WO2021256274A1

WO2021256274A1 - Methane gas decomposition method and methane gas decomposition apparatus

Info

Publication number: WO2021256274A1
Application number: PCT/JP2021/021148
Authority: WO
Inventors: 優一大塚; 尊宏平岡; 謙介中村
Original assignee: ウシオ電機株式会社
Priority date: 2020-06-17
Filing date: 2021-06-03
Publication date: 2021-12-23
Also published as: JP2021194617A

Abstract

Provided is a technique with which methane can be decomposed in a simple manner even if a gas to be processed contains methane at a low concentration such as a few hundred ppm or less.　In this methane gas decomposition method, a gas to be processed comprising methane gas mixed in air is heated by being circulated in an atmospheric pressure plasma space formed by a dielectric barrier discharge, and thus methane is decomposed.

Description

Methane gas decomposition method, methane gas decomposition equipment

The present invention relates to a methane gas decomposition method and a methane gas decomposition apparatus.

Currently, as global warming progresses, carbon dioxide, methane, chlorofluorocarbons, etc. are known as greenhouse gases that cause global warming. It emits the most carbon dioxide, followed by methane. The global warming potential of methane is said to be about 20 to 70 times that of carbon dioxide, and it has a great impact on global warming. In addition, the methane concentration has been increasing in recent years, and the technology for efficiently decomposing the emitted methane may be necessary in view of the global environment in the future.

Conventionally, as a technique for decomposing methane, there is a methane removal system disclosed in Patent Document 1 below. FIG. 10 is a drawing schematically showing the configuration of the methane removal system.

The methane removal system 100 is a system that decomposes methane contained in the gas to be treated, and includes a gas pipe 102 to be treated, a catalyst 104 for removing methane oxidation, a plasma generation means 105, and a control means 106.

The processed gas pipe 102 is set inside the processed gas passage 102a for discharging the processed gas Eg from the processed gas discharge source 101. The catalyst 104 for removing methane oxidation is housed in a layer in a catalyst accommodating portion 104a set in the gas passage 102a to be treated, and is arranged to remove the gas Eg to be treated flowing in the gas passage 102a to be treated. ing.

The plasma generating means 105 includes an external electrode 108a, an internal electrode 108b, and a power supply source 107. The external electrode 108a is arranged on the outer peripheral surface of the gas pipe 102 to be treated so as to surround the catalyst accommodating portion 104a, and has a tubular shape. The internal electrode 108b is arranged at a position corresponding to the catalyst accommodating portion 104a in the gas passage 102a to be treated. One terminal of the power supply source 107 is electrically connected to the internal electrode 108b. The other terminal of the power supply source 107 and the external electrode 108a are grounded.

The plasma generation means 105 supplies electric power to the internal electrode 108b by the electric power supply source 107 to generate atmospheric pressure plasma at a place where the catalyst 104 for removing methane oxidation accommodated in the catalyst accommodating portion 104a exists. As a result, the gas to be treated Eg flowing in the gas passage 102a to be treated is turned into plasma in the catalyst accommodating portion 104a, and the generated plasma activates the catalyst 104 for removing methane oxidation. Then, the methane contained in the gas to be treated Eg is decomposed into carbon dioxide by the synergistic effect of the action of the activated catalyst for removing methane oxidation 104 and the action of the generated plasma.

Japanese Unexamined Patent Publication No. 2019-155242

As a source of methane released to the environment, it is said that the largest amount of methane emitted is from livestock, etc. as flatulence and burp. For example, the amount of methane emitted from one cow is said to be about 300 liters a day. On the other hand, the atmospheric concentration of methane emitted from livestock such as cattle raised in the barn is about 20 ppm.

Patent Document 1 describes that methane could be decomposed by passing a gas to be treated having a methane concentration of 3000 ppm. However, it cannot be said that the technology for almost completely decomposing low-concentration methane such as several hundred ppm or less has been sufficiently established at this time.

When decomposing methane from a gas to be treated containing a high concentration of methane (for example, several thousand ppm or more and several percent or less), it is considered that the method of simply burning the gas to be treated is the simplest and most effective. However, in the case of such a combustion treatment method, the residual gas often contains methane of several hundred ppm or less, and the combustion treatment is performed on the gas to be treated containing methane gas at such a low concentration. Therefore, decomposing methane is not a realistic method from the viewpoint of the amount of methane decomposition with respect to the amount of input energy.

In view of the above problems, an object of the present invention is to provide a technique capable of decomposing methane by a simple method even for a gas to be treated containing methane at a low concentration of several hundred ppm or less.

In the methane gas decomposition method according to the present invention, the temperature of the gas to be treated is raised by passing the gas to be treated, which is a mixture of methane gas in air, in the atmospheric pressure plasma space formed by the dielectric barrier discharge. It is characterized by decomposing methane.

