Patent Publication Number: US-2023158452-A1

Title: Gas treatment method and gas treatment device

Description:
TECHNICAL FIELD 
     The present invention relates to a gas treatment method and a gas treatment device, and in particular, to a method and device for treating gas to be treated in which a target substance to be treated exhibiting volatility at room temperature is mixed with air. 
     BACKGROUND ART 
     With global warming in progress, a greenhouse gas that contributes to global warming is known to include carbon dioxide, methane, and chlorofluorocarbon gas. The amount of carbon dioxide emissions is the largest and the amount of methane emissions is the second largest. The global warming potential of methane is considered to be about 20 to 70 times that of carbon dioxide, thus its impact on global warming is significant. In addition, the concentration of methane has been increasing in recent years, hence technology of efficiently decomposing methane that has been emitted will be necessary in view of the future of the global environment. 
     Patent Document 1 discloses a methane removal system as a conventional technology of decomposing methane.  FIG.  14    is a drawing schematically illustrating a configuration of the methane removal system. 
     A methane removal system  100  is a system that decomposes methane contained in gas to be treated, and includes a tube for gas to be treated  102 , a catalyst for removing methane by oxidation  104 , a plasma generation means  105 , and a control means  106 . 
     A tube for gas to be treated  102  is configured inside a channel for gas to be treated  102   a  through which gas to be treated Eg is exhausted from an emission source of gas to be treated  101 . A catalyst for removing methane by oxidation  104  that has in layers is accommodated in the catalyst container  104   a  that is configured inside the channel for gas to be treated  102   a , and is used to remove the gas to be treated Eg that flows inside the channel for gas to be treated  102   a.    
     The plasma generation means  105  includes an external electrode  108   a , an internal electrode  108   b , and a power supply source  107 . The external electrode  108   a  is disposed around the outer circumference of the tube for gas to be treated  102  in a manner that surrounds the catalyst container  104   a , and has a cylindrical shape. The internal electrode  108   b  is disposed to be located at a position corresponding to the catalyst container  104   a  in the channel for gas to be treated  102   a . The power supply source  107  has one terminal electrically connected to the internal electrode  108   b . The other terminal of the power supply source  107  and the external electrode  108   a  are grounded. 
     The plasma generation means  105  supplies power to the internal electrodes  108   b  by the power supply source  107  to generate atmospheric pressure plasma at a position where the catalyst for removing methane by oxidation  104  accommodated in the catalyst container  104   a  exists. This allows the gas to be treated Eg circulating in the channel for gas to be treated  102   a  to transform into plasma in the catalyst container  104   a . The generated plasma activates the catalyst for removing methane by oxidation  104 . The methane contained in the gas to be treated Eg is decomposed into carbon dioxide by the synergy of the action of the catalyst for removing methane by oxidation  104  that has been activated and the action of the generated plasma. 
     CITATION LIST 
     Patent Documents 
     Patent Document 1: JP-A-2019-155242 
     SUMMARY OF INVENTION 
     Technical Problem 
     The largest source of methane emissions released into the environment is reportedly livestock and other animals in the form of farts and burps. For example, the amount of methane emissions released from a single cow is estimated to be about 300 liters per day. On the other hand, the atmospheric concentration of methane released from cattle and other livestock raised in cattle sheds is about 20 ppm. 
     The Patent Document 1 discloses that methane was able to be decomposed by allowing gas to be treated having a methane concentration of 3000 ppm to pass through the system. However, the technology that can decompose methane having low concentrations such as several hundred ppm or less, has yet to been sufficiently established at the present moment. 
     In the case of decomposing methane from gas to be treated containing methane at high concentrations (e.g., several thousand ppm or more, or several percent or less), a method of simply combusting gas to be treated is considered to be the simplest and most effective method. However, in the case of such a combustion process, the residual gas often contains methane at several hundred ppm or less. Decomposing methane by combustion process for the treated gas containing methane at such low concentrations is impractical method from the viewpoint of the decomposition amount of methane with respect to the amount of energy input. 
     Incidentally, in addition to the viewpoint of global warming, there exists a demand of treating air containing substances that cause odors. For example, the above-mentioned cattle sheds emit gas with odors such as ammonia, hydrogen sulfide, methyl mercaptan, etc., in addition to methane. Hence, the atmospheric gas from cattle sheds comes to contain these substances in low concentrations. Moreover, for gas exhausted from locations other than cattle sheds, there exists a similar demand of treating air containing odor causing substances at low concentrations. 
     In view of the above issues, it is an object of the present invention that provides a technology that can decompose, by a simple method, gas to be treated containing target substance to be treated at low concentrations such as several hundred ppm or less. 
     Solution to Problem 
     A gas treatment method according to the present invention includes: 
     a process (a) of allowing gas to be treated in which a target substance to be treated is mixed with air to pass through inside a housing, the target substance to be treated exhibiting volatility at room temperature and belonging to at least one substance selected from a group consisting of carbon compounds, nitrogen compounds, and sulfur compounds; 
     a process (b) of introducing ozone into a space through which the gas to be treated flows inside the housing at 200° C. or lower; 
     a process (c) of stirring the gas to be treated after the process (b); and 
     a process (d) of heating the gas to be treated to 300° C. or higher after executing the process (c). 
     The details will be described later; however, the diligent study of the present inventors confirmed that the decomposition rate of the target substance to be treated contained in the gas to be treated at low concentrations such as several hundred ppm or less is improved by introducing ozone into the gas to be treated at 200° C. or less, stirring the gas to be treated, and then performing heat treatment. 
     Stirring the gas to be treated after the introduction of ozone increases the probability of contact of the gas to be treated with ozone. Subsequently conducting heat treatment makes O( 3 P) that is produced by the thermal decomposition of ozone easier to come into contact with the gas to be treated, increasing the decomposition rate of the target substance to be treated contained in the gas to be treated. 
     The target substance to be treated may belong to at least one substance selected from a group consisting of methane, acetylene, ethylene, ethane, propane, methylamine, ammonia, hydrogen sulfide, and methyl mercaptan. 
     The process (c) may be a process of allowing the gas to be treated to pass through a stirring area in which a cross-sectional area of a gas channel is varied, or a stirring area in which a wind shield plate is provided in the middle of the gas channel. 
     This enables the gas to be treated to be stirred while decreasing the velocity of the gas to be treated flowing toward the outlet through which the gas that has been treated is exhausted after ozone has been introduced. 
     The stirring area may have a length longer than the length of the area through which the gas to be treated flows when the process (b) is executed, in a direction of flow of the gas to be treated. 
     The above method makes the gas to be treated easier to come into contact with ozone before executing the heating process (c). As a result, O( 3 P) that is produced by the thermal decomposition of ozone in the heating process (c), which is to be executed later, is made easier to come into contact with the gas to be treated. 
