Abstract:
Oil-rich coal, as a special coal resource with abundant hydrogen-rich structures, shows great potential in realizing the clean and efficient utilization of coal. This review systematically summarizes the research progress of solvent extraction and direct liquefaction technologies for oil-rich coals. In terms of solvent extraction technology, the focus is on exploring the mechanisms, solvent system optimization, and applications in obtaining high-value chemicals, liquid fuels, and multifunctional carbon materials through methods such as low-temperature extraction, thermal extraction, biomass co-extraction, and supercritical extraction. Among them, low-temperature extraction technology features mild conditions and can effectively retain the main structure of oil-rich coals, making it an important means to study its macromolecular structure. Thermal extraction technology enhances the swelling and cracking of coal by moderately increasing the reaction temperature, demonstrating broad application prospects in the preparation of liquid fuel precursors. Biomass co-extraction technology leverages the hydrogen-rich properties and synergistic cracking effect of biomass, not only improving the hydrogen-carbon ratio of extraction products but also reducing the environmental pressure during the reaction process. Supercritical extraction technology utilizes the excellent mass transfer and permeability of supercritical fluids (such as CO
2 and ethanol) to achieve green and efficient extraction processes. In the part of direct liquefaction, the effects of key factors including solvent, catalyst, temperature, pressure, and atmosphere on liquefaction efficiency are systematically analyzed, and the core mechanisms of hydrogen donation by solvents (free radical mechanism, hydrogen shuttling mechanism, and hydrogenolysis mechanism) are elaborated. As the core medium of liquefaction reaction, the solvent plays a crucial role in dissolution, hydrogen donation, and hydrogen transfer. Current research often combines methods such as density functional theory to rationally design and optimize hydrogen-donating solvents. In terms of catalysts, iron-based catalysts are the most widely used due to their low cost, abundant resources, and strong adaptability. The research on these catalysts focus on the regulation of active phases, optimization of dispersion, and enhancement of synergistic effects. Doping with elements such as Co and Ni can further improve catalytic performance, significantly increasing coal conversion rates and liquefied oil yields. In addition, appropriate temperatures help balance the generation of free radicals and hydrogenation rate, avoiding excessive condensation. Moderately increasing pressure can enhance the solubility of hydrogen, promoting the combination of hydrogen free radicals with coal-cracking free radicals. The research on alternative atmospheres such as CO and methane provides new pathways for reducing process costs and improving economic efficiency. Among the solvent hydrogen donation mechanisms, the hydrogenolysis mechanism is not been widely recognized due to its extremely low conversion rates. Current research primarily focuses on the free radical mechanism and the hydrogen shuttling mechanism. The free radical mechanism includes stepwise mechanism and synergistic mechanism, with the synergistic mechanism being dominant due to its lower reaction energy barrier. The hydrogen shuttling mechanism involves three paths: gas hydrogen → coal, solvent hydrogen → coal, and gas hydrogen → solvent then → coal. As a key carrier, the solvent mediates the secondary distribution of hydrogen, and isotope tracer studies have confirmed its directional migration patterns among different products. This paper systematically summarizes the current status of extraction and liquefaction technologies for oil-rich coal, deeply analyzes the existing technical bottlenecks and core scientific issues, aiming to provide theoretical references and technical support for the optimization and upgrading of low-carbon and high-value conversion technologies for oil-rich coal, thereby promoting technological development in related fields.