Soot Yield Research Articles

Numerical investigation into the decoupling effects of hydrogen blending on flame structure and soot formation in a laminar ethylene diffusion flame

Numerical calculations were conducted to explore the various effects of hydrogen blending on flame properties and soot behaviors in an ethylene coflow diffusion flame, based on a fully step-by-step decoupling method, by introducing several virtual species into the gas-phase mechanism. Results show that the concentration of OH increases under the chemical effect of hydrogen due to an enhanced rate of H2 + O ↔ OH + H. The soot yield, primary number density, and average primary number per aggregate decrease under dilution effect while these increase under chemical effect. The enhancements of hydrogen-abstraction-carbon-addition (HACA) rates and polycyclic aromatic hydrocarbon (PAH) condensation rates are responsible for soot mass addition under chemical effect. Both the oxidation rates by O2 and OH are delayed under the chemical effect because of lower concentrations of O2 and OH in the sooting zone. The overall effect of higher surface growth rates and delayed oxidation rates results in an increased soot volume fraction (SVF).

Roles of Ion-Exchangeable Sodium in the Conversion Process of Tar to Soot during Rapid Pyrolysis of Two Brown Coals in a Drop-Tube Reactor.

In this work, two series of brown coals (including acid-washed coal and ion-exchangeable Na-loaded coal) were pyrolyzed in a drop-tube reactor. The experimental results revealed that soot and tar yields of Na-loaded coals were significantly lower than that of acid-washed coals. Gasified Na can reduce the formation of big soot agglomerates. During coal primary pyrolysis, ion-exchangeable Na can reduce the amount and aromaticity of primary tar. Na released with volatiles can catalyze the cracking of aliphatic and aromatic compounds, inhibit the polymerization between aromatic rings, and promote the combination of soot/tar with oxygen-containing substances, resulting in the decrease of graphite crystallite size and the increase of amorphous carbon content. Na can also reduce the organization degree of soot by forming intercalation compounds.

Effects of iron on the soot formation during coal pyrolysis

The addition of metal to fuel can effectively control the soot emission. However, there are few studies on the effect of metallic additive to coal on soot emissions. In this study, the effect of iron on the characteristics of coal-derived soot was analyzed. Soot particles were obtained at different temperatures from coal pyrolysis in a drop tube furnace without and with the addition of iron nitrate (Fe(NO3)3) into the coal, respectively. The microstructure and functional group information of soot derived from coal were obtained through Raman spectroscopy, infrared spectroscopy and XPS. The results showed that the yield of soot derived from coal pyrolysis with iron addition was reduced by 48–68% than that derived from coal pyrolysis without iron addition at different temperature. With the iron addition, the number of holes in the graphite layers in soot produced by coal pyrolysis was increased, while the valued of sp2/sp3 and the length of graphite layer were decreased. At the same time, the C–O and COO-functional groups in the soot increased, and the carbonyl content decreased. These phenomena indicated that the addition of iron can significantly inhibit the soot formation in the coal pyrolysis process, increase the defects of graphite layers in soot, inhibit the carbonyl formation, and promote the formation of C–O during the process of soot formation.

Experimental study on combustion and yield characteristics of dimethyl carbonate/n-heptane blends in the cone calorimeter

A detail investigation of the combustion behavior of a representative ester-alkane blended fuel was conducted with the aim to get better knowledge of the fire hazard of the blended fuels. Experiments have been carried out in a cone calorimeter with ISO 5660 standard procedures. The blended fuel is burned in a fuel pan with 80 mm in diameter and 20 mm in lip, and the initial fuel volume is 20 mL. The hazard parameters such as heat release rate, effective combustion heat, combustion yield, and combustion efficiency were studied using a cone calorimeter. The actual combustion reaction equation of dimethyl carbonate (DMC)/n-heptane blends was calculated, and the thermal chemistry method based on the stoichiometry of reaction equation is adopted to evaluate the oxygen consumption method for the DMC/n-heptane blends. A formula is put forward to describe the burning rate of respective component in the blends during burning process, and the burning rate of DMC in the blends is steady during the test. The CO yield, carbon conversion ratio, and energy conversion efficiency data of the blends are calculated and compared, the result shows that the carbon conversion is close to the energy conversion, and this means that soot yield is the major contributor to incomplete combustion of blends.

Impact of Sodium on the Formation Mechanism and Physicochemical Properties of Coal-Derived Soot

Ion-exchanged Na (INa) and adsorbed Na (ANa) concentrations have significant impact on the physicochemical properties of coal-derived aerosol, and product yields of soot and tar. INa effectively re...

