Coal ash comes from burning coal.
The term "coal ash" is used to describe the waste products or materials left after burning coal. The United States has 26 coal-producing states supporting 45 percent of the country's production of electrical power, according to the website Renewables-info.com. These products have a variety of uses, ranging from the construction industry to agriculture.
Bottom Ash
The byproduct left at the bottom of a coal-burning furnace is called "bottom ash." It is a coarse material, similar to what you find after burning charcoal in a barbecue grill. The commercial use for bottom ash is limited, but it can be a substitute for the aggregate used in concrete. The size of bottom ash ranges from fine sand to gravel.
Fly Ash
The finest coal ash material is called "fly ash." This byproduct comes from the coal residue that gets transported by the exhaust gases from the combustion chamber. The fine powder of coal left throughout the furnace contains noncombustible coal leftovers plus carbon. Fly ash can be used as a substitute for the cement used to mix concrete.
Gypsum
Coal-burning utilities have an air pollution control system that removes the sulfur in the smoke of a burning coal furnace. The air pollution control system catches a kind of coal ash called "flue gas desulfurization gypsum." The gypsum is yellow in color and is not as fine as fly ash. Calcium sulfate within the gypsum residue is used for agricultural purposes, as well as in wallboard, such as sheet rock.
Slag
Boiler slag is the very coarse type of coal ash. Some utility companies use cyclone boilers to burn the coal. The residue found at the bottom of the boiler -- large remnants of coal -- is the boiler slag. Roof shingles, sand substitute for sand-blasting, asphalt substitute for road paving and traction-control material placed on icy road are common commercial uses of this type of coal ash.
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