JPS6043170B2 - Catalyst for removing nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a catalyst for removing nitrogen oxides (hereinafter abbreviated as NOx) contained in gas.

In particular, the present invention adds ammonia to a harmful gas containing NOx and sulfur compounds (mainly sulfur dioxide, hereinafter abbreviated as SOx) at the same time and causes a catalytic reaction.
The present invention provides a catalyst that efficiently reduces NOx to harmless nitrogen without being poisoned by SOx, and has excellent durability. Thermal power plants and other industrial facilities use heavy oil and coal containing sulfur and nitrogen compounds as fuel sources for boilers and other sources, and use air as an oxygen source, so their exhaust gases contain NOx and SOx. It is a cause of environmental pollution. Therefore, research and development on techniques for removing SOx and NOx from the exhaust gas are being advanced in various fields, and many specific methods have been proposed. These methods can be roughly divided into wet and dry methods, but among the above harmful substances, desulfurization technology as a method for removing SOx has seen rapid technological progress since the regulations for SOx emissions preceded the regulations for NOx emissions, and wet desulfurization is the main method. It has been established as However, the denitrification method as a NOx removal method has not yet become a completed technology, and various methods are being proposed one after another. Developmental research on these proposed NOx removal technologies can be broadly classified into adsorption methods, absorption methods, and catalytic reduction methods.

The mass adsorption method uses synthetic zeolite, activated carbon, or ion-exchange resin as an adsorbent to adsorb and remove grains, but because of the limited adsorption capacity, it is difficult to avoid the effects of coexisting gases such as sulfur compounds and water vapor. However, there are problems such as the need to frequently repeat the regeneration of the adsorbent and the equipment being large-scale due to the generally low exhaust gas processing capacity.

Next, absorption methods include the oxidation absorption method, which oxidizes and absorbs NOx, and the reduction absorption method, which reduces and absorbs NOx. Another method is to absorb with an alkaline aqueous solution containing an oxidizing agent such as potassium permanganate, and another method is to oxidize with ozone or catalytic oxidation and then absorb with an alkaline aqueous solution. In addition, the reduction absorption method uses sodium sulfite, sodium thiosulfate,
Contact with an aqueous solution containing a reducing agent such as sodium sulfide
This is a method of reducing Ox to N2 and removing it. These absorption methods have poor absorption efficiency when dealing with large amounts of exhaust gas because the concentration of NOx is dilute, resulting in large-scale equipment.Furthermore, consideration must be given to wastewater treatment of the aqueous solution used for absorption. There are many problems in practical application of the process.
In contrast to these methods, the catalytic reduction method is a method in which exhaust gas is treated with various reducing agents to reduce NOx to harmless nitrogen, and is promising because it solves the shortcomings of the above methods.

