JPH06288224A - Exhaust emission control device - Google Patents

Abstract

PURPOSE:To provide an exhaust emission control device capable of efficiently regenerating a filter as well as capturing environmentally harmful particulates contained in exhaust gas by the filter. CONSTITUTION:This is an exhaust emission control device having a plural number of filter units 3 connected to an exhaust gas pipe 1 through a directional control valve 2, each of the filter units 3 has one or a plural number of filters 4 coated with a catalyzer and an electric heater 5, the catalyzer holds a ctalyzer activated species consisting of fine powder of a composite oxide expressed by a usual expression Ln1-xAxMO3 on a porous inorganic oxide, the particulates scavenged on the filters 4 are burnt by heating of the electric heater 5 and accordingly the filters 4 are regenerated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for trapping fine particles in exhaust gas, which is an environmental problem. More specifically, it traps particulate carbonaceous material (particulates) in the exhaust gas of a diesel engine and efficiently The present invention relates to an exhaust gas purifying device for burning.

[0002]

2. Description of the Related Art An attempt has already been made to purify exhaust gas by collecting and burning particulates in exhaust gas from a diesel engine. For example, the actual exploitation Sho 61-551
No. 15 shows an example. The device disclosed in the publication is
A heat radiating member extending in the axial direction is provided in the honeycomb filter, and an electric heater is arranged in front of the honeycomb filter. In this known example, the exhaust gas entering from the inlet side of the honeycomb filter passes between the cells forming the honeycomb filter and flows to the outlet side. During such exhaust gas flow, particulates in the exhaust gas are collected by the filter.

As the amount of collected particulates increases, the pressure loss due to the filter increases, and when the pressure loss exceeds a predetermined value, the electric heater is energized to burn the particulates. However, when the particulates are forcibly combusted in a part of the exhaust pipe in use, it is not preferable to do so while the vehicle is running, and the engine is stopped.

Since the combustion of particulates requires a high temperature of 600 ° C. or higher, energization of the electric heater is
The battery installed in the vehicle with the engine stopped does not make it in time and requires a separate power supply.

Therefore, an attempt has been made to connect a generator directly connected to a diesel engine to an electric heater so as to secure a necessary amount of electricity. This attempt is quite useful.
However, the increase in exhaust gas pressure loss due to particulates trapped in the filter has already put a heavy burden on the diesel engine, and in addition, it is not the best way to impose the burden on the diesel engine on the generator. Absent.

An improvement to this attempt was to connect a pair of filter traps to the exhaust pipe and stop the flow of exhaust gas from the switching valve to the other filter trap while passing the exhaust gas to one filter trap and collect it. The particulates are burned with an electric heater.

According to this improvement plan, before the pressure loss due to the collected particulates is significantly increased, the exhaust gas is caused to flow to another filter trap by the switching valve, and the particulates can be burned. The burden is small.

In this improvement plan, the heat of the electric heater is hard to be transmitted to the filter, and the influence of the outside temperature causes
It has the drawback that the temperature rise in the filter cannot be expected quickly and the particulate combustion efficiency is poor.

Therefore, an object of the present invention is to capture environmentally harmful fine particles contained in exhaust gas with a filter, and
It is an object of the present invention to provide an exhaust gas purifying apparatus capable of efficiently regenerating a filter, and particularly having a function of efficiently burning particulates contained in exhaust gas of a diesel engine or the like.

[0010]

As a result of earnest research in view of the above problems, the present inventor has found that a plurality of foam type filter units coated with a particulate combustion catalyst are connected through a switching valve and collected in a filter. The present invention has been completed by discovering that the particulates can be effectively burned and removed by causing a part of the exhaust gas to flow through a switching valve when the particulates thus burned are burned by an electric heater.

That is, the first exhaust gas purifying apparatus of the present invention has a plurality of filter units connected to the exhaust gas pipe via a switching valve, and each filter unit is one or more filters coated with a catalyst. And an electric heater, wherein the catalyst is a porous inorganic oxide having the general formula Ln
_1-X A _X MO ₃ (however, Ln is La, Pr, Nd, 1 or two or more elements selected from Sm and Gd, A is one or selected from Li, Na, K and Cs 2 or more elements, M is V, C
It carries one or more elements selected from r, Mn, Fe, Co, Ni and Ti, and catalytically active species composed of fine powder of complex oxide represented by 0 ≦ x ≦ 1) The particulate matter collected on the filter is burned by the heating of the electric heater, whereby the filter is regenerated.

