U.S. patent application number 12/552096 was filed with the patent office on 2011-03-03 for high efficiency nox reduction system and method.
This patent application is currently assigned to CUMMINS INTELLECTUAL PROPERTIES, INC.. Invention is credited to Neal W. CURRIER, Aleksey YEZERETS.
Application Number | 20110047970 12/552096 |
Document ID | / |
Family ID | 43622809 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110047970 |
Kind Code |
A1 |
YEZERETS; Aleksey ; et
al. |
March 3, 2011 |
HIGH EFFICIENCY NOx REDUCTION SYSTEM AND METHOD
Abstract
An exhaust gas stream aftertreatment system and method achieves
high efficiency NOx reduction in exhaust gas emissions by arranging
a selective catalytic reduction (SCR) catalyst element upstream
from a NOx adsorber catalyst. In an exemplary embodiment, a
reductant is introduced into the exhaust gas stream, exposing the
exhaust gas stream containing the reductant to a selective
catalytic reduction (SCR) catalyst element, and exposing the
exhaust gas stream that was exposed to the SCR catalyst element to
a NOx adsorber catalyst element. The NOx adsorber catalyst element
can be regenerated by temporarily reducing an oxygen concentration
in an exhaust gas stream to reduce the .lamda. of the exhaust gas
stream. While the oxygen concentration is reduced, reductant is
introduced into the exhaust gas stream at a rate that achieves a
fuel-rich condition, and the fuel rich exhaust stream is exposed to
the NOx adsorber catalyst element.
Inventors: |
YEZERETS; Aleksey;
(Columbus, IN) ; CURRIER; Neal W.; (Columbus,
IN) |
Assignee: |
CUMMINS INTELLECTUAL PROPERTIES,
INC.
Minneapolis
MN
|
Family ID: |
43622809 |
Appl. No.: |
12/552096 |
Filed: |
September 1, 2009 |
Current U.S.
Class: |
60/274 ; 60/276;
60/286; 60/295; 60/301 |
Current CPC
Class: |
F01N 3/0871 20130101;
Y02T 10/12 20130101; Y02T 10/24 20130101; F01N 2560/021 20130101;
F01N 3/0842 20130101; F01N 13/009 20140601; F01N 3/208 20130101;
F02D 41/0275 20130101; F01N 2610/02 20130101 |
Class at
Publication: |
60/274 ; 60/301;
60/295; 60/276; 60/286 |
International
Class: |
F01N 3/10 20060101
F01N003/10; F01N 3/023 20060101 F01N003/023; F01N 11/00 20060101
F01N011/00; F01N 9/00 20060101 F01N009/00 |
Claims
1. A method of reducing NOx in an exhaust gas stream emitted from
an internal combustion engine, comprising: introducing a reductant
into the exhaust gas stream; exposing the exhaust gas stream
containing the reductant to a selective catalytic reduction (SCR)
catalyst element; and exposing the exhaust gas stream that was
exposed to the SCR catalyst element to a NOx adsorber catalyst
element.
2. The method of claim 1, further comprising intermittently
regenerating the NOx adsorber catalyst by temporarily reducing the
oxygen mass flux in the exhaust gas stream and increasing a rate
that the reductant is introduced into the exhaust stream while the
oxygen mass flux is reduced to create a net reducing condition in
the NOx adsorber catalyst element.
3. The method of claim 2, wherein temporarily reducing the oxygen
mass flux in the exhaust gas stream comprises choking the air
intake of the engine.
4. The method of claim 2, wherein the intermittent regeneration of
the NOx adsorber catalyst is performed periodically.
5. The method of claim 2, further comprising sensing an amount of
NOx in the exhaust stream downstream from the NOx adsorber
catalyst, and triggering the intermittent regeneration when the
sensed amount indicates the adsorber catalyst is at least one of
full and approaching a saturation point.
6. The method of claim 2, further comprising sensing an amount of
reductant in the exhaust stream downstream from the NOx adsorber
catalyst, and triggering the intermittently regeneration when the
sensed amount of reductant indicates the adsorber catalyst is at
least one of full and approaching a saturation point.
7. The method of claim 1, wherein the reductant is ammonia or
urea.
8. The method of claim 2, wherein an intermittent regeneration
further comprises reversing the order in which the exhaust gas
stream is exposed to the SCR catalyst element and the NOx adsorber
catalyst element.
