The process of burning coal to generate electricity inevitably produces large amounts of ash – both in the form of bottom ash and fly ash. As the names suggest, bottom ash collects at the bottom whereas fly ash rises up through the chimney along with other flue gases. Together, these two forms of ash comprise the bulk of the waste produced during coal-combustion.
To give you an idea of just how much of this ash is produced, the U.S. alone produces about 131 million tons annually, by 460 coal-fired power plants (as of 2008). This includes other coal-combustion by-products such as flue gas desulfurization materials (FGD) and boiler slag. However, fly ash comprises roughly 60 percent of the total, with bottom ash, FGD, and boiler slag at 12, 24, and 4 percent respectively.
There have been gradual steps made toward finding commercial large-scale uses for these massive quantities of ash, but even today, there is only about 40 to 50 percent being recycled. The rest is either stored wet (to keep toxic dust from being carried by the wind) in ash ponds, or dry in landfills. Note here the Tennessee Valley Authority Fly Ash containment failure on the 23rd of December, 2008.
Fly ash can be reused in a number of ways, with some of the major ones being:
- As a substitute for cement and sand, up to about 30% by mass.
- Substitute for clay and aggregate in brick-making.
- Embankments, land and mine reclamation, and other structural fills.
- Soil stabilization (soft soil in particular).
- Fertilizer, ice control, toothpaste, composite building materials, and other various commercial products.
Finding a way to fully recycle such things as fly ash is but a small step in the direction of complete sustainability, but it’s certainly a start. Of course, in order to even consider the environmental benefits of recycling fly ash, one has to overlook the burning of huge amounts of coal that creates the problem in the first place. Ideally, we would nip the whole problem in the bud and find a way to get by only using renewable and clean sources of energy.
But we can only each do what we can. Here’s a short and informative video on why and how bricks are made using fly ash.