U.S. patent application number 10/660264 was filed with the patent office on 2004-03-11 for nox reduction using zeolite catalysts.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Brusasco, Raymond M., Merritt, Bernard T., Penetrante, Bernardino M., Vogtlin, George E..
Application Number | 20040045285 10/660264 |
Document ID | / |
Family ID | 30444247 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040045285 |
Kind Code |
A1 |
Penetrante, Bernardino M. ;
et al. |
March 11, 2004 |
NOx reduction using zeolite catalysts
Abstract
A high-surface-area (greater than 600 m.sup.2/g), large-pore
(pore size greater than 6.5 angstroms), basic zeolite having a
structure such as an alkali metal cation-exchanged Y-zeolite is
employed to convert NO.sub.x contained in an oxygen-rich exhaust to
N.sub.2 and ON.sub.2. Preferably, the invention relates to a
two-stage method and apparatus for NO.sub.x reduction in an
oxygen-rich engine exhaust that includes a plasma oxidative stage
and a selective reduction stage. The first stage employs a
non-thermal plasma treatment of NO.sub.x gases in an oxygen-rich
exhaust and is intended to convert NO to NO.sub.2 in the presence
of O.sub.2 and added hydrocarbons. The second stage employs a
lean-NO.sub.x catalyst including the basic zeolite at relatively
low temperatures to convert such NO.sub.2 to environmentally benign
gases that include N.sub.2, CO.sub.2, and H.sub.2O.
Inventors: |
Penetrante, Bernardino M.;
(San Ramon, CA) ; Brusasco, Raymond M.;
(Livermore, CA) ; Merritt, Bernard T.; (Livermore,
CA) ; Vogtlin, George E.; (Fremont, CA) |
Correspondence
Address: |
Alan H. Thompson
Assistant Laboratory Counsel
Lawrence Livermore National Laboratory
P.O. Box 808, L-703
Livermore
CA
94551
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
30444247 |
Appl. No.: |
10/660264 |
Filed: |
September 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10660264 |
Sep 10, 2003 |
|
|
|
09478576 |
Jan 6, 2000 |
|
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Current U.S.
Class: |
60/286 ; 422/171;
422/177; 60/295; 60/301 |
Current CPC
Class: |
B01D 53/32 20130101;
B01D 2255/2022 20130101; B01D 53/9418 20130101; Y02A 50/2325
20180101; Y02T 10/24 20130101; B01D 53/9472 20130101; B01D 2251/208
20130101; B01D 2255/50 20130101; Y10S 423/10 20130101; B01D
2255/2027 20130101; B01D 2258/012 20130101; Y02T 10/12 20130101;
Y02A 50/20 20180101 |
Class at
Publication: |
060/286 ;
422/177; 422/171; 060/295; 060/301 |
International
Class: |
B01D 053/34; F01N
003/10; F01N 003/00; B01D 050/00; B32B 027/02 |
Goverment Interests
[0002] The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
Claims
We claim:
1. An apparatus comprising a catalytic converter, said apparatus
comprising: an oxygen rich engine-exhaust gas inlet; a diesel fuel
inlet; and a reductive stage convert of NO.sub.x connected to
receive a mixture of NO.sub.x from the engine-exhaust gas inlet and
diesel fuel from the diesel fuel inlet, said convert comprising an
alkali metal cation-exchanged faujasite-type zeolite catalyst that
further serves to convert NO.sub.x to gases that include N.sub.2,
CO.sub.2, and H.sub.2O.
2. The apparatus of claim 1 further comprising a plasma converter
upstream of said catalyst capable of converting at least a portion
of said NO.sub.x to NO.sub.2.
3. The apparatus of claim 1 wherein said zeolite comprises an
X-zeolite or Y-zeolite.
4. The apparatus of claim 1 wherein said zeolite wherein said
zeolite comprises a pore volume above about 0.20 ml/gram.
5. The apparatus of claim 1 wherein said zeolite comprises a pore
volume above about 0.30 ml/gram.
6. The apparatus of claim 1 wherein said zeolite comprises a pore
size greater than about 6.5 angstroms.
7. The apparatus of claim 1 wherein said zeolite comprises a
silicon/aluminum ratio in the range of about 1 to about 3.
8. The apparatus of claim 1 wherein said zeolite comprises a
silicon/aluminum ratio in the range of about 1 to about 3.
9. A vehicle with reduced NO.sub.x engine exhaust emissions,
comprising: a fuel supply of diesel fuel; an internal combustion
engine operably connected to receive a major portion of said fuel
supply of diesel fuel and to propel a vehicle, and having an
oxygen-rich exhaust comprising NO.sub.x; a first reactor operably
connected to receive pulsed inletted minor portions of said fuel
supply of diesel fuel, said first reactor comprising a catalyst
that further comprises an alkali metal cation-exchanged
faujasite-type zeolite for NO.sub.x reduction gas treatment and
wherein said first reactor is further operably connected to receive
said oxygen-rich exhaust comprising NO.sub.x, and operably
connected to output therefrom a product comprising N.sub.2 that has
been converted from said NO.sub.x and noncombusted hydrocarbons
from said diesel fuel, and a second reactor for collection and
combustion of said noncombusted hydrocarbons connected to receive
said product of the first reactor with said NO.sub.x and connected
to receive said noncombusted hydrocarbons, and operably connected
to output a second exhaust with reduced NO.sub.x emissions.
