Novel supercritical CO₂ coal upgrading technology: Experimental and simulation investigation

The development and utilization of low-rank coal is one of the important issues in the coal industry. The high moisture content of coal greatly limits its application. Supercritical CO₂ (SC-CO₂) extraction technology can effectively dry and dewater coal by utilizing the high solubility and diffusion capacity of SC-CO₂. In contrast to existing single-condition studies, we used SC-CO₂ to improve the quality of coal under a wide range of temperature and pressure through experimental and molecular dynamics simulation methods. In this work, we used a SC-CO₂ extraction experimental system to extract the water of Zhundong coal at extraction temperature and pressure varied from 50 °C to 200 °C and 8 MPa to 16 MPa, respectively. Thereafter, the alterations in pore structure, surface functional group, microcrystalline structure, and combustion characteristics of Zhundong coal were examined before and after SC-CO₂ extraction. The findings demonstrated that the water content of coal was significantly reduced with the extraction temperature increasing and the extraction pressure decreasing, and the water content was reduced by 99.13 % in the highest case. The coal samples’ specific surface area increased with decreasing extraction temperature and increasing extraction pressure. The number of functional groups on the coal surface diminished slightly, but the types of functional groups remained unchanged. Besides that, the thermogravimetric analysis results demonstrated that the average burning rate of the SC-CO₂-upgraded coal samples was significantly improved, which has a positive impact on the development of the coal industry. In addition, we explored the dissolution and diffusion of water in SC-CO₂ through molecular dynamics simulation to clarify the mechanism of the dewatering process. The simulation results showed that the diffusion coefficients of water increased when the temperature was increased and the pressure was decreased.