U.S. patent application number 12/395988 was filed with the patent office on 2010-09-02 for nox emission control system for hydrocarbon fueled power source.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to BYONG KWON CHO, JONG H. LEE.
Application Number | 20100221164 12/395988 |
Document ID | / |
Family ID | 42667200 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100221164 |
Kind Code |
A1 |
LEE; JONG H. ; et
al. |
September 2, 2010 |
NOX EMISSION CONTROL SYSTEM FOR HYDROCARBON FUELED POWER SOURCE
Abstract
A method of reducing NO.sub.x in a lean burn engine exhaust
stream from a hydrocarbon burning engine may be first passing the
exhaust stream over a thrifted diesel oxidation catalyst that
substantially completes the oxidation of carbon monoxide to carbon
dioxide and the oxidation of hydrocarbons (HC) to carbon dioxide
and water. Next, separate additions of ozone and ammonia or urea
may be introduced to the exhaust gas stream upstream of the
catalytic reduction reactor at temperatures below 250 degrees
Celsius. The additions of ozone and ammonia or urea modify the
exhaust gas composition to improve the performance of NO.sub.x
reduction catalysts in the catalytic reduction reactor. At
temperatures above 250 degrees, the ozone addition may be reduced
or eliminated, while the ammonia addition can be controlled as a
function of the amount of NOx in the exhaust stream and the
temperature of the catalytic reduction reactor.
Inventors: |
LEE; JONG H.; (ROCHESTER
HILLS, MI) ; CHO; BYONG KWON; (ROCHESTER HILLS,
MI) |
Correspondence
Address: |
General Motors Corporation;c/o REISING ETHINGTON P.C.
P.O. BOX 4390
TROY
MI
48099-4390
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
DETROIT
MI
|
Family ID: |
42667200 |
Appl. No.: |
12/395988 |
Filed: |
March 2, 2009 |
Current U.S.
Class: |
423/239.2 ;
422/105; 422/170; 423/239.1 |
Current CPC
Class: |
F01N 3/103 20130101;
B01D 53/90 20130101; B01D 2255/1023 20130101; B01D 2255/20738
20130101; Y02A 50/2341 20180101; F01N 3/021 20130101; B01D 2255/504
20130101; F01N 2900/1402 20130101; B01D 2251/2067 20130101; F01N
13/009 20140601; B01D 2255/20761 20130101; B01D 53/9472 20130101;
B01D 2251/104 20130101; F01N 2560/026 20130101; B01D 2255/1021
20130101; F01N 3/208 20130101; B01D 2251/2062 20130101; F01N
2610/08 20130101; F01N 2900/1602 20130101; B01D 53/9418 20130101;
F01N 2240/38 20130101; Y02A 50/20 20180101; Y02T 10/24 20130101;
Y02T 10/12 20130101; F01N 2610/02 20130101; B01D 53/944
20130101 |
Class at
Publication: |
423/239.2 ;
423/239.1; 422/170; 422/105 |
International
Class: |
B01D 53/86 20060101
B01D053/86; B01D 53/56 20060101 B01D053/56 |
Claims
1. An emission system for treating a NO.sub.x-containing exhaust
stream comprising: an exhaust conduit; a catalytic reduction
reactor comprising a selective catalytic reduction catalyst
connected to the exhaust conduit; a single zone catalytic oxidation
reactor comprising a thrifted diesel oxidation catalyst that
includes a single washcoat composition, said single zone catalytic
oxidation reactor located upstream of said catalytic reduction
reactor connected to the exhaust conduit; and an ozone generator
connected to the exhaust conduit at a location upstream of said
catalytic reduction reactor.
2. The emission system of claim 1 further comprising: an ammonia or
urea injection device located upstream of said catalytic reduction
reactor.
3. The emission system of claim 1 further comprising: a diesel
particulate filter.
4. The emission system of claim 1 further comprising: a temperature
sensor for measuring said temperature of said SCR catalyst, said
temperature sensor being coupled to said ozone generator; and a
NO.sub.x sensor coupled to said ozone generator, said NO.sub.x
sensor located with the NO.sub.x-containing exhaust stream at a
location upstream from said catalytic reduction reactor.
