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

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


Robert S. Donovan 

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, fly ash, bottom ash, boiler slag. Here, a quick breakdown of what these terms mean and how they're different.

Coal ash

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. Coal ash is commonly divided into two subcategories based on particle size.

Fly ash

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

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.

Boiler slag

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!

This article was originally published on onEarth, which is no longer in publication. onEarth was founded in 1979 as the Amicus Journal, an independent magazine of thought and opinion on the environment. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. This article is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the article was originally published by and link to the original; the article cannot be edited (beyond simple things such grammar); you can’t resell the article in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select articles individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our articles.

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