U.S. patent application number 11/998073 was filed with the patent office on 2009-05-28 for selective nox catalytic reduction system including an ammonia sensor.
Invention is credited to Mark A. Shost.
Application Number | 20090133383 11/998073 |
Document ID | / |
Family ID | 40668566 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090133383 |
Kind Code |
A1 |
Shost; Mark A. |
May 28, 2009 |
Selective NOx catalytic reduction system including an ammonia
sensor
Abstract
An improved SCR system for controlling NOx levels in internal
combustion engine exhaust, comprising a least one ammonia sensor
disposed at an intermediate longitudinal location in an SCR
catalyst and in communication with a System Control Module (SCM).
The ammonia measurement permits calculation of ammonia storage on
catalyst sites via a stored SCM algorithm. Locating the ammonia
sensor midway in the catalyst allows for optimum control of NOx
reduction and permits the portion of the catalyst downstream of the
sensor to be treated as a slip catalyst, thus minimizing or
eliminating the need for a second slip catalyst and housing, and
reducing the size, volume, complexity, and cost of an SCR system.
In-brick ammonia sensor permits the system to manage engine exhaust
to a desired NOx conversion level and ammonia slip target value,
thus minimizing the rate of consumption of ammonia while meeting
required limits for NOx emissions.
Inventors: |
Shost; Mark A.; (Northville,
MI) |
Correspondence
Address: |
DELPHI TECHNOLOGIES, INC.
M/C 480-410-202, PO BOX 5052
TROY
MI
48007
US
|
Family ID: |
40668566 |
Appl. No.: |
11/998073 |
Filed: |
November 28, 2007 |
Current U.S.
Class: |
60/276 ; 60/286;
60/299; 60/301 |
Current CPC
Class: |
F01N 13/0097 20140603;
F01N 3/2066 20130101; F01N 13/0093 20140601; Y02T 10/12 20130101;
F01N 2610/02 20130101; Y02T 10/24 20130101; F01N 2560/021
20130101 |
Class at
Publication: |
60/276 ; 60/299;
60/301; 60/286 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Claims
1. A system for control of nitrogen oxide concentrations in a
stream of exhaust gas from an internal combustion engine,
comprising: a) a first catalytic element disposed in said stream of
exhaust gas for catalytically reducing nitrogen oxides to elemental
nitrogen; b) a second catalytic element disposed in said stream of
exhaust gas downstream of said first catalytic element for further
catalytic treatment of said exhaust gas; c) a system for injecting
an ammoniacal chemical reductant into said stream of exhaust gas
ahead of said first and second catalytic elements; and d) an
ammonia sensor disposed in said stream of exhaust gas between said
first and second catalytic elements for sensing ammonia
concentrations in said stream of exhaust gas.
2. A system in accordance with claim 1 wherein said further
catalytic treatment is selected from the processes consisting of
reducing nitrogen oxides, oxidizing ammonia, and combinations
thereof.
3. A system in accordance with claim 1 wherein said first and
second catalytic elements are disposed in a single housing.
4. A system in accordance with claim 1 wherein said first and
second catalytic elements are provided in a single monolithic
catalytic brick, and wherein said ammonia sensor is disposed at an
intermediate longitudinal location of said brick, which location
defines portions of said brick upstream of said ammonia sensor as
said first catalytic element and portions of said brick downstream
of said ammonia sensor as said second catalytic element.
5. A system in accordance with claim 1 wherein said first catalytic
element and second catalytic element are abuttingly disposed and
have a well formed at the location of said abutting, and wherein
said ammonia sensor is disposed in said well.
6. A system in accordance with claim 1 wherein said first catalytic
element and said second catalytic element have a gap therebetween,
and wherein said ammonia sensor is disposed in said gap.
7. A system in accordance with claim 1 wherein said ammoniacal
chemical reductant is ammonia.
8. A system in accordance with claim 7 wherein said ammonia is
derived by decomposition of urea.
9. A system in accordance with claim 8 wherein said urea is
provided in the form of an aqueous solution.
10. A system in accordance with claim 1 further comprising a
programmable controller responsive to signals from said ammonia
sensor for regulating rate of injecting of said ammoniacal chemical
reductant into said exhaust gas stream.
11. A system in accordance with claim 1 further comprising a second
ammonia sensor disposed in said stream of exhaust gas after said
second catalytic element.
