Patent Publication Number: US-2016236145-A1

Title: Flue gas treatment method and denitration/so3 reduction apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a flue gas treatment method and to a denitration and 
     SO 3  reduction apparatus, and more specifically relates to a flue gas treatment method and to a denitration and SO 3  reduction apparatus for treatment of a combustion flue gas including sulfur trioxide. 
     BACKGROUND ART 
     In recent years, a flue gas treatment method and a flue gas treatment apparatus for treating combustion flue gases generated from various combustion furnaces have been strongly desired in order to prevent air pollution. Such flue gases contain nitrogen oxides (NO x ) and a large amount of sulfur oxides (SO x ). In treating NO x , a method has been applied in which NO x  is brought into contact with a denitration catalyst to be decomposed into nitrogen (N 2 ) and water (H 2 O). Among SO x , sulfur trioxide (SO 3 ) is corrosive, and is a factor that inhibits the continuous long-term operation for treating flue gases due to clogging by ash inside flue gas treatment facilities such as an air preheater and an electric precipitator, dew point corrosion, and the like caused due to SO 3 . 
     For a method of treating such SO 3 , a method has been known in which ammonium (NH 3 ) is charged to a combustion flue gas, then the combustion flue gas is brought into contact with a denitration catalyst constituted by ruthenium (Ru) carried on titania (TiO 2 ), and thereby NO x  is reduced and generation of SO 3  in combustion flue gases is prevented (e.g., Patent Literature 1). However, even in the exemplary case recited in Patent Literature 1, when ammonia is consumed by a denitration reaction in a treatment of denitrating a combustion flue gas, an oxidation reaction expressed by the following expression (1) predominantly progresses, and therefore the concentration of SO 3  may increase. In addition, although a method of reducing SO 3  in a combustion flue gas by using a reductant constituted by carbon monoxide (CO) and hydrocarbon has been known, but expensive iridium (Ir) is used in the catalyst (e.g., Patent literature 2). 
     [Chemical Formula 1] 
       SO 2 +1/2O 2 →SO 3    (1)
 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] JP 3495591 
     [Patent Literature 2] JP 3495527 
     SUMMARY OF INVENTION 
     Technical Problem 
     Under these circumstances, an object of the present invention is to provide a flue gas treatment method and a denitration and SO 3  reduction apparatus that reduce treatment costs, reduce NO x  contained in a combustion flue gas, and reduce the concentration of SO 3  more efficiently compared with prior art. 
     Solution to Problem 
     In order to achieve the above-described object, according to an aspect of the present invention, a flue gas treatment method is provided, in which a 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon) is added to a combustion flue gas including SO 3  as well as NO x  as a first additive, and then the combustion flue gas is brought into contact with a catalyst which includes an oxide constituted by one or more of elements selected from the group consisting of Ti, Si, Zr, and Ce and/or a mixed oxide and/or a complex oxide constituted by two or more of elements selected from the group as a carrier but not including a noble metal, and thereby SO 3  is treated by reduction to SO 2 . 
     In this aspect, it is made possible to denitrate NO x  in the flue gas, prevent oxidation of SO 2 , reduce the concentration of SO 3  during the treatment of the combustion flue gas, and reduce the costs for the material of the catalyst without having to use an expensive catalyst containing Ru and the like. In descriptions given herein and in the claims, the term “and/or” is used, as is provided by JIS Z 8301, to collectively express a combination of two terms used in parallel to each other and either one of the two terms, i.e. to collectively express the three possible meanings that can be expressed by the two terms. 
     It is preferable that the first additive be one or more selected from the group consisting of 2C-5C olefinic hydrocarbon (unsaturated hydrocarbon), 2C-5C paraffinic hydrocarbon (saturated hydrocarbons), alcohols, aldehydes, and aromatic compounds. Further, it is preferable that the 2C-5C olefinic hydrocarbon (unsaturated hydrocarbon) be one or more selected from the group consisting of C 2 H 4 , C 3 H 6 , C 4 H 8 , and C 5 H 10 . In addition, the C 4 H 8  and C 5 H 10  may be a geometric isomer or a racemic body of either one thereof. 
     By using the above-described additive, oxidation of SO 2  can be more efficiently suppressed and the concentration of SO 3  during the treatment of the combustion flue gas can be more efficiently reduced compared with the case of using NH 3  as the first additive. 
     It is preferable that the carrier include a mixed oxide and/or a complex oxide including one or more selected from the group consisting of TiO 2 —SiO 2 , TiO 2 —ZrO 2 , and TiO 2 —CeO 2 . 
     With the above-described carrier, the performance of reduction of SO 3  into SO 2  can be dramatically improved by using a mixed oxide and/or complex oxide with TiO 2  and with an amount of solid acid higher than a predetermined value. 
     In addition, the catalyst may be a catalyst in which a metal oxide including one or more selected from the group consisting of V 2 O 5 , WO 3 , MoO 3 , Mn 2 O 3 , MnO 2 , NiO, and Co 3 O 4  is carried on the complex oxide as the carrier. In addition, the catalyst may be a catalyst in which one or more selected from the group consisting of Ag, Ag 2 O, and AgO carried on a carrier constituted by one or more selected from the group consisting of the oxide, the mixed oxide, and the complex oxide. 
     In addition, a metallosilicate-base complex oxide, in which at least a part of Al and/or Si in a zeolite crystal structure is substituted with one or more selected from the group consisting of Ti, V, Mn, Fe, and Co may be coated onto the catalyst. 
     With the above-described catalyst, SO 3  in the combustion flue gas can be reduced at a high reduction rate by using the first additive, and the SO 3  reduction reaction would not be inhibited even if NH 3  coexists. 
     It is preferable that NH 3  be added as a second additive simultaneously as the first additive is added and simultaneously perform the reduction of SO 3  and the denitration when performing a treatment for reducing SO 3  to SO 2 . 
     In this aspect, the first additive can be added by partially reforming the ammonia supply line equipment provided to the existing denitration apparatus to contribute to reduction of SO 3  in the combustion flue gas. 
     It is preferable that the treatment for reducing SO 3  into SO 2  be performed in a temperature range of 250° C. to 450° C. . In addition, it is preferable that the treatment for reducing SO 3  into SO 2  be performed in a temperature range of 300° C. to 400° C. 
     By employing the temperature ranges, the treatment for reducing SO 3  in the combustion flue gas to SO 2  by using an existing denitration apparatus and under denitration treatment conditions for a high activity of the catalyst as a denitration catalyst. 
     According to another aspect of the present invention, the present invention is a denitration and SO 3  reduction apparatus. The denitration and SO 3  reduction apparatus according to the present invention includes a first injection device configured to obtain add a first additive to a combustion flue gas containing SO 3  as well as NO x ; and a catalyst layer including a catalyst through which the combustion flue gas is allowed to flow, and in the denitration and SO 3  reduction apparatus, the first additive is a 3C-5C olefinic hydrocarbon (unsaturated hydrocarbon), the catalyst does not include a noble metal and includes an oxide including one or more of elements selected from the group consisting of Ti, Si, Zr, and Ce and/or a mixed oxide and/or a complex oxide including two or more of elements selected from the group as a carrier, and the SO 3  reduction apparatus is configured to perform a treatment for reducing SO 3  into SO 2 . 
