WO2015051363A1

WO2015051363A1 - Method and system for removing gaseous mercury in flue gases

Info

Publication number: WO2015051363A1
Application number: PCT/US2014/059288
Authority: WO
Inventors: Joo-Youp LEE; Zhouyang LIU
Original assignee: University Of Cincinnati
Priority date: 2013-10-04
Filing date: 2014-10-06
Publication date: 2015-04-09

Abstract

The present invention is directed to methods and systems for removing elemental mercury in a mercury-containing flue gas by oxidizing the elemental mercury with a catalyst composite to form one or more water soluble, oxidized mercury compounds. The catalyst composites are useful over a wide temperature range from about 150°C to about 400°C and are highly resistant to various gases present in fossil fuel combustion flue gases. The catalyst composite includes an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (TiO₂), gamma-alumina (Al₂O₃), or a combination thereof. The catalyst composite may further include a ruthenium oxide and/or other Deacon catalysts adsorbed on the surface of the support material.

Description

METHOD AND SYSTEM FOR REMOVING GASEOUS MERCURY IN FLUE GASES

GOVERNMENT LICENSE RIGHTS

[0001] This invention was made with government support under NSF CAREER Grant #1151017 awarded by National Science Foundation. The government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] Pursuant to 37 C.F.R. § 1.78(a), this application claims the benefit of and priority to prior filed, co-pending Provisional Application Serial No. 61/877,040 filed October 4, 2013, which is expressly incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0003] The present invention relates generally to pollution control, and more specifically is directed to catalytic methods and catalytic systems useful for oxidizing elemental mercury to substantially reduce the amount of mercury released into the environment by coal-fired utility plants and from other sources.

BACKGROUND

[0004] Mercury and its compounds are significant environmental pollutants and major threats to human life and natural ecosystems. Mercury is of significant environmental concern because of its toxicity, persistence in the environment, and bioaccumulation in the food chain. The toxicity of soluble Hg ions and elemental Hg even in very dilute concentrations has been widely reported in the literature. Mercury is released readily into the environment from natural and anthropogenic sources. Because of its physical and chemical properties, mercury can also be transported regionally through various environmental cycles. Atmospheric deposition of mercury is reported to be the primary cause of elevated mercury levels in fish, which is a potential threat to pregnant women and young children. [0005] The annual global mercury emission is estimated at 5,000 tons. The United States accounts for approximately 3% of such mercury emissions although this persistent pollutant travels globally via jet stream and gets converted to methyl mercury in the environment with high neurodevelopment toxicity. In the United States, coal-fired power utility plants are the biggest source of mercury emissions into the air, emitting a total of about fifty metric tons of mercury into the atmosphere annually, which is approximately thirty-three percent of all mercury emissions from the United States. Coal-fired combustion flue gas streams are of particular concern because of their composition that includes trace amounts of acid gases, such as SO₂, NO_x, and HC1 plus CO₂ and oxygen contents. Other sources of mercury emissions may include the chloralkali industry, metal sulfide or smelting, gold refining, cement production, fossil fuel combustion and incineration of sewage sludge or municipal garbage or the like.

[0006] The major chemical forms of the metal in the combustion flue gases are the elemental Hg(0) (zero valent) and the oxidized mercury, i.e., Hg(I) and Hg(II). Elemental mercury (Hg(0)) vapor is found predominantly in coal-fired boiler flue gas. Mercury can also be bound to fly ash in the flue gas. Mercury speciation (elemental or oxidized) and concentration is dependent on the source (e.g. the characteristics of the fuel being burned), process conditions and the constituents in the ensuing gas streams (e.g., (¾ HQ, SO₂, NO_x). The thermodynamically stable predominant form of mercury in the flue gases from coal-fired utilities is the elemental one (Hg(0)). However, the oxidized ¾(¾ may be the major species from waste incinerators.

[0007] Unlike the oxidized forms, mercury in its zero valent state is difficult to remove due its high volatility and low water solubility. One method to remove elemental mercury from flue gas involves the injection of bromine-treated, powdered, activated carbon into the flue gas stream, for mercury adsorption with subsequent removal in a particulate collector. However, activated carbon is expensive, and the method generally requires high carbon-to- mercury mass ratios. Additionally, the quality of captured fly ash can be degraded.

[0008] Another method utilizes oxidative sorbents containing metal impregnated silicates that facilitate mercury removal by using oxidative reactions of elemental mercury and sequential adsorption of Hg(I) or Hg(II) on the surfaces of the one or more silicates, as disclosed in U.S. Patent No. 7,858,061, which is incorporated by reference herein in its entirety. The silicates are reported to contain active metal salts (Cu(I), Cu(II), Fe(II), Fe(III), Ni(II), or Zn(II)) on the surface that oxidize elemental mercury. However, the oxidative sorbents also include activated carbon to achieve high selectivity in removing elemental mercury.

[0009] But on December 21, 201 1, the U.S. Environmental Protection Agency (EPA) announced the Mercury and Air Toxics Standards (MATS) rule that would further limit mercury, acid gases, and other toxic heavy metal emissions from coal- and oil-fired utility, industrial, commercial, and institutional power plants. The new rule, which was effective on April 16, 2012, is projected to reduce mercury emissions by greater than 90%. In July 2010, prior to the MATS, the U.S. EPA also issued a new proposed rule, the Transport Rule, which replaces the 2005 Clean Air Interstate Rule (CAIR) and will start to regulate sulfur dioxide (SO₂) and nitrogen oxide (NO_x) emissions from power plants in 28 states from 2012. The U.S. EPA estimates that a total 272.2 gigawatts (GW) of flue gas desulfurization (FGD) and 217.6 GW of selective catalytic reduction (SCR) units of a total 373 GW to be generated from coal combustion would be operative by 2020 in order to meet the Transport Rule requirements (according to the TR SB Limited Trading model).

[0010] The use of powder river basin (PRB) subbituminous coal, which generates higher percentages of elementary mercury (Hg(0)) vapor, is increasing, and the proposed Transport Rule is very likely to increase the installation of wet FGD systems (>95% for SO₂ control on a basis of total electricity generation) and SCR units for large coal-fired power plants. In this context, heterogeneous Hg(0) oxidation using catalysts or oxidants is highly expected to play a critical role in future mercury emissions control in the U.S.

