Method for producing methane

Abstract

Provided is a methane producing method which includes: supplying a raw material gas comprising hydrogen gas, oxygen gas, and carbon dioxide gas to a reaction vessel having a catalyst provided thereto, and using heat comprising reaction heat from catalytic combustion of the hydrogen gas to start a methanation reaction; and continuing the methanation reaction.

Description

Technical Field

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

Background Art

[0002] Conventionally, it has been studied to suppress carbon dioxide emissions by converting carbon dioxide, which is abundant in industrial exhaust gases such as thermal power plants and steel mills, into methane. For this purpose, a reaction called a methanation reaction that produces methane and water from carbon dioxide and hydrogen, as shown in the following reaction formula, is used (for example, Patent Document 1). CO 2 +4H 2 →CH 4 +2H 2 O

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Exhaust gas containing carbon dioxide may also contain oxygen. For example, exhaust gas discharged from a thermal power plant generally contains about 4 to 15% by volume of oxygen gas. When oxygen is present in the exhaust gas, the catalytic metal binds to oxygen to form a metal oxide, so the catalyst is likely to be deactivated. Therefore, in the case of a methanation reaction using exhaust gas containing oxygen as a raw material gas, it has been necessary to remove oxygen from the exhaust gas in advance. However, from the viewpoint of simplifying the manufacturing equipment, etc., it is desirable to be able to omit the step of removing oxygen. Also, it is desirable to be able to save the energy cost required for heating to start the reaction.

[0005] Therefore, an object of one aspect of the present invention is to efficiently produce methane from a raw material gas containing carbon dioxide and oxygen without the need to remove oxygen in advance, and to reduce the energy cost required for heating therefor.

Means for Solving the Problems

[0006] One aspect of the present invention provides a method for producing methane, including supplying a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas to a reactor provided with a catalyst, and starting a methanation reaction by heat including reaction heat generated by catalytic combustion of the hydrogen gas, and continuing the methanation reaction.

[0007] Another aspect of the present invention provides a production system for producing methane by a methanation reaction from a raw material gas containing carbon dioxide, hydrogen, and oxygen. The production system according to one aspect of the present invention includes a reactor, a catalyst provided in the reactor, a hydrogen supply line for supplying hydrogen gas to the reactor, an air supply line for supplying air containing oxygen gas to the reactor, and an exhaust gas supply line for supplying exhaust gas containing carbon dioxide gas to the reactor. The catalyst includes a catalyst that functions as a catalyst for both combustion of hydrogen gas and methanation reaction, or includes a first catalyst for hydrogen gas combustion and a second catalyst for methanation reaction.

Effects of the Invention

[0008] According to one aspect of the present invention, methane can be efficiently produced from a raw material gas containing carbon dioxide and oxygen without the need to remove oxygen in advance, and the energy cost required for heating therefor can be reduced.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0010] Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0011] One embodiment of a method for producing methane is to supply a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas to a reactor provided with a catalyst, and to start a methanation reaction by heat including the reaction heat generated by the catalytic combustion of hydrogen gas, and to continue the methanation reaction.

[0012] FIG. 1 is a configuration diagram showing an example of a manufacturing system for producing methane by the above method. The manufacturing system 1 shown in FIG. 1 includes a reactor 10 provided with a catalyst, an air supply line 21 for supplying air containing oxygen gas and nitrogen gas, a hydrogen supply line 22 for supplying a gas containing hydrogen gas, and an exhaust gas supply line 23 for supplying exhaust gas containing carbon dioxide gas and oxygen gas. The air may be air taken in from the atmosphere. That is, part or all of the oxygen gas in the raw material gas may be the gas supplied by introducing air. The exhaust gas may further contain nitrogen gas.

[0013] As the catalyst, a first catalyst (H 2 combustion catalyst) for hydrogen gas combustion and a second catalyst (methane synthesis catalyst) for the methanation reaction are provided in the reactor 10. The reactor 10 includes a first reaction chamber 11 and a second reaction chamber 12 capable of heat exchange with each other. The first catalyst is provided in the first reaction chamber 11, and the second catalyst is provided in the second reaction chamber 12.

[0014] The reactor 10 forms a gas flow path through which the gas introduced into the first reaction chamber 11 passes through the second reaction chamber 12 and is then discharged outward from the reactor 10. An air supply line 21, a hydrogen supply line 22, and an exhaust gas supply line 23 are connected to this gas flow path. A raw material gas containing carbon dioxide gas, hydrogen gas, and oxygen gas is introduced into the gas flow path of the reactor 10 by the gases supplied from the air supply line 21, the hydrogen supply line 22, and the exhaust gas supply line 23.

