Effects of Flame Configuration and Soot Aging on Soot Nanostructure

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Effects of flame configuration and soot aging on soot nanostructure and reactivity in n-butanol-doped ethylene diffusion flames Yaoyao Ying, and Dong Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00042 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Effects of flame configuration and soot aging on soot nanostructure and reactivity in n-butanol-doped ethylene diffusion flames Yaoyao Ying1,2, Dong Liu1,2,* 1

MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and

Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China 2

Advanced Combustion Laboratory, School of Energy and Power Engineering, Nanjing

University of Science and Technology, Nanjing 210094, People’s Republic of China Keywords: Flame configuration; Soot aging; Nanostructure; Reactivity; n-Butanol

ABSTRACT: Soot has received considerable attentions since it is a major pollutant in exhaust gas from fossil fuel combustion, and it causes adverse climate and health effects. This work focused on soot morphology, nanostructure and reactivity variations regarding the soot collected in different flame configurations of n-butanol-doped ethylene inverse diffusion flame (IDF) and normal diffusion flame (NDF) at different heights above burner surface (HAB = 20 mm, 30 mm, and 40 mm). The effects of flame configuration and soot aging on structural and reactivity


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characteristics were analyzed with emphasis on the differences of young and mature soot generated in IDF and NDF without and with n-butanol addition at specific positions, respectively. The effects of fuel-side n-butanol addition on IDF and NDF soot structures and reactivity were also evaluated. The soot obtained using thermophoretic sampling technique was analyzed by transmission electron microscopy (TEM) to investigate soot morphology evolutions along the centerline and the boundary of the flames at different axial locations. Moreover, soot samples collected by quartz plate were characterized by thermogravimetric analyzer (TGA), highresolution transmission electron spectroscopy (HRTEM), Raman spectroscopy, elemental analyzer, and surface area and porosimetry analyzer. It showed the soot generated in IDF and NDF could have different nanostructure and reactivity relying on the flame configurations and soot aging. The oxidation reactivity for soot in IDF without and with n-butanol addition slightly decreased with the increase of collection height. While as the collection height rose in ethylene and ethylene/n-butanol NDF, the finial mass loss percent of soot and the average oxidation rate increased. The n-butanol addition in IDF and NDF generally enhanced soot oxidation reactivity. The structural analysis via HRTEM and Raman indicated that soot from IDF with and without nbutanol addition was young, which presented amorphous particles with irregular shapes. Whereas, the ethylene NDF and ethylene/n-butanol NDF soot were composed of well-defined spherical particles and showed a typical core-shell structure. Furthermore, HRTEM photographs displayed an evident discrepancy between the oxidation modes of soot from ethylene and ethylene/n-butanol IDF and NDF at HAB = 30 mm. High correlations between the nanostructure and reactivity for the soot of ethylene IDF and NDF without and with n-butanol addition were found. With an increase in the degree of crystallization in soot nanostructure, the soot reactivity decreased.


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INTRODUCTION Soot (also known as black carbon particles) emitted by many combustion applications such as

internal combustion engines and furnaces burning with hydrocarbon fuels is a major pollutant to the urban environment, and can cause serious environmental problems.1 More importantly, soot presents a significant hazard to human health, and may cause carcinogenic, cardiac or respiratory disease, especially when its size is in the nanometer range.2-4 Due to the damaging effects of soot and the increasingly stringent soot particles emission regulations, understanding soot formation process for distinct flame conditions is essential. Moreover, a wide range of studies have been motivated in attempts to reduce soot emissions. Since understanding soot formation and growth processes are important to aid researchers and engineers in designing efficient combustion devices, the nanostructure and reactivity of soot particles for different flame conditions are closely involved with the understanding of soot formation. Fuel composition can affect soot characteristics, and molecular growth of condensed species is important in soot inception.5 Soot reactivity is connected to the physical and chemical features of the particles, which can be affected by combustion conditions and fuel molecular structures.6 Vander Wal and Tomasek7 illustrated that the soot particles derived from acetylene, benzene and ethanol had different nanostructures which resulted in corresponding variations of oxidation rates. The soot generated by benzene pyrolysis exhibited amorphous structure and was more reactive than the crystallite structured soot derived from acetylene pyrolysis, showing that soot nanostructures accounted for the differences in oxidation rates. In addition, Vander Wal8 further explored the impact of soot nanostructure covering fringe length, tortuosity and interval space of the carbon lamellae on soot oxidation behavior, and illustrated that soot which showed higher oxidation rate had shorter fringe length, and larger tortuosity and interval space.


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Numerous recent studies have also been conducted to investigate the relation between soot nanostructure and oxidation rate, and all concluded soot reactivity strongly depended on initial soot nanostructure.9-18 Furthermore, it has been illustrated that soot nanostructure is influenced by formation conditions for example residence time, flame temperature, and initial fuel property.19 With the increasing studies paying attention to the fuel blends, oxygenated fuel additives such as biodiesel,10,14,20 alcohols,20-26 and ethers25-27 were widely used to be doped in conventional fossil fuels. The blended fuels showed a positive impact on reducing soot emissions. As mentioned above, alcohols have been studied extensively because of their outstanding effect on soot reduction as a favorable fuel additive candidate. Butanol is a quite promising biofuel which provides many benefits comparing to ethanol, including lower water absorption, higher energy content, and better miscibility with present fuels.28 Butanol also has a higher cetane number and is less corrosive than methanol and ethanol.29 As a result, n-butanol has been widely studied as fuel additions in diesel engines on the combustion and soot emission behaviors, and the results showed n-butanol blends had great potential in lowering soot emissions.30-32 Besides, Ghiassi et al.18 researched the impact of nanostructure on soot surface reactivity in a two-stage burner with n-butanol addition and showed that the oxidation reactivity was related to the order degree of the layer planes. Moreover, particles in early stages of the soot formation process are intriguing research targets. The detailed inspection of young soot can improve the understanding of soot evolution and model development. Dobbins et al.33 explored the chemical development of young soot from the centerline of ethene normal diffusion flame (NDF) by laser microprobe mass spectrometry. Schenk et al.34 investigated PAHs formation in opposed-flow flames and nascent soot


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morphology in premixed flames of C4 fuels. The results showed the differences in nascent soot morphology in premixed flames were due to the residence time and the chemical property of the fuel. Moreover, Schenk et al.35 used Helium-ion microscopy (HIM) to analyze soot morphology and geometrical characteristics of premixed ethylene flames and revealed that the nascent soot possessed no well-defined morphologies. Alfè et al.36 studied the nanostructure organization and chemical properties of young and mature soot probed from premixed flames burning aliphatic and aromatic fuels. They found the soot nanostructures and chemical natures were depended on the fuel property and soot aging, and established a correlation between soot nanostructure and bulk features. In recent years, inverse diffusion flame (IDF) was used to study the formation and emission of PAHs and young soot in ventilated conditions because large samples of young soot could be obtained without invading the flames.37-58 Blevins et al.46 concluded that particles produced in ethylene IDF resembled precursor particles and seemed to be young soot which was on the border of carbonizing. Santamaría et al.57 performed a study on the chemical and morphological characterizations of soot and soot precursors from ethylene and benzene IDF. The results presented that soot particles produced in ethylene IDF were in the incipient period of soot formation, while the particles from benzene IDF had a high degree of crystallinity. The flame structure, soot evolution and carbonization of young soot particles were experimentally measured to investigate the soot inception and growth processes, and the effects of oxygen enrichment in ethylene IDF.50,58 Ying et al.59 investigated soot nano-characteristics and oxidation rate of ethylene IDF individually adding three pentanol isomers and showed that the pentanol isomer additions delayed the growth of carbonization and enhanced the oxidation reactivity of the particles. Very recently, Ying et al.60 performed the comprehensive investigations of soot nanostructures and oxidation behavior in ethylene IDF and NDF doped by butanol and focused


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on the butanol isomers influences, but that study did not consider the effects of collection height and soot aging, and was also in the absence of in-flame soot sampling analyses. However, very few of these studies have fully compared the nanostructures and reactivity of young and mature soot for different flame configurations at different axial positions as well as flames with butanol additions. Hence, the major objective of the present work was to probe soot nanostructure and oxidation characteristics regarding the soot formed both in ethylene IDF and NDF with n-butanol additive, and the focus here was put on two key aspects: (1) the effect of flame configuration on soot physicochemical properties and evolutions along flame centerlines and wings, and (2) the effect of soot aging on soot structure characteristics and oxidation reactivity. The in-flame soot particles were collected by thermophoretic sampling technique along flame centerlines and wings for soot morphology evolution investigation. In addition, quartz plate collection method was also used to collect abundant soot particles at different heights above burner surface in IDF and NDF for various characterization methods. The soot aging effects on structural and reactive properties were analyzed centering on young and mature soot generated from IDF and NDF with and without n-butanol addition at specific positions, respectively. To gain a deep understanding on soot oxidation process of IDF and NDF soot, the partially oxidized soot was investigated. Furthermore, the relationship between nanostructure and bulk properties of soot from pure ethylene flames as well as ethylene/n-butanol mixture flames was estimated as well. 2.

