Structural changes of char particles during chemically controlled oxidation

The structural changes-particle diameter, shape, porosity, fine structure-during the oxidation of highly porous synthetic char (Spherocarb) particles at kinetically controlled conditions were investigated to provide insight on the factors that govern fragmentation during combustion. More than 100 experiments were conduced in an electrodynamic chamber in the temperature range of 700-1000 K, for particles of initial diameters of 150-250 μm and densities of 0.45-1.0 g/cm³ for carbon conversions up to 99%. Particle mass, density, size, and shape were measured continuously. Similar experiments were conducted in a thermogravimetric analyzer (TGA) to provide samples for determination of the fine structure of the chars using high-resolution transmission electron microscopy (HRTEM). For a subset of particles that were analyzed in detail, the total porosity increased from 0.60-0.76 to 0.83-0.88, when allowance is made for the observed particle shrinkage. Although the maximum porosity surpassed the value of ca. 0.7 required for percolative fragmentation in a material of uniform porosity, little fragmentation was observed. The explanation for this is that the particles have a bimodal pore size distribution and the porosity of neither the macroporous nor the microporous regions exceeded the critical porosity. The maximum final macroporosity was calculated to be 0.55-0.60. The HRTEM images showed an increased order in the microporous regions, which accounts for the densification of the chars responsible for particle shrinkage and the maintenance of particle integrity.