U.S. patent application number 11/059498 was filed with the patent office on 2006-08-17 for integrated nox and pm reduction devices for the treatment of emissions from internal combustion engines.
This patent application is currently assigned to Eaton Corporation. Invention is credited to Dawn Marie Becher, Fred Joseph Begale, Karen Evelyn Bevan, Haoran Hu, Wayne Scott Kaboord, James Edward JR. McCarthy, Subbaraya Radhamohan, Johannes W. Reuter, Vishal Singh.
Application Number | 20060179825 11/059498 |
Document ID | / |
Family ID | 36814236 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060179825 |
Kind Code |
A1 |
Hu; Haoran ; et al. |
August 17, 2006 |
Integrated NOx and PM reduction devices for the treatment of
emissions from internal combustion engines
Abstract
One concept of the inventors relates to a system and method in
which a particulate filter comprises at least about 40% by weight
of an NOx adsorbant. The filter can be used as both an NOx trap and
a particulate filter. By constructing the filter elements using a
substantial amount of NOx adsorbant, a large volume of NOx
adsorbant can be incorporated into the particulate filter, which
substantially reduces the volume and expense of an exhaust system
that includes both a catalytic diesel particulate filter and an NOx
trap having a large quantity of NOx adsorbant. In a preferred
embodiment, the filter also oxidizes NO to NO.sub.2. In another
preferred embodiment, an SCR catalyst is position downstream of the
filter elements.
Inventors: |
Hu; Haoran; (Novi, MI)
; Radhamohan; Subbaraya; (Novi, MI) ; Bevan; Karen
Evelyn; (Northville, MI) ; McCarthy; James Edward
JR.; (Canton, MI) ; Reuter; Johannes W.;
(Ypsilanti, MI) ; Singh; Vishal; (Farmington
Hills, MI) ; Kaboord; Wayne Scott; (Oregon, WI)
; Begale; Fred Joseph; (Oconomowoc, WI) ; Becher;
Dawn Marie; (Random Lake, WI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
36814236 |
Appl. No.: |
11/059498 |
Filed: |
February 16, 2005 |
Current U.S.
Class: |
60/297 ; 60/295;
60/311 |
Current CPC
Class: |
B01D 2279/30 20130101;
B01D 2253/108 20130101; B01D 2253/102 20130101; B01D 2259/4566
20130101; Y02A 50/20 20180101; B01D 2251/60 20130101; B01D
2255/2063 20130101; F01N 2610/02 20130101; B01D 46/0036 20130101;
F01N 13/0097 20140603; Y02A 50/2325 20180101; F01N 3/0842 20130101;
B01D 46/2418 20130101; F01N 2570/14 20130101; F01N 3/0821 20130101;
B01D 2258/012 20130101; F01N 3/035 20130101; F01N 2240/20 20130101;
F01N 2510/065 20130101; B01D 2257/402 20130101; F01N 3/2066
20130101; F01N 2330/06 20130101; Y02C 20/10 20130101; B01D
2259/40088 20130101; F01N 2330/14 20130101; F01N 3/025 20130101;
F01N 13/009 20140601; F01N 3/0878 20130101; Y02T 10/12 20130101;
B01D 2251/30 20130101; B01D 2253/104 20130101; F01N 3/0814
20130101; Y02T 10/24 20130101; F01N 2240/30 20130101; B01D 2251/40
20130101; B01D 2257/404 20130101; F01N 2330/22 20130101; B01D
2259/40086 20130101; F01N 2370/24 20130101; F01N 2330/10 20130101;
F01N 3/027 20130101; F01N 2410/12 20130101; B01D 53/04 20130101;
B01D 53/9431 20130101; F01N 3/0871 20130101; Y02A 50/2344 20180101;
F01N 2430/06 20130101; B01D 46/0063 20130101; F01N 3/0231 20130101;
F01N 3/0835 20130101; F01N 13/017 20140601; B01D 53/944 20130101;
F01N 2330/12 20130101 |
Class at
Publication: |
060/297 ;
060/295; 060/311 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/02 20060101 F01N003/02 |
Claims
1. A particulate filter, comprising: a body having an entrance and
an exit; and filter elements adapted to filter particulate matter
from gas flowing between the entrance and the exit; wherein the
filter elements comprise at least about 40% by weight of an NOx
adsorbant.
2. The particulate filter of claim 1, wherein the filter elements
comprise at least about 60% by weight NOx adsorbant.
3. The particulate filter of claim 1, wherein the filter elements
further comprise a catalyst for the reduction of NOx.
4. The particulate filter of claim 3, wherein the catalyst for the
reduction of NOx is also effective for oxidizing NO to
NO.sub.2.
5. The particulate filter of claim 1, wherein the filter elements
are formed by combining NOx adsorbant particles into a cohesive
mass.
6. The particulate filter of claim 1, wherein the filter elements
comprise large pores formed by interstices between NOx adsorbant
particles.
7. The particulate filter of claim 1, further comprising an
effective amount of an SCR catalyst.
8. The particulate filter of claim 7, wherein the SCR catalyst is
contained in a wash coat on an exit side of the filter
elements.
9. A power generation system comprising the particulate filter of
claim 1.
