c) Research between the 1960s and 1970s :
Hargraves (1963a) conducted a series of tests on the
Bulli seam coal at Metropolitan Colliery, NSW. Using a simplified shrinkage
apparatus as shown
in the Figure, he was able to determine the sorption coefficient of expansion
of the Bulli coal to be approximately 1.75 x 10-3cm/cm at 1
MPa. The test was conducted using 25.4 mm (1 in) cubes subjected
to CO2 gas pressures.
Gunther (1968) investigated swelling of different ranks
of coal using both methane and carbon dioxide, and calculated the swelling
coefficients
for different types of coal, ranging from low rank, high volatile coals
to anthracite. He observed greater swelling of coal in CO2 than
in CH4. The reported swelling coefficients ranged from 1.90
x10-8 to 4.76 x 10-8 MPa-1 (2.76 to 6.90 E-6 psi-1).
Czaplinski (1971) examined the relationship between
the kinetics of sorption and the swelling of coal. Tests were conducted
parallel and perpendicular
to beddings. He showed that at low pressures, the sorption of carbon dioxide
was faster than the development of swelling strain, but at higher pressures
(> 4 MPa) the two occur simultaneously.
Czaplinski’s (1971) studies, stated that “the
delay in coal dilation at the initial low pressure levels, causes the gas
to enter the
coal macro-pores, causing a minimum of volume change. Increased swelling
of coal takes place when the gas, at higher pressures, is forced into the
micro-pores.”
Ettinger (1977) carried out extensive studies on the swelling of different
ranks of coal from deposits in Russia, Ukraine and other former Soviet
republics. Table below depicts data on coals from the Donetsk basin and
other regions of the Ukraine. The stresses due to swelling cause the release
of free energy in coal which, according to Ettinger; contributes to the
onset of outburst.
Coal deposit |
Yield of volatile substances vg,
%
|
Swelling of
coal,
%
|
Swelling stress
P0,
MPa
|
Relative free energy of stress state , J/kg |
Komsomlets pit,
Mazurka seam,
(fat coal)
|
30.6 |
0.14 |
21.5 |
0.5* 102 |
Karl Marx pit,
Mazurka seam,
(coking coal)
|
18.2 |
0.17 |
25.5 |
0.88* 102 |
Kondratevka pit, Derezovka seam,
(lean coal)
|
11.4 |
0.24 |
42.3 |
1.5* 102 |
contd.. |
