U.S. patent application number 09/736418 was filed with the patent office on 2001-08-16 for exhaust-gas cleaning system with nitrogen oxide accumulator catalyst and sulphur oxide trap and operating method therefor.
Invention is credited to Boegner, Walter, Gunther, Josef, Holzt, Hans-Peter, Krutzsch, Bernd, Renfftlen, Stefan, Schon, Christof, Voightlander, Dirk, Weibel, Michel, Wenninger, Gunter.
Application Number | 20010013223 09/736418 |
Document ID | / |
Family ID | 7932720 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013223 |
Kind Code |
A1 |
Boegner, Walter ; et
al. |
August 16, 2001 |
Exhaust-gas cleaning system with nitrogen oxide accumulator
catalyst and sulphur oxide trap and operating method therefor
Abstract
A method for operating an exhaust-gas cleaning system having an
NO.sub.X adsorption accumulator and an SO.sub.X trap, includes, in
normal operating phases, feeding the exhaust gas to be cleaned
first via the SO.sub.X trap and then via the NO.sub.X adsorption
accumulator. The normal operating phases are from time to time
interrupted by desulphurization phases for desulphurizing the
SO.sub.X trap. Means are provided for controlling the direction of
flow of the exhaust stream so that it optionally passes firstly via
the SO.sub.X trap and then via the NO.sub.X adsorption accumulator,
or firstly via the NO.sub.X adsorption accumulator and then via the
SO.sub.X trap, so that during the desulphurization phases the
exhaust gas can be passed firstly via the NO.sub.X adsorption
accumulator and then via the SO.sub.X trap.
Inventors: |
Boegner, Walter; (Remseck,
DE) ; Gunther, Josef; (Affalterbach, DE) ;
Holzt, Hans-Peter; (Esslingen, DE) ; Krutzsch,
Bernd; (Denkendorf, DE) ; Renfftlen, Stefan;
(Eislingen, DE) ; Schon, Christof; (Remshalden,
DE) ; Voightlander, Dirk; (Korntal-Munchingen,
DE) ; Weibel, Michel; (Stuttgart, DE) ;
Wenninger, Gunter; (Stuttgart, DE) |
Correspondence
Address: |
CROWELL & MORING LLP
INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Family ID: |
7932720 |
Appl. No.: |
09/736418 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
60/295 ; 60/297;
60/301 |
Current CPC
Class: |
B01D 53/944 20130101;
F01N 3/2093 20130101; F01N 3/085 20130101; Y02T 10/20 20130101;
B01D 53/9481 20130101; B01D 53/949 20130101; F01N 3/22 20130101;
Y02T 10/12 20130101; F01N 3/0885 20130101; F01N 13/0097
20140603 |
Class at
Publication: |
60/295 ; 60/301;
60/297 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 1999 |
DE |
199 60 430.4-43 |
Claims
What is claimed is:
1. A method for operating an exhaust-gas cleaning system comprising
an NO.sub.X adsorption accumulator and an SO.sub.X trap, said
method comprising: in a normal operating phase, passing an exhaust
gas first through the SO.sub.X trap and then through the NO.sub.X
adsorption accumulator; interrupting the normal operating phase
from time to time by desulphurization phases for desulphurizing the
SO.sub.X trap, wherein during the desulphurization phase, the
exhaust gas is passed first through the NO.sub.X adsorption
accumulator and then through the SO.sub.X trap.
2. A method according to claim 1, further comprising, during the
desulphurization phase, feeding secondary air into the exhaust gas
downstream of the SO.sub.X trap, and feeding the exhaust gas that
is enriched with secondary air to an oxidation catalytic
converter.
3. A method for operating an exhaust-gas cleaning system comprising
an NO.sub.X adsorption accumulator and an SO.sub.X trap, said
method comprising: passing an exhaust gas first through the
SO.sub.X trap and then through the NO.sub.X adsorption accumulator
during a lean phase operation of the system; passing the exhaust
gas first through the NO.sub.X adsorption accumulator and then
through the SO.sub.X trap during a desulphurization phase for
desulphurizing the SO.sub.X trap.
