Mechanical and compositional characteristics of steam-treated Douglas fir (Pseudotsuga menziesii L.) during pelletization

Rheological properties of pelletizing steam-treated Douglas fir at four different severities were investigated. Steam-treated wood pellets exhibited three compression regions that became more distinct with increasing treatment severity. Hydrolysis of hemicelluloses and condensations of lignin led to an increase in elasticity of pellets made from steam treated feedstock. The increased treatment duration improved the dimensional stability of pellets. The increase in particle surface roughness and concentration of mono-sugars and extractives contributed to the increase in required energy for extruding steam treated pellets from the die. The maximum breaking force, Meyer hardness and the hardness modulus increased with steam treatment. The increased hardness and dimensional stability of steam-treated pellets can be attributed to the binding role of mono-sugars released from Douglas fir during steam treatment.