The diligent research of the present inventor has proved that methane can be decomposed in a gas to be treated containing methane at an extremely low concentration of 100 ppm according to the above method.

Also, unlike corona discharge, in the case of dielectric barrier discharge, it is possible to generate discharge in a wide range within the region surrounded by the dielectric. Therefore, almost all of the introduced gas to be treated can be passed through the atmospheric pressure plasma space. As a result, the amount of gas to be treated that passes through without plasma treatment can be reduced as much as possible.

In the above method, the gas to be treated may be allowed to flow into the atmospheric pressure plasma space in a state of being heated to 200 ° C. or higher.

Through the diligent research of the present inventor, it was confirmed that the decomposition rate of methane is improved by passing the gas to be treated containing methane gas through the atmospheric pressure plasma space at a high temperature of 200 ° C. or higher. That is, according to the above method, the decomposition rate of methane gas can be increased.

In the above method, the gas to be treated may be passed through the atmospheric pressure plasma space in a humidified state.

Through the diligent research of the present inventor, it was confirmed that the decomposition rate of methane is improved by passing the gas to be treated including methane gas through the atmospheric pressure plasma space in a humidified state. That is, according to the above method, the decomposition rate of methane gas can be increased.

The methane gas decomposition apparatus according to the present invention is
A tube body including a tube wall made of a dielectric and a tube body
A pair of electrodes arranged across the tube wall,
A power supply that applies a high frequency voltage to the pair of electrodes,
A gas inlet for introducing a gas to be treated, which is a mixture of methane gas in air inside the tube, and a gas inlet.
With a gas discharge port that discharges the gas to be processed that has passed through the atmospheric pressure plasma space formed inside the tube body by applying the high frequency voltage to the pair of electrodes to the outside of the tube body. It is characterized by being equipped with.

According to the above-mentioned methane gas decomposition device, in the space inside the tube through which the gas to be processed flows, the direction orthogonal to the tube axis direction of the tube and the direction from the tube wall toward the axis (for convenience, "diameter" An atmospheric pressure plasma space is formed over almost the entire area of "direction"). Therefore, since almost all of the introduced gas to be treated passes through the atmospheric pressure plasma space, the methane contained in the gas to be treated is efficiently decomposed.

In the above methane gas decomposition device, various designs are possible for the arrangement of electrodes and the shape of the tube.

For example, the pair of electrodes includes a first electrode formed on the outer surface of the tube wall and a second electrode formed inside the tube body by extending along the tube axis direction of the tube body. It may be configured to include.

Further, for example, the pair of electrodes includes a first electrode formed on a part of the outer surface of the tube wall and the tube wall having a position facing the first electrode via the inside of the tube body. It may be configured to include a second electrode formed on a part of the outer surface.

Further, for example, the pair of electrodes has a first electrode formed on a part of the outer surface of the tube wall and the tube wall at a position separated from the first electrode in the tube axis direction of the tube body. It may be configured to include a second electrode formed on a part of the outer surface of the above.

Further, the tube wall of the tube body located on the outside of the end portion of the atmospheric pressure plasma space formed inside the tube body on the gas discharge port side in the tube axis direction of the tube body is 230. It is preferably ℃ or higher.

According to this configuration, the temperature of the gas to be treated flowing in the atmospheric pressure plasma space can be set to 200 ° C. or higher, and the decomposition rate of methane can be increased. The temperature of the tube wall can be controlled, for example, by the electric power applied to the electrodes from the power source. In addition, another heater may be provided.

According to the present invention, methane can be decomposed by a simple method even for a gas to be treated containing methane at a low concentration of several hundred ppm or less.

It is a top view which shows typically the structure of 1st Embodiment of a methane gas decomposition apparatus. FIG. 3 is a cross-sectional view taken along the line A1-A1 in FIG. 1A. It is a top view schematically showing another configuration of 1st Embodiment of a methane gas decomposition apparatus. FIG. 2 is a cross-sectional view taken along the line A1-A1 in FIG. 2A. It is sectional drawing which shows typically another structure of 1st Embodiment of a methane gas decomposition apparatus. It is sectional drawing which shows typically another structure of 1st Embodiment of a methane gas decomposition apparatus. It is a top view schematically showing another configuration of 1st Embodiment of a methane gas decomposition apparatus. It is a top view which shows typically the structure of the 2nd Embodiment of a methane gas decomposition apparatus. FIG. 3 is a cross-sectional view taken along the line A1-A1 in FIG. 3A. It is a top view schematically showing another configuration of the 2nd Embodiment of a methane gas decomposition apparatus. FIG. 3 is a cross-sectional view taken along the line A1-A1 in FIG. 4A. It is a top view which shows typically the structure of the 3rd Embodiment of a methane gas decomposition apparatus. FIG. 5 is a cross-sectional view taken along the line A1-A1 in FIG. 5A. It is sectional drawing which shows typically another structure of the 3rd Embodiment of a methane gas decomposition apparatus. It is a top view which shows typically the structure of the 4th Embodiment of a methane gas decomposition apparatus. 6 is a cross-sectional view taken along the line A1-A1 in FIG. 6A. FIG. 6 is a cross-sectional view taken along the line A2-A2 in FIG. 6A. It is a top view schematically showing another configuration of the 4th Embodiment of a methane gas decomposition apparatus. It is a top view which shows typically the method of the cooling process in an Example. It is a top view which shows typically the method of heat treatment in an Example. It is a top view which shows typically the method of the humidification treatment in an Example. It is a graph which shows the relationship between the thermal decomposition rate of ozone, and temperature. It is a drawing which shows typically the structure of the conventional methane removal system.

