Patent Publication Number: US-2023147787-A1

Title: Gas treatment method and gas treatment device

Description:
TECHNICAL FIELD 
     The present invention relates to a gas treatment method and a gas treatment device. Particularly, the present invention relates to a gas treatment method and a gas treatment device used to treat a gas to be treated by irradiating the gas to be treated with ultraviolet light, with the gas to be treated containing a substance that is a type of volatile organic compound (VOC) and that is subjected to treatment. 
     BACKGROUND ART 
     Conventionally, some technologies utilize oxidation catalysts or photocatalysts as decomposition and removal devices for formaldehyde (see Patent Documents 1 and 2 below). 
     The following Patent Document 3 discloses a xenon excimer lamp designed to emit light at 172 nm, which is a shorter wavelength than the above low-pressure mercury lamp. 
     PRIOR ART DOCUMENT 
     Patent Documents 
     Patent Document 1: JP-A-2004-163055 
     Patent Document 2: JP-A-2000-102596 
     Patent Document 3: JP-A-2007-335350 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Ultraviolet light with a main emission wavelength of 180 nm or less, as described in Patent Document 3, has conventionally been used for optical cleaning and surface modification in manufacturing processes of semiconductors and liquid crystal panels, and has been supposed to be irradiated in an atmosphere of an inert gas such as nitrogen or clean dry air. 
     In recent years, gases possibly affect the global environment and humans have become a problem, and certain regulations are imposed on emissions discharged from places such as automobiles and factories. Even indoors, many places where there is a possibility of using certain chemicals, such as laboratory facilities and medical sites, have VOC emission regulations. 
     However, knowledge has not been acquired until now about treating a VOC-containing gas to be treated by using ultraviolet light with the main emission wavelength of 180 nm or less. 
     It is an object of the present invention to provide a gas treatment method and a gas treatment device that enable efficient treatment of a VOC-containing gas to be treated by using ultraviolet light with the main emission wavelength of 180 nm or less. 
     Means for Solving the Problems 
     A gas treatment method of the present invention is a method for treating a gas to be treated containing a mixture of air and a substance that is a type of volatile organic compound (VOC) and that is subjected to treatment by causing the gas to be treated to flow through a treatment space, 
     in which a light source designed to emit ultraviolet light having a main emission wavelength of 160 nm to 180 nm is located in the treatment space, and the gas to be treated is passed through a gap with a separation distance of 10 mm or less from a light-emitting area of the light source at a flow velocity of 23 m/s or less. 
     As described later in the “MODE FOR CARRYING OUT THE INVENTION” section, according to the inventor&#39;s intensive research, it is confirmed that when the gas to be treated is irradiated with ultraviolet light having a main emission wavelength of 160 nm to 180 nm, the decomposition performance of the substance to be treated decreases as the flow velocity of the gas to be treated increases if it exceeds 23 m/s. 
     Additionally, the ultraviolet light having a main emission wavelength of 160 nm to 180 nm is easily absorbed by oxygen due to its short wavelength. This leads to a situation in which the ultraviolet light hardly reaches the gas to be treated flowing at places over 10 mm distant from the light-emitting area of the light source and the VOC is hardly decomposed. 
     According to the method described above, the VOC-type substance contained in the gas to be treated and subjected to treatment can be efficiently decomposed. 
     In this specification, the “main emission wavelength” refers to a wavelength Xi in the wavelength range Z(λi) that accounts for 40% or more of a total integrated intensity of the emission spectrum when the wavelength range Z(λ) of ±10 nm is defined for a certain wavelength X. in the emission spectrum. For a light source, such as an excimer lamp in which a predetermined luminescent gas is enclosed, that has a considerably narrow half bandwidth and displays light intensities only at specific wavelengths, a wavelength with the highest relative intensity (the main peak wavelength) can be, in most cases, regarded as the main emission wavelength. 
     In the method described above, the flow velocity of the gas to be treated may be 0.3 m/s or more. 
     By setting the flow velocity of the gas to be treated is 0.3 m/s or more, the gas to be treated is allowed to produce a turbulent flow in the treatment space. This increases the time during which the gas to be treated is irradiated with the ultraviolet light from the light source, thereby improving the decomposition efficiency of the substance subjected to treatment. 
     The substance subjected to treatment may include at least one type selected from the group consisting of formaldehyde and toluene. The group for the substance subjected to treatment may include methanol, isopropyl alcohol (IPA), acetone, methyl isobutyl ketone (MIBK), and ethyl acetate, other than the above-exemplified substances. 
