Geology Reference
In-Depth Information
from vent 5 (Figure 11.4.3E) exhibits an even greater degree of devolatilization (e.g., notice the reduced proportion of
four-ring PAHs compared to the coal tar from vent 1 and also exhibits lower proportions of (FL0+PY0)/(FP2+FP3)).
Despite the apparently high degree of devolatilization in the vent 5 tar, the distillate range two- and three-ring PAHs are
enriched in naphthalene (N0) and phenanthrene (P0) and exhibit a strong pyrogenic pattern, which argues for the
presence of a relatively unweathered tar. Collectively, this mixture of weathered pyrogenic HPAHs and unweathered
pyrogenic LPAH in the vent 5 coal-tar sample demonstrates the presence of a heavily weathered coal tar mixed (giving
rise to the prominent four- to six-ring PAHs) with a less weathered coal tar (giving rise to the prominent two- and three-
ring PAHs with strong pyrogenic distributions; Figure 11.4.3E).
The PAH patterns for the carbonization products of coal produced under controlled heating conditions
(Figure 11.4.4) allows for further interpretation of the naturally produced coal-fire residues shown in Figure
11.4.3. The original coal feedstock consists of weakly pyrogenic two- and three-ring PAHs with lower proportions
of petrogenic four-ring PAHs (Figure 11.4.4A), which is typical of high-volatile bituminous coals (Stout and
Emsbo-Matttingly, 2008). The coal distillate generated at 500°C contains a nearly identical petrogenic two- to four-
ring PAHs as the parent coal (not shown). Similarly, the PAHs in the coal distillate generated at 1000°C also
resembled the parent coal (Figure 11.4.4B). However, the coal-tar samples generated at 500°C (Figure 11.4.4C)
and 1000°C (Figure 11.4.4D) contain nearly identical pyrogenic two- to six-ring PAHs, which are markedly
different from the parent coal and the coal distillates. The relative abundance of the PAH homologs decreases with
increasing ring number in the coal-tar samples. Also, the ratio of RET/PA4 decreases slightly during distillation
(Figure 11.4.4B) and decreases dramatically during carbonization (Figure 11.4.4C, D) indicating the thermal
instability of RET. Importantly, the extractable PAHs from the resulting coke residue predominantly contains
two-ring pyrogenic PAHs dominated by naphthalene (Figure 11.4.4E). This naphthalene enrichment is likely
attributable to the sequestered vestiges of the coal cracking process that occur when the final coal vapors fail to
completely migrate out of the carbonized coal bed. The more highly volatile two-ring PAHs that successfully
migrate to the surface frequently condense upon contact with cooler surficial soils and air; for example, the vent 5
coal-tar sample from the coal-fire site (Figure 11.4.3E).
On a quantitative level, the Breathitt high-volatile bituminous coal contained 144mg/kg EPAPAHs or 630mg/kg
45PAHs (Figure 11.4.3A). The PAH concentrations of the carbonized coals from sites 1 and 2 and the coal tar at
vent 1 increased by more than 2 orders of magnitude (Figure 11.4.3B
D). The coal-tar residue from vent 5
contained only double the PAH concentration measured in the source coal (Figure 11.4.3E). The difference
between the vent 1 and vent 5 samples likely reflected mixing/dilution with ambient soil.
-
The high-volatile bituminous reference coal contained 19 700mg/kg EPAPAHs or 83 700mg/kg 45PAHs
(Figure 11.4.4A). The coal distillates generated at 500°C (same as Figure 11.4.4B) and 1000°C (Figure 11.4.4B)
contained about half of the PAHs of the original coal sample. However, the coal tars generated at 500°C (Figure
11.4.4C) and 1000°C (Figure 11.4.4D) contained 5
10 times more PAHs than the original coal. These data suggest that
the concentration PAHs associated with coal distillates are significantly lower than the concentration of PAHs derived
from coal tar. Essentially, these disparate PAH production rates demonstrate that the PAH composition of carbonization
residues is principally attributed to the coal-tar fraction and not to the coal distillate fraction. The amount of coal tar
remaining in (or extractable from) the coke residue is relatively low (EPAPAHs = 8100mg/kg and 45 PAHs = 12 000
mg/kg) because the minerals and graphitic carbon effectively dilute or inhibit the extraction of the coal-tar residue.
-
Saturated Hydrocarbon Residues
T
he transformation of saturated hydrocarbons in coal and its by-products provides another line of evidence demon-
strating the effects of thermal exposure and subsequent volatilization experienced during coal fires. Unlike the high-
resolution hydrocarbon fingerprints (GC/FID) that primarily resolve the dominant hydrocarbon features (e.g., plant
waxes and UCM profiles), the saturated hydrocarbon fingerprints reflected in the
85 extracted ion chromatograms
obtained using GC/MS reveal more clearly the full spectrum of normal alkanes and acyclic isoprenoid hydrocarbons
(Figure 11.4.5). In addition to plant waxes, the Breathitt high-volatile bituminous coal contains abundant pristane and a
homologous series of lighter alkanes (
m
/
z
-C
20
) (Figure 11.4.5A). The distribution of these compounds was not as
clearly evident in the high-resolution fingerprint for this coal (Figure 11.4.1A). Upon heating in the Ruth Mullins coal
fire, the odd-carbon preference among the plant waxes evident in the parent coal disappears (Figure 11.4.5B
n
-C
8
to
n
E).
In addition, the lighter alkanes are depleted in the carbonized coal by-products due to devolatilization.
The progressively more extreme degree of devolatilization that was evident in the PAH profiles (Figure 11.4.3) is
-
