Gas decomposition method, gas decomposition apparatus, and gas decomposition system

Abstract

To provide a method for decomposing a gas to be treated, in which dinitrogen monoxide is decomposed while inhibiting exhaustion of NOx.SOLUTION: In a method for decomposing a gas to be treated, the gas to be treated contains at least dinitrogen monoxide and carbon dioxide. A gas concentration of the carbon dioxide contained in the gas to be treated is higher than any of the gas concentrations contained in the gas to be treated excluding the carbon dioxide. In the method for decomposing the gas to be treated, dinitrogen monoxide in the gas to be treated is decomposed by subjecting the gas to be treated to energy for exciting the gases contained in the gas to be treated.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to a method for decomposing a gas to be treated, a gas decomposition apparatus, and a gas decomposition system. [Background technology]

[0002] Since the Industrial Revolution, the average temperature of the Earth 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 widely emitted, followed by methane, and then nitrous oxide.

[0003] Meanwhile, 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. Given these circumstances, the impact of nitrous oxide emissions on global warming cannot be ignored.

[0004] Nitrous oxide is emitted not only during the disposal 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.

[0005] Nitrous oxide is decomposed to prevent it from being released into the atmosphere. High-temperature combustion and catalytic methods have been used to decompose nitrous oxide. However, high-temperature combustion requires 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 are also undesirable as a measure against global warming.

[0006] Another method for decomposing nitrous oxide is to use light (see Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2024-022455 [Patent Document 2] Japanese Patent Publication No. 2024-021127 Summary of the Invention [Problem to be solved by the invention]

[0008] When gas containing nitrous oxide (hereinafter sometimes abbreviated as "NO") is irradiated with light, the NO can be decomposed into nitrogen gas and oxygen gas, but during this decomposition process, NOx is produced from some of the NO. In this specification, NOx is an expression that includes nitric oxide (hereinafter sometimes abbreviated as "NO"), nitrogen dioxide (hereinafter sometimes abbreviated as "NO"), nitrogen trioxide (hereinafter sometimes abbreviated as "NO"), and dinitrogen pentoxide (hereinafter sometimes abbreviated as "N2O5").

[0009] Because NOx has adverse effects on humans and animals, one approach is to convert the NOx produced during the decomposition of N2O into nitric acid (hereinafter referred to as "HNO3") to render it harmless. Obtaining HNO3 from N2O has the advantage of being able to fix N2O without releasing it into the atmosphere, and also of producing and using nitric acid from N2O. Nitric acid is a useful substance in various industrial fields, including the chemical industry, so obtaining nitric acid from N2O has high business value. If nitric acid can be obtained by decomposing emitted N2O, the amount of nitric acid produced for the purpose of manufacturing nitric acid itself can also be reduced. Since the nitric acid manufacturing process also emits a large amount of N2O, the amount of N2O emitted during the production of nitric acid itself can also be reduced.

[0010] The chemical reaction that decomposes N2O to produce HNO3 will be described in detail later, but the reaction pathway that leads to the decomposition of N2O to produce HNO3 is complex. The inventors have noticed that there is a risk that the decomposition of N2O may stop midway through the reaction pathway, resulting in the emission of NOx instead of HNO3. As mentioned above, NOx has adverse effects on humans and animals, so it is necessary to suppress the emission of NOx as much as possible.

[0011] Therefore, an object of the present invention is to provide a method, an apparatus and a system for decomposing a gas to be treated, which appropriately decomposes N2O while suppressing the emission of NOx. [Means for solving the problem]

[0012] 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.

[0013] Details will be given later, but the background to arriving at the above decomposition method will be briefly explained. As a result of intensive research, the inventors of the present invention have discovered that if the temperature inside the decomposition reactor for decomposing N2O is high, the reaction stops midway through the reaction pathway leading to the generation of HNO3 from N2O, making it more likely that NOx will remain. When the temperature inside the decomposition reactor is high, ozone (hereinafter sometimes referred to as "O3") breaks down into oxygen molecules (O2) and oxygen atoms O( 3 P), which reduces O3 in the decomposition reactor. O3 is a substance necessary for converting NO produced by decomposing N2O to HNO3. O( 3 P) is an oxygen atom in the ground state, called "triplet oxygen." When O3 decreases, NOx tends to remain without reaching HNO3.

[0014] 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 NO is emitted. In this method, energy for exciting the gas is given to the gas to be treated, which contains a higher concentration of CO2 than NO.

[0015] The energy converts CO2 into a large amount of CO and oxygen atoms O( 1 D) or O( 3 P) is generated (see equations (24) and (25) below). 1 D) is a highly reactive excited state oxygen atom called "singlet oxygen." When the temperature inside the decomposition reactor that decomposes N2O is high, there is a shortage of O3 inside the decomposition reactor, and the reaction is likely to stop with the NO generated from N2O. However, the large amount of CO generated from CO2 reduces NO to N2. Also, the large amount of oxygen atoms O( 3 P) oxidizes NO to NO2, which is then converted to nitrate by hydroxyl radicals (described later). In this way, the remaining NOx is reduced.