10. The power generation system of claim 9, further comprising a
fuel reformer configured to supply reformed fuel for regenerating
the NOx adsorbant.
11. A vehicle comprising the power generation system of claim
9.
12. A method of cleaning a diesel-powered vehicle's exhaust,
comprising: passing the exhaust through a particulate filter
comprising at least about 40% by weight NOx adsorbant to
substantially reduce both a particulate matter and an NOx content
of the exhaust; and intermittently regenerating the NOx adsorbant
by creating a reducing atmosphere within the particulate filter;
wherein the particulate filter is mounted on the vehicle.
13. The method of claim 12, wherein the particulate filter
comprises filter elements shaped from material comprising the NOx
adsorbant.
14. The method of claim 12, wherein regenerating the NOx adsorbant
comprises supplying syn gas to the particulate filter.
15. The method of claim 12, wherein the particulate filter
comprising at least about 60% by weight of the NOx adsorbant.
16. The method of claim 12, further comprising passing the exhaust
over an effective amount of an ammonia SCR catalyst.
17. The method of claim 16, wherein the ammonia SCR catalyst is
supported by the particulate filter.
18. The method of claim 17, wherein the ammonia SCR catalyst
comprises an effective amount of catalyst selected from the group
consisting of TiO.sub.2, WO.sub.3, V.sub.2O.sub.5, and MoO.sub.3
and combinations thereof.
19. The method of claim 12, further comprising oxidizing NO to
NO.sub.2 within the filter.
20. The method of claim 16, further comprising oxidizing NO to
NO.sub.2 within the filter.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of pollution
control devices for internal combustion engines, especially diesel
engines and lean burn gasoline engines.
BACKGROUND OF THE INVENTION
[0002] NO.sub.x emissions from vehicles with internal combustion
engines are an environmental problem recognized worldwide. Several
countries, including the United States, have long had regulations
pending that will limit NO.sub.x emissions from vehicles.
Manufacturers and researchers have put considerable effort toward
meeting those regulations. In conventional gasoline powered
vehicles that use stoichiometric fuel-air mixtures, three-way
catalysts have been shown to control NO.sub.x emissions. In diesel
powered vehicles and vehicles with lean-burn gasoline engines,
however, the exhaust is too oxygen-rich for three-way catalysts to
be effective.
[0003] Several solutions have been proposed for controlling NOx
emissions from diesel powered vehicles and lean-burn gasoline
engines. One set of approaches focuses on the engine. Techniques
such as exhaust gas recirculation and homogenizing fuel-air
mixtures can reduce NOx emissions. These techniques alone, however,
will not solve the problem. Another set of approaches remove NOx
from the vehicle exhaust. These include the use of lean-burn
NO.sub.x catalysts, lean NO.sub.x traps (LNTs), and selective
catalytic reduction (SCR).
[0004] Lean-burn NOx catalysts promote the reduction of NO.sub.x
under oxygen-rich conditions. Reduction of NOx in an oxidizing
atmosphere is difficult. It has proved challenging to find a
lean-burn NO.sub.x catalyst that has the required activity,
durability, and operating temperature range. Lean-burn NO.sub.x
catalysts also tend to be hydrothermally unstable. A noticeable
loss of activity occurs after relatively little use. Lean burn NOx
catalysts typically employ a zeolite wash coat, which is thought to
provide a reducing microenvironment. The introduction of a
reductant, such as diesel fuel, into the exhaust is generally
required and introduces a fuel economy penalty of 3% or more.
Currently, peak NOx conversion efficiency with lean-burn catalysts
is unacceptably low.
[0005] A lean NOx trap (LNT) is an NOx adsorber combined with a
catalyst for NOx reduction. The adsorber removes NOx from lean
exhaust. Periodically, the adsorber is regenerated by creating a
reducing environment. In the reducing environment, NOx is reduced
over the catalyst. The adsorbant is generally an alkaline earth
oxide adsorbant, such as BaCO.sub.3 and the catalyst can be a
precious metal, such as Ru.
[0006] SCR involves the reduction of NOx by ammonia. The reaction
takes place even in an oxidizing environment. The NOx can be
temporarily stored in an adsorbant or ammonia can be fed
continuously into the exhaust. SCR can achieve NOx reductions in
excess of 90%, however, there is concern over the lack of
infrastructure for distributing ammonia or a suitable precursor.
SCR also raises concerns relating to the possible release of
ammonia into the environment.
[0007] U.S. Pat. No. 6,560,958 describes an LNT system in which
hydrogen-rich synthesis gas (syn gas), including H.sub.2 and CO, is
used as a reductant to regenerate the adsorbant. The syn gas is
produced from diesel fuel in a plasma converter. Periodically, the
LNT is taken offline from the exhaust system and supplied with the
syn gas. A dual adsorber system is also described.
[0008] U.S. Pat. No. 6,732,507 describes a hybrid exhaust treatment
system using an LNT and an SCR reactor in series. The SCR reactor
captures ammonia produced by the LNT during regeneration and uses
the captured ammonia to increase the extent of NOx conversion.
