Methane production method and methane production device

Abstract

Provided is a methane production method for producing methane from carbon dioxide and hydrogen, wherein the method includes a methanation step in which, with a mixed gas of carbon dioxide and hydrogen introduced into a reactor packed with a methanation catalyst, non-equilibrium plasma is generated inside the reactor to decompose the mixed gas and generate radicals and provoke a methanation reaction, the methanation catalyst is disposed in a fixed-bed catalyst layer housed in the reactor, the methanation reaction is provoked without supplying heat to the reactor from outside, and the methanation reaction is continued without supplying heat to the reactor from outside.

Description

Methane production method and methane production apparatus

[0001] The present invention relates to a method and an apparatus for producing methane. This application claims priority to Japanese Patent Application No. 2023-213961, filed on December 19, 2023, the contents of which are incorporated herein by reference.

[0002] Carbon dioxide is considered to be one of the greenhouse gases that contributes to global warming, and technologies to convert carbon dioxide into useful substances are being investigated in order to realize a carbon-neutral society.Methanation, which produces methane from carbon dioxide and hydrogen, is known as a technology for converting carbon dioxide into the useful substance methane.

[0003] The methanation reaction is generally carried out in the presence of a heated methanation catalyst. To further optimize the methanation reaction, Non-Patent Document 1 discloses a method using plasma. Non-Patent Document 1 discloses the use of an electric furnace as an external heating device, which heats the catalyst and initiates the methanation reaction.

[0004] Dae-Yeong Kim, Hyungwon Ham Xiaozhong Chen, Shuai Liu, Haoran Xu, Bang Lu, Shinya Furukawa, Hyun-Ha Kim, Satoru Takakusagi, Koichi Sasaki, and Tomohiro Nozaki: Cooperative Catalysis of Vibrational Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation: J. Am. Chem. Soc. 2022,144,31,14140-14149

[0005] In order to popularize carbon dioxide conversion technology by methanation, it is important to reduce production costs. The method disclosed in Non-Patent Document 1 is useful in that it can significantly improve the carbon dioxide conversion efficiency, but there is room for improvement in terms of further reducing the installation costs and operating costs of methane production equipment.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a methane production method and a methane production apparatus that can efficiently produce methane and reduce costs.

[0007] That is, the present invention includes the following aspects: [1] A methane production method for producing methane from carbon dioxide and hydrogen, comprising a methanation step of generating non-equilibrium plasma inside a reactor filled with a methanation catalyst, introducing a mixed gas of carbon dioxide and hydrogen into the reactor, decomposing the mixed gas to generate radicals, and causing a methanation reaction, wherein the methanation catalyst is disposed in a fixed-bed catalyst layer contained in the reactor, and in the methanation step, the methanation reaction is caused to occur without supplying heat to the reactor from the outside, and the methanation reaction is continued without supplying heat to the reactor from the outside. [2] The method for producing methane according to [1], wherein the methanation catalyst comprises a carrier and a metal catalyst supported on the carrier, the carrier containing an oxide containing one or more elements selected from the group consisting of cerium, zirconium, yttrium, aluminum, silicon, magnesium, potassium, calcium, sodium, and lanthanum, and the metal catalyst containing one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium, and iridium. [3] The method for producing methane according to [1] or [2], wherein the generation of non-equilibrium plasma is stopped after the methanation reaction has started. [4] The method for producing methane according to any one of [1] to [3], wherein the generation of non-equilibrium plasma is stopped after the methanation reaction has started, and the non-equilibrium plasma is re-generated based on the internal temperature of the reactor or the amount of methane produced. [5] The method for producing methane according to any one of [1] to [4], wherein the methanation step is carried out in the absence of oxygen.[6] A methane production apparatus for producing methane from carbon dioxide and hydrogen, comprising: a reactor filled with a methanation catalyst; a gas supply unit that supplies the carbon dioxide and the hydrogen to the reactor; a gas discharge unit that discharges a reaction gas containing the methane from the reactor; and plasma generation means that generates non-equilibrium plasma inside the reactor and decomposes a mixed gas of the carbon dioxide and the hydrogen to generate radicals, wherein the reactor has a reaction vessel and a fixed-bed catalyst layer that is housed in the reaction vessel and has the methanation catalyst disposed therein; and the methane production apparatus does not have an external heat source that supplies heat from the outside to the reactor. [7] The methane production apparatus according to [6], wherein the methanation catalyst comprises a carrier and a metal catalyst supported on the carrier, the carrier containing an oxide containing one or more elements selected from the group consisting of cerium, zirconium, yttrium, aluminum, silicon, magnesium, and potassium, calcium, sodium, and lanthanum, and the metal catalyst contains one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium, and iridium.

