Self-heating Potential of Coal Inferred from Elemental Data – A Case Study of the Witbank Coalfield of South Africa

Initiation of self-heating
is governed by a coal’s ability
to oxidize at low temperatures. Aliphatics undergo low-temperature
oxidation and are the main constituents of vitrinite. Conversely,
the chief constituents of inertinite, aromatics, oxidize at elevated
temperatures. In this study, published elemental data for the Witbank
Coalfield are used to calculate the H/C ratio and aromatic fraction
(fa). The parameters are correlated with
petrographic composition. An increased proportion of aliphatics in
vitrinite suggests that it has higher heat generating capacity (HG)
at low temperatures than inertinite of similar rank. When the heat
retention capacity (HR) exceeds HG, self-heating initiates, resulting
in oxidation of aromatics. Sustained self-heating, initiated by oxidation
of vitrinite macerals, may trigger ignition. Spontaneous combustion
in vitrinite-poor coals is indicative of a coal body with high HR.
For Witbank coals, the likelihood of self-heating is classed as high-
and low-risk corresponding to vitrinite- and inertinite-rich coals,
respectively. By implication, spontaneous combustion tests quantify
HG.