A second exhaust gas purifying apparatus of the present invention is
It has a plurality of filter units connected to the exhaust gas pipe via a switching valve, each filter unit has one or more filters coated with a catalyst, and an electric heater, the catalyst is porous General formula for inorganic oxides Ln ₂ CuO
₄ (However, Ln is selected from La, Pr, Nd, Sm and Gd 1
Or a combination of two or more elements), a catalytically active species consisting of a fine powder of a complex oxide represented by the following is supported, and the particulates collected on the filter are burned by the heating of the electric heater. It is characterized in that the filter is regenerated.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a sectional view of an exhaust gas purifying apparatus according to an embodiment of the present invention. In the exhaust gas purifying apparatus of the present invention, a plurality of filter units 3 are connected in the middle of the exhaust gas pipe 1 via the switching valve 2. Each filter unit has one or more filters 4 and the same number of electric heaters 5 as the filters.

The filter 4 has a hollow cylindrical shape. The electric heater 5 is installed in the hollow portion of the cylindrical filter 4. The exhaust gas first enters the hollow portion of the filter 4, passes through the filter 4, and exits the outer peripheral portion of the cylinder.

As the filter 4 installed in the exhaust gas purifying device, a porous one having excellent heat resistance, thermal shock resistance and the like is used, but a porous material made of ceramic or metal which has been proposed as an exhaust gas converter up to now. Quality filters can be used. Further, it may be a filter in which a casing is filled with a porous pellet or granular material, or a filter in which a casing is filled with a heat-resistant fibrous material. Considering durability and the like, a ceramic filter is preferably used, and a foam-type or honeycomb-type filter can be preferably used. Considering the ease of manufacturing and the like, it is preferable to use a foam filter made of ceramic.

As the ceramics forming the filter, alumina, silica, zirconia, titania and their composite inorganic oxides, and mullite, cordierite and other composite inorganic oxides can be used. Preferred ceramic materials include cordierite, mullite, alumina and composites thereof.

The porosity of the filter is 40 in order to keep the pressure loss when the exhaust gas passes through the filter within an allowable range.
~ 90%. If the porosity is less than 40%, it becomes difficult for exhaust gas to pass through the filter, and if it exceeds 90%, the strength of the filter decreases. The density is determined by the material of the filter and the porosity, but for a diesel engine, it is generally preferably 0.3 to 0.7 g / ml. If the density is less than 0.3 g / ml, the strength is too low to be practical, and the collection efficiency of carbon fine particles becomes low. On the other hand, when it exceeds 0.7 g / ml, the flow of the exhaust gas is not smooth and the pressure loss is high.

In the purifying apparatus of the present invention, it is particularly preferable to use a foam type filter. In this case, it is preferable that the porosity of the filter is 60 to 90% and the density is 0.3 to 0.6 g / ml. The pore diameter is preferably 250 to 900 μm. If the pore size exceeds 900 μm, the contact area with the exhaust gas becomes too small and the collection rate decreases. The arithmetic mean of the preferable pore sizes is 250 to 500 μm.

In the purifying apparatus of the present invention, a high-density thin layer can be provided on the outlet side of the filter (the outer peripheral surface of the cylindrical filter) to enhance the trapping function. The thickness of this thin layer is 5 to 2000 μm. Preferred thickness is 10-50μ
m. The thin layer preferably has a porosity of 40 to 70% and a density of 0.4 to 0.8 g / ml. The pore size is 3 ~
The thickness is preferably 80 μm. The preferred arithmetic mean of pore size is 2
It is set to 0 to 30 μm. The material of this thin layer has the same selection range as the filter body, but it is preferable to use the same material as the filter body.

In the present invention, the inner and outer peripheral surfaces of the ceramic filter made of the above-mentioned materials are coated with a catalyst to promote the combustion and removal of particulates. When a thin layer is provided on the outlet side of the filter, the surface of the thin layer can be coated with a catalyst. The catalyst comprises a porous inorganic oxide having a large surface area and a catalytically active species supported on the inorganic oxide. Examples of the porous inorganic oxide include silica, alumina, titania, zirconia, titania-alumina, silica-alumina, zirconia-alumina, silica-titania, silica-zirconia, titania-zirconia and the like. Of these, alumina-based ceramics (alumina or composite ceramics of alumina and other oxides), zirconia,
It is preferable to use a ceramic material such as titania or titania-zirconia.

As the catalytically active species supported on the above-mentioned porous inorganic oxide, Ln _1-X A _X MO ₃ or Ln ₂ CuO can be used.
₄ (However, Ln is selected from La, Pr, Nd, Sm and Gd 1
One or more elements, A is one or more elements selected from Li, Na, K and Cs, M is V, Cr, Mn, Fe, C
1 or 2 or more elements selected from o, Ni and Ti, 0
Fine powder of the complex oxide catalyst represented by ≦ x ≦ 1) is used.

In the composite oxide represented by the above general formula, the composition ratio of oxygen is usually represented by 3. However, it may actually be slightly different depending on the constituent metals and use conditions.
_{1-X A X MO 3 +} a (a <0) or _{Ln 2 CuO 4 + a (a} <0) oxygen-deficient or _{Ln 1-X A X MO 3} + a (a> 0) , or Ln ₂ CuO
It may be an oxygen-excess type of _{4 + a} (a> 0).

Generally, a complex oxide represented by Ln _1-X A _X MO ₃ , particularly an oxide having a perovskite structure, is known as a catalyst for catalytically decomposing NOx at high temperature, but in the presence of oxygen. Since the NOx decomposing property deteriorates, it has previously been considered impossible to decompose NOx in an oxidizing atmosphere in exhaust gas from diesel engines. However, the present inventors have found that Ln _1-X A _X MO ₃ or
If the complex oxide represented by Ln ₂ CuO ₄ is used as the catalytically active species, the particulates can be efficiently removed.

The above-mentioned composite oxide catalyst is prepared by mixing oxides, hydroxides, carbonates, nitrates, sulfates, acetates, oxalates, chlorides, etc. of the respective metal elements at a desired ratio and finally mixing them. To
It can be prepared by firing at 850 ° C. or higher for 5 hours or longer. Examples of the mixing method of the above substances include a method of mixing each substance in a solid state, a method of evaporating a mixed aqueous solution of each metal salt to dryness, and a mixed aqueous solution of each metal salt with an aqueous alkaline solution such as aqueous ammonia. A so-called coprecipitation method for decomposition can be used.

Of the above general formulas, a perovskite type structure may be obtained when prepared by the above-mentioned preparation method so as to have the composition of the formula Ln _1-X A _X MO ₃ . In addition, M
When V alone is used as the above, a perovskite structure is not obtained. When Ln / M = 1/1, M
When Cr, Mn, Fe, Co, Ni or Ti is used as the material, it has a perovskite structure.

In La _1-X Sr _X MO ₃ , when Co is used as M, a perovskite structure is taken if 0 ≦ x ≦ 0.6, and a perovskite structure crystal is taken if 0.6 ≦ x <1. It becomes a mixture with a crystal having a brown-milarite-like structure, and when x = 1, it has a brown-milarite-like structure. In La _1-X Sr _X MO ₃ , M as M
When n and Fe are used, in the whole range of x, that is, 0 ≦ x ≦
Although it has a perovskite structure at 1, when x approaches 1, it may have a structure similar to brown-milarite depending on the oxygen composition.

The composite oxide catalyst thus prepared is used after being made into a fine powder.

As the method of supporting the above-mentioned catalyst, there are a method of directly supporting it on a porous filter by a known impregnation method, a coprecipitation method, a kneading method, a sol-gel method and the like, and a method of supporting it on a porous surface. There is a method of supporting a catalytically active species on a ceramic layer having a large surface area.

