Abstract:
A simplified submerged chain conveyor system for handling ash produced by large-scale coal fired boilers. The system incorporates an endless chain conveyor system moving a coal/ash aqueous mixture within a conveyor segment having a hydraulically closed duct. The system is adapted for retrofit applications of existing coal-fired boiler installations. In each embodiment the chain conveyor elevates the aqueous ash solution to dewater the ash. In several embodiments the boiler ash hopper is partially flooded with water and the system moves the ash mixture through a water column to the dewatering section. In one embodiment the ash mixture is not submersed but is subjected to water sprays before reaching the dewatering section. Great flexibility is provided in locating and positioning the conveyor system. One unit may be implemented to provide ash handling for multiple boilers. Embodiments are described operable in continuous or batch mode processes.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a system for handling ash, and particularly to a simplified submerged chain conveyor system called a submerged grind conveyor (“SGC”) for removing bottom ash from large-scale coal fired boilers. 
       BACKGROUND OF THE INVENTION 
       [0002]    The following description of general background of the present invention makes reference to the appended drawing  FIGS. 1 through 4  which show prior art systems. The combustion process of coal in power utility fired boilers produces two types of waste products; 1) ash particles that are small enough to be entrained in the flue gas referred to as fly ash, and 2) relatively large ash particles that overcome drag in the combustion gases and drop to the bottom of the boiler referred to as bottom ash. Typically, bottom ash is either collected in a water impoundment or in a dry bottom. Water impounded ash, referred to as wet bottom ash, is typically collected in individual water filled hoppers, as shown in  FIG. 1  which illustrates a typical bottom ash-to-pond system  10 , or in a closed loop recirculation system  26  shown in  FIG. 2 , or in a water filled trough with a submerged drag chain system  12  as shown in  FIG. 3 . In the system of  FIG. 1 , ash is discharged each shift in a batch process from hoppers  14  through bottom gate  16  on the side of the hoppers  14 . Ash grinders  18  are provided to reduce ash particle size to less than about 3 in. (typically) to allow conveyance in a pipe as an ash/water slurry. The slurry is discharged into a storage pond  20  where the ash settles out over time. Surge tank  30  is provided to handle transient surges in the slurry flow. Numerous pumps  22  and valves  24  are provided for moving the slurry through system  10  (these elements are also shown in  FIGS. 2, 3 and 4 ). 
         [0003]    Closed loop recirculation system  26  shown in  FIG. 2  is a modified form of system  10  and provides closed loop dewatering system and uses a settlement unit referred to by applicant as a “Hydrobin®” unit  28  as shown in FIG.  2 . In the system  26  shown in  FIG. 2 , bottom ash  11  is discharged from hoppers  14  into the grinder  18  and is then pumped as an ash-water slurry to remotely located Hydrobin® dewatering bins  28  which provide a two-stage settling process necessary to clarify the water enough for recycling. Settled ash is drained of water through screens in the dewatering bins  28 . Surge tank  30  and settling tank  32  handle the drained water and provide further clarification and separation of coal ash from the water. Clarified water is recycled back to convey the next batch of ash slurry. Dewatered ash slurry is hauled away from the plant site. 
         [0004]    Systems and  10  and  26  are so-called wet sluicing systems which operate successfully but have a number of drawbacks, principally requiring large amounts of transport water that requires sophisticated treatment as well as significant capital expenditures. 
         [0005]    The submerged mechanical drag conveyor system  12  (or submerged chain conveyor or “SCC”) is illustrated in  FIGS. 3 and 4 , and is typically applied to provide continuous ash removal. Bottom ash continuously falls into the SCC  12  through hopper discharge  42  and settles onto a chain-and-flight conveyor system referred to as a submerged drag chain unit  34 . Unit  12  forms an open trough which is filled with water to quench the dry ash as it falls into the unit from the boiler. The chain of unit  34  moves continuously and carries away ash which is dewatered as it moves along an inclined section  36  and is transported via a conveyor  44  and into a bottom ash silo  38 , and is later discharged into a truck to transport the material off-site. Make-up water is added to offset water loss with wet ash being removed from the system and due to evaporation. Mill reject hoppers  40  are provided to process such material which is directed onto chain conveyor inclined section  36  for processing along with the bottom ash slurry stream. The submerged drag chain conveyor unit  34  is positioned directly beneath the boiler ash hopper discharge  42 . The boiler throat being rectangular shaped requires the orientation of the submerged drag chain conveyor unit  34  and boiler ash hopper discharge  42  to be substantially parallel to the major axis of the boiler throat. Another view of submerged drag chain conveyor unit  12  is shown in  FIG. 4  which further illustrates the conveyor drive unit  46  and take-up unit  48  which provide proper conveyor chain tensioning. In this prior art system, one of the units  12  shown in  FIGS. 3 and 4  is provided for each boiler ash hopper discharge  42 . 
