U.S. patent application number 11/582039 was filed with the patent office on 2007-03-15 for lnt-scr packaging.
This patent application is currently assigned to Eaton Corporation. Invention is credited to James Edward JR. McCarthy.
Application Number | 20070056268 11/582039 |
Document ID | / |
Family ID | 37853654 |
Filed Date | 2007-03-15 |
United States Patent
Application |
20070056268 |
Kind Code |
A1 |
McCarthy; James Edward JR. |
March 15, 2007 |
LNT-SCR packaging
Abstract
A power generation system comprises a diesel engine and a fuel
reformer configured to receive the engine exhaust. Two or more
separate LNT bricks are configured in a parallel valveless
arrangement wherein each simultaneously receives a separate portion
of the exhaust leaving the fuel reformer. The LNTs are each adapted
and configured to simultaneously store NO.sub.x when the exhaust
from the fuel reformer is lean and to simultaneously reduce stored
NO.sub.x and regenerate when the exhaust from the fuel reformer
contains reformate. This parallel multi-brick arrangement reduces
the effective length to width ratio of the LNTs as a group without
the packaging difficulties associated with a single LNT having an
equivalently reduced length to width ratio. Axial temperature
gradients that develop in the LNTs during desulfation are thereby
mitigated.
Inventors: |
McCarthy; James Edward JR.;
(Canton, MI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
37853654 |
Appl. No.: |
11/582039 |
Filed: |
October 17, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11223589 |
Sep 10, 2005 |
|
|
|
11582039 |
Oct 17, 2006 |
|
|
|
Current U.S.
Class: |
60/286 ; 60/295;
60/301 |
Current CPC
Class: |
F01N 13/0097 20140603;
F01N 2240/30 20130101; F01N 13/02 20130101; Y02T 10/24 20130101;
F01N 3/0885 20130101; F01N 2610/02 20130101; F01N 3/2066 20130101;
F01N 3/0814 20130101; F01N 3/2825 20130101; F01N 13/04 20130101;
Y02T 10/12 20130101 |
Class at
Publication: |
060/286 ;
060/295; 060/301 |
International
Class: |
F01N 3/10 20060101
F01N003/10; F01N 3/00 20060101 F01N003/00 |
Claims
1. A power generation system, comprising: a diesel engine operative
to produce an exhaust containing NO.sub.x a fuel reformer
configured to receive the exhaust and operative to produce
reformate when the fuel reformer is sufficiently warm, the exhaust
is rich, and the exhaust contains diesel fuel; two or more separate
LNT bricks configured in a parallel valveless; arrangement wherein
each simultaneously receives a separate portion of the exhaust
leaving the fuel reformer, the LNTs each being adapted and
configured to simultaneously store NO.sub.x when the exhaust from
the fuel reformer is lean and to simultaneously reduce stored
NO.sub.x and regenerate when the exhaust from the fuel reformer is
rich and contains reformate.
2. The power generation system of claim 1, wherein the two or more
separate LNT bricks comprise at least three separate LNT
bricks.
3. The power generation system of claim 1, wherein each LNT brick
is a monolith from about 7 cm to about 15 cm in length.
4. The power generation system of claim 1, wherein: an equivalent
diameter to equivalent length ratio of the two or more separate LNT
bricks is at least about two; the equivalent diameter is obtained
by dividing the total frontal area of the two or more separate LNT
bricks by pi, taking the square root, and multiplying by two; and
the equivalent length is obtained by dividing the total volume of
the two or more separate LNT bricks by the total frontal area of
the two or more separate LNT bricks.
5. The power generation system of claim 4, wherein the equivalent
diameter to equivalent length ratio of the two or more separate LNT
bricks is at least about three.
6. The power generation system of claim 4, wherein the equivalent
diameter to equivalent length ratio of the two or more separate LNT
bricks is at least about four.
7. A method of operating a power generation system, comprising:
operating a diesel engine to produce an exhaust containing NO.sub.x
and SO.sub.x; channeling the exhaust through a plurality of LNTs
that adsorb and store a first portion of NO.sub.x and a portion of
the SO.sub.x from the exhaust; passing the exhaust from the
plurality of LNTs through one or more SCR catalysts that reduce a
second portion of NO.sub.x in the exhaust by reaction with ammonia
under lean conditions; generating a first control signal to
denitrate a first one or more of the LNTs; in response to the
control signal, supplying rich exhaust to the first one or more of
the LNTs, whereby adsorbed NO.sub.x in the first one or more LNTs
is reduced producing ammonia-containing exhaust; passing the
ammonia containing exhaust through one or more of the SCR
catalysts, whereby the one or more ammonia-SCR catalysts adsorb and
store ammonia; generating a second control signal to desulfate one
or more of the LNTs; and in response to the second control signal,
desulfating a second one or more LNTs by heating the second one or
more LNTs and making the exhaust supplying the second one or more
LNTs rich such that over the course of the desulfation, the
temperatures in the second one or more LNTs increase in the
direction of the exhaust flow; wherein the LNTs each comprise a
separate brick and each LNT simultaneously receives a separate
portion of the exhaust.
