U.S. patent application number 12/020416 was filed with the patent office on 2008-10-02 for method and apparatus for regenerating nox adsorbers.
Invention is credited to Richard Ancimer, Olivier Lebastard.
Application Number | 20080236146 12/020416 |
Document ID | / |
Family ID | 4171224 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080236146 |
Kind Code |
A1 |
Ancimer; Richard ; et
al. |
October 2, 2008 |
Method And Apparatus For Regenerating NOx Adsorbers
Abstract
In a method of regenerating a NOx adsorber, the NOx adsorber is
used to treat exhaust gases created during the combustion of
gaseous fuels in general. Methane is introduced into a reformer or
exhaust line in which hydrogen generated during reforming is used
to regenerate the NOx absorber.
Inventors: |
Ancimer; Richard;
(Vancouver, CA) ; Lebastard; Olivier; (Burnaby,
CA) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET, SUITE 3400
CHICAGO
IL
60661
US
|
Family ID: |
4171224 |
Appl. No.: |
12/020416 |
Filed: |
January 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11096053 |
Mar 30, 2005 |
7386977 |
|
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12020416 |
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PCT/CA2003/001462 |
Oct 2, 2003 |
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11096053 |
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Current U.S.
Class: |
60/286 ;
60/295 |
Current CPC
Class: |
B01D 53/9454 20130101;
B01J 38/04 20130101; B01D 2251/208 20130101; Y02T 10/20 20130101;
B01D 53/8612 20130101; F01N 2240/36 20130101; F01N 3/0885 20130101;
F01N 2610/04 20130101; Y02T 10/12 20130101; B01D 53/96 20130101;
B01D 2251/202 20130101; F01N 2240/30 20130101; F01N 3/0871
20130101; B01D 53/92 20130101; B01J 38/10 20130101; Y02T 10/22
20130101 |
Class at
Publication: |
60/286 ;
60/295 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2002 |
CA |
2,406,386 |
Claims
1. A method for regenerating a NOx adsorber used to remove NOx from
exhaust gases generated by combustion of a fuel in a combustion
chamber of an internal combustion engine, the method comprising:
(a) carrying said exhaust gases from said combustion chamber to an
exhaust line; (b) reducing an oxygen concentration of a quantity of
exhaust gases; (c) creating a regeneration mix comprising: (1) said
quantity of exhaust gases; and (2) hydrogen and carbon monoxide
reformed from a first quantity of a gaseous hydrocarbon; (d)
directing the regeneration mix into the NOx adsorber to regenerate
the NOx adsorber; wherein said quantity of exhaust gases is
introduced in the reforming process of said first quantity of the
gaseous hydrocarbon.
2. The method of claim 1 wherein the gaseous hydrocarbon comprises
methane.
3. The method of claim 2 wherein said hydrogen and carbon monoxide
are reformed in an exhaust gas environment.
4. The method of claim 2 further comprising oxidizing a second
quantity of methane to reduce the oxygen concentration of the
quantity of exhaust gases.
5. The method of claim 4 further comprising oxidizing hydrogen to
reduce the oxygen concentration of the quantity of exhaust
gases.
6. The method of claim 4 wherein the first quantity of methane
comprises a residual unoxidized portion of the second quantity of
methane.
7. The method of claim 2 wherein creating the regeneration mix
occurs within the exhaust line.
8. The method of claim 2 further comprising reforming the first
quantity of methane off-line.
9. The method of claim 2 further comprising heating the first
quantity of methane prior to reforming the first quantity of
methane.
10. The method of claim 9 wherein the heating is performed by
operating a heater.
11. The method of claim 9 wherein the heating is performed by
permitting heat exchange with the exhaust gases.
12. The method of claim 4 further comprising heating the second
quantity of methane by oxidation of hydrogen whereby oxidation of
the second quantity of methane is initiated.
13. The method of claim 2 further comprising by-passing a second
quantity of exhaust gas around the NOx adsorber during a
regeneration cycle.
14. The method of claim 13 further comprising utilizing emissions
gas recirculation during the regeneration cycle.
15. The method of claim 13 further comprising warming an oxidizer
prior to commencement of the regeneration cycle by directing a
third quantity of methane into the oxidizer.
16. An aftertreatment system for treating NOx within exhaust gases
produced during combustion of a fuel within a combustion chamber of
an internal combustion engine system, the aftertreatment system
comprising: (a) an exhaust line connected to carry exhaust gases
from the combustion chamber to a NOx adsorber; (b) an oxidizer
connectable to receive a first quantity of exhaust gases at a
location downstream of the combustion chamber and upstream of the
NOx adsorber, the oxidizer reducing an oxygen concentration of the
first quantity of the exhaust gases; (c) a reformer upstream of the
NOx adsorber, the reformer generating hydrogen from a gaseous
hydrocarbon; (d) a first gas line connected to deliver to the
reformer a first quantity of the gaseous hydrocarbon from a gaseous
hydrocarbon store; and (e) a regeneration line directing a
regeneration mix to a location in the exhaust line upstream of the
NOx adsorber, the regeneration mix comprising the hydrogen and the
carbon monoxide from the reformer and the first quantity of exhaust
gases from the oxidizer wherein the reformer is disposed upstream
of the NOx adsorber and downstream of the oxidizer such that an
output of the oxidizer is connected to an input of the reformer and
on output of the reformer is connected to the regeneration
line.
17. (canceled)
18. The aftertreatment system of claim 16 wherein the gaseous
hydrocarbon comprises methane.
19. (canceled)
20. The aftertreatment system of claim 16 wherein the reformer and
the oxidizer are combined into a partial oxidation catalyst.
21. The aftertreatment system of claim 20 wherein the oxidation
catalyst comprises a metal substrate.
22. The aftertreatment system of claim 20 wherein the first
quantity of methane is a residual quantity of the second quantity
of methane not consumed in the oxidation catalyst.
23. The aftertreatment system of claim 22 wherein the oxidation
catalyst comprises a metal substrate.
