Methane production method

Abstract

Provided is a methane production method capable of stably and efficiently producing methane. A methane production method according to one embodiment of the present invention comprises: a step for preparing a raw material gas by adding hydrogen gas to a gas mixture containing carbon dioxide gas and oxygen gas; and a step for supplying the raw material gas to a gas flow path included in a methane production device. In the step for adding hydrogen gas to the gas mixture, the addition amount A of the hydrogen gas satisfies formula (1).　(1): A = {z × (c1 / 100) × x} + {z × (c2 / 100) × y} (In formula (1), A represents the addition amount of hydrogen gas. z represents the flow rate of the gas mixture. c1 represents the concentration of oxygen in the gas mixture. c2 represents the concentration of carbon dioxide in the gas mixture. x represents 2.0. y represents a numerical value of 4.0 or more.)

Description

Methane production method

[0001] The present invention relates to a method for producing methane.

[0002] In recent years, from the viewpoint of reducing environmental load, studies have been conducted on recovering carbon dioxide and reusing it as a raw material for carbon compounds. For example, a method for producing methane has been proposed in which a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas is supplied to a reactor equipped with a catalyst, and a methanation reaction is initiated and continued by heat including reaction heat from catalytic combustion of the hydrogen gas (see, for example, Patent Document 1).

[0003] International Publication No. 2021 / 045101

[0004] Exhaust gas emitted from various industrial facilities may contain carbon dioxide gas and oxygen gas. Therefore, it has been considered to use a mixed gas obtained by adding hydrogen gas to the exhaust gas as a feedstock gas for the methane production method described in Patent Document 1. However, the composition of exhaust gas emitted from industrial facilities may change over time. Adding hydrogen gas to the exhaust gas and supplying it to a reactor may result in unstable methane production. More specifically, when the oxygen concentration in the exhaust gas increases, the amount of hydrogen gas burned out of the added hydrogen gas increases, and the amount of hydrogen gas used in the methanation reaction decreases. This reduces the methane production efficiency. Furthermore, when the oxygen concentration in the exhaust gas decreases, the amount of hydrogen gas burned decreases, resulting in a decrease in the heat of reaction generated. As a result, the carbon dioxide methanation reaction may not be able to continue stably. The main object of the present invention is to provide a methane production method that can produce methane stably and efficiently.

[0005] [1] A methane production method according to one embodiment of the present invention produces methane from a feed gas containing carbon dioxide gas, oxygen gas, and hydrogen gas. The methane production method includes the steps of: preparing a feed gas by adding hydrogen gas to a mixed gas containing carbon dioxide gas and oxygen gas; and supplying the feed gas to a gas flow path of a methane production apparatus. In the step of adding hydrogen gas to the mixed gas, the amount A of hydrogen gas added satisfies the following formula (1): A = {z × (c1 / 100) × x} + {z × (c2 / 100) × y} (1) (In formula (1), A represents the amount of hydrogen gas added, z represents the flow rate of the mixed gas, c1 represents the oxygen concentration in the mixed gas, c2 represents the carbon dioxide concentration in the mixed gas, x represents 2.0, and y represents a value equal to or greater than 4.0.) [2] In the methane production method described in [1] above, y in formula (1) may be 10.0 or less. [3] The methane production method according to [1] or [2] above may include, in this order, a step of measuring an oxygen concentration c1 in the mixed gas and a step of measuring a carbon dioxide concentration c2 in the mixed gas before the step of adding hydrogen gas to the mixed gas. [4] In the methane production method according to any one of [1] to [3] above, the methane production apparatus may include a honeycomb substrate and a catalyst layer. The honeycomb substrate has partition walls defining a plurality of cells. At least some of the plurality of cells include the gas flow channels. The catalyst layer is provided on the surfaces of the partition walls. The catalyst layer includes a methanation catalyst. The methanation catalyst may be capable of promoting a first reaction of reacting oxygen gas with hydrogen gas to produce water and a second reaction of reacting carbon dioxide gas with hydrogen gas to produce methane. [5] In the methane production method according to any one of [1] to [4] above, the methanation catalyst may include a transition metal as an active component. [6] In the method for producing methane according to the above [5], the transition metal may include Co, Fe, Pt, Ru, Rh, Pd, Ni, Cu, Ag, Au, Ir, or a combination thereof. [7] In the method for producing methane according to the above [5] or [6], the transition metal may include Ni.[8] In the method for producing methane according to any one of [5] to [7] above, the methanation catalyst may further contain a support supporting the transition metal. The support may contain cerium oxide, silicon oxide, zirconium oxide, yttrium oxide, aluminum oxide, or a composite oxide thereof. [9] In the method for producing methane according to [8] above, the support may contain cerium oxide.

[0006] According to an embodiment of the present invention, methane can be produced stably and efficiently.