     A gas treatment device according to the present invention is a gas treatment device that treats gas to be treated in which a target substance to be treated is mixed with air, the target substance to be treated exhibiting volatility at room temperature and belonging to at least one substance selected from a group consisting of carbon compounds, nitrogen compounds, and sulfur compounds, the gas treatment device includes: 
     a housing; 
     a gas inlet through which the gas to be treated is introduced into the housing; 
     an ozone introduction unit that introduces ozone into a gas channel through which the gas to be treated flows inside the housing; 
     a stirring area that is disposed downstream from a location of the ozone introduction unit, and that stirs the gas to be treated flowing through the gas channel; 
     a heating area that is disposed downstream from a location of the stirring area, and that heats the gas to be treated flowing through the gas channel; and 
     a gas outlet through which gas that has been treated having passed through the heating area is exhausted from the housing. 
     In the above gas treatment device, the gas to be treated and ozone that is introduced by the ozone introduction unit come into contact with each other at a high probability when passing through the stirring area, and then are introduced into the heating area. As a result, O( 3 P) produced by the thermal decomposition of ozone comes into contact with the gas to be treated at a high probability, allowing the target substance to be treated contained in the gas to be treated to decompose. Then, the gas that has been treated in which the target substance to be treated is decomposed with high efficiency is exhausted from the gas outlet. 
     The gas channel in the stirring area in the direction of flow of the gas to be treated may have a length longer than a length from a location at which ozone is introduced by the ozone introduction unit to the stirring area in the direction of flow of the gas to be treated. 
     The above configuration enables the gas to be treated passing through the stirring area to come into contact with ozone at higher probability. 
     The gas channel may be configured to include a heated wall whose surface is heated to 300° C. or higher in the heating area. 
     An example of a more specific configuration includes an aspect in which the entire heating area is mounted in the heating furnace. Another example includes an aspect in which a heat transfer member such as a metal tube is disposed on the inner wall surface of the gas channel located in the heating area, and this heat transfer member is heated. Yet another example includes an aspect in which a heated plate member having an opening is mounted in the gas channel, and the gas to be treated flows through the opening. 
     The gas channel may be bent in the heating area. 
     In the above configuration, when the gas to be treated flows through the heating area, its flow velocity is reduced at a bent section, thereby extending the time for the gas to be treated to flow through the heating area. This allows the heating time of the gas to be treated containing ozone to be longer, thereby increasing the time for O( 3 P), which is obtained by the thermal decomposition of ozone, to come into contact with the gas to be treated, in other words, the reaction time between the target substance to be treated contained in the gas to be treated and O( 3 P). This further increases the decomposition rate of the target substance to be treated. 
     The gas channel may be configured in a manner that a cross-sectional area of the gas channel changes in the stirring area. 
     In the above configuration, when the gas to be treated flows through the location in which the cross-sectional area of the gas channel changes, turbulence is readily generated due to the pressure difference. This turbulence causes the gas to be treated and ozone to be sufficiently stirred and mixed, making the ozone and the gas to be treated readily come into contact with each other. 
     The gas channel may include a wind shield plate with which the gas to be treated flowing through the gas channel collides in the stirring area. 
     In the above configuration, when the gas to be treated collides with the wind shield plate, the direction of airflow changes, readily generating turbulence at the position. This turbulence causes the gas to be treated and ozone to be sufficiently stirred and mixed, making the ozone and the gas to be treated readily come into contact with each other. 
     The ozone introduction unit may include a light source that is disposed in the gas channel, and that emits ultraviolet light having a main peak wavelength of less than 200 nm, and ozone may be generated from part of the gas to be treated by irradiating the gas to be treated with the ultraviolet light from the light source. 
     In the above configuration, ozone can be generated from gas to be treated without separately supplying gas as an ozone generation source. In particular, when the light source is configured to be an excimer lamp using Xe as its luminescent gas, the main peak wavelength is near 172 nm (160 nm or more and less than 180 nm), thereby also providing a secondary effect that no NO x  is generated during ozone generation. 
     The ozone introduction unit may include an atmospheric pressure plasma generator that is disposed in the gas channel, and ozone may be generated from part of the gas to be treated by allowing the gas to be treated to pass through a space of atmospheric pressure plasma generated by the atmospheric pressure plasma generator. 
     In the above configuration, ozone can be generated from gas to be treated without separately supplying gas as an ozone generation source. 
     The ozone introduction unit may include an ozone generator that is disposed in a channel different from the gas channel, and ozone gas generated from the ozone generator may be supplied into the gas channel. 
     Advantageous Effects of Invention 
     According to the present invention, gas to be treated containing a target substance to be treated at low concentrations such as several hundred ppm or less, can be decomposed by a simple method. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view schematically illustrating a configuration of a first embodiment of a gas treatment device. 
         FIG.  2 A  is a side view schematically illustrating an example of the configuration of an excimer lamp as an ozone introduction unit. 
         FIG.  2 B  is a cross-sectional view taken along the line A 1 -A 1  in  FIG.  2 A . 
         FIG.  3    is a graph indicating the spectrum of ultraviolet light emitted from an excimer lamp filled with luminescent gas containing Xe and the absorption spectrum of oxygen (O2) in a superimposed manner. 
         FIG.  4 A  is a drawing schematically illustrating an example of the configuration of a stirring area. 
         FIG.  4 B  is a drawing schematically illustrating another example of the configuration of a stirring area. 
         FIG.  4 C  is a drawing schematically illustrating yet another example of the configuration of a stirring area. 
         FIG.  5    is a cross-sectional view schematically illustrating another configuration of the first embodiment of the gas treatment device. 
         FIG.  6    is a graph illustrating a relationship between the thermal decomposition rate (half-life) of ozone and temperature. 
         FIG.  7    is a graph illustrating a relationship between the reaction rate in formulas (7) and (9), and temperature. 
         FIG.  8 A  is a drawing schematically illustrating another example of the configuration of a heating area. 
         FIG.  8 B  is a drawing schematically illustrating yet another example of the configuration of a heating area. 
         FIG.  9 A  is a cross-sectional view schematically illustrating another example of the configuration of an excimer lamp as an ozone introduction unit. 
         FIG.  9 B  is a plan view schematically illustrating another example of the configuration of an excimer lamp as an ozone introduction unit. 
         FIG.  9 C  is a cross-sectional view taken along the line A 2 -A 2  in  FIG.  9 B . 
         FIG.  10 A  is a cross-sectional view schematically illustrating a configuration of a second embodiment of the gas treatment device. 
         FIG.  10 B  is another cross-sectional view schematically illustrating the configuration of the second embodiment of the gas treatment device. 
         FIG.  11 A  is a cross-sectional view schematically illustrating a configuration of an atmospheric pressure plasma generator as an ozone introduction unit. 
         FIG.  11 B  is a cross-sectional view taken along the line A 3 -A 3  in  FIG.  11 A . 
         FIG.  12    is a drawing schematically illustrating a structure of an experimental system used in Examples. 
         FIG.  13    is a graph illustrating a relationship between the concentrations of methane and ozone in the gas that has been treated in Examples 1 through 5 and the preset temperatures of an electric furnace. 
