Methane production method

Abstract

To provide a methane production method in which a conversion ratio of carbon dioxide into methane is high.SOLUTION: There is provided a methane production method configured to: supply a raw material gas having a molar ratio (H2 / CO2) of carbon dioxide and hydrogen of 7-12, into a reaction vessel in which a temperature is set to 310-390°C; bring the raw material gas into contact with a catalyst which activates a methanation reaction for producing methane.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Currently, reducing carbon dioxide emissions, which have a negative impact on the environment as a greenhouse gas, is an urgent issue, and in addition to technologies that reduce carbon dioxide emissions themselves through fuel conversion and the use of new energy sources, there is a growing need for carbon dioxide separation and capture technologies and carbon dioxide fixation technologies.

[0003] As a carbon dioxide fixation technology, for example, a methanation reaction in which methane is produced from carbon dioxide and hydrogen is known (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-172595 Summary of the Invention [Problem to be solved by the invention]

[0005] The methanation reaction is a reversible reaction, and if unreacted carbon dioxide is contained in the product gas after the reaction, the chemical equilibrium state shown in the following formula (1) is reached.

[0006] [ka]

[0007] In the above formula (1), if the water vapor contained in the product gas is condensed and removed, the reaction will proceed to the right, and it is generally thought that the methane production rate can be improved. However, in reality, it is difficult to achieve a conversion rate of carbon dioxide to methane of 95% or more. Therefore, an object of the present invention is to provide a method for producing methane with a high conversion rate of carbon dioxide to methane. [Means for solving the problem]

[0008] The inventors have discovered that by setting a reactor within a specific temperature range and then supplying a raw material gas into the reactor in which the molar ratio of carbon dioxide to hydrogen (H2 / CO2) is controlled within a specific range, and subjecting the raw material gas to a methanation reaction, it is possible to produce methane from carbon dioxide with an extremely high conversion rate of 95% or more.

[0009] That is, the present invention provides the following [1] to [4]. [1] A method for producing methane, comprising the steps of supplying a raw material gas having a carbon dioxide to hydrogen molar ratio (H2 / CO2) of 7 to 12 into a reactor set at a temperature of 310 to 390°C and bringing the raw material gas into contact with a catalyst that activates a methanation reaction to produce methane. [2] The method for producing methane according to [1] above, wherein the molar ratio of carbon dioxide to hydrogen (H2 / CO2) in the raw material gas is greater than 7.5 and not more than 10.5. [3] The method for producing methane according to [1] or [2] above, wherein the set temperature of the reactor is 315 to 375°C. [4] The method for producing methane according to any one of [1] to [3] above, wherein the catalyst for activating the methanation reaction is one or more selected from nickel, ruthenium, zirconium, platinum, and rhodium. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a method for producing methane with a high conversion rate of carbon dioxide to methane. DETAILED DESCRIPTION OF THE INVENTION

[0011] The methanation reaction uses a feed gas containing carbon dioxide and hydrogen. According to the stoichiometric formula shown in formula (1) above, 1 mole of methane and 2 moles of water should be produced from 1 mole of carbon dioxide and 4 moles of hydrogen. However, even if the H2 / CO2 molar ratio in the feed gas is the theoretical value of 4, not all of the carbon dioxide and hydrogen will react to produce 1 mole of methane. In other words, the composition of the gas discharged from the reactor is a mixed gas consisting of methane and water vapor produced in the methanation reaction and unreacted carbon dioxide and hydrogen.

[0012] The present inventors have found that the carbon dioxide concentration in the product gas can be significantly reduced by increasing the H2 / CO2 molar ratio in the feed gas beyond the theoretical value of 4 and setting the reactor temperature within a specific temperature range. More specifically, the inventors have found that when the methanation reaction is carried out with the H2 / CO2 molar ratio in the feed gas within a specific range of 7 to 12 and the reactor temperature set at 310 to 390°C, the conversion rate of carbon dioxide to methane increases to 95% or more, and a product gas consisting almost entirely of the two components hydrogen and methane is obtained. They have also found that high-purity methane can be produced by separating hydrogen from this product gas.

[0013] The molar ratio of carbon dioxide to hydrogen (H2 / CO2) in the raw material gas is 7 to 12, but from the viewpoint of improving the conversion rate of carbon dioxide to methane, it is preferably more than 7.5 and not more than 10.5, more preferably 7.7 or more and 10.3 or less, and even more preferably 8 or more and 10 or less.

