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Isothermal water vapor adsorption and desorption on eight Australian bituminous coals, varying in rank, was measured as a function of vapor pressure. The isotherms were measured within the temperature range of 15 °C to 40 °C and at relative vapor pressures up to ca. 0.9. These data enabled the effects of high- and low-pressure hysteresis to be separated and quantified, and the calculation of the net heat of water adsorption as a function of water coverage. High-pressure hysteresis is attributed to “ink bottle” type pore structure, which diminishes as coal rank increases. Application of a model to isothermal adsorption data has enabled the relative amounts of water binding to primary and secondary adsorption sites on coal to be quantified. Proximity of the primary sites to one another and the distribution of water between primary and secondary adsorption sites provide an explanation for the changes in the net heat of water adsorption as a function of the water coverage on coal.

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