Abstract:
A desulfurization device releases exhaust gas into the atmosphere without reduction in CO 2  recovery rate and without mercury components. Because the absorbent of the desulfurization device is drawn from an absorbent reservoir by a circulating pump and sprayed through spray nozzles into a desulfurization-absorption unit and is mainly circulated outside the wall of a water seal tube by a stirrer in the absorbent reservoir, the flow of the absorbent that falls from the desulfurization-absorption unit into the water seal tube flows in a single direction from top to bottom and hinders the ascension of gas bubbles. Intermixing of the gas for oxidizing the sulfur dioxide with the desulfurization device exhaust gas is thereby prevented, efficient CO 2  recovery is possible without reduction in the CO 2  concentration recovered from the exhaust gas after desulfurization and mercury in the combustion exhaust gas is absorbed in the absorbent of the desulfurization device.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a wet flue gas desulfurization device (which may be simply referred to as a desulfurization device hereinafter) which is provided to the thermal power generation boiler plant etc. which performs oxygen combustion, and recovers CO 2  in combustion exhaust gas and removes mercury by a CO 2  recovery device on the downstream side of the desulfurization device, a combustion system and the combustion method including the desulfurization device. 
       BACKGROUND OF THE INVENTION 
       [0002]      FIG. 7  shows an example of a thermal power generation boiler plant, which uses coal as fuel and performs oxygen combustion on the presumption that CO 2  in combustion exhaust gas is recovered by a CO 2  recovery device arranged on the downstream side of the boiler, in thermal power generation boiler plants. 
         [0003]    The thermal power generation boiler plant shown in  FIG. 7  is constituted of a boiler  13 , a denitration device  14 , a heat exchanger  15 , a dust collection device  16 , a desulfurization device  3 , a CO 2  recovery device  17 , a circulation line  18 , an oxygen production device  19 , an oxygen supply line  20 , and others. 
         [0004]    The boiler  13  performs oxygen combustion of fuel  25  such as coal supplied through a fuel supply system and thereby generates exhaust gas. At this time, oxygen is supplied from the oxygen supply line  20  or the like by the oxygen production device  19 . 
         [0005]    The denitration device  14  decomposes NOx (nitrogen oxide) contained in a gas discharged from the boiler  13 . The gas discharged from the denitration device  14  is adjusted to 200° C. to 160° C. by the heat exchanger  15 . 
         [0006]    Smoke dust is removed from the gas by the dust collection device  16 . A part of the gas subjected to dust removal by the dust collection device  16  is introduced into the desulfurization device  3 , and SO 2  is removed. Further, the CO 2  recovery device  17  recovers CO 2  from the exhaust gas from which SO 2  has been removed. 
         [0007]    A part of the gas subjected to the dust removal passes through the circulation line  18 , is reheated to 200° C. by the heat exchanger  15 , and then supplied to the boiler  13 . 
         [0008]    Furthermore,  FIG. 8  shows a configuration of a desulfurization device in an oxygen combustion system according to the conventional technology. The desulfurization device  3  is mainly constituted of a spray nozzle  4  which sprays a desulfurization absorbent liquid  6  (which may be simply referred to as absorbent hereinafter), an absorbent circulation pump  5  which supplies the desulfurization absorbent  6  to the spray nozzle  4 , a desulfurization absorption unit  26  with a mist eliminator  8 , an oxidation gas supply unit  9  for a sulfurous acid generated in the desulfurization absorbent  6 , a stirrer  10  configured to stir the desulfurization absorbent  6 , an absorbent reservoir  11  configured to store the desulfurization absorbent and oxidize the sulfurous acid, and others. The desulfurization device  3  in the conventional system sprays an alkaline absorbent containing limestone and the like, whereby sulfur oxides such as SO 2  contained in boiler exhaust gas  1  are removed. To oxidize a sulfurous acid content (which may be simply referred to as sulfurous acid hereinafter) such as calcium sulfite generated by absorbing SO 2  in the boiler exhaust gas  1 , oxidization gas  27 , i.e. air is supplied from the oxidation gas supply unit  9 . When the sulfurous acid is oxidized, it is recovered as gypsum. 
       PRIOR ART DOCUMENTS 
       [0009]    Patent Document 1: Unexamined Patent Application Publication No. 2007-147162 
         [0010]    Patent Document 2: Unexamined Patent Application Publication No. 2007-147161 
         [0011]    Patent Document 3: Unexamined Patent Application 
         [0012]    Patent Document 4: Unexamined Patent Application Publication No. 5-71726 
       DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0013]    The oxidation gas (oxidation air)  27  supplied to the absorbent reservoir  11  changes to gas bubbles, disperses into the desulfurization absorbent  6 , ascends toward liquid surface  6   a , and contributes to an oxidation reaction of the sulfurous acid until it reaches the liquid surface  6   a . When the oxidation gas  27  shifts to a gas phase  6   b  in the desulfurization absorption unit  26  after reaching the liquid surface  6   a , the oxidation gas  27  hardly contributes to the oxidation reaction of the sulfurous acid. 
