U.S. patent application number 11/708551 was filed with the patent office on 2008-08-21 for hc mitigation to reduce nox spike.
This patent application is currently assigned to Eaton Corporation. Invention is credited to Jiyang Yan.
Application Number | 20080196398 11/708551 |
Document ID | / |
Family ID | 39705483 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080196398 |
Kind Code |
A1 |
Yan; Jiyang |
August 21, 2008 |
HC mitigation to reduce NOx spike
Abstract
Systems and methods are disclosed for ameliorating NOx slip from
a lean NOx trap by reducing the amount of hydrocarbons reaching the
lean NOx trap during the early stages of, or in a period
immediately preceding, a rich regeneration. In one embodiment, a
hydrocarbon absorber is configured downstream from a fuel reformer,
but upstream from the lean NOx trap, in order to reduce the
quantity of hydrocarbons that reach the lean NOx trap during lean
reformer warm-up and rich regeneration phases. In another
embodiment, the fueling rate to a fuel reformer configured in an
exhaust line upstream from the lean NOx trap is limited to reduce
NOx slip.
Inventors: |
Yan; Jiyang; (Troy,
MI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
39705483 |
Appl. No.: |
11/708551 |
Filed: |
February 20, 2007 |
Current U.S.
Class: |
60/299 |
Current CPC
Class: |
Y02A 50/20 20180101;
Y02T 10/12 20130101; Y02T 10/20 20130101; Y02A 50/2325 20180101;
F01N 3/10 20130101; F01N 3/0807 20130101 |
Class at
Publication: |
60/299 |
International
Class: |
F01N 3/10 20060101
F01N003/10 |
Claims
1. A power generation system, comprising: a diesel engine operative
to produce exhaust; an exhaust system configured to receive the
exhaust from the diesel engine; a lean NOx trap configured within
an exhaust line of the exhaust system; a hydrocarbon absorber
configured within the exhaust system upstream from the lean NOx
trap; a fuel reformer configured to supply syn gas for regenerating
the lean NOx trap; wherein the exhaust system configuration
requires the syn gas to pass through the hydrocarbon absorber in
order to reach the lean NOx trap.
2. The power generation system of claim 1, wherein the fuel
reformer is configured to receive most of the exhaust from the
engine.
3. The power generation system of claim 1, wherein the exhaust
system does not contain any valves other than exhaust gas
recirculation valves.
4. The power generation system of claim 1, wherein the hydrocarbon
absorber is functional to absorb at least about 25% of the
hydrocarbons with more than three hydrocarbons in the exhaust
passing through the hydrocarbon absorber during a regeneration of
the lean NOx trap while at a temperature of 275.degree. C.
5. The power generation system of claim 1, wherein the hydrocarbon
absorber is functional to adsorb hydrocarbons at 275.degree. C. and
release most of the stored hydrocarbons at 450.degree. C.
6. The power generation system of claim 1, wherein the hydrocarbon
absorber comprises an effective amount of a zeolite for adsorbing
hydrocarbons from the exhaust.
7. The power generation system of claim 2, wherein the reformer is
of a type that is not effective for reforming diesel fuel contained
in the exhaust at 350.degree. C., but is effective for reforming
diesel fuel contained in the exhaust at 600.degree. C.
8. The power generation system of claim 1, further comprising an
ammonia-selective catalytic reduction catalyst configured in the
exhaust system downstream from the lean NOx trap.
9. The power generation system of claim 1, wherein the hydrocarbon
absorber has a significantly greater thermal mass than the fuel
reformer.
10. A vehicle comprising the power generation system of claim
1.
11. A method of operating a power generation system, comprising:
operating a diesel engine to produce an exhaust comprising NOx;
passing at least a portion of the exhaust from the engine to a lean
NOx trap through a fuel reformer; using the lean NOx trap,
adsorbing NOx from the exhaust under lean conditions; preparing to
regenerate the lean NOx trap by injecting diesel fuel into the
exhaust upstream from the fuel reformer, whereby the injected fuel
combusts in the fuel reformer under lean conditions to heat the
fuel reformer over a period; as the fuel reformer heats, trapping
hydrocarbons slipping past the fuel reformer in a hydrocarbon
absorber downstream from the fuel reformer, but upstream from the
lean NOx trap; regenerating the lean NOx trap by increasing the
fuel injection rate to form syn gas under rich conditions within
the fuel reformer.
12. The method of claim 11, wherein the fuel reformer is heated to
at least about 500.degree. C. under lean conditions.
13. The method of claim 12, wherein over the course of the lean NOx
trap regeneration, the fuel reformer is heated by at least
200.degree. C., but the lean NOx trap is heated by less than about
100.degree. C.
14. The method of claim 12, wherein the hydrocarbon absorber is
eventually heated to a temperature at which it releases the
hydrocarbons it adsorbed during the period of heating the fuel
reformer under lean conditions.
15. The method of claim 11, further comprising oxidizing the
adsorbed hydrocarbons within the hydrocarbon absorber under lean
conditions.
16. The method of claim 11, wherein at least about 5% of the
hydrocarbon injected to heat the fuel reformer under lean
conditions slips past the fuel reformer without being oxidized.
17. The method of claim 11, wherein the hydrocarbon absorber
functions to substantially reduce the amount of NOx released from
the lean NOx trap without being reduced.
18. The method of claim 11, wherein the hydrocarbon absorber
functions to substantially reduce hydrocarbon poisoning of an
ammonia selective catalytic reduction catalyst configured
downstream from the lean NOx trap.
