Geology Reference
In-Depth Information
However, coal is not composed of elemental carbon. On a dry, mineral matter free basis, coal contains between 60
and 90% carbon. The rest of the coal
is composed of hydrogen, oxygen, nitrogen, and sulfur. For
example, the stoichiometric combustion of coal can be written as (Chaiken, 1977):
“
molecule
”
CH
1
:
18
N
0
:
15
O
0
:
35
S
0
:
005
þ
1
:
12O
2
þ
4
:
15N
2
→
CO
2
þ
0
:
58H
2
O
þ
0
:
005SO
2
þ
4
:
157N
2
þ
HEAT
:
This reaction produces 138.4 kcal/mol. Combustion reactions are exothermic. Depending on the rank of the coal,
combustion produces from 5 to 10 kcal/g of coal or between 6 and 16 000 Btu/lb.
The oxidation of coal occurs constantly. The temperature of the coal is a function of the rate of heat generation
versus the rate of heat loss. Since the rate of heat generation is an exponential function of temperature and the rate
of heat loss is a linear function of temperature, as the temperature increases, the reaction rate increases faster than
the heat loss (Kanury, 1975). Ignition is a function of the amount of energy released by a reaction and the rate at
which it is released, as well as the rate at which energy is transferred from the reacting mass to the surroundings.
The reaction rate is a function of the concentration of reactants, carbon and oxygen, the surface area, particle size,
temperature, and activation energy.
Sources of Ignition
There are two types of ignition: forced and spontaneous. Forced ignition sources include lightning, brush and forest
fires, improperly controlled man-made fires and spontaneous combustion in adjacent materials like trash. Sponta-
neous combustion in coal or coal refuse is related to the oxidation of the coal to form CO
2
, CO, and H
2
O (Kim,
1977). Spontaneous combustion may be the initial cause of a fire which is then spread by conduction or convection
to other areas of a mine (Banerjee, 1985; CMRS, 1991).
The oxidation of pyrite and the adsorption of H
2
O on the coal surface also are exothermic reactions that increase
the probability of spontaneous combustion. Thermophilic bacteria may also contribute to raising the temperature of
the coal (Chaiken et al., 1983).
Factors Favoring Propagation of Coal Fires
An abandoned mine or waste bank is a physical environment that favors the accumulation of heat. In bituminous-
coalfields, the depth of overburden, the degree of fracturing, and the nature of the overlying strata are the primary
geologic factors (Dalverny and Chaiken, 1991).
In underground mines that used a room-and-pillar mining system, a relatively large proportion (30
50%) of
the coal is left in place. The roof coals and carbonaceous shales are also left in the mine. The tonnage of
combustible material remaining in the mine may exceed that extracted during mining. Older mines had several
entries at the outcrop for drainage, ventilation, and access. Fires usually started at the outcrop and propagated
along the outcrop or through interconnected workings. Heat could move by convection through the mine or by
conduction into the overburden. The overburden served as an insulator, preventing the transfer of heat away
from the combustible material. As the overburden became warmer or as the coal pillars failed, the overburden
subsided, creating a system of cracks and fractures through which smoke and fumes left the mine and fresh air
entered the mine (Figure 1.2.2). Under these conditions, most abandoned mine fires exhibit smoldering
combustion, involving relatively small amounts of coal at any given time, with little visible flame. They can
continue to burn in an atmosphere with as little as 2% oxygen (Scott, 1944). Such fires can burn for extended
periods of time (10
-
-
80 years) and are difficult to extinguish (Dalverny and Chaiken, 1991; Kim et al., 1992;
Leitch, 1940).
In abandoned surface mines, the coal outcrop may be left exposed when stripping operations are terminated, or
coal refuse may be left in contact with the outcrop. In either case, fires are not unusual. If a stripping operation
involved the barrier pillar of an abandoned mine, it is possible for a fire to propagate into the mine (Kim and
Chaiken, 1993).