5. The emission system of claim 4, wherein said ozone generator
generates ozone that is introduced to the NO.sub.x containing
exhaust stream when the temperature of said selective catalytic
reduction catalyst is below a temperature as sensed by said
temperature sensor at which said selective catalytic reduction
catalyst converts NO.sub.x gases in the NO.sub.x-containing exhaust
stream to nitrogen and water at its maximum efficiency.
6. The emission system of claim 4, wherein said ozone generator
introduces a sufficient amount of ozone to the NO.sub.x-containing
exhaust stream to oxidize nitrogen oxide to nitrogen dioxide to
achieve a substantially equimolar ratio of nitrogen oxide and
nitrogen dioxide in said NO.sub.x-containing exhaust stream prior
to said NO.sub.x-containing exhaust stream entering said catalytic
reduction reactor when the NO:NO.sub.2 ratio in the NO.sub.x
containing exhaust stream as sensed by said NO.sub.x sensor is
greater than about 1:1.
7. The emission system of claim 4, wherein said ozone generator
introduces a sufficient amount of ozone to the NO.sub.x-containing
exhaust stream to oxidize nitrogen oxide to nitrogen dioxide to
achieve a substantially equimolar ratio of nitrogen oxide and
nitrogen dioxide in said NO.sub.x-containing exhaust stream prior
to said NO.sub.x-containing exhaust stream entering said catalytic
reduction reactor when the temperature of said selective catalytic
reduction catalyst as sensed by said temperature sensor is below
said temperature at which said selective catalytic reduction
catalyst converts NO.sub.x gases in the NO.sub.x-containing exhaust
stream to nitrogen and water at its maximum efficiency and when the
NO:NO.sub.2 ratio in the NO.sub.x-containing exhaust stream as
sensed by said NO.sub.x sensor is greater than about 1:1.
8. The emission system of claim 2 further comprising: a temperature
sensor for measuring said temperature of said SCR catalyst, said
temperature sensor being coupled to said ammonia or urea injection
device and said ozone generating device; and a NO.sub.x sensor
coupled to said ammonia or urea injection device and said ozone
generating device, said NO.sub.x sensor located within the
NO.sub.x-containing exhaust stream at a location upstream from said
catalytic reduction reactor.
9. The emission system of claim 1, wherein the single washcoat
composition of said diesel oxidation catalyst varies from about 100
percent palladium to about 50 volume percent palladium and 50
volume percent platinum.
10. The emission system of claim 9, wherein said diesel oxidation
catalyst comprises said single washcoat composition applied to a
ceramic substrate material at about 10-100 g/ft.sup.3.
11. The emission system of claim 1, wherein said selective
catalytic reduction catalyst comprises a washcoat applied to a
substrate material, said washcoat comprising a base metal as the
active material contained in a zeolite material.
12. The emission system of claim 11, wherein said base metal is
selected from the group consisting of copper and iron.
13. A method for treating nitrogen oxides, comprising NO and
NO.sub.2, in an exhaust stream from a lean-burn combustion source,
the method comprising: (a) providing an exhaust system for treating
said exhaust stream comprising: a catalytic reduction reactor
having a selective catalytic reduction catalyst; a catalytic
oxidation reactor having a thrifted diesel oxidation catalyst, said
catalytic oxidation reactor located upstream of said catalytic
reduction reactor; an ozone generator located upstream of said
catalytic reduction reactor; (b) determining a temperature of said
selective catalytic reduction catalyst; and (c) passing a stream of
ambient air through said ozone generator to generate a quantity of
ozone sufficient to react with a quantity of NO in the exhaust
stream to form NO.sub.2 and thereby achieve about an equimolar
amount of NO and NO.sub.2 in the exhaust stream upstream of said
catalytic reduction reactor, wherein said ozone generator only
passes said stream when said determined temperature is below a
temperature at which said selective catalytic reduction catalyst
converts NO.sub.x gases in the exhaust stream to nitrogen and water
at its maximum efficiency.
14. The method of claim 13 further comprising: (d) providing an
ammonia or urea injector device; and (e) injecting a quantity of
ammonia or urea to the exhaust stream from said ammonia or urea
injector device at a position upstream of said catalytic reduction
reactor, wherein said amount of said quantity of ammonia or urea is
sufficient to react with a quantity of NOx to form N.sub.2, wherein
said ammonia or urea injector device only injects said quantity of
ammonia or urea when said determined temperature is above a
temperature at which said selective catalytic reduction catalyst
converts NO.sub.x gases in the exhaust stream to nitrogen and
water.