12. An internal combustion engine, comprising a system for control
of nitrogen oxide concentrations in a stream of exhaust gas from
said engine, wherein said system includes a first catalytic element
disposed in said stream of exhaust gas for catalytically reducing
nitrogen oxides to elemental nitrogen, a second catalytic element
disposed in said stream of exhaust gas downstream of said first
catalytic element for further catalytic treatment of said exhaust,
a system for injecting an ammoniacal chemical reductant into said
stream of exhaust gas ahead of said first and second catalytic
elements, and an ammonia sensor disposed in said stream of exhaust
gas between said first and second catalytic elements for sensing
ammonia concentrations in said stream of exhaust gas.
13. An internal combustion engine in accordance with claim 12
wherein said further catalytic treatment is selected from the
processes consisting of reducing nitrogen oxides, oxidizing
ammonia, and combinations thereof.
14. An internal combustion engine in accordance with claim 12
wherein said engine is selected from the group consisting of
spark-ignited and compression-ignited.
Description
TECHNICAL FIELD
[0001] The present invention relates to systems for controlling the
level of nitrogen oxides (NOx) in internal combustion engine
exhaust; more particularly, to such systems for catalytically
reducing NOx to N.sub.2 by reaction with ammonia; and most
particularly, to an improved system having an ammonia sensor
disposed within the catalyst for feedback control to manage exhaust
levels of NOx and ammonia.
BACKGROUND OF THE INVENTION
[0002] Reducing and controlling engine emissions of oxides of
nitrogen are important considerations in modern internal combustion
engines, both spark-ignited and compression-ignited. NOx emissions
are an element of smog production, and emissions limits mandated by
state governments and/or the federal government are likely to
become even more stringent in the future.
[0003] One known approach to reducing NOx emissions is to reduce
NOx formation by reducing combustion temperatures, such as by
recirculation of exhaust gas into the engine firing chambers to
dilute the combustion mixture. Even under the best of control,
however, untreated engine exhaust typically contains an
unacceptable level of NOx. Thus, another approach is to strip NOx
from the exhaust via one or more aftertreatment devices.
[0004] Aftertreatment systems are known in the art which can
convert NOx to elemental N.sub.2 by selective catalytic reduction
(SCR) in the presence of a suitable reductant, for example, ammonia
(NH.sub.3) in accordance with the following equations:
NO+NO.sub.2+2NH.sub.3.fwdarw.2N.sub.2+3H.sub.2O (Eq. 1)
4NO+O.sub.2+4NH.sub.3.fwdarw.4N.sub.2+6H.sub.2O (Eq. 2)
2NO.sub.2+O.sub.2+4NH.sub.3.fwdarw.3N.sub.2+6H.sub.2O (Eq. 3)
Typically, ammonia is provided ("dosed") to the catalyst via
decomposition of urea (typically aqueous urea) in accordance with
the following equations:
CO(NH.sub.2).sub.2 (aqueous).fwdarw.CO(NH.sub.2).sub.2+H.sub.2O
(Eq. 4)
CO(NH.sub.2).sub.2.fwdarw.NH.sub.3+HNCO (Eq. 5)
HNCO+H.sub.2O.fwdarw.NH.sub.3+CO.sub.2 (Eq. 6)
or a net reaction of:
CO(NH.sub.2).sub.2+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2 (Eq. 7)
[0005] It will be recognized that specific molar match of ammonia
to NOx is desired to convert all NOx while slipping no excess
ammonia to atmosphere. In practice, this has proved to be very
difficult to achieve. For example, in a prior art SCR system first
and second catalyst bricks are required, typically disposed in two
separate sequential housings. The first brick is designated as the
SCR catalyst and the second brick is designated as the "slip"
catalyst for oxidizing residual exhaust ammonia, which is known to
happen due to one or more of three causes:
[0006] First, excess ammonia in the tailpipe exhaust can be due to
incomplete reaction of the SCR as shown in Eqs. 1-3.
[0007] Second, in the SCR catalytic reaction mechanism, NOx reacts
with ammonia stored on the catalyst. Ammonia storage capacity is
highly dependent on temperature of the catalyst, with capacity at
low temperatures being significantly greater than at higher
temperatures. Because of this effect, even when dosing is greatly
reduced or even stopped completely during hot exhaust transients,
unreacted ammonia can be desorbed from the SCR catalyst and pass
into the tailpipe exhaust.
[0008] Lastly, dosing at low temperatures can lead to solid or
liquid urea deposits in the exhaust system which, upon subsequent
heating, can lead to additional ammonia release unaccounted for in
the dosing control.