19. A method of operating a power generation system, comprising:
operating a diesel engine to produce an exhaust comprising NOx;
passing the exhaust from the engine to a lean NOx trap through a
fuel reformer; adsorbing NOx from the exhaust under lean conditions
using the lean NOx trap; regenerating the lean NOx trap by
injecting diesel fuel into the exhaust upstream from the fuel
reformer to create rich conditions within the fuel reformer,
whereby a portion of the injected diesel fuel is reformed into syn
gas that regenerates the lean NOx trap; and while regenerating the
lean NOx trap, adsorbing hydrocarbons from the exhaust downstream
from the fuel reformer but upstream from the lean NOx trap in a
hydrocarbon absorber.
20. The method of claim 19, wherein at least about 5% of the
hydrocarbon injected to create rich conditions within the fuel
reformer slips past the fuel reformer without being completely
oxidized or converted to syn gas.
21. The method of claim 19, wherein the hydrocarbon absorber
functions to substantially reduce the amount of NOx released from
the lean NOx trap without being reduced during the regeneration of
the lean NOx trap.
22. The method of claim 19, wherein the hydrocarbon absorber
functions to reduce the amount of NOx released from the lean NOx
trap without being reduced to nitrogen or ammonia to below 10% of
the amount of NOx removed from the lean NOx trap over the course of
the regeneration.
23. The method of claim 19, wherein the hydrocarbons adsorbed by
the hydrocarbon absorber are desorbed as a result of the
hydrocarbon absorber becoming heated during the later part of the
lean NOx trap regeneration.
24. The method of claim 19, further comprising oxidizing the
adsorbed hydrocarbons within the hydrocarbon absorber under lean
conditions.
25. The method of claim 19, wherein the hydrocarbon absorber
functions to substantially reduce hydrocarbon poisoning of an
ammonia selective catalytic reduction catalyst configured
downstream from the lean NOx trap.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to diesel power generation
systems with exhaust aftertreatment.
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 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) catalysts, 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 proven challenging to find a
lean-burn NO.sub.x catalyst that has the required activity,
durability, and operating temperature range. A reductant such as
diesel fuel must be steadily supplied to the exhaust for lean NOx
reduction, introducing a fuel economy penalty of 3% or more.
Currently, peak NOx conversion efficiencies for lean-burn NOx
catalysts are unacceptably low.
[0006] SCR generally refers to selective catalytic reduction of NOx
by ammonia. The reaction takes place even in an oxidizing
environment. The NOx can be temporarily stored in an adsorbent 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] To clarify the state of a sometimes ambiguous nomenclature,
one should note that in the exhaust aftertreatment art the terms
"SCR catalyst" and "lean NOx catalyst" can be used interchangeably.
Often, however, the term "SCR" is used to refer just to
ammonia-SCR, in spite of the fact that strictly speaking
ammonia-SCR is only one type of SCR/lean NOx catalysis. Commonly,
when both ammonia-SCR catalysts and lean NOx catalysts are
discussed in one reference, SCR is used in reference to ammonia-SCR
and lean NOx catalysis is used in reference to SCR with reductants
other than ammonia, such as SCR with hydrocarbons.
[0008] LNTs are devices that adsorb NOx under lean exhaust
conditions and reduce and release the adsorbed NOx under rich
conditions. An LNT generally includes a NOx 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 including Pt and Rh. In lean exhaust, the catalyst
speeds oxidizing reactions that lead to NOx adsorption. In a
reducing environment, the catalyst activates reactions by which
hydrocarbon reductants are converted to more active species, the
water-gas shift reaction, which produces more active hydrogen from
less active CO, and reactions by which adsorbed NOx is reduced and
desorbed. In a typical operating protocol, a reducing environment
will be created within the exhaust from time-to-time to regenerate
(denitrate) the LNT.
[0009] An LNT can produce ammonia during denitration. Accordingly,
it has been proposed to combine LNT and ammonia-SCR catalysts into
one system. Ammonia produced by the LNT during regeneration is
captured by the SCR catalyst for subsequent use in reducing NOx,
thereby improving conversion efficiency over a stand-alone LNT with
no increase in fuel penalty or precious metal usage. U.S. Pat. No.
6,732,507 describes such a system. U.S. Pat. Pub. No. 2004/0076565
describes such systems wherein both components are contained within
a single shell or disbursed over one substrate.
[0010] An SCR catalyst can be used to address the problem of
ammonia release from the LNT during regeneration, but there is
another issue in that some NOx is released without being reduced.
The release occurs primarily at the beginning of LNT regeneration.
The resulting sharp and transient increase in exhaust NOx
concentration is often referred to as an NOx release spike. Several
theories have been proffered to explain this release spike. These
theories have led to diverse proposals for potential solutions.
[0011] U.S. Pat. Pub. No. 2004/0076565 proposes that the NOx spike
results from a sudden increase in LNT temperature due to reaction
of reductant with oxygen stored in the LNT. The proposed solution
is to reduce the oxygen storage capacity of the LNT.
[0012] WO 2005/049984 proposes that the NOx spike results from
violent reactions between oxygen-containing exhaust gases and
reductant rich exhaust gases mixing within the interstices of the
LNT at the beginning of the regeneration. The proposed solution is
a near stoichiometric phase in between rich and lean phases. Oxygen
carrying exhaust gas is to be flushed from the LNT during the near
stoichiometric phase by an exhaust gas that contains little or no
reductant.
[0013] U.S. Pat. No. 5,740,669 proposes that the NOx spike results
from the exhaust conditions occurring within the LNT during the
transition period between lean and rich phases. During the
transition period, the exhaust is though to be sufficiently rich to
cause NOx to release, but not sufficiency rich to reduce all the
released NOx. The proposed solution is to regenerate the LNT only
when the LNT is below a predetermined temperature, whereby NOx is
not so readily released.
[0014] U.S. Pat. No. 5,778,667 suggests that the NOx spike results
from an imbalance between the rate of release of NOx and the
availability of HC and CO reductants. The proposed solution is to
introduce ammonia, which is used to reduce the released NOx
downstream from the NOx absorber.
