Investigation on the intra-particle anisotropic transport properties of a beech wood particle during pyrolysis

Abstract

In the present study, the influence of the dynamic and anistropic pore microstructure of wood and char samples on the intra-particle flow permeability and tortuosity was investigated. To this end, a beech wood sphere was pyrolysed at different temperatures (100 °C, 200 °C, 300 °C, 400 °C, and 500 °C) and characterised, after each pyrolysis step, by X-ray micro-computed tomography (μ-CT). From the μ-CT images, the structural geometry of the particle at the different conversion degrees achieved at each temperature level was extracted. The porosity evolution was characterised, accounting for pores larger than 15 μm, which was the limit of resolution for μ-CT imaging in this study. The structural geometry was divided in subdomains and used for CFD (computational fluid dynamics) simulations, where the pressure loss at different velocities and in different directions with respect to the main pores (vessel cells) was determined and used to estimate the dynamic and anisotropic permeabilities. The permeability differed by an order of magnitude in the direction of the main pores (vessel cells) in comparison to the perpendicular directions, supporting the need to develop permeability tensors for improved simulations of the pyrolysis process at particle level, accounting for the coupled effects of microstructure, transport, and reaction.