Catalyst for treating an exhaust gas, an exhaust system and a method

Abstract

A catalyst for treating an exhaust gas comprising SO.sub.2, NO.sub.x and elemental mercury in the presence of a nitrogenous reductant comprises a composition containing oxides of: (i) Molybdenum (Mo) and/or Tungsten (W); (ii) Vanadium (V); (iii) Titanium (Ti), and (iv) an MFI zeolite, wherein the composition comprises, based on the total weight of the composition: (i) 1 to 6 wt % of MoO.sub.3 and/or 1 to 10 wt % WO.sub.3; and (ii) 0.1 to 3 wt % V.sub.2O.sub.5, and (iii) 48.5 to 94.5 wt % TiO.sub.2; and (iv) 35 to 50 wt % MFI zeolite.

Claims

1. A catalyst for treating an exhaust gas comprising SO.sub.2, NO.sub.x and elemental mercury in the presence of a nitrogenous reductant, the catalyst comprising a composition containing oxides of: (i) Molybdenum (Mo) and/or Tungsten (W); (ii) Vanadium (V); (iii) Titanium (Ti); and (iv) an MFI zeolite, wherein the composition comprises, based on the total weight of the composition: (i) 1 to 6 wt % of M.sub.oO3 and/or 1 to l0 wt % WO.sub.3; and (ii) 0.1 to 3 wt % V.sub.2O.sub.5; and (iii) 48.5 to 94.5 wt % TiO.sub.2; and (iv) 35 to 50 wt % MFI zeolite.

2. The catalyst according to claim 1, wherein the composition does not comprise phosphorus.

3. The catalyst according to claim 1, wherein the composition consists of the MFI zeolite and the oxides of Mo, W, V, and Ti.

4. The catalyst according to claim 1, wherein the catalyst is a plate catalyst.

5. The catalyst according to claim 1, wherein the MFI zeolite is an H-MFI zeolite.

6. The catalyst according to claim 1, wherein the MFI zeolite is an H-ZSM-5 zeolite.

7. A method for treating an exhaust gas comprising SO.sub.2, NO.sub.x and elemental mercury, the method comprising: contacting a flow of exhaust gas with the catalyst of claim 1 in the presence of a nitrogenous reducing agent to thereby provide a treated exhaust gas.

8. The method according to claim 7, wherein the nitrogenous reducing agent is ammonia, hydrazine or an ammonia precursor selected from the group consisting of urea ((NH.sub.2).sub.2CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate and ammonium formate.

9. The exhaust system of claim 8, wherein the combustion source is a furnace or a boiler, of a coal or oil power plant, a cement plant or a waste incinerator.

10. A exhaust system for a combustion source for treating an exhaust gas comprising SO.sub.2, NO.sub.x and elemental mercury, the system comprising a conduit for carrying a flowing exhaust gas, a source of nitrogenous reductant, a catalyst according to claim 1 disposed in a flow path of the exhaust gas and means for metering nitrogenous reductant into a flowing exhaust gas upstream of the catalyst.

11. The exhaust system according to claim 10, the system further comprising a wet or a dry scrubber for recovering oxidized mercury from the treated exhaust gas.

Description

EXAMPLES

(1) The present invention will next be specifically described in detail by way of the following non-limiting examples.

Preparation of Comparative Example 1 (Ref. 1.2% V.SUB.2.O.SUB.5./TiMo)

(2) A catalyst comprising 1.2 wt. % V.sub.2O.sub.5 and 2.7 wt. % MoO.sub.3 on TiO.sub.2 was prepared by combining titania with ammonium metavanadate and ammonium heptamolybdate to with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Comparative Example 2 (Ref. 2.2% V.SUB.2.O.SUB.5./TiMo)

(3) A catalyst comprising 2.2 wt. % V.sub.2O.sub.5 and 2.7 wt. % MoO.sub.3 on TiO.sub.2 was prepared by combining titania with ammonium metavanadate and ammonium heptamolybdate with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Example 1 (1.2% V.SUB.2.O.SUB.5./TiMo+16% H-MFI)

(4) A catalyst comprising 1.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 16 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

P Preparation of Example 2 (1.2% V.SUB.2.O.SUB.5./TiMo+40% H-MFI)

(5) A catalyst comprising 1.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 40 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Example 3 (2.2% V.SUB.2.O.SUB.5./TiMo+13% H-MFI)

(6) A catalyst comprising 2.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 13 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a to plate-type catalyst.

Preparation of Example 4 (2.2% V.SUB.2.O.SUB.5./TiMo+16% H-MFI

(7) A catalyst comprising 2.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 16 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst. Two samples (Example 4A and 4B) were tested and are reported in the Table below.

Preparation of Example 5 (2.2% V.SUB.2.O.SUB.5./TiMo+19% H-MFI)

(8) A catalyst comprising 2.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 19 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Example 6 (2.2% V.SUB.2.O.SUB.5./TiMo+40% H-MFI)

(9) A catalyst comprising 2.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 40 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Comparative Example 71.2% V.SUB.2.O.SUB.5./TiMo+16% MOR

(10) A catalyst comprising 1.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 16 wt. % mordenite Zeolite (MOR) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and mordenite Zeolite (MOR) with clay, fibers and organic binders and then kneading into a paste. Crystalline ammonium heptamolybdate tetrahydrate was added directly into the paste, and the mixture was further kneaded. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Example 81.2% V.SUB.2.O.SUB.5./TiMo+16% FeMFI

(11) A catalyst comprising 1.2 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 16 wt. % Zeolite (Fe-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Fe-Zeolite (Fe-MFI) with clay, fibers and organic binders and then kneading into a paste. Crystalline ammonium heptamolybdate tetrahydrate was added directly into the paste, and the mixture was further kneaded. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Comparative Example 30.6% V.SUB.2.O.SUB.5./TiMo

(12) A catalyst comprising 0.6 wt. % V.sub.2O.sub.5 and 2.7 wt. % MoO.sub.3 on TiO.sub.2 was prepared by combining titania with ammonium metavanadate and ammonium heptamolybdate with clay, fibers and organic binders and then kneading into a paste. Crystalline ammonium heptamolybdate tetrahydrate was added directly into the paste, and the mixture was further kneaded. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

Preparation of Example 90.6% V.SUB.2.O.SUB.5./TiMo+16% H-MFI

(13) A catalyst comprising 0.6 wt. % V.sub.2O.sub.5, 2.7 wt. % MoO.sub.3 on TiO.sub.2 and 16 wt. % Zeolite (H-MFI) was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Zeolite (H-MFI) with clay, fibers and organic binders and then kneading into a paste. Crystalline ammonium heptamolybdate tetrahydrate was added directly into the paste, and the mixture was further kneaded. The paste was laminated onto stainless-steel mesh to a thickness of 0.8 mm and calcined to form a plate-type catalyst.

General Procedure for Evaluating NO.SUB.z., SO.SUB.S .& Hg Conversion

(14) Each catalyst plate was first cut into strips with the dimensions of 25 mm400 mm. Four of these strips were then mounted vertically in a reaction tube and a synthetic gas mixture was passed through the reaction tube. The synthetic gas mixture for NOR, SON, and Hg testing were different for each test and the compositions and conditions of these synthetic gas mixtures are provided in the table below. 1. Hg Testing: The compositions of inlet and outlet gases to and from the reactor were determined by on-line FTIR spectroscopy, which analyzes for multiple compounds simultaneously. The FTIR sample cell temperature was kept at about 230 C. to avoid water condensation and salt formation inside the instrument. The Hg concentrations were analyzed at both the inlet and outlet of the reactor using a commercial Continuous Emissions Monitor (CEM) that uses Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS). The Hg conversion was calculated using the inlet and outlet concentrations of elemental Hg. 2. NOx Testing: The percent NO.sub.x removal was determined through measurement of NO.sub.x concentration at the inlet and outlet of a catalyst layer by means of a chemiluminescent NO.sub.x analyzer. 3. SO.sub.x Testing: Percent SO.sub.2 oxidation was determined through measurement of SO.sub.3 concentration at the outlet of the catalyst layer by wet chemistry.

(15) TABLE-US-00001 TABLE 1 test parameters Conditions Hg Test NO.sub.x Test SO.sub.x Test Temperature ( C.) 380 350 400 Total Flow (L/min) 30 74 16.6 Area Velocity (m/hr) 22.5 55.5 12.5 Linear Velocity (m/s) 0.69 1.71 0.39 Space Velocity (hr.sup.1) 6237 15400 3470 NH.sub.3 (ppm) 24 400 0 NO (ppm) 60 400 0 SO.sub.2 (ppm) 425 500 500 O.sub.2 (%) 6 5 5 H.sub.2O (%) 11 10 10 HCl (ppm) 15 0 0 Hg (ug/m.sup.3) 10 0 0

(16) TABLE-US-00002 TABLE 2 results of the testing wt % wt % Hg conv. SO.sub.x conv. kNO.sub.x V.sub.2O.sub.5 H-MFI (%) (%) (m/hr) Comparative 1.2 0 41.5 1.2 42.5 Example 1 (Ref 1.2) Example 1 1.2 16 57.0 1.35 42.4 (15073) Example 2 1.2 40 54.01 1.05 37.6 (15418) Comparative 2.2 0 55.6 2.1 46.7 Example 2 (Ref 2.2) Example 3 2.2 13 63.9 2.0 46.6 (15312) Example 4A 2.2 16 66.4 2.6 45.4 (15220) Example 4B 2.2 16 69.1 2.0 43.7 (15287) Example 5 2.2 19 62.6 2.2 46.3 (15313) Example 6 2.2 40 65.1 1.4 42.5 (15417) Comparative 1.2 16 52.8 0.9 39.7 Example 7 (MOR) Example 8 1.2 16 56.3 3.8 42.6 (Fe-MFI) Comparative 0.6 0 37 0.55 23.0 Example 3 (Ref. 0.6) Example 9 0.6 16 37.5 0.62 29.0 (160065)

(17) As is clear from test data of the catalysts, the catalyst of the present invention has good performance; i.e., maintains high levels of NO.sub.x removal and high mercury oxidation activity, and low levels of SO.sub.2 oxidation.

(18) As demonstrated, the inventive Examples 1-6 have improved Hg oxidation activity and comparable or better SO.sub.x and NO.sub.2 activity than those comparative compositions without the MFI zeolite. As demonstrated by Example 7, the use of the MFI zeolite, especially H-ZSM5 zeolite, is much more effective than mordenite. As demonstrated to by Example 8, the use of the H-MFI zeolite is better than iron promoted-MFI zeolite because the latter demonstrates excessive SO.sub.x oxidation.

(19) Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.

(20) For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.