[0009] U.S. Patent Application Publication No. 2004/0052699
describes an exhaust treatment device in which the functionalities
of a catalytic particulate filter and a NOx adsorber-catalyst are
combined into a single device. In one embodiment, a wash coat
comprising an NOx adsorbant is applied to a surface of a filter
element.
[0010] There continues to be a long felt need for reliable,
affordable, and effective systems for removing NOx and particulate
matter from the exhaust of diesel and lean-burn gasoline
engines.
SUMMARY OF THE INVENTION
[0011] One concept of the inventors relates to a system and method
in which a particulate filter comprises at least about 40% by
weight of an NOx adsorbant. The filter can be used as both an NOx
trap and a particulate filter. By constructing the filter elements
using a substantial amount of NOx adsorbant, a large volume of NOx
adsorbant can be incorporated into the particulate filter, which
substantially reduces the volume and expense of an exhaust system
that includes both a catalytic diesel particulate filter and an NOx
trap having a large quantity of NOx adsorbant. In a preferred
embodiment, the filter also oxidizes NO to NO.sub.2. In another
preferred embodiment, an SCR catalyst is position downstream of the
filter elements.
[0012] The forgoing summary encompasses certain of the inventors'
concepts. Its primary purpose is to present these concepts in a
simplified form as a prelude to the more detailed description that
follows. The summary is not a comprehensive description of what the
inventors have invented. Other concepts of the inventors will
become apparent to one of ordinary skill in the art from the
following detailed description and annexed drawings. Moreover, the
detailed description and annexed drawings draw attention to only
certain of the inventors' concepts and set forth only certain
examples and implementations of what the inventors have invented.
Other concepts of the inventors and other examples and
implementations of their concepts will become apparent to one of
ordinary skill in the art from that which is described and/or
illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic illustration of a particulate filter
incorporating an SCR catalyst.
[0014] FIG. 2 is a schematic illustration of a particulate filter
incorporating an SCR catalyst in a different way.
[0015] FIG. 3 is a schematic illustration of a power generation
system.
[0016] FIG. 4 is a schematic illustration of another power
generation system.
[0017] FIG. 5 is a schematic illustration of an exhaust treatment
system.
[0018] FIG. 6 is a schematic illustration of a particulate filter
incorporating an SCR catalyst and an NOx adsorbant.
[0019] FIG. 7 is a schematic illustration of another particulate
filter incorporating an NOx adsorbant.
[0020] FIG. 8 is a schematic illustration of another particulate
filter incorporating an NOx adsorbant and an SCR catalyst.
DETAILED DESCRIPTION OF THE INVENTION
[0021] A particulate filter is a relatively large device and is
only one of several devices that may be required in a diesel
exhaust system to meet emissions control regulations. Incidental to
its main function, which is to physically screen particulate matter
from exhaust gases, a typical particulate filter occupies a large
volume and presents a large contact area to the exhaust gases. The
space taken up by a particulate filter and/or the filter's large
contact area can be used to facilitate a second function. In one
aspect of the invention, that function is that of an SCR catalyst
bed.
[0022] FIG. 1 is a schematic illustration of an exemplary
particulate filter/SCR catalyst 10. The device 10 comprises filter
elements 11 and catalyst elements 12. The filter elements 11 are
porous and the structure of the device 10 generally causes exhaust
gases to pass through the filter elements 11. The catalyst elements
12 comprise an ammonia SCR catalyst. In FIG. 1, the ammonia SCR
catalyst is formed into a porous wash coat that lies over the
external surfaces of the filter elements 11. Optionally the SCR
wash coat covers only the inlet side of the filter elements 11 and
optionally the SCR wash coat covers only the outlet side of the
filter elements 11.
[0023] FIG. 2 illustrates another particulate filter/SCR catalyst
14. In this embodiment, the ammonia SCR catalyst forms a wash coat
that conforms to the high internal surface area of filter elements
15, whereby the SCR catalyst is disposed within the filter elements
15. The high internal surface area of the filter elements 15 and
the flow of exhaust gases through those filter elements provides a
high degree of contacting between the exhaust gases and the
catalyst, thereby making efficient use of the catalyst and avoiding
the need for a separate ammonia SCR catalyst device where an
ammonia SCR catalyst is desired.
[0024] A particulate filter/SCR catalyst can have any of the
configurations suitable for a diesel particulate filter. Examples
of suitable configurations include monolithic wall flow filters,
which are typically made from ceramics, especially cordierite or
SiC, blocks of ceramic foams, monolith-like structures of porous
sintered metals or metal-foams, and wound, knit, or braided
structures of temperature resistant fibers, such as ceramic or
metallic fibers. Typical pore sizes for the filter elements are
about 10 .mu.m or less, although larger pores may be initially
formed in anticipation of pore sizes being reduced by the
application of a catalyst-containing wash coat. On the other hand,
the ammonia SCR catalyst can be incorporated into the filter
material.
