JP4499511B2 - Method for treating exhaust gas containing nitrogen oxides - Google Patents

Description

  The present invention relates to a method for treating exhaust gas containing nitrogen oxides, and in particular, efficiently reduces and removes nitrogen oxides in exhaust gases using a reducing agent such as ammonia and urea in a low temperature range of 150 ° C. or higher and lower than 200 ° C. On how to do.

  Currently, as a method of removing nitrogen oxides in exhaust gas discharged from thermal power plants, garbage incinerators, etc., nitrogen oxides in exhaust gas are catalytically reduced on the catalyst using a reducing agent such as ammonia or urea. A selective catalytic reduction (SCR) process that decomposes into harmless nitrogen and water is common. As a catalyst for removing nitrogen oxide (denitration catalyst) used for this, an oxide carrier containing titanium such as a titania carrier, a binary composite oxide carrier made of titanium (Ti) and silicon (Si), vanadium (V ), Tungsten (W), molybdenum (Mo) and other metal oxide catalysts are put into practical use. These catalysts are used at a high temperature of 200 ° C. or higher, usually 250 ° C. or higher. It is designed to exhibit an efficient denitration function. On the other hand, in recent years, utilization of thermal recycling of waste has been studied, and thermal energy obtained by burning waste is used for various purposes. There is an increasing demand to remove nitrogen oxides in the gas discharged from these various thermal recycling facilities, but the exhaust gas temperature of this type of facility is as low as 200 ° C. or less, and the conventional high temperature type described above is used. There is a problem that a denitration catalyst cannot sufficiently perform its function.

  Conventionally, various catalyst systems have been proposed as this low-temperature denitration catalyst. Among them, a catalyst comprising a titanium oxide as a carrier and a base metal oxide such as manganese (Mn) as a main active component. As a catalyst, a catalyst in which a manganese oxide is supported on a titanium oxide support whose content of nitrate radical is as low as 0.1% by mass or less is used, and in the presence of this catalyst, ammonia is used in a temperature range of 150 to 300 ° C. A method of denitration treatment has been proposed (see Patent Document 1). However, the purpose of the manganese catalyst is an oxidation catalyst for the purpose of converting NO into NO2, and it is not described that it has a denitration function.

  Further, as a catalyst for denitration in the presence of ammonia or the like in the temperature range of 200 to 500 ° C., a binary composite oxide composed of Ti and Si is used as a support, and V, W, Mo, Mn, Cu, Cr, Ce and Sn There has been proposed a catalyst comprising an oxide of at least one element selected from the group consisting of (see Patent Document 2). However, this document does not disclose the denitration performance when the catalyst system is used in a temperature range below 200 ° C.

  Further, in the temperature range of 200 to 250 ° C., in the presence of a reducing agent such as hydrocarbons and ammonia, a nitrogen oxide (NO) oxidation catalyst such as Ti—Mn or Ti—Cr upstream of the exhaust gas flow. And a method of denitration using a catalyst device in which a denitration catalyst is arranged on the downstream side has been proposed (see Patent Document 3). However, in this document, there is no specific description regarding the catalyst composition, the catalyst preparation method, and the like for the NO oxidation catalyst such as Ti—Mn and Ti—Cr, and it is not possible to specify what the catalyst is.

JP-A-9-155190 (Claims) Japanese Patent Publication No. 5-87291 (Claims, Examples 9, 10, and 11) JP-A-8-103636 (Claims)

  An object of the present invention is to use nitrogen oxides in exhaust gas in the low temperature range of 150 ° C. or more and less than 200 ° C. using a reducing agent in the presence of a catalyst, and while suppressing decomposition of ammonia, It is to provide a method for reducing and removing well.

  According to the study by the present inventors, it has been found that the above problem can be solved by using a catalyst comprising (A) a composite oxide of titanium and silicon and (B) an oxide of manganese as a catalyst. The present invention has been completed.

  That is, the present invention provides an exhaust gas treatment method in which nitrogen oxides in exhaust gas are reduced and removed with ammonia in the presence of a catalyst, and the exhaust gas is treated at a temperature of 150 ° C. or higher and lower than 200 ° C. (A) a composite of titanium and silicon. A method for treating exhaust gas containing nitrogen oxides, characterized by contacting with a catalyst comprising an oxide and an oxide of (B) manganese.