         [0006]    The handling of ash from large-scale coal burning boilers is subject to ever increasingly stringent governmental regulations, including the US EPA&#39;s federal ELG (Effluent Limitations Guidelines) rules. These rules treat different forms of water streams found in bottom ash handling systems in different ways. For example, these rules preclude the discharge into the environment of ash transport water such as used in the pond system  10  shown in  FIG. 1  and in the closed loop hydraulic system  26  shown in  FIG. 2 , and ash basins. Water streams not subject to these ELG requirements (presently) include quench water used in submerged chain conveyor systems  12  and other minor discharges. Retrofitting existing coal-fired boilers to modern ash handling system frequently involves a considerable capital expense. Operators of these systems will often decommission boilers in view of the significant expenses associated with retrofits. 
         [0007]    With the above considerations in mind, boiler operators are often faced with difficult decisions regarding continuing the lifetime of existing installations. Installation of a conventional submerged drag chain system  12  as illustrated in  FIG. 3  ordinarily requires removal of existing bottom ash hoppers  14  and replaced with a rectangular shape trough hopper  42  that can accept a continuous flow of ash. As mentioned previously, make-up water must be added to offset water loss. The water temperature is relatively high in these systems and therefore a cooling system is provided, such as through recirculation to a pond or installation of heat exchangers. Existing SCC systems  12  provide the benefits of not requiring transport water and the equipment cost is relatively low. In addition, maintenance and operating costs are relatively low as compared with wet sluicing systems. However, significant disadvantages are associated with the major reworking of the boiler mentioned above and the significant space requirements of such systems including orientation constraints. Since the system  12  is situated directly beneath the boiler without any isolation valves, a break in the SCC chain or other maintenance issue may require boiler shutdown in order to repair the fault. 
         [0008]    This invention is related to embodiments of simplified submerged chain conveyor systems which are adaptable for retrofit applications which avoid the disadvantages mentioned previously. Several embodiments of the invention are illustrated and described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a typical bottom ash-to-pond ash handling system in accordance with the prior art. 
           [0010]      FIG. 2  illustrates a typical closed loop recirculation system for ash slurry handling in accordance with the prior art. 
           [0011]      FIGS. 3 and 4  illustrate a typical bottom ash submerged drag chain conveyor system in accordance with the prior art. 
           [0012]      FIG. 5  illustrates a SGC in accordance with the present invention. 
           [0013]      FIG. 6  illustrates an embodiment of SGC illustrated in  FIG. 5  operated in a continuous high water level configuration. 
           [0014]      FIGS. 6A and 6B  illustrate a variation of the SGC embodiment illustrated in  FIG. 6 . 
           [0015]      FIG. 7  illustrates an embodiment of SGC illustrated in  FIG. 5  operated in a high water level with drain configuration. 
           [0016]      FIG. 8  illustrates an embodiment of SGC illustrated in  FIG. 5  operated in a dry ash hopper and low water level in SGC configuration. 
           [0017]      FIG. 9  illustrates an embodiment of SGC illustrated in  FIG. 5  operated in a dry ash hopper and water spray configuration. 
           [0018]      FIG. 10  illustrates a bottom carry arrangement of SGC. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Now with reference to  FIGS. 5 through 10 , embodiments of the present invention will be described.  FIG. 5  represents the basic configuration of the SGC in accordance with the present invention, generally designated by reference number  50 . In describing SGC  50  certain components are common with the prior art systems described previously, and the same reference numbers are used to designate them. SGC  50  includes receiving section  52  directly connected with the existing ash hopper  14 . Bottom gate  16  can be opened or closed to connect or isolate ash hopper  14  from receiving section  52 . Clinker grinder  18  is provided for the purposes of reducing particle size to less than about 2 inch (typically) to fit in a smaller conveyor cross-section. SGC  50  forms an elongated closed duct  54  which extends from receiving section  52 . Duct  54  has a generally horizontal section  56  and an inclined portion  58 . Horizontal section  56  is primarily provided to adapt the system to existing plant installation space constraints. Closed duct  54  is fully enclosed hydraulically on all sides for some embodiments of operating configurations which will be described. Duct  54  preferably has a generally rectangular cross-section, is water proof, has removal covers at appropriate places with special seals and a double strand drag chain  60  moving inside the duct in an endless manner between sprocket  62  near receiving section  52  and drive sprocket  64  at the terminal end of inclined section  58 . A mechanism is provided for adjusting tension in conveyor  60  which may operate at either of sprocket  62  or  64 . Drag chain conveyor  60  forms a lower carrying section  66  which moves accumulated ash from receiving section  52  along horizontal section  56  and up inclined section  58 , with an upper return section  68  completing the endless chain loop. Inclined section  58  typically extends at an angle of around 30 to 40° which is intended to provide optimized ash dewatering while providing efficiency of ash transport. SGC  50  can be easily installed with existing boilers since the existing hoppers  14  are utilized and only a sluice line is replaced (when replacing a hydraulic transport system). Similarly, the bottom ash gate  16  and clinker grinder  18  normally provided are left in place. Maintaining bottom ash gate  16  provides maintenance isolation between the boiler and SGC  50  allowing maintenance operations without requiring the associated boiler to be taken off line. Since ash is loaded into the conveyor at a single, approximately square or round point location in receiving section  52  rather than along a long, rectangular shaped opening below the boiler throat, the orientation of the SGC  50  can be rotated 360° in any direction from a plan view. One option is to use a single conveyor arranged in the same manner as a conventional SCC to pick up multiple single loading points. Alternatively multiple smaller conveyors can be used for each single loading point if pre-existing structures occlude a conventional arrangement. This provides great flexibility for space-congested retrofit applications. Another advantage of the point loading configuration is that secondary isolation valves can be installed between the grinder  18  and the conveyor  60  for an additional level of personnel safety when performing conveyor maintenance while the boiler remains operational. 
