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Gas environment
There are several gases associated with coal deposits, with methane (CH4), carbon dioxide (CO2) and CH4/CO2 mixtures prominent, and nitrogen (N2), ethane (C2H6) and helium (He) to a lesser extent. Gas type, gas pressure, and gas composition affect the way the gas stored in coal, as such the volume of gas contained in coal varies with the above and also with temperature, pressure, coal structures and coal composition (mineralogy). In Australia, the gas content of some coal seams can be as much as 50m3/t (average 10-15m3/t) depending on gas type and therefore are classified as gassy seams.
In general the level of carbon dioxide gas contained in coal at any given pressure is higher than methane and other gases. The relationship between pressure and methane gas content is shown in the figure opposite. Drainage of gas from coal seam occurs prior to mining, and is generally carried out by borehole drilling into the coal seam, and above and below coal seam as needed. Gas drainage can be carried out underground or from the surface, pre and post mining as needed. In Australia the critical values of gas content for outbursts, known as the threshold levels (THL), are 9m3/t for methane and 6m3/t for cardon dioxide. The coal gas deposit pressures can be as much as 5MPa, particularly in the Bulli seamof the Southern Coalfields of NSW.
