Abstract:
Disclosed is a method for removing unburned carbon from fly ash at low cost and within a short time. The method comprises the steps of adding a collecting agent to fly ash directly, agitating/mixing the mixture in a mixer ( 5 ), adding water to the resulting mixed material in a mixing vessel ( 7 ) to yield a slurry, applying a shearing force to the slurry in a submerged stirrer ( 9 ), and performing flotation separation of unburned carbon in a flotator ( 15 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    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. 
         [0003]    2. Description of the Related Art 
         [0004]    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. 
         [0005]    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. 
         [0006]    In order to expand the range of applications and amount of fly ash that is usable, it is necessary to improve the quality by removing the unburned carbon from fly ash, which will then lead to the expansion of applications such as a cement admixture, for example. 
         [0007]    Therefore, the applicant has invented the method shown in  FIG. 8 , where slurry is generated by adding water to fly ash  60  in a mixing tank  62 , a shearing force is applied to the slurry in a submerged stirrer  66  and then the unburned carbon in fly ash is efficiently removed in a floatation unit  72  (Refer to Patent document 1). 
         [0000]    Patent document 1: Japan Patent No. 3613357 
         [0008]    In the method described in the above referred to patent document 1, as shown in  FIG. 9(   a ), in order to enable for unburned carbon  91 , which is adherent to fly ash  90  or loose, to reach a capturing agent  92  as shown in  FIG. 9(   b ) by adding shearing force in a submerged stirrer  66  thereto after adding an oil-based capturing agent  92  to slurry that includes a large amount of water  93  that has no affinity to oil content, it has been necessary to either add more energy to the shearing force to eliminate the water  93 , or to add larger amount of capturing agent  92  than the amount of unburned carbon  91  content. 
         [0009]    However, in either method, the issue exists that the cost of removing unburned carbon from the fly ash becomes too expensive, and this is because a driving power for the submerged stirrer  66  increases and longer time mixing is required. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Taking these issues into consideration, the present invention was achieved with the purpose of providing a method for removing unburned carbon from fly ash at low cost and in a short time. 
         [0011]    In order to achieve the above purpose, the present invention relates to a method for removing unburned carbon in fly ash wherein a mixture is generated by adding 0 to 40 wt % of water and a capturing agent to fly ash and mixing it, and a slurry is generated by adding more water to this mixture, then a shearing force is then applied to the slurry, and the unburned carbon in the fly ash is separated by floatation by supplying air while adding a foaming agent to the slurry to which the shearing force is applied and stirring. 
         [0012]    According to the present invention, as the capturing agent is added to the fly ash that contains water content to the extent that it does not become slurry and is stirred and mixed, it is possible to reduce the driving energy required for stirring and mixing as it is not necessary to remove so much water from among fly ash and capturing agent. 
         [0013]    It is desirable for the mixer to be a high speed flow type mixer or a ribbon blender. 
         [0014]    Regarding as the high-speed flow type mixer, paddle-type mixer, an Eirich and a Henshel mixer are all available. 
         [0015]    According to the method for removing unburned carbon in fly ash relating to the present invention, since it is possible to reduce the driving energy of the submerged stirrer and to lower the residual volume of unburned carbon by optimizing the amount of added capturing agent, it is possible to obtain a high quality fly ash at low cost and in a short time. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]      FIG. 1  shows a schematic diagram of a plant system according to the embodiment of the present invention. 
           [0017]      FIG. 2  shows a cross sectional view illustrating the structure of a mixer according to the first embodiment of the present invention. 
           [0018]      FIG. 3  shows a cross sectional view illustrating the structure of a mixer according to the second embodiment of the present invention. 
           [0019]      FIG. 4  shows a cross sectional view illustrating the structure of a mixer according to the third embodiment of the present invention. 
           [0020]      FIG. 5  shows a cross sectional view illustrating the structure of a mixer according to the fourth embodiment of the present invention. 
           [0021]      FIG. 6  shows a cross sectional view illustrating the structure of a modified example of a mixer according to the fourth embodiment of the present invention. 
           [0022]      FIG. 7  shows a schematic diagram of a system in case that two mixers according to the first embodiment of the present invention are connected in series. 
           [0023]      FIG. 8  shows a schematic diagram of a plant system according to the conventional embodiment prior to the present invention. 
           [0024]      FIG. 9  shows a diagram that explains the state of the slurry before and after the shearing force is applied, wherein (a) shows the state before the shearing force is applied and (b) shows the state after the shearing force is applied. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Here the best form of the embodiment according to the present invention is described in reference to  FIG. 1 .  FIG. 1  is a schematic diagram of a plant system for implementing the present invention. 
