Bottom ash refers to the non combustible constituents of coal with traces of combustibles embedded in forming clinkers and sticking to hot side walls of furnace during the furnace working. The clinkers fall by themselves into the water or sometimes by poking manually, and get cooled.
The clinker lumps get crushed to small sizes by clinker grinders mounted under water and fall down into a trough from where a water ejector takes them out to to a sump. From here it is pumped out by suitable rotary pumps to dumping yard far away. In another arrangement a continuous link chain scrapes out the clinkers from under water and feeds to clinker grinders outside the bottom ash hopper.