         [0020]    Under certain conditions it may be necessary to limit the feed of ash into the SGC  50  to prevent over-filling. The present invention accomplishes this by monitoring the conveyor drive torque during operation. Torque monitoring can be achieved in several ways, including but not limited to an output of electric motor current or hydraulic pressure. At a pre-determined high set point for the output, simple logic can be used to close an upstream feed valve or stop a preceding conveyor, thereby stopping the feed of additional ash. The conveyor whose drive has reached the high set point can continue to run until sufficiently emptied of ash, as represented by a low set point for the drive output parameter. At this point the signal would then initiate the re-opening of a valve or re-starting of an upstream conveyor to begin feeding ash again. Torque control in this manner also provides benefits for chain size selection and wear life. Because the amount of ash accumulated in the conveyor can be controlled, much smaller chain sizes can be used compared to a conventional SCC in which large masses of ash could pile on top of the chain and flight mechanism. Indeed, the chain size of conventional SCCs is dictated by the amount of ash that could accumulate on top of the chain and flight mechanism rather than the conveying capacity of the machine. Despite the large pile of ash accumulated on the chain in a conventional SCC, the removal rate remains constant based on the dimensions of the flight bars. Therefore, a SGC  50  as described in the present invention can provide an equivalent conveying capacity to a conventional SCC with a given flight bar size while using smaller chain. The use of smaller chain provides considerable cost savings. Additionally, chain wear life is prolonged by lower link-to-link stresses realized by reduced ash loading. 
         [0021]    The basic system described for SGC  50  can be operated in various configurations, each providing certain features for optimization for a particular plant application. Also, the duct widths, the flight design and the flight distances and the chain speed are flexible and can be adapted to the requirements. 
         [0022]    Such configurations are next described with reference to  FIGS. 6-9 .  FIG. 6  illustrates an operational approach referred to as “continuous high water level” system  70 . In this configuration, a high water level is maintained in the ash hopper  14  which is normally filled with water and also in the SGC  50 . Dark shading in  FIG. 6  indicates the presence of water and its level.  FIG. 6  illustrates the normal water column height maintained in system  70 . As shown, ash hopper bottom gate  16  is normally open resulting in a water column height designated by horizontal line  72  in both the hopper  14  and in SGC  50 . A portion of inclined section  58  extends above water column height  72 , designated as section  74 , and over that inclined distance dewatering of the collected ash particles occurs. It is noted that since system  70  is typically intended to operate continuously there is a relatively small mass flow rate of ash falling into SGC  50  (as compared with batch type operation). Accordingly, as compared with batch type systems, conveyor  60  runs at a relatively slow rate. This slow rate of advancement of the drag chain conveyor  60  provides more residence time of ash particles in the dry inclined section  74 , enhancing dewatering. 
         [0023]    The water level height  72  of system  70  may be in the range similar to that for conventional wet sluicing systems; namely, around 15-25 feet above grade. In operation, SGC  50  operated in accordance with configuration  70  runs continuously with bottom gate  16  normally open, thus water level  72  will not change. It is likely that flushing nozzles (not shown) may be needed within the inclined internal surfaces of hopper  14  to clear away accumulated ash solids. 
         [0024]    SGC  50  operated in accordance with configuration  70  provides significant advantages over prior art systems. It is highly adaptable to existing plant configurations, avoids the necessity of high volumes of transport water, and can be installed with a comparatively low capital expenditure and operated at expected low maintenance cost. 
