WO2025234172A1 - Gas decomposition method, gas decomposition device, and gas decomposition system - Google Patents

Abstract

Provided is a method for decomposing a treatment target gas, the method involving decomposing dinitrogen monoxide while suppressing emission of NOx.　This method is for decomposing a treatment target gas, the method comprising decomposing dinitrogen monoxide in the treatment target gas by providing, to the treatment target gas, energy for exciting a gas contained in the treatment target gas. The treatment target gas contains at least dinitrogen monoxide and carbon dioxide. The gas concentration of the carbon dioxide contained in the treatment target gas is higher than the concentration of each gas contained in the treatment target gas excluding the carbon dioxide.

Description

Gas decomposition method, gas decomposition device, and gas decomposition system

This invention relates to a method for decomposing a gas to be treated, a gas decomposition device, and a gas decomposition system.

Since the Industrial Revolution, the Earth's average temperature has been rising, making measures to combat global warming an urgent issue. Greenhouse gases known to cause global warming include carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons. Of these gases, carbon dioxide is the most highly emitted, followed by methane, and then nitrous oxide.

On the other hand, when looking at global warming potential (GWP), it has been reported that the GWP of methane is 25 times that of carbon dioxide, and that of nitrous oxide is 298 times that of carbon dioxide. For these reasons, the impact of nitrous oxide emissions on global warming cannot be ignored.

Nitrous oxide is emitted not only during the treatment of human and livestock waste and agricultural activities, but also during industrial activities such as the manufacture of chemical products and the burning of waste. In order to curb global warming, it is necessary to prevent the release of nitrous oxide into the atmosphere.

Nisous oxide is decomposed to prevent it from being released into the atmosphere. High-temperature combustion and catalytic methods have traditionally been used to decompose nitrous oxide. However, high-temperature combustion methods require a large amount of energy to burn the gas. Using fossil fuels to secure large amounts of energy increases carbon dioxide emissions, making it undesirable as a measure against global warming. Catalytic methods also require the gas to be heated to high temperatures. Furthermore, ammonia must be procured for use as a catalyst and reducing agent, and there are also issues with wastewater treatment after treatment. Therefore, catalytic methods also cannot be considered desirable as a measure against global warming.

Another method for decomposing nitrous oxide is to use light (see Patent Documents 1 and 2).

Japanese Patent Application Laid-Open No. 2024-022455 Japanese Patent Application Laid-Open No. 2024-021127

When gas containing nitrous oxide (hereinafter sometimes referred to as "N ₂ O") is irradiated with light, the N ₂ O can be decomposed into nitrogen gas and oxygen gas, but NOx is produced from some of the N ₂ O during this decomposition process. In this specification, NOx is an expression that includes nitrogen monoxide (hereinafter sometimes referred to as "NO"), nitrogen dioxide (hereinafter sometimes referred to as "NO ₂ "), nitrogen trioxide (hereinafter sometimes referred to as "NO ₃ "), and dinitrogen pentoxide (hereinafter sometimes referred to as "N ₂ O ₅ ").

Since NOx has a harmful effect on humans and animals, it is possible to consider converting the NOx produced during the decomposition process of N ₂ O into nitric acid (hereinafter sometimes referred to as "HNO ₃ ") in order to render it harmless. Obtaining HNO ₃ from _{N 2} O has the advantage of being able to fix N ₂ O without releasing it into the atmosphere, and the advantage of producing nitric acid from N ₂ O for use. Nitric acid is a useful substance in various industrial fields, including the chemical industry, so obtaining nitric acid from N ₂ O has high value from a business perspective. If nitric acid can be obtained by decomposing emitted N ₂ O, the amount of nitric acid produced for the purpose of producing nitric acid itself can also be reduced. Since a large amount of N ₂ O is also emitted in the nitric acid production process, the amount of N ₂ O emitted when producing nitric acid for the purpose of producing nitric acid itself can also be reduced.

The chemical reaction of decomposing _N2O to produce _HNO3 will be described in detail below, but the reaction pathway leading to the decomposition of _N2O to produce _HNO3 is complex. The inventors have noticed that the decomposition of _N2O may stop midway through the reaction pathway, resulting in the N2O not reaching _HNO3 but being emitted in the form of NOx. As mentioned above, NOx has adverse effects on humans and animals, so it is necessary to suppress NOx emissions as much as possible.

Therefore, an object of the present invention is to provide a method for decomposing a gas to be treated, a decomposition device, and a decomposition system that appropriately decompose N ₂ O while suppressing the emission of NOx.

In the method for decomposing a gas to be treated disclosed in the present specification,
the gas to be treated contains at least nitrous oxide and carbon dioxide, and the gas concentration of carbon dioxide contained in the gas to be treated is higher than the concentrations of any other gases contained in the gas to be treated except for carbon dioxide;
The dinitrogen monoxide in the gas to be treated is decomposed by providing the gas to be treated with energy for exciting the gas contained therein.

Details will be described later, but the background to arriving at the above decomposition method will be briefly explained. As a result of extensive research, the inventors discovered that when the temperature inside the decomposition reactor for decomposing _N2O is high, the reaction stops midway through the reaction pathway leading from _N2O to _HNO3 , making it more likely that NOx will remain. When the temperature inside the decomposition reactor is high, ozone (hereinafter sometimes referred to as " _O3 ") is decomposed into oxygen molecules ( _O2 ) and oxygen atoms O( ^3P ), reducing the amount of _O3 inside the decomposition reactor. _O3 is a substance necessary for converting NO produced by the decomposition of _N2O to _HNO3 . O( ^3P ) is an oxygen atom in the ground state, known as "triplet oxygen." When _O3 is reduced, NOx is more likely to remain without reaching _HNO3 .

Based on the above realization, the inventor of the present invention has devised a method for adding a large amount of carbon dioxide (hereinafter, sometimes referred to as " _CO2 ") to the gas to be treated from which _N2O is discharged. In this method, energy for exciting the gas is applied to the gas to be treated, which contains a higher concentration of _CO2 than _N2O .

This energy generates a large amount of CO and oxygen atoms O( ^1D ) or O( ^3P ) from _CO2 (see formulas (24) and (25) described below). O( ^1D ) is a highly reactive excited state oxygen atom called "singlet oxygen." If the temperature inside the decomposition reactor that decomposes _N2O is high, there will be a shortage of _O3 inside the decomposition reactor, and the reaction will likely stop with NO generated from _N2O . However, the large amount of CO generated from _CO2 reduces NO to _N2 . In addition, the large amount of oxygen atoms O( ^3P ) generated from _CO2 oxidizes NO to _NO2 , which is then converted to nitrate by hydroxyl radicals described below. In this way, the remaining NOx is reduced.

On the other hand, when the temperature inside the decomposition reactor that decomposes _N2O is low, the reduction of NO by CO becomes difficult. However, the large amount of _O3 present inside the decomposition reactor promotes the nitration of NO. In this way, the differences in the properties of _O3 and CO complement each other, allowing the reaction to proceed without being stopped by NO generated from _N2O , regardless of whether the reaction field inside the decomposition reactor is high or low. This can be said to be a synergistic effect of the treated gas that is a mixture of _N2O and _CO2 .

