Geology Reference
In-Depth Information
and experiences of the authors. These studies demonstrate that some of the geotechnical and environmental
problems caused by coal fires are as follows:
Sterilization.
Lock up of coal reserves and resources.
Generation of noxious gas that may sink to the bottom of an opencast mine because it is denser than air. The
noxious gas may have a detrimental effect on mine workers and those living in surrounding communities
(Walker, 1999; Stracher, 2004a,b; Finkelman et al., 2002).
Residual ash or red shale produced from burning coal, observed by the authors to influence the stability of benches
and slopes in opencast-mining operations. This has resulted in the failure of open-pit slopes that has delayed
mining, damaged mine infrastructure, and has been anecdotally reported to have caused occasional loss of life.
Ground fissures and sinkholes, or localized subsidence, especially in association with abandoned shallow mines.
Destruction of flora and fauna habitats.
In addition to coal seams, colliery spoil heaps (spoil tips) and carbonaceous shale are susceptible to combustion.
This may present problems during reclamation, particularly if hot spots are encountered and long-term smoldering
has occurred. Some of these hot spots have been reported to be at temperatures of 600
-
900 °C (Bell, 1996).
Colliery-spoil heap fires produce residual deposits including ash, burnt shale, red shale, and clinker. These have
been observed by the authors to accumulate up to several meters thick. Subsurface cavities present in some
colliery spoil heaps that have undergone combustion could present a localized subsidence problem, either naturally
or by a load exerted onto the pile like heavy-engineering machinery or even a person walking on the pile.
Natural coal fires occur in the geological record. For example, coal fires occurred during the Pliocene in the Power
River basin of Wyoming and western North Dakota (Heffern and Coates, 2004) and in the Pleistocene in northwest
China (Kroonenberg and Zhang, 1997). These fires were dated by uranium
-
thorium/helium ratios found in detrital
clinker and by sedimentary strata relationships (Stracher, 2007a).
Coal fires, especially when they are burning underground, may be difficult to locate and investigate and many
cannot be extinguished. The investigation, monitoring, control, and mitigation of coal fires may be very expensive
and time consuming. Where the cost to manage and control these fires outweighs the economic benefits of coal
mining, this may result in closure of the mine. For example, in China, financial losses of approximately US
$125
250 million were reported and it is expected that US$651 million is required to control the fires (Stracher,
2004a). Stracher (2004) also documents that in the Jharia coalfield in India, almost 1500 million tons of coal are
locked up in about 70 separate coal fires. In addition, a total surface area of around 10 km
2
in India is estimated to
be affected by underground fires (Michalski and Gray, 2001).
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Origin of Spontaneous Combustion
T
here are several factors that may influence the generation of coal fires in mines and colliery spoil heaps (Mohan,
1996). These include the rank of the coal, the surface area of the exposed coal, moisture content of the coal, and
the presence of volatile gases. According to Mohan (1996), the rate of oxidation increases above 40 °C and may be
self-sustaining for bituminous coals at temperatures greater than 70
80 °C and for lignites at temperatures as low as
50 °C. The pyrite content of coal is also a significant factor in promoting combustion. Michalski et al. (1990)
suggest that if the pyrite content exceeds 2%, an exothermic reaction during the oxidation of pyrite may cause the
coal to ignite. It was also noted by Michalski et al. (1990) that the volatile content of the coal is important for
promoting coal fires. Once coal fires have started, their perpetuation is influenced by a constant air supply. Van
Vuuren (1995) calculated that approximately 6000
-
7000 m
3
of air is required for the combustion of 1 ton of coal.
Furthermore, Voracek (1997), noted that an air flow velocity of 0.3
-
0.4m/min, with a minimum oxygen content of
7%, will be sufficient to maintain oxidation and burning of the coal.
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Spontaneous Combustion: Life and Human Health
C
oal fires are destructive because they consume finite coal resources, may destroy floral and floral habitats, and
they negatively impact people who live and work near the fires. Potentially harmful gas components may be
emitted from coal seam and spoil-heap fires. These include acids, aerosols, toxic-particulate matter, carbon