The methane gas decomposition method and the methane gas decomposition apparatus according to the present invention will be described with reference to the drawings. It should be noted that the following drawings are schematically shown, and the dimensional ratios on the drawings do not always match the actual dimensional ratios. Moreover, the dimensional ratios do not always match between the drawings.

[First Embodiment]
1A and 1B are drawings schematically showing the configuration of the first embodiment of the methane gas decomposition apparatus. FIG. 1B is a sectional view taken along line A1-A1 in FIG. 1A.

The methane gas decomposition device 1 includes a tube body 3 and a pair of electrodes (5a, 5b). In this embodiment, the tubular body 3 exhibits a double tubular structure. More specifically, as shown in FIG. 1B, the tube body 3 has a cylindrical shape and is arranged coaxially with the outer tube 3a located on the outside and the outer tube 3a inside the outer tube 3a. It has an inner tube 3b having a cylindrical shape having an inner diameter smaller than that of 3a.

One electrode 5a (corresponding to the "first electrode") is arranged on the outer wall of the outer tube 3a. In this embodiment, the electrode 5a has a mesh shape. Further, a rod-shaped electrode 5b (corresponding to the "second electrode") extending along the tube axial direction d1 of the tube body 3 is inserted inside the inner tube 3b. A discharge space S1 having a ring shape (here, an annular shape) when viewed from the pipe axis direction d1 is formed between the outer tube 3a and the inner tube 3b.

The outer tube 3a and the inner tube 3b are made of a dielectric material such as quartz glass or ceramics. The electrodes (5a, 5b) are made of a metal material such as stainless steel, aluminum, copper, tungsten, or nickel.

As shown in FIG. 1A, the pipe body 3 has two openings, each of which corresponds to the gas introduction port 11 and the gas discharge port 12. The gas introduction port 11 is an opening for introducing the treated gas G1 supplied from the gas supply source 20 in which methane gas is mixed with air into the inside of the pipe body 3. The gas supply source 20 is a mechanism for sending the gas to be treated G1 in the space requiring treatment of the atmosphere containing methane gas to the methane gas decomposition device 1, and is composed of, for example, a blower or a duct.

The gas introduction port 11 is provided as an opening at a part of the outer pipe 3a, and is connected to the discharge space S1 located outside the inner pipe 3b. That is, the gas to be processed G1 supplied from the gas supply source 20 flows into the discharge space S1 through the gas introduction port 11.

The gas discharge port 12 is arranged at a position separated from the gas introduction port 11 with respect to the pipe axial direction d1. In the present embodiment, the gas discharge port 12 is arranged at a position separated from the region where the electrode 5a is formed with respect to the pipe axis direction d1 with respect to the gas introduction port 11. Like the gas introduction port 11, the gas discharge port 12 is provided as an opening at a part of the outer pipe 3a, and is connected to the discharge space S1 located outside the inner pipe 3b.

The methane gas decomposition device 1 includes a power source 6. The power supply 6 is connected to the

electrodes

5a and 5b, and has a configuration in which a predetermined voltage is applied between both electrodes (5a, 5b). The applied voltage and frequency supplied from the power supply 6 are within a range in which a dielectric barrier discharge can be caused in the tube 3 by applying a voltage between the electrodes (5a, 5b). Just do it. Specifically, the applied voltage supplied from the power supply 6 is preferably in the range of 3 kVpp or more and 50 kVpp or less. The frequency of the applied voltage supplied from the power supply 6 is preferably in the range of 1 kHz or more and 1000 kHz or less, and more preferably in the range of 1 kHz or more and 150 kHz or less. The reason why the upper limit is preferably 150 kHz is that the frequency detected by the noise terminal voltage in the EMC standard is 150 kHz or more. In this way, a high frequency voltage is applied between both electrodes (5a, 5b) from the power supply 6.

