Abstract:
Apparatus for separating ash particles from a flue gas. The apparatus includes a screen that has a plurality of semi-elliptical cylinder surfaces. The semi-elliptical cylinder surfaces having holes through which said flue gas flows and through which the ash particles will not pass. The screen has a single layer for performing the separation in a manner such that the ash particles fall away from the screen and collect outside of the screen. A method of reducing velocity of a flue gas passing through screening apparatus for separating flue gas from ash particles. The method includes replacing a first screen of the screening apparatus with a second screen that has a plurality of semi-elliptical cylinder surfaces.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a Divisional of U.S. patent application Ser. No. 13/633,717, entitled Apparatus and Methods for Large Particle Ash Separation From Flue Gas Using Screens Having Semi-Elliptical Cylinder Surfaces, filed Oct. 2, 2012, the disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure is related to the control of particulate emissions from industrial plants such as coal fired plants. More specifically, the present disclosure is related to the separation of large particle ash from flue gas utilizing screens. 
       BACKGROUND 
       [0003]    Coal is a primary source of energy today and is commonly used as fuel to produce electricity. A byproduct of producing electricity in a coal combustion process is nitrous oxide (NOx), which is emitted with a flue gas from coal burning electrical generating plants. This nitrous oxide is considered a pollutant to the atmosphere. Catalytic reactors are used to address this type of pollution by reducing the nitrous oxide concentration in the flue gas. Ash is another byproduct of burning coal and typically comprises silicon dioxide, calcium oxide, carbon and many other constituents depending on the makeup of the coal being burned. The combustion ash particles are usually small (up to 300 micro meters in diameter) and usually suspended in the flue gas. However, the combustion ash particles can form large particle ash (LPA), which may have a diameter exceeding 1 centimeter. LPA formation can be traced to combustion conditions in the boiler and clay like fly ash deposits on superheater tubes and backpass. The catalytic reactors are equipped with a plate or honeycomb-type catalyst and may have a pitch or opening ranging up to 8 millimeters. LPA particles are larger than the catalyst opening and therefore clogs up the catalytic reactor. As such, methods for separating ash from flue gas have been developed. 
         [0004]    For example, U.S. Pat. No. 7,531,143, entitled “Arrangement for separating coarse ash out of a flue gas stream,” discloses screens with pleated arrangements for separating ash particles from flue gas. In practice, these screens experience blockage in certain areas of the pleated arrangement, which creates high velocity zones that cause damage to the screens. As a result, these pleated screens have to be replaced frequently or perform inefficiently and increase pressure drop in the system. It should also be noted that channels through which flue gas flows are large and, at least for this reason, screens used in these channels to separate ash from flue gas are also large and can be relatively costly. 
         [0005]    U.S. Pat. No. 7,556,674, entitled “Method and device for the separation of dust particles,” discloses a system involving a baffle arrangement for deflecting ash particles from the flue gas towards hoppers, which collect the ash particles. This system requires a long duct to settle out the ash particles. The length of the duct makes this system relatively expensive. 
         [0006]    The pleated screen design and baffle arrangement utilizing gravimetric forces do not economically remove large particle ash. Another problem in the art is that the ash particles in the flue gas erode structural duct members and separation screens. High flue gas velocities combined with hard ash particles will lead to significant metal wastage of this equipment. In summary, existing systems and methods for screening ash particles from flue gas are associated with high operating costs and high capital expenditure. 
       BRIEF SUMMARY 
       [0007]    The current disclosure is directed to apparatus and methods for separating ash particles from a flue gas using a screen having a plurality of semi-elliptical cylinder surfaces. According to embodiments, the semi-elliptical cylinder surfaces have holes through which the flue gas flows and through which the ash particles will not pass. The semi-elliptical cylinder shape ensures a uniform velocity profile at the screen surface. Further, the semi-elliptical cylinder shaped screen arrangement allows for the strategic exposure of the internal hole walls of the screen to the incoming ash and flue gas. Depending on the concave or convex configuration of the screen having semi-elliptical cylinder surfaces, more or less wall material is exposed to the ash particles. Furthermore, the semi-elliptical cylinder shaped screen arrangement increases the surface area of the screen and reduces flue gas velocity at the screen surface and overall pressure drop over the screen. Further yet, in embodiments, the semi-elliptical screen arrangement assures that a coated-surface of the screen is maximized in the concave position of the screen thus extending the utilization life of the screen material. 