EXPERIMENTAL SECTION 2.1. Burner Setup. A co-annular laminar inverse diffusion flame burner, same with our

previous works,59,60 was set up for this work. The inner diameters of the centric tube, the midterm tube and the exterior tube were 3.86 mm, 15.30 mm and 32.36 mm, respectively.


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Perforated plates made of stainless steel were placed in the tubes to provide uniform and stable gas flows. In the IDF configuration, the centric tube was used to supply the oxidizer flow and the midterm tube was used for the fuel flow (ethylene or ethylene/n-butanol mixtures). Nitrogen shield was formed by the exterior tube to avoid the fuel contacting with surrounding air. In order to generate an NDF, the fuel stream was still transported by the midterm tube, but the exterior tube was for the oxidizer stream. A high performance pump (Gilson, Minipuls Evolution, France) served as the liquid fuel delivery system to send liquid fuel to an evaporator at constant volumetric flow rates. From the pump to the evaporator, the liquid fuel was transported through a stainless steel tube with an outer diameter 1/16". The vaporized fuel was then conveyed by N2 passing through the evaporator to the fuel delivery tube. The whole liquid fuel transport route was winded by heating tapes to maintain constant temperature at 150 oC (423 K) to avoid vapor fuel condensation along the flow route. Two thermocouples were embedded along the transport route mainly in the evaporator and in the middle of the fuel transport route to assure that adequate heating achieved. The preheating was adopted for pure ethylene IDF and NDF as well to keep the same boundary conditions in all the flames. The diagrammatic drawing of experimental system was presented in Figure 1. 2.2. Flame Conditions. In the present study, ethylene was used as the base fuel to produce the IDF and NDF because ethylene has been extensively utilized in soot formation investigations. The n-butanol-doped ethylene flame was obtained by substituting part of ethylene. Both in IDF and NDF, 15% volume fraction of ethylene fuel were replaced by n-butanol. The flame conditions were demonstrated in Table 1. The flow rates for all gas streams were dominated by high precision digital mass flow controllers (Sevenstar, CS200A). In order to obtain a better


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comparison, the initial visible flame heights of both ethylene inverse and normal diffusion flames were adjusted to close to each other. The flame height was determined using the enlarged direct flame photos taken by Nikon D3000 camera with a vertical calibrated scale. All the flames were approximately 45 mm in visible height as shown in Figure 2. Flame temperatures were measured using an uncoated, B-type thermocouple (Omega Engineering, Inc.) by the rapid thermocouple insertion method. The thermocouple was installed on an electric cylinder and the spatial resolution accuracy of the electric cylinder was ± 0.08 mm. The wire diameter of the thermocouple was 0.2 mm, with a bead diameter of about 0.6 mm. Measured maximum temperatures and temperature radial distributions at specific heights above burner surface (HAB = 20 mm, 30 mm, 40 mm) of the four flames were presented in Figure 3. The flame temperatures were corrected for radiation losses from the thermocouple surface.61 The accuracy of the temperature measurements was within ± 50 K. The maximum temperature of each flame was very close. 2.3. Soot Sampling. Thermophoretic sampling (TS) technique was employed to capture soot particles from the different flames for morphology analysis. The thermophoretic sampling device consisted of an electric cylinder (FESTO, EGC-50-100-TB-KF-0H-GK), a motor controller (FESTO, CMMP-AS-55-M-LS-TM) and a TEM grid (200 mesh carbon coated square Cu grid with the diameter of 3 mm and thickness of 30 µm). The particles were driven thermophoretically to the cold grid surface due to the temperature gradient.62 A high-speed camera (MIKROTRON, Eosens mini champion) was utilized to trace the motion of the grids inside and outside the flame as well as the residence time. The grid was inserted rapidly into the flame at 1 m/s, and stayed in the flame for about 40 ms. To minimize the contamination from the annular region when the grid was passing through the flame, the transit time of the grid was less


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than 10 ms. Sampling positions were on the centerline and the boundary of the flames at three different heights above burner surface (HAB = 20 mm, 30 mm, and 40 mm). The purpose was to compare the morphology evolution of the particles on the flame wing with that of the centerline. In addition, for the analysis of the soot structural and reactivity characteristics (e.g. fringe analysis, and oxidation reactivity), soot was collected from the three corresponding heights (HAB = 20 mm, 30 mm, and 40 mm) by a quartz plate with a diameter of 95 mm, which was similar with the collection method in previous studies.60,63-65 The large plate was used to collected soot since thermophoretic sampling cannot gather abundant samples for oxidation analysis and other diagnostics. A large plate was also employed in many previous studies to form different stagnation flames to track the development of the particle size distribution function of nascent soot.66-74 The large plate sampling is effective to obtain sufficient soot for analytical characterization. The soot was accumulated upon the quartz plate which was cooled via circulating water. Soot collection time was 30 min. Soot mass was shown in Table S1 (Supporting Information) as obtained by quartz plate collection in the four flames at different heights above burner surface. The soot was then scratched from the plates and crushed into powder to carry out the following characterizations. 2.4. Thermogravimetric Analysis (TGA). The soot oxidation tests were conducted by a NETZSCH STA 449 F3 Jupiter thermogravimetric analyzer and the weight changes were recorded by the recording software automatically through the whole process. The oxidation procedure kept same with our previous works59,60 and was set in the build-in software of the analyzer. Briefly, 10 mg soot was firstly heated up in an argon flow (100 ml/min) from 50 °C (323 K) to 300 °C (573 K) and then maintained for 60 min to remove volatile compounds. Then, soot was heated up to 500 °C (773 K). Afterwards, the argon flow was substituted by an oxygen


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mixture flow consisting of 22% O2 and 78% argon. The mixture flow was kept constant (100 ml/min) for 150 min during the isothermal oxidation process. The oxidation process then automatically stopped when the setting oxidation procedure was finished. A new oxidation procedure would start when the analyzer cooled down to the ambient temperature. The soot mass loss during oxidation process was normalized regarding the weight after pretreatment. The percent of the remaining mass ( wt % ) was calculated as: wt % =

mo − m , where mo was the mo − ml

initial soot mass, m was the soot mass at a certain time during the oxidation procedure, and m l was the remaining soot mass when oxidation was ceased. The normalized soot mass loss curves were compared to interpret soot oxidation behavior. For a series of TGA tests (each soot sample was conducted for at least 3 times), the experimental uncertainty was ± 6.5% error with 95% confidence. TGA mass loss curves of different oxidation temperature conditions were used to evaluate the soot apparent kinetic parameters as well.60 The oxidation temperatures were 520 °C (793 K), 530 °C (803 K), 540 °C (813 K), and 550 °C (823 K). To extract the soot rate constants, an Arrhenius-type equation was adopted as follows: −

where k c = A exp(

dm = k c mc pOn 2 dt

(1)

− Ea ) was the apparent rate constant, mc was a constant, p O2 was the partial RT

pressure of oxygen, and n was the reaction order.65 Afterwards, activation energy Ea and preexponential factor A can be estimated from the slope and intercept of the plot ln(−d(

m )/dt ) mo

against 1/T. After that, the average rate constant kc was calculated by the determined Ea and A.


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2.5. Transmission Electron Microscopy (TEM) and High-Resolution Transmission Electron Microscopy (HRTEM). The thermophoretic soot was detected utilizing a TEM (Philips, Tecnai 12), operating at 120 kV. The exposure time was 0.4 s. The soot morphology was analyzed by coupling two magnifications: 18500 × and 97000 ×. The soot sampled by quartz plate was directly adopted for structural analysis to better correlations between general soot nanostructure and oxidation behavior. The soot morphologies and nanostructures were characterized using an FEI Tecnai G2 F20 S-Twin TEM operating at 200 kV (resolution 0.14 nm) with low-magnification and high-magnification, respectively. All images (2004 × 1336 pixels) were gained in a Gatan Digital Micrograph with a Gatan 832 CCD camera. The exposure time was 0.3 s. For morphology and nanostructure analysis, the soot was dissolved in ethanol by ultrasonication and the suspension was dropped to the TEM grid, and then the grids were dried by an infrared lamp. For each analyzed sample, at least four wide apart positions of the TEM grid were observed, and more than twenty pictures were acquired. For each sample, five images with clear organization of carbon layers were chosen for image analysis. In this paper, only typical images were shown.

