Abstract:
A method of reducing particulate matter and mercury emissions in a combustion flue gas includes, in an exemplary embodiment, combusting a fuel resulting in generation a flue gas flow, cooling the flue gas flow within a duct, positioning a flow conditioning apparatus within the duct, enhancing a reaction rate of the mercury and carbon-containing fly ash particles by directing the flue gas flow through the flow conditioning apparatus to mix the carbon-containing fly ash particles and mercury within the flue gas flow and to facilitate at least one of oxidation of the mercury and binding the mercury to the carbon-containing fly ash particles, collecting a portion of the carbon-containing fly ash particles in the flow conditioning apparatus, and directing the flue gas flow to a particulate collection device to remove the remaining portion of the fly ash particles from flue gas flow.

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention relates generally to combustion furnaces, and more particularly, to a method and apparatus to reduce emissions of trace elements and particulate matter from a combustion furnace flue gas. 
     During a typical combustion process within a furnace or boiler, for example, a flow of combustion exhaust gas, or flue gas, is produced. Known combustion exhaust gases contain combustion products including, but not limited to, carbon, fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or trace metals, for example, mercury, generated as a result of combusting solid and/or liquid fossil fuels. 
     Volatile metal mercury is one air pollutant produced through coal combustion. Mercury released from coal during combustion is readily aerosolized and can become airborne. Airborne mercury may travel globally prior to being deposited onto soil and water. Mercury released in the environment is a persistent and toxic pollutant that may accumulate in the food chain. For example, mercury can be transformed within microorganisms into methylmercury. Consumption of contaminated fish is the major route of human exposure to methylmercury. 
     Mercury emissions from coal-fired power plants are the subject of governmental regulation. The control of mercury emissions is complicated by the several forms mercury may take within combustion flue gas. For example, at combustion temperatures, mercury is present in flue gas in its elemental form, Hg 0 , which may be difficult to control because elemental mercury is mostly non-reactive and easily volatized. Mercury reacts with halogens, predominately chlorine, present in coal and released into flue gas during combustion as flue gas cools below 1000° F. Such reactions may convert mercury to its highly reactive, oxidized form, Hg +2 . Mercury may also be absorbed in fly ash and/or other particulate matter present in the flue gas to form particulate-bound mercury. 
     Because mercury can take several forms, known control technologies do not effectively control mercury emission for all coal types and for all combustion configurations. Some known mercury control technologies take advantage of mercury&#39;s reactivity with carbon and use carbon as a mercury sorbent to remove mercury from flue gas. Carbon may be formed in-situ during the combustion process as a result of incomplete coal combustion or may be injected into mercury-containing flue gas, usually in the form of activated carbon. Further, carbon in the presence of chlorine may increase the oxidation of elemental mercury. In the flue gas, mercury can be converted to its oxidized form, Hg +2 , and react with chlorine-containing species to form mercury chloride (HgCl 2 ). As such, the extent of mercury oxidation in flue gas is generally higher for coals with a higher chlorine content, such as bituminous coals, and lower for coals with a lower chlorine content, such as low-rank coals. 
     Particulate matter is another major pollutant produced by fossil fuel combustion. Various pollutant control techniques when implemented primarily for removal of nitrogen oxides (NOx), e.g., low-NOx burners (LNB) and combustion modifications such as air and fuel staging could lead to increased amounts of particulate matter in the exhaust gas at the furnace exit. Higher particulate matter loadings at the furnace exit increase loads placed on particulate control devices such as electrostatic precipitators and baghouses, and can lead to increased particulate matter emissions into atmosphere. Sometimes combustion modifications are implemented in such a way as to intentionally increase the amount of carbon in fly ash. While unburned carbon (also loosely referred to as loss-on-ignition, or LOI) can serve as effective capturing agent for gaseous pollutants such as mercury, increased levels of LOI in fly ash negatively affect fly ash properties and can lead to additional difficulties associated with fly ash collection, use, and disposal. 
     Combustion fly ash can be used as a cement additive. Stringent requirements exist both in Europe and North America for the maximum allowed carbon content in fly ash sold as a cement additive. The limitations are typically based on the foaming index of the cement. Carbon distribution in fly ash as a function of ash particle size is usually not uniform. Depending on the grinding characteristics of the coal fuel, unburned carbon content can be higher in the larger ash particles (50 to 200 microns or larger), because these particles are generated as a result of incomplete combustion of the largest coal particles. Conversely, typical size of activated carbon sorbent particles is about 3 to 20 microns because it is advantageous to increase particle surface area and improve gas-solid contact for higher removal efficiency. Therefore, depending on plant configuration and operation, high carbon fractions can be presented mostly in larger fly ash particles, or smaller particles, or both. Most often, the highest amounts of captured mercury are associated with particles having high carbon content. There is growing concern about re-use of mercury-laden fly ash, because captured mercury can later escape. 
