Characterization Techniques Coupled With Mathematical Tools for Deepening Asphaltenes Structure

Asphaltenes are the heavy fraction of fossil fuels, whose characterization remains a very difficult and challenging issue due to the still-persisting uncertainties about their structure and/or composition and molecular weight. Asphaltene components are highly condensed aromatic molecules having some heteroatoms and aliphatic functionalities. Their molecular weights distribution spans in a wide range, from hundreds to millions of mass units, depending on the diagnostic used, which is mainly due to the occurrence of self-aggregation. In the present work, mass spectrometry along with size exclusion chromatography and X-ray diffraction analysis have been applied to asphaltenes for giving some further insights into their molecular weight distribution and structural characteristics. Relatively small polycyclic aromatic hydrocarbons (PAHs) with a significant degree of aliphaticity were individuated by applying fast Fourier transform (FFT) and double bond equivalent (DBE) number analysis to the mass spectra. X-ray diffraction (XRD) confirmed some aliphaticity, showing peaks specific of aliphatic functionalities. Size exclusion chromatography indicated higher molecular weight, probably due to the aliphatic substituents. Mass spectrometry at high laser power enabled observing a downward shift of molecular masses corresponding to the loss of about 10 carbon atoms, suggesting the occurrence of aryl-linked core structures assumed to feature asphaltenes along with island and archipelago structures

Thermal treatment under high-vacuum of tars relevant in combustion and material science

The composition of tars, typically derived from coal and heavy fuel processing or formed in fuel-rich combustion, determines their transformation into carbons relevant in combustion and environmental fields as well as for material production. The speciation of the huge number of aromatic components of tars, usually found in form of viscous black liquid or solid, is not straightforward because of the tar complexity and high molecular weight, spanning from few hundreds up to thousands of Da. To this regard, the pre-separation of tar in lighter and heavier fractions simplifies the further characterization of its composition. The present work reports a fractionation method of a typical sample of combustion-formed tar based on moderate heating in high-vacuum conditions (10-6 mbar). It was preliminarily tested on a single polycyclic aromatic hydrocarbon, coronene, and on synthetic mixtures of polycyclic aromatic hydrocarbons, presumed to be the basic aromatic moieties of tar components. Lighter components obtained by condensation/deposition as thin films and/or crystals, as well as the heavier residue, were analysed by optical microscopy and spectroscopy. The separation procedure allowed to get more information on the components distribution also inferring the self-organization in cluster assembly and/or crystal forms

One-step synthesis of carbon nanoparticles and yellow to blue fluorescent nanocarbons in flame reactors

We propose a one-step synthesis for the large production of carbon nanoparticles (CNP) based on a flame reactor synthesis approach in which the key tunable parameter is the fuel composition. Specifically, the properties of CNP including fluorescent nanocarbons could be varied by fueling a premixed flame with mixtures ranging from ethylene to benzene. CNP characterization was carried out through Raman and UV–Visible spectroscopy defining their in-plane crystallite size and optical band gap. By simply changing the relative percentages of ethylene/benzene in the fuel it is shown how CNP properties can be tuned, fixing the optical band gap and changing the crystallinity and particle size, or viceversa fixing the crystallinity and changing the optical band gap and particle size. Fluorescent nanocarbons presenting typical carbon nanodot characteristics could be obtained by extraction and simple filtration (<20 nm) of CNP. By changing the fuel composition, nanocarbons fluorescing from yellow to blue and showing a quantum yield moving from 3 to 10% could be easily produced. The properties of CNP and of their fluorescent fraction (<20 nm) are directly defined by the flame reactor conditions without any further modifications that is a key advantage for large-scale production of CNP and especially of fluorescent nanocarbons

Soot formation in premixed heptane-toluene flames

In this work soot formation was studied in a series of laminar premixed flames fueled with binary heptane-toluene mixtures. The equivalence ratio (φ = 2), the cold gas velocity and the total carbon flow rate were kept constant changing accordingly the oxygen content and the diluent (N2) concentration. A very similar temperature up to 80% of toluene concentration was obtained and, hence, a direct comparison between particle formation from such different fuels. In-situ optical techniques (laser induced incandescence – LII) and ex-situ particle size distribution (PSD) measured downstream of the flame front have furnished data on the total amount and size of combustion generated particles derived from heptane/toluene combustion. LII showed an increase of total volume fraction of soot as the toluene percentage increased. PSD profiles showed that particles with sizes less than 10 nm decrease as toluene percentage in the feed mixture increases, disappearing for toluene percentages above 60%. Conversely, large aggregates grow towards sizes larger than 100 nm when toluene concentration is increased. Bulk material was also sampled thermophoretically from the flames by means of fast insertion of quartz plate. UV–Visible and Raman spectroscopy was performed in order to obtain information on the aromaticity of the particles that was found to increase with toluene percentage. These results follow what already found for other aliphatic-aromatic binary mixtures. However, the heptane/toluene mixture peculiarities different from other mixtures will have to be taken into account for the future studies on surrogates and other synthetic fuels

Fingerprints of polycyclic aromatic hydrocarbons (PAHs) in infrared absorption spectroscopy

We have analyzed a set of 51 PAHs whose structures have been hypothesized from mass spectrometry data collected on samples extracted from carbon particles of combustion origin. We have obtained relationships between infrared absorption signals in the fingerprint region (mid-IR) and the chemical structures of PAHs, thus proving the potential of IR spectroscopy for the characterization of the molecular structure of aromatic combustion products. The results obtained here for the spectroscopic characterization of PAHs can be also of interest in Materials Science and Astrophysics

On the Application of Electron Energy-Loss Spectroscopy for Investigating Nanostructure of Soot from Different Fuels

Soot is characterized by a multiscale structural organization; the only diagnostic tool that can give access to it is the transmission electron microscope (TEM). However, as it is a diffraction-based technique, TEM images only conjugate aromatic systems and, thus, it is particularly useful to combine it with electron energy-loss spectroscopy (EELS), which is able to provide quantitative information about the relative abundance of sp3 and sp2 hybridized carbon. In this paper, a method for the EELS spectrum analysis of carbonaceous materials, recently developed for electron-irradiated graphite and glassy carbon composition analysis, has been applied for the first time on soot samples, in order to test its performance in soot nanostructure study in combination with TEM and high-resolution TEM (HRTEM). Soot samples analyzed were collected in the soot inception region of premixed flames of different hydrocarbon fuels. EELS, in agreement with TEM and HRTEM, showed a quite disordered and heterogeneous structure for young soot, with a relatively low sp2 content and slight presence of fullerene-like structures, more evident in the case of methane soot hinting to the effect of more saturated aliphatic fuels on soot characteristics at soot inception

Mechanical and thermal treatments of municipal solid waste organic fraction in small dehydration units

Landfilling, composting and incineration represent the most common processes used for the disposal of organic fraction of municipal solid waste (OFMSW). The evaluation of the best available technology used for the exploitation of these processes has to take into account the environmental impact of the technology itself and of the connected activities in addition to peculiar local aspects related to lands availability, legal constrains and climatic conditions of the specific country. Different technological solutions and management strategies have been proposed aimed to reduction of Green House Gas (GHG) emissions and leachate release as well as to increase energy recovery efficiency of existing disposal plants. The aim of the present study is to identify a feasible OFMSW disposal processing chain at apartment or condominium scale capable to perform a proper dehydration and volume reduction of the organic waste thus producing a material directly available for Waste to Energy processes. Four sub-processes have been identified: mechanical treatment, separation of the aqueous phase, drying and effluent gas treatment. The first three sub-processes have optimized on the basis of characteristic times, energy consumption and adaptability to domestic scale