Impacts of intrinsic alkali and alkaline earth metals on chemical structure of low-rank coal char: Semi-quantitative results based on FT-IR structure parameters

The intrinsic alkali and alkaline earth metals (AAEMs) have obvious catalysis on pyrolysis of low-rank coal. Due to the differences in dispersion degree and chemical form, the catalytic effects of AAEMs with diverse occurrence states may be different. The chemical structures of coal and char are the most direct evidences to reveal the catalysis of AAEMs. However, quantitatively characterizing and evaluating these structures, especially the asymmetric functional groups, has always been an unsolved problem. In this study, the FT-IR spectra of Zhundong coal and char containing different forms of intrinsic AAEMs were fitted, the functional group structures of these samples were semi-quantitatively characterized by a series of original infrared structural parameters, and the volatile products during pyrolysis were detected using TG-FTIR. The comprehensive analysis of functional group structures, weight-loss rates and volatile product concentrations indicates that in the char, the water-soluble and ion-exchangeable AAEMs hinder the release/decomposition of –COO⁻ and aliphatic C–H groups, and promote the thermal cracking of aromatic C–H groups. The inhibition of ion-exchangeable AAEMs on release/decomposition of Ar–O⁻ groups and dehydrogenation/polycondensation of aromatic clusters in char is also confirmed. The infrared structural parameter method proposed in this paper is expected to be a reasonable, effective, comprehensive and universal method for quantitatively characterizing the functional group structures of coal and char, and improve the current method system of coal quality analysis, accordingly bring a new tool for the evaluation and communication of chemical structures of coal and char.