JPS629368B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method and a catalyst for catalytically reducing nitrogen oxides in exhaust gas with ammonia, and particularly relates to a method for removing nitrogen oxides in exhaust gas by catalytic reduction with ammonia, and particularly relates to a method for removing nitrogen oxides from exhaust gas by catalytic reduction with ammonia. In removing nitrogen oxides from smoke, titanium oxide, V, Fe,
It is characterized by using a catalyst made of at least one oxide selected from W, Cr, and Cu and coated with molybdenum oxide. Nitrogen oxides emitted from various stationary sources are
Along with sulfur oxides, it is a major air pollutant. The most effective technique for removing nitrogen oxides is to add ammonia to the exhaust gas and selectively reduce and remove it. Until now, transition metal oxide catalysts containing titanium oxide as a main component have been developed and put into practical use for boiler exhaust gas using petroleum-based fuels. The above transition metal oxide catalyst containing titanium oxide as a main component is known to have excellent durability against sulfur oxides, particularly SO ₃ in exhaust gas, and to have high activity even at relatively low temperatures. This is a transition metal oxide (e.g. V, Fe,
Cr, Mn, Cu, etc.) are not expressed by catalysts. However, when nitrogen oxides are removed from exhaust gases that contain large amounts of sulfur oxides and dust, such as sintering machine exhaust gas in steel plants, coal-fired boilers, and glass melting furnace exhaust gases, the titanium oxides mentioned above are removed. The activity of the transition metal oxide catalyst as a component decreases over time, making it impossible to operate for a long period of time. The dust in the exhaust gas mentioned above contains 1 to 5% by weight of alkali and alkaline earth metal compounds such as potassium, sodium, magnesium, and calcium, and in some cases, 10% by weight.
Contains about % by weight. When such dust comes into contact with the catalyst, the alkali or alkaline earth metal compounds gradually migrate into the catalyst and accumulate therein. Moreover, alkali or alkaline earth metal compounds are poisonous components of nitrogen oxide removal catalysts, and there are detailed reports on the poisoning of V ₂ O ₅ catalysts using alumina and titania as carriers by alkalis (e.g. Bito, et al.: Proceedings of the 37th Spring Annual Meeting of the Chemical Society of Japan, p. 310, (1978)). The present invention provides a catalyst for removing nitrogen oxides that is not poisoned and has high activity when removing nitrogen oxides from exhaust gas in which dust containing the above-mentioned alkali or alkaline earth metal compounds coexists; The purpose of this invention is to provide a method for removing nitrogen oxides using this method. In other words, a mixture catalyst consisting of titanium oxide and at least one oxide selected from V, W, Fe, Cr, and Cu is coated with molybdenum oxide, thereby converting dust into a catalyst. It is characterized by using a highly active catalyst for removing nitrogen oxides, which prevents the migration of alkali and alkaline earth metal compounds. Alternatively, the catalyst may be formed into granules and a moving bed reactor may be used, or the catalyst may be formed into a monolith (for example, honeycomb, etc.).
By molding the dust into a pipe or plate shape and using a parallel flow type reactor, the contact between the dust and the catalyst can be reduced and the movement of alkali and alkaline earth metal compounds from the dust to the catalyst can be prevented. The present invention relates to a method for removing nitrogen oxides characterized by the following. The present inventors conducted detailed research on the poisoning of catalysts by dust containing alkali and alkaline earth metal compounds. As a result, contact between the dust and the catalyst results in the formation of alkali metal compounds, especially K,
Na moves and diffuses into the catalyst. It has been found that this is accelerated by the presence of sulfur oxides in the exhaust gas, and in the absence of sulfur oxides, the diffusion rate of the alkali metal compound becomes significantly slow. For example, a TiO ₂ V ₂ O ₅ catalyst has a diameter of 5 mm and a length of 5 mm.
The molded catalyst was brought into contact with 50 g each of dust in the sintering machine exhaust gas containing 10% by weight of K, and the gas was passed through the catalyst at 240/h at 350° C. for 50 hours. Table 1 shows the K content and catalytic activity in 1 mm of the catalyst surface layer after that.
Here, k/k ₀ is the reaction rate of a catalyst that does not contain K.
It is normalized by k ₀ , and when the reaction temperature is 350°C, space velocity, and catalyst shape are the same, it is obtained by the following formula. k/k ₀ =ln(1-η)/ln(1-η ₀ ) η, η ₀ : Nitrogen oxide removal efficiency (-)