[0016] On the other hand, when the temperature inside the decomposition reactor that decomposes NO is low, the reduction of NO by CO becomes less effective. 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 NO, 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, which is a mixture of NO and CO2.

[0017] 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 NO2 is converted into NO3 and then N2O5 is generated, 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.

[0018] The energy may be light energy having a main emission wavelength of 160 nm or more and less than 200 nm.

[0019] The energy may be electron energy generated by converting the gas to be treated into plasma.

[0020] The gas to be treated may be brought into contact with a catalyst.

[0021] The catalyst may be a three-way catalyst used to promote the reduction of nitric oxide produced by the decomposition of nitrous oxide.

[0022] The catalyst may be a two-way catalyst used to promote oxidation of at least one of nitric oxide produced by decomposition of dinitrogen monoxide and carbon monoxide produced from carbon dioxide.

[0023] A gas containing oxygen may be additionally supplied to the gas to be treated after the energy is applied.

[0024] 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.

[0025] The decomposition device may be provided with a gas concentration adjusting 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 concentration of any other gas other than carbon dioxide contained in the gas to be treated.

[0026] The energy may be light energy having a main emission wavelength of 160 nm or more and less than 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 near 172 nm.

[0027] The energy may be electron energy that converts the gas to be treated into plasma, and the energy source may be an electrode that supplies the electron energy.

[0028] The decomposition device may include a three-way catalyst that promotes the reduction of nitric oxide produced by the decomposition of nitrous oxide.

[0029] The decomposition device may be provided with a two-way catalyst used to promote oxidation of at least one of nitric oxide produced by decomposition of the dinitrogen monoxide and carbon monoxide produced from the carbon dioxide.

[0030] 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.

[0031] 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. [Effects of the Invention]

[0032] This makes it possible to provide a method, apparatus, and system for decomposing gas to be treated that can decompose NO regardless of the temperature of the reaction field in the decomposition reactor, thereby appropriately decomposing NO while suppressing NOx emissions. Providing such a gas decomposition method, apparatus, and system will greatly contribute to achieving Goal 13 of the United Nations' Sustainable Development Goals (SDGs), "Take urgent action to combat climate change and its impacts." [Brief explanation of the drawings]

[0033] [Figure 1A] FIG. 1 is a diagram showing a first embodiment of a gas decomposition apparatus. [Figure 1B] FIG. 1B is a cross-sectional view taken along line S1-S1 of FIG. 1A. [Figure 2] FIG. 1 is a diagram showing the mechanism of NO decomposition. [Figure 3] Optical absorption spectra of N2O and CO2. [Figure 4] FIG. 1 is a diagram showing an example of a method for treating nitric acid. [Figure 5A] FIG. 2 is a diagram showing a second embodiment of a gas decomposition apparatus. [Figure 5B] FIG. 5B is a cross-sectional view taken along line S2-S2 of FIG. 5A. [Figure 5C] FIG. 5C is an enlarged view of the P1 region of FIG. 5B. [Figure 6]FIG. 10 is a diagram showing a modified example of the second embodiment of the gas decomposition apparatus. [Figure 7A] FIG. 10 is a diagram showing a third embodiment of a gas decomposition apparatus. [Figure 7B] 7B is a cross-sectional view taken along line S3-S3 in FIG. 7A. [Figure 8A] FIG. 10 is a diagram showing a modified example of the third embodiment of the gas decomposition apparatus. [Figure 8B] 8B is a cross-sectional view taken along line S4-S4 in FIG. 8A. [Figure 9A] FIG. 10 is a diagram showing a fourth embodiment of a gas decomposition apparatus. [Figure 9B] 9B is a cross-sectional view taken along line S6-S6 in FIG. 9A. [Figure 10A] FIG. 10 is a diagram showing a first modified example of the fourth embodiment of the gas decomposition apparatus. [Figure 10B] 10B is a cross-sectional view taken along line S7-S7 in FIG. 10A. [Figure 11] FIG. 10 is a diagram showing a second modified example of the fourth embodiment of the gas decomposition apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0034] The embodiments will be described with reference to the drawings as appropriate. Note that all drawings, except for 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 the actual dimensional ratios.

[0035] First Embodiment [Outline of gas decomposition unit] 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.

[0036] A first embodiment of a gas decomposition apparatus is shown in Figure 1A. Figure 1B is a cross-sectional view taken along line S1-S1 in Figure 1A. In gas decomposition apparatus 10 shown as the first embodiment, the energy source that provides energy for exciting gases contained in gas G1 is light source 1 that emits ultraviolet light L1.

[0037] 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 to face 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.