  According to the method of the present invention, nitrogen oxides in exhaust gas can be efficiently reduced and removed in a low temperature range of 150 ° C. or higher and lower than 200 ° C. without losing ammonia or the like by decomposition.

The catalyst used in the present invention is a catalyst comprising (A) a composite oxide of titanium and silicon (hereinafter referred to as Ti-Si composite oxide) and (B) an oxide of manganese.

  Both the Ti-Si composite oxide and Ti-Zr composite oxide of the component (A) are generally well known and can be easily prepared according to conventionally known methods.

  As the titanium source, in addition to titanium oxide, any inorganic and organic compound can be used as long as it can be baked to produce a titanium oxide. For example, an inorganic titanium compound such as titanium tetrachloride or titanium sulfate, or an organic titanium compound such as titanium oxalate or tetraisopropyl titanate can be used.

  As the silicon source, colloidal silica, water glass, fine silicon, inorganic silicon compounds such as silicon tetrachloride, and organic silicon compounds such as tetraethyl silicate can be used.

  As the zirconium source, inorganic zirconium compounds such as zirconium chloride and zirconium sulfate, and organic zirconium compounds such as zirconium oxalate can be used.

The Ti—Si composite oxide can be prepared, for example, by the following procedures (a) to (d).
(A) Silica sol and ammonia water are mixed, a sulfuric acid aqueous solution of titanium sulfate is added to cause precipitation, and the obtained precipitate is washed and dried, and then baked at 300 to 700 ° C.
(B) A sodium silicate aqueous solution is added to a titanium sulfate aqueous solution and reacted to cause precipitation. The obtained precipitate is washed and dried, and then baked at 300 to 700 ° C.
(C) Ethyl silicate (tetraethoxysilane) is added to a water-alcohol solution of titanium tetrachloride and then hydrolyzed to form a precipitate. The resulting precipitate is washed and dried, and then 300 to 700 ° C. Bake with.
(D) Ammonia is added to a water-alcohol solution of titanium oxide chloride (titanium oxytrichloride) and ethyl silicate to cause precipitation. The resulting precipitate is washed and dried, and then calcined at 300 to 700 ° C. To do.

Among the above methods, the method (a) is particularly preferable. Specifically, the ammonia source, the silicon source and the titanium source are each in an aqueous solution or a sol state, and each amount is a predetermined amount (the ammonia source is NH ₃ , the silicon source is SiO ₂ and the titanium source are converted to TiO ₂ . Next, an ammonia source and a silicon source were mixed, and while maintaining the mixed solution at 10 to 100 ° C., a titanium source was dropped into the mixed solution and kept at pH 2 to 10 for 1 to 50 hours. A silicon coprecipitate is produced, and this precipitate is filtered, washed thoroughly, dried at 80 to 140 ° C. for 10 minutes to 3 hours, and calcined at 300 to 700 ° C. for 1 to 10 hours. A Ti—Si composite oxide can be obtained.

  The Ti—Zr composite oxide may be prepared in accordance with the above-described method for preparing the Ti—Si composite oxide, and may be prepared using a water-soluble zirconium compound or the like as the zirconium source instead of the silica source.

The content of silicon or zirconium oxide in the Ti-Si composite oxide or Ti-Zr composite oxide is 0.5 to 60 mol%, preferably 1.5 to 60 mol%, relative to the titanium oxide. , more preferably from 1.5 to 45 mol% (titanium, in terms of each silicon and zirconium as _TiO 2, SiO ₂ and ZrO _2).

  As the manganese source of component (B), in addition to manganese oxide, any inorganic and organic compounds can be used as long as they generate oxides by firing. For example, manganese-containing hydroxides, ammonium salts, oxalates, halides, sulfates, nitrates, carbonates, and the like can be used.

