Abstract:
An inert impingement surface that is pliable and which, when set in motion, works to shed itself of particle deposits. The pliable surface may be formed of a plurality of coactive members that co-operate with one another to clean-off the particle deposits from each other&#39;s surfaces. The coactive members may be in the form of chains, rods, cables, wires, granular inert materials such as gravel or metal balls or other impact means. The pliable surface may also be formed of a heat-resistant material which is caused to experience a continuing shape change, thereby casting-off the particle deposits adhering to the surface.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to flue gas desulfurization, and more particularly to an impingement surface for the removal of wetted particles or droplets from a flue gas stream.  
         [0003]     2. Description of the Related Art  
         [0004]     The desulfurization of flue gas, particularly flue gas from power plants, has been the subject of considerable study. Air quality laws, both at the federal and state level, have set increasingly stringent emission standards especially for such known pollutants as sulfur oxides. The sulfur oxides, principally present as sulfur dioxide, are found in the flue gases discharged by coal and oil-fired electric power generating plants, refuse-to-energy plants, and the waste gases from other industrial processes. In addition, sulfur-containing gases, notably sulfur dioxide, may be formed in the partial combustion or gasification of sulfur containing fuels, such as coal or petroleum residuals. The control of air pollution resulting from the discharge of sulfur dioxide into the atmosphere has thus become increasingly urgent.  
         [0005]     Sulfur oxides are produced in significant quantity by the combustion of coal or fuel oil, and a popular choice for utilities burning low sulfur fuels, for smaller industrial processes, and for refuse-to-energy plants for purposes of removing sulfur dioxide from flue gases has been the dry scrubber because of its relatively lower capital costs and its simplicity of operation. In dry scrubbing, sometimes referred to as spray drying or spray absorption, an aqueous alkaline solution or slurry is finely atomized via a mechanical, dual fluid, or rotary type atomizer and sprayed into hot flue gas to remove sulfur oxides and other contaminants. Another technical solution to reduce the emission of sulfur oxides during the production of steam for industrial uses or utility power generation is the circulating fluidized bed (CFB) reactor or combustor. In fluidized bed combustion, combustion takes place in a mixture of particles including the combustible material and limestone or dolomite which is suspended in an upwardly flowing gas stream within a furnace. The process is characterized by high heat transfer rates in the furnace, but with low combustion temperatures. The limestone undergoes a chemical reaction known as calcination within the furnace, forming calcium oxide (CaO), a sorbent which reacts with the sulfur oxides in the gas to form calcium sulfate CaSO 4 . CaSO 4  is then removed from the system as a solid for disposal. In the CFB process, a circulating inventory of solids circulates throughout the furnace and heat transfer portions and since not all of the CaO particles react with the sulfur oxides in the gas, unreacted sorbent is carried along with the flue gas exiting from the CFB reactor.  
         [0006]     Because of equipment space and cost limitations, the size of the dry scrubber may be such that the distance between the fluid atomizers and the absorber chamber outlet does not provide sufficient gas residence or retention time in the chamber to evaporate all of the droplets and the liquid from the wetted particles. This dry scrubber size limitation raises the likelihood of wetted particles and droplets hitting the wall of the absorber chamber and causing deposits to adhere to the internal surface of the chamber and such deposits may accumulate and interfere with the proper operation of the dry scrubber and necessitate a shutdown of the equipment to allow for a clean out of the absorber chamber internal walls. The prior art, as disclosed in U.S. Pat. No. 4,888,158 to Downs, sought to overcome this problem by introducing an inert impingement surface in the absorber chamber designed to remove only the unevaporated droplets and wetted particles from the gas stream while allowing the dry particles to pass to the particulate collection device. A shortcoming of this prior art inert impingement device is its rigid one-piece construction, which has made it difficult to remove the dried particle deposits from its surface, notwithstanding the use of rappers, sootblowers and mechanical scrappers.  
       SUMMARY OF THE INVENTION  
       [0007]     The aforementioned prior problem is overcome, to a large extent, through the practice of the present invention which provides an inert impingement surface that is pliable and which, when set in motion, works to shed itself of dried particle deposits. In its broadest aspect, the present invention is drawn to an apparatus for conducting a gas-liquid reaction on the surface of wetted particles and/or droplets contained in a gas flowing therethrough.  
