Importance of wood anatomy for pine wood biochar properties: Insights from multiscale characterisation with synchrotron X-ray microtomography and N2/CO2 gas sorption analyses

Biochar performance in applications like soil amendment depends on properties including porosity, surface area, and pore structure, which are influenced by both process conditions and feedstock characteristics such as wood anatomy. However, their effects across micro- and nanometre length scales remain poorly understood. This study therefore investigates how feedstock and process conditions affect properties of pine wood biochar produced in industrially relevant pilot-scale continuous reactor for torrefaction (291–315°C, 6–12 min) and pyrolysis (350–400°C, 25 min). High-resolution X-ray microtomography combined with AI-assisted image analysis was used to quantify earlywood–latewood distribution, porosity, pore size, cell wall thickness, and micrometre-scale surface area, while N 2 and CO 2 sorption probed nanometre-scale structure. The wood microstructure was largely preserved across treatments, resulting in highly anisotropic pore networks. Porosity remained high (33–77%), primarily influenced by earlywood–latewood variability rather than temperature. From torrefaction to pyrolysis, cell wall thickness and pore diameter decreased, while microporosity (<∼0.7 nm) and accessible surface area increased significantly above 350°C. Discrepancies between N 2 and CO 2 measurements indicate differences in pore accessibility, likely associated with ultramicroporosity, pore constrictions and/or partially inaccessible pores. Water holding capacity ranged from 1.7 to 5.3 times dry weight, with highest average at 350°C, although differences were not significant. Combined use of microtomography and gas sorption provides a multiscale framework for linking biochar structure across micro- and nanometre scales to performance-relevant properties. These findings demonstrate that wood anatomy dominates over process conditions under mild thermal treatment, highlighting new possibilities for tailoring process design through consideration of wood anatomy.