A shrinking model for combustion/gasification of char based on transport and reaction time scales

Abstract

The objective of this contribution is to introduce a shrinking model for gasification of char formed by pyrolysis of biomass or coal. The model describing gasification of a charred particle is based on one-dimensional and transient differential conservation equations for mass, momentum, species and energy. Shrinkage is estimated by a physically based approach of the Thiele modulus or second Damköhler number. It represents a ratio of diffusion time to reaction time and primarily allows distinguishing between a reacting and shrinking core behaviour. Thus, this approach does not require any empirical correlations and is employed to a reacting bed of solid particles. Within the reacting core domain, depleted solid material increases porosity of the solid matrix, while the shrinking core behaviour contributes to a recessing volume.