JP2000015099A - NOx-containing exhaust gas purification catalyst and NOx-containing exhaust gas purification method - Google Patents

Abstract

(57) Abstract: An improved silver-carrying alumina catalyst for purifying NOx-containing exhaust gas and a method for purifying NOx-containing exhaust gas using the catalyst are provided. In A NOx-containing exhaust gas purifying silver on alumina catalyst, SiO ₂ in alumina 180ppm or less, Na ₂ O is 100ppm or less, a silver-supported alumina for NOx-containing exhaust gas purification, characterized in that chlorine is 50ppm or less A method for purifying NOx-containing exhaust gas, comprising contacting the catalyst and the NOx-containing exhaust gas with the catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silver-carrying alumina catalyst for purifying NOx-containing exhaust gas and a method for purifying NOx-containing exhaust gas using the catalyst.

[0002]

2. Description of the Related Art When incinerating industrial waste or municipal waste, NOx (nitrogen oxide) or CO, SOx, hydrogen chloride, Odors and the like are generated. Various measures have been taken against exhaust gases including these, and further research and development have been promoted. In this regard, exhaust gases emitted from automobiles, thermal power generation, cogeneration systems using gas engines, etc. and GHP systems The same applies to.

[0003] Among the harmful components in various exhaust gases, various methods are known as a so-called flue gas denitration technology, particularly for the treatment of NOx. Among them, the catalytic reduction method of purifying using a catalyst in the treatment is usually performed using N
It is of particular interest because it is an innocuous changed Ox to finally N _2. There are various catalysts used in the catalytic reduction method, and one of them is a silver-supported alumina catalyst, that is, a catalyst in which silver is supported on alumina.

Among the silver-carrying alumina catalysts, alumina is effectively used not only as a catalyst itself but also as a carrier for carrying silver as a catalyst metal. In a denitration reaction using an alumina catalyst carrying silver, for example,
As disclosed in JP-A-92124, nitrogen oxides in an exhaust gas can be removed by adding a gaseous alkane and / or alkene to a combustion exhaust gas containing excess oxygen. However, with respect to alumina used as a carrier in the catalyst of the publication, only the specific surface area is 30 m ² /
g or more, and the effect of alumina itself was conventionally considered to be small [Miyadera, Journal of the Japan Institute of Energy, Vol. 73, No. 11, pages 987-993 (199)
4)].

The present inventors have further pursued the catalytic properties and practicality of the silver-supported alumina catalyst on the premise of the above-mentioned points and the like. Was observed to be significantly different. FIG. 1 illustrates the process leading to the present invention.
This graph shows the measured NOx purification rates of commercially available catalysts in which 2 wt% of Ag is supported on various types of alumina.
The test conditions were as follows: 4.5 ml of a silver-carrying alumina catalyst using various aluminas, NO = 91 ppm, C ₃
H ₈ = 303 ppm, O ₂ = 9.1%, CO = 910 pp
m, CO ₂ = 6.4%, steam (steam) = 9.1%,
N ₂ = balance gas was flowed under the condition of space velocity of 33000 h ⁻¹ , and the reaction temperature was 500 ° C.

Here, the NOx purification rate is the NOx purification rate before the treatment.
Assuming that the concentration is X and the NOx concentration after treatment is Y, the following equation (1)
It is calculated by: In this regard, the same applies to the NOx purification rate in the examples described later.

[Equation 1]

As shown in FIG. 1, the specific surface area of alumina and NO
It can be seen that there is not always a correlation with the x purification rate. For example, as is clear from the comparison between silver-carrying alumina 1 and silver-carrying alumina 4, silver-carrying alumina 1 having a small specific surface area of alumina is superior to silver-carrying alumina 4 having a large alumina specific surface area. Although the alumina specific surface areas of the silver-carrying alumina 4 and the silver-carrying alumina 5 are both as large as 250 m ² / g, the NOx conversion of the silver-carrying alumina 4 is superior to that of the silver-carrying alumina 5. Further, the alumina specific surface area of the silver-carrying alumina 9 is as large as 250 m ² / g, but the NOx conversion is only about 11%.

[0008]

Then, when the cause was further pursued, it was found that there was an influence of impurities derived from raw materials and manufacturing methods, and that it was influenced by specific three components of SiO ₂ , Na ₂ O and chlorine. Do you get it. That is, the present invention relates to a catalyst for purifying NOx-containing exhaust gas comprising a silver-carrying alumina catalyst, wherein SiO ₂ , N
It is an object of the present invention to provide a silver-carrying alumina catalyst for purifying NOx-containing exhaust gas in which a ₂ O and chlorine are simultaneously reduced to a predetermined value or less, and a method for purifying NOx-containing exhaust gas using this catalyst.

