Residual carbon from pulverized-coal-fired boilers. 2. Morphology and physicochemical properties

The morphology and bulk physicochemical properties of residual carbon in eight fly ash samples from commercial power plants were investigated. Enriched carbon samples extracted from the bulk fly ash were characterized by high-depth-of-field optical microscopy, reflected-light microscopy, scanning electron microscopy, elemental analysis (C, H, O), and CO₂ adsorption. The crystalline structure of the carbon was characterized by X-ray diffraction, optical reflectance, and high-resolution transmission electron microscopy fringe imaging. The results were compared with measurements on laboratory-generated chars in the early-to-intermediate stages of combustion. Compared with those chars, the residual carbon is of similar elemental composition, petrographic composition and surface area but higher crystallinity. The fuel-related mechanisms that can contribute to carbon carryover in boilers are discussed, including inertinite persistence, mineral matter encapsulation and char deactivation by pregraphitization, as well as the implications for utilization of residual carbon.