Nanostructure Of Soot Particles Research Articles

Morphology and nanostructure of soot particles from diesel engine under transient and steady-state operating conditions with a microalgae fuel component, dioctyl phthalate biofuel

In order to adhere to strict emissions standards, it is necessary to reduce diesel particulate emissions under real driving conditions. This can be achieved by reducing the production of soot particles and optimising the combustion process within the combustion chamber. This investigation examines the impact of engine operating conditions (transient and steady-state) on the characterisation of the soot particles. Soot samples from a common rail direct injection (CRDI), turbocharged six-cylinder diesel engine without any aftertreatment devices were collected on TEM grids. This investigation utilised both pure diesel and diesel-dioctyl phthalate (DOP) blends (D90DOP10 and D80DOP20). To correlate the soot characterisation results, diesel particulate emissions were also measured using a fast particle analyser (DMS500). The results indicate that emissions from transient cycles are higher in comparison to steady-state engine operation. Smaller primary particle diameter with a compact structure were observed for transient cycles when compared to steady-state operating conditions. Parameters such as number of primary particles within a single aggregate, radius of gyration, and soot aggregate area were also studied. Nanostructure analysis reveals that the soot particles from transient cycles could have lower oxidation reactivity and longer fringes with less curvature and interplanar spacing.

Diesel soot combustion in air-NO environment: Evolution of soot physical properties and fragmentation characteristics

Since the oxidation activity of nitrogen oxides on soot is obviously higher than that of O2, it is one of the most effective means to improve soot combustion in diesel particulate filter (DPF) by fully utilizing the oxidation activity of nitrogen oxides in diesel exhaust. This paper investigated the physical properties (i.e. morphology, primary particle diameter, fractal dimension and nanostructure) and oxidation-induced fragmentation characteristics of diesel exhaust soot particles during oxidation degrees (0 %, 20 %, 50 % and 80 %) in different atmospheres (air, air-1000 ppm NO and air-2000 ppm NO). The results showed that during the oxidation process the variation trends of soot morphology in air and air-NO environments were similar, while the number and size of primary particles in an aggregate decreased and the fractal dimension of the aggregate increased with the presence of NO in air atmosphere. With the progress of oxidation, the nanostructure of soot particles became more ordered, while this variation trend was slowed down when NO was added to the air atmosphere. This is because soot particles oxidized in air-NO atmospheres showed less probability of internal oxidation but more external oxidation than those in air atmosphere. Over the oxidation process, the soot aggregate fragmentation rate presented a decreasing variation trend under each oxidizing atmosphere, with a higher aggregate fragmentation rate and more apparent variations in air-NO atmospheres. Moreover, in the air atmosphere, the probability of primary soot particle fragmentation showed a consistent upward trend, while the addition of NO slowed down this trend and showed an upward trend in stages 1(0 % ∼ 20 %) and 2(20 % ∼ 50 %), but a downward trend in stage 3(50 % ∼ 80 %). This suggests that the addition of NO reduces the probability of oxidants (especially O2) entering the particles, which would lead to a decrease in the probability of primary soot fragmentation caused by internal oxidation.

Effects of ammonia on morphological characteristics and nanostructure of soot in the combustion of diesel surrogate fuels

The morphological characteristics and nanostructure of soot particles in pure n-heptane (C7H16) and n-heptane/ammonia co-flow diffusion flames were analyzed and compared using thermophoretic sampling and transmission electron microscopy (TEM) observation combining with quantitative image information extraction methods. The results showed that the overall formation and evolution of soot particles in NH3-doped n-heptane flames along the flame centerline were similar with that without NH3-doping. However, compared to n-heptane flame, the peak average diameter of primary soot particles and the peak gyration radius of soot aggregates in NH3-doped flames were reduced by about 45% and 37%, respectively, which indicated that the growth of both primary soot particles via surface reaction/condensation and soot aggregates via coagulation were significantly decreased. Meanwhile, the fractal dimension of soot aggregates was lower with NH3 addition as the structure of soot aggregates was looser and tended to be more chain-like. After NH3 doping, the peak average fringe length inside soot particles was decreased by 13%, and the inter-fringe spacing and tortuosity of soot were increased by 8% and 3%, respectively. This represented a more disordered microcrystal structure and lower degree of graphitization of soot particles, meaningfully indicating a higher oxidation reactivity.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 3. The morphology and nanostructure of soot particles