Effects of Na and Fe on the formation of coal-derived soot in a two-stage flat-flame burner

Coal-combustion-generated soot can be a source of particulate emissions and significantly affects the temperature profile inside the practical boiler. In this work, we experimentally investigated the effects of sodium (Na) and iron (Fe) on soot formation in pulverized coal combustion. Two kinds of coal samples with similar properties were used, with one rich in Na while the other rich in Fe. The water washed sample for each coal was ultilized to further explore the mineral effects. A novel two-stage flat-flame burner was designed to flexibly simulate the reducing-to-oxidizing atmosphere which a real coal particle stream undergoes in the boiler. The ex-situ sampling indicates the existence of mineral species, including Na and Fe, doped in the mature soot agglomerates. In-situ Laser-Induced Incandescence (LII) diagnostics, incorporated with on-line measurements of the particle size distribution (PSD), gives quantitative concentrations of primary soot particles. The experiment results show a reduced soot yield after water washing for both coal samples, elucidating a promoting effect of Na on soot formation in pulverized coal combustion. A similar effect is inferred for Fe, since the water washed Fe-rich coal produces more soot than the water washed Na-rich coal even though their residual Na contents in ash are close. It is postulated that both Na and Fe form nano-sized particles at the early stage of coal combustion and serve as effective nuclei for the soot precursor, the tar. Besides, the catalytic effect of Na on soot oxidation is confirmed because of the significant soot reduction in the combustion of Na-rich coal when the oxygen mole fraction in the inner region increases.

Numerical simulation of the variation of obscuration ratio at the fire early phase with various soot yield rate

In this paper, the Fire Dynamics Simulator (FDS) is used to simulate the fire detection process in a general chamber under various fire scenario. Firstly, the grid independence of the fire simulation has been analyzed. Then, mounts of the smoke detectors at various locations are adopted in the fire scenarios to investigate the fire detection process, meanwhile, the influence of the soot yield rate is also deeply discussed. According to the experimental data, an appropriate soot yield rate is raised, which is in good agreement with the prior value. The numerical simulation result indicates that in the axis above the fire source the obscuration ratio is decrease with the height away from the fire source, however the obscuration ratio is increase abruptly. What's more, on the 3.0 m height above the fire source, the obscuration ratio is decrease along the distance to the fire source. In summary, the research provides basic simulation parameter for the fire detect process simulation, and it is valuable for the selecting and setting of fire alarm system.

Impacts of Organic Structures and Inherent Minerals of Coal on Soot Formation during Pyrolysis

The pyrolysis of four pairs of raw and acid-washed coals under N2 atmosphere was carried out in a drop tube reactor at 1250 °C. The results show that both organic structures and metal elements have an important influence on the formation of soot. The total area of aromatic and aliphatic hydrogen absorption bands is positively correlated with soot yield. Aromatic compounds have a greater contribution to soot and tar formation. The absorption band area of oxygen structures in coal FTIR spectra is negatively correlated with the soot conversion rate of tar. During pyrolysis, metal substances in coal can catalyze the dehydrogenation and deoxygenation of tar, reduce the content and stability of the aliphatic compound, and catalyze aromatic ring rupturing. More importantly, gasified metals can inhibit the polymerization reaction of aromatic compounds.

Numerical simulations of yield-based sooting tendencies of aromatic fuels using ReaxFF molecular dynamics

We present the first ReaxFF Molecular Dynamics (MD) simulations to quantitatively predict the sooting tendencies of various fuels. The specific sooting tendency metric used in this work is the Yield Sooting Index (YSI), which quantifies the effects of fuel molecular structure on soot yield. YSI has been experimentally measured and numerically simulated using computational fluid dynamics for a large range of fuels, but there is no existing reactive MD framework for YSI simulations. To adopt the experimental YSI concept, a multi-stage simulation procedure is designed using ReaxFF. As a proof-of-concept, toluene and phenol are selected as test fuels, since both have relatively well-understood reaction pathways. The ReaxFF YSI simulations are shown to capture key reaction events for both fuels selected that are consistent with existing chemical kinetic understanding. Toluene is shown to mostly retain its original aromatic ring structure and directly grow to larger aromatic compounds with multiple rings. On the other hand, the aromatic growth process from phenol is accompanied by carbon-loss reactions with CO release. In addition, a quantitative YSI formulation is also derived in ReaxFF and the ReaxFF-predicted YSI values are compared with measurement data and reasonably good agreement is achieved. The results reported in this work demonstrates that the ReaxFF-based framework can potentially be used to quantitatively predict the relative sooting tendencies, especially for fuels with unknown or poorly-known chemistry, to understand their sooting properties and in search for soot relevant reaction pathways from these fuels.