     Efficiencies of most available mercury emission control technologies depend on the mercury speciation in flue gas. Oxidized mercury is water-soluble and may be removed from flue gas using known wet desulfurization systems (wet scrubbers). At least some particulate-bound mercury may be removed from flue gas using known particulate collection systems. Elemental mercury is more difficult to remove than oxidized mercury and/or particulate-bound mercury because elemental mercury is unreactive and, as such, cannot be removed from flue gas with wet desulfurization systems or particulate collection system. 
     In some known systems, because the concentration of mercury in the flue gas is very small (typically less than 10 parts per billion or ppb), diffusion of mercury from the surrounding flue gases may limit the mercury removal process. Most of the flue gases produced in known systems flow in substantially laminar flow patterns and are characterized by slow diffusion rates. Because of the flow characteristics of the flue gas, some known mercury emission reduction systems have attempted to optimize the use of the sorbent by modifying the number and design of sorbent injection lances to achieve sorbent coverage within the flue duct. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method of reducing particulate matter and mercury emissions in a combustion flue gas emitted from a combustion unit. The method includes combusting a fuel resulting in generation a flue gas flow, the flue gas flow containing mercury and carbon-containing fly ash particles, cooling the flue gas flow within a duct, and positioning a flow conditioning apparatus within the duct. The method also includes enhancing a reaction rate of the mercury and carbon-containing fly ash particles by directing the flue gas flow through the flow conditioning apparatus to mix the carbon-containing fly ash particles and mercury within the flue gas flow and facilitate at least one of oxidation of the mercury and binding the mercury to the carbon-containing fly ash particles. The method further includes collecting a portion of the carbon-containing fly ash particles in the flow conditioning apparatus, and directing the flue gas flow to a particulate collection device to remove the remaining portion of the fly ash particles from flue gas flow. 
     In another aspect, a pollution reduction system for a combustion unit is provided. The combustion unit includes a combustion zone configured to generate a flue gas flow that includes at least carbon-containing fly ash particles and mercury. The pollution reduction system includes a particulate control device, a duct configured to channel the flue gas flow from the combustion zone to the particulate control device, with the duct coupled to the particulate control device, and a flow conditioning apparatus positioned upstream of the particulate control device. The flow conditioning device is located at least partially inside the duct. The flow conditioning device includes a first portion positioned inside the duct, and a second portion located below the first portion. The first portion includes a plurality of vanes, and the second portion includes a particle collection chamber positioned below the plurality of vanes. 
     In another aspect, a combustion power plant is provided. The power plant includes a combustion furnace including a combustion zone configured to generate a flue gas flow that includes at least carbon-containing fly ash particles and mercury, a duct coupled to the combustion zone for channeling the flue gas therethrough, a particulate control device coupled to the duct, and a flow conditioning apparatus positioned upstream of the particulate control device. The flow conditioning device is located at least partially inside the duct. The flow conditioning device includes a first portion positioned inside the duct, and a second portion located below the first portion. The first portion includes a plurality of vanes, and the second portion includes a particle collection chamber positioned below the plurality of vanes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an exemplary power plant system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of an exemplary power plant system  10 . In the exemplary embodiment, system  10  is supplied with fuel  12  in the form of coal  14 . More specifically, the coal  14  can be bituminous coal, lignite coal, and/or any other suitable coal that enables system  10  to function as described herein. Alternatively, fuel  12  may be any other suitable fuel, such as, but not limited to, oil, natural gas, biomass, waste, or any other fossil or renewable fuel. In the exemplary embodiment, coal  14  is supplied to system  10  by a coal supply means  16 , for example, a conveyor, and is processed in a coal mill  18 . Coal  14  is pulverized in coal mill  18  to form coal particles having a predetermined and selectable fineness. 