9. A NOx reduction system comprising a gas flow circuit for
treating an exhaust gas stream emitted from the outlet of an
internal combustion engine, said gas flow circuit comprising: a
reductant introduction port; a selective catalytic reduction (SCR)
catalyst element positioned downstream from the reductant
introduction port; and a NOx adsorber catalyst element positioned
downstream from the SCR catalyst element.
10. The system of claim 9, further comprising a controller that
monitors whether an intermittent regeneration triggering event
occurs, and initiates regeneration of the NOx adsorber catalyst
element after sensing a triggering event.
11. The system of claim 10, wherein regeneration of the NOx
adsorber catalyst element includes temporarily reducing the oxygen
mass flux in the exhaust gas stream exiting the internal combustion
engine and increasing a rate of injection of a reductant injected
from the reductant injection port while the oxygen mass flux is
reduced.
12. The system of claim 10, wherein said intermittent regeneration
triggering event comprises monitoring at least one sensor
positioned downstream from the NOx adsorber catalyst and initiating
the regeneration of the NOx adsorber catalyst when a sensed amount
indicates the adsorber catalyst is at least one of full and
approaching a saturation point.
13. The system of claim 12, wherein the sensor is a NOx sensor.
14. The system of claim 12, wherein the sensor is an ammonia
sensor.
15. The system of claim 10, further comprising a valve that can be
switched from a first state in which the SCR catalyst element is
connected upstream from the NOx adsorber catalyst, and a second
state in which the NOx adsorber catalyst is connected upstream from
the SCR catalyst element.
16. The system of claim 15, wherein the controller switches the
valve to the second state after determining that a triggering event
has occurred.
17. A method of regenerating a NOx adsorber catalyst element in an
exhaust aftertreatment system including a selective catalytic
reduction (SCR) catalyst element positioned upstream from the NOx
adsorber catalyst element, comprising: temporarily reducing an
oxygen concentration in an exhaust gas stream to reduce the .lamda.
of the exhaust gas stream; while the oxygen concentration is
reduced, introducing a reductant into the exhaust gas stream at a
rate that achieves a fuel-rich condition; and exposing the
fuel-rich exhaust gas stream to the NOx adsorber catalyst.
18. The method of claim 17, further comprising sensing an amount of
NOx in the exhaust gas stream downstream from the NOx adsorber
catalyst, and triggering regeneration when the sensed amount
indicates the adsorber catalyst is at least one of full and
approaching a saturation point.
19. The method of claim 17, further comprising sensing an amount of
reductant in the exhaust stream downstream from the NOx adsorber
catalyst, and triggering regeneration when the sensed amount of
reductant indicates the adsorber catalyst is at least one of full
and approaching a saturation point.
20. The method of claim 17, wherein regeneration of the NOx
adsorber catalyst is performed periodically.
Description
FIELD OF THE INVENTION
[0001] A system and method for reducing nitrogen oxides in exhaust
gas are disclosed.
BACKGROUND
[0002] Nitrogen oxides (NOx), which include nitric oxide (NO) and
nitrogen dioxide (NO.sub.2), are formed during the high temperature
and pressure combustion of an air and fuel mixture in an internal
combustion engine. These oxides cause a number of concerns related
to the environment, such as a source of ground-level ozone or smog,
acid rain, excess aqueous nutrients, and can readily react with
common organic chemicals, and even ozone, to form a wide variety of
toxic products. Since the 1970's, government legislation has
required increasing reductions of NOx in exhaust gas emissions.
[0003] To comply with increasingly stringent government mandates,
industry has developed several NOx reduction technologies to treat
post combustion exhaust, of which NOx adsorber catalyst and
selective catalytic reduction (SCR) technologies are actively
pursued.
[0004] The NOx adsorber catalyst, sometimes referred to as an NAC,
a lean NOx trap (LNT), a NOx storage reduction catalyst (NSR), and
a NOx storage catalyst (NSC), performs the process of adsorbing, or
"trapping" NOx using an alkali or alkaline earth storage material,
such as a barium salt (e.g., barium carbonate), and reducing the
bound nitrogen oxides using a catalytic surface comprised of
precious metal (e.g., platinum, palladium, rhodium, and
combinations thereof). More specifically, under lean air/fuel ratio
that typifies diesel exhaust, the storage material M adsorbs
NO.sub.2 as a nitrate, MNO.sub.3. After the NOx adsorber catalyst
reaches or approaches a certain level of saturation, it is
regenerated by creating or producing a fuel-rich environment, which
can be either stoichiometric air/fuel mixture or fuel-rich of a
stoichiometric mixture. A practical way of creating such an
environment typically involves two steps: 1) reducing oxygen
contained in the exhaust, and 2) consuming the residual of the
remaining oxygen and injecting some extra fuel in the cylinder.