24. The aftertreatment system of claim 20 wherein the oxidizer
comprises a methane oxidation catalyst, wherein the first gas line
is connected to direct a second quantity of methane to the
oxidation catalyst.
25. The aftertreatment system of claim 24 wherein the oxidation
catalyst comprises a metal substrate.
26. The aftertreatment system of claim 25 wherein the first
quantity of methane is a residual quantity of the second quantity
of methane not consumed in the oxidation catalyst.
27. The aftertreatment system of claim 26 wherein the oxidation
catalyst comprises a metal substrate.
28. The aftertreatment system of claim 27 further comprising a
second gas line connected to deliver the second quantity of methane
to the oxidation catalyst.
29. The aftertreatment system of claim 28 wherein the oxidation
catalyst comprises a metal substrate.
30. The aftertreatment system of claim 18 further comprising a heat
exchanger for transferring heat from the exhaust gases to the
reformer.
31. The aftertreatment system of claim 18 further comprising a
heater for heating the reformer.
32. The aftertreatment system of claim 18 further comprising a
heater for heating at least one of the reformer and the first
quantity of the exhaust gases upstream of the oxidizer.
33. The aftertreatment system of claim 18 wherein the reformer is
off-line, and the aftertreatment system comprises: a heater for
heating at least one of a second oxidizer and a quantity of air,
and an air line directing the heated air to the reformer.
34. The aftertreatment system of claim 33 wherein the second
oxidizer and the reformer are combined in a partial oxidation
catalyst.
35. The aftertreatment system of claim 33 wherein the first gas
line is connected to introduce a second quantity of methane into
the air line upstream of the oxidation catalyst.
36. The aftertreatment system of claim 33 wherein the heated second
oxidizer oxidizes methane in the air, the first gas line directing
the methane to the second oxidizer.
37. The aftertreatment system of claim 33 further comprising a
second gas line connected to direct a second quantity of methane
from the hydrocarbon store to the air line downstream of the
oxidizer and upstream of the reformer.
38. The aftertreatment system of claim 18 further comprising a
by-pass line for directing a second quantity of exhaust gas around
the NOx adsorber.
39. The aftertreatment system of claim 18 wherein the fuel is a
gaseous fuel.
40-46. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/096,053, filed on Mar. 30, 2005, which is,
in turn, a continuation of International Application No.
PCT/CA2003/001462, having an international filing date of Oct. 2,
2003, entitled "Method And Apparatus For Regenerating NOx
Adsorbers". International Application No. PCT/CA2003/001462 claimed
priority benefits, in turn, from Canadian Patent Application No.
2,406,386 filed Oct. 2, 2002. Each of the '053 application and
International Application No. PCT/CA2003/001462 is also hereby
incorporated by reference herein in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to a method and apparatus for
regenerating NOx absorbers used in internal combustion engines.
BACKGROUND OF THE INVENTION
[0003] Emissions controls for internal combustion engines are
becoming increasingly important in transportation and energy
applications. Emissions control is becoming especially important
for diesel engines. One pollutant of concern is nitrogen oxides
(NOx). NOx are generated by the combustion of fuel in internal
combustion engines.
[0004] Aftertreatment systems for reducing NOx are important in all
types of combustion processes. One NOx treatment system is a lean
NOx adsorber (LNA). LNA systems need to be periodically
regenerated. That is, over time, a reductant is needed to treat NOx
traps to permit further NOx removal to take place. It is generally
desirable to ensure that regeneration takes place during less than
5% of the operating time of the engine. As such, it is important to
ensure that an efficient means of regeneration is employed.
[0005] As discussed in, by way of example, PCT/International
Publication No. WO 00/76637, there are a variety of reductants
available for NOx trap regeneration. By way of example, many
hydrocarbons, carbon monoxide (CO) and hydrogen can be used as
reductants. Hydrogen is especially effective as a reductant (see
U.S. Pat. No. 5,953,911). Also, hydrogen is advantageous in regard
to the emissions generated. When hydrogen is used as a
reductant--water and nitrogen result. Other carbon-based reductants
tend to generate other emissions. For example, the use of CO, as a
reductant, produces the greenhouse gas carbon dioxide.
[0006] Hydrogen is difficult to store and is generally not readily
available. However, hydrocarbons are readily available since
internal combustion engines typically use hydrocarbons as fuel. As
hydrocarbons comprise hydrogen atoms, they provide a possible
source of hydrogen. A hydrocarbon fuel may be passed through a
reformer to yield hydrogen.
[0007] In a diesel-fuelled compression ignition engine, the
hydrocarbon in abundance is diesel fuel. Diesel fuel is not an
ideal source of hydrogen, however. It is relatively high in sulfur
and therefore may create sulfur management issues in relation to
the reformer. Also, partial oxidation of diesel fuel to provide
hydrogen, using a partial oxidation catalyst (POX), requires
temperatures in excess of 800.degree. C. Aside from the excess
energy often needed to generate such temperatures, such
temperatures substantially limit the materials suitable for use in
devices for reforming diesel fuel. As such, an operating strategy
wherein diesel is reformed or partially oxidized to provide
hydrogen has not yet generated significant industry acceptance.
[0008] As noted above, hydrocarbon reforming requires sulfur
management. Sulfur contamination can impair the performance of
Ni-based reformers by poisoning such reformers.
[0009] NOx emissions may also be reduced by managing the combustion
process. NOx emissions can be reduced by using certain gaseous
fuels in place of heavy hydrocarbons. Examples of such fuels
include natural gas, methane and propane. Even with gaseous fuel,
however, NOx emissions are not insignificant.
[0010] Developments in gaseous combustion processes have also
attempted to address NOx emissions problems. Spark ignited gaseous
fuel engines, wherein a premixed charge of air and gaseous fuel is
ignited within the combustion chamber, have resulted in further
reductions of NOx. However, there have been corresponding penalties
in performance of such engines when compared to diesel-fuelled
compression-ignition engines.
