Methane production method and methane production apparatus

Abstract

To provide a methane production method that efficiently produces and can reduce cost and a methane production apparatus.SOLUTION: A method for producing methane from carbon dioxide and hydrogen, which includes a methanation process where a mixed gas of carbon dioxide and hydrogen is introduced into a reactor filled with a methanation catalyst, generating non-equilibrium plasma inside the reactor to decompose the mixed gas, thereby producing radicals and enabling the methanation reaction. The methanation catalyst is arranged in a fixed bed catalytic layer housed within the reactor, and in the methanation process, the methanation reaction is caused without supplying heat from outside to the reactor, continuously allowing the methanation reaction to proceed without external heat supply to the reactor.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method and an apparatus for producing methane. [Background Art]

[0002] Carbon dioxide is considered one of the greenhouse gases that cause global warming, and technologies for converting carbon dioxide into useful substances are being studied toward the realization of a carbon-neutral society. As a technology for converting carbon dioxide into methane, which is a useful substance, a methanation reaction that produces methane from carbon dioxide and hydrogen is known.

[0003] The methanation reaction is generally carried out in the presence of a heated methanation catalyst. Further, in order 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, and initiates the methanation reaction by heating the catalyst. [Prior Art Literature] [Non-Patent Literature]

[0004] [Non-Patent Document 1] 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 Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation:J.Am.Chem.Soc.2022,144,31,14140-14149 [Summary of the Invention] [Problems that the invention aims to solve]

[0005] Reducing manufacturing costs is a challenge in promoting carbon dioxide conversion technology through methanation. The method disclosed in Non-Patent Document 1 is useful in that it can significantly improve the conversion efficiency of carbon dioxide, but there is room for improvement in terms of further saving on the installation and operating costs of methane production equipment.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a methane production method and a methane production apparatus that can efficiently produce methane and reduce costs. [Means for solving the problem]

[0007] In other words, the present invention includes the following embodiments. [1] A methane production method for producing methane from carbon dioxide and hydrogen, comprising a methanation step in which a mixed gas of carbon dioxide and hydrogen is introduced into a reactor filled with a methanation catalyst, a non-equilibrium plasma is generated inside the reactor to decompose the mixed gas and generate radicals, and a methanation reaction occurs, wherein the methanation catalyst is arranged in a fixed-bed catalyst layer housed in the reactor, and in the methanation step, the methanation reaction occurs without supplying heat to the reactor from the outside, and the methanation reaction continues 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 comprising 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 comprising one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium and iridium. [3] The methane production method according to [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 according to any one of [1] to [3], 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 temperature inside the reactor or the amount of methane produced. [5] A 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 for supplying the carbon dioxide and hydrogen to the reactor; a gas discharge unit for discharging the reaction gas containing the methane from the reactor; and a plasma generating means for generating a non-equilibrium plasma inside the reactor and decomposing the mixed gas of carbon dioxide and hydrogen to generate radicals, wherein the reactor comprises a reaction vessel and a fixed-bed catalyst layer housed in the reaction vessel and on which the methanation catalyst is arranged, and the reactor does not have an external heat source for supplying heat from the outside. [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 contains an oxide comprising 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 comprises one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium and iridium. [Effects of the Invention]

[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. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram of an example of a methane production apparatus according to the present invention. [Figure 2]This is a schematic diagram of an example of a methane production apparatus according to the present invention. [Figure 3] This graph shows the methane production methods of Examples 1-3 and Comparative Example 1, with the temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis. [Figure 4] This graph shows the methane production methods of Example 4 and Comparative Example 2, with the temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis. [Figure 5] This graph shows the methane production methods of Example 4 and Comparative Example 2, with the temperature of the fixed-bed catalyst layer on the horizontal axis and the methane selectivity on the vertical axis. [Modes for carrying out the invention]

[0010] <Method and apparatus for producing methane> This invention relates to a method for producing methane from carbon dioxide and hydrogen. Figure 1 is a schematic diagram of an example of a methane production apparatus of the present invention. The methane production apparatus 1 shown in Figure 1 includes a reactor 10 packed 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 the reaction gas containing methane from the reactor 10, and a plasma generating means 13 that generates a non-equilibrium plasma inside the reactor 10. The plasma generating means 13 includes a first electrode 13a and a second electrode 13b. The reactor 10 comprises 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 methane catalyst. The manufacturing apparatus 1 does not have an external heat source that supplies heat to the reactor 10 from the 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 13a and the second electrode 13b, electric wires, an earth ground, a detection unit 16 that detects components in the reaction gas containing methane, a control means 17 that controls the on / off of the plasma generation means 13 according to the amount of methane, and a temperature detection means 18 that detects the temperature of the fixed-bed catalyst layer 12.

