Abstract:
One embodiment described herein relates to a system for removing pollutants from a flue gas. The system includes a selective catalytic reduction (SCR) system having a SCR reactor containing a NO x  reducing catalyst and one or more SCR protective devices. At least one of the SCR protective devices is connected to a rapping hammer system that actively remove fly ash collected on the SCR protective devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a device and method of cleaning protective screens used in a Selective Catalytic Reduction (SCR) system. 
     2. Description of the Related Art 
     Selective Catalytic Reduction (SCR) systems are increasingly being applied to coal-fired power stations to reduce nitrogen oxide (NO x ) emissions. SCR systems commonly include a SCR reactor that contains a NO x  reducing catalyst that converts NO x  present in flue gases emitted from a combustion source into by-products of nitrogen and water. Many power station installations place the SCR reactor system in a “high dust” location between the combustion source and a particle collection system. Generally, these installations have ductwork that directs or diverts the particle-laden flue gases from the combustion source to the SCR reactor system and then to an air preheater. 
     The dust loading ability of these SCR systems, located in such “high dust” locations, is an important consideration in their design and use. In particular, the NO x  reducing catalyst composition and construction thereof should be designed to withstand erosion and potential chemical degrading effects of the fly ash and other particles in the flue gases. Similarly, the ductwork to and from the SCR reactor system and the associated internal structures within the SCR system should be designed to withstand this erosive environment. For example, certain aspects of the ductwork design parameters, such as the duct&#39;s gas velocity, may be closely monitored to insure proper operation. In particular, undesirable operating results such as unwanted fly ash drop out should be prevented or minimized by selection of proper operating design parameters. 
     The NO x  reducing catalyst construction in the SCR reactor also requires proper design considerations. Generally, the NO x  reducing catalyst is constructed in a manner that has gas channels whereby the flue gases can pass through such channels to maximize contact with the catalyst surface thereby maximizing the reduction of NO x . The gas channels of the NO x  reducing catalyst typically have a diameter in the range of about 5 to 7 mm. However, particles in the flue gas (hereinafter referred to as “fly ash”) generally have a wide range of sizes (e.g. from 1-2 microns up to 7 mm and larger). 
     The larger particles of fly ash, sometimes referred to as “popcorn ash” or large particle ash (“LPA”), may pose problems with the NO x  reducing catalyst. For instance, when the gas channel diameter is 5-7 mm and the fly ash particles are larger than 7 mm, the large fly ash particles may lodge within the channels and block the flow of flue gas through the catalyst. Even fly ash particles smaller than 7 mm have been shown to plug the catalyst channels because of the irregular shape of those particles. If just one irregular shaped fly ash particle gets lodged in the catalyst channels, other fly ash particles cannot pass through the channel, thereby blocking the channel. 
     This blockage decreases the overall NO x  reduction capability of the system because once a gas channel is blocked, that reaction channel in the NO x  reducing catalyst becomes ineffective. Once many reaction channels become blocked, fly ash accumulation on the NO x  reducing catalyst surface increases rapidly. Over time, the surface of the NO x  reducing catalyst can eventually become so covered with fly ash that the SCR system cannot meet its NO x  reduction target. Also, the resulting increase in catalyst pressure drop will require the system to be cleaned. For SCR units without a gas bypass capability, this build-up may require the combustion source to be shut down as well. 
     A known practice to mitigate this ash or dust build-up over the NO x  reducing catalyst has been to place one or more mesh screens over the NO x  reducing catalyst. The openings in the mesh screens are selected to be slightly smaller than the diameters of the channels in the NO x  reducing catalyst. Thus, large fly ash particles are stopped from entering the channels in the NO x  reducing catalyst. While this method can keep the actual catalyst channels clean, its ability to lengthen the time between outages for cleaning is uncertain. Cleaning is still necessary for this method because the quantity of large fly ash particles entering the SCR reactor remains unchanged and these fly ash particles are now collected on the screens instead of on the catalyst or within its channels. Large fly ash particles may accumulate on the screens, thereby creating blockages which will then start collecting smaller fly ash particles. It is therefore possible to have mounds of fly ash on each screen. 
     Mounds of fly ash that are collected on the screens can significantly increase the pressure drop across the SCR system and may lead to localized areas of high velocity, which have been known to cause erosion within the catalyst. The accumulation of fly ash on the screens will also affect gas distribution and gas velocity into the NO x  reducing catalyst. This in turn will reduce the efficiency of the SCR system. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention relates to a system for removing pollutants from a flue gas. The system includes a selective catalytic reduction (SCR) system that has a SCR reactor containing a NO x  reducing catalyst and one or more SCR protective devices located upstream of the SCR reactor wherein the one or more SCR protective devices substantially prevent large particles in the flue gas from entering the SCR reactor or otherwise impeding the flow of flue gas therethrough. The system also includes a mechanical rapping system for impacting the SCR protective device to dislodge therefrom accumulated large particles. 
     Another aspect of the invention relates to a method of removing accumulated fly ash from an SCR protective device. The method includes the steps of connecting a rapping hammer system that has at least one hammer and at least one rotating shaft to an SCR protective device, rotating the rotating shaft to turn the at least one hammers, and contacting the at least one hammer to the SCR protective device, whereby accumulated fly ash present on the SCR protective device is removed. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purposes of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  shows SCR protective devices placed at various points in ductwork upstream of the SCR reactor. 
         FIG. 2  shows a rapping hammer assembly within a flue gas stream. 
         FIG. 3  shows a rapping hammer assembly outside of a flue gas stream. 
         FIG. 4  shows a side view of the rapping hammer assembly. 
         FIG. 5  shows a side view of a SCR protective device. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The term “SCR protective device” as used in the present specification and claims refers to any device that prevents appreciable quantities of large fly ash particles (LPA) and other large particulate material in flue gases from entering the NO x  reducing catalyst channels or accumulating on other SCR catalyst surfaces. One example of an SCR protective device is a wire mesh screen that has openings that are slightly smaller than the diameters of the NO x  reducing catalyst channels. Typically, the SCR protective device is a screen that is surrounded by a supporting frame. 
     It is noted that while the SCR protective device prevents appreciable quantities of fly ash from entering the NO x  reducing catalyst channels, it does not hinder the flow of the gas from entering the NO x  reducing catalyst. 
     Now referring to the figures in which like numerals correspond to like parts, and in particular to  FIG. 1 , an SCR protective device  20  may be placed in various locations upstream of the SCR reactor  22 . One embodiment described herein relates to the active removal of accumulated fly ash on any SCR protective device  20  placed upstream of SCR reactor  22  as well as any SCR protective device that is placed directly over the catalyst material. In one embodiment, SCR protective device  20  can be placed in a sloped or angled orientation in relation to a flue duct wall (“ductwork”)  35 . In another embodiment, SCR protective device  20  can be placed in a perpendicular orientation in relation to flue duct wall  35 . 
     As shown in  FIG. 2 , one embodiment of the invention has a mechanical rapping system  24  operatively connected to SCR protective device  20 . Mechanical rapping system  24  generally includes a rapping hammer assembly  26  and a control unit  28 . Rapping hammer assembly  26  includes hammers  30  that are attached to a rotating shaft  32 . Hammers  30  can be made of any material suitable to contact SCR protective device  20 . Examples of such materials include, but are not limited to: metal, plastic, rubber, concrete, and any other suitable synthetic or naturally occurring material. The weight and size of hammers  30  will vary depending on the system, the amount of fly ash, and the size of SCR protective device  20 . Hammers  30  can be replaced from time to time, or as necessary with hammers that weigh more or less than the typical hammers used in the system. Additionally, hammers  30  can be replaced with hammers that are larger or smaller than the typical hammers used in the system. 
     Hammers  30  contact SCR protective device  20  with a hitting, rapping, or striking motion of sufficient force to cause at least a portion of fly ash that has accumulated on the SCR protective device to slough off and be removed therefrom. It is contemplated that hammers  30  can contact any portion of SCR protective device  20 , including any surrounding supporting frame. 
     Rotating shaft  32  is attached to hammers  30 . Preferably, rotating shaft  32  is made of steel; however one skilled in the art will recognize that other materials, such as plastic, or other synthetic or naturally occurring material may be used for the rotating shaft. 
     Rotating shaft  32  is typically rotated by control unit  28  thereby causing hammers  30  to contact SCR protective device  20 . Rapping hammer assembly  26  may be operated by an electric or battery operated motor located in control unit  28 . Alternatively, rapping hammer assembly  26  could be operated by pneumatic cylinders or magnetic impulse devices, or by any other power source that would allow hammers  30  to contact SCR protective device  20  in a forceful motion to remove accumulated fly ash. 
     Typically, control unit  28  is connected to rapping hammer assembly  26  via rotating shaft  32 . The motor, or other power means, actuates the movement of hammers  30 . 
     In one embodiment of the present invention, control unit  28  includes a user interface  33  such as a desktop computer, a laptop computer, a monitor, or other display device that allows a user to vary the settings of rapper hammer assembly  26 . User interface  33  would allow the user to control several variables, including but not limited to, the pressure of hammers  30  striking SCR protective device  20 , the amount of times the hammers strike the SCR protective device in a specific time period, and/or the continuity of the hammer strikes on the SCR protective device. These variables would vary and are specific to each plant. Control of these variables will facilitate the removal of at least a portion of any fly ash accumulated on SCR protective device  20 . 