Ammonia (NH3) fuel has been widely considered a competitive alternative fuel and hydrogen carrier due to its “non-carbon and hydrogen-rich” nature. In this study, the thermophoretic sampling combined with transmission electron microscopy (TEM) analysis was conducted to investigate the effects of NH3 addition with varying ratios (between 0% and 60% by volume fraction while the remainder is argon) on the evolution of morphology and nanostructures of soot particles along the flame axis in the ethylene (40% by volume fraction) co-flow diffusion flames. Results showed that the flame visible height increased with the addition of NH3, and the flame temperature decreased at the height above burner (HAB) below 30 mm while increased at HAB > 30 mm. With NH3 addition, the size of soot (primary particles and aggregates) and the number of soot primary particles contained in the soot aggregates decreased significantly. In addition, the investigation of soot nanostructures showed that the carbon layers with shorter fringe length (Fl) and higher tortuosity (Ft) would be oxidized before it arrived to the flame tip in the NH3-doped flame, resulting in the longer Fl and higher Ft. In general, the inter-fringe spacing (Fd) increased with NH3 addition. The evolution of soot morphology and nanostructures indicated that NH3 would delay the process of particle inception and surface growth.

Impact of multi-injection strategies on morphology, nanostructure and oxidation reactivity of diesel soot particles

The objective of the present study was to investigate and optimize the multi-injection parameters that were able to match the diesel particulate filter (DPF) after-treatment system for reduction of particulate matter (PM) generated from diesel engines. Three representative multi-injection strategies, including main plus post-injection (designated as M+Po), one pilot injection plus M+Po (Pi1+M+Po) and two pilot injections plus M+Po (Pi2+Pi1+M+Po), were selected to generate soot particles in a diesel engine. The morphology and nanostructure of soot particles, including primary particle size, fringe length, thermal stability of organic carbon (OC) and graphitization degree, were analyzed by transmission electron microscopy (TEM) and Raman spectroscopy. The OC content and oxidation reactivity were quantified by thermalgravimetric analyzer (TGA). Results revealed that pilot injection led to the formation of soot particle with smaller primary particle size, more ordered nanostructure, and higher graphitization degree, ascribing to the better air-fuel mixing, higher cylinder temperature, as well as longer residence time. On the contrary, post injection (e.g., M+Po) resulted in soot particles with more OC exhibiting larger size and more ambiguous boundary. Overall, the soot particles with more disordered nanostructure and more OC content showed better oxidation reactivity. Interestingly, the soot particle morphology and nanostructure were observed to be varying under TEM electron irradiation, which might be due to the OC heating dissolution.

Soot particle morphology and nanostructure with oxygenated fuels: A comparative study into cold-start and hot-start operation.

This study investigates the morphology and nanostructure of soot particles during cold-start and hot-start engine operation of a diesel engine using oxygenated fuels. The soot samples were analysed using transmission electron microscopy. The oxygen content in the fuel was varied between 0 and 12%. The results showed that the primary particles during cold-start have significantly smaller size when compared to hot-start engine operation. The addition of oxygenated fuels also resulted in smaller sized primary particles. Smaller radius of gyration and higher fractal dimension of soot aggregates during cold-start would mean smaller aggregate size with a more compact structure. Shorter fringes with a higher inter-fringe spacing for cold-start would mean lower graphitisation of soot particles that could be related to higher oxidation reactivity of soot particles.

Analysis of morphology, nanostructure, and oxidation reaction of soot particulates from CI engines with dimethoxymethane–diesel blends under different loads and speeds

Dimethoxymethane (DMM)–diesel blended fuels can simultaneously reduce exhaust emissions of soot and nitrogen oxide (NOX); several studies have been conducted in this regard. However, the influence of additive DMM on the production of inception and precursors of particulates, especially the relation between oxidation, morphology, and the nanostructure of soot particles has not been extensively investigated. In this study, a transmission electron microscope (TEM) and a thermogravimetric analyzer are introduced to acquire TEM images and conduct temperature-programmed-oxidation experiments. Aiming to study the influence of DMM addition on soot oxidation, morphology, and nanostructure, tests are conducted at different rotational speeds (1400 rpm and 2200 rpm), two engine loads (0.6 MPa and 1.2 MPa), and three fuels (D100, DMM6.4, and DMM13). The results show that the diameter distributions of all samples display a similar distribution, with the range of sample diameters being from 10 to 45 nm, and the addition of DMM reduces the dp (primary particle diameters) and the Df (fractal dimension), indicating a decreased structural compactness of aggregates, compared with diesel. Moreover, with increasing load and speed, La (the length of the fringe) increases and d (the distance between adjacent layer planes) decreases. Furthermore, with the addition of DMM, a more regular and higher degree of graphitization within soot particles can be observed in comparison to D100. The nanostructure influences the oxidation reaction of graphene segments with a line relation, leading to a difference in soot oxidation property.

