U.S. patent application number 10/593494 was filed with the patent office on 2007-06-28 for exhaust gas purification apparatus, exhaust gas purification method, and sulfur component trapping agent for internal combustion engine.
This patent application is currently assigned to BABCOCK-HITACHI K.K.. Invention is credited to Kazutoshi Higashiyama, Hidehiro Iizuka, Masato Kaneeda, Yuichi Kitahara, Osamu Kuroda.
Application Number | 20070144143 10/593494 |
Document ID | / |
Family ID | 34993762 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070144143 |
Kind Code |
A1 |
Kaneeda; Masato ; et
al. |
June 28, 2007 |
Exhaust gas purification apparatus, exhaust gas purification
method, and sulfur component trapping agent for internal combustion
engine
Abstract
An object of the present invention is to provide a new exhaust
gas purification apparatus for an internal combustion engine
operated under a condition of an air fuel ratio leaner than a
theoretical air fuel ratio, a method for purification of exhaust
gas and an exhaust gas purification catalyst, which is suitable for
suppressing degradation of the NOx purification catalyst by sulfur
components. An exhaust gas purification apparatus for an internal
combustion engine, which comprises an exhaust gas passage for an
internal combustion engine into which exhaust gas of lean air fuel
ratio and rich or stoichiometric air fuel ratio flows, a NOx
trapping catalyst that functions to trap NOx in the exhaust gas
when the air fuel ratio is lean, a sulfur component trapping agent
for trapping sulfur components in the exhaust gas, which is
disposed before the NOx trapping catalyst, and a catalyst for
oxidizing the sulfur components, which is disposed before the
sulfur component trapping agent, wherein the sulfur component
trapping agent has a trapping rate of 85 % or more of an amount of
inflow sulfur in a trapping test at a flow rate of 150 ppm
SO.sub.3-5% O.sub.2--balance being N.sub.2 gas per 1.5 moles of the
sulfur trapping agent at 300.degree. C. and a space velocity of
30,000/h for 1 hour; and the sulfur component trapping agent has a
release rate of sulfur amount of 5 % or less of sulfur trapped in
the sulfur component trapping agent in a release test under a flow
of a 3000 ppm H.sub.2-600 ppm C3H6-3000 ppm O.sub.2-3.5 %
CO--balance being N.sub.2 gas at a temperature elevation rate of
10.degree. C. /min from 250 to 750.degree. C. at an sulfur
component trapping agent entrance, after the trapping test.
Inventors: |
Kaneeda; Masato;
(Hitachinaka, JP) ; Iizuka; Hidehiro; (Mito,
JP) ; Higashiyama; Kazutoshi; (Naka, JP) ;
Kitahara; Yuichi; (Hitachinaka, JP) ; Kuroda;
Osamu; (Hitachi, JP) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
BABCOCK-HITACHI K.K.
Chiyoda-ku, Tokyo
JP
101-0021
|
Family ID: |
34993762 |
Appl. No.: |
10/593494 |
Filed: |
March 24, 2004 |
PCT Filed: |
March 24, 2004 |
PCT NO: |
PCT/JP04/04053 |
371 Date: |
September 20, 2006 |
Current U.S.
Class: |
60/277 ; 60/286;
60/295; 60/301 |
Current CPC
Class: |
F01N 3/106 20130101;
F01N 3/085 20130101; Y02A 50/20 20180101; F01N 3/035 20130101; F01N
2570/14 20130101; Y02T 10/12 20130101; F01N 13/009 20140601; F01N
13/0097 20140603; F01N 3/0807 20130101; F01N 3/0814 20130101; F01N
3/0842 20130101; F01N 2510/0682 20130101 |
Class at
Publication: |
060/277 ;
060/301; 060/286; 060/295 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10; F01N 7/00 20060101
F01N007/00 |
Claims
1. An exhaust gas purification apparatus disposed in an exhaust gas
passage of an internal combustion engine having a NOx purification
catalyst, which comprises a sulfur component trapping agent for
trapping sulfur components, which is arranged before the NOx
trapping catalyst and a catalyst for oxidizing the sulfur
components, disposed before the sulfur component trapping agent,
wherein the sulfur component trapping agent does not substantially
release the trapped sulfur components under the conditions of the
internal combustion engine.
2. An exhaust gas purification apparatus for an internal combustion
engine, which comprises an exhaust gas passage for an internal
combustion engine into which exhaust gas of lean air fuel ratio and
rich or stoichiometric air fuel ratio flows, a NOx trapping
catalyst that functions to trap NOx in the exhaust gas when the air
fuel ratio is lean, a sulfur component trapping agent for trapping
sulfur components in the exhaust gas, which is disposed before the
NOx trapping catalyst, and a catalyst for oxidizing the sulfur
components, which is disposed before the sulfur component trapping
agent, wherein the sulfur component trapping agent has a trapping
rate of 85% or more of an amount of inflow sulfur in a trapping
test at a flow rate of 150 ppm SO.sub.3-5% O.sub.2--balance being
N.sub.2 gas per 1.5 moles of the sulfur component trapping agent at
300.degree. C. and a space velocity of 30,000/h for 1 hour; and the
sulfur component trapping agent has a release rate of sulfur amount
of 5% or less of sulfur trapped in the sulfur component trapping
agent in a release test under a flow of a 3000 ppm H.sub.2--600 ppm
C.sub.3H.sub.6-3000 ppm O.sub.2--3.5% CO--balance being N.sub.2 gas
at a temperature elevation rate of 10.degree. C./min from 250 to
750.degree. C. at an sulfur component trapping agent entrance,
after the trapping test.
3. An exhaust gas purification apparatus for an internal combustion
engine, which comprises an exhaust gas passage for an internal
combustion engine into which exhaust gas of lean air fuel ratio and
rich or stoichiometric air fuel ratio flows, a NOx trapping
catalyst that functions to trap NOx in the exhaust gas when the air
fuel ratio is lean, a sulfur component trapping agent for trapping
sulfur components in the exhaust gas, which is disposed before the
NOx trapping catalyst, and a catalyst for oxidizing the sulfur
components, which is disposed before the sulfur component trapping
agent, wherein the sulfur component trapping agent has a trapping
rate of 60% or more of an amount of inflow sulfur in a trapping
test at a flow rate of 150 ppm H.sub.2S--0.5% O.sub.2--balance
being N.sub.2 gas at 300.degree. C. of the sulfur trapping agent
and a space velocity of 30,000/h for 1 hour.
4. An exhaust gas purification apparatus for an internal combustion
engine, which comprises a NOx trapping catalyst for trapping NOx,
which is disposed in an exhaust gas passage, a sulfur component
trapping agent disposed before the NOx trapping catalyst for
trapping sulfur components, and a catalyst disposed before the
sulfur component trapping agent for oxidizing the sulfur
components, wherein the sulfur component trapping agent contains at
least one of alkali metals and alkaline earth metals and a total
amount of Pt, Pd and Rh is at least 0.4% by weight of the sulfur
component trapping agent.
5. The exhaust gas purification apparatus according to claim 1,
wherein the sulfates contained in the sulfur component trapping
agent has a melting temperature or decomposition temperature of
750.degree. C. or higher.
6. The exhaust gas purification apparatus according to to claim 1,
wherein the sulfur component trapping agent is disposed below the
engine.
7. The exhaust gas purification apparatus according to claim 1,
which further comprises a filter disposed in upstream of the NOx
trapping catalyst, wherein an upstream side of the filter is
provided with a catalyst for oxidizing the sulfur components and a
downstream side of the filter is provided with the sulfur component
trapping agent.
8. The exhaust gas purification apparatus according to claim 1,
which further comprises a filter disposed at upstream of the NOx
trapping catalyst, wherein the sulfur component trapping agent if
formed on part of the filter, and the catalyst for oxidizing sulfur
components is formed on another part of the filter.
9. The exhaust gas purification apparatus according to claim 4,
wherein an amount of the alkali metals or the alkaline earth metals
is 1 to 4 moles or less in terms of (alkali metals /2+alkaline
earth metals).
10. The exhaust gas purification apparatus according to claim 1,
wherein the catalyst for oxidizing sulfur components contains at
least one of Pt, Pd and Rh.
11. The exhaust gas purification apparatus according to claim 1,
wherein the NOx trapping catalyst contains at least one of alkali
metals and alkaline earth metals and at least one of noble metals,
and has a function to trap SOx under a lean air fuel condition and
a function to release SOx in a rich or stoichiometric air fuel
condition by heating the catalyst to 500.degree. C. or higher.
12. The exhaust gas purification apparatus according to claim 1,
wherein the sulfur component trapping agent is replaceable with
another.
13. A sulfur component trapping agent containing an ingredient for
trapping sulfur components in an exhaust gas, wherein the
ingredient has a trapping rate of 85% or more of an amount of
inflow sulfur in a trapping test at a flow rate of 150 ppm
SO.sub.3-5% O.sub.2--balance being N.sub.2 gas per 1.5 moles of the
sulfur trapping agent at 300.degree. C. and a space velocity of
30,000/h for 1 hour; and the sulfur component trapping agent has a
release rate of sulfur amount of 5% or less of sulfur trapped in
the sulfur component trapping agent in a release test under a flow
of a 3000 ppm H.sub.2--600 ppm C.sub.3H.sub.6--3000 ppm
O.sub.2--3.5% CO--balance being N.sub.2 gas at a temperature
elevation rate of 10.degree. C./min from 250 to 750.degree. C. at
an sulfur component trapping agent entrance, after the trapping
test.
14. The sulfur component trapping agent according to claim 13,
wherein the sulfur component trapping agent contains at least one
selected from the group consisting of alkali metals, alkaline earth
metals, Ce, Al, Y, La and Ni.
15. A sulfur component trapping agent for trapping sulfur in an
exhaust gas, which comprises a honeycomb substrate made of
cordierite or metal, a porous support, and a sulfur trapping agent
supported on the porous support, wherein the sulfur trapping agent
contains 1 to 4 moles of at least one of alkali metals and alkaline
earth metals in (molar number of alkali metals /2+molar number of
alkaline earth metals) as conversion of elements, and the total
amount of Pt+Pd+Rh is 0.4% by weight or more per the sulfur
component agent.
16. A method of purification of an exhaust gas from an internal
combustion engine, which uses the sulfur component trapping agent
according claim 13.
17. A method of purification of an exhaust gas for an internal
combustion engine, which comprises oxidizing sulfur components in
the exhaust gas, trapping and accumulating the sulfur components in
a sulfur component trapping agent, and purifying NOx in the exhaust
gas with a NOx purifying catalyst.
18. The method of purification of an exhaust gas according to claim
16, which comprises a step for releasing the sulfur components from
the NOx purifying catalyst, wherein the releasing step is carried
out by changing the air fuel ratio to rich or stoichiometric and
elevating temperature of the NOx purifying catalyst to 500.degree.
C. or higher.
19. A method of diagnosis of degradation of a sulfur component
trapping agent in an exhaust gas purification apparatus comprising
a NOx purification catalyst, a sulfur component trapping agent
disposed before the NOx purification catalyst, and a sulfur
component oxidizing catalyst disposed before the sulfur component
trapping agent, which comprises measuring NOx purification rates
before and after a step of releasing a sulfur component from the
NOx purification catalyst and diagnosing a degradation of the
sulfur component trapping agent based on a difference or ratio of
the NOx purification rates.
20. A system for diagnosis of degradation of a sulfur component
agent in an exhaust gas purification apparatus comprising a NOx
purification catalyst for trapping NOx, a sulfur component trapping
agent disposed before the NOx purification catalyst for trapping
sulfur components, a sulfur component oxidizing catalyst disposed
before the sulfur component trapping agent, which comprises means
for diagnosing the sulfur component trapping agent in accordance
with the diagnosis method defined in claim 19 for every sulfur
component releasing step, and means for indicating replacement of
the sulfur component trapping agent when the sulfur component
trapping agent is degraded to a predetermined level.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purification
apparatus and exhaust gas purification method for an internal
combustion engine operated under a lean burn condition, wherein
fuel is leaner than a theoretical air/fuel ratio.
BACKGROUND ART
[0002] In recent years, there have been attention to lean burn
engines for lean fuel combustion in terms of an air fuel ratio. The
air fuel ratio represents a ratio of air to fuel in gas.
[0003] Three way catalysts that have been used for exhaust gas
purification of engines for theoretical air fuel ratio
(stoichiometry) combustion have difficulty in purifying NOx. Thus,
exhaust gas purifying catalysts for lean burn engines are
investigated. One of them is disclosed in Japanese patent laid-open
11-319564(Patent publication 1) is disclosed. The publication
discloses one of NOx absorbing metal oxides selected from alkali
metals, alkaline earth metals and rare earth metals and a noble
metal supported on a porous support. According to the publication,
NOx is effectively purified in a lean state of the air fuel ratio
by using the catalysts. However, it has been known that since the
exhaust gas from the lean burn engines contains SOx derived from
sulfur contained in gasoline, activity of the catalysts is
deteriorated by SOx, which reacts with components in the NOx
trapping catalysts. M-O+SO.sub.2+O.sub.2.fwdarw.M-SO.sub.4 (1)
[0004] (M: alkali metals or alkaline earth metals)
[0005] When the catalytic activity by sulfur component happens, it
has been contemplated that the catalysts are exposed to gas of high
temperature and rich or stoichiometric s air fuel ratio so as to
carry out a reaction shown by the formula (2), thereby to
regenerate the catalyst by desorbing the sulfur component (S
purge). M-SO.sub.4.fwdarw.M-O+SO.sub.2+O.sub.2 (2)
[0006] (M: alkali metals or alkaline earth metals)
[0007] In addition to the S purge, a method for controlling
absorption amount of SOx is disclosed in Japanese patent laid-open
8-192051 (Patent document 2) wherein composite oxides of Ti and Zr
are used as a support whereby the activity of the NOx trapping
catalysts is maintained even if sulfur components are present in
the exhaust gas.
[0008] Further, Japanese patent laid-open 11-169708 (Patent
document 3) discloses a SOx trapping agent is disposed before the
NOx trapping catalyst, thereby to trap SOx at the time of lean air
fuel ratio and release SOx at the time of rich air fuel ratio,
whereby an amount of inflow of sulfur components is reduced to
suppress the sulfur poisoning to the NOx trapping catalysts or NOx
purification catalyst.
DESCRIPTION OF INVENTION
[0009] The S purge makes fuel cost worse because it employs rich
air fuel ratio. When a large amount of sulfur components adheres to
the NOx trapping catalyst or NOx purification catalyst, an amount
of a reducing agent for removing sulfur components as the amount of
adhered sulfur components increases. As a result, a degree of rich
air fuel ratio becomes large (the air fuel ratio becomes small),
which makes the fuel cost worse. Further, once a large amount of
sulfur components adhere to the NOx trapping catalyst, sulfur
components that strongly react with the NOx trapping agent are hard
to be removed thereby to lower the purification performance.
[0010] On the other hand, at the time of regeneration of NOx
catalysts, sulfur components react with hydrogen, etc contained in
the exhaust gas and exhausted as hydrogen sulfide (H.sub.2S) as
shown in the formula 3, which makes the exhaust gas smell.
M-SO.sub.4+H.sub.2.fwdarw.M-O+H.sub.2S+O.sub.2 (3)
[0011] According to the technologies disclosed in Japanese patent
laid-open 8-192051 (Patent document 2), adhesion of sulfur
components to the NOx trapping catalyst at the time of lean air
fuel ratio is suppressed and NOx trapping performance becomes
lower.
[0012] According to the technologies disclosed in Japanese patent
laid-open 11-169708 (Patent document 3), though an amount of inflow
of sulfur components at the time of lean air fuel ratio, the sulfur
components are released from the SOx trapping agent disposed in
front of the NOx trapping catalyst at the time of rich air fuel
ratio, and the NOx catalyst is sulfur poisoned by the released
sulfur. Accordingly, Spurge is necessary, which leads to the above
problem, however.
[0013] Further, regeneration of sulfur trapping agent is necessary
at the time of rich air fuel ratio where sulfur components are
released, sulfur components are discharged though a NOx trapping
rate of NOx trapping catalyst increases.
[0014] In addition to the above, since development for increasing a
NOx purification rate under lean burn conditions has been made, an
exhaust gas purification systems that does not discharge sulfur
components has never been investigated. Since regeneration of NOx
trapping catalyst and sulfur trapping agent release sulfur
components, these sulfur components should be controlled.
[0015] An object of the present invention is to provide a sulfur
component trapping agent that removes the above-mentioned
disadvantages and can trap sulfur components for a long time, an
exhaust gas purification apparatus and an exhaust gas purification
method.
[0016] The present invention relates to an exhaust gas purification
apparatus for an internal combustion engine capable of lean burn
operation at an air fuel ratio leaner than the theoretical air fuel
ratio (18 or more), which comprises an exhaust gas passage of an
internal combustion engine into which exhaust gas of an air fuel
ratio of 14.7 or less flows, an NOx purification catalyst for
trapping NOx in the exhaust gas at the time of lean air fuel ratio,
and a sulfur component trapping agent, disposed in a previous stage
of or before the NOx tapping catalyst, for capable of trapping
sulfur components in the exhaust gas, wherein the sulfur trapping
agent does not desorb (or does not substantially release) at the
time of rich or stoichiometric air fuel ratio. The sulfur
components include sulfur and sulfur compounds, which are derived
from gasoline, light oil, lubricants, etc, and are present in the
exhaust gas.