[0007] Fig. 1 is a schematic perspective view of a methane production apparatus used in a methane production method according to one embodiment of the present invention. Fig. 2 is a schematic cross-sectional view of the methane production apparatus of Fig. 1. Fig. 3 is a schematic configuration diagram of a methane production system capable of carrying out a methane production method according to one embodiment of the present invention.

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In addition, in order to clarify the description, the drawings may show the width, thickness, shape, etc. of each part more schematically than in the embodiment, but these are merely examples and do not limit the interpretation of the present invention.

[0009] A. Overview of the Methane Production Method A methane production method according to one embodiment of the present invention is capable of producing methane from a feed gas containing carbon dioxide gas, oxygen gas, and hydrogen gas, and includes a hydrogenation step and a reaction step. In the hydrogenation step, hydrogen gas is added to a mixed gas containing carbon dioxide gas and oxygen gas to prepare a feed gas. The mixed gas is typically an exhaust gas emitted from various industrial products or industrial facilities (hereinafter referred to as industrial products / facilities). Examples of industrial products / facilities include combustion devices, thermal power plants, and factories. In the reaction step, the feed gas prepared in the hydrogenation step is supplied to a gas flow path of the methane production apparatus. In the hydrogenation step, the amount A of hydrogen gas added satisfies the following formula (1): A = {z × (c1 / 100) × x} + {z × (c2 / 100) × y} (1) (In formula (1), A is the amount of hydrogen gas added [unit: m ３ / hr], z is the flow rate of the mixed gas [unit: m ３ / hr]. c1 represents the oxygen concentration in the mixed gas [unit: vol %]. c2 represents the carbon dioxide concentration in the mixed gas [unit: vol %]. x represents 2.0. y represents a value of 4.0 or more.) According to this method, the amount of hydrogen gas added to the mixed gas is adjusted to satisfy the above formula (1), so even if the composition of the mixed gas fluctuates, an appropriate amount of hydrogen gas according to the composition of the mixed gas can be added to the mixed gas. Therefore, when the prepared raw material gas is supplied to the gas flow path of the methane production apparatus, the first reaction represented by the following formula (I) and the second reaction represented by the following formula (II) can be stably and smoothly progressed. O ２ +2H ２ →2H ２ O...(I) CO ２ +4H ２ →CH ４ +2H ２ O... (II) The first reaction is a combustion reaction of hydrogen gas, in which oxygen and hydrogen react to produce water. The second reaction is a methanation reaction of carbon dioxide gas, in which carbon dioxide and hydrogen react to produce methane. Because both the first and second reactions are exothermic reactions, the generated heat (reaction heat) can be effectively utilized to advance and continue the reaction process. Therefore, methane gas can be efficiently produced in an energy-saving manner in the reaction process, and as a result, methane can be produced stably and efficiently. Such a methane production method can be suitably applied to the continuous industrial production of methane.

[0010] In the above formula (1), y is, for example, 10.0 or less, preferably 7.0 or less, and more preferably 5.0 or less. When y in the above formula (1) is equal to or less than this upper limit, the amount of hydrogen gas remaining unreacted in the reaction step can be reduced. As a result, the cost and energy required for separating the produced methane gas from the unreacted hydrogen gas can be reduced, and the methane gas can be efficiently separated from the hydrogen gas.

[0011] The methane production method typically further includes an oxygen concentration measurement step and a carbon dioxide concentration measurement step before the hydrogenation step. In the oxygen concentration measurement step, the oxygen concentration c1 in the mixed gas is measured. In the carbon dioxide concentration measurement step, the carbon dioxide concentration c2 in the mixed gas is measured. This makes it possible to accurately calculate the amount A of hydrogen gas to be added to the mixed gas in the hydrogenation step, and to improve the methane conversion rate in the reaction step.

[0012] The oxygen concentration measurement step may be performed before or after the carbon dioxide concentration measurement step. In one embodiment, the methane production method includes an oxygen concentration measurement step and a carbon dioxide concentration measurement step, in this order. The first reaction (combustion reaction of hydrogen gas) shown in the above formula (I) usually proceeds prior to the second reaction (methanation reaction of carbon dioxide gas) shown in the above formula (II). In one embodiment, the second reaction (methanation reaction of carbon dioxide gas) shown in the above formula (II) proceeds after the completion of the first reaction (combustion reaction of hydrogen gas) shown in the above formula (I). Therefore, if the oxygen concentration c1 in the mixed gas is measured before the carbon dioxide concentration c2, the amount of hydrogen gas (z × c1 / 100 × x) required for the first reaction (combustion reaction of hydrogen gas) can be accurately calculated from the oxygen concentration c1. As a result, in the reaction step, the first reaction (combustion reaction of hydrogen gas) can be completed early, and sufficient reaction time can be ensured for the second reaction (methanation reaction of carbon dioxide gas).

