Abstract Combustion attracts a high scientific interest due to the number of applications that range from power generation to automotive and airplane engines. In general, combustion-based technology poses significant challenges when it comes to maximize the performance while reducing emissions. Incomplete combustion processes release, among other pollutants, soot particulate into the atmosphere. The environmental impact of these by-products have led to the development of experimental techniques that contribute to a better understanding of the fundamental physical and chemical combustion processes. The characterization of adsorbed volatile organic compounds on these particles may help to track the emitting source and to identify the processes leading to their incomplete conversion.
For this reason we pay special attention to simpler combustion sources, by investigating the formation of large hydrocarbons inside laboratory flames at different combustion stages (from below the soot inception up until the oxidation regions). In this context, two methane low pressure premixed flames are stabilized in conditions that result in similar concentration profiles of known polycyclic aromatic hydrocarbons (PAH), and molecular clustering leading to soot nucleation beginning at the same reaction time, or equivalently at the same height above the burner (HAB). After the soot inception zone, the gas phase and particulate matter coexist all along the flame height for the two premixed flames. Alternatively two methane diffusion flames are stabilized at atmospheric pressure in similar aerodynamic conditions. However, in stark contrast to the previous case in which soot and molecular precursors coexist all along the reaction coordinate, the diffusion flames show a net stratification of the soot (downstream the flame tip) and molecular precursors (near the fuel injection point) regions that are sufficiently far to be probed independently and with little interference from each other.
Many theoretical and experimental evidences point to the role of PAH as soot precursors. Therefore, understanding their chemistry will lead to a better understanding of the soot formation process. This work proposes a unique approach and methodology for the characterization of soot particle surface and condensable gas phase collected from laboratory flames at different (HAB). Secondary ion mass spectrometry (SIMS) is used for identifying the compounds adsorbed on the surface of soot samples. The high mass resolution ~7000 allows attributing PAH and oxygenated hydrocarbons as the main contributors to the chemical reactions leading to soot nucleation. Additionally, statistical methods as principal component analysis (PCA) and hierarchical clustering analysis (HCA) are used for data interpretation. These detailed analyses provide an innovative approach when dealing with complex mass spectra of soot. Once the sampling conditions were taken into account, we succeeded to identify similar conditions of soot evolution in flames where the growth/oxidation mechanisms exist (for one premixed flame and the two diffusion flames). Multivariate data analysis aids to separate the species involved in the soot nucleation phase suggesting that a broad range of PAH are participating to this process.