15. The method of claim 13, wherein (c) passing a stream of ambient
air through said ozone generator comprises: coupling a NO.sub.x
sensor within the exhaust stream upstream of said catalytic
reduction reactor; coupling said NO.sub.x sensor to said ozone
generator; measuring the concentration of NO and NO.sub.2 in the
exhaust stream using said NO.sub.x sensor; and determining a
quantity of ozone to generate and introduce to the exhaust stream
using said ozone generator, wherein said quantity of ozone is
sufficient to react with a quantity of NO in the exhaust stream to
form NO.sub.2 and thereby achieve about an equimolar amount of NO
and NO.sub.2 in the exhaust stream upstream of said catalytic
reduction reactor.
16. The method of claim 13, wherein (a) providing an exhaust system
comprises: forming a catalytic reduction reactor having a selective
catalytic reduction catalyst; forming a catalytic oxidation reactor
having a thrifted diesel oxidation catalyst; providing an ozone
generator; coupling said ozone generator to said catalytic
reduction reactor and said catalytic oxidation reactor to said
exhaust stream to form an exhaust system, wherein said catalytic
oxidation reactor is upstream of said catalytic reduction reactor
within said exhaust stream.
17. The method of claim 16, wherein forming a catalytic oxidation
reactor having a thrifted diesel oxidation catalyst comprises:
forming a washcoat including a diesel oxidation catalyst; and
applying said washcoat to a ceramic substrate material.
18. The method of claim 17, wherein the composition of said diesel
oxidation catalyst varies from about 100 percent palladium to about
50 volume percent palladium and 50 volume percent platinum.
19. The method of claim 16, wherein forming a catalytic reduction
reactor having a selective catalytic reduction catalyst comprises:
forming a washcoat including an selective catalytic reduction
catalyst wherein said selective catalytic reduction catalyst
comprises a base metal as the active material contained in a
zeolite material, wherein said base metal is selected from the
group consisting of copper and iron; and applying said washcoat to
a substrate material.
20. A method for treating nitrogen oxides, comprising NO and
NO.sub.2, in an exhaust stream from a lean-burn combustion source,
the method comprising: (a) providing an exhaust system for treating
said exhaust stream comprising: a catalytic reduction reactor
having a selective catalytic reduction catalyst; a single zone
catalytic oxidation reactor having a thrifted diesel oxidation
catalyst that includes a single washcoat composition, said single
zone catalytic oxidation reactor located upstream of said catalytic
reduction reactor; an ozone generator located upstream of said
catalytic reduction reactor; (b) determining a temperature of said
selective catalytic reduction catalyst; and (c) passing a stream of
ambient air through said ozone generator to generate a quantity of
ozone sufficient to react with a quantity of NO in the exhaust
stream to form NO.sub.2 and thereby achieve about an equimolar
amount of NO and NO.sub.2 in the exhaust stream upstream of said
catalytic reduction reactor, wherein said ozone generator only
passes said stream when said determined temperature is below a
temperature at which said selective catalytic reduction catalyst
converts NO.sub.x gases in the exhaust stream to nitrogen and water
at its maximum efficiency.
Description
TECHNICAL FIELD
[0001] The technical field generally relates to treatment of
exhaust gas from a hydrocarbon fueled power source, operated with a
fuel lean combustion mixture.
BACKGROUND
[0002] Diesel engines, some gasoline fueled engines and many
hydrocarbon fueled power plants are operated at higher than
stoichiometric air to fuel mass ratios for improved fuel economy.
Such lean-burning engines and other power sources, however, produce
a hot exhaust with a relatively high content of oxygen and nitrogen
oxides (NO.sub.x). In the case of diesel engines, the temperature
of the exhaust from a warmed up engine is typically in the range of
200 degrees to 400 degrees Celsius, and has a typical composition,
by volume, of about 17% oxygen, 3% carbon dioxide, 0.1% carbon
monoxide, 200 ppm hydrocarbons, 200 ppm NO.sub.x and the balance
nitrogen and water. These NO.sub.x gases, typically comprising
nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are difficult to
reduce to nitrogen (N.sub.2) because of the high oxygen (O.sub.2)
content in the hot exhaust stream.
SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0003] Exemplary embodiments include a method and apparatus for
reducing NO.sub.x in a lean burn engine exhaust stream over a wide
variety of temperatures, including during warm-up conditions.
[0004] One exemplary embodiment includes a method wherein an
exhaust stream from a hydrocarbon burning engine such as diesel
engine may be first passed over a catalytic oxidation reactor
having a thrifted diesel oxidation catalyst (DOC), which
substantially completes the oxidation of carbon monoxide to carbon
dioxide and the oxidation of hydrocarbons (HC) to carbon dioxide
and water.
[0005] Next, separate additions of ozone and ammonia or urea may be
introduced to the exhaust gas stream upstream of a catalytic
reduction reactor. The ozone addition, via a controllable ozone
generator, converts much of the NO content of the exhaust to
NO.sub.2 before the exhaust stream reaches the reduction catalyst
reactor. The ammonia or urea participate in the reduction of NO and
NO.sub.2 to N.sub.2. The additions of ozone and ammonia or urea
therefore modify the exhaust gas composition to improve the
performance of NO.sub.x reduction catalysts (i.e., SCR catalysts)
in the catalytic reduction reactor, which reduces NO.sub.x to
nitrogen and water, including during engine and exhaust warm-up
temperatures (i.e., cold start conditions) below about 250 degrees
Celsius.
[0006] Other exemplary embodiments will become apparent from the
detailed description provided hereinafter. It should be understood
that the detailed description and specific examples, while
disclosing exemplary embodiments, are intended for purposes of
illustration only and are not intended to limit the scope of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Exemplary embodiments of the invention will become more
fully understood from the detailed description and the accompanying
drawings, wherein:
[0008] FIG. 1 is a schematic flow diagram of an exhaust system for
a lean-burn engine;
[0009] FIG. 2 is a perspective view of the catalytic oxidation
reactor according to one exemplary embodiment;
[0010] FIG. 3 is a perspective view of the catalytic reduction
reactor according to one exemplary embodiment; and
[0011] FIG. 4 is a graphical illustration comparing NOx conversion
percentage versus plasma energy density for one exemplary
embodiment.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0012] The following description of the embodiment(s) is merely
exemplary (illustrative) in nature and is in no way intended to
limit the invention, its application, or uses. A flow diagram of an
exhaust system 10 for a hydrocarbon burning engine is illustrated
according to one exemplary embodiment in FIG. 1. An exhaust stream
or conduit 12 from the exhaust manifold of an engine operating at
an air-to-fuel mass ratio well above the stoichiometric ratio is to
be treated to reduce the NO.sub.x (mainly a mixture of NO and
NO.sub.2) content to nitrogen (N.sub.2). When the exhaust stream 12
is from a gasoline-fueled engine operated, for example, at an air
to fuel ratio of greater than 14 (i.e., A/F>14), the exhaust gas
contains some unburned hydrocarbons (HC), NO.sub.x, carbon monoxide
(CO), carbon dioxide (CO.sub.2), water (H.sub.2O) and nitrogen
(N.sub.2). The exhaust stream 12 from a diesel engine contains the
same gaseous constituents plus suspended diesel particulates
(composed of high molecular weight hydrocarbons deposited on carbon
particles).
[0013] Such hydrocarbon containing exhaust streams 12 may be passed
through a catalytic oxidation reactor 14 having a thrifted diesel
oxidation catalyst (DOC) 15, which substantially completes the
oxidation of carbon monoxide to carbon dioxide and the oxidation of
hydrocarbons to carbon dioxide and water. There is typically
abundant oxygen in the exhaust gas stream 12 for these
reactions.
[0014] Ambient air alone, or alternatively air combined with
exhaust (shown as AIR in FIG. 1), may be blown or drawn through an
ozone generator 16 such as a hyperplasma ozone generator 16. The
plasma generated in the air stream converts some of the oxygen
molecules to ozone (O.sub.3). The amount of ozone generated is
related to the level of electric power applied to the ozone
generator 16. Other activated oxygen species may also be generated.
