Carbon Dioxide Treatment Equipment

Abstract

To provide a carbon dioxide processing apparatus capable of increasing utilization efficiency in CO2.SOLUTION: A carbon dioxide processing apparatus 1 of the embodiment comprises: a carbon dioxide electrolysis part 2 that electrolyzes carbon dioxide to produce carbon monoxide; and a fuel cell part 4 to which an oxygen-carbon dioxide-containing gas G6 discharged from the carbon dioxide electrolysis part 2 is supplied to reduce the oxygen concentration in the oxygen-carbon dioxide-containing gas G6. The gas with reduced oxygen concentration in the fuel cell part 4 is returned upstream to the carbon dioxide electrolysis part 2.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE An embodiment of the present invention relates to a carbon dioxide treatment device. [Background technology]

[0002] Carbon dioxide (CO2), which is mainly generated by burning fossil fuels, is considered to be the main cause of global warming due to the greenhouse effect. Since suppressing the release of such CO2 into the atmosphere would enable efficient removal of the causes of global warming, activities to suppress CO2 release are being actively undertaken. Various efforts are being made not only in Japan but also around the world, such as the spread and acceleration of renewable energy, the elimination of engines from automobiles, and the efficiency of energy use. At the same time, reusing CO2 as a resource is also important from the perspective of reducing CO2. For example, if CO2 is reduced in some way, it can be regenerated into carbon compounds similar to fuels and chemicals derived from fossil resources, making it possible to both reuse and reduce CO2.

[0003] One approach is to use surplus renewable energy to electrolyze CO2 to produce carbon monoxide (CO). For example, a CO2 electrolysis device that reduces CO2 to produce carbon compounds such as CO is suitable for converting CO2 to carbon compounds. If a CO2 electrolysis device is driven by renewable energy such as solar or wind power, it is possible to reduce the use of fossil resources and convert CO2 to carbon compounds such as CO while minimizing the generation of new CO2. In the CO2 electrolysis reaction, the main reaction is the reaction that converts CO2 to CO, and CO is used as a raw material for various industrial products such as fuel.

[0004] In CO2 electrolysis devices, a side reaction that produces O2 occurs, and CO2 may be compounded with O2 during this process. If a mixed gas containing O2 and CO2 is burned to render it harmless and released into the atmosphere, not only will the CO2 not be effectively utilized, but the CO2, which causes global warming, will be re-released. Ideally, CO2 would be separated and captured from the mixed gas containing O2 and CO2 and returned to the CO2 electrolysis device, but it is difficult to efficiently separate CO2 from the mixed gas. Therefore, there is a demand for a CO2 treatment device that can efficiently remove or consume O2, which has a negative effect on the reduction section of the CO2 electrolysis device, and return the remaining CO2 to the CO2 electrolysis device. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6870956 [Patent Document 2] JP 2021-147679 A Summary of the Invention [Problem to be solved by the invention]

[0006] The problem to be solved by the present invention is to provide a carbon dioxide treatment device that makes it possible to increase the efficiency of CO2 utilization. [Means for solving the problem]

[0007] A carbon dioxide treatment device according to an embodiment includes a carbon dioxide electrolysis unit that electrolyzes carbon dioxide to produce carbon monoxide, and a fuel cell unit that is supplied with an oxygen-carbon dioxide containing gas discharged from the carbon dioxide electrolysis unit and that reduces the oxygen concentration in the oxygen-carbon dioxide containing gas, and the gas whose oxygen concentration has been reduced in the fuel cell unit is returned upstream of the carbon dioxide electrolysis unit. [Brief description of the drawings]

