Kinetic Model of the Reduction Zone of a Downdraft Biomass Gasifier

The model has been developed by considering two distinct zones in the downdraft gasifier: pyro-oxidation zone and reduction zone. The process in the pyro-oxidation zone has been solved based on the chemical equilibrium consideration considering water gas shift reaction and methanation reaction. The composition of different species (H2, CO, CO2, CH4, H2O, N2 and Char) and the temperature of the gas which flows from the pyro-oxidation to the reduction zone are obtained. Reduction zone has been modeled based on chemical kinetics of reduction reactions by char considering a large number of elemental control volumes along the length of the zone to predict final gas composition. The species and energy balances have been performed across each control volume considering the finite rate of chemical reactions. The contributions of ash in the fuel and heat loss from the downdraft gasifier have been considered in the energy balance equation. In the present modified model pressure drop across the reduction zone has been calculated considering a packed bed of char particles of varying particle diameter due to the char reduction through the zone. Effect of equivalence ratio and moisture content of the biomass have been studied based on the modified model.