Soot Nanostructure Research Articles

Oxidation behaviors and nanostructure of particulate matter produced from a diesel engine fueled with n-pentanol and 2-ethylhexyl nitrate additives

Experiments were performed on a high pressure common-rail diesel engine fueled with D100 (diesel fuel), DP15 (15% n-pentanol and 85% diesel, v/v), DP30 (30% n-pentanol and 70% diesel, v/v), DP30E1 (DP30 and 1000 ppm EHN) and DP30E2 (DP30 and 2000 ppm EHN), respectively. Particulates were sampled from the engine exhaust tailpipe and further analyzed with transmission electron microscope (TEM), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). Results showed that an increase in soot oxidation reactivity was observed with increasing n-pentanol concentration in diesel/n-pentanol mixtures. This trend was in line with the poorer structural ordering of soot particles generated from blended fuels by both TEM and RS analyses. Regarding the impact of EHN addition to DP30 fuel, the soot oxidation reactivity decreased as the fuel was changed from DP30 to DP30E1, and then increased as the fuel was changed from DP30E1 to DP30E2. Notably, the soot from DP30 + EHN additive was easier to be oxidized than that from DP30. Similarly, the trends in soot oxidation reactivity with EHN additive can qualitatively capture the variations in soot nanostructure. The correlation between soot reactivity and nanostructure in this study was nonlinear, which infers that soot oxidation reactivity would be also influenced by other features like active surface area and surface functional group. It can be suggested that the application of DP30 would significantly affect the regeneration performance of diesel particulate filter (DPF); when the baseline fuel is DP30, the impacts of EHN additives on regeneration performance of DPF are relatively slight.

Spatial distribution of metallic heteroatoms in soot nanostructure mapped by aberration-corrected STEM-EELS

Soot from the thermochemical conversion of solid and liquid fuels can be infused with metallic heteroatoms originating from the fuel – these heteroatoms alter the nanostructure and the reactivity of the soot. Here, we investigate the spatial distribution of metallic heteroatoms in soot generated by biomass gasification, using aberration-corrected Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). The technique allowed for the mapping of heteroatom distribution in soot at the nanoscale, and thereby for the direct correlation of heteroatom concentration with the graphitic nanostructure. Spherical soot particles were coated with a thin layer of silicon, possibly in the form of quartz that may be linked to minor distortions of the nanostructure of the graphitic shell of the particles. Further results on non-spherical soot and inorganic-carbon fused aggregates suggest that the chemistry of formation was affected by the presence of gaseous ash-forming elements, especially calcium, with carbon-oxygen functional groups forming as intermediates in the graphite-inorganic reaction; i.e., prior to the formation of the thermodynamically stable carbonate bonds. The analytical approach demonstrated here can potentially help select fuel additives or aid in the design of fuel blends that minimize the formation of similar, hybrid carbon nanoparticles in combustion or gasification systems.

Soot properties in ethylene inverse diffusion flames blended with different carbon chain length alcohols

This study was devoted to the characterization of soot properties in ethylene inverse diffusion flames with different carbon length alcohols, which mainly contained the soot morphology, soot nanostructure, and oxidation reactivity. Ethanol, n-butanol, n-hexanol, and n-octanol were selected as the fuel additives. For all blended flames, a 30% volume fraction of ethylene was substituted by different alcohol additives, and the flow rates of the alcohols were adjusted to achieve the same number of carbon atoms. A combination of the thermophoretic sampling technique and the quartz plate sampling method was applied to gain soot features. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and thermogravimetric analysis (TGA) were used. The results showed that soot production was reduced with the addition of all alcohols, and the minimum soot yield was observed with ethanol addition. The soot-reducing ability gradually decreased as the alcohols move higher. The sizes of the primary particles and soot aggregates increased slightly faster with the increasing carbon chain length. Furthermore, the soot generated from higher alcohol-doped flame has a higher degree of graphitization with longer fringe length and smaller fringe tortuosity. The four different alcohol additives all could improve soot oxidation reactivity. There was a correlation between soot oxidation reactivity and the carbon chain length of the alcohols that the longer carbon chain of these alcohol additives led to lower soot reactivity.

