U.S. patent application number 12/160488 was filed with the patent office on 2010-08-19 for exhaust gas-purifying apparatus and exhaust gas-purifying method.
Invention is credited to Masahiko Ogai, Juji Suzuki.
Application Number | 20100205936 12/160488 |
Document ID | / |
Family ID | 38006580 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100205936 |
Kind Code |
A1 |
Suzuki; Juji ; et
al. |
August 19, 2010 |
EXHAUST GAS-PURIFYING APPARATUS AND EXHAUST GAS-PURIFYING
METHOD
Abstract
An exhaust gas purifying apparatus, including an NO.sub.x
occlusion reduction catalyst (2) and a filter catalyst (3) arranged
in series, is used, and the exhaust gas is allowed to flow from the
NO.sub.x occlusion reduction catalyst (2) to the filter catalyst
(3) in a normal flow process, and, in a recovery process for
allowing exhaust gas added with a reducing agent to flow, the flow
direction of the exhaust gas is reversed toward the NO.sub.x
occlusion reduction catalyst (2) from the filter catalyst (3).
Since the exhaust gas is heated by reaction heat of the filter
catalyst (3), the NO.sub.x occlusion reduction catalyst (2) can
recover from sulfur poisoning in the exhaust gas at a low
temperature. Thereby, overheating of the filter catalyst (3) is
prevented. Therefore, recovery from sulfur poisoning can be
improved, and also deterioration and breakage of the filter
catalyst can be prevented.
Inventors: |
Suzuki; Juji; (Aichi-ken,
JP) ; Ogai; Masahiko; (Aichi-ken, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
38006580 |
Appl. No.: |
12/160488 |
Filed: |
January 10, 2007 |
PCT Filed: |
January 10, 2007 |
PCT NO: |
PCT/JP2007/050549 |
371 Date: |
April 13, 2010 |
Current U.S.
Class: |
60/274 ;
60/295 |
Current CPC
Class: |
F01N 3/2093 20130101;
F01N 3/0233 20130101; Y02T 10/22 20130101; F01N 3/0885 20130101;
F01N 3/0821 20130101; Y02T 10/12 20130101; B01D 53/9431 20130101;
B01D 2258/012 20130101; F01N 3/035 20130101; F01N 2240/36 20130101;
F01N 13/0097 20140603; F01N 3/0253 20130101; F01N 3/0842 20130101;
B01D 53/944 20130101 |
Class at
Publication: |
60/274 ;
60/295 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F01N 3/035 20060101 F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 17, 2006 |
JP |
2006-008916 |
Claims
1. An exhaust gas purifying apparatus, comprising: a reducing agent
supply device supplying a reducing agent to exhaust gas; a first
catalyst comprising an NO.sub.x occlusion reduction catalyst
obtained by forming a catalyst layer on a surface of a honeycomb
substrate having a straight flow structure, the catalyst layer
including a porous oxide support and an NO.sub.x occluding material
and noble metal supported thereon; a second catalyst obtained by
forming a catalyst layer at least on a surface of a filter
substrate having a wall flow structure and including a porous oxide
support and at least a noble metal supported thereon; a container
having at least the first catalyst and the second catalyst arranged
in series; and a direction change device changing a flow direction
of exhaust gas in the container between a normal flow direction and
a reverse flow direction, wherein the normal flow direction, the
first catalyst is provided upstream side of an exhaust gas flow
direction and the second catalyst is provided downstream side
thereof, and in the reverse flow direction, the second catalyst is
provided upstream side of the exhaust gas flow direction and the
first catalyst is provided downstream side thereof.
2. The apparatus according to claim 1, wherein the second catalyst
further comprises an NO.sub.x occluding material supported on the
catalyst layer.
3. The apparatus according to claim 1, wherein, in the container,
the first catalyst, the second catalyst, and a third catalyst are
arranged in sequence, the third catalyst obtained by forming a
catalyst layer on a surface of a honeycomb substrate having a
straight flow structure, the catalyst layer including a porous
oxide support and a noble metal supported thereon, and the
direction change device changes the flow direction of the exhaust
gas between a normal flow direction in a sequence of the first
catalyst, the second catalyst, and then the third catalyst and a
reverse flow direction in a sequence of the third catalyst, the
second catalyst, and then the first catalyst.
4. The apparatus according to claim 3, wherein the third catalyst
is at least one selected from among an NO.sub.x occlusion reduction
catalyst, a three-way catalyst, and an oxidation catalyst.
5. The apparatus according to claim 3, wherein the first catalyst
comprises an oxidation catalyst or a three-way catalyst disposed at
an upstream side thereof and a NO.sub.x occlusion reduction
catalyst disposed at a downstream side thereof when the first
catalyst is provided upstream side of the exhaust gas flow
direction, and the third catalyst comprises an oxidation catalyst
or a three-way catalyst disposed at an upstream side thereof and an
NO.sub.x occlusion reduction catalyst disposed at a downstream side
thereof when the third catalyst is provided upstream side of the
exhaust gas flow direction.
6. An exhaust gas purifying method using the exhaust gas purifying
apparatus of claim 1, comprising a normal flow process for allowing
exhaust gas to normally flow and a recovery process for allowing
exhaust gas in a high-temperature rich atmosphere added with a
reducing agent to flow in a pulsing manner so that a
sulfur-poisoned NO.sub.x occluding material is reduced to thereby
recover an NO.sub.x occlusion function, in which a flow direction
of the exhaust gas in the normal flow process is changed in the
recovery process.
7. The method according to claim 6, further comprising a
regeneration process for allowing the exhaust gas in a lean
atmosphere added with the reducing agent to flow in a pulsing
manner to thereby generate combustion heat which is then used to
combust PM accumulated in the second catalyst, thus regenerating a
PM capture function, in which the flow direction of the exhaust gas
in the normal flow process is changed in the regeneration
process.
8. The method according to claim 7, wherein the regeneration
process precedes the recovery process.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an exhaust gas-purifying
apparatus, comprising at least an NO.sub.x occlusion reduction
catalyst and a filter catalyst arranged in series, and an exhaust
gas purifying method using the same.
