Evolution of porosity and thermal conductivity during char oxidation

Measurements of the natural convection drag and the photophoretic force have been conducted forSpherocarb char particles as a function of carbon conversion. These forces were obtained by measuring the balancing voltage with and without laser heating during the reaction of single particles in an electrodynamic balance. The photophoretic force was determined by subtraction of the calcuulated natural convection force, after an initial transient corresponding to about 5% carbon conversion during which the natural convection force was dominant. The particle conductivity inferred from the photophoretic force was found to decrease by more than one order of magneitude as the reaction progressed, qualittively in agreement with models of the dependence of conductivity on porosity. Confirmation of the temperature gradient across the particle was provided by the development of asphericity in the particles when heated from below, but not when heated uniformly. The simutaneous measurements of the mass, diameter, and particle conductivity as a function of carbon conversion provide a critical test of pore evolution models since the reaction rate is dependent on the accessibility of the internal surface area to the reactant gas through the open pore structure and the thermal conductivity is dependent on the connectivity of the solid structure. Induction periods were observed before the reaction rate accelerated and the particle conductivity declined, confirming the influence of pore structure on both. Particles could be reacted to a high conversion of greater than 95% without any evidence of fragmentation, providing further insight on the connectivity of the solid surfaces.