U.S. patent application number 11/423979 was filed with the patent office on 2007-12-20 for apparatus and method for nox reduction.
Invention is credited to Nikolai Alexeev, Kalle Arve, Kari Eranen, Dmitry Murzin, Alexander Peschkoff, Alexander Rabinovich, Andrei Samokhin.
Application Number | 20070289291 11/423979 |
Document ID | / |
Family ID | 38832843 |
Filed Date | 2007-12-20 |
United States Patent
Application |
20070289291 |
Kind Code |
A1 |
Rabinovich; Alexander ; et
al. |
December 20, 2007 |
Apparatus and Method for NOx Reduction
Abstract
Apparatus and method for NOx reduction. A reducing catalyst is
provided on a monolith or other suitable catalytic converter
element. A multi-mode fuel processor of liquid hydrocarbon fuel is
capable of delivering a required quantity and composition of a
reducing agent while operating in a desired sequence of the
following modes: partial oxidation, incomplete pyrolysis,
evaporation, combustion, and atomization. Temperature sensors
detect the catalyst temperature and means are provided to introduce
the reducing agent into the exhaust stream at a rate correlated to
the measured temperature. Means also provided to implement a
predetermined control algorithm.
Inventors: |
Rabinovich; Alexander;
(Swampscott, MA) ; Samokhin; Andrei; (Moscow,
RU) ; Alexeev; Nikolai; (Moscow, RU) ; Murzin;
Dmitry; (Turku, FI) ; Eranen; Kari; (Masku,
FI) ; Arve; Kalle; (Turku, FI) ; Peschkoff;
Alexander; (London, GB) |
Correspondence
Address: |
CHOATE, HALL & STEWART LLP
TWO INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Family ID: |
38832843 |
Appl. No.: |
11/423979 |
Filed: |
June 14, 2006 |
Current U.S.
Class: |
60/286 ; 60/295;
60/297; 60/301 |
Current CPC
Class: |
Y02T 10/24 20130101;
F01N 2610/04 20130101; F01N 3/206 20130101; F01N 2240/30 20130101;
F01N 3/2073 20130101; F01N 3/2066 20130101; F01N 2240/28 20130101;
Y02T 10/12 20130101; F01N 13/009 20140601; F01N 3/0253 20130101;
F01N 3/035 20130101 |
Class at
Publication: |
60/286 ; 60/295;
60/297; 60/301 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10 |
Claims
1. A system for reducing oxides of nitrogen (NOx) in the exhaust
stream produced by an internal combustion engine operating at lean
air/fuel ratios into non polluting emissions comprising: (a) a
reducing catalyst on a monolith or other suitable catalytic
converter; (b) a multi-mode fuel processor of liquid hydrocarbon
fuel such as diesel capable of delivering a required quantity and
composition of a reducing agent while operating in a desired
sequence of at least the following modes: partial oxidation,
incomplete pyrolysis, evaporation, combustion, atomization; (c)
means to introduce said reducing agent into said exhaust stream;
and (d) means to implement a predetermined control algorithm.
2. A system according to claim 1, further comprising means to
introduce atomized liquid hydrocarbon fuel into said exhaust stream
in addition to the amount of reducing agent already delivered by
said fuel processor.
3. A system according to claim 2 wherein the atomized liquid
hydrocarbon fuel is introduced into the hot stream of reducing
agent generated by the fuel processor.
4. A system according to claim 1, further comprising temperature
sensors.
5. A system according to claim 1 wherein a multi-mode fuel
processor of liquid hydrocarbon fuel is a plasma fuel reformer.
6. A system according to claim 1, further comprising an on-board
source of hydrogen.
7. A system according to claim 1, further comprising a diesel
particulate filter (DPF).
8. A system according to claim 1, further comprising means to cool
gases upstream of the said catalyst.
9. A system according to claim 1, further comprising means to cool
gases upstream of the said catalyst if the temperature of said
catalyst is above a predetermined level.
10. A system according to claim 9, further comprising control means
such that said means to cool gases are activated only when a high
catalyst temperature is detected or conditions are determined that
are expected to lead to high catalyst temperatures.
