Abstract:
A method of reducing the pressure drop in a downflow/upflow wet flue gas desulfurization (WFGD) system and of improving overall sulfur dioxide collection efficiency by converting the downflow/upflow WFGD system to an upflow single-loop WFGD system. The method includes the replacing of the downflow quencher and related duct work with a bypass for connecting the incoming flue gas duct with the upflow absorber, and the adding of a quenching zone in the absorber comprised of spray headers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates, in general, to wet flue gas desulfurization (WFGD) systems and, in particular, to a new and useful method of reducing the pressure drop in a downflow/upflow WFGD system and improving its collection efficiency by converting it to an upflow single-loop WFGD system.  
           [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. Because coal and oil-fired electrical power generating plants can discharge large quantities of sulfur oxides as combustion by-products, much effort has focused on the desulfurization of flue gas to reduce power plant sulfur dioxide emissions to permissible levels.  
           [0005]    Thus, sulfur oxides, principally present as sulfur dioxide, are found in the flue gases discharged by coal and oil-fired and other fossil fuel-fired electrical 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.  
           [0006]    The most common flue gas desulfurization process used with coal and oil-fired electrical generating power plants is known as “wet scrubbing”. In this process the sulfur dioxide-containing flue gas is scrubbed with an aqueous alkaline solution or slurry reagent comprised of lime, limestone, soda ash, or other chemicals including sodium, magnesium and calcium compounds and may include any number of additives to enhance removal, control chemistry, and reduce chemical scale.  
           [0007]    The technology for wet scrubbing provides gas-liquid contact in a number of differently configured systems. One of the more prominent of these systems is comprised of a downflow quencher and an upflow absorber. The hot flue gas to be treated enters the quencher which is equipped with a venturi scrubber or spray headers connected to a slurry or water source to produce droplets that promote rapid cooling of the hot flue gas as it flows downwardly through the quencher. After leaving the quencher, the cooled flue gas discharges into a lateral passageway and flows therethrough and then upwardly through the absorber where it is scrubbed with an alkaline slurry reagent where the gas flow is countercurrent to and in intimate contact with the slurry reagent. The slurry reagent is introduced into the absorber through spray header nozzles and flows over packing or trays. Mist eliminators are included near the absorber outlet to remove additional moisture from the flue gas.  
           [0008]    While the downflow/upflow WFGD system generally provides the sulfur dioxide removal effect, it experiences a pressure loss higher than that of a contemporary single-loop WFGD system of the same capacity. It, then, follows that the downflow/upflow WFGD system requires more fan power and more pump power than the single-loop WFGD system. This, in turn, increases the operating and maintenance costs of a downflow/upflow WFGD system when compared to a single-loop WFGD system of the same capacity.  
           [0009]    In other words, the present invention makes it possible to decrease the flow resistance of the flue gas and thereby reduce the operating and maintenance costs.  
           [0010]    As noted, the trend in pollution control has been towards increased stringency, such that many facilities face the need to upgrade or retrofit their existing pollution control equipment to achieve better performance. In addition owners/operators are often interested in upgrading or retrofitting existing pollution control equipment to realize the benefit of lower operational and maintenance costs from improved efficiency. In many situations, the retrofitting or upgrading of an air pollution control system is difficult due to space and/or power consumption considerations. A benefit of the present invention is that it addresses both of these conditions by conforming the retrofit to the existing space and by lowering fan power and pump power requirements through a decrease in pressure loss across the pollution control system, and improved effectiveness in the removal of sulfur dioxide from the flue gases. The present invention can provide pressure drop reductions across the system of about 5 inches water gage.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides a method of reducing the pressure drop in a downflow/upflow WFGD system by converting it to an upflow single-loop WFGD system. The downflow/upflow system includes a downflow quencher and an upflow absorber and a lateral flow passageway therebetween. The downflow quencher is comprised of a venturi scrubbing device mounted in the duct work used to convey the incoming flue gas through the quencher for discharge into the lateral passageway for flow therethrough to the absorber. As a practical matter, venturi scrubbing devices, even those claimed to utilize very fine droplets, actually utilize droplets which are much larger than the optimal size. The primary methods heretofore utilized in improving the collection efficiency of a venturi scrubber have been to decrease the size of the throat or to increase the overall rate at which gas flows through the system. Both of these methods increase the differential velocities between the contaminant particles and the liquid droplets as they pass through the throat of the venturi scrubber. This causes more interactions between particles and droplets to occur, thereby improving contaminant removal. However, increasing the collection efficiency in this manner comes at a cost of significantly higher energy input into the system, thereby resulting in higher operating costs. The extra energy is expended due either to the increased overall resistance attributable to the reduced throat diameter or to the increased overall gas flow rate through the venturi scrubber. In either case, the pressure drop across the venturi is increased and greater fan and pumping capacity is required.  