8. The method of claim 7, wherein the exhaust is divided among the
plurality of LNTs by static structures that do not move in response
to either control signal.
9. The method of claim 7, wherein the temperatures of the second
one or more LNTs increase in the direction of flow during
desulfation due to reactions involving residual oxygen carried by
the exhaust during desulfation.
10. The method of claim 7, wherein the temperatures of the second
one or more LNTs increases in the direction of flow during
desulfation by reactions between reductants and oxygen stored in
the LNTs.
11. The method of claim 7, wherein supplying rich exhaust to the
first one or more of the LNTs comprises injecting hydrocarbons into
the exhaust and passing the exhaust through a fuel reformer.
12. The method of claim 11, further comprising heating the fuel
reformer in response to the first control signal in preparation for
supplying rich exhaust to the first one or more of the LNTs and
wherein the fuel reformer comprises an effective amount of a steam
reforming catalyst.
13. The method of claim 7, wherein the first one or more LNTs and
the second one or more LNTs each comprise all the LNTs.
14. The method of claim 13, wherein a single fuel reformer is
configured to supply rich exhaust to all the LNTs.
15. The method of claim 14, wherein the fuel reformer is configured
to receive all the exhaust from the diesel engine and the fuel
reformer produces reformate by steam reforming reactions.
16. The method of claim 7, wherein there are three or more LNTs
each comprising a monolith brick from about 7 cm to about 15 cm in
length.
17. The method of claim 7, wherein the plurality of LNTs
collectively have an equivalent diameter to equivalent length ratio
of at least about three; the equivalent diameter is obtained by
dividing the total frontal area of the two or more separate LNT
bricks by pi, taking the square root, and multiplying by two; and
the equivalent length is obtained by dividing the total volume of
the two or more separate LNT bricks by the total frontal area of
the two or more separate LNT bricks.
18. The method of claim 7, wherein the equivalent diameter to
equivalent length ratio of the two or more separate LNT bricks is
at least about four.
Description
PRIORITY
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 11/223,589, filed Sep. 10, 2005.
FIELD OF THE INVENTION
[0002] The present invention relates to pollution control devices
for diesel engines.
BACKGROUND
[0003] NO.sub.x and particulate matter (soot) 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 and particulate matter emissions from trucks and
other diesel-powered vehicles. Manufacturers and researchers have
put considerable effort toward meeting those regulations. Diesel
particulate filters (DPFs) have been proposed for controlling
particulate matter emissions. A number of different solutions have
been proposed for controlling NO.sub.x emissions.
[0004] In gasoline-powered vehicles that use stoichiometric
fuel-air mixtures, NO.sub.x emissions can be controlled using
three-way catalysts. In diesel-powered vehicles, which use
compression ignition, the exhaust is generally too oxygen-rich for
three-way catalysts to be effective.
[0005] One set of approaches for controlling NO.sub.x emissions
from diesel-powered vehicles involves limiting the creation of
pollutants. Techniques such as exhaust gas recirculation and
partially homogenizing fuel-air mixtures are helpful in reducing
NO.sub.x emissions, but these techniques alone are not sufficient.
Another set of approaches involves removing NO.sub.x from the
vehicle exhaust. These approaches include the use of lean-burn
NO.sub.x catalysts, selective catalytic reduction (SCR), and lean
NO.sub.x traps (LNTs).
[0006] Lean-burn NO.sub.x catalysts promote the reduction of
NO.sub.x under oxygen-rich conditions. Reduction of NO.sub.x in an
oxidizing atmosphere is difficult. It has proven 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
NO.sub.x 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 NO.sub.x conversion efficiencies for lean-burn
NO.sub.x catalysts are unacceptably low.
[0007] SCR generally refers to selective catalytic reduction of
NO.sub.x by ammonia. The reaction takes place even in an oxidizing
environment. The NO.sub.x can be temporarily stored in an adsorbent
or ammonia can be fed continuously into the exhaust. SCR can
achieve high levels of NO.sub.x 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.
[0008] To clarify the state of a sometimes ambiguous nomenclature,
in the exhaust aftertreatment art, the terms "SCR catalyst" and
"lean NO.sub.x catalyst" are occasionally used interchangeably.
Where the term "SCR" is used to refer just to ammonia-SCR, as it
often is, SCR is a special case of lean NO.sub.x catalysis.
Commonly, when both types of catalysts are discussed in one
reference, SCR is used with reference to ammonia-SCR and lean
NO.sub.x catalysis is used with reference to SCR with reductants
other than ammonia, such as SCR with hydrocarbons.
[0009] LNTs are devices that adsorb NO.sub.x under lean exhaust
conditions and reduce and release the adsorbed NO.sub.x under rich
exhaust conditions. A LNT generally includes a NO.sub.x adsorbent
and a catalyst. The adsorbent is typically an alkaline earth
compound, such as BaCO.sub.3 and the catalyst is typically a
combination of precious metals, such as Pt and Rh. In lean exhaust,
the catalyst speeds oxidizing reactions that lead to NO.sub.x
adsorption. In a reducing environment, the catalyst activates
reactions by which adsorbed NO.sub.x is reduced and desorbed. In a
typical operating protocol, a reducing environment will be created
within the exhaust from time-to-time to remove accumulated NO.sub.x
and thereby regenerate (denitrate) the LNT.