11. The method of reducing oxides of nitrogen (NOx) in the exhaust
stream by controlling the system described in claim 1 via the
following steps: (a) Determining temperature of the catalyst and/or
temperature of exhaust stream; (b) Changing the control parameters
and/or operating mode of said fuel processor on the basis of said
measurements in accordance with a predetermined control
algorithm.
12. The method according to claim 11, wherein the fuel processor is
specifically operating in the combustion mode to provide for fast
light off of the catalyst.
13. The method according to claim 11, wherein a setpoint value for
a reducing agent feed is based on the engine's parameters.
14. The method according to claim 11, wherein the output of said
fuel processor is also delivered upstream of DPF.
15. The method according to claim 11, wherein a diesel internal
combustion engine is equipped with exhaust gas recirculation, and
wherein the exhaust gas is partially directed to said fuel
processor.
16. The method of claim 11 wherein in order to decrease a response
time only one control parameter (namely fuel flow rate) is changed
when switching from one operation mode to another whilst other
control parameters (primarily air flow rate) remains constant.
17. A system according to claim 11 wherein the fuel processor's
mode of operation is changing from complete combustion to partial
oxidation to incomplete pyrolisys to vaporization inversely
proportional to catalyst temperature increase.
18. A method according to claim 11, wherein at least one of the
control parameters is processed by a dedicated signal
processor.
19. A method according to claim 11, wherein at least one of the
control parameters is processed by a main engine control unit or
EMS.
20. The method of reducing oxides of nitrogen (NOx) in the exhaust
stream by controlling the system described in claim 1 using a
predetermined control algorithm based on an engine map.
21. A system according to claim 1, wherein the reducing catalyst is
an SCR catalyst comprising silver on alumina.
22. A system according to claim 1, wherein the reducing catalyst is
an SCR catalyst comprising a combination of silver and one or more
other metals on a metal oxide (e.g. Al.sub.2O.sub.3 or SiO.sub.2)
or on a zeolite (MFI, MOR, BEA or Y).
23. A system according to claim 1, wherein the catalyst is an SCR
catalyst comprising silver or any other catalyst showing increased
low temperature reduction activity in the presence of hydrogen and
hydrocarbons.
24. A system according to claim 1, wherein such fuels as
bio-diesel, ethanol, gasoline, propane, methane and biofuels or
their mixtures are used as a source of hydrocarbons in the reducing
agent.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of emission
control devices for internal combustion engines, especially diesel
engines and lean burn gasoline engines.
BACKGROUND OF THE INVENTION
[0002] NOx emissions from vehicles with internal combustion engines
are an environmental problem recognized worldwide, Many countries,
including the United States, are introducing ever-tightening
regulations that limit NOx emissions from vehicles.
[0003] Manufacturers and researchers have put considerable effort
toward meeting those regulations. In conventional gasoline powered
vehicles that use stoichiometric fuel-air mixtures, three-way
catalysts have been shown to control NOx emissions. In diesel
powered vehicles and vehicles with lean-burn gasoline engines,
however, the exhaust is too oxygen-rich for three-way catalysts to
be effective.
[0004] Several solutions have been proposed for controlling NOx
emissions from diesel powered vehicles and lean-burn gasoline
engines. One set of approaches focuses on the engine. Techniques
such as exhaust gas recirculation, homogenizing fuel-air mixtures
and fuel switching can reduce NOx emissions. These techniques
alone, however, do not eliminate NOx emissions.
[0005] Another set of approaches removes NOx from the vehicle
exhaust. These include lean NOx traps (LNTs) and selective
catalytic reduction (SCR).
[0006] The LNTs are NOx adsorbers combined with catalysts for NOx
reduction. The adsorber removes NOx from the exhaust stream. The
LNTs have to be frequently regenerated by introducing a reducing
environment. In the reducing environment, NOx are desorbed and
reduced over a suitable catalyst.
