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Ozone is a prospective additive for enhancing and controlling combustion under lean or very lean conditions, and reduces NOx and particulate matter emissions simultaneously. Typically, in studying the effects of ozone on combustion pollutants, the focus is on the final yield of pollutants, while its detailed effects on the soot formation process remain unknown. Here, the formation and evolution profiles of soot containing morphology and nanostructures in ethylene inverse diffusion flames with different ozone concentration additions were experimentally studied. The surface chemistry and oxidation reactivity of soot particles were also compared. The soot samples were collected by a combination of the thermophoretic sampling method and deposition sampling method. High-resolution transmission electron microscopy analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis were applied to obtain the soot characteristics. The results showed that soot particles experienced inception, surface growth, and agglomeration in the ethylene inverse diffusion flame within a flame axial direction. The soot formation and agglomeration were slightly advanced since the ozone decomposition contributed to promoting the production of free radicals and active substances in the ozone added flames. The diameter of primary particles in the flame with ozone addition was larger. With the increase of ozone concentration, the content of soot surface oxygen increased and the ratio of sp2/sp3 decreased. Furthermore, the addition of ozone increased the volatile content of soot particles and improved soot oxidation reactivity. [ABSTRACT FROM AUTHOR]

This study focused on the effects of n-butanol addition on the combustion characteristics of n-heptane counterflow diffusion flame under pressures of 1, 3, and 5 atm by a detailed kinetic simulation. The added n-butanol volume fraction ranged from 0 to 50%. The mass averaged velocity of fuel streams was selected to ensure momentum flux balance and was approximately constant for the investigated flames. Flame structures and mole fraction profiles impacted by n-butanol addition for major species, free radicals, and intermediate species were analyzed by concentrating on the formations of soot precursors and oxygenated air pollutants. The results showed that with the addition of n-butanol, the flame temperature decreased and the formation of the main soot precursors such as C2H2 and C6H6 was inhibited. This can be attributed to the reduced rate of production of these species. The flame temperature increased significantly, and the profile moved towards the fuel side with the increasing pressure. Moreover, the production of C2H2 and C6H6 was observably promoted as the pressure increased. The concentrations of free radical H, O, and OH decreased significantly as the pressure increased but slightly decreased with the increasing n-butanol addition, which might have been caused by the chemical effect of n-butanol. [ABSTRACT FROM AUTHOR]

Ammonia is considered one of the most competitive fuels due to its carbon neutrality. The chemical effects of NH 3 are distinguished by kinetic analysis via adding NH 3 as reactive NH 3 and fictitious inert NH 3. The flame temperature and the mole fraction profiles affected by the chemical effects of NH 3 addition for important species and soot are analyzed, with special emphasis on soot and its important precursor polycyclic aromatic hydrocarbons (PAHs). The results illustrate that NH 3 addition inhibits the production of A1-A4. The chemical effects of ammonia decrease the hydrogen abstraction–C 2 H 2 –addition (HACA) surface growth rate and PAH condensation rate, which further reduces soot volume fraction and average particle diameter D 63. The ammonia decomposition pathways interact with ethylene decomposition pathways via the four reactions: NH 3 + C 2 H 5 = C 2 H 6 + NH 2 , HCN + C 2 H 5 = C 2 H 6 + CN, NH 2 + C 2 H 4 = C 2 H 3 + NH 3 , and CH 2 CH 2 NH 2 = C 2 H 4 + NH 2. The dilution and thermal effects of NH 3 are dominant effects on soot reduction, while the chemical effects further inhibit soot formation. • The chemical effects of NH 3 addition on ethylene diffusion flames are studied. • A1-A4 are inhibited by NH 3 additions mainly due to dilution and thermal effects. • NH 3 additions decrease the soot volume fraction and soot diameter D 63. • The combined paths of NH 3 and ethylene decomposition are revealed. • HACA surface growth rate and PAH condensation rate are reduced by NH 3 addition. [ABSTRACT FROM AUTHOR]

Soot is a sort of critical pollutants from incomplete combustion of carbonaceous fuel. Photocatalytic decomposition of soot from the isooctane and 2,5-dimethylfuran (DMF) flames was carried out under vacuum ultraviolet irradiation. The change of PEG addition led to different morphology of titanium dioxide films as the photocatalysts, which might affect photocatalytic soot decomposition. The photodegradation process of soot was analyzed by digital cameras, the field emission scanning electron microscope (FESEM) analysis, and the higher resolution transmission electron microscope (HRTEM) analysis. The photocatalytic degradation time of soot from DMF flames is far fewer than that of soot from isooctane flames. Loose deposition of DMF soot is beneficial for the photocatalytic decomposition process due to the blocking to incident light reducing. Nanostructures of soot particles from different flames were obtained and analyzed by HRTEM. Soot samples that were collected by DMF flames exhibited higher oxidation reactivity and more surface oxygen than that from isooctane flames. The above factors which might affect the photocatalytic degradation time of soot were analyzed to compare differences. [ABSTRACT FROM AUTHOR]