     According to another aspect of the present invention, the catalyst layer includes a first catalyst layer arranged on a back stream side of the first injection device and configured to reduce the concentration of SO 3 ; and a second catalyst layer arranged on a back stream side of a second injection device arranged close to the first injection device and configured to add NH 3  to the combustion flue gas as a second additive, the second catalyst layer being configured to perform denitration, and in this aspect, the first catalyst layer is arranged on a front stream side or a back stream side of the second catalyst layer. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present invention, a flue gas treatment method and a denitration and SO 3  reduction apparatus are provided which are configured to denitrate NO x  in the combustion flue gas and reduce the concentration of SO 3  in the combustion flue gas at the same time at treatment costs lower than those conventionally. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a denitration and SO 3  reduction apparatus according to the present invention, which is a first embodiment. 
         FIG. 2  is a schematic diagram showing a denitration and SO 3  reduction apparatus according to the present invention, which is a second embodiment. 
         FIG. 3  is a schematic diagram showing a denitration and SO 3  reduction apparatus according to the present invention, which is a third embodiment. 
         FIG. 4  is a graph showing variation of the concentration of SO 3  in a combustion flue gas in Example 1 of the present invention. 
         FIG. 5  is a graph showing variation of the concentration of SO 3  in a combustion flue gas in Example 2 of the present invention. 
         FIG. 6  is a graph showing a rate of reduction of SO 3  in a combustion flue gas and a denitration rate in Example 2 of the present invention. 
         FIG. 7  is a graph showing a rate of reduction of SO 3  and a denitration rate obtained by using a catalyst of Example 3 of the present invention. 
         FIG. 8  is a graph showing a relationship between a solid acid amount and an SO 3  reduction rate in Example 3 of the present invention. 
         FIG. 9  is a graph showing a rate of reduction of SO 3  and a denitration rate obtained by using a catalyst of Example 4 of the present invention. 
         FIG. 10  is a graph showing a rate of reduction of SO 3  obtained by using a catalyst of Example 5 of the present invention. 
         FIG. 11  is a graph showing a denitration rate obtained by using the catalyst of Example 5 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A denitration and SO 3  reduction apparatus and a flue gas treatment method according to the present invention will be described below with reference to embodiments shown in the attached drawings. A flue gas generated by combusting an oil-derived fuel or a coal-derived fuel in a boiler and in an oxygen atmosphere will be herein referred to as a “combustion flue gas”. In addition, the stream of gas will be herein referred to as a “front stream” or “back stream” in relation to the direction of flow of a combustion flue gas. 
     [Denitration and SO 3  Reduction Apparatus] 
     First Embodiment 
       FIG. 1  shows a first embodiment, in which the denitration and SO 3  reduction apparatus according to the present invention is arranged on a back stream side of a boiler. Referring to  FIG. 1 , a denitration and SO 3  reduction apparatus  5  is arranged on a back stream side of a flue gas chimney  2  of a boiler which generates a flue gas in the furnace  1 . 
     The boiler burns an externally supplied fuel in the furnace  1  and discharges a combustion flue gas generated by the burning into the flue gas chimney  2 . The denitration and SO 3  reduction apparatus  5 , which is arranged on a back stream side of the flue gas chimney  2 , simultaneously performs a NO x  denitration treatment and an SO 3  reduction treatment for the combustion flue gas that flows through the flue gas chimney  2 . In the descriptions given herein and in the claims, the treatment for reducing SO 3  into SO 2  will be referred to as the “SO 3  reduction treatment”. 
     An ECO 3, which is arranged in the combustion flue gas chimney  2  in which the combustion flue gas is circulated, performs heat exchange between boiler feed water and the combustion flue gas that flow through the inside of the ECO 3. More specifically, the ECO 3 increases the temperature of the boiler feed water by using the thermal inertia of the combustion flue gas and thereby improves the efficiency of combustion in the boiler. An ECO bypass  4  is arranged so that one end thereof is in communication with the front stream side of the ECO 3 and the other end thereof is in communication with the back stream side of the ECO 3, and feeds the combustion flue gas before being fed into the ECO 3 to the side of an inlet of the denitration and SO 3  reduction apparatus  5 , bypassing the ECO 3. In addition, the ECO bypass  4  controls the temperature of the combustion flue gas to be fed into the denitration and SO 3  reduction apparatus  5  within a predetermined temperature range appropriate for denitration and reduction reactions. 
     The denitration and SO 3  reduction apparatus  5  is arranged in the combustion flue gas chimney  2 , and at least includes a first injection device  6 , a second injection device  7 , and a catalyst layer  8 . The denitration and SO 3  reduction apparatus  5  adds a first additive and a second additive to the flue gas and allows the combustion flue gas including the additives to flow through the catalyst layer  8 . The denitration and SO 3  reduction apparatus  5  performs an SO 3  reduction treatment by using the catalyst layer  8 , the first injection device  6 , and the second injection device  7 . It is preferable that the denitration and SO 3  reduction apparatus  5  be configured so as to add the first additive and the second additive at the same time. 
     The first injection device  6  is arranged on a front stream side of the denitration and SO 3  reduction apparatus  5  and on a back stream side of the ECO bypass  4 , and adds the first additive to the combustion flue gas including SO 3  as well as NO x . More specifically, the first injection device  6  collaborates with the catalyst layer  8  to reduce SO 3  in the combustion flue gas. 
     The first additive injected from the first injection device  6  is an SO 3  reductant primarily for reduction of SO 3  into SO 2 , and hydrocarbon constituted by carbon element (C) and/or hydrogen element having a capability of reducing SO 3  in an oxygen atmosphere can be used. More specifically, the first additive is an additive constituted by one or more selected from the group consisting of: an olefinic hydrocarbon (unsaturated hydrocarbon) expressed by a general formula: C n H 2n  (n is an integer of 2 to 5), a paraffinic hydrocarbon (saturated hydrocarbon) expressed by a general formula: C m H 2m+2  (m is an integer of 2 to 5), alcohols such as methanol (CH 3 OH) and ethanol (C 2 H 5 OH), aldehydes such as acetaldehyde (CH 3 CHO) and propionaldehyde (C 2 H 5 CHO), and aromatic compounds such as toluene (C 6 H 5 CH 3 ) and ethyl benzene (C 6 H 5 C 2 H 5 ). 
     For the 2C-5C olefinic hydrocarbon (unsaturated hydrocarbon), it is preferable to use one or more selected from the group consisting of C 2 H 4 , C 3 H 6 , C 4 H 8 , and C 5 H 10 , and it is more preferable to use one or more selected from the group consisting of ≧3C olefinic hydrocarbons including C 3 H 6 , C 4 H 8 , and C 5 H 10 . For the C 4 H 8  and C 5 H 10 , a geometric isomer or a racemic body of either one of them can be used. Examples of ≧4C unsaturated hydrocarbons include 1-butene (1-C 4 H 8 ); 2-butenes (2-C 4 H 8 ) such as cis-2-butene and trans-2-butene; isobutene (iso-C 4 H 8 ); 1-pentene (1-C 5 H 10 ); and 2-pentenes (2-C 5 H 10 ) such as cis-2-pentene and trans-2-pentene. With this configuration, by reactions expressed by the following expressions (2) to (8), the present invention can contribute to the reduction of SO 3  in an oxygen atmosphere and reduce the concentration of SO 3  in the combustion flue gas. 
     [Chemical Formula 2] 
       SO 3 +CH 3 OH+3/4O 2 →SO 2 +1/2CO+1/2CO 2 +2H 2 O   (2)
 
