EP2058480A1

EP2058480A1 - Exhaust gas purifying system

Info

Publication number: EP2058480A1
Application number: EP08165383A
Authority: EP
Inventors: Yoshifumi Kato
Original assignee: Toyota Industries Corp
Current assignee: Toyota Industries Corp
Priority date: 2007-11-09
Filing date: 2008-09-29
Publication date: 2009-05-13
Also published as: JP2009115064A; US20090120075A1

Abstract

An exhaust gas purifying system removes particulate matter from exhaust gas from an engine (1) through an exhaust passage (4). The system includes a filter (5), a reforming catalyst (6) and an injector (8). The filter (5) is provided in the exhaust passage (4) and collects the particulate matter in the exhaust gas. The reforming catalyst (6) is provided in the exhaust passage (4) and is in directly or indirectly contact with the filter (5). The reforming catalyst (6) generates the heat of reaction by reforming fuel. The heat of reaction is transferred to the filter (5) through the contact. The injector (8) supplies the fuel to the reforming catalyst (6).

Description

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas purifying system, and more particularly to a system for removing particulate matter from exhaust gas of a diesel engine.
In a conventional exhaust gas purifying system, a filter is provided in an exhaust pipe to collect particulate matter in exhaust gas of a diesel engine. In order to prevent particulate matter accumulation on the filter and thereby to prevent an increase in the flow resistance of the filter, particulate matter needs to be removed from the filter so that the filter is regenerated. In a known system, for example, disclosed in Japanese Unexamined Patent Application Publication No. 59-155523 , hydrocarbon is supplied to a filter that contains oxidation catalysts, The hydrocarbons are oxidized or burned off on the filter, thereby removing particulate matter on the filter. The system has a fuel injection unit at an exhaust pipe disposed upstream of the filter with respect to the flow of exhaust gas. The fuel injection unit is connected through a fuel passage to a diesel fuel tank. The fuel passage has a reformer that contains reforming catalysts. The reformer produces hydrocarbons highly reactive with the oxidation catalysts of the filter by reforming diesel fuel. The reformer is connected to a bypass passage branching from the exhaust pipe. Exhaust gas flowing in the exhaust pipe is introduced through the branch passage into the reformer to preheat the reforming catalysts, thereby accelerating the reforming reaction of the diesel fuel in the reforming catalysts.
The system disclosed in the reference No. 59-155523 , however, requires two catalytic reactions for removal of particulate matter on the filter, Specifically, the system requires reforming of diesel fuel in the reforming catalyst and oxidation of hydrocarbons in the oxidation catalysts of the filter, which prevents efficient removal of particulate matter from the filter.
The present invention is directed to an exhaust gas purifying system that efficiently removes particulate matter from a filter.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, an exhaust gas purifying system removes particulate matter from exhaust gas from an engine through an exhaust passage. The system includes a filter, a reforming catalyst and an injector. The filter is provided in the exhaust passage and collects the particulate matter in the exhaust gas. The reforming catalyst is provided in the exhaust passage and is in directly or indirectly contact with the filter. The reforming catalyst generates the heat of reaction by reforming fuel. The heat of reaction is transferred to the filter through the contact. The injector supplies the fuel to the reforming catalyst.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

