Wet bottom furnace

1. Slag-tap boiler, burns pulverized coal. 50 percent of the ash is retained in the furnace as boiler slag.
2. Cyclone boiler, burns crushed coal. 70 to 80 percent of the ash is retained as boiler slag. The rest of the ash leaves as fly ash.

If the ash fusion temperature is less than the furnace temperature then that type of furnace is called a wet bottom furnace.

1. Both boiler types have a solid base with an orifice that can be opened to permit the molten ash that has collected at the base to flow into the ash hopper below.
2. The ash hopper in wet-bottom furnaces contains quenching water.
3. When the molten slag comes in contact with the quenching water, it fractures instantly, crystallizes, and forms pellets.
4. The resulting boiler slag, often referred to as “black beauty,” is a coarse, hard, black, angular, glassy material.
5. At intervals, high-pressure water jets wash the boiler slag from the hopper pit into a sluiceway which is then conveys it to a collection basin for dewatering, possible crushing or screening, and either disposal or reuse.