Abstract:
A combustion apparatus includes: a combustion device operable to produce flue gas having large particulate ash entrained therein; an outlet duct coupled in flow communication with the combustion device so as to receive the flue gas; a screen positioned at least partially in the outlet duct downstream of the combustion device, the screen effective to trap large particulate ash while permitting flow of the flue gas therethrough; and apparatus operable to remove large particulate ash from the screen while at least a portion of the screen remains inside the duct.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Provisional Patent Application No. 61/241,071, filed Sep. 10, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to NOx emissions reduction in combustion processes, and more particularly to the improvement of performance of Selective Catalytic Reduction (SCR) systems. 
         [0003]    Fossil fuel combustion processes, such as electric power generation, produce numerous types of atmospheric pollutants as by-products, in particular oxides of nitrogen (NOx). Environmental regulations are becoming increasingly more stringent, requiring power plants to reduce stack emissions even further. Therefore, it is desirable to improve and enhance the methods and equipment currently used for removing pollutants from gas streams. 
         [0004]    One particular type of emissions-reduction equipment is a Selective Catalytic Reduction (SCR) system. SCR systems are the only NOx reduction technology capable of achieving greater than 90% reductions of NOx emissions. A major issue with SCR systems is the accumulation of large particle ash (LPA) deposits originating from the combustion process on the SCR system, which impedes the performance of the SCR system. 
         [0005]    Current technology utilizes fixed screens to prevent the LPA deposits from contacting the SCR system. A drawback of this technology is the build up of LPA particles on the screens, which cause performance of the overall system to degrade over time as LPA accumulates on the screen. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    These and other shortcomings of the prior art are addressed by the present invention, which provides a catalyst protection system incorporating moving screens that can be cleaned continuously, to prevent build up of build up of LPA. 
         [0007]    According to one aspect of the invention, a combustion apparatus includes: a combustion device operable to produce flue gas having large particulate ash entrained therein; an outlet duct coupled in flow communication with the combustion device so as to receive the flue gas; a screen positioned at least partially in the outlet duct downstream of the combustion device, the screen effective to trap large particulate ash while permitting flow of the flue gas therethrough; and apparatus operable to remove large particulate ash from the screen while at least a portion of the screen remains inside the duct. 
         [0008]    According to another aspect of the invention, a method of removing large particulate ash from a gas stream includes: passing a combustion gas stream having large particulate ash entrained therein through a duct; collecting large particulate ash on a screen positioned at least partially in the duct, while permitting flow of the flue gas therethrough; and removing the ash from the screen at a location external to the duct while at least a portion of the screen remains inside the duct. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a schematic block diagram of a combustion process incorporating a catalyst ash protector constructed according to an aspect of the present invention; and 
           [0011]      FIG. 2  is an enlarged view of a portion of an outlet duct and catalyst ash protector shown in  FIG. 1 ; and 
           [0012]      FIG. 3  is a view the outlet duct and catalyst ash protector of  FIG. 1  in a vertical orientation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts an exemplary combustion process incorporating a catalyst protection system constructed according to an aspect of the present invention. As shown, the combustion process  10  comprises a combustion device  12 , such as a fossil-fuel-fired boiler, that uses air to combust fuel, such as a fossil fuel, for example, coal. The combustion device  12  produces a gas stream in the form of flue gas that exits the combustion device  12  through an outlet duct  14  (represented schematically by an arrow in  FIG. 1 ). The flue gas produced within the combustion device  12  includes air; gaseous products of combustion, such as water vapor, carbon dioxide, oxides of nitrogen (NOx) and sulfur, halides, organic compounds, mercury, selenium, and other trace metal vapors; and aerosols, including particulate matter or fly ash and in some cases sulfuric acid mist; and large particulate ash (LPA). As used herein, the term “large particulate ash” refers to solid-phase combustion byproducts in the form of particles having a normal size of about 6 mm (0.23 in.) to about 10 mm (0.39 in.). 
         [0014]    A selective catalyst reduction (SCR) unit  16  is positioned downstream of the combustion device  12 . The SCR is of a known type which functions to reduce oxides of nitrogen (NOx) in the flue gas to nitrogen and oxygen. Downstream of the SCR unit  16  is an air preheater  18  which serves to recover heat from the flue gas by transferring the heat to the combustion air which is sent to the combustion device  12  along with the fuel supply. Finally, downstream of the air preheater  18  is a particulate collection device  20 , such as a wet or dry electrostatic precipitator or a baghouse, which removes aerosols and particulate matter from the flue gas. The flue gas exiting the particulate collection device  20  exits to a stack  22  and thence to the atmosphere. It should be appreciated that this combustion process is only exemplary, and the concepts of the present invention may be used with combustion processes having various configurations of specific equipment. 
         [0015]    As noted above, the SCR unit  16  is susceptible to blockage or clogging by large particulate ash (LPA). Accordingly, a catalyst ash protector  24  is positioned in the outlet duct  14  between the combustion device  12  and the SCR unit  16 . It serves to protect downstream selective catalyst reduction unit (SCR)  18  from blockage by removing LPA. 