By the way, there are various catalytic reduction methods depending on the reducing agent used, but there are two types: non-selective catalytic reduction, which also reduces the ions coexisting in the exhaust gas, and selective catalytic reduction, which selectively reduces only NOx. Broadly classified. In the former case, methane, hydrocarbons such as u℃, hydrogen, or carbon monoxide are used as reducing agents, but generally 2 to 6% oxygen coexists in exhaust gas from boilers, etc. There are many problems in reducing NOx, such as requiring a large amount of reducing agent, which is not economical, and furthermore, the reaction heat accompanying the reduction reaction being large, making it difficult to control the reaction temperature. On the other hand, in the latter case,
Using ammonia as a reducing agent selectively reduces only NOx, which is economical as only a small amount of reducing agent is needed, and the heat of reaction is extremely low, and what is produced by the reaction is harmless. This method is very advantageous compared to the above-mentioned methods. This selective catalytic reduction method using ammonia as a reducing agent is a generally well-known method, but the problem with this method is whether a catalyst suitable for the composition and properties of the exhaust gas to be treated can be obtained. be. The first physical property that the catalyst must have in this method is that the exhaust gas contains oxygen, sulfur compounds, carbon dioxide, water vapor, etc., but it is not affected by these coexisting gases, and only NOx is selectively produced. The second reason is that the treatment temperature is 400°C considering thermal efficiency since the temperature at the outlet of the economizer attached to the boiler system is generally about 400°C. Third, it must be able to perform the reduction at the lowest possible temperature; and thirdly, it must work satisfactorily at a high space velocity (this space velocity is related to the volume of the catalytic reactor, and the larger the volume, the more compact the device can be. On the other hand, there is also an increase in pressure drop, so a high standard is required in terms of performance as well as the problem of catalyst shape.)Fourthly, there is soot dust in the exhaust gas, and this soot dust is mostly composed of carbon. However, it is necessary that the catalyst is not poisoned even if soot and dust that contains various heavy metals including iron adheres to the catalyst. Although there are other characteristics that a catalyst should have in addition to these four points, the above four points are the important characteristics. Currently, the desulfurization method is almost established as a wet process as an exhaust gas purification system, and when considering the removal of NOx from the boiler exhaust gas after desulfurization, it is necessary to introduce the exhaust gas whose temperature has been lowered in the wet process further to the dry catalytic reduction process. Considering the amount of heat required, it is most desirable to perform direct dry catalytic reduction before desulfurization.
It will be of utmost importance to develop catalysts that are not poisoned by sulfur oxides such as sulfur oxides.

In the selective catalytic reduction method using ammonia as the reducing agent,
Conventionally well-known catalysts include catalysts in which noble metals such as platinum and palladium are supported on activated alumina, but these catalysts do not support sulfur compounds, especially sulfur compounds such as oxygen, carbon dioxide, and water vapor, which coexist in exhaust gas. depending on NO
It has the disadvantage that the contact activity decreases over time due to the influence of the reducing activity of x.

On the other hand, the present inventors investigated a catalyst in which copper oxide, etc., which is known as a heavy metal catalyst, is supported on activated alumina, etc., and found that it is poisoned by sulfur compounds, especially sulfur oxides, that coexist in exhaust gas. Catalysts of this type are known as adsorbents for removing sulfur oxides from exhaust gases, for example as disclosed in British Patent No. 1,089,716. According to research conducted by the present inventors, sulfur content accumulates in the catalyst over time, and copper oxide changes to copper sulfate. Furthermore, a part of activated alumina changes to aluminum sulfate, and these It has been found that these changes can eventually affect the catalytic activity and cause it to deteriorate over time. That is,
Activated alumina cannot be used as a support in its original form; if it is used, it must be used in a form that does not accumulate sulfur content, or even if it accumulates, the components of the catalyst are not affected at all. It was confirmed that this was necessary.

In view of the above points, the present inventors added ammonia to the exhaust gas, and the inventors added ammonia to the exhaust gas to remove coexisting sulfur compounds, oxygen, carbon dioxide gas, water vapor, soot, etc., without being particularly affected by sulfur compounds, at a relatively low temperature and in a high space. As a result of research aimed at creating a catalyst that efficiently reduces and removes NOx to harmless nitrogen at a high rate and is durable, we found that titanium oxide (TlO2) with a surface area of 3077-1 V or more, which can be obtained by the manufacturing method specified below as catalyst component A. ) based on the finished catalyst, preferably 40 to 95% by weight, and further contains manganese (Mn) and tin (Sn), manganese and cadmium (Cd), iron (Fe) and nickel as catalyst B components. (N1), iron and bismuth (Bi), chromium (Cr) and nickel, chromium and antimony (Sb), cobalt (CO) and molybdenum (MO
) and cobalt and lead (Pb) in an amount of 1 to 7%, preferably 5 to 6%, which overcomes the above-mentioned drawbacks. The present invention was completed by discovering that an excellent NOx purification rate can be maintained over a long period of time.

One of the features of the present invention is the use of high surface area TiO2 obtained by the manufacturing method specified below. This Ti
Catalysts using O2 are affected by the sulfur content, and it is surprising that they maintain stable catalytic activity for a long period of time.
(a) Titanium ammonium sulfate obtained from titanium tetrachloride, ammonium sulfate and sulfuric acid [(Nl
il) 2●SO4・TiO●SOeH2O] thermal decomposition method.

