About Coal Ash

What Are Coal Combustion Products?

Coal combustion products (CCP) are the materials produced when we burn coal to generate electricity. They include fly ash,bottom ashboiler slag,flue gas desulfurization gypsum, and other power plant byproducts. The term “product” was coined by the U.S. Environmental Protection Agency to promote recycling these and other industrial byproducts.

Using CCP rather than disposing of them conserves natural resources, reduces greenhouse gas emissions, and saves taxpayers significant costs.

When selecting materials for construction we have the choice of substituting recycled materials for conventional products to achieve sustainable design and drastically reduce a myriad of environmental impacts. In most cases these materials cost less, are available locally, and are technically equivalent or superior to virgin materials.

A multibillion dollar industry has “risen from the ashes” since the 1940s when the U.S. Bureau of Reclamation began using concrete made with fly ash to construct dams. Today, coal combustion products are incorporated into a wide range of building materials as well as engineered composite materials, such as metal alloys and plastics to provide strength without adding weight. Fly ash, bottom ash, FGD gypsum, and boiler slag are known worldwide as coal combustion “products” to emphasize the benefits of recycling over landfilling.

See our CPP production & use reports in the publications section of this site.

Fly Ash

Fly ash is a fine, powdery substance that “flies up” from the coal combustion chamber (boiler)and is captured by emissions controls, such as an electrostatic precipitatororfabric filter “baghouse,” and scrubbers.

This material is virtually identical in its composition to volcanic ash with pozzolanic properties ideal for concrete that built the structures of Ancient Rome we appreciate 2,000 years later. More than half of the concrete produced today in the U.S. uses fly ash in some quantity as a substitute for traditional cement. Among the world’s most renowned bridges, skyscrapers, roads, dams, and a wide range of other construction projects were built using high-performance fly ash concrete mixes to achieve superior strength and longevity. Builders routinely use 40 percent fly ash mixes. That amount can reach 70 percent or more in massive walls, girders, dams, and foundations.

Among fly ash concrete’s major environmental advantages:

  • Eliminates the need to mine virgin materials and thus prevents the associated impacts, while also conserving limited material resources;
  • Conserves land otherwise used for disposal;
  • Does not require kilning and therefore protects the atmosphere from carbon dioxide release; for every ton of fly ash used to replace traditional cement a ton of carbon dioxide is saved from entering the Earth’s atmosphere; over the past decade fly ash recycling eliminated nearly a billion tons of greenhouse gas;
  • Conserves water because fly ash uses very little compared with traditional cement.

Besides concrete, fly ash is also used in composite materials, such as in aluminum metal alloys for lightweight auto parts and synthetic lumber for outdoor decks and fences.

Please visit www.coalashfacts.org for more factual information on fly ash and other coal combustion products.

Applications

  • Raw material in concrete products and grout
  • Feed stock in the production of cement
  • Fill material for structural applications and embankments
  • Ingredient in waste stabilization and/or solidification
  • Ingredient in soil modification and/or stabilization
  • Component of flowable fill
  • Component in road bases, sub-bases, and pavement
  • Mineral filler in asphalt

Bottom Ash

Bottom ash is agglomerated ash particles, formed in pulverized coal furnaces, that are too large to be carried in the flue gases and impinge on the furnace walls or fall through open grates to an ash hopper at the bottom of the furnace. Physically, bottom ash is typically grey to black in color, is quite angular, and has a porous surface structure.

Bottom ash can be used as a replacement for aggregate and is usually sufficiently well-graded in size to avoid the need for blending with other fine aggregates to meet gradation requirements. The porous surface structure of bottom ash particles make this material less durable than conventional aggregates and better suited for use in base course and shoulder mixtures or in cold mix applications, as opposed to wearing surface mixtures. This porous surface structure also makes this material lighter than conventional aggregate and useful in lightweight concrete applications.

Applications

  • Filler material for structural applications and embankments
  • Aggregate in road bases, sub-bases, and pavement
  • Feed stock in the production of cement
  • Aggregate in lightweight concrete products
  • Snow and ice traction control material

Boiler Slag

Boiler slag is the molten bottom ash collected at the base of slag tap and cyclone type furnaces that is quenched with water. When the molten slag comes in contact with the quenching water, it fractures, crystallizes, and forms pellets. This boiler slag material is made up of hard, black, angular particles that have a smooth, glassy appearance.

Boiler slag is generally a black granular material.

Boiler slag particles are uniform in size, hard, and durable with a resistance to surface wear. In addition, the permanent black color of this material is desirable for asphalt applications and aids in the melting of snow. Boiler slag is in high demand for beneficial use applications however, supplies are decreasing because of the removal from service of aging power plants that produce boiler slag.

Applications

  • Component of blasting grit and roofing granules
  • Mineral filler in asphalt
  • Fill material for structural applications and embankments
  • Raw material in concrete products
  • Snow and ice traction control material

Flue Gas Desulfurization Gypsum

The second most common use of CCPs is flue gas desulfurization (FGD) gypsum in wallboard. This material is produced by emissions control systems (scrubbers) that remove sulfur and oxides from power plant flue gas streams. Residues vary, but the most common uses in construction applications are FGD gypsum (or “synthetic” gypsum). This material is used in more than 40 percent of the gypsum panel products (wallboard) manufactured in the U.S. FGD gypsum can be used in cement production and geotechnical applications. It can also be used in self-leveling floor applications. The agricultural industry also uses this material to treat undesirable soil conditions and to improve crop performance.

Applications

  • Raw material for wallboard
  • Fill material for structural applications and embankments
  • Feed stock in the production of cement
  • Raw material in concrete products and grout
  • Ingredient in waste stabilization and/or solidification