[0017] The words "does not desorb" are used to mean that a damaging
amount of sulfur components are not released. For example, the
sulfur component trapping agent exhibit a trapping rate of 85% or
more with respect to an inflow amount of sulfur components when gas
composed of 150 ppm SO.sub.2-5% O.sub.2--balance being N.sub.2 is
flown through the sulfur component trapping agent at an SV of
30,000/h and at 300.degree. C. per 1.5 mol of the sulfur component
trapping agent, and after the above test, the sulfur trapping agent
exhibits an amount of 5% or less of sulfur components released from
the sulfur component trapping agent per an amount of trapped sulfur
components under conditions that a gas composition is 2000
ppmH.sub.2-500 ppmC.sub.3H.sub.6-3000 ppmO.sub.2-3.5% CO--balance
being N.sub.2, which is flown through the sulfur components
trapping and temperatures are elevated from 300 to 750.degree. C.
at a rate of 10.degree. C. /min.
[0018] Another feature of the present invention resides in
providing an exhaust gas purification apparatus that installs a
sulfur component trapping agent capable of sufficiently trapping
sulfur components under rich or stoichiometric conditions.
[0019] A still another feature of the present invention resides in
that a sulfur component trapping agent is disposed in the previous
stage of the NOx trapping catalyst so as to suppress reduction of
catalytic activity by the sulfur adhesion and a catalyst for
oxidizing sulfur in the exhaust gas is disposed in the previous
stage of the sulfur component trapping agent. The sulfur trapping
agent includes a support that holds sulfur components in the
exhaust gas.
[0020] As is disclosed in Japanese laid-open 11-169708, bonding
power of SO.sub.2 to the sulfur component trapping agent is weaker
than that of SO.sub.3, and the sulfur components are hard to be
trapped. This is because the number of oxygen atoms in SO.sub.3 is
large and because there is imbalance of electrons. Thus, acidity of
the SO.sub.3 is high. Accordingly, the sulfur components in the
exhaust gas are more easily trapped in the form of SO.sub.3 than
SO.sub.2, as shown in the formula 4.
SO.sub.2+O.sub.2.fwdarw.SO.sub.3 (4)
[0021] Further, when SO.sub.2 gas enters the sulfur trapping agent,
there is a possibility of reaction represented by the formula (5)
in addition to the formula (2) thereby to produce sulfites as well
as sulfates. M-O+SO.sub.2.fwdarw.M-SO.sub.3 (5)
[0022] (M: metal atom for the sulfur trapping agent)
[0023] In general, sulfites are less stable than sulfates, and thus
decomposition temperature of the sulfites is low. Accordingly, if
sulfites are produced dominantly, a temperature of the exhaust gas
elevates, and the temperature of the sulfur trapping agent
elevates, decomposition reaction of the sulfites takes place
thereby to release sulfur components from the sulfur trapping agent
and to cause a problem that a NOx trapping catalyst disposed at a
post stage may be poisoned by sulfur components.
[0024] By disposing a catalyst for oxidizing sulfur components in
the exhaust gas in front of the sulfur component trapping agent,
sulfur components are trapped as SO.sub.3. As examples of sulfur
component oxidizing catalysts, noble metals such as Rh, Pt, Pd,
etc, and any other catalysts that can oxidize sulfur can be
employed.
[0025] Although a sulfur component trapping agent supported
together with sulfur component oxidizing component are supported on
the same support may trap the sulfur components, such sulfur
trapping agents are not proper if the sulfur oxidizing catalysts
oxidize sulfur components and accelerate decomposition, which leads
to release of sulfur from the sulfur component trapping agent.
[0026] The sulfur component trapping agent according to the present
invention is featured by that the sulfur trapping agent is an oxide
or carbonate of the metals and is capable of forming sulfates or
sulfites that hardly release or discharge trapped sulfur components
even when a temperature of the sulfur trapping agent elevates. The
sulfur trapping agent is featured by sulfates of alkali metals such
as Li, Na, K, Rb, Cs, etc, alkaline earth metals such as Mg, Ca,
Sr, Ba, etc, or Ce, Al, Y, La, Ni, etc, which form sulfates having
high melting points or high decomposition temperatures, and is also
featured by not substantially containing such components as Ph, Pt,
Pd, etc.
[0027] If the components such as noble metals that effect reaction
of decomposition of sulfites are contained in the SOx trapping
agent, the sulfur components trapped as the sulfites in the SOx
trapping agent may be released in accordance with the formula (2)
in a rich air fuel ratio. Accordingly, it is preferable that any
components that contribute to decomposition reaction of sulfites
are not contained in the sulfur component trapping agent. A
preferable total amount of the noble metals should be 0.4% by
weight or less, and more preferably, 0.3% by weight or less.
[0028] The present invention also relates to a filter, disposed in
an exhaust gas passage of an internal combustion engine, for
purification of exhaust gas from an internal combustion engine, a
part of which carries a sulfur oxidizing catalyst and the other
part of which carries a sulfur component trapping agent. The
present invention relates to an exhaust gas purification apparatus
using the exhaust gas purification filter. The exhaust gas
purification apparatus is featured by disposing the filter in such
a manner that the exhaust gas is introduced into the sulfur
component oxidizing catalyst side and discharged from the sulfur
component trapping agent side.
[0029] The sulfur component oxidizing catalyst may be supported on
the surface of the upstream side of the filter, which is a plate or
shaped flat filter, as same as in diesel particulate filters.
[0030] The sulfur component trapping agent or the exhaust gas
purification apparatus according to the present invention can
suppress degradation of the NOx purification catalyst for the
internal combustion engine, which is operated under the lean burn
condition of which the air fuel ratio is leaner than that of
theoretical air fuel ratio.
[0031] Further, the present invention can provide an internal
combustion engine for automobiles and an exhaust gas purification
apparatus that has good fuel consumption, suppressed degradation of
a NOx purification catalyst caused by sulfur component and a small
amount of sulfur component emission.
BRIEF DESCRIPTION OF DRAWINGS
[0032] FIG. 1 is an arrangement of a NOx trapping catalyst, sulfur
component trapping agent and sulfur component oxidizing
catalyst.
[0033] FIG. 2 is a graph showing a SOx trapping rate with respect
to disposition and non-disposition of the sulfur oxidizing
catalyst.
[0034] FIG. 3 is a graph showing release of H.sub.2S from sodium
sulfate.
[0035] FIG. 4 is a graph showing release of SO.sub.2
LiPt/Al.sub.2O.sub.3.
[0036] FIG. 5 is a graph showing a sulfur release rate with respect
to a content of Rh+Pt+Pd.
[0037] FIG. 6 is a diagrammatic view of a DPF wherein the sulfur
component oxidizing catalyst is supported on the upper-stream side
of DPF and the sulfur component trapping agent is supported on the
down-stream side of the DPF.
[0038] FIG. 7 shows an arrangement of the sulfur component trapping
agent, which is disposed immediately after the engine.
[0039] FIG. 8 is a graph showing degradation of NOx trapping
catalyst 1 depending on time.
[0040] FIG. 9 is a graph showing relationship between a sulfur
trapping rate by the sulfur component trapping agent and sulfur
purge frequency.
[0041] FIG. 10 is a graph showing an amount of sulfur component on
the NOx trapping catalyst depending on gas atmosphere.
[0042] FIG. 11 is a graph showing regeneration of the sulfur
component trapping agent by sulfur purging.
[0043] FIG. 12 is a diagrammatic view of the exhaust gas
purification apparatus according to the present invention.
BEST EMBODIMENTS FOR PRACTICING THE INVENTION
(Investigations of Sulfur Component Trapping Agents)
[0044] As the sulfur component trapping agents, components that
hardly release and discharge sulfur components even when a
temperature of exhaust gas changes, which leads to temperature
change of the sulfur component trapping agent were
investigated.