[0012] Examples of the water removal means 14 include cooling separation means and solid adsorption means. The cooling separation means is a means for cooling a reaction gas containing methane to a temperature equal to or lower than the dew point of water, and removing generated water droplets. The solid adsorption means is a means for adsorbing water onto a solid adsorbent such as silica gel.

[0013] The methanation step is a step in which, in a state where 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 that converts carbon dioxide into methane is caused. A reaction gas containing methane is discharged from the reactor 10 through the methanation step.

[0014] In non-equilibrium plasma, the electron temperature is much higher than the ion temperature, and some electrons have extremely high energy. For this reason, using the high energy of electrons, a large amount of active species (radicals) are generated. In addition, heat is generated by the non-equilibrium plasma.

[0015] When non-equilibrium plasma is generated inside the reactor 10 in a state where the mixed gas of carbon dioxide and hydrogen is introduced into the reactor 10, vibrationally excited CO₂, H₂, and H₂ + , CO₂ + and other ions, as well as many radicals such as atomic hydrogen and atomic oxygen are generated; the heat generated by the non-equilibrium plasma and the radicals activate the catalyst, and the methanation reaction starts in the mixed gas when the catalyst temperature exceeds about 200°C.

[0016] The production apparatus 1 does not have an external heat source that supplies heat from the outside to the reactor 10. That is, in the methanation step, the methanation reaction is caused without supplying heat from the outside to the reactor 10, and the methanation reaction is continued without supplying heat from the outside to the reactor 10. Thus, we define "auto-methanation" as a process that generates a methanation reaction without supplying heat to the reactor from the outside, and continues the methanation reaction without supplying heat to the reactor from the outside. When referring to the "methation reaction," it means the reaction represented as "CO2 + 4H2 → CH4 + 2H2O".

[0017] In this embodiment, the presence of oxygen causes it to react with the hydrogen in the raw material and be converted into water, and the presence of water further suppresses the suppression of the methanation reaction. Therefore, it is preferable to carry out the automethanation process in the absence of oxygen.

[0018] In this specification, "mixed gas" refers to a mixed gas of carbon dioxide and hydrogen, and means the raw material gas for the methane production method of the present invention. The mixed gas may be a mixture of carbon dioxide and hydrogen, or it may contain components other than carbon dioxide and hydrogen. For example, when using exhaust gas emitted from power plants or factories as a carbon dioxide source, it may contain components other than carbon dioxide and hydrogen, as long as they do not become catalytic poisons. Alternatively, a mixed gas of biogas and hydrogen may be used. Biogas mainly consists of CH4 and CO2, and although it originally contains CH4, its value as a fuel is increased by converting the CO2 in biogas to CH4, so it is widely used as a raw material for methanation.

[0019] In one example of a methanation reaction using automethanation, a gas mixture is introduced into a reactor, and when the non-equilibrium plasma acts on the reaction, the methanation reaction begins when the temperature inside the reactor reaches approximately 200°C. Due to the reaction heat generated by the methanation reaction, the temperature inside the reactor rises without external heating, and the methanation reaction continues. When the temperature exceeds approximately 250°C, the conversion rate of carbon dioxide approaches a value close to thermal equilibrium.

[0020] Generally, methanation reactions using a methanation catalyst require heating to around 300°C to 400°C to initiate the reaction, resulting in a reaction temperature in the high-temperature range of 300°C or higher. This heating to initiate the reaction is typically carried out 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 the reaction temperature of the methanation reaction is around 250°C, enabling methane production in a low temperature range below 300°C.

[0021] Automethanation is a reaction that continues due to the heat of reaction generated by the exothermic methanation reaction; therefore, if the introduction of the mixed gas continues, the methanation reaction will continue. When the introduction of the mixed gas is stopped, automethanation stops after all the carbon dioxide in the mixed gas has been converted to methane. Since the methanation reaction requires four times the amount of hydrogen as carbon dioxide, automethanation can be stopped by reducing the amount of hydrogen in the gas mixture to a ratio of less than four times the amount of carbon dioxide, or by increasing the amount of carbon dioxide.

