About 37 percent of U.S. electricity comes from coal, and burning it leaves coal ash (coal combustion products) at an average of 130 million tons a year. In 2013 that meant 14.5 million tons of bottom ash, 53.4 million tons of fly ash and 35.1 million tons of flue gas desulfurization products, and coal use and ash production are expected to stay at about current levels for decades.
Figure 1: How coal combustion makes coal ash: bottom ash, fly ash and flue gas desulfurization products, which are stored or reused. USGS.
What is in it
Fly ash is fine particles of clays, quartz, iron oxides, glass formed when minerals melt at combustion temperatures, and unburned carbon. Its main constituents are silicon, aluminium and iron, with smaller and more variable amounts of calcium, magnesium, potassium, sulfur, titanium and phosphorus. It also holds small amounts of trace elements, among them chromium, nickel, zinc, arsenic, selenium, cadmium, antimony, mercury and lead, and uranium at 10 to 30 ppm, near the top of the range found in natural rocks such as granite and black shale.
The trace elements come from the coal itself. Most stay in the ash when coal burns, so they become more concentrated in the ash than in the coal, at parts-per-million to parts-per-billion levels that vary with the coal, the combustion process and how the ash is collected. Many, such as chromium, arsenic and mercury, can be toxic at low concentrations, which is why states and the federal government monitor them in rivers and lakes used for drinking water.
Storing and reusing ash
Figure 2: How coal ash and its elements can escape from impoundments, through rain and snow soaking into groundwater, soil, lakes and rivers, or through liner failure. USGS.
In the United States, coal ash goes into impoundments or landfills, which are costly and limited by nearby space. Rain and snow soaking through leaky sites can carry ash and its elements into the environment, and a failed impoundment can do widespread, lasting harm. The Tennessee Valley Authority Kingston spill in December 2008 released about 5.4 million cubic yards of ash into the Emory and Clinch Rivers; the Eden, North Carolina, spill in February 2014 released up to 39,000 tons into the Dan River. The EPA has identified other impoundments that could do significant harm if they failed.
The alternative is reuse, mainly in concrete and other building materials and secondarily as structural fill; about 43 percent of coal ash is reused. Studies found little leaching from ash-amended concrete, and radon from ash-containing building materials is generally much lower than from ordinary rocks and soils. After the Kingston spill, river water was not enriched in trace elements because of dilution, but sediments held elevated arsenic and mercury; in North Carolina lakes receiving discharges from power plants and ash ponds, arsenic, nickel, antimony and selenium in water and sediment exceeded EPA water-quality standards.
USGS research
The USGS analysed fly ash from three power plants: two burning Appalachian Basin coal with high and relatively low sulfur (samples A and B), and one burning low-sulfur Powder River Basin coal from Wyoming and Montana (sample C).
Figure 3: Trace elements in the three fly ash samples compared with average U.S. coals. USGS.
- Sample C had lower concentrations of most trace elements than A and B, and A and B differed, especially in arsenic.
- Electron microscopy showed two components, glassy spheres and a non-crystalline, carbon-rich surface coating of nanometer-scale structures, with vanadium, zinc, molybdenum, thallium and lead associated with the coating in sample A.
- Synchrotron X-rays showed that 89% of the arsenic in sample A was the more oxidised As 5+ and 11% the more toxic As 3+, and that its chromium was mostly Cr 3+, which does not dissolve readily and is a nutrient, with a very small share of Cr 6+, which dissolves in water and causes cancer.
Figure 4: Fly ash under the electron microscope: (A) glassy spheres and their surface coating; (B) a close-up of the coating, without crystalline form. USGS.
When elements leach out
Surface waters are usually oxygen-rich (oxic) and bottom waters and sediments oxygen-poor (anoxic), which changes how trace elements behave. The USGS exposed arsenic- and chromium-free fresh water to fly ashes A and B under both conditions for 28 days (672 hours):
- Arsenic dissolved from both ashes under both conditions, rising over time to 30 to 150 ppb, above the EPA's drinking-water limit of 10 ppb. Arsenic exposure can damage skin and the circulatory system and raise cancer risk.
- Chromium dissolved only from ash A in oxic water, and stayed below the EPA limit of 100 ppb; little or none came from ash B.
Figure 6: Dissolution of (A) arsenic and (B) chromium from fly ashes A and B in simulated oxic and anoxic fresh water at pH 7. USGS.
Regulation and next steps
The EPA classifies coal ash as a non-hazardous solid waste under Subtitle D of the Resource Conservation and Recovery Act. Its final rule on coal combustion residuals requires minimum federal standards for siting and designing ash landfills and impoundments, routine structural and groundwater checks, clean-up of contaminated groundwater, and closure of facilities, to reduce failures and protect aquifers; it does not cover beneficial reuse.
With coal use expected to continue, more research is needed on how environmentally sensitive trace elements move during ash handling, reuse and spills, and what that means for health and the environment.
Sources
- Amrika Deonarine, Allan Kolker and Michael Doughten, Trace elements in coal ash, U.S. Geological Survey Fact Sheet 2015–3037. https://pubs.usgs.gov/publication/fs20153037
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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