Study on the Pyrolysis Characteristics of a Typical Low Rank Coal with Hydrothermal Pretreatment

Coal pyrolysis characteristics are closely related to its structure and composition, and revealing the relationship between them is essential for deep understanding of the pyrolysis mechanism. Therefore, in this work, Shendong coal (SDC) was treated by hydrothermal pretreatment (HTP) to change its structure and composition, and the influence of the changes on pyrolysis characteristics was investigated. The changes in physicochemical structure and composition of treated coal samples were characterized by N2 isothermal adsorption, Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance, X-ray diffraction, and inductively coupled plasma optical emission spectrometry. The pyrolysis experiments of raw and treated coal samples were carried out in a fluidized bed reactor. Then, the pyrolysis product distribution, the gas composition, the tar chemical composition and properties, and the char gasification behaviors were comprehensively studied. The experimental results showed the effectiveness of HTP in surface area and pore volume increase, oxygen removal and transfer, hydrogen radical introduction, and water-soluble inorganic metal removal for SDC. Correspondingly, the distribution and composition of pyrolysis products of treated coal samples changed. The pyrolysis gas quality was improved with more formation of H2 and CH4. The tar yield and N-hexane insoluble component increased. More oxygen was enriched in macromolecular tar instead of gas and char products, enhancing the char gasification activity. In addition, the structure–activity relationship between the changes in coal structure and composition and the pyrolysis characteristics was revealed in this work.