Influence of torrefaction with microwave and furnace heating on pyrolysis of poplar sawdust.

Microwave heating is different from furnace heating in terms of heat transfer direction and heating mechanism. The torrefaction with microwave or furnace heating might further impact evolution of the pyrolytic products in further pyrolysis. This was investigated herein via thermal pretreatment of sawdust at 150 to 250 °C and the subsequent pyrolysis at 600 °C. The results showed that the microwave pretreatment led to more pronounced pyrolysis at 200/250 °C, which enhanced aromatic degree of semi-char and created difficulty for its further cracking in pyrolysis at 600 °C but not affected gasification. This led to the formation of more biochar and much more gases but less bio-oil with less heavy components than that from furnace heating plus pyrolysis. Additionally, the microwave pretreatment could enhance deoxygenation but the enhanced aromatic degree of the semi-char created difficulty for creating micropores and development of the pore structures (176.2 m2g−1 with 67.3% micropores from microwave pretreatment versus 441.1 m2g−1 with 78.5% micropores from furnace pretreatment). The characterization of the pyrolysis of semi-char with in-situ IR technique confirmed that the microwave pretreatment substantially removed the aliphatic structures and made the biochar with more fragmented morphology. [Display omitted] • Microwave pretreatment (MP) at 200/250 °C induces aromatization of semi-char. • MP forms more biochar/gases but less bio-oil than furnace pretreatment (FP) in semi-char pyrolysis. • MP at 250 °C hinders formation of micropores in pyrolysis but forms more mesopores. • In-situ DRIFTS measurement indicates less aliphatic structures in semi-char from MP. • MP plus pyrolysis lead to biochar of more fragmented morphologies. [ABSTRACT FROM AUTHOR]