Photolysis method and system for carbon dioxide

Abstract

To provide a photolysis method of carbon dioxide that can decompose more carbon dioxide with less energy input.SOLUTION: A photolysis method of carbon dioxide includes at least one of the following methods: a method comprising a step of reducing water from a gas containing the carbon dioxide and water, and a step of irradiating the gas containing the reduced water with excimer light; a method of irradiating a gas containing carbon dioxide and 5% to 20% of oxygen with excimer light; and a method of irradiating a gas containing carbon dioxide and 0.4% to 4.0% of hydrogen with excimer light.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and system for photolysis of carbon dioxide. [Background technology]

[0002] In recent years, the increase in the amount of fossil fuels burned, such as oil and coal, has led to an increase in the concentration of carbon dioxide (hereinafter referred to as "CO2") in the atmosphere, accelerating global warming. Therefore, reducing CO2 emissions is an urgent issue, especially in industrialized countries.

[0003] One method for reducing CO2 emissions is to convert CO2 into an organic substance and chemically fix it. Patent Document 1 below discloses a method in which excimer light emitted from a light source that uses a dielectric barrier discharge is irradiated onto a gas containing CO2, photodecomposing the CO2 to carbon monoxide (hereinafter sometimes referred to as "CO"), and then reacting this CO with sodium hydroxide to fix it into sodium formate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-011407 Summary of the Invention [Problem to be solved by the invention]

[0005] In the technology of photodecomposing CO2 by irradiating it with excimer light, the important factor is the photodecomposition efficiency of CO2. If the photodecomposition efficiency of CO2 is improved, more CO2 can be decomposed with less input energy.

[0006] The present invention provides a method and system for photodecomposing CO2 that can decompose a larger amount of CO2 with less input energy. [Means for solving the problem]

[0007] The photodecomposition method of CO2 to solve the above problems is as follows: (1) A method comprising the steps of reducing water from a gas containing CO2 and water, and irradiating the water-reduced gas with excimer light; (2) A method of irradiating a gas containing CO2 and 5% or more and 25% or less of oxygen (hereinafter sometimes referred to as "O2") with excimer light; and (3) A method of irradiating a gas containing CO2 and 0.4% or more and 4.0% or less of hydrogen (hereinafter sometimes referred to as "H2") with excimer light; The present invention includes at least one of the following methods.

[0008] The method (1) above, which includes the steps of reducing the amount of water from a gas containing CO2 and water and irradiating the water-reduced gas with excimer light, will be described in detail below. However, the inventors have discovered that if the gas to be photolyzed contains water, the water will hinder the photolysis of CO2. Therefore, they have devised a method of reducing the amount of water contained in the gas before photolysis. This increases the efficiency of photolysis of CO2.

[0009] The method of irradiating excimer light onto a gas containing CO and 5% or more and 25% or less of O as described in (2) above will be described in detail later. The excimer light photodecomposes a large amount of O to produce excited atomic oxygen, O( 1 D) to increase O( 1 D) reacts with CO2, increasing the rate of CO2 production. In other words, the photodecomposition efficiency of CO2 is increased. Furthermore, the gas obtained by reducing the water content of a gas containing CO2 and water may contain 5% or more and 25% or less O2. Note that this oxygen concentration of 25% is close to the oxygen concentration in air, but the gas to be photodecomposed may have a component ratio close to that of atmospheric air. However, if the gas contains more CO2 than atmospheric air, the photodecomposition efficiency will be higher. The CO2 concentration of the gas to be photodecomposed may be, for example, 1% or more, more preferably 5% or more, and even more preferably 10% or more.

[0010] The method of irradiating excimer light onto a gas containing CO2 and 0.4% or more and 4.0% or less H2 described in (3) above will be described in detail later. However, H2 combines with OH in preference to CO combining with OH originating from water. Therefore, the reverse reaction of CO and OH combining to return to CO2 can be suppressed. Suppressing the reverse reaction leads to an improvement in the photodecomposition efficiency of CO2.

[0011] The water in "OH originating from water" is contained in the supplied CO2 gas itself. Furthermore, even if the supplied CO2 gas itself does not contain water, water vapor contained in the atmosphere may enter from the outside and mix with the CO2 gas, resulting in CO2 gas containing trace amounts of water unintentionally (details will be described later). Therefore, even if the supplied CO2 gas itself does not contain water, H2 may substantially contribute to improving the photodecomposition efficiency of CO2 by suppressing the above-mentioned reverse reaction.

[0012] If the supplied CO2 gas itself contains a large amount of water, after the water reduction process, the water-reduced gas containing 0.4% to 4.0% H2 can be photodecomposed with excimer light. In addition, by setting the upper limit of the hydrogen concentration to 4.0%, the risk of explosion due to the oxidation reaction of hydrogen can be reduced.

[0013] The temperature of the gas in the atmosphere irradiated with the excimer light may be 400K or less. As will be described in detail later, when the gas temperature drops, the generation rate of ozone (hereinafter sometimes referred to as "O3") increases and the thermal decomposition of O3 is suppressed. In this case, the atomic oxygen in the ground state, O( 3 P) production is suppressed. As a result, O( 3 This suppresses the reaction in which CO recombines with P and CO, turning CO back into CO2. Suppressing the reverse reaction leads to improved efficiency in CO2 photodecomposition.

