Note: Descriptions are shown in the official language in which they were submitted.
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Emission control system for lean burn engines
and method for operating the system
Description
The invention relates to an emission control system for the cleaning of the
exhaust gases
of lean burn engines with two or more cylinders and to a method for operating
the
system. In the context of this invention, lean bum engines refer to diesel
engines and
lean bum petrol engines.
WO 2004/020807 Al describes a two-flow emission control system for a diesel
engine
with a plurality of cylinders. The emission control system comprises a first
exhaust leg
for the exhaust gases of a first group of cylinders and a second exhaust leg
for the
exhaust gases of a second group of cylinders. A nitrogen oxide storage
catalyst is
arranged in each exhaust leg. The two exhaust legs are combined downstream of
the
storage catalysts at a confluence to form a common exhaust leg. The common
exhaust
leg contains an oxidation catalyst. The compositions of the exhaust gases in
the first and
second exhaust legs are adjusted independently by the electronic engine
control system,
such that the exhaust gas is enriched in one leg for regeneration of the
storage catalyst,
while the exhaust gas in the other leg is lean. Enrichment and depletion are
adjusted
such that a lean exhaust gas is present after the combination of the two
exhaust gas
streams in the common exhaust leg, and any possible slippage of the reducing
agent is
oxidized over the oxidation catalyst.
This emission control system has a crucial drawback: during the regeneration
of a
nitrogen oxide storage catalyst, considerable amounts of ammonia can be
generated
when the storage catalyst is regenerated for longer than necessary. This risk
exists
particularly in the case of aged storage catalysts. The ammonia formed flows
together
with the other exhaust gases over the oxidation catalyst in the combined
exhaust leg and
is oxidized again to nitrogen oxides, which reduces the control performance of
the
exhaust gas system with regard to the nitrogen oxide. This is the case
especially at
relatively high exhaust gas temperatures in the case of oxidation catalysts
with high
oxidation activity. Such catalysts are used in order to oxidize the
hydrocarbons and
carbon monoxide present in the exhaust gas at as early as possible a stage at
low
exhaust gas temperatures, for example after a cold start.
US 6,047,542 describes an exhaust gas system for an engine which possesses a
first and
a second cylinder group. The first cylinder group is connected to a three-way
catalyst.
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The second group of cylinders and the three-way catalytist are connected via a
common
exhaust leg to an ammonia-absorbing and -oxidizing catalyst. When the first
group of
cylinders is operated under rich conditions, the second group of cylinders is
operated
under lean conditions. The three-way catalytic converter converts the nitrogen
oxides
present in the rich exhaust gas of the first cylinder group to ammonia, which
reduces the
nitrogen oxides emitted by the second cylinder group over the ammonia-
absorbing and -
oxidizing catalyst in the common exhaust leg. A nitrogen oxide storage
catalyst inserted
in the exhaust leg between the second cylinder group and the common exhaust
leg
reduces the amount of nitrogen oxide which flows into the ammonia-absorbing
and -
oxidizing catalyst.
WO 2006/008625 describes an exhaust gas treatment system for a lean burn
engine with
an SCR reactor downstream of an NOx adsorber (nitrogen oxide adsorber). The
nitrogen oxide adsorber is regenerated with syngas from a fuel reforming
reactor. The
nitrogen oxide adsorber preferably possesses a catalytic function for
converting the
nitrogen oxides during the regeneration. The SCR reactor increases the
conversion of
the nitrogen oxides by storing the ammonia formed during the regeneration and
using
the stored ammonia to convert the nitrogen oxides during the lean operating
mode of the
engine. To reduce slip of ammonia, an oxidation catalyst is arranged
downstream of the
SCR reactor.