         [0008]     In accordance with the invention, the pliable surface may be formed of a plurality of coactive members that co-operate with one another to clean-off the dried particle deposits from each other&#39;s surfaces. It has been found that solid objects which bounce-off one another cause a wiping effect and, thus, prevent caking or hardening of dried particle deposits formed on their surfaces. This impact mechanical action provides a self-cleaning arrangement. While the coactive members, which co-operate to form the pliable surface, may be in the form of chains, rods, cables, wires, granular inert materials such as gravel or metal balls, etc., or other impact means, hereinafter the invention will be described specifically with reference to chains and granular inert materials for simplicity. However, it should be understood that the use of such description is for the sake of conciseness and readability and is in no way intended to limit the scope of the invention to the coactive members herein described. The pliable surface may also be formed of a heat resistant material which is caused to experience a continuing shape change, thereby casting-off the dried particle deposits adhering to the surface.  
         [0009]     In one embodiment of the invention, the pliable surface is made up of coactive members comprising a series of metal links or rings connected to or fitted into one another so as to move freely and, thus, form the pliable or flexible surface, for example, a chain. In this embodiment, the pliable surface is preferably formed from a series of such chains suspended side by side from a support structure so as to hang in the absorber chamber. Conventional rappers or other devices are provided so as to cause the chains to move and hit each other and, thus, clean-off the dried particle deposits.  
         [0010]     In another embodiment of the invention, the pliable surface is made up of coactive members comprised of granular inert material which forms a bed supported by a perforated plate. The bed is set in motion by a rotating paddle agitator causing the coactive members to move about and collide or bounce-off each other and, thus, clean-off the dried particle deposits. A conventional mechanical or pneumatic conveyor may recirculate the coactive members to a predetermined level above the bed and cause the recirculated coactive members to cascade through the gas stream and onto the bed. In this embodiment, the bed and the cascading coactive members form the pliable surface.  
         [0011]     In still another embodiment of the invention, the pliable surface is made up of coactive members comprised of granular inert material which forms a bed agitated by a rotating paddle agitator. The bed is set in motion by a hollow drive shaft causing the coactive members to move about and collide or bounce-off each other and, thus, clean-off the dried particle deposits. The drive shaft projects above the bed, and has an opening at the lower portion of the bed. A conventional rotary screw conveyor extends coaxially through the shaft to pick-up the coactive members at the shaft opening and recirculate them to a predetermined level above the bed and cause the recirculated coactive members to cascade through the gas stream onto the bed. In this embodiment, the moving bed and the cascading coactive members form the pliable surface.  
         [0012]     In a further embodiment of the invention, the pliable surface is made up of cascading coactive members comprised of granular inert material moving about and colliding as they cascade through the gas stream. A gaseous medium sweeps the dried particle deposits off the cascaded coactive members, and the latter are then recirculated to the spray drier at a predetermined level above the gas outlet.  
         [0013]     In still a further embodiment of the invention, the pliable surface is made up of a flexible heat resistant material preferably structured as a hollow cylinder in its undeformed state. Biasing rollers engage either the outer surface, or the inner surface, or both surfaces of the hollow cylinder and rotate around its circumference while forcing a continuing shape change to deform it into a shape similar to a frustum of a cone which causes the dried particle deposits to fall-off the surface thereof. A rotating paddle agitator is positioned within a bed of granular inert material, particles of which move about the bed and collide with one another and, thus, clean-off the dried particle deposits. In this embodiment the flexible surface and the moving bed form the pliable surface.  
         [0014]     The common thread, in each of the above embodiments of the invention, is that the surface which is impinged by the liquid droplets and the wetted particles is pliable or flexible and therefore changeable and, thus, facilitates the removal of dried particle deposits from the surface. Preferably, the surface is a dedicated surface which is located at the gas outlet, upstream of any other surface facing a flow of wetted particles and/or droplets.  
         [0015]     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic sectional side view of a spray absorber in accordance with one embodiment which incorporates the present invention;  
         [0017]      FIG. 2  is a schematic sectional side view of a spray absorber in accordance with another embodiment which incorporates the present invention;  
         [0018]      FIG. 3  is a schematic sectional side view of a spray absorber in accordance with still another embodiment which incorporates the present invention;  
         [0019]      FIG. 4  is a schematic sectional side view of a spray absorber in accordance with a further embodiment which incorporates the present invention;  
         [0020]      FIG. 5  is a schematic sectional side view of a spray absorber in accordance with yet another embodiment which incorporates the present invention; and  
         [0021]      FIG. 6  is a schematic cross-sectional view taken along line  6 - 6  of  FIG. 5 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The present invention has application to spray absorbers which are widely used in power, chemical and other industries, and is not limited to the illustrated embodiments. It is to be understood that, in these embodiments, the dimensions, shapes and relative arrangements of the various components are given solely to illustrate the present invention and not to limit the scope thereof.  
         [0023]     Reference will hereinafter be made to the accompanying drawings wherein like reference numerals throughout the various figures denote like elements.  
