Geology Reference
In-Depth Information
Shallow, abandoned room and pillar mine workings
Trees
Figure 5.1.2. Airborne, pre-dawn, thermal image at high spatial resolution, showing near-surface room and pillar
mining in Southern Africa. The mine is possibly affected by underground coal fires. After Lamb 2000; photo by
Anglo American PLC 2000.
Each coal fire will have its own unique characteristics and as far the authors are aware, there seems to be no approved
general methodology for investigating and controlling all types of coal fires. However, a number of techniques are
recognized. These involve depriving the fire of oxygen, building barriers to isolate the burn front, saturating the coal
with water, or dosing the coal with water and grout-slurry mixes. These techniques require significant financial
resources which unfortunately are not always available in many countries where these fires occur.
Underground Coal Fires
Gases that are not reactive were used in the nineteenth century to extinguish some underground coal-seam fires. For
example, at the Clackmannan Mine in Scotland, a mixture of CO
2
,N
2
,SO
2
, and steam was pumped into a mine for
about one month to extinguish a fire (Adamus, 2002; Morris, 1987). In the twentieth century, nitrogen was used to
extinguish fires underground (and to control spontaneous combustion in spoil piles) and to protect rescue workers.
Adamus (2002) provides an example of the use of nitrogen at the Doubrava Mine, in the Czech Republic. At this
mine, a fire was initially exacerbated by a methane and coal dust explosion. It was subsequently controlled by the
sealing of both the downcast and upcast shafts with an airtight plug of sand and clay. For 35 days after this,
approximately 17000m
3
of nitrogen was pumped into the mine to extinguish the fire.
Burning-coal seams may be extinguished by the injection of grout, pulverized fuel ash, or foam into boreholes
which are drilled to intercept the workings (Bell, 1996). The boreholes need to be drilled on a closely spaced grid
around the hot spots. Grout injection begins at the lowest stratigraphic horizon where burning occurs. This process
then proceeds toward the higher levels of the mine workings or area of burning. Before the grouting takes place,
any abandoned mine shafts, ground fissures, boreholes, or areas of ground collapse (crown holes) would need to be
sealed to prevent the flow of air into the mine workings (Bullock and Bell, 1997).
According to Michalski and Gray (2001), in the Jharia coalfield in India, about 30 million tons of fuel ash is
produced each year from thermal power stations. They suggested this may be injected into underground mines or
dumped into opencast mines for its disposal and to contribute to the control or prevention of coal-mine fires.
Coal fires in shallow abandoned mine workings may be extinguished by flooding (Bullock and Bell, 1997). This
requires impoundment with dams or fast pumping rates to prevent re-ignition. It is also necessary to monitor water
levels to reduce the chance of re-ignition during a decline in the levels. The force of water flowing through a mine
may affect ground water flow and thus contaminate aquifers, streams, and lakes. The stability of mine pillars, and
therefore subsidence, may also be influenced by large volumes of water being pumped into mine workings.
Controlled explosions have been used with variable success in China to extinguish coal-mine fires (Bell and
Donnelly, 2006). However, this method has to be used carefully because rock fissures and fragmentation may
enable the storage or passage of air, promoting combustion (Figure 5.1.3).
Where coal seams are burning at shallow depth, the fire may be contained by excavating a trench into the seam
ahead of the fire and then backfilling (South Africa example, below). However, this depends on the ability to
accurately monitor and forecast the direction of burning and the depth of the coal seam.