           [0012]    The method according to the present invention replaces the duct work, the quencher and, except for an alternate embodiment hereinafter described, the lateral passageway with a bypass that conveys the incoming flue gas directly to the absorber. The quenching zone is transferred to the absorber and replaced by a spray level. The spray level includes a plurality of spray nozzles mounted on headers arranged parallel to one another. The nozzles spray an aqueous slurry of sulfur dioxide-reducing reagent within the spray zone to contact the flue gas while descending through the absorber counter-currently to the flow of flue gas, the slurry reagent is collected in the absorber sump or reaction tank and a portion of it is recycled for contact with the flue gas flowing through the absorber. The piping used to supply the slurry reagent to the quencher in the replaced duct work may be rerouted to the spray nozzle headers located in the absorber. The replacement of the bypassed quencher with a level of spray nozzles improves overall sulfur dioxide removal from the flue gases flowing through the system. An awning is mounted over the absorber inlet to prevent the slurry reagent from entering the inlet, and to initially deflect the incoming flue gas in a downward direction thereby achieving a more uniform distribution of the flue gas in its upward flow through the absorber. The bypass is configured to have a lesser number of turns than the duct work thereby reducing pressure losses. The front wall of the absorber is extended below the absorber inlet and becomes the front wall of the sump so as to accommodate the replacement of the lateral passageway with the bypass and the connecting of the bypass with the absorber. An overflow conduit is added to the front wall of the sump to maintain a desired or preset level of slurry reagent and contaminant particles in the sump, with any excess slurry reagent and contaminant particles being discharged through downcomers to a holding tank. The bypass, the awning, and the front wall of the sump are fabricated from alloys that are corrosion-resistant to both oxidizing and reducing media, and are resistant to localized corrosion attack.  
           [0013]    An optional standby quencher may also be provided in the bypass for emergency use.  
           [0014]    Flow guide elements may be mounted in the bypass such as turning vanes around corners so as to promote laminar flow of the flue gases, particularly around sharp corners in the duct work, and thus further reduce pressure losses.  
           [0015]    The lateral passageway need not be replaced, provided that it is restructured in that its flue gas inlet opening located on the roof is closed off and replaced by a flue gas inlet opening on the front wall. The bypass is then connected to the portion of the passageway front wall bordering the relocated flue gas inlet opening. A set of headers and nozzles may have to be added on the gas side of the passageway roof as part of the restructuring so as to provide a spray of alkaline solution to primarily prevent the overheating of the roof.  
           [0016]    It should be noted that removal of the duct work and the venturi scrubber type quencher will not only reduce fan power and pump power requirements due to reduced pressure drop across the system, but also result in the elimination of the costly maintenance associated with the venturi scrubber throat. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention will be better understood and its advantages will be more appreciated from the following detailed description, especially when read in light of the accompanying drawings, wherein:  
         [0018]    [0018]FIG. 1 is a schematic sectional side view of a downflow/upflow WFGD system known in the art;  
         [0019]    [0019]FIG. 2 is a schematic sectional side view of an upflow single-loop WFGD system derived from the system shown in FIG. 1 after utilizing a method according to the present invention;  
         [0020]    [0020]FIG. 3 is a schematic sectional side view of an alternate embodiment of the present invention, and depicts an emergency quencher mounted in the bypass;  
         [0021]    [0021]FIG. 4 is a schematic sectional side view of another alternate embodiment of the present invention, and depicts turning vanes mounted in the bypass;  
         [0022]    [0022]FIG. 5 is a schematic sectional side view of a further alternate embodiment of the present invention, and depicts the bypass connected to the lateral passageway; and  
         [0023]    [0023]FIG. 6 is a schematic sectional side view of still another alternate embodiment of the present invention, and depicts an arrangement for accommodating a partitioned sump. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The present invention has preferred application to utility boiler flue gases which is the embodiment that will be described for purposes of illustrating the invention and its advantages. However, the invention is not limited to the illustrated embodiment, and effluents from all types of combustion sources, utilizing packed or other types of scrubbing apparatus, and a wide range of reagents in co-current and/or countercurrent applications are envisioned.  