[0007] U.S. Pat. No. 6,560,958 describes an LNT system in which
hydrogen-rich gas, including hydrogen and carbon monoxide, is used
as a reductant to regenerate the adsorber. By "reductant" is meant
a reducing agent. The reductant is produced from diesel fuel in a
plasma converter. However, NOx reduction efficiency of a typical
LNT is lower than that of an SCR system. Moreover, LNTs are prone
to sulfur poisoning that further reduces their effectiveness.
Additionally, constant regeneration of LNTs leads to thermal aging
that results in a significant drop of an already low NOx reduction
efficiency.
[0008] Also, the '958 patent describes a dual-branch adsorber
system, whereby during regeneration of a LNT in one branch, all or
part of the exhaust flow can be diverted to the other branch. Apart
from increasing the system cost, such a method requires the use of
at least one exhaust valve. Experience with similar EGR valves
suggests durability and reliability present substantial challenges
for such valves.
[0009] Another solution for controlling NOx emissions is
represented by selective catalytic reduction (SCR) that involves
the selective reduction of NOx to N.sub.2 by a suitable reductant.
Ammonia is the most widely-used reductant, typically in a form of a
urea solution. The reaction takes place even in an oxidizing
environment. The reductant is fed continuously into the exhaust.
SCR can achieve NOx reductions in excess of 90%, however, there is
concern over the lack of infrastructure for distributing
ammonia/urea or a suitable precursor. SCR also raises concerns
relating to the possible release of unreacted ammonia into the
environment (so called "ammonia slip"). More importantly,
urea/ammonia needs to be regularly refilled by a driver. Such a
change in driver habits is difficult to enforce and, therefore, it
leads to a serious compliance issue.
[0010] To overcome the disadvantages of SCR based on urea/ammonia,
it was proposed to use hydrocarbons as a reducing agent (HC--SCR).
For example, an HC--SCR process using propane as the reducing agent
is discussed in U.S. Pat. No. 5,993,764. U.S. Pat. No. 5,824,621
discusses ethanol as a reducing agent and U.S. patent application
No. 20050002843 discusses usage of gasoline, kerosene, bio-diesel
oils etc. for the same purpose. However, the use of those
hydrocarbons for the HC--SCR process requires, as is the case with
urea-SCR, an additional tank for a reductant and a dedicated
reductant delivery infrastructure in the case of propane and
ethanol.
[0011] Using vehicle's primary fuel, such as diesel or gasoline
(petrol), as a reductant or a source thereof represents a more
practical, cost-effective, and environmentally sound approach.
Thus, U.S. Pat. No. 6,202,407 describes an apparatus and a method
for catalytically reducing NOx emissions, particularly emissions
from diesel engine exhaust, by intermittently injecting diesel fuel
into engine exhaust, including an option for using non-thermal
plasma for converting NO to NO.sub.2.
[0012] U.S. Pat. No. 5,727,385 describes a system in which an
HC--SCR catalyst is configured upstream of an LNT. The two
components together are said to provide higher NOx conversion than
either of the components individually. U.S. Pat. No. 6,677,264
describes a combined LNT/HC--SCR catalyst.
[0013] None of these SCR systems that use alternative reductants
can match the NOx reduction efficiency of SCR based on
urea/ammonia. Also, the use of hydrocarbons is associated with high
fuel penalties. Moreover, the use of hydrocarbons for an SCR
process could result in soot and sulfur deposition on a hot
catalyst surface that would lead to a gradual decrease of catalytic
activity.
[0014] Recently, it was found that the addition of a small,
controlled amount of hydrogen to hydrocarbons has a remarkably
positive effect on the NOx reducing efficiency of the HC--SCR
process. The research indicated that hydrogen has a direct role in
the reaction mechanism by either promoting the formation and
storage of organic C.dbd.N compositions which can then readily
reduce NOx, and/or by removing compositions which act as a poison
to the SCR reaction at low temperatures. Thus, for example, it has
been shown that an addition of 1% of H.sub.2 to hydrocarbons
(octane) increases NOx conversion from 5% to 70% at 300.degree. C.
(temperature of the catalyst), and from 40% to 90% at 350.degree.
C. ("On the mechanism of the selective catalytic reduction of NO
with higher hydrocarbons over a silver/alumina catalyst", Kari
Eranen, Fredrik Klingstedt, Journal of Catalysis 227 (2004)
328-343).
