JPS6372342A - Catalyst for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To efficiently remove nitrogen oxide in arsenic-containing flue gas, by forming a catalyst for removing nitrogen oxide in the exhaust gas by containing titanium and vanadium so that vanadium is present in the surface layer part of the catalyst at high concn. CONSTITUTION:Vanadium is supported by honeycomb shaped titanium according to an impregnation method or a roll coating method so as to be present in the surface layer thereof at high concn. to form a catalyst for removing nitrogen oxide in exhaust gas. The ratio of titanium and vanadium is pref. 99.9-92.0:0.1-8.0 on an oxide basis. Titanium and vanadium are pref. used in a form of oxide, sulfate or nitrate. When titanium is used in a form of titanium oxide, titanium oxide wherein a crystal has a particle size of 150-250Angstrom is pref. used. Further, the shape of the catalyst can be wet to a pellet, a globule or a honeycomb shape.

Description

[Detailed description of the invention] The present invention relates to a catalyst for removing nitrogen oxides.

In recent years, attempts have been made to make nitrogen oxides in combustion exhaust gas harmless by adding a reducing gas to the combustion exhaust gas and reducing the nitrogen oxides to harmless components such as nitrogen in the presence of a catalyst. - carbon oxide,
When hydrocarbons, hydrogen, etc. are used as reducing gases, they react with oxygen in the processing gas, so a large amount of reducing gas is required, and there are drawbacks such as a large amount of heat generation and ammonia by-product. Attempts have been made to use ammonia, which does not have these drawbacks, as a reducing gas and react selectively with nitrogen oxides.

Various catalysts have been proposed and used.

However, in all cases, the exhaust gases used were gases that did not contain arsenic compounds. (Even if it existed, it was in very small amounts.) However, depending on the type of coal, the exhaust gas contains a considerable amount of arsenic or arsenic compounds (hereinafter referred to as arsenic). When conventionally used known catalysts are used to detoxify nitrogen oxides in exhaust gas containing arsenic, they are affected by arsenic.
It was found that the desired effect was no longer achieved.

As a result of various studies, the inventors of the present invention found that it is extremely effective to use a catalyst that contains titanium and vanadium and has a high concentration of vanadium in its surface layer, and has completed the present invention.

The catalyst according to the present invention (hereinafter referred to as the present catalyst) will be explained in detail below.

Although this catalyst must contain titanium and vanadium as elements, tungsten and zirconium may also be added. If the oxidation rate of S02 in the exhaust gas is a problem, tungsten can be added to reduce the amount of vanadium contained in S02.
It is effective in suppressing the oxidation rate of 02.

Furthermore, since titanium oxide has a high arsenic adsorption property and zirconium oxide has a low adsorption property for arsenic, covering the surface of titanium oxide with zirconium is effective in attenuating the deterioration of denitrification performance caused by arsenic.

The ratio of titanium and vanadium is 99.9 to 9 in terms of oxide.
It is sufficient if the content is in a ratio of 2.0 to 0.1 to 8.0.

If vanadium is outside the above ratio, the denitrification performance will deteriorate if the vanadium content is too small, and even if vanadium is added at a ratio of 8.0 or more, no positive effect will be brought about on the denitrification performance.

The ratio of titanium to tungsten is 98 to 7 in terms of oxide.
It is sufficient if it is included in a ratio of 0:2.0 to 30. If the amount of tungsten is less than 2, the denitrification performance will be low, and if you try to improve the performance with other vanadium, the oxidation rate of 802 in the exhaust gas will increase, causing secondary problems. When 802 is present in , it is necessary to have WO2 with a low SO3 oxidation rate of 2.0 or more. On the other hand, if there are 30 or more, the economic burden becomes large in comparison to the effect, which is not preferable.

The purpose of zirconium is to increase the resistance to arsenic by coating a titanium carrier with a high arsenic adsorption property with zirconium, which is expensive but has a low arsenic adsorption property and is comparable to titanium as a denitrification catalyst carrier. The ratio should be 99.5 to 95:0.5 to 10 in terms of oxides; even if it is present in excess of 10, it will only increase the economic burden and negative effects such as blocking the pores of the catalyst. It is.