[0011] Some types of gaseous-fuelled compression ignition engine
are capable of being fuelled by gaseous fuels instead of diesel
without sacrifices in performance or efficiency. In particular,
gaseous fuel combustion engines, herein referred to as a high
pressure direct injection gas engines, are known in the art. High
pressure directly injected gaseous fuel, ignited by an ignition
source such as a small quantity of pilot fuel introduced within the
engine combustion chamber, yields an improvement over
diesel-fuelled engines by reducing the emissions levels of NOx
depending on the gaseous fuel chosen. Although such direct
injection gaseous fuelled engines have the added benefit of
maintaining diesel performance, where that is not usually the case
with spark ignited gaseous fuelled engines, there is often a
penalty in NOx emissions when such engines are compared directly to
spark-ignited engines. Combustion of a high pressure directly
injected quantity of gaseous fuel results in diffusion combustion
where the bulk of the combustion is believed to occur in a local
near-stoichiometric reaction zone. The temperature and resulting
NOX formation are relatively high (compared to the temperature and
resulting NOx formation resulting from lean burn SI premixed
combustion).
[0012] Ultimately, for both spark-ignited engines as well as high
pressure direct injection compression-ignition engines, there is a
need to further manage NOx levels when gaseous fuels are used.
[0013] This invention addresses some of the issues discussed
above.
SUMMARY OF THE INVENTION
[0014] The following invention manages the above problems noted
regarding NOx adsorber regeneration in gaseous-fuelled internal
combustion engines.
[0015] One aspect of the invention provides a method for
regenerating a NOx adsorber. The NOx adsorber is used to remove NOx
from exhaust gases generated by combustion of a fuel in a
combustion chamber of an internal combustion engine. The method
comprises directing exhaust gases from a combustion chamber into an
exhaust line. The NOx adsorber is disposed in the exhaust line. The
method reforms a first quantity of a gaseous hydrocarbon to
generate hydrogen and creates a regeneration mix comprising: a
quantity of the exhaust gases wherein an oxygen concentration of
the quantity of exhaust gases has been reduced, and the hydrogen.
The method directs the regeneration mix into the NOx adsorber.
[0016] Another aspect of the invention provides an aftertreatment
system for treating NOx within exhaust gases. The exhaust gases are
produced during combustion of a fuel within a combustion chamber of
an internal combustion engine system. The aftertreatment system
comprises: an exhaust line connected to carry exhaust gases from
the combustion chamber to a NOx adsorber disposed in the exhaust
line; an oxidizer capable of providing an oxidized quantity of
exhaust gas; a reformer capable of using a gaseous hydrocarbon to
generate hydrogen; a first gas line connected to carry a first
quantity of the gaseous hydrocarbon from a gaseous hydrocarbon
store to the reformer, and a regeneration line connected to carry a
regeneration mix to the exhaust line upstream of the NOx adsorber.
The regeneration mix comprises the hydrogen and the oxidized
quantity of exhaust gas.
[0017] Further aspects of the invention and features of specific
embodiments of the invention are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In drawings which illustrate non-limiting embodiments of the
invention:
[0019] FIG. 1 shows a schematic of a NOx management system
according to one embodiment of the invention.
[0020] FIG. 2 shows a graphical representation of selected
properties of exhaust gas at various points in the system shown in
FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0021] A method of regenerating a NOx adsorber is disclosed where
the NOx adsorber is used to treat exhaust gases created during the
combustion of gaseous fuels in general. Methane is introduced into
a reformer or exhaust line wherein hydrogen generated during
reforming is used to regenerate the NOx absorber. Reforming the
methane may also produce CO. The CO may also be used to regenerate
the NOx absorber.
[0022] FIG. 1 is a schematic showing a regeneration system
according to one embodiment of the invention. An engine exhaust
line 10 carries exhaust gases flowing in the direction of arrow 20
from an engine block 11 to a NOx aftertreatment system. In the
aftertreatment system, an exhaust line 27 carries exhaust gases to
a NOx absorber 46 as indicated by arrow 23. NOx adsorber 46 has an
inlet 49 and an outlet 51. Gases exiting outlet 51 are delivered to
an outlet line 21 where they flow in the direction of arrow 31.
[0023] A by-pass line 12 is provided to carry a proportion of the
exhaust gases around adsorber 46 while absorber 46 is being
regenerated. The exhaust gases may be directed through by-pass line
12 as indicated by arrow 18 by opening by-pass valve 14 and closing
valve 25. By-pass valve 14 may be disposed anywhere along by-pass
line 12. In this embodiment, by-pass line 12 branches off from
exhaust line 27 at a junction 16 and rejoins exhaust line 27 at a
point 48 downstream from NOx adsorber 46.
[0024] A reformer line 22 branches off of exhaust line 27 at
junction 16. A valve 13 controls the flow of exhaust gases into
reformer line 22. An oxidation catalyst 42 and a reformer 44 are
connected in series in reformer line 22. Any gases flowing in
reformer line 22 flow into the catalyst input 41 of oxidation
catalyst 42, out of the catalyst outlet 43 of oxidation catalyst
42, into the reformer inlet 45, through reformer 44 and out of
reformer outlet 47. The direction of flow is indicated by arrow 56.
Gases flowing in reformer line 22 rejoin exhaust line 27 at a
junction 53 upstream from NOx adsorber 46.
[0025] Methane gas may be introduced at methane junctions, 24 and
40, which are disposed on either side of oxidation catalyst 42 in
reformer line 22. Junction 40 is upstream of reformer 44 and
downstream of catalyst 42. Junction 24 is upstream from oxidation
catalyst 42.
[0026] First methane junction 24 connects reformer line 22 with
upstream methane line 26. Upstream valve 28 is disposed in upstream
methane line 26. Upstream methane line 26 and downstream methane
line 30 are connected to a main methane junction 32 into which main
methane line 34 feeds. Methane store 36 flows into main methane
line 34 as indicated by arrow 50. Downstream valve 38 is disposed
in downstream methane line 30. Downstream line 30 then joins
reformer line 22 at second methane junction 40. Methane can flow
through upstream and downstream methane lines 26 and 30 to reformer
line 22, as indicated by arrows 52 and 54.