[0011] To date, noble metals and metal oxides have been primarily studied for heterogeneous catalytic Hg(0) oxidation. Noble metal-based catalysts have shown limited success in the absence of HC1 or chlorine gas. Au and Pd catalysts oxidize Hg(0) vapor primarily by C¾ gas, but Pt catalyst requires HC1 and oxygen gases for Hg(0) oxidation, suggesting different mechanisms. Recently, various metal oxide-based Hg(0) catalysts including Fe₂0₃, V2O5, M0O₃, (¾(¾, MnO_¾ Ce0₂, C0₃O4 and RUO2 have been studied on various supports for the development of a Hg(0)-specific or modified SCR catalyst.

However, even in the presence of HC1 gas, many of these metal oxide catalysts exhibited limited Hg(0) oxidation at a low HC1 level typically found in sub-bituminous or lignite coal combustion flue gas (e.g., <10 ppm) due to the competitive adsorption of multiple gases including S0₂, NH₃, HC1, and Hg(0) onto metal oxide sites.

[0012] Thus, in view of the foregoing, a need exists for new methods, systems, and/or catalyst composites for efficiently oxidizing elemental mercury present in mercury- containing fluids to substantially reduce mercury emissions into the environment.

SUMMARY OF THE INVENTION

[0013] In accordance with an embodiment of the present invention, a method for removing elemental mercury in an elemental mercury-containing flue gas is provided. The method includes contacting the elemental mercury-containing flue gas with a catalyst composite at a temperature within a range from about 150°C to about 400°C in the presence of oxygen and at least one of a hydrogen halide or a molecular halogen, wherein the elemental mercury is converted into one or more water soluble, oxidized mercury compounds. The catalyst composite includes an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (T1O2), gamma-alumina (AI2O₃), or a combination thereof. In accordance with another embodiment, the catalyst composite further includes a ruthenium oxide adsorbed on the surface of the support material.

[0014] In another embodiment, a system for removing elemental mercury in a combustion exhaust gas is provided. The system includes a combustion chamber for combustion of a fossil fuel source; a catalyst chamber comprising a catalyst composite, wherein the combustion chamber is fluidly coupled to an inlet of the catalyst chamber to allow flow of combustion exhaust gas from the combustion chamber into the catalyst chamber; and a scrubber for removing the one or more water soluble, oxidized mercury compounds from the mercury-containing flue gas. An inlet of the scrubber is fluidly coupled to an outlet of the catalyst chamber to allow flow of the combustion exhaust gas from the outlet of the catalyst chamber into the scrubber. The catalyst composite includes an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (T1O2), gamma-alumina (AI2O₃), or a combination thereof, wherein the catalyst composite converts elemental mercury into one or more water soluble, oxidized mercury compounds. In accordance with another embodiment, the catalyst composite further includes a ruthenium oxide adsorbed on the surface of the support material. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description given below, serve to describe the invention.

[0016] FIGS. 1A-1C are schematics of coal-fired power plants showing three exemplary configurations for incorporating a catalyst composite for oxidizing elemental mercury in the coal-fired power plant, in accordance with embodiments of the invention.

[0017] FIG. 2 is schematic of an experimental set up for a fixed bed catalyst chamber for evaluating the catalyst composites, in accordance with an embodiment of the invention.

[0018] FIG. 3 is schematic of an experimental set up for a structured catalyst chamber for evaluating the catalyst composites, in accordance with an embodiment of the invention.

[0019] FIG. 4 is a performance curve showing an absence of mercury break-through upon doubling sulfur dioxide (SO₂) presence in a simulated coal flue gas comprising 12% CO₂, 7% H₂0, 3% 0₂, 1,000-2,000 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 100 ppmv HCl, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention.

[0020] FIG. 5 is a performance curve for mercury oxidation over time in a simulated bituminous coal reduced flue gas comprising 12% CO₂, 7% H₂0, 3% O₂, 1,000 ppmv SO₂, 20 ppmv NH₃, 20 ppmv NO, 10 ppmv HCl, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention.

[0021] FIG. 6 is a performance curve for mercury oxidation over time in a simulated bituminous coal reduced flue gas comprising 12% CO₂, 7% H₂0, 6% O₂, 1,000 ppmv SO₂, 0 ppmv NH₃, 20 ppmv NO, 10 ppmv HCl, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention.

[0022] FIG. 7 is a performance curve comparing mercury oxidation for various catalytic conditions as a function of HBr concentration in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 6% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HCl, and 20 ppbv Hg(0) in the presence of a) an empty sand bed; b) 10 wt% Fe/DT51 catalyst composite dispersed in a sand bed; c) (10 wt% Fe + 0.5 wt% Ru)/DT51 catalyst composite dispersed in a sand bed; and d) empty sand bed without HC1 gas, at 350°C, in accordance with an embodiment of the invention.

[0023] FIG. 8 is a performance curve comparing mercury oxidation as a function of temperature from 150°C to 400°C in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite, in accordance with an embodiment of the invention.

[0024] FIG. 9 is a performance curve comparing mercury oxidation as a function of temperature from 150°C to 400°C in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a (10 wt% Fe + 0.5 wt% Ru)/DT51 catalyst composite, in accordance with an embodiment of the invention.

[0025] FIG. 10 is a performance curve comparing mercury oxidation as a function of HBr concentration from 1 ppm to 5 ppm in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, and 20 ppbv Hg(0) in the presence of a (10 wt% Fe + 0.5 wt% Ru)/DT51 catalyst composite at 150°C, in accordance with an embodiment of the invention.

[0026] FIG. 11 is graph showing X-Ray Diffraction (XRD) patterns, obtained using Cu Ka radiation with a wavelength of 1.5406 A, of an anatase-tititania support with and without Fe or Fe/Ru active metal component(s).

[0027] FIG. 12 is a graph showing X-ray Absorption Near Edge Structure (XANES) data showing iron oxide speciation of a 10% Fe-loaded anatase-titania catalyst composite, in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0028] Embodiments of the present invention are directed to methods and systems for removing elemental mercury in a mercury-containing flue gas by oxidizing the elemental mercury with a catalyst composite to form one or more water soluble, oxidized mercury compounds. The catalyst composites are useful over a wide temperature range from about 150°C to about 400°C and are highly resistant to various gases present in fossil fuel combustion flue gases. In accordance with embodiments of the present invention, the catalyst composite includes an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (T1O₂), gamma-alumina (AI₂O₃), or a combination thereof. In accordance with another embodiment, the catalyst composite further includes a ruthenium oxide and/or other Deacon catalysts adsorbed on the surface of the support material.

