Combined Transport and Kinetic Modeling of Downdraft Biomass Gasifier

Kinetics-free equilibrium models can predict the exit gas composition of the biomass gasifier, given the solid composition and the equilibrium temperature, but they cannot be used for reactor design. Hence, there is a need to develop a combined transport and kinetic model which takes into account of the kinetics of homogeneous and heterogeneous chemical reactions, transport of volatiles produced, heat and mass transfer between solid and gaseous phase and pyrolysis reactions. Taking into account of the importance of downdraft biomass gasifier and its commercial applications, it is essential to have a complete model for such a configuration. In the present study, a transient onedimensional model is developed for the throated close-top downdraft biomass gasifier. The model takes into account of the drying, pyrolysis, secondary tar reactions, homogeneous gas reactions and heterogeneous combustion/gasification reactions. The experimental data obtained in our earlier study are used to validate the simulation results of the complete combined transport and kinetic model. The fraction of initial moisture content, air flow rate, temperature of the drying & pyrolysis zone, and chemical composition of the biomass are required as input data for the model to predict the composition of producer gas. The variation of molar fraction of producer gas components with time is predicted and compared with the experimental data. It is concluded from the present study that the developed model can predict the performance of the biomass gasifier, a priori. The results of this study are also useful in the design of a downdraft biomass gasifier.

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