WO2023074013A1

WO2023074013A1 - Method and system for photodecomposition of carbon dioxide

Info

Publication number: WO2023074013A1
Application number: PCT/JP2022/009946
Authority: WO
Inventors: 優一大塚; 貴紀鮫島; 章弘島本; 謙介中村; 良徳相浦
Original assignee: ウシオ電機株式会社
Priority date: 2021-10-29
Filing date: 2022-03-08
Publication date: 2023-05-04
Also published as: JP2023066624A

Abstract

Provided is a method for photodecomposition of carbon dioxide, whereby it becomes possible to decompose a larger amount of carbon dioxide with a smaller input energy. The method for photodecomposition of carbon dioxide includes at least one method selected from a method including a step for reducing water from a gas containing the carbon dioxide and water and a step for irradiating the water-reduced gas with excimer light, a method including irradiating a gas containing the carbon dioxide and 5% to 20% inclusive of oxygen with excimer light, and a method including irradiating a gas containing the carbon dioxide and 0.4% to 4.0% inclusive of hydrogen with excimer light.

Description

Carbon dioxide photodecomposition method and photodecomposition system

The present invention relates to a carbon dioxide photodecomposition method and a photodecomposition system.

In recent years, as the amount of fossil fuels such as petroleum and coal burned has increased, the concentration of carbon dioxide (hereinafter sometimes referred to as “CO ₂ ”) in the atmosphere has increased, and global warming is progressing. Therefore, the reduction of _CO2 emissions is an urgent issue, especially in industrialized countries.

One method of reducing _CO2 emissions is to convert _CO2 into organic matter and chemically fix it. In Patent Document 1 below, a gas containing _CO2 is irradiated with excimer light emitted from a light source using a dielectric barrier discharge, thereby photolyzing _CO2 into carbon monoxide (hereinafter, "CO"). ) and reacting this CO with sodium hydroxide to immobilize it on sodium formate.

Japanese Patent Application Laid-Open No. 2021-011407

In the technology of irradiating excimer light to _CO2 to photodecompose _CO2 , an important factor is the photodecomposition efficiency of _CO2 . As the photodegradation efficiency of _CO2 improves, more _CO2 can be decomposed with less input energy.

The present invention provides a _CO2 photolysis method and photolysis system that can decompose more _CO2 with less input energy.

The method for photodegradation of _CO2 to solve the above problems is
(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 CO ₂ and 5% or more and 25% or less oxygen (hereinafter sometimes referred to as “O ₂ ”) with excimer light, and (3) CO ₂ and 0.4% or more and 4.0% or less hydrogen (hereinafter sometimes referred to as “H ₂ ”), a method of irradiating excimer light to a gas,
includes at least one method among

The details of the method (1) including the step of reducing water from the gas containing CO ₂ and water and the step of irradiating the water-reduced gas with excimer light will be described later, but the present inventors It has been found that when water is included in the photolytic gas, the water interferes with the photolysis of _CO2 . Therefore, they devised a way to reduce the amount of water in the gas before performing photolysis. This increases the photodegradation efficiency of _CO2 .

The method of irradiating excimer light to the gas containing _CO2 and 5% or more and 25% or less of O2 _in (2) above will be described in detail later, but the excimer light emits a large amount of _O2 . The decomposition increases O( ¹ D), which is atomic oxygen in an excited state. O( ¹ D) reacts with CO ₂ to increase the production rate of CO. That is, the photodecomposition efficiency of _CO2 is increased. Also, the gas obtained by reducing water from the gas containing CO ₂ and water may contain 5% or more and 25% or less of O ₂ . The value of the oxygen concentration of 25% is a value close to the oxygen concentration in the air, but the gas to be photodecomposed may have a composition ratio close to that of the air in the atmosphere. However, gases containing more CO ₂ than atmospheric air are more efficient at photodegradation. The CO ₂ concentration of the photodecomposed gas is, for example, preferably 1% or more, more preferably 5% or more, and even more preferably 10% or more.

A method of irradiating excimer light to the gas containing CO ₂ and 0.4% or more and 4.0% or less H ₂ in (3) above will be described in detail later, but H ₂ is CO combines with OH in preference to combining with OH originating from water. Therefore, the reverse reaction of returning to _CO2 due to the combination of CO and OH can be suppressed. Suppression of the reverse reaction leads to improved photodecomposition efficiency of _CO2 .

The water in the “water originated OH” 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 can enter from the outside and mix with the _CO2 gas, resulting in unintentionally trace amounts of water. ( _details below). Therefore, even if the supplied CO ₂ gas itself does not contain water, H ₂ may substantially contribute to improving the photolysis efficiency of CO ₂ by suppressing the reverse reaction.

If the supplied _CO2 gas itself contains a large amount of water, a water-reduced gas containing 0.4% or more and 4.0% or less _H2 after the water-reducing step. may be photolyzed with excimer light. Also, 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.