[0015] The raw material gas does not necessarily always contain all of carbon dioxide gas, hydrogen gas, and oxygen gas during the production of methane. For example, in a method of producing methane by the production system 1, a raw material gas containing oxygen gas and nitrogen gas supplied from the air supply line 21 and hydrogen gas supplied from the hydrogen supply line 22 is supplied to the reactor 10. After the catalytic combustion of hydrogen gas starts in the first reaction chamber 11, carbon dioxide gas is introduced into the reactor 10 from the exhaust gas supply line 23, whereby a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas may be supplied to the reactor 10. The supply to the reactor 10 may be started in the order of hydrogen gas from the hydrogen supply line 22 and oxygen gas from the air supply line 21.

[0016] When the catalytic combustion of hydrogen gas starts in the first reaction chamber 11, nitrogen gas as a carrier gas is sent to the second reaction chamber 12 with heat, together with the remaining hydrogen gas and the generated water. As a result, the temperature of the second catalyst (methane synthesis catalyst) in the second reaction chamber 12 rises. When the temperature of the second catalyst exceeds, for example, a predetermined temperature of 100 °C (or 220 °C), it is determined that the methanation reaction of carbon dioxide can be started. At startup when the catalytic combustion of hydrogen gas starts, usually, a raw material gas is formed by the gases supplied from the air supply line 21 and the hydrogen supply line 22. The temperature of the gas near the catalyst may be regarded as the temperature of the catalyst.

[0017] After it was confirmed that the methanation reaction of carbon dioxide could be initiated, carbon dioxide gas was supplied from the exhaust gas supply line 23 to the reactor 10 and introduced into the second reaction chamber. The exhaust gas from the exhaust gas supply line 23 may be directly introduced into the second reaction chamber 12 without passing through the first reaction chamber 11. In the second reaction chamber 12, the methanation reaction of carbon dioxide is initiated by the reaction heat including the heat from the catalytic combustion of hydrogen gas, and then the methanation reaction continues. The product gas containing methane generated by the methanation reaction is discharged out of the reactor 10.

[0018] The production system 1 may include a cooling device 20 provided on the downstream side of the reactor 10. By the cooling device 20, the water generated together with methane by the methanation reaction is removed. The production system 1 may further include a separation device 30 provided on the downstream side of the reactor 10 for separating methane and nitrogen. By these devices, the concentration of methane in the product gas can be increased. The recovered methane can be used as a fuel such as city gas.

[0019] Another embodiment of the method for producing methane is implemented using the production system 2 shown in FIG. 2. Instead of providing the first reaction chamber 11 and the second reaction chamber 12 as in the production system 1 shown in FIG. 1, the production system 2 is provided with a single reaction chamber 13, and a catalyst (third catalyst) that functions as a catalyst for both hydrogen gas combustion and methanation reaction is provided in the reaction chamber 13.

[0020] The raw material gas does not necessarily always contain all of carbon dioxide gas, hydrogen gas, and oxygen gas during the production of methane. For example, in the method of producing methane by the production system 2, a raw material gas containing oxygen gas and nitrogen gas supplied from the air supply line 21 and hydrogen gas supplied from the hydrogen supply line 22 is supplied to the reactor 10. After the catalytic combustion of hydrogen gas starts in the reaction chamber 13, carbon dioxide gas may be introduced into the reactor 10 from the exhaust gas supply line 23, and thereby a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas may be supplied to the reactor 10. The supply to the reactor 10 may be started in the order of hydrogen gas from the hydrogen supply line 22 and oxygen gas from the air supply line 21.

[0021] When the catalytic combustion of hydrogen gas starts in the reaction chamber 13, heat is generated, and the temperature of the reaction chamber 13 and the third catalyst rises. When the temperature of the third catalyst exceeds, for example, a predetermined temperature of 100 °C (or 220 °C), it is determined that the methanation reaction of carbon dioxide can be started. At the start-up when the catalytic combustion of hydrogen gas starts, usually, a raw material gas is formed by the gases supplied from the air supply line 21 and the hydrogen supply line 22. The temperature of the gas near the catalyst may be regarded as the temperature of the catalyst.

[0022] After it is confirmed that the methanation reaction of carbon dioxide can be started, carbon dioxide gas is supplied from the exhaust gas supply line 23 to the reactor 10. In the reaction chamber 13, the methanation reaction of carbon dioxide is started by the reaction heat including the heat from the catalytic combustion of hydrogen gas, and then the methanation reaction continues. The product gas containing methane generated by the methanation reaction is discharged out of the reactor 10.