[0029] The catalyst composites in accordance with embodiments of the present invention, in part, function as "Deacon catalysts" in the catalytic oxidation of elemental mercury present in fossil fuel combustion gases. The Deacon reaction, which is shown in Reaction (1) below, represents the conversion of a hydrogen halide (e.g., hydrogen chloride) and molecular oxygen to a molecular halogen (e.g., chlorine) and water over a Deacon catalyst. At one time it was proposed that chlorine production correlates with Hg(0) oxidation. However, the homogeneous reaction between chlorine (gas) and Hg(0) vapor is known to be slow, and thus is not enough to explain the extent of observed Hg(0) oxidation. The production of chlorine was also significantly inhibited by higher concentrations of SO₂ gas over metal oxides.

Recently, it was supposed that the CI atom adsorbed onto coordinatively unsaturated ruthenium atoms might be enough and responsible for high Hg(0) conversion, suggesting a heterogeneous catalytic reaction. The adsorption of HC1 gas followed by the formation of CI atoms onto metal oxide surfaces is thought to be the key to successful Hg(0) oxidation. In any event, the elemental mercuiy Hg(0) is oxidized to create water-soluble, oxidized mercury in the flue gas stream, which may then be removed from the flue gas stream.

Reaction (1) 2HC1 + ½ 0₂ ^ C1₂ + H₂0, AH⁰ = -28.4 kJ/mol

[0030] Accordingly, methods and systems for eliminating or substantially reducing elemental mercury from flue gas using the catalyst composite are provided herein. The catalyst composite is positioned in an oxidation zone of a fossil fuel combustion and provides for the oxidation of elemental mercury. Advantageously, the catalyst composites of the present invention do not substantially cause the formation of sulfur trioxide from sulfur dioxide in the flue gas. Accordingly, the oxidation zone is does not need to be maintained at a temperature of less than 200°C. As a result, the present catalyst composites may be used over a wide temperature range, e.g., from about 150°C to about 400°C for oxidizing elemental mercury in flue gases generated from the combustion of mercury containing fossil fuels, (e.g., coal).

[0031] With reference to FIGS. 1A-1C, schematic illustrations of a traditional coal-fired power plants 10a- 10c showing non-limiting, examplary configurations for incorporating the catalyst composite for oxidizing elemental mercury in the coal-fired power plant are provided. The basic components of the coal-fired power plants 10a- 10c include a combustion chamber 15, a selective catalytic reduction (SCR) unit 20, a fly ash removal unit 25, a flue- gas desulfurization (FDG) unit 30, and an emission stack 35. In a typical coal-fired process, coal is combusted in the presence of air, which generates flue gas. Coal, such as bituminous, sub-bituminous, and lignite coals, and other fossil fuels, such as crude oil, contain variable amounts of mercury. Accordingly, the flue gas created from combustion of coal, oil, or other fossil fuels contains elemental mercury and oxidized mercury, as well as other undesirable constituents, such as nitrogen oxides, sulfur dioxide, particulate matter such as fly ash, and hydrogen chloride gas. [0032] As shown in FIGS. 1A-1C, the flue gas 17 created from coal combustion carried out in the combustion chamber 15 and then fed to the SCR unit 20. The SCR unit 20 functions to reduce nitrogen oxides (NOx) in the flue gas 17 and can be considered to create a reduced flue gas 21. Generally, the SCR unit 20 is run at elevated temperatures of between about 300°C and about 400°C. As shown, the SCR unit 20 may add a gaseous reductant 23, such as anhydrous ammonia, aqueous ammonia or urea, to the flue gas 17. Within the selective catalytic reduction unit 20, the nitrogen oxides, the reductant 23, and oxygen are converted over the catalyst to nitrogen and water.

[0033] The reduced flue gas 21 is fed to a fly ash removal unit 25, such as a baghouse, electrostatic precipitator, inertial separator, fabric filter or other known device. At the fly ash removal unit 25, particulate matter 27 such as fly ash, along with pollutants or toxins adsorbed on the particulate matter 27, is removed from the reduced flue gas 21 producing a flue gas stream 28.

[0034] With particular reference to FIGS. 1A and IB, the flue gas stream 28 exits the fly ash removal unit 25 and then fed to a flue gas desulfurization unit 30, such as a wet scrubber as shown . In the flue gas desulfurization unit 30, sulfur dioxide and water-soluble, oxidized mercury are separated and removed from the flue gas stream 28 creating a scrubbed flue gas 37. For example, within a wet scrubber-type flue gas desulfurization unit, a water stream 32 containing calcium carbonate or calcium hydroxide is brought into contact with the flue gas stream 28. Water soluble compounds in the flue gas stream 28, including oxidized mercury and sulfur dioxide, are dissolved into the water stream 32 and exit the flue gas desulfurization unit 30 in a liquid stream 34. The scrubbed flue gas 37 may then be safely emitted into the air through an emission stack 35.

[0035] While the illustrated coal-fired power plants 10a- 10c include components in a defined sequence, embodiments of the present invention are not particularly limited thereto. Instead, other embodiments may include alternate arrangements. For example, the power plant may be operated without the SCR unit 20, or an adsorption-based desulfurization unit may be used in place of the wet flue gas desulfurization unit 30. However, in accordance with embodiments of the present invention, prior to discharging the scrubbed flue gas 37 to the outdoor air, at least one of the flue gas 17, the reduced flue gas 21, the flue gas stream 28, and/or the scrubbed flue gas 37 are contacted with the catalyst composite, as described in more detail below.

[0036] The catalyst composite may be housed within a catalyst chamber 40, which may be positioned within, or separate from, the SCR unit 20. In either arrangement, the catalyst chamber 40 may be positioned upstream or downstream of the SCR unit 20. For example as shown in FIG. 1A, the catalyst chamber 40 may be positioned within the SCR unit 20, with the catalyst composite positioned upstream of the SCR catalyst. Alternatively as shown in FIG. IB, the catalyst chamber 40 may be positioned within the SCR unit 20 with the catalyst composite positioned downstream of the SCR catalyst. When the catalyst chamber 40 is configured separately, i.e., a standalone unit, from the SCR unit 20, it may again be positioned upstream or downstream of the SCR unit 20.

