Modeling and simulation of a downdraft biomass gasifier 1. Model development and validation

An ‘EQB’ computer program for a downdraft gasifier has been developed to predict steady state performance. Moving porous bed of suction gasifier is modeled as one-dimensional (1-D) with finite control volumes (CVs), where conservation of mass, momentum and energy is represented by fluid flow, heat transfer analysis, drying, pyrolysis, oxidation and reduction reaction modules; which have solved in integral form using tri-diagonal matrix algorithm (TDMA) for reaction temperatures, pressure drop, energetics and product composition. Fluid flow module relates the flow rate with pressure drop, while biomass drying is described by mass transfer 1-D diffusion equation coupled with vapour–liquid-equilibrium relation. When chemical equilibrium is used in oxidation zone, the empirically predicted pyrolysis products (volatiles and char) and kinetic modeling approach for reduction zone constitutes an efficient algorithm allowing rapid convergence with adequate fidelity. Predictions for pressure drop and power output (gasifier) are found to be very sensitive, while gas composition or calorific value, temperature profile and gasification efficiency are less sensitive within the encountered range of gas flow rate.

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