U.S. patent application number 11/179750 was filed with the patent office on 2007-01-18 for hybrid system comprising hc-scr, nox-trapping, and nh3-scr for exhaust emission reduction.
This patent application is currently assigned to Eaton Corporation. Invention is credited to Haoran Hu.
Application Number | 20070012032 11/179750 |
Document ID | / |
Family ID | 37660407 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012032 |
Kind Code |
A1 |
Hu; Haoran |
January 18, 2007 |
Hybrid system comprising HC-SCR, NOx-trapping, and NH3-SCR for
exhaust emission reduction
Abstract
An exhaust aftertreatment system is provided with a first SCR
catalyst, a NOx adsorber-catalyst, and an ammonia-SCR catalyst. The
first catalyst is generally a hydrocarbon-SCR catalyst, but can be
a carbon monoxide-SCR catalyst or a hydrogen-SCR catalyst. The
first catalyst is functional to reduce NOx in lean exhaust using
the corresponding reductant. The NOx adsorbant-catalyst is
functional to adsorb NOx and to produce ammonia during
regeneration. The ammonia SCR catalyst is configured to adsorb
ammonia so produced and is functional to subsequently use that
ammonia to reduce NOx in lean exhaust. The first SCR catalyst is
useful to reduce the frequency with which the NOx adsorber-catalyst
needs to be regenerated, and can thereby extends the life of that
catalyst. In one embodiment, reductant for the first SCR catalyst
is stored during regeneration of the NOx adsorber-catalyst and is
used to convert additional NOx in a subsequent lean phase.
Inventors: |
Hu; Haoran; (Novi,
MI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
44114-2584
|
Family ID: |
37660407 |
Appl. No.: |
11/179750 |
Filed: |
July 12, 2005 |
Current U.S.
Class: |
60/286 ; 60/295;
60/301 |
Current CPC
Class: |
Y02T 10/40 20130101;
Y02T 10/26 20130101; Y02T 10/12 20130101; F01N 3/0842 20130101;
Y02T 10/24 20130101; B01D 53/9422 20130101; B01D 53/9472 20130101;
F01N 2560/026 20130101; B01D 2255/912 20130101; B01D 53/9418
20130101; F01N 3/0885 20130101; F01N 2240/25 20130101; B01D
2255/911 20130101; F01N 3/2073 20130101; F01N 3/2033 20130101; Y02A
50/20 20180101; B01D 53/944 20130101; B01D 2258/012 20130101; F01N
11/002 20130101; F01N 2240/30 20130101; Y02A 50/2341 20180101; B01D
2255/91 20130101; F01N 13/0097 20140603; Y02T 10/47 20130101; F01N
3/0835 20130101; F01N 2610/03 20130101; F01N 3/0231 20130101; F01N
3/0871 20130101; F01N 2570/18 20130101 |
Class at
Publication: |
060/286 ;
060/295; 060/301 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10 |
Claims
1. A power generation system comprising an exhaust aftertreatment
system, comprising: an effective amount of a first SCR catalyst
selected from the group consisting of hydrocarbon-SCR catalysts,
carbon monoxide-SCR catalysts, and hydrogen-SCR catalysts; an
effective amount of a NOx adsorbant-catalyst; and an effective
amount of an ammonia-SCR catalyst; wherein the first SCR catalyst
is functional to reduce NOx in lean exhaust using a reductant
selected from the group consisting of hydrocarbons, carbon
monoxide, and hydrogen; the NOx adsorbant-catalyst is functional to
adsorb NOx from lean exhaust and to produce ammonia during
regeneration; and the SCR catalyst is configured to adsorb ammonia
so produced and is function to use that ammonia to reduce NOx in
exhaust.
2. The system of claim 1, wherein the first SCR catalyst is
configured upstream of the ammonia-SCR catalyst
3. The system of claim 1, wherein the first SCR catalyst is a
hydrocarbon-SCR catalyst
4. The system of claim 1, further comprising an in-line reformer
configured upstream of the first SCR catalyst, the NOx-adsorber
catalyst, and the ammonia-SCR catalyst.
5. The system of claim 1, further comprising a in-line reformer
configured downstream of the first SCR catalyst, but upstream of
the NOx-adsorber catalyst, and the ammonia-SCR catalyst.
6. The system of claim 5, wherein the power generation system is
configured to inject diesel fuel into the exhaust before the first
SCR catalyst, which is a hydrocarbon-SCR catalyst.
7. The system of claim 6, wherein the hydrocarbon-SCR catalyst is
adapted to store hydrocarbon and effectively use it to reduce NOx
after diesel fuel injection into the exhaust ceases.
8. The system of claim 1, wherein the first SCR catalyst is
functional to adsorb the reductant during regeneration of the NOx
adsorber-catalyst and is functional to subsequently use that
reductant to reduce NOx in lean exhaust.
9. The system of claim 8, wherein the reductant is produced by the
reformer.