The gas to be treated may further contain water vapor. When hydroxyl radicals generated from water vapor react with _NO2 , nitric acid is generated. As will be described in detail later, when _N2O5 is generated from _NO2 via _NO3 , nitric _acid can be generated without relying on water vapor by bubbling _N2O5 in water, for example _. Therefore, nitric acid can be generated even if the gas to be treated does not contain water vapor.

The energy may be light energy whose main emission wavelength is greater than or equal to 160 nm and less than 200 nm.

The energy may be electron energy generated by converting the gas to be treated into plasma.

The gas to be treated may be brought into contact with a catalyst.

The catalyst may be a three-way catalyst used to promote the reduction of nitric oxide produced by the decomposition of nitrous oxide.

The catalyst may be a two-way catalyst used to promote the oxidation of at least one of the nitric oxide produced by the decomposition of nitrous oxide and the carbon monoxide produced from the carbon dioxide.

Oxygen-containing gas may be additionally supplied to the gas to be treated after the energy has been applied.

The decomposition device for a gas to be treated disclosed in the present specification comprises:
a gas supply port for introducing the gas to be treated into a decomposition device, the gas to be treated containing at least nitrous oxide and carbon dioxide, and a gas concentration of carbon dioxide contained in the gas to be treated being higher than the concentrations of any other gases contained in the gas to be treated except for carbon dioxide;
an energy source that provides energy to the gas to be treated that is introduced into the decomposition device from the gas supply port in order to decompose the nitrous oxide in the gas to be treated, for exciting gas contained in the gas to be treated;
Equipped with.

The decomposition device may also include a gas concentration adjustment unit connected to the gas supply port that adjusts the amount of gas components contained in the gas to be treated so that the gas concentration of carbon dioxide contained in the gas to be treated is higher than the concentrations of all gases other than carbon dioxide contained in the gas to be treated.

The energy is light energy whose main emission wavelength is between 160 nm and 200 nm, and the energy source may be a light source that radiates the light energy. The light source may be an excimer lamp. The main wavelength may be 172 nm or close to 172 nm.

The energy is electron energy that converts the gas to be treated into plasma, and the energy source may be an electrode that supplies the electron energy.

The decomposition device may be equipped with a three-way catalyst that promotes the reduction of nitric oxide produced by the decomposition of nitrous oxide.

The decomposition device may be equipped with a two-way catalyst used to promote the oxidation of at least one of the nitric oxide produced by the decomposition of nitrous oxide and the carbon monoxide produced from the carbon dioxide.

the decomposition device includes an oxygen supply port for adding an oxygen-containing gas to the gas to be treated after the energy is applied;
The two-way catalyst may be disposed so as to come into contact with the gas to be treated to which an oxygen-containing gas has been added.

The system for decomposing a gas to be treated disclosed in this specification comprises:
the decomposition device for the gas to be treated;
a gas supply source connected to the gas supply port for supplying the gas to be treated into the decomposition device.

This makes it possible to provide a method, apparatus, and system for decomposing gas to be treated that can decompose N ₂ O regardless of the temperature of the reaction field in the decomposition reactor, thereby appropriately decomposing N ₂ O while suppressing NOx emissions. Providing such a gas decomposition method, apparatus, and system will greatly contribute to achieving Goal 13 of the United Nations-led Sustainable Development Goals (SDGs), which is to "take urgent action to combat climate change and its impacts."

FIG. 1 is a diagram showing a first embodiment of a gas decomposition apparatus. 1B is a cross-sectional view taken along line S1-S1 in FIG. 1A. FIG. 1 is a diagram showing the N ₂ O decomposition mechanism. 1 shows the optical absorption spectra of N ₂ O and CO ₂ . FIG. 1 is a diagram showing an example of a method for treating nitric acid. FIG. 2 is a diagram showing a second embodiment of a gas decomposition apparatus. 5B is a cross-sectional view taken along line S2-S2 of FIG. 5A. FIG. 5C is an enlarged view of the P1 region of FIG. 5B. FIG. 10 is a diagram showing a modified example of the second embodiment of the gas decomposition apparatus. FIG. 10 is a diagram showing a third embodiment of a gas decomposition apparatus. This is a cross-sectional view taken along line S3-S3 in Figure 7A. FIG. 10 is a diagram showing a modified example of the third embodiment of the gas decomposition apparatus. This is a cross-sectional view taken along line S4-S4 in Figure 8A. FIG. 10 is a diagram showing a fourth embodiment of a gas decomposition apparatus. This is a cross-sectional view taken along line S6-S6 in Figure 9A. FIG. 10 is a diagram showing a first modified example of the fourth embodiment of the gas decomposition apparatus. This is a cross-sectional view taken along line S7-S7 in Figure 10A. FIG. 10 is a diagram showing a second modified example of the fourth embodiment of the gas decomposition apparatus.

The embodiments will be explained with reference to the drawings as appropriate. Note that all drawings, excluding graphs, are schematic illustrations, and the dimensional ratios in the drawings do not necessarily correspond to the actual dimensional ratios, and the dimensional ratios between the drawings do not necessarily correspond to each other.

First Embodiment
[Outline of gas decomposition device]
The gas decomposition device has a gas supply port for introducing a gas to be treated into the decomposition device, and an energy source for imparting energy to a gas contained in the gas to be treated. The gas G1 to be treated is a mixed gas containing at least _N2O gas and _CO2 gas.

A first embodiment of a gas decomposition device is shown in Figure 1A. Figure 1B is a cross-sectional view taken along line S1-S1 in Figure 1A. In the gas decomposition device 10 shown as the first embodiment, the energy source that provides energy to excite the gas contained in the gas G1 is a light source 1 that emits ultraviolet light L1.

The gas decomposition device 10 has a decomposition reactor 2. The light source 1 is disposed within the decomposition reactor 2. The decomposition reactor 2 has a gas supply port 3i and a gas exhaust port 3o. The gas supply port 3i and the gas exhaust port 3o are disposed opposite each other with the light source 1 in between. In this specification, the light L1 emitted from the light source 1 is illustrated by a solid arrow pointing outward from the light source 1.

Gas G1 is supplied into the decomposition reactor 2 from the gas supply port 3i. Light L1 emitted from the light source 1 is irradiated onto the gas G1 in the decomposition reactor 2. Light L1 triggers a chain reaction of chemical reactions, described below, within the decomposition reactor 2 as a reaction field, decomposing N ₂ O. Gas G2, irradiated with light and resulting from the chemical reaction, is discharged from the gas discharge port 3o. By continuously performing these steps, the N ₂ O contained in the gas G1 can be continuously decomposed.

The light source 1 is electrically connected to the control unit 5, and the light source 1 is turned on when power is supplied from the control unit 5 to the light source 1. Details of the light source 1 will be described later. The light intensity of the light source 1 may be set taking into consideration the degree of decomposition of _N2O .

From the perspective of increasing decomposition efficiency, it is more preferable to design the decomposition reactor 2 so that light L1 reaches sufficiently inside the reactor 2. For this reason, the distance D1 (see Figure 1A or 1B) between the surface of the light source 1 and the inner wall of the reactor 2 is relatively narrow. The distance D1 may be, for example, 500 mm or less, and preferably 300 mm or less. The distance D1 is set to an appropriate distance so that the light is not excessively attenuated. This makes it possible to reduce the amount of gas that passes through the reactor 2 without being irradiated with light L1.

