U.S. patent application number 13/843906 was filed with the patent office on 2014-09-18 for catalytic reduction of nox.
The applicant listed for this patent is Diamler AG, HJS Emission Technology GmbH & Co. KG, Johnson Matthey Public Ltd. Co.. Invention is credited to Anders ANDREASSON, Guy Richard CHANDLER, Claus Fnednch GOERSMAN, Georg HUELHWOHI, James Patrick WARREN.
Application Number | 20140271385 13/843906 |
Document ID | / |
Family ID | 51527821 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140271385 |
Kind Code |
A1 |
ANDREASSON; Anders ; et
al. |
September 18, 2014 |
Catalytic Reduction of NOx
Abstract
A system for NO.sub.x, reduction in combustion gases, especially
from diesel engines, incorporates an oxidation catalyst to convert
at least a portion of NO to NO.sub.2, particulate filter, a source
of reductant such as NH.sub.3 and an SCR catalyst. Considerable
improvements in NO.sub.x conversion are observed.
Inventors: |
ANDREASSON; Anders; (Vastra
Frolunda, SE) ; CHANDLER; Guy Richard; (Cambridge,
GB) ; GOERSMAN; Claus Fnednch; (Royston, GB) ;
WARREN; James Patrick; (Cambridge, GB) ; HUELHWOHI;
Georg; (Soest, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Public Ltd. Co.
Diamler AG
HJS Emission Technology GmbH & Co. KG |
Reading
Stuttgart
Menden/Sauerland |
|
GB
DE
DE |
|
|
Family ID: |
51527821 |
Appl. No.: |
13/843906 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
422/169 |
Current CPC
Class: |
Y02T 10/26 20130101;
F01N 2570/18 20130101; Y02T 10/12 20130101; Y02A 50/20 20180101;
F01N 2610/02 20130101; F01N 3/2046 20130101; F01N 3/106 20130101;
Y02A 50/2325 20180101; F01N 3/0231 20130101; F01N 13/009
20140601 |
Class at
Publication: |
422/169 |
International
Class: |
F01N 3/28 20060101
F01N003/28 |
Claims
1. A system for treating an exhaust gas stream, comprising: an
oxidation catalyst, a particulate trap downstream of the oxidation
catalyst, a reductant fluid injector downstream of the particulate
trap, and an SCR catalyst downstream of the reductant fluid
injector; wherein the oxidation catalyst enhances the NO.sub.2
content of the exhaust gas stream such that the ratio of NO to
NO.sub.2 in the exhaust gas stream exiting the oxidation catalyst
is lower than the ratio of NO to NO.sub.2 in the exhaust gas stream
entering the oxidation catalyst; wherein the oxidation catalyst
adjusts the ratio of NO to NO.sub.2 in the exhaust gas stream such
that more particulate matter is combusted in the particulate trap
and more NO.sub.x is removed from the exhaust gas stream in the SCR
catalyst than in the absence of the oxidation catalyst; wherein the
particulate trap traps particulates contained in the exhaust gas
stream; and wherein the reductant fluid injector injects
nitrogenous reductant fluid into the gas stream.
2. A system according to claim 1, wherein the exhaust gas stream is
the exhaust from a diesel engine.
3. A system according to claim 1, wherein the exhaust gas stream is
cooled before reaching the SCR catalyst.
4. A system according to claim 1, wherein the oxidation catalyst
adjusts the ratio of NO to NO.sub.2 in the exhaust gas stream to a
level pre-determined to be optimum for the SCR catalyst.
5. A system according to claim 1, wherein the SCR catalyst is
maintained at a temperature from 160.degree. C. to 450.degree.
C.
6. A system according to claim 1, wherein the SCR catalyst includes
a component selected from the group consisting of a transition
metal and a rare-earth metal.
7. A system according to claim 1, wherein the SCR catalyst includes
a component selected from the group consisting of copper and
vanadium.
8. A system according to claim 4, wherein the ratio of NO to
NO.sub.2 leaving the oxidation catalyst is adjusted to about
4:3.
9. A system according to claim 1, wherein the reductant fluid is
urea.
10. A system according to claim 9, further comprising a clean-up
catalyst downstream of the SCR catalyst to remove NH.sub.3 or
derivatives thereof.
11. A system according to claim 1, wherein the space velocity of
the exhaust gas over the SCR catalyst is in the range 40,000 to
70,000 h-1.