[0009] The slip catalyst acts to oxidize excess ammonia, converting
it into elemental nitrogen, per the following mechanism:
4NH.sub.3+3O.sub.2.fwdarw.2N.sub.2+6H.sub.2O (Eq. 8)
Less desirably, excess ammonia may also be further oxidized back
into NOx, per the following mechanisms:
4NH.sub.3+5O.sub.2.fwdarw.4NO+6H.sub.2O (Eq. 9)
2NH.sub.3+2O.sub.2.fwdarw.N.sub.2O+3H.sub.2O (Eq. 10)
[0010] A prior art SCR system may further include, after the SCR or
slip catalyst, an NOx sensor which is additionally cross-sensitive
to ammonia, and the system is close-loop controlled to maintain
slipped ammonia below a predetermined level by regulating the
dosing rate of aqueous urea.
[0011] Such a prior art SCR system has several shortcomings. First,
significant inadequacies in the prior art require a slip catalyst
element and volume of slip catalyst to offset lack of optimal
dosing control. Second, due to hysteresis the system cannot be
responsive to abrupt changes in NOx load or catalyst temperature,
which occur frequently in actual engine usage. Third, when the
sensor is placed after the slip catalyst, ammonia sensed by the
ammonia sensor is by definition lost to atmosphere, as the system
has no means for absorbing or oxidizing slipped ammonia after the
sensor.
[0012] What is needed in the art is an improved method and
apparatus for determining and controlling the ammonia level
resident in the exhaust leaving the SCR catalyst.
[0013] What is further needed in the art is an improved method and
apparatus for eliminating the need for a slip catalyst or
minimizing slip catalyst volume as well as minimizing the amount of
slip ammonia lost to atmosphere.
[0014] It is a principal object of the present invention to
optimize the consumption of urea in an SCR system to a targeted
ammonia slip average and/or slip maximum while meeting required
targets for NOx emissions.
[0015] It is a still further object of the present invention to
reduce the size, volume, complexity, and cost of an SCR system.
SUMMARY OF THE INVENTION
[0016] Briefly described, an improved SCR system in accordance with
the invention comprises a least one ammonia sensor disposed at an
intermediate longitudinal location in an NOx-reducing SCR catalyst
and in communication with an Engine Control Module (ECM). Locating
the ammonia sensor within the catalyst allows for optimal NOx
reduction and permits the downstream NOx catalyst to be treated as
an effective slip catalyst, thus minimizing or eliminating the need
for a second slip catalyst and housing, and reducing the size,
volume, complexity, and cost of the SCR system. Continuous
measurement of ammonia concentration in exhaust at an intermediate
location in the SCR catalyst permits calculation of ammonia storage
amounts on active catalyst sites throughout the catalyst brick, and
thus permits the engine exhaust to be managed to a desired NOx
conversion level and ammonia slip target value. Further, placing
the ammonia sensor closer to the point of urea introduction reduces
hysteresis in the SCR system, allowing faster response to NOx load
changes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0018] FIG. 1 is an elevational view, partially in cutaway, of the
exhaust treatment catalysts in a prior art SCR;
[0019] FIG. 2 is a schematic drawing of an SCR system improved in
accordance with the present invention;
[0020] FIG. 3 is a graphical representation of key factors in SCR
control; and
[0021] FIGS. 4a, 4b, and 4c are elevational views, partially in
cutaway, of three alternative embodiments of the improved SCR
catalytic converter shown in FIG. 2.
[0022] Corresponding reference characters indicate corresponding
parts throughout the several views. The exemplifications set out
herein are not to be construed as limiting the scope of the
invention in any manner.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The benefits and advantages of the present invention may be
better appreciated by first considering a prior art SCR system.
[0024] Referring to FIG. 1, a catalyst assembly 10 in a prior art
SCR system comprises a Selective Catalytic Reduction (SCR) unit 14
and an optional ammonia slip catalyst 16 connected in sequence to
treat raw engine exhaust 18 from an internal combustion engine 20,
especially a diesel or gas engine operated with excess oxygen in
the exhausted gases, and to discharge treated engine exhaust 22 to
atmosphere 24.
[0025] SCR 14 includes a first selective catalyst 30 disposed in a
first housing 32 for selectively reducing NOx to N.sub.2 in the
presence of NH.sub.3 and O.sub.2, as described above, in known
fashion. An atomizing nozzle 34 prior to SCR 14 receives a
reductant solution 38 containing urea and/or ammonia from a source
40 and sprays atomized reductant solution 42 into exhaust 18.
[0026] Slip catalyst 16, a second selective catalyst, is disposed
in a second housing 46 for receiving intermediate exhaust 48
containing excess ammonia which is oxidized by catalyst 16 to
minimize the amount of ammonia slipped to atmosphere 24 in treated
exhaust 22. An exhaust gas sensor 50 sensitive to both NOx and
NH.sub.3 is disposed between first and second SCR housings 32,46
and monitors levels of NOx and NH.sub.3 in intermediate exhaust 48
and communicates with a System Control Module 54 for feedback
control of dosing rate of reductant solution 38 from source 40
based upon the sensed NOx levels in the exhaust. (Description and
illustration of the storage and dosing apparatus is omitted for
clarity.) An optional ammonia sensor 56 may be provided after slip
catalyst 16 to monitor actual slip ammonia levels in treated
exhaust 22.