         [0008]    Embodiments of the disclosure include methods of reducing the velocity of a flue gas passing through screening apparatus used for separating flue gas from ash particles. The methods may include replacing a first screen of the screening apparatus with a second screen comprising a plurality of semi-elliptical cylinder surfaces. The semi-elliptical cylinder surfaces have holes through which the flue gas flows and through which the ash particles will not pass. 
         [0009]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0011]      FIGS. 1A and 1B  show a screen having a plurality of semi-elliptical cylinder surfaces according to embodiments of the disclosure; 
           [0012]      FIGS. 2A and 2B  show a screen having a plurality of semi-elliptical cylinder surfaces according to embodiments of the disclosure; 
           [0013]      FIGS. 3A and 3B  show a screen having a plurality of semi-elliptical cylinder surfaces according to embodiments of the disclosure; 
           [0014]      FIG. 4  shows a system for separating ash particles from flue gas according to embodiments of the disclosure; 
           [0015]      FIG. 5A  shows a prior art system for separating ash particles from flue gas; 
           [0016]      FIG. 5B  shows a prior art flat screen; 
           [0017]      FIGS. 6A and 6B  show flat screens; 
           [0018]      FIGS. 7A to 7C  show a semi-elliptical cylinder surface according to embodiments of the disclosure, and 
           [0019]      FIGS. 8A to 8C  show screens according to embodiments of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 5A  shows prior art system  50  for separating large ash particles from flue gas. Ash particles, as discussed herein are large and may be about 1-2 centimeters in diameter. System  50  represents a typical coal fired power plant boiler arrangement. At boiler  500 , coal is mixed with air (from preheater  505 ) and burned. The burning coal causes an increase in temperature in boiler  500  such that water injected into boiler  500  is vaporized to steam. As mentioned above, the burning coal produces ash particles  105 , which flows with hot flue gas  104  through duct  501 . Duct  501  leads to screen  506 . Screen  506  has holes having a diameter such that flue gas is allowed to pass through screen  506 . However, at least some ash particles are too big to pass through the holes of screen  506 . Because these ash particles are too big to pass through the holes of screen  506 , they accumulate in hopper  502 . Flue gas passes through screen  506  and enters duct  503 . Duct  503  channels flue gas  104  to Selective Catalytic Reduction (SCR) Catalyst  504 , which removes nitrous oxide from flue gas  104 . The flue gas  104  leaving SCR Catalyst  504  may then be discharged into the atmosphere or cleaned further before discharge into the atmosphere. 
         [0021]    Screen  506  is a pleated screen as known in the art and described for example, in U.S. Pat. No. 7,531,143. After screen  506  has been in operation for some time, ash particulate matter lodges in section  507   a  of pleat  507  and blocks that section. This blockage may cause the flue gas to flow through triangular pleat  507  at a non-uniform velocity. This non-uniform velocity can cause deterioration of screen material and rupturing of the screen. When this happens, particulate matter that should be screened passes through screen  506 . 
         [0022]    Screens that are essentially flat panels, such as flat screen  508  shown in  FIG. 5B , do not have the problem described above with respect to pleated screens. However, flat panel screens, such as flat screen  508 , causes a high pressure drop in the order of 0.5 to 1 inch water column, as flue gas passes through it. This high pressure drop is consistent with the high velocity at which flue gas flows through flat screen  508 . 
         [0023]      FIGS. 1A-3B  show screens having a plurality of semi-elliptical cylinder surfaces according to embodiments of the disclosure.  FIG. 1A  shows screen  10  having semi-elliptical cylinder surfaces  100 - 1  to  100 - 3 . In embodiments, screen  10  is made from a single layer of material such as metal, plastic, composites and combinations thereof. Semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  have holes  101  that allow flue gas  104  to flow through screen  10  but does not allow ash particles  105  to flow through screen  10 . That is, ash particles  105  have a cross sectional area or a diameter that is greater than the cross sectional area or diameter of holes  101 , respectively. In embodiments, holes  101  have a cross sectional area of about 1 mm 2  -200 mm 2 . Whatever the size of holes  101 , ash particles  105  are larger than holes  101 . It should be noted that, in practice, some ash particles are small enough to pass through, and do pass through, holes  101 . 
