Mature Soot Research Articles

Evolution of physicochemical characteristics of soot in a single coal combustion flame

This study experimentally investigated the evolution of the nanomorphology, graphite crystals, oxidation reactivity, and functional groups of soot in a single coal combustion flame. Nascent (sample #1), young (sample #2), and mature (sample #3) soot were collected from a Shenhua single coal combustion flame. The soot samples were then tested and analyzed using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy to determine their physicochemical characteristics. The results showed that the nascent soot was an individual particle, the mature soot was a highly agglomerated soot aggregate, and the young soot was a transitional particle. The primary particles of sample #1 comprised several randomly orientated individual polycyclic aromatic hydrocarbon (PAH) crystal sheets, whereas the primary particles of samples #2 and #3 were typical core-shell graphite structures. The sp2/sp3 content, volatile organic fraction, and oxidation rate constant of samples #1, #2, and #3 decreased, resulting in successively weakened reactivity. The infrared spectrum absorption of 2800–3000 and 1700–900 cm−1 indicated that the hydrogen in the three coal-derived soot samples was mainly aromatic rather than aliphatic. Furthermore, the proportion of solo and duo aromatic out-of-plane (OPLA) CH bending in samples #1, #2, and #3 increased, whereas the proportion of trio-quatro OPLA CH bending decreased.

Study on the application of laser diagnosis technology in the rapid real time measurement of soot

Based on previous soot studies, this paper proposes a new method to quantify the evolution of the soot generation mechanism in flames. The soot distribution in inverse diffusion flame at different fuel gas velocities was quantitatively measured by laser-induced incandescent (LII) technique, and the soot structure evolution was analyzed by Raman spectroscopy and field emission transmission electron microscopy (FETEM). Laser-induced breakdown spectroscopy (LIBS) and numerical simulation were used to investigate the association between the intensity ratio of hydrocarbon atoms of at different positions of the flame and soot formation. Meanwhile, a detailed chemical kinetic simulation of the formation process of soot was carried out, and the flame structure was divided in more detail. The flame state in different regions and the key reactions of soot formation were analyzed. The results presented that the volume fraction of soot measured by LII is consistent with the height-dependent graphitization of the flame and is identical to the yellow definition of the flame. The variation of the C/H atomic intensity ratio reflected the soot formation in flame. The CH intensity ratio in the flame axial direction showed a bimodal distribution, corresponding to the blue phase of soot precursor formation and the yellow phase of soot formation, respectively. In addition, the CH atomic intensity ratio and the CH atomic molar ratio showed a parabolic transition from incipient to mature soot in the soot production region. The CH intensity ratios at different heights are linear in the radial direction, and the main core region of soot production can be obtained when the CH intensity ratio is 0.10 ∼ –0.13. With radial distance increases, it is clear results that the most sensitive reaction of (precursor) A1 changes from C6H5OH + H = A1 + OH to C2H3 + HCO=C3H3 + OH. It reveals that with the increase of yellow light, the soot structure along the flame center line is gradually graphitized, and the graphitization degree is the highest at the axial height of 18.0De ∼ 22.4De. This is consistent with the distribution of CH atomic intensity ratio. The present results demonstrate that LIBS can quickly achieve the initial measurement of soot and provide a new method for the study of flame soot during its evolution.

Intumescent closed-cell porous molecular engineering towards high flame retardancy and ultra-low smoke release epoxy resin

Wide applications of high-flammable epoxy resin (EP) bring potential fire risks with huge heat and smoke hazards. Although effective strategies for flame retardancy and heat suppression have been developed, there are still few measures for reducing smoke release synchronously. Herein, 2-aminobenzimidazole modified ammonium polyphosphate intumescent flame-retardant co-curing agent (ABMZ-APP), possessing excellent char-forming performance, is designed and endows EP/ABMZ-APP thermosets with excellent flame retardancy, heat suppressing, and smoke suppressing performance. It is worth mentioning that the total smoke production (TSP), and maximum smoke density (DSmax) of EP with 20 wt% ABMZ-APP decrease by 73.2%, and 67.6%, respectively. Deep investigation illustrates EP/ABMZ-APP suppresses the smoke by obstructing soot-precursor transfer through the special porous char structure, rather than altering the chemical changes in nucleation and growth processes of mature soot. Meanwhile, the closed-cell porous char layers, acting as physical barriers, obstruct the heat/fuels transfer simultaneously which is beneficial for suppressing sustained burning.