The present study demonstrated the novel control on nanostructures and reactivity of soot by CO 2 local micro-injection in ethylene inverse diffusion flames. Both global and local sampling methods were employed to capture soot particles at different flame positions. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were applied to investigate detailed soot characteristics. The results indicated that, when CO 2 was added using micro-injection in flames at micro flow rates, the soot exhibited turbostratic structures with amorphous characters and had shorter fringe length and larger fringe tortuosity related with higher oxidation reactivity compared to soot from the pure ethylene flame, which inferred that CO 2 addition could be beneficial to enhance soot oxidation and make the soot to have a low carbonization. Especially, the injection of CO 2 in flames caused an obvious effect on delaying and decreasing in soot nucleation and growth rates during soot evolution process. The properties of soot from the global quartz plate sampling were quite similar for different local injection positions. However, it was worth noting that the direct addition of CO 2 through the oxidizer pipe at same flow rates had no significant effects on soot characteristics. • Effects of local CO 2 micro-injection on soot formation were studied. • Soot from CO 2 injected flames has amorphous structures. • CO 2 injection could delay soot nucleation and inception processes. • The oxidation reactivity of soot increased with increasing CO 2 flow rates. • Adding CO 2 through the oxidizer side pipe has no distinct effect. [ABSTRACT FROM AUTHOR]

Abstract The micro and nanostructure evolutions and reactivity of nascent soot from n -butanol-doped ethylene inverse diffusion flames in CO 2 , N 2 , and He atmospheres were studied and compared. The thermophoretic sampling technique was employed to capture soot particles directly at different positions along the flame boundary. Transmission electron microscopy was applied to obtain the particle morphology evolution. Moreover, high-resolution transmission electron microscopy analysis, Raman spectroscopy analysis, thermogravimetric analysis, and X-ray photoelectron spectroscopy were performed to study the nanostructure and oxidation reactivity of soot from the quartz plate sampling. It was found that soot inception rate was reduced in CO 2 atmosphere, indicating CO 2 could inhibit soot formation through inception process. However, He promoted soot formation since the soot agglomerate amount was larger than those in flames with CO 2 and N 2. High correlations were found between soot nanostructure and reactivity. The soot from CO 2 atmosphere had shortest fringe length and largest fringe tortuosity related with highest oxidation reactivity. On the contrary, soot from He diluted flame exhibited prevalent fullerenic-like nanostructures with evident large or small shells, and also had a higher carbonization degree resulting in lower oxidation reactivity. The surface oxygen content related to soot oxidation reactivity. Graphical abstract Image [ABSTRACT FROM AUTHOR]

The jet impinging flame is a typical flame-wall interaction configuration. This paper presented an experimental study of laboratory jet impinging ethylene diffusion flames with the special emphasis on characteristics of soot forming on the impinging plate. The soot distribution, nanostructure and reactivity which corresponded with different flame structures were investigated comparatively based on parametric variations of nozzle to plate distance (H), Reynolds number (Re) and collecting time (T). The results showed that impinging flame structures could be classified as cool central core, envelope, and disc flames based on different nozzle to plate distances and Reynolds numbers. A series of concentric soot rings for jet impinging flames were formed on the plate surface. Inner ring soot was the film-like material with the amorphous structure, and outer ring soot consisted of suborbicular primary particles with the classic core-shell structure. For soot at Re = 600 and T = 5 min, the augment of the nozzle to plate distance reduced the content of film-like soot at the inner ring because the extension of the jet free region could promote the oxidation of soot. The crystallization degree of outer ring soot was non-monotonic with the increase of the nozzle to plate distance. It might attribute to the reduction of air entrainment of the flame layer in the wall jet region. Inner ring soot at H = 10 mm had the most disordered carbon atoms with the shortest fringe length and the highest fringe tortuosity. It also presented the highest reactivity. Because of the high degree of crystallization with the longest fringe length and the smallest tortuosity, the reactivity of soot sampled at H = 30 mm was the lowest. For soot sampled with Reynolds numbers of 300, 600 and 900 at H = 10 mm and T = 5 min, inner ring soot at Re = 600 had the most amorphous structure related with the shortest fringe length and the largest tortuosity. The crystallization degrees of soot from outer rings increased with Reynolds numbers. The increase of the impinging flame layer might promote the air entrainment which could accelerate the soot oxidation. In addition, the oxidation reactivity of soot sampled at different Reynolds numbers were in a sequence of Re = 600 inner ring > Re = 300 inner ring > Re = 900 inner ring > Re = 300 outer ring > Re = 600 outer ring > Re = 900 outer ring, which was in consistent with variations of the fringe length and tortuosity. The reactivity of soot from both inner and outer rings decreased with the increase of collecting times at H = 10 mm and Re = 600, which might result from the increase of the plate temperature. The results also confirmed the relationship between the soot structure and oxidation reactivity that soot with the higher degree of crystallization was more difficult to oxidize. [ABSTRACT FROM AUTHOR]