[0015] U.S. Pat. No. 6,718,756 suggests that the NOx spike is
caused by CO in the exhaust, which both releases NOx and reduces
NOx, but at rates that do not match. It is said that increasing the
CO supply rate will not ameliorate the spike, because CO increases
the release rate as well as the reduction rate. The proposed
solution is to supply a reductant that does not cause NOx release.
The preferred reductant is fuel, which can be supplied to the
exhaust by injection into engine cylinders during exhaust
strokes.
[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 is practical for reducing NOx emissions from 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] This disclosure relates to limiting the release of unreduced
NOx from an LNT. The inventor has obtained data showing that the
NOx release spike is associated with hydrocarbon reductants
contacting the LNT immediately prior to or during the early stages
of LNT regeneration. The data includes evidence for NOx release
even under lean conditions when significant amounts of hydrocarbons
reach the LNT. This can happen, for example, when hydrocarbons used
to heat a fuel reformer upstream from an LNT slip past the fuel
reformer to the LNT.
[0018] One of the inventor's concepts is to position a hydrocarbon
absorber upstream from an LNT to limit the amount of hydrocarbon
that reaches the LNT while the LNT is at relatively low
temperatures. In one embodiment, the hydrocarbon absorber stores
the hydrocarbons until it and the LNT have heated, whereupon the
hydrocarbons desorb. In another embodiment, the hydrocarbon
absorber stores the hydrocarbons until they are oxidized.
[0019] One power generation system based on this concept comprises
a diesel engine, an exhaust line configured to receive exhaust from
the diesel engine, a lean NOx trap configured within the exhaust
line, a fuel reformer configured to supply syn gas for regenerating
the lean NOx trap, and a hydrocarbon absorber configured within the
exhaust line upstream from the LNT, but downstream from the fuel
reformer. The system configuration requires the syn gas to pass
through the hydrocarbon absorber in order to reach the LNT.
[0020] One method of operating a power generation system based on
this concept comprises operating a diesel engine to produce an
exhaust comprising NOx, passing the exhaust from the engine to an
LNT through a fuel reformer, adsorbing NOx from the exhaust under
lean conditions using the LNT, and preparing to regenerate the LNT
by injecting diesel fuel into the exhaust. The injected fuel
combusts in the fuel reformer under lean conditions to heat the
fuel reformer. As the fuel reformer heats, hydrocarbons slipping
past the fuel reformer are trapped in a hydrocarbon absorber
downstream from the fuel reformer, but upstream from the LNT. The
exemplary method further involves regenerating the LNT by
increasing the fuel injection rate to form syn gas under rich
conditions within the fuel reformer. In a related method, the
hydrocarbons are absorbed during at least the first part of the
rich phase, with absorption during the lean warm-up phase being
optional. In particular embodiments of these methods, some
absorption is likely to occur during the lean warm-up phase and
some during the rich regeneration phase.
[0021] Another of the inventors concepts is to limit the amount of
hydrocarbon that slips to the LNT by suitably limiting the fueling
rate of a fuel reformer that supplies syn gas to the LNT. In one
embodiment, the fueling rate is limited during a lean reformer
warm-up phase. In another embodiment, the fueling rate is limited
during the early stages of a rich regeneration phase. Preferably,
the limitations to fueling rate are functions of LNT temperature,
as the LNT's ability to process hydrocarbons will increase with
increasing LNT temperature. Optionally, the fueling rates are
regulated using feedback from an approximation of either NOx slip
rate from the LNT or hydrocarbon slip rate from the fuel reformer.
An approximation can be obtained from a model-based estimate.
[0022] One method of operating a power generation system based on
this concept comprises operating a diesel engine to produce exhaust
comprising NOx, passing at least a portion of the exhaust from the
engine to an LNT through a fuel reformer, adsorbing NOx from the
exhaust under lean conditions using the LNT, preparing to
regenerate the LNT by heating the fuel reformer by at least
100.degree. C. to a temperature of at least about 500.degree. C. by
injecting fuel at rates that leave the exhaust entering the fuel
reformer lean, and then injecting fuel at rates that leave the
exhaust entering the fuel reformer rich, whereby the fuel reformer
produces syn gas that regenerates the LNT. The fuel injection rate
during the lean warm-up phase is limited to limit the amount of
hydrocarbon that slips from the fuel reformer during that phase.
Optionally, the fuel injection rate is also limited during a first
part of the rich regeneration phase to limit the amount of
hydrocarbon that slips from the fuel reformer during that
period.
[0023] Another method of operating a power generation system based
on this concept comprises operating a diesel engine to produce
exhaust comprising NOx, passing at least a portion of the exhaust
from the engine to an LNT through a fuel reformer, adsorbing NOx
from the exhaust under lean conditions using the LNT, and then
injecting fuel at rates that leave the exhaust entering the fuel
reformer rich, whereby the fuel reformer produces syn gas that
regenerated the LNT. The fuel injection rate is limited during at
least an initial portion of the regeneration phase to limit the
amount of hydrocarbon that slips from the fuel reformer.
Hydrocarbon absorption can be used in conjunction with limiting
fuel injection rates to mitigate hydrocarbon slip to the LNT.
[0024] 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
[0025] FIG. 1 is schematic illustration of an exemplary power
generation system configured to mitigate NOx spike by hydrocarbon
absorption.
[0026] FIG. 2 is schematic illustration of an exemplary power
generation system configured to mitigate NOx spike by limiting
hydrocarbon slip from a fuel reformer.
[0027] FIG. 3 is a plot showing LNT fueling rates, NOx
concentrations entering an LNT, and NOx concentrations exiting an
LNT for two test cases.
[0028] FIG. 4 is a plot showing LNT temperatures, HC concentrations
entering the LNT, and CO concentrations entering the LNT for the
two test cases of FIG. 3.
