Method for producing methane or propane by recycling co2

Abstract

To provide a carbon-neutral methanation process that does not require an external hydrogen supply, when recycling CO2 generated through oxidation reactions of hydrocarbons.SOLUTION: A method for producing methane or propane by recycling CO2 generated through oxidation reactions of hydrocarbons includes: (1) a step of generating hydrogen and / or CO2 through oxidation reactions of hydrocarbons; (2) a step of generating carbon and hydrogen through thermal decomposition of hydrocarbons; and (4) a step of reacting the hydrogen generated in step (1) and / or step (2) with CO or CO2 generated in step (1) to produce methane or propane; further included is either: (I) a step (5) of using the methane or propane produced in step (4) as the hydrocarbon in step (1); or (II) using the unreacted hydrocarbons from step (2) as the hydrocarbon used in step (1).SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a method for producing methane or propane by recycling CO2 generated by oxidation of city gas or the like.

Background Art

[0002] Methanation is positioned as a "next-generation thermal energy industry" in the "Green Growth Strategy for Carbon Neutrality in 2050" formulated in June 2021, regarded as an important field expected to grow, and the development of basic technologies is underway.

[0003] In France in 2018 and in Japan in 2021, some manufacturers started demonstrating a system for recycling CO2 generated by the following formula (a) in a factory site for oxidizing city gas by a conventional methanation reaction represented by the following formula (b) (Non-Patent Document 1, Non-Patent Document 2). CH4 + 2O2 → CO2 + 2H2O (a) 4H2 + CO2 → CH4 + 2H2O (b) However, as is clear from the above formula (b), in the conventional methanation reaction, 4 Nm 3 of hydrogen is required to produce 1 Nm 3 of methane for carbon-neutral use (Patent Document 1), and it is necessary to produce this hydrogen in the workplace or supply it from outside.

[0004] When producing hydrogen in the workplace, for example, renewable energy devices such as solar power generation, water electrolysis hydrogen production devices, storage facilities, piping, etc. are required (Patent Documents 2, 3, 4, 5, 6), but the amount of CO2 generated in the workplace is usually enormous and cannot cover the hydrogen demand. Therefore, it has to be produced and received outside the workplace, in which case, furthermore, a transport vehicle, compression equipment, storage equipment, a driver on the supply side, and storage equipment on the receiving side are required. All of these are initial cost factors.

[0005] The initial cost is ultimately reflected in the running cost, and the cost of hydrogen itself is expected to be 30 yen per Nm 3 of CIF in 2030 and 20 yen in 2050. Therefore, in the conventional methanation reaction, the cost of hydrogen utilization (120 yen / Nm 3 ·CH4 in 2030 and 80 yen / Nm 3 ·CH4 in 2050) will be added on top of the price when using simple methane oxidation up to now, and it can be said that the weight of the running cost of hydrogen is an issue even in the future (Non-Patent Document 3).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0008] If a carbon-neutral methanation process can be constructed with a cheaper raw material without using hydrogen as a raw material, it is considered that it will contribute to the popularization of methanation itself.

[0009] In view of the above situation, an object of the present invention is to provide a carbon-neutral methanation process that does not require an external hydrogen supply in the recycling of CO2 generated through the oxidation reaction of hydrocarbons.

Means for Solving the Problems

[0010] One aspect of the present invention made to achieve the above object is a method for producing methane or propane by recycling CO2 generated through the oxidation reaction of hydrocarbons, comprising: step (1) of generating hydrogen and / or CO2 through the oxidation reaction of hydrocarbons; step (2) of generating carbon and hydrogen by thermal decomposition of hydrocarbons; and step (4) of reacting hydrogen and CO generated in step (1) and / or step (2) or CO2 generated in step (1) to produce methane or propane, further satisfying the following condition (I) or (II), which is a method for producing methane or propane. (I) Further includes step (5) of using the methane or propane generated in step (4) as the hydrocarbon in step (1). (II) The hydrocarbon used in step (1) is the unreacted product of the hydrocarbon used in step (2). According to such a method, the supply of renewable hydrogen is not consistently essential in the methanation / propaneation process, and it is sufficient to use only hydrocarbons as raw materials for producing methane or propane and using them in a carbon-neutral manner. Furthermore, if a part of the generated carbon monoxide and / or carbon is sold, the hydrocarbon cost can be compressed.

[0011] In the method for producing methane or propane described above, step (2) is a step of generating carbon and hydrogen by thermal decomposition of methane in an amount 1.5 times or more in molar ratio or propane in an amount 5 / 6 times or more with respect to the recycled CO2. According to such a method, a continuous supply of renewable hydrogen is not essential in the methanation process, and it is sufficient to use only methane or propane as a raw material for producing methane or propane and using it in a carbon-neutral manner. Furthermore, if a part of the generated carbon monoxide and / or carbon is sold, the methane cost can be compressed.

[0012] In the method for producing methane or propane described above, when CO is used in step (4), it is separately prepared in step (3) of reacting the CO2 generated in step (1) with the carbon generated in step (2) to generate CO, and it is preferable that the reaction of step (2) and the reaction of step (3) proceed sequentially in a substantially anhydrous environment. According to such a method, by preventing the reverse water-gas shift reaction (the following formula (c)) in which the hydrogen of the product is oxidized to water, deactivation of the catalyst (catalyst poisoning) due to the reaction of the catalyst with water can be prevented, and hydrogen required for the methanation process can be supplied with high efficiency. Also, after discharging the generated carbon, the carbon remaining in the reaction furnace can be gasified (CO) and cleaned up, and it can be said to be superior to the case where only methane thermal decomposition is performed in that it can not only perform methanation but also handle the case where CO and hydrogen are to be output. CO2 + H2 → CO + H2O ····(c)

[0013] In the method for producing methane or propane described above, it is preferable that step (4) is a step of reacting the CO2 generated in step (1) with the hydrogen generated in step (1) and / or step (2) to produce methane or propane. According to such a method, the amounts of methane and propane to be thermally decomposed are sufficient to be 2 times and 2.5 times, respectively, in molar ratio with respect to the recycled CO2, and the conversion of CO2 to CO becomes unnecessary, so the apparatus configuration is simplified compared to the case of conversion.

