Lattice Boltzmann simulations of diffusion in steam-exploded wood

Diffusion of large molecules throughout the porous microstructure of wood pretreated with steam explosion was investigated by using the lattice Boltzmann method for simulations. Wood samples were investigated with high-resolution X-ray tomography to effectively reconstruct an accurate geometry of the structural changes that ensue after pretreatment. Samples of approximately 1mm(3) with voxel sizes from 0.5 to 1 mu m were examined with X-ray imaging. These large volumes, relative to what reasonably can be simulated, were divided into sub-volumes and were further reconstructed into geometries suited for the LBM simulations. The transient development of the concentration was investigated, and the effective diffusion coefficient at steady state was computed. Diffusion rates were found to increase significantly in the transversal direction due to the steam explosion pretreatment. The increase was observed both in the time needed for solutes to diffuse throughout the pores and in the effective diffusion coefficient. A shorter diffusion pathway and a higher connectivity between pores were found for the pretreated samples, even though the porosity was similar and the pore size distribution narrower than the native sample. These results show that local mass transport depends not only on porosity but also, in a complex manner, on pore structure. Thus, a more detailed analysis of pore space structure using tomography data, in combination with simulations, enables a more general understanding of the diffusional process.