13
SPONTANEOUS COMBUSTION LIABILITY
OF COALS AND COAL-SHALES IN THE SOUTH
AFRICAN COAL MINES
Spontaneous combustion is one of the major challenges in the
South African coal mining sector. The event involves a range
of complex physical and chemical processes, caused by the
chemical reaction between coal, coal-shale and oxygen.
Coals and coal-shales tend to undergo low-temperature
oxidation
under
favourable
atmospheric
conditions.
The frequent occurrence of self-heating of coal-shales has been
reported in South African coal mines to be the likely cause of
spontaneous combustion. The study has established that
in highwalls and a selected band of coal seams, evidence of
self-heating potentially leading to spontaneous combustion
is found within coal-shales. The study developed a device
to predict the spontaneous combustion liability of some
coal-shales that could not be determined by the existing
Wits-Ehac Index and obtained a liability index referred to as
the Wits-Coaltech (CT) Index. The influence and relationship of
the intrinsic properties (obtained from proximate, ultimate and
petrographic analysis) of coal-shale in relation to coal affecting
spontaneous combustion liability has been established a using
statistical method.
The linear regression analysis indicates that the spontaneous
combustion liability index shows absolute linear relationships
with the intrinsic factors, and, thus, identifies the major
intrinsic factors affecting their liability toward spontaneous
combustion. It was found that a definite positive or negative
correlation coefficient exists between the intrinsic factors and
spontaneous combustion liability.
The study shows no single factor is sufficient to influence the
coal and coal-shale spontaneous combustion liability, but a
cumulative effect of favourable parameters may influence
whether a coal and coal-shale will be liable to spontaneous
combustion or not.
A set of models to predict the spontaneous combustion liability
were derived. The best significant correlation along with the
most appropriate model as indicated by R-squared values,
the coefficient of correlations and standard error was used to
predict the incident of spontaneous combustion.
The distribution of the maceral group into sub-categories has
been revealed to have reasonable influences on spontaneous
combustion liability. The quantity and mineral constituents
in coal and coal-shale samples were evaluated by chemical
procedures and optimised by XRD analysis. The XRD analysis
confirms the presence of mineral constituents in the samples,
as identified by the XRF.
Furthermore, the study has established a comparative analysis
between the dependence of spontaneous combustion liability
index on coals and coal-shales intrinsic properties which
may serve as a point of reference when comparing their
characteristics.
The Coaltech sponsored PhD thesis is almost complete and
was submitted for examination during June, 2018. This study
has produced 11 publications so far and Coaltech’s funding has
been acknowledged in each one of these listed publications
below.
M. Onifade & B. Genc
