Anisotropic Gas Permeability of Beech Wood from X-ray Microtomography and Pore Network Modeling: The Role of Vessels, Fibers, and Pyrolytic Conversion

Abstract

Gas transport within biomass particles is a key factor controlling volatile release, secondary reactions, and product formation during pyrolysis. However, the influence of the anisotropic microstructure and its evolution during thermal conversion remains insufficiently understood. In this study, we combine high resolution X-ray micro-computed tomography with a pore network model (PNM) to compute the anisotropic gas permeability of beech wood at different stages of pyrolysis. Darcy permeabilities are obtained for the three major anatomical directions (radial, tangential, and longitudinal). Pore-resolved computational fluid dynamics (CFD) simulations on the same porous domains are used to derive a geometric correction factor for gas transport in the PNM. This correction brings the PNM close to the accuracy of the pore-resolved CFD simulations at significantly reduced computational cost. The PNM is then applied to quantify the contribution of the different cell types of the wood microstructure to gas transport. The results show that gas transport occurs predominantly in vessels larger than 15 μm. The estimated permeability of the extracted network of interconnected, mostly longitudinally oriented vessels is in the range 10^–11 m² to 10^–10 m² and is nearly unaffected by pyrolysis, whereas the network of fibers smaller than 15 μm yields values of 10^–17 m² to 10^–14 m². A detailed pore-scale analysis reveals that the opening and enlargement of smaller pores have a negligible effect in the longitudinal direction, where flow is dominated by the larger vessels. Perpendicular to this main transport direction, however, permeability increases substantially, as thermal decomposition enhances pore connectivity, particularly among the fibers. By linking microstructural evolution to directional transport properties, the method supports the parameterization of predictive multiscale models for biomass pyrolysis.