Patent Publication Number: US-2015076079-A1

Title: Method and system for seawter foam control

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of removing foam comprising heavy metals generated during treatment of effluent seawater from a seawater based flue gas desulfurization system seawater aeration basin. 
     The present invention further relates to a system for removing foam comprising heavy metals generated during treatment of effluent seawater from a seawater based flue gas desulfurization system seawater aeration basin. 
     BACKGROUND OF THE INVENTION 
     Process gases containing sulfur dioxide, SO 2 , are generated in many industrial processes. One such industrial process is combustion of a fuel such as coal, oil, peat, waste, or the like, in a combustion plant such as a power plant. In such a power plant, a hot process gas often referred to as a flue gas, is generated. The generated flue gas contains pollutants such as for example acid gases, such as for example sulfur dioxide, SO 2 . It is necessary to remove as much of the generated acid gases as possible from the flue gas before the flue gas may be emitted into ambient air. Another example of an industrial process that generates a process gas containing pollutants is electrolytic production of aluminum from alumina. In that process, a process gas or flue gas containing sulfur dioxide, SO 2 , is generated within venting hoods of electrolytic cells. 
     WO 2008/105212 discloses a boiler system comprising a boiler, a steam turbine system, and a seawater scrubber for flue gas desulfurization. The boiler generates, by combustion of a fuel, high-pressure steam utilized in the steam turbine system to generate electric power. Seawater is collected from the ocean, and is utilized as a cooling medium in a condenser of the steam turbine system. The seawater is then utilized in the seawater based flue gas desulfurization scrubber useful for absorbing sulfur dioxide, SO 2 , from flue gas generated in the boiler. Sulfur dioxide, SO 2 , is absorbed in seawater contacted in the seawater based flue gas desulfurization scrubber and forms sulfite and/or bisulfite ions. Effluent seawater from the seawater based flue gas desulfurization scrubber is forwarded to a seawater aeration basin for treatment. In the seawater aeration basin, air is bubbled through effluent seawater forwarded from the seawater based flue gas desulfurization scrubber for oxidation of sulfite and/or bisulfite ions therein to sulfate ions. The sulfite and/or bisulfite ions therein are so oxidized to sulfate ions by means of oxygen gas contained in the bubbled air. The resulting inert sulfate ions in the treated effluent seawater may then be release back to the ocean. 
     One problem with effluent seawater treatment in a seawater aeration basin is the generation of foam on the effluent seawater surface. At the present time, such generated foam on the surface of the effluent seawater is typically released back to the ocean. However, this generated foam carries a relatively high concentration of heavy metals unsuitable for release back to the ocean. A method and system for controlling seawater foam in effluent seawater aeration basins is needed to reduce or prevent the release of heavy metals back into the ocean. 
     SUMMARY OF THE INVENTION 
     An object of the present disclosure is to provide a system for controlling seawater foam generation on the surface of effluent seawater, generated in the removal of sulfur dioxide from a flue gas by contacting the flue gas containing sulfur dioxide with seawater, and treated in seawater aeration basins. This stated object is achieved using a seawater aeration basin aeration fan. The high pressure side, i.e., the blower side, of the aeration fan is used to blow air to aerate effluent seawater in the seawater aeration basin. Adding ductwork to the low pressure side, i.e., the suction side of the aeration fan makes the aeration fan operable to suck generated foam from the surface of the effluent seawater undergoing treatment in the seawater aeration basin for foam storage, treatment and environmentally conservative disposal. 
     An advantage of this system is that the system achieves the stated objective with minimal added operational and/or capital expenses associate therewith. Using part of the aeration fan&#39;s low pressure energy results in minimal high pressure blower efficiency loss. Likewise, in most applications, foam control is achieved with system operation in a non-continuous mode. Operating the system in a non-continuous mode likewise reduces any aeration fan efficiency losses attributed thereto, again reducing associated costs. 
     Another object of the present disclosure is to provide a method for controlling seawater foam generation on the surface of effluent seawater, generated in the removal of sulfur dioxide from a flue gas by contacting the flue gas containing sulfur dioxide with seawater, and treated in seawater aeration basins. This stated object is achieved using an effluent seawater seawater aeration basin aeration fan. The high pressure side, i.e., the blower side, of the aeration fan is used to blow air or aerate effluent seawater in the seawater aeration basin. Adding ductwork to the low pressure side, i.e., the suction side of the aeration fan makes the aeration fan operable to suck generated foam from the surface of the effluent seawater for storage, treatment and environmentally conservative disposal. 
