Background The Dananhu mining area boasts abundant coal resources, serving as a significant coal-fired power generation base in Xinjiang, China. However, the high sodium (Na) content in coals within the mining area tends to cause fouling and slagging during coal combustion, severely affecting the clean and efficient utilization of coal resources in the area.

Methods and Results epresentative samples collected from the major mineable coal seams were subjected to proximate and ash-composition analyses. The analytical results show that the coal samples had low ash and sulfur contents and high volatile content. SiO₂ predominated in the ash components, followed by CaO, Al₂O₃, and Fe₂O₃. Additionally, Na₂O exhibited an average mass fraction of 4.83% in the ash composition, indicating typical high-Na coals. The Na content in the coal ashes displayed distinct vertical differentiation, showing a pronounced decreasing trend with an increase in the coal seam depth. The correlations between proximate analysis indices and ash components indicate that Na in the coals primarily occurs as organic Na, followed by water-soluble Na. The geological factors influencing Na enrichment in the coals were analyzed by combining the swamp environment and plant types during the coal-forming period, the stratigraphic evolutionary patterns after coal formation, and the groundwater characteristics. The analytical results reveal that high-salinity groundwater serves as a primary source of Na in coals within the mining area, with the groundwater conditions determining the Na content in the coals.

Conclusions Further investigation into the mechanisms underlying Na enrichment in the coals reveals that Na in the coals originated from magmatic rocks in the periphery of the mining area. After rock weathering and leaching, the produced Na was transported into sags by surface runoff and precipitation. Then, it infiltrated into groundwater through rock fractures and finally entered the coal seams. Consequently, the groundwater salinity increased. During the long-term interactions between high-salinity groundwater and coal seams, the organic functional groups in the coals were progressively bound with the Na+ in groundwater, leading to the formation of organic Na, while water-soluble Na occurred in substantial pore structures within the coals. Given the actual conditions of the mining area, two Na removal methods are applicable: washing with Na-removing agents and the co-combustion of coals with additives. Specifically, low-concentration organic acid and ammonium salt solutions, as Na-removing agents, can accurately remove organic and water-soluble Na in the coals. Additionally, adding an appropriate proportion of low-Na coals for co-combustion or incorporating a proper quantity of Na-fixing agents can effectively mitigate or eliminate the slagging phenomenon during coal combustion.