         [0024]     Referring to the embodiments in  FIGS. 1 through 5  of the drawings, there is shown a vertical co-current down flow dry scrubber module  10  including a shell  14 , a flue gas inlet  16 , a flue gas outlet  18 , and a chamber  20  situated therebetween. Generally, a hot flue gas stream  12  enters the dry scrubber module  10  through the inlet  16  and past one or more fluid atomizers  22 , located at the upper end of the absorber chamber  20 , for spraying a wetting liquid  24  into the flue gas. The wetting liquid  24  may comprise a finely atomized alkali solution or slurry reagent which is dispersed into the flue gas stream  12  to absorb sulfur oxides and other contaminants entrained in the flue gas stream  12 . Alternatively, the wetting liquid  24  may comprise only water which is being sprayed into the flue gas stream  12 , since it is understood that the particulates which may be contained within the flue gas  12  can comprise unreacted sulfur sorbents which are present in sufficient quantity and with sufficient alkalinity which will react with the water and absorb sulfur oxides from the flue gas stream  12 . These particulates may be contained within the flue gas stream  12  due to the fact that the flue gas stream  12  may be the exhaust flue gas from a circulating fluidized bed (CFB) reactor (not shown) which still contains a significant amount of solids particles and unreacted sorbent (e.g., CaO particles). The treated flue gas stream  12  continues to travel downwardly and eventually exits the absorber chamber  20  through the outlet  18 , and at a right angle with respect to the downward direction of flow within the chamber  20 . After exiting through the outlet  18 , the flue gas stream  12  is directed to a particulate collection device (not shown) such as a baghouse or an electrostatic precipitator. It is understood that a baghouse may be generally preferred over an electrostatic precipitator because there can be further sulfur absorption taking place between the filter cake which deposits upon the surfaces of the bag filters and the flue gas stream  12  being passed therethrough.  
         [0025]     Refer now to the embodiment of the invention shown at  FIG. 1  and, in particular, the lower portion of the shell  14  which includes a hopper  34  located at the bottom of the absorber chamber  20 . In this embodiment of the invention, the pliable wetted particle and droplet impingement surface  26  is comprised of coactive members  28 , schematically shown, and preferably intended to be in the form of metal links or rings connected to or fitted into one another to constitute a chain. However, it should be understood that rods, cables, wires, and the like, are suitable substitutes provided they are free to move and, thus, able to form a pliable surface. A series of chains are suspended side by side from a support structure  29 , and conventional rappers or other devices (not shown), are connected to the support structure  29  to cause the chains to move, as schematically shown at arrows  30 , and hit each other and, thus, clean-off the dried particle deposits impinged thereon by the passing flue gas stream  12 . Although the direction of movement  30  is schematically shown in  FIG. 1  as being up-and-down, persons skilled in the art will appreciate that any type or direction of motion of the coactive members  28  which will cause particles adhering thereto to be dislodged, may be employed. The coactive members may be moved towards or away from (relative to) the support structure  29 , or towards or away from (relative to) each other. The degree of motion may be selected to cause the coactive members  28  to strike the support structure  29 , or each other, or both. Some of the coactive members  28  may be held stationary while other coactive members  28  are moved relative thereto, or by a combination of the aforementioned approaches. Similarly, the coactive members  28  may be mounted on eccentric cams to cause them to move in a manner which would shake them and dislodge particles adhering to the members  28 , such as by side-to-side motion and striking adjacent members  28 , or by striking the support structure  29 , or both. If desired, a rotating paddle agitator means as described in connection with several of the following embodiments disclosed in the present specification may be employed, the paddles of the agitator striking the coactive members  28  to dislodge adhering particles therefrom. The dry deposits which are removed from the coactive members  28  forming the pliable surface  26  are collected in the hopper  34  and discharged through the hopper outlet  32  to a conventional rotary screw conveyor  36  to be transported to an alkaline slurry preparation facility or to a landfill. Preferably the dedicated surface is located at the gas outlet, upstream of any other surface facing a flow of wetted particles and/or droplets. This feature can be accomplished by suitable design and/or location of the one or more fluid atomizers  22 , located at the upper end of the absorber chamber  20 , for spraying the wetting liquid  24  into the flue gas.  