Spectral investigation of soot absorption properties during laser-induced incandescence measurements

The effect of rapid laser irradiation on the optical properties of mature soot particles is investigated by performing wavelength-resolved extinction measurements in the visible spectral region. In particular, the spectral behavior of the absorption properties is explored during laser irradiation, at the peak of the incandescence signal (prompt LII) and few nanoseconds after the peak, and finally when particles have reached an equilibrium condition with the surrounding gas. A significant variation of the absorption coefficient of the laser-irradiated soot particles compared to that of the pristine ones is observed already at the LII peak. Such variation keeps evolving with time until it reaches a final permanent value. Results are presented in relation to the laser fluence used for irradiation and discussed with the aim of stressing the need of knowing the entity of the modification of soot absorption properties during laser irradiation for a correct interpretation of LII data.

Sooting propensity and maturity of gasoline/anisole blends in a laminar coflow diffusion flame

Liquid oxygenated biofuels such as anisole have been used as additives, since they increase the octane number and improve spark-ignition engines’ performance. In this study, the sooting propensity of a gasoline surrogate, anisole, and gasoline/anisole mixture is quantified by measuring bidimensional fields of soot volume fraction, temperature and radiative heat flux in a coflow laminar flame using well-established non-intrusive optical diagnostics. The maturity of soot particles is also measured using innovative multiwavelength line-of-sight attenuation and emission measurements. Experimental measurements shows that increasing anisole content in fuel mixture results in a greatly enhanced soot volume fraction, while overall temperature is slightly decreased. In addition, maps of soot maturity index are reported based on the spectral dependence of soot optical properties with the amount of immature soot. Mature soot is found near the outer edge of the flame for the three flames, and mature soot content was found to decrease with increased anisole addition to the fuel mixture. In addition, high temperature zones were found to correlate with the presence of more graphitic compounds within the soot particles for the investigated fuels. The comprehensive dataset reported in this study provides rich information for a better understanding of soot formation processes, and for the development and validation of soot models in coflow laminar diffusion flames using complex fuels of interest for practical applications.

The effect of ammonia addition on soot nanostructure and composition in ethylene laminar flames

The formation of soot in ammonia (NH3) combustion with a hydrocarbon such as ethylene (C2H4) is investigated using the analysis of particles’ nanostructure and surface chemical composition to identify the mechanisms by which NH3 suppresses soot growth. Young and mature soot particles were extracted from laminar diffusion co-flow flames of NH3C2H4 blends up to 50% NH3. The soot nanostructure is examined using lattice fringe analysis of high-resolution transmission electron microscopy (HRTEM) images, Raman Spectroscopy, and Electron Energy Loss Spectroscopy (EELS). The chemical composition of the soot surface is analyzed using X-ray Photoelectron Spectroscopy (XPS). With increasing NH3 addition, reduced planar growth of the soot graphitic layers is observed through the analysis of the soot nanostructure which indicates the suppression of carbon addition at the edge sites of the layer planes. It is also found that the carbon sp2/sp3 bonding ratio increases which shows a nanostructure with less carbon sp3 bonding at defect sites. By exploring the chemical composition, the nitrogen content of the soot surface increases with NH3 addition which is caused by the bonding of nitrogenated species with carbon at defect and edge sites of the carbon layer, reducing the potential for carbon addition, and thus soot growth. The results also show an increased risk of formation of nitrogenated polyaromatic hydrocarbons (N-PAHs) on the soot surface by NH3 addition. The study presents a novel investigation of the impact of NH3 on the soot surface chemistry and particles nanostructure which explains the role of NH3 in suppressing soot growth and reveals the potential for new toxic characteristics for the soot emissions formed in NH3-hydrocarbon cofired systems.