[0014] The main wavelength of the excimer light may be 172 nm or close to 172 nm. CO2 molecules absorb light with a main wavelength close to 172 nm well, so excimer light with a main wavelength close to 172 nm can efficiently decompose CO2. Furthermore, excimer light with a main wavelength close to 172 nm can be obtained by turning on a xenon excimer lamp. Xenon excimer lamps are light sources that can be mass-produced stably, so light with a main wavelength close to 172 nm is highly cost-effective.

[0015] In this specification, "near 172 nm" refers to a region within the range of 172 nm ± 5 nm. In this specification, "main 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 (main peak wavelength) can usually be considered to be the main wavelength.

[0016] The concentration of water contained in the gas after the water reduction may be 100 ppm or less.

[0017] The water contained in the gas may be reduced by adsorbing the water onto zeolite.

[0018] The gas may be irradiated with the excimer light while being sent to the space irradiated with the excimer light, thereby enabling photodecomposition of a large amount of CO2.

[0019] The CO2 photolysis system to solve the above problems is: (4) a device for reducing water from a gas containing carbon dioxide and water before photolysis; (5) a source of oxygen for supplying oxygen to the carbon dioxide-containing gas before photolysis so that the oxygen concentration is 5% or more and 25% or less; (6) a supply source for supplying hydrogen to the gas containing carbon dioxide before photolysis so that the hydrogen concentration is 0.4% or more and 4.0% or less; At least one of the following is provided.

[0020] Specifically, the CO2 photolysis system equipped with (4) above includes: A first treatment chamber including an inlet for introducing the gas containing carbon dioxide and water, a water removal device for reducing water from the gas, and an outlet for discharging the gas from the water removal device; a second processing chamber connected to the exhaust port of the first processing chamber and into which the gas exhausted from the exhaust port is introduced; an excimer lamp that irradiates the gas in the second processing chamber with excimer light, In the second processing chamber, the excimer light photodecomposes the carbon dioxide contained in the gas present therein.

[0021] The CO2 photolysis system further comprises: a hydrogen source upstream of the second treatment chamber for supplying hydrogen to the gas; a hydrogen regulating valve for regulating the amount of hydrogen supplied; and a control unit that controls the hydrogen regulating valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less. In other words, the CO2 photolysis system having the above (4) may be configured in combination with the above element (6).

[0022] The CO2 photolysis system further includes an oxygen source upstream of the second treatment chamber for supplying oxygen to the gas. an oxygen regulating valve for regulating the amount of oxygen supplied; and a control unit that controls the oxygen regulating valve so that the oxygen concentration in the gas is 5% or more and 25% or less. In other words, the CO2 photolysis system having the above (4) may be configured in combination with the above element (5).

[0023] Specifically, the CO2 photolysis system equipped with (5) above includes: an inlet for introducing the gas containing carbon dioxide; an excimer lamp that emits excimer light; a processing chamber connected to the inlet, in which the excimer light photodecomposes the carbon dioxide present therein; an oxygen source upstream of the processing chamber that supplies oxygen to the gas; an oxygen regulating valve for regulating the amount of oxygen supplied; and a control unit that controls the oxygen regulating valve so that the oxygen concentration in the gas is 5% or more and 25% or less.

[0024] Specifically, the CO2 photolysis system equipped with (6) above includes: an inlet for introducing the gas containing carbon dioxide; an excimer lamp that emits excimer light; a processing chamber connected to the inlet, in which the excimer light photodecomposes the carbon dioxide present therein; a hydrogen source upstream from the processing chamber that supplies hydrogen to the gas; a hydrogen regulating valve for regulating the amount of hydrogen supplied; and a control unit that controls the hydrogen regulating valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less.

[0025] The excimer lamp may be disposed inside the processing chamber or the second processing chamber where the carbon dioxide is photodecomposed.

[0026] The photolysis system includes a cooler disposed upstream of the treatment chamber or the second treatment chamber in which the carbon dioxide is photolyzed, the cooler cooling the gas; a temperature sensor for measuring the temperature of the gas cooled by the cooler; The cooling device may further include a control unit that controls the cooler so that the measured value of the temperature sensor is 400K or less.

[0027] The excimer lamp may be such that the light-emitting gas sealed in the light-emitting tube is xenon gas.

[0028] The water removal device may be equipped with zeolite. [Effects of the Invention]

[0029] This makes it possible to provide a method and system for photodecomposing carbon dioxide that can decompose a larger amount of carbon dioxide with a smaller input of energy.

[0030] Providing a carbon dioxide photolysis method and photolysis system that can decompose more carbon dioxide with less input energy will make a significant contribution to achieving Goal 13 of the United Nations' Sustainable Development Goals (SDGs), which is to "take urgent action to combat climate change and its impacts." [Brief explanation of the drawings]

[0031] [Figure 1] FIG. 1 shows a first embodiment of a carbon dioxide photolysis system. [Figure 2] 1 is a graph showing the relationship between the water content in gas and the CO production rate. [Figure 3] 1 is a graph showing the relationship between the hydrogen concentration in gas and the CO production rate. [Figure 4] 1 is a graph showing the relationship between gas temperature and CO production rate. [Figure 5] 1 is a variation of the second treatment chamber of the photolysis system. [Figure 6] FIG. 1 shows a second embodiment of a carbon dioxide photolysis system. [Figure 7] FIG. 10 shows a third embodiment of a carbon dioxide photolysis system. [Figure 8] 1 is a graph showing the relationship between the oxygen concentration in gas and the CO production rate. [Figure 9] This is a flow diagram from CO2 capture to fixation. DETAILED DESCRIPTION OF THE INVENTION

[0032] The embodiments will be described with reference to the drawings as appropriate. Note that all drawings, excluding graphs and flow charts, 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.

[0033] First Embodiment [Photolysis system overview] 1 is a diagram showing a first embodiment of a CO2 photolysis system. The photolysis system 10 has a first treatment chamber 1 and a second treatment chamber 2 connected downstream of the first treatment chamber 1. The photolysis system 10 has a function of decomposing carbon dioxide contained in gas G1.

[0034] The first treatment chamber 1 is a treatment chamber that reduces the amount of water contained in the gas G1. The gas G1 is a gas containing CO2 and water. The first treatment chamber 1 includes an inlet 4i for introducing the gas G1, a water remover 3, and an outlet 4o for discharging the gas from the water remover. The water remover 3 removes at least a portion of the water contained in the gas G1. As a result, the gas G1 becomes a dry gas with a reduced amount of water. In FIG. 1, different symbols are used to distinguish the gas treated by the water remover 3 from the gas before treatment. That is, the gas before treatment by the water remover 3 is referred to as "gas G1," and the gas after treatment by the water remover 3 is referred to as "gas G2." Furthermore, as described below, the gas after treatment in the second treatment chamber 2 is referred to as "gas G3" to distinguish it from the gas G2. Specific embodiments of the water remover 3 will be described later.

[0035] In this specification, the concept of water to be removed includes gaseous and atomized liquid water. In other words, the gas G1 before being treated by the water removal device 3 may contain, for example, atomized liquid water in addition to water vapor. The gas G2 after being treated by the water removal device 3 contains only water vapor or does not contain any water vapor or liquid water.

[0036] The carbon dioxide content of gas G1 is not particularly limited, but is preferably 50% or more, more preferably 70% or more, and even more preferably 80% or more. In this specification, when the percentage of a fluid is expressed using "%," it means "vol%." Furthermore, gases other than carbon dioxide contained in gas G1 are preferably gases that are inert to the light irradiated onto the gas, except for gases of the same type as additional gas G4 or additional gas G5 described below. For example, when the light irradiated onto the gas has a wavelength of around 172 nm, nitrogen gas is an example of an inert gas.

[0037] The second treatment chamber 2 is connected to the exhaust port 4o of the first treatment chamber 1 via a connecting pipe 7. In other words, the second treatment chamber 2 is connected to the rear of the first treatment chamber 1. A gas G2, which is obtained by reducing the water content of the gas G1 in the first treatment chamber 1, is supplied to the second treatment chamber 2. The second treatment chamber 2 is a treatment chamber for photolyzing the gas G2. In this embodiment, an excimer lamp 5 is disposed inside the second treatment chamber 2, and when the excimer lamp 5 is turned on, the gas G2 in the second treatment chamber 2 is irradiated with excimer light L1. As a result, CO2 contained in the gas G2 is photolyzed to generate CO. Details of the excimer lamp 5 will be described later. The processed gas G3, which contains CO2 generated in the second treatment chamber 2, is discharged from the exhaust port 6 of the second treatment chamber 2. Photolysis can be continuously performed by irradiating the gas G2 with the excimer light L1 while sending the gas G2 to a space irradiated with the excimer light L1 (i.e., the second treatment chamber 2).

[0038] Returning to FIG. 1 , in the photolysis system 10 of this embodiment, a supply pipe 8 for supplying additional gas G4 is connected to a connecting pipe 7 that connects the first treatment chamber 1 and the second treatment chamber 2. A supply source 11 of the additional gas G4 is connected to the supply pipe 8. A flow valve 12 and a flow meter (not shown) are disposed midway along the supply pipe 8. The flow valve 12 is controlled by a control unit 14. The additional gas G4 will be described later. Note that it is sufficient that the additional gas G4 is supplied upstream of the second treatment chamber 2, so the additional gas G4 may be supplied not only between the first treatment chamber 1 and the second treatment chamber 2, but also upstream of the first treatment chamber 1.

[0039] [CO generation mechanism] The photolysis system and photolysis method of the present invention photolyzes CO2 with excimer light to generate CO. The mechanism by which CO2 is generated from CO2 by photolysis will be described. When CO2 is irradiated with excimer light hν (for example, light with a main wavelength of 172 nm), CO2 absorbs the excimer light hν, dissociating the C=O bond of the CO2 molecule, and the following reaction occurs: CO2+hν → CO +O( 3 P) …(1) CO2+hν → CO +O( 1 D) …(2)

[0040] Equations (1) and (2) show that excimer light hν directly generates CO from CO2. The O( 3 P) represents the ground state atomic oxygen. O( 1 D) represents excited atomic oxygen.

