Estimation of the propensity of remnant underground coal pillars to spontaneously combust during opencast mining at a colliery in the Witbank coalfield
Date
2017
Authors
Gemmell, Graham Barry
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Abstract
Spontaneous combustion of coal may occur when coal is mined, stored or transported and is influenced by a combination of intrinsic and /or extrinsic factors. While it is unusual for intact seams to burn in the highwall, the most common occurrence is when surface mines extract seams previously partially mined by underground bord and pillar operations.
The aim of the study is to provide a predictive model (matrix) of the spontaneous combustion potential of remnant pillars at Colliery X. A number of different thermal, chemical and petrographic tests (coal factors) will be undertaken to determine their individual and collective impacts on the sponcom predictive model. The primary geology at the mine is conformable with that of the Witbank Coalfield. Battacharyya (1982) described 3 main factors in the spontaneous combustion of coal, mining factor,
coal factor and geological factor which have an aggregate effect.
Some of the main historical and present theories of sponcom are the pyrite theory, the bacterial theory, the oxidation theory and the humidity theory. It is important to note that no single factor is responsible for spontaneous combustion. 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. Fires can start at
outcrops and move through interconnected workings with heat transfer by conduction (into the
overburden) or convection (between panels).The overburden can also insulate the burning coal seam.
Geological factors such as depth of overburden, the degree of fracturing, and the nature of the
overlying strata vary between coalfields.
A coal seam fire or mine fire is the underground smouldering of a coal deposit, often in a coal mine.
Such fires have economic, social and ecological impacts In order to extinguish a fire, one of three
elements, fuel, oxygen, or energy, must be removed. The components of the fire triangle can be
further subdivided into conventional mine control techniques and more or less unconventional or
unproven mine fire control techniques. The thermal techniques discussed include the crossing point
temperature, thermogravimetric analyses and oxygen absorption. Macerals, the microscopically
identifiable organic constituents of coal, are one of the three basic parameters that define coal. The
other two parameters are the coal rank and the mineral matter Vitrinite is the principal maceral group
of the No.5 seam and inertinite dominates the No.2 and No.4 seams.
The results obtained from the 22 drill-core samples and 2 ROM samples were matched to the existing
borehole dataset (2296 boreholes) based on similarity of heat value (figure 3.11). A total of 24 test
results (thermal, chemical and petrographic) from borehole A and borehole B were thus assigned to
the borehole database which has approximately 1500 samples for each seam. By linking the
laboratory datasets (borehole A and B) and the existing borehole database used for resource
modelling, the sponcom variables could be modelled in a similar way to the coal resources. The overall risk matrix was calculated on a full seam basis by combining 15 variable scores, each
variable having a score of 0, 1 or 2 (low-mod-high probability). The overall results from this research
produced clear and unambiguous contour plans of different factors effecting sponcom of coal using
single variable and combined variable datasets. In conclusion, it appears that the acceptability of a
method for determining spontaneous heating characteristics of coal mainly depends upon how closely
it predicts the spontaneous heating behaviour in the field conditions
Description
A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2016
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Citation
Gemmell, Graham Barry (2017) Estimation of the propensity of remnant underground coal pillars to spontaneously combust during opencast mining at a colliery in the Witbank coalfield, University of the Witwatersrand, <https://hdl.handle.net/10539/25542>