[0003] 2. Description of the Related Art
[0004] A catalyst for efficiently removing NO.sub.x from vehicle
exhaust gas is known as an NO.sub.x occlusion reduction catalyst.
Such an NO.sub.x occlusion reduction catalyst is formed by
supporting an NO.sub.x occluding material, selected from among
alkali metals, alkali earth metals on the like, and noble metal, on
an oxide support such as alumina. In an oxygen-excess lean
atmosphere, NO.sub.x is adsorbed on an NO.sub.x occluding material
in the form of nitrate or nitrite. The exhaust gas in a rich
atmosphere is allowed to flow in a pulsing manner, thereby
decomposing nitrate or nitrite. The emitted NO.sub.x is reduced and
purified by a reduction component that is abundantly present in the
atmosphere.
[0005] However, under the NO.sub.x occlusion reduction catalyst,
sulfur oxide (SO.sub.x) present in the exhaust gas reacts with the
NO.sub.x occluding material, undesirably causing sulfur poisoning
related to the deterioration of NO.sub.x occlusion performance. The
sulfur-poisoned NO.sub.x occluding material is present in the form
of sulfate or sulfite, which has a higher decomposition temperature
than nitrate or nitrite.
[0006] According to conventional technologies, treatment for
recovering the NO.sub.x occlusion function of the sulfur-poisoned
NO.sub.x occluding material is performed. The recovery treatment is
a process of allowing exhaust gas in a high-temperature rich
atmosphere added with a reducing agent to flow in a pulsing manner
to thereby reduce and decompose the sulfur-poisoned NO.sub.x
occluding material.
[0007] For example, Japanese Patent No. 2605580 discloses a method
of reducing and desorbing SO.sub.x through the inflow of rich gas
having a low oxygen concentration. According to this method,
SO.sub.x is found to be more easily removed under higher
temperature conditions. Also, Japanese Patent Application
Publication No. H08-061052 discloses a method of heating a catalyst
to 800.about.900.degree. C. in order to emit SO.sub.x from a
sulfur-poisoned NO.sub.x occluding material. In addition, Japanese
Patent Application Publication No. 2000-230447 discloses a method
of decreasing the temperature of the recovery process by supplying
a large amount of reduction gas such as CO.
[0008] However, in the case where the exhaust gas added with the
reducing agent is supplied to the NO.sub.x occlusion reduction
catalyst, the temperature at the inlet of the catalyst is lower
than that at the outlet thereof, and thus the NO.sub.x occlusion
function is insufficiently recovered near the inlet. Accordingly,
Japanese Patent Application Publication No. 2002-013413 describes
the reversed exhaust gas flow of the NO.sub.x occlusion reduction
catalyst upon recovery from sulfur poisoning. In a normal state,
the temperature at the outlet of the catalyst is higher than that
at the inlet thereof due to the reaction heat of the catalyst.
Hence, when the exhaust gas flow direction is reversed upon
recovery treatment, the original outlet having a high temperature
becomes an inlet, and thus such heat is used to recover the
NO.sub.x occlusion function. On the other hand, when the original
inlet becomes an outlet, it is brought into contact with the
exhaust gas having a high temperature due to the reaction heat of
the catalyst, and therefore the NO.sub.x occlusion function may
also be recovered at the original inlet.
[0009] Further, in the exhaust gas from a diesel engine, since
harmful components are discharged in a particulate form
(particulate material: carbon particles, sulfur particles such as
sulfate, high-molecular-weight hydrocarbon particle and the like,
hereinafter, referred to as "PM"), as compared with the gasoline
engines, diesel engines have difficulty in purifying the exhaust
gas.
[0010] The exhaust gas purifier for diesel engines developed to
date is largely classified into a trap type exhaust gas purifier
(wall flow) and an open type exhaust gas purifier (straight flow).
In particular, the trap type exhaust gas purifier is known to be a
clogged ceramic honeycomb body (diesel PM filter, hereinafter
referred to as "DPF"). The DPF, in which both open ends of the
cells of a ceramic honeycomb structure (e.g., a checkered pattern)
comprises inlet cells, outlet cells and cell partition walls. The
inlet cells each are clogged at the downstream side of the exhaust
gas flow direction. The outlet cells each adjoin the respective
inlet cells and clogged at the upstream side of the exhaust gas
flow direction. The cell partition walls each partition the
respective inlet cells and outlet cells. Through the fine pores in
the cell partitions, the exhaust gas is filtered and PM is thus
captured, consequently making it possible to suppress the discharge
of PM.
[0011] However, in the DPF, since the loss of exhaust gas pressure
is increased due to the accumulation of PM, the accumulated PM
needs to be periodically removed using any means in order to
regenerate the DPF. Thus, in the case where the loss of exhaust
pressure is increased, there are proposed conventional methods of
flowing high-temperature exhaust gas or performing a heating
process using a burner or an electrical heater to combust the
accumulated PM in order to regenerate the DPF. In such a case,
however, as the accumulated amount of the PM increases, the
combustion temperature also increases, undesirably producing heat
stress. This heat stress often causes damage to DPF.
[0012] Recently, a continuous regenerative type DPF (filter
catalyst) has been developed by forming a coating layer such as an
alumina on the surface of the cell partition of the DPF and
supporting noble metal such as platinum (Pt) on the coating layer.
According to this filter catalyst, since the captured PM is
oxidized and combusted through the catalytic reaction of noble
metal, the PM may be combusted simultaneously with or successively
to the capture thereof, thereby making it possible to recover the
filter function. Further, the catalytic reaction occurs at a
relatively low temperature and the PM may be combusted even though
the collected amount thereof is small. Thereby, the DPF is
advantageously prevented from breakage due to low heat stress
applied thereto.
