Method of extending the useful life of an aged SCR catalyst bed in an exhaust system of a stationary source of NOX

Abstract

A method of extending the useful life of an aged selective catalytic reduction (SCR) catalyst bed, which catalyses the conversion of oxides of nitrogen (NO.sub.x) to dinitrogen (N.sub.2) in the presence of a nitrogenous reductant, in the exhaust gas after treatment system of a stationary source of NO.sub.x so that the exhaust gas emitted to atmosphere from the system continues to meet proscribed limits for both NO.sub.x and ammonia emissions, which method comprising the step of retrofitting an additional honeycomb substrate monolith or a plate-type substrate comprising a catalyst (ASC) for converting ammonia in exhaust gas also containing oxygen to nitrogen and water downstream of the aged SCR catalyst bed, wherein the kNO.sub.x of the honeycomb substrate monolith comprising the catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water is greater than or equal to 80 m/hr between 300 and 400 C. inclusive, wherein kNOx of a sample of the catalyst, which has been aged at 450 C. in 10% H.sub.2O (as steam) in air for 48 hours, is determined by a SCR activity test in a laboratory scale reactor using a gas composition of 50 ppm CO, 30 ppm NO, 36 ppm NH.sub.3, 15% O.sub.2, 8% water, 3% CO.sub.2, balance N.sub.2.

Claims

1. A method of extending the useful life of an aged selective catalytic reduction (SCR) catalyst bed, which catalyses the conversion of oxides of nitrogen (NO.sub.x) to dinitrogen (N.sub.2) in the presence of a nitrogenous reductant, in the exhaust gas after treatment system of a stationary source of NO.sub.x so that the exhaust gas emitted to atmosphere from the system continues to meet proscribed limits for both NO.sub.x and ammonia emissions, which method comprising the step of retrofitting a honeycomb substrate monolith or a plate-type substrate comprising a catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water downstream of the aged SCR catalyst bed, wherein a kNO.sub.x of the honeycomb substrate monolith comprising the catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water is greater than or equal to about 80 m/hr between about 250 and about 400 C. inclusive, wherein the kNOx of a sample of the catalyst, which has been aged at 450 C. in 10% H.sub.2O (as steam) in air for 48 hours, is determined by a SCR activity test in a laboratory scale reactor using a gas composition of 50 ppm CO, 30 ppm NO, 36 ppm NH.sub.3, 15% O.sub.2, 8% water, 3% CO.sub.2, balance N.sub.2.

2. The method according to claim 1, wherein the kNO.sub.x of the honeycomb substrate monolith or the plate-type substrate comprising the catalyst is less than or equal to about 300 m/hr.

3. The method according to claim 1, wherein the kNOx of the honeycomb substrate monolith or the plate-type substrate comprising the catalyst is about 90<kNOx<about 300 m/h between about 300 and about 400 C.

4. The method according to claim 1, wherein the catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water has an sNOxNOx out divided by (NH.sub.3 in minus NH.sub.3 out)<about 20% below about 400 C., wherein the sNOx is determined using the same conditions defined in claim 1 for determining kNOx.

5. The method according to claim 4, wherein the catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water has an sNOx<about 10% below about 350 C.

6. The method according to claim 1, wherein the space velocity at which the exhaust gas contacts the retrofitted catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water is from 50,000 to 500,000 h.sup.1.

7. The method according to claim 1, wherein the retrofitted catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water comprises a platinum group metal supported on a refractory oxide support and a selective catalytic reduction catalyst.

8. The method according to claim 7, wherein the SCR catalyst is one of (i) vanadia supported on titania in the anatase form and promoted with tungsta or molybdena (ii) a metal promoted molecular sieve; or (iii) a mixture of vanadia supported on titania in the anatase form and promoted with tungsta or molybdena and a metal promoted molecular sieve.

9. The method according to claim 7, wherein the molecular sieve has a Framework Type Code that is CHA, AEI, AFX, BEA, MOR, MFI or FER.

10. The method according to claim 7, wherein the metal in the metal promoted molecular sieve is copper and/or iron.

11. The method according to claim 7, wherein the honeycomb substrate monolith or the plate-type substrate has an axial length and is coated with a first washcoat layer comprising the platinum group metal supported on a refractory oxide support and a second washcoat layer comprising the SCR catalyst.

12. The method according to claim 11, wherein the first layer is disposed in a first zone coated from a first end of the honeycomb substrate monolith and the second layer is disposed in a second zone coated from a second end of the honeycomb substrate monolith or the plate-type substrate, wherein the second layer is disposed upstream from the first layer.

13. The method according to claim 12, wherein the first zone is less than the axial length of the honeycomb substrate monolith or the plate-type substrate and the second layer of the second zone at least partially overlies the first layer.

14. The A method according to claim 12, wherein the second layer extends to the entire axial length of the honeycomb substrate monolith or the plate-type substrate.

15. The method according to claim 11, wherein the first layer extends to the entire axial length of the honeycomb substrate monolith and the second layer overlies the first layer and extends to the entire axial length of the honeycomb substrate monolith or the plate-type substrate.

16. The method according to claim 1, wherein the temperature at which the exhaust gas contacts the SCR catalyst bed is about 200 C. to about 450 C.

17. The method according to claim 1, wherein the nitrogenous reductant is ammonia (NH.sub.3).

18. The method according to claim 1, wherein an alpha ratio of ammonia molecules to NO.sub.x molecules contacting the aged SCR catalyst bed is about 0.90 to about 2.00.