Under these fuel-rich conditions, the NO.sub.x adsorber catalyst
promotes the decomposition and release of the nitrate as NO and
catalyzes the reduction of NO to nitrogen and carbon dioxide.
[0005] The SCR process reduces NOx to diatomic nitrogen (N.sub.2)
and water (H.sub.2O) using a catalyst and anhydrous ammonia
(NH.sub.3) or aqueous NH.sub.3, or a precursor that is convertible
to NH.sub.3, such as urea. Typical SCR catalysts are a honeycomb or
plate ceramic carrier (e.g., titanium oxide) and oxides of base
metals (e.g., vanadium and tungsten), zeolites and other precious
metals. Unreacted NH.sub.3 is sometimes released from the SCR, and
known as "NH.sub.3 slip." To reduce NH.sub.3 slip, an NH.sub.3 slip
catalyst can be provided downstream from the SCR.
SUMMARY
[0006] Embodiments consistent with the claimed invention relate to
an exhaust gas stream aftertreatment system and method that
achieves high efficiency NOx reduction in exhaust gas emissions by
arranging a selective catalytic reduction (SCR) catalyst element
upstream from a NOx adsorber catalyst.
[0007] In an exemplary embodiment consistent with the claimed
invention, a method of reducing NOx in an exhaust gas stream
emitted from an internal combustion engine includes introducing a
reductant into the exhaust gas stream, exposing the exhaust gas
stream containing the reductant to a selective catalytic reduction
(SCR) catalyst element, and exposing the exhaust gas stream that
was exposed to the SCR catalyst element to a NOx adsorber catalyst
element.
[0008] In another embodiment consistent with the claimed invention,
a NOx reduction system includes a gas flow circuit for treating an
exhaust gas stream emitted from an internal combustion engine. The
gas flow circuit comprises a reductant introduction port, an SCR
catalyst element positioned downstream from the reductant
introduction port, and a NOx adsorber catalyst element positioned
downstream from the SCR catalyst element.
[0009] In yet another aspect, a method of regenerating a NOx
adsorber catalyst element in an exhaust aftertreatment system
including an SCR catalyst element positioned upstream from the NOx
adsorber catalyst element includes temporarily reducing an oxygen
concentration in an exhaust gas stream to reduce the .lamda. of the
exhaust gas stream. While the oxygen concentration is reduced,
reductant is introduced into the exhaust gas stream at a rate that
achieves a fuel-rich condition. The fuel-rich exhaust gas stream is
then exposed to the NOx adsorber catalyst element.
[0010] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and exemplary only and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention that together with the description serve to explain
the principles of the invention. In the drawings:
[0012] FIG. 1 is a diagram of an exhaust treatment system in
accordance with an exemplary embodiment.
[0013] FIG. 2 is a set of graphs showing .lamda. values and ammonia
injection rates versus time in the exhaust stream at different
points along the exhaust treatment system of FIG. 1.
[0014] FIG. 3A is a flowchart of processes for treating exhaust gas
during normal operation in accordance with an exemplary
embodiment.
[0015] FIG. 3B is a flowchart of processes for treating exhaust gas
during regeneration in accordance with an exemplary embodiment.
[0016] FIG. 4A is diagram of an exhaust treatment system including
a valve configured for normal operation in accordance with
exemplary embodiments.
[0017] FIG. 4B is a diagram of the exhaust treatment system of FIG.
4A with the valve is configured for a regeneration operation.
DETAILED DESCRIPTION
[0018] The various aspects are described hereafter in greater
detail in connection with a number of exemplary embodiments to
facilitate an understanding of the invention. However, the
invention should not be construed as being limited to these
embodiments. Rather, these embodiments are provided so that the
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art.
[0019] Typical demonstrated efficiency of current NOx reduction
technologies, such as NOx adsorber catalyst and SCR using urea or
ammonia (NH.sub.3) is limited to, at best, about 80 to 85% over the
typical regulatory testing cycles. To achieve a target of 0.2
g/hp-hr NOx level at the tailpipe, for example, at an 80% NOx
conversion efficiency the engine-out NOx levels would need to be
reduced to 1 g/hp-hr. To meet such stringent NOx emission levels,
exhaust gas recycling (EGR) or other mechanisms are utilized to
reduce NOx exiting the engine (i.e., "engine-out NOx levels").
However, EGR negatively affects an engine's fuel efficiency. At the
same time, regulatory adjustment factors and engineering margins
have been driving engine-out NOx to even lower levels. Thus, a
different way to increase NOx conversion efficiency downstream of
the engine would allow for both cleaner exhaust gases and improved
engine performance. For example, if the downstream NOx conversion
efficiency can be increased to 95%, this would permit engine-out
NOx levels on the order of 4 g/hp-hr, which can provide margins for
more fuel-efficient engine operation, improved passive regeneration
of a diesel particle filter (DPF), and possible improvements to
engine durability due to reduced EGR flow rates or elimination of
EGR.
[0020] To provide more efficient reduction of NOx downstream of an
engine, embodiments consistent with the claimed invention position
a NOx adsorber catalyst downstream from an SCR catalyst. With this
arrangement, the overall NOx reduction efficiency of an SCR system
can be substantially improved by using the NOx adsorber catalyst to
capture NOx slip from the SCR catalyst, oxidize the NH.sub.3 slip
from the SCR, and capture the NOx produced by the SCR process. This
SCR-LNT sequential arrangement also can allow for more aggressive
urea/NH.sub.3 injection strategies, which result in higher SCR
efficiency. Regeneration of the NOx adsorber catalyst can be
accomplished using NH.sub.3 to create a net reducing environment
because NH.sub.3 can be a hydrogen carrier in the NOx adsorber
catalyst regeneration. For example, see L. Cumaranatunge et al.,
"Ammonia is a hydrogen carrier in the regeneration of
Pt/BaO/Al.sub.2O.sub.3 NOx traps with H.sub.2," Journal of
Catalysis, 246 (2007) 29-34.
[0021] While use of a LNT upstream from an SCR is known and has
been commercialized, the LNT-SCR sequential system is fundamentally
different from systems or methods consistent with the claimed
invention utilizing the SCR to LNT arrangement. Namely,
conventional LNT-SCR systems are NOx adsorber catalyst (LNT)-based
system, with a downstream SCR catalyst merely enhancing its
conversion efficiency at some conditions. When LNT is not working
in the conventional LNT-SCR system, the SCR also does not work.
There is no urea injection in those types of systems. In contrast,
systems and methods consistent with the claimed subject matter are
primarily SCR-based ones, with LNT acting as a NO.sub.x and
NH.sub.3 slip control catalyst. These and other differences will
become apparent in the following description of exemplary
embodiments.
[0022] FIG. 1 is a diagram of an exhaust aftertreatment system 100
and according to an exemplary embodiment. As shown in FIG. 1, the
exhaust aftertreatment system 100 includes a gas flow circuit that
includes an exhaust gas channel 112 provided to an exhaust gas
outlet of an internal combustion engine 110. The exhaust gas
channel 112 can have one or more segments that connect the engine
110 to a diesel exhaust fluid (DEF) port 120, an SCR catalyst
element 130, and a LNT element 140, although additional
aftertreatment elements (not shown) can be included in the exhaust
aftertreatment system 100.
[0023] The exhaust aftertreatment system 100 can include a
controller 150, which can be an electronic control unit (ECU) of
the engine 110 that monitors the performance of the engine 110 and
other elements of the exhaust aftertreatment system 100. The
controller 150 can be a single unit or plural control units that
collectively perform these monitoring and control functions. The
controller 150 utilizes sensors to determine whether the exhaust
aftertreatment system 100 is functioning properly. As shown in FIG.
1, the controller 150 can be connected to the DEF port 120, which
can include an injecting device such as an atomizer to inject a
reductant into the channel 112 downstream from the exhaust gas
outlet of the engine 110. The reductant can be an NH.sub.3 source,
such as anhydrous NH.sub.3 aqueous NH.sub.3, or a precursor that is
convertible to NH.sub.3 such as urea. The controller 150 can
control a timing and amount of reductant injected into the exhaust
stream by the DEF port 120.
