Changes of Zhundong coal properties by hydrothermal upgrading and its impacts on CO₂ gasification

The Zhundong high sodium coal was hydrothermally upgraded at 150-350℃ in an autoclave. The inductively coupled plasma mass spectrometer (ICP-MS), nitrogen adsorption isotherm (BET) and X-ray diffraction (XRD) were used to investigate the changes of coal properties and the impacts on the CO₂ gasification characteristics. The results indicate that the coal quality is increased after the hydrothermal upgrading. The removal effect of sodium is obvious, reaching to more than 95% at 300-350℃. The pore structure of the chars changes significantly, the specific surface area and total pore volume increase initially at 150-300℃ and then decrease at 300-350℃. The crystalline structure tends to be aromatic and graphitized, and the chemical structure becomes dense, orderly and stable. Additionally, the gasification reactivity exhibits a decreasing trend, especially for the chars treated at 300-350℃. The CO₂ gasification reactivity during the process of hydrothermal upgrading is comprehensively influenced by different factors containing coal rank, sodium content, physical pore structure and chemical microcrystalline structure.