U.S. patent application number 11/712798 was filed with the patent office on 2007-10-18 for catalytic reduction of nox.
Invention is credited to Anders Andreasson, Guy Richard Chandler, Claus Friedrich Goersmann, James Patrick Warren.
Application Number | 20070240402 11/712798 |
Document ID | / |
Family ID | 10826538 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070240402 |
Kind Code |
A1 |
Andreasson; Anders ; et
al. |
October 18, 2007 |
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, a 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;
(Frolunda, SE) ; Chandler; Guy Richard;
(Cambridge, GB) ; Goersmann; Claus Friedrich;
(Cambridge, GB) ; Warren; James Patrick;
(Cambridge, GB) |
Correspondence
Address: |
Monte R. Browder;2nd Floor Suite
314 Shell Road
Carney's Point
NJ
08069
US
|
Family ID: |
10826538 |
Appl. No.: |
11/712798 |
Filed: |
February 28, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10886778 |
Jul 8, 2004 |
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11712798 |
Feb 28, 2007 |
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09601964 |
Aug 9, 2000 |
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10886778 |
Jul 8, 2004 |
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Current U.S.
Class: |
60/274 |
Current CPC
Class: |
F01N 2570/14 20130101;
Y02T 10/24 20130101; Y02T 10/26 20130101; B01J 23/30 20130101; F01N
2260/022 20130101; F01N 2330/06 20130101; Y02A 50/20 20180101; F01N
3/2046 20130101; Y02T 10/22 20130101; F01N 3/2066 20130101; F01N
9/00 20130101; F01N 13/009 20140601; F01N 2610/00 20130101; B01D
53/9445 20130101; B01D 2258/012 20130101; B01J 35/0006 20130101;
F01N 3/32 20130101; F01N 2610/03 20130101; F01N 2610/02 20130101;
Y02A 50/2345 20180101; F01N 3/0231 20130101; F01N 2330/02 20130101;
Y02T 10/12 20130101; B01D 53/9431 20130101; Y02A 50/2324 20180101;
F01N 3/0253 20130101; F01N 2370/04 20130101; F01N 2510/06 20130101;
F01N 2370/00 20130101; F01N 2250/02 20130101; Y02A 50/2325
20180101; Y02T 10/40 20130101; F01N 2270/02 20130101; Y02A 50/2344
20180101; B01J 23/42 20130101; B01D 2255/1021 20130101; F01N 3/2882
20130101; F01N 3/106 20130101; Y02T 10/47 20130101; F01N 2260/024
20130101 |
Class at
Publication: |
060/274 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 1998 |
GB |
9802504.2 |
Claims
1.-14. (canceled)
15. A method of reducing NO.sub.x in a gas stream containing NO and
particulates, comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream; adding a
reductant fluid to the converted gas stream to form a gas mixture;
and passing the gas mixture over an SCR catalyst.
16. The method of claim 15, wherein the ratio of NO to NO.sub.2 in
the gas mixture is from about 4:1 to 1:3 by volume.
17. The method of claim 15, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
18. The method of claim 17, wherein the gas stream comprises
exhaust from a heavy duty diesel engine.
19. The method of claim 17, wherein the gas stream comprises
exhaust from a light duty diesel engine.
20. The method of claim 17, wherein the gas stream comprises
exhaust from a gasoline direct injection engine.
21. The method of claim 17, wherein the gas stream comprises
exhaust from a compressed natural gas engine.
22. The method of claim 15, wherein the SCR catalyst is selected
from the group consisting of transition metal/zeolite catalysts,
rare earth-based catalysts and transition metal catalysts.
23. The method of claim 15, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
24. The method of claim 15, wherein the oxidation catalyst
comprises a platinum catalyst.
25. The method of claim 24, wherein the platinum catalyst is
deposited on a ceramic through-flow honeycomb support.
26. The method of claim 24, wherein the oxidation catalyst
comprises platinum deposited on a metal through-flow honeycomb
support.
27. The method of claim 24, wherein the platinum catalyst comprises
a Pt/Al.sub.2O.sub.3 catalyst.
