Geology Reference
In-Depth Information
In general, the following, often interrelated, properties of coal affect spontaneous combustion (e.g., Güney, 1968;
Kaymakçi and Didari, 2002):
(1) moisture content and volatile matter content,
(2) particle size and available surface area,
(3) mineral matter type and pyrite content in particular,
(4) coal rank, and
(5) petrographic composition (coal type)
Chakravorty and Kolada (1988) and Morris and Atkinson (1988) reviewed the factors which contribute to the
potential for spontaneous combustion of coal. Among the intrinsic factors which cannot be controlled, Chakravorty
and Kolada (1988) noted high volatile matter and high moisture, both indicative of low-rank coals; high pyrite; and
high liptinite as the important coal properties. Chandra and Prasad (1990) found that the higher volatile matter
Indian coals in their study had the highest susceptibility to spontaneous combustion.
Particle size and available surface area are important because adsorption is an important process in coal oxidation.
Higher adsorption, and the monolayer capacity in particular, influences the amount of moisture that can be retained
in the coal and moisture is frequently cited as the main control on spontaneous combustion (Allardice and Evans,
1971). The larger the monolayer capacity, the smaller is the rate of increase of wetting the outer core of the particle
during condensation of moisture. It appears that there is a range of values for monolayer capacity that may lead to
ignition; at higher and lower values, the particle does not ignite.
Fractured and faulted thick coal seams with pyrite and organic shale partings are particularly susceptible to
spontaneous combustion. Falcon (1986) and Misra and Singh (1994) also considered open-structure fusinite to
be a factor owing to the enhanced surface area. Beamish et al. (2001), Beamish and Blazak (2005), Beamish and
Hamilton (2005), and Beamish (2005) correlated R
70
, the average rate of temperature rise from 40°C to 70°C in an
adiabatic oven with O
2
flow of 50 mL/min, with a number of properties for New Zealand and Australian coals.
Beamish and Blazak (2005) and Beamish and Hamilton (2005) found R
70
to decrease with an increase in mineral
matter, with the mineral matter acting as a heat sink, and an increase in the moisture content. Subbituminous coals
have the highest self-heating rate (Beamish et al., 2001; Beamish, 2005). In general, it has been widely recognized
that lower rank coals are more susceptible to spontaneous combustion than higher rank coals because they have
high percent of reactive macerals (vitrinite and liptinite).
Coal type (maceral and microlithotype composition) influences (1) the reactivity between oxygen and coal and (2)
the adsorption behavior of the coal, two processes very important during coal oxidation. Higher reactivity of coal
results in a sharper increase in particle temperature and thus the ignition delay is shorter. If the coal reactivity is
lower than a critical value, the rate of heat generation remains lower than the rate of heat dissipation; the coal
particle fills up with moisture and ignition is not achieved. Therefore, coal rich in reactive macerals (liptinite and
vitrinite) has higher susceptibility to spontaneous combustion.
These and other parameters have been used in modeling of spontaneous combustion (Bhattacharyya, 1971; Arisoy
and Akgün, 1994; Bhat and Agarwal, 1996; Krishnaswamy et al., 1996a, b, c). These studies provide valuable
information about significance of the used parameters for various coals.
Important Terms
alginite
liptite
ankerite
lithotypes
bimaceral microlithotypes
macerals
bituminite
macrinite
bituminous
meta-anthracite
colcite
micrinite
carbominerites
microlithotypes
carbonates
mineral matter
cellulose
monomaceral microlithotypes
clarite
montmorillonite
clarodurite
oxides