[0038] 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, which then decomposes the NO. The gas G2 irradiated with light and subjected to the chemical reaction is discharged from the gas discharge port 3o. By continuously performing these steps, the NO contained in the gas G1 can be continuously decomposed.

[0039] 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 NO.

[0040] From the viewpoint of increasing the decomposition efficiency, it is more preferable to design the decomposition reactor 2 so that the light L1 reaches sufficiently inside the decomposition reactor 2. Therefore, the distance D1 (see FIG. 1A or 1B) between the surface of the light source 1 and the inner wall of the decomposition 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 can reduce the amount of gas that passes through the decomposition reactor 2 without being irradiated with the light L1.

[0041] 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 be used. Details of the catalyst 8 will be described later.

[0042] [N2O decomposition mechanism] The decomposition mechanism of N2O by light L1 will be described with reference to FIG. 2. The gas decomposition device of this embodiment decomposes N2O by ultraviolet light, so 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 N2O: direct decomposition of N2O by ultraviolet light, and decomposition of O( 1 D) Indirect decomposition of N2O by

[0043] The direct decomposition of NO by ultraviolet light is explained below. Direct decomposition is shown in chemical reaction R1 in Figure 2. When NO is irradiated with ultraviolet light with a wavelength of 340 nm or less, NO is decomposed into N and O( 1 D). N2O+hν(≦340nm) → N2+O( 1 D) (1)

[0044] In this specification, the notation "hν(≦Wnm)" (where "W" represents a number) represents the energy of light with a wavelength of W (nm) or less. For example, this indicates that the decomposition reaction of formula (1) occurs with ultraviolet light with a wavelength of 340 nm or less. Looking at formula (1) alone, it would be sufficient to use ultraviolet light with a wavelength of 340 nm or less, but the absorption cross section of ultraviolet light with a wavelength of 200 nm or more for NO is small. To make the decomposition reaction of formula (1) more efficient, it is better to use light with a wavelength of less than 200 nm, which has a relatively large absorption cross section. This also applies to other light-based decomposition reactions described below.

[0045] UV-generated O( 1The indirect decomposition of NO by the NO decomposition reaction in Eq. (1) is explained. 1 The chemical reaction for decomposing N2O by O( 1 When D) comes into contact with NO, NO is produced according to the following equation (2): NO+O( 1 D) → 2NO (2)

[0046] In addition, the O( 1 D) may produce oxygen molecules (O2) and nitrogen molecules (N2) according to equation (3). NO+O( 1 D) → O2 + N2 (3) The main part of equation (3) is shown in chemical reaction R3 in Figure 2.

[0047] The O( required for the reactions of equations (2) and (3) 1 D) is produced from NO by the formula (1). However, in the reaction field, there may be O produced by the formula (3), O originally contained in the gas G1 itself, or O produced by the reaction described below. In such cases, O( 1 D) is produced. O2 + hν(≦175 nm) → O( 3 P)+O( 1 D) (4) O3 + hν(≦411 nm) → O( 1 D) + O2 (5) Equation (4) is shown as chemical reaction R4 and chemical reaction R5 in Figure 2. The main part of equation (5) is shown as chemical reaction R6 in Figure 2.

[0048] 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" in the chemical reaction formula represents a third body. N2O+hν(≦340nm) → N2+O( 1 D) (1) O2 + hν(≦175 nm) → O( 3 P)+O( 1 D) (4) O2 + hν(≦242 nm) → O( 3 P)+O( 3 P) ···(6) O( 1 D)+M → O( 3 P)+M (7) O2+O( 3 P) + M → O3 + 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 the ultraviolet light. The main part of equation (7) is shown as chemical reaction R7 in Figure 2. The main part of equation (8) is shown as chemical reaction R8 in Figure 2. As shown in Figure 2, the arrow (broken line) of chemical reaction R17, which is the reverse reaction of chemical reaction R8, indicates that O3 is converted to O2 and O( 3 There may also be a reaction in which O3 is decomposed into O3 (P). Chemical reaction R17 will be explained in detail later in the section "Lack of O3 due to high-temperature reaction fields."

[0049] Thus, we have demonstrated the direct decomposition of NO by the energy hν of ultraviolet light and the decomposition of O( 1 D) explained the indirect decomposition of N2O. Usually, both direct and indirect decomposition occur. The ratio of direct and indirect decomposition varies depending on the gas composition of gas G1.

[0050] NO produced by chemical reaction R2 in equation (2) is converted into O( 3 P) causes the reaction of equation (9) to produce nitrogen dioxide (hereinafter sometimes referred to as "NO2"). NO+O( 3 P) → NO2 (9) The main part of equation (9) is included in chemical reaction R9 in Figure 2.

[0051] 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 formulas (10) and (11). NO + OH → HNO2 (10) HNO2+OH → NO2+H2O ···(11) Equation (10) is shown as chemical reaction R10 in Figure 2. Equation (11) is shown as chemical reaction R11 in Figure 2.