[0014] In the above-mentioned method for producing methane or propane, when methane is thermally decomposed in the above-mentioned step (2), the amount of methane used is preferably three times or more by molar ratio relative to the amount of CO2 to be recycled, and the surplus amount of methane produced in the above-mentioned step (4), i.e., the amount of methane that exceeds the amount of methane consumed through the oxidation reaction, is preferably used in the step (2). 3 methane required for carbon neutral use is produced and doubled in molar ratio (minimum 2 Nm 3 ), but for simplicity's sake, if we consider LNG to be methane, then the methane concentration can be reduced to 1 Nm 3 The CIF price for LNG is 13.3 yen / Nm 3 When H2 is assumed (Non-Patent Document 4), 45.2 yen / Nm 3 -CH4 (Non-Patent Document 5, Non-Patent Document 6; the calorific value of LNG and hydrogen is calculated using the lower heating value (LHV)). From the perspective of methane cost, methane costs 2 Nm 3 Since it is necessary, 45.2 x 2 = 90.4 yen / 2 Nm 3 This is the amount of hydrogen needed to supply hydrogen to methanation in the conventional method, which is 4 Nm 3 Cost 4 x 30 yen = 120 yen / 4Nm 3 This represents an overwhelming reduction in costs compared to the cost in 2030. Furthermore, since procuring hydrogen from outside the facility would entail costs for land transportation of hydrogen and for facilities to receive it, this system maintains an economic advantage in terms of running costs. Furthermore, if part of the carbon produced is sold, the cost of hydrogen equivalent to the calorific value of LNG can be reduced to 24 yen / Nm 3 1 Nm of methane when H2 is assumed (Non-Patent Document 3) 3 CIF price (81.6 yen / Nm 3 CH4: Even though the calorific value of LNG and hydrogen is calculated using the lower heating value (LHV), methane 2Nm 3 Cost: 81.6 x 2 = 163.2 yen / 2Nm 3 This reduces the amount of hydrogen required for methanation to 4Nm3 by 11Nm3. 3 The cost is 4 x 20 yen = 80 yen / 4Nm 3 Even assuming a future like this (2050), we will still be able to maintain our economic advantage.

[0015] The above method for producing methane or propane is preferably such that the reaction in step (2) is carried out using a structural catalyst having a catalytic action for methane pyrolysis reaction, and the reaction in step (3) is carried out in the presence of carbon generated in the reaction of step (2). This has the advantage that the same catalyst can be used for each reaction in the same reaction vessel without catalyst replacement. Further, by carrying out the reaction in step (3) in the presence of carbon generated in the reaction of step (2), the generated carbon can be effectively consumed and utilized as a raw material for CO.

Advantages of the Invention

[0016] According to the method for producing methane or propane of the present invention, methane or propane can be produced by a methanation / propaneation process, and it is sufficient to use only hydrocarbons as raw materials for carbon-neutral utilization. Furthermore, if a part of the generated carbon is sold, the hydrocarbon cost can be compressed.

Brief Description of the Drawings

[0017]

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Embodiments for Carrying Out the Invention

[0018] Hereinafter, the "terms" used in this specification will be defined. In this specification, "hydrocarbon" refers to linear or branched saturated hydrocarbons having 1 to 12 carbon atoms such as methane, ethane, propane, octane, etc. and linear or branched unsaturated hydrocarbons having 1 to 12 carbon atoms such as ethylene, propylene, acetylene, etc. In this specification, "in a substantially anhydrous environment" means an environment in which water or water vapor (steam) is not artificially included in or input to the raw material for the purpose of participating in the reaction. Therefore, it is allowed that water enters accidentally or unavoidably due to the reaction of the raw material with impurities, the reaction of multiple impurities with each other, etc. in a "substantially anhydrous environment", but it is preferable to reduce the moisture and humidity of the raw material by contacting it with a desiccant, etc. As used herein, "sequentially proceeding the reactions" means proceeding a plurality of reactions in order and / or alternately, in the desired direction in the case of a reversible reaction. As used herein, "structural catalyst" refers to a catalyst in which the structure itself functions as a catalyst or a catalyst based on the structure. As a catalyst based on the structure, it generally refers to one obtained by impregnating a substrate having a shape such as a honeycomb in a slurry containing a catalyst component, but it may also be one in which a non-supported catalyst layer (plated layer, sprayed layer) exposed by spraying, plating, etc. is formed on the structure. As used herein, "structure" means an object having a certain shape even without a mold maintaining the shape. The shape of the structure is not particularly limited, and examples include a plate (including not only a flat plate but also those obtained by arbitrarily processing the flat plate such as bending, folding, punching, notching, embossing, etc.), a rod, a cylinder, a porous body, a honeycomb (monolith type), a felt, a mesh, a fabric, or an expanded metal.

[0019] The method for producing methane or propane according to one embodiment of the present invention is a method for producing methane or propane by recycling CO2 generated through the oxidation reaction of hydrocarbons. As used herein, "recycling CO2" means that it is finally possible to produce and secure methane in an equimolar amount to the generated CO2 or propane in a 1 / 3 molar amount. Therefore, for recycling, it is not necessary to burn and reuse the produced methane or propane in the factory, reuse them as raw materials for the SDRM reaction module or the methane oxidation module, or use them for combustion as a heat source for the SDRM reaction module or the methane oxidation module. Supplying them to a city gas conduit, a cylinder, etc. for use in other applications also falls under recycling.

[0020] The method for producing methane or propane according to one embodiment of the present invention includes a step (1) of generating CO2 through the oxidation reaction of hydrocarbons. In this specification, "through the oxidation reaction of hydrocarbons" means via any one-step or multi-step reaction that includes a reaction in which hydrocarbons can be regarded as reducing agents and one or more other chemical substances as oxidizing agents. Such reactions include not only the combustion reactions of hydrocarbons represented by the following formulas (d), (e), and (f), but also the steam reforming reactions of hydrocarbons represented by the following formulas (g) and (h). C n H 2n+2 +((3n + 1) / 2)O2 → nCO2 + (n + 1)H2O ···· (d) C n H 2n +(3n / 2)O2 → nCO2 + nH2O ···· (e) C n H 2n-2 +((3n - 1) / 2)O2 → nCO2 + (n - 1)H2O ···· (f) CH4 + H2O → CO + 3H2 ···· (g) CO + H2O → CO2 + H2 ···· (h)