28. The method of claim 15, wherein the reductant fluid is selected
from the group consisting of NH.sub.3, urea, ammonium carbamate,
hydrocarbons, and diesel fuel.
29. The method of claim 15, wherein the reductant fluid comprises
NH.sub.3.
30. The method of claim 15, wherein the reductant fluid comprises
urea.
31. The method of claim 15, wherein the reductant fluid comprises
ammonium carbamate.
32. The method of claim 15, wherein the reductant fluid is injected
into the gas stream.
33. The method of claim 32, wherein the supply of the reductant
fluid is controlled using a mass controller.
34. The method of claim 32, wherein the reductant fluid is injected
into the gas stream through an injection ring.
35. The method of claim 34, wherein the injection ring is an
annular injection ring mounted in an exhaust pipe of a vehicle.
36. The method of claim 15, wherein the SCR catalyst is maintained
at a temperature of from 160.degree. C. to 450.degree. C.
37. The method of claim 36, wherein the temperature of the catalyst
is maintained using a cooling means.
38. The method of claim 37, wherein the cooling means comprises
water injection.
39. The method of claim 37, wherein the cooling means comprises air
injection.
40. The method of claim 15, wherein the ratio of the reductant
fluid to NO added to the converted gas stream comprises 0.6:1 to
1:1.
41. The method of claim 40, wherein the reductant fluid comprises
NH.sub.3.
42. The method of claim 15, wherein the ratio of the reductant
fluid to NO.sub.2 added to the converted gas stream comprises 0.8:1
to 4:3.
43. The method of claim 42, wherein the reductant fluid comprises
NH.sub.3.
44. The method of claim 41, further comprising removing any
NH.sub.3 and derivatives thereof downstream of the SCR
catalyst.
45. The method of claim 44, wherein the removing any NH.sub.3 and
derivatives thereof downstream of the SCR catalyst includes
incorporation of a clean-up catalyst downstream of the SCR
catalyst.
46. A method according to claim 15, wherein a space velocity of the
exhaust gas over the SCR catalyst is in the range 40,000 to 70,000
hr.sup.-1.
47. A method of reducing NO.sub.x in a gas stream containing NO and
particulates, comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream; removing at
least a portion of the particulates from the converted gas stream;
adding a reductant fluid to the converted gas stream to form a gas
mixture; and passing the gas mixture over an SCR catalyst.
48. The method of claim 47, wherein the particulates are removed
from the converted gas stream using a particulate filter or
particulate trap.
49. The method of claim 48, wherein the particulates are removed
without causing accumulation and resulting blockage and back
pressure problems.
50. The method of claim 48, wherein the particles are removed from
the particulate trap by combustion in the presence of NO.sub.2.
51. The method of claim 48, wherein the particulate trap comprises
a wall-flow filter.
52. The method of claim 48, wherein the particulate trap is
manufactured from ceramic.
53. The method of claim 48, wherein the particulate trap is
manufactured from woven knitted heat resistant fabrics.
54. The method of claim 48, wherein the particulate trap is
manufactured from non-woven heat resistant fabrics.
55. The method of claim 48, wherein the ratio of NO to NO.sub.2 in
the gas mixture is from about 4:1 to 1:3 by volume.
56. The method of claim 48, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
57. The method of claim 56, wherein the gas stream comprises
exhaust from a heavy duty diesel engine.
58. The method of claim 56, wherein the gas stream comprises
exhaust from a light duty diesel engine.
59. The method of claim 56, wherein the gas stream comprises
exhaust from a gasoline direct injection engine.
60. The method of claim 48, wherein the SCR catalyst is selected
from the group consisting of transition metal/zeolite catalysts,
rare earth-based catalysts and transition metal catalysts.
61. The method of claim 48, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
62. The method of claim 48, wherein the oxidation catalyst
comprises a platinum catalyst.
63. The method of claim 62, wherein the platinum catalyst is
deposited on a ceramic through-flow honeycomb support.
64. The method of claim 62, wherein the platinum catalyst is
deposited on a metal through-flow honeycomb support.
