In Kentucky, estimates of coal tonnage use an 1,800 tons/acre-foot conversion
factor, which is derived from the average density of bituminous coal determined
by the U.S. Geological Survey (Wood et al., 1983). The actual density of coal
varies depending on its rank (degree of alteration) and composition (maceral
content and mineral impurities). Mined coals in Kentucky (in both the Eastern
and Western Kentucky Coal Fields) are of bituminous rank. The composition
of Kentucky coals is highly variable. Because of this variability, many samples
are taken in mines to calculate an accurate density of the coal as it is mined.
This value is important for determining how the coal is washed (cleaned, beneficiated)
to remove mineral impurities that lower the coal's heating value. Hence, the
conversion factor a specific mine or coal companies might use to estimate
the tonnage of a particular seam may differ from 1,800 tons/acre-foot depending
on the coal and reason for the calculation.
If the purpose of the calculation is to obtain a general estimate for the
tonnage of coal on a property or a regional average resource estimate, site-specific
information (the details of coal composition) are not needed and an average
coal density is commonly used for the calculation.
Relative coal density is measured as specific gravity. Specific gravity is
the ratio of the mass of an object (solid or liquid) to the mass of an equal
volume of water at 4ºC (39ºF). For reference, the specific gravity
of water = 1. Materials with specific gravities of less than 1 float in water.
Specific gravities that are greater than 1, sink in water. Because specific
gravity is measured relative to water, specific gravity, the weight of water,
and the area of the seam can be used to determine tonnage.
According to the U.S. Geological Survey (Wood et al., 1983), the average specific
gravity of unbroken (solid) bituminous coal is 1.32.
Specific gravity of bituminous coal = 1.32
1 ft3 of water = 62.6 lbs
1 acre = 43,560 ft2
1 ton = 2,000 lbs
(1.32 * 62.6 lbs. * 43,560 ft2) / 2,000 lbs/ton = 1,799.72 tons/acre foot,
which is rounded off to 1,800 tons/ acre foot
Because specific gravity varies with rank, different ranks of coal have different
Anthracite: specific gravity = 1.47, so the conversion factor is 2,000 tons/acre
Bituminous: specific gravity = 1.32, which is 1,800 tons/acre foot
Subbituminous: specific gravity = 1.30, which is 1,770 tons/acre foot
Lignite: specific gravity = 1.29, which is 1,750 tons/acre foot
The specific gravity of coal is important during coal cleaning because many
of the common mineral impurities in coal have much higher specific gravity
(density) than coal. For example, the specific gravity of pyrite (fool's gold),
a common impurity in coal, and the main source of sulfur in coal, is 4.9 to
5.2. Because pyrite is much denser than coal, density separation techniques
can be used during coal cleaning (washing, beneficiation) to remove much of
the pyrite from coal.
Coal Cleaning (Preparation, beneficiation)
Coal is cleaned at preparation plants. The main goal of preparation is to
remove mineral impurities in the coal that lower the coal's heating value
and increase sulfur emissions when the coal is combusted. There are several
types of density separation processes in preparation plants but the most common
involves floating crushed coal in a fluid. The fluid's density is adjusted
for the density (specific gravity) of the coal being processed. Materials
such as powdered magnetite or diesel fuel are added to water to increase the
water's specific gravity (relative density) to just more than the specific
gravity of the crushed coal. If the fluid's specific gravity is greater than
the coal and less than the mineral impurities, the coal floats and the impurities
sink. In modern preparation plants the additives can be added and removed
throughout the preparation process for the specific gravity of the coals being
processed. The floating coal is removed, rinsed with water to remove the additives
in the fluid, and dried. The mineral impurities are separated by their size.
Coarse material is disposed of as gob and usually used as fill at the property
or adjacent mine sites. Fine material is suspended in water and disposed of
in slurry ponds. The magnetite and water are recycled through the preparation
plant. All plants are regulated by State and Federal environmental regulations.
Wood, G.H., Kehn, J., Carter, T.M., and Culberston, W.C., 1983, Coal resources
classification system of the U.S. Geological Survey: U.S. Geological Survey
Circular 891, 65 p.