The combustion of cylindrical pellets of a market-available, packaging-derived fuel, obtained from a mono-material collection of polyethylene terephthalate (PET) bottles, was investigated by means of two batchwise operated bench-scale fluidized bed combustors. After devolatilization, the fixed carbon was present in the bed in three phases: an A-phase, made of aggregates of sand and char, an S-phase, made of individual carbon-covered sand particles and an F-phase, made of elutriable carbon fines, abraded by the surfaces of A- and S-phases. A simple model was developed in order to quantify the role of different phenomena affecting the overall fixed carbon conversion. The algorithm was based on a network of paths representing the generation of the three phases from the parent fuel, the crumbling of A-phase, the attrition of S-phase, the combustion of the fixed carbon as well as the elutriation of carbon fines. The calculated data compared well with the experimental results. The role of the comminution phenomena was negligible. The kinetic parameters and the bed temperature appeared to have a remarkable influence on the fixed carbon combustion efficiency.