65. The method of claim 62, wherein the platinum catalyst comprises
a Pt/Al.sub.2O.sub.3 catalyst.
66. The method of claim 48, wherein the reductant fluid is selected
from the group consisting of NH.sub.3, urea, ammonium carbamate,
hydrocarbons, and diesel fuel.
67. The method of claim 66, wherein the reductant fluid comprises
NH.sub.3.
68. The method of claim 66, wherein the reductant fluid comprises
urea.
69. The method of claim 66, wherein the reductant fluid comprises
ammonium carbamate.
70. The method of claim 48, wherein the SCR catalyst is maintained
at a temperature of from 160.degree. C. to 450.degree. C.
71. The method of claim 70, wherein the temperature of the catalyst
is maintained using a cooling means.
72. The method of claim 71, wherein the cooling means comprises
water injection.
73. The method of claim 71, wherein the cooling means comprises air
injection.
74. The method of claim 48, wherein the ratio of the reductant
fluid to NO added to the gas stream comprises 0.6:1 to 1:1.
75. The method of claim 54, wherein the reductant fluid comprises
NH.sub.3.
76. The method of claim 48, wherein the ratio of the reductant
fluid to NO.sub.2 added to the gas stream comprises 0.8:1 to
4:3.
77. The method of claim 76, wherein the reductant fluid comprises
NH.sub.3.
78. The method of claim 75, further comprising removing any
NH.sub.3 and derivatives thereof downstream of the SCR
catalyst.
79. The method of claim 64, wherein the removing any NH.sub.3 and
derivatives thereof downstream of the SCR catalyst includes
incorporation of a clean-up catalyst downstream of the SCR
catalyst.
80. A method according to claim 48, wherein the space velocity of
the exhaust gas over the SCR catalyst is in the range 40,000 to
70,000 hr.sup.-1.
81. A method of reducing NO.sub.x in a gas stream containing NO and
particulates, comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream having a ratio of
NO to NO.sub.2 adjusted according to the type of SCR catalyst to
improve NO.sub.x reduction; adding a reductant fluid to the
converted gas stream to form a gas mixture; and passing the gas
mixture over an SCR catalyst.
82. The method of claim 81, wherein the ratio of NO to NO.sub.2 in
the gas mixture is from about 4:1 to 1:3 by volume.
83. The method of claim 81, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
84. The method of claim 81, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
85. The method of claim 81, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
86. The method of claim 85, wherein the platinum catalyst comprises
a Pt/Al.sub.2O.sub.3 catalyst.
87. The method of claim 81, wherein the reductant fluid comprises
urea.
88. A method of reducing NO.sub.x in a gas stream containing NO and
particulates, comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream having a ratio of
NO to NO.sub.2 adjusted according to the type of SCR catalyst to
improve NO.sub.x reduction; removing at least a portion of the
particulates from the converted gas stream; adding a reductant
fluid to the converted gas stream to form a gas mixture; and
passing the gas mixture over an SCR catalyst.
89. The method of claim 88, wherein the ratio of NO to NO.sub.2 in
the gas mixture is from about 4:1 to 1:3 by volume.
90. The method of claim 88, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
91. The method of claim 88, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
92. The method of claim 88, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
93. The method of claim 92, wherein the platinum catalyst comprises
a Pt/Al.sub.2O.sub.3 catalyst.
94. The method of claim 88, wherein the reductant fluid comprises
urea.
95. A method of improving NO.sub.x conversion in an SCR system,
comprising: passing the gas stream over an oxidation catalyst
thereby converting at least a portion of the NO in the gas stream
to NO.sub.2 to form a converted gas stream; adding a reductant
fluid to the converted gas stream to form a gas mixture; and
passing the gas mixture over an SCR catalyst.
96. The method of claim 95, wherein the ratio of NO to NO.sub.2 in
the gas mixture is from about 4:1 to 1:3 by volume.
97. The method of claim 95, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
98. The method of claim 95, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
99. The method of claim 95, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
100. The method of claim 99, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
101. The method of claim 95, wherein the reductant fluid comprises
urea.
