JPH11128688A - Purification of waste gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying waste gas by which NOx of a low conc. in waste combustion gas after burning can be removed with high efficiency and high reliability and superior durability is ensured. SOLUTION: Waste combustion gas from an internal combustion engine operated in a lean air-fuel ratio is brought into contact with a catalyst-contg. layer and NOx is removed with kerosene or gasoline as a reducing agent. The catalyst contained in the catalyst-contg. layer is a catalyst layer consisting of alumina and silver or a catalyst coated structure obtd. by coating the inner surfaces of at least through holes in a substrate having a monolithic structure made of a refractory material having many through holes with a catalyst consisting of alumina and silver. The alumina has a porous structure in which the relation between pore radius and pore diameter measured by a gaseous nitrogen adsorption method represents a specified value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to exhaust gas used for purifying nitrogen oxides contained in combustion exhaust gas of mobile and stationary internal combustion engines of automobiles, boilers, gas engines, gas turbines, ships and the like. Regarding the purification method, more specifically, nitrogen oxides in exhaust gas discharged from an internal combustion engine operated at a lean air-fuel ratio at a high space velocity,
The present invention also relates to an exhaust gas purification method capable of purifying with high efficiency.

[0002]

2. Description of the Related Art Various combustion exhaust gases emitted from internal combustion engines such as automobiles contain nitrogen oxides (NOx) such as nitric oxide and nitrogen dioxide together with water and carbon dioxide as combustion products. Have been. NOx not only adversely affects the human body, particularly the respiratory system, but also is one of the causes of acid rain, which is regarded as a problem from the viewpoint of global environmental protection. Therefore, development of a denitration technology for efficiently removing nitrogen oxides from these various exhaust gases is desired.

On the other hand, in view of the prevention of global warming, lean-burn internal combustion engines have recently attracted attention. A conventional gasoline engine for an automobile has an air-fuel ratio (A / F) = 1.
Combustion at a controlled stoichiometric ratio near 4.7. For exhaust gas treatment, carbon monoxide, hydrocarbons and NOx in the exhaust gas are converted to alumina mainly containing platinum, rhodium, palladium and ceria. A three-way catalyst system has been employed in which three harmful components are simultaneously removed by contact with a catalyst.

However, the three-way catalyst system cannot be applied to exhaust gas purification of a lean burn gasoline engine operated at a lean air-fuel ratio because it is an absolute condition that the engine is operated at a stoichiometric ratio. . In addition, diesel engines are originally lean burn engines, but strict regulations are being imposed on both the suspended particulate matter and NOx in the exhaust gas.

Conventionally, as a method of reducing and removing NOx in an oxygen-excess atmosphere, a technique of using NH ₃ that is selectively adsorbed on a catalyst even in a small amount as a reducing gas has already been established. It is industrialized as a method for denitration of exhaust gas from boilers and diesel engines that are the source. However, this method requires a special device to recover unreacted reducing agent and uses ammonia, which has a strong odor and is harmful, which is particularly dangerous as a technology for denitration of exhaust gas from mobile sources such as automobiles. There is no applicable.

In recent years, it has been reported that a NOx reduction reaction can be promoted by using unburned hydrocarbons remaining in a lean combustion exhaust gas in an oxygen-excess atmosphere as a reducing agent. Have been developed and reported. For example, there are many reports that alumina or a catalyst in which a transition metal is supported on alumina is effective for a NOx reduction reaction using a hydrocarbon as a reducing agent. Also, Japanese Patent Application Laid-Open No. 4-284848 discloses O.M. 1-4% by weight of Cu, Fe, Cr, Zn, Ni,
V-containing alumina or silica-alumina
An example of use as an x reduction catalyst has been reported.

Further, when a catalyst in which Pt is supported on alumina is used, the NOx reduction reaction proceeds in a low temperature range of about 200 to 300 ° C., as disclosed in JP-A-4-267946 and JP-A-5-68855. Kaihei 5-1039
No. 49, for example. However, when these supported noble metal catalysts are used, the combustion reaction of hydrocarbons as a reducing agent is excessively promoted, and N ₂ O which is one of the substances causing global warming is produced as a by-product, Harmless N ₂
However, it has a drawback that it is difficult to selectively advance the reduction reaction to.