Size and light absorption of miniature-inverted-soot-generator particles during operation with various fuel mixtures

The Argonaut miniature inverted soot generator (MISG) serves as a convenient and stable source of soot black-carbon aerosol particles. The MISG performance has been previously characterized for ethylene and propane, in a simple two-flow setup of fuel and air. These fuels produce soot size distributions with geometric median diameters (GMDs) of 150–200 nm. These sizes are substantially larger than the soot particles emitted from typical internal-combustion and aviation gas turbine engines. Here, we sought to produce substantially smaller soot particles, representative of aviation gas turbine engine emissions, by using five different fuels (acetylene, methane, ethylene, propane, and dimethyl ether) and by enriching the combustion air with oxygen. We developed a rapid screening protocol for each fuel, to allow the full range of stable flame conditions to be explored. We monitored the size distribution, mass concentration, and mass absorption cross section (MAC) of denuded MISG soot in real time. We considered only operating conditions where stable soot mass concentrations above 3 μg/m3 were emitted, corresponding to a non-flickering and open-tipped flame. For virtually all operating conditions, the measured MAC was consistent with literature values for mature soot. The only exception was a propane-acetylene mixture, for which the measured MAC was much lower than literature values. The observed GMDs fell into two groups, and we did not identify any trends leading to the mixtures falling into either group. The two groups were at approximately 100 nm and 160 nm. Therefore, while this study identified conditions that reduced the soot particles to 100 nm, it did not identify conditions where even smaller soot particles could be produced. This study has demonstrated that the soot properties produced by the MISG are highly repeatable and relatively insensitive to fuel composition.

Effect of maturity on soot volume fraction measurements using the AC-LII technique in a laminar coflow ethylene diffusion flame

The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (fv) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E(m,λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E(m,λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured fv. In this study, a correction to the AC-LII measured fv with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E(m,λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals SLII and soot temperature Teff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured fv in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E(m,λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured fv is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII fv results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured fv and largely eliminate the difference in the measurements of fv by AC-LII and MW-LOSA/EMI.

Optical properties of incipient soot

The exact knowledge of the optical properties of soot nanoparticles is fundamental for several aspects including the correct determination of the soot concentration in combustion environments using optical diagnostics and the correct estimation of the environmental impact of the emitted particles. Although extensive researches over the years have led to a substantial agreement on the optical properties of mature soot particles, the optical properties of the incipient soot nanoparticles are still uncertain. From the particle inception point to the formation of large and more mature soot particles, the evolution of the optical properties must account for variations due to the size and the physicochemical transformation of the investigated particles. This work aims to determine the refractive index and optical properties of inception particles formed in lightly sooting flames. A previous determination based on in-situ light absorption and scattering measurements is revisited taking advantage of particle size measurements by differential mobility analysis. The spectral dependencies of the optical properties are derived by the Kramers-Krӧnig analysis of the ex-situ VUV-NIR light absorption measurements. Results confirm the strong decrease in the absorptivity in the vis-NIR region of inception particles with unimodal size distribution and d63∼3 nm, and confirm a strong size dependency of soot optical properties. The thermal-optical analysis of the sampled particles shows that the particle mass absorption coefficient also correlates with organic carbon content.

Raman Spectroscopy of Nascent Soot Oxidation: Structural Analysis During Heating

The effect of oxidation on nascent soot particles is investigated in this work by means of Raman spectroscopy. Three different soot particle samples are produced in a lightly sooting ethylene/air laminar premixed flame and collected at different residence times. The samples are characterized by a different size of primary particles and a different degree of aging and hence graphitization, and they are representative of the early evolution of nascent soot in flames. Oxidation of particles is induced by exposing the samples to a high temperature oxidation treatment in air over a range of temperature 25–500°C. To this aim, a programmable heating microscope stage is used. Thermo-optical transmission (TOT) measurements are used to characterize the chemical composition of pristine particles in terms of organic and elemental carbon content, also providing the pyrolytic carbon fraction of the organic carbon. The TOT results show that the organic fraction is constant for the three conditions. On the other hand, the pyrolytic carbon fraction decreases and the elemental carbon increases when moving from the sample composed of just-nucleated particles to the sample mostly made of mature soot particles. Raman spectroscopy reveals that the thermal oxidation treatment performed on the sample of just-nucleated particles, with the highest organic carbon content, results in a reduction of the amorphous carbon component. Conversely, the sample of mature soot, with the highest elemental carbon content, shows an increase in the amorphous carbon phase after oxidation, which is attributed to fragmentation or the formation of point defects by O2 oxidation. Finally, the thermal oxidation procedure produces a strong reduction in the photoluminescence signal detected from Raman spectra.

