The effects of the addition of dicyclopentadienyl iron, ferrocene, on soot particle inception and growth were studied in atmospheric pressure, premixed ethylene flames with C/O = 0.71-0.83 at 0.015-0.46% ferrocene by weight of the fuel. The rate of surface growth of soot in undoped and doped flames was measured using laser light scattering and light extinction techniques to determine the particle size and number density as a function of residence time in the flame. The final soot yields in the ferrocene doped flames were enhanced by factors of 13.5-1.2 over the range of flame C/O ratios studied. This enhancement decreased with increasing C/O ratio for the same concentration of iron in the flames.
The spatial distribution of the elements iron and carbon in the particles was determined with Auger spectroscopy. The iron was found to be concentrated in the cores of the particles. The results of a steady-state one-dimensional nucleation model which indicated that the iron oxide particles would nucleate well in advance of the soot inception time were consistent with this elemental stratification and the ob~rvation of the onset of carbon deposition in doped flames at earlier residence times than in undoped flames of the same C/O ratio. The iron oxide particle sizes and number densities predicted from the model indicated that these particles were both numerous and small enough to be ~ed particles on which carbon deposition could take place.
The importance of the activity of the surfaces of the solid in determining the rate of surface growth of soot was investigated. Lower rates of surface growth measured in doped flames than in undoped flames of the same C/O ratio at early residence times indicated that the nucleated iron oxide particles were a less active surface on which carbon deposition took place than were the young soot particles. The rate of surface growth decreased by a factor of two over the residence time in the flames in all undoped flames and in doped flames of C/O ratio above 0.80. For the doped flames of C/O ratio below 0.80, the rate of surface growth did not decay in the late residence time regions of the flames. The break between the doped flames which did and did not display the persistent growth rates correlates with the relative magnitudes of the characteristic diffusion times for iron through carbon based on the final particle size formed in the flames and the particle growth time.