Origin of sulfur in coal

This chapter reviews research on the abundance of sulfur in major coal basins in the U.S., the forms of sulfur in coals, the distribution of sulfur in coal lithotypes and macerals, and the nature of sulfur-containing organic compounds in coal. Next, the origin of sulfur in coal is reviewed based on the evidence from the distribution and speciation of sulfur in peat, and from stratigraphic, isotopic, and trace element data. Finally, the origin of sulfur in coal is explained by a geochemical model. 

Sulfur isotopic measurements can shed light on the origin of sulfur in coal. The 34S/32S ratio depends on the source of sulfur and the geologic processes involved during coal formation. For example, isotopic compositions are different for the two principal sources of sulfur in coal 1) the sulfur preserved from the precursor plant material, and 2) the sulfur derived from the bacterial reduction of dissolved sulfate in ambient waters. Plant assimilation of sulfur results in a slight depletion of 34S (4-4.5%c) relative to the 834S in the dissolved sulfate source (102.103). In contrast, the dissimilatory bacterial reduction of sulfate results in a large isotopic fractionation sulfide sulfur can be depleted as much as 60%o in the heavy isotope (89.104-106). 

Use of Trace Elements in Studying the Origin of Sulfur in Coal, The... 

Geochemical studies of sulfur in coal provide information on the forms, distribution, and origin of sulfur in coal. The variation of sulfur content in coal is controlled by geologic conditions during coal formation. This is... 

In order to determine the origin of sulfur in coals, Hackley and Anderson (18) studied the sulfur isotopic composition of various coals from the western United States. Coals from the Powder River Basin, Wyoming were found to have anomalously low values (-18.7 to 3.9 parts per mil). The unusual values of these coals suggest that original plant material and isotopically light secondary sulfur from bacterial activity were the major contributors to the sulfur content of these coals. 

Casagrande, D. J., Gronli, K., and Sutton, N. (1980). The distribution of sulfur and organic matter in various fractions of peat Origins of sulfur in coal. Geochim. Cosmochim. Acta 44, 24-32. 

Casagrande, D.J., Siefert, K., Berschinski, C. and Sutton, N., 1977. Sulfur in peat-forming systems of the Okefenokee Swamp and Florida Everglades origins of sulfur in coal. Geochim. Cosrhochim. Acta, 41 161—167. 

Variations in the amount of ash arise from the retention of sulfur that originates from the pyrite. Sulfur in ash is usually determined as sulfate (ASTM D-1757 ASTM D-5016), and the method may give abnormally high amounts of sulfur. This is due to the sulfur retention from pyrite (and marcasite). If the forms of sulfur in coal are known (ASTM D-2492), the amount of pyrite retention can be estimated (see also ASTM D-3174, Note 2). Nevertheless, sulfur retention will give rise to anomalous results. 

Altschuler, Z.S., Schnepfe, M.M., Silber, C.C. and Simon, F.O. (1983) Sulfur diagenesis in Everglades peat and origin of pyrite in coal. Science, 221, 221-227. 

The origin of sulfur in most coals is believed to be sulfate ion, derived fi om seawater. During the earliest stages of coal formation, bacterial decomposition of the coalforming plant deposits occurs. Some of these bacteria reduce sulfate to sulfide. This immediately reacts with iron to form pyrite, the principal inorganic form of sulfur in coals. It is also incorporated into the organic portion of the coal. The amount and form of sulfur in coals depend much more on the coal s depositional environment than on its age or rank. In this sense, it is largely a coal-type parameter, not a rank parameter. 

Mass spectrometric investigations of isotopes in coal and coal minerals have also been very limited in scope. Rafter (25) published sulfur isotope data on 27 New Zealand coal samples but did not draw any conclusions from these data. Smith and Batts (26) determined the isotopic composition of sulfur in a number of Australian coals and concluded that, from this type of data, one might deduce the origin of the organically... 

It is widely recognized today that coal desulfurization efficiency depends on the distribution of sulfur in the original coal to pyritic and organic sulfur. Pyritic sulfur can be desulfurized, but it is more difficult to remove organic sulfur. 

A new approach for the chemical removal of pyritic sulfur from coal is described. The process is based on the discovery that aqueous ferric salts selectively oxidize the pyritic sulfur in coal to chemical forms which can be removed by vaporiza-tion, steam, or solvent extraction. Data for removal of the pyritic sulfur from four major coals (Lower Kittanning, Illu nois No. 5, Herrin No. 6 and Pittsburgh) are presented together with a discussion of the process chemistry. The effect of variables, such as coal particle size, acid and iron concern tration, reaction time, and temperature are discussed. The results show that near complete removal of pyritic sulfur can be obtained under mild conditions, resulting in a reduction of the total sulfur content of the coals from 40 to 80%, depending on the original pyritic sulfur content. 

The oxides of sulfur create global pollution problems because they have longer lifetimes in the atmosphere than the oxides of nitrogen. Some of the SO2 and SO3 in the air originates from biological processes and from volcanoes, but much comes from the oxidation of sulfur in petroleum and in coal burned for fuel. If the sulfur is not removed from the fuel or the exhaust gas, SO2 enters the atmosphere as a stable but reactive pollutant. Further oxidation by radicals leads to sulfur trioxide ... 

FIGURE 10.20 Potential routes to the origin of organic sulfur in coal. (From Whitehurst, D.D. et al., Coal Liquefaction The Chemistry and Technology of Thermal Processes, Academic Press, Inc., New York, 1980.)... 

Pollutant distributions between char, tar, and gases are expected to be a function of pyrolysis conditions and, hence, are process-dependent. Khan developed models to predict the distribution of sulfur in the products based on pyrolysis data for 32 different coal samples devolatilized in a fixed-bed reactor. It was observed that the total sulfur content in the pyrolysis products at 500 C could be correlated with the total sulfur (weight percent of original coal) content. The relationships are as follows ... 

Present theories of the origin of acid rain indicate that we can limit acid rain by reducing sulfur dioxide emissions and moving to low-sulfur fuels but, only about 20% of the world s petroleum reserves are low in sulfur. Switching U.S. midwestern power plants to low-sulfur coal could cause economic problems since much of the coal from the Midwest and Appalachia has a high sulfur content. Most of the electric power generated in the Midwest uses high-sulfur coal and it would cost tens of billions of dollars to scrub the sulfur out of coal. 

---