[0024] The exhaust gas stream treated with the reductant by the DEF
port 120 is provided to an inlet of the SCR 130 at a location
downstream of the DEF port 120. While the exhaust gas stream
traverses the SCR catalyst element, the gas stream is exposed to
the SCR catalyst. After exiting an outlet the SCR 130, the exhaust
gas stream flows into the LNT element 140, where it is exposed to a
NOx adsorber catalyst in the LNT element 140. The LNT 140 can
comprise a housing defining an inlet at one end thereof configured
to receive exhaust gas exiting the SCR 130, an outlet at an
opposite end thereof, and a chamber between the inlet and the
outlet that houses the catalyst. Alternatively, more than one
element of the gas flow circuit of the exhaust aftertreatment
system 100 can be contained within a single housing. For example,
the SCR and LNT can be contained within the same housing in which
the SCR element is provided upstream of the LNT element so the
exhaust gas stream flows through a portion of the housing including
the SCR element before flow through a portion of the housing
including the LNT element.
[0025] Under normal operating conditions, the SCR element 130 is
the primary element of the exhaust aftertreatment system for
removing NOx from the exhaust gas stream. During normal operating
conditions, the controller 150 receives signals from various
sensors, such as the throttle sensor and air temperature sensor on
the engine (not shown) or a NOx or ammonia sensor 160 provided in
the exhaust aftertreatment system. From the sensed conditions, the
controller 150 can determine various parameter values of engine 110
and elements of the exhaust aftertreatment system. More
specifically, the controller 150 will cause the DEF port 120 to
inject reductant at a rate needed to operate the SCR catalyst
element 130 for a current operating condition of the engine 110.
NOx that slips through the SCR catalyst element 130 can be captured
by the LNT element 140.
[0026] Additionally, the LNT element 140 can oxidize NH.sub.3 (if
any) slipping from the SCR catalyst element 130 to N.sub.2, which
is benign, or to NOx, which the LNT element 140 also can trap and
process. This allows the SCR catalyst 112 to be overdosed with
reductant (e.g., NH.sub.3), which can result in higher SCR
conversion efficiency. For example, an ammonia dosing scheme will
have a respective ammonia/NOx ratio (ANR), where an ANR equal to 1
would represent a stoichiometric ratio. As an example, typical
average ANR values used under normal SCR system operation are
between about 0.8 and 1.5 or higher, although it is to be
understood that instantaneous ANR values can be greater or smaller
than these averages. The LNT element acting as a slip clean-up
catalyst in the described embodiments, enables the use more
aggressive reductant schemes in which ANR values can be
substantially higher than 1, for example, up to 5 or 10.
[0027] From time-to-time, the LNT element 140 must be actively
regenerated because NOx adsorber capture efficiency declines as a
function of the accumulation, or "storage" of NOx as a nitrate on
the catalytic surface of the LNT element 140. By rapidly increasing
the rate at which the reductant fluid is injected into the exhaust
stream, the stored NOx on the catalytic surface of the LNT element
140 can be converted in an active regeneration process that
promotes decomposition and release of the nitrate as NO. For
example, a sharp increase in the rate at which the reductant fluid
injected by the DEF port 120, and/or an introduction of additional
reductant fluid at some other point upstream from the LNT element
140, can create a fuel-rich, net reducing condition necessary for
regeneration of the LNT element 140. The reducing condition causes
the release of NOx through decomposition of nitrates on the
catalytic surface of the LNT 140 upon introduction of the
reductant, which leads to reduction of the regenerated NOx to
N.sub.2. The regeneration process can be further facilitated by
reducing oxygen mass flux in the exhaust gas stream. For example,
an air intake of the engine 110 can be choked to reduce the oxygen
content in the exhaust gas stream prior to rapidly increasing the
rate that reductant is injected into the stream.
[0028] FIG. 2 includes a set of graphs showing characteristics of
the exhaust gas stream at different points along the exhaust gas
flow path before, during, and after regeneration of the LNT 140.
Graphs 212 and 232 depict exemplary oxygen stoichiometry value, or
".lamda. value", versus time at different points along the exhaust
stream. Graph 222 depicts levels of NH.sub.3 injected by the DEF
port 120 during the same time period covered by graphs 212 and 232.
