What Is Coal Ash and How Dangerous Is It?

Two coal power plant smokestacks against a clear sky.

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Coal ash refers to the hazardous byproducts of coal combustion at coal-based power plants — namely, fly ash, bottom ash, and boiler slag — which contain toxic materials such as arsenic and lead. It is a highly controversial type of industrial waste, given that the U.S. Environmental Protection Agency (EPA) didn’t begin regulating its disposal until 2015

In its natural state, coal is mildly hazardous. It can emit fine particulate pollution when sitting uncovered in stockpiles or being transported by trains, especially during windy weather conditions. But when coal is combusted or burned — such as in a power plant, when coal is burned in a boiler; heat from the furnace converts boiler water to steam; and the steam spins turbines to turn generators — it releases a hazardous brew of toxic pollutants into the air, including:

  • sulfur dioxide (SO2), which contributes to acid rain and respiratory illnesses,
  • nitrogen oxides (NOx), which contribute to smog and respiratory illnesses, and
  • carbon dioxide (CO2), a primary greenhouse gas that contributes to human-caused global warming.

Coal’s non-gaseous remnants, coal ash, contain arsenic, lead, mercury, and other heavy metals that are known to cause cancer, developmental disorders, and reproductive issues.  

The American Coal Ash Association estimates that in 2019, nearly 79 million tons of coal ash were generated. Consider this along with the fact that, from 1950 to 2015, coal was the largest fossil-fuel-based source of electricity generation in the United States (in 2016, it became the second-largest energy source behind natural gas), and you’ll have some idea of how much coal ash currently plagues the planet. According to the EPA, it’s the second-largest waste material in the United States behind household trash.

What Are the Byproducts of Coal Ash?

Coal ash consists of multiple coal combustion byproducts, including fly ash, flue-gas gypsum, bottom ash, and boiler slag, which accumulate in the bellies of coal-fired power stations.

Fly Ash

Roughly half of coal’s combustion leftovers take the form of “fly ash,” a light-colored, powdery residue that resembles wood ash. Fly ash is so fine and featherlight that it flies up into a power plant’s smokestacks. In the past, fly ash was released into the air this way, but laws now require that fly ash emissions be captured by filters.

Flue-Gas Gypsum

Flue-gas gypsum is produced when emission scrubbers inside of a coal power plant's exhaust stacks remove sulfur and oxides from the gas streams. It is the second most common coal combustion byproduct.

Bottom Ash

As its name suggests, bottom ash is the heavier portion of coal ash. Rather than floating into the exhaust stacks, it clumps together and settles at the bottom of the boiler’s furnace. Bottom ash comprises about 10% of coal ash waste.

Boiler Slag

The portions of coal ash that melt under the intense heat of combustion and then cool to form glassy, obsidian-like pellets are called boiler slag. Traces of boiler slag can be found in smokestack filters, as well as along the furnace bottom.

Exactly How Dangerous Is Coal Ash?

Aerial view of a coal ash cleanup site.
An aerial view of a coal ash pit cleanup site.

Savannah River Site / Flickr / CC BY 2.0

Coal ash is stored near power plants, both in open-air landfills (“ash pits”) and ponds of water or impoundments (“ash ponds”). The problem with this storage system is contaminants within coal ash can leach into the soil, rivers, lakes, and groundwater. This is especially dangerous for those living next to the over 310 active coal ash pits, as well as the over 735 active coal ash pond disposal sites across the United States. It's so dangerous, in fact, that if you live near a wet ash pond and get your drinking water from a well, you may have as much as a one in 50 chance of getting cancer from drinking arsenic-contaminated water, notes the EPA.   

A December 2008 coal ash spill in Kingston, Tennessee, which resulted in more than one billion gallons of coal ash slurry damaging homes and flowing into tributaries of the Tennessee River, highlighted the environmental and human health dangers of coal ash. In response, the EPA proposed the “Disposal of Coal Combustion Residuals from Electric Utilities” rule for regulating the disposal of coal ash, in June 2010. The EPA finalized the rule under the Obama administration in October 2015, but because the rule designated coal ash as “no-hazardous solid waste,” it wasn’t the win environmentalists had hoped it would be.

Although the EPA still labels coal as non-hazardous, this doesn’t negate the scientific fact that coal ash contains hazardous chemical compounds. Nor does it void the Coal Ash Rule’s aim of protecting communities against coal ash toxicity, or of holding companies in violation of coal ash regulations accountable.

Can Coal Ash Be Recycled?

One option for reducing the amount of coal ash that’s dumped in ash pits and ponds is to recycle and reuse it as other materials. The key to safely recycling such toxic materials is a process known as encapsulation, which binds the coal ash on a molecular level and thus minimizes the leaching of toxic chemicals. Fly ash, for example, binds together and solidifies when mixed with water, making it an ideal ingredient for cement and grout. Encapsulated gypsum is commonly used to create drywall.

Similarly, slag and bottom ash have been cleared by the EPA to be used as filler during the construction of roadways and embankments; however, these are unencapsulated uses of coal ash — uses where coal ash still poses some level of risk to the surrounding environment.  

Clearly, coal ash recycling is an imperfect science. Even so, it sometimes offers the least environmentally problematic option for coal ash disposal. 

View Article Sources
  1. "Electricity Explained: Electricity in the United States." U.S. Energy Information Administration.

  2. "What are Coal Combustion Products (CCPs)?" American Coal Ash Association.

  3. "Coal Ash, Fly Ash, Bottom Ash, and Boiler Slag" NRDC

  4. "Frequent Questions About the 2015 Coal Ash Disposal Rule." EPA.

  5. "Coal Ash Rule." Environmental & Energy Law Program at Harvard Law School, 2017.