A catalyst 8 is supported on the inner wall of the decomposition reactor 2. The catalyst 8 may be a three-way catalyst or a two-way catalyst. Both three-way and two-way catalysts may also be used. Details of the catalyst 8 will be described later.

[N ₂ O decomposition mechanism]
The decomposition mechanism of N ₂ O by light L1 will be described with reference to Figure 2. Since the gas decomposition device of this embodiment decomposes N ₂ O by ultraviolet light, the decomposition mechanism when light energy is used will be described, but the decomposition mechanism when the gas to be treated is converted into plasma is basically the same. There are two methods for decomposing _{N 2} O: direct decomposition of N ₂ O by ultraviolet light and indirect decomposition of N ₂ O by O( ¹ D) generated by ultraviolet light.

The direct decomposition of N ₂ O by ultraviolet light will now be explained. Direct decomposition is shown in chemical reaction R1 in Figure 2. When N ₂ O is irradiated with ultraviolet light with a wavelength of 340 nm or less, N ₂ O is decomposed to produce N ₂ and O( ¹ D) according to the following formula (1).

N ₂ O + hν (≦340 nm) → N ₂ + O ( ¹ D) ... (1)

In this specification, the notation "hv (≦Wnm)" (where "W" represents a number) represents the energy of light having a wavelength of W (nm) or less. For example, this indicates that the decomposition reaction of formula (1) occurs with ultraviolet light having a wavelength of 340 nm or less. Looking at formula (1) alone, it would be sufficient to use ultraviolet light having a wavelength of 340 nm or less, but the absorption cross section of ultraviolet light having a wavelength of 200 nm or more for N ₂ O is small. To more efficiently carry out the decomposition reaction of formula (1), it is advisable to use light having a wavelength of less than 200 nm, which has a relatively large absorption cross section. This also applies to other decomposition reactions by light, which will be described later.

The indirect decomposition of N ₂ O by O( ¹ D) generated by ultraviolet light will now be described. The chemical reaction of decomposing N ₂ O by O( ¹ D) generated by the N ₂ O decomposition reaction of formula (1) is shown as chemical reaction R2 in Figure 2. When O( ¹ D) comes into contact with N ₂ O, NO is generated according to the following formula (2).

_N2O +O( ^1D ) → 2NO...(2)

Furthermore, O( ¹ D) produced by the formula (1) may produce oxygen molecules (O ₂ ) and nitrogen molecules (N ₂ ) according to the formula (3).

_N2O +O( ^1D ) → _O2 + _N2 ...(3)

The main part of formula (3) is shown as chemical reaction R3 in FIG.

O( ^1D ) required for the reactions of formulas (2) and (3) is produced from _N2O according to formula (1). However, in the reaction field, there may be _O2 produced according to formula (3), _O2 originally contained in the gas G1 itself, or _O3 produced by the reaction described below. In such cases, O( ^1D ) is also produced according to the following formulas (4) and (5).

O ₂ + hν (≦175 nm) → O ( ³ P) + O ( ¹ D) ... (4)
O ₃ + hν (≦411 nm) → O ( ¹ D) + O ₂ ... (5)

Equation (4) is shown as chemical reaction R4 and chemical reaction R5 in Figure 2. A key part of equation (5) is shown as chemical reaction R6 in Figure 2.

The _O3 required for the reaction of formula (5) can be produced through the following reactions of formulas (1), (4), (6), (7), and (8) (formulas (1) and (4) are shown again). In this specification, "M" included in the chemical reaction formula represents a third body.

N ₂ O + hν (≦340 nm) → N ₂ + O ( ¹ D) ... (1)
O ₂ + hν (≦175 nm) → O ( ³ P) + O ( ¹ D) ... (4)
O ₂ + hν (≦242 nm) → O ( ³ P) + O ( ³ P) ... (6)
O ( ¹ D) + M → O ( ³ P) + M ... (7)
O ₂ + O ( ³ P) + M → O ₃ + M ... (8)

Equation (6) is shown as chemical reaction R5 in Figure 2. Chemical reaction R5 corresponds to either equation (4) or equation (6) depending on the wavelength of ultraviolet light. A key part of equation (7) is shown as chemical reaction R7 in Figure 2. A key part of equation (8) is shown as chemical reaction R8 in Figure 2. Note that as shown in Figure 2, an arrow (dashed line) indicating chemical reaction R17, which is the reverse reaction of chemical reaction R8, may also exist, in which _O3 is decomposed into _O2 and O( ^3P ). Chemical reaction R17 will be explained in detail later in the section "Deficient _O3 due to a high-temperature reaction field."

The direct decomposition of N ₂ O by the energy hν of ultraviolet light and the indirect decomposition of N ₂ O by O( ¹ D) generated by ultraviolet light have been described above. Usually, both direct and indirect decomposition occur. The ratio at which direct and indirect decomposition occur varies depending on the gas composition of gas G1.

The NO produced by chemical reaction R2 of formula (2) reacts with O( ³ P) produced in the reaction field to produce nitrogen dioxide (hereinafter sometimes referred to as “NO ₂ ”) as shown in formula (9).

NO+O ( ³ P) → NO ₂ ...(9)

The main part of formula (9) is included in chemical reaction R9 in FIG.

In addition, when a hydroxyl radical (hereinafter sometimes referred to as "OH") is present in the reaction field, _NO2 is produced from NO via the reactions shown in formulas (10) and (11).

NO+OH → HNO ₂ ...(10)
HNO ₂ +OH → NO ₂ +H ₂ O (11)

Equation (10) is shown as chemical reaction R10 in Figure 2. Equation (11) is shown as chemical reaction R11 in Figure 2.

OH in the reaction field is produced from water. That is, when the gas G1 contains water (including water vapor or mist, hereinafter sometimes referred to as "H ₂ O"), OH is produced in the decomposition reactor 2 according to formula (12).

_H2O +hν(≦242nm) → OH+H...(12)

In addition, when ozone is present in the reaction field, _NO2 is produced from NO via the reaction of formula (13).

NO + O ₃ +M → NO ₂ +O ₂ +M ... (13)

The main part of formula (13) is included in chemical reaction R9 in FIG.

_NO2 reacts with O( ^3P ) in the reaction field to produce _NO3 as shown in formula (14). If ozone is present in the reaction field, _NO2 reacts with ozone to produce _NO3 as shown in formula (15).

NO ₂ + O ( ³ P) + M → NO ₃ + M ... (14)
NO ₂ +O ₃ +M → NO ₃ +O ₂ +M ... (15)

The main part of the reactions of formulas (14) and (15) is shown as chemical reaction R12 in FIG.

The _NO3 produced by chemical reaction R12 and the _NO2 present in the reaction field undergo _the reaction of formula (16) to produce _N2O5 .

NO ₃ + NO ₂ → N ₂ O ₅ ...(16)

The reaction of formula (16) is shown as chemical reaction R13 in FIG.

In addition, _NO3 produced by chemical reaction R12 may react with NO or O( ^3P ) present in the reaction field, causing the reaction of formula (17) or formula (18), and returning to _NO2 .

