Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals

The effect of coal composition, particularly the organic fraction, upon gas sorption has been investigated for Bowen Basin and Sydney Basin, Australia coals. Maceral composition influences on gas retention and release were in vestigated using isorank pairs of hand-picked bright and dull coal in the r ank range of high volatile bituminous (0.78% R-o max) to anthracite (3.01% R-o max). Adsorption isotherm results of dry coals indicated that Langmuir volume (V-L) for bright and dull coal types followed discrete, second-order polynomial trends with increasing rank. Bright coals had a minimum V-L at 1.72% R-o max and dull coals had a minimum V-L at 1.17% R-o max. At low ran k, V-L was greater in bright coal by about 10 cm(3)/g, but as rank increase d, the bright and dull trends converged and crossed at 1.65% R-o max. At ra nks higher than 1.65% R-o max both bright and dull coals followed similar t rends. These competing trends mean that the importance of maceral compositi on on V-L varies according to rank. In high volatile bituminous coals, incr eases in vitrinite content are associated with increases in adsorption capa city. At ranks higher than medium to low volatile bituminous, changes in ma ceral composition may exert relatively little influence on adsorption capac ity. The Langmuir pressure (P-L) showed a strong relationship of decreasing P-L with increasing rank, which was not related to coal type. It is sugges ted that the observed trend is related to a decrease in the heterogeneity o f the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed samples show that dull coals desorb more rapidly than bright coals and that desorption rate i s also a function of rank. Coals of lower rank have higher effective diffus ivities. Mineral matter was found to have no influence on desorption rate o f these finely crushed samples. The evolution of the coal pore structure wi th changing rank is implicated in diffusion rate differences. (C) 1999 Else vier Science B.V. All rights reserved.