Torrefaction after pelletization (TAP): Analysis of torrefied pellet quality and co-products

Highlights: • Torrefied pellet yield reduced from 89 to 52% between 230 and 290 °C. • Torrefied pellets produced at 270 °C had similar fuel properties as coal. • TAP pathway improved the hydrophobicity of wood pellets. • Pellet hardness and durability decreased with increased torrefaction temperature. • Tor-liquid was rich in organic acids, and levoglucosan and can be a co-product. - Abstract: Torrefaction is a thermal pretreatment method to increase the energy density and to decrease the grinding energy of biomass. In this study, the torrefaction of two types of commercially available wood pellets was carried out in the temperature range of 230–290 °C for 30 min residence time. Torrefaction produces both the solid fuel known as torrefied pellets and the volatile stream known as torgas. The torgas was further separated into condensable liquids, known as tor-liquid and non-condensable gases. The products yield, its compositions and fuel properties at various torrefaction temperatures were also determined. The increase in temperature reduced the torrefied pellet yield from 89 to 52%, while increased the condensable liquid yield from 5 to 23% within the torrefaction study condition. The heating value (24 MJ kg⁻¹) and the volumetric energy density (12.5 GJ m⁻³) of torrefied pellets at 270 °C were comparable to that of coal for biopower. The increase in torrefaction temperature improved the hydrophobicity (resistance to water uptake), but adversely reduced the pellet density, hardness, and durability of torrefied pellets causing high susceptibility to breakage and fine generation during handling, transport, and storage. Further research is required to improve the quality of torrefied pellets for safe handling and storage. The non-condensable fraction of torgas was mainly composed of carbon dioxide, carbon monoxide, and traces of methane. The condensable liquid was rich in organic acids, ketones, furfural, and levoglucosan, which could be potentially transformed into high-value chemicals and other commercially viable products.