PACS FORMATION AND INTERACTION IN SEMIPRACTICAL FLAMES OF LIQUID FUELS

Results of experiments carried out in semipractical turbulent diffusio n 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, whil e soot particles were subjected to particle size analysis by electron microscopy and quantitative image analysis. Thirty-eight PACs were ide ntified in oil and 22 in emulsion flames. High concentrations of some of the PACs detected in the flame (e.g., naphthalene, pyrene) are attr ibuted to their presence in the fuel. Other compounds (e.g., fluorene) were formed very quickly in the initial combustion stages due to pyro synthesis and decomposition of higher hydrocarbons. Apart from PACs co ntaining only carbon and hydrogen elements, PACs containing heteroatom s 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 n ozzle). At a distance over 0.5 m from the burner, PAC destruction pred ominated. This was caused by their direct transformation to soot and d ecomposition due to oxidation and dehydrogenation (one-ring aromatic s pecies and aliphatic hydrocarbons from C-8 to C24 were mainly formed). Distinctly higher PAC concentrations were found during the combustion of oil, especially in the high-temperature flame zones. This was conn ected with more intensive pyrolysis/pyrosynthesis processes in fuel oi l flames and then the faster PAC formation. The explosive burning of w ater-emulsion droplets also influences this behavior. The results indi cated that the fuel oil combustion in the form of water-emulsion lower s PAC formation in flames and their emissions to the atmosphere. An ev ident decrease in pyrene and fluorene concentrations was observed, in particular in water-emulsion flames where microexplosion of droplets i n the fuel-atomized stream intensified droplet evaporation and fuel va por mixing with oxidizer, and thus the oxidation process. Although flu orene decomposition was significant in the radial direction, this proc ess was more intensive with the growth of the distance along the flame aids. It was confirmed that the temperature-time history that the pyr olyzing fuel undergoes is a very important physical parameter affectin g PAC formation/destruction in semipractical oil flames. It was also s howed that PACs can fulfill the role of precursors for soot particles formed in turbulent diffusion heavy liquid hydrocarbon-air flames, pla ying an essential part in their formation and growth during combustion .