JPH0240374B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a nitrogen oxide reduction and purification catalyst comprising a titanium compound, barium sulfate, and water-insoluble vanadyl sulfate. Nitrogen oxides (NO _x ), such as nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ), are emitted from fixed combustion equipment in various factories, including boilers, power plants, and steel mills, as well as carbon dioxide along with NO _x . NO _x in exhaust gas containing sulfur oxides (SO _x ) such as sulfur (SO ₂ ) and sulfur trioxide (SO ₃ ) and dust,
Many methods of reducing and purifying nitrogen oxides in the presence of reducing substances such as ammonia and catalysts for reducing and purifying nitrogen oxides used in this process are already known. For example, the Japanese Patent Publication No. 56-1981, which is filed by the present applicant,
Publication No. 32020 describes a catalyst consisting of barium sulfate and vanadyl sulfate which is insoluble in water.
The catalyst described in this publication has excellent water resistance and SO _x resistance, low activity for oxidizing SO ₂ to SO ₃ , and a high NO _x removal rate at relatively low temperatures. However, according to the research of the present inventors, a catalyst consisting of barium sulfate and water-insoluble vanadyl sulfate has a high temperature even at a relatively low temperature of around 300°C when ammonia is used in an amount equal to or more than the amount of NO in the exhaust gas. This shows the NO _x removal rate, but if the amount of ammonia used is reduced, for example, NH ₃ /
If NO is kept at around 0.8 (molar ratio),
It was found that there were problems in that the NO _x removal activity decreased and the amount of unreacted ammonia remaining in the exhaust gas after denitrification (purification) increased. Furthermore, Japanese Patent Publication No. 56-30067 filed by the present applicant describes a catalyst consisting of water-insoluble vanadyl sulfate and titanium dioxide, but this catalyst can also be used for a long time by reducing the amount of ammonia used. It was found that after a period of use, the initial NO _x removal activity gradually decreased, and there were also problems in that SO ₃ in the exhaust gas was easily oxidized to SO ₃ . In addition, the temperature of the exhaust gas varies greatly depending on the operating conditions of the fixed combustion equipment, so in order to withstand these changes in conditions and maintain stable NO _x removal activity,
In addition, in order to lower the ammonia concentration in the exhaust gas after denitrification, a catalyst is required that has high NO _x removal activity over a wide temperature range even if the amount of ammonia used is small. There are still points to be improved in the catalysts described in Japanese Patent Publications No. 56-32020, Japanese Patent Publication No. 56-30067, etc. The present inventors have conducted intensive research with the aim of improving the above-mentioned problems without impairing the excellent properties of a catalyst composed of barium sulfate and water-insoluble vanadyl sulfate.
The inventors have discovered that a catalyst capable of achieving the objective can be obtained by manufacturing a catalyst using titanium hydroxide as a component, leading to the present invention. The present invention relates to a solution of a vanadium compound in which the valence of vanadium is reduced to a valence lower than 5 by adding a reducing substance to a vanadium compound in the presence of water, sulfuric acid or ammonium sulfuric acid. The present invention relates to a method for producing a nitrogen oxide reduction and purification catalyst consisting of a titanium compound, barium sulfate, and water-insoluble vanadyl sulfate, which comprises mixing a salt, barium sulfate, and titanium hydroxide, and then calcining the mixture. Compared to the catalyst described in Japanese Patent Publication No. 56-32020, the catalyst obtained by the present invention has a NH ₃ /NO (molar ratio) of 1 or less and has a high performance when used at relatively low temperatures.
It has a high NO _x removal activity and has the characteristics of low ammonia remaining in the exhaust gas after denitrification.