2.6. Fringe Analysis. To relate the soot nanostructure with the oxidation behavior, soot nanostructure characteristics including fringe length and tortuosity were analyzed by applying fringe analysis algorithms. The measurement of physical length of carbon layer planes was considered as fringe length, while the curvature of the fringes caused by odd numbered ring structures was represented by fringe tortuosity.75 The fringe tortuosity was defined as the ratio of the fringe length to the distance between the two endpoints. HRTEM images were quantified via a homemade MATLAB software,60 using the algorithms developed by Yehliu et al.76,77 In this work, fringes shorter than 0.5 nm were eliminated.36 For each condition, at least three HRTEM


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images were analyzed. The distributions of fringe length and curvature were concluded from the obtained length and tortuosity results, which were fit to a 2-parameter lognormal PDF with mean value and standard deviation.65 The largest standard deviations of the acquired fringe length and tortuosity were 0.05 nm and 0.02, respectively.

2.7. Raman Spectroscopy. Raman spectra of the initial and partially oxidized soot was measured using a confocal Raman microscope of Horiba Jobin-Yvon LabRAM HR800, which was equipped with an Ar ion laser with an excitation wavelength of λ = 514.5 nm and source power of 10 mW. The initial and partially oxidized soot was in powder form. The partially oxidized soot was obtained when the TGA procedure achieved the expected degree of oxidation. Wavelength calibrations were conducted with a silicon film using the first-order phonon band of Si at 520 cm-1 (Raman shift). The microscope was manipulated with a 50× objective lens. It was equipped with a XY stage for selecting the sample area of interest and a CCD camera for detecting. All Raman spectra were detected from 800-3500 cm-1 with a grating of 1800 grooves/mm. To prevent alteration of the carbon structure caused by the laser-induced heating impacts, the laser power was reduced to record for concordant spectra without unwanted extra influences. For each sample, the Raman spectra were recorded with an exposure time of 40 s from at least three positions to ensure better meaningful results. All spectra were processed via LabSpec5 software.

2.8. Elemental and Surface Area Analysis. Elemental analysis of the collected soot was performed by the combustion method using an Elementary vario EL cube analyzer. The contents of C and H were detected directly, while the O content presented in soot was determined by difference with a 100% base.57,60 The soot surface area and pore parameters were measured by a


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BELSORP-max surface area and porosimetry analyzer using the method of isothermal N2 gas adsorption. The surface area was determined by the method of Brunauer-Emmett-Teller (BET).

3.

RESULTS AND DISCUSSION 3.1. Initial Soot Morphology. Figure 4 represents a series of TEM photographs of soot

particles extracted thermophoretically along the centerline and boundary of ethylene IDF at three different heights above burner surface. On the centerline, soot sampled at HAB = 20 mm shows both transparent and opaque aggregated particles. The aggregate sizes and concentrations are small. The primary particle size and the aggregate increase where the high particle concentration leads to cluster formation (a grouping of a number of aggregates) with HAB increase. The size of agglomerate still increases when the particle growth has stopped as extracted from HAB = 30 mm and 40 mm due to a slight cluster-cluster aggregation (CCA) via collisional growth.51 The soot morphology variations on the boundary at the specific heights are similar with that on the centerline. Both transparent and opaque particles are obvious. However, the aggregate sizes and concentrations on the boundary are smaller than that along the centerline at the corresponding heights. This can be attributed to the flame temperature differences as Figure 3 shown. The flame temperature on the boundary is much lower than that on the centerline, which results in the reduction of the aggregate growth.51,78 Furthermore, the particles generated on the boundary have relatively short residence time, which are transported to the fuel side by convective and thermophoretic forces. Thus, the aggregate volumes of the particles are small due to less surface growth.61 Figure 5 depicts TEM images of soot extracted from ethylene/n-butanol IDF on the centerline and boundary at specific heights. Figures 5a-f are micrographs of soot particles sampled on the centerline at HAB = 20 mm, 30 mm, and 40 mm, which shows the aggregate sizes and


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concentrations increase with the increase of collection height. The soot particles grow along the centerline as the residence time rises. TEM images reveal translucent and liquid-like precursor particles which are transformed from semitransparent tarlike material.46,51 The term tarlike material refers to disorganized and unstructured material with a lower carbonization degree.31,51 The material subsequently experiences carbonization process by coagulation and coalescent collisions. While on the boundary of the flame at HAB = 20 mm, most of the particles are single and nearly uniform diameter particles as shown in Figure 5g, and simultaneously a few aggregates containing only several particles appear. Figure 5j shows small clumps of particles with a higher magnification at the corresponding height with Figure 5g. These TEM images resemble the micrographs shown in Fig. 2a-d of a previous study, which are considered as young soot.46 With the increase of sampling height, the sizes and concentrations of aggregates grow. Comparing Figure 4 with 5, it reveals that the soot particles in ethylene/n-butanol IDF are in small aggregates and less carbonized.46 This feature can be attributed to the addition of n-butanol in ethylene IDF, which reduces the degree of carbonization. While the pure ethylene IDF soot carbonizes more rapidly and produces larger aggregates by CCA.51 Figure 6 presents the soot morphology development along the centerline and boundary of ethylene NDF. The images display the soot growth and oxidation processes in flame. The primary particle diameter and aggregate number density significantly change with the sampling heights. The particle number density per agglomeration increases in the upper region of the ethylene NDF. At HAB = 20 mm on the centerline, the particles are presented in small soot aggregates consisting of several particles. In higher regions, aggregates consisting of dozens or hundreds of particles with lengths of a few micrometers are observed. The aggregates grow due to the collision of independent aggregates after the collision process.79 Figures 6d-f clearly show


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that the particle size changes significantly with HAB. The particles sampled near the flame tip are the smallest. The soot particles at HAB = 40 mm are almost spherical particles with uniform diameter, which is resulted from the effect of soot oxidation.62 Similar to the soot particles on the centerline for ethylene NDF, an obvious alteration of particle size and number of soot aggregate appears along the flame boundary with HAB. The particle concentration per aggregate increases as the sampling height rises. At a distance near the flame tip, HAB = 40 mm, the soot consists of hundreds of small particles with nearly uniform diameter and spheroidal shape. Comparing Figures 6a-c and Figures 6g-i, the number of particle aggregate and soot concentration collected on the boundary are much higher than that on the centerline at the corresponding heights. This characteristic may be resulted from the temperature differences between the centerline and boundary because soot formation processes are significantly influenced by temperature.80,81 The soot inception rate becomes higher as oxygen content increases.82 As a result, a higher degree of aggregates is found on the flame boundary. Figure 7 displays the soot morphology evolution along the centerline and boundary at different heights for ethylene/n-butanol NDF. Figures 7a-c and Figures 7g-I present the soot number density and agglomeration degree along the centerline and boundary of the butanol-doped NDF at different HAB. Similar to the ethylene NDF, the primary particle size changes as the sampling height increases. An increase in particle concentration per aggregate is observed as the collection position approaches the flame tip. At HAB = 20 mm, the soot samples are composed of a few nearly round particles. The particle aggregates grow larger at HAB = 30 mm, which are resulted from particle-particle collisions, as well as aggregate-aggregate collisions.79 The particle size of n-butanol-doped ethylene NDF is larger than that from ethylene NDF at this height. Surface

growth of the particles accounts for this phenomenon, which happens with the addition of mass


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on the surface. Surface growth along the centerline is controlled by PAHs condensation,83 while on the wings is commanded by H-Abstraction-C2H2-Addition (HACA) mechanism.84 This may contribute to PAH concentration increase with n-butanol addition which ultimately results in surface growth. At the upper sampling height, HAB = 40 mm, the primary particle diameter decreases due to soot oxidation process. Another interesting feature of the ethylene/n-butanol NDF soot collected at HAB = 40 mm is that the soot particle size is slightly larger than that from ethylene NDF at the same height. This can be explained that the n-butanol-doped NDF is a bit higher than ethylene NDF, and the temperature at the collection position is slightly lower. Thus, the degree of soot oxidation for the ethylene/n-butanol NDF reduces, which results in a light larger particle size. To construct a compact correlation between general soot nano-characteristics and reactivity, structural analysis for quartz plate collected soot including morphology and nanostructure at different HAB in the flames was carried out before TGA experiments. Figure 8 shows the typical TEM photographs of soot derived from ethylene IDF and ethylene/n-butanol IDF at HAB = 20 mm, 30 mm, and 40 mm, respectively. The TEM images shown in Figures 8a-f are all at a resolution of 100 nm and the micrographs shown in Figures 8g-l have a 50 nm reference marker for better comparisons. TEM photographs of IDF with and without n-butanol all show heterogeneous characteristics with both liquid-like amorphous particles and tightly coalesced primary particles as shown in Figures 8a-f. The amorphous particles show no clear boundary between primary particles, which may contain condensable species.46,57 The amount of the amorphous particles in n-butanol-doped IDF is a little more than that in ethylene IDF collected at the coincident height. With the increase of collection height, the amorphous particles decrease in both ethylene IDF and ethylene/n-butanol IDF. The amorphous particles showing irregular


