System and Process for Efficient SCR at High NO2 to NOx Ratios

Abstract

Disclosed herein is a system for the removal of volatile organic compounds, carbon monoxide and nitrogen oxides from off-gas even at high NO.sub.2 to NO.sub.x ratios, wherein the amount of NO.sub.2 within NO.sub.x is higher than or equal to 50 mol-%, comprising a source of ammonia, means for introducing ammonia into a catalytic article having an SCR functionality; a catalytic article having both an oxidation and an SCR functionality, the catalytic article comprising a catalyst substrate and a catalyst composition comprising at least one platinum group metal and/or at least one platinum group metal oxide, at least one oxide of titanium and at least one oxide of vanadium, wherein the washcoat is located in and/or on the walls of the catalyst substrate: means for measuring the amount of NO.sub.x and/or the ammonia slip between the outlet end of the catalytic article and the stack or at the stack, at least one carbon monoxide source, and means for introducing carbon monoxide into the catalytic article. Optionally, an SCR catalytic article can be placed upstream of downstream of the cata-lytic article having both an oxidation and an SCR functionality. Also disclosed is a method for the removal of volatile organic compounds, carbon monoxide and nitrogen oxides from off-gas introducing carbon monoxide in order to keep the amount of NOx and/or the ammonia slip between the outlet end of the catalytic article and the stack or at the stack at predetermined values. The method makes use of the system according to the invention. The system and the method can be used for the cleaning of flue gas.

Claims

1. A system for the removal of volatile organic compounds, carbon monoxide and nitrogen oxides from off-gas comprising a) a source of ammonia b) means for introducing ammonia into a catalytic article having an SCR functionality; c) a catalytic article having both an oxidation and an SCR functionality, the catalytic article comprising a catalyst substrate and a catalyst composition comprising at least one platinum group metal and/or at least one platinum group metal oxide, at least one oxide of titanium and at least one oxide of vanadium, wherein the washcoat is located in and/or on the walls of the catalyst substrate, d) means for measuring the amount of NOx and/or the ammonia slip between the outlet end of the catalytic article and the stack or at the stack, e) at least one carbon monoxide source, and f) means for introducing carbon monoxide into the catalytic article.

2. The system according to claim 1, wherein the source of ammonia is selected from anhydrous ammonia, aqueous ammonia or an ammonium precursor selected from an aqueous urea solution, an aqueous ammonium formate solution and an aqueous ammonium carbamate solution and mixtures thereof.

3. The system according to claim 1, wherein the catalyst substrate is selected from flow-through substrates, wall-flow substrates, corrugated substrates, ceramic candle filters, bag filters, or catalyst pellets.

4. The system according to claim 1, wherein the means for introducing ammonia or an aqueous ammonia precursor into the off-gas is a means for introducing anhydrous ammonia.

5. The system according to claim 1, wherein the catalyst substrate is a corrugated substrate, and the catalyst composition comprises 50 to 10,000 ppmw (parts per million per weight) of at least one platinum group metal calculated as the pure precious metal and based on the total weight of the catalytic article, wherein the platinum group metal is palladium, and 60 to 90 wt.-% of at least one oxide of titanium, calculated as TiO.sub.2 and based on the total weight of the catalytic article, wherein the at least one oxide of titanium is titanium dioxide, and 0.1 to 17 wt.-% of at least one oxide of vanadium, calculated as V.sub.2O.sub.5 and based on the total weight of the catalytic article, wherein the at least one oxide of vanadium is vanadium pentoxide, and wherein the total weight of the catalytic article is the sum of the amounts of the at least one platinum group metal, the amount of the at least one oxide of titanium, the amount of the at least one oxide of vanadium, and the amount of the catalyst substrate.

6. The system according to claim 5, wherein the catalyst composition additionally comprises 0.001 to 10 wt.-% of at least one oxide of tungsten, calculated as WO.sub.3 and based on the total weight of the catalytic article.

7. The system according to claim 1, wherein the catalytic article having both an oxidation and an SCR functionality is the only catalytic article having an SCR functionality, and wherein the means for introducing ammonia are located directly upstream of said catalytic article.

8. The system according to claim 1, wherein an SCR catalytic article is present upstream of the catalytic article having both an oxidation and an SCR functionality, and wherein the SCR catalytic article only has a selective catalytic reduction functionality, but no oxidation functionality, and wherein the means for introducing ammonia are located directly upstream of the SCR catalytic article.

9. The system according to claim 1, wherein an SCR catalytic article is present downstream of the catalytic article having both an oxidation and an SCR functionality, and wherein the SCR catalytic article only has a selective catalytic reduction functionality, but no oxidation functionality, and wherein the means for introducing ammonia according to b) are either located upstream the DFC or between the DFC and the SCR catalytic article.

