The kinetics of oxidation of Diesel soots and a carbon black (Printex U) by O2 with reference to changes in both size and internal structure of the spherules during burnout

The rates of oxidation of two soots, produced from burning either ultra low sulphur Diesel or biodiesel in an engine, were measured at 450–550 °C, with oxygen concentrations of 2.7–24.4 vol%; Printex U was also studied. These carbons were first heated in argon to remove any volatile material; the resulting particles were found to burn in two stages. An initial, fast, transient reaction consumed almost 20% of the carbon in a soot particle. The rates of oxidation, during the second part of burnout were consistent with a model assuming these soots are comprised of porous spherules, which burn throughout their interiors. The overall rates in this second stage of burning were half-order with respect to O2, with an apparent activation energy of 145 ± 8 kJ mol−1. For the two Diesel soots, [CO2] and [CO] in the off-gases decreased, whilst the carbon burned, but the rate of oxidation of Printex U increased to a second maximum. This was consistent with the spherules in this carbon having pores, which grew and intersected, whilst the carbon was consumed. Thus the interiors of spherules of both Diesel soots and Printex U must have been accessible to O2, whilst they burned.