Carbon Dioxide Conversion Device and Carbon Dioxide Conversion Method

Abstract

To provide a carbon dioxide conversion apparatus that can enhance the utilization efficiency of CO2.SOLUTION: A carbon dioxide conversion apparatus 1 of an embodiment of the present invention comprises: an electrochemical reaction unit 5 including a cathode unit 2 for reducing carbon dioxide, an anode unit 3 for oxidizing water, and a diaphragm 4; a carbon dioxide supply unit 6 for supplying carbon dioxide to the cathode unit 2; a liquid fuel synthesis unit 7 for synthesizing liquid fuel using first emissions including the carbon monoxide discharged from the cathode unit 2; an off-gas reaction unit 8 for reacting second emissions including carbon dioxide and oxygen discharged from the anode unit 3 and third emissions including at least one of the carbohydrate, carbon monoxide, and hydrogen discharged from the liquid fuel synthesis unit 7; and a cooling unit 9 for cooling fourth emissions including the carbon dioxide and water discharged from the off-gas reaction unit 8, and for sending them to the carbon dioxide supply unit 6 or to its upstream after removing the water from the fourth emissions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a carbon dioxide conversion device and a carbon dioxide conversion method. [Background technology]

[0002] Carbon dioxide (CO2), primarily generated by burning fossil fuels, is considered the main cause of global warming due to the greenhouse effect. Large-scale sources of CO2 include thermal power plants and steel mills. By removing CO2 from the exhaust gases emitted from these sources and suppressing its release into the atmosphere, it is possible to efficiently eliminate the cause of global warming. It has been proposed to store the CO2 removed from the exhaust gases underground to ensure its isolation from the atmosphere. On the other hand, if CO2 is reduced by some means, it can be recycled into carbon compounds similar to those derived from fossil fuels and chemicals.

[0003] A CO2 electrolysis device, comprising an anode section that oxidizes water (H2O) to produce oxygen (O2) and a cathode section that reduces CO2 to produce carbon compounds, is suitable for converting CO2 into carbon compounds. If the CO2 electrolysis device is powered by renewable energy such as solar or wind power, the use of fossil resources can be reduced, and it is possible to convert CO2 into carbon compounds while minimizing the generation of new CO2.

[0004] The CO2 electrolysis apparatus described above can directly produce carbon compounds such as carbon monoxide (CO), methanol (CH3OH), and ethylene glycol (C2H6O2) from CO2. However, it is also possible to produce higher value-added carbon compounds using these as intermediate products. For example, by synthesizing hydrogen (H2) and CO produced by the CO2 electrolysis apparatus in a liquid fuel synthesis reactor, common liquid fuels such as gasoline and diesel can be produced. In carbon dioxide conversion apparatuses that include such CO2 electrolysis apparatuses and liquid fuel synthesis reactors, there is a need to improve the efficiency of CO2 utilization. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6870956 [Patent Document 2] Patent No. 6896748 [Patent Document 3] Patent No. 5767497 [Patent Document 4] Japanese Patent Publication No. 2021-147679 [Overview of the project] [Problems that the invention aims to solve]

[0006] The problem that this invention aims to solve is to provide a carbon dioxide conversion device and a carbon dioxide conversion method that enable increased CO2 utilization efficiency. [Means for solving the problem]

[0007] The carbon dioxide conversion apparatus of the embodiment comprises an electrochemical reaction section having a cathode section for electrolytically reducing carbon dioxide, an anode section for electrolytically oxidizing water, and a diaphragm disposed between the cathode section and the anode; a carbon dioxide supply section for supplying carbon dioxide to the cathode section; a liquid fuel synthesis section for synthesizing a liquid fuel using a first emission discharged from the cathode section, which contains at least carbon monoxide; an off-gas reaction section for reacting a second emission discharged from the anode section, which contains carbon dioxide and oxygen, with a third emission discharged from the liquid fuel synthesis section, which contains at least one selected from hydrocarbons, carbon monoxide, and hydrogen; and a cooling section for cooling a fourth emission discharged from the off-gas reaction section, which contains carbon dioxide and water, removing water from the fourth emission, and sending the fourth emission from which the water has been removed to the carbon dioxide supply section or upstream thereof. [Brief explanation of the drawing]

[0008] [Figure 1]This is a diagram showing a carbon dioxide conversion device according to an embodiment. [Figure 2] This figure shows a first modified example of the carbon dioxide conversion device shown in Figure 1. [Figure 3] This figure shows a second modified example of the carbon dioxide conversion device shown in Figure 1. [Modes for carrying out the invention]

[0009] The carbon dioxide conversion apparatus and carbon dioxide conversion method of the embodiments will be described below with reference to the drawings. In each embodiment shown below, substantially identical components are denoted by the same reference numerals, and their descriptions may be partially omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the ratio of thickness of each part, etc., may differ from those in reality. In the following description, the symbol "~" indicates a range between the respective upper and lower limits. In this case, each range includes both the upper and lower limits.

