Energy Input and Quality of Pellets Made from Steam-Exploded Douglas Fir (Pseudotsuga menziesii)

Ground softwood Douglas fir ( Pseudotsuga menziesii ) was treated with pressurized saturated steam at 200−220 °C (1.6−2.4 MPa) for 5−10 min in a sealed container. The contents of the container were released to the atmosphere for a sudden decompression. The steam-exploded wood particles were dried to 10% moisture content and pelletized in a single-piston−cylinder system. The pellets were characterized for their mechanical strength, chemical composition, and moisture sorption. The steam-treated wood required 12−81% more energy to compact into pellets than the untreated wood. Pellets made from steam-treated wood had a breaking strength 1.4−3.3 times the strength of pellets made from untreated wood. Steam-treated pellets had a reduced equilibrium moisture content of 2−4% and a reduced expansion after pelletization. There was a slight increase in the high heating value from 18.94 to 20.09 MJ/kg for the treated samples. Steam-treated pellets exhibited a higher lengthwise rigidity compared to untreated pellets.