     A gas treatment device of the present invention is configured to treat a gas to be treated containing a mixture of air and a substance that is a type of volatile organic compound (VOC) and that is subjected to treatment by causing the gas to be treated to flow through a treatment space. The gas treatment device includes: 
     a gas inlet to introduce the gas to be treated into the treatment space; 
     a gas outlet to exhaust the gas to be treated that is treated in the treatment space; and 
     a light source located in the treatment space, the light source being configured to emit ultraviolet light having a main emission wavelength of from 160 nm to 180 nm, in which the gas to be treated is passed through a gap with a separation distance of 10 mm or less from a light-emitting area of the light source at a flow velocity of 23 m/s or less. 
     The flow velocity of the gas to be treated may be 0.3 m/s or more. 
     By setting the flow velocity of the gas to be treated is 0.3 m/s or more, a turbulent flow of the gas to be treated is generated in the treatment space, which ensures that the gas to be treated remains in the treatment space for a longer period and allows the ultraviolet light to be irradiated for the time necessary for the decomposition of the substance subjected to treatment. 
     The light source may be an excimer lamp that includes a tubular body enclosed with a luminescent gas containing xenon (Xe). In this case, the light source emits ultraviolet light having a main emission wavelength of 172 nm. 
     The gas treatment device may include a wind shielding member arranged to surround the tubular body with the gap left or to interpose the tubular body with the gap left. The gas treatment device may include a blower arranged between the gas inlet and the gas outlet to control the flow velocity of the gas to be treated. In this case, with the blower, the gas to be treated is controlled to be introduced into the treatment space at a flow velocity of 23 m/s or less. 
     Effect of the Invention 
     According to the present invention, the VOC-type substance contained in the gas to be treated and subjected to treatment can be efficiently decomposed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view showing a configuration of a gas treatment device according to an embodiment of the present invention. 
         FIG.  2    is a schematic cross-sectional view of an excimer lamp. 
         FIG.  3    is a schematic plan view of an excimer lamp and a wind shielding member, viewed along a direction d 1 . 
         FIG.  4    is a superimposed graph of an emission spectrum of an excimer lamp enclosed with a luminescent gas containing Xe and absorption spectra of oxygen (O 2 ) and ozone (O 3 ). 
         FIG.  5    is a graph showing a relationship between distance from a surface of an excimer lamp and a concentration of hydroxy radical (.OH) contained in a gas to be treated when there is no wind shielding member. 
         FIG.  6    is a schematic cross-sectional view of another excimer lamp. 
         FIG.  7    is a schematic cross-sectional view of another excimer lamp. 
         FIG.  8    is a schematic view showing a configuration of an experimental system. 
         FIG.  9    is a graph showing a relationship between the flow velocity of a gas to be treated and the VOC removal ratio. 
         FIG.  10    is a schematic view showing a configuration of a gas treatment device according to another embodiment of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of a gas treatment device and a gas treatment method of the present invention will be described with reference to the drawings as appropriate. 
     Regarding the drawings shown below, size proportions in the drawings do not always coincide with actual size proportions. 
       FIG.  1    is a schematic view showing a configuration of a gas treatment device according to an embodiment. A gas treatment device  1  has a housing  3  that incorporates a hollow-shaped treatment space  8 . The housing  3  includes a gas inlet  5  and a gas outlet  7  located at a place apart from the gas inlet  5  in a direction d 1 . A gas to be treated G 1  introduced from the gas inlet  5  travels in the direction d 1  and is treated in the treatment space  8 . After that, the gas is exhausted as a treated gas G 2  from the gas outlet  7 . 
     The gas treatment device  1  treats the gas to be treated G 1 , which contains a mixture of air and a substance that is a type of VOC and that is subjected to treatment, and converts the gas G 1  into a treated gas G 2  by decreasing a concentration of the contained substance subjected to treatment. Examples of the substance of the VOC type subjected to treatment include formaldehyde, toluene, methanol, isopropyl alcohol (IPA), acetone, methyl isobutyl ketone (MIBK), and ethyl acetate. The gas treatment device  1  can be used to treat a gas containing at least one type of these substances subjected to treatment. 
     The gas treatment device  1  includes an excimer lamp  10  located in the treatment space  8 .  FIG.  2    is a schematic cross-sectional view of the excimer lamp  10 , cross-sectioned at a plane orthogonal to the direction d 1 . 