[0037] With reference to FIG. 1C, in an embodiment where inventive catalyst composite is configured as a stand alone unit separate from the SCR unit 20, the catalyst chamber 40 may be positioned in a lower operating temperature region, e.g., between the fly ash removal unit 25 and the flue gas desulfurization unit 30. The flue gas 17 is generally at an elevated temperature, e.g., above about 300°C. For the illustrated plant 10c shown in FIG. 1C, the temperature of the flue gas stream 28 after passing through the fly ash removal unit 25 is about 200°C or less, e.g., about 150°C. Accordingly, the elemental mercury present in the flue gas stream 28 is oxidized in the presence of the catalyst composite to provide oxidized flue gas stream 29, which then passes onto the flue gas desulfurization unit 30. [0038] In accordance with embodiments of the present invention, the catalyst composite includes an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (Ti0₂), gamma-alumina (AI2O₃), or a combination thereof. The weight percent ratio of iron to the support material may be in a range from about l : 100 (about 1 wt%) to about 1 :5 (about 17 wt%). The weight percent of the active metal component on the support material may be in a range that includes about 1 wt% up to and including about 17 wt%, wherein the wt% is based on the entire weight of the catalyst composition. For example, the weight percent of the active metal component on the support material may be about 0.5 wt%, about 1 wt%, about 3 wt%, about 5 wt%, about 7 wt%, about 10 wt%, about 15 wt%, or about 17 wt%, or in a range between any combination of the foregoing.

[0039] The iron oxide may be derived from an iron oxide precursor selected from the group consisting of iron (II) and iron (III) salts and compounds, which after adsorption onto the surface of the support material followed by a thermal treatment process (i.e., calcination) in presence of air or oxygen is converted to the iron oxide, such as Fe₂0₃, FeO, or Fe₃0₄. Non-limiting examples of suitable iron (II) and iron (III) salts and compounds include FeCl₂, FeBr₂, Fel₂, Fe(OH)₂, Fe(OAc)₂, Fe₃(P0₄)₂, FeCl₃, FeBr₃, Fel₃, FeO(OH), Fe(N0₂)₃, Fe- EDTA, and combinations thereof. According to an embodiment, the catalyst composite includes iron oxide on anatase-titania. According to another embodiment, the catalyst composite includes iron oxide on gamma-alumina support.

[0040] In accordance with another embodiment, the active metal component may further include a ruthenium oxide adsorbed on the surface of the support material. The ruthenium may be co-deposited with iron on the support material or applied in a separate deposition step. In one aspect, a weight percent ratio of iron to ruthenium in the active metal component may be in a range from about 100: 1 to about 4: 1. For example, weight ratio of iron to ruthenium may be about 95: 1, about 90: 1, about 80: 1, about 60: 1, about 50: 1, about 40: 1, about 30: 1, about 20: 1, about 15: 1, about 10: 1, or about 5: 1, or in a range between any combination of the foregoing. The ruthenium oxide may be derived from a ruthenium oxide precursor, which after adsorption onto the surface of the support material followed by a thermal treatment process in presence of air or oxygen is converted to the ruthenium oxide, such as Ru0₂. Non-limiting exemplary ruthenium oxide precursors include Ραι(¾, RuBr₂, RuBr₃, ruthenium acetylaccetonate, Ru(NH₃)₆Cl₂, Ru₃(CO)i₂, or combinations thereof. In one embodiment, the iron is deposited and calcined on the support material prior to adsorbing and calcining ruthenium onto the support material.

[0041] In accordance with another embodiment, the catalyst composition may further include additional active metal components adsorbed on the support material. Additional active metal components and/or precursors include various halide and oxides of copper, chromium, cobalt, cerium, manganese, palladium, iridium, or combinations thereof. As noted above, the additional active metal percursor(s) may be co-adsorbed and calcined onto the support material with the iron oxide precursor, or the additional active metal precursor may be added after the iron deposition and calcination steps. Exemplary active metal components and/or precursors include copper compounds of Q1CI₂, CuCl, CuBr₂, Cul, CuCr0₄;

chromium compounds of CrBr₃, CrCi₃, Cr₂03, Cr₃0₄; cobalt compounds of C0CI₂, C0CI₃, CoBr₂, C0I₂; cerium compounds of CeCi₃, CeBr₃, CeC^, Ce₂0₃; manganese compounds of MnBr₂, MnCl₂, MnCl₃, MnO, Mn0₂, Mn₂0₃, Mn₃0₄; palladium compounds of PdCl₂, PdBr₂, Pd½; iridium compounds of ¾ ΙΓ(¾, IrCl₄, IrBr₂, IrBr₃, IrBr₄; or combinations thereof.

[0042] In accordance with embodiments of the present invention, the support material of the catalyst composite includes anatase-titania (T1O₂), gamma-alumina (AI₂O₃), or a combination thereof. It should be appreciated that other forms of titania and alumina are not specifically excluded as minor constituents of the support material. For example, in an embodiment, catalyst composite support may include minor quantities of rutile and/or brookite, and/or include minor quantities of alpha-alumina. Optionally, the support material can also include minor quantities of other refractory metal oxides. With respect to the support material, "minor quantities" is based on a total weight percent of the support material(s) and means less than 50 wt%. For example, in an embodiment, the support material comprises greater than 75 wt% of anatase-titania (T1O₂), gamma-alumina (AI₂O₃), or a combination thereof. In another embodiment, the support material comprises greater than 95 wt% of anatase-titania (T1O₂), gamma-alumina (AI₂O₃), or a combination thereof.

[0043] The surface area of the support material used in accordance with embodiments of the invention may be at least 50 m²/g. However, it is preferred that the support material has a surface area of at least 80 m²/g. The surface area can be measured by the (Brunauer, Emmett, and Teller) "BET method" as described by Kantro, D. L., Brunauer, S., and Copeland, L. E. in "BET Surface Areas: Methods and Interpretations" in The Solid-Gas Interface, Vol. 1 (E. A. Flood, Ed.), Marcel Dekker, New York, 1967.

[0044] Exemplary anatase-titania support materials include CristalACTiV™ DT-51 (Crystal Corp., Hunt Valley, MD) having an average surface area of about 90 m²/g, or Aeroxide® P25 (Evonik, Parsippany, NJ). Exemplary gamma-alumina support materials include product number 43855 (Alfa Aesar, Ward Hill, MA) having an average surface area of about 220 m²/g, medium pore diameter 70 A, and a total pore volume 0.62 cc/g.

