ASH HANDLING

ASH HANDLING

The purpose of the ash and slag removal system is to collect, cool down and remove the slag (bottom ash), boiler ash and fly ash formed in a combustion of the fuel on the grate and separated from the flue gas on the heat surfaces and bag house filter to an extraction point for storage and use.

Bottom ash (slag) is the solid residue remaining after the waste fuel is incinerated on the grate. The bottom ash discharger is used to cool and discharge this solid residue that accumulates at the end of the grate and drops down into the discharge pool. Siftings, particles that fall through the grate during incineration, are also collected into this pool. The cooling water in the pool serves as an air seal for the furnace, preventing flue gas emissions and uncontrolled air leaks into the furnace. An apron conveyor is used to extract the bottom ash as well as any bulky objects from the pool.

The water used for cooling is separated from the bottom ash by gravity at the conveyor and it drops back into the discharge pool. Top-up water is required to maintain the water level in the discharger pool. The top-up water from blowdown water tank or raw water tank replaces water lost as moisture in the removed slag as well as the evaporation losses.

Fly ash comprises of the particles formed in the combustion that are transported out of the combustion chamber with the flue gas. Some of the fly ash accumulates on the heat transfer surfaces forming layers that must be removed using a cleaning system, such as mechanical rapping. Rest of the fly ash is separated from the flue gas in a bag house filter installed in the flue gas treatment (FGT) system after the boiler.

The fly ash removed from the heat transfer surfaces is collected in ash hoppers and discharged onto a drag chain conveyor through a rotary airlock feed valve. The hopper and the valve maintain the gas-tightness of the boiler during ash discharge.

The fly ash and FGT residue separated from the flue gas in the bag house filter are collected from the ash hoppers with a screw conveyor and led to a pneumatic conveyor through a rotary airlock feeder. The conveyor transports the solids to the ash handling and storage. Fly ash and FGT residue can also be collected and stored separately.