     The excimer lamp  10  includes a tubular body  14  extending along the direction d 1 . More specifically, the tubular body  14  includes an outer tube  14   a  having a cylindrical shape and located outside, and inner tube  14   b  located inside the outer tube  14   a  coaxially with the outer tube  14   a  and having a cylindrical shape with a smaller inner diameter than the outer tube  14   a . Both the tubes ( 14   a ,  14   b ) of the tubular body  14  are made of a dielectric substance such as synthetic fused silica. 
     The outer tube  14   a  and the inner tube  14   b  are both sealed at the end of the direction d 1  (not shown), and a light-emitting space with an annular shape, when viewed from the direction d 1  is formed between them. A luminescent gas  13 G that forms excimer molecules by electric discharging is sealed in the light-emission space. In one specific example, the luminescent gas  13 G is a gas that contains both xenon (Xe) and neon (Ne) in a predetermined ratio (e.g., 3:7) and may further include oxygen and hydrogen in minute quantities. 
     The excimer lamp  10  illustrated in  FIG.  2    includes a first electrode  11  disposed on an outer wall surface of the outer tube  14   a  and a second electrode  12  disposed on an inner wall surface of the inner tube  14   b . The first electrode  11  has a mesh shape or a line shape. The second electrode  12  has a film shape. The second electrode  12  may have a mesh shape or a line shape in a similar way to the first electrode  11 . 
     In an example shown in  FIG.  1   , the excimer lamp  10  includes a base  19  at each end in the direction d 1 . The bases  19  are made of a ceramic material (an inorganic material) such as steatite, forsterite, sialon, or alumina and have a function of fixing the ends of the tubular body  14 . Through a hole formed in the base  19  or an outer edge of the base  19 , a power feeder (not shown) is laid to connect to the electrodes ( 11 ,  12 ). 
     When an alternating-current voltage at a high frequency, for example, approximately from 50 kHz to 5 MHz is applied to between the first electrode  11  and the second electrode  12  of the excimer lamp  10  from a lighting power source (not shown) via the power feeder, the voltage is applied to the luminescent gas  13 G via the tubular body  14 . This produces discharge plasma in a discharge space filled with the luminescent gas  13 G and causes atoms of the luminescent gas  13 G to be excited and get into an excimer state. When the atoms transition to a ground state, excimer light is generated. When the luminescent gas  13 G is the gas that contains xenon (Xe) described above, the excimer light is ultraviolet light L 1  with a peak wavelength near 172 nm. 
     The gas treatment device  1  of the present embodiment includes a wind shielding member  20 .  FIG.  3    is a schematic plan view of the excimer lamp  10  and the wind shielding member  20 , viewed along the direction d 1 . The wind shielding member  20  includes a wind shielding surface  23  that has an opening around a center of the surface. The excimer lamp  10  is inserted through inside the opening of the wind shielding member  20  to create a gap  21  between a light-emitting area of the excimer lamp  10  and the wind shielding surface  23 . A dimension of the gap  21  is 10 mm or less and preferably is 8 mm or less. On the other hand, if the gap  21  is too thin, the treatment capacity of the gas treatment device may decrease since the gas to be treated G 1  introduced from the gas inlet  5  is less likely to flow into the downstream space after colliding with the wind shielding member  20 . Therefore, the dimension of the gap  21  is preferably 2 mm or more, and is more preferably 3 mm or more. 
     In the excimer lamp  10  shown in  FIG.  2   , the light-emitting area corresponds to the wall surface of the tubular body  14  that is not hidden beneath the first electrode  11  but is exposed because of the mesh shape or the line shape of the first electrode  11 . 
       FIG.  4    is a superimposed graph of an emission spectrum of an excimer lamp enclosed with a luminescent gas containing Xe and absorption spectra of oxygen (O 2 ) and ozone (O 3 ). In  FIG.  4   , the horizontal axis represents wavelengths, the left vertical axis represents relative values of light intensity of the excimer lamp, and the right vertical axis represents absorption coefficients of oxygen (O 2 ) and ozone (O 3 ). 
     When the luminescent gas in the excimer lamp  10  is a gas containing Xe, as shown in  FIG.  4   , the main emission wavelength of ultraviolet light L 1  emitted from the excimer lamp  10  is in a range of  160  nm to  180  nm (hereinafter referred to as a “first wavelength range λ 1 ”). As shown in  FIG.  4   , the light in the first wavelength range λ 1  is absorbed by oxygen (O 2 ) to a large extent. 
     When the gas to be treated G 1  is irradiated with the ultraviolet light L 1  emitted in the first wavelength range λ 1  from the excimer lamp  10  and the ultraviolet light is absorbed by oxygen (O 2 ), a reaction in the following formula (1) proceeds. In formula (1), O( 1 D) represents an oxygen atom in an excited state and displays high reactivity. O( 3 P) is an oxygen atom in a ground state. In formula (1), hv(λ 1 ) represents the absorption of light in the first wavelength range 
       O 2   +hv (λ 1 )→O( 1   D )+O( 3 P)   (1)
 
