A comprehensive study on the transformation of chemical structures in the plastic layers during coking of Australian coals

•The changes in the chemical structures take place in the plastic layers during coking.•The combination of Synchrotron IR, 13C NMR and Micro-GC is effective.•The increase of bridge bonds and looped structures across the plastic layer during coking is evident.•The amount of weak bridge bonds and looped structures is responsible for the generation of coal fluidity during coking. The changes in chemical structures over the plastic layer region during the coking of coals have a significant impact on coke formation and coke quality. This paper employed the Solid-state 13Carbon Nuclear Magnetic Resonance (13C NMR), and the Synchrotron attenuated total reflection 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 significantly 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 fluidity exhibited cross-linking structures with a high degree of branching and aromaticity, while those from high fluidity 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, methylene and hydrogen were strongly correlated to the cross-linking reaction and side-chain elimination in the thermoplastic region, which is reflected by the release profiles of methane and hydrogen gas during the plastic layer stage.