         [0025]    Reference will hereinafter be made to the accompanying drawings wherein like reference numerals throughout the various figures denote like elements.  
         [0026]    Referring now to the drawings, and particularly to FIG. 1, The downflow/upflow WFGD system  10 , illustrated herein, is known in the art and embodies a flue gas duct  11  for receiving incoming flue gas  12 , such as from a coal-fired utility or industrial boiler (not shown), and preferably cleaned of particulates such as by an electrostatic precipitator (not shown) or a fabric filter (not shown). The flue gas  12  is conveyed from the duct  11  by duct work  17 , located between the cut points  52  and  53 , and through the quencher  14  which is mounted in the duct work  17 . The quencher  14  comprises a venturi scrubber  16 , and as is known, venturi scrubber  16  is formed of an inlet cone  18 , a throat  20  and an outlet cone  22 .  
         [0027]    As is also known, as the flue gas  12  travels through the venturi scrubber  16  it is accelerated by the reduced cross section of inlet cone  18  and throat  20 , and it is then decelerated by the increased cross section of outlet cone  22 . The process of accelerating and decelerating the flue gas flow facilitates interactions between the droplets of scrubbing fluid and the and acid gases particles in the flue gas  12 , such that a portion of the contaminants particles are captured by the droplets and removed from the flue gas  12 .  
         [0028]    In the illustrative example, an alkaline slurry reagent is supplied via conduit  24  to the venturi scrubber  16  and sprayed into the flue gas stream through spray nozzles  25  mounted on spray headers  23 . The nozzles  25  provide a uniform spray of relatively coarse droplets suspended in concurrent or cross-current contact with the flue gas  12  in the throat  20 . The disposition of the sprays relative to the downwardly converging walls of the inlet cone  18  is such as to provide a wash along the lower regions of these walls to keep them relatively clean and to prevent the buildup of deposits on the wall surface.  
         [0029]    After leaving the venturi scrubber  16 , the flue gas, the sprayed slurry reagent and the captured contaminant particles flow co-currently downward and are discharged downwardly through the inlet opening  27  of a lateral passageway  30 . The inlet opening  27  is located on the roof  29  of the passageway  30  and adjacent its front wall  47 . The flue gas, the sprayed slurry, and the captured contaminant particles flow over and in contact with the slurry reagent and contaminants  31  collected in the reaction tank or sump  32 . The slurry reagent and contaminants in the sump  32  are maintained at a desired or preset level with any excess slurry reagent and contaminants being discharged through downcomers  33  to a holding tank  35 . The quenched and partially scrubbed flue gas  12  enters the absorber  26  through the inlet opening  28 . Thus, the flue gas makes a 180° turn as it flows downwardly through the quencher  14 , laterally through the passageway  30  and upwardly through the absorber  26 . In its upward flow through the absorber  26 , the flue gas  12  passes through a perforated tray  21  that promotes gas-liquid contact, and is generally of the type disclosed by the present applicant in U.S. Pat. No. 4,263,021. Thence, the flue gas  12  flows through a spray zone  34  that comprises spray levels  36   a  and  36   b  where additional gas-liquid contact is achieved. The spray levels  36   a  and  36   b  include spray nozzles  40  mounted on a set of headers  38 . An alkaline slurry reagent is supplied to the headers  38  via manifolds, not shown, and conduit  43 . A disengagement zone  42  is provided above spray level  36   a  before the flue gas  12  reaches the mist eliminator  44 . The mist eliminator  44  is equipped with chevrons  45  to remove additional moisture from the flue gas  12 . The scrubbed flue gas  12  leaves the mist eliminator  44  and exits from the absorber  26  through outlet  46  into the flue duct  48  for discharge through a stack (not shown).  