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

[0023] In the methane production method of this embodiment, if the temperature of the reactor 10 rises rapidly, 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 time. After the methanation reaction has started and the generation of non-equilibrium plasma has been stopped, non-equilibrium plasma may be generated again. For example, if the temperature of the reactor 10 continues to decrease after the non-equilibrium plasma has been stopped, auto-mettanation will proceed by generating non-equilibrium plasma again.

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

[0025] In one aspect of the present invention, when a metal catalyst (for example, one or more selected from the group consisting of ruthenium, rhodium, and platinum) is used, the reaction heat generated after the start of the methanation reaction makes it easier to maintain an efficient methanation reaction. For this reason, even if the generation of non-equilibrium plasma is stopped after the start of the methanation reaction is confirmed, the methanation reaction may continue to proceed in the reactor 10 at room temperature.

[0026] The component composition in a reactor under 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 the mixed gas (moles) - Amount of carbon dioxide in the produced gas (moles)] / Amount of carbon dioxide in the mixed gas (moles) × 100

[0027] According to the methane production method using automethanation, an external heat source is not required to initiate the methanation reaction. Therefore, methane can be produced without the use of separate heating or cooling equipment, significantly reducing the installation cost of methane production equipment. Furthermore, since 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] Using renewable energy sources such as solar and wind power to generate plasma will further expand the use of renewable energy and contribute even more to reducing CO2 emissions.

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

[0030] From the viewpoint of using glass or ceramics such as alumina as the dielectric material for the plasma generating means described later, the reaction vessel 11 of the reactor 10 is preferably made of ceramics.

[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 thermocouples, infrared thermal imaging cameras, etc., can be used.

[0032] By detecting the temperature of the fixed-bed catalyst layer 12 using the temperature detection means 18, the state of the methanation reaction, such as whether the methanation reaction has started or whether it has reached thermal equilibrium, can be monitored.

[0033] The fixed-bed catalyst layer 12 comprises 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 support material is more preferably one or more selected from the group consisting of cerium oxide (CeO2), zirconium oxide (ZrO2), yttrium oxide (Y2O3), aluminum oxide (Al2O3), silicon oxide (SiO2), magnesium oxide (MgO), calcium oxide (CaO), sodium oxide (Na2O), and lanthanum(III) oxide (La2O3).

[0034] From the viewpoint of facilitating the generation of non-equilibrium plasma, the support material is preferably a material that does not have electrical conductivity, and more preferably one or more selected from the group consisting of silicon oxides such as silicon oxide, 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, but is, for example, 0.5% by mass or more and 20% by mass or less based on the mass of the support. The total amount of catalyst contained in the fixed-bed catalyst layer 12 can be adjusted as appropriate 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 filling it with pellet-shaped catalyst, or it may be formed by supporting a metal catalyst on a carrier molded into a three-dimensional shape through which the mixed gas can pass. The size of the catalyst can be adjusted as appropriate depending on the shape of the pellets, etc. A carrier molded into a three-dimensional shape that allows a gas mixture to pass through is, for example, a porous carrier.

[0038] While a fluidized bed catalyst layer, for example, offers the advantage of uniform reaction, it requires ensuring proper flow of the powder catalyst, severely limiting the conditions for setting the flow rate of the mixed gas. For instance, the flow rate of the mixed gas required to ensure the flow of the powder catalyst may differ from the flow rate of the mixed gas required to achieve automethanation. Furthermore, the methanation reaction (CO2 + 4H2 → CH4 + 2H2O) is a reaction in which 5 moles of gas are reduced to 3 moles. Even if the gas flow rate is sufficient near the reactor inlet, it decreases to about 60% near the reactor outlet. In the methanation reaction, where the volume change of the gas is large, the powder catalyst becomes difficult to flow. For this reason, it may be difficult to achieve both powder flow and auto-methanation. On the other hand, fixed-bed catalyst layers do not require consideration of powder flow, and conditions such as the flow rate of the mixed gas and the amount of catalyst packed in order to achieve automethanation can be set with a high degree of freedom, thus accommodating changes in reactor size and load.

