Abstract:
A stepped, perforated tray is provided for increasing the available gas-slurry contact area in a flue gas desulfurization chamber. The tray redirects the horizontal flow upwardly through the tray for increased surface area and contact time with slurry for removing SO x  or other contaminants from a fossil fuel combustion flue gas before releasing the flue gas to the atmosphere. The tray is adaptable for use in vertical tower systems as well.

Description:
FIELD AND BACKGROUND OF INVENTION 
     The present invention relates generally to the field of industrial combustion processes and in particular to a new and useful apparatus and method for removing contaminants from combustion gases prior to release into the atmosphere. 
     Fossil fuel combustion is used in industrial processes for many different purposes. Coal and natural gas are commonly burned to heat steam in electric power generation plants, for example. Unfortunately, fossil fuel combustion produces several contaminants which have been found to be detrimental to the environment. In particular, sulfur and nitrogen oxide compounds are major components of “acid rain”, which is harmful to plants. 
     In recognition of the harm caused by SOx and NOx compounds, different combustion gas cleaning systems have been developed to remove these components of combustion flue gases prior to release of the flue gases into the atmosphere. 
     Flue gas desulfurization systems are one such flue gas cleaning system. For a general description of the characteristics of flue gas desulfurization systems, the reader is referred to Chapter 35 of  Steam/Its Generation and Use , 40th Edition, The Babcock &amp; Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein. 
     Flue gas desulfurization systems and other liquid-gas contact processes have been designed and constructed using a perforated metal tray to produce and support a liquid-gas mixing or contact zone in which the vertically flowing gas passes up through the perforations as a liquid slurry or solution containing the reagent is falling down through the same perforations. An example of such a system is described in U.S. Pat. No. 4,263,021 for a “Gas-Liquid Contact System” assigned to the Babcock &amp; Wilcox Company, which is hereby incorporated by reference as though fully set forth herein. 
       FIG. 1  herein illustrates the prior art flue gas cleaning system of U.S. Pat. No. 4,263,021. A gas, such a flue gas  40 , is passed upwardly from inlet  55  at velocities of 5-20 feet per second through an upright tower  50  in counter-current contact with liquid, such as liquid slurry  65  which is introduced near the top through one or more spray headers  68  and discharged from the bottom of the tower. One or more horizontally disposed perforated plates, each forming a tray  60 , is positioned intermediate the height of the tower  50 . Each plate is provided with a plurality of upright partitions attached to the plate and arranged to subdivide the upper plate surface into a plurality of generally equal-area open-topped compartments. 
     With a proper coordination of liquid and gas flow rates, plate perforation arrangement and spacing dimensions, the gas and liquid will form gasified liquid masses in the compartments leading to stabilized liquid holdup encouraging both intimate contact and sufficient contact time for adequate chemical interchange between the media for absorption purposes. The cleaned gases  80  continue rising through tower  50  to mist eliminator  70  before exiting through outlet  75 , while contaminants removed from the gases are disposed of with the discharged liquid. 
     A second known type of flue gas desulfurization system is illustrated by  FIG. 2 , in which horizontally flowing flue gas  40  is treated with slurry  65  introduced from headers  67  mounted in the top of the desulfurization chamber  51 . The slurry  65  is essentially sprayed “cross-currently,” i.e. in cross flow, perpendicular to the flow of flue gas  40 . The cleaned gas  80  leaves the chamber  51  after passing through mist eliminator  70  adjacent to outlet  75 . Liquid slurry with contaminants is drained from the bottom or lower portion  53  of chamber  51  in any known manner. 
     These horizontal systems do not use a gas-liquid contact device such as the perforated tray as described above. Horizontal systems like that of  FIG. 2  have been plagued with performance problems and limitations due to poor mixing of the gas and liquid. Stratification occurs where lighter flue gas seeks the top and heavier liquid reagent moves to the bottom of the reaction chamber without good mixing or sufficient contact time. 
     Horizontal flue gas desulfurization systems are sometimes required in retrofit applications due to space constraints. And, in new plants, a horizontal system is sometimes preferred for a variety of reasons including available space or height limitations. 