DETAILED DESCRIPTION
[0029] The power generation system 100 schematically illustrated by
FIG. 1 is one example of a power generation configured to mitigate
NOx slip during LNT regeneration via hydrocarbon absorption. The
system 100 comprises an engine 101, an exhaust line 102, and a
controller 107. Within the exhaust line 102 are configured a fuel
reformer 104, a hydrocarbon absorber 105, and a LNT 106 in that
order respectively with respect to the direction of exhaust flow
from the engine 101. The controller 107 is operative to selectively
control injection of fuel into the exhaust line 102 through the
fuel injector 103. The controller 107 may be an engine control unit
(ECU) for the engine 101 or a separate control unit. Generally the
fuel injector 103 injects the same fuel used to power the engine
101. Generally this fuel is a diesel fuel and the engine 101 is a
compression ignition engine, although the invention is applicable
to systems using other types of fuels and engines. The diesel fuel
can be any type of hydrocarbon-based fuel suitable for a
compression ignition engine.
[0030] The engine 101 is operational to produce lean
exhaust-comprising NOx. The LNT 106 is functional to absorb a
substantial portion of the NOx from this exhaust during normal
operation of the system 100 (a lean phase). During normal operation
of the system 100, the exhaust generally contains from about 4 to
about 20% oxygen. In the system 100, the exhaust flow path is
static. All the exhaust reaching the LNT 106 first passes through
the fuel reformer 104, regardless of whether the LNT 106 is
undergoing regeneration.
[0031] From time-to-time, the LNT 106 must be regenerated to remove
accumulated NOx (denitrated) in a rich phase. Denitration generally
involves heating the reformer 104 to an operational temperature and
then using the reformer 104 to produce reformate. The reformer is
generally heated by injecting fuel into the exhaust upstream from
the fuel reformer 104 at a sub-stoichiometric rate, whereby the
exhaust remains lean and most of the injected fuel completely
combusts in the reformer 104. This may be referred to as a lean
warm-up phase. Once combustion has heated the reformer 104, the
fuel injection rate can be increased to make the exhaust rich,
whereupon the reformer 104 consumes most of the oxygen from the
exhaust and produces reformate by partial oxidation and steam
reforming reactions. The reformate thus produced reduces NOx
adsorbed in the LNT 106.
[0032] It is generally desirable to make the lean warm-up phase as
short as possible. The engine 101 is generally a diesel engine
comprising a turbocharger, and the fuel reformer 104 is generally
configured downstream from the turbocharger. The exhaust
temperatures at this point are in the range from about 110 to about
550.degree. C. Commonly, the exhaust temperatures are in the lower
part of this range. As the fuel reformer is heated to its minimum
operational temperature, which is typically from about 500 to about
600.degree. C., the exhaust passing through the reformer 104 is
heated to approximately the reformer temperature. Thus, the longer
the heating phase, the more exhaust must be heated, and the greater
the fuel cost of heating the fuel reformer 104.
[0033] A rapid warm-up is also desirable in terms of providing a
quick response to a signal to regenerate the LNT 106. Quicker
responses facilitate optimization of LNT regeneration scheduling.
Quicker responses also facilitate taking advantage of conditions
conducive to LNT regeneration, which conditions may be transient
and occur somewhat unpredictably. The heating rate can generally be
increased by increase fuel injection rate, at least up to the point
that injected fuel makes up a stoichiometric amount for complete
combustion with the oxygen contained in the exhaust
[0034] While increasing the fuel injection rate increases the
heating rate of the fuel reformer 104, fuel slips from the fuel
reformer 104 during heating and the amount of injected fuel that
slips also fuel generally increases with increasing fuel injection
rate. Fuel slips due to various factors, such as inefficiencies in
mixing, limited mass transfer rates, and limited catalyst activity.
A certain amount of injected fuel slips past the fuel reformer 104
even when the fueling rate is well below stoichiometric. If this
hydrocarbon reaches the LNT 106, it can caused unreduced NOx to be
released immediately preceding LNT regeneration
[0035] In certain respects, it is also desirable to maximize the
fuel injection rate during the rich regeneration phase. Over the
course of regeneration, fuel must be consumed to remove excess
oxygen from the exhaust. While some of the energy produced by this
combustion drives steam reforming reactions, the efficiency is
limited. As a result, the fuel penalty for regeneration decreases
up to a point as the fuel injection rate, and consequently the
regeneration rate, increases. When the fuel injection rate is set
to minimize fuel penalty for regeneration, some hydrocarbon slips
from the fuel reformer 104. If this hydrocarbon reaches the LNT
106, it can caused unreduced NOx to be released, particularly at
the beginning of LNT regeneration.
[0036] In the system 100, most of the fuel slipping during the lean
warm-up and rich regeneration phases is absorbed and stored by the
hydrocarbon absorber 105. Absorbing fuel in this manner
substantially reduces the amount of fuel that slips to the LNT 106
during the lean warm-up and rich regeneration phases. The
efficiency of the hydrocarbon absorber 105 is generally greater at
lower temperatures, which is convenient in that the tendency of
hydrocarbons to cause unreduced NOx release from the LNT 106 is
higher at lower temperatures. Accordingly, the hydrocarbon absorber
105 substantially reduces NOx slip rates from the LNT 106 and
permits the fuel reformer 104 to be heated faster and the LNT 106
to be regenerated faster while keeping NOx slip from the LNT 106
within an acceptable range.
[0037] Any suitable mechanism can be used to remove stored
hydrocarbon from the hydrocarbon absorber 105 in order to restore
its absorption capacity. One possible mechanism is temperature
swing adsorption. Over the course of regeneration, the hydrocarbon
absorber 105 may become heated through the action of the fuel
reformer 104. If the hydrocarbon absorber 105 is saturated with
absorbed hydrocarbons, the hydrocarbon absorber 105 will release
some hydrocarbon as it is heated. This should not occur until the
LNT 106 is at least partially regenerated. Preferably, it does not
occur until the LNT 106 has also heated significantly. Both
reducing the amount of stored NOx in the LNT 106 and heating the
LNT 106 reduce the tendency of the LNT 106 to release NOx upon
exposure to hydrocarbons.