It is to be appreciated that the graphs of FIG. 2 are simplified
for ease of illustrating concepts, and that the levels of reductant
solution and .lamda. value depicted can vary to some degree
depending on how the engine 110 is operating. Further, while the
graphs depict some events occurring simultaneously, it is to be
understood that a small time delay (not shown) can occur for events
at locations more downstream of others.
[0029] Referring again to FIG. 1, plural dotted arrows, each having
an indicator designating one of the graphs 212, 222 and 232 of FIG.
2 are positioned at the different points along the flow path
corresponding to exemplary exhaust gas characteristics depicted by
the graphs 212, 222, and 232. Starting from the engine 110, the
regeneration process can occur while choking air entering an intake
of the engine 110 from time t.sub.rb until a later time t.sub.re.
This choking is shown in graph 212 where the value of the exhaust
exiting the engine 110 (i.e., the "engine.sub.out .lamda.") is
reduced from a fuel-lean "normal" operating condition L to a more
rich condition while the air intake of the engine 110 is choked
during the time period from t.sub.rb to t.sub.re. As shown in graph
212, the choking process reduces air in the value of exhaust gas
stream in the channel 112 exiting the outlet of the engine 110 from
the fuel-lean value L to a "near rich" value, NR, in which the
.lamda. value is reduced, but remains above a stoichiometric level
of 1.
[0030] As shown by graph 222 of FIG. 2, prior to regeneration, a
reductant solution is injected into the exhaust stream by the DEF
port 120 at a level N that is sufficient for operating the SCR 130.
Typically, a value of N can be about 300 ppm, although N can take
on any value of a range of values, for example a range of about 50
to 1000 ppm, because instantaneous amounts of reductant introduced
into the exhaust stream can vary often in accordance with operating
conditions. During regeneration event, the rate of injected
reductant fluid (e.g., from the DEF port 120) can be rapidly
increased and held at a level R between time t.sub.ib and t.sub.ie
within the time period from t.sub.rb to t.sub.re when the engine
110 is choked. For example, the level of NH.sub.3 introduced can be
increased from about 300 ppm used to operate the SCR to
concentrations of at least 3000 to 5000 ppm during the rich pulse R
(assuming about 0.5% residual oxygen). Thus, the reductant
concentration can increase an order of magnitude, or about 10-50
times the normal concentrations of NH3 for SCR. In a case where
residual O.sub.2 is greater than about 2%, the reductant
concentration can increase to four or more times the numbers for
the 0.5% O.sub.2 case. Hence, an amount R of reductant introduced
during a regeneration event can be about 10 to 1000 times the
amount N used to normally operate an SCR catalyst element. Further,
the amount R need not be held to a single value during the entire
regeneration event, but may increase or decrease in correspondence
with an amount N used to operation of the engine around the time of
regeneration.
[0031] At about the same time the reductant level is increased, as
shown by the graph 232, the value of the exhaust entering LNT 140
(i.e., the "LNT.sub.in .lamda.") breaks through the stoichiometry
line at .lamda.=1 to create a fuel-rich condition in the exhaust
gas stream, although it is to be understood that reductant can be
introduced at a rate at or near stoichiometry to produce a
fuel-rich reducing condition. It is during this time interval of a
large increase in the DEF injection (i.e., when the exhaust gas is
rich) that regeneration of the LNT 140 actually occurs. Because the
LNT 140 can be regenerated without necessarily injecting extra fuel
into the exhaust stream or over-injecting fuel into the cylinders
of the engine 110 as in conventional LNT aftertreatment systems,
performance of the overall engine and exhaust aftertreatment system
can be improved.
[0032] Embodiments of system and methods for reducing NOx described
herein operate differently from known generic NH.sub.3 slip
catalysts. One difference is embodiments have a mode of operation
that involves an intermittent (e.g., periodic) cleaning up of a NOx
adsorber catalyst (LNT) using NH.sub.3 as the reductant. More
specifically, the NOx adsorber catalyst removes both NOx and
NH.sub.3 slip from an SCR by accumulating them, and then
periodically the NOx adsorber catalyst must be cleaned by first
choking the engine to some degree to reduce the air-fuel ratio and
injecting a massive amount of a reductant, such as NH.sub.3, and/or
introducing additional fuel into exhaust stream by some mechanism,
such as an injection port, to regenerate it. By first reducing
oxygen in the exhaust gas exiting the engine, the amount of
NH.sub.3 that would be required as a reductant for a regeneration
event can be reduced to a practical level. By contrast, known slip
catalysts work continuously to remove NH.sub.3 slippage from an
SCR. Another difference is the use of a reductant (e.g., NH.sub.3)
to create a net reducing condition and eliminate or reduce the
necessity of injecting fuel in the way a known lean NOx trap is
operated.
