Investigation of critical equivalence ratio and chemical speciation in flames of ethylbenzene–ethanol blends

Abstract

This work investigates five different one-dimensional, laminar, atmospheric pressure, premixed ethanol/ethylbenzene flames (0%, 25%, 50%, 75% and 90% ethanol by weight) at their soot onset threshold ( ϕ critical). Liquid ethanol/ethylbenzene mixtures were pre-vaporized in nitrogen, blended with an oxygen–nitrogen mixture and, upon ignition, burned in premixed one-dimensional flames at atmospheric pressure. The flames were controlled so that each was at its visual soot onset threshold, and all had similar temperature profiles (determined by thermocouples). Fixed gases, light volatile hydrocarbons, polycyclic aromatic hydrocarbons (PAH), and oxygenated aromatic hydrocarbons were directly sampled at three locations in each flame. The experimental results were compared with a detailed kinetic model, and the modeling results were used to perform a reaction flux analysis of key species. The critical equivalence ratio was observed to increase in a parabolic fashion as ethanol concentration increased in the fuel mixture. The experimental results showed increasing trends of methane, ethane, and ethylene with increasing concentrations of ethanol in the flames. Carbon monoxide was also seen to increase significantly with the increase of ethanol in the flame, which removes carbon from the PAH and soot formation pathways. The PAH and oxygenated aromatic hydrocarbon values were very similar in the 0%, 25% and 50% ethanol flames, but significantly lower in the 75% and 90% ethanol flames. These results were in general agreement with the model and were reflected by the model soot predictions. The model predicted similar soot profiles for the 0%, 25% and 50% ethanol flames, however it predicted significantly lower values in the 75% and 90% ethanol flames. The reaction flux analysis revealed benzyl to be a major contributor to single and double ring aromatics (i.e., benzene and naphthalene), which was identified in a similar role in nearly sooting or highly sooting ethylbenzene flames. The presence of this radical was significantly reduced as ethanol concentration was increased in the flames, and this effect in combination with the lower carbon to oxygen ratios and the enhanced formation of carbon monoxide, are likely what allowed higher equivalence ratios to be reached without forming soot.