Titanium, vanadium, tungsten, and zirconium are effective in the form of oxides, sulfates, and nitrates. When using titanium in the form of titanium oxide,
It is preferable to use titanium oxide having 150 to 250 crystallites.

It is sufficient that vanadium and, if necessary, tungsten and zirconium are present in the surface layer at a high degree of 1 gi. The concentration in the catalyst surface layer up to 200μ is 1.5 times or more the concentration in the entire catalyst.

Manufacturing methods include impregnation method, brush coating method, roll coating method,
Although the method is not particularly limited, such as a spraying method, it is preferable to proceed with drying and supporting at the same time. For honeycomb substrates, it is preferable to use an impregnation method and dry immediately after impregnation.

Titanium oxide has a strong affinity for arsenic in exhaust gas and easily adsorbs arsenic, and its adsorption capacity is proportional to the specific surface area of titanium oxide, so the specific surface area can be reduced within a range that does not significantly affect denitrification performance. Therefore, the number of titanium oxide crystallites is preferably 150 to 250. Furthermore, in order to improve the denitrification performance, which decreases slightly due to enlarging the crystallites of titanium oxide, without increasing the oxidation rate of S02, it is preferable to have vanadium present in a high concentration in the surface layer of the catalyst. be. In other words, by making vanadium (tungsten, zirconium if necessary) exist in a high concentration in the surface layer, it is possible to improve the initial performance, suppress the performance deterioration caused by arsenic, and extend the life of the catalyst. Since the oxidizing ability depends on the amount of vanadium in the total catalyst, it does not have the adverse effect of oxidizing SO□ and is extremely effective as a catalyst for removing nitrogen oxides from arsenic-containing exhaust gas.

The shape of the catalyst may be any shape such as a pellet shape, a spherical shape, or a honeycomb shape.

The method for removing nitrogen oxides is carried out in the presence of a reducing agent as described above, and ammonia is usually preferred as the reducing agent.

In order to remove nitrogen oxides from a mixed gas containing nitrogen oxides using this catalyst, 0% of the nitrogen oxides contained in the mixed gas must be removed. Ammonia is added in an amount of 5 to 5 times the mole, preferably 1 to 2 times the mole, and is passed through a reaction bed filled with a catalyst. Any of a moving bed, a fluidized bed, a fixed bed, etc. can be used as the reaction bed. The reaction temperature may range from 200 to 500°C, preferably from 250 to 400°C.

Also, the space velocity of gas is 1.000 to 100,000 hr.
, preferably in the range of 3,000 to 300,000 hr.

The catalyst can be used to treat any gas containing nitrogen oxides, but especially boiler exhaust gas, i.e.
~1,000 ppm of nitrogen oxides, mainly nitric oxide, as well as 200-2.000 ppm of sulfur oxides, mainly dioxide, sulfur, 1-10 volumes of oxygen,
5-20 volume ffl Q, 6 carbon dioxide gas, 5-20 volume%
It can be suitably used to remove nitrogen oxides from exhaust gas containing H of 1:0,01 ppm or more in addition to water vapor.

This will be explained in detail below using examples.

Example 1 Metatitanic acid obtained from a titanium oxide production process using a sulfuric acid method was neutralized, filtered and washed with water to obtain cake-like metatitanic acid. This meta-titanium acid is equivalent to 800 kg of titanium oxide.
) was added with 8 kg of 67.5% nitric acid to partially peptize the metatitanic acid, and the sol solution was spray-dried and then calcined at 450° C. for 3 hours. Thereafter, it was cooled and pulverized to obtain titanium oxide powder with an average size of 2 μm.

300 fl of an aqueous solution in which 100 kg of ammonium paratungstate was dissolved in a monoethanolamine aqueous solution, 100 kg of polyvinyl alcohol 5 [g and E glass chopped strands (fiber length 5 mm, fiber diameter 9 μm, manufactured by Nitto Boseki Oki), and 800 kg of the above titanium oxide powder. Hon fishing 1
00fl and kneaded them with a kneader.

