Abstract:
An apparatus and method for removing mercury from an incoming gas stream contaminated with mercury so as to deliver an outgoing gas stream free of the removed mercury introduces ozone into the gas stream to deposit mercury carried by the gas stream on separator elements so as to remove the deposited mercury from the gas stream. The deposited mercury is washed off of the separator elements, and the removed mercury is collected for further processing.

Description:
FIELD OF INVENTION 
     The present invention relates generally to the reduction of contaminants emitted into the atmosphere as a result of commercial and industrial processes and pertains, more specifically, to apparatus and method for the removal of mercury from industrial exhausts. 
     BACKGROUND 
     The continuing pursuit of more stringent regulations pertaining to the control of contaminants emitted into the ambient atmosphere has led to the requirement for more effective treatment of emissions emanating from commercial and industrial processes. 
     One of the more difficult to deal with toxic substances encountered in industrial exhausts is mercury. Mercury, in the form of mercury vapor, is prevalent in industrial exhausts, and the removal of mercury from industrial exhausts has received much attention in efforts to attain effective results with economy. 
     The present invention accomplishes effective removal of mercury from industrial exhausts, with economy, through the utilization of a known physical characteristic of mercury. As reported in page 343 of a text entitled BASICS OF GENERAL CHEMISTRY, by B. V. Nekrasov, published by Chemical Publishing, Moscow, 1969, in the presence of even minute traces of ozone, mercury will tend to lose its liquidity and will be deposited on the wall of the vessel within which it is contained, in the manner of a thin film. The present invention provides method and apparatus which takes advantage of that physical characteristic to accomplish the removal of mercury from industrial exhausts. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention attains several objects and advantages, some of which are summarized as follows: Removes mercury from industrial exhausts with increased effectiveness, efficiency and economy; enables the effective removal of mercury from industrial exhausts utilizing currently available industrial exhaust treatment apparatus with relatively simple and inexpensive modifications in equipment design and operation; facilitates the safe handling and disposal of mercury removed from industrial exhausts; takes advantage of characteristics of current electrostatic precipitators and, in particular, condensing wet electrostatic precipitators, to accomplish the removal of mercury from industrial exhausts with increased effectiveness, efficiency and economy; allows relatively simple and economical modification of existing facilities for the effective removal of mercury from industrial exhausts, and subsequent safe handling and disposal of the removed mercury; provides a process and an integrated apparatus for the removal of a wide range of particulates and contaminants, including mercury, from contaminated gas streams; provides an integrated apparatus and process for effective and reliable treatment of contaminated gas streams over a relatively long service life. 
     The above objects and advantages, as well as further objects and advantages, are attained by the present invention which may be described briefly as apparatus for removing mercury from an incoming gas stream contaminated with mercury so as to deliver an outgoing gas stream free of the removed mercury, the apparatus comprising: an inlet for receiving the incoming gas stream and directing the incoming gas stream to a path of travel through the apparatus; an outlet located along the path of travel for delivering the outgoing gas stream; a separator having separator elements located along the path of travel between the inlet and the outlet such that the gas stream is conducted through the separator and to the separator elements; an ozone supplier for introducing ozone into the gas stream to deposit mercury carried by the gas stream on the separator elements so as to remove the deposited mercury from the gas stream; a washer located along the path of travel for washing the deposited mercury from the separator elements, whereby the outgoing gas stream is rendered free of the removed mercury; and a collector for collecting the removed mercury for further disposition. 
     In addition, the present invention includes a method for removing mercury from an incoming gas stream contaminated with mercury so as to deliver an outgoing gas stream free of the removed mercury, the method comprising: directing the incoming gas stream along a predetermined path of travel extending between an inlet for the incoming gas stream and an outlet for the outgoing gas stream; conducting the gas stream through a separator having separator elements located along the path of travel such that the gas stream is conducted through the separator and to the separator elements; introducing ozone into the gas stream to deposit mercury carried by the gas stream on the separator elements so as to remove the deposited mercury from the gas stream; washing the deposited mercury from the separator elements, whereby the outgoing gas stream is rendered free of the removed mercury; and collecting the removed mercury from the separation elements for further disposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood more fully, while still further objects and advantages will become apparent, in the following detailed description of preferred embodiments of the invention illustrated in the accompanying drawing, in which: 
         FIG. 1  is a diagrammatic, longitudinal cross-sectional view of an apparatus constructed and operated in accordance with the present invention. 