The temperature of the gas in the atmosphere irradiated with the excimer light may be 400K or lower. Although the details will be described later, when the temperature of the gas decreases, the production rate of ozone (hereinafter sometimes referred to as “O ₃ ”) increases and the thermal decomposition of O ₃ is suppressed. Then, the generation of O( ³ P), which is atomic oxygen in the ground state, is suppressed. As a result, the reaction in which O( ³ P) and CO recombine and CO returns to CO ₂ can be suppressed. Suppression of the reverse reaction leads to improved photodecomposition efficiency of _CO2 .

The main wavelength of the excimer light may be 172 nm or around 172 nm. Since CO ₂ molecules absorb light with a dominant wavelength around 172 nm well, excimer light with a dominant wavelength around 172 nm can efficiently decompose CO ₂ . Also, excimer light whose main wavelength is around 172 nm is obtained by lighting a xenon excimer lamp. Since the xenon excimer lamp is a light source that can be stably mass-produced, light with a dominant wavelength near 172 nm is highly cost-effective.

In this specification, "near 172 nm" refers to a region within the range of 172 nm ± 5 nm. As used herein, the term “main wavelength” means that when a wavelength region Z(λ) of ±10 nm for a certain wavelength λ is defined on the emission spectrum, the total integrated intensity in the emission spectrum is 40% or more. indicates the wavelength λi in the wavelength region Z(λi) indicating the integrated intensity of . In a light source that emits light of the "principal wavelength", such as a xenon excimer lamp, has an extremely narrow half-value width and exhibits high light intensity only at a specific wavelength, usually the light intensity is relatively The highest wavelength (the dominant peak wavelength) may be considered the dominant wavelength.

The concentration of water contained in the gas after reducing the water may be 100 ppm or less.

The amount of water contained in the gas may be reduced by allowing zeolite to adsorb the water.

The excimer light may be irradiated while sending the gas to the space irradiated with the excimer light. This allows a large amount of _CO2 to be photolyzed.

A CO ₂ photolysis system for solving the above problems is
(4) a device for reducing water from a gas containing carbon dioxide and water prior to photolysis;
(5) a supply source for supplying oxygen to a gas containing carbon dioxide before photolysis so that the oxygen concentration is 5% or more and 25% or less;
(6) a supply source for supplying hydrogen to a 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

Specifically, the CO ₂ photolysis system comprising (4) above is
a first processing chamber including an inlet for introducing the gas containing carbon dioxide and water, a water removal device for removing water from the gas, and an outlet for discharging the gas from the water removal device;
a second processing chamber connected to the discharge port of the first processing chamber and into which the gas discharged from the discharge 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 photolyzes the carbon dioxide contained in the gas present therein.

The _CO2 photolysis system further comprises, upstream from the second processing chamber, a hydrogen source for supplying hydrogen to the gas;
a hydrogen regulating valve that adjusts the supply amount of the hydrogen;
and a control unit that controls the hydrogen adjustment valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less. That is, the CO ₂ photolysis system provided with (4) above may be combined with the elements (6) above.

The _CO2 photolysis system further comprises, upstream from the second processing chamber, an oxygen source for supplying oxygen to the gas;
an oxygen regulating valve that adjusts the amount of oxygen supplied;
A control unit that controls the oxygen control valve so that the oxygen concentration in the gas is 5% or more and 25% or less. That is, the CO ₂ photolysis system provided with (4) may be combined with the elements of (5) above.

Specifically, the CO ₂ photolysis system comprising (5) above is
an inlet for introducing the gas containing carbon dioxide;
an excimer lamp for emitting excimer light;
a processing chamber connected to the inlet, wherein the excimer light photolyzes the carbon dioxide present therein;
an oxygen supply source upstream from the processing chamber for supplying oxygen to the gas;
an oxygen regulating valve that adjusts 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.

Specifically, the CO ₂ photolysis system comprising (6) above is
an inlet for introducing the gas containing carbon dioxide;
an excimer lamp for emitting excimer light;
a processing chamber connected to the inlet, wherein the excimer light photolyzes the carbon dioxide present therein;
a hydrogen supply source upstream from the processing chamber for supplying hydrogen to the gas;
a hydrogen regulating valve that adjusts the supply amount of the hydrogen;
a control unit that controls the hydrogen adjustment valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less.

The excimer lamp may be arranged inside the processing chamber or the second processing chamber that photolyzes the carbon dioxide.

The photolysis system is arranged upstream of the processing chamber or the second processing chamber that photolyzes the carbon dioxide, and a cooler that cools the gas;
a temperature sensor that measures the temperature of the gas cooled by the cooler;
and a control unit that controls the cooler so that the measured value of the temperature sensor is 400K or less.

In the excimer lamp, the luminous gas enclosed in the arc tube may be xenon gas.

The water removal device may include zeolite.

As a result, it is possible to provide a carbon dioxide photodecomposition method and a photodecomposition system that can decompose a larger amount of carbon dioxide with less input energy.

