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.

Frequently Asked Questions

A percentage of the coal used in utility boilers does not combust. This non-combusted material is primarily in the form of carbon and is essentially unused or wasted energy. Unburned carbon is collected with the fly ash and follows the fly ash either to beneficial use or landfill. The energy value of the unburned carbon is therefore wasted contributing to the inefficiency of the electricity producing process.

Wasted energy in the form of unburned carbon is the fuel for the Carbon Burn-Out process. The CBO process tailors the combustion conditions to recover the energy (unburned carbon) in the fly ash. This recovered heat is returned to the electricity making process in the form of hot water.

The amount of energy recovered by the Carbon Burn-Out process is dependent on the carbon content and quantity of the fly ash. In any case, returning the heat to the electricity making process increases the electricity making efficiency by a measurable percentage.

A Carbon Burn-Out facility typically uses approximately 7,000 square feet of space. Most generating stations can easily accommodate a CBO facility. In most utility operations space is a premium. Landfill operations normally consume several square miles of land area. The CBO process is designed to drastically reduce the need for space intensive landfill operations thereby conserving precious natural resources.

Carbon Burn-Out can accommodate carbon contents as high as 90% and as low as 8%. A minimum of 8% carbon is typically needed for the process to work solely with the fuel value of the fly ash. For ash streams lower than 8% support fuel may be needed.

No, Carbon Burn-Out is specifically designed for use on fly ash.

Facilities using the CBO process are reporting that virtually 100% of the fly ash produced at the generating station has been placed in the marketplace. In fact, several of our customers have indicated demand exceeding the fly ash producing capacity of the generating station.

CBO produced fly ash meets or exceeds ASTM Class F specifications.

In markets where CBO fly ash is available, our customers have reported that CBO fly ash is the material of choice for use as a replacement for portland cement. CBO processed fly ash gives Ready mix operators the benefits of a consistently produced fly ash allowing improved quality and performance.

In many instances utilities add moisture to the fly ash to enhance handling properties prior to fly ash transportation and disposal. The CBO process was originally designed to use dry fly ash as the feed. Therefore, the CBO process eliminates the need for fly ash conditioning.

Construction of a CBO facility will take approximately 12 to 18 months.

Sizing of the Carbon Burn-Out System is critical to the success of the ash treatment program.

The size of the Carbon Burn-Out facility is governed by the amount of carbon combusted. For example a CBO system processing 100,000 tons per year fly ash containing 16% carbon is roughly the same size as a Carbon Burn-Out unit processing 200,000 tons per year at 8% carbon.

Certain design features of the CBO system allow for variation in carbon content in the + or – 25% range while maintaining designed throughput. Once this “system flex” is exceeded feed rate is adjusted to accommodate the carbon content change.

The Carbon Burn-Out process removes all of the ammonia contained in fly ash. The ammonia is decomposed to nitrogen and water resulting in ash containing no ammonia residues.

Commercial CBO operations operate 24/7 using two employees per shift.

The size of the Carbon Burn-Out system is determined by the amount of carbon combusted. By the addition of modules the size of the CBO unit is virtually unlimited. Units are sized to match the utilities fly ash production and the anticipated demand for fly ash in the service area.

Carbon Burn-Out Process

Carbon Burn-Out Process

Technical Information

Technical Data

Carbon Burn-Out