Particulate Oxidation Kinetics: Surface Area Dependence

Andrea  Strzelec, Oak Ridge National Laboratory

Diesel particulate produced in a light duty engine from different biodiesel-conventional fuel blends was subjected to physical and chemical measurements in order to better understand the mechanisms by which fuel-related changes to oxidation reactivity are brought about. Surface area evolution with extent of oxidation reaction was measured for PM from ULSD and biodiesel. Biodiesel particulate has a significantly larger surface area for the first 40% of carbon conversion, at which point the samples become quite similar. Oxidation characteristics of nascent PM and the fixed carbon portion were measured by temperature programmed oxidation and it was noted that increased biodiesel blending lowered the light-off temperature as well as the temperature where the peak rate of oxidation occurred. A shift in the oxidation profiles of all fuels was seen when the mobile carbon fraction was removed, leaving only the fixed carbon, however the trend in temperature advantage of the biofuel blending remained. Effective Arrhenius parameters for fixed carbon oxidation were directly measured with isothermal, differential oxidation experiments. Normalizing the reaction rate to the total carbon surface area available for reaction allowed for the definition of a single reaction rate with constant activation energy (113 ± 6 kJ/mol) for the oxidation of PM, independent of its fuel source. A kinetic model incorporating the surface area dependence of fixed carbon oxidation rate and the impact of the mobile carbon fraction was constructed and validated against experimental data.

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