Characterisation of the parameters influencing the storage and drainage of gas in coal

Adsorption and desorption characteristics of coal samples of the Bulli seam were examined with respect to coal particle size, gas type, and temperature. Adsorption and desorption tests were conducted by an indirect gravimetric method to determine the gas content in coal. Various coal particle sizes, 0.21 mm, 0.30 mm, 1.18 mm, 2.36 mm, 4.75 mm, 6.70 mm, 8.00 mm and 15 mm cubical blocks were examined in addition to the 54 mm diameter coal core. The coal samples were subjected to gas pressure increase, in steps of 500 kPa, up to 4 000 kPa. The coal samples were tested with N₂, CO₂ and CH₄ gas, as single and as mixed gases, at temperatures of 21, 24 and 28 °C. The reported results suggest that the adsorbed mass in coal depends strongly on gas type, sorption time, temperature, and particle size. Saturation time appears to be dependent on gas type but independent of particle size. The minimum saturation time with N₂ and CO₂ gas was found to be approximately two day (2 880 min) while with CH₄ gas it took one day (1 440 min) longer to reach an equilibrium pressure. In addition, there appears to be a hysteresis between adsorption and desorption curves in all the coals tested and with all three gases (N₂, CO₂ and CH₄) investigated. The residual adsorbed mass in coal at atmospheric pressure was found to be about 20-50%, of the maximum gas content at 4 000 kPa, depending on gas type. Experiments were also conducted to examine the effect of displacing the adsorbed CO₂ and CH₄ gases and their binary mixture in coal by the injection of N₂ gas. To study the feasibility of removing the initially adsorbed gas from coal with another gas, experiments were conducted using an in-house built multi-function outburst research rig. The results revealed that CO₂ gas desorption increased by almost 30% as a result of N₂ gas injection, which was about double that obtained without N₂ gas injection. This finding has a significant bearing in solving the drainage difficulties experienced at rich CO₂ gas concentration zones such as at West Cliff Colliery, area 5, where this study was focused.