JP2743336B2 - Catalyst for reducing nitrogen oxides and method for removing nitrogen oxides from exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for catalytically reducing and removing nitrogen oxides (hereinafter referred to as NOx) contained in exhaust gas using a reducing agent such as ammonia in the presence of a catalyst, and a catalyst used therefor. Furthermore, high temperature, for example, 50
A catalyst capable of efficiently decomposing NOx in exhaust gas into nitrogen and water in the presence of a reducing agent in a temperature range of 0 ° C. or higher and having excellent durability and heat resistance, and a method for removing NOx using the same. .

[0002]

2. Description of the Related Art Exhaust gas discharged from gas turbines, open hearths for steelmaking, and the like has a high temperature of 500 to 650 ° C., and therefore, NOx in such exhaust gas is used even in a high temperature range of 500 ° C. or more. There is a demand for a denitration catalyst having sufficient activity and excellent heat resistance. A catalyst suitable for use in such a high-temperature region is proposed in, for example, JP-A-2-83039. However, when NOx treatment in exhaust gas was performed at a reaction temperature of 500 ° C. or higher using a conventional denitration catalyst, satisfactory results were not obtained due to insufficient denitration activity and the like.

[0003]

The object of the present invention is to remedy the disadvantages of the prior art;
It is an object of the present invention to provide a catalyst capable of efficiently reducing and removing NOx in exhaust gas over a long period of time with high denitration activity in a temperature range of 00 ° C. or higher, and a method of removing NOx using the catalyst.

[0004]

The catalyst of the present invention comprises a group IIa and a group IIIa of the periodic table.
A fired titanium oxide containing at least one element selected from the group a, and the fired product carries tungsten and at least one element component selected from phosphorus, boron, zinc and lead. It is.