[0025] An ammonia SCR catalyst is one that effectively catalyzes a
reaction such as: 4NO+4NH.sub.3+O.sub.24N.sub.2+6H.sub.2O in lean
exhaust. Catalysts for this reaction will also reduce other species
of NOx. NO.sub.x includes, without limitation, NO, NO.sub.2,
N.sub.2O, and N.sub.2O.sub.2. Examples of SCR catalysts include
oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd,
Pt, Rh, Rd, Mo, and W. Other examples of ammonia SCR catalyst
include zeolites, such as ZSM-5 or ZSM-11 substituted with metal
ions such as cations of Cu, Co, Ag, Zn, or Pt, and activated
carbon. A preferred catalyst is a combination of TiO.sub.2, with
one or more of WO.sub.3, V.sub.2O.sub.5, and MoO.sub.3, for example
about 70 to about 95% by weight TiO.sub.2, about 5 to about 20% by
weight WO.sub.3 and/or MoO.sub.3, and 0 to about 5% by weight
V.sub.2O.sub.3. Catalysts of this type are commercially available
and can be tailored by the manufacturer for specific applications.
The typical temperature range in which these catalysts are
effective is from about 230 to about 500.degree. C. If the
temperature is too high, the ammonia decomposes before reducing
NOx.
[0026] FIG. 3 is an exemplary power generation system 20 employing
a particulate filter/SCR catalyst 24, which can have the structure
of either the particulate filter/SCR catalyst 10 or the particulate
filter/SCR catalyst 14. The power generation system 20 comprises an
internal combustion engine 21, which is typically a compression
ignition diesel engine, an ammonia supply 22, and an optional
oxidation catalyst 23. The ammonia supply 22 provides ammonia for
the NOx reduction reaction in the device 10. The optional oxidation
catalyst 23 converts NO to NO.sub.2 to facilitate continuous
removal of accumulated soot from the device 10. Converting NO to
NO.sub.2 also facilitates the reduction of NO.sub.x by NH.sub.3
over the SCR catalyst.
[0027] Any suitable method can be used to remove accumulated soot
from the particulate filter/SCR catalyst 24. Two general approaches
are continuous and intermittent regeneration. An example of
continuous regeneration depends on the reaction of soot with
NO.sub.2. Soot will react with NO.sub.2 at a lower temperature than
with O.sub.2. The optional oxidation catalyst 23 can comprise a
transition metal, preferably platinum, and catalyzes a reaction of
NO with O.sub.2 to form NO.sub.2. The combined filter/SCR catalyst
24 can contain a catalyst to further lower the effective
temperature for soot oxidation. Examples of catalysts for the
oxidation of soot by NO.sub.2 include oxides of Ce, Zr, La, Y, and
Nd. A soot oxidation catalyst is preferably concentrated on the
inlet side of the filter elements 15, where soot accumulates.
[0028] While FIG. 3 illustrates The oxidation catalyst 23 in a
separate brick upstream of a filter 24 containing an SCR catalyst,
an oxidation catalyst and an SCR catalyst can be distributed in any
suitable fashion within an exhaust system comprising a combined
filter/SCR catalyst according to the present invention. In one
embodiment, the two catalyst are co-dispersed, but generally they
are dispersed separately with one upstream of the other. The
advantage of having the oxidation catalyst upstream of the SCR
catalyst is that it converts NO to NO.sub.2, which facilitates the
ammonia SCR reaction. The disadvantage of having the oxidation
catalyst upstream of the SCR catalyst is that, in some
configurations, the oxidation catalyst may oxidize ammonia.
[0029] In one embodiment, the SCR catalyst is upstream of the
oxidation catalyst. For example, the SCR catalyst can be formed in
a washcoat on the inlet side of the filter, while the oxidation
catalyst is contained in an underlying coating.
[0030] In another embodiment, the oxidation catalyst is upstream of
the SCR catalyst, for example in a brick upstream from the filter
as in FIG. 3, and ammonia is supplied between the oxidation
catalyst and the SCR catalyst. In a further embodiment, the system
contains a NOx adsorbant, and the oxidation catalyst is upstream of
the adsorbant and the filter. In a still further embodiment, the
NOx adsorbant is associated with a catalyst that is effective for
converting NO to NO.sub.2 and the NOx adsorber/catalyst acts as the
oxidation catalyst. A NOx adsorber catalyst can for a separate
brick upstream of the filter/SCR catalyst or be incorporated within
the filter/SCR catalyst as described more fully below.
[0031] An example of an intermittent regeneration process is one
where the filter/SCR catalyst 24 is heated to a temperature where
soot reacts with oxygen. The process can be controlled by measuring
the pressure drop across the filter/SCR catalyst 24 and initiating
the regeneration process based on the pressure exceeding a critical
value. The filter/SCR catalyst 24 can be heated by any suitable
method. Examples of suitable heating methods may include electrical
resistance heating and a fuel burner located upstream of the
filter/SCR catalyst 24. Electrical resistance can involve applying
a voltage directly to the filter/SCR catalyst or to resistance
wires permeating the device. Soot oxidation is exothermic. It may
be possible to initiate the soot oxidation reaction in a localized
area of the filter/SCR catalyst 24 and have the reaction propagate
through the rest of the device.
[0032] The ammonia supply 22 can be any suitable ammonia source.
Examples of ammonia sources include reservoirs, such as reservoirs
of ammonia, ammonium carbomate, or urea, and ammonia plants, such
as plants that form ammonia from H.sub.2 and N.sub.2 or from
H.sub.2 and NOx. N.sub.2 can be obtained from air and H.sub.2 can
be produced by a fuel reformer. Ammonia, whatever its source, is
optionally stored in one or more adsorption beds, such as molecular
sieve adsorption beds, and desorbed as needed.