[0052] OH in the reaction field is generated from water. That is, when gas G1 contains water (including water vapor or mist, hereinafter sometimes referred to as "H2O"), it is generated in decomposition reactor 2 according to formula (12). H2O+hν(≦242nm) → OH+H ···(12)

[0053] In addition, when ozone is present in the reaction field, NO2 is produced from NO via reaction (13). NO+O3+M → NO2+O2+M ···(13) The main part of equation (13) is included in chemical reaction R9 in Figure 2.

[0054] NO2 reacts with O( 3 When ozone is present in the reaction field, NO2 reacts with ozone to produce NO3 via the reaction shown in formula (15). NO2+O( 3 P) + M → NO3 + M (14) NO2+O3+M → NO3+O2+M ···(15) The main part of the reactions of equations (14) and (15) is shown as chemical reaction R12 in FIG.

[0055] The NO3 produced by chemical reaction R12 and the NO2 present in the reaction field undergo the reaction shown in equation (16), producing N2O5. NO3 + NO2 → N2O5 (16) The reaction of equation (16) is shown as chemical reaction R13 in Figure 2.

[0056] In addition, NO3 produced by chemical reaction R12 reacts with NO or O( 3 P), resulting in reaction (17) or (18), which may return to NO2. NO3 + NO → NO2 + NO2 (17) NO3+O( 3 P) → NO2 + O2 (18) The reactions of equations (17) and (18) are shown as chemical reaction R14 in FIG.

[0057] N2O5, together with water vapor or water mist present in the reaction field, undergoes the reaction of equation (19) to produce HNO3. N2O5 + H2O → 2HNO3 (19) The reaction of formula (19) is shown as chemical reaction R15 in Figure 2. Note that the reaction of formula (19) can also be induced by contacting N2O5 with water, for example by bubbling a gas containing N2O5 in water.

[0058] When OH is present in the reaction field, NO2 may reach nitric acid without going through chemical reactions R12, R13, and R15, as shown in formula (20). NO2 + OH → HNO3 (20) The reaction of equation (20) is shown as chemical reaction R16 in Figure 2.

[0059] The series of reactions described above that convert N2O to nitric acid assumes that there is sufficient O3 present in the reaction field. If there is not enough O3 present in the reaction field, nitrification 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.

[0060] [O3 shortage due to NOx cycle] The NOx cycle reaction will now be explained. NO is converted to NO2 according to the formulas (9) and (13) shown below. On the other hand, NO2 generates NO by reacting with oxygen atoms O according to the formula (21) or (22) below, or by being photolyzed according to the formula (23). NO+O( 3 P) → NO2 (9) NO+O3+M → NO2+O2+M ···(13) NO2+O( 1 D) → NO + O2 (21) NO2+O( 3 P) → NO+O2 (22) NO2 + hν(≦398 nm) → NO + O( 3 P) ···(23)

[0061] The reaction shown in the previous paragraph to produce NO2 from NO and the reaction to produce NO from NO2 can be repeated. This is called the NOx cycle reaction. O3 is consumed continuously during the NOx cycle reaction process. When the amount of O3 in the reaction field decreases due to the consumption of O3, the reactions of equations (13) and (15) begin to slow down. This means that the chemical reactions of R9 and R12 become more difficult to occur, and NO and NO2 remain in the reaction field.

[0062] [O3 shortage due to high temperature reaction field] The occurrence of the NOx cycle reaction is not the only cause of the shortage of O3. The inventors have noticed that if the temperature inside the decomposition reactor that decomposes N2O is high, the reaction stops midway through the complex reaction pathway that leads to nitric acid. In more detail, if the temperature of the reaction field is high, O3 is converted into O2 and O( 3 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), so if O3 decreases, NO does not reach HNO3, resulting in an increase in NOx.

[0063] [The significance of using CO2] The inventors have found that using CO2 is an effective way to deal with the residual NOx caused by the decrease in O3 in the reaction field. CO2 is produced by the reaction of CO and oxygen atoms (O( 3 P) or O( 1 D)) to generate. CO2 + hν(≦166 nm) → CO+O( 1 D) (24) CO2 + hν(≦227 nm) → CO+O( 3 P) ···(25)

[0064] The CO produced by the reactions (24) and (25) undergoes the reaction (26) in the presence of a three-way catalyst. 2NO + 2CO → N2 + 2CO2 (26)

[0065] Reaction (26) is a reaction that reduces NO to nitrogen. Reaction (26) converts the NO remaining in the reaction field into nitrogen gas. Furthermore, when NO decreases, the reactions of equations (9) and (13) decrease, and NO2 also decreases. Furthermore, when NO decreases, the NOx cycle reaction also decreases, and the shortage of O3 is alleviated.