4. An exhaust-gas cleaning system, comprising: at least one
SO.sub.X trap; at least one NO.sub.X adsorption accumulator
arranged in series with the at least one SO.sub.X trap in an
exhaust system; and means for controlling the direction of exhaust
gas flow so that it passes first through the SO.sub.X trap and then
through the NO.sub.X adsorption accumulator or first through the
NO.sub.X adsorption accumulator and then through the SO.sub.X
trap.
5. An exhaust-gas cleaning system according to claim 4, wherein
said means for controlling the direction of the exhaust gas flow
comprises a flap, a valve, or a slide.
6. An exhaust-gas cleaning system according to claim 4, further
comprising: an oxidation catalytic converter arranged on the
opposite side of the SO.sub.X trap from the NO.sub.X adsorption
accumulator; and a secondary-air supply device having a
secondary-air inlet point that opens into the exhaust system
between the SO.sub.X trap and the oxidation catalytic
converter.
7. A motor vehicle internal-combustion engine comprising the
exhaust-gas cleaning system of claim 4.
Description
BACKGROUND AND SUMMARY OF INVENTION
[0001] This application claims the priority of German application
No. 199 60 430.4, filed Dec. 15, 1999, the disclosure of which is
expressly incorporated by reference herein.
[0002] The present invention relates to a method for operating an
exhaust-gas cleaning system which comprises a nitrogen oxide
adsorption accumulator and a sulphur oxide trap and to an
exhaust-gas cleaning system which can be operated using this
method.
[0003] Exhaust-gas cleaning systems with a nitrogen oxide
(NO.sub.X) adsorption accumulator, such as an NO.sub.X adsorber
catalytic converter, are used in particular in motor vehicles with
an internal-combustion engine in order to minimize the nitrogen
oxide emissions. In operating phases involving conditions which are
unfavourable to nitrogen oxide reduction, such as in lean-burn
mode, it is known for nitrogen oxide to be temporarily stored in
the NO.sub.X accumulator by an adsorption process. This stored
nitrogen oxide can then be converted back to nitrogen in a suitable
subsequent reduction operating phase, such as in rich-burn mode.
The terms lean-burn and rich-burn mode are understood to mean
operation with oxygen-rich and low-oxygen exhaust-gas composition,
respectively, for example as a result of an engine being operated
with a lean or rich engine air ratio (i.e., with an air/fuel ratio
for the air/fuel mixture burnt in the engine which is above or
below the stoichiometric value).
[0004] The SO.sub.X trap prevents sulphur poisoning of the NO.sub.X
adsorber. This is because, particularly when the engine is in
lean-burn mode, sulphur which is contained in standard fuels and
engine oils leads to sulphur dioxide being present in the exhaust
gas. The sulphur dioxide is taken up by the NO.sub.X adsorber as a
result of sulphates being formed, thereby reducing the capacity of
this adsorber to store NO.sub.X. The SO.sub.X trap prevents this by
trapping the sulphur oxides before they reach the NO.sub.X
adsorption accumulator. It is subjected to a desulphurization
treatment at the latest when its SO.sub.X storage capacity is
exhausted. To achieve effective desulphurization, it is known to
set elevated exhaust-gas temperatures of, for example, over
600.degree. C. and a rich exhaust air ratio, i.e. a ratio which is
below the stoichiometric level.
[0005] A further special method for periodically desulphurizing a
nitrogen oxide or sulphur oxide accumulator of an exhaust-gas
cleaning system using secondary-air supply means is described in
German patent application 199 22 962, which is not a prior
publication. In this method, during part of the desulphurization
period, the exhaust-gas cleaning system is operated in such a way
that, after a predeterminable desulphurization temperature has been
reached, the accumulator air ratio (i.e., the air ratio in the
exhaust gas supplied to the accumulator which is to be
desulphurized), is made to oscillate between an oxidizing
atmosphere, on the one hand, and a reducing atmosphere, on the
other hand, by correspondingly changing the amount of secondary air
supplied.
