Three-stage steady-state model for biomass gasification in a dual circulating fluidized-bed

A three-stage steady state model (TSM) was developed for biomass steam gasification in a dual circulating fluidized-bed (DFB) to calculate the composition of producer gas, carbon conversion, heat recovery, cost efficiency, and heat demand needed for the endothermic gasification reactions. The model was divided into three stages including biomass pyrolysis, char–gas reactions, and gas–phase reaction. At each stage, an empirical equation was estimated from experimental data. It was assumed that both unconverted char and additional fuel were completely combusted at 950 °C in the combustor (riser) and the heat required for gasification reactions was provided by the bed material (silica sand). The model was validated with experimental data taken from the literature. The parametric study of the gasification temperature (T) and the steam to fuel ratio (γ) was then carried out to evaluate performance criteria of a 1.8 MW DFB gasifier using woodchips as a feedstock for the electric power generation. Effective operating conditions of the DFB gasifier were proposed by means of the contour of the solid circulation ratio, the heat recovery, the additional fuel ratio and the cost efficiency with respect to T and γ.