It is preferable to apply a voltage to the power supply 6 so that the electrode 5a has a ground voltage and the electrode 5b has a high voltage. This reduces the risk of electric shock due to the high voltage of the electrodes exposed to the outside.

When the above voltage is applied from the power supply 6 between both electrodes (5a, 5b), a dielectric barrier discharge occurs in the tube body 3. That is, a dielectric barrier discharge is generated with respect to the gas to be treated G1 flowing in the discharge space S1 to generate plasma. That is, the discharge space S1 forms the atmospheric pressure plasma space.

Oxygen contained in the gas to be treated G1 passes through the atmospheric pressure plasma space to show the reaction of the following equation (1). In the following equation (1), AP means that energy from atmospheric pressure plasma is applied.
O ₂ + AP → O + O ‥‥‥ (1)

A part of the oxygen atom O generated by the formula (1) reacts with the oxygen molecule contained in the gas to be treated G1 to generate _{ozone (O 3) by the following formula (2).} In addition, M in the formula (2) indicates the third body of the reaction (the same applies hereinafter).
O + O ₂ + M → O ₃ + M ‥‥ (2)

Methane contained in the gas to be treated G1 ^{reacts with O (3} P) in the O atom and is converted into a methyl radical (CH ₃ ) by the following equation (3). _{Further, the water vapor (H 2} O) contained in the gas to be treated G1 is decomposed by plasma to generate hydroxyl radical (OH) by the following equation (4). Further, the methane contained in the gas to be treated G1 also reacts with the hydroxyl radical (OH) as shown in the following equation (5). As the hydroxyl radical used in the reaction of the formula (5), the hydroxyl radical generated by the formula (4) is more dominant, but a part of the hydroxyl radical generated by the formula (3) is also included.
CH ₄ + O ( ³ P) → CH ₃ + OH ‥‥ (3)
H ₂ O + AP → H + OH ‥‥‥ (4)
CH ₄ + OH → CH ₃ + H ₂ O ‥‥ (5)

The methyl radical (CH ₃ ) obtained by the decomposition is further decomposed into more stable carbon monoxide (CO) and carbon dioxide (CO _{2) through various reactions.}

That is, when the gas to be treated G1 flows through the discharge space S1 (atmospheric pressure plasma space), it is converted into the treated gas G2 in which the concentration of methane contained in the gas to be treated G1 is reduced. After this treatment, the gas G2 is discharged from the gas discharge port 12. This result will be described later with reference to Examples.

In the present embodiment, the electrode 5a arranged on the tube wall of the outer tube 3a is not limited to the mesh shape. For example, as shown in FIGS. 2A and 2B, the electrode 5a may have a block shape arranged so as to cover the tube wall of the outer tube 3a in the circumferential direction. 2A and 2B are drawings schematically showing the configuration of the methane gas decomposition apparatus of this other aspect according to FIGS. 1A and 1B, and FIG. 2B corresponds to the cross-sectional view taken along the line A1-A1 in FIG. 2A. do.

In the example shown in FIG. 2B, it is assumed that the electrode 5a has a block shape having a cylindrical opening in the center, and the tubular body 3 is inserted into the opening. However, the electrode 5a may have a curved side surface and may be configured to cover the outer wall of the outer tube 3a of the tubular body 3 in the circumferential direction (see FIG. 2C). Further, the electrode 5a does not necessarily have to completely cover the tube wall of the outer tube 3a in the circumferential direction, and may be configured not to cover a part of the tube wall of the outer tube 3a, for example, as shown in FIG. 2D. No.

Further, as shown in FIG. 2E, the electrode 5a may have a configuration in which a mesh shape and a block shape are combined.

[Second Embodiment]
The second embodiment of the methane gas decomposition apparatus will be described focusing on the parts different from the first embodiment. In each of the following embodiments, the same elements as those in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted as appropriate.

3A and 3B are drawings schematically showing the configuration of the second embodiment of the methane gas decomposition apparatus. FIG. 3B is a sectional view taken along line A1-A1 in FIG. 3A.

In the present embodiment, unlike the first embodiment, the tube body 3 is composed of a single tube body. The electrode 5a is arranged on the outer wall of the tube body 3, and the electrode 5b is arranged so as to extend along the tube axis direction d1 of the tube body 3 at a position inside the tube body 3. The discharge space S1 is formed inside the tube body 3.

Also in the configuration of this embodiment, when a voltage is applied from the power source 6 to both electrodes (5a, 5b), a dielectric barrier discharge occurs in the tube body 3, and an atmospheric pressure plasma space is formed in the discharge space S1. do. The methane contained in the gas to be treated G1 is decomposed in the same manner as in the first embodiment by passing through the atmospheric pressure plasma space, and is discharged from the gas discharge port 12 as the treated gas G2 having a reduced methane content concentration. To.

