Geology Reference
In-Depth Information
This reaction exemplifies the fact that moisture is conducive to spontaneous combustion (DOE, 1993).
According to the DOE (1993), many spontaneous fires start in storage facilities including open-air stockpiles, coal
bunkers, and silos. The DOE attributes combustion to numerous factors. These include improperly loaded and
compacted storage in facilities, which promotes the diminution of coal into highly combustible fines, and storage
for prolonged periods of time, which promotes exothermic oxidation reactions in high-sulfur coals. Spontaneous
fires in storage bins have been extinguished by injecting N
2
or CO
2
into the bins. Water has also been used, but
with extreme caution, because of the risk of steam explosions. The DOE (1993) has published guidelines for
preventing spontaneous combustion in coal storage facilities. These include recommendations for the shape,
height, and composition of stockpiles.
The significance of spontaneous coal fires in storage facilities was conveyed in the testimony of John Dilley to the
US Senate (1912). Dilley, of Southampton, England, was a survivor and fireman in the engine department on the
RMS Titanic that sank about 640 km south of Newfoundland on Monday, April 15, 1912 (US Senate, 1912).
According to Dilley, the bottom of a coal bunker containing hundreds of tons of coal was on fire from the time the
ship departed Southampton for New York City on April 10, until the iceberg that the Titanic hit on the evening of
April 14 tore open this bunker and the incoming water put out the fire.
Although coal fires often start at storage facilities where they can be extinguished relatively easily, larger and more
problematic fires frequently start in the excavations of the abandoned workings or workings of a coal mine
(PDEP, 1965). Fires in abandoned underground workings are the most problematic because they are difficult or
impossible to locate precisely. According to the latest estimate of the US Department of the Interior, more than
$651 million is needed to control these fires in the United States alone (Office of Surface Mining, 1999).
Coal Gas
Carbon dioxide concentrations exceed those of other gases produced by coal fires; however, CO
2
concentrations
are not entirely controlled by the coal itself. The concentrations are also determined by Mitchell (1996, p. 70): (1)
blackdamp (chokedamp) produced by the respiration of miners and working animals (formerly used in the United
States), cellulose-decomposing monerans in wooden supports, and the oxidation of wood, pyrite, or coal; (2) CO
2
associated with coal, gypsum, potash, and salt-bearing strata encountered during mining; and (3) chemical
reactions between acidic water and carbonate rocks, rock dust, or inclusions in strata.
Carbon monoxide, hydrogen, and the hydrocarbons ethylene (C
2
H
4
), propylene (C
3
H
6
), and acetylene (C
2
H
2
) are
monitored as coal-fire detector gases because they are released sequentially as temperature increases during
heating. Consecutive temperatures at which release begins in medium-volatile bituminous coal, for example, are
about 110°C (CO), 170°C (H
2
), 240°C (C
2
H
4
), and 300°C (C
3
H
6
) (Chamberlain, 1971). Combustion occurs
between about 110 and 170°C, and flames appear at about 200°C (Chamberlain and Hall, 1973).
The late D.W. Mitchell (1996, pp. 45
97), a leading authority on mine fires, discussed gas collection
devices, concentration detectors, and the complexities associated with using coal-fire detector and wood-fire
detector gases such as formaldehyde (CH
2
O), formic (CH
2
O
2
) and acetic (C
2
H
4
O
2
) acid, and glyoxal (C
2
H
2
O
2
)
for identifying mine fires.
-
61, 64
-
Other gases associated with coal fires include (1) N
2
, Ar, and trace amounts of other noble gases, all mainly from
air; (2) O
2
mostly from air, occasionally from coal strata; (3) hydrogen sulfide or stinkdamp (H
2
S), oxides of
sulfur (SO
x
) and nitrogen (NO
x
) from oxidation and combustion processes; and (4) the hydrocarbons ethane
(C
2
H
6
), propane (C
3
H
8
), and methane (CH
4
) released in increasing amounts during heating and consumed once
flames erupt (Mitchell, 1996; Alvarez et al., 1997; WCI, 2000; PDEP, 2001a; ITC, 2003). Methane is the most
combustible gas associated with blasting and burning coal, but CO and H
2
also contribute to combustion hazards
(PDEP, 2001a). Methane, a component of virgin coal, accumulates in underground mines, especially in poorly
ventilated shafts. Oxygen supports combustion and the amount necessary for burning is a function of the
proportion of any combination of methane, carbon monoxide, and hydrogen to other gases. Five to fifteen percent
methane in air, for example, is highly combustible (PDEP, 2001a).
Condensation products, associated with burning coal, form as gas exhaled from surficial vents, cracks, coal seams,
and culm banks first cools and then condenses (Christie, 1926; Rost, 1937; Limacher, 1963; Lapham et al., 1980;
