JPH05115722A - Exhaust gas filter - Google Patents

Abstract

PURPOSE:To enhance the heat resistance and crystallization temp. of an exhaust gas filter for a diesel engine vehicle and to extend the cycle life thereof. CONSTITUTION:A ceramic fiber based on aluminum oxide and silicon oxide is bonded by an inorg. binder based on aluminum oxide and silicon oxide and having an m.p. lower than that of the ceramic fiber to form a honeycomb structure. Cells 1a, 1b are formed using a material wherein a crystal nucleus forming agent is added to a ceramic fiber and this material is wound to constitute an exhaust gas filter. The generation of a white powder from the exhaust gas filter can be suppressed and the heat resistance of the fiber is enhanced to a large extent. As result, the exhaust gas filter can be baked at high temp. and the mechanical strength and heat shock resistance of the exhaust gas filter are enhanced and the life thereof can be extended.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to fine particles (particulates) contained in exhaust gas of a diesel engine or the like.
It relates to an exhaust gas filter for removing.

[0002]

2. Description of the Related Art In recent years, air pollution due to NOx and suspended particulate matter (hereinafter abbreviated as fine particles) in metropolitan areas has advanced. Of these, the fine particles floating in the atmosphere
Black smoke emitted from diesel vehicles is 2
It is said to occupy 0 to 30%. Moreover, it contains mutagenic and carcinogenic components such as polycyclic aromatic hydrocarbons. Therefore, as the most effective measure against the black smoke, there is an exhaust gas purifying apparatus that traps particles contained in the exhaust gas in the exhaust system and then performs self-regeneration. This exhaust gas purification device is provided in the exhaust system, and mainly comprises a filter for capturing fine particles in exhaust gas (hereinafter, abbreviated as exhaust gas filter) and a regenerating device for burning the captured fine particles. The regeneration method includes, for example, an electric heater method and a burner method, but each regeneration method has advantages and disadvantages, and a decisive method is not yet available.

The mainstream of conventional exhaust gas filters is a ceramic monolithic wall flow type particulate matter filter (hereinafter referred to as a ceramic monolithic type filter) as disclosed in US Pat. No. 4,364,761. there were.

As shown in FIG. 1, this type of ceramic monolithic filter has a honeycomb structure having cells 1a and 1b arranged in parallel to the length direction, and among two adjacent cells 1a and 1b. The cell 1b has a structure in which the inlet side is filled with the inflow side plug 2 and the cell 1a is filled with the outflow side plug 3 in the outlet side. Therefore, the exhaust gas containing fine particles flows into the cell 1a whose outlet side is clogged, passes through the porous wall, and is pushed to the adjacent cell 1b whose inlet side is clogged. At this time, since the fine particles cannot pass through the porous wall, the exhaust gas is filtered when passing through the porous wall, and the fine particles are trapped in the filter.

As the amount of trapped particulates increases, the porous walls become clogged with particulates and the back pressure of the diesel engine exhaust system increases. Therefore, it is necessary to suppress the increase in the load on the engine due to the increase in back pressure by removing the fine particles when the amount of the captured fine particles exceeds a certain amount.

The fine particles consist of a fixed carbon component and a soluble organic component (SOF) which can be dissolved in an organic solvent (dichloromethane is often used), both of which are flammable and may be slightly heated depending on the type of engine and load conditions. Although there is a difference, it will burn if heated to a temperature of about 600 ° C or higher. Therefore, conventionally, a method of regenerating the filter by reburning these fine particles using a heating means such as an electric heater or a burner has been attempted.

A conventional exhaust gas filter uses cordierite, which is a typical low thermal expansion ceramic, so as to withstand the temperature gradient generated during regeneration. However, cordierite ceramic has a drawback that the low back porosity of the diesel engine exhaust system causes a heavy load on the engine due to its low porosity. In addition, there is also a drawback that if the temperature rises too much during heating and regeneration, it will melt. Therefore, Japanese Patent Laid-Open No. 2-4
In the 3022 publication, an attempt was made to prevent an increase in back pressure and melting of the filter by using a plate-like body obtained by binding and sintering silica-alumina fiber with sericite clay as a filter element. ..