[0033] An intermittent regeneration event can be triggered by
either a physical or virtual sensor (not shown) recognizing that
the NOx adsorber catalyst is full, or approaching a saturation
point. For example, FIG. 1 shows a NOx and/or NH.sub.3 sensor 160
located after the outlet of the NOx adsorber catalyst that can
trigger regeneration based on feedback information related to
sensed NOx and/or ammonia concentrations. Alternatively,
intermittent regeneration can be triggered using a time based
sensor, for example, a timer that triggers regeneration processes
in a periodic manner (e.g., once every minute, once every ten
minutes etc.). Time between intermittent regeneration may be
determined by any, or a combination of controlling variables, such
as sensed operating environment (e.g., city versus highway), heavy
duty versus light duty operation, and "real time" or recent driving
history. A scheme can be designed to regenerate the NOx adsorber
catalyst at such a frequency, manner, and/or extent to achieve a
prescribed emissions criterion while minimizing a performance
penalty.
[0034] FIG. 3A depicts an exemplary process flow 300 performed by
an embodiment of an exhaust aftertreatment system including an LNT
element located downstream from an SCR catalyst element. In process
310, an emitted exhaust gas stream, for example, from a diesel
internal combustion engine, is treated with a reductant solution
containing urea and/or ammonia by introducing the reductant
solution into the exhaust gas stream. For example, a rate at which
a reductant is introduced into the exhaust gas can be increased
from an amount that would be required to operate the SCR element
when operating the engine under normal conditions. The exhaust gas
stream including the reductant is then exposed to an SCR catalyst
in process 312. In process 314, the exhaust gas stream that was
exposed to the SCR catalyst is exposed to the to a NOx adsorber
catalyst.
[0035] FIG. 3B illustrates an exemplary process flow 320 that can
occur when the controller determines a regeneration event has been
triggered. At process 322, the air mass flux in the exhaust is
reduced to lower the exhaust gas .lamda. value and create a
near-rich condition. For example, the air intake of the engine can
be choked to lower the air/fuel ratio to a lean level in which the
oxygen stoichiometry is close to 1. While the .lamda. value is
lowered, an amount of reductant introduced into the exhaust gas
stream in process 324 to create a fuel-rich, net reducing condition
for the exhaust gas. In process 326, the exhaust gas having the
fuel rich, net reducing condition is exposed to a NOx adsorber
catalyst. At the end of a regeneration event, process 320 reduces
the rate that the reductant solution is introduced into the exhaust
gas stream and increases the air mass present in the exhaust gas
stream to levels that are adequate for at the current engine
operating condition.
[0036] The end of a regeneration event can be commanded based on
the pre-set timer value (e.g., two to four seconds), an open loop
virtual sensor such as one based on the exhaust conditions at the
time of regeneration and/or integrated since the last regeneration
event, or a closed loop virtual sensor, such as one based on
measurements of some catalyst properties, such as detection of
depletion of oxygen storage capacity using a .lamda.-sensor.
[0037] With reference now to FIGS. 4A and 4B, an embodiment of a
system 400 for reducing NOx in exhaust gas includes a mechanism
that reverses the exhaust gas flow through a segment of the system
400 to facilitate precise control of NH.sub.3 slip during
regeneration of the NOx adsorber catalyst, and thus an overall
increase in the efficiency of operation.
[0038] FIG. 4A depicts the direction of exhaust gas flow during a
normal operating mode. As indicated by the arrows 402, a reductant
solution (e.g., NH.sub.3 or aqueous urea) is injected by a DEF port
420 into an exhaust gas flowing from an engine 410 at a level
sufficient for operating an SCR catalyst element 430 downstream of
the engine 410. After the exhaust gas stream is exposed to the SCR
catalyst element 430, the exhaust gas is exposed to a catalyst of a
LNT element 440. NH.sub.3 slip from the SCR catalyst element 430 is
removed by the LNT element 440. A controller 460 controls the DEF
420, the engine 410 and a four port, or four-way valve 470. The
controller 460 can set the valve 470 at one of two possible states.