[0066] Incidentally, the reaction in equation (26) occurs more easily the higher the temperature of the reaction field. This combines well with the phenomenon that the higher the temperature of the reaction field, the more O3 is reduced, resulting in a synergistic effect. In other words, since O3 is less likely to be reduced in a relatively low-temperature reaction field, the "N2O decomposition mechanism" described above promotes the production of nitric acid. In contrast, in a relatively high-temperature reaction field, O3 is more likely to be reduced, so nitric acid is not produced and the amount of residual NOx increases. However, the reduction of NO by CO, as shown in equation (26), occurs, reducing the amount of residual NOx. In other words, in a reaction field containing a large amount of CO2, NOx can be reduced regardless of the temperature.

[0067] Since the reactions of equations (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.

[0068] In equations (24) and (25), CO2 is converted to an oxygen atom (O( 1 D) or O( 3 P)) is also produced, but oxygen atoms (O( 3 Oxygen atoms (O( P)) also contribute to the production of ozone through chemical reactions R7 and R8 (see equations (7) and (8)), to the production of NO2 through chemical reaction R9 (see equation (9)), and to the production of NO3 through chemical reaction R12 (see equation (14)). 1 D)) also contributes to the decomposition of N2O through chemical reactions R2 and R3 (see equations (2) and (3)). Therefore, the significance of using CO2 is not only to reduce NOx by utilizing the reducing action of CO produced from CO2, but also to promote nitration by utilizing the oxygen atoms produced from CO2.

[0069] [light source] The light source 1 of this embodiment preferably emits light L1 whose main emission wavelength is in the range of 160 nm or more and less than 200 nm. FIG. 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 FIG. 3, curve C1 represents the absorption cross section of N2O, and curve C2 represents the absorption cross section of CO2.

[0070] 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 be 200 nm or less. Curve C1 shows that light is absorbed even at wavelengths above 200 nm, and NO 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 NO 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.

[0071] The light source 1 of this embodiment uses a xenon excimer lamp that emits excimer light with a peak or main emission wavelength of 172 nm or near 172 nm. When the wavelength is 172 nm, the light absorption of curve C1 (NO) is higher than the light absorption of curve C2 (CO) by ΔA. In Figure 3, ΔA is the difference between the light absorption of curve C1 and the light absorption of curve C2, which is approximately 10 times the light absorption of curve C2. This is a preferable wavelength of light that can sufficiently decompose NO while decomposing a large amount of CO.

[0072] 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 a wavelength λi in a 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 a specific wavelength, 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.

[0073] As shown in FIG. 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-saving effect. The shape of the arc tube is not limited to a cylindrical shape, and light source 1 is not limited to a xenon excimer lamp but may be, for example, a low-pressure mercury lamp. Light source 1 may also be an excimer lamp in which a gas other than xenon is sealed. Light source 1 may also be a solid-state light source such as an LED or LD.

[0074] [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 NO and CO. 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).

[0075] 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)

[0076] In addition, water reacts with O( 1 D) produces hydroxyl radicals. H2O+O( 1 D) → 2OH (28)

[0077] 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 → HNO2 (10) HNO2+OH → NO2+H2O ···(11) NO2 + OH → HNO3 (20)

[0078] As shown in Figure 2, R10, R11, and R16 merely form additional reaction pathways for producing HNO from N2O, and do not constitute essential reaction pathways such that nitration would not be possible without these reaction pathways. The fact that they form additional reaction pathways indicates that gas G1 does not necessarily have to contain OH, and thus HO.

[0079] In chemical reaction R15, as shown in formula (19), H2O is essential to generate HNO3 from N2O5, but as mentioned above, this H2O can be added by submerging a gas containing N2O5 in water using the bubbling method. Chemical reaction R15 can be achieved even if the gas G1 supplied from the gas supply port 3i does not contain H2O. Formula (19) is shown again. N2O5+H2O → HNO3+HNO3···(19)

[0080] A case where the gas G1 contains oxygen gas (O2) will be described. As described above, the gas G1 itself may contain O2. By making the gas G1 supplied from the gas supply port 3i contain O2, more O ( 1 D) and O( 3 P) is generated. 3 P) through chemical reaction R8, increasing the amount of O3 produced. As mentioned above, O( 3 P) and O3 are important species for the nitrification of N2O.

[0081] A case where the gas G1 contains nitrogen gas (N2) 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 hinder 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.

[0082] As can be seen from the fact that the gas G1 may contain water vapor, oxygen gas, and nitrogen gas, the gas G1 supplied from the gas supply port 3i may contain air. This air may be CDA (Clean Dry Air) or atmospheric air containing water vapor.

[0083] 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 O( 1 D), O( 3 The saturated hydrocarbon may be an alkane. The number of carbon atoms in the alkane may be 10 or less, 6 or less, or 4 or less. The saturated hydrocarbon may be methane. Since methane, like NO, is a greenhouse gas, being able to decompose methane and NO simultaneously is preferable from the standpoint of environmental conservation. The gas G1 may contain multiple saturated hydrocarbons.

