Impact of sulphur and trace element geochemistry on the utilization of a marine-influenced coal—case study from the South Wales Variscan foreland basin

Abstract

Incremental channel samples through two high volatile A bituminous coal seams, the Amman Rider and Bute seams, below and above the Vanderbeckei Marine Band at the Westphalian A/B boundary, have been analysed using proximate, petrographic, inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF) and scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) techniques. The results show that both coals are vitrinite-rich and indicate a similar history of palaeomire development, with a period of falling water tables towards the middle plies of each seam. Mean sulphur contents in the coal with the marine roof, the Amman Rider seam (4.19 wt.%) are almost 4 times higher than in the coal with a non-marine roof, the Bute seam (1.09 wt.%), implying that high sulphur contents are derived from sea water sulphate. In both coals, sulphur is predominantly in the form of pyritic sulphur. Both syngenetic, framboidal and euhedral crystals, and epigenetic vein and cleat-fill pyrite are present. Relatively constant levels of organic sulphur at around 2% have been recorded in the Amman Rider seam. Sulphur levels in the basal and top plies of each seam are at least twice that of the ply with the lowest sulphur content in the seam, reflecting the stages in palaeomire development most strongly influenced by sulphate-containing waters. Thorium/Uranium ratios are below 2.9 in the coal with a marine roof, the Amman Rider seam, indicating an enrichment of U from sea water, most marked at the top and bottom of the seam. The seam without a marine roof shows Th/U ratios between 2.5 and 4.9, indicating mild marine enrichment of U. The concentrations of environmentally significant trace elements (ESEs) are correlated with either the ash or sulphur contents, or both. The correlation between S and Pb, As, Cu, Ni in the coal with a marine roof implies an origin of these ESEs from sea water and their mode of occurrence in sulphide minerals. Most ESEs in this coal are present at levels at least twice that of the global average in coals, the exceptions being Pb and Zn which have concentrations less than half the global average. By contrast, the coal without a marine roof and with lower sulphur contents has most ESEs present at concentrations less than a third of the global average, the exceptions being Ni and Cu. Diagenetic hydrothermal activity is documented in the middle ply of the coal with a marine roof. This hydrothermal system redistributed the sulphur, Pb and As in this ply. The principal utilization of bituminous coals in the South Wales coalfield is as feed coals in power station boilers. The resultant concentration of most ESEs in the solid residues from power plant coal combustion theoretically can be alleviated by careful blending of feed coals with low and high ESE contents.