Molecular structure model construction and pyrolysis characterization of four typical coals: a case study

ABSTRACT

This paper constructed the molecular structures of four types of coal with different ranks and investigated the correlation between coal molecular structure and pyrolysis characteristics by using ¹³C NMR, XPS, TG-FTIR, and GC-MS. The results demonstrate that the structural model formulas of lignite (HM), weakly caking coal (RN), long-flame coal (CY), and gas coal (QM) are C₁₀₄H₁₁₁NO₂₀S, C₉₃H₁₀₉NO₁₃, C₁₆₇H₁₉₁NO₁₅S, and C₁₄₂H₁₂₉N₃O₉S, respectively. The aromatic structural units of HM and RN consist of benzene and naphthalene, which are connected by ether bonds or aliphatic chains. The molecular structures of HM and RN contain a large amount -OH and -COOH, which are easily broken to generate gases, such as H₂O, CO, and CO₂ during pyrolysis. The gas yield of RN is 28.90%, which is higher than the others. The ether bonds in HM breakage to form groups are easily combined with each other to form tar, while the aliphatic chains in RN fracture to form fragments to generate gas. The light component (C_≦10) is the dominant form of tar in HM and RN, with a content of 95.62% and 87.78%, respectively. Phenols are the dominant components in HM tar; alcohols are the primary components in RN tar. In CY, the aromatic structures consisted of benzene, naphthalene, anthracene, and phenanthrene; the aliphatic structures mainly included cycloalkanes and long-chain alkanes. The QM structural units are dominated by phenanthrene and naphthalene, with the highest aromaticity. The pyrolysis gas in CY and QM is composed of CH₄; the tar component consists of alcohols, phenols, and alkanes. The coke yield in RN is the lowest at 52.84%, and QM is the highest at 80.20%.

KEYWORDS:

• Molecular structure
• pyrolysis
• ¹³C NMR
• GC-MS
• TG-FTIR

Acknowledgments

The authors acknowledge the financial assistance of the Key Program of Shaanxi Natural Science Foundation (2018JZ5011), and Joint Program of Shaanxi Natural Science Foundation (2019JLM-44).

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.