WO 2004/090296 discloses, in Figure 1, a single-line exhaust gas
aftertreatment unit. It
comprises, in the exhaust gas flow direction, downstream of an internal
combustion
engine, in the full flow of the exhaust gas line, in succession, a reforming
unit which
simultaneously acts as a particulate filter, a nitrogen oxide storage catalyst
and an SCR
catalyst as emission control components. In the reforming unit, hydrogen is
obtained by
steam reforming, partial oxidation of hydrocarbons and/or mixed forms thereof.
US 6,732,507 B1 likewise describes a single-line nitrogen oxide aftertreatment
system
in which a nitrogen oxide adsorber is combined with an SCR catalyst. The
nitrogen
oxide aftertreatment system is operated alternately with rich and lean
air/fuel mixtures.
The SCR catalyst stores the ammonia generated by the nitrogen oxide adsorrber
during
the regeneration in the rich exhaust gas and converts, with the ammonia
stored, the
nitrogen oxides which have not been adsorbed during the lean operation of the
nitrogen
oxide adsorber to harmless products. The SCR catalyst possesses a first end
which is
directly connected to the second end of the nitrogen oxide adsorber.
It is an object of the present invention to modify the known exhaust gas
system of
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WO 2004/020807 A 1 in such a way that the ammonia formed in an unwanted manner
in
the regeneration of the storage catalysts is neither oxidized to nitrogen
oxides nor
released to the environment. Furthermore, the slip of reducing agents during
the
regeneration should also be rendered harmless.
This object is achieved by the exhaust gas system according to the main claim.
Claim 5
describes a method for operating the exhaust gas system.
The invention relates to lean burn engines with at least two cylinders. The
lean burn
engines preferably have four, six or more cylinders which are combined in a
first group
and in a second group of cylinders, each of which can be supplied
independently with
an air/fuel mixture.
The engines may be configured as in-line engines in which all cylinders are
arranged in
series in a single cylinder bank. Alternatively, each group of cylinders may
be combined
in a separate cylinder bank.
The emission control system of these lean burn engines comprises a first
exhaust leg for
the exhaust gases of the first group of cylinders and a second exhaust leg for
the exhaust
gases of the second group of cylinders. Each exhaust leg contains at least one
nitrogen
oxide storage catalyst. The two exhaust legs are combined downstream of the
storage
catalysts at a confluence to form a common exhaust leg. This emission control
system is
characterized in that the common exhaust leg contains an SCR catalyst.
The invention utilizes the storage action of SCR catalysts for ammonia in
order to store
any ammonia formed in the regeneration of the nitrogen oxide storage
catalysts.
When the nitrogen oxides are stored in the nitrogen oxide storage catalysts
and the
storage catalysts are regenerated, there may be unwanted slip of nitrogen
oxides during
the storage phase. This slip of nitrogen oxides can be converted to nitrogen
with the
ammonia stored by the SCR catalyst. Moreover, an SCR catalyst has sufficient
oxidation activity in order to convert any slip of reducing agents
(hydrocarbons, carbon
monoxide and hydrogen) with the oxygen content of the exhaust gases to
harmless
components.
In the context of this invention, a nitrogen oxide storage catalyst is
understood to mean
a catalyst which oxidizes the nitrogen monoxide present in a lean exhaust gas
to
nitrogen dioxide during a storage phase and then stores it in the form of
nitrates. The
mode of operation of nitrogen oxide storage catalysts is described in detail
in the SAE
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document SAE 950809. For oxidation of nitrogen monoxide, a storage catalyst
usually
contains platinum, with or without palladium, as catalytically active
components. The
materials used for storage of the nitrogen oxides as nitrates include basic
oxides,
carbonates or hydroxides of alkali metals, alkaline earth metals and rare
earth metals;
preference is given to using basic compounds of barium and of strontium.
After exhaustion of its storage capacity, a storage catalyst has to be
regenerated during a
regeneration phase. To this end, the exhaust gas is briefly enriched, for
example by
operating the engine with a rich air/fuel mixture. In the rich exhaust gas,
the nitrogen
oxides are desorbed again and reduced to nitrogen with the aid of the rich
exhaust gas
constituents over the catalytically active components. For this purpose, the
storage
catalyst usually contains rhodium in addition to platinum.