         [0026]    This system is mainly composed of mixer  5  that adds a capturing agent to fly ash to mix, mixing tank  7  that generates slurry by adding water to the mixed fly ash and capturing agent, submerged stirrer  9  that applies high shearing force to the slurry, adjusting tank  12  that generates air bubbles by adding a foaming agent to the slurry to which the high shearing force was applied, floatation unit  15  that separates unburned carbon by supplying air while stirring the slurry, and adhering the unburned carbon in the fly ash to the air bubble and floating, solid-liquid separator  17  that separates and recovers the fly ash from the sediment separated in floatation unit  15 , and hydro-extractor  23  that recovers the unburned carbon by dehydrating the floatating substances after being separated in the floatation unit  15 . 
         [0027]    Next, the equipments composing the plant system are described in detail below. 
         [0028]    Fly ash tank  1  is a tank that stores fly ash discharged from a coal fired power plant (not shown in the drawings). 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. 
         [0029]    Volumetric feeder  2  is a machine that feeds a certain volume of fly ash from the fly ash stored in the fly ash tank  1  to the mixer  5 , into which a rotary valve, for example, is utilized. 
         [0030]    Capturing agent tank  3  is a tank that stores the capturing agent that is fed to the mixer  5  via pump  4 . As the capturing agent, any of kerosene, diesel oil or heavy oil can be used. 
         [0031]    Mixer  5  is a high-speed flow type mixer used to add the capturing agent directly to the fly ash, to stir and mix them and also to apply shearing force to the mixture. 
         [0032]    Here, since the amount of capturing agent is very small compared to the amount of fly ash, it is desirable to add water for atomizing the capturing agent by less than the 40 wt % that is the limit to the mixture not becoming slurry, or it is desirable to add water by 5 to 15 wt % to the fly ash with regard to suppressing the generation of dust from the fly ash and that the adhesive force among fly ashes should not be too large. 
         [0033]      FIG. 2  shows the first embodiment of this mixer.  FIG. 2  shows a cross sectional view illustrating the structure of the mixer according to the first embodiment. 
         [0034]    Mixer  30 A according to this embodiment is that is so called “paddle-type” mixer and has the structure in which rotation shaft  33  that has multiple L-shape stirring paddles  32  is inserted down into vertically placed hollow cylindrical vessel  31  from above. At the top of the hollow cylindrical vessel  31 , volumetric feeder  2  that feeds a specified amount of fly ash F from the fly ash tank  1  and atomizer  34  that sprays capturing agent G containing water supplied via the pump  4  from capturing agent tank  3  are arranged. 
         [0035]    Fly ash F fed to mixer  30 A is stirred and mixed with capturing agent G by stirring paddle  32  that is rotated by electric motor  35 , then it is applied a shearing force and discharged from the discharge port not shown in the diagram. 
         [0036]    This mixer  30 A has the feature that the structure is simple and it is easy to handle. 
         [0037]      FIG. 3  shows the second embodiment.  FIG. 3  shows a cross sectional view illustrating the structure of mixer according to the second embodiment, and the same parts as those of  FIG. 2  are designated by similar numbers. 
         [0038]    Mixer  30 , according to this embodiment, is that is so called as “Eirich Mixer” (product of Japan Eirich Co.), that has the structure in which agitator  38  having small stirring paddle  37  is inserted at the eccentric position inside the hollow cylindrical vessel  36  placed obliquely from the above. Vessel  36  and agitator  38  rotate mutually in reverse direction. At the top in the vicinity of the bottom surface of vessel  36  placed obliquely, scraper  39  is arranged. 
         [0039]    Fly ash F and capturing agent G fed to mixer  30 B rotate together with the vessel  36  and a high shearing force is applied by stirring paddle  37  of agitator  38  that rotates in reverse direction at the eccentric position. And, since fly ash F and capturing agent G that are carried up to the top of vessel  36  due to its rotation are reversed by the scraper  39 , the vertical mixing in the vessel  36  is accelerated. 
         [0040]    In such a way, this mixer  30 B has the feature that it is possible to execute dense and uniform mixing and, at the same time, to apply high shearing force. 
         [0041]      FIG. 4  shows the third embodiment of the mixer.  FIG. 4  shows a cross sectional view of the mixer according to the third embodiment, and the same parts as those of  FIG. 2  are designated by similar numbers. 