[0039] The plasma generating means 13 comprises a pair of electrodes (a first electrode 13a and a second electrode 13b). In the methane production apparatus 1, the reaction vessel 11 is made of dielectric material, and when a high voltage is applied to the first electrode 13a and the second electrode 13b, a non-equilibrium plasma is generated by dielectric barrier discharge so as to cover the surface of the catalyst pellet placed between the first electrode 13a and the second electrode 13b. The second electrode 13b is preferably provided on the outer circumference of the reaction vessel 11. The plasma generation means 13 is not limited to dielectric barrier discharge and may be changed to other means as long as it can combine a non-equilibrium plasma with 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, 5kV or more and 20kV or less. The pressure in the plasma generation field is, for example, between 500 hPa and 2000 hPa. The temperature of the plasma generation field is kept at ambient temperature (for example, between 15°C and 30°C) and does not exceed the ambient temperature. To serve as a dielectric, the reaction vessel 11 is preferably made of ceramics such as quartz glass, soda glass, or alumina.

[0041] Figure 2 is a schematic diagram of an example of the methane production apparatus of the present invention. The methane production apparatus 2 shown in Figure 2 includes a reactor 20 that is filled with a methane 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 the reaction gas containing methane from the reactor 20, and a plasma generating means 23 that generates a non-equilibrium plasma inside the reactor 20. The plasma generating means 23 includes a first electrode 23a and a second electrode 23b. The reactor 20 has a reaction vessel 20 made of a dielectric material, and a fixed-bed catalyst layer 22 housed in the reaction vessel 21, on which a methane catalyst is placed. The manufacturing apparatus 2 does not have an external heat source that supplies heat to the reactor 20 from the 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 23a and the second electrode 23b, electric wires, an earth ground, a detection unit 26 that detects components in the reaction gas containing methane, a control means 27 that controls the on / off of the plasma generation means 23 according to the amount of methane, and a temperature detection means 28 that detects the temperature of the fixed-bed catalyst layer 22.

[0043] The reaction vessel 21 has a closed bottom, and a gas supply pipe 25 is located inside the reaction vessel 21. The gas supply pipe 25 has an open bottom, and a fixed-bed catalyst layer 22 is formed between the reaction vessel 21 and the gas supply pipe 25. Preferably, the fixed-bed catalyst layer 22 is formed by filling it with, for example, pellet-type catalyst, so that the filled portion faces vertically. The reaction vessel 21 has an upper pipe wall 21a on its upper end side, and the gas supply pipe 25 is supported by the upper pipe wall 21a, passing through it.

[0044] The gas supply pipe 25 is a pipe that introduces the mixed gas into the reaction vessel 21. The mixed gas introduced through the gas supply pipe 25 is discharged from the bottom 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 a non-equilibrium plasma is generated, the methanation reaction is initiated by the action of the catalyst. Subsequently, the methanation reaction continues in the reactor 20 at room temperature, and the gas containing the produced methane and water flows to the top of the reaction vessel 21 and is discharged from the gas discharge pipe L12 located above the reaction vessel 21.

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

[0046] The plasma generating means 23 comprises 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 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, a non-equilibrium plasma is generated by dielectric barrier discharge so as to cover the surface of the pellet. The second electrode 23b is preferably provided on the outer circumference of the reaction vessel 21.

[0047] Methane production apparatus 2 is described in the same way as methane production apparatus 1, except for the features mentioned above. [Examples]

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

[0049] <Example 1> A methane production apparatus 2 with the configuration shown in Figure 2 was used. A glass reaction vessel 21 with a diameter of 30 mm and a height of 60 mm was used. The catalyst used was Ru / Al2O3 (manufactured by N.E. Chemicat Co., Ltd.) containing 2% by mass of ruthenium. The reactor 20, which was not equipped with a heating or cooling device, was kept at 25°C, and the pressure inside the reactor 20 was maintained between 200 hPa and 1000 hPa.

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

[0051] As the raw material gas, a mixed gas of carbon dioxide and hydrogen (carbon dioxide:hydrogen = 1:4) is used at a flow rate of 1500 cm³. 3 While introducing the solution into the reaction vessel 21 at a rate of / 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, generating a non-equilibrium plasma by dielectric barrier discharge.

