Leveraging the Leftovers

Coal ash is one of those catch-all terms that refers to the leftover material from the burning of coal to produce energy. Just remember those images of Dad cleaning out the bottom of the charcoal grill. In a very basic way, that’s coal ash.

Now think big!

There’s a $50 billion industry that has developed over the years around the use of coal ash in building materials. It turns out there are different types of coal ash, such as fly ash, which is collected at the tops of chimneys and in the scrubbers that keep it from flying into the atmosphere. There’s also bottom ash and boiler slag, and each of which varies not only by where it is found in the burning process inside the power plant but also by its composition, burning properties, and other factors.

The point is, not all forms of coal ash are the same and so it can’t all be used the same way.

The good news is that a good part of the more than 100 million tons of coal ash that power companies in the United States produce each year can be recycled into building materials. Researchers at the University of North Carolina Charlotte are looking to discover more about coal ash to determine what are the best options for dealing with the material.

“Realistically, there are two things that can be done with it,” says John Daniels, Associate Professor and Interim Chair of the Civil and Environmental Engineering Department at the William States Lee College of Engineering at UNC Charlotte. “You can re-use it or dispose of it, that’s really it.”

Daniels admits using coal ash in as many ways as possible makes the most sense, because there is already so much coal ash available and more is being produced everyday.

UNC Charlotte researchers recently used fly ash as a component in a geopolymer concrete. The new material uses no Portland cement, the most common type of bonding material used in concrete.

The group is also evaluating the use of coal ash in what’s called a “flowable fill.” That’s concrete that is used in irregularly shaped areas where structural fills are required. They’ve found that the ash-based mixture flows and fills in better than traditional cement and is just as strong.

For now, the primary use of coal ash is in concrete. The Environmental Protection Agency endorses the use of ash-made concrete. It is strong and durable, and most importantly, the potentially toxic materials in ash that could contaminate water are locked inside. Coal ash also replaces Portland cement, which, as I mentioned earlier, is the bonding material in concrete.

The coal ash spill at Duke Energy’s Dan River plant has sparked new interest in possible uses for coal ash, which is a good thing because as Daniels says, “Coal is used to produce almost half of the world’s energy and that’s not expected to change through at least the first half of the century.”

- Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!


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