[0041] Excited atomic oxygen, O( 1 D) is highly active. Therefore, O( 1 D) reacts with CO2 to produce CO. CO2+O( 1 D) → CO + O2…(3) Equation (3) shows that excimer light hν indirectly produces CO from CO2.

[0042] The ground state atomic oxygen, O( 3 P) may undergo the following reactions: CO+O( 3 P) + M → CO2 + M … (4) Equation (4) shows that even if CO2 is decomposed to produce CO according to equation (1), CO does not convert to O( 3 P) to form CO2, which is the reverse reaction. In equation (4), M represents a third body.

[0043] The rate at which equations (1) and (2) occur varies depending on the wavelength of the excimer light hν. The shorter the wavelength, the higher the light energy, so the reaction in equation (1) decreases and the reaction in equation (2) increases. Therefore, the shorter the wavelength, the faster the O( 1 D) tends to increase and CO2 decomposition progresses.

[0044] In addition, O( 1 D) and O( 3 In addition to being produced by equations (1) and (2), O2 is also produced from O2 produced by equation (3), or from HO, which will be described later. X It can also be produced in the process of a cyclic chain reaction.

[0045] [Effect of water on photodecomposition] As a result of extensive research into the photolysis of CO2, the inventors have discovered the relationship shown in Figure 2 between the water content in the gas for photolysis and the CO production rate by photolysis.

[0046] Figure 2 is a graph showing the relationship between the water content in a CO2-containing gas and the rate of CO2 generation by photolysis, as determined by simulation. The CO2 generation rate is the percentage of CO2 in the total gas mixture, expressed in vol%. The simulation conditions were set as follows: Gas G2 was a gas composed of CO2 and water vapor. The gas temperature was set to 400 K. For photolysis, a xenon excimer lamp emitting excimer light with a peak wavelength of 172 nm was used. The average distance between the xenon excimer lamp and the gas being photolyzed and the flow rate of Gas G2 were assumed to be constant throughout the simulation.

[0047] Figure 2 shows that the CO generation rate for gas that does not contain any water (water content of 0 ppm) is 8%, while the CO generation rate for gas that contains 6 ppm of water is only 3%. In other words, the CO generation rate decreased by 5% when the gas contained only 6 ppm of water.

[0048] Therefore, based on the results of the above verification, it can be understood that reducing the water content of the carbon dioxide-containing gas is likely to lead to an improvement in the photolysis efficiency. Furthermore, it was found that even if the supplied carbon dioxide-containing gas itself does not contain water, water vapor contained in the atmosphere can infiltrate through tiny gaps in the treatment chamber or the piping path leading to the treatment chamber. As a result, the infiltrated water vapor mixes with the carbon dioxide-containing gas, resulting in a carbon dioxide-containing gas containing a small amount of water. As mentioned above, even a small amount of water, less than 10 ppm, reduces the CO2 production rate. In other words, the risk of water interfering with the photolysis of CO2 exists even when the carbon dioxide-containing gas itself is unlikely to contain water.

[0049] According to the inventors' extensive research, it is believed that the phenomenon in which the photodecomposition of CO2 is hindered is generally caused by the following chemical reaction. CO + OH → CO2 + H … (5) Equation (5) shows that even if CO2 is decomposed to produce CO according to equation (3), OH reacts with CO, turning CO back into CO2. In other words, it is important to suppress the production of OH so that the reaction of equation (5) does not occur.

[0050] The OH in equation (5) originates from water (sometimes called "H2O"). However, it is thought that the majority of the OH in equation (5) is generated by a chain reaction, rather than simply OH generated via H2O → OH + H. The OH generated by a chain reaction is typically thought to be generated by the following four reaction formulas: O3+H → OH+O2…(6) HO2 + H → OH + OH …(7) HO2+O3→ OH+O2+O2…(8) H2O2+H → H2O+OH …(9)

[0051] Of these, the following contribute to the generation of H2O2 in equation (9): H2+HO2→ H2O2+H (10) It is thought that this is the case.

[0052] The factors that contribute to the generation of HO2 in equations (7), (8) and (10) are: HCO + O2 → CO + HO2 (11) It is thought that this is the case.

[0053] The main factors that contribute to the HCO production in equation (11) are: CO + H + M → HCO + M (12) In equation (12), M represents a third body.

[0054] In other words, when light energy is applied to H2O mixed with CO2, a large amount of OH is generated through the reaction shown in equations (6) to (12). The large amount of OH generated by the mechanism described above inhibits the photodecomposition of CO2 through the reaction shown in equation (5). Therefore, even if only a small amount of water, less than 10 ppm, is mixed in, it is thought that the photodecomposition of CO2 will be inhibited.

[0055] [Water removal device] As a result of the above analysis, the present inventors discovered a method for increasing the CO production rate by reducing the water contained in the gas present in the photolysis atmosphere.