Formation and characteristics of soot from pyrolysis of ethylene blended with furan fuels

As two renewable oxygenated biofuels, 2,5-dimethylfuran and 2-methylfuran (DMF and MF) have been considered to be two of the most potential fuels in the future due to the development of the second-generation biosynthetic technologies. The atmosphere pyrolysis experiments with 0%, 25%, 50%, 75% and 100% replacement of ethylene by DMF/MF at 1173 and 1273 K were conducted. Collected soot samples were characterized by high resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TGA) to acquire their internal structure and oxidation reactivity. Results showed that soot mass was positively related with DMF addition ratios and the reaction temperature. Soot production was also enhanced when the MF addition ratio gradually increased from 0% to 75%. The influences of DMF addition can promote soot formation more obviously than MF. Temperatures showed a more significant influence on soot morphology than fuel types and DMF/MF addition ratios. For a fixed addition ratio of DMF/MF, soot showed liquid-like substances at 1173 K. At 1273 K, approximately round particles formed and linked together in chains. Moreover, at 1273 K, the aggregates obtained by adding MF contained more single particles, longer carbon chains, and larger projection area compared with the aggregates by adding same MF. For nanostructures, as the addition ratios of DMF/MF increased, as well as the reaction temperature improved, the fringe length of the carbon layer increased, the average fringe tortuosity decreased, and the stacking arrangement of soot became more orderly, the soot oxidation reactivity was lower. Under a same addition ratio and reaction temperature, soot obtained by adding DMF possessed slightly longer fringe length, smaller fringe tortuosity and lower oxidation reactivity than those of the soot obtained by adding MF. High correlation between fringe parameters and soot oxidation reactivity was discovered. The more ordered soot nanostructure, the longer fringe length, and the smaller fringe tortuosity could make the oxidation reactivity of soot get lower.

Supercritical extraction and microwave activation of wood wastes for enhanced syngas production and generation of fullerene-like soot particles

This work demonstrated that supercritical carbon dioxide extraction is effective as a pre-treatment technology to generate soot particles with the fullerene-like structure and increase syngas yield from extracted residues during coupled microwave activation with gasification. Supercritical carbon dioxide extraction removes over half of the fatty and resin acids from needles and branches, whereas the extraction of needles generates greater yields of value-added compounds. The high yields of extractives indicate the effective conversion of waste wood for the sustainable production of value-added chemicals. The wood extraction did not influence the solid residue yields during pyrolysis/gasification emphasizing the significant potential of integrating the extraction process into the holistic biorefinery. Interestingly, supercritical carbon dioxide extraction had a significant effect on the structure and quality of soot particles formed. The differences in the extractives composition led to the formation of needle soot particles with a porous and less ordered nanostructure, whereas the soot branches obtained a ring graphitic structure. The greater yields of steroids and terpenes during the extraction of needles compared to the branches pretreatment indicated the influence of the extractives type on the soot nanostructure.

The effect of elevated reactant temperatures on soot nanostructures in a coflow diffusion ethylene flame

The relationship between soot surface growth, soot nanostructure and reactant temperature (Tr) in a coflow diffusion ethylene flame was investigated with multiple experimental techniques. The Tr was raised by heating the coflow air. Three cases, with 300K, 473K, and 673K Tr, respectively, were studied. Laser-induced Incandescence revealed that increasing Tr promotes soot formation. Although soot primary particle diameter (dp) also increases with Tr, the increase in dp slows down after 473K Tr, suggesting that there is a deceleration in soot surface growth. Transmission Electron Microscopy (TEM) imaging showed that increased Tr promotes soot aggregation and yields larger and more mature primary particles. The assessment of the Selected Area Electron Diffraction (SAED) patterns indicated that, at 673K Tr, there is a growth of lattice planes. Raman spectroscopy revealed further structural details. By assessing the band intensity ratios, soot for the Tr of 673K has more curved nanostructures. The deceleration of soot surface growth may be explained by surface aging, which is characterized by an increase in curved nanostructures.

Experimental investigation on soot formation characteristics of n-heptane/butanol isomers blends in laminar diffusion flames

Biobutanol is a highly competitive alternative biofuel for fossil fuels in the future. However, there is no consensus on the soot formation of the four butanol isomers. The soot formation characteristics of the pure n-heptane (B00) and the volumetric mixture 50% n-heptane/50% butanol isomer (H50B50) flames were investigated on a coflow burner. The quantitative soot volume fraction, qualitative OH radical concentration, morphological parameters and nanostructures of soot particles were measured by the two-color laser induced incandescence (TC-LII) method, the planar laser induced fluorescence (PLIF) method, and transmission electron microscope (TEM) analysis. Compared with B00 flame, the soot loading of n-heptane/n-butanol (N50), n-heptane/s-butanol (S50) and n-heptane/i-butanol (I50) flames decreased by 62%, 40% and 33%, respectively, whereas the n-heptane/t-butanol (T50) flame increased by 57%. The oxidation of soot particles was jointly affected by the region and the peak concentration of OH radical. As the flame developed, the primary particle diameter initially increased and then decreased in the order T50 > I50 > S50 > N50. Regarding soot nanostructure, the fringe tortuosity and inter-fringe spacing of soot sampled in the three flames decreased in the order I50 > S50 > T50.

Comparative Study on Soot Reduction, Soot Nanostructure and Oxidation Reactivity of n-heptane/DMC and Isooctane/DMC Inverse Diffusion Flames

Dimethyl carbonate (DMC) is an environmentally oxygenated compound which can be used efficiently for soot reduction. This paper compared the soot reduction, soot nanostructure and oxidation reactivity from inverse diffusion flames (IDFs) of the hydrocarbon fuels, namely n-heptane and isooctane doped with DMC. Effects of DMC additions on soot reduction were discussed. DMC addition is more effective for the soot reduction of n-heptane/DMC IDF than isooctane/DMC IDF. The morphology and nanostructures of soot particles were investigated by Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM), and the soot graphitization and oxidation reactivity were analyzed by X-ray Diffraction (XRD) and Thermogravimetric Analyzer (TGA), respectively. The results of HRTEM images showed that many larger aggregates were observed for the structures of soot particles from IDFs with DMC additions. The soot particles exhibited more liquid-like material, more amorphous, higher disorganized layers, and less graphitic than that of IDFs without DMC additions. With increasing of DMC blending rate, soot particles changed younger to have shorter fringe length, higher tortuosity, and greater fringe separation. Based on the XRD and TGA results, the degree of the soot graphitization level decreased; the soot mass lost significantly faster, and the soot become more reactive.

Morphology and size of soot from gas flares as a function of fuel and water addition

A large-scale, laboratory turbulent diffusion flame was used to study the effects of fuel composition on soot size and morphology. The burner and fuels are typical of those used in the upstream oil and gas industry for gas flaring, a practice commonly used to dispose of excess gaseous hydrocarbons. Fuels were characterized by their carbon-to-hydrogen ratio (from 0.264 to 0.369) and their volumetric higher heating value (HHVv) (from 35.8 to 75.2 MJ/m3). Transmission electron microscopy (TEM) was used to assess primary particle and aggregate size, showing that the scaling of primary particle size to aggregate size was roughly the same for all of the considered fuels (dp = 16.3(da,100 [nm]/100)0.35). However, fuels with higher HHVv produced substantially larger soot aggregates. A scanning mobility particle sizer (SMPS) was also used (i) to measure mobility diameter distributions and (ii) in tandem with a centrifugal particle mass analyzer (CPMA) to determine the two-dimensional mass-mobility and effective density-mobility distributions using a new inversion approach. The new approach was shown to improve internal consistency of inferred morphological parameters, though with a shift relative to median-based analysis of the tandem data. Raman spectroscopy was used to quantify the degree of graphitization in the soot nanostructure. The addition of water to the fuel consistently reduced the soot yields but did not affect other morphological parameters. Larger aggregates also tended to have larger primary particles and higher Raman D/G ratios suggesting larger graphitic domains.

The impact of cyclic fuels on the formation and structure of soot

This paper investigates the impact of cyclic fuels on the nanostructure, nucleation and overall production of soot in an n-heptane (C7H16) laminar coflow diffusion flame. The fuels selected to dope the n-heptane flame are cyclopentene (C5H8), cyclohexene (C6H10) and methylcyclohexane (C7H14). These fuels were chosen for their differences in their structure and sooting tendency. The flame structure was studied with Differential Mobility Spectrometry (DMS) for particle size distribution determination, two-colour ratio pyrometry to calculate the soot volume fraction and soot temperature. The soot nanostructure was investigated using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The addition of cyclic fuels was found to promote the formation of soot nanoparticles earlier in flames. In addition, the soot volume fraction was increased significantly by the addition of the cyclic fuels, especially by the addition of cyclopentene. The addition of 20% of cyclopentene increased the soot volume fraction by a factor of 2. HRTEM results suggest a significant influence of cyclopentene on the soot nanostructure; cyclopentene addition promotes the incorporation of five-membered rings (pentagonal rings) leading to highly curved fringes. This suggests cyclopentene could be used as a fuel to promote curvature in different carbonaceous materials to modify their properties.