         [0042]    Mixer  30 C according to this embodiment is that is so called as “Henshel Mixer” (product of Mitsui Miike Kakoki, Co.) that has the structure in which L-shape stirring paddle  44  installed to the top of rotary shaft  43  is arranged near the bottom surface of the vertically placed hollow cylindrical vessel  42 . Rotary shaft  43  is connected to driver  45  passing vertically through the bottom surface of the vessel  42 . At the lower side surface of the vessel  42 , discharge port  46  is attached obliquely downward. 
         [0043]    Fly ash F and capturing agent G fed to this mixer  30 C are discharged through discharge port  46  after convection stirring and mixing are executed on them and a shearing force is applied to them with stirring paddle  44  that rotates at high speed. 
         [0044]    This mixer  30 C has the feature that it is possible to execute mixing and applying a shearing force in a short time. 
         [0045]      FIG. 5  shows the fourth embodiment of the mixer.  FIG. 5  shows a pattern diagram illustrating the structure of the mixer of the fourth embodiment according to the present invention. 
         [0046]    Mixer  50  according to the present invention is that is so called as “Ribbon Blender”. This mixer is composed of main body  51  that takes a rectangular solid of which bottom surface is shaped in convex and like a semicyclinder, and rotary shaft  52  installed in parallel to the direction of axis inside the main body  51 . At rotary shaft  52 , ribbon like paddles  53   a  and  53   b  that transfer the objective substance mutually in reverse direction are installed so as to spirally surround the rotary shaft  52 . The top surface of the main body  51  is connected to volumetric feeder  2  that feeds a given amount of fly ash, and to the inner-top part of the main body  51 , atomizing tube  54  for spraying capturing agent G is installed in parallel to the direction of axis. 
         [0047]    Here, the atomizing tube  54  may be arranged inside the rotary shaft  52  as shown in  FIG. 6 . 
         [0048]    Fly ash F fed to this mixer  50  is moved like a shape of 8 looking at from the side view, and is stirred and mixed together with capturing agent G sprayed from the atomizing tube  54  installed at the upper-top part or inside of the rotary shaft, and finally is discharged from discharge port  56 . 
         [0049]    This mixer  50  has the feature that the structure is simple and it is possible to process high volume of fly ash. 
         [0050]    In any mixers according to the embodiments described above, it is desirable to compose mixers with multiple stages by connecting multiple mixers in series so as to enable to continuously and effectively mix of fly ash and capturing agent. 
         [0051]      FIG. 7  shows a configuration example that serially connects mixer  5  with multiple stages.  FIG. 7  corresponds to the case where two mixers  30 A according to the first embodiment are connected, in which similar numbers are given to the same parts as those of  FIG. 2 . 
         [0052]    Fly ash F and capturing agent G fed to the first stage mixer are mixed in the mixer  30 A and, after a shearing force is applied to, the mixture is fed to the second stage mixer through discharge port  57 . Mixture F fed to the second stage mixer is mixed again with capturing agent G fed through tube  58  and a shearing force is applied again. 
         [0053]    Here, a method for connecting the mixers is not limited to the one shown in  FIG. 7 , but it is of course good to connect three or more mixers depending on an objective performance, or to connect a plurality of the mixers according to other embodiments to the ones of the same embodiment or to the ones of the different embodiments. 
         [0054]    Mixing tank  7  adjusts slurry by mixing fly ash and capturing agent that are mixed in mixer  5  by mixing with water fed from water feeder  6 . 
         [0055]    Submerged stirrer  9  applies a high shearing force to the slurry that is adjusted in the mixing tank  7  by stirring with high speed to modify unburned carbon&#39;s surface. 
         [0056]    Adjusting tank  12  is a tank in which foaming agent fed through pump  11  from foaming agent tank  10  is added and mixed to the slurry discharged from submerged stirrer  9 , by which the slurry becomes to a state that is possible to easily generate air bubbles. 
         [0057]    Floatation unit  15  is a machine that separates unburned carbon by supplying air to the slurry fed through pump  13  while stirring to adhere the unburned carbon to air bubbles, and to float the air bubbles to the surface. As the method for supplying air used at this time, such methods that absorb air by rotation of stirrer in floatation unit  15 , or although it is not shown in diagrams, that forcibly inject air by air supplying equipment (such as blower) are desirable. 
         [0058]    The unburned carbon separated as the floating substances in the floatation unit  15  is transferred through pipe  22  to hydro-extractor  23 . The slurry from which unburned carbon is separated and recovered as the sediment in the floatation unit  15  is transferred through pump  16  to solid-liquid separator  17 . 
