Biomass containing organic materials could come from a number of sources such as from agricultural residues, sustainable forests, waste food, and industry by-products. Also, being a renewable source of energy, it has the significant potential to reduce greenhouse gas emissions releasing from the fossil fuel based technologies. Therefore, energy from biomass is becoming a favourable technology to convert solid fuel to valuable gas and one of the effective approaches is gasification. In this research, a three dimensional (3D) computational fluid dynamics (CFD) steady-state thermochemical model is developed to simulate biomass (rubber wood) gasification in a downdraft gasifier. Simulated CFD model includes all the four zones (drying, pyrolysis, oxidation and reduction) of gasifer. For optimising the gasifier temperature and syngas composition, a sensitivity analysis of homogeneous oxidation reactions is carried out, with the model identifying the suitable kinetic reactions for gasification. Predicted CFD modelling results are compared with those from the kinetic modelling and experimental results, where a good agreement is obtained. The effect of gasifier temperature, equivalence ratio (ER) and biomass feed rate on the syngas production is studied. Further, the effect of volatile composition and rate of Boudouard reaction at different ERs along the gasifier height is investigated. (C) 2019 Elsevier Ltd. All rights reserved.