[0045] Since the temperature of the exhaust gas varies from room
temperature to 650.degree. C., the sulfates of the sulfur component
trapping agents should preferably have a melting temperature or
decomposition temperature of 750.degree. C. or or higher. When the
melting temperature or decomposition temperature of the sulfates is
750.degree. C., decomposition of sulfates hardly occurs and release
and discharge of sulfur components are prevented. Table 1 shows
melting points or decomposition temperatures of various sulfates
(Refer to Chemistry Encyclopedia: Basic Version II, Japan Chemical
Society, Maruzen Publishing Co.). TABLE-US-00001 TABLE 1 Melting
points and decomposition temperatures Melting point Element for a
sulfur (*; decomposition trapping agent Sulfate temperature) Li
Li.sub.2SO.sub.4 1256 Na Na.sub.2SO.sub.4 884 K K.sub.2SO.sub.4
1069 Rb Rb.sub.2SO.sub.4 1060 Cs Cs.sub.2SO.sub.4 1010 Mg
MgSO.sub.4 1185 Ca CaSO.sub.4 1450 Sr SrSO.sub.4 1605 Ba BaSO.sub.4
1580 Mn MnSO.sub.4 700 Ce Ce.sub.2(SO.sub.4).sub.3 920* Al
Al.sub.2(SO.sub.4).sub.3 770 La La.sub.2(SO.sub.4).sub.3 1150* Fe
Fe.sub.2(SO.sub.4).sub.3 480* Y Y.sub.2(SO.sub.4).sub.3 1000* Zn
ZnSO.sub.4 600* Co CoSO.sub.4 735 Zr
Zr(SO.sub.4).sub.2.cndot.4H.sub.2O 380* Cu CuSO.sub.4 200 Ni
Ni.sub.2SO.sub.4 848
[0046] From Table 1, it is considered that sulfates of alkali
metals such as Li, Na, K, Rb, Cs, etc, alkaline earth metals such
as Mg, Ca, Sr, Ba, etc and Ce, Al, Y, La, Ni, etc have high melting
point or decomposition temperatures and they are suitable as the
sulfur component trapping agents.
[0047] When the sulfur component trapping agent is the alkali
metals or alkaline earth metals, for example, sulfates of the
alkali metals or alkaline earth metals are produced by reaction
according to the formula (1) when SOx contained in the exhaust gas
in the lean air fuel ratio contacts with the sulfur component
trapping agents.
[0048] As is discussed above, when the noble metals are present in
the sulfur component trapping agents, release of sulfur components
tends to occur at the time of rich air fuel ratio. Accordingly, the
noble metals such as Rh, Pt, Pd, etc should preferably be not
contained in the sulfur component trapping agents. A total amount
of Rh+Pt+Pd should be 0.4% by weight or less per a weight of the
sulfur component trapping agent. An amount of the sulfur component
trapping agents depends on kinds of fuels used and required
performance.
[0049] Estimation on the total sulfur components exhausted from a
diesel car for a running at a speed of 16000 km was made. If a fuel
consumption is 20 km/L, an amount of light oil is 8000 L. On the
other hand, if a specific gravity of the light oil is 0.85 g/cc and
a concentration of sulfur components in the light oil is 10 ppm, a
total sulfur exhausted is 2.1 moles. Accordingly, in order to trap
all of the sulfur, 4.2 moles of Na.sub.2CO.sub.3 is required
thereby to convert Na.sub.2CO.sub.3 to Na.sub.2SO.sub.4, when
Na.sub.2CO.sub.3 is used as the sulfur component trapping
agent.
[0050] On the other hand, when CaO is used as the sulfur component
trapping agent, 2.1 moles of CaO is required to convert CaO to
CaSO.sub.4. Accordingly, when the alkali metals are used, at least
4.2 moles are necessary, and when the alkaline earth metals are
used, at least 2.1 moles the alkali metals are necessary.
[0051] Although only the sulfur trapping components can be used in
the sulfur component trapping agent, the sulfur trapping components
may be supported on a porous support. The porous support may
function to enhance dispersion of the sulfur trapping components.
When the porous support is used, an amount of the sulfur trapping
component, which is 0.2 mole part or less in a metallic element
conversion per 1.9 mole parts of the porous support, is
insufficient for sulfur component trapping performance if a large
amount of sulfur components is generated. If the amount of the
sulfur trapping component is 0.8 mole part or larger, the sulfur
component trapping agent tends to cohere on the porous support
thereby to decrease the sulfur trapping capacity. Accordingly, an
amount of the sulfur trapping components is preferably 0.2 to 0.8
mole part per 1.9 mole parts of the porous support.
[0052] In the specification, the words "mole part" are used to
represent concentration rates of respective components in
conversion of moles; if a supported amount of component B is 0.5
mole part per 1 mole part of component A, a mole ratio of the
amount of B component to that of A component is 0.5, irrespective
of an absolute amount of A component.
[0053] The porous support may be supported on a substrate; in this
case, a preferable supporting amount of the support is 30 to 400 g
per 1 L of the substrate for the sulfur component trapping
performance. If the supporting amount of the porous support is less
than 30 g, the performance of the porous support will be
insufficient, and if the supporting amount of the porous support is
larger than 400 g, the specific surface area of the porous support
itself becomes small. The above are not preferable.
[0054] If the amount of the porous support is too small, a sulfur
trapping amount becomes small, and if the amount id too large, the
specific surface area of the porous support becomes small.
[0055] As the porous supports, there are metal oxides or composite
oxides of alumina, titania, silica, silica-alumina, zirconia,
magnesia, etc. Alumina is particularly suitable because it is good
in heat resistance and has a function of good dispersion of the
sulfur trapping components thereon.
[0056] Various shapes of the sulfur component trapping agents and
sulfur oxidizing catalysts may be employed in accordance with
applications. Honeycomb structures produced by coating the sulfur
component trapping agent and the sulfur component oxidizing agent
on a honeycomb substrate made of cordierite, SiC, stainless, etc,
or pellets, plates, granules, powders may be used. In the case of
honeycomb structures, cordierite is the most preferable. However,
if there is a possibility of temperature elevation of the catalyst,
metallic substrates, which hardly react with the catalysts, would
bring about good results. Further, it is possible to produce
honeycombs made of only the sulfur component trapping agents and
the sulfur component oxidizing agents.
(Investigations on NOx Purification Catalyst Components)
[0057] Any NOx trapping catalysts that trap and purify NOx can be
used; NOx trapping catalysts comprise at least one selected from
alkali metals and alkaline earth metals and a noble metal, which
trap NOx at high efficiency and high NOx purification
performance.
[0058] Methods for preparing the catalysts include physical
preparation methods and chemical reaction preparing methods such as
impregnation methods, mixing methods, co-precipitation methods,
sol-gel methods, ion-exchange methods, evaporation methods,
etc.
[0059] As starting materials for the exhaust gas purification
catalysts, various compounds, metals and metal oxides such as
nitrate compounds, chelate complex compounds, hydroxides,
carbonates, organic compounds, etc can be used.
(Investigations on Trapping Amounts in Rich Air Fuel Ratio)
[0060] Most of sulfur components in the exhaust gas are present in
a form of SOx when the exhaust gas is in a lean air fuel ratio. On
the other hand, when the air fuel ratio is rich, it is conceived
that the sulfur components are present mainly as H.sub.2S. Since a
period of the rich air fuel ratio is shorter than that of the lean
air fuel ratio, a large reduction in trapped sulfur components is
not required unless the trapped sulfur components are released in
the lean air fuel ratio. The reduction rate is defined as an amount
of sulfur present after the sulfur trapping agent/an amount of
sulfur flowing into the sulfur trapping agent.times.100 (%).
[0061] When sulfur components are present in the form of H.sub.2S,
there is a case where degradation of NOx trapping catalysts by
sulfur components is small. Considering that the NOx trapping
catalysts are degraded by the reaction represented by the formula
(1), degradation of the NOx catalyst is less problematic even if
the reduction in the trapped sulfur components is low, when the
amount of the sulfur components that enter the NOx trapping
catalysts at the time of rich air fuel ratio is small is
lowered.
[0062] Accordingly, if there is almost no lean air fuel ratio
condition in a running mode of the internal combustion engine, it
is possible to reduce an amount of sulfur component trapping agents
of the exhaust gas purification apparatus thereby to lower the cost
of the apparatus and to save space thereof.
[0063] Such sulfur component trapping agents as to trap 60% of the
sulfur components entering the sulfur component tripping agents are
preferably used. The trapping rate of 85% or more is more
preferable. That is, when a temperature of the sulfur component
trapping agent is set to 300.degree. C. and gas consisting of 150
ppm H.sub.2S-0.5% O.sub.2--balance being N.sub.2 is flown at a
space velocity SV of 30,000/h for 1 hour, a sulfur component
trapping agent having a trapping rate of 60% or more should
preferably be used; 85% is more preferable.