During the regeneration, the rich constituents of the exhaust gas are
converted to
harmless components in an exothermic reaction with the nitrogen oxides stored
in the
catalyst and with any oxygen stored and residual oxygen still present. This
heats the
exhaust gas during the regeneration over the storage catalyst. The exhaust gas
is heated
additionally by virtue of the fact that the air content in the cylinder is
greatly reduced by
throttling of the engine during the rich operation and hence the exhaust gas
is not cooled
by high air excess as in lean operation. The two effects together can lead to
a
temperature increase in the exhaust gas over the storage catalyst of 50 to 150
C during
the regeneration.
Storage phase and regeneration phase alternate regularly. The storage phase
usually
lasts between 60 and 120 seconds, whereas the duration of the regeneration
phase is
only between 1 and 10% of that of the storage phase and hence is only a few
seconds.
The short regeneration time increases the risk that the storage catalyst
regenerates for
longer than required, i.e. is supplied with rich exhaust gas. Under these
conditions, the
storage catalyst forms ammonia from the nitrogen oxides.
Oxidation catalysts refer here to those catalysts which oxidize hydrocarbons
and carbon
monoxide to carbon dioxide and water in the lean exhaust gas. For this
purpose,
oxidation catalysts comprise, as the catalytically active component, platinum
with or
without palladium. These oxidation catalysts also oxidize ammonia to nitrogen
and
nitrogen oxides.
SCR catalysts are understood to mean catalysts which convert nitrogen oxides
selectively to nitrogen under lean exhaust gas conditions with addition of
ammonia as a
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reducing agent. These catalysts contain acidic oxides and can store ammonia.
Typical
SCR catalysts contain, for example, vanadium oxide and/or tungsten oxide on
titanium
oxide. Alternatively, it is also possible to use copper- and/or iron-exchanged
zeolites.
Usually, such catalysts do not contain any catalytically active platinum group
metals
5 since these metals would oxidize the ammonia in the lean exhaust gas to
nitrogen
oxides. For the inventive emission control system, preference is given to
using SCR
catalysts which comprise zeolites. Zeolites have a particularly high storage
capacity for
ammonia and for hydrocarbons. They are therefore outstandingly suitable for
the
storage and conversion of these components of the exhaust gas with nitrogen
oxides.
The storage effect of the SCR catalysts for ammonia depends very greatly on
the
temperature. In particular after ageing of the catalysts in real operation,
the storage
effect declines very greatly above 300 C and is barely perceptible any longer
at
temperatures above 400 C. Therefore, particularly at high exhaust gas
temperatures,
there is the risk that too much ammonia metered in leaves the SCR catalyst
with the
exhaust gas before it can react with the nitrogen oxides. In order to prevent
this, a so-
called ammonia barrier catalyst is usually arranged downstream of the SCR
catalyst. In
the simplest case, this is an oxidation catalyst which, however, can also
reoxidize the
ammonia to nitrogen oxides under unfavourable operating conditions.
The nitrogen oxide storage catalysts, oxidation catalysts and SCR catalysts
used in the
context of this invention are known to those skilled in the art. The catalysts
are
preferably applied in the form of a coating to inert honeycombs of ceramic or
metal.
It is an advantage of the SCR catalyst in the common exhaust leg that it
barely, if at all,
oxidizes any nitrogen monoxide which breaks through to nitrogen dioxide - in
contrast
to the oxidation catalyst. This property is particularly important with regard
to the
expected emissions legislation for the emission of nitrogen dioxide. Nitrogen
monoxide
harms the environment to a lesser degree than nitrogen dioxide.