[0056] In this embodiment, before photolysis of CO2, a water remover 3 is used to reduce water from gas G1 to generate gas G2. In this embodiment, the water remover 3 uses a synthetic zeolite with high water absorption properties (e.g., Union Carbide's "molecular sieve") to adsorb and reduce water. However, the type of zeolite is not particularly limited. Also, water-absorbing materials other than zeolite (e.g., silica gel, calcium chloride, or MOF (metal-organic framework)) may also be used. Furthermore, the water remover is not limited to a water-absorbing material, and for example, a water remover using a compressor may also be used. If gas G1 contains atomized water, the water may be reduced by using a filter that allows water vapor to pass through but prevents the passage of atomized water. Furthermore, the water remover 3 may be a combination of the above-mentioned multiple water removal mechanisms.

[0057] The water concentration in the gas G2 after the water reduction is preferably 100 ppm or less. When the water concentration is 100 ppm or less, the photodecomposition efficiency is high. The water concentration in the gas G2 can be detected using a moisture meter.

[0058] [Addition of hydrogen gas] The photolysis system 10 of this embodiment supplies hydrogen gas as the additional gas G4. The supply source 11 is a supply source of hydrogen gas. The significance of adding hydrogen gas to the gas G2 will be explained with reference to FIG.

[0059] Figure 3 is a graph showing the relationship between the hydrogen concentration in the gas and the rate of CO production by photolysis, obtained by simulation. In this specification, the CO production rate refers to the proportion of CO in the total amount of the mixed gas, and is expressed in "vol%". The simulation conditions were set as follows: The gas was a mixed gas containing N2 and CO2, with a CO2 ratio of 4% in the total gas, and 500 ppm of water vapor. The temperature of the mixed gas was set to 500 K. For the photolysis, a xenon excimer lamp emitting excimer light with a peak wavelength of 172 nm was used, with a light intensity of 30 mW / cm. 2 The gas was assumed to be irradiated with excimer light for 1000 seconds at 1000 s. The average distance between the xenon excimer lamp and the gas being photolyzed and the flow rate of gas G2 were assumed to be constant throughout the simulation.

[0060] As shown in Figure 3, when the hydrogen concentration is increased from 0% (0% hydrogen, hereafter sometimes referred to as "H2") to 0.2%, the CO production rate temporarily drops. This phenomenon is thought to be caused by the increase in the amount of OH according to equation (13) or (14) shown below, which converts CO back to CO2 according to the reaction of equation (5). H2+O( 3 P) → OH+H (13) H2+O( 1 D) → OH+H (14)

[0061] However, Figure 3 shows that when 0.4% or more hydrogen is added to the gas, the CO production rate increases with photolysis. This phenomenon is thought to be mainly due to H2 combining with OH, consuming OH, prior to the reaction of CO combining with OH in equation (5). The reaction of H2 combining with OH and consuming OH is shown below. H2+OH → H2O+H (15) As the reaction of formula (15) increases, OH decreases and reaction (5) is suppressed. In other words, the decrease in the CO2 decomposition efficiency is suppressed.

[0062] Therefore, it is preferable that the gas contains 0.4% or more hydrogen. Furthermore, since the lower explosion limit of hydrogen is 4.0%, it is preferable to set the hydrogen concentration to 4.0% or less to reduce the risk of explosion. The concentration of hydrogen contained in gas G2 can be detected, for example, using a commercially available portable hydrogen concentration meter.

[0063] [Gas temperature control] Returning to FIG. 1 , the photolysis system 10 of this embodiment includes a gas cooler 9 and a temperature sensor 13. In this embodiment, the gas cooler 9 is provided midway through the connecting pipe 7 and cools the gas G2 passing through the connecting pipe 7. The configuration of the gas cooler 9 is not particularly limited. The gas cooler 9 is, for example, configured as a heat exchanger that air-cools the gas by flowing the gas through a pipe built into a number of fins. The temperature sensor 13 is attached to the second treatment chamber 2 and measures the temperature of the gas G2 in the second treatment chamber 2. The control unit 14 is electrically connected to the temperature sensor 13 and the gas cooler 9. The control unit 14 cools the gas G2 using the gas cooler 9 according to the measurement result of the temperature sensor 13. In this embodiment, the temperature sensor 13 measures the temperature in the second treatment chamber 2, but it may also measure the temperature of the gas G2 in the connecting pipe 7 or the temperature of the gas G1 upstream of the water removal device 3. Furthermore, the additional gas G4 may be cooled in advance before being mixed with the gas G2, thereby cooling the gas after being mixed with the additional gas G4.

[0064] Figure 4 is a graph showing the relationship between the temperature of a CO2-containing gas and the proportion of CO produced, i.e., the CO production rate, obtained by simulation. The CO production rate is the proportion of the amount of CO in the entire mixed gas, and is expressed in "vol%." The simulation conditions were set as follows: Gas G2 was 100% CO2 gas, and contained no water or hydrogen. For photolysis, a xenon excimer lamp emitting excimer light with a wavelength of 172 nm was used, with a light intensity of 30 mW / cm. 2 It was assumed that the gas would be irradiated with excimer light for 1000 seconds. At 300K, which is near room temperature, the CO generation rate reaches 69%. However, as the gas temperature increases, the CO generation rate decreases. When the gas temperature is 400K, the CO generation rate is 57%. When the gas temperature is 500K, it was found that the CO generation rate falls below 10%. Figure 4 shows that if the gas temperature is 400K or below, a CO generation rate of over 50% can be maintained.

