Geology Reference
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also increases (Kim and Douglas, 1973). In a laboratory study of temperature-dependent desorption from coals and
coal wastes (Kim, 1991), a ratio (
1) of higher molecular weight hydrocarbon gases (C 2 to C 5 ), determined as total
hydrocarbon gases minus methane, to total hydrocarbons was defined as
R
1
:
01
½
THC
½
CH 4
R
1
¼
1000
þ c
ð
16
:
4
:
1
Þ
½
THC
where [THC] is the concentration of total hydrocarbons in parts per million (ppm), [CH 4 ] is the concentration of
methane in ppm, and
is a constant, 0.01 ppm. The ratio was found to increase with increasing temperature and
decrease with decreasing temperature. The concentration of desorbed gas and the ratio were related to the rank of
the coal. For bituminous samples, the value of
c
1 is interpreted according to the empirical scale (Table 16.4.1.).
The hydrocarbon ratio is sensitive, unaffected by dilution, and requires relatively simple analytical methods (Kim,
1988).
R
The MFD methodology, in addition to the hydrocarbon ratio, uses communication testing to define hot and cold
subsurface zones (Dalverny et al., 1990). An exhaust fan is used to impose a pressure gradient which controls the
direction of airflow. This assumes that sufficiently large negative pressure (vacuum) applied to underground
regions will cause the gases in the mine atmosphere to flow from some distance toward the point of suction
(Figure 16.4.1.). Pressure and temperature measurements are taken at the base of boreholes under both baseline
(no suction) and communication (suction) conditions; in-mine gas samples are obtained under the same conditions
(Figure 16.4.2.). Repeated sampling at all points in the borehole array provides data to determine the presence or
absence of fire along pathways between sampling points.
For each test, a quadrant indicating heating, cooling, or no change is placed on a straight line drawn through the
borehole and the suction point. Reiteration of the tests using various boreholes as suction points produces a
composite map of heated and cold zones. The tests can be repeated, sometimes with the drilling of additional
boreholes, until a cold boundary can be defined.
Four field studies supported the use of the Bureau
s MFD methodology to define remote combustion zones. Each
of the trials presented different challenges, but each indicated the value of the methodology.
'
Carbondale
The Carbondale mine-fire site in Pennsylvania
s Northern Anthracite Field was located adjacent to an apartment
complex (Justin and Kim, 1987). The complex had been built on an area excavated and backfilled during a
previous fire-control project. Anomalous snow melt (Figure 16.4.3.) indicated that combustion was occurring in the
unreclaimed portion of the mine. According to drillers
'
'
logs, there were two anthracite seams in the area. Thirty-
four cased boreholes had been installed at the 3.4 ha site. They extended 16
30.5m beneath the surface through
both coal seams. The objective of the MFD project was to determine the proximity of heated zones to the apartment
complex (Kim et al., 1992).
-
Since anthracite has a lower concentration of higher hydrocarbons adsorbed on the coal surface, methane emission
was found to be sufficiently temperature dependent to allow its use as a combustion indicator for anthracite. The
Table 16.4.1
Values of diagnostic ratio (R1) for bituminous coal.
R
1 value
Estimated coal temperature
Interpretation
0
-
50
<
30°C
Normal
50
-
100
30
-
100°C
Possible heated coal
>
100
>
100°C
Heated coal
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