[0010] Creating a reducing environment for LNT regeneration
involves eliminating most of the oxygen from the exhaust and
providing a reducing agent. Except when the engine can be run
stoichiometric or rich, a portion of the reductant reacts within
the exhaust to consume oxygen. The amount of oxygen to be removed
by reaction with reductant can be reduced in various ways. If the
engine is equipped with an intake air throttle, the throttle can be
used. However, at least in the case of a diesel engine, it is
generally necessary to eliminate some of the oxygen in the exhaust
by combustion or reforming reactions with reductant that is
injected into the exhaust.
[0011] The reactions between reductant and oxygen can take place in
the LNT, but it is generally preferred that the reactions occur in
a catalyst upstream from the LNT, whereby the heat of reaction does
not cause large temperature increases within the LNT at every
regeneration.
[0012] Reductant can be injected into the exhaust by the engine
fuel injectors or by separate injection devices. For example, the
engine can inject extra fuel into the exhaust within one or more
cylinders prior to expelling the exhaust. Alternatively, or in
addition, reductant can be injected into the exhaust downstream of
the engine.
[0013] U.S. Pat. No. 7,082,753 (hereinafter "the '753 patent")
describes an exhaust treatment system with a fuel reformer placed
in the exhaust line upstream from a 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.
[0014] The operation of a fuel reformer can be modeled in terms of
the following three reactions:
0.684CH.sub.1.85+O.sub.2.fwdarw.0.684CO.sub.2+0.632H.sub.2O (1)
0.316CH.sub.1.85+0.316H.sub.20.fwdarw.0.316CO+0.608H.sub.2 (2)
0.316CO+0.316H.sub.2O.fwdarw.0.316CO.sub.2+0.316H.sub.2 (3) wherein
CH.sub.1.85 represents an exemplary reductant, such as diesel fuel,
with a 1.85 ratio between carbon and hydrogen. Reaction (1) is
exothermic complete combustion by which oxygen is consumed.
Reaction (2) is endothermic steam reforming. Reaction (3) is the
water gas shift reaction, which is comparatively thermal neutral
and is not of great importance in the present disclosure, as both
CO and H.sub.2 are effective for regeneration.
[0015] The inline reformer of the '753 patent is designed to be
rapidly heated and to then catalyze steam reforming. Temperatures
from about 500 to about 700.degree. C. are said to be required for
effective reformate production by this reformer. These temperatures
are substantially higher than typical diesel exhaust temperatures.
The reformer is heated by injecting fuel at a rate that leaves the
exhaust lean, whereby Reaction (1) takes place. After warm up, the
fuel injection rate is increased to provide a rich exhaust.
[0016] Depending on such factors as the exhaust oxygen
concentration, the fuel injection rate, and the exhaust
temperature, the inline reformer of the '753 patent tends to either
heat or cool as reformate is produced. In theory, heating can be
limited by increasing the fuel injection rate and thereby
increasing the rate of endothermic reaction (2). In practice, due
to differences in the locations at which reactions (1) and (2)
occur and limitations on one more of heat transfer rates, reformer
reaction rates, and the efficiency with which an LNT can use
reformate, the reformer cannot always be cooled in this manner. As
an alternative, the '753 patent suggests pulsing the fuel injection
to the reformer during LNT regenerations. The reformer cools
between fuel pulses and thereby remains within an acceptable
operating temperature range.
[0017] During denitrations, much of the adsorbed NO.sub.x is
reduced to N.sub.2, although a portion of the adsorbed NO.sub.x is
released without having been reduced and another portion of the
adsorbed NO.sub.x is deeply reduced to ammonia. The NO.sub.x
release occurs primarily at the beginning of the regeneration. The
ammonia production has generally been observed towards the end of
the regeneration.
[0018] U.S. Pat. No. 6,732,507 proposes a hybrid system in which a
SCR catalyst is configured downstream from the LNT in order to
utilize the ammonia released during denitration. The LNT is
provided with more reductant over the course of regeneration than
is required to remove the accumulated NO.sub.x in order to
facilitate ammonia production. The ammonia is utilized to reduce
NO.sub.x slipping past the LNT and thereby improves conversion
efficiency over a stand-alone LNT.
[0019] U.S. Pat. Pub. No. 2004/0076565 (hereinafter "the '565
publication") also describes hybrid systems combining LNT and SCR
catalysts. In order to increase ammonia production, it is proposed
to reduce the rhodium loading of the LNT. In order to reduce the
NO.sub.x release at the beginning of the regeneration, it is
proposed to eliminate oxygen storage capacity from the LNT.
