e) Studies in the 1990s
Stefanska (1990), a Polish researcher, tested some coal samples using
both methane and carbon dioxide under pressures ranging from 0.5 MPa to
5 MPa. It was found that factors such as coal rank and moisture content
affected coal sorption behaviour as well as coal matrix changes.
Harpalani (1989) conducted a series of sorption tests and found a relationship
between gas sorption and changes in permeability. He confirmed that by
desorption of coal gas, the coal matrix shrinks and the permeability increases.
Milewska-Duda, Cegarska-Stefanska and Duda (1994) studied
the swelling of the coal matrix due to methane sorption at pressures from
0.5 MPa up
to 4.5 MPa and at 298 K temperature. They claimed that coal matrix expansion
caused by gas sorption plays an important role in determining mining method
so far as outburst phenomena are concerned. Adjoining figure indicates
that at lower pressures the empirical measurements were in agreement with
the theoretical data, but deviations were observed at the higher pressures.
The compression of cleats and pores was recognized as the reason for this
difference. As can be seen from Figure, at the lower pressures the amount
of expansion was dependent on the direction and expansion of the cleats.
The expansion perpendicular to the bedding was more than in the parallel
direction. At higher pressures the difference decreased
again as a result of the cleats and pores closing.
Seidle and Huitt (1995) measured the shrinkage of the
coal matrix with respect to desorption and the changing permeability of
coal.
They postulated that
coal matrix shrinkage was more correlated to gas content than to gas pressure.
They found that the amount of volumetric change depends on coal rank and
sorbed gas type. It should be mentioned that their samples were placed
in an oven at 48oC, which is significantly higher than the temperature
of 20oC that is commonly used for tests. However they did not
clarify the reasons for the more rapid desorption than adsorption. In
adsorption it takes a longer time for the gas molecules to get adsorbed
in macropores and reach the end of their path in the pore space and micropores,
however, in desorption the molecules that are in the vicinity of the free
surface and in macropores are easily desorbed and leave their places immediately.
Then the inner molecules will be able to quickly flow toward the surface
until equilibrium is established.
contd.. |
