A pilot downdraft gasifier is investigated experimentally using almond shell biomass as feedstock. Experimental temperature profiles along the different zones of the gasifier are measured and overlapping regions between the different gasifier stratified zones is proved: we identified two main zones (drying - pyrolysis zone and partial oxidation-reduction zone) instead of four stratified zones as proposed in the literature. In light of this experimental finding, a two dimensional mathematical model is developed for the combined partial oxidation-reduction zone based on conservation equations coupled to the heterogeneous and homogeneous chemistry. Partial oxidation and thermal cracking mechanisms of tar are proposed based on the available kinetic data. The model is developed for the quasi-steady state period of the experiment and used to simulate the heat and mass transport fields within the computational domain, analyze the interaction between the heterogeneous and homogeneous reactions and evaluate the performance of the gasifier in terms of tar conversion and syngas composition. Validation against the mean experimental temperature data and the producer gas composition at the outlet of the reactor are presented and a satisfactory agreement is observed. The influence of pyrolysis gas composition, air flow rate and bed porosity on the process and its outputs is also investigated. (C) 2017 Elsevier Ltd. All rights reserved.