[0021] The oxidation reaction of hydrocarbons may be an exothermic (combustion) reaction or an endothermic reaction, and depending on the reaction, the use of a catalyst may or may not be required. In the combustion reactions of the above formulas (d), (e), and (f), a catalyst may not be used, but examples of cases where it can be used for the complete combustion of residual methane contained in the exhaust gas include Pt / SnO2-based catalysts, Pt-Ir / SnO2-based catalysts, catalysts in which cobalt and palladium are supported on a carrier, and the like. When using a catalyst, the reaction temperatures of the above formulas (d), (e), and (f) are usually set to 340°C to 500°C. Examples of catalysts used in the steam reforming reaction of the above formula (g) include catalysts in which a catalytically active component containing nickel, cobalt, molybdenum, rhodium, ruthenium, aluminum, zirconium, magnesium, or oxides thereof is supported on a porous carrier. Specifically, Ru / Al2O3 catalysts in which ruthenium is supported on an alumina carrier, Ni / Al2O3 catalysts in which nickel is supported on an alumina carrier, and the like can be used. When using a catalyst, the reaction temperatures in the above formulas (g) and (h) are usually set at 500 to 900 °C.

[0022] The source of the hydrocarbon to be oxidized is preferably derived from oil fields, crude oil, natural gas, city gas, or biogas. When the hydrocarbon is methane, the source is preferably derived from natural gas, city gas, or biogas. City gas contains a high concentration of methane (CH4), can be easily obtained from existing laid pipelines, and can be easily converted into a high-concentration methane-containing source by passing through a desulfurizer. In view of the global trend towards carbon negativity, carbon-neutral natural gas-derived or biogas-derived methane, i.e., methane obtained by separating CO2 from biogas or methane containing CO2, may be used.

[0023] As the oxidant for combustion, in addition to air, pure oxygen, oxygen-enriched air obtained by adding oxygen to air, etc. can be considered. Pure oxygen can be utilized from the pure hydrogen and pure oxygen generated by water electrolysis using renewable energy such as solar power generation (installed within the factory) in addition to purchasing it as an industrial gas. Alternatively, renewable energy can be purchased from grid power, and pure hydrogen and pure oxygen can be produced by a water electrolysis device (installed within the factory), and the pure oxygen therein can be utilized. Note that the pure hydrogen produced by the water electrolysis device can be used as fuel for the heating device 35 for pyrolysis described later.

[0024] The method for producing methane or propane according to an embodiment of the present invention includes step (2) of producing carbon and hydrogen by causing a methane pyrolysis reaction represented by the following formula (i) or a propane pyrolysis reaction represented by the following formula (j) to proceed substantially in an anhydrous environment by pyrolyzing methane or propane prepared separately from the hydrocarbon to be oxidized. CH4 → C + 2H2 ···· (i) C3H8 → 3C + 4H2 ···· (j)

[0025] The reaction temperature of the above formula (i) or (j) usually requires a high temperature of 1000°C to 1200°C in the case of no catalyst, but when a catalyst is used, it is usually set at 600°C to 800°C or higher. The reaction temperature in this specification is a value obtained by measurement with a thermocouple.

[0026] When proceeding with the reaction represented by the above formula (i) or (j), a catalyst may or may not be used, but when proceeding at the above reaction temperature, it is preferable to use a catalyst.

[0027] The reaction of the above formula (i) or (j) is usually carried out using a catalyst having a catalytic action on the methane pyrolysis reaction. As the catalyst having a catalytic action on the methane pyrolysis reaction, a catalyst provided with an exposed nickel-containing layer having a thickness of about 1 μm to 200 μm and a support layer made of iron, copper, nickel, steel, cast iron, iron-nickel alloy, or copper alloy having a thickness of about 0.5 mm to 10 mm is preferably adopted, and a catalyst provided with an intermediate layer containing copper having a thickness of about 1 to 1000 μm between the support layer and the nickel-containing layer is more preferably adopted. The support layer may be the structure itself before laminating the nickel-containing layer, or may be a layer laminated on the structure. Here, "exposed" means that it is sufficient if methane molecules can come into contact, and is not limited to those that are visually exposed. The nickel-containing layer may be a non-supported nickel-containing layer. "Non-supported" means that the nickel-containing component as a catalyst component does not exist as particles dispersed on a porous support such as activated carbon or porous oxide, but exists in an organized manner. "Organized" means that the particles may be welded in a partial region, may be welded in the entire region, or may be cooled and solidified after being melted as a whole. Note that the catalyst having a catalytic action on the methane pyrolysis reaction is not limited to a structured catalyst, and may be a powder catalyst that cannot maintain a certain shape without a mold that maintains the shape.

[0028] Methane (CH4), which is prepared separately from the hydrocarbon to be oxidized used in the above formula (i) or formula (j), is preferably derived from city gas. City gas contains methane (CH4) at a high concentration, can be easily obtained from existing laid pipelines, and can be easily converted into a high-concentration methane-containing source by passing through a desulfurizer. In view of the global trend towards carbon negativity, methane derived from biogas, i.e., methane obtained by separating CO2 from biogas, may be used, but there are limitations in terms of supply volume. Methane derived from coal gas may be used, but coal gas is originally a mixed gas and has the drawback that methane and CO2 cannot be supplied separately.

[0029] The amount of methane to be pyrolyzed is usually 1.5 times or more in molar ratio with respect to the recycled CO2. Strictly speaking, the lower limit of 1.5 times can also be increased or decreased based on the knowledge established for subsequent reactions using syngas or knowledge to be established in the future, operating conditions, the ratio of carbon and carbon monoxide as by-products of hydrogen, etc. Preferred lower limit values of the molar ratio are 1.8 times, 2 times, 2.5 times, 3 times. The amount of propane to be pyrolyzed is usually 5 / 6 times or more in molar ratio with respect to the recycled CO2. Strictly speaking, the lower limit of 5 / 6 times can also be increased or decreased based on the knowledge established for subsequent reactions using syngas or knowledge to be established in the future, whether propane is generated or methane is generated in subsequent reactions (the lower limit of the molar ratio in the case of generating methane is 1 time as shown in Figure 12 stoichiometrically), operating conditions, the ratio of carbon and carbon monoxide as by-products of hydrogen, etc. Preferred lower limit values of the molar ratio are 1 time, 2 times, 2.5 times, 3 times. The molar ratio can be set, for example, at ±50%, ±20%, ±5%, etc. with respect to the lower limit value.