[0013] B. Details of the methane production method according to one embodiment will now be described in detail. In one embodiment, the methane production method includes a flow rate measurement step and a separation / purification step in addition to the oxygen concentration measurement step, carbon dioxide concentration measurement step, hydrogenation step, and reaction step described above.

[0014] B-1. Flow rate measurement step A mixed gas containing carbon dioxide gas and oxygen gas is typically discharged from the above-mentioned industrial facilities and then transported toward the gas flow path of a methane production plant. In the flow rate measurement step, the flow rate z [unit: m ３ / hr] is measured by any suitable method.

[0015] The flow rate measuring step may be performed before the oxygen concentration measuring step and the carbon dioxide concentration measuring step, after the oxygen concentration measuring step and the carbon dioxide concentration measuring step, or between the oxygen concentration measuring step and the carbon dioxide concentration measuring step. In one embodiment, the flow rate measuring step is performed before the oxygen concentration measuring step and the carbon dioxide concentration measuring step.

[0016] The flow rate z of the mixed gas can be measured using, for example, a thermal flow meter, a differential pressure flow meter, or a Coriolis flow meter. The temperature of the mixed gas is, for example, 25°C to 500°C, and preferably 50°C to 300°C.

[0017] In the oxygen concentration measurement step, the oxygen concentration c1 [unit: vol%] in the mixed gas is measured by any appropriate method. In one embodiment, the oxygen concentration measurement step is performed after the flow rate measurement step and before the carbon oxide concentration measurement step.

[0018] The oxygen concentration c1 in the mixed gas may be measured by, for example, a zirconia method, a laser method, a porcelain method, or an electrode method. The oxygen concentration c1 in the mixed gas is, for example, 0.5% by volume to 10% by volume, and preferably 1.0% by volume to 5.0% by volume.

[0019] B-3. ​​Carbon dioxide concentration measurement step In the carbon dioxide concentration measurement step, the carbon dioxide concentration c2 [unit: vol%] in the mixed gas is measured by any appropriate method. In one embodiment, the carbon dioxide concentration measurement step is performed after the oxygen concentration measurement step and before the hydrogenation step.

[0020] The carbon dioxide concentration c2 in the mixed gas is, for example, 0.1% by volume to 20% by volume, and preferably 1.0% by volume to 15% by volume, by way of example, non-dispersive infrared absorption.

[0021] B-4. Hydrogen Addition Step In one embodiment, in the hydrogen addition step, the amount of hydrogen gas added to the mixed gas A [unit: m ３ Next, hydrogen gas is added to the mixed gas at the same pressure as the mixed gas and in the calculated amount A. As a result, carbon dioxide gas, oxygen gas, and hydrogen gas are mixed in an appropriate ratio to prepare a source gas.

[0022] B-5. Reaction Step In the reaction step, the raw material gas prepared in the hydrogenation step is supplied to a gas flow path of the methane production apparatus.

[0023] B-5-1. Methane Production Apparatus A methanation reaction catalyst is typically disposed in the gas flow path of a methane production apparatus. The methanation reaction catalyst can promote the first reaction and the second reaction described above.

[0024] The methanation catalyst contains any suitable metal element as an active component. The methanation catalyst may contain a metal element in a metallic state, a salt of the metal element, or an oxide of the metal element. The methanation catalyst preferably contains a metal element in a metallic state.

[0025] Examples of metal elements include alkali metals, alkaline earth metals, and transition metals, and preferably transition metals. Specific examples of transition metals include Co, Fe, Pt, Ru, Rh, Pd, Ni, Cu, Ag, Au, and Ir. These transition metals may be used alone or in combination. In one embodiment, the methanation catalyst contains Ni. When the methanation catalyst contains Ni, the first reaction (combustion reaction of hydrogen gas) and the second reaction (methanation reaction of carbon dioxide gas) can be further promoted.

[0026] The methanation catalyst may further contain a support in addition to the above-described active component. The support is capable of supporting the active component (typically a transition metal). The support is composed of any appropriate inorganic material depending on the application. Examples of inorganic materials include oxides, carbides, nitrides, sulfides, halides, hydrogen compounds, and hydroxides. The inorganic materials may be used alone or in combination.

[0027] Among such inorganic materials, oxides are preferred. Specific examples of oxides include cerium oxide, silicon oxide, zirconium oxide, yttrium oxide, aluminum oxide, and composite oxides thereof. In one embodiment, the support contains cerium oxide. When the above-mentioned active component (particularly Ni) is supported on a support containing such an oxide, the activity of the methanation reaction catalyst can be stably improved.

[0028] When the methanation catalyst contains an active component and a support, the content of the active component is 0.01 to 80 parts by mass, preferably 1 to 50 parts by mass, and more preferably 1 to 20 parts by mass, relative to 100 parts by mass of the support. When the content of the active component is within this range, the activity of the methanation catalyst can be improved more stably.