     O( 3 P) generated by the formula (1) reacts with oxygen (O 2 ) contained in the gas to be treated G 1  and generates ozone (O 3 ) in accordance with the formula (2). 
       O( 3 P)+O 2 →O 3    (2)
 
     Part of O( 1 D) displaying high reactivity reacts with moisture (H 2 O ) contained in the gas to be treated G 1  and generates a hydroxy radical (.OH) in accordance with the formula (3). 
       O( 1   D )+H 2 →·OH+.OH   (3)
 
     O( 1 D) and the hydroxyl radical (.OH) displaying high reactivity are generated by the reactions described above, and the substance of the VOC type contained in the gas to be treated G 1  and subjected to treatment is thereby efficiently decomposed. 
     Meanwhile, as described above with reference to  FIG.  4   , the ultraviolet light L 1  in the first wavelength range λ 1  is absorbed by oxygen (O 2 ) to a large extent. Thus, despite the introduction of the gas to be treated G 1  from the gas inlet  5 , the ultraviolet light L 1  is absorbed by oxygen contained in the gas to be treated G 1  flowing in a vicinity of the excimer lamp  10  if the wind shielding member  20  is not disposed inside the housing  3 . Consequently, the gas to be treated G 1  flowing through a place apart from the excimer lamp  10  cannot be irradiated with the ultraviolet light L 1  while maintaining a high light intensity. 
       FIG.  5    is a graph showing a relationship between distance from a surface of the excimer lamp  10  and relative illumination intensity of the ultraviolet light L 1  emitted from the excimer lamp  10  when the excimer lamp  10  is placed in the housing  3  without the wind shielding member  20  and is emitting the light while the gas to be treated G 1  is flowing through the housing. In detail,  FIG.  5    corresponds to the results calculated by simulation based on the spectral data of excimer lamp  10  and the absorption coefficient of oxygen (O2) shown in  FIG.  4   , and the distance through which the ultraviolet light L 1  is transmitted, assuming that the ultraviolet light L 1  is exponentially attenuated along with an increase in the distance through which the ultraviolet light L 1  is transmitted. In  FIG.  5   , relative illumination intensities at different places are plotted on a graph on the condition that the illumination intensity of the ultraviolet light L 1  at a place of zero distance through which the ultraviolet light is transmitted, i.e., at the surface of the excimer lamp  10 , is 100%. 
     According to the formulas (1) and (3), it is found that an amount of the hydroxy radical (.OH) generated is proportional to an amount of O( 1 D), and the amount of O( 1 D) is proportional to the quantity of the light with which the gas is irradiated. In other words,  FIG.  5    shows a relationship between distance from the surface of the excimer lamp  10 , i.e., the light-emitting area, and the amount of the hydroxy radical (.OH) generated. According to  FIG.  5   , it is observed that a concentration of the hydroxy radical (.OH) decreases with an increase in the distance from the surface (the light-emitting area) of the excimer lamp  10 . It is observed that the concentration of the hydroxyl radical (.OH) is extremely low at a place approximately over 10 mm distant from the surface (the light-emitting area) of the excimer lamp  10 . According to  FIG.  5   , it is observed that the tendency is shown similarly regardless of humidity of the gas to be treated G 1  and the concentration of the hydroxyl radical (.OH) generated rises with an increase in the humidity of the gas to be treated G 1 . 
     As described above, according to the gas treatment device  1  of the present embodiment, since the wind shielding member  20  is located inside the housing  3 , a region where the gas to be treated G 1  flows through is limited by the wind shielding member  20 . More specifically, the gas to be treated G 1  introduced from the gas inlet  5  collides with the wind shielding surface  23 , a direction in which the gas is traveling changes, and the gas flows through the gap  21  with a dimension of 10 mm or less. This allows the gas to be treated G 1  to be guided to a vicinity of the light-emitting area of the excimer lamp  10 . Consequently, the gas to be treated G 1  located at the area is irradiated with the ultraviolet light L 1  with a high rate from the excimer lamp  10 , and the probability of generation of O( 1 D) and OH, which display high reactivity, is increased. 