[0064] The present invention provides an internal combustion engine
provided with the sulfur component trapping agent. The
above-mentioned DPF is provided with a sulfur component trapping
agent as an example for trapping sulfur components. Further, an
internal combustion engine equipped with the above-mentioned sulfur
component trapping agent and the catalyst in the exhaust duct is
operated under a lean air fuel ratio, and then the operation is
switched to a rich or stoichiometric air fuel ratio, followed by
switching to lean air fuel ratio to perform purification of the
gas. If poisoning of the NOx trapping catalyst proceeds, a
temperature at the entrance of the NOx trapping catalyst is
elevated and gas of a stoichiometric or rich air fuel ratio is
flown thereby to release sulfur components from the NOx trapping
catalyst and regenerate the catalyst.
EMBODIMENTS
[0065] In the following embodiments of the present invention will
be described. The present invention is not limited to these
embodiments.
(Preparation Methods of NOx Catalyst)
[0066] After a slurry containing alumina powder and a precursor of
alumina adjusted with nitric acid was coated on a cordierite
honeycomb (400 cells/in.sup.2) and the coating was dried thereby to
produce an alumina-coated honeycomb with an amount of alumina of
1.9 moles per 1 L of an appearance volume.
[0067] After the alumina-coated honeycomb was impregnated with a
first impregnation component containing cerium nitrate, the
honeycomb was dried at 120.degree. C., followed by calcining it at
600.degree. C. for 1 hour. Then the Ce carrying honeycomb was
impregnated with a second impregnation component mixture containing
dinitrosodiamine platinum nitrate solution, dinitrosodiamine
palladium nitric acid solution, rhodium nitrate solution and
potassium acetate, and the honeycomb was dried at 200.degree. C.,
followed by calcining it at 600.degree. C. for 1 hour. Then, the
honeycomb impregnated with Ce, Rh, Pt, Pd and K was impregnated
with a third impregnating component mixture containing potassium
acetate, sodium nitrate, lithium nitrate and titanium sol.
Thereafter, the honeycomb was dried at 200.degree. C., followed by
calcining it at 600.degree. C. for 1 hour. The potassium acetate in
the second and third impregnation mixture was the same. At the
final step, the honeycomb was treated in an electric oven at
700.degree. C. for 5 hours.
[0068] According to the above-mentioned procedure, a NOx trapping
catalyst 1 was prepared wherein the composition was 190 g of
alumina, 27 g of Ce, 12.4 g of Na, 15.6 g of K, 0.4 g of Li, 4.3 g
of Ti, 0.139 g of Rh, 2.792 g of Pt and 1.35 g of Pd, in conversion
of elements.
(Preparation of Sulfur Component Trapping Agent)
[0069] Sulfur component trapping agents were prepared from Li, Na,
K, Cs, Mg, Ca, Sr, Ba, La and Fe, respectively. The alkali metals
were used in the form of carbonates, alkaline earth metals were in
the form of oxides, and La.sub.2O.sub.3 and FeO. Amounts of Li, Na,
K, Cs, Mg, Ca, Sr, Ba, La and Fe were 1.5 moles per 1 liter of the
honeycomb. In accordance with the same manner as in the preparation
of the NOx trapping catalyst 1, sulfur component trapping agents
were prepared wherein the above components were impregnated in the
cordierite honeycombs to produce sulfur component trapping agents
A, B, C, D, E, F, G, H, I and J.
Embodiment 1
Installation of Sulfur Component Oxidizing Catalyst
[0070] As shown in FIG. 1, the sulfur component trapping agent B
(Na.sub.2CO.sub.3) was installed before the NOx trapping catalyst 1
in an engine exhaust gas duct. Further, a sulfur component
oxidizing catalyst was disposed before the sulfur component
trapping agent to constitute a purification apparatus. As the
sulfur component oxidizing catalyst, Al.sub.2O.sub.3 with Pt
prepared in the same manner as in the method for preparing the NOx
trapping catalyst was used. The sulfur component oxidizing catalyst
comprises 190 g of alumina per 1 liter of the honeycomb, 2.792 g of
Pt, in conversion of elements.
[0071] In FIG. 1, a comparative embodiment 1 had no sulfur
component oxidizing catalyst. SOx containing lean gas was supplied
to the sulfur component trapping agents to evaluate sulfur trapping
rates by the sulfur component trapping agents.
[0072] Temperatures of the sulfur component oxidizing catalysts and
sulfur component trapping agents were 400 degrees and 300.degree.
C., respectively. A flow rate of Sox containing lean gas was 3
liters/min and concentration of SO.sub.2 was 150 ppm. The flow time
was 1 hour.
[0073] The composition of the lean gas supplied to the sulfur
component trapping agents is shown in Table 2. The sulfur component
trapping rates were calculated by the equation (6) below. Sulfur
component trapping rate (%)=(Sulfur amount of sulfur components
trapped by the sulfur component trapping agent, mole)/an inflow
amount of sulfur to the Sulfur component trapping agent,
mole).times.100(%) (6)
[0074] TABLE-US-00002 TABLE 2 Lean gas composition Gas components
Gas concentration (%) Gas amount (mmol/h) O.sub.2 5 401.8 N.sub.2
balance balance SO.sub.3 150 ppm 1.215
[0075] FIG. 2 shows sulfur component trapping rates in cases where
the sulfur component oxidizing catalyst was disposed and was not.
It is apparent from FIG. 2 that the disposition of the sulfur
component oxidizing catalyst exhibits higher sulfur trapping rates
with the sulfur component trapping agent.
[0076] As is disclosed in Japanese patent laid-open 11-169708, it
has been known that Pt functions as a sulfur component oxidizing
catalyst. Accordingly, when the sulfur component oxidizing catalyst
is disposed, SO.sub.2 oxidation reaction takes place as represented
by the formula (4), and the sulfur components entering the sulfur
component trapping agent are mainly SO.sub.3. Since the bonding
force of SO.sub.2 to the sulfur component trapping agent is weaker
than that of SO.sub.3, SO.sub.2 is hardly trapped. Thus, sulfur
components in the form of SO.sub.3 are more easily trapped than in
the form of SO.sub.2 gas. Accordingly, the disposition of the
sulfur component oxidizing catalyst before the sulfur component
trapping catalyst improves the sulfur component trapping rates, as
is apparent from FIG. 2.
Embodiment 2
Evaluation of Sulfur Component Trapping Performance of the Sulfur
Component Trapping Agent
[0077] In the system shown in FIG. 1, amounts of sulfur trapped by
the sulfur component trapping agent were calculated in a
thermodynamic method when SOx containing lean gas was supplied to
the sulfur component trapping agent. As calculation software, MALT2
(thermodynamic database for personal computer; Japan Society of
thermodynamics) was used.
[0078] A temperature of the sulfur component trapping agent was
300.degree. C. A flow rate of the SOx containing lean gas was 3
L/min. The sulfur components in the gas were oxidized to SO.sub.3
by the sulfur component oxidizing catalyst disposed before the
sulfur component trapping agent. Thus, a concentration of SO.sub.3
was estimated as 150 ppm. A period of flow time was 1000 hours. Gas
components other than SO.sub.3 were shown in Table 3. The sulfur
trapping rates were calculated by the equation (6). TABLE-US-00003
TABLE 3 Sulfur trapping rates of the sulfur component trapping
agents A, B, C, D, E, F, G, H, I and J. Sulfur component Sulfur
component trapping agent trapping rate (%) A Li.sub.2CO.sub.3 100 B
Na.sub.2CO.sub.3 100 C K.sub.2CO.sub.3 100 D Cs.sub.2CO.sub.3 100 E
MgO 100 F CaO 100 G SrO 100 H BaO 100 I La.sub.2O.sub.3 100 J FeO
0
[0079] From the above table, it is apparent that when the sulfur
component trapping agents A to I are used, the sulfur trapping
rates exceed 85%, which are sufficiently high. Accordingly, it is
possible to sufficiently suppress the sulfur components entering
the NOx trapping catalyst 1 disposed after the sulfur component
trapping agent. Further, the sulfur component trapping agents A to
I were converted into sulfates of alkali metals, alkaline earth
metals and La.sub.2O.sub.3 by virtue of trapping sulfur components.
The sulfur trapping reactions of the alkali metals and alkaline
earth metals with sulfur components may be represented by the
following formulae.
M.sub.2CO.sub.3+SO.sub.2+1/2O.sub.2.fwdarw.M.sub.2SO.sub.4+CO.sub.2
(7)
[0080] (M: alkali metal) M'O+SO.sub.2+1/2O.sub.2.fwdarw.M'SO.sub.4
(8)
[0081] (M': alkaline earth metal)
[0082] The melting points or decomposition temperatures of the
sulfates of alkali metals, alkaline earth metals and La sulfate are
higher than 750.degree. C. ; thus, once the sulfates are formed,
they are not decomposed unless the temperature becomes above
750.degree. C. Accordingly, if the temperature of the sulfur
component trapping agents is below 750.degree. C., the sulfur
components trapped by they are not released again even when a
normal temperature rise of the exhaust gas.