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appearances demonstrate that particles mass grows by surface deposition and coalescence when the particles travel through the flame.57 The amorphous particles resemble the precursor particles just starting to carbonize, which resemble the particles found by Blevins et al.,46 claiming as young soot. The low resolution TEM images also show that soot agglomeration degree grows as the collection height approaches the flame tip. This tendency is consistent with that observed in the morphology analysis of thermophoretically sampled soot. In Figures 8g-l, the variation of the primary particle size is not evident. The amorphous carbon particles with irregular shapes and the coalesced particles are difficult to measure the sizes due to the tight bundle of primary particles. In contrast, the soot samples from NDF present distinctive morphology, as shown in Figure 9. The soot particles obtained from ethylene NDF and ethylene/n-butanol NDF both appear as chain-like aggregates made up of hundreds of almost rounded particles with clear boundary. The primary particle size is significantly influenced by the collection height within the flame. The sizes of the particles are strongly reduced in the upper part of the flames, especially the soot samples collected at HAB = 40 mm as shown in Figures 9c and f. The diameter of ethylene NDF soot is much smaller than that of ethylene/n-butanol NDF soot at this location. It is because the flame height grows with the addition of n-butanol, and the particles collected at the same height with ethylene NDF are less oxidized and the diameter of the particles is larger. The reduction in particle size clearly presents the effect of particle oxidation. Furthermore, the soot density of the aggregates increases with HAB in ethylene and ethylene/n-butanol NDF. The soot aggregation is caused by impendent aggregates collision after the collision process.62,79 The mean size of nearly spherical particles in aggregates produced in ethylene/n-butanol NDF is slightly larger than that from ethylene NDF. The surface growth may be caused by the doped n-butanol as the


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oxygenated additions in diffusion flames can result in an increase of soot formation.85 The variation trends of the particle size and soot number density in agglomeration coincide with that obtained by thermophoretic sampling, which reveals that the sampling duration in the present study not strongly and obviously influences the soot morphology. The profound differences between the soot morphology of IDF and NDF are because of the different flame configurations. In an NDF, soot forms on the fuel side, and then transports from the fuel flow toward the flame. Then, soot inception, coagulation, and growth are followed by oxidation as they go through the flame.44 Thus, the particle sizes initially increase and then decrease. Whereas in an IDF, a central air stream is surrounded by a fuel jet.86 The soot particles are formed at the fuel side during the formation process, and then are transported away from the flame by thermophoretic forces.44,58 The IDF soot escapes oxidation since soot never travels through the flame but inversely moves outward to cooler zones of the fuel flow.39,48,53 Therefore, they can avoid significant oxidation and carbonization, presenting characteristics of nascent and young soot.

3.2. Initial Soot Nanostructure and Fringe Analysis. Figure 10 illustrates typical HRTEM photographs of the soot from IDF without and with n-butanol addition at three different heights above burner surface, based upon surveying more than twenty images for each soot sample. All the HRTEM photographs are at a resolution of 5 nm for visual comparisons. As seen in Figures 10g-l, the IDF soot consists of amorphous particles with irregular shapes according to Figure 8. The amorphous particles appear as very disordered carbon and show very little or no crystallinity, in agreement with previous works of the observations of young soot nanostructures.36,87 In Figures 10a-f, the small and tightly bundled primary particles present short and curved lamellae, which show a trend to fullerenic nanostructure. The lamellae are not tightly packed and exhibit little stacking order. The soot particles gathered in ethylene and ethylene/n-


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butanol IDF at three different axial positions all lack large shells of evident fullerenic structures or graphitic structures, and the nanostructures are strongly heterogeneous with both disordered carbon and fullerenic carbon layers, which agrees with the results shown in recent work.59,60 The discrepancy among the six kinds of soot samples is not significant by visual comparison. A distinctive difference in nanostructures between the soot particles produced in IDF and NDF, in other words, between young and mature soot, could be seen with visual observations from Figures 10 and 11. In Figure 11, soot generated in ethylene and ethylene/n-butanol NDF exhibits the classic core-shell structure.7,88 The inner core of the particles is composed of a small part of several carbon layers oriented in a random pattern, which is marked by yellow arrow. Whereas, the outer shell comprises carbon lamellae oriented in a concentric mode and primarily parallel to the adjacent layer. The result is consistent with the recent work.60 The NDF soot consists primarily of extended, straight and nearly parallel organized fringes, which suggests a higher degree of graphitization.75,89 The particles generated in ethylene NDF with and without n-butanol addition show that the size of the soot primary particles strongly changes with the collection height increases within flame. The particle size increases from HAB = 20 mm to 30 mm may be attributed to the particle surface growth, and then decreases from HAB = 30 mm to 40 mm due to oxidation. The soot from NDF exhibits a crystallite organization in structure,9,65,90 an indicator of graphitic-like structure. The result above illustrates that the young soot obtained in both ethylene and ethylene/n-butanol IDF seems to be less carbonized and graphitized than those mature ones produced in ethylene and ethylene/n-butanol NDF.53 To have a deeper understanding of the soot nanostructure, the carbon layers in the primary soot particles at different HAB in IDF and NDF are studied via fringe analysis algorithms. The extracted skeletons of carbon layers in the regions of interest (ROI) of Figures 10a-f and 11a-f


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are shown in Figure 12. The differences in nanostructure of the segments are unclear by visual comparison of the skeleton figures. Therefore, quantification of these HRTEM images described nanostructure order as fringe length and tortuosity are given via these lognormal fitting curves. The lognormal fitting curves of fringe length and tortuosity resulting from the fringe analysis are shown in Figures 13 and 14. Figure 13 shows the extracted fringe length and tortuosity distributions across the six kinds of soot which are gathered from three different heights above burner surface in ethylene IDF and ethylene/n-butanol IDF. The resemblance of the distributions verifies that the initial IDF soot nanostructure is very similar especially in fringe length. With the increase of the collection height in ethylene IDF with and without n-butanol addition, the fringe length slightly increases while the fringe tortuosity decreases. Comparing the soot structure of ethylene/n-butanol IDF with ethylene IDF at the same sampling height, the variation of the fringe length is very small. However, the distribution of fringe tortuosity is broadened with the nbutanol addition at the corresponding height. Fringe length and tortuosity distributions in Figure 14 present that the NDF soot particles have a wider range of length and a narrower distribution of tortuosity than IDF soot, which indicates a character with longer average fringe length and lower average fringe curvature in NDF soot. This finding coincides with the recent work.60 The fringe length distribution slightly shrinks and the fringe tortuosity range enlarges as the collection height shifts to the upper region in NDF. Moreover, with the addition of n-butanol in NDF, the variations of fringe length and fringe tortuosity show a similar trend as the fringe length becomes shorter and the fringe tortuosity longer, except the soot collected at HAB = 20 mm. The variations of the distributions for the fringe parameters reveal that the IDF soot is less graphitized than the NDF soot, and generally the n-butanol addition in flame could slightly


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decrease crystallite order of the generated soot. Median fringe length and curvature values with standard deviations from the fitting curves are listed in Table 2.

3.3. Morphology and Nanostructure of Partially Oxidized Soot. To gain deep knowledge about what happened to the soot during isothermal oxidation, partially oxidized (extent 30% and 70%) samples of ethylene and ethylene/n-butanol IDF and NDF collected at HAB =30 mm were studied by HRTEM. The samples were acquired when soot lost approximately 30% and 70% mass of the initial loadings in TGA experiments, respectively. The process of preparing TEM grids was the same with the initial soot samples. Figures 15 and 16 show the TEM images of approximately 30% and 70% partially oxidized soot, which were initially produced at HAB = 30 mm in IDF and NDF without and with n-butanol addition. Comparing with the initial morphology of IDF soot, the aggregates of the partially oxidized IDF soot are quite large and are composed of a range of much smaller size particles. Figure 15 shows that the amorphous particles disappear and the primary particles shrink slightly and agglomerate more tightly especially the 70% partially oxidized soot, which reflects the development of densification due to the oxidation reactions inducing reorganization of the fine structure.91,92 Figure 16 provides the images of 30% and 70% partially oxidized NDF soot. A reduction of primary particles sizes of the NDF soot is seen as well. Close inspection of the interior of the 70% partially oxidized NDF soot shows hollow interiors with thick outer boundary which forms the structure similar to capsule and the hollow shell-like particles appear together within aggregates. Figure 17 shows the images of nanostructure of approximately 30% and 70% partially oxidized IDF and NDF soot without and with n-butanol addition to trace the variations through oxidation process. Figures 17a-d represent the oxidation sequences of IDF soot. The IDF soot is dense with the disappearance of amorphous particles showing irregular shapes which is