[0013] In addition, There is known a filter catalyst, in which a
coating layer is further supported with an NO.sub.x occluding
material selected from among alkali metals, alkali earth metals,
and rare earth elements. According to such a filter catalyst, in
the same lean atmosphere as in the NO.sub.x occlusion reduction
catalyst, NO.sub.x is occluded on the NO.sub.x occluding material
and NO.sub.x emitted in a rich atmosphere is reduced, thereby
increasing NO.sub.x purification performance highly. Therefore, in
the case where the exhaust gas of a diesel engine is purified using
the above filter catalyst, a system in which a reducing agent is
intermittently added to the exhaust gas to thereby form a rich
atmosphere is adopted. However, this filter catalyst having the
NO.sub.x occluding materials supported thereon has low HC oxidation
activity of noble metal. Thus various methods of attaching a
reducing agent to a filter catalyst or decreasing accumulation of
PM need to be researched.
[0014] For example, Japanese Patent Application Publication No.
2002-021544 discloses a purification technologies, in which an
oxidation catalyst or an NO.sub.x occlusion reduction catalyst is
disposed at the upstream side of a filter catalyst, and HC is
supplied to the exhaust gas through the post spray of fuel into a
combustion chamber or through the addition of fuel to the exhaust
gas. It is also described that the reaction heat of the oxidation
catalyst or NO.sub.x occlusion reduction catalyst make PM
accumulated in the DPF or filter catalyst combusted and NO.sub.x
reduced and purified.
[0015] In this way, in the case where the reducing agent, such as
light oil, is added to the exhaust gas, the reducing agent need to
be added before the NO.sub.x occlusion capability of the NO.sub.x
occlusion reduction catalyst is saturated, in order to recover the
ability to adsorb NO.sub.x. Accordingly, even upon acceleration or
deceleration at a low speed, the reducing agent need to be added at
relatively short time intervals. In such a case, however, since the
temperature of the exhaust gas is relatively low and decreases
further by the addition of the reducing agent, it is thus difficult
to react the reducing agent with NO.sub.x. Hence, the added
reducing agent is attached to the filter catalyst in an unreacted
state, and the supported catalyst metal is poisoned, and thus the
activity thereof is decreased. Furthermore, while the PM adheres to
the attached reducing agent, front end cells are undesirably
clogged.
[0016] In the case of the filter catalyst, ash is increasingly
accumulated in the catalyst, undesirably increasing the loss of
exhaust gas pressure.
[0017] In the case where the NO.sub.x occlusion reduction catalyst
is disposed at the upstream side of the filter catalyst, there is a
need for recovery treatment from sulfur poisoning with respect to
the NO.sub.x occlusion reduction catalyst. When the temperature of
exhaust gas flowing into the NO.sub.x occlusion reduction catalyst
is, for example, 300.degree. C., the temperature distribution of
each catalyst is as illustrated in FIG. 9. In order to perform
sufficient recovery treatment, a temperature not lower than
650.degree. C. is required. Thus, when such high-temperature
exhaust gas flows into the upstream NO.sub.x occlusion reduction
catalyst, the temperature of the downstream filter catalyst is
increased, and noble metal supported on the filter catalyst may be
deteriorated due to grain growth. Further, the PM accumulated in
the filter catalyst may be combusted all at once, undesirably
causing heat stress. As a result, a filter catalyst may be
damaged.
SUMMARY OF THE INVENTION
[0018] Accordingly, The present invention has been made to solve
the above-described problems occurring in the prior art, and
Embodiments of the present invention provide an exhaust gas
purifying apparatus, comprising an NO.sub.x occlusion reduction
catalyst disposed upstream thereof and a filter catalyst disposed
downstream thereof, thus improving recovery from sulfur poisoning,
preventing deterioration and breakage of the filter catalyst.
[0019] The exhaust gas purifying apparatus for achieving the above
object of the present invention is characterized in that it
comprises a reducing agent supply device supplying a reducing agent
to exhaust gas, a first catalyst comprising an NO.sub.x occlusion
reduction catalyst obtained by forming a catalyst layer on a
surface of a honeycomb substrate having a straight flow structure,
the catalyst layer including a porous oxide support and an NO.sub.x
occluding material and noble metal supported thereon, a second
catalyst obtained by forming a catalyst layer at least on a surface
of filter substrate having a wall flow structure and including a
porous oxide support and at least a noble metal supported thereon,
a container having at least the first catalyst and the second
catalyst arranged in series, and a direction change device changing
the flow direction of the exhaust gas in the container between a
normal flow direction and a reverse flow direction, wherein in the
normal flow direction, the first catalyst is disposed upstream of
the exhaust gas flow direction and the second catalyst is disposed
downstream thereof, in the reverse flow direction, the second
catalyst is disposed upstream of the exhaust gas flow direction and
the first catalyst is disposed downstream thereof.
[0020] In the container, the first catalyst, the second catalyst,
and a third catalyst obtained by forming a catalyst layer
consisting of a porous oxide support and noble metal supported
thereon on the surface of a honeycomb substrate having a straight
flow structure are sequentially arranged in series, and the
direction change device preferably changes (reverses) the flow
direction of the exhaust gas between a normal flow direction in a
sequence of the first catalyst, the second catalyst, and then the
third catalyst and a reverse flow direction in a sequence of the
third catalyst, the second catalyst, and then the first
catalyst.
[0021] In addition, the exhaust gas purifying method according to
the present invention is characterized in that it comprises a
normal flow process for allowing exhaust gas to typically flow and
a recovery process for allowing exhaust gas in a high-temperature
rich atmosphere, added with a reducing agent, to flow in a pulsing
manner so that a sulfur-poisoned NO.sub.x occluding material is
reduced to thereby recover the NO.sub.x occlusion function, using
the exhaust gas purifying apparatus of the present invention. In
the recovery process, the flow direction of the exhaust gas upon
the normal flow process is changed (reversed).
[0022] Further, the method of the present invention also includes a
regeneration process for allowing the exhaust gas in a lean
atmosphere added with a reducing agent to flow in a pulsing manner
to thereby generate combustion heat which is then used to combust
PM accumulated in the second catalyst so as to regenerate the PM
collection function. In the regeneration process, the flow
direction of the exhaust gas upon the normal flow process is
changed (reversed).