[0008] According to the present invention, it is possible to provide a methane production method and a methane production apparatus that can efficiently produce methane and reduce costs.

[0009] FIG. 1 is a schematic diagram of an example of a methane production apparatus of the present invention. FIG. 2 is a schematic diagram of an example of a methane production apparatus of the present invention. FIG. 3 is a graph showing the temperature of the fixed bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis for the methane production methods of Examples 1 to 3 and Comparative Example 1. FIG. 4 is a graph showing the temperature of the fixed bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis for the methane production methods of Example 4 and Comparative Example 2. FIG. 5 is a graph showing the temperature of the fixed bed catalyst layer on the horizontal axis and the methane selectivity on the vertical axis for the methane production methods of Example 4 and Comparative Example 2.

[0010] <Methane Production Method and Methane Production Apparatus> The present invention relates to a methane production method for producing methane from carbon dioxide and hydrogen. FIG. 1 is a schematic diagram of an example of the methane production apparatus of the present invention. The methane production apparatus 1 shown in FIG. 1 includes a reactor 10 filled with a methanation catalyst. The methane production apparatus 1 includes a gas supply unit L1 that supplies carbon dioxide and hydrogen to the reactor 10, a gas discharge unit L2 that discharges a reaction gas containing methane from the reactor 10, and plasma generation means 13 that generates non-equilibrium plasma inside the reactor 10. The plasma generation means 13 includes a first electrode 13a and a second electrode 13b. The reactor 10 includes a reaction vessel 11 made of a dielectric material and a fixed-bed catalyst layer 12 housed in the reaction vessel 11 and containing a methanation catalyst. The production apparatus 1 does not include an external heat source that supplies heat to the reactor 10 from outside.

[0011] The methane production apparatus 1 may optionally include a water removal means 14, a power supply that supplies high voltage to the first electrode 13 a and the second electrode 13 b, electric wires, an earth ground, a detection unit 16 that detects components in the reaction gas including methane, a control means 17 that controls on / off of the plasma generation means 13 depending on the amount of methane, and a temperature detection means 18 that detects the temperature of the fixed-bed catalyst layer 12.

[0012] The water removal means 14 may be, for example, a cooling separation means or a solid adsorption means. The cooling separation means is a means for cooling the reaction gas containing methane to a temperature below the dew point of water and removing the resulting water droplets. The solid adsorption means is a means for adsorbing water onto a solid adsorbent such as silica gel.

[0013] The methanation process is a process in which a mixed gas of carbon dioxide and hydrogen is introduced into the reactor 10, non-equilibrium plasma is generated inside the reactor 10, the mixed gas is decomposed to generate radicals, and a methanation reaction occurs in which carbon dioxide is converted to methane. By the methanation process, a reaction gas containing methane is discharged from the reactor 10.

[0014] In non-equilibrium plasma, the electron temperature is much higher than the ion temperature, and some electrons have very high energy. This uses the high energy of the electrons, generating many activated species (radicals). Non-equilibrium plasma also generates heat.