Compared to the catalyst described in Japanese Patent Publication No. 56-30067, it is possible to prevent a decrease in NO _x removal activity when using NH ₃ /NO less than 1, and also has a low SO ₂ oxidation activity. It has its features. In the present invention, examples of the vanadium compound having a pentavalent vanadium include ammonium metavanadate, metavanadate, vanadium pentoxide, and the like, with ammonium metavanadate being particularly preferred. In addition, the reducing substance may be any substance that can reduce a pentavalent vanadium compound to a valence smaller than pentavalent, generally tetravalent, and examples thereof include organic carboxylic acids such as oxalic acid, citric acid, and tartaric acid. Of these, oxalic acid is preferred. When preparing a solution of a vanadium compound in which the valence of vanadium is 5 by adding a reducing substance in the presence of water to reduce the valence of vanadium to a valence lower than 5, for example, A method of dissolving a pentavalent vanadium compound such as ammonium metavanadate in water and adding a reducing substance such as oxalic acid to reduce the pentavalent vanadium compound also dissolves the reducing substance. The reduction may be carried out by adding a pentavalent vanadium compound to water for reduction. In addition, the sulfuric acid or ammonium salt of sulfuric acid used in the present invention includes concentrated sulfuric acid, ammonium sulfate,
Examples include acidic ammonium sulfate, ammonium sulfite, ammonium persulfate, etc. Among them, ammonium sulfate is preferred because it is inexpensive and has good reproducibility of the intended catalyst. Sulfuric acid or an ammonium salt of sulfuric acid is necessary to form vanadyl sulfate, which is insoluble in water.The amount of sulfur used is 1 to 1 gram atom of vanadium in the pentavalent vanadium compound used. An amount of 2 gram atoms is preferred, and there is no particular advantage to using more than 2 gram atoms. In the present invention, the mixing order of the solution of a vanadium compound obtained by reducing the valence of vanadium to a valence lower than 5, sulfuric acid or ammonium sulfate salt, barium sulfate, and titanium hydroxide is not particularly limited, and even if the mixing order is different. There is no significant difference in catalyst performance. The mixing ratio of the vanadium compound solution, barium sulfate and titanium hydroxide is such that water-insoluble vanadyl sulfate (β-VOSO ₄ ) is present in the catalyst.
is 1 to 35% by weight, preferably 5 to 30% by weight, barium sulfate is 65 to 95% by weight, preferably 70 to 95% by weight, and titanium compound is 1 to 15% by weight in terms of titanium dioxide (TiO ₂ ), preferably It is appropriate that the amount is within the range of 2 to 10% by weight. The paste-like or suspension-like mixture obtained by mixing is dried and shaped as required, and then fired. Drying is generally suitably carried out in an air atmosphere at a temperature of 90 to 200°C. Calcination of the mixture is carried out at 200-450℃, preferably at 250℃
It is appropriate to carry out the firing at a temperature of -400°C, and the firing time is generally about 1 to 24 hours, preferably about 3 to 16 hours. Further, the firing atmosphere is not particularly limited, and may be an atmosphere containing any of sulfur dioxide gas, ammonia, water vapor, nitrogen, oxygen, etc., but an oxygen-containing gas atmosphere such as air is economical.
It is also suitable. The vanadium compound used as a raw material by calcination is converted into water-insoluble vanadyl sulfate (β
-VOSO ₄ ), so the catalyst contains almost no other vanadium compounds other than vanadyl sulfate, which is insoluble in water, but if a small amount (about 5% by weight or less of the total vanadium compounds) There is no problem even if a compound is included. According to an infrared absorption spectrum, vanadyl sulfate, which is insoluble in water, exhibits characteristic absorption peaks at 940 ^-1 cm and 510 ^-1 cm, which are not observed in water-soluble vanadyl sulfate (α-VOSO ₄ ). In addition, the titanium hydroxide used as a raw material is a complex titanium compound containing titanium hydroxide and titanium dioxide, although it is not clear from X-ray diffraction spectra what kind of titanium compound it is in the catalyst. It is presumed that this is the case. The catalyst obtained by calcination has a bluish green color. As mentioned above, the catalyst according to the method of the present invention is
Also, as is clear from the experimental results in the later stages,
It has many superior effects compared to conventionally known catalysts, and is suitable for purifying exhaust gas containing _NOx , especially exhaust gas containing _SOx and dust along with _NOx . Next, Examples and Comparative Examples will be shown. In each example, testing for NO _x removal activity
Fill 20ml into a 30mmφ stainless steel U-shaped reaction tube, maintain it at temperatures of 300℃, 320℃ and 340℃ in a salt bath, and add 300ppm of NO to the reaction tube.
