d) Research in the 1980’s
Reucroft and Patel (1983) measured the surface area and porosity of coal
with respect to sorption with different gases and vapours, in a similar
way to the tests conducted by Mahajan (1982), which measured the porosity
as a function of volumetric change. The following equation was used for
measuring the volumetric changes of coal:
 Density of the sorbate
in the liquid state, g/cm3
 Density of coal
in the liquid sorbate, g/cm3
V2 = Volume fraction
of coal in the swollen coal sample at equilibrium
qw = Mass of coal at
equilibrium/initial mass.
Q= Swelling parameter (swollen volume/ unswollen volume)
Reucroft and Patel (1983) found that the apparent surface area was higher
with CO2 in comparison with other gases such as Nitrogen. Using
the weighting method, they estimated the volumetric changes of coal, but
without mentioning in which direction it happened and by how much.
Later, Reucroft and Patel (1986) investigated the swelling
of coal, which was induced by gas, in order to obtain a better understanding
of the internal
structure and surface area of coal. They observed the swelling of various
coal samples exposed to different gases such as He, N2 and CO2.
The coal samples 1 cm long and 0.4 cm in diameter showed an increase in
their length between 0.36% and 1.31% when exposed to carbon dioxide. Insignificant
changes were observed when the coal was tested in both nitrogen and helium
under the same conditions. Further experiments were conducted under a low
pressure of 0.14 MPa, which is rather low compared with the existing in-situ
coal gas pressures.
Gray (1987) examined the relationship between shrinkage, fluid pressure
/effective stress and coal permeability. Gray stated, “shrinkage
of the coal occurs on desorption, leading to an effective stress reduction.
This opposes the effective stress increase that would normally be expected
with a lowering of fluid pressure. Because permeability is a function of
the effective stress, it may increase or decrease with stress changes associated
with drainage”. This implied that shrinkage of the coal matrix associated
with desorption opens up the cleat and results in an increase in the permeability
of the coal.
Sethuraman (1987) studied the effect of gas pressure on the changing dimensions
of coal. He found that there was a linear correlation between swelling
and pressure. For methane pressures up to 1.5 MPa, there was a reported
volume increase between 0.75% and 4.18%. He also found that the lower the
carbon content, the higher the swelling of coal.
contd.. |