[0009]

The present invention provides (1) NOx
In a silver-carrying alumina catalyst for purifying contained exhaust gas, the specific surface area of alumina is 100 m ² / g or more, and SiO ₂ in alumina is 180 ppm or less, and Na ₂ O is 100% or less.
A silver-carrying alumina catalyst for purifying NOx-containing exhaust gas, which is characterized in that the content thereof is not more than 50 ppm and the content of chlorine is not more than 50 ppm.

[0010] The present invention is, (2) a NOx-containing exhaust gas, and a specific surface area of the alumina is 100 m ² / g or more, and, SiO ₂ in alumina 180ppm or less, Na ₂
Provided is a method for purifying NOx-containing exhaust gas, which is brought into contact with a NOx-containing exhaust gas-purifying silver-carrying alumina catalyst in which O 2 is 100 ppm or less and chlorine is 50 ppm or less.

[0011]

In DETAILED DESCRIPTION OF THE INVENTION NOx-containing supported silver-alumina catalysts for purification of exhaust gas of the present invention, the specific surface area of alumina 100 m ² / g or more, more preferably 140 m ² / g or more, SiO ₂ in alumina 180 ppm or less, preferably 150 ppm or less, more preferably 100 p
pm or less, Na ₂ O is 100ppm or less, preferably not more than 95 ppm, chlorine 50ppm or less, alumina is used and more preferably 30ppm or less.

Thus, the alumina used in the present invention is:
In addition to the above requirements for the specific surface area, the above SiO ₂ , Na ₂
It is essential that the above quantitative conditions for the three components of O and chlorine are simultaneously satisfied. Alumina can be obtained by various methods. As the alumina of the present invention, any alumina which satisfies the above conditions with respect to specific surface area, SiO ₂ , Na ₂ O and chlorine is used regardless of the production method.

Alumina used in the catalyst of the present invention includes, for example, (1) a method of rapidly dehydrating aluminum hydroxide obtained from bauxite by a Bayer method at a high temperature,
(2) Gibbsite or boehmite is used as a raw material and treated with an aqueous caustic sorter to produce sodium aluminate, which is separated from insoluble iron oxide, aluminosilicate, etc.
A method of filtering and washing Al (OH) ₃ obtained by adding l (OH) ₃ seeds, followed by heat dehydration, and (3) a method of neutralizing sodium aluminate with aluminum sulfate to obtain an alumina hydrate (4) a method in which aluminum metal is placed in an aqueous solution of aluminum chloride and the resulting mixture is heated to obtain an aluminum hydroxide sol; (5) an aluminum alkoxide is dispersed in water, a lower alcohol, or a hydrocarbon such as benzene; It is obtained by a method of obtaining an alumina hydrate by hydrolysis.

The alumina of the present invention can be obtained by using boehmite, pseudo-boehmite, bayerite, or amorphous aluminum hydroxide powder or gel as it is, or by heat dehydration at 400 to 800 ° C. Γ-type, η
-, Δ-, χ-, or a mixture thereof. For example, in the above methods (1) to (5), the specific surface area, SiO ₂ , Na ₂ O, and chlorine are the above conditions. It is obtained by considering to be

The silver-carrying alumina catalyst of the present invention is constituted by supporting silver on alumina satisfying the above conditions.
The amount of silver supported on alumina is in the range of 0.5 to 10% by weight, preferably 1 to 5% by weight. The method of supporting silver is not particularly limited as long as silver can be uniformly supported on alumina as a metal. Preferably, an impregnation method, a wet kneading method, or the like can be applied. As an example, one embodiment of the impregnation method is described as follows. Silver is made into an aqueous solution of nitrate, chloride, acetate, or another form, and alumina, such as powder, is added thereto, immersed and stirred appropriately, and the resulting mixture is added to the alumina. Impregnated with silver compound. Thereafter, it can be manufactured by drying and firing in a conventional manner.

The catalyst may be used in any form such as powder, granule, granule (including sphere), pellet (cylindrical or annular), tablet (tablet), or honeycomb (monolith). Can be used as a shape. In the present invention, since it is necessary to allow the combustion exhaust gas to pass therethrough, if the catalyst obtained by supporting silver on alumina is in the form of a powder, in order to prevent the catalyst from escaping from the catalyst layer filled with the powder. It is desirable that the particles be sized or granulated in a predetermined particle size range, or that they be pressed and extruded. Among them, in the case of extrusion molding, it is cut into a predetermined length and pelletized before use.