10. The vehicle of claim 9 wherein said zeolite comprises an
X-zeolite or Y-zeolite.
11. The vehicle of claim 9 wherein said first reactor is adapted to
receive said minor portion of said fuel supply in an amount less
than 10% of said fuel supply of a diesel fuel requirement that
initially produces said diesel engine exhaust prior to said
injecting.
12. The vehicle of claim 11 wherein said minor portion of said fuel
supply of diesel fuel comprises less than 5% of said fuel supply of
diesel fuel.
13. The vehicle of claim 9 further comprising a plasma converter
upstream of said catalyst capable of converting at least a portion
of said NO.sub.x to NO.sub.2.
14. The vehicle of claim 9 wherein said zeolite comprises a pore
volume above about 0.20 ml/gram.
15. The vehicle of claim 9 wherein said zeolite comprises a pore
volume above about 0.30 ml/gram.
16. The vehicle of claim 9 wherein said zeolite comprises a pore
size greater than about 6.5 angstroms.
17. The vehicle of claim 9 wherein said zeolite comprises a
silicon/aluminum ratio in the range of about 1 to about 3.
18. The apparatus of claim 9 wherein said zeolite comprises a
silicon/aluminum ratio in the range of about 1 to about 3.
Description
RELATED APPLICATION
[0001] This application is a division of U.S. application Ser. No.
09/478,576, filed Jan. 6, 2000, entitled "NO.sub.x Reduction Using
Zeolite Catalysts."
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to NO.sub.x reduction, more
particularly to reduction of NO.sub.x by selective catalytic
reduction technology including plasma-assisted catalytic reduction
(PACR) technology, and more particularly to systems for chemically
reducing NO.sub.x to N.sub.2 and other benign gases in oxygen-rich
environments.
[0005] 2. Description of Related Art
[0006] The control of NO.sub.x emissions from vehicles is a
worldwide environmental problem. Gasoline engine vehicles can use
newly developed three-way catalysts (i.e., reactors) to control
such emissions, because their exhaust gases lack oxygen. But
so-called "lean-burn" gas engines, and diesel engines too, have so
much oxygen in their exhausts that conventional catalytic systems
are effectively disabled. Lean-burn, high air-to-fuel ratio,
engines are certain to become more important in meeting the
mandated fuel economy requirements of next-generation vehicles.
Fuel economy is improved since operating an engine
stoichiometrically lean improves the combustion efficiency and
power output. But excessive oxygen in lean-burn engine exhausts can
inhibit NO.sub.x removal in conventional three-way catalytic
converters. An effective and durable catalyst for controlling
NO.sub.x emissions under net oxidizing conditions is also critical
for diesel engines.
[0007] Catalysts that promote the reduction of NO.sub.x under
oxygen-rich conditions are generally known as lean-NO.sub.x
catalysts. Difficulty has been encountered in finding lean-NO.sub.x
catalysts that have the activity, durability, and temperature
window required to effectively remove NO.sub.x from the exhaust of
lean-burn engines. Prior art lean-NO.sub.x catalysts are
hydrothermally unstable. A noticeable loss of activity occurs after
relatively little use, and even such catalysts only operate over
very limited temperature ranges.
[0008] Such catalysts that can effectively reduce NO.sub.x to
N.sub.2 in oxygen-rich environments have been the subject of
considerable research. (For instance, see, U.S. Pat. No. 5,208,205,
issued May 4, 1993, to Subramanian, et al.) One alternative is to
use catalysts that selectively reduce NO.sub.x in the presence of a
reductant, e.g., selective catalytic reduction (SCR) using ammonia
as a reductant.
[0009] However, another viable alternative that involves using
co-existing hydrocarbons in the exhaust of mobile lean-burn
gasoline or diesel engines as a reductant is a more practical,
cost-effective, and environmentally sound approach. The search for
effective and durable SCR catalysts that work with hydrocarbon
reductants in oxygen-rich environments is a high-priority issue in
emissions control and the subject of intense investigations by
automobile and catalyst companies, and universities, throughout the
world.
[0010] In the presence of hydrocarbons, catalysts that selectively
promote the reduction of NO.sub.x under oxygen-rich conditions are
known as lean-NO.sub.x catalysts, and more specifically--SCR
lean-NO.sub.x catalysts. Selective catalytic reduction is based on
the reaction of NO with hydrocarbon species activated on the
catalyst surface and the subsequent reduction of NO.sub.x to
N.sub.2. More than fifty such SCR catalysts are conventionally
known to exist. These include a wide assortment of catalysts, some
containing base metals or precious metals that provide high
activity. Unfortunately, just solving the problem of catalyst
activity in an oxygen-rich environment is not enough for practical
applications. Like most heterogeneous catalytic processes, the SCR
process is susceptible to chemical and/or thermal deactivation.
