Page 252 - Advanced Mine Ventilation

230 Advanced Mine Ventilation

Figure 14.2 Lost gas estimation graph.
A hermetically sealed modified ball mill (Bleuler Mill) [10] is used for this purpose. A
measured quantity of the coal core or drill cutting is put in the mill and crushed. The
released gas is measured by the same setup that was used for desorbed gas measurements.
The total gas content of the coal sample is obtained by adding the three components,
i.e., desorbed gas, lost gas, and residual gas. The coal sample is next weighed and sent
to a laboratory for a proximate analysis which yields the moisture, ash, volatile matter,
and fixed carbon contents of coal. The weight of coal is calculated on a dry, ash-free
basis. The total gas content of the coal sample is divided by the weight of the coal sam-
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ple (dry, ash-free) to get the final gas content of coal in ft /t (1 cm /gm ¼ 32 ft /t).
Table 14.1 shows the gas content and gas composition data for some typical US
coal seams.
14.1.5 Gas Isotherms and Indirect Methods of Gas Content
Determination

At constant temperature, each coal seam shows a measurable relationship between the
total gas adsorbed (or desorbed) and the confining pressure. Fig. 14.3 shows typical
gas isotherms for five US coal seams.