One of the present applicants has previously found that the use of a catalyst containing silver with a hydrocarbon as a reducing agent in an oxygen-excess atmosphere causes the NOx reduction reaction to proceed selectively. -281844. Even after this disclosure was made, a similar NOx reduction and removal technique using a catalyst containing silver was disclosed in JP-A-4-354536.
JP-A-5-92124, JP-A-5-92124
No. 125, JP-A-5-317647, JP-A-6-182156 and JP-A-6-277454
No., for example.

[0009]

However, in the alumina-supported silver catalyst and the exhaust gas purifying method using the same described in these conventional publications, propylene, light oil and the like are used as a reducing agent, so that SOx In addition, the denitration performance in the coexistence of water vapor is still insufficient for practical use, and there is also a problem in durability. Further, when alcohol is used as the reducing agent, the durability performance in the coexistence of SOx and steam is high, but there are drawbacks such as a problem of a product treatment by a side reaction and a high cost.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks of the prior art. It is an object of the present invention to remove NOx in lean combustion exhaust gas with high efficiency and high reliability, and to improve durability. An object of the present invention is to provide an exhaust gas purifying method having excellent performance.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies on a method for purifying exhaust gas having high denitration performance and excellent durability in a lean combustion region where SOx and steam coexist. Alternatively, the present inventors have found that the problem can be solved by contacting a catalyst layer composed of alumina and silver or a catalyst-containing layer composed of the catalyst-coated structure thereof with an exhaust gas containing nitrogen oxides using gasoline as a reducing agent. It was completed.

That is, in a first embodiment of the present invention for solving the above-mentioned problems, a combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio is brought into contact with a catalyst-containing layer to convert kerosene or gasoline as a reducing agent. A method for removing nitrogen oxides, wherein the catalyst contained in the catalyst-containing layer is an exhaust gas purification method in which the catalyst is a catalyst layer composed of alumina and silver.

A second embodiment of the present invention is a method of removing nitrogen oxides by using kerosene or gasoline as a reducing agent by bringing combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio into contact with a catalyst-containing layer. Wherein the catalyst contained in the catalyst-containing layer is a catalyst-coated structure in which at least the inner surface of the through-hole is coated with a catalyst made of alumina and silver in a support substrate having an integral structure made of a refractory material having many through-holes. It is characterized by an exhaust gas purification method composed of a body.

Still further, a third embodiment of the present invention provides:
The alumina used in the first and second embodiments is characterized in that the relationship between the pore radius and the pore volume measured by the nitrogen gas adsorption method is the sum of the pore volumes occupied by pores having a pore radius of 300 Å or less. Let X be the value, and let Y be the total value of the pore volume occupied by pores having a pore radius of 25 Å or more and less than 100 Å, and a pore radius of 100
When the total value of the pore volume occupied by the pores not less than Å and not more than 300 Å is defined as Ζ, Y is X
And an exhaust gas purifying method having a pore structure in which Ζ is 20% or less of X.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention and its operation will be more specifically described below.

[Structure of Catalyst and Production Method Thereof] Alumina, which is one of the main components of the catalyst used in the exhaust gas purification method of the present invention, is classified, for example, into boehmite, pseudoboehmite, vialite, or norstrandite in terms of mineralogy. The aluminum hydroxide powder or gel is heated and dehydrated in air or vacuum at 300 to 800 ° C., preferably 400 to 900 ° C., to obtain crystallographically γ-type, η-type,
It is preferable from the viewpoint of denitration performance that the phase transition is made to alumina classified into δ-type, χ-type or a mixed type thereof. Alumina having another crystal structure, for example, α-type alumina, has an extremely small specific surface area and poor solid acidity, and is therefore unsuitable as a catalyst component used in the present invention.