10. The system according to claim 1, wherein the at least one carbon monoxide source is selected from uncleaned exhaust gas released by a power plant and/or hydrocarbons and/or carbon monoxide from external sources.

11. The system according to claim 1, said system additionally comprising means for measuring the amount of CO emitted between the outlet end of the catalytic article and the stack or at the stack.

12. A method for the removal of volatile organic compounds, carbon monoxide and nitrogen oxides from off-gas, comprising the steps of a) introducing ammonia into the off-gas, b) introducing the off-gas into a catalytic article having both an oxidation and an reduction functionality, the catalytic article comprising a catalyst substrate and a catalyst composition comprising at least one platinum group metal and/or at least one platinum group metal oxide, at least one oxide of titanium and at least one oxide of vanadium, c) measuring the amount of NOx and/or the ammonia slip between the outlet end of the catalytic article and the stack or at the stack, d) introducing carbon monoxide into the catalytic article to decrease the amount of NOx and/or the ammonia slip in the stack measured under step c), and e) measuring the amount of NOx and/or the ammonia slip between the outlet end of the catalytic article and the stack or at the stack after the introduction of carbon monoxide.

13. The method according to claim 12, wherein the source of ammonia is selected from anhydrous ammonia, aqueous ammonia or an ammonium precursor selected from an aqueous urea solution, an aqueous ammonium formate solution and an aqueous ammonium carbamate solution and mixtures thereof.

14. The method according to claim 12, wherein the catalyst substrate in step b) is selected from flow-through substrates, wall-flow substrates, corrugated substrates, ceramic candle filters, bag filters or catalyst pellets.

15. The method according to claim 12, wherein steps c) and e) additionally comprise measuring the amount of CO emitted between the outlet end of the catalytic article and the stack or at the stack.

16. The method according to claim 12, wherein the catalytic article having both an oxidation and an SCR functionality is the only catalytic article having an SCR functionality, and wherein the means for introducing ammonia are located directly upstream of said catalytic article.

17. The method according to claim 12, wherein an SCR catalytic article is present upstream of the catalytic article having both an oxidation and an SCR functionality, and wherein the SCR catalytic article only has a selective catalytic reduction functionality, but no oxidation functionality, and wherein the means for introducing ammonia are located directly upstream of the SCR catalytic article.

18. The method according to claim 12, wherein an SCR catalytic article is present downstream of the catalytic article having both an oxidation and an SCR functionality, and wherein the SCR catalytic article only has a selective catalytic reduction functionality, but no oxidation functionality, and wherein the means for introducing ammonia according to b) are either located upstream the DFC or between the DFC and the SCR catalytic article.

19. A method for the cleaning of flue gas released by gas turbines or by nitric acid plants, which comprises using the system according to claim 1.

20. A method for the cleaning of flue gas released by gas turbines or by nitric acid plants, which comprises using the method according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0121] The ΔNO.sub.x [%] values versus NH.sub.3/NO.sub.x [ppm/ppm] for the various NO.sub.2 contents are shown in FIG. 1.

[0122] The NO/NO.sub.x [%] values versus NH.sub.3/NO.sub.x [ppm/ppm] for the various NO.sub.2 contents are shown in FIG. 2.

[0123] FIG. 3 shows ΔNO.sub.x [%] versus NH.sub.3/NO.sub.x for measurements 2a), 2b) and 2c).

[0124] FIG. 4 shows ΔNO.sub.x [%] versus NH.sub.3/NO.sub.x for measurements 2a), 2b) and 2d).

[0125] FIG. 5 shows a comparison for ΔNO.sub.x versus NH.sub.3/NO.sub.x for Comparative Example 1 and Embodiment 1 at 0% and 100% NO.sub.2, respectively, is shown in FIG. 5.

EMBODIMENTS

Comparative Example 1: DeNOx Efficiency of a Standard SCR Catalytic Article De-Pending on the NH.SUB.3./NO.SUB.x .Ratio

[0126] The SCR kinetics of a standard SCR catalyst were tested to examine what happened if the fraction of NO.sub.2 in off-gas varied between 0 and 100 mol-%. of total NO.sub.x. The standard catalyst was a vanadia-titania-tungsten catalytic article comprising 3 wt.-% V.sub.2O.sub.5, 3 wt.-% WO.sub.3, 70-80 wt.-% TiO.sub.2 and about 15 wt.-% of a silica-alumina binder on a corrugated substrate. The catalyst was free of precious metals or precious metal oxides.

[0127] Measurement Conditions:

[0128] 35 ppmvd NO.sub.x, wherein ppmvd stands for “parts per million per volume and dry-based” 10% dry O.sub.2

[0129] Temperature:

[0130] The measurements were performed for 0, 25, 50, 75 and 100% NO.sub.2 at 560 F (=293.3° C.) and for 0% NO.sub.2 at 700 F (=371.1° C.)

