Three forms of sulfur occur in coal: (1) organic, (2) inorganic, and (3) elemental. Understanding the forms of sulfur in a coal is important because coal preparation can reduce the inorganic (pyritic and sulfate) sulfur in coal, but not the organically bound sulfur in coal.
The testing procedures and guidelines for determining the weight percent contributions of these forms of sulfur to the total sulfur content of a coal are outlined in ASTM method D2492-02 (American Society for Testing and Materials, 2013, p. 458–462). Parts of this analysis are similar to the ultimate analysis for total sulfur, but rather than running whole coal samples through a sulfur analyzer, the sample is incrementally leached with hydrochloric (HCl) and nitric acids (HNO3) to preferentially remove sulfate and sulfide compounds, respectively. After the weight losses from the acid leaches are recorded, the remaining sample is then analyzed to determine organic sulfur content. Alternatively, the weight percent of monosulfide and sulfate can be measured directly using infrared absorption in the analyzer. Pyritic sulfur is measured indirectly by measuring the iron in a sample after chemical treatment using flame atomic absorption spectrometry. Measurements of these forms of sulfur in combination with the total sulfur value can be used to determine the organic, inorganic, and elemental sulfur contributions in the original coal sample (Rait and Aruscavage, 1989).
(1). Organic S weight % = total S weight % – (monosulfide S weight % + sulfate S weight % + pyritic S weight %)
(2). Inorganic S weight % = (sulfate S weight % + pyritic S weight %)
(3). Elemental S weight % = monosulfide S weight %