[0029] The mass of the methanation catalyst per unit volume of the gas flow passage is, for example, 30 g / L or more, preferably 50 g / L or more, while the upper limit of the mass of the methanation catalyst per unit volume of the gas flow passage is typically 1000 g / L or less.

[0030] As shown in FIGS. 1 and 2 , in one embodiment, the methane production apparatus 1 has a flow-through structure. The methane production apparatus 1 includes a honeycomb substrate 11 and a catalyst layer 12. The honeycomb substrate 11 includes partition walls 13 that define a plurality of cells 14. At least some of the cells 14 include gas flow channels 15. In the illustrated example, all of the cells 14 include the gas flow channels 15. The catalyst layer 12 is provided on the surface of the partition walls 13. The catalyst layer 12 includes the methanation catalyst described above. With this configuration, the feed gas can be efficiently brought into contact with the methanation catalyst in the catalyst layer when the feed gas passes through the gas flow channels. This allows the first reaction (combustion reaction of hydrogen gas) and the second reaction (methanation reaction of carbon dioxide gas) to proceed smoothly, further improving the methane conversion rate.

[0031] The honeycomb substrate 11 has any appropriate shape (overall shape). Examples of the shape of the honeycomb substrate 11 include a cylindrical shape with a circular bottom, an elliptical cylindrical shape with an elliptical bottom, a rectangular prism with a polygonal bottom, and a cylindrical shape with an irregular bottom. In one embodiment, the honeycomb substrate 11 has a cylindrical shape. The outer diameter and length of the honeycomb substrate 11 can be appropriately set depending on the purpose.

[0032] In the illustrated example, the honeycomb substrate 11 includes outer walls 16 and partition walls 13 located inside the outer walls 16. The outer walls 16 and the partition walls 13 may be integrally formed or may be separate bodies. In the illustrated example, the outer walls 16 and the partition walls 13 are integrally formed.

[0033] The outer wall 16 has a cylindrical shape. The thickness of the outer wall 16 is set arbitrarily and appropriately. The thickness of the outer wall 16 is, for example, 1 mm to 10 mm, or, for example, 2 mm to 8 mm.

[0034] As described above, the partition walls 13 define a plurality of cells 14. The cells 14 extend in the longitudinal direction (axial direction) of the honeycomb substrate 11 from the first end face E1 (inlet end face) to the second end face E2 (outlet end face) of the honeycomb substrate 11 (see FIG. 2 ). The cells 14 have any appropriate shape in a cross section perpendicular to the longitudinal direction of the honeycomb substrate 11. Examples of the cross-sectional shape of the cells include a triangle, a rectangle, a pentagon, a polygon having hexagons or more, a circle, and an ellipse. The cross-sectional shapes and sizes of the cells may all be the same, or at least some of them may be different. Among such cross-sectional shapes of the cells, a rectangle is preferred, and a square or rectangle is more preferred.

[0035] The cell density of the honeycomb substrate 11 is, for example, 40 cpsi or more, preferably 50 cpsi or more, and more preferably 100 cpsi or more. On the other hand, the cell density of the honeycomb substrate 11 is, for example, 1000 cpsi or less, and preferably 900 cpsi or less. When the cell density is in this range, the raw material gas can be brought into contact with the catalyst layer more efficiently. In this specification, the "cell density of the honeycomb substrate" means the cell density of the cross section in the length direction (direction in which the cells extend) of the honeycomb substrate, and "cpsi" means the cell density of 6.4516 cm of the cross section. ２ This refers to the number of cells per square inch.

[0036] In the illustrated example, the partition walls 13 have first partition walls 13a and second partition walls 13b that are perpendicular to each other, and the first partition walls 13a and the second partition walls 13b define a plurality of cells 14. The cross-sectional shape of the cells 14 is quadrangular except for the portions where the first partition walls 13a and the second partition walls 13b contact the outer wall 16. The configuration of the partition walls is not limited to the above-described partition walls 13. The partition walls may have first partition walls extending in the radial direction and second partition walls extending in the circumferential direction, which define a plurality of cells.

[0037] The thickness of the partition walls 13 can be set arbitrarily and appropriately. The thickness of the partition walls 13 is typically thinner than the thickness of the outer walls 16. The thickness of the partition walls 13 is, for example, 0.0508 mm or more, preferably 0.0635 mm or more. On the other hand, the thickness of the partition walls 13 is, for example, 1.52 mm or less, preferably 1.27 mm or less. When the thickness of the partition walls is in this range, the mechanical strength of the honeycomb substrate can be sufficient, and the cell density can be adjusted to the above-mentioned range. The thickness of the partition walls is measured, for example, by cross-sectional observation using a scanning electron microscope (SEM).

