Detailed analysis of kinetic reactions in soot oxidation by simulated diesel exhaust emissions

Abstract

Development of an advanced diesel particulate filter system requires efficient regeneration strategies, which, in turn, require a comprehensive understanding of soot oxidation kinetics. A goal of this study is first to define the effect of inert balance gases and heating rates used for thermogravimetric analyses on soot oxidation, with applying different analytic methodologies to the calculation of kinetic parameters. We identified an inert gas-independent activation energy value of surrogate soot to be 155kJ/mole, evaluated at a heating rate of 1°C/min, and proposed the differential method for evaluating kinetic parameters. Using the results, we conducted subsequent non-isothermal soot oxidation experiments to characterize the effects of reactant gases (NO, NO2, CO2, and O2) in the various compositions of gas mixtures simulating diesel exhaust emissions. Notable results include the following. With no O2 in the mixtures, NO gases rarely affected soot oxidation, while CO2 effects also appeared to be minor. In the presence of a constant NO2 concentration, the increased O2 concentration (particularly up to 4%) significantly enhanced soot oxidation. In the case of a constant O2 concentration, increasing the NO2 concentration promoted soot oxidation in a fairly low temperature range of 200–580°C. These O2 and NO2 effects turned out to be quite significant and temperature dependent. Overall, a higher degree of oxidation was observed with the mixture composition simulated for the higher engine load. The Arrhenius plots exhibited two distinct linear regression lines as NO2 was present in the mixture, where the low temperature linearity range was extended with increasing NO2 concentration. The activation energy quite significantly decreased from 153 to 39kJ/mole in the low temperature range, as NO2 concentration increased from 0 to 1250ppm. In the high temperature range, however, the activation energy changed only slightly between 153 and 159kJ/mole, indicating negligible dependence on mixture compositions.