| Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor -> Monitor Keywords |
|
|
  Catalytic adsorbents for mercury removal from flue gas and methods of manufacture thereforRelated Patent Categories: Catalyst, Solid Sorbent, Or Support Therefor: Product Or Process Of Making, Solid Sorbent, Free Carbon Containing, And Specified Adde Active Sorbent MaterialThe Patent Description & Claims data below is from USPTO Patent Application 20060205592. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates generally to catalytic adsorbents for use in the removal of mercury from flue gas streams and methods of manufacturing such catalytic adsorbents. BACKGROUND OF THE INVENTION [0002] The toxicity of mercury to humans and the environment has long been known. It is known for example that mercury exposure can cause neurological damage in humans. A particularly devastating example of the harmful effects of mercury occurred in Minamata, Japan in the 1950's where organic mercury byproducts of acetaldehyde production were discharged into the local bay. The byproducts were consumed and metabolized by fish. By consuming fish in the bay, wide spread neurological damage and birth defects among the local population were reported. [0003] Coals used for generating electric power often contain about 0.1 ppm mercury. In the United States alone, about 50 tons of mercury are discharged as vapor in stack gas every year. Through chemical and biological processes, this mercury can become concentrated in fish by many thousand fold, thereby entering human food supplies at harmful levels. [0004] The effort to remove trace mercury from air, water, natural gas, and other industrial streams has a long history, however; removing mercury from coal burning flue gas streams is a very different problem. [0005] Prior art techniques for removing mercury from air or hydrocarbons at room temperature generally have limited relevance to removing mercury from flue gas streams. Mercury has a high atomic weight and adsorption temperature is a significant issue. At room temperature, the dispersion interaction with carbon is sufficient to immobilize mercury atoms. At about 300.degree. F. (the temperature of many flue gas streams), however, physical adsorption is no longer able to hold down the volatile elemental mercury. [0006] In addition, sufficient contact time with rapidly moving flue gas streams is another issue for mercury removal. The total time for flue gas, from generation by combustion to exit through the stack, is often less than 10 seconds. Either as injected powder, where adsorbent fly amid flue gas is for about 2 seconds, or as filter cake on bags in a bag house, the contact time between flue gas and activated carbon captured by the filter is less than one second. [0007] The demand on reactivity and reaction kinetics by flue gas cleaning can not be properly tested by conventional packed beds. Conventional packed beds are insufficient for flue gas cleaning because the volume of flue gas is so large, the cost for compressing it to push it through a packed bed is prohibitive. [0008] Further issues relating to the removal of mercury from flue gas include the small, yet potentially toxic, concentration levels of mercury in the flue gas streams. The concentration of mercury in flue gas streams is in .mu.g/m.sup.3 whereas the concentration of mercury in many other industrial processes is on the order of mg/m.sup.3. Much early work considered effluents containing mercury in the 5 .mu.g/m.sup.3 range (that is not much lower than the initial concentration of mercury in the flue gas) as fully purified. [0009] Above all, prior art techniques consider the adsorption of mercury as an event between the adsorbent and the mercury. While this is true in air or hydrocarbon streams at room temperature, flue gas contains highly polar and reactive components that can play both an interfering and enabling role for mercury removal. One model composition used for flue gas contains about: 6% O.sub.2, 12% CO.sub.2, 8% H.sub.2O, 1600 ppm SO.sub.2, 400 ppm NO, 50 ppm HCl, 20 ppm NO.sub.2, and 12 .mu.g/m.sup.3 elemental Hg. [0010] Prior art attempts to remove mercury from flue gas have included various techniques. One approach has focused on adding halogen salts into coal prior to combustion such that the combustion process generates hydrogen halide gases and then injecting powder carbon downstream into the flue gas at a lower temperature. Some mercury is captured by interaction between the hydrogen halide gases, activated carbon and mercury. Another approach has been to add hydrogen halides or elemental halogen together with activated carbon to a lower temperature flue gas. [0011] U.S. Pat. No. 1,984,164 to Karlsruhe proposes carbon or silica gel or other adsorbents impregnated with elementary halogen for removal of mercury from room air. Other prior art attempts have included adding halide salts to coal before combustion since these salts are known to be very stable. The combustion process oxidizes halides to halogen and further reacts with hydrogen to yield hydrogen halides. For example, U.S. Pat. No. 5,435,980 to Felsvang et al. suggest adding chloride or a chlorine containing material into the coal before or during combustion or adding HCl into flue gas upstream of or in the drying-absorption zone. [0012] U.S. Patent Application No. 2004/0003716 Al to Nelson, Jr. discloses a method for removing mercury and mercury containing compounds from combustion gas by injecting an adsorbent into the flue stream. The sorbent is prepared by treating a carbonaceous substrate with a bromine containing gas. Bromine gas is known to be highly toxic by inhalation, ingestion or skin contact. HBr is also known to be corrosive. In addition, bromine and HBr compounds are reactive and can easily be added onto alkenes. Further, bromine is reactive with aromatics. [0013] U.S. Pat. No. 6,533,842 B1 to Maes et al. disclose powder adsorbents which contain about 40% carbon, 40% calcium hydroxide, 10% cupric chloride and 10% KI.sub.3 impregnated carbon to remove mercury from a high temperature, high moisture gas stream. [0014] In December 2000, the United