FTIR-based study on the effect of deashing on the molecular structure of Hefeng sub-bituminous coal and its liquefaction residue

ABSTRACT

Mineral presents an important effect on the structure and reactivity of coal, and deashing pretreatment is one of the most effective ways to remove mineral from coal. However, deashing pretreatment can change the molecular structure of coal, thus affecting its clean and efficient utilization. To understand the effect of deashing on the molecular structure of Hefeng coal (HF) and its liquefaction residue (LR), HF, LR, Hefeng deashed coal (HF_AC), and its liquefaction residue (LR_AC) were investigated by Fourier transform infrared spectroscopy (FTIR). Results showed that the deashing treatment could destroy the chemical structure of the coal, while the removal of mineral is not conducive to the liquefaction process of HF. After deashing treatment, the relative content of OH–π hydrogen bonds and self-associated –OH hydrogen bonds declined by 22.40% and 32.10%, respectively. For the aliphatic structure, the content of aliphatic –CH₂ changed obviously, among which the asymmetric –CH₂ reduced by 12.23%, while the symmetric –CH₂ added by 15.27%. For the oxygen-containing functional groups, the carboxyl group increased by 118.95%, and the relative content of C–O group reduced by 49.35%. The 5-substitutions of benzene ring almost disappeared after the deashing process, and the 2-substitutions of benzene ring remarkably increased by 148%, which are predominant in the aromatic structure. After liquefaction, compared to HF, the characteristic peaks ascribed to hydroxyl structure, aliphatic structure, and C=O in LR almost vanished, while the intensity of these peaks in LR_AC was stronger than that in LR. The relative content of 5- and 4-substitutions of benzene rings dropped by 33.9%, while the content of 2- and 3-substitutions of benzene rings were increased by 45.47% in LR_AC. Structural parameters from functional groups indicated that the length of aliphatic chain was shortened in HF_AC and LR_AC, and the aromaticity and the condensation degree of aromatic ring were decreased.

KEYWORDS:

• FTIR
• deashing
• sub-bituminous coal
• liquefaction residue
• molecular structure

Acknowledgments

The project was supported by the special project for regional collaborative innovation of Xinjiang Uyghur Autonomous Region (2022E01057), the National Natural Science Foundation of China (22068035), the Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2019I003), the Scientific Research Program of Xinjiang Energy Co., LTD “Composition/structure characteristics and catalytic cracking of organic matter in middle-low rank coal in Xinjiang” and “Composition and structure analysis of low-middle rank coal in Xinjiang and its thermochemical transformation”, the Tianchi project for introducing high-level talents to Xinjiang Uyghur Autonomous Region (China) and the Key Laboratory of Coal Processing and Efficient Utilization from Ministry of Education and High Quality Development Special Project for Science and Technology Supporting Industry from Changji, Xinjiang, China (2022Z04) “Research, Development and Application of Key Technologies for Improving Quality and Efficiency in Coal Chemical Industry.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the the National Natural Science Foundation of China [22068035]; the Scientific Research Program of the Higher Education Institution of Xinjiang [XJEDU2019I003]; Tianchi project for introducing high-level talents to Xinjiang Uyghur Autonomous Region (China) the Key Laboratory of Coal Processing and Efficient Utilization from Ministry of Education the special project for regional collaborative innovation of Xinjiang Uyghur Autonomous Region [2022E01057]; the Scientific Research Program of Xinjiang Energy Co.