10. The system of claim 1, wherein the first SCR catalyst is
combined with the NOx-adsorber catalyst.
11. The system of claim 1, wherein the ammonia-SCR catalyst is
combined with the NOx-adsorber catalyst.
12. A method of treating NOx-containing lean exhaust, comprising:
in a first phase: contacting the exhaust with a first SCR catalyst
selected from the group consisting of hydrocarbon-SCR catalysts,
and carbon monoxide-SCR catalysts to reduce a portion of the NOx by
reactions with hydrocarbons or CO contained in the exhaust;
contacting the exhaust with a NOx adsorber-catalyst to remove
another portion of the NOx from the exhaust by adsorption; and
contacting the exhaust with an ammonia-SCR catalyst to reduce a
further portion of the NOx by reactions with stored ammonia; and in
a second phase: enriching the exhaust to reduce NOx stored in the
NOx adsorber-catalyst and in the process produce ammonia that
becomes stored in the SCR catalyst.
13. The method of claim 12, wherein the ammonia-SCR catalyst is
downstream of the first SCR catalyst.
14. The method of claim 12, wherein: the exhaust is enriched in the
second phase by injecting diesel fuel; and the enriched exhaust is
processed through a reformer upstream of the first SCR
catalyst.
15. The method of claim 14, wherein the diesel fuel is injected at
a rate in excess of a stoichiometric rate for producing reformate,
whereby insufficient oxygen is available in the reformer to
completely reform all the injected diesel fuel.
16. The method of claim 12, wherein: the exhaust is enriched in the
second phase by injecting diesel fuel upstream of the first SCR
catalyst; and the enriched exhaust is processed through a reformer
downstream of the first SCR catalyst.
17. The method of claim 12, wherein at least about 15% of the NOx
is reduced by the first SCR catalyst.
18. The method of claim 12, wherein at least about 30% of the NOx
is reduced by the first SCR catalyst.
19. A method of treating NOx-containing lean exhaust, comprising:
in a first phase: contacting the exhaust with a first SCR catalyst
selected from the group consisting of hydrocarbon-SCR catalysts,
carbon monoxide-SCR catalysts, and hydrogen-SCR catalysts to reduce
a portion of the NOx by reactions with a stored first reductant
selected from the group consisting of hydrocarbons, carbon
monoxide, and hydrogen; contacting the exhaust with a NOx
adsorber-catalyst to remove another portion of the NOx from the
exhaust by adsorption; and contacting the exhaust with an
ammonia-SCR catalyst to reduce a further portion of the NOx by
reactions with stored ammonia; and in a second phase: making the
exhaust rich; storing the first reductant in the first SCR
catalyst; and reducing NOx stored in the NOx adsorber-catalyst and
in the process producing ammonia that becomes stored by the
SCR-catalyst.
20. The method of claim 19, wherein making the exhaust rich
comprises injecting diesel fuel into the exhaust and processing the
diesel fuel through a reformer to produce reformate while removing
excess oxygen from the exhaust.
21. The method of claim 20, wherein the first SCR catalyst is a
hydrocarbon-SCR catalyst and the first reductant is
hydrocarbon.
22. The method of claim 21, wherein the hydrocarbon-SCR catalyst is
configured upstream of the reformer.
23. The method of claim 20, wherein the first reductant is produced
by the reformer.
24. The method of claim 23, wherein the diesel fuel is injected at
a rate in excess of a stoichiometric rate for producing reformate,
whereby insufficient oxygen is available in the reformer to
completely reform all the injected diesel fuel.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to pollution control systems
and methods for diesel engines.
BACKGROUND
[0002] NO.sub.x emissions from diesel engines are an environmental
problem. Several countries, including the United States, have long
had regulations pending that will limit NO.sub.x emissions from
trucks and other diesel-powered vehicles. Manufacturers and
researchers have already put considerable effort toward meeting
those regulations.
[0003] In 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, which use
compression ignition, the exhaust is generally too oxygen-rich for
three-way catalysts to be effective.
[0004] Several solutions have been proposed for controlling NOx
emissions from diesel-powered vehicles. One set of approaches
focuses on the engine. Techniques such as exhaust gas recirculation
and partially homogenizing fuel-air mixtures are helpful, but these
techniques alone will not eliminate NOx emissions. Another set of
approaches remove NOx from the vehicle exhaust. These include the
use of lean-burn NO.sub.x catalysts, selective catalytic reduction
(SCR), and lean NO.sub.x traps (LNTs).
[0005] 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. Currently, peak NOx
conversion efficiencies for lean-burn catalysts are unacceptably
low. 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.
[0006] Ammonia-SCR refers to selective catalytic reduction of NOx
by ammonia. Often, this is referred to simply as SCR. 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 high levels of NOx
reduction, but there is a disadvantage in the lack of
infrastructure for distributing ammonia or a suitable precursor.
