A Theoretical Heterogeneous Model of Wood Pyrolysis

Abstract

A model is developed for the pyrolysis and combustion of wood, explicitly including the fibrous heterogeneity of the substance. The heterogeneity enters the equations through the fiber diameter and the interfiber material treated as undergoing charring. The emphasis is on the condensed phase. The vapor phase details are not included in this paper. The energy flux needed to drive the pyrolysis is externally prescribed; an externally heated slab or a slab burning under its own flame resulting from the products of pyrolysis mixing with an oxidizer (air) provide two examples. The method of matched asymptotic expansions is used to analytically derive a closed form expression for the time independent rate of regression of the charring plane, including the nonlinear Arrhenius degradation term for the pyrolysis. The differences between predictions from previous homogeneous models and this heterogeneous model are discussed. Some remarks concerning the apparent negative temperature rate region indicated by many thermogravimetric analyses and the fibrous nature of wood are presented