On the effect of pressure on soot nanostructure: A Raman spectroscopy investigation

Although the majority of existing combustion devices operate at high pressure conditions, most of our understanding of the soot formation process and soot physicochemical properties rely on studies performed at atmospheric pressure. Pressure is known to have nonlinear effects on combustion processes and a significant influence on soot formation; soot loading increases with increasing combustion pressure. Soot characteristics directly affect soot oxidation and optical/radiative properties, and it is desirable to have a better insight into them under high-pressure conditions. Due to scarcity of information on soot primary particles, aggregate morphology, and soot nanostructure relevant to high-pressure combustion, there are challenges in predicting soot oxidation and radiation, particularly at engine-relevant conditions.In this study, we perform Raman spectroscopy measurements on soot sampled from a set of laminar diffusion flames of ethylene at various pressures, ranging from atmospheric pressure to 12 bar. Our results show an increase in soot maturity as the pressure increases within the range of investigated pressures. In the examined co-flow flames, pressure seems to have an indirect influence on soot nanostructure through an earlier inception of soot, resulting in longer residence times of the carbon soot particles in the hot and reactive flame environment. It is found that soot maturity, tracked through the size of graphitic domains, La, increases linearly with residence times. The longer residence time of soot in high-pressure flames could be the main cause of the higher degree of graphitization observed, which suggests a greater resistance to oxidation with increasing pressure.

Impacts of advanced diesel combustion operation and fuel formulation on soot nanostructure and reactivity

Advanced diesel combustion, accomplished via a single pulse fuel injection and high levels of exhaust gas recirculation (referred to as “PCCI”, partially-premixed charge compression ignition), is shown to be a path to reduce oxides of nitrogen and particulate matter simultaneously. This is well established in the literature. Less established is the extent to which such dilute combustion processes influence soot formation and affect soot that is emitted from diesel engines under such combustion modes. This work focuses on characterization of the nanostructure and oxidative reactivity of soot generated by a light-duty turbodiesel engine operating under a PCCI combustion mode, a dilute, low-temperature combustion process. Previous work on a type of PCCI combustion, referred to as high-efficiency clean combustion (HECC), showed soot samples having a fullerenic nanostructure, characterized by high levels of tortuosity of the fringe layers as seen in transmission electron micrograph images and as quantified using an image processing algorithm. Thermogravimetric analysis of the HECC mode soot samples showed that they displayed higher rates of oxidation than soot samples from a conventional diesel combustion mode. The present work returns to PCCI combustion, considering the timing of the main fuel injection, and the effects of operating on fuels rich in n-alkanes, particularly a fuel produced from a low temperature Fischer-Tropsch process (LTFT) and a renewable diesel fuel (RD) produced via hydrodeoxygenation of a plant oil. PCCI combustion conditions yield soot that shows higher reactivity compared to soot from the conventional combustion mode regardless of fuel type. LTFT and RD fuels produce soots with lower reactivity compared to ULSD. Soots produced from PCCI combustion have higher surface oxygen concentration and higher proportion of amorphous carbon. In addition, TEM images show that PCCI soots from all three fuels have smaller primary particle and particle aggregate sizes, and smaller graphene layers. These properties explain the higher reactivity of soot from PCCI combustion. The less reactive soots, which are produced from LTFT and RD fuel under conventional combustion, show internal burning during oxidation. However, soots with higher reactivity which are produced from late injection PCCI combustion and ULSD show shrinking core oxidation, likely because of their overall amorphous structure.