[0027] In the NOx aftertreatment system of FIGS. 1 and 2, exhaust
gas is generated by combustion events within one or more combustion
chambers disposed upstream of engine exhaust line 10 in engine
block 11. Exhaust gas results from the combustion of natural gas.
The gaseous fuel is in general either directly injected into the
combustion chamber or pre-mixed with a quantity of air to create a
fumigated charge. In each case, spark ignition, glow plug or
compression ignition are utilized to initiate the combustion
process within the combustion chamber.
[0028] During normal operation of the engine valves 14 and 13 are
closed and exhaust gas flows along exhaust line 27. The exhaust gas
passes through NOx adsorber 46 which removes NOx.
[0029] Eventually NOx adsorber 46 will become saturated. During
normal operation, NOx adsorber under lean operating conditions will
drive NOx to (NO.sub.3).sub.2 by way of:
NO+1/2O.sub.2 (Pt).fwdarw.NO.sub.2 (1)
XO+2NO.sub.2+1/2O2.fwdarw.X(NO.sub.3).sub.2 (2)
[0030] When NOx adsorber 46 is saturated with X(NO.sub.3).sub.2, it
must be regenerated. A regeneration cycle begins. During the
regeneration cycle, a proportion of the exhaust gases are diverted
to flow through bypass line 12 while the reformer generates H.sub.2
and CO from methane. The H.sub.2 and CO pass through NOx adsorber
46 where they remove NOx.
[0031] A rich environment within exhaust line 27 is preferred to
help ensure that H.sub.2 and CO regenerate NOx adsorber 46.
Therefore, in oxidation catalyst 42 the following occurs:
CH.sub.4+2O.sub.2 CO.sub.2+2H.sub.2O
[0032] in reformer 44 the following occurs:
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2
[0033] and in NOx adsorber, regeneration takes CO and H.sub.2
created and, in such a rich environment:
X(NO.sub.3).sub.2.fwdarw.XO+2NO+ 3/2O.sub.2 (3)
X(NO.sub.3).sub.2.fwdarw.XO+2NO.sub.2+1/2O.sub.2 (4)
NO+H.sub.2.fwdarw.H.sub.2O+1/2N.sub.2 (5)
2NO.sub.2+4H.sub.2.fwdarw.N.sub.2+4H.sub.2O (6)
NO+CO (Rh).fwdarw.N.sub.2+CO.sub.2 (7)
[0034] where X is in a washcoat.
[0035] The regeneration cycle is started by opening by-pass valve
14 to allow some exhaust gas to be routed around NOx adsorber 46
and opening valve 13 to allow some exhaust gas to be routed into
reformer line 22. In general, a percentage of the total exhaust gas
is routed through reformer line 22.
[0036] Depending on the amount of exhaust gas introduced through
reformer line 22 during regeneration, a controller commands
upstream valve and/or downstream valve, 28 and 38, to direct a
quantity of methane (or natural gas) through these valves ensuring
that a quantity of the gas is provided on the upstream side of
oxidation catalyst 42 and/or the downstream of oxidation catalyst
42. As natural gas is, overwhelmingly, methane with a few
additional heavier hydrocarbons, C2 and C3 hydrocarbons in general,
it can, where natural gas is fueling the engine, be retrieved from
the fuel storage tanks. That is, methane store 36 may be the engine
fuel tanks oxidation catalyst 42 and/or the downstream of oxidation
catalyst 42. As natural gas is, overwhelmingly, methane with a few
additional heavier hydrocarbons, C2 and C3 hydrocarbons in general,
it can, where natural gas is fueling the engine, be retrieved from
the fuel storage tanks. That is, methane store 36 may be the engine
fuel tanks.
[0037] Note that by-pass line 12 may branch off of exhaust line 27
at any point prior to junction 53.
[0038] The purpose of oxidation catalyst 42 is to burn off excess
oxygen within the exhaust gas (a rich environment is needed to
drive the initial reaction releasing (NO.sub.3).sub.2 (see
equations (3) and (4) above). The regeneration process is not very
tolerant to excess oxygen within the regeneration stream. Also, as
the reformation process is highly endothermic, it is beneficial to
heat the exhaust stream just prior to the introduction of this
stream with methane through reformer 44. Catalyst 42 provides a
dual function.
[0039] Preferably a metal substrate, rather than, for example, a
ceramic substrate, is utilized as it improves thermal response to
reformer 44 and oxidation catalyst 42. The quicker the thermal
response the quicker the regeneration process can be completed
reducing the amount of untreated exhaust gas allowed to flow
through by-pass line 12. This improves, as well fuel usage-the use
of natural gas in regeneration that could otherwise be used to
drive the engine.
[0040] The upstream quantity of methane to be introduced prior to
oxidation catalyst 42 may be adjusted in response to the properties
of the exhaust gas flowing out of block 11. By way of example, in
one operating situation where the flow of exhaust is constant, low
O.sub.2 content and/or an exhaust gas at a relatively high
temperature will dictate a reduced flow of methane through upstream
valve 28 and into exhaust line 22 prior to oxidation catalyst 42. A
relatively low exhaust gas temperature from the combustion chamber
and into the upstream portion of exhaust line 22 and/or a high
concentration of oxygen will dictate a higher flow of methane
through valve 28. The quantity of methane introduced upstream may
also be controlled in response to exhaust flow. This may depend, by
way of example, on the amount which passes through by-pass line 12
or the amount generated during combustion in light of engine
operating conditions or the combustion process.
[0041] In an in-line set-up as shown in FIG. 1, it is important
that the exhaust gas introduced into oxidation catalyst 42 should
have a temperature above a given minimum temperature to ensure that
the catalyst is "lit-off" initially. One way of managing this is to
choose a combustion strategy or combustion timing that ensures
either relatively late heat release, as might be the case with
spark ignited engines, or a delayed or second direct injection of
fuel into the combustion chamber late in the power stroke where
regeneration is required. This may also reduce NOx levels with
associated benefits during regeneration as a quantity of exhaust
gas is directed through the by-pass line without NOx treatment. A
reduced NOx level has benefits here. Other strategies are well
known to persons skilled in the art.
