The effect of temperature on soot properties in premixed methane flames

The effect of flame temperature on soot properties was studied in premixed methane/oxygen flames burning at a constant mixture composition (C/O = 0.60, Φ = 2.4) and different cold-gas flow velocities (4 and 5 cm s ^-1). Temperature and concentration profiles of stable gases and condensed phases combustion products were measured along the flame axis. It was found that the high flame temperature conditions cause a larger decomposition of methane into hydrogen and C ₂-C ₄ hydrocarbons, thereby reducing the formation of benzene and condensed phases including condensed species and soot. Soot properties were studied by UV-Visible absorption spectroscopy, thermogravimetry and H/C elemental analysis. A description of soot nanostructural organization was also performed by means of high-resolution transmission electron microscopy. Different properties and nanostructures were found to develop in the soot, depending on the temperature and on soot aging associated. Soot dehydrogenation occurred to a larger extent in the high flame temperature conditions. As soot dehydrogenates the mass absorption coefficients of soot exhibited an increasing trend along the flame axis. However, mature soot retained a relatively high H/C ratio and low absorption coefficients with respect to other less hydrogenated fuels even in high temperature conditions. This indicates that the aromatization/dehydrogenation of soot in premixed flames is more dependent on the fuel characteristics rather than on the flame temperature.

Generally, it was assessed that mature soot produced from diverse hydrocarbon fuels with similar flame temperatures and flame types possess a different chemical composition and structure. To this regard the H/C atomic ratio and mass absorption coefficients appeared to be signatures of soot properties and structural evolution.