RESIDUAL CARBON FROM PULVERIZED-COAL-FIRED BOILERS .2. MORPHOLOGY ANDPHYSICOCHEMICAL PROPERTIES

The morphology and bulk physicochemical properties of residual carbon in eight fly ash samples from commercial power plants were investigate d. Enriched carbon samples extracted from the bulk fly ash were charac terized by high-depth-of-field optical microscopy, reflected-light mic roscopy, scanning electron microscopy, elemental analysis (C, H, O), a nd CO2 adsorption. The crystalline structure of the carbon was charact erized by X-ray diffraction, optical reflectance, and high-resolution transmission electron microscopy fringe imaging. The results were comp ared with measurements on laboratory-generated chars in the early-to-i ntermediate stages of combustion. Compared with those chars, the resid ual carbon is of similar elemental composition, petrographic compositi on and surface area but higher crystallinity. The fuel-related mechani sms that can contribute to carbon carryover in boilers are discussed, including inertinite persistence, mineral matter encapsulation and cha r deactivation by pregraphitization, as well as the implications for u tilization of residual carbon.