[0014] The feed gas may be a mixture of carbon dioxide and hydrogen in a predetermined molar ratio and supplied to the reactor, or carbon dioxide and hydrogen may be supplied to the reactor via separate pipes so that the carbon dioxide and hydrogen in the feed gas have a predetermined molar ratio in the reactor. In order to adjust the H2 / CO2 molar ratio in the feed gas, it is preferable to install valves and sensors capable of adjusting the flow rates of carbon dioxide and hydrogen.

[0015] The flow rate of the raw material gas can be selected appropriately, but from the viewpoint of promoting the methanation reaction, the total flow rate of carbon dioxide and hydrogen is preferably 0.02 to 1.00 m / sec, more preferably 0.05 to 0.50 m / sec. The space velocity (SV) of the raw gas is set to 500 to 50,000 h from the viewpoint of promoting the methanation reaction. -1 is preferable, and 2000 to 20,000 hours -1 is more preferred.

[0016] The carbon dioxide may be carbon dioxide filled in a commercially available cylinder, but is preferably carbon dioxide derived from atmospheric carbon dioxide or exhaust gas. Examples of exhaust gas include combustion exhaust gas and calcination exhaust gas. Examples of combustion exhaust gas include exhaust gas generated by the combustion of fuel in thermal power plants, boilers, etc., and exhaust gas generated by the incineration of garbage in waste disposal plants. Examples of calcination exhaust gas include cement kiln exhaust gas generated by the calcination of clinker raw materials in cement kilns, and biomass exhaust gas generated by methane fermentation of biomass. Since exhaust gas contains sulfur oxides, nitrogen oxides, etc. in addition to carbon dioxide, the exhaust gas may be treated with a desulfurization device or a denitrification device to remove sulfur oxides and nitrogen oxides. Examples of hydrogen include, but are not limited to, hydrogen stored in a commercially available cylinder and hydrogen obtained by electrolyzing water.

[0017] The reactor is not particularly limited as long as it can pass the raw material gas and discharge the produced gas, but examples thereof include a cylindrical body. The shape of the cylindrical body may be either cylindrical or rectangular. A gas inlet pipe for supplying the raw material gas is installed at one end of the reactor, and a gas outlet pipe for discharging the produced gas is installed at the other end. The size of the reaction tube can be appropriately selected depending on the production scale, etc., but in the case of a cylindrical tube, the inner diameter is, for example, 0.005 to 10 m and the length is 0.5 to 100 m. The material of the reaction tube is not particularly limited as long as it has high heat resistance, and examples thereof include metal, glass, and quartz.

[0018] The reactor may be, for example, one equipped with a gas flow pipe filled with a methanation catalyst and a heating furnace capable of heating the gas flow pipe, such as an atmospheric flow reactor that is a fixed gas flow pipe. In this case, the raw material gas flowing through the gas flow pipe comes into contact with the methanation catalyst filled in the gas flow pipe and heated by the heating furnace, thereby producing methane from the raw material gas.

[0019] The set temperature of the reactor is 310 to 390°C, but from the viewpoint of improving the conversion rate of carbon dioxide to methane, it is preferably 315°C or higher, more preferably 320°C or higher, and preferably 380°C or lower, more preferably 375°C or lower, even more preferably 370°C or lower, even more preferably 365°C or lower, even more preferably 360°C or lower, and even more preferably 355°C or lower. The set temperature here refers to the set temperature of a temperature controller that controls the heating temperature of the reactor.

[0020] The catalyst for activating the methanation reaction is not particularly limited as long as it can produce methane from carbon dioxide and hydrogen, and examples thereof include nickel (Ni), ruthenium (Ru), platinum (Pt), rhodium (Rh), and zirconium (Zr). Note that the catalyst can be used alone or in combination of two or more. The catalyst may also be supported on a carrier, and examples of the carrier include various metal oxides, aluminosilicates, and silica. Among these, metal oxides are preferred, and transition metal oxides are more preferred. Examples include CeO2, ZrO2, and Y2O3. The carrier may be used alone or in combination of two or more.

[0021] The catalyst supported on a carrier may be used by being coated on a metal plate from the viewpoint of ensuring the space velocity of the raw material gas. The shape of the metal plate is not particularly limited as long as it ensures the space velocity of the raw material gas and has a large surface area, and examples thereof include a spiral shape and a honeycomb shape. When the catalyst is coated on a spiral metal plate, for example, it can be produced by forming a metal plate member made of aluminum, stainless steel, or the like into a spiral shape and coating it with a paste-like catalyst.

[0022] Furthermore, in the methanation reaction, from the viewpoint of improving the conversion rate of carbon dioxide to methane, it is preferable to determine the length of the catalyst packed bed and the flow rate of the raw material gas so that the contact time between the catalyst that activates the methanation reaction and the raw material gas is 0.25 seconds or more. If the contact time is less than 0.25 seconds, the contact between the raw material gas and the catalyst becomes insufficient, and the conversion rate of carbon dioxide to methane tends to decrease.