         [0014]    At a time of supplying the oxidation gas (the oxidation air)  27  into the desulfurization absorbent  6 , the oxidation air  27  is miniaturized as much as possible in order to relatively increase a contact area of the oxidation air  27  and the desulfurization absorbent  6 , and prolong a residence time. However, after the oxidation gas reaches the liquid surface  6   a , generation of the oxidation air  27  which shifts to the gas phase  6   b  in the desulfurization absorption unit  26  is unavoidable. 
         [0015]    Therefore, to sufficiently effect the oxidation reaction of the sulfurous acid in the absorbent  6  in the absorbent reservoir  11 , the oxidation gas (the oxidation air)  27  which is greatly excessive as compared with a stoichiometrically required amount of oxygen must be supplied from the oxidation gas supply unit  9 . Therefore, a considerable amount of the oxidation gas (the oxidation air)  27  is discharged into the exhaust gas  2 , and the exhaust gas  2  is diluted. 
         [0016]    As described above, it has not been taken into consideration in the conventional technology that CO 2  concentration in the exhaust gas  2  decreases. 
         [0017]    For example, if the concentration of CO 2  contained in the exhaust gas  1  supplied to an inlet of the desulfurization device  3  is 90%, the concentration of CO 2  contained in the exhaust gas  2  at an outlet of the desulfurization device  3  decreases to approximately 80% because of dilution by the oxidation air. Therefore, there is a problem that CO 2  recovery efficiency in the CO 2  recovery device  17  decreases. For example, when air (N 2 :79%, O 2 :21%) is used as the oxidation gas  27 , the oxidation gas  27  gets mixed in with the exhaust gas  2  at the outlet of the desulfurization device, the CO 2  concentration decreases by approximately 10%, and hence a CO 2  recovery rate in the CO 2  recovery device  17  decreases by approximately 5%. 
         [0018]    Additionally, in the coal-fired boiler  13 , mercury contained in coal  25  is discharged into the exhaust gas. Apart of the mercury is taken into the desulfurization absorbent  6  in the form of, mercury chloride, etc. during a process of an exhaust gas treatment. The mercury taken into the desulfurization absorbent  6  may be discharged again (or re-discharged hereinafter) into the gas phase  6   b  in the desulfurization absorption unit  26  in the form of, e.g., metallic mercury depending on conditions. 
         [0019]    Therefore, the re-discharged mercury scatters and reaches the CO 2  recovery device  17  on the downstream side together with the exhaust gas  2  at the outlet of the desulfurization device. The CO 2  recovery device  17  has a process of compressing the exhaust gas  2 , and there is concern that corrosion of constituent devices advances at an accelerated pace under the presence of the mercury. Besides, discharge of the harmful mercury to the outside of a plant system or into the air is not preferable either. 
         [0020]    It is an object of the present invention to provide a wet flue gas desulfurization device, a combustion system using the wet flue gas desulfurization device, and a combustion method that prevent exhaust gas from being diluted with oxidation gas, avoid a reduction of CO 2  recovery rate in a CO 2  recovery device on the downstream side, and also prevent a mercury component in the exhaust gas taken into a desulfurization absorbent from scattering together with the exhaust gas at the outlet of the desulfurization device, reaching the CO 2  recovery device on the downstream side, or being directly discharged to the outside of a plant system or into the air. 
       MEANS FOR SOLVING PROBLEMS 
       [0021]    The problem of the present invention is solved by the following solving means. 
         [0022]    A first aspect of the present invention provides a wet flue gas desulfurization device comprising: a desulfurization absorption unit ( 26 ) that allows gas-liquid contact of treatment target exhaust gas ( 1 ) and a desulfurization absorbent ( 6 ) containing a lime content; and a desulfurization absorbent reservoir ( 11 ) that accommodates the desulfurization absorbent ( 6 ) after the gas-liquid contact, wherein the desulfurization absorbent reservoir ( 11 ) is partitioned by a wall surface so as to insulate the inside from outside air and has: an oxidation gas supply unit ( 9 ) that supplies oxidation gas for oxidizing a sulfurous acid content in the desulfurization absorbent ( 6 ) through the wall surface; and an oxidation gas discharge unit ( 12 ) that is provided at a position higher than a liquid level of the desulfurization absorbent ( 6 ) and discharges the excess oxidation gas to the outside, and the desulfurization absorption unit ( 26 ) has a water seal tube ( 7 ) having a lower end opening portion ( 7   b ) at a position that is lower than the liquid level of the desulfurization absorbent ( 6 ) in the desulfurization absorbent reservoir ( 11 ) and close to the center of the desulfurization absorbent reservoir ( 11 ) apart from the wall surface where the oxidation gas supply unit ( 9 ) is provided. 