         [0016]      FIGS. 2 and 3  illustrate the catalyst ash protector  16  in more detail. The catalyst ash protector  16  may be used with ducting in any orientation. In  FIG. 2 , the outlet duct  14  is shown in a horizontal orientation, and in  FIG. 3 , it is shown in a vertical orientation. The main component is a screen  26  which extends across the outlet duct  14 . The screen  26  may be mounted in any orientation (e.g. vertical or horizontal), and need not be mounted perpendicular to the direction of flow through the outlet duct  14 . 
         [0017]    The screen  26  comprises a plurality of holes that allow the gas to pass through the screen  26 . The size, shape, and position of the holes in the screen  26  can be varied to optimize the performance of the screen  26  and the gas pressure drop across the screen  26 . In general, the holes should be less than about 10 mm (0.39 in.) in diameter so as to effectively trap LPA particles. Preferably, the holes have a diameter ranging from about 0.1 mm (0.004 in.) to about 10 mm (0.4 in.), more preferably about 1 mm (0.04 in.) to about 8 mm (0.3 in.). Alternatively, the screens may be a felted cloth or metal material with a high gas permeability, for example, approximately 100 cfm/ft 2  of screen or greater. It should be appreciated that the shape of the holes and the hole pattern may also be varied as desired. For example, the holes could be simple squares or other shapes such a diamond, rectangles, etc. The hole pattern or pitch in the screen may also vary. The materials of construction used for the screen  26  may also be varied and selected based upon the gas composition, such as the corrosivity of the gas, and to optimize the structural integrity of the screen  26 . For example, the screen  26  may be constructed from metal wire mesh; woven cloth made from glass, ceramic, carbon, or polymer yarn; or holes punched in a metal or polymer sheet. Given the various options for constructing the screen  26 , it should be appreciated that the term “screen” is used throughout generically and should not be construed as limited to, for example, a metal wire screen. 
         [0018]    The screen  26  may be wrapped around two rollers  28  and  30  near the boundaries of the outlet duct  14 . The rollers  28  and  30  may be mounted inside or outside the outlet duct  14 . An electric motor  32  or other suitable drive mechanism is provided to drive the roller  28  (and thus the screen  26 ) such that, when operating, each portion of the screen moves along a continuous path from one roller  28 , across the outlet duct  14  to the other roller  30 , and back again to the first roller  28 . A hopper  34  may be connected or positioned under a portion of the outlet duct  14  such that the screen  26  may move from the outlet duct  14  through the hopper  34  and back into the outlet duct  14 . The hopper  34  may be filled with water or a wash solution to clean the screen  26  as it rotates through the hopper  34 . When operated in the position shown in  FIG. 3  the hopper  34  may include a slot or opening to allow the screen  26  to pass through its side wall. Alternatively, the hopper  34  could incorporate spray nozzles (not shown) to enhance the cleaning of the screen  26 . The hopper  34  may be fed with water or a wash solution by a feed line (not shown) and dirty water or spent wash solution may be discharged from the hopper  34  through a discharge line (not shown). 
         [0019]    In operation, the screen  26  may be rotated through the hopper  34  as desired. For example, such rotation may be done on a regularly scheduled basis, either manually or automatically. Alternatively, rotation may be done continuously or only on an as-needed basis depending upon, for example, the gas pressure drop across the screen  26 . Other alternative apparatus and methods of cleaning the screen  26  include air jets, vacuum cleaning, mechanical brushing or scraping, vibration cleaning, or configuration of the screen and surrounding hardware so that LPA may fall off by gravity. Box  35  in  FIG. 2  represents schematically LPA removal apparatus positioned to access the screen  26 . This may be used in addition to or instead of the hopper  34 . As another alternative the screen may be fed from a roll of screen material and move in only one direction onto another roller on the other side. The screen can still be cleaned on the rollers as described previously. Once the roll of screen is depleted, the direction of the screen is simply reversed. Alternatively the spent screen roll can also be disposed of or refurbished off site as needed. 
         [0020]    In addition to plugging or blocking by LPA particles, the SCR system  16  is also subject to poisoning and loss of effectiveness from vapor phase contaminants in the flue gas stream such as arsenic, selenium, and nickel. To address this problem, alkaline material such as lime may be injected into the flue gas upstream of the catalyst.  FIG. 1  shows a schematic view of an injector  36  positioned for this purpose. The alkaline material would be provided as a powder or particulate having a size similar to that of the LPA, for example greater than 0.1 mm (0.004 in.), preferably greater than 1.0 mm (0.04 in.). In the outlet duct  14 , this material is believed to bind with the vapor phase contaminants, especially arsenic. The alkaline material may then be removed by the catalyst ash protector  24  in the same way the LPA is removed. 
         [0021]    The moving screen catalyst ash protector described herein has several advantages as opposed to currently used stationary screens which are subject to plugging. The moving screen will avoid misdistribution of the flue gas, high pressure drops across the screen  26 , and screen deterioration, which can only be rectified during a plant outage. 
         [0022]    The foregoing has described a catalyst protection system for a combustion process and a method for its operation. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only.