     An advantage of this method is that the method achieves the stated objective with minimal added operational and/or capital expenses associate therewith. Using part of the aeration fan&#39;s low pressure energy results in minimal high pressure blower efficiency loss. Likewise, in most applications, foam control is achieved operating in accordance with the subject method in a non- continuous mode. Operating in a non-continuous mode likewise reduces any aeration fan efficiency losses attributed thereto, again reducing associated costs. 
     In summary, the subject disclosure provides a system and method for controlling foam comprising heavy metals generated on the surface of effluent seawater during aeration of the effluent seawater in a flue gas desulfurization system associated seawater aeration basin. As such, the subject foam control system comprises a seawater aeration basin fluidly connected to an aeration fan operable to blow an oxygen containing gas into effluent seawater in the seawater aeration basin for aeration of the effluent seawater and operable to suction foam from a surface of effluent seawater in the seawater aeration basin. Suction of foam from the surface of the effluent seawater using the aeration fan may be conducted on a continuous, scheduled periodic, detector initiated “as-needed” periodic or other non-continuous basis. A foam collection tank is used for collection, storage and optionally treatment of the foam comprising heavy metals such as mercury, suctioned from the surface of the effluent seawater. As the suctioned foam comprises heavy metals such as mercury, treatment of the suctioned foam in the foam collection tank comprises removing at least a portion of heavy metals therefrom. For this purpose, suctioned foam may be stored for a time in the foam collection tank prior to and/or after treatment thereof. Alternatively, the suctioned foam may be collected and optionally stored for a time in the foam collection tank prior to transport of the suctioned foam for treatment elsewhere in the power plant or offsite. 
     The subject method for controlling foam comprising heavy metals comprises providing a seawater aeration basin fluidly connected to an aeration fan operable to blow an oxygen containing gas into effluent seawater for aeration of the effluent seawater in the seawater aeration basin and operable to suction foam comprising heavy metals such as mercury from a surface of effluent seawater in the seawater aeration basin. Suctioning of foam from the surface of the effluent seawater using the aeration fan may be conducted on a continuous, scheduled periodic, detector initiated “as-needed” periodic or other non-continuous basis. A foam collection tank is used for collection, storage and optionally treatment of the suctioned foam comprising heavy metals such as mercury, suctioned from the surface of the effluent seawater. As the suctioned foam comprises heavy metals such as mercury, treatment of the suctioned foam in the foam collection tank comprises removing at least a portion of heavy metals therefrom. For this purpose, suctioned foam may be stored for a time in the foam collection tank prior to and/or after treatment thereof. Alternatively, the suctioned foam may be collected and optionally stored for a time in the foam collection tank prior to transport of the suctioned foam for treatment elsewhere in the power plant or offsite. 
     Further objects and features of the present disclosure will be apparent from the following description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be disclosed in more detail with reference to the appended drawings described below. 
         FIG. 1  is a schematic side cross-section view of a power plant with apparatus according to the present disclosure. 
         FIG. 2  is a schematic side cross-section view illustrating an enlarged seawater based flue gas desulfurization system seawater aeration basin according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic side cross-section view illustrating a power plant  10 . The power plant  10  comprises a boiler  12  to which a fuel F, such as coal, oil, or the like, is supplied from a fuel source  14  through a fluidly connected feeding pipe  16  to boiler  12  for combustion therein. Fuel F is combusted in boiler  12  in the presence of oxygen O, supplied to boiler  12  via a fluidly connected oxygen supply duct  20  from an oxygen source  18 . The oxygen O supplied to boiler  12  may, for example, be supplied in the form of air, and/or in the form of a mixture of oxygen gas and recirculated power plant  10  flue gas FG. In such a case, boiler  12  would be what is commonly called an “oxy-fuel” boiler. The combustion of the fuel F generates a hot process gas in the form of a flue gas FG. Sulphur species contained in fuel F, upon combustion of the fuel F, form sulphur dioxide, SO 2 . As such, power plant  10  flue gas FG includes as a portion thereof sulphur dioxide. 
