CA1200539A

CA1200539A - Cleansing of exhaust gases

Info

Publication number: CA1200539A
Application number: CA000421261A
Authority: CA
Inventors: Enrique Santiago; Wilhelm Wolsing
Original assignee: Zeuna Starker GmbH and Co KG; Engelhard Kali Chemie GmbH
Current assignee: BASF Catalysts Germany GmbH; Arvin Industries Deutschland GmbH
Priority date: 1982-02-17
Filing date: 1983-02-09
Publication date: 1986-02-11
Also published as: AU1085883A; EP0086367A2; AU552796B2; EP0086367A3; DE3205673A1; DE3363221D1; EP0086367B1; JPS58143112A; ATE19539T1; JPS6340248B2

Abstract

Abstract of the Disclosure Purification of the exhaust gases of diesel engines, particularly for motor vehicles, by a device with a plurality of macrocells in a substrate of porous ceramic. The macro-cells preferably are small channels (10,11) which are mutually parallel, in a monolithic ceramic body (8). Some of the channels (10,11) are open upstram and others are open down-stream. The channels (11) which are open upstream are filled with an insert body (13) of compacted metal threads, which support catalytic material. Such a device, connected into the exhaust line of a diesel engine, forms a carbon filter, which is distinguished by high filter action with a relatively lower exhaust-gas counterpressure and a relatively low carbon combustion temperature.

Description

~'Z~ 539 CLEANSING OF EXHAUST GASES
1 Back~round of the Invention This invention relates to the cleansing or purification of exhaust gases, particularly for diesel engines in motor vehicles.
Althou~h the exhaust gases of diesel engines contain car-bon particles, which are recognized as noxious, they otherwise contain only relatively small ounts of carbon monoxide, hydro-carbon compounds, and nitro~en oxide. With known exhaust gas filters ~or diesel engines the primary objective, therefore, has been to intercept the carbon particles and burn them. In so doinq, certain emission limits must be observed as to the hydro-carbons must be observed as to the hydrocarbon component in the exhaust gas. These limits are prescribed by national directive such as those of the Environmental Protection Agency (EPA) o~
the United States of America.
Environmental protective directives specify the allow-able emission of carbon particles for passenger vehicles at a level such as 0.6 g/mi ~grams per mile) for the model year 1982 and at 0.2 g/mi for the model year 1985. The present objective for the year 1990 is to reduce carbon particles by 75~, relative to the total diesel exhaust. The purpose of these directives is to achieve a considerable reduction in environmental pollu-tion. It has been calculated that, for example, in the year 1985, about 8 million motor vehicles will be produced equipped with diesel engines, and that a diesel passenger car without an exhaust gas filter will expel, over a driving distance of 100,000 km., an average of 30 to 40 kg. of carbon particles into the environment.
Technical experts have agreed that the emission ~imits for 1985, mentioned above, can be achieved only by additional filtering of the diesel exhaust gas. It has been ~enerally assumed that suitable filter devices require equipment for regeneration, i.e., the collection and burning of incident car-bon Particles as soon as the exhaust gas temperature has reached the combustion temperature of carbon, which i5 about ~50 to 50~C.

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1 ~ well~knowll ~arbon filter for diesel exhaust gases is of monolithic porous ceramic, formed ~y mutually parallel, thin-walled channels running over its entire length. On their frontal sides, the channels are closed and opened in a checkerboard pattern, and every channel has an open end and a closed end. ~hen such a ceramic filter is used with diesel exhaust qases, the exhaust which enters the channels that are open on the inlet side is forced to flow throu~h the porous channel walls into the neighboring channels, which are o~en on the outlet side. Since about 50~ of the carbon particles have a si~e exceeding 0.5mm, they consequently cannot pass through the channel walls and collect in the channels that are open upstream. The carbon collected in such a filter is not burned off before its ignition tempera-ture is reached, and only then if there is sufficient concen-tration of oxygen in the exhaust gas.
Between the two such regeneration processes,in such a filter, there is an in¢reased loadin~, and associated with this, a not inconsiderable rise in the counterpressure of the exhaust gas. If the load is especially high, the carbon burn~off can be quite intense and can occur with the generation of undesirably high temperatures. Under unfavorable conditions - there is even a risk of exceeding the melting point of the ceramic material. Although a high degree of filterinq,of about 70%,can be achieved with such a filter, with only 30%
of the particles in the exhaust gas actually exhausted -to the outside, the burn~off conditions need improvement. This is in part because of the high counterpressure of the exhaust gas, which even rises gradually from its basic level as the operating time increases, and in part because of the sudden and severe temperature rise following ignition of the carbon.
A more homogeneous burn~off of carbon is achieved by another well-~nown filter device as disclosed in DE-OS 30 24 491, where the exhaust gases are transported through a chamber with a catalyst. Here, the catalyst is a substrate 3~
~3-1 of a thread-like metallic material in a meshed, woven, or fis-sured~form, which is coated with catalytic material consisting of metals, metal alloys, metal oxides. The catalyst causes a catalytic oxidation of the smoke-generating particles in the exhaust gases. Due to the action of the catalyst, the oxida-tion occurs below temperatures above 405C,beginning in fact at tem?erat7~res as low as 200~C. The resulting homogenization of the combustion of the carbon particles in the diesel exhaust gas makes it possible to o~erate the catalytic filter at a relatively low counterpressure of the exhaust gas. The operating temperature of 400C, which is usual with diesel engines, is quite adequate for the combustion process in the catalyst; the only precondition for the catalytic oxidation is an excess of air in the exhaust gas and the presence of hydrocarbons which burn exothermally under the action of the catalyst. The resulting filter efficiency, however, is lower than with the ceramic filter discussed above, namely about ~0 to 50%.