Influence of rapid laser heating on differently matured soot with double-pulse laser-induced incandescence

For accurate laser-induced incandescence (LII) measurements of soot properties it is of great importance to understand the nature of the physical processes involved during rapid laser heating. In this work, we investigate how well-characterized differently matured fresh soot from a soot generator responds to rapid laser heating. For this purpose, a double-pulse LII setup is used with 10 μs time separation between the pulses using various combinations of two common LII wavelengths (532 and 1064 nm). Detection is performed at two wavelength bands for fluorescence analysis, and additionally elastic light scattering is used for mass loss analysis during heating. We investigate how the LII signal changes with pre-heating laser energy, specifically by fluence curve analysis to estimate the influence of thermal annealing, sublimation and laser-induced fluorescence interference. It is shown that extensive absorption enhancement occurs for all types of soot as the soot is thermally annealed, which is manifested through decreasing dispersion coefficient ξ and an increasing absorption coefficient E(m,λ). When comparing young and mature soot, a much larger impact of sublimation can be observed in the fluence curves of the mature soot. Also, we observe an enhanced contribution of laser-induced fluorescence for the young soot when performing LII measurements using 532 nm, which is suggested to originate from vaporized carbon fragments with an aromatic structure. This work further shows the potential of utilizing double-pulse arrangements for increasing the detectability of poorly absorbing soot, but also it highlights the impact of laser heating on soot, which may be important to avoid interferences when performing soot diagnostics.

Characterisation of the transition type in optical band gap analysis of in-flame soot

Semiconductor materials are characterised by a band gap between conduction and valence band, which can either be of direct or indirect nature depending on the crystal momentum. In case of applying the so-called Tauc method to infer the band gap value, both transition types are described by different values for the transition type exponent. In the case of soot particles, the optical band gap is an important quantity during the evolution process to describe the optical characteristics mainly linked to the underlying structural properties. In this work, an approach to infer the optical band gap value together with the transition type exponent is used to shed light on the electronic nature and optical properties of soot during its formation process. In situ UV–Vis-NIR extinction measurements as well as ex situ Raman spectroscopy are performed in a non-premixed ethylene/air flame at two different positions in the flame reflecting different stages of the soot formation process. The results reveal two important tendencies: first, an increase in the band gap energy of young soot is observed, possibly caused by the curvature of initially planar polycyclic aromatic hydrocarbon monomers due to the presence of odd-numbered rings in the hexagonal aromatic lattice. Second, a decrease of the transition type exponent value can be observed, indicating a shift from an indirect transition to a more direct band structure. Further, the indirect band gap assumption for mature soot and its band gap value close to 0.2 eV can be confirmed experimentally. The Raman results provide additional insight in the atomic fine structure of the soot particles regarding the defect density and the loss of amorphous character with increasing maturity.

Insight into soot formed in coal combustion flame: Evolution of physiochemical structure, oxidation reactivity

Combustion experiments of a bituminous coal were performed on a flat-flame burner that provided a realistic low-O2 combustion atmosphere. Soot samplings were performed via both thermophoresis and dilution-filtering sampling methods along the flame centerline. The morphology, size, fractional- and nanostructure evolution, and especially the detailed oxidation characteristics of soot of different maturities in flame were comprehensively characterized. Results showed that liquidus tar was formed in the flame bottom which acted as soot precursor, and a direct evidence was obtained that the solid soot particles in the coal flame were mostly derived from the liquidous tar via carbonization. In the nanostructure of the mature soot particles, both of fullerenic, amorphous and diamond phases were detected, indicating a multiple nucleation process during the soot formation in coal flame. Furthermore, it was found that soot in the bright flame core was more difficult to be oxidized than the soot upstream and downstream it. And finally based on the Raman spectrum and particle micromorphology analysis, the non-linear change of the oxidation reactivity of the soot along the flame centerline was revealed to be controlled firstly by the gradual graphitization with increasing of the flame temperature and then by the change of soot aggregate size.