     The gas treatment device  1  shown in  FIG.  1    includes two pieces of the wind shielding members  20  at places apart from each other in the direction d 1 , but there may be three or more wind shielding members  20 , or even just one. Even if the gas treatment device  1  includes one piece of the wind shielding member  20 , the gas to be treated G 1  introduced from the gas inlet  5  passes through the gap  21  and just after that, flows toward the gas outlet  7 . As a result, the gas to be treated G 1  is sure to pass through the vicinity of the light-emitting area of the excimer lamp  10 , albeit temporarily. Thus, the gas passing through the vicinity of the area is irradiated with the ultraviolet light L 1 , and this increases the probability of generation of O( 1 D) and OH, which display high reactivity. 
     The shape of the excimer lamp  10  is not limited to that in  FIG.  2   , but the shape may form a structure shown in  FIG.  6    or  FIG.  7   .  FIG.  6    is a schematic cross-sectional view of an excimer lamp  10  having what is called a “single-tube structure”, cross-sectioned at a plane orthogonal to the direction d 1 . Being different from the excimer lamp  10  shown in  FIG.  2   , the excimer lamp  10  shown in  FIG.  6    includes a tubular body  14  of one tube. The tubular body  14  is sealed off at each end (not shown) in a lengthwise direction, i.e., in the direction d 1 . A luminescent gas  13 G is enclosed in a space inside the tubular body. A second electrode  12  is placed inside (in an interior of) the tubular body  14 , and a first electrode  11  having a net shape or a line shape is placed on an outer wall surface of the tubular body  14 . 
       FIG.  7    is a schematic cross-sectional view of an excimer lamp  10  having what is called a “flat-tube structure”, illustrated in a similar way to  FIG.  6   . The excimer lamp  10  shown in  FIG.  7    includes a tubular body  14  of one tube being rectangular when viewed in the lengthwise direction, i.e., in the direction d 1 . The excimer lamp  10  includes a first electrode  11  disposed on one outer surface of the tubular body  14  and a second electrode  12  disposed on a place that is another outer surface of the tubular body  14  and that is opposite to the first electrode  11 . The first electrode  11  and the second electrode  12  each have a mesh shape (a net shape) or a linear shape so as not to hinder the ultraviolet light L 1  generated in the tubular body  14  from being emitted outside the tubular body  14 . 
     The shape of the excimer lamp  10  cross-sectioned at a plane orthogonal to the direction d 1  is not limited to the circles shown in  FIGS.  2  and  6    and to the rectangle shown in  FIG.  7   . The excimer lamp may have any of other various shapes in cross-section. 
     It is observed that by causing the gas to be treated G 1  to flow through the vicinity of the light-emitting area of the excimer lamp  10  at a flow velocity of 23 m/s or less, the gas treatment device  1  decomposes the VOC contained in the gas to be treated G 1  with further increased efficiency. A description will be given below with reference to examples. 
     EXAMPLES 
     A description is given below based on experimental data.  FIG.  8    is a schematic view showing a configuration of an experimental system imitating the gas treatment device  1 . An experimental system  2  includes a housing  3  that has an internal treatment space  8 , as well as conduits ( 51 ,  52 ,  53 ), joined to the housing  3 . A VOC generator  30  volatilizes VOCs to produce the gas to be treated G 1 , which is obtained by mixing formaldehyde into the air. The gas to be treated G 1  is taken into the conduit  51  from a gas inlet  5 , flows through the conduit  51 , and is introduced into the treatment space  8 . The gas to be treated G 1  is treated in treatment space  8  by being irradiated with ultraviolet light L 1 . After that, the gas as a treated gas G 2  passes through the conduits  52 ,  53  and is exhausted from a gas outlet  7  to outside the system. 
