The study of soot formation and oxidation in flames has long been undertaken by many researchers worldwide for many decades and is still receiving greater considerations at present and throughout the future. Several reasons for such great concerns are evident. First, the emissions of soot cause an environmental degradation of the air quality; giving rise to serious health hazards, lowering the efficiency of practical combustion systems and increasing deposition on the surfaces of furnaces. This lowers the efficiency and may form hot spots with their serious effects on the lifetime of the equipment and/or increased maintenance cost. On the other hand, the existence of soot may be demanded to enhance radiation heat transfer in part of the combustor volume, provided that it should be fully oxidized before being emitted to the environment.
The present study included a fundamental experimental investigation that aimed to assess the effects of different fuel additives on the formation and oxidation regimes of a well-defined flame configuration, namely vertical laminar natural gas (NG) diffusion flames. These additives included (i) a diluent (Argon) that suppresses the formation of soot and (ii) a soot promoter (Acetylene) that accelerates and intensifies the soot formation.
The axial and radial distributions of the mean gas temperature and soot volume fractions were presented and analyzed for selective cases. The digital visual images were analyzed to yield the variations in the lengths of the soot inception zone, soot surface growth zone and soot oxidation zones for all cases.
The results indicated that, the soot inception zone (deep dark parabolic shape) occurred at the immediate vicinity of the burner; whereby molecular diffusion took place with partial pyrolysis and oxidation of the fuel molecules, resulting in increased number of fine soot particles. Rapid temperature rise was recorded within this zone; indicating that combustion followed a premixed mode due to molecular mixing between fuel and entrained air molecules.
The temperature within the soot surface growth zone (orange color) continued rising but at a lower rate that reflected the domination of diffusion combustion mode. Limited partial oxidation may be anticipated within this zone due to the relatively high temperature, which was not high enough to cause luminosity of the soot particles.
The progressive increase of Argon (diluent) and/or NG decreased the lengths of both the inception and surface soot growth zone, while the increase of Acetylene in the fuel mixture would ultimately lead to almost its disappearance due to its accelerating effect on soot formation.
The soot oxidation zone was characterized by high luminosity and it began after the fuel was largely consumed. The increased percentages of Acetylene in the fuel mixture would lead to extending the length of this zone to ultimately occupy the whole visible flame length; where the luminosity becomes independent of the amount of soot.