U.S. patent application number 12/873127 was filed with the patent office on 2012-03-01 for exhaust treatment system and method of operation.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Stanlee Teresa Buddle, Dan Hancu, Ashish Balkrishna Mhadeshwar, Daniel George Norton, Benjamin Hale Winkler.
Application Number | 20120047877 12/873127 |
Document ID | / |
Family ID | 45695290 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120047877 |
Kind Code |
A1 |
Winkler; Benjamin Hale ; et
al. |
March 1, 2012 |
EXHAUST TREATMENT SYSTEM AND METHOD OF OPERATION
Abstract
An exhaust treatment system is provided. Method of increasing
activation of NOx reduction catalyst using two or more reductant is
discussed. The exhaust treatment system includes an exhaust source,
a reductant source, a nitrogen oxide (NOx) reduction catalyst, a
sensor, and a controller. The reductant source includes a first
reductant and second reductant, and is disposed to inject a
reductant stream into an exhaust stream from the exhaust source.
The NOx catalyst is disposed to receive both the exhaust stream and
reductant stream. The sensor is disposed to sense a system
parameter related to carbon loading of the catalyst and produce a
signal corresponding to the system parameter. The controller is
disposed to receive the signal and to control dosing of the
reductant stream based at least in part on the signal.
Inventors: |
Winkler; Benjamin Hale;
(Albany, NY) ; Norton; Daniel George; (Niskayuna,
NY) ; Mhadeshwar; Ashish Balkrishna; (Storrs, CT)
; Hancu; Dan; (Clifton Park, NY) ; Buddle; Stanlee
Teresa; (Gloversville, NY) |
Assignee: |
GENERAL ELECTRIC COMPANY
SCHENECTADY
NY
|
Family ID: |
45695290 |
Appl. No.: |
12/873127 |
Filed: |
August 31, 2010 |
Current U.S.
Class: |
60/276 ;
60/286 |
Current CPC
Class: |
F01N 3/0885 20130101;
F01N 2610/03 20130101; F01N 3/206 20130101; F01N 3/2006 20130101;
Y02T 10/12 20130101; F01N 3/208 20130101; Y02T 10/26 20130101; Y02T
10/24 20130101; F01N 2610/04 20130101 |
Class at
Publication: |
60/276 ;
60/286 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/20 20060101 F01N003/20 |
Claims
1. An exhaust treatment system comprising: an exhaust source; a
reductant source disposed to inject a reductant stream into an
exhaust stream from the exhaust source, the reductant source
comprising a first reductant and second reductant; a nitrogen oxide
(NOx) reduction catalyst disposed to receive the exhaust stream and
the reductant stream; a sensor disposed to sense a system parameter
related to carbon loading of the catalyst, and produce a signal
corresponding to the system parameter; and a controller disposed to
receive the signal and to control dosing of the reductant stream
based at least in part on the signal.
2. The system of claim 1, wherein the controller is disposed to
control the dosing by varying a ratio of the second reductant to
the first reductant, a flow rate of first reductant, a flow rate of
second reductant, or a combination of any of the foregoing.
3. The system of claim 2, wherein the first reductant comprises
diesel, biodiesel, ultra low sulphur diesel, Fischer-Tropsch fuel,
kerosene, or any combinations thereof.
4. The system of claim 2, wherein the second reductant comprises
ethanol, methanol, isopropyl alcohol, n-propanol, n-butanol, methyl
tert-butyl ether, E85, gasoline, or any combinations thereof.
5. The system of claim 2, wherein the controller is configured to
cause a ratio of the second reductant to the first reductant in the
reductant stream to be greater than 0.5 when an estimated carbon
loading exceeds a predetermined value.
6. The system of claim 5, wherein the controller is configured to
cause a ratio of the second reductant to the first reductant in the
reductant stream to be greater than 3 when an estimated carbon
loading exceeds a predetermined value.
7. The system of claim 6, wherein the predetermined value is
greater than about 1 weight percent of the catalyst.
8. The system of claim 1, wherein the system parameter related to
carbon loading comprises an exhaust stream composition parameter,
an exhaust stream temperature parameter, an exhaust stream flow
parameter, an exhaust source parameter, a time parameter, or a
combination thereof.
9. The system of claim 8, wherein the exhaust stream composition
parameter comprises a concentration of NOx in the exhaust stream
and a space velocity of the catalyst.
10. The system of claim 9, wherein the predetermined NOx
concentration value is in the range of 1 to 2000 ppmV.
11. The system of claim 9, wherein the catalyst space velocity is
between 1000 hr.sup.-1 and 200,000 hr.sup.-1.
12. The system of claim 8, wherein the exhaust stream temperature
parameter comprises temperature of the catalyst and temperature of
the exhaust stream.
13. The system of claim 12, wherein the exhaust temperature is
between about 200.degree. C. and about 650.degree. C.
14. The system of claim 8, wherein the exhaust stream flow
parameter comprises a molar, mass, and volumetric flow rate of the
exhaust stream, reductant stream, and air stream.
15. The system of claim 14, wherein a ratio of the molar flow rate
of carbon in the reductant stream to the molar flow rate of
nitrogen in NOx in the exhaust stream is between 0 and 12.
16. The system of claim 8, wherein the exhaust source parameters
comprises source speed, torque, and source power.
17. The system of claim 1, wherein the first reductant is a fuel
and the second reductant is an oxygenate.
18. The system of claim 17, wherein the second reductant comprises
ethanol.
19. The system of claim 1, wherein the catalyst comprises a
hydrocarbon selective catalytic reduction (HC-SCR) catalyst.