[0015] When a non-equilibrium plasma is generated inside the reactor 10 while a mixed gas of carbon dioxide and hydrogen is introduced into the reactor 10, vibrationally excited CO ２ and H ２ , H ２ ＋ , CO ２ ＋ The heat and radicals generated by the non-equilibrium plasma activate the catalyst, and when the catalyst temperature exceeds approximately 200°C, the methanation reaction begins in the mixed gas.

[0016] The production apparatus 1 does not have an external heat source that supplies heat to the reactor 10 from the outside. That is, the methanation step causes the methanation reaction to occur without supplying heat to the reactor 10 from the outside, and continues the methanation reaction without supplying heat to the reactor 10 from the outside. In this way, the methanation reaction is defined as "automethanation," which causes the methanation reaction to occur without supplying heat to the reactor from the outside, and continues the methanation reaction without supplying heat to the reactor from the outside. Note that when describing the "methanation reaction," the term "CO ２ +4H ２ →CH ４ +2H ２ O" means the reaction represented by

[0017] In this embodiment, the automethanation step is preferably carried out in the absence of oxygen, since the presence of oxygen reacts with the hydrogen in the feedstock to convert it to water, and the presence of water also inhibits the methanation reaction.

[0018] In this specification, the term "mixed gas" refers to a mixed gas of carbon dioxide and hydrogen, and refers to the raw material gas for the methane production method of the present invention. The mixed gas may be a mixed gas consisting of carbon dioxide and hydrogen, or may contain components other than carbon dioxide and hydrogen. For example, when exhaust gas emitted from a power plant, factory, etc. is used as the carbon dioxide source, it may contain components other than carbon dioxide and hydrogen to the extent that they do not poison the catalyst. Furthermore, a mixed gas of biogas and hydrogen may be used as the mixed gas. Biogas is CH ４ and CO ２ The main component is originally CH ４ However, the CO in biogas ２ CH ４ It is widely used as a raw material for methanation because its added value as a fuel increases when converted into

[0019] In one example of a methanation reaction by automethanation, a mixed gas is introduced into a reactor, and when the heat and radicals generated by decomposition of the mixed gas by non-equilibrium plasma act, the methanation reaction begins when the temperature inside the reactor reaches approximately 200°C. The heat of reaction generated by the methanation reaction increases the temperature inside the reactor without external heating, and the methanation reaction continues. When the temperature exceeds approximately 250°C, the carbon dioxide conversion rate reaches a value roughly close to thermal equilibrium.

[0020] Generally, a methanation reaction using a methanation catalyst requires heating to approximately 300°C to 400°C to initiate the reaction, resulting in a high reaction temperature of 300°C or higher. Heating to initiate the reaction is usually performed by supplying heat from an external heat source. In contrast, the present invention does not require an external heat source to initiate the reaction, and methane can be produced at a methanation reaction temperature of approximately 250°C, a low temperature range of less than 300°C.

[0021] Automethanation continues due to the heat of reaction generated by the exothermic methanation reaction, so the methanation reaction will continue as long as the introduction of the mixed gas is continued. If the introduction of the mixed gas is stopped, all of the carbon dioxide in the mixed gas is converted to methane and then automethanation stops. Because the methanation reaction requires four times the amount of hydrogen as carbon dioxide, automethanation can also be stopped by reducing the amount of hydrogen or increasing the amount of carbon dioxide in the mixed gas at a ratio such that the amount of hydrogen is less than four times the amount of carbon dioxide.

[0022] The flow rate of the mixed gas may be adjusted appropriately depending on the size and number of the reactors 10 .

[0023] In the methane production method of this embodiment, if the temperature of the reactor 10 rises suddenly, it is determined that the methanation reaction has started. After the start of the methanation reaction is confirmed, the generation of non-equilibrium plasma may be stopped at any timing. After the start of the methanation reaction, the generation of non-equilibrium plasma may be stopped and then the non-equilibrium plasma may be generated again. For example, if the temperature of the reactor 10 continues to decrease after the non-equilibrium plasma is stopped, the non-equilibrium plasma may be generated again, thereby allowing auto-methanation to proceed.