The ozone containing stream 18 may be added to the exhaust stream
12 upstream of catalytic reduction reactor 22 and downstream of the
catalytic oxidation reactor 14 and may be used for oxidation of NO
to NO.sub.2. The input power of the ozone generator 16 may be
controlled by the amount of NO.sub.x, or any of the components of
NO.sub.x as described above, in the exhaust stream 12 that is to be
oxidized, or by the temperature of the downstream catalytic
reduction reactor 22, or by both the amount of NOx in the exhaust
stream and the temperature of the catalytic reduction reactor 22,
as will be described in further detail below.
[0015] One non-limiting example of a non-thermal ozone generator 16
that may be utilized herein is described in U.S. Pat. No. 7,090,811
to Cho et. al., entitled "Method of Reducing NO.sub.x in Diesel
Engine Exhaust", and herein incorporated by reference.
[0016] In addition to ozone addition for NO oxidation, ammonia
(NH.sub.3) or urea may also be added to exhaust stream 12. Ammonia
can be stored in a suitable form (such as liquid ammonia or as
urea) on-board a lean burn engine vehicle, or near-by a stationary
engine, collectively referred to herein as an ammonia injector
device 17, and added as stream 20 to the ozone-treated exhaust
stream 13 upstream of catalytic reduction reactor 22. The ammonia
or urea participate in the reduction of NO and NO.sub.2 to N.sub.2.
While the introduction of ammonia or urea from the injection device
17 is shown downstream of the addition of ozone stream 18 as in
FIG. 1, alternative exemplary arrangements may introduce the
ammonia stream 20 to the exhaust stream 12 prior to the
introduction of ozone stream 18.
[0017] The exhaust stream 19 treated with ozone and/or ammonia or
urea then enters the catalytic reduction reactor 22. The catalytic
reduction reactor 22 includes a selective catalytic reduction (SCR)
catalyst 24 that may function primarily to substantially reduce NO,
N.sub.2O and NO.sub.2 (i.e. NO.sub.X) to N.sub.2 and water.
[0018] Finally, the exhaust stream 25 flows through a diesel
particulate filter 26 to remove any remaining particulate matter
and exits through a tailpipe (not shown) or similar type device to
the atmosphere. In alternative exemplary arrangements, the diesel
particulate filter 26 may be placed after the catalytic oxidation
reactor 14 to filter the exhaust stream 12 prior to entering the
catalytic reduction reactor 22. The diesel particulate filter may
be formed from various materials, including cordierite or
silicone-carbide, which traps particulate matter.
[0019] The catalytic oxidation reactor 14 replaces the dual zone
type catalytic oxidation reactor, which is often used with an SCR
catalyst. In a dual zone type catalytic oxidation reactors, the
exhaust stream first passes through a platinum- and
palladium-containing front side, which oxidizes hydrocarbons and
carbon monoxide to carbon dioxide, and subsequently passes through
a platinum-only containing rear side, which oxidizes NO to
NO.sub.2.
[0020] The catalytic oxidation reactor 14, by contrast, is a single
zone type catalytic oxidation reactor that may be substantially
smaller and oxidizes hydrocarbons and carbon monoxide to carbon
dioxide. This smaller size may allow faster warm-up of the
downstream SCR catalyst 24, which may lead to improved NO.sub.x
reduction and enhanced fuel economy.
[0021] As shown best in FIG. 2, the DOC catalytic material 15 may
be formed from a washcoat 32 applied to a conventional ceramic
substrate material 34 such as cordierite, which may allow for
easier manufacturing. From a compositional standpoint, the amount
of platinum per unit volume of the washcoat 32, and hence the DOC
catalytic material 15, may be substantially decreased, or even
eliminated, as compared with the DOC catalytic material in the dual
zone type catalytic oxidation reactors, which may lead to increased
cost savings. In addition, by applying a single washcoat 32 over
the entirety of the substrate material 34, as opposed to
application of two distinct washcoats to the front side and back
side of the ceramic substrate as in dual zone DOC's, additional
manufacturing costs and material costs may be realized.
[0022] In one group of exemplary embodiments, the composition of
the DOC catalytic material 15 of the washcoat 32 may vary from
about 100 percent palladium to about 50 volume percent palladium
and 50 volume percent platinum. In these exemplary embodiments, the
washcoat 32 may be coated onto the substrate 34 at about 10-100
g/ft.sup.3. The washcoat 32 may include other support
materials.
