De Filippo, Andrea (2008) Characterization of carbonaceous nanoparticle size distributions (1-10 nm) emitted from laboratory flames, diesel engines and gas appliances. [Tesi di dottorato] (Inedito)

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Abstract

Commercially available SMPS and a differential mobility analyser (DMA) designed to enable detection of particles as small as 1 nm were used to provide further information about nanoparticles emitted from premixed flames, engines and common burners, with particular attention to the 1-10 nm range. Premixed laboratory flames were studied to examine carbonaceous nanoparticle formation in fuel rich burning conditions near the onset of soot formation. Nanoparticles were measured at different carbon/oxygen (C/O) ratio and at different heights above the burner surface in order to evaluate their behaviour in high temperature conditions. The measured size distributions showed that the first particles observed in flames have a size of ~2nm (Mode I), consistent with previous in situ measurements by light scattering and extinction (LSE), size distributions determined by Atomic Force Microscopy (AFM) of particles deposited by thermophoresis on mica substrates and off-line size measurements of material captured in water samples from the same flames. A larger 3-7 nm particle mode (Mode II) was measured in richer flames and later in the flame, which was not previously distinguished by optical measurements, which can only give the d6-3 diameter (ratio of the 6th and 3rd moments of the size distribution) or in the size distributions determined by AFM, which had lower resolution. This larger mode is also not observed in water samples collected from flames even when their concentration is as large as or larger than the 2nm mode, presumably because the larger particles are hydrophobic. The evolution of these two nanoparticles modes was studied in a flame with C/O = 0.65 at different height above the burner surface. The results were consistent with the conceptual framework for particle inception, advanced in earlier works based on UV-visible optical measurements, and for particle coagulation which seemed to increase because of the appearance of mode II, in agreement with the size-dependent coagulation rate addressed in previous studies based on optical measurement and atomic force microscopy (AFM). To begin addressing the question of whether or not such small particles are also found in the exhausts/emissions of applied combustion systems were they may be inhaled by hu-mans or interact with the atmosphere, size distributions of nanoparticles generated by a burner, a test engine and diesel vehicles were measured. The emissions from a test bench single-cylinder engine and from two modern light-duty diesel vehicles run with commercial ultralow sulphur fuel indicated the presence of a “solid” particle nucleation mode (~ 10nm or lower) which accompanied normal soot emissions (~ 60 nm). In diesel vehicle tests, this mode, most prominent at idle, was highly sensitive to the level of exhaust gas recirculation (EGR), non-volatile to temperature higher than 400 C and electrically charged. All of these characteristics suggest that these solid particles were formed during combustion and not by lower temperature condensation processes as the exhaust cools. An important conclusion of these tests was also that the specific diesel particulate filters employed removed the “solid” nucleation mode and the soot with efficiency comparable to soot. Tests on bench single-cylinder engines again indicated that the smallest nanoparticles were highly influenced by the EGR levels but also the type of injections and the type of fuel employed have an effect on the emitted particulate. Preliminary measurements on domestic gas appliances burning methane also showed the presence of nanoparticles in the size range 1-3 nm in the plume near the burner, in agreement with optical absorption measurements and time resolved fluorescence polarization anisotropy (TRFPA) analysis on water samples from the same flames.

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