[0045] In accordance with another embodiment of the invention, a method of making a catalyst composite for oxidizing elemental mercury is provided. The method comprises, comprises impregnating an active metal compound or precursor onto the support material, followed by calcination. Immobilization of the active metal compound onto the substrate material may be accomplished by suspending, grinding, or otherwise contacting the active metal compound and the substrate material. For example, the active metal compound/precursor and the substrate material may be combined in the presence of a liquid medium, such as water, one or more organic solvents, or an aqueous solution, and then followed by removal of the liquid medium. In another embodiment, the active metal compound/precursor, the substrate, and a binder material are mixed together and upon calcination, the binder material is eliminated, for example by combustion, evaporation, or sublimation.

[0046] The catalyst composites, prepared in accordance with embodiments of the present invention, may be incorporated into a system for removing elemental mercury from an elemental mercury-containing flue gas in any manner presently known or developed in the future. In an embodiment, the catalyst composite may be incorporated into a catalyst chamber in a fixed bed configuration, e.g., in a granular packed sand bed. In another embodiment, the catalyst composite may be formed into a structured catalyst, such as a honeycomb, plate, or corrugated array. In an embodiment, the catalyst composite may be or in a monolithic or honeycomb form, although a moving bed or other arrangement could be utilized. In certain embodiments, a honeycomb form of the catalyst composite will have a plurality (e.g., 4 or more) of channels per inch in order to optimize surface per unit volume.

[0047] In certain embodiments, a second catalyst that is also active in mercury oxidation may be positioned in the oxidation zone. For example, the second catalyst may be a supported catalyst comprising one or more metals from group VIII or the noble metals of the periodic table. Alternatively, a metal oxide or mixed metal oxide with activity for mercury oxidation can be utilized either self-supported or provided on a refractory metal oxide support.

[0048] In addition to the oxidation of a portion of the elemental mercury initiated by the Deacon reaction, another portion of the elemental mercury may be removed from the flue gas 17, reduced flue gas 21, and/or flue gas stream 28 by absorption. Specifically, elemental mercury contacting the catalyst composite (or an adsorbent material) may be adsorbed on the surface of the composite. The adsorption of elemental mercury on the catalyst composite may occur before and/or during the Deacon reaction. The adsorbed mercury on the catalyst composite can then removed by a catalyst regenerator (not shown) for removal of other deposits, such as ash on the catalyst composite surface.

[0049] In accordance with an embodiment of the present invention, at least 90% of elemental mercury present in a "simulated coal reduced flue gas" may be oxidized to one or more water soluble, oxidized mercury compounds using the catalyst composite in the presence of the simulated coal reduced flue gas matrix at 350°C. The flue gas 17 is formed as a result of combustion of a fossil fuel source, such as coal, in a furnace or boiler 20. While coal flue gas 17 can vary in composition and temperature, a simulated bituminous coal flue gas composition includes: 12% C0₂, 3% 0₂, 7% H₂0, 2,000 ppmv S0₂, 100 ppmv HCl, 200- 500 ppmv NO, and 20 ppbv Hg(0). A simulated bituminous coal flue gas composition after passing through an SCR unit (reduced flue gas) includes: 12% C0₂, 3% 0₂, 7% H₂0, 2,000 ppmv S0₂, 100 ppmv HCl, 5 ppmv NO, 5 ppmv NH₃, and 20 ppbv Hg(0). A simulated lignite/sub-bituminous coal flue gas composition includes: 12% C0₂, 3% 0₂, 10% H₂0, 500 ppmv S0₂, 5 ppmv HCl, 1-2 ppm HBr, 200-500 ppmv NO, and 20 ppbv Hg(0). A simulated lignite/sub-bituminous coal flue gas composition after passing through an SCR unit (reduced flue gas) includes: 12% C0₂, 3% 0₂, 10% H₂0, 500 ppmv S0₂, 5 ppmv HCl, 1 ppmv HBr, 5 ppmv NO, 5 ppmv NH₃, and 20 ppbv Hg(0).

[0050] In accordance with an embodiment of the present invention, at least 90% of elemental mercury present in a "simulated coal reduced flue gas" may be oxidized to one or more water soluble, oxidized mercury compounds using the catalyst composite in the presence of the simulated coal reduced flue gas matrix at 350°C. Accordingly, a general "simulated coal reduced flue gas" composition for evaluating the catalyst composite at at 350°C includes 12% C0₂, 3% 0₂, 7% H₂0, 2,000 ppmv S0₂, 100 ppmv HC1, 5 ppmv H₃, 5 ppmv NO, and 20 ppbv Hg(0).

[0051] In accordance with another embodiment of the present invention, at least 90% of elemental mercury present in a "simulated coal reduced flue gas" may be oxidized to one or more water soluble, oxidized mercury compounds using the catalyst composite in the presence of the simulated coal reduced flue gas matrix at 150°C. Accordingly, the "simulated coal reduced flue gas" composition for evaluating the catalyst composite at 150°C includes 12% C0₂, 3% 0₂, 10% H₂0, 500 ppmv S0₂, 5 ppmv HC1, 3 ppmv HBr, 5 ppmv NH₃, 5 ppmv NO, and 20 ppbv Hg(0).

[0052] The method of removing elemental mercury in a mercury-containing fluid is premised on contacting the mercury-containing fluid with the catalyst composite in the presence of oxygen and at least one of a hydrogen halide or a molecular halogen, wherein the elemental mercury is converted into one or more water soluble, oxidized mercury compounds. The one or more water soluble, oxidized mercury compounds can be removed from the mercury-containing fluid by absorption or by solubilizing in or reacting with an aqueous solution. Exemplary and non-limiting examples of adsorbent materials include carbon, silica gel, or other adsorbents impregnated with halogens or halides.

[0053] The one or more water soluble, oxidized mercury compounds can be removed from the mercury-containing fluid by solubilizing in or reacting with one or more aqueous solutions. According to embodiments of the present invention, in addition to water, the one or more aqueous solutions may include acids or bases, and may further or alternatively include oxidizing agents, reducing agents, or other mercury-reactive reagents. Non-limiting examples of a combination of aqueous solutions permits speciation of the oxidized mercury compounds, include potassium chloride (KC1) impinger solution (e.g., 1M); a sodium hydroxide scrubbing solution (e.g., 1M), and/or an acidic potassium permanganate KMnO solution (e.g. 4% KMn0₄ in 10% (v/v) H₂S0₄). Exemplary and non-limiting techniques for implementing solubilizing or reacting the oxidized mercury with an aqueous solution includes wet scrubbers, such as those typically used in flue gas desulphurization (FGD) units.