[0023] Conventionally, in order to obtain high-purity methane, a method has been used in which carbon dioxide gas in exhaust gas is once separated and recovered to form high-purity carbon dioxide, and this is reacted with hydrogen gas. The apparatus for separating and recovering carbon dioxide is relatively large-scale. In the case of the methanation reaction of high-purity carbon dioxide, the reaction heat is likely to be localized in the catalyst, which may cause thermal runaway. The method according to the present embodiment is advantageous in that these problems can be avoided.

[0024] FIG. 3 is a configuration diagram showing still another example of a production system for producing methane. Also in the production system 1 shown in FIG. 3, similar to the production system of FIG. 1, a first reaction chamber 11 and a second reaction chamber 12 capable of heat exchange with each other are provided in the reactor 10. However, the reactor 10 has a first gas flow path through which the gas introduced into the first reaction chamber 11 passes through the second reaction chamber 12 and is then discharged out of the reactor, and a second gas flow path through which the gas introduced into the first reaction chamber 11 is discharged out of the reactor 10 without passing through the second reaction chamber 12. It has a configuration different from that of the production system of FIG. 1 in that it is formed. The hydrogen supply line is branched into two, the hydrogen supply line 22A and the exhaust gas supply line 23 are connected to the first flow path, and the hydrogen supply line 22B and the air supply line 21 are connected to the second flow path.

[0025] In the production system 1 of FIG. 3, gas flow paths separated into a first gas flow path and a second gas flow path are formed in the reactor 10. During startup until the catalytic combustion of hydrogen gas is started, the raw material gas is supplied to the second flow path. At startup, it is necessary to supply oxygen gas at a relatively high concentration, and as a result, a large amount of moisture is generated. By circulating the generated moisture through the second gas flow path and discharging it out of the reactor 10, the amount of moisture flowing into the second reaction chamber 12 can be suppressed. The fact that less moisture flows from the first reaction chamber 11 into the second reaction chamber 12 is advantageous for an efficient methanation reaction. Further, instead of the exhaust gas containing carbon dioxide, high-purity carbon dioxide gas may be used. In that case, it is possible to prevent the high-purity carbon dioxide gas from being diluted by the air supplied from the air supply line 21.

[0026] When producing methane using the production systems exemplified in FIGS. 1 to 3 or other production systems, the concentration of hydrogen gas and oxygen gas in the raw material gas can be controlled so that the methanation reaction starts and then continues autonomously. For example, based on the temperature of the second catalyst (methane synthesis catalyst) or the third catalyst, the concentration of hydrogen gas and oxygen gas in the raw material gas may be controlled. When the temperature of the methane synthesis catalyst increases due to the heat including the reaction heat of the catalytic combustion of hydrogen gas, the concentration of oxygen gas required for the start and continuation of the methanation reaction can be lowered. Based on the relationship between the temperature of such a catalyst and the concentration of each gas, the concentration of hydrogen gas and oxygen gas can be appropriately controlled.

[0027] When the methanation reaction starts, the combined reaction heat of the methanation reaction and the reaction heat of the catalytic combustion of hydrogen gas may cause the temperature of the catalyst and gas in the reactor to rise excessively, resulting in a decrease in the methane yield. Therefore, after the methanation reaction starts, the concentration of oxygen gas in the raw material gas may be decreased until the temperature of the second catalyst (methane synthesis catalyst) or the third catalyst reaches a predetermined temperature, and then controlled so that the temperature of the second catalyst (methane synthesis catalyst) or the third catalyst is maintained at a predetermined temperature or higher. The predetermined temperature here depends on the catalyst, but may be, for example, 100°C to 250°C or 220 to 250°C. By decreasing the concentration of oxygen gas, the temperature of the second catalyst or the third catalyst can be decreased. For example, after the methanation reaction starts, the supply of air from the air supply line may be cut off. In that case, the supply amount of hydrogen gas may be adjusted to compensate for the consumption due to the catalytic combustion of hydrogen gas caused by oxygen gas in the exhaust gas.

[0028] Before the methanation reaction starts, in the raw material gas introduced into the reactor, the concentration of hydrogen gas is controlled, for example, in the range of 6 to 40% by volume, and the concentration of oxygen gas is controlled, for example, in the range of 0.1 to 30% by volume, 3 to 30% by volume, 0.1 to 20% by volume, 3 to 20% by volume, 0.1 to 10% by volume, or 3 to 10% by volume. During the steady operation in which the methanation reaction starts and then continues, in the raw material gas introduced into the reactor, the concentration of hydrogen gas is controlled, for example, in the range of 30 to 60% by volume, and the concentration of oxygen gas is controlled, for example, in the range of 0.1 to 20% by volume or 0.1 to 6% by volume. These concentrations are values based on the total volume of the raw material gas supplied to the reactor. The raw material gas usually contains an inert gas such as nitrogen gas as a carrier gas. The raw material gas may further contain a small amount of other gases derived from air or the like.

