COAL 



135 



Figure 15. — Comparison on moist, mineral-matter-free basis 

 of heat values and proximate analyses of coal of different 

 ranks. 



content has been made, but classification on the 

 basis of ash content has not been made, because ash 

 is a more highly variable component than sulfur. 

 In recent years, information on trace elements in 

 coal has increased somewhat, but classification ac- 

 cording to trace-element content is not yet possible. 



SULFUR 



Sulfur is an undesirable element in coal. It lowers 

 the quality of coke and of the resulting iron and 

 steel products. It contributes to corrosion, to the 

 formation of boiler deposits, and to air pollution. 

 Its presence in spoil banks inhibits the growth of 

 vegetation. As sulfuric acid, it is the main deleteri- 

 ous compound in acid mine waters, which contribute 

 to stream pollution. 



The sulfur content of coal in the United States 

 ranges from 0.2 to about 7.0 percent, but the aver- 

 age in all coal is 1.0-2.0 percent. Most of the sulfur, 

 perhaps 40-80 percent, occurs as a constituent of 

 pyrite and marcasite (FeSa). The remainder occurs 

 as hydrous ferrous sulfate (FeSi'THjO), derived 

 by weathering of pyrite, as gypsum (CaS04'2H20), 



and as organic sulfur in combination with the coal- 

 forming vegetal material (Walker and Hartner, 

 1966). 



The percentage of sulfur and of pyritic sulfur is 

 highest in bituminous coals of Pennsylvanian age 

 in the Appalachian and Interior coal basins. The 

 percentage is relatively low, generally less than 1 

 percent, in subbituminous coal and lignite of the 

 Rocky Mountain and Northern Great Plains regions. 

 This relation is shown clearly in table 26. 



Table 26. — Distribution, in percent, of identified ' United 

 States coal resources according to rank and sulfur content ' 



Sulfur content (in percent) 



Rank Low Medium Hiffh 



0-1 1.1-3.0 3 + 



Anthracite 97.1 2.9 



Bituminous coal 29.8 26.8 43.4 



Subbituminous coal 99.6 .4 



Lignite 90.7 9.3 



All ranks 65.0 15.0 20.0 



^Identified resources: Specific, identified mineral deposits that may or 

 may not be evaluated as to extent and srrade, and whose contained min- 

 erals may or may not be profitably recoverable with existing technology 

 and economic conditions. 



'From DeCarlo, Sheridan, and Murphy (1966). 



The conspicuously large percentage of low-sulfur 

 coal in the United States, shown on the last line 

 of table 26, is primarily due to the fact that the 

 resources of low-sulfur subbituminous coal and lig- 

 nite concentrated in the Rocky Mountain and North- 

 ern Great Plains regions represent about 54 percent 

 of total identified resources. 



RESEARCH ON REMOVAL OF SULFUR 



Pyrite and marcasite have a high specific gravity, 

 and most of this material can be removed from coal 

 by various washing and cleaning procedures. The 

 other forms of sulfur have lower specific gravities 

 and are more intimately mixed with the coal, and 

 consequently are less easily removed. Between 60 

 and 65 percent of all coal mined in the United States 

 is cleaned to remove pyritic and inert material be- 

 fore use. However, in spite of such large-scale clean- 

 ing, the average sulfur content of all coal used in 

 the United States is still nearly 2 percent. 



Current efforts to reduce the sulfur content of 

 coal and of flue gas take many forms : 



1. Much research is in progress on methods to re- 



move SO2 and SO3 from flue gas. This can be 

 done by several well-known chemical processes, 

 and the technical problems inherent in the 

 large-scale commercial application of chemical 

 processes are likely to be solved in the near 

 future. 



2. Meanwhile, the search for low-sulfur coal has 



