further drying leads to intercalation compound 'coal-MOH' formation. This work has demonstrated that reorganization of over-molecular structure for all investigated coals is found. Emphasis is put on brown coal (Cd"' 73.6%) and anthracite (Cd"' 95.2%) as extreme representatives of rank coals.
98100038
Structural comparison of hazardous and nonhazardous coals based on gas sorption experiments
Lakatas, _I. er al. DGMK Tagungsber., 1997, 9702, (Proceedings ICCS '97, Volume I), 79-82. COz and CxHx sorption at ambient temperature was compared and the results were used for characterizing the difference of the structure of hazardous and non-hazardous coals. Hazardous coals were found to be more microporous or contain more closed pores than non-hazardous ones, but this difference could not be enlarged and attributed to one petrographical component by producing the density fractions. Gas sorption isobars (N?, CH.,, CxHx) are proposed to make a distinction between fine pure structure of coals.
98~00039
Structural rearrangement of strained coals Larsen, J. W. et al. Energy Fuels, 1997, 11, (5), 998-1002. Native coals are strained and glassy. This strain is relieved when coals are swollen as the coal structure rearranges to a lower free energy and more highly non-covalently associated state. Testing was carried out on four coals ranging in carbon content from 77% to 84%. They were warmed in the weak swelling solvent chlorobenzene at 132ยฐC for 2 weeks, and at intervals samples were withdrawn. After chlorobenzene evaporation, the pyridine extractability of the treated coals had decreased, sometimes by a factor of two. The pyridine swelling of Pittsburgh No. 8 coal was sharply reduced. The extractability and swelling decreases with time demonstrate that changes in coal structure occurred with the rearranged coal being more associated. The rearranged Pittsburgh No. 8 coal was studied by differential scanning calorimetry.
The results showed that over the 2-week chlorobenzene reflux period, the heat capacity decreased by a factor of 2; the coal had rearranged to a more highly associated, more rigid structure. X-ray diffraction studies show_ enhanced i,ltensity for a regular structural feature occurring at about 20 A with no other alterations, including the aromatic face-to-face stacking. The conditions defy increases in covalent bonding as the source of the observed changes. The driving force for the rearrangement is believed to be the release of stored elastic strain. Coal swelling provides the macromolecules with the opportunity to undergo conformational rearrangements and to adopt a lower free energy more highly associated structure.
In the case of high-rank Upper Freeport coal behaviour is contrasting to lower rank coal, as it apparently rearranges to a less associated structure.