NO ₃ + NO → NO ₂ + NO ₂ ...(17)
NO ₃ + O ( ³ P) → NO ₂ + O ₂ ... (18)

The reactions of equations (17) and (18) are shown as chemical reaction R14 in FIG.

N ₂ O ₅ , together with water vapor or water mist present in the reaction field, undergoes the reaction of formula (19) to produce HNO ₃ .

_{N2O5 + H2O} → _2HNO3 ...(19)

The reaction of formula (19) is shown as chemical reaction R15 in Fig. 2. Note that the reaction of formula (19) may be induced by contacting _{N2O5 with} water, for example, by bubbling _a gas containing _N2O5 in water.

When OH is present in the reaction field, NO ₂ may reach nitric acid without going through chemical reactions R12, R13, and R15, as shown in equation (20).

NO ₂ +OH → HNO ₃ ...(20)

The reaction of formula (20) is shown as chemical reaction R16 in FIG.

The series of reactions described above that convert _N2O to nitric acid presupposes the presence of sufficient _O3 in the reaction field. If there is not enough _O3 in the reaction field, nitration will not occur and NOx, especially NO and _NO2, will remain. There are several factors that can cause a lack of _O3 in the reaction field, as explained below.

[ _O3 deficiency due to NOx cycle]
The NOx cycle reaction will now be described. NO is converted to _NO2 according to the following formulas (9) and (13). On the other hand, _NO2 generates NO by reacting with oxygen atoms O according to the following formula (21) or (22), or by being photodecomposed according to the formula (23).

NO+O ( ³ P) → NO ₂ ...(9)
NO + O ₃ +M → NO ₂ +O ₂ +M ... (13)
NO ₂ + O ( ¹ D) → NO + O ₂ ... (21)
NO ₂ + O ( ³ P) → NO + O ₂ ... (22)
NO ₂ + hν (≦398 nm) → NO + O ( ³ P) ... (23)

The reaction shown in the previous paragraph, which produces _NO2 from NO and the reaction which produces NO from _NO2 , can be repeated. This is called the NOx cycle reaction. _O3 is consumed continuously during the NOx cycle reaction. When the amount of _O3 in the reaction field decreases due to the consumption of _O3 , the reactions of equations (13) and (15) become sluggish. This means that the chemical reaction between R9 and R12 becomes difficult to occur, and NO and _NO2 remain in the reaction field.

[Lack of _O3 due to high temperature reaction field]
The occurrence of the NOx cycle reaction is not the only cause of the _O3 shortage. The inventors discovered that if the temperature in the decomposition reactor for decomposing _N2O is high, the reaction stops midway through the complex reaction pathway leading to nitric acid. Specifically, when the temperature of the reaction field is high, _O3 decomposes into _O2 and O( ^3P ), as shown in chemical reaction R17 in Figure 2, resulting in a decrease in _O3 . As mentioned above, _O3 is an essential substance for converting NO to _NO2 according to equation (13) and _NO2 to _NO3 according to equation (15). Therefore, if _O3 is reduced, NO does not reach _HNO3 , resulting in an increase in NOx.

[The significance of using _CO2 ]
The inventors have found that using _CO2 is an effective way to deal with the residual NOx that occurs when _O3 in the reaction field decreases. _CO2 generates CO and oxygen atoms (O( ^3P ) or O( ^1D )) according to equation (24) or (25).

CO ₂ + hν (≦166 nm) → CO + O ( ¹ D) ... (24)
CO ₂ + hν (≦227 nm) → CO + O ( ³ P) ... (25)

The CO produced by the formulas (24) and (25) undergoes the reaction of the formula (26) in the presence of a three-way catalyst.

2NO+2CO → N ₂ +2CO ₂ ...(26)

The reaction of formula (26) is a reaction that reduces NO to nitrogen. The reaction of formula (26) converts the NO remaining in the reaction field into nitrogen gas. Furthermore, when NO decreases, the reactions of formulas (9) and (13) decrease, and therefore _NO2 also decreases. Furthermore, when NO decreases, the NOx cycle reaction also decreases, and the shortage of _O3 is alleviated.

Incidentally, the reaction of formula (26) occurs more easily as the temperature of the reaction field increases. This combines well with the phenomenon that the higher the temperature of the reaction field, the more _O3 decreases, resulting in a synergistic effect. In other words, since _O3 is less likely to decrease in a relatively low-temperature reaction field, the above-mentioned " _N2O decomposition mechanism" promotes the production of nitric acid. In contrast, in a relatively high-temperature reaction field, _O3 is more likely to decrease, so nitric acid is not produced and the residual amount of NOx increases. However, the reduction of NO by CO shown in formula (26) occurs, reducing the residual amount of NOx. In other words, in a reaction field containing a large amount of _CO2 , NOx can be reduced regardless of the temperature.

Since the reactions of formulas (24) and (25) occur only in a portion of the _CO2 , it is better for the _CO2 concentration in the gas G1 to be treated to be high. It is better for the _CO2 gas concentration contained in the gas G1 to be treated to be higher than the concentrations of all gases other than _CO2 contained in the gas G1 to be treated. In this case, the _CO2 concentration will be higher than the _N2O concentration.

In formulas (24) and (25), oxygen atoms (O( ^1D ) or O( ^3P )) are also produced from _CO2 . However, oxygen atoms (O( ^3P )) also contribute to the production of ozone through chemical reactions R7 and R8 (see formulas (7) and (8)), the production of _NO2 through chemical reaction R9 (see formula (9)), and the production of _NO3 through chemical reaction R12 (see formula (14)). Oxygen atoms (O( ^1D )) also contribute to the decomposition of _N2O through chemical reactions R2 and R3 (see formulas (2) and (3)). Therefore, the significance of using _CO2 is not only to reduce NOx by utilizing the reduction action of CO produced from _CO2 , but also to promote nitration by utilizing oxygen atoms produced from _CO2 .

[light source]
The light source 1 of this embodiment preferably emits light L1 whose main emission wavelength is greater than or equal to 160 nm and less than 200 nm. Figure 3 shows the absorption spectrum of light in a medium. The horizontal axis represents wavelength, and the vertical axis represents absorption cross-section (unit: ^cm2 ·molecule ^-1 ). In Figure 3, curve C1 represents the absorption cross-section of _N2O , and curve C2 represents the absorption cross-section of _CO2 .

Curve C2 shows high absorption at wavelengths of 200 nm or less. In other words, to generate oxygen atoms from _CO2 , it is preferable that the wavelength of light is 200 nm or less. Curve C1 shows that light is absorbed even at wavelengths above 200 nm and _N2O is decomposed into nitrogen and oxygen atoms (see formula (1) above), but decomposition is promoted at wavelengths of 200 nm or less. If the absorption of curve C2 becomes too high, there is a risk that the light absorption of _N2O will be hindered, so a wavelength of 165 nm or more at which the absorption of curve C2 does not become too high is preferable.