         [0023]    A second aspect of the present invention provides a combustion system comprising: an oxygen production device ( 19 ); a combustion device ( 13 ) that combusts fuel by using oxygen produced by the oxygen production device ( 19 ); a denitration device ( 14 ); a heat exchanger ( 15 ) that preheats combustion gas used in the combustion device ( 13 ); a dust collection device ( 16 ); a wet flue gas desulfurization device ( 3 ) according to the first aspect; and a CO 2  recovery device ( 17 ), wherein the denitration device ( 14 ), the heat exchanger ( 15 ), the dust collection device ( 16 ), the wet flue gas desulfurization device ( 3 ), and the CO 2  recovery device ( 17 ) being sequentially arranged on a flow path of exhaust gas discharged from the combustion device ( 13 ) from the upstream side toward the downstream side, wherein a circulation line ( 18 ) which is branched from the exhaust gas flow path before a clarifying treatment performed by the wet flue gas desulfurization device ( 3 ) and through which the exhaust gas is recirculated to the combustion device ( 13 ) is provided, and an oxygen supply line ( 21 ) through which the oxygen produced by the oxygen production device ( 19 ) is supplied to an oxidation gas supply unit ( 9 ) of the wet flue gas desulfurization device ( 3 ) is provided, and an oxidation gas discharge unit ( 12 ) of the wet flue gas desulfurization device ( 3 ) is connected to the circulation line ( 18 ). 
         [0024]    A third aspect of the present invention provides a combustion method comprising: producing oxygen by an oxygen production device ( 19 ), using the obtained oxygen for combustion of fuel in a combustion device ( 13 ), performing a denitration treatment, a preheating treatment of combustion air, a dust collection treatment, and a desulfurization treatment in a wet flue gas desulfurization device ( 3 ) according to the first aspect with respect to exhaust gas discharged from the combustion device ( 13 ), then carrying out a CO 2  recovery treatment; recirculating the exhaust gas before the wet flue gas desulfurization treatment to the combustion device; and supplying the oxygen produced by the oxygen production device ( 19 ) to an absorbent reservoir ( 11 ) of the wet flue gas desulfurization device ( 3 ) as oxidation gas, and returning the oxidation gas from an oxidation gas discharge unit ( 12 ) of the wet flue gas desulfurization device ( 3 ) to the combustion device ( 13 ). 
       (Operation) 
       [0025]    To make an operation of the present invention understandable, a structural example shown in  FIG. 1  and  FIG. 3  will now be described. 
         [0026]    A gas phase  6   b  in an absorbent reservoir  11  of a desulfurization device  3  is isolated from a desulfurization absorption unit  26  by a partition wall  7   a  of a water seal tube  7 . 
         [0027]    A desulfurization absorbent  6  is sucked from the absorbent reservoir  11  by the absorbent circulation pump  5 , sprayed to the desulfurization absorption unit  26  through a spray nozzle  4 , and circulated mainly on the outer side of a wall surface of a water seal tube  7  by a stirrer  10  in the absorbent reservoir  11 . Therefore, since the desulfurization absorbent  6 , which falls from the desulfurization absorption unit  26  of the desulfurization device  3  and flows into the water seal tube  7 , flows in one direction from the upper side toward the lower side, there are characteristics that a sedimentation rate of the desulfurization absorbent  6  is high in the water seal tube  7  and gas bubbles hardly ascend. 
         [0028]    Therefore, the partition wall  7   a  of the water seal tube  7  functions as a barrier with respect to a movement that oxidation gas  27  supplied from the oxidation gas supply unit  9  to the desulfurization absorbent  6  flows toward the desulfurization absorption unit  26 , and this movement is also restricted by a downward flow of the desulfurization absorption  6  inside of the water seal tube  7 . 
         [0029]    Therefore, the oxidation gas  27  of a sulfurous acid generated in the absorbent of the desulfurization device  3  is prevented from getting mixed in with the exhaust gas  2  at the outlet of the desulfurization device, and CO 2  concentration at an inlet of a CO 2  recovery device  17  (see  FIG. 3 ) arranged on the downstream side of the desulfurization device  3  is prevented from decreasing, thereby effecting highly efficient CO 2  recovery. 