[0008]

However, in such a conventional structure, in order to prevent the melting of the filter, silica-alumina clay having a high melting point must be used as the filter material. In Japanese Unexamined Patent Publication (Kokai) No. 2-43022, sericite clay is used and fired and sintered at a temperature of 1250 ° C.

However, 100 silica-alumina fibers are used.
Mullite (3Al ₂ O ₃
・ 2SiO ₂ ) crystals are deposited. As a result, crystal growth of mullite occurs in the fiber, the surface of the fiber becomes brittle, mullite powder is separated from the surface layer by a slight external force such as wind pressure and vibration, and the mechanical strength of the filter decreases. there were. Further, there is a risk that an operator of a vehicle equipped with a filter inhales this powder into the body by inhalation and accumulates it in the body, which may impair health. Further, when the exhaust gas filter is mounted on a diesel turbo engine vehicle, there is a risk that the mullite powder is sucked into the engine by the turbocharger and wears the engine.

The surface layer of the conventional silica-alumina fiber fired at 1000 ° C. or higher is composed of mullite crystals and aluminosilicate glass, and if fired at 1260 ° C. or higher, the aluminosilicate glass is softened, so that it is 1250 ° C. It was necessary to bake below. Therefore, quartz in the sericite clay does not sufficiently dissolve into the liquid phase, and sufficient mechanical strength cannot be obtained after firing, so that there is a problem that the filter is destroyed by thermal stress during heating and regeneration.

The present invention is intended to solve such a problem, and is a crystal of mullite crystals when a plate-like body obtained by binding and sintering silica-alumina fibers with silica-alumina clay is used as a filter element. By suppressing growth,
The object of the present invention is to suppress the exfoliation of mullite powder, to enable the exhaust gas filter to be fired at high temperature, and to increase the mechanical strength of the exhaust gas filter.

[0012]

In order to solve this problem, the present invention relates to a ceramic fiber mainly composed of aluminum oxide and silicon oxide, an inorganic material mainly composed of aluminum oxide and silicon oxide and having a melting point lower than that of the ceramic fiber. An exhaust gas filter mainly comprising a honeycomb structure formed by binding with a binder, wherein a crystal nucleating agent is contained in the ceramic fiber.

Further, Cr ₂ O ₃ is used as a crystal nucleating agent. Further, the content rate of chromium oxide is 1.5 to
It is within the range of 3.5 wt%.

Further, as the ceramic fiber, an aluminosilicate glass or aluminoborosilicate glass, mullite (3Al ₂ O ₃ .2SiO ₂ ) crystal, and a fiber mainly containing a crystal nucleating agent are used.

[0015]

According to this structure, by containing the crystal nucleating agent in the silica-alumina fiber, a large number of crystal nuclei are formed in the crystal nucleation temperature range when heating for heating the exhaust gas filter. The crystal nuclei can suppress the crystal growth of the mullite crystal. In addition, a homogeneous mullite microcrystalline phase is generated on the surface of the exhaust gas filter to prevent softening of the surface of the silica-alumina fiber and improve the heat resistance of the fiber. As a result, the heat resistance and heat shock resistance of the exhaust gas filter are improved, and the cycle life is extended.

[0016]

An embodiment of the present invention will be described below with reference to the drawings.

First, the ceramic fiber used in this example will be described. The exhaust gas filter of the present embodiment is required to have high heat resistance as its constituent material in order to heat and incinerate fine particles. Typical high heat resistant ceramic fibers include alumina fibers, aluminosilicate fibers, aluminoborosilicate fibers, and mullite fibers. Of these, alumina fibers have a large thermal expansion coefficient of alumina themselves, and thus are inferior in thermal shock resistance (strength against a sudden temperature gradient). Mullite fibers are polycrystalline and weak in strength, so they are not suitable as constituent materials for exhaust gas filters. For the above reasons
Aluminosilicate fiber and aluminoborosilicate fiber were selected as constituent materials. Of these, for example, as aluminosilicate fibers, both SiO ₂ and Al ₂ O ₃ are about 50
A fiber having a composition of wt% is commercially available as a heat resistant fiber having a temperature of 1260 ° C.

This heat-resistant 1260 ° C. aluminosilicate fiber shows a large exothermic peak at around 980 ° C. when subjected to differential thermal analysis. This exothermic peak is mullite (3Al ₂
It corresponds to the crystallization temperature of O ₃ · 2SiO ₂ ) crystals. Therefore, when the aluminosilicate fiber is heated, it is in an amorphous state at room temperature, but at the temperature of 950 ° C. or lower, mullite crystal nuclei already start to precipitate, and the crystal growth reaches a peak at around 980 ° C. Mullite crystals are Al ₂ O ₃ : SiO
_Since the ratio of ₂ is 72:28, the amount of precipitation is limited from the aluminosilicate fiber in which Al ₂ O ₃ : SiO _{2 is} approximately 50:50. Therefore, at a temperature of 980 ° C. or higher, the SiO ₂ rich aluminosilicate is Needle-like mullite crystals are dispersed in the glass.

In order to suppress the crystal grain growth of needle-like crystals of mullite, it is sufficient to increase the amount of crystal nuclei produced and to deposit a large amount of fine crystals. Generally, as a means for suppressing the crystal grain growth of crystallized glass, a method of adding a metal oxide serving as a crystal nucleating agent is adopted. As the crystal nucleating agent, a metal oxide such as Cr ₂ O ₃ , TiO ₂ , and ZrO ₂ which is easily precipitated as crystals from a relatively glassy state is used. In this example, C was used as the crystal nucleating agent.
Two types, r ₂ O ₃ and ZrO ₂ , were examined. First, C
Differential thermal analysis was performed on the aluminosilicate fiber containing r ₂ O _{3, the} aluminosilicate fiber containing ZrO ₂ , and the conventional aluminosilicate fiber. As a result, as shown in (Table 1), it was confirmed that the heat generated by the crystallization of the aluminosilicate fiber containing Cr ₂ O ₃ was much smaller than that of the other two types. When the fibers after firing were observed with an electron microscope, precipitation of needle-shaped crystals with an average length of about 0.5 μm was observed in the conventional product and the aluminosilicate fiber containing ZrO ₂ , but the aluminosilicate fiber containing Cr ₂ O ₃ was observed. In, it was confirmed that the crystals were fine crystals having an average length of 0.1 μm or less. From this result, it is found that Cr ₂ O ₃ is the most excellent crystal nucleating agent for the aluminosilicate fiber.

[0020]

[Table 1]

Next, the inorganic binder used in this embodiment will be described. When using the aluminosilicate fiber as a constituent material of the exhaust gas filter as in this example,
Since sufficient strength cannot be obtained with fibers alone, it is necessary to add an inorganic binder. The properties required of this inorganic binder are high heat resistance, a coefficient of thermal expansion close to that of the ceramic fiber alone, and compatibility with the ceramic fiber. Therefore, glass or ceramic containing SiO ₂ , Al ₂ O ₃ is suitable as the inorganic binder. Moreover, in order to obtain high-strength adhesion without breaking the shape of the ceramic fiber, it is necessary to use one having a melting point (or softening point) lower than the melting point (or softening point) of the ceramic fiber. For that purpose, those containing a small amount of alkali metal oxides and alkaline earth metal oxides are desirable. In particular, when the step of aggregating in an aqueous solution is included as in the present embodiment, it is preferable to use clay mineral because the inorganic binder is required to have hydration properties. Typical clay minerals include kaolinite, pyrophyllite, montmorillonite, and sericite. Among them, sericite best satisfies the above conditions. Of course, by improving the cohesiveness, it is possible to use alkali metal oxide (alkaline earth metal oxide) -SiO ₂ -Al ₂ O ₃ -based glass and feldspar minerals such as potassium feldspar and plagioclase. It will be possible. These inorganic binders are dispersed between the ceramic fibers and fired. As the firing temperature, it is necessary to select a temperature higher than the temperature at which the inorganic binder vitrifies (glass softens) and lower than the temperature at which the ceramic fibers melt.

