Sooting tendencies

Most of the early work and more recent efforts on soot formation under premixed conditions were conducted with air as the oxidizer. These early results reported data as a critical sooting equivalence ratio fic, in which the higher the fic, the lower the tendency to soot. As shown in Table 8.5, the trend observed followed the order 

The most extensive early data of sooting under laminar diffusion flame conditions, as measured by the smoke height method, were obtained by Schalla et al. [57, 58], The general trend observed followed the order 

TABLE 8.5 Critical Sooting Equivalence Ratios ( / c) Premixed with Air of Various Fuels 

FIGURE 8.13 Critical sooting equivalence ratios ( / c) based on CO and H20 stoichiometry of various hydrocarbon fuels as a function of calculated adiabatic flame temperature Tf. 

FIGURE 8.14 Critical sooting equivalence ratio l c at 2200K as a function of the number C—C bonds in hydrocarbon fuels. +, 0, and - indicate ethane/l-octane mixtures in molar ratios of 5 to 1, 2 to 1 and 1 to 2, respectively x, acetylene/benzene at a molar ratio of 1 to 3. The O symbol for 2 to 1, falls on top of the butene symbol. 

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Estimates of fuel sooting tendency have been made using various types of flames and chemical systems. In the context to be used here, the term sooting tendency generally refers to a qualitative or quantitative measure of the incipient soot particle formation rate. In many cases, this tendency varies strongly with the type of flame or combustion process under investigation. This variation is important because the incipient soot particle formation rate determines the soot volume fraction formed in a combustion system. 

FIGURE 17 Sooting tendency of some hydrocarbon fuels plotted as the log of the reciprocal of the fuel mass flow rate at the smoke height versus the reciprocal of the calculated adiabatic flame temperature. 

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... 

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