[0054] EXAMPLES

[0055] Catalyst composite preparation:

[0056] A wetness incipient method can be used to impregnate the active metal components onto anatase-Ti0₂ and/or gamma-A^Os, followed by calcination.

[0057] 10 wt% Fe/anatase-Ti0₂ catalyst composite:

[0058] A 1.6 grams portion of iron (III) nitrate-nonahydrate (Fe( 0₃)3*9 H₂O) ((Sigma Aldrich, St. Louis, MO) is dissolved in 2 mL of deionized water at room temperature. The solution is used to impregnate the iron onto 2 grams of anatase-Ti0₂ (CristalACTiV™ DT- 51, surface area ~ 90 m²/g, Crystal Corp., Hunt Valley, MD) by combining the iron nitrate solution and the anatase-Ti0₂ and mixing throughly. Water was then removed by drying the wet catalysts at 120°C under a vacuum for 2 hours, followed by calcination in air at 500°C for 5 hours to provide the 10 wt% Fe/anatase-Ti0₂ catalyst composite.

[0059] (10 wt% Fe + 0.5 wt% Ru)/anatase-Ti0₂ catalyst composite:

[0060] A 0.021 gram portion of ruthenium (III) chloride (RuCl₃) (Sigma Aldrich, St. Louis, MO) is dissolved in 2 mL of deionized water at room temperature. The solution is used to impregnate Ru onto a 10 wt% Fe/anatase-Ti0₂ catalyst composite by combining the ruthenium chloride solution and the 10 wt% Fe/anatase-Ti0₂ catalyst composite and mixing thoroughly. Water may be removed by drying the wet catalysts at 105°C under a vacuum for 2 hours, followed by calcinations in air at 400 °C for 2 hours and 500 °C for 2 hours in air to provide the (10 wt% Fe + 0.5 wt% Ru)/anatase-Ti0₂ catalyst composite.

[0061] Catalyst Composite Characterization [0062] As shown in FIG. 1 1, X-Ray Diffraction (XRD) patterns were obtained using Cu Ka radiation with a wavelength of 1.5406 A (X'Pert Pro MPD X-ray diffractometer). An aluminum holder was used to support the catalyst samples. The scanning range was from 10° to 60° (2Θ) with a step size of 0.02° and a step time of 0.5 sec.

[0063] As shown in FIG. 12, Fe K edge X-ray Absorption Fine Structure (XAFS) spectroscopic experiments were performed using the 10-BM Beamline at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL, Argonne, IL). A Si (1 11) monochromator was used, and an energy calibration was performed using Fe metal foil. The reference iron oxides of FeO, Fe2C>3, and Fe₃0₄ and samples were dispersed on a Kapton (polyimide) tape and the tape was folded for the measurements. X-ray Absorption Near Edge Structure (XANES) data were collected in a fluorescence mode, and the fluorescent data were obtained using a 13 -element Ge detector.

[0064] The XANES of 10%(wt) Fe/DT51 and the iron oxide standards are shown in Figure 12. From the Fe near edge, it shows that the iron is in the form of Fe2C>3 for the 10%(wt) Fe/DT51 sample. The Fe edge X-ray absorption of other catalysts including l%(wt) Fe/DT51, 3%(wt) Fe/DT51, 5%(wt) Fe/DT51, 7%(wt) Fe/DT51, 5%(wt)

Fe+0.5%(wt)Pvu/DT51, 10%(wt)Fe+l%(wt)Ru/DT51 and spent 5%(wt) Fe+0.5%(wt) Ru/DT51 were also measured (data not shown). For all the catalysts analyzed, Fe2C>3 was found to be the closest iron oxide species, indicating that different Fe and Ru loadings do not change the Fe speciation, and the catalyst is stable under simulated flue gas conditions.

[0065] Lab-scale fixed-bed reactor tests with simulated flue gases:

[0066] As shown in FIG. 2, an experimental mercury oxidation set-up 200 that includes a fixed-bed catalyst reactor and that may be used to evaluate the catalyst composite performances is shown in FIG. 2. The experimental mercury oxidation set-up 200 includes an elemental mercury source 210, a carrier gas supply 215, a simulated coal flue gas source 220, a fixed-bed reactor 230, a glass-fiber filter 240, a first wet scrubbber 250, an on-line mercury analyzer 260, a second wet scrubber 270, and a discharge stack 280, which discharges to a disposal unit.

[0067] About 25 mg of a catalyst composite was mixed with 1 g silica sand and then loaded in a fixed-bed reactor with an inner diameter of 12 mm. The height of the catalyst bed was around 2 cm. The quartz reactor was placed in a tube furnace maintained between about 100°C to about 400°C. The mercury source 210 provides Hg(0) vapor, which was generated from a mercury permeation tube (Dynacal Hg(0) permeation tube, VICI Metronics, Inc.) and the inlet Hg(0) concentration was controlled at 20 ppbv at a flow rate of 100 mL/min. The simulated flue gas source 220 provided a mixture including about 7-10 vol% water vapor, about 3 vol% to about 6 vol% (¾, about 10 vol% to about 12 vol% CO2, about 500 to about 2,000 ppmv SO2, about 5 to about 500 ppmv NO, about 0 to about 100 ppmv HCl, about 0 to about 5 ppmv HBr, about 0 to about 20 ppmv NH₃, and N2 carrier gas was used to obtain a flow rate of about 1 L/min. The concentrations of these individual gases were controlled by mass flow controllers. Hg(0) concentration was monitored by an online Hg(0) analyzer (VM- 3000, Mercury Instruments, Inc.), and Hg²⁺ can be collected using a KC1 impinger solution 252 and a NaOH impinger solution 254 and the amount was measured using a separate mercury analyzer (a cold vapor atomic absorption mercury analyzer (Model 400A, Buck Scientific), which is not shown in FIG. 2). The discharge of the on-line mercury analyzer 260 was subjected to a final oxidizing scrubber by passing through two KMn0₄ solutions 272, 274. When the online analyzer was not used, the Ontario Hydro Method (ASTM- D26784) using wet impinger solutions was used to obtain mercury speciation results.

[0068] As shown in FIG. 3, an experimental mercury oxidation set-up 300 that includes a structured catalyst reactor and that may be used to evaluate the catalyst composite performances. The experimental mercury oxidation set-up 300 includes an elemental mercury source 310, a carrier gas supply 315, a simulated coal flue gas source 320, a heat exchanger 325, a structured catalyst reactor 330, an on-line mercury analyzer 360, a propane heater 365, a fluidized bed scrubber 370, an induced fan 380, a damperand 390, which controls the exhaust discharge.