[0030] The method for producing methane or propane according to an embodiment of the present invention may include step (3) of producing carbon monoxide by sequentially proceeding the Boudouard reaction represented by the following formula (k) in which CO2 generated through the oxidation reaction of hydrocarbons is reacted with the carbon generated in step (2) substantially in an anhydrous environment. C + CO2 → 2CO ····(k)

[0031] When using a catalyst, the reaction temperature of the above formula (k) is usually set at 400°C or higher. However, in order to make the reaction proceed with the CO generation side being dominant, it is preferably 650°C or higher, more preferably exceeding 650°C, and even more preferably 800°C or higher.

[0032] When proceeding with the reaction represented by the above formula (k), a catalyst may or may not be used. However, when proceeding at the above reaction temperature, it is preferable to use a catalyst.

[0033] The reaction of the above formula (k) is usually carried out in the presence of the carbon generated by the reaction of the above formula (i) or (j). The carbon generated here only needs to remain to some extent, and a part of it may be removed. The carbon generated here mainly tends to be in the form of tip-growth (with nickel nanoparticles attached as white dots at the tips as shown in the SEM image of FIG. 3). The generated carbon containing these catalyst particles may not only serve as a raw material but also have a catalytic effect. However, regardless of the form and role in which it exists, in implementing the production method of the present invention, it only needs to exist. That is, what has the catalytic effect does not restrict the technical scope of the production method of the present invention in any way.

[0034] Of the carbon generated in step (2), the surplus that was not used in step (3) can compress the methane cost by being valorized and sold. Since the shape of the carbon generated (spherical, fibrous) differs depending on the type of the structural body catalyst and the reaction conditions, although it cannot be generally stated, in the case of generating spherical graphite with different particle sizes, the electric conductivity of the generated carbon has been obtained to be at the same level as that of Ketjen black, and it is also conceivable to sell it as carbon black as it is or after removing metal fine particles, etc. (The price of carbon black is 142 yen / kg (2020 = 76 yen / Nm 3 ·CH4), source URL: https: / / www.meti.go.jp / statistics / tyo / seidou / result / gaiyo / resourceData / 02_kagaku / nenpo / h2dbb2020k.pdf#page=13). Also, by mixing the generated carbon with the thermoplastic resin PPS, it is conceivable to sell it as a high thermal conductivity resin, sell and use it as a carbon additive in the casting process, etc., but the application is not particularly limited.

[0035] Of the carbon monoxide generated in step (3), when a surplus that is not used in the subsequent step (4) is generated, it can be appropriately used for casting, acrylic acid production, gas fuel, reduction of metal oxides, calcium oxide, calcium phosphate, or silicon dioxide.

[0036] In the methane / propane production method according to another embodiment of the present invention, without performing step (3), the CO2 generated in step (1) can be used in step (4). Whether to perform step (3) or not may be selected according to the conditions of the methane pyrolysis reaction, etc.

[0037] The method for producing methane or propane according to an embodiment of the present invention includes a Fischer-Tropsch reaction in which hydrogen generated in the above step (2) and CO generated in the above step (3), represented by the following formula (l), react to produce methane. When step (3) is not performed, it includes step (4) of producing methane by reacting hydrogen generated in the above step (1) or step (2) with CO2 generated in the above step (1), represented by the following formula (m). 3H2+CO→CH4+H2O····(l) 4H2+CO2→CH4+2H2O····(m)

[0038] When using a catalyst, the reaction temperature of the above formulas (l) and (m) is usually set at 100 to 300°C. However, in order to avoid a runaway reaction and promote the reaction so that the methane production side is dominant, low temperature and high pressure are advantageous. On the other hand, in order to exhibit catalyst activity and increase the conversion rate of CO, there is a situation where temperature increase is necessary. The preferable upper limit is 260°C, and the more preferable upper limit is 250°C. The preferable lower limit is 130°C, and the more preferable lower limit is 170°C. In addition, when setting the above reaction temperature, the surplus waste heat generated when heating to proceed the reaction of the above formula (l) and the reaction of the above formula (m) can be utilized.

[0039] In carrying out the reactions represented by the above formulas (l) and (m), a catalyst may or may not be used. However, when the reaction is carried out at the above reaction temperature, it is preferable to use a catalyst. Examples of usable catalysts include those in which at least one element selected from the group consisting of iron, ruthenium, rhodium, nickel, and cobalt is supported as an active ingredient on a carrier material selected from the group consisting of Al2O3, ZrO2, TiO2, SiC, SiO2, ZnO, oxides of Group IIA metals, oxides of transition metals of Groups IIIB, IVB, VB, and VIB, oxides of rare earth metals, aluminosilicates, zeolites, MOFs (metal-organic frameworks), and mixtures thereof, and catalysts having the catalytic action of the above-described methane pyrolysis reaction. Commercially available products include the N110 series (manufactured by JGC Catalysts and Chemicals Ltd.), core-shell type catalysts in which Ni particles are dispersed in a porous matrix described in Patent No. 6203375 (manufactured by IHI Corporation), etc., which are conventionally known catalysts used for the removal of carbon monoxide by methanation, and core-shell type catalysts containing nickel in the core part and zirconium in the shell part described in the pamphlet of International Publication No. 2022 / 065468 (manufactured by Mitsui Mining & Smelting Co., Ltd.), etc., which are conventionally known catalysts used for the removal of carbon dioxide by methanation. These conventionally known catalysts can be employed.

[0040] As a hydrogen supply source, usually, the hydrogen generated in the above step (1) or step (2) is utilized. However, when there is a shortage, etc., electrolytic hydrogen from renewable energy may be used.

[0041] Hereinafter, a specific system configuration example when methane is used as the hydrocarbon will be described. The CO₂ recycling system 1 shown in Fig. 6 is mainly designed to be installed within the premises of urban gas consumers, biogas producers, etc. It consists of a methane oxidation module 3, a dehydrator 7 for removing H₂O from the mixed gas of CO₂ and H₂O discharged from the methane oxidation module 3, an SDRM reaction module 11 for receiving the CO₂ discharged from the dehydrator 7, the urban gas, and the methane supplied from 8, a methanation module 15 that uses the H₂ and CO discharged from the SDRM reaction module 11 as raw materials and discharges CH₄ and H₂O, and a dehydrator 17 for removing H₂O from the mixed gas of CH₄ and H₂O.

