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• Char structure was investigated via characterization of its extraction and residue. • Effects of pellet sizes on char structure were weakened by temperatures. • Polymerisation of extract is account for the structural difference of pellet chars. • Aromatisation degree of pellet char is related to its surface-area-to-volume ratio. Char structure can reflect the pyrolysis characteristics and affect the efficiency of gasification and combustion processes, while biomass pellet size and shape are critical for the heat and mass transfer, which in turn influence greatly char structure. The aim of this study is to experimentally study and compare the effect of aspect ratio (diameter/length) on char structure during the pyrolysis of sawdust pellet. Cylindrical sawdust pellets with similar masses and volumes but different diameters (6, 8, 12, 16 mm) were pyrolysed at various temperatures. The chars were separated into two phases (liquid extraction, solid residue) via microwave-assisted extraction. Experimental results show that the pellet with a low aspect ratio had a large char yield because of intra-particle volatile-char interaction especially at low temperature, while there was an optimum aspect ratio due to the change of residence time. The pellet with a high aspect ratio had an intense polymerisation, resulting in the formation of fused aromatic rings in char and residue. By increasing aspect ratio, there was a lowest surface area. This means that less heat entered from surface, and low thermal conductivity caused a slow decomposition. Due to large heat and mass transfer residence, more aromatic molecules with small weight formed in extracts of char, especially at high temperature. Besides, an increase in temperature weakened the differences in structural features of the pellet char caused by different aspect ratios. [ABSTRACT FROM AUTHOR]

• Fe/activated char shows high performance on bio-oil upgrading. • The fresh catalysts favor the production of aromatic hydrocarbons. • The regenerated catalysts by active agents facilitate the production of phenols. • The catalyst regeneration promotes the formation of γ-Fe 2 O 3 on the char surface. Char-supported metals as catalysts has attracted much attention due to the renewability and structural controllability of char. However, such catalysts can not be regenerated by the conventional calcination method, which is widely used in industy. In this study, the catalytic performance of char-supported Fe on bio-oil upgrading during pyrolysis of pine wood was first investigated. It was found that the typical components (acids, ketones, furans, etc) in bio-oil were completely converted to aromatic hydrocarbons over char-supported Fe, in which char acted not only as a support but also as a catalyst for the converison of bio-oil to phenols. Then, the spent char-supported Fe catalyst was regenerated by CO 2 or steam gasification. The results showed that only phenols could be produced over both CO 2 and steam regenerated catalysts, which was caused by the changes in carbon structure and surface functional groups of char as well as the species of Fe. The CO 2 regenerated catalyst was rich in C O groups and α-Fe 2 O 3 , while the H 2 O regenerated one mainly contained O C O groups and Fe/FeO. [ABSTRACT FROM AUTHOR]