     System  10  includes a coal-fired furnace  20  that includes a combustion zone  22  and heat exchangers  24 . Combustion zone  22  includes a primary combustion zone  26 , a reburning zone  28 , and a burnout zone  30 . In another embodiment, combustion zone  22  does not include reburning zone  28  and/or burnout zone  30  such that furnace  20  is a “straight fire” furnace. Fuel  12  enters furnace  20  through a fuel inlet  32 , and air  34  enters furnace  20  through an air inlet  36 . In primary combustion zone  26 , the fuel/air mixture is ignited to create combustion gases  38 . 
     Fuel  12  and air  34  are supplied to primary combustion zone  26  through one or more main injectors and/or burners  40 . Main burners  40  receive a predetermined amount of fuel  12  from fuel inlet  32  and a predetermined quantity of air  34  from air inlet  36 . Burners  40  may be tangentially arranged in each corner of furnace  20 , wall-fired, or have any other suitable arrangement that enables furnace  20  to function as described herein. Within primary combustion zone  26 , combustion gases  38  are formed, and may include, but is not limited to, carbon, carbon containing fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or trace metals, for example, mercury. Combustion products not contained in combustion gases  38  may include solids and may be discharged from furnace  20  as waste. 
     Combustion gases  38  flow from primary combustion zone  26  towards reburning zone  28 . In reburning zone  28 , a predetermined amount of reburn fuel  42  is injected through a reburn fuel inlet  44 . Reburn fuel  42  is supplied to inlet  44  from fuel inlet  32 . Although reburn fuel  42  and fuel  12  are shown as originating at a common source, such as fuel inlet  32 , reburn fuel  42  may be supplied from a source other than fuel inlet  32 , and/or may be a different type of fuel than fuel  12 . For example, fuel  12  entering through fuel inlet  32  may be, but is not limited to, pulverized coal, and reburn fuel  42  entering through a separate reburn fuel inlet may be natural gas. In the exemplary embodiment, the amount of reburn fuel  42  injected is based on a desired stoichiometric ratio within reburning zone  28 . More specifically, in the exemplary embodiment, the amount of reburn fuel  42  creates a fuel-rich environment in reburning zone  28 . As such, less of the carbon in fuel  12  and in reburn fuel  42  is combusted, which facilitates increasing the Loss on Ignition (LOI) and facilitates creating a more reactive, high-carbon content fly ash entrained in combustion gases  38 . 
     Combustion gases  38  flow from reburning zone  28  into burnout zone  30 . Overfire air  46  is injected into burnout zone  30  through an overfire air inlet  48  and, a predetermined quantity of overfire air  46  is injected into burnout zone  30 . In the exemplary embodiment, overfire air inlet  48  is in flow communication with air inlet  36 . In another embodiment, overfire air  46  may be supplied to furnace  20  through an inlet  48  that is separate from air inlet  36 . The quantity of overfire air  46  is selected based on a desired stoichiometric ratio within burnout zone  30 . More specifically, in the exemplary embodiment, the quantity of overfire air  46  is selected to facilitate completing combustion of fuel  12  and reburn fuel  42 , which facilitates reducing pollutants in combustion gases  38 , such as, but not limited to, nitrogen oxides (NOx), and/or carbon monoxide (CO). 
     Flue gas  50  exits combustion zone  22  and may include carbon, carbon-containing fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or trace metals, for example, mercury. Flue gas  50  exits combustion zone  22  and enters heat exchangers  24 . Heat exchangers  24  transfer heat from flue gas  50  to a working fluid in a known manner. More specifically, the heat transfer heats the fluid, such as, for example, heating water to generate steam. The heated fluid, for example, the steam, is used to generate power, typically by known power generation methods and systems, such as, for example, a steam turbine. Power may be supplied to a power grid or any suitable power outlet. 
     Flue gas  50  flows from heat exchangers  24  to a duct or convective pass  52 . As flue gas  50  flows through duct  52 , the gas  50  is cooled to a temperature that is less than the combustion temperature. More specifically, in the exemplary embodiment, flue gas  50  within duct  52  is cooled convectively, conductively, and/or radiantly by ambient air and/or any other suitable cooling means, including evaporative cooling. In the exemplary embodiment, the cooling fluid at least partially surrounds duct  52  to facilitate cooling flue gases  50  therein. In an alternative embodiment, the cooling fluid is vented into duct  52  to facilitate cooling flue gases  50 . In another alternative embodiment, system  10  includes cooling fluid at least partially surrounding duct  52  and cooling fluid vented into duct  52  to facilitate cooling flue gases  50 . In the exemplary embodiment, flue gas  50  is cooled to a temperature that enables mercury to react with the carbon in the fly ash, for example, a temperature below 350° F. As such, mercury is oxidized, and captured by, carbon, chlorine, and/or any other suitable mercury-reactive elements and/or compounds in flue gas  50 . 