20. The system of claim 19, wherein the catalyst comprises silver
and a templated metal oxide substrate.
21. An exhaust treatment system, comprising: an exhaust source; a
reductant source comprising a first storage place and a second
storage place, wherein the first storage place is disposed to
inject a fuel to the exhaust source and further configured to
inject fuel to an exhaust stream emitted from the exhaust source,
and the second storage place disposed to inject an oxygenate into
the exhaust stream; a nitrogen oxide reduction catalyst disposed to
receive the exhaust stream, fuel, and the oxygenate; a sensor
disposed to sense a system parameter related to carbon loading of
the catalyst, and produce a signal; and a controller disposed to
receive the signal, compare the signal with predetermined points,
estimate the carbon loading, and increase a ratio of oxygenate to
fuel in the exhaust stream, for a calculated duration of time,
wherein the system parameter is a post-catalyst NOx concentration,
a space velocity of the catalyst, temperature of the catalyst,
temperature of the exhaust stream, a flow rate of the exhaust
stream, or combinations thereof.
22. The system of claim 21, wherein the first storage place
comprises a fuel comprising diesel, biodiesel, ultra low sulphur
diesel, Fischer-Tropsch fuel, kerosene or any combinations
thereof.
23. The system of claim 21, wherein the second storage place
comprises an oxygenate comprising ethanol, methanol, isopropyl
alcohol, n-propanol, n-butanol, methyl tert-butyl ether, gasoline,
E85, or any combinations thereof.
24. The system of claim 21, wherein the ratio of oxygenate to fuel
is greater than 3.
25. An exhaust treatment system comprising: an exhaust source; a
reductant source disposed to inject a reductant stream into an
exhaust stream from the exhaust source, the reductant source
comprising a first reductant and second reductant; a nitrogen oxide
(NOx) reduction catalyst disposed to receive the exhaust stream and
the reductant stream; a sensor disposed to sense a system parameter
related to carbon loading of the catalyst, and produce a signal
corresponding to the system parameter; and a controller disposed to
receive the signal and to control temperature of the exhaust stream
and dosing of the reductant stream based at least in part on the
signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to co-pending US Patent
Application, Docket Number 242310-2, Ser. No., ______, entitled
"EXHAUST TREATMENT SYSTEM AND METHOD OF OPERATION" filed
contemporaneously herewith, which application is hereby
incorporated by reference.
BACKGROUND
[0002] The invention relates generally to an exhaust treatment
system and method of operating the exhaust treatment system.
[0003] Exhaust streams generated by the combustion of fossil fuels
in, for example, furnaces, ovens, and engines, contain nitrogen
oxides (NOx) that are undesirable pollutants. There is a growing
need to have efficient and robust exhaust treatment systems to
treat the NOx emissions.
[0004] In selective catalytic reduction (SCR) using hydrocarbons
(HC), hydrocarbons serve as the reductants for NOx conversion.
Hydrocarbons employed for HC-SCR include relatively small molecules
like methane, ethane, ethylene, propane and propylene as well as
longer linear hydrocarbons like hexane, octane, etc. or branched
hydrocarbons like iso-octane. The injection of several types of
hydrocarbons has been explored in some heavy-duty diesel engines to
supplement the HC in the exhaust stream. From an infrastructure
point of view, it would be advantageous to employ an on-board
diesel fuel as the hydrocarbon source for HC-SCR.
[0005] The use of fuels, including gasoline or diesel fuel as SCR
reductants, leads to a number of disadvantages when trying to clean
up the exhaust gases. During the combustion process, the catalyst
may get poisoned by some part of the exhaust gas, such as sulfur
dioxide (SO.sub.2), or from the formation of base metal compounds
from the components of a catalyst composition. The NOx absorption
performance of the catalyst declines as the poisoning of the
catalyst increases. Therefore, it is desirable to have an exhaust
treatment system and method of operation that will help to mitigate
poisoning and increase the catalyst performance.
BRIEF DESCRIPTION
[0006] One embodiment is an exhaust treatment system. The exhaust
treatment system includes an exhaust source, a reductant source, a
nitrogen oxide (NOx) reduction catalyst, a sensor, and a
controller. The reductant source includes a first reductant and a
second reductant, and is disposed to inject a reductant stream into
an exhaust stream from the exhaust source. The NOx catalyst is
disposed to receive both the exhaust stream and reductant stream.
The sensor is disposed to sense a system parameter related to
carbon loading of the catalyst and produce a signal corresponding
to the system parameter. The controller is disposed to receive the
signal and to control dosing of the reductant stream based at least
in part on the signal.
[0007] Another embodiment is an exhaust treatment system. The
exhaust treatment system includes an exhaust source, a reductant
source including a first storage place and a second storage place,
a nitrogen oxide (NOx) reduction catalyst, a sensor, and a
controller. The first storage place is disposed to inject a fuel to
the exhaust source and into an exhaust stream emitted from the
exhaust source and the second storage place is disposed to inject
an oxygenate into the exhaust stream. The NOx reduction catalyst is
disposed to receive the exhaust stream, fuel, and oxygenate. The
sensor is disposed to sense a system parameter related to carbon
loading of the catalyst and produce a signal and the controller is
disposed to receive the signal, compare the signal with
predetermined points, estimate the carbon loading, and increase a
ratio of oxygenate to fuel in the exhaust stream, for a calculated
duration of time. The system parameter used herein is a
post-catalyst NOx concentration, a space velocity of the catalyst,
temperature of the catalyst, temperature of the exhaust stream, a
flow rate of the exhaust stream, or any combinations thereof.