[0023] Water vapor is produced in the methanation reaction, and from the viewpoint of improving the conversion rate of carbon dioxide to methane, it is preferable to provide a water vapor removal facility in the gas exhaust pipe of the reactor and actively remove the water vapor.

[0024] The product gas discharged from the gas discharge pipe of the reactor after the methanation reaction is composed almost entirely of two components: hydrogen and methane. Therefore, methane and hydrogen can be easily separated from the product gas using a hydrogen separation membrane or the like. The separated hydrogen may be reused as a feed gas in another methanation reaction. The separated hydrogen may also contain carbon dioxide, carbon monoxide, and methane, and may be used as a feed gas as is.

[0025] In this way, the production method of the present invention makes it possible to produce methane from carbon dioxide at an extremely high conversion rate of 95% or more. [Example]

[0026] The following examples will explain the present invention in more detail, but the present invention is not limited to the examples below.

[0027] (1) Experimental equipment Electric furnace: One-zone electric furnace with temperature control (As One tubular electric furnace TMF-500N, effective length 300 mm) Normal pressure flow reaction tube used for methanation reaction: quartz tube with inner diameter of 8 mm and length of 600 mm Catalyst installation location: One catalyst is installed inside the atmospheric pressure flow reactor tube at the location where the thermocouple for temperature control of the electric furnace is installed.

[0028] Manufacturing Example 1 Preparation of catalysts for activating methanation reactions Using CeO2 (reagent, Kanto Chemical Co., Ltd.), 10 wt% Ni(NO3)2·6H2O (98%, Fujifilm Wako Pure Chemical Industries, Ltd.) was supported by evaporation to dryness. After support, the support was calcined in air at 500°C for 2 hours to separate the nitric acid component. After cooling, the calcined product was added with distilled water and ground in a mortar to produce a Ni / CeO2 paste. This Ni / CeO2 paste was then applied to the surface of a spirally twisted aluminum plate (JIS A1100 H14, 7 mm wide x 100 mm long, 0.8 mm thick). The plate was repeatedly dried with cold air and immersed in water until a sufficient amount of Ni / CeO2 catalyst was supported to achieve the desired methanation reaction.

[0029] Examples 1 to 8 and Comparative Examples 1 to 10 Prior to methane production, the catalyst precursor was reduced by heating the reactor to 500°C for 1 hour while flowing hydrogen at 200 mL / min. A moisture trap was then installed at the reactor outlet, followed by a gas sampling hole for analyzing the gas composition after the methanation reaction. A wet gas meter was also installed at the reactor outlet to measure the gas flow rate. Next, carbon dioxide and hydrogen were supplied to the reactor via mass flow controllers to carry out the methanation reaction. The methanation reaction was carried out by varying the H2 / CO2 molar ratio in the feed gas and the temperature of the electric furnace as shown in Table 1. The gas discharged from the reactor outlet was quantitatively analyzed for hydrogen, nitrogen, oxygen, methane, carbon monoxide, and carbon dioxide using a gas chromatograph (Micro990, Agilent Technologies, USA). The conversion rate of carbon dioxide to methane was then calculated using the following formula. The results are also shown in Table 1.

[0030] Conversion rate of carbon dioxide to methane (%) = (X + Y) / (X + Y + Z) x 100 (wherein X represents the CH4 concentration (vol%), Y represents the CO concentration (vol%), and Z represents the CO2 concentration (vol%).)

[0031] [Table 1]

[0032] Table 1 shows that by controlling the molar ratio of carbon dioxide to hydrogen (H2 / CO2) in the raw material gas to 7 to 12 and then supplying it to a reactor set at a temperature of 310 to 390°C for the methanation reaction, it is possible to produce methane from carbon dioxide with an extremely high conversion rate of over 95%.

Claims

1. The molar ratio of carbon dioxide to hydrogen (H 2 / CO 2 ) is 7 to 12 into a reactor set at a temperature of 310 to 390°C, and bringing the raw material gas into contact with a catalyst that activates a methanation reaction to produce methane.

2. The molar ratio of carbon dioxide to hydrogen in the raw material gas (H 2 / CO 2 2. The method for producing methane according to claim 1, wherein the β-glucan content is greater than 7.5 and not more than 10.

5.

3. The method for producing methane according to claim 1, wherein the set temperature of the reactor is 315 to 375°C.

4. The method for producing methane according to any one of claims 1 to 3, wherein the catalyst for activating the methanation reaction is one or more selected from nickel, ruthenium, zirconium, platinum, and rhodium.

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