U.S. patent application number 13/843870 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 Friedrich Goersmann, Georg Huethwohl, James Patrick Warren.
Application Number | 20140260200 13/843870 |
Document ID | / |
Family ID | 51493221 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140260200 |
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) ; Goersmann; Claus Friedrich; (Royston, GB)
; Warren; James Patrick; (Cambridge, GB) ;
Huethwohl; 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: |
51493221 |
Appl. No.: |
13/843870 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
60/274 |
Current CPC
Class: |
F01N 13/009 20140601;
F02B 37/168 20130101; B01D 53/9431 20130101; F01N 3/32 20130101;
B01D 2255/20776 20130101; Y02T 10/26 20130101; Y02T 10/24 20130101;
B01D 2251/2062 20130101; B01D 53/9459 20130101; F01N 3/106
20130101; F01N 3/2046 20130101; Y02A 50/2345 20180101; Y02T 10/144
20130101; B01D 2255/20723 20130101; Y02T 10/12 20130101; B01D
53/9409 20130101; B01D 2255/20707 20130101; Y02A 50/20 20180101;
F01N 3/035 20130101; F01N 3/18 20130101; F01N 3/2066 20130101; B01D
2258/012 20130101; Y02A 50/2325 20180101 |
Class at
Publication: |
60/274 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Claims
1-12. (canceled)
13. A method comprising: (a) passing an exhaust gas from a diesel
engine over an oxidation catalyst to provide an adjusted gas
stream, the exhaust gas comprising a first content level by volume
of NO, a first content level by volume of NO.sub.2, and particulate
matter, and the adjusted gas stream comprising a second content
level by volume of NO that is lower than the first content level of
NO, a second content level by volume of NO.sub.2, and the
particulate matter; (b) passing the adjusted gas stream through a
particulate trap that results in trapping at least a portion of the
particulate matter on the particulate trap; (c) combusting a
significant portion of the trapped particulate matter such that
there is no significant accumulation of particulate matter in the
particulate trap in the presence of the adjusted gas stream at a
combustion temperature that is lower than the temperature necessary
to combust the trapped particulate matter in the presence of the
exhaust gas such that there is no significant accumulation of
particulate matter in the particulate trap, to create a further
adjusted gas stream comprising a third content level by volume of
NO and a third content level by volume of NO.sub.2 that is lower
than the second content level of NO.sub.2; (d) injecting a
reductant fluid comprising urea into the further adjusted gas
stream; (e) mixing the further adjusted gas stream with the
injected reductant fluid to form a further adjusted gas stream
mixed with reductant fluid; and (f) passing the further adjusted
gas stream mixed with reductant fluid over an SCR catalyst to
provide a final adjusted gas stream comprising a fourth content
level by volume of NO and a fourth content level by volume of
NO.sub.2; wherein the second content level of NO.sub.2 is
sufficiently higher than the first content level of NO.sub.2 such
that when a portion of the second content level of NO.sub.2 in the
adjusted gas stream is consumed during the combustion of the at
least a portion of the trapped particulate matter, the resulting
third content level of NO.sub.2 is still sufficiently high for use
with the SCR catalyst to provide the final adjusted gas stream
where the total combined volume of the fourth content level of NO
with the fourth content level of NO.sub.2 is lower than the total
combined volume of the first content level of NO with the first
content level of NO.sub.2, and the total combined volume of the
fourth content level of NO with the fourth content level of
NO.sub.2 is lower relative to the respective total combined volume
of NO with NO.sub.2 in a final exhaust stream that would result
from carrying out steps b-f starting with the exhaust gas instead
of the adjusted gas stream.
14. The method of claim 13, wherein the diesel engine is a vehicle
engine.
15. The method of claim 13, wherein the diesel engine is a heavy
duty diesel truck engine.
16. The method of claim 13, wherein the diesel engine is a
turbocharged heavy duty diesel truck engine.
17. The method of claim 16, further comprising cooling the further
adjusted gas stream.
18. The method of claim 17, wherein the further adjusted gas stream
is cooled by air supplied by the turbocharger.
19. The method of claim 13, wherein the oxidation catalyst converts
less than all of the NO in the exhaust gas to NO.sub.2.
20. The method of claim 16, wherein the further adjusted gas stream
mixed with reductant fluid is at least 225.degree. C. when passed
over the SCR catalyst, and the final adjusted gas stream has more
than 90% less NO.sub.x content by volume than the exhaust gas.
21. The method of claim 20, wherein the final gas stream has at
least 67% less particulate matter content by volume than the
exhaust gas.