Providing a carbon dioxide photolysis method and photolysis system that can decompose more carbon dioxide with less input energy is one of the United Nations Sustainable Development Goals (SDGs) Goal 13 "Climate change and its impacts. This will make a significant contribution to “taking urgent measures to mitigate

1 shows a first embodiment of a carbon dioxide photolysis system; FIG. 4 is a graph showing the relationship between water content in gas and CO production rate. 4 is a graph showing the relationship between hydrogen concentration in gas and CO production rate. 4 is a graph showing the relationship between gas temperature and CO production rate. Fig. 3 is a modified example of the second processing chamber of the photolysis system; Fig. 2 shows a second embodiment of a carbon dioxide photolysis system; FIG. 13 illustrates a third embodiment of a carbon dioxide photodecomposition system; 4 is a graph showing the relationship between oxygen concentration in gas and CO production rate. FIG. ₂ is a flow diagram from CO2 capture to immobilization.

The embodiment will be described with reference to the drawings as appropriate. In addition, all drawings other than graphs or flow diagrams are schematic illustrations, and the dimensional ratios on the drawings do not necessarily match the actual dimensional ratios, and the dimensional ratios between the drawings are not the same. not necessarily match.

<First Embodiment>
[Outline of photolysis system]
FIG. 1 shows a first embodiment of a CO ₂ photolysis system. The photolysis system 10 has a first processing chamber 1 and a second processing chamber 2 connected downstream of the first processing chamber 1 . This photodecomposition system 10 has a function of decomposing carbon dioxide contained in the gas G1.

The first processing chamber 1 is a processing chamber for reducing the amount of water contained in the gas G1. Gas G1 is a gas containing _CO2 and water. The first processing chamber 1 includes an inlet 4 i for introducing gas G<b>1 , a water removal device 3 , and an outlet 4 o for discharging the gas from the water removal device 3 . The water removal device 3 removes at least part of the water contained in the gas G1. This makes the gas G1 a dry gas with a reduced amount of water in it. In FIG. 1, different symbols are used to distinguish between the gas treated by the water removal device 3 and the gas before treatment. That is, the gas before being treated by the water removal device 3 is called "gas G1", and the gas after being treated by the water removal device 3 is called "gas G2". Furthermore, as will be described later, the gas after being processed in the second processing chamber 2 will be referred to as "gas G3" from the viewpoint of distinguishing it from gas G2. A specific aspect of the water removal device 3 will be described later.

In this specification, the water to be removed is a concept that includes gaseous and liquid mist water. In other words, the gas G1 before being treated by the water removal device 3 may contain, for example, liquid water in the form of mist in addition to water vapor. The gas G2 after being treated by the dewatering device 3 contains only water vapor or does not contain water vapor and liquid water at all.

The content of carbon dioxide in the 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 addition, in this specification, "vol%" is intended when the ratio of the fluid is displayed using "%". Further, the gas contained in the gas G1 other than carbon dioxide is preferably inert to the light with which the gas is irradiated, except for the gas of the same kind as the additional gas G4 or the additional gas G5, which will be described later. For example, when the light with which the gas is irradiated has a wavelength of about 172 nm, nitrogen gas can be used as the inert gas.

The second processing chamber 2 is connected to the discharge port 4 o of the first processing chamber 1 through a connecting pipe 7 . That is, the second processing chamber 2 is connected to the rear stage of the first processing chamber 1 . The second processing chamber 2 is supplied with a gas G2 obtained by reducing water in the gas G1 in the first processing chamber 1 . The second processing chamber 2 is a processing chamber for photolyzing the gas G2. In this embodiment, the excimer lamp 5 is arranged inside the second processing chamber 2 , and when the excimer lamp 5 is turned on, the gas G2 in the second processing chamber 2 is irradiated with the excimer light L1. As a result, the CO ₂ contained in the gas G2 is photolyzed to produce CO. Details of the excimer lamp 5 will be described later. The treated gas G3 containing CO produced in the second processing chamber 2 is discharged from the outlet 6 of the second processing chamber 2. Photolysis can be continuously performed by irradiating the excimer light L1 while sending the gas G2 to the space (that is, the second processing chamber 2) irradiated with the excimer light L1.

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 connection pipe 7 connecting the first processing chamber 1 and the second processing 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 arranged in the middle of the supply pipe 8 . The flow valve 12 is controlled by the controller 14 . Additional gas G4 will be described later. Since the additional gas G4 may be supplied to the upstream of the second processing chamber 2, the additional gas G4 may be supplied not only between the first processing chamber 1 and the second processing chamber 2 but also between the first processing chamber 1 and the upstream of the first processing chamber 1. may be supplied to

[CO generation mechanism]
The photolysis system and photolysis method of the present invention photolyzes _CO2 with excimer light to produce CO. The mechanism of generating CO 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 and dissociates the C=O bond of the _CO2 molecule, causing the following reactions: .
CO ₂ + hν → CO + O( ³ P) (1)
CO ₂ + hv → CO + O ( ¹ D) (2)

Equations (1) and (2) express that excimer light hν directly produces CO from CO ₂ . O( ³ P), generated in equation (1), represents atomic oxygen in the ground state. O( ¹ D) produced in equation (2) represents atomic oxygen in an excited state.

