Torrefaction performance and energy usage of biomass wastes and their correlations with torrefaction severity index

Weight loss has been suggested as an important indicator of torrefaction severity for describing the performance of this thermal pretreatment process. The present study establishes a correlation of torrefaction performance, severity, and energy usage for the applications of this pretreatment for solid fuel (biochar) production. Three different biomass wastes, namely, spent coffee grounds, Chinese medicine residue, and microalga residue, have been torrefied in nitrogen at a temperature range of 200–300 °C for 15–60 min to produce biochars. Their properties such as the enhancement factor of higher heating value, energy yield, decarbonization, dehydrogenation, deoxygenation, and atomic O/C and H/C ratios have been investigated. The obtained results indicate that the introduced dimensionless parameter (the torrefaction severity index in terms of the weight loss of biomass) correlates sufficiently with the biochar properties; the exception is the O/C ratio as a consequence of the intrinsic difference in components between the utilized wastes. A new parameter (the upgrading energy index) was also defined to account for the energy efficiency of the torrefaction system. This index follows a decreasing trend with increasing torrefaction severity index. This shows that biochar quality or heating value is intensified at a higher torrefaction severity; however, the energy efficiency is lowered.