Updraft Fixed-bed Gasification of Softwood Pellets: Mathematical Modelling and Comparison with Experimental Data

This paper presents a one dimensional steady state mathematical model for the simulation of a small scale fixed-bed gasifier. The model is based on a set of differential equations describing the entire gasification process of softwood pellets and is solved by a two step iterative method. The main features of the model are: homogeneous and heterogeneous combustion and gasification reactions, one step global pyrolysis kinetics and drying, heat and mass transfer in the solid and gas phases as well as between phases, heat loss, particle movement and shrinkage within the bed. The model is used to simulate a laboratory scale fixed-bed updraft gasifier. Good agreement is achieved between prediction and measurements for the axial temperature profiles. The prediction of the composition and particularly of the tar content of the producer gas is imprecise due to insufficient data regarding the product composition of primary pyrolysis of softwood pellets. Therefore, the tar ratio of the primary pyrolysis reaction has been reduced and the remaining reported tar ratio undergoes a secondary reaction forming lighter gases and finally the model is in good agreement with experimental data.