Abstract:
A particulate separator includes a plurality of perforated screens which remove ash particulate from flue gas. A rapper assembly dislodges ash deposits on the perforated screens by exerting a vibrating force thereon. Shafts, which are operatively connected to a rapping device, are received in the openings of perforated screens and welded thereto. The rapper exerts a force on the shaft, which, in turn, vibrates the perforated screen. The vibration of the perforated screens causes the ash deposits to fall off into the hopper.

Description:
FIELD AND BACKGROUND OF INVENTION 
     The present invention relates generally to coal-fired power plants and in particular to an assembly for ash separation from flue gas. 
     It is well-known that coal and other solid fuels generate significant quantities of ash as a byproduct of combustion. The ash typically collects at the bottom of the furnace, but significant quantities of ash becomes entrained in the flue gas stream exiting the furnace. Hoppers for the collection of ash from the flue gas stream are typically positioned downstream from the furnace in order to accumulate the ash entrained in the flue gas stream. 
     The carry over of ash particulate into equipment located downstream of the furnace, is a severe problem. The catalytic converter reduces the NOx levels in flue gas. The catalytic converter typically includes packings of narrow channels having catalyzer surfaces. The ash in the flue gas accumulates on the catalyzer surfaces which results in clogging of the catalyzer channels and the channel walls increasingly losing their catalytic effect. 
     In addition to a reduction of the catalytic effect, the ash accumulation on the catalyzer channel wall results in pressure loss due to the reduction of the flow cross-section. The ash particulate lodges into the surfaces of the downstream equipment and results in plugging of air passages and increased pressure differentials. It often becomes necessary to shut down the furnace and clean, repair or replace the catalytic converter. The replacement of the catalytic converters is extremely expensive, and in particular on the one hand due to the unavoidable shutdown of the power station, and on the other hand due to the procurement and assembly costs of the catalytic converters. 
     Conventional power plants employ an ash hopper upstream from the catalytic converter to collect ash particulate from the flue gas stream. Screens can be positioned across the flue gas stream at the outlet duct of the ash hopper in order to increase ash removal. The screens can cover the entire duct, or cover only a portion of the duct and be staggered with additional partial screens. The screens are typically positioned vertically across the horizontal section of the outlet duct downstream of the hopper, or can be tipped away from the hopper at their upper edges. In either the vertical arrangement of the screens or where the screens are tipped away from the hopper, the pressure of flue gas stream tends to hold the ash in position on the screen once the ash has impacted the screen surface. The holding of the ash against the screen results in plugging of the screen and decreased flow of the flue gas stream. The plugging of the screens also increases power plant operating cost. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a system for improved removal of ash from an ash laden flue gas. 
     It is another object of the invention to provide a system for ash removal which includes a means for removing ash deposits on the ash removal device. 
     It is also an object of the invention to provide a system for ash removal which protects the catalytic converter from obstructions in a simple and effective manner. 
     Accordingly, the invention comprises a particle separator having a chamber with an inlet for receiving flue gas and an outlet for exhausting the flue gas. The chamber houses a plurality of perforated screens. Each screen has at least one opening. A rapper assembly is mounted to a chamber wall. The rapper assembly includes a rapper and a shaft which is operatively connected to the rapper. The shaft extends through an opening in the chamber wall and the openings of the perforated screens. Thus, the shaft directly contacts each of the perforated screens. The rapper moves the shaft, which in turn, vibrates the perforated screens to dislodge the ash particulate therefrom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a front view of the rapper assembly of a first embodiment of an ash screen system according to this invention; 
         FIG. 2  is a schematic view of a second embodiment of an ash screen system according to this invention; 
         FIG. 3  is a side sectional view of the first embodiment of an ash screen system according to this invention; 
         FIG. 4  is a front view of a first embodiment of an ash screen system according to this invention; 
         FIG. 5  is a side view of an opening of the perforated screen of an ash screen system according to this invention; and 
         FIG. 6  is a side view of the perforated screen of an ash screen system according to this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 3 , a boiler (not shown) is connected via a flue gas channel  3  to a particulate separation chamber  5 . The flue gas channel  3  transfers the ash laden gas to the particulate separation chamber  5 . The particulate separation chamber  5  has a housing or frame  12  which defines an inlet opening  11  and an outlet opening  13 . 
     A particulate separator  10  is housed inside the particulate separation chamber  5 . The particulate separator  10  includes screens  14  which are connected to the frame  12  by welds, nut and bolt connection or by other known connectors. 
     The screens  14  of the particulate separator  10  are positioned across the inlet opening  11  and extend vertically downward from the edge of the inlet opening  11 . The screens  14  are preferably dimensioned to extend across both of the inlet  11  and outlet  13  openings. The screens  14  can be arranged in a multitude of formations—horizontally or vertically aligned, parallel or at angle to one another and/or in staggered formation. 
     The screens  14  have perforations  16  to allow the flow of flue gas through the particulate separation chamber  5 . The screens  14  are preferably plates and composed of materials, such as steel, which can withstand the temperature and constant abrasion of ash laden flue gas. By utilizing a perforated screen, instead of a woven wire screen element, the rapping force can be applied directly to the planar surface of the screen  14 . The force used can be less, and at lower frequencies, that would typically be required when a frame holding a woven wire screen element is employed. 