[0023] The exhaust gas purifying apparatus of the present invention
comprises the direction change device changing the flow direction
of the exhaust gas in the container having the first catalyst,
composed of an NO.sub.x occlusion reduction catalyst, and the
second catalyst, serving as a filter catalyst, arranged in series,
between the normal flow direction in which the first catalyst is
disposed upstream of the exhaust gas flow direction and the second
catalyst is disposed downstream thereof and the reverse flow
direction in which the second catalyst is disposed upstream of the
exhaust gas flow direction and the first catalyst is disposed
downstream thereof. Further, in the recovery process of the exhaust
gas purifying method of the present invention, the flow direction
of the exhaust gas upon the normal flow process is changed
(reversed).
[0024] Thus, in the case where the exhaust gas flows in a normal
flow direction toward the second catalyst from the first catalyst
in the normal flow process, the flow direction of the exhaust gas
in the recovery process becomes the direction toward the first
catalyst from the second catalyst. Upon the normal flow process,
the amount of sulfur poisoning is increased toward the upstream
side at a low temperature, and is larger in the first catalyst than
in the second catalyst and also is larger at the upstream side of
the first catalyst than at the downstream side thereof.
Accordingly, in the case where the exhaust gas flow direction is
changed (reversed) in the recovery process so that the second
catalyst is provided upstream with respect to the exhaust gas flow
direction, the exhaust gas is further heated by the reaction heat
of the second catalyst, and therefore the temperature of the first
catalyst becomes higher than that of the second catalyst.
Consequently, in the NO.sub.x occlusion reduction catalyst
constituting the first catalyst, recovery from sulfur poisoning is
improved.
[0025] In the normal flow process, the second catalyst, which is
downstream, has a higher temperature than the first catalyst, and
the temperature of the downstream side of the second catalyst is
higher than that of the upstream side thereof. That is, even though
the temperature of the exhaust gas flowing into the second catalyst
is decreased to 650.degree. C. or less upon the recovery process,
the temperature of the exhaust gas flowing into the first catalyst
is high, and thus the first catalyst may sufficiently recover from
sulfur poisoning. Hence, it is possible to inhibit the
deterioration and breakage of the second catalyst (filter catalyst)
due to heat.
[0026] In the case where the second catalyst (filter catalyst)
further includes the NO.sub.x occluding materials supported
thereon, the amount of sulfur poisoning is larger at the upstream
side thereof than at the downstream side thereof upon the normal
flow process. When the exhaust gas flow direction is changed
(reversed) upon the recovery process, the temperature of the
exhaust gas at the outlet of the second catalyst is sufficiently
increased even at the low temperature of the exhaust gas flowing
into the second catalyst, and thus the recovery treatment of the
second catalyst may be realized, and the deterioration and breakage
of the second catalyst (filter catalyst) due to heat may be
inhibited at the same time.
[0027] In the normal flow process, ash accumulated in the second
catalyst (filter catalyst) is blown by the exhaust gas reversely
flowing upon the recovery process, and is then passed through the
first catalyst to thereby discharge it. Thereby, excessive ash
accumulation in the second catalyst (filter catalyst) may be
prevented, and the increase in the loss of exhaust pressure may be
inhibited.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other objects and features of the present
invention will become apparent from the following description of
preferred embodiments, given in conjunction with the accompanying
drawings, in which:
[0029] FIG. 1 is a schematic view illustrating the exhaust gas
purifying apparatus according to a first embodiment of the present
invention, in which exhaust gas flows in a normal flow
direction,
[0030] FIG. 2 is a schematic view illustrating the exhaust gas
purifying apparatus according to the first embodiment of the
present invention, in which exhaust gas flows in a reverse flow
direction,
[0031] FIG. 3 is a schematic view illustrating the exhaust gas
purifying apparatus according to a second embodiment of the present
invention, in which exhaust gas flows in a normal flow
direction,
[0032] FIG. 4 is a schematic view illustrating the exhaust gas
purifying apparatus according to the second embodiment of the
present invention, in which exhaust gas flows in a reverse flow
direction,
[0033] FIG. 5 is a graph illustrating the concentration
distribution of sulfur,
[0034] FIG. 6 is a graph illustrating the concentration
distribution of ash,
[0035] FIG. 7 is a schematic view illustrating the exhaust gas
purifying apparatus according to a third embodiment of the present
invention, in which exhaust gas flows in a normal flow
direction,
[0036] FIG. 8 is a schematic view illustrating the exhaust gas
purifying apparatus according to a fifth embodiment of the present
invention, in which exhaust gas flows in a normal flow direction,
and
[0037] FIG. 9 is a schematic view illustrating a general catalyst
temperature distribution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Various embodiments of the present invention will now be
described in detail with reference to the accompanying
drawings.
[0039] According to the present invention, the apparatus for
purifying exhaust gas comprises a first catalyst and a second
catalyst. The first catalyst comprises an NO.sub.x occlusion
reduction catalyst, that is, may be composed of only an NO.sub.x
occlusion reduction catalyst, or of a three-way catalyst or an
oxidation catalyst and an NO.sub.x occlusion reduction catalyst
that have been applied and divided.
[0040] The NO.sub.x occlusion reduction catalyst is obtained by
forming a catalyst layer, consisting of a porous oxide support and
an NO.sub.x occluding material and noble metal supported thereon,
on the surface of a honeycomb substrate having a straight flow
structure. Alternatively, a conventional NO.sub.x occlusion
reduction catalyst may be used. Examples of the honeycomb substrate
include a monolithic honeycomb substrate, formed of heat resistant
ceramics such as cordierite, or a metallic honeycomb substrate
formed of a metal foil. The porous oxide is selected from among
alumina, titania, zirconia, silica, ceria, composite oxides formed
of a plurality of species thereof, and mixtures thereof.
[0041] The NO.sub.x occluding material is at least one selected
from among alkali metals, alkali earth metals, and rare earth
elements, and a mixture of alkali metal and alkali earth metal is
preferably useful. The NO.sub.x occluding material is preferably
supported in a range of an amount of 0.05.about.1 mol per liter of
the honeycomb substrate. The noble metal is selected from among Pt,
Rh, Pd, Ru, and Ir, and Pt having high oxidation activity is
particularly useful. The noble metal is preferably supported in a
range of an amount of 0.1.about.5 g per liter of the honeycomb
substrate.