         FIG. 2  is a diagrammatic, longitudinal cross-sectional view of another apparatus constructed and operated in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and especially to  FIG. 1  thereof, an apparatus constructed in accordance with the present invention is illustrated generally at  10  and is seen to include a housing  12  which extends vertically from a lower bottom end  14  to an upper top end  16 . An inlet is shown in the form of a port  20  located adjacent the bottom end  14  and receives an incoming gas stream, as indicated at  22 , laden with contaminants, including particulates, corrosive gases and mercury. The incoming gas stream  22  is directed upwardly along a vertical path of travel  24  into a separator which includes a scrubber section  30 , passing first into a first stage scrubber in the form of a liquid distribution scrubber  32  having a scrubbing matrix  34  comprised of separator elements illustrated as a plurality of transverse bars  36 . 
     A moisture supplier in the scrubber section  30  includes a liquid distributor in the form of a spray header  40  having a plurality of spray nozzles  42  located immediately above the matrix  34  for spraying water  44  downwardly into the gas stream  22  as the gas stream  22  travels upwardly along the path of travel  24  through the matrix  34 . The turbulence induced by the bars  36  of the matrix  34  accomplishes thorough mixing of the incoming gas stream  22  with the water  44 , and larger particulates, in sizes of about two microns and above in diameter, are entrained within water droplets  46  which drop downwardly, by gravity, into a reservoir  50  at the bottom of the housing  12 . In addition to the removal of these larger particulates, gas absorption takes place to remove some toxic gases from the gas stream  22 . Water  44  is drawn from the reservoir  50 , through a passage  52 , by a pump  54  which circulates the water to the spray header  40  through a water circuit  56  including a water conduit  58 . A bleed line  60  communicates with the water conduit  58  through a bleed control valve  62  to periodically direct portions of water  44  out of the water circuit  56  for removal of the solids collected in the water  44 . An overflow drain  63  maintains the water  44  in reservoir  50  at a predetermined level. A control valve  64  controls the flow of water in the water circuit  56 , and a heat exchanger  65 , which includes an input  66  for a heat exchange medium and an output  67 , and a control valve  68  for controlling circulation of the heat exchange medium through the heat exchanger  65 , is placed in the water circuit  56  for purposes to be described fully hereinafter. 
     A demister  70  is located above the scrubbing matrix  34 , and a spray header  72  is placed beneath the demister  70  for periodically washing the demister  70  with water supplied at an input  74 . A first interstage drain  76  drains wash water accumulated in a first interstage collector shown in the form of first interstage pan  78 . Upon leaving the first stage scrubber, the gas stream  22  continues moving upwardly in scrubber section  30  of the separator, passing through the demister  70  and entering a second stage scrubber of the separator, in the form of a packed bed scrubber  80  which includes separator elements in the form of a bed  82  of packing. The packing is preferably a collection of loose materials to provide a large surface area. The larger surface area attracts water by surface tension to provide for greater interaction with the gas stream. A cooler in the form of a cooling coil  84  is embedded in the packing of bed  82  and conducts a cooling medium in the form of cooling water through a conduit  85  established by the cooling coil  84  through the bed  82 , for purposes to be described more fully below. A washer includes a first washer element in the form of a spray header  86  having a plurality of spray nozzles  88  located immediately above the packed bed  82  for spraying washing water  89  downwardly into bed  82  of packing, for purposes to be described hereinafter. 
     The gas stream  22  continues upwardly along path of travel  24  out of the scrubber section  30  and into a wet electrostatic precipitator section  90  of the separator wherein the gas stream  22  passes through a condensing wet electrostatic precipitator  92 . 
