In this paper, a simulation package called GASDS is introduced. It is particularly suited to evaluate the pyrolysis, gasification and combustion of biomass and coal feedstocks. The aim of this work is to describe the package from a numerical point of view and its interface. Additionally, experimental results for a countercurrent fixed-bed biomass gasification reactor are reproduced. The influence of reactor and particle discretizations are investigated with respect to accuracy and computational time. Some differences are present between experimental and simulation results. In order to improve the agreement between simulation and experimental results it is suggested to improve the kinetic scheme of the solid phase and gas-solid reactions. The negligible differences in terms of predictions, instead, do not justify the adoption of finer discretizations for the particle and reactor, which imply longer computational times.