         [0030]    In accordance with the present invention and with particular reference to FIG. 2, and as shown in FIG. 1, a duct section or duct work  17  is disconnected from the flue gas duct  11  at a cut point  52  and from the inlet  27  of the passageway  30  at a cut point  53 . The duct work  17  which includes the quencher  14  is thus removed from operation as part of converting the downflow/upflow WFGD system  10  into an upflow single-loop WFGD system  15 , shown in FIG. 2, and may be dismantled. As part of this conversion, and as shown in FIG. 2, a duct or bypass  56  is installed between flue gas duct  11  and the absorber  26 . The bypass  56  has one end connected to the flue gas duct  11  at the cut point  52 , shown in FIG. 1, and the other end connected to the portions of the absorber front wall  39  and the sump front wall  37  bordering on the inlet  28  of the absorber  26 . The passageway  30 , shown in FIG. 1, is thus removed from operation as part of the conversion, and may be dismantled. The bypass  56  receives the incoming flue gas  12  from the duct  11  and conveys it to the inlet  28  of the absorber  26 .  
         [0031]    Also as part of the conversion, the function performed by the quencher  14 , shown in FIG. 1, is transferred to a quenching zone  58  located in the absorber  26  between spray level  36   b  and the inlet  28  of absorber  26 . The quenching zone  58  consists of a spray level  60 . The spray level  60  is comprised of a set of headers  64  and spray nozzles  66 . An alkaline slurry reagent is supplied to the spray nozzles  66  through headers  64  via conduit  24  that is disconnected from the quencher  14 , shown in FIG. 1, and rerouted and reconnected through a manifold (not shown) to the headers  64 . Alternatively, a new conduit, not shown, may be installed to supply the alkaline slurry reagent to the spray nozzles  66 .  
         [0032]    Further as part of the conversion, an awning  72 , generally of the type disclosed by the present applicant in U.S. Pat. No. 5,281,402, is mounted over the inlet  28  of the absorber  26  to prevent the slurry reagent from entering the bypass  56 , and to initially deflect the flue gas  12  in a downward direction as it enters the absorber  26  so as to achieve better distribution of the flue gas  12  in its subsequent upward flow through the absorber  26 . As it flows upwardly through the absorber  26 , the flue gas  12  passes through a perforated tray  21  that promotes gas-liquid contact, and thence through a spray zone  34  that comprises spray levels  36   a  and  36   b  where additional gas-liquid contact is achieved. The spray levels  36   a  and  36   b  include spray nozzles  40  mounted on a set of headers  38 . An alkaline slurry reagent is supplied to the headers  38  via manifolds, not shown, and conduit  43 . The spray nozzles  40  produce a spray of relatively coarse droplets suspended in countercurrent contact with the flue gas  12  for several seconds. A majority of the sulfur dioxide absorption from the flue gas occurs during this short contact time. A disengagement zone  42  is provided above spray level  36   a  before the flue gas  12  reaches the mist eliminator  44 . The purpose of the zone  42  is to allow disengagement and return of the largest slurry droplets by gravity to the spray zone  34 . The mist eliminator  44  design in most wet scrubbers uses chevrons  45  to remove additional moisture from the flue gas  12 . Chevrons  45  are closely spaced corrugated plates that collect slurry deposits by impaction. The scrubbed flue gas  12  leaves the mist eliminator  44  and exits from the absorber  26  through outlet  46  into the flue duct  48  for discharge through a stack (not shown). Because the flue gas  12  leaving the absorber  26  is saturated with water vapor, surface condensation is inevitable. This condensate can become severely acidic and calcium salts can deposit on the walls. Two approaches are used to minimize these effects, flue gas reheat (not shown), and flue duct and stack lining (not shown). In the latter approach, the flue duct  48  is lined with corrosion resistant materials, and the stack is lined with acid resistant brick or other suitable material. A drainage system (not shown) is also included to accommodate the condensed water vapor.  
