Geology Reference
In-Depth Information
land plants or microorganisms (Peters et al., 2007). The triterpane pattern of the coal distillate (Figure 11.4.8B)
resembles the parent coal, although again several subtle differences attributable to thermal exposure are evident.
These differences include the relative abundances of BNH/Tm, M/H31-S, Tm/H, and NH/H that are all slightly
lower in the native coal compared to the coal distillate. These same changes were evident in the coal-fire sample set
described above. When carbonized efficiently at 500°C (Figure 11.4.8C) many of the biomarkers degrade leading
to markedly lower overall abundances and changes in the relative abundances of individual triterpanes; for
example, the ratio of Ts/Tm and NH/H increase while M/H31-S decreases. Triterpane biomarkers experience
additional losses at higher temperature (1000°C) (Figure 11.4.8D) before reaching almost complete destruction in
the coke sample (Figure 11.4.8E). Based on personal experience, the complete destruction of triterpane biomarkers
is rarely observed in either coal fire or industrial coal carbonization plants. The retention of triterpane signatures in
the course of coal fires (Figure 11.4.7) and industrial coal tars is the norm, not the exception, which is likely due to
the fact that it is very difficult to carbonize coal and prevent the commingling of coal distillate and coal-tar vapors.
Nevertheless, it is important to know that it is possible to destroy triterpane biomarkers under extreme heating
conditions typical of industrial coal carbonization between 500 and 1000°C.
Conclusions
S
ophisticated analytical methods exist for the chemical characterization of the extractable, semivolatile hydro-
carbons that occur in the nonvolatile by-products produced in the course of coal fires, namely, carbonized coal
residues and coal tars. The composition of extractable hydrocarbons in these coal fire by-products resemble those
found in the products produced in the course of the industrial carbonization of coal (e.g., coal tar and coke).
Both, coal fires and industrial coal carbonization plants produce by-products with hydrocarbon signatures imbued
with information about the native (unburned) parent coal, the conditions of carbonization, and the weathering of the
by-products in the environment (particularly volatilization). This information can prove useful in environmental
investigations that involve coal carbonization by-products. Both coal fires and industrial coal carbonization involve
incomplete and variably efficient carbonization that yields high concentrations of semivolatile hydrocarbons that
are readily isolated and measured in modern chemistry laboratories. Of particular utility in the characterization
of carbonization by-products are (1) the high-resolution hydrocarbon fingerprints that identify the dominant
semivolatile compounds, (2) the distributions and concentrations of PAHs and related sulfur-containing aromatics
(S-PAHs) that gauge the degree of pyrogenicity (the severity of heating) and the extent of weathering in the
environment, (3) the saturated hydrocarbons that reflect the character of the condensed-distillate fraction produced
during carbonization, and (4) the triterpane biomarkers that help associate a parent coal with its carbonization by-
products. The growing collection of reference samples from both coal fires and industrial carbonization processes
helps recognize and interpret hydrocarbon signatures when evaluating the fate, transport, and potential adverse
effects of coal carbonization by-products in the environment.
Acknowledgments
T
he analytical chemistry discussed in this chapter was produced by the dedicated chemists of the Alpha Woods
Hole Laboratory in Raynham, Massachusetts. The authors offer special thanks to Mr. Nicholas Corso (general
manager), Elizabeth Porta (director), Norman Lauriano (GC/FID chemist), and Andrew Cram (GC/MS chemist).
Important Terms
Alkylated PAHs
PAHs with hydrocarbon side chains; e.g., 1-methylnaphthalene, 2-ethylphenanthrene,
3-propylpyrene, and 4-butylchrysene.
Anoxic
An environment with little to no oxygen.
Aromatics
Compounds with a conjugated system of fused carbon rings exhibiting high stability
from very stable
-bonds (pi-bonds).
Π
Capillary column
A narrow bore column made of fused silica.