This paper explored the effects of butanol isomers, n -butanol, iso -butanol, sec -butanol, and tert -butanol, as fuel-side additives on soot nanostructure and reactivity in ethylene inverse diffusion flames (IDF) and normal diffusion flames (NDF). The variations of structures and oxidation rates among the soot samples derived from ten different flames were studied and compared using high resolution transmission electron spectroscopy (HRTEM), Raman spectroscopy, thermogravimetric analyzer (TGA), surface area and porosimetry analyzer, and elemental analyzer. Results demonstrated that soot from IDF with the additions of butanol isomers had irregular shapes with film-like materials and highly heterogeneous nanostructures, which showed both amorphous carbon and fullerenic lamellae. The shells presented more prevalently in tert -butanol-doped IDF soot, which had the longest fringe length and smallest fringe tortuosity. Whereas in the NDF as butanol isomers were individually added, the soot samples were aggregated by several tens or hundreds nearly rounded particles. All the NDF soot exhibited a typical core-shell structure with extended and planar lamellae. Soot from sec -butanol added NDF presented the lowest degree of graphitization with the shortest fringe length and largest fringe tortuosity. The TGA results revealed that the oxidation rates of the IDF soot particles were much higher than that of the NDF soot. Moreover, when butanol isomers were individually added in IDF, soot produced with n -butanol (1.34E-03 s −1 ) showed the highest reactivity with the least degree of crystallization in structure, followed by iso -butanol (1.31E-03 s −1 ), sec -butanol (1.29E-03 s −1 ) and tert -butanol (1.14E-03 s −1 ) in the sequence of reactivity. However, the oxidation rates of the soot samples generated from butanol isomers-doped NDF were in the order of sec -butanol (4.26E-04 s −1 ), tert -butanol (3.70E-04 s −1 ), n -butanol (3.52E-04 s −1 ) and iso -butanol (3.23E-04 s −1 ) from the highest to lowest. The results confirmed a relationship on structure-property between soot nanostructure and reactivity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effects of plasma on flame characteristics and soot formation were investigated. • The soot concentration variation with the non-thermal plasma were explored. • Soot graphitization degree decreased and disorder degree increased with plasma on. • Soot produced by the plasma-flame interaction exhibited a stronger reactivity. The study of soot characteristics in plasma-flame interaction was necessary to improve combustion efficiency and reduce pollutant emissions. This work experimentally investigated effects of plasma on soot formation, evolution and characteristics in ethylene diffusion flames by both optical diagnostic and sampling methods. The optical diagnostic method was employed to capture the two-dimensional distribution of soot temperatures and concentrations. Moreover, the nanostructure, crystallite properties and oxidation reactivity of soot from diffusion flames with plasma addition were obtained using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray diffraction spectroscopy. The flame shapes and luminosity varied depending on the electrical parameters and oxygen concentration. It was found that the overall temperature increased somewhat with plasma activation, but remained roughly constant when the discharge frequency or applied voltage increased. The soot concentration immediately reduced with plasma on and then displayed different trends as the discharge frequency or applied voltage increased. The variation in soot concentration was caused by the combination of several parameters, including flame behavior, soot temperature and the effect of the electric field by energetic and chemically active species. The generated soot showed typical chain-like aggregates without plasma, but more film-like materials were presented with plasma acting on flames. The soot graphitization degree decreased notably with plasma generation resulting in higher oxidation reactivity because the shorter residence time and the plasma activation were simultaneously obtained, which both illustrated an inhibition effect of plasma on soot emissions. The disordered carbon content related to soot oxidation reactivity. [ABSTRACT FROM AUTHOR]

A detailed kinetic analysis of chemical effects of hydrogen addition on laminar premixed stoichiometric methane-air flames was conducted at atmospheric pressure. Flame structures and mole fraction profiles affected by chemical effects of hydrogen addition for major species, free radicals and intermediate species are analyzed with particular emphasis on the formations of soot precursor and oxygenated air pollutants. The results illustrate that chemical effects of hydrogen additive lead the methane profile to move towards the upstream side and suppress the formation of acetylene and ketene. The concentrations of free radical H, O and OH increase as methane is replaced by hydrogen mainly due to its chemical effects. In contrast, although the chemical effects of hydrogen addition facilitate the productions of formaldehyde and acetaldehyde, the hydrogen dilution and thermal effects on reducing mole fractions of both species are more significant. As a consequence, the total effects of hydrogen addition lead to a decrease in formaldehyde and acetaldehyde concentrations. Compared to formaldehyde and acetaldehyde, NO mole fraction diminishes in a similar fashion with increased hydrogen additive that the decrease of NO concentration caused by hydrogen dilution and thermal effects is larger than the increase due to its chemical effects. [ABSTRACT FROM AUTHOR]