Many lean-NO.sub.x catalysts are too susceptible to high
temperatures, water vapor and sulfur poisoning (from SO.sub.x).
Catalyst deactivation is accelerated by the presence of water vapor
in the stream and water vapor suppresses the NO reduction activity
even at lower temperatures. Also, sulfate formation at active
catalyst sites and on catalyst support materials causes
deactivation. Practical lean-NO.sub.x catalysts must overcome these
problems simultaneously before they can be considered for
commercial use.
[0011] Some hydrocarbons may be better reductants or better
NO.sub.x to N.sub.2 promoters. Many lean-NO.sub.x catalysts have
been tested with propylene as the reductant. A disadvantage of such
an embodiment is that two different supplies of hydrocarbons must
be maintained aboard a diesel-powered vehicle. The preferred
embodiment is the use of fuels, such as No. 1 or 2 diesel fuels, as
reductants with the lean-NO.sub.x catalyst to reduce NO.sub.x and
concurrently provide fuel for the upstream exhaust-generating
engine. Thus, only one uncombusted source of hydrocarbons needs to
be maintained aboard the vehicle. Most of the lean-NO.sub.x
catalysts that have been shown to be efficient with propylene as
reductant are not efficient when used with the heavy hydrocarbons
present in diesel fuel. There is a great need to find a
lean-NO.sub.x catalyst that can reduce NO.sub.x efficiently using
heavy hydrocarbons similar to those present in diesel fuel.
[0012] The U.S. Federal Test Procedure for cold starting gasoline
fueled vehicles presents a big challenge for lean-NO.sub.x
catalysts due to the low-temperature operation involved. Diesel
passenger car applications are similarly challenged by the driving
cycle that simulates slow-moving traffic. Both tests require
reductions of CO, hydrocarbons, and NO.sub.x at temperatures at or
below 200.degree. C. when located in the under-floor position.
Modifications of existing catalyst oxidation technology are
successfully being used to address the problem of CO and
hydrocarbon emissions, but a need still exists for improved
NO.sub.x removal.
SUMMARY OF THE INVENTION
[0013] The present invention provides a method for reducing
NO.sub.x emissions and a vehicle with reduced NO.sub.x emissions.
The present invention also provides a system for attachment to an
engine with an oxygen rich exhaust for the reduction of NO.sub.x
emissions.
[0014] Briefly, in a lean NO.sub.x selective catalytic reduction
system of the present invention, NO.sub.x (usually in the form of
NO and preferably NO.sub.2) is reacted on a high-surface-area,
large-pore, basic catalyst, such as an alkali metal-exchanged
X-zeolite or Y-zeolite, and converted to environmentally benign
products. The invention preferably comprises a non-thermal plasma
gas treatment of exhaust NO to produce NO.sub.2 which is then
combined with the selective catalytic reduction treatment, e.g., a
SCR lean NO.sub.x catalyst, to enhance NO.sub.x reduction in
oxygen-rich vehicle engine exhausts. An engine controller can
continually or periodically run brief fuel-rich conditions that
provide hydrocarbon reductants for a reaction that catalyzes the
NO.sub.2 (produced by a plasma) into benign products such as
N.sub.2. By using a plasma, the SCR lean NO.sub.x catalyst may
contain less or essentially no precious metals, such as Pt, Pd and
Rh, for reduction of the NO.sub.2 to N.sub.2.
[0015] Accordingly, an advantage of the present invention is that a
method for NO.sub.x emission reduction is provided that is
inexpensive and efficient. The plasma-assisted lean-NO.sub.x/basic
zeolite catalyst system can not only remove the dependence on
precious metal lean-NO.sub.x catalysts, but allows for relatively
more efficient compliance with NO.sub.x emission reduction
laws.
[0016] Furthermore, not only does the plasma-assisted lean
NO.sub.x/basic zeolite catalyzed process improve the activity,
durability, and temperature window of SCR/lean-NO.sub.x catalysis,
but it also allows the combustion of fuels containing relatively
high sulfur contents with a concomitant reduction of NO.sub.x,
particularly in an oxygen-rich vehicular environment. The present
invention allows the use of a lean NO.sub.x catalyst to reduce
NO.sub.x emissions in engine exhausts containing relatively high
concentrations of sulfur, such as greater than 20 ppmw sulfur
(calculated as S).
[0017] Still another advantage of the present invention is that an
efficient method for NO.sub.x emissions reduction at relatively low
temperatures is provided using heavy hydrocarbons as the
reductant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a block diagram of a vehicle embodiment of the
invention.
[0019] FIG. 2 is a flow chart of the method of the invention.
