Biomass gasification is regarded as one of the most promising energy recovery technologies. The gases produced from the biomass gasification can be used for heat and power production. Alternatively, they can be used for the production of liquid biofuel. Fixed bed downdraft gasification reactors are easier to use in small scale gasification processes. A three-dimensional Computational Particle Fluid Dynamic (CPFD) model has been developed to gain a better understanding of the gasification process in a fixed bed downdraft gasification reactor. The processes considered in the model are volatilization, partial combustion and gasification. Using the model, the product gas composition is calculated. The computational results are compared with published results from experimental measurements in a fixed bed gasification reactor. There are reasonable agreements between computational and experimental results confirming that the model can be used for further investigation of the fixed bed biomass gasification reactor. The model is used to investigate the effect of the air–fuel ratio on the product gas composition.