In the impregnation method, the catalyst is supported by immersing the heat resistant porous filter in an aqueous solution of metal carbonate, nitrate, acetate, chloride, hydroxide or the like which forms the catalyst. A method of immersing the filter in a solution of a compound containing a plurality of base metal-based metals such as alkali ferrocyanide to impregnate the catalyst is also possible. After the immersion, it is preferable to dry at about 70 ° C. and raise the temperature step by step at 100 to 600 ° C. for firing. Firing is an oxygen atmosphere,
It is preferably carried out under a nitrogen atmosphere or under a flow of hydrogen gas. When performing under a nitrogen atmosphere or under a flow of hydrogen gas, the oxidation treatment is performed last.

As the precipitation method, there is a method in which a catalyst-supported ceramic is prepared by a coprecipitation method using an aqueous solution of a nitrate of a metal element or the like which constitutes ceramics and an aqueous solution of a metal salt serving as a catalyst.

When the catalytically active species are directly supported on the porous filter, the amount of the catalytically active species supported is preferably 5 to 15% by weight based on 100% by weight of the filter.

When a porous ceramic layer of titania or the like is formed on the surface of the filter that is the base material of the filter and the catalyst is supported on this porous ceramic layer, the porous ceramic layer that becomes the catalyst supporting layer is formed. Layers 2 to 2 of the filter
It is preferably 0% by weight. More preferably 5 to 15
Weight% Further, the catalytically active species is preferably set so as to be 5 to 15% by weight of the above filter.

In the above-mentioned indirect loading method, the carrier layer can be formed on the filter by a wash coat method, a sol-gel method or the like. The wash coating method is a method of forming a carrier layer on the filter by immersing the filter in the slurry of the above-mentioned porous carrier and drying it. In the case of supporting the catalyst by this method, (a) first, a porous carrier layer is formed on the filter by a wash coating method, and then the catalyst is supported on the carrier layer by a known impregnation method or precipitation method, (B) A method of forming a porous layer supporting the catalyst on the filter by wash-coating with a suspension of the ceramic powder supporting the catalytic active species in advance can be adopted.

There are the following two methods for supporting the catalyst by the sol-gel method. The first method is to hydrolyze an organic salt (for example, an alkoxide) of a metal forming the ceramics for the carrier layer, coat the obtained sol on a filter, and form a film of colloidal particles by contact with water vapor or the like. This is followed by drying and firing to form a catalyst carrier layer on the filter, and finally carrying a catalytically active species.
For example, as the ceramics for the carrier layer, titania (TiO 2
₂ ) is used to support a catalytically active species, first, an alkoxide of Ti (for example, Ti (o-isoC ₃ H
₇ ) In the alcohol solution of ₄ ), add CH ₃ COOH, HNO
A coating solution containing an acid such as ₃ , HCl is generated.
The filter is dipped in this coating solution, pulled up, and then reacted with water vapor or water to cause gelation. Next, when the filter is dried and fired, a titania film is formed on the pore surface of the filter. Next, an aqueous solution of a carbonate, a nitrate, an acetate, a hydroxide or the like of the catalytically active species is impregnated, dried again and calcined to carry the catalyst.

The second method is a method of simultaneously coating the carrier layer ceramics and the catalytically active species on the filter. For example, first add C to an alcohol solution of Ti alkoxide.
An acid such as H ₃ COOH, HNO ₃ , or HCl and an aqueous solution of a salt of a catalytically active metal species are added to form a coating liquid. Then, after immersing the filter in the coating liquid, it is reacted with water vapor or water to form a sol, and further gelation. After that, the filter is dried and baked,
A coating layer made of titania supporting a catalyst is formed.

As the salt of the catalytically active metal species, any kind such as carbonate, nitrate, acetate, hydroxide and the like can be used as long as it is soluble in water. In addition, it is preferable to add a dispersant such as ethylene glycol for the purpose of uniformly dispersing the salt of the catalyst metal in the alcohol solution of the alkoxide.

In the sol-gel method, the acid is added as a catalyst for the hydrolysis reaction during gelation. However, the hydrolysis reaction can be promoted by adding an alkali instead of the acid.