[0042] Similarly, the flow of methane through downstream valve 38
may be controlled in response to the quantity of methane needed
within the exhaust gas entering reformer 44. After the exhaust gas
has passed through oxidation catalyst 42 its properties are
changed. There will be less oxygen within the gas and less methane.
This is because oxidation of methane occurs within catalyst 42.
This consumes oxygen. As methane serves to provide the source for
H.sub.2 and CO--preferred components in the regeneration
process--the quantity of methane needed within reformer 44 is
determined by the amount present within the exhaust stream upstream
from reformer 44. The amount of methane preferred is determined by
that present in the gases which are exiting oxidation catalyst 42,
and the H.sub.2 and CO concentrations preferred in light of this
initial quantity of methane present (that is, methane not oxidized
within catalyst 42).
[0043] Once forced through oxidation catalyst 42, the exhaust gas,
supplemented with methane via downstream valve 38, is forced
through reformer 44. Reformer 44 utilizes the high temperature of
exhaust gas heated in oxidation catalyst 42, if any, and the
combustion chamber to drive reformation of methane within reformer
44 in reformer line 22 to provide H.sub.2 and CO downstream of
reformer 44. This stream is directed into exhaust line 27 and NOx
adsorber 46 where H.sub.2 and CO regenerate NOx adsorber 46.
[0044] Note that downstream line 30 is optional. The regeneration
controller could provide sufficient methane through upstream line
26 to ensure that enough methane remains after passing through
oxidation catalyst 42. In some embodiments of the invention,
catalyst 42 and reformer 44 may be combined in a POX and methane
may be introduced upstream from the POX. A POX may be provided as a
single component within the aftertreatment system. A POX may be
used in conjunction with a stand-alone upstream oxidation catalyst
or may provide functions of both catalyst 42 and reformer 44.
[0045] Oxidation catalyst 42 may be any oxidization catalyst
suitable to drive up the temperature of the exhaust gas and any
added methane from methane source 36 such that it is at a suitable
temperature for reforming. By way of example, oxidation catalyst
may convert HC/CO to C0.sub.2/H.sub.20:
C.sub.xH.sub.y+(x+(y/4))O.sub.2
(Pt).fwdarw.xCO.sub.2+(y/2)H.sub.2O
C.sub.xH.sub.y+(x+(y/4))O2 (Pd).fwdarw.xCO.sub.2+(y/2)H.sub.2O
CO+1/2O.sub.2.fwdarw.CO.sub.2
[0046] By way of example only, washcoats are typically zeolite
based or Al.sub.2O.sub.3. Other suitable washcoat formulations may
also be used.
[0047] Reformer 44, as well, can be that found in the art. Reformer
44 is preferably suitable to convert methane with water to CO and
H.sub.2. By way of example, reformer 44 may comprise a steam
reforming catalyst such as a Ni-based catalyst within washcoat
materials including calcium aluminate or Al.sub.2O.sub.3.
[0048] NOx adsorber 46 typically adsorbs and stores of NOx in the
catalyst washcoat while operating under lean conditions where
NO.sub.2 would be released and reduced to N.sub.2 under rich
operating conditions where a regeneration mixture, that includes
hydrogen and rich exhaust gas, is passed through the adsorber. As
noted above, the following shows typical operation of the NOx
adsorber under lean conditions:
NO+1/2O2 (Pt).fwdarw.NO.sub.2
XO+2NO.sub.2+1/2O.sub.2.fwdarw.X(NO.sub.3).sub.2
[0049] and under rich conditions:
X(NO.sub.3).sub.2.fwdarw.XO+2NO+ 3/2O.sub.2
X(NO.sub.3).sub.2.fwdarw.XO+2NO.sub.2+1/2O.sub.2
NO+CO (Rh).fwdarw.1/2N.sub.2+CO.sub.2
2NO.sub.2+4H.sub.2N.sub.2+4H.sub.2O
[0050] where X is in the washcoat and is typically an alkali (for
example, K, Na, Li, Ce), an alkaline earth (for example, Ba, Ca,
Sr, Mg) or a rare earth (for example, La, Yt).
[0051] Upon completion of regeneration, by-pass valve 14 and valve
13 are closed as are both upstream and downstream valves 28, 38.
All exhaust gas is then driven through NOx adsorber 46. Once
saturated, the whole cycle repeats and the NOx adsorber is
regenerated as noted above.
[0052] The time needed for regeneration and/or the amount of
exhaust gas routed through by-pass line 12 during regeneration
needs to be considered so as not to release excessive levels of NOx
during the regeneration process. The longer the period of time
needed for regeneration, the more cumulative exhaust gas flows
through by-pass line 12. Preferably, regeneration cycles should be
kept to less than 5% of operating time of the engine. Also, a
greater volume of exhaust gas routed through by-pass line 12,
results in a greater quantity of exhaust gas not passed through NOx
adsorber 46--by-pass line 12 does not generally include a separate
NOx adsorber. A second NOx adsorber or other NOx management system
could be disposed in this line to treat this exhaust gas, this NOx
management system adds costs to the overall system.
[0053] Likewise, there is a fuel penalty where the flow, volume or
length of time through the by-pass line is limited during
regeneration. The less gas passes through bypass line 12 during
regeneration, the more methane that may be needed to heat exhaust
gas for reforming methane. A greater exhaust gas mass volume needs
to be heated that much more before reforming begins. Also, where
more exhaust gas is forced through the exhaust line during
regeneration, the greater quantity of oxygen within the exhaust
line. As it is preferable to burn this off prior to regeneration,
the result is additional combustion of methane in the regeneration
cycle.
[0054] One method of operating with the preferred embodiment
discussed that helps to reduce regeneration time, is to allow the
controller to open valve 13 prior to closing valve 25. This should
allow a flow of exhaust gas through reformer line 22, lighting off
oxidation catalyst 42 and warming the reformer line upstream of
reformer 44 to warm this reformer prior to a regeneration cycle.
When valve 25 is closed at the beginning of a regeneration cycle,
there is less time needed to heat reformer line 22 and less time
before regeneration can commence. In other words the regeneration
process is initiated by opening valve 13 prior to a regeneration
cycle.
