A Preliminary Study on the Influence of the Pelletizing Process on the Off-Gassing Behavior of Wood Pellets

It is a well established fact that during storage of wood pellets emissions of carbon monoxide (CO), carbon dioxide (CO2) and a large quantity of volatile organic compounds (VOCs) can be detected. These off-gases have been reported to originate from autoxidation reactions of woods own fatty acids. Different wood species are known to show a different propensity to exhibit high off-gassing potential, e.g. pine pellets tend to be more reactive than spruce pellets. However, even among pellet samples made from the same wood species, considerable variations in off-gassing potential can be observed. Earlier attempts to correlate these variations with variations in the extractive content of the investigated pellets have proven unsuccessful, which lead to the question on the impact of the pelletizing conditions on the off-gassing potential of the produced pellets. In this study data on the changes of the off-gassing potential dependent on the pelletizing parameters have been investigated. To this effect a pelletizing experiment was carried out where two distinct sets, set A and set B, of spruce shavings have been obtained. Both sets were characterised for moisture content, extractive content and composition and off-gassing potential. Afterwards set B was divided into two fractions, B and B2. As a next step, a pelletizing experiment utilizing a laboratory pellet press was carried out where shavings from set A and B were pelletized using the same settings for the pelletizing. Shavings from the B2 set, however, were pelletized on the same pellet press keeping all pressing parameters on the same settings as before but for the raw material throughput through the pellet press, which was lowered by half. The reduction of the raw material throughput lead to a prolonged retention time of the pelletizing material within the pellet press, which also results in a prolonged thermal treatment of the material during pelletizing. The produced pellets were characterised for moisture content, extractive content and composition and off-gassing potential just as the raw material. A comparison of the results clearly indicates the similarities between the off-gassing potential, as well as the extractive content and composition, from pellets A and B, whereas pellets B2 show a distinctly different off-gassing potential. This demonstrates the up to now neglected importance of the pelletizing process on the off-gassing potential of the pellet product and demands further investigation.
Proceedings of the 23rd European Biomass Conference and Exhibition, 1-4 June 2015, Vienna, Austria, pp. 682-685