Abstract:
The object is to remove unburned carbon in a fly ash in a stable and economically advantageous manner. A fly ash, water and a trapping agent are mixed together in a hybrid mixer ( 2 ), a shearing force is applied to the mixture to prepare a slurry containing surface-modified unburned carbon within a short time, a foaming agent is added to the slurry, and then the unburned carbon is separated by performing flotation separation in a flotator ( 11 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for removing unburned carbon from fly ash, in particular a method for removing unburned carbon contained in fly ash discharged from coal fired power plants. 
     2. Description of the Related Art 
     Coal can be stably utilized as an energy source in the long term as the ratio of its proven reserves to annual production is more than 200 years. Therefore, the ratio of coal fired power generation to total power generation has been increasing year by year and the amount of coal ash generated (hereinafter referred to as “fly ash”) is expected to increase in the future. 
     In such circumstances, the large amount of fly ash needs to be efficiently utilized from the viewpoints of environmental conservation and the effective utilization of resources. 
     At present, fly ash is used as a cement admixture after having the unburned carbon removed from it. The quality of fly ash can be improved by increasing the removal rate of unburned carbon, which then enables the amount of fly ash used to be increased. 
     Therefore, the applicant has invented the method shown in  FIG. 8 , where slurry is generated by adding water to fly ash  61  in a mixing tank  62 , a shearing force is applied to the slurry in a submerged stirrer  63  and then the unburned carbon in fly ash is efficiently removed in a floatation unit  67  (Refer to Patent document 1).
     Patent document 1: Japan Patent No. 3613347   

     In the method described in Patent document 1, however, there has been concern that during the process of preparing slurry by adding water to fly ash, the required amount cannot be attained because fly ash adheres inside the feeding pipe and mixing tank, and moreover operation must be stopped because of the clogging of the pipe. 
     Specifically, when slurry  89  is attempted to be generated by stirring fly ash  87  and water  88  in the mixing tank  62  shown in  FIG. 9 , there has been problems that the fly ash  87  and the water  88  is insufficiently mixed because of the fly ash  87  adhering around the rotary shaft  81  of stirring blades  82  and that the fly ash  87  becomes wet because of water or water vapor in the mixing tank  52 , resulting in clogging of a feeding pipe  84 . 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in view of such problems and the objective of the invention is to provide a method for efficiently and easily removing unburned carbon from fly ash. 
     The invention according to Claim  1  and in order to achieve the above-mentioned objective is a method for removing unburned carbon from fly ash, wherein slurry is prepared by mixing fly ash and water in a hybrid mixer; a shearing force is applied to the slurry while adding a capturing agent; the unburned carbon is separated from fly ash by flotation in such a way that the slurry to which a shearing force was applied is stirred while supplying air thereto after adding a foaming agent the slurry. 
     The invention according to Claim  2  is a method for removing unburned carbon from fly ash, wherein slurry is prepared by mixing fly ash and water in a hybrid mixer; a shearing force is applied to the slurry while adding a capturing agent and a foaming agent thereto; the unburned carbon is separated from fly ash by flotation by stirring the slurry to which a shearing force was applied while supplying air thereto. 
     The invention according to Claim  3  is the method for removing unburned carbon from fly ash described in Claim  1  or  2 , wherein the hybrid mixer is composed of a lateral cylindrical mixer body, a cylindrical body that communicates with one end of the mixer body and a volumetric feeder that communicates with the cylindrical body and is erected thereon; the mixer body is provided with a rotary shaft on which a plurality of stirring blades are mounted at constant intervals, baffle plates disposed on the inner surface of the mixer body in such a way that each babble plate is located between the stirring blades, supply ports that supply water and a capturing agent into the mixer body respectively and an exhaust air port and a discharge port on the other end of the mixer body; the cylindrical body is provided with a screw feeder coupled to the rotary shaft; and wherein fly ash is supplied to the cylindrical body by the volumetric feeder, the supplied fly ash is fed into the mixer body by the screw feeder, the fed fly ash is mixed and stirred between the stirring blades and the baffle plates after adding the water and the capturing agent thereto, and then is discharged through the discharge port. 
     The invention according to Claim  4  is the method for removing unburned carbon from fly ash described in Claim  3 , wherein the volumetric feeder is composed of a hopper for receiving fly ash and a rotary valve for cutting off the predetermined amount of the fly ash from the hopper. 
     The invention according to Claim  5  is the method for removing unburned carbon from fly ash described in Claim  3 , wherein the volumetric feeder is a screw feeder erected on the cylindrical body. 
     The invention according to Claim  6  is the method for removing unburned carbon from fly ash described in Claim  3 , wherein the cylindrical body is provided with a first screw feeder coupled to the rotary shaft and a second screw feeder disposed in parallel with the first screw feeder and having a communication part with the mixer body as the end thereof. 
     The invention according to Claim  7  is the method for removing unburned carbon from fly ash described in Claim  3 , wherein the cylindrical body is provided with a screw feeder that is rotated by a driving means different from that of the rotary shaft. 
     According to a method for removing unburned carbon from fly ash in accordance with the present invention, fly ash does not adhere inside a pipe or a mixing tank and furthermore it does not clog the pipe. 
     In addition, both the preparation of slurry and the application of a shearing force to the slurry can be performed in a single machine, thus enabling stable operation of the machine to be maintained, as well as reducing the cost of equipment and the area for installation. Therefore, an economically advantageous method for removing unburned carbon from fly ash can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a plant system according to an embodiment of the present invention. 
         FIG. 2  is a cross sectional view illustrating a structure of a hybrid mixer shown in  FIG. 1 , according to a first embodiment. 
         FIG. 3  is a plane view illustrating an example of a stirring blade of a hybrid mixer. 
         FIG. 4  is a cross sectional view illustrating a structure of a hybrid mixer shown in  FIG. 1 , according to a second embodiment. 
         FIG. 5  are cross sectional views illustrating a structure of a hybrid mixer shown in  FIG. 1 , according to a third embodiment. 
         FIG. 6  is a cross sectional view illustrating a structure of a hybrid mixer shown in  FIG. 1 , according to a fourth embodiment. 
         FIG. 7  is a schematic diagram of a plant system according to another embodiment of the present invention. 
         FIG. 8  is a cross sectional view illustrating a structure of a hybrid mixer shown in  FIG. 7 . 
         FIG. 9  is a schematic diagram of a conventional plant system. 
         FIG. 10  is a cross sectional view of a mixing tank according to a conventional plant system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment according to the present invention will be described in reference to  FIG. 1 .  FIG. 1  shows a schematic diagram of a plant system for implementing the present invention. 
     The system is mainly composed of a hybrid mixer  2  that prepares slurry from fly ash and water and applies a high shearing force to the slurry while adding a capturing agent thereto; a floatation unit  11  that separates unburned carbon from fly ash by adding a foaming agent to the slurry generated in the hybrid mixer  2  to generate air bubbles and then making the unburned carbon in the fly ash attach to those air babbles and float on with them; a solid-liquid separator  14  that separates the fly ash from sediment separated by the floatation unit  11  to recover it; and a dehydrator  20  that recovers the unburned carbon by dehydrating floating substances separated by the floatation unit  11 . 
     Main pieces of equipment composing the system are described in detail below. 
     A fly ash tank  1  is provided for storing and supplying fly ash discharged from a coal fired power plant (not shown). Unburned carbon that remains unburned upon combustion of coal in a boiler of a coal fired power plant adheres to or is contained in the fly ash. 
     A capturing agent tank  5  is provided for storing and supplying a capturing agent and the capturing agent is supplied via a capturing agent pump  6 . As the capturing agent, kerosene, diesel oil or heavy oil may be used. 
     The hybrid mixer  2  is provided for mixing fly ash supplied via a volumetric feeder  3  from the fly ash tank  1  and water supplied from a water feeder  4  to prepare slurry, as well as for adding the capturing agent from the capturing agent tank  5  to the slurry and stirring them, thus applying a high shearing force to the slurry and modifying the surface of the unburned carbon. 
       FIG. 2  shows a hybrid mixer according to a first embodiment.  FIG. 2  shows a cross sectional view illustrating a structure of the hybrid mixer  2  according to the first embodiment and also shows a volumetric feeder  3 . 
     The hybrid mixer  2  is composed of a lateral cylindrical mixer body  30 ; a cylindrical body  31  that communicates with the mixer body  30  at its one end; and a volumetric feeder  3  composed of a hopper  50  and a rotary valve  51 . 