[0024] The temperature of the reactor 10 may be measured by directly measuring the temperature inside the reactor 10 using, for example, a thermocouple, or by measuring the temperature inside the reactor 10 from the outside using an infrared thermal imaging camera.

[0025] In one embodiment of the present invention, when a metal catalyst (e.g., one or more selected from the group consisting of ruthenium, rhodium, and platinum) is used, the heat of reaction generated after the methanation reaction starts tends to maintain an efficient methanation reaction. Therefore, even if the generation of non-equilibrium plasma is stopped after the start of the methanation reaction is confirmed, the methanation reaction may continue in the reactor 10 at room temperature.

[0026] The component composition in the reactor at thermal equilibrium can be calculated using the Gibbs free energy minimization method. The carbon dioxide conversion rate is calculated using the following formula: Carbon dioxide conversion rate (%) = [amount of carbon dioxide in mixed gas (moles) - amount of carbon dioxide in produced gas (moles)] / amount of carbon dioxide in mixed gas (moles) × 100

[0027] The methane production method using automethanation does not require an external heat source to start the methanation reaction, so methane can be produced without using a separate heating or cooling device, significantly reducing the installation costs of the methane production equipment.In addition, because automethanation proceeds using the reaction heat from the methanation reaction, there is no need to provide additional energy to continue the reaction, significantly reducing operating costs.

[0028] If renewable energy such as solar power or wind power is used as the electricity required to generate plasma, the use of renewable energy will expand further, and CO ２ This will further contribute to reducing emissions.

[0029] Even when the supply of carbon dioxide or hydrogen is low or fluctuates, this method can produce methane from carbon dioxide in the required amount when needed using simple equipment and electrical energy (i.e., plasma), making it applicable as a methane production system that can handle small-scale distribution and load fluctuations. The scale of the methane production system of the present invention is not particularly limited, but it can also be expanded by connecting small-scale distributed methane production systems in parallel. Therefore, the length of the fixed-bed catalyst layer 12 (the length indicated by symbol S1 in FIG. 1 ) provided in the reactor 10 is, for example, 1 cm to 50 cm, or 1 cm to 20 cm. The diameter of the reactor (inner diameter of the glass tube) can be set between 2 cm and 10 cm.

[0030] The reaction vessel 11 provided in the reactor 10 is preferably made of glass or ceramics such as alumina, from the viewpoint of being made of a dielectric material for the plasma generating means described later.

[0031] The methane production apparatus 1 preferably includes a temperature detection means 18 for detecting the temperature of the fixed-bed catalyst layer. The temperature detection means 18 is not particularly limited, and a thermocouple, an infrared thermal imaging camera, or the like can be used.

[0032] By detecting the temperature of the fixed bed catalyst layer 12 with the temperature detection means 18, it is possible to monitor the state of the methanation reaction, such as whether the methanation reaction has started or whether thermal equilibrium has been reached.

[0033] The fixed-bed catalyst layer 12 includes a catalyst. The catalyst includes a carrier and a metal catalyst supported on the carrier. The carrier preferably contains an oxide containing one or more elements selected from the group consisting of cerium, zirconium, yttrium, aluminum, silicon, magnesium, potassium, calcium, sodium, and lanthanum. The carrier preferably contains cerium oxide (CeO ２ ), zirconium oxide (ZrO ２ ), yttrium oxide (Y ２ O ３ ), aluminum oxide (Al ２ O ３ ), silicon oxide (SiO ２ ), magnesium oxide (MgO), calcium oxide (CaO), sodium oxide (Na ２ O), lanthanum(III) oxide (La ２ O ３ It is more preferable that the compound is one or more selected from the group consisting of:

[0034] From the viewpoint of facilitating the generation of non-equilibrium plasma, the support is preferably a material that does not have electrical conductivity, and more preferably is one or more selected from the group consisting of silicon oxide, which is an oxide of silicon, aluminum oxide, and magnesium oxide.