[0008] Another embodiment is an exhaust treatment system. The
exhaust treatment system includes an exhaust source, a reductant
source, a nitrogen oxide (NOx) reduction catalyst, a sensor, and a
controller. The reductant source includes a first reductant and a
second reductant, and is disposed to inject a reductant stream into
an exhaust stream from the exhaust source. The NOx catalyst is
disposed to receive both the exhaust stream and reductant stream.
The sensor is disposed to sense a system parameter related to
carbon loading of the catalyst and produce a signal corresponding
to the system parameter. The controller is disposed to receive the
signal and to control temperature of the exhaust stream and dosing
of the reductant stream based at least in part on the signal.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a schematic diagram of an exhaust treatment system
in accordance with one embodiment of the invention;
[0010] FIG. 2 is a process map of an exhaust treatment methods in
accordance with one example of the invention.
DETAILED DESCRIPTION
[0011] The systems and methods described herein include embodiments
that relate to controlling reductant stream dosing for enhancing
the performance of an exhaust treatment system comprising a
catalyst that promotes reduction of nitrogen oxide (NOx). Such
catalysts are referred to herein as NOx catalyst.
[0012] In the following specification and the claims that follow,
the singular forms "a", "an" and "the" include plural referents
unless the context clearly dictates otherwise.
[0013] As used herein, without further qualifiers, a catalyst is a
substance that may cause a change in the rate of a chemical
reaction without itself being consumed in the reaction.
"Activation" of a catalyst relates to increasing the performance
efficiency of the catalyst at a given set of operating conditions.
"Deactivation" of a catalyst, a decrease in the performance
efficiency, may happen because of certain system and operating
conditions including temperature of operation, time of operation,
and exposure of catalyst to carbonaceous materials. "Dosing"
includes composition and amount of reductants supplied to a
reductant stream. Dosing may be changed by changing a ratio of
different reductants in the reductant stream or by changing the
flow rates of reductants of the reductant stream. Approximating
language, as used herein throughout the specification and claims,
may be applied to modify any quantitative representation that may
permissibly vary without resulting in a change in the basic
function to which it is related. Accordingly, a value modified by a
term such as "about" is not to be limited to the precise value
specified. In some instances, the approximating language may
correspond to the precision of an instrument for measuring the
value. All temperatures given herein are for atmospheric
pressure.
[0014] In one embodiment, an exhaust treatment system 10 is
provided as shown in FIG. 1. The exhaust treatment system 10
comprises an exhaust source 12, a reductant source 14, a NOx
catalyst 30, a sensor 40 and a controller 50. An exhaust treatment
system 10 reduces undesirable emissions in an exhaust stream 16
generated by the exhaust source 12, such as a combustion engine. A
combustion engine is any engine that accepts fuel, performs an
action by burning the fuel, and emits an exhaust stream. In one
embodiment, the combustion engine is an internal combustion engine
in which the combustion of a fuel occurs with an oxidizer in a
combustion chamber resulting in an expansion of the high
temperature and pressure gases that may be applied to move a
movable component of the engine. Examples of combustion engines
include gasoline engines, diesel engines, and turbine engines,
reciprocating engines, rotary engines, and any engine that produces
exhaust gases.
[0015] The internal combustion engine may be part of any of a
variety of mobile or fixed/stationary assets, for example, an
automobile, locomotive, or power generator. Different engines have
different combustion characteristics and the exhaust stream
components differ from one engine to another. Such differences may
include variations in NO.sub.x levels, presence of sulfur, oxygen
level, steam content, and the presence or quantity of other
species. Changes in the operating parameters of the engine may also
alter the exhaust flow characteristics. Examples of differing
operating parameters may include temperature and flow rate of fuel
and air. The exhaust treatment system 10 may be used to reduce
NO.sub.x to nitrogen at a desirable rate and at a desirable
temperature appropriate for the given system and operating
parameters.
[0016] A reductant source 14 supplies reductants to the exhaust
treatment system 10. Reductant source 14 may include one or more
storage places (such as tanks or compartments) for reductants or
may include one or more points of origins for continuous supply of
reductants. The reductant source 14 may be designed to supply two
or more reductants. In one embodiment, the reductant source 14 is a
combination of different storage places for the different
reductants. In one embodiment, different storage places of the
reductant source 14 may exist separately from one another within
the exhaust treatment system 10. In another embodiment, the storage
places of the reductant source 14 may co-exist, for example, where
the first and second reductants are to be stored as a mixture, in
one storage place. For the ease of description, the reductant
source 14 is described herein throughout the specification as an
example of a reductant source comprising a first reductant and
second reductant. However, the application is in no way limited in
scope to the use of only two reductants in the reductant source or
only one storage place for the different reductants.
[0017] The reductant source 14 is disposed to inject a reductant
stream 18 into the exhaust stream 16. NOx catalyst 30 is disposed
to receive the exhaust stream 16 and the reductant stream 18. The
reductant stream 18 may contain one or more reductants at any given
point of time during the operation of the exhaust treatment system.
The reductants in the reductant stream 18 may be of different kinds
that are used to reduce the exhaust gases such as NOx. In one
embodiment, reductant comprises diesel fuel, ultra low sulfur
diesel (ULSD), biodiesel fuel, Fischer-Tropsch fuel, gasoline,
ethanol, methanol, isopropyl alcohol, n-propanol, n-butanol, methyl
tert-butyl ether, E85, kerosene, or any combinations thereof.