[0010] Figure 1 shows a carbon dioxide conversion device 1 of an embodiment. The carbon dioxide conversion device 1 shown in Figure 1 comprises an electrochemical reaction unit 5 having a cathode unit 2, an anode unit 3, and a diaphragm 4; a CO2 supply unit 6 that supplies carbon dioxide (CO2) to the cathode unit 2; a liquid fuel synthesis unit 7 that synthesizes a carbon-containing liquid fuel using a first emission discharged from the cathode unit 2; an off-gas reaction unit 8 that reacts a second emission discharged from the liquid fuel synthesis unit 7 with a third emission discharged from the anode unit 3; and a cooling unit 9 that cools the fourth emission discharged from the off-gas reaction unit 8, removes water (H2O) from the fourth emission, and supplies the H2O-free fourth emission to the CO2 supply unit 6 or upstream thereof.

[0011] The CO2 supply unit 6 is connected to the cathode unit 2, and CO2-containing gas G1 is supplied from the CO2 supply unit 6 to the cathode unit 2. In the CO2-containing gas G1 supplied from the CO2 supply unit 6 to the cathode unit 2, the CO2 concentration is preferably in the range of 80% to 100% by volume. When using exhaust gas G2 containing CO2 emitted from a thermal power plant, waste incinerator, steel mill, etc., a CO2 separation and recovery unit 10 may be provided to separate and recover CO2 from the exhaust gas G2, and CO2 gas G3 with a higher CO2 concentration may be supplied to the CO2 supply unit 6. The CO2 supply unit 6 is configured to supply an appropriate flow rate of CO2 to the cathode unit 2 by combining the CO2 gas G3 recovered in the CO2 separation and recovery unit 10 and the CO2 gas G4 from which H2O has been removed in the cooling unit 9 described later. For this reason, the CO2 supply unit 6 has a means for adjusting the amount of CO2, and may also have a container for temporarily storing CO2.

[0012] The electrochemical reaction unit 5 is a CO2 electrolytic device having an electrolytic cell, and comprises a cathode unit 2, an anode unit 3, and a diaphragm 4. The cathode unit 2 is equipped with a reducing electrode (cathode), and the anode unit 3 is equipped with an oxidizing electrode (anode), and at least the anode unit 3 is filled with electrolyte. The anode unit 3 is connected to an electrolyte supply unit 11 that supplies electrolyte L1. The cathode unit 2 may be configured to circulate CO2 gas, or to circulate or be filled with an electrolyte containing CO2. Power supplies (not shown) are connected to the reducing electrode and the oxidizing electrode. The cathode unit 2 and the anode unit 3 are connected to hydrogen ions (H + ) and hydroxide ions (OH - The electrochemical reaction section 5 is separated by a diaphragm 4, such as an ion exchange membrane, which is capable of moving ions such as ions. The electrochemical reaction section 5 may consist of a single electrolytic cell, or it may have a configuration in which multiple electrolytic cells are stacked and integrated.

[0013] In the electrochemical reaction section 5, in the cathode section 2, the supplied CO2 is electrolyzed and reduced. Carbon monoxide (CO) is generated by the reduction of CO2. Also, in the anode section 3, H2O in the electrolyte is electrolyzed and oxidized. Oxygen (O2) is generated by the oxidation of H2O. As shown in the following equation (1), an oxidation reaction of H2O in the electrolyte occurs, electrons are lost, and oxygen (O2) and hydrogen ions (H + ) are generated. A part of the generated hydrogen ions (H + ) moves to the cathode section 2 through the diaphragm 4. 2H2O → 4H + +O2+4e - …(1) When the hydrogen ions (H + ) generated in the anode section 3 reach the cathode section 2, as shown in the following equation (2), a reduction reaction of CO2 occurs and carbon monoxide (CO) is generated. 2CO2+4H + +4e - → 2CO+2H2O …(2)

