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 optionally Tungsten (W); and (ii) Vanadium (V); and (iii) Titanium (Ti); and (iv) Phosphorus (P), wherein, with respect to the total metal atoms in the composition, the composition comprises: (i) Mo in an amount of less than 2 at. %, and optionally up to 9 at. % W; (ii) from 2.5 to 12 at. % V; (iii) from 85 to 96 at. % Ti, and wherein the composition comprises (iv) P in an atomic ratio to the sum of atoms of Mo, W and V of from 1:2 to 3:2. The values expressed must total 100%.

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); (ii) Vanadium (V); (iii) Titanium (Ti); and (iv) Phosphorus (P), wherein, with respect to the total metal atoms in the composition, the composition comprises: (i) from 0.5 to 1.95 at. % Mo and no Tungsten (W); (ii) from 2.5 to 12 at. % V; (iii) from 85 to 96 at. % Ti, and wherein the composition comprises (iv) P in an atomic ratio to the sum of atoms of Mo and V of from 1:2 to 3:2.

2. The catalyst according to claim 1, wherein the composition comprises: (i) from 1.75 to 1.95 at. % Mo; (ii) from 3 to 11 at. % V; (iii) from 87 to 95 at. % Ti; and wherein the composition comprises (iv) P in an atomic ratio to the sum of atoms of Mo and V of from 9:10 to 7:5.

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

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

5. A method for treating an exhaust gas comprising SO.sub.2, NOx 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.

6. The method according to claim 5, 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, or a mixture of two or more thereof.

7. A exhaust system for a combustion source for treating an exhaust gas comprising SO.sub.2, NOx and elemental mercury, the system comprising a conduit for carrying a flowing exhaust gas, a source of nitrogenous reductant, a catalyst of 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.

8. The exhaust system according to claim 7, the system further comprising a wet or a dry scrubber for recovering oxidized mercury from the treated exhaust gas.

9. The exhaust system of claim 7, wherein the combustion source is a furnace or a boiler, of a coal or oil power plant, a cement plant or a waste incinerator.

Description

EXAMPLES

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

(2) The following methods were conducted to provide the examples in the table below. In table 1, the amounts are by wt. % (weight percent). In Table 2 the amounts are by at. % (atomic percent):

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

(3) 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 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.

Comparative Example 28.1% V.SUB.2.O.SUB.5.-6.4% MoO.SUB.3.-9.4% P.SUB.2.O.SUB.5./TiO.SUB.2

(4) A catalyst comprising 8.1 wt. % V.sub.2O.sub.5, 6.4 wt. % MoO.sub.3 and 9.4 wt. % P.sub.2O.sub.5 on TiO.sub.2 according to U.S. Pat. No. 8,535,628 was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and Ammoniumhydrogenphosphate 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.

Example 18.1% V.SUB.2.O.SUB.5.-2.3% MoO.SUB.3.-6% P.SUB.2.O.SUB.5./TiO.SUB.2

(5) A catalyst comprising 8.1 wt. % V.sub.2O.sub.5, 2.3 wt. % MoO.sub.3 and 6 wt. % P.sub.2O.sub.5 on TiO.sub.2 was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and ammoniumhydrogenphosphate 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.

Example 23.1% V.SUB.2.O.SUB.5.-2.3% MoO.SUB.3.-4.9% P.SUB.2.O.SUB.5./TiO.SUB.2

(6) A catalyst comprising 3.1 wt. % V.sub.2O.sub.5, 2.3 wt. % MoO.sub.3 and 4.9 wt. % P.sub.2O.sub.5 on TiO.sub.2 was prepared by combining titania with ammonium metavanadate, ammonium heptamolybdate and ammoniumhydrogenphosphate 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.x., SO.SUB.x .& Hg conversion

(7) 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 NO.sub.x, SO.sub.x, 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.

(8) TABLE-US-00003 TABLE 1 kNOx SOx conv. (m/hr) (%) Hg conv. Sample Catalyst (350 C.) (400 C.) (%) Comp. 1 1.2% V.sub.2O.sub.5 42.5 1.2 41.5 2.7% MoO.sub.3 Balance TiO.sub.2 Comp 2 8.1% V.sub.2O.sub.5 33.9 0.79 60.8 6.4% MoO.sub.3 9.4% P.sub.2O.sub.5 Balance TiO.sub.2 Ex. 1 8.1% V.sub.2O.sub.5 34.2 1.3 70 2.3% MoO.sub.3 6% P.sub.2O.sub.5 Balance TiO.sub.2 Ex. 2 3.1% V.sub.2O.sub.5 33.1 1.2 60.5 2.3% MoO.sub.3 4.9% P.sub.2O.sub.5 Balance TiO.sub.2

(9) TABLE-US-00004 TABLE 2 Sample V at. % Mo at. % Ti at. % P/(V + Mo + W) Comp. 2 9.5 4.7 85.8 1.0 Ex. 1 11.0 1.9 87.1 0.9 Ex. 2 3.6 1.8 94.6 1.4

(10) As is clear from test data of the catalysts, the catalyst of the present invention has good performance; i.e. it maintains good levels of NO.sub.x removal and relatively high mercury oxidation activity, and relatively low levels of SO.sub.2 oxidation. Surprisingly these benefits are seen with the claimed relatively low levels of Mo and optionally relatively lower levels of V.sub.2O.sub.5, reducing the cost of producing an effective catalyst composition for treating exhaust gases as described herein.

(11) 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.

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