[0006] Patent DE 197 47 222 C1 describes a method for operating an
exhaust-gas cleaning system in which on demand, from time to time,
an NO.sub.X accumulator catalytic converter is desulphurized, i.e.,
accumulated sulphate is removed. The exhaust-gas cleaning system
comprises secondary-air supply means having a secondary-air line
and a secondary-air pump which can be controlled, for example, by
an engine management system.
[0007] A problem with the conventional periodic regeneration of the
SO.sub.X trap is that the NO.sub.X accumulator catalytic converter
which follows it is exposed to the sulphur components released from
the SO.sub.X trap, and in unfavourable circumstances this may lead
to its ability to store NO.sub.X being partially blocked by the
accumulation of sulphates. DE 198 02 631 A1 discloses an
exhaust-gas cleaning system in which this problem is combatted by
providing a bypass around the NO.sub.X accumulator catalytic
converter in the exhaust system, via which bypass the exhaust gas
is guided past the NO.sub.X accumulator catalytic converter while
the SO.sub.X accumulator catalytic converter is desulphurized.
[0008] The present invention is based on the technical problem of
providing an exhaust-gas cleaning system and an operating method
which enable the SO.sub.X trap to be desulphurized with the minimum
possible outlay and without there being any risk of sulphur
poisoning of the nitrogen oxide accumulator catalytic
converter.
[0009] In the method according to the present invention, the
direction of flow of the exhaust gas during the desulphurizing
phases is selected to be different from during the normal operating
phases. During the normal operating phases, the exhaust gas which
is to be cleaned is passed firstly via the SO.sub.X trap and then
via the NO.sub.X adsorption accumulator. The sulphur oxide
constituents of the exhaust gas, which are undesirable in the
nitrogen oxide adsorption accumulator, remain trapped in the
SO.sub.X trap, provided that the storage capacity of the SO.sub.X
trap has not yet been exhausted and there is as yet no saturation
behaviour. If the desulphurization phase is selected, the
exhaust-gas flow is switched over in such a way that it passes
through the two exhaust-gas cleaning components, namely the
SO.sub.X trap and NO.sub.X adsorption accumulator, in the reverse
direction (i.e., is passed firstly via the NO.sub.X adsorption
accumulator and then via the SO.sub.X trap) . This prevents sulphur
compounds released during the desulphurization of the SO.sub.X trap
from polluting the NO.sub.X adsorption accumulator.
[0010] In an embodiment of the present invention, secondary air is
fed into the exhaust gas downstream of the SO.sub.X trap during the
desulphurization phase. The secondary air is used to oxidize
undesirable sulphur compounds, such as H.sub.2S and COS, which are
released in the SO.sub.X trap, for which purpose an oxidation
catalytic converter is connected downstream of the SO.sub.X
trap.
[0011] An exhaust-gas cleaning system is equipped with an SO.sub.X
trap and an NO.sub.X adsorption accumulator and can be operated
according to the present invention. For this purpose, it has means
for reversing the direction of flow of the exhaust gas in such a
manner that the exhaust-gas stream can optionally be passed firstly
through the SO.sub.X trap and then through the NO.sub.X adsorption
accumulator or, conversely, firstly through the NO.sub.X adsorption
accumulator and then through the SO.sub.X trap.
[0012] A configuration of the exhaust-gas cleaning system according
to the present invention provides, as further components, an
oxidation catalytic converter and secondary-air supply means.
[0013] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 depicts an exhaust-gas cleaning system in a normal
operating phase; and
[0015] FIG. 2 depicts the exhaust-gas cleaning system from FIG. 1
in a desulphurization operating phase.
DETAILED DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1 and 2 diagrammatically depict a part of an
exhaust-gas cleaning system which is of interest in the context of
the present invention, having an SO.sub.X trap 1, which is also
referred to as an SO.sub.X accumulator catalytic converter, and a
catalytically active nitrogen oxide adsorber, i.e., an NO.sub.X
accumulator catalytic converter 2. In the example shown, the
SO.sub.X trap 1 and the NO.sub.X accumulator catalytic converter 2
are connected in series in a common housing 3 which is arranged in
an associated section 4 of an exhaust system which carries the
exhaust gas 5 which is to be cleaned. Alternatively, the two
exhaust-gas cleaning components 1, 2 may be arranged in separate
housings but in series in the exhaust-gas system section 4.
