The effect of butanol isomers on the formation of carbon particulate matter in fuel-rich premixed ethylene flames

Abstract The effect of the butanol isomers on carbon particulate matter formation was studied by substituting up to 20% of the total carbon of ethylene, fed to premixed flames with different equivalence ratios, with the four butanol isomers. Soot and condensed-phase nanostructures were tracked by means of particle size distribution (PSD) measurements and laser induced emission spectroscopy, namely fluorescence and incandescence. Butanol isomers, especially t-butanol, significantly reduced the total amount and the size of the soot particles, whereas a negligible effect was detected on condensed-phase nanostructures. PSDs were measured along with the aromaticity and functionalities of the carbon particulate matter thermophoretically sampled in the highest equivalence ratio condition. No significant differences were found among the different butanol isomers neither in the soot aggregate size, as measured by size exclusion chromatography, nor in the aromaticity, as evaluated by Raman and UV–vis spectroscopy, of the particulate matter. Conversely, FTIR analysis showed that carbon particulate matter produced from 1-butanol and t-butanol-doped flames contained larger amounts of oxygen in form of C = O, C–O–C and OH functionalities. However, most of the differences in the oxygen functionalities disappeared after dichloromethane (DCM) treatment, suggesting that these oxygenated moieties belong to the condensed-phase nanostructures, soluble in DCM, rather than to soot particles

Particle formation in premixed ethylene-benzene flames: An experimental and modeling study

Abstract In this work soot formation was studied in laminar premixed flames of binary ethylene-benzene mixtures varying throughout the composition range from pure ethylene to pure benzene keeping constant the equivalence ratio (φ = 2) and obtaining a very similar maximum temperature (Tmax around 1750 K). In such way, it was possible to study for the first time the effect of binary aliphatic-aromatic fuel mixtures composition on the sooting behavior in comparable combustion conditions. In-situ optical techniques (laser induced incandescence and fluorescence) and ex-situ particle size distribution (PSD) measured downstream of the flame front, as well as modeling by means of a multi-sectional method, were applied. PSD profiles showed that particles with sizes less than 10 nm decrease as benzene percentage in the feed mixture increases, disappearing for benzene percentages above 30%. Conversely, large aggregates grow towards sizes larger than 100 nm when benzene concentration is increased. A non-linear effect of the benzene content in the binary fuel mixture on soot particle concentration was observed by laser induced incandescence, and confirmed by the multi-sectional model. In particular soot formation was found to increase more than linear up to 50% then leveled off to reincrease linearly from 80% to 100%. On the contrary, particles smaller than 10 nm at the end of the flame rapidly decreased for benzene percentages larger than 30%. From reaction rate analysis, the formation of gas-phase polycyclic aromatic hydrocarbons (PAH) and high-molecular mass aromatics precursors was found to be significantly large already for fuel mixtures featured by low benzene amounts (from 10 up to 40–50%). The enhanced aromatic precursor formation, combined with the abundance of acetylene mainly coming from the dehydrogenation of ethylene as predominant component of the binary fuel mixture, appeared to be responsible for the non-linear effect of ethylene-benzene composition on particle formation, particularly significant up to 40–50% of benzene. This finding has a considerable importance as regards the exploitation of highly-aromatic fuels as well as to foresee the soot emission for effect of the aromatic presence in natural and synthetic fuels used in practical combustion systems

Blue, green and yellow carbon dots derived from pyrogenic carbon: Structure and fluorescence behaviour

Fluorescence lifetimes and quantum yields featuring polycyclic aromatic hydrocarbons (PAHs) and other organics constituting pyrogenic carbon particulate matter (PM) are seldom measured. In this work, PM sampled in a fuel-rich ethylene flame was firstly separated in organic carbon (OC), soluble in dichloromethane, and refractory organic carbon (ROC), soluble in N-methyl pyrrolidinone, and then analyzed by size exclusion chromatography (SEC) coupled with online UV and fluorescence detection, and by offline fluorescence spectroscopy and mass spectrometry. It was found that three classes of differently light emitting carbon dots (CDs) could be bottom-up synthesized in the same flame system by selecting appropriately the residence time. Actually, OC presented blue fluorescence regardless the residence time, whereas ROC sampled at low and high residence time emitted fluorescence in the green (green CDs) and in the yellow (yellow CDs) region, respectively. The SEC molecular weight of all CDs presented similar trimodal distributions, centered around 300, 1000 and 10,000 u. For the first time fluorescence lifetimes and quantum yields of pyrogenic CD fractions were measured as additional parameters useful for discriminating the fluorescent components and inferring their structural properties, with the support of mass spectrometry. The different spectroscopic features of CDs could be associated to different compositional characteristics as the polydispersity of molecular components featuring blue CDs, opposed to the oligomer-like nature of green and yellow CDs. Pyrogenic CDs showed different fluorescence emission ranges, quantum yield and lifetimes, appealing for their possible applications in the fields of imaging, electronics and sensors