12. A system according to claim 9, further comprising a NH.sub.3
clean-up catalyst downstream of the SCR catalyst.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/204,634, filed Aug. 5, 2011, which was a continuation of
U.S. application Ser. No. 12/380,414, filed Feb. 27, 2009, which is
a continuation of U.S. application Ser. No. 10/886,778, filed Jul.
8, 2004, which is a divisional application of U.S. application Ser.
No. 09/601,964, filed Jan. 9, 2001, now U.S. Pat. No. 6,805,849,
which is the U.S. National Phase of International Application No.
PCT/GB1999/000292, filed Jan. 28, 1999, and which claims the
benefit of priority from British Application No. 9802504.2, filed
Feb. 6, 1998. These applications, in their entirety, are
incorporated herein by reference.
SUMMARY OF THE INVENTION
[0002] The present invention concerns improvements in selective
catalytic reduction of NO.sub.x in waste gas streams such as diesel
engine exhausts or other lean exhaust gases such as from gasoline
direct injection (GDI).
BACKGROUND OF THE INVENTION
[0003] The technique named SCR (Selective Catalytic Reduction) is
well established for industrial plant combustion gases, and may be
broadly described as passing a hot exhaust gas over a catalyst in
the presence of a nitrogenous reductant, especially ammonia or
urea. This is effective to reduce the NO.sub.x content of the
exhaust gases by about 20-25% at about 250.degree. C., or possibly
rather higher using a platinum catalyst, although platinum
catalysts tend to oxidise NH.sub.3 to NO.sub.x, during higher
temperature operation. We believe that SCR systems have been
proposed for NO.sub.x reduction for vehicle engine exhausts,
especially large or heavy duty diesel engines, but this does
require on-board storage of such reductants, and is not believed to
have met with commercial acceptability at this time.
[0004] We believe that if there could be a significant improvement
in performance of SCR systems, they would find wider usage and may
be introduced into vehicular applications. It is an aim of the
present invention to improve significantly the conversion of
NO.sub.x in a SCR system, and to improve the control of other
pollutants using a SCR system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a graph plotting percentage NO.sub.x conversion
against temperature resulting from Test 1.
[0006] FIG. 2 is a graph plotting percentage NO.sub.x conversion
against temperature resulting from Test 2.
[0007] FIG. 3 is a graph plotting percentage NO.sub.x conversion
against temperature resulting from Test 3.
[0008] FIG. 4 is a bar graph showing percentage conversion of
pollutants [NO.sub.x, particulates, hydrocarbons (HC) and carbon
monoxide (CO)] resulting from Test 4.
DETAILED DESCRIPTION OF THE INVENTION
[0009] Accordingly, the present invention provides an improved SCR
catalyst system, comprising in combination and in order, an
oxidation catalyst effective to convert NO to NO.sub.2, a
particulate filter, a source of reductant fluid and downstream of
said source, an SCR catalyst.
[0010] The invention further provides an improved method of
reducing NO.sub.x in gas streams containing NO and particulates
comprising passing such gas stream over an oxidation catalyst under
conditions effective to convert at least a portion of NO in the gas
stream to NO.sub.2, removing at least a portion of said
particulates, adding reductant fluid to the gas stream containing
enhanced NO.sub.2 to form a gas mixture, and passing the gas
mixture over an SCR catalyst.
[0011] Although the present invention provides, at least in its
preferred embodiments, the opportunity to reduce very significantly
the NO.sub.x emissions from the lean (high in oxygen) exhaust gases
from diesel and similar engines, it is to be noted that the
invention also permits very good reductions in the levels of other
regulated pollutants, especially hydrocarbons and particulates.
[0012] The invention is believed to have particular application to
the exhausts from heavy duty diesel engines, especially vehicle
engines, e.g., truck or bus engines, but is not to be regarded as
being limited thereto. Other applications might be LDD (light duty
diesel), GDI, CNG (compressed natural gas) engines, ships or
stationary sources. For simplicity, however, the majority of this
description concerns such vehicle engines.