[0020] In addition to accumulating NO.sub.x, LNTs accumulate
SO.sub.x. SO.sub.x is the combustion product of sulfur present in
ordinarily fuel. Even with reduced sulfur fuels, the amount of
SO.sub.x produced by combustion is significant. SO.sub.x adsorbs
more strongly than NO.sub.x and necessitates a more stringent,
though less frequent, regeneration. Desulfation requires elevated
temperatures as well as a reducing atmosphere. In the case of a
lean-burn gasoline engine, the temperature of the exhaust can
generally be elevated by engine measures. In the case of a diesel
engine, however, it is generally necessary to provide additional
heat. Typically, this heat can be provided through the same types
of reactions as those used to remove excess oxygen from the
exhaust. Once the LNT is sufficiently heated, the exhaust is made
rich by measures like those used for LNT denitration. If an inline
reformer is used to make the exhaust rich for LNT desulfation, it
may be necessary to pulse the fuel injection over the course of
desulfation to prevent the fuel reformer from overheating.
[0021] In spite of advances, a long felt need exists for an
affordable and reliable exhaust treatment system that is durable,
has a manageable operating cost (including fuel penalty), and is
practical for reducing NO.sub.x emissions from diesel engines to an
extent that meets U.S. Environmental Protection Agency (EPA)
regulations effective in 2010 and other such regulations.
SUMMARY
[0022] One of the inventor's concepts relates to a power generation
system, comprising a diesel engine and a fuel reformer configured
to receive the exhaust from the diesel engine. Two or more separate
LNT bricks are configured in a parallel valveless arrangement so
that each simultaneously receives a separate portion of the exhaust
leaving the fuel reformer. The LNTs are each adapted and configured
to simultaneously store NO.sub.x when the exhaust from the fuel
reformer is lean and to simultaneously reduce stored NO.sub.x and
regenerate when the exhaust from the fuel reformer contains
reformate. This parallel multi-brick arrangement reduces the
effective length to width ratio of the LNTs as a group without the
packaging difficulties that occur when equivalently reducing the
length to width ratio with a single LNT brick.
[0023] A small length to width ratio is particularly useful in this
system for reducing axial temperature gradients within the LNTs
during desulfation. When fuel injection is pulsed to limit the
inline reformer temperature, it has been observed that significant
axial temperature gradients develop within the downstream LNTs;
their temperatures increase along the direction of flow.
Desulfation rates are highly sensitive to temperature. Having the
temperatures increasing along the direction of flow can
substantially prolong desulfation and concomitant thermal
degradation of the LNTs, particularly considering that sulfur
deposits primarily at the fronts of the LNTs, where the LNTs are
coolest. Reducing the length to width ratio ameliorates these
gradients. Multiple LNT bricks in a parallel valveless arrangement
are largely equivalent to a single LNT with a very small length to
width ratio, but can be packaged more easily than the single
brick.
[0024] Another concept relates to a method of operating a power
generation system. The method comprises operating a diesel engine
to produce an exhaust containing NO.sub.x and SO.sub.x. The exhaust
is channeled through a plurality of LNTs, each comprising a
separate brick and each receiving a separate portion of the exhaust
flow. The LNTs adsorb and store a first portion of NO.sub.x and a
portion of the SO.sub.x from the exhaust. The exhaust from these
LNTs is passed through one or more SCR catalysts that reduce a
second portion of NO.sub.x in the exhaust by reactions with ammonia
under lean conditions. The method further comprises generating a
first control signal to denitrate one or more of the LNTs. In
response to the control signal, a rich exhaust is supplied to the
one or more of the LNTs, whereby adsorbed NO.sub.x is reduced
producing ammonia-containing exhaust. The ammonia containing
exhaust is passed through one or more of the SCR catalysts, whereby
the SCR catalysts adsorb and store ammonia. A second control signal
to desulfate one or more of the LNTs is also eventually generated.
In response to the second control signal, one or more LNTs are
regenerated by heating them and making the exhaust supplying them
rich. The manner of making the exhaust rich is such that the
temperatures in the LNTs being desulfated increase in the direction
of the exhaust flow. The provision of multiple LNTs each receiving
a separate portion of the exhaust flow mitigates the temperature
gradients that develop in the LNTs during desulfation.
[0025] 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 be conveyed to one of ordinary skill
in the art by the following detailed description together with the
drawings. The specifics disclosed herein may be generalized,
narrowed, and combined in various ways with the ultimate statement
of what the inventor claims as his invention being reserved for the
claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic illustration of an exemplary power
generation system.
[0027] FIG. 2 is a plot of temperatures and reductant concentration
in a comparison power generation over the course of LNT
desulfation.
[0028] FIG. 3 is a schematic of the power generation system that
produced the data plotted in FIG. (2).
[0029] FIG. 4 is a schematic illustration of another exemplary
power generation system.
[0030] FIG. 5 is a schematic illustration of yet another exemplary
power generation system.
[0031] FIG. 6 is a schematic illustration of a further exemplary
power generation system.
DETAILED DESCRIPTION
[0032] FIG. 1 is a schematic of an exemplary power generation
system 100 embodying one of the inventor's concepts. The power
generation system 100 comprises an engine 101 and an exhaust
aftertreatment system 102. The exhaust aftertreatment system 102
includes a controller 103, a fuel injector 104, a fuel reformer
105, a plurality of lean NO.sub.x-traps (LNT) 106 (including at
least two LNTs 106 more specifically identified as 106a and 106b),
and a plurality of ammonia-SCR catalysts 107. The controller 103
may be an engine control unit (ECU) that also controls the exhaust
aftertreatment system 102 or may include several control units that
collectively perform these functions.