     Precipitator  92  includes an inlet area  93  extending transversely across the wet electrostatic precipitator section  90 , and a plurality of electrode assemblies  94  arranged in a matrix  96 , the matrix  96  extending across the inlet area  93  and the electrode assemblies  94  being powered by a source  98  of high voltage, in a now conventional manner. To that end, the voltage source  98  is connected to discharge electrodes  100  of the electrode assemblies  94  through a support assembly which includes support members  101  and a support frame in the form of a bus frame  102  supported by insulator members in the form of insulators  104  placed in corresponding chambers  106 , forming a wet electrostatic precipitator as is known in the art. The bus frame  102  is suspended below the insulators  104  by the support members  101 , and the discharge electrodes  100  are suspended downwardly from the bus frame  102  such that each discharge electrode  100  passes through the center of a corresponding collection electrode  110  having a tubular wall  112  connected to the source  98  so that the discharge electrodes  100  carry an electrostatic charge of given polarity and the collection electrodes  110  carry an electrostatic charge having a polarity opposite to the given polarity. In the illustrated embodiment, the discharge electrodes  100  carry a negative charge, while the collection electrodes  110  carry a positive charge. 
     A coolant jacket  120  surrounds the electrode assemblies  94  and, more specifically, the tubular walls  112  of the collection electrodes  110  surrounding the discharge electrodes  100  in the matrix  96  so as to enable circulation of a coolant  122  around the outside of the tubular walls  112 , in contact with the outside surfaces  114  of the tubular walls  112 , to maintain the temperature of the tubular walls  112  at a level most conducive to condensation of the moisture carried by the gas stream  22  on the inside surfaces  116  of the tubular walls  112  as the gas stream  22  passes through the interior of the tubular walls  112 . Coolant  122  is introduced into the coolant jacket  120  at inlet  124  located adjacent the top end  126  of the coolant jacket  120  and is circulated to outlet  128  adjacent the bottom end  130  of the coolant jacket  120 . A second washer element in the form of a spray header  140  having a plurality of spray nozzles  142  is located immediately above the condensing wet electrostatic precipitator  92  and is supplied with wash water through an inlet valve  144 , for purposes to be described below. Coolant  122  may be water or another fluid with heat conducting properties. 
     Incoming gas stream  22  at the inlet port  20  is at an elevated temperature and is contaminated with mercury vapor, as well as with particulates and toxic gases. Larger particulates and toxic gases are removed from the gas stream  22  as the gas stream  22  passes through the scrubber  32  of the separator. At the same time, the gas stream  22  is cooled by water  44  distributed by the spray header  40 , preferably to a temperature of about 80° F., before being passed to the packed bed scrubber  80  of the separator. The cooling medium passed through the conduit  85  provided by cooling coil  84  further cools the gas stream  22  so that the temperature of the gas stream  22  as the gas stream  22  is passed to the condensing wet electrostatic precipitator  92  is about 60° F. 
     Mercury vapor carried in the gas stream  22  will condense at 70° F., the temperature reached within the packed bed  82 , and condensed mercury will accumulate within the packing in bed  82 . In addition, any mercury droplets entrained in the gas stream  22  emerging from the packed bed  82  will be carried to the precipitator  92 . 
     Within the electrode assemblies  94  of the electrostatic precipitator  92 , the discharge electrodes  100  have relatively sharp points  150 . As is known in electrostatic precipitators, a strong electrostatic field is generated in each electrode assembly  94 , between the discharge electrode  100  and the collection electrode  110 , and the sharp points  150  cause corona discharges which, in turn, generate ozone so that, in effect, each electrode assembly  94  serves as an ozone supplier for introducing ozone into the gas stream  22 . As the gas stream  22  passes between the discharge electrode  100  and the collection electrode  110  of each electrode assembly  94 , mercury droplets carried in the gas stream  22  migrate, together with entrained moisture and other particulates, to the inside surface  116  of the tubular wall  112 . The cooled inside surface  116  enables condensation of the moisture from the saturated gas stream  22 , establishing a film of condensate  160  on the inside surface  116 . At the same time, ozone generated by the corona discharge is absorbed in the condensate  160 . The condensate  160 , with the entrained mercury and the absorbed ozone, runs down the tubular wall  112  and the mercury thus separated from the gas stream  22  is delivered to the packed bed  82 . As indicated above, the mercury, in the presence of the ozone, is deposited, in a thin film of elemental mercury, upon the cooling coil  84  and upon the packing in bed  84 . Any mercury which may become separated from the gas stream  22  and deposited upon the inside surface  116  of the tubular wall  112  of an electrode assembly  94  as a result of exposure to ozone in the electrode assembly  94  and which is not carried away by the condensate  160  to the packed bed  84  is washed periodically from the tubular wall  112  by wash water delivered through the spray nozzles  142  of spray header  140 . 
