Recycling and Reusing Coal Ash
According to the U.S. Energy Information Administration, about 39 percent of all
electricity generated in the United States comes from coal.
Coal-fired power plants burn coal to make steam, and the steam turns turbines to
generate electricity. All coal naturally contains inorganic matter from the rocks and
minerals in the coal seam where it was mined. When that coal is combusted in a
boiler, the inorganic matter in the coal becomes coal ash. According to the U.S.
Environmental Protection Agency (EPA), coal combustion residual material is one
of the largest waste streams generated in the United States.
Duke Energy is committed to exploring opportunities to recycle ash produced
at operating coal plants (production ash) and the ash currently stored in basins
(ponded ash).
Fast facts about Duke Energy’s beneficial reuse
Ash produced in 2015
Percent of ash reused in concrete
products and structural fill projects
Fleetwide
3.0 million tons
63%
North Carolina
1.3 million tons
38%
Commercial construction products and engineered structural fills
Fly ash – a fine material similar to the consistency of talcum powder – can be reused in concrete products and projects,
including roads, bridges and buildings. It also can be used as structural fill in projects, such as building embankments or filling
trenches, when native soil is not readily available or not strong enough to support a structure. Using fly ash in place of Portland
cement in concrete can increase the overall strength of the concrete by up to 30 percent.
Bottom ash – a coarser, granular material collected from the bottom of coal-fired boilers – is typically used to replace sand or
gravel in the manufacturing process for concrete blocks and pipe. It is also well-suited for use in engineered structural fills,
embankments and road base.
Construction projects that use coal ash, such as structural fills, are engineered to meet specific design requirements. National
transportation and construction codes and standards, and state specifications, require coal ash to meet certain criteria when
it’s used in a project. Currently, the N.C. Department of Transportation limits replacement of cement with fly ash up to 30
percent. State regulators review and approve projects that use recycled materials such as coal ash to ensure protection of the
environment and public health.
Complexities and limitations affecting the coal ash market
Several key factors affect the marketability of coal ash:
• Chemical composition – Coal ash must undergo chemical analysis and testing before it is considered for reuse or recycling
applications. Consistency in the quality of the ash is key to its desirability as an ingredient in concrete.
• Transportation and logistics – The distance coal ash must be hauled from the power plant to the manufacturing or structural
fill site will affect the cost-benefit evaluation of reuse. The lack of a cement kiln in North Carolina causes an additional
challenge in the state.
• Suitable project size – Coal ash is an economically feasible alternative to other fill materials for larger-scale structural fill projects.
www.duke-energy.com
February 2016
Processing fly ash for beneficial reuse in concrete and potential technology options
In order to be suitable for concrete, coal ash must
consistently have a carbon content below 2 percent. Boiler
design and many environmental controls, including scrubber
technology, installed at operating coal plants cause coal ash
to have higher carbon content, making the ash unsuitable
for reuse without additional processing.
Duke Energy is exploring a variety of technologies that
can process ash to remove carbon, including thermal
beneficiation and electrostatic separation. Other
technologies that beneficially reuse a significant volume of
coal ash include surface mine reclamation, brick production
and plasma technology. While asphalt filler and highway
applications reuse a smaller volume of ash, both are also
viable options.
The company continues to review other technologies. In 2015, the company selected the Electric Power Research Institute
(EPRI) to conduct a comprehensive study of the coal ash recycling market and available technologies.
The findings of this study will be available in mid-2016 and will help complement the ongoing work of a dedicated
Duke Energy team to identify safe and viable options for coal ash.
Regulating coal ash reuse
Any beneficial reuse option Duke Energy chooses must meet all state and federal regulations and be a prudent investment
for our customers.
Ideas and questions?
Feel free to contact us at CoalAshReuse@duke-energy.com.
Sources
U.S. Environmental Protection Agency
American Coal Ash Association
U.S. Energy Information Administration
Duke Energy statistics