[0018] In one embodiment, the reductant source 14 comprises a first
storage place 22 and a second storage place 24. In one embodiment,
a first storage place 22 comprises a fuel tank disposed to supply
fuel to the exhaust source 12 and to inject fuel through a
reductant stream 18 into an exhaust stream 16 emitting from the
exhaust source. Fuel may be of different kinds that are used to run
the exhaust sources 12. In one embodiment, fuel comprises a
material selected from the group consisting of diesel fuel, ultra
low sulfur diesel (ULSD), biodiesel fuel, Fischer-Tropsch fuel,
gasoline, kerosene, and any combination thereof. The second storage
place 24 comprises an oxygenate tank disposed to inject oxygenates
into the exhaust stream 16 through the reductant stream 18. In one
embodiment, the second reductant comprises an oxygenate. An
oxygenate is a component, generally comprising reductants suitable
for enhancing NOx reduction efficiency of the catalysts or for
regeneration of the catalysts. An oxygenate is a liquid organic
species containing oxygen as a part of its chemical structure.
Examples of the oxygenates that may be used as a reductant in the
present exhaust treatment system include, but not limited to
ethanol, methanol, isopropyl alcohol, n-propanol, n-butanol, methyl
tert-butyl ether, E85, gasoline, and any combinations thereof.
[0019] NOx catalyst 30 is used to reduce NOx content of an exhaust
stream 16. A NOx catalyst capable of substantially reducing NOx
through selective catalytic reduction (SCR) using hydrocarbons (HC)
is known as an HC-SCR catalyst. It is desirable to use NOx
catalysts that may influence NOx reduction across a wide range of
temperatures and operating conditions. In one embodiment, a NOx
catalyst 30 is a catalyst composition comprising a metal disposed
upon a mesoporous inorganic oxide substrate. As used herein,
without further qualifiers, "mesoporous" refers to a material
containing pores with diameters in a range of from about 2
nanometers to about 50 nanometers.
[0020] The substrate may include an inorganic material. Suitable
inorganic materials may include, for example, oxides, carbides,
nitrides, hydroxides, carbonitrides, oxynitrides, borides, or
borocarbides. In one embodiment, the inorganic oxide may have
hydroxide coatings. In one embodiment, the inorganic oxide may be a
metal oxide. The metal oxide may have a hydroxide coating. Other
suitable metal inorganics may include one or more metal carbides,
metal nitrides, metal hydroxides, metal carbonitrides, metal
oxynitrides, metal borides, or metal borocarbides. Metallic cations
used in the foregoing inorganic materials may be transition metals,
alkali metals, alkaline earth metals, rare earth metals, or the
like.
[0021] In one embodiment, the catalyst substrate includes oxide
materials. In one embodiment, the catalyst substrate includes
alumina, zirconia, silica, zeolite, or any mixtures comprising one
or more of these materials. Suitable substrate materials may
include, for example, aluminosilicates, aluminophosphates,
hexaaluminates, zirconates, titanosilicates, titanates, or a
combination of two or more thereof. In one embodiment, the metal
oxide is an aluminum oxide. In other embodiments, other substrates
may be suitable and may be selected based on end-use parameters. In
one embodiment, the composition of an HC-SCR NOx catalyst includes
a templated metal oxide substrate having a plurality of pores, and
a catalyst material comprising a catalyst metal disposed on the
substrate, as described in US Patent Application 20090074641A1.
[0022] Suitable catalyst metal may include one or more of gallium,
indium, rhodium, palladium, ruthenium, and iridium. Other suitable
catalyst metal includes transition metal elements. Suitable
catalyst metal also includes one or more of platinum, gold, and
silver. In one embodiment, the catalyst metal comprises silver. In
one particular embodiment, the catalyst metal is substantially 100%
silver.
[0023] Typically, soot, sulfur-containing compounds, and unreacted
hydrocarbons adsorb on the surface of the catalyst during operation
of the exhaust source 12. The adsorbed species block the active
surface of the NOx catalyst 30 from the exhaust stream 16, thereby
reducing the efficiency of the NOx catalyst 30. Measuring
efficiency reduction of the NOx catalyst 30 at certain points and
taking actions to improve efficiency of the catalysts may enhance
the catalyst performance over a period. At some point, the
efficiency of the NOx catalyst 30 may be reduced to a point that
the pollutant gases, such as NO.sub.x, are not sufficiently removed
from the exhaust stream to meet predetermined exhaust control
specifications. A periodic or need-based reactivation of the NOx
catalyst 30 may return the NOx content emitted from the exhaust
treatment system 10 to an acceptable level.
[0024] In one embodiment, diesel fuel is a convenient reductant for
reducing NOx from a diesel engine exhaust, because diesel is
readily available as a fuel and in a diesel-engine-powered system,
such as a locomotive, the diesel fuel is already stored on board.
However, other reductants, such as ethanol, are sometimes more
active than diesel for SCR of NOx. For example, NOx reduction on a
silver-templated alumina catalyst is higher in the presence of
ethanol than in the presence of diesel at some operating
conditions. In an example where diesel is used as a reductant for
NOx, more active reductants such as ethanol may be used in
combination with or instead of diesel to boost the NOx conversion
in operating conditions where diesel may not meet desired NOx
conversion or in situations where catalyst activity has degraded
over time. However, using ethanol at all times as a sole reductant
instead of diesel would, for example, inconveniently increase the
tank size of ethanol to be carried on board a vehicle employing the
exhaust treatment system.