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presented in initial IDF soot. There is no distinct difference between the primary particles with and without n-butanol during the oxidation process. The diameters of the partially oxidized IDF soot are roughly a few nanometers. It is very difficult to distinguish individual particle due to the overlapping of particles. The soot particles show roughened surfaces which is indicative for oxidation. In contrast, partially oxidized NDF soot, Figures 17e-h, shows quite different oxidation behavior. The soot particles exhibit hollow interiors as illustrated by the yellow arrow with completely formed shells, similar with the partially oxidized diesel engine soot.11 The hollow, shell-like particles appearing throughout aggregates are quite different from the partially oxidized IDF soot which possesses a solid structure with no void. The unique finding indicates a different burning mode - internal burning - which is reported in previous studies.11,92 The remarkable alteration in NDF soot nanostructure can be seen at 70% conversion level. Most of the structure and content inside are missing and the lamellae are merely observed along the particle perimeters which are indicated by white arrows. As the initial NDF soot nanostructure shown in Figure 11, the inner core of the soot is composed of disordered carbon, which burns out more quickly than the outer lamellae placed in a concentric structure. The oxidation reactions in the two soot samples appear to promote realignment of the layers. The skeleton images of partially oxidized soot are presented in Figure S1 in Supporting Information. The partially oxidized IDF soot exhibits distorted carbon layers in the particles, while in partially oxidized NDF soot especially with 70% oxidation, the absent internal structure and content are clearly reflected. The fringe analysis results extracted from the partially oxidized soot are presented in Figure 18. It shows the general variations of the fringe length and curvature that the fringe length range becomes slightly wider (corresponding to longer average value in the lamella length) and the fringe curvature shifts to a narrower distribution (corresponding to


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decreasing average tortuosity in the lamella) during the oxidation process. The fringe tortuosity changing of the ethylene IDF soot is a little different as it increases at the level of 30% oxidation and then decreases with further oxidation.

3.4. Soot Raman Spectra. Raman spectroscopy has been extensively utilized to detect the structural differences in carbon-based materials.12,64,93-96 The first-order Raman spectra of carbonaceous stuffs exhibit two intense bands, namely the G (or graphite) band and the D (or defect) band.94 The G band corresponds to the crystalline graphite feature. Conversely the D band represents the defects in graphite structure and other disordered structures.97,98 Raman D/G peak integrated intensity ratio (ID/IG) is usually employed to estimate the disorder extent in carbon materials. Following the recommendations by Sadezky et al.,98 a five peak curve fitting means was applied to qualitatively analyze these Raman spectra. The curve fitting process kept same with our previous work.60 The G-peak positions of IDF soot range in 1582-1590 cm-1 are higher than that designated to graphitic materials, ~1580 cm-1, which indicates the presence of disordered carbon in soot.11 The integrated intensity ratio ID/IG was used as indicator of carbonization degree as well as reactivity of soot. Raman spectra of the initial soot collected from IDF and NDF at different HAB are displayed and curve fittings are performed, as shown in Figures S2 and S3. All spectra are normalized to the peak intensity to have a clear comparison. The G-peak appears because of the stretching modes of the sp2 site, while the appearance of the D-band is due to the manifestation of the in-plane vibrational mode of the sp2 domains at the surface.11,99 Figure 19 shows the ID/IG ratios of the soot samples in different IDF and NDF conditions at specific heights above burner surface. The ID/IG value decreases with the increasing collection height in both ethylene and ethylene/n-butanol IDF. ID/IG ratios increase at the corresponding


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collection height with n-butanol addition. For the soot from NDF with and without n-butanol, ID/IG values are in the same range and the difference is not significant. With the collection height moves up in NDF, ID/IG ratio increases slightly. Overall, the ID/IG ratios of IDF soot are briefly higher than those of NDF soot, implying that the IDF soot is younger and in lower graphitization degree than the NDF soot. Consistent with HRTEM conclusions, the IDF soot with disordered carbon in nanostructure is less carbonized, while the soot from NDF showing the higher degree of crystallization has smaller ID/IG ratio. Raman spectra of the partially oxidized soot samples of IDF and NDF at HAB = 30 mm are displayed in Figure S4 and curve fittings are also performed. Figure 20 presents the variation of ID/IG ratio of the soot during oxidation process. The ID/IG ratios of all the four soot samples decrease with the increasing oxidation conversion. This tendency is prospective because the oxidation progression preferentially burns the disorganized part of the soot.11 The most significant change is observed in ethylene/n-butanol IDF soot because it has the most disordered carbon as observed in HRTEM images.

3.5. Elemental Analysis and BET Surface Area. The results of elemental analysis shown in Table 3 illustrates that the extracts of the soot generated in IDF without and with n-butanol at different axial positions are mainly composed of C, H, and O. The consequences provide the elemental distribution in soot particles along with H/C atomic ratios, which gives the further information on the degree of soot carbonization.36,46,57 The carbon content of the extracts taken from different positions in IDF is very close to each other, between 94% and 96%. As the collection height increased, the hydrogen content in the IDF soot slightly reduces which changes from 3% to 2%. The element concentrations for the IDF soot do not vary apparently. A further analysis illustrates that the atomic H/C ratio decreases slightly due to the increasing collection


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height, which implies that soot collected at higher position in flame has more graphitic organization. Table 4 presents the results of the NDF soot collected at distinct heights above burner surface via elemental analysis. The differences of carbon and hydrogen contents among the six NDF soot samples are not significant. However, the hydrogen content for NDF soot is less than that for IDF soot. This could be attributed to the NDF soot undergo the process of carbonization in the high temperature region, which causes a reduction in the hydrogen concentration.33,57 As a result, the soot collected at NDF achieves a quite low H/C ratio. The lower H/C would contribute to a less content of positive sites which is accessible for the attack of oxygen. The results of BET surface area and pores for IDF and NDF soot at different HAB in flames are demonstrated in Tables 5 and 6. The BET values of the IDF soot range from 24 to 103 m2/g. While the surface area of NDF soot ranges from 82 to 223 m2/g. The BET value shows a decrease in IDF soot as the increasing collection height. However, in NDF soot, the BET value tends to increase with the collection position going higher. The surface area of the particles is involved in the reactivity of the soot samples.100 Large surface area can promote the contact of soot and oxidizer molecules which subsequently may accelerate oxidation process.

3.6. Isothermal Oxidation. Figures 21 presents the normalized mass loss curves of the soot produced in IDF and NDF to illustrate soot oxidation behaviors. Figures 21a and b illustrate that the variations of soot oxidation reactivity for IDF with and without n-butanol addition are very minute, showing that the soot reactivity slightly decreases with the increase in collection height. The effect of n-butanol addition enhances soot oxidation reactivity in a quite small way. Figures 21c and d give the mass loss curves of ethylene and ethylene/n-butanol NDF soot gathered at different axial heights. It is observed that the total mass conversion percentage increases as the


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collection height moves towards upper regions of the two flames. The initial reaction rates of the soot collected at HAB = 40 mm are lower than that of soot gathered at HAB = 20 mm and 30 mm because these particles have experienced oxidation in the flames. Nevertheless, the finial high conversion of the soot from HAB = 40 mm within the same reaction time may be attributed to the large surface area of soot.100 With n-butanol addition in NDF, the reactivity of soot collected at HAB = 30 mm and 40 mm increases, but it decreases for the soot from HAB = 20 mm. As shown in Figure 2, the normal diffusion flame becomes taller with n-butanol addition as well as the soot inception zone. The soot collected from ethylene/n-butanol NDF at HAB = 20 mm appears more counteractive to be oxidized regarding the ethylene NDF soot from HAB = 20 mm. The soot oxidation reactivity of IDF with and without n-butanol addition is larger than that for NDF soot during the isothermal oxidation process, which shows the same trend with the recent work.60 The reason accounting for this phenomenon may be the significant differences of flame configurations and temperatures. The soot generated in IDF is young and less carbonized which is clearly demonstrated by structural analyses by HRTEM and Raman spectra. The IDF soot is more reactive since disordered carbon is more accessible to oxygen. Furthermore, the lower H/C ratio shown in Table 4 testifies the lower oxidation reactivity of the NDF soot in spite of the similar fringe extent and curvature with respect to the IDF soot. Additional isothermal TGA experiments were carried out to calculate the apparent kinetic parameters for the ethylene and ethylene/n-butanol IDF and NDF samples collected at different axial locations. The Arrhenius plots for pure ethylene, ethylene/n-butanol IDF and pure ethylene, ethylene/n-butanol NDF soot samples are presented in Figures S5 and S6. The oxidation kinetic parameters containing activation energies and pre-exponential factors as well as the rate constants estimated are listed in Tables 7 and 8. These can only be treated as apparent kinetic


