Coal Ash in Spill Could Not Have Been Used in Concrete

The ash from the Tennessee Valley Authority’s (TVA) Kingston Fossil Plant that is now endangering the local community following a December 22 breach in a holding pond is most likely an 80/20 mix of fly ash (from smokestack pollution control systems) and bottom ash. To be used in concrete, fly ash must be low in residual carbon, as defined by ASTM Standard C 618. The boilers at the Kingston plant run at relatively low temperatures to reduce emissions of nitrogen oxides. A side effect of this lower temperature is that the coal is not burned as completely and the ash retains more carbon.

There are technologies for post-processing the fly ash to remove carbon, but those are not used at Kingston. Any marketable fly ash for use in concrete or as a filler in products such as carpet backing would have been protected from weather and stored in silos. Unmarketable ash is flushed through pipelines into holding ponds, including the one that just burst, with water as the carrier.

The ash from the Tennessee Valley Authority’s (TVA) Kingston Fossil Plant that is now endangering the local community following a December 22 breach in a holding pond is most likely an 80/20 mix of fly ash (from smokestack pollution control systems) and bottom ash. To be used in concrete, fly ash must be low in residual carbon, as defined by ASTM Standard C 618. The boilers at the Kingston plant run at relatively low temperatures to reduce emissions of nitrogen oxides. A side effect of this lower temperature is that the coal is not burned as completely and the ash retains more carbon.

There are technologies for post-processing the fly ash to remove carbon, but those are not used at Kingston. Any marketable fly ash for use in concrete or as a filler in products such as carpet backing would have been protected from weather and stored in silos. Unmarketable ash is flushed through pipelines into holding ponds, including the one that just burst, with water as the carrier.

David Goss, executive director of the American Coal Ash Association, told EBN that about 10% of the coal burned in a power plant typically ends up as a combination of fly ash and bottom ash. Nationwide, the amount of ash produced from coal-fired power plants each year, according to the U.S. Environmental Protection Agency, is about 90 million tons—an amount that would fill three trains stretching from New York to San Francisco. Trace metals and other impurities in the coal are concentrated in the ash, more so in recent decades as air-pollution controls have reduced the amount of toxicants that go out the smokestack. These hazardous constituents can leach out and cause ecological and health problems when the ash is loose—Goss argues that they do not present a threat when the ash is bound up in concrete.

If the TVA plant were generating fly ash that has value as a concrete additive, it would very likely be “type F” fly ash, which has pozzolanic properties (meaning that it contributes to the curing of cement), but has no cementitious properties of its own. The chemical composition of the bituminous coal found in the Appalachian Mountains creates type F fly ash, while subbituminous coal from the western states tends to create the more cementitious type C.

– Nadav Malin