[0014] In the cathode section 2, in addition to the reduction reaction of CO2, hydrogen (H2) may be by-produced by the reduction reaction of water (H2O) as shown in the following equation (3). 2H2O → 4H + +O2+4e - …(3) Furthermore, in the anode section 3, in addition to the generation of O2 by the oxidation of H2O, CO2 may be by-produced. For example, as shown in the following equation (4), the supplied CO2 in the cathode section 2 is electrolyzed to generate CO and carbonate ions (CO3 2- ). As shown in the following equation (5), CO2 and O2 are generated in the anode section 3 from the generated carbonate ions. 2CO2+2e - → CO+CO3 2- …(4) CO3 2- → CO2+0.5O2+2e - …(5)

[0015] The CO and H2 generated in the cathode section 2 are supplied to the liquid fuel synthesis section 7 as a mixed gas (first emission) G5. The gas containing O2 and CO2 generated in the anode section 3 becomes the off-gas for the anode section 3 as a mixed gas (second emission) G6. This off-gas G6 containing O2 and CO2 is supplied to the off-gas reaction section 8.

[0016] In the liquid fuel synthesis unit 7, CO and H2 are reacted using a catalyst to produce hydrocarbon compounds with one or more carbon atoms in the molecule. The Fischer-Tropsch synthesis reaction, in which CO and H2 are reacted under conditions of 1-4 MPa and 200-300°C in the presence of a metal catalyst, is an example of a liquid fuel synthesis reaction; however, other synthesis reactions may also be applied. The H2 required for the synthesis reaction may be supplied from the H2 supply unit 12 in addition to the H2 contained in the mixed gas (second emission) G3. In this case, it is preferable to determine the composition of the mixed gas G5 in advance so that the ratio of CO to H2 in the liquid fuel synthesis unit 7 is appropriate, and to supply an appropriate amount of H2 from the H2 supply unit 12.

[0017] In the liquid fuel synthesis section 7, when reaction conditions corresponding to the Fischer-Tropsch synthesis reaction described above are applied, hydrocarbon compounds with approximately 10 to 20 carbon atoms are produced as carbon-containing liquid fuels. These are mixtures of components similar to those of kerosene and diesel fuel, and may be separated by distillation, isomerized, etc. On the other hand, the liquid fuel synthesis section 7 also produces light components that cannot become components of liquid fuel, such as methane (CH4) and ethane (C2H6). Such lower hydrocarbons complete their reaction in this form and do not contribute to the synthesis reaction of liquid fuel, so they are discharged as the second emission G7. At this time, small amounts of unreacted CO and H2 in gaseous form are discharged along with the lower hydrocarbons.

[0018] The emissions G7 discharged from the liquid fuel synthesis unit 7 are, for example, combustible off-gas containing CH4, C2H6, CO, and H2. An example of off-gas G5 is a mixed gas having a composition of 53.49% by volume of CH4, 15.56% by volume of CO, and 27.29% by volume of H2. The off-gas G7 discharged from the liquid fuel synthesis unit 7 can be any combustible gas containing at least one selected from lower hydrocarbons with 1 to 3 carbon atoms, such as CH4 and C2H6, CO, and H2. However, off-gas G7 generally contains lower hydrocarbons, CO, and H2, even if the content of each component increases or decreases.

[0019] As mentioned above, in the anode section 3, H2O is oxidized to produce O2, and CO2 is also produced as a by-product. Therefore, the off-gas G6 from the anode section 3, which contains O2 and CO2, is discharged. The CO2 concentration in the off-gas G6 from the anode section 3 is approximately 40-80% by volume, depending on the operating conditions of the electrochemical reaction section (CO2 electrolysis device) 5. If such off-gas G6 from the anode section 3 is released into the atmosphere, there are concerns about environmental impacts such as global warming, and it will also reduce the utilization efficiency of the recovered CO2.

[0020] Methods for separating CO2 from a mixture of CO2 and O2 include chemical absorption, which is used for separating CO2 from natural gas and from exhaust gases of thermal power plants. However, because the O2 concentration in mixed gas G6 is high (20-60% by volume), the CO2 absorbent used in chemical absorption deteriorates rapidly, significantly reducing its economic viability. Similarly, when using organic CO2 separation membranes, membrane deterioration is a concern. On the other hand, in the pressure swing method, which uses inorganic materials such as zeolites as adsorbents, the risk of adsorbent deterioration is low, but several steps are required to remove O2 and increase the purity of CO2, resulting in excessive equipment requirements.