[0042] The second catalyst is a filter catalyst obtained by forming
a catalyst layer consisting of a porous oxide support and at least
a noble metal supported thereon on at least the surface of a filter
substrate having a wall flow structure. The filter substrate is
composed of inlet cells clogged at the downstream side of the
exhaust gas flow direction, outlet cells adjacent to the inlet
cells and clogged at the upstream side of the exhaust gas flow
direction, and porous cell partitions having a plurality of fine
pores and sectioning the inlet cells and the outlet cells. As the
filter substrate, a conventional DPF made of heat resistant
ceramics such as cordierite or silicon carbide may be used.
[0043] The fine pores of the cell partition of the filter substrate
are preferably distributed to have porosity of 40.about.80% and an
average diameter of 10.about.50 .mu.m. In the case where the
porosity or average diameter falls outside of the above range, PM
capture efficiency is decreased and the loss of exhaust pressure
may be increased.
[0044] On at least the surface of the filter substrate, a catalyst
layer, including a porous oxide support and at least the noble
metal supported thereon, is formed. Further, the catalyst layer is
preferably formed on the inner surface of the fine pores of the
cell partition. The porous oxide is selected from among alumina,
titania, zirconia, silica, ceria, composite oxides formed of
plurality of species thereof, and mixtures thereof.
[0045] The noble metal is one or more selected from among noble
metals of a platinum group, including Pt, Rh, Pd, Ru, and Ir. The
noble metal is preferably supported in a range of an amount of
0.1.about.5 g per liter of the filter substrate. If the amount is
less than the lower limit, the activity is very low and thus is
unusable. On the other hand, if the amount exceeds the upper limit,
the activity is saturated and the cost is increased.
[0046] Preferably, the catalyst layer of the second catalyst
further includes a NO.sub.x occluding material which is selected
from among alkali metals, alkali earth metals, and rare earth
elements, and which is supported thereon, as in the first catalyst.
Due to the NO.sub.x occluding material included in the catalyst
layer, NO.sub.x purification activity is increased. The NO.sub.x
occluding material is preferably supported in a range of an amount
of 0.05.about.1 mol per liter of the filter substrate. If the
amount is less than the lower limit, the activity is very low and
thus unusable. On the other hand, if the amount exceeds the upper
limit, the catalyst metal is covered, and thus the activity thereof
is decreased.
[0047] In order to form the catalyst layer on the filter substrate,
porous oxide powder is formed into a slurry along with a binder
component, such as alumina sol, and water. Subsequently, the slurry
is attached to the partition and then burned. Subsequently, noble
metal and an NO.sub.x occluding material are supported thereon.
Although the attachment of the slurry to the cell partition may be
realized using a typical dipping process, the slurry may be
forcibly charged in the fine pores of the cell partition through
air blowing or suction. Here, the slurry remaining after being
charged in the fine pores is preferably removed.
[0048] The catalyst layer is formed in a range of an amount of
30.about.200 g per liter of the filter substrate. If the amount of
the catalyst layer is less than the lower limit, the durability of
the noble metal or NO.sub.x occluding material is decreased. On the
other hand, if the amount exceeds the upper limit, the loss of
pressure is excessively increased and thus is unusable.
[0049] The exhaust gas purifying apparatus of the present invention
comprises a reducing agent supply device supplying a reducing agent
to the exhaust gas. For example, an injector directly supplying a
liquid reducing agent, such as light oil, to the exhaust gas may be
used. In addition, a reducing agent may be supplied to the exhaust
gas under a fuel rich condition based on the air-to-fuel ratio in
the combustion chamber. In addition, active HC, resulting from
partial oxidation of a liquid reducing agent, such as light oil,
may be supplied to the exhaust gas. To this end, it is preferred
that an oxidation catalyst be disposed in the uppermost end of the
exhaust gas flow direction in the recovery process.
[0050] Also, in the container, a third catalyst is further
connected to the first catalyst and the second catalyst in series,
and thus the first catalyst, the second catalyst, and the third
catalyst, in that order, are preferably arranged. The third
catalyst is preferably at least one selected from among an NO.sub.x
occlusion reduction catalyst, a three-way catalyst, and an
oxidation catalyst, and preferably includes at least the NO.sub.x
occlusion reduction catalyst.
[0051] For example, in the case where the NO.sub.x occlusion
reduction catalyst is used as the third catalyst, the same
purification performance may be assured even when the normal flow
process is performed without changing the direction after the
recovery process. Thus, in the normal flow process, since the
second catalyst (filter catalyst) may be used in two directions, PM
and ash accumulated in the fine pores of the cell partition are
always discharged when the flow direction is changed, and an
increase in the loss of pressure may be effectively inhibited.
[0052] In addition, in the case where the third catalyst is
composed of the three-way catalyst or oxidation catalyst, the
exhaust gas primarily flows into the third catalyst in the recovery
process. Since the third catalyst has high oxidation activity, heat
generated when part of the reducing agent in the exhaust gas is
oxidized is responsible for drastically increasing the temperature
of the exhaust gas. Ultimately, even though low-temperature exhaust
gas is supplied, recovery from sulfur poisoning remains high,
thereby improving such capability.
[0053] In addition, in the case where the first catalyst is
provided upstream side of the exhaust gas flow direction, the first
catalyst is preferably composed of an oxidation catalyst or a
three-way catalyst disposed at the upstream side of the exhaust gas
and an NO.sub.x occlusion reduction catalyst disposed at the
downstream side thereof. On the other hand, in the case where the
third catalyst is provided upstream side of the exhaust gas flow
direction, the third catalyst is preferably composed of an
oxidation catalyst or three-way catalyst disposed at the upstream
side of the exhaust gas and an NO.sub.x occlusion reduction
catalyst disposed at the downstream side thereof. In such cases,
both functions are realized, so that the increase in the loss of
exhaust pressure is inhibited and recovery from sulfur poisoning is
improved.
[0054] In the exhaust gas purifying method of the present
invention, the exhaust gas atmosphere in a normal flow process may
be either a lean-burn atmosphere or an alternating lean/rich
atmosphere. In the latter case, the rich atmosphere may also be
provided in a recovery process or in a regeneration process, as
described below.