A further advantage of the inventive emission control system is the fact that
the SCR
catalyst, caused by the design of the system, has a large distance between the
nitrogen
oxide storage catalysts and the SCR catalyst. The exhaust leg between the
storage
catalysts and the SCR catalyst may be 0.5 to 1.5 metres. In the course of flow
through
this exhaust leg, the exhaust gas cools down by about 50 C per metre of
exhaust leg. A
further crucial advantage of the process according to the invention is that
storage and
regeneration phases of the two cylinder groups are offset in time with respect
to one
another, as a result of which the exhaust gas in the common exhaust line has
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temperature variations of lesser magnitude and lower maximum temperatures in
storage/regeneration operation than would be the case directly downstream of
the
nitrogen oxide storage catalysts in the individual exhaust lines. This leads
to the effect
that the SCR catalyst, over wide operating ranges of the engine, has a
temperature at
which the catalyst has a high storage effect for ammonia and is thus capable
of
converting the nitrogen oxides which break through the storage catalysts
during the lean
phase to harmless products with the ammonia stored.
In a specific embodiment of the invention, there is an oxidation catalyst
downstream of
the SCR catalyst in the common exhaust leg. SCR catalyst and oxidation
catalyst may
be arranged in series in separate housings. In this arrangement, the exhaust
gas must
heat the two separate catalysts to operating temperature. This is additionally
complicated by heat losses between the two catalysts. It is therefore
preferred for
thermal reasons to apply both catalysts in the form of coatings to a common
honeycomb
as a support of the coatings. This combined SCR and oxidation catalyst may be
designed as a so-called zone catalyst, which means that the SCR catalyst is
applied on
an inflow part of the honeycomb and the oxidation catalyst on an outflow part
of the
honeycomb.
It is particularly preferred, however, to apply the oxidation catalyst in the
form of a first
layer to a honeycomb and to apply the SCR catalyst as a second layer to this
first layer.
This arrangement has an outstanding barrier effect for ammonia and
additionally also
converts residual nitrogen oxides.
In further embodiments of the invention, the nitrogen oxide storage catalysts
may be
connected upstream of oxidation catalysts or three-way catalysts, for example
in a
position close to the engine, in order to reduce cold start emissions and to
promote the
oxidation of nitrogen monoxide to nitrogen dioxide in normal operation.
Combination
with a diesel particulate filter is also possible.
According to the invention, the emission control system described here is
operated as
follows: lean exhaust gas flows through the two nitrogen oxide storage
catalysts during
a storage phase, and rich exhaust gas during a regeneration phase, storage
phase and
regeneration phase alternating cyclically. The regeneration phase of one of
the two
storage catalysts is initiated whenever the other storage catalyst is in its
storage phase.
Lean and rich exhaust gas are adjusted with respect to one another so as to
result in a
lean exhaust gas after the combination of the exhaust gases in the combined
exhaust leg.
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Lean and rich exhaust gases are preferably obtained by operating the cylinders
assigned
to the two storage catalysts with lean or rich air/fuel mixtures and releasing
them into
the corresponding exhaust legs.
Alternatively, the engine can also be operated constantly with lean air/fuel
mixture. In
this case, the exhaust gas in the two exhaust legs is enriched in each case by
injecting
reducing agents for regeneration of the storage catalysts. Suitable reducing
agents are,
for example, fuel or other hydrocarbons. This mode of operation may be
advantageous
particularly in diesel engines.
The correct running of these operations is preferably monitored by an
electronic engine
control system. This engine control system regulates the compositions of the
exhaust
gases in the two exhaust legs independently of one another. It supplies, for
example, the
first group of cylinders assigned to the first exhaust leg with lean air/fuel
mixture during
the storage phase and initiates the regeneration of the nitrogen oxide storage
catalyst in
the second exhaust leg during this phase by briefly supplying the second group
of
cylinders assigned to the second exhaust leg with rich air/fuel mixture. This
operation is
repeated periodically in the reverse sequence in each case.