The excited state of atomic oxygen, O( ¹ D), is highly active. Thus, O( ¹ D) reacts with CO ₂ to produce CO.
CO ₂ + O ( ¹ D) → CO + O ₂ (3)
Equation (3) expresses that the excimer light hν indirectly produces CO from _CO2 .

The ground-state atomic oxygen O( ³ P) produced by formula (1) may undergo the following reactions.
CO+O( ³ P)+M→CO ₂ +M (4)
Equation (4) indicates that even if CO is generated by decomposing CO ₂ according to Equation (1), a reverse reaction occurs in which CO combines with O( ³ P) to form CO ₂ . In addition, in this formula (4), M indicates the third body.

The rate at which formulas (1) and (2) occur differs depending on the wavelength of the excimer light hν. Since the shorter the wavelength, the higher the light energy, the reaction of formula (1) decreases and the reaction of formula (2) increases. Therefore, the shorter the wavelength, the more O( ¹ D) tends to increase and the decomposition of CO ₂ progresses.

Note that O( ¹ D) and O( ³ P) are generated by the formulas (1) and (2), as well as generated from the O ₂ generated by the formula (3), or It may also be produced during the HO _X cycle chain reaction.

[Effect of water on photolysis]
As a result of intensive research on the photolysis of CO ₂ by the present inventors, it was found that there is a relationship shown in FIG. discovered.

FIG. 2 is a graph obtained by simulation of the relationship between the content of water in gas containing CO ₂ and the production rate of CO produced by photolysis. The CO production rate is the ratio of the amount of CO in the total mixed gas, and is intended as "vol%". The simulation conditions were set as follows. Gas G2 was a gas composed of CO ₂ and water vapor. The gas temperature was 400K. For photolysis, the use of a xenon excimer lamp that emits excimer light with a peak wavelength of 172 nm was assumed. The average distance between the xenon excimer lamp and the gas to be photolyzed and the flow rate of gas G2 were assumed to have constant values during the simulation.

From FIG. 2, it is confirmed that the CO production rate of gas containing 6 ppm of water is only 3%, while the CO production rate of gas containing no water (water content is 0 ppm) is 8%. be. In other words, it can be seen that the CO production rate decreased by 5% when the gas contained only 6 ppm of water.

Therefore, based on the results of the above verification, it is understood that reducing water from the carbon dioxide-containing gas tends to lead to an improvement in the photolysis efficiency. Moreover, even if the supplied carbon dioxide-containing gas itself does not contain water, it has been found that water vapor contained in the atmosphere penetrates through minute gaps in the processing chamber or the piping route leading to the processing chamber. rice field. As a result, the infiltrated water vapor is mixed with the carbon dioxide-containing gas, resulting in a carbon dioxide-containing gas containing a small amount of water. Then, as described above, even a small amount of water less than 10 ppm lowers the CO production rate. Thus, the risk that water interferes with the photodegradation of _CO2 exists even if the carbon dioxide-containing gas itself may not contain water.

According to the intensive research of the present inventors, it is considered that the phenomenon in which the photodecomposition of CO ₂ is hindered is generally caused by the following chemical reaction.
CO + OH → CO ₂ + H (5)
Equation (5) expresses that even if CO is generated by decomposing CO ₂ by Equation (3), OH reacts with CO to return CO to CO ₂ . In other words, it is important to suppress the generation of OH so as not to cause the reaction of formula (5).

OH represented in formula (5) originates from water (sometimes referred to as “H ₂ O”). However, it is considered that most of the OH represented in the formula (5) is OH generated by a chain reaction rather than simply H ₂ O→OH+H. OH produced by the chain reaction is typically thought to be produced by the following four reaction formulae.
O ₃ +H → OH + O ₂ (6)
HO ₂ +H → OH + OH (7)
_HO2 + _O3- >OH+ _O2 + _O2 (8)
H ₂ O ₂ +H → H ₂ O + OH (9)

Of these, what contributes to the generation of H ₂ O ₂ in formula (9) is
_H2 + _HO2- > _H2O2 +H (10 ₎
It is considered to be

(7), (8) and (10) contribute to the generation of HO ₂ by
HCO+O ₂ →CO+HO ₂ (11)
It is considered to be

(11) Contributing to HCO generation in the formula is generally
CO + H + M → HCO + M (12)
It is considered to be In addition, in (12) Formula, M shows a third body.

In other words, when H ₂ O is mixed with CO ₂ and light energy is applied, a large amount of OH is produced via the formulas (6) to (12). Then, a large amount of OH produced by the above mechanism hinders the photodecomposition of CO ₂ through the reaction of formula (5). Therefore, it is believed that even a minute amount of water, less than 10 ppm, would interfere with the photodegradation of CO ₂ .