[0023] As best shown in FIG. 3, the SCR catalyst 24 is formed from
a washcoat 36 including a base metal as the active material
contained in a zeolite material and other support materials coupled
to a conventional substrate material 38 such as cordierite. The
base metal aids in converting NO and NO.sub.2 to N.sub.2 and water
which is discharged through the tailpipe (not shown) as an
emission. The NO.sub.X conversion rate of the base metal reaction
is generally considered the rate limiting step of the system 10 in
the conversion of exhaust gases to suitable tailpipe emissions such
as N.sub.2 and water.
[0024] Examples of base metals that may be used in the exemplary
embodiments include but are not limited to copper and iron coupled
within a zeolite structure. One exemplary SCR catalyst includes
Cu/ZSM-5 catalyst particles containing about 2.5 weight percent of
copper.
[0025] Maximum NOx reduction performance of the SCR catalyst 24 is
often achieved at a substantially equimolar ratio (1:1 ratio) of NO
and NO.sub.2 in the exhaust stream 19, especially at lower
temperatures (such as start up or warm up conditions for the
engine) where the SCR catalyst 24 does not convert NO.sub.x to
N.sub.2 at its maximum efficiency. In addition, at the 1:1 ratio,
the detrimental effects of high space velocity and SCR catalyst 24
aging can be minimized.
[0026] In one group of exemplary embodiments, the amount of ozone
generated within the ozone generator 16 and introduced into the
exhaust stream 13 may be precisely controlled to achieve the
desired substantially equimolar ratio of NO and NO.sub.2 in the
exhaust gas to increasing NO.sub.X conversion at temperatures below
which the SCR catalyst 24 works at maximum efficiency, typically
under start up or warm up conditions.
[0027] For example, where the SCR catalyst 24 utilizes copper or
iron as the base metal such as the Cu/ZSM-5 catalyst material,
maximum efficiency for the SCR catalyst 24 may not be achieved
until the SCR catalyst 24 is heated to about 250 degrees Celsius.
At about 250 degrees Celsius and above, the SCR catalyst 24 may
function at a high enough efficiency to convert all the NO.sub.x
gases to N.sub.2 without the need for ozone supplementation to the
exhaust stream 13.
[0028] In one exemplary embodiment, the ozone generator 16 may be
coupled to a sensor, such as a NO.sub.x sensor 28 or similar
device, which determines the relative amounts of NO and NO.sub.2 in
the NO.sub.X exhaust gas 13 prior to entering the catalytic
reduction reactor 22. In addition, or in the alternative, the ozone
generator 16 may be coupled to a catalytic reduction reactor
temperature sensor 30 that measures the temperature of the SCR
catalyst 24 in the catalytic reduction reactor 22.
[0029] The ozone generator 16 therefore may adjust the amount of
ambient air and/or exhaust converted to ozone, and hence the amount
of NO to be oxidized by the ozone to NO.sub.2 in the exhaust stream
12, by adjusting the level of electrical power supplied to the
ozone generator 16 as a function of either the composition of the
NO.sub.x exhaust gas prior to entering the catalytic reduction
reactor 22 as measured by the NO.sub.x sensor 28, the temperature
of the SCR catalyst 24 as measured by the temperature sensor 30, or
more preferably as a function of both the composition of the
NO.sub.x exhaust gas 13 prior to entering the catalytic reduction
reactor and the temperature of the SCR catalyst 24.
[0030] Thus, in one exemplary embodiment, wherein the system 10
includes the NO.sub.x sensor 28 but no temperature sensor 30, when
the exhaust stream 13 has a high content of NO relative to NO.sub.2
prior to entering the catalytic reduction reactor 22, the
electrical power of the ozone generator 16 may be increased or
maintained in an on position (i.e. a "plasma on" position) to
increase the amount of ozone generated. Conversely, when the
NO.sub.x sensor 28 senses that the NO content is lower (i.e. at
around a 1:1 ratio of NO to NO.sub.2 or less), the electrical power
to the ozone generator 16 may be decreased or turned off (i.e. a
"plasma off" position) to decrease or eliminate the amount of ozone
generated.