  The method for preparing the catalyst comprising the above components (A) and (B) of the present invention may be appropriately selected from known methods usually employed in this field, such as a normal impregnation supporting method, a kneading method and a dipping method. it can. For example, after obtaining a powder of a mixture of components (A) and (B), it is molded into a desired shape. In that case, each component may be prepared by mixing in the form of powder or slurry, or may be prepared by coprecipitation from a mixture of solutions of each salt. In addition, as a method of supporting the component (B) on the component (A), a method of adding the salt of the component (B) or a solution thereof to the powder or slurry mixture of the component (A), or from the component (A) A method of impregnating and supporting a solution of the salt of component (B) on the resulting molded body can be used.

About the composition of the catalyst of this invention, in the case of the catalyst containing component (A) and (B), component (B) is 0.1-40 mass% on the mass reference | standard of component (A), Preferably it is 1- 40% by mass [Ti-Si composite oxide, Ti-Zr composite oxide is the total mass, and manganese is converted as MnO ₂ ]. When the content of the component (B) is less than 0.1% by mass of the component (A), the denitration activity is low. On the other hand, when the content exceeds 40% by mass, no significant improvement in activity is observed, and in some cases the activity is It may decrease.

The total pore volume of the catalyst of the present invention measured by mercury porosimetry is preferably in the range of 0.2 to 0.6 cm ³ / g. If the total pore volume of the catalyst is smaller than 0.2 cm ³ / g, the denitration activity is low, while if it exceeds 0.6 cm ³ / g, the mechanical strength of the catalyst is lowered, which is not preferable. BET specific surface area of the catalyst of the present invention is 30~250m ² / g, preferably, from the 40 to 200 m ² / g. If the specific surface area of the catalyst is less than 30 m ² / g, the denitration activity will be low. On the other hand, if it exceeds 250 m ² / g, no significant improvement in activity will be observed, and in some cases the accumulated amount of catalyst poisoning components will increase. Thus, the catalyst life may be adversely affected.

Therefore, in the catalyst of this invention, a component (B) is included in the ratio of 0.1-40 mass% of a component (A), or a component (C) is further 0.1-25 mass% of a component (A). In particular, a catalyst having a total pore volume measured by mercury porosimetry in the range of 0.2 to 0.6 cm ³ / g and a specific surface area by the BET method in the range of 30 to 250 m ² / g is particularly preferable. Used for.

  The shape of the catalyst of the present invention is not particularly limited, and may be used by molding into a desired shape selected from a plate shape, a corrugated plate shape, a net shape, a honeycomb shape, a columnar shape, a cylindrical shape, and the like. In addition, it may be used by being supported on a carrier having a desired shape selected from a plate shape, corrugated plate shape, net shape, honeycomb shape, columnar shape, cylindrical shape made of silica, cordierite, titania, stainless steel, etc. .

  The catalyst of the present invention is used for treating various exhaust gases containing nitrogen oxides. The composition of the exhaust gas is not particularly limited. It is suitably used for the treatment of exhaust gas containing.

The denitration treatment of the present invention can be performed according to a generally known method except that the catalyst of the present invention is used as a catalyst. Specifically, exhaust gas containing nitrogen oxides may be brought into contact with the catalyst of the present invention together with an amount of ammonia necessary for reducing and removing nitrogen oxides. The conditions at this time are not particularly limited, and can be carried out under conditions generally used for denitration treatment. Specifically, it may be appropriately determined in consideration of the type and properties of exhaust gas, the required decomposition rate of nitrogen oxides, and the like. Incidentally, the space velocity of the exhaust gas when performing the denitration process of the present invention is usually, 100～100000Hr ^- a ¹ (STP), preferably 200～50000Hr ^- a ¹ (STP). 100 hr ^- it is less than ^1, the processing apparatus becomes inefficient because too large, whereas 100000Hr ^{- 1} and more than decomposition efficiency is lowered.

  The temperature of the exhaust gas when performing the denitration treatment of the present invention is 150 ° C. or more and less than 200 ° C., preferably 170 to 200 ° C. If the exhaust gas temperature is lower than 150 ° C., the denitration efficiency is lowered, which is not preferable. The reducing agent (also referred to as ammonia or the like) according to the present invention is capable of reducing nitrogen oxides, preferably ammonia and / or urea, and more preferably ammonia in consideration of the exhaust gas treatment temperature.