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

In this embodiment, the water removal device 3 is used to reduce the water from the gas G1 to generate the gas G2 before the photolysis of _CO2 . In this embodiment, the water removal device 3 uses synthetic zeolite with high water absorption performance (for example, Union Carbide's "Molecular Sieve") to adsorb and reduce water. However, the type of zeolite is not particularly limited. Also, a water absorbing material other than zeolite (for example, silica gel, calcium chloride, MOF (metal organic framework), etc.) may be used. Moreover, it is not limited to the water removing device using the water absorbing material, and for example, a water removing device using a compressor may be used. If the gas G1 contains a mist of water, the water may be reduced by using a filter that allows water vapor to pass but blocks the passage of the mist. Moreover, the water removal device 3 may combine a plurality of water removal mechanisms described above.

The concentration of water contained in the gas G2 after reducing the water is preferably 100 ppm or less. When the water concentration is 100 ppm or less, the photodegradation efficiency is high. Note that the concentration of water contained in the gas G2 can be detected by a moisture meter.

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

FIG. 3 is a graph obtained by simulation of the relationship between the concentration of hydrogen in gas and the production rate of CO produced by photolysis. As used herein, the CO production rate is the ratio of the amount of CO in the entire mixed gas, and is intended to be "vol%". The simulation conditions were set as follows. The gas was a mixed gas containing N ₂ and CO ₂ , the ratio of CO ₂ in the total gas being 4%, and water vapor of 500 ppm. The temperature of the mixed gas was set at 500K. A xenon excimer lamp that emits excimer light with a peak wavelength of 172 nm was used for photolysis, and it was assumed that the gas was irradiated with the excimer light at a light intensity of 30 mW/cm ² for 1000 seconds. The average distance between the xenon excimer lamp and the gas to be photolyzed and the flow rate of gas G2 were assumed to have constant values during the simulation.

From FIG. 3, by adding hydrogen concentration of about 0.2% from a state of 0% in which hydrogen (hereinafter sometimes referred to as “H ₂ ”) is not contained in the gas, the CO generation rate temporarily descend. It is considered that this phenomenon is caused by the increase in the amount of OH according to formula (13) or (14) shown below and the reaction of formula (5) returning CO to CO ₂ .
H ₂ +O( ³ P)→OH+H (13)
H ₂ +O( ¹ D)→OH+H (14)

However, from FIG. 3, it was found that the addition of 0.4% or more hydrogen to the gas increased the CO production rate with photolysis. This phenomenon is considered to be mainly due to the fact that H ₂ combined with OH and consumed OH prior to the reaction in which CO combined with OH in formula (5). The reaction in which _H2 combines with OH and consumes OH is shown below.
H ₂ +OH → H ₂ O + H (15)
As the reaction of formula (15) increases, OH decreases and the reaction of (5) is suppressed. That is, it suppresses the deterioration of the decomposition efficiency of CO ₂ .

Therefore, the gas should contain 0.4% or more of hydrogen. Moreover, since the lower limit of the explosion limit of hydrogen is 4.0%, it is preferable to set the hydrogen concentration to 4.0% or less in order to reduce the risk of explosion. The concentration of hydrogen contained in the gas G2 can be detected by, for example, a commercially available portable hydrogen concentration meter.

[Gas temperature control]
Returning to FIG. 1 , the photolysis system 10 of this embodiment has a gas cooler 9 and a temperature sensor 13 . In this embodiment, the gas cooler 9 is provided in the middle of 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 composed of, for example, a heat exchanger that air-cools the gas by flowing the gas through piping built in a large number of fins. A temperature sensor 13 is attached to the second processing chamber 2 and measures the temperature of the gas G2 inside the second processing chamber 2 . The controller 14 is electrically connected to the temperature sensor 13 and the gas cooler 9 respectively. The controller 14 cools the gas G<b>2 with 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 processing chamber 2, but it may also measure the temperature of the gas G2 in the connecting pipe 7, or the gas in the upstream of the water removal device 3. The temperature of G1 may be measured. Moreover, the gas after mixing with the additional gas G4 may be cooled by pre-cooling the additional gas G4 before mixing with the gas G2.

FIG. 4 is a graph obtained by simulation of the relationship between the temperature of gas containing CO ₂ and the ratio of the amount of CO produced, that is, the CO production rate. The CO production rate is the ratio of the amount of CO in the entire mixed gas, and is intended as "vol%". The simulation conditions were set as follows. Gas G2 was 100% _CO2 gas and was taken as a gas without water and hydrogen. For photolysis, a xenon excimer lamp that emits excimer light with a wavelength of 172 nm was used, and it was assumed that the gas was irradiated with the excimer light at a light intensity of 30 mW/cm ² for 1000 seconds. At 300 K, which is near room temperature, the CO production rate reaches 69%. However, the CO production rate decreases as the temperature of the gas increases. At a gas temperature of 400K, the CO yield is 57%. It was found that at a gas temperature of 500K, the CO yield was below 10%. From FIG. 4, it was found that if the gas temperature is 400 K or less, the CO production rate exceeding 50% can be maintained.

