The thermodynamic and kinetic modeling of char reduction reactions in a downdraft (biomass) gasifier has been presented. Mass and energy balance are coupled with equilibrium relations or kinetic rate parameters (using varying char reactivity factor) in order to predict status of un-converted char in addition to gas composition, calorific value, conversion efficiency, exit gas temperature, endothermic heat absorption rate and gasifier power output. Both modeling predictions are compared against experimental data for their validity. The influence of char bed length and reaction temperature in reduction zone has been examined. CO and H-2 component, calorific value of product gas and the endothermic heat absorption rate in reduction zone are found to be sensitive with reaction temperature, while char bed length is less sensitive to equilibrium predictions. For present case, all char conversion takes place at critical reaction temperature of 932 K for equilibrium, while for kinetic modeling critical reaction temperature and critical char bed length of 950 K and similar to 25 cm have been identified, comparing the predictions. The critical reaction temperatures and critical char bed length also depend on inlet components composition and initial temperature supplied to the reduction reaction zone model. (c) 2008 Elsevier Ltd. All rights reserved.