         [0030]    Mercury in the combustion exhaust gas discharged by combustion of the coal is absorbed by the absorbent in the desulfurization device  3 , and it may be re-discharged into a gas phase depending on conditions. As described above, in a coal-fired boiler  13  (see  FIG. 3 ), a countermeasure must be taken to prevent the re-discharged mercury from scattering together with the exhaust gas  2  at the outlet of the desulfurization device  3 , reaching the CO 2  recovery device  17  on the downstream side, or being directly discharged to the outside of a plant system or into the air. 
         [0031]    In the present invention, as described above, there are characteristics that a flow of the absorbent, which falls from the desulfurization absorption unit  26  of the desulfurization device  3  and flows into the water seal tube  7 , becomes a flow in one direction from the upper side toward the lower side, a sedimentation rate of the absorbent is high in the water seal tube  7 , and gas bubbles hardly ascend. Further, since the water seal tube  7  is inserted in the vicinity of a bottom portion of the absorbent reservoir  11  below the water surface, the absorbent in the desulfurization device  3  can be easily allowed to flow down to the absorbent reservoir  11 , the falling absorbent contributes to stirring of the desulfurization absorbent  6  in the absorbent reservoir  11 , and hence the number of the stirrers  10  can be reduced or the size of the stirrer  10  can be miniaturized, and the cost reduction can be expected. 
         [0032]    Furthermore, for example, even if the mercury moves into the oxidation gas  27  and is discharged into the gas phase, the mercury can be prevented from scattering together with the exhaust gas  2  at the outlet of the desulfurization device and reaching the CO 2  recovery device  17  on the downstream side. 
         [0033]    The gas phase portion  6   b  in the absorbent reservoir  11  is isolated from the gas phase portion in the desulfurization absorption unit  26  through the partition wall  7   a  of the water seal pipe  7 , and hence the mercury re-discharged here can be easily removed by a mercury removal device  23  connected to an oxidation gas outlet pipe  12 . Therefore, the re-discharged mercury does not diffuse into the air or to the outside of the system. 
         [0034]    When the oxidation gas  27  supplied to the absorbent reservoir  11  is substituted by high-oxygen gas produced in an oxygen production device  19 , since an amount of supplied gas can be reduced as compared with that in a situation where the oxidation gas is air, the number of air blowers for oxidation can be reduced or a capacity of each blower can be decreased. Furthermore, a liquid level  6   a  of the desulfurization absorbent reservoir  11  rises, and a problem, e.g., overflowing to the outside of the desulfurization absorbent reservoir  11  can be avoided. 
         [0035]    As described above, in the conventional technology, to sufficiently effect an oxidation reaction of the sulfurous acid in the absorbent in the absorbent reservoir  11 , it is necessary to supply from the oxidation gas supply unit  9  oxidation air which is greatly excessive as compared with a stoichiometrically required amount of oxygen. 
         [0036]    When the oxidation air which is the oxidation gas  27  supplied from the oxidation gas supply unit  9  is substituted by high-oxygen gas produced by an oxygen production device  19 , a supply amount of the oxidation gas can be greatly reduced. That is, it is possible to reduce the supply amount of the gas which corresponds to an amount of the components other than oxygen, e.g., nitrogen which does not at least contribute to the oxidation reaction from air. 
         [0037]    Therefore, supply power of, a compressor, etc. that is used for supplying the oxidation gas from the oxidation gas supply unit  9  can be reduced. Furthermore, since a diameter of a pipe in the oxidation gas supply unit  9  can be reduced without (greatly) increasing a pressure loss, gas bubbles can be further miniaturized. As a synergetic effect obtained from this miniaturization, the oxidation reaction of the sulfurous acid can readily advance, and hence it is possible to reduce a supply amount of the oxidation gas beyond an amount obtained by simply eliminating nitrogen from air. 
         [0038]    Moreover, stirring and miniaturization of the gas bubbles by the stirrer  10  used in the desulfurization absorbent reservoir  11  can easily advance. 
         [0039]    Even if the oxidation gas is discharged into the exhaust gas through an opening portion at a lower end (a lower end opening portion  7   b  of the water seal tube  7 ) of the desulfurization absorption unit  26 , an amount of the discharge is alleviated, and the exhaust gas is hardly diluted. 
         [0040]    Additionally, the excess oxygen discharged into the gas phase  6   b  without being used for oxidation of the sulfurous acid can be returned to a circulation line of the oxygen combustion system, and it can be used as combustion gas in the boiler  13 . Therefore, the oxygen generated by the oxygen production device  19  can be fully effectively used. 
       EFFECTS OF THE INVENTION 
       [0041]    According to the present invention, the following effects can be obtained.