[0015] A hydrogen rich gas--a mix that contains a controlled amount
of hydrocarbons and hydrogen--could be produced in plasma fuel
reformers. Such reformers are ideally suited for the SCR
application: they can produce on-demand without any catalyst
hydrogen rich gas in required quantities and of required
composition, and offer a wide dynamic range of operation.
[0016] It should be noted that the '958 patent mentions an SCR
application, suggesting the use of a hydrogen-rich gas. However,
the patent teaches that the reducing agents in the hydrogen rich
gas should be hydrogen and carbon monoxide. More specifically, the
'958 patent suggests the use of an additional catalyst or water
shift reaction in order to increase the concentration of hydrogen.
There are no known catalysts that can selectively reduce NOx to
N.sub.2 using a gas mixture based on hydrogen and carbon monoxide.
In the HC--SCR process a high concentration of hydrogen leads to
its oxidation by free oxygen contained in exhaust gas. High
concentration of hydrogen could also generate excessive amount of
heat on a catalytic surface and eventually cause sintering and
melting of the catalyst.
[0017] More importantly, although the patent '958 fully describes
an LNT application, it does not teach how to use that invention for
an SCR application, nor does it disclose any specific control
strategy. SCR requires a much more complex and precise control than
LNT. Several parameters, including O/C ratio and the catalyst
temperature, need to be closely monitored as they, being linked to
engine operation, are changing constantly. To achieve the desired
optimum performance, specifically high NOx conversion efficiency
and low fuel penalty, an SCR system should operate in several modes
and provide for a continuous control of these parameters.
[0018] Also, SCR is very efficient for NOx reduction as long as the
exhaust temperature is within the active temperature range of the
catalyst (e.g. >300.degree. C.). Unfortunately diesel exhaust
temperatures are often lower than those required for good catalyst
efficiency (i.e., below "light-off" level). This is especially true
for light duty diesel applications such as diesel vehicles which
operate at light load for the most part, resulting in very low
exhaust temperatures (150-250.degree. C.). Even heavy-duty diesel
vehicles operate under conditions which result in exhaust
temperatures below the optimum temperatures for SCR catalysts.
[0019] In such cases, for example during a cold start, it is
possible to improve NOx reduction efficiency by heating catalytic
converters to rapidly reach the operating temperature for selective
catalytic reduction. However, the use of an electric heater for
such a purpose leads to significant electric power consumption.
[0020] Therefore, there continues to be a long felt need for an
affordable and reliable exhaust aftertreatment system that is
durable, has a manageable operating cost (including fuel penalty),
and can practically be used to reduce NOx emissions across the
spectrum of diesel engines to a satisfactory extent in the sense of
meeting U.S. Environmental Protection Agency (EPA) regulations
effective in 2010 and other similar regulations. Such an improved
system should be able to use liquid hydrocarbon fuel as a reductant
or a source of a reductant, while matching NOx conversion
efficiencies of urea- or ammonia-based SCR systems across all
engine cycles.
SUMMARY OF THE INVENTION
[0021] The following presents a simplified summary in order to
provide a basic understanding of some aspects of the invention.
This summary is not an extensive overview of the invention. The
primary purpose of this summary is to present some concepts of the
invention in a simplified form as a prelude to the more detailed
description that is presented later.
[0022] It is an object of the present invention to provide a safe,
reliable and efficient SCR system for reducing NOx emissions from
an internal combustion engine.
[0023] It is another object of the invention to eliminate the
problems associated with the use of ammonia or urea as a reductant
in a mobile SCR system.
[0024] It is another, more specific, object of one aspect of the
invention to provide a simple, robust SCR system capable of rapid
response time in order to meet transient conditions prevailing in
diesel engines.
[0025] It is a yet further and more specific object of the
invention to provide thermal management of an SCR system in order
to improve its overall NOx reduction efficiency.