[0033] FIG. 4 is a schematic illustration of an exemplary power
generation system 30 employing the filter/SCR catalyst 24 in a
different configuration. The exemplary power generation system 30
comprises the internal combustion engine 21, a NOx trap 31, an
optional reductant supply 32, an optional oxidation catalyst 23,
and the filter/SCR catalyst 24. The NOx trap 31 is regenerated
intermittently. Regeneration generally comprises supplying
reductant to the NOx trap 31. The reductant can be obtained from
the optional reductant supply 32, although reductant can also be
obtained by running the engine 21 rich for a period of time. During
regeneration, ammonia and some NOx are released from the NOx trap
31. The ammonia reacts to reduce NOx in the filter/SCR catalyst 24.
The filter/SCR catalyst 24 can include an ammonia adsorbant to
buffer the ammonia. The filter/SCR catalyst 24 thereby improves NOx
removal, reduces ammonia emissions, and reduces the amount of
reductant required. The ammonia supply 22 can also be incorporated
in the power generation system 30 to reduce a further portion of
NOx remaining in the exhaust entering the device 10.
[0034] The NOx trap 31 comprises a NOx adsorption bed and a
catalyst effective for reducing NOx in a reducing environment. In
some cases, the catalyst contributes to the adsorbant function and
is necessarily provided in the adsorbant bed. In other cases, the
catalyst is optionally provided in a separate bed downstream of the
adsorption bed. The adsorption bed comprises an effective amount of
an adsorbent for NOx in an oxidizing (lean) environment. The NOx
trap 31 desorbs and/or reduces NOx in a reducing environment,
provided that the lean NOx trap is in an appropriate temperature
range.
[0035] The NOx trap 31 can be incorporated into the filter/SCR
catalyst 24. For example, the NOx adsorbant and catalyst can be
coated over the inlet passages of the combined filer/SCR catalyst
24. FIG. 6 provides a schematic illustration of a combined SCR
catalyst, NOx adsorbant, and particulate filter 50. An NOx
adsorber/catalyst 52 coat the inlet sides of filter elements 51,
while an SCR catalyst 53 coats the outlet sides of the filter
elements 51. The adsorber/catalyst 52 preferably enriches the ratio
of NO.sub.2 to NO in the NO.sub.x it does not adsorb.
[0036] The adsorption bed can comprise any suitable adsorbant
material. Examples of adsorbant materials include molecular sieves,
such as zeolites, alumina, silica, and activated carbon. Further
examples are oxides, carbonates, and hydroxides of alkaline earth
metals such as Mg, Ca, Sr, and Be or alkali metals such as K or Ce.
Still further examples include metal phosphates, such as phoshates
of titanium and zirconium.
[0037] Molecular seives are materials having a crystalline
structure that defines internal cavities and interconnecting pores
of regular size. Zeolites are the most common example. Zeolites
have crystalline structures generally based on atoms tetrahedrally
bonded to each other with oxygen bridges. The atoms are most
commonly aluminum and silicon (giving aluminosilicates), but P, Ga,
Ge, B, Be, and other atoms can also make up the tetrahedral
framework. The properties of a zeolite may be modified by ion
exchange, for example with a rare earth metal or chromium.
Preferred zeolites generally include rare earth zeolites and
Thomsonite. Rare earth zeolites are zeolites that have been
extensively (i.e., at least about 50%) or fully ion exchanged with
a rare earth metal, such as lanthanum. For NOx traps generally, a
preferred adsorbant is an alkaline metal or an alkaline earth metal
oxide loaded with a precious metal.
[0038] The adsorbant is typically combined with a binder and either
formed into a self-supporting structure or applied as a coating
over a substrate, which can be a particulate filter. A binder can
be, for example, a clay, a silicate, or a cement. Portland cement
can be used to bind molecular sieve crystals. Generally, the
adsorbant is most effective when a minimum of binder is used.
Preferably, the adsorbant bed contains from about 3 to about 20%
binder, more preferably from about 3 to about 12%, most preferably
from about 3 to about 8%.
[0039] Devices according to the present invention are generally
adapted for use in vehicle exhaust systems. Vehicle exhaust systems
create restriction on weight, dimensions, and durability. For
example, an adsorption bed for a vehicle exhaust system must be
reasonably resistant to degradation under the vibrations
encountered during vehicle operation.
[0040] When the adsorbant bed is not part of the filter, the
adsorbant bed can have any suitable structure. Examples of suitable
structures may include monoliths, packed beds, and layer screening.
A packed bed is preferably formed into a cohesive mass by sintering
the particles or adhering them with a binder. When the bed has an
adsorbant function, preferably any thick walls, large particles, or
thick coatings have a macro-porous structure facilitating access to
micro-pores where adsorption occurs. A macro-porous structure can
be developed by forming the walls, particles, or coatings from
small particles of adsorbant sintered together or held together
with a binder.