Also in the present embodiment, the electrode 5a arranged on the outer wall of the tube 3 is not limited to the mesh shape, and for example, as shown in FIGS. 4A and 4B, the electrode 5a covers the outer wall of the tube 3 in the circumferential direction. It may have a block shape arranged in the above. 4A and 4B are drawings schematically showing the configuration of the methane gas decomposition apparatus of this other aspect according to FIGS. 3A and 3B, and FIG. 4B corresponds to the cross-sectional view taken along the line A1-A1 in FIG. 4A. do. Also in this case, as in the first embodiment, the electrode 5a may have a curved surface shape and may be configured to cover the outer wall of the tube 3 in the circumferential direction, or may be a part of the outer wall of the tube 3. It may be a configuration that does not cover.

[Third Embodiment]
The third embodiment of the methane gas decomposition apparatus will be described focusing on the parts different from the first embodiment.

5A and 5B are drawings schematically showing the configuration of the third embodiment of the methane gas decomposition apparatus. FIG. 5B is a sectional view taken along line A1-A1 in FIG. 5A.

In the present embodiment, unlike the first embodiment, the tube body 3 is composed of a single tube body. Further, in the present embodiment, the tube body 3 has a rectangular tubular shape having a pair of flat surfaces (7a, 7b) facing each other when viewed from the tube axis direction d1 (see FIG. 5B).

In this embodiment, both of the pair of electrodes (5a, 5b) are arranged on the outer wall of the tube body 3. One electrode 5a is arranged on the flat surface 7a, and the other electrode 5b is arranged on the flat surface 7b. That is, the mutual electrodes (5a, 5b) are separated by the tube body 3. The electrodes (5a, 5b) may have a mesh shape or a film shape.

In the present embodiment, the pipe body 3 may have a circular shape when viewed from the pipe axis direction d1. In this case, as shown in FIG. 5C, both electrodes (5a, 5b) extend in the tube axial direction d1 while exhibiting a shape along the curved surface of the outer wall of the tube 3, and mutually pass through the tube 3. Separated.

[Fourth Embodiment]
The fourth embodiment of the methane gas decomposition apparatus will be described focusing on the parts different from the first embodiment.

6A to 6C are drawings schematically showing the configuration of the fourth embodiment of the methane gas decomposition apparatus. 6B is a sectional view taken along line A1-A1 in FIG. 6A, and FIG. 6C is a sectional view taken along line A2-A2 in FIG. 6A.

In the present embodiment, unlike the first embodiment, the tube body 3 is composed of a single tube body. Further, in the present embodiment, the

electrodes

5a and 5b are respectively arranged on the outer wall of the tube body 3, and both are arranged alternately in a spiral shape when viewed in a direction orthogonal to the tube axis direction d1. ing. That is, as shown in FIGS. 6A to 6C, the place where the electrode arranged on the pipe wall on the + d2 side of the pipe body 3 becomes the electrode 5a and the place where the electrode 5b becomes, depending on the position related to the pipe axis direction d1. Is a mode in which and is changed. The same applies to the electrodes arranged on the tube wall on the −d2 side of the tube body 3.

As another embodiment, as shown in FIG. 7, the

electrodes

5a and 5b are arranged in a state of being formed along the surface of the tube wall of the tube body 3 and in a state of being separated from each other along the tube axial direction d1. It does not matter if it is done. In this case, the

electrodes

5a and 5b are arranged so as to be separated from each other along the tube axis direction d1.

In the case of this configuration, the space sandwiched between the

electrodes

5a and 5b in the tube axial direction d1 forms the atmospheric pressure plasma space inside the tube body 3. That is, the atmospheric pressure plasma space is discretely arranged along the tube axis direction d1. However, even with such a configuration, when the gas to be treated G1 flows through each atmospheric pressure plasma space, it is decomposed in the same manner as in the first embodiment, and the gas is discharged as the treated gas G2 having a reduced methane content concentration. It is discharged from the outlet 12.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples.

[Explanation of experimental conditions]
The experimental conditions will be described below.

(Experimental system)
An experiment was conducted using the methane gas decomposition apparatus 1 described above in the first embodiment. The dimensions of the experimental system are as follows.
Outer tube 3a: inner diameter 12 mm, wall thickness 1 mm, made of quartz glass Inner tube 3b: inner diameter 6.5 mm, wall thickness 0.85 mm, made of quartz glass Electrode 5a: arranged in a region of 300 mm along the tube axial direction d1, nickel Electrode 5b: Inserted along the axis of the inner tube 3b and arranged in a region including at least the region where the electrode 5a is formed, made of SUS (stainless quartz).

