Coal Ash, Fly Ash, Bottom Ash, and Boiler Slag

A top-to-bottom tour of fossil fuel sludge.

January 09, 2015

Coal ash (n.): a collection of residuals produced during the combustion of coal

Fly ash (n.): a light form of coal ash that floats into the exhaust stacks

Bottom ash (n.): the heavier portion of coal ash that settles on the ground in the boiler

Boiler slag (n.): melted coal ash

Few people have heard of coal ash. But U.S. fossil fuel plants produce 140 million tons of the stuff every year in the process of combusting coal, making it the nation’s second-largest waste stream behind household trash. Every now and then we get a reminder of just how much coal ash is around. In 2008, a coal ash pond broke through its dam in Kingston, Tennessee, covering hundreds of acres in toxic sludge. In early 2014, a spill released more than 30,000 tons of coal ash into North Carolina’s Dan River. The U.S. Environmental Protection Agency finally got around to regulating the waste last month, but the rules are a major disappointment to environmentalists and the millions of Americans living near coal ash pits and ponds.

Perhaps one of the reasons for coal ash’s relatively low profile is that it goes by so many names. Coal ash is a general term—it refers to whatever waste is leftover after coal is combusted, usually in a coal-fired power plant. It contains arsenic, mercury, lead, and many other heavy metals. Not something you’d want to sprinkle on a salad.

Coal ash is commonly divided into two subcategories based on particle size. The most voluminous and well-known constituent is fly ash, which makes up more than half of the coal leftovers. Fly ash particles are the lightest kind of coal ash—so light that they “fly” up into the exhaust stacks of the power plant. Filters within the stacks capture about 99 percent of the ash, attracting it with opposing electrical charges. Fly ash is recyclable. The fine particles bind together and solidify, especially when mixed with water, making them an ideal ingredient in concrete and wallboard. The coal ash versions of these products are actually stronger than those made from virgin materials. The recycling process also renders the toxic materials within fly ash safe for use.

Bottom ash is the coarser component of coal ash, comprising about 10 percent of the waste. Rather than floating into the exhaust stacks, it settles to the bottom of the power plant’s boiler. Bottom ash not quite as useful as fly ash, although power plant owners have tried to develop “beneficial use” options, such as structural fill and road-base material. This isn’t a good idea, because the bottom ash remains toxic when recycled. On a Virginia golf course in 2007, for example, a hill composed of coal ash leaked heavy metals into the groundwater.

Finally, there’s boiler slag, the melted form of coal ash that can be found both in the filters of exhaust stacks and the boiler at the bottom. Even this foul sludge has its uses. Boiler slag can be included in roofing shingles (a reasonably safe application) and in structural fill (not as good, remember the golf course?).

Coal ash isn’t going anywhere anytime soon. The EPA’s recent rules still leave it up to states to manage the waste, and unfortunately, much of it will likely continue to end up in our groundwater and streams. At least now you can call it by name when you see it on a hike. I spy something...boiler slag!


onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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