Another concern relates to the possible release of ammonia into the
environment.
[0007] LNTs are NOx adsorbers combined with catalysts for NOx
reduction. The adsorbant is typically an alkaline earth oxide
adsorbant, such as BaCO.sub.3 and the catalyst is typically a
precious metal, such as Pt or Ru. In lean exhaust, the catalyst
speeds oxidizing reactions that lead to NOx adsorption. Accumulated
NOx is removed by creating a rich environment within the LNT
through the introduction of a reductant. In a rich environment, the
catalyst activates reactions by which adsorbed NOx is reduced and
desorbed, preferably as N.sub.2.
[0008] A LNT must periodically be regenerated to remove accumulated
NOx. This type of regeneration may be referred to as denitration in
order to distinguish desulfation, described below. The conditions
for denitration can be created in several ways. One approach uses
the engine to create a rich fuel-air mixture. For example, the
engine can inject extra diesel fuel into the exhaust of one or more
cylinders prior to expelling the exhaust. Reductant may also be
injected into the exhaust downstream of the engine. In either case,
a portion of the reductant is generally expended to consume excess
oxygen in the exhaust.
[0009] Reductant can consume excess oxygen by either combustion or
reforming reactions. Typically, the reactions take place upstream
of the LNT over an oxidation catalyst or in a reformer. The
reductant can also be oxidized directly in the LNT, but this tends
to result in faster thermal aging.
[0010] U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system
with a fuel reformer placed inline with the exhaust and upstream of
an LNT. The reformer includes both oxidation and reforming
catalysts. The reformer both removes excess oxygen and converts the
diesel fuel reductant into more reactive reformate.
[0011] In addition to accumulating NOx, LNTs accumulate SOx. SOx is
the combustion product of sulfur present in ordinarily diesel fuel.
Even with reduced sulfur fuels, the amount of SOx produced by
diesel combustion is significant. SOx adsorbs more strongly than
NOx and necessitates a more stringent, though less frequent,
regeneration. Desulfation requires elevated temperatures as well as
a reducing atmosphere. The elevated temperatures required for
desulfation can be produced by oxidizing reductant.
[0012] A NOx adsorber-catalyst can produce ammonia during
denitration. The ammonia can be captured by a downstream SCR
catalyst for subsequent use in reducing NOx, thereby improving
conversion efficiency over a stand-alone NOx adsorber-catalyst with
no increase in fuel penalty or precious metal usage. U.S. Pat. No.
6,732,507 describes a system with an ammonia SCR catalyst
configured downstream of a LNT for this purpose. U.S. Pat. Pub. No.
2004/0076565 describes such systems wherein both components are
encased by a single shell and/or co-disbursed. WO 2004/090296
describes such a system wherein there is an inline reformer
upstream of the NOx adsorber-catalyst and the SCR catalyst.
[0013] U.S. Pat. No. 5,727,385 describes a system in which a
hydrocarbon-SCR (HC--SCR) catalyst is configured upstream of an
LNT. The two components together are said to provide higher NOx
conversion than either of the components individually.
[0014] U.S. Pat. No. 6,677,264 describes a combined LNT/HC--SCR
catalyst. The catalyst comprises two layers on a support. The first
layer is a NOx adsorber-catalyst and the second layer is an HC--SCR
catalyst having a HC-storing function provided by a zeolite. The
HC-storage function is intended to concentrate hydrocarbon
reductants in the vicinity of the catalyst and thereby increase
activity.
[0015] U.S. Pat. No. 6,202,407 describes an HC--SCR catalyst that
has a hydrocarbon-storing function. In one embodiment, a diesel
fuel reductant supply is pulsed and the catalyst continues to show
activity for extended periods between the pulses.
[0016] In spite of advances, there continues to be a long felt need
for an affordable and reliable exhaust treatment system that is
durable, has a manageable operating cost (including fuel penalty),
and can practically be used to reduce NOx emissions across the
spectrum of diesel engines to a satisfactory extent in the sense of
meeting U.S. Environmental Protection Agency (EPA) regulations
effective in 2010 and other such regulations.
SUMMARY
[0017] One concept relates to an exhaust aftertreatment system. The
system comprises a first SCR catalyst, a NOx adsorber-catalyst, and
an ammonia-SCR catalyst. The first catalyst is generally a
hydrocarbon-SCR catalyst, but can be a carbon monoxide-SCR catalyst
or a hydrogen-SCR catalyst. The first catalyst is functional to
reduce NOx in lean exhaust using the corresponding reductant. The
NOx adsorbant-catalyst is functional to adsorb NOx from lean
exhaust and to produce ammonia during regeneration. The ammonia SCR
catalyst is configured to adsorb ammonia so produced and is
functional to subsequently use that ammonia to reduce NOx in lean
exhaust. The first SCR-catalyst is useful to reduce the frequency
with which the NOx adsorber-catalyst needs to be regenerated and
can thereby extends the life of that catalyst. In one embodiment,
reductant for the first SCR-catalyst is stored during regeneration
of the NOx adsorber-catalyst and is used by the first SCR-catalyst
to convert additional NOx during a subsequent lean phase.