         FIG.  14    is a drawing schematically illustrating a configuration of a conventional methane removal system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a gas treatment method and a gas treatment device according to the present invention will be described with reference to the drawings. Note that the following drawings are schematically illustrated, and a dimensional ratio on the drawing does not always match an actual dimensional ratio. Furthermore, the dimensional ratios between the drawings do not always the same. 
       FIG.  1    is a cross-sectional view schematically illustrating a configuration of a first embodiment of a gas treatment device. A gas treatment device  1  is a device that treats gas to be treated G 1  in which a target substance to be treated is mixed with air, in the device and exhausts it as gas that has been treated G 2 . In  FIG.  1   , the direction of flow of the gas to be treated G 1  is indicated as d 1 . 
     The target substance to be treated belongs to at least one substance selected from a group consisting of carbon compounds, nitrogen compounds, and sulfur compounds. Examples of the target substance to be treated include methane, acetylene, ethylene, ethane, propane, methylamine, ammonia, hydrogen sulfide, methyl mercaptan, which exhibit volatility at room temperature and exhibit an odor. 
     The gas treatment device  1  includes a housing  2 , a gas inlet  3   a  through which the gas to be treated G 1  is introduced into the housing  2 , and a gas outlet  3   b  through which gas that has been treated G 2 , which is gas after the gas to be treated G 1  has been treated in the housing  2 , is exhausted from the housing  2 . The housing  2  is provided with a gas channel  5  through which the gas to be treated G 1  (or the gas that has been treated G 2 ) is allowed to flow. 
     The gas to be treated G 1  that is introduced into the housing  2  from the gas inlet  3   a  is expected to be at room temperature or below approximately 100° C.; the temperature thereof will not exceed 200° C. no matter how high it is. In contrast, the gas to be treated G 1  that has been introduced into the heating area  30 , which will be described below, is heated to 300° C. or higher. 
     In the present embodiment, the gas treatment device  1  houses an excimer lamp  11 . This excimer lamp  11  constitutes an ozone introduction unit that introduces ozone into the gas channel  5 . In the following description, the area where ozone is introduced into the gas channel  5  from the ozone introduction unit is referred to as the “ozone introduction area  10 ”. 
     Ozone Introduction Unit 
       FIGS.  2 A and  2 B  are drawings schematically illustrating an example of the configuration of an excimer lamp  11 .  FIG.  2 A  corresponds to a side view of the excimer lamp  11 , and  FIG.  2 B  corresponds to a cross-sectional view taken along the line A 1 -A 1  in  FIG.  2 A . 
     The excimer lamp  11  illustrated in  FIGS.  2 A and  2 B  is provided with a tube body  13  and a pair of electrodes ( 14   a ,  14   b ). In the present embodiment, the tube body  13  has a double-tube structure. More specifically, as shown in  FIG.  2 B , the tube body  13  includes an outer tube  13   a  and an inner tube  13   b . The outer tube  13   a  is located on the outside and has a cylindrical shape. The inner tube  13   b  is located coaxially with the outer tube  13   a  inside the outer tube  13   a , and has a cylindrical shape. The inner diameter of the outer tube  13   a  is larger than the outer diameter of the inner tube  13   b.    
     In the excimer lamp  11  shown in  FIGS.  2 A and  2 B , presented is an example in which the tube body  13  is disposed such that a direction of its tube axis is aligned with a direction of flow d 1  of the gas to be treated G 1 . However, the direction of the tube axis of the tube body  13  is not necessarily parallel to the direction of flow d 1  of gas to be treated; this drawing is merely an example. Hereinafter, for convenience, the direction of flow d 1  of the gas to be treated G 1  is assumed to be parallel to the direction of the tube axis of tube body  13 . The sign “d 1 ” is also used for the direction of the tube axis, which is same as the direction of flow, in the viewpoint of avoiding an increase in the number of signs. 
     The outer tube  13   a  and the inner tube  13   b  are both sealed at their ends in the direction of the tube axis d 1  (not shown). A luminescent space with a circular shape (in this case, an annular shape) is formed between them when viewed from the direction of the tube axis d 1 . This luminescent space is filled with luminescent gas  15 G that forms excimer molecules by electrical discharge. 
     An electrode  14   a  is provided on the outer wall of the outer tube  13 A. In the present embodiment, this electrode  14   a  has a mesh shape or a linear shape. Also, a rod-shaped electrode  14   b  extending along the direction of the tube axis of the tube body  13  is inserted inside the inner tube  13   b.    
     The outer tube  13   a  and inner tube  13   b  exhibit transparency to ultraviolet light L 1 , and are made of a dielectric material such as synthetic quartz glass, for example. The electrodes ( 14   a ,  14   b ) are made of a metal material including stainless steel, aluminum, copper, tungsten, titanium, nickel. 
     When a high-frequency AC voltage of approximately 1 kHz to 5 MHz is applied between the electrodes ( 14   a ,  14   b ) via a power feed line from a power supply (not shown), the voltage is applied to the luminescent gas  15 G via the tube body  13 . This generates a discharge plasma in the discharge space in which the luminescent gas  15 G is sealed, which causes the atoms of the luminescent gas  15 G to be excited to the excimer state, producing excimer luminescence during the transition of these atoms to the ground state. 
     The material of the luminescent gas  15 G determines the wavelength of the ultraviolet light L 1  emitted from the tube body  13 . When gas containing xenon (Xe) is used as the luminescent gas  15 G, the excimer luminescence is the ultraviolet light L 1  having a main peak wavelength near 172 nm. 
     The wavelength of the ultraviolet light L 1  can be changed by using different substances as the luminescent gas  15 G. Examples of the luminescent gas  15 G include ArBr (main peak wavelength being near 165 nm), ArCl (main peak wavelength being near 175 nm), and F 2  (main peak wavelength being near 153 nm). Here, the case in which the luminescent gas  15 G is gas containing Xe is described. 
     As mentioned above, in the excimer lamp  11  of the present embodiment, the electrode  14   a  provided on the outer wall of the outer tube  13   a  has a mesh shape. Hence, there are openings in the electrode  14   a , through which the ultraviolet light L 1  is extracted toward the outside of the outer tube  13   a . This ultraviolet light L 1  is irradiated to the gas to be treated G 1  flowing through the gas channel  5 . 
       FIG.  3    is a graph indicating the spectrum of ultraviolet light L 1  emitted from an excimer lamp  11  filled with luminescent gas  15 G containing Xe and the absorption spectrum of oxygen (O2) in a superimposed manner. In  FIG.  3   , the horizontal axis represents the wavelength, the left vertical axis represents the relative value of the light intensity of the ultraviolet light L 1 , and the right vertical axis represents the absorption coefficient of oxygen (O2). 
     When gas containing Xe is used as the luminescent gas  15 G of the excimer lamp  11 , as shown in  FIG.  3   , the ultraviolet light L 1  emitted from the excimer lamp  11  has a main peak wavelength of 172 nm, and the bandwidth in a range of approximately from 155 nm to 190 nm. 
     As described above, the gas to be treated G 1  is air that is mixed with the target substance to be treated. Hence, when ultraviolet light L 1  having a wavelength emitted from excimer lamp  11  is irradiated to the gas to be treated G 1  and absorbed by oxygen (O 2 ), the reaction of the Formula (1) below proceeds. In Formula (1), O( 1 D) represents an oxygen atom in the excited state and exhibits extremely high reactivity, while O( 3 P) represents an oxygen atom in the ground state. The reactions in Formulas (1) and (2) occur in response to the wavelength component of the ultraviolet light L 1 . Specifically, Formula (1) is the reaction that occurs when the shorter wavelength component of the ultraviolet light L 1  is absorbed by oxygen (O 2 ). More precisely, Formula (1) is the reaction that occurs when ultraviolet light L 1  having a wavelength λ of less than 175 nm is absorbed by oxygen (O 2 ). Formula (2) is the reaction that occurs when ultraviolet light L 1  having a wavelength λ of 175 nm or more and less than 242 nm is absorbed. 
       O 2   +h ν(λ)→O( 1 D)+O( 3 P)  (1)
 