[0013] We have surprisingly found that a "pre-oxidising" step,
which is not generally considered necessary because of the low
content of CO and unburnt fuel in diesel exhausts, is particularly
effective in increasing the conversion of NO.sub.x to N.sub.2 by
the SCR system. We also believe that minimising the levels of
hydrocarbons in the gases may assist in the conversion of NO to
NO.sub.2. This may be achieved catalytically and/or by engine
design or management. Desirably, the NO.sub.2/NO ratio is adjusted
according to the present invention to the most beneficial such
ratio for the particular SCR catalyst and CO and hydrocarbons are
oxidized prior to the SCR catalyst. Thus, our preliminary results
indicate that for a transition metal/zeolite SCR catalyst it is
desirable to convert all NO to NO.sub.2, whereas for a rare
earth-based SCR catalyst, a high ratio is desirable providing there
is some NO, and for other transition metal-based catalysts gas
mixtures are notably better than either substantially only NO or
NO.sub.2. Even more surprisingly, the incorporation of a
particulate filter permits still higher conversions of
NO.sub.x.
[0014] The oxidation catalyst may be any suitable catalyst, and is
generally available to those skilled in art. For example, a Pt
catalyst deposited upon a ceramic or metal through-flow honeycomb
support is particularly suitable. Suitable catalysts are, e.g.,
Pt/Al.sub.2O.sub.3 catalysts, containing 1-150 g Pt/ft.sup.3
(0.035-5.3 g Pt/litre) catalyst volume depending on the NO.sub.2/NO
ratio required. Such catalysts may contain other components
providing there is a beneficial effect or at least no significant
adverse effect.
[0015] The source of reductant fluid conveniently uses existing
technology to inject fluid into the gas stream. For example, in the
tests for the present invention, a mass controller was used to
control supply of compressed NH.sub.3, which was injected through
an annular injector ring mounted in the exhaust pipe. The injector
ring had a plurality of injection ports arranged around its
periphery. A conventional diesel fuel injection system including
pump and injector nozzle has been used to inject urea by the
present applicants. A stream of compressed air was also injected
around the nozzle; this provided good mixing and cooling.
[0016] The reductant fluid is suitably NH3, but other reductant
fluids including urea, ammonium carbamate and hydrocarbons
including diesel fuel may also be considered. Diesel fuel is, of
course, carried on board a diesel-powered vehicle, but diesel fuel
itself is a less selective reductant than NH.sub.3 and is presently
not preferred.
[0017] Suitable SCR catalysts are available in the art and include
Cu-based and vanadia-based catalysts. A preferred catalyst at
present is a V.sub.2O.sub.5/WO.sub.3/TiO.sub.2 catalyst, supported
on a honeycomb through-flow support. Although such a catalyst has
shown good performance in the tests described hereafter and is
commercially available, we have found that sustained high
temperature operation can cause catalyst deactivation. Heavy duty
diesel engines, which are almost exclusively turbocharged, can
produce exhaust gases at greater than 500.degree. C. under
conditions of high load and/or high speed, and such temperatures
are sufficient to cause catalyst deactivation.
[0018] In one embodiment of the invention, therefore, cooling means
is provided upstream of the SCR catalyst. Cooling means may
suitably be activated by sensing high catalyst temperatures or by
other, less direct, means, such as determining conditions likely to
lead to high catalyst temperatures. Suitable cooling means include
water injection upstream of the SCR catalyst, or air injection, for
example utilizing the engine turbocharger to provide a stream of
fresh intake air by-passing the engine. We have observed a loss of
activity of the catalyst, however, using water injection, and air
injection by modifying the turbocharger leads to higher space
velocity over the catalyst which tends to reduce NO conversion.
Preferably, the preferred SCR catalyst is maintained at a
temperature from 160.degree. C. to 450.degree. C.
[0019] We believe that in its presently preferred embodiments, the
present invention may depend upon an incomplete conversion of NO to
NO.sub.2. Desirably, therefore, the oxidation catalyst, or the
oxidation catalyst together with the particulate trap if used,
yields a gas stream entering the SCR catalyst having a ratio of NO
to NO.sub.2 of from about 4:1 to about 1:3 by volume, for the
commercial vanadia-type catalyst. As mentioned above, other SCR
catalysts perform better with different NO/NO.sub.2 ratios. We do
not believe that it has previously been suggested to adjust the
NO/NO.sub.2 ratio in order to improve NO reduction.