(b) A method in which an organic titanate ester such as titanium tetrachloride, titanium nitrate or tetraisopropyl titanate is hydrolyzed and then fired.

(c) A method in which commercially available reagent grade titanium sulfate or titanium oxide sulfate is hydrolyzed and then calcined.

Among the TiO2 powders obtained in this way, as shown in the following examples, the present invention particularly has a surface area of at least 3
Good results are obtained by using one with 0 dIy.

It is also possible to appropriately mix two or more types of TlO2 with different surface areas, and as a result, prepare and use TlO2 with a preferable surface area. Although the crystal form of TiO2 to be used does not matter, it is generally preferable to use the anatase type, which has a higher surface area than the rutile type, and in the case of a mixed type of both types, one with a high anatase content is preferable. In the present invention, T
The catalyst component used with lO2 is manganese (Mn)
and tin (Sn), manganese and cadmium (Cd), iron (
Fe) and nickel (Ni), iron and bismuth (Bj),
Chromium (Cr) and nickel, chromium and antimony (Sb)
), a pair of oxides selected from the group consisting of cobalt (CO) and molybdenum (MO), and cobalt and lead (Pb), and the starting materials are oxides, hydroxides, )
It can be selected from inorganic acid salts, organic acid salts, oxyacids or their salts, and especially ammonium salts, oxalates, nitrates, sulfates, phosphates, and halides.

The method for preparing the catalyst of the present invention is not particularly limited, and general methods may be employed, but one example is as follows.

For example, in the case of TiO2-MnO2-SnO2 based catalyst,
A predetermined amount of TiO2 powder, manganese nitrate, and tin chloride is dispersed into a slurry with an aqueous solution, and if necessary, an appropriate molding aid such as carboxymethyl cellulose or starch is added, and the mixture is thoroughly kneaded with a kneader. Molded, then dried at 120-15σC,
It is obtained by calcination at 00-600°C, preferably 400-500°C for several hours in a stream of air. Alternatively, TiO2 may be made into spherical or cylindrical pellets in advance, and the manganese component or tin component may be impregnated or baked onto the pellets. The shape of the catalyst is not limited to the above-mentioned pellets, but may be suitably selected from a plate shape, a ribbon shape, a corrugated plate shape, a donut shape, a lattice shape, and other integrally molded shapes. Further, the above method can be appropriately employed for catalyst components other than Mn, Sn. The composition of the exhaust gas to be treated using the catalyst of the present invention is industrial exhaust gas with a high sulfur oxide content.
0-2500ppm1 oxygen 1-20%, carbon dioxide 1-1
5%, water vapor 5-15% and NOx (mainly NO) 10
The content is in the range of 0 to 1000 ppm.

Normal boiler exhaust gas falls within this range, but the gas composition is not particularly limited. For example, it is possible to treat NOx-containing exhaust gas that does not contain sulfur compounds. In addition, treatment conditions vary depending on the type and properties of the exhaust gas, but first, the amount of ammonia (NH3) added is
0.5 to 3 parts (molar ratio) per 1 part. preferable. For example, in the exhaust gas composition of a boiler, most of the NOx is NO, but NOI:. The equivalence relationship of NH3 is 3:
2, so the molar ratio of NOx to NOx is 1:1, which is slightly in excess of the equivalent amount.
The vicinity of is particularly preferable. Excessive NH3 is not discharged as unreacted material. This is because you must be very careful. Next, the reaction temperature is preferably 200 to 400°C, particularly 280 to 3500°C, and the space velocity is 1000°C.
A range of 3000 to 30000-1 is particularly suitable. The present invention will be explained in more detail below using Examples and Comparative Examples, but the present invention is not limited only to these Examples.

Comparative Example 1 Commercially available anatase type TlO2 powder (BET surface area 10d
200y and manganese nitrate Mn(NO3)2.6H
Mix with 160cc of aqueous solution containing 20176fI,
It was well kneaded in the kneader.