In the alumina, the relationship between the pore radius and the pore volume measured by the nitrogen gas adsorption method is as follows.
Let X be the total value of the pore volume occupied by pores of 0 Å or less, and 10 if the pore radius is 25 Å or more.
The total value of the pore volume occupied by pores smaller than 0 Å is defined as Y, and 3 for a pore radius of 100 Å or more.
Assuming that the total value of the pore volume occupied by pores of not more than 00 Å is Ζ, it is necessary to have a pore structure in which Y is 70% or more of X and Ζ is 20% or less of X. is there. When alumina not satisfying the above conditions was used as a carrier in the catalyst used in the method according to the present invention, the catalyst constituted by this was insufficient in denitration performance in the presence of SOx and steam. Therefore, it can be said that alumina having the above-mentioned crystal structure and pore characteristics is suitable as the alumina effective as a component of the catalyst in the present invention.

The catalyst used in the exhaust gas purifying method according to the present invention is as follows. The catalyst in the present invention,
The catalyst is a catalyst composed of alumina and silver having the specific pore structure described above. The state of silver contained in the catalyst is not particularly limited, and examples thereof include a metal state, an oxide state, and a mixed state thereof. In particular, the composition of the combustion exhaust gas of an internal combustion engine such as an automobile changes each time depending on the operation state, and thus the catalyst is exposed to a reducing atmosphere and an oxidizing atmosphere. Therefore, it is assumed that the state of the active metal constituting the catalyst changes depending on the atmosphere. The starting material of silver is not particularly limited.

The method for producing the catalyst comprising alumina and silver is not particularly limited, and any of the conventional methods such as an adsorption method, a pore filling method, an incipient wetness method, an evaporation to dryness method, and a spray method Any known method such as an impregnation method, a kneading method, a physical mixing method, and a combination thereof can be arbitrarily adopted. In this case, after a silver source is supported on alumina or an alumina precursor substance, drying and firing are performed. Further, a method for producing a catalyst containing an active metal species at the time of producing alumina or an alumina carrier having a specific pore structure as described above, for example, mixing an alcohol solution of aluminum alkoxide and an alcohol solution of a silver source, followed by heating and adding water An alkoxide method for decomposition or a coprecipitation method in which an alkali is added to an aqueous solution of a mixture of an aluminum source and a silver source to cause precipitation are applicable.

The content of silver in the above catalyst in terms of metal is not particularly limited, but is 1 to 10% by weight, preferably 2 to 8% by weight in terms of denitration performance. If the amount of silver carried is less than 1% by weight, the effect of the denitration performance is insufficient, and if it exceeds 10% by weight, the combustion reaction of kerosene or gasoline as a reducing agent proceeds preferentially, and the NOx removal performance decreases.

The drying temperature is not particularly limited, and drying is usually performed at about 80 to 120 ° C. The firing temperature is 300 to 1000 ° C, preferably 400 to 900 ° C.
It is about. The atmosphere at this time is not particularly limited,
Each atmosphere such as in air, inert gas, or oxygen may be appropriately selected according to the catalyst composition. In addition, each atmosphere may be alternately changed at regular intervals.

In the first embodiment of the present invention, when a catalyst-containing layer for purifying exhaust gas is formed, the catalyst-containing layer is formed by molding the above-mentioned catalyst into a predetermined shape or flowing a desired exhaust gas in a powder state. Filling a certain space.
When forming the catalyst into a molded body, the shape is not particularly limited, for example, spherical, cylindrical, honeycomb, spiral, granular,
Various shapes such as a pellet shape and a ring shape can be adopted. These shapes, sizes, and the like may be arbitrarily selected according to use conditions.

Next, the catalyst-containing layer according to the second embodiment of the present invention will be described. The catalyst-containing layer referred to here is a catalyst coating in which at least the inner surface of the through-hole of a refractory material having a large number of through-holes is coated with a catalyst containing silver on an alumina carrier. It is a structure.

The support substrate is provided with a large number of through holes along the flow direction of the exhaust gas. In a cross section perpendicular to the flow direction, the porosity is usually 60 to 90%, preferably 70 to 90%. And the number is one square inch (5.0
30 to 700, preferably 200 to 6, 6 cm ² )
00. The catalyst is coated on at least the inner surface of the through-hole, but may be coated on the end face or side face of the supporting substrate.

