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Microwave pyrolysis features with different heating mechanisms and heat transfer directions from traditional pyrolysis with furnace heating, which might impact evolution of volatiles and nature of biochar in varied ways. This was investigated in this study by pyrolysis of wet and dried banana peel with microwave heating and furnace heating at 400 and 600 °C in a fixed-bed reactor. The results showed that microwave pyrolysis could drastically promote gasification of volatiles to form significantly more gases like H 2 via dehydrogenation and CO via cracking and decarbonylation. This was especially evident in the pyrolysis of wet banana peel with abundant water as the heat transfer medium. The dominance of cracking/gasification of volatiles in microwave pyrolysis was due to the rapid heating and the minimized heat transfer limitation, which suppressed the secondary condensation and thus diminished the formation of bio-oil and biochar. Microwave pyrolysis also favored the cracking of the cellulose/hemicellulose-derived aliphatic organics via cracking but promoted the formation of phenolics via cracking of lignin in banana peel. Moreover, the excessive cracking reactions in the microwave pyrolysis led to the biochar of more excellent thermal stability, lower abundance of oxygen-containing functionalities like C O and more rugged morphology than that with furnace heating source. [Display omitted] [ABSTRACT FROM AUTHOR]

Pyrolysis of biomass typically involves intensive dehydration, cracking, secondary condensation, etc. It is known that these reactions could be accelerated or suppressed with externally added acidic/basic sites presenting on surface of a biomass feedstock, which is expected to impact evolution of pyrolytic products and pyrolysis kinetics. This was investigated herein by pyrolysis of the poplar sawdust loaded with H 2 SO 4 or NaOH at 600 °C. The results indicated that H 2 SO 4 presence enhanced charring of the volatiles including sugars, sugar-derivatives and some lignin-derivatives via dehydration, polymerization, etc. This enhanced biochar production while diminished bio-oil formation and abundance of most of the organics in bio-oil including carboxylic acids, heavy tar (i.e. π-conjugated aromatics), etc. Nevertheless, H 2 SO 4 did not lead to substantial removal of hydrogen and oxygen in the biochar, but increased the activation energy (Ea) of the pyrolysis from 162.6 kJ/mol in pyrolysis of unloaded sawdust to 194.7 kJ/mol. In converse, NaOH promoted cracking/gasification of biochar and the volatiles including heavy tar, forming dominantly gases. NaOH presence also made structures of the semi-char (biochar precursor) more fragmented, reducing the Ea for pyrolysis to 135.7 kJ/mol. The in-situ IR characterization of the pyrolysis process showed that H 2 SO 4 promoted removal of -OH via dehydration or transformation to C O and the further condensation of C O, but not dehydrogenation of -C-H. NaOH promoted mainly deoxygenation and cracking of the sugary structures but not dehydration of -OH to carbonyls. [Display omitted] • H 2 SO 4 enhances charring to form more biochar, while NaOH promotes cracking to gas. • Both H 2 SO 4 and NaOH diminish tar formation via condensation to solids or cracking. • Charring-H 2 SO 4 reserves H and O in biochar, while cracking-NaOH enhances aromatization. • Activation energy in pyrolysis is enhanced with H 2 SO 4 , while decreased with NaOH. • DRIFTS shows H 2 SO 4 does not promote dehydrogenation, while NaOH suppresses dehydration. [ABSTRACT FROM AUTHOR]

Bio-oil is a complex mixture of organics and each group of chemicals has a varied response to pyrolysis temperatures. This was investigated via the staged pyrolysis of poplar sawdust, cellulose, and lignin by consecutively increasing temperature to 250, 350, 450, and 550 °C, respectively. The results showed that 350 °C was the boundary temperature for formation of majority of organics from sawdust and cellulose and more than half of organics from lignin. The further increase of temperature to 550 °C mainly promoted the aromatization of reaction intermediates from aliphatic components. The staged pyrolysis also facilitated the charring reactions, generating the bio-oil with more light organics including anhydrate sugars, ketones, aldehydes, and phenolics. The enhanced charring reactions promoted the carbonization reactions via deoxygenation, making biochar carbon-rich and thermally more stable. Additionally, the staged pyrolysis also impacted the morphologies and evolution of the functionalities of biochar via influencing the volatile-char interactions. [Display omitted] • Staged pyrolysis of various feedstocks forms the majority of bio-oil at 350 °C. • Increasing temperature promotes mainly aromatization of reaction intermediates. • Staged pyrolysis of cellulose/sawdust produces more sugars, ketones/aldehydes. • Staged pyrolysis forms more light organics in bio-oil via charring reactions. • Staged pyrolysis forms biochar of higher C content, stability, and hydrophilicity. [ABSTRACT FROM AUTHOR]