       (1) Since the oxidation gas  27  of the sulfurous acid generated in the desulfurization absorbent  6  does not get mixed in with the gas  2  at the outlet of the desulfurization device, there can be obtained the effect of avoiding a reduction in CO 2  concentration in the gas at the inlet of the CO 2  recovery device  17  and a reduction in recovery rate of CO 2 .   (2) The metallic mercury re-discharged from the desulfurization absorbent  6  can be prevented from getting mixed in with the gas  2  at the outlet of the desulfurization device. Further, there can be obtained the effect of avoiding discharge of the mercury re-discharged from the desulfurization absorbent  6  to the outside of the system or into the air and outflow of the same to the CO 2  recovery device on the downstream side.   (3) When oxygen having high concentration is used as the oxidation gas of the sulfurous acid and the excess oxygen, which was not used for oxidation of the sulfurous acid, is supplied to the circulation line  18  through which the oxygen is supplied to the combustion device  13 , a supply gas amount can be reduced more than that in a situation where air is used as the oxidation gas. Therefore, gas supply power can be reduced. In addition, a rise of a desulfurization absorbent surface can be avoided, a size of the absorbent reservoir can be reduced, and a facility cost can be decreased. Further, since the excessively supplied oxygen can be used as the combustion gas for the boiler  13 , excess oxygen is not used, and the effect of reducing facility cost can be obtained.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1  is a view showing a configuration of a desulfurization device in an oxygen combustion system according to an embodiment of the present invention; 
           [0046]      FIG. 2  is a an arrow view of a circular cross section ( FIG. 2(   a )) and an arrow view of a square cross section ( FIG. 2(   b )) taken along a line A-A′ in  FIG. 1 ; 
           [0047]      FIG. 3  is a view showing a configuration of an oxygen combustion system according to an embodiment of the present invention; 
           [0048]      FIG. 4  is a view showing a configuration of an oxygen combustion system according to an embodiment of the present invention; 
           [0049]      FIG. 5  is a view showing a configuration of an oxygen combustion system according to an embodiment of the present invention; 
           [0050]      FIG. 6  is a view showing a configuration of an oxygen combustion system according to an embodiment of the present invention; 
           [0051]      FIG. 7  is a view showing a configuration of an oxygen combustion system according to a conventional technology; and 
           [0052]      FIG. 8  is a view showing a configuration of a desulfurization device according to the conventional technology. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    An embodiment according to the present invention will now be described with reference to the accompanying drawings. 
         [0054]    A description on structures and operations common to the conventional technology will be omitted. 
       Desulfurization Device 
       [0055]      FIG. 1  shows a configuration of a desulfurization device in an oxygen combustion system according to this embodiment. 
         [0056]    A desulfurization device  3  has an integral configuration constituted of a desulfurization absorbent unit  26  and an absorbent reservoir  11 , and a lower end of the desulfurization absorption unit  26  is provided below a water surface (a liquid surface  6   a ) of an absorbent in the desulfurization absorbent reservoir  11  to form a so-called water seal tube  7 . It is to be noted that the desulfurization absorbent unit  26  means a void tower region where a desulfurization reaction is effected by boiler exhaust gas  1  which is introduced into the desulfurization device  3  and a desulfurization absorbent  6  sprayed from a spray nozzle  4  and a water phase region which corresponds to the inside of the water seal tube  7  inserted in the absorbent reservoir  11  where the desulfurization absorbent  6  is stored and extends to a lower end opening portion  7   b , and the absorbent reservoir  11  means a region constituted of a liquid phase  6   a  where the desulfurization absorbent  6  that has flowed out from the water seal tube  7  convects and a gas phase  6   b.    
         [0057]    The desulfurization device  3  is mainly constituted of the spray nozzle  4  that sprays the desulfurization absorbent  6  with respect to an upward flow of the boiler exhaust gas  1 , an absorbent circulation pump  5  which supplies the desulfurization absorbent  6  to the spray nozzle  4 , the water seal tube  7  which leads the downwardly flowing desulfurization absorbent  6  into the absorbent reservoir  11 , an oxidation gas supply unit  9  which leads oxidation gas into the desulfurization absorbent  6  in the absorbent reservoir  11 , a stirrer  10  that stirs the desulfurization absorbent  6  in the absorbent reservoir  11 , an oxidation gas outlet pipe  12  that communicates with the outside through the gas phase  6   b  in the absorbent reservoir  11 , a mist eliminator  8  provided in the vicinity of an exhaust gas outlet of the desulfurization absorption unit  26 , and others. 
         [0058]    A cross-sectional area of an opening portion at a lower end of the desulfurization absorption unit  26  is set to be equal to or smaller than a cross-sectional area of the absorbent reservoir  11 , and the water seal tube  7  having the opening portion  7   b  at a lower portion is provided at a lower end portion of the desulfurization absorption unit  26 . 
