Geology Reference
In-Depth Information
Because oxygen supply has a strong effect on the smoldering spread rate (Belcher et al., 2010; Rein, 2009;
Ohlemiller, 2002) and some methods of smothering are cheap compared to other techniques (Kim, 2010) , it might
be assumed that smothering by oxygen displacement is the most efficient method to suppress the fire. However, the
size and thermal properties of coal seams indicates that the short term efficiency of this method is low as detailed
below.
Coal seams can be on the order of 100m in length and 1-10 m thick. Coal has a low thermal conductivity (e.g.,
k = 0.13W·m
1
·K
1
), high density (e.g.,
= 1300-1500 kg·m
3
) and high specific heat capacity (e.g., C
p
= 0.8-1.6
kJ·kg
1
K
1
) (Babrauskas, 2003). This results in a system with a high thermal inertia which means it has the
ability to retain heat for long periods of time. Simple calculations show that a 1 m diameter sphere of coal
(equivalent to approximately 5000 kg) will take on the order of one year to cool to from 900°C, a typical peak
value for smoldering, to 50°C with an ambient ground temperature of 10°C. For larger coal masses (and taking into
account surrounding rock which will also have been heated by the coal combustion), natural cooling will take
longer. Therefore in order for smothering to be effective, oxygen must be excluded from the reaction zone for
extended periods of time. It is thought that poorly maintained exclusion methods will fail within 1-3 years (Kim,
2004). For these reasons, smothering is a long-term, secondary approach to suppress subsurface fires.
ρ
Suppression by forced cooling, the objective of this paper, could offer results in shorter time scales and with
potentially higher efficiency. As this is the first time that experiments of this nature with smoldering coal have been
conducted in the laboratory, the findings are intended to be a first step into the topic. Extrapolation of these results
to larger field scale requires further research and a larger effort especially in understanding the subsurface
distribution of the suppression agent. However, results at the small scale are required before understanding at
the large scale is possible.
Small-Scale Experimental Work
Apparatus
100mm
3
box constructed from insulating board and open at the top.
Coal samples occupy all the free volume of the box. A U-shaped electrical heater of length 290 mm is introduced
on one side of the sample as an ignition source. The setup is based on that of (Rein et al., 2008) for studying peat
fires. Opposite the heater, a window measuring 95
T
he tests were conducted in a 100
100
30mm
2
is created on the wall while the bottom has 25 holes of
diameter 6 mm. The purpose of these was to allow air flow through the coal sample to assist combustion. K-type
thermocouples were placed at 7 locations in the sample: one on the heater and six spaced in three rows of two
thermocouples at distances 20, 50, and 80mm from the igniter at a depth of 50mm. Figure 18.1.2 shows the
Igniter
Coal
Insulation board
Window
Thermocouples
Figure 18.1.2.
Image of the experimental apparatus.