The impact of chemical composition of oxygenated fuels on morphology and nanostructure of soot particles

Over the years, oxygenated fuels such as biodiesel and alcohol fuels have been useful in reducing particulate matter (PM) emissions of diesel engines. The presence of oxygen in the fuel impacts the soot oxidation process and thus the morphology and nanostructural characteristics of soot particles change. The nanostructure characteristics of soot particles hold an importance for their oxidation reactivity and toxicity. Higher reactivity and exposure of soot particles to oxygen will affect the regeneration and filtration efficiency of diesel particulate filters. In this study, we investigate the impact of oxygen functional groups on morphology and nanostructure of soot particles by blending different types of essential oil in diesel. The test fuels were prepared by mixing orange oil, tea tree oil, eucalyptus oil and coconut biodiesel to keep an overall oxygen content of 0 and 2.2%. Transmission electron microscopy was used to investigate the physical changes in the nanostructure of soot particles. The primary particle diameter decreases, and fractal dimension of soot aggregates increases for oxygenated fuels. Microscopy reveals that the arrangement of carbon lamellae within the soot particles changes significantly with use of oxygenated as observed by the changes in fringe length, fringe tortuosity and fringe separation distance. Lower fringe length, higher fringe tortuosity and fringe separation distance shows disordered arrangement of carbon layers with more possibility of oxygen attack. The different structural characteristics for oils having same oxygen content shows that formation and structure of soot particles strongly depend upon chemical structure and composition of the fuel.

Comparison of manual and automatic approaches for characterisation of morphology and nanostructure of soot particles

This study is an attempt to compare the manual and automatic approaches for analysis of the morphology and nanostructure of soot particles. The automated methodology is composed of two parts: soot particle analysis (morphology characterisation) and soot fringe analysis (nanostructure characterisation) such as fringe length, fringe tortuosity and fringe separation distance. In this study the application of Fast Fourier Transform (FFT) is introduced, which omits the manual selection of fringe pairs for measurement of fringe separation distance and aids fast computation. The parameters which effect the performance of automated approach were identified and optimised. The automated approach was applied to analyse images from different fuels. Using the optimised parameters, the performance of the automatic approach was tested against results obtained by the manual approach. The statistical comparison between two approaches showed similar results but with better accuracy for automated approach (lower standard error) and thus validates the functionality of the proposed methodology. The application of FFT in computation of fringe separation distance reduces the computation time and avoids biases of operator in selecting fringe pairs.

HRTEM evaluation of primary soot particles originated in a small-bore biofuel compression-ignition engine

Nanostructure of soot particles from a small-bore compression-ignition engine was investigated by high-resolution transmission electron microscopy (HRTEM). Four test fuels namely conventional diesel, waste cooking oil (WCO) biodiesel, Jatropha biodiesel, and Karanja biodiesel were studied. Lacey carbon TEM grids were utilized to capture soot particles from engine exhaust gas. An in-house image processing algorithm was developed to measure primary particle diameter, fringe length, fringe tortuosity, and fringe spacing. The HRTEM image revealed the presence of thicker absorbed hydrocarbon layers surrounding biodiesel soot primary particles than those of diesel soot. The primary particle size of WCO biodiesel was smaller than diesel, on the other hand, Jatropha biodiesel and Karanja biodiesel showed slightly larger particles. In terms of nano-structure analysis, WCO biodiesel and Jatropha biodiesel particles exhibited shorter fringe length than diesel, while the Karanja biodiesel particles showed the longest fringe length of 0.91 nm. Fringe tortuosity of biodiesels was smaller than diesel because of relatively lower portion of core area, where highly curved fringes existed. The soot particles from biodiesels exhibited larger fringe spacing than diesel, especially the Karanja biodiesel showed the longest fringe spacing of 0.67 nm compared to 0.55 nm in case of diesel.

