Geology Reference
In-Depth Information
Lines of Evidence
T
he thermal transformation of coal and its by-products can be evaluated on a molecular level using multiple lines
of chemical fingerprinting evidence (Figure 11.1.1). Ideally, the chemical results are interpreted in the context of
the field, historical, geotechnical, hydrological, and physical (e.g., maceral) data. In keeping with the objective of
this chapter, however, the line of evidence of greatest emphasis is the molecular composition of the extractable
semivolatile hydrocarbons associated with authentic samples of coal and coal carbonization by-products, particu-
larly liquid coal tar. Coal-tar source investigations benefit greatly from the collection and analysis of uncarbonized
(parent) coal and carbonization residues (e.g., coke, coal tar, soot or soils containing mixtures of these).
The lines of chemical evidence are generally evaluated from the major to minor constituents by mass. The
measurement of total organic carbon (TOC) provides a commonly employed tool for normalizing the concen-
trations of solvent extractable compounds from coal and coal by-product samples. This normalization minimizes
the effect of variable mineral and moisture content. The solvent extractable compounds can include both
hydrocarbons and nonhydrocarbons. The measurement of total extractable material (TEM) describes the mass
of hydrocarbons and nonhydrocarbon extracted from the sample using an organic solvent, such as dichloro-
methane (DCM). This measurement is primarily used to assure that the optimal hydrocarbon mass is loaded onto
the various chromatography columns as described below. The measurement of total extractable hydrocarbons
(TEHs) is the mass of semivolatile hydrocarbons eluting between
-C
44
. The TEH fraction is isolated
from the TEM through the chemical adsorption of more polar nonhydrocarbon compounds (see below). The
instrument used to measure the concentration of TEH also produces a high-resolution hydrocarbon
n
-C
9
and
n
”
or source signature, that chromatographically depicts the dominant hydrocarbons in the sample. The PAHs
comprise a fraction of the TEH that consists of two to six benzene rings with various alkyl side chains. Related
to PAHs are numerous condensed aromatic compounds containing various heteroatomic (nitrogen, sulfur, and
oxygen
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fingerprint,
NSO) rings or functional groups (side chains). As a class, the PAHs and related heterotomic compounds
are dominant features of coal tars but only minor components of coal (see below). The specific PAH patterns for
different coals, however, can provide source specific information regarding the thermal maturity or rank of the
coal. In addition to PAHs, the TEH of coal and coal by-products also include various saturated hydrocarbons. The
concentration and chromatographic
—
of saturated hydrocarbons, such as normal alkanes, acyclic
isoprenoid hydrocarbons, and normal alkylcyclohexanes can also help identify coal rank or type as well as
distinguish coal distillates in coal tars. Finally, the TEH also includes geochemical biomarkers that are always
minor constituents of coal and coal by-products; however, these recalcitrant compounds can differentiate coals
from different geologic formations and feedstocks from which coal tars originate. Collectively, the multiple lines
of chemical evidence derived from the molecular characterization of extractable hydrocarbons in coals and coal
by-products help differentiate coals and coal-tar sources while revealing details about the geologic origin and
effects of carbonization. The specific analytical methods and interpretative strategies utilized in providing source
signatures based upon the semivolatile hydrocarbons extractable from coal and coal by-products, particularly coal
tar, comprise the remainder of this chapter.
“
fingerprint
”