[0041] Preferably an NOx adsorption bed has a large capacity for
adsorbing a NOx species at a typical exhaust temperature and NOx
partial pressure. Preferably, the adsorbant can adsorb at least
about 3% of a NOx species by weight adsorbant at a typical exhaust
temperature and 1 torr partial pressure of the NOx species, more
preferably at least about 5% by weight adsorbant, and still more
preferably at least about 7% by weight adsorbant. The weight of
adsorbant does not include the weight of any binders or inert
substrates. Depending on the application, a typical exhaust
temperature may be 350.degree. C.
[0042] The NOx adsorbant bed preferably comprises a catalyst for
the reduction of NOx in a reducing environment. The catalyst can
be, for example, one or more precious metals, such as Au, Ag, and
Cu, group VIII metals, such as Pt, Pd, Ru, Ni, and Co, Cr, Mo, or
K. A typical catalyst includes Pt and Rh, although it may be
desirable to reduce or eliminate the Rh to favor the production of
NH.sub.3 over N.sub.2. Effective operating temperatures are
generally in the range from about 200 to about 450.degree. C. Lower
temperatures may also be desirable in terms of favoring the
production of NH.sub.3 over N.sub.2.
[0043] The catalyst of the NOx trap 31 can also serve the function
of the optional oxidation catalyst 23: providing NO.sub.2 for
continuous oxidation of soot in the device 10. A further option is
to provide the NOx trap 31 with an additional catalyst for the sole
purpose of oxidizing some of the escaping NO to NO.sub.2. Such a
catalyst is preferably concentrated near the outlet of the NOx trap
31.
[0044] The reductant source 32 can supply any suitable reductant.
Examples of suitable reductants include synthesis gas (syn gas),
hydrocarbons, and oxygenated hydrocarbons. Syn gas includes H.sub.2
and CO. The reductant can be a fuel for the internal combustion
engine 21. The fuel can be injected into the exhaust.
[0045] The reductant source 32 is preferably a fuel reformer
producing simple hydrocarbons, such as syn gas. Simple hydrocarbons
are generally more reactive than more complex hydrocarbons in
regenerating the NOx trap 31. A fuel reformer can be a catalytic
reformer, a steam reformer, an autothermal reformer, or a plasma
reformer. A reformer catalyst is one that favors the production of
CO and H.sub.2 (syn gas) and small hydrocarbons over complete
oxidation of the diesel fuel to form CO.sub.2 and H.sub.2O.
Examples of reformer catalysts include oxides of Al, Mg, and Ni,
which are typically combined with one or more of CaO, K.sub.2O, and
a rare earth metal such as Ce to increase activity. A reformer
would generally be supplied with a fuel for the internal combustion
engine 21. The reformer would also be supplied with an oxygen
source, such as air or lean exhaust. Lean exhaust can be drawn from
a high pressure portion of the exhaust system, such as from a
manifold upstream of a turbine used in a turbo charge system. A
fuel reformer is optionally placed directly in the exhaust
stream.
[0046] During regeneration, sufficient reductant must be provided
to consume free oxygen in the exhaust while leaving enough
reductant left over to regenerate the NOx trap 31. The reaction of
free oxygen can take place either before the NOx trap 31 or in the
NOx trap 31. In one embodiment, the reaction with oxygen takes
place in a fuel reformer provided in the exhaust stream. In another
embodiment, the reductant is injected in two parts. A first part is
a fuel directly injected into the exhaust to consumer excess
oxygen. A second part is syn gas, which is less efficient for
consuming excess oxygen, but more efficient for reducing NOx.
[0047] Any suitable strategy can be used to control the
regeneration of the NOx trap 31. As opposed to a simple periodic
regeneration scheme, the control scheme can involve determination
of one or more of the following parameters: the time at which a
regeneration cycle is initiated, the duration of a regeneration
cycle, and the reductant concentration during a regeneration
cycle.
[0048] One method of determining when to initiate a regeneration
cycle involves measuring the NOx concentration downstream of the
device 10. When this concentration exceeds a target level,
regeneration begins.
[0049] During regeneration, some NOx desorbs from the NOx trap 31.
Particularly during the first part of the regeneration cycle, some
NOx escapes the NOx trap 31 un-reacted. If there is no stored
ammonia in the device 10 and no ammonia supply 22, the escaping NOx
is released into the atmosphere. To avoid this, in one embodiment
regeneration begins while ammonia remains in the device 10.
[0050] Regeneration can be initiated based on the concentration of
ammonia in the device 10 falling to a critical value. This approach
involves maintaining an estimated of the amount of ammonia in the
device 10. Maintaining this estimate generally involves measuring
ammonia and NOx concentrations between the NOx trap 31 and the
device 10.
[0051] Another control strategy is simply focused on increasing
ammonia production during regeneration of the NOx trap 31. When an
NOx trap is saturated with NOx, relatively little ammonia
production is observed. Over the course of a regeneration cycle for
a saturated NOx trap, as the amount of NOx in the trap decreases,
ammonia production increases. By starting the regeneration cycle
prior to saturation, the production of ammonia in favor of N.sub.2
can be increased. Accordingly, in another embodiment, regeneration
begins when the NOx trap 31 reaches a certain level of saturation,
which is preferably in the range from about 5 to about 50%
saturation, more preferably from about 10 to about 30% saturation.
