Gasification thermochemical processes of biomass in a 20 kW downdraft gasifier are investigated using a robust two-dimensional (2D) computational fluid dynamics (CFD) modelling method. The model includes all the four zones of the gasifier namely drying, pyrolysis, oxidation and reduction. A step-by-step approach is proposed to evaluate the composition of different gas species as a result of the volatile break-up during gasification. However, selecting suitable chemical reactions for the CFD modelling becomes challenging as the commonly used reactions in kinetic study showed discrepancy in predicting the synthesis gas compositions. A revised set of chemical mechanisms is therefore proposed in the study and the robustness of the approach is examined with results validated against data from literature. The study reports how the air equivalence ratio (ER) affects the gasifier temperature and also the composition of producer gases. The model is then applied to investigate the syngas production of various biomass feedstocks sourced from Scottish agricultural sites. (C) 2018 Elsevier Ltd. All rights reserved.