MACROMOLECULAR NETWORK STRUCTURE OF COALS: INTERPRETATION OF EQUILIBRIUM AND DYNAMIC SWELLING EXPERIMENTS
Abstract
The macromolecular structures of twenty-one coals were investigated via equilibrium and dynamic solvent swelling experiments. The variables were the carbon content of the coal sample, ranging from 69 to 94%C (dmmf); the solvent used, including pyridine, methanol, ethanol, n-propanol, 2-butanone, acetone, and cyclohexane; the size of the coal particles, which were 1-2 cm('3) sections, 20-30 mesh, 50-60 mesh and 80-100 mesh particles; the temperature of swelling, either 35, 60 or 80(DEGREES)C; and the coal pretreatment, which included pyridine-extraction, atmospheric oxidation and the methylation of -OH and -COOH functionalities. As support data, the pore volumes of coal samples were determined via analysis of mercury porosimetry experiments and the glass transition temperatures of the samples were determined via analysis of differential scanning calorimetry thermograms. The equilibrium swelling data were analyzed by application of a modified Gaussian network equation. We concluded that the effective number of repeating units between crosslinks, N, at 35(DEGREES)C is 7.0 to 8.5, corresponding to a molecular weight between crosslinks of 990 to 1280, over the carbon content range of 69.94 to 82.48%C (dmmf). At 86.01%C the value of N drops to 4.6, corresponding to a molecular weight between crosslinks of 780, and continues to decline to a value of 2.2 (molecular weight between crosslinks of 814) at 91.54%C. At higher temperatures, the increased swelling proved that thermodynamic interaction increased and pyridine attacked the hydrogen-bonded structure of coal. Approximately 60% of the crosslinked network structure of a coal sample of 79.82%C was found to be the result of hydrogen bonds due to the presence of hydroxyl and carboxyl groups. It was determined via analysis of dynamic solvent swelling data that coals are glassy macromolecular systems at room temperatures, a conclusion that is consistent with the glass transition temperatures of 300 to 330(DEGREES)C. It was also found that (i) transport of pyridine into coal was controlled by relaxation processes and did not proceed by Fickian diffusion; (ii) the transport mechanism changed from Case-II to anomalous to Fickian as the particle size of the coal was reduced; and (iii) the value of the relaxation constant depended on the carbon content of the coal and was on the order of 10('-4) g/(cm('2)-hr).
Degree
Ph.D.
Subject Area
Chemical engineering
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