Effect of particle irregularity and particle size distribution on the morphology of packed beds of biochar particles

The heat and mass transfer in packed bed reactors (PBRs) are strongly influenced by the random packing of particles, making a thorough understanding of the packed bed structure crucial for optimal reactor design. This study investigates the impact of particle shape irregularities and size distributions on packing and transport properties using X-ray microtomography (XMT) imaging. Key morphological parameters, including void fraction and tortuosity, are extracted and analyzed. Two pore network models (PNMs)- one using cylindrical throats and another based on dense graph approach- are compared, with the dense graph model more accurately reflecting empirical tortuosity distributions. Results reveal that in monodispersed beds, void fraction decreases for particle diameters below 2 mm, nearing theoretical minimums for spherical packings, while tortuosity aligns with established models despite particle sphericity ranging between 0.6 and 0.8. In contrast, highly polydispersed beds exhibit lower void fractions compared to monodispersed beds, yet their tortuosity distributions remain similar. Visualization indicates small particles fill voids without blocking flow paths, preventing substantial tortuosity increases. These findings enhance understanding of packed bed behavior and provide valuable insights for designing biochar-based PBRs.