Pyrolysis of the aromatic-poor and aromatic-rich fractions of bio-oil: Characterization of coke structure and elucidation of coke formation mechanism

Coke formation is one major problem during thermal conversion of bio-oil and its main components. Fundamental knowledge about the evolution of the structure of cokes is a prerequisite towards a deep understanding of coking of bio-oil. This study investigates the structure (morphology, elemental composition, O-containing functional groups and aromatic structures) of cokes generated from the pyrolysis of aromatic-rich fraction (ARF) and the aromatic-poor fraction (APF) of bio-oil. The effects of interactions of ARF and APF on properties of the coke formed during the pyrolysis of bio-oil are also studied. The results show that the cokes from the pyrolysis of APF (APF-cokes) are sponge-like while the cokes from the pyrolysis of ARF (ARF-cokes) have a dense structure. The matrix of cokes from the pyrolysis of the whole bio-oil (oil-cokes) is similar to the matrix of ARF-cokes, while its surface is similar to that of APF-cokes, which should be due to the interactions between different bio-oil fractions. The APF-cokes contain more C-O, O-H and C=O functional groups than the ARF-cokes due to the higher O content of APF. Moreover, the interactions between ARF and APF can promote more O-containing species to be transformed as C-O, O-H and C=O functional groups in the oil-cokes. The aromatic rings of ARF-cokes and APF-cokes can be cracked to form smaller ring systems at 300-500 degrees C, while it is opposite for the oil-cokes because the aromatic structures formed via the interactions between ARF and APF are more stable. At higher temperatures (> 500 degrees C), the interactions (e.g. self-gasification) lead to the highly condensed cokes, while the secondary cokes, which are spherical particles, are preferentially consumed by the steam.