[0017] The exhaust-gas cleaning system shown has means for
reversing the direction of flow of the exhaust gas flowing through
the exhaust system section 4 in which the SO.sub.X trap 1 and the
NO.sub.X accumulator catalytic converter 2 are arranged. These
means include a branching junction 6, at which the two connections
of the exhaust system section 4 which includes the SO.sub.X trap 1
and the NO.sub.X accumulator catalytic converter 2 meet an
exhaust-gas supply line 7 and an exhaust-gas discharge line 8. In
the branching junction 6 there is a controllable flap 9 which can
be switched between a normal operating position shown in FIG. 1 and
a desulphurization position shown in FIG. 2.
[0018] In the normal operating position shown in FIG. 1, the flap 9
passes the exhaust gas supplied by the supply line 7 into the
left-hand part, as seen in FIG. 1, of the exhaust system section 4,
so that in that section it firstly flows via the SO.sub.X trap 1
and then via the NO.sub.X accumulator catalytic converter 2, from
where it returns to the branching junction 6 and, through the
flow-guiding effect of the flap 9, is passed into the discharge
line 8. In its position shown in FIG. 2, by contrast, the flap 9
passes the supplied exhaust gas into the right-hand part of the
exhaust system section 4 which includes the two exhaust-gas
cleaning components 1, 2, so that the exhaust gas flows firstly via
the NO.sub.X accumulator catalytic converter 2 and then via the
SO.sub.X trap 1, from where it returns to the branching junction 6
and, in turn, is passed into the discharge line 8 under the action
of the flap 9.
[0019] As an alternative to the flap 9, it is possible to use any
other conventional flow-guiding means which can be switched between
two positions in order: in one position, to guide the exhaust gas
in one direction and, in the other position, to guide the exhaust
gas in the other direction through the exhaust system section 4
which includes the SO.sub.X trap 1 and the NO.sub.X accumulator
catalytic converter 2 (e.g., a corresponding four-way valve or a
suitable slide or opener element). Furthermore, it will be
understood that, depending on the particular requirements, further
exhaust-gas cleaning components, in particular further catalytic
converter bodies, may be provided in addition to the exhaust-gas
cleaning components shown in the present case, for example a
three-way catalytic converter or a nitrogen oxide reduction
catalytic converter may be provided in the exhaust-gas discharge
line 8. The exhaust-gas cleaning system which has been constructed
in this way and is suitable in particular for cleaning the exhaust
gases from a spark-ignition or diesel engine of a motor vehicle,
allows the following advantageous method of operation.
[0020] During so-called normal operating phases, the flap 9 is set
into its normal operating position shown in FIG. 1. The exhaust gas
5 which is to be cleaned then firstly reaches the SO.sub.X trap 1,
which adsorbs any sulphur compounds which are contained in the
exhaust gas, after which the exhaust gas from which the sulphur has
been removed is fed to the NO.sub.X accumulator catalytic converter
2. The function of the NO.sub.X accumulator catalytic converter
then differs according to whether, during the normal operating
phase, it is being run in adsorption mode or in desorption mode. In
the adsorption mode, the NO.sub.X accumulator catalytic converter 2
adsorbs nitrogen oxides contained in the exhaust gas, while in
desorption mode it releases the nitrogen oxides which have been
adsorbed and temporarily stored again, and these nitrogen oxides
are then reduced to form nitrogen, for which purpose the NO.sub.X
accumulator catalytic converter 2 can simultaneously act as a
nitrogen oxide reduction catalytic converter, or a catalytic
converter of this type may be connected downstream as a separate
catalytic converter body, as is standard procedure for the person
skilled in the art. It is known the NO.sub.X accumulator catalytic
converter 2 is preferably operated as long as possible in
adsorption mode, during which the exhaust gas may be of lean
composition, until the NO.sub.X accumulator catalytic converter 2
has reached a certain level of loading, after which the accumulated
nitrogen oxides are released again during a short desorption phase.