[0008] [Figure 1]FIG. 1 is a diagram showing a carbon dioxide treatment device according to a first embodiment. [Diagram 2] FIG. 2 is a diagram showing an example of the configuration of a fuel cell section used in the carbon dioxide treatment device of the first embodiment. [Diagram 3] FIG. 4 is a diagram showing a carbon dioxide treatment device according to a second embodiment. [Figure 4] FIG. 13 is a diagram showing a carbon dioxide treatment device according to a third embodiment. [Diagram 5] FIG. 13 is a diagram showing an example of the configuration of a fuel cell section used in a carbon dioxide treatment device according to a third embodiment. [Figure 6] FIG. 13 is a diagram showing a carbon dioxide treatment device according to a fourth embodiment. [Figure 7] FIG. 13 is a diagram showing a carbon dioxide treatment device according to a fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the carbon dioxide treatment device of the embodiment will be described with reference to the drawings. In each embodiment shown below, substantially the same components are given the same reference numerals, and some of the description may be omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each part, etc. may differ from the actual ones. In the following description, the symbol "~" indicates a range between the respective upper and lower limit values. In such cases, each range includes the upper and lower limit values.

[0010] (First embodiment) Fig. 1 is a diagram showing a carbon dioxide treatment device 1 of the first embodiment. The carbon dioxide (CO2) treatment device 1 shown in Fig. 1 includes a carbon dioxide (CO2) electrolysis unit 2, a CO2 supply unit 3 that supplies CO2 to the CO2 electrolysis unit 2, and a fuel cell unit 4 to which oxygen (O2)-carbon dioxide (CO2)-containing gas discharged from the CO2 electrolysis unit 2 is supplied.

[0011] The CO2 electrolysis unit 2 is a CO2 electrolysis device having an electrolytic cell, and includes a reduction unit (cathode unit) 5 and an oxidation unit (anode unit) 6. The reduction unit 5 includes a reduction electrode (cathode), and the oxidation unit 6 includes an oxidation electrode (anode). At least the oxidation unit 6 is filled with or flows through an electrolyte. The reduction unit 5 may be configured to pass CO2 gas through, or may be configured to pass or be filled with an electrolyte containing CO2. In the reduction unit 5 and the oxidation unit 6, for example, a solution using water (H2O) can be used as the electrolyte, for example, an aqueous solution containing any electrolyte. The aqueous solution containing an electrolyte can be, for example, hydrogen carbonate ions (HCO3 - ), carbonate ion (CO3 - ), phosphate ion (PO4 2- ), borate ion (BO3 3- ), sodium ion (Na + ), potassium ion (K + ), calcium ion (Ca 2+ ), lithium ion (Li + ), Cesium ion (Cs + ), magnesium ion (Mg 2+ ), chloride ion (Cl - ), hydroxide ion (OH - ) and the like.

[0012] The reduction unit 5 is supplied with a CO2-containing gas G1 from the CO2 supply unit 3, as described in detail later. The oxidation unit 6 is supplied with an electrolytic solution (aqueous solution containing an electrolyte). A power source (not shown) is connected to the reduction electrode and the oxidation electrode. The reduction unit 5 and the oxidation unit 6 are connected to, for example, hydrogen ions (H + ), hydroxide ion (OH - ), carbonate ion (CO3 2- ), hydrogen carbonate ion (HCO3 - The CO2 electrolysis unit 2 (CO2 electrolysis device having an electrolytic cell) may be composed of a single electrolytic cell, or may have a configuration in which a plurality of electrolytic cells are stacked and integrated.

[0013] The CO2 supply unit 3 is connected to the reduction unit (cathode unit) 5, and the CO2-containing gas G1 is supplied from the CO2 supply unit 3 to the reduction unit 5. In the CO2-containing gas G1 supplied from the CO2 supply unit 3 to the reduction unit 5, the CO2 concentration is preferably in the range of 80% by volume or more and 100% by volume. When using an exhaust gas G2 containing CO2 discharged from a thermal power plant, a waste incineration plant, a steelworks, etc., a CO2 capture unit 8 that separates and captures CO2 from the exhaust gas G2 may be provided, and a CO2 gas G3 with an increased CO2 concentration may be supplied to the CO2 supply unit 3. The CO2 supply unit 3 is configured to combine the CO2 gas G3 captured in the CO2 capture unit 8 and the CO2 gas G4 captured in the fuel cell unit 4 described later and supply an appropriate flow rate of CO2 to the reduction unit 5. For this reason, the CO2 supply unit 3 has a CO2 amount adjustment means, and may also have a container or the like for temporarily storing CO2.