         [0059]    The lower end opening portion  7   b  of the water seal tube  7  is provided at a position lower than the liquid level (a liquid surface)  6   a  of the desulfurization absorbent during a desulfurizing operation of the absorbent reservoir  11 . 
         [0060]    Further, the lower end opening portion  7   b  is opened at a position that is apart from a sidewall surface of the absorbent reservoir  11  in which the oxidation gas supply unit  9  is provided and close to the center of the desulfurization absorbent reservoir  11 , and it is opened at the bottom portion of the water seal tube  7  in the absorbent reservoir  11  of the oxidation gas supply unit  9 . 
         [0061]    Since the oxidation gas outlet pipe  12  communicates with the outside from the sidewall portion of the absorbent reservoir  11  through the gas phase  6   b  in the absorbent reservoir  11 , the oxidation gas  27  that is not used for oxidation of the sulfurous acid contained in the absorbent in the absorbent reservoir  11  is discharged to the outside of the system without getting mixed in with the exhaust gas  2  at the outlet of the exhaust gas desulfurization device. 
         [0062]      FIG. 2(   a ) shows a configuration when each of the water seal tube  7  and the desulfurization absorbent reservoir  11  has a circular shape as seen in a planar view as an embodiment showing a cross-sectional arrow view taken along a line A-A′ in  FIG. 1 . Droplets sprayed from the spray nozzle  4  pass through the water seal tube  7  and are supplied to the absorbent  6  in the desulfurization absorbent reservoir  11 . The oxidation gas  27  is supplied to the absorbent  6  from the oxidation gas supply unit  9 . 
         [0063]    Further,  FIG. 2(   b ) shows an example where each of the water seal tube  7  and the desulfurization absorbent reservoir  11  has a shape other than the circular shape, e.g., a square shape as the embodiment showing a cross-sectional arrow view taken along the line A-A′ in  FIG. 1 . As described above, it is not necessary to be fixated on the cross-sectional shape in the arrow view taken along the line A-A′ in particular and, when an area of the cross section of the water seal tube  7  is relatively small, a space to which the oxidation gas  27  is supplied increases, and the oxidation efficiency of the sulfurous acid is enhanced. 
         [0064]    In a conventional desulfurization device  3 , oxidation gas  27  gets mixed in with the exhaust gas  2  at the outlet of the desulfurization device. For example, when air is used as the oxidation gas  27 , since CO 2  concentration is reduced by approximately  10 %, a CO 2  recovery rate in a CO 2  recovery device  17  decreases by approximately 5%. On the other hand, when the desulfurization device  3  having the water seal tube  7  according to the present invention is used, the excess oxidation gas  27  that is not used for oxidation of the sulfurous acid generated in the desulfurization absorbent does not get mixed in with the exhaust gas  2  at the outlet of the desulfurization device, and hence the concentration of CO 2  at the inlet of the CO 2  recovery device  17  can be prevented from being lowered. Therefore, the decrease in CO 2  recovery rate of the CO 2  recovery device  17  can be avoided. 
         [0065]    A mercury removal device  23  is connected to the oxidation gas outlet pipe  12 . At a time of oxidizing the sulfurous acid generated from the desulfurization absorbent  6  with air, gas containing high-concentration oxygen, which was not used for the oxidation, is discharged into the atmosphere from the oxidation gas outlet pipe  12 . At the moment of desulfurization, metallic mercury may be re-discharged due to, e.g., an influence of the sulfurous acid generated in the desulfurization absorbent  6  in some cases. 
         [0066]    The mercury removal device  23  connected to the oxidation gas outlet pipe  12  captures the metallic mercury by using, e.g., a gold chip (a metal which forms mercury and amalgam and is fixed, e.g., gold (Au)), whereby the metallic mercury can be prevented from being discharged into air. 
         [0067]    As the configuration shown in  FIG. 1 , a configuration that the desulfurization absorption unit  26  of the desulfurization device  3  is supported right over the absorbent reservoir  11  by, e.g., a non-illustrated steel construction is shown, but the present invention is not necessarily restricted to the configuration where both the members are arranged in the vertical direction in this manner. As long as it has a configuration that the oxidation gas  27  hardly flows out toward the desulfurization absorption unit  26  through the water seal tube  7 , e.g., these members may be arranged in parallel. 
       Oxygen Combustion System 
       [0068]      FIG. 3  shows an embodiment of a combustion system using the desulfurization device  3  according to the present invention. This embodiment exhibits a system configuration including an exhaust gas treatment system when the oxidation gas  27  of the desulfurization device  3  depicted in  FIG. 1  is high-concentration oxygen. 
