Soot Particle Coagulation Research Articles

Aerosol emissions from a marine diesel engine running on different fuels and effects of exhaust gas cleaning measures

The emissions of marine diesel engines have gained both global and regional attentions because of their impact on human health and climate change. To reduce ship emissions, the International Maritime Organization capped the fuel sulfur content of marine fuels. Consequently, either low-sulfur fuels or additional exhaust gas cleaning devices for the reduction in sulfur dioxide (SO2) emissions became mandatory. Although a wet scrubber reduces the amount of SO2 significantly, there is still a need to consider the reduction in particle emissions directly.We present data on the particle removal efficiency of a scrubber regarding particle number and mass concentration with different marine fuel types, marine gas oil, and two heavy fuel oils (HFOs). An open-loop sulfur scrubber was installed in the exhaust line of a marine diesel test engine. Fine particulate matter was comprