Catalytic reduction of NO
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 , tparticulate filter, a source of reductant such as NH 3 and an SCR catalyst. Considerable improvements in NO x conversion are observed.
1. A method of reducing particulates and NO x in an exhaust gas stream of a diesel engine, comprising: (1) converting at least a portion of the NO in the gas stream to NO 2 on an oxidation catalyst; (2) passing the gas stream through a particulate trap to trap at least a portion of the particulates from the gas stream; (3) reducing the proportion of NO 2 in the gas stream by combusting a portion of the trapped particulates in the presence of NO 2 ; (4) adding reductant fluid to the gas stream to form a mixed composition; and (5) passing the mixed composition over the SCR catalyst under NO x reduction conditions.
2. The method of claim 1 , wherein: the ratio of NO to NO 2 in the gas stream, prior to adding the reductant fluid, is from about 4:1 to about 1:3 by volume.
3. The method of claim 1 , wherein: the particulate trap comprises a wall-flow filter.
4. A method of reducing particulates and NO x in an exhaust gas stream of a diesel engine, comprising: (1) converting at least a portion of the NO in the gas stream to NO 2 on an oxidation catalyst; (2) passing the converted gas stream through a particulate trap to trap at least a portion of the particulates from the gas stream; (3) reducing the proportion of NO 2 in the converted gas stream by combusting a portion of the trapped particulates in the-presence of NO 2 ; (4) adding reductant fluid to the gas stream to form a gas mixture; and (5) passing the gas mixture over the SCR catalyst under NO x reduction conditions.
5. The method of claim 4 , wherein: the ratio of NO to NO 2 in the gas stream, prior to adding the reductant fluid, is from about 4:1 to about 1:3 by volume.
6. The method of claim 4 , wherein: the particulate trap comprises a wall-flow filter.
7. The method of claim 1 , wherein the reductant fluid is selected from the group consisting of ammonia, ammonium carbamate and urea.
8. The method of claim 1 , wherein the reductant fluid in NH 3 .
9. The method of claim 1 , wherein the reductant fluid in urea.
10. The method of claim 1 , wherein the reductant fluid being added to the gas stream prior to entering the SCR catalyst is urea.
11. The method of claim 1 , wherein the oxidation catalyst is a platinum catalyst carried on a through-flow honeycomb support.
12. The method of claim 1 , wherein the gas stream is cooled before reaching the SCR catalyst.
13. The method of claim 1 , wherein the NO to NO 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 the oxidation catalyst.
14. The method of claim 11 , wherein the NO to NO 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 the oxidation catalyst.
15. The method of claim 14 , wherein the ratio of NO:NO 2 leaving the oxidation catalyst is adjusted to about 4:3.
16. The method of claim 1 , wherein the SCR catalyst is maintained at a temperature from 160° C. to 450° C.
17. The method of claim 1 , wherein the SCR catalyst includes a component selected from the group consisting of a transition metal and a rare-earth metal.
18. The method of claim 1 , wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.
19. The method of claim 1 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
20. The method of claim 1 , further comprising the step of contacting the gas mixture leaving the SCR catalyst with a clean-up catalyst to remove NH 3 or derivatives thereof.
21. The method of 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 .
22. The method of claim 1 , wherein the particulates are removed without causing accumulation and resulting blockage and back pressure problems.
23. The method of claim 1 , wherein the particles are removed from the particulate trap by combustion in the presence of NO 2 .
24. The method of claim 1 , wherein the particulate trap is manufactured from ceramic.
25. The method of claim 1 , wherein the particulate trap is manufactured from woven knitted heat resistant fabrics.
26. The method of claim 1 , wherein the particulate trap is manufactured from non-woven heat resistant fabrics.
27. The method of claim 1 , wherein the gas stream comprises exhaust from a heavy duty diesel engine.
28. The method of claim 1 , wherein the gas stream comprises exhaust from a light duty diesel engine.
29. The method of claim 1 , wherein NH 3 or derivatives thereof which pass unreacted or as by-products through the SCR catalyst are removed by means of a clean-up catalyst arranged downstream of the SCR catalyst.
30. The method of claim 4 , wherein the reductant fluid is selected from the group consisting of ammonia, ammonium carbamate and urea.
31. The method of claim 4 , wherein the reductant fluid is NH 3 .
32. The method of claim 4 , wherein the reductant fluid is urea.
33. The method of claim 4 , wherein the reductant fluid being added to the gas stream prior to entering the SCR catalyst is urea.
34. The method of claim 4 , wherein the oxidation catalyst is a platinum catalyst carried on a through-flow honeycomb support.
35. The method of claim 4 , wherein the gas stream is cooled before reaching the SCR catalyst.
36. The method of claim 4 , wherein the NO to NO 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 the oxidation catalyst.
37. The method of claim 34 , wherein the NO to NO 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 the oxidation catalyst.
38. The method of claim 37 , wherein the ratio of NO:NO 2 leaving the oxidation catalyst is adjusted to about 4:3.
39. The method of claim 4 , wherein the SCR catalyst is maintained at a temperature from 160° C. to 450° C.
40. The method of claim 4 , wherein the SCR catalyst includes a component selected from the group consisting of a transition metal and a rare-earth metal.
41. The method of claim 4 , wherein the SCR catalyst is selected from the group consisting of transition metal/zeolite catalysts, rare earth-based catalysts and transition metal catalysts.
42. The method of claim 4 , wherein the SCR catalyst comprises a transition metal/zeolite catalyst.
43. The method of claim 4 , further comprising the step of contacting the gas mixture leaving the SCR catalyst with a clean-up catalyst to remove NH 3 or derivatives thereof.
44. The method of claim 4 , wherein the space velocity of the exhaust gas over the SCR catalyst is in the range 40,000 to 70,000 h −1 .
45. The method of claim 4 , wherein the particulates are removed without causing accumulation and resulting blockage and back pressure problems.
46. The method of claim 4 , wherein the particles are removed from the particulate trap by combustion in the presence of NO 2 .
47. The method of claim 4 , wherein the particulate trap is manufactured from ceramic.
48. The method of claim 4 , wherein the particulate trap is manufactured from woven knitted heat resistant fabrics.
49. The method of claim 4 , wherein the particulate trap is manufactured from non-woven heat resistant fabrics.
50. The method of claim 4 , wherein the gas stream comprises exhaust from a heavy duty diesel engine.
51. The method of claim 4 , wherein the gas stream comprises exhaust from a light duty diesel engine.
52. The method of claim 4 , wherein NH 3 or derivatives thereof which pass unreacted or as by-products through the SCR catalyst are removed by means of a clean-up catalyst arranged downstream of the SCR catalyst.