     Produced flue gas FG flows from the boiler  12 , via a fluidly connected duct  22 , to a particulate collection device  24 , in the form of a fabric filter or electrostatic precipitator. The particulate collection device  24 , such as an electrostatic precipitator as described in U.S. Pat. No. 4,502,872, serves to remove dust and/or ash particles entrained in the flue gas FG. Alternatively, a fabric filter such as that described in U.S. Pat. No. 4,336,035, may be used for particulate collection of flue gas dust and/or ash from the flue gas FG. As an alternate embodiment, particulate collection device  24  may be arranged downstream of a seawater based flue gas desulfurization system  28 . As still another embodiment, particulate collection device  24  may be eliminated from the system with particulate removal occurring solely in a seawater based flue gas desulfurization system  28 . 
     According to the present embodiment illustrated in  FIG. 1 , the flue gas FG from which most of the ash and/or dust particles have been removed, flows from the particulate collection device  24  via a fluidly connected duct  26  to a seawater based flue gas desulfurization system  28 . The seawater based flue gas desulfurization system  28  comprises a wet scrubber tower or absorber  30 . An inlet  32  is arranged at a lower portion  34  of the absorber  30 . The duct  26  is fluidly connected to the inlet  32 , such that flue gas FG flowing from particulate collection device  24  via duct  26  may enter interior  36  of absorber  30  via inlet  32 . 
     After entering interior  36 , flue gas FG flows vertically upward through absorber  30 , as indicated by arrow FG. Central portion  38  of absorber  30  is equipped with a number of spray arrangements  40  arranged vertically one above each other. For purposes of simplicity in the embodiment illustrated in  FIG. 1 , there are three such spray arrangements  40 . Typically, there are 1 to 20 such spray arrangements  40  in an absorber  30 . Each spray arrangement  40  comprises a supply pipe  42  and a number of nozzles  44  fluidly connected to the respective supply pipe  42 . Seawater SW supplied via the respective supply pipes  42  to the nozzles  44  is atomized by means of the nozzles  44  contacting in interior  36  of absorber  30  flue gas FG flowing therethrough. As such, contact between the seawater SW and flue gas FG enables seawater SW absorption of sulphur dioxide, SO 2 , from the flue gas FG within interior  36  of absorber  30 . 
     A pump  46  is arranged for pumping seawater SW via fluidly connected suction pipe  48  from seawater supply or ocean  50 , and forwarding the seawater SW via fluidly connected pressure pipe  52  to fluidly connected supply pipes  42 . 
     In accordance with an alternative embodiment, the seawater SW supplied by pump  46  to supply pipes  42  may be seawater SW previously utilized as cooling water in steam turbine systems (not shown) associated with the boiler  12  prior to supply of such seawater SW to absorber  30 . 
     In accordance with an alternative embodiment, the seawater based flue gas desulfurization system  28  may comprise one or more layers of a packing material  58  arranged in interior  36  of absorber  30 . The packing material  58 , which may be made from plastic, steel, wood, or another suitable material, enhances gas-liquid contact. With packing material  58 , the nozzles  44  would merely distribute seawater SW over packing material  58 , rather than atomizing the seawater SW. Examples of packing material  58  include Mellapak™ available from Sulzer Chemtech AG, Winterthur, CH, and Pall™ rings available from Raschig GmbH, Ludwigshafen, 
     DE. 
     Seawater SW atomized by means of nozzles  44  in interior  36  of absorber  30  flows downwardly in absorber  30  and absorbs sulphur dioxide from the flue gas FG flowing vertically upwardly in interior  36  of absorber  30 . Absorption of sulphur dioxide by the seawater SW in interior  36  forms effluent seawater ES collected in lower portion  34  of absorber  30 . Effluent seawater ES collected in lower portion  34  of absorber  30  is forwarded via a fluidly connected effluent pipe  54  to seawater aeration basin  56 . 