The light source 1 of this embodiment uses a xenon excimer lamp that emits excimer light with a peak wavelength or main emission wavelength of 172 nm or near 172 nm. When the wavelength is 172 nm, the light absorption of curve C1 ( _N2O ) is higher by ΔA than the light absorption of curve C2 ( _CO2 ). In Figure 3, ΔA is the difference corresponding to the light absorption of curve C1 being approximately 10 times the light absorption of curve C2, and this is light of a preferred wavelength that can sufficiently decompose _N2O while decomposing a desired amount of CO2 from a large amount of _CO2 .

In this specification, "near 172 nm" refers to a region within the range of 172 nm ±5 nm. In this specification, "main emission wavelength" refers to the wavelength λi in the wavelength range Z(λi) that shows an integrated intensity of 40% or more of the total integrated intensity in the emission spectrum, when a wavelength range Z(λ) of ±10 nm from a certain wavelength λ is defined on the emission spectrum. When the light source that emits light of the "main wavelength" has an extremely narrow half-width and shows high light intensity only at specific wavelengths, such as a xenon excimer lamp, the wavelength with the relatively highest light intensity (peak wavelength) can usually be considered to be the main wavelength.

As shown in Figure 1B, light source 1 has a cylindrical arc tube, and xenon gas is sealed inside 1i of the arc tube. Excimer lamps are light sources that can be mass-produced stably and have a significant cost-cutting effect. The shape of the arc tube is not limited to being cylindrical, and light source 1 is not limited to a xenon excimer lamp; for example, it can be a low-pressure mercury lamp. Light source 1 can also be an excimer lamp filled with a gas other than xenon. Light source 1 can also be a solid-state light source such as an LED or LD.

[Gas to be treated]
The gas to be treated will now be described in detail. As described above, the gas G1 to be treated supplied from the gas supply port 3i is a mixed gas containing at least _N2O and _CO2 . The gas G1 may also contain water, oxygen gas, nitrogen gas, air, or saturated hydrocarbons (particularly, alkanes having 10 or less, 6 or less, or 4 or less carbon atoms).

The case where the gas G1 contains water will be described. In this specification, water is a concept that includes water vapor, which is a gas, and mist-like water, which is a liquid. When the gas G1 contains water, the water is irradiated with light L1, and hydroxyl radicals are generated according to formula (27).

_H2O +hν(≦242nm) → H+OH...(27)

Furthermore, water generates hydroxyl radicals together with O( ¹ D) present in the reaction field according to formula (28).

_H2O +O( ^1D ) → 2OH...(28)

OH promotes the conversion of NO to _NO2 in the above-mentioned chemical reactions R10 and R11 (see equations (10) and (11)). OH also promotes the nitration of _NO2 in the above-mentioned chemical reaction R16 (see equation (20)). Equations (10), (11), and (20) are shown again below.

NO+OH → HNO ₂ ...(10)
HNO ₂ +OH → NO ₂ +H ₂ O (11)
NO ₂ +OH → HNO ₃ ...(20)

As shown in Fig. 2, R10, R11 and R16 merely form additional reaction pathways for producing _HNO3 from _N2O , and do not constitute essential reaction pathways such that nitration would not be possible without these reaction pathways. The fact that they merely form additional reaction pathways indicates that gas G1 does not necessarily have to contain OH, and therefore _H2O .

In chemical reaction R15, as shown in formula (19), _H2O is essential to generate _{HNO3 from N2O5 ,} but as described above, this _H2O can _be added by submerging a gas containing _N2O5 in water using the bubbling method, and chemical reaction R15 can be achieved even if the gas G1 supplied from gas supply port 3i does not contain _H2O . Formula (19) is shown again.

_N2O5 + _H2O → _HNO3 + _HNO3 ... ₍ 19)

A case where the gas G1 contains oxygen gas ( _O2 ) will be described. As described above, the gas G1 itself may contain _O2 . By including _O2 in the gas G1 supplied from the gas supply port 3i, more O( ^1D ) and O( ^3P ) are produced through chemical reactions R4 and R5. In addition, the amount of _O3 produced from O( ^3P ) via chemical reaction R8 is increased. As described above, O( ^3P ) and _O3 are important substances for the nitration of _N2O .

A case where the gas G1 contains nitrogen gas (N ₂ ) will be described. Nitrogen gas itself does not directly contribute to the series of chemical reactions shown in FIG. 2 , but it does not significantly impede the series of chemical reactions. Therefore, the gas G1 may contain nitrogen gas. Furthermore, the gas G1 may contain an inert gas other than nitrogen gas.

As can be seen from the fact that the gas G1 supplied from the gas supply port 3i may contain water vapor, oxygen gas, and nitrogen gas, it may also contain air. This air may be CDA (Clean Dry Air) or atmospheric air containing water vapor.

The saturated hydrocarbons that may be contained in the gas G1 supplied from the gas supply port 3i will be described. The saturated hydrocarbons are decomposed by O( ¹ D), O( ³ P), or OH produced by irradiation with ultraviolet light. The saturated hydrocarbons may be alkanes. The number of carbon atoms in the alkanes may be 10 or less, 6 or less, or 4 or less. The saturated hydrocarbons may be methane. Since methane, like N ₂ O, is a greenhouse gas, being able to decompose methane and N ₂ O simultaneously is preferable from the standpoint of environmental conservation. The gas G1 may contain multiple saturated hydrocarbons.

[Nitric acid treatment method]
FIG. 4 shows an example of a method for treating nitric acid. In FIG. 4, gas G2 containing nitric acid produced in the gas decomposition device 10 and discharged from the decomposition reactor 2 passes through an exhaust pipe 11 connected to the gas exhaust port 3o and comes into contact with water W1 in a container 12. The nitric acid contained in the gas G2 dissolves in the water W1 to form an aqueous nitric acid solution, thereby trapping the nitric acid. Nitric acid is a raw material for ammonium nitrate and is a useful substance in the chemical industry, agriculture, and other fields. Therefore, the aqueous nitric acid solution in the container 12 may be recovered. Alternatively, if the concentration of the aqueous nitric acid solution is low enough to be discharged, it may be discharged into a sewer system without being recovered. Nitric acid discharged into a sewer system is converted to NO ₃ ⁻ through biodegradation and ultimately to N ₂ through N 2 . While FIG. 4 illustrates a method for recovering nitric acid by dissolving it in water W1, a method for recovering nitric acid by simply cooling the gas G2 to liquefy it is also acceptable. Because the boiling point of nitric acid is approximately 83°C, nitric acid liquefies when the gas is cooled.

[catalyst]
The catalyst 8 will now be described in detail. As described above, the catalyst 8 may be a three-way catalyst or a two-way catalyst.

The three-way catalyst will now be described. The three-way catalyst uses carbon monoxide contained in gas G1 to promote the reduction of nitrogen oxides. Gas G1 contains a high concentration of _CO2 as well as _N2O . Therefore, light L1 generates NO from _N2O and CO from _CO2 . Gas G1 uses CO to reduce NO to generate _N2 according to equation (29). At the same time, CO is oxidized to generate _CO2 . The three-way catalyst promotes the reaction of equation (29), in which oxidation and reduction occur simultaneously.