     In the exemplary embodiment, a predetermined amount of sorbent  54  is injected into duct  52  to react with flue gas  50 . Sorbent  54  is injected into duct  52  through a sorbent injector  56 . In an alternate embodiment, sorbent  54  is not injected to duct  52 , but rather mercury entrained in flue gas  50  reacts only with elements and/or compounds present within flue gas  50 . The sorbent  54  is selected to facilitate oxidation and/or capture of mercury, for example, activated carbon. Alternatively, sorbent  54  may be any other suitable element and/or compound that facilitates oxidation and/or capture of mercury. 
     In the exemplary embodiment, a flow conditioning apparatus  58  is positioned within duct  52  downstream from sorbent injector  56 , and upstream of a particulate control device  60 . Flow conditioning apparatus  58  includes a first portion  62  and a second portion  64 . First portion  62  includes a plurality of vanes  66  mounted within duct  52  that alter the flow of flue gas  50  to increase the mixing of smaller fly ash particles and/or sorbent particles  54  with flue gas  50  to improve the removal of mercury from flue gas  50 . Some of the vanes can be adjustable vanes  68  that are capable of changing the angle of adjustable vanes  68  in relation to the flow of flue gas  50  to permit tuning of the efficiency of adjustable vanes  68  in response to changing operating characteristics of the combustion furnace, fuel quality, and sorbent characteristics. 
     Second portion  64  includes a particle collection chamber  70  positioned below vanes  66  of first portion  62 . The impingement on vanes  66  turbulence created in the flow of flue gas  50  by vanes  66  causes larger and/or heavier fly ash particles to drop out of the flow of flue gas  50  and be collected in collection chamber  70 . The larger and heavier fly ash particles usually contain a high proportion of unburned carbon. The removal of these larger particles in flow conditioning apparatus  58  reduces the total particulate matter mass collected by particulate control device  60  and decreases the carbon content of the fly ash collected in particulate control device  60 . In another embodiment, an electric field can be used in conjunction with vanes  66  to enhance the removal of particulate matter by flow conditioning apparatus  58  In another embodiment, an electric field can be used in conjunction with vanes  66  to enhance the removal of particulate matter by flow conditioning apparatus  58 . In an alternative embodiment that does not include sorbent injector  56 , flow conditioning apparatus  58  is positioned downstream from heat exchangers  24 , and upstream of particulate control device  60 . Further, in the exemplary embodiment, particulate control device  60  may be, for example, but not limited to, an electrostatic precipitator, a cyclone, or a baghouse, used to collect particles including those containing oxidized mercury and/or particulate-bound mercury. 
     In an alternative embodiment, system  10  may also include an ash burnout unit (not shown) and/or a mercury collection unit (not shown) coupled to particulate control device  60  and/or collection chamber  70 . The ash burnout unit facilitates the removal of carbon from collected particle matter, causing desorption of mercury from the fly ash. The mercury collection unit is optionally coupled to the ash burnout unit and may include activated carbon, or any other suitable reagent, for capturing mercury desorbed by the ash burnout unit. System  10  may further include a wet scrubber (not shown) and/or a dry scrubber (not shown) positioned downstream of particulate control device  60  for removing oxidized mercury and/or particulate-bound mercury from flue gas  50  and/or for removing other compounds and/or elements from flue gas  50 , such as, for example, sulfur dioxide. At least partially decontaminated flue gas  50  exits system  10  as exhaust gases  72  discharged through an exhaust stack  74 . 
     During operation of system  10 , fuel  12 , air  34 , reburn fuel  42 , and/or overfire air  46  are injected and combusted in combustion zone  22  to form flue gases  50  that include, but are not limited to, carbon, carbon containing fly ash, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur, chlorine, and/or trace metals, for example, mercury. Flue gases  50  flow from combustion zone  22  through heat exchangers  24 , and into duct  52 . In the exemplary embodiment, the flow of flue gases  50  through duct  52  is substantially laminar, except where the geometry of duct  52  causes minor turbulence. 
