Coal fired power stations operating tap slag boilers release most of the ashes resulting from the combustion as slag at the boiler bottom. There is a number of such power stations in Europe operative. These boilers are also known as wet bottom boilers.
There are two types of wet-bottom boilers: the slag-tap boiler and the cyclone boiler. The slag-tap boiler burns pulverized coal and the cyclone boiler burns crushed coal. In each type, the bottom ash is kept in a molten state and is tapped off as a liquid. 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. The ash hopper in wet-bottom furnaces contains quenching water. When the molten slag comes in contact with the quenching water, it fractures instantly, crystallizes, and forms pellets. The resulting boiler slag, often referred to as "black beauty," is a coarse, hard, black, angular, glassy material.
When pulverized coal is burned in a slag-tap furnace, as much as 50 percent of the ash is retained in the furnace as boiler slag. In a cyclone furnace, which burns crushed coal, some 70 to 80 percent of the ash is retained as boiler slag, with only 20 to 30 percent leaving the furnace in the form of fly ash.
Slag is tapped off the boiler bottom outlet.
Wet-bottom boiler slag is a term that describes the molten condition of the ash as it is drawn from the bottom of the slag-tap or cyclone furnaces. 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.(4) During 1995, the utility industry in the United States generated 2.3 million metric tons (2.6 million tons) of boiler slag.
The molten and then crystallized slag is washed out of the power station into a slag pond. The wet ash is extracted from the pond and left in the open to loose most of the quenching water. Thereafter the slag is dried in a rotating dryer to dry and de-agglomerate bigger particles before screening in to five different particle sizes. This end-product is either bagged in 50 kg bags or supplied in bulk by tankers. The application is blast cleaning.
DIRK Blastgrit is still produced and sold by the Sakresiv Group in Europe.