[0038] The hydrocarbon absorber 105 can also release stored
hydrocarbons without heating. When the hydrocarbon absorber 105 is
saturated with hydrocarbons, an equilibrium exists between the
exhaust hydrocarbon concentration and the activity of the
hydrocarbons absorbed on the LNT 106. When the hydrocarbon
concentration in the exhaust is reduced, hydrocarbons will begin to
desorb. Preferably, most of this desorption occurs during the
subsequent lean phase. Hydrocarbons desorbed from the hydrocarbon
absorber 105 during a lean phase will generally be oxidized over
the LNT 106.
[0039] Another mechanism of removing stored hydrocarbons to which
the system 100 can be adapted is oxidation. Stored hydrocarbons can
be oxidized by providing the hydrocarbon absorber 105 with an
oxidation catalyst. In a lean phase following a rich phase
regeneration phase, adsorbed hydrocarbons can be oxidized using
oxygen contained in the exhaust. Examples of suitable oxidation
catalyst include precious metals, such as Pt and Pd. If the
hydrocarbon absorber 105 is provided with an oxidation catalyst,
preferably the hydrocarbon absorber 105 has little or no oxygen
storage capacity so that excessive amounts of reductant intended
for the LNT 106 are not consumed over the hydrocarbon absorber
105.
[0040] The engine 101 is generally a medium or heavy duty diesel
engine. The inventor's concepts are applicable to light duty diesel
and lean burn gasoline power generation systems, but the problem
addressed by the inventor does not occur as often or to as great an
extent in these systems. Minimum exhaust temperatures from lean
burn gasoline engines are generally higher than minimum exhaust
temperatures from light duty diesel engines, which are generally
higher than minimum exhaust temperatures from medium duty diesel
engines, which are generally higher than minimum exhaust
temperatures from heavy duty diesel engines. Lower exhaust
temperatures lead to lower LNT temperatures. LNTs are more prone to
release NOx on exposure to hydrocarbons at lower temperatures as
compared to high temperatures. A medium duty diesel engine is one
with a displacement of at least about 4 liters, typically about 7
liters. A heavy duty diesel engine is one with a displacement of at
least about 10 liters, typically from about 12 to about 15
liters.
[0041] The exhaust line 102 is provided with an exhaust line fuel
injector 103 to create rich conditions for LNT regeneration. The
inventor's concepts are applicable to other method's of creating a
reducing environment for regenerating the LNT 106, but NOx spike
due to hydrocarbon slip is more of an issue when hydrocarbons are
injected directly into the exhaust line 102. For example, NOx slip
is less likely to occur when diesel fuel is injected into the
exhaust within the engine cylinders, whereby high temperatures
within the cylinders can decompose the diesel fuel. NOx slip is
also less likely to occur if lighter reductants, such as propane,
are injected into the exhaust line instead of diesel fuel.
Nevertheless, it is preferred that the reductant is the same as the
fuel used to power the engine 101. It is also preferred that the
reductant be injected into the exhaust line 102, rather than into
the cylinders of engine 101, in order to avoid oil dilution caused
by fuel passing around piston rings and entering the oil gallery.
Additional disadvantages of cylinder reductant injection include
having to alter the operation of the engine 101 to support LNT
regeneration, excessive dispersion of pulses of reductant, forming
deposits on any turbocharger configured between the engine 101 and
the exhaust line 102, and forming deposits on any EGR valves.
[0042] The exhaust line 102 is preferably configured without
exhaust valves or dampers. In particular, the exhaust line 102 is
preferably configured without valves or dampers that could be used
to vary the distribution of exhaust among a plurality of LNTs 104.
The inventor's concepts are applicable to aftertreatment systems
with exhaust valves or dampers, but hydrocarbon slip and resulting
NOx release is more easily avoided when exhaust valves or dampers
are used. By using exhaust valves or dampers to reduce the exhaust
flow to a fuel processor 102, the residence time can be increased.
Increasing the residence time allows a greater extent of reaction
to be achieved for a given catalyst loading, which would reduce
hydrocarbon slip. Nevertheless, it is preferred that the exhaust
line 102 be configured without valves or dampers because these
moving parts are subject to failure and can significantly decrease
the durability and reliability of an exhaust aftertreatment
system.
[0043] Even when the exhaust line 102 is free from exhaust valves
or dampers, an exhaust line upstream of the exhaust line 102 may
still contain an exhaust valve, such as an exhaust gas
recirculation (EGR) valve in an EGR line. Exhaust valves are
particularly problematic when they are configured within a main
exhaust line to divert a majority of the exhaust flow as compared
to exhaust valves configured to control the flow through a side
branch off a main exhaust line. Exhaust valves for larger conduits
are more subject to failure than exhaust valves for smaller
conduits.
[0044] The fuel reformer 104 preferably comprises an effective
amount of catalyst to catalyze steam reforming reactions at
600.degree. C. Rh in particular, when provided in sufficient
amounts in a suitable wash coat formulation, can be effective to
catalyze steam reforming at temperatures from about 500 to about
700.degree. C. In a typical formulation for the fuel reformer 104,
Rh is combined with at least one other precious metal, such as Pt
or Pd.
[0045] Preferably the fuel reformer 104 is designed to have a low
thermal mass, whereby at least a part of the fuel reformer 104 can
be easily heated to steam reforming temperatures for each
regeneration of the LNT 106. Low thermal mass is typically achieved
by constructing the fuel reformer 104 using a thin metal substrate.
A thin metal substrate has a thickness that is about 100 .mu.m or
less, preferably about 50 .mu.m or less, and still more preferably
about 25 .mu.m or less.
