Lukashova Lidiya Sergeevna
Discriminatory Mine Fire Source Detection
The Individual Task
"The computerised subsystem of diagnostics and monitoring
of a condition of the mountain machine
(on an example of coal-mining combine 1KDK500)"
POSSIBILITY
OF LOCATING HEAT SOURCE OF SPONTANEOUS
COMBUSTION OF COAL BY THE ELECTRIC POTENTIAL
In spontaneous combustion of coal a temperature distribution is always created around the heat source. This temperature distribution can be measured, it will be possible to locate the source. Also, it is known that a corresponding distribution of electric potential is produced in a porous medium like an aggregation of broken rocks, if a temperature distribution exists in the medium. Considering the corelationship of the two potentials, the authors performed model experiments to investigate whether the measurement of the electric potential distribution can be used for locating the heat source. The results of the experiments show that a distribution of electric potential is always created corresponding to the conditions of the heat source and that the electric potential reflect the change in the intensity and distribution of the heat source more sensitively than the temperature. It is concluded that there is a possibility that the measurement of potential distribution can be used for locating the heat source of spontaneous combustion of coal.
MECHANISM OF SELF-POTENTIAL
It
is known that there are various self-potentials such as diffusion potential,
streaming potential, redox potential and thermo-electromotive potential.
However, the self-potential developed by spontaneous combustion of coal
has never been studied. In the field of geothermal prospecting, however,
a large number of studies are reported. In the geothermal system, it is
thought that the self-potentials are caused by the thermoelectric effect
and the streaming potential, because there is a heat source of high temperature
and the convection of fluid in the depth of underground. Generally, the
electric current flows with the fluid in a porous medium like an aggregation
of broken rocks. In consequence, a potential difference(streaming potential)
is produced. If a temperature gradient is maintained across a porous medium
like an aggregation of broken rocks, a corresponding voltage gradient
will appear across it. This phenomenon is known as thermoelectric coupling,
and may be caused by differential thermal diffusion of ions in the pore
fluid and of electrons and donor ions in the rock matrix. The ratio of
the voltage to the temperature difference V is called the thermoelectric
coupling coefficient.
Data presented by Nourbehecht for a variety of rock types give thermoelectric
coupling from -0.09 to +1.36 mV/°C, with an average value of about 0.27
mV/°C. Dorfman et al obtained coefficients ranging from about 0.3 to 1.5
mV/°C. In a related experiment by Dorfman et al, a point heat source of
49°C inserted near the center of a limestone block measuring about 2?1?1
m was seen to immediately generate a surface potential field of about
+20 mV amplitude centered over the heat source.
The application of self-potential method has started in 1970's in geothermal
prospecting. Since then self-potentials have been found in many geothermal
areas. In most cases the potential difference is about from 50 mV to 2
V and the spatial distribution is about from 100 m to 10 km.
Basically the mechanism of self-potential development can be applied to
the spontaneous combustion system of coal, because they have a common
characteristic that there is a heat source of high temperature in underground
and it generates a convective motion of fluid.
DonNTU | Master's Portal | Searching System of DonNTU