A one-dimensional steady model for downdraft biomass gasifiers

A one-dimensional stationary model of biomass gasification in a fixed bed downdraft gasifier is presented in this paper. The model is based on the mass and energy conservation equations and accounts for the energy exchange between the solid and gaseous phases and the heat transfer by radiation between the solid and gaseous species and the reactor walls. The downdraft gasifier is discretized as a sequence of layers where the following typical sub-processes take place: biomass drying (on the top of the reactor), pyrolysis (in the center), oxidation of char and tar cracking (on the bottom of the reactor). Nine species are considered as participating the gaseous reactions: O2, N2, H2O, CO2, H2, CO, CH4, H2S, TAR. The model is solved by dividing the system of differential algebraic equations into two sub-systems: one made of differential equations, solved through algorithms suitable for the solution of stiff problems, one made of non-linear algebraic equations solved through the Newton-Raphson algorithm. The model is suitable of being used as a tool to study the influence of process parameters, such as biomass type, moisture content, gasifier geometry, composition and inlet temperature of the gasifying agent, biomass particle diameter.