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

The magnetic effect on soot properties in ethylene inverse diffusion flames (IDF) was investigated in this study. The investigation involved the use of high-resolution transmission electron microscopy analysis (HRTEM), Raman spectroscopy analysis, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction spectroscopy (XRD) to examine the nanostructure and graphitization degree of soot collected from flame exhaust gas at various height above burner surface (HAB) under both magnetic and non-magnetic conditions. Increasing the magnetic flux density resulted in modifications to soot properties. When the sampling height was HAB = 4 cm, increasing magnetic flux density resulted in longer fringe lengths, lower fringe tortuosity, less amorphous carbon content and surface oxygen content for the collected soot from flame exhaust gas. These results indicated higher graphitization degrees, which may be attributed to longer residence time and oxidation of soot particle in flames. Moreover, when the sampling height increased from HAB = 4 cm to 6 cm under non-magnetic condition, the nanostructure of soot in the flame exhaust gas was more organized, possible due to the oxidation of soot at the region between HAB = 4 and 6 cm region. However, with the magnetic field on, the crystalline structure of soot in the flame exhaust gas became increasingly disordered as the sampling height increased. This suggested that the soot may also be affected by the magnetic effects in addition to oxidation at the region between HAB = 4 and 6 cm region. The magnetic effect could trigger the transformation of graphitic carbon to amorphous carbon in soot particles as they passed through the magnetic field. With more elaborated magnetic fields, the magnetic effect on soot could contribute to strategies controlling soot emissions and removal. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Flame structure gradually becomes irregular in large inlet velocity. • More soot production is obtained with the enlargement of combustor size. • Different effects on soot characteristics between combustors are found. • Notable distinctions in nanostructure of soot due to the scale effect are observed. • Different combustion efficiencies between combustors are detected. Abstract Soot formation, evolution and characteristics of diffusion ethylene/air flames in ψ-shaped mesoscale combustors of two different diameters with the variations of excess air ratio and flow rate were experimentally investigated. The variation in nanostructure and oxidation reactivity of soot was compared based on the results of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that with an increasing excess air ratio and flow rate, the unventilated flames with bifurcated shapes were observed in both types of combustors due to the deteriorated mixing process in large flow velocity. Different effects on characteristics of soot from two combustors with the same variation of flow rate were found. For the variation in ethylene flow rate of 60–100 ml/min at excess air ratio of 0.5, the oxidation reactivity of soot from the combustor with d = 4 mm first decreased and then slightly increased, while it decreased all the time as for the soot from the combustor with d = 6 mm. Moreover, the significant distinctions in soot nanostructure due to the scale effect were observed. The soot from the combustor with d = 4 mm exhibited the partial amorphous structure aggregated by a large amount of PAHs with high reactivity. Whereas among the soot from the combustor with d = 6 mm, a typical fullerene-like structure which represented the simultaneous existence of PAHs and graphitic parts was found. The soot graphitization degree and production increased notably with the enlargement of combustor size because the higher combustion temperature and longer residence time were simultaneously obtained, which both were beneficial to soot formation and growth rate. Significantly, the lower combustion efficiency was obtained in the combustor of d = 6 mm than 4 mm at α = 0.5 because of the larger soot production and more existence of unburned gas. But the higher combustion efficiency was found for the 6 mm than 4 mm at α = 1 in the case of almost no soot generation, which was due to the longer residence time. [ABSTRACT FROM AUTHOR]

Carbon particles (Soot) have been the one of primary pollutants inevitably with the combustion of fossil fuels. A better understanding on incipient soot was useful to build the models of soot formation and even control soot emissions. The present work focuses on the nanostructure and oxidation reactivity of nascent carbon particles (soot) formed from 2,5-dimethylfuran (DMF)/n-heptane non-swirling and swirling inverse diffusion flames (IDFs). The nascent soot samples were derived from three different fuels: 100% n-heptane, 50% n-heptane/50% DMF and 100% DMF. In addition, the effects of swirling combustion and collection time on characteristics of nascent soot were investigated in detail using the high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that nascent soot from pure n-heptane flames presented the film-like morphology and nanostructure of amorphous nature, while the nearly primary particles with more well-organized nanostructure were found in pure DMF soot. Swirling combustion increased the entire flame front areas and diameters due to the existence of tangential velocity at the outlet of burner. Moreover, swirling combustion could enhance the mixability of the oxidizer stream and fuel stream, leading to more young soot within the fuel stream being oxidized in the inception stage. However, the collection time on soot characteristics exhibited quite negligible impacts in comparison to the swirling effects. [ABSTRACT FROM AUTHOR]