[0035] The metal catalyst is more preferably one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium and iridium, and even more preferably nickel or ruthenium.

[0036] The content of the metal catalyst contained in the catalyst is not particularly limited and is, for example, 0.5 mass % to 20 mass % based on the mass of the carrier. The total amount of catalyst contained in the fixed-bed catalyst layer 12 may be adjusted appropriately depending on the size of the reactor.

[0037] The fixed-bed catalyst layer 12 has voids through which the mixed gas can pass. The fixed-bed catalyst layer 12 may be formed by packing a pellet-shaped catalyst, or may be formed by supporting a metal catalyst on a carrier formed into a three-dimensional shape through which the mixed gas can pass. The size of the catalyst may be adjusted appropriately depending on the shape of the pellets, etc. The carrier formed into a three-dimensional shape through which the mixed gas can pass is, for example, a porous carrier.

[0038] As a catalyst layer configuration, for example, a fluidized bed catalyst layer has the advantage that the reaction tends to proceed uniformly, but it is necessary to ensure appropriate flow of the powder catalyst, and the setting conditions for the flow rate of the mixed gas are extremely limited. For example, the flow rate of the mixed gas to ensure the flow of the powder catalyst may differ from the flow rate of the mixed gas to achieve automethanation. In addition, the methanation reaction (CO ２ +4H ２ →CH ４ +2H ２ Methanation reaction (O) is a reaction in which 5 moles of gas are reduced to 3 moles. Even if the gas flow rate near the reactor inlet is sufficient, the gas flow rate near the reactor outlet decreases to about 60%. In methanation reactions, which involve large changes in gas volume, the powder catalyst becomes less fluid. This can make it difficult to ensure powder fluidity while achieving auto-methanation. On the other hand, a fixed-bed catalyst bed does not require consideration of powder fluidity. Conditions for achieving auto-methanation, such as the mixed gas flow rate and catalyst loading, can be set with a high degree of freedom, allowing for adaptation to reactor size and load fluctuations.

[0039] The plasma generating means 13 includes a pair of electrodes (first electrode 13a and second electrode 13b). In the methane production apparatus 1, the reaction vessel 11 is made of a dielectric material, and when a high voltage is applied to the first electrode 13a and the second electrode 13b, non-equilibrium plasma is generated by dielectric barrier discharge so as to cover the surface of the catalyst pellets placed between the first electrode 13a and the second electrode 13b. The second electrode 13b is preferably provided on the outer periphery of the reaction vessel 11. The plasma generating means 13 is not limited to dielectric barrier discharge and may be changed to another means as long as it can combine non-equilibrium plasma and a catalyst.

[0040] The pair of electrodes are arranged, for example, parallel to the flow direction of the mixed gas. The distance between the first electrode 21 and the second electrode 23 is not particularly limited and is, for example, 0.5 mm or more and 10 cm or less. The voltage applied to the first electrode 13a and the second electrode 13b is, for example, 5 kV or more and 20 kV or less. The pressure of the plasma generation field is, for example, 500 hPa or more and 2000 hPa or less. The temperature of the plasma generation field is maintained at room temperature (for example, 15°C or more and 30°C or less) that does not exceed the ambient temperature. In order to be a dielectric, the reaction vessel 11 is preferably made of quartz glass, soda glass, alumina, or other ceramics.