[0025] Two or more reductants may be used to optimize the NOx
conversion over a NOx catalyst 30. For example, a reductant that
gives good NOx conversion at a comparatively low temperature may be
used in combination with another reductant that gives good NOx
conversion at comparatively high temperature to extend the
operating range of the SCR catalyst. In one embodiment, ethanol is
more active as a reductant than diesel at lower operating
temperatures of the engine exhaust. A controller may be implemented
to preferentially inject more oxygenate (such as ethanol in the
current example) in situations where higher oxygenate content in
the reductant stream 18 would improve the performance of the system
10.
[0026] It is desirable to enhance reduction efficiency of the NOx
catalyst without undue increase of weighted fuel penalty of the
system. In an embodiment where the first reductant is engine fuel
and second reductant is an oxygenate, fuel penalty is determined by
measuring quantity of fuels and oxygenates used, and the relative
value of the fuels and oxygenates. Weighted fuel penalty value is
determined by relative price and the size constraints and refilling
frequency of the oxygenate fuel tank. For example, it may be
possible to operate a controller to minimize NOx by adjusting the
reductant flow rates, and adjusting the oxygenate/fuel ratio.
However, doing so may lead to unnecessary oxygenate expenditure,
thereby increasing requirement of the tank volume of oxygenate and
thereby weighted fuel penalty. Therefore, striking a balance
between the reduction efficiency increment of the NOx catalyst and
the weighted fuel penalty is desirable.
[0027] The second reductant can be used to enhance catalyst
efficiency through two methods, named herein as catalyst
reactivation and catalyst regeneration. In catalyst reactivation,
the presence of second reductant facilitates the NOx reduction of
the catalyst, thereby making the catalyst more active for NOx
reduction at certain system and operating conditions.
[0028] For example, during the engine start-up time, the
temperature of the exhaust stream and/or the catalyst may be less
than the optimum temperature required for the NOx catalyst to
effectively reduce NOx using only diesel (first reductant). In such
case, ethanol, or some other oxygenate (second reductant) that is
more active as reductant than diesel at lower temperature ranges,
may be injected as the sole reductant or in a mixture with the
diesel. As the catalyst warms up, the amount of oxygenate used may
be decreased while increasing the amount of diesel reductant. A
sensor 40 may be employed to measure the temperature and a
controller 50 may be used to compare the temperature with an
available data set, to analyze the amount of oxygenate required to
maintain the required NOx reduction, and to inject that amount of
oxygenate to the diesel reductant stream.
[0029] Similarly, two or more reductants may be used to optimize
the NOx conversion over the catalyst's lifetime. A more active
second reductant may be used to boost the NOx conversion on a NOx
catalyst 30 that typically uses a less active first reductant, as
the catalyst loses activity over its operating lifetime. For
example, the NOx catalyst 30 performance may decrease due to aging,
and in such cases, continuous injection of a measured amount of
more active second reductant may help in keeping the NOx reduction
in the required levels. For example, the use of oxygenate
reductant, such as ethanol, may be increased over the lifetime of
the catalyst as the catalyst activity degrades, to boost the NOx
conversion across part of or all of the operating range as
necessary to meet the requirements for NOx reduction.
[0030] In catalyst regeneration, the second reductant restores the
NOx reduction efficiency of the catalyst thereby making the
catalyst substantially regain its original catalytic activity.
Therefore, regeneration of catalyst is a revival of the catalyst to
perform to a predetermined level at a given set of operating
conditions by, for instance, removing undesirable deposits from the
catalyst. In one embodiment, the regeneration restores greater than
about 80% of the initial performance of the catalyst at similar
operating conditions. For example, if a SCR catalyst is reducing
NOx using diesel as a reductant, and the efficiency of NOx
reduction of the catalyst reduces over time, regenerating the
catalyst will increase the catalytic efficiency to about 80% of its
original efficiency or greater while using engine fuel as the
reductant. In a further embodiment, the catalyst performance after
regeneration is greater than about 90% of the initial performance
at similar operating conditions. By regeneration, in one
embodiment, the catalyst performance is restored to the initial
level at similar operating conditions.
[0031] In one embodiment, restoration of catalyst reduction
efficiency by the second reductant is through reaction of the
reductant with the deposits such as carbon and burning off the
deposits from the catalyst surface.
[0032] The second reductant may be a single reductant or a mixture
of reductants formulated based on factors such as, but not limited
to, reduction efficiency, economic advantage, and environmental
effects. In one example, hydrogen is used as a co-reductant with
ethanol and/or diesel to improve NOx conversion at low
temperatures.
[0033] During exhaust treatment, different applications may demand
different levels of catalyst performance. Further, measurement of
catalyst performance during operation at certain operating
conditions may not be straightforward. Catalyst performance at any
point of time may depend on a combination of different factors,
including, but not limited to, age of the catalyst, temperature of
exhaust stream, product of the exhaust, and /or volume of the
exhaust. For example, a system may have 100 ppm NOx as the catalyst
output, with 150 ppm at the catalyst inlet, translating to about
33% NOx conversion. This conversion may be termed as an efficient
performance in some conditions, such as where the exhaust
temperature is about 250.degree. C., and the reductant is a ULSD at
a carbon to nitrogen ratio (C:N) of 1:1. However, the same
performance may be termed as unsatisfactory under other conditions,
such as where if the temperature of the exhaust stream is at about
375.degree. C. and the reductant dosing comprises a C:N of 6:1.