On the role of potassium as a tar and soot inhibitor in biomass gasification

The work investigates in a drop tube furnace the effect of potassium on carbon conversion for three different types of fuels: an ash lean stemwood, a calcium-rich bark and a silicon-rich straw. The study focuses on an optimal method for impregnating the biomass with potassium. The experiments are conducted for 3 different impregnation methods; wet impregnation, spray impregnation, and dry mixing to investigate different levels of contact between the fuel and the potassium. Potassium is found to catalyse both homogenous and heterogeneous reactions. All the impregnation methods showed a significant effect of potassium on heterogeneous reactions (char conversion). The fact that dry mixing of potassium in the biomass shows an effect reveals the existence of a gas-induced mechanism that supply and distributes potassium on the char particles. Concerning the effect of potassium on homogenous reactions, it is found that potassium in the gas phase leads to much lower yields of C2 hydrocarbons, heavy tars and soot. The results indicate that potassium reduces the likelihood of light aromatic to progress toward heavier polyaromatic hydrocarbons clusters, thereby inhibiting the formation of soot-like material. A moderate interaction between the added potassium and the inherent ash forming elements is also observed: Potassium has a smaller effect when the fuel is naturally rich in silicon. The combined results are of interest for the design of a gasification process that incorporates recirculation of naturally occurring potassium to improve entrained flow gasification of biomass.

Coupling Effects of Chemical Kinetics and Start of Injection Strategies on Soot Emission in a Light-Duty Direct-Injection Diesel Engine

In this reported work, the numerical simulation of 3-D in-cylinder combustion and soot formation was carried out with ANSYS FLUENT 18.1 software. Here, two different combustion models (namely MCDS1 and MCDS2) were integrated to demonstrate the effects of aromatic components on soot formation. While both the surrogate models include straight-and branched-alkanes as its constituent hydrocarbon species, MCDS2 contains additional aromatic components to better represent the diesel fuel properties. Here, the start of injection timing was varied, i.e. at −6°, −2°, and +2° after the top dead centre, to study its effect on the soot emission performance after appropriate model validations. Clear reduction in soot yield was observed with advanced injection timing of -6° after top dead centre, in which the soot oxidation rate was found to be higher with earlier start of injection timing. This may be attributed to the higher in-cylinder temperature which enhances the soot oxidation rate. As compared to MCDS2, it was observed that the combustion of MCDS1 has consistently led to higher soot yield across all the cases. This can be attributed to its higher content of straight-and branched-alkanes.

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O2+N2) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.

The Effect of Hydrolysis on Properties of Soot and Tar During the Pyrolysis of Sewage Sludge

In this study, we evaluate the effect of hydrolysis treatment on the properties of tar and soot produced during the pyrolysis of sewage sludge at different operating temperatures (500–800 °C). The pyrolysis experiments were performed in a two-stage tubular fixed-bed reactor at atmospheric pressure. Raw sludge and hydrolysis-treated sludge (at pressure of 0.4 MPa and temperature 200 °C) samples were collected from local municipal sewage sludge treatment plant. The tar and soot samples obtained from pyrolysis at different temperatures were characterized by gas chromatography–mass spectrometry and scanning electron microscopy with energy dispersive X-ray spectroscopy, respectively. The results showed that increase in the pyrolysis temperature decreases the tar and soot yield for both raw and hydrolysis-treated sludge. The minimum tar yield was achieved with hydrolysis-treated sludge at 800 °C. The soot from raw sludge pyrolysis was of greater yield at high temperature than hydrolysis-treated sludge pyrolysis. The tar from pyrolysis of hydrolysis-treated sludge had more aliphatic character as compared to raw sewage sludge. The formation of heavy polycyclic aromatic hydrocarbons during pyrolysis of hydrolysis-treated sludge was lower at high temperatures than that of raw sludge. The aromatic hydrocarbons yield of hydrolysis-treated sludge was higher as compared to raw sludge at 800 °C.