Embodiment 3
Evaluation on Decomposition Sulfates of Sulfur Component Trapping
Agents
[0083] Decomposition of sulfates of the sulfur component trapping
agents was evaluated. Sulfates/Al.sub.2O.sub.3 prepared by drying
and mixing various sulfates and Al.sub.2O.sub.3 were used. A
concentration of the sulfur component trapping agents per 10 grams
of alumina was 0.04 mol in conversion of elements.
[0084] As samples of sulfates of the sulfur component trapping
agents, sulfates of alkali metals, alkaline earth metals, and
sulfates of Ce, Al, La, Y and Ni were chosen.
[0085] Powders of the above sulfur component trapping agents were
granulated into 0.85 to 1.70 mm diameter. The temperature of the
sulfur trapping agents was kept at 300.degree. C. and the rich gas
shown in Table 4 was flown through the trapping agents, while
elevating a temperature of from 250 to 750.degree. C.
Concentrations of sulfur components (SO.sub.2+H.sub.2S) released
from the trapping agents were measured. A space velocity SV of the
gas was 30,000/h. TABLE-US-00004 TABLE 4 Gas composition
Composition (Rich air fuel ratio) N.sub.2 Balance H.sub.2 3000 ppm
CO 3.5% O.sub.2 3000 ppm C.sub.3H.sub.6 600 ppm
[0086] Decomposition performance of the sulfur component trapping
agents was calculated in accordance with the following equation.
Sulfur component release rate (%)=(an amount of sulfur released
until 750.degree. C. (mol)/(an amount of sulfur trapped by the
trapping agent mol).times.100(%) (9)
[0087] The results are shown in Table 5 below. TABLE-US-00005 TABLE
5 Evaluation on decomposition performance of sulfates of sulfur
trapping agents Sulfur trapping Sulfur component agent Sulfate
release rate (%) Li Li.sub.2SO.sub.4 1 Na Na.sub.2SO.sub.4 0 K
K.sub.2SO.sub.4 0 RB Rb.sub.2SO.sub.4 0 Cs Cs.sub.2SO.sub.4 0 Mg
MgSO.sub.4 2.5 Ca CaSO.sub.4 2.5 Sr SrSO.sub.4 2 Ba BaSO.sub.4 1.5
Ce Ce.sub.2(SO.sub.4).sub.3 4 Al Al.sub.2(SO.sub.4).sub.3 4.5 La
La.sub.2(SO.sub.4).sub.3 3 Y Y.sub.2(SO.sub.4).sub.3 3 Ni
NiSO.sub.4 4.5
[0088] It is apparent from Table 5 that the sulfur component
release rates of the sulfur component trapping agents are 5% or
less; the sulfur components trapped by the sulfur component
trapping agents are hardly released and the trapping agents are
suitable for the sulfur component trapping agents.
Embodiment 4
Influence of Coexistence of Noble Metals
[0089] Influence of coexistence of noble metals and the sulfate
(Na.sub.2SO.sub.4) in the trapping agent B on decomposition of the
sulfate was evaluated.
[0090] Na.sub.2SO.sub.4 was impregnated with a Pd solution, and the
impregnated Na.sub.2SO.sub.4 was dried at 160.degree. C., followed
by calcining it at 600.degree. C. for 1 hour. The resulting was
mixed with dried Al.sub.2O.sub.3 to prepare a Pd containing
catalyst (Na.sub.2SO.sub.4--Pd/Al.sub.2O.sub.3). A catalyst
(Na.sub.2SO.sub.4/Al.sub.2O.sub.3) was prepared in the same manner
as in the above, except for not containing Pd. The above catalysts
were used in the evaluation. Concentration of Na and Pd were 0.04
mol of Na and 0.15 g of Pd per 10 g of Al.sub.2O.sub.3, in
conversion of elements.
[0091] Powders of the above catalysts were granulated into 0.85 to
1.70 mm in diameter. The temperature of the catalysts was kept at
300.degree. C. and rich gas was flown through the catalysts, while
elevating temperatures from 250 to 800.degree. C. to measure
amounts of sulfur components (SO.sub.2+H.sub.2S) released from the
catalysts. The space velocity SV of the gas was 30,000/h.
[0092] FIG. 3 shows the results. In the case of Pd impregnation,
when the temperature of the catalysts exceeds 500.degree. C.,
release of sulfur components was observed. This means decomposition
of the sulfates. As the temperature of the catalysts elevates,
amounts of released sulfur components increased; the amount of
released sulfur components around 750.degree. C. was over 200
ppm.
[0093] On the other hand, in the case of the catalyst not
containing Pd, release of sulfur component was not observed when
the temperature was elevated to 800.degree. C. In the case of Pd
impregnation, an amount of released sulfur until 800.degree. C. was
9% per an amount of sulfur, which was originally contained in the
catalyst. From the above fact, when Pd is impregnated,
decomposition of sodium sulfate takes place at a temperature of
500.degree. C. or higher.
[0094] If the noble metals and the sulfur component trapping agents
are in contact, release of the trapped sulfur components occurs
when the rich gas flows thereby to bring about poisoning of the NOx
trapping catalyst 1. Accordingly, it is preferable that the sulfur
component trapping agents do not contain noble metals.
Embodiment 5
Influence of Coexistence of Noble Metals
[0095] Decomposition performance of lithium sulfate using a sulfur
component trapping agent comprising LiPt/Al.sub.2O.sub.3, which was
prepared in the same manner as of the NOx trapping catalyst, was
evaluated. Additive amounts were 190 g of alumina per 1 L of the
honeycomb, 1.8 g of Li and 2.792 g of Pt, in conversion of
elements.
[0096] A temperature of the LiPt/Al.sub.2O.sub.3 catalyst was set
to 300.degree. C., and a lean model gas whose composition is shown
in Table 6 was flown for 1 hour thereby to cause the catalyst to
absorb sulfur components. Considering that the sulfur component
trapping agent is installed in the system shown in FIG. 1, the
sulfur components to be flown through the trapping agent
LiPt/Al.sub.2O.sub.3 was SO.sub.3.
[0097] While the rich gas whose composition is shown in Table 6 was
being flown through the catalyst, the temperature of the gas was
elevated from 300 to 700.degree. C. ; then concentrations of
SO.sub.2 released from the LiPt/Al.sub.2O.sub.3 catalyst were
measured. The space velocity SV of the gas was 30,000/h at lean and
rich conditions.
[0098] FIG. 4 shows the results. A peak is observed around
400.degree. C. It is assumed that the decomposition reaction of
sulfates takes place around that temperature.
[0099] On the other hand, decomposition temperatures of
Li.sub.2SO.sub.4 and Al.sub.2(SO.sub.4).sub.3 are 1256.degree. C.
and 770.degree. C., respectively. Accordingly, it is conceivable
that the decomposition of sulfate, which took place at 400.degree.
C., was caused by Pt added to the catalyst. That is, when the noble
metals and the sulfur component trapping agents are in contact,
release of trapped sulfur components takes place at the time of
flowing the rich gas thereby to bring about poisoning of the NOx
trapping catalyst. Accordingly, the sulfur component trapping
agents should not contain the noble metals. TABLE-US-00006 TABLE 6
Gas composition Gas composition Lean Rich N.sub.2 balance balance
CO.sub.2 10% 12% H.sub.2O 10% 10% H.sub.2 0 ppm 3000 ppm CO 1000
ppm 6000 ppm O.sub.2 5% 5000 ppm C.sub.3H.sub.8 500 ppm 600 ppm
SO.sub.3 150 ppm 0 ppm NO 600 ppm 1000 ppm
Embodiment 6
Quantitative Influence of Coexistence of Noble Metals
[0100] A KNaRhPtPd/Al.sub.2O.sub.3, which was prepared in the same
manner as in the method of preparing the NOx trapping catalyst 1,
was used as the sulfur component trapping agent, and its sulfur
component release performance was evaluated. Compositions of the
trapping agents were 190 g of alumina per 1 L of the honeycomb,
12.4 g of Na and 15.6 g of K, in conversion of elements, and noble
metals (Rh+Pt+Pd), wherein the total amounts of noble metals were
0% by weight, 0.3% by weight and 0.7% by weight per the weight of
the sulfur component trapping agent. Weight ratios of Rh, Pt and Pd
were 1:20:10.
[0101] After the entrance temperature of the
KNaRhPtPd/Al.sub.2O.sub.3 was set to 300.degree. C., the model gas
shown in Table 7 was fed though the sulfur component trapping agent
for 2 hours, then the temperature was elevated to 650.degree. C.,
followed by letting the rich model gas flow for 10 min. Sulfur
release rates were measured.