[0018] Another concept relates to a method of treating
NOx-containing lean exhaust. The method includes a first phase in
which: the exhaust is contacted with a first SCR catalyst to reduce
a portion of the NOx by reactions with a first reductant; the
exhaust is contacted with a NOx adsorber-catalyst to remove another
portion of the NOx from the exhaust by adsorption; and the exhaust
is contacted with an ammonia-SCR catalyst to reduce a further
portion of the NOx by reactions with stored ammonia. In a second
phase, the environment of the NOx-adsorber catalyst is made rich,
whereby stored NOx is reduced. In the process, ammonia is generated
and becomes stored in the SCR catalyst. The first reductant can be
provided and stored in the second phase, or can be limited to that
normally present in the exhaust.
[0019] A further concept relates to a method of treating
NOx-containing lean exhaust. The method includes a first phase
comprising: contacting the exhaust with a first SCR catalyst to
reduce a portion of the NOx by reactions with a stored reductant
selected from the group consisting of hydrocarbons, carbon
monoxide, and hydrogen; contacting the exhaust with a NOx
adsorber-catalyst to remove another portion of the NOx from the
exhaust by adsorption; and contacting the exhaust with an
ammonia-SCR catalyst to reduce a further portion of the NOx by
reactions with stored ammonia. The method also include a second
phase comprising: making the exhaust rich; storing the first
reductant in the first SCR catalyst; and reducing NOx stored in the
NOx adsorber-catalyst and in the process producing ammonia that
becomes stored by the SCR-catalyst.
[0020] The primary purpose of this summary has been to present
certain of the inventor's concepts in a simplified form to
facilitate understanding of the more detailed description that
follows. This summary is not a comprehensive description of every
one of the inventor's concepts or every combination of the
inventor's concepts that can be considered "invention". Other
concepts of the inventor will become apparent to one of ordinary
skill in the art from the following detailed description and
annexed drawings. The concepts disclosed herein may be generalized,
narrowed, and combined in various ways with the ultimate statement
of what the inventor claim as his invention being reserved for the
claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic illustration of an exemplary power
generation system.
[0022] FIG. 2 is a schematic illustration of another exemplary
power generation system.
[0023] FIG. 3 is a schematic illustration of a further exemplary
power generation system.
[0024] FIG. 4 is a schematic illustration of a further exemplary
power generation system.
[0025] FIG. 5 is a schematic illustration of a further exemplary
power generation system.
[0026] FIG. 6 is a schematic illustration of a further exemplary
power generation system.
DETAILED DESCRIPTION
[0027] FIG. 1 is a schematic illustration of an exemplary power
generation system 5. The system 5 comprises an engine 9 and an
exhaust aftertreatment system 7. The exhaust aftertreatment system
7 includes a controller 10, a fuel injector 11, a reformer 12, a
diesel particulate filter (DPF) 13, a first selective catalytic
reduction (SCR) catalyst 15 (illustrated as an hydrocarbon-SCR
catalyst), a lean NOx-trap (LNT) 14, an ammonia SCR catalyst 16,
and a clean-up oxidation catalyst 17. The controller 10 may be an
engine control unit (ECU) that also controls the exhaust
aftertreatment system 7 or may include two or more control units
that collectively perform these functions. The first SCR catalyst
is a catalyst selected from the group consisting of HC--SCR
catalysts, CO--SCR catalyst, and hydrogen-SCR catalysts.
[0028] The engine 9 is operational to produce lean exhaust that
contains NOx and particulate matter. During normal operation, the
fuel injector 11 and the fuel reformer 12 are inactive and exhaust
from the engine 9 passes these devices substantially unchanged. The
DPF 13 removes most of the particulates from the exhaust. The first
SCR catalyst 15 reduces a portion of the NOx by reaction with a
first reductant species, if that species is available. The first
reductant species is generally one present in the engine exhaust,
although in one embodiment the first SCR catalyst 15 has a storage
capacity for the first reductant. In the later case and the first
reductant may be available from storage. The LNT 14 adsorbs another
portion of the NOx. The ammonia-SCR catalyst 16 reduces a further
portion of the NOx by reaction with ammonia, if ammona is
available. Where ammonia is available, it is generally ammonia
stored in the ammonia-SCR catalyst from a previous regeneration of
the LNT 14. The oxidation catalyst 17 may serve to oxidize CO and
unburned hydrocarbons remaining in the exhaust.
