Gas content: direct method
The direct method of estimating the content of gas in
a body of coal consists of drilling a core sample from an area of the coal
seam and enclosing this sample as quickly as possible in an airtight bomb.
It is customary to use the full length of the core sample as variations
can occur depending on the quality of the coal seam. The coal bomb is then
connected to a water displacement apparatus and the quantity of gas released
with time is measured and recorded as Q2. This method is continued
until the gas emitted from the sample is less than 0.05 cm3/gram
per day for five consecutive days (McCulloch et al, 1975) or the gas emitted
is less than 10 cm3/day for seven consecutive days (Diamond & Levine,
1981).
After the gas emission has reduced to the required rates,
the mass of the sample is measured and the sample is placed once again
in a sealed bomb and crushed using steel rods or balls to release any remaining
gas. This gas is measured once again using the water displacement method
and the quantity is measured and recorded as Q3.
To account for the quantity of gas lost between drawing
the sample from the coal seam and placing the sample in the sealed bomb,
the results of gas emitted vs. the square root of time are plotted on a
graph. By making the assumption that gas release follows the square root
law, the release at time zero can be found by extrapolating the other results.
Q1 can then be found by noting the time taken to seal the sample
in the bomb and recording the gas content. The lost time should be taken
as from the start of drilling to the time of sealing in a horizontal drill
hole, or for vertical drill holes lost time is the time taken for the pressure
in the core to equal the hydraulic head.
The total gas content of the sample can be calculated
as
Q = Q1 + Q2 + Q3 |