[0029] In addition to the reaction heat generated by the catalytic combustion of hydrogen gas, the methanation reaction may be started and continued by heat including heat supplied from an external heat source. In this case, the amount of heat supplied from the external heat source is controlled so that the methanation reaction starts and continues. As exemplified in the examples described later, when the concentration of oxygen gas in the raw material gas decreases, the appropriate amount of heat supplied from the external heat source tends to increase in order for the methanation reaction to start and continue. Based on such a relationship between the concentration of oxygen gas and the amount of heat supplied from the external heat source, the amount of heat supplied from the external heat source can be appropriately controlled.

[0030] The amount of heat supplied from the external heat source is adjusted, for example, by the set temperature of the heat source. The heat source is not particularly limited, and may be, for example, an electric heater that generates heat by resistance heating or the like, or a heat medium heated to a predetermined temperature.

[0031] The catalyst provided in the reactor may contain a catalyst (third catalyst) that functions as a catalyst for both hydrogen gas combustion and methanation reaction, or may contain a first catalyst for hydrogen gas combustion and a second catalyst for methanation reaction. The first catalyst and the second catalyst may be the same or different.

[0032] The catalyst for hydrogen gas combustion may be a catalyst containing a carrier and a catalyst metal supported on the carrier. Examples of the carrier include metal oxides such as alumina (Al 2 O 3 ), ceria (CeO 2 ), zirconia (ZrO 2 ), yttria (Y 2 O 3 ), magnesia (MgO) and titania (TiO 2 ), and their composite metal oxides. Furthermore, composite metal oxides such as silica alumina (SiO 2 ·Al 2 O 3 ), various perovskites, and zeolites are included. Examples of the catalyst metal include noble metals such as Rh, Pd, Pt, and Ru, and base metals such as Ni, Co, Cu, Mn, and Fe. The catalyst metal may contain two or more metals.

[0033] The catalyst for the methanation reaction (catalyst for methane synthesis) may also be a catalyst containing a carrier and a catalyst metal supported on the carrier. Examples of the carrier include metal oxides such as alumina (Al 2 O 3 ), ceria (CeO 2 ), zirconia (ZrO 2 ), yttria (Y 2 O 3 ), magnesia (MgO) and titania (TiO 2 ), and their composite metal oxides. Furthermore, composite metal oxides such as silica alumina (SiO 2 ·Al 2 O 3 ), various perovskites, and zeolites are included. Examples of the catalyst metal include base metals such as Ni, Co, Cu, Mn, and Fe, and noble metals such as Pd, Rh, and Ru. The catalyst metal may contain two or more metals.

[0034] The reactor may contain granular catalyst. Alternatively, a so-called monolith catalyst, which is a catalyst structure having a metal formed body and a catalyst layer containing a catalyst formed on the metal formed body, may be contained in the reactor. The method according to this embodiment may be accompanied by a significant temperature rise of the catalyst, and although the methanation reaction is an exothermic reaction, by providing a catalyst layer on a metal formed body with a high heat dissipation effect, it is easier to avoid excessive heat accumulation compared to, for example, a catalyst having a ceramic carrier. The metal formed body may be a plate-like body or a plate-like body processed into a twisted shape. The plate-like body processed into a twisted shape is a plate-like formed body that extends along an axis while being twisted in a direction of rotation around a certain axis. A monolith catalyst having a twisted plate-like body exhibits excellent characteristics in terms of contact with gas and heat dissipation, and thus, when this is used, it is easy to achieve an excellent methane yield even with a relatively small amount of catalyst. The metal formed body may be, for example, a formed body of aluminum.

Examples

[0035] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

[0036] 1. Catalyst preparation Powder of cerium oxide was added to an aqueous solution of nickel nitrate hexahydrate, and after stirring, water was evaporated from the dispersion while heating. The remaining powder was held in a dryer at 150 °C overnight, and then calcined in an electric furnace at 500 °C for 2 hours in air to obtain a powder of catalyst particles (Ni / CeO 2 ) containing cerium oxide and nickel supported thereon. The content of nickel in this catalyst particle is 10% by mass based on the mass of cerium oxide. The powder of the catalyst particles was compression-molded, and the obtained molded body was pulverized. Granular catalyst with a particle size of 30 - 60 mesh was obtained from the pulverized product using a sieve.

