This study uses simulations with detailed chemistry to characterize the conversion of the various fuel components-noncondensable gas mixtures, soot, char fines; and coarse char and to identify the factors that determine unburned carbon emissions (a.k.a. LOI) from CFBCs. It covers virtually the entire operating domain for commercial CFBCs with 20 test cases in 11 full-scale CFBCs for a range of coal quality from brown coals through anthracite. Soot burnout was complete for all cases except one. LOI was entirely determined by incomplete burnout of coarse char particles, except that an anthracite produced fines that burned slower than coarse char. Simulations for a Chinese CFBC accurately depicted how hotter furnace temperatures and load variations affect LOI. The intrinsic char oxidation reactivity determines whether or not char fines and coarse char can effectively compete with noncondensable fuel mixtures for the available O-2 at lower elevations, and thereby influence NO production. More reactive chars compete more effectively. Char burning mechanisms determine which segment of the char PSD contributes to LOI, in conjunction with size variations in the transit times of char particles across CFBC risers. Relevant burning mechanisms comprise chemical reaction control, mediation by pore transport and film diffusion, thermal annealing, and ash encapsulation. Depending on the magnitude of the intrinsic char oxidation reactivity, any segment of the char PSD can make the largest contributions to flyash LOT.