Consider why the CO production rate decreases as the temperature of the gas increases. As described above, according to equations (1) and (2), CO ₂ is decomposed to produce O( ³ P) and O( ¹ D), and O( ³ P) and O( ¹ D) are , used for the production of _O3 . Equations (1) and (2) are shown below.
CO ₂ + hν → CO + O( ³ P) (1)
CO ₂ + hv → CO + O ( ¹ D) (2)
However, as the temperature of the gas rises, _O3 is less likely to be produced. Alternatively, even if _O3 is produced temporarily, _O3 is thermally decomposed into _O2 and O( ^3P ). Since O( ³ P) consumes CO and increases CO ₂ as shown in the formula (4), it is thought that the decomposition rate of CO ₂ decreases. Therefore, the lower the temperature of the gas, the better.

[Excimer lamp]
In this embodiment, the excimer lamp 5 (see FIG. 1) is a xenon excimer lamp that emits excimer light with a main wavelength of 172 nm. Light with a dominant wavelength of 172 nm can dissociate the C═O bonds of CO ₂ molecules. A xenon excimer lamp has xenon gas as a light emission gas inside an arc tube. The xenon gas is excited by a dielectric barrier discharge. Then, when the excited state xenon gas returns to the ground state xenon gas, excimer light L1 having a main wavelength of 172 nm is emitted. The power supplied to the excimer lamp 5 is controlled by a control unit 14, and the control unit 14 controls lighting and extinguishing of the excimer lamp.

[Modification]
2 shows a variant of the second processing chamber 2 of the photolysis system. FIG. 5 is an enlarged view of the second processing chamber 2 of the photolysis system. In this modification, two second processing chambers (2a, 2b) are arranged outside the excimer lamp 5 so as to sandwich the excimer lamp 5 therebetween. The excimer light emitted from the excimer lamp 5 is irradiated toward the interior of each of the second processing chambers (2a, 2b) to process the gas G2 flowing inside the second processing chambers (2a, 2b). Alternatively, a gas (for example, nitrogen) for cooling the excimer lamps 5 may be introduced between the two second processing chambers (2a, 2b) and the excimer lamps 5 to cool the excimer lamps 5. FIG. 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 be suppressed.

<Second embodiment>
FIG. 6 is a diagram showing a second embodiment of a _CO2 photolysis system. It should be noted that in the following description, descriptions of parts common to the above-described first embodiment will be omitted as appropriate. The photolysis system 20 of the present embodiment differs from the photolysis system 10 of the first embodiment in that the first processing chamber 1 having the water removal device 3 is not provided. The photolysis system 20 is common to the photolysis system 10 of the first embodiment in that it includes a second processing chamber 2 for photolysis in which the excimer lamp 5 is arranged.

The photolysis system 20 of this embodiment includes a connection pipe 7 that connects the introduction port 4i of the gas G1 and the inlet of the second processing chamber 2 for photolysis. As in the first embodiment, the connection pipe 7 is connected to a supply pipe 8 for supplying an additional gas G4 (that is, hydrogen) to the gas G1, and the supply pipe 8 is connected to a supply source 11 of the additional gas G4. It is As for the gas G1, the gas G1 itself supplied to the inlet 4i contains water. Alternatively, even if the gas G1 itself supplied to the inlet 4i does not contain water, the gas G1 in the second processing chamber 2 contains a small amount of water (for example, 10 ppm or less) due to infiltration of atmospheric water vapor. . A flow valve 12 and a flow meter (not shown) are arranged in the middle of the supply pipe 8 . The flow valve 12 is controlled by the controller 14 . This embodiment shows that the photodecomposition efficiency of CO ₂ increases with the addition of hydrogen gas even without a water removal device.

<Third Embodiment>
FIG. 7 shows a third embodiment of a _CO2 photolysis system. It should be noted that in the following description, descriptions of portions common to the above-described first embodiment or second embodiment will be omitted as appropriate. Unlike the photolysis system 10 of the first embodiment, the photolysis system 30 of the present embodiment does not include the first processing chamber 1 having the water removal device 3, and the photolysis system 20 of the second embodiment , in that hydrogen gas is not added. The photolysis system 30 is similar to the photolysis system 10 of the first embodiment in that it includes a second processing chamber 2 for photolysis in which the excimer lamp 5 is arranged.

The photolysis system 30 of this embodiment includes a connection pipe 7 that connects the introduction port 4i of the gas G1 and the inlet of the second processing chamber 2 for photolysis. A supply pipe 28 for supplying an additional gas G5 to the gas G1 is connected to the connection pipe 7, and the supply pipe 28 is connected to a supply source 21 of the additional gas G5. A flow valve 22 and a flow meter (not shown) are arranged in the middle of the supply pipe 28 . The flow valve 22 is controlled by the controller 14 .