[0027] Referring now to FIG. 2, an exemplary improved SCR system
100 in accordance with one aspect of the present invention
comprises a dosing apparatus assembly 102, a catalyst assembly 110,
and a system controller 154.
[0028] Dosing apparatus assembly 102 includes a tank 158 for
storing reductant solution 38, a high level sensor 160, a
temperature sensor 164, and a solution heater 166 in communication
with system controller 154 which may be an Engine Control Module. A
supply line 168 leading from a solution filter 170 and containing a
pressure sensor 172 is connected to catalyst assembly 110 at a
solenoid pump/injector 174 for supplying reductant solution 38 to
atomizer 134. Preferably, supply line 168 is heated by an electric
heater 176.
[0029] Some elements of catalyst assembly 110 are similar to their
counterparts in prior art catalyst assembly 10. Assembly 110
comprises a Selective Catalytic Reduction (SCR) unit 114 comprising
both a selective NOx reduction first catalyst 130 and a second
catalyst 116. Catalysts 130,116 may be disposed in separate
sequential housings as in the prior art but preferably are disposed
in a common housing 132, all elements being connected in sequence
to treat raw engine exhaust 18 from an internal combustion engine
20, especially a diesel engine, and to discharge treated engine
exhaust 122 to atmosphere 24.
[0030] Preferably, catalysts 116,130 are provided as porous or
channeled monoliths known in the art, and used herein, as
"bricks".
[0031] Selective catalyst 130 is disposed in housing 132 for
selectively reducing NOx to N.sub.2 in the presence of NH.sub.3 and
O.sub.2, as described above. Atomizing nozzle 134 sprays atomized
reductant solution 42 into exhaust 18.
[0032] (Note that an SCR system in accordance with the invention
contemplates provision of ammonia from any source. For exemplary
purposes, the present discussion refers only to urea as the source,
and to liquid urea dosing as the means of introduction, but it
should be understood that all other ammoniacal chemical reductants
and apparatus for supplying them having the net effect of providing
ammonia to the SCR catalysts are fully comprehended by the
invention.)
[0033] Second catalyst 116, preferably also disposed in housing 132
and downstream of selective NOx catalyst 130, receives treated
exhaust which may contain slip ammonia and unreacted NOx.
[0034] In a first embodiment (see FIG. 4a), catalyst 116a is an
extension of selective NOx catalyst 130a, and slip ammonia is
controlled as described below.
[0035] In a second and third embodiments (see FIGS. 4b and 4c),
second catalysts 116b,116c are selective NOx catalysts similar to
catalysts 130b,130c. However, optionally, catalysts 116b,116c may
also have ammonia-oxidizing capability like a prior art ammonia
slip catalyst. In this configuration, excess ammonia is oxidized by
catalysts 116b,116c, and NOx passing beyond catalysts 130b,130c is
also reduced by catalysts 116b,116c, thereby minimizing the amount
of ammonia and NOx slipped to atmosphere 24 in treated exhaust
122.
[0036] An important feature of the present invention is an ammonia
sensor 180 disposed between first and second catalysts 130,116 and
in communication with a System Control Module (SCM) 154 for
feedback control of dosing rate of reductant solution from tank
158. As used herein, the term "ammonia sensor" should be taken to
mean any device capable of determining the ammonia content in
exhaust gases. Note that prior art catalyst assembly 10 becomes a
functional embodiment of the present invention when provided with
an ammonia sensor 180 in place of NH.sub.3/NOx sensor 50 in
accordance with the present invention, even though catalysts 30 and
16 are disposed in separate housings.
[0037] Exemplary ammonia sensors are disclosed in the prior art;
see, for example, U.S. Pat. No. 7,069,770 B2, and Published US
Patent Application Nos. 20060200969 A1 and 20070080074 A1, the
relevant disclosures of which are incorporated herein by reference.
Such sensors are presented as only exemplary, and any other ammonia
sensing device is fully anticipated by the invention.
[0038] The present invention affords at least two distinct
advantages.
[0039] First, sensing ammonia rather than nitrogen oxides in the
treated exhaust permits direct feedback flow control of
ammonia-generating reductant via an algorithm programmed into SCM
154. The algorithm factors in at least the exhaust temperature at
the first catalyst inlet (temperature sensor 182), the exhaust
temperature at the second catalyst outlet (temperature sensor 184),
the flow rate of reductant, and the instantaneous concentration of
exhaust ammonia, as well as various engine operating parameters, to
calculate the new flow rate.