       SO 3 +C 2 H 5 OH+5/42O 2 →SO 2  +CO+CO 2 +2/5H 2 O   (3)
 
       SO 3 +C 2 H 4 +5/2O 2 →SO 2 +CO+CO 2 +2H 2 O   (4)
 
       SO 3 +3/2C 3 H 6 +41/83O 2 →SO 2 +9/4CO+9/4CO 2 +6H 2 O   (5)
 
       SO 3 +3/2C 3 H 8 +63/8O 2 →SO 2 +9/4CO+9/4CO 2 +6H 2 O   (6)
 
       SO 3 +3/2C 4 H 8 +13/2O 2 →SO 2 +3CO+3CO 2 +6H 2 O   (7)
 
       SO 3 +C 5 H 10 +23/4O 2 →SO 2 +5/2CO+5/2CO 2 +5H 2 O   (8)
 
     If C 3 H 6  is used as the first additive, it is useful if the load of the first additive be 0.1 to 2.0 by molar ratio. If the molar ratio of the first additive is less than 0.1, the oxidation of SO 2  may become predominant and thus SO 3  may increase, and in contrast, if the molar ratio of the first additive is more than 2.0, then a large amount of unreacted excessive C 3 H 6  may be discharged. By controlling the amount of the first additive in the above-described range, the performance of eliminating SO 3  in the combustion flue gas can be improved. The effect of removing SO 3  can be obtained outside the range specified above. 
     The second injection device  7  is arranged close to the first injection device  6 , and adds NH 3  to the combustion flue gas as the second additive. The second injection device  7  is arranged on a front stream side of the denitration and SO 3  reduction apparatus  5  and on a back stream side of the ECO bypass  4 , and injects the second additive for denitration of NO x  to the combustion flue gas. The second injection device  7  collaborates with the catalyst layer  8  to denitrate NO x . 
     The catalyst layer  8  is constituted by a catalyst which denitrates the combustion flue gas. It is preferable that the shape of the catalyst arranged in the catalyst layer  8  be a honeycomb shape so that the catalyst can efficiently function also as a denitration catalyst and reduce the pressure drop that may occur during treatment of the combustion flue gas. The honeycomb structure is not limited to a structure with a rectangular section, and can include various shapes such as circular, elliptical, triangular, pentagonal, and hexagonal shapes. 
     The catalyst arranged in the catalyst layer  8  is a catalyst in which the active component is carried on a carrier which is an oxide, a mixed oxide, and/or a composite oxide. More specifically, examples of the carrier include an oxide of one or more of elements selected from the group consisting of titanium (Ti), silicon (Si), zirconium (Zr), and cerium (Ce) and/or a mixed oxide and/or a composite oxide of two or more of elements selected from the above group. In other words, the carrier at least includes the following form.
         An oxide constituted by one or more of titania (TiO 2 ), silica (SiO 2 ), zirconia (ZrO 2 ), and cerium oxide (Ce 2 O 3 )   A mixed oxide or a complex oxide constituted by two, three, or four of titanium (Ti), silicon (Si), zirconium (Zr) and cerium (Ce)   A mixture constituted by two, three, or four of the above oxide   A mixture of one of the above mixtures and one of the above mixed oxides or complex oxides       

