EFFECT OF FUEL MOLECULAR STRUCTURE AND FUNCTIONALITIES ON COMBUSTION-GENERATED NANOPARTICLES

Atmospheric pollution is one of the most discussed topics of recent decades. Short and long-term effects on climate, flora, fauna, and human health are visible now. Thus a great slice of the research world is addressing its forces towards the study of possible ways to put down pollutant emissions. Many different routes are followed in order to address this purpose: after treatment of car systems, fuel reformulation or substitution, severe legislation, etc. Despite of the efforts, a definitive solution has not yet been found. In particular it seems that all mechanisms adopted until now to reduce emissions of carbonaceous particulate leave unaltered or in certain cases enhance the concentration of fine and ultrafine particles. Understanding how fuels break down and react in a combustion environment is essential to be able to act on the particles reduction. This thesis has indeed the purpose to investigate how the fuel features and its amount influence the production of nano- and soot particles in lab-scale reactors performed in different conditions. Two classes of compounds have been studied in this work: furanic fuels and alkylbenzenes. Furanic fuels derive from green source, lignocellulosic biomass, and are seen as possible engine fuels substitute because of their properties and costs, but a deep study on their effect on nanoparticle production has to be carried on. Alkylbenzenes are components of real engine fuels and in spite of some their advantageous features as anti-knocking and the high energy content, they are strong soot precursors enhancer. An experimental study has been conducted in laboratory atmospheric flames performed in two different mixing configurations: laminar premixed and opposed-flow diffusion flames. For the analysis of the particle production in flame laser induced emissions and temperature measurements have been performed as in-situ diagnostic techniques, while particles size distribution functions have been obtained by differential mobility analysis system as ex-situ technique. Results have shown for alkylbenzenes a strong dependence of their sooting propensity on the length and branching of the side alkyl chain, and a particular behavior of xylenes in oxidative environment has been observed. Furanic fuels have shown a very different behavior depending on the mixing flame configuration passing from the role of soot reducer in premixed flame, to the role of soot enhancer in opposed-flow diffusion flame, especially in pyrolytic environment. It is worth noting that in premixed flame configuration it was found that although furans behave as soot reducer, they do not have effect on nanoparticles concentrations; they even entail particles enhancement in certain conditions. A comparison with other oxygen-containing compounds previously studied in the same operating conditions has been also reported to better understand the role of the fuel molecular structure. Finally it was verified that the different techniques are in agreement among them allowing to draw definite conclusions about the effect of these fuels on particulate formation in the explored conditions

Effect of furanic biofuels on particles formation in premixed ethylene-air flames: An experimental study

Furanic fuels have been recently investigated because they are considered possible fossil fuel substituents due to their high energy density, their renewable origin and consequently low green-house gas emissions. Few data are available about the behaviors of these fuels in laboratory flame reactors. We have recently studied their sooting tendency in a counter-flow diffusion flame showing a different propensity to form particulate matter depending on the richness of the combustion conditions. In this paper, we explore their tendency in forming particulate matter in atmospheric pressure premixed flames burning mixtures of ethylene and furanic fuels at four different equivalence ratios, namely 2.01, 2.16, 2.31 and 2.46, thus covering the range from slightly sooting to fully sooting conditions. Liquid furan, 2-methylfuran and 2,5-dimethylfuran have been added to ethylene as 10% and 20% of the total carbon fed to the flame. In-situ spectroscopy, namely laser UV-induced emission, has been used as diagnostic tool to detected different classes of precursor nanoparticles by changing the detection wavelength from the UV to the visible. Laser induced incandescence has instead been used to detect soot particles. Equivalence ratios, the total carbon flow rate and cold gas velocity have been kept constant, allowing the combustion temperature to be almost the same also when furans were added. LII signal decreases significantly when furans are added showing a strong effect of furanic fuels in decreasing the amount of soot formed in atmospheric pressure premixed flames. Particularly the reduction increases as the equivalence ratio increases. On the other side, LIF UV and LIF VIS slightly decrease when furans are added. The absence of a significant reduction of the LIF UV signal when furans are added suggests that small particle formation is not effectively inhibited by these additives. This behavior is similar to what found for ethanol and dimethyl ether addition to ethylene premixed flames, showing a similarity in the behavior of these oxygenated fuels

Effect of alkylated aromatics on particle formation in diffusion flames: An experimental study

The sooting tendency of four C8H10 isomers, specifically ethylbenzene, ortho-, meta- and para-xylene, has been studied in atmospheric pressure, counter-flow diffusion flames of ethylene, whose fuel stream was doped with 10%, 20% and 30% of C8H10 isomers. In-situ spectroscopy, namely laser UV-induced emission, is used as diagnostic tool to detected different types of combustion-formed nanoparticles by changing the detection wavelength from the UV to the visible. Laser induced incandescence has been also measured to detect soot particles. Experimental results are compared to those obtained in an ethylene/toluene flame operated in the same conditions to understand the effect of the number and position of the alkyl groups and of the alkyl chain lengths on particulate formation. The experimental results show that the effect of the alkyl chain on soot tendency strongly depends on the combustion environment. In low temperature and pyrolytic zones of the flame, the long chain promotes nanoparticle formation. Aromatics containing non-adjacent methyl groups are more stable and hardly react forming small quantities of nanoparticles. In high temperature and oxidative zones, the behavior is the same but nanoparticles are produced in lower amount, especially for higher percentages of aromatic fuel addition. Regarding sooting tendency, in the pyrolytic region of the flame the presence of a longer alkyl chain gives a much stronger soot production, whereas in oxidative region the differences among the additives are smaller. The strongest increase of soot production occurs moving from the methyl to ethyl group addition to the aromatic ring showing that, in all the experimental conditions we have examined, a shorter chain and the presence of methyl groups positioned far from each other on the aromatic ring give the lowest sooting tendency

Characterization and Inflammatory Potential of sub-10nm Particles from Gas Cooking Appliances

Combustion generated ultrafine particles are believed to have an effect on human health. Their presence in the atmosphere is mainly attributed to outdoor sources, but they may also form indoor. Gas cooking is a widely diffused indoor activity commonly considered environmentally clean, and without emissions of particulate matter. However, even bluish flames of natural gas may produce considerable number concentrations of sub-10nm particles if operating conditions deviate from stoichiometry and mixing at atomic level. These particles negligibly account for particulate mass but, due to their very low sizes, they can deposit far inside the airways and on skin and potentially reach target organs being dangerous although present in low mass concentrations. We have characterized the exhausts of a domestic cooktop burner measuring stable compounds, gas-phase aromatic compounds and particulate matter and collected nanoparticles for in vitro toxicological studies and for the analysis of their possible inflammatory effects. Combustion exhausts, including polycyclic aromatic hydrocarbons (PAH) and nanoparticles, have been sampled above a mid-range cooktop burner fed with network natural gas. Tests have been performed in a free flame and by putting a pot on the burner in order to simulate operating conditions closer to those of the real life. Speciation of PAHs and the distribution of the particles generated during combustion has been measured. Results of measurements show that the cooktop burner flames produce and emit low concentrations of PAHs and huge number concentrations of sub-10nm particles. Tests on cell viability performed with crystal violet assay shows no significative reduction in cell number after 24h of treatment, both with nanoparticles collected in a “free flame” and in the operating conditions with “a pot on the fire”. It is interesting to note a little positive effect in increasing cell number (+20%) at the lowest concentration. No relevant overexpression or downregulation is noted on the secretion of the 27Plex Panel of Human Cytokine, performed with Bio-Plex 200 system