[0046] A small size also facilitate rapid heating. Preferably, the
total supported catalyst volume of fuel reformer 104 is only about
60% of the engine displacement or less, more preferably about 50%
of the engine displacement or less. If the fuel reformer 104 is in
an exhaust branch processing only a partial exhaust stream, the
preferred size would be reduced in proportion to the fraction of
the exhaust being treated.
[0047] Steam reforming temperatures are at least about 500.degree.
C., which is generally above the exhaust temperature. The fuel
reformer 104 can be configured to be heated by any suitable means,
but preferably the fuel reformer 104 can be warmed up and operated
using diesel fuel from the injector 103. Preferably, the fuel
reformer 104 can be heated in this manner stating from an initial
temperature of 275.degree. C. while the exhaust from the engine 101
remains at 275.degree. C. More preferably, the fuel reformer 104
can be heated in this manner and operated from initial exhaust and
reformer temperatures of 225.degree. C., and still more preferably
from exhaust and reformer temperatures of 175.degree. C. These
properties can be achieved by providing the fuel reformer 104 with
effective amounts of precious metals, such as Pt and/or Pd, for
catalyzing oxidation of diesel fuel at the starting
temperatures.
[0048] Having the fuel reformer 104 operate at steam reforming
temperatures reduces the total amount of precious metal catalyst
required by the exhaust aftertreatment system 100. Less precious
metal catalyst is required when reforming at steam reforming
temperatures as compared to reforming diesel fuel at exhaust
temperatures regardless of whether reforming is through partial
oxidation and stream reforming or exclusively though partial
oxidation reactions.
[0049] Having the fuel processors operate at least partially
through steam reforming reactions significantly increases the
reformate yield and reduces the amount of heat generation. In
principal, if reformate production proceeds through partial
oxidation reforming as in the reaction:
CH.sub.1.85+0.5O.sub.2.fwdarw.CO+0.925H.sub.2 (1)
1.925 moles of reformate (moles CO plus moles H.sub.2) could be
obtained from each mole of carbon atoms in the fuel. CH.sub.1.85 is
used to represent diesel fuel having a typical carbon to hydrogen
ratio. If reformate production proceeds through steam reforming as
in the reaction:
CH.sub.1.85+H.sub.2O.fwdarw.CO+1.925H.sub.2 (2)
2.925 moles of reformate (moles CO plus moles H.sub.2) could in
principle be obtained from each mole of carbon atoms in the fuel.
In practice, yields are lower than theoretical amounts due to the
limited efficiency of conversion of fuel, the limited selectivity
for reforming reactions over complete combustion reactions, the
necessity of producing heat to drive steam reforming, and the loss
of energy required to heat the exhaust. Preferably, the fuel
reformer 104 comprises enough steam reforming catalyst that at
600.degree. C., with an 8 mol % exhaust oxygen concentration from
the engine 101 and with sufficient diesel fuel to provide the
exhaust with an overall fuel to air to fuel ratio of 1.2:1, at
least about 2 mol % reformate is generated by steam reforming, more
preferably at least about 4 mol %, and still more preferably at
least about 6 mol %. For purposes of this disclosure, functional
descriptions involving diesel fuel are tested on the basis of the
No. 2 diesel fuel sold in the United States, which is a typical
diesel fuel. The overall fuel to air ratio is calculated on the
basis of fuel injected by the engine and fuel injected into the
exhaust line 103. Preferably, the fuel reformer 104 operates nearly
auto-thermally during LNT regeneration. Nearly auto-thermal
operation means the fuel reformer 104 heats at half or less the
rate it would if it operated entirely by partial oxidation
reforming, more preferably one quarter or less.
[0050] The inventor's concepts are also applicable to systems
having exhaust line fuel reformers that operate at exhaust line
temperatures essentially without steam reforming. Hydrocarbon slip
and NOx release may be even a greater problem in such systems due
to lower conversion rates unless a very large amount of catalyst is
used. The principle difference is that such a system generally does
not use a lean warm-up phase, thus hydrocarbon slip mitigation is
generally entirely during the rich regeneration phase, primarily at
the beginning of that phase.
[0051] Each of the fuel reformer 104, the hydrocarbon absorber 105,
and the LNT 106 is required to have a sufficiently large mass
transfer coefficient while not introducing excessive back pressure
that could adversely affect the engine 101. Generally this means
that size of each of these devices is at least about 0.5 times the
displacement of the engine 101. Typical sizes are from about 1.0 to
about 2.0 times the engine displacement 101. Each of these device
can be provided in one or more bricks. Multiple catalyst bricks can
be configured in series or in parallel with respect to the exhaust
flow. Parallel bricks can be provided in separate parallel exhaust
conduits. Preferably, at least the bulk of the exhaust from the
engine 101 passes through the fuel reformer 104, the hydrocarbon
absorber 105, and the LNT 106.
[0052] The catalyst bricks of which the fuel reformer 104, the
hydrocarbon absorber 105, and the LNT 106 are comprised can have
any suitable structures and composition. Preferred structures are
monoliths. The substrates can be, for example, metal, ceramic, or
silicon carbide.
[0053] If the system 100 is designed for temperature swing
desorption of adsorbed hydrocarbons from the hydrocarbon absorber
105, hydrocarbon absorber 105 is preferably constructed with an
appropriate thermal mass. If the heat release from the fuel
reformer 104 is relatively low during rich regeneration, a
hydrocarbon absorber may suitably be constructed using thin metal
walls. If the heat release from the fuel reformer 104 is relatively
large during rich regeneration, a ceramic substrate may be more
suitable. The hydrocarbon absorber 105 may also serve as a thermal
buffer between the fuel reformer 104 and the LNT 106, preventing
excessive temperature increases with the LNT 106 during each rich
regeneration. Optionally, the hydrocarbon absorbent is only coated
on the upstream portion of such a thermal buffer, which can be
advantageous in that the upstream portion may undergo larger
temperature swings than the downstream portion.