     Among them, the mixed oxide or the complex oxides selected from the group consisting of TiO 2 —SiO 2 , TiO 2 —ZrO 2 , and TiO 2 -CeO 2  is preferable, and the complex oxides selected from the above group is more preferable. 
     The complex oxide can be prepared by a process in which an alkoxide compound, a chloride, a sulfate, or an acetate of the above-described elements is mixed, then the resulting mixture is further mixed with water and then stirred in the form of an aqueous solution or sol for hydrolysis. The complex oxide may also be prepared by a known coprecipitation process instead of the above-described sol-gel process. 
     The active component is a metal oxide constituted by one or more selected from the group consisting of vanadium oxide (V 2 O 5 ), tungsten oxide (WO 3 ), molybdenum oxide (MoO 3 ), manganese oxide (Mn 2 O 3 ), manganese dioxide (MnO 2 ), nickel oxide (NiO), and cobalt oxide (Co 3 O 4 ). The active component may be one or more selected from the group consisting of silver (Ag), silver oxide (Ag 2 O), and silver monoxide (AgO). With this configuration, an active metal carried by the catalyst acts as an active site, and thus the denitration of NO x  such as NO and NO 2  can be efficiently performed in an oxygen atmosphere and the reduction of SO 3  in an excess oxygen atmosphere can be performed. It is preferable that the active component, among these metal oxides, include tungsten oxide (WO 3 ). 
     In addition, for the catalyst, a catalyst prepared by coating or impregnating a metallosilicate-base complex oxide in which at least a part of the aluminum element (Al) and/or silicon element (Si) in a zeolite crystal structure is substituted with one or more selected from the group consisting of titanium element (Ti), vanadium element (V), manganese element (Mn), iron element (Fe), and cobalt element (Co). The above-described metallosilicate can be prepared by using a hydrothermal synthesis method in which at least a part of water glass and silicon element that are the silicon source are mixed with a source of metal element to be replaced and a structure directing agent, and the mixture is placed in an autoclave and processed by using the hydrothermal synthesis method under a high temperature and high pressure. 
     [Flue Gas Treatment Method] 
     A first embodiment of the flue gas treatment method according to the present invention will be described by describing its mode of operation of the denitration and SO 3  reduction apparatus according to the above-described first embodiment. The flue gas treatment method of the present embodiment at least performs an SO 3  reduction treatment. 
     In the SO 3  reduction treatment, the first additive for reducing SO 3  and NH 3  that is the second additive for reducing NO x  are injected from the first injection device  6  and the second injection device  7  to the combustion flue gas including SO 3  as well as NO x  on a front stream thereof. By allowing the combustion flue gas including the charged additives to flow through the catalyst layer  8  constituted by the denitration catalyst on a back stream side thereof, the NO x  denitration treatment and the SO 3  reduction treatment are carried out at the same time. In the treatment, it is preferable that the first additive and the second additive be added to the combustion flue gas at the same time. 
     It is preferable that the SO 3  reduction treatment be carried out in a temperature range of 250° C. to 450° C., and it is more preferable to carry out the SO 3  reduction treatment in a temperature range of 300° C. to 400° C. If the SO 3  reduction treatment is carried out at a temperature below 300° C., the denitration treatment may not be sufficiently completed, and in contrast, if the SO 3  reduction treatment is carried out at a temperature above 400° C., the reduction of SO 3  may not be sufficient. 
     According to the present embodiment, NO x  in the combustion flue gas including SO 3  and/or NO x  generated when burned in the boiler can be removed by denitration, oxidation of SO 2  can be prevented, and thus the concentration of SO 3  can be reduced during treatment of the combustion flue gas, and addition, costs for the material of the catalyst can be reduced because no expensive catalyst is used. Further, the above-described effect of the present embodiment can be achieved merely by additionally installing a first injection device configured to inject the first additive for reducing SO 3  on a front stream side of an existing denitration apparatus. Accordingly, the SO 3  reduction treatment can be carried out at low cost. 
     [Denitration and SO 3  Reduction Apparatus] 
     Second Embodiment 
     A second embodiment of the denitration and SO 3  reduction apparatus according to the present invention will be described in detail with reference to  FIG. 2 . In the present embodiment, the components that are the same as those of the above-described first embodiment of the denitration and SO 3  reduction apparatus will be provided with the same reference numerals and symbols, and detailed descriptions thereof will not be repeated. A denitration and SO 3  reduction apparatus  15  according to the present embodiment includes a catalyst layer segmented into a first catalyst layer and a second catalyst layer and a first injection device is arranged between them, and the denitration and SO 3  reduction apparatus  15  is different from the denitration and SO 3  reduction apparatus  5  according to the above-described first embodiment at this point. 
     Referring to  FIG. 2 , the denitration and SO 3  reduction apparatus  15  is arranged in the combustion flue gas chimney  2 , and at least includes s first injection device  16  configured to add the first additive to the combustion flue gas; a second injection device  17  configured to add the second additive to the combustion flue gas; and a catalyst layer a catalyst configured to denitrate the combustion flue gas. The catalyst layer is constituted by a first catalyst layer  18  configured to reduce the concentration of SO 3  and a second catalyst layer  19 , which is arranged on a front stream side of the first catalyst layer  18  and configured to perform denitration. The denitration and SO 3  reduction apparatus  15  adds the second additive from the second injection device  17  to the combustion flue gas that enters therein from the flue gas chimney  2 , and then allows the combustion flue gas containing the second additive to flow through the second catalyst  19 . In addition, the denitration and SO 3  reduction apparatus  15  adds the first additive from the first injection device  16  to the combustion flue gas that has gone through the second catalyst  19  and then allows the combustion flue gas containing the first additive to flow through the first catalyst layer  18 . 
     The first injection device  16  is arranged in the combustion flue gas chimney  2  on a front stream side of the first catalyst layer  18  and on a back stream side of the second catalyst layer  19 . The first catalyst layer  18  is arranged on a back stream side of the second catalyst layer  19 . In addition, the first injection device  16  injects the first additive for reducing the concentration of SO 3  to the combustion flue gas. 
     The second injection device  17  is arranged in the combustion flue gas chimney  2  on a front stream side of the second catalyst layer  19 . In addition, the second injection device  17  injects the second additive for denitration of NO x  to the combustion flue gas. For the second injection device  17  and the second catalyst layer  19 , a denitration apparatus installed in an existing plant can be employed, for example. 
     For the first additive injected from the injection device  16  and the catalyst installed in the catalyst layer  18 , an additive and a catalyst similar to those of the first embodiment can be applied. For the second additive injected from the injection device  17  and the catalyst installed in the second catalyst layer  19 , not only an additive and a catalyst similar to those of the first embodiment but also a publicly known catalyst (e.g., V 2 O 5 —TiO 2 ) can be applied. 
     [Flue Gas Treatment Method] 
     A second embodiment of the flue gas treatment method according to the present invention will be described by describing a mode of operation of the above-described second embodiment of the denitration and SO 3  reduction apparatus. The flue gas treatment method of the present embodiment at least performs an SO 3  reduction treatment. 
     In the SO 3  reduction treatment, as a pretreatment to a combustion flue gas at least including NO x  and SO 3 , NH 3  that is the second additive is added from the second injection device  17  to the combustion flue gas, and the combustion flue gas is brought into contact with the denitration catalyst in the second catalyst  19  arranged on a back stream side of the second injection device  17 . Then, as a posttreatment, an additive for SO 3  is added from the first injection device  16  to the combustion flue gas, and the combustion flue gas is brought into contact with the catalyst for SO 3  in the first catalyst layer  18  arranged on a back stream side of the first injection device  16 . 
     For the treatment temperature for the SO 3  reduction treatment, temperature ranges similar to those of the first embodiment can be employed. 
     According to the denitration and SO 3  reduction apparatus and the flue gas treatment method of the second embodiment, SO 3  can be more efficiently treated on a back stream side of the existing denitration apparatus, and in addition, the catalyst can be easily exchanged according to degradation of the function of the catalyst used for the denitration and the reduction of SO 3 , respectively. 
     [Denitration and SO 3  reduction Apparatus] 
     (Third Embodiment) 
     A third embodiment of the denitration and SO 3  reduction apparatus according to the present invention will be described in detail with reference to  FIG. 3 . In the present embodiment, the components that are the same as those of the first and the second embodiments are provided with the same reference numerals and symbols, and detailed descriptions thereof will not be repeated. A denitration and SO 3  reduction apparatus  25  according to the present embodiment is different from the denitration and SO 3  reduction apparatus  15  according to the second embodiment in such a point that a first injection device and a first catalyst layer are arranged on a front stream side of a second injection device and a second catalyst layer. 
     Referring to  FIG. 3 , the denitration and SO 3  reduction apparatus  25  is arranged in the combustion flue gas chimney  2 , and at least includes a first injection device  26 , a second injection device  27 , a first catalyst layer  28 , and a second catalyst layer  29 . The denitration and SO 3  reduction apparatus  25  adds a second additive from the first injection device  26  to a combustion flue gas that enters from the flue gas chimney  2 , and then allows the combustion flue gas containing the second additive to flow through the first catalyst layer  28 . In addition, the denitration and SO 3  reduction apparatus  25  adds the second additive from the second injection device  27  to the combustion flue gas that has flown through the first catalyst layer  28 , and then allows the combustion flue gas containing the second additive to flow through the second catalyst layer  29 . 
