In this study, a downdraft gasifier model is coupled with a single-particle model to analyze the effects of particle geometries such as slab, cylindrical, and spherical on different parameters. Simulations were performed using particles in a thermally thick regime with larger particle sizes of 0.003-0.05 m, which are generally used in commercial gasifiers. This combination of the downdraft gasifier model and single particle model was implemented using Comsol Multiphysics software program. The results obtained are in good agreement with those of obtained in previous studies. The low thermal conductivity of biomass (k(g)), small particle size (d(p)), high gas temperature (T-g), higher molar fraction of oxygen in primary air (X-O2), and high mass-transfer coefficient (k(m)) favor the formation of carbon monoxide (CO), hydrogen (H-2), high tar conversion, and higher lower heating value. To maximize the CO composition and tar conversion, the initial gas temperature (T-g) is more crucial. The least sensitive parameter is the thermal conductivity of biomass (k(B)) in relation to product composition of CO, carbon dioxide (CO2), H-2, and methane (CH4). The sensitivity for all the parameters is found to be the highest for the spherical geometry and is least for the slab geometry. (C) 2017 Elsevier Ltd. All rights reserved.