Modeling the vaporization of ash constituents in a coal-fired boiler

Emissions of fine particulate and trace toxic metals are being subjected to increasing regulation. This paper addresses how the emission of these compounds can be influenced by changes in combustion conditions, particularly those selected to minimize NOx emissions. The vaporization and condensation of refractory oxides dominate submicron aerosol formation during the combustion of bituminous coals. The vaporization of these oxides is augmented by the reduction of refractory oxides to suboxides or metals.We used available drop tube furnace data to develop and test models for the vaporization of aluminum and iron. The vaporization of alumina is found to occur primarily via reduction of the alumina to Al2O. Although other paths are recognized to be important, the vaporization of iron is approximated by a mechanism involving FeO reduction. The models for aluminum and iron vaporization, together with previously developed models for the other refractory metals, are incorporated into a computational fluid dynamics code to determine the impact of different oxidation-temperature histories in a utility boiler. The vaporization of refractory oxides is calculated for conditions corresponding to the operation of the boiler before and after retrofitting with low-NOx burners and overfire air ports. The results show that the vaporization of refractory oxides is diminished under low-NOx operating conditions and that the different oxygen-temperature histories of particles lead to significant differences in the vaporization of oxides for particles originating from different burners. The vaporization is found to occur primarily in regimes where temperature and CO concentration are high.