Although titania has been described as an example of the ceramics for the carrier layer in the above, other ceramics can be similarly loaded by the sol-gel method. For example, when a catalytically active species is supported on alumina, an alkoxide of aluminum is used, and the same method as in the case of titania described above is performed. The same applies when other porous carriers are used.

According to the sol-gel method, it is possible to support the catalyst in the filter very uniformly, and
The catalytic activity is enhanced and the exhaust gas purification ability is improved.

Electric heater 5 used in the purifying apparatus of the present invention
Should be one that is not easily oxidized at high temperature. Specific examples include a corrosion-resistant nichrome wire, a Fe-Cr-Al alloy-based high-temperature heating element, and a kanthal wire. The arrangement shape of the heater is not particularly limited as long as the filter can be uniformly and efficiently heated.

Next, the operation will be described. As shown in FIG. 1, when a gas containing particulates or the like flows from the upstream side 1 to the downstream side of the exhaust passage, the gas passes through the switching valve 2 and passes through the filter 4 from the inner peripheral side to the outer peripheral side. During that time, the particles are captured by the filter 4. The pressure loss increases as the particles gradually accumulate. When the back pressure of the exhaust gas reaches a predetermined value, the switching valve 2 is switched to reduce the flow rate of the exhaust gas, the filter 4 is uniformly heated by the electric heater 5, and the particulates collected in the filter 4 are burned. Is done. The back pressure of the exhaust gas at which the regeneration process starts is 500 to 1500 mmAq. The exhaust gas flow rate during the regeneration process depends on the size of the purification device, but it is 2
It is preferably from 0 to 150 liters / minute. The operating time of the electric heater is preferably 2 to 20 minutes. At this time, the temperature of the filter 4 is preferably 300 to 600 ° C, and more preferably 400 to 500 ° C.

As described above, by combining the electric heater and the filter with catalyst, the temperature does not rise rapidly during regeneration, and stable particulate combustion removal can be performed. At the same time, combustion of fine particles is promoted by the catalyst carried on the filter 4, and hydrocarbons, nitrogen oxides, and carbon monoxide are also purified.

In FIG. 1, the exhaust passage may be arranged in the opposite direction to that of the above embodiment. In that case, the gas passes through the filter 4 from the outer peripheral side to the inner peripheral side, but the effects are the same as those described above. At this time, the electric heater 5 is arranged only on the inner and outer peripheral surfaces or the outer peripheral surface of the cylindrical filter.

Although the exhaust gas purifying apparatus of the present invention has been described with reference to FIG. 1 as an example, the present invention is not limited to this and can take various forms without departing from the gist of the present invention. For example, as shown in FIG. 2, the filter unit 3 may be composed of two filters.

The present invention will be described in more detail by the following examples. Example 1 La (NO ₃ ) ₃ , KNO ₃ , and Co (NO ₃ ) ₂ were weighed and mixed to have a composition of La _0.9 K _0.1 CoO ₃ , and the resulting aqueous solution was evaporated to dryness, 1 at 850 ° C
Firing was performed for 0 hours to prepare a composite oxide having the above composition. A slurry comprising fine powder of this composite oxide and titania sol was prepared, and a cylindrical foam type cordierite filter (volume 1300 cm ³ , density 0.42 g / ml,
It was coated with a porosity of 80%), dried, and then fired stepwise to 700 ° C. to prepare a filter having a catalyst layer on the inner and outer peripheral surfaces. The coating amount of the catalytically active species was 10% by weight, and the coating amount of titania was 10%, based on the weight of the filter.
% By weight.

Next, as shown in FIG. 2, the two filters are set as a set in the filter unit, and 2200c is set.
It was connected via a switching valve in the middle of the exhaust pipe of the diesel engine of c. When the engine speed is 2000 rpm, the particulates are collected and the pressure loss is 1000 mmA.
When it becomes q, the regeneration process starts. The amount of exhaust gas entering the filter during regeneration was 30 liters / minute (200 ° C.). The amount of electricity supplied to the electric heater during the regeneration step was 700W.
The regeneration step time was 5 minutes. In this way, the purification device was able to perform stable collection and regeneration.