[0033] During lean operation (a lean phase), the LNTs 106 adsorb a
first portion of the NO.sub.x from the exhaust. The ammonia-SCR
catalysts 107 may have ammonia stored from a previous regeneration
of the LNTs 106 (a rich phase). If the ammonia-SCR catalysts 107
contain stored ammonia, they remove a second portion of the
NO.sub.x from the lean exhaust.
[0034] From time to time, the LNTs 106 must be regenerated in a
rich phase to remove accumulated NO.sub.x (denitrated). Denitration
may involve heating the reformer 105 to an operational temperature
and then injecting fuel using the fuel injector 104 to make the
exhaust rich. The fuel reformer 105 uses the injected fuel to
consume most of the oxygen from the exhaust while producing
reformate. The reformate thus produced reduces NO.sub.x adsorbed in
the LNTs 106. Some of this NO.sub.x is reduced to NH.sub.3, most of
which is captured by the ammonia-SCR catalysts 107 and used to
reduce NO.sub.x during a subsequent lean phase.
[0035] From time to time, the LNTs 106 must also be regenerated to
remove accumulated sulfur compounds (desulfated). Desulfation
involves heating the reformer 105 to an operational temperature,
heating the LNTs 106 to a desulfating temperature, and providing
the heated LNTs 106 with a rich atmosphere. Desulfating
temperatures vary, but are typically in the range from about 500 to
about 800.degree. C., with optimal temperatures typically in the
range of about 650 to about 750 .degree. C. Below a minimum
temperature, desulfation is very slow. Above a maximum temperature,
the LNTs 106 may be damaged.
[0036] The primary means of heating the LNTs 106 is heat convection
from the reformer 105. To generate this heat, fuel can be supplied
to the reformer 105 under lean conditions, whereby the fuel
combusts in the reformer 105. Once the reformer 105 is heated, the
fuel injection rate can be controlled to maintain the temperature
of the reformer 105 while the LNTs 106 are heating.
[0037] The LNTs 106 can also be heated in part by combustion within
them. Heating the LNTs 106 in part in this way reduces the peak
temperatures at which the reformer 105 must be operated. One method
of achieving combustion within the LNTs 106 is to design and
operate the fuel reformer 105 in such a way that a portion of the
fuel supplied to the fuel reformer 105 slips to the LNTs 106. For
example, the catalyst loading of the fuel reformer 105 or its mass
transfer coefficient can be kept low to facilitate this mechanism.
Another method of achieving combustion in the LNTs 106 is to use
rapid cycling between rich and lean phases. Oxygen for the lean
phases can mix with fuel or reformate from the rich phases to
combust in the LNTs 106. This mixing and combustion can be
facilitated by a capacity of the LNTs 106 to adsorb reductants or
oxygen.
[0038] Even when the LNTs 106 are not specifically designed to
adsorb either reductants or oxygen, it has become evident that when
fuel is pulsed to the fuel reformer 105 in order to maintain its
temperature over the course of a desulfation, reductant and oxygen
mix and combust in the LNTs 106. Data regarding this phenomenon are
provided in FIG. 2.
[0039] The data in FIG. 2 were gathered for a power generation
system 300 configured as illustrated in FIG. 3. In the system 300
of FIG. 3, two LNT bricks 106a and 106b are arranged in series. The
LNTs 106 are provided in two separate bricks in the system 300 to
give a target total LNT volume using conventionally sized LNT
bricks. During desulfation, the fuel injection is pulsed to give
the reformate concentration profile illustrated by line 201 (CO)
and line 202 (H.sub.2) in FIG. 2. Line 203 plots temperature
readings obtained from a thermocouple in the LNT brick 106a 2.5 cm
from its entrance. Line 204 plots temperature readings obtained
from a thermocouple in the LNT brick 106i a 2.5 cm from its exit.
Line 205 plots temperature readings obtained from a thermocouple in
the LNT brick 106b 2.5 cm from its exit. Both LNTs were about 24 cm
long and 15 cm in diameter. The plots show that peak temperatures
increase along the direction of flow, with peak temperatures near
the exit of the two brick system being about 150.degree. C. higher
than peak temperatures near the front of the system.
[0040] The inventor's concept is to replace a series arrangement of
LNTs such as illustrated by FIG. 3 with a parallel arrangement of
LNTs such as illustrated by FIG. 1. By reducing the collective
lengths of the LNTs 106, the axial temperature gradients can be
ameliorated. Temperatures still increase along the direction of
flow when fuel injection is pulsed, but to a lesser degree. Axial
conduction through the substrates of the LNT bricks smoothes the
temperature profiles. The area available for this transport is
increased and the distance over which heat must be transported is
reduced when the LNTs 106 are arranged in parallel.
[0041] For simplicity of representation, FIG. 1 shows only two
separate LNT bricks arranged in parallel. Preferably, however, more
than two separate LNT bricks are used in order to achieve a very
small overall effective length to width ratio for the LNT in
comparison to the length to width ratios of the individual LNT
bricks. Preferably, three or more LNTs bricks are used. More
preferably, four or more separate LNT bricks are used.