A gas to be treated G1 composed of a mixed gas of air and methane (methane concentration is 100 ppm) is introduced from the gas introduction port 11, and 300 W of electric power is supplied from the power source 6 to each electrode (5a, 5b) and a pipe. An atmospheric pressure plasma space was formed inside the body 3. Specifically, a high frequency voltage having an applied voltage of 15 to 18 kVpp, a frequency of 40 to 50 kHz, and a pulse width of 1.8 μsec was applied between both electrodes (5a, 5b).

The treated gas G2 discharged from the gas discharge port 12 is introduced into a sampling bag (manufactured by GL Sciences), and FTIR (Fourier transform infrared spectroscopy) is applied to the treated gas G2 contained in the sampling bag. Component analysis was performed using.

(Processed gas G1)
As the gas to be treated G1, a gas having the components of the conditions shown in Table 1 by FTIR analysis was adopted. _{Note that Table 1 does not describe nitrogen (N 2} ), oxygen (O ₂ ), and argon (Ar) contained in the air in the gas to be treated G1. In addition, "NO _y " in Table 1 is a notation that includes NO, NO ₂ , N ₂ O, N ₂ O ₅ , HNO ₂ , and HNO _3.

(Temperature measurement)
In the experiment, the temperature T1 of the tube body 3 and the temperature T2 of the treated gas G2 were measured, respectively. As the temperature T1, a value measured by a thermocouple was adopted as the temperature of the outer wall of the pipe body 3 outside the formation portion of the electrode 5a at the position closest to the gas discharge port 12 in the pipe axis direction d1.

(Cooling process)
In some experiments, cooling water W1 having a water temperature of 15 ° C. was passed through the inner pipe 3b in order to cool the gas to be treated G1 (see FIG. 8A).

(Heat treatment)
In some experiments, in order to raise the temperature of the gas to be treated G1, the gas to be treated G1 passed through the electric furnace 30 set at 500 ° C. was introduced into the gas inlet 11 (see FIG. 8B). ).

(Humidification treatment)
In some experiments, in order to humidify the gas to be treated G1, the gas to be treated G1 was supplied to the water in the airtight container 31 in which the water W2 was stored through the introduction pipe 32. Then, the gas to be treated G1 extracted from the outlet pipe 33 which passed through the water and was installed at a position above the water surface was introduced into the gas introduction port 11 (see FIG. 8C).

[inspection]
Under the experimental conditions of Examples 1 to 9 shown in Table 2, the treated gas G1 was passed through the methane gas decomposition apparatus 1 under different flow rate conditions, and the treated gas discharged from the gas discharge port 12 was discharged. The component analysis of G2 was performed. The results are shown in Table 3.

(Result analysis 1)
According to Table 3, it was confirmed that the concentration of methane contained in the treated gas G2 could be reduced as compared with the treated gas G1 in all the examples. It was confirmed that, under the same conditions, the smaller the flow rate of the gas to be treated G1 introduced into the methane gas decomposition apparatus 1, the larger the amount of methane decomposed.

In particular, in Examples 1 to 4 in which the cooling, heating, and humidifying treatments were not performed, when the mutual results were compared, the case of Example 1 in which the flow rate of the gas to be treated was 3 L / min, which was the smallest. It was confirmed that almost 100% of the methane contained in the gas to be treated G1 could be decomposed. The description of "almost 100%" here means that it is below the detection limit and does not consider the order of 0.01 ppm or less (10 ppb or less).

It was also confirmed that in Example 4 in which the flow rate was the highest at 20 L / min, methane contained in the gas to be treated G1 could be decomposed by about 20%. That is, from the results of Examples 1 to 4, it was confirmed that methane can be decomposed even when the concentration of methane contained in the gas to be treated G1 is as low as 100 ppm.

(Result analysis 2: Impact of cooling treatment)
The influence on the decomposition rate of the gas to be treated G1 was verified depending on the presence or absence of the cooling treatment. Examples 3 and 5, and Examples 4 and 6 correspond to a group of examples in which conditions other than the presence or absence of cooling treatment are shared. As described above, the cooling treatment was performed by allowing the cooling water W1 having a water temperature of 15 ° C. to flow through the inner pipe 3b provided in the methane gas decomposition device 1.

According to Table 3, when Example 3 and Example 5 in which the flow rates of the gas to be treated G1 are both 10 L / min are compared, Example 5 in which the cooling treatment for the gas to be treated G1 is subjected to the cooling treatment is compared with Example 3. It was confirmed that the decomposition performance of methane was significantly reduced. Similarly, comparing Example 4 and Example 6 in which the flow rates of the gas to be treated G1 are both 20 L / min, Example 6 in which the cooling treatment for the gas to be treated G1 is cooled is methane as compared with Example 4. It was confirmed that the decomposition performance was significantly reduced.

Further, in Examples 5 and 6 in which the cooling treatment was performed, O ₃ was detected as compared with Examples 3 and 4 in which the cooling treatment was not performed.

