Soot Yield Research Articles

Evolution in size and structural order for incipient soot formed at flame temperatures greater than 2100 K

Soot formed in flames hotter than conventional combustion applications is expected to undergo unique formation processes and develop a carbon structure distinct from typical soot. A complementary experimental and modeling approach is reported here to assess flame temperature and equivalence ratio effects for soot formed in the higher-temperature regime (1950 K < Tf < 2250 K). Computations using three separate combustion chemistry models show that predictions of polycyclic aromatic hydrocarbon (PAH) concentration profiles for the higher-temperature flames are more sensitive to the choice in mechanism rather than specific flame conditions. As for material properties, the Raman signatures transition from a typical soot spectrum to features observed in disordered sp2 carbon materials. The defect distance extracted from the Raman bands nearly doubles from values typically reported for soot as the flame temperature exceeds 2200 K. Higher concentrations of gas-phase precursors may facilitate development of an ordered carbon structure as indicated by the relatively high defect distance observed for the highest equivalence ratio series. Particle size distributions measured by mobility sizing show size and yield of soot decreases with increasing flame temperature and the bimodal distribution falls within the ultra-fine range for all flame conditions. This is especially promising if the significant transformation in carbon structure inferred from the evolution in Raman spectra enables development of functional high-surface area sp2 carbon materials. Namely, the current observations indicate that the flame-formed carbon structure evolves towards high-defect sp2 carbon with size and carbon structure that can be tuned to some extent.

Soot-based Global Pathway Analysis: Soot formation and evolution at elevated pressures in co-flow diffusion flames

One of the major concerns in high pressure combustion is its high soot yield. An exact and comprehensive mechanism behind this phenomenon, from a chemical kinetics perspective, is still elusive. In this study, a series of pressurized (1–16 atm) co-flow ethylene diffusion sooting flames are simulated with detailed finite-rate chemistry and molecular transport. The experimental maximum soot volume fraction and its scaling law with pressure are well reproduced by the simulations. To extract kinetic information from the complex sooting reacting system, a Soot-based Global Pathway Analysis (SGPA) method is developed to identify the dominant Global Pathways (GPs) from fuel to soot by considering carbon element flux from gaseous species to soot. Using SGPA, the dominance and sensitivity of soot chemical pathways at elevated pressures are revealed. It is found that increasing pressure shifts the first ring Polycyclic Aromatic Hydrocarbon (PAH) formation from C3H3 recombination to reactions involving C2H2. At 1 atm, the production of C2H2 for surface growth is purely controlled by the H-abstraction of C2H4 and C2H3. In contrast, at elevated pressures, the production of C2H2 for surface growth is also influenced by many other reactions including some third body reactions. The SGPA method reveals that the mismatch of predicted PAH with the experimental data at 12 atm is majorly caused by the rate coefficient uncertainty of the reaction C2H2 + A1CH2 = C9H8 + H. Based on the analysis by SGPA, the mechanism reduction based on Directed Relation Graph with Error Propagation (DRGEP) with A2 and C2H2 as the target species deleted significant species such as C9H8, C9H7, incurring inaccurate soot field prediction. It is also found that the combined dominance of GPs with heavier PAH species (A4-A7) is even greater than the most dominant GP at the flame wing regions, indicating that heavier PAH species play critical roles for soot nucleation and condensation, especially at the flame wing regions.

Sooting propensity dependence on pressure of ethylbenzene, p-xylene, o-xylene and n-octane in laminar diffusion flames