     Optionally if needed, fresh seawater SW may be added to the effluent seawater ES flowing through effluent pipe  54  to seawater aeration basin  56 . To this end, an optional pipe  60  may be fluidly connected to pressure pipe  52  to forward a flow of fresh seawater SW to fluidly connected effluent pipe  54  forwarding effluent seawater ES to seawater aeration basin  56 . Hence, an intermixing of fresh seawater SW and effluent seawater ES may occur in effluent pipe  54 . As another optional alternative (not illustrated), the fresh seawater SW forwarded via pipe  60  may be forwarded directly to seawater aeration basin  56  mixing with the effluent seawater ES therein. As a still further option (not illustrated), residual waters and/or condensates generated in the boiler  22  or steam turbine systems (not shown) associated therewith could be mixed with the effluent seawater ES in seawater aeration basin  56 . 
     The absorption of sulphur dioxide in interior  36  of absorber  30  is assumed to occur according to the following reaction: 
       SO 2 (g)+H 2 O=&gt;HSO 3   − (aq)+H + (aq)   [eq. 1.1a]
 
     The bisulphite ions, HSO 3   − , may, depending on the pH value of the effluent seawater ES, dissociate further to form sulphite ions, SO 3   2− , in accordance with the following equilibrium reaction: 
       HSO 3   − (aq)&lt;=&gt;SO 3   2− (aq)+H + (aq)   [eq. 1.1b]
 
     Hence, as an effect of the absorption of sulfur dioxide, the effluent seawater ES will have a lower pH value as an effect of the hydrogen ions, H + , generated in the absorption of sulfur dioxide, than that of the fresh seawater SW from the ocean  50 , and will contain bisulphite and/or sulphite ions, HSO 3   −  and SO 3   2− , respectively. Bisulphite and/or sulphite ions are oxygen demanding substances, and the release thereof to the ocean  50  is restricted. 
     In the seawater aeration basin  56 , the bisulphite and/or sulphite ions, HSO 3   −  and/or SO 3   2− , are oxidized by reacting the same with oxygen A, in accordance with the following reactions: 
       HSO 3   − +H + +1/2O 2 (g) =&gt;SO 4   2− +2H +   [eq. 1.2a]
 
       SO 3   2− +2H + +1/2O 2 (g)=&gt;SO 4   2− +2H +   [eq. 1.2b]
 
     The seawater aeration basin  56  includes an aeration fan  62  operative for blowing, via fluidly connected ductwork  64 , an oxygen containing gas, such as air, into the effluent seawater ES therein. The aeration fan  62  and the ductwork  64  together form an oxygen supply system  66  for supplying oxygen A to the effluent seawater ES in the seawater aeration basin  56 . A more detailed description of the seawater aeration basin  56  is provided hereinafter with reference to  FIG. 2 . 
       FIG. 2  illustrates the seawater aeration basin  56  in more detail. Effluent seawater ES is supplied to the seawater aeration basin  56  portion of seawater treatment system  80  via fluidly connected effluent pipe  54  at a first end  90 , being an inlet end of seawater aeration basin  56 . The effluent seawater ES flows, generally horizontally as indicated by arrow S, from the first end  90  to a second end  92 , being an outlet end of seawater aeration basin  56 . As effluent seawater ES flows from the first end  90  to second end  92  a foam HM is generated on a surface  82  of effluent seawater ES near second end  92 . Foam HM carries a relatively high concentration of heavy metals such as mercury and the like. At the second end  92 , treated effluent seawater TS overflows from seawater aeration basin  56  via fluidly connected overflow pipe  68 . 
     Aeration system  80  further includes the oxygen supply system  66  with aeration fan  62  and ductwork  64 , The ductwork  64  comprises a number of outlets  94  within interior  84  of seawater aeration basin  56 . Aeration fan  62  blows oxygen A through ductwork  64  for release from outlets  94  below effluent seawater ES surface  82  in seawater aeration basin  56 . The ductwork  64  extends along the seawater aeration basin  56 , between the first end  90  and the second end  92  thereof. Oxygen A blown by high pressure side  86  of aeration fan  62  and released from outlets  94  mixes with the effluent seawater ES in seawater aeration basin  56 . Oxygen A is dispersed in and mixed with effluent seawater ES to oxidize bisulphite and/or sulphite ions present therein to form inert sulfates in treated seawater TS prior to environmental release of the treated seawater TS via overflow pipe  68  into ocean  50 . 