[Table] Also, when dust and catalyst come into contact in exhaust gas where sulfur oxides coexist, the catalyst has a higher sulfur oxide adsorption capacity or absorption capacity, for example, if it contains Fe, Cu, or V, It was found that the movement and diffusion of alkali metal compounds was rapid and the activity was significantly reduced. As a result, by coating a catalyst consisting of titanium oxide and at least one oxide of V, W, Cr, Fe, and Cu with Mo oxide, which has a low ability to adsorb sulfur oxides, it is possible to coexist in dust. It was found that the catalyst is highly active and is not poisoned by alkali or alkaline earth metal compounds. A catalyst consisting of titanium oxide and at least one or more oxides from V, W, Cr, Fe, and Cu,
Prepared by conventional kneading methods. In preparing the catalyst, titanium oxide may be titanium oxide having a large specific surface area, or titanium oxide that can be turned into titanium oxide by heat treatment, such as orthotitanic acid or metatitanic acid. Further, titanium tetrachloride, titanium sulfate, titanium tetraisopropoxy, etc., which yield the above-mentioned titanic acid through hydrolysis treatment, may be used.
Oxides of V, W, Cr, Fe, and Cu can be used as raw materials in any form, such as nitrates, sulfates, oxides, and halides, as long as they provide oxides in the range of 300 to 600°C. Good too. Preferably,
A water-soluble salt is desirable for homogeneous dispersion with titanium oxide or its precursor. To coat molybdenum oxide, the homogeneous mixture is thoroughly mixed with a water-soluble molybdenum salt (for example, ammonium molybdate) in an aqueous dispersion state into a powder that has been calcined at a temperature in the range of 300 to 600°C. This mixture is dried or calcined at a temperature of 300 to 600°C, and the powder is formed into granules or honeycomb shapes by tabletting, extrusion, rolling granulation, etc., or plated. Apply to the shaped substrate. The molded product thus obtained is heated to 300 to 600°C, preferably 300 to 450°C.
By performing the calcination treatment at .degree. C., the catalyst targeted by the present invention can be obtained. Further, the object of the present invention is to contain one or more types selected from V, W, Fe, Cr, and Cu in a titanium oxide support formed into a granular, plate-like, or honeycomb shape, or a substantial titanium oxide support. After drying and firing at a temperature of 300 to 600°C, impregnating with a solution containing molybdenum (e.g. ammonium molybdate aqueous solution) and drying, 300 to 600°C, preferably 300°C. A calcination treatment may be performed at ~450°C. Also, titanium oxide, V, W,
The object of the present invention can also be achieved by impregnating a shaped catalyst made of one or more oxides selected from Fe, Cr, and Cu with a solution containing molybdenum, drying, and calcination. It can be achieved. The amount of molybdenum oxide may be sufficient to coat the surface of the catalyst, and the desired effect of the present invention can be achieved at 3% by weight or more. In order to maintain high activity of the catalyst, it is important that the molybdenum oxide coating be thin, and it is preferably 30% by weight or less. Furthermore, parallel flow using a moving bed reactor using a catalyst formed into granules according to the method described above or a catalyst formed into a monolith shape (for example, honeycomb shape, plate shape, pipe shape, etc.) is possible. If the nitrogen oxide removal reaction is carried out in a type reactor, there will be less dust deposited on the catalyst layer, and the effects of the present invention will be more pronounced. The content of the present invention will be specifically explained below with reference to Examples. Examples 1 to 5 Metatitanic acid slurry 5Kg (1.5Kg as _TiO2 )
into a kneader, and add each of the following substances to it. Ammonium metavanadate 140g Example 1
Ammonium metavanadate 156.6g, ammonium paratungstate 540g
…Example 2 Triiron tetroxide 149.6g …Example 3 Chromium nitrate nonahydrate 833.6g …Example 4 Copper nitrate 504g …Example 5 This was heated until the moisture content was approximately 30%. Heat and knead,
Dry at 140°C for 5 hours and grind. The particle size of the powder was such that about 90% by weight could pass through a 60 mesh. This powder is calcined at 450°C for 2 hours.
The catalyst composition at this time was as follows in terms of atomic ratio. Ti:V=94:6 Example 1 Ti:V:W=84:6:10 Example 2 Ti:Fe=9:1 Example 3 Ti:Cr=9:1 Example 4 Ti:Cu=9: 1 Example 5 1.5 kg of this powder is placed in a kneader, 184 g of ammonium molybdate powder is added thereto, kneaded for 1 hour at a moisture content of 40%, and heated and kneaded until the moisture content is approximately 30%. This is dried and ground. The particle size at this time was such that 90% of the particles could pass through a 90 mesh sieve. This was molded into a cylindrical tablet with a diameter of 5 mm and 5 mm, and calcined at 450°C for 3 hours. The rate of catalyst poisoning by the alkali in the dust was evaluated as follows. The dust collected from the sintering machine exhaust gas is mixed with potassium sulfate to a concentration of 30% by weight, dried, and pulverized. this dust 30
g and 30 g of the above catalyst were mixed and filled into a glass reaction tube with a diameter of 60 mm. The catalyst layer was externally heated to 350°C in an electric furnace. The distribution gas composition was dry gas: SO ₂ 500ppm SO ₃ 100ppm O ₂ 3% CO ₂ 12% H ₂ O 12% N ₂ balance The dry gas supply rate was 240/h, and the distribution was carried out for 50 hours. Thereafter, the dust and catalyst were separated, and the nitrogen oxide removal rate of the catalyst was measured. The reaction temperature was 350°C, the space velocity was 10000 h ^-1 , and the gas composition was as follows. NO 200ppm NH ₃ 240 SO ₂ 500 SO ₃ 10 O ₂ 3% CO ₂ 12 H ₂ O 12 N ₂ balance The dry gas supply rate was 240/h. The results are shown in Table 2. Comparative Examples 1 to 5 Examples 1 to 5 except for the molybdenum oxide coating operation
A catalyst was produced and molded in the same manner as in 5. Also,
The rate of poisoning by alkali in the dust was determined in the same manner as in Examples 1 to 5. The results are shown in Table 2.

