Trace Elements in West Virginia Coals

Glossary

Ash Yield: Mineral matter, not ash, is naturally present in coal. Mineral matter content of coal may vary from trace amounts up to 50 weight percent by definition. Ash remains after the complete combustion of coal and the amount of ash, on a whole coal weight percent basis, is correctly referred to as weight percent ash yield.

Btu: Heating values are measures of the amount of heat produced during the combustion of any fuel ¹.  Standard "rulers" such as Btu's, calories, kilocalories or joules are used to express the amount of heat given off.
America uses the British system of Btu or British thermal unit to measure the heating value of coals expressed as Btu's per pound of coal.  By definition, one Btu is the quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit from 63 and 64° F^*. The WVGES published the following comparisons between Btu's of important West Virginia coals ¹. One pound of Pittsburgh coal from Marshall County (10,000 Btu/lb.) would raise the temperature of 10,000 pounds of water (= twenty-two 55-gallon barrels) from 63 to 64°F. One pound of No.5 Block coal from Kanawha County (13,000 Btu/lb.) would raise the temperature of  28 barrels of water from 63 to 64°F. And 1 pound of Pocahontas No.3 coal from McDowell County (15,000 Btu/lb.) would raise the temperature of 33 barrels of water from 63 to 64°F.
^* By convention a standard temperature range of 63° to 64° F is used in all calculations involving volume.

Mineral Matter: Minerals make up the majority of  the ash forming constituents in coal.  Also present are inorganic elements intimately intermixed with the organic majority of the coal referred to as inherent mineral matter. Minerals in West Virginia coals are dominated by clay minerals, kaolinite (Al₄(Si₄O₁₀)(OH)₈) (35% of the mineral matter) and 35% illite (KAl₂(OH)₂AlSi₃O₁₀) along with 18% quartz (SiO₂), 7% pyrite, and 3% calcite based on analyses of 1400 samples at the WVGES. Ash does not equate exactly with the amount of mineral matter in coal because clay minerals lose water upon ashing, calcite loses CO₂ and pyrite loses sulfur. The Parr formula (mineral matter = 1.08 x ash + 0.55 x total sulfur) is used to estimate the amount of mineral matter in coal.

Parts-Per-Million (ppm): For consistency trace element analyses are given in parts-per-million (ppm) on a whole coal basis. These values are very small and represent the amount of the pure element in the coal.  For example 1 ppm lead on a whole coal basis means there is 1 pound of lead in 1 million pounds (or 500 tons) of coal.

Rank: Rank is a classification of coal based on the degree of metamorphism from lignite to anthracite coal ². Coal rank increases across the state of West Virginia from west to east and from northwest to southeast. At any location within the state the rank of the coal is a reflection of the final stage of metamorphism that occurred over millions of years caused by temperature, duration and depth of burial of the coal by overlying sedimentary rocks ³. Rank is an important coal parameter influencing coal combustion characteristics (especially Btu) and coking properties as well as numerous other technological properties of coal.  Coal rank is measured in several ways, but the standard method is based on volatile matter (or fixed carbon) on a moisture, mineral-matter-free (m,mmf) basis and Btu ².
 

Classifications of West Virginia Coals by Rank

Rank	Fixed Carbon (m,mmf)	Volatile Matter (m,mmf)	Gross Calorific Value (Btu/lb)
semianthracite (sa)	>86% - 92%	>8% - 14%
low volatile bituminous (lvb)	>78% - 86%	>14% - 22%
medium volatile bituminous (mvb)	>69% - 78%	>22% - 31%
high volatile bituminous A (hvAb)	<69%	>31%	<14,000
high volatile bituminous B (hvBb)			>13,000 - 14,000
high volatile bituminous C (hvCb)			>11,500 - 13,000

SEM: Scanning Electron Microscopy.  The scanning electron microscope can show minerals in coal ranging from large to extremely small (much less than 1 µm = 1/1000 mm = 4/10,000 inch) sizes.  To identify the minerals the SEM must be equipped with an Energy Dispersive X-ray detector (EDX) which can identify the elements in the mineral and proportionally the amounts of each element in the mineral.  From the chemical composition the mineral can be identified, for example pyrite has the chemical formula FeS₂, indicating the mineral contains only iron and sulfur and in a 1:2 ratio.  An EDX spectrum of suspected pyrite in the Pittsburgh coal shows just this ratio. Identification by elemental analysis alone however cannot positively identify a mineral, for example the chemical formula for marcasite is the same FeS₂ and other methods must be used to separate pyrite from marcasite.
Sulfur forms:  Sulfur is known to occur in both mineral matter and the organic fraction of coal.  Wet chemical separation techniques applied in coal laboratories results in the somewhat artificial partitioning of sulfur in coal into three sulfur forms or sulfur "species", pyritic sulfur, sulfate sulfur and organic sulfur. Pyritic sulfur is technically sulfur bound up in the mineral pyrite (FeS₂) in coal, but also includes sulfur in marcasite (also FeS₂), chalcopyrite, galena, sphalerite and other sulfides in coal soluble in nitric acid, but not hydrochloric acid.. Sulfate sulfur is bound up in sulfate minerals in coal easily dissolved by hydrochloric acid.  These sulfates are generally the result of oxidation of pyrite in the coal and thus are an indicator of weathering of the coal before or after mining.  Organic sulfur is, of course, sulfur organically bound in the coal and is not soluble in hydrochloric or nitric acid.  Sulfur has long been known to be part of the organic molecules of coal, but some organic sulfur determined is always extremely small sulfide minerals (mainly pyrite) in the matrix of the coal not accessible by wet chemical techniques.

Whole coal basis:  Most trace elements, as well as some other analyses, are determined on the ash of the coal because ashing (burning) concentrates the elements and ash is more conducive to standard chemical analysis techniques.  Some elements (sulfur, selenium, mercury and chlorine) must be determined on the whole coal because they are highly volatile, and burn off during ashing. Calculating all trace elements and other analyses to a whole coal basis allows direct comparisons of values on a consistent basis.

References:
    1. Smith, C., (1980)
    2. ASTM, D121-94 (1995)
    3. Stach, E., et al., (1975)
 

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Page last revised: March 1, 2002

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