[0069] To make a Fe/Ti0₂ honeycomb catalyst, 5 kg of powdered Ti0₂ (containing 9wt% W0₃ and 10wt% Si0₂), 4 kg of Fe(N0₃)₃-9H₂0, and 0.25 kg of organic binder (methylcellulose) are well mixed. Then 3 kg of water is added and the mixture is kneaded using a kneader. The mixture is then extruded into a flow-through honeycomb catalyst body. Subsequently, the catalyst body is dried in an oven by increasing the temperature to 100 °C slowly and calcined at a temperature of 500 °C for 5 hours in air to form a honeycomb catalyst body. To make a Fe+Ru/Ti0₂ honeycomb catalyst, RuCl₃ can be either added to Fe(N0₃)₃-9H₂0 together, or be impregnated after the calcination of the Fe/Ti0₂ catalyst followed by another calcination step at temperatures between 400-500 °C to form Ru0₂.

[0070] The mercury source 310 provides Hg(0) vapor, which was generated from a mercury permeation tube (Dynacal Hg(0) permeation tube, VI CI Metronics, Inc.) and the inlet Hg(0) concentration was controlled at 20 ppbv. The simulated flue gas source 320 and the propane burner 365 combined streams to provided a mixture including about 7-10 vol% water vapor, about 3 vol% to about 6 vol% 0₂, about 10 vol% to about 12 vol% C0₂, about 200 to about 2,000 ppmv S0₂, about 2-10 ppm S0₃, about 10-100 ppm CO, about 0 to about 5 ppmv HC1, about 0 to about 5 ppmv HBr, about 5 ppm ppmv NH₃, about 5 ppmv NO, and N₂ carrier gas was used to obtain a flow rate of about 50-150 L/min. The concentrations of these individual gases were controlled by mass flow controllers. Hg(0) concentration was monitored by an online Hg(0) analyzer (VM-3000, Mercury Instruments, Inc.), and Hg²⁺ can be collected using a KC1 impinger solution 252 and a NaOH impinger solution 254 and the amount was measured using a separate mercury analyzer 260 (a cold vapor atomic absorption mercury analyzer (Model 400A, Buck Scientific)). The discharge of the on-line mercury analyzer 260 was subjected to a final oxidizing scrub by passing through two KMn0₄ solutions 272, 274. When the online analyzer was not used, the Ontario Hydro Method (ASTM-D26784) using wet impinger solutions was used to obtain mercury speciation results.

[0071] Results:

[0072] FIG. 4 is a performance curve showing an absence of mercury break-through upon doubling sulfur dioxide (SO₂) presence in a simulated coal flue gas comprising 12% CO₂, 7% H₂0, 3% 0₂, 1,000-2,000 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 100 ppmv HCl, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention. Under the simulated bituminous coal flue gas conditions with 100 ppmv HCl, greater than 98% Hg(0) oxidation can be achieved by a 10 wt%

Fe/DT51 catalyst. When the SO₂ gas concentration increases from 1,000 ppmv to 2,000 ppmv, it does not affect the Hg(0) oxidation performance.

[0073] FIG. 5 is a performance curve for mercury oxidation over time in a simulated bituminous coal reduced flue gas comprising 12% CO₂, 7% H₂0, 3% O₂, 1,000 ppmv SO₂, 20 ppmv NH₃, 20 ppmv NO, 10 ppmv HCl, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention. Even under a high 20 ppmv N¾ slip condition, the 10 wt% Fe/DT51 catalyst can maintain greater than 90% Hg(0) oxidation capability and did not show a sign of the performance degradation during the test period.

[0074] FIG. 6 is a performance curve for mercury oxidation over time in a simulated bituminous coal reduced flue gas comprising 12% CO₂, 7% H₂0, 6% 0₂, 1,000 ppmv SO₂, 0 ppmv NH₃, 20 ppmv NO, 10 ppmv HCl, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite at 350°C, in accordance with an embodiment of the invention. Compared with the performance shown in FIG. 5, the 10 wt% Fe/DT51 catalyst can maintain greater than 95% Hg(0) oxidation performance when there is no NH₃ present in the simulated flue gas. The results indicate that the performance depends on NH₃ concentrations.

[0075] FIG. 7 is a performance curve comparing mercury oxidation for various catalytic conditions as a function of HBr concentration in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 6% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HC1, and 20 ppbv Hg(0) in the presence of a) an empty sand bed; b) 10 wt% Fe/DT51 catalyst composite dispersed in a sand bed; c) (10 wt% Fe + 0.5 wt%

Ru)/DT51 catalyst composite dispersed in a sand bed; and d) empty sand bed without HC1 gas, at 350°C, in accordance with an embodiment of the invention. The results show enhanced Hg(0) oxidation performances in the presence of the catalysts compared to homogeneous reactions in an empty bed reactor in terms of different HBr concentrations. An addition of 0.5 wt% Ru further improves the Hg(0) oxidation performance, requiring lower HBr concentrations to achieve greater than 90% Hg(0) oxidation.

[0076] FIG. 8 is a performance curve comparing mercury oxidation as a function of temperature from 150°C to 400°C in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a 10 wt% Fe/DT51 catalyst composite, in accordance with an embodiment of the invention. Under the simulated flue gas conditions, the performances of the 10 wt% Fe/DT51 catalyst increase with an increase in temperature between 150°C and 400°C.

[0077] FIG. 9 is a performance curve comparing mercury oxidation as a function of temperature from 150°C to 400°C in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, 5 ppmv HBr, and 20 ppbv Hg(0) in the presence of a (10 wt% Fe + 0.5 wt% Ru)/DT51 catalyst composite, in accordance with an embodiment of the invention. Compared with FIG. 8, an addition of 0.5 wt% Ru improves the 10 wt% Fe/DT51 catalyst performance between 150°C and 400°C, particularly at lower temperatures.

[0078] FIG. 10 is a performance curve comparing mercury oxidation as a function of HBr concentration from 1 ppm to 5 ppm in a simulated sub-bituminous and lignite coal reduced flue gas comprising 12% C0₂, 7% H₂0, 3% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, and 20 ppbv Hg(0) in the presence of a (10 wt% Fe + 0.5 wt% Ru)/DT51 catalyst composite at 150°C, in accordance with an embodiment of the invention. At 150°C, the performance of the (10 wt%Fe + 0.5 wt% Ru)/DT51 catalyst increases with an increase in HBr gas concentration. The catalyst can achieve greater than 90% Hg(0) oxidation with greater than about 3 ppmv HBr gas.