[0065] Let us consider why the CO production rate decreases as the gas temperature increases. As mentioned above, according to equations (1) and (2), when CO2 is decomposed, O( 3 P) and O( 1 D) and O( 3 P) and O( 1 D) is used to generate O3. Equations (1) and (2) are shown below. CO2+hν → CO +O( 3 P) …(1) CO2+hν → CO +O( 1 D) …(2) However, as the gas temperature rises, O3 becomes less likely to be produced. Even if O3 is temporarily produced, it may not be possible for O3 to be mixed with O2 and O( 3 P) is thermally decomposed into O( 3 As shown in equation (4), CO2 is consumed and CO2 is increased, so it is thought that the decomposition rate of CO2 decreases. Therefore, a low gas temperature is preferable.

[0066] [Excimer lamp] In this embodiment, a xenon excimer lamp that emits excimer light with a main wavelength of 172 nm is used as the excimer lamp 5 (see FIG. 1). Light with a main wavelength of 172 nm can dissociate the C=O bond of CO2 molecules. The xenon excimer lamp contains xenon gas as a light-emitting gas inside the arc tube. The xenon gas is excited by a dielectric barrier discharge. When the excited xenon gas returns to the ground state, it emits excimer light L1 with a main wavelength of 172 nm. The power supplied to the excimer lamp 5 is controlled by a control unit 14, which also controls the on / off of the excimer lamp.

[0067] [Variations] A modified example of the second treatment chamber 2 of the photolysis system is shown. FIG. 5 is an enlarged view of the second treatment chamber 2 of the photolysis system. In this modified example, two second treatment chambers (2a, 2b) are arranged outside the excimer lamp 5, sandwiching the excimer lamp 5. The excimer light emitted from the excimer lamp 5 is irradiated toward the interior of each second treatment chamber (2a, 2b) and treats the gas G2 flowing inside the second treatment chamber (2a, 2b). Alternatively, a gas (e.g., nitrogen) for cooling the excimer lamp 5 may be introduced between the two second treatment chambers (2a, 2b) and the excimer lamp 5 to cool the excimer lamp 5. By introducing the gas for cooling the excimer lamp 5, the excimer lamp 5 is cooled and the temperature rise of the gas G2 can also be suppressed.

[0068] Second Embodiment 6 is a diagram showing a third embodiment of a CO2 photolysis system. In the following description, parts common to the first embodiment will be omitted as appropriate. The photolysis system 20 of this embodiment differs from the photolysis system 10 of the first embodiment in that it does not include a treatment chamber 1 having a water removal device 3. The photolysis system 20 is common to the photolysis system 10 of the first embodiment in that it includes a photolysis treatment chamber 2 having an excimer lamp 5 disposed therein.

[0069] The photolysis system 20 of this embodiment includes a connecting pipe 7 connecting the gas G1 inlet 4i to the inlet of the photolysis treatment chamber 2. As in the first embodiment, the connecting pipe 7 is connected to a supply pipe 8 that supplies an additional gas G4 (i.e., hydrogen) to the gas G1, and the supply pipe 8 is connected to a supply source 11 of the additional gas G4. Regarding the gas G1, the gas G1 supplied to the inlet 4i itself contains water. Alternatively, even if the gas G1 supplied to the inlet 4i does not contain water, the gas G1 in the treatment chamber 2 contains a trace amount of water (e.g., 10 ppm or less) due to the infiltration of water vapor from the atmosphere. A flow valve 12 and a flow meter (not shown) are disposed in the supply pipe 8. The flow valve 12 is controlled by a control unit 14. This embodiment demonstrates that the addition of hydrogen gas can improve the photolysis efficiency of CO2, even without a water removal device.

[0070] Third Embodiment 7 is a diagram showing a third embodiment of a CO2 photolysis system. In the following description, explanations of parts common to the first or second embodiment will be omitted as appropriate. The photolysis system 30 of this embodiment differs from the photolysis system 10 of the first embodiment in that it does not include a treatment chamber 1 having a water removal device 3, and from the photolysis system 20 of the second embodiment in that it does not add hydrogen gas. The photolysis system 30 is common to the photolysis system 10 of the first embodiment in that it includes a photolysis treatment chamber 2 having an excimer lamp 5 disposed therein.

[0071] The photolysis system 30 of this embodiment includes a connecting pipe 7 that connects the inlet 4i of the gas G1 to the inlet of the photolysis processing chamber 2. A supply pipe 28 that supplies an additional gas G5 to the gas G1 is connected to the connecting pipe 7, and a supply source 21 of the additional gas G5 is connected to the supply pipe 28. A flow valve 22 and a flow meter (not shown) are disposed midway along the supply pipe 28. The flow valve 22 is controlled by the control unit 14.

