METAL ENHANCED SOOT AND PAH FORMATION

Iron and manganese have been added to laminar, sooting, premixed ethylene/oxygen/nitrogen flames with fuel equivalence ratios ranging from 2.22 to 2.49. The iron concentration was 200 ppm in the fuel, while the manganese concentration was 140 ppm in the fuel (mole basis). The additives increased the amount of soot formed under all conditions, up to a factor of 3. The iron additive also caused up to a sixfold increase in the fluorescence coefficient, while the manganese additive had little effect on fluorescence. The fluorescence coefficient was approximately proportional to the total tar concentration. The additives did not cause a detectable change in the concentration of any of the C1-C4 species, and did not have a measurable effect on soot particle inception or soot particle collision rates. At high temperatures, equilibrium predicts both iron and manganese were present in the gas phase, predominantly as free metal atoms. As the flame temperature decreased, iron was predicted to precipitate out of the gas phase as metallic iron, while manganese was predicted to form solid MnO. Metallic iron on the surface of the growing soot particles was hypothesized to catalyze the addition of acetylene to soot. A reaction probability between acetylene and an iron site of about 0.023 fit the measurements reasonably well. Predictions of the fraction of the particle surface covered by iron ranged from about 0.5%-3%. Metallic iron on the surface of the soot was also hypothesized to catalyze the formation of PAH from acetylene.