Removal mechanism of sulfur compounds from Zhundong coal using supercritical carbon dioxide-cosolvent systems

Sulfur compounds in coal release sulfur oxides during thermochemical conversion, causing environmental pollution in the form of acid rain when discharged into the air. There is thus a significant need to develop efficient, environmentally friendly coal desulfurization technologies. This work studied Zhundong coal desulfurization using supercritical carbon dioxide-cosolvent systems experimentally and computationally. The desulfurization effects were investigated by adding nine different kinds of cosolvents. The sulfur content, pore structure, and functional group changes of the coal samples after extraction were analyzed using several experimental techniques. The sulfur composition of the extraction residuals was also analyzed by gas chromatography-mass spectrometry. The experiments showed that cosolvents enhanced sulfur extraction from Zhundong coal, with desulfurization rates ranging from 3.59% to 23.68%. Among the tested systems, the methanol-containing system exhibited superior synergistic desulfurization performance, achieving the highest sulfur removal while maintaining relatively low coal weight loss. In contrast, the NMP-containing system caused the highest weight loss of 13.65%, which may be related to the dissolution or release of low molecular weight organic fragments. After extraction, the specific surface area and pore volume of the coal samples changed only slightly, suggesting that the overall pore structure was largely retained. Quantum chemical calculations of dipole moments and electrostatic potential distributions showed that acetic acid had a strong affinity for high-valent sulfur compounds due to its pronounced local charge difference, whereas methanol's small molecular size and strong permeation favored access to more sulfur sites. These results provide guidance for selecting efficient cosolvents for supercritical carbon dioxide-assisted coal desulfurization.