102. A method of improving NO.sub.x conversion in an SCR system,
comprising: passing the gas stream over an oxidation catalyst
thereby converting at least a portion of the NO in the gas stream
to NO.sub.2 to form a converted gas stream; removing at least a
portion of the particulates from the converted gas stream; adding a
reductant fluid to the converted gas stream to form a gas mixture;
and passing the gas mixture over an SCR catalyst.
103. The method of claim 102, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
104. The method of claim 102, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
105. The method of claim 102, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
106. The method of claim 102, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
107. The method of claim 106, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
108. The method of claim 102, wherein the reductant fluid comprises
urea.
109. A process for reducing NO.sub.x present in a lean exhaust gas
from an internal combustion engine by selective catalytic reduction
on a reduction catalyst using ammonia, comprising oxidizing some of
the NO present in the exhaust gas to NO.sub.2 so that the ratio of
NO to NO.sub.2 in the exhaust gas is from about 4:1 to 1:3 by
volume before contact with the reduction catalyst, wherein
oxidation of the NO present in the exhaust gas takes place in the
presence of an oxidation catalyst, passing the exhaust gas,
together with ammonia, over said reduction catalyst, wherein the
reduction catalyst comprises a transition metal/zeolite
catalyst.
110. The process according to claim 109, wherein the oxidation
catalyst comprises platinum on aluminum oxide.
111. The process according to claim 110, wherein the oxidation
catalyst is deposited on a honeycomb carrier.
112. A method of reducing levels of regulated pollutants in a gas
stream comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream; adding a
reductant fluid to the converted gas stream to form a gas mixture;
and passing the gas mixture over an SCR catalyst.
113. The method of claim 112, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
114. The method of claim 112, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
115. The method of claim 112, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
116. The method of claim 112, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
117. The method of claim 116, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
118. The method of claim 112, wherein the reductant fluid comprises
urea.
119. The method according to claim 112, wherein the regulated
pollutants comprise particulates.
120. The method according to claim 112, wherein the regulated
pollutants comprise hydrocarbons.
121. A method of reducing levels of regulated pollutants in a gas
stream comprising: passing the gas stream over an oxidation
catalyst thereby converting at least a portion of the NO in the gas
stream to NO.sub.2 to form a converted gas stream; removing at
least a portion of the particulates from the converted gas stream;
adding a reductant fluid to the converted gas stream to form a gas
mixture; and passing the gas mixture over an SCR catalyst.
122. The method according to claim 121, wherein the regulated
pollutants comprise particulates.
123. The method according to claim 121, wherein the regulated
pollutants comprise hydrocarbons.
124. The method of claim 121, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
125. The method of claim 121, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
126. The method of claim 121, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
127. The method of claim 121, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
128. The method of claim 127, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
129. The method of claim 121, wherein the reductant fluid comprises
urea.
130. A method of improving NO.sub.x conversion in an SCR system,
comprising: passing the gas stream over an oxidation catalyst
thereby converting at least a portion of the NO in the gas stream
to NO.sub.2 to form a converted gas stream; removing at least a
portion of the particulates from the converted gas stream;
minimizing the level of hydrocarbons in the gas stream; adding a
reductant fluid to the converted gas stream to form a gas mixture;
and passing the gas mixture over an SCR catalyst.
131. The method of claim 130, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
132. The method of claim 130, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
133. The method of claim 130, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
134. The method of claim 130, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
135. The method of claim 134, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
136. The method of claim 130, wherein the reductant fluid comprises
urea.
137. A method of reducing NO.sub.x in a gas stream comprising:
passing the gas stream over an oxidation catalyst thereby
incompletely converting NO in the gas stream to NO.sub.2 to form a
converted gas stream; adding a reductant fluid to the converted gas
stream to form a gas mixture; and passing the gas mixture over an
SCR catalyst.
138. The method of claim 137, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
139. The method of claim 137, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
140. The method of claim 137, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
141. The method of claim 137, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
142. The method of claim 141, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
143. The method of claim 137, wherein the reductant fluid comprises
urea.