[0031] In another exemplary embodiment, wherein the system 10 does
not include a NO.sub.x sensor 28 but includes a temperature sensor
30, the amount of electrical power to the ozone generator 16 is
increased or placed in a "plasma on" position when the temperature
of the SCR catalyst 24 is below the temperature which the SCR
catalyst works at maximum efficiency, while the electrical power to
the ozone generator 16 is decreased or switched to a "plasma off"
position when the temperature of the SCR catalyst 24 is at or above
the temperature in which it works at maximum efficiency. For
example, when the SCR catalyst 24 is Cu/ZSM-5 as described above,
the ozone generator 16 is in a "plasma on" position or higher
electrical power position when the SCR catalyst is below about 250
degrees Celsius to pump ozone into the exhaust stream 13, and is
switched to a "plasma off" position or lower electrical power
position when the temperature reaches 250 degrees Celsius or
greater, where the Cu/ZSM-5 catalyst is capable of converting
NO.sub.x at its maximum efficiency regardless of NO or NO.sub.2
content.
[0032] In yet another exemplary embodiment, wherein the system 10
includes both a NO.sub.x sensor 28 and a temperature sensor 30, the
amount of electrical power to the ozone generator 16 may be
increased, or placed in a "plasma on" position, when the
temperature of the SCR catalyst 24 is below which it converts
NO.sub.x at its maximum efficiency and when the exhaust stream 13
has a high content of NO relative to NO.sub.2 prior to entering the
catalytic reduction reactor 22. When the temperature of the SCR
catalyst 24 is above the temperature in which it converts NO.sub.x
at its maximum efficiency regardless of the NO.sub.x content, or
when the NO to NO.sub.2 content is at a 1:1 ratio or lower at a
temperature below SCR catalyst maximum efficiency, the ozone
generator 16 is placed in a "plasma off" position or lower
electrical power position to limit or eliminate the amount of ozone
entering the exhaust stream.
[0033] In still another exemplary embodiment, the afore-mentioned
NO.sub.x sensor 28 and temperature sensor 30 could also be coupled
to the ammonia or urea injector 17 and thus used to separately
control the introduction of ammonia or urea into the exhaust stream
13.
[0034] FIG. 4 illustrates the effect of plasma energy density on
the NO.sub.x conversion performance of a plasma-assisted
NH.sub.3/SCR system according to one exemplary embodiment, where a
sidestream plasma device was used as the ozone generator and
wherein a 2.5% Cu/ZSM-5 was used as the SCR catalyst 24. The
exhaust stream 12 contained 190 ppm NOx with the NO.sub.2/NOx ratio
of 0.08. An NH.sub.3 stream 20 was injected to the exhaust stream
13 using the NH.sub.3 injector 17, resulting in the NH.sub.3
concentration of 190 ppm in the exhaust stream 19. The temperature
of the catalytic reduction reactor 22 was 210.degree. C. The
beneficial effect of the sidestream air plasma increased the
NO.sub.x conversion performance from 16% with plasma off to -85%
with plasma on.
[0035] The exemplary embodiments illustrate a NO.sub.x reduction
system may have many advantages over conventional systems. The
thrifted DOC 14 of the exemplary embodiments is smaller than
traditional DOC, which may allow for faster warm-up of the SCR
catalyst 24, which may in turn provide increased conversion of
NO.sub.x, on a percentage basis, associated with the faster warm-up
while the system is in operation. Moreover, by reducing or
eliminating the use of platinum in the thrifted DOC, a cost savings
may be realized. Further, a smaller DOC with a single washcoat, as
compared with a dual zone washcoat, may be easier to manufacture
with reduced raw material costs.
[0036] In addition, the use of an ozone generator 16 during warm-up
periods, in conjunction with or separate from the introduction of
ammonia, may provide a method for producing NO.sub.2 more reliably,
which may allow for an increase in the percent conversion of
NO.sub.x at temperatures below 250 degrees Celsius. Further, by
providing a controllable ozone generator, an electric device, may
provide a method for most efficiently and reliably converting NO to
NO.sub.2 throughout the lifetime of a vehicle.
[0037] The above description of embodiments of the invention is
merely exemplary in nature and, thus, variations thereof are not to
be regarded as a departure from the spirit and scope of the
invention.
* * * * *