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

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

Thermo-optical-transmission OC/EC and Raman spectroscopy analyses of flame-generated carbonaceous nanoparticles

Elemental and organic carbon content of flame-generated soot nanoparticles produced under different operating conditions of equivalence ratio and particle residence time is determined by thermo-optical-transmission measurements, while particle carbon structure is characterized by Raman spectroscopy analysis. The sampled soot nanoparticles are produced in slightly-rich, almost bluish, flames and comprise small 2–4 nm nascent soot nanoparticles, often neglected in the primary carbonaceous emissions from combustion systems, and larger 4–15 nm soot particles.The application of the NIOSH-like protocol for the thermo-optical-transmission analysis allows the separation of the total carbon (TC) into fractions of organic carbon (OC), pyrolytic carbon (PC), and elemental carbon (EC). The lowest amount of elemental carbon is measured in young soot nanoparticles and it is progressively larger in mature soot particles. The organic carbon has the opposite trend and shows that it is the “pyrolyzable” carbon fraction of the young soot nanoparticles that transforms into elemental carbon in the primary soot.Raman spectroscopy showed consistent results as compared to the thermo-optical-transmission analysis indicating a analogous trend in the graphitization order of the different soot particles. Most importantly, the logarithm of the intensity of the measured photoluminescence background shows a linear dependence with the OC/EC ratio.Mass absorption coefficients (MAC) were retrieved by the thermo-optical-transmission analysis for the four soot samples; the estimated MAC coefficient ranged from 2 to 6 m2/g, dependently from the relative percentage of EC component. The data are in excellent agreement with currently available literature data and extend the determination of MACs to carbonaceous particles with a larger OC/TC content.

Impact of the maturation process on soot particle aggregation kinetics and morphology

Mature soot aggregates exhibit a morphology which is currently mimicked by diffusion-limited aggregation (DLCA) codes, i.e. with a sticking probability = 1 when two aggregates collide. Nevertheless, nascent soot particles may grow in the reaction-limited aggregation regime (RLCA), i.e. with a sticking probability ≪ 1. Yet, it remains to be seen how fast the transition from RLCA to DLCA occurs for soot particles and what is the impact on aggregation kinetics, particle size distribution and morphology. This work intends to fill this gap by exploring the aggregation of soot particles formed in a laminar premixed flame through numerical simulations. Results show that the transition from RLCA to DLCA is very fast and produces a moderate impact on soot formation dynamics and morphology. However, soot particles mass bulk density is found to play an important role and should be considered in future simulations of soot formation in flames.

Laser-induced incandescence (2\u03bb and 2C) for estimating absorption efficiency of differently matured soot

Two-wavelength and two-color laser-induced incandescence (2λ–2C-LII) was used to study the absorption properties of three types of cold soot of different maturity from a mini-CAST soot generator. LII fluence curve analysis allowed for estimating absorption wavelength dependence in terms of dispersion coefficients ξ by the use of two excitation wavelengths (532 and 1064 nm). The estimated ξ (based on E(m, λ) ∝ λ1−ξ) spanned from ~ 1.2 for the mature soot, up to 2.3 for the young soot. The results for the mature soot showed good agreement with previous measurement using multi-wavelength extinction. For the young soot, however, some discrepancy was observed suggesting a weaker wavelength dependence (lower ξ) from the LII fluence analysis. Furthermore, an estimation of the E(m, λ) for the different types of soot was done from the experimental fluence curves with temperature analysis in the low-fluence regime and simulations using an LII model. Additionally, uncertainties and limitations were discussed. Finally, it should be pointed out that caution has to be taken when interpreting 2λ-LII results to obtain quantitative absorption properties of less mature soot, which may be influenced by thermal annealing during the laser pulse and by absorption from non-refractory species externally/internally mixed with the soot.