_NH3 240ppm, _SO2 700ppm, _SO3 50ppm, _H2O10
%, O ₂ 5% and the remaining N ₂ (NH ₃ /NO = 0.8, molar ratio), space velocity
After 24 hours, the NO _x content in ^the gas at the inlet and outlet of the reaction tube was measured using a chemiluminescent NO _x analyzer, and calculated according to the following formula:
The NO _x removal rate (%) was determined. NO _x removal rate ( _% ) = X ₁ - X ₂ / X ₁ × 100 X ₁ = _{NO x concentration} in the gas at the inlet of the reaction tube The water-insoluble vanadyl sulfate was confirmed by measuring the valence of vanadium, infrared absorption spectrum, and X-ray diffraction spectrum. Vanadium compounds of high valence were mixed in. Example 1 10.8 g of ammonium metavanadate was added to 150 ml of water, heated to about 70°C, and mixed with 15 oxalic acid while stirring.
18 g of ammonium sulfate was added to the vanadium compound solution, and then 175 g of barium sulfate and 10 g of titanium hydroxide (calculated as titanium dioxide) were added and kneaded to make a paste, and then heated at 150°C. 5mmφ×4
Formed into mmH pellets at 380°C in an air atmosphere.
The mixture was calcined for 4 hours to obtain a catalyst. The catalyst had a bluish green color. This catalyst consists of 7.5% by weight of water-insoluble vanadyl sulfate (β-VOSO ₄ ), 87.5% by weight of barium sulfate (BaSO ₄ ), and 5% by weight of a titanium compound (calculated as titanium dioxide). The results of the NO _x removal activity test are shown in Table 1. Example 2 Barium sulfate, titanium hydroxide and ammonium sulfate were kneaded with a small amount of water, and then a solution of a vanadium compound obtained by reducing ammonium metavanadate with oxalic acid was added and mixed, and the firing temperature was 400°C. A catalyst consisting of water-insoluble vanadyl sulfate, barium sulfate, and a titanium compound was produced in the same manner as in Example 1, and an NO _x removal activity test was conducted. The results are shown in Table 1. Example 3 A solution of a vanadium compound obtained by reducing ammonium metavanadate with oxalic acid was mixed with barium sulfate, then ammonium sulfate was added, and finally titanium hydroxide was added, and the firing temperature was changed to 400°C. A catalyst consisting of water-insoluble vanadyl sulfate, barium sulfate, and a titanium compound was produced in the same manner as in Example 1, and tested for NO _x removal activity. The results are shown in Table 1. Example 4 Concentrated sulfuric acid (98%) instead of ammonium sulfate
13.6g was used, and the firing atmosphere was changed from air to NH ₃ 2%.
A catalyst consisting of water-insoluble vanadyl sulfate, barium sulfate, and a titanium compound was produced in the same manner as in Example 1, except that the gas was changed to a gas consisting of ₁ % O ₂ and the remainder N 2, and an NO _x removal activity test was conducted. Ivy. The results are shown in Table 1. Example 5 Same as Example 1 except that 15.7 g of acidic ammonium sulfate (NH ₄ HSO ₄ ) was used instead of ammonium sulfate and the firing atmosphere was changed from air to a gas consisting of 1% NH ₃ and 99% N ₂ A catalyst consisting of water-insoluble vanadyl sulfate, barium sulfate, and a titanium compound was prepared and tested for NO _x removal activity. The results are shown in Table 1. Examples 6 and 7 Water-insoluble vanadyl sulfate, barium sulfate, and titanium compounds were prepared in the same manner as in Example 1, except that the amounts of starting materials used were changed so that the composition of the catalyst was as shown in Table 1. Produce a catalyst consisting of
A NO _x removal activity test was conducted. The results are shown in Table 1. Example 8 A catalyst was produced in the same manner as in Example 1, except that the amount of ammonium sulfate used was changed to 24 g, and NO _x
A removal activity test was conducted. The results are shown in Table 1. Comparative Example 1 A catalyst consisting of water-insoluble vanadyl sulfate and barium sulfate was produced in the same manner as in Example 1, except that titanium hydroxide was not used and the amount of barium sulfate used was 85 g, and NO _x removal was performed. An activity test was conducted.