Fig. 1 is a schematic view of an exhaust gas purifying system according to a first embodiment of the present invention;
Fig. 2 is a longitudinal cross-sectional view of a reformer of the exhaust gas purifying system of Fig. 1;
Fig. 3 is a cross-sectional view taken along the line III-III in Fig. 2;
Fig. 4 is a longitudinal crass-sectional view of a reformer according to a second embodiment of the present invention;
Fig. 5 is a longitudinal cross-sectional view of a reformer according to a third embodiment of the present invention; and
Fig. 6 is a cross-sectional view taken along the line VI-VI in Fig. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe the first embodiment of the present invention with reference to Figs. 1 through 3. Fig. 1 shows an exhaust gas purifying system according to the first embodiment. A diesel engine 1 includes a cylinder head 1A connected to an intake manifold 2 and an exhaust manifold 3. The intake manifold 2 introduces air into the diesel engine 1, and the exhaust manifold 3 emits exhaust gas out of the diesel engine 1. The exhaust manifold 3 is connected to an exhaust pipe 4 as an exhaust passage. Exhaust gas of the diesel engine 1 flows through the exhaust manifold 3 and the exhaust pipe 4, as indicated by arrow A in Fig. 1. The exhaust pipe 4 is connected to a reformer 7 accommodating therein a filter 5 and a reforming catalyst 6 and supplied with diesel fuel from an injector 8. The filter 5 collects particulate matter in exhaust gas (diesel particulate, hereinafter referred to as PM). The reforming catalyst 6 reforms diesel fuel used as a fuel of the diesel engine 1. The injector 8 is connected to a diesel fuel tank (not shown In drawings) through a fuel passage 9 and extends into the reformer 7, thereby injecting diesel fuel into the reformer 7.
As shown in Fig. 2, the reformer 7 is a hollow cylindrical case made of a metal and having tapered longitudinal ends connected to the exhaust pipe 4. The reformer 7 has therein a partition wall 7A as a heat transfer member. The partition wall 7A is a hollow cylindrical member made of a metal such as stainless steel and accommodates therein the cylindrical filter 5 so that an outer peripheral surface 5A of the filter 5 is in contact with an inner peripheral surface 7B of the partition wall 7A. That is, the partition wall 7A is in surface contact with the filter 5. The filter 5 is a wall-flow filter having a honeycomb structure formed of a porous ceramic such as cordierite and removes PM from exhaust gas flowing downstream therethrough. The filter 5, the reforming catalyst 6, the reformer 7, the partition wall 7A and the injector 8 are components of the exhaust gas purifying system.
The reforming catalyst 6 has a hollow cylindrical shape and disposed outside the partition wall 7A in the reformer 7. An inner peripheral surface 6A of the reforming catalyst 6 is in contact with an outer peripheral surface 7C of the partition wall 7A, and an outer peripheral surface 6B of the reforming catalyst 6 is in contact with an inner peripheral surface 7D of the reformer 7. That is, the partition wall 7A is in surface contact with the reforming catalyst 6. The injector 8 extends into the reformer 7 through an outer peripheral surface 7G and the inner peripheral surface 7D of the reformer 7. The injector 8 is disposed at such a position that allows diesel fuel to be injected upstream of the reforming catalyst 6 in the reformer 7. The reforming catalyst 6 is, for example, a rhodium (Rh) containing catalyst, where diesel fuel is reacted with oxygen (O₂) and water vapor (H₂O) in exhaust gas, thereby reforming the diesel fuel so as to produce carbon monoxide (CO), hydrogen (H₂), and hydrocarbon (HC). This reforming of diesel fuel in the reforming catalyst 6 occurs through exothermic reaction at a temperature of about 700 to 800 degrees Celsius. That is, the reforming catalyst 6 produces the heat of reaction at a temperature of about 700 to 800 degrees Celsius by reforming diesel fuel. The filter 5 is disposed inside the reforming catalyst 6. That is, the filter 5 is surrounded along the entire circumference thereof by the reforming catalyst 6, and the partition wall 7A is in directly contact with the filter 5 and the reforming catalyst 6, as shown in Fig. 3. Therefore, the heat of reaction generated when the reforming catalyst 6 reforms diesel fuel is transferred efficiently to the filter 5 via the partition wall 7A.
As shown in Fig. 2, the partition wall 7A has at the upstream end thereof an opening 7E formed between the outer peripheral surface 7C and the inner peripheral surface 7D and extending circumferentially. The partition wall 7A also has at the downstream end an opening 7F formed between the outer peripheral surface 7C and the inner peripheral surface 7D and extending circumferentially. The exhaust gas introduced from the exhaust pipe 4 into the reformer 7 flows mostly in the space surrounded by the partition wall 7A and passes through the filter 5 out of the reformer 7. The rest of the exhaust gas flows through the opening 7E into the space formed between the outer peripheral surface 7C and the inner peripheral surface 7D, passes through the reforming catalyst 6, and then flows through the opening 7F out of the reformer 7 while joining the exhaust gas passing through the filter 5.
As described above, the filter 5 and the reforming catalyst 6 are disposed parallel to each other, and the partition wall 7A is in directly contact with the filter 5 and the reforming catalyst 6 to transfer the heat therebetween in the reformer 7. That is, the filter 5 and the reforming catalyst 6 are in indirectly contact with each other, and are in physically contact with each other. Since the thermal conductivity of a solid or the partition wall 7A is larger than that of a gas, the heat of reaction generated at the reforming catalyst 6 is efficiently transferred to the filter 5 via the partition wall 7A. In addition, a part of the exhaust gas introduced into the reformer 7 is branched through the opening 7E and passes through the reforming catalyst 6 provided at the space between the outer peripheral surface 7C and the inner peripheral surface 7D in the reformer 7.
