GEOMETRIC EFFECTS ON MASS-TRANSFER DURING THERMOGRAVIMETRIC ANALYSIS - APPLICATION TO REACTIVITY OF DIESEL SOOT

The measurement of soot reactivity by thermogravimetric analysis is complicated by oxygen mass transfer limitations. The large surface area of the solid imposes local oxygen depletion, even with very small bed masses. An analytical solution has been developed to the equations describing flow above, and reaction within, a bed of soot contained within a rectangular crucible. Solutions are derived for a two-dimensional situation where the sides of the bed are either open to, or shielded from oxygen. The output of the model is modified by incorporating external mass transfer rates, evaluated for a number of bed configurations using the CFD package FLUENT. The influences of bed mass, the proximity of crucible walls and of heat release from the exothermic reaction are considered. It was found that the mass transfer coefficient is independent of sample mass and only slightly influenced by the walls. The depth of the soot bed is the most sensitive variable determining the derived reactivity. For satisfactory results, a small mass of sample carefully loaded into the crucible is recommended. New values for the reactivity of a Diesel soot previously examined are presented for the temperature range 600 degrees-800 degrees C.