[0042] The methane oxidation module 3 is a device or its component that oxidizes methane and discharges CO₂. Examples of the methane oxidation module 3 include conventionally known gas turbine generators, combined cycle generators, diesel generators, biogas generators, gas turbines for pump drive, boilers, chillers, absorption chillers, solid oxide fuel cells (Enefarm), steam reformers, etc.

[0043] The dehydrator 7 removes moisture when water is generated through the oxidation reaction of methane in the methane oxidation module 3 (for example, when oxygen is used as the oxidant) or when water is used as the oxidant for methane, etc., to prevent catalyst poisoning and the like caused by water mixing into the subsequent SDRM reaction module 11. The dehydrator 17 removes the water generated simultaneously with the production of methane in the methanation module 15 to prevent a decrease in thermal efficiency caused by water mixing into the subsequent methane oxidation module 3 and catalyst poisoning and the like caused by water mixing into the SDRM reaction module 11. As the dehydrators 7 and 17, a method by cooling and condensation, a separation method using membranes such as polyimide membranes, ceramic membranes, zeolite membranes, etc., or a method using adsorbents such as silica gel, molecular sieves, zeolites, activated carbon, etc. can be adopted. Note that when a decrease in oxidation efficiency and conversion rate is acceptable or when an improved catalyst is used, the dehydrator 7 and the dehydrator 17 may be omitted.

[0044] The methanation module 15 is a device or its component that supplies H2 and CO discharged from the SDRM reaction module 11 and discharges CH4 and H2O. As an example of the methanation module 15, a methane synthesis unit or the like corresponding to the latter stage of SOEC methanation can also be diverted.

[0045] The SDRM reaction module 11 shown in FIG. 7 includes a reactor 33 having a methane gas supply port 42, a CO2 supply port 44, a hydrogen gas discharge port 46, and a CO gas discharge port 48, a heating device 35 inserted and fixed into the reactor 33 from above, a structural catalyst 37 having a catalytic action for methane pyrolysis reaction disposed so as to surround the heating device 35 in the reactor 33, a first on-off valve 39 for managing the supply of CH4 from the CH4 supply source 47 to the methane gas supply port 42, a second on-off valve 41 for managing the supply of CO2 from the CO2 supply source 49 to the CO2 supply port 44, a third on-off valve 43 for managing the discharge of hydrogen from the hydrogen gas discharge port 46, and a fourth on-off valve 45 for managing the discharge of CO from the CO gas discharge port 48.

[0046] As the heating device 35, an internal heating type that is inserted into the reactor 33 and heated from the inside is adopted. Specifically, a recuperative burner using hydrogen as fuel is adopted, but biogas may be used as fuel instead. Also, a heater that heats the reactor 33 from the outside (external heating type) can be adopted.

[0047] As the structural catalyst 37 having a catalytic action for methane pyrolysis reaction, a structure obtained by electrolytic plating a copper layer as an intermediate layer containing copper, a nickel layer as a nickel-containing layer, and a nickel plate catalyst as a support layer (structure) in this order is adopted.

[0048] As the first to fourth on-off valves 9, 41, 43, and 45, a partition valve capable of switching between fully open and fully closed is adopted, but any other valve such as a ball valve can be adopted instead. Particularly when flow rate adjustment is required, a butterfly valve or a ball valve can also be adopted. Note that the switching operation of the on-off valve is manual, but it may be automatically performed using a solenoid valve or the like.

[0049] Between the CH4 supply source 47 and the first on-off valve 39, a desulfurizer 54 for removing trace sulfur-based compounds contained in the city gas is provided. As the desulfurizer 54, a conventionally known solid catalyst or adsorbent can be used.

[0050] As the H2 purification device 36, a pressure swing adsorption method (PSA method) is adopted, but a purification method such as a zeolite membrane, a DDR membrane, or palladium alloy membrane permeation can also be adopted instead.

[0051] As the CO purification device 38, an exchangeable solid absorbent having the property of absorbing CO2 in which an amine is supported on a porous support is adopted, but a liquid absorbent, a membrane separation method, or a PSA method can also be adopted instead. Note that when the required purity is low, the purification devices 36 and 38 may not be provided.

[0052] On the flow path connecting the gas discharge port of the reactor and the fourth on-off valve 45, or on the flow path connecting the fourth on-off valve 45 and the CO purification device 38, a CO detector (not shown) is provided. The CO detector is an instrument capable of specifically detecting the carbon monoxide concentration or an index reflecting this change (for example, a potential change, an absorbance change, etc.). In this embodiment, an NDIR type gas sensor is adopted as the CO detector, but the measurement method is not particularly limited. Also, a detector capable of simultaneously measuring the concentration of gases other than carbon monoxide may be used.

[0053] Compressors 40a and 40b are provided at the subsequent stages of the purification devices 36 and 38, respectively.

[0054] As an actual operation method, when the first on-off valve 39 and the third on-off valve 43 are open, the second on-off valve 41 and the fourth on-off valve 45 are closed. When the second on-off valve 41 and the fourth on-off valve 45 are closed, the first on-off valve 39 and the third on-off valve 43 are switched to be closed for operation. As the opening and closing procedure of each valve, the second on-off valve 41 is closed → the fourth on-off valve 45 is closed → the first on-off valve 39 is opened → the third on-off valve 43 is opened to perform the methane pyrolysis shown in formula (i) or the propane pyrolysis reaction shown in formula (j), and the first on-off valve 39 is closed → the third on-off valve 43 is closed → the second on-off valve 41 is opened → the fourth on-off valve 45 is opened to perform the CO2 reduction reaction shown in formula (k).

[0055] According to the CO2 circulation utilization system 1 configured as described above, instead of requiring the SDRM reaction module 11, the solid oxide type electrolysis cell device that was required when reducing CO2 and / or water in the methanation process, the renewable energy power generation device such as solar power generation, the reduction device for the generated metal oxide that was required when using a metal when reducing CO2 and / or water in the methanation process, the water electrolysis hydrogen production device when generating hydrogen from water in the methanation process, the hydrogen storage facility, the hydrogen piping, etc. are not required. Also, when manufacturing and receiving hydrogen outside the business premises, the transport vehicle, compression facility, storage facility required on the supply side, and the driver, and the storage facility required on the receiving side are not required either.