Referring to Fig. 1, the reformer 7 is connected at the downstream side thereof to a NOx storage reduction (NSR) catalyst 10, and the NOx storage reduction catalyst 10 is connected at the downstream side thereof to a selective catalytic reduction (SCR) catalyst 11. The NSR catalyst 10 contains therein alkaline earth metals like barium (Ba) as a storage material. In lean exhaust gas, that is, in an oxidizing atmosphere with high oxygen concentration wherein the injector 8 injects no diesel fuel, the NSR catalyst 10 temporarily stores nitrogen oxides (hereinafter referred to as NOx) in the exhaust gas, In rich exhaust gas, that is, in a reducing atmosphere with low oxygen concentration wherein the injector 8 injects diesel fuel, the NSR catalyst 10 releases the stored NOx for reduction to nitrogen (N₂) and produces ammonia (NH₃), Specifically, NSR catalyst 10 reduces the stored NOx to nitrogen by using carbon monoxide, hydrogen, and hydrocarbon produced by the reforming catalyst 6 as the reducing agent. In the SCR catalyst 11, the remaining NOx in the exhaust gas is reacted with ammonia produced by the NSR catalyst 10, thereby being reduced to nitrogen.
The following will describe the operation of the exhaust gas purifying system according to the first embodiment in a lean condition wherein the injector 8 injects no diesel fuel into the reformer 7. As shown in Fig. 1, exhaust gas of the diesel engine 1 flows through the exhaust manifold 3 and the exhaust pipe 4 into the reformer 7. The exhaust gas flows mostly in the space surrounded by the partition wall 7A (see Fig. 2) and passes through the filter 5 out of the reformer 7, so that PM in the exhaust gas is collected by the filter 5. The rest of the exhaust gas flows through the opening 7E into the space between the outer peripheral surface 7C and the inner peripheral surface 7D. The exhaust gas then passes through the reforming catalyst 6, but reforming reaction in the reforming catalyst 6 does not occur because the injector 8 injects no diesel fuel into the reformer 7. After passing through the reforming catalyst 6, the exhaust gas flows through the opening 7F out of the reformer 7.
The exhaust gas emitted from the reformer 7 passes through the NSR catalyst 10 and the SCR catalyst 11. Since the injector 8 injects no diesel fuel into the reformer 7, the exhaust gas from the reformer 7 is in the oxidizing atmosphere. Therefore, the NSR catalyst 10 stores NOx in the exhaust gas but produces no ammonia, and no reaction occurs in the SCR catalyst 11. As described above, in the lean condition, PM in exhaust gas is collected by the filter 5, and NOx in the exhaust gas is stored on the NSR catalyst 10. Therefore, exhaust gas emitted out of the system contains neither PM nor NOx.
The following will describe the operation of the exhaust gas purifying system according to the first embodiment in a rich condition wherein the injector 8 injects diesel fuel into the reformer 7. When the amount of PM accumulated on the filter 5 becomes a predetermined level, the injector 8 injects diesel fuel into the reformer 7 thereby to supply diesel fuel to the reforming catalyst 6. In the reforming catalyst 6, the diesel fuel is reacted with oxygen in exhaust gas introduced through the opening 7E into the space between the outer peripheral surface 7C and the inner peripheral surface 7D, thereby being reformed so as to produce carbon monoxide. This reforming of diesel fuel in the reforming catalyst 6 occurs through exothermic reaction at a temperature of about 700 to 800 degrees Celsius. Since the partition wall 7A is in directly contact with the filter 5 and the reforming catalyst 6, the heat of reaction generated at the reforming catalyst 6 is efficiently transferred to the filter 5 through the contact, thereby heating the filter 5. When the filter 5 is heated to the PM combustion temperature, the accumulated PM on the filter 5 is burned off, and the filter 5 is regenerated. That is, the heat of reaction generated at the reforming catalyst 6 is used for heating the PM on the filter 5.
The exhaust gas introduced into the space between the outer peripheral surface 7C and the inner peripheral surface 7D flows through the opening 7F out of the reformer 7 along with carbon monoxide, hydrogen, and hydrocarbon produced by the reforming catalyst 6. The exhaust gas from the reformer 7 then passes through the NSR catalyst 10. The NSR catalyst 10 releases the NOx previously stored in the oxidizing atmosphere for reduction to nitrogen and produces ammonia, Specifically, NSR catalyst 10 reduces the stored NOx to nitrogen by using carbon monoxide, hydrogen, and hydrocarbon serving as reducing agent produced by the reforming catalyst 6, and produces ammonia. In the SCR catalyst 11, NOx remaining in the exhaust gas that is not reduced in the NSR catalyst 10 is reacted with ammonia produced by the NSR catalyst 10, thereby being reduced to nitrogen.
According to the first embodiment, since the partition wall 7A being in directly contact with the filter 5 and the reforming catalyst 6 is provided in the reformer 7, the heat of reaction generated at the reforming catalyst 6 is efficiently transferred to the filter 5 via the partition wall 7A. Therefore, the filter 5 is heated to the PM combustion temperature without using any heating means other than the heat of reaction at the reforming catalyst 6, and the accumulated PM is efficiently removed from the filter 5. Additionally, the heat of reaction at the reforming catalyst 6 is efficiently transferred to the filter 5 because the filter 5 is surrounded along the entire circumference thereof by the reforming catalyst 6. Therefore, the filter 5 is heated to the PM combustion temperature, and the accumulated PM is removed from the filter 5 more efficiently than heretofore.
The following will describe an exhaust gas purifying system according to the second embodiment of the present invention. In the second embodiment, the filter is in directly contact with the reforming catalyst without providing a partition wall therebetween as a heat transfer member, but the other components and structures are substantially the same as those of the first embodiment. Therefore, the following description will use the same reference numbers for the common elements or components in both embodiments, and the description of such elements or components in Figs. 1 through 3 for the second embodiment will be omitted. Fig. 4 shows a reformer 17 according to the second embodiment. As with the reformer 7 of the first embodiment, the reformer 17 is provided by a hollow cylindrical case made of a metal. The reformer 17 accommodates therein a hollow cylindrical partition wall 17A so that a downstream end 17B of the partition wall 17A is disposed at a middle position as viewed in longitudinal direction of the reformer 17, thereby being divided radially into two spaces.