       O 2   +h ν(λ)→O( 3 P)+O( 3 P)  (2)
 
     Part of the O atoms generated in Formulas (1) and (2) react with the oxygen (O 2 ) contained in the gas to be treated G 1 , thus which generates ozone (O 3 ) in accordance with Formula (3) below. In Formula (3), note that M indicates a third body. (the same applies hereinafter) 
       O+O 2 +M→O 3 +M  (3)
 
     This reaction enables ozone to be introduced into the gas to be treated G 1 , thus the gas to be treated G 1  changes to gas containing ozone. In other words, the excimer lamp  11  constitutes the ozone introduction unit in the gas treatment device  1  of the present embodiment. 
     From this viewpoint, the wall surface of the gas channel  5  through which the gas to be treated G 1  flows is preferably made of ozone-resistant materials, for example stainless steel or glass. When the wall of the gas channel  5  is constituted by part of the housing  2 , the housing  2  itself may be constituted by the aforementioned material. 
     The O( 1 D) generated by the reaction in the above Formula (1) may react with water (H 2 O) contained in the air to form hydroxyl radical in accordance with the Formula (4) below. 
       O( 1 D)+H 2 O→⋅OH+⋅OH  (4)
 
     When the ultraviolet light L 1  has a spectrum in the wavelength range of 185 nm or less, water vapor (H 2 O) contained in the gas to be treated G 1  absorbs the ultraviolet light L 1  to produce hydroxyl radical (OH) in accordance with Formula (5) below. 
       H 2 O+ h ν(λ)→H+⋅OH  (5)
 
     Since extremely reactive, hydroxyl radical (OH) may react with part of the target substance to be treated in the gas to be treated G 1  and decompose it. However, hydroxyl radical (⋅OH) alone can decompose a very limited amount of the target substance to be treated contained in the gas to be treated G 1 . As an example, when methane (CH 4 ) is contained as the target substance to be treated, the hydroxyl radical (⋅OH) may cause the reaction in Formula (6) below to decompose methane. 
       CH 4 +⋅OH→CH 3 +H 2 O  (6)
 
     In addition, part of the atomic oxygen, O( 1 D) and O( 3 P), which are produced in the reactions of Formula (1) and (2), may react with the substance to be treated and decompose it, but still has a limited decomposition amount. As an example, when methane is contained as the target substance to be treated, methane may be decomposed by the reaction in Formula (7) below. Note that O( 1 D) and O( 3 P) are collectively denoted as O in Formula (7). 
       CH 4 +O→CH 3 +⋅OH  (7)
 
     In accordance with the reactions shown in Formulas (6) and (7) above, part of the target substance to be treated undergoes the decomposition reaction with radicals, but this reaction alone makes it difficult to sufficiently decompose the target substance to be treated. 
     Hence, the gas treatment device  1  of the present embodiment includes a stirring area  20  that stirs the gas to be treated G 1  flowing through the gas channel  5 , and a heating area  30  that heats the gas to be treated G 1  flowing through the gas channel  5 . Both of the area are disposed downstream from a location of the area (ozone introduction area  10 ) in which ozone from the ozone introduction unit (excimer lamp  11 ) is introduced into the gas to be treated G 1 . Hereinafter, each of these area will be described. 
     (Stirring Area  20 ) 
     In the example of the gas treatment device  1  shown in  FIG.  1   , the stirring area  20  is constituted by arranging a plurality of wind shield plates  21  at separate locations along the direction of flow d 1  in the gas channel  5 . The gas to be treated G 1  that collides with the wind shield plate  21  changes the direction of airflow due to the collision. Then, this causes part of the gas to be treated G 1  to generate turbulence. As a result, the gas to be treated G 1  is stirred with each other. 
     The wind shield plate  21 , similar to the wall surface of the gas channel  5  through which the gas to be treated G 1  flows, may be made of an ozone-resistant material, such as stainless steel, and glass. 
     Various methods can be employed to achieve the stirring area  20 , in addition to providing the wind shield plates  21  in the gas channel  5 . Examples may include, as shown in  FIG.  4 A , a method in which the shape of the gas channel  5  is set in a manner that the cross-sectional areas ( 5   a ,  5   b ) in the gas channel  5  are different at the position in the direction of flow d 1 . In this case, the direction of airflow of the gas to be treated G 1  also changes at the locations in which the cross-sectional area of the gas channel changes, stirring the gas to be treated G 1  with each other. 
     Another example includes, as shown in  FIG.  4 B , a method in which one or more bent sections  5   c  are provided in the stirring area  20  in the gas channel  5 . In this case, the direction of airflow of the gas to be treated G 1  also changes at the locations at which the bent sections  5   c  are formed, stirring the gas to be treated G 1  with each other. 
     Yet another example includes, as shown in  FIG.  4 C , a method in which, while being the same in the stirring area  20 , a cross-sectional area of the gas channel  5   e  is different in size compared to a cross-sectional area of the gas channel  5   d  in the ozone introduction area  10  that is located upstream from the stirring area  20 . Since the cross-sectional area of the gas channel changes at a location at which the gas to be treated G 1  flows from the ozone introduction area  10  into the stirring area  20 , the direction of airflow of the gas to be treated G 1  changes, thereby stirring the gas to be treated G 1  with each other. 
     As described above, the stirring area  20  is provided for the purpose of stirring the gas to be treated G 1  after the ozone has been introduced by the ozone introduction area  10 . In other words, allowing the gas to be treated G 1  flow through the stirring area  20  increases the probability of contact of the gas to be treated G 1  with ozone. From the viewpoint of increasing this probability of contact, the stirring area  20  preferably has a longer length than the ozone introduction area  10  in the direction of flow d 1  of the gas to be treated. 
     Incidentally, part of ozone (O 3 ) introduced by the ozone introduction area  10  may undergo the following decomposition reaction shown in Formula (8) below, when flowing through the stirring area  20  together with the gas to be treated G 1 . 
       O 3 →O( 3 P)+O 2   (8)
 
