Geology Reference
In-Depth Information
Durain
Bright clarain
Kaolinite-rich tonstein
Illitic clay
0
15
cm
Figure 3.1.5. In-mine profile of portion of Fire Clay coal bed, eastern Kentucky, with bright clarain and durain
above illitic clay and tonstein partings.
refractory organic material in oxygen and carbon. Humic acids are formed during this process. Burial of the
plant material or degraded plant material below the groundwater table effectively stops oxidation or attack from
aerobic organisms. Anaerobic bacteria may continue to decompose the plant matter until it reaches a depth or
conditions unsuitable for their viability. Biochemical degradation ends with the polymerization of humic acids
at the rank of subbituminous coal.
Physicochemical coalification that follows biochemical degradation is caused by the amount of heating of the coal
and, to a lesser extent, the pressure conditions during the coal
'
s history. Temperature increase has been traditionally
thought of as the product of greater depth of burial (and the prevailing geothermal gradient) (Hilt, 1873), but recent
studies have led to the recognition that the flow of hydrothermal fluids through coal seams can account for the
heating (Hower and Gayer, 2002; Harrison et al., 2004). Physicochemical coalification causes major changes in the
coal properties, including changes in the thermal and mechanical properties (details in Taylor et al., 1998).
Coal Petrology and Spontaneous Combustion
A
mong many factors that trigger spontaneous combustion, oxidation of coal at ambient temperature is the major
one (Banerjee et al., 1990; Goodarzi and Gentzis, 1990). The oxidation of coal is an exothermic reaction, the exact
mechanisms of which are not fully understood. At very low temperatures, reaction between coal and oxygen is
physical (adsorption), and it changes into chemisorptions starting at ambient temperature (Postrzednik et al., 1988).
The rate of oxygen consumption is very high during the first few days, following exposure of a fresh coal to the
atmosphere. Afterwards, it decreases slowly unless generated heat is allowed to accumulate in the environment.