  The sulfur oxide (SOx) concentration in the exhaust gas is preferably 1% or less. This is because when the SOx concentration in the exhaust gas exceeds 1%, the catalyst activity deteriorates greatly.

  The invention is further illustrated by the following examples, which illustrate advantageous embodiments of the invention.

Catalyst preparation example 1

After adding 21.3 kg of SNOWTEX-20 (silica sol manufactured by Nissan Chemical Co., Ltd., containing about 20% by mass of SiO ₂ ) to 700 L of 10% by mass ammonia water, stirring and mixing, a sulfuric acid solution of titanyl sulfate (TiO ₂₎ (125 g / liter, sulfuric acid concentration 550 g / liter) was gradually added dropwise with stirring. The obtained gel was allowed to stand for 20 hours, filtered, washed with water, and then dried at 150 ° C. for 10 hours. This was baked at 500 ° C. to obtain a powder. The composition of the obtained powder is TiO ₂ : SiO ₂ = 8.5: 1.5 (molar ratio), and clear intrinsic peaks of TiO ₂ and SiO ₂ are recognized in the X-ray diffraction chart of the powder. In addition, it was confirmed by a broad diffraction peak that it was a composite oxide of titanium and silicon (Ti-Si composite oxide) having an amorphous microstructure.

  A solution prepared by dissolving 2.1 kg of ammonium metavanadate, 2.4 kg of oxalic acid and 0.6 kg of monoethanolamine in 7 liters of water and 10% methylamine aqueous solution (three Add 3 liters of tungsten oxide (400 g / liter), add organic binder (0.5 kg of starch) as a molding aid, mix, mix well with blender while adding appropriate amount of water, then use continuous kneader The mixture was kneaded and extruded into a honeycomb shape. The shape was formed into a lattice shape having an opening of 4.35 mm, a thickness of 0.6 mm, and a length of 500 mm. Next, the obtained molded product was dried at 80 ° C. and then fired at 450 ° C. in an air atmosphere for 5 hours to obtain a vanadium / tungsten (V · W) -supported Ti—Si composite oxide molded body.

Next, the V · W-supported Ti—Si composite oxide compact was impregnated with an aqueous manganese nitrate [Mn (NO ₃ ) ₂ .6H ₂ O] solution (300 g-Mn / liter), and then dried at 120 ° C. The catalyst (a) was obtained by calcination at 3 ° C. for 3 hours. The composition of the catalyst (a) is Ti—Si composite oxide: V ₂ O ₅ : WO ₃ : MnO ₂ = 67.5: 3.75: 3.75: 25 (mass ratio) (component (B) / Component (A) = 37.0% by mass, Component (C) / Component (A) = 11.1% by mass).

Catalyst preparation example 2

The V · W-supported Ti—Si composite oxide molded body obtained in Catalyst Preparation Example 1 was impregnated with a manganese nitrate [Mn (NO ₃ ) ₂ .6H ₂ O] aqueous solution (200 g-Mn / liter), and then 120 ° C. And dried at 420 ° C. for 3 hours to obtain a catalyst (b). The composition of the catalyst (b) is Ti—Si composite oxide: V ₂ O ₅ : WO ₃ : MnO ₂ = 76.5: 4.25: 4.25: 15 (mass ratio) (component (B) / Component (A) = 19.6% by mass, Component (C) / Component (A) = 11.1% by mass).

Catalyst preparation example 3

After adding 21.3 kg of SNOWTEX-20 (silica sol manufactured by Nissan Chemical Co., Ltd., containing about 20% by mass of SiO ₂ ) to 700 L of 10% by mass ammonia water, stirring and mixing, a sulfuric acid solution of titanyl sulfate (TiO ₂₎ (125 g / liter, sulfuric acid concentration 550 g / liter) was gradually added dropwise with stirring. The obtained gel was allowed to stand for 20 hours, filtered, washed with water, and then dried at 150 ° C. for 10 hours. This was baked at 500 ° C. to obtain a powder. The composition of the obtained powder is TiO ₂ : SiO ₂ = 8.5: 1.5 (molar ratio), and clear intrinsic peaks of TiO ₂ and SiO ₂ are recognized in the X-ray diffraction chart of the powder. In addition, it was confirmed by a broad diffraction peak that it was a composite oxide of titanium and silicon (Ti-Si composite oxide) having an amorphous microstructure.