     When the above reaction occurs, highly reactive O( 3 P) reacts with the target substance to be treated contained in the gas to be treated G 1  flowing through the stirring area  20 , decomposing part of the target substance to be treated. In other words, the gas flowing through the stirring area  20  may have a mixture of the gas to be treated G 1  that has been introduced from the gas inlet  3   a , and the gas that has been treated G 2  being in a state in which part of the target substance to be treated contained in the gas to be treated G 1  has been decomposed. However, the rate of the ozone (O 3 ) decomposition reaction in Formula (8) that occurs in the stirring area  20  is sufficiently slow compared with the rate at which the gas to be treated G 1  flows toward the heating area  30 . Hence, when the gas to be treated G 1  flows through the stirring area  20 , the O( 3 P) that has been generated in the stirring area  20  can only decompose the substance to be treated to a very small extent. In the above viewpoint, the gas flowing from the stirring area  20  toward the heating area  30  is referred to as “gas to be treated G 1 ”. 
     (Heating Area  30 ) 
     As described above, the gas treatment device  1  shown in  FIG.  1    is provided with a heating area  30  in which the gas to be treated G 1  is heated at a position downstream from the stirring area  20 . Here, part of the housing  2  constitutes a heating furnace  31 , and the gas channel  5  is located in this heating furnace  31 . The component constitutes the heating area  30 . 
     However, the heating area  30  does not necessarily need to be mounted in the same housing as the housing  2  into which the gas to be treated G 1  flows. For example, as shown in  FIG.  5   , the configuration in which a housing  2   a  into which the gas to be treated G 1  flows is communicated with the heating furnace  31  through the gas channel  5 . In this case, the heating furnace  31  can be considered to constitute a kind of housing  2   b ; and the housing  2   a , the housing  2   b , and the gas channels  5  communicating these housings ( 2   a ,  2   b ) can be considered to form the single housing  2 . 
     In other words, “housing  2 ” in the present specification is not limited to that exhibiting a single box shape; however, it is a concept entirely encompassing a structure that prevents the gas to be treated G 1  and the gas that has been treated G 2  from leaking out between the gas inlet  3   a  provided on one side and the gas outlet  3   b  provided on the other side. In other words, “housing  2 ” includes a structure in which a plurality of housings ( 2   a ,  2   b , . . . ) are provided and each housing ( 2   a ,  2   b , . . . ) is communicated with each other via the gas channels  5 . 
     In the heating area  30 , the gas to be treated G 1  flowing through the gas channel  5  is heated to a temperature of 300° C. or more. When being higher temperature, ozone (O 3 ) is known to make the reaction of the above Formula (8) proceed more due to thermal decomposition.  FIG.  6    is a graph of the relationship between the thermal decomposition rate (half-life) of ozone and temperature.  FIG.  6    shows that ozone has a half-life of approximately 1000 seconds at 100° C., and a half-life of less than 0.1 seconds at 300° C. or higher. In other words, when ozone is heated at 300° C. or higher, the reaction of Formula (7) proceeds on the half of the ozone present in 0.1 second or less, thus generating O( 3 P). 
     On the other hand, since the generated O( 3 P) is highly reactive, undergoes a reaction of changing to oxygen (O 2 ) in accordance with Formula (9) below if other substance M is present in the surroundings. Note that O( 3 P) is denoted as O, similar to Formula (7). Formula (7) is restated below for the sake of explanation. 
       O+O+M→O 2 +M  (9)
 
       CH 4 +O→CH 3 +⋅OH  (7)
 
       FIG.  7    is a graph illustrating the temperature dependence of the reaction rates of Formula (7) and Formula (9) above. As described above, increasing the temperature increases the reaction rate of Formula (8) and the amount of production of atomic oxygen (O). However, being highly reactive, atomic oxygen fails to contribute to decomposing target substance to be treated, and undergoes a reaction of changing to oxygen (O 2 ) in accordance with Formula (9). This reaction rate gradually decreases as the temperature increases. 
     In contrast, the reaction of Formula (7), i.e., the rate of the reaction in which the target substance to be treated is decomposed by atomic oxygen (O) increases as the temperature increases. When the target substance to be treated is methane (CH 4 ), atomic oxygen (O) reacts with the C—H bond to form the methyl radical (CH 3 ). In other words, the decomposition amount of methane in the gas to be treated G 1  increases as the temperature of the gas to be treated G 1  increases. 
     From the viewpoint of making the generated atomic oxygen (O) contribute more to decomposing the target substance to be treated, the gas to be treated G 1  is preferably heated to 300° C. or higher in the heating area  30 . It is more preferably heated to 350° C. or higher. Heating the gas to be treated G 1  to 350° C. or higher allows the reaction rate of Formula (7) to exceed that of Formula (9), thereby, further accelerating the decomposition of the target substance to be treated. 
     In general, a solid body has higher heating efficiency than gas. For this reason, the heating area  30  is set in a manner that the inner wall of the gas channel  5  present in the heating area  30  is heated. In other words, the gas channel  5  includes a heated wall. For example, in  FIGS.  1  and  5   , disposing the gas channel  5  in the heating furnace  31  supplies heat from the heating furnace  31  to the gas channel  5 , making the temperature of the wall surface of gas channel  5  increase. Then, allowing the gas to be treated gas G 1  to pass in the gas channel  5  with its wall surface being heated in this manner, increases the temperature of the gas to be treated G 1 . 
     The result of  FIG.  7    indicates that the temperature of the gas to be treated G 1  preferably increases as rapidly as possible. If the rate of temperature rise is slow, the reaction in which the generated atomic oxygen (O) changes to oxygen (O 2 ) in accordance with Formula (9) is likely to occur while the temperature of the gas to be treated G 1  is increasing. 
     From the above viewpoint, the heating area  30  preferably has an aspect in which the gas to be treated G 1  impinges multiple times at a certain velocity against the wall surface of the gas channel  5 , which is heated to high temperature. The examples shown in  FIGS.  1  and  5    illustrate a configuration in which the gas channel  5  is bent multiple times in the heating area  30 . In this configuration, the gas to be treated G 1  proceeds toward the gas outlet  3   b  while repeatedly and intermittently colliding the wall surface of the gas channel  5 , thus the gas to be treated G 1  can be rapidly heated to 300° C. or higher while flowing through the heating area  30 . 
     From the similar viewpoint, as shown in  FIG.  8 A , the heating area  30  may be provided with a plurality of heated plates  32  in a manner that the heated plates  32  are arranged to obstruct the direction of flow d 1  while being separated from each other in the gas channel  5  in the direction of flow. The heated plates  32  are made of a material that can be heated to high temperature including metal materials such as stainless steel, aluminum, copper, silver, or ceramic, and are heated to 500° C., for example, by heat transfer from an external heat source. In this case, the gas to be treated G 1  flowing through the heating area  30  proceeds toward the gas outlet  3   b  while repeatedly and intermittently colliding on the heated plates  32 , thus the gas to be treated G 1  can be heated rapidly in the same manner. Note that the heated plates  32  may be provided with a coating layer made of a material such as fluorine or ceramic, on its front surface to prevent degradation and rust due to ozone. 
     As yet another example, as shown in  FIG.  8 B , the heating area  30  can be configured such that the plurality of heated plates  33  each being provided with a plurality of flow holes  34  are arranged at positions spaced apart along the direction of flow d 1 . In this case, the position of the flow holes  34  is preferably set in a manner that the flow holes  34  of the adjacent heated plates  33  are not aligned each other along the direction of flow d 1 . In this case, the gas to be treated G 1  flowing through the heating area  30  also proceeds toward the gas outlet  3   b  via the flow holes  34  while repeatedly and intermittently colliding on the heated plates  33 , thus the gas to be treated G 1  can be heated rapidly in the similar manner. 
     (Another Example of the Configuration of an Excimer Lamp) 
     Note that the excimer lamp  11  provided as the ozone introduction unit in the present embodiment is not limited to the structure illustrated in  FIGS.  2 A and  2 B . 
     For example, as shown in  FIG.  9 A , the excimer lamp  11  may be provided with the single tube body  13 .  FIG.  9 A  is a schematic cross-sectional view of the excimer lamp  11  illustrated in accordance with  FIG.  2 B . The tube body  13  is sealed at its ends in the longitudinal direction, in other words, the direction of the tube axis d 1  (not shown), and the luminescent gas  15 G is sealed in the inner space of the tube body  13 . The electrode  14   a  that has a mesh shape or a linear shape is provided on the outer wall surface of the tube body  13 , and the rod-shaped electrode  14   b  is provided inside the tube body  13 . 
     As another example, the configuration in which both of the electrodes ( 14   a ,  14   b ) are arranged on the outer wall surface of the tube body  13  of the excimer lamp  11  can also be employed.  FIGS.  9 B and  9 C  are drawings schematically illustrating the structure of the excimer lamp  11  in this another example of the configuration.  FIG.  9 B  corresponds to a plan view, and  FIG.  9 C  corresponds to a cross-sectional view taken along the line A 2 -A 2  in  FIG.  9 B . The excimer lamp  11  shown in  FIGS.  9 B and  9 C  includes the single tube body  13 , and both of the electrodes ( 14   a ,  14   b ) are arranged on the outer wall surface of the tube body  13  such that they are opposite each other via the tube body  13 . The electrodes ( 14   a ,  14   b ) exhibits a mesh shape or a linear shape to allow the ultraviolet light L 1  generated inside the tube body  13  to be extracted to the outside of the tube body  13 . 
     Second Embodiment 
     A second embodiment of the gas treatment device according to the present embodiment will be described, focusing on the point that differs from that of the first embodiment. Elements that are common to the first embodiment will be assigned to the same symbol and their descriptions will be omitted as appropriate. 
     The gas treatment device  1  of the present embodiment differs from that of the first embodiment in the configuration of the ozone introduction unit. 
       FIG.  10 A  is a cross-sectional view schematically illustrating the configuration of the second embodiment of the gas treatment device of the present invention in accordance with  FIG.  1   . In the present embodiment, the housing  2  is provided with a gas inlet  3   c  through which ozone source gas G 3  is introduced, in addition to the gas inlet  3   a  through which the gas to be treated G 1  is introduced. The ozone source gas G 3  can be air that does not contain the target substance to be treated. 
     In this configuration, ozone gas G 4  is generated from the ozone source gas G 3  by excimer lamp  11  as an ozone introduction unit. This ozone gas G 4  is introduced into the gas channel  5  through which the gas to be treated G 1  flows. Since ozone is introduced into the gas to be treated G 1  in the ozone introduction area  10 , thus the gas to be treated G 1  changes to gas containing ozone. 
     In the above viewpoint, the gas treatment device  1  according to the present invention can employ any ozone generation method provided that the gas treatment device  1  is configured such that ozone is introduced into the gas channel  5  through which the gas to be treated G 1  flows in the ozone introduction area  10 . Examples can include a configuration such that, as shown in  FIG.  10 B , an ozone cylinder  18  containing the ozone gas G 4  is accommodated in the housing  2 , and the ozone gas G 4  supplied from this ozone cylinder  18  is introduced into the gas channel  5  through which the gas to be treated G 1  flows. 
     Other Embodiment 
     In the embodiments described above, the excimer lamp  11  was referred to be described, as an example, as a device that generates ozone from air. However, instead of this excimer lamp  11 , an atmospheric pressure plasma generator can be employed. 
       FIGS.  11 A and  11 B  are drawings illustrating the schematic structure of the atmospheric pressure plasma generator.  FIG.  11 A  corresponds to a side view of the atmospheric pressure plasma generator  19 , and  FIG.  11 B  corresponds to a cross-sectional view taken along the line A 3 -A 3  in  FIG.  11 A . 
     The atmospheric pressure plasma generator  19  differs from the excimer lamp  11  in that the luminescent gas  15 G is not sealed in the tube body  13 . In  FIG.  11 A , the gas to be treated G 1  is made to pass through the tube body  13 . Other configurations are common to the excimer lamp  11  described above. 
     When the voltage is applied to both of the two electrodes ( 14   a ,  14   b ) from a power supply (not shown), a dielectric barrier discharge is generated in the tube body  13 . This generates dielectric barrier discharge in the gas to be treated G 1  flowing through the discharge space S 1 , allowing the gas to be treated G 1  to transform into plasma. 
     Oxygen (O2) in the air contained in the gas to be treated G 1  flows through the space of atmospheric pressure plasma, and undergoes the following reaction indicated in Formula (10) below. In Formula (10) below, AP means that energy is added by atmospheric pressure plasma. 
       O 2 +AP→O+O  (10)
 