[0079] Table 1 : Hg(0) Oxidation of simulated reduced flue gas* at 150 °C and 350 °C using (11% Fe + 1% Ru)/DT51 catalyst composite.

Temperature HC1 HBr Hg(0) oxidation

(°C) (ppmv) (ppmv) (%)

150 0 0 (empty**) 22

150 0 0 57

150 5 0 39

150 0 2 89

150 5 2 93

150 0 3 96

150 5 3 98

350 0 0 (empty**) 10

350 0 0 26

350 5 0 87

350 0 2 98

350 0 1 (empty**) 56

350 0 1 96

350 5 1 96

350 5 2 97

* Simulated reduced flue gas: 12 vol% C0₂, 10 vol% H₂0, 3 vol% 0₂, 500 ppmv S0₂, 5 ppmv NH₃, 5 ppmv NO, and 20 ppbv Hg(0), with variable hydrogen halides.

** Empty: empty bed with no catalyst. [0080] The Hg(0) oxidation performances of the (10 wt% Fe + 1 wt% Ru)/DT51 catalyst under the simulated flue gas conditions with different halogen halide levels are shown in Table 1. HBr has a much stronger positive effect on Hg(0) oxidation than HC1, and Hg(0) oxidation depends primarily on HBr concentrations. At 350°C, low HBr concentrations are required to achieve greater than 90% Hg(0) oxidation.

[0081] While the present invention has been illustrated by a description of one or more embodiments thereof and while these embodiments have been described in considerable detail, they are intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described.

Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.

Claims

What is claimed is:

1. A method of removing elemental mercury in an elemental mercury-containing flue gas, comprising:

contacting the elemental mercury-containing flue gas with a catalyst composite at a temperature within a range from about 150°C to about 400°C in the presence of oxygen and at least one of a hydrogen halide or a molecular halogen, wherein the catalyst composite comprises:

an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (T1O2), gamma-alumina (AI2O₃), or a combination thereof;

wherein the elemental mercury is converted into one or more water soluble, oxidized mercury compounds.

2. The method of claim 1, further comprising:

removing the one or more water soluble, oxidized mercury compounds from the mercury-containing flue gas by solubilizing in or reacting with an aqueous solution.

3. The method of claim 1, wherein a weight percent ratio of iron to the support material is in a range from 1 : 100 to about 1 :5.

4. The method of claim 1, wherein the iron oxide is derived from an iron oxide precursor selected from the group consisting iron (II) and iron (III) salts and compounds, which after adsorption onto the surface of the support material followed by a thermal treatment process in presence of air or oxygen is converted to iron oxide.

5. The method of claim 4, wherein the iron (II) and iron (III) salts and compounds are selected from the group consisting of FeCi2, FeBr₂, Fe , Fe(OH)₂, Fe(OAc)2, Fe₃(P0₄)2, FeCl₃, FeBr₃, Fel₃, FeO(OH), Fe( 0₂)₃, Fe-EDTA, and combinations thereof.

6. The method of claim 1, wherein the support material comprises anatase-titania.

7. The method of claim 1, wherein the active metal component further comprises a ruthenium oxide adsorbed on the surface of the support material.

8. The method of claim 7, wherein a weight percent ratio of iron to ruthenium is in a range from about 100: 1 to about 4: 1.

9. The method of claim 7, wherein the ruthenium oxide is derived from a ruthenium oxide precursor, which after adsorption onto the surface of the support material followed by a thermal treatment process in presence of air or oxygen is converted to the ruthenium oxide.

10. The method of claim 1, wherein contacting the elemental mercury-containing flue gas with the catalyst composite is performed at a temperature within a range from about 150°C to about 300°C.

11. The method of claim 1, wherein contacting the elemental mercury-containing flue gas with the catalyst composite is performed at a temperature within a range from about 300°C to about 400°C.

12. The method of claim 1, wherein the at least one of hydrogen halide or the molecular halogen is present in the elemental mercury-containing flue gas in an amount equal to about 1 parts per million by volume or more.

13. The method of claim 1, wherein the conversion of the elemental mercury into one or more water soluble, oxidized mercury compounds is about 90 percent or more in the presence of a simulated reduced flue gas mixture comprising 12 vol% CO2, 3 vol% O2, 7 vol% H₂0, 2,000 ppmv S0₂, 100 ppmv HC1, 5 ppmv NH₃, 5 ppmv NO, and 20 ppbv Hg(0) at 350°C.

14. The method of claim 1, wherein the catalyst composite is further supported in a honeycomb, plate, or corrugated array within a catalyst chamber.

15. A system for removing elemental mercury in a combustion exhaust gas, comprising:

a combustion chamber for combustion of a fossil fuel source;

a catalyst chamber comprising a catalyst composite comprising an active metal component comprising an iron oxide adsorbed on the surface of a support material comprising anatase-titania (T1O2), gamma-alumina (AI2O₃), or a combination thereof, wherein the combustion chamber is fluidly coupled to an inlet of the catalyst chamber to allow flow of combustion exhaust gas from the combustion chamber into the catalyst chamber, wherein the catalyst composite converts elemental mercury into one or more water soluble, oxidized mercury compounds in the presence of oxygen and at least one of a hydrogen halide or a molecular halogen within a temperature in a range from about 150°C to about 400°C; and a scrubber for removing the one or more water soluble, oxidized mercury compounds from the mercury-containing flue gas, wherein an inlet of the scrubber is fluidly coupled to an outlet of the catalyst chamber to allow flow of the combustion exhaust gas from the outlet of the catalyst chamber into the scrubber.

16. The system of claim 15, wherein the scrubber comprises:

an aqueous solution for removing the one or more water soluble, oxidized mercury compounds from the mercury-containing flue gas by solubilizing in or reacting with the aqueous solution; or

a fixed or an injected adsorbent material for removing the one or more water soluble, oxidized mercury compounds from the mercury-containing flue gas.

17. The system of claim 15, wherein a weight percent ratio of iron to the support material is in a range from 1 : 100 to about 1 :5.

18. The system of claim 15, wherein the support material comprises anatase-titania.

19. The system of claim 15, wherein the active metal component further comprises a ruthenium oxide (RuC^) adsorbed on the surface of the support material.

20. The system of claim 15, wherein a weight percent ratio of iron to ruthenium is in a range from about 100: 1 to about 4: 1.