Evolution of the plastic phase structure in metallurgical coals with different metamorphic degrees and its relationship with coke thermal strength

The study on the plastic phase structure during coking process is crucial for a deeper understanding of the relationship between coal quality and coke quality. This article used a semi-automatic plastic layer analyzer to conduct pyrolysis coking experiments. The plastic stage samples of four metallurgical coals, including Wuguantun (WGT) gas coal, Qianjiaying (QJY) fat coal, Lvjiatuo (LJT) coking coal, and Tangshan (TS) 1/3 coking coal were obtained by quenching method. Subsequently, based on the changes in probe resistance during the testing process of the plastic layer analyzer, the quenched plastic phase was cut and divided into different stages. The microscopic structures were characterized by the combination of Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), Raman Spectroscopy and Solid-State Nuclear Magnetic Resonance (¹³CNMR). Based on the microscopic characterization, the relationship between the raw metallurgical coal, plastic phase structure and the coke thermal strength was discussed initially. The results demonstrated the plastic phase was generated, developed, and solidified, the trends of various chemical structural parameters for the four selected coal samples were roughly the same. In the FT-IR structural parameters, the length or branching degree of aliphatic side chains (CH₂/CH₃) decreased, while the aromatic hydrogen ratio (f_ar^H) and the degree of aromatic ring condensation (DOC) increased. In the XRD structural parameters, the interlayer spacing of graphite (d_002,a) gradually decreased, while the stack height (L_c,a), number of layers (N) and diameter of microcrystalline layers (L_a) increased. The Raman structural parameters showed a decrease in the content of defective carbon (A_D1/A_G) and an increase in the content of graphite carbon (A_G/A_all). The ¹³CNMR structural parameters indicated a continuous decrease in the content of aliphatic carbon (f_al) and the average chain length of methylene (C_n), while the content of aromatic carbon (f_ar) and the content of aromatic bridgehead carbon (χ_b) continually increased. QJY and TS had longer fatty side chain lengths or shorter branching degrees (1.50<CH₂/CH₃<2.50), moderate degrees of aromatic ring condensation (0.08<DOC<0.15) and smaller graphite interlayer spacing (3.60 Å<d_002,a<3.80 Å). At the same time, they had thicker stack heights (L_c,a>20.30 Å), smaller microcrystalline layer diameters (17.90 Å<L_a<20.00 Å), lower defect carbon content (A_D1/A_G<2.60 Å) and high graphite carbon content (A_G/A_all>21.00 Å), lower aliphatic carbon content (f_al<36.00) and higher aromatic carbon content (f_ar>63.00). These two types of metallurgical coal exhibited continuous changes in various chemical structural parameters during the plastic stage, this means that their molecular functional group structure, microcrystalline structure, graphitized structure, and carbon skeleton structure could maintain stable evolution, which is beneficial for enhancing coke thermal strength. LJT had excessively long fatty side chain lengths (CH₂/CH₃>2.90) and higher stacking layers (N>6.20 Å), its microcrystalline structure collapsed during the solidification of plastic stage, resulting in a worse coke thermal strength. WGT had a low degree of aromatic ring condensation (DOC<0.07), large graphite interlayer spacing and microcrystalline layer diameter (d_002,a>3.80 Å, L_a>26.00 Å), thin stacking height (L_c,a<20.30 Å) and high defect carbon content (A_D1/A_G>2.90). Its graphitized structure collapsed during the solidification of the plastic stage, resulting in the lowest thermal strength of its coke.