[0056] The CO₂ recycling system 101 shown in Fig. 8 is an off-site installation type system mainly assumed to be installed dispersedly in the site 102 of a city gas supplier, the site 104 of a city gas consumer, etc. It includes a methane oxidation module 103 installed in the site 104 of a city gas consumer, a dehydrator 107 for removing H₂O from the mixed gas of CO₂ and H₂O discharged from the methane oxidation module 103, a transportation means 106 for transporting the CO₂ discharged from the dehydrator 107 into the site 102 of a city gas supplier, an SDRM reaction module 111 installed in the site 102 of a city gas supplier for receiving the CO₂ supplied from the transportation means 106 and the methane supplied from the city gas pipeline 108, a methanation module 115 for supplying H₂ and CO discharged from the SDRM reaction module 111 as raw materials and discharging CH₄ and H₂O, and a dehydrator 117 for removing H₂O from the mixed gas of CH₄ and H₂O.

[0057] According to the CO₂ recycling system 101 configured as described above, although a CO₂ compression / liquefier 109, a CO₂ shipping facility (not shown), a CO₂ receiving facility (not shown), a CO₂ storage facility 110, a CO₂ transport truck 106, a driver (not shown), etc. are required, compared with the conventional methanation process that requires hydrogen supply, the same merits as those of the on-site installation type CO₂ recycling system 1 can be obtained.

[0058] The difference between the CO₂ recycling system 121 shown in Fig. 9 and the CO₂ recycling system 1 shown in Fig. 6 is that not only the H₂ discharged from the SDRM reaction module 11 but also hydrogen derived from renewable energy is added from the outside to the methanation module 15.

[0059] According to the CO₂ recycling system 121 configured as described above, compared with the CO₂ recycling system 1 shown in Fig. 6, the hydrogen derived from renewable energy supplied from the outside can be halved from 4 unit volumes to 2 unit volumes, and the cost can also be reduced from 30 yen / Nm 3 (in 2030) × 4 Nm 3 = 120 yen to 60 yen, and 20 yen / Nm 3 (in 2050) × 4 Nm3 It has advantages such as being able to reduce the natural gas (city gas, methane) supplied from the outside from 80 yen to 40 yen, halving the volume from 2 unit volumes to 1 unit volume, halving the amount of generated carbon, and facilitating the market development and monetization of the generated carbon.

[0060] The difference between the CO₂ recycling system 131 shown in Fig. 10 and the CO₂ recycling system 1 shown in Fig. 6 is that a methane pyrolysis module 41 is used instead of the SDRM reaction module 11, the generated hydrogen is supplied to the methanation module 65, and the CO₂ discharged from the dehydrator 7 is directly supplied to the methanation module 65. In the methane pyrolysis module 41, the structural catalyst 37 having the catalytic action of the methane pyrolysis reaction used in the SDRM reaction module 11 can be diverted. In the methanation module 65, a core-shell type catalyst (manufactured by Mitsui Mining & Smelting Co., Ltd.) containing nickel in the core part and zirconium in the shell part described in the pamphlet of International Publication No. 2022 / 065468 can be adopted.

[0061] According to the CO₂ recycling system 131 configured as described above, compared with the CO₂ recycling system 1 shown in Fig. 6, the amount of methane to be pyrolyzed is only twice the molar ratio to the CO₂ to be recycled, and since the conversion of CO₂ to CO becomes unnecessary, there is no need to provide the second on-off valve 41, the CO₂ supply port 44 in the system for introducing CO₂, the CO gas discharge port 48, the fourth on-off valve 45, the purification device 38, and the compressor 40b in the system for discharging CO. There is an advantage that the device configuration is simplified compared to the case of conversion.

[0062] The differences between the CO₂ recycling system 141 shown in Fig. 11 and the CO₂ recycling system 131 shown in Fig. 10 are as follows: the methane supply from the pipeline 8 to the methane pyrolysis module 41 is abolished, and instead, propane is supplied from the propane cylinder 18 to the propane pyrolysis module 31; the methanation module 65 is replaced with a propanation module 55; and propane from the propane cylinder 18 and propane from the propanation module 55 are used in the propane oxidation module 53. Note that as the propane oxidation module 53, propane pyrolysis module 31, and propanation module 55, basically the same configurations as the methane oxidation module 3, methane pyrolysis module 41, and methanation module 65 can be used.

[0063] According to the CO₂ recycling system 141 configured as described above, compared with the CO₂ recycling system 131 shown in Fig. 10, it is a suitable configuration for recycling CO₂ generated by the combustion utilization of propane in a factory located in the suburbs without a city gas pipeline to produce propane.

[0064] The difference between the CO₂ recycling system 151 shown in Fig. 12 and the CO₂ recycling system 141 shown in Fig. 11 is that methane is circulated by using the methanation module 65 instead of the propanation module 55 and is used for combustion in the methane oxidation module 3.

[0065] According to the CO₂ recycling system 151 configured as described above, although a relatively larger amount of propane in terms of molar ratio is required for the CO₂ to be recycled compared to the case of using the CO₂ recycling system 141 shown in Fig. 11 (the former is 5 / 6 times or more, while the latter is 1 time or more), since methane is recycled, the CO₂ emissions from combustion are less than those of propane, and it is excellent from the perspective of CO₂ emissions.

[0066] The difference between the CO₂ recycling system 161 shown in Fig. 13 and the CO₂ recycling system 131 shown in Fig. 10 is that the methane pyrolysis module 41 is installed not after the methane oxidation module 3 but between the pipeline 8 and the methane oxidation module 3, and the supply to the methanation module 65 is also retained, and the recycled methane is returned to the methane pyrolysis module 41.

[0067] According to the CO₂ recycling system 161 with the above configuration, when the methane decomposition rate is 1, it is hydrogen-only combustion, and when the methane decomposition rate is less than 1, it is hydrogen-mixed combustion. Therefore, compared with the CO₂ recycling system 131 shown in Fig. 10, to ensure the same calorific value as the conventional methane-only combustion, an excessive amount of methane-hydrogen mixed gas is required (the calorific value of methane (LHV) = 35900 kJ / Nm 3 , the calorific value of hydrogen (LHV) = 10780 kJ / Nm 3 ). However, since the separation and purification of unreacted methane are not required, there is an advantage that the device is simplified.

