KINETIC MEASUREMENT AND MODELING OF CARBON OXIDATION

Oxidation of uncatalyzed and Ca-catalyzed soot was investigated in terms of the overall reaction rate as well as the CO and CO2 evolution rates. Temperature-programmed desorption was also studied to provide a better understanding of the carbon oxidation process. Both CO and CO2 are primary products during carbon oxidation but are generated via different mechanisms. CO formation is believed to involve a wide range of activation energies due to the complex structure of carbon. The rate of CO formation was found to be of fractional order with respect to the O2 partial pressure, and to be insensitive to Ca catalysis. It is postulated that CO2 is formed at sites which are different from the CO sites. The number of CO2 sites is very small for the uncatalyzed carbon and is increased by Ca addition. The rate of CO2 formation was found to be first order with respect to the O2 partial pressure and can be effectively catalyzed by Ca. The CO/CO2 ratio decreases from near unity for noncatalytic carbon oxidation to approximately 0.01 for catalytic carbon oxidation at 663 K. A carbon oxidation model was formulated which quantitatively describes the experimental results for both catalyzed and uncatalyzed carbons and leads to the following conclusions: (1) The controlling factor for a noncatalytic carbon oxidation is the carbon structure which may be appropriately represented by the site energy distribution of that carbon. (2) The controlling factor for a Ca-catalyzed carbon oxidation is the Ca dispersion on the carbon surface. The CO/CO2 ratio provides a measure of the importance of catalysis in the oxidation.