As the material of the refractory support substrate, ceramics such as α-type alumina, mullite, cordierite and silicon carbide, and metals such as austenitic and ferritic stainless steels are used. The shape may be a conventional one such as a honeycomb or a foam, but a honeycomb-shaped supporting substrate made of cordierite or stainless steel is preferred.

As a method for coating the support substrate with a catalyst,
A so-called ordinary wash coat method or a sol-gel method, in which the catalyst of the present invention sized to a certain particle size is coated on the inner surface of the support substrate with or without a binder, can be applied. Alternatively, the support substrate may be coated with alumina in advance, and then the catalyst active layer of the present invention may be subjected to a treatment to form a catalyst coating layer. The amount of the catalyst layer coated on the support substrate is not particularly limited, but is preferably about 50 to 250 g / liter, more preferably about 100 to 200 g / liter per unit volume of the support substrate.

An exhaust gas purifying method according to the first to third embodiments of the present invention will be described. According to the exhaust gas purification method of the present invention, the catalyst layer of the first embodiment or the second
Using the catalyst-coated structure of the embodiment of the present invention and the excess from near the stoichiometric amount required to completely oxidize the reducing components such as carbon monoxide, kerosene or gasoline and hydrogen in the exhaust gas with the oxidizing components such as NOx and oxygen. NOx is reduced and decomposed into nitrogen and water by contacting the exhaust gas containing oxygen, and kerosene or gasoline is also oxidized to water and carbon dioxide. The usage state of kerosene or gasoline used as the reducing agent may be the unburned portion of the engine fuel, external addition, or a combination thereof. Also, like diesel engine exhaust gas, the H
When the C / NOx ratio is low, a high NOx removal rate can be achieved by using a system in which kerosene is externally added to exhaust gas and then brought into contact with the catalyst-containing layer of the present invention. Here, any hydrocarbon can be used as long as it is a fuel for a diesel engine.

The amount of kerosene or gasoline to be added from the outside may be appropriately selected depending on the NOx concentration in the exhaust gas.
0, preferably about 2 to 15.

In the exhaust gas purification method according to the present invention, the gas space velocity (SV) for purifying the combustion exhaust gas of the internal combustion engine operated in the lean air-fuel ratio range is not particularly limited, but is SV5,000 h. 200,0 if ^-1 or more
It is preferably at most 00h ^-1 .

[0030] In the exhaust gas purifying method of the present invention, the inlet temperature of the catalyst-containing layer is 300 ° C or higher and 550 ° C.
It is preferable to set the following. If the temperature is lower than 300 ° C., the combustion of the reducing agent is insufficient, and the deterioration due to SOx is large. On the other hand, if the temperature exceeds 550 ° C., the oxidation reaction of hydrocarbons proceeds preferentially, so that the NOx reduction activity decreases.

[0031]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples. However, the present invention is not limited to the following examples. (1) Selection of Alumina for Exhaust Gas Purifying Catalyst In order to select an alumina carrier to be used, a to c of the present invention are used for various γ-type aluminas having specific surface areas and pore distributions as shown in Table 1. Alumina falling within the range of the catalyst, and d to g are aluminas outside the scope of the present invention. Note that a
The pore distribution of ｇg of alumina was measured by a soapmatic manufactured by Carlo Elba.

[0032]

[Table 1] ───────────────────────────── Alumina specific surface area pore distribution (m ² / g) Y / X ( %) Z / X (%) ─────────────────────────────a 241 83.2 2.4 b 219 87.0 3.9 c 174 88.4 4.4 d 199 47.0 0.7 e 177 68.5 4.9 f 241 51.0 45.9 g 266 71.1 22.7} ──────────────────────

(2) Preparation of Catalyst Layer Hereinafter, preparation examples of preparation of each catalyst for constituting the catalyst layer of the present invention are shown as reference examples. (A) Production of catalyst: [Reference Example 1] 100 g of alumina hydrate, which is a precursor of γ-type alumina a in Table 1, was prepared by adding silver nitrate to 5.36.
After immersing in a 300 ml aqueous solution containing g for 10 hours, the mixture was evaporated to dryness at 80 ° C. This was air-dried at 110 ° C. and then calcined in air at 550 ° C. for 3 hours to obtain Catalyst 1 (Reference Example 1). The metal content of Ag in the catalyst 1 was 4.5% by weight with respect to the entire catalyst.

