Abstract:
A batch slag handling system for gasifiers and the like which operate under pressure, has a lockhopper receiving the slag under pressure and dispensing it after depressurization. The slag is ground and combined with water for transport to a slag sump where it is dewatered and then removed from the site. Sluice water is provided by a closed loop system.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. The Filed of the Invention  
           [0002]    The present invention pertains to an improved slag handling system and, in particular, to a system which obviates the use of an expensive and unreliable drag conveyor.  
           [0003]    2. The Prior Art  
           [0004]    All coal and coke gasification systems must have slag removal systems to discharge the ash and nonvolatile materials which are unavoidable by-products of such processes. One present slag removal system incorporates a slag drag conveyor which receives slag directly from a lockhopper onto a conveyor belt which conveys the slag to a slag containment vessel (such as a truck, train, pit, etc). The slag producing sections of these gasification processes are in a harsh environment exposed to both erosive materials and corrosive chemicals. This harsh environment has caused the drag conveyors, with their many moving parts, to be failure prone, maintenance intensive, and thus unreliable for slag removal. The drag conveyors are very expensive, in and of themselves, and therefor spare or backup systems are too costly to be kept on site for emergency use. The unreliable nature of this type of slag removal equipment can lead to downtime for an entire gasification plant and thereby reduced onstream time/capacity factors. One known drag conveyor was such a major weak link in a gasification process that it was eventually bypassed by using an emergency slag dump line. In order to improve the reliability of gasification processes, an improved method of slag handling, which is environmentally acceptable, economical to maintain and operate, and safe to operate, is necessary.  
           [0005]    Coal-fired boilers in other industries generate ash/slag material which is similar to, but not exactly the same as, the slag which results from gasification processes. However, unlike gasifiers, the slag producing portions of conventional boilers usually do not operate under pressure and therefor can have continuous removal of slag from the system. There are variations of sluicing systems used in these coal-fired boiler plants.  
           [0006]    It is believed that the present invention can overcome at least some of the above discussed problems by significantly reducing unit downtime of coal and coke gasification plants and thereby improve capacity factors for potential customers. It will allow higher onstream times by reducing downtime for maintenance and repair of the slag removal system. The cost of the system should be considerably less than for a drag conveyor system, especially considering that plant maintenance costs will be substantially less.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides for the removal of slag from a gasification system operated under pressure by using a lock hopper to receive, depressurize and dispense batches of slag. The slag passes through a discharger, where it is ground to sufficiently small size to pass through the rest of the system without causing any jamming. The ground slag is passed to an eductor where it is mixed with water, from a closed loop sluice water system, and sent to a slag pit. The water level in the slag pit is monitored and returned to the closed loop sluice water system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which the single FIGURE is a schematic diagram of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0009]    The subject system  10  is preferably used in conjunction with, and as part of, a known coal or coke gasification plant, of which only the slag receiving sump  12  has been shown. The sump  12  usually has therein grinding means (not shown) to break up the slag it receives from the gasifier operation. The slag handling portion of the subject system has a lockhopper  14  with a first pressure lock  16  connecting the output of sump  12  to the input of lockhopper  14  and a second pressure lock  18  serving for its output. A slag discharger  20  is connected between the second pressure lock  18  and slag grinder  22 , where the slag is ground and reduced in size so as not to plug the downstream equipment. The ground slag is passed through pipe  26  to eductor  24  where it is mixed with water and sent through pipe  28  to the sump pit  30 .  
         [0010]    The system also includes a closed loop sluice water portion in which tank  32  serves as the primary source of sluice water. A sluice water pump  34  is connected to an output of tank  32  and by distribution piping  36  through valve  38  to eductor  24 , valve  40  to discharger  20 , valve  42  back to the tank  32 , and valve  44  to a grey water treatment facility (not shown). Forming the return portion of the closed loop is sump piping  46  having pump  48  connected to the sump pit drain line  50 , valve  52  connected to a return line  54  to the sump pit  30 , and valve  56  to the sluice water tank  32 . Valves  52  and  56  are controlled by sump level sensing and control means  58 . The sluice water tank  32  includes level control means  60  and inlet valve  62  connected to a make up water source (not shown). Valve  44  connects the close loop to a gray water treatment facility (not shown) to grey water to dispose of overly contaminated water. A control  64  controls the operation of the pressure locks  16 ,  18 , and valves  38 ,  40 ,  42 , as described below. The discharger  20  preferably is equipped with a vent  66  connected to vapor recovery means (not shown).  
