Oxidation Kinetics of Monodisperse Spherical Carbonaceous Particles of Variable Properties

Abstract

Synthetic chars of variable physical and chemical properties have been developed to study char oxidation mechanisms and rates. The char particles were spherical and monodisperse, with sizes ranging from a few microns to several tens of microns. The particles were made from a carbon-yielding polymer and pore-forming additives. The surface areas of the chars made from different additives varied by more than two orders of magnitude and the porosities varied by a factor of five. The pore size distributions included both micro and transitional pores. X-ray studies revealed that all chars were amorphous when heat treated to temperatures up to 1600 K in an inert atmosphere. However, upon oxidation at 1600 K, the carbon matrix underwent partial graphitization. This transformation was particularly pronounced for some of the polymer pore-former chars. Combustion experiments showed that the total surface area of the chars increased dramatically with conversion, revealing the existence of a vast network of micropores. Apparent oxidation rates were higher for the chars that contained transitional pores in a microporous matrix. When compared with the rates reported in the literature for coal derived chars, the calculated intrinsic rates were lower at intermediate particle temperatures (800-1600 K) but comparable at elevated temperatures (1800-2300 K). As the temperature was increased further, the intrinsic rates decreased consistent with the Nagle and Strickland-Constable kinetic mechanism [l].