The connection between NOx and soot in oxygen-enriched propane flames

Soot and nitrogen oxide are two harmful pollutants that are formed during combustion and strict emission legislation continues to motivate research on how they can be controlled. The current paper investigates how the formation of nitrogen oxide and soot is influenced by increasing the oxygen content in the oxidizer. Propane is combusted (80 kWth) applying oxygen-enriched air. In-flame measurement of temperature as well as soot and gas concentrations are performed with a combination of intrusive and non-intrusive equipment. The results show that increasing the oxygen content in the oxidizer from 21% to 27% increases the formation of nitrogen oxide while soot formation remains relatively low. At 30% however, soot formation increases by order of magnitudes and the emission of nitrogen oxide drops significantly. Detailed reaction modeling is performed and the shift in soot formation is captured by the model. Although the exact reason for the shift is not completely understood, the model shows that for a flame after the shift, soot inception initiates earlier in an environment where the concentration of acetylene and PAHs are relatively high, leading to fast surface growth. High temperatures in the early combustion sequence caused by higher oxygen content is believed to be the controlling factor, rather than the increased oxygen content itself.