[0042] Preferably, the equivalent diameter to equivalent length
ratio of the LNTs 106 collectively is at least about two, more
preferably at least about three, and still more preferably at least
about four. Equivalent diameter and equivalent length are
calculated on the basis of a single cylindrical LNT brick having
the same total frontal area and total volume as the LNTs 106
collectively. The equivalent diameter is obtained by dividing the
total frontal area of the LNTs 106 by pi, taking the square root,
and multiplying by two. The equivalent length is obtained by
dividing the total volume of the LNTs 106 by the total frontal area
of the LNTs 106.
[0043] Each of the LNTs 106 is preferably a separate monolith
brick. A monolith is a structure providing an array of parallel
passages. A brick is a cohesive unit, for example, an extruded
structure or a structure formed by rolling one or more stacked
sheets of metal into a cylinder. Monolith bricks generally have
aspect ratios from about 0.5 to about 2.0, with a 1.0 aspect ratio
being typical. These dimensions provide structural stability.
Bricks with aspect ratios greater than 2.0 are less strong and are
more difficult to manufacture and effectively can. Typical
diameters and lengths of monolith bricks range from about 15 cm to
about 36 cm. According to the present concept, shorter bricks are
preferable, e.g., bricks from about 7 cm to about 15 cm in
length.
[0044] Each brick preferably provides a high degree of axial heat
conduction per unit of surface area. Combustion that produces heat
occurs at a rate proportional to the surface area whether the rate
of combustion is kinetically or mass transfer rate controlled. For
high porosity monoliths, increasing the wall thickness increases
the degree of axial heat conduction. Metal conducts heat better
than ceramic. A preferred LNT brick according to the inventor's
concept is constructed with relatively thick metal walls. A thick
metal wall is about 100 .mu.m or thicker, preferably about 200
.mu.m or thicker, more preferably about 400 .mu.m or thicker.
[0045] The benefit of arranging LNTs 106 in parallel can be
realized whether or not the LNTs 106 are desulfated one at a time.
In the power generation system 100, the LNTs 106 are desulfated
simultaneously using a single reductant source. One advantage of
the power generation system 106 is that it can be constructed and
operated without exhaust system valves. Exhaust valves are
undesirable because they lack durability and reliability. Mobile
dampers are within the scope of valves for the purpose of this
description. The system 106 divides the flow among the various
branches passively; the division of flow is independent of the
control signals that trigger regeneration.
[0046] FIG. 4 is a schematic of an exemplary power generation
system 400 illustrating a second embodiment of the inventor's
concept. The most significant difference between this embodiment
and that exemplified by the power generation system 100 is that in
the power generation system 400 each LNT 106 is provided with an
independent mechanism for making the exhaust supplying it rich, in
this case a separate inline reformer 105 for each of the exhaust
branches 109. This configuration allows one or more of the LNTs 106
to be regenerated independently of the others.
[0047] A significant advantage of independently regenerating the
LNTs 106 is that rich exhaust from LNTs 106 being regenerated can
be combined with lean exhaust from LNTs 106 not being regenerated.
Oxygen from the lean exhaust can be used to oxidized residual
reductants, slipping NO, and H.sub.2S in the rich exhaust.
[0048] NO tends to slip from the LNTs 106 being regenerated,
particularly at the start of a regeneration. Some of this NO may be
reduced in the SCR catalysts 107. Some, however, is not so reduced
either because of limitations on the catalyst efficiency or on the
amount of available ammonia. NO is environmentally more harmful
than NO.sub.2. Oxidizing untreated NO to NO.sub.2 improves the
overall performance of the exhaust treatment system.
[0049] H.sub.2S may slip from the LNTs 106 during desulfation.
H.sub.2S has an offensive odor even in very small concentrations.
By oxidizing this H.sub.2S to SO.sub.2, the unpleasant odor can be
avoided.
[0050] Additional benefits are realized if the SCR catalysts 107
are arranged after the point in the exhaust line where the lean and
rich flows are combined. FIG. 5 is a schematic of an exemplary
power generation system 500 in which the flow is combined while the
SCR catalyst 107 still consists of multiple separate bricks in a
parallel arrangement. This embodiment realizes the benefits of a
combined flow and an arrangement of SCR catalysts 107 that fits
compactly with the arrangement of LNTs 106 contemplated by the
inventor.
[0051] One benefit of combining the flows of separately regenerated
LNTs 106 prior to supplying the combined flow to SCR catalysts 107
is that ammonia produced by the LNTs 106 during the regenerations
is distributed to SCR catalysts 107 more evenly in time. This more
even distribution in time increases the efficiency with which the
ammonia is used. In the case of a single LNT 106 followed by a
single SCR catalyst 107, the ammonia concentration in the SCR
catalyst 107 is highest immediately following regeneration.
Immediately following regeneration, NO.sub.x slip from the LNT 106
is generally at its lowest. As a result, much of the ammonia
remains in the SCR catalyst 107 for an extended period prior to
being used to reduce NO.sub.x. Over this period, a significant
portion of the stored ammonia can be lost to decomposition. By
staggering the regenerations and spreading out the times over which
the LNT bricks 106 are regenerated and ammonia is produced, the
average time that ammonia must be stored in the SCR catalysts 107
is significantly reduced, which results in increased ammonia
utilization.
