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.