Geology Reference
In-Depth Information
monoxide, carbon dioxide, sulfur dioxide, benzene, toluene, numerous other toxins, and hydrogen sulfide
(Stracher, 2004; Stracher and Taylor, 2004). These gas components may be hazardous to animal and human life
if inhaled in certain concentrations (Anon, 1973). The emissions potentially harmful to health may be accelerated
by disturbing a burning-spoil heap, for example during an attempt to extinguish a fire.
Carbon monoxide is one of the most dangerous gas components, as it cannot be detected by taste, smell, or
irritation and may be present in potentially harmful concentrations. Sulfur-based gas components are more easily
detected and may also cause health problems, if inhaled. In Pennsylvania, United States, over 200 coal fires have
generated gases that have destroyed large tracts of vegetation while high concentrations of sulfur and other
pollutants nucleated on the ground. These contribute to acidic stream and ground water flows (Stracher and Taylor,
2004). Illnesses reported from exposure to coal-fire gas include carbon monoxide poisoning, pulmonary disease,
cancer, strokes, arsenosis, and bronchitis (Stracher and Taylor, 2004; Finkelman et al., 2002; Pone et al., 2007).
Coal Seam and Colliery-Spoil Heap Fires
A
ir flowing into shallow abandoned mine workings, via mine entries (shafts and adits), subsidence induced
ground fissures, and collapsed workings (crown holes), may promote self-ignition (South African example,
below). In addition, combustion may be influenced by ventilation within the mine. In pillar and stall workings,
the sides of support pillars may be fractured and thereby increase the surface area of coal available for oxidation. If
exothermic reactions occur, this may cause the coal to self-ignite. Partially burnt-coal pillars may collapse, leading
to the development of subsidence troughs and fissuring of the ground surface, thereby further accentuating the
problem by permitting additional air to enter the mine workings.
It is often desirable for colliery spoil heaps to become rehabilitated as this type of dereliction can generate urban
blight (Barnsley, England example, below). If, however, these have undergone combustion or are susceptible to
combustion, this may present a series of challenges that have to be overcome. The rehabilitation of colliery spoil
heaps often involves the removal and reinstatement of large volumes of rock and/or soil. Each colliery spoil heap
has a different shape, moisture content, coal/carbonaceous shale content, sulfide (pyrite) mineralogy, porosity,
permeability, method of tipping, and bulk density particulate type and size, all reflecting the geology of the mine
from where it was derived and the method of tipping.
During the placement of the colliery spoil, coarse discard often consists mainly of gravel and cobble-sized
materials, but subsequent breakdown on weathering reduces the particle sizes. The extent to which breakdown
occurs depends on the type of parent material. For example, mudstones, shales, and seatearth (a layer of
sedimentary rock which underlies a coal seam) exhibit rapid disintegration to gravel and some seatearths may
disintegrate within a few cycles of wetting and drying. Once buried within a spoil heap, coarse discard undergoes
little further reduction in size. Hence, older and surface samples of spoil contain a higher proportion of fines than
those at depth.
Spontaneous combustion of colliery spoil may cause long-term smoldering. Fissures in spoil heaps should be
avoided during rehabilitation and workers should wear lifelines if they walk over areas deemed unsafe, since the
ground may suddenly collapse. Subsurface cavities in spoil heaps may be formed by spontaneous combustion, the
roofs of which may be incapable of supporting a person or machine. When steam comes in contact with hot
carbonaceous material, water-gas is formed. If this is mixed with air, over a wide range of concentrations, it
becomes potentially explosive. If a cloud of coal dust is formed near burning spoil when reworking a heap, then
this can ignite and explode. Damping with a spray may prove useful in the latter case.
Control and Preventation of Spontaneous Combustion
and Coal Fires
B
efore the options to control, prevent, or mitigate spontaneous combustion can be determined, the factors that
control coal fires should first be understood. Spontaneous combustion may be monitored and investigated by a site
investigation. This may include, for example, temperature measurement monitoring in boreholes, geophysical
surveys, and the acquisition of airborne thermal imagery (Lamb, 2000) (Nottinghamshire, England example,
below) (Figure 5.1.2).