[0055] In the exhaust gas purifying method of the present
invention, the temperature of the exhaust gas containing the
reducing agent upon the recovery process preferably ranges from 650
to 700.degree. C. If the temperature is above the upper limit,
grain growth of the noble metal in the downstream catalyst results,
or PM accumulated in the second catalyst is rapidly combusted,
undesirably resulting in a broken filter substrate. On the other
hand, if the temperature is below the lower limit, recovery from
sulfur poisoning is decreased.
[0056] In the exhaust gas purifying method of the present
invention, a regeneration process is preferably further performed
by allowing the exhaust gas in the lean atmosphere added with the
reducing agent to flow in a pulsing manner to thereby generate
combustion heat which is then used to combust the PM accumulated in
the second catalyst, thus regenerating the PM capture function.
Although the accumulated PM may be combusted through the recovery
process, since the recovery process is not conducted in concurrence
with the regeneration process all of the time, the regeneration
process is preferably carried out in the case where the loss value
of exhaust pressure, determined by continuous detection, falls
within a predetermined range. As such, in the regeneration process,
the reducing atmosphere is typically weaker than in the recovery
process. Hence, it is preferred that the regeneration process
precede the recovery process. In the case where the recovery
process precedes the regeneration process, the temperature is
drastically increased and thus the honeycomb substrate may be
cracked, or may be melted and damaged. Consequently, the
regeneration process, which acts to gradually increase the
temperature, preferably precedes the recovery process.
[0057] Further, upon the regeneration process, the flow direction
of the exhaust gas in the normal flow process is preferably
changed. The amount of PM capture by the filter substrate is larger
at the upstream side (inlet cell) of the exhaust gas flow direction
upon the normal flow process. Also, a liquid reducing agent is
attached to an opening portion of the passage of the inlet cell,
and also a large amount of PM may adhere thereto. Therefore, in the
case where the flow direction of exhaust gas is changed in the
regeneration process, the PM and ash may be blown by the flow of
exhaust gas, resulting in increased regeneration efficiency.
EXAMPLE
[0058] Below, the present invention is described in detail through
the following examples and comparative examples.
Example 1
[0059] FIGS. 1 and 2 schematically illustrate the exhaust gas
purifying apparatus according to the present invention. In the
exhaust gas purifying apparatus, an NO.sub.x occlusion reduction
catalyst 2 (hereinafter referred to as NSR2) as a first catalyst
and a filter catalyst 3 (hereinafter, referred to as DPNR3) as a
second catalyst are sequentially arranged in series in a catalytic
converter 1. As such, DPNR3 is an NO.sub.x occlusion reduction type
filter catalyst. An exhaust pipe 100 from an exhaust manifold is
divided into two passages, that is, a first passage 101 and a
second passage 102, in front of the catalytic converter 1. Then,
the first passage 101 and the second passage 102 are combined with
each other again. That is, the first passage 101 and the second
passage 102 are respectively disposed at either side of the
catalytic converter 1, and are then connected to each other.
Further, in the divided portion of the exhaust pipe 100, a first
valve 200 for switching the exhaust gas from the exhaust pipe 100
to the first passage 101 or the second passage 102 is disposed. In
addition, in the first passage 101, a second valve 201 for turning
on or off communication between the opening of the catalytic
converter 1 and the first passage 101 is disposed. In addition, in
the second passage 102, a third valve 202 for turning on or off
communication between the other opening of the catalytic converter
1 and the second passage 102 is disposed. In addition, in the
exhaust pipe 100, an injector 103 for adding light oil to the
exhaust gas is further provided.
[0060] The NSR2 comprises a cordierite-based honeycomb substrate
(0.8 L, cell number 400/in.sup.2) having a straight flow structure
and 270 g/L of a catalyst layer formed thereon, the catalyst layer
including K, Ba, Li, and 5 g/L of Pt, supported thereon. Further,
the DPNR3 comprises a cordierite-based filter substrate (2.0 L,
cell number 300/in.sup.2) having a wall flow structure and 150 g/L
of a catalyst layer, the catalyst layer including K, Ba, Li, and 5
g/L of Pt, supported thereon. The catalyst layer is formed not only
on the surface of the cell partition but also on the inner surface
of fine pores thereof.
[0061] <Test>
[0062] The exhaust gas purifying apparatus thus constructed was
mounted to the exhaust system of a diesel engine for direct spray,
having 2 L of exhaust air volume, on an engine bench. Further,
while controlling lean and rich atmospheres to supply a rich spike
for 0.2 sec at intervals of 30 sec, a normal flow process for 100
hours (about 5000 km) was conducted under conditions simulating 11
Lap driven by actual automobiles. At intervals of 10 hours during
the normal flow process, a recovery process for adding light oil to
the exhaust gas at a flow rate of 1000 cm.sup.3/min for 200 sec
using the injector 103 was performed.
[0063] In the normal flow process, as illustrated in FIG. 1, the
second passage 102 is closed by the first valve 200, the
communication between the first passage 101 and the catalytic
converter 1 is allowed by the second valve 201, and the
communication between the second passage 102 and the catalytic
converter 1 is blocked by the third valve 202. Accordingly, the
exhaust gas flows into the catalytic converter 1 from the first
passage 101, sequentially passes through the NSR2 and DPNR3, and is
then discharged from the second passage 102.
[0064] In the recovery process, as illustrated in FIG. 2, the first
passage 101 is closed by the first valve 200, communication between
the second passage 102 and the catalytic converter 1 is allowed by
the third valve 202, and communication between the first passage
101 and the catalytic converter 1 is blocked by the second valve
201. Thus, the exhaust gas flows into the catalytic converter 1
from the second passage 102, sequentially passes through the DPNR3
and NSR2, and is then discharged from the first passage 101.
[0065] In the case where the exhaust gas flow direction shown in
FIG. 1 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 2 is set in a reverse flow direction, the
exhaust gas flow direction in the present example is given in Table
1 below.
