A one-dimensional mathematical model has been developed to predict behavior of a spouted bed reactor during the gasification of char obtained from pyrolysis of pistachio shells leading to the production of activated carbons. The pores overlapping phenomenon is considered in the model with no auxiliary empirical correlation used. This model is capable of the prediction of BET surface area of the produced activated carbons besides the gas-solid hydrodynamics and distribution of gas and solid temperature and concentration in the bed. The results revealed that the one-dimensional model well predicts values for char burn-off, BET surface area, and bed temperature in gasification reactions in spouted bed reactor. This is due to the excellent mixing and high circulation rates of the materials inside a spouted bed which reduces radial distributions.