     The conduit  52  is provided with a VOC measuring instrument  41  to measure the concentration of the VOC contained in the gas discharged from the treatment space  8 . In conduit  53 , a blower  42  is provided to control the flow velocity of the gas to be treated G 1 . 
       FIG.  9    is a graph showing a relationship between flow velocity of the gas to be treated G 1  and VOC removal ratio in both a case where the VOC generator  30  generates formaldehyde as a VOC and a case where toluene is generated as a VOC. A VOC removal ratio Y1 is a value defined by: 
         Y 1=( A 1 −A 2)/ A 1 
     where A1 is a reference concentration that is a concentration of the VOC contained in the gas to be treated G 1  before the gas is irradiated with the ultraviolet light L 1 , and A2 is a post-treatment concentration that is a concentration of the VOC contained in the treated gas G 2 . 
     For the reference concentration A1, a concentration of the VOC read out by the VOC measuring instrument  41  when the gas to be treated G 1  flowed from the gas inlet  5  through the system with the excimer lamp  10  being unlit was used. For the post-treatment concentration A2, the value measured similarly, with the excimer lamp  10  being lit, was used. 
     Detailed conditions specified for the experimental system  2  were as described below. 
     When the VOC was formaldehyde, settings were configured such that the concentration of formaldehyde contained in the gas to be treated G 1  owing to the VOC generator  30  was 15 ppm. When the VOC was toluene, settings were configured such that the concentration of toluene contained in the gas to be treated G 1  owing to the VOC generator  30  was 10 ppm. 
     The excimer lamp  10  used for the system was an excimer lamp of the flat-tube structure shown in  FIG.  7   . Specifically, the dimensions of a rectangular section of the excimer lamp were 11 mm×20 mm when viewed along the direction d 1 , and a length in the direction d 1  was 145 mm. 
     A wind shielding member  20  was installed at a place 100 mm away from an end of the excimer lamp  10  adjacent to the gas inlet  5  in the direction d 1 . The wind shielding member  20  was a plate-shaped member made of a fluorine resin such as polytetrafluoroethylene (PTFE) and had an opening such that a gap  21  of 6 mm was maintained from electrodes ( 11 ,  12 ) when a tubular body  14  of the excimer lamp  10  was inserted into the opening. With reference to  FIG.  7   , the gap  21  formed between an end face of the tubular body  14  in a lateral direction and the wind shielding member  20  was 6 mm or less. 
     All of an inner diameter of a gas-flowing part of the housing  3  and inner diameters of the conduits ( 52 ,  53 ) were Φ100. 
     The flow velocity of the gas to be treated G 1  was measured with the testo 425 hot wire anemometer (made by Testo K. K.). Flow velocity values on the horizontal axis in  FIG.  9    were values determined by calculating velocity values of the gas to be treated G 1  passing through the gap  21  of 3 mm based on velocity values of the treated gas G 2  flowing into the pipe  53  of Φ100, which were measured by the method above. More specifically, a flow rate Q of the gas to be treated G 1  (and the treated gas G 2 ) is determined by the product of a cross-sectional area S of the gas-flowing region and a flow velocity v. Since the product of the cross-sectional area S and the flow velocity v is constant while the flow rate Q is constant, a flow velocity v2 at a place to be obtained is determined based on a cross-sectional area S 1  of the gas-flowing region at a place where the flow velocity v is measured, a flow velocity v1 that is measured, and a cross-sectional area S 2  of the gas-flowing region at the place to be obtained. 
     Values plotted on the graph shown in  FIG.  9    are as shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Flow velocity 
                 Removal Ratio [%] 
                   
