Probing the physicochemical properties of wood pellet surfaces in an axial direction with confocal Raman microscopy

Abstract

The pelletization of lignocellulosic biomass is essential for producing energy-efficient and easily transportable products due to high density and low moisture content. A hard, glossy surface is often associated with high-quality pellets; however, the composition and formation of this surface layer remain insufficiently understood. This study examines the surface and what components it contains using confocal Raman imaging. Four pellet types were analyzed: pure cellulose and three spruce-based variants. Scanning electron microscopy (SEM) revealed a glossy layer in all the samples, with thicknesses of 2.1 ± 0.3 µm for cellulose, 2.9 ± 1.3 µm for spruce, 4.8 ± 1.5 µm for spruce from cross-laminated timber (CLT), and 5.7 ± 1.1 µm for the spruce–starch pellet. The glossy upper layer was further characterized using confocal Raman imaging with a 532 nm laser. The results of Raman imaging indicate that biopolymer migration reduced crystallinity of the cellulose structure near the pellet surface, reflected by a decreased I ₃₈₀ / I ₁₀₉₆ ratio. A method to evaluate the migration of lignin – and potentially hemicellulose or lipophilic extractives – to the surface of spruce-based pellets is also proposed. These findings provide new insights into the factors contributing to pellet surface gloss and may support improved pellet production.