Comparative study on characteristics of soot from n-decane and RP-3 kerosene normal/inverse diffusion flames

RP-3 kerosene is the most important hydrocarbon fuel used in civilian airplanes in China. Up to date, there are few experimental literatures on soot formation and oxidation of RP-3 kerosene available. In this experimental study, the evolution, nanostructure, and oxidation reactivity of the soot particles for RP-3 kerosene in normal and inverse diffusion flames, were investigated. The n-decane was selected as a base fuel for the comparison. Thermophoretic techniques followed by transmission electron microscopy were used to study the evolution of the soot particles in the different locations at the centerline and radial stream of the flames. Nanostructure, lattice parameters, and oxidation reactivity of the soot particles were examined by high-resolution transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis. The results showed that the size of RP-3 kerosene flame soot particles was bigger than n-decane flame at 20 mm and 30 mm height above the burner surface for normal diffusion flame. In inverse diffusion flame, RP-3 kerosene flame had bigger soot aggregates than n-decane flame. Moreover, RP-3 kerosene flame nanostructure had larger fringe length and shorter fringe tortuosity than n-decane flame in normal and inverse diffusion flames, which reflect lesser oxidation reactivity of the RP-3 kerosene flame. X-ray diffraction spectrum and thermogravimetric curves displayed good agreements with the high-resolution transmission electron microscopy images. Finally, the nanostructure of soot particles had a strong relation to their oxidation reactivity.

The Effect of Fuel Property on Nanostructure of Soot Particles from Diesel Engine

The Effect of Fuel Property on Nanostructure of Soot Particles from Diesel Engine

Relating aerosol mass spectra to composition and nanostructure of soot particles

The composition and carbon nanostructure of soot are important parameters influencing health and climate effects, and the efficacy of soot mitigation technologies. We used laser-vaporization, electron-ionization aerosol mass spectrometry (or SP-AMS) to systematically investigate relationships between aerosol mass spectra, carbon nanostructure (HRTEM), and composition (thermal-optical carbon analysis) for soot with varying physicochemical properties. SP-AMS refractory black carbon concentrations (based on C≤5+ clusters) were correlated to elemental carbon (r = 0.98, p < 10−8) and equivalent black carbon (aethalometer) concentrations. The SP-AMS large carbon (C≥6+, midcarbons and fullerene carbons) fraction was inversely correlated to fringe length (r = −0.97, p = 0.028) and linearly correlated to the fraction of refractory organic carbon that partially pyrolize during heating (r = 0.89, p < 10−4). This refractory organic carbon material was incompletely detected with conventional aerosol mass spectrometry (flash vaporization at 600 °C). This suggests that (SP-AMS) refractory carbon cluster analysis provides insight to chemical bonding and nanostructures in refractory carbon materials, lowcarbons (C≤5+) indicate mature soot and large carbons indicate refractory organic carbon and amorphous nanostructures related to C5-components. These results have implications for assessments of soot particle mixing state and brown carbon absorption in the atmosphere and enable novel, on-line analysis of engineered carbon nanomaterials and soot characteristics relevant for climate and health.

Relationships between the electrical properties and nanostructure of soot particles in a laminar inverse diffusion flame

This paper studies the electrical properties of the soot particles generated from an n-heptane inverse diffusion flame and the relationship between the electrical properties and the nanostructure. The electrical conductivity of isolated soot particle and work function of soot particles were measured by a PeakForce tunneling atomic force microscopy and a Kelvin-probe force microscopy. The nanostructure of soot particles was evaluated using X-ray diffraction and X-ray photoelectron spectroscopy. Different electrical conductivity distributions were found as the soot particles gradually matured, and at the same time the mean electrical conductivity of soot particles exhibited a nearly exponential increase. There existed a percolation threshold at which the mean electrical conductivity sharply increased by approximately two orders of magnitude. Similarly, the work function of soot particles increased during soot maturation process, which implies that it becomes harder for the electrons to escape from the soot samples. The crystallite width had a positive correlation with the logarithm of electrical conductivity and the work function for soot particles, while the interlayer spacing presented a negative correlation with the logarithm of electrical conductivity and work function. These results suggest that the electrical conductivity and work function can serve as indicators of ordering degree of soot particles. Moreover, the dependence of electrical properties on the nanostructure demonstrates the potential of better controlled flame conditions for producing tailored soot particles for a variety of applications.

Quantitative Image Analysis of Carbon Nanostructure of Particles Produced from Combustion Process.

In the present study, the soot particles produced from diffusion flames burning biodiesel fuel were thermophoretically sampled and the carbon nanostructure of soot particles were imaged using a high resolution transmission electron microscopy (HRTEM). The HRTEM images of soot particles were then quantitatively analyzed using a digital image processing algorithm developed and implemented as part of this work. The HRTEM interpretations with an aid of image processing support feasibility of use of the developed image processing technique for carbon nanostructure quantification.