Abstract Oxygenated fuel represents an attractive alternative as an additive for reducing soot emissions. Dimethyl carbonate (DMC) is an oxygenated compound which is a good option to reduce soot, but the detailed characteristics of soot produced from combustion of hydrocarbon fuels blended with DMC are still lacking. The present research studied the nanostructure and reactivity of soot particles in ethylene/DMC normal and inverse diffusion flames. High resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to analyze the nanostructure and reactivity of soot. It was found that DMC addition was effective in decreasing the average weights of soot formed in flames. The results of HRTEM images showed that soot particles obtained with DMC addition showed liquid-like material and tight bonding, and exhibited more highly disorganized layers, which give it higher reactivity than soot obtained without DMC addition. Furthermore, HRTEM was used to analyze soot fringe characteristics consisting of fringe tortuosity, fringe length, and fringe separation. XRD was used to crosscheck the results for fringe separation, and was consistent with HRTEM results. In addition, the mass loss curve of TGA experiments showed that DMC addition could enhance the reactivity of soot particles. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

This paper presents nanostructure and reactivity of carbon particles (soot) from co-pyrolysis of biodiesel surrogate methyl octanoate blended with n-butanol at temperatures from 1023 K to 1223 K in a quartz tube flow reactor, as n-butanol concentration increases from 0 to 50% (mole fraction) in the mixed fuels. The soot structure was characterized by low and high resolution transmission electron spectroscopy (LRTEM and HRTEM) and X-ray diffraction (XRD). The soot reactivity was investigated by thermogravimetric analyzer (TGA). When the pyrolysis temperature increased, as well as the rise of n-butanol addition, soot presented the higher degree of graphitization with the larger fringe length and lower fringe tortuosity. TGA results revealed that the higher amount addition of n-butanol can induce the formations of less reactive soot, and soot yielded at lower temperatures was more reactive than that at high temperatures. In addition, the high correlations between soot reactivity with its nanostructure were given in details. [ABSTRACT FROM AUTHOR]

In this paper, the influence of the edge effect on the simultaneous reconstruction of axisymmetric flame temperature and soot volume fraction profiles by a single CCD camera was investigated in detail. The reconstruction accuracy of the flame temperature profile and soot volume fraction was insensitive to the measurement error of the coefficient matrix. When the signal to ratio (SNR) of the measurement system for both the radiation intensity and coefficient matrix was as low as 46 dB, the reconstruction accuracy for both temperature and soot volume fraction was acceptable and was more influenced by the radiation intensity measurement error. The reconstruction of the flame temperature and soot volume fraction was greatly influenced by the edge effect. When the flame edge with weak radiation signals was ignored during the reconstruction, the relative reconstruction error for the temperature and soot volume fraction increased from the flame center to the edge, and reached an unacceptable value at the reconstruction boundary, especially for the soot volume fraction. The flame image boundary could be chosen as the unified reconstruction boundary to reconstruct the two-dimensional distribution of the temperature and soot volume fraction with satisfactory accuracy. The low soot volume fraction could influence the reconstruction accuracy for both the temperature and soot concentration in non-sooting regions. Moreover, there was no obvious regularity between the reconstruction accuracy of the temperature and soot volume fraction and the extension of the reconstruction boundary. [ABSTRACT FROM AUTHOR]

Summary: Soot particulate emitting from diesel engines are serious to both human health and environment. Efficient catalyst is vital in diesel particulate filter technology to decrease soot oxidation temperature. Cu1−xKxFe2O4 (x = 0, 0.05, 0.1, and 0.15) catalysts are synthesized and analyzed with XRD, Raman, BET, SEM, TEM, TG, EDS, XPS, catalytic activity tests, and kinetic analysis. The results suggests that many factors affect the catalytic activity, and the contents of oxygen vacancies are dominant. A large amount of oxygen vacancies which facilitate to the soot oxidation are created with the assistance of K+, and they increase with the increasing doping contents. Excessive surface concentration of K+ may lead to the partially covering of active sites of the catalysts. Therefore, Cu0.9K0.1Fe2O4 catalyst exhibits the best catalytic activity in soot oxidation, which decreases the Tmax of soot by 279°C. This decrease makes it highly promising for application in diesel particulate filter. [ABSTRACT FROM AUTHOR]

