h i g h l i g h t s Thermal decomposition and severity of microalgae residues in torrefaction are studied. Microalgae residues are thermally degraded in a thermogravimetric analyzer. Torrefaction severity index (TSI) is defined to indicate torrefaction degree. The curvature of TSI profile reveals the sensitivity of biomass thermal degradation. Biomass upgraded with high temperature and short duration is more effective. g r a p h i c a l a b s t r a c t Tem pera ture (0 C) 200 225 250 275 300 Torrefa ction time (min) 0 20 40 60 Torrefaction severity index 0 0.2 0.4 0.6 0.8 1 Intermediate curvature High curvature Low curvature Curvature = 0 Torrefaction severity index C. sp. JSC4 residue C. sorokiniana CY1 residue Torrefaction in N2 200-300 °C 0-60 min: Torrefaction and pyrolysis Microalgae residues Torrefaction kinetics Thermogravimetric analysis Torrefaction severity index (TSI) a b s t r a c t To figure out the torrefaction characteristics and weight loss dynamics of microalgae residues, the thermogravimetric analyses of two microalgae (Chlamydomonas sp. JSC4 and Chlorella sorokiniana CY1) residues are carried out. A parameter of torrefaction severity index (TSI) in the range of 0–1, in terms of weight loss ratio between a certain operation and a reference operation, is defined to indicate the degree of biomass thermal degradation due to torrefaction. The TSI profiles of the two residues are similar to each other; therefore, the parameter may be used to describe the torrefaction extents of various bio-mass materials. The curvature of TSI profile along light torrefaction is slight, elucidating its slight impact on biomass thermal degradation. The sharp curvature along severe torrefaction in the initial pretreat-ment period reveals that biomass upgraded with high temperature and short duration is more effective than using low temperature with long duration.