Effects of torrefaction after pelleting (TAP) process on strength and fuel characteristics of binderless bio-pellets

Production of fuel pellets from torrefied biomass is an approach that has gained importance in recent years. As an alternative, effects of torrefaction after the production of biomass pellets (TAP process) were investigated in this study. For this purpose, binderless fuel pellets were prepared using ash tree (AT), hazelnut shell (HS), and refuse-derived fuel (RDF) and then subjected to torrefaction at temperatures of 230–290 °C. The effects of post-torrefaction treatment on fuel quality, strength, water resistance, and combustion properties of bio-pellets were investigated. It was determined that the bio-pellets burned more stably and did not show excessive thermal reactivity after torrefaction. In addition, the water resistance of the bio-pellets was greatly improved as the torrefaction process changed the hydrophilic structure of the biomass to the hydrophobic structure. Pellets produced from raw biomass dispersed immediately when thrown into water, while torrefied pellets remained intact in water for more than 10 days. However, some decrease was observed in compressive strength and hardness properties, which indicate the mechanical strength of the pellets. Therefore, in order to justify the effects of the TAP process, it is necessary to consider a trade-off between the water resistance and the mechanical strength of the bio-pellets. Graphical abstract: [Figure not available: see fulltext.]