Effect of Heat Conduction Along the Reduction Zone on the Performance Prediction of a Downdraft Biomass Gasifier

ABSTRACT

A model of downdraft biomass gasifier has been presented considering thermodynamic equilibrium of the species at the exit of the pyro-oxidation zone and kinetically controlled reduction reactions in the reduction zone to predict the gasifier performance. The reduction zone has been divided into a large number of small control volumes and the species and energy balance equations have been solved across each control volume taking into account the chemical reaction rates. The contribution of ash towards the mass and energy balance has been considered. The char reactivity factor (CRF) in the reduction zone, representing the number of active sites on the char and its degree of burn out has been considered based on the reduction temperature and species formation in available literature. In the reduction zone, both advection and conduction contribute towards the energy transport process. The conduction takes place through the solid particles present in the bed as well as through the gaseous phase. Performance of the gasifier has been studied considering the effect of heat conduction and neglecting heat conduction during energy transport. It has been observed from the results that there is only a little change in the predicted performance when the heat conduction in the reduction zone has been considered. Temperature at the exit is somewhat increased whereas char flow rate at the exit is reduced.

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