         [0069]    The high-concentration oxygen (e.g., gas having oxygen concentration of  95 % or above obtained by separating nitrogen from air) is supplied from an oxygen production device  19  to the oxidation gas supply unit  9  of the desulfurization device  3 , and the gas containing oxygen that was not used for oxidation is supplied to a circulation line  18  from the oxidation gas outlet pipe  12  at a time of oxidizing the sulfurous acid generated from the desulfurization absorbent  6 . 
         [0070]    It is to be noted that a starting point of the circulation line  18  may be set at any position on an exhaust gas flow path from a denitration device  14  to the CO 2  recovery device  17  shown in  FIG. 3 , and a heat exchanger  15  does not have to be interposed along the way. 
         [0071]    Further, although a conformation that an oxygen supply line  20  is connected to the circulation line  18  from the oxygen production device  19  is shown, a connecting region is not restricted to the circulation line  18 . The present invention is not restricted to the single circulation line  18 , the plurality of circulation lines  18  may be provided, or the circulation line  18  may branch to be connected to a supply system leading to fuel  25 , and the circulation line  18  is not limited to this conformation as long as it is configured to use oxygen produced by the oxygen production device  19  and the combustion exhaust gas as the combustion gas for the boiler  13 . 
         [0072]    When the oxidation gas  27  is air, since the oxygen concentration in the air is approximately 21%, the gas that is not used for oxidation such as N 2  contained in the air is excessively supplied. Since the desulfurization absorbent  6  in the absorbent reservoir  11  contains a large quantity of gas bubbles, a liquid level of the absorbent rises, and overflow to the outside of the device is apt to occur. Avoiding this overflow results in an increase in size of the absorbent reservoir  11 , and the facility cost is raised. 
         [0073]    On the other hand, when gas containing high-concentration oxygen is used as the oxidation gas  27 , a supply amount of the oxygen can be reduced to approximately ⅕ of a supply amount of the air, gas bubbles in the desulfurization absorbent  6  in the absorbent reservoir  11  are reduced, and a height of the liquid level  6   a  is also lowered. 
         [0074]    Therefore, the size of the absorbent reservoir  11  can be reduced as compared with that in the conventional desulfurization device  3 , and the facility cost of the desulfurization device  3  can be decreased. 
         [0075]    At this time, assuming that SO 2  concentration in the exhaust gas  1  at the inlet of the desulfurization device  3  is 500 to 10,000 ppm, an amount of O 2  used for the oxidation of the sulfurous acid contained in the desulfurization absorbent  6  is 0.2 to 2.5% with respect to a mount of O 2  used for combustion. 
         [0076]    As described above, in the oxidation gas  27  supplied to the desulfurization absorbent  6  with respect to the oxidation reaction of the sulfurous acid, it is unavoidable to produce gas that reaches the liquid level  6   a  of the absorbent reservoir  11  and changes to the gas phase  6   b.    
         [0077]    To sufficiently effect the oxidation reaction of the sulfurous acid in the absorbent in the absorbent reservoir  11 , it is necessary to supply from the oxidation gas supply unit  9  the oxidation gas that is excessive as compared to an amount of oxygen stoichiometrically required. 
         [0078]    In the conventional desulfurization device and combustion system, if the oxygen is supplied from the oxidation gas supply unit  9 , an amount of the oxygen excessively supplied is wastefully discharged together with the exhaust gas at the outlet of the desulfurization device  3 . 
         [0079]    However, in the desulfurization device  3  and the combustion system according to this embodiment, a substantially all amount of the oxidation gas  27  excessively supplied is discharged to the gas phase  6   b . Since the gas phase  6   b  is isolated by the wall surface of the water seal tube  7 , it does not get mixed in with the exhaust gas  2  at the outlet of the desulfurization device  3 . 
         [0080]    Since the gas phase  6   b  communicates with the oxidation gas outlet pipe  12  and this pipe  12  is connected to the circulation line  18 , the excessive oxygen in the desulfurization device  3  can be all used as the combustion gas in the boiler  13  through these paths. 
         [0081]    In this manner, the amount of the oxygen supplied from the oxygen production device  19  to the desulfurization device  3  can be minimized. Furthermore, since the air is not supplied to the exhaust gas in the desulfurization system according to this embodiment, the CO 2  concentration in the exhaust gas at the outlet of the boiler  13  can be prevented from being lowered. 
         [0082]    As shown in  FIG. 3 , when the mercury removal device  23  is connected to the oxidation gas outlet pipe  12  and the metallic mercury is removed, the metallic mercury re-discharged from the desulfurization absorbent  6  is not recirculated, and impurities can be prevented from entering the CO 2  recovery device  17 . 
