Methods for producing methane

Abstract

To provide a novel method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides, which enables the stable production of methane. [Solution] A method is provided for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides. The method comprises a first step of reducing the concentration of nitrogen oxides in the mixed gas by contacting the mixed gas with water or an aqueous solution; a second step of recovering carbon dioxide from the mixed gas after contact with water or an aqueous solution; and a third step of reacting the recovered carbon dioxide with hydrogen to produce methane. The concentration of nitrogen oxides in the mixed gas after the first step is 100 ppm by volume or less.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing methane. [Background technology]

[0002] In recent years, there has been research into recovering carbon dioxide from exhaust gases and using the recovered carbon dioxide to produce methane (for example, Patent Document 1). In the process of recovering carbon dioxide from exhaust gases, the exhaust gas may be pre-treated by a desulfurization device and a denitrification device in order to remove acidic gases beforehand (for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-172595 [Patent Document 2] Japanese Patent Publication No. 2013-244454 [Overview of the project] [Problems that the invention aims to solve]

[0004] This disclosure provides a novel method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides, with the aim of providing a method that can stably produce methane. [Means for solving the problem]

[0005] The inventors have discovered that, in a process of recovering carbon dioxide from a mixed gas containing carbon dioxide and nitrogen oxides, nitrogen oxides can be removed from the mixed gas beforehand without significantly reducing the carbon dioxide concentration by performing a predetermined treatment. Further investigation by the inventors has led them to discover that by recovering the carbon dioxide after the predetermined treatment and reacting the recovered carbon dioxide with hydrogen, methane can be stably produced over a long period without reducing the methane concentration, thus completing the invention disclosed herein.

[0006] This disclosure provides a method for producing methane [1] to [7]. [1] A method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides, A first step involves bringing the mixed gas into contact with water or an aqueous solution to reduce the concentration of nitrogen oxides in the mixed gas, A second step of recovering carbon dioxide from the mixed gas after contact with the water or aqueous solution, A third step involves reacting the recovered carbon dioxide with hydrogen to produce methane, Equipped with, The concentration of nitrogen oxides in the mixed gas after the first step is 100 ppm by volume or less. method. [2] The first step is to bring the mixed gas into contact with an alkaline aqueous solution. The method described in [1]. [3] The alkaline aqueous solution contains at least one selected from the group consisting of carbonate ions and bicarbonate ions. The method described in [2]. [4] The alkaline aqueous solution contains ammonia, The method described in [2]. [5] Between the second step and the third step, further comprising a step of reducing the concentration of nitrogen oxides in the gas containing carbon dioxide recovered in the second step, The method described in any of [1] to [4]. [6] The mixed gas further contains sulfur oxides, The first step is a step of reducing the concentrations of nitrogen oxides and sulfur oxides in the mixed gas by contacting with water or an aqueous solution. The method according to any one of [1] to [5]. [7] Further comprising a step of reducing the concentrations of nitrogen oxides and sulfur oxides in the gas containing carbon dioxide recovered in the second step, between the second step and the third step. The method according to [6]. [8] The mixed gas is exhaust gas. The method according to any one of [1] to [7].

Advantages of the Invention

[0007] According to the present disclosure, there is provided a method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides, and a novel method capable of stably producing methane is provided.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is a schematic diagram showing an embodiment of a methane production apparatus (methanation apparatus) for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an embodiment of a methane production unit. [Figure 3] FIG. 3 is a schematic diagram showing an apparatus for removing nitrogen oxides from the mixed gas of the examples.

Embodiments for Carrying Out the Invention

[0009] Hereinafter, this embodiment will be described in detail with reference to the drawings. However, the present disclosure is not limited to the following embodiments.

[0010] <Method and Equipment for Producing Methane> A method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides includes a first step of reducing the concentration of nitrogen oxides in the mixed gas by bringing the mixed gas into contact with water or an aqueous solution, a second step of recovering carbon dioxide from the mixed gas after contact with water or the aqueous solution, and a third step of reacting the recovered carbon dioxide with hydrogen to produce methane. The method may further include a step (step 2A) of reducing the concentration of nitrogen oxides in the gas containing carbon dioxide recovered in the second step, between the second step and the third step.