         [0026]     Refer now to the embodiment shown in  FIG. 2  and, in particular, the components that are associated with the pliable wetted particle and droplet impingement surface  26 . In this embodiment, surface  26  is made up of coactive members  27  that are comprised of granular inert material and forming a bed  31  supported by a grid  33 , the latter having openings which are sized to be smaller than the coactive members  27  and, thus, preventing them from passing therethrough. The bed  31  is set in motion by a rotating paddle agitator  35  which causes the coactive members  27 , i.e., the granular inert material, to move about and collide or bounce-off each other, thereby wiping or cleaning-off the dried particle deposits resulting from the wetted particles and droplets impinged thereon by the passing flue gas stream  12 . A drive shaft  37  is secured to the paddle agitator  35  to impart rotation thereto. The dried particle deposits which have been cast-off or removed from the coactive members  27  are carried-off by the flue gas stream  12  as it passes through the bed  31  and are then conveyed through the gas outlet  18  to a baghouse or electrostatic precipitator (not shown). A clean-out pipe  39  is provided at the bottom of chamber  20  for the removal of dried particle deposits and other particles which may have precipitated out of the gas stream  12  exiting the chamber  20 . This embodiment of the invention includes a system  40  for recirculating the coactive members  27  which have shed the dried particle deposits. The system  40  includes a conduit  41  which delivers the coactive members  27  to a conventional rotary screw conveyor  36  which recirculates or transports the coactive members  27  to a predetermined level above the rotating bed  31 , and discharges them through a conduit  43  into the chamber  20  thereby causing the coactive members  27  to cascade through the flue gas stream  12  and onto the agitated bed  31  while being impinged by wetted particles and droplets from the flue gas stream  12 . In accordance with this embodiment of the invention, the coactive members  27  moving about the agitated bed  31  and cascading through the flue gas stream  12  form the pliable surface  26 .  
         [0027]     Refer now to the embodiment covered by  FIG. 3  and, in particular, the components that are associated with the pliable wetted particle and droplet impingement surface  26  which, in this embodiment, is made up of coactive members  27  that are comprised of granular inert material and forming a bed  31  which is agitated by a rotating paddle agitator  42 . A drive shaft  44  is secured to the agitator  42  to impart rotation thereto. A portion of the drive shaft  44  is hollow and projects coaxially to a predetermined level through and above the agitator  42  and the bed  31 , and is formed with an opening  46  located at the elevation of the rotating paddle agitator  42  near the bottom of the bed  31 . The bed  31  is set in motion by the rotating paddle agitator  42  which causes the granular inert material that makes up the coactive members  27  to move about and collide or bounce-off each other, thereby wiping-off the dried particle deposits resulting from the wetted particles and droplets impinged thereon by the passing flue gas stream  12 . The dried particle deposits which have been cast-off or removed from the coactive members  27  are carried-off by the flue gas stream  12  as it passes through and across the moving bed  31 , and are then conveyed through the gas outlet  18  to a baghouse or electrostatic precipitator (not shown). A clean-out pipe  45  is provided underneath the bed  31  for the removal of dried particle deposits and other particles which may have precipitated out of the flue gas stream  12  exiting the chamber  20 , or for removing, when necessary, coactive members  27  from the bed  31 . A supply pipe  47  is provided above the bed  31  for the addition, when necessary, of coactive members  27  to the bed  31 . This embodiment includes a system  48  for recirculating the coactive members  27  which have shed the dried particle deposits. The system  48  includes a conventional rotary screw conveyor  36  which extends up through the hollow drive shaft  44 . The conveyor  36  picks-up coactive members  27  from the bed  31  through a shaft opening  46  and recirculates or transports the coactive members  27  to the top of the drive shaft  44  where they are discharged and caused to cascade through the flue gas stream  12  onto the moving bed  31  while being impinged by wetted particles and droplets from the flue gas stream  12 . In accordance with this embodiment of the invention, the coactive members  27  moving about the agitated bed  31  and cascading through the flue gas stream  12  form the pliable surface  26 .  
         [0028]     Refer now to the embodiment covered by  FIG. 4  and, in particular, the components that are associated with the pliable wetted particle and droplet impingement surface  26  which, in this embodiment, is made up of coactive members  27  that are comprised of granular inert material. This embodiment of the invention includes a system  50  for recirculating the coactive members  27  which have shed the dried particle deposits. The system  50  includes a chute  52  located above the gas outlet  18  for discharging the coactive members  27  and, thus, causing them to cascade through the flue gas stream  12  and into the hopper  34  while being impinged by wetted particles and droplets from the flue gas stream  12 . The cascaded coactive members  27  are discharged from the hopper  34  through a conduit  54  onto a conventional rotary screw conveyor  36 , and the latter discharges the coactive members  27  into a conduit  56  which intersects a conduit  58 . A gaseous medium  57  is caused to flow through the conduit  58 , either by suction resulting from the negative pressure in the gas outlet duct  60  or by an external pressure source (not shown). The gaseous medium  57  flows across the intersecting coactive members  27  and sweeps-off the dried particle deposits which are then discharged into the flue gas stream  12  for collection in the baghouse or the electrostatic precipitator (not shown). The swept coactive members  27  are discharged into a hopper  62  to be picked-up by pneumatic conveyor  64  and recycled to the chute  52 . In accordance with this embodiment of the invention, the coactive members cascading through the flue gas stream  12  and intersecting the gaseous medium  57  form the pliable surface  26 .  
