Successful deployments of treatment technologies for substandard coal combustion products (CCP) signal decades of Class F fly ash supplies for North American concrete producers. Such assurance stands regardless of specification-grade ash availability reflecting power generating station coal to natural gas conversions—or the pace of a broader energy sector shift from fossil fuels to alternative sources.
With 6 percent to 15 percent carbon content, high LOI (loss on ignition) CCP has traditionally been impounded at coal-fired power generating station sites for interim or longer periods, while lacking an economical path to recycling in concrete mixes or as cement kiln chemical compound and fuel feedstock. Along with state and federal regulators, cement and concrete market conditions are altering impounded material math and driving the practice of ash harvesting.
One of the most effective technologies for bringing harvested ash to ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete performance is proving out at three Duke Energy power stations in North Carolina. The Charlotte-based utility giant reports 2020-2023 shipments of 2.5 million-plus tons of Class F ash processed from Staged Turbulent Air Reactor (STAR) installations at its Buck, Cape Fear and H.F. Lee stations. Duke Energy enlisted The SEFA Group, a Heidelberg Materials North America subsidiary based in Lexington, S.C., to build and operate STAR facilities as part of a larger agreement with North Carolina regulators on recycling or permanent storage of 120 million tons of impounded CCP at 14 stations. Each equipped for annual output of 250,000-400,000 tons, the Buck, Cape Fear and H.F. Lee stations are modeled on large scale STAR installations SEFA Group has provided Duke Energy peers in South Carolina and Maryland.
PROVEN PATH
Duke Energy is the first among CCP managers to tackle 100 percent ponded ash versus a dry and wet material mix. The SEFA-operated STAR installations have supported harvested ash output at higher volumes than concrete-grade ash emanating from the utility’s current coal-fired power plant fleet in the Carolinas.
SEFA Group developed the STAR process to alter chemical composition whereby ash meets the ASTM C618 silica, alumina and iron oxides content threshold of 70 percent or more and has carbon content below 1 percent. The technology and equipment at Buck, Cape Fear and H.F. Lee entail:
- Ash excavation from ponds or basins, followed by screening and hauling to feed sheds;
- Feeding of material into external heat exchangers, which dry the ash using excess reactor heat;
- Pneumatic conveying of material to the STAR reactors, where high temperatures quell the carbon; and,
- Cooling of processed ash, then storage silo transfer.
Prior to the STAR deployments, notes Duke Energy Manager of Coal Ash Recycling John Halm, “We lacked the ability to dry and reduce excess carbon from ash that was stored wet in basins, and this limited options for beneficial use. SEFA created a breakthrough technology to resolve that. The beauty of this process is that it allows the consistent production of larger volumes of ash to meet market demand while utilizing previously impounded basin ash that would have required further disposal. Using higher concentrations of ash in concrete reduces the embodied carbon footprint of the finished product and helps our customers achieve their long-term sustainability goals by reducing the need to mine natural resources like limestone to produce cement. For every ton of coal ash recycled to produce concrete, about one ton of greenhouse emissions is avoided.”
The STAR facilities are engineered for harvested ash volumes upward of 17 million tons, based on the estimated 5.3 million to 5.8 million tons of ponded CCP at each of the Buck, Cape Fear and H.F. Lee stations. All three are on track to sunset by December 2035, the state deadline for their basin closures. Through 2028, about 2 million tons of CCP material from a fourth Duke Energy station, Weatherspoon, is being delivered to cement producers, who will realize the ash’s Btu value and clinker-augmenting chemicals in the kiln phases. Over the next 15 years, Duke Energy plans transfer of nearly 80 million tons of CCP from legacy storage to permanent landfills meeting State of North Carolina guidelines—building on the 40 million-plus tons of material recycled or transferred thus far.
Turbulent Air Reactors. The STAR technology embodies the premier process for converting the marginal material to high performance, ASTM C618 Class F fly ash.
PLAYING IT SAFE
STAR output has been of such purity and consistency that concrete producers—among them this month’s cover story subject, Concrete Supply Co. (pages 44-45)—hardly need to concern themselves with which Duke Energy station their ash originated. Carolinas or neighboring state contractors and agencies have been quick to credit the Buck, Cape Fear and H.F. Lee projects as stabilizing Class F fly ash supplies across their region for nearly four years.
Strong sales of material exhibiting optimal chemistry and gradation are only part of the success story: SEFA Group earned the Duke Energy 2023 Environmental, Health and Safety Supplier of the Year award, a salute to 184,000 hours of ash handling, processing and transportation with zero OSHA recordable injuries.
“The energy sector must transition for tomorrow in a way that protects our communities today,” said Duke Energy Senior Vice President, Enterprise Safety & Generation Services Jessica Bednarik, addressing the company’s Sixth Supplier Exchange Forum last year. “The SEFA Group is helping make this a reality with their support in recycling coal by-products for beneficial reuse and their commitment to employee safety.”
“This award recognizes the outstanding record of our Transportation and CCP Plant Operations team members at Duke Energy sites,” added SEFA Group Chief Operating Officer Jim Clayton. “We are proud of continuous efforts to make the safety of employees, customers and communities our highest priority.”