Journal of Analytical and Applied Pyrolysis 152 (2020) 104947 Available online 7 October 2020 0165-2370/© 2020 Elsevier B.V. All rights reserved. A comprehensive study on the transformation of chemical structures in the plastic layers during coking of Australian coals Yunze Hui a , Lu Tian b, 1 , Soonho Lee a , Yixin Chen a , Arash Tahmasebi a , Merrick Mahoney a , Jianglong Yu a, b, * a Chemical Engineering and International Collaborative Centre for Carbon Futures, University of Newcastle, Callaghan, NSW 2308, Australia b Key Laboratory of Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China A R T I C L E INFO Keywords: Cross-linking structures Coking coal Synchrotron IR Plastic layer 13 C NMR ABSTRACT The changes in chemical structures over the plastic layer region during the coking of coals have a signifcant impact on coke formation and coke quality. This paper employed the Solid-state 13 Carbon Nuclear Magnetic Resonance ( 13 C NMR), and the Synchrotron attenuated total refection Fourier transform infrared (ATR-FTIR) microspectroscopy (Synchrotron IR) to study the transformation of the chemical structures in plastic layer samples. The light gases (mainly methane and hydrogen) released from coking process were analyzed using micro gas chromatography (micro-GC) connected to a small coking reactor heated in an electric furnace that simulated the formation of the plastic layers. The results show clearly that the total aromaticity increased consistently in the plastic layers for all coals tested, while the amounts of side-chains decreased signifcantly during the plastic layer. There was a clear trend showing that the total number of bridge bonds and the looped structures, indicating that the degree of cross-linking would increase through the plastic layer. The plastic layer samples from low fuidity exhibited cross-linking structures with a high degree of branching and aromaticity, while those from high fuidity coals formed cross-linking structures with a relatively low degree of aromaticity and branching but with a large number of bridge bonds and looped structures. The transferable methyl, meth- ylene and hydrogen were strongly correlated to the cross-linking reaction and side-chain elimination in the thermoplastic region, which is refected by the release profles of methane and hydrogen gas during the plastic layer stage. 1. Introduction When single coking coals or blends are heated inside a coke oven, coking coals undergo a complex and dramatic chemical structure changes during coking process, during which plastic layers form in the coal bed where the temperature is in the range of somewhat 350 ◦ C–550 ◦ C [1–8]. These chemical structure transformations in the plastic layers are realized through complex pyrolysis reactions such as bond cleavage of aliphatic structures (including bridges and looped structures, and side-chains), cross-linking reactions, the re-polymerization and ring condensation of aromatic structures, and accompanied by the generation and release of volatile matters. While the changes in the chemical structures of plastic layers during coking are strongly infuenced by the parent coking coal properties, such as the maceral composition (e.g., the content of vitrinite and inertinite), coal rank, and coal fuidity, they play a crucial role in the subsequent coke formation and coke quality [9–11]. In the literature [12], the ar- omatic structures cross-linked back into the molecular due to the transferable radicals in the thermoplastic range during coking. For instance, the cross-linking structure changes in the thermoplastic region signifcantly infuences the coke tensile strength [13]. Furthermore, the quality of coke is critically governed by the formation of cross-linking structures and ordered carbon structures during coking process [2, 14–16]. The changes in the density of the cross-linking during coking was responsible for the changes of microporosity of plastic layers during coking [17], which was correlated to the formation of volatile matter * Corresponding author at: Chemical Engineering and International Collaborative Centre for Carbon Futures, University of Newcastle, Callaghan, NSW 2308, Australia; and School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China. E-mail address: jianglong.yu@newcastle.edu.au (J. Yu). 1 Co-frst author. Contents lists available at ScienceDirect Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap https://doi.org/10.1016/j.jaap.2020.104947 Received 17 August 2020; Received in revised form 5 October 2020; Accepted 5 October 2020