Apply the obtained slurry to a stainless steel locking plate and press 4T.
A molded body of nφ×4wm was prepared, dried at 12 (generations) for 3 hours, and then fired at 450° C. for 6 hours.

The composition of the obtained catalyst was TlO2:MnO2=80:2
It was 0 (weight ratio).

Example 1 Titanium tetrachloride TiCl457OOy was gradually dropped into water to make a 60% aqueous solution, and 2940y of sulfuric acid was added to this aqueous solution with stirring.

On the other hand, prepare a saturated aqueous solution containing 3940f of ammonium sulfate heated to 100°C, and add this saturated aqueous solution to the above NC.
Add titanium ammonium sulfate [(NH4)2
●SO4●TiO●SO, ●H2O] were precipitated. This was separated and fired at 500℃ for 10 hours, and then heated to 700℃.
After firing for one hour, titanium ammonium sulfate was thermally decomposed to obtain 2300y of TiO2. The surface area of this TlO2 was 34.4y1y. Obtained TiO2 powder 20
Add 180 cc of water to 0 V, mix well, and mold, dry, and bake this white slurry in the same manner as in Comparative Example 1.
A Tlα molded body of φ×4 size was obtained.

This molded body 80y was mixed with manganese nitrate Mn (NO3) 2°6
H2033y and tin chloride SnCl・2H2015f
The sample was immersed in 100 cc of an aqueous solution containing 100 ml of water, concentrated to dryness on a hot water bath, and then calcined at 450° C. for 6 hours. The composition of the obtained catalyst was TiO2:MnO2:SnO2=80:
The ratio was 10:0 (weight ratio).

Comparative Example 2 0.532y of hexachloroplatinic acid was dissolved in 70cc of water, and commercially available spherical activated alumina (average particle size 4wnφ, BET
It was concentrated and impregnated and supported on a surface area of 120dh and 100y.

After drying, reduction firing was performed at 450° C. for 3 hours in a hydrogen stream. The amount of platinum supported on the obtained catalyst was 0.2% by weight.

Comparative Example 3 Ferric nitrate Fe(NO3)3-9H20101y in 1 part water
The same spherical activated alumina 80V as in Comparative Example 2 was concentrated and impregnated and supported at 450℃ after drying.
After firing for a period of time, an iron-supported catalyst was obtained.

Comparative Example 4 Instead of the activated alumina used in Comparative Example 2, a silica carrier (SlO296%, surface area 10, Iyl4mφ×4
A platinum supported catalyst was obtained in the same manner as in Comparative Example 2 except that m) 100y was used.

Comparative Example 5 Using the silica carrier used in Comparative Example 4, an iron supported catalyst was obtained in accordance with Comparative Example 3 except for the same.

Example 2 A desktop activity test was conducted on each of the catalysts obtained in Example 1 and Comparative Examples 1 to 5 in the following manner, and the initial activities were compared.

30 cc of each catalyst was filled in a stainless steel reaction tube with an inner diameter of 2'' immersed in a molten salt bath, and a gas of the composition shown in Table 1 below, which was made by adding ammonia to synthesis gas similar to boiler exhaust gas, was fed at 2.5 f/min. Flow velocity (air velocity 5000F1r-
In 1), it was introduced into the catalyst layer, and the relationship between the reaction temperature and the NOx reduction rate (purification rate, %) was determined.

Note that a chemical CLD7S model manufactured by Yanagimoto was used for the NOx analysis. The results obtained are shown in Table 2.

- As is clear from Table 2, the noble metal catalyst of Comparative Example 2 has low-temperature activity, the catalyst using an alumina carrier of Comparative Example 3 and the catalyst of the present invention show equivalent initial activity, and Comparative Examples 4-
The silica-supported catalyst of No. 5 was shown to exhibit significantly lower levels. Example 3 Each of the catalysts obtained in Example 1 and Comparative Examples 1 to 3 was subjected to a life durability test using actual gas in the following manner.