The evolution of soot morphology and nanostructure in laminar diffusion flame of surrogate fuels for diesel

The morphology and nanostructure of soot particles in laminar diffusion flame has been investigated in this work. The fuels considered were diesel surrogate fuels, including n-heptane, n-butanol and the volumetric mixture 50% n-heptane/50% n-butanol (H50B50). Thermophoretic sampling and TEM (transmission electron microscopy) analysis were utilized to study the morphology and nanostructure of soot particles at different points on the centerline inside the flames. The soot morphological characteristics include primary particle diameter, projected area of aggregate, and fractal dimension. The nanostructure is quantitatively described by fringe length, tortuosity and inter-fringe spacing.Overall, n-butanol flame produced smaller primary particles and aggregates, followed by H50B50, and n-heptane flames. The primary particle diameter shows a trend of increasing first and then decreasing due to soot growth and oxidation respectively. The fractal dimension of aggregates almost keeps increasing along the centerline, indicating more and more compact soot structure. As the particles travel along the flame, a graphitic ‘onion-like’ structure develops where the core remains amorphous and the outer shell graphitises. In the process of soot growth, the fringes become shorter and less curved, meanwhile inter-fringe spacing increases due to electron repulsion between the adjacent carbon layers. Subsequently, the degree of graphitization would increase, concretely, fringe length increases, while tortuosity and inter-fringe spacing decrease. At the flame tip, most microcrystalline carbon layers in the outer shell are oxidized, which causes a sharp increase in the tortuosity, but does not have obvious effect on the other two parameters. In addition, for mature soot near the flame tips, particles in n-butanol flame has the maximum fringe length and tortuosity while the minimum inter-fringe spacing.

HRTEM evaluation of soot particles produced by the non-premixed combustion of liquid fuels

The morphology and nanostructure of soot particles as a function of fuel type in a laminar diffusion flame were studied. The fuels considered were heptane, toluene, the volumetric mixture 50% heptane – 50% toluene and a commercial gasoline. The structural features of soot sampled in the flame were inferred using HRTEM. An image analysis algorithm was developed to calculate the fringe length, tortuosity and inter-fringe spacing from HRTEM images. For the first time, the performance of the algorithm is tested against artificially generated TEM images with known geometries.Heptane flame produced the smaller primary particles followed by the heptane/toluene mixture and gasoline, toluene flame formed the largest particles. Toluene soot exhibits the longer and less curved fringes and more stacked layers. Heptane soot develops a similar degree of order as the aromatic flames but stronger oxidation at the tip reduces the fringe length and increases the tortuousity. There is no significant difference between the nanostructure of heptane/toluene mixture and the gasoline. The main differences in the nanostructure all the soot sampled was the degree of order of the PAHs within each particle. The fringe length was found to be similar for all the fuels, around the size of circumpyrene (C42 H16).

Nanostructure of Soot Collected from Ethanol Droplet Flames in Microgravity

The nanostructure of soot particles collected from spherically symmetric ethanol droplet flames were analyzed using a high resolution transmission electron microscopy (HRTEM). Nanostructure properties, including fringe length and curvature of carbon lamellae, were measured for soot particles collected in various inert environments. The sampling experiments were performed in the reduced gravity environments produced in the NASA 2.2 sec Droptower at the Glenn Research Center in Cleveland, Ohio. Microgravity droplet combustion experiments provide unique opportunities to vary the residence times over a large range and to independently vary the temperature. In this study, the time-temperature histories experienced by the soot particles were adjusted by means of inert substitutions (argon vs. helium) and variations in the initial droplet diameters (ranging from 1.6 to 2.2 mm). The variations in the initial droplet diameter were found to affect only the residence time necessary for soot inception and growth, whereas inert substitutions modified both residence time and temperature. The measurements of soot nanostructure properties indicated that the higher temperatures produced in the argon inert environment produced more graphitic nanostructures, while the lower temperatures produced in the helium inert environment produced more amorphous nanostructures at the inner core of the soot primary particle. The variations in the initial droplet diameter produced distinct soot nanostructures on the periphery of the soot particle. The higher residence times experienced for the largest initial droplet diameter experiments produced longer carbon lamellae with negligible curvature, while the lower residence times for the smallest initial droplet diameter experiments produced shorter carbon lamellae with higher degrees of curvature. These experimental results provide important foundational understanding of the influence of residence time and temperature on the soot nanostructure that has not been studied previously.