[0014] In the CO2 electrolysis section 2, the supplied CO2 is electrolyzed and reduced in the reduction section 5. Carbon monoxide (CO) and the like are generated by the reduction of CO2. Furthermore, in the oxidation section 6, for example, H2O in the electrolyte is electrolyzed and oxidized. Oxygen (O2) is generated by the oxidation of H2O. Various reactions are considered as the oxidation-reduction reaction in the CO2 electrolysis section 2. An example of an oxidation-reduction reaction is described below. For example, as shown in the following formula (1), an oxidation reaction of H2O in the electrolyte occurs, electrons are lost, and oxygen (O2) and hydrogen ions (H + ) may be generated. The hydrogen ions (H + A part of the oxidized carbon dioxide moves to the reduction section 5 through the diaphragm 7. 2H2O → 4H + +O2+4e - …(1) The hydrogen ions (H + When the CO2 reaches the reduction unit 5, a reduction reaction of the CO2 may occur to generate carbon monoxide (CO), as shown in the following formula (2). 2CO2+4H + +4e - → 2CO+2H2O …(2)

[0015] Furthermore, in the reduction unit 5, hydrogen (H2) and hydroxide ions (OH - ) is produced, or CO and hydroxide ions (OH - ) may be generated. 4H2O+4e - → 2H2+4OH - …(3) 2CO2+2H2O+4e - → 2CO+4OH - …(4)

[0016] The hydroxide ion (OH - ) reacts with CO2 to produce bicarbonate ions (HCO3 - ) and the resulting bicarbonate ions (HCO3 - ) is further converted into hydroxide ions (OH - ) reacts with carbonate ions (CO3 2- ) may be generated. 2CO2+2OH - → 2HCO3 - …(5) 2HCO3 - +2OH - → 2CO3 2- +2H2O …(6)

[0017] Furthermore, the carbonate ions (CO3 2- ) reaches the oxidation section 6, the hydrogen ions (H + ) may react with CO2 according to the following equation (7): 4H + +2CO3 2- → 2CO2+2H2O …(7) As shown in the above formulas (1) and (7), in the oxidation section 6, in addition to the production of O2, CO2 may be produced as a by-product.

[0018] The CO and unreacted CO2 produced in the reduction section 5 are sent as a mixed gas G5 to, for example, a CO purification section (not shown) or to a chemical synthesis section such as a liquid fuel synthesis section. The mixed gas G5 may further contain H2 as a by-product. The liquid fuel synthesis section may be a synthesis device that uses a catalyst to react CO and H2 to produce a hydrocarbon compound having one or more carbon atoms in the molecule. For example, the Fischer-Tropsch synthesis reaction is used for the reaction of CO and H2.

[0019] In addition, the O2 and CO2 generated in the oxidation unit 6 are discharged as a mixed gas G6. The mixed gas G6 may contain a small amount of CO. Furthermore, the mixed gas G6 may contain H2O. The CO2 concentration in the mixed gas G6 discharged from the oxidation unit 6 is often about 40 to 80 volume %, depending on the operating conditions of the CO2 electrolysis unit 2, etc. If such a mixed gas G6 is discharged into the atmosphere, there are concerns about the impact on the environment, such as global warming, and the utilization efficiency of the captured CO2 will be reduced.