     As the gases  50  cool in duct  52 , mercury reacts with carbon within the flue gases  50  to form oxidized mercury. Mercury may also react with elements and/or compounds within flue gas  50  to form particulate-bound mercury. In the exemplary embodiment, sorbent  54  is injected into cooling flue gas  50  such that mercury within flue gas  50  reacts with sorbent  54  to form oxidized and/or particulate bound mercury. For reactions to occur between mercury and other reactive elements and/or compounds within flue gas  50  and/or sorbent  54 , mercury must collide with such reactive particles. The rate of mercury oxidation is affected by the number of collisions between mercury and other reactive particles in flue gas  50  and/or sorbent  54 . Further, mercury reactions occur at temperatures cooler than the combustion temperature, such as, but not limited to, temperatures below 350° F. Adsorption of mercury on a surface of a carbon-containing particle is relatively fast process, and, as such, mercury in the nearest proximity to carbon containing particles is adsorbed first. 
     In the exemplary embodiment, vanes  66  create a substantially turbulent flow in the flow of flue gas  50 . Turbulence in flue gas  50  increases the number of collisions between mercury and other particles, which increases the mercury chemical reaction rate within flue gas  50  and/or between flue gas  50  and sorbent  54 . As such, as the number of collisions between mercury and other particles increases, the possibility that mercury will oxidize or become particulate-bound also increases. As a result of the collisions and reactions caused by turbulence in flue gas  50 , the percentage of oxidized mercury and particulate-bound mercury in flue gas  50  is increased while the percentage of elemental mercury in flue gas  50  is decreased. 
     The above-described method and apparatus facilitates removing mercury from combustion exhaust gas by improving natural mercury capture on fly ash and improving sorbent utilization. The diffusion rate of mercury atoms to carbon particles within the flue gas is greater in substantially turbulent flow in comparison to a substantially laminar flow; therefore, increasing flue gas flow turbulence facilitates improving mercury absorption on carbon within the flue gas, and, more specifically, on the carbon-containing fly ash within the flue gas. Furthermore, the efficiency of mercury removal using sorbent injection is increased when the sorbent is substantially uniformly distributed across a flue duct cross-section because the uniform distribution facilitates utilizing the mercury removal capacity of the sorbent. Turbulence in the flue gas flow facilitates increasing the uniformity of the distribution of the sorbent across the flue duct cross-section. Turbulence in the flue gas flow facilitates decreasing the requirements for the amount of sorbent injected for mercury control by facilitating improving the mixing of carbon-contain fly ash, sorbent, and mercury within the flue gas flow. Because turbulence in the flue gas flow facilitates increasing mercury absorption on sorbent, the sorbent is utilized more effectively, and the amount of sorbent to achieve the same mercury removal efficiency is decreased. 
     Further, because flow turbulization also facilitates improving mercury absorption on carbon-containing fly ash, requirements for sorbent injection are reduced in comparison to coal-fired power plants that do not include a flow conditioning apparatus for turbulizing the flue gas flow. The efficiency of natural mercury capture on carbon-containing fly ash and the efficiency of sorbent utilization can be increased by introducing turbulent mixing of fly ash, sorbent, and/or mercury-containing flue gas. Such mixing at the location downstream of sorbent injection and upstream of particulate control device facilitates increasing the amount of mercury the particulate control device removes from the flue gas flow. 
     In addition, the above-described method and apparatus provides for increased efficiency of gaseous pollutant (such as mercury) removal, while simultaneously improving operation of the particulate collection system and improving characteristics of the fly ash collected by particulate collection device  60 . The above-described method and apparatus facilitates tuning of the combustion process to reduce amount of carbon contained in the fly ash while maintaining or improving gaseous pollutant removal efficiency, achieving better combustion efficiency and improving characteristics of fly ash collected by particulate control device  60 . Further, the above-described method and apparatus provide at least two separate streams of collected particulate matter: 1) the coarser fraction removed by the flow conditioning apparatus  58 , and 2) the finer fraction collected by particulate control device  60 . The coarser fraction includes a significant portion of the total unburned carbon and potentially can be used as a fuel. The finer fraction has a reduced level of unburned carbon, improving its desirable characteristics such as carbon content, foam index, flowability, etc. 
     Exemplary embodiments of a method and apparatus for removing particulate matter and mercury from combustion exhaust gas are described above in detail. The method and apparatus are not limited to the specific embodiments described herein, but rather, components of the method and apparatus may be utilized independently and separately from other components described herein. For example, the flow conditioning apparatus may also be used in combination with other pollution control systems and methods, and is not limited to practice with only the coal-fired power plant as described herein. Rather, the above-described method and apparatus can be implemented and utilized in connection with many other pollutant emission reduction applications. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.