In the case of a monolithic body, (1) alumina is supported on a honeycomb-structured base material by, for example, wash coating, and silver is impregnated on the alumina-supported base material by, for example, an impregnation method. A) The silver is supported on the alumina by, for example, an impregnation method, and then the silver-supported alumina is applied to the honeycomb body by, for example, a wash coat. The use of the above-mentioned silver compound at the time of impregnation, drying and baking by a conventional method are the same as described above. In any of the above embodiments, a monolith body made of metal or ceramic can be used. Examples of metals include iron-aluminum-chromium alloys and stainless steels, and examples of ceramics include cordierite.

The present invention is one type of catalytic reduction denitration method,
In addition to the catalyst, a separate reducing agent is required. In the case of the silver-supported alumina catalyst according to the present invention, the type of the reducing agent is not particularly limited, and various conventionally known reducing agents can be used. In particular, when the exhaust gas of a lean burn gas engine is an exhaust gas from a gas engine of a cogeneration system or a GHP system, a small amount of unburned lower hydrocarbons is contained in the exhaust gas. Lower hydrocarbons can also be used.

FIG. 2 is a diagram schematically showing an example of application of the catalyst of the present invention to purification of exhaust gas from a gas engine of a cogeneration system or a GHP system. Fuel gas such as city gas is supplied to the gas engine C together with air from the introduction section A through the combustion air introduction section B.
And the exhaust gas generated by the combustion here is led out from a conduit D. E is a catalyst layer, in which the silver-carrying alumina catalyst according to the present invention is filled and arranged. NOx in the exhaust gas from the conduit D is denitrified in the catalyst layer E and discharged from the outlet F.

When the form of the catalyst is granular, granular (including spherical), pellet (cylindrical or annular), tablet (tablet), or the like, for example, a mesh or It is carried out in the form of arranging perforated plates and filling them with the catalyst therebetween. FIG. 3 schematically shows one or two examples of the use of the present catalyst in the form of a monolith as an enlarged sectional view. In order to set this in the catalyst layer as shown in FIG. 2, it is arranged such that its cross-sectional opening faces the flow direction (→) in FIG.

[0021]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but it goes without saying that the present invention is not limited to the examples.

Alumina obtained in various steps including commercially available ones were prepared, trace components of each alumina were analyzed, and an evaluation test of NOx purification rate in exhaust gas was performed using each alumina. The amount of Ag supported on each alumina was 2 wt%.
After impregnation so that
It is fired for a time to obtain a powdery solid, formed into pellets, crushed, and sized to have a particle size of 24 to 42 mesh.

As a test apparatus, a fixed bed flow type reactor as shown in FIG. 2 was used.
ml. The test exhaust gas (exhaust gas to be treated) is N
O = 91 ppm, C ₃ H ₈ = 303 ppm, O ₂ = 9.1
%, CO = 910 ppm, CO ₂ = 6.4%, steam (steam) = 9.1%, N ₂ = a gas containing a balance amount was used. Among them, C ₃ H ₈ acts as a reducing agent. The space velocity (SV) is 132000 h ^-1 , and the processing temperatures are 450 ° C., 500 ° C., 525 ° C., and 550, respectively.
C. and 600.degree. Table 1 shows several examples of the results.

[0024]

[Table 1]

As shown in Table 1, N in the alumina of Comparative Example 1
Although the a ₂ O content is very low at 40 ppm and the Cl content at 10 ppm or less, the NOx purification rate is zero at 450 ° C.
It only works slightly after 00 ° C. This is recognized because the specific surface area is as small as 96 m ² / g. Further, the amount of Na ₂ O and the amount of Cl in the alumina of Comparative Example 2 are the same as those of Comparative Example 1, but the NOx purification rate is somewhat higher than that of Comparative Example 1, but still lower. The point that the NOx purification rate in Comparative Example 2 was somewhat higher than that in Comparative Example 1 was that the specific surface area was 285 m ² /
g and large points are considered to be contributing, but NOx
The reason why the purification rate is still low is considered to be based on the fact that the amount of impurity SiO _{2 in} alumina exceeds 180 ppm and is as high as 214 ppm.