The degree of saturation can be estimated from measurements or a
model-based estimate of the amount of NOx in the exhaust and a
model for the NOx trap 31's adsorption efficiency and capacity.
Preferably, the control scheme is effective whereby the fraction of
adsorbed NOx converted to ammonia is at least about 20%, more
preferably at least about 40%.
[0052] Using the foregoing control method, the amount of ammonia
released from the NOx trap 31 may exceed the amount of NOx passing
through the NOx trap 31. This excess ammonia can be used to reduce
a stream of exhaust that bypasses the NOx trap 31. The ability to
produce excess ammonia allows an NOx trap to function as the
ammonia supply 22. Similarly, excess ammonia production is useful
in a system with two or more adsorbers as described more fully
below.
[0053] In another embodiment, regeneration is timed to control a
ratio between total ammonia and NOx released by the NOx trap 31.
The ratio may be targeted at one to one (a stoichiometric ratio),
whereby the ammonia produced by the NOx trap 31 is just enough to
reduce the NOx passing through to the device 10. Preferably,
however, the ratio is slightly less, whereby ammonia slip can be
avoided. A lesser amount of ammonia is preferably from about 60 to
about 95% of a stoichiometric amount. The amount may also be
reduced by an efficiency factor accounting for the fact that,
depending on the structure, catalyst loading, and temperature of
the device 10, a significant fraction of the NOx supplied to the
device 10 may not react with ammonia even when adequate ammonia is
available. Feedback control can be used to obtain the target ratio.
In particular, the time between regeneration cycles can be
shortened to increase ammonia production and lengthened to decrease
ammonia production, with the ultimate goal of creating a balance
between ammonia production and NOx emission from the NOx trap
31.
[0054] A control strategy can also be used to determine when to
terminate a regeneration cycle, as opposed to the alternative of
terminating the regeneration cycle after a fixed or pre-determined
period of time. Typically, the amount of NOx in the NOx trap 31 can
be determined from vehicle operating conditions and a few
measurements. The amount of reductant required to regenerate the
NOx trap 31 can then be calculated. Nevertheless, it can be
advantageous to use feedback control to determine when to conclude
a regeneration cycle. In a preferred embodiment, a regeneration
cycle is terminating according to measurements of the ammonia
concentration downstream of the NOx trap 31.
[0055] As a regeneration cycle progresses, the ammonia
concentration downstream of an NOx trap 31 first increases, then
decreases. The regeneration cycle can be terminated at any
recognizable point in the ammonia concentration curve. Most
preferably, the regeneration cycle is ended upon the ammonia
concentration falling below a target value following a peak. As the
ammonia concentration is falling, progressively more unused
reductant is slipping through the NOx trap 31. Therefore, the
target value is a design choice reflecting a trade-off between
maximizing ammonia production and minimizing reductant slip.
[0056] Another control strategy relates to the rate at which
reductant is injected. Reductant injection rate can be targeted to
a particular equivalence ratio. An equivalence ratio is based on
the fuel-air mixture as supplied to the engine 21, with a
stoichiometric ratio having an equivalence ratio of one. Additional
reductant injected into the exhaust downstream of the engine 21 is
figured into the equivalence ratio just as if it were supplied to
the engine 21.
[0057] In one embodiment, the reductant injection rate is maximized
subject to a limit on reductant breakthrough. Generally, increasing
the equivalence ratio increases the ammonia production rate and
minimizes the regeneration time. Where the reductant is injected
into the exhaust, reducing the regeneration time reduces the fuel
penalty. During regeneration, reductant must be supplied to consume
free oxygen in the exhaust. This reductant is in excess of the
reductant used to reduce NOx. The total amount of oxygen to consume
depends on the length of the regeneration cycle. If the
regeneration cycle is shorter, the molar flow of oxygen that must
be reduced is less.
[0058] In a preferred embodiment, the reductant breakthrough rate
is determined by an oxidizable species sensor downstream of the
device 10. All oxidizable species can be considered reductant. For
purposes of control, the breakthrough rate is preferably expressed
as a fraction of the injection rate in excess of the injection rate
required to consume free oxygen. For example, if doubling the
excess injection rate over the amount required to consume free
oxygen only doubles the breakthrough rate, the fractional
conversion of reductant has not decreased at all. In one
embodiment, the reductant injection rate is controlled to give from
about 50 to about 95% conversion of reductant in excess of the
amount required to consume free oxygen, in another embodiment from
about 70 to about 90% conversion.
[0059] Another method of reducing the fuel penalty is to employ a
dual adsorber system as schematically illustrated by the exhaust
system 40 of FIG. 5. The exhaust system 40 has twin lean NOx traps
31A and 31B, the filter/SCR catalyst 24, and an optional clean-up
oxidation catalyst 41 all contained in a single housing 42. The
exhaust flow can be diverted to one or the other NOx traps by a
damper 43. Injection ports 44A and 44B are configured to provide
reductant to one or the other of the NOx traps 31A and 31B. Sample
ports 45A and 45B are provided to sample the outflows of the NOx
traps 31A and 31B respectively for purposes of control. Rather than
use sample ports, sensors can be placed inside the housing 42. The
outflows of NOx traps 31A and 31B combine after passing through
baffling device 46, which is designed to promote mixing of the two
streams. After passing through the filter/SCR catalyst 24, the
exhaust is treated by the oxidation catalyst 41 to convert escaping
ammonia and reductant to more benign species.
