ON WHY DO DIFFERENT CARBONS SHOW DIFFERENT GASIFICATION RATES - A TRANSIENT ISOTOPIC CO2 GASIFICATION STUDY

A suite of nine different carbons have been investigated by step-response experiments (SRE) using (CO2)-C-13 at 10 and 100 kPa and, predominantly, at 1200 and 1250 K. Although the gasification activities vary by a factor of twenty, all carbons behave similarly in their CO desorption patterns upon a step change from a reactive to an inert gas phase. This CO desorption can be described by a two-stage exponential decay, irrespective of the type and structure of the carbon used. The amounts of desorbable surface oxygen complexes differ between the various carbons and increase with increasing CO2 pressure and temperature. The complexes are restored by a reversed step from an inert to a reactive atmosphere. Clear indications are found that a distribution in desorption activation energies exists. The variation in gasification cation rate between the carbons and as a function of the CO2 pressure and temperature is mainly due to the variation in desorbable surface oxygen complex concentration. The transient CO desorption is best accounted for by a recent model of Chen, el al., where two types of oxygen complexes contribute to the gasification rate. The largest contribution comes from highly reactive intermediates that contribute five times more than the more stable complexes, but which are present in 3-4 times smaller amounts. The associated rate constants are fairly similar for the different carbons, averaged 0.6 and 0.034 s(-1) respectively, and exhibit only a weak temperature dependency due to the surface heterogeneity. Correlation of the gasification rate with the total amounts of desorbable surface complexes is incompatible with the decay curves. The use of (CO2)-C-13 demonstrated that carbon from the gas phase reactant is inserted in the carbon structure according to an activated process, which is evidence of the reversibility of the gasification reaction. Three carbons show a sharp maximum in the decaying CO desorption curve at 100 kPa CO2, a peculiar phenomenon not yet explained.