[0034] In general, there is competition for consumption of the
reductant(s) by both direct oxidation (combustion) and through
reduction of NOx. Additionally, there is a tradeoff for higher C:N
ratios between increased availability of reductant for NOx
reduction and increased carbon deposits on the catalyst. At lower
temperatures (<350.degree. C.), the rate of direct oxidation
decreases faster than the rate of the consumption by the reduction
of NOx. The rate of reductant desorption decreases at lower
temperatures, causing the catalyst surface coverage by the
reductant to increase. Therefore, at lower temperatures, lower
carbon to NOx ratios generally achieve optimal performance. At
higher temperatures (>350.degree. C.) the rate of reductant
direct oxidation increases, and the rate of reductant desorption
from the catalyst increases. Therefore, at higher temperatures,
higher carbon to NOx ratios generally achieve optimal
performance.
[0035] Therefore, it is desirable to have a "trigger point"
triggering a reactivation or regeneration when a predicted NOx
output from the catalyst under given operating conditions
approaches a predetermined value (within some tolerance) such as,
for example, a regulation limit.
[0036] In one embodiment, a sensor 40 is disposed to sense a system
parameter of the exhaust treatment system 10 and to produce a
signal 42 corresponding to the system parameter. A controller 50 is
disposed to receive the signal 42 and to control dosing of the
reductant stream 18 based at least in part on the signal 42.
However, trigger points for controller actions may be designed
based on data obtained before and/or during operation of the
exhaust source.
[0037] A system parameter is any parameter that affects the quality
of treated exhaust 60 coming out from the exhaust treatment system
10 after the NOx catalyst 30 reduction treatment. A system
parameter may be an in-situ parameter determined during operation
of the exhaust source 12 and/or a pre-determined parameter
determined based on the laboratory tests. System parameters may
include, for example, exhaust stream 18 composition parameters,
exhaust stream 18 temperature parameters, exhaust stream flow
parameters, exhaust source parameters, and time parameters.
Examples of an exhaust stream composition parameter include
concentration of NO.sub.x in the exhaust stream, and space velocity
of the catalyst. The concentration of NOx in the exhaust stream may
be a pre-catalyst NOx concentration or a post-catalyst NOx
concentration. In one particular embodiment, a post--catalyst
concentration of NOx, that is, the NOx concentration of the gases
down-stream of the catalyst, is used as a system parameter. Space
velocity is herein defined as the normalized ratio of exhaust flow
rate to the volume of the catalyst. In one embodiment, a
predetermined NOx concentration may vary in the range from about 1
ppmV to about 2000 ppmV. In one embodiment, a catalyst space
velocity is in the range from about 1000 hr.sup.-1 to about 200,000
hr.sup.-1. Examples of an exhaust stream 18 temperature parameter
include temperature of the NOx catalyst, and temperature of the
exhaust stream. In one embodiment, temperature of the NOx catalyst
can be increased using a heater 70. In one embodiment, the exhaust
stream 18 temperature parameter is the temperature of the exhaust
stream. The NOx catalyst 30 may get influenced by temperature of
the exhaust stream and therefore change its NOx reduction
characteristics based on the temperature of the exhaust stream. In
one embodiment, the exhaust temperature is between about
200.degree. C. and about 650.degree. C. Examples of an exhaust
stream flow parameter include respective flow rates of the exhaust
stream, reductant stream, and air stream. The flow rates may be
measured as molar, mass, or volumetric flow rates. In one
embodiment, ratio of a molar flow rate of carbon in the reductant
stream to the molar flow rate of nitrogen in NOx in the exhaust
stream is between about 0 and about 12. Examples of the exhaust
source parameter include source speed, torque, and source
power.
[0038] One measure of catalyst performance at a given point of time
at certain operating conditions is the deposit of carbonaceous
materials on the NOx catalyst. During operation of the exhaust
source, carbonaceous materials such as carbon are deposited on the
catalyst. The amount of carbonaceous material deposited (also
called "carbon loading") may be estimated by measuring some or all
of the system parameters and correlating these parameters and
carbon loading with data previously developed under controlled
conditions, as in a laboratory, for example. A time average
estimated carbon loading may predict a catalyst deactivation or
decrease in catalyst efficiency. In one embodiment, carbon loading
of the catalyst is a measure of catalyst deactivation. A dosing
strategy that achieves desired conversion of NOx with minimum
weighted fuel penalty based on the operating conditions may be
estimated and controlled using the controller.
[0039] Some data relating to carbon loading of a catalyst may be
obtained in controlled tests. The exhaust conditions may be
simulated in the laboratory and the catalyst performance may be
documented at different operating conditions. Trigger points may be
formulated based on the analysis of carbon loading at different
conditions in the laboratory tests. Information obtained from
sensors about the system parameters during operation of the exhaust
source may aid to judge the operating conditions and carbon loading
and thereby catalyst performance and deactivation.
[0040] Different dosing strategies can be applied to the reductant
stream to increase the catalyst activation. The dosing strategy
includes changing one or more of first reductant dosing rate,
second reductant dosing rate, ratio of second reductant to first
reductant, first reductant flow rate, and second reductant flow
rate. In one embodiment, once the controller receives the system
parameter signals, the controller determines which dosing strategy
yields the desired conversion at the minimum weighted fuel penalty
for each set of operating conditions such as space velocity,
temperature, oxygen concentration, and carbon loading.
[0041] A periodic or need-based reactivation of the NOx catalyst 30
during operation may be realized by following different methods.
