This paper provides a numerical analysis of a downdraft fixed bed reactor to examine the effect that the different operating modes have on temperature profile, velocity profile, gas composition and particle residence time. First, pine wood sawdust is considered as the feedstock to the reactor. Simulations of the reactor operating modes are performed using the commercial CFD code Ansys-Fluent coupled with the Discrete Phase Method (DPM). So different chemical reactions are considered in the reactor depending on the operating mode: a) pyrolyzer, b) gasifier or c) combustor. A two-dimensional modelling for the geometry is used in conjunction with 0.001 mm diameter spherical particles. The interaction of particles (discrete phase) with the continuous phase is performed by modifying the transport equations. For the purpose of model validation, experimental data of temperature profiles and syngas composition are used. Besides, the influence of concentration of an oxidant agent is numerically investigated. Results show that the maximum temperatures reached in combustion, gasification, and pyrolysis processes are, respectively, 2207.91 (K), 1559.47 (K) and 1097.55 (K). Finally, simulated gasification temperature showed a standard deviation of 5.2% from experimental one.