[0084] [Nitric acid treatment method] FIG. 4 shows an example of a method for treating nitric acid. In FIG. 4, gas G2 containing nitric acid generated in a gas decomposition device 10 and discharged from a decomposition reactor 2 passes through an exhaust pipe 11 connected to a gas exhaust port 3o and comes into contact with water W1 in a container 12. As a result, the nitric acid contained in the gas G2 dissolves in the water W1 to form an aqueous nitric acid solution, and as a result, the nitric acid can be trapped. 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. Also, if the concentration of the aqueous nitric acid solution is low enough to be discharged, it may be discharged into a sewer without being recovered. The nitric acid discharged into the sewer is biodegraded to NO3 - It is eventually converted into N2 through this process. Figure 4 shows a method of recovering it by dissolving it in water W1, but it is also possible to recover it by simply cooling the gas G2 to liquefy the nitric acid. Since the boiling point of nitric acid is approximately 83°C, when the gas is cooled, the nitric acid turns from gas to liquid.

[0085] [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.

[0086] We will now explain the three-way catalyst. 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 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 → N2 + 2CO2 (29)

[0087] The higher the temperature of the reaction field, the more likely reaction (29) occurs. 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 produced from N2O. However, reaction (29) becomes more active, and the CO produced 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, and since it can be converted to CO2 by a three-way catalyst, the three-way catalyst of Gas G1 contributes to its detoxification.

[0088] On the other hand, when the temperature inside the decomposition reactor that decomposes NO is low, the nitration of NO by O can be promoted. As a result, whether the reaction site inside the decomposition reactor is high temperature or low temperature, the reaction can proceed to nitric acid or N2 without stopping with the NO generated from NO. As a result, NOx can be reduced.

[0089] Examples of materials that can be used for the three-way catalyst include rhodium, ruthenium, iridium, palladium, and platinum. It is also preferable to use a catalyst made of an iron-cobalt composite oxide or a tungsten-substituted vanadium oxide catalyst in which tungsten atoms are dispersed in vanadium oxide, because these catalysts exhibit catalytic performance even in a relatively low-temperature environment of around 150°C.

[0090] 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 gas G1. When gas G1 contains at least one of nitrogen oxides and carbon monoxide, the two-way catalyst promotes the oxidation of at least one of nitrogen oxides and carbon monoxide using oxygen in gas G1. When light L1 is used to produce NO from N2O and CO from CO2, the two-way catalyst promotes the reactions of equations (30) and (31). 2CO+O2 → 2CO2 (30) 2NO + O2 → 2NO2 (31)

[0091] Regarding nitrogen oxides, if NO can be converted to NO2 using 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.

[0092] Possible materials for the binary catalyst include, for example, iron, platinum, and platinum.

[0093] [How to use the gas decomposition device] The 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. Carbon dioxide is also emitted in the same way. 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 burns carbon and hydrocarbons with oxygen contained in air. Therefore, it is desirable to adjust the concentration of carbon dioxide contained in 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.

[0094] The wavelength, light intensity, and irradiation time of the light may be selected to provide the gas to be treated with energy for exciting the gas, thereby promoting the decomposition of NO. The gas decomposition device 10 may include a gas concentration adjusting unit connected to the gas supply port 3i, which adjusts the amounts of each gas component contained in the gas G1 so that the gas concentration of CO2 contained in the gas G1 is higher than the concentrations of all gases contained in the gas G1 except for CO2.

[0095] 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.

[0096] 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.

[0097] 5B is a cross-sectional view taken along line S2-S2 in FIG. 5A. The catalytic contactor 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.

[0098] FIG. 5C is an enlarged view of region P1 in FIG. 5B. Ceramic walls 23 are arranged in a lattice pattern. A space 24 is located in the center of each cell 27, and the space 24 functions as a passageway for gas G2. A catalyst 8 is located outside the space 24 and inside the ceramic wall 23. The above-mentioned three-way catalyst and two-way catalyst can be used as the catalyst 8. 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, and when a two-way catalyst is used, it oxidizes CO and converts it into CO2.

[0099] In this embodiment, in order to increase the contact area between the catalyst 8 and the gas G2, the catalyst contact portion 25 is disposed in the widened portion of the pipe 29, and the catalyst 8 is installed on the wall surfaces of the numerous cells 27. However, the catalyst contact portion 25 may not have numerous cells 27. Furthermore, even when the catalyst contact portion 25 having numerous cells 27 is employed, the cross-sectional shape of the ceramic walls (cell shape) may have any shape. For example, in FIG. 5C, the ceramic walls 23 are arranged in a rectangular lattice pattern, but the ceramic walls 23 may be arranged to form a honeycomb structure.

[0100] [Transformation] 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 unit 25. An oxygen gas supply pipe 41 is connected between the gas decomposition device 20 and the catalytic contact unit 25 shown in FIG. 6, and oxygen gas is supplied upstream of the catalytic contact unit 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, supplying oxygen gas from the oxygen gas supply pipe 41 promotes the oxidation of CO and its conversion to CO2 in the catalytic contact unit 25. This reduces NOx and improves safety for humans and animals.