     The first injection device  26  is arranged in the combustion flue gas chimney  2  on a front stream side of the first catalyst layer  28  and on a front stream side of the second catalyst layer  29 . The first catalyst layer  28  is arranged on a front stream side of the second catalyst layer  29 . The first injection device  26  injects the first additive for reducing the concentration of SO 3  to the combustion flue gas. The second injection device  27  is arranged in the combustion flue gas chimney  2  on a front stream side of the second catalyst layer  29 . In addition, the second injection device  27  injects the second additive for denitration of NO x  to the combustion flue gas. Similarly to the second embodiment, for the second injection device  27  and the second catalyst layer  29  also, a denitration apparatus installed in an existing plant can be applied. 
     For the first additive injected from the injection device  26  and the catalyst installed in the catalyst layer  28 , an additive and a catalyst similar to those of the first and the second embodiments can be applied. In addition, for the second additive injected from the injection device  27  and the catalyst installed in the second catalyst layer  29  also, not only an additive and a catalyst similar to those of the first embodiment but also a publicly known denitration catalyst (e.g., V 2 O 5 —TiO 2 ) can be applied. 
     [Flue Gas Treatment Method] 
     A third embodiment of the flue gas treatment method according to the present invention will be described by describing a mode of operation of the denitration and SO 3  reduction apparatus according to the above third embodiment. The flue gas treatment method of the present embodiment at least performs a SO 3  reduction treatment. 
     In the SO 3  reduction treatment, as a pretreatment to a combustion flue gas at least including NO x  and SO 3 , an additive for SO 3  is added from the first injection device  26 , and the combustion flue gas is brought into contact with the catalyst for SO 3  in the first catalyst layer  28  arranged on a back stream side of the first injection device  26 . Then, as a posttreatment, NH 3  is added from the second injection device  27  to the combustion flue gas as the second additive, and the combustion flue gas is brought into contact with the denitration catalyst in the second catalyst  29  arranged on a back stream side of the second injection device  27 . 
     For the treatment temperature for the SO 3  reduction treatment, temperature ranges similar to those of the first and the second embodiments can be employed. 
     According to the denitration and SO 3  reduction apparatus and the flue gas treatment method of the third embodiment, SO 3  can be more efficiently treated on a back stream side of the existing denitration apparatus, and in addition, the catalyst can be easily exchanged according to degradation of the function of the catalyst used for the denitration and the reduction of SO 3 , respectively. 
     EXAMPLES 
     The effects of the present invention will be clarified by more specifically describing the present invention with reference to examples. The flue gas treatment method and the denitration and SO 3  reduction apparatus according to the present invention are not limited by the following examples by any means. 
     Example 1 
     By using a different catalyst, the effect of the first additive (SO 3  reductant) for reducing SO 3  into SO 2  were examined. 
     (Preparation of Catalyst A) 
     A catalyst A including ruthenium (Ru), which functions as a catalyst for reducing SO 3  into SO 2 , was prepared. An aqueous solution of ruthenium chloride (RuCl 3 ) was impregnated with an anatase type titania powder including 10 wt. % tungsten oxide (WO 3 ) per 100 wt. % titania (TiO 2 ), thereby 1 wt. % Ru was carried on the powder per 10.0 wt. % of anatase type titania powder, and the resultant was evaporated and dried. Then the residue was fired at 500° C. for 5 hours, and the obtained powder was used as the catalyst A. 
     (Preparation of Catalyst B) 
     A catalyst B was prepared as a typical catalyst having a denitration function by ammonia. Ti(O-iC 3 H 7 ) 4 , a Ti alkoxide, and Si(OCH 3 ) 3 , a Si alkoxide, were mixed at a ratio of 95:5 (wt. %) (as TiO 2 , SiO 2 , respectively), the mixture was added to 80° C. water for hydrolysis, then the reaction mixture was stirred and matured, the produced sol was filtered, and the obtained gelled product was washed, dried, and heated and fired at 500° C. for 5 hours to obtain a powder of TiO 2 —SiO 2  complex oxide (TiO 2 —SiO 2  powder). Ammonium metavanadate (NH 3 VO 3 ) and ammonium paratungstate ((NH 4 ) 10 H 10 W 12 O 46 .6H 2 O) were impregnated into the complex oxide by using a 10 wt. % aqueous solution of methylamine, 0.6 wt. % V 2 O 5  and 8 wt. % WO 3  were carried per 100 wt. % complex oxide, the resultant was evaporated and dried, and then heated and fired at 500° C. for 5 hours. The obtained powder was used as the catalyst B. 
     (Preparation of Catalyst C) 
     A catalyst C, a typical catalyst having a function of denitration by ammonia, was prepared. Ti(O-iC 3 H 7 ) 4 , a Ti alkoxide, and Zr(Oi-C 4 h 9 ) 4 , a Zr alkoxide, were mixed at a ratio of 95:5 (wt. %) (as TiO 2 , ZrO 2 , respectively), the mixture was added to 80° C. water for hydrolysis, then the reaction mixture was stirred and matured, the produced sol was filtered, and the obtained gelled product was washed, dried, and heated and fired at 500° C. for 5 hours to obtain a powder of TiO 2 —ZrO 2  complex oxide (TiO 2 -ZrO 2  powder). Ammonium paratungstate ((NH 4 ) 10 H 10 W 12 O 4 .6H 2 O) was impregnated into the complex oxide by using a 10 wt. % aqueous solution of methylamine, 8 wt. % WO 3  was carried per 100 wt. % complex oxide, the resultant was evaporated and dried, and then heated and fired at 500° C. for 5 hours. The obtained powder was used as the catalyst C. 
     (Preparation of Catalyst D) 
     A catalyst D containing titania (TiO 2 ) only was prepared. A powder of anatase type titania of the same amount as the catalyst A was fired at 500° C. for 5 hours to prepare a powder of catalyst D. 
     (Preparation of Test Examples 1 to 5) 
     80 wt. % of water was added respectively to 20 wt. % of catalysts A to D, and the mixture was pulverized by wet ball mill to obtain wash coat slurry. Then a monolith base material (pitch: 7.4 mm, wall thickness: 0.6 mm) produced by Cordierite was coated with the slurry by dipping, and the obtained product was dried at 120° C. and then fired at 500° C. The amount of the coating was 100 g per surface area of 1 m 2  of the base material. A case of using the catalyst A and ammonia (NH 3 ) was used as the SO 3  reductant was used as Test Example 1. On the other hand, a case of using the catalyst A and propylene (C 3 H 6 ) was used as the SO 3  reductant was used as Test Example 2. A case of using the catalyst B and C 3 H 6  was used as the SO 3  reductant was used as Test Example 3. A case of using the catalyst C and C 3 H 6  as the SO 3  reductant was used as Test Example 4. In addition, a case of using the catalyst D and C 3 H 6  as the SO 3  reductant was used as Test Example 5. 
     (SO 3  Removal Test I) 
     By bench-scale testing in which an actual machine is assumed, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas containing the SO 3  reductant was allowed to flow through the catalyst of the respective Test Examples installed in the denitration and SO 3  reduction apparatus, and thereby variation of the concentration of SO 3  (ppm) in the combustion flue gas in terms of 0.03 to 0.8 (1/AV (m 2 ·h/Nm 3 ) after the combustion flue gas had flowed through the catalyst layer was examined. The test results and the test conditions are shown in  FIG. 4 . The concentration of SO 3  was analyzed by a deposition titration method after the sampling was done. In the drawing, “AV” denotes the area velocity (total contact area by gas amount/catalyst), and “1/AV” means the total contact area of the catalyst in relation to the gas amount. The unit of 1/AV is denoted as m 2 ·h/Nm 3 . 
       FIG. 4  shows variation of the concentration of SO 3  (ppm) in terms of 0.03 to 0.08 m 2 ·h/Nm 3  in Test Examples 1 to 5. Referring to  FIG. 4 , in Test Example 1, the concentration of SO 3  at the inlet of the catalyst layer did not substantially vary. In Test Example 2, the concentration of SO 3  at the inlet of the catalyst layer decreased from about 100 ppm to about 40 ppm at 0.06 m 2 ·h/Nm 3 . On the other hand, in Test Example 3, the concentration of SO 3  at the inlet of the catalyst layer decreased from about 100 ppm to about 20 ppm at 0.08 m 2 ·h/Nm 3 . In Test Example 4, the concentration of SO 3  at the inlet of the catalyst layer decreased from about 100 ppm to about 20 ppm at 0.08 m 2 ·h/Nm 3 . In addition, also in Test Example 5, the concentration of SO 3  at the inlet of the catalyst layer decreased from about 100 ppm to about 25 ppm at 0.08 m 2 ·h/Nm 3 . 
     It was observed that in Test Example 1 in which the catalyst A containing Ru was used and NH 3  was used as the SO 3  reductant, the concentration of SO 3  at the inlet of the catalyst layer did not substantially vary. It was observed that in Test Example 2 in which the catalyst A containing Ru was used and C 3 H 6  was used as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas decreased. In addition, it was observed that in Test Example 3 using the catalyst B not including expensive Ru, by using C 3 H 6  as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas remarkably decreased. In addition, it was observed that in Test Example 4 using the catalyst C, by using C 3 H 6  as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas remarkably decreased. Furthermore, it was observed that also in Test Example 5 using the catalyst D, by using C 3 H 6  as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas remarkably decreased. From these results, it was found that by using C 3 H 6  as the SO 3  reductant, concentration of SO 3  in the combustion flue gas could be reduced. 
     It was found that instead of using the catalyst A including Ru, by using a hydrocarbon including hydrogen element (H) and carbon element (C) such as C 3 H 6 , concentration of SO 3  in the combustion flue gas at the inlet of the catalyst layer could be reduced more even if a normal denitration catalyst was used, compared with the case of using NH 3  as the SO 3  reductant. Next, based on the elementary reaction model on the surface of the catalyst described in the following items 1 to 4, it was estimated that to obtain these results, sulfonation occurring due to the reaction on the catalyst between the matter obtained by decomposition of hydrocarbon and SO 3  was important. 
     1. Hydrocarbon adsorption reaction 
       Hydrocarbon (C x H y )+surface→C x H y −surface
 