     Ash laden flue gas enters the particulate separation chamber  5  through the inlet opening  11  and strikes the surface of the screens  14 . The screens  14  collect ash particulate in the flue gas as the gas passes through the chamber  5 . The ash entrained in the flue gas strikes the screens  14  and is deflected downward into an ash hopper  30 . However, ash particulate also remains on the screens  14  and accumulates over time. As the ash deposits accumulate, the efficiency of the particular separator  10  decreases. 
     In order to clean the ash deposits from the surfaces of the screens  14 , rapper assemblies  20 , as shown in  FIGS. 1 and 4 , are mounted to the separation chamber frame  12 . As will be understood by one skilled in the art, a greater or lesser number of rapper assemblies  20 , at varying locations, could alternatively be employed. The rapping of the screens  14  is by direct contact on the screens  14  themselves, rather than on the frame of the screen. Direct rapping on the screens  14  will provide enhanced cleaning of the collection surface. 
     Each rapper assembly  20  generally includes a rapper  22 , a shaft  24 , a shaft mounting plate  26  and a seal support plate  28 . The rapper assembly also includes a control system (schematically shown) which controls the operation of the rapper  22 . The control system can operate the rappers individually or simultaneously in groups and controls the frequency of impact hits and striking force for each rapper  22 . 
     The rapper is positioned on the frame  12  of the separation chamber  5 . The rapper  22  may be a pneumatic or electromechanical hammer of a variety well known in the industry. One such electromechanical hammer is disclosed in U.S. Pat. No. 5,079,459, which is hereby incorporated by reference. The invention may employ a commercially available rapper, such as SR-A1 model, which is available from A.V.C. Associates, Inc. 
     The shaft  24  is preferably comprised of high temperature mechanical tubing with preferably has a 2.5 inch outer diameter and a 0.25 inch wall. The seal support plate  26  operatively connects the rapper  22  to the shaft  24  in axial alignment. The seal support plate  28  is preferably comprised of stainless steel and has a thickness of ⅜ inches. 
     The shaft mounting plate  26  is mounted, preferably via a seal weld, to the particulate separator chamber wall. The mounting plate  26  supports the shaft  24  at the point where the shaft  24  extends into the chamber  5 . The shaft  24  extends through an opening (not shown) of the mounting plate  26 . 
     The rapper assembly preferably also includes a roller support bracket  25  which is mounted to the chamber frame  12 . The roller support bracket  25  includes a roller  27  which supports the horizontal movement of the shaft  24  toward and away from the screen  14 . 
     The screens  14  include openings  17  which receive the shafts  24 . The shafts  24  extend through the openings  17  of the screen  14  across the width of the particulate separation chamber. The shafts  24  are preferably welded to plates  19  (shown in  FIG. 6 ) positioned on both sides of the screens  14  around the openings  17 . 
     The openings  17  in each screen  14  are preferably aligned with the openings  17  of the adjacent screens  14  to facilitate entry and removal of the shafts  24  from screen to screen. The openings  17  are preferably positioned at the center of the screens  14 . 
     As shown in  FIG. 5 , the screens  14  have preferably a rectangular shape and are approximately thirteen feet in length and four feet in width. However, the screens  14  can have any dimensions and shape which are suitable for mounting inside the particulate separation chamber  5 . 
     During operation of the rapper assembly  20 , the rapper  22  transmits shock energy through the seal support plate  28  to the shaft  24 . The axial alignment of the rapper  22  and the shaft  24  increases the efficiency of shock energy to the screens  14 . 
     The shafts  24  vibrate the screens  14  which causes deposited ash particles to fall off from the screens  14  and to pass into the ash funnel  7 . 
     The rapper  22  is preferably a spring assisted electromagnetic device which is oriented in a horizontal plane. The rapper  22  includes a coil (not shown) which is supplied with electrical current for creating a magnetic field. The magnetic field compresses an internal rapper spring thereby storing potential energy. When an electrical signal is suspended, the stored energy of the spring is released and accelerates an internal hammer (not shown). The hammer, in turn, moves the shaft  24  via the seal support plate  28 , thereby imparting a shaking force on the screens  14 . 
     The impact energy is regulated by the rapper controller which can be set to control the amount of electrical energy supply in a signal cycle. The controller can also be programmed to vary the frequency of the rapping. 
     In another embodiment (which is schematically shown in  FIG. 2 ), the proposed invention employs six separate rappers, three mounted on opposite sides of the air separation chamber  5 , to rap the screens  14 . Each individual rapper  22  is attached to a shaft which extends through each of the screens  14 . Three rappers  22  are provided and the shafts  24  of each rapper are spaced vertically along the height of the same screens  14 . The shafts  24  which extend through the same screen are preferably vertically aligned. 
     In an alternative embodiment, the rapper may be connected via a bracket or series of brackets or other connecting means to the plates without having openings for the shafts that extend through the screen. 
     The invention capitalizes upon the ability to vary the force, duration, timing, etc. of the various rapping elements employed on such a planar screen element. If ash deposition is more severe on an upper portion of the screen, for example, the rapping can be adjusted to focus on that area. By having individual control over individual rappers, more efficient and targeted cleaning can be accomplished. The invention can rap an upper region, then a middle region, and then a lower region, in that order if desired, or alternative orders can be used. Any possible permutation of order, timing and/or duration and amount of force can be used to achieve cleaning of the screen elements can be employed. 
     While a specific embodiment of the invention has 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 without departing from such principles.