[0101] The second embodiment and its modified example have been described above. In the above description, instead of providing the catalyst 8 inside the gas decomposition apparatus 20, the catalyst 8 is arranged inside the piping 29 downstream of the gas decomposition apparatus 20. However, it is also possible to arrange the catalyst 8 inside the gas decomposition apparatus 20 as in the first embodiment, and also arrange the catalyst 8 inside the piping 29 downstream of the gas decomposition apparatus 20 as in the second embodiment.

[0102] Third Embodiment A third embodiment of the gas decomposition apparatus will now be described. The energy source in the gas decomposition apparatus 30 shown in Fig. 7A as the third embodiment is an energy source that provides electronic energy for exciting gas contained in the gas to be treated to generate plasma, and the electronic energy is provided by applying a high-frequency voltage between electrodes that sandwich the gas to be treated.

[0103] As shown in FIG. 7A, the closed cylindrical tubular body 33 has two openings, which correspond to a gas supply port 3i and a gas exhaust port 3o. The gas supply port 3i is an opening for introducing gas G1, which is a gas to be treated, into the inside of the tubular body 33. The gas exhaust port 3o is an opening for discharging treated gas G2. The gas exhaust port 3o is located at a position separated 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.

[0104] The two electrodes (35a, 35b) of the gas decomposition apparatus 30 will be described. The outer electrode 35a is a mesh-like electrode provided along the outer wall surface of the tubular body 33. The inner electrode 35b is a rod-like electrode that extends linearly inside the tubular body 33 along the axial direction d1 of the tubular body 33. The inner electrode 35b is disposed from the outside to the inside of the tubular body 33, penetrating the tubular body 33. Both electrodes (35a, 35b) are electrically connected to a power source 6.

[0105] Fig. 7B is a cross-sectional view taken along the line S3-S3 in Fig. 7A. As shown in Fig. 7B, a space SP1 is formed inside the tubular body 33 located between the two electrodes (35a, 35b). When a voltage is applied between the two electrodes (35a, 35b), a dielectric barrier discharge occurs inside the tubular body 33, and an atmospheric pressure plasma space is formed in the space SP1.

[0106] The applied voltage supplied from the power supply 6 may be in a range that allows a dielectric barrier discharge to occur 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. The frequency of the applied voltage supplied from the power supply 6 is preferably in the range of 1 kHz or more and 1000 kHz or less, and more preferably in the range of 1 kHz or more and 150 kHz or less. The reason why the upper limit is preferably 150 kHz is that the frequency detected in the noise terminal voltage in 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).

[0107] Preferably, the power supply 6 applies a voltage such that the outer electrode 35a is at ground voltage and the inner electrode 35b is at high voltage, thereby reducing the risk of electric shock caused by the electrode exposed to the outside being at high voltage.

[0108] 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. NO 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.

[0109] The tube 33 is made of a dielectric material such as quartz glass or ceramics. The electrodes (5a, 5b) are made of a metal material such as stainless steel, aluminum, copper, tungsten, or nickel. While the tube 33 is cylindrical, it is not limited to a cylindrical shape. The tube 33 may be rectangular or, in particular, flat. While the outer electrode 35a is a mesh-like electrode provided along the outer wall surface of the tube 33, it is not limited to this shape. 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 only partially cover the wall surface of the tube 33. Similarly, the shape of the inner electrode 35b is not limited to the above-described shape.

[0110] [Transformation] A modified version of the third embodiment will be described. Gas decomposition apparatus 40 shown in FIG. 8A has tubular body 3 with 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, tubular body 33 includes outer tube 33a, which has a cylindrical shape and is located on the outside, and inner tube 33b, which is arranged coaxially with outer tube 33a inside outer tube 33a and has a cylindrical shape with a smaller inner diameter than outer tube 33a. The end of inner tube 33b may be open to allow the same gas as the atmosphere in which gas decomposition apparatus 40 is placed to flow into inner tube 33b, or the end of inner tube 33b may be sealed to allow a gas different from the atmosphere to flow into inner tube 33b.

[0111] 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 outside the outer tube 33a. A space SP1 having an annular shape (here, a circular ring shape) when viewed from the tube axis direction d1 is formed between the outer tube 3a and the inner tube 3b. The gas supply port 3i and the gas discharge port 3o communicate with the space SP1 located outside the inner tube 33b. That is, the gas G1 flows into the space SP1 through the gas supply port 3i.

[0112] <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.

[0113] 9A and 9B, gas decomposition apparatus 50 has a double-pipe structure in which inner pipe 54 is disposed inside outer pipe 53. Inside inner pipe 54 is gas flow path 52 through which gas G1, which is the gas to be treated, flows along the direction in which the double pipe extends.