     The mixer body  30  is provided for mixing fly ash and water to prepare slurry, as well as for adding a capturing agent to the slurry, mixing and stirring the slurry with rotary blades thus applying a high shearing force to the slurry. For this purpose, a plurality of stirring blades  33  are erected at constant intervals in the outward radial direction on a rotary shaft  34  disposed in the center of the mixer body  30 , and baffle plates  38  are erected in the inward radial direction on the inner surface of the mixer body  30  in such a way that each baffle plate  30  is located between two adjacent stirring blades  33 . 
     As shown in  FIG. 3 , for example, a discoid shape stirring blade  33  provided with turbine blades  37  on its surface can increase the efficiency of mixing and stirring, although various shapes as the stirring blade may be considered. 
     The cylindrical body  31  is provided for supplying fly ash to the mixer body  30  and has a lateral screw feeder  32  coupled to the rotary shaft  34  inside. 
     On the axial prolongation of the cylindrical body  31  and the mixer body  30 , provided are an electric motor  35  that is a driving means for rotating the rotary shaft  34  and a reduction gear  36 . 
     The hopper  50  and the rotary valve  51  composing the volumetric feeder  30  are erected almost vertically on the upper part of the cylindrical body  31 . 
     Operation of the hybrid mixer  2  according to the embodiment will be described below. 
     Fly ash from the fly ash tank  1  is received in the hopper  50  and the predetermined amount of the fly ash is cut off by the rotary valve  51  and fed to the lateral screw feeder  32  in the cylindrical body  31 . With the rotation of the lateral screw feeder  32  driven by the electric motor  35 , the fly ash is gradually supplied into the mixer body  30 , where the fly ash is mixed with water B supplied through a medium liquid supply port  39  from a water feeder  4  to generate slurry  40 . 
     To the generated slurry  40 , a capturing agent C is added through a capturing agent supply port  44  from the capturing agent tank  5  and the slurry  40  to which the capturing agent was added is mixed and stirred vigorously with the stirring blades  33 , thus applying a high shearing force to the slurry  40 . When the slurry  40  being mixed and stirred, the fly ash does not adhere to the rotary shaft  34  since the slurry  40  is pressed against the inner surface of the mixer body  30  due to centrifugal force of the rotary shaft  34 , resulting in a gap  41  around the rotary shaft  34 . 
     The slurry  40  gradually moves downward while being mixed and stirred between the stirring blades  33  and the baffle plates  38  as the baffle plates  38  prevents the slurry  40  from short-passing, and finally being discharged as slurry D through a discharge port  43 . Gas components such as air moving along with the fly ash will be discharged outside through an exhaust air port  42 . 
     Due to such a structure, the fly ash before being supplied into the mixer body  30  is separated from water and air containing water vapor by the lateral screw feeder  32 . Therefore, there is little possibility that the fly ash may become wet and clog the rotary valve  51 , the cylindrical body  31  and the like. Furthermore, as described above, this structure can prevent the disadvantageous case where the generated amount of slurry  40  is insufficient because of the fly ash adhering to the rotary shaft  34 . 
       FIG. 4  shows a hybrid mixer  2  according to a second embodiment.  FIG. 4  shows a cross sectional view of the hybrid mixer  2  according to the second embodiment and the same symbols are given to the same parts as those shown in  FIG. 2 . 
     The hybrid mixer  2  according to the present embodiment differs from the hybrid mixer  2  according to the first embodiment shown in  FIG. 2 , in that an inclined screw feeder  53  having a solid supply port  52  on its upper part composes a volumetric feeder  3 . 
     The inclined screw feeder  53  is erected in an inclined state or almost vertically on a cylindrical body  31  and an electric motor  54  for rotating the inclined screw feeder  53  is disposed on its uppermost part. 
     Operation in a mixer body  30  is the same as that of the first embodiment. The method for supplying fly ash to the mixer body  30  will be described below. 
     Fly ash A from a fly ash tank  1  is supplied through the solid supply port  52  to the inclined screw feeder  53 , and then fed to a cylindrical body  31  in a lower part with the rotation of the screw feeder  53 . In the cylindrical body  31 , fly ash is fed to a lateral screw feeder  32  and is gradually supplied into the mixer body  30  with the rotation of the lateral screw feeder  32 . 