[0068] (Experimental Example 1 - Manufacture of SDRM Reaction Module and Methane Pyrolysis) The SDRM reaction module 21 shown in Fig. 1 was obtained by electroplating Cu to a thickness of approximately 1 μm on both sides of a Ni plate and then electroplating Ni to a thickness of approximately 10 μm. The metal plate catalyst 27 (size: width 30 mm × length 300 mm × thickness 0.6 mm) was housed in the reaction furnace 23 (capacity Φ30 * 300 mm, 0.2 L) in a state of being suspended from the catalyst suspension stage 24. Methane at room temperature was flowed from the methane supply port 32 provided in the upper part of the peripheral wall of the furnace 23 at a rate of 10 mL / min from top to bottom while heating the outer periphery of the furnace 23 with a heater (not shown) until the furnace temperature reached 800°C, and it was maintained for 8 hours a day. The change in the hydrogen concentration discharged from the hydrogen gas discharge port 36 was observed. For safety reasons, after 8 hours of continuous operation each day, the furnace core was cooled and heated again from room temperature to 800°C the next day. The time-dependent change in the hydrogen concentration is shown in Fig. 2, and the SEM image of the catalyst with the product attached and the measurement results of EDS are shown in Fig. 3. Note that the furnace internal temperature was measured by the thermocouple 25 inserted so as to penetrate the upper lid of the furnace and reach the central part. The hydrogen concentration was measured by attaching a Wheatstone bridge type gas thermal conductivity gas analyzer (zero gas: city gas 13A, span gas: hydrogen 100%, gas flow rate: 0.2 L / min, manufactured by Chino Corporation) to the product gas discharge pipe for discharging the atmosphere provided at the lower end of the furnace side wall. The SEM image and EDS were obtained by secondary electron detection (incident voltage 15.0 kV, WD 12.5 mm, magnification x5,000, irradiation current mode Std.-PC, vacuum mode HighVac.) using a scanning electron microscope (product number: JCM-7000, manufactured by JEOL Ltd.). As can be seen from Fig. 2, from the first day to the second day, the hydrogen concentration (conversion rate) due to the thermal decomposition reaction of methane continued to increase, and at the end of the second day, the hydrogen concentration reached 80%, which is the theoretically equilibrium concentration, and thereafter, it remained stable at about 80% after the sixth day. From Fig. 3, it was found that the product was fibrous and mainly composed of C and Ni. Also, from the examination of elemental mapping conducted separately, the white dot-like substances visible at the tip or in the middle of the fiber were Ni fine particles.

[0069] (Experimental Example 2 - Demonstration of the Boudouard reaction in the presence of residual carbon and examination of the reaction temperature) After the experiment of Example 1, the supply of methane was stopped, and the weight of the generated carbon in the reaction furnace was measured. After measuring the weight of the generated carbon, the metal catalyst plate with the generated carbon adhering thereto and the generated carbon were returned to the reaction furnace again. At that time, in order to remove the air in the reaction furnace, the reaction furnace was filled with nitrogen gas at room temperature at a flow rate of 0.4 L / min for 30 minutes to purge the air remaining in the furnace. Next, instead of stopping the supply of nitrogen gas, while supplying CO2 at room temperature at a flow rate of 0.4 L / min, heating was performed until the furnace internal temperature was near the constant heater temperature, and the CO concentration level and the CO2 concentration level were measured until equilibrium was reached. The relationship between the furnace internal temperature and the equilibrium concentration levels of CO and CO2 is shown in Fig. 4. At this time, the concentrations of CO and CO2 were measured with a NDIR type gas sensor (gas flow rate: 0.5 L / min, manufactured by AWIT) by sampling the product gas from the discharge pipe for discharging the atmosphere provided at the lower end of the furnace side wall. As can be seen from FIG. 4, as the heater temperature rises from 400°C to 800°C, CO rises from 0% to nearly 85%, while CO2 decreases from 100% to 15%. The equilibrium concentration at 800°C is C + CO2⇔ 2CO The theoretical concentration of 87% (RTlnK=-ΔG°, ΔG°=-17.5kJ·mol -1 Calculated from 800°C, data source: Report of the Central Research Institute of Electric Power Industry M19002 (Basic Characteristics of Molten Carbonate Direct Carbon Fuel Cells Using Cylindrical Cells). It is suggested that the Boudouard reaction occurs, in which the carbon formed and CO2 on the catalyst react to form CO.

[0070] (Experimental Example 3 - Time-dependent change of Boudouard reaction) In Example 1, the supply of methane was stopped, and nitrogen gas at room temperature was filled into the reactor with the metal catalyst plate with deposited carbon formed at a flow rate of 0.4 L / min for 30 minutes to purge methane and hydrogen remaining in the furnace. Next, simultaneously with stopping the supply of nitrogen gas, CO2 at room temperature was supplied into the furnace at a flow rate of 0.4 L / min. The moment of switching from nitrogen to CO2 was set as 0 seconds, and the time-dependent changes in the CO and CO2 concentration levels were observed in seconds. The results are shown in FIG. 5. As can be seen from FIG. 5, reduction started 22 seconds after switching from nitrogen to CO2, and the CO concentration stabilized at 83% after 1 minute and 41 seconds. From this, although there is a temperature drop at the beginning of introducing CO2, it is considered that the CO reduction proceeds rapidly as the furnace temperature returns to near 800°C soon after, and the concentration of the supplied CO2 is converted to CO with almost no increase. In this state, it is considered that there is still carbon formed in the furnace that can reduce CO2.

[0071] It should be noted that the embodiments of the present invention are not limited to the above embodiments at all, and not all of the configurations described in the above embodiments are essential requirements of the present invention. The present invention can take various modified forms and the like as long as it belongs to the technical scope without departing from its technical idea.

[0072] In an alternative embodiment of the CO2 recycling system 1, a CO2 storage tank can be provided between the dehydrator 7 and the SDRM reaction module 11. By providing the CO2 storage tank, when the demand for methane is high, it can be stored, and when the demand for methane is low, it can be released, so that the supply amount of CO to the methanation module 15 can be adjusted so that it does not become more than 33% excessive or less than the hydrogen production amount.

[0073] In an alternative embodiment of the CO2 recycling system 1, assuming that air is used as the oxidant for combustion in the methane oxidation module 3, a device for separating CO2 from CO2, nitrogen, nitrogen oxides, etc. contained in the combustion gas can be provided downstream of the methane oxidation module 3.