[Addition of oxygen gas]
In this embodiment, the additional gas G5 is oxygen gas. The supply source 21 is an oxygen gas supply source. The significance of adding oxygen gas to gas G1 will be described with reference to FIG. FIG. 8 is a graph obtained by simulation of the relationship between the oxygen concentration in gas and the production rate of CO produced by photolysis. The simulation conditions were set as follows. The gas was CO ₂ gas with 500 ppm water vapor and was taken as a hydrogen-free gas. The gas temperature was 300K. A case was assumed in which a xenon excimer lamp that emits excimer light with a peak wavelength of 172 nm was used and the gas was irradiated with the excimer light at a light intensity of 30 mW/cm ² for 1000 seconds. The average distance between the xenon excimer lamp and the gas to be photolyzed and the flow rate of gas G2 were assumed to have constant values during the simulation.

The relationship between the oxygen concentration and the CO production rate will be discussed below with reference to FIG. According to FIG. 8, by adding 2% oxygen to the gas G1 from the state where the gas G1 does not contain _O2 (the state where the oxygen concentration is 0%), the CO production rate temporarily decreases. was confirmed. This decrease is believed to occur due to the following mechanism. First, since the amount of O ₂ that bonds with HCO increases, the amount of HO ₂ increases according to equation (11). It is thought that the increased HO ₂ produces a large amount of OH, and the produced CO is returned to CO ₂ by the reaction of formula (5). Relevant formulas are listed below.
HCO+O ₂ →CO+HO ₂ (11)
HO ₂ +H → OH + OH (7)
_HO2 + _O3- >OH+ _O2 + _O2 (8)
CO + OH → CO ₂ + H (5)

However, when oxygen gas is added so that the oxygen concentration contained in gas G1 exceeds 2%, the amount of _O2 or _O3 in the gas increases. Then, these O ₂ or O ₃ absorb the excimer light hν, dissociate the O ₂ molecules or O ₃ molecules, and generate a large amount of O( ¹ D). O( ¹ D) combines with CO ₂ to produce CO according to equation (3). Equation (3) is shown again below.
CO ₂ + O ( ¹ D) → CO + O ₂ (3)

When the oxygen concentration in the gas is 5% or more, the CO production rate when irradiated with ultraviolet rays increases compared to when the gas does not contain oxygen. Therefore, it is preferable to add oxygen gas in order to increase the CO production rate by irradiating the gas with ultraviolet rays. CO production increases with increasing oxygen concentration 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 by, for example, a commercially available portable oximeter.

Embodiments of the _CO2 photolysis method and photolysis system have been described above. The above-described embodiment is merely an example of the present invention, and the present invention is not limited to the above-described embodiment. Various modifications or improvements can be made to the above-described embodiments, and the above-described embodiments can be combined without departing from the spirit of the present invention.

For example, the photolysis system 10 of the first embodiment may not include at least one of the supply source 11 for supplying hydrogen and the gas cooler 9 . The photolysis system 20 of the second embodiment may not have the gas cooler 9 . The photolysis system 30 of the third embodiment may not have the gas cooler 9 or may additionally have the water removal device 3 . Any one of the following: reducing water from a gas containing carbon dioxide and water, adding hydrogen to the gas, adding oxygen to the gas, and cooling the temperature of the gas Decomposition efficiency is improved.

[How to apply the photolysis system]
FIG. 9 is a flow diagram from CO ₂ capture to immobilization. An example of a CO ₂ fixation method to which the above-described CO ₂ photolysis system 10 can be applied will be described with reference to FIG. 9 .

In step S1, a fossil fuel combustion engine (eg, a thermal power plant) emits _CO2 .

In step S2, the high-concentration _CO2 generator converts the emitted low-concentration _CO2 into high-concentration _CO2 . By increasing the concentration of CO ₂ , CO ₂ can be efficiently decomposed in the post-process. It is known that high concentrations of _CO2 can be achieved with adsorbents and physical membranes. Moreover, as described above, it may be left in order to improve the decomposition efficiency of CO ₂ in the post-process. The step S2 for enriching _CO2 is not a mandatory step.

In step S3, the high-concentration CO ₂ is guided to the photolysis system (10, 20, 30) described above as the gas G1 to be treated, and the CO ₂ is decomposed in the system (10, 20, 30), produces CO. The treated gas G3 after photolysis includes CO produced, residual CO ₂ and O ₂ , and H ₂ or O ₂ .

At step S4, the treated gas G3 is separated to remove carbon monoxide. As a method for separating carbon monoxide, for example, a method of adsorbing carbon monoxide to a porous material can be used. The gas from which carbon monoxide has been removed is again led to the high-concentration CO ₂ generator (step S2), and the same process is repeated thereafter.

In step S5, CO is led to a liquid containing sodium hydroxide. The CO is then reacted with NaOH to produce sodium formate. Formic acid produced from sodium formate is an organic substance with a very simple structure, and is used as a raw material in synthetic organic chemistry as well as a hydrogen source in fuel cells.

