Geologic Factors Affecting the Abundance, Distribution, and Speciation of Sulfur in Coals

Abstract

Sulfur in coals is derived partly from original plant materials and partly from ambient fluids in the coal-forming environment. Low-sulfur (LS) coal seams (less than 1% total sulfur content), such as Tertiary coals of the Powder River Basin, USA, were deposited in an alluvial environment and the peat was not influenced by seawater. High-sulfur (HS) coal seams (greater than 3% total sulfur content) are usually associated with marine strata. Seawater influence was the primary cause of the high sulfur in the coal. For example, in the Illinois Basin, high-sulfur Herrin Coal is overlain by marine Anna Shale and Brereton Limestone, but, medium-sulfur (MS) coal (1%−3% total sulfur content) occurs in well-defined areas of the Herrin seam where the coal is directly overlain by thick (>6 m) Energy Shale. This thick nonmarine gray shale was an effective barrier to seawater diffusion into peat. Superhigh-organic-sulfur (SHOS) coals have been reported, including Rasa coal from Croatia, which has a total sulfur content of 10.8% and organic sulfur content of 10.5% (dry basis). A SHOS coal of Late Permian age from Guidin, Guizhou, China contains 9.2–10.5% total sulfur content and 8.2–9.2% organic sulfur content (dry basis). Guidin coal was derived from peat deposited on a coastal carbonate platform. Profound seawater influence and iron deficiency are key factors in the formation of SHOS Guidin coal. The sulfur content of coal macerals is highly variable. Sporinite embedded in vitrinite is higher in organic sulfur than surrounding vitrinite in Illinois Basin coals. A high efficiency of organic sulfur formation in sporinite may be resulted from abundant functional groups in sporinite precursors which reacted with active sulfur species (hydrogen sulfide, elemental sulfur and polysulfides) during early diagenesis. The thiophenic fraction of organic sulfur increases with increasing carbon content of coal. It is interpreted that organic sulfur species initially formed during early diagenesis are mostly thiols and sulfides, which convert to thiophenic compounds during coal maturation. Thus the thiophenic fraction of organic sulfur increases with coal rank.