[0038] The partition walls 13 may or may not have pores. The porosity of the partition walls 13 can be appropriately set depending on the purpose. The porosity of the partition walls 13 is, for example, 70% or less, and preferably 65% ​​or less. On the other hand, the porosity of the partition walls 13 is, for example, 0% or more. The porosity is measured by, for example, mercury intrusion porosimetry.

[0039] The bulk density of the partition walls 13 can be appropriately set depending on the purpose. The bulk density of the partition walls 13 is, for example, 1.0 g / cm ３ ~3.0 g / cm ３ and preferably 2.0 g / cm ３ ~3.0 g / cm ３ The bulk density is measured by, for example, the Archimedes method.

[0040] Examples of materials that can be used to form the honeycomb substrate 11 include ceramic materials. Specific examples of ceramic materials include zirconia-based materials, alumina-titanium carbide-based composite materials, Si-SiC-based composite materials, aluminum nitride, aluminum oxide, silicon nitride, silicon carbide, zirconia, cordierite, and mullite. These ceramic materials can be used alone or in combination. Of these ceramic materials, cordierite and Si-SiC-based composite materials are preferred.

[0041] The thermal conductivity of such a honeycomb substrate 11 is, for example, 1 W / m·K or more, preferably 5 W / m·K or more, and more preferably 8 W / m·K or more. If the thermal conductivity of the honeycomb substrate (each of the partition walls and outer walls) is equal to or greater than this lower limit, the thermal uniformity of the methane production apparatus in the reaction step can be improved, and the methane conversion rate can be stably improved. On the other hand, the upper limit of the thermal conductivity of the honeycomb substrate 11 is not limited, but is typically 1000 W / m·K. The thermal conductivity is measured, for example, using an optical alternating current thermal diffusivity measuring device.

[0042] The catalyst layer 12 is formed on the surface of the partition wall 13. In the methane production apparatus 1, gas flow paths 15 are formed in portions (typically central portions) of the cross section of the cells 14 where the catalyst layer 12 is not formed. The catalyst layer 12 may be formed on the entire inner surface of the partition wall 13 (i.e., so as to surround the gas flow path 15) as in the illustrated example, or may be formed on a portion of the surface of the partition wall. When the catalyst layer 12 is formed on the entire inner surface of the partition wall 13, the methane conversion rate can be improved more stably.

[0043] The gas flow channels 15 are spaces formed inside the cells 14, and extend from a first end face E1 (inlet end face) to a second end face E2 (outlet end face) similarly to the cells 14 (see FIG. 2). The cross-sectional shape of the gas flow channels 15 may be the same as that of the cells 14, preferably a quadrangle, and more preferably a square or rectangle. The cross-sectional shapes and sizes of the gas flow channels 15 may all be the same, or at least some may be different.

[0044] As described above, the catalyst layer 12 contains a methanation catalyst. The methanation catalyst contained in the catalyst layer 12 may have any appropriate shape. The methanation catalyst is typically in a particulate form. Hereinafter, particulate methanation catalysts may be referred to as catalyst particles. In one embodiment, the catalyst layer 12 contains aggregates formed by aggregating a plurality of catalyst particles. The aggregates of the plurality of catalyst particles may form mesopores in the catalyst layer 12.

[0045] The content of the methanation catalyst in the catalyst layer 12 is, for example, 10% by mass to 100% by mass, and preferably 50% by mass to 100% by mass. When the content of the methanation catalyst is within this range, the mass of the methanation catalyst per unit volume of the gas flow channel can be stably adjusted within the above range.

[0046] The catalyst layer 12 may contain an additive in addition to the methanation catalyst. Examples of the additive include a filler, a binder, and a heat transfer material. The additives may be used alone or in combination. The additive is added in an amount of, for example, 0.1 to 90 parts by mass, and preferably 0.1 to 50 parts by mass, per 100 parts by mass of the methanation catalyst.

[0047] The thickness of the catalyst layer 12 is, for example, 0.1 μm to 3000 μm, and preferably 20 μm to 300 μm. The average pore diameter of the catalyst layer 12 is, for example, 0.01 μm to 30 μm, and preferably 1 μm to 20 μm. The porosity of the catalyst layer 12 is, for example, 1% to 80%, and preferably 2% to 50%.

[0048] B-5-2. Details of the Reaction Step In one embodiment, before the feed gas is supplied, the methane production apparatus is heated to a reaction initiation temperature. The reaction initiation temperature is, for example, 100°C or higher, preferably 150°C or higher. On the other hand, the upper limit of the reaction initiation temperature is typically 600°C.

