Torrefaction of Densified Woody Biomass: The Effect of Pellet Size on Thermochemical and Thermophysical Characteristics

Thermal pretreatment by torrefaction is known to produce improved properties in biomass fuels, but the role of pellet size on key properties in commercially available hardwood derived fuel has not been investigated to date. In this study, densified Australian woody biomass was torrefied over different temperatures (250–300 °C), time (30 and 60 min) and mean pellet size (7 ± 0.5 mm and 15 ± 0.5 mm). Benchmarking, relative to raw fuel properties, is reported in terms of proximate analysis, high heating value (HHV) and water immersion tests as well as FTIR and thermal stability (TGA). Significant differences of 23.55% in mass loss (ML) and 10% in hygroscopic behaviour were observed at varied pellet size. FTIR analysis of the samples identified reduction of polar species such as the hydroxyl (-OH) functional group during torrefaction. This increased the hydrophobicity of torrefied pellets. Torrefaction of larger sized pellets was also accompanied with lower hemicellulose and cellulose degradation. A correlation predicted the HHV of the torrefied pellets which fits well with the actual HHV’s of the wide body of literature with an average difference of less than 1 MJ/kg. Pellet sizing was found to impact the fuel properties only at milder torrefaction conditions. As such, with the increase in torrefaction severity, the effect of pellet size became insignificant. The outcomes emphasise the need to describe pellet size distributions when reporting torrefaction performance indicators, particularly if commercial scale torrefaction is used at higher temperatures and longer times. Graphical abstract: [Figure not available: see fulltext.]