               
            
           
           
               
               
               
            
               
                 [m/s] 
                 Formaldehyde 
                 Toluene 
               
               
                   
               
            
           
           
               
               
               
            
               
                 12 
                 99.9 
                 99.7 
               
               
                 23 
                 99.8 
                 99.5 
               
               
                 27 
                 98.6 
                 98.4 
               
               
                 31 
                 93.5 
                 92.9 
               
               
                 35 
                 81.6 
                 80.3 
               
               
                 38 
                 65.4 
                 63.2 
               
               
                   
               
            
           
         
       
     
     Unexpectedly, according to  FIG.  9   , it is observed that whereas the VOC removal ratio is substantially 100% for the flow velocity of the gas to be treated G 1  passing through the gap  21  in a range of 23 m/s or less, the VOC removal ratio declines with a rise in the flow velocity in a range exceeding 23 m/s. Further, it is observed that the tendency of decline in the VOC removal ratio is conspicuous when the flow velocity of the gas to be treated G 1  passing through the gap  21  is higher than 35 m/s. Similar results were obtained for both VOC substances, formaldehyde and toluene. Moreover, results identical to those in this example were obtained for the structure of the excimer lamp  10  in the form of the one shown in  FIG.  2   . 
     This is thought to be because even if the gas to be treated G 1  passes through the gap  21  to flow through the vicinity of the light-emitting area of the excimer lamp  10  if the velocity of the gas is too fast, it will immediately spread to a location far from the light-emitting area of the excimer lamp  10  after passing through the gap  21 , and the reaction described in formulas (1) to (3) above will not occur sufficiently. 
     On the other hand, it is thought that the VOC is efficiently decomposed if the flow velocity of the gas to be treated G 1  passing through the gap  21  is within the range of 23 m/s or less because the gas to be treated G 1  can flow in the vicinity of the light-emitting area of the excimer lamp  10  during a period for which the reactions in the formulas (1) to (3) described above are occurring. 
     According to similar calculations, it is preferred that the flow velocity of the gas to be treated G 1  passing through the gap  21  be 8 m/s or less when the dimension of the gap  21  is  7  mm. It is preferred that the flow velocity of the gas to be treated G 1  passing through the gap  21  be 6.7 m/s or less when the dimension of the gap  21  is 8 mm. It is preferred that the flow velocity of the gas to be treated G 1  passing through the gap  21  be 5.7 m/s or less when the dimension of the gap  21  is 9 mm. It is preferred that the flow velocity of the gas to be treated G 1  passing through the gap  21  be 5 m/s or less when the dimension of the gap  21  is 10 mm. 
     On condition that a viscosity coefficient of the gas to be treated G 1  is μ [N·s/m 2 ], a diameter of the pipe is L [m], the density is ρ [kg/m 3 ], and the flow velocity is v [m/s], the Reynolds number Re is defined by the following equation (4): 
       Re=(ρ· v·L )/μ  (4)
 
     When the Reynolds number Re is 2300 or more, the flow of the fluid is assessed as a turbulent flow. If the inner diameter of the conduit ( 52 ,  53 ) Φ=100 mm is adopted as the value of L, the viscosity coefficient μ is 1.82×10 −5  [Pa·s], and the density ρ is 1.20×10 −3  [g/cm3], it is confirmed that the gas to be treated G 1  produces a turbulent flow inside the conduits ( 52 ,  53 ) when the flow velocity v≥0.32 m/s. Assuming that the gas treatment device  1 , which is joined to a typical conduit with a diameter of or more, is used, the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 0.3 m/s or more. As a result, a large portion of the gas to be treated G 1  is apt to flow to the vicinity of the light-emitting area of the excimer lamp  10 . 
     [Other Embodiments] 
     Other embodiments will now be described. 
     &lt;1&gt; In the embodiment described above, the wind shielding member  20  is a plate-shaped member that has an opening around the center thereof, and the excimer lamp  10  as a light source is inserted in the opened part to form the gap  21 . However, a plurality of wind shielding members  20  without openings may be arranged to interpose the tubular body  14  of the excimer lamp  10 , so that the length of the gap  21  formed between the wind shielding members  20  and the light-emitting area of the excimer lamp  10  is less than or equal to 10 mm. 
     &lt; 2 &gt; With reference to  FIG.  10   , an opened space inside a housing  3  may be partly made thin without wind shielding member  20  such that the length of the gap  21  formed between an inner wall of the housing  3  and the light-emitting area of the excimer lamp  10  is less than or equal to 10 mm. 
     In this case, with L=10 mm in the above formula (4), the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 3.5 m/s or more. In a case where L=9 mm, the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 3.9 m/s or more. In a case where L=8 mm, the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 4.4 m/s or more. In a case where L=7 mm, the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 5 m/s or more. In a case where L=6 mm, the gas to be treated G 1  is apt to produce a turbulent flow in the gas treatment device  1  when the flow velocity of the gas to be treated G 1  is 5.8 m/s or more. 
     &lt; 3 &gt; In the above embodiment, the case where the longitudinal direction of the excimer lamp  10  matches the flow direction d 1  of the gas to be treated G 1  is described, but the arrangement mode of the excimer lamp  10  is arbitrary. 
     Furthermore, in the above embodiment, the case where the gas treatment device  1  has an excimer lamp  10  as the light source is described. However, the light source is not limited to the excimer lamp  10  as long as it emits ultraviolet L 1  having a main emission wavelength of 160 nm to 180 nm. 
     &lt;4&gt; In the experimental system  2  referred to in  FIG.  8   , the flow velocity of the gas to be treated G 1  is controlled by the blower  42 , but the method of controlling the velocity of the gas to be treated G 1  flowing through the gap  21  to 23 m/s or less is arbitrary. 
     &lt;5&gt; According to  FIG.  9    and Table 1, it is observed that the VOC removal ratio further declines when the flow velocity of the gas to be treated G 1  exceeds 35 m/s. When the flow velocity of the gas to be treated G 1  is 35 m/s or less, a removal ratio of 80% or more is achieved for both formaldehyde and toluene. 
     Depending on the environmental standards, a gas containing VOC (the gas to be treated G 1  shown herein) is permitted to be exhausted from a duct to outside the system in some cases, if the system is capable of removing the VOC from the gas at a ratio of 80% or more. In such a case, the flow velocity of the gas to be treated G 1  may be set at 35 m/s or less. Naturally, given further increasing the VOC removal ratio beyond 80%, the flow velocity of the gas to be treated G 1  is preferably set to 31 m/s or less and is particularly preferably set to 23 m/s or less. 
     DESCRIPTION OF REFERENCE SIGNS 
       1  Gas treatment device 
       2  Experimental system 
       3  Housing 
       5  Gas inlet 
       7  Gas outlet 
       8  Treatment space 
       10  Excimer lamp 
       11  First electrode 
       12  Second electrode 
       13 G Luminescent gas 
       14  Tubular body 
       14   a  Outside tube 
       14   b  Inside tube 
       19  Base 
       20  Wind shielding member 
       21  Gap 
       23  Wind shielding surface 
       30  VOC generator 
       41  VOC measuring instrument 
       42  Blower 
       51 ,  52 ,  53  Conduit 
     L 1  Ultraviolet light