[0011] FIG. 1 is a schematic diagram showing an embodiment of a methane production apparatus (methanation apparatus) for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides. The methane production apparatus 5 shown in FIG. 1 mainly includes a gas-liquid contact part 1 for carrying out the first step, a carbon dioxide recovery part 2 for carrying out the second step, a methane production part 3 for carrying out the third step, and pipes connecting them. The methane production apparatus 5 may further include a carbon dioxide purification part 2A for carrying out step 2A, between the carbon dioxide recovery part 2 and the methane production part 3.

[0012] The gas-liquid contact part 1 can be an apparatus for carrying out the first step. The gas-liquid contact part 1 is provided with water or an aqueous solution. In the gas-liquid contact part 1, water or the aqueous solution is brought into contact with a mixed gas G0 containing carbon dioxide and nitrogen oxides. By contact with water or the aqueous solution, a part of the nitrogen oxides in the mixed gas G0 dissolves in water or the aqueous solution, and as a result, the concentration of nitrogen oxides in the mixed gas G0 decreases. The mixed gas G1 after contact with water or the aqueous solution (after the first step) is discharged from the gas-liquid contact part 1 and sent to the carbon dioxide recovery part 2 through a pipe.

[0013] The mixed gas G0 introduced into the gas-liquid contact part 1 contains carbon dioxide (CO2) and nitrogen oxides (NO X ). The mixed gas G0 contains sulfur oxides (SO XMixed gas G0 may further contain other acidic gases other than nitrogen oxides and sulfur oxides, and may further contain other gases such as nitrogen. Mixed gas G0 is carbon dioxide (CO2) and nitrogen oxides (NO2). X The exhaust gas may contain carbon dioxide (CO2), nitrogen oxides (NOx), etc. X ), and sulfur oxides (SO X The exhaust gas may contain ) ). The exhaust gas may be, for example, exhaust gas discharged from a factory such as a cement plant. The mixed gas G0 may be dehumidified beforehand.

[0014] The concentration of carbon dioxide in mixed gas G0 may be, for example, 8.0% by volume or more, or 12% by volume or more, and 30% by volume or less, or 20% by volume or less, based on the volume of mixed gas G0. The concentration of nitrogen oxides in mixed gas G0 may be greater than 100 ppm by volume, and 1000 ppm by volume or less, or 800 ppm by volume or less, based on the volume of mixed gas G0.

[0015] The gas-liquid contact section 1 may be any processing apparatus capable of bringing a gas and a liquid into contact. The gas-liquid contact section 1 may be, for example, a wet scrubber comprising a processing tower provided with a gas inlet and a gas outlet, a filling section for filling a packing material placed in the processing tower, a water storage section for storing an alkaline aqueous solution placed in the processing tower, a water spraying means for spraying the alkaline aqueous solution placed in the processing tower onto the packing material in the filling section, and a circulation means for circulating the alkaline aqueous solution from the water storage section to the water spraying means. Other examples of the gas-liquid contact section 1 include, for example, an apparatus equipped with means for bubbling a gas in a liquid, and an apparatus equipped with means for directly spraying a liquid into a gas.

[0016] Water or an aqueous solution is disposed in the gas-liquid contact part 1. Since carbon dioxide has the property of being difficult to dissolve in water, by using water or an aqueous solution, nitrogen oxides can be efficiently removed from the mixed gas G0 without significantly reducing the concentration of carbon dioxide in the mixed gas G0. In the present disclosure, the term "water" is used to mean not only pure water but also ordinary water (e.g., tap water or industrial water) containing inevitable trace amounts of impurities. The proportion of water in the aqueous solution may be 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more, 99% by mass or more, 99.9% by mass or more, or 99.99% by mass or more, based on the mass of the aqueous solution, and may be 100% by mass or less. When the mixed gas G0 further contains sulfur oxides, the sulfur oxides can also be absorbed and removed by water or an aqueous solution in the gas-liquid contact part 1. That is, the concentration of sulfur oxides in the mixed gas G0 can also be reduced by the gas-liquid contact part 1.

