Microstructural evolution of char under oxidation induced by uneven heating

Single spherical char particles were intentionally irradiated nonuniformly in an electrodynamic chamber, in the temperature range 600-1000 K. The char particles were irradiated from one side (bottom) and consequently heated unevenly. Nonuniform shrinkage of an initially spherical char particle has been observed during oxidation. The features of nonuniform shrinkage are rather peculiar: (1) Up to 40%-60% conversion, the particle shrank uniformly. Then spatial preferential consumption initiated, indicating the threshold nature of the phenomenon. (2) Preferential consumption, in most experiments, was observed to start from the top of the particle. (3) Above the threshold conversion, a disk structure became clear. (4) At very high conversion the particle became like a center-hole doughnut. A model was developed to explain these features by nonuniform transformation of the micropore structure due to oxidation of the unevenly heated particle. The threshold nature of the phenomenon was also explained in terms of the dynamic stability of a particle. The threshold of nonuniform shrinkage corresponds to the transition of the particle to a stable position. After initiation of nonuniform shrinkage, the macroporosity distribution inside the particle becomes 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 controlled) is an indication of the fundamental microstructural transformations in the course of oxidation.