Sooting tendencies diffusion flames

The presence of halogen additives substantially increases the tendency of all fuels to soot under diffusion flame conditions [69], The presence of H atoms increases the soot pyrolysis rate because the abstraction reaction of H + RH is much faster than R + RH, where R is a hydrocarbon radical. Halogenated compounds added to fuels generate halogen atoms (X) at modest temperatures. The important point is that X + RH abstraction is faster than H + RH, so that the halogen functions as a homogeneous catalyst through the system... 

Order the following compounds in their tendency to soot under diffusion flame conditions vinyl acetylene, ethene, phenyl acetylene, benzene, and acetylene. 

In addition to CN and ON, the smoke point (SP), which is the maximum smoke-free laminar diffusion flame height, has been employed widely to evaluate the tendency of different fuels to form soot. This tool was first applied to kerosenes, later diesel, and then jet engine fuels.19,20 Researchers have tried to relate smoke points of pure compounds to their molecular structure. It was found that the inverse of smoke point, which measures the potential of a fuel to form soot, increases from n-paraffins to iso-paraffins to alkylbenzenes to naphthalenes.21,22 Since smoke points vary with experimental conditions, the concept of a threshold soot index (TSI), which is calculated from the smoke point, molecular weight, and experimental constants, has been used to compare the soot-formation tendencies of different fuel molecules.23... 

Sampling in inverse coannular diffusion flames [62] in which propene was the fuel has shown the presence of large quantities of allene. Schalla et al. [57] also have shown that propene is second to butene as the most prolific sooter of the n-olefins. Indeed, this result is consistent with the data for propene and allene in Ref. 72. Allene and its isomer methylacetylene exhibit what at first glance appears to be an unusually high tendency to soot. However, Wu and Kem [111] have shown that both pyrolyze relatively rapidly to form benzene. This pyrolysis step is represented as alternate route C in Fig. 8.23. 

It has been known for many years that molecular structure of a fuel has a direct bearing on the tendency of that fuel to smoke, i.e., to form carbon or soot in a flame. For example, in 1954 Schalla (41), reporting on a study of diffusion flames, indicated that the rate at which hydrocarbons could be burned smoke free varied in the order n-paraffins — mono-olefins — alkynes — aromatics. This same phenomena has been reconfirmed by many authors in a variety of systems and always in the same general order (j6, J3, J 5, 1 7, J 9, 26, 43, 45). Paraffins have the least tendency to smoke, whereas the naphthalene series have the greatest tendency to smoke. 

Frenklach et al. [78], who evaluated the sooting tendency of fuels by shock tube pyrolysis at various temperatures, found that the sooting rate increased with temperature, reached a maximum, and then decreased. The maxima occur in the range 1900-2300 K. The pyrolysis in cofiow diffusion flames is initiated at temperatures much lower than the stoichiometric temperature, so that the soot forms prior to reaching the maximum temperatures Frenklach et al. created in their shock tube experiments. In shock tubes the fuel is instantaneously exposed to very high temperatures thus the precursors that form decompose and the soot... 

The results presented in Fig. 17 for diffusion flames and those from shock tubes clearly indicate that fuel structure does indeed play a role in a fuel s tendency to form particulates—in significant contrast to the results observed in premixed flames. One may conclude, then, that a fundamental knowledge of a fuel s pyrolysis chemistry [51, 76] will allow one to predict its relative tendency to soot with respect to the results presented in Fig. 17. For example, cyclohexadienes are known to dehydrogenate to benzene during pyrolysis and, indeed, the data in... 

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