_{From the results in Table 3, it is inferred that ozone (O 3} ) generated from the gas to be treated G1 by the atmospheric pressure plasma does not directly contribute to the decomposition of _{CH 4.} _{It is also shown that CH 4} can hardly be decomposed by the atmospheric pressure plasma alone.

Further, as shown in the results of Table 3, in Examples 3 and 4, ozone was not present in the treated gas G2 because ozone (O ₃ ) was thermally decomposed by the following equation (6). It has been considered due to a change in O ⁽³ P).
O ₃ → O ( ³ P) + O ₂ ‥‥ (6)

FIG. 9 is a graph showing the relationship between the thermal decomposition rate (half-life) of ozone and the temperature. According to FIG. 9, it can be seen that the half-life of ozone is about 1000 seconds at 100 ° C. and the half-life of ozone is about 1 second at 200 ° C. That is, it can be seen that when ozone is heated to 200 ° C. or higher, ozone is decomposed within 1 second ^{to generate O (3 P).}

That is, from the above results, in Examples 5 and 6, the temperature T1 of the outer wall of the tube 3 outside the formation portion of the electrode 5a at the position closest to the gas discharge port 12 is less than 200 ° C. In view of this (see Table 2), it is probable that the temperature of the treated gas G2 did not exceed 200 ° C., and ozone that was not thermally decomposed remained in the treated gas G2. On the other hand, in Examples 3 and 4, in view of the fact that the temperature T1 of the outer wall of the tube 3 outside the formation portion of the electrode 5a at the position closest to the gas discharge port 12 is 230 ° C. or higher (see Table 2). ), It is considered that the temperature of the gas to be treated G1 at the time of passing through the atmospheric pressure plasma space has reached 200 ° C. or higher, so the ozone generated when passing through this region is O ( ³ P). ^{After being decomposed, it is considered that this O (3} P) contributed to the decomposition of methane according to the above-mentioned equation (3).

When an atmospheric pressure plasma space is formed by the dielectric barrier discharge occurring in the discharge space S1 in the tube body 3, the heat generated by the collision of electrons, ions, neutral particles, molecules, etc. in the plasma space is generated. , The temperature inside the space is raised by the heat of the dielectric loss due to the dielectric barrier discharge. That is, the atmospheric pressure plasma space and the tube 3 form a heating source for the gas to be treated G1. From the above results, at least in the atmospheric pressure plasma space, the temperature of ozone is raised to the extent that ozone is thermally decomposed within 1 second, or at the latest within a few seconds. As a result, it can be seen that methane can be efficiently decomposed by simply passing the gas to be treated G1 into the atmospheric pressure plasma space.

It is also confirmed that when the atmospheric pressure plasma space is cooled as in Examples 5 and 6, the decomposition rate of ozone decreases, and the decomposition rate of methane decreases. However, in Examples 5 and 6, decomposition of methane was confirmed, although it was slight as compared with Examples 1 to 4.

In view of the above results, if the temperature T1 of the portion of the tube 3 corresponding to the outside of the end position on the gas discharge port 12 side of the atmospheric pressure plasma space is 230 ° C. or higher, the temperature is within the atmospheric pressure plasma space. It is considered that the temperature of the gas to be processed G1 passing through the above can be set to 200 ° C. or higher.

(Result analysis 3: Effect of heat treatment)
The effect on the decomposition rate of the gas to be treated G1 was verified depending on the presence or absence of heat treatment. Examples 2 and 7, Examples 3 and 8, and Examples 4 and 9 correspond to a group of examples in which conditions other than the presence or absence of heat treatment are shared. In the heat treatment, as described above, the gas G1 to be treated is passed through the electric furnace 30 set at 500 ° C., and then the gas G1 to be treated is introduced into the methane gas decomposition apparatus 1 from the gas introduction port 11. Made by.

According to Table 3, comparing Example 2 and Example 7 in which the flow rates of the gas to be treated G1 are both 5 L / min, Example 7 in which the heat treatment to the gas to be treated G1 is heat-treated is higher than that in Example 2. Furthermore, it was confirmed that the decomposition performance of methane was improved. More specifically, in the case of Example 2, 24.0 ppm of methane remained in the treated gas G2, whereas according to Example 7 in which the heat-treated gas G1 was heat-treated. The concentration of methane contained in the treated gas G2 was below the detection limit.

Similarly, comparing Example 3 and Example 8 in which the flow rates of the gas to be treated G1 are both 10 L / min, Example 8 in which the heat treatment to the gas to be treated G1 is heat-treated is more methane than Example 3. It was confirmed that the disassembly performance was improved. Similarly, comparing Example 4 and Example 9 in which the flow rates of the gas to be treated G1 are both 20 L / min, Example 9 in which the heat treatment to the gas to be treated G1 is heat-treated is more methane than Example 4. It was confirmed that the disassembly performance was improved.