Next, this kneaded material was molded into a honeycomb shape using a screw extruder equipped with a honeycomb extrusion nozzle. After allowing the formed honeycomb to air dry for a sufficient amount of time,
It was dried with ventilation at 00°C for 5 hours. After that, both ends in the axial direction were trimmed and fired in an electric furnace at 450°C for 3 hours to obtain a cell pitch of 7.4mm, wall thickness of 1.35mm, and outer diameter of 150+on+X.
A honeycomb catalyst for denitration (equivalent diameter of 5.9 mm) with a length of 150 mm and an axial length of 500 an was obtained.

Next, 19.2 kg of oxalic acid and 7.7 kg of ammonium metavanadate were added with water to prepare a vanadium impregnating solution of 401 and 150 g/Ω.

After that, 7.4g of water was added to 1g of impregnating liquid to yield 17.9g.
This diluted solution was kept at 60°C and impregnated with the catalyst.

Immediately after impregnation, the inside of the catalyst cell kept at 60°C was placed in a circulating ventilation dryer, and the temperature was raised to 100°C in 1 hour.
It was dried for 5 hours. Thereafter, it was fired at 450°C for 3 hours.

Comparative Example 1 A catalyst prepared in the same manner as in Example 1 was impregnated with a diluted vanadium impregnating solution at room temperature in the same manner as in Example 1. The impregnated plate was placed in the ventilation dryer described in Example 1 at room temperature for 2.5 hours at room temperature.
Air dry for an hour. Thereafter, the temperature was raised to 100°C over 5 hours, and the mixture was dried at 100°C for 5 hours. Further at 450℃ 3
Baked for an hour.

Example 2 Cell pitch 7.4 and wall thickness 1.35 obtained in Example 1
II11. Outer diameter 150mm x 150mm.

A denitrification honeycomb catalyst having an axial length of 500 mm was impregnated with an aqueous solution of 240.9 g/Ω of zirconium oxychloride, which was kept at 1M. Thereafter, it was dried by the drying method of Example 1 and then baked at 450° C. for 3 hours. After cooling, this catalyst was impregnated into a mold with a 17.9 g/N diluted vanadium solution kept at 60°C in the same manner as in Example 1, and dried in the same manner as in Example 1.
It was baked at 0°C for 3 hours.

Example 3 The concentration distribution of vanadium in the catalysts obtained in Example 1 and Comparative Example 1 was measured using an X-ray microanalyzer (ASM manufactured by Kansei Corporation) after gold deposition.
-8X).

The nI constant conditions are shown in Table 1 below.

Table 1 The results are shown in Figures 1 and 2.

Furthermore, ■20. ■ Entire catalyst, ■ Approximately 20 from the surface layer
The powder was divided into 0 μm scraped pieces and analyzed.

The results are shown in Table 2.

Table 2 Example 4 Each catalyst for removing nitrogen oxides obtained in the above Examples and Comparative Examples was cut into 3 cells x 3 cells 300 mm in length, and the catalyst contained 200 ppm of nitrogen oxides and 200 pl of ammonia).
A mixed gas consisting of 10% 111 water vapor, 12% carbon dioxide, 800 ppm sulfur dioxide, 25 ppm arsenite gas, and the balance nitrogen was heated at a temperature of 380°C and a space velocity of 4.
The nitrogen oxide (NOX) removal rate and the sulfur dioxide (So□) oxidation rate were determined by APJ. The results are shown in Table 2. Note that the nitrogen oxide removal rate (%) and the sulfur dioxide oxidation rate (%) were determined by the following formulas.

Nitrogen oxide removal rate (%) - (catalyst layer inlet NOx concentration - catalyst layer outlet NOxa degree) / (catalyst layer inlet NOx concentration) X1
00 Sulfur dioxide oxidation rate (%) - (SO concentration at catalyst layer inlet - SO2 concentration at catalyst layer outlet) / (soot layer inlet (S02 + 803
) Concentration) x 100 The results are shown in Table 3.

Table 3

Claims (1)

[Claims] Catalyst for removing nitrogen oxides from exhaust gas containing arsenic, containing titanium and vanadium, with a high concentration of vanadium in the surface layer.