FUNDAMENTAL STUDIES ON GAS-SOLID REACTIONS: PORE STRUCTURE AND REACTIVITY OF COAL CHARS
SANDMANN, CHARLES WILLIAM, JR.
Doctor of Philosophy
The combustion and gasification of five char samples was investigated. Four chars were produced by pyrolysis and demineralization of two coals, a Texas lignite and Illinois #6. The fifth sample was a commercially obtained activated carbon. All samples were substantially free of ash impurities eliminating possible effects of ash interaction on the reaction rate data. Optical microscopy and digital image processing were used to obtain a quantitative characterization of the macropore structure, and adsorption of gaseous nitrogen and carbon dioxide produced a fully micropore characterization. Oxygen chemisorption followed by temperature programmed desorption indicated a range of active sites occupying only a small fraction of the total surface area. Characterization and reaction rate determination were performed using a newly developed computer control and data acquisition system. This sytem produced more consistent data than previously available while allowing more data to be taken with less operator effort. The change in reactivities of the five samples during gasification with oxygen at 400-500$\sp\circ$C and with carbon dioxide at 850-950$\sp\circ$C were measured as a function of conversion. A wide range of behavior was observed in the shapes of the reaction rate curves for the five samples using two reaction gases. For each char-gas set of experiments, however, the normalized rate curves were identical within experimental error. Significant differences were found to exist for the same char reacting with the two different gases. A discrete numerical model was developed to simulate the evolution of the surface area and pore structure of the samples. The discrete model can simulate more complex phenomena than other available models, including diffusional limitations and structured systems. Data from the physical characterization were used to analyze the reaction rate curves. A non-random structure was indicated in some of the samples, which was utilized differently by the two gases. Different active sites are used by the two gases, and these are probably distributed differently between the random and ordered structures of the chars.