Gas content: gravimetric method
The gravimetric adsorption /desorption method is determining
the maximum amount of gas the coal can retain at various predetermined
confining gas pressures and to assess the desorption
characteristics of various gas types in coal. The apparatus, described
by Lama and Bartosiewicz (1982),a d later by Aziz and Li Ming (1999), consisted
of a number of cylindrical pressure vessels, known as pressure 'bombs'.
Coal samples are sealed in 'bombs' and pressurized to saturation level
at various determined pressures up to 5 MPa.
To maintain a constant environment for the test, the
bombs are immersed in a water bath, but are isolated from the water bath
by copper sleeves
to keep them dry. A thermostatically controlled water bath (with a stirrer)
allows the coal samples to be kept at the desired temperature. The lid
of each bomb has generally two types of valves, an isolation and a quick
release valve, and are connected to a gas supply cylinder via a manifold
and pressure regulator.
To evacuate the gas, a vacuum
pump, connected to the manifold, applies a suction (sub atmospheric pressure)
to the line, expelling any
residual gases or air from the system and air. With this approach, it is
possible to bring the pressure to near zero absolute pressure. Pressure
release valves enable the release of pressure and regulates the pressure
in each bomb. The whole
system capacity is designed to measure the adsorption rate up to 5 MPa
pressure (absolute) and a temperature up to 40oC .The bomb lid
was attached to the body by six bolts with the bomb being sealed perfectly
using an '0' ring in the top of the bomb.
A
precision balance should be used to weigh the bombs for determining accurately
the level of gases charged in each bomb at the given pressure level. Usually
a precision balance with resolution down to 0.00001 gm. Following the
completion of the bomb calibration, the coal samples are then placed
in the bombs
and sealed. The bombs are then weighed and connected to the gas cylinder
via the manifold. The pressure in the line is monitored using an on line
pressure indicator. Each bomb is pressurized until equilibrium at the required
pressure is reached. The gas pressure in each bomb is maintained constant
until saturation is achieved. At saturation stage, the bombs are re-weighed
to determine the total weight of the bomb, coal and gas. This process is
repeated for each bomb at each predetermined pressure level. Normally,
gas saturation readings are recorded at each of 0.2, 0.5, 1, 2, 3 and 4
MPa pressures respectively.
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