Fluidization of ultrafine particles differs considerably, and stable spouting is difficult to achieve. However, the stability of a spouted bed with ultrafine powders could be improved by adding coarse particles. In this work, the aggregation process and flow behavior of ultrafine powders in a spouted bed are simulated and analyzed under varying operating conditions by a two-fluid model coupled with the population balance equation. The effect of additional coarse particles is determined and analyzed by the proposed model. The simulation results indicate that the agglomerate diameter decreases with the addition of noncohesive coarse particles. Furthermore, the effect of operating parameters on the diameter distribution, solid volume fraction, and mixing behavior is analyzed. (C) 2017 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.