Microscopic investigation of unburnt char in fly ash

Abstract

A microscopic method to characterize unburnt char particles in fly ash is proposed. Polished samples of fly ash in epoxy resin to which a few drops of blue pigment have been added are observed by incident light microscopy. The char particles are classified into four types (vesicular, dense, mixed and mineral-rich) and categorized into five volumetric size classes (>10 6, 10 6−10 5, 10 5−10 4, 10 4−10 3, <10 3μm 3) Percentages of individual types and classes are calculated and can be illustrated diagrammatically. To test the feasibility of the proposed method, eight fly ash samples were analysed. Reasonable relationships between the level of unburnt char, particle size and proportions of char types were recognized. When the total carbon content in a fly ash was high, the proportion of coarse classes was also high, and there was a broad inverse correlation between the total carbon and total percentage of the dense char. The proposed microscopic method may be useful in understanding various technical problems related to coal combustion in pulverized-fuel fired furnaces.