TABLE-US-00001 TABLE 1 Process Normal Normal Normal Flow Recovery
Flow Recovery Flow . . . Flow Normal Reverse Normal Reverse Normal
. . . Direction
[0066] After the completion of the operation, the NO.sub.x
purification efficiency in the normal flow process was measured.
The results are given in Table 5 below.
Example 2
[0067] As illustrated in FIGS. 3 and 4, the exhaust gas purifying
apparatus of the present example is the same exhaust gas purifying
apparatus of Example 1, with the exception that NSR2, DPNR3, and
NSR2 are sequentially arranged in a catalytic converter 1. The NSR2
provided at each of the two sides of DPNR3 is the same as the case
in which the NSR2 of Example 1 is halved. As in Example 1, such an
exhaust gas purifying apparatus was mounted to the 2 L exhaust
system of a diesel engine for direct spray, and the same normal
flow process and recovery process were conducted.
[0068] After the recovery process, the normal flow process was
conducted under unchanged valve set conditions, and then the
recovery process was conducted with the exhaust gas flow direction
changed. That is, in the state shown in FIG. 3, the normal flow
process was performed for 10 hours, after which the recovery
process was performed in the state shown in FIG. 4, in which the
valves were converted. Thereafter, the normal flow process was
conducted for 10 hours under unchanged conditions, and then the
recovery process was conducted in the state shown in FIG. 3, in
which the valves were converted again, after which the normal flow
process was conducted for 10 hours under unchanged conditions.
These procedures were repeated.
[0069] In the case where the exhaust gas flow direction shown in
FIG. 3 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 4 is set in a reverse flow direction, the
exhaust gas flow direction of the present example is given in Table
2 below.
TABLE-US-00002 TABLE 2 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Reverse
Reverse Normal Normal Reverse Reverse . . . Direction
[0070] Further, in the recovery process, when the exhaust gas flow
direction is any one among the normal flow direction and the
reverse flow direction, the exhaust gas heated in the NSR2 flows
into the DPNR3. Thus, in the recovery process, a regeneration
process for combusting the PM accumulated in the DPNR3 may be
further performed. Accordingly, Table 2 is also given like Table 3
below.
TABLE-US-00003 TABLE 3 Process Normal Regener- Normal Regener-
Normal Flow ation Recovery Flow ation Recovery Flow . . . Flow
Normal Reverse Reverse Reverse Normal Normal Normal . . .
Direction
[0071] Thereafter, the NO.sub.x purification efficiency was
measured using the method of Example 1. Further, after measurement
of the NO.sub.x purification efficiency, the NSR2 and DPNR3 were
decomposed and the amount of sulfur poisoning was determined
through element analysis. In addition, from the difference in
weight compared to before the test, the amount of ash after the
combustion of PM was calculated. The amounts of sulfur poisoning
and ash were measured at the ends of the inlets and outlets of the
catalysts. The results are given Table 5 and FIGS. 5 and 6.
Comparative Example 1
[0072] The normal flow process and recovery process were conducted
in the same manner as in Example 2 using that the exhaust gas
purifying apparatus of Example 2, with the exception the state
shown in FIG. 3 was maintained and thus the exhaust gas flow was
not reversed.
[0073] In the case where the exhaust gas flow direction of FIG. 3
is set in a normal flow direction and the exhaust gas flow
direction of FIG. 4 is set in a reverse flow direction, the exhaust
gas flow direction in the present comparative example is given in
Table 4 below. That is, in the present comparative example, in any
one among the normal flow process and the recovery process, the
exhaust gas flow direction is maintained in a normal flow
direction.
TABLE-US-00004 TABLE 4 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Normal
Normal Normal Normal Normal Normal . . . Direction
[0074] Further, the NO.sub.x purification efficiency was measured
through the method of Example 1 and the amounts of sulfur poisoning
and ash were measured through the method of Example 2. The results
are given in Table 5 and FIGS. 5 and 6.
[0075] <Evaluation>
TABLE-US-00005 TABLE 5 No. NO.sub.x Purification (%) Example 1 80
Example 2 85 Comparative Example 1 64
[0076] According to the exhaust gas purifying method in the
examples, even after the diesel engine was driven for 100 hours,
the NO.sub.x purification efficiency was higher than in Comparative
Example 1. In Example 2, the amount of sulfur poisoning was much
lower than in Comparative Example 1. From this, in the examples,
high NO.sub.x purification efficiency is considered to have been
realized by virtue of high recovery from sulfur poisoning.
[0077] Further, in Comparative Example 1, related to the
conventional exhaust gas purifying method, the amount of sulfur
poisoning was higher toward the upstream side, and the sulfur
poisoning of NSR2 in the uppermost end was not recovered even
through the recovery process. As in Example 2, the exhaust gas flow
direction was changed to thereby perform the recovery process,
whereby the NSR2 on both sides of DPNR3 and the DPNR3 equally
recovered from sulfur poisoning, and the ash amount of the DPNR3
was decreased.
Example 3
[0078] As illustrated in FIG. 7, the exhaust gas purifying
apparatus of the present invention is the same exhaust gas
purifying apparatus as in Example 1, with the exception that NSR2,
DPNR3, and an oxidation catalyst 4 (hereinafter referred to "CCo4")
are sequentially arranged in a catalytic converter 1. As such, the
NSR2 is the same as the case in which the NSR2 of Example 1 is
halved. Further, CCo4 is composed of a cordierite-based honeycomb
substrate (2.0 L, cell number of 400/in.sup.2) having a straight
flow structure and 160 g/L of a catalyst layer formed thereon, the
catalyst layer including 5 g/L of Pt supported thereon.
[0079] Such an exhaust gas purifying apparatus was mounted to the 2
L exhaust system of a diesel engine for direct spray, as in Example
1, and a normal flow process and a recovery process were performed,
as in Example 2.
[0080] In the case where the exhaust gas flow direction shown in
FIG. 7 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 4 is set in a reverse flow direction, the
exhaust gas flow direction of the present example is the same as in
Example 2, and is given in Table 6 below.