[0131] 5% H.sub.2O

[0132] 35 Nm/Hr A.sub.v

TABLE-US-00001 TABLE 1 ΔNO.sub.x [%] versus NH.sub.3/NO.sub.x [ppm/ppm] for the various NO.sub.2 contents 0% NO.sub.2 25% NO.sub.2 50% NO.sub.2 NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] 0 0 0 0 0 0 0.41 34.44 0.41 37.54 0.40 34.20 0.85 71.86 0.85 75.80 0.84 74.90 1.28 83.45 1.26 87.17 1.27 87.23 1.66 85.32 1.68 88.34 1.68 88.80 2.55 87.03 2.53 89.34 2.54 89.90 0% NO.sub.2 at 700 F. 75% NO.sub.2 100% NO.sub.2 (=371.1° C.) NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] 0 0 0 0 0 0 0.41 22.96 0.41 22.34 0.41 42.41 0.84 43.85 0.84 28.50 0.84 77.70 1.27 50.89 1.26 34.11 1.26 88.44 1.68 54.10 1.69 39.32 1.69 90.50 2.52 60.39 2.53 44.83 2.53 91.98

[0133] The results are shown in FIG. 1.

TABLE-US-00002 TABLE 2 NO/NO.sub.x [%] versus NH.sub.3/NO.sub.x [ppm/ppm] for the various NO.sub.2 contents 25% NO.sub.2 50% NO.sub.2 NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] NO/NO.sub.x[%] [ppm/ppm] NO/NO.sub.x[%] 0 74.63 0 51.87 0.41 87.79 0.40 47.46 0.85 89.15 0.84 37.15 1.26 82.74 1.27 30.29 1.68 81.11 1.68 33.13 2.53 79.13 2.54 37.01 75% NO.sub.2 100% NO.sub.2 NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] NO/NO.sub.x[%] [ppm/ppm] NO/NO.sub.x[%] 0 28.85 0 11.38 0.41 16.22 0.41 4.87 0.84 6.52 0.84 4.07 1.27 5.88 1.26 3.93 1.68 6.04 1.69 6.65 2.52 6.39 2.53 5.99

[0134] The results are shown in FIG. 2.

Embodiment 1: DeNOx Efficiency of a DFC According to the Present Invention De-Pending on the NH.SUB.3./NO.SUB.x .Ratio

[0135] The SCR kinetics of a DFC according to the present invention were tested. The DFC comprised 400 ppm Pd, 4 wt.-% V.sub.2O.sub.5, 3 wt.-% WO.sub.3 and 78 wt.-% TiO.sub.2, rest binder (SiO.sub.2/Al.sub.2O.sub.3) on a corrugated substrate.

[0136] Measurement Conditions:

[0137] 35 ppmvd NO.sub.X

[0138] 10% dry O.sub.2

[0139] Temperature:

[0140] The measurements were performed for 0 and 100% NO.sub.2 at 700 F (=371.1° C.)

[0141] 5% H.sub.2O

[0142] 35 Nm/Hr A.sub.v

[0143] The measurements were performed with [0144] a) 0% NO2, [0145] b) 100% NO2 [0146] c) 100% NO2 repeat [0147] d) 100% NO.sub.2 and 400 ppm CO

TABLE-US-00003 TABLE 3 ΔNO.sub.x [%] versus NH.sub.3/NO.sub.x [ppm/ppm] for the measurements 2a) to 2d) 2a) 0% 2b) 100% NO.sub.2 NO.sub.2 NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] 0 0 0 0 0.48 45.11 0.47 23.01 1.00 80.20 0.94 29.16 1.51 85.46 1.41 31.79 2.01 86.87 1.88 33.87 2.99 87.86 2.81 38.14 2c) 100% 2d) 100% NO.sub.2 NO.sub.2 + 400 repeat ppm CO NH.sub.3/NO.sub.x NH.sub.3/NO.sub.x [ppm/ppm] ΔNO.sub.x[%] [ppm/ppm] ΔNO.sub.x[%] 0 0 0 0 0.47 17.49 0 −5.47 0.94 24.71 0.46 42.62 1.42 28.02 0.94 78.06 1.88 30.10 1.40 83.91 2.83 34.51 1.88 85.33 3.27 36.42 2.82 86.16 3.76 37.96 4.22 39.95 4.69 40.59

[0148] The results for 2a), 2b) and 2c) and for 2a), 2b) and 2d), respectively, are shown in FIGS. 3 and 4.

[0149] The negative value for ΔNO.sub.x during in measurement 2d) is due to NO production across the catalyst while CO is present.

[0150] A comparison for ΔNO.sub.x versus NH.sub.3/NO.sub.x for Comparative Example 1 and Embodiment 1 at 0% and 100% NO.sub.2, respectively, is shown in FIG. 5.