Effect of 5-membered bicyclic hydrocarbon additives on nanostructural disorder and oxidative reactivity of diffusion flame-generated diesel soot

Soot emission from diesel engines is of serious concern, as it is carcinogenic and remains suspended in air for a long time to cause adverse effects on human health and the environment. A way to reduce soot emission is by altering soot nanostructures by introducing fringe curvatures to enhance reactivity and increase oxidation. Such disorder in soot nanostructure can be initiated by the introduction of 5-membered rings into 6-membered, graphite-like soot structures. This study investigates the effect of the addition of norbornane, a saturated, 5-membered bicyclic hydrocarbon additive to diesel on the physicochemical properties, sooting propensity, structural disorders, and the oxidative reactivity of soot. The physicochemical analysis revealed that the threshold sooting index (TSI) of the blend is reduced to 29.5 at an optimum blending percentage of 10% norbornane-90% Diesel (NBD) as compared to the TSI of 37 for diesel. The analyses using XRD, Raman, HRTEM, and EDX indicated that the addition of this additive resulted in an increased soot nanostructural disorder, smaller PAH size, increased fringe curvature, and significantly greater aliphatic content in soot as compared to an unsaturated 5-membered bicyclic additive, dicyclopentadiene (DCPD). The reactivity studies confirmed that NBD soot is easily oxidized in air, since it requires a lower initial activation energy (90 kJ/mol) as compared to DCPD (120 kJ/mol) and pure diesel soots (170 kJ/mol). Thus, norbornane, a saturated 5-membered bicyclic compound, which is available as a by-product of polymer industry and in crude oil, can serve as a potential fuel additive for designing advanced fuels.

Quantitative high-resolution transmission electron microscopy nanostructure analysis of soot oxidized in diesel spray flame periphery

In order to better understand the in-flame diesel soot oxidation processes, soot particles at the oxidation-dominant periphery of diesel spray flame were sampled via newly developed suck type soot sampler employing a high-speed solenoid valve, and their morphology and nanostructure were observed and analyzed via high-resolution transmission electron microscopy. A single-shot diesel flame for the soot sampling experiment was achieved in a constant-volume vessel under a diesel-like condition (9.5 kg/m3, 2.5 MPa, 1070 K, and 21%O2). Morphology of soot aggregates, inner-core/outer-shell structure of primary particles within the aggregates, and carbon crystallite nanostructures within the primary particles were compared between the soot aggregates sampled at the diesel flame core near the central axis and the oxidation-dominant flame periphery. The morphology observation and the inner-core/outer-shell structure characteristics obtained by newly employed concentricity analysis showed that the flame core soot exhibits graphitic primary particles with clear outlines and boundaries similarly observed for engine exhaust soot. Each primary particle contained a well-defined inner core surrounded by thick graphitic outer shell. On the contrary, the flame periphery soot exhibited smaller primary particles with unclear and lumpy outlines containing multiple obscured inner cores surrounded by thinner outer shell. The analysis of carbon crystallite nanostructures within the primary particles showed that the in-flame soot nanostructure shifts toward amorphous from the flame core to the periphery. On the contrary, the engine soot nanostructure in the literature shifts toward graphitic from the in-cylinder TDC to the exhaust, exhibiting the opposite trend with the in-flame soot. These results suggest that the engine exhaust soot has not experienced the rapid in-flame oxidation by OH radicals and is therefore considered not to be the remains of incomplete or partial oxidation, but the runaways escaped from the flame core to the exhaust without being attacked by the in-flame OH radicals.

A comprehensive study of fuel composition, combustion and soot nanostructure characteristics of a diesel/light hydrocarbons premixed charge compression ignition engine

The present research concentrates on the fuel properties and engine operating conditions on combustion and emission characteristics especially for morphology and nano-structure of a light hydrocarbons PCCI engine. The experiments were conducted by using gas chromatography-mass spectrometry, thermogravimetric analysis and high-resolution transmission electron microscopy. Additionally, the performance, combustion and gaseous pollutant emissions characteristics were also performed. The results indicate that the carbon chain length of main compounds in light hydrocarbons sample is shorter and the volatile matter content is higher. TG/DTG curves of light hydrocarbons move to a lower temperature region compared to diesel, which is due to lower decomposition temperature and higher volatility of light hydrocarbons. The performance characteristics show that BSFC is higher and BTE is lower in light hydrocarbons PCCI engine compared to conventional diesel engine. The combustion and emission characteristics show that the in-cylinder peak pressure of light hydrocarbons PCCI engine is reduced by 18.1%, and the pressure rise rate and heat release rate present a consistent variation trend compared to conventional diesel engine. Light hydrocarbons PCCI engine exhibits higher THC emission with respect to different engine load, while CO and NOx emissions are lower. Compared with conventional diesel engine, the soot aggregate of light hydrocarbons PCCI engine exhibits more sparse, and it appears more chain-like. Additionally, the primary particles in light hydrocarbons PCCI engine represent more ordered graphitic and fullerenic nanostructure.