[0041] FIG. 2 is a schematic diagram of an example of a methane production apparatus of the present invention. The methane production apparatus 2 shown in FIG. 2 includes a reactor 20 filled with a methanation catalyst. The methane production apparatus 2 includes a gas supply unit 25 that supplies carbon dioxide and hydrogen to the reactor 20, a gas discharge unit L12 that discharges a reaction gas containing methane from the reactor 20, and plasma generation means 23 that generates non-equilibrium plasma inside the reactor 20. The plasma generation means 23 includes a first electrode 23a and a second electrode 23b. The reactor 20 includes a reaction vessel 21 made of a dielectric material and a fixed-bed catalyst layer 22 housed in the reaction vessel 21 and containing a methanation catalyst. The production apparatus 2 does not include an external heat source that supplies heat to the reactor 20 from outside.

[0042] The methane production apparatus 2 may optionally include a water removal means 24, a power supply that supplies high voltage to the first electrode 23 a and the second electrode 23 b, electric wires, an earth ground, a detection unit 26 that detects components in the reaction gas including methane, a control means 27 that controls on / off of the plasma generation means 23 depending on the amount of methane, and a temperature detection means 28 that detects the temperature of the fixed-bed catalyst layer 22.

[0043] The bottom of the reaction vessel 21 is closed, and a gas supply pipe 25 is disposed inside the reaction vessel 21. The bottom of the gas supply pipe 25 is open, and a fixed-bed catalyst layer 22 is formed between the reaction vessel 21 and the gas supply pipe 25. The fixed-bed catalyst layer 22 is preferably formed by filling the catalyst with, for example, pellets, so that the filling portion faces in the vertical direction. The reaction vessel 21 has an upper pipe wall 21a at its upper end, and the gas supply pipe 25 is supported by the upper pipe wall 21a while passing through the upper pipe wall 21a.

[0044] The gas supply pipe 25 is a pipe for introducing the mixed gas into the reaction vessel 21. The mixed gas introduced through the gas supply pipe 25 is discharged from the lower part of the gas supply pipe 25 and introduced into the fixed-bed catalyst layer 22. When the mixed gas is introduced into the fixed-bed catalyst layer 22 and non-equilibrium plasma is generated, a methanation reaction begins due to the action of the catalyst. Thereafter, the methanation reaction continues in the reactor 20 at room temperature, and the produced gas containing methane and water flows upward in the reaction vessel 21 and is discharged from the gas discharge pipe L12 provided above the reaction vessel 21.

[0045] The length of the fixed bed catalyst layer 22 (the length indicated by reference symbol S2 in FIG. 2) included in the reactor 20 is, for example, 1 cm to 100 cm, or 1 cm to 50 cm. The outer diameter of the fixed bed catalyst layer 22 may be adjusted appropriately depending on the size of the reactor.

[0046] The plasma generating means 23 includes a pair of electrodes (a first electrode 23a and a second electrode 23b). In the methane production apparatus 2, the reaction vessel 21 is made of a dielectric material, and the gas supply pipe 25 also serves as the first electrode 23a. The gas supply pipe 25 is, for example, a metal pipe. When a high voltage is applied to the first electrode 23a and the second electrode 23b, non-equilibrium plasma is generated by dielectric barrier discharge so as to cover the surface of the pellets. The second electrode 23b is preferably provided on the outer periphery of the reaction vessel 21.

[0047] The methane production apparatus 2 is similar to the methane production apparatus 1 except for the above-mentioned features.

[0048] The present invention will now be described in more detail with reference to examples.

[0049] Example 1 A methane production apparatus 2 having the configuration shown in FIG. 2 was used. A glass reaction vessel 21 having a diameter of 30 mm and a height of 60 mm was used. The catalyst was Ru / Al containing 2 mass % of ruthenium. ２ O ３ (manufactured by N.E. Chemicat Co., Ltd.) was used. The reactor 20, which was not equipped with a heating device or a cooling device, was kept at 25° C., and the inside of the reactor 20 was kept at 200 hPa to 1000 hPa.