The invention is illustrated in detail by Figures I to 5. The figures show:
Figure 1: Emission control system according to the invention with an SCR
catalyst in
the common exhaust leg
Figure 2: Emission control system according to the invention with an SCR
catalyst in
the common exhaust leg and an oxidation catalyst arranged downstream
thereof
Fiwe 3: Emission control system according to the invention with an SCR
catalyst and
an oxidation catalyst on a honeycomb in the common exhaust leg
Figure 4: Emission control system according to the invention with a
combination
catalyst composed of a layer of an SCR catalyst atop a layer of an oxidation
catalyst in the common exhaust leg
Figure 5: Schematic diagram of the air ratios k against time in the first and
second
exhaust leg and in the common exhaust leg
Figures l to 4 show four embodiments of the emission control system. The same
reference numeral denotes components of the same type. Reference numeral (1)
denotes
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a lean burn engine with two cylinder banks (2) and (2'). The exhaust gases of
these
cylinder banks are released into the two exhaust legs (3) and (3'). At the
confluence (4),
the two exhaust legs (3) and (3') are combined to form a common exhaust leg
(5). For
storage and conversion of the nitrogen oxides emitted by the lean burn engine
(1), the
nitrogen oxide storage catalysts (6) and (6') are arranged in the exhaust legs
(3) and
(3').
According to the invention, an SCR catalyst (7) is present in the common
exhaust leg
(5). It stores the unwanted ammonia which forms in the course of regeneration
of the
storage catalysts. The correct running of storage phase and regeneration phase
is
] 0 preferably monitored by an electronic engine control system. This engine
control
system and the necessary sensors for the determination of the air ratio in the
two
exhaust legs are not shown in the figures for the sake of simplicity.
Electronic engine
control systems and the necessary sensors for operation of a lean burn engine
with
nitrogen oxide storage catalysts are known to those skilled in the art. For
the operation
of the inventive exhaust gas system according to the method described, the
control
programme has to be adjusted correspondingly.
In a specific embodiment of the invention, as shown in Figure 2, an oxidation
catalyst
(8) is inserted into the common exhaust leg downstream of the SCR catalyst
(7).
Figure 3 shows the design of the emission control system in a preferred
embodiment of
the invention. In the common exhaust leg (5), the SCR and oxidation catalysts
are now
arranged in direct succession. This combination of SCR and oxidation catalysts
can be
configured, for example, as zone catalyst on a single continuous honeycomb.
Figure 4 shows a further embodiment of the invention. In this embodiment, the
catalyst
(9) in the common exhaust leg is designed as a combined SCR and oxidation
catalyst.
The catalyst has an oxidation catalyst as a first layer on an inert honeycomb.
The SCR
catalyst is applied as a second layer to this oxidation catalyst. This second
layer is in
direct contact with the exhaust gas.
Figure 5 shows, in schematic form, the air ratio lambda (X) against time in
the first
exhaust leg (curve a)) relative to that in the second exhaust leg (curve b))
and in the
common exhaust leg (curve c)). The air ratio X is the air/fuel ratio
normalized to
stoichiometric conditions.
The dotted reference line in Figure 5 shows in each case the stoichiometric
ratio X = 1.
The storage phase with k > 1 (lean exhaust gas) alternates regularly with the
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regeneration phase with k < I in the two exhaust legs. The regeneration phase
is
significantly shorter than the storage phase. The phase position of the two
lambda
curves a) and b) relative to one another is substantially variable, provided
that the
exhaust gas in the combined exhaust leg is always lean (curve c)), i.e. has a
lambda
value greater than 1.
A particular advantage of the proposed emission control system and of the
method for
operation thereof is the fact that unwanted ammonia which forms in the
regeneration of
the storage catalysts is stored by the downstream SCR catalyst. The ammonia
stored
selectively converts nitrogen oxides which pass through the storage catalysts
during the
regeneration phase to nitrogen. This additionally reduces nitrogen oxide
emission.
