An advanced, comprehensive thermochemical equilibrium model of a downdraft biomass gasifier

A stoichiometric model is formulated for predicting the syngas yield from the reduction zone of a downdraft biomass gasifier. It incorporates the thermodynamic equilibrium of the global gasification reaction, predicts the concentration of the minor gasification products of hydrogen sulphide and ammonia as the sulphur-based and nitrogen-based contaminants, respectively, and implements a new empirical correlation, formulated using existing pertinent experimental data, to account for the mass tar yield. The governing set of model equations is solved in a fully coupled manner, with the boudouard reaction employed to predict char output and the ammonia synthesis reaction used to predict ammonia production. The model does not require the use of correction factors and satisfactorily predicts the concentration of methane, a shortcoming that has tended to plague existing equilibrium models. The syngas composition, tar and char yields, gasification temperature, cold gas efficiency and lower heating value are obtained for various biomass feedstock with a specific ultimate analysis, for different equivalence ratios and varying moisture content. Where possible, predictions are compared with corresponding experimental data and found to be in very good agreement.