Systematic errors in the use of loss-on-ignition to measure unburned carbon in fly ash

The accuracy of the loss-on-ignition (LOI) test to determine the content of unburned carbon in fly ash samples was evaluated by thermogravimetry. Evaluations were performed on fly ash from a stoker boiler, a pulverized coal boiler, an atmospheric fluidized bed boiler and a pressurized fluidized bed boiler. The fly ashes from the fluidized bed boilers were found to contain significant quantities of portlandite [Ca(OH)2] and carbonate originating from limestone or dolomite injected into the boilers for control of sulfur emissions. Dehydration of the portlandite and calcination of the carbonate upon heating produce weight losses that are confused with oxidation of unburned carbon in the LOI test. The result is gross overestimation of the unburned carbon content of fluidized bed combustor fly ashes. Even the fly ash from the pulverized coal boiler contained enough carbonate from the coal to produce an unacceptably large error in the LOI analysis.