(}~i3~

1 Summary of the Invention By contrast with the prior art t the present invention provides a filter for diesel en~ine exhaust gases which i5 com-parable in filtering act-on with the well-known ceramic filters, 5 but simultaneously achieves a uniform particle combustion with relatively low exhaust-gas counterpressure at a relatively low temperature.
According to the invention, this is achieved by the use of a large number of macrocells in a substrate of porous lOceramic, connected into the flow of the exhaust gas. The macro-cells always have an opening connected with the flow of the exhaust gas, and all or some of them are wholly or partly filled with a gas-permeable catalyst support, having a surface coated with catalytic material.
In accordance with one aspect of the invention, the catalytic material can be a metal, alloy or intermetallic com-pound with an oxidizin~ effect.
In accordance with another aspect of the invention, the gas permeable catalyst support is of metal threads in the 20form of wire or tape and is compacted to an insert correspond-ing to the hollow shape of the macrocells.
In accordance with still another aspect of the inven-tion, the macrocells are formed in a monolithic ceramic body.
The resulting filter combines the advantages of uni-25form catalytic combustion of exhaust gases and the partialoxidation of particles at tem~eratures lower than 450C, with the advantage of filtering even the finest carbon particles in the areas of the walls of the macrocells of a ceramic sub-strate, where the macrocells contain the catalyst. The carbon 30particles that are collected in the pores of the walls and on their surface, and which have ~assed the catalyst without com-bustion, are burned uniformly and at a temperature below 450C, in contrast to the well-known ceramic filter. Evidently, there is a reduction in energy because of the catalytic reaction.

r~ 3~3 1 This result is furthered by the fact that the catalyst support, which is present preferably in the form of metal threads, fills the cavity of the macrocells to the walls, so that the catalytic energy can act directly,or in the form of radiation,on the porous walls of the macrocells, where-the volatile carbon particles have been trapped. The catalytic energy is essentially generated in the oxidation of hydro-carbons, which are present in the exhaust gas either in free form or as deposited solid carbon particles.
Accordin~ to another aspect of the invention, the cells, which are filled with catalyst, are designed as lonqi-tudinally extended channels, open on one side, in a mono-lithic ceramic body. The channels are alternatively open and closed upstream or downstream respectively, more or less accordin~ to standard ceramic filter construction of cellular ceramic discussed above.
However, in contrast with the well-known ceramic filters, the channel cross sections in the present invention are con-siderably larger. Instead of 50 to 200 open channels per square inch. the ceramic structure accordin~ to the invention provides only for about 5 to 15 channels per square inch, that is about 0.5 to 4 channels per s~uare centimeter. These channels are filled with insert catalytic bodies. The cata-lyst support consists of metal threads in the form of wire or tape material, which is pre-compressed in accord with the hollow shape of the channels that are filled.
Also according to the invention, the wall thickness of the channels is less than 1 mm, preferably between 0.3 and 017 mm.
A preferred embodiment of the invention provides that only one complete channel cross section exists per square centimeter frontal surface of the ceramic body. It has already been noted that the catalytic insert bodies desirably have a cross section correspondinq with the hollow shape of the channels. Although the bodies can fill the channels wholly (}~3~

