| The kinetics of gasification of pure and catalyst-containing graphite pellets were investigated using a thermogravimetric apparatus. The catalysts used included Cs(,2)CO(,3), Li(,2)CO(,3), K(,2)CO(,3), and Na(,2)CO(,3). Pellets of different weights were pressed from mixtures of Ultrapure (UCP-1-M) graphite powder catalyst amounting to 1 mol % cation of finely divided alkali carbonate. These samples were reacted isothermally in the temperature range of 808(DEGREES)C to 1038(DEGREES)C in CO(,2), or CO(,2)-He, or CO(,2)-CO-He, or CO(,2)-CO-N(,2) gaseous mixtures of various compositions at 1 atm total pressure. In pure CO(,2), the relative activities of the catalysts was ranked as: Cs(,2)CO(,3) > Li(,2)CO(,3) > K(,2)CO(,3) > Na(,2)CO(,3). The smallest pellets, weighing 150 mg and containing either Li(,2)CO(,3) or K(,2)CO(,3), were used for an in-depth study of the effect of gas composition. At all the temperatures investigated, the reaction appeared independent of the CO(,2)-concentration and was inhibited by CO. Reaction mechanisms involving the formation of AGO (alkali-graphitic-oxide) surface-complexes and their decomposition into either Li(,2)O for Li(,2)CO(,3), and K(g) for K(,2)CO(,3), followed by the recarbonation of the oxide or the akali vapor are thought to constitute the reaction paths. An analysis based on the assumption of steady state concentrations of AGO-complexes and alkali metal vapor pressure, when applicable, provided rate equations that agree very well with the experimental results.;The changes in B.E.T. surface area and porosity during the course of the reaction were monitored up to 35% burnoff using Quantachrome Surface Area and Mercury Porosimetry instruments. A decrease in surface area at low burnoff followed by sharp increase was recorded with K(,2)CO(,3)-catalyzed graphite pellets while the surface area of Li(,2)CO(,3)-catalyzed pellets increased at first but then decreased very slowly with burnoff. In situ observation of the gasifying pellets using Hot stage microscopy failed to reveal the reason for the differences. They were explained on the basis a moving front of alkali-vapor hypothesis and the exfoliation of the graphite sheets as the latter reacted with CO(,2) gas.;The external gas mass-transport coefficients and effectiveness factors (ratio of the measured rates to the true chemical reaction rates) for the reaction of porous pellets were calculated. A method was developed for correcting the measured rate constants for the external mass-transport or pore-diffusion effects. |