[0052] Another benefit is that the environment of the SCR catalysts
107 can be maintained continuously lean. SCR catalysts function
more effectively in the presence of oxygen. Maintaining a
continuously lean environment in the SCR catalyst 107 can improve
its performance and reduce NO.sub.x slip during regenerations.
[0053] In the exemplary power generation systems 100, 400, and 500,
the exhaust is made rich using inline reformers 105. The concepts,
however, extend to methods of making the exhaust rich that do not
include or entirely rely upon inline reformers. The engine 101 can
be used remove excess oxygen from the exhaust: the engine 101 could
be operated with a stoichiometric or rich fuel-air mixture, if the
engine is of such a design that this is possible. Reformate or
another reductant other than diesel fuel can be injected into the
exhaust. Excess oxygen can be removed by combustion of reductant in
a device other than a fuel reformer 105, such as an oxidation or
three-way catalyst. In addition, it should be noted that diesel
fuel can be injected into the exhaust by an engine fuel injector
rather than by an exhaust line fuel injector.
[0054] At least one DPF will typically be included in a diesel
exhaust aftertreatment system. The DPF can be placed at any
suitable location. Examples of suitable locations are upstream from
the fuel reformer 105, between the fuel reformer 105 and the LNTs
106, between the LNTs 106 and the SCR catalysts 107, and downstream
from the SCR catalysts 107. A potential advantage of placing the
DPF upstream from the LNTs 106 is that NO.sub.x concentrations are
high, facilitating continuous regeneration. A potential advantage
of placing the DPF downstream from the fuel reformer 105 is that
oxidation of NO to NO.sub.2 in the fuel reformer 105 can facilitate
DPF regeneration. Also, if placed downstream from the fuel reformer
105, the fuel reformer 105 can be used to heat the DPF for
intermittent regeneration.
[0055] If the DPF is placed between the fuel reformer 105 and the
LNTs 106, the DPF can provide a thermal mass ameliorating
temperature excursion in the LNTs 106 during denitrations. Repeated
exposure to high temperatures can reduce the life of the LNTs 106.
Between the LNTs 106 and the SCR catalysts 107, the DPF can have a
similar effect: protecting the SCR catalysts 107 from desulfation
temperatures; some SCR catalysts undergo degradation if exposed to
desulfation temperatures. Downstream from the SCR catalysts 107 may
be a preferred location if the DPF has a catalyst that could
oxidize NH.sub.3. The preferred location for the DPF depends on the
type of DPF and other particulars of the various system
components.
[0056] FIG. 6 provides a schematic illustration of an exemplary
power generation system 600 comprising an exhaust treatment system
602 in which a DPF 108 is configured. Other components of the
system 600 are the same as described for the system 500. The DPF
108 is placed downstream from the LNTs 106 at a point where the
exhaust flow is unified. This configuration allows a continuously
lean environment to be maintained in the DPF 108, provided the LNTs
106 are not all regenerated simultaneously. The environment in the
SCR catalyst 107 would also be continuously lean. A lean
environment allows the DPF 108 to be regenerated simultaneously
with desulfation of one or more of the LNTs 106. Heat from the
desulfations helps achieve soot combustion. Consumption of oxygen
in one or more of the LNTs 106 reduces the risk the DPF 108 will
overheat at internal hot spots.
[0057] A DPF can be a wall flow filter or a pass through filter and
can use primarily either depth filtration or cake filtration. Cake
filtration is the primary filter mechanism in a wall flow filter.
In a wall flow filter, the soot-containing exhaust is forced to
pass through a porous medium. Typical pore diameters are from about
0.1 to about 1.0 .mu.m. Soot particles are most commonly from about
10 to about 50 nm in diameter. In a fresh wall flow filter, the
initial removal is by depth filtration, with soot becoming trapped
within the porous structure. Quickly, however, the soot forms a
continuous layer on an outer surface of the porous structure.
Subsequent filtration is through the filter cake and the filter
cake itself determines the filtration efficiency. As a result, the
filtration efficiency increases over time.
[0058] In contrast to a wall flow filter, in a flow through filter
the exhaust is channeled through macroscopic passages and the
primary mechanism of soot trapping is depth filtration. The
passages may have rough walls, baffles, and bends designed to
increase the tendency of momentum to drive soot particles against
or into the walls, but the flow is not forced though micro-pores.
The resulting soot removal is considered depth filtration, although
the soot is generally not distributed uniformly with the depth of
any structure of the filter. A flow through filter can also be made
from temperature resistant fibers, such as ceramic or metallic
fibers, that span the device channels. A flow through filter can be
larger than a wall flow filter having equivalent thermal mass
[0059] Diesel particulate filters must be regenerated from
time-to-time to remove accumulated soot. Two general approaches to
DPF regeneration are continuous and intermittent regeneration. In
continuous regeneration, a catalyst is provided upstream from the
DPF to convert NO to NO.sub.2. N0.sub.2 can oxidize soot at typical
diesel exhaust temperatures and thereby effectuate continuous
regeneration. Intermittent regeneration involves heating the DPF to
a temperature at which soot combustion is self-sustaining in a lean
environment. Typically this is a temperature from about 400 to
about 600.degree. C., depending in part on what type of catalyst
coating has been applied to the DPF to lower the soot ignition
temperature.
