Study on the Structure and Product Characteristics of Lignite Treated with Tetrahydronaphthalene, Water, and Their Mixture in a Bench-Scale Autoclave

In coal liquefaction, preparing a mixture of hydrogen-donor solvents and coal is essential. During slurry preheating, lignite undergoes dehydration, pyrolysis, and subsequent thermal conversion. Therefore, investigating the characteristics of the three-phase products of lignite in individual organic solvents, water, and their mixtures is crucial. This study examines the characteristics of three-phase products of lignite treated with tetrahydronaphthalene (THN), water, and their mixtures in an autoclave. The methodology includes comprehensive characterization of solid and gas products, along with an analysis of the metal ion content in wastewater. The results show that THN is more effective than water or their mixtures in removing oxygen-containing functional groups from lignite. Lignite treated with water exhibits a significant reduction in the number of methoxy groups. The decomposition of macromolecular structures in lignite during thermal treatment predominantly occurs through the cleavage of aliphatic linkages, which facilitates the release of the soluble fractions. In contrast, water has a negligible effect on this process. Gas production is highest with THN treatment, primarily producing CO₂, with minor amounts of CH₄ and CO. All treatments increase the concentrations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions in wastewater, with Mg²⁺ being the dominant ion. These findings provide valuable insights into the coal liquefaction process, particularly in slurry preheating, lignite dehydration, and pyrolysis.