[0055] Alternatively, the flow rate within reformer line 22 can be
set to ensure a certain amount of exhaust gas is always flowing
through reformer line 22 eliminating the need for valve 13. As
oxidation catalyst 42 and reformer 44 are disposed in this line,
the flow rate with valve 25 opened could be regulated by valve 25.
In other words, a maximum flow rate through valve 25 could reduce
the flow rate through reforming line 22 to a negligible amount. As
valve 25 is increasingly restricted, increased flow through
reforming line 22 could be secured as necessary to allow reforming
line 22 to heat up appropriately prior to a regeneration cycle.
[0056] Balancing fuel efficiency and emissions limits, preferably
80% by-pass represents the upper limit of the exhaust gas volume
directed through by-pass valve 14 during regeneration.
[0057] By way of example, typical properties can be described for
exhaust gas exiting from the combustion chamber at various points
along the system embodiment described above and shown in FIG. 1
during a regeneration cycle where valve 14 and valve 13 are opened
and valve 25 is close.
[0058] Prior to regeneration, the main difference across the system
is the reduction of NOx. By way of example this reduction may range
from 100 to 500 ppm exiting from the combustion chamber. After
being directed through the NOx adsorber, the NOx concentration may
be reduced to less than 50 ppm.
[0059] Once the controller directs regeneration of the NOx adsorber
opening up by-pass valve 14 and reformer valve 13 and upstream
and/or downstream valves 28 and 38, typical properties of the gas
at various points along the regeneration line can be summarized in
the following table. As would be understood by a person skilled in
the art, the table provides only one example of typical ranges of
operating condition during regeneration:
TABLE-US-00001 TABLE 1 Typical Properties of Gas Along Regeneration
Line After Exit Entry Exit Adsorber Junction 15 Junction 24 43 45
47 51 Line 12 O.sub.2 2-8 2-8 1.9-8 1.9-8 0 0 2-8 (%) C0.sub.2 5-8
5-8 5-10 5-10 5-10 5-10 5-8 (%) H.sub.2O (%) 5-15 5-15 5-18 5-18
5-18 5-18 5-15 CH.sub.4 500 3000- <500 <500 <500 <500
500 (%) 50000 NOx 100- 100- 100- 100- 100- <1000 100-500 (ppm)
500 500 500 500 500 CO <1000 <1000 <100 <100 <20000
0 <1000 (ppm) S0.sub.2 <5 <5 <5 <5 <5 <5 <5
(ppm) P 1 1 1 1 1 1 1 (bar) T 400-600 350-600 600-700 600-700
400-600 250-400 350-600 (.degree. C.)
[0060] The selection of the amount of methane introduced across
upstream valve 28 and downstream valve 38 may be determined by,
amongst other things, the temperature downstream from oxidation
catalyst 42, the oxygen content within the exhaust stream and the
amount of hydrogen desired for regeneration. Typically, by way of
example, 9500 ppm of methane is typically needed per 100.degree. C.
exhaust gas temperature rise. An appropriate temperature for the
resulting exit gas out of catalyst 42 at exit 43 is 650.degree. C.
The preferred range is between 600.degree. C. to 700.degree. C. A
sensor prior to oxidation catalyst 42 may be used to allow a
controller to direct upstream valve 28 to meet the methane demands
for a given temperature demand.
[0061] Open loop control may be used to determine the quantity of
natural gas to direct upstream of oxidation catalyst without need
for temperature measurements. Similarly, the properties of the NOx
adsorber will dictate how a controller directs downstream valve 38
to supply methane to the reformer to ensure that the necessary
amount of H.sub.2 is available at reformer exit 47 to facilitate
regeneration of NOx adsorber 46.
[0062] The following has been shown to be a typical trend found in
systems according to this invention during regeneration cycles.
Typically, at junction 16, which represents the exhaust gas upon
exit from the combustion chamber, the methane concentration is
relatively low. At inlet 41 of the embodiment shown in FIG. 1, a
quantity of methane is introduced from upstream line 26. The
exhaust gas temperature cools by the time it reaches inlet 41. Upon
exit from oxidation catalyst 42 at outlet 43, the methane
concentration falls with a corresponding rise in the exhaust
temperature. The temperature may be driven to something near
700.degree. C. The temperature preferred for this embodiment is in
the range of 600 to 700.degree. C. By the time the exhaust gas has
been directed through to inlet 45 of reformer 44, a second quantity
of methane is provided to the exhaust line through downstream line
30. The temperature of the exhaust gas has fallen slightly due, in
part, to the addition of methane, however, it remains at or above
700.degree. C. The H.sub.2O:CH.sub.4 ratio at this point, prior to
entry into the reformer, should be higher than 2:1 and preferably
higher than 2.5:1 (that is, more water for same amount of methane).
This ration at reformer inlet 45 helps to prevent coking and
improve efficiency of reformation process.
[0063] At reformer outlet 47, the methane concentration falls again
with a consequential rise in hydrogen and CO concentrations. The
temperature falls across the reformer by approximately 100.degree.
C. as this reformer includes a partial oxidation catalyst, in this
embodiment, wherein the POX generates heat prior to or during the
endothermic reforming process. The temperature fall is a typical
trade-off between the endothermic reforming process and exothermic
oxidation of the fuel.
[0064] At inlet 49 to NOx adsorber 46, the temperature of the
exhaust gas falls. Across NOx adsorber 46 and by the time the
exhaust gas is delivered from the adsorber, the hydrogen
concentration is reduced to negligible levels. The methane
concentration is largely unaffected across the NOx adsorber. Any
remaining methane can be oxidized out prior to expulsion from the
exhaust system. As well, additional remaining hydrogen can be
removed with a downstream clean-up catalyst.
[0065] Efficiencies are provided by a ready on-board supply of
natural gas where this is the fuel used to drive the engine. As
natural gas is for the most part methane, its use as the fuel
ensures a ready supply of a hydrogen source.
