Energy & Fuels 2005, 19, 1493-1502
1493
Comparison of Heating Losses and Macro Thermogravimetric Analysis Procedures for Estimating Unburned Carbon in Combustion Residues Stuart C. Burris,* Dong Li, and John T. Riley Department of Chemistry, Materials Characterization Center, Western Kentucky University, Bowling Green, Kentucky 42101 Received December 21, 2004. Revised Manuscript Received April 28, 2005
One of the most important indices for evaluating the combustion efficiencies of boilers, as well as the commercial value of the produced fly ash, is the unburned carbon in fly ash. The most common method currently used by combustion engineers to estimate the amount of unburned carbon in fly ash is to equate it to the dry loss on ignition (LOI) value. There seems to be no reported systematic study linking LOI values with the true carbon content of ashes and combustion residues. In this study, the LOI values for 35 combustion residues were determined at 500, 750, and 950 °C, using a macro thermogravimetric analyzer. The carbon contents of the combustion residues and the residues from the LOI determinations were then measured. For the samples in this study, it was determined that temperatures of >790 °C should be used to achieve complete carbon burnoff. For low-percentage-carbon combustion residues, there is very poor agreement between the unburned carbon contents and the LOI values. This is especially true if the samples are exposed to the atmosphere for extended periods of time, because the combustion residues readily absorb moisture and acidic gases. For high-percentage-carbon combustion residues, there is good agreement between the unburned carbon and the LOI values, especially if the residues are relatively fresh.
Introduction Unburned carbon in fly ash is one of the most important indices for evaluating the combustion efficiencies of boilers, as well as the commercial value of the produced fly ash. The principal components of most fly ashes are SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, and TiO2, in addition to carbon. The unburned carbon that is associated with fly ash may range from a few tenths of a percent to ∼20%. If the amount of unburned carbon is