[0029] From time-to-time, the LNT 14 must be regenerated to remove
accumulated NOx. The need or desirability of regenerating the LNT
14 can be determined in any suitable fashion. In one example,
regeneration is begun based on the NOx removal rate falling below
acceptable limits. An acceptable limit may involve one or more of a
peak NOx concentration, an instantaneous brake-specific NOx
emission rate, an average brake-specific NOx emission rate over a
period, or a NOx conversion efficiency. In another example, the NOx
production rate of the engine 9 is determined by measurement,
model, or a combination of the two. The amount of NOx accumulated
in the LNT 14 is estimated from this data and compared against an
estimated storage capacity for the LNT 14. Regeneration is
initiated when the LNT 14 has reached a certain percentage of its
estimated storage capacity. In further examples, without
limitation, regeneration is based on miles driven or fuel consumed.
In general, the time at which to regenerate is determined by the
controller 10 and the regeneration process is managed by the
controller 10.
[0030] The foregoing control processes generally utilize one or
more sensors. A sensor may be used to determine the NOx rate
supplied to the LNT 14. Such a sensor is preferably placed just
downstream of the HC--SCR catalyst. If the adsorption of the LNT is
modeled, a temperature sensor for the LNT 14 is generally used. If
the regeneration is based on a NOx conversion efficiency or
emission rate, a NOx sensor with generally be placed downstream of
the LNT 14, optionally also downstream of the SCR catalyst 16.
Additional sensors may be employed to facilitate control over
regeneration.
[0031] Regeneration involves starting the reformer 12, consuming
excess oxygen in the exhaust, and supplying reformate to the LNT
14. Starting the reformer 12 generally involves heating the
reformer 12 to a minimum operational temperature, typically in the
range from about 600 to about 700.degree. C. The reformer 12 can be
heated by supplying it with diesel fuel at a rate that leaves
excess oxygen in the exhaust to fully combust the fuel. Diesel fuel
can be injected into the exhaust stream by the fuel injector 11 or
the engine 9. Once the reformer 12 is started, diesel fuel is
injected at a rate that provides a rich exhaust composition and the
diesel fuel that is not combusted by the reformer 12 is generally
converted to reformate. The reformer 12 consumes excess oxygen by
reforming and/or combusting diesel fuel. The exhaust carries the
reformate to the LNT 14.
[0032] In this example, the LNT 14 is regenerated with reformed
diesel fuel (reformate). Another suitable reductant can be used
instead. Examples of suitable reductants include gasoline, diesel
fuel, ammonia, and ammonia precursors.
[0033] In one embodiment, the first reductant is present in the
exhaust during regeneration of the LNT 14 and is stored by the
first SCR catalyst 15. The first reductant can be the same or
different from the reductant used to regenerate the LNT 14. For
example, the reductant used by the first SCR catalyst 15 may be
hydrocarbon while the LNT 14 is regenerated primarily with hydrogen
and CO.
[0034] Where the reductant for the first SCR catalyst 15 is
hydrocarbon and the first SCR catalyst 15 has a hydrocarbon storage
capacity, it may be desirable to operate the reformer 12 in a way
that ensures a large quantity of hydrocarbon is made available to
the SCR catalyst 15 for storage. Specifically, it may be desirable
to inject diesel fuel into the exhaust at a rate that provides
diesel fuel to the reformer 12 at a rate in excess of the
stoichiometric rate based on oxygen sources in the exhaust. Oxygen
sources in the exhaust are O.sub.2, H.sub.2O and CO.sub.2. A
stoichiometric feed ideally results in all the oxygen source being
consumed by reactions that produce syn gas, such as:
CH.sub.1.85+0.5 O.sub.2.fwdarw.CO+0.93 H.sub.2 (1) wherein
CH.sub.1.85 represents diesel fuel with a 1.85 ratio between carbon
and hydrogen. Similar equations can be written for consuming water
and carbon dioxide. If diesel fuel is injected at greater than the
stoichiometric rate, some hydrocarbons must necessarily break
through the reformer 12 to where they can be adsorber by the first
SCR catalyst 15. Ideally, most of the diesel fuel breaking through
the reformer 12 is broken down into smaller hydrocarbon molecules
that are more easily stored and used.
[0035] The system 7 can achieve high NOx conversions without the
first SCR-catalyst 16. The value of this catalyst is in reducing
the burden on the LNT 14. By removing some of the NOx, the
SCR-catalyst 16 reduces the frequency with which the LNT 14 needs
to be denitrated and/or desulfated in order to meet emission
requirements. The inventor believes this will extend the life of
the LNT 14.
[0036] Without a reductant storage capacity, the first SCR catalyst
15 is expected to remove at least about 10% of the NOx in the
exhaust, more preferably at least about 15%, still more preferably
at least about 20%. With reductant storage capacity, the first SCR
catalyst 15 more easily meets these goals and can potentially meet
loftier goals, such as removing at least about 30% or at least
about 50% of the NOx produced by the engine 9 during a normal
driving cycle.
