A STUDY OF CHAR GASIFICATION REACTIONS (COAL CONVERSION, COMBUSTION, CATALYTIC, MODELING, PORE STRUCTURE)
BALLAL, GIRISH DAMODAR
Doctor of Philosophy
Structural, kinetic and catalytic effects relevant to the combustion and gasification of coal-derived chars were investigated. A Texas lignite, an anthracite and two bituminous coals, Pittsburgh #8 and Illinois #6, were pyrolyzed in a nitrogen atmosphere to prepare chars. Optical microscopy, mercury porosimetry and gas adsorption techniques using nitrogen, CO(,2) and CO, were employed for pore structure characterization. The micropores present in the lignite char were accessible to all adsorbents. However, a substantial fraction of the pore space in other chars was inaccessible to nitrogen at 77(DEGREES)K. The lignite char also exhibited the fastest rates of gaseous diffusion, followed in order of decreasing diffusivities by the Illinois #6, Pittsburgh #8 and anthracite chars. The changes in reactivities and pore structures of chars were measured experimentally during their reaction with oxygen (400-550(DEGREES)C) and CO(,2) (800-1000(DEGREES)C). For a particular char-gas system, the normalized rate-conversion pattern was invariant with respect to temperature and gaseous concentration. In the case of lignite and Pittsburgh #8 chars, the rate-conversion pattern was similar during reaction with oxygen and CO(,2). Adsorption experiments on partially reacted chars indicated that the micropores in the lignite char were accessible to both reactants. The micropores in the Illinois #6 char were, however, not accessible during its reaction with oxygen. The evolution of pore structure during reaction was modeled by using a probabilistic approach which accounts for overlapping pores with different shapes and sizes. A priori predictions of pore structure evolution are obtainable from the structural measurements on unreacted char. The model predictions were validated using the experimental data reported in literature and the data obtained in this work. The kinetics of gasification of the lignite and the Pittsburgh #8 chars was studied using a Langmuir-Hinshelwood type kinetic expression to correlate the experimental data. CO was found to inhibit the reaction substantially. The effect of a potassium carbonate catalyst on the reaction of these two chars was also investigated. A marked change in the reactivity-conversion pattern was observed when the catalyst impregnated char samples were reacted with oxygen and CO(,2). Substantial increases in reaction rates were observed, and the enhancement was approximately proportional to the catalyst loading.