2NO+2CO → N ₂ +2CO ₂ ...(29)

The reaction of formula (29) occurs more easily the higher the temperature of the reaction field. If the temperature of the reaction field for decomposing _N2O is high, there will be a shortage of _O3 , and the reaction will stop with the NO generated from _N2O . However, the reaction of formula (29) becomes more active, and the CO generated from _CO2 reduces NO to _N2 . This makes it possible to reduce the amount of NO that does not become nitrate, even if the temperature of the reaction field is high. Furthermore, CO is also harmful to the human body, but since it can be converted to _CO2 by a three-way catalyst, the three-way catalyst of gas G1 contributes to its detoxification.

On the other hand, when the temperature inside the decomposition reactor that decomposes _N2O is low, the conversion of NO to nitrate by _O3 can be promoted. As a result, whether the reaction field inside the decomposition reactor is high temperature or low temperature, the reaction can proceed to nitric acid or _N2 without stopping at NO generated from _N2O . As a result, NOx can be reduced.

For example, rhodium, ruthenium, iridium, palladium, or platinum can be used as materials for the three-way catalyst. It is also preferable to use a catalyst composed of iron-cobalt composite oxide or a tungsten-substituted vanadium oxide catalyst in which tungsten atoms are dispersed in vanadium oxide, as these catalysts exhibit catalytic performance even in relatively low-temperature environments of around 150°C.

The two-way catalyst will now be described. The two-way catalyst is an oxidation catalyst that promotes the oxidation of both substances contained in the gas G1. When the gas G1 contains at least one of nitrogen oxides and carbon monoxide, the two-way catalyst promotes the oxidation of at least one of the nitrogen oxides and carbon monoxide using oxygen in the gas G1. When NO is produced from _N2O and CO is produced from _CO2 by light L1, the two-way catalyst promotes the reactions of formulas (30) and (31).

2CO+ _O2 → _2CO2 ...(30)
2NO+O ₂ → 2NO ₂ ...(31)

Regarding nitrogen oxides, if NO can be converted to _NO2 according to equation (31), nitration is possible even in the absence of ozone in the reaction field, as long as hydroxyl radicals are present. Therefore, the binary catalyst contributes to the nitration of NO. Furthermore, CO, which is harmful to humans and animals, can be rendered harmless by converting it to _CO2 .

Possible materials for the binary catalyst include, for example, iron, platinum, and platinum.

[Method of using the gas decomposition device]
A method of using the gas decomposition device 10 will now be described. Nitrous oxide is emitted, for example, from soil on agricultural and livestock farms, waste management areas and septic tanks, sewage systems and sewage treatment facilities, garbage disposal plants, biomass factories, and chemical plants. The same is true for carbon dioxide. Carbon dioxide itself is emitted simply by burning carbon and hydrocarbons. However, it is unlikely that a mixed gas containing a high concentration of carbon dioxide, for example, a mixed gas in which carbon dioxide is the highest concentration among the gases, would be directly emitted from equipment (including transportation equipment such as automobiles) that combusts carbon and hydrocarbons with oxygen contained in air. Therefore, it is desirable to adjust the concentration of carbon dioxide contained in the gas G1 to a high level. For example, a gas decomposition system may be constructed that includes equipment that emits gas at a high concentration of carbon dioxide or a gas supply source that supplies high-concentration carbon dioxide.

In order to provide the gas to be treated with energy for exciting the gas, it is preferable to select the wavelength, light intensity, and irradiation time of the light to promote the decomposition of N ₂ O. The gas decomposition device 10 may include a gas concentration adjustment unit that is connected to the gas supply port 3i and adjusts the amount of _each gas component contained in the gas G1 so that the gas concentration of CO ₂ contained in the gas G1 is higher than the concentration of any other gas contained in the gas G1 except for CO 2.

Second Embodiment
A second embodiment of a gas decomposition apparatus will be described. The following description will focus on differences from the first embodiment, and descriptions of commonalities with the first embodiment will be omitted. The same applies to the third and subsequent embodiments described below.

The gas decomposition device 20 shown in Figure 5A does not have a catalyst in the decomposition reactor 2. Instead, it has a catalyst unit 21 downstream of the decomposition reactor 2. The catalyst unit 21 incorporates a catalyst contact section 25 in the widened portion of the pipe 29. Gas G2 discharged from the gas outlet 3o of the decomposition reactor 2 flows into the catalyst contact section 25.

Figure 5B is a cross-sectional view taken along line S2-S2 in Figure 5A. The catalytic contact section 25 has a large number of cells inside the pipe 29. Each cell 27 is surrounded by a ceramic wall. Each cell 27 is elongated in the gas flow direction and is arranged side by side along the cross section of the pipe 29.

5C is an enlarged view of region P1 in FIG. 5B. Ceramic walls 23 are arranged in a lattice pattern. A space 24 is provided in the center of each cell 27, and the space 24 functions as a flow path for gas G2. A catalyst 8 is disposed outside the space 24 and inside the ceramic walls 23.
The catalyst 8 can be either a three-way catalyst or a two-way catalyst. The role of the catalyst contact portion 25 will be explained below. When a three-way catalyst is used as the catalyst, it reduces NO and converts it into nitrogen gas, thereby reducing NOx. When a two-way catalyst is used, it oxidizes CO and converts it into _CO2 .

In this embodiment, in order to increase the contact area between the catalyst 8 and the gas G2, the catalyst contact portion 25 is arranged in the widened portion of the pipe 29, and the catalyst 8 is installed on the wall surface of the numerous cells 27. However, the catalyst contact portion 25 may not have numerous cells 27. Furthermore, even if a catalyst contact portion 25 having numerous cells 27 is used, the cross-sectional shape of the ceramic walls (cell shape) can be any shape. For example, in Figure 5C, the ceramic walls 23 are arranged in a rectangular lattice pattern, but the ceramic walls 23 may also be arranged to form a honeycomb structure.

[Variations]
A modified version of the second embodiment will be described with reference to FIG. 6 . Differences between this modified version and the second embodiment will be mainly described. A two-way catalyst is used in the catalytic contact section 25. An oxygen gas supply pipe 41 is connected between the gas decomposition device 20 and the catalytic contact section 25 shown in FIG. 6 , and oxygen gas is supplied upstream of the catalytic contact section 25 from an oxygen gas supply port 42 provided in the pipe 29. If either NO or CO remains in the gas G2 discharged from the gas outlet 3o, oxygen gas is supplied from the oxygen gas supply pipe 41 to promote the oxidation of CO and its conversion to _CO2 in the catalytic contact section 25. This reduces NOx and improves safety for humans and animals.

The second embodiment and its modified form have been described above. In the above description, instead of providing the catalyst 8 inside the gas decomposition device 20, the catalyst 8 is arranged inside the piping 29 downstream of the gas decomposition device 20. However, it is also possible to arrange the catalyst 8 inside the gas decomposition device 20 as in the first embodiment, and also arrange the catalyst 8 inside the piping 29 downstream of the gas decomposition device 20 as in the second embodiment.

Third Embodiment
7A shows a gas decomposition apparatus according to a third embodiment of the present invention. The energy source in the gas decomposition apparatus 30 shown in FIG. 7A is an energy source that provides electron energy for exciting gas contained in the gas to be treated to generate plasma, and the electron energy is provided by applying a high-frequency voltage between electrodes that sandwich the gas to be treated.