[Display omitted] • Polyoxymethylene dimethylether (PODE 3) and dimethoxymethane (DMM) were substitutes. • Effect of DMM and PODE 3 additions to isooctane on soot reduction was examined. • Soot particle features obtained by different isooctane flames were investigated. • 40% PODE 3 / i -octane showed more amorphous soot and least graphitization degree. • Soot characteristics via HRTEM, XRD and Raman spectra were strongly correlated. Polyoxymethylene dimethyl ethers (PODE n , n = 1–3) such as dimethoxymethane (DMM) and PODE 3 are considered highly promising alternative fuels due to their lack of C–C bonds and significant oxygen contents. Particularly, PODE 3 -blends with diesel have been proven to substantially reduce soot emissions and fuel consumption rates under optimized engine conditions, although the impact on soot nanostructure properties remains unclear. This study explored the effects of DMM and PODE 3 on soot characteristics in isooctane (i -octane) inverse diffusion flames, focusing on soot morphology, soot nanostructure, soot nanocrystallite, and soot spectral features. To compare soot particles behavior, DMM and PODE 3 blended as the isooctane substitute (by 20% and 40% volumetric fractions) were examined with various diagnostic techniques, including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).The results showed that DMM and PODE 3 contributed to a reduction in sooting tendency from i -octane flames, whereas the average soot mass decreased significantly as the PODE 3 ratio increased, consistent with a higher oxygen concentration. Therefore, 20% PODE 3 / i -octane exhibited growing disorganized arrangements of soot particles with shorter fringes, smaller crystallite sizes, and a relatively lower degree of graphitization than 20% and 40% DMM additions. However, XPS analysis indicated that soot produced from PODE 3 / i -octane mixtures had less oxygen atoms because PODE 3 has multiple –CH 2 O- chain groups that can convert oxygen into carbon monoxide with little soot production. Moreover, HRTEM, XRD, and Raman spectra analyses of 40% PODE 3 -doped flames confirmed an exceptional correlation with turbostratic soot structure, as manifested by the shortest fringe length, longest fringe tortuosity, more amorphous soot with the smallest crystallite dimensions, and least degree of graphitization. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is considered as a promising alternative fuel due to its zero-carbon content. Impacts of various NH 3 additions on soot nanostructure and reactivity during 1-butene pyrolysis are investigated from low to high temperatures in this work. Soot morphology and nanostructure are characterized by (high-resolution) transmission electron microscopy, Raman spectroscopy and X-ray diffraction. Thermogravimetric analysis is adopted to evaluate soot reactivity, as well as X-ray photoelectron spectroscopy and elemental analysis to explore the chemical composition of soot. Results show the soot nanostructure is highly correlated with its reactivity, but they do not vary monotonically with NH 3 blending. Soot particle size, fringe length and graphitization degree are ranked as 0%NH 3 > 50%NH 3 > 30%NH 3 for various NH 3 additions, while fringe tortuosity, oxidation reactivity and H/C ratio vary in the opposite order. The maturity of soot improves significantly with an increase in pyrolysis temperature. However, the impact of doping ammonia on the soot nanostructure gradually weakens, indicating that young soot is more sensitive to ammonia. • Non-linear changes in soot nanoscale features have been found with NH 3 blending. • NH 3 can inhibit soot formation by reducing available carbon atoms. • Doping of N-containing species might contribute to the growth of carbon layers. • Young soot was more sensitive to NH 3 addition. • Correlations between soot nanostructure and reactivity with NH 3 were also revealed. [ABSTRACT FROM AUTHOR]

This study explored the impact of non-thermal plasma and CO 2 on the flame soot characteristics within the diffusion flames. We analyzed on flame structures that were diluted with either CO 2 or N 2 , temperature distributions, and soot characteristics, both in the presence and absence of plasma. Due to the higher specific heat capacity of CO 2 compared to N 2 , the optical observations consistently showed lower temperatures in flames diluted with CO 2 as compared to those diluted with N 2. The inclusion of plasma and carbon dioxide resulted in the lowest soot concentration, indicating that plasma coupled with CO 2 has a synergistic inhibitory effect on soot emissions. The findings revealed that when CO 2 was used to dilute the flames and the oxygen concentration was low, the soot nanostructure appeared amorphous. Raman results showed that the level of graphitization observed in soot particles from CO 2 dilution flames was lower than that from N 2 dilution flames. In the presence of plasma and CO 2 , the soot obtained exhibited the shortest fringe length and the highest fringe tortuosity. Significant correlations were observed between the nanostructure of soot and its reactivity. The combined application of plasma and CO 2 proved to be effective in reducing the soot carbonization degree. [Display omitted] • Effects of plasma and CO 2 addition on flame characteristics were investigated. • Coupled plasma and CO 2 effects on soot concentration were explored. • Soot graphitization degree decreased with plasma and CO 2 addition. • Soot produced by the coupled plasma with CO 2 exhibited the highest reactivity. [ABSTRACT FROM AUTHOR]

In general, when charge-coupled device (CCD) cameras are used to reconstruct key flame parameters via inverse radiation analysis, the uncertainty of the camera lens optical center will introduce difficulties for practical applications and also reduce the reconstruction accuracy. This paper investigated the effect of the camera lens optical center location on the accuracy for concurrently reconstructing the distributions of temperature and soot volume fraction in an optically thin, axisymmetric flame with a single CCD camera via inverse radiation analysis. The impact of the ray number and measurement error on the precision of reconstruction was analyzed. Overall, the measurement error played a crucial role in reconstruction accuracy while the increase of the ray number could improve the reconstruction accuracy. When the lens optical center used for reconstruction was inconsistent with the actual one, the reconstructed errors can remain in an acceptable range. In summary, the effect of the lens optical center uncertainty on the reconstruction accuracy could be small. The conclusion obtained here can be useful for the practical reconstruction process. [ABSTRACT FROM AUTHOR]

