This study focuses on developing a dynamic two dimensional Computational Fluid Dynamics (CFD) model of a moving bed updraft biomass gasifier. The model uses inlet air at room temperature as the gasifying medium and a fixed batch of biomass. The biomass batch is initially ignited by a heat source which is removed after a certain amount of time. This model operates by the heat emitted by combustion reactions, until the fuel is finished. Since the operation is batch wise, model is transient and takes into consideration the effect of bed movement as a result of shrinkage. The CFD model is capable of simulating the movement of interface between solid packed bed and gas free board and this motion is also presented. The model is validated by comparing the simulation results with experimental data obtained from a laboratory scale updraft gasifier operated in batch mode with Gliricidia. The developed model is used to find the optimum air flow rate that maximizes the cumulative CO production. It is found that from the simulation study for the particular experimental gasifier, a flow rate of 7 m(3)/h maximizes the CO production. The maximum cumulative CO production was 6.4 m(3) for a 28 kg batch of Gliricidia. (C) 2016 Elsevier Ltd. All rights reserved.