JP2003314242A - Ceramic filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-priced and reliable ceramic filter to be used for a long time by preventing generation of a crack to be caused by local combustion of PM (particulate matters) deposited in the filter. <P>SOLUTION: On an exhaust-emission inlet surface (gas inlet surface) of the exhaust gas purification filter, holes (through holes) are made for communication between the inlet surface and the outlet surface so as to let out purified exhaust gas. The area of the through holes on the gas-inlet surface is 1-10% of the gas-inlet surface. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic filter for purifying exhaust gas from diesel engines used in automobiles, trains, power generation equipment, ships and the like.

[0002]

2. Description of the Related Art Particulate matter containing carbon as a main component (Particulate) is contained in exhaust gas of a diesel engine.
Matter, hereinafter abbreviated as PM) is contained in a large amount. Since the emission of PM into the atmosphere causes environmental pollution, a technique for incorporating a ceramic filter for trapping PM into the exhaust system of a diesel engine is being studied.

Most of the ceramic filters have a cylindrical shape having a large number of honeycomb walls along the exhaust gas flow direction, and among them, the so-called wall flow type has become the mainstream. . The wall-flow type is one in which one end of the gas inflow side or the outflow side is alternately sealed with respect to a large number of cells divided by the honeycomb wall, and the exhaust gas has pores formed in the honeycomb wall. It has a structure that allows it to pass therethrough, and exhibits a function of trapping PM on the honeycomb wall when exhaust gas passes through the pores.

However, in actual use, it is difficult for the ceramic filter to permanently collect PM due to an increase in exhaust resistance, and it is necessary to regenerate the PM once it has been collected. Techniques such as a continuous regeneration system using an oxidation catalyst and a heating regeneration system using a heater or a burner are known.

[0005]

In the conventional ceramic filter, the high temperature (> 600 ° C.) exhaust gas generated during the above-mentioned regeneration treatment or when the diesel engine is operated under a high load condition in actual use is rapidly exposed to the exhaust gas. The PM accumulated in the ceramic filter is locally burned by being exposed to the heat, causing a large temperature distribution in the ceramic filter, and as a result, the ceramic filter itself often breaks. .

For the purpose of solving the above problem, Japanese Patent Laid-Open No. 3-121213 proposes a method of reducing the temperature distribution by dividing the filter into a plurality of parts along the filter axis. However, along with the division, if the division is separately manufactured from the beginning, the number of products to be manufactured is increased, and if the division is performed in the post-processing, a division processing step is newly required. In addition, a process of finally connecting the divided single bodies with a sealing material is additionally required, which causes a problem of increased manufacturing cost.

[0007] Further, Japanese Utility Model No. 2548279 discloses a filter provided with a central hole extending over the entire length and a slit which communicates with the central hole and which is open on the outer peripheral surface and extends over the entire length. It is difficult to sufficiently prevent the temperature difference from increasing in the filter because the size of the pores is unclear, and there is a problem that the mechanical strength of the filter decreases due to the formation of slits, which is not satisfactory in practice. .

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances of the prior art, and an object of the present invention is to eliminate unnecessary use in a ceramic filter used for purifying exhaust gas of a diesel engine. (EN) A highly reliable ceramic filter that can be used for a long period of time by preventing the manufacturing cost from unnecessarily increasing as a result of the manufacturing process, and by preventing the PM accumulated in the filter from cracking due to local combustion. Especially.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a ceramic filter for purifying exhaust gas, wherein the surface of the ceramic filter into which the exhaust gas flows (gas inflow surface) is cleaned and flows out. It has a hole (through hole) connecting with (gas outflow surface), and the area of the through hole in the gas inflow surface is 1 to 10% of the area of the gas inflow surface. It is a ceramic filter.

Further, the present invention is the above ceramic filter, wherein the ceramic filter is made of silicon carbide or cordierite.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The ceramic filter of the present invention is a hole (through hole) for connecting an exhaust gas inflow surface and an outflow surface.
Is to have. In the conventional filter having no through hole, the inside has a characteristic that the temperature becomes higher than the outer peripheral portion due to heat accumulation due to the difference in heat dissipation caused by the structure. Therefore, PM local combustion is also likely to occur inside the high temperature, and the temperature difference between the inside and the outer peripheral portion is further expanded, and large thermal stress is generated, which leads to damage of the filter. In the present invention, a temperature difference in the filter is reduced by forming a through hole having a specific size in advance in a portion where the temperature tends to rise due to the structure, and as a result, cracking due to PM local combustion can be prevented. it can.

