Soot oxidation kinetics under pressurized conditions

Abstract

The oxidation kinetics, under different pressures, of soot samples obtained from different liquid fuels and two standards (a commercial black carbon sample and a reference diesel soot) was studied. Soot samples were generated in a flat-flame, premixed burner under heavily-sooting conditions and captured on a water-cooled stabilization plate located above the burner surface. The collected soot was oxidized using a high-pressure thermogravimetric analyzer (HTGA). TGA operation was optimized to reduce mass transfer effects by adjusting the oxidizer flow rate and initial sample mass. Further corrections for mass transfer were accomplished by computing the effectiveness factors for intraparticle, interparticle, and external mass transfer. Two pressures were evaluated (1 and 10atm) and O2 concentration was varied between 10 and 21%.There was not a significant difference in the activation energies of the soot samples for either pure components or as a mixture, with exception of soot from oxygen-containing fuels, and activation energies for both standards were within the range reported by others.Pressure did not change the activation energies when mass transfer corrections were applied.Soot nanostructure for the soot collected from the flat flame “nascent”, and partially oxidized at 1, 10 and 40atm was studied by High-Resolution Transmission Electron Microscopy (HRTEM). The lattice fringe length and fringe tortuosity were estimated to correlate the nanostructure and the soot reactivity, and the nascent soot nanostructure was similar across the five fuels. n-butanol/n-dodecane soot showed a significant change in nanostructure with increasing pressure; however, there was no apparent change in the nanostructure (length or tortuosity) for the other fuels. Corroborating the HRTEM data, surface carbon oxidation studies by X-ray photoelectron spectroscopy (XPS) revealed that the sp2/sp3 content for the oxygenated fuel blend was six times lower than the other fuels leading to lower activation energy and external surface lamellae break-up.