[0054] If the system 100 is designed for temperature swing
adsorption and desorption of hydrocarbons from the hydrocarbon
absorber 105, the system is preferably designed for the temperature
of the hydrocarbon absorber 105 to increase by about 50 to about
300.degree. C. over the course of a typical regeneration of the LNT
106 from a starting temperature in the range from about 250 to
about 300.degree. C. More preferably, the system 100 is designed
for the hydrocarbon absorber 104 to increase from about 100 to
about 200.degree. C. over the course of the rich regeneration. A
temperature difference of this magnitude can effectively restore
the absorption capacity of a suitable constructed hydrocarbon
absorber 104.
[0055] The hydrocarbon absorber 105 can operate through any
suitable hydrocarbon absorption mechanism. Suitable hydrocarbon
absorption mechanisms include capillary condensation, hydrogen
bonding, and Lewis acid interaction. These are all relatively low
activation energy mechanisms, whereby absorption can be easily
reversed. Any suitable material can be used as a hydrocarbon
absorbent. Hydrocarbon absorbents can be found, for example, among
zeolites.
[0056] The hydrocarbon absorber 105 preferably functions under at
least some lean fuel reformer warm-up conditions to adsorb at least
about 25% of the exhaust line injected hydrocarbons having more
than three carbon atoms and slipping past the fuel reformer 104
with the LNT at a temperature of 275.degree. C., more preferably at
least about 50%, and still more preferably at least about 70%.
[0057] Preferably the hydrocarbon absorber 105 has the capacity to
absorb at 2.75.degree. C. an amount of hydrocarbons that is enough
to heat the fuel reformer 104 by about 10.degree. C., optionally an
amount that is at least enough to heat the fuel reformer 104 by
about 30.degree. C., and optionally an amount that is at least
enough to heat the fuel reformer 104 by about 50.degree. C.
[0058] On the other hand, the capacity, size, and cost of the
hydrocarbon absorber 105 can be limited to the capacity to absorb
at 275.degree. C. an amount of hydrocarbons that is no more that
enough to heat the fuel reformer 104 by about 200.degree. C.,
optionally limited to an amount that is no more than enough to heat
the fuel reformer 104 by about 100.degree. C., and optionally
limited to an amount that is no more than enough to heat the fuel
reformer 104 by about 50.degree. C.
[0059] The other approach to mitigating NOx spike, which can be
used in conjunction with or separately from the hydrocarbon
absorber described above, is to limit the amount of HC that slips
from the fuel reformer 104 by limiting the fueling rate of the fuel
reformer 104. This concept can comprise limiting the fuel injection
rate during a lean warm-up phase and/or limiting the fuel injection
rate during the early stages of a rich regeneration phase.
[0060] FIG. 2 provides a schematic illustration of an exemplary
power generation system 200 in which the inventor's concept of
limiting fuel injection rates to limit NOx slip can be implemented.
The power generation system 200 includes some of the same
components as the system 100 including the engine 101, the exhaust
line 102, the fuel injector 103, the fuel reformer 104, and the LNT
106. The power generation system 200 also includes a controller 207
configured to limit the fuel injection rate through the fuel
injector 103 based on the inventors concepts. The system 200 may
also include a thermocouple 208 for measuring a temperature of the
fuel reformer 104, a thermocouple 209 for measuring a temperature
of the LNT 106, and a NOx sensor 210 for detecting NOx slipping
from the LNT 106. Each of the thermocouples and sensor 210 is
optional and each can optionally be used to provide feedback to the
controller 207.
[0061] The controller 207 can limit the fuel injection rate in any
suitable fashion. Options include feedback and feed forward
control. Feed back control can be based on NOx slip rate, wherein
fueling rates are reduced in response to NOx slip rate from the LNT
106 increasing beyond a set limit. A set limit on NOx slip rate
could be a not-to-exceed limit set by government regulation or a
customer-determined limit. Feed back control can also be based on
hydrocarbon slip rates, wherein fueling rates are reduced in
response to HC slip rate from the fuel reformer 104 increasing
beyond a set limit. A set limit could be, for example, about 5% or
the injected amount, about 1000 ppm, the particular hydrocarbon
slip rate limit optionally being temperature dependent. HC slip
rates are generally not measured outside of experimental set-ups.
In practice, HC slip rates for feedback control can be model based
estimates. A typical model would be of the fuel reformer 104 and
would include as inputs exhaust conditions, which can be related to
the engine speed-load point, and a temperature reading from the
thermocouple 208.
[0062] Optionally, limits on fueling rates can be established in
advance based on test system measurements of HC or NOx slip rates.
Limits determined in advance can be used to set target fueling
rates correlated to one or more system variables. Examples of
suitable variables to consider include variables relating to
exhaust flow rate, exhaust oxygen concentration, a temperature of
the fuel reformer 104, and a temperature of the LNT 106. These
limits can be used to set fueling rates or fueling rate
targets.
[0063] Regardless of how the fuel injection rate is controlled,
when limiting the fuel injection rate to mitigate NOx slip, the
amount of hydrocarbon slipping from the fuel reformer 104 is
preferably made less at low temperatures. At higher temperatures,
more hydrocarbon slip is permissible in that the LNT 106 is able to
more effectively use hydrocarbons to reduce NOx. In general, it is
desirable to maximize fuel injection rates during lean reformer
warm-up and rich syn gas production subject to certain limits. When
the limit is related to mitigating unreduced NOx release from the
LNT 106, the amount of slip allowed is preferably permitted to
increase with increasing temperature of the LNT 106 and preferably
required to be less as the temperature of the LNT 106 becomes less.
The hydrocarbon slip can be made less in terms of concentration or
flow rate.
