Characterizing the Internal Structure of Biomass Pellets with Three-Dimensional Imaging

Pelleting is a common technique used to process loose biomass into a densified form. Pelleting packs the feedstock material tightly while forming a complex pore network between the solid constituents, which allows access of water and other substances to the interior parts of pellets. The characteristics of this pore system depend on the raw material and pelleting parameters. In pelleted recycled and organic fertilizers, the properties of the pore system partly determine the degradation of the pellets in soil and consequently the release of nutrients to plant available form. The fertilizer pellet structural properties have rarely been studied quantitatively at the pore level, whereby we conducted a three-dimensional imaging study using X-ray tomography to quantitatively analyse the pore characteristics of six pellet types, comprising three raw materials and two compression ratios. The image analysis revealed that both the raw material and compression ratio affected the internal pore structure of pellets. These findings suggest that the internal porous structure of fertilizer pellets can be tailored on length scales which affect the interaction of fertilizer with the surrounding soil and consequently affect the agronomic performance of the fertilizer.