Comparative study on microwave and conventional hydrothermal pretreatment of bamboo sawdust: Hydrochar properties and its pyrolysis behaviors

•Two pretreatments affect differently the hydrochar properties and its pyrolysis behaviors.•Hydrochar by conventional hydrothermal pretreatment shows higher thermal stability.•Microwave hydrothermal pretreatment removes more acetyl.•Hydrochar by microwave hydrothermal pretreatment produces more glucopyranose and less acids. Comparative study on microwave and conventional hydrothermal pretreatment of bamboo sawdust was carried out in this study. Microwave and conventional hydrothermal pretreatment both improved the hydrochar properties and its pyrolysis behaviors. Proximate and elemental analyses show that the properties of hydrochar from microwave hydrothermal pretreatment are better than conventional hydrothermal pretreatment in terms of calorific value and oxygen content except for 150°C. Microwave hydrothermal pretreatment removes more acetyl groups in hemicellulose compared to conventional hydrothermal pretreatment, which may be attributed to the hot spot effect of microwave irradiation. The peaks of thermogravimetric and derivative thermogravimetric curves of pretreated samples always shifted to higher temperature region. Also, the conventional hydrothermal pretreated samples are more thermally stable than those by microwave heating. In addition, the glucopyranose content in pyrolysis vapors of microwave hydrothermal pretreated bamboo sawdust (190°C) was 9.82% higher than that from conventional hydrothermal pretreated bamboo sawdust. However, the acids content from microwave hydrothermal pretreated bamboo sawdust (150°C) was 4.12% lower. In this regard, microwave hydrothermal pretreatment is more suitable for upgrading the pyrolysis oil quality than conventional hydrothermal pretreatment.