[0017] The aqueous solution may be, for example, an alkaline aqueous solution containing water and a base. In one embodiment, the alkaline aqueous solution may contain at least one selected from the group consisting of carbonate ions (CO3 2- ) and hydrogen carbonate ions (HCO3 - ). Carbon dioxide tends to be even more difficult to dissolve in an alkaline aqueous solution containing carbonate ions and / or hydrogen carbonate ions. Therefore, by using an alkaline aqueous solution containing carbonate ions and / or hydrogen carbonate ions, the concentration of carbon dioxide in the mixed gas G0 tends to be more easily maintained at a higher level.

[0018] When the alkaline aqueous solution contains carbonate ions (CO3 2- ) and hydrogen carbonate ions (HCO3 -When at least one selected from the group consisting of ) is included, a higher total concentration of carbonate ions and bicarbonate ions tends to make it easier to maintain a higher concentration of carbon dioxide in the mixed gas G0. Therefore, the total concentration of carbonate ions and bicarbonate ions may be 0.10 mol / L or higher, 0.15 mol / L or higher, 0.20 mol / L or higher, 0.25 mol / L or higher, 0.30 mol / L or higher, 0.35 mol / L or higher, 0.40 mol / L or higher, or 0.45 mol / L or higher, based on the volume of the alkaline aqueous solution, and may be 1.5 mol / L or lower or 1.0 mol / L or lower. Examples of bases used to prepare the alkaline aqueous solution include sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), potassium carbonate (K2CO3), and potassium bicarbonate (KHCO3).

[0019] In other embodiments, the alkaline aqueous solution may contain ammonia. Carbon dioxide tends to be even less soluble in alkaline aqueous solutions containing ammonia. When the alkaline aqueous solution contains ammonia, the concentration of ammonia may be between 10 mol / L and 15 mol / L, based on the volume of the alkaline aqueous solution.

[0020] From the viewpoint of maintaining a high carbon dioxide concentration in the mixed gas G0 and efficiently removing nitrogen oxides, the pH of the alkaline aqueous solution may be 6 to 14 or 7.5 to 12.

[0021] The alkaline aqueous solution may further contain an oxidizing agent. An alkaline aqueous solution containing an oxidizing agent can contribute to more efficient removal of nitrogen oxides while maintaining a high carbon dioxide concentration in the mixed gas G0. The oxidizing agent may be water-soluble. Examples of oxidizing agents include potassium permanganate (KMnO4) and hydrogen peroxide (H2O2). The concentration of the oxidizing agent in the alkaline aqueous solution can be adjusted based on the nitrogen oxide removal rate, etc. The concentration of the oxidizing agent in the alkaline aqueous solution may be 0.0050 mol / L or more or 0.01 mol / L or more, and 0.3 mol / L or less or 0.1 mol / L or less, based on the volume of the alkaline aqueous solution.

[0022] The temperature of the water or aqueous solution in contact with the mixed gas G0 at the gas-liquid contact section 1 may be, for example, 10°C to 60°C. The temperature of the mixed gas G0 introduced into the gas-liquid contact section 1 may be, for example, 20°C to 50°C.

[0023] Mixed gas G O The gas may be continuously introduced into the gas-liquid contact section 1. In that case, the ratio (space velocity) of the flow rate (mL / h) of the mixed gas G0 introduced into the gas-liquid contact section 1 to the volume (mL) of water or aqueous solution is 50h -1 More than 250h -1 The following is acceptable:

[0024] The concentration of nitrogen oxides in the mixed gas G0 decreases upon contact with water or an aqueous solution. However, it is not necessary for the entire amount of nitrogen oxides in the mixed gas G0 to be removed. The percentage of nitrogen oxides removed at the gas-liquid contact point 1 (nitrogen oxide removal rate) of nitrogen oxides in the mixed gas G0 may be, for example, 1.0 volume% or more, 3.0 volume% or more, or 10 volume% or more, and may be 100 volume% or less, 90 volume% or less, 80 volume% or less, or 70 volume% or less.