[0026] It is a yet further and more specific object of the
invention to provide an SCR system that enables integration with an
engine management system (EMS), thereby avoiding the need for an
extra controller. It is a yet further, more specific object of the
invention to provide an SCR system with a dedicated controller
including the features necessary for SCR, using either dedicated
sensors or sharing inputs and outputs where possible with the
EMS.
[0027] These and other objects are achieved by the present
invention, which provides an improved method and a modular
apparatus for NOx reduction.
[0028] One aspect of the invention relates to an exhaust treatment
system having a suitable SCR catalyst, a multi-mode plasma fuel
processor, temperature sensors, an optional controller, a
predetermined controlling algorithm, and reductant delivery
means.
[0029] Another aspect of the invention relates to an exhaust
treatment system also including an additional source of hydrogen.
Yet another aspect of the invention relates to an exhaust treatment
system that also includes a diesel particulate filter (DPF).
[0030] Additional aspects of the invention relate to control of an
exhaust treatment system. One aspect of the control strategy
provides for maintaining catalyst temperature within a
predetermined range to offer, in particular, improvements in
emission control during start-up of diesel engines and thus offer
the potential to control emissions to meet expected future
regulations.
[0031] Other aspects, advantages and novel features of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a schematic illustration of an embodiment of the
invention.
[0033] FIG. 2 is a schematic illustration of another embodiment of
the invention.
[0034] FIG. 3 is a schematic illustration of yet another embodiment
of the invention.
[0035] FIG. 4 is a schematic illustration of still another
embodiment of the invention.
[0036] FIG. 5 is a schematic illustration of an embodiment of the
invention using exhaust gas in the plasma fuel reformer.
[0037] FIG. 6 is a schematic illustration of an embodiment of the
invention in which the fuel is injected into the reformate gas
stream.
[0038] FIG. 7 is a schematic illustration of an embodiment of the
invention including an alternate source for hydrogen.
[0039] FIG. 8 is a graph of NO to N.sub.2 conversion (%) vs.
temperature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] The proposed system includes an internal combustion engine
or a turbine as a source of emissions, a multi-mode plasma fuel
processor, selective catalytic reduction (SCR) catalyst and means
to implement a predetermined control algorithm.
[0041] The SCR catalyst suitable for this system is usually a
silver/alumina catalyst. The fuel processor is able to process
liquid hydrocarbon fuel such as diesel or petrol to deliver the
reducing agent in required quantity and of required composition
while operating in a certain sequence of the following modes in
accordance with a predetermined control algorithm: partial
oxidation (POX), incomplete pyrolysis, evaporation, combustion,
atomization.
[0042] The fuel processor could be based on one of the plasmatrons
described in a U.S. patent application No. 11/330,515 entitled
"Plasma Reformer With Extended Volume Discharge" and U.S. patent
application No. 11/352,138 entitled "High Enthalpy Low Power Plasma
Reformer".
[0043] The system can also be equipped with a diesel particulate
filter (DPF) installed before or after the SCR catalyst. The system
can further be equipped with temperature sensors. The system can
also be equipped with a source of hydrogen such as an electrolysis
apparatus and/or an on-board hydrogen storage device.
[0044] The fuel processor should also be able to: [0045] (a)
Increase or decrease the total throughput at approximately the same
composition in response to engine requirements (from idle to high
speed). The dynamic range of the fuel processor shall be as high as
1:20. [0046] (b) Operate in one steady state mode providing a
certain amount of the reducing agent composed of hydrocarbons and a
small amount of hydrogen sufficient for NOx reduction. At higher
engine speeds, the amount of reducing agent delivered by the fuel
processor can be increased; in addition, atomized diesel fuel can
be injected into the exhaust stream in close proximity to the SCR
catalyst.
[0047] Possible soot formation could be controlled by changing O/C
ratio to a higher value. The O/C ratio could also control the
concentration of hydrocarbons in the reducing agent. For example,
by changing O/C ratio from 1 to 1.3 the concentration of
(C.sub.2H.sub.2+C.sub.2H.sub.4+C.sub.2H.sub.6) decreased from 3.78
vol. % to 1.82 vol. %. The diesel flow rate to the fuel reformer
decreased by 25% (from 0.8 g/s to 0.6 g/s). The hydrogen
concentration remained the same.