[0021] Therefore, in the carbon dioxide conversion device 1 of this embodiment, off-gas G7 discharged from the liquid fuel synthesis unit 7 and off-gas G6 discharged from the anode unit 3 are mixed and, for example, burned in the off-gas reaction unit 8. A combustor such as a boiler or gas turbine is used in the off-gas reaction unit 8. When the combustible components contained in off-gas G7 and the O2 contained in off-gas G4 are stoichiometrically balanced, the majority of the exhaust gas discharged from the off-gas reaction unit 8 becomes a mixed gas G8 of CO2 and H2O. When the mixed gas G8 is cooled, the water vapor H2O condenses into water, which can be easily separated from the gaseous CO2. The H2OL2 separated from the mixed gas G8 may be returned to the anode unit 3 and reused.

[0022] In the cooling unit 9, H2O in the mixed gas G8 is cooled and removed, and the cooled gas (CO2 gas) G4, which has become high-purity CO2, can be reused as CO2 by returning it to the CO2 supply unit 6. This makes it possible to significantly improve the CO2 utilization efficiency. If the CO2 concentration in the cooled gas G4 is low, it may be returned to the CO2 separation and recovery unit 10 located upstream of the CO2 supply unit 6, which also improves the CO2 utilization efficiency. Since the O2 concentration in the cooled gas G4 is significantly lower than the O2 concentration in the mixed gas G6, even when a chemical absorption method is used for the CO2 separation and recovery unit 10, the deterioration of the absorbent is suppressed, and there is little risk of economic problems arising.

[0023] In the off-gas reaction section 8, the combustion reactions of CH4, C2H6, CO, and H2 contained in the off-gas G7 of the fuel synthesis section 7 proceed based on the following reaction equations (6) to (9). Note that the combustion of CH4 and C2H6 may occur in either form or in any other form. CH4 + 2O2 → CO2 + 2H2O …(6) 2C2H6 + 7O2 → 4CO2 + 6H2O …(7) 2CO + O2 → 2CO2…(8) 2H2 + O2 → 2H2O …(9)

[0024] In the combustion reaction equations (6) to (9) described above, if the combustible component derived from the off-gas G7 of the liquid fuel synthesis unit 7 and the O2 derived from the off-gas G6 of the anode unit 3 are not stoichiometrically balanced, O2 and auxiliary fuel may be supplied from the O2 supply unit 13 and the auxiliary fuel supply unit 14, respectively, as shown in Figure 2. For this adjustment, it is preferable to know in advance, using a measuring device, the composition of the off-gas G7 of the liquid fuel synthesis unit 7 and the off-gas G6 of the anode unit 3 supplied to the off-gas reaction unit 8. The auxiliary fuel supplied from the auxiliary fuel supply unit 14 is preferably a carbon-containing compound or H2, and more preferably CH4.

[0025] One form of the off-gas reaction unit 8 is a combustion device such as a boiler, which may obtain heat through the reaction, i.e., combustion, between the off-gas G7 of the liquid fuel synthesis unit 7 and the off-gas G6 of the anode unit 3. Furthermore, it is preferable to control the supply amounts of the off-gas G7 of the liquid fuel synthesis unit 7 and the off-gas G6 of the anode unit 3 so that the combustion temperature of the off-gas reaction unit 8 does not exceed the limits of the constituent materials. In addition, as shown in Figure 3, a portion of the CO2 gas G9 may be supplied from the CO2 supply unit 6 to adjust the combustion temperature. If the heat obtained from the combustor, such as a boiler, which serves as the off-gas reaction unit 8, is used within the carbon dioxide conversion device 1 of the embodiment, it is possible to further increase efficiency.

[0026] As described above, according to the carbon dioxide conversion device 1 of this embodiment, the off-gas G6 containing a mixed gas of CO2 and O2 discharged from the anode 3 of the electrochemical reaction unit (CO2 electrolyzer) 5 is converted into CO2 and H2O in the off-gas reaction unit 8 using combustible by-products, which are off-gas G7 from the liquid fuel synthesis unit 7, thereby increasing the CO2 utilization efficiency. Furthermore, by cooling the mixture of CO2 and H2O G8 discharged from the off-gas reaction unit 8, H2O can be separated. This allows the purified CO2 gas G4 to be returned to the CO2 supply unit 6 or upstream thereof. Therefore, a carbon dioxide conversion device 1 capable of increasing CO2 utilization efficiency can be provided.