As shown in FIG. 7A, the closed cylindrical tube 33 has two openings, corresponding to the gas supply port 3i and the gas exhaust port 3o. The gas supply port 3i is an opening for introducing gas G1, which is the gas to be treated, into the inside of the tube 33. The gas exhaust port 3o is an opening for exhausting the treated gas G2. The gas exhaust port 3o is located at a distance from the gas supply port 3i in the tube axis direction d1. In this embodiment, the gas exhaust port 3o is located at a position separated from the gas supply port 3i in the tube axis direction d1, across the region where the outer electrode 35a is formed, with respect to the gas supply port 3i as the reference.

The two electrodes (35a, 35b) of the gas decomposition device 30 will now be described. The outer electrode 35a is a mesh-like electrode provided along the outer wall surface of the tube body 33. The inner electrode 35b is a rod-shaped electrode that extends linearly inside the tube body 33 along the tube axis direction d1 of the tube body 33. The inner electrode 35b is disposed from the outside of the tube body 33 to the inside of the tube body 33, penetrating the tube body 33. Both electrodes (35a, 35b) are each electrically connected to the power source 6.

Fig. 7B is a cross-sectional view taken along the line S3-S3 in Fig. 7A. As also shown in Fig. 7B, a space SP1 is formed inside the tube 33 located between the two electrodes (35a, 35b).
When a voltage is applied between the two electrodes (35a, 35b), a dielectric barrier discharge occurs within the tube 33, and an atmospheric pressure plasma space is formed within the space SP1.

The applied voltage supplied from the power supply 6 may be in a range that is capable of generating a dielectric barrier discharge within the tube body 3 by applying a voltage between the electrodes (35a, 35b). 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. Furthermore, 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 because the frequency detected in the noise terminal voltage under the EMC standard is 150 kHz or more. In this way, a high-frequency voltage is applied from the power supply 6 between both electrodes (35a, 35b).

It is preferable that the power supply 6 applies a voltage so that the outer electrode 35a is at ground voltage and the inner electrode 35b is at high voltage. This reduces the risk of electric shock caused by the electrode exposed to the outside being at high voltage.

As described above, the decomposition mechanism of the gas decomposition device of this embodiment is basically the same as the decomposition mechanism when light energy is used. _N2O molecules contained in gas G1 are decomposed into nitrate in the atmospheric pressure plasma space by the same mechanism as the decomposition mechanism described above.

The tube 33 is made of a dielectric material such as quartz glass, ceramics, etc. The electrodes (5a, 5b) are made of a metal material such as stainless steel, aluminum, copper, tungsten, nickel, etc. The tube 33 is cylindrical, but is not limited to a cylindrical tube.
The tube 33 may have a rectangular cylindrical shape, or in particular, a flat cylindrical shape. Although the outer electrode 35a is a mesh-like electrode provided along the outer wall surface of the tube 33, the shape is not limited to this. The outer electrode 35a may be, for example, a metal sheet or metal film provided along the outer wall surface of the tube 33, or may be a block-like electrode located outside the tube 33. The outer electrode 35a does not necessarily need to completely cover the wall surface of the tube 33 in the circumferential direction, and may be configured to not cover part of the wall surface of the tube 33. The shape of the inner electrode 35b is similarly not limited to the above-mentioned shape.

[Variations]
A modified version of the third embodiment will be described. In the gas decomposition apparatus 40 shown in FIG. 8A, the tubular body 3 has a double-tube structure. FIG. 8B is a cross-sectional view taken along line S4-S4 in FIG. 8A. More specifically, as shown in FIG. 8A, the tubular body 33 includes an outer tube 33a having a cylindrical shape and located on the outside, and an inner tube 33b having a cylindrical shape and a smaller inner diameter than the outer tube 33a, which is arranged coaxially with the outer tube 33a inside the outer tube 33a. The end of the inner tube 33b may be open to allow the same gas as the atmosphere in which the gas decomposition apparatus 40 is placed to flow into the inner tube 33b, or the end of the inner tube 33b may be sealed to allow a gas different from the atmosphere to flow into the inner tube 33b.

A rod-shaped inner electrode 35b extending linearly along the tube axis direction d1 of the tube body 33 is inserted inside the inner tube 33b. Another outer electrode 35a is provided on the outside of the outer tube 33a. A space SP1 that is ring-shaped (here, circular) when viewed from the tube axis direction d1 is formed between the outer tube 3a and inner tube 3b. The gas supply port 3i and gas exhaust port 3o are connected to the space SP1 located outside the inner tube 33b. In other words, gas G1 flows into the space SP1 through the gas supply port 3i.

<Fourth embodiment>
Fig. 9A shows a gas decomposition apparatus 50 according to a fourth embodiment. Fig. 9B is a cross-sectional view taken along line S6-S6 in Fig. 9A. In gas decomposition apparatus 50, the energy used to excite the gases contained in the mixed gas is both ultraviolet light and plasma generated by high-frequency voltage.

As shown in Figures 9A and 9B, the gas decomposition device 50 has a double-pipe structure in which an inner pipe 54 is disposed inside an outer pipe 53. Inside the inner pipe 54 is a gas flow path 52 that allows the gas G1 to be treated to flow in the direction in which the double pipe extends.

The outer electrode 55a is disposed outside the outer wall of the outer tube 53, and the inner electrode 55b is disposed inside the inner wall of the inner tube 54. The inner electrode 55b and the outer electrode 55a may preferably have a mesh shape.
The outer tube 53 is shorter than the inner tube 54, and both ends of the outer tube 53 are sealed. A space 58 between the outer tube 53 and the inner tube 54 is filled with a light-emitting gas such as xenon gas. When a voltage is applied between the outer electrode 55 a and the inner electrode 55 b, the space 58 becomes a discharge space, generating light L1 that is radiated into the gas G1 flowing through the gas flow path 52 (see FIG. 9A ).

When a voltage is applied between the inner electrode 55b and the outer electrode 55a, atmospheric pressure plasma AP is generated in the gap between the inner wall of the inner tube 54 and the inner electrode 55b. When gas G1 flows through this gap, the gas molecules contained in gas G1 are excited by the atmospheric pressure plasma AP. In this way, the gas molecules that make up gas G1 are excited by the plasma formed by the ultraviolet light and high-frequency voltage.

The inner tube 54 is made of a material that transmits the luminous gas, such as quartz. The luminous gas passes through the inner tube 54 and reaches the inner gas flow passage 52. The gas decomposition device 50 has a gas supply port 3i at one end of the inner gas flow passage 52 and a gas exhaust port 3o at the other end of the inner gas flow passage 52. Gas G1 is supplied from the gas supply port 3i to the inner gas flow passage 52, light L1 emitted from the light source 1 is irradiated onto the gas G1 to be treated, and the gas G2 after light irradiation is continuously discharged from the gas exhaust port 3o. This allows continuous decomposition of _N2O and _CO2 in the gas G1.

The outer tube 53 is made of, for example, quartz. In the gas decomposition device 50, a reflective film that reflects light L1 may be formed on the inner wall surface of the outer tube 53. The luminous gas is emitted toward the outside of the outer tube 53, and if a reflective film is formed on the inner wall surface of the outer tube 53, light L1 that would otherwise be directed toward the outside of the outer tube 53 is reflected back inside, increasing the light intensity within the inner gas flow path 52.

