Effect of storage on extractives from particle surfaces of softwood and hardwood raw materials for wood pellets

When wood pellet raw material (dried sawdust and shavings) is stored for six months, wood pellet producers can observe an increased pellet mill capacity and pellet quality. To investigate the storage effect on the raw material and the possibilities for accelerating the effect, the effect of storage on surface extractives of the raw material was studied. Softwood and hardwood raw material was stored for six months, and the effect of the storage on the composition and concentration of the extractives on particle surface peelings was analyzed. Furthermore, the effect of storage at elevated temperatures was analyzed. The acetone extracts of peelings were analyzed by gas chromatography mass spectrometry (GC-MS), and multivariate regression modelling on the chemical composition was made to compare the effects of storage at elevated temperatures to the normal storage. The analysis showed that the extractive concentrations of the surface material were initially high and that these concentrations further increased during storage. Compound analyses showed that the concentrations of triterpenoid and steroidal structures increased during the storage period, whereas the concentrations of fatty acids and diterpenoid structures decreased during storage. It is suggested that this reflects oxidation and polymerization reactions of the fatty acids and diterpenoid structures. Modelling of the softwood storage proved good linear correlation of specific compound concentration changes to storage, whereas hardwood showed large uncertainties. It was shown that storage at higher temperatures accelerated the changes in extractive concentrations for softwood, whereas it was non-significant for hardwood. The study indicates that the improvement for the wood pellet production connected to raw material storage could be related to changes in the amount and composition of surface extractives for softwoods, whereas the effect for hardwoods is less certain.