[0020] Methods for separating CO2 from a mixture of CO2 and O2 include the chemical absorption method, which is used to separate CO2 from natural gas and CO2 from exhaust gas from thermal power plants. However, since the O2 concentration in the mixed gas G6 is high at 20 to 60 volume %, the CO2 absorbing solution used in the chemical absorption method deteriorates rapidly, significantly reducing the economic efficiency. Similarly, when using an organic CO2 separation membrane, there is a concern about the deterioration of the membrane. On the other hand, in the pressure swing method, which uses inorganic substances such as zeolite as an adsorbent, there is a low risk of the adsorbent deteriorating, but in order to remove O2 and increase the purity of CO2, several steps are required, which results in excessive equipment size.

[0021] Therefore, in the carbon dioxide treatment device 1 of the embodiment, the mixed gas G6 discharged from the oxidation unit 6 is sent to the air electrode unit 9 of the fuel cell unit 4, and O2 in the mixed gas G6 is consumed in the fuel cell unit 4. The fuel cell unit 4 has an air electrode unit (cathode unit) 9 having an air electrode, an anode unit 10 having an anode, and a solid electrolyte layer 11 such as a solid oxide electrolyte layer or a solid polymer electrolyte membrane arranged between the air electrode and the anode. The air electrode and the anode are each formed of a porous electric conductor. The electrolyte layer 11 is formed of a dense ion conductor that does not conduct electricity. The fuel cell unit 4 has a cell in which the air electrode, the anode, and the solid electrolyte layer are stacked, and may be configured as such a single cell, or may have a configuration in which a plurality of cells are stacked and integrated. The specific configuration of the fuel cell unit 4 is not particularly limited, and may be any of a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), a solid oxide fuel cell (SOFC), a molten carbonate fuel cell (MCFC), and an alkaline electrolyte fuel cell (AFC).

[0022] In the fuel cell section 4, the reaction at the air electrode section 9 is as shown in the following formula (8), and the reaction at the fuel electrode section 10 is as shown in the following formula (9). 1 / 2O2+2H + +2e - → H2O …(8) H2 → 2H + +2e - …(9) As shown in formula (7), oxygen supplied to the air electrode 9 is converted into hydrogen ions (H + ) to generate H2O. In this way, O2 is consumed in the air electrode section 9 of the fuel cell section 4. Therefore, mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 is sent to the air electrode section 9, and O2 in the mixed gas G6 is used in the air electrode section 9. H2 is supplied to the fuel electrode section 10 from a hydrogen supply section (not shown).

[0023] The CO2 in the mixed gas G6 supplied to the air electrode section 9 does not adversely affect the air electrode section 9, and is discharged as it is as exhaust gas G4 from the air electrode section 9. When the CO2 concentration in the exhaust gas G4 is high, it can be sent directly to the CO2 supply section 3 located upstream of the CO2 electrolysis section 2. Since O2 in the mixed gas G6 adversely affects the reduction section 5 of the CO2 electrolysis section 2, when the O2 concentration in the exhaust gas G4-2 from the air electrode section 9 of the fuel cell section 4 has not been sufficiently reduced, the exhaust gas G4 may be sent to the CO2 recovery section 8. Furthermore, when the O2 concentration in the exhaust gas G4 from the air electrode section 9 has not been sufficiently reduced, as shown in FIG. 2, a cathode recycle may be applied in which the gas discharged from the air electrode section (cathode section) 9 is returned to the air electrode section 9. This allows the O2 concentration in the exhaust gas G4 to be, for example, less than 5% while maintaining the utilization rate of O2 by the air electrode section 9.

[0024] In this way, the mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 is sent to the air electrode section 9 of the fuel cell section 4, and O2 in the mixed gas G6 is used in the air electrode section 9, whereby CO2, which is the remaining gas in the mixed gas G6, can be sent to the CO2 supply section 3 as exhaust gas G4. This makes it possible to reuse CO2, which is the remaining gas in the mixed gas G6, in the CO2 electrolysis section 2 while eliminating any adverse effects on the reduction section 5 of the CO2 electrolysis section 2. Furthermore, electricity generated in the fuel cell section 4 can be used as part of the power for the CO2 electrolysis section 2, making it possible to further improve the efficiency of the CO2 treatment device 1.

