Geology Reference
In-Depth Information
general source area of the fire can be estimated. Such surveys are useful when repeated periodically under similar
weather conditions, the same time of day (typically in the predawn hours) and in the same season. Repeat surveys
will show new as well as retired areas of hot gas venting. TIR surveys most prominently highlight the area of
venting gases and may not capture the total spatial extent of an underground mine fire.
Mine fires in active surface or subsurface workings typically are located quickly and abatement measures implemented
in a timely manner. Underground mine fires in abandoned mines are more difficult to locate, track, and abate. These
mine fires require integrating a host of technologies to effectively locate and assess the scope of the fire problem.
Where little is known about an underground fire other than smoke exiting an entry or vent, the first task is to determine
the potential limits of the fire. These limits can be determined through an understanding of the coal geology, mining
history, pattern of mine workings, and known barriers to the spread of fire both above and below ground. ATIR survey
will quickly locate the limits of venting along an outcrop or from fissures extending into the mine workings.
Mine Atmosphere
O
n a volumetric basis, normal (fresh) air consists of 20.94% oxygen (O
2
), 78.08% nitrogen (N
2
), 0.93% argon
(Ar), 0.03% carbon dioxide (CO
2
), and 0.02% other gases, including neon (Ne), methane (CH
4
), helium (He), and
other trace gases (Weast, 1986, p. F-156). Mine atmosphere compositions may depart from this norm due to a
variety of underground phenomena, including mine fires. The composition of the mine atmosphere varies within a
mine due to a variety of underground conditions, and these variations, in conjunction with temperatures can assist
in differentiating areas that are burning from those that are not. The composition of the mine atmosphere can
change with time and present conditions will not necessarily last indefinitely. Roof falls and other ground
adjustments may redirect air flow so as to dilute or drive off areas of low O
2
content so flaming combustion
cannot occur. Reactions
may chemically alter the constituents of the mine
atmosphere. Like temperature, the composition of the mine atmosphere is transient and must be monitored on a
periodic basis to obtain useful information.
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some of them subtle, others violent
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Oxidation is a chemical process that is widespread in mine workings and involves the combination of oxygen with
another substance to form a compound. The composition and properties of the compound may vary depending
upon the rate and temperature of the oxidation process. Here, oxidation refers to slow, low-temperature processes,
such as the weathering of coal or pyrite or the decay of wood whereas the term combustion refers to the rapid
chemical combination of oxygen with the combustible elements of a fuel wherein appreciable heat is given off,
sometimes with light.
Oxidation of coal can result in the phenomenon of spontaneous combustion where the coal is not exposed to an
external heat source, but environmental conditions allow for the accumulation of heat and the continued
oxidation of the coal. The oxidation process releases CO
2
, CO, and heat. In addition, the oxidation of pyrite
and the adsorption of water on the coal surface are also heat-generating processes and add to the accumulating
heat. As the temperature rises, sustainable self-heating of coal begins when the rate of heat gain increases faster
than the rate of heat loss. Once self-heating begins and the environmental conditions that induce the process are
maintained, the temperature of the coal will continue to rise until the ignition temperature is reached whereby
flaming combustion is initiated. Figure 2.4.1 shows the cross-over point whereby accumulating heat leads to
accelerated oxidation which results in ignition and flaming combustion. As long as the heat of oxidation is
removed faster than it can accumulate or the oxygen content is insufficient to support combustion, then there will
be no fire.
The sustainable self-heating temperature of coal can vary substantially, but mostly depends on the rank of the coal
and conditions that favor the retention of heat. The minimum temperature in which a coal will self-heat is about
35
°
C (95
°
F) for lignite, subbituminous, and high-volatile bituminous coal (Smith and Lazzara, 1987, p. 25), 135
°
C
(275
°
F) for bituminous coal and 140
°
C (284
°
F) for anthracite coal (Kim and Chaiken, 1990, p. 3). Kim and
Chaiken (1993) report that normal ignition temperature of coals they investigated range between 400 and 500
°
C
(753
932
°
F), though some poorer quality coals in Asia have also reported to ignite well below the boiling point of
water (Chakrabarty, 2009, p. 3 and Coalfire.org, 2009). The ignition temperature is influenced by pressure,
velocity, geometry of the burning zone, catalysts, air
-
fuel mixture, ignition source, and moisture content. In
combustion, a portion of the fuel may remain unburned, leaving carbon in the ash; or the carbon may burn
incompletely to form CO as well as CO
2
. The quantity of air theoretically required for complete combustion is
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