CATALYTIC REDUCTION OF NOx
A system for NO x reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO 2 , particulate filter, a source of reductant such as NH 3 and an SCR catalyst and methods for operating such a system are disclosed. Considerable improvements in NO x conversion and soot reduction are observed even at relatively low operating temperatures.
1 . A method for treating a diesel engine exhaust gas stream for introduction to an SCR catalyst, comprising:
passing the exhaust gas stream over an oxidation catalyst and a particulate trap under conditions effective to convert NO in the exhaust gas stream to NO 2 such that there is sufficient NO 2 to combust a substantial portion of the particulates from the gas stream in the particulate trap; and
whereby the resulting gas stream has a ratio of NO to NO 2 of about 4:1 to 1:3 when the exhaust gas exits the particulate trap.
2 . The method of claim 1 , wherein the exhaust gas stream is between 225° C. and 425° C.
3 . The method of claim 1 , wherein the gas stream is between 225° C. and 350° C.
4 . The method of claim 1 , wherein the gas stream is between 250° C. and 425° C.
5 . The method of claim 2 , wherein the diesel engine is a heavy duty diesel truck engine configured for mobile use.
6 . The method of claim 5 , wherein the heavy duty diesel truck engine is turbocharged.
7 . The method of claim 6 , further comprising injecting a metered dose of urea reductant into the exhaust gas stream after the exhaust gas stream exits the particulate trap and before the exhaust gas stream enters the SCR catalyst system.
8 . The method of claim 7 , wherein the oxidation catalyst comprises a Pt catalyst deposited upon a through-flow honeycomb support.
9 . The method of claim 8 , wherein the SCR catalyst comprises a transitional metal/zeolite catalyst.
10 . The method of claim 9 , further comprising passing the exhaust gas over a clean-up catalyst to remove NH 3 from the exhaust gas.
11 . The method of claim 10 , wherein the oxidation catalyst and particulate trap are sized and configured such that under the majority of normal on-road operational conditions of the heavy duty diesel truck engine the exhaust gas stream has a ratio of NO to NO 2 of about 4:1 to 1:3 when the exhaust gas exits the particulate trap.
12 . The method of claim 11 , further comprising adjusting the amount of urea injected into the exhaust gas stream based at least in part upon the NO:NO 2 ratio of the NO x in the exhaust gas stream.
13 . The method of claim 12 , wherein under the majority of normal on-road operational conditions of the heavy duty diesel truck engine the exhaust gas stream has at least 90% less NO x as it exits the SCR catalyst than the exhaust gas stream contained before it entered the oxidation catalyst.
14 . A method for treating a diesel engine exhaust gas stream, comprising:
passing the exhaust gas stream over an oxidation catalyst under conditions effective to convert a sufficient amount of NO in the gas stream to NO 2 such that there is sufficient NO 2 to:
(i) combust, when operated at a temperature between 225° C. and 425° C., a substantial portion of particulates from the gas stream in a particulate trap; and
(ii) reduce a substantial portion of NO x exiting the particulate trap in an SCR catalyst.
15 . The method of claim 14 , wherein the diesel engine is a heavy duty diesel truck engine configured for mobile use.
16 . The method of claim 15 , wherein the heavy duty diesel truck engine is turbocharged.
17 . The method of claim 16 , further comprising injecting a metered dose of urea reductant into the exhaust gas stream after the exhaust gas stream exits the particulate trap and before the exhaust gas stream enters the SCR catalyst system.
18 . The method of claim 17 , further comprising adjusting the amount of urea injected into the exhaust gas stream based at least in part upon the NO:NO 2 ratio of the NO x in the exhaust gas stream.
19 . The method of claim 18 , wherein the SCR catalyst comprises a transitional metal/zeolite catalyst.
20 . The method of claim 19 , further comprising passing the exhaust gas over a clean-up catalyst to remove NH 3 from the exhaust gas.
21 . The method of claim 20 , wherein the oxidation catalyst and particulate trap are sized and configured to provide an exhaust gas stream with a ratio of NO to NO 2 of about 4:1 to 1:3 at the exit of the particulate trap when the exhaust gas is at a temperature between 225° C. and 425° C.
22 . The method of claim 21 , wherein the oxidation catalyst comprises a Pt catalyst deposited upon a through-flow honeycomb support.
23 . The method of claim 22 , wherein the exhaust gas stream has at least 90% less NO x as it exits the SCR catalyst than the exhaust gas stream contained before it entered the oxidation catalyst when the exhaust gas stream is at a temperature between 225° C. and 425° C.