[0037] 2. Methanation reaction (Test 1) 0.3 g of the obtained catalyst was filled into a glass reaction tube (inner diameter 8.0 mm, outer diameter 10.0 mm) of an atmospheric pressure fixed bed flow-through microreactor to form a catalyst layer. The reaction tube was placed in an electric furnace. Supply lines for carbon dioxide, hydrogen, oxygen, and nitrogen gas were connected to the reaction tube. A mass flow controller and a flow meter for controlling the flow rate were attached to each supply line.

[0038] The temperature of the electric furnace in which the reaction tube was placed was maintained at 25 °C, and nitrogen (N 2 ) gas was passed through the catalyst layer at a flow rate of 180 mL / min. In this state, when hydrogen (H 2 ) gas was further passed through at a flow rate of 200 mL / min and oxygen (O 2 ) gas was further passed through at a flow rate of 20 mL / min in this order, the temperature of the catalyst layer rapidly increased due to the reaction heat of the catalytic combustion of hydrogen. Subsequently, while maintaining the flow rates of nitrogen gas, hydrogen gas, and oxygen gas, carbon dioxide (CO 2 ) gas was further supplied at a flow rate of 40 mL / min, and the methanation reaction started and the temperature of the catalyst layer rose to 500 °C, and the methanation reaction continued. The CO 2 concentration of the product gas flowing out from the reaction tube during the methanation reaction was quantified, and the conversion rate from CO 2 to methane (CO 2 conversion) was determined from the measurement results.

[0039] (Tests 2 - 6) Methanation reaction tests similar to Test 1 were conducted except that the flow rate of oxygen gas was changed to 0 mL / min, 2 mL / min, 4 mL / min, 8 mL / min, or 12 mL / min. However, since the start of the methanation reaction was not confirmed at the set temperature of 25 °C of the electric furnace in which the reaction tube was placed, the set temperature of the electric furnace was adjusted so that the methanation reaction started and continued. Table 1 shows the set temperatures of the electric furnace at which the start and continuation of the methanation reaction were confirmed. The CO 2 concentration of the product gas flowing out from the reaction tube during the methanation reaction was quantified, and the conversion rate from CO 2 to methane (CO 2 conversion rate) was determined from the measurement results.

[0040]

Table 1

[0041] Figure 4 shows the relationship between the conversion rate of CO 2 and the set temperature of the electric furnace at which the methanation reaction starts and continues. By referring to the relationship shown in Figure 4 and appropriately controlling the concentration of each component of the raw material gas and, if necessary, the heating conditions by an external heat source, the methanation reaction can be efficiently carried out using a raw material gas containing O 2 . Note that the conversion rate of CO 2 in Test 5 where the raw material gas contains 2% by volume of O 2 is slightly lower than the conversion rate of CO 2 in Test 6 where the raw material gas does not contain O 2 . However, the yield of CH 2 with respect to CO 4 is 73% in Test 5 and 75% in Test 6, and it can be said that both are at an equivalent level.

Explanation of Reference Signs

[0042] 1, 2... manufacturing system, 10... reactor, 11... first reaction chamber, 12... second reaction chamber, 13... reaction chamber.

Claims

1. Supplying a raw material gas containing hydrogen gas, oxygen gas, and carbon dioxide gas to a reactor provided with a catalyst, and starting a methanation reaction with heat including the reaction heat generated by the catalytic combustion of the hydrogen gas; continuing the methanation reaction, A method for producing methane, comprising supplying a raw material gas containing the hydrogen gas and the oxygen gas to the reactor, and after the catalytic combustion of the hydrogen gas starts, supplying a raw material gas further containing the carbon dioxide gas to the reactor.

2. The method according to claim 1, wherein the concentrations of the hydrogen gas and the oxygen gas in the raw material gas are controlled so that the methanation reaction starts and continues.

3. The method according to claim 2, wherein the concentrations of the hydrogen gas and the oxygen gas in the raw material gas are controlled based on the temperature of the catalyst.

4. The method according to claim 3, wherein the concentration of the oxygen gas in the raw material gas is decreased until the temperature of the catalyst reaches a predetermined temperature after the methanation reaction starts, and then controlled so that the temperature of the catalyst is maintained at or above the predetermined temperature.

5. The method according to any one of claims 1 to 4, wherein part or all of the carbon dioxide gas is a gas derived from exhaust gas.

6. The method according to any one of claims 1 to 5, wherein part or all of the oxygen gas is a gas supplied by introducing air containing oxygen gas.

7. The method according to any one of claims 1 to 6, wherein a catalyst structure having a metal formed body and a catalyst layer containing the catalyst formed on the metal formed body is accommodated in the reactor.

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