μCT based quantification of pellet char morphology: Effects of biomass pelletization and fluidized bed pyrolysis

Spruce wood pellets were produced with flat dies of different press-channel diameter-to-length ratios (1:3, 1:4, 1:5) and pyrolyzed at 900 °C for 4 min in a fluidized bed (FLB) and, for comparison, in a control setup (CS) where hot gas flowed around the pellets. The study includes (a) implementing a μCT radial porosity analysis to relate pellet-char structure to mechanical stability across distinct gas–solid contacting modes; (b) developing a μCT-based sand correction to separate entrained quartz from pellet char, reconciling image- and density-derived porosities; and (c) providing μCT evidence of fines enrichment toward the pellet core prior to pyrolysis, consistent with central-cavity formation under FLB conditions. FLB-pyrolysis yielded degraded pellet chars with pine cone-like morphology and large central cavities; μCT-resolved porosity increased by 6–12× relative to the wood pellets, depending on initial density. CS-pyrolysis produced chars that retained cylindrical shape and radial porosity distributions similar to untreated pellets, albeit at higher absolute porosity. The sand-mass correction indicated small fractions that minimally affected partial porosity but biased density-derived values. Across both conditions, extensive carbonization and loss of inter-particle bonding led to strength ranked 1:5 > 1:4 > 1:3, mirroring initial pellet quality. • Biomass pellet density determines char density and radial porosity pattern. • μCT enables 3D quantification of cavity and pore evolution in pellet chars. • Fluidized-bed pyrolysis forms pine-cone-like chars with large central voids. • Gas convection and particle fines drive cavity formation in pellet center. • μCT–porosity metrics linked to char mechanical integrity under stress.