Fungal colonisation and moisture uptake of torrefied wood, charcoal, and thermally treated pellets during storage

Storage is a challenging stage in the supply chain of any solid biofuel, as they readily absorb moisture. Increased moisture content (MC) bears many negative consequences, such as biological degradation, reduced heating value and worker health problems. A five-month storage trial served to determine how certain properties of torrefied wood, charcoal and thermally treated pellets change when exposed to natural weathering in a covered and uncovered storage area. Biological degradation and changes in MC and composition were recorded. The pellets also underwent a durability test. Different fungi were isolated from the stored samples and the genus of selected isolates was identified with internal transcribed spacer polymerase chain reaction. Significant changes were detected in the carbon content of the wood material following the trial. The samples in the uncovered storage area had absorbed substantial amounts of water and, after incubation, 99 % of these samples showed visible fungal growth, compared to only 20 % of the covered samples. The pellets showed varying responses to storage in terms of durability and moisture absorption, with the steam explosion pellets possessing more favourable properties than torrefied and untreated pellets.