Abstract
The advantages of well-defined experimental conditions as provided by a plug flow reactor (PFR) are used to make an approach toward a phenomenological and quantitative description of soot mass growth in flames and combustion systems. On the basis of gravimetrically determined residence time-resolved soot concentration profiles, a quantitative correlation is proposed including the effects of temperature, stoichiometry, and residence time as the main parameters of influence. Gaseous hydrocarbon fuels, such as methane, propane, and acetylene, have been investigated concerning their differences and parallels in soot formation. It was shown that a simple autocatalytical approach and an assumed superposed deactivation process of the soot surface is sufficient to interpret experimental results in a wide range of temperature, fuel, residence time, and stoichiometry conditions. Under approximately isothermal conditions, as realized in the PFR, soot growth rate increases with time up to a maximum value after a certain reaction time that is dependent exclusively on temperature. With increasing residence times beyond that maximum, soot growth decreases and finally ceases. The influence of temperature on these profiles, in particular the reaction time at which the maximum growth rate appears, is given in terms of an Arrhenius expression, independent of fuel specification. The activation energy was evaluated at 192 kJ/mol. Stoichiometry was found relevant for the final soot concentration after long residence times. An expression using the C/O ratio as a correlating parameter is proposed in accordance with recent literature. The remaining influence of temperature on the “final” soot concentration is discussed more qualitatively, with respect to an additional influence of a subdivided stoichiometry as realizable in the plug flow reactor, to try to distinguish premixed and turbulent diffusive combustion. A final soot yield decrease was observed when increasing the temperature from about 1200 to 1650°C.
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