Generation of fine chars from Australian black coals in pressurized fluidized bed combustion

Abstract

Five Australian black coals were characterized and the most significant mechanisms for the in-bed generation of fine char particles were studied under pressurized fluidized bed combustion (PFBC) conditions of 1.6 MPa, 850°C, U = 0.9 m/s, 7% O2, and coal particle size 4 to 4.75 mm in a novel batch-fed, bench-scale pressurized fluidized bed of 1.3 mm silica sand particles. The generation of elutriable fine char particles in our PFBC tests was attributed to attrition, fragmentation, and combustion residues of in-bed char particles. Both primary fragmentation and secondary fragmentation mainly produced large particles, which remained in the bed, but also generated some elutriable small pieces, contributing to carbon elutriation from PFBC. However, attrition was the dominant contributor to the generation of elutriable fine char particles. The specific char attrition rates were approximately constant during the early stage of burnout (for larger char particles > ∼2 mm), because of the stable development of detachable asperities on the external surface of char particles, when combustion was controlled by the external mass transfer of oxygen. The specific attrition rates increased at the later stage of burnout, because of the more effective intraparticle diffusion of oxygen into the pores of smaller particles and the development of porous structure within the surface layer, giving a higher specific attrition rate.