Geology Reference
In-Depth Information
presence of PAH-rich residues consistent with variably weathered coal tar. The variability in the coal-tar signatures
observed at the vents likely reflect the sequential layering of less weathered and more weathered coal-tar residues.
Alternatively, the existence of variously weathered coal tars may represent different mixtures of coal tar and
condensates from older (cooked) and newer (leading edge) zones within the coal-fire area.
Unlike an uncontrolled coal fire, the industrial coal carbonization process used for the production of metallurgical
coke, manufactured gas, and coal-tar chemical feed stocks occurs in a carefully controlled fashion to assure the
quality and quantity of the by-products (Morgan, 1926; Rhodes, 1945). Modern studies of this process demonstrate
the molecular changes that occur during the destructive distillation of coal (Emsbo-Mattingly et al., 2003a, b). For
example,
high-volatile bituminous coal feedstock predominantly
contains normal alkanes, acyclic isoprenoid hydrocarbons, and a wide UCM (Figure 11.4.2A). The hydrocarbons
comprising a carbonized coal distillate produced at 500°C in a glass-lined container with no catalytic surfaces or
oxygen predominantly contains alkylated naphthalenes, a homologous series of normal alkane polymers (
the extractable hydrocarbon in a
“
typical
”
n
-C
9
to
n
-
C
36
) with no odd-carbon preference, and a broad-boiling UCM (
-C
36
) (Figure 11.4.2B). The hydro-
carbons comprising a carbonized coal distillate produced at 1000°C exhibits nearly identical features as were
produced at 500°C (Figure 11.4.2C). Thus, the temperature of formation for a distillate (500°C vs. 1000°C) does
not appear to alter the relative distribution of alkylated naphthalenes and prominence of the UCM in the distillate. It
is also notable that this distribution of alkyl naphthalene and prominence of a UCM is not atypical for hydrocarbons
extracted from uncombusted bituminous coal samples (Figure 11.3.1C, D), perhaps indicating that the longer
heating at lower temperatures typical of geological processes can generate the same hydrocarbon distributions
produced at higher temperatures over shorter times during controlled carbonization conditions in which catalytic
reactions were inhibited. By contrast, pyrogenic PAHs with an absence of any prominent UCM form readily when
the same coal is heated to 500°C in an industrial coke oven (Figure 11.4.2D). In this case, a thick coal bed was
efficiently heated in an oven lined with refractory brick and metal tubing that collectively provided numerous
catalytic surfaces in the form of mineral matter and purified metals. Under these heating conditions, the molecular
signature of crude coal tar consists of two- to six-ring pyrogenic PAHs with relative abundances that decline as
molecular weight increases (Figure 11.4.2D). Little to no UCM is evident. The coal tar produced in a coke oven at
1000°C resembles the coal tar produced at 500°C (Figure 11.4.2E). Thus, the hydrocarbons comprising distillates
or tars produced during the carbonization of coal under controlled conditions are more greatly affected by the
potential for reactivity with catalytic surfaces than by the carbonization temperature. The critical difference
between the formation of coal distillates and tars appears to be the presence of catalytic surfaces. These results
indicate that the variable proportions of pyrogenic PAH and petrogenic UCM among the coal-fire samples (Figure
11.4.1) likely reflect the variable production or mixing of coal tars and coal distillates.
n
-C
12
to
n
PAH Transformations
A
lthough the dominant hydrocarbons provide significant insight to the hydrocarbons produced during coal fires (see
above), the PAH analytical data help specifically reflect the dominant aromatic hydrocarbon signatures in carbonized
coals and coal tars. The uncombusted high-volatile bituminous coal from the Breathitt Formation is rather typical of
many high-volatile bituminous coals (Stout and Emsbo-Mattingly, 2008). It contains only weakly pyrogenic two- to
three-ring PAHs, petrogenic four-ring PAHs, and diagenetic retene (Figure 11.4.3A). However, the carbonized
equivalent of this coal from site 1 at the Ruth Mullins coal fire contains a greater abundance of three- to six-ring
PAHs that exhibit a strongly pyrogenic distribution (Figure 11.4.3B). The low proportion of two-ring PAHs relative
to three- and four-ring PAHs in the carbonized coal is attributable to devolatilization of most two- and some three-
ring PAHs associated with a long thermal exposure period. Compared to site 1, the carbonized coal from site 2
contains even lower proportions of two- and three-ring PAHs relative to four-ring PAHs (Figure 11.4.3C), which is
likely attributable to even more extreme or longer thermal exposure. Interestingly, the more devolatilized carbonized
coal from site 2 exhibits some petrogenic characteristics (e.g., PA0
PA4) due to the prefer-
ential loss of less alkylated homologs (e.g., PA0 and PA1) with higher vapor pressures versus more alkylated
homologs (e.g., PA2, PA3, PA4) with lower vapor pressures. These data also demonstrate that retene (RET), a
C4-alkylated phenanthrene isomer, is less thermally stable than other PA4 isomers (see change in RET/PA4 ratio
among Figure 11.4.3A
PA1
PA2
PA3
<
<
>
>
C). This thermal instability is a feature shared by many immature coal constituents (e.g.,
terpenoids, partially condensed aromatics, and, to a lesser degree, plant waxes). The PAHs found in the coal-tar sample
from vent 1 (Figure 11.4.3D) resembles the carbonized coal from site 2 with respect to the degree of devolatilization
(dashed line). However, the coal tar exhibits lower proportions of fluoranthene and pyrene relative to the alkylated
fluoranthenes and pyrenes indicating more extreme devolatilization than in the carbonized coal samples. The coal tar
-