[0048] FIG. 1 describes a system in which a plasma fuel processor
or reformer 10 is installed upstream of a DPF 12 followed by an SCR
catalyst 14. The advantage of such arrangement is that the fuel
processor 10 could periodically operate in a lean combustion mode
(at O/C>3) oxidizing particulates collected in the DPF 12 from a
diesel engine 16.
[0049] An alternative embodiment is depicted in FIG. 2. Atomized
diesel fuel 18 can be injected into the exhaust stream in close
proximity to the SCR catalyst 14 if an increased amount of reducing
agent is needed. One possible modification of this scheme is
injection of additional fuel 20 into the hot stream of the reducing
agent generated by the fuel processor 10 (FIG. 3). The high
temperature of the reducing agent (800-1000.degree. C.) could
provide conditions for vaporization and incomplete pyrolysis of the
additionally injected fuel 20 thus increasing the overall amount of
the reducing agent.
[0050] Yet another embodiment is shown in FIG. 4. In this variation
a part 22 of the engine exhaust stream after the DPF 12 is diverted
into the fuel processor 10. The advantages of utilizing exhaust gas
as an oxidizer instead of air for plasma fuel reforming are:
[0051] (a) High temperature of exhaust gas (up to 300-500.degree.
C.) improves conditions for fuel atomization, vaporization and
mixing; it also improves ignition stability.
[0052] (b) Since exhaust gas contains much less oxygen compared to
air (.about.6 vol. % instead of 21 vol. %), a much higher amount
(over .times.3) of exhaust gas is needed to achieve the same O/C
ratio. Using higher volume of an oxidizer improves fuel atomization
and plasma discharge stability. More importantly, under such
conditions a significant part of NOx emissions would be reduced to
N.sub.2 within the plasma fuel processor.
[0053] (c) High concentration of water vapor (up to 12%) in exhaust
gas could prevent soot formation during partial oxidation or
incomplete pyrolysis modes of operation.
[0054] FIG. 5 describes yet another modification of the previous
arrangement. For an additional flexibility, both air 24 and exhaust
gas 22 are used as an oxidizer. This embodiment could be used, for
example, when the plasma fuel processor 10 operates in combustion
mode.
[0055] Yet another embodiment is shown in FIG. 6, where both air 24
and exhaust gas 22 are used as an oxidizer, and an additional fuel
26 is injected into a hot stream of the reducing agent generated by
the fuel processor 10. With this arrangement the reducing agent
would contain products of incomplete pyrolysis. Additional air
could prevent soot formation by changing the O/C ratio.
[0056] FIG. 7 describes yet another system arrangement. For an
additional flexibility, a source of hydrogen 28 such as an
electrolysis apparatus or an on-board hydrogen storage device can
be installed and used in addition to or instead of the fuel
processor 10 under certain conditions.
[0057] FIG. 8 provides a comparison of NOx to N.sub.2 reduction
activities over Ag/Al.sub.2O.sub.3 catalyst achieved by the
proposed multi-mode emission control system (solid line) and a
conventional HC--SCR system based on diesel (dashed line). The
almost complete conversion of NOx to N.sub.2 occurs at much lower
temperature (250 C. instead of 450 C. compared to using only diesel
as a reducing agent).
[0058] It should be noted that in all described embodiments the use
of additional oxidation catalyst (not shown) might be required.
This catalyst could be installed after the SCR catalyst for
oxidation of unreacted H.sub.2, CO and HC.
[0059] The present invention must not be regarded as being limited
to the examplary embodiments described above as several further
modifications are feasible without departing from the scope of the
claims contained herewith.
[0060] The present invention also relates to a method of
controlling the system, especially its NOx reduction efficiency. As
opposed to a simple single-mode SCR operating scheme offered by the
prior art whereby the system is controlled primarily by varying the
amount of the reducing agent, the present invention in several of
its aspects contemplates control over various operating modes and
parameters. More specifically, the proposed control strategy is
based on choosing the most appropriate operating mode or a sequence
thereof for any given condition to provide an optimum combination
of the quantity, composition, thermal properties and feed rate in
respect of the reducing agent.
