In December 2008, at a Tennessee Valley Authority (TVA) electric generating plant in Kingston, Tenn., one dike of a containment pond ruptured spilling more than 5.4 million cubic yards of coal ash slurry, inundating the Emory River and covering more than 300 acres with wet fly ash. Cleanup is expected to cost more than $1 billion.
The long-term consequences of this accident, based more on politics than science, could change the way fly ash is used or disposed of in the United States for the foreseeable future. If fly ash is declared a hazardous waste, the damage caused to the concrete industry is impossible to overstate.
The fact that this spill was an environmental disaster is not in dispute. Although four homes were destroyed and more than 20 were damaged, no people were injured and, despite dire predictions from environmental activists, water quality in the area did not significantly deteriorate. According to the TVA, “All EPA, TDEC (Tennessee Department of Environment and Conservation), and TVA water treatment facility sampling results … continue to meet water quality standards for drinking water.”
Disposal and beneficial use
The Environmental Protection Agency (EPA) currently does not regulate fly ash under Subtitle D of the Resource Conservation and Recovery Act (RCRA). This means the individual states have jurisdiction over its disposal, unlike RCRA Subtitle C hazardous wastes, which are regulated by the federal government. This approach has been in place since 1980 under the so-called Bevill exclusion, which excluded “solid waste from the extraction, beneficiation, and processing of ores and minerals” from regulation as hazardous waste under Subtitle C of RCRA. In 2000, the EPA determined that coal combustion products (CCP), which includes fly ash, bottom ash, and flue-gas desulfurization material, were exempt due to the desire to avoid placing “barriers on the beneficial uses of coal combustion waste so they can be used in applications that conserve natural resources and reduce disposal costs.”
Before December 2008, when a dike ruptured at the TVA's Kingston, Tenn., power plant spilling 5.4 million cu. yds. of fly ash sludge.
Credit: Tennessee Valley Authority
The U.S. Energy Information Administration (EIA) estimates that nearly half of the electricity generated in the United States comes from coal, resulting in about 130 million tons of CCP. About 55%, or 72 million tons, is fly ash and about half the fly ash generated currently is used for beneficial purposes, mostly in concrete. This so-called beneficial use not only saves landfill space but reduces the amount of cement being produced and thus the amount of CO2 generated.
The ash that isn't used, though, ends up in landfills or containment ponds. To handle the ash, some electric utilities mix it with water and pump the slurry into containment ponds. Unfortunately, these ponds have not always been well designed or maintained, which is what led to the Tennessee disaster and the current political situation, and why the EPA's administrator Lisa Jackson pledged during her confirmation hearing last January to come up with new disposal regulations for CCPs by the end of 2009. The EPA doesn't trust the states to properly regulate fly ash but has no legal way to regulate it federally other than to designate the material as hazardous under Subtitle C, even though the toxicity of the material does not qualify it as hazardous.
Unfortunately for the nation, Jackson's pledge may have created a greater environmental problem than she intended. Although the EPA has for many years promoted the use of fly ash, even sponsoring the Coal Combustion Products Partnership to increase the beneficial use of fly ash, it now appears that some sort of hazardous designation is likely.
Fly ash in concrete
Credit: Tennessee Valley Authority
Perhaps the single greatest contributor to making concrete more environmentally friendly is the use of fly ash as a cement replacement. Concrete's greatest environmental liability is the inclusion of portland cement because of its energy-intensive manufacturing process. The typical equation cited by the Portland Cement Association is that approximately 1 ton of CO2 is released for every ton of cement manufactured. Using fly ash in a concrete mix reduces the amount of cement needed thereby reducing the carbon footprint. At the same time, an industrial waste product is consumed rather than sending it to a landfill. The Electric Power Research Institute estimates that beneficial use of CCPs annually saves 159 trillion BTU, 32 billion gallons of water, 11 million tons of CO2 normally released into the atmosphere, and 51 million cubic yards of landfill space.
Fly ash isn't just filler in concrete. As a very fine grained pozzolan, it reacts with the calcium hydroxide generated by the hydration reaction of cement and water to take on cementitious properties of its own. This makes concrete stronger, less permeable, and reduces the alkalinity, which can in turn reduce the danger of alkali silica reactivity in the aggregate—and the need in some areas to import nonreactive aggregate.
Fly ash concrete has better workability and pumpability. It also hydrates more slowly, which reduces the heat of hydration—critical to reducing cracking in mass concrete placements. All of this makes fly ash concrete more durable than plain portland cement concrete. In addition, concrete with fly ash costs less, because fly ash is typically cheaper than cement.