         [0059]    Solid-liquid separator  17  is a machine that separates the slurry to fly ash and water. Separated fly ash is transferred to dryer  18  as a cake, and separated water is returned through circulation tube  27  by pump  26  to mixing tank  7  to be recycled as water for generating slurry. 
         [0060]    Dryer  18  is a machine to dry the fly ash as a cake with hot air generated by air heating furnace  21 . The fly ash after drying from which unburned carbon is separated becomes fly ash  20  as a commercial product and utilized as cement admixture, etc. 
         [0061]    Bag filter  19  is a machine that recovers the fine powder of fly ash generated during the drying process in the dryer  18  by conducting the filtration dust collection, and the recovered fly ash also becomes fly ash  17  as a commercial product. 
         [0062]    Hydro-extractor  23  is a machine that dehydrates the unburned carbon separated as the floating substances in the floatation unit  15 . Examples of hydro-extractor  23  include, filterpress and the like. In this case, the floating substances are dehydrated by pressurizing with a filter. 
         [0063]    Since unburned carbon  25  after being dehydrated can be used as a fuel, a part of this is supplied to air heating furnace  21  to generate hot air used in the dryer  18 . 
         [0064]    The water separated in the hydro-extractor  23  is transferred to the circulation tube  27  to be recycled in the mixing tank  7  similarly to the water separated from the solid-liquid separator  17 . 
         [0065]    Next, a method for separating unburned carbon from fly ash that uses the above described system is described in reference to  FIG. 1 . 
         [0066]    Fly ash is cut out from fly ash tank  1  by the volumetric feeder  2  and input to the mixer  5 , and it is stirred and mixed with a capturing agent fed from capturing agent tank  3 . Then, a shearing force is applied to this mixture. 
         [0067]    At this time, in the mixer  5 , since powdered fly ash is directly stirred and mixed, there exists no water differently from the case of slurry, driving energy to eliminate water from the mixture of fly ash and capturing agent is not required and, at the same time, added capturing agent can easily reach to the fly ash. Also, since a shearing force is applied to the unburned carbon contained in fly ash while stirring, and a part of shearing force that should be applied in the submerged stirrer  9 , which is a downstream process, can be borne on the unborned carbon, it is possible to reduce the driving energy of the submerged stirrer  9  to this extent. 
         [0068]    The amount of capturing agent added in the mixer  5  is set to 0.05 to 10 wt % for fly ash when kerosene is used as a capturing agent. 
         [0069]    Due to the very small amount of capturing agent for fly ash, water for atomizing capturing agent should be added by less than 40 wt % for fly ash, or more preferably, by 5 to 15 wt %. 
         [0070]    Since a shearing force of 10 to 50 kWh/m 3  per unit weight is applied, or more preferably, 20 to 40 kWh/m 3  is applied to fly ash, it is possible to reduce the driving energy of the submerged stirrer  9  by about 50%. 
         [0071]    In this way, the mixture of fly ash and capturing agent that is stirred and mixed and to which a shearing force is applied is transferred to the mixing tank  7 . 
         [0072]    In the mixing tank  7 , slurry is generated by adding water to the mixture from water feeder  6  to mix. Concentration of generated slurry at this time is desirable to be in the range of 10 to 30 wt %. 
         [0073]    Then, a high shearing force is applied to this slurry by mixing and stirring up in the submerged stirrer  9 . Owing to the high shearing force, the surface of the unburned carbon contained in the slurry is modified, an affinity to capturing agent  37  is increased and a floatability to separate by floatation of the unburned carbon is increased in floatation unit  15  in the later stage process. 
         [0074]    Next, the slurry applied high shearing force is transferred to adjusting tank  12  to add a foaming agent and mix so as to make the slurry to easily generate air bubbles. The slurry is stirred and supplied air in the floatation unit  15 , then the unburned carbon contained in fly ash  36  is separated while adhering to air bubbles with capturing agent  37  and floating. 
         [0075]    Since the unburned carbon separated as the floating substances by this means contains much water, it is dehydrated in the hydro-extractor  23  so that it may be utilized as a fuel. 
         [0076]    Fly ash  20  as a commercial product can be also obtained by recovering the slurry from which the unburned carbon was separated as the sediment and by drying in dryer  18  after separating the water in the solid-liquid separator  17 . The yield of fly ash  20  as a commercial product can be increased by recovering the fine powder of fly ash in the dryer  18  by using the bag filter  19 . 
         [0077]    As for the water separated in the solid-liquid separator  17  and the hydro-extractor  23 , it is transferred through the circulation tube  27  to the mixing tank  7  to recycle for generating slurry.