Physical properties of selected block Argonne Premium bituminous coal related to CO2, CH4, and N2 adsorption

CO2, CH4, and N-2 adsorption and gas-induced swelling were quantified for block Blind Canyon, Pittsburgh #8 and Pocahontas Argonne Premium coals that were dried and structurally relaxed at 75 degrees C in vacuum. Strain measurements were made perpendicular and parallel to the bedding plane on similar to 7x7x7 mm(3) coal blocks and gravimetric sorption measurements were obtained simultaneously on companion coal blocks exposed to the same gaseous environment The adsorption amount and strain were determined after equilibration at P <= 1.8 MPa. There is a strong non-linear correlation between strain and the quantity of gas adsorbed and the results for all gases and coals studied follow a common pattern. The dependence of the coal matrix shrinkage/swelling coefficient (C-gc) on the type and quantity of gas adsorbed is seen by plotting the ratio between the strain and the adsorbate concentration against the adsorbate concentration. In general, C-gc increases with increasing adsorbate concentration over the range of similar to 0.1 to 1.4 mmol/g. Results from the dried block coals are compared to CO2 experiments using native coals with an inherent level of moisture as received. The amount Of CO2 adsorbed using native coals (assuming no displacement of H2O by CO2) is significantly less than the dried coals. The gas-induced strain (S) and adsorption amount (M) were measured as a function of time following step changes in CO2, CH4, and N-2 pressure from vacuum to 1.8 MPa. An empirical diffusion equation was applied to the kinetic data to obtain the exponent (n) for time dependence for each experiment. The data for all coals were pooled and the exponent (n) evaluated using an ANOVA statistical analysis method. Values for (n) near 0.5 were found to be independent on the coal, the gas or type of measurement (e.g., parallel strain, perpendicular strain, and gas uptake). These data support the use of a Fickian diffusion model framework for kinetic analysis. The kinetic constant k was determined using a unipore diffusion model for each experiment and the data were pooled for ANOVA analysis. For dry coal, statistically significant differences for k were found for the gases (CO2>N-2>CH4) and coals (Pocahontas>Blind Canyon>Pittsburgh #8) but not for the method of the kinetic measurement (e.g., strain or gas uptake). For Blind Canyon and Pittsburgh #8 coal, the rate Of CO2 adsorption and gas-induced strain for dry coal was significantly greater than that of the corresponding native coal. For Pocahontas coal the rates of CO2 adsorption and gas-induced strain for dry and native coal were indistinguishable and may be related to its low native moisture and minimal amount of created porosity upon drying. (C) 2008 Elsevier B.V. All rights reserved.