MORPHOLOGY OF CHROMIUM EMISSIONS FROM A LAMINAR HYDROGEN DIFFUSION FLAME

The morphology and size distribution of chromium oxide particles has b een studied in laminar hydrogen diffusion flames. Nitrogen was added t o vary the flame temperatures. Two sources of chromium compounds were introduced: chromium nitrate and chromium hexacarbonyl. In order to cl arify the morphology of chromium oxide particles that were produced in the flame, a detailed investigation was carried out by scanning elect ron microscopy (SEM) after sampling with an isokinetic, dilution sampl ing probe. The morphology of the particles varied with the flame tempe rature and with the chromium source. The particles were characterized by porous structures, cenospheres, and agglomerated dense particles wh en chromium nitrate solution was added to the flames. At low to modera te temperatures, porous sintered cenospheric structures were formed, i n some cases with a blow hole. At higher temperatures, an agglomerated cluster, which was composed of loosely sintered submicron particles w as observed. SEM analysis revealed that the size distribution of chrom ium oxide particles was bimodal; submicron sizes were generated by the mechanism of cenosphere fragmentation. Chromium oxides formed from ad ding chromium hexacarbonyl to a hydrogen flame did not exhibit cenosph eric structures. At high temperatures, a film of crystalline material was deposited onto a filter; at low temperatures very small agglomerat ed chains were found. The morphology of the metal particles is strongl y dependent on the form in which the metal enters a flame. (C) 1998 by The Combustion Institute