[0061] Implementing this control strategy generally involves
estimation of the catalytic activity by measuring the catalyst
temperature using temperature sensors or other suitable means.
Also, the control parameters can be based on an engine map and,
therefore, can be modified or adjusted via the EMS or any other
suitable controller.
[0062] One aspect of the invention improves over conventional
methods by first operating the fuel processor in a combustion mode
to enable fast "light off" of the SCR catalyst. According to this
aspect of the present invention, the system operates in this mode
until the temperature of the SCR catalyst reaches its activation
point (i.e. NOx reduction efficiency over 60% and/or temperature
over 150.degree. C.). Activation of the catalyst can be determined
by temperature sensors.
[0063] As an additional step, with a view of improving overall NOx
reduction efficiency of the system, when the catalyst is nearing
its activation temperature (i.e. is from about 10 to about 50% or
more preferably from about 20 to about 30% of the target
temperature) the fuel processor may intermittently alternate
between the combustion and POX modes in a predetermined sequence,
for example 5 sec of combustion followed by 5 sec of POX, or 10 sec
of combustion followed by 5 sec of POX, or 5 sec of combustion
followed by 10 sec of POX. Such a change of modes could be
accomplished by changing fuel flowrate at the constant air
flowrates. In addition to the simplicity of such an approach, the
liquid fuel is not compressible and mode change could be done very
fast,
[0064] According to yet another aspect of the present invention,
once the catalyst reaches its activation point, the fuel processor
is automatically switched into POX mode. It would operate in that
mode until the temperature of the catalyst reaches the range of
300.degree. C. -500.degree. C. after which the fuel processor would
be automatically switched into the evaporation mode or fuel
atomization mode or an intermittent alternating sequence
thereof.
[0065] As an option, in any of these modes an additional fuel may
be injected either into the hot stream of the reducing agent
generated by the fuel processor or into the exhaust stream in close
proximity to the SCR catalyst in order to increase the overall
volume of the reducing agent and improve NOx reduction efficiency
of the system.
[0066] The fuel that is injected may be in liquid or gaseous form.
It may be the same fuel that is used to propel the vehicle, for
example diesel or petrol, or any other suitable hydrocarbon
fuel.
[0067] Another method of the present invention relates to the use
of an on-board source of hydrogen such as an electrolysis apparatus
or a hydrogen storage device. Such a source can be used in addition
to the fuel processor or instead of it in conjunction with an
additional injection of diesel fuel into the exhaust stream.
Alternatively, for additional flexibility a hydrogen storage could
also be used to store the hydrogen rich gas produced by the fuel
processor.
[0068] According to yet another aspect of the present invention,
the fuel processor can be operated in several modes intermittently,
with the ratio of on/off cycles with the mode being 1:1, 2:1, 1:2,
3:1, 1:3, 4:1, 1:4, 5:1 or 1:5. A predetermined sequence of
alternating modes allows to reduce the fuel penalty and improve NOx
reduction efficiency.
[0069] According to yet another aspect of the present invention,
should the temperature of the catalyst drop below or start dropping
towards its activation point, the fuel processor should operate in
a combustion mode as described above.
[0070] According to yet another aspect of the present invention,
should the temperature of the catalyst rise above or start
approaching a certain predetermined point (typically 650.degree.
C.-700.degree. C.), means to cool the exhaust stream and/or the
reducing agent will be automatically activated.
[0071] The invention has been shown and described with respect to
certain aspects, examples, and embodiments. While a particular
feature of the invention may have been disclosed with respect to
only one of several aspects, examples, or embodiments, the feature
may be combined with one or more other features of the other
aspects, examples, or embodiments as may be advantageous for any
given or particular application.
[0072] A well-controlled temperature in the exhaust system improves
NOx reduction efficiency as well as minimizes the risk of an
incomplete reaction of the reducing agent or the risk of the
reducing agent being destroyed by excessively high temperatures.
Also, an appropriate temperature and thermal mass management
provides for an increased duty cycle of a catalyst.
* * * * *