[Table] Example 6 A moving bed reactor was filled with a catalyst prepared and molded in the same manner as in Example 1, and the space velocity was 10000 h ^-1 , the amount of treated gas was 2000 Nm ³ /h, the reaction temperature was 350°C, and NH ₃ / NOx
= 1.0, the sintering machine exhaust gas was treated. exhaust gas composition,
The composition of the dust was as follows. NOx 150-250ppm SOx 900-1200 CO 0.1-1% CO ₂ 3-5 H ₂ O 5-10 Dust 30-50mg/Nm ³ S 12% by weight Na 2 K 11 Ca 4 Fe 21 Si 2 C 2 It showed a high nitrogen oxide removal rate without being poisoned by K or Na over 2000 hours. The results are shown in the figure. Comparative Example 6 A catalyst prepared and shaped in the same manner as in Comparative Example 1 was tested in the same manner as in Example 6. Poisoning by the alkali in the dust was significant, and the activity of the catalyst was greatly reduced. The results are shown in the figure. Example 7 Ammonium metavanadate adjusted to 5% by weight with V ₂ O ₅ and 1.2 times the weight of this were added to a titanium oxide honeycomb carrier (100 x 100 x 500 mm, water absorption rate 33%) with ventilation holes of 5 mm x 5 mm. It is impregnated with an aqueous solution containing oxalic acid, dried at 120°C for 4 hours, and fired at 350°C for 2 hours. This was impregnated with an aqueous ammonium molybdate solution adjusted to 10% by weight with MoO ₃ , dried at 120°C for 4 hours, and fired at 450°C for 2 hours. A parallel flow reactor packed with the obtained catalyst was tested. The test conditions other than the space velocity of 5000 h ^-1 were the same as in Example 6. As shown in the figure, the results show that there is almost no decrease in catalytic activity due to alkali poisoning in the dust. Comparative Example 7 Using the same method as Comparative Examples 1 to 5, Ti:V:Mo=
A catalyst having a composition of 84:6:10 was obtained. Poisoning by alkali in the dust was carried out in the same manner as in Examples 1-5. As a result, the nitrogen oxide removal rate is
The activity was 88%, which was lower than that of the Mo oxide-coated catalyst of Example 1.

[Brief explanation of the drawing]

The drawing is a curve diagram showing changes in activity in a test for removing nitrogen oxides from sintering machine exhaust gas.

Claims (1)

[Claims] 1. Ammonia is added to exhaust gas in which dust containing nitrogen oxides, sulfur oxides, and alkali and alkaline earth metal compounds coexists, and ammonia is catalytically treated on a catalyst. In the method of converting and removing nitrogen oxides into nitrogen, the catalyst includes 25 to 96% by weight of titanium oxide and 1 to 72% by weight of at least one oxide of V, W, Cr, Fe, and Cu. 3 to 30% by weight of Mo oxide
A method for removing nitrogen oxides from exhaust gas, characterized by using an oxide-coated catalyst (excluding a mixture catalyst). 2. In claim 1, the catalyst is in the form of granules, and the reaction vessel moves part or all of the catalyst intermittently or continuously so that the catalyst in the exhaust gas is transferred into the catalyst packed bed. A method for removing nitrogen oxides from exhaust gas, characterized by using a moving bed type that prevents the accumulation of solids. 3. A method for removing nitrogen oxides from exhaust gas according to claim 1, which comprises using a monolith-shaped catalyst to prevent solid matter in exhaust gas from accumulating in the catalyst layer. .