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Ash Handling in Captive Power Plants [Feb 2012]

Ash handling is one of the complex issue in every industry. This article focuses on the various procedures of handling ash in captive power plants
Ash generation in different industries is in different forms, and accordingly the thought is given for its disposal at different levels. Though ash is a saleable item; every type of ash is not saleable. The quality & grade of ash depend upon the fuel.
Coal ash from boilersThis comes in three categories: fine, course and gravel. Detailed study of the same is as under:
Fine AshAsh coming out of the FBC boilers is normally fine and course. These days, the pollution department has fixed the norms of emission and ash content in the flue gases. Because of this either bag filter or ESP are installed. These equipment collect fine ash, and it can be up to 200 mesh.
Course AshAsh coming out from the plenum chambers and drum bottom of FBC boilers is normally course. This contains particles of coal and has the calorific value up to 700 Kcal/kg.
Gravel AshThis ash is generated by gasifires and travelling grate stoker boilers.
HANDLING OF ASH IN BOILERSAs mentioned above, the boilers generate three types of ash. There are different ways of handling them. The procedures are being discussed below separately.
FBC BoilersAsh of FBC boilers are normally handled by pneumatic process. There are three different processes in practice. Depending upon the requirement and on individuals comfort, the application is used.
Dense phase system: This system is used at many places. Ash from different points is collected in one main pipeline and from there it is conveyed to silo. In this process, ash moves in to the pipeline in compressed form and pneumatic pressure is used to push it along the length of pipeline.
Pneumatic system by vacuum process: This is the most widely used system. The ash is collected from multiple points by pipelines and delivered in to one main line. One suction blower is installed at the silo, which constantly sucks the air through pipeline. As the system is put in to the operation, air suction valve opens and the rotary valve starts delivering ash in to the pipeline. Because of the suction ash laden air moves in the pipeline. The major part is collected in the cyclone separator and balance up to 30 microns is collected in pulse jet bag filter (Figure 1). In this system, only one line is blown every time for two minutes and ash is constantly delivered in to cyclone. As per the number of collection points and quantity of ash per point, cycle is set in to logic. The complete system is operated by PLC logic.  Figure 1Ash which has accumulated in various collection hoppers is pneumatically conveyed to a centrifugal receiver atop the ash storage silo. Vacuum forces providing the conveyance air flow are created by a blower (exhauster) located near the silo.
Coarser ash particulates collecting at the bottom of the receiver are transferred to the silo via a double dump gate air lock assembly. Finer ash particles and the induced conveying air flow report through the ‘vortex finder’ pipe at the top of the centrifugal receiver to a pulse jet, bag type dust collector located nearby. Ash particulates released by periodic blow down of the bags collect below, where this material is transferred to the silo via a double dump gate air lock assembly. Overall solids recovery typically exceeds 99.9 per cent for 2 micron and larger particulates.
Pneumatic ash handling by pressure process: This is also a very common system and used at many installations. As mentioned in the figure, the ash is conveyed by pneumatic process and the media used is compressed air. The operation of the system is as under. Figure 2As the system is put in the operation, compressor, pulse jet bag filter, and rotary valves below cyclone separator and bag filter are turned ON. As per the logic of the system, one gate valve and one rotary air lock also turn ON. The ash starts falling in to the pipeline and compressed air pushed it in to cyclone separator. The major ash is collected in to cyclone and balance is collected in to bag filter.
Handling of Ash in Stoker Fired BoilersAsh coming out of the stoker fired boilers is normally handled by submerged ash conveyors. Normally ash comes in lumps from the bed, and have temperature up to 500 deg C. It is as such not possible to handle it manually, and hence this kind of equipment becomes necessary.
In such type of equipment, the hot ash falls in to a trough which is filled with water. Chain with drag slats or belt is used to convey the wet ash out of water. Both kinds of equipment are being shown in figures below.        Both the equipment have their own applicability and versatility. Once the ash comes out, it is either sent for storage in silo or trolley is mounted directly below the discharge of conveyor. Ash collected from under the drum, cyclone separators and bag filters are also dropped on to the same conveyor. This type of equipment can handle ash up to 25 tons per hour.

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