Moisture adsorption rate and durability of commercial softwood pellets in a humid environment

Abstract

Durability, a crucial parameter in determining the quality of a wood pellet, is affected by moisture adsorption from the air. The conditions in a confined space in which pellets are stored can be characterised as active or passive. In the active case, the interstitial air within the pile circulates around the pellets, whereas in the passive case, the interstitial air is stagnant. These two cases represent stored pellets in a ventilated silo (active) or in a ship s hold (passive). Commercial wood pellets were exposed to humid air in a humidity chamber with circulating air set at 95% relative humidity (RH) and 30\xa0°C for up to 24\xa0h. The wood pellets were also exposed to a range of RH values (11–90%) for four weeks, created using saturated salt solutions. The durability of pellets, volumetric swelling, and pellet density were measured. For pellets exposed to 95% RH, the durability of pellets, initially at 99%, dropped to below 80% after 10\xa0h. A similar drop in durability was observed for pellets stored above saturated salt solutions but after a longer storage period. The density of pellets initially increased when pellets had a moisture content of 0.05–0.08 decimal dry basis (db). The pellets expanded by 20–80% volumetrically when the moisture content exceeded 0.15 (db), resulting in a marked decrease in pellet density.