PAHs and fullerenes as structural and compositional motifs tracing and distinguishing organic carbon from soot

Examining the features distinguishing organic carbon from soot is crucial for understanding the source, the effect on the environment and their respective role in aerosol chemistry and soot formation. Beside to the obvious PAH picking-out in the low-mass mode (C number 40) of organic carbon, separated by carbon particulate matter extraction from young and mature soot thermophoretically sampled in premixed flames, was done by laser-desorption-time-of-flight mass spectrometry, exploiting the laser power increase. The perusal of organic carbon mass spectra through mathematical tools in comparison to aromatic and alkyl-substituted PAH-laden samples and the persistence of high-mass mode at high laser power led to exclude the contribution of dimers and alkyl-bridged PAHs attributing the second mode to both fully-benzenoid and cyclopenta-PAHs. Profound differences between mass spectra of organic carbon and soot were noticed as neither molecules nor radicals of PAHs could be drawn out from soot, even at high laser power, and only small radicals and carbon clusters like fullerenes were observed, especially for young soot. These inferences evidenced the importance of analysing separately organic carbon and soot especially if insights into soot particles nucleation are to be obtained. In the case of benzene flame, already at the inception, soot consists of strongly tangled aromatic motifs crosslinked each other, presumably deriving from reactive coagulation/clustering of relatively small aromatic hydrocarbons/radicals early formed. In methane and ethylene flames, coalesced liquid-like material composed of soot and PAHs is formed and transformed later on undergoing some carbonization and molecular growth, respectively

Optical Properties of Organic Carbon and Soot Produced in an Inverse Diffusion Flame

The carbonaceous matter (soot plus organic carbon) sampled downstream of an ethylene inverse diffusion flame (IDF) was chemically and spectroscopically analyzed in detail. In particular, the H/C ratio, the UV-Visible absorption coefficient and Raman parameters were measured and found to be representative of a highly disordered sp2 -rich carbon as the early soot sampled in a premixed flame. In contrast, the optical band gap was found to be relatively low (0.7eV), closer to the optical band gap of graphite than to that of medium-sized polycyclic aromatic hydrocarbons (\u3e2eV) which are widely considered to be soot precursors and are mostly contained in the organic carbon. The significance of the optical band gap as signature of different structural levels (nano-, micro- and macro-structure) of sp2 -rich aromatic disordered carbons was critically analyzed in reference to their molecular weight/size distribution. The relevance of the optical band analysis to the study of the soot formation mechanism was also highlighted

Wet Chemical Method for Making Graphene-like Films from Carbon Black

Reduction of strongly oxidized carbon black by hydrazine hydrate yields water-insoluble graphene-like sheets that undergo to self-assembling in thin film on surfaces after drying. The height of a drop-casted graphene-like film was determined by atomic force microscopy (AFM) to be around 20 nm, corresponding to approximately 25 graphene-like layers. The oxidized carbon black and the corresponding reduced form were carefully characterized

Controlling steps in the low-temperature oxidation of n-heptane and iso-octane

The low-temperature oxidation of n-heptane and iso-octane in mixtures with air in a jet-stirred-how reactor have been compared under suitable high-pressure conditions, such that the two mixtures of hydrocarbon and air showed comparable fuel conversions and phenomenologies. The large presence of aldehydes in the products of the low-temperature oxidation of n-heptane was attributed to a degenerate chain-branching path involving the addition of molecular oxygen to heptylhydroperoxy radicals and isomerization by internal H-atom abstraction. The latter step is particularly favored in linear alkanes where easy-to-abstract H-atoms are available. On the other hand, cyclic ethers and fuel-conjugate olefins were the dominant products of the low-temperature oxidation of iso-octane. This is due to a lack of H-atoms for internal abstraction; this limits the degenerate chain-branching route and favors the propagation path toward species having the same skeleton of the fuel, such as cyclic ethers and fuel-conjugate olefins. The prevalence of a degenerate chain-branching path for n-heptane compared with the propagation reactions in iso-octane oxidation is responsible for the different autoignition tendencies of n-heptane and iso-octane. (C) 1998 by The Combustion Institute