[0037] From time-to-time, the LNT 14 must be desulfated to remove
accumulated SOx. The need or desirability of desulfation can be
determined in any suitable fashion. In one example, a NOx sensor
downstream of the LNT 14 is used to determine whether the NOx
removal rate measured following denitration has fallen below an
acceptable limit. In other examples, without limitation, the SOx
accumulation is estimated based on miles driven, fuel consumed, or
number of denitatrions. When SOx has accumulated to an appropriate
degree, a full or partial desulfation is initiated. In general, the
time at which to desulfate is determined by the controller 10 and
the desulfation process is managed by the controller 10.
[0038] Desulfation of the LNT 14 involves starting the reformer 12,
heating the LNT 14 to a desulfating temperature, and providing the
heated LNT 14 with a reducing atmosphere. A typical desulfation
temperature is in the range from about 650 to about 750.degree. C.
Below the minimum temperature, desulfation is very slow. Above the
maximum temperature, the LNT 14 may be damaged. Preferably, the
desulfation temperature is at least about 670.degree. C., more
preferably at least about 710.degree. C.
[0039] Normal desulfation temperatures may gradually inactivate the
LNT 14. For this reason, measures that reduce the frequency with
which desulfation is required or that reduce the required duration
for desulfation processes are desirable. One approach is to
over-design the system 7, whereby extended intervals between
desulfation are permissible. Long periods between desulfation
result in high sulfur concentrations. Desulfating with a higher
sulfur concentration may allow desulfation to proceed more quickly
in the sense of a rate based on grams sulfur removed per minute.
Desulfation may be terminated when desulfation efficiency has
dropped rather than when all possible sulfur has been removed. Less
time at desulfation temperatures and fewer temperature cycles are
expected to extend the life of the LNT 14.
[0040] U.S. EPA 2007 standards will limit engine NOx production to
1.2 g/bhp-hr over emissions test cycles. U.S. EPA 2010 standards
will limit tailpipe NOx emissions to 0.2 g/bhp-hr over the test
cycles. An exhaust aftertreatment system with an average NOx
conversion of 83% could adapt a vehicle with an engine meeting the
U.S. EPA 2007 standard to satisfy the U.S. EPA 2010 standard for
NOx tailpipe emissions. Allowing a 15% margin, a typical NOx
conversion target is 87%.
[0041] In one embodiment, the system 7 is overdesigned to meet a
conversion target when the LNT 14 is sulfur-poisoned to 50% of its
original efficiency, e.g., twice the NOx break-through rate at full
efficiency. In another embodiment, the system 7 is overdesigned to
meet the conversion target when the LNT 14 is sulfur-poisoned to
40.degree. of its original efficiency. In a further embodiment, the
LNT 14 meets the target when sulfur-poisoned to 30% of its original
efficiency.
[0042] Desulfation temperatures are generally obtained by operation
of the reformer 12. It may not be possible to operate the reformer
12 continuously through the duration of a regeneration cycle
without overheating the reformer 12 or the LNT 14. In such a case,
the fuel supply to the reformer 12 can be pulsed. Pulsing allows
devices to cool between fuel pulses.
[0043] The first SCR catalyst 15 and/or the ammonia-SCR catalyst 16
may be damaged by desulfation temperatures. FIG. 3 is a schematic
illustration of a system 30 showing one method one of avoiding such
damage. A device providing a large thermal mass has been placed
between the LNT 14 and the and the SCR catalysts. In this example,
the device is the DPF 13, although another device or an inert
thermal mass could be used instead. The DPF 13 damps temperature
pulse transmitted by the exhaust from the LNT 14 to downstream
devices.
[0044] In order for such damping to be effective, high temperatures
must not be maintained in the LNT 14 for an overly long time. If
necessary, the temperature of the LNT 14 can be pulsed during
desulfation. In between pulses, the LNT 14 cools to below
desulfation temperatures. Provided the pulses are short enough and
the thermal mass between the devices is large enough, the SCR
catalysts will experience peak temperatures closer to the average
cycle temperature than the peak temperatures experienced by the LNT
14.
[0045] FIG. 4 is a schematic of a system 40 illustrating another
approach to protecting the first SCR catalyst 15 from desulfation
temperatures. In this example, the first SCR catalyst 15 is placed
upstream of the reformer 12. An addition possible potential
advantage of this configuration is that oxygen remains available to
the first SCR catalyst 15 during regeneration of the LNT 14. Oxygen
is required for some SCR catalysts to be effective. The presence of
oxygen upstream of the reformer 12 assures the first SCR catalyst
15 will continue to be active through regeneration.
[0046] FIG. 5 is a schematic of a system 50 illustrating a further
possible improvement. In this example, the fuel injector 11 has
been placed upstream of the first SCR catalyst 15, whereby injected
fuel passes through the first SCR catalyst 15 before entering the
reformer 12. For this configuration, the first SCR catalyst 15 is
generally a HC--SCR catalyst. During periods of fuel injection, a
very high concentration of reductant is available for reducing NOx.