         [0029]     Turning now to the embodiment covered by  FIG. 5  and, in particular, the components that are associated with the pliable wetted particle and droplet impingement surface  26  which, in this embodiment, includes a heat resistant material preferably structured as a flexible or pliable hollow cylinder  66 , in its undeformed state, and located in the chamber  20 . A hollow drive shaft  70  projects through the chamber  20 , and has an atomizer in the form of pipe  22  extending through it and terminating in a nozzle  25  for spraying a finely atomized alkali solution, slurry reagent, or simply water  24  into the flue gas stream  12  to absorb sulfur oxides and other contaminants entrained in the flue gas stream  12 . As described earlier, when water  24  is sprayed into the flue gas stream  12 , solids or particulates which may be contained within the flue gas  12  comprising unreacted sulfur sorbents having sufficient alkalinity will react with the water and absorb sulfur oxides. These particulates may be contained within the flue gas  12  due to the fact that the flue gas  12  may be the exhaust flue gas from a circulating fluidized bed (CFB) reactor (not shown) which still contains a significant amount of solids particles and unreacted sorbent (e.g., limestone particles). The drive shaft  70  is provided with at least a pair of diametrically opposed radial arms  76 , each of which supports a biasing roller  78  engaging the pliable hollow cylinder  66 . The length of each of the radial arms  76  is sized to cause the rollers  78  to bias or force the engaged section of the flexible hollow cylinder  66  inwardly into a frustum shape. The rotating drive shaft  70  causes the biasing rollers  78  to rotatably move about the circumference of the pliable hollow cylinder  66  thereby forcing it to experience a continuing shape change and, thus, causing the dried particle deposits to fall-off the surface of the frustum  66 . Although  FIG. 5  illustrates externally engaging biasing rollers  78 , and the flexible structure  66  in its deformed state, it is understood that rollers  78  may engage either the outer surface, or the inner surface, or both surfaces of the hollow cylinder  66  rotate around its circumference while forcing a continuing shape change to deform it into a shape similar to a frustum of a cone which causes the dried particle deposits to fall-off the surface thereof. The frustum  66  is supported from the shell  14  by link structures  80  which are designed to support and locate the cylinder  66  during its continuing shape change into a frustum  66  caused by the rollers  78 . A bed  31  of coactive members  27  is located below the frustum  66  and may again be agitated by a rotating paddle agitator  42  which is secured to a drive shaft  68  to impart rotation thereto. The bed  31  is set in motion or agitated by the rotating paddle agitator  42  which causes the granular inert material that make up the coactive members  27  to move about and collide or bounce-off each other, thereby wiping-off the dried particle deposits resulting from the wetted particles and droplets impinged thereon by the passing flue gas stream  12 . The dried particle deposits, which have been cast off by the frustum  66  and by the coactive members  27 , are carried-off by the flue gas stream  12  as it passes through the frustum  66  and through and across the moving bed  31 , and are then conveyed through the gas outlet  18  to a baghouse or electrostatic precipitator (not shown). A clean-out pipe  45  is provided underneath the bed  31  for the removal of dried particle deposits and other particles which may have precipitated out of the gas stream  12  exiting the chamber  20 , or for removing, when necessary, coactive members  27  from the bed  31 . A supply pipe  47  is provided above the bed  31  for the addition, when necessary, of coactive members  27  to the bed  31 . In accordance with this embodiment of the invention, the frustum  66  and the coactive members  27  moving about the agitated bed  31  form the pliable surface  26 .  
         [0030]     Referring now to  FIG. 6 , there is shown the shell  14  enclosing the chamber  20  which houses the hollow pliable frustum  66 . The drive shaft  70  includes the diametrically opposed radial arms  76  which support the biasing rollers  78 . The pliable hollow frustum  66  is forced to continuously change shape to a generally elliptical cross-section as the biasing rollers  78  are rotatably driven about the circumference of the frustum  66  by the drive shaft  70 .  
         [0031]     Although the present invention has been described above with reference to particular means, materials and embodiments, it is to be understood that this invention may be varied in many ways without departing from the spirit and scope thereof, and therefore is not limited to these disclosed particulars but extends instead to all equivalents within the scope of the following claims.