What Is Coal Ash?

Coal Combustion Residuals (CCRs), more commonly known as coal ash, is the waste material or by-products left over when coal is burned to produce electricity in coal-fired power plants.  All coal ash contains toxic components normally found deep in the earth.  These include arsenic, mercury, lead, chromium, cadmium, and selenium.  The levels of toxic components can vary because it depends on where the power plant got its coal.

Coal ash ​may be disposed of or, in some instances, recycled for future use as a raw material.  There are four types of coal combustion waste: fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) residue.

Fly Ash
Fly ash is a very fine, powdery material composed mostly of silica, and is quite similar to volcanic ash.  Fly ash is carried up into the power plant's smoke stacks with the flue gases from the furnace/combustion chamber, and is captured by air pollution control equipment.

Bottom Ash
Bottom ash is composed of coarse, jagged, slightly porous particles that, unlike fly ash, are too large and heavy to be carried up into the power plant's smoke stacks.  Instead, it falls through grates into a ash hopper at the bottom of the furnace/combustion chamber.

Boiler Slag
Boiler slag is molten (melted) bottom ash that is formed only in certain types of coal furnaces,  When boiler slag is doused with water, it turns into hard, black, glass-like pellets.

Flue Gas Desulfurization (FGD) Residue
Flue gas desulfurization (FGD) residue is a by-product of the air pollution control equipment required at coal-fired power plants for reducing sulfur dioxide emissions.  FGD residue is initially released as a wet sludge/slurry that is later dried to form a powder.

Recycling of Coal Ash

When recycled as a raw material, coal ash is referred to as Coal Combustion Products (CCPs), a term devised by the U.S. Environmental Protection Agency to encourage the beneficial and proper reuse of this industrial waste.  US EPA sees several advantages in recycling coal ash, such as fewer landfills needed for ash disposal, revenue gained from the sale of coal ash, employing coal ash as a substitute for limited natural resources or more expensive materials, and improved end product or application performance.

Fly ash is used extensively today in making concrete, especially high-performance concrete mixes for bridges, skyscrapers, roads, and dams.  Bottom ash can replace other construction aggregate in road bases, pavement, and lightweight concrete.  Boiler slag is in demand for use in asphalt, blasting grit, and roofing materials.  Flue gas desulfurization (FGD) residue is dried to form a powdered material for use in the manufacture of drywall needed by the construction industry.

Disposal of Coal Ash

Coal Ash Pollution

Federal and State Regulation of Coal Ash

Disposal of Coal Combustion Residuals from Electric Utilities (and Independent Power Producers)

Coal Ash Pollution Prevention Act

Sources and Further Reading

1.  Coal ash basics: What is coal ash? (2019, February 05). Retrieved from https://www.epa.gov/coalash/coal-ash-basics#01

2.  CCP FAQs: Is coal ash hazardous? (n.d.). Retrieved from https://www.acaa-usa.org/aboutcoalash/ccpfaqs.aspx#Q9

3.  Coal ash basics: Why is coal ash reused? (2019, February 05). Retrieved from https://www.epa.gov/coalash/coal-ash-basics#04

4.  Coal ash reuse. (2019, July 15). Retrieved from https://www.epa.gov/coalash/coal-ash-reuse

5.  What are CCPs? (n.d.). Retrieved from https://www.acaa-usa.org/what-are-ccps.aspx

6.  Fly ash. (n.d.). Retrieved from https://www.acaa-usa.org/AboutCoalAsh/WhatareCCPs/FlyAsh.aspx

7.  Bottom ash. (n.d.). Retrieved from https://www.acaa-usa.org/AboutCoalAsh/WhatareCCPs/BottomAsh.aspx

8.  Boiler slag. (n.d.). Retrieved from https://www.acaa-usa.org/aboutcoalash/whatareccps/boilerslag.aspx

9.  Flue gas desulfurization gypsum. (n.d.). Retrieved from https://www.acaa-usa.org/aboutcoalash/whatareccps/fluegasdesulfurizationgypsum.aspx