     2. Hydrocarbon decomposition reaction (hydrogen abstraction reaction) 
       C x H y −surface→C x H y−1 (surface-coordination)+H−surface
 
     3. Reaction with SO 3(g) (conversion into sulfonic acid) 
       C x H y−1 (surface-coordination)+SO 3(g) →SO 2 +C x H y−1 —SO 3− H −  surface)
 
       4. Decomposition of SO 3    
       C x H y−1 —SO 3− H −  surface→SO 2 +CO 2 +CO
 
     Example 2 
     By using a hydrocarbon with a different composition as the first additive (SO 3  reductant), the effect of reducing SO 3  into SO 2  depending on the composition of the  2   0  hydrocarbon compound was examined. 
     (Preparation of Test Examples 6 to 10) 
     Similarly to Example 1, the catalyst B was coated on a monolith base material produced by Cordierite. The amount of the coating was 100 g per surface area of 1 m 2  of the base material. The case in which C 3 H 6  was used as the SO 3  reductant was used as Test Example 6, the case in which propane (C 3 H 8 ) was used as the SO 3  reductant was used as Test Example 7, the case in which methanol (CH 3 OH) was used as the SO 3  reductant was used as Test Example 8, and the case in which ethanol (C 2 H 5 OH) was used as the SO 3  reductant was used as Test Example 9. In addition, for comparison with the other Test Examples, the case in which ammonia (NH 3 ) was used as the SO 3  reductant was used as Test Example 10. 
     (SO 3  removal Test II) 
     Similarly to Example 1, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas containing the SO 3  reductant was allowed to flow through the catalyst layer using the SO 3  catalyst installed in the denitration and SO 3  reduction apparatus, and thereby variation of the concentration of SO 3  in the combustion flue gas at 0.04 to 0.08 m 2 ·h/Nm 3  after the combustion flue gas had flowed through the catalyst layer. The variation of the concentration of SO 3  before and after the combustion flue gas had flowed through the catalyst layer was examined. The test conditions were the same as those of Example 1. The test results and the test conditions are shown in  FIG. 5 . 
       FIG. 5  shows variation of the concentration of SO 3  (ppm) in the combustion flue gas at 0.04 to 0.08 m 2 ·h/Nm 3  in Test Examples 6 to 10. Referring to  FIG. 5 , in Test Examples 6 to 9, the concentration of SO 3  in the combustion flue gas at the inlet of the catalyst layer decreased. In contrast, in Test Example 10, the concentration of SO 3  in the combustion flue gas at the inlet of the catalyst layer did not decrease. In Test Examples 5 and 6 in which C 3 H 6  and C 3 H 8  were used as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas decreased more compared with the Test Examples 8 and 9 in which CH 3 OH and C 1 H 5 OH were used as the SO 3  reductant. In addition, in Test Example  6  in which C 3 H 6  was used as the SO 3  reductant, the effect of reducing the concentration of SO 3  was the most remarkable. 
     Then, further, by using a hydrocarbon having a different composition as the first additive (SO 3  reductant), the effect of reducing SO 3  into SO 2  and the denitration effect depending on the composition of the hydrocarbon compound were examined. 
     (Preparation of Catalyst E) 
     A catalyst E was prepared in a similar manner as the case of preparing the catalyst B except that the ratio of TiO 2  and SiO 2  was changed to 88:12 (wt. %), that the amount of V 2 O 5  was 0.3 wt. %, and that the amount of WO 3  was 9 wt. %. 
     (Preparation of Test Examples 11 to 18) 
     Similarly to Example 1, the catalyst E was coated onto the monolith base material produced by Cordierite. The case in which methanol (CH 3 OH) was used as the SO 3  reductant was used as Test Example 11, the case in which ethanol (C 2 H 5 OH) was used as the SO 3  reductant was used as Test Example 12, and the case in which propane (C 3 H 8 ) was used as the SO 3  reductant was used as Test Example 13. In addition, the case in which ethylene (C 2 H 4 ) was used as the SO 3  reductant was used as Test Example 14, the case in which propylene (C 3 H 6 ) was used as the SO 3  reductant was used as Test Example 15, the case in which 1-butene (1-C 4 H 8 ) was used as the SO 3  reductant was used as Test Example 16, the case in which 2-butene (2-C 4 H 8 ) was used as the SO 3  reductant was used as Test Example 17, the case in which isobutene (iso-C 4 H 8 ) was used as the SO 3  reductant was used as Test Example 18. 
     (SO 3  Removal Test III) 
     By using Test Examples 11 to 18, similarly to Example 1, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas containing the SO 3  reductant was allowed to flow through the catalyst layer using the SO 3  catalyst installed in the denitration and SO 3  reduction apparatus, and thereby the variation of the concentration of SO 3  and the denitration rate before and after the combustion flue gas had flowed through the catalyst layer was examined. The SO 3  reduction rate and the denitration rate were determined in the following manner. The test results and the test conditions are shown in  FIG. 6 . 
       SO 3  reduction rate (%)=(1−concentration of SO 3  at catalyst layer outlet/concentration of SO 3  at catalyst layer inlet)×100
 
       Denitration rate (%)=(1−concentration of NO x  at catalyst layer outlet/concentration of NO x  at catalyst layer inlet)×100
 
       FIG. 6  shows the SO 3  reduction rate (%) and the denitration rate (%) at 0.080 m 2 ·h/Nm 3  in Test Examples 11 to 18. Referring to  FIG. 6 , in Test Example 11 using an alcohol, the SO 3  reduction rate was 5.0%, and in Test Example 12 using an alcohol, the SO 3  reduction rate was 6.0%. In contrast, the SO 3  reduction rate of Test Example 13 using a saturated hydrocarbon was 10.0%, while in Test Example 14 using an unsaturated hydrocarbon was 20.0%, which were high values. Further, in Test Examples 15 to 18 using ≧3C unsaturated hydrocarbons, the SO 3  reduction rate of Test Example 15 was 58.0%, the SO 3  reduction rate of Test Example 16 was 50.2%, the SO 3  reduction rate of Test Example 17 was 54.2%, and the SO 3  reduction rate of Test Example 18 was 63.5%, which were very high values. 
     In Test Examples 11 and 12 using alcohols, the denitration rate of Test Example 11 was 92.6%, and the denitration rate of Test Example 12 was 93.2%. In Test Examples 13 and 14 using a saturated hydrocarbon and an unsaturated hydrocarbon, the denitration rate of Test Example 13 was 94.1%, and the denitration rate of Test Example 14 was 94.0%, which were high values. In Test Examples 15 to 18 using ≧3C unsaturated hydrocarbons, the denitration rate of Test Example 15 was 95.1%, the denitration rate of Test Example 16 was 92.1%, the denitration rate of Test Example 17 was 92.3%, and the denitration rate of Test Example 18 was 91.8%, which were sufficiently high values. 
     From the results of Examples 1 and 2, it was found that by using a hydrocarbon including the elements H and C as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas at the inlet of the catalyst layer decreased. It was also found that by using C 3 H 8 , C 2 H 4 , C 3 H 6 , or C 4 H 8 , i.e., a saturated hydrocarbon or an unsaturated hydrocarbon, as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas decreased more compared with the case of using alcohols such as CH 3 OH and C 2 H 5 OH. Further, it was found that among them, by using C 2 H 4 , C 3 H 6 , or C 4 H 8 , i.e., an unsaturated hydrocarbon, as the SO 3  reductant, the concentration of SO 3  in the combustion flue gas could be efficiently decreased. It was found that by using ≧3C unsaturated hydrocarbons as the SO 3  reductant, in particular, the concentration of SO 3  in the combustion flue gas remarkably decreased. It was estimated that this was because the decomposition activity of ≧3C unsaturated hydrocarbons is high and the intermediate body thereof has high reactivity with SO 3 . 
     Example 3 
     A catalyst having another composition was prepared and the effect of reducing SO 3  into SO 2  and the denitration rate depending on the catalyst composition was examined. 
     (Preparation of Catalyst F) 
     Ti(O-iC 3 H 7 ) 4 , a Ti alkoxide, and Ce(OCH 3 ) 4 , a Ce alkoxide, were mixed at a ratio of 88:12 (wt. %) (as TiO 2 , CeO 2 , respectively), the mixture was added to 80° C. water for hydrolysis, then the reaction mixture was stirred and matured, the produced sol was filtered, and the obtained gelled product was washed, dried, and heated and fired at 500° C. for 5 hours to obtain a powder of TiO 2 —Ce 2 O 3  complex oxide (TiO 2 —Ce 2 O 3  powder). The obtained powder was used as the catalyst F. 
     (Preparation of Catalyst G) 
     A catalyst including zirconia (ZrO 2 ) only was prepared. A powder of zirconium oxychloride (ZrOCl 2 ) was fired at 500° C. for 5 hours, and the obtained powder was used as the catalyst G. 
     (Preparation of Catalyst H) 
     A catalyst including cerium oxide (Ce 2 O 3 ) only was prepared. A powder of cerium nitrate (Ce(NO 3 ) 2 ) was fired at 500° C. for 5 hours, and the obtained powder was used as the catalyst H. 
     (Preparation of Test Examples 19 to 24) 
     80 wt. % water was added respectively to the TiO 2 —SiO 2  powder of the catalysts D and B, the TiO 2 —ZrO 2  powder of the catalyst C, and the catalysts F, G, and H, of which the amount was 20 wt. %, respectively, and the mixture was pulverized by wet ball mill to obtain wash coat slurry, and then the slurry was coated onto a ceramics base material including kaolinite as its main component, and the obtained pieces were used as Test Examples 19 to 24. Table 1 shows the composition of the respective Test Examples. In Table 1, for the coating amount average value, an average value was used which was obtained by measurement of 2 samples by using values calculated by dividing a carriage amount, which had been obtained based on the difference in the weight before and after the coating by the surface area of the base material. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Test Example Composition 1 
               