Synergistic effect of biomass and polyurethane waste co-pyrolysis on soot formation at high temperatures

Soot is an important toxic pollutant generated during high-temperature incineration of solid waste (i.e., biomass and plastic waste) under air-lean conditions, and has a great impact on flame radiation. The main objective of this work is to study the synergistic effect of biomass and polyurethane co-pyrolysis on soot formation at high temperatures (1100–1250 °C). The effects of temperature, biomass species, and co-pyrolysis ratio on the yield, morphology, composition and reactivity of soot particles are studied. Results show that under controlled co-pyrolysis conditions, the measured soot yield from co-pyrolysis of biomass and polyurethane is lower than the theoretical value by weight average, while the particle size distribution tends to concentrate on a smaller diameter range. The degree of synergistic effect increases with the increasing biomass ratio (0–50 wt%) and decreasing pyrolysis temperature. Wood in co-pyrolysis presents a stronger synergistic effect on soot yields than straw co-pyrolysis does. Degree of synergistic effect on soot oxidation reactivity depends much on the biomass addition ratio but less on pyrolysis temperature. At 10 wt% straw addition ratio, co-pyrolysis exerts a negative synergistic effect on soot oxidation reactivity, while the synergistic effect turns significantly positive when the straw addition ratio increases to 50 wt%.

An experimental study on the gas and soot formation in ethanol pyrolysis

Ethanol is an alternative fuel to oil with high oxygen content, and its utilisation is helpful in solving the issues of petroleum shortage. Pyrolysis has always existed in the process of ethanol utilisation so it is very important to understand the process of gas production and soot formation during ethanol pyrolysis. Also, excessive soot will affect the effective use of ethanol, so the analysis of soot production at various conditions is significant, and soot rate is a key factor to predict and control soot yield. In the paper, pyrolysis experiments have been carried out in a quartz tube reactor in the temperature range of 700°C to 1,100°C, and a system combing a Fourier infrared spectrometer and a hydrogen analyser is used to detect the gas composition in the outlet of the reactor. By the oxidation using oxygen, the amount of soot deposited inside the reactor can also be measured. It is observed that temperature, ethanol concentrations and residence time can all affect the formation of gases and soot significantly. Also, the production of soot is obviously increasing during ethanol pyrolysis, especially for temperature over 1,025°C. By the carbon deposition rate under different items, an effective way to reduce soot have been obtained. Moreover, the impacts of the additions of methane, water vapour and oxygen on the formation of soot during ethanol pyrolysis have also been investigated. It is found that the amount of soot formed reduces as water vapour or oxygen is added, while the presence of methane exhibits a converse effect. [Received: December 12, 2015; Accepted: June 18, 2017]

An experimental and simulation study of the effect of acetone and propane additives on soot formation in acetylene pyrolysis behind reflected shock waves

Experimental investigations and detailed kinetic simulations of the formation of soot particles during pyrolysis of mixtures of acetylene with acetone and propane behind reflected shock waves are performed. Acetone and propane additives are found to substantially promote the process of soot formation as compared with that in acetylene–argon mixtures. Detailed kinetic simulations closely reproduce our own experimental results and published data. The kinetic model of soot formation is comprised of 4782 direct and reverse reactions involving 372 species. The predictive possibility of the kinetic model of soot formation is tested by describing the effect of acetone and propane additives to acetylene–argon mixtures on soot formation. All the kinetic parameters of the unified kinetic model are kept constant. The indicated additives enhance the soot yield because polyyne-dominated pathway of soot nucleation, characteristic of unseeded acetylene–argon mixtures, is augmented by the aromatic pathway, typical of most hydrocarbons.

The Role of Methyl Radical in Soot Formation

ABSTRACT It was noted a significant increase in soot formation with the addition of methane in the pyrolysis of acetylene in shock tubes. Qualitative kinetic analysis revealed an increase of propargyl formation due to methyl/methylene and acetylene recombination. Here, we study the role of methyl radicals in the soot formation process more carefully. Mixtures of acetylene with various sources of methyl radicals – methane, diacetyl and dimethyl ether (DME) – were studied under shock tube pyrolysis conditions over a temperature range of 1650–2250 K and a pressure range of 3.5–4.5 bar. Laser light extinction was used for soot volume fraction and induction time of the particle inception measurements. A more than threefold increase in soot volume fraction with the addition of 1% of methane and 2% of acetylene diluted in argon was observed. Additions of diacetyl and DME resulted in less significant increases in soot volume fraction. All additions of methane, diacetyl and DME reduced the induction time of particle inception. The numerical modeling of the initial gas phase part of the process confirmed our hypothesis that in all cases the increase in the soot yield was proportional to the appearance of an additional amount of CH3 radicals, which promoted the formation of propargyl, benzene and pyrene.