In a first state illustrated in FIG. 4A, the exhaust gas flows into
the valve 470, through the SCR catalyst element 430, through the
LNT element 440 downstream from the SCR catalyst element 430, and
then again through the valve 470 before exiting the system.
[0039] FIG. 4B shows the system 400 operating during a regeneration
operating mode. In this operating mode, the controller 460 sets the
valve 470 in a second state, which changes the order that the
exhaust stream flows through the SCR and NOx adsorber catalyst
elements. More particularly, valve 470 is set so that the LNT 440
is connected upstream from the SCR catalyst element 430, as
indicated by arrows 404. During regeneration, exhaust gas rich in
reductant fluid (e.g., urea or NH.sub.3) is first sent through the
LNT 440 and thereafter through the SCR catalyst element 430
because, while controlling NH.sub.3 slip under normal operating
conditions is less difficult when only several hundred ppm of
NH.sub.3 is injected into the exhaust flow (i.e., a percentage of
NH.sub.3 that would slip through would be low), increasing an
amount of reductant to a relatively high level at the time of
regeneration, for example, going from 300 ppm to 2% or more
concentration of NH.sub.3, presents a higher risk of large NH.sub.3
slip. By switching the order of elements during a regeneration
event, the SCR catalyst element 430 can better act as a slip
control device for NH.sub.3 passing through the LNT element
440.
[0040] Embodiments described herein having the NOx adsorber
catalyst arranged downstream from the SCR can allow for more
efficient operation of the engine because use of EGR, which reduces
engine efficiency (e.g., fuel efficiency, horsepower, and engine
performance) can either be reduced significantly or eliminated
altogether. Additionally, a reduction in EGR also decreases overall
operating costs. Also, positioning a NOx adsorber catalyst after
the SCR in the normal operating mode allows for higher
concentrations of ammonia during a normal operating mode of the
engine, and thus greater SCR efficiency.
[0041] It will be appreciated that the embodiments described and
shown herein may be modified in a number of ways. For instance, in
any embodiment consistent with the claimed invention, the exhaust
system can include other elements such as a diesel oxidation
catalyst (DOC) and/or a DPF in the stream of the exhaust gas flow
from the engine. Additionally, a reductant fluid injection point
may be provided at any location after the engine and upstream of
the SCR. It will be appreciated that a reductant other than those
described above can be used, such as gaseous NH3 or some source of
bound NH3 other than urea solution.
[0042] Additional embodiments can include, but are not limited to
one or more additional reductant (e.g., urea or NH.sub.3) injection
location upstream of the NOx adsorber catalyst. Embodiments also
can include alternative way of regenerating the NOx adsorber
catalyst. In addition, LNT regeneration can be accomplished in a
traditional way, albeit much less frequently since it is used only
as a slip catalyst, by achieving rich conditions in-cylinder or
with a supplemental fuel doser. Even with such a classical mode of
LNT regeneration, an application of a LNT can provide a combined
reductant (e.g., NH.sub.3) slip and NOx slip catalyst for an SCR
catalyst element.
[0043] Further, while the embodiments are depicted as separate
devices in the figures, other embodiments can include an SCR and
NOx adsorber catalysts being zone-coated on the same substrate or
the SCR and NOx adsorber catalysts arranged as layers on the same
substrate. Hence, while embodiments described above include a
separate chamber for each of the SCR and NOx adsorber catalyst
(LNT), other embodiments can include both the SCR and LNT in a same
chamber. The exhaust in all such embodiments would flow in a
direction from the SCR to the LNT within the chamber. Furthermore,
while the exemplary embodiments are sometimes described in the
context of a diesel internal combustion engine, the same concepts
can be applied in a lean burn gasoline powered internal combustion
engine.
[0044] Although a limited number of exemplary embodiments is
described herein, one of ordinary skill in the art will readily
recognize that there could be variations to any of these
embodiments and those variations would be within the scope of the
appended claims. Thus, it will be apparent to those skilled in the
art that various changes and modifications can be made to the NOx
reduction system and method described herein without departing from
the scope of the appended claims and their equivalents.
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