[0025] Second Embodiment Next, a CO2 processing device 1 of a second embodiment will be described with reference to Fig. 3. The CO2 processing device 1 shown in Fig. 3 includes a first catalytic combustor 12 provided in a pipe P1 connecting the oxidation section 6 of the CO2 electrolysis section 2 and the air electrode section 9 of the fuel cell section 4, and a second catalytic combustor 13 and a cooling section 14 provided in a pipe P2 connecting the air electrode section 9 of the fuel cell section 4 and the CO2 supply section 3. The rest of the configuration is the same as that of the CO2 processing device 1 of the first embodiment.

[0026] As described above, the mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 may contain a trace amount of CO in addition to O2 and CO2. The mixed gas G6 is sent to the first catalytic combustor 12 before being sent to the air electrode section 9. In the first catalytic combustor 12, the CO in the mixed gas G6 is removed by reacting with O2 in the first catalytic combustor 12 as shown in the following formula (10). 2CO+O2 → 2CO2…(10) By removing CO from the mixed gas G6 in the first catalytic combustor 12, it is possible to improve the efficiency of O2 usage in the air electrode unit 9 of the fuel cell unit 4. Furthermore, although not shown, a cooling device that functions as a dehumidifier may be provided in the piping between the oxidizer 6 and the air electrode unit 9. This makes it possible to remove H2O contained in the mixed gas G6.

[0027] Furthermore, the exhaust gas G4 discharged from the air electrode unit 9 of the fuel cell unit 4 may contain several percent of O2 depending on the operation of the fuel cell unit 4 and its control method. Such exhaust gas G4 containing O2 is sent to the second catalytic combustor 13 before being sent to the CO2 supply unit 3. The second catalytic combustor 13 is supplied with a required amount of H2, lower hydrocarbons such as CH4, and reducing agents (auxiliary fuels) such as CO. When H2 is supplied as a reducing agent, O2 in the exhaust gas G4 reacts with the supplied H2 and is removed as H2O, as shown in the following formula (11). When lower hydrocarbons such as CH4 or CO are supplied, O2 is burned and consumed according to the following formula (12) or formula (13). 2H2+O2→ 2H2O …(11) CH4+2O2→ CO2+2H2O …(12) 2CO+O2 → 2CO2…(13)

[0028] Between the air electrode 9 and the CO2 supply unit 3, a cooling unit 14 is provided that functions as a dehumidifier. The exhaust gas G7 discharged from the second catalytic combustor 13 is sent to the cooling unit 14, where H2O in the exhaust gas G7 is cooled and removed. In addition, in order to increase the efficiency of removing O2 from the exhaust gas G4, it is preferable that the air-fuel ratio in the second catalytic combustor 13 is less than 1 (incomplete combustion region), for example, about 1.3. This makes it possible to further reduce the O2 concentration in the gas G8 returned to the CO2 supply unit 3, and further make it approximately zero. Although H2, CH4, CO, etc. remain in the gas G8, they have a smaller adverse effect on the oxidation unit 6 of the CO2 electrolysis unit 2 than O2. As a result, the gas G8 containing high-purity CO2 can be returned to the CO2 supply unit 3. Note that, as the combustors (12, 13), the catalytic combustors 12, 13 are effective because of their excellent efficiency, but are not limited thereto.

[0029] (Third embodiment) Next, the CO2 processing device 1 of the third embodiment will be described with reference to Fig. 4. The CO2 processing device 1 shown in Fig. 4 has a two-stage fuel cell section 4, i.e., a first fuel cell section (first fuel cell) 4A and a second fuel cell section (second fuel cell) 4B. The fuel cell section 4 is not limited to a two-stage fuel cell, and may have three or more stages of fuel cells. The rest of the configuration is the same as that of the CO2 processing device 1 of the second embodiment.

[0030] In the CO2 processing device 1 of the third embodiment, a mixed gas G6 having an O2 concentration of, for example, 20 to 60% by volume is introduced into the first fuel cell unit 4A of the first stage. Although the O2 concentration is slightly high in the operating conditions of the first fuel cell unit 4A, a general fuel cell can be applied to the first fuel cell unit 4A. From the air electrode unit 9 of such a first fuel cell unit 4A, a gas G41 having an O2 concentration of, for example, less than 10% (for example, about 7%) is discharged. It is inefficient to send such exhaust gas G41 directly to the second catalytic combustor 13. Therefore, in the third embodiment, the first fuel cell unit 4A and the second fuel cell unit 4B are applied, and a gas G42 having an O2 concentration of, for example, less than 5% can be discharged from the second fuel cell unit 4B. The introduced O2 is H generated by the reaction of the fuel electrode unit 10 as described in detail in the first embodiment. + It reacts with this to produce H2O, which is emitted as exhaust gas G42.

[0031] In order to discharge the gas G42 having an O2 concentration of less than 5% in the first fuel cell section 4A and the second fuel cell section 4B, for example, a cascade structure can be applied as shown in FIG. 5. The fuel cell section 4 shown in FIG. 5 includes two first fuel cell sections 4A1 and 4A2 and one second fuel cell section 4B. The two first fuel cell sections 4A1 and 4A2 are each supplied with a mixed gas G6 discharged from the oxidation section 6. In the two first fuel cell sections 4A1 and 4A2, O2 is consumed, and gases G411 and G412 having an O2 concentration of less than 10% are discharged, for example. These exhaust gases G411 and G412 are combined and sent to the second fuel cell section 4B as a gas having an O2 concentration of less than 20%, for example. In the second fuel cell section 4B, O2 in the gas is consumed, and the gas is discharged as a gas G42 with a reduced O2 concentration. By applying such a configuration, it is possible to discharge gas G42 with a reduced O2 concentration while maintaining the oxygen utilization rate in each fuel cell portion.

[0032] Although the second fuel cell unit 4B cannot obtain a large electric output, it is effective in exerting the function as an oxygen remover. The O2 concentration in the gas G42 discharged from the air electrode unit 9 of the second fuel cell unit 4B is preferably, for example, less than 1%. Note that the gas G41 discharged from the air electrode unit 9 of the first fuel cell unit 4A may be introduced, for example, into a cooling unit for dehumidification before being sent to the second fuel cell unit 4B. The gas G42 discharged from the air electrode unit 9 of the second fuel cell unit 4B can be sent to the CO2 supply unit 3 and reused well by, for example, making the O2 concentration less than 1%. The exhaust gas G42 may be sent to the CO2 supply unit 3 via the second catalytic combustor 13 and the cooling unit 14 as in the second embodiment. This makes it possible to remove even a trace amount of O2 contained in the exhaust gas G42, and to introduce the gas G8 containing CO2 with a concentration of 99% or more into the CO2 supply unit 3.

[0033] (Fourth embodiment) Next, a fourth embodiment of the CO2 processing device 1 will be described with reference to Fig. 6. The CO2 processing device 1 shown in Fig. 6 includes a CO purification section 15 to which a mixed gas G5 discharged from the reduction section 5 of the CO2 electrolysis section 2 is introduced, and an H2 separation section 16 to which a CO purification residual gas (mixed gas) G9 discharged from the CO purification section 15 is introduced. The rest of the configuration is the same as that of the CO2 processing device 1 of the second embodiment.

[0034] As described above, the mixed gas G5 discharged from the reduction unit 5 of the CO2 electrolysis unit 2 may contain H2 as a by-product of the reduction unit 5 in addition to CO and CO2. Furthermore, in order to increase the utilization efficiency of CO in the mixed gas G5, a CO purification unit 15 for purifying CO in the mixed gas G5 may be installed. For example, the CO purification unit 15 may be a method of absorbing CO into a toluene solution of CuAlCl4 and separating and recovering CO at high temperature and reduced pressure (COSORB method), a method of adsorbing CO under pressure by utilizing the adsorption and desorption of CO due to pressure fluctuation of the adsorbent, and desorbing CO under reduced pressure to separate and recover CO (PSA method), etc., but is not limited thereto.

