Abstract:
In a method for reducing lime consumption in DFGD systems, a slaker is provided wherein lime and water is introduced to produce a slaked lime slurry. The slurry is introduced into a spray dryer wherein the slurry is atomized. A flue gas stream is also introduced into the spray dryer and reacts with the atomized slurry, causing calcium sulfate and sulfite, as well as unreacted lime particulate to form. A portion of the particulate is filtered and sent back in a substantially dry form to the slaker to form part of the slurry.

Description:
FIELD 
       [0001]    The disclosure herein relates generally to dry flue gas desulfurization, and more particularly to a method and system whereby recycle material, including unreacted lime, is introduced back into the system. 
       BACKGROUND 
       [0002]    Dry flue gas desulfurization (DFGD) covers a range of technologies that include the absorption of sulfur dioxide (SO 2 ) by reaction with alkaline materials. These materials can be dry particles, dry particles in a humid environment, or moist particles in the process of drying. The absorption of SO 2  with moist particles is typically accomplished using a spray dryer. 
         [0003]    Spray drying is a DFGD process where a lime slurry is prepared by slaking lime. The slaked lime is usually stored in a tank and then transferred to the spray dryer. A spray dryer is a large, empty vessel where flue gas is directed at high velocity toward lime slurry atomizers. The atomizer uses high energy to divide the slurry into extremely fine droplets and dispense them into the high velocity gas stream. The result is three activities that occur at essentially the same time, albeit incompletely. Sulfur dioxide is absorbed into the droplets, the sulfur dioxide reacts with the lime to form a mixture of calcium sulfite and calcium sulfate, and the droplet dries leaving the reaction products suspended in the gas. Historically, the resulting particulates are carried out of the spray dryer with the flue gas for separation and disposal. 
         [0004]    DFGD systems that only use fresh lime (often referred to a “once-through lime”) use a large quantity of lime as compared to DFGD systems that recycle at least a portion of lime already used in the system. However, known lime recycle systems are expensive and time consuming because of the need to combine the used lime with dilution water. A system that reduces fresh lime consumption while also reducing cost and time in lime recycling is desired. 
       SUMMARY 
       [0005]    In one aspect disclosed herein there is provided a method for reducing lime consumption in DFGD systems that includes introducing lime and water into a slaker to produce a slaked lime slurry. The slaked lime slurry is subsequently introduced into a spray dryer where prior to the introduction of the slurry into the spray dryer, water is added to the slurry. Once introduced into the spray dryer, the slurry is atomized A flue gas stream is also introduced into the spray dryer and reacts with the atomized slaked lime slurry. This reaction causes sulfur dioxide in the flue gas to be absorbed into droplets that form part of the atomized slurry. The sulfur dioxide reacts with the slaked lime in the slurry to form calcium sulfite and calcium sulfate particulate. 
         [0006]    At least a portion of the flue gas and the particulate are transferred from the spray dryer to a filter wherein at least a portion of the particulate is separated from the flue gas. The separated particulate is substantially dry. At least a portion of the substantially dry particulate separated in the filter, which also contains some unreacted lime, is transferred back to the slaker where it is mixed with the water and lime during a slaking operation, thereby forming part of the slaked lime slurry. 
         [0007]    The above described and other features are exemplified by the following figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Referring now to the figures, which are exemplary embodiments, and wherein the like elements are number alike: 
           [0009]      FIG. 1  schematically illustrates a DFGD system; 
           [0010]      FIG. 2  schematically illustrates a DFGD system; 
           [0011]      FIG. 3  schematically illustrates a DFGD system; and 
           [0012]      FIG. 4  schematically illustrates a DFGD system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A general DFGD system is shown in  FIG. 1  and generally designated by the reference number  10 . Water and lime are fed via conduits  12  and  14  respectively, into a slaker  16  where a slaking operation is performed. From the slaker  16 , the slaked lime slurry flows into a tank  18  for storage. The slaked lime slurry is pumped, via pump  20  through conduit  22  to a spray dryer  24 . Prior to entering the spray dryer  24  additional water can be added to the slaked lime slurry via conduit  26 . A flue gas stream  28  is also fed into the spray dryer  24 . Inside the spray dryer  24 , the slaked lime slurry is atomized and comes into contact with the flue gas stream where it reacts therewith in the above-described manner. Particulate and flue gas exit the spray dryer  24  via conduit  30  and are collected by a filter  32 . A portion of the particulate also exits the spray dryer  24  via conduit  34  for disposal. Particulate is separated from the flue gas in the filter  32  and exits therefrom via conduits  36  for subsequent disposal. 