Example 2 As in Example 1, La (NO ₃ ) ₃ , KNO ₃ , Co
(NO ₃ ) ₂ was used to prepare a composite oxide having a composition of La _0.8 K _0.2 CoO _{3 and} the same was coated on a similar filter.
A filter having a catalyst layer was prepared. The particulates were removed under the same conditions as in Example 1, and stable collection and regeneration could be performed.

Example 3 As in Example 1, La (NO ₃ ) ₃ , NaNO ₃ , C
A composite oxide having a composition of La _0.9 Na _0.1 CoO ₃ was prepared using o (NO ₃ ) ₂ and coated on the same filter to prepare a filter having a catalyst layer. The particulates were removed under the same conditions as in Example 1, and stable collection and regeneration could be performed.

Example 4 In the same manner as in Example 1, La (NO ₃ ) ₃ and copper acetate were used to prepare a composite oxide having a composition of La ₂ CuO ₄ , which was coated on the same filter to form a catalyst layer. A filter having The particulates were removed under the same conditions as in Example 1, and stable collection and regeneration could be performed.

[0050]

As described above, the exhaust gas purifying apparatus of the present invention uses an electric heater in combination with a filter with a catalyst to incinerate and remove combustible fine particles such as particulates captured by the filter, It can be efficiently regenerated without clogging.

The purifying apparatus of the present invention can be mainly used for exhaust gas treatment of diesel engines.

[Brief description of drawings]

FIG. 1 is a sectional view showing an exhaust gas purifying apparatus according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of a filter and an electric heater used in the exhaust gas purifying apparatus of the present invention.

[Explanation of symbols]

 1 ... Exhaust gas pipe 2 ... Switching valve 3 ... Filter unit 4 ... Filter 5 ... Electrothermal heater

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display area F01N 3/02 E

Claims (6)

[Claims] 1. An exhaust gas purifying apparatus for collecting and combusting and removing particulates contained in exhaust gas discharged from a diesel engine, comprising a plurality of filter units connected to an exhaust gas pipe via a switching valve, Each filter unit has one or more filters coated with a catalyst and an electric heater, and the catalyst is a porous inorganic oxide having the general formula Ln _1-X A _X MO ₃ (where Ln is L
a, Pr, Nd, Sm and one or more elements selected from Gd, A is one or more elements selected from Li, Na, K and Cs, M is V, Cr, One or more elements selected from Mn, Fe, Co, Ni and Ti, and catalytically active species composed of fine powder of a complex oxide represented by 0 ≦ x ≦ 1) are supported. An exhaust gas purifying apparatus characterized in that particulates collected on a filter are burned by the heating of the electric heater, whereby the filter is regenerated. 2. An exhaust gas purifying apparatus for collecting and combusting and removing particulates contained in exhaust gas discharged from a diesel engine, comprising a plurality of filter units connected to an exhaust gas pipe via a switching valve, Each filter unit has one or more filters coated with a catalyst and an electric heater, and the catalyst is a porous inorganic oxide of the general formula Ln ₂ CuO ₄ (where Ln is La, Pr,
Nd, Sm, and Gd are one or more elements selected from), and catalytically active species consisting of fine powder of a complex oxide represented by the above are supported, and the particulates collected on the filter are An exhaust gas purifying apparatus, which burns when heated by the electric heater and thus regenerates a filter. 3. The exhaust gas purifying apparatus according to claim 1, wherein the filter is a cylindrical body, and a flow direction of exhaust gas in the filter is a direction from an inner peripheral surface of the cylindrical body toward an outer peripheral surface thereof. An exhaust gas purifying device characterized by being present. 4. The exhaust gas purifying apparatus according to claim 1, wherein when the filter is regenerated, the exhaust gas is caused to flow at a flow rate of 20 to 150 liters / minute by controlling the switching valve. Exhaust gas purification device. 5. The exhaust gas purifying apparatus according to any one of claims 1 to 4, wherein the filter is a porous foam-type ceramic material. 6. The exhaust gas purifying apparatus according to any one of claims 1 to 5, wherein the outer peripheral surface on the outlet side of the filter has a fine structure of a thin layer.