TABLE-US-00006 TABLE 6 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Reverse
Reverse Normal Normal Reverse Reverse . . . Direction
[0081] Using the method of Example 1, the NO.sub.x purification
efficiency was measured, and also the HC purification efficiency
was measured in the normal flow process. Further, using the method
of Example 2, the amounts of sulfur poisoning and ash were
measured. The results are shown in Table 10 below. Since the
amounts of sulfur poisoning and ash are the same as in Example 2, a
figure related thereto is omitted.
Example 4
[0082] The normal flow process and recovery process were conducted
in the same manner as in Example 2 using the exhaust gas purifying
apparatus of Example 3, with the exception that the exhaust gas
flow direction was changed only in the recovery process. That is,
after the recovery process, the exhaust gas flow direction was
changed again and thus the normal flow process was conducted. In
the subsequent recovery process, the exhaust gas flow direction was
changed. That is, in the state shown in FIG. 7, a normal flow
process was performed for 10 hours. Thereafter, respective valves
were converted to thereby set the state corresponding to FIG. 4,
and thus the recovery process was performed, and then the normal
flow process was performed for 10 hours in the state shown in FIG.
7. These procedures were repeated.
[0083] In the case where the exhaust gas flow direction shown in
FIG. 7 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 4 is set in a reverse flow direction, the
exhaust gas flow direction of the present example is shown in Table
7 below.
TABLE-US-00007 TABLE 7 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Reverse
Normal Reverse Normal Reverse Normal . . . Direction
[0084] Using the method of Example 3, the NO.sub.x purification
efficiency and the HC purification efficiency were measured, and
the amounts of sulfur poisoning and ash were measured using the
method of Example 2. The results are shown in Table 10 below. Also,
since the amounts of sulfur poisoning and ash are the same as in
Example 2, a figure related thereto is omitted.
Example 5
[0085] As illustrated in FIG. 8, the exhaust gas purifying
apparatus of the present example is the same exhaust gas purifying
apparatus as in Example 1, with the exception that CCo4, NSR2,
DPNR3, NSR2, and CCo4 are sequentially arranged in a catalytic
converter 1. The NSR2 and CCo4 are the same as the case in which
each of the NSR2 and CCo4 of Example 4 is halved. In the present
example, although the NSR2 and CCo4 are separately formed, the
respective catalyst layers of NSR and CCo4 may be formed on a
single honeycomb substrate.
[0086] Such an exhaust gas purifying apparatus was mounted to a 2 L
exhaust system of a diesel engine for direct spray as in Example 1,
and a normal flow process and a recovery process were conducted
according to the method of Example 3.
[0087] In the case where the exhaust gas flow direction shown in
FIG. 8 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 4 is set in a reverse flow direction, the
exhaust gas flow direction of the present example is shown in Table
8 below.
TABLE-US-00008 TABLE 8 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Reverse
Reverse Normal Normal Reverse Reverse . . . Direction
[0088] Using the method of Example 3, NO.sub.x purification
efficiency and the HC purification efficiency were measured, and
the amounts of sulfur poisoning and ash were measured using the
method of Example 2. The results are shown in Table 10 below. Also,
since the amounts of sulfur poisoning and ash are the same as in
Example 2, a figure related thereto is omitted.
Comparative Example 2
[0089] The normal flow process and recovery process were conducted
in the same manner as in Example 3 using the exhaust gas purifying
apparatus of Example 3, with the exception that the state shown in
FIG. 7 was maintained and thus the exhaust gas flow direction was
not reversed.
[0090] In the case where the exhaust gas flow direction shown in
FIG. 7 is set in a normal flow direction and the exhaust gas flow
direction shown in FIG. 4 is set in a reverse flow direction, the
exhaust gas flow direction of the present comparative example is
given in Table 9 below. In the present comparative example, in any
one of the normal flow process and the recovery process, the
exhaust gas flow direction was maintained in a normal flow
direction.
TABLE-US-00009 TABLE 9 Process Normal Normal Normal Normal Flow
Recovery Flow Recovery Flow Recovery Flow . . . Flow Normal Normal
Normal Normal Normal Normal Normal . . . Direction
[0091] Using the method of Example 3, the NO.sub.x purification
efficiency and the HC purification efficiency were measured, and
the amounts of sulfur poisoning and ash were measured using the
method of Example 2. The results are shown in Table 10 below. Also,
since the amounts of sulfur poisoning and ash are the same as in
Comparative Example 1, a figure related thereto is omitted.
[0092] <Evaluation>
TABLE-US-00010 TABLE 10 No. NO.sub.x Purification (%) HC
Purification (%) Example 3 75 71 Example 4 82 75 Example 5 85 74
Comparative Example 2 64 70
[0093] According to the exhaust gas purifying method of the
examples, even after the diesel engine was driven for 100 hours,
the NO.sub.x purification efficiency and HC purification efficiency
were higher than in Comparative Example 2. In respective examples,
the amount of sulfur poisoning was much lower than in Comparative
Example 2. Consequently, high NO.sub.x purification efficiency is
considered to have been realized due to high recovery from sulfur
poisoning in the examples.
[0094] In Comparative Example 2, since the amount of sulfur
poisoning was increased toward the upstream side, the sulfur
poisoning of NSR2 was not recovered even through the recovery
process. However, when the exhaust gas flow direction was reversed
to perform the recovery process as in Example 3, the NSR2 and DPNR3
equally recovered from sulfur poisoning, and the ash amount of the
DPNR3 was decreased.
[0095] However, when comparing the examples, the NO.sub.x
purification efficiency of Example 3 was slightly lower. This is
because the rich spike is consumed in the CCo4 due to the flow of
the exhaust gas in the sequence of CCo4, DPNR3, NSR2 in the normal
flow process upon measurement. Thus, it is preferred that the NSR2
be disposed on both sides of the DPNR3.
[0096] As described hereinbefore, the exhaust gas purifying
apparatus and the exhaust gas purifying method, according to the
present invention, can be applied not only to the purification of
exhaust gases from diesel engines but also to the purification of
exhaust gases from gasoline engines, gas engines, boilers, etc.
[0097] While the invention has been shown and described with
respect to preferred embodiments thereof, it will be understood by
those skilled in the art that various changes and modification may
be made without departing from the spirit and scope of the
invention as defined in the following claims.
* * * * *