[0025] The gas-liquid contact section 1 allows for efficient removal of nitrogen oxides from the mixed gas G0 without significantly reducing the concentration of carbon dioxide in the mixed gas G0. The concentration of carbon dioxide in the intermediate mixed gas G1 introduced into the carbon dioxide recovery section 2 after contact with water or aqueous solution (after the first step) may be, for example, 8.0 vol% or more, 9.0 vol% or more, 10 vol% or more, 11 vol% or more, 12 vol% or more, 13 vol% or more, 14 vol% or more, or 15 vol% or more, based on the volume of the mixed gas G1, and may be 30 vol% or less or 20 vol% or less. The concentration of nitrogen oxides in the mixed gas G1 may be 100 vol ppm or less, based on the volume of the mixed gas G0, and may be 80 vol ppm or less, 60 vol ppm or less, 40 vol ppm or less, or 20 vol ppm or less.

[0026] The carbon dioxide recovery unit 2 may be a device for carrying out the second process. The carbon dioxide recovery unit 2 recovers carbon dioxide from the mixed gas G1. The mixed gas G2, after the carbon dioxide has been recovered, is discharged from the carbon dioxide recovery unit 2. Meanwhile, the recovered carbon dioxide is discharged from the carbon dioxide recovery unit 2 and sent via piping to the methane production unit 3 or the carbon dioxide purification unit 2A.

[0027] The carbon dioxide recovery unit 2 is not particularly limited as long as it is a known device capable of recovering carbon dioxide from the mixed gas G1. The carbon dioxide recovery unit 2 may be, for example, a device equipped with an absorbent liquid that absorbs carbon dioxide. By recovering carbon dioxide from a mixed gas containing carbon dioxide at a relatively high concentration in the carbon dioxide recovery unit 2, carbon dioxide can be obtained efficiently. Prior removal of nitrogen oxides can suppress deterioration of the carbon dioxide recovery unit 2 and contribute to stable carbon dioxide recovery.

[0028] The carbon dioxide purification unit 2A may be an apparatus for carrying out process 2A. The carbon dioxide purification unit 2A is not particularly limited as long as it is a known apparatus capable of reducing the concentration of nitrogen oxides in the gas containing carbon dioxide recovered in the carbon dioxide recovery unit 2. The carbon dioxide purification unit 2A may be, for example, an apparatus similar to the gas-liquid contact unit 1, or it may be a different apparatus. By passing through the carbon dioxide purification unit 2A, the carbon dioxide contained in the gas can be further purified. If the gas containing carbon dioxide recovered in the carbon dioxide recovery unit 2 also contains sulfur oxides, the sulfur oxides can also be absorbed and removed in the carbon dioxide purification unit 2A. That is, the carbon dioxide purification unit 2A can reduce the concentration of sulfur oxides in the gas containing carbon dioxide recovered in the carbon dioxide recovery unit 2.

[0029] The methane production unit 3 may be a device primarily for carrying out the third step. The methane production unit 3 is configured to produce methane by reacting, for example, carbon dioxide recovered in the carbon dioxide recovery unit 2 or carbon dioxide purified in the carbon dioxide purification unit 2A with hydrogen supplied from an external source. In the methane production unit 3, for example, the Sabatier reaction (see the reaction equation below), which produces methane by reacting carbon dioxide and hydrogen via a catalyst, may be used. Prior removal of nitrogen oxides can suppress deterioration of the methane production unit 3 (e.g., deterioration of the catalyst) and contribute to stable methane production. CO2 + 4H2 → CH4 + 2H2O ΔH 289K = -164.9 kJ / mol

[0030] Figure 2 is a schematic cross-sectional view showing one embodiment of a methane production unit. The methane production unit 3 shown in Figure 2 includes a reactor 3a and an insulating material 3b. The reactor 3a includes an inner cylindrical portion 3a1 and an outer cylindrical portion 3a2. The inner cylindrical portion 3a1 and the outer cylindrical portion 3a2 are cylindrical in shape and may be arranged concentrically when viewed from their central axis. The space between the inner cylindrical portion 3a1 and the outer cylindrical portion 3a2 is configured as a containment space for accommodating a catalyst CT.

[0031] The catalyst CT may be, for example, a cylindrical pellet with a diameter and height of several millimeters. The catalyst CT may be composed of a material that has high activity in the methanation reaction of carbon dioxide, and may be composed of Ni or a Ni alloy supported on a catalyst support. Examples of catalyst supports include Al2O3, Cr2O3, SiO2, MgAl2O4, TiO2, and ZrO2.