     As noted above, effluent seawater ES flows, generally horizontally as indicated by arrow S, from the first end  90  to the second end  92 , being an outlet end of seawater aeration basin  56 . As effluent seawater ES flows from the first end  90  to second end  92  foam HM is generated by turbulence and aeration on surface  82  of effluent seawater ES. As a result of effluent seawater ES flowing from the first end  90  to second end  92  foam HM builds or collects near second end  92 . Foam HM carries a relatively high concentration of heavy metals such as mercury and the like. 
     To prevent foam HM from flowing from seawater aeration basin  56  via overflow pipe  68  into ocean  50 , low pressure side  88 , i.e., the suction side, is fluidly connected via suction duct  96  to a foam collection tank  98 . Also fluidly connected to foam collection tank  98  is foam duct  100 . Foam duct  100  fluidly connects to foam collection tank  98  via outlet  102 . Opposite outlet  102  of foam duct  100  is free open end  104 . Free open end  104  of foam duct  100  is operable to remove via suction from aeration fan  62  foam HM from surface  82 . Foam HM so removed from surface  82  collects in foam collection tank  98  for storage and treatment prior to disposal. Collection of foam HM using aeration fan  62  may be conducted on a continuous, scheduled periodic, detector initiated “as-needed” periodic or like non-continuous basis. 
     As an option, foam HM collected in foam collection tank  98  may be treated to remove at least a portion of the relatively high concentration of foam heavy metals prior to release of the treated foam TF. As such, treated foam TF may be released from the foam collection tank  98  via a fluidly connected treated foam pipe  106  fluidly connected to overflow pipe  68  for return of treated foam TF to the ocean  50 . 
     Alternatively, foam HM collected in foam collection tank  98  may be transported to additional power plant  10  equipment (not shown) for removal of at least a portion of the relatively high concentration of foam heavy metals prior to release of the treated foam TF or use of the treated foam TF elsewhere in the power plant  10  or offsite. 
     In summary, the subject disclosure provides a system and method for controlling foam comprising heavy metals generated on the surface of effluent seawater during aeration of the effluent seawater in a flue gas desulfurization system associated seawater aeration basin. As such, the subject foam control system comprises a seawater aeration basin fluidly connected to an aeration fan operable to blow an oxygen containing gas into effluent seawater in the seawater aeration basin for aeration of the effluent seawater and operable to suction foam from a surface of effluent seawater in the seawater aeration basin. Suction of foam from the surface of the effluent seawater using the aeration fan may be conducted on a continuous, scheduled periodic, detector initiated “as-needed” periodic or other non-continuous basis. A foam collection tank is used for collection, storage and optionally treatment of the foam comprising heavy metals such as mercury, suctioned from the surface of the effluent seawater. As the suctioned foam comprises heavy metals such as mercury, treatment of the suctioned foam in the foam collection tank comprises removing at least a portion of heavy metals therefrom. For this purpose, suctioned foam may be stored for a time in the foam collection tank prior to and/or after treatment thereof. Alternatively, the suctioned foam may be collected and optionally stored for a time in the foam collection tank prior to transport of the suctioned foam for treatment elsewhere in the power plant or offsite. 
     The subject method for controlling foam comprising heavy metals comprises providing a seawater aeration basin fluidly connected to an aeration fan operable to blow an oxygen containing gas into effluent seawater for aeration of the effluent seawater in the seawater aeration basin and operable to suction foam comprising heavy metals such as mercury from a surface of effluent seawater in the seawater aeration basin. Suctioning of foam from the surface of the effluent seawater using the aeration fan may be conducted on a continuous, scheduled periodic, detector initiated “as-needed” periodic or other non-continuous basis. A foam collection tank is used for collection, storage and optionally treatment of the suctioned foam comprising heavy metals such a mercury, suctioned from the surface of the effluent seawater. As the suctioned foam comprises heavy metals such as mercury, treatment of the suctioned foam in the foam collection tank comprises removing at least a portion of heavy metals therefrom. For this purpose, suctioned foam may be stored for a time in the foam collection tank prior to and/or after treatment thereof. Alternatively, the suctioned foam may be collected and optionally stored for a time in the foam collection tank prior to transport of the suctioned foam for treatment elsewhere in the power plant or offsite. 
     While the present invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may 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 embodiments 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. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 
     It will be appreciated that numerous modifications of the embodiments described above are possible within the scope of the appended claims.