PARTICLE SIZE-DENSITY RELATION AND CENOSPHERE CONTENT OF COAL FLY-ASH

The results are reported of detailed physical characterization of six ashes from coals representative of those burned in US power plants. Centrifugal separation was used to classify the ashes into six density categories in the range < 1.6 to > 3.2 g cm-3. The size distributions of all density classes were determined in the range 1-200 mum. For most of the density classes, log-normal functions, truncated outside the measurement limits, described the size distributions quite well. For all six ashes, the median diameter initially decreased and then increased with increasing particle density. The influence of particle structure on this large variation (up to sixfold) in size is discussed. Centrifugal separation using a liquid of density 2.2 g cm-3 was used to estimate the mass fraction of cenospheres (i.e. particles with trapped interior 'bubbles') in the ashes. This fraction varied from < 5 to > 95 wt%. The cenosphere content was apparently uncorrelated with coal rank but was positively correlated with the total mineral content of the coal. The median diameters of the cenospheric fractions were found to be two to three times those of the non-cenospheric (solid) fractions. The density-size data were used to determine the Fe2O3 distribution in the ashes.