States Environmental Protection Agency (EPA) made its regulatory decision that mercury emissions from coal-fired electric generating plants need to be controlled. [0015] In the field of the mercury removal from flue gas streams, it would therefore be desirable to provide adsorbents having improved adsorbent characteristics in the flue gas temperature range and that can be economically and efficiently manufactured. BRIEF SUMMARY OF THE INVENTION [0016] The present invention provides catalytic adsorbents in which a halide salt is dispersed on activated carbon and the oxidation catalytic activity of the activated carbon promotes the formation of mercury halide. At the same time, the adsorbent qualities of activated carbon retain the mercury halides thus formed. The present invention recognizes that while the halide salts are stable and harmless at room temperature, these doped activated carbon compounds form mercury halogen compounds at elevated temperatures typical of those found in flue gas streams, and in the presence of reactive components typical of flue gas. These mercury halogen compounds are retained on the surface of the activated carbon. Moreover, the increased adsorbent capacity and faster rate of adsorption result in a need for smaller quantities of adsorbent relative to an undoped activated carbon formed from the same starting material. [0017] A catalytic adsorbent composition for removal of mercury from a flue gas stream thus includes an activated carbon having a dopant (i.e, a halide salt) dispersed thereon. The cation of the dopant used for the halide salt in accordance with the present invention can be an alkaline, alkaline earth, or transition metal (e.g., Na, Ca, Mg, Cu and K). The anion involved can be bromide or chloride. Particularly preferred dopants include, but are not limited to, NaCl, CaCl.sub.2, CuCl.sub.2, CuBr.sub.2, NaBr, KBr, CaBr.sub.2 and MgBr.sub.2. [0018] The halide salt is inert with respect to mercury and the activated carbon at room temperature. At elevated temperatures (e.g., 200-570.degree. F.) and in the presence of typical flue gas compositions, mercury halogen compounds are formed and retained on the activated carbon. While not intending to be bound by any theory, it is believed that any or all of the following or a combination of the following may occur. An oxidant (for example, oxygen form the flue gas or oxidant on the activated carbon) oxidizes the mercury and the anion of the dopant provides a counter ion for the mercury ion as oxidized by the oxidant. Alternatively, the oxidant oxidizes the anion in the salt and the oxidized anion in turn oxidizes the mercury to form a mercury halogen compound on the activated carbon. In addition or in the alternative, acidic gases present in the flue gas react with the dopant salt to yield a hydrogen halide. The hydrogen halide is then oxidized by an oxidant and yields a halogen compound. The halogen compound then reacts with the mercury to form a mercury halogen compound that are then adsorbed by the activated carbon. [0019] The present invention also provides methods of manufacturing such doped activated carbon adsorbents that are both economical and safe. The catalytic adsorbents of the present invention can be made from a variety of methods. In one embodiment, the catalytic adsorbents can be formed by placing an activated carbon in an aqueous solution containing a halide salt to form a mixture, stirring the mixture until a homogeneous slurry is formed and drying the activated carbon such that water from the aqueous solution evaporates and the halide salt is dispersed on the surface of the activated carbon. [0020] In another exemplary method of manufacture, the catalytic adsorbents can be made by injecting a presoaked carbonaceous feedstock into a reaction chamber together with oxidizing gases such as air and/or steam. The carbonaceous feedstock and the oxidizing gases are injected into the reaction chamber under conditions and for a residence time sufficient to form a powder activated carbon having a dopant dispersed on the surface of the powder activated carbon. In this method, the reaction chamber can be a batch type reactor such as a tube furnace or a reactor designed for continuous mode operation (e.g., a fluidized bed reactor). The dopant is formed of a cation selected from the group including an alkaline metal, an alkaline earth metal, and a transition metal (e.g, Na, K, Mg, Ca and Cu) while the anion is selected from bromide and chloride. In some embodiments, the dopant may be selected from the group including: NaCl, KCl, CaCl.sub.2, CuCl.sub.2, CuBr.sub.2, NaBr, KBr, CaBr.sub.2 and MgBr.sub.2. [0021] The catalytic adsorbents of the present invention are suitable for use in the removal of mercury from a gas stream containing an oxidant and/or acidic gases at an elevated temperature such as a flue gas stream exiting a boiler or combustion process. In this process, the catalytic adsorbents of the present invention are injected into the flue gas stream for an in-flight mode of mercury capture. As discussed above, the dopant is inert with respect to the mercury at room temperature. At flue gas temperatures and in the presence of the activated carbon, oxidant and/or acidic gases, however, the dopant effectively removes mercury from the flue gas stream. The mercury is retained on the activated carbon in the form of mercury halogen compounds and can be separated from the flue gas stream together with the flyash. Continue reading... Full patent description for Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor or other areas of interest. ### Previous Patent Application: Adsorbent for removing mercury using sulfided iron compounds containing oxygen and method of producing same Next Patent Application: Heat-sensitive recording material Industry Class: Catalyst, solid sorbent, or support therefor: product or process of making ### FreshPatents.com Support Thank you for viewing the Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor patent info. IP-related news and info Results in 0.10664 seconds Other interesting Feshpatents.com categories: Tyco , Unilever , Warner-lambert , 3m |
||