[0035] The remaining gas after separation and recovery of CO in the CO purification section 4, that is, the CO separation residual gas G9, contains H2, CO2, and a trace amount of CO that is discharged without being separated. The residual gas G9 containing H2, CO2, and a trace amount of CO is supplied to the H2 separation section 16. For example, a separation device having an H2 separation membrane can be applied to the H2 separation section 16. The H2 gas G10 separated and recovered in the H2 separation section 16 is used as a part of the H2 to be supplied to the fuel electrode section 10 of the fuel cell section 4. The CO2 in the remaining gas after separation and recovery of H2 in the H2 separation section 16 can be sent to the CO2 supply section 3 and reused. The mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 consumes O2 in the air electrode section 9 of the fuel cell section 4, as in the second embodiment, and is then sent to the CO2 supply section 3 via the second catalytic combustor 13 and the cooling section 14.

[0036] According to the CO2 processing device 1 of the fourth embodiment, O2 in the mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 is used in the fuel cell section 4, and the remaining CO2 can be reused in the CO2 electrolysis section 2. In addition, CO in the mixed gas G5 discharged from the reduction section 5 of the CO2 electrolysis section 2 can be refined to increase the utilization efficiency, and H2 in the mixed gas G5 can be used in the fuel electrode section 10 of the fuel cell section 4. As described above, electricity generated in the fuel cell section 4 can also be used as part of the power for the CO2 electrolysis section 2. These features make it possible to realize even greater efficiency in the CO2 processing device 1.

[0037] Fifth embodiment Next, the CO2 processing device 1 of the fifth embodiment will be described with reference to Fig. 7. The CO2 processing device 1 shown in Fig. 7 includes a second processing system having a combustion unit 17 in addition to a first processing system for the mixed gas G6 discharged from the oxidation unit 6 of the CO2 electrolysis unit 2 having the fuel cell unit 4. The second processing system includes a pipe P2 connected to a pipe P1 that supplies the mixed gas G5 to the fuel cell unit 4, a combustion unit 17 connected to the pipe P2, and a pipe P4 that sends the exhaust gas G9 from the combustion unit 17 to the cooling unit 14. The configuration other than these is the same as that of the CO2 processing device 1 of the second embodiment.

[0038] The first and second treatment systems include a first switching mechanism having a valve V1 provided in the pipe P1 and a valve V2 provided in the pipe P2, and a second switching mechanism having a valve V3 provided in the pipe P3 connecting the second catalytic combustor 13 and the cooling unit 14 and a valve V4 provided in the pipe P4. The CO2 treatment device 1 of the fifth embodiment is configured to be capable of switching between the first and second treatment systems depending on, for example, the composition of the mixed gas G6 discharged from the oxidation unit 6 of the CO2 electrolysis unit 2.

[0039] For example, when the O2 concentration in the mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2 is not suitable for use in the fuel cell section 4, the first switching mechanism and the second switching mechanism switch the destination of the mixed gas G6, and the mixed gas G6 is sent to the combustion section 17. The combustion section 17 can be, for example, a catalytic combustor, or a combustor such as a boiler or a gas turbine. In the combustion section 17, the exhaust gas G4 discharged from the air electrode section 9 of the fuel cell section 4 is combusted, thereby removing O2 from the exhaust gas G4. The combustion section 17 is supplied with a required amount of reducing agent (auxiliary fuel) such as H2, lower hydrocarbons such as CH4, and CO. The H2, lower hydrocarbons such as CH4, or CO supplied as reducing agents combust O2 according to the above-mentioned formula (11), formula (12), or formula (13).