1 or partially, it is preferred for them to extend over the entire length of their channels.
To attain a desirable filtering action oE 60 to 80~
it may be sufficient in accordance with the invention, that only channels which are open upstream are provided with an insert body. The prime consideration is that only a small fraction of the carbon particles, and of this fraction only those wi-th the smallest dimensions, can travel from the open upstream channel, through its walls, into a channel that is open downstream. Another advantage is that, except for the customary fastening of the ceramic monolith, no special fas-tening is necessary for the insert bodies in the interior channels. They are held at the channel walls through surface friction of the woven wire material. This provides a clear improvement over the well-known catalytic filter of the prior artfor which expensive devices are required to fasten the catalyst support to the interior of a combustion chamber~
With respect to exhaust-gas counterpressure, a clear reduction is achieved compared to the well-known ceramic filter which operates without any catalytic action. ~nly the catalytic combustion effects continuous carbon burn-off in the area of the catalyst insert bodies. The result-ing energy is again directly used to burn off the carbon particles that have been caught at the channel walls7 parti-cles which were able to pass the catalyst insert body withouthaving been burned, so that no carbon collects. Consequently the channel walls or the channels themselves do not become plugged. They always remain free for the unhindered passage of the exhaust gases. As a result, the exhaust-gas counter-pressure has a desirably low level of about 0.1 to 0.2 bar.

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1 Description of the Drawings Other aspects of the invention will become apparentafter considering several illustrative embodiments taken in conjunction with the drawing which shows an axial longi-tudinal section through a filter device that is connectedto a diesel enqine.

r s(}~39 1 Detailed Description As shown in the drawing, a cylinder head 1 of a diesel engine is connected to an exhaust gas manifold 2. The latter is in turn connected to the outlet 3 of the engine. Prom the exhaust manifold 2, a pipe socket 4 feeds the exhaust gases throuqh a connection sleeve 5 into the housing 6 of an exhaust gas filter according to the invention. The flow direction of the exhaust gases is indicated by arrows. Down-stream of the exhaust gases in the filter,the housin~ 6 has a sleeve 7 to connect it to the remaining exhaust line.
In the interior of the filter housing 6, a monolitic ceramic body 8 is supported in shock-proof fashion by a woven wire casing 9, which is situated between the housing 6 and the circumferential surface of the ceramic body. A gas tight ring lOa is integrated into the woven wire casing 9 so that no exhaust gases can pass through. The monolithic-ceramic body 8 is~ofiporous ceramic material, e.g. Kordierit ceramic, which is well known. The monlith has axial channels 10,11, which are mutually parallel, and bound one another by thin ~ walls. These channels are closed either at the upstream or at the downstream end, alternately as viewed in cross section and in checkerboard fashion as in a top view on the ~rontal side of the monlith. The channels 11, which are o~en upstream, and the channels 10, which are open downstream, are closed respectively downstream and upstream by a floor 12, which likewise consists of ceramic material. The floor 12 can also be thicker than the channel walls. The exhaust gases which flow into the channels 11, which are open upstream, will diffuse, as in~icated by the arrows, through the channel walls into the neiahboring channels 10, which are open on the exhaust side.
It is now important that the channels 10~11 are filled with catalyst inserts 13, which are placed into the channels from their open ends. The catalyst inserts 13 are form-pressed woven wire bodies, with the wire being coated with a catalyt-3~
g 1 ically effective oxidation sur~ace. The inserts 13 have across section coxresponding to the hollow shape of the channels, or example, square, so that they can be inserted and can be fixed within the channels without special mounting. The inserts in the channels 10, which are o~en do~stream, have been shown without cross-hatchinq only to make the drawing clearer. However, it is also possible to leave the downstream open channels 10 empty, i.e. not to place any inserts 13 in them.
The catalytic combustion of the carbon particles in the exhaust gas takes place in the region of the catalyst inserts 13. Here the hydrocarbon compon~nts which are deposited as the carbon particles are burned with the oxygen contained in the exhaust gas, to form carbon dioxide and water, i.e.
pure carbon is burned to carbon dioxide. The particles which have not alrea~y been burned in the region of the catalyst inserts 13 are deposited in the pores of the channel walls and on their surfaces. There they are likewise burned by the energy derived from the catalytic combustion.
As a result only a very small portion of the carbon particles contained in the exhaust gas, namely those with very small dimensions, will pass through the channel walls into the channels that are open on the exhaust side. From their =, if no catalytic after-buring takes place there, they pass into the exhaust gas flowing out of the filter. The proportion o these particles lies between 20 and 30%.
Naturally, a cer-tain exhaust-gas temperature is required for the catalytic reaction in the area of the carbon filter.
As a consequence of the catalytic combustion, however, this temperature is reached by all types of diesel engines, even those with relatively low combustion temperatures. Normally, the temperature in the exhaust gas emerging ~rom the enqine combusiton sapce is about 400C. The catalytic xeactions, however, already occur beginning at 200C.
The woven wire material of the insert body 13 prefer-~.~?,rt(~s3~

