Sulfur in coals: A review of geochemistry and origins

Abstract

Geochemical studies of sulfur in coals comprise several major aspects relating to the nature and origin of sulfur in coals, including the abundance and distribution of sulfur in coal seams, abundance of sulfur in coal lithotypes and macerals, characteristics and geochemical significance of sulfur-containing organic compounds, sulfur isotopic studies relating to the sources of sulfur in coals, and sedimentary environments controlling the geochemistry of sulfur in coal. A review of the evidence suggests that the variation of sulfur in coals is closely related to the depositional environments of coal seams. For low-sulfur coal (< 1% S), sulfur is derived primarily from parent plant material. For medium-sulfur (1 to < 3% S) and high-sulfur (≥ 3% S) coals, there are two major sources of sulfur: 1) parent plant material, and 2) sulfate in seawater that flooded peat swamps. Abundances of sulfur in coal are largely controlled by the degree of seawater influence during peat accumulation and by postdepositional changes (diagenesis). In high-sulfur coals, seawater sulfate diffuses into the peat, which is subsequently reduced by bacteria into hydrogen sulfide, polysulfides, and elemental sulfur. Reaction of hydrogen sulfide with ferrous iron generates fine pyrite crystals and mackinawite [FeS0.9]. Mackinawite reacts with elemental sulfur to converts to greigite [Fe3S4] and then to framboidal pyrite. The reduced sulfur species in the peat (hydrogen sulfide, elemental sulfur and polysulfides) react with the organic matter to form organic sulfur compounds. During coal diagenesis, nodular pyrite forms. Permineralized peat was formed during diagenesis which contains appreciable fraction of pyrite. After coal is solidified, pyrite can deposit in the cleats from circulating groundwater. Epigenetic pyrite veins may be deposited from basinal fluids. Thus, pyrite forms during various stages of coal formation from peat to coal, as well as late epigenetic activity. The relationships between sulfur abundance in coal seams and depositional environments of coals were reviewed for cases from the U.S., China, U.K., Germany, Hungary, Turkey, Indonesia, and Brazil. In most cases, low-sulfur coals formed in a fluvial environment and high-sulfur coals were deposited in seawater-influenced environments. There are exceptions. For example, Turkish lignites formed in freshwater environments are high-sulfur. Sulfur sources other than seawater are needed for these high-sulfur coals. The superhigh-organic-sulfur (SHOS) coals are highly enriched in organic sulfur but depleted in pyritic sulfur. The SHOS coals were deposited in sulfur-rich, iron-poor environments, such as carbonate platforms or in an iron-poor and clastic-starved environment in which algae accumulate. Speciation of organic sulfur compounds in coal appears to be related to coal rank; thiophenic compounds are more abundant in bituminous coal and anthracite than in low-rank coals.