[0049] Next, the raw material gas is supplied to the gas flow path of the methane production apparatus heated to the reaction initiation temperature. This causes the raw material gas to flow into the gas flow path and come into contact with the methanation catalyst heated to the reaction initiation temperature. Typically, the first reaction (hydrogen combustion reaction) described above proceeds preferentially over the second reaction (carbon dioxide methanation reaction). At this time, reaction heat is generated by the first reaction and is used to continue the first reaction and to start the second reaction (carbon dioxide methanation reaction). Once the second reaction starts, reaction heat is also generated by the second reaction. As a result, the first and second reactions are continued using the reaction heat, and the second reaction continues stably even after the first reaction is completed. Therefore, in one embodiment, external heating of the methane production apparatus is stopped.

[0050] When external heating of the methane production apparatus is stopped, the temperature (reaction temperature) in the gas flow passage of the methane production apparatus is maintained at, for example, 250°C to 450°C by the heat of reaction. Therefore, in one embodiment, the methane production apparatus is capable of methanation. When the reaction temperature is in this range, the second reaction (the methanation reaction of carbon dioxide) can be continued more stably.

[0051] As a result, carbon dioxide and hydrogen react in the gas flow path of the methane production apparatus to produce methane gas. Thereafter, methane-containing gas is continuously discharged from the gas flow path of the methane production apparatus. The methane conversion rate in the reaction step is the percentage of the amount of methane gas contained in the methane-containing gas relative to the amount of carbon dioxide contained in the raw material gas, and is, for example, 65% or more, preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more. On the other hand, the methane conversion rate is, for example, 100% or less, or, for example, 95% or less. When the methane conversion rate is in this range, it is possible to suppress the remaining unreacted carbon dioxide gas in the methane-containing gas. Therefore, it is possible to reduce the energy and cost required for the separation / purification step. The pressure range of the methane-containing gas is, for example, the same as the reaction pressure range described above. The temperature range of the methane-containing gas is, for example, the same as the reaction temperature range described above.

[0052] The methane-containing gas may contain water vapor in addition to methane gas. The content of methane gas in the methane-containing gas is, for example, 1% by volume or more, preferably 10% by volume or more.

[0053] The methane-containing gas may contain, in addition to methane gas and water vapor, a raw material gas (oxygen, hydrogen, and / or carbon dioxide) remaining in the reaction step. In one embodiment, the amount A of hydrogen gas added to the mixed gas in the hydrogen addition step satisfies the above formula (1), so that the raw material gas can be sufficiently consumed in the reaction step. Therefore, the amount of raw material gas mixed into the methane-containing gas can be reduced.

[0054] B-6. Separation / Purification Step In the separation / purification step, typically, methane is separated and purified from the methane-containing gas discharged from the gas flow path by any appropriate method. Examples of methods for separating and purifying methane include membrane separation using a methane separation membrane and pressure swing adsorption (PSA) using an adsorbent. A single separation and purification method may be performed, or two or more types may be combined. Among the methods for separating and purifying methane, membrane separation is preferred. This makes it possible to recover high-purity methane. The methane recovered in the separation / purification step may be in a gaseous state or a liquid state. The purity of the methane is, for example, 50% to 100% by volume.

[0055] C. Methane Production System Next, a methane production system 100 capable of implementing a methane production method according to one embodiment will be described with reference to Figure 3. The methane production system 100 includes an industrial product / facility 2, a mixed gas supply unit 3, a hydrogen addition unit 4, a methane production apparatus 1, a methane recovery unit 5, and a control unit 10.

[0056] C-1. Industrial Product / Facility The industrial product / facility 2 is any of various industrial products or industrial facilities that discharge a mixed gas containing carbon dioxide gas and oxygen gas. The industrial product / facility 2 is not particularly limited. The industrial product / facility 2 typically has an exhaust port that discharges the mixed gas.

[0057] C-2. Mixed Gas Supply Unit In one embodiment, the mixed gas supply unit 3 includes a first gas supply line 31. The first gas supply line 31 is typically a pipe for supplying the mixed gas discharged from the industrial product / facility 2 to the methane production apparatus 1. The upstream end of the first gas supply line 31 in the supply direction of the raw material gas is connected to the industrial product / facility 2 so as to communicate with the exhaust port. The downstream end of the first gas supply line 31 in the supply direction of the mixed gas is connected to the methane production apparatus 1 so as to communicate with the end of the gas flow path 15 on the first end face E1 side (see FIG. 2 ).

[0058] In one embodiment, the first gas supply line 31 is provided with a flow meter 32 , an oxygen concentration meter 33 , and a carbon dioxide concentration meter 34 .

[0059] The flow meter 32 is capable of performing the flow rate measurement process described above. The flow meter 32 is capable of measuring the flow rate z of the mixed gas passing through the first gas supply line 31. Typical examples of the flow meter 32 include a thermal flow meter, a differential pressure flow meter, and a Coriolis flow meter.