[0020] FIG. 3 is a cross sectional diagram representing a NO.sub.x
reduction unit of the invention.
[0021] FIG. 4 is a bar graph illustrating the percentage of
NO.sub.x reduction in a diesel engine exhaust by catalysts
containing gamma-alumina, sodium Y-zeolite, calcium Y-zeolite or
hydrogen Y-zeolite and hydrocarbon addition to the exhaust at 260
degrees C., with and without plasma assistance.
[0022] FIG. 5 is a bar graph illustrating the percentage of
NO.sub.x reduction in a diesel engine exhaust by catalysts (i.e.,
reactors) containing gamma-alumina, sodium Y-zeolite, sodium
beta-zeolite or sodium ZSM5-zeolite and hydrocarbon addition to the
exhaust at 260 degrees C., with and without plasma assistance.
[0023] FIG. 6 is a bar graph illustrating the percentage of
NO.sub.x reduction in a diesel engine exhaust by catalysts
containing gamma-alumina or sodium Y-zeolite and hydrocarbon
addition to the exhaust at 200 degrees C., with and without plasma
assistance.
[0024] FIG. 7 is a plot graph of the amount of NO.sub.x reduced as
a function of time, where a catalyst has been exposed to
hydrocarbon injection prior to time zero and the hydrocarbon
injection is stopped at time zero.
DETAILED DESCRIPTION OF THE INVENTION
[0025] In the present invention a basic catalyst containing or
consisting essentially of an alkali-metal-exchanged,
high-surface-area, large-pore, faujasite-type zeolite, is employed
to convert NO.sub.x contained in an oxygen-rich engine exhaust to
N.sub.2. Preferably, the invention relates to a two-stage method
for NO.sub.x reduction in an oxygen-rich engine exhaust that
comprises a plasma oxidative stage and a selective reduction stage.
The first stage employs a non-thermal plasma treatment of NO.sub.x
gases in an oxygen-rich exhaust and is intended to convert NO to
NO.sub.2 in the presence of O.sub.2 and hydrocarbons. The second
stage employs a lean NO.sub.x catalyst usually comprising a basic
faujasite-type zeolite at relatively low temperatures to convert
such NO.sub.2 to environmentally benign gases that include N.sub.2,
CO.sub.2, and H.sub.2O. By preconverting NO to NO.sub.2 in the
first stage with a plasma, the efficiency of the second stage for
NO.sub.x reduction is enhanced. For example, an internal combustion
engine exhaust is connected by a pipe to a first chamber in which a
non-thermal plasma converts NO to NO.sub.2 in the presence of
O.sub.2 and hydrocarbons, such as diesel fuel, kerosene or
propylene. A flow of such hydrocarbons (C.sub.xH.sub.y) is input
from usually a second pipe into at least a portion of the first
chamber (optionally on an intermittent basis). The NO.sub.2 from
the plasma treatment proceeds to a second chamber to contact a
preferred alkali-metal-exchanged Y-zeolite lean NO.sub.x selective
reduction catalyst that converts NO.sub.2 to N.sub.2, CO.sub.2, and
H.sub.2O. The hydrocarbons and NO.sub.x are simultaneously reduced
while passing through the lean-NO.sub.x selective reduction
catalyst. The method allows for enhanced NO.sub.x reduction in
vehicular engine exhausts, particularly those having relatively
high sulfur contents. More specific embodiments are discussed
below.
[0026] FIG. 1 illustrates a vehicle embodiment of the present
invention, and is referred to herein by the general reference
numeral 10. The vehicle 10 is provided with a fuel tank 12 that
supplies an internal combustion engine 14 and a NO.sub.x reduction
unit 16 via fuel tank outlet line 13. The fuel used may be #2
diesel oil and the engine 14 may be a diesel type common to busses
and trucks. The engine 14 has an output of exhaust gas that is both
rich in oxygen and oxides of nitrogen (NO.sub.x), e.g., NO and
NO.sub.2. Oxygen-rich exhausts are typical of diesel engines and
lean-burn gasoline engines. Such NO.sub.x in the exhaust is
environmentally undesirable. The exhaust and a hydrocarbon such as
unused fuel from fuel tank 12, are input to the NO.sub.x reduction
unit 16 via exhaust outlet 14a and fuel tank bleed line 15,
respectively. Hydrocarbons in the fuel and a selective catalytic
reduction (SCR) system containing a high-surface-area, large-pore,
basic catalyst, such as an alkali metal-exchanged X-zeolite or
Y-zeolite, are used in a one-step conversion of
hydrocarbons+NO.sub.x->N.sub.2, CO.sub.2 and H.sub.2O by the
NO.sub.x reduction unit 16. A muffler 18 may optionally be used to
quiet the otherwise noisy cleaned exhaust produced in NO.sub.x
reduction unit 16 via optional exhaust outlet 17. An oxidative
system (not shown), which is usually catalytic, can be employed to
oxidize and remove unused hydrocarbon (diesel fuel) from NO.sub.x
reduction unit 16 prior to final exhaust emission from the
vehicle.