A stainless steel reaction tube with an inner diameter of 28 mm was filled with 90 cc of each catalyst immersed in a molten salt bath, and a portion of the boiler exhaust gas was introduced.

Most of the dust entrained in the exhaust gas was removed using an alundum packed bed and a glass wool layer, and then introduced into the catalyst bed. The average composition of this boiler exhaust gas is NOx3OOppm. ．． SOxl4OOppm
1O23.5% by volume, CO213.5% by volume, H2Ol
4% by volume and the balance was nitrogen gas, and 300 ppm of ammonia was added thereto for catalytic reaction.

Space velocity (SV) is 500011r-1, reaction temperature is 3
The temperature was adjusted to 30°C. The results obtained are shown in Table 3.

As is clear from Table 3 above, the catalyst of the present invention has a
It was found that the activity of the catalysts of Comparative Examples 2 and 3 deteriorated very significantly, whereas the catalysts of Comparative Examples 2 and 3 showed almost no deterioration of their activity even after passing the catalyst.

Examples 4 to 10 Using TiO2 molded bodies prepared in the same manner as in Example 1, according to the preparation method of Example 1,
Catalysts were prepared with different heavy metal components as shown in Table 4.

In Examples 4 to 10, catalysts were prepared using two types of heavy metal components.

As heavy metal components, ferric chloride, manganese nitrate, lead nitrate, cobalt acetate, cadmium chloride, nickel nitrate, chromic anhydride, antimony trichloride, ammonium molybdate, and bismuth nitrate were used in preparing the catalyst. A desktop activity test similar to that in Example 2 was conducted for each of the catalysts obtained, and the results shown in Table 4 were obtained. Example 11~
12 For TIO2 powder obtained in the same manner as in Example 1, the amounts of manganese nitrate and stannic chloride were changed to MnO2/
The method of Example 1 was followed except that SnO2 = 1 (weight ratio), each catalyst shown in Table 5 was used (Example 11), and ferric chloride and nickel nitrate were replaced with Fe2O.
3/NiO=1 (weight ratio) to obtain each catalyst shown in Table 5 (Example 12).

The initial NOx purification activity of each of the catalysts obtained above was measured at 325° C. in the same manner as in Example 2, and the results shown in Table 5 were obtained.

As is clear from Table 5, it was shown that heavy metal components other than titanium showed a high level when the weight amount was in the range of 1 to 7% based on the total catalyst weight. Example 13 Of the synthetic exhaust gas shown in Table 1, SO2 was
A desktop activity test was conducted on the catalyst of Example 1 using the same gas composition except that ppm was changed from 100 ppm, and results similar to those shown in Table 2 were obtained.

Claims (1)

[Claims] 1. As a catalyst for selectively reducing nitrogen oxides by adding ammonia into a harmful gas containing nitrogen oxides and catalytically reacting the gas mixture, the following catalyst component A is used: Titanium dioxide (TiO_2) with a surface area of 30 m^2/g or more obtained by the method of a), (b) or (c)
) per finished catalyst, and as catalyst component B, manganese (Mn) and tin (Sn), manganese and cadmium (Cd), iron (Fe) and nickel (Ni).
), one set selected from the group consisting of iron and bismuth (Bi), chromium (Cr) and nickel, chromium and antimony (Sb), cobalt (Co) and molybdenum (Mo), and cobalt and lead (Pb). 1 to 7 oxides per finished catalyst
A catalyst composition for removing nitrogen compounds, characterized in that it contains 0% by weight. (a) Titanium ammonium sulfate obtained from titanium tetrachloride, ammonium sulfate, and sulfuric acid [(NH
_4)_2・SO_4・TiO・SO_4・H_2O]
pyrolysis method. (b) A method in which an organic titanate ester such as titanium tetrachloride, titanium nitrate or tetraisopropyl titanate is hydrolyzed and then fired. (c) A method in which commercially available reagent grade titanium sulfate or titanium oxide sulfate is hydrolyzed and then calcined.