[0114] An outer electrode 55a is disposed outside the outer wall of the outer tube 53, and an inner electrode 55b is disposed inside the inner wall of the inner tube 54. The inner electrode 55b and the outer electrode 55a may be mesh-shaped. 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. By applying a voltage between the outer electrode 55a and the inner electrode 55b, the space 58 becomes a discharge space, generating light L1 which is radiated into the gas G1 flowing through the gas flow path 52 (see FIG. 9A).

[0115] 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 constituting gas G1 are excited by the plasma formed by ultraviolet light and high-frequency voltage.

[0116] 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 outlet 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 discharged from the gas outlet 3o. This allows for continuous decomposition of NO and CO in the gas G1.

[0117] Outer tube 53 is made of, for example, quartz. In gas decomposition device 50, a reflective film that reflects light L1 may be formed on the inner wall surface of outer tube 53. The light-emitting gas is emitted toward the outside of outer tube 53, and if a reflective film is formed on the inner wall surface of outer tube 53, light L1 that would otherwise be directed toward the outside of outer tube 53 is reflected back inside, increasing the light intensity in inner gas flow path 52.

[0118] [First transformation form] 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. The gas decomposition apparatus 60 has multiple gas flow paths. The first is a gas flow path 52 formed within the inner tube 54, as in the gas decomposition apparatus 50. The second 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.

[0119] 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 gas molecules contained in the gas G1 flowing through the outer gas flow passage 72. In this way, the gas molecules flowing through each gas flow passage (52, 72) are excited by the ultraviolet light and high-frequency voltage. Since the gas G1 can be treated in each gas flow passage (52, 72), a large amount of gas can be treated, improving the utilization efficiency of the light L1 and the atmospheric pressure plasma AP.

[0120] [Second transformation form] 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 efficiency of use of light L1.

[0121] 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.

[0122] The above describes various embodiments of the gas decomposition method and gas decomposition apparatus, as well as appropriate variations thereof. The above-described embodiments and variations thereof are merely examples of the present invention, and the present invention is not limited to the above-described embodiments. Various changes or modifications 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. [Explanation of symbols]

[0123] 1:Light source 2: Decomposition reactor 3: Body 3a: Outer tube 3b: Inner tube 3i, 4i: Gas supply port 3o, 4o: Gas outlet 5: Control section 5a,5b: Electrode 6: Power supply 8: Catalyst 10, 20, 30, 40, 50, 60, 70: Gas decomposition device 11: Discharge pipe 12: Container 21: Catalytic unit 23: Wall 24: Space 25: Catalyst contact part 29: Piping 31: Gas supply port 32: Gas outlet 33: Body 33a,53,71:Outer tube 33b,54: Inner tube 35a, 55a: Outer electrode 35b,55b: Inner electrode 41: Oxygen-containing gas supply piping 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

1. 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.

2. 2. The decomposition method according to claim 1, wherein the gas to be treated further contains water vapor.

3. 2. The decomposition method according to claim 1, wherein the energy is light energy having a main emission wavelength in the range of 160 nm to less than 200 nm.

4. 2. The decomposition method according to claim 1, wherein the energy is electron energy obtained by converting the gas to be treated into plasma.

5. 5. The decomposition method according to claim 1, wherein the gas to be treated is brought into contact with a catalyst.

6. 6. The decomposition method according to claim 5, wherein the catalyst is a three-way catalyst used to promote the reduction of nitric oxide produced by the decomposition of dinitrogen monoxide.

7. 6. The decomposition method according to claim 5, wherein the catalyst is a two-way catalyst used to promote oxidation of at least one of nitric oxide produced by decomposition of the dinitrogen monoxide and carbon monoxide produced from the carbon dioxide.

8. 8. The decomposition method according to claim 7, further comprising the step of: supplying an oxygen-containing gas to the gas to be treated after the energy has been applied.

9. 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:

10. The decomposition apparatus according to claim 9, further comprising a gas concentration adjusting unit connected to the gas supply port, which 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 any other gases contained in the gas to be treated except for carbon dioxide.

11. 10. The decomposition device according to claim 9, wherein the energy is light energy having a main emission wavelength in the range of 160 nm to less than 200 nm, and the energy source is a light source that radiates the light.

12. 10. The decomposition apparatus according to claim 9, wherein the energy is electron energy obtained by converting the gas to be treated into plasma, and the energy source is an electrode that supplies the electron energy.

13. The decomposition device according to any one of claims 9 to 12, characterized in that the decomposition device is provided with a three-way catalyst that promotes reduction of nitric oxide produced by decomposition of the dinitrogen monoxide.

14. The decomposition device according to any one of claims 9 to 12, characterized in that the decomposition device comprises a two-way catalyst used to promote oxidation of at least one of nitric oxide produced by decomposition of the dinitrogen monoxide and carbon monoxide produced from the carbon dioxide.

15. 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.

16. 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.

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