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parameters since they reflect the macroscopic experiment results. The oxidation reactions are usually influenced by chemical surface limitation and diffusion limitation factors, and the obtained experiment result reveals the actual reactions with both chemical surface and diffusion effects. Thus, the calculated kinetic parameters are not real kinetic quantities. As presented in Table 7, the activation energies for the IDF soot range from 128 to 145 kJ/mol. The activation energies for soot in ethylene/n-butanol IDF are smaller than those for ethylene IDF soot collected at the coincident height above burner surface. Both in ethylene IDF and ethylene/n-butanol IDF, the apparent rate constant slightly decreases as the collection height increased. Moreover, the apparent rate constants increase when n-butanol is added to ethylene IDF with the comparison of soot samples gathered at the same height. These oxidation behaviors are consistent with those demonstrated in Figures 21a and b. The apparent rate constants of NDF particles listed in Table 8 present an opposite pattern that soot particles collected at higher positions demonstrate larger apparent rate constants. The values of the soot collected at the corresponding height with n-butanol addition increase, except that gathered at HAB = 20 mm. The observations coincide with the mass loss curves shown in Figures 21c and d. These results are consistent with HRTEM and Raman results that IDF soot with the more amorphous structure has the higher reactivity because the disorganized lamellae were easier to be attacked by oxygen and the NDF soot with the larger degree of graphitization shows the lower reactivity. In addition, the results further suggest that, with n-butanol addition, the soot produced in NDF would oxidize more quickly, while soot derived from IDF shows a very tiny increase in reactivity.

3.7. Relations of Soot Nanostructure and Reactivity. To build correlations between soot structure and oxidation behavior, fringe length and tortuosity of the soot collected at different axial positions in ethylene and ethylene/n-butanol IDF and NDF are drawn versus the apparent


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rate constants. Figure 22 demonstrates that as fringe length decreases and fringe curvature increases, the apparent rate constant increases in all IDF and NDF. Former researches have revealed the analogous relation between soot nanostructure and reactivity.9,14,60,65 It is wellknown that the reactivity of basal carbon atoms is much lower than that of edge carbon atoms, because the basal carbon atoms have only shared pi electrons consisting chemical bonds, while the edge carbon atoms can form bonds with chemisorbed oxygen owing to the unpaired sp2 electrons availability.7 Generally, fringe length increase causes a reduction in edge carbon atom concentration, and then the rate of edge carbon atoms to basal carbon atoms decrease, which results in a decrease in oxidation reactivity. In the present study, the soot gathered in ethylene IDF from HAB = 20 mm with the shortest fringe length 0.75 nm shows the highest apparent rate constant 8.88E-04 s-1; in ethylene/n-butanol doped IDF, the soot collected at HAB = 20 mm with the shortest fringe length 0.73 nm has the highest apparent rate constant 9.60E-04 s-1. As the collection height goes up towards the flame tip, the fringe length in IDF soot becomes slightly longer while the homologous soot apparent rate constant decreases briefly. The soot samples from HAB = 40 mm in ethylene IDF and ethylene/n-butanol IDF showing a longer fringe length 0.76 nm have lower apparent rate constants 7.79E-04 s-1 and 8.63E-04 s-1, respectively. The NDF soot demonstrates the same regular pattern that the soot reactivity decreases as the fringe length increases. In pure ethylene NDF, the soot sampled at HAB = 40 mm showing the shortest fringe length 0.81 nm presents the highest apparent rate constant 2.50E-04 s-1, which are followed by soot collected at HAB = 30 mm (2.29E-04 s-1), and at HAB = 20 mm (2.19E-04 s-1) in the order of reactivity due to the relevant increase in fringe length of 0.86 nm and 0.87 nm. In ethylene/nbutanol NDF, the soot samples illustrate the same trend. The ethylene/n-butanol NDF soot shows a sequence in reactivity as HAB = 40 mm (2.72E-04 s-1) > HAB = 30 mm (2.39E-04 s-1) > HAB


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= 20 mm (2.01E-04 s-1), corresponding with the fringe length increase order as HAB = 40 mm (0.80 nm) < HAB = 30 mm (0.84 nm) < HAB = 20 mm (0.94 nm). In addition, the fringe curvature also impacts their oxidation rate.7 Fringe curvature contributes to the orbitals overlap and then weakens the C-C bonds for easier oxidative attack.7 An increase in fringe tortuosity involves in an increase in oxidation rate as tortuous carbon lamellae are oxidized more quickly than flat lamellae.13 In IDF, the soot samples generated at HAB = 20 mm with and without nbutanol addition both showing the higher apparent rate constant 9.60E-04 s-1 and 8.88E-04 s-1 have the higher fringe tortuosity 1.12 and 1.11, respectively. While the soot particles from HAB = 40 mm in ethylene and ethylene/n-butanol IDF both with the lowest fringe tortuosity 1.10 present the lowest apparent rate constant 7.79E-04 s-1 and 8.63E-04 s-1, respectively. In NDF, the samples illustrate the same relationship that the higher fringe tortuosity indicates the higher reactivity. The soot sampled at HAB = 20 mm in NDF with and without n-butanol addition present the smallest apparent rate constant 2.01E-04 s-1 and 2.19E-04 s-1 due to the lowest fringe tortuosity 1.08 and 1.09, respectively. While the soot samples collected at HAB = 40 mm with and without n-butanol addition demonstrate the highest apparent rate constant 2.72E-04 s-1 and 2.50E-04 s-1, which both show the highest fringe tortuosity 1.11. The results shown in Figure 22 coincide with the explanation of increasing reactivity due to the increase in the fractions of edge carbon atoms and curved carbon layers.

4.

CONCLUSION The comprehensive study focused on soot nanostructures and oxidation behaviors regarding

the soot produced at different heights above burner surface in ethylene IDF and NDF with nbutanol. Morphology development of the soot along the flame centerline and boundary in the direction of axis was reconstructed. The effects of flame configurations and soot aging on


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structural and reactive characteristics were performed with the soot sampled from different axial positions in flames. The effects of n-butanol as an additive on soot nanostructure and reactivity were determined as well. The following main conclusions were obtained. (1) The soot particles in ethylene/n-butanol IDF were in small aggregates and less carbonized compared with ethylene IDF soot thermophoretically sampled at the corresponding height, which indicated that n-butanol addition reduced the degree of soot carbonization. (2) In ethylene and ethylene/n-butanol NDF, particles were agglomerated and carbonized as they traveled through the flame, and the particle size decreased and the degree of agglomeration increased. (3) The soot particles generated in ethylene and ethylene/n-butanol IDF at three different axial positions all contained amorphous particles with irregular shapes. The nanostructure of IDF soot was strongly heterogeneous as disordered carbon and fullerenic nanostructure appeared. (4) Soot obtained in NDF with and without n-butanol addition showed chain-like clusters of almost rounded particles with clear boundary. The particle size increased from HAB = 20 mm to 30 mm because of the surface growth, and then decreased from HAB = 30 mm to 40 mm caused by oxidation. The ethylene and ethylene/n-butanol NDF soot possessed a classic core-shell structure, which consisted of randomly oriented carbon layers in the central core and concentrically oriented planar crystallites in the outer shell. The typical structure implied that NDF soot was more graphitized and mature than IDF soot. (5) Upon partial oxidation, HRTEM pictures of IDF and NDF soot presented different oxidation modes. Partially oxidized NDF soot, showing hollow and shell-like particles, exhibited internal burning. (6) With the increase of the collection height in ethylene IDF with and without n-butanol


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addition, the fringe length slightly increased and the fringe tortuosity decreased, while the fringe length slightly decreased and the fringe tortuosity increased as the collection height moved towards higher region in NDF. Moreover, generally the n-butanol addition in flame could slightly decrease crystallite order of the generated soot. (7) From the Raman spectra, the IDF soot samples were more disordered in structure than the NDF soot, which confirmed that IDF soot particles were younger. Moreover, n-butanol addition in IDF and NDF increased soot disordered fraction from the ID/IG ratio comparison. (8) The soot oxidation reactivity for IDF with and without n-butanol addition slightly decreased with the increase in collection height, while it increased for ethylene and ethylene/n-butanol NDF soot as the collection height rose. The n-butanol addition in IDF and NDF enhanced the reactivity except the soot collected at HAB = 20mm in ethylene/n-butanol NDF. In summary, results here confirmed that the different structures and properties of soot particles were dependent on the flame configurations and soot aging. The results indicated a close relation between the nanostructure and reactivity for the soot particles. With an increase in the degree of crystallization in soot nanostructure, the soot reactivity decreased. Furthermore, n-butanol showed a potential impact on reducing soot carbonization and enhancing oxidation as an additive in future practical application. ASSOCIATED CONTENT

Supporting Information. Table S1. Soot mass from n-butanol-doped IDF and NDF at different heights above burner surface. (Unit for the weight is mg).

Figure S1. Skeletonized images for Figure 17 via fringe analysis: left column shows the ROI and extracted skeletons of partially oxidized IDF soot, right column shows the ROI and extracted


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skeletons of partially oxidized NDF soot: first row at 30% partial oxidation, second row at 70% partial oxidation, third row at 30% partial oxidation with n-butanol addition, forth row at 70% partial oxidation with n-butanol addition.

Figure S2. Five band curve fitting for Raman spectra of IDF soot: left column shows the ethylene IDF soot, right column shows the ethylene/n-butanol IDF soot; first row at 20 mm, second row at 30 mm, and third row at 40 mm.

Figure S3. Five band curve fitting for Raman spectra of NDF soot: left column shows the ethylene NDF soot, right column shows the ethylene/n-butanol NDF soot; first row at 20 mm, second row at 30 mm, and third row at 40 mm.

Figure S4. Five band curve fitting for Raman spectra of partially oxidized IDF and NDF soot collected at HAB = 30 mm: left column shows the partially oxidized IDF soot, right column shows the partially oxidized NDF soot: first row at 30% partial oxidation, second row at 70% partial oxidation, third row at 30% partial oxidation with n-butanol addition, forth row at 70% partial oxidation with n-butanol addition.

Figure S5. Arrhenius plot of intrinsic rate constants for IDF soot with and without n-butanol addition at different heights above burner surface: left column shows the ethylene IDF soot, right column shows the ethylene/n-butanol IDF soot; first row at 20 mm, second row at 30 mm, and third row at 40 mm.

Figure S6. Arrhenius plot of intrinsic rate constants for NDF soot with and without n-butanol addition at different heights above burner surface: left column shows the ethylene NDF soot, right column shows the ethylene/n-butanol NDF soot; first row at 20 mm, second row at 30 mm, and third row at 40 mm. AUTHOR INFORMATION


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Page 34 of 80

Corresponding Author *E-mail: [email protected] (D. Liu).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work is supported by the National Natural Science Foundation of China (51576100), the Jiangsu Province Graduate Innovative Program (KYZZ16_0184), and the Jiangsu Provincial Project of “Six Talent Summit” (2014-XNY-002). ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51576100), the Jiangsu Province Graduate Innovative Program (KYZZ16_0184), and the Jiangsu Provincial Project of “Six Talent Summit” (2014-XNY-002). REFERENCES (1) Jacobson, M.Z. Nature 2001, 409, 695-697. (2) Neumann, H.-G. Chemosphere 2001, 42, 473-479. (3) Ribeiro, N. M.; Pinto, A. C.; Quintella, C. M.; da Rocha, G. O.; Teixeira, L. S. G.; Guarieiro, L. L. N.; do Carmo Rangel, M.; Veloso, M. C. C.; Rezende, M. J. C.; da Cruz, R. S.; de Oliveria, A. M.; Torres, E. A.; de Andrade, J. B. Energy Fuels 2007, 21, 2433-2445. (4) Echavarria, C. A.; Jaramillo, I. C.; Sarofim, A. F.; Lighty, J. S. Combust. Flame 2012, 159, 2441-2448.


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Table Captions Table 1. Flame conditions of IDF and NDF. Table 2. Median fringe parameters for IDF and NDF soot via fringe analysis. Table 3. Elemental analysis results for soot in IDF at different HAB. Table 4. Elemental analysis results for soot in NDF at different HAB. Table 5. BET result and pore parameters for IDF soot collected at different HAB. Table 6. BET result and pore parameters for NDF soot collected at different HAB. Table 7. Oxidation kinetic parameters and rate constants of IDF soot. Table 8. Oxidation kinetic parameters and rate constants of NDF soot.


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Figure Captions Figure 1. Diagrammatic drawing of experimental system. Figure 2. (a) ethylene IDF, (b) ethylene/n-butanol IDF, (c) ethylene NDF, (d) ethylene/n-butanol NDF.

Figure 3. Measured flame temperatures for different flame conditions: (a) maximum temperatures at different heights, (b) measured temperature radial profiles at HAB = 20 mm, (c) measured temperature radial profiles at HAB = 30 mm, (b) measured temperature radial profiles at HAB = 40 mm.

Figure 4. Representative TEM images of thermophoretically extracted soot morphologies of ethylene IDF at different heights, (a-f) on the centerline, (g-l) on the boundary.

Figure 5. Representative TEM images of thermophoretically extracted soot morphologies of ethylene/n-butanol IDF at different heights, (a-f) on the centerline, (g-l) on the boundary.

Figure 6. Representative TEM images of thermophoretically extracted soot morphologies of ethylene NDF at different heights, (a-f) on the centerline, (g-l) on the boundary.

Figure 7. Representative TEM images of thermophoretically extracted soot morphologies of ethylene/n-butanol NDF at different heights, (a-f) on the centerline, (g-l) on the boundary.

Figure 8. Typical TEM pictures of soot in IDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) n-butanoldoped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm.

Figure 9. Typical TEM pictures of soot in NDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i)


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ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) n-butanoldoped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm.

Figure 10. Typical HRTEM pictures of soot in IDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) nbutanol-doped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm. [yellow arrow marks fullerenic-like structure].

Figure 11. Typical HRTEM pictures of soot in NDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) nbutanol-doped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm. [yellow arrow marks the core].

Figure 12. Skeletonized images for Figure 10a-f and 11a-f via fringe analysis: left column shows the ROI and extracted skeletons of ethylene and ethylene/n-butanol IDF soot, right column shows the ROI and extracted skeletons ethylene and ethylene/n-butanol NDF soot; the rows show the variation with height in the flame, and the last three rows show the n-butanol-doped soot, first and forth row at 20 mm, second and fifth row at 30 mm, third and sixth row at 40 mm.

Figure 13. Lognormal distributions of fringe parameters for initial IDF soot with and without nbutanol addition.

Figure 14. Lognormal distributions of fringe parameters for initial NDF soot with and without nbutanol addition.

Figure 15. Representative images of morphology of IDF soot collected at HAB = 30 mm with partial oxidation: (a,e) 30% partially oxidized ethylene IDF soot, (b,f) 70% partially oxidized


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ethylene IDF soot, (c,g) 30% partially oxidized ethylene/n-butanol IDF soot, (d,h) 70% partially oxidized ethylene/n-butanol IDF soot.

Figure 16. Representative images of morphology of NDF soot collected at HAB = 30 mm with partial oxidation: (a,e) 30% partially oxidized ethylene NDF soot, (b,f) 70% partially oxidized ethylene NDF soot, (c,g) 30% partially oxidized ethylene/n-butanol NDF soot, (d,h) 70% partially oxidized ethylene/n-butanol NDF soot.

Figure 17. Representative images of nanostructure of IDF and NDF soot collected at HAB = 30 mm with partial oxidation: first row at 30% partial oxidation, second row with 70% partial oxidation, third row at 30% partial oxidation with n-butanol addition, forth row at 70% partial oxidation with n-butanol addition. [yellow arrow marks internal void, white arrows indicate lamellae on the particle perimeter].

Figure 18. Lognormal distributions of fringe parameters for initial and partially oxidized IDF and NDF soot at HAB = 30 mm.

Figure 19. ID/IG ratios for soot collected in flames at specific heights above burner surface: (a) ethylene and ethylene/n-butanol IDF, (b) ethylene and ethylene/n-butanol NDF.

Figure 20. Variations in ID/IG ratio during the oxidation process of IDF and NDF soot collected at HAB = 30 mm: (a) ethylene and ethylene/n-butanol IDF, (b) ethylene and ethylene/n-butanol NDF.

Figure 21. TGA results of soot in IDF and NDF at different HAB. Figure 22. Correlations between soot apparent rate constant and median fringe length and tortuosity: (a,b) IDF soot, (c,d) NDF soot.


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TABLES Table 1. Flame conditions of IDF and NDF.