Japanese Patent Application Laid-Open No. 2-241 filed earlier by the present applicant
No. 543, as a NOx reduction catalyst in exhaust gas, a mixture of hydrated titanium oxide and a compound of at least one element selected from Group IIa and Group IIIa of the periodic table and calcining the mixture. A catalyst supporting a catalytically active component has been proposed, and this catalyst has excellent characteristics as an industrial catalyst. However, this catalyst is used for removing NOx contained in combustion exhaust gas discharged from a thermal power plant or the like, and the temperature range of the applied exhaust gas is in the range of 250 to 400 ° C.
Satisfactory results were not obtained in the higher temperature range. Conventionally, in a reaction region lower than 500 ° C., in addition to active components such as tungsten and vanadium, denitration catalysts containing components such as phosphorus, boron, zinc and lead are more denitrification than catalysts not containing these components. Activity tended to be low. However, the present inventors unexpectedly found that these phosphorus,
A denitration catalyst containing components such as boron, zinc, and lead exhibits high denitration activity. Further, these components and a tungsten component contain at least one element selected from Group IIa and Group IIIa of the periodic table. The catalyst obtained by supporting the carrier obtained by calcining the hydrous titanium oxide is 500 ° C.
The present inventors have also found that NOx in exhaust gas can be efficiently reduced and removed over a long period of time with high denitration activity even at the above high temperatures, and have completed the present invention. Hereinafter, the catalyst used in the present invention will be described. The hydrous titanium oxide containing at least one element selected from the group IIa and group IIIa of the periodic table used in the production of the catalyst of the present invention is obtained by mixing with a titanium raw material such as titanium sulfate, titanyl sulfate, titanium tetrachloride and the like. Precipitating with at least one elemental compound selected from Table IIa group and IIIa group by a neutralization method or the like,
Alternatively, a hydrous titanium oxide and a group IIa or III
It is obtained by mixing with at least one elemental compound selected from group a. The “hydrous titanium oxide” in the present invention is a compound also referred to as titanium hydroxide or hydrated titanium oxide, specifically, titania sol, metatitanic acid,
Orthotitanic acid and the like are included. In the catalyst of the present invention,
A titanium compound containing 50 wt% or more, preferably 80 wt% or more, of titanium oxide can be used. In particular, a titanium oxide slurry obtained as an intermediate in a titanium oxide production process by a sulfuric acid method is optionally used. The washed one is suitable as a starting material of the present invention. Also,
Examples of the compound of an element selected from Group IIa and Group IIIa of the periodic table include sulfates, nitrates, and the like containing these elements.
Carbonates, acetates, oxalates, ammonium salts, chlorides, hydroxides and the like, among which Mg, Ba, La, C
Of the above compounds containing at least one of e and Y, water-soluble compounds are particularly preferred. In the catalyst of the present invention, the compound of at least one element selected from the group IIa and group IIIa of the periodic table is contained in the hydrated titanium oxide. The conversion is preferably in the range of 1.0 to 10% by weight, more preferably 1.5 to 5.0% by weight of the mixture. 1.0
If the amount is less than 10% by weight, satisfactory heat resistance and durability cannot be provided. If the amount is 10% by weight or more, the moldability of the mixture is deteriorated, and the SOx oxidizing ability of the catalyst is also increased. Among compounds containing elements of groups IIa and IIIa of the periodic table, M
Compounds containing at least one of g, Ba, La, Ce, and Y are suitable for achieving the object of the present invention.
In particular, the use of a compound containing at least one of Mg, Ba, La, and Ce makes it possible to obtain a catalyst having a small surface area of the obtained carrier, high denitration performance, low SOx oxidizing ability, and excellent durability. It is especially effective on the above. In the method of mixing the hydrated titanium oxide and the compound, the compound or an aqueous solution thereof is added to an aqueous slurry of the hydrated titanium oxide, and the mixture is mixed in order to improve the moldability of the powder obtained by firing the mixture. It is desirable that the slurry be adjusted to a pH of 6 or more, preferably 8 to 10 and then stirred at a temperature of 40 ° C. or more, preferably 50 to 95 ° C. for 0.5 to 5 hours. The mixture thus obtained is then calcined, the calcining being carried out at a temperature above 500 ° C., preferably 550-700 ° C., for 1-7 hours.
When the calcination temperature is lower than 500 ° C., the powder obtained by calcination has poor fluidity and moldability, so that the denitration catalyst obtained therefrom cannot have sufficient heat resistance and durability.
The catalyst of the present invention is prepared by supporting at least one element component selected from tungsten, phosphorus, boron, zinc and lead as a catalytically active component on the powder obtained from the above-mentioned calcination step. As the tungsten element component, tungsten trioxide, ammonium metatungstate, ammonium paratungstate, etc. can be used. As the phosphorus element component, phosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, dipentoxide, etc. Phosphorus and the like, boron element components such as boric acid, ammonium boride and boron oxide, zinc element components such as zinc chloride, sulfate and acetate, and lead element components as lead chloride and lead acetate;
Lead carbonate, lead nitrate, lead oxide and the like can be used. The amount of tungsten as an active component in the catalyst is in the range of ₃ to 30% by weight, preferably ₅ to 25% by weight as WO3, and at least one elemental component selected from phosphorus, boron, zinc and lead is an oxide. It is desirably in the range of 7 wt% or less, preferably in the range of 0.3 to 5 wt%.
The catalyst of the present invention may optionally contain a third active ingredient in addition to the above active ingredient. The catalyst of the present invention is prepared by supporting a catalytically active component on the powder obtained from the above-mentioned calcination step by an optional method. For example, the catalytically active component can be formed by molding a calcined powder into a carrier having an appropriate shape and dimensions in advance, and then supporting the carrier by any method. Further, a compound containing a catalytically active component and the above calcined powder are kneaded in the presence of water, and then extruded into, for example, a honeycomb shape, and then dried and calcined in the usual manner to prepare a catalyst. You can also. In molding, a molding aid, inorganic fiber, etc. can be blended as required, and the molded product is fired at 500 to 700 ° C. for 1 to 1
Usually, it is performed for about 0 hours. The catalyst of the present invention produced by the above method, ammonia, urea, hydrocarbon,
NOx-containing exhaust gas and a high temperature, preferably 500 to 650 ° C, more preferably 550 to 550 ° C in the presence of a reducing agent such as CO.
It is used in contact at a temperature range of 600 ° C. The type of exhaust gas to be treated using the catalyst of the present invention is not particularly limited, but is used for high-temperature exhaust gas of 500 ° C. or more such as gas discharged from gas turbines, steel furnaces, etc. Then it is particularly useful. As the conditions for treating exhaust gas using the catalyst of the present invention, reaction conditions conventionally used for reducing and removing NOx from exhaust gas in the presence of a reducing agent such as ammonia can be adopted.

