An evaluation of the sequential extraction method for quantifying sulfur fractions in coals from the Illinois Basin.

摘要

Coal is a combustible sedimentary rock composed of a complex heterogeneous mixture of mostly organic constituents and minor inorganic phases. Coal is a vital energy resource providing more than half of the electric power generated in the United States. However, coal combustion is responsible for a significant portion of anthropogenic release of different toxic elements including sulfur into the environment. Therefore, deciphering the residence of the different fractions of sulfur in coal is essential.;In this study, eight different sulfur fractions from Pennsylvanian-age coal samples collected from the Murphysboro, Mount Rorah, Springfield (No. 5), and Herrin (No. 6) coal seams from the Illinois Basin were separated using a wet sequential chemical extraction procedure in order to evaluate the coal quality and to test the efficiency of this technique. The average weight percent of sulfur in each seam was 1.98%, 2.1%, 2.26%, and 2.4%, respectively, showing that the coal samples were of medium-sulfur-type. Among the eight different sulfur fractions extracted, kerogen sulfur was found to be the most abundant, followed by sulfate sulfur, fulvic acid sulfur, pyritic sulfur, and elemental sulfur. However, XRD and coal petrography revealed the significant amounts of pyrite still present in the coal sample even after pyritic sulfur extraction, indicating that the finely disseminated pyrite in the coal was not completely removed during the sequential extraction.;The sulfur isotopic study showed the average delta34S values of pyritic sulfur and sulfate sulfur in the Murphysboro coals as 7.82‰ and 2.44‰ and that of Mount Rorah coals were 10.68‰ and 7.87‰, respectively. The heavier delta34S values of pyritic sulfur compared to the sulfate sulfur can be explained by a bacterial sulfate reduction (BSR) model in a closed system where most of the sulfate reservoir was consumed at the top of the seam. Similarly, the average delta34S values of elemental sulfur for the same coals (8.05‰ and 14.54‰, respectively) were also heavier than the sulfate sulfur which suggests the pyrite oxidation followed by disproportionation of intermediate sulfur species. The delta 34S values of handpicked pyrite samples and the mercury concentration for the Herrin (No.6) and Springfield (No. 5) coals indicated at least two stages of hydrothermal inputs into these coal seams.;SEM/EDS and petrographic microscopy of the Illinois coal samples revealed the presence of different syngenetic and epigenetic sulfur-containing minerals such as framboidal pyrite, euhedral pyrite, galena, anhydrite, anglesite, and also non-sulfur containing minerals such as calcite and clay. Based on these results, it can be said that sulfur in Illinois coals is present in different phases extractable by wet sequential chemical extraction however; care should be taken during each individual extraction step to obtain better results.