            
           
           
               
               
               
               
            
               
                   
                 Catalyst 
                 Coating amount 
                 Charged material 
               
               
                   
                 composition 
                 average value 
                 shape 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Test Example 19 
                 TiO 2   
                 106 
                 3 × 3 × 60 × 2 
               
               
                 Test Example 20 
                 TiO 2 —SiO 2   
                 108 
               
               
                 Test Example 21 
                 TiO 2 —ZeO 2   
                 127 
               
               
                 Test Example 22 
                 TiO 2 —Ce 2 O 3   
                 127 
               
               
                 Test Example 23 
                 ZrO 2   
                 102 
               
               
                 Test Example 24 
                 Ce 2 O 3   
                 103 
               
               
                   
               
            
           
         
       
     
     (SO 3  Removal Test VI) 
     The capability of reducing SO 3  when propylene (C 3 H 6 ) was used as the SO 3  reductant was examined for the respective Test Examples. Similarly to Example 2, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas including the SO 3  reductant was allowed to flow through the catalyst layer using the SO 3  catalyst installed in the denitration and SO 3  reduction apparatus, and thereby variation of the concentration of SO 3  before and after the combustion flue gas had flowed through the catalyst layer was examined. The test results and the test conditions are shown in  FIG. 7 . 
       FIG. 7  shows the SO 3  reduction rate (%) and the denitration rate (%) at 0.080 m 2 ·h/Nm 3  in Test Examples 19 to 24. Referring to  FIG. 7 , in Test Examples 19, 23, and 24 in which an oxide including a single component, the SO 3  reduction rate of Test Example 19 was 16.5%, the SO 3  reduction rate of Test Example 23 was 23.1%, and the SO 3  reduction rate of Test Example 24 was 11.1%. On the other hand, in Test Examples 20 to 22 in which a complex oxide containing TiO 2  was used, the SO 3  reduction rate of Test Example 20 was 52.2%, the SO 3  reduction rate of Test Example 21 was 47.3%, and the SO 3  reduction rate of Test Example 22 was 46.6%. 
     In addition, in Test Examples 19, 23, and 24 in which an oxide including a single component was used, the denitration rate of Test Example 19 was 32.8%, the denitration rate of Test Example 23 was 6.7%, and the denitration rate of Test Example 24 was 19.1%. On the other hand, in Test Examples 20 to 22 in which a complex oxide containing TiO 2  was used, the denitration rate of Test Example 20 was 60.4%, the denitration rate of Test Example 21 was 39.3%, and the denitration rate of Test Example 22 was 42.3%. 
     From these results, in all of Test Examples 19 to 24, the concentration of SO 3  in the combustion flue gas at the inlet of the catalyst layer decreased. In Test Examples 20 to 22 in which the TiO 2 —SiO 2  powder, TiO 2 —ZrO 2  powder, or the TiO 2 —Ce 2 O 3  powder was used, the SO 3  reduction rate was higher than that in Test Examples 19, 23, and 24 in which the TiO 2  powder, ZrO 2  powder, or the Ce 2 O 3  powder was used. In addition, in Test Examples 19, 23, and 24 in which an oxide including a single component was used, the reduction rate of Test Example 19 in which the TiO 2  powder was used was high, and the reduction rate of Test Example 23 in which the ZrO 2  powder was used was the highest. In addition, among Test Examples 20 to 22 in which the complex oxide was used, the SO 3  reduction rate of Test Example 19 using the TiO 2 —SiO 2  powder was the most remarkable. From these results, it was found that by using a complex oxide containing TiO 2 , in particular, the SO 3  reduction rate could be high. It was estimated that the above results were obtained due to increase of the solid acid amount, which occurred due to the use of the complex oxide. 
     (Determination of the Solid Acid Amount) 
     Then the relationship between the solid acid amount and the SO 3  reduction rate was examined. The solid acid amount in Test Examples 19 to 24 was measured by a pyridine thermal adsorption desorption method. More specifically, the same amount of 25 mg of a powder of quartz was added to the respective Test Example and the mixture was fixed in a quartz glass tube with Kaowool. The quartz glass tube was installed in an electric furnace installed in FID gas chromatography, then the reaction mixture was treated under the condition of the temperature of 450° C. for 30 minutes in a helium (He) gas stream. Then the electric furnace was maintained at 150° C., pyridine was injected by 0.5 μl for 4 to 6 times until saturation in terms of pulse was obtained, and the pyridine was adsorbed to the respective Test Examples. Then the temperature of the electric furnace was raised at the rate of 30° C./min, the desorbed pyridine was measured by FID gas chromatography, and the solid acid amount of the respective Test Examples was determined based on the obtained peak value in TPD spectrum. 
       FIG. 8  shows the relationship between the solid acid amount (μmol/g.cata) and the SO 3  reduction rate (%) measured in the respective Test Examples 19 to 24. Referring to  FIG. 8 , the larger the solid acid amount of the catalyst was, the higher the SO 3  reduction rate was. In particular, in Test Examples in which the solid acid amount was 200 μmol/g.cata to 300 μmol/g.cata, the SO 3  reduction rate was high. From these results, it was found that it became more effective for reduction of SO 3  as the solid acid amount increased. 
     Example 4 
     A catalyst having yet another composition was prepared and the effect of an active metal for reducing SO 3  into SO 2  and the denitration rate were examined. 
     (Preparation of Catalyst H) 
     Ti(O-iC 3 H 7 ) 4 , a Ti alkoxide, and Si(OCH 3 ) 3 , a Si alkoxide, were mixed at a ratio of 95:5 (wt. %) (as TiO 2 , SiO 2 , respectively), the mixture was added to 80° C. water for hydrolysis, then the reaction mixture was stirred and matured, the produced sol was filtered, and the obtained gelled product was washed, dried, and heated and fired at 500° C. for 5 hours to obtain a powder of TiO 2 —SiO 2  complex oxide (TiO 2 —SiO 2  powder). The obtained powder was used as the catalyst H. 
     (Preparation of Test Examples 25 to 32) 
     80 wt. % water was added to 20 wt. % catalyst H, and the mixture was pulverized by wet ball mill to obtain wash coat slurry, and then the slurry was coated onto a ceramic base material including kaolinite as its main component, and the obtained piece was used as Test Example 25. In addition, a predetermined amount of the respective solution of sulfate or nitrate used as the raw material of V 2 O 5 , MoO 3 , Ag, WO 3 , Mn 2 O 3 , NiO, and Co 3 O 4 , respectively, was added to the catalyst H, the solution was impregnated and the component was carried, then the obtained product was coated onto the ceramic base material similarly to Test Example 25, and the resultant product was used as Test Examples 26 to 32. The coating amount for each Test Example was measured similarly to Example 3, i.e., about 100 g/m 2 . Table 2 shows the composition of the respective Test Examples. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Test Example Composition 2 
               
            
           
           
               
               
               
            
               
                   
                 Catalyst composition 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Active 
                   
                 Load of active 
               
               
                   
                 Component 
                 Carrier 
                 component 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Test Example 25 
                 — 
                 TiO 2 —SiO 2   
                 — 
               