One example of a method to estimate the NOx catalyst 30 performance
is to maintain reference databases and use the measured system
parameters during operation for comparison. For example, a
reference database relating various system parameters and the
carbon loading at those conditions may be maintained. Another
reference database relating the reactivation effects of different
second reductants on the NOx catalyst 30 may be maintained. The
system parameters during service are measured and used as inputs to
estimate carbon loading and to determine the appropriate dosing
strategy by comparing measured data with the reference
databases.
[0042] In one embodiment, the catalyst may be regenerated by
increasing temperature of the catalyst to a level that is
sufficient to desorb or oxidize carbonaceous materials deposited
over the catalyst 30. Temperature of the catalyst may be increased,
for example, by directly heating the catalyst 30 or by increasing
temperature of the gases coming into contact with the catalyst 30.
In one embodiment, a heater 70 is employed to heat catalyst 30
and/or the gases. Temperature may be increased for a predetermined
amount of time, to clean the surface of the catalyst. Examples of
temperatures at which the HC-SCR catalyst may be regenerated
include, but are not limited to, between approximately 400.degree.
C. to 600.degree. C., 425.degree. C. to 500.degree. C., and
440.degree. C. to 460.degree. C. In one embodiment, the catalyst is
regenerated by operating at changed dosing strategy and increased
temperature simultaneously for a predetermined amount of time. For
example, the dosing may be controlled to be about 3:1 ratio of
oxygenate to fuel while the temperature experienced by the catalyst
is in the range of about 400.degree. C. to about 600.degree. C.
This may enhance oxidation of carbonaceous materials from the
catalyst surface and thereby realize an effective regeneration.
[0043] By regenerating the catalyst periodically, or whenever
catalyst activity is reduced below a certain required level, in
accordance with the embodiments described herein, the overall
lifetime and performance of the catalyst may be improved.
[0044] In one embodiment, a method of treating exhaust is provided.
The method comprises producing an exhaust stream 16 from an exhaust
source 12 and injecting a reductant stream 18 from a reductant
source 14 to the exhaust stream 18. The reductant source 14
comprises a first reductant and second reductant. As described
earlier, the reductant source 14 may be a combination of different
storage places for the different reductants. Reductant source 14
may include one or more storage places (tanks) for reductants or
may include one or more points of origins for continuous supply of
reductants. The reductant source 14 may be designed to supply two
or more reductants. In one embodiment, different storage places of
the reductant source 14 may exist separately from one another
within the exhaust treatment system 10. In another embodiment, the
storage places of the reductant source 14 may co-exist, for
example, where the first and second reductants are to be stored as
a mixture, in one storage place. A nitrogen oxide (NOx) reduction
catalyst 30 is disposed to receive the exhaust stream 16 and the
reductant stream 18. A sensor 40 is operated to sense a system
parameter related to carbon loading of the catalyst 30 producing a
signal 42 corresponding to the system parameter and sending the
signal 42 to a controller 50. The controller 50 controls a dosing
of the reductant stream 18 based at least in part on the signal
42.
[0045] In one embodiment, the sensor 40 is operated to assess a
system parameter, and a controller 50 is operated to receive and
analyze signals 42 coming from the sensor 40 and reduce NOx in the
exhaust by adjusting dosing of the reductant stream 18. The sensor
40 may communicate with the exhaust source 12, reductant source 14,
NOx catalyst 30, post NOx treatment exhaust gases 60, and/or
controller 50.
[0046] In one embodiment, the controller 50 controls the dosing by
changing a ratio of second reductant to first reductant. Normally,
in an embodiment including a fuel as a first reductant and an
oxygenate as a second reductant, the ratio of second reductant to
first reductant in the reductant stream is increased to decrease
the carbon loading on the catalyst. In one embodiment, the
controller 50 causes a ratio of the second reductant to the first
reductant in the reductant stream to be greater than 0.5 when an
estimated carbon loading exceeds a predetermined value. In another
embodiment, controller 50 causes a ratio of the second reductant to
the first reductant in the reductant stream to be greater than 3
when an estimated carbon loading exceeds a predetermined value.
[0047] In one embodiment, the carbon loading is estimated by the
controller 50 based on system parameter information received from
the sensor 40 and using system parameter information as input to
estimate the corresponding carbon loading and required dosing
strategy by using preloaded correlations in the form of transfer
functions or look-up tables. In one embodiment, the carbon loading
is estimated by analyzing the signal corresponding to a system
parameter comprising an exhaust stream composition parameter, an
exhaust stream temperature parameter, an exhaust stream flow
parameter, an exhaust source parameter, a time parameter, or any
combination of these parameters.
[0048] Regeneration is a temporary event when compared to
reactivation, and is used to "recover" the catalyst to enable the
catalyst to function more efficiently compared to pre-regeneration
state of the catalyst. In regeneration, the second reductant
promotes the burning off of the deposits on the catalyst and clears
the catalyst areas for reaction with the exhaust gases. In one
embodiment, the regeneration is carried out by passing a
significant amount of second reductant for a certain duration of
time, so that the reaction between the second reductant and the
carbon loading leads to carbon burning off. In a further
embodiment, second reductant is solely used for an estimated time.
During this time, the second reductant functions as a reductant as
well as a chemical regenerator. In one embodiment, a regeneration
is carried out for a fixed point of time. In one embodiment,
calculated duration of time for regeneration is in the range from
about 10 minutes to about 60 minutes.
