Fly ash from pulverized coal power plants can be a valuable mineral admixture in concrete and other high-value applications. However, plant technologies in use today to reduce NOx emissions and enhance electrostatic precipitator performance make meeting applicable quality standards an ongoing challenge.
- Use of low-NOx burners, pursuant to Clean Air Act requirements, has driven residual carbon levels in fly ash higher. Treatment of such ash is needed to maintain, and even improve, its quality to meet customer demands for low loss on ignition.
- Ammonia injection is widely applied in selective catalytic reduction (SCR) and selective non-catalytic (SNCR) flue gas treatment systems to meet more stringent NOx standards than can be met solely with low-NOx burners. Removal of ammonia is required for any ash containing more than approximately 100 parts per million if it is to be used in concrete applications.
To remedy these effects, Boral provides Carbon Burn-Out (CBO), a technology and process in which residual carbon in fly ash is combusted to produce a consistent low-carbon, low loss-on-ignition (LOI), high-quality pozzolan. As a side benefit, and without modification to the CBO, ammonia on the ash is reduced to non-detectable levels.
The process may be used to beneficiate high-carbon fly ash either directly from the power plant or from fly ash that has been stored in landfills or ponds.
Founded on the time-proven fluid bed combustion technology, the CBO process is continuous and fueled solely by residual carbon. CBO tailors the combustion conditions to recover the wasted energy of the unburned carbon in the fly ash. Heat is recovered from both the flue gas and the hot product ash.
Recovered heat is typically returned to the host power plant by heating a portion of the power plant’s condensate stream. This portion of the condensate stream bypasses existing feedwater heaters, thereby reducing the amount of extraction steam required, which in turn increases the quantity of steam available to the turbine-generator.
To maximize product availability and minimize labor expense, CBO facilities include an ash storage dome as well as automated truck loading and weighing. This system is available for loading around-the-clock and requires neither plant operators nor scale house attendants.
The CBO process was first commercialized early in 1999 when the Wateree Carbon Burn-Out plant began operations. Owned by South Carolina Electric and Gas, this full-scale facility met or exceeded its design parameters for ash processed and heat recovered. A second CBO facility located at Santee-Cooper’s Winyah power station was placed online in 2002.