Alkylbenzenes constitute a significant portion of middle-distillate transportation fuels, and they are judged as the key contributors to the extent of soot production and consequent exhaust particulate emissions in combustion engines. Because of the uncertainties involved in engine experiments due to boundary condition variation among test platforms and the difficulties involved in controlling engine variables independently, benchmark information on pressure dependence of sooting propensities of alkylbenzenes is essential to understand their influence on particulate emissions and provide a more consistent interpretation of engine data. One of the approaches to acquire such benchmark data is to investigate laminar diffusion flames doped with alkylbenzenes under tractable conditions at elevated pressures. Here, we study the influence of doping a methane laminar diffusion flame with alkylbenzenes having eight carbon atoms, namely ethylbenzene, p-xylene, and o-xylene and compare their sooting characteristics to that of n-octane at pressures up to 10 bar. Liquid hydrocarbons added to methane replaced 3% of the carbon in the base fuel keeping the carbon mass flow in the fuel stream fixed, at all pressures considered, to have tractable measurements. The flames of base fuel methane and those doped with liquid C8 hydrocarbons were stabilized on a laminar co-flow diffusion burner positioned inside a combustion chamber capable of elevated pressures and has suitable optical access. Radially resolved temperatures and soot volume fractions were evaluated from spectral emission of soot radiation from the flames. Soot yields, inferred from the radial distributions of soot volume fractions, of ethylbenzene, p-xylene, and o-xylene doped flames were found to be a factor of about 2.6–3.2 higher than that of n-octane at 2 bar; however, this difference gradually disappeared with increasing pressure and soot yields were almost the same above 6 bar. It appears that pressure dependence of sooting propensities of ethylbenzene, p-xylene, and o-xylene is weaker than that of n-octane within the pressure range of 2 to 10 bar.

Impact of torrefaction on entrained-flow gasification of pine sawdust: An experimental investigation

Entrained-flow gasification of raw and torrefied pine sawdust (PS) was conducted at 1300 °C and different equivalence ratios (ERs). The effects of torrefaction conditions on gasification performance as well as the relationship between soot yield and the volatile content of feedstocks were investigated. An additional burning procedure after gasification was used to quantify the carbon deposits (CD) inside the reactor tube, which completed the mass balance calculation and thus enabled reliable conclusions to be made. The results of the entrained-flow gasification tests indicated that PS torrefied at 280 °C exhibited a better gasification performance than raw PS and PS torrefied at other temperatures. Moreover, it was found that torrefaction pretreatment also reduced soot yield. These results indicated that the total yield of soot and CD is closely related to the content of volatile matter in feedstocks.

Effects of CO/H2/N2 addition on the soot morphology and nanostructure in laminar co-flow ethylene diffusion flame

This study investigated the effect of CO/H2/N2 addition on the morphological evolutions and nanostructures of soot generated from a laminar co-flow ethylene diffusion flame through utilizing the method of thermophoretic sampling and transmission electron microscopy (TEM). The additions were introduced into the inner tube along with the fuel stream, and the volume fractions of the additions are 10%, 20% and 30%. Specifically, the parameters of soot production, primary particle diameter (Dp), fractal dimension (Df), fractal pre-factor (kg) and the nanostructures were determined and analyzed. A reduction in soot production was observed with increasing proportion of the additives, demonstrating the addition of these diluent gases could inhibit the soot formation. Among the three additives, N2 is the most effective in reducing soot yield. The size analysis of soot particles and aggregates indicated the addition of CO or N2 could diminish the primary particle diameter and fractal parameters. The addition of H2 could reduce the primary particle diameter at the bottom flame while yields greater primary particles at 40 mm HAB. This result demonstrated that H2 addition may weaken the oxidation occurring on soot particles. High resolution transmission electron microscopy (HRTEM) analysis was also developed and the results showed that the addition of CO or H2 results in greater fringe length, smaller fringe tortuosity and inter-fringe spacing for soot particles, demonstrating the more orderly and compact lattice structure. In particular, the nanostructure parameters of soot particles in H2 enriched flame vary more significantly than those in CO enriched flame, demonstrating the soot nanostructure is more graphitized in H2 enriched flame. Different from the case of CO or H2 addition, the lattice characteristics for soot in the flames with N2 addition present inverse variation tendency.

Formation of nascent soot during very fuel-rich oxidation of ethylene at low temperatures

Soot formation during very fuel-rich oxidation of ethylene (with equivalence ratio, φ, ranging from 3 to infinity) at relatively low temperatures (1473–1673 K) was studied in a flow reactor, in which temperature was well controlled through an external heater, and φ of the reactants was adjusted by adding different amounts of O2 (0–6000 ppm) at the inlet of the reactor. Detailed sooting properties including soot volume fraction, number density, particle size distribution (PSD), and morphology were examined. The results show that at a temperature of 1673 K, the total soot yield increases monotonously with φ. However, at 1473 and 1573 K, the soot volume fraction increases first and then decreases as φ increases from 3 to infinity, which means a small amount of oxygen can promote soot formation. The PSD and number density results indicate that this promotion effect is more prominent at the nucleation stage and weakens as soot grows at longer residence times. In addition, soot morphology is affected mostly by temperature rather than φ, and the decrease of temperature leads to larger primary particles and less fractal aggregates. Kinetic modeling results indicate that the concentrations of PAHs and soot increase because a small amount of O2 addition can enhance the C3 + C3 pathways through the generation of reactive oxygenated species.