         [0024]    The flow of flue gas/ash particles mixture  104 / 105  to semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  and the screening of ash particles  105  do not cause a buildup of ash particles, as occurs with respect to pleat  507  of screen  506 . For instance, screen  10  does not have an “apex shaped portion”—section  507   a  that traps large ash particles. Furthermore, when ash particles  105  hit screen  10 , screen  10 &#39;s elliptical shape cause ash particles  105  to fall away from screen  10  under the influence of gravitational forces. In other words, screens as disclosed herein are designed to block the flow of large particle ash in a manner such that the ash particles collect in or on other equipment apart from the screens. This is unlike filters, which are designed to trap particulates within the filter itself. Referring again to  FIG. 1A , ash particles  105  are collected away from screen  10  in, for example, a hopper. Because screen  10  is not designed to have a buildup of ash particles  105  on it, the velocity of flue gas across semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  (and screen  10  as a whole) is uniform. 
         [0025]    In addition to not being susceptible to blockages, the shape of semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  reduce the velocity of flue gas  104  by increasing the surface area of the screen as compared with the surface area of flat screen  508 . The pressure drop may be calculated from the following formula: 
         [0000]      ΔP=fv 2  
       where f=friction factor and v=the velocity of the flue gas
 
In embodiments of the disclosure, screen  10  may also include screen side sections  102 . Sections  102  may also have holes  101  for separating ash particles  105  from flue gas  104 . In embodiments, screen side sections  102  may be a solid plate without holes.
       
 
         [0027]    As can be seen from  FIG. 1B , semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  have foci points  103 - 1  to  103 - 3  respectively. The foci points are the points representing the focal line, based on the elliptical shape, at which light rays would focus (or substantially focus) when the semi-elliptical surfaces are exposed to light if semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  are reflective. It should be noted, however, that this disclosure does not require reflective surfaces, which is mentioned here only to explain what is meant by focal point in the context of elliptical shapes. Semi-elliptical cylinder surfaces  100 - 1  to  100 - 3  may be any type of ellipse (e.g. a circle or a parabolic ellipse). The type of ellipse will determine the exact location of foci points  103 - 1  to  103 - 3 . Notably, in the embodiment shown in  FIGS. 1A and 1B , flow direction F of flue gas/ash particular mixture  104 / 105  is from the side of screen  10  on which foci points  103 - 1  to  103 - 3  lie. In other words,  FIGS. 1A and 1B  show a concave configuration. 
         [0028]    Screen  20  have the features of screen  10 , except that, as shown in  FIGS. 2A and 2B , screen  20  has semi-elliptical cylinder surfaces  200 - 1  to  200 - 3  with a convex configuration because foci points  203 - 1  to  203 - 3  are on the opposite side of flow F (i.e. flow of flue gas/ash particle mixture  104 / 105 ). The semi-elliptical convex shaped surfaces of screen  20  are also not prone to blockages (thereby facilitating uniform velocity distribution of flue gas  104 ) and has a higher surface area than flat screen  508  (assuming screen  508  has the same perimeter as screen  20 ). The higher surface area of screen  20  reduces flue gas velocity as compared with flat screen  508 . The perimeter of the screens illustrated herein is 2h+2w, where h is the height and w is the width of the screens as illustrated in  FIGS. 1A ,  2 A and  5 B. For purposes of discussion and comparison, all the screens described herein are assumed to have the same perimeter. In embodiments of the disclosure, screen  20  may also include screen side sections  202 . Sections  202  may also have holes  201  for separating ash particles  105  from flue gas  104 . In embodiments, screen side sections  202  may be a solid plate without holes. It should be noted that any number of sides  202  may have holes or may be a solid plate without holes. In embodiments, holes  201  have a cross sectional area of about 1 mm 2  -200 mm 2 . 