19. The method according to claim 1, wherein the stationary source of NO.sub.x is a power station, an industrial heater, a cogeneration power plant, a combined cycle power generation plant, a wood-fired boiler, a stationary diesel engine, a stationary natural gas-fired engine, a marine propulsion engine, a diesel locomotive engine, an industrial waste incinerator, a municipal waste incinerator, a chemical plant, a glass manufacturing plant, a steel manufacturing plant or a cement manufacturing plant.

20. The method according to claim 19, wherein the exhaust gas after treatment system comprises a heat recovery steam generator (HRSG).

21. The method according to claim 1, wherein the SCR catalyst is a washcoat coated onto a substrate or is a component of an extruded honeycomb body.

22. The method according to claim 21, wherein the SCR catalyst is one of: i. vanadia supported on titania in the anatase form and promoted with tungsta or molybdena; ii. a metal promoted molecular sieve; or iii. a mixture of vanadia supported on titania in the anatase form and promoted with tungsta or molybdena and a metal promoted molecular sieve.

Description

EXAMPLES

Example 1

(1) A dual layered Ammonia Slip Catalyst (ASC) was prepared on a ceramic 230 cells per square inch (cpsi) honeycomb substrate monolith having 7 mil (thousandths of an inch) cell wall thickness. A first (lower) layer was coated directly onto the substrate having a continuous alumina washcoated layer along the entire axial length of the substrate. The resulting coated part was dried and calcined. Next Pt was impregnated into the washcoated alumina layer from a platinum nitrate solution to a loading of 5 g/ft.sup.3 Pt. The resulting part was then dried and calcined. Finally, a second washcoat layer comprising Cu impregnated CHA zeolite mixed with binders was applied in a continuous layer covering 100% of Pt alumina layer along the entire axial length of the substrate and the resulting part was then dried and calcined. A cylindrical core of 2 inches in diameter and 3.3 inches in length was cut from the finished honeycomb substrate monolith coated with the dual layered ASC.

Example 2 (SCR Test)

(2) The cylindrical ASC core of Example 1 was degreened at 450 C. in 10% H.sub.2O (as steam) in air for 48 hours then activity tested in a laboratory scale reactor for SCR activity. The gas composition fed to the catalyst for the activity test was 50 ppm CO, 30 ppm NO, 36 ppm NH.sub.3, 15% O.sub.2, 8% water, 3% CO.sub.2, and balanced by N.sub.2. CO, NOx, and NH.sub.3 conversions were measured with the reactor held at steady state temperature points ranging from 200 to 450 C. The Gas Hourly Space Velocity (GHSV) over the ASC volume was 180,000 h.sup.1.

(3) The CO conversion, NOx conversion, and NH.sub.3 conversion are each shown in Table 1 for the various temperatures. The kNOx, defined as kNOx=(Area Velocity)*ln (1NOx Conversion/100), are also shown.

(4) TABLE-US-00001 TABLE 1 Temp C. NO Conv (%) CO Conv (%) NH.sub.3 Conv (%) kNOx 200 40 41 35 46 252 64 56 73 94 305 73 64 84 120 352 75 67 87 125 402 71 70 87 113 452 59 73 86 80

Example 3 (NH.SUB.3 .Oxidation Test)

(5) The cylindrical ASC core of Example 1 was degreened at 450 C. in 10% H.sub.2O in air for 48 hours then activity tested in a laboratory scale reactor for NH.sub.3 oxidation activity. The gas composition fed to the catalyst was 50 ppm CO, 30 ppm NH.sub.3, 10% 02, 4% water, 3% CO.sub.2, and balanced by N.sub.2. CO, NOx, and NH.sub.3 conversions were measured with the reactor held at steady state temperature points ranging from 200 to 450 C. The Gas Hourly Space Velocity (GHSV) over the ASC volume was 120,000 h.sup.1.

(6) The results show CO conversion and NH.sub.3 conversion in Table 2. The sNOx, defined as (NOx out divided by (NH.sub.3 in minus NH.sub.3 out)), is also shown.

(7) TABLE-US-00002 TABLE 2 Temp C. NH.sub.3 Conv. (%) CO Conv. (%) sNOx (%) 200 44 46 2 250 78 72 3 298 88 80 4 345 91 83 6 401 92 86 8 451 92 88 18

Example 4 (SCR Test w/High Ammonia to NOx Ratio (ANR))

(8) The cylindrical ASC core of Example 1 was degreened at 450 C. with 10% H.sub.2O in air for 48 hours then activity tested in a laboratory scale reactor for SCR activity. The gas composition fed to the catalyst for the activity test was 50 ppm CO, 30 ppm NO, 54 ppm NH.sub.3, 15% 02, 8% water, 3% CO.sub.2, and balanced by N.sub.2. CO, NOx, and NH.sub.3 conversions were measured with the reactor held at steady state temperature points ranging from 200 to 450 C. The Gas Hourly Space Velocity (GHSV) over the ASC volume was 180,000 h.sup.1.

(9) The CO conversion, NOx conversion, and NH.sub.3 conversion are shown in Table 1 for the various temperatures. The kNOx, defined as kNOx=(Area Velocity)*ln(1NOx Conversion/100), are also shown.

(10) TABLE-US-00003 TABLE 3 Temp C. NO Conv CO Conv NH.sub.3 Conv kNOx 200 46 46 30 56 253 67 59 70 101 305 73 66 81 120 353 76 69 86 128 402 73 71 87 118 452 63 74 88 90