A cylindrical filter 15 is provided downstream of the partition wall 17A so that an upstream end 15A of the filter 15 is in contact with the downstream end 17B of the partition wall 17A. As with the filter 5 of the first embodiment, the filter 15 is a wall-flow filter and removes PM from exhaust gas flowing downstream therethrough. A hollow cylindrical reforming catalyst 16 is disposed in the space formed between an outer peripheral surface 15B of the filter 15 and an inner peripheral surface 17C of the reformer 17. An inner peripheral surface 16A of the reforming catalyst 16 is in contact with the outer peripheral surface 15B, and an outer peripheral surface 16B of the reforming catalyst 16 is in contact with the inner peripheral surface 17C. That is, the reforming catalyst 16 is in surface contact with the filter 15. In the reforming catalyst 16, as with the reforming catalyst 6 of the first embodiment, diesel fuel is reacted with oxygen and water vapor in the exhaust gas so as to produce carbon monoxide, hydrogen, and hydrocarbon, thereby reforming the diesel fuel. This reforming of diesel fuel in the reforming catalyst 16 occurs through exothermic reaction at a temperature of about 700 to 800 degrees Celsius. That Is, the reforming catalyst 16 produces the heat of reaction at a temperature of about 700 to 800 degrees Celsius by reforming diesel fuel. The heat of reaction generated at the reforming catalyst 16 is directly transferred to the filter 15 in close contact therewith.
The partition wall 17A has at the upstream end thereof an opening 17E formed between an outer peripheral surface 17D of the partition wall 17A and the inner peripheral surface 17C and extending circumferentially. The exhaust gas introduced from the exhaust pipe 4 into the reformer 17 flows mostly in the space surrounded by the partition wall 17A and passes through the filter 15 out of the reformer 17, The rest flows through the opening 17E into the space formed between the inner peripheral surface 17C and the outer peripheral surface 17D, passes through the reforming catalyst 16, and then flows out of the reformer 17. According to the second embodiment, since the filter 15 is in directly contact with the reforming catalyst 16, the heat of reaction generated at the reforming catalyst 16 is directly transferred to the filter 15. That is, the heat of reaction generated at the reforming catalyst 16 is used for heating the PM on the filter 15. Therefore, the filter 15 is heated to the PM combustion temperature without using any heating means other than the heat of reaction at the reforming catalyst 16, and the accumulated PM is efficiently removed from the filter 15, as with the first embodiment.
The following will describe an exhaust gas purifying system according to the third embodiment of the present invention. Fig. 5 shows a reformer 27 according to the third embodiment, The reformer 27 has a heating member 21 made of a metal such as stainless steel. The heating member 21 is a component of the exhaust gas purifying system. The heating member 21 is composed of a hollow cylindrical frame 21A and a mesh body 21C disposed inside the frame 21A. The heating member 21 is disposed upstream of the filter 5 in the space inside the partition wall 7A so that an outer peripheral surface 21 B of the frame 21 A is in contact with the inner peripheral surface 7B of the partition wall 7A. As shown in Fig. 5, the mesh body 21C has therein a plurality of passages extending in longitudinal direction of the reformer 17 to allow the exhaust gas to flow downstream therethrough. Therefore, the exhaust gas introduced into the space of the partition wall 7A passes through the filter 5 after passing through the mesh body 21C. The partition wall 7A is in directly contact with the reforming catalyst 6 and the heating member 21.
According to the third embodiment, the heating member 21 allowing the exhaust gas to flow therethrough is disposed upstream of the filter 5 in the space of the partition wall 7A so that the partition wall 7A is in directly contact with the reforming catalyst 6 and the heating member 21. Therefore, the heating member 21 is heated by the heat of reaction generated at the reforming catalyst 6, thereby increasing the temperature of the exhaust gas passing through the mesh body 21C of heating member 21. That is, the filter 5 disposed downstream of the heating member 21 is heated by the exhaust gas passing through the mesh body 21, as well as by the heat of reaction transferred from the reforming catalyst 6 via the partition wall 7A. Therefore, the filter 5 is heated more efficiently to the PM combustion temperature, and the accumulated PM is removed more efficiently from the filter 5.
The above embodiments may be modified in various ways as exemplified below.
In the above embodiments, the reformer is divided radially into the two spaces by the partition wall. Alternatively, cylindrical partition walls having different diameters may be concentrically disposed in the reformer so that the filters and the reforming catalysts are disposed alternately, thereby constituting a multilayer structure.
In the embodiments, the opening is formed at the partition wall of the reformer, and a part of exhaust gas constantly flows through the upstream opening into the space formed between the outer peripheral surface of the partition wall and the inner peripheral surface of the reformer. Alternatively, the reformer may have a valve operable to open the upstream opening to allow the exhaust gas to flow through the opening only when diesel fuel needs to be reformed.
In the first and second embodiments, the filter is surrounded along the entire circumference thereof by the reforming catalyst. Alternatively, the filter may be surrounded along only the partial circumference thereof by the reforming catalyst.
Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims.
An exhaust gas purifying system removes particulate matter from exhaust gas from an engine (1) through an exhaust passage (4). The system includes a filter (5), a reforming catalyst (6) and an injector (8). The filter (5) is provided in the exhaust passage (4) and collects the particulate matter in the exhaust gas. The reforming catalyst (6) is provided in the exhaust passage (4) and is in directly or indirectly contact with the filter (5), The reforming catalyst (6) generates the heat of reaction by reforming fuel. The heat of reaction is transferred to the filter (5) through the contact. The injector (8) supplies the fuel to the reforming catalyst (6).

