A Transient, Two-Dimensional Model of Biomass Pyrolysis

Abstract

A two-dimensional, unsteady mathematical model of a thermally degrading cellnlosic particle is presented. Heat mass and momentum transfer are modeled by the usual conservation equations for porous media. Chemical processes are described through a multi-step, lumped-parameter model. The dynamics of particle conversion, simulated for sizes above 0.5 × 10^-2 m, show a process largely affected by the grain structure of the solid, with convective heat transfer predominating over conduction along the solid grain. A sensitivity analysis has shown that, among physical properties, the most important role is played by the char/solid thermal conductivities. The degradation process becomes slower as the these are made lower. However, in the first case, the reaction front enlarges and the lower reaction temperature causes an increase in the char yield. On the contrary, as the solid thermal conductivity is decreased, the thickness of the pyrolysis region shrinks and, as a consequence of the larger reaction temperatures, lower final char yields are obtained.