Morphology of chromium emissions from a laminar hydrogen diffusion flame

The morphology and size distribution of chromium oxide particles has been studied in laminar hydrogen diffusion flames. Nitrogen was added to vary the flame temperatures. Two sources of chromium compounds were introduced: chromium nitrate and chromium hexacarbonyl. In order to clarify the morphology of chromium oxide particles that were produced in the flame, a detailed investigation was carried out by scanning electron microscopy (SEM) after sampling with an isokinetic, dilution sampling probe. The morphology of the particles varied with the flame temperature and with the chromium source. The particles were characterized by porous structures, cenospheres, and agglomerated dense particles when chromium nitrate solution was added to the flames. At low to moderate temperatures, porous sintered cenospheric structures were formed, in some cases with a blow hole. At higher temperatures, an agglomerated cluster, which was composed of loosely sintered submicron particles was observed. SEM analysis revealed that the size distribution of chromium oxide particles was bimodal; submicron sizes were generated by the mechanism of cenosphere fragmentation. Chromium oxides formed from adding chromium hexacarbonyl to a hydrogen flame did not exhibit cenospheric structures. At high temperatures, a film of crystalline material was deposited onto a filter; at low temperatures very small agglomerated chains were found. The morphology of the metal particles is strongly dependent on the form in which the metal enters a flame. (C) 1998 by The Combustion Institute