         [0029]    Screen  30 , shown in  FIGS. 3A and 3B , is a combination of the features of screen  10  ( FIGS. 1A to 1B ) with the features of screen  20  ( FIGS. 2A and 2B ).  FIGS. 3A and 3B  show screen  30  having semi-elliptical cylinder surfaces  300 - 1  to  300 - 4 . Screen  30  has both concave and convex semi-elliptical cylinder surfaces. As shown in  FIG. 3B , semi-elliptical cylinder surfaces  300 - 1  to  300 - 4  have foci points  303 - 1  to  303 - 4  respectively, which are on different sides of flow F (i.e. flue gas/ash particle mixture  104 / 105 ). Semi-elliptical cylinder surfaces  300 - 1  to  300 - 4  have holes  301  that allow flue gas  104  to flow through screen  30  but does not allow ash particles  105  to flow through screen  30 . That is, ash particles  105  have a cross sectional area or a diameter that is greater than the cross sectional area or diameter of holes  301 , respectively. 
         [0030]    Again, screens with semi-elliptical surfaces in concave and convex orientation are not prone to blockages and facilitate uniform velocity distribution of flue gas  104 . Further, the surface area of screen  30  is relatively larger than the surface area of flat screen  508 , of screen  10  and of screen  20 , which all have the same perimeter. This larger surface area of screen  30  increases flue gas velocity as compared to flat screen  508 , screen  10  and screen  20 . In embodiments of the disclosure, screen  30  may also include screen side sections  302 . Sections  302  may also have holes  301  for separating ash particles  105  from flue gas  104 . In embodiments, screen side sections  302  may be a solid plate without holes. In embodiments, holes  301  have a cross sectional area of about 1 mm 2  -200 mm 2 . 
         [0031]      FIG. 4  shows a system for separating ash particles from flue gas according to embodiments of the disclosure. System  40  shows a coal fired power plant boiler arrangement. Boiler  400  (using air from preheater  405 ) burns coal to produce steam as described above with respect to boiler  500 . The ash produced by burning the coal flows with hot flue gas (as mixture  104 / 105 ) through duct section  401 . Duct section  401  leads to screen  10 . Screen  10  is located across the lumen of duct section  401  such that flue gas  104  has to pass through screen  10  to enter duct section  403 . In other words, screen  10  extends from wall to wall of the duct ( 401  and  403 ) such that there is screening across all of the lumen of the duct. 
         [0032]    As described above, screen  10  has semi-elliptical cylinder surfaces  100 - 1  to  100 - 3 . Holes  101  of screen  10  have a diameter or cross sectional area such that flue gas  104  is allowed to pass through screen  10  but ash particles  105  are too big to pass through holes  101  of screen  10 . Because ash particles  105  are too big to pass through holes  101 , ash particles  105  fall away from screen  10  and accumulate in hopper  402 . At the same time, flue gas  104  passes through screen  10  into duct section  403 , which channels flue gas  104  to SCR Catalyst  404  (a destination equipment). SCR Catalyst  404  removes nitrous oxide from the flue gas. Flue gas  104  is then discharged into the atmosphere or cleaned further before discharge into the atmosphere. In embodiments, screens  20  and  30  can be used in system  40  instead of screen  10  or in addition to screen  10 . Any combination of screens  10 ,  20  or  30  may be used in embodiments of the disclosure. Furthermore, screens  10 ,  20  and  30  may be used in a system that includes other types of separation equipment, such as baffle arrangements, deflector plates, other types of screens and the like. 
         [0033]    According to embodiments of the disclosure, the pressure drop in large ash particle separator systems may be reduced by using the screen designs disclosed herein. For instance, flue gas velocity may be reduced by replacing a flat screen or a pleated screen, in the separator system (such as the system shown in  FIG. 5 ), with a screen that includes a plurality of semi-elliptical cylinder surfaces, such as screens  10 ,  20 ,  30  or combinations thereof. In embodiments, this change can produce a reduction in flue gas velocity of about up to 20 percent and a reduction of pressure drop of about 40 percent. In embodiments, this change can produce a reduction in flue gas velocity of about up to 40 percent and a reduction of pressure drop of about 60 percent. In embodiments, the screens as disclosed herein may be used to replace separation systems such as baffle based systems, deflector based systems, prior art screen systems and the like. 