     In the hybrid mixer  2  according to the embodiment, the two screw feeders  32  and  58  ensure that the fly ash can be supplied to the mixer body  30  without the fly ash being deposited to inner walls. 
       FIG. 5  show a hybrid mixer  2  according to a third embodiment.  FIG. 5  show cross sectional views illustrating a structure of the hybrid mixer  2  according to the third embodiment;  FIG. 5(   a ) is its side cross sectional view and  FIG. 5(   b ) is its top cross sectional view. The same symbols are given to the same parts as those shown in  FIG. 2 . Some parts of the structure are omitted. 
     The hybrid mixer  2  according to the present embodiment, as seen in  FIG. 5(   a ), has the same side cross sectional view as that of the first embodiment shown in  FIG. 2 . However, it differs from the hybrid mixer of the first embodiment, as shown in  FIG. 5(   b ), in that a second screw feeder  45  is horizontally provided in parallel with a lateral screw feeder  32  inside a cylindrical body  31 . 
     This second lateral screw feeder  45  extends from the end of the cylindrical body  31  to the vicinity of the inlet of a mixer body  30  and is coupled to the lateral screw feeder  32  by a rotary shaft  34  and a gear  46  so that both lateral screw feeders are interlocked. As the means for coupling, a belt may be used. 
     Fly ash supplied via a rotary valve  51  from a hopper  50  is thrown between the two lateral screw feeders  32  and  45 . 
     This structure ensures that the fly ash from the hopper  50  can be supplied to the mixer body  30  by the second lateral screw feeder  45  even if, for example, fly ash becomes wet and adheres to the lateral screw feeder  34 , resulting in clogging. 
       FIG. 6  shows a hybrid mixer  2  according to a fourth embodiment.  FIG. 6  shows a cross sectional view illustrating a structure of the hybrid mixer  2  according to the fourth embodiment and the same symbols are given to the same parts as those shown in  FIG. 2 . 
     The hybrid mixer  2  according to the present embodiment is characterized in that a lateral screw feeder  32  in a cylindrical body  31  is not coupled to a rotary shaft  34  on which stirring blades  33  are mounted in a mixer body  30  and they rotates separately in such a way that the lateral screw feeder  32  is rotated by an electric motor  35  and the rotary shaft  34  is rotated by an electric motor  49 . 
     Since this structure enables slurry  40  to be prepared in the mixer body  30  regardless of the amount of fly ash A supplied to a hopper  50 , the amount and concentration of the slurry can be easily adjusted depending on the operating status of plant equipment on the downstream side that processes slurry D to which a shearing force was applied, thus helping to streamline the operation of the plant. 
     An adjusting tank  7  is provided for adding a foaming agent supplied via a pump  9  from a foaming agent tank  8  to the slurry discharged from the hybrid mixer  2  and mixing them with stirring blades, thus causing the slurry to be in a state liable to generate air bubbles. 
     A floatation unit  11  is provided for separating unburned carbon in such a way that it stirs slurry fed via a pump  10  to take in air in the atmosphere, generating air bubbles and causing the unburned carbon to attach to those air bubbles and float on with them. Furthermore, the amount of air bubbles to be generated can be adjusted by blowing air into through the bottom part of the floatation unit  11  from an air feeder  12 . 
     Unburned carbon separated as a floating substance in the floatation unit  11  is fed through a pipe  19  to a dehydrator  20  while slurry after having unburned carbon separated is recovered as sediment in the floatation unit  11  and fed via a pump  13  to a solid-liquid separator  14 . 
     The solid-liquid separator  14  is provided for separating the slurry into fly ash and water. The separated fly ash is fed to a drier  15  as a cake while the separated water is returned through a circulation pipe  24  by a pump  23  to the hybrid mixer  2  to be reused as water for generating slurry. 
     A drier  15  is provided for drying the fly ash as a cake with hot air generated in a hot air furnace and the dried fly ash becomes fly ash  17 , i.e. a product after having unburned carbon separated and is used as a cement admixture and the like. 
     A bag filter  16  is provided for filtering and collecting fine powder of fly ash generated during the drying process in the drier  15  to recover it. The recovered fly ash also becomes the fly ash  17  as a product. 
     A dehydrator  20  is provided for dehydrating the unburned carbon separated as a floating substance in the floatation unit  11 . The dehydrator  20  includes a filter press by way of example. In a filter press, floating substances would be dehydrated through compression by a filter. 