An embodiment of the present invention will be described more specifically below. The structure of the exhaust gas filter of one embodiment of the present invention is shown in FIGS.

20 parts by weight of aluminosilicate fibers chopped to a length of 0.1 to 10 mm with an average fiber diameter of about 3 μm are sufficiently dispersed and suspended in 1000 parts by weight of water. This aluminosilicate fiber has Cr as a crystal nucleating agent.
₂ O ₃ , containing 40 to 60 wt% SiO ₂ and A
_{l 2 O 3 40~60wt%, Cr} 2 O 3 0.01~5wt%
The range of is desirable. If more SiO ₂ is much Al ₂ O ₃ is small, decreases the heat resistance temperature of the filter, Al ₂ O ₃ is more SiO ₂ is the thermal expansion coefficient becomes large and small, the cause causing thermal destruction during thermal regeneration of the filter Become. Also Cr ₂
A proper content of O ₃ is 1.5 to 3.5 wt%,
If it is added in an amount of 3.5% or more, workability becomes poor, and if it is 1.5% or more, it does not function sufficiently as a crystal nucleating agent. On the other hand, as a ceramic raw material powder, 15 parts by weight of sericite clay was mixed with 5 parts of water.
Suspend to 0 parts by weight. The fiber suspension and the suspension of the ceramic raw material powder are mixed with stirring. Next, as an organic binder, 1 part by weight of an emulsion liquid of vinyl acetate-acrylic copolymer was added and sufficiently stirred and mixed, and then a polymer coagulant was added to add aluminosilicate fiber, sericite clay, and an organic binder. Coagulate with each other to form a floc suspension. The flocculated suspension thus obtained is diluted with water to 3000
After diluting to a weight part, a sheet is prepared by papermaking with a normal Fourdrinier paper machine.

On the other hand, crushed aluminosilicate fiber 2
A plug material in which 0 parts by weight and 15 parts by weight of sericite clay are made into a paste with polyvinyl alcohol is prepared.

The above sheet is divided into two parts, one of which is formed into a corrugated shape by a corrugating machine having two gear-shaped rolls, and the crushed aluminosilicate fiber and sericite clay are injected into one end while the plug raw material is injected. An adhesive obtained by kneading with an organic sizing agent is applied to the top of the corrugate, and the other flat sheet is attached. The plug raw material injected at this time becomes the inflow side plug 2 after firing. The corrugated board-shaped molded product obtained here is applied to the corrugated top portion and the other flat sheet is attached. The adhesive is applied to the corrugated top of the corrugated board-shaped molded body obtained here, and the plug raw material is injected into the other end and rolled up into a cylindrical shape to obtain a honeycomb structured molded body. The plug raw material injected at this time becomes the outflow side plug 3 after firing. Here, at the time of winding, since the core is wound up, the center of the molded body becomes hollow when removed from the winding machine. Therefore, the plug raw material is injected into the inflow side. This plug material forms the core 4 after firing.
When this formed body is fired in an electric furnace at 1350 ° C. for 2 hours, organic matters disappear, aluminosilicate fibers are fused by sericite clay, and an exhaust gas filter having a fiber ceramic honeycomb structure is obtained. Since the aluminosilicate fiber of this example does not soften even if fired at 1350 ° C, it can be fired at 1350 ° C at which quartz in the sericite clay is sufficiently dissolved in the liquid phase.

When a conventional exhaust gas filter was cut with a saw and the filter was hit after cutting, a large amount of white powder was generated, but in the exhaust gas filter of the present invention, generation of powder was hardly seen.

