Skip to main content

What is Coal Ash?

Posted by on Wednesday, April 28, 2010 in Ask an Engineer, Spring 2010.

The term “coal ash” has been in the media a lot in the past year. Vanderbilt Engineering asked a School of Engineering research team to explain the term and tell us why it is important. The team consists of Professors David S. Kosson, Florence Sanchez and Andrew Garrabrants, all of the Department of Civil and Environmental Engineering, along with collaborators Peter Kariher (Arcadis Inc.); Hans van der Sloot and Paul Seignette (Energy Research Centre of the Netherlands); and Susan Thorneloe (project director for the Environmental Protection Agency’s Office of Research and Development).

Where does coal ash come from?

Coal combustion for energy use results in the formation of several types of residuals, collectively referred to as coal combustion residues (CCRs). Coal ash refers to both bottom ash, the material remaining within the boiler chamber after coal combustion (which is continuously removed), and fly ash, the fine material suspended in the combustion gases that is typically separated in the first stage of air pollution control.

Why is it in the news?

A large quantity of fly ash was stored wet in a diked landfill operated by the Tennessee Valley Authority in Kingston, Tenn. The dike failed in December 2008, resulting in several hundred acres, including several homes, water bodies and transportation routes, being inundated with fly ash sludge. This also focused attention on the broader issues of coal combustion, including design of containment facilities for CCRs, potential environmental impacts from the disposal or use of CCRs, and the role of coal fired power plants in air pollution and climate change.

Is it toxic? Is it environmentally dangerous?

The presence of specific environmental contaminants in CCRs has been of ongoing public and environmental concern. These contaminants include mercury, aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, molybdenum, selenium and thallium.

Leaching, the extraction of contaminants from solids by contacting water, followed by transport and contamination of water resources, is a primary path to potential impact on human health and the environment. So how CCRs are managed is of great interest.

And how are CCRs managed?

Current management practices include disposal as either wet or dry/moist materials in landfills, collection in wet impoundments, and use in cement, concrete, wallboard, highway base layers and other applications. In the United States, approximately 44 percent (or approximately 61 million tons of the 136 million tons produced annually) of all CCRs produced are reused in commercial and engineering applications.

What more can be done?

The key is to test CCRs and to use or dispose of CCRs in contexts that are protective of human health and the environment. Many CCRs can be used safely in a range of applications. Many current disposal conditions are safe. Many also have been identified as being of potential concern. Some disposal and use applications for CCRs also have resulted in adverse environmental impacts. It is important to have a flexible decision structure that guides utilities and environmental regulators to make decisions that are protective and economic. When done safely, use of CCRs in construction applications also reduces the demand for other materials (e.g., Portland cement, fine aggregate) and the environmental consequences of their production and use.

Team members Andrew Garrabrants, David Kosson and Florence Sanchez
Team members Andrew Garrabrants, David Kosson and Florence Sanchez

How does your work come into this topic?

Vanderbilt School of Engineering’s contributions in this area—working in collaboration with our partners—have been the development of robust methodologies for leaching assessment and the characterization of more than 70 CCRs. This provides a detailed understanding of the effects of coal type and facility configuration on those CCR characteristics important to assessing potential environmental impacts. The EPA and Department of Energy have been key sponsors and collaborators. Additional collaborators include The Energy Research Centre of The Netherlands (ECN) and Arcadis Inc. Another contribution is LeachXS, software developed by Vanderbilt, ECN and DHI to facilitate data evaluation from leaching tests and assessment of potential environmental impacts. A simplified version, LeachXS Lite, is being made available through the EPA to facilitate distribution and use of the data developed as part of this program.

For more information, email The report, Characterization of Coal Combustion Residues from Electric Utilities—Leaching and Characterization Data, EPA-600/R-09/151 (Dec. 2009), authored by D. Kosson, F. Sanchez, P. Kariher, L.H. Turner, R. Delapp and P. Seignette, is available from