Abstract:
Gas emission and outburst phenomena in underground coal mines continue to be hazards. The aim of this study has been to investigate the gas/coal system, in particular the relationships between coal type, pore nature and gas sorption of a range of coal ranks. Fundamental associations between these parameters and gas emission and outburst phenomena are presented, using examples from operating mines in the Bowen Basin, Queensland, Australia and Huntly Coalfield, New Zealand. Petrographic analysis of coal from the Bowen Basin shows bright coal to be Vitriniterich and dull coal to be inertinite-rich. These fundamental differences in maceral composition have significant effects on the chemical and physical properties of the coal. In particular, vitrinite-rich coal has a higher volatile matter content than isorank dull coal. Physical measurements of density, porosity and surface area show vitriniterich coal is predominantly microporous and inertinite-rich coal is predominantly macroporous. Dull coal rich in inertodetrinite has the highest mesoporosity. The effect of these differences in the pore nature of coal is apparent in the gas sorption behaviour of the coal. Bright coal has a higher sorption capacity than dull isorank coal, which relates to a higher degree of microporosity and larger internal surface area. Gas desorption from bright coal is extremely slow and is described by a unipore diffusion model. In contrast, gas desorption from dull coal is described by a bidisperse pore diffusion model. Initial desorption from dull coal is very rapid, with over 50% of the stored gas released in minutes from coal lumps 8 mm in diameter, followed by a much slower phase of desorption. An exception to this is dull coal rich in inertodetrinite, which desorbs gas in a similar manner to bright coal. Gas desorption from coal in the working face is complex. Different desorption rates can be expected related to coal type (bright, dull or banded), which can result in significant gas content gradients ahead of the face, sufficient to initiate an outburst. Practical application of this has been found at No.2 Mine, Collinsville, Australia, where bright (vitrinite-rich) lower portions of the working seam have retained higher gas contents than the duller (inertinite-rich) upper parts; the lower parts of the seam have a greater tendency for outbursting. Similarly, at Leichhardt Colliery, Blackwater, Australia, lenses of inertodetrinite-rich coal surrounded by vitrinite-rich coal have combined to render the working seam extremely outburst-prone. Gas contents of Subbituminous coal from Huntly Coalfield, New Zealand, are low, due to the competing effects of moisture at sorption sites. Conversely, gas pressures are relatively high, due to the shape of the adsorption isotherm for this coal. Gas desorption is very rapid, related to the high degree of macroporosity inherent to the coal. Major gas emissions, therefore, appear as "blowers" rather than outbursts in this situation. Outburst-proneness is increased by the presence of coal with Iow strength, and a high gas retention capacity resulting from slow desorption in-situ. The problem is exacerbated further when these factors are coupled with high face advance rates, that increase the gas content gradient in the face area. High rank Bowen Basin coals rich in vitrinite or inertodetrinite have both Iow strength and slow desorption rates. Consequently, these coals are frequently associated with outburst-prone conditions.