Reference Examples 2 to 11 Similarly to Reference Example 1 except that alumina hydrate which is a precursor substance from which γ-type aluminas b to g shown in Table 1 were obtained was used. Catalyst 2 (Reference Example 2), Catalyst 3 (Reference Example 3),
Catalyst 4 (Reference Example 4), Catalyst 5 (Reference Example 5), Catalyst 6 (Reference Example 6), and Catalyst 7 (Reference Example 7) were obtained. Reference Example 1
Catalyst 8 (Reference Example 8) and Catalyst 9 were prepared in the same manner as in Reference Example 1 except that the silver content was changed to 0% by weight, 2% by weight, 3% by weight, and 8% by weight. (Reference Example 9), Catalyst 10 (Reference Example 10) and Catalyst 11 (Reference Example 11) were obtained.

(B) Preparation of honeycomb catalyst: [Reference Example 12] 60 g of the above-mentioned powder catalyst 1 was mixed with 8 g of alumina sol (Al ₂ O ₃ solid content 10% by weight) and water 12
The mixture was charged into a ball mill pot together with 0 ml and wet-pulverized to obtain a slurry. Into this slurry, a 1 inch diameter, 2.5 inch length bored from a commercially available 400 cpsi (cell / inch ² ) cordierite honeycomb substrate
After immersing the inch-shaped cylindrical core and pulling it up, excess slurry was removed by air blow and dried. Then 500
Baked at 30 ° C. for 30 minutes, coated with 150 g of solid content in terms of dry weight per liter of honeycomb, and 4.5% Ag / Al ₂
A honeycomb catalyst 12 having an O ₃ composition (Reference Example 12) was obtained.

The results of evaluating the denitration performance of the catalyst layers for purifying exhaust gas formed using the catalysts of Reference Examples 1 to 12 under various conditions will be described below. Example 1 After the catalyst 1 was molded under pressure, it was pulverized and sized to a particle size of 350 to 500 μm, filled in a stainless steel reaction tube having an inner diameter of 15 mm to form a catalyst layer, and this was fixed under a normal pressure fixed bed. It was mounted on a flow reactor.

[Performance Evaluation Example 1] The temperature of the exhaust gas in the reaction tube was maintained at 420 ° C.
O: 750 ppm, kerosene (C ₁ ): 4500 ppm, O
_{2: 10%, SO 2:} lppm, H 2 O: 10%, the balance space, a mixed gas consisting of _{N 2} velocity 75,000h
^-1 . In the analysis of the gas composition at the outlet of the reaction tube, the concentrations of NO and NO ₂ were measured with a chemiluminescence NOx meter, and the N ₂ O concentration was measured with a gas chromatograph / thermal conductivity detector equipped with a Porapak Q column. . The denitration rate was defined by the following equation 1. Further, N ₂ O and NO ₂ were hardly generated in any of the catalyst layers of the present invention.

[0038]

(Equation 1)

[Examples 2 to 6 and Comparative Examples 1 to 5] The catalysts 2, 3, and 9 to 11 of Reference Examples 2, 3, and 9 to 11 and the catalysts 4 to 8 of Reference Examples 4 to 8 were used in Example 1, respectively. An evaluation test using a model gas was performed in the same manner as above, except that the catalyst 1 was used. Catalyst layers using catalysts 2, 3, and 9 to 11 are referred to as Examples 2 to 6, respectively, and catalyst layers using catalysts 4 to 8 are referred to as:
These were Comparative Examples 1 to 5, respectively. Table 2 shows Examples 1 to 6 above.
The initial denitration performance of the catalyst layers of Comparative Examples 1 to 5 is shown as a denitration rate (%).

