Relationship between torrefaction severity, product properties, and pyrolysis characteristics of various biomass

Torrefaction of biomass improves the fuel quality via mild thermal decomposition of the lignocellulosic structure. Establishing common relationships for key characteristics of various types of torrefied biomass can benefit the process design and reaction severity assessment of samples in a commercial plant. In this study, the properties of torrefied biomass were correlated with the mass yield for the experimental results of five biomass samples (namely, wood chips, wood pellets, kenaf, rice straw, and rice husk) processed between 190°C and 310°C with a mass loss of up to 57.0% on a dry, ash-free (daf) basis. Compared with existing studies, improved curve-fit quality (R² = 0.954-0.994) was achieved in terms of the volatile matter/fixed carbon ratio, elemental composition, and heating value based on an assessment of the property ratio between raw and torrefied biomass for the mass yield on a daf basis. A new methodology was proposed to derive the torrefaction severity from the pyrolysis characteristics using thermogravimetric analysis, with R² = 0.951. Additionally, the amount of fixed carbon increased by up to 44% compared to that of raw biomass during torrefaction, indicating cross-linking and polymerization reactions. The energy density of torrefied biomass varies widely between 1.7 and 10.4 GJ/m³ depending on the compaction degree of the raw biomass and torrefaction severity. The grindability of torrefied biomass was similar to coal when the mass loss reached 13% daf for woody samples and kenaf.