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

[0053] <Example 2> The flow rate of the mixed gas is 3000 cm³. 3 Methane was produced in the same manner as in Example 1, except that the volume was changed to / min. A temperature distribution occurs in the fixed-bed catalyst layer, and Figure 3 shows a graph with the highest temperature on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0054] <Example 3> The flow rate of the mixed gas is 5000 cm³.3 Methane was produced using the same method as in Example 1, except that the volume was changed to / min. Figure 3 shows a graph with the temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0055] <Comparative Example 1> Without using the plasma generating means 23, and with the reaction vessel 21 heated from the outside, the flow rate of the mixed gas was set to 3000 cm³. 3 Methane was produced using the same method as in Example 1, except that it was introduced at a rate of / min. Figure 3 shows a graph with the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0056] Figure 3 shows that in Examples 1-3, heat and radicals were simultaneously supplied into the reactor 20 by dielectric barrier discharge, causing the temperature of the catalyst layer to rise from room temperature to approximately 200°C, at which point the methanation reaction began. Since the methanation reaction is an exothermic reaction, the reaction heat further accelerated the methanation reaction, raising the catalyst temperature to approximately 300°C.

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

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

[0059] Focusing on the carbon dioxide conversion rate at around 250°C, Comparative Example 1 showed a conversion rate of approximately 5%, while Example 1 showed approximately 75%, Example 2 approximately 65%, and Example 3 approximately 35%. This confirms that Examples 1-3 showed a significantly higher carbon dioxide conversion rate than Comparative Example 1.

[0060] <Example 4> The catalyst was prepared by the following method. A mixture was obtained by adding urea in an aqueous solution of Ni(NO3)3 in an amount of 10 equivalents relative to Ni and dissolving it, then adding Al2O3. The resulting mixture was heated to 90°C and stirred for 5 hours. After stirring, the resulting solid was filtered and dried, and reduced under a hydrogen stream at 600°C for 1 hour to obtain Ni / Al2O3 containing 6% by mass of nickel.

[0061] Ni / Al2O3 was used as the catalyst, and the flow rate of the mixed gas was 500 cm³. 3 Methane was produced in the same manner as in Example 1, except that the volume was changed to / min. Figure 4 shows a graph with the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the carbon dioxide conversion rate on the vertical axis.

[0062] Figure 4 shows that heat and radicals were simultaneously supplied into the reactor 20 by dielectric barrier discharge, causing the temperature of the catalyst layer to rise from room temperature to approximately 170°C, at which point the methanation reaction began. The exothermic reaction of methanation increased the catalyst temperature, and at 250°C, the conversion rate of carbon dioxide reached approximately 40%.

[0063] <Comparative Example 2> Ni / Al2O3 was used as the catalyst, and the flow rate of the mixed gas was 500 cm³. 3 Methane was produced using the same method as in Comparative Example 1, except that the volume was changed to / min. Figure 4 shows a graph with 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 selectivity of methane was measured using the following formula. Methane selectivity (%) = [Amount of methane in the generated gas (moles)] / [Amount of methane in the generated gas (moles) + Amount of carbon monoxide in the generated gas (moles)] × 100

[0065] Figure 5 shows graphs for Example 4 and Comparative Example 2, with the maximum temperature of the fixed-bed catalyst layer on the horizontal axis and the methane selectivity on the vertical axis.

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

[0067] 1, 2: Methane production apparatus, 10, 20: Reactor, 11, 21: Reaction vessel, 12, 22: Fixed-bed catalyst layer, 13, 23: Plasma generating means, L1, 25: Gas supply section, L2, L12: Gas discharge section

Claims

1. A method for producing methane from carbon dioxide and hydrogen, The system includes a methanation step in which a mixed gas of carbon dioxide and hydrogen is introduced into a reactor filled with a methanation catalyst, and a non-equilibrium plasma is generated inside the reactor, which is kept at room temperature, to decompose the mixed gas and generate radicals, thereby causing a methanation reaction. The methanation catalyst is placed in a fixed-bed catalyst layer housed in the reactor. In the methanation process, the methanation reaction is initiated without supplying heat to the reactor from the outside, 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 temperature inside the reactor or the amount of methane produced. A method for producing methane, wherein the methanation reaction is continued without supplying heat to the reactor from an external source.

2. The methane production method according to claim 1, wherein the methanation catalyst comprises a carrier and a metal catalyst supported on the carrier, the carrier 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, and the metal catalyst contains one or more metal elements selected from the group consisting of nickel, ruthenium, rhodium, platinum, palladium, and iridium.

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

Images