Development of the micro- and ultramicroporous structure of coals with rank as deduced from the accessibility to water

Low-pressure adsorption experiments with carbon dioxide on dry and moisture-equilibrated coal samples from the Ruhr basin, Germany at 273, respectively, 275 K have been carried out in order to obtain information about pore structure, its interaction with water and development with rank. The range of rank of the samples varies between 0.76 and 2.23% vitrinite reflectance VRr. The evaluation of structural parameters was achieved by using the theory of micropore filling. Results show the well-known parabolic curvature for microporosity but a linear increase of ultramicroporosity with rank. The determined water uptake of the coals, achieved with the ASTM 1402 D method, correlates very well with the pore space ≈>1 nm, which suggests that water is not able to penetrate the aromatic structures of the macromolecular network. The results of this study are in general agreement with investigations of Lu et al. [Lu L, Sahajwalla CK, Harris D. Carbon 2001;39:1821] and Rouzaud and Oberlin [Rouzaud JN, Oberlin A. Advanced methodologies in coal characterization, vol. 15. Amsterdam: Elsevier; 1990. p. 311].