     Part of the oxygen atoms (atomic oxygen) O generated in Formula (10) reacts with oxygen (O2) contained in the gas to be treated G 1 , generating ozone (O3) in accordance with Formula (3) above. Formula (3) is restated below. 
       O+O 2 +M→O 3 +M  (3)
 
     In other words, when the gas to be treated gas G 1  flows through the atmospheric pressure plasma generator  19 , ozone is introduced to the gas to be treated G 1 , and then the gas to be treated G 1  changes to gas containing ozone. 
     The atmospheric pressure plasma generator  19  is not limited to the structures shown in  FIGS.  11 A and  11 B . The atmospheric pressure plasma generator  19  can employ, for example, a structure similar to that of the excimer lamp  11  that is described above with reference to  FIGS.  9 A to  9 C . In this case also, the tube body  13  may not be filled with the luminescent gas  15 G, and the gas to be treated G 1  may be made to flow through the tube body  13 . 
     In addition, unlike the excimer lamp  11 , the atmospheric pressure plasma generator  19  eliminates the need for extracting light to the outer wall surface of the tube body  13 , hence the electrodes  14  that have been formed on the outer wall surface of the tube body  13  do not necessarily have a mesh shape or a linear shape. 
     Furthermore, instead of the excimer lamp  11  shown in  FIG.  10 A  described in the second embodiment, the atmospheric pressure plasma generator  19  can also be employed. 
     However, when ozone is generated by the atmospheric pressure plasma generator  19 , NO x  components are unavoidably included due to the high energy from the plasma. Hence, in this method, NO x  components are partly included in the gas that has been treated G 2 . When there are circumstances in which no NO x  component is made to be included in the gas that has been treated G 2 , the excimer lamp  11  or the ozone cylinder  18  can preferably be used as the ozone introduction unit. 
     EXAMPLES 
     The present invention will be described with reference to Examples; however the invention is not limited to these Examples. 
     Description of Experimental Condition 
     Hereinafter, the experimental condition will be described. 
     Experimental System 
     Experiments were conducted using an experimental system modeling the gas treatment device  1  of the first embodiment.  FIG.  12    is a drawing schematically illustrating the structure of the experimental system. 
     A housing  42  accommodating the excimer lamp  11  therein shown in  FIG.  9 A  was prepared as the ozone introduction area  10 . The excimer lamp  11  had an outer diameter of 26 mm and a luminescent length of 200 mm. The housing  42  was a glass cylinder having an inner diameter (diameter) of 45 mm and a wall thickness of 3 mm, and its length was 350 mm. The excimer lamp  11  was disposed in the housing  42  with the tube axis of the tube body  13  being substantially aligned with the center axis of the housing  42 . In the excimer lamp  11 , the tube body  13  was filled with the luminescent gas  15 G containing Xe, and the both of the electrodes ( 13   a ,  13   b ) were subject to a high frequency voltage having an input power of 40 W and an applied voltage of 8 KVpp from a power supply (not shown), thereby emitting ultraviolet light L 1  having a main peak wavelength of 172 nm. 
     A glass tube  43  having a diameter smaller than the inner diameter of the housing  42  was prepared as the stirring area  20 . The glass tube  43  had an inner diameter of 6 mm and a length of 1000 mm. 
     The heating furnace  31  constituted by an electric furnace was prepared as the heating area  30 . The glass tube  43  as the stirring area  20  was connected with a heat-resistant glass tube  44  that was inserted in the heating furnace  31  at the front stage of the heating furnace  31 . The heat-resistant glass tube  44  had an inner diameter of 6 mm and a length of 500 mm. The heating furnace  31  had a length of 300 mm along the direction of flow d 1 . 
     The heat-resistant glass tube  44  extended from the outlet of the heating furnace  31  was connected with a glass tube for cooling  45 . The glass tube for cooling  45  had an inner diameter of 6 mm and a length of 1000 mm. 
     The glass tube for cooling  45  was connected with a sampling bag  46  via a flow tube  52 . The flow tube  52  was constituted by a silicone tube, and had an inner diameter of 6 mm and a length of 3 m. 
     The gas to be treated G 1  constituted by a mixed gas of air and methane (methane concentration of 100 ppm) was introduced from the gas inlet  3   a , and allowed to pass through the ozone introduction area  10 , the stirring area  20 , and the heating area  30  in sequence. The resultant gas that had been treated G 2  was introduced into the sampling bag  46  (manufactured by GL Sciences). Then, the gas that had been treated G 2 , which was contained in the sampling bag  46 , underwent FTIR (Fourier Transform Infrared Spectroscopy) in order to analyze the components. 
     Gas to be Treated G 1   
     The gas to be treated G 1  having the components of conditions indicated in Table 1 below by FTIR analysis was employed. Note that Table 1 does not include nitrogen (N 2 ), oxygen (O 2 ), and argon (Ar) that are contained in the air in the gas to be treated G 1 . In addition, “NO y ” in Table 1 is a notation that collectively includes NO, NO 2 , and N 2 O, which are generally referred to as NO R , as well as N 2 O 5 , HNO 2 , and HNO 3 . 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 CH 4   
                 CO 
                 CO 2   
                 H 2 O 
                 NO y   
                 O 3   
               
