Photoionization Study of Soot Precursor Nanoparticles in Laminar Premixed Ethylene/Ethanol Flames

Abstract

The effect of ethanol addition in ethylene laminar premixed flames on the characteristics of soot precursor nanoparticles was investigated by aerosol photoionization technique. The fifth harmonic of a Nd:YAG laser, 213 nm (5.82 eV), was used as ionization source, while a differential mobility analyzer (DMA) coupled to a Faraday cup electrometer was used for ionized particle classification and detection. Carbonaceous nanoparticles in the nucleation mode, i.e., with sizes ranging from 1 to 10 nm, show a photoionization charging efficiency clearly dependent on the percentage of ethanol used as dopant in the flame. In particular, we observed that the more ethanol was added as fuel, the lower the particle photo-charging efficiency was. This result indicates a modification in the nanoparticle chemical structure as the amount of ethanol is increased. These experimental evidences may be explained by a decrease of aromaticity of the particles and/or by the presence of oxygen bonds within the nanoparticles

Chemical Features of Particles Generated in an Ethylene/Ethanol Premixed Flame

“…Particles were sampled along the flame axis for size distribution measurements using a stainless-steel probe placed horizontally in the flame. The probe has an internal diameter of 8 mm, a wall thickness of 0.5 mm, and a pinhole diameter of 0.8 mm, which is larger than those generally used in other works. ,, The large pinhole has the advantage of reducing the clogging of the hole, a problem often encountered in fully sooting flames when smaller pinholes are used. Although the perturbation of the flame for the effect of the probe and the large pinhole was evident, the system has proven to be reliable for the determination of the particle size distributions (PSDs), even in fully sooting flames with equivalence ratios as high as 2.46. ,, With the large pinhole, the dilution ratio in the probe line decreases and the possibility of coagulation of particles and condensation of gas-phase PAHs onto the particle surface increases.…”

“…The concentration of the smaller size particles is not significantly reduced, and in some cases, it is even increased by the addition of ethanol . Also, it appears that the oxidation and pyrolysis of ethanol as well as the environment in which it reacts have an impact on the amount, morphology, and chemical characteristics of the formed particles. , …”

“…Also, in comparison to the particle size distributions of ethanol-doped and neat ethylene flames, it has been found that, in slightly sooting flames, the addition of ethanol does not significantly affect particle coagulation, suggesting that the main effect relies on the main oxidation pathways that lead to the formation of particle precursors. Moreover, the presence of ethanol induces some modifications in the photoionization energy and presumably in the chemical features of the smaller particles …”

Oxygen driven soot formation

“…acetylene). Recent work has shown some evidence that oxygen concentration levels can impact the volume of particulates and soot formed [18], that oxygenated fuels can alter the amount of soot production [19], the chemical composition of nanoparticles [20,21], and that PAHs ranging from one to several hundred atomic mass units (u) are likely oxygenated in laminar premixed and counterflow diffusion flames [22][23][24].…”

“…In order to investigate the impact of oxygen on the chemical pathways of PAH evolution and on the propensity to initiate particle formation, in this paper we report a detailed analysis of ethylene-air and ethylene/ethanol-air premixed laminar flames [16] since recent work has suggested the addition of ethanol alters the particle size distributions [20,21]. The chosen flames make for an interesting case study to investigate the mechanisms leading to the formation of nanoparticles because they present similar measured temperature profiles as well as the same equivalence ratio and carbon flow rate, thus ensuring a similar gas-phase environment between the flames.…”