[0020] The present invention incorporates a particulate trap
downstream of the oxidation catalyst. We discovered that soot-type
particulates may be removed from a particulate trap by "combustion"
at relatively low temperatures in the presence of NO.sub.2. In
effect, the incorporation of such a particulate trap serves to
clean the exhaust gas of particulates without causing accumulation,
with resultant blockage or back-pressure problems, whilst
simultaneously reducing a proportion of the NOR. Suitable
particulate traps are generally available, and are desirably of the
type known as wall-flow filters, generally manufactured from a
ceramic, but other designs of particulate trap, including woven
knitted or non-woven heat-resistant fabrics, may be used.
[0021] It may be desirable to incorporate a clean-up catalyst
downstream of the SCR catalyst, to remove any NH.sub.3 or
derivatives thereof which could pass through unreacted or as
by-products. Suitable clean-up catalysts are available to the
skilled person.
[0022] A particularly interesting possibility arising from the
present invention has especial application to light duty diesel
engines (car and utility vehicles) and permits a significant
reduction in volume and weight of the exhaust gas after-treatment
system, in a suitable engineered system.
EXAMPLES
[0023] Several tests have been carried out in making the present
invention. These are described below, and are supported by results
shown in graphical form in the attached drawings.
[0024] A commercial 10 litre turbocharged heavy duty diesel engine
on a test-bed was used for all the tests described herein.
Test 1--(Comparative)
[0025] A conventional SCR system using a commercial
V.sub.2O.sub.5/WO.sub.3/TiO.sub.2 catalyst, was adapted and fitted
to the exhaust system of the engine. NH.sub.3 was injected upstream
of the SCR catalyst at varying ratios. The NH.sub.3 was supplied
from a cylinder of compressed gas and a conventional mass flow
controller used to control the flow of NH.sub.3 gas to an
experimental injection ring. The injection ring was a 10 cm
diameter annular ring provided with 20 small injection ports
arranged to inject gas in the direction of the exhaust gas flow.
NO.sub.x conversions were determined by fitting a NO.sub.x analyser
before and after the SCR catalyst and are plotted against exhaust
gas temperature in FIG. 1. Temperatures were altered by maintaining
the engine speed constant and altering the torque applied.
[0026] A number of tests were run at different quantities of
NH.sub.3 injection, from 60% to 100% of theoretical, calculated at
1:1 NH.sub.3/NO and 4:3 NH.sub.3/NO.sub.2. It can readily be seen
that at low temperatures, corresponding to light load, conversions
are about 25%, and the highest conversions require stoichiometric
(100%) addition of NH.sub.3 at catalyst temperatures of from 325 to
400.degree. C., and reach about 90%. However, we have determined
that at greater than about 70% of stoichiometric NH.sub.3
injection, NH.sub.3 slips through the SCR catalyst unreacted, and
can cause further pollution problems.
Test 2 (Comparative)
[0027] The test rig was modified by inserting into the exhaust pipe
upstream of the NH.sub.3 injection, a commercial platinum oxidation
catalyst of 10.5 inch diameter and 6 inch length (26.67 cm diameter
and 15.24 cm length) containing log Pt/ft.sup.3 (=0.35 g/litre) of
catalyst volume. Identical tests were run, and it was observed from
the results plotted in FIG. 2, that even at 225.degree. C., the
conversion of NO.sub.x has increased from 25% to >60%. The
greatest conversions were in excess of 95%. No slippage of NH.sub.3
was observed in this test nor in the following test.
Test 3
[0028] The test rig was modified further, by inserting a
particulate trap before the NH.sub.3 injection point, and the tests
run again under the same conditions at 100% NH.sub.3 injection and
a space velocity in the range 40,000 to 70,000 hr.sup.-1 over the
SCR catalyst. The results are plotted and shown in FIG. 3.
Surprisingly, there is a dramatic improvement in NO.sub.x
conversion, to above 90% at 225.degree. C., and reaching 100% at
350.degree. C. Additionally, of course, the particulates, which are
the most visible pollutant from diesel engines, are also
controlled.
Test 4
[0029] An R49 test with 80% NH.sub.3 injection was carried out over
a V.sub.2O.sub.5/WO.sub.3/TiO.sub.2 SCR catalyst. This gave 67%
particulate, 89% HC and 87% NO.sub.x conversion; the results are
plotted in FIG. 4.
[0030] Additionally tests have been carried out with a different
diesel engine, and the excellent results illustrated in Tests 3 and
4 above have been confirmed.
[0031] The results have been confirmed also for a non-vanadium SCR
catalyst.
* * * * *