Effects of large aromatic precursors on soot formation in turbulent non-premixed sooting jet flames

In this paper, we computationally study the effects of large Polycyclic Aromatic Hydrocarbons (PAH) (more than two aromatic rings) on soot yield and distribution in turbulent flames, when they are considered as nucleating species for soot formation. This is examined in two turbulent non-premixed sooting jet flames using ethylene and a jet fuel (JP-8) surrogate as fuels. For each flame, two Large-Eddy Simulations (LES) are performed with two different soot nucleation strategies. In the first strategy, a range of PAH from naphthalene to cyclopenda[cd]pyrene are considered as nucleating species, while in the second strategy, naphthalene is considered as the only soot nucleating species and the effects of larger PAH are represented entirely by naphthalene. Flamelet-based chemistry-tabulation is used for the major thermochemical quantities, such as density, temperature, and major species mass fractions. Turbulence-chemistry interactions for PAH are accounted for by transporting their mass fractions and using a recently developed PAH relaxation model for their source term closure. The effects of large PAH on soot are highlighted by comparing the PAH profiles, soot nucleation rate, and soot volume fraction distributions obtained from both simulations for each test flame. These results are also compared against experimental measurements, when available. From these comparisons, it is first shown that naphthalene is predominantly formed along the flame centerline, and larger PAH species with more than two aromatic rings are primarily formed away from the centerline. Further, it is found that these larger PAH species only contribute to around of the overall soot nucleation rate along the flame centerline but their contributions are more substantial (up to ) away from the centerline. Finally, it is shown that representing the effects of these large PAH species by naphthalene leads to an under-prediction of around for the soot volume fraction magnitude along the flame centerline. Away from the centerline, this under-prediction can be as much as 100

Characterization of the incipient smoke point for steam-/air-assisted and nonassisted flares

ABSTRACTFlares are important safety devices for pressure relief; at the same time, flares are a significant point source for soot and highly reactive volatile organic compounds (HRVOCs). Currently, simple guidelines for flare operations to maintain high combustion efficiency (CE) remain elusive. This paper fills the gap by investigating the characteristics of the incipient smoke point (ISP), which is widely recognized as the condition for good combustion. This study characterizes the ISP in terms of 100–% combustion inefficiency (CE), percent opacity, absorbance, air assist, steam assist, air equivalence ratio, steam equivalence ratio, exit velocity, vent gas net heating value, and combustion zone net heating value. Flame lengths were calculated for buoyant and momentum-dominated plumes under calm and windy conditions at stable and neutral atmosphere. Opacity was calculated using the Beer–Lambert law based on soot concentration, flame diameter, and mass-specific extinction cross section of soot. The calculated opacity and absorbance were found to be lognormally distributed. Linear relations were established for soot yield versus absorptivity with R2 > 0.99 and power-law relations for opacity versus soot emission rate with R2 ≥ 0.97 for steam-assisted, air-assisted, and nonassisted flares. The characterized steam/air assists, combustion zone/vent gas heating values, exit velocity, steam, and air equivalence ratios for the incipient smoke point will serve as a useful guideline for efficient flare operations.Implications: A Recent EPA rule requires an evaluation of visible emissions in terms of opacity in compliance with the standards. In this paper, visible emissions such as soot particles are characterized in terms of opacity at ISP. Since ISP is widely recognized as most efficient flare operation for high combustion efficiency (CE)/destruction efficiency (DE) with initial soot particles formed in the flame, this characterization provides a useful guideline for flare operators in the refinery, oil and gas, and chemical industries to sustain smokeless and high combustion efficiency flaring in compliance with recent EPA regulations, in addition to protecting the environment.

Decomposition of pyrene by steam reforming: The effects of operational conditions and kinetics

In this paper, experimental and numerical studies on the decomposition pyrene, a main component of tar, under the steam reforming condition were conducted. The effects of operational conditions of reaction temperature TR and steam to carbon molar ratio (S/C) on the yield of main light gases and soot are assessed. The associated global reaction rate and main reaction pathways were analyzed. The results revealed that pyrene was mostly converted into soot instead of light gases in the steam reforming. The yields of CO2 and H2 significantly increased with TR, whereas those of CO and CH4 remained relatively stable. The water shift reaction of exhaust gases at low temperature zone should be taken into account to accurately predicted the yield of light gases. The conversion of pyrene rapidly happened when S/C was <2 and became gradually after. The global reaction rate of pyrene decomposition was obtained. Sensitivity analysis showed that the secondary cracking of soot with steam occurred at elevated temperatures. The reaction pathway of pyrene steam reforming could be divided into two stages with different products during the steam reforming process.