[Performance Evaluation Example 2 (Example 7)] In the performance evaluation example 1, the honeycomb catalyst 12 of Reference Example 12
It was machined into a cylindrical shape having a length of 24 mm and a length of 24 mm, and filled in a stainless steel reaction tube having an inner diameter of 15 mm. Performance evaluation example 1 except that the space velocity of the gas to be fed was 13,000 h ^-1
A model gas evaluation test similar to that described above was performed, and this was designated as Example 7. The results are shown in Table 2 together with the results of Performance Evaluation Example 1.

[0041]

[Table 2] ────────────────────────────── Examples and comparative examples Catalyst Denitration rate (%) ───── １ Example 1 Catalyst 1 93 Example 2 Catalyst 2 86 Example 3 Catalyst 3 86 Comparative Example 1 Catalyst 4 34 Comparison Example 2 Catalyst 5 22 Comparative Example 3 Catalyst 6 43 Comparative Example 4 Catalyst 7 44 Comparative Example 5 Catalyst 8 46 Example 4 Catalyst 9 91 Example 5 Catalyst 10 93 Example 6 Catalyst 11 84 Example 7 Catalyst 12 90 ───────────────────────────

Table 2 shows that Examples 1 to 7 have an initial performance of 80.
% Or more, showing excellent performance as compared with Comparative Examples 1 to 5.

[Performance Evaluation Example 3 (Example 8 and Comparative Example 6)
-8)] A similar activity test was performed in Example 1 except that kerosene (C ₁ ) as a reducing agent was changed to gasoline in Example 1 of performance evaluation. In addition, the same activity test was carried out except that kerosene as a reducing agent was replaced with light oil, propane and propylene in Performance Evaluation Example 1, and Comparative Examples 6 to 8 were obtained.

[0044]

[Table 3] ─────────────────────── Reducing agent Denitration rate (%) ──────────────── Example 1 Kerosene 93 Example 8 Gasoline 94 Comparative Example 6 Light Oil 68 Comparative Example 7 Propane 12 Comparative Example 8 Propylene 35分 か っ It can be seen that Example 1 using kerosene as the reducing agent and Example 8 using gasoline have higher performance than Comparative Examples 6 to 8 using gas oil, propane and propylene as the reducing agent. Was.

[0045]

As described above, according to the exhaust gas purifying method of the present invention, nitrogen oxides contained in lean combustion exhaust gas can be reduced and purified at a high denitration rate and excellent in durability performance. It is useful for purifying nitrogen oxides in the discharged combustion exhaust gas.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Atsushi Katakei 3-18-5, Chugoku, Ichikawa-shi, Chiba Sumitomo Metal Mining Co., Ltd. Central Research Laboratory (72) Inventor Masaki Funabiki 678 Ichihonmatsu, Numazu-shi, Shizuoka N・ Echem Cat Co., Ltd. Numazu Plant

Claims (3)

[Claims] 1. A method for removing nitrogen oxides by contacting combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio with a catalyst-containing layer using kerosene or gasoline as a reducing agent,
An exhaust gas purifying method, wherein the catalyst contained in the catalyst-containing layer is a catalyst layer made of alumina and silver. 2. A method for removing nitrogen oxides by bringing combustion exhaust gas of an internal combustion engine operated at a lean air-fuel ratio into contact with a catalyst-containing layer and using kerosene or gasoline as a reducing agent.
The catalyst contained in the catalyst-containing layer is constituted by a catalyst-coated structure in which at least the inner surface of the through-hole is coated with a catalyst made of alumina and silver in an integral support substrate made of a refractory material having a large number of through-holes. An exhaust gas purification method characterized in that: 3. The alumina has a relationship between a pore radius and a pore volume measured by a nitrogen gas adsorption method, the pore radius of which is 30.
Let X be the total value of the pore volume occupied by pores of 0 Å or less, and 10 if the pore radius is 25 Å or more.
The total value of the pore volume occupied by pores smaller than 0 Å is defined as Y, and 3 for a pore radius of 100 Å or more.
When the total value of the pore volume occupied by the pores of not more than 00 Å is Z, Y has a pore structure of 70% or more of X and Z has a pore structure of 20% or less of X, The exhaust gas purifying method according to claim 1 or 2, wherein