         [0033]    Additionally as part of the conversion, the front wall  39  of the absorber  26  is extended below the inlet  28  of the absorber  26  and becomes the front wall  37  of the sump  32 . An overflow conduit  41  is added to the front wall  37  of the sump  32  to maintain a desired or preset level of slurry reagent spent slurry and contaminant particles  31  in the sump  32 , with any excess slurry reagent and contaminants  31  being discharged through downcomers  33   a  and  33   b  to the holding tank  35 .  
         [0034]    Turning now to FIG. 3, there is shown an alternate embodiment depicting fragmented portions of the flue gas duct  11  and the absorber  26 , the bypass  56 , the awning  72 , and the direction of flow of the flue gas  12  through the duct  11 , the bypass  56  and the absorber  26 . In accordance with this embodiment, a standby quencher  76  is mounted in the bypass  56  for emergency use. For example, the quencher  76  may consist of a set of headers  78  and spray nozzles  80 . An alkaline solution or water is supplied via conduit  82  to a manifold  84  and thence through headers  78  to the spray nozzles  80 . Control apparatus, not shown, may be provided to automatically activate the standby quencher  76  whenever the flue gas  12  being conveyed through the bypass  56  exceeds a desired or preset temperature.  
         [0035]    In FIG. 4, there is shown another alternate embodiment of the present invention depicting fragmented portions of the flue gas duct  11  and the absorber  26 , the bypass  56 , the awning  72 , and the direction of flow of the flue gas  12  through the duct  11 , the bypass  56  and the absorber  26 . In accordance with this embodiment, flow guiding means in the form of turning vanes  74  are mounted in the corner  88  of bypass  56  to direct the flow of flue gas  12  around the corner  88  and to promote uniform flow of the flue gas  12  and thus reduce the pressure drop across the bypass  56  by reducing the turning losses at the corner  88 .  
         [0036]    In FIG. 5, there is shown a further alternate embodiment of the present invention depicting fragmented portions of the bypass  56  and the absorber  26 , and the direction of flow of the flue gas  12  through the bypass  56  and the absorber  26 . In accordance with this embodiment, the bypass  56  does not replace the lateral passageway  30  of FIG. 1, instead, it discharges the flue gas  12  into passageway  30  which then conveys it to the absorber  26 . The retained passageway  30  has been restructured to include the closing of the inlet opening  27  located in the roof  29  of passageway  30  and shown in FIG. 1, or the installation of a new roof without an inlet opening, and the making of an inlet opening  86  in the front wall  47  of passageway  30  to receive the flue gas  12  being discharged from the bypass  56  which is connected to the portion of the front wall  47  bordering the opening  86 . A set of headers  90  and spray nozzles  92  may have to be added to the gas side of the roof  29 , as part of the restructuring of passageway  30 , to prevent the flue gas  12  from overheating the roof  29 . An alkaline solution is supplied by a conduit  94  through a manifold, not shown, and thence through headers  90  to the spray nozzles  92 . Control apparatus, not shown, may be provided to create a shield of alkaline spray protecting the roof  29  whenever the flue gas exceeds a desired or preset temperature. The flue gas  12  entering the passageway  30  flows over and in contact with the slurry reagent and contaminants  31  collected in the sump  32 . The excess slurry reagents and contaminants  31  in the sump  32  are discharged through downcomers  33  into the holding tank  35 .  
         [0037]    In FIG. 6, there is shown still another embodiment of the present invention depicting fragmented portions of the bypass  56 , the awning  72  and the absorber  26 , and the direction of flow of the flue gas  12  through the bypass  56  and the absorber  26 . In accordance with this embodiment, the sump  32  is divided into sections  32   a  and  32   b . The partition  49  that divides the sump  32  into sections  32   a  and  32   b  is provided with an opening  51  which enables excess slurry reagent and contaminants  31  in section  32   b , beyond that being discharged through downcomer  33   b  to the holding tank  35 , to flow from section  32   b  to section  32   a  and thence through the overflow conduit  41  located in the front wall  37  of the sump  32 . The excess slurry agent and contaminants  31  are discharged from the overflow conduit  41  through downcomer  33   a  into the holding tank  35 .  
         [0038]    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.