[0102] Considering that the sulfur component trapping agent is
installed in the system shown in FIG. 1, the sulfur components
entering the sulfur component trapping agent in lean air fuel ratio
was SO.sub.3. The space velocity SV of the gas in lean and rich
periods was 30,000/h. TABLE-US-00007 TABLE 7 (Gas composition) Gas
composition Lean rich N.sub.2 Balance balance CO.sub.2 10% 12%
H.sub.2O 10% 10% H.sub.2 0 ppm 3000 ppm CO 1000 ppm 3.5% O.sub.2 5%
3000 ppm C.sub.3H.sub.6 500 ppm 600 ppm SO.sub.3 300 ppm 0 ppm NO
600 ppm 1000 ppm
[0103] The sulfur component release rates were calculated in
accordance with the equation (10). Sulfur component release rate
(%)=An amount of sulfur released from the sulfur trapping agent in
a rich period (mol)/an amount of sulfur released from the sulfur
trapping agent in a lean period (mol).times.100(%) (10)
[0104] FIG. 5 shows the results. When the amount of Rh+Pt+Pd is
0.4% by weight or less, the release rate of sulfur component is
less than 2%, and the sulfur component trapping agent exhibits a
high sulfur component retention performance. Accordingly, in order
to expect a high sulfur component retention performance, the total
amount of Rh+Pt+Pd should be 4% by weight or less.
Embodiment 7
Support on a Filter
[0105] As a method of arranging the sulfur component oxidizing
catalysts before and after the sulfur component trapping agent, a
filter can be utilized. As the filter, there may be exemplified
diesel particulate filters (DPF), which have been used for removing
particle material (PM).
[0106] For example, the sulfur component oxidizing catalyst is
supported on a surface of the upper stream side of the DPF and the
sulfur component trapping agent is supported on a surface of the
downstream side of the filter. By this arrangement, it is possible
to oxidize the exhaust gas to form SO.sub.3 when the exhaust gas
enters DPF so that the SO.sub.3 can be trapped by the sulfur
component trapping agent. In this case, if the noble metals are
used as the sulfur component oxidizing catalyst, decomposition of
sulfates of the sulfur component trapping agent does not take place
and flowing-out of the trapped sulfur does not occur even when the
air fuel ratio becomes rich, because the noble metals are not in
contact with the sulfur component trapping agent.
[0107] When the sulfur component oxidizing catalyst and the sulfur
component trapping agent are disposed with a distance, there may be
a problem that the formed SO.sub.3 may adhere to a wall of the gas
passage thereby to damage the gas passage; on the other hand, in
the case of filters, the above problem may be removed, because the
formed SO.sub.3 is immediately trapped. Since the both faces of the
filters can be utilized, the space can be reduced, compared with
the case where the sulfur component oxidizing catalyst and the
sulfur component trapping agent are separately arranged.
[0108] The present invention can be applied to diesel exhaust gas,
wherein PM contained in the gas includes solid substance such as
soot, sulfates and mists, which contain sulfur components. When the
filters are used, these solid and liquid sulfur components as well
as sulfur components in the gas phase are removed thereby to
remarkably increase trapping performance of the sulfur component
trapping agent.
[0109] Any filters may be employed as long as they perform the
above functions. Materials for the filters may include cordierite,
stainless steel, SiC, etc. Various shapes of the filters may be
employed. For Example, cross sections thereof may be circular,
rectangular, elliptic, etc.
[0110] The sulfur component oxidizing catalyst and sulfur component
trapping agent may be supported on filters by any proper manners
such as wholly or partially supported on the filter surface in
accordance with objects expected.
[0111] FIG. 6 shows a sectional view of a DPF (diesel particulate
filter), which employs the sulfur component oxidizing catalyst and
the sulfur component trapping agent. The sulfur component oxidizing
catalyst is supported on the upstream side face of the filter and
the sulfur component trapping agent is supported on the downstream
side face of the filter. When exhaust gas enters DPF, it is
oxidized by the sulfur component oxidizing catalyst to form
SO.sub.3 and the resulted SO.sub.3 is contacted with the sulfur
component trapping agent. When the noble metals are used as the
sulfur component oxidizing catalyst, the sulfates of the sulfur
component trapping agent by the noble metals in the rich air fuel
ratio condition does not occur and release of sulfur components
does not occur as well, because the noble metals do not contact
with the sulfur component trapping agent.
[0112] When the sulfur component oxidizing catalyst and sulfur
component trapping agent are disposed separately, produced SO.sub.3
adheres to the passage walls thereby to degrade the passage
material. However, in the above-mentioned filter PDF there is no
problem because the formed SO.sub.3 is immediately trapped.
[0113] Since the both faces of the filter are utilized, the space
for the filter is smaller than the case where the catalyst and the
trapping agent are separately disposed.
[0114] The exhaust gas from the diesel engines contains solid
matters such as soot, sulfates, etc and mists. These substances
contain the sulfur components. Since the filters can remove these
solid or liquid matters as well as sulfur components in the gas
phase, the sulfur component trapping performance is remarkably
improved.
Embodiment 8
Installment of the Sulfur Trapping Agent Just Below an Engine
[0115] FIG. 7 shows a diagrammatic view of an arrangement where the
honeycomb type sulfur component trapping agent is disposed just
below the engine. The words "just below the engine" are used to
mean a position which is as close to the engine as possible. For
example, the position is within 1 m from an exhaust manifold
entrance port. The NOx trapping catalyst may be disposed
vertically.
[0116] In disposing the sulfur component trapping agent beneath a
floor, a space is necessary in the floor. On the other hand, in
disposing the sulfur trapping agent below the engine the space in
the floor is not necessary so that a car room becomes wide.
[0117] The exhaust gas from an engine contain, which condenses. If
the sulfates are formed on the sulfur component trapping agent, the
sulfates dissolve into condensed water, which may be discharged as
sulfuric acid into the downstream. This phenomenon leads to
corrosion of the passage and degradation of the NOx trapping
catalyst disposed after the sulfur component trapping agent.
[0118] When the sulfur component trapping agent is disposed just
below the engine, a temperature of the sulfur component trapping
agent tends to elevate thereby to prevent condensation of water.
When the sulfur component trapping agent disposed just below the
engine is a honeycomb type structure, the condensed water does not
stay in the honeycomb structure to prevent the above problem,
because a direction of the gas passages is vertical the ground.
Embodiment 9
Estimation of S Purge Frequency
[0119] It is difficult to completely prevent sulfur components from
entering the NOx trapping catalyst disposed after the sulfur
component trapping agent. Accordingly, degradation of performance
of the NOx trapping catalyst to which the sulfur components adhere
after a lapse of a long period of time even when the sulfur
component trapping agent is disposed; thus the NOx trapping
catalyst should be subjected to S purge.
[0120] An SO.sub.2 containing model lean gas whose composition is
shown in Table 6 was flown through the NOx trapping catalyst 1 so
as to evaluate degradation of the catalyst by sulfur components.
The temperature at the entrance of the catalyst was set to
300.degree. C., and the space velocity of the gas was 30,000/h.
FIG. 8 shows degradation rates of catalytic activity of the
catalyst with respect to treatment time. The NOx purification rate
was calculated by the following equations. NOx purification rate
(%)={(an amount of NOx entered the catalyst within 1 min. after
switching to lean condition)-(an amount of NOx flowing-out from the
catalyst within 1 min. after switching to lean condition)}/an
amount of NOx flowing-out from the catalyst within 1 min. after
switching to lean condition).times.100 (11)
[0121] From the above results, it is apparent that when the
SO.sub.2 containing lean model gas is flown through the catalyst
for 1 hour, the catalytic activity lowers to 40%, which
necessitates S purge.
[0122] When a lean model gas containing 300 ppm of SO.sub.2 is
flown through the catalyst for 1 hour, an amount of entering sulfur
components is 2.4 mmoles. Estimation of an amount of sulfur
components entering the catalyst in a conventional exhaust gas
purification apparatus is made under conditions that a
concentration of sulfur components in gasoline is 10 ppm and fuel
consumption is 10 km/L, the above entering amount of sulfur
components is equivalent to a running for 90.6 km. Accordingly, one
S purge is required for every 90.6 km running.
[0123] If the S concentration in the exhaust gas is lowered by the
sulfur component trapping agent in the system shown in FIG. 1 to
thereby lower the S concentration of the gas entering the catalyst
1 to 1.5 ppm, the S purge is required for every 9060 km. In
accordance with the above theory, relationship between calculation
results of trapping rates of sulfur components in lean condition
and frequency of S purge is shown in FIG. 9. The sulfur component
trapping rate was calculated by the equation (6). The S purge
frequency represents a distance between two S purges.
