If properly managed, fly ash can be put to beneficial reuse, reducing the environmental footprint that it produces.
Savannah Cooper
In December 2008, an embankment at the Tennessee Valley Authority’s Kingston Fossil Plant collapsed, releasing 5.4 million cubic yards of stored wet fly ash.1 The spill, the largest of its kind to date, covered 12 homes, damaged 42 residential properties and contaminated the Emory River, in addition to washing out a road, rupturing a major gas line and destroying trees and power lines.
Fly ash, also known as flue ash, is a green material that originates from one of the earth’s largest sources of air pollution—coal. More than 50 percent of electricity in the U.S. comes from coal-fired power plants.2 Because these plants are so widely used, fly ash is one of the most abundant industrial by-products on Earth.
Fly ash is produced by coal-fired power plants. In these plants, coal is pulverized and blown with air into the boiler’s combustion chamber where the coal immediately ignites, generating heat and creating a molten mineral residue. The boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. The coarser ash particles, known as bottom ash or slag, fall to the bottom of the combustion chamber. The lighter, finer ash particles, known as fly ash, remain suspended in the flue gas.
The disposal of fly ash presents a challenge because of the staggering amount produced by coal-fired power plants and because of the heavy metals, such as arsenic, that are contained in fly ash. The most common way to dispose of fly ash is to place it in a specially designed landfill built to prevent these heavy metals from leaching into drinking water supplies. While fly ash does not generate methane emissions, the transportation of fly ash to landfills results in carbon dioxide emissions because of the combustion of fossil fuels in the vehicles. Storing fly ash can also be potentially hazardous if an embankment collapses and the fly ash spills, like it did in Tennessee in 2008.
Reusable Fly Ash
Fortunately, fly ash can be recycled and reused, and over 22 million tons of fly ash are used annually in a variety of engineering applications.3 Properly managed, fly ash can be put to beneficial reuse, reducing the environmental footprint that it produces. Power companies will continue to burn coal and produce fly ash, so many in the industry feel that it makes sense to put that seeming waste to good use, especially if recycling the ash can save money and energy in the construction sector.
Cement Production
The United States EPA recently determined that coal ash from power plants is safe for use in cement production.4 Making cement is an incredibly energy-intensive process; concrete production contributes 5 to 8 percentof all the greenhouse gases produced in the world. However, concrete made with fly ash can reduce carbon emissions by as much as 90 percent, while also developing a more durable infrastructure.5
Fly ash can be used as a substitute material for Portland cement, the primary ingredient in concrete. For every ton of fly ash used in place of Portland cement, about one ton of carbon dioxide is prevented from entering the earth’s atmosphere.6 Fly ash also requires less water than Portland cement, reduces energy consumption and limits the need for landfill space. The energy saved by using fly ash in cement is equivalent to the electricity consumed by an average home in 24 days.
Paving Material and CLSM
In addition to its use in cement production, fly ash can also be combined with lime and aggregate to produce a quality stabilized base course, a type of paving material. Road bases are known as pozzolanic-stabilized mixtures (PSMs) and have the advantage of increased energy efficiency, providing a durable mixture and lower costs than other base materials.
Another use for fly ash is in flowable fill, or controlled low strength material (CLSM)—a mixture of fly ash, water and Portland cement that is designed to function in the place of conventional backfill materials, such as soil, sand and gravel. There are numerous benefits to using flowable fill, including decreased excavation costs, improved safety at job sites and reduced labor costs.
Fills and Embankments
Fly ash can also be used in structural fills and embankments, from small fills for road shoulders to large fills for highway embankments. Fly ash is more cost-effective than soil and rock, and its ease of handling and compaction reduces both construction time and equipment costs. Fly ash can also be used for soil improvement applications, such as soil stabilization and drying, and as a mineral filler in hot mix asphalt (HMA). Mineral filers increase the stiffness of the asphalt mortar matrix, improving the rutting resistance of pavements and the durability of the mix.
Recent developments in fly ash use promise even greater cost savings and environmental benefits. High volume fly ash concrete (HVFAC) is a concrete in which fly ash comprises more than 30 percent of the total cementitious materials.7 HVFAC has a lower cost and is more durable than conventional concrete.
Fly ash, if properly managed, can be recycled and put to beneficial reuse, avoiding the hazards that result from storage of the material. One of the most abundant industrial by-products on Earth, fly ash can be recycled and reused in a variety of applications, including cement production, paving material, flowable fill and in structural fills and embankments. The number of ways in which fly ash can be beneficially reused make it a valuable material, allowing companies in the coal sector to save both money and energy.
For more information, contact Worldwide Recycling Equipment Sales, LLC at (660) 263-7575 or [email protected].
Notes
- TVA Kingston Fossil Fuel Plant Release, http://www.epakingstontva.com/default.aspx
- Boyer, Mark. “How Fly Ash Concrete Works.” http://home.howstuffworks.com/home-improvement/construction/materials/fly-ash-concrete.htm
- U.S. Department of Transportation, Federal Highway Administration, http://www.fhwa.dot.gov/pavement/recycling/fach01.cfm
- “Coal Ash Declared Safe for Recycling by EPA,” Bloomberg News, http://www.bloomberg.com/news/2014-02-07/coal-ash-declared-safe-for-recycling-by-epa.html
- Boyer, Mark. “How Fly Ash Concrete Works.” http://home.howstuffworks.com/home-improvement/construction/materials/fly-ash-concrete.htm
- “Coal Ash Facts,” American Coal Ash Association Educational Foundation, http://www.coalashfacts.org/
- U.S. Department of Transportation, Federal Highway Administration, http://www.fhwa.dot.gov/pavement/recycling/fach01.cfm