     A duct  170  provides a conduit for conducting a portion of the outgoing gas stream  22  at outlet  172  of the apparatus  10  to a manifold  176  located at an ozone introduction location placed immediately below the packed bed  84 . A return blower  178  delivers return gas enriched with ozone to manifold  176  for introduction into the packed bed  82  through distributors  180  in the manifold  176 . Exposure of the mercury accumulated in the packed bed  82  to ozone deposits elemental mercury on the cooling coil  84  and the packing of bed  82 . The elemental mercury thus separated from the gas stream  22  is then washed from the packed bed scrubber  80  by wash water distributed to the packed bed scrubber  80  through the spray nozzles  88  of the spray header  86  located immediately above the packed bed  82 . The wash water with the separated mercury is collected in a second interstage collector, shown in the form of second interstage pan  182  and is drained through a second interstage drain  184  for further disposition. The removed mercury is handled readily for safe disposal. 
     In the embodiments of the invention illustrated in  FIG. 2 , an apparatus constructed in accordance with the present invention is illustrated generally at  210  and is seen to include a housing  212  which extends vertically from a lower bottom end  214  to an upper top end  216 . An inlet is shown in the form of a port  220  located adjacent the bottom end  214  and receives an incoming gas stream, as indicated at  222 , laden with contaminants, including mercury. The incoming gas stream  222  is directed upwardly along a vertical path of travel  224  into a separator which includes a scrubber section  230  comprising a packed bed scrubber  234  having separator elements in the form of a bed  236  of packing. A washer includes a washer element in the form of a spray header  240  having a plurality of spray nozzles  242  located immediately above the packed bed  236  for spraying washing water  244  downwardly into bed  236  of packing, for purposes to be described below. Washing water  244  is supplied to spray header  240  by a supply line  246  and a supply pump  248  communicating with a reservoir  250  located adjacent the bottom end  214  of the housing  212 . A further spray header  260  is located above the spray header  240  and includes a plurality of spray nozzles  262  for spraying water into outgoing gas stream  264 , under the control of a valve  266 , as the gas stream  264  moves toward an outlet  268  adjacent the top end  216  of the housing  212 . A mist eliminator  270  is placed between the further spray header  260  and the outlet  268 . 
     Incoming gas stream  222  at the inlet port  20  is contaminated with mercury. An ozone injector  280  is placed at the inlet port  20  and is supplied with ozone by an ozone generator  282  so as to introduce ozone into the incoming gas stream  222 . As the mercury and ozone laden gas stream  222  passes through the scrubber  234  of the separator, mercury carried in the gas stream  222  will be deposited on the packing in bed  236 . The deposited mercury thus separated from the gas stream  222  then is washed from the packed bed scrubber  234  by wash water distributed to the packed bed scrubber  234  through the spray nozzles  242  of the spray header  240  located immediately above the packed bed  236 . The wash water with the separated deposited mercury is collected in reservoir  250  and is drained through a drain  290  for further disposition. The removed mercury is handled readily for safe disposal. 
     It will be seen that the present invention attains the several objects and advantages summarized above, namely: Removes mercury from industrial exhausts with increased effectiveness, efficiency and economy; enables the effective removal of mercury from industrial exhausts utilizing currently available industrial exhaust treatment apparatus with relatively simple and inexpensive modifications in equipment design and operation; 
     facilitates the safe handling and disposal of mercury removed from industrial exhausts; takes advantage of characteristics of current electrostatic precipitators and, in particular, condensing wet electrostatic precipitators, to accomplish the removal of mercury from industrial exhausts with increased effectiveness, efficiency and economy; allows relatively simple and economical modification of existing facilities for the effective removal of mercury from industrial exhausts, and subsequent safe handling and disposal of the removed mercury; provides a process and an integrated apparatus for the removal of a wide range of particulates and contaminants, including mercury, from contaminated gas streams; provides an integrated apparatus and process for effective and reliable treatment of contaminated gas streams over a relatively long service life. 
     While certain novel features of the present invention have been shown and described, it will be understood that various omissions, substitutions and changes in the forms and details of the device illustrated and in its operation can be made by those skilled in the art without departing from the spirit of the invention.