This study attempts to investigate the effect of gas volume change caused by chemical reactions on the flow hydrodynamics in a fluidized bed reactor. Various 2D simulations of ozone decomposition and the reverse reaction are conducted with a Eulerian-Lagrangian code-MFIX-DEM for a small-scale bubbling fluidized bed. The effect of particle size is studied by simulating two types of bed material with different sizes. Both transient and time-averaged flow behaviors are analyzed. The influences of gas volume change due to chemical reactions are studied with respect to bed expansion, superficial gas velocity, bubble characteristics including bubble size and frequency. The fluidized bed is then divided into bubble, shell and emulsion phases to further analyze the behavior of emulsion phase and gas flow distribution in different phases. A profound impact on the flow hydrodynamics by the gas volume change is found. For the small systems studied, the volume change in gas flow demonstrates the most significant influence on the gas flow in the shell phase for coarse particles and the bubble phase for fine particles. (c) 2011 Elsevier B.V. All rights reserved.