Coal Ash

Coal ash is the noncombustible waste product left over from the burning of coal. It consists of airborne particles called “fly ash” as well as heavier particles called “bottom ash” that settle on the floors of coal-fired furnaces. Fly ash, which was once released into the atmosphere by coal-burning electric utilities, is now typically “scrubbed” from exhaust gases through a variety of mechanisms (such as bag filters, cyclone separators, or electrostatic precipitators) installed within smokestacks. Once the dry ash has been recovered, water is typically added to form a slurry, allowing for pipeline transport and reducing the potential for the finely grained ash to become airborne after capture. The coal ash or slurry can then be disposed of in a landfill; however, since such a solution entails not only transport costs but also the additional expense of paying the municipality in charge of the landfill, it is more frequently pumped into nearby impoundment ponds already owned by the utility. The coal-burning industries of the United States produced an estimated 131 million tons of coal ash annually as of 2010. Since the total weight of ash produced increases with each passing year—a phenomenon attributable to new coal-fired plants being brought into operation to meet increasing electricity demand as well as to improvements in scrubbing technology—the disposal of this waste material presents a serious and growing matter of ecological concern.