     Due to the harm caused by flue gas contaminants and the fact that a 100% efficient flue gas desulfurization system has not yet been created, there is always a need for improved cleaning systems which remove a greater fraction of contaminants from flue gases. Further, systems which are more cost efficient to manufacture and more easily retrofit into existing fossil fuel combustion plants are highly desirable. A more effective horizontal flue gas desulfurization system is very desirable due to the lag in effectiveness between horizontal and vertical systems. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to create improved mixing of slurry and flue gas in an area of a horizontal flue gas desulfurization system with limited cross section in the horizontal direction of gas flow. 
     It is another object of the invention to improve gas-slurry contact in a horizontal flue gas desulfurization system and increase the available area for this contact. 
     Yet another object of the invention is to provide a horizontal desulfurization system having stepped slurry sprays introduced via headers at varying elevations and distances in a flue. 
     A further object of the invention is to overcome performance problems associated with horizontal flue gas desulfurization systems. 
     A still further object of the invention is to reduce pressure drop and power consumption in vertical flue gas desulfurization systems. 
     Accordingly, a variable vertical cross-section, stepped absorption tray is provided in a flue gas desulfurization system for creating a zone of increased flue gas and slurry mixing. In one embodiment, the tray includes a combination “Z” support, guide vane and liquid baffle which simplify fabrication and construction of the tray. The combined functions of the “Z” support, acting simultaneously as a structural support, a gas guide vane and a liquid retaining device, helps to minimize cost and improve the ease of installation in new or retrofit applications. 
     This invention thus solves the performance problem, which has plagued these types of horizontal liquid-gas reaction chambers caused by poor liquid-gas mixing due to stratification where the flue gas remains at the top of the absorption chamber and the liquid seeks the bottom. 
     In one embodiment the invention provides a flue gas desulfurization unit comprising a chamber having a bottom, a sidewall and an inlet and an outlet defining a flowpath therethrough. A stepped, perforated tray defining a plurality of open top compartments having perforated floors is positioned within the chamber spanning the flowpath. Means for spraying a slurry into the flue gas are located above the stepped tray. 
     In another embodiment the invention provides a horizontal flue gas desulfurization unit with increased flue gas contact area, comprising a chamber having an inlet, an outlet, a pair of side walls, and a bottom. A stepped, perforated tray, positioned within the chamber, extends between the side walls forming a plurality of open top compartments having perforated floors. The tray is stepped downwardly from the inlet toward the outlet. 
     In yet another embodiment the invention provides a flue gas desulfurization system comprising a chamber having a bottom, at least one side wall, a gas inlet and a gas outlet. A stepped tray, defining a plurality of open top compartments with perforated floors, steps downwardly from adjacent the gas inlet. Means for spraying a slurry against a flow of flue gases flowing upwardly through the perforated floors of the compartments are located above the stepped tray. 
     In a further embodiment the invention provides a flue gas desulfurization system comprising a chamber having one or more side walls, a gas inlet and a gas outlet defining a flowpath therethrough. A stepped tray defines a plurality of open top compartments positioned within and extending across the chamber from the one or more side walls. The tray includes a support having a vertical section and a lower flange. A first perforated plate is connected to the vertical section adjacent the lower flange. A second perforated plate, vertically and horizontally spaced from the first perforated plate, is connected to the vertical section above the lower flange. Stepped means for spraying a slurry against a flow of flue gases flowing upwardly through the first and second perforated plates are located above the stepped trays. 
     In a still further embodiment the invention comprises a tray for use in a gas-liquid contact device, comprising a plurality of vertically and horizontally spaced, horizontally disposed perforated plates. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same: 
         FIG. 1  is a sectional side elevation view of a prior art vertical flue gas desulfurization tower; 
         FIG. 2  is a sectional side elevation view of a prior art horizontal flue gas desulfurization chamber; 
         FIG. 3  is a sectional side elevation view of a horizontal flue gas desulfurization chamber according to the invention; 
         FIG. 4  is a perspective view of a baffle system used to increase vertical cross-sectional area in the chamber of  FIG. 3 ; 
         FIG. 5  is a perspective view of an alternate support and plate for making the tray of  FIG. 4 ; and 
         FIG. 6  is a sectional side elevation view of a vertical flue gas desulfurization tower incorporating the baffle system of FIG.  4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements,  FIG. 3  illustrates a horizontal flue gas desulfurization chamber  51  having a perforated, variable vertical cross-section, stepped absorption tray  100  of the invention for increasing size of the gas-liquid contact region inside the chamber  51 . The tray  100  is arranged stepped downwardly from the entrance  52  of the chamber  51  toward a mist eliminator  70  adjacent the outlet  75 . 
