Geology Reference
In-Depth Information
11.1. Source Identification
Stephen D. Emsbo-Mattingly
Scott A. Stout
Coal
Tar
Thermal transformation of coal to coke.
Coke
Photo by Stephen D. Emsbo-Mattingly, 2003.
Source Signatures
C
oal contains thousands of organic compounds and macromolecules derived from ancient biomass and reformed
by biogeochemical processes over geological time at moderate temperatures (
400°C). Many factors control the
specific chemical composition of coal; for example, different types of coal are formed depending upon the species
of prehistoric terrestrial plants and the conditions in the ancient depositional environment (e.g., mineral content and
oxygen concentration) in which the plant debris accumulated. Different ranks of coal are formed depending upon
the geologic conditions (temperature and pressure) and duration of burial. The collective process of coalification
imbues the coal bed with physical and chemical features that can differentiate coals and coal by-products from
different formations.
<
Source signatures can be broadly classified by properties that differentiate one material from another. Recognizing
source signatures is an important component of any investigation of coal and coal-derived by-products in the
environment, including those associated with coal fires. In most cases, different coals exhibit distinct physical or
microscopic properties that allow coals of different type and rank to be distinguished by long established (mostly)
physical testing methods. However, as modern chemical testing methods improve, the number of source signatures
based upon diagnostic molecular features of different coal types and ranks greatly expands. By using multiple lines
of physical and chemical evidence, the ability to recognize and define independent source signatures increases and
the distinctions among coals of different types and ranks and the distinction of coal from other fossil fuels (e.g.,
petroleum) becomes more resolved or specific. Similarly, these same multiple lines of evidence help differentiate
coal and coal-derived by-products formed during the carbonization of coal via natural (e.g., coal fires) or industrial
(e.g., coking) processes. Of particular use in the chemical source signatures of coal and coal-derived by-products
are semivolatile hydrocarbons, which are the focus of this chapter.
Semivolatile hydrocarbons are solvent-extractable compounds found in coals and coal-derived by-products that
include a broad range of compounds. We have functionally defined semivolatile hydrocarbons as solvent-
extractable compounds that elute between about
-C
44
)onagas
chromatograph (GC) equipped with a non-polar silicone capillary column. The semivolatile hydrocarbons include
normal alkanes, acyclic isoprenoids, aromatics, sesqui-, di-, and triterpanes, regular and rearranged steranes,
mono- and triaromatic steranes, and many other compound groups.
n
-nonane (
n
-C
9
) and
n
-tetratetracontane (
n
The specific nature of these compounds in fossil fuels and other forms of ancient organic matter, including coals,
were originally investigated by geochemists in order to identify and locate oil-bearing formations (Peters et al.,
2007 and references therein). Numbering in the thousands of individual compounds, the relative abundances or
absolute concentrations of diagnostic semivolatile hydrocarbons can differentiate fossil fuels (e.g., coals and crude
oils) and various derived products, such as coke, coal tar, kerosene, diesel fuel, heating fuel, heavy fuel oil, lube oil,
asphalt, and many others (Stout et al., 2002; Douglas et al., 2007). By contrast, volatile hydrocarbons, which
include hydrocarbons and other compounds that elute before approximately
n
-dodecane (
n
-C
12
), are very useful for
