Ultra-fine soot investigation in flames

Abstract

In-flame soot particle size and concentration analysis techniques were developed for high concentrations of ultra-fine soot particles (particle diameters<< 1000 nm) using a Scanning Mobility Particle Sizer (SMPS). The SMPS particle size results were compared to results obtained by thermo phoretic sampling and Transmission Electron Microscopy (TEM). SMPS particle concentration results were compared to mass measurements obtained by filter collection and gravimetric techniques. The SMPS analysis techniques were then used to investigate soot oxidation in fuel rich flames. Soot sampling techniques for polycyclic aromatic hydrocarbon (PAH) analysis were also developed and used to obtain P AH information on soot generated from methane/oxygen premixed flames. The SMPS analysis was performed for SMPS inlet flowrates of 1 lpm and 2 lpm while keeping a constant ratio of inlet to sheath air flowrate of 1 : 10. The 2 lpm flowrate results gave much smaller particle diameters compared to the 1 lpm flowrate results. The shift in particle diameters was around 6 nm. This shift can not be explained by diffusion losses alone because, in some cases, an increase in mean particle diameter is accompanied by an increase in number concentration. The cause of this shift is unknown at this point, but the problem seems to reside in the SMPS instrument itself. The SMPS manufacturer has recently confirmed the shift when sampling polydispersed aerosols. Comparison of the SMPS results with mass measurements is very good for the 1 lpm case, but not for the 2 lpm case. Conversely, comparison of the SMPS results with TEM measurements is very good for the 2 lpm case, but not good for the 1 lpm case. Therefore, the true particle distribution has not been determined conclusively from this analysis. More investigation into the diameter shift phenomenon needs to be done before mass and TEM comparisons can be conclusive. Oxidation was studied in fuel rich ethylene/air/nitrogen flames using a Jet-stirred Reactor/Plugflow Reactor (JSR/PFR) system with oxygen injection at the beginning of the PFR section. An initial amount of oxygen injection was found to increase soot particle size and number concentration most likely due to the increased temperature resulting from the oxygen and combustion products reacting. Further increasing the amount of oxygen injection reduced soot particle number concentration, and eventually decreased the particle mean diameter. P AH analysis of methane/oxygen flame-generated soot revealed that cyclopenta[cd]pyrene, a know mutagen, was the most abundant species aside form pyrene. Cyclopenta[cd]pyrene concentration relative to pyrene increased significantly with soot flame residence time. Other known mutagens were detected in the soot samples including benzo[a]pyrene and some oxy-PAH.