The high concentration of reductant may also facilitate hydrocarbon
storage for use in reduction even after fuel injection stops. The
configuration of FIG. 5 is particularly suited to an HC--SCR
catalyst that is adapted to store long chain hydrocarbons. An
HC--SCR catalyst with a hydrocarbon storage function may be better
adapted to store or use either long or short chain
hydrocarbons.
[0047] The first SCR catalyst 15 can be either an HC--SCR catalyst,
a CO--SCR catalyst, or a H.sub.2-SCR catalyst. Examples of HC--SCR
catalysts include transitional metals loaded on refractory oxides
or exchanged into zeolites. Examples of transition metals include
copper, chromium, iron, cobalt, nickel, cadmium, silver, gold,
iridium, platinum and manganese, and mixtures thereof. Exemplary of
refractory oxides are alumina, zirconia, silica-alumina, and
titania. Useful zeolites include ZSM-5, Ba/Y, and Na/Y. Preferred
zeolites have Si:Al ratios greater than about 20. Specific examples
of zeolite-based HC--SCR catalysts include Cu-ZSM-5, Fe-ZSM-5, and
Co-ZSM-5. A CeO.sub.2 coating may reduce water and SO.sub.2
deactivation of these catalysts. Cu/ZSM-5 is effective in the
temperature range from about 300 to about 450.degree. C. Specific
examples of refractory oxide-based catalysts include
alumina-supported silver. Two or more catalysts can be combined to
extend the effective temperature window.
[0048] Where a hydrocarbon-storing function is desired, zeolites
can be effective. U.S. Pat. No. 6,202,407 describes HC--SCR
catalysts that have a hydrocarbon storing function. The catalysts
are amphoteric metal oxides. The metal oxides are amphoteric in the
sense of showing reactivity with both acids and bases. Specific
examples include gamma-alumina, Ga.sub.2O.sub.3, and ZrO.sub.2.
Precious metals are optional. Where precious metals are used, the
less expensive precious metals such as Cu, Ni, or Sn can be used
instead of Pt, Pd, or Rh.
[0049] In the present disclosure, the term hydrocarbon is inclusive
of all species consisting essentially of hydrogen and carbon atoms,
however, a HC--SCR catalyst does not need to show activity with
respect to every hydrocarbon molecule. For example, some HC--SCR
catalysts will be better adapted to utilizing short-chain
hydrocarbons and HC--SCR catalysts in general are not expected to
show substantial activity with respect to CH.sub.4.
[0050] Examples of CO--SCR catalysts include precious metals on
refractory oxide supports. Specific examples include Rh on a
CeO.sub.2-ZrO.sub.2 support and Cu and/or Fe ZrO.sub.2 support.
[0051] Examples of H.sub.2-SCR catalysts also include precious
metals on refractory oxide supports. Specific examples include Pt
supported on mixed LaMnO.sub.3, CeO.sub.2, and MnO.sub.2, Pt
supported on mixed ZiO.sub.2 and TiO.sub.2, Ru supported on MgO,
and Ru supported on Al.sub.2O.sub.3.
[0052] The engine 9 is preferably a compression ignition diesel
engine, however, the various concepts of the invention are
applicable to power generation systems with lean-burn gasoline
engines or any other type of engines that produces an oxygen rich,
NOx-containing exhaust. For purposes of the present disclosure, NOx
consists of NO and NO.sub.2.
[0053] The fuel injector 11 can be of any suitable type. It can
inject the fuel co-current, cross-current, or counter-current to
the exhaust flow. Preferably, it provides the fuel in an atomized
or vaporized spray. The fuel may be injected at the pressure
provided by a fuel pump for the engine 9. Preferably, however, the
fuel passes through a pressure intensifier operating on hydraulic
principles to at least double the fuel pressure from that provided
by the fuel pump to provide the fuel at a pressure of at least
about 4 bar.
[0054] A fuel reformer is a device that converts heavier fuels into
lighter compounds without fully combusting the fuel. A fuel
reformer can be a catalytic reformer, a steam reformer, an
autothermal reformer, or a plasma reformer. Preferably, the
reformer 12 is a partial oxidation catalytic reformer. A partial
oxidation catalytic reformer comprises a reformer catalyst.
Examples of reformer catalysts include precious metals, such as Pt,
Pd, or Ru, and oxides of Al, Mg, and Ni, the later group being
typically combined with one or more of CaO, K.sub.2O, and a rare
earth metal such as Ce to increase activity. A reformer is
preferably small in size as compared to an oxidation catalyst or a
three-way catalyst designed to perform its primary functions at
temperatures below 500.degree. C. A partial oxidation catalytic
reformer is generally operative at temperatures from about 650 to
about 850.degree. C.