In this study, the differences of nanostructure and oxidation reactivity of the nascent soot formed in n-heptane/2,5-dimethylfuran (DMF) inverse diffusion flames (IDF) with/without influence of magnetic fields were studied, and the effects of DMF-doped and magnetic fields were discussed. Morphology and nanostructures of the soot samples were investigated using high-resolution transmission electron spectroscopy and X-ray diffraction, and the oxidation reactivity characteristics were analyzed by thermogravimetric analyzer. Results demonstrated that both additions of DMF-doped and magnetic fields could promote soot production and modify the soot nanostructure and oxidation reactivity in IDF. Soot production increased along with the increase of DMF-doped. With DMF blends, more clustered soot particles and typical core-shell structures with well-organized fringes were exhibited compared with that formed from the pure n-heptane IDF. With effects of magnetic fields, the precursor formation and the oxidization of soot were promoted, soot production was enhanced. Soot particles became relatively more mature with typical core-shell structure, thicker shell, longer fringe lengths, smaller fringe tortuosity, higher graphitization degree and lower oxidation reactivity. With magnetic force pointed to the central line and the inner direction of IDF under the conditions of N pole and S pole of the magnet facing the flame, oxygen was trapped, having an increased residence time to get more chance to react with the fuel molecules to cause more soot to be yielded and oxidized. That resulted in the soot precursor promotion, soot production enhancement, and soot part-oxidization and graphitization. [ABSTRACT FROM AUTHOR]

• Critical sooting transition point was identified with a novel optical method. • Sooting transition chemistry of iso -octane/DEE flames was firstly studied. • DEE addition delayed the transition point but accelerated the transition process. • The major effect mechanism of DEE addition was the dilution of carbon flow. • The chemical effect of DEE addition mainly reflected in the promotion of C 2 H 2. This study studied the effects of the addition of diethyl ether (DEE) in the fuel stream on sooting transition process in iso -octane counterflow diffusion flames with optical and chemical diagnostics. Compared with the baseline iso -octane flame, two fuel/oxygen/nitrogen flames were studied with a 20 % DEE/ iso -octane and a 40 % DEE/ iso -octane mixture as the fuel, while holding constant fuel mole fraction. Optical diagnosis was used for the investigation of the effect of DEE addition on macroscopic sooting transition point with a high-resolution digital camera based on image processing and transition point extraction methods. Online gas chromatograph (GC) was used for all flames to investigate the flame structure and measure quantitative mole fraction profiles of stable and reactive species formed in the sooting transition process, especially the changes caused by partial substitution of iso -octane by DEE. Meanwhile, the experimental results were compared with predictions from a model suitable for both iso -octane and DEE. Sooting transition point was delayed with the increase of DEE addition. Although the overall flame chemical structure did not change significantly with the addition of DEE, it was found that some primary decomposition products and precursors had more significant effects. In most cases, experimental trends could be predicted and explained well by the model. It was noted that the dilution of carbon flow caused by partial substitution was the main reason for the inhibition of soot formation and the delay of sooting transition point. Secondly, on the chemical effect, DEE addition could produce a large amount of C 2 H 2 which could contribute to the production of C 3 H 3 and the formation of large PAHs. [ABSTRACT FROM AUTHOR]

Macroscopic transition processes of soot formation for 1-butene, isobutene, n-butane and isobutane counterflow diffusion flames from sooting-free to heavy-sooting were studied by varying fuel concentration (X F) in the fuel stream. A novel optical diagnostic algorithm was proposed to classify different sooting transition stages in flames. Effects of saturated/unsaturated bonds and linear/branched structures of fuel molecules on the sooting transition were investigated in details. Flame species were detected and quantified by on-line gas chromatography and also kinetic analysis was performed. KL factor was obtained by optical diagnostics to qualitatively compare soot concentrations. Results showed that X F required for 1-butene flame to reach the critical state of macroscopic sooting transition was the lowest, followed by isobutene, isobutane and n-butane flames. Fuels with higher sooting tendencies could achieve entire sooting transition process with a narrower range of X F. Linear butane was more difficult to achieve the sooting transition than branched butane, while linear 1-butene was easier to form soot than branched butene, reflecting the difference between isomeric structures acting on C4 alkenes and alkanes. These might be largely due to the close association of allyl (C 3 H 5 -A) with 1-butene. Higher C3 species content in isobutane flame made it have higher sooting tendency than n-butane. • Novel optical method was developed to identify flame sooting transition stages. • Close association with allyl brought the highest sooting tendency of 1-butene. • Linear/branched butanes performed opposite order of sooting tendencies to butenes. • Benzene formation mechanism in different sooting transition stages was revealed. [ABSTRACT FROM AUTHOR]