[0066] An inline external heater could be used to help light off
oxidation catalyst 42 and promote reformation in reformer 44 and
oxidation of exhaust gas during regeneration. This could be used to
help heat the exhaust gas to encourage oxidation and/or to the
reformer operate efficiently. While, it is preferable that the
majority of heat is provided by the exhaust gas, things such as, by
way of example and not limited to:
[0067] transient response,
[0068] efficiency considerations,
[0069] combustion strategies that utilize a quick heat release,
[0070] valve timing,
[0071] cylinder design that takes advantage of a large expansion
ratio,
[0072] may release exhaust gas that could benefit from such a
heater in order to initiate oxidation prior to or during
regeneration.
[0073] Also, where such a heater is employed in-line, oxidation
catalyst 42 may benefit from in-line proximate generation of
hydrogen. A heat exchanger could direct a quantity of heat from
outlet 47 or heat from gases unused after regeneration out of
outlet 51 back through to a point along reformer line 22 upstream
of catalyst 42. This could be used to help reduce the load on such
heater after it initially lights the catalyst off.
[0074] The preferred embodiment discussed above, may also be used
without that part of line 27 joining by-pass line 12 to reformer
line 22. In such an embodiment, exhaust gas would constantly flow
through catalyst 42 and reformer 44 before reaching NOx adsorber
46. During regeneration, by-pass valve 14, would open allowing flow
through by-pass line 12 and a process would begin to create a rich
environment prior to NOx adsorber 46 by lighting off oxidation
catalyst which would, in turn, initiate reformation of methane as
described above. The regeneration cycle would operate as described
above, as valve 25 was closed during regeneration effectively
eliminating this part of exhaust line 27.
[0075] While this variation on the in-line embodiment reduces
complexity by removing aftertreatment piping and valves 25 and 13,
it does not provide the flexibility of the embodiment described
above. It could however, utilize a control strategy that would
introduce methane into the line and cause the oxidation catalyst to
begin heating exhaust gases for a period prior to regeneration. It
would be preferred that the heat of the exhaust gas or quantity of
methane not reach a level that it would begin significant
reformation prior to the regeneration cycle. However, as with the
embodiment of pre-heating discussed above, by heating the catalyst
for a period prior to regeneration, the regeneration time can be
reduced.
[0076] As would be understood by a person skilled in the art, where
a Ni-catalyst is used care should be taken to avoid formation of
any carbonyl, such as nickel carbonyl, in the exhaust environment
in which reforming is taking place. Temperature, CO and oxygen
concentration, as two examples, should be managed to ensure that
carbonyl problems are avoided. For combustion that results in
relatively low exhaust temperature, such as diffusion combustion of
natural gas, Ni-based catalyst should preferably be avoided. A POX
might be substituted.
[0077] FIG. 2 shows a schematic of a NOx treatment system according
to a second embodiment of the invention. Oxidation catalyst 300 and
reformer 302 are both removed from their in-line configuration
found in the embodiment of FIG. 1. As with the above embodiment,
reformer 302 can be a POX as well. Heater 304 is introduced to
assist the reforming process. Air or O.sub.2 reservoir 307 is
provided. Natural gas source 306 along with upstream line 308 and
downstream line 310 are provided each of which branches off of
methane line 311. As well, oxidation entry 312 and exit 314 and
reformer entry and exit 316, 318, are shown. Introduction line 320
runs from exit 318 through to a NOx downstream junction 321 or NOx
upstream junction 323 on exhaust line 322 through NOx upstream line
329 and NOx downstream line 331, as the case may be. Second
oxidation catalyst 325 is provided. Methane line 333 provides a
route to direct methane from natural gas source 306 through to
exhaust line 322 upstream of oxidation catalyst 325. By-pass line
326 and by-pass valve 328 are shown where by-pass line branches off
of NOx line 322 at by-pass junction 340. Engine out 341 directs
exhaust gas to the aftertreatment system. Along NOx line 322,
catalyst inlet 342 and catalyst outlet 344 are shown on either side
of catalyst 325. NOx inlet 346 and NOx outlet 348 are also provided
on either side of NOx adsorber 324. System out 350 is shown beyond
the junction where by-pass line 326 rejoins with NOx line 322.
[0078] Supplemental heat conductor or exchanger 327 downstream of
catalyst 325 may used to direct excess heat from just prior to NOx
adsorber 324 back to heater 304.
[0079] In the off-line embodiment of FIG. 2, reforming is done in
an environment free of exhaust gas. This embodiment is adaptable to
natural gas fueled applications that utilize a combustion process
or engine design that results in exhaust temperatures that are not
high enough to initiate oxidation across a catalyst. In such a
case, an off-line system can be controlled independent of exhaust
gas temperatures.
[0080] Off-line reformer 302 generates and directs hydrogen to the
exhaust gas stream during regeneration.
[0081] An additional component to this system is heater 304. This
heater can be used with a quantity of air (or O.sub.2) from air
reservoir 307 and methane source 306 to feed upstream line 308 or
downstream line 310 into reform line 320 prior to or after
oxidation catalyst 300. The resulting gas from outlet 314 and into
inlet 316, which may or may not include an additional quantity of
methane from source 306 through downstream line 310, is used to
reform methane within the stream to create a gaseous mixture,
including hydrogen and CO, from outlet 318. This mixture is then
directed to junction 321 in exhaust line 322 where exhaust gases
from an internal combustion engine are directed through NOx
adsorber. The regeneration mixture is directed through the NOx
adsorber.
[0082] The reformed hydrogen and CO can be directed upstream or
downstream of oxidation catalyst 325. Utilizing oxidation catalyst
325 by introducing the inlet from the off-line reformer upstream of
catalyst 325 may provide efficiencies as exhaust gas temperature
can be supplemented by oxidizing the exhaust gas with a quantity of
hydrogen. This may help to heat the exhaust gas enough to light off
oxidation catalyst 325. Additional methane remaining after the
reforming process within line 320 and directed to line 329 can be
used to reduce the oxygen concentration within the exhaust line.
Also, methane line 333 can be used to direct methane to oxidation
catalyst 325 to help light-off this catalyst and reduce the oxygen
concentration found in the exhaust gases flowing through exhaust
line 322 during a regeneration cycle.
