In fluidized bed methanation reactors, gas volumetric flow reduction due to the reaction can cause serious problems for fluidization quality, and in extreme cases, unwanted defluidization may occur. Note that the fluidization quality is closely linked to the expansion of emulsion phase, this paper present a numerical study on the emulsion phase expansion of Geldart A type of particles in fluidized bed methanation reactors by use of the well-established CFD-DEM model. The effects of operation conditions such as reaction rate, superficial gas velocity, pressure and bed inventory are discussed and compared qualitatively with experiments in literature. It is found that a higher gas velocity, based on the reactant gas at the inlet, is required to fully fluidize the particles and maintain good fluidization quality in a methanation reactor. Fast bubbles, considered as an important feature in the fluidization of Geldart A type of particles, were observed. Close check shows that the gas circulation around fast bubbles is responsible for the worsened gas exchange between gas bubbles and emulsion phase and consequently the apparent contraction of emulsion phase for Geldart A type of particles. Higher pressure and lower bed height can improve emulsion phase expansion in the methanation fluidized bed reactors, which is in accordance with previous experimental findings. The superficial gas velocity affects the emulsion phase expansion in a complicated way, where two mechanisms, i.e., large bubble size and short gas residence time, will work together. (C) 2015 Elsevier B.V. All rights reserved.