[0049] The decision whether to reactivate or regenerate a catalyst
at any given situation may depend on certain conditions. Weighted
fuel penalty is one of the factors to be considered for deciding on
reactivation or regeneration. The second reductant used for the
reactivation may be the same as or different from the second
reductant used for regeneration. If same reductant is used as a
second reductant for reactivation as well as regeneration, the
amount of fuel used over a certain period of time may be a factor
to consider. If the reductants used are different in each case,
then the cost of second reductant along with the tank capacity be
compared for deciding one method against another. The factors such
as catalyst type and carbon loading of the catalyst may also be
considered in choosing between reactivation or regeneration of the
catalyst at a given point of time.
[0050] A reactivation of the catalyst may be carried out in an
intermittent manner or in a continuous manner. In a continuous
manner, a small amount of second reductant or oxygenate may be
supplied along with the first reductant or fuel throughout the
operation of the exhaust source. Ratio of the reductants or flow
rates of the reductants may be varied such that desired NOx
conversions are realized at different points of time during
operation.
[0051] In one embodiment, the controller 50 changes dosing for the
reductant stream when an estimated carbon loading is greater than a
predetermined value of about 1 weight percent of the catalyst. In
one embodiment, the controller 50 changes dosing for the reductant
stream when an estimated carbon loading is greater than a
predetermined value of about 5 weight percent of the catalyst. In
one embodiment, when the carbon loading is in the range of about 1
to about 5 weight percent, the controller 50 controls the dosing to
reactivate the catalyst. In one embodiment, when the carbon loading
is greater than 5 weight percent of the catalyst, the controller 50
controls the dosing to regenerate the catalyst. The dosing may be
achieved by varying the ratio of second reductant and first
reductant through changing the flow rate of first reductant and/or
second reductant.
[0052] FIG. 2 shows an example process map of an exhaust treatment
employing the sensor 40 and controller 50. The system parameters
(operating conditions) are monitored either continuously or
periodically to asses the carbon loading on the catalyst 30, in
anticipation of a trigger signal. When the controller 50 receives a
trigger signal 42, the controller calculates the optimal dosing
strategy. Based on the dosing strategy and predetermined parameter
relationships, the controller decides whether a reactivation
process is sufficient to reactivate the catalyst and achieve
required NOx reduction performance out of it, or a regeneration is
necessary. In one embodiment, the decision about using reactivation
or regeneration is based on the estimated carbon loading on the
catalyst 30. In one embodiment, if the carbon loading is more than
about 5 wt % of the catalyst 30, a regeneration is selected by the
controller 50 over a reactivation. If the reactivation is found to
be sufficient, the controller 50 initiates an elected dosing
strategy for reactivation and the system parameters are monitored
by the sensors to assess carbon loading. If a regeneration is found
to be preferred for the effective performance of the catalyst, the
controller 50 initiates an elected dosing strategy for regeneration
and the system parameters are monitored by the sensors to assess
carbon loading. In one embodiment, the regeneration may be
terminated by withdrawing the elected dosing strategy for
regeneration. Termination may be based on the time passed during
regeneration or based on an estimated decrease in carbon loading.
The regeneration time and the dosing levels are calculated and
administered by the controller 50 while the sensor 40 continues to
monitor system parameters to give feedback about catalyst
performance to the controller 50.
[0053] In one embodiment, temperature of the exhaust stream 16 or a
combination of the exhaust stream 16 and reductant stream 18 is
controlled by the controller 50 by using a heater 70. The carbon
loading of the catalyst 30 is expected to decrease and the catalyst
regeneration is expected to increase by increasing the temperature
of the catalyst environment. Depending on the catalyst involved and
its temperature zone of optimum activity, reactivation of catalyst
also may be helped by the temperature increment. In one embodiment,
temperature of an exhaust stream 16 flowing to the catalyst 30 is
controlled to be in the range of about 450.degree. C. to about
650.degree. C. In one embodiment, controller 50 activates the
heater and/or dosing of reductant stream 18, depending on the
carbon loading on the catalyst 30 and/or calculated weighted fuel
penalty for the reactivation or regeneration of the catalyst
30.
[0054] In one embodiment, temperature of the exhaust stream 16 is
increased to greater than about 400.degree. C., along with
controlling the dosing. The changed dosing and/or increase in
temperature may be in effect for a predetermined duration of time.
The time duration may also vary depending on the temperature of the
exhaust stream or catalyst and designed dosage of the reductant
stream. For example, in one embodiment using silver on mesoporous
alumina as the NOx catalyst, if the increment in temperature is
greater than about 550.degree. C., then time duration for the
regeneration is less than about 30 minutes. Similarly, if ratio of
the oxygenate to fuel is greater than 5, time duration required for
the regeneration is typically less than about 30 minutes.
[0055] One technical advantage of this invention over using only a
highly active reductant, such as ethanol, is that less reductant
may be required because the engine fuel such as diesel, already on
board, may be used at higher exhaust temperatures. This system may
also have advantages over a system that only uses engine fuel,
because it may achieve higher NOx conversions at lower and middle
temperatures by injecting less diesel and more ethanol. Further, as
the catalyst 30 loses activity over time, the NOx conversion may be
boosted by injecting a greater proportion of ethanol.
[0056] The embodiments described herein are examples of
composition, system, and methods having elements corresponding to
the elements of the invention recited in the claims. This written
description may enable those of ordinary skill in the art to make
and use embodiments having alternative elements that likewise
correspond to the elements of the invention recited in the claims.
The scope of the invention thus includes composition, system and
methods that do not differ from the literal language of the claims,
and further includes other compositions and articles with
insubstantial differences from the literal language of the claims.
While only certain features and embodiments have been illustrated
and described herein, many modifications and changes may occur to
one of ordinary skill in the relevant art. The appended claims
cover all such modifications and changes.
* * * * *