[0027] FIG. 2 illustrates a method embodiment (shown generally as
30) of the present invention for NO.sub.x removal in oxygen-rich
exhaust flows. The NO.sub.x reduction unit 16 of FIG. 1 represents
an implementation of method 30. A step 32 converts the NO.sub.x in
an oxygen-rich exhaust flow to N.sub.2 by mixing hydrocarbon
molecules (e.g., engine fuel) into the oxygen-rich exhaust flow and
passing the (normally vaporous) mixture through or over the basic
zeolite-containing SCR catalyst. Exemplary large pore zeolites have
relatively large pore volumes greater than about 0.20 ml/gram and
preferably greater than about 0.30 ml/gram, relatively large pore
sizes greater than 5 angstroms and a silicon/aluminum ratio from
about 1.0 to about 3.0, with an Si/Al ratio of above about 1.5 to
about 3.0 being highly preferred. Exemplary large pore zeolites
include L-zeolite, Omega zeolite, ZSM-3, and X and Y zeolite.
Although any SCR catalyst having a basic nature and a zeolite
having a pore size diameter of at least 6.5 angstroms and
preferably at least 7.5 angstroms, and/or a surface area from about
600 to about 1000 m.sup.2/g can be employed, catalysts containing,
for instance, a basic faujasite-type zeolite, including all forms
of alkali-metal-exchanged X-zeolites and Y-zeolites, are preferred.
The zeolite preferably should have a large number of exchangeable
cations of which at least 90 percent are alkali metal ions selected
from the group consisting of lithium, sodium, potassium, rubidium
and cesium. It is highly preferred that the SCR catalyst, i.e., a
NO.sub.x reducing catalyst, contain essentially no supported metals
deposited onto the zeolite supports; however, if such supported
metals are employed, it is particularly preferred that such metals
be a relatively inexpensive, non-noble metal such as copper,
nickel, tin and the like, rather than expensive platinum, palladium
or rhodium.
[0028] Furthermore, complex hydrocarbons and mixtures of
hydrocarbons, such as diesel fuel, can optionally be reduced to
simpler hydrocarbon molecules by cracking the complex hydrocarbon
molecules with a plasma processor or other cracking means. In a
subsequent step 34, an oxidizing catalyst, typically any
conventional oxidizing catalyst, is used to convert the unused
hydrocarbons and O.sub.2 to more benign products such as
CO.sub.2.
[0029] Alternatively, a simple hydrocarbon may be supplied to the
NO.sub.x reduction unit 16. Some hydrocarbons may be better
reductants or better NO.sub.x to N.sub.2 promoters. A disadvantage
of such an embodiment is that two different supplies of
hydrocarbons must be maintained aboard the vehicle 10. An advantage
of a preferred embodiment of the present invention is that fuels,
such as No. 1 or 2 diesel fuels, can serve as reductants with the
basic, large pore zeolite SCR catalyst to reduce NO.sub.x and
concurrently provide fuel for the upstream exhaust-generating
engine. Thus, only one uncombusted source of hydrocarbons can be
maintained aboard the vehicle. Nevertheless, other hydrocarbons
which may be used, at least in part, as a reductant with the basic
zeolite SCR catalyst include kerosene, propane, propylene, cracked
No. 1 diesel fuel, and cracked No. 2 diesel fuel. Since a preferred
embodiment of the invention relates to NO.sub.x reduction in
industrial diesel fuel-burning engines, stationary or in vehicles,
where diesel fuel No. 2 is combusted, a highly preferred
hydrocarbon reductant added to the NO.sub.x-polluted exhaust is No.
2 diesel fuel.
[0030] FIG. 3 illustrates a NO.sub.x reduction unit (shown
generally as 50) of the present invention. The NO.sub.x reduction
unit 50 is similar to the NO.sub.x reduction unit 16 of FIG. 1 and
similar in operation to the NO.sub.x reduction method 30 of FIG. 2.
The NO.sub.x reduction unit 50 comprises a cylindrical housing 52
including an adjacent insulative bulkhead 62 with an atomized
hydrocarbon inlet 54, an engine exhaust inlet 56 and a processed
exhaust outlet 58. The housing 52 need not be cylindrical and can
take the form of an exhaust manifold attached to an engine. The
temperature on the catalyst can be optimized by adjusting the
proximity of the NO.sub.x reduction unit 50 to the engine. The
catalyst temperature should be less than 400 degrees C., more
particularly less than 260 degrees C. and most preferably in the
range from about 175 degrees C. to about 225 degrees C.
