On the effect of pressure on soot nanostructure: A Raman spectroscopy investigation
Introduction
Soot nanoparticles are known to have detrimental effects on human health, climate, and air quality [1], [2], [3]. As a consequence of these concerns, significant research efforts have been put into understanding and explaining the chemical and physical processes behind the mechanisms of soot formation in fuel-rich flames. Although the majority of existing combustion technologies operate at pressure conditions ranging from moderate to high, most of our understanding of the soot formation process relies on studies performed at atmospheric condition. Pressure is known to strongly affect flame burning properties and soot formation [4]. It is established that pressure significantly increases soot loading [4]; as the pressure is increased in a combustion chamber, more soot is formed and released. However the amount of soot in an exhaust also depends on the extent of oxidation in the chamber. Nevertheless, soot concentration is not the only parameter to be considered. Soot particle size distribution, morphology, and nanostructure are also important parameters since they directly affect the soot oxidation rate as well as particle optical properties [5,6]. However, probing and characterizing soot from high pressure environments is not a trivial task, and only a few attempts have been made so far [7], [8], [9], [10], [11], [12], [13], [14]. Therefore, a clear understanding of the effect of pressure on particle size/morphology and nanostructure is far from being established firmly.
Recently, in an attempt to probe the dependence of soot nanostructure on pressure, we used Raman spectroscopy to characterize soot particles collected from several methane laminar diffusion flames operated under a variety of pressure conditions [10]. It was demonstrated that soot nanostructure was primarily affected by soot residence times rather than pressure [10]. Soot particles were shown to graphitize at an increasing rate when collected at increasing distances from the burner rim, while no differences were observable by changing the pressure of the system [10].
Compared to methane flames, ethylene flames are known to display significant differences in terms of soot formation and properties. In addition to the considerably different soot propensities, the dissimilar chemical structures of the two fuels may also affect their soot characteristics, such as particle size, morphology and structure [9,11,[15], [16], [17]. Thus, to complement our previous work [10] and achieve a better understanding of the effect of pressure on soot nanostructures, we carried out Raman spectroscopy measurements in this work on soot samples extracted from ethylene diffusion flames at various pressures.
Section snippets
Experimental
Soot samples were collected thermophoretically from a set of nitrogen diluted ethylene laminar diffusion flames operated under various pressures. A high pressure combustion chamber capable of stabilizing laminar diffusion flames on a coflow burner was used. The high-pressure combustion chamber, housing the burner and the integrated thermophoretic soot sampling system has been thoroughly described elsewhere [8], [9], [10], [11], [12]. Briefly, the burner, installed in the high-pressure chamber,
Results and discussion
To track the evolution of soot particles, the ethylene laminar diffusion flames operated at several pressures, i.e., 3, 6 and 10 bar, have been characterized by SSE measurements. Two-dimensional depictions of the soot volume fraction (SVF) measurements for flames operated under these three pressures are reported in Fig. 2.
Soot volume fractions increase significantly with increasing pressures, which is also illustrated by the trend of increasing maximum soot yields shown in Fig. 3. For all the
Conclusions
In this study, we investigated nanostructure of soot particles collected in a set of ethylene laminar diffusion flames at different pressures from 1 to 10 bar. Soot volume fraction and temperature measurements by spectral soot emission diagnostic have been performed for flames operated under different pressures. The flame conditions corresponding to soot inception and maximum volume fraction were selected, and the corresponding soot particle nanostructure is analyzed by Raman spectroscopy.
The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by Natural Sciences and Engineering Research Council of Canada (RGPIN-2017-06063) and by the “Accordo di Programma CNR-MSE Ricerca di Sistema Elettrico” under the project “MIcro co/tri generazione di Bioenergia Efficiente e Stabile (Mi-Best)”.
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