[0050] An infrared thermal imaging camera (product name: TH5104, manufactured by NEC Sanei Corporation) was used to measure the temperature of the fixed-bed catalyst layer. The amount of carbon dioxide in the produced gas discharged from the gas discharge pipe L12 was measured, and the carbon dioxide conversion rate was calculated using the following formula: Carbon dioxide conversion rate (%) = [amount of carbon dioxide (moles) in the mixed gas - amount of carbon dioxide (moles) in the produced gas] / amount of carbon dioxide (moles) in the mixed gas × 100

[0051] As a mixed gas, which is a raw material gas, a mixed gas of carbon dioxide and hydrogen (carbon dioxide: hydrogen = 1:4) was supplied at a flow rate of 1500 cm ３ While introducing the gas into the reaction vessel 21 at a rate of 1 / min (calculated at 25° C. and 1013 hPa), a high voltage of 5 to 20 kV was applied to the first electrode 23a and the second electrode 23b, and non-equilibrium plasma was generated by dielectric barrier discharge.

[0052] The temperature of the fixed-bed catalyst layer was measured simultaneously with the supply of the mixed gas and the generation of non-equilibrium plasma, and the maximum temperature and carbon dioxide conversion rate of the fixed-bed catalyst layer were measured. Figure 3 shows a graph in which the temperature of the fixed-bed catalyst layer is on the horizontal axis and the carbon dioxide conversion rate is on the vertical axis.

[0053] Example 2: The flow rate of the mixed gas was 3000 cm ３ Methane was produced in the same manner as in Example 1, except that the flow rate was changed to / min. A temperature distribution occurs in the fixed-bed catalyst layer, and a graph showing the maximum temperature on the horizontal axis and the carbon dioxide conversion rate on the vertical axis is shown in Figure 3.

[0054] Example 3: The flow rate of the mixed gas was 5000 cm ３ Except for changing the temperature of the fixed-bed catalyst layer to / min, methane was produced in the same manner as in Example 1. A graph showing the temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis is shown in Figure 3.

[0055] Comparative Example 1: Without using the plasma generating means 23, the reaction vessel 21 was heated from the outside, and the flow rate of the mixed gas was set to 3000 cm ３ 3 is a graph showing the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0056] 3, it was confirmed that in Examples 1 to 3, heat and radicals were simultaneously supplied to the inside of reactor 20 by the dielectric barrier discharge, the temperature of the catalyst layer rose from room temperature, and the methanation reaction started when it reached approximately 200°C. Because the methanation reaction is an exothermic reaction, the methanation reaction was further accelerated by the heat of reaction, and the temperature of the catalyst rose to approximately 300°C.

[0057] In Examples 1 and 2, the carbon dioxide conversion rate was roughly close to thermal equilibrium when the temperature exceeded 250°C. Even if the dielectric barrier discharge was stopped in this state, as long as the supply of the mixed gas continued, the methanation reaction continued at room temperature, confirming that auto-methanation could be achieved.

[0058] On the other hand, in Comparative Example 1, the methanation reaction was observed to start at around 300°C.

[0059] Focusing on the carbon dioxide conversion rate at around 250°C, it was confirmed that the carbon dioxide conversion rate in Comparative Example 1 was approximately 5%, while that in Example 1 was approximately 75%, that in Example 2 was approximately 65%, and that in Example 3 was approximately 35%. This indicates that the carbon dioxide conversion rate in Examples 1 to 3 was significantly higher than that in Comparative Example 1.

[0060] Example 4 A catalyst was prepared by the following method. ３ ) ３ 10 equivalents of urea was added to the aqueous solution of Ni and dissolved. ２ O ３ The resulting mixture was heated to 90°C and stirred for 5 hours. After stirring, the resulting solid was filtered, washed, dried, and reduced in a hydrogen stream at 600°C for 1 hour to obtain a Ni / Al mixture containing 6 mass% of nickel. ２ O ３ obtained.