This study reported comprehensive optical diagnostics for flame behavior and soot emission under directly coupling of the plasma with the flame at specific heights. The morphometric parameters of unstable flames under the direct plasma coupling were quantified. We also proposed an optical method to eliminate plasma luminescence from the field of view to extract flame intensity. At the same discharge height, with the increase of discharge frequency, the flame height decreased, while the flame horizontal extension distance and deflection angle increased. The relationships between flame morphologic and electrical parameters were obtained. The experimental results suggested a one-to-one correspondence between plasma action and flame deflection or shortening. When the plasma interacted with the flame, the overall temperature and soot emission of the flame decreased compared with that without plasma. At the same discharge height, the flame temperature and soot decreased further with the increase of discharge frequency. The soot emission changed more remarkably with the discharge frequency for higher discharge heights. With the increase of voltage, the flame temperature and soot increased at the lower discharge heights. The analysis of the experiment data demonstrated that the variation of soot emission was caused by the cooperation of multiple factors. • The effect of the plasma on a local flame was investigated. • A quantification method of flame structure was proposed for the plasma coupling. • The decoupling method of plasma and flame luminescence was developed. • The soot concentration of flames above electrodes decreased with plasma coupling. [ABSTRACT FROM AUTHOR]

Boron has been of considerable interest as fuel for propellants and explosives due to its high gravimetric and volumetric calorific values. In this study, the effects of different mixed modes, blending ratios, and packing densities on ignition and combustion characteristics of boron-aluminum mixture were explored using flash ignition facility and thermobalance. The results showed that the boron nanoparticles can be ignited upon exposure to the flash with a certain amount of aluminum nanoparticles addition and the flame luminous intensity increased with content of the added aluminum. The boron-aluminum mixture in loose contact performed better combustion characteristics than those in tight contact because of more access to the oxygen and less heat dissipation into the bulk sample. The main combustion products were detected and analyzed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The temperatures of the ignited nanoparticles were measured and mostly distributed between 1273 K and 1473 K. The burning time decreased significantly with the increase of aluminum addition at the high packing density condition. The boron combustion efficiency increased with the addition of aluminum content in loose contact. The thermogravimetric experimental results showed that the aluminum addition could decrease the primary ignition temperature of boron. Moreover, the photothermal effect and plasmon resonance effect of aluminum were the potential flash ignition mechanism of the boron-aluminum mixture. [ABSTRACT FROM AUTHOR]

• The enhanced soot oxidation activity is ascribed to the formation of surface oxygen vacancies. • The low initial temperature (T ig = 308 °C) and the low temperature of maximum CO 2 emission (T max = 389 °C) were achieved. • DFT calculations were performed to correlate with the activity test results. Soot particle exhausted from diesel engines are causing severe health and environmental problems. Alkali metals (Li, Na, K, Cs) doped CuFe 2 O 4 catalysts were synthesized to eliminate soot residue. XRD, Raman, SEM, H 2 -TPR and temperature-programmed oxidation (TPO) were employed to investigate the effect of alkali metals addition on CuFe 2 O 4. Density functional theory (DFT) calculations were used to explore the inner reaction mechanism. The results revealed that the doping of all alkali metals facilitated the formation of oxygen vacancies which are the main active sites for soot oxidation. The CuFe 2 O 4 doped by K showed smallest crystal size and the most oxygen vacancies. Therefore, Cu 0.9 K 0.1 Fe 2 O 4 catalyst performed best catalytic activity in soot oxidation with the T ig of 308 °C and T max of 389 °C. [ABSTRACT FROM AUTHOR]

• As fuel flow rate increased, the primary particle diameters were increased. • Dimethyl carbonate suppressed soot precursors formation in inception zone. • With RP-3 kerosene flow rate increased, soot particles had turbostratic nanostructure. • With dimethyl carbonate addition ratio increased, soot particles emitted from flame was still young. • Physical properties of soot particles had a strong effect on oxidation reactivity. RP-3 kerosene fuel is the dominant hydrocarbon fuel used in civilian aeroplanes in China. In this study, the effects of fuel flow rates and dimethyl carbonate additions on the formation and evolution of young soot particle collected from RP-3 kerosene in inverse diffusion flames were studied comprehensively by several methods. Physical properties (nanostructures) of soot particles were evaluated by low-resolution transmission electron (TEM) microscopy, high resolution transmission electron microscopy (HRTEM), X-ray Diffraction (XRD) spectroscopy, and Raman spectroscopy. Chemical properties (oxidation reactivity and surface oxygen contents) of soot particles were studied over thermogravimetric analyzer (TGA) and X-ray photoelectron spectroscopy (XPS). When the fuel flow rate increased and dimethyl carbonate addition ratio decreased, the soot particles and aggregates were found to form a lot and grow faster with bigger primary particle diameter. The HRTEM, XRD, and Raman analyses have shown that the disorder and turbostratic structure with shorter fringe length and longer tortuosity were increased, which indicated higher oxidation reactivity rates, as the fuel flow rate and dimethyl carbonate addition ratio increased. The surface oxygen contents were found to decrease as the fuel flow rate increased, while were shown to increase as the dimethyl carbonate addition ratio increased. [ABSTRACT FROM AUTHOR]