144. A method of reducing volume and/or weight of an exhaust gas
after-treatment system of a light duty diesel engine comprising:
attaching an SCR system to the light duty diesel engine, the SCR
system providing reduction of NO.sub.x in the exhaust gas by:
passing the exhaust gas over an oxidation catalyst thereby
converting at least a portion of the NO in the exhaust gas to
NO.sub.2 to form a converted exhaust gas; adding a reductant fluid
to the converted gas stream to form a gas mixture; and passing the
gas mixture over an SCR catalyst.
145. The method of claim 144, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
146. The method of claim 144, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
147. The method of claim 144, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
148. The method of claim 144, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
149. The method of claim 148, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
150. The method of claim 144, wherein the reductant fluid comprises
urea.
151. A method of reducing NO.sub.x in a gas stream containing NO
and particulates, comprising: passing the gas stream over an
oxidation catalyst thereby converting at least a portion of the NO
in the gas stream to NO.sub.2 to form a converted gas stream;
removing at least a portion of the particulates from the converted
gas stream; and passing the gas mixture over an SCR catalyst.
152. The method of claim 151, wherein the particulates are removed
from the converted gas stream using a particulate filter or
particulate trap.
153. The method of claim 151, wherein the ratio of NO to NO.sub.2
in the gas mixture is from about 4:1 to 1:3 by volume.
154. The method of claim 151, wherein the gas stream comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
155. The method of claim 151, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
156. The method of claim 151, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
157. The method of claim 156, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
158. The method of claim 151, wherein the reductant fluid comprises
urea.
159. A method of reducing pollutants, including particulates and
NO.sub.x, in a gas stream, comprising passing said 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 thereby
enhancing the NO.sub.2 content of the gas stream, removing at least
a portion of said particulates in a particulate trap, reacting
trapped particulate with NO.sub.2, adding reductant fluid to the
gas stream to form a gas mixture downstream of said trap, and
passing the gas mixture over an SCR catalyst under NO.sub.x
reduction conditions.
160. A method according to claim 159, wherein said gas stream is
the exhaust from a diesel, GDI or DNG engine.
161. A method according to claim 159, wherein the gas stream or gas
mixture is cooled before reaching the SCR catalyst.
162. A method according to claim 159, wherein the NO to NO.sub.2
ratio of the gas mixture is adjusted to a level pre-determined to
be optimum for the SCR catalyst, by oxidation of NO over said
oxidation catalyst.
163. A method according to claim 159, wherein the SCR catalyst is
maintained at a temperature from 160.degree. C. to 450.degree.
C.
164. A method according to claim 159, wherein the SCR catalyst
includes a component selected from the group consisting of a
transition metal and a rare-earth metal.
165. A method according to claim 164, wherein the transition metal
is selected from the group consisting of copper and vanadium.
166. A method according to claim 159, wherein the SCR catalyst is
V.sub.2O.sub.5/WO.sub.3/TiO.sub.2.
167. A method according to claim 162, wherein substantially all NO
is converted to NO.sub.2.
168. A method according to claim 162, wherein the ratio of
NO:NO.sub.2 is adjusted to about 4:3.
169. A method according to claim 159, wherein the reductant fluid
is a hydrocarbon.
170. A method according to claim 159, wherein the reductant fluid
is selected from the group consisting of ammonia, ammonium
carbamate and urea.
171. A method according to claim 168, comprising contacting the gas
mixture leaving the SCR catalyst with a clean-up catalyst to remove
NH.sub.3 or derivatives thereof.
172. A method according to claim 171, wherein the space velocity of
the exhaust gas over the SCR catalyst is in the range 40,000 to
70,000 hr.sup.-1.
173. An SCR system for reducing NO.sub.x in exhaust gases
comprising: an oxidation catalyst that converts at least a portion
of NO in the exhaust gas to NO.sub.2; a reductant fluid source
downstream from the oxidation catalyst; and an SCR catalyst
downstream from the reductant fluid source.
174. The system of claim 173, wherein the ratio of NO to NO.sub.2
in the exhaust gas just prior to the SCR catalyst is from about 4:1
to 1:3 by volume.