[0083] Excess heat generated by such oxidation catalyst 325 or the
oxidation of hydrogen can be transferred through heat exchanger 327
back to heater 304 thereby lessening the load on heater 327. The
advantages of an in-line oxidation catalyst can be utilized where,
following initial heating of off-line catalyst 300 and reformer
302, oxidation catalyst 325 may incorporate reformer 302 into
exhaust line 322 through heat exchanger 327. That is, after initial
heating, methane may be directed through to catalyst 325 where
after being initially heated may be lit off in catalyst 325 to
provide heat for reformer 302 through heat exchanger 327 and
oxidation of the exhaust gas. Similarly, a separate line from
exhaust line 322 to off-line catalyst prior to catalyst 312 may be
used wherein catalyst 325 is by-passed. This may be appropriate
where the off-line catalyst has been initially lit off, or exhaust
gas could be used to help lit off the catalyst. In this case, the
off-line catalyst effectively behaves in the same manner as the
in-line apparatus discussed above.
[0084] Additionally, during a regeneration cycle, it may be
advantageous to direct H.sub.2 and CO through to line 331 while
by-passing all exhaust gas through line 326 (valve 328 may, in such
case, need to be disposed in line 322). There are advantages in
some circumstances arising from limiting the need to remove oxygen
from the exhaust gas and simplifying the system by providing an
opportunity to remove catalyst 325. While heat from an inline
catalyst would not be available to provide heat to the off-line
reformer or off-line catalyst, heat from the by-pass line or
upstream of NOx adsorber 324 provided by the exhaust gas could be
used.
[0085] Many of the same considerations mentioned in relation to the
first embodiment discussed need to be addressed in this embodiment.
That is, the oxygen concentration into exhaust line 322 from reform
line 320 needs to be controlled so as to ensure that regeneration
is effective. Also, the balance of methane introduced into upstream
line 308 and downstream line 310 needs to be controlled to ensure
adequate supply of hydrogen and CO. Also, the same considerations
related to control of valve 328 leading to by-pass line 326 are
needed to optimize fuel efficiency and emissions.
[0086] With reference to the second embodiment of the subject
invention, typical operating conditions along points along exhaust
line 322 during regeneration of NOx adsorber 324 are described as
follows.
[0087] A reduction in the oxygen concentration occurs across
catalyst 325. H.sub.2 and CO are provided through junction 321 at
inlet 346 of NOx adsorber 324. Regeneration across NOx adsorber 324
depletes H.sub.2 and CO concentrations. Once by-pass exhaust is
mixed back in with exhaust line 322 by the time system out 350 is
reached, the oxygen concentration spikes back up.
[0088] As noted above, H.sub.2 is introduced into exhaust line 322
prior to oxidation catalyst 325 helping to light off this catalyst.
This reduces the amount of methane that might have to be used to
oxidize oxygen with the line, as is preferred prior to
regeneration. Excess heat from oxidation of H.sub.2 and methane,
the later across catalyst 325, allows heat to be redirected back to
heater 304, if desired. The heat demanded of heater 304 can be
corresponding reduced where H.sub.2and methane oxidation are
managed to generate an adequate excess of heat for reformation to
take place.
[0089] Note for the reformers discussed above, steam is required in
order to generate H.sub.2and CO for regeneration. This need tends
to be met as exhaust gas has sufficient quantities of water. For
off-line reformers, water levels may need to be supplemented.
However, POX catalyst should be able to reform without the need for
supplemental water. Other reformers could be used as understood by
a person skilled in the art.
[0090] A further advantage may be realized where a fuel is used
that combines methane and hydrogen as two major components. By way
of example, natural gas with 20% hydrogen might be appropriate.
Such a fuel could then be utilized in the embodiments discussed
wherein the hydrogen introduced with the fuel prior to the
oxidation catalyst could help to light off those catalysts.
Further, by providing a quantity of hydrogen into the exhaust
stream, the burden on the reformer is reduced. A smaller reformer
may be adequate to provide the total hydrogen required for
regeneration.
[0091] Exhaust gas recirculation (EGR) can also be utilized to help
reduce NOx emissions during regeneration when a by-pass line is
opened. Increased EGR rates during regeneration can reduce NOx
generated in the combustion chamber resulting in less NOx flowing
through the by-pass line and into the atmosphere. Further,
increases in EGR may also be used to reduce the concentration in
oxygen in the exhaust gas during regeneration, reducing, in turn
the burden on the oxidation catalyst to reduce oxygen during a
regeneration cycle as well as reduce the amount of methane needed
to burn off oxygen.
[0092] Where the fuel or supply of methane is from CNG, where
sulfur levels tend to be higher than is the case for LNG
(generally, the two options for providing methane), the combustion
of the fuel resulting in the exhaust gas found in an in-line
reformer will help to dilute any methane source introduced into the
line prior to reformation as combustion of the gas results in the
conversion of sulfur products to less harmful by-products (such as
sulfur dioxide) from a contamination point of view. As such, the
relatively high concentration of sulfur within methane will be
diluted by exhaust gas reducing the potential for poisoning of the
reformer. Where an off-line reformer is used, CNG may not be an
appropriate source of methane, as the dilution noted above is not
available, for example, a CNG source for methane may not be
appropriate due to the high concentrations of problematic sulfur
and the resulting poisoning of the reformer or a filter may be
needed upstream to remove the sulfur. However, as many such
applications benefit from utilizing LNG (for example, by increasing
the range of gaseous fueled vehicles), where this source of methane
has almost no sulfur, it will not generally result in a sulfur
poisoning problem.
[0093] While methane is the preferred source for hydrogen, as would
be understood by a person skilled in the art, other lighter
hydrocarbons, generally, gaseous hydrocarbons, could be used
including but not limited to other gaseous hydrocarbons such as
ethane, propane and butane.
[0094] For the purposes of the application, reformers contemplate,
but are not limited to, steam reformers and POX.
[0095] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the scope of the present disclosure, particularly in
light of the foregoing teachings.
* * * * *