[0064] Fuel injection rates may be limited during a lean warm-up
phase to avoid excessive thermal stress during heating of the fuel
reformer 104, but the fueling rates may be limited to an even
greater extent to implement the inventor's concepts. For example,
the fuel injection rate might be limited to about 100.degree. C./s
or less to prevent damage to the fuel reformer 104. According the
inventor's concept, however, the fuel injection rate might limit
the fuel reformer heating rate to about 50.degree. C./s or less,
about 20.degree. C./s or less, or even to about 10.degree. C./s or
less.
[0065] Fuel injection rates during a rich regeneration phase might
be limited based on the amount of fuel the fuel reformer 104 can
effectively process, but the fueling rates may be limited to an
even greater extent to implement the inventor's concepts. For
example, US 2004/0050037 describes operating an exhaust line fuel
reformer to produce an exhaust containing only about 4% syn gas
during LNT regeneration. According to the present concept, fueling
rates may be so limited that syn gas concentrations are only about
3% or less, ore even about 2% or less, at least during the early
stages of an LNT regeneration.
[0066] An LNT is a device that adsorbs NOx under lean exhaust
conditions and reduces and releases NOx under rich exhaust
conditions. An LNT as defined herein comprises a NO.sub.x adsorbent
and a precious metal catalyst in intimate contact on the surfaces
of a porous support. The support is typically a monolith, although
other support structures can be used. The monolith support is
typically ceramic, although other materials such as metal and SiC
are also suitable for LNT supports. The supported catalyst volume
of the LNT 104 is typically from about one to about four times the
displacement of the engine 101. The supported catalyst volume is
the volume of the support, which includes voids within the support
and the volume occupied by the adsorbent and catalyst. Preferably,
the total supported catalyst volume of the LNT 104 is no greater
than about two times the maximum displacement of the engine 101. An
LNT 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 Ba or alkali
metals such as K or Cs.
[0067] An ammonia SCR catalyst is commonly configured downstream
from the LNT 104. An ammonia SCR catalyst is a catalyst functional
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
certain oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe,
Ni, Mo, W, and Ce, and certain zeolites, for example five or
six-member ring zeolites, such as ZSM-5 ZSM-11, and Mordenite,
exchanged with metal ions such as cations of Fe, Cu, Co, Ag, or Zn.
Preferably, an ammonia-SCR catalyst is designed to tolerate
temperatures required to desulfate the LNT 104. Ammonia SCR
catalysts are generally susceptible to hydrocarbon poisoning and
the methods used herein to prevent NOx slip from the LNT 104 are
also useful in preventing hydrocarbon poisoning of a downstream SCR
catalyst.
[0068] FIGS. 3 and 4 provide plots of data from two runs on a test
apparatus having exhaust after treatment devices as illustrated in
FIG. 2. The test runs related to a startup period in which the
exhaust after treatment devices were warming. The inlet NOx
concentrations to the LNT 106 from one of the runs is plotted by
line 303. The inlet NOx concentrations were essentially the same
for both runs. The runs differ in the fueling injection profiles,
plotted by lines 301 and 302. In the base case corresponding to
line 301, fuel was injected to warm-up the fuel reformer 104 under
lean conditions and then injected at an increased provide syn gas
to regenerate the LNT 106 in a rich phase twice within the plotted
period, first beginning at about 265 seconds and second beginning
at about 410 seconds. In the comparison case plotted by line 302,
fuel was also injected to provide two fuel reformer warm-up/rich
regeneration phases. In addition, the comparison case included a
period, beginning at about 280 seconds, over which hydrocarbons
were injected at a rate leaving the exhaust lean in order to warm
the LNT 106. Some hydrocarbon slip from the fuel reformer 104
occurred during this period.
[0069] NOx release rates from the LNT 106 are plotted by lines 305
and 306. Line 305 corresponds to the base case and line 306
corresponds to the comparison case. In each case, there is a NOx
release spike associated with the second lean warm-up/rich
regeneration phase. Much less NOx is released by the first
regeneration. This is due to the temperature of the LNT 106, which
is plotted by line 403 for the base case and line 404 for the
comparison case. The LNT 106 is at about 200.degree. C. for the
first regeneration in each case and undergoes little or no
regeneration.
[0070] A significant feature of these plots is that NOx is released
by the LNT 106 in the comparison case during the lean warm-up phase
from about 280 and to about 335 seconds into the experiment as
shown by line 306, but not in the base case as shown by line 305.
The hydrocarbons reaching the LNT 106 in the comparison case are
plotted by line 402. About 3000 ppm hydrocarbon reaches the LNT 106
in the comparison case over this period. The hydrocarbons reaching
the LNT 106 in the base case are plotted by line 401. Relatively
little hydrocarbon reaches the LNT 106 over this period in the base
case. There is little difference between the temperature of the LNT
106 over this period in the comparison case as compared to the base
case. Accordingly, the NOx release correlates with the hydrocarbon
slip, but not with the LNT temperature.
[0071] NOx release from the LNT 106 is also evident during the
regeneration phases. During the early parts of the regenerations
when HC slip from the fuel reformer 104 is high, NOx slip from the
LNT 106 is also high, provided the LNT 106 is above some minimum
temperature. As CO production become significant, NOx slip
decreases. CO concentrations entering the LNT 106 are plotted by
line 405 for the base case and line 406 for the comparison case.
The onset of CO production is nearly coincident with the end of HC
slip and the end of NOx slip in each case.
[0072] There is some offset in the times for the data plotted in
FIGS. 3 and 4 due to the different ways each type of data is
obtained. In particular, the fuel injection rate data had little
offset from real time, but the concentration measurements had a few
seconds offset due to the delays inherent in the sensor readings.
These delays were not of sufficient magnitude to alter the
conclusions discussed above.
[0073] 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.
* * * * *