Loading of Ni2+ in Coal by Hydrothermal Treatment to Conduct Catalytic Pyrolysis Under the Context of In Situ Pyrolysis

Identifying suitable catalyst types and efficient loading methods remains a key research challenge for implementing the in situ catalytic pyrolysis of tar-rich coal. This study investigated a lignite and a gas coal, employing NiCl2 solution for Ni2+ catalyst loading via room-temperature impregnation and hydrothermal treatment on coal particles sized 6–13 mm. The efficiency of Ni2+ loading through hydrothermal treatment and the characteristics of pyrolysis product distribution and composition before and after treatment were examined. The results indicated that after NiCl2 solution impregnation, the Ni2+ content in lignite increased from nearly undetectable to over 20 mg/g, whereas in gas coal, it only rose to less than 2 mg/g. Ion exchange is hypothesized to be a primary pathway for Ni2+ loading into coal. After hydrothermal treatment at 170 °C, the Ni2+ loadings in lignite and gas coal reached 33.6 and 1.45 mg/g, respectively. The loaded Ni2+ exhibited distinct catalytic effects on the two coals. For lignite, Ni2+ catalyzed the deoxygenation of oxygen-containing compounds and the aromatization of aliphatic hydrocarbons. For gas coal, hydrothermal treatment with NiCl2 solution at 170 and 220 °C promoted hydrogen transfer reactions, resulting in an increase in tar yield from 10.67% to 11.30% and 11.64%, respectively. Also, the H2 yield decreased, accompanied by a decrease in aromatic hydrocarbons and an increase in phenolic compounds within the tar.