Spark discharge-generated soot: Varying nanostructure and reactivity against oxidation with molecular oxygen by synthesis conditions

Spark discharge generators enable a fast, reliable, continuous, and highly reproducible production of nanoparticle aerosols along with a widely tunable size distribution. Graphite spark discharge generators have been particularly applied for studies of instrument calibration, filter testing, soot reactivity studies, and in toxicological risk evaluation of soot. Until now, it has been consensus that soot produced in spark discharge generators is highly reactive because of its high surface area and its amorphous nanostructure. In this paper, we will demonstrate that the properties of soot produced in spark discharge generators strongly depend on the used carrier gas (i.e. argon, nitrogen). Soot produced in argon (spark discharge argon soot, SDS-Ar) as carrier gas is composed of a major fraction with high reactivity exhibiting a temperature at maximum oxidation rate Tmax between 796 K and 803 K. Soot produced in nitrogen (spark discharge nitrogen soot, SDS-N2) has a similar size distribution as SDS-Ar, whereas the main fraction is less reactive (963 K < Tmax < 983 K). This can be derived from temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA) and also correlates well with a considerable higher BET surface area of SDS-Ar (679 m2 g−1) compared with SDS-N2 (426 m2 g−1). C/H/N Elemental Analysis, High-Resolution Transmission Electron Microscopy (HRTEM), Raman- and FTIR-spectroscopy complement the measurements, all leading to the observation that SDS-N2 has a more graphitic nanostructure than the highly amorphous nanostructure of SDS-Ar (Raman spectra SDS-Ar: I(D, disordered bands)/I(G, graphite band) = 0.76, SDS-N2: I(D)/I(G) = 1.0 or elemental analysis SDS-Ar: H/C = 0.25, SDS-N2: H/C = 0.10). These findings imply that studies applying spark discharge soot have to be critically viewed with respect to the used carrier gas. Spark-over field strength, Paschen's law, the energy release within the arc, decaying plasma or cooling rates, as well as dissociation/ionization processes are discussed to explain the observations above. Finally, we want to emphasize that it is possible to vary the reactivity towards oxidation and nanostructure of spark discharge soot simply by applying different mixtures of the carrier gas.

Improved HRTEM image processing methods and the application on soot nanostructure analysis for GDI engine

A novel algorithm based on finding Longest common subsequence is coupled to the conventional method to automatically calculate the interlayer spacing of soot. The Gabor filter-based method (GFBM) is also improved by revising the parameter setting and response surface calculation. Firstly, the results of the two methods are compared and correlated. It is found that the interlayer spacings from the two methods are highly positively correlated. The fringe interlayer spacing decreases with the modulation strength increasing; the modulation strength can be regarded as an indicator of the degree of carbonization. The nanostructures of soot particles that are emitted from the gasoline direct injection (GDI) engine analyzed. The results reveal that the GDI engine soot particles possess a core-shell nanostructure. However, the shell is not entirely connected, and the stacks are surrounded by amorphous structures. The distribution of the interlayer spacing does not follow a normal distribution or log-normal distribution. The soot particles from the idle condition have a maximum interlayer spacing. The smallest interlayer spacing and a mid-level modulation strength were at 2000 r/min-3 bar, while at 2000 r/min-5 bar case, the highest modulation strength, and the second smallest interlayer spacing have been detected. The results of this study indicate that the interlayer spacing of soot particles increases with engine load and decreases with the increase of engine speed.