In the present invention, the cross-sectional area of the through hole at the gas inflow surface is 1 to the area of the gas inflow surface.
It is preferably 10% (the total of the cross-sectional area of the through hole and the area of the gas inflow surface is 100%). If the through-hole area is less than 1% of the inflow area, it is too small to remove the high temperature portion generated in the structure, and the effect of the invention is not exhibited.
When the content exceeds%, a gas impermeable material such as ceramic fiber or metal is filled and used as a sealing material in the through hole, so that a problem of exhaust resistance occurs due to an increase in gas impermeable region.

The through hole is not formed by post-processing,
By preparing a dedicated mold, through-hole formation can be handled at the same time in the normal molding process, so the manufacturing efficiency is the same as the conventional filter without a through-hole filter, and there is no additional step such as bonding of separate units. Manufacturing cost increase can be suppressed. As the molding method, an extrusion molding method is generally used, but other molding methods such as a press molding method and a casting molding method can be used.

The outer shape and the shape of the through hole are preferably circular, that is, cylindrical. Further, it is more preferable that the outer shape and the through hole are concentric. This is because the thermal stress is concentrated on the corners and cracks are likely to occur in the square shape, and the shape in which the outer shape and the through hole are not concentric may reduce the mechanical strength in the vicinity of the outer shape and the through hole wall. .

Further, when the outer peripheral wall thickness and the through hole wall thickness of the ceramic filter are at least greater than the wall constituting the cell, during the manufacturing process or during the handling of the ceramic filter after commercialization, or the product. Since it is possible to prevent abnormalities such as crushing of cells in actual use, it is preferable.

In the present invention, regarding the material constituting the ceramic filter, the high temperature (> 60) generated when the diesel engine is operated under a high load condition in actual use.
In consideration of being rapidly exposed to exhaust gas (0 ° C.), any ceramic may be used as long as it has a heat resistance of about 1000 ° C. or higher. , Cordierite is preferably selected.

First, silicon carbide has a high melting point of 2000 ° C. or higher, and the temperature reached by PM combustion (900 to 1000).
It is stable even at (° C). Further, since silicon carbide has conductivity, it is possible to use a heating method by self-energization for the regeneration treatment.

Cordierite is stable even at the temperature reached by PM combustion and has a thermal expansion coefficient of 1 pp.
It is possible to effectively reduce the thermal stress caused by a temperature difference as small as m / K, and to more reliably achieve the object of the present invention.

[0019]

The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited thereto.

(Example 1) 1 part of methylcellulose was added to 100 parts by mass of a ceramic raw material consisting of 60 parts by mass of silicon carbide, 32 parts by mass of silicon nitride and 8 parts by mass of carbon black.
5 parts by mass was added, and the mixture was dry-mixed for 10 minutes with a Henschel mixer, and further 30 parts by mass of water was added and kneaded. The obtained kneaded product is extrusion-molded by an extrusion molding machine using a mold designed so that the cross-sectional shape of the product becomes the shape shown in Fig. 1, cut into a desired length, and then heated with hot air at 80 ° C. It was dried for 30 minutes to obtain a structure.

Thereafter, 90 parts by mass of silicon carbide and 7 parts of metallic silicon
100 parts by weight of a ceramic raw material composed of 3 parts by weight of carbon black and 2 parts by weight of methyl cellulose, 0.8 parts by weight of a cationic dispersant, and 25 parts by weight of water were used as a sealing material for a wall flow type ceramic filter. So that the mouths of the cells on both end faces of the structure were filled. After that, it is heated to 400 ° C. to remove organic components, and fired at 2200 ° C. by the electric current sintering method to obtain an outer diameter of 150 m.
A cylindrical ceramic filter having m and a length of 140 mm (cell density 200 cpi ² ; the number per 1 inch square) was obtained. The through hole formed on the central axis has a diameter of 45.
It had a cylindrical shape of mm.

