Experiments were carried out to investigate the agglomeration mechanism of neutral sulfite semi-chemical (NSSC) black liquor combusted in a bench-scale fluidized bed. The combustion experiments were focused on the influence or different operating conditions at temperatures ranging from 600 to 800 degrees C and at additive dosages from 0 to 30% with different additives (kaolin, calcium carbonate, and alumina). Defluidization resulting from agglomeration occurred frequently in the experiments. Samples of the bed material collected after the experiments were characterized by scanning electron microscopy, energy dispersive X-ray detection and X-ray diffractometry. There were explainable differences in the experimental phenomena and surface characteristics of the sinters depending on different operating conditions in this study, which can also be found in other studies. The mechanism has also explained that the additive is effective in solving the agglomeration problem. The experimental results indicate that the formation of the agglomeration in the fluidized bed burning NSSC black liquor depends mainly on the temperature, additive dosage, and additive species.