[0006]

EXAMPLES The present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. Example 1 1.26 kg of barium acetate was added to 84 kg of a 30 wt% concentration of metatitanic acid slurry as TiO ₂ , and after stirring for 1 hour, the pH was adjusted to 6.0 with aqueous ammonia. Next, the solid content was separated by filtration, dried, and calcined at 650 ° C. for 5 hours. The obtained powder contained 2.9% by weight of BaO as BaO. 50 kg by weight of WO _{3 in} 5 kg of this calcined powder
1.6 kg of an aqueous solution of ammonium metatungstate containing tungsten and 2.0 liters of ion-exchanged water were added.
Kneaded for hours. Further, 350 g of boron oxide (B ₂ O ₃ ) and 150 g of methyl cellulose were added and kneaded while heating for 1 hour. Next, after moisture control, the kneaded product was extruded into a honeycomb shape, dried sufficiently, and dried at 650 ° C. for 5 minutes.
After calcination for a time, a honeycomb-shaped denitration catalyst (A) was obtained. Example 2 Triammonium phosphate [(NH3) was added to a kneaded mixture obtained by adding the same amount of an aqueous solution of ammonium metatungstate to powder composed of titanium oxide and barium oxide obtained in the same manner as in Example 1.
_{4) 3 PO 4 · 3H 2} O ] 430g and methylcellulose 2
Then, the mixture was kneaded for 1 hour. Then, after moisture adjustment, the mixture was extruded into a honeycomb having the same shape as in Example 1, dried,
The catalyst was obtained by calcining at 650 ° C. for 5 hours. This is designated as catalyst (B). Example 3 84 k of hydrated titanium oxide having a concentration of 30% by weight as TiO ₂
2.9 kg of La (NO ₃ ) ₂ .6H ₂ O was added to
After stirring for an hour, ammonia water was added to adjust the pH to 9.0,
After further stirring for 10 hours, the mixture was filtered, dried, and dried at 650 ° C.
For 10 hours. Lanthanum is L in the obtained powder.
The content was 4.8% by weight as a ₂ O ₃ . 1.5 kg of ammonium paratungstate aqueous solution was added to 5 kg of this calcined powder.
And 3 liters of deionized water, monoethanolamine 5
00g was added and kneaded for 5 hours. Next, 4.2 kg of an aqueous solution containing 8 wt% of zinc sulfate was added to the kneaded material, and the mixture was kneaded while heating. Next, after performing moisture control, the kneaded product was extruded into a honeycomb shape, dried sufficiently, and then dried at 650 ° C.
For 5 hours to obtain a honeycomb catalyst (C). Example 4 Instead of boron oxide of Example 1, lead oxide (Pb ₃ O ₄ ) 40 was used.
0 g and 300 g of (NH ₄ ) ₃ PO ₄ .3H ₂ O were added, and 150 g of methylcellulose and 150 g of carboxymethylcellulose were added as plasticizers and kneaded with heating for 2 hours. Thereafter, a honeycomb-shaped denitration catalyst (D) was obtained in the same manner as in Example 1. Reference Example 1 (Reference Catalyst) 1.8 kg of ammonium paratungstate was added to 55 kg of a 30 wt% hydrated titanium oxide slurry as TiO _2.
Was added and stirred for 2 hours. Next, after dehydrating the slurry, the slurry was fired at 500 ° C. for 5 hours to prepare a titanium oxide powder. This titanium oxide powder contained 8.4% by weight of WO ₃ . This titanium oxide powder containing tungsten oxide 1
To 0 kg, 1.86 kg of an aqueous solution containing 7% by weight of ammonium metavanadate, 4 liters of ion-exchanged water, and 400 g of methylcellulose were added and kneaded with heating.
Next, after adjusting the water content, the mixture was extruded into a honeycomb shape, dried, and calcined at 630 ° C. for 3 hours to prepare a catalyst (E). Reference Example 2 (Reference Catalyst) As TiO ₂ , lanthanum nitrate [LA (NO ₃ ) ₂ .6H ₂ was added to 55 kg of a 30% by weight hydrated titanium oxide slurry.
O] 1.0 kg and then 50 wt% as WO ₃
