The formation of soot and polycyclic aromatic hydrocarbons in diffusion flames— part one

Abstract

The amounts of chloroform soluble material, polycyclic aromatic hydrocarbons (PCAH) and carbonaceous residue (CR), in the dry soot recovered from an ethylene diffusion flame have been determined and the effects on these of changes in the oxygen index (i.e. the mole fraction of oxygen in the mixture) of oxygen-nitrogen and oxygen-argon mixtures supplied to the flame have been investigated. In the oxygen-nitrogen mixtures, there is a rapid decline in the amount of PCAH in the soot as the oxygen index increases from 0·18 to 0·26. The amount of soot reaches a maximum at an oxygen index of about 0·26 and above this value it declines rapidly being then composed almost entirely of carbonaceous material. The decline in the amounts of individual PCAH in the soot with increase in oxygen index has been investigated. Substitution of nitrogen by argon leads to higher temperatures in the reaction and pyrolysis zones and at lower oxygen indices the effect is to favour the formation of CR and to lead to a reduction in the amount of PCAH in the soot. At higher oxygen indices, the amounts of soot (actually CR) are less. From the point of view of air pollution, the results emphasize the undesirability of either a general, or local, depletion of oxygen.