         [0014]    Another DFGD system is shown in  FIG. 2  and is generally designated by the reference number  110 . DFGD system  110  is similar to the above-described system  10  with like elements being identified with like reference numbers preceded by the numeral  1 . One difference between systems  10  and  110  is that not all of the particulate collected by the filter  132  is disposed. At least a portion of the particulate separated by the filter  132  is transferred via conduit  140  into a storage silo  142 . The particulate is then transferred to a feed tank  144  via conduit  146 . Water is added to the feed tank  144  via conduit  148 . Additionally, a portion of the slaked lime slurry from pump  120  is provided to the feed tank  144  via a conduit  122   a . The mixture of slaked lime slurry, water and particulate exit the feed tank  144  where additional slaked lime slurry may be added via a conduit  122   b  prior to introduction to a pump  150 . The mixture exits the pump  150  in a conduit  151 , where a portion of the mixture is provided to the feed tank  144  via a conduit  152 , and the remaining mixture of slaked lime slurry, water and particulate is provided to the spray dryer  124  via a conduit  153 . 
         [0015]    As shown in  FIG. 3 , a DFGD system generally designated by the reference number  200  includes conduits  212  and  214  that feed water and lime into a lime slaker  216  where a slaking operation is performed. From the slaker  216 , the slaked lime slurry flows into a tank  218  for storage. The slaked lime slurry is pumped, via pump  220  and conduit  222  to a spray dryer  224 . 
         [0016]    Prior to entering the spray dryer  224  additional water can be added to the slaked lime slurry via conduit  226 . A flue gas stream  228  is also fed into the spray dryer  224 . The flue gas  228  is produced by combustion of a fuel in a boiler (not shown), which may be, for example, a circulating fluidized bed boiler (CFB boiler). 
         [0017]    Inside the spray dryer  224 , the slaked lime slurry is atomized and comes into contact with the flue gas stream where it reacts therewith in the above-described manner. Particulate and flue gas exit the spray dryer  224  via conduit  230  and flow into a filter  232 . A portion of the particulate also exits the spray dryer  224  via conduit  234  for disposal. Excess slurry is recycled back to the tank  218  prior to entering the spray dryer  224  via conduit  235 . 
         [0018]    Particulate is separated from the flue gas  228  in the filter  232 . The separated particulate exits the filter  232  via conduits  236  with a portion of the filtered particulate going to disposal via conduit  238  and another portion of the particulate being transferred via conduit  240  to a storage receptacle, which in the illustrated embodiment is shown as a storage silo  242 . The particulate, which contains unreacted lime, is transferred via conduit  246  from the storage silo  242 , back to the slaker  216  where the particulate is mixed with the water and lime feed during a slaking operation. The particulate from the storage silo is used to form the slurry that is fed via conduit  222  to the spray dryer  224 . 
         [0019]    Combustion of certain types of coals, such as, for example, Power River Basin (PRB) coal, produce particulates in the flue gas  228  including, for example, CaO (lime), Na 2 O, K 2 O and MgO. These particulates, when added to the slaker  216 , convert to hydroxides, which react with SO 2 , thereby reducing the amount of fresh lime consumed in the slaker. Use of a CFB boiler produces a flue gas  228  having particulates that are rich in CaO. These particulates, when added to the slaker  216 , convert to calcium hydroxide, which reacts with SO 2 , thereby reducing the amount of fresh lime consumed in the slaker. 
         [0020]    Another DFGD system is shown in  FIG. 4  and is generally designated by the reference numeral  300 . DFGD system  300  is similar to the above-described system  110 . One difference between the DFGD system  300  and the system  110  is a portion of the particulate transferred from the storage silo  342  to the feed tank  344  is transferred to the slaker  316  via a conduit  346   a.  The remaining portion of the particulate from the storage silo  342  is transferred to the feed tank  344  via a conduit  346   b.  The particulate is mixed with water added to the feed tank via conduit  348  and a portion of the slaked lime slurry via the conduit  322   a.  The mixture formed in the feed tank  344  exits the feed tank to a pump  350 , wherein a portion of the mixture is sent to the spray dryer via a conduit  353  where it is atomized. A separate portion of the mixture is recycled back to the feed tank  344  via conduit  352  where it is mixed with the water, slaked lime slurry and particulate. 
         [0021]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.