     In one embodiment of the invention, hammers  30  continuously strike SCR protective device  20 . In another embodiment, hammers  30  strike SCR protective device  20  at predetermined times. In yet another embodiment, a sensor or measuring device  34 , such as a differential pressure transmitter, may be employed to determine when a certain amount of fly ash accumulates on SCR protective device  20 . Once a certain amount of fly ash accumulates on SCR protective device  20 , hammers  30  will be activated and will strike the SCR protective device. 
     As shown in  FIG. 2 , at least a portion of rapping hammer assembly  26  is within ductwork that has a flue gas stream flowing through it. Typically, in this embodiment control unit  28  is located outside flue duct wall  35 . A wall seal  36  prevents the flue gas from escaping from flue duct wall  35 . 
     In another embodiment, as shown in  FIG. 3 , SCR protective device  20  includes a plurality of contact elements  38  that protrude from the SCR protective device. Contact elements  38  also protrude at least partially outside flue duct wall  35 . Contact elements  38  may be made of any material that is suitable to be contacted with hammers  30 . Examples of appropriate materials include, but are not limited to, metal, plastic, rubber, concrete, and other synthetic or naturally occurring materials. Contact elements  38  provide a surface which hammers  30  can impact instead of hitting SCR protective device  20  directly. 
     Typically, rapping hammer assembly  26  is not directly connected to SCR protective device  20 . As shown in  FIG. 3 , hammers  30  strike contact elements  38  which protrude outside flue duct wall  35 . In this embodiment, rapping hammer assembly  26  is outside flue duct wall  35  and is not exposed to the flue gas. 
       FIG. 4  shows a side view of  FIG. 3 . As seen in this figure, the flow of the flue gas  40  travels towards and goes through SCR protective device  20 . Fly ash and other particulates present in the flue gas are captured by SCR protective device  20 . Hammers  30  move in a semi-circular direction  42  toward contact elements  38 , connected to SCR protective device  20 . Rotating shaft  32  rotates hammers  30  toward contact elements  38 . 
     As one skilled in the art will recognize, there may be one or more mechanical rapping systems  24  attached to one SCR protective device  20 . The number of hammers  30  per rapping hammer assembly  26  may vary to optimize the point(s) at which SCR protective device  20  is impacted by the hammers. Additionally, one of ordinary skill in the art will recognize that one or more contact elements  38  may be connected to SCR protective device  20 . 
     Once hammers  30  have struck SCR protective device  20  in an effective manner, very little fly ash will remain on the SCR protective device. However, it may be necessary to repeat the contact of hammers  30  to the SCR protective device  20  more than once. Therefore, rapping hammer assembly  26  may be programmed or monitored so hammers  30  strike contact elements  38  numerous times within a certain time period. Alternatively, rapping hammer assembly  26  may repeatedly contact SCR protective device  20  for continuous fly ash removal. In another alternative embodiment, sensor  34  may be used to measure or detect an amount of fly ash present on SCR protective device  20 . Once the amount of fly ash reaches a certain level, rapping hammer assembly  26  can be activated, thereby causing hammers  30  to strike contact elements  38 . 
     The manner in which hammers  30  contact SCR protective device  20  will vary from system to system. The action of hammers  30  contacting SCR protective device  20  will allow fly ash particles to slough off and continue through the system. Rapping hammer systems applied to SCR protective devices installed upstream of the catalyst bed dislodge fly ash particles back into the flue gas stream or move the fly ash along SCR protective device  20  to a discharge point. Alternatively, dislodging fly ash can be transported along SCR protective device  20  to an ash collection hopper (not shown). 
     While the invention is directed to the use of a mechanical rapping system on SCR protective devices, one skilled in the art will recognize that this mechanical rapping system can alternatively be employed on any item or device, including SCR protective devices that are designed to improve fly ash knockout in hoppers upstream of the SCR reactor. These items or devices include, but are not limited to economizer outlet “bull noses,” kicker plates, splitters, and other similar items. 
     When a mechanical rapping system is used in connection with SCR protective devices upstream of the SCR reactor, the dislodged fly ash particles may be removed back into the flue gas stream or may be removed to a discharge point or fly ash collection hopper.  FIG. 5  shows a side view of SCR protective device  20  and a path which a fly ash particle may take once it contacts the SCR protective device. After the fly ash particles are dislodged from SCR protective device  20 , a portion of the particles may fall by gravity to an ash collection hopper installed below the screen. Some of the dislodged particles may be carried by the flue gas stream back to SCR protective device  20 . When an SCR protective device  20  is installed on a slope, as shown in  FIG. 5 , the ash particles will eventually work their way to the edge of the SCR protective device where they can be dislodged into a discharge pipe  44 , vacuumed out from time to time, or removed by a device that provides a gas seal  46 , e.g. a cyclone or loop seal. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.