Water vapour adsorption on coal

Water vapour adsorption isotherm and kinetic studies were performed on three coal samples originating from the Czech Republic. Measurements were conducted at three temperatures: 25, 35 and 45 °C. The adsorption isotherms fit the Dent model. For a description of the kinetic process linear driving force model, and in the case of a sample with higher concentration of carboxylic groups, a double exponential model provides a good fit. The Dent model enables us to distinguish water adsorbed on primary and secondary sites. For low relative pressures, adsorption on primary sites dominates, while with increasing pressure adsorption on secondary sites gains in significance. The percentage of secondary site adsorption depends on the concentration of oxygen-containing functional groups, because a higher concentration of functional groups supports bridging of the water molecules. The concentration of the functional groups also has an impact on the rate constant. A sample with a higher concentration of functional groups has a lower rate constant. However, a lower value of the rate constant is related to greater water uptake of the sample and its transfer during adsorption.