[0060] While the engine 9 is preferably a compression ignition
diesel engine, the various concepts of the inventor are applicable
to power generation systems with lean-burn gasoline engines or any
other type of engine that produces an oxygen rich,
NO.sub.x-containing exhaust. For purposes of the present
disclosure, NO.sub.x consists of NO and NO.sub.2.
[0061] The power generation system can have any suitable type of
transmission. A transmission can be a conventional transmission
such as a counter-shaft type mechanical transmission, but is
preferably a CVT. A CVT can provide a much larger selection of
operating points than can a conventional transmission and generally
also provides a broader range of torque multipliers. The range of
available operating points can be used to control the exhaust
conditions, such as the oxygen flow rate and the exhaust
hydrocarbon content. A given power demand can be met by a range of
torque multiplier-engine speed combinations. A point in this range
that gives acceptable engine performance while best meeting a
control objective, such as minimum oxygen flow rate, can be
selected. In general, a CVT prevents or minimizes power
interruptions during shifting.
[0062] Examples of CVT systems include hydrostatic transmissions,
rolling contact traction drives, overrunning clutch designs,
electrics, multispeed gear boxes with slipping clutches, and V-belt
traction drives. A CVT may involve power splitting and may also
include a multi-step transmission.
[0063] A preferred CVT provides a wide range of torque
multiplication ratios, reduces the need for shifting in comparison
to a conventional transmission, and subjects the CVT to only a
fraction of the peak torque levels produced by the engine. These
can be achieved using a step-down gear set to reduce the torque
passing through the CVT. Torque from the CVT passes through a
step-up gear set that restores the torque. The CVT is further
protected by splitting the torque from the engine, and recombining
the torque in a planetary gear set. The planetary gear set mixes or
combines a direct torque element transmitted from the engine
through a stepped automatic transmission with a torque element from
a CVT, such as a band-type CVT. The combination provides an overall
CVT in which only a portion of the torque passes through the
band-type CVT.
[0064] The fuel reformer 105 is a device that converts heavier
fuels into lighter compounds without fully combusting the fuel. The
fuel reformer 105 can be a catalytic reformer or a plasma reformer.
Preferably, the fuel reformer 105 is a partial oxidation catalytic
reformer comprising a steam reforming catalyst. Examples of
reformer catalysts include precious metals, such as Pt, Pd, and Rh,
and oxides of Al, Mg, and Ni, the latter group being typically
combined with one or more of CaO, K.sub.2O, and a rare earth metal
such as Ce to increase activity. The fuel reformer 105 is
preferably small compared to an oxidation catalyst that is designed
to perform its primary functions at temperatures below 450.degree.
C. The reformer 105 is generally operative at temperatures within
the range of about 450to about 1100.degree. C.
[0065] The LNTs 106 can comprise any suitable NO.sub.x-adsorbing
material. Examples of NO.sub.x adsorbing materials include oxides,
carbonates, and hydroxides of alkaline earth metals such as Mg, Ca,
Sr, and Ba or alkali metals such as K or Cs. Further examples of
NO.sub.x-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 NO.sub.x-absorbing material is an
alkaline earth oxide. The adsorbent is typically combined with a
binder and either formed into a self-supporting structure or
applied as a coating over an inert substrate.
[0066] The LNTs 106 also comprise a catalyst for the reduction of
NO.sub.x in a reducing environment. The catalyst can be, for
example, one or more transition metals, such as Au, Ag, and Cu,
group VIII metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A
typical catalyst includes Pt and Rh. Precious metal catalysts also
facilitate the adsorbent function of alkaline earth oxide
adsorbers.
[0067] Adsorbents 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 NO.sub.x adsorbent bed for a vehicle
exhaust system must be reasonably resistant to degradation under
the vibrations encountered during vehicle operation.
[0068] The ammonia-SCR catalysts 107 are catalysts functional to
catalyze reactions between NO.sub.x and NH.sub.3 to reduce NO.sub.x
to N.sub.2 in lean exhaust. Examples of SCR catalysts include
oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd,
Pt, Rh, 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. Preferably, the ammonia-SCR catalysts 107
are designed to tolerate temperatures required to desulfate the
LNTs 106.
[0069] Although not illustrated in any of the figures, a clean-up
catalyst can be placed downstream from the other aftertreatment
device. A clean-up catalyst is preferably functional to oxidize
unburned hydrocarbons from the engine 101, unused reductants, and
any H.sub.2S released from the LNTs 106 and not oxidized by the
ammonia-SCR catalyst 107. Any suitable oxidation catalyst can be
used. To allow the clean-up catalyst to function under rich
conditions, the catalyst may include an oxygen-storing component,
such as ceria. Removal of H.sub.2S, when required, may be
facilitated by one or more additional components such as NiO,
Fe.sub.2O.sub.3, MnO.sub.2, CoO, and CrO.sub.2.
[0070] 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.
* * * * *