Sooting characteristics of partially-premixed flames of ethanol and ethylene mixtures: Unravelling the opposing effects of ethanol addition on soot formation in non-premixed and premixed flames

Abstract

Oxygenated biofuels such as bioethanol and biodiesel are widely used as clean fuel additives in practical combustion devices to reduce emissions of greenhouse gases and soot particles. Literature data showed that the effects of the oxygenates may differ under different combustion conditions. In particular, ethanol addition was seen to monotonically decrease soot formation in ethylene premixed flames. However, doping a small amount of ethanol would enhance soot formation in ethylene non-premixed flames. Although this dependence of the effects of ethanol doping on flame configuration has been observed experimentally, detailed kinetic explanation is yet to be presented. In this work, we performed a combined experimental and numerical study to explore the underlying mechanisms responsible for the reversal of the effects of ethanol addition on soot formation in partially premixed flames with increasingly higher level of partial premixing ratio. We showed experimentally that the soot synergistic effect of ethylene and ethanol as observed in non-premixed flames became weaker and eventually disappeared with the increase of the premixing ratio. These new findings suggest the important role of oxygen-related reactions in explaining the opposing effect of ethanol addition in non-premixed and premixed flames. Numerical simulations show that the presence of oxygen in partially premixed flames (PPF) enhance the formation of C1 species from ethylene; as a result, ethylene PPF exhibit low sensitivity to the additional C1 species produced from ethanol decomposition, which is the main reason for ethanol’s soot enhancing effect in non-premixed flames of ethylene/ethanol mixtures. The opposing effect of ethanol addition on soot formation in non-premixed and premixed flames has finally been elucidated with kinetic information provided by our results on partially premixed flames.