COALBED METHANE SORPTION RELATED TO COAL COMPOSITION

Gas sorption by coal is closely related to its physical and chemical p roperties, which are, in turn, governed by coal type and rank. The rol e of coal type (sensu maceral composition) is not fully established bu t it is clear that coal type may affect both adsorption capacity and d esorption rate. Adsorption capacity is closely related to micropore (p ores <2 nm) development, which is rank and maceral dependent. Adsorpti on isotherms indicate that in most cases bright (vitrinite-rich) coals have a greater adsorption capacity than their dull (often inertinite- rich) equivalents. However, no differences, or even the opposing trend , may be observed in relation to coal type. Desorption rate investigat ions have been performed using selected bright and dull coal samples i n a high pressure microbalance. Interpretation of results using unipor e spherical and bidisperse pore models indicate the importance of the pore structure. Bright, vitrinite-rich coals usually have the slowest desorption rates which is associated with their highly microporous str ucture. However, rapid desorption in bright coals may be related to de velopment of extensive, unmineralised fracture systems. Both macro-and micro-pore systems are implicated in the more rapidly desorbing dull coals. Some dull, inertinite-rich coals may rapidly desorb due to a pr edominance of large, open cell lumina. Mineral matter is essentially n onadsorbent to coal gases and acts as a simple diluent. However, miner al-rich coals may be associated with more rapid desorption. Coal rank and type (maceral composition) per se do not appear to be the critical factors in controlling gas sorption, but rather the influence they ex ert over pore structure development. (C) 1998 Elsevier Science B.V.