[0031] The exhaust and hydrocarbons are mixed in a chamber 66
between an insulative bulkhead 72, which separates inlets 54 and
56, and insulative bulkhead 80 on which a catalytic converter 78 is
mounted. The exhaust and hydrocarbon parameters may be made
independently variable and microcomputer controlled to accommodate
a variety of exhaust flow rates being processed. Another
parameter--temperature--is a feature of the invention. The mixture
of added hydrocarbons (particularly additive diesel fuel) is passed
over or through catalytic converter 78 at temperatures normally
less than 400 degrees C., more particularly less than 260 degrees
C. and most preferably in the range from about 175 degrees C. to
about 225 degrees C., normally within the boiling temperatures of
the reductant hydrocarbons, e.g., diesel fuel. In a preferred
embodiment, hydrocarbons in a concentration above about 1000 ppm
C1(where C1 refers to a carbon atom) of the exhaust are added to
the exhaust and passed over the basic zeolite SCR catalyst at a
temperature less than about 225 degrees C.
[0032] Optionally, a preprocessor 70 is constructed as a concentric
metal tube that pierces the bulkhead 72. The preprocessor 70 can
crack the complex hydrocarbons provided from the inlet 54 into
simpler hydrocarbons using, for instance, a non-thermal plasma,
such as that disclosed in U.S. Pat. No. 5,711,147, issued to
Vogtlin et al, the disclosure of which is incorporated by reference
herein in its entirety. Furthermore, both the hydrocarbons and a
non-thermal plasma from a plasma converter (not shown) can be mixed
in chamber 66 and used to convert NO in the flow from the engine
exhaust inlet 56 into NO.sub.2. Optionally, porous bulkhead 64 can
be positioned within chamber 66 to concentrate NO.sub.2 with the
hydrocarbons in the area of the catalyst surface of catalytic
converter 78.
[0033] However, in the principal thrust of the invention, catalytic
converter 78, mounted on bulkhead 80, provides for the selective
catalytic reduction of the exhaust NO.sub.x (predominantly NO.sub.2
if the NO.sub.x reduction is plasma-assisted or otherwise) to more
environmentally benign molecules, such as N.sub.2, CO.sub.2 and
H.sub.2O, using the added hydrocarbon reductant mixed with the
exhaust in chamber 66. Oxygen also enhances the selective catalytic
reduction of NO by hydrocarbons.
[0034] In general, catalysts having a high-surface-area,
large-pore, basic zeolite surface and/or framework structure having
a pore size of about 7.5 angstroms in diameter are utilized in the
invention; however, any basic SCR catalyst, i.e., a basic surfaced
zeolitic lean-NO.sub.x catalyst, can be employed in the catalytic
converter. The catalytic converter 78 may preferably be configured
as a bed of alkali-metal-exchanged Y-zeolite. The catalytic
converter 78 may also be configured as a wash coat of
alkali-metal-exchanged Y-zeolite on a substrate. The term "alkali
metal" is used as a descriptor of the elements of Group IA of the
Periodic Table of the Elements (lithium, sodium, potassium,
rubidium, cesium).
[0035] An oxidative system, usually an oxidation catalyst 82 can be
mounted on a bulkhead 84 and provides for the burning of any excess
(unused) hydrocarbons not consumed by the catalytic converter 78.
Preferably, the flow of hydrocarbons into the inlet 54 is
controlled to minimize such excess hydrocarbons that must be burned
by the oxidation catalyst 82.
[0036] Conventional catalysts that are active in selective
catalytic reduction of NO by hydrocarbons usually have surface
acidity, e.g., they possess surface hydroxyl groups. The simplest
surface on which prior art selective catalytic reduction by
hydrocarbons is observed is, for example, the amorphous, acidic
form of alumina, known as .gamma.-Al.sub.2O.sub.3. In contrast to
such useful prior art SCR catalysts, the present invention
surprisingly employs the basic zeolite having a much higher surface
area, large pore structure and a basic nature. The term "basic", as
it is associated with the zeolites, refers to having the
characteristic of a base; e.g., when placed in a solution, a basic
material will have a pH consistent with a base rather than an acid
and, if a catalyst, will catalyze chemical reactions that are
catalyzed by bases. The basic property can be prepared by having a
large number of exchangeable cations of which at least 90 percent
are alkali metal ions selected from the group consisting of
lithium, sodium, potassium, rubidium and cesium. Alkali metal
cation exchanged zeolites are particularly suitable for the
adsorption of acidic gases and for the catalysis of base-catalyzed
reactions. The basic strength increases as the aluminum content of
the aluminosilicate framework structure of the zeolite increases.
The basic strength also increases as the cation size goes up in the
alkali metal ion series. The basicity may be increased further by
preparing a composition according to U.S. Pat. No. 5,194,244, the
disclosure of which is incorporated by reference herein in its
entirety, wherein the sum of the amount of the alkali metal in the
compounds plus any alkali metal cation exchanged into the zeolite
is in excess of that required to provide a fully alkali metal
cation exhanged zeolite. Highly effective catalysts include such
alkali metal catalysts having the above-described relatively large
pore sizes and relatively large pore volumes, particularly
catalysts having the large pore faujasite-type zeolites.
