A two dimensional modeling is developed in the reduction zone of a fixed bed downdraft biomass gasifier based on mass, energy and momentum conservation equations written for the solid and fluid phases and coupled with chemical kinetics. Kinetics parameters are derived from previous works and an effectiveness factor was used in the reaction rate correlation to quantify the mass transfer resistance in the bed. The obtained numerical results are compared with experimental and numerical data from literature and a reasonable agreement is observed. Fields of temperature, gaseous concentrations are investigated for the two-dimensional domain. Results show that the solid and fluid inlet temperatures to the reduction zone and the reactivity of the bio-char including the effectiveness factor are the main variables affecting the conversion of char to syngas in the gasification zone of the fixed bed reactor. (C) 2013 Elsevier Ltd. All rights reserved.