Investigation of sooting flames by color-ratio pyrometry with a consumer-grade DSLR camera.

The accurate measurement of temperature in sooty flames remains a challenging task. In this study, the procedure for calibration and use of a consumer-grade digital single-lens reflex (DSLR) camera for the measurement of temperature in sooty flames using color-ratio pyrometry (CRP) is elucidated. Owing to the necessity of acquiring RAW images for CRP, investigations conducted thus far have been limited to stable flames. In this work, the potential of a CANON EOS 550D DSLR camera for measuring temperature during a transient process such as a droplet combustion event at frame rates up to 50fps is demonstrated. The spectral response curves of the camera-lens system were obtained with the help of a tunable laser source and a laser power meter, which were subsequently used to generate lookup tables for blackbody as well as soot radiation. Soot radiation was assumed to vary with wavelength (λ) as λ-α, where α is the soot dispersion exponent. The blackbody lookup table was validated against blackbody calibration data from 1023 to 1773K. Experiments were conducted on a candle flame as well as a McKenna flat flame burner with ethylene-air mixtures of equivalence ratios 2.1 and 2.3. For estimating temperatures using CRP, soot particles were classified as nascent and mature soot with soot dispersion exponent (α) values of 4 and 1.38, respectively. The CRP results were found to yield a decent match with thermocouple measurements as well as data reported in the literature.

Evolution of structure and oxidation reactivity from early-stage soot to mature soot sampled from a laminar coflow diffusion flame of ethylene

This work investigated the structure and oxidation reactivity of soot sampled from a laminar coflow diffusion flame of ethylene. A capillary-nozzle-hybrid sampling method was developed to extract soot from five sampling positions along flame axis, covering both early-stage and mature soot samples. The results reveal that residence time plays an important role in modifying surface functional groups. Oxygenated and aliphatic groups gradually disappear, soot structure becomes more organized. As a consequence, the rate of mass losses is impaired during thermo-chemical conversion. The derivative thermogravimetry (DTG) results show that oxidation of early-stage soot can be separated into low-temperature (low-T) conversion and carbonaceous substances oxidation processes. The former process including both volatile organic fraction (VOF) releasing and early oxidation reactions generates the first maximum mass loss rate at about 510 °C, while the latter forms the second maximum mass loss rate at about 600 °C. Recognizing that the two processes are partially merged, the distributed activation energy model (DAEM) was introduced to decouple the bimodal behavior of DTG curves. The DAEM results reveal that with increased degree of soot maturity, relative contribution from low-T conversion process decreases abruptly, and DTG curve eventually becomes unimodal and can be well simulated by considering only carbonaceous substances oxidation process.

Revealing soot maturity based on multi-wavelength absorption/emission measurements in laminar axisymmetric coflow ethylene diffusion flames

A novel diagnostic is proposed to characterize the maturity of soot particles in a laminar axisymmetric coflow ethylene diffusion flame in terms of the spectral dependence of soot absorption function. The method relies on the combination of line-of-sight attenuation (LOSA) and emission measurements at four wavelengths (500, 532, 660 and 810 nm). The analysis of the measured signals enables the determination of soot temperature, soot volume fraction, soot maturity and the contribution of soot scattering to extinction. The analysis of extinction and emission measurements considers the spatial variation of soot optical properties. The introduction of a maturity index allows the evaluation of soot maturity based on the spectral variation of the soot absorption function. The maturity index is correlated with the organic or the mature soot content and finally in terms of the absolute value of absorption function at 810 nm. The methodology is validated using a set of synthetic spectral LOSA and emission signals representing experimental measurements based on numerical results obtained using the CoFlame code. A sensitivity analysis of the Abel inversion is also performed to properly address the effect of deconvolution procedure. Finally, the proposed method is applied to analyze the experimental data of spectrally-resolved LOSA and emission acquired in a laminar axisymmetric coflow ethylene diffusion flame established on a Gülder burner. The two-dimensional distributions of soot temperature, soot volume fraction, soot maturity, and the ratio of total scattering to absorption are determined. Mature soot particles are found on the top of the flame in the centerline region and also in the outer edge of the flame wing displaying strong gradients.