[0037] In FIG. 4, at incoming exhaust temperatures of 260 degrees
C. to a catalytic converter, the percentage of total NO.sub.x
reduction in an exhaust from a diesel engine is compared in the
presence of four different SCR lean-NO.sub.x catalysts, using an
additional portion of proplyene as the reductant. The catalysts
contain pellets of pure .gamma.-Al.sub.2O.sub.3, pellets of
Y-zeolites, including sodium Y-zeolite, calcium Y-zeolite, and
hydrogen Y-zeolite. The NO.sub.x reduction is attributed the
combination of additive hydrocarbon (proplyene) concentration and
the activity of the catalyst. The concentrations of NO and NO.sub.2
(NO.sub.x) are detected and quantified by both chemiluminescence
and infrared absorbance. The NO.sub.x reduction is presumably due
to increased N.sub.2, since the amount of N.sub.2O and any other
oxides of nitrogen, like HONO.sub.2, is negligible compared to the
reduction in NO.sub.x concentration. The maximum NO.sub.x reduction
shown in FIG. 4 can be increased by increasing the amount of
additive diesel fuel, increasing the catalyst amount and/or
decreasing the exhaust gas flow rate.
[0038] In the eight experiments (data summarized in FIG. 4) that
are conducted in view of the scheme of FIG. 3 (four with and four
without plasma assistance), the incoming engine-exhaust gas is at a
temperature of about 260.degree. C. The propylene reductant, which
provides 3000 ppm (C.sub.1), is initially injected in each
experiment through inlet 54 to the NO.sub.x-containing gas exhaust
stream inletted through inlet 56 in chamber 66. After passing
through the respective SCR catalysts in catalytic converter 78 in
each experiment, less than about 20% of the NO.sub.x is reduced at
the 260 degree C. temperature while consistently higher NO.sub.x
reductions are observed for each catalyst when initial plasma
assistance is provided. The highest total NO.sub.x reduction at the
260 degree C. temperature is achieved after passing through the
catalytic converter when the catalyst comprises the gamma alumina,
and the zeolites provide lower amounts of NO.sub.x reduction. The
data obtained at 260 degrees exhibits comparatively high NO.sub.x
reduction in the presence of the conventional gamma alumina
catalyst.
[0039] In FIG. 5, at incoming exhaust temperatures of 260 degrees
C. to a catalytic converter, the percentage of total NO.sub.x
reduction in an exhaust from a diesel engine is compared in the
presence of four different SCR lean-NO.sub.x catalysts, using an
additional portion of proplyene as the reductant. The catalysts
contain pellets of pure .gamma.-Al.sub.2O.sub.3, pellets of sodium
cation exchanged zeolites, including sodium Y-zeolite, sodium
beta-zeolite, and sodium ZSM-5-zeolite. Among the sodium cation
exchanged zeolites, the Y-type zeolite exhibits the highest
NO.sub.x reduction. But again, the data obtained at 260 degrees
exhibits comparatively high NO.sub.x reduction in the presence of
the conventional gamma alumina catalyst.
[0040] However, a startling discovery of the invention is observed
when the above experiment is run at less than about 250 degrees C.,
e.g., at about 200 degrees C. The high-surface-area, large-pore,
basic zeolite-containing catalysts provide greater NO.sub.x
reduction activity than the conventional alumina catalysts at a
lower exhaust (and conversion) temperature. Furthermore, at the
relatively low temperatures, the basic, large pore
zeolite-containing catalysts are able to provide such activity in
the presence of a heavier hydrocarbon reductant than the propylene
employed with gamma-alumina at 260 C.
[0041] Such a surprising effect can be applied to diesel engine
NO.sub.x reduction control, particularly since the exhaust
temperatures of the experiments are relatively low, yet within the
range of typical industrial diesel exhaust temperatures and the
additive hydrocarbon reductant concentrations indicate greater than
50% NO.sub.x reduction with less than a 5% fuel penalty for the
overall combustion system. For instance (as data summarized in FIG.
6), at an exhaust temperature of 200 degrees C. and about 2,500 ppm
(C.sub.1) of additive kerosene provides activity with the gamma
alumina catalyst that effects less than 20% (and with plasma
assistance less than 30%) NO.sub.x reduction. In the invention, at
the same 200 degree C. temperature and 2,500 ppm (C.sub.1) additive
kerosene concentration provides activity with the basic large-pore
zeolite-containing catalyst (as shown in FIG. 6) that effects
greater than 50% (and with plasma assistance greater than 60%)
NO.sub.x reduction. A 2,500 ppm (C.sub.1) additive kerosene or
diesel fuel concentration is only about a 2.2% fuel penalty.
Furthermore, such a NO.sub.x reduction improvement from about 10%
(and with plasma assistance about 30%) NO.sub.x reduction with the
basic zeolite-containing catalyst is clearly unpredicted and
unexpected. Accordingly, even at such a low exhaust temperature as
200 degrees C., the results illustrated in FIG. 6 clearly suggest
that relatively high percentages of NO.sub.x reduction can be
achieved at concentrations of less than (or equal to) 3000 ppm
(C.sub.1) additive hydrocarbon, and even in concentrations greater
than 3000 ppm (C.sub.1) that are still less than a 5% fuel
penalty.