[0040] Second, placing the ammonia sensor ahead of second catalyst
116 allows the algorithm also to calculate the instantaneous
ammonia loading already in both catalyst bricks 116,130, as a part
of calculating a new reductant flow rate, and further allows
ammonia-oxidizing catalysts 116b,116c to oxidize ammonia sensed by
sensor 180. Thus, the algorithm is able to minimize consumption of
reductant and formation of atmospheric slip ammonia while
maximizing conversion of NOx to N.sub.2 in treated exhaust 122.
Note that an optional second ammonia sensor 186 after second
catalyst 116 may also be included to monitor tailgas ammonia levels
in treated exhaust 122 directly, to confirm proper operation of the
control system 100.
[0041] Referring to FIG. 3, the sensing and control advantages of
the present invention are shown. The actual concentrations of
ammonia and NOx in tailgas exhaust 122 are shown as a function of
the rate of urea dosing. As ammonia concentration (curve 188)
increases, NOx concentration (curve 190) decreases. By severely
overdosing the system with urea, the concentration of NOx can be
reduced to near zero (SCR efficiency=100%) but with a large amount
of slip ammonia being released to atmosphere. Use of an ammonia
sensor permits correlation of curves 184,186 at all points such
that actual levels of ammonia and NOx in exhaust 122 are known, and
the system may be controlled to any desired value of either one. On
the other hand, in prior art systems (FIG. 1) herein a tailgas
sensor 50 (FIG. 3 curve 192) is sensitive to both NOx and NH.sub.3,
the sensor output is the same for two values of the curve, e.g.,
points 194,196, and therefore cannot distinguish whether to further
increase or to decrease urea flow.
[0042] A third advantage of the present invention is that both
first catalyst 130 and second catalyst 116 may be disposed together
in a single housing 132, thus reducing the cost and volume of an
SCR system.
[0043] Referring to FIGS. 4a-4c, various configurations of
catalysts and ammonia sensor are possible within the scope of the
present invention. In each case, the first and second catalysts
130,116 have NOx-reducing capability, and optionally second
catalysts 116 may also have ammonia-oxidizing capability.
[0044] FIG. 4a shows a single monolithic brick catalyst disposed in
housing 132 and having a well 198 formed at an intermediate
longitudinal location for receiving ammonia sensor 180, which
location defines portions of the brick upstream of sensor 180 as
SCR catalyst 130a and portions downstream of sensor 180 as slip
catalyst 116a.
[0045] FIG. 4b shows SCR catalyst 130b and slip catalyst 116b
abuttingly disposed in housing 132 and having a well 198 formed at
the abutting location for receiving ammonia sensor 180.
[0046] FIG. 4c shows SCR catalyst 130c and slip catalyst 116c
sequentially disposed in housing 132 and having a gap 199 formed
therebetween for receiving ammonia sensor 180.
[0047] In operation, ammonia, preferably formed from a urea
solution, is supplied (dosed) via nozzle 134 and exhaust 18 into
SCR catalyst assembly 114 at a controlled dispensing rate. Ammonia
reacts with NOx at reaction sites on first and second catalysts
130,116, reducing NOx in exhaust 122 to a predetermined level.
Ammonia sensor 180 senses the level of ammonia entrained in exhaust
gas leaving first catalyst 130 and sends a signal to SCM 154, which
is being provided with various relevant engine operating
parameters. SCM 154 is programmed with an algorithm describing
exhaust ammonia levels as a function of engine operating data over
the range of engine speeds and temperatures in which the engine can
operate, as well as test-bed generated performance data showing
ammonia storage three-dimensionally within first and second
catalysts 130,116 as a function of at least exhaust temperature,
exhaust flow rate, and NOx level. The SCM determines whether the
level of ammonia being experienced in the exhaust by sensor 180 is
within predetermined limits and also predicts, from the present
urea dosing rate and the expected near-term engine conditions,
whether the present ammonia level is a) insufficient, b) correct,
or c) excessive to meet present and anticipated needs in order to
meet desired levels of NOx and ammonia in tailpipe exhaust 122. The
SCM then adjusts the liquid urea dosing rate accordingly and begins
another calculation cycle.
[0048] While the invention has been described by reference to
various specific embodiments, it should be understood that numerous
changes may be made within the spirit and scope of the inventive
concepts described. Accordingly, it is intended that the invention
not be limited to the described embodiments, but will have full
scope defined by the language of the following claims.
* * * * *