Nanosecond laser irradiation of soot particles: Insights on structure and optical properties

In spite of the advances in laser diagnostics in combustion, the effect of rapid laser irradiation on the physical/chemical properties of soot particles is far from being comprehensively understood. Optical properties, particle nanostructure and aggregate size of laser-irradiated soot particles are analyzed in this paper. Carbonaceous particles sampled from nitrogen-quenched diffusion flames of ethylene and methane are irradiated on-line by a 1064-nm short laser pulse. Wavelength-resolved extinction measurements in the visible range are used to follow their transformation by varying the laser energy density. A variation of the extinction coefficient of the irradiated particles compared to the extinction coefficient of the pristine ones is observed, especially for ethylene soot. The particle nanostructures are analyzed by Raman spectroscopy and the effect of laser irradiation on aggregate structure is evaluated by measuring particle size distributions. The results indicate that both soot nanostructure and optical properties are strongly dependent on the laser energy density when irradiated by a laser source. This is significant for ethylene soot, while for methane soot the degree of variation of such properties is less pronounced, at least in the investigated heating conditions.

Formation and nanoscale-characteristics of soot from pyrolysis of ethylene blended with ethanol/dimethyl ether

Ethanol and dimethyl ether (DME) have been considered to be two of the most potential additives for conventional hydrocarbon fuels. This paper focused on the nanoscale characteristics of soot from ethylene pyrolysis with ethanol and DME additions. The pyrolysis experiments were conducted in a α-alumina tube flow reactor at 1273 K, 1373 K and 1473 K, with the replacement of 0%, 50% and 100% (mole fraction) ethylene by the two oxygenated fuels. The gas-phase kinetic modeling was also performed to explore and understand the soot formation process. The main pathways and some key soot precursors in the pyrolysis have been obtained. Soot samples were characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to acquire their internal structure and oxidation reactivity. Results showed that the mass of collected soot diminished with the increase of the replacement of ethylene by ethanol/DME. The effects of DME to inhibit the formation of soot were more obvious. The least amount of soot was collected in the pyrolysis of pure DME. Peak mole fraction of C2H2, C4H2, C4H4 and C5H5 also decreased with the increase of replacement of ethylene by ethanol/DME, displaying the same tendency with the variation trend of soot mass in the different pyrolysis conditions. According to TEM and HRTEM results, the additions of ethanol and DME could decrease the growth rate of soot contrasted with the pyrolysis of pure ethylene. Soot collected from the pyrolysis of pure DME at 1273 K and 1373 K showed a typical amorphous structure with short, highly-curved and turbulent fringe. With the reduction of the replacement of ethylene by DME, mature soot with longer and more ordered fringe formed at 1373 K and 1473 K. The sequence of the mean fringe tortuosity of soot samples was 100% ethylene<50% DME<100% ethanol<50% ethanol< 100% DME. The order was the same as the sequence of oxidation reactivity. Furthermore, with the increase of temperature, the mass of soot increased. More mature soot with higher degree of graphization, longer fringe length, smaller fringe tortuosity and lower oxidation reactivity was obtained. High correlation between soot nanostructure and soot oxidation reactivity was found.

Nanostructure and reactivity of soot from biofuel 2,5-dimethylfuran pyrolysis with CO2 additions

This paper investigated the nanostructure and oxidation reactivity of soot generated from biofuel 2,5-dimethylfuran pyrolysis with different CO2 additions and different temperatures in a quartz tube flow reactor. The morphology and nanostructure of soot samples were characterized by a low and a high resolution transmission electron spectroscopy (TEM and HRTEM) and an X-ray diffraction (XRD). The oxidation reactivity of these samples was explored by a thermogravimetric analyzer (TGA). Different soot samples were collected in the tail of the tube. With the increase of temperature, the soot showed a smaller mean particle diameter, a longer fringe length, and a lower fringe tortuosity, as well as a higher degree of graphization. However, the variation of soot nanostructures resulting from different CO2 additions was not linear. Compared with 0%, 50%, and 100% CO2 additions at one fixed temperature, the soot collected from the 10% CO2 addition has the highest degree of graphization and crystallization. At three temperatures of 1173 K, 1223 K, and 1273 K, the mean values of fringe length distribution displayed a ranking of 10% CO2 > 100% CO2 > 50% CO2 while the mean particle diameters showed the same order. Furthermore, the oxidation reactivity of different soot samples decreased in the ranking of 50% CO2 addition > 100% CO2 addition > 10% CO2 addition, which was equal to the ranking of mean values of fringe tortuosity distribution. The result further confirmed the close relationship between soot nanostructure and oxidation reactivity.