A PRELIMINARY STUDY ON THE INFLUENCE OF THE PELLETIZING PROCESS ON THE OFF-GASSING BEHAVIOR OF WOOD PELLETS

摘要

It is a well established fact that during storage of wood pellets emissions of carbon monoxide (CO),rncarbon dioxide (CO_2) and a large quantity of volatile organic compounds (VOCs) can be detected. These off-gasesrnhave been reported to originate from autoxidation reactions of woods own fatty acids. Different wood species arernknown to show a different propensity to exhibit high off-gassing potential, e.g. pine pellets tend to be more reactivernthan spruce pellets. However, even among pellet samples made from the same wood species, considerable variationsrnin off-gassing potential can be observed. Earlier attempts to correlate these variations with variations in the extractiverncontent of the investigated pellets have proven unsuccessful, which lead to the question on the impact of thernpelletizing conditions on the off-gassing potential of the produced pellets. In this study data on the changes of the offgassingrnpotential dependent on the pelletizing parameters have been investigated. To this effect a pelletizingrnexperiment was carried out where two distinct sets, set A and set B, of spruce shavings have been obtained. Both setsrnwere characterised for moisture content, extractive content and composition and off-gassing potential. Afterwards setrnB was divided into two fractions, B and B2. As a next step, a pelletizing experiment utilizing a laboratory pellet pressrnwas carried out where shavings from set A and B were pelletized using the same settings for the pelletizing. Shavingsrnfrom the B2 set, however, were pelletized on the same pellet press keeping all pressing parameters on the samernsettings as before but for the raw material throughput through the pellet press, which was lowered by half. Thernreduction of the raw material throughput lead to a prolonged retention time of the pelletizing material within thernpellet press, which also results in a prolonged thermal treatment of the material during pelletizing. The producedrnpellets were characterised for moisture content, extractive content and composition and off-gassing potential just asrnthe raw material. A comparison of the results clearly indicates the similarities between the off-gassing potential, asrnwell as the extractive content and composition, from pellets A and B, whereas pellets B2 show a distinctly differentrnoff-gassing potential. This demonstrates the up to now neglected importance of the pelletizing process on the offgassingrnpotential of the pellet product and demands further investigation.