By heating the gas to be treated G1, the temperature of the gas to be treated G1 becomes high at the time of passing through the atmospheric pressure plasma space (in the discharge space S1), and as a result, the decomposition of ozone described in the above equation (6) is performed. It is considered that the rate of the reaction and the decomposition reaction of methane described in the above equation (3) was accelerated, and the decomposition rate of methane was improved.

(Result analysis 4: Effect of presence / absence of humidification treatment)
The effect on the decomposition rate of the gas to be treated G1 was verified depending on the presence or absence of the humidification treatment. Examples 3 and 10 correspond to a group of examples in which conditions other than the presence or absence of humidification treatment are shared. In the humidification treatment, as described above with reference to FIG. 8C, the treated gas G1 is placed in the water in the airtight container 31 in which the water is stored before the treated gas G1 is introduced into the methane gas decomposition apparatus 1. It was done by letting it pass.

According to Table 3, comparing Example 3 and Example 10 in which the flow rates of the gas to be treated G1 are both 10 L / min, Example 10 in which the gas to be treated G1 is humidified is higher than that in Example 3. Furthermore, it was confirmed that the decomposition performance of methane was improved. More specifically, in the case of Example 3, 52.9 ppm of methane remained in the treated gas G2, whereas according to Example 10 in which the treated gas G1 was humidified. The concentration of methane contained in the treated gas G2 was 31.9 ppm.

Therefore, as the amount of water contained in the gas to be treated G1 increases due to the humidification treatment, the amount of hydroxyl radicals (OH) decomposed and generated by plasma according to the above equation (4) increases. As a result, it is considered that the reaction amount of the above formula (5) increased and the decomposition rate of methane was improved.

1: Methane gas decomposition device 3: Tube body 3a: Outer tube 3b: Inner tube 5a: Electrode (first electrode)
5b: Electrode (second electrode)
6: Power supply 7a: Flat surface 7b: Flat surface 11: Gas inlet 12: Gas discharge port 20: Gas supply source 30: Electric furnace 31: Container 32: Introduction pipe 33: Outlet pipe 100: Methane removal system 101: Processed Gas discharge source 102: Gas pipe 102a to be treated: Gas path 104 to be treated: Catalyst 104a for removing methane oxidation: Catalyst accommodating portion 105: Plasma generating means 106: Control means 107: Power supply source 108a: External electrode 108b: Internal electrode G1 : Processed gas G2: Treated gas S1: Discharge space W1: Cooling water W2: Water d1: Pipe axis direction

Claims

A methane gas decomposition method characterized by raising the temperature by passing a gas to be treated, which is a mixture of methane gas in air, into an atmospheric pressure plasma space formed by a dielectric barrier discharge to decompose methane.
The methane gas decomposition method according to claim 1, wherein the gas to be treated is passed through the atmospheric pressure plasma space in a state of being heated to 200 ° C. or higher.
The methane gas decomposition method according to claim 1 or 2, wherein the gas to be treated is passed through the atmospheric pressure plasma space in a humidified state.
A tube body including a tube wall made of a dielectric and a tube body
A pair of electrodes arranged across the tube wall,
A power supply that applies a high frequency voltage to the pair of electrodes,
A gas inlet for introducing a gas to be treated, which is a mixture of methane gas in air inside the tube, and a gas inlet.
When the high frequency voltage is applied to the pair of electrodes, the gas to be treated that has passed through the atmospheric pressure plasma space formed inside the tube is discharged to the outside of the tube. A methane gas decomposition device characterized by being equipped with.
The pair of electrodes includes a first electrode formed on the outer surface of the tube wall and a second electrode inserted inside the tube body along the tube axis direction of the tube body. The methane gas decomposition apparatus according to claim 4, wherein the methane gas decomposition apparatus is provided.
The pair of electrodes is one of a first electrode formed on a part of the outer surface of the tube wall and an outer surface of the tube wall at a position facing the first electrode via the inside of the tube body. The methane gas decomposition apparatus according to claim 4, further comprising a second electrode formed in the portion.
The pair of electrodes are the first electrode formed on a part of the outer surface of the tube wall and the outer surface of the tube wall at a position separated from the first electrode in the tube axis direction of the tube body. The methane gas decomposition apparatus according to claim 4, further comprising a second electrode formed in a part thereof.
The tube wall of the tube body located on the outside of the end portion of the atmospheric pressure plasma space formed inside the tube body on the gas discharge port side in the tube axis direction of the tube body is 230 ° C. or higher. The methane gas decomposition apparatus according to any one of claims 4 to 7, wherein the methane gas decomposition apparatus is characterized by the above.