a) Studies prior to 1950
In the early 1930’s Briggs and Sinha (1933) tested
the swelling and shrinkage of different varieties of UK (Scottish) coals
with both firedamp
and carbon dioxide, over various pressures ranging from atmospheric to
2.07 MPa (300lbs). Using the specially constructed apparatus shown in opposite
Figure.The change in sample size was monitored only along its axis, which
was cut parallel with the samples’ bedding. The axial elongation
of the coal samples ranged from 0.06% to 0.3% in firedamp and 5% in carbon
dioxide gas. Once the gas pressure was removed most samples returned to
near original size, however there was an elongation of 0.14% for anthracite.
The elongation or shrinkage of the coal was measured by a micrometer, with
resolution down to+/- 0.0021 mm. The researchers did not address the radial
shrinking or swelling of coal samples in CH4, but recognised
that coal can expand in all directions when absorbing gas, reacting less
strongly with methane than with carbon dioxide. Coal also absorbs moisture
with greater eagerness than methane, and if coal charged with gas is placed
in water, much of the gas will be expelled and replaced with water. Others
with interest in the field in the pre 1950s period include Meehan (1927),
Kvalnes and Gaddy (1931), Audibert (1935) and Coppens (1937).
b) Research studies between 1950 and 1960:
During the 1950s Moffat and Weale (1955) attempted to
define a correlation between the sorption mechanism and the isotherm diagram.
They interpreted
the sorption mechanism by measuring the coal matrix expansion caused by
methane sorption under pressure. Tests were conducted on coal samples parallel
and perpendicular to the bedding planes, and were combined to determine
the bulk expansion of coal. A constant volume sorption apparatus(opposite
figure) was used for the test. Electrical strain gauges, attached to the
surface of suitably cut blocks of coal, with connections from the pressure
vessels to a Wheatstone bridge
were used to monitor coal volume change. The tests were made in methane
gas at different pressures ranging between 0 and 70 MPa. Tests were made
on different coal types ranging from low rank coal to anthracite. Tests
conducted perpendicular to the bedding plane attained a linear expansion
ranging from 0.2% to 1.6%, at 15.0 to 20 MPa (150 to 200 atm). Higher
rank coal expansion was generally less than for low rank coals. Less
expansion was reported on coals tested parallel to the bedding plane.
Similar observations were also reported by Audibert (1942) and confirmed
by De braaf,
Itz and Mass (1952).
contd.. |