The results are shown in Table 1. Comparative Example 2 A catalyst consisting of water-insoluble vanadyl sulfate, barium sulfate, and titanium dioxide was produced in the same manner as in Example 1, except that 10 g of titanium dioxide (anatase type) was used instead of titanium hydroxide.
A NO _x removal activity test was conducted. The results are shown in Table 1.

[Table] * Titanium dioxide conversion Reference Example 1 and Comparative Reference Example 1
in model gas in NO _x removal activity test.
NO _x removal activity tests were conducted on the catalysts of Example 1 and Comparative Example 1 using the same test method, except that NH ₃ /NO = 0.8 was changed as shown in Table 2 and the reaction temperature was maintained at 300°C. The results of the measurement and the measurement of the NH ₃ concentration remaining in the gas at the outlet of the reaction tube are shown in Table 2 as Reference Example 1 (catalyst of Example 1) and Comparative Reference Example 1 (catalyst of Comparative Example 1). show.

[Table] Comparative Example 3 38.0g of ammonium metavanadate in 100ml of water
Add 57g of oxalic acid and warm to 80℃, and while stirring, add 57g of oxalic acid.
64.3 g of ammonium sulfate was added to this solution of vanadium compound, and then titanium dioxide (TiO ₂ anatase type) was added to reduce the vanadium.
Add 300g of powder, mix thoroughly, dry while kneading to the extent that it can be extruded with an extruder, and mix it thoroughly with an extruder.
Extruded into a string of mmφ and heated at 110℃ in an air atmosphere.
After drying for 15 hours, cut into 5 mm length and SO ₂ =
0.5%, 390℃ in air atmosphere containing NH ₃ = 0.2%
The mixture was calcined for 20 hours to produce a catalyst. A long-term NO _x removal activity test was conducted on this catalyst.
An SO ₂ oxidation activity test was conducted. The results are shown in Tables 3 and 4 together with the results for the catalyst of Example 1. Note that the NO _x removal activity test was conducted using Examples 1 to 8, except that the reaction temperature was maintained at 300°C.
The same test method as in Comparative Examples 1 and 2 was used. In addition, in the SO ₂ oxidation activity test, 20 ml of catalyst was
Fill a mmφ stainless steel U-shaped reaction tube,
A model gas consisting of 0.1% SO ₂ , 5% O ₂ and the balance N ₂ was introduced into the reaction tube at a space velocity of 350°C.
The SO ₂ concentration in ^the gas at the inlet and outlet of the reaction tube was measured using an SO ₂ analyzer, and the oxidation activity (%) of SO ₂ was determined using the following formula. Oxidation activity of _SO2 (%) = _SO2 concentration in inlet gas - _SO2 concentration in outlet gas / _SO2 concentration in inlet gas x 100

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Claims (1)

[Claims] 1. A solution of a vanadium compound in which the valence of vanadium is lower than 5 by adding a reducing substance to a vanadium compound in the presence of water, sulfuric acid or an ammonium salt of sulfuric acid, sulfuric acid A method for producing a catalyst for nitrogen oxide reduction and purification comprising a titanium compound, barium sulfate, and water-insoluble vanadyl sulfate, which comprises mixing barium and titanium hydroxide and then calcining the mixture.