[0042] Zeolites are crystalline aluminosilicate materials. The
range of silicon-to-aluminum ratio varies a great deal between
different zeolites. For example, the ZSM-5 zeolite can only
tolerate low levels of aluminum substitution, the smallest
silicon-to-aluminum ratio being around 10. Faujasite-type zeolites,
such as X-zeolite and Y-zeolite can accommodate much higher levels
of aluminum substitution and accordingly have ratios less than 10.
FIG. 5 illustrates that, for the same type of cation (in this case
sodium), the higher aluminum content provides a relatively higher
percentage of NO.sub.x reduction. The incorporation of aluminum
into the zeolite structure has two major consequences. First, the
replacement of silicon by aluminum results in a net negative charge
for the zeolite framework. The basic sites are described as oxygen
anion bound to aluminum cation, AlO.sub.4.sup.-. The negative
charge of the site AlO.sub.4.sup.- is neutralized by monovalent
(e.g. Na.sup.+, K.sup.+), divalent (e.g. Ca.sup.2+, Sr.sup.2+) or
trivalent (e.g. La.sup.3+) cations. FIG. 4 illustrates that the
monovalent cations from the alkali metal ion series (e.g. sodium)
provides a relatively higher percentage of NO.sub.x reduction
compared to the divalent cations from the alkaline earth metal ion
series (e.g. calcium). The basic strength of the AlO.sub.4.sup.-
sites may be correlated to the ionic radius of the monovalent
cation, i.e., LiY<NaY<KY<RbY<CsY. The basic strength
may also increase by increasing the silicon-to-aluminum ratio of
the zeolite. The X-zeolite and the Y-zeolite are synthetic
counterparts of the naturally occurring mineral faujasite., i.e.,
faujasite-type zeolites. Zeolites X and Y have the same framework
structure but different silicon-to-aluminum ratios than natural
faujasite. By convention, the zeolite is labeled X if the
silicon-to-aluminum ratio is greater than or equal to 1 and less
than 1.5, and labeled Y if the silicon-to-aluminum ratio is greater
than or equal to 1.5 and less than 3. The second consequence of the
inclusion of aluminum in the zeolite framework is that the material
becomes hydrophilic.
[0043] The most preferred pore size diameter of about 7.5 angstroms
in the zeolite employed in the invention is an important parameter
that determines the variety of organic and inorganic molecules than
can be absorbed by the zeolite. Molecules can be absorbed provided
their dimensions are comparable with those of the pore size. For
increased reactivity with the large molecules present in heavy
hydrocarbons in diesel, the relatively large pore size is
preferred. In addition to having high surface area, the X-zeolite
and Y-zeolite have the largest pore sizes, typically about 7.5
angstroms in diameter, among the zeolites. The remarkably stable
and rigid framework structure of the X and Y zeolites also contains
the largest void space of any known zeolite and amounts to greater
than 40, and even greater than about 50 volume percent of the
crystal.
[0044] In one embodiment illustrating the present invention in view
of the block diagram of FIG. 1, an engine exhaust 14a is treated
for NO.sub.x reduction in the presence of a basic large-pore
zeolite-containing SCR catalyst in NO.sub.x reduction unit 16. An
initial concentration of 2000 ppm C.sub.1 of kerosene hydrocarbon
vapor from a periodic pulse controller (i.e., a gas flow controller
adapted to periodically or intermittently inject gas or the like)
is injected into NO.sub.x reduction unit 16 from, through, or
within, for example, fuel tank bleed line 15 to reduce NO.sub.x by
55 ppm. Such a pulse of hydrocarbon vapor injected for
approximately 5 minutes is then stopped for 25 minutes; however,
NO.sub.x reduction continues to occur as the hydrocarbon vapor
concentration falls to less than 300 ppm in about the first 5
minutes of stoppage. Although the NO.sub.x reduction declines over
about the entire 25 minutes of stoppage from 55 ppm NO.sub.x
reduction to a 40 ppm NO.sub.x reduction, nevertheless the NO.sub.x
species continues to be removed (reduced) from the exhaust stream
even though the hydrocarbon level is essentially negligible (e.g.,
less than 100 ppm) over the stopped-pulse period (e.g., about 25
minutes). Such a treatment indicates that NO.sub.x removal can be
achieved essentially in the absence or minimus of added hydrocarbon
vapor, particularly under suitable catalytic NO.sub.x reduction
conditions including employment of the basic, large pore
faujasite-type catalyst. FIG. 7 exhibits a summary of the data for
such an embodiment for the single hydrocarbon pulse and single
stop.
[0045] Changes and modifications in the specifically described
embodiments can be carried out without departing from the scope of
the invention which is intended to be limited only by the scope of
the claims.
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