The exhaust gas filter of this example was attached to the exhaust gas purifying apparatus shown in FIG. In the exhaust gas purifying apparatus of the present embodiment, the exhaust gas filter 5 is housed in the case 13 while being fixed by a heat insulating cushion material 6 made of a thermally expansive ceramic fiber. When collecting the particulates in the exhaust gas, the exhaust gas discharged from the engine 8 is sent to the exhaust gas filter 5 by switching the valve 10 to the exhaust gas filter 5 side. With the exhaust gas filter 5,
The fine particles in the exhaust gas are removed, and the exhaust gas containing no fine particles is sent to the outside of the vehicle or the turbocharger through the exhaust gas discharge pipe 14. When the amount of accumulated particulates exceeds a certain value, the valve 10 is switched to the bypass 12 side and the exhaust gas filter 5 is regenerated. When the heater 7 is energized to heat the exhaust gas filter 5, fine particles near the inflow port in the exhaust gas filter 5 are ignited. When the particles are ignited, the heater 7
Is stopped, air is sent from the air pump 11, the flame is propagated in the outlet direction, and combustion of the particles is promoted. When the combustion of the fine particles is completed, the valve 10 is switched to the exhaust gas filter 5 side, and the collection of the fine particles is started again.

Using the exhaust gas purifying apparatus of this embodiment, the engine 8 was operated for 2 hours to measure the increase in back pressure when fine particles were deposited. The increase in back pressure was 60 mmHg, which was not different from the conventional product. .. Further, when the exhaust gas filter of the present example and the conventional exhaust gas filter were subjected to 1000 cycles of repeated deposition of fine particles and heat regeneration,
In the conventional exhaust gas filter, melting loss occurs due to excessive temperature rise in 250 cycles, and black smoke is now observed in the exhaust gas. In the exhaust gas filter of this embodiment, black smoke is generated 1
It was not seen even after 000 cycles.

Although an example using aluminosilicate fiber has been described in the present embodiment, the same effect can be obtained by using aluminoborosilicate fiber.

[0030]

As is apparent from the above description of the embodiments, according to the present invention, a ceramic fiber mainly composed of aluminum oxide and silicon oxide is used, which is mainly composed of aluminum oxide and silicon oxide and has a melting point higher than that of the ceramic fiber. Is an exhaust gas filter mainly composed of a honeycomb structure formed by bonding with a low inorganic binder, by configuring the exhaust gas filter by including a crystal nucleating agent in the ceramic fiber, of the white powder from the exhaust gas filter Generation can be suppressed and heat resistance of the fiber is significantly improved. As a result, the exhaust gas filter can be fired at a high temperature, the mechanical strength of the exhaust gas filter is improved, the thermal shock resistance is improved, and the life of the exhaust gas filter can be extended.

[Brief description of drawings]

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

FIG. 2 is a perspective view of the exhaust gas filter.

FIG. 3 is a configuration diagram of an exhaust gas purification apparatus using the same exhaust gas filter.

[Explanation of symbols]

 1a, 1b Cell 2 Inflow side plug 3 Outflow side plug 4 Core 5 Exhaust gas filter 6 Insulation cushion material 7 Heater 8 Engine 9 Exhaust gas introduction pipe 10 Valve 11 Air pump 12 Bypass 13 Case 14 Exhaust gas exhaust pipe

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location F01N 3/02 301 C 7910-3G

Claims (4)

[Claims] 1. An exhaust gas mainly composed of a honeycomb structure formed by bonding ceramic fibers mainly composed of aluminum oxide and silicon oxide, mainly composed of aluminum oxide and silicon oxide, with an inorganic binder having a lower melting point than said ceramic fibers. An exhaust gas filter comprising a crystal nucleating agent in the ceramic fiber. 2. The exhaust gas filter according to claim 1, wherein the crystal nucleating agent is chromium oxide. 3. The content of chromium oxide is 1.5 to 3.5 w.
The exhaust gas filter according to claim 2, wherein the range is t%. 4. The ceramic fiber mainly comprises aluminosilicate glass or aluminoborosilicate glass, mullite (3Al ₂ O ₃ .2SiO ₂ ) crystal, and a crystal nucleating agent. Exhaust gas filter.