               
                   
                   
               
             
            
               
                   
                 Gas to be 
                 100.0 
                 0.0 
                 5.8 
                 2906.4 
                 0.0 
                 0.0 
               
               
                   
                 treated 
                   
                   
                   
                   
                   
                   
               
               
                   
                 G1 [ppm] 
               
               
                   
                   
               
            
           
         
       
     
     Examples 1 Through 5 
     In Examples 1 through 5, the conditions were made such that the preset temperatures of the electric furnace were different, and the other conditions were the same. 
     Comparative Example 1 
     In Comparative Example 1, the condition was such that no heating treatment was performed by turning off the power of the electric furnace. In other words, Comparative Example 1 corresponds to the case in which no heating treatment was performed on the gas to be treated G 1  after ozone had been introduced. 
     Comparative Example 2 
     In Comparative Example 2, the condition was such that the preset temperature of the electric furnace was the same as in Example 1, and no power is applied to the excimer lamp  11 . In other words, Comparative Example 2 corresponds to the case in which the heat treatment was performed on the gas to be treated G 1  without introducing ozone. 
     Verification 
     Table 2 summarizes the experimental conditions under which each of Examples 1 through 5 and Comparative Examples 1 and 2 were conducted. In Table 2, note that the “temperature of the gas that has been treated T 2 ” is the temperature of the gas that has been treated G 2  measured at the outlet of the electric furnace. Under the experimental conditions shown in Table 2, the gas to be treated G 1  having the components of the conditions shown in Table 1 was supplied from the gas cylinder  41  through the flow tube  51  toward the housing  42  at a flow rate of 10 L/min. The resultant gas that had been treated G 2  was introduced into the sampling bag  46 . The components of the gas that had been treated G 2  were analyzed. The results are shown in Table 3. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Flow 
                   
                   
                   
                 Temper- 
                 Temper- 
               
               
                   
                 rate of  
                   
                   
                   
                 ature 
                 ature of  
               
               
                   
                 gas to 
                   
                   
                   
                 of 
                 gas that 
               
               
                   
                 be 
                 Ozone 
                   
                   
                 electric 
                 has been 
               
               
                   
                 treated 
                 intro- 
                   
                   
                 furnace 
                 treated 
               
               
                   
                 [L/min] 
                 duction 
                 Heating 
                 Stirring 
                 T1 [° C.] 
                 T2 [° C.] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 1 
                 10 
                 Yes 
                 Yes 
                 Yes 
                 400 
                 244 
               
               
                 Example 2 
                 10 
                 Yes 
                 Yes 
                 Yes 
                 450 
                 310 
               
               
                 Example 3 
                 10 
                 Yes 
                 Yes 
                 Yes 
                 500 
                 367 
               
               
                 Example 4 
                 10 
                 Yes 
                 Yes 
                 Yes 
                 600 
                 382 
               
               
                 Example 5 
                 10 
                 Yes 
                 Yes 
                 Yes 
                 700 
                 455 
               
               
                 Com- parative 
                 10 
                 Yes 
                 No 
                 Yes 
                 — 
                 79 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
               
               
                 Com- parative 
                 10 
                 No 
                 Yes 
                 No 
                 500 
                 367 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 CH 4   
                 CO 
                 CO 2   
                 H 2 O 
                 NO y   
                 O 3   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Gas to be treated 
                 100.0 
                 0.0 
                 5.8 
                 2906.4 
                 0.0 
                 0.0 
               
               
                 G1 [ppm] 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Gas that 
                 Example 1 
                 96.5 
                 17.2 
                 21.7 
                 1007.2 
                 0.0 
                 2343.5 
               
               
                 has been 
                 Example 2 
                 96.0 
                 14.2 
                 15.8 
                 859.6 
                 0.0 
                 2039.5 
               
               
                 treated 
                 Example 3 
                 94.2 
                 14.0 
                 13.9 
                 912.3 
                 0.0 
                 1796.5 
               
               
                 G2 [ppm] 
                 Example 4 
                 86.8 
                 18.9 
                 12.6 
                 1106.3 
                 0.0 
                 1463.8 
               
               
                   
                 Example 5 
                 77.3 
                 25.8 
                 11.9 
                 1050.0 
                 0.0 
                 985.6 
               
               
                   
                 Comparative 
                 97.1 
                 8.2 
                 11.5 
                 1643.5 
                 0.0 
                 2401.1 
               
               
                   
                 Example 1 
                   
                   
                   
                   
                   
                   
               
               
                   
                 Comparative 
                 99.5 
                 0.0 
                 6.8 
                 899.9 
                 0.0 
                 2.2 
               
               
                   
                 Example 2 
               
               
                   
               
            
           
         
       
     
