MicroCT imaging to determine coordination number and contact area of biomass particles in densified assemblies

Coordination number, denoting the number of neighboring particles contacting one central particle, and the corresponding contact area can provide crucial information about the arrangement of particles for the formation of a stable assembly. Towards that goal, two temperatures (75 °C and 90 °C) and two pressures (60 MPa and 100 MPa) were selected to form densified assemblies of biomass particles. Lengths of selected needle-shaped particles, that were chosen as central particles to determine the coordination number, were from 2.2 to 3.0 mm (2.66 ± 0.19 mm). The range of coordination number for switchgrass particles was from 9 to 12 for Treatment A (75 °C and 60 MPa), 12 to 14 for Treatment B (75 °C and 100 MPa), 6 to 7 for Treatment C (90 °C and 60 MPa), and 8 to 10 for Treatment D (90 °C and 100 MPa). The irregular shape of the biomass particle makes the standard scale of coordination number inapplicable, where the value of 14 tends to imply a fully densified assembly. The associated contact area of the particles can provide crucial complementary information leading to a thorough understanding of the arrangement of neighboring biomass particles. The percentage of contact area for Treatments A, B, C, and D were 58.3 ± 8.8, 81.7 ± 4.5, 36.2 ± 4.2, and 40.3 ± 2.1, respectively. Both, coordination number and associated percentage contact area, were highest for Treatment B and lowest for Treatment C. It was observed that higher pressure during the densification process results in higher coordination number and contact area, which also correlates to higher density and strength of assemblies. Coordination number and percentage contact area of all specimens were found to be positively correlated (R² = 0.872). In addition, the correlation between microscale properties, i.e., coordination number and associated percentage area, and macroscale properties, i.e., densities and strength of assemblies, were also studied (R² > 0.85).