Development of a semi-empirical equilibrium model for downdraft gasification systems

This study proposes a stoichiometric equilibrium model (SEM) to predict the chemical composition of the syngas, yield of tar and char produced by a downdraft gasifier for various wood based fuels for different equivalence ratios (ER). Equivalence ratio is a measure of the amount of external air supplied to the gasifier and one of the crucial operating variables in biomass gasification. Global stoichiometric equilibrium approach is used to model a downdraft gasifier. In order to predict dry gas composition, three different SEM models have been developed. Those models were validated and modified through comparison with the large amount of data collected from various sources. We have introduced two correction factors for altering the equilibrium constant of methanation and the water-gas shift reaction as a function of gasification temperature, equilibrium temperature and ER. Those correction factors were obtained by comparison of the theoretical models with experimental data of downdraft gasification of woody biomass from literature by using Levenberg-Marquadt algorithm. The modified model shows a clear enhancement in the prediction of the concentration for all gaseous species in the producer gas and tar yield. Furthermore, sensitivity analysis for SEM models has been performed using different higher heating value correlation for woody biomass.