Thermal decomposition dynamics and severity of microalgae residues in torrefaction

•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.

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 biomass 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 pretreatment period reveals that biomass upgraded with high temperature and short duration is more effective than using low temperature with long duration.

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