[0040] As described above, in order to increase the efficiency of removing O2 from the exhaust gas G6, it is preferable that the air-fuel ratio in the combustion section 17 is less than 1 (incomplete combustion region), for example, about 1.3. This allows the O2 concentration in the gas G8 returned to the CO2 supply section 3 to be further reduced, and can even be made approximately zero. Although H2, CH4, CO, etc. remain in the gas G8, they have a smaller adverse effect on the oxidation section 6 of the CO2 electrolysis section 2 than O2. As a result, the gas G7 containing high-purity CO2 can be returned to the CO2 supply section 3. In the CO2 processing device 1 of the fifth embodiment, the first processing system and the second processing system can be switched and used depending on, for example, the composition of the mixed gas G6 discharged from the oxidation section 6 of the CO2 electrolysis section 2. Therefore, O2 in the mixed gas G6 can be removed more efficiently, and the CO2 gas G8 from which O2 has been removed can be returned to the CO2 supply section 3 and reused.

[0041] The configurations of the above-mentioned embodiments can be applied in combination with each other, and some of them can be replaced. Although several embodiments of the present invention have been described here, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0042] 1...carbon dioxide treatment device, 2...CO2 electrolysis section, 3...CO2 supply section, 4, 4A, 4B...fuel cell section, 5...reduction section, 6...oxidation section, 8...CO2 recovery section, 9...air electrode section, 10...fuel electrode section, 12...first catalytic combustor, 13...second catalytic combustor, 14...cooling section, 17...combustion section.

Claims

1. a carbon dioxide electrolysis unit that electrolyzes carbon dioxide to generate carbon monoxide; a fuel cell unit to which the oxygen-carbon dioxide containing gas discharged from the carbon dioxide electrolysis unit is supplied and which reduces the oxygen concentration in the oxygen-carbon dioxide containing gas, The gas whose oxygen concentration has been reduced in the fuel cell section is returned upstream of the carbon dioxide electrolysis section.

2. The carbon dioxide treatment device according to claim 1 , wherein the fuel cell section comprises two or more stages of fuel cells connected in series.

3. 3. The carbon dioxide treatment device according to claim 2, wherein, of the two or more stages of fuel cells, the fuel cell arranged downstream is configured to operate at a lower oxygen concentration than the fuel cell arranged upstream.

4. 4. The carbon dioxide treatment device according to claim 1, further comprising a first combustion section disposed between the carbon dioxide electrolysis section and the fuel cell section, and supplied with the oxygen-carbon dioxide containing gas.

5. 5. The carbon dioxide treatment device according to claim 1, further comprising a second combustion section to which the gas discharged from the fuel cell section is supplied.

6. The carbon dioxide treatment device according to claim 5 , wherein the second combustion section is configured to be supplied with at least one reducing agent selected from the group consisting of hydrogen, carbon monoxide, and lower hydrocarbons.

7. The carbon dioxide treatment device according to claim 6 , wherein the reducing agent is supplied to the second combustion section so that the second combustion section becomes an incomplete combustion region.

8. 8. The carbon dioxide treatment device according to claim 1, wherein the power generated in the fuel cell section is supplied to the carbon dioxide electrolysis section.

9. The present invention further includes a combustion unit arranged in parallel with the fuel cell unit so as to be supplied with the oxygen-carbon dioxide containing gas discharged from the carbon dioxide electrolysis unit, 9. The carbon dioxide treatment device according to claim 1, wherein the oxygen-carbon dioxide containing gas is supplied by switching between the fuel cell section and the combustion section.

10. The carbon dioxide treatment device according to claim 9 , wherein the combustion section is configured to be supplied with at least one reducing agent selected from the group consisting of hydrogen, carbon monoxide, and lower hydrocarbons.

11. The carbon dioxide treatment device according to claim 10 , wherein the reducing agent is supplied to the combustion section so that the combustion section is in an incomplete combustion region.

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