[0027] The configurations of each embodiment described above can be applied in combination, and can also be partially 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, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as described in the claims. [Explanation of symbols]

[0028] 1...Carbon dioxide conversion unit, 2...Cathode section, 3...Anode section, 4...Diaphragm, 5...Electrochemical reaction section, 6...CO2 supply section, 7...Liquid fuel synthesis section, 8...Off-gas reaction section, 9...Cooling section, 10...CO2 separation and recovery section, 11...H2 supply section, 12...O2 supply section, 13...Auxiliary fuel supply section, G1...CO2-containing gas, G2...Exhaust gas, G3...CO2 gas, G4...CO2 gas, G5...First emission, G6...Second emission (off-gas), G7...Third emission (off-gas), G8...Fourth emission.

Claims

1. an electrochemical reaction unit including a cathode unit that electrolyzes and reduces carbon dioxide, an anode unit that electrolyzes and oxidizes water, and a diaphragm disposed between the cathode unit and the anode; a carbon dioxide supply unit that supplies carbon dioxide to the cathode unit; a liquid fuel synthesis unit that synthesizes a liquid fuel using a first discharged product that is discharged from the cathode unit and includes at least carbon monoxide; an off-gas reaction section that reacts a second effluent discharged from the anode section and containing carbon dioxide and oxygen with a third effluent discharged from the liquid fuel synthesis section and containing at least one selected from hydrocarbons, carbon monoxide, and hydrogen; a cooling section that cools a fourth effluent discharged from the off-gas reaction section and containing carbon dioxide and water, removes water from the fourth effluent, and sends the fourth effluent from which water has been removed to the carbon dioxide supply section or upstream thereof; A carbon dioxide conversion device comprising:

2. The carbon dioxide conversion apparatus of claim 1 , wherein the off-gas reactor comprises a combustor.

3. 3. The carbon dioxide conversion apparatus according to claim 1, further comprising an oxygen supply section that supplies oxygen to the off-gas reaction section.

4. 4. The carbon dioxide conversion apparatus according to claim 1, further comprising an auxiliary fuel supply section that supplies auxiliary fuel to the off-gas reaction section.

5. 5. The carbon dioxide conversion apparatus according to claim 1, wherein the first effluent comprises the carbon monoxide and hydrogen.

6. 6. The carbon dioxide conversion apparatus according to claim 1, further comprising a hydrogen supply unit that supplies hydrogen to the liquid fuel synthesis unit.

7. The carbon dioxide conversion apparatus according to claim 1 , wherein the second effluent includes the hydrocarbons, the carbon monoxide, and the hydrogen.

8. a step of supplying carbon dioxide to the cathode section and electrolyzing and reducing the carbon dioxide using an electrochemical reaction section including a cathode section, an anode section, and a diaphragm disposed between the cathode section and the anode, and supplying an electrolytic solution containing water to the anode section and electrolyzing and oxidizing the electrolytic solution; synthesizing a liquid fuel from a first discharged product discharged from the cathode section and containing at least carbon monoxide using a liquid fuel synthesis section; a step of reacting a second effluent discharged from the anode section and containing carbon dioxide and oxygen with a third effluent discharged from the liquid fuel synthesis section and containing at least one selected from hydrocarbons, carbon monoxide, and hydrogen, using an off-gas reaction section; cooling a fourth effluent discharged from the off-gas reaction section and containing carbon dioxide and water, removing water from the fourth effluent, and sending the fourth effluent from which water has been removed to the carbon dioxide supply section or upstream thereof; A carbon dioxide conversion method comprising:

9. 9. The carbon dioxide conversion method according to claim 8, wherein the step of reacting the second effluent with the third effluent comprises a step of combusting at least one selected from the hydrocarbons, the carbon monoxide, and the hydrogen contained in the third effluent using at least the oxygen contained in the second effluent.

10. 10. The carbon dioxide conversion method according to claim 8 or claim 9, wherein in the step of reacting the second effluent with the third effluent, oxygen is supplied to the off-gas reaction section so as to increase a combustion rate of at least one selected from the hydrocarbons, the carbon monoxide, and the hydrogen contained in the third effluent.

11. 11. The carbon dioxide conversion method according to claim 8, wherein in the step of reacting the second effluent with the third effluent, an auxiliary fuel is supplied to the off-gas reaction section so as to increase a reaction ratio of the oxygen contained in the second effluent.

12. 12. The carbon dioxide conversion method according to claim 8, wherein a portion of the carbon dioxide supplied to the cathode section is supplied to the off-gas reaction section so that the internal temperature of the off-gas reaction section is equal to or lower than a specified value.

13. 13. The carbon dioxide conversion method according to claim 8, wherein the second effluent contains 40% by volume or more and 80% by volume or less of the carbon dioxide.

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