         [0034]      FIGS. 6A and 6B  show flat screens  600 . Flat screen  600  includes hexagonal shaped holes  602 , formed by material M. Material M is typically a metal such as iron, aluminum, steel and the like. However, material M could also be plastic, ceramic composites and the like. When ash particles hit material M of screen  600 , they erode material M. In order for material to withstand the effect of processes (such as abrasion) brought about by ash particles hitting material M and the flow of flue gas  104  and particles (that are small enough) through holes  602 , material M may be specially formulated or coated with other substances so that it is resistant to these impacts. Specially formulating or coating material M extends the life of screen  600 . Material M has top section  603  and internal section  604 . Coating top sections  603  can be done easily using a spraying device because these sections are fully exposed. Internal section  604 , however, has to be coated by the use of a spray device at an angle. For example, spray device  605 , as shown in  FIG. 6B , is positioned in a manner such that when material M is sprayed with appropriate coating, the coating contacts section  604 . 
         [0035]      FIGS. 7A to 7C  show screen  700  according to embodiments of the disclosure. Screen  700 , may also be made of material M and may have a similar hexagonal structure as screen  600 . However, in screen  700 , these hexagonal structures are in an elliptical cylindrical surface. When ash particles hit material M of screen  600 , they erode material M. To reduce this erosion, high efficiency ash removal systems must ensure uniform flow conditions as well as use high performance erosion resistant surface treatments. Like screen  600 , material M of screen  700  may be specially formulated or coated with other substances so that it is resistant to these impacts. Specially formulating or coating material M extends the life of screen  700 . The coating material may include ceramic metal composite. 
         [0036]      FIG. 7C  shows screen  700  and the views that  FIGS. 7A and 7B  represent.  FIG. 7B  is a view from the side where focal point  705  is located while  FIG. 7A  is a view from the opposite side. Screen  700  has holes  702 , which are similar to holes  602  shown in  FIGS. 6A and 6B . In screen  700  ( FIG. 7A ), however, internal sections  704  are more exposed than internal sections  604  because the bending of screen  700  into the semi-elliptical cylinder shape pushes internal surface  704  outwards. In this way, screen  700  presents a configuration in which it is much easier to coat internal sections  704  with a selected coating material than coating sections  604 . 
         [0037]      FIG. 7B  shows screen  700  from the side of focal point  705 . That is, internal sections  704  are less exposed than internal sections  604 . Thus, if screen  700  is used in the concave configuration (i.e. flow of flue gas is from the side on which focal point  705  is located) less of internal sections  704  are exposed as compared to internal sections  604 . Thus, in addition to making it easier to coat internal surfaces, configuring screens to have semi-elliptical cylinder surfaces can decrease the surface area that will be exposed to the effects of abrasion from ash particles hitting screen  700 . Consequently, in the concave configuration shown in  FIG. 7B , it might not be necessary to provide any or as much coating of internal sections  704  as compared to the internal coating needed for internal sections  604 . 
         [0038]    Therefore, for different applications of ash separation, a determination may be made as to which of the concave or convex semi-elliptical cylinder designs is more efficient. For example, a determination may be made whether the reduction in cost due to the ease of coating internal sections  704  outweighs the increased exposure of internal section  704  if screen  700  is used in a convex configuration. In sum, screens with semi-elliptical cylinder surfaces provide flexibility in designing screen separating systems. 
         [0039]    The screens disclosed herein offer plants (that separate ash particles from flue gas) much more versatility in designing flue gas/ash particle separation systems as compared to traditional screens (e.g. flat screens) and other separation mechanisms. For instance, by changing a flat screen of a particular perimeter to a screen with semi-elliptical cylinder surfaces and the same perimeter as the flat screen, one can change the screen surface area exposed to the flue gas/ash particle mixture. 
         [0040]    Screens having both concave and convex semi-elliptical cylinder surfaces provide a further benefit in the art. Specifically, in order to achieve a particular surface area of screen, less depth is required for screens with both concave and convex semi-elliptical cylinder surfaces. This feature is illustrated by comparing  FIGS. 8A and 8B  with  FIG. 8C .  FIGS. 8A and 8B  show concave and convex orientation screens with depth distance “d”.  FIG. 8C  shows a screen with both concave and convex orientation with depth “½d.” Thus,  FIG. 8C  provides the same surface area as  FIGS. 8A and 8B  with half the depth. 
         [0041]    In sum, embodiments of the disclosure involves screens that are longer lasting and operates at a lower pressure drop, at lower velocity for the flue gas and with more uniform velocity distribution of the flue gas. Further, embodiments of the screens disclosed offers more versatility as compared to traditional screens. 
         [0042]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.