22. A method comprising: (a) passing an exhaust gas from a diesel
engine over an oxidation catalyst to provide an adjusted gas
stream, the exhaust gas comprising a first content level by volume
of NO, a first content level by volume of NO.sub.2, and particulate
matter, and the adjusted gas stream comprising a second content
level by volume of NO that is lower than the first content level of
NO, a second content level by volume of NO.sub.2, and the
particulate matter; (b) passing the adjusted gas stream through a
particulate trap that results in trapping at least a portion of the
particulate matter on the particulate trap; (c) combusting a
significant portion of the trapped particulate matter in the
presence of the adjusted gas stream to reduce a combustion
temperature necessary to stop significant accumulation of
particulate matter in the particulate trap relative to the
combustion temperature of a significant portion of the particulate
matter in the presence of the exhaust gas necessary to stop
significant accumulation of particulate matter in the particulate
trap, and to create a further adjusted gas stream comprising a
third content level by volume of NO and a third content level by
volume of NO.sub.2 that is lower than the second content level of
NO.sub.2; (d) injecting a reductant fluid comprising urea into the
further adjusted gas stream; (e) mixing the further adjusted gas
stream with the injected reductant fluid to form a further adjusted
gas stream mixed with reductant fluid; and (f) passing the further
adjusted gas stream mixed with reductant fluid over an SCR catalyst
to provide a final adjusted gas stream comprising a fourth content
level by volume of NO and a fourth content level by volume of
NO.sub.2; wherein the second content level of NO.sub.2 is
sufficiently higher than the first content level of NO.sub.2 such
that when a portion of the second content-level of NO.sub.2 in the
adjusted gas stream is consumed during the combustion of the at
least a portion of the trapped particulate matter, the resulting
third content level of NO.sub.2 is still sufficiently high for use
with the SCR catalyst to provide the final adjusted gas stream
where the total combined volume of the fourth content level of NO
with the fourth content level of NO.sub.2 is lower than the total
combined volume of the first content level of NO with the first
content level of NO.sub.2, and the total combined volume of the
fourth content level of NO with the fourth content level of
NO.sub.2 is lower relative to the respective total combined volume
of NO with NO.sub.2 in a final exhaust stream that would result
from carrying out steps b-f starting with the exhaust gas instead
of the adjusted gas stream.
23. The method of claim 22, wherein the diesel engine is a vehicle
engine.
24. The method of claim 22, wherein the diesel engine is a heavy
duty diesel truck engine.
25. The method of claim 22, wherein the diesel engine is a
turbocharged heavy duty diesel truck engine.
26. The method of claim 25, further comprising cooling the further
adjusted gas stream.
27. The method of claim 26, wherein the further adjusted gas stream
is cooled by air supplied by the turbocharger.
28. The method of claim 22, wherein the oxidation catalyst converts
less than all of the NO in the exhaust gas to NO.sub.2.
29. The method of claim 25, wherein the further adjusted gas stream
mixed with reductant fluid is at least 225.degree. C. when passed
over the SCR catalyst, and the final adjusted gas stream has more
than 90% less NO.sub.x content by volume than the exhaust gas.
30. The method of claim 29, wherein the final gas stream has at
least 67% less particulate matter content by volume than the
exhaust gas.
31. A method comprising: (a) passing an exhaust gas from a diesel
engine over an oxidation catalyst to provide an adjusted gas
stream, the exhaust gas comprising a first content level by volume
of NO, a first content level by volume of NO.sub.2, and particulate
matter, and the adjusted gas stream comprising a second content
level by volume of NO that is lower than the first content level of
NO, a second content level by volume of NO.sub.2, and the
particulate matter; (b) passing the adjusted gas stream through a
particulate trap that results in trapping at least a portion of the
particulate matter on the particulate trap; (c) combusting a
significant portion of the trapped particulate matter such that
there is no significant accumulation of particulate matter in the
particulate trap in the presence of the adjusted gas stream at a
combustion temperature that is lower than the temperature necessary
to combust the trapped particulate matter in the presence of the
exhaust gas such that there is no significant accumulation of
particulate matter in the particulate trap, to create a further
adjusted gas stream comprising a third content level by volume of
NO and a third content level by volume of NO.sub.2 that is lower
than the second content level of NO.sub.2; (d) injecting a
reductant fluid comprising urea into the further adjusted gas
stream; (e) mixing the further adjusted gas stream with the
injected reductant fluid to form a further adjusted gas stream
mixed with reductant fluid; and (f) passing the further adjusted
gas stream mixed with reductant fluid over an SCR catalyst to
provide a final adjusted gas stream comprising a fourth content
level by volume of NO and a fourth content level by volume of
NO.sub.2; wherein the second content level of NO.sub.2 is
sufficiently higher than the first content level of NO.sub.2 such
that when a portion of the second content-level of NO.sub.2 in the
adjusted gas stream is consumed during the combustion of the at
least a portion of the trapped particulate matter, the resulting
third content level of NO.sub.2 is still sufficiently high for use
with the SCR catalyst to provide the final adjusted gas stream
where the total combined volume of the fourth content level of NO
with the fourth content level of NO.sub.2 is lower than the total
combined volume of the first content level of NO with the first
content level of NO.sub.2, and the total combined volume of the
fourth content level of NO with the fourth content level of
NO.sub.2 is lower relative to the respective total combined volume
of NO with NO.sub.2 in a final exhaust stream that would result
from carrying out steps b-f starting with the exhaust gas instead
of the adjusted gas stream; and wherein the further adjusted gas
stream mixed with reductant fluid is at least 225.degree. C. when
passed over the SCR catalyst, and the final adjusted gas stream has
more than 90% less NO.sub.x content by volume and at least 67% less
particulate matter content by volume than the exhaust gas.
32. The method of claim 31, wherein the diesel engine is a vehicle
engine.
33. The method of claim 31, wherein the diesel engine is a heavy
duty diesel truck engine.
34. The method of claim 31, wherein the diesel engine is a
turbocharged heavy duty diesel truck engine.
35. The method of claim 34, further comprising cooling the further
adjusted gas stream.
36. The method of claim 35, wherein the further adjusted gas stream
is cooled by air supplied by the turbocharger.
37. The method of claim 31, wherein the oxidation catalyst converts
less than all of the NO in the exhaust gas to NO.sub.2.
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/GB 1999/000292, 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.R.
[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.
* * * * *