Coal ash is formed when coal is burned in boilers that generate steam for power generation and industrial applications. TENORM is generated when burning removes organic constituents, leaving minerals and concentrating trace quantities of naturally occurring radionuclides:
- their radioactive decay products including radium. (The amount radium in coal can vary by more than two orders of magnitude depending upon the type of coal and where it was mined.)
In addition to TENORM, coal ash contains silicon, aluminum, iron, and calcium; these elements make up about 80 to 90 percent of all of the constituents of coal ash.
The table below lists typical radiation levels in coal ash:
|Wastes||Radiation Level [pCi/g]|
The Radiation in TENORM Summary Table provides a range of reported concentrations, and average concentration measurements of NORM associated with various waste types and materials.
How Fly Ash is Generated
In a typical coal-fired boiler, coal is burned and the heat is extracted to generate steam. The steam is used to drive a turbine, which in turn, drives an electrical generator. Low pressure steam leaving the turbine is fed to a cooling system which extracts any residual heat and condenses the steam back to water.
Condensed water is collected and recycled back to the boiler where the process is repeated.
The coal combustion process generates ash equal to about 10 percent of the original volume of the coal. About 95 percent of the ash is retained:
Fly ash is entrained with hot flue gases, trapped by stack filters, and accounts for about 74% of the ash generated
Bottom ash, too large or heavy to be entrained, settles to the bottom of the boiler and accounts for about 20% of the ash generated
Boiler slag, formed when the ash melts under the intense heat, collects at the bottom of the boiler and in exhaust stack filters, and accounts for about 6% of the ash generated.
Stack filtration devices, such as electrostatic precipitators, baghouses, and scrubbers are routinely used to reduce the emission of fly ash to the atmosphere by at least 95 percent. A small fraction of the fly ash produced, typically 2-5 percent, is released into the air.
The amount of ash produced depends on the mineral content of the coal and the type of boiler. The typical natural content of minerals in coal used in the U.S. ranges from about 3 percent to 30 percent, with an average of about 10 percent depending upon the region from which the coal was mined. Some utilities are selecting coal with lower mineral (or ash) content to meet particulate emission standards. In other instances, utilities reduce the ash content by washing the coal. Washing can reduce the ash content by as much as 50 to 70 percent.
The average yearly generation of coal ash is about 61 million metric tons (MT). In 1990, the combustion of coal in utility and industrial boilers generated 61.6 million MT of coal ash and slags and 17.2 million MT of sludges.
Disposal and Reuse
Typically 70 to 80 percent of coal ash is disposed of in dry landfills. (Sluiced ashes and sludges are first dewatered in ash ponds then landfilled.) A landfill for a typical coal fired power plant (500-1000 Megawatts) requires about 30 to 60 hectares (74 to 148 acres). These landfills range from about 4 to 80 hectares (10 to 197 acres) and may be as much as 9 m (30 ft) deep.
The remaining coal ash (roughly 20 million MT per year) is used as additives in a variety of applications depending upon the characteristics of the ash. The following list shows the application of the ashes and sludges that were used
Fly Ash, 75% of use is in concrete and cement and in concrete blocks. It is typically substituted for cement in concrete at about 10-30%. It is also used as a filler for asphalt at a rate of about 12%.
Sludge, 57% of use is in wallboard, as roadbase, and other miscellaneous applications, but the total volume used is minimal compared to the total production.
Boiler Slag, 54% of use is as blasting grits and roof granules
Bottom Ash, 30% of use is for snow and ice control and other miscellaneous applications.
Since the early 1970s, all three types of coal ash have been used in construction projects. Coal ash is used to level out uneven terrain or applied as a stable fill for building construction. Typical applications include sites where shopping malls, housing developments, and industrial parks are planned for construction. Other projects have included the construction of road embankments, runways, public transportation system structures, and soil stabilization.
Other emerging applications of fly ash include the construction and sinking of artificial reefs, metal (aluminum and iron) extraction via direct acid leaching, and as a filler in paints and plastics. Examples of products which may contain fly ash include paints and undercoatings, auto bodies and boat hulls, PVC pipes, battery cases, bowling balls, utensils and tool handles, vinyl floor covering, and shower stalls.