[First Variation]
FIG. 10A shows a gas decomposition apparatus 60 according to a first modified example of the fourth embodiment. FIG. 10B is a cross-sectional view taken along line S7-S7 in FIG. 10A. The gas decomposition apparatus 60 has a triple-tube structure in which an intermediate tube 73 is disposed within an outer tube 71, and an inner tube 54 is disposed within the intermediate tube 73. There are multiple gas flow paths. First, there is a gas flow path 52 formed within the inner tube 54, similar to that of the gas decomposition apparatus 50. Second, there is a gas flow path 72 formed between the outer tube 71 and the intermediate tube 73, which is not present in the gas decomposition apparatus 50. Because the gas flow path 72 is located outside the gas flow path 52, the gas flow path 72 is sometimes referred to as the "outer gas flow path 72," and the gas flow path 52 is sometimes referred to as the "inner gas flow path." Light L1 is also irradiated onto the gas G1 flowing through the outer gas flow path 72.

There is a gap not only between the inner electrode 55b and the inner tube 54, but also between the outer electrode 55a and the intermediate tube 73. Therefore, the atmospheric pressure plasma AP acts not only between the inner electrode 55b and the inner tube 54, but also between the outer electrode 55a and the intermediate tube 73, exciting the gas molecules contained in the gas G1 flowing through the outer gas flow path 72. In this way, the gas molecules flowing through each gas flow path (52, 72) are excited by ultraviolet light and high-frequency voltage. Because the gas G1 can be processed in each gas flow path (52, 72), a large amount of gas can be processed, improving the utilization efficiency of the light L1 and the atmospheric pressure plasma AP.

[Second Modification]
11 shows a gas decomposition apparatus 70 according to a second modified example of the fourth embodiment. The gas decomposition apparatus 70 differs from the gas decomposition apparatus 60 according to the first modified example of the fourth embodiment in that the inner gas flow passage 52 formed within the inner tube 54 is connected to an outer gas flow passage 72. The gas G2 processed in the inner gas flow passage 52 turns back and passes through the outer gas flow passage 72, where it is processed again. This allows the gas to be processed to be processed more effectively, improving the utilization efficiency of the light L1.

In this modified embodiment, the gas first passes through the inner gas flow passage 52 and then the outer gas flow passage 72, but it may also be configured so that the gas first passes through the outer gas flow passage 72 and then the inner gas flow passage 52.

The above describes various embodiments of the gas decomposition method and gas decomposition apparatus, as well as appropriate variations thereof. The above embodiments and their variations are merely examples of the present invention, and the present invention is in no way limited to the above-described embodiments. Various modifications or improvements can be made to the above-described embodiments, and the above-described embodiments or variations can be combined, without departing from the spirit of the present invention.

1: Light source 2: Decomposition reactor 3: Tube body 3a: Outer tube 3b: Inner tube 3i, 4i: Gas supply port 3o, 4o: Gas exhaust port 5: Control unit 5a, 5b: Electrode 6: Power supply 8: Catalyst 10, 20, 30, 40, 50, 60, 70: Gas decomposition device 11: Exhaust pipe 12: Container 21: Catalyst unit 23: Wall 24: Space 25: Catalyst contact portion 29: Pipe 31: Gas supply port 32: Gas exhaust port 33: Tube body 33a, 53, 71: Outer tube 33b, 54: Inner tube 35a, 55a: Outer electrode 35b, 55b: Inner electrode 41: Oxygen-containing gas supply pipe 42: Oxygen-containing gas supply port 52, 72: Gas flow path 57: Reflective film 58: Space 73: Intermediate tube L1: Light AP : Atmospheric pressure plasma SP1 : Space

Claims (16)

A method for decomposing a gas to be treated, comprising:
the gas to be treated contains at least nitrous oxide and carbon dioxide, and the gas concentration of carbon dioxide contained in the gas to be treated is higher than the concentrations of any other gases contained in the gas to be treated except for carbon dioxide;
A decomposition method comprising: decomposing dinitrogen monoxide in a gas to be treated by applying energy to the gas to be treated for exciting gases contained in the gas to be treated. The decomposition method described in claim 1, wherein the gas to be treated further contains water vapor. The decomposition method described in claim 1, characterized in that the energy is light energy having a main emission wavelength of 160 nm or more and less than 200 nm. The decomposition method described in claim 1, characterized in that the energy is electron energy generated by converting the gas to be treated into plasma. The decomposition method described in any one of claims 1 to 4, characterized in that the gas to be treated is brought into contact with a catalyst. The decomposition method described in claim 5, characterized in that the catalyst is a three-way catalyst used to promote the reduction of nitric oxide produced by the decomposition of nitrous oxide. The decomposition method described in claim 5, characterized in that the catalyst is a dual catalyst used to promote the oxidation of at least one of the nitric oxide produced by the decomposition of nitrous oxide and the carbon monoxide produced from the carbon dioxide. The decomposition method described in claim 7, characterized in that an oxygen-containing gas is additionally supplied to the gas to be treated after the energy has been applied. A decomposition device for a gas to be treated, comprising:
The decomposition device includes:
a gas supply port for introducing the gas to be treated into a decomposition device, the gas to be treated containing at least nitrous oxide and carbon dioxide, and a gas concentration of carbon dioxide contained in the gas to be treated being higher than the concentrations of any other gases contained in the gas to be treated except for carbon dioxide;
an energy source that provides energy to the gas to be treated that is introduced into the decomposition device from the gas supply port in order to decompose the nitrous oxide in the gas to be treated, for exciting gas contained in the gas to be treated;
A decomposition apparatus comprising: The decomposition apparatus of claim 9, further comprising a gas concentration adjustment unit connected to the gas supply port, which adjusts the amounts of gas components contained in the gas to be treated so that the gas concentration of carbon dioxide contained in the gas to be treated is higher than the concentrations of all gases contained in the gas to be treated other than carbon dioxide. The decomposition device described in claim 9, characterized in that the energy is light energy having a main emission wavelength of 160 nm or more and less than 200 nm, and the energy source is a light source that radiates the light. The decomposition apparatus described in claim 9, characterized in that the energy is electron energy that converts the gas to be treated into plasma, and the energy source is an electrode that supplies the electron energy. The decomposition device described in any one of claims 9 to 12, characterized in that it is equipped with a three-way catalyst that promotes the reduction of nitric oxide produced by the decomposition of nitrous oxide. The decomposition device described in any one of claims 9 to 12 is characterized in that it is equipped with a two-way catalyst used to promote the oxidation of at least one of the nitric oxide produced by the decomposition of the dinitrogen monoxide and the carbon monoxide produced from the carbon dioxide. the decomposition device includes an oxygen supply port for adding an oxygen-containing gas to the gas to be treated after the energy is applied;
15. The decomposition apparatus according to claim 14, wherein the two-way catalyst is disposed so as to come into contact with the gas to be treated to which a gas containing oxygen has been added. The decomposition device according to any one of claims 9, 11 and 12;
a gas supply source connected to the gas supply port for supplying the gas to be treated into the decomposition device.