Geology Reference
In-Depth Information
FEOER
MINE
SHARP MT.
NORTH
1600
1400
1200
1000
SOUTH
Topographic relief
1
= 1000
Vertical exaggeration
2×
A
B
800
0
1000
2000 ft
0
300
Scale
600 m
Figure 16.3.2. Generalized cross section of coal beds in the anthracite region. From Deul and Kim 1988, p. 121.
In a discussion of factors influencing the initiation and propagation of coal fires, it is apparent that all factors
are related to the three elements: fuel, oxygen, and energy. The amount of combustible material, its particle
size, surface area, and tendency to spontaneous combustion are fuel-related factors. The presence of fractures
through which air can be drawn into the fire zone, circulation caused by the fire, and changes in barometric
pressure control the amount of available oxygen. The rate of heat generation versus the rate of heat loss, the
heat-generating reactions (oxidation of coal, oxidation of pyrite, surface adsorption of water vapor, bacterial
activity) and the insulation provided by adjacent strata control the amount of energy within the system.
Natural Barriers
N
atural barriers to subsurface fire propagation basically affect the availability of fuel and the generation and
retention of heat energy. Faults with vertical displacement disrupt the continuity of the coal bed and limit the
amount of fuel. Boundary pillars are regarded as natural barriers to fire propagation because solid unfractured
coal seams are believed to be less likely to burn. However, the surface of the pillar and any fractured or faulted
areas can be combustion zones. The water table serves as a barrier by limiting the amount of oxygen on the coal
and by absorbing energy released by the fire. In the absence of these natural barriers, a subsurface fire can, in an
extended time period, burn from outcrop to outcrop.
The mass of surrounding rock, because of its low heat conductivity, serves as an insulator, but it is not a barrier to
increased combustion. Normal heat transfer occurs by conduction or radiation to the overburden. Overlying roof
coal and carbonaceous shale with carbon contents as low as 25% can conduct or adsorb heat and serve as a
pathway for the spread of the fire. Even if no combustion occurs in the roof, heat transfer by conduction through the
overburden is an extremely slow cooling mechanism. For example, the combustion of 1 ton of medium-volatile
bituminous coal releases 7565 thermochemical calories (30 million BTU). If this energy is simply adsorbed by the
roof rock, it would raise the temperature of 75 tons of rock, ~25.5 m
3
(900 ft
3
), to 500°C. Depending on the rock,
the extent of combustion, and the length of time the fire has been burning, the amount of heat stored in the coal and
adjacent strata can be in excess of a 252 trillion thermochemical calories (approximately a billion BTU). If all
combustion ceases, it would take 10
20 years for this amount of heat to dissipate by conduction through the
overburden. Long-term cooling rates, measured in mines have been found to be as low as 0.1°C per year (Hansen
et al., 1990).
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