[0055] The NOx adsorber-catalyst 14 can comprise any suitable
NOx-adsorbing material. Examples of NOx adsorbing materials include
oxides, carbonates, and hydroxides of alkaline earth metals such as
Mg, Ca, Sr, and Be or alkali metals such as K or Ce. Further
examples of NOx-adsorbing materials include molecular sieves, such
as zeolites, alumina, silica, and activated carbon. Still further
examples include metal phosphates, such as phosphates of titanium
and zirconium. Generally, the NOx-adsorbing material is an alkaline
earth oxide. The adsorbant is typically combined with a binder and
either formed into a self-supporting structure or applied as a
coating over an inert substrate.
[0056] The LNT 14 also 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. Precious metal catalysts also facilitate the adsorbant
function of alkaline earth oxide adsorbers. Typical effective
operating temperatures for a LNT are in the range from about 300 to
about 450.degree. C.
[0057] The LNT 12 may be combined with one or both of the SCR
catalysts. FIG. 2 provides and example in which the LNT 14, the
HC--SCR catalyst 15, and, the ammonia-SCR catalyst 16 are replaced
by a combined HC--SCR/NOx-adsorber catalyst 22 and a combined
ammonia-SCR/NOx-adsorber catalyst 23. Combining adsorbants and
catalysts in this manner can increase efficiencies. Combining may
involve layering or codispersing.
[0058] Adsorbant and catalysts 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, a NOx adsorbant bed for a vehicle exhaust
systems must be reasonably resistant to degradation under the
vibrations encountered during vehicle operation.
[0059] An adsorbant bed or catalyst brick can have any suitable
structure. Examples of suitable structures may include monoliths,
packed beds, and layered 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.
[0060] The ammonia-SCR catalyst 16 is a catalyst effective to
catalyze reactions between NOx and NH.sub.3 to reduce NOx to
N.sub.2 in lean exhaust. Examples of ammonia-SCR catalysts include
oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd,
Pt, Rh, Rd, Mo, W, and Ce, 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.
[0061] The particulate filter 13 can have any suitable structure.
Examples of suitable structures 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. Optionally, one or more of the
the LNT 14, the first SCR catalyst 15, or the ammonia SCR catalyst
16 is integrated as a coating on the DPF 13.
[0062] From time-to-time, the DPF 13 is regenerated to remove
accumulated soot. The DPF 13 can be of the type that is regenerated
continuously or intermittently. For intermittent regeneration, the
DFP 13 is heated, using the reformer 12 for example. The DPF 13 is
heated to a temperature at which accumulated soot combusts with
O.sub.2. This temperature can be lowered by providing the DPF 13
with a suitable catalyst. After the DPF is heated, soot is
combusted in an oxygen rich environment. Regeneration of the DPF 13
can be combined with desulfation of the LNT 14 by heating and then
switching between lean and rich conditions.
[0063] For continuous regeneration, the DPF 13 may be provided with
a catalyst that promotes combustion of soot with NO.sub.2. Examples
of catalysts that promote the oxidation of soot by NO.sub.2 include
oxides of Ce, Zr, La, Y, and Nd. To completely eliminate the need
for intermittent regeneration, it may be necessary to provide an
additional oxidation catalyst to promote the oxidation of NO to
NO.sub.2 and thereby provide sufficient NO.sub.2 to combust soot as
quickly as it accumulates. Where regeneration is continuous, the
DPF 13 is suitably placed upstream of the reformer 13 as
illustrated in FIG. 6. An advantage of this configuration is an
additional potion of NOx is removed by the DPF 13, further reducing
demands on the downstream aftertreatment devices.
[0064] Where the DPF 13 is not continuously regenerating, the DPF
13 is preferably positioned downstream of the reformer 12. Placing
the DPF 13 immediately downstream of the reformer 12 facilitates
protecting downstream devices, such as the LNT 12, from temperature
excursions during denitration. Placing the DPF 13 between the LNT
12 and downstream SCR catalysts can be useful in protecting those
downstream devices during desulfation, as described previously.
[0065] The clean-up oxidation catalyst 17 is preferably functional
to oxidize unburned hydrocarbons from the engine 9, unused
reductants, and any H.sub.2S released from the NOx
adsorber-catalyst 13 and not oxidized by the SCR catalyst 15. Any
suitable oxidation catalyst can be used. A typical oxidation
catalyst is a precious metal, such as platinum. To allow the
clean-up catalyst 17 to function under rich conditions, the
catalyst may include an oxygen-storing component, such as ceria.
Removal of H.sub.2S, where required, may be facilitated by one or
more additional components such as NiO, Fe.sub.2O3, MnO.sub.2, CoO,
and CrO.sub.2.
[0066] The invention as delineated by the following claims has been
shown and/or described in terms of certain concepts, components,
and features. While a particular component or feature may have been
disclosed herein with respect to only one of several concepts or
examples or in both broad and narrow terms, the components or
features in their broad or narrow conceptions may be combined with
one or more other components or features in their broad or narrow
conceptions wherein such a combination would be recognized as
logical 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 described herein.
* * * * *