an anode chamber by a membrane electrode assembly including an electrolyte membrane, and a cathode and an anode provided on either side of the electrolyte membrane and connected to a power source; a first passage through which the electrolytic solution flows from the carbon dioxide gas absorption section to the anode chamber; a second passage for flowing the electrolyte from the anode chamber to the cathode chamber; a third passage for allowing the electrolytic solution to flow from the cathode chamber to the carbon dioxide gas absorbing section; a gas-liquid separator provided in the second passage for separating the electrolytic solution from carbon dioxide gas; The electrolytic reduction device is a carbon dioxide recovery system that uses the carbon dioxide separated from the mixed gas in the carbon dioxide separation device as part of a raw material.

15. the carbon dioxide gas absorption unit includes a housing extending vertically and storing the electrolytic solution at a bottom portion, a gas inlet provided at a lower portion of the housing and through which the mixed gas is supplied, a gas outlet provided at an upper end of the housing and through which the mixed gas is discharged, a nozzle provided at an upper portion of the housing and spraying the electrolytic solution, a fourth passageway for supplying the electrolytic solution stored at the bottom portion of the housing to the nozzle, a tray provided in the housing between the gas inlet and the nozzle and for temporarily storing the electrolytic solution supplied from the nozzle, and an electrolytic solution outlet provided in the tray, the electrolyte outlet is connected to the first passage; the third passage is connected to the fourth passage, a first flow control valve is provided in the third passage; a second flow control valve is provided in the fourth passage closer to the bottom of the housing than the third passage, the power source, the first flow control valve, and the second flow control valve are controlled by a control device; If the pH of the electrolytic solution stored in the bottom portion of the housing is pH1, the pH of the electrolytic solution flowing in the portion of the fourth passage closer to the nozzle than the third passage is pH2, the pH of the electrolytic solution flowing through the third passage is pH3, and the pH of the electrolytic solution flowing through the first passage is pH4, The carbon dioxide capture system according to claim 14 , wherein the control device controls the power source, the first flow control valve, and the second flow control valve so as to satisfy pH3≧pH2≧pH1≧pH4≧5.5.

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