  An organic binder (0.5 kg of starch) was added to 10 kg of the Ti—Si composite oxide, and after mixing well with a blender while adding an appropriate amount of water, the mixture was sufficiently kneaded with a continuous kneader and extruded into a honeycomb shape. The shape was formed into a lattice shape having an opening of 4.35 mm, a thickness of 0.6 mm, and a length of 500 mm. Next, the obtained molded product was dried at 80 ° C. and then fired at 450 ° C. for 5 hours in an air atmosphere to obtain a Ti—Si composite oxide molded body.

Next, the Ti—Si composite oxide molded body was impregnated with a manganese nitrate [Mn (NO ₃ ) ₂ .6H ₂ O] aqueous solution (300 g-Mn / liter), then dried at 120 ° C., and at 420 ° C. for 3 hours. Firing was performed to obtain a catalyst (c). The composition of the catalyst (c) was Ti—Si composite oxide: MnO ₂ = 75: 25 (mass ratio) (component (B) / component (A) = 33.3 mass%).

Catalyst preparation example 4

The Ti—Si composite oxide compact obtained in Catalyst Preparation Example 3 was impregnated with an aqueous manganese nitrate [Mn (NO ₃ ) ₂ .6H ₂ O] solution (200 g-Mn / liter), and then dried at 120 ° C. And calcined at 420 ° C. for 3 hours to obtain a catalyst (d). The composition of the catalyst (d) was Ti—Si composite oxide: MnO ₂ = 85: 15 (mass ratio) (component (B) / component (A) = 17.6 mass%).

Catalyst preparation example 5

The V · W-supported Ti—Si composite oxide molded body obtained in Catalyst Preparation Example 1 was directly used as a comparative catalyst (e). The composition of the catalyst (e) was Ti—Si composite oxide: V ₂ O ₅ : WO ₃ = 90: 5: 5 (mass ratio).

Activity tests were carried out on the catalysts (a), (b) (reference catalyst) , (c), (d) (the present invention catalyst) obtained in Catalyst Preparation Examples 1 to 5 and the comparative catalyst (e). went. Each honeycomb-type catalyst was cut into an outer shape of 15.5 mm (3 × 3 cells) and a length of 126 mm, and this was packed in a catalyst reactor having a diameter of 35 mm so as to be balanced with respect to the gas flow direction. Synthetic gas 0.5Nm ³ / h having the following composition was supplied to the catalyst layer together with ammonia (NH ₃ ).
(Gas composition) NOx: 350 ppm, NH ₃ : 350 ppm, O ₂ : 15%, SO ₂ : 0 ppm, H ₂ O: 10%, N ₂ : Balance (gas temperature) 150 ° C, 180 ° C, 195 ° C
The NOx concentration at the outlet of the reactor was measured, and the NOx removal rate was determined according to the following formula.
NOx removal rate = [(reactor inlet NOx concentration) − (reactor outlet NOx concentration)] ÷ (reactor inlet NOx concentration) × 100
The results are shown in Table 1.

Claims (3)

  In a method for treating exhaust gas in which nitrogen oxides in exhaust gas are reduced and removed with ammonia in the presence of a catalyst, the exhaust gas is treated at a temperature of 150 ° C. or higher and lower than 200 ° C. (A) a composite oxide of titanium and silicon and (B) A method for treating exhaust gas containing nitrogen oxides, characterized by contacting with a catalyst comprising an oxide of manganese. Method of processing an exhaust gas total pore volume measured by mercury porosimetry of catalyst including nitrogen oxides according to claim 1, wherein the 0.2~0.6cm ³ / g. The method for treating exhaust gas containing nitrogen oxides according to claim 1 or 2, wherein the catalyst has a specific surface area of 30 to 250 m ² / g by BET method.