IDF NDF

n-Butanol (ml/min) / 0.42 / 0.09

C 2 H4 0.70 0.59 0.15 0.13

Gas flow rate (l/min) Air N2 (Carrier)

N2 (Shield)

0.70

0.70

13.00

6.00

0.40

/


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Table 2. Median fringe parameters for IDF and NDF soot via fringe analysis. Soot Ethylene IDF

Ethylene/n-butanol IDF

Ethylene NDF

Ethylene/n-butanol NDF

HAB (mm)

Fringe length (nm)

Fringe tortuosity

20

0.75 ± 0.01

1.11 ± 0.01

30

0.76 ± 0.03

1.11 ± 0.01

40

0.76 ± 0.02

1.10 ± 0.02

20

0.73 ± 0.04

1.12 ± 0.02

30

0.75 ± 0.01

1.11 ± 0.01

40

0.76 ± 0.03

1.10 ± 0.01

20

0.87 ± 0.03

1.09 ± 0.01

30

0.86 ± 0.05

1.10 ± 0.01

40

0.81 ± 0.02

1.11 ± 0.01

20

0.94 ± 0.04

1.08 ± 0.01

30

0.84 ± 0.02

1.10 ± 0.01

40

0.80 ± 0.02

1.11 ± 0.01


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Table 3. Elemental analysis results for soot in IDF at different HAB. Soot

wt.%C

wt.%H

wt.%O

H/C

IDF-20 mm

94.78

2.62

2.60

0.33

IDF-30 mm

95.19

2.12

2.69

0.27

IDF-40 mm

95.29

2.00

2.71

0.25

IDF+n-butanol-20 mm

94.60

2.62

2.78

0.33

IDF+n-butanol-30 mm

95.53

2.54

1.93

0.32

IDF+n-butanol-40 mm

94.91

2.38

2.71

0.30


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Table 4. Elemental analysis results for soot in NDF at different HAB. Soot

wt.%C

wt.%H

wt.%O

H/C

NDF-20 mm

95.07

1.04

3.89

0.13

NDF-30 mm

93.82

0.71

5.47

0.09

NDF-40 mm

92.53

0.62

6.85

0.08

NDF+ n-butanol-20 mm

94.79

0.75

4.46

0.09


94.31

0.59

5.10

0.07


93.06

0.52

6.42

0.07


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Table 5. BET result and pore parameters for IDF soot collected at different HAB. Soot

BET surface area

Pore volume

Mean pore diameter

(m2/g)

(cm3/g)

(nm)

IDF-20 mm

103

0.29

11

IDF-30 mm

64

0.19

12

IDF-40 mm

69

0.27

16

IDF+n-butanol-20 mm

41

0.14

13

IDF+n-butanol-30 mm

27

0.15

22

IDF+n-butanol-40 mm

24

0.09

14


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Table 6. BET result and pore parameters for NDF soot collected at different HAB. BET surface area

Pore volume

Mean pore diameter

(m2/g)

(cm3/g)

(nm)

NDF-20 mm

91

0.31

13

NDF-30 mm

102

0.35

14

NDF-40 mm

223

1.38

25

NDF+n-butanol-20 mm

82

0.24

12

NDF+n-butanol-30 mm

87

0.32

15

NDF+n-butanol-40 mm

167

0.63

15

Soot


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Table 7. Oxidation kinetic parameters and rate constants of IDF soot. Soot

Ea (kJ/mol)

A (s-1)

k500c (s-1)

R2

IDF-20 mm

138 ± 3

1.97E+06

8.88E-04

0.99

IDF-30 mm

134 ± 4

0.95E+06

8.37E-04

0.99

IDF-40 mm

145 ± 6

4.75E+06

7.79E-04

0.99

IDF+ n-butanol -20 mm

136 ± 4

1.40E+06

9.60E-04

0.99


128 ± 5

0.42E+06

8.91E-04

0.99


142 ± 3

0.49E+06

8.63E-04

0.99


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Table 8. Oxidation kinetic parameters and rate constants of NDF soot. Soot

Ea (kJ/mol)

A (s-1)

k500c (s-1)

R2

NDF-20 mm

173 ± 3

1.05E+08

2.19E-04

0.99

NDF-30 mm

180 ± 3

3.14E+08

2.29E-04

0.99

NDF-40 mm

205 ± 9

1.70E+10

2.50E-04

0.95

NDF+ n-butanol -20 mm

193 ± 5

0.22E+10

2.01E-04

0.98


174 ± 6

1.34E+08

2.39E-04

0.96


177 ± 8

2.35E+08

2.72E-04

0.96


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FIGURES

Figure 1. Diagrammatic drawing of experimental system.


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Figure 2. (a) ethylene IDF, (b) ethylene/n-butanol IDF, (c) ethylene NDF, (d) ethylene/n-butanol NDF.


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Figure 3. Measured flame temperatures for different flame conditions: (a) maximum temperatures at different heights, (b) measured temperature radial profiles at HAB = 20 mm, (c) measured temperature radial profiles at HAB = 30 mm, (b) measured temperature radial profiles at HAB = 40 mm.


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Figure 4. Representative TEM images of thermophoretically extracted soot morphologies of ethylene IDF at different heights, (a-f) on the centerline, (g-l) on the boundary.


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Figure 5. Representative TEM images of thermophoretically extracted soot morphologies of ethylene/n-butanol IDF at different heights, (a-f) on the centerline, (g-l) on the boundary.


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Figure 6. Representative TEM images of thermophoretically extracted soot morphologies of ethylene NDF at different heights, (a-f) on the centerline, (g-l) on the boundary.


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Figure 7. Representative TEM images of thermophoretically extracted soot morphologies of ethylene/n-butanol NDF at different heights, (a-f) on the centerline, (g-l) on the boundary.


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Figure 8. Typical TEM pictures of soot in IDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) n-butanoldoped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm.


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Energy & Fuels

Figure 9. Typical TEM pictures of soot in NDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) n-butanoldoped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm.


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Energy & Fuels

Figure 10. Typical HRTEM pictures of soot in IDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) nbutanol-doped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm. [yellow arrow marks fullerenic-like structure].


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Energy & Fuels

Figure 11. Typical HRTEM pictures of soot in NDF with and without n-butanol at different heights above burner: (a,g) ethylene soot at HAB = 20 mm, (b,h) ethylene soot at HAB = 30 mm, (c,i) ethylene soot at HAB = 40 mm, (d,j) n-butanol-doped soot at HAB = 20 mm, (e,k) nbutanol-doped soot at HAB = 30 mm, (f,l) n-butanol-doped soot at HAB = 40 mm. [yellow arrow marks the core].


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Figure 12. Skeletonized images for Figure 10a-f and 11a-f via fringe analysis: left column shows the ROI and extracted skeletons of ethylene and ethylene/n-butanol IDF soot, right column shows the ROI and extracted skeletons ethylene and ethylene/n-butanol NDF soot; the rows show the variation with height in the flame, and the last three rows show the n-butanol-doped soot, first and forth row at 20 mm, second and fifth row at 30 mm, third and sixth row at 40 mm.


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Figure 13. Lognormal distributions of fringe parameters for initial IDF soot with and without nbutanol addition.


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Energy & Fuels

Figure 14. Lognormal distributions of fringe parameters for initial NDF soot with and without nbutanol addition.


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Figure 15. Representative images of morphology of IDF soot collected at HAB = 30 mm with partial oxidation: (a,e) 30% partially oxidized ethylene IDF soot, (b,f) 70% partially oxidized ethylene IDF soot, (c,g) 30% partially oxidized ethylene/n-butanol IDF soot, (d,h) 70% partially oxidized ethylene/n-butanol IDF soot.


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Figure 16. Representative images of morphology of NDF soot collected at HAB = 30 mm with partial oxidation: (a,e) 30% partially oxidized ethylene NDF soot, (b,f) 70% partially oxidized ethylene NDF soot, (c,g) 30% partially oxidized ethylene/n-butanol NDF soot, (d,h) 70% partially oxidized ethylene/n-butanol NDF soot.


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Figure 17. Representative images of nanostructure of IDF and NDF soot collected at HAB = 30 mm with partial oxidation: first row at 30% partial oxidation, second row with 70% partial


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oxidation, third row at 30% partial oxidation with n-butanol addition, forth row at 70% partial oxidation with n-butanol addition. [yellow arrow marks internal void, white arrows indicate lamellae on the particle perimeter].


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Figure 18. Lognormal distributions of fringe parameters for initial and partially oxidized IDF and NDF soot at HAB = 30 mm.


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Figure 19. ID/IG ratios for soot collected in flames at specific heights above burner surface: (a) ethylene and ethylene/n-butanol IDF, (b) ethylene and ethylene/n-butanol NDF.


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Figure 20. Variations in ID/IG ratio during the oxidation process of IDF and NDF soot collected at HAB = 30 mm: (a) ethylene and ethylene/n-butanol IDF, (b) ethylene and ethylene/n-butanol NDF.


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Figure 21. TGA results of soot in IDF and NDF with and without n-butanol at different heights above burner surface.


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Figure 22. Correlations between soot apparent rate constant and median fringe length and tortuosity: (a,b) IDF soot, (c,d) NDF soot.


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Effects of Flame Configuration and Soot Aging on Soot Nanostructure

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