[0072] [Oxygen gas addition] In this embodiment, the additional gas G5 is oxygen gas. The supply source 21 is a supply source of oxygen gas. The significance of adding oxygen gas to the gas G1 will be explained using FIG. 8. FIG. 8 is a graph obtained by simulation of the relationship between the oxygen concentration in the gas and the production rate of CO produced by photolysis. The simulation conditions were set as follows: The gas was CO2 gas containing 500 ppm water vapor and did not contain hydrogen. The gas temperature was set to 300 K. A xenon excimer lamp emitting excimer light with a peak wavelength of 172 nm was used, and the light intensity was 30 mW / cm. 2 The gas was assumed to be irradiated with excimer light for 1000 seconds at 1000 s. The average distance between the xenon excimer lamp and the gas being photolyzed and the flow rate of gas G2 were assumed to be constant throughout the simulation.

[0073] Below, we will consider the relationship between oxygen concentration and CO production rate with reference to Figure 8. Figure 8 confirms that adding 2% oxygen to gas G1, when gas G1 does not contain O2 (oxygen concentration 0%), temporarily reduces the CO production rate. This reduction is thought to be caused by the following mechanism. First, the amount of O2 that combines with HCO increases, which increases HO2 according to equation (11). The increased HO2 is thought to generate a large amount of OH, which converts the generated CO back into CO2 through the reaction of equation (5). The relevant equations are listed again below. HCO + O2 → CO + HO2 (11) HO2 + H → OH + OH …(7) HO2+O3→ OH+O2+O2…(8) CO + OH → CO2 + H … (5)

[0074] However, if oxygen gas is added so that the oxygen concentration in the gas G1 exceeds 2%, the amount of O2 or O3 in the gas increases. Then, this O2 or O3 absorbs the excimer light hν, dissociates O2 molecules or O3 molecules, and produces a large amount of O( 1 D) is generated. 1D) combines with CO2 to produce CO according to equation (3). Equation (3) is shown below: CO2+O( 1 D) → CO + O2…(3)

[0075] When the oxygen concentration in the gas is 5% or higher, the CO2 generation rate when irradiated with ultraviolet light increases compared to when the gas does not contain oxygen. Therefore, in order to increase the CO2 generation rate when irradiating the gas with ultraviolet light, it is preferable to add oxygen gas. The CO2 generation rate increases as the oxygen concentration increases until the oxygen concentration reaches 25%. Air may be used as the additional gas G5. The concentration of oxygen contained in the gas can be detected, for example, using a commercially available portable oxygen concentration meter.

[0076] The above describes the embodiments of the photolysis method and photolysis system for CO2. The above embodiments are merely examples of the present invention, and the present invention is not limited to the above embodiments. Various changes or modifications can be made to the above embodiments, or the above embodiments can be combined, without departing from the spirit of the present invention.

[0077] For example, the photolysis system 10 of the first embodiment does not have to include at least one of the hydrogen supply source 11 and the gas cooler 9. The photolysis system 20 of the second embodiment does not have to include the gas cooler 9. The photolysis system 30 of the third embodiment does not have to include the gas cooler 9, and may additionally include a water removal device 3. The photolysis efficiency is improved even when any one of reducing water from a gas containing carbon dioxide and water, adding hydrogen to the gas, adding oxygen to the gas, and cooling the gas temperature is performed.

[0078] [How to apply the photodecomposition system] Fig. 9 is a flow diagram from CO2 capture to fixation. With reference to Fig. 9, an example of a CO2 fixation method to which the above-described CO2 photolysis system 10 can be applied will be described.

[0079] In step S1, a fossil fuel combustion engine (eg, a thermal power plant) emits CO2.

[0080] In step S2, the discharged low-concentration CO2 is converted to high-concentration CO2 in a high-concentration CO2 generator. By converting the CO2 to high concentration, it can be efficiently decomposed in subsequent processes. It is known that high-concentration CO2 can be achieved using adsorbents or physical membranes. Also, as mentioned above, it is acceptable to leave the CO2 behind in order to improve the efficiency of decomposition in subsequent processes. Step S2, for converting the CO2 to high concentration, is not an essential step.

[0081] In step S3, the highly concentrated CO2 is introduced as the gas to be treated G1 into the photolysis system (10, 20, 30) described above, where the CO2 is decomposed in the system (10, 20, 30) to generate CO. The processed gas G3 after photolysis contains, in addition to the generated CO, remaining CO2 and O2, and H2 or O2 introduced as an additional gas in the photolysis system (10, 20, 30).

[0082] In step S4, the treated gas G3 is separated to extract carbon monoxide. For example, carbon monoxide can be separated by adsorbing it onto a porous material. The gas from which carbon monoxide has been removed is again introduced into the high-concentration CO2 generator (step S2), where the same process is repeated.

[0083] In step S5, the CO is introduced into a liquid containing sodium hydroxide. Then, the CO is reacted with NaOH to produce sodium formate. The formic acid produced from sodium formate is an organic substance with a very simple structure, and is used as a raw material in organic synthetic chemistry and as a hydrogen source in fuel cells.

