Formation of Soot in Oxygen-Enriched Turbulent Propane Flames at the Technical Scale

Soot is an important component for heat transfer in combustion processes. However, it is also a harmful pollutant for humans, and strict emissions legislation motivates research on how to control soot formation and release. The formation of soot is known to be triggered by high temperature and high pressure during combustion, and it is also strongly influenced by the local stoichiometry. The current study investigates how the formation of soot is affected by increasing the oxygen concentration in the oxidizer, since this affects both the temperature profile and partial pressures of reactants. The oxygen-to-fuel ratio is kept constant, i.e., the total flow rate of the oxidizer decreases with increasing oxygen concentration. Propane is combusted (80 kWth) while applying oxygen-enriched air, and in-flame measurements of the temperature and gas concentrations are performed and combined with available soot measurements. The results show that increasing the oxygen concentration in the oxidizer from 21% to 27% slightly increases soot formation, due to higher temperatures or the lower momentum of the oxidizer. At 30% oxygen, however, soot formation increases by orders of magnitude. Detailed reaction modeling is performed and the increase in soot formation is captured by the model. Both the soot inception rates and surface growth rates are significantly increased by the changes in combustion conditions, with the increase in soot inception being the most important. Under atmospheric conditions, there is a distinct threshold for soot formation at around 1200 °C for equivalence ratios >3. The increase in temperature, and the slower mixing that results from the lower momentum of the oxidizer, have the potential to push the combustion conditions over this threshold when the oxygen concentration is increased.