[0060] The oxygen concentration meter 33 is capable of performing the oxygen concentration measurement process described above. The oxygen concentration meter 33 is capable of measuring the oxygen concentration c in the mixed gas passing through the first gas supply line 31. In the illustrated example, the oxygen concentration meter 33 is provided on the first gas supply line 31 on the opposite side of the flow path meter 32 from the industrial product / facility 2. Representative oxygen concentration meters 33 include zirconia type, laser type, porcelain type, and electrode type.

[0061] The carbon dioxide concentration meter 34 is capable of carrying out the carbon dioxide concentration measurement process described above. The carbon dioxide concentration meter 34 is capable of measuring the carbon dioxide concentration c2 in the mixed gas passing through the first gas supply line 31. In the illustrated example, the carbon dioxide concentration meter 34 is provided on the first gas supply line 31 on the opposite side of the flow meter 32 with respect to the oxygen concentration meter 33. A representative example of the carbon dioxide concentration meter 34 is a gas sensor using a non-dispersive infrared absorption method.

[0062] C-3. Hydrogen Addition Unit In one embodiment, the hydrogen addition unit 4 includes a hydrogen production unit 42 , a second gas supply line 41 , and a pump 43 .

[0063] The hydrogen production unit 42 is capable of producing hydrogen gas. Examples of the hydrogen production unit 42 include a water electrolysis device that produces hydrogen gas by electrolyzing water, an ammonia decomposition device that produces hydrogen gas by cracking ammonia, and a dehydrogenation device that releases hydrogen gas from a liquid organic carrier.

[0064] The second gas supply line 41 is typically a pipe for supplying hydrogen gas produced in the hydrogen production unit 42 to the first gas supply line 31. An upstream end of the second gas supply line 41 in the supply direction of hydrogen gas is connected to the hydrogen production unit 42. A downstream end of the second gas supply line 41 in the supply direction of hydrogen gas is connected to the gas supply line 31. In the illustrated example, the downstream end of the second gas supply line 41 is connected to a portion of the gas supply line 31 between the carbon dioxide concentration meter 34 and the methane production apparatus 1.

[0065] The pump 43 is provided on the second gas supply line 41. The pump 43 is capable of adjusting the flow rate of the hydrogen gas passing through the second gas supply line 41. The pump 43 has any appropriate configuration.

[0066] C-4. Methane Production Apparatus The methane production apparatus 1 is capable of carrying out the reaction step described above. In one embodiment, the methane production apparatus 1 includes the honeycomb substrate 11 described above and the catalyst layer 12 described above. Therefore, a description of the methane production apparatus 1 will be omitted. The methane production apparatus 1 is connected to the methane recovery unit 5 via a connecting line 6. The connecting line 6 is typically a pipe for supplying the methane-containing gas discharged from the methane production apparatus 1 to the methane recovery unit 5. The upstream end of the connecting line 6 in the supply direction of the methane-containing gas is connected to the methane production apparatus 1 so as to communicate with the end of the gas flow channel 15 on the second end face E2 side (see FIG. 2 ). The downstream end of the connecting line 6 in the supply direction of the methane-containing gas is connected to a methane separation device 51, which will be described later.

[0067] C-5. Methane Recovery Section The methane recovery section 5 is capable of carrying out the separation / purification process described above. In one embodiment, the methane recovery section 5 includes a methane separation device 51, a methane recovery line 52, and a discharge line 53. The methane separation device 51 is capable of separating methane from a methane-containing gas. Examples of the methane separation device 51 include a membrane separator equipped with a methane separation membrane and a PSA gas adsorption device. The methane recovery line 52 is typically a pipe through which the methane separated in the methane separation device 51 can pass. The upstream end of the methane recovery line 52 in the direction in which methane passes is connected to the methane separation device 51. The downstream end of the methane recovery line 52 in the direction in which methane passes is typically connected to a methane storage tank (not shown). The discharge line 53 is typically a pipe for discharging the residual gas after methane separation. The upstream end of the discharge line 53 in the direction in which the residual gas is discharged is connected to the methane separation device 51.

[0068] C-6. Control Unit The control unit 10 can perform the hydrogen addition process described above in both the mixed gas supply unit 3 and the hydrogen addition unit 4. The control unit 10 can control the operation of the methane production system 100. The control unit 10 includes, for example, a central processing unit (CPU), ROM, and RAM. The control unit 10 stores a program for calculating the amount of hydrogen gas added A based on the above-described formula (1). In one embodiment, the control unit 10 is communicatively connected to the flow meter 32, the oxygen concentration meter 33, the carbon dioxide concentration meter 34, and the pump 43 of the hydrogen addition unit 4. Therefore, the control unit 10 can receive the flow rate z of the mixed gas from the flow meter 32, the oxygen concentration c1 in the mixed gas from the oxygen concentration meter 33, and the carbon dioxide concentration c2 in the mixed gas from the carbon dioxide concentration meter 34. In the illustrated example, the control unit 10 calculates the amount of hydrogen gas added A based on the above-described formula (1) when it receives the flow rate z, oxygen concentration c1, and carbon dioxide concentration c2 of the mixed gas. Thereafter, the control unit 10 adjusts the output of the pump 43 to adjust the flow rate of the hydrogen gas flowing through the second gas supply line 41 to the addition amount A. As a result, the hydrogen gas is supplied at the addition amount A to the mixed gas flowing through the first gas supply line 31.