         [0011]    Slag accumulates in the lockhopper  14 , according to normal gasifier operation, by periodic actuation of pressure lock  16 . The pressure lock  18  is likewise be periodically actuated, but only when pressure lock  16  is closed, to dump the accumulated slag into discharger  20 . Some sluice water is admitted to the discharger through valve  40  and some vapor is discharged through vent  66 . The discharger then discharges the partially cooled and depressurized slag to slag grinder  22  where it is reduced in size sufficiently so as to not cause clogging problems downstream. Ground slag is then be fed to the sluicing water eductor  24  where it is mixed with sluice water and hydraulically transferred to the slag pit  30 .  
         [0012]    The slag pit  30  is constructed to promote efficient dewatering of the slag. Slag pit water will be pumped by pump  48  through piping  46  to sluice water tank  32 , where residence time can be provided for solids settling. High volume pump  34  provides sluice water through valve  38  and the eductor  24  to the slag pit  30 .  
         [0013]    Level control system  58  maintains a minimum water level in the slag pit  30  by selectively actuating valves  52  and  56  and pump  48 . Level control system  60  maintains a sufficient quantity of water in the sluice water tank  32 , by actuating valve  62 , to assure a full slag dump cycle.  
         [0014]    The total closed loop sluice water system preferably is sized to maintain a water balance. Occasional excess water is passed to a grey water treatment system (not shown) through valve  44 .  
         [0015]    The discharger  20  is a commercially available piece of equipment and a suitable example is the Roplex Discharger manufactured by the Hindon Corporation of Charleston, S.C. It is designed with a unique internal configuration and a bottom dump rotary plow which provides uniform discharge feed and eliminates vessel plugging. The discharger  20  discharges into slag grinder  22  which reduces slag size to dimensions which will not plug downstream equipment in the path to the slag pit  30 .  
         [0016]    The slag pit will preferably have multiple slag entry points. When a section of the pit becomes full, an alternate entry location will be selected and opened. The pit will be designed for efficient dewatering of the slag piles. After a predetermined period, to allow for additional dewatering, the dewatered slag can be loaded into trucks and hauled off site.  
         [0017]    The low end of the slag pit will collect water runoff from the incoming slag. The slag pit water pump  48  pumps the water from the slag pit sump to either recirculate it to the pit through valve  52  or to the sluice water tank  32  through valve  56 . System design should enable the slag water pump  48  to run continuously to reduce on/off operation pump stress and to prevent solids from settling in the lines  46 ,  50 ,  54  and pump  48 . If the sump level becomes low, the slag pit sump level control  58  will open the water return valve  52  and close the water valve  56  to the sluice water tank  32  to maintain the minimum sump level required to prevent loss of suction to the pump  48 . If the sump level drops below a low-low level point, the pump  48  will shut down.  
         [0018]    The sluice water tank  32  normal operating range will provide adequate water supply to sustain the sluicing system through a complete slag lock hopper dump cycle. A level control system  60  will maintain the proper level in the sluice water tank, providing make-up water through valve  62  during low level conditions and rejecting excess water through valve  44  to a grey water treatment system (not shown) during high level conditions. The tank  32  will provide residence time for additional solids settling. This will help to protect the downstream, high volume, sluice water pump  34  and the slag eductor  24  from unnecessary erosion. Solids settling will also provide a cleaner source of water for rejection to the grey water system. Accumulated solids will need to be cleaned out periodically, or a cone bottom tank can be used incorporating a solids removal system. If the closed sluice water system requires chemical additions for water quality, the tank  32  will provide a suitable injection/mixing point.  
         [0019]    The sluicing water control valves  38 ,  40 , and  42  will operate in conjunction with the interlock/timing system of the lock hopper  14 . When the lock hopper  14  is in the collect mode, the sluice water valve  38  to the slag eductor  24  and the flush water valve  40  to the slag tank  20  will be closed. Sluice water return valve  42  to the tank  32  will be open. System design should enable the sluice water pump  34  to run continuously to reduce on/off operation pump stress and to prevent solids settling in the lines and pump. When the lock hopper  14  completes the depressurization step, valve  38  will open to provide sluice water to the system and valve  42  will close. The flush water valve  40  will open to allow the necessary flush of water to the discharger  20 . This flush will help slag move through the discharger  20 , through the slag grinder  22  and into the eductor  24 . At the completion of the sluicing cycle, a timing control system will open valve  42  and close valves  38  and  40 .  
         [0020]    The present invention may be subject to many modifications and changes, which will become apparent to one skilled in the art, without departing from the spirit or essential characteristics thereof. Thus the above described embodiment should be considered in all respects as illustrative and not restrictive of the scope of the present invention as defined by the appended claims.