[0084] The organic compounds immobilized by the CO2 immobilization method may be consumed in society and then emitted again as CO2. The above-mentioned CO2 immobilization method is only an example, and CO2 may be immobilized into other organic compounds, etc. [Explanation of symbols]

[0085] 1: First treatment chamber 2, 2a, 2b: Second treatment chamber, or treatment chamber (for photolysis) 3:Water removal device 4i: Inlet (of the first treatment chamber or the treatment chamber for photolysis) 4o: Outlet (of the first treatment chamber) 5: Excimer lamp 6: Outlet (of the photolysis chamber or second treatment chamber) 7: Connecting pipe 8: Supply pipe 9: Gas cooler 10: Photolysis system 11: Source 12: Flow valve 13: Temperature sensor 14: Control section 20: Photolysis system 21: Source 22: Flow valve 28: Supply pipe 30: Photolysis system G1, G2, G3: Gas G4, G5: Additional gas L1: Excimer light

Claims

1. 1. A method for photodecomposing carbon dioxide, comprising: reducing water from the gas containing carbon dioxide and water; and photodecomposing the carbon dioxide by irradiating the water-reduced gas with excimer light.

2. 2. The photolysis method according to claim 1, wherein the gas contains 0.4% or more and 4.0% or less of hydrogen.

3. 2. The photolysis method according to claim 1, wherein the gas contains 5% to 25% oxygen.

4. 1. A method for photodecomposing carbon dioxide, comprising: A photodecomposition method comprising a step of photodecomposing the carbon dioxide by irradiating a gas containing the carbon dioxide and 5% or more and 25% or less of oxygen with excimer light.

5. 1. A method for photodecomposing carbon dioxide, comprising: A photolysis method comprising a step of photodecomposing the carbon dioxide by irradiating a gas containing the carbon dioxide and 0.4% or more and 4.0% or less of hydrogen with excimer light.

6. 6. The photolysis method according to claim 1, wherein the temperature of the gas in the atmosphere irradiated with the excimer light is 400K or less.

7. 6. The photolysis method according to claim 1, wherein the excimer light has a main wavelength of 172 nm.

8. 4. The photodecomposition method according to claim 1, wherein the concentration of water contained in the gas after the water reduction is 100 ppm or less.

9. 6. The photolysis method according to claim 1, wherein the excimer light is irradiated while the gas is being fed into a space irradiated with the excimer light.

10. 1. A carbon dioxide photolysis system comprising: A first treatment chamber including an inlet for introducing the gas containing carbon dioxide and water, a water removal device for reducing water from the gas, and an outlet for discharging the gas from the water removal device; a second processing chamber connected to the exhaust port of the first processing chamber and into which the gas exhausted from the exhaust port is introduced; an excimer lamp that irradiates the gas in the second processing chamber with excimer light, A photolysis system, wherein the second treatment chamber photodecomposes the carbon dioxide contained in the gas present therein using the excimer light.

11. a hydrogen supply source upstream of the second treatment chamber that supplies hydrogen to the gas; a hydrogen regulating valve for regulating the amount of hydrogen supplied; 11. The photolysis system according to claim 10, further comprising: a control unit that controls the hydrogen regulating valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less.

12. an oxygen supply source upstream of the second treatment chamber for supplying oxygen to the gas; an oxygen regulating valve for regulating the amount of oxygen supplied; 11. The photolysis system according to claim 10, further comprising: a control unit that controls the oxygen regulating valve so that the oxygen concentration in the gas is 5% or more and 25% or less.

13. 1. A carbon dioxide photolysis system comprising: an inlet for introducing the gas containing carbon dioxide; an excimer lamp that emits excimer light; a processing chamber connected to the inlet, in which the excimer light photodecomposes the carbon dioxide present therein; an oxygen source upstream of the processing chamber that supplies oxygen to the gas; an oxygen regulating valve for regulating the amount of oxygen supplied; a control unit that controls the oxygen regulating valve so that the oxygen concentration in the gas is 5% or more and 25% or less.

14. 1. A carbon dioxide photolysis system comprising: an inlet for introducing the gas containing carbon dioxide; an excimer lamp that emits excimer light; a processing chamber connected to the inlet, in which the excimer light photodecomposes the carbon dioxide present therein; a hydrogen source upstream from the processing chamber that supplies hydrogen to the gas; a hydrogen regulating valve for regulating the amount of hydrogen supplied; a control unit that controls the hydrogen regulating valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less.

15. A photolysis system, characterized in that the excimer lamp is disposed inside the second treatment chamber according to any one of claims 10 to 12 or inside the treatment chamber according to claim 13 or 14.

16. a cooler that cools the gas, the cooler being disposed upstream of the second treatment chamber according to any one of claims 10 to 12 or the treatment chamber according to claim 13 or 14; a temperature sensor for measuring the temperature of the gas cooled by the cooler; A photolysis system comprising: a control unit that controls the cooler so that the measurement value of the temperature sensor is 400K or less.

17. 15. The photolysis system according to claim 10, wherein the excimer lamp has a light-emitting gas sealed in a light-emitting tube of the excimer lamp, which gas is xenon gas.

18. The photolysis system according to any one of claims 10 to 12, characterized in that the water removal device comprises zeolite.

Images