[0061] Ni / Al as catalyst ２ O ３ The flow rate of the mixed gas was set to 500 cm ３ 4 is a graph showing the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0062] 4, it was confirmed that heat and radicals were simultaneously supplied to the inside of reactor 20 by the dielectric barrier discharge, and the temperature of the catalyst layer rose from room temperature, and the methanation reaction began when it reached approximately 170°C. The heat generated by methanation caused the catalyst temperature to rise, and at 250°C the carbon dioxide conversion rate reached approximately 40%.

[0063] Comparative Example 2: Ni / Al catalyst ２ O ３ The flow rate of the mixed gas was set to 500 cm ３ 4 is a graph showing the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0064] For Example 4 and Comparative Example 2, the methane selectivity was measured using the following formula: Methane selectivity (%) = [amount of methane (moles) contained in the produced gas] / [amount of methane (moles) contained in the produced gas + amount of carbon monoxide (moles) contained in the produced gas] × 100

[0065] FIG. 5 is a graph showing the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the methane selectivity on the vertical axis for Example 4 and Comparative Example 2.

[0066] In Example 4, the carbon dioxide conversion rate at 250°C was about 40%, while it was 5% in Comparative Example 2. In Example 4, the carbon dioxide conversion rate was about 8 times higher than in Comparative Example 2, which utilized a thermal reaction.

[0067] 1, 2: methane production apparatus, 10, 20: reactor, 11, 21: reaction vessel, 12, 22: fixed-bed catalyst layer, 13, 23: plasma generation means, L1, 25: gas supply section, L2, L12: gas discharge section

Claims

1. A methane production method for producing methane from carbon dioxide and hydrogen, comprising a methanation step of introducing a mixed gas of carbon dioxide and hydrogen into a reactor filled with a methanation catalyst, generating non-equilibrium plasma inside the reactor to decompose the mixed gas to generate radicals and cause a methanation reaction, wherein the methanation catalyst is disposed in a fixed bed catalyst layer contained in the reactor, and wherein the methanation step causes the methanation reaction to occur without supplying heat to the reactor from the outside, and continues the methanation reaction without supplying heat to the reactor from the outside.

2. The method for producing methane according to claim 1, wherein the methanation catalyst comprises a carrier and a metal catalyst supported on the carrier, the carrier containing an oxide containing one or more elements selected from the group consisting of cerium, zirconium, yttrium, aluminum, silicon, magnesium, potassium, calcium, sodium and lanthanum, and the metal catalyst containing one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium and iridium.

3. A method for producing methane according to claim 1 or 2, wherein the generation of the non-equilibrium plasma is stopped after the methanation reaction has started.

4. A method for producing methane as described in claim 1 or 2, wherein the generation of the non-equilibrium plasma is stopped after the methanation reaction has started, and the non-equilibrium plasma is generated again based on the internal temperature of the reactor or the amount of methane produced.

5. The method for producing methane according to claim 1 or 2, wherein the methanation step is carried out in the absence of oxygen.

6. A methane production apparatus for producing methane from carbon dioxide and hydrogen, comprising: a reactor filled with a methanation catalyst; a gas supply unit for supplying the carbon dioxide and the hydrogen to the reactor; a gas discharge unit for discharging a reaction gas containing the methane from the reactor; and plasma generating means for generating non-equilibrium plasma inside the reactor and decomposing a mixed gas of the carbon dioxide and the hydrogen to generate radicals, wherein the reactor has a reaction vessel and a fixed bed catalyst layer contained in the reaction vessel and having the methanation catalyst disposed therein; and the methane production apparatus does not have an external heat source for supplying heat from the outside to the reactor.

7. The methane production apparatus according to claim 6, wherein the methanation catalyst comprises a carrier and a metal catalyst supported on the carrier, the carrier containing an oxide containing one or more elements selected from the group consisting of cerium, zirconium, yttrium, aluminum, silicon, magnesium, and potassium, calcium, sodium, and lanthanum, and the metal catalyst containing one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium, and iridium.

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