     The tray  100  provides a region for improved mixing of slurry  65  and flue gas  40  within chamber  51 . Chamber  51  comprises an area with an otherwise limited cross-section in the horizontal direction of gas flow. Installing a stepped, perforated tray  100  retains the gas-slurry mixture within a contact region. The flue gas  40  may pass upwardly through the tray  100  into contact with slurry  65  sprayed from slurry spray headers  63 . The liquid slurry  65  in turn passes downwardly through tray  100  after mixing with flue gases  40  and removing a substantial portion of contaminant gases, such as SOx and NOx. Liquid slurry with contaminants is drained from the bottom or lower portion  53  of chamber  51  in any known manner. 
     Further, tray  100  locally redirects the flow of flue gas  40  vertically, thereby increasing the available contact area and slowing the flue gases to improve gas-slurry contact. The tray  100  effectively changes the gas-slurry contact from cross-current (perpendicular paths) to counter-current (opposing directions), which greatly improves the effectiveness of the desulfurization chamber  51 . 
     As will be understood, a horizontal flue gas desulfurization chamber  51  is typically much longer than it is wide or high, so that the cross-sectional area of the gas flow path is much less than the top-down area of the chamber  51 . The stepped tray  100  takes advantage of the length of the chamber  51  to produce a substantially increased surface area for gas-slurry contact by redirecting the gas flow vertically through the perforated plates  120  of tray  100 , as shown in FIG.  4 . 
     Referring again to  FIG. 3 , to take full advantage of the increased gas-slurry contact area provided by the tray  100 , slurry spray headers  63  are also preferably arranged stepped at varying elevations along the length of tray  100 . The stepped spray headers  63  ensure the stepped tray  100  remains fully flooded with slurry  65  and provide counter-current spray in the chamber  51 . 
     After the flue gases  40  pass through tray  100  and sprays of slurry  65 , the cleaned flue gases with some slurry entrained therein passes through mist eliminator  70 . Mist eliminator  70  functions in a known manner to remove entrained liquid slurry droplets and return the slurry to the other pool of slurry being discharged from the chamber  51 . 
     A preferred construction for tray  100  is illustrated by FIG.  4 . As shown, tray  100  is formed by connected perforated plates  120  and “Z” supports  110 , which simultaneously perform the function of supports, guide vanes and liquid baffles. The Z supports  110  significantly simplify fabrication and construction. The combined function of structural support, gas guide vane and liquid retaining device helps to minimize cost and improve the ease of installation. 
     Each Z support  110  has a pair of upper and lower flanges  112 ,  111  connected to the upper and lower ends, respectively, of vertical section  114 . The flanges  111 ,  112  direct the oncoming flow of flue gas  40  from a horizontal flow to a vertical flow, through perforated plates  120 . The leading edges  111   a  of the lower flanges  111  scoop oncoming flue gases  40  and direct the flue gases  40  in conjunction with the adjacent lower portions of vertical sections  114 . Upper flanges  112  similarly guide the flue gases  40  back to a horizontal downstream flow after exiting the perforated plates  120 . 
     The flow disruption created by the flue gas redirection with the Z supports  110  can be adjusted by changing the size of the Z supports  110  to maximize the gas-slurry contact time while preventing the loss of flue gas velocity from having a significant negative impact on the performance of downstream systems. 
     Each perforated plate  120  has a plurality of holes or perforations  125  through the plate  120  sized to permit flue gases to flow through at a minimum velocity, while used slurry drains downwardly. The Z supports  110  and perforated plates  120  extend between side walls  56  of the chamber  51 , so that substantially all the area between the chamber walls is occupied by the tray  100  along its length, spanning the flow path of flue gases  40 . 
     The Z supports are mounted with the corner  113  of their lower flange  111  secured to a front edge of one of the perforated plates  120 , and a second perforated plate  120  connected to about the middle of the vertical section  114 . Thus, the Z supports  110  and perforated plates  120  form a descending staircase of open top compartments with perforated floors defined by the side walls  56 , and at least a portion of the vertical sections  114  of each adjacent Z support  110 . 
       FIG. 5  illustrates an alternative L-shaped support  110   a  for use with tray  100 . The L-shaped support  110   a  has only lower flange  111 , and the upper edge of vertical section  114  is free. This support will not redirect gas flow to one side or the other as well as it leaves the tray  100 . 
