Biomass gasification in a fluidized bed reactor is a complex process involving intricately multi-scale and multi-physics processes and may be affected by many operating parameters. In this work, it is numerically investigated at particle scale using a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach. The flow dynamics, turbulence, heat transfer via convective, conductive, and radiative paths, and heterogeneous and homogeneous reactions are considered. The effects of key operating parameters on gasification performance are studied in terms of biomass particle diameter, initial bed temperature, steam to biomass (S/B) ratio, inlet gas flow rate, and feed position. The simulation results show biomass particle diameter has an insignificant influence on the production of H-2 and CH4 while it slightly affects the production of CO and CO2. Higher initial bed temperature leads to a larger decrease of gas temperature at bed exit and benefits the production of H-2 and CO. Increasing S/B ratio increases H-2 and CO2 production while decreases CH4 and CO production. Larger inlet gas flow rate leads to a shorter gas residence time and the decrease of S/B ratio. Biomass particles fed from the side wall give rise to higher temperature, lower H-2 and CO2, and higher CO and CH4 than that fed from the center bottom. Changing feed positions on the side wall has a slight influence on gas temperature while it has an insignificant influence on product compositions. (c) 2020 Elsevier Ltd. All rights reserved.