Die Porenstruktur von Kohlen

Investigations with respect to quantitative changes in pore structure with rank were carried out on 10 dry and moist coal samples from the Ruhr basin (Germany). The samples were characterised by means of geochemical and organic-petrograpic methods. A variety of parameters which describe the relationship between pore structure and rank were thereby calculated. These parameters include: micropore volumes, pore volume distributions, pore mean diameter, fractal dimension, radius of gyration, specific surfaces (LANGMUIR- BET-, CI-, fractal surfaces), characteristic adsorption energies and heterogeneity factors using SANS (Small Angle Neutron Scattering)- and gas adsorption analysis. In order to determine the influence of moisture, samples were moisture-equilibrated with the ASTM 1402 D procedure. Further on, Fourier Transformation Infrared Spectroscopy was carried out to perform functional group analysis with respect to their bonds. Additionally, gaschromatographic investigations on coal gas samples were carried out. All samples show CH4-contents > 90 vol.-%. Furthermore, appreciable amounts of CO2, N2, C2H6, and C3H8 were detectable. The results of the pore structure investigations were correlated with the "two component model" (LARSEN et al. 1985, ROUZAUD et al. 1990), the commonly accepted model for the description of chemical and physical changes in coals with rank. In brief, a congruent image concerning the results of this work and the "two component model" could be achieved. The "two component model" views the structure of coal as a macromolecular network, consisting of covalent and non-covalent bonded aromatic structures, in which dissolved molecules, bonded through physisorption – the so-called molecular component – are located. It could be shown that the decisive development of the meso- and macroporous structure (> 20 Å) in the range of rank approximately 0.7–1.1% VRr is governed by the conversion of the molecular component. This process is mainly characterised by the loss of oxygen functional groups. Additionally, it could be shown that only coals within this range have a broad size distribution. Coals with higher maturity up to 1.7% VRr show minor meso- and macroporosity, whereas above 1.7% VRr porosity is reduced to the macroporous range. The specific surfaces and fractal dimensions of the meso- and macroporous structures show an exponential decay with rank, which is proved by the calculated pore volume distributions and radii of gyration. For dry samples microporosity in relation to rank exhibits a minimum at approx. 1.4% VRr. Moist coals, on the other hand, display a relatively continuous increase in micropore volume with rank. The difference between the degrees of microporosity of dry and moist samples is due to the point that in moist coals, the adsorption space is reduced to the aromatic Crystallite-/BSU-structures. Although the effective diameter of water molecules is relatively small (2.8 Å), they are not able to penetrate the ultramicroporous spacings (< 4 Å) between the aromatic Crystallite-/BSU-structures due to the formation of clusters. Their linear accretion with increasing rank could be correlated with the "Simplified Coal Model" from LU et al. (2001), which predicts a linear increase of aromatic bonded carbon with rank. The filling process of the pore structure with water is principally governed physical adsorption. Functional groups are only of importance at low surface coverage. The gas adsorption capacity for supercritical CH4 in relation to rank was determined for dry and moist coals at 313 K up to 15 MPa. The collected isotherms correlate very well with the LANGMUIR model, which predicts a monomolecular surface coverage. It could be shown that in the presence of water, the adsorption of supercritical CH4 is controlled by the aromatic Crystallite-/BSU-structures. In dry samples, according to in situ conditions, the adsorption of supercritical CH4 is under the assumption of a monomolecular surface coverage reduced up to 35%.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2004

Author dc:creator

• Prinz, Dirk

Contributors dc:contributor

• Littke, Ralf

• info:eu-repo/classification/ddc/540
• Ruhrgebiet
• Kohle
• Gesteinsprobe
• Porosität
• Inkohlungsgrad
• Chemie
• Neutronenstreuung
• Adsorption

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-120860%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59044