1 ably is of a corrosion-resistant metal alloy, with nickel and chromium portions. Such materials are well know. The catalytically active coating, which is composed of the most various metals, their alloys, and their intermetallic com-pounds, is also well known.
As far as the cermaic monolith is concerned, thefigure in the drawing corresponds to only one embodiment.
Smaller and larger channel cross sections may be used. The density of the channels, measured per unit area on the frontal side, desirably lies between 0~5 and 4 channels per square centimeter of frontal surface and the drawing corresponds to the upper limit of this range. The wall thickness between the channels is approximately between 0.3 and 0.7 mm. The prosity of the ceramic material is between about 35 and 55%, with an average pore diameter of 10 to 35 um.
While various aspects of the invention have been set --- forth by the drawing and specification, it is to be under-stood that the foregoing detailed description is for illustra-tion only and that various changes in parts, as well as the substititution of equivalent constituents for those shown and describea may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A device for cleansing the exhaust gases of diesel engines for motor vehicles, characterized by macro-cells in a substrate of porous ceramic, positioned in the flow of exhaust gas, with said macrocells having an opening connected with the stream of exhaust gas, and some or all being wholly or partly filled with a gas-permeable catalyst support whose surface is coated with catalytic material

2. A device according to claim 1, characterized in that the catalytic material comprises metals, alloys, and intermetallic compounds which have an oxidizing effect.

3. A device according to claim 2,characterized in that the gas-permeable catalyst support comprises metal threads in the form of wire or tape material which are compacted to indi-vidual insert bodies corresponding to the shape of the hollow space of the macrocells.

4. A device according to claim 3, characterized in that the macrocells are formed in a monolithic ceramic body.

5. A device according to claim 4, characterized in that the macrocells are longitudinally extended channels open on one side.

6. A device according to claim 5, characterized in that the macrocells are formed by a bundle of parallel channels, aligned in the flow direction of the exhaust gases, which are alternately open either upstream or downstream and which are alternately closed respectively at the end that is down-stream or upstream, so that the openings and closures of the channels are distributed over the frontal sides of the ceramic body in checkerboard fashion, and at least the channels which are open upstream are provided with an insert body.

7. A device according to claim 5, characterized in that the wall thickness between neighboring channels of the ceramic body has a maximum value less than 1.0 mm and the number of channels per square centimeter frontal surface of the ceramic body is in the range from about 0.5 to 4.

8. A device according to claim 7, characterized in that only one complete channel cross section is provided per sguare centimeter frontal surface of the ceramic body.

9. A device according to claim 7, characterized in that the wall of the channels is in the range between about 0.3 and 0.7 mm. thick.

10. A device according to claim 5, characterized in that the channels have a square cross section.

11. A device according to claim 1, characterized in that the insert bodies have a cross section corresponding to the hollow shape of the channels and length corresponding to the total or partial longitudinal extend of the channels.

12. A device according to claim 1, characterized in that the gas-permeable catalyst support comprises metal threads in the form of wire or tape material, which is com-pacted to individual insert bodies corresponding to the shape of the hollow space of the macrocells.

13. A device according to claim 1, characterized in that the macrocells are formed in a monolithic ceramic body.

14. A device according to claim 2, characterized in that the macrocells are formed in a monolithic ceramic body.

15. A device according to claim 4, characterized in that the macrocells are formed by a bundle of parallel chan-nels aligned in the flow direction of the exhaust gases, which are alternately open either upstream of downstream and which are alternately closed respectively at the end that is down-stream or upstream, so that the openings and closures of the channels are distributed over the frontal sides of the ceramic body in checkerboard fashion, and at least the channels which are open upstream are provided with an insert body.

16. A device according to claim 6, characterized in that the wall thickness between neighboring channels of the ceramic body has a maximum value less than 1.0 mm and the number of channels per square centimeter frontal surface of the ceramic body is in the range from about 0.5 to 4.

17. A device according to claim 6, characterized in that the channels have a square cross section.

18. A device according to claim 7, characterized in that the channels have a square cross section.

19. A device according to claim 8, characterized in that the channels have a square cross section.

20. A device according to claim 9, characterized in that the channels have a square cross section.

21. A device according to claim 2, characterized in that the insert bodies have a cross section corresponding to the hollow shape of the channels and length corresponding to the total or partial longitudinal extend of the channels.