Branch Wood Proves a Poor Substitute for Stem Wood in Spruce Pellet Production

Pellets made from Norway spruce branches perform markedly worse than pellets made from stem wood, according to a laboratory study in which researchers pressed pellets from four distinct fractions of a single debarked tree. The work, published open access in Discover Applied Sciences by Magnus Ståhl, Jonas Berghel, Gunnar Henriksson and Stefan Frodeson, examined heartwood, sapwood, juvenile wood and branch material from one Picea abies stem and compared how each behaved in single pellet pressing.

The rising demand for wood fuel pellets and growing competition for conventional raw materials have intensified interest in alternative forest-derived feedstocks, the authors note. Juvenile wood and branches are among the residues that could expand the raw material base, but branches differ chemically and morphologically from stem wood, and their use may influence both the energy needed to pelletize the material and the quality of the resulting pellets.

In the experiments, pellets were produced in single pellet press systems at moisture contents between 6 and 12 percent on a wet basis. Energy inputs were quantified separately as compression work, friction work and total work, allowing the researchers to distinguish the mechanical demands of each feedstock fraction. Pellet quality was then evaluated through measurements of solid density, hardness and mechanical durability.

The results showed that pellet quality was broadly comparable among the stem-derived fractions, meaning heartwood, sapwood and juvenile wood performed similarly under the tested conditions. Branch material, however, told a different story: pellets produced from branches showed markedly inferior properties, including durability and hardness, when processed under conditions optimized for softwood. The branch-derived feedstock also exhibited pronounced differences in cell morphology and chemical composition relative to the stem wood fractions.

In several respects, the authors report, pelletizing branch material appears more similar to pellet production from hardwood than from softwood, a distinction with practical consequences for mills designed around softwood sawdust. Because the study used material from a single tree and a single pellet press technique, the findings describe the mechanical behaviour of these specific fractions under controlled conditions rather than full-scale production outcomes. The paper discusses the implications for utilizing juvenile wood and branches and presents recommendations for the industry, but the central message is that branch wood cannot simply be swapped into softwood pelletizing without accounting for its distinct structure and chemistry.

Subject of Research: Single pellet pressing of heartwood, sapwood, juvenile wood and branch fractions from one Norway spruce tree

Article Title: Mechanical behaviour of Norway spruce fractions in single pellet pressing and consequences for using juvenile wood and branches

Article References: Ståhl, M., Berghel, J., Henriksson, G., & Frodeson, S. (2026). Mechanical behaviour of Norway spruce fractions in single pellet pressing and consequences for using juvenile wood and branches. Discover Applied Sciences, 8(11), Article 1279. https://doi.org/10.1007/s42452-026-09802-8

Image Credits: AI Generated

DOI: 10.1007/s42452-026-09802-8

Keywords: Norway spruce, wood pellets, single pellet pressing, juvenile wood, branches, pellet durability, biofuel, forest residues, Picea abies, compression energy, Mechanical, behaviour