         [0025]    A variation of “continuous high water level” system  70  shown in  FIG. 6  is shown in  FIGS. 6A and 6B . These figures illustrate that hopper  14  may be configured to have multiple separate pantlegs  15 . In this representation a small SGC  71  can be provided for each of pantlegs  15  and can be configured to be completely submerged, including in the discharge section. A second SCC  73  is then used to transport the ash above the water level, in this case oriented to transport the ash in a direction 90° from the first SGCs  71 . This configuration provides further flexibility to arrange the conveyors to reach a clear space in a congested area. 
         [0026]    Now with reference to  FIG. 7 , a second operational configuration of SGC  50  is shown, referred to as “high water level with drain configuration”  76 . Configuration  76  differs from configuration  70  previously described in that it is operated in a batch process mode and thus ash hopper bottom gate  16  only opens during an ash pull cycle. Hopper  14  is normally maintained with a high water height  72 . When bottom gate  16  opens, water and ash drains from the hopper  14  into SGC  50  into an overflow box  78  which could be placed anywhere along the length of the SGC  50 . Overflow water can be moved to an adjacent tank or sump (not shown) for temporary storage until the ash pull cycle is complete. Ash is removed from the full hopper for 1-2 hours, for example. It is likely that hopper wall flushing nozzles will be needed which would run at the end of a pull cycle to clear accumulated ash solids adhering to the walls of the hopper. At the end of the flush, the bottom gate  16  is again closed and hopper  14  begins to accumulate ash until the next pull cycle occurs which may take place, for example 8-12 hours later. 
         [0027]    As shown in  FIG. 7 , the water level maintained in SGC designated as level  80  is much lower than hopper water level  72 . This provides a lower height inclined section  58  provided for dewatering of the ash. This can result in a more compact SGC  50  installation for particular applications. 
         [0028]    Now with reference to  FIG. 8 , another configuration for operating SGC  50  is illustrated, referred to as a “dry ash hopper and low water level in SGC” configuration  84 . In this application, ash hopper  14  is dry and the water level  86  within the hopper and SGC  50  is maintained just above clinker grinder  18 , which provides cooling for the grinder. In this system, water level  86  is a lower than prior embodiments. Operation of system  84  is typically continuous in that ash hopper bottom gate  16  remains open continuously and ash falls into grinder  18  as it is produced. Air cannons, water sprays or other systems (not illustrated) may be employed to clear the walls of hopper  14  of accumulated ash. Configuration  84  provides a relatively small volume of water in SGC  50  which may require auxiliary cooling systems. The low water height maintained in system  84  permitting a short run of inclined section  58  further provides opportunities for installation in very tight installation space allotments. 
         [0029]    Now with reference to  FIG. 9 , another configuration for operating SGC  50  is illustrated, referred to as a “dry ash hopper and water spray” configuration  88 . This configuration is essentially a dry system in that the ash is not submerged in water. However, the requirement of cooling the ash remains. For this purpose a series of water sprays  90  is provided. Operation here would typically be continuous with bottom gate  16  open with SGC  50  operated continuously. Here it should be noted that conveyor unit  60  does not provide a submerged drag chain and therefore this configuration is better described as a dry chain conveyor or DCC since the ash is no longer submerged. 
         [0030]    The several embodiments of the present invention permit a large volume of ash to be stored within the existing separate ash storage hoppers  14 , which allows lower-cost type conveyors to be used. Conventional SCC&#39;s can allow large ash piles to accumulate above the conveyor mechanism. Accordingly, the chain and drive system must be designed to accommodate removal of such a large ash pile which normally requires a large chain and drive size. With the SGCs  50  of the present invention, the existing ash hopper  14  contains the ash pile if prolonged storage is needed. The conveyor  50  itself has a small volume in which ash can be stored, so the design condition for worst-case ash loading is much smaller, allowing the use of smaller chain and drive size. 
         [0031]    In preferred embodiments of SGC  50  of the present invention a “bottom carry” configuration can be used. In part this is enabled since the use of clinker grinder  18  reduces the size of ash particles.  FIG. 10  illustrates the bottom carry arrangement in which the chains flights  66  and  68  are directly supported by the chains. The double strand chains are guided in replaceable wear bars and are guided by “U” channels  69 . A bottom carry arrangement is an improvement because all the runs of the chain are contained and the submerged water bath is positioned to clean the return run of chain and deposit ash into the bottom run by gravity.  FIG. 10  further illustrates in the cross-section of SGC that it provides a hydraulically closed vessel having sidewalls  100 , bottom plate  102  and upper lid  104 . Additionally, a bottom carry conveyor can be configured with a fully submerged chain tension arrangement, thereby eliminating the need for a tall vertical tower for chain tensioning where the dry return run of chain must elevate above the water level to re-enter the submerged portion of the conveyor. In this way, a bottom carry conveyor can be configured to operate such that its internals are completely submerged as indicated by conveyor  71  in  FIGS. 6A and 6B . 
         [0032]    While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.