Soot Formation during the Decomposition of Chlorine-Containing Hydrocarbons with an AC Plasma Torch

The process of decomposition of organochlorine compounds by a three-phase ac plasma torch with vortex arc stabilization is studied. The plasma torch had two zones for introducing the plasma-forming media: the near-electrode and arc-burning zones. The near-electrode zone was fed with a shielding gas, and the arc-burning was fed with water vapor, methane, and vapors of organochlorine compounds. The resulting products were analyzed using mass spectrometry, XRD, scanning electron microscopy, and IR spectroscopy. In an experiment with chlorobenzene and carbon tetrachloride, in the gaseous and liquid fractions, only one chlorine-containing compound, HCl, was detected. However, in the experiment with chlorobenzene, the soot yield was 0.98 wt % of the raw material and the chlorine content in the soot was 1.61%. IR spectral data confirm the presence of the Cl–O and Cl–С bonds.

Methodology for DB construction of input parameters in FDS-based prediction models of smoke detector

The input parameters of the Heskestad and Cleary models—which are numerical models included in fire dynamics simulator (FDS)—are measured, and a sensitivity analysis is conducted on the effects of individual and common input parameters of the numerical models on the detection time. The input parameters are applied to the FDS, and the results predicted the activation time of the detector within +5 s. Compared to the individual input parameters, the obscuration per meter (OPM), which is a common input parameter, significantly affected the detection time. Finally, additional input parameters that correspond to combustion properties, such as the soot yield and mass specific extinction coefficient, are discovered to have a greater impact on the detection time than the input parameters in the detector’s numerical models. Considering various smoke detectors and combustibles, this study’s findings will contribute to the efficient use of resources to build a database of input parameters.

Probing the low-temperature chemistry of methyl hexanoate: Insights from oxygenate intermediates

Understanding the combustion of methyl esters is crucial to elucidate kinetic pathways and predict combustion parameters, soot yields, and fuel performance of biodiesel, however most kinetic studies of methyl esters have focused on smaller, surrogate model esters. Methyl hexanoate is a larger methyl ester approaching the chain length of methyl esters found in biodiesel and has not received as much research attention as other smaller esters. The purpose of this work is to present the first atmospheric pressure combustion data of methyl hexanoate, CH3CH2CH2CH2CH2COOCH3. Mixtures of 2% methyl hexanoate in O2 and N2 are studied using a plug flow reactor at atmospheric pressure, wall temperatures from 573 to 973 K, residence times from roughly 1-2 s., and fuel equivalence ratios of 1, 1.5, and 2. Exhaust gases are analyzed by a gas chromatograph-mass spectrometer system and species mole fractions are presented. The literature model shows satisfactory agreement with the experimental species profiles and improvements for future mechanistic studies are suggested. In particular, this work proposes new unimolecular decomposition pathways of methyl hexanoate to form methanol or methyl acetate. Furthermore, the experiment detected three unsaturated esters that are direct products of the low temperature oxidation chemistry and it provides more insight into branching ratios for the formation of methyl hexanoate radicals and for the decomposition of hydroperoxyalkyl radicals.

Soot properties in ethylene inverse diffusion flames blended with different carbon chain length alcohols

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.

Seashell waste-derived materials for secondary catalytic tar reduction in municipal solid waste gasification

Catalytic tar removal from producer gas is critical for the economic feasibility of Municipal Solid Waste (MSW) gasification in the waste-to-energy(WtE) approach. Nickel- and noble-metal catalysts have the highest tar cracking activities, but they increase costs, use scarce materials, and generate dangerous byproducts. To overcome these drawbacks, naturally occurring materials should be used for tar cracking. In this paper, two nanomaterials, synthesized from oyster and mussel waste shells respectively, are used to clean syngas from MSW in a secondary tar cracking unit. We observed that they reform class 1 tar (heavy tars that condense at high temperatures at very low concentrations) into class 3 tar (light hydrocarbons that are not important in condensation) and benzene. Although both catalysts’ composition and textural properties were identical, crystallite size and especially specific surface area variation was enough to generate a change in product selectivity. A larger crystallite size and SSA shows a soot yield reduction of 95% with respect to the non-catalytic case, simultaneously increasing the H2/CO at 1000 °C.

Modelling soot formation during biomass gasification

Biomass gasification offers a significant potential to close the loop of agriculture and many other activities that produce biomass residues. However, the presence of tars and impurities, such as soot, in the gasification products is still a major bottleneck causing operational problems, particularly blocking or fouling of equipment and low syngas quality. Numerical modelling is a very attractive solution to enhance the understanding of the physical and chemical phenomena occurring in gasifiers and thereby to overcome these barriers. In addition, understanding the biomass gasification fundamentals in laboratory-scale devices, such as drop tube furnaces (DTF), is crucial to evaluate its relevance for industrial applications. The main objective of this study is to develop a model able to predict soot formation during the gasification of biomass in a DTF. The model developed is based on a kinetic-diffusion approach and was implemented in Python using the Cantera reaction kinetics library. The model is validated against experimental data that assessed the effect of the temperature and steam to biomass (S/B) ratio on char and soot yields and syngas composition during the gasification of wheat straw in the DTF. Overall, the model developed captures with a rather good accuracy the evolution of the char yields, but with a less accuracy the evolution of the soot yields and syngas composition with the temperature and S/B ratio.

Unsteady RANS Numerical Study on Soot Productivity in a Turbulent Jet Flames for various Reynolds Number Configurations

This paper presents a numerical simulation on turbulent non-premixed C 2 H 4 /H 2 /N 2 – air jet flames for three different Reynolds number (Re) configurations to compute soot volume fraction (SVF). In this work, a chemical equilibrium diffusion combustion flamelet model is used along with the second-moment closure (SMC) transient linear pressure strain Reynolds Stress Method (RSM). A two-equation semi-empirical Brookes-Moss Hall (BMH) model as a soot source term chooses C 6 H 6 as soot precursor, and hydroxyl (OH) as the oxidation radical for soot oxidation is considered for this study. A probability density function (PDF) based temperature, mixture fraction (Z), and P-1 radiation model are invoked along with the combustion turbulent soot interaction. All the above-chosen models are solved for three different mixing configurations given by Mahmoud and co-workers (2018) in their experimental study. The numerical results obtained with the precursor (C 6 H 6 ) in the soot model, along with the turbulent combustion chemistry interaction, are compared with the experimental data at various normalized radial and axial flame positions and reasonable agreement for SVF is observed. It is also seen that there is a strong linear dependency of total soot yield with fuel flow rate, exit jet diameter, and is almost independent on Re. Further, it is seen that soot productivity decreases with an increase in the Froude number (Fr), which agrees with experimental findings.

Self-consistent estimates of emission factors of carboncontaining pollutants from a typical gas flare

This study proposes an approach for estimating the emission of soot, carbon monoxide (CO) and carbondioxide (CO ) from a typical gas flare. The estimations depend on the quantity and varying composition of the 2 natural gas, flame dynamics (represented by the fire Froude number, Fr ) and the equivalence ratio, f, of the fuel- f air mixture. Soot emission estimates are presented as a function of fire Froude number for gases used in labbased test in order to validate the scheme and for two real-world fuel gas compositions. The mass-weighted carbon-hydrogen ratio (C:H) of the fuel gas compositions are 0.25 and 0.29 which are two extreme cases in terms of density. The soot yield of the lab-based test case was scaled up to estimate the soot yield of a full scale flare using the Richardson number as the scaling parameter. When all other variables are held constant at values characteristics of real-world flares, a difference of 16 % in the fuel-gas density, as indicated by the carbonhydrogen ratio, results in an increase of the emission factors (EF) of soot, CO and CO by factors of ~3, ~1.4 2 3 and ~1.7, measured in g/m , respectively. For both fuel gas compositions, the ratio of EF to EF at the fuel- soot CO lean region f &lt; 1) is higher. The ratio lies in the range 0.031 – 0.13 and 0.0012 – 0.0055 for the fuel-lean (f &lt; 1) and fuel-rich (f &gt; 1) regions, respectively. The approach proposed and results obtained may be adopted to generate emissions inventories of emission species associated with gas flaring on regional and global scales.&#x0D; Keywords: gas flaring; soot; natural gas; emission factor; black carbon; equivalence ratio

Effects of benzene, cyclo-hexane and n-hexane addition to methane on soot yields in high-pressure laminar diffusion flames

Effects of doping high pressure methane diffusion flames with benzene, cyclo-hexane and n-hexane were investigated to assess the sooting propensity of three hydrocarbons with six carbons at elevated pressures. Amount of liquid hydrocarbons added to methane constituted 7.5% of the total carbon content of the fuel stream. The pressure range investigated extended up to 10 bar and the experiments were carried out in a high pressure combustion chamber capable of establishing stable laminar diffusion flames with various fuels at elevated pressures and was used in similar experiments previously. Temperatures and soot volume fractions were measured using the spectral soot emission technique capturing spectrally-resolved line-of-sight intensities which were subsequently inverted using an Abel type algorithm to obtain radial distributions assuming that the flames are axisymmetric. The total mass carbon flow of the fuel stream was kept constant at 0.524 mg/s in neat methane, benzene-doped methane, cyclo-hexane-doped methane, and n-hexane-doped methane flames to have tractable measurements at all pressures. Measured maximum soot volume fractions and evaluated maximum soot yields showed that benzene-doped methane flame had the higher values than cyclo-hexane doped methane flames which in turn had higher values than n-hexane doped methane flames at all pressures. Sooting propensity dependence of the three hydrocarbons on pressure can be ranked as, in descending order, n-hexane, cyclo-hexane, and benzene; however, the difference between pressure dependencies of n-hexane and cyclo-hexane was within the measurement error margins. Ratio of soot yields of benzene to n-hexane doped flames changed from about 2 at 2 bar to 1.2 at 10 bar; the ratio of benzene to cyclo-hexane doped flames showed similar trends.

Mechanistic insights into effect of feeding rate on soot formation during rapid pyrolysis of biomass model components in a drop-tube furnace at high temperature

This paper reports the significant effect of feeding rate on soot formation during rapid pyrolysis of water-washed cellulose and acid-washed lignin (denoted as W-cellulose and A-lignin, respectively) in a drop-tube furnace at 1300 °C and a residence time of ∼0.75 s in argon. Soot produced during W-cellulose pyrolysis has a unimodal distribution, with only a fine mode that its modal diameter increases from 0.043 to 0.246 µm as the feeding rate increases from 40 to 280 mg/min. However, at feeding rates of 12–200 mg/min, soot produced from A-lignin pyrolysis has a bimodal distribution with two fine modes located at 0.077 µm and 0.246 µm, respectively. As the A-lignin feeding rate further increases from 200 to 280 mg/min, the fine mode at 0.077 µm disappears and the particle size distribution of soot becomes unimodal with only a fine mode with diameter of 0.246 µm. Increasing feeding rate increases the yield and particle size of total soot but decreases the yield of non-mature soot for W-cellulose at feeding rates < 280 mg/min and A-lignin at feeding rates < 40 mg/min. The results suggest that a high feeding rate produces high concentration of soot precursors and enhances the collisions for increased formation of incipient soot to large mature soot. As the feeding rate further increases to be more than 40 mg/min, the soot yield from A-lignin pyrolysis levels off, the particle sizes of overall soot decease and the yields of non-mature soot increase. Under such conditions, further results show that soot growth is limited by the availability of soot-forming carbonaceous materials (including small gas molecules e.g. C2H2 and polycyclic aromatic hydrocarbons).

Pressure dependence of sooting characteristics of m-xylene and n-octane doped laminar methane diffusion flames from 2 to 10 bar

The work reported here contributes to a wider effort that focuses on the issue of whether single ring aromatics and their same carbon number alkane counterparts have the same pressure dependence of their sooting characteristics. Influence of doping methane base fuel with either m-xylene or n-octane in laminar high-pressure co-flow diffusion flames was studied from 2 to 10 bar pressure in current work. Two liquid hydrocarbons added to the base fuel methane contributed 3% of the total carbon mass flow in the fuel stream, which was kept fixed at 0.524 mg/s at all pressures considered, to have tractable measurements. The methane flames doped with either m-xylene or n-octane were stabilized on a co-flow diffusion burner installed inside of a high pressure combustion chamber with optical access. Temperatures and soot volume fractions were inferred from spectrally-resolved measurements of soot radiation. Line of sight soot emission intensities were inverted at each measurement plane along the flame axis to obtain radially-resolved temperature and soot volume fraction. Soot yields, calculated from soot volume fractions, of m-xylene doped flames were found to be higher than those of n-octane doped flames in the pressure range of 2–10 bar; however, the ratio of soot yields of m-xylene to n-octane decreased with pressure, from 2.9 at 2 bar to 1.2 at 10 bar. This finding implies that m-xylene and n-octane, both with eight carbons in their molecular structure, do not have the same sooting propensity with increasing pressure; m-xylene’s pressure dependence is weaker than that of n-octane within the pressure range of 2–10 bar.

Effect of different forms of Na and temperature on soot formation during lignite pyrolysis

Current research on coal-derived soot lacks understanding of the role of Na, and also lacks of comparative studies between physically adsorbed Na (ANa) and ion-exchangeable Na (INa). In this study, different analytical methods were used to study the yield, particle size distribution and surface/bulk chemical characteristics of the soot generated by the pyrolysis of acid-washed Yimin lignite loaded with blank, ANa and INa in a drop-tube reactor. The results showed that: small-molecular aromatic compounds and aliphatic substances in lignite volatiles began to form soot around 1175 °C; and the oxygen-containing substances had the strongest inhibiting effect on soot formation near 1250 °C. As a result, “inflexion points” were caused in curves of soot yield with the changing temperature. Different forms of Na had different effects on soot generation characteristics. Some INa organically combine with char or volatiles. INa on char greatly reduced primary tar release, and INa in volatiles promoted the combination of volatiles with oxygen-containing and sulfur-containing substances. Without the chemical connection with coal matrix, ANa was partially gasified during pyrolysis and had no obvious effects on primary tar release. Vaporized Na (both ANa and INa) had multiple effects on soot formation. At a low temperature, Na+ promoted the aggregation of small-molecular aromatic compounds and aliphatic substances, contributing to the soot formation. However, when the temperature was higher than 1100 °C, vaporized Na usually presented an inhibiting effect on soot formation. Generally, vaporized Na can improve volatile reaction activity and reduce the temperature of two “inflection points”.

Effect of dividing single flare tip into multiple tips on soot reduction

Several methods are applied for soot reduction in industrial flares. However, they usually involve other issues such as flame safety and other pollutant formations; therefore, they should be handled carefully. Splitting flare tip into multiple branches is an idea that is thought to reduce soot formation by increasing the contact surface and providing better mixing. In this study, changing a single tip of an industrial flare into multiple tips was investigated by Computational Fluid Dynamic software Ansys Fluent 18. Reynolds Average Navier–Stokes approach was used to model the fluid flow, and its turbulence was modeled by realizable k–e model. The steady laminar flamelet model was chosen as the combustion model. Soot formation was modeled using Moss–Brookes approach, and validated by comparing the simulation results with available experimental data. Effects of tip diameter, number of branches, and distance between branches on soot and NOx yields, and flame stability were studied. According to the results, increasing the number of branches having larger diameters by taking flame stability limitations into consideration, and choosing optimum distances between branches caused significant soot reduction without noticeable changes in the NOx formation.

Effect of phosphorus on soot formation and flame retardancy in fires

The mechanism by which phosphorus imparts flame retardancy to a burning polymeric solid remains controversial because of the complex coupling of gas and condensed phase reactions. To address that, an organophosphorous compound was mixed into epoxy at different concentrations and burned under diffusion flame conditions. The yield of carbon monoxide and soot was found to increase in the presence of phosphorus, significantly reducing the combustion efficiency. The reduction in heat release was attributed to a heterogeneous mechanism that promotes soot formation. Phosphorus does not exhibit a noticeable effect on soot morphology. Element analysis reveals that 75% of phosphorus in the virgin sample is incorporated into soot particles. More aromatic structures were identified under the influence of phosphorus. Soot surface reactivity was found to decrease as phosphorus loading increases. The main flame retardancy mechanism of phosphorus is through promoting soot formation in the gas phase. It is speculated that it increases direct condensation of aromatic structures in the soot formation path and decreases the number of the active sites on the particle surface for soot oxidation. The increased soot yield cools the flame by radiative heat loss, further hindering the dehydrogenation process that converts nascent soot to mature soot.