Abstract:
A method of halogenating coal prior to combustion, wherein the coal is treated with a halogen prior to pulverization and the pulverized halogenated coal is delivered to a burner as fuel for a combustion process.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to combustion of fossil fuels in a boiler or furnace, and more specifically to a method of halogenating a fossil fuel prior to combustion to enhance removal of mercury from flue gas.  
         [0002]     Coal fired utility plants are necessary to satisfy the energy needs of many countries. Heat energy derived from combusting fossil fuels such as coal produces steam utilized to drive energy producing turbines. A byproduct of the combustion process is a flue or combustion gas which, after a cleaning process, exits into the atmosphere.  
         [0003]     Fossil fuels such as coal may contain varying levels of mercury. Upon combustion mercury may enter the flue gas as a solid particulate or as a gas in the vapor phase as elemental mercury or in one of many oxidized forms. Flue gas scrubbers are readily able to remove a substantial portion of mercury in the oxidized form, however current scrubber technology is relatively ineffective in removing elemental mercury from flue gas.  
         [0004]     Environmental regulations strictly regulate flue gas emissions for pollutants such as sulfur and nitrogen oxides, and the U.S. EPA is considering implementing mercury emission standards. In order to meet such proposed environmental regulations, a need exists for a new and more effective means of controlling mercury emissions arising from the combustion of fossil fuels.  
       SUMMARY OF INVENTION  
       [0005]     A general object of the present invention is drawn to a method for enhancing mercury removal from flue gas. A more particular objective of the present invention involves a method for halogenating a fossil fuel such as coal prior to combustion to suppress the formation of elemental mercury in the vapor phase and favor its conversion to an oxidized form.  
         [0006]     In a first aspect, the present invention provides a method for halogenating a fossil fuel such as coal with chlorine or compounds thereof. After chlorination the coal is fed to a pulverizer which further admixes the coal and chlorine thereby more uniformly dispersing the chlorine amongst the pulverized coal particles. Once pulverized the coal is fed to at least one burner and combusted in the presence of oxygen. The admixed chlorine readily oxidizes the elemental mercury contained within the coal during combustion, thereby suppressing formation of mercury in the elemental form. The oxidized mercury is then removed downstream of the burner utilizing a scrubber or other mercury removal means readily known to one of ordinary skill in the art.  
         [0007]     In another aspect, the invention comprises the steps of treating mercury containing coal with a chlorine containing compound, pulverizing the treated coal, conveying the pulverized treated coal to a pulverized coal burner, utilizing the burner to combust the pulverized treated coal, wherein the chlorine containing compound oxidizes a substantial portion of the mercury contained in the coal during combustion, and creating a flue gas comprising oxidized mercury.  
         [0008]     In yet another aspect, the invention comprises the steps of providing a burner with a mercury containing coal, utilizing the burner to combust the coal, and combusting the coal in the presence of a halogen containing compound, wherein elemental mercury is oxidized during combustion.  
         [0009]     In yet another aspect a flue gas monitoring means is utilized to monitor the mercury concentration of flue gas exiting the boiler. The monitoring means then relays a signal to a regulating means, which regulates the level of chlorine treatment applied to the coal prior to pulverization in order to maximize mercury oxidation.  
         [0010]     In yet another aspect of the present invention a halogen other than chlorine, such as bromine, iodine, fluorine, or compounds thereof, may by used to halogenate the fossil fuel.  
         [0011]     The present invention, due to its ease of incorporation and benefit in mercury removal, provides an important advantage over prior art methods due to unexpected advantages and cost savings associated with forgoing the need to install expensive mercury vapor removal means in order to lower mercury emissions. Further, as a single pulverizer may provide coal to numerous burners, substantial economic savings are observed by chlorinating the coal prior to combustion and utilizing existing piping between the pulverizer and burners thereby forgoing the need to install additional piping or ductwork to each burner.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1 . is a schematic diagram of an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring now to  FIG. 1 , a schematic diagram of an embodiment of the present invention is shown. A fossil fuel, such as coal, is provided from a coal silo onto a belt feeder, such as a gravimetric feeder or other belt feeding apparatus. The belt feeder in turn provides the coal to the pulverizer wherein the coal is pulverized to a predetermined particle size.  
         [0014]     Prior to being pulverized, preferably while the coal is on the belt feeder or alternatively dropping into the pulverizer, the coal is treated with a liquid or solid chlorine reagent, preferably sodium chloride or calcium chloride. Spray nozzles located above and/or around the belt may be provided to treat the coal with liquid chlorine reagent prior to pulverization. Reagent flow rate may be control manually by valves or alternatively utilize automated means. Alternatively, reagent may be supplied to coal in the form of solid pellets, powder, or granules utilizing a solids feeder in place of the spray nozzle. Preferably the coal is treated with between about 2 ppmw and 1200 ppmw of halogen, and more preferably between about 200 ppmw and 600 ppmw, wherein ppmw refers to pounds of halogen per one million pounds of a wet fossil fuel such as wet coal.  
         [0015]     Treatment comprises the steps of providing the coal with sufficient chlorine to substantially suppress the formation of elemental mercury vapor, and pulverizing the coal in the presence of the chlorine reagent. During pulverization the coal and chlorine reagent are further admixed so as to provide more uniform distribution of chlorine amongst the pulverized coal particles.  
         [0016]     Upon exiting the pulverizer, the treated coal particles are fed through a series of coal pipes to a burner, wherein the coal is combusted, preferably at a temperature between about 1050° C. and about 2000° C., in the presence of oxygen. During combustion the mercury contained within the coal oxidizes in the presence of chlorine to favor the production of an oxidized form of mercury, such as mercuric chloride, over elemental mercury in the vapor form.  
         [0017]     Resulting mercury concentration in the flue gas exiting the boiler is decreased because scrubbers and other flue gas cleansing means are more readily able to remove mercury in the oxidized form, and formation of mercury in the elemental form is suppressed in favor of the oxidized form due to the presence of a halogen such as chlorine.  
         [0018]     In an alternative embodiment, a single reagent storage means is used to supply chlorine reagent to multiple belt feeders utilizing control valves and piping that allows the treatment rate to any belt feeder to be controlled independently of the treatment rate to another belt feeder.  
         [0019]     In yet another alternative embodiment, an automated means for controlling the treatment rate may be utilized. In this embodiment, a mercury detection means is installed in the flue gas stack to monitor the level of mercury in the flue gas as it exits into the atmosphere. An electronic signal is then sent to a regulating means which conveys instructions to adjust the flowrate of the chlorine reagent treating the coal to a level which optimizes low mercury emissions.  
         [0020]     In yet another embodiment, the chlorine reagent may be replaced with another halogen reagent such as bromine, iodine, fluorine, etc. . . . or compounds thereof.  
         [0021]     In yet another embodiment a combination of more than one halogen reagent may be used such as reagent mixture comprising both bromine and chlorine, or any other number of halogen reagent combinations incorporating more than one halogen reagent as would be known by one of ordinary skill in the art.  
         [0022]     While the specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise as appreciated by one of ordinary skill in the art without departing from the scope of the present invention.