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Electric conductivity and selfheating ability of hard coal |
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Electric conductivity and selfheating ability of hard coal Author: A.Kotyrba
On heating up the coal, many kinds of physico-chemical processes affecting the electric conductivity of coal may arise. It can be assumed that also the self heating ability of coal is dependent on its electric conductivity. The coal's behaviour differs from that of other rocks and minerals in that the coal temperature related conductivity variation coefficient is positive (an increase in electric conductivity with an increase in temperature) whereas the majority of solid bodies including rocks and minerals show the opposite. The aim of work is dependence between electric conductivity and self heating ability of hard coal. This relation has been tested on the coal samples base for which standard classes of self heating have been determined due to polish standard regulations. The input data base consisted of electrical resistivity time series recorded by geo-electrical multi channel apparatus WAG-Olduring heating in laboratory oven. Those series after transformation of time values to temperature values (fig. 1) were used to determine empirical relations between specific electrical resistivity of coal samples (being an analog of conductivity a) and temperature. The obtained relationships indicate that each of tested coal samples has a slightly different thermal characteristics. However basically all the curve shapes are similar, which could be easily visible if the recorded data were plotted using the double logarithmic scale (fig. 3, 4). After the initial curvilinear drop in resistivity (an increase of conductivity), a slow rise until the initial resistivity level is reached can be observed. Then the resistivity rapidly rises with temperature rises. Overall, the dependence of coal resistivity on temperature can be divided into the two following, temperature related phases: Phase I - A drop in electric resistivity over temperature range from 18 to about 80°C (for all tested samples). Al the samples differ in resistivity gradients. Phase II - An increase in electric resistivity observed for all samples over the temperature range above the threshold values of 60 to 80°C. Only one of the samples does not show any variation in resistivity near the temperature threshold value. The curve slopes vary from phase to phase. In the majority of samples much steeper slopes can be observed along the phase II type curve segment than along the phase I type curve segment. It indicates that the approximative relations may be described by polynomial functions (equation 3). In presented data, 4th order polynomials were used. Correlation coefficients are very high and for analysed data R2 vary from 0.89 to 0.98. In next step the correlation between selected resistivity changes parameter RR (the ratio of resistivity at a heating oven temperature 500°C to initial resistivity at 18°C) and the standard (due to polish regulations) self heating parameters (Sza - coal spontaneous combustion index in temperature 190°C; Sza1 - coal spontaneous combustion index in temperature 237°C; A - coal oxidation activation energy) have been plotted and analysed (fig. 5, 6, 7). The best correlation was obtained using the exponential curve approximation procedure. The correlation coefficients are good for all the studied relationships (R2 0.7). The relationship between the RR parameter and the Sz" combustion index appears to be statistically the best (R2 = 0.81). The coal oxidation activation energy, the energy needed for coal to start the combustion process is proportional to the temperature related electric resistivity gradient. This means that if this energy is low in coal, then the temperature related electric conductivity decline rate will also be comparatively low. The obtained results can create a basis for the new application of geo-electrical methods to the studies on the problem of combating of fire hazards in coal mines. The work has been completed within join project sponsored by The Polish - American Maria Skłodowska-Curie Fund II (Kotyrba and Zakolski, 1996).
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2011