     Analysis of Result 
     Table 3 confirms that, in all cases, the concentration of methane in the gas that had been treated G 2  was reduced compared to that in the gas to be treated G 1 . It was also confirmed that when the preset temperature of the electric furnace (heating furnace  31 ) was higher, the decomposition amount of methane became larger and the concentration of ozone mixed in the gas that had been treated G 2  was lowered.  FIG.  13    is a graph illustrating a relationship between the preset temperature of the electric furnace and the amount of methane and ozone remaining in the gas that has been treated G 2 , based on the results of Examples 1 through 5. In  FIG.  13   , the horizontal axis represents the preset temperature of the electric furnace, the left vertical axis represents the concentration of methane in the gas that has been treated G 2 , and the right vertical axis represents the concentration of ozone in the gas that has been treated G 2 . 
     In contrast, in Comparative Example 1, in which no heat treatment was performed, although being lower than that in the gas to be treated G 1 , the concentration of methane in the gas that had been treated G 2  remains at higher concentrations with respect to those in each of Examples 1 through 5. In the case of Comparative Example 1, the ozone concentration is higher than those in Examples 1 to 5, indicating that ozone is not thermally decomposed compared to Examples 1 through 5. In other words, in Comparative Example 1, atomic oxygen (O) was not sufficiently generated, thus resulting in the insufficient decomposition amount of methane. 
     In the case of Comparative Example 2, in which the heat treatment was performed without introducing ozone, the methane contained in the gas that has been treated G 2  was barely decomposed. 
     Description of simulation conditions Next, simulations were conducted to verify the difference in the decomposition efficiency of methane with and without the stirring area. 
     A cylinder (inner diameter: 50 mm) having the excimer lamp  11  therein was set as the ozone introduction area  10 . The excimer lamp  11  having an outer diameter of 10 mm and a luminescent length of 300 mm was set. The heating area  30  having a length of 150 mm was set at the rear stage of the ozone introduction area  10 . 
     When a mixed gas (methane concentration of 100 ppm) of air and methane as the gas to be treated G 1  is made to flow through the introduction area  10  at a flow rate of 10 LPM, the methane concentration contained in the gas that has been treated G 2  was calculated. Specifically, the concentration of methane in the gas that has been treated G 2  was calculated in the case in which the heating area  30  is provided immediately after the ozone introduction area  10  (condition #1) and in the case in which the stirring area  20  having a cylinder is provided between the ozone introduction area  10  and the heating area  30  (Condition #2). Comparing these results can confirm the degree to which the presence of the stirring area  20  affects the capability of decomposition and production of methane. 
     Two patterns were set for the heating area  30  such that the case in which the ambient temperature is 450° C. and the case in which the ambient temperature is 700° C. The heating area  30  was simulated as an area in which the cylinder through which the mixed gas (gas to be treated G 1 ) flows is disposed in the electric furnace, the area being similar to the ozone introduction area  10 . 
     Condition #1 was set as follows. The ozone introduction area  10  was set to an area with a length of 300 mm in the longitudinal direction (the direction of flow of the gas to be treated G 1 ) in the cylinder having an inner diameter of 50 mm. The heating area  30  was set to an area with a length of 150 mm in the longitudinal direction in the cylinder having an inner diameter of 50 mm, the area being disposed immediately after the ozone introduction area  10 . 
     Condition #2 was set as follows. The ozone introduction area  10  was set to an area with a length of 300 mm in the longitudinal direction in the cylinder having an inner diameter of 20 mm. In the cylinder having an inner diameter of 20 mm, the stirring area  20  was set to an area with a length of 300 mm in the longitudinal direction immediately after the ozone introduction area  10 , and furthermore the heating area  30  was set to be an area with a length of 150 mm disposed immediately after the stirring area  20 . 
     As described above, the length of the heating area  30  in the longitudinal direction is set to the same value (150 mm) in order to make the heating conditions common in Condition #1 and Condition #2. 
     The results of verification are shown in  FIG.  4   . 
     
       
         
           
               
               
            
               
                   
                   
               
               
                   
                 Decomposition amount of methane 
               
               
                   
                 [ppm] 
               
            
           
           
               
               
               
            
               
                   
                 450° C. 
                 700° C. 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 #1 (without stirring area) 
                 5.1 
                 11.8 
               
               
                 #2 (with stirring area) 
                 8.1 
                 17.5 
               
               
                 Difference of decomposition rate 
                 3.0% 
                 5.7% 
               
               
                   
               
            
           
         
       
     
     Note that the “difference in decomposition rate” in Table 4 refers to the value that is calculated by dividing the value of difference in decomposition amount of methane with and without the stirring area by the initial methane concentration (100 ppm). 
     Table 4 confirms that, when compared under the same heating temperature, the decomposition amount of methane on the treatment under Condition #2 (with the stirring area) is higher than that on the treatment under Condition #1 (without the stirring area). 
     The reasons for this are inferred as follows. In Condition #1, since no stirring area  20  is provided between the ozone introduction area  10  and the heating area  30 , the mixed gas is introduced into the heating area  30  with insufficient stirring of the ozone and methane. This causes atomic oxygen that has been produced by the thermal decomposition of ozone to disappear at a higher rate before colliding with methane. In contrast, in condition #2, since ozone is introduced to the gas to be treated G 1  and then the mixed gas flows through the stirring area  20 , the stirring of ozone and methane is promoted. The mixed gas in the state that ozone and methane have been stirred through the stirring area  20  is introduced into the heating area  30 , thereby increasing the probability of collision between the atomic oxygen that has been obtained by the thermal decomposition of ozone and the methane. 
     Table 4 also confirms that increasing the heating temperature widens the difference of decomposition capability of methane between Condition #2 (with the stirring area) and Condition #1 (without the stirring area). 
     The reasons for this are inferred as follows. Since the thermal decomposition of ozone is accelerated as the mixed gas becomes at high temperatures, the amount of atomic oxygen produced by the thermal decomposition of ozone in the case of a heating temperature of 700° C. is higher than that in the case of a heating temperature of 450° C. However, as described above, the probability that atomic oxygen disappears without colliding with methane is higher in the case of Condition #1, in which no stirring area  20  is provided, compared to Condition #2, in which the stirring area  20  is provided. As a result, setting the higher heating temperature increases the amount of atomic oxygen that disappears without colliding with ozone, which is caused by the absence of the stirring area  20 . 
     In other words, with the heating temperature in the heating area  30  being set higher, providing the stirring area  20  can significantly enhance the performance of decomposition of methane. 
     As described above, introducing ozone and undergoing heating treatment after stirring enables the target substance to be treated that has been contained in the gas to be treated G 1  to be in contact with atomic oxygen (O) at a high probability, increasing the decomposition rate of the target substance to be treated. In particular, the above results confirm that methane that has been mixed with air at an extremely low concentration of 100 ppm can also be decomposed at a high rate. 
     In addition, the effect of providing the stirring area  20  is particularly confirmed to be noticeable in the equipment in which the thermal decomposition of ozone can be effectively performed. 
     REFERENCE SIGNS LIST 
     
         
           1  Gas treatment device 
           2  Housing 
           2   a  Housing 
           2   b  Housing 
           3   a  Gas inlet 
           3   b  Gas outlet 
           3   c  Gas inlet 
           5  Gas channel 
           5   c  Bent section 
           5   d  Cross-sectional area of gas channel 
           5   e  Cross-sectional area of gas channel 
           5   f  Cross-sectional area of gas channel 
           10  Ozone introduction area 
           11  Excimer lamp 
           13  Tube body 
           13   a  Outer tube 
           13   b  Inner tube 
           14  Electrode 
           14   a  Electrode 
           14   b  Electrode 
           15 G Luminescent gas 
           18  Ozone cylinder 
           19  Atmospheric pressure plasma generator 
           20  Stirring area 
           21  Wind shield plate 
           30  Heating area 
           31  Heating furnace 
           32  Heated plate 
           33  Heated plate 
           34  Flow hole 
           40  W Power 
           41  Gas cylinder 
           42  Housing 
           43  Glass tube 
           44  Heat-resistant glass tube 
           45  Glass tube for cooling 
           46  Sampling bag 
           51  Flow tube 
           52  Flow tube 
           100  Methane removal system 
           101  Emission source of gas to be treated 
           102  Tube for gas to be treated 
           102   a  Channel for gas to be treated 
           104  Catalyst for removing methane by oxidation 
           104   a  Catalyst container 
           105  Plasma generation means 
           106  Control means 
           107  Power supply source 
           108   a  External electrode 
           108   b  Internal electrode 
         G 1  Gas to be treated 
         G 2  Gas that has been treated 
         G 3  Ozone source gas 
         G 4  Ozone gas 
         L 1  Ultraviolet light 
         D 1  Direction of flow