3.5 kg of aqueous solution of ammonium metatungstate containing tungsten and 600 g of polyvinyl alcohol were added and kneaded with heat for 2.5 hours. Next, after adjusting the water content, the mixture was extruded into a honeycomb shape, dried, and calcined at 650 ° C. for 5 hours to prepare a catalyst (F). Reference Example 3 (Reference catalyst) Powder 5 of calcined titanium oxide having a specific surface area of 65 m ² / g
800 g of magnesium chloride (MgCl ₂ ) and 550 g of ammonium paratungstate were added to the kg, 4.0 liters of ion-exchanged water, 300 g of polyvinyl alcohol,
100 g of glycerin was added and kneaded. Next, after adjusting the water content, the mixture was extruded into a honeycomb shape, dried, and dried.
Calcination was carried out at 5 ° C. for 5 hours to obtain a catalyst (G). Example 5 50% aqueous solution of 10% by weight titanium sulfate as TiO ₂
1.0 kg of an aqueous solution of cerium nitrate of 4.8% by weight as Ce ₂ O ₃ was added to g, and the mixture was heated to a temperature of 75 ° C. Next, aqueous ammonia was added to the aqueous solution to adjust the pH to 9.0, thereby preparing a precipitate of cerium-containing hydrous titanium oxide. After the precipitate was dehydrated and washed, it was dried at 110 ° C. for 1 day and calcined at 600 ° C. for 5 hours. The composition of this fired product is
It contained 9.5% by weight of Ce ₂ O _{3 and} 90.4% by weight of TiO ₂ . 50 to 5.5 kg of this titanium oxide powder
2.5% by weight aqueous solution of ammonium metatungstate
g, 1.5 liters of ion-exchanged water, 500 g of B ₂ O _3, 150 g of methylcellulose and 50 g of glycerin,
After kneading for 3 hours and adjusting the water content, the mixture was extruded into a honeycomb shape. After fully drying, bake at 650 ° C for 5 hours,
A denitration catalyst (H) was obtained. Example 6 The catalyst (A) produced in Examples 1 to 5 and Reference Examples 1 to 3
To H), the denitration performance was evaluated under the following method and conditions. A honeycomb catalyst (mesh size: 6.0 mm, wall thickness: 1.2 mm) was cut into 16 cell squares and 400 mm in length, filled into a quartz reaction tube having an inner diameter of 55 mmφ, and filled with gas having the following composition only in the lattice space of the catalyst. Was flowed at a predetermined reaction temperature for 10 hours, and then the denitration rate was determined. The gas flow rate is
It was 2.304 Nm ³ / Hr. Reaction temperature is 400
4, 500, 600 and 650 ° C. The denitration rate (%) was calculated based on the following equation by measuring the NOx concentration before and after the reaction with a chemiluminescence analyzer of the ECL-77A type manufactured by Yanagimoto Seisakusho. (NOx concentration before reaction-NOx concentration after reaction) Denitration rate (%) = ─────────────────── × 100 NOx concentration before reaction Gas composition NOx 500 ppm NH ₃ 600 ppm H Table 1 shows the evaluation results of the ₂ O 10% O ₂ 10% N ₂ balance catalysts (A to G).

[Table 1] From the above results, the catalysts A to D and H correspond to the reference catalysts EG.
At a high reaction temperature of 500 to 650 ° C. as compared with

[0007]

The catalyst of the present invention has an excellent property of having a high denitration activity at a high temperature for a long period of time, and by using this catalyst, NOx in exhaust gas can be efficiently removed.

Claims (3)

(57) [Claims] 1. A hydrated titanium oxide containing at least one element selected from the group IIa and IIIa of the periodic table is calcined, and the calcined product is selected from tungsten and phosphorus, boron, zinc and lead. A catalyst for reducing nitrogen oxides, wherein the catalyst supports at least one element component. 2. The nitrogen oxide reduction catalyst according to claim 1, which is used for removing nitrogen oxides in exhaust gas having a high temperature of 500 ° C. or higher. 3. A method for removing nitrogen oxides from exhaust gas, comprising contacting the exhaust gas containing nitrogen oxides with the catalyst for reducing nitrogen oxides according to claim 1 in the presence of a reducing agent.