Petrographical and geochemical characterization of the inertinite-rich coal from the Ningdong Coalfield, Northwest Ordos Basin, China

Abstract

This study deals with the coal petrographical and geochemical characteristics of the #2 and #6 coal seams in the Ningdong Coalfield, Northwest Ordos Basin. Proximate analysis, sulfur forms, ash composition, trace element contents along with macral analysis, and X-ray diffraction were performed on the coal samples. The Ningdong coal was deposited in a large continental (freshwater) basin, which resulted in very low ash yield and total sulfur contents (7.63 and 0.35% on average, respectively), with organic sulfur being the dominant sulfur form. The average random vitrinite reflectance of #2 and #6 seams are 0.49 and 0.60%, respectively, indicating a low metamorphic grade. Inertinite is the most abundant group in the Ningdong coal (50.1% on average), whereas, the vitrinite and liptinite contents are 44.2 and 2.4%, on average, respectively. SiO2 is the major coal ash component (35.57%), whereas, the Al2O3, CaO, and Fe2O3 contents also exceed 10% of the crystalline phases. Trace element contents are low in general; a lot of them are less than the respective World hard coal values. At the same time, the contents of Sc, Cr, Mn, Cu, Y, La, Tl, Pb, Th, U, Cs, Yb, and Hf are close to the average values of the World hard coal, with Mn and Sr even slightly enriched in the #6 and #2 seam, respectively. Analysis shows that Si, Al, K, Ti, Li, Cr, Nb, Pb, Th, Cd, In, Hf, and Ta are mainly hosted in the exogenous minerals and Ca, Mg, and Na mainly come from the endogenous minerals. The flat and wide basin structure along with the paleomire supply with fresh water are the major reasons of the special petrographical and geochemical feature of the Ningdong coal.