Fly ash

Fly ash is a coal combustion product. It is part of a set of products that makes up the most abundant waste materials worldwide.[1] If not collected, this waste material is blown out with the flue gas in a coal fired power plant. Fly ash exists after combustion because ash adheres to coal, making up between 1-15% of its weight. About 90% of the ash is fly ash, while 10% is bottom ash.[2] Fly ash is composed of tiny, airborne particles and is thus considered to be a type of particulate matter or particle pollution.[3] Below is a comparison of fly ash and bottom ash, to show the size difference of the particles and the difference in texture.

To prevent the ash from completely escaping when the coal is burned, electrostatic precipitators are used to collect the fly ash. Additionally, other stack filtration devices such as baghouses and scrubbers are used to reduce the emission of fly ash. These methods are able to catch most, but not all, of the ash as they are unable to capture particles that are less than a micrometer in diameter. Overall, about 1% of the particles are released into the air.[5] These small particles escape the flue stacks into the air.[2] The captured fly ash is analyzed to determine if it contains chemicals that are dangerous to human health. If it is deemed harmful, it is disposed of. If the fly ash is found to be nonhazardous, it is recycled and used in the production of concrete or asphalt.

Along with the health and environmental impacts of fly ash, an additional issue is that it exists in massive quantities. The amount of coal burned and its ash content dictates the total amount of coal ash generated each year. The amount of coal ash has been increasing since 1996.[1]

Health Effects

Fly ash can have a different chemical makeup depending on where the coal was mined.[6] Broadly speaking, fly ash is a pollutant, and it contains acidic, toxic, and radioactive matter.[2] This ash can contain lead, arsenic, mercury, cadmium, and uranium.[7] The EPA found that significant exposure to fly ash and other components of coal ash increases a persons risk of developing cancer and other respiratory diseases. Wet ash ponds can pollute groundwater and if ingested, the arsenic contaminated water increases a person's risk of developing cancer. Inhalation or ingestion of the toxins in fly ash can have impacts on the nervous system, causing cognitive defects, developmental delays, and behavioral problems while also increasing a person's chance of developing lung disease, kidney disease, and gastrointestinal illness.[6]

Disposal and Recycling

The sheer amount of fly ash that exists is a problem. The EPA estimates that 140 million tons of coal ash are generated each year, with fly ash making up a majority of this ash.[6]

Some power plants dispose of coal ash in surface impoundments - known as wet ash ponds because the ash is kept wet to ensure it does not fly away - or in landfills, while others get rid of the ash by putting it into a waterway. It can also be recycled into several different materials. Fly ash has long been used as an additive in cement, grout, and concrete or to stabilize road beds as a fill material.[7] About 43% of the fly ash in the USA is recycled.[7] Re-purposing this coal ash reduces greenhouse gas emissions and reduces the amount of space taken up in landfills by this ash. Making concrete and bricks released a significant amount of greenhouse gases. Using fly ash in the manufacturing process reduces these emissions. There are also economic benefits of re-purposing fly ash, including reduced costs associated with its disposal, increased revenue from the sale of new coal ash products, and savings from using coal ash instead of other more costly materials. Although re-purposing this ash reduces environmental effects, there are critics who say that the use of fly ash in building materials is dangerous, as there could be air contamination from leaching chemicals.[1]

Environmental Effects

Figure 3. Piles of fly ash.[8]

When ash is disposed in dry landfills or wet ponds, there are associated environmental effects. Wet surface impoundments account for a fifth of coal ash disposal. These wet impoundments can be an issue if they do not have proper liners for the landfill or pond to prevent leaking and leaching. Both leaking and leaching lead to groundwater contamination. Leaching is a process that occurs when fly ash is wet, and it simply means that the toxic components of the ash dissolve out and percolate through water. This groundwater contamination can be harmful to human heath if the groundwater is a source of drinking water.[6] In addition to leaching, fly ash toxics are able to travel through the environment as a result of erosion, runoff, or through the air as fine dust. The fact that the chemicals in the ash can escape and move through the environment is what makes fly ash harmful.[6]

For Further Reading


  1. 1.0 1.1 1.2 A.Alva, T. Punshon, K.Sajwan,I.Twardowska. Coal Combustion Byproducts and Environmental Issues, 1st Ed. Springer Link, 2006
  2. 2.0 2.1 2.2 J.Fay, D. Golomb. Energy and the Environment, 1st ed. New York, U.S.A.: Oxford, 2002.
  3. SourceWatch. (July 28, 2015). Particulates and Coal [Online]. Available:
  4. 4.0 4.1 Photo taken by a member of the Energy Education team.
  5. EPA. (July 14, 2015). Fly Ash [Online]. Available:
  6. 6.0 6.1 6.2 6.3 6.4 PSR. (July 14, 2015). Coal Ash: Hazardous to Human Health [Online]. Available:
  7. 7.0 7.1 7.2 Ed Dodge. (July 13, 2015). Can Coal Fly Ash Waste Be Put to Good Use? [Online]. Available:
  8. Derek Dye. (July 13, 2015). Fly-Ash Removal [Online]. Available:

Authors and Editors

Jordan Hanania, Ashley Sheardown, Kailyn Stenhouse, Jason Donev
Last updated: January 4, 2019
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