Kinetics of Water Adsorption/Desorption on Bituminous Coals

Abstract

The kinetics of water adsorption and desorption on nine bituminous coals of various ranks have been studied, mainly at 26 °C. To quantify the sorption kinetics, isobaric water uptake vs time data have been fitted by an empirical stretched-exponential model. This model provides good fits to water uptake data throughout the entire equilibration process at relative water vapor pressures (relative to saturation pressure) in the range of 0−0.9 for all coals investigated. This represents a distinct advantage oversimplified Fickian models which are valid for only a portion of the data within the short and long time limits. A flow rate kinetic parameter has been introduced to quantify the rate of water transfer from outside the coal particle to the intraparticle pore structure. Most of the bituminous coals investigated in this study have similar flow rates at relative pressures in the range of 0.2−0.9. However, for two coals of rank approaching that of semianthracites, the flow rate is significantly less than for all other bituminous coals investigated. It is believed this difference is caused by differences in the coal structure. Variations between coals in the flow rate at low relative pressures (0−0.2) appear to be caused by variations in the primary adsorption site density which is largely determined by the coal oxygen content. The density of oxygen containing functional groups also influences the flow rate at low relative pressures during water desorption.