Formation of nascent soot during very fuel-rich oxidation of ethylene at low temperatures

Soot formation during very fuel-rich oxidation of ethylene (with equivalence ratio, φ, ranging from 3 to infinity) at relatively low temperatures (1473–1673 K) was studied in a flow reactor, in which temperature was well controlled through an external heater, and φ of the reactants was adjusted by adding different amounts of O₂ (0–6000 ppm) at the inlet of the reactor. Detailed sooting properties including soot volume fraction, number density, particle size distribution (PSD), and morphology were examined. The results show that at a temperature of 1673 K, the total soot yield increases monotonously with φ. However, at 1473 and 1573 K, the soot volume fraction increases first and then decreases as φ increases from 3 to infinity, which means a small amount of oxygen can promote soot formation. The PSD and number density results indicate that this promotion effect is more prominent at the nucleation stage and weakens as soot grows at longer residence times. In addition, soot morphology is affected mostly by temperature rather than φ, and the decrease of temperature leads to larger primary particles and less fractal aggregates. Kinetic modeling results indicate that the concentrations of PAHs and soot increase because a small amount of O₂ addition can enhance the C₃ + C₃ pathways through the generation of reactive oxygenated species.