Key Role of Modes of Occurrence of Inorganic Elements in Direct Coal Liquefaction: Migration, Transformation and Impacts on Liquefaction (by) Products

Direct coal liquefaction (DCL) is a critical approach to clean and efficient coal utilization. With international energy supplies becoming increasingly unstable due to geopolitical conflicts, the conversion of coal-to-liquid technology plays critical roles in securing energy safety. Inorganic elements in coal are highly variable in content and modes of occurrence. In particular, the modes of occurrence of each element largely determine their migration and transformation behavior during liquefaction, which in turn directly affects products distribution, oil quality, process stability and environmental risks. This paper summarizes the modes of occurrence of major mineral elements (Al, Si, Fe and alkali and alkaline earth metals (AAEMs)) and hazardous trace elements (Hg, As, Pb, Cd and Cr) in low-rank coals, and then discusses their redistribution and speciation evolution during liquefaction. Inorganic elements and minerals exert distinct effects during coal liquefaction. Iron (Fe), cobalt (Co), and nickel (Ni) generally promote hydrogenation and bond cleavage, whereas AAEMs inhibit liquefaction by enhancing radical cross-linking. Pyrite facilitates hydrogen transfer and hydrocracking, while clay minerals may promote condensation and coke formation. For toxic trace elements, mercury (Hg) generally exhibits high volatility and is readily released into the gas phase. Arsenic (As) and cadmium (Cd) show more complex transformation behaviors governed by their associations with sulfides, organic matter, and mineral phases. Lead (Pb) and chromium (Cr) mostly remain in the solid residue due to their low volatility and strong associations with silicates. Overall, this review highlights the importance of the modes of occurrence of inorganic elements in process optimization and environmental risk management in DCL.