A Fundamental Study of Char Combustion: Changes in Particle Morphology during Oxidation

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This thesis presents a study of the morphological changes that occur in selected coal chars during oxidation at low temperature (725K-875K) and at high temperature (1400K-1600K). Gas adsorption and mercury porosimetry were the primary means by which these changes were monitored. An attempt was made to relate the observed reactivity of the char in oxygen to the evolving porous structure of the char. Initial pore structure was varied by using three different raw coals: a lignite, a subbituminous and high volatile A bituminous coal. In the case of the bituminous coal, pore structure was varied further by using different pyrolysis temperatures. Of course, while there were differences in the physical structure of the chars, there were differences in the chemical structure as well. In order to account for this, the chemical nature of the chars was monitored, using elemental analysis and oxygen chemisorption. The results of this study indicate that, at low temperatures, the rate of oxidation of the subbituminous and bituminous chars is proportional to the BET surface area beyond 20% conversion. The lignite char did not show such simple behavior because of the presence of large amounts of ash. For the high-temperature case, reaction appeared to be confined to the exterior of the particle and to the interior of the macropores. Time-temperature histories of individual lignite char particles were obtained with a two-color pyrometer. A simplified model of single- particle-char combustion was used in conjunction with statistical analysis to infer kinetic parameters from the experimental time-temperature traces.