Claims

An exhaust gas purifying system for removing particulate matter from exhaust gas from an engine (1) through an exhaust passage (4), comprising:
a filter (5) provided in the exhaust passage (4) and collecting the particulate matter in the exhaust gas;

a reforming catalyst (6) provided in the exhaust passage (4) and generating the heat of reaction by reforming fuel; and

an injector (8) supplying the fuel to the reforming catalyst (6);
characterized in that the reforming catalyst (6) is in directly or indirectly contact with the filter (5), so that the heat of reaction generated at the reforming catalyst (6) is transferred to the filter (5).
The exhaust gas purifying system according to claim 1, further comprising a heat transfer member (7A) being in directly contact with the filter (5) and the reforming catalyst (6) to transfer the heat therebetween.
The exhaust gas purifying system according to claim 1 or claim 2, wherein the reforming catalyst (6) circumferentially surrounds the filter (5).
The exhaust gas purifying system according to any one of claims 1 through 3, further comprising a heating member (21) provided upstream of the filter (5) and allowing the exhaust gas to flow downstream therethrough, the heating member (21) receiving the heat of reaction generated at the reforming catalyst (5).
The exhaust gas purifying system according to claim 2, wherein the heat transfer member (7A) is in surface contact with the reforming catalyst (6) and the filter (5).
The exhaust gas purifying system according to claim 5, wherein the heat transfer member (7A) has a hollow cylindrical shape, the reforming catalyst (6) is disposed outside of the heat transfer member (7A), and the filter (5) is disposed inside the heat transfer member (7A).
The exhaust gas purifying system according to claim 2, wherein the heat transfer member is a partition wall (7A) to separate the filter (5) and the reforming catalyst (6).