[0069] D. Modifications In one embodiment, the methane production method includes an oxygen concentration measuring step and a carbon dioxide concentration measuring step. However, if it is possible to calculate the oxygen concentration and the carbon dioxide concentration of the exhaust gas emitted from an industrial product or facility, the methane production method does not need to include the oxygen concentration measuring step and the carbon dioxide concentration measuring step.

[0070] For example, if the industrial product or facility is a combustion device, the oxygen concentration and carbon dioxide concentration of the exhaust gas can be calculated from the amount of air supplied to the combustion device and the amount of fuel consumed in the combustion device. More specifically, the amount of carbon contained in the fuel supplied to the combustion device is measured. Then, the amount of carbon dioxide produced when all of the measured carbon is combusted is calculated. The amount of oxygen supplied to the combustion device (supplied oxygen amount) is calculated from the amount of air supplied to the combustion device. Next, the amount of oxygen required for fuel combustion (consumed oxygen amount) is calculated from the various components (e.g., carbon, hydrogen, oxygen, sulfur) contained in the fuel supplied to the combustion device. The amount of consumed oxygen is then subtracted from the amount of supplied oxygen to calculate the amount of remaining oxygen (residual oxygen amount). Next, the flow rate of the exhaust gas from the combustion device is measured, and the oxygen concentration and carbon dioxide concentration of the exhaust gas are calculated from the calculated amounts of carbon dioxide and residual oxygen. If such exhaust gas is used as a mixed gas in one embodiment of the methane production method, the hydrogenation process can be performed without the oxygen concentration measurement process and the carbon dioxide concentration measurement process.

[0071] The methane production method according to the embodiment of the present invention can be used for producing methane, and is particularly suitable for continuous production of methane using exhaust gas emitted from various industrial products or industrial facilities as a raw material.

[0072] REFERENCE SIGNS LIST 1 methane production device 11 honeycomb substrate 12 catalyst layer 13 partition wall 14 cell 15 gas flow path

Claims

1. A methane production method for producing methane from a raw material gas containing carbon dioxide gas, oxygen gas, and hydrogen gas, comprising: a step of adding hydrogen gas to a mixed gas containing carbon dioxide gas and oxygen gas to prepare the raw material gas; and a step of supplying the raw material gas to a gas flow path of a methane production apparatus, wherein in the step of adding hydrogen gas to the mixed gas, an amount A of hydrogen gas added satisfies the following formula (1): A={z×(c1 / 100)×x}+{z×(c2 / 100)×y} (1) (In formula (1), A represents the amount of hydrogen gas added; z represents the flow rate of the mixed gas; c1 represents the oxygen concentration in the mixed gas; c2 represents the carbon dioxide concentration in the mixed gas; x represents 2.0; and y represents a numerical value of 4.0 or more).

2. The method for producing methane according to claim 1, wherein y in formula (1) is 10.0 or less.

3. A methane production method according to claim 1 or 2, comprising, prior to the step of adding hydrogen gas to the mixed gas, a step of measuring an oxygen concentration c1 in the mixed gas and a step of measuring a carbon dioxide concentration c2 in the mixed gas, in this order.

4. The methane production method according to claim 1 or 2, wherein the methane production apparatus comprises: a honeycomb substrate having partition walls that form a plurality of cells, at least some of the plurality of cells including the gas flow path; and a catalyst layer provided on the surface of the partition walls, the catalyst layer including a methanation reaction catalyst, wherein the methanation reaction catalyst is capable of promoting a first reaction of producing water by reacting oxygen with hydrogen, and a second reaction of producing methane by reacting carbon dioxide with hydrogen.

5. The reactor for producing methane according to claim 1 or 2, wherein the methanation catalyst contains a transition metal as an active component.

6. The methane production reactor of claim 5, wherein the transition metal comprises Co, Fe, Pt, Ru, Rh, Pd, Ni, Cu, Ag, Au, Ir, or a combination thereof.

7. The methane production reactor according to claim 6, wherein the transition metal comprises Ni.

8. The reactor for producing methane according to claim 5, wherein the methanation catalyst further comprises a support that supports the transition metal, and the support comprises cerium oxide, silicon oxide, zirconium oxide, yttrium oxide, aluminum oxide, or a composite oxide thereof.

9. The methane production reactor of claim 8, wherein the support comprises cerium oxide.

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