[0124] From FIG. 9, it is apparent that when the S trapping rate
exceeds 85%, the S purge frequency exceeds 604 km. Accordingly,
this value is 6 times or more of that where the sulfur component
trapping agent is not disposed. Thus, the advantage of the sulfur
component trapping agent is remarkable.
Embodiment 10
Evaluation of Poisoning Depending on Gas Atmosphere
[0125] Lean gas and rich gas whose compositions are shown in Table
8 were flown through the NOx trapping catalyst 1 for 1 hour. A
temperature at the entrance of the catalyst was 300.degree. C. and
the space velocity was 30,000/h. In the lean gas the sulfur
components present in the form of SO.sub.2, and in the rich gas the
sulfur components present in the form of H.sub.2S. TABLE-US-00008
TABLE 8 Released gas from NOx trapping catalyst Gas composition
lean Rich N.sub.2 balance balance CO.sub.2 10% 12% H.sub.2O 10% 10%
H.sub.2 0 ppm 3000 ppm CO 1000 ppm 3.5% O.sub.2 5% 3000 ppm
C.sub.3H.sub.6 500 ppm 600 ppm NO 600 ppm 1000 ppm SO.sub.2 150 ppm
0 ppm H.sub.2S 0 ppm 150 ppm
[0126] FIG. 10 shows amounts of sulfur adhered to the NOx trapping
catalyst 1. When the catalyst is poisoned by SO.sub.2 (lean gas),
the amount of adhesion of S is about 4 times that in case of
poisoning by H.sub.2S (rich gas). Accordingly, poisoning by the
lean gas goes faster than in the case of the rich gas.
[0127] Suppose the sulfur component trapping agent is used in the
system shown in FIG. 1. If a sulfur component trapping rate is 90%,
10% of sulfur components outflows from the sulfur component
trapping agent. In view of facts that 90% of the sulfur component
trapping arte is sufficient and the S poisoning by lean gas is 4
times that by rich gas, (100-10.times.4=60%) or more of the
trapping rate of the S component trapping trapped by the sulfur
component trapping agent in case of rich gas is preferable.
Embodiment 11
Spurge
[0128] Elevation of temperature of the exhaust gas is necessary at
the time of S purge. If the elevation is too much and the
temperature exceeds the melting point or decomposition temperature
of the sulfates, release of sulfur from the sulfur component
trapping agent takes place. Accordingly, S purge should be carried
out at a temperature as low as possible. For example, the S purge
should preferably be carried out at a temperature of 500 to
700.degree. C. at the NOx trapping catalyst entrance.
[0129] In the arrangement shown in FIG. 1, used were the Pt added
Al.sub.2O.sub.3 catalyst in Embodiment 1 as the sulfur component
oxidizing catalyst and the trapping agent B as the sulfur component
trapping agent. Under the same conditions as in embodiment 8, the
NOx trapping catalyst 1 was subjected to S purge in the rich gas
flow after the activity of the NOx trapping catalyst was decreased
to 40%. The rich gas whose composition is shown in Table 6 was
supplied for 10 min to the NOx trapping catalyst 1 of which
activity was decreased to 40% at a temperature of 300.degree. C. at
the NOx trapping catalyst entrance. A temperature of the NOx
trapping catalyst during the rich gas flow was 650.degree. C.
[0130] FIG. 11 shows the results. The NOx purification rate was
calculated by the equation (11). Activity of the NOx trapping
catalyst 1 was regenerated by flowing rich gas for 10 min to the
activity before supply of the sulfur components.
[0131] In view of the fact that in embodiment 3, decomposition of
the trapping agent B did not occur until 800.degree. C. in addition
to the above fact, it is apparent that release of the sulfur
components from the sulfur component trapping agent does not occur
thereby to regenerate the NOx trapping catalyst, when the
temperature of the sulfur component trapping agent is set to
750.degree. C. and the temperature of the NOx trapping catalyst 1
is set to 650.degree. C. at the time of S purge.
Embodiment 12
Diagnosis of Degradation of Sulfur Component Trapping Agent
[0132] Since the sulfur component trapping agent of the present
invention is not one that is used and regenerated as in the
conventional sulfur trapping materials, it is necessary to replace
the sulfur component trapping agent with new one or a part of the
exhaust gas purification apparatus is replaced. The present
invention provides a system for indicating information on timing of
replacement of the sulfur component trapping agent, wherein the
degree of degradation of the sulfur component trapping agent is
estimated by the following method. When the sulfur component
trapping agent degrades, degradation of the NOx trapping catalyst
disposed after the sulfur component trapping agent proceeds.
Therefore, there is a correlation between the degree of degradation
of the sulfur component trapping agent and poisoning speed of the
NOx trapping catalyst by sulfur.
[0133] When S purge is applied to the NOx trapping catalyst, the
degree of regeneration of activity of the NOx trapping catalyst
greatly poisoned by sulfur is large. Accordingly, the degradation
of the NOx trapping catalyst by sulfur can be estimated by the
degree of regeneration of activity caused by S purge, and from that
result degree of degradation of the sulfur component trapping agent
can be estimated.
[0134] That is, when NOx purification rates after regeneration and
before regeneration of the catalyst are measured and when a
difference or a ratio between them becomes over a certain value,
there is provided an apparatus for diagnosing degradation that
indicates replacement of the sulfur component trapping agent and a
replacement supporting system.
[0135] In order to make the replacement easy, the sulfur component
trapping agent and a part of the NOx purification catalyst may be
made detachable. According to the embodiment, the exhaust gas
purification apparatus of the present invention can be used for a
long time keeping effective purification performance.
Embodiment 13
Constitution of Internal Combustion Engine
[0136] FIG. 12 shows a schematic diagram of an embodiment of an
internal combustion engine equipped with the exhaust gas
purification apparatus of the invention.
[0137] The exhaust gas purification apparatus of the invention
comprises an engine 99, which is capable of lean burn combustion,
an air-intake system comprising an air flow sensor 2, a throttle
valve 3, etc; an exhaust gas system comprising an oxygen
concentration sensor or A/F sensor 7, a gas temperature sensor 8
for a NOx trapping catalyst entrance, a gas temperature sensor 9
for a sulfur component trapping agent entrance, a temperature
sensor 10 for a sulfur component trapping agent, the sulfur
component trapping agent 12, a NOx trapping catalyst 13, a sulfur
component oxidizing catalyst 14, a temperature sensor 15 for the
sulfur component oxidizing catalyst, a temperature sensor 16 for
the sulfur component oxidizing catalyst entrance, etc, a control
unit (ECU) 11, etc. ECU is constituted by I/O for an interface of
input and output, LSI, a calculation device MPU, memory devices RAM
and ROM that memory a large number of control programs, a timer
counter, etc.
[0138] The above-mentioned exhaust gas purification apparatus works
as follows. An intake air amount into the engine is measured by the
air flow sensor 2 after filtering with an air cleaner 1'. It flows
through the throttle valve 3 and an injector 5 where the air is
injected with fuel; then the mixed gas is supplied to the engine
99. Signals from the air flow sensor and other sensors are input
into the engine control unit ECU.
[0139] Running conditions of the internal combustion engine and
conditions of the NOx trapping catalyst, etc are evaluated by ECU
to decide air fuel ratio and injection time, etc of the injector 5
are controlled to concentration of the fuel to a predetermined
values. The mixed gas intaken into cylinders is ignited by ignition
plugs 6, which are controlled by signals from ECU 11 to combust
air/fuel mixture.
[0140] Combustion exhaust gas is introduced into an exhaust gas
purification system. The exhaust gas purification system is
provided with the exhaust gas purification catalyst 13 for lean
burn combustion, wherein NOx, HC and CO in the exhaust gas are
purified by three way function of the catalyst in stoichiometric
combustion running. In lean combustion running, NOx is purified by
its NOx trapping function and HC and CO are purified by burning
them by the catalyst. When the exhaust gas contains SOx, the sulfur
components are oxidized by the sulfur component oxidizing catalyst
and then almost all of the oxidized sulfur components is removed,
followed by introducing the gas into the catalyst 13. If the
catalyst 13 is partially poisoned by Sox to lower the NOx trapping
performance, the NOx trapping performance of the catalyst 13, which
is normally monitored during the lean combustion is recovered by
shifting the air fuel ratio of the gas to a rich side in response
to judging signals and control signals from ECU. At the same time,
the temperature of the catalyst 13 may be elevated in response to
the judging signals and control signals from ECU to cause the
sulfur components to release from the catalyst 13 so that the
catalyst 13 is regenerated.
[0141] According to the above-described operation, exhaust gases
over the entire operation conditions including lean and
stoichiometric (including rich) operations are effectively
purified.
* * * * *