Boiler Slag sample as taken from an ash impoundment - Enlarge Photo
Boiler Slag 'Needles' as viewed via a microscope - Enlarge Photo
Boiler slag is a by-product produced from a wet-bottom boiler, a special type of boiler designed to keep bottom ash in a molten state before it is removed. These types of boilers (slag-tap and cyclone boilers) are much more compact than pulverized coal boilers used by most large utility generating stations. They can burn a wide range of fuels and generate a higher proportion of bottom ash than fly ash (50 to 80% bottom ash vs. 15 to 20% bottom ash for pulverized coal boilers). These are some of the reasons they are typically used by industrial manufacturing plants and smaller utilities.
With wet-bottom boilers, the molten ash is withdrawn from the boiler and allowed to flow into quenching water. The rapid cooling of the slag causes it to immediately crystallize into a black, dense, fine-grained glassy mass that fractures into angular particles, which can be crushed and screened to the appropriate sizes for several uses.
Since boiler slag is angular, dense and hard, it is often used as a wear-resistant component in surface coatings of asphalt in road paving. Finer-sized boiler slag can be used as blasting grit and is commonly used for coating roofing shingles. Other uses include raw material for the manufacture of cement and in colder climates, it is spread onto icy roads for traction control. Because there are so many uses and such a limited supply, we use essentially all of the boiler slag produced in the U.S. and even import some from other countries.
Products from Boiler Slag can be used in:
roofing shingles -- landscaping -- traction control material for icy roads ...
"Coal combustion products are the solid residues generated by coal-burning electric utilities in the production of electricity. In 1998, electricity accounted for about 35% of the primary energy use in the United States and was produced by electric power generators designed to convert different fuel types into electricity. More than one-half of the electricity in the United States was generated by burning coal. (Courtesy of USGS)".