MICROSTRUCTURAL EVOLUTION OF CHAR UNDER OXIDATION-INDUCED BY UNEVEN HEATING

Single spherical char particles were intentionally irradiated nonunifo rmly in an electrodynamic chamber, in the temperature range 600-1000 K . The char particles were irradiated from one side (bottom) and conseq uently heated unevenly. Nonuniform shrinkage of an initially spherical char particle has been observed during oxidation. The features of non uniform shrinkage are rather peculiar: (1) Up to 40%-60% conversion, t he particle shrank uniformly. Then spatial preferential consumption in itiated, indicating the threshold nature of the phenomenon. (2) Prefer ential consumption, in most experiments, was observed to start from th e top of the particle. (3) Above the threshold conversion, a disk stru cture became clear. (4) At very high conversion the particle became li ke a center-hole doughnut. A model was developed to explain these feat ures by nonuniform transformation of the micropore structure due to ox idation of the unevenly heated particle. The threshold nature of the p henomenon was also explained in terms of the dynamic stability of a pa rticle. The threshold of nonuniform shrinkage corresponds to the trans ition of the particle to a stable position. After initiation of nonuni form shrinkage, the macroporosity distribution inside the particle bec omes nonuniform. Macroporosity was shown to increase near the particle 's center; this eventually creates a hole at high conversion. The good agreement of modeling results with experimental observation confirms the notion that nonuniform shrinkage in regime I (kinetically controll ed) is an indication of the fundamental microstructural transformation s in the course of oxidation.