The alumina in Comparative Example 3 had a specific surface area of 13
9 m ² / g, which is larger than Comparative Example 1. However, the amount of SiO _{2, the} amount of Na ₂ O, and the amount of Cl in alumina were both large, and thus the NOx purification rate was somewhat higher than that of Comparative Example 1.
Still low. This point is almost the same in Comparative Example 4. The alumina of Comparative Example 5 had a large specific surface area of 250 m ² / g and the amount of Na ₂ O in the alumina was as small as 13 ppm, but the NOx purification rate was 2.1% at 450 ° C. and 10.2 even at 600 ° C. % And low. This means that the amount of SiO ₂ is 514 pp
It is recognized that this is based on the fact that the amounts of both impurities are as high as 210 ppm for m and Cl.

On the other hand, the NOx purification rate in Example 1 was 17.6% at a temperature of 450 ° C. and 60.7% at a temperature of 500 ° C.
%, 65.9% at 525 ° C. This is because the specific surface area of the alumina of Example 1 was 24.
0 m ² / g, and the amount of SiO ₂ in alumina 64
ppm, the amount of Na ₂ O is 81 ppm, and the amount of Cl is 10 ppm or less.

The NOx purification rates in Examples 2 and 3 are as follows:
Although slightly lower than that of Example 1, each of them has excellent NOx
Shows purification ability. SiO in alumina in Example 2
₂ ppm 86 ppm, Na ₂ O content 40 ppm, Cl content 10 p
pm or less, and the SiO ₂ content in the alumina in Example 3 was 86 ppm, the Na ₂ O content was 67 ppm, and the Cl content was 10 ppm.
It is recognized that these points contribute to the NOx purification ability. In this regard, in particular, NOx in the gas to be treated
In view of the fact that the amount is as small as 91 ppm, the excellent NOx purification effect of the silver-supported alumina catalyst of the present invention is apparent.

[0029]

According to the present invention, an effective and excellent NOx purifying effect can be obtained. In particular, even when the NOx concentration in the exhaust gas is extremely low, for example, 91 ppm, an excellent purification effect can be obtained. Therefore, a very small amount of N in the combustion exhaust gas from a gas engine or the like can be obtained.
Ox can also be effectively denitrated and purified.

[Brief description of the drawings]

FIG. 1 is a graph showing actual measured values of a NOx purification rate by a catalyst supporting Ag on various commercially available aluminas in a process leading to the present invention.

FIG. 2 is a diagram showing an example of an installation mode of a NOx purification catalyst layer according to the present invention.

FIG. 3 is a diagram (a schematic cross-sectional view) showing an example of a monolith catalyst applied in the present invention.

[Explanation of symbols]

 A Fuel gas introduction section B Air introduction section C Gas engine D Conduit E Catalyst layer F Outgoing section of treated exhaust gas

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Uchida 3-2-15 -106 F-term (reference) 4D048 AA06 AB10 AC02 BA03X BA34X BA41X BB01 BB17 BB20 CA04 4G069 AA01 AA03 AA09 BA01A BA01B BA18 BB04A BB04B BB10B BC32A BC32B BD02A BD05A CA02 CA03 CA06 CA13 DA06 EA01Y EA02X EA02Y EA19 EC03X EC03Y EC04X EC05X FB14 FB20 FB30 FB57 FC08

Claims (5)

[Claims] In a NOx-containing silver-carrying alumina catalyst for purifying exhaust gas, the specific surface area of alumina is 100 m ² / g or more, and SiO ₂ in alumina is 180 ppm or less, Na ₂ O is 100 ppm or less, and chlorine is 50 ppm. A silver-carrying alumina catalyst for purifying NOx-containing exhaust gas, characterized in that: 2. The method according to claim 1, wherein the silver-carrying alumina catalyst is granular, granular,
The silver-carrying alumina catalyst for purifying NOx-containing exhaust gas according to claim 1, wherein the catalyst is formed in a pellet or tablet shape. 3. A silver-carrying alumina catalyst for purifying NOx-containing exhaust gas according to claim 1, wherein said silver-carrying alumina catalyst is a catalyst constituted as a monolith body. 4. The silver-carrying alumina catalyst for purifying NOx-containing exhaust gas according to claim 3, wherein said monolith body is made of ceramic or metal. 5. An exhaust gas containing NOx, wherein the specific surface area of alumina is 100 m ² / g or more, and SiO 2 in alumina is used.
₂ , 180 ppm or less, Na ₂ O is 100 ppm or less,
NOx characterized by being brought into contact with a silver-carrying alumina catalyst for purifying NOx-containing exhaust gas having a chlorine content of 50 ppm or less.
Purification method of contained exhaust gas.