               
                 Test Example 26 
                 V 2 O 5   
                   
                 3.0/3.5 
               
               
                 Test Example 27 
                 MoO 3   
                   
                 3.0/5.5 
               
               
                 Test Example 28 
                 Ag 
                   
                 0.7/1.0 
               
               
                 Test Example 29 
                 WO 3   
                   
                 3.0/8.9 
               
               
                 Test Example 30 
                 Mn 2 O 3   
                   
                 3.0/3.0 
               
               
                 Test Example 31 
                 NiO 
                   
                 3.0/2.9 
               
               
                 Test Example 32 
                 Co 3 O 4   
                   
                 3.0/3.1 
               
               
                   
               
            
           
         
       
     
     (SO 3  Removal Test V) 
     The capability of reducing SO 3  when propylene (C 3 H 6 ) was used as the SO 3  reductant was examined for the respective Test Examples. Similarly to Example 2, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas including the SO 3  reductant was allowed to flow through the catalyst layer using the SO 3  catalyst installed in the denitration and SO 3  reduction apparatus, and thereby variation of the concentration of SO 3  before and after the combustion flue gas had flowed through the catalyst layer was examined. The test results and the test conditions are shown in  FIG. 9 . 
       FIG. 9  shows the SO 3  reduction rate (%) and the denitration rate (%) at 0.1 m 2 ·h/Nm 3  in Test Examples 24 to 32. Referring to  FIG. 9 , the SO 3  reduction rate of Test Example 25 was 52.2%. On the other hand, the SO 3  reduction rate of Test Example 26 was 11.4%, the SO 3  reduction rate of Test Example 27 was 44.5%, and the SO 3  reduction rate of Test Example 28 was 45.8%. The SO 3  reduction rate of Test Example 29 was 56.0%, the SO 3  reduction rate of Test Example 30 was 48.3%, the SO 3  reduction rate of Test Example 31 was 41.8%, and the SO 3  reduction rate of Test Example 32 was 39.7%. 
     The denitration rate of Test Example 25 was 60.4%. On the other hand, the denitration rate of Test Example 26 was 94.4%, the denitration rate of Test Example 27 was 82.4%, the denitration rate of Test Example 28 was 55.5%, the denitration rate of Test Example 29 was 73.4%, the denitration rate of Test Example 30 was 50.9%, the denitration rate of Test Example 31 was 46.2%, and the denitration rate of Test Example 32 was 44.3%. 
     From these results, it was verified that the SO 3  reduction effect and the denitration effect could be obtained by using C 3 H 6  as the SO 3  reductant for all Test Examples in which V 2 O 5 , MoO 3 , Ag, WO 3 , Mn 2 O 3 , MnO 2 , NiO, or Co 3 O 4  was carried. In Test Examples 27 to 32 in which MoO 3 , Ag, WO 3 , Mn 2 O 3 , MnO 2 , NiO, or Co 3 O 4  was carried, a high SO 3  reduction effect was observed. The SO 3  reduction effect and the denitration effect were observed particularly in Test Example 29 among them, in which WO 3  was carried. From this result, it was found that a catalyst impregnated with WO 3  was effective. 
     Example 5 
     A catalyst having yet another composition was prepared and both the SO 3  reduction capability and the denitration capability were evaluated. 
     (Preparation of Test Examples 33 to 37) 
     A catalyst I, in which V 2 O 5 —WO 3  was carried on TiO 2 , was prepared in a similar manner as the case of preparing the catalyst B except that 0.3 wt. % V 2 O 5  was carried by using ammonium metavanadate and 9 wt. % WO 3  was simultaneously carried by using ammonium paratungstate, per 100 wt. % of complex oxide, and the catalyst I was used as Test Example 33. 
     A catalyst J, in which V 2 O 5 —WO 3  was carried on a TiO 2 —SiO 2  complex oxide, was prepared in a similar manner as the case of preparing the catalyst B except that 0.3 wt. % V2O5 was carried and 9 wt. % WO 3  was simultaneously carried, per 100 wt. % of complex oxide, and the catalyst J was used as Test Example 33. The catalyst J was coated with metallosilicate at 25 g/m 2  to obtain a catalyst K, and the obtained catalyst K was used as Test Example 35. The catalyst B was used as Test Example 36. A catalyst L, in which V 2 O 5 —WO 3  was carried on TiO 2 , was prepared in a similar manner as the case of preparing the catalyst B except that 0.7 wt. % V 2 O 5  was carried and 9 wt. % WO 3  was simultaneously carried, per 100 wt. % of complex oxide, and the catalyst L was used as Test Example 37.  FIG. 3  shows the composition of the respective Test Examples. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Test Example Composition 3 
               
            
           
           
               
               
            
               
                   
                 Catalyst composition 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Active 
                   
               
               
                   
                 Coating layer 
                 Component 
                 Carrier 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Test Example 33 
                 — 
                 V 2 O 5 —WO 3   
                 TiO 2   
               
               
                 Test Example 34 
                 — 
                 V 2 O 5 —WO 3   
                 TiO 2 —SiO 2   
               
               
                 Test Example 35 
                 Metallosilicate 
                 V 2 O 5 —WO 3   
                 TiO 2 —SiO 2   
               
               
                 Test Example 36 
                 — 
                 WO 3   
                 TiO 2   
               
               
                 Test Example 37 
                 — 
                 WO 3   
                 TiO 2 —SiO 2   
               
               
                   
               
            
           
         
       
     
     (SO 3  Removal Test VI) 
     The capability of reducing SO 3  when propylene (C 3 H 6 ) was used as the SO 3  reductant was examined for the respective Test Examples. Similarly to Example 2, the SO 3  reductant was added to the combustion flue gas, and the combustion flue gas including the SO 3  reductant was allowed to flow through the catalyst layer using the SO 3  catalyst installed in the denitration and SO 3  reduction apparatus, and thereby variation of the concentration of SO 3  before and after the combustion flue gas had flowed through the catalyst layer and the denitration rate were examined. The test results and the test conditions are shown in  FIGS. 10 and 11 . 
       FIG. 10  shows the SO 3  reduction rate (%) at 0.1 (1/AV: m 2 ·h/Nm 3 ) in Test Examples 33 to 37. Referring to  FIG. 10 , the SO 3  reduction rate of Test Example 33 was 33.3%. On the other hand, the SO 3  reduction rate of Test Example 34 was 58.4%, the SO 3  reduction rate of Test Example 35 was 75.6%, the SO 3  reduction rate of Test Example 36 was 68.6%, and the SO 3  reduction rate of Test Example 37 was 79.9%. 
       FIG. 11  shows the rate of denitration (%) from the combustion flue gas at 0.10 (1/AV: m 2 ·h/Nm 3 ) in Test Examples 33 to 37. Referring to  FIG. 11 , the denitration rate of Test Example 33 was 95.3%, the denitration rate of Test Example 34 was 95.1%, the denitration rate of Test Example 35 was 91.1%, the denitration rate of Test Example 36 was 91.4%, and the denitration rate of Test Example 37 was 91.8%. 
     From these results, it was found that in all these Test Examples, both the high SO 3  reduction capability and the high denitration capability could be obtained. In addition, in Test Examples 34 to 37, a high performance of reducing SO 3  to SO 2  higher than that of Test Example 33 was observed as expected. 
     INDUSTRIAL APPLICABILITY 
     According to the flue gas treatment method and the denitration and SO 3  reduction apparatus of the present invention, it is made possible to denitrate NO x  in the combustion flue gas and reduce the concentration of SO 3  in the combustion flue gas at the same time at treatment costs lower than those conventionally. 
     REFERENCE SIGNS LIST 
     
         
           1  Furnace 
           2  Flue gas chimney 
           3  ECO 
           4  ECO bypass 
           5 ,  15 ,  25  Denitration and SO 3  reduction apparatus 
           6 ,  16 ,  26  First injection device 
           7 ,  17 ,  27  Second injection device 
           8  Catalyst layer 
           18 ,  28  First catalyst layer 
           19 ,  29  Second catalyst layer