PACs formation and interaction in semipractical flames of liquid fuels

Abstract

Results of experiments carried out in semipractical turbulent diffusion flames of heavy fuel oil and its water-emulsion (E10) doped with 10% water are reported. Variations in axial and radial concentrations of Polycyclic Aromatic Compounds (PACs) and their interactions with soot particles were studied. PACs were identified/determined by GC-MS, while soot particles were subjected to particle size analysis by electron microscopy and quantitative image analysis. Thirty-eight PACs were identified in oil and 22 in emulsion flames. High concentrations of some of the PACs detected in the flame (e.g., naphthalene, pyrene) are attributed to their presence in the fuel. Other compounds (e.g., fluorene) were formed very quickly in the initial combustion stages due to pyrosynthesis and decomposition of higher hydrocarbons. Apart from PACs containing only carbon and hydrogen elements, PACs containing heteroatoms of nitrogen, sulfur and oxygen were also found (e.g., 10-azabenzo(a)pyrene, dibenzo(b,d)thiophene, anthraquinone). PACs were mainly formed in the high-temperature fuel-rich region (0.2–0.3 m from the burner nozzle). At a distance over 0.5 m from the burner, PAC destruction predominated. This was caused by their direct transformation to soot and decomposition due to oxidation and dehydrogenation (one-ring aromatic species and aliphatic hydrocarbons from C 8 to C 24 were mainly formed). Distinctly higher PAC concentrations were found during the combustion of oil, especially in the high-temperature flame zones. This was connected with more intensive pyrolysis/pyrosynthesis processes in fuel oil flames and then the faster PAC formation. The explosive burning of water-emulsion droplets also influences this behavior. The results indicated that the fuel oil combustion in the form of water-emulsion lowers PAC formation in flames and their emissions to the atmosphere. An evident decrease in pyrene and fluorene concentrations was observed, in particular in water-emulsion flames where microexplosion of droplets in the fuel-atomized stream intensified droplet evaporation and fuel vapor mixing with oxidizer, and thus the oxidation process. Although fluorene decomposition was significant in the radial direction, this process was more intensive with the growth of the distance along the flame axis. It was confirmed that the temperature-time history that the pyrolyzing fuel undergoes is a very important physical parameter affecting PAC formation/destruction in semipractical oil flames. It was also showed that PACs can fulfill the role of precursors for soot particles formed in turbulent diffusion heavy liquid hydrocarbon-air flames, playing an essential part in their formation and growth during combustion.