It does not matter if the organic compound after being immobilized by the CO ₂ immobilization method is consumed by society and discharged again as CO ₂ . In addition, the above-described CO ₂ fixation method is merely an example, and other organic compounds may be used for fixation.

1:

first treatment chamber

2, 2a, 2b: second treatment chamber or treatment chamber (for photolysis) 3: dewatering device 4i: (of first treatment chamber or treatment chamber for photolysis) Inlet 4o: Outlet 5 (of first treatment chamber): Excimer lamp 6: Outlet 7 (of treatment chamber for photolysis or second treatment chamber): Connection pipe 8: Supply pipe 9: Gas cooler 10: photolysis system 11: supply source 12: flow valve 13: temperature sensor 14: controller 20: photolysis system 21: supply source 22: flow valve 28: supply pipe 30: photolysis system G1, G2, G3: gas G4, G5: additional gas L1: excimer light

Claims

A method for photolysis of carbon dioxide, comprising:
reducing water from the gas comprising carbon dioxide and water;
and a step of photolyzing the carbon dioxide by irradiating a water-reduced gas with excimer light.
The photolysis method according to claim 1, wherein the gas contains 0.4% or more and 4.0% or less of hydrogen.
The photolysis method according to claim 1, wherein the gas contains 5% or more and 25% or less of oxygen.
A method for photolysis of carbon dioxide, comprising:
A photodecomposition method, comprising the step of photolyzing carbon dioxide by irradiating a gas containing carbon dioxide and 5% or more and 25% or less of oxygen with excimer light.
A method for photolysis of carbon dioxide, comprising:
A light characterized by comprising a step of photolyzing the carbon dioxide by irradiating the gas containing the carbon dioxide and 0.4% or more and 4.0% or less of hydrogen with excimer light. decomposition method.
The photodecomposition method according to any one of claims 1 to 5, wherein the temperature of the gas in the atmosphere irradiated with the excimer light is 400K or less.
The photodecomposition method according to any one of claims 1 to 5, characterized in that the main wavelength of the excimer light is 172 nm.
The photolysis method according to any one of claims 1 to 3, characterized in that the concentration of water contained in the gas after reducing the water is 100 ppm or less.
The photolysis method according to any one of claims 1 to 5, characterized in that the excimer light is irradiated while sending the gas into a space irradiated with the excimer light.
A carbon dioxide photolysis system comprising:
a first processing chamber including an inlet for introducing the gas containing carbon dioxide and water, a water removal device for removing water from the gas, and an outlet for discharging the gas from the water removal device;
a second processing chamber connected to the discharge port of the first processing chamber and into which the gas discharged from the discharge port is introduced;
an excimer lamp that irradiates the gas in the second processing chamber with excimer light,
The photolysis system, wherein the second processing chamber photolyzes the carbon dioxide contained in the gas present therein with the excimer light.
a hydrogen supply source that supplies hydrogen to the gas upstream from the second processing chamber;
a hydrogen regulating valve that adjusts the supply amount of the hydrogen;
11. The light according to claim 10, further comprising a control unit that controls the hydrogen adjustment valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less. decomposition system.
an oxygen supply source upstream from the second processing chamber for supplying oxygen to the gas;
an oxygen regulating valve that adjusts 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.
A carbon dioxide photolysis system comprising:
an inlet for introducing the gas containing carbon dioxide;
an excimer lamp for emitting excimer light;
a processing chamber connected to the inlet, wherein the excimer light photolyzes the carbon dioxide present therein;
an oxygen supply source upstream from the processing chamber for supplying oxygen to the gas;
an oxygen regulating valve that adjusts the amount of oxygen supplied;
and a controller that controls the oxygen control valve so that the oxygen concentration in the gas is 5% or more and 25% or less.
A carbon dioxide photolysis system comprising:
an inlet for introducing the gas containing carbon dioxide;
an excimer lamp for emitting excimer light;
a processing chamber connected to the inlet, wherein the excimer light photolyzes the carbon dioxide present therein;
a hydrogen supply source upstream from the processing chamber for supplying hydrogen to the gas;
a hydrogen regulating valve that adjusts the supply amount of the hydrogen;
and a control unit that controls the hydrogen adjustment valve so that the hydrogen concentration in the gas is 0.4% or more and 4.0% or less.
The excimer lamp is arranged inside the second processing chamber according to any one of claims 10 to 12 or inside the processing chamber according to claim 13 or 14. decomposition system.
a cooler for cooling the gas, arranged upstream of the second processing chamber according to any one of claims 10 to 12 or upstream of the processing chamber according to claim 13 or 14;
a temperature sensor that measures the temperature of the gas cooled by the cooler;
and a controller that controls the cooler so that the measured value of the temperature sensor is 400K or less.
The photolysis system according to any one of claims 10 to 14, characterized in that the excimer lamp is characterized in that the luminous gas enclosed in the arc tube is xenon gas.
Photolysis system according to any one of claims 10 to 12, characterized in that the water removal device comprises zeolite.