The desorption operation is known to generally include the setting
of a rich exhaust-gas composition, for example by suitably altering
the air/fuel mixture which is burnt by the combustion device
generating the exhaust gas and/or by injecting a reducing agent,
such as ammonia, into the exhaust gas upstream of the NO.sub.X
accumulator catalytic converter 2.
[0021] Therefore, during normal operation outlined above, the
SO.sub.X trap 1 is positioned upstream of the NO.sub.X accumulator
catalytic converter 2 and prevents sulphur poisoning of the latter.
If, after a prolonged period of normal operation, the SO.sub.X trap
1 has reached a certain degree of saturation, normal operation is
interrupted by a so-called desulphurization phase. For this
purpose, the flap 9 is moved into its desulphurization position
shown in FIG. 2, with the result that the incoming exhaust gas is
then passed firstly via the NO.sub.X accumulator catalytic
converter 2 and only then into the SO.sub.X trap 1. Moreover, the
standard desulphurization conditions are established, with regard,
for example, to composition and temperature of the exhaust-gas
stream. If, despite the SO.sub.X trap 1, some sulphur has been
accumulated in the NO.sub.X accumulator catalytic converter 2, this
sulphur is also removed during the desulphurization operation. The
desulphurization operation releases the sulphur which has been
accumulated in sulphate form in the SO.sub.X trap 1, so that the
ability of this trap to take up sulphur is restored.
[0022] The reversal of flow of the exhaust gas in the exhaust
system section 4 during the desulphurization phase with respect to
the direction of flow of the exhaust gas in normal operation means
that the exhaust gas which leaves the SO.sub.X trap 1 during the
desulphurization operation and contains the released sulphur or
corresponding sulphur compounds does not enter the NO.sub.X
accumulator catalytic converter 2. This prevents the possibility of
sulphur which is released in the SO.sub.X trap 1 during the
desulphurization from being accumulated in the NO.sub.X accumulator
catalytic converter 2.
[0023] Since emission of the sulphur compounds, such as H.sub.2S
and/or COS, which enter the exhaust gas during the desulphurization
of the SO.sub.X trap 1 to the environment is generally undesirable,
it is possible to provide an oxidation catalytic converter 10 as
one possible remedy. This oxidation catalytic converter is arranged
downstream of the SO.sub.X trap 1, as seen in the direction of flow
of the exhaust gas during desulphurization, in the same exhaust
system section 4, as indicated by dashed lines in FIG. 2, or in the
adjoining exhaust-gas discharge line 8. To enable the problematical
sulphur compounds to be converted optimally in the oxidation
catalytic converter 10 despite the fact that the composition of the
exhaust gas supplied in the oxidation catalytic converter 10 is
generally selected to be rich for the desulphurization of the
SO.sub.X trap 1, for this situation secondary-air supply means 11
of conventional type are additionally provided, as diagrammatically
indicated by dashed lines in FIG. 2. At a secondary-air inlet point
12 which lies between the SO.sub.X trap 1 and the oxidation
catalytic converter 10, the secondary-air supply means 11 feed air
into the exhaust system section 4 during a desulphurization
operation. As a result, sufficient oxygen to convert the sulphur
compounds which have been released is admixed with the exhaust gas
flowing to the oxidation catalytic converter 10.
[0024] It will be understood that, to control operation of the
exhaust-gas cleaning system, a suitably designed control unit (not
shown) is provided, which, for example, may simultaneously be used
to control the combustion device which generates the exhaust gas, a
measure which is inherently common to the person skilled in the art
and therefore requires no further explanation.
[0025] It is clear from the above explanation of an advantageous
example that the operating method according to the present
invention and the exhaust-gas cleaning system according to the
present invention are suitable for cleaning exhaust gases not only
from a motor vehicle engine but also from a stationary combustion
device. The exhaust-gas cleaning allows periodic adsorption and
desorption of an NO.sub.X adsorption accumulator; protection for
the NO.sub.X adsorption accumulator from sulphur poisoning by an
associated SO.sub.X trap; and periodic desulphurization of the
SO.sub.X trap without the NO.sub.X adsorption accumulator being
exposed to sulphur compounds which are released.
[0026] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *