Surface Growth Processes Research Articles

Numerical study of the effects of surface nitrogen-containing on soot formation process in ammonia/ethylene counterflow diffusion flames

In this paper, a modified soot kinetic model was developed to explore the potential inhibitory effects of nitrogen-containing functional groups on the soot surface growth process. In the modified soot kinetic model, the formation process of cyano functional groups on the soot surface was simulated through the reaction of hydrogen cyanide (HCN) and the surface open sites, and the surface cyano functional groups were assumed to no longer participate in the subsequent hydrogen-abstraction-acetylene-addition (HACA) reaction. The prediction performance of soot volume fraction (SVF) and average particle size (D63) in ammonia doping ethylene counterflow diffusion flames was compared between the traditional soot kinetic model and the modified kinetic soot model. The results show that both models reflected the suppression effects of ammonia doping on soot formation observed in the experiments. However, compared to the traditional soot kinetic model, the modified soot kinetic model exhibited better performances in predicting SVF and D63 with prediction deviations reduced by 52.7 % and 2.3 nm, respectively. The simulation results showed the formation of surface cyano functional groups led to a nitrogen content of up to 0.84 % in the soot, which is consistent with previous experimental measurements. In-depth analysis shows that the formation of cyano functional groups on the soot surface decreased the proportion of hydrogen sites and further diminished the number of open sites, resulting in reductions in the rates of HACA surface growth and the soot mass growth by up to 22 % and 29 %, respectively. It is proposed that the nitrogen-containing functional groups on the surface of soot in ammonia-doped hydrocarbon flames inhibit the formation of soot, providing an alternative pathway to the well-known gas-phase suppression mechanism.

Modeling of Soot Emission in Turbulent Diffusion Flames Impinging on a Cold Surface

ABSTRACT A hydrocarbon flame impinging on a cold surface produces an enhanced soot emission compared to the identical free flame. Cooking with biomass fuels is one practical process where soot emissions due to flame impingement has important environmental and health consequences. This problem was examined in a simplified system in which a turbulent non-premixed ethylene flame impinged on a cold surface. Previous experiment on this configuration examined the influence of a number of parameters on soot emission. The present paper uses numerical modeling to investigate the mechanism of soot emission enhancement in this configuration. Variations in the height of the surface above the fuel jet inlet showed going through a maximum, replicating the data. Since soot behavior can be affected by both thermal quench and flow field disruption, we decoupled the processes by replacing the cold surface with an adiabatic surface. These results indicated that most of the emission enhancement was due to the flow field disruption and not due to thermal quench. The modeling suggests that the presence of the surface changes the scalar dissipation rate, which influences C2H2 concentration, a key species in the soot surface growth process. Specifically, the surface causes a spike in the scalar dissipation rate adjacent to the surface, which significantly influences C2H2 concentration and results in increased soot production. The surface also forces the flame to spread radially, allowing more O2 to penetrate the flame immediately, enhancing the soot oxidation process. The combination of the enhanced soot production and oxidation produces the observed behavior.

Dynamics of the surface growth resulted from sedimentation of spheres in a Hele–Shaw cell containing a low-viscosity fluid

In this paper, we investigate the dynamics of surface growth resulting from sedimentation of spherical granular particles in a fluid environment, using experiments and simulations. In the experimental part, spherical polystyrene particles are poured down from the top of a vertical Hele–Shaw cell and form a 1 + 1-dimensional growing surface. The surface roughness is obtained from the images, and the growth and roughness exponents are measured. In the numerical simulation part, the surface growth process is simulated using the discrete element method, considering the interactions between the grains, and the exponents are calculated. In this method, unlike conventional simulation models, instead of a discrete deposition law, the dynamics of the individual particles throughout the process are obtained, considering different forces acting on the particles. Finally, the simulation results are compared with the experiment, and we see a very good agreement between them. We find different values for the exponents using different methods, indicating that the system is multi-affine and does not obey the scaling laws of affine models.

Surface growth approach for bulk reconstruction in the AdS/BCFT correspondence

Abstract In this paper, we extend the surface growth approach for bulk reconstruction into the AdS spacetime with a boundary in the AdS/BCFT correspondence. We show that the geometry in the entanglement wedge with a boundary can be constructed from the direct growth of bulk extremal surfaces layer by layer. In addition, we find that the surface growth configuration in BCFT can be connected with the defect multi scale entanglement renormalization ansatz (MERA) tensor network. Furthermore, we also investigate the entanglement of purification within the surface growth process, which not only reveals more refined structure of entanglement entropy in the entanglement wedge but also suggests a selection rule for surface growth in the bulk reconstruction.

Effect of simultaneous H2 and NH3 addition on soot formation in co-flow diffusion CH4 flame

Simultaneous blending of hydrogen (H2) and ammonia (NH3) to hydrocarbon fuels can tackle the safety issues of H2 and improve burning efficiency of NH3. While this strategy brings challenges for soot prediction due to the promotion effect of H2 and the suppression effect of NH3, and the interactions between H2 and NH3. In this study, the simultaneous addition of NH3 and H2 on soot formation was experimentally and numerically investigated in a co-flow diffusion CH4 flame. The interactions between NH3 and H2, and how they impacted different soot formation processes were comprehensively revealed using a detailed soot sectional model. The decrease of peak SVF in CH4 flame caused by NH3 was 0.013 ppm, about 31.6 % smaller than that in CH4/H2 flame (0.019 ppm), indicating that the inhibitive effect of NH3 on soot formation was promoted by H2. The existence of H2 promoted the suppression effect of NH3 on soot nucleation, condensation and HACA processes in the CH4 flame. Compared with CH4/NH3 flame, the pyrolysis rates of NH3, NH2 and NH in the CH4/NH3/H2 flame were higher since more H and OH radicals were generated via H2 decomposition. This led to a larger consumption rate of H and OH radicals, which decreased the reaction rates of CH4+OH=CH3+H2O and C2H4+OH=C2H3+H2O, and promoted the combination of NO and C2H. Both factors accounted for a stronger suppression effect of NH3 on the formation of A1 in CH4/H2 flame than that in CH4 flame, and thus a stronger inhibitive effect on soot inception and condensation. Compared with the CH4 flame, NH3 resulted in a larger decline of H and OH radicals mole fractions in the CH4/H2 flame, which explained the stronger suppression effect of NH3 on the HACA surface growth process in the CH4/H2 flame.

Effect of CH4 Addition on Soot Formation in C2H4 Diffusion Flame

Abstract Studying the effect of co-combustion of multiple fuels on soot formation has become a hot spot in the investigation of soot particles. In this paper, the influence of methane blending on soot formation in ethylene flame combustion is studied experimentally and numerically. The visible spectrum of flame image processing technology was used for the in situ measurement of laminar flame temperature and carbon smoke volume points in the experiment. The effects of different methane blending ratios on particle nucleation, coalescence, surface growth, and oxidation process of soot were analyzed based on the piecewise particle dynamics soot model of polycyclic aromatic hydrocarbons (PAHs) using CoFlame Code. Results indicate that the synergistic effect promoted the increasing rate of nucleation and addition reaction of hydrogen extraction at a low methane blending ratio, and the increase in the total mass of soot was mainly due to the PAH condensation rate. The total amount of soot generation gradually decreases with increasing blending ratio. The overall trend of condensation, surface growth rate, and soot nucleation in the flame decreases with increasing blending ratio. And the nucleation rate gradually shifts from a single peak to a double peak and increases slightly at the initial stage of the flame combustion reaction. It is worth mentioning that the change of three PAH precursors (secondary benzo(a)pyrenyl, benzo(a)pyrene, and benzo(ghi)fluoranthene) and the temperature explains the change of nucleation rate from unimodal to bimodal.

Analysis of the mechanism of the effect of N–H–O impurities on diamond growth under HPHT

In this work, diamond crystals were synthesized with N–H–O impurities by the temperature gradient method (TGM) under high pressure and high temperature (HPHT) conditions using FeNi alloy as the metal solvent (MS). The results indicated that the spontaneous nucleation rate and proportion, growth characteristics, surface growth texture, and the impurity concentration of diamond crystals changed drastically upon changing the impurity content in the system. Mutual diffusion between the MS and carbon source (CS) was also blocked, which seriously inhibited the growth rate of diamond crystals. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses corroborated that the zero-valent iron (Fe0) and nickel (Ni0) contents declined after N–H–O impurities were introduced. The newly formed graphite can be found in the MS, but the ferric carbide disappears. XPS also confirmed shifts in the binding energy of FeNi MS peaks, more iron oxide and nickel oxide were identified in MS, hindering the mass transfer process. CO and NO were absorbed on the surface of MS, which hindered the surface processes of diamond growth. The formation of intermediates (Fe3C) was impeded during diamond growth and blocked the spontaneous nucleation of diamond. All of these phenomena contributed to a poor growth rate and changed the surface growth process of diamond crystals.

Importance of resonance-stabilized radicals in soot formation mechanism of diphenyl ether pyrolysis

Soot formation during fuel pyrolysis can pose environmental and human health hazards. As a typical coal-related model compound, diphenyl ether (DPE) represents the specific structures in coal. In this study, soot formation mechanism of DPE pyrolysis was systematically investigated from theoretical principle. The molecular dynamics (MD) simulations show that the variety and number of DPE pyrolysis products increase with the raising temperature. In addition, the higher temperature also promotes the soot formation. Quantum mechanics (QM) calculations indicate that the energy barrier of CO removal from the phenoxy radical is lower than that of the phenyl isomerization reaction, leading to the easier depletion of the phenoxy radical. It is consistent with the result observed from the MD simulation that the phenoxy is consumed more rapidly than phenyl. The main pathways for initial ring formation are the isomerization of polyyne-like chains and the decomposition of DPE molecules which form the ring molecules of C6H5 and C5H5. C2H2 molecule and resonance-stabilized radical (RSR) species like C3H3, C6H5 and C5H5 are precursors for the growth of polycyclic aromatic hydrocarbons (PAHs), which have an important role in the growth process of soot. Subsequently, larger PAHs aggregate with each other to form incipient soot particles which will undergo surface growth and graphitization processes. We hope this work can provide worthy insights into the mechanism of soot formation in DPE and the suppression of carbon soot.

Numerical Predictions of Soot Formation in Kerosene/Air Jet Diffusion Flame

Soot volume fraction in a turbulent diffusion flame burning kerosene/air is simulated using steady flamelet approach. The soot inception model based on the formation rate of three and two ring aromatics is used, which further accounts for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy are solved in conjunction with k-? turbulence model in ANSYS-FLUENT. The fairly detailed reaction mechanism for kerosene/air is coupled to the turbulent flow field by steady laminar flamelet approach, while the soot is calculated using Moss-Brookes model. The predictions are found to match well with the measurements where no significant effects are observed with the inclusion of higher order hydrocarbon as precursors and surface growth.

Simulation research on surface growth process of positive and negative frequency detuning chromium atom lithographic gratings

Chromium atom photolithography gratings are a promising technology for the development of nanoscale length standard substances due to their high accuracy, uniformity, and consistency. However, the inherent difference between the interaction of positive and negative frequency detuning standing wave field and the atoms can cause a difference in the adjacent peak-to-valley heights of the grating in positive and negative frequency detuning chromium atom lithography, which greatly reduces its accuracy. In this study, we performed a controlled variable growth simulation using the semi-classical theoretical model and Monte Carlo method with trajectory tracking and ballistic deposition methods to investigate the influence of key experimental parameters on the surface growth process of positive and negative frequency detuning atomic lithography gratings. We established a theoretical model based on simulation results and summarized empirical equations to guide the selection of experimental parameters. Our simulations achieved uniform positive and negative frequency detuning atomic lithography gratings with a period of 1/4 of the wavelength corresponding to the atomic transition frequency, and adjacent peak-to-valley heights differing by no more than 2 nm, providing an important theoretical reference for the controllable fabrication of these gratings.

Effects of H2 addition on soot formation in counterflow diffusion flames of propane: A comparative analysis with He and N2 addition

Co-firing hydrogen (H2) with hydrocarbon fuels is widely seen as an effective pathway to reduce soot emissions. Nevertheless, the soot-reducing potential of H2 can be strongly dependent on the type of the co-fired hydrocarbon and the combustion conditions. In extreme cases H2 enrichment could even promote soot production in hydrocarbon flames, thus highlighting the necessity of an in-depth understanding on the roles of H2 addition. The present study aims to clarify the effects of H2 addition on soot formation in non-premixed flames of propane. The effects of He and N2 addition are included for a cross-comparison to identify the different effects of H2 (i.e., direct chemical roles, dilution or transport effects associated with fuel stream's physical properties). The technique of laser-induced incandescence (LII) was used for soot volume fraction measurement, and accompanying soot modeling analysis was performed for additional kinetic insights into the experimental results. The measurements and modeling results consistently show that H2 is a more effective soot inhibitor than the chemical inert additive of He. Considering the comparative transport properties between H2 and He, we concluded that H2 has additional chemically-inhibiting effects on soot formation; kinetic analysis indicates that the chemical roles of H2 mainly occur in the stages of soot precursor formation and soot nucleation process. Another interesting finding is that while both He and N2 are chemically inert, He has notably stronger soot-reducing ability than N2, suggesting the favorable physical properties of He (i.e., the high diffusivity) in reducing soot formation. The major contributions of this work include: 1) experimentally identify the different effects of H2 (chemical, dilution and physical) on soot formation in counterflow flames of propane, 2) unravel the chemical roles of H2 in the different sooting stages including soot precursors formation, soot inception and surface growth process, 3) provide new datasets for mechanism validation and refinement related to sooting characteristics of H2/hydrocarbon mixture fuels.

Soot growth mechanism in C2H2 combustion with H2 addition: A reactive molecular dynamics study

A numerical investigation of the effect of H2 addition on soot formation was conducted in C2H2 combustion process from the molecular level. The results showed that the chemical effect of H2 (CE-H2) inhibited the soot formation of C2H2 combustion. With the gradual increase of H2 doping ratio, the soot morphology became sparser and the soot particle size decreased. The CE-H2 significantly inhibited the surface growth and coalescence of soot. Addition of H2 led to an increase in the H2 concentration in the combustion process, which inhibited the dehydrogenation reaction of the primary soot particle with C2H2, free radicals, and polycyclic aromatic hydrocarbons (PAHs). The CE-H2 increased the consumption rate of C2H2 and the formation rate of H2O, and it decreased the formation rate of CO. Moreover, the addition reaction of free radicals and PAHs played an important role in the soot growth process, which effectively promoted the soot surface growth process. And the coalescence process significantly increased the soot mass and affected the soot size and morphology although its frequency was relatively low.

History-dependent attachment of Pseudomonas aeruginosa to solid–liquid interfaces and the dependence of the bacterial surface density on the residence time distribution

This study investigates how the recent history of bacteria affects their attachment to a solid–liquid interface. We compare the attachment from a flowing suspension of the bacterium, Pseudomonas aeruginosa PAO1, after one of two histories: (a) passage through a tube packed with glass beads or (b) passage through an empty tube. The glass beads were designed to increase the rate of bacterial interactions with solid–liquid surfaces prior to observation in a flow cell. Analysis of time-lapse microscopy of the bacteria in the flow cells shows that the residence time distribution and surface density of bacteria differ for these two histories. In particular, bacteria exiting the bead-filled tube, in contrast to those bacteria exiting the empty tube, are less likely to attach to the subsequent flow cell window and begin surface growth. In contrast, when we compared two histories defined by different lengths of tubing, there was no difference in either the mean residence time or the surface density. In order to provide a framework for understanding these results, we present a phenomenological model in which the rate of bacterial surface density growth, , depends on two terms. One term models the initial attachment of bacteria to a surface, and is proportional to the nonprocessive cumulative residence time distribution for bacteria that attach and detach from the surface without cell division. The second term for the rate is proportional to the bacterial surface density and models surface cell division. The model is in surprisingly good agreement with the data even though the surface growth process is a complex interplay between attachment/detachment at the solid–liquid interface and cell division on the surface.

A nanoparticle formation model considering layered motion based on an electrical explosion experiment with Al wires

To study the evolution of nanoparticles during Al wire electrical explosion, a nanoparticle formation model that considered layered motion was developed, and an experimental system was set up to carry out electrical explosion experiments using 0.1 mm and 0.2 mm Al wires. The characteristic parameters and evolution process during the formation of nanoparticles were calculated and analyzed. The results show that the maximum velocities of the innermost and outermost layers are about 1200 m·s−1 and 1600 m·s−1, and the velocity of the middle layer is about 1400 m·s−1, respectively. Most of the nanoparticles are formed in the temperature range of 2600 K‒2500 K. The characteristic temperature for the formation of Al nanoparticles is ∼2520 K, which is also the characteristic temperature of other parameters. The size distribution range of the formed nanoparticles is 18 to 110 nm, and most of them are around 22 nm. The variation of saturated vapor pressure determines the temperature distribution range of particle nucleation. There is a minimum critical diameter of particles (∼25 nm); particles smaller than the critical diameter can grow into larger particles during surface growth. Particle motion has an effect on the surface growth and aggregation process of particles, and also on the distribution area of larger-diameter particles. The simulation results are in good agreement with the experiments. We provide a method to estimate the size and distribution of nanoparticles, which is of great significance to understand the formation process of particles during the evolution of wire electrical explosion.

Soot volume fraction and size measurements over laminar pool flames and pre-vaporised non-premixed flames of biofuels, methyl esters and blends with diesel

Previous studies have demonstrated that methyl ester-based biodiesels produce significantly less soot compared to petroleum diesel in standard diffusion flames. However, the roles of the oxygen content and the degrees of unsaturation (DoU) of the biodiesels on their sooting propensity have not been independently analysed. In the present study, the relationship between soot yield of biodiesels and the DoU of the fuel is systematically analysed in laminar pool flames and pre-vaporised diffusion flames. The spatial distribution of soot volume fraction of the flames fuelled with four biodiesel fuels, two methyl esters and their blends with diesel are quantitatively measured using laser induced incandescence. All six biofuels have different DoU but similar oxygen mass fractions, so that the effect of the DoU on soot formation could be separately considered. The soot yield using biodiesels was measured to be lower than that of diesel fuel by a factor up to 7.5 in pool flames and by a factor of 4.0 in vapour flames. In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. The more saturated the fuel molecule is, the lower the soot yield. The soot reduction effect of the blending ratio with biodiesel was found to be non-linear, and different in pool and vapour flames. The morphology, size, and number density of soot particles were analysed using scanning electronic microscopy. The results suggest that the shape of primary soot particles is close to spherical and the structure of the point-contact aggregates is similar in all cases. Biofuels not only produce smaller particle sizes, but also fewer particles compared to diesel. Chemical kinetic simulations suggest that blending of biodiesels reduces the soot yield primarily by slowing the soot surface growth process, and secondarily by prohibiting the soot nucleation and inception. Both the nucleation (C3H3 and benzene) and growth species (C2H2) for soot formation take place more quickly and intensely for the combustion of more unsaturated fuels, which results in the correspondingly larger soot yield.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 2. Further insights into soot inception, growth and oxidation

Ammonia, as an alternative fuel, is attracting unprecedented attention due to the absence of CO2 emission during its combustion. Effects of ammonia co-firing on soot formation in ethylene laminar diffusion flames have been studied in the first part of this series of papers, addressing the soot volume fraction change and reaction kinetics of gaseous soot precursors. This paper further investigates the effect of ammonia on evolution behaviors of soot particle, namely soot inception, growth and oxidation in the ethylene co-flow diffusion flame. The soot volume fraction (SVF) and spatial distribution in flames without/with ammonia addition (5 and 20 vol%) were measured using laser-induced incandescence (LII) method. A newly constructed C2H4/NH3/PAHs kinetic model consisting of 163 components and 1018 reactions and the CoFlame codes dedicated to simulating laminar co-flow diffusion flame coupled with fixed sectional soot particle model were used to model the flames and soot inception, surface growth, and oxidation processes in them. Numerical results well captured the experimental observation on both peak SVF and flame height, and confirmed that ammonia inhibited the soot formation in the flame, showing a peak SVF decrease of 2–4 % per 1 % ammonia addition. The numerical results revealed that the ammonia addition inhibited all soot inception, surface growth via HACA (hydrogen-abstraction-acetylene-addition) and PAH condensation, and oxidation via OH/O2 processes. Most importantly, inhibition of HACA reactions induced by ammonia was identified as the main reason for reducing soot formation. Kinetic analysis revealed that adding ammonia created a new pathway to consume H radical while decreasing the rate of H radical-generating reactions, leading to a decrease in H radical and ultimately a reduction in the HACA reaction rate.

Continuous soot surface growth over carbene active site through spin density migration and spontaneous dehydrogenation: A DFT study

Surface growth process contributes the majority of soot mass, but the detailed mechanism remains vague, especially at post-flame zone where reactive radicals like H· are insufficient to continuously generate active sites. In this work, the reactivity of carbene active center on soot surface towards different categories of hydrocarbons was evaluated through DFT calculation, and the self-sustaining mechanisms for continuous further growth were proposed. Acetylene, aromatic molecules (benzene and naphthalene) and resonance stabilized radicals (RSRs, cyclopentadienyl and indenyl) were selected as representative reactants. Potential energy surfaces were obtained at the M06-2X/cc-pVTZ level of theory, and spin density distributions were presented to analyze the evolution of active sites. The surface growth process was divided into two stages, i.e., initial capture and further growth. During initial capture process, carbene active site exhibits high reactivity towards all kinds of representative reactants, and the energy barriers are lower than 10 kJ/mol except for benzene (26.17 kJ/mol). Meanwhile, the initial capture processes are exothermic and capturing RSRs shows the largest exothermicity. After the carbene site was occupied through initial capture process, the graphene flake still tends to retain the radical-like behavior and shows open-shell character, providing possibilities for further surface growth. During further growth process, spin density migration mechanism combined with spontaneous dehydrogenation step could achieve continuous surface capture without involving exogenous radicals. For acetylene, the unpaired electrons will preferentially locate at the terminal carbon of the growing chain to continuously refresh the active site. Extra dehydrogenation steps are required for the further growth involving aromatics and RSRs, otherwise the surface growth steps would be less exothermic and more reversible. Covalent bonding of these sp2-hybridized hydrocarbon species and the carbene site will create a vulnerable tertiary CH bond. Spontaneous dehydrogenation through the breakage of the tertiary CH bond can reproduce active site with localized spin density, after which the further growth process will be energetically and kinetically more feasible.

One-step in situ growth of a simple and efficient pore-sealing coating on micro-arc oxidized AZ31B magnesium alloy

Micro-arc oxidation (MAO) is an environmentally friendly and effective way to generate a dense and homogeneous coating with high corrosion resistance on magnesium alloys. However, the inherent porous structure of MAO coating can lead to the invasion of corrosive media to the substrate and thus reduce its corrosion resistance. Herein, we reported a novel one-step in situ growth of a pore-sealing coating on MAO-coated AZ31B alloy by hydrothermal treatment in Na3PO4 solution. The surface morphology, chemical composition and growth process of the pore-sealing coating were thoroughly studied by FESEM, XRD, FTIR and XPS. The micro-pores and micro-cracks of the MAO coating can be fully covered by the pore-sealing coating with spherical structure, which is mainly composed of Mg(OH)2 and Mg3(PO4)2. The corrosion resistance of the coatings prepared at different hydrothermal time was evaluated by electrochemical test and immersion test. Compared with the bare AZ31B substrate, the current density of the composite coating with hydrothermal treated for 18 h decreased by three orders of magnitude from 8.40 × 10−5 A/cm2 to 1.59 × 10−8 A/cm2, indicating a remarkable enhancement of the corrosion resistance. The MAO/Mg(OH)2/Mg3(PO4)2 composite coating shows high adhesive strength with the substrate, especially the outermost Mg3(PO4)2 layer with the inner Mg(OH)2 layer. In addition, the in situ growth and corrosion protection mechanisms of the pore-sealing composite coating are also proposed.

Investigation of conditional source-term estimation coupled with a semi-empirical model for soot predictions in two turbulent flames

The modelling of soot formation is investigated for two turbulent flames, at atmospheric and 3 atm pressure conditions. For the first time, a semi-empirical soot formulation that accounts for soot inception, coagulation, surface growth, and oxidation processes is coupled with the turbulent combustion model, Conditional Source-term Estimation (CSE) using Reynolds-Averaged Navier–Stokes equations. Detailed chemistry is included and an optically thin radiation model is considered. Non-adiabatic chemistry tabulations are created. Good agreement with the experiments is found for turbulent mixing and temperature fields in both flames, with some discrepancies believed to be due to the turbulence modelling approach. At 1 atm, the soot volume fractions are in reasonable agreement with the experiments, but typically smaller than the measurements with the centerline peak locating closer to the fuel exit. At 3 atm, good agreement between the numerical predictions and experimental data is achieved for the soot volume fraction within the experimental error. The centerline peak location is observed slightly farther downstream. Possible sources of discrepancies are examined and comparison with previously published numerical results is also undertaken. Differential diffusion and modified soot chemistry constants may bring further improvement. Without any particular tuning of soot chemistry, soot modelling within CSE is shown to be a promising approach.

Effect of ion-exchangeable calcium on carbonaceous particulate matter formation during coal pyrolysis

This work investigated the effect of ion-exchangeable calcium on the mass yield, size distribution and structural properties of carbonaceous particulate matter from coal pyrolysis. The raw, acid washed and calcium-exchanged coal samples were pyrolyzed in a flat-flame burner at 1600 K. The particulate matter was collected with a three-stage dilution sampler. Then, the particle size distributions were measured by a low pressure impactor (DLPI+) and a scan mobility particle sizer (SMPS). The results show that, with the addition of ion-exchangeable calcium, the soot yield reduces, and the size growth of soot particles is inhibited. Transmission electron microscopy (TEM) analysis shows that ion-exchangeable calcium diminishes the sizes of disordered cores in soot primary particles, accelerates the graphitization of soot and makes the soot aggregates more compact. The chemical structures derived from Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) indicate that the graphitization degree of the soot is first enhanced and then inhibited with increasing calcium loading amount. Additionally, the inorganic species in the size-segregated soot particles were characterized. The fractions of calcium and sodium in soot show opposite trend with particle size. Unlike Na, which nucleates before soot inception, Ca tends to participate in both the soot nucleation and surface growth processes, and is enriched at larger particle size.