Underground » Ventilation, Gas Drainage and Monitoring
Coals at depths greater than 200 to 400m usually contain significant quantities of gas, which is predominantly methane (CH4 ). However, many coals contain large amounts of carbon dioxide (CO2 ) and mixtures of CH4 and CO2. Safe underground coal mining requires management of these gases, so it is important to understand gas storage, release and transport mechanisms within the coal.
A large body of work exists in relation to CH4 but problems relating to CO2 and mixtures of these two gases have not been studied intensively. Particular problems relating to gas mixtures are the relative release rates of the two gases, the mechanisms of mixed gas adsorption by the coal and the mechanisms of mixed gas release.
Coals from mixed gas mines at Dartbrook and South Bulli have been studied with the aim of understanding problems particular to mixed gases. Changes in gas composition during desorption were of particular interest.
Large petrographic and geochemical variations were found in the coals sampled, which indicated that coal sampling was a significant problem. However, type (maceral) trends found from other studies could be confirmed. Increases in vitrinite content are associated with increases in adsorption capacity for both methane and carbon dioxide.
Firm data during the earlier stages of bomb desorption was not obtained. However, some results suggested that early desorbed gas at Dartbrook is slightly depleted in methane, by about 2%, while that at South Bulli is slightly enriched in methane by about 2%. Residual gas compositions showed a consistent pattern of enrichment in CO2 at both South Bulli and Dartbrook. Variations in residual gas content are controlled by methane adsorption properties, despite its enrichment in CO2 .
Gas composition and relative flow rates from in-seam drainage holes were compared to bomb desorption studies in terms of desorption rates and gas composition variations with time. Trends in the in-seam drainage holes could not be fully explained by reference to the bomb studies due to the unrepresentative nature of the bomb sample compared to the entire length of the hole.
Adsorption isotherms of the pure gases gave expected results with adsorption of CO2 being 2 to 3 time greater than that of CH4. Adsorbed CH4 contents at Dartbrook were lower than at South Bulli, which is related to coal rank. CO2 adsorption isotherms were similar for both mines.
Mixed gas adsorption and desorption isotherms were conducted using a mixture of abut 50:50 CH4:CO2 . Results obtained were unexpected and shed new insights into mixed gas sorption processes by coals. During mixed gas adsorption, the CH4 component of the isotherm approximated the pure CH4 isotherm while CO2 adsorption was unrelated to its pure gas isotherm. During desorption, neither component of the mixed gas followed a trend related to the pure gas isotherm. These results cannot be explained using accepted models of layer adsorption but require pore filling models.
During adsorption, CH4 gains access to many of the adsorption sites before CO2 and prevents large quantities of CO2 from adsorbing. The more rapid access of CH4 is related to its smaller molecular mass, which allows more rapid diffusion into the pore spaces. Under geological conditions, methane would normally be present before introduction of CO2 into the system. This initial CH4 may prevent the coal from becoming fully saturated with respect to CO2 .
During desorption, CH4 is released more readily from its adsorption site and is replaced by CO2. The larger adsorption coefficient of CO2 allows it to displace the CH4 when the opportunity arises. Carbon dioxide is thereby preferentially retained by the coal during the desorption.