175. The system of claim 173, wherein the exhaust gas comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
176. The system of claim 173, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
177. The system of claim 173, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
178. The system of claim 177, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
179. The system of claim 173, wherein the reductant fluid comprises
urea.
180. The system of claim 173, wherein the ratio of the reductant
fluid to NO in the exhaust gas stream just prior to the SCR
catalyst comprises 0.6:1 to 1:1.
181. The system of claim 173, wherein the ratio of the reductant
fluid to NO.sub.2 in the exhaust gas stream just prior to the SCR
catalyst comprises 0.8:1 to 4:3.
182. The system of claim 173 further comprising a clean-up catalyst
downstream of the SCR catalyst.
183. The system of claim 173, wherein a space velocity capacity of
the SCR catalyst is in the range of 40,000 to 70,000 hr.sup.-1.
184. An SCR system for reducing NO.sub.x in exhaust gases
comprising: an oxidation catalyst that converts at least a portion
of NO in the exhaust gas to NO.sub.2; a particulate filter or
particulate trap downstream from the oxidation catalyst; a
reductant fluid source downstream from the particulate filter or
particulate trap; and an SCR catalyst downstream from the reductant
fluid source.
185. The system of claim 184, wherein the ratio of NO to NO.sub.2
in the exhaust gas just prior to the SCR catalyst is from about 4:1
to 1:3 by volume.
186. The system of claim 184, wherein the exhaust gas comprises
exhaust from sources selected from the group consisting of: heavy
duty diesel engines, light duty diesel engines, gasoline direct
injection engines, compressed natural gas engines, ships, and
stationary sources.
187. The system of claim 184, wherein the SCR catalyst comprises a
transition metal/zeolite catalyst.
188. The system of claim 184, wherein the oxidation catalyst
comprises a platinum catalyst deposited on a ceramic through-flow
honeycomb support.
189. The system of claim 184, wherein the platinum catalyst
comprises a Pt/Al.sub.2O.sub.3 catalyst.
190. The system of claim 184, wherein the reductant fluid comprises
urea.
191. The system of claim 184, wherein the ratio of the reductant
fluid to NO in the exhaust gas stream just prior to the SCR
catalyst comprises 0.6:1 to 1:1.
192. The system of claim 184, wherein the ratio of the reductant
fluid to NO.sub.2 in the exhaust gas stream just prior to the SCR
catalyst comprises 0.8:1 to 4:3.
193. The system of claim 184 further comprising a clean-up catalyst
downstream of the SCR catalyst.
194. The system of claim 184, wherein a space velocity capacity of
the SCR catalyst is in the range of 40,000 to 70,000 hr.sup.-1.
Description
[0001] This application is a divisional application of U.S. patent
application Ser. No. 09/601,964, filed Jan. 9, 2001, which is the
U.S. national phase application of International Application No.
PCT/GB99/00292, filed Jan. 28, 1999, and claims priority of British
Patent Application No. 9802504.2, filed Feb. 6, 1998.
[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).
[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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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/A12O3 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.
[0011] 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.
[0012] The reductant fluid is suitably NH.sub.3, 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.
[0013] 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. 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
utilising 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.sub.x
conversion. Preferably, the preferred SCR catalyst is maintained at
a temperature from 160.degree. C. to 450.degree. C.
[0014] 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 vol, 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.sub.x reduction.
[0015] 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 NO.sub.x. 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] A commercial 10 litre turbocharged heavy duty diesel engine
on a test-bed was used for all the tests described herein.
Test 1--(Comparative)
[0020] 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.
[0021] 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)
[0022] 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 10 g 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
[0023] 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
[0024] An R49 test with 80% NH.sub.3 injection was carried out over
a V.sub.2O.sub.5/WO3/TiO.sub.2 SCR catalyst. This gave 67%
particulate, 89% HC and 87% NO.sub.x conversion; the results are
plotted in FIG. 4.
[0025] Additionally tests have been carried out with a different
diesel engine, and the excellent results illustrated in Test 3 and
4 above have been confirmed.
[0026] The results have been confirmed also for a non-vanadium SCR
catalyst.
* * * * *