[0060] One advantage of a dual adsorber system is that reducing
agent does not need to be wasted consuming free oxygen in the
exhaust during regeneration. Another advantage is that the reducing
agent does not need to be diluted with the exhaust. This increases
the concentration of the reducing agent and thereby the efficiency
with which it reacts. A further advantage is that the residence
time of the reducing agent in the NOx traps 31A and 31B can be
increased. The residence time can be increased both because the
residence time is not limited by the exhaust flow rate and because
more time can be taken to regenerates the NOx traps. A longer
residence time allows for a higher conversion efficiency for a
given amount of catalyst.
[0061] Additional advantages can be realized when the outflows of
the NOx traps are combined. One advantage is that excess reductant
from the NOx traps and ammonia slipping from the filter/SCR
catalyst 24 can be reduced by the oxidation catalyst 41 without
injecting oxygen. In a system that does not have a unified flow,
there is no free oxygen in the exhaust downstream of the NOx traps
during regeneration. Air must be injected or another oxygen source
provided to oxidize unconverted hydrocarbons and NH.sub.3. With a
unified flow, ample oxygen is generally supplied by the
exhaust.
[0062] Another advantage of a unified flow is that the ammonia
production rate from one of the NOx traps 31A and 31B can be
controlled to match the NOx flow rate from the other of the traps,
whereby the NOx and NH.sub.3 rates into the filter/SCR catalyst 24
remain in an approximately fixed proportion. Total ammonia
production can be controlled through the frequency of regeneration
and the reductant concentration and the rate of ammonia production
can be controlled through the rate at which reductant is
supplied.
[0063] To allow a unified flow, the pressure of the reductant
injection must be regulated to a level above that of the exhaust at
the point where the streams join. This can be accomplished without
extra pumps, even when the reductant is syn gas. For example, syn
gas can be generated from exhaust drawn from a high pressure point
in the exhaust system and fuel drawn from a common rail. The feeds
can be reacted while remaining at an elevated pressure.
[0064] According to another concept of the invention, a particulate
filter also acts as a NOx adsorber. The filter elements of the
device are made with the adsorbant material. FIG. 7 provides a
schematic illustration of an exemplary wall-flow particulate
filter/NOx adsorber 70 in which the filter elements 71 comprise the
adsorbant material. Preferably, the filter elements 71 comprise at
least about 40% by weight adsorbant, more preferably at least about
60% by weight adsorbant, and still more preferably at least about
80% by weight adsorbant. In a preferred embodiment, the filter
elements 71 are made up of adsorbant-containing particles bound
together by sintering or held together with a binder. The filter
pores are spaces between the particles. The adsorbant-containing
particles together with any binder can be extruded to form a
monolith structure of a wall flow filter. Alternating passages can
be plugged at either end to direct the flow through the filter
walls. The porosity of the filter can be controlled through the
particle sizes. For example, particle sizes in the range from about
3 to about 20 .mu.m may be appropriate. The particles themselves
may have micro-porosity to allow effective utilization of the
entire adsorbant mass.
[0065] The combined adsorbant/diesel particulate filter can be used
in conjunction with an ammonia SCR catalyst. The catalyst can be
placed downstream of the device or incorporated into the device.
The catalyst can be incorporated into the device by coating the
internal surfaces of the filter elements with an SCR
catalyst-containing wash coated. Alternatively a highly porous wash
coat can be used that lies on top of the filter elements. Where the
later type of wash coat is used, it can be selectively applied to
the outlet side of the device. FIG. 8 provides an example 80, in
which an SCR catalyst is provided in a wash coat 72 on the outlet
side of the filter elements 71.
[0066] Another way of integrating the SCR catalyst is to
co-disperse it with the adsorbant. For example, fine particles of
an NOx-adsorbant, such as a NOx-adsorbing zeolite impregnated with
a NOx trap catalyst, can be combined with fine particles of an SCR
catalyst, for example an ammonia SCR catalyst zeolite or an
ammonia-adsorbing zeolite impregnated with an ammonia SCR catalyst.
The mixed particles can be combined in a wash coat over a
supporting structure or formed into a self-supporting structure by
sintering or binding.
[0067] Ammonia-adsorbing zeolites include faujasites and rare earth
zeolites. Faujasites include X and Y-type zeolites. Rare earth
zeolites are zeolites that have been extensively (i.e., at least
about 50%) or fully ion exchanged with a rare earth metal, such as
lanthanum.
[0068] The invention as delineated by the following claims has been
shown and/or described in terms of certain concepts, aspects,
embodiments, and examples. While a particular feature of the
invention may have been disclosed with respect to only one of
several concepts, aspects, examples, or embodiments, the feature
may be combined with one or more other concepts aspects, examples,
or embodiments where such combination would be recognized as
advantageous by one of ordinary skill in the art. Also, this one
specification may describe more than one invention and the
following claims do not necessarily encompass every concept,
aspect, embodiment, or example contained herein.
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