     The tray  100  and stepped slurry headers  63  solve the performance problem of horizontal liquid-gas reaction chambers, such as flue gas desulfurization chamber  51 , caused by poor liquid-gas mixing as a result of stratification where the lighter flue gas  40  remains at the top of the absorption chamber  51  and the liquid slurry  65  seeks the bottom. 
     Use of the variable vertical cross section stepped absorption tray  100  effectively minimizes or eliminates gas-slurry stratification in horizontal gas flow liquid-gas contact devices by creating a uniform pressure drop across the flow cross section. Further, the variable vertical cross section stepped absorption tray  100  provides improved liquid gas contact area by creating a counter-current liquid-gas interchange where only a cross-current interchange was present. 
     Use of the variable vertical cross section stepped absorption tray  100  in a horizontal flue gas desulfurization chamber  51  results in increased absorber efficiency and performance due to improved gas-slurry contact without increasing the size of the existing absorber vessel or increasing the liquid to gas ratio. In fact, it may be possible to reduce the liquid to gas ratio due to the greatly improved gas-slurry contact. This is because a uniform head of liquid is created within the compartments defined by the tray  100  and chamber side walls  56  through which all flue gas  40  intended for treatment must pass in order to exit the chamber  51 . The significant effect of this is to produce an increased liquid-gas contact area as well as increased liquid-gas contact time thus increasing the efficiency of the system for a given liquid to gas ratio. 
     The presence of a variable vertical cross section stepped absorption tray  100  may also result in increased mist eliminator  70  efficiency and decreased maintenance from the improved flow distribution to the mist eliminator  70 . 
     The retrofit installation of stepped trays  100  in the many existing horizontal flow scrubbers will allow the owner/operators of these systems to increase SOx removal without increasing operating costs due to the increased efficiency. Cleaner air with no increase in operating costs is the result. 
     Use of the variable vertical cross section stepped absorption tray  100  may be applied to other types of liquid-gas systems using other reagents and gases than traditional wet limestone flue gas desulfurization systems. 
     Without installing tray  100  in existing horizontal systems, the only alternative for improving efficiency of a horizontal gas flow scrubber would be a new flue gas desulfurization chamber  51  of increased size to thereby decrease gas velocity and/or accommodate more slurry sprays thereby increasing liquid to gas ratio. This is essentially a new scrubber and not cost effective for owner operators who have maintained their horizontal gas flow systems and desire the benefit of increased performance efficiency from their existing systems. And, the cross-sectional area of the horizontal flow path cannot be increased as much as by using tray  100 , nor will the benefits of counter-current gas-slurry contact be realized. 
     Materials used to make the tray  100  components include stainless and/or nickel alloy steel for the perforated plates  120  and Z supports  110 . Alternative materials such as plastics or fiberglass can also be used. Packing can be used to create the reaction surface on tray  100  and resistance required. The packing needs to be positioned in such a way as to step across the horizontal distance of the absorber chamber creating the variable vertical cross-section thereby improving liquid-gas contact. 
     The connection between the perforated plates  120  and Z supports  110  can be by bolting or welding when these parts are fabricated from metal. For fiberglass reinforced plastic construction, the attachment may be bolted, glued or fused. 
     The stepped spray headers  63  can be constructed of rubber lined and coated steel piping, various plastics, refractory or stainless steel. The liquid spray need not be limited to slurry  65  but may be used to introduce other reagents such as liquid solutions, dry solids or gaseous products as needed for the process requirements or to improve the cleaning efficiency of the system. 
     And, as illustrated in  FIG. 6 , the tray  100  can be used in vertical flue gas desulfurization systems as well. The stepped tray  100  can be applied to typically cylindrical tower  50  systems that utilize vertical gas flow by installing the tray steps extending from side wall  56  across the inlet  55  for flue gas  40 . In this embodiment, the tray  100  functions as a turning vane for the inlet  55  thereby reducing pressure drop and lowering power consumption. An additional benefit is reduced pumping power consumption due to the lower elevation of sprays from the stepped configuration, since the entire tower  50  can be made shorter as the tray  100  increases contact surface area along a stepped diagonal rather than horizontally or vertically. 
     While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles. For example, perforations may be included only in the horizontal or in both vertical and horizontal directions, depending upon the desired gas distribution effect.