Geology Reference
In-Depth Information
combustion, spontaneous combustion in higher rank coals can be supposed from the number of fires in which no
other cause is suggested (Kim and Chaiken, 1993).
Natural barriers to subsurface fire propagation include faults where vertical displacement disrupts the continuity of
the coal bed. Boundary pillars are considered natural barriers to fire propagation because solid coal seams do not
burn readily. The H 2 O table serves as a barrier by limiting the amount of oxygen 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 (Kim and Chaiken, 1993).
Hazards
The primary hazards of coal fires are toxic fumes and subsidence. These fires also can affect the conservation of coal
resources, ignite surface fires, and affect the value of adjacent property. Fumes from a fire zone frequently contain coal
distillates, mercaptans carbonyl sulfides, which create noxious and unpleasant odors in addition to CO 2 and CO.
Subsidence occurs when a fire consumes a portion of the coal, removing support from the overlying strata. The surface
expression of the subsurface fire may be a small vent, a fracture line, a slight depression or a relatively large sinkhole.
At least 60 deaths have been attributed to accidents at burning-coal refuse piles (McNay, 1971).
The potentially most serious problem associated with coal fires is the migration of toxic fumes from the fire
through overlying strata into homes or other enclosed surface structures. A fire produces CO, CO 2 ,H 2 O, and
consumes oxygen. CO is the most serious hazard. This colorless, odorless gas readily combines with the
hemoglobin of the blood, which normally transports oxygen; it replaces oxygen and forms carboxyhemoglobin.
The effect of CO exposure increases with duration of exposure, higher humidity, and lower barometric pressure.
The rate of effect also increases with increased physical exertion. Other factors in individual response to CO
exposure are age (very old and very young), pregnancy, heart disease, poor circulation, anemia, asthma, lung
impairment, or the presence of drugs/alcohol in the blood (Plunkett, 1976).
The hazards of mine fires are insidious. They are not like hurricanes, tornados, earthquakes, or floods, in which a
single catastrophic event affects a large number of people. Fires in abandoned mines and waste banks are protracted
events; they can have a moderate effect on people for 20 years or more. The most widespread effect is the
environmental degradation caused by noxious odors and fumes. A more serious, but less prevalent, effect is
subsidence and/or fume migration into surface structures (Kim and Chaiken, 1993).
Control of Coal Fires
I n order to to extinguish a fire, one of the three elements, fuel, oxygen, or energy, must be removed. Fuel is
removed when it is consumed or when it is physically separated from the burning mass. Oxygen removal depends
on either the introduction of an inert atmosphere or on the isolation of the fire zone from sources of fresh air. Heat
removal can be accomplished by moving a heat-absorbing agent (usually an inert gas or H 2 O) through the mine. To
prevent reignition of the fire, all coal and heated rock must be cooled below the reignition temperature. Even very
small isolated areas where the coal is oxidizing at a high rate can serve as reignition points if the control measure
fails and oxygen becomes available. It is generally assumed that if the temperature is below 100 o C, the chance of
reignition is small (Kim and Chaiken, 1993).
Conventional Methods of Extinguishing Subsurface Fires
Excavation (loading out, daylighting, dig and quench, stripping) is a fuel-removal method that is the most
successful of the AML fire-control techniques (Chaiken, 1984). It involves physically removing the burning
material and cooling it to extinguish the fire. The hot material is cooled either by spraying it with H 2 Oorby
spreading it out on the ground and allowing it to cool in air. H 2 O is preferred as the heat removal medium. It is also
used to protect equipment from high temperatures and to suppress dust. If properly applied, excavation is the surest
method of extinguishing wasted-coal fires. However, exposure of smoldering coal to an unlimited supply of
oxygen can produce a sudden increase in fire activity (Figure 1.2.4). Excavation also involves exposure of men and
equipment to a hazardous environment (Figure 1.2.5) The recurrence of excavated fires is usually due the failure to
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