         [0083]    It is to be noted that the mercury re-discharged to the gas phase  6   b  in the absorbent reservoir  11  is returned to the circulation line  18  even though the mercury removal device  23  is omitted, and hence discharge into the atmosphere or the outside of the system can be avoided. 
         [0084]    Additionally,  FIG. 4  shows another embodiment having a configuration in which the desulfurization device  3  and the absorbent reservoir  11  depicted in  FIG. 1  are incorporated in the oxygen combustion system. The exhaust gas treatment device is mainly constituted of a boiler  13 , a denitration device  14 , a heat exchanger  15 , a dust collection device  16 , a desulfurization device  3 , a CO 2  recovery device  17 , a circulation line  18 , an oxygen production device  19 , an oxygen supply line  20 , and others. 
         [0085]    Fuel  25  such as coal is combusted with oxygen in the boiler  13 , and exhaust gas is generated. At this time, the oxygen is supplied to the boiler  13  from the oxygen supply line  20  and the like by the oxygen production device  19 . The denitration device  14  decomposes NOx (nitrogen oxide) contained in the gas discharged from the boiler  13 . Further, a temperature of the gas discharged from the denitration device  14  is adjusted to 200° C. to 160° C. by the heat exchanger  15 , and smoke dust is removed by the dust collection device  16 . A part of the gas subjected to the dust removal is supplied to the desulfurization device  3 , then SO 2  is removed, and CO 2  is recovered by the CO 2  recovery device  17 . Furthermore, a part of the exhaust gas discharged from the dust collection device  16  passes through the circulation line  18  without being supplied to the desulfurization device  3 , and it is reheated to 200° C. by the heat exchanger  15  and then supplied to the boiler  13 . At a time of oxidizing the sulfurous acid, which is generated from the desulfurization absorbent  6  in the absorbent reservoir  11  of the desulfurization device  3 , by using air, the air that is not used for oxidation is discharged into the atmosphere from the oxidation gas outlet pipe  12 . 
         [0086]      FIG. 5  shows another embodiment of the oxygen combustion system according to the present invention. This embodiment exhibits a configuration of an exhaust gas treatment system when the oxidation gas  27  of the desulfurization device  3  according to the conventional technology depicted in  FIG. 8  is high-concentration oxygen. It is to be noted that the desulfurization device  3  in  FIG. 5  may be the desulfurization absorbing tower shown in  FIG. 1 . In the exhaust gas treatment system shown in  FIG. 5 , when high-concentration oxygen is used as the oxidation gas  27 , since a supply amount of the oxygen can be reduced to approximately ⅕ of a supply amount of air, a blower is no longer necessary, or a capacity of the blower can be reduced, and hence the facility cost of the desulfurization device  3  can be decreased. Furthermore, since air does not get mixed in with exhaust gas, a reduction in CO 2  concentration of the exhaust gas at the outlet of the boiler  13  can be avoided. 
         [0087]      FIG. 6  shows another embodiment of an oxygen combustion system according to the present invention. This embodiment has a configuration that oxygen is supplied from an oxygen production device  19  to an oxidation gas supply unit  9  provided in an absorbent reservoir  11  in the desulfurization device  3  according to the present invention depicted in  FIG. 1  and an oxidation gas outlet pipe  12  is connected to a circulation line  18 . As a result, when excess oxidation oxygen discharged from the oxidation gas outlet pipe  12  is supplied to the circulation line  18 , the excess oxidation oxygen at a time of combusting coal with oxygen in a boiler  13  can be reused, and hence a consumption of the oxygen generated by the oxygen production device  19  can be minimized. 
       EXPLANATIONS OF LETTERS OR NUMERALS 
       [0088]      1  boiler exhaust gas 
         [0089]      2  exhaust gas at the outlet of the desulfurization device 
         [0090]      3  desulfurization device 
         [0091]      4  spray nozzle 
         [0092]      5  absorbent circulation pump 
         [0093]      6  desulfurization absorbent liquid 
         [0094]      7  water seal tube 
         [0095]      8  mist eliminator 
         [0096]      9  oxidation gas supply unit 
         [0097]      10  stirrer 
         [0098]      11  absorbent reservoir 
         [0099]      12  oxidation gas outlet pipe 
         [0100]      13  boiler 
         [0101]      14  denitration device 
         [0102]      15  heat exchanger 
         [0103]      16  dust collection device 
         [0104]      17  CO 2  recovery device 
         [0105]      18  circulation line 
         [0106]      19  oxygen production device 
         [0107]      20  oxygen supply line 
         [0108]      21  oxygen supply line 
         [0109]      23  mercury removal device 
         [0110]      25  fuel such as coal 
         [0111]      26  desulfurization absorption unit 
         [0112]      27  oxidation gas (gas bubbles of air or oxygen)