Geochemical research status of sodium element in high-sodium coal in Xinjiang

The coal resource in Xinjiang Uygur Autonomous Region, China, which is dominated by low-ash, low-sulfur, and low-phosphorus coals for power generation and chemical industry, is abundant and has become the main battlefield for coal development and utilization in my country. However, due to the special geological environment and the action of groundwater, a large amount of sodium is enriched in some coal seams in Xinjiang. The clean and efficient utilization of the high-sodium coal in Xinjiang has been constrained because of slagging and fouling. Xinjiang high-sodium coal and its distribution and occurrence characteristics of sodium have become a hot spot in the field of basic coal chemistry research in the past decade. This paper systematically summarized research results of the occurrence state of sodium in coal, summarized the determination method of sodium content in coal, analyzed the key geological factors controlling sodium enrichment in coal, and discussed the migration law of sodium and its influencing factors in the process of coal utilization. Previous research demonstrated that sodium in coal dominantly occurs as inorganic forms, with minor organic forms. The sequential chemical extraction further indicated that sodium in coal is mainly water-soluble. At present, the determination procedure for coal sodium is conducted through combustion and ashing and digestion under certain conditions. However, because sodium in Xinjiang coal is mainly water-soluble, a sequential extraction can reduce the loss of sodium in sample preparation and the sodium content can be detected more precisely.The enrichment of sodium in coal mainly includes the accumulation of biological components, the input of clastic sediments, the precipitation of solution in peat pores, and the action of seawater in the later stage of coal formation. Previous studies on high-sodium coal at home and abroad focused on the influence of seawater intrusion,but the enrichment of sodium in high-sodium coal in Xinjiang needs to focus on the surface water leaching and groundwater driving effect. In the process of coal processing and utilization, the occurrence form of sodium and the way of coal utilization determines its release pathway. In-depth study of the geochemical control mechanism of sodium enrichment in coal and the migration mechanism of sodium in the process of coal combustion is of great significance for the rational development and utilization of high-sodium coal.