[0074] In an alternative embodiment of the SDRM reaction module 11, a pipe for returning methane, which is the raw material gas separated by the purification device 36, to the reactor 33 or the methane supply port 42 may be provided between the purification device 36 and the compressor 40a, and a pipe for returning CO2, which is the raw material gas separated by the purification device 38, to the reactor 33 or the CO2 supply port 44 may be provided between the purification device 38 and the compressor 40b.

[0075] In an alternative embodiment of the SDRM reaction module 11, a compressor may also be provided upstream (closer to the gas discharge port) of the purification devices 36 and 38.

[0076] In an alternative embodiment of the SDRM reaction module 11, a main valve capable of discharging the gas remaining in the reactor to the atmosphere or a main valve that discharges the gas to a flow path separate from each flow path of the generated gas can be provided at a position closer to the gas discharge port of the reactor than the third on-off valve and the fourth on-off valve. By performing a purge operation with the main valve using the raw material gas, the purity of the generated gas controlled by the third on-off valve and the fourth on-off valve on the downstream side of the main valve can be improved.

[0077] In another alternative embodiment of the SDRM reaction module 11, the number of gas supply ports and gas discharge ports is reduced by one each, and the flow paths from the gas supply port to the supply source of each raw material gas and from the gas discharge port to the third and fourth on-off valves are combined halfway, and at each branch point, instead of the first and second on-off valves, one supply-side three-way valve is used, and instead of the third and fourth on-off valves, one discharge-side three-way valve can also be adopted.

[0078] In an alternative embodiment of the SDRM reaction module 11, a plurality of the devices described in FIG. 7 (for example, two devices, device A and device B) are prepared. First, in device A, the reaction of formula (i) or (j) and in device B, the reaction of formula (k) are temporarily carried out. When solid carbon has accumulated in device A, it is conceivable to switch so that the reaction of formula (k) is carried out in device A and the reaction of formula (i) or (j) is carried out in device B. According to such a device and method, carbon monoxide and hydrogen can be produced simultaneously and continuously. Also, if at least three devices are prepared, even if there is some device or piping trouble in one device, operation stoppage can be prevented.

Industrial Applicability

[0079] The method according to the present invention is such that when a customer who recovers and recycles CO2 generated by using natural gas, city gas, or biogas at home and abroad installs it on-site (for example, within the factory site of each manufacturer where CO2 is generated or within the site of an LNG thermal power plant), or when an energy supplier installs it off-site (for example, within the site of a city gas factory), there is no need to purchase hydrogen, and by valorizing carbon and selling it, the methane increment cost can be reduced, so it becomes a method with lower running costs. Also, especially for energy suppliers, not only can the decrease or disappearance of the city gas sales volume, which was inevitable in the conventional methanation system using hydrogen, be avoided, but there is also an advantage in that the city gas sales volume can be increased by 1.5 to 3 times, so the industrial applicability is great. Also, when the customer who recovers and recycles CO2 is especially a cement factory or the like, since it is often located in the suburbs without a city gas conduit, the applicability of this method that can utilize propane cylinders is great.

Description of Symbols

[0080] 1, 101, 121, 131, 141, 151, 161 CO₂ Recycling System 3, 103 Methane Oxidation Module 53 Propane Oxidation Module 7, 17, 107, 117 Dehydrator 8, 108 Pipeline 11, 21, 111 SDRM Reaction Module 31 Propane Pyrolysis Module 41 Methane Pyrolysis Module 15, 65, 115 Methanation Module 55 Propaneation Module 18 Propane Cylinder 24 Catalyst Hanging Stand 25 Thermocouple 33, 23 Reactor 54 Desulfurizer 35 Heating Device 36 H₂ Purification Device 37, 27 Structured Catalyst 38 CO Purification Device 39 First On - Off Valve 40a, 40b Compressor 41 Second On - Off Valve 42 Methane Supply Port 43 Third On - Off Valve 44 CO₂ Supply Port 45 Fourth On - Off Valve 46 Hydrogen Gas Outlet 47 CH₄ Supply Source 48 CO Gas Outlet 49 CO₂ Supply Source 102 Premises of Urban Gas Supplier 104 Premises of Urban Gas Customer 106 Transportation Means 109 CO₂ Compression and Liquefaction Unit 110 CO₂ Storage Facility

Claims

Claim 1 CO produced through the oxidation reaction of hydrocarbons ２ A method for producing methane or propane by recycling Step (1) of generating hydrogen and / or CO through the oxidation reaction of hydrocarbons ２ and a step (2) of producing carbon and hydrogen by thermal decomposition of a hydrocarbon, and Hydrogen generated in step (1) and / or step (2) and CO, or CO generated in step (1) ２ A method for producing methane or propane, comprising step (4) of reacting the above with each other to produce methane or propane, and further satisfying the following condition (I) or (II). (I) further comprising a step (5) of using methane or propane produced in step (4) as the hydrocarbon in step (1). (II) The hydrocarbon used in step (1) is an unreacted substance of the hydrocarbon used in step (2). Claim 2 The step (2) is a step of generating carbon and hydrogen by thermal decomposition of methane in an amount of 1.5 times or more in molar ratio or propane in an amount of 5 / 6 times or more with respect to CO ２ The method for producing methane or propane according to claim 1, wherein the step is a step of generating carbon and hydrogen by thermal decomposition of methane in an amount of 1.5 times or more in molar ratio or propane in an amount of 5 / 6 times or more with respect to CO Claim 3 When CO is used in step (4), the CO generated in step (1) ２ is separately prepared in step (3) for generating CO by reacting with the carbon generated in step (2), and the reaction of step (2) and the reaction of step (3) are sequentially carried out in a substantially anhydrous environment. The method for producing methane or propane according to claim 1. Claim 4 The step (4) reacts CO generated in the step (1) ２ with hydrogen generated in the step (1) and / or step (2) to produce methane or propane. The method for producing methane or propane according to claim 1, which is a step of producing methane or propane. Claim 5 When pyrolyzing methane in the step (2), the recycled CO ２ is in an amount of 3 times or more in molar ratio to The method for producing methane or propane according to claim 1, wherein the excess of methane produced in step (4) is used in step (2). Claim 6 The method for producing methane or propane according to claim 3, wherein the reaction in step (2) is carried out using a structural catalyst having a catalytic action for methane thermal decomposition reaction, and the reaction in step (3) is carried out in the presence of carbon produced in the reaction in step (2).

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