Next, a mixed powder consisting of silver powder, silicon powder, nickel oxide powder and silicon carbide powder was used for 1100 under vacuum condition.
After baking at 0 ° C., electrodes for conductive heating were formed on both end faces of the ceramic filter and side faces of each 10 mm from the end faces. After fixing a metal electrode ring to which lead wires were connected on this electrode, alumina silicate fibers were filled in the through holes, and further an alumina silicate fiber mat was wound around the ceramic filter and set in a metal can. At this time, the wiring connected to the above-mentioned electrode ring is taken out of the can body and connected to a 24V battery power source.
It was subjected to a PM local combustion test described below.

Example 2 A Henschel mixer was prepared by adding 25 parts by mass of carbon powder and 10 parts by mass of methyl cellulose to 100 parts by mass of a ceramic raw material composed of 44 parts by mass of aluminum hydroxide, 37 parts by mass of talc, and 19 parts by mass of fused silica. Were dry mixed for 10 minutes, and 30 parts by mass of water was further added and kneaded. Example 1 using the obtained kneaded product
After being molded and dried by the method described above, it was baked at 1400 ° C. for 20 hours to obtain a cordierite structure.

Further, 44 parts by mass of aluminum hydroxide, 37 parts by mass of talc, 19 parts by mass of fused silica and 50 parts by mass of water were kneaded in the same manner as in Example 1, and 14
Outer circumference of 150m by firing at 00 ℃ for 5 hours
A cylindrical ceramic filter having a length of m and a length of 140 mm (cell density of 200 cpi ² ) was obtained. The through hole formed on the central axis had a cylindrical shape with a diameter of 45 mm. An alumina silicate fiber mat was wrapped around this ceramic filter and set in a metal can body, and subjected to a PM local combustion test in the same manner as in Example 1.

Comparative Example 1 A ceramic filter having the shape shown in FIG. 2 was prepared through the same operation as in Example 1 except that a different mold was used in the extrusion process, and was used as a comparative example.

Comparative Example 2 A ceramic filter having the shape shown in FIG. 2 was prepared through the same operation as in Example 2 except that a different mold was used in the extrusion process, and used as a comparative example.

(PM Local Combustion Test) The metal can bodies set with the filters of Examples 1 and 2 and Comparative Examples 1 and 2 were connected to an exhaust line of a diesel engine having a displacement of 1500 cc, and the engine was rotated at 1500 rpm and a load was applied. Continuous operation under the condition of 20%, 15g in the ceramic filter
Collected PM.

For Example 1 and Comparative Example 1, 24V
The battery power source was switched on, an electric current was passed through the filter itself, which was conductive, and the Joule heat generated raised the ceramic filter temperature to around 600 ° C. to induce PM local combustion. On the other hand, in Example 2 and Comparative Example 2, the engine speed and load were 3500 rp, respectively.
m, 100% and the exhaust gas temperature was raised to around 600 ° C. to induce PM local combustion. The above operation was repeated 10 times, and the presence or absence of abnormalities such as cracks was visually observed each time.

As a result, in Comparative Example 1, large cracks were generated after the first test, and in Comparative Example 2, small cracks were observed after the second test. On the other hand, in each of Examples 1 and 2 according to the present invention, no crack was observed even after repeating 10 times, and high reliability was obtained.

[0030]

Since the ceramic filter of the present invention has a simple cross-sectional shape having only a through hole of a specific size, it can be manufactured at a low cost because it can be manufactured without unnecessary steps. In addition to having the characteristics of being producible, the structure described above can prevent the generation of cracks due to the local combustion of PM deposited in the ceramic filter, and as a result, it can withstand long-term use and is highly reliable. It has features and is very useful in industry.

[Brief description of drawings]

FIG. 1 is a sectional view of a ceramic filter according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a ceramic filter according to a comparative example.

[Explanation of symbols]

1 ... Sealed cell 2 ... Open cell 3 ... Through hole

Claims (3)

[Claims] 1. A ceramic filter for purifying exhaust gas, wherein a surface (gas inflow surface) of the ceramic filter where the exhaust gas flows in is connected to a surface (gas outflow surface) where the exhaust gas is purified and flows out. A ceramic filter having a hole (through hole) which is formed, and the area of the through hole on the gas inflow surface is 1 to 10% of the area of the gas inflow surface. 2. The ceramic filter according to claim 1, wherein the ceramic filter is made of silicon carbide. 3. The ceramic filter according to claim 1, wherein the ceramic filter is made of cordierite.