THE EFFECTS OF HEAT AND MASS-TRANSFER IN THERMOGRAVIMETRICAL ANALYSIS- A CASE-STUDY TOWARDS THE CATALYTIC-OXIDATION OF SOOT

A convenient model has been developed which qualitatively describes th e heat and oxygen mass transport within a TGA sample crucible. The maj or conclusion from this model and from experimental observations was t hat for exothermal reactions, heat- and mass-transfer limitations can significantly influence the results of thermogravimetrical analysis. T herefore, it is crucial to use small, diluted samples, in particular f or quantitative thermo-analytical measurements. It is recommended that the occurrence of heat- and mass-transport limitations be assessed by variation of the sample mass by at least a factor of ten. The model p rovides an explanation for the double-peak-shaped curves which are oft en observed when studying the catalysed oxidation of a model soot. The first peak is attributed to a thermal runaway reaction, which occurs when the sample is relatively large and when it is not diluted with si licon carbide. It is shown that the occurrence of the two peaks is cau sed by a combination of heat- and mass-transport limitations within th e TGA sample.