     Dehydrated unburned carbon  22  can be used as fuel and a part of the unburned carbon  22  is supplied to the hot air furnace  18  as fuel and used for generating hot air for the drier  15 . 
     Processes after the drier  15  can be left out depending on the required degree of dryness of fly ash. 
     The water separated in the dehydrator  20  is fed to a circulation pipe  24  and reused in the hybrid mixer  2  in the same manner as the above-mentioned water separated in the solid-liquid separator  14 . 
     A method for separating unburned carbon from fly ash by using the above-mentioned system will now be described with reference to  FIGS. 1 and 2 . 
     Fly ash A is thrown from the fly ash tank  1  to the hopper  50  in the volumetric feeder  3  and the predetermined amount of the fly ash is cut off by operating the rotary valve  51 , supplied to the cylindrical body  31  and then supplied to the mixer body  30  by the lateral screw feeder  32 . 
     Water B is supplied from the water feeder  4  through a medium liquid supply port  39  to the mixer body  30  to prepare slurry  40 . The concentration of the slurry is preferably within a range of 10 to 40 weight percent. 
     After that, a capturing agent C is added from the capturing agent tank  5  through a capturing agent port  44  to the slurry  40 . If, for example, kerosene is used as the capturing agent, the amount of the capturing agent is preferably within a range of 0.01 to 3.0 weight percent of solid content in slurry. 
     A high shearing force is applied to the slurry  40  by sufficiently mixing and stirring them with the stirring blades  33 . This high shearing force enables the surface of unburned carbon contained in solid-liquid mixture to be modified, thus increasing the affinity of the unburned carbon to a capturing agent, resulting in improving the floatability of the unburned carbon in the floatation unit  11  used as a later process. 
     In order efficiently to mix and stir the slurry, the total volume of the fly ash, water and the capturing agent is preferably within a range of 40 to 90 percent of the internal volume of the mixer body  30 . 
     Thus, the slurry D to which a high shearing force was applied is fed to the adjusting tank  7  where adding a foaming agent and mixing them cause the slurry D to be in a state liable to generate air babbles. Then in the floatation unit  11 , the unburned carbon is separated by stirring the slurry to takes in air, thus making the unburned carbon and the capturing agent attach to air bubbles and float on with them. 
     Since the unburned carbon separated as a floating substance in this manner contains a large amount of moisture, it is dehydrated in the dehydrator  20  for fuel use. 
     In addition, the slurry after having the unburned carbon separated is recovered as sediment; water is separated from the slurry in the solid-liquid separator  14 ; and the obtained slurry is dried in the drier  15  to obtain the fly ash  17  as a product. The yield of the fly ash  17  as a product can be increased by recovering fly ash remaining in fine powder form in the drier  15 , using the bag filter  16 . 
     The water separated in the solid-liquid separator  14  and the dehydrator  20  is fed through the circulation pipe  24  to the hybrid mixer  2  to be reused as water for generating slurry. 
       FIG. 7  shows a schematic diagram of a plant system according to another embodiment of the present invention and the same symbols are given to the same parts as those shown in  FIG. 1 . 
     In the present embodiment, an adjusting tank  7  is not required since a foaming agent is added in the hybrid mixer  2 . 
       FIG. 8  shows an example of a structure of the hybrid mixer  2  according to the embodiment.  FIG. 8  shows a cross sectional view illustrating a structure of the hybrid mixer  2  according to the embodiment and the same symbols are given to the same parts as those shown in  FIG. 2 . 
     This hybrid mixer  2  has the same basic structure as that of the hybrid mixer  2  shown in  FIG. 2 . However it differs from the hybrid mixer shown in  FIG. 2 , in that a mixer body  30  is provided with a foaming agent supply port  49  for adding a foaming agent from a foaming agent tank  8 . Although the basic structure is described as the hybrid mixer  2  of the first embodiment shown in  FIG. 2 , it should be understood that the hybrid mixer  2  according to the second to fourth embodiments shown  FIGS. 4 to 6  may also be provided with the foaming agent supply port  49 . 
     By using the hybrid mixer  2  having such a structure, adding a foaming agent and mixing them can be performed in the hybrid mixer  2 , thus enabling an adjusting tank  7  to be eliminated and plant equipments to be further streamlined.