[0032] As shown in Figure 2, carbon dioxide recovered in the carbon dioxide recovery unit 2 or purified in the carbon dioxide purification unit 2A, along with hydrogen supplied from an external source, are introduced from above the reactor 3a. These then flow downstream while coming into contact with the catalyst CT inside the reactor 3a. In this process, carbon dioxide and hydrogen react via the catalyst CT to produce methane in the reactor 3a. To improve the reaction rate of methane, the methane production unit 3 may be configured with multiple reactors 3a to carry out the reaction in multiple stages. The produced methane is discharged from below the reactor 3a. The hydrogen may be, for example, hydrogen stored in a cylinder, or hydrogen obtained by the electrolysis of water.

[0033] In this way, methane can be produced using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides. The produced methane can then be used as thermal energy, for example, by combustion. [Examples]

[0034] The present disclosure will be described in more detail below with reference to examples. However, the present disclosure is not limited to these examples.

[0035] (Example 1) [Methane production test] <Removal of nitrogen oxides> Figure 3 is a schematic diagram showing an apparatus for removing nitrogen oxides from a mixed gas in an embodiment. The apparatus shown in Figure 3 is a wet scrubber as the gas-liquid contact part. The apparatus (wet scrubber) shown in Figure 3 consists of a processing tower 10 provided with a gas inlet 11 and a gas outlet 12, a filling section 21 for filling the processing tower 10 with packing material, a water storage section 22 for storing an alkaline aqueous solution 20 placed in the processing tower 10, a water spraying means 23 for spraying the alkaline aqueous solution 20 placed in the processing tower 10 onto the packing material in the filling section 21, and a circulation means 24 for circulating the alkaline aqueous solution 20 from the water storage section 22 to the water spraying means 23. The gas inlet 11 is for introducing the mixed gas G0, and the gas outlet 12 is for discharging the mixed gas G1 after it has come into contact with the alkaline aqueous solution 20. As the packing material, for example, a resin packing material is used. An alkaline aqueous solution 20 is sprayed into the filling section 21 by a water spraying means 23, and the alkaline aqueous solution 20 and the mixed gas G0 come into countercurrent contact in the filling section 21. A demister (mist separator) 13 is provided near the gas outlet 12.

[0036] 100 L of alkaline aqueous solution 20 (base: NaHCO3 (concentration: 1.0 mol / L), oxidizing agent: KMnO4 (concentration: 0.013 mol / L)) was stored in the water storage means 31. Factory exhaust gas A was used as the mixed gas G0. The mixed gas G0 was introduced into the treatment tower 10 from the gas inlet 11 at a flow rate of 300 L / min. In the filling section 21 where the alkaline aqueous solution 20 was sprayed by the water spraying means 23, the alkaline aqueous solution 20 and the mixed gas G0 were brought into countercurrent contact to obtain the mixed gas G1 after contact with the alkaline aqueous solution 20. A portion of each of the mixed gas G0 and mixed gas G1 was separated and analyzed using a multi-gas analyzer (VA-5112 model, manufactured by Horiba, Ltd.) to determine the nitrogen oxides (NOx) in the mixed gas G0 and mixed gas G1. X The concentration of ) was measured. The results are shown in Table 1.

[0037] <Carbon dioxide capture> Carbon dioxide (CO2) was recovered from the resulting mixed gas G1 using a chemical absorption method with amines.

[0038] <Methane production> The recovered carbon dioxide (CO2) and hydrogen (H2) stored in cylinders were introduced into a methanation unit (manufactured by Hitachi Zosen Corporation) to produce methane by reacting the carbon dioxide (CO2) and hydrogen (H2). The carbon dioxide (CO2) was introduced at a flow rate of 1.66 L / min, and the hydrogen (H2) at a flow rate of 6.68 L / min. The reaction between carbon dioxide (CO2) and hydrogen (H2) was carried out under external heater temperature of 230°C and back pressure of 500 kPa. Continuous operation was performed for 50 hours after the start of the reaction, and the methane production concentration was measured every 10 hours. The results are shown in Table 1. Note that the values ​​in Table 2 represent the concentration ratio at each elapsed time, with the methane production concentration at 10 hours after the start of the reaction being set to 1.

[0039] (Example 2) Except for changing factory exhaust gas A, which is used as mixed gas G0, to factory exhaust gas B, which is different from factory exhaust gas A, nitrogen oxides were removed, carbon dioxide was recovered, and methane was produced in the same manner as in Example 1. The results are shown in Table 2.

[0040] (Example 3) Except for changing factory exhaust gas A, which is used as mixed gas G0, to factory exhaust gas C, which is different from factory exhaust gas A, nitrogen oxides were removed, carbon dioxide was recovered, and methane was produced in the same manner as in Example 1. The results are shown in Table 2.

[0041] (Comparative Example 1) Except for changing the factory exhaust gas A, which is used as mixed gas G0, to a different factory exhaust gas D, and not performing nitrogen oxide removal, carbon dioxide recovery and methane production were carried out in the same manner as in Example 1. The results are shown in Table 2.

[0042] (Reference example 1) Methane was produced in the same manner as in Example 1, except that nitrogen oxides were not removed and carbon dioxide was not recovered, and liquefied carbon dioxide was used as the carbon dioxide (CO2). The results are shown in Table 2. It is presumed that liquefied carbon dioxide does not contain nitrogen oxides.

[0043] [Table 1]

[0044] [Table 2]

[0045] As shown in Table 2, in Examples 1-3, where the nitrogen oxide concentration in mixed gas G1 after contact with the alkaline aqueous solution was 100 ppm by volume or less, the difference between the methane concentration ratio after 10 hours from the start of the reaction and the methane concentration ratio after 50 hours from the start of the reaction was smaller compared to Comparative Example 1, where the nitrogen oxide concentration was greater than 100 ppm by volume. This suggests that the deterioration of the methane production unit (e.g., catalyst deterioration) was suppressed. These results confirm that the method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides according to this disclosure can stably produce methane. [Explanation of Symbols]

[0046] 1...Gas-liquid contact section, 2...Carbon dioxide recovery section, 2A...Carbon dioxide purification section, 3...Methane production section, 3a...Reactor, 3a1...Inner cylinder section, 3a2...Outer cylinder section, 3b...Insulation material, 5...Methane production apparatus, 10...Processing tower, 11...Gas inlet, 12...Gas outlet, 13...Demister (mist separator), 20...Alkaline aqueous solution, 21...Filling section, 22...Water storage section, 23...Water spraying means, 24...Circulation means, CT...Catalyst, G0...Mixed gas (untreated), G1...Mixed gas (after contact with water or aqueous solution), G2...Mixed gas (after carbon dioxide recovery).

Claims

1. A method for producing methane using carbon dioxide recovered from a mixed gas containing carbon dioxide and nitrogen oxides, A first step of reducing the concentration of nitrogen oxides in the mixed gas by bringing the mixed gas into contact with water or an aqueous solution, A second step of recovering carbon dioxide from the mixed gas after contact with the water or aqueous solution, A third step involves reacting the recovered carbon dioxide with hydrogen to produce methane, Equipped with, The concentration of nitrogen oxides in the mixed gas after the first step is 100 ppm by volume or less. method.

2. The first step is to bring the mixed gas into contact with an alkaline aqueous solution. The method according to claim 1.

3. The aforementioned alkaline aqueous solution contains at least one selected from the group consisting of carbonate ions and bicarbonate ions. The method according to claim 2.

4. The aforementioned alkaline aqueous solution contains ammonia. The method according to claim 2.

5. Between the second step and the third step, the method further includes a step of reducing the concentration of nitrogen oxides in the gas containing carbon dioxide recovered in the second step. The method according to any one of claims 1 to 4.

6. The aforementioned mixed gas further contains sulfur oxides, The first step is to reduce the concentration of nitrogen oxides and sulfur oxides in the mixed gas by bringing it into contact with water or an aqueous solution. The method according to any one of claims 1 to 4.

7. Between the second step and the third step, the method further includes a step of reducing the concentrations of nitrogen oxides and sulfur oxides in the gas containing carbon dioxide recovered in the second step. The method according to claim 6.

8. The aforementioned mixed gas is exhaust gas. The method according to any one of claims 1 to 4.

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