Coal quality characterization and its relationship with geological process of the Early Permian Huainan coal deposits, southern North China

• Different subsidence rates cause different coal-forming environment influencing quality of coals.
• Split coal seams 4-1 and 4-2 exhibit significant differences in coal characteristics.
• Vertical variation of trace elements is distinct among different coal seams.
• Host rocks of the coal beds are significant carriers of some elements.

The Huainan coalfield situated in the southern margin of North China Plate contains coal deposits of Carboniferous–Permian age. The 1 and 3 seams of the Early Permian Shanxi Formation and seams 4-1, 4-2, 5-1, 7-2 and 8 of the Late Early Permian Lower Shihezi Formation were analyzed for coal petrography, mineralogy and geochemical parameters. Collected data were designed to offer basic information on the assessment of coal quality and further to reveal possible factors controlling on coal quality. The shift of basin subsidence center from Huaibei coal-bearing basin to Huainan coal accumulation region caused the change of coal-forming environment (groundwater table, detrital matters and swamp style) which may further cause the differences in coal quality. The average vitrinite contents, ash yields and mineral matters are higher in the Lower Shihezi Formation coals than in the Shanxi Formation coals. The contents of vitrinite sharply decrease upward within coal seam 4, whereas the inertinite contents, ash yields and mineral matters show opposite trend which may be attributed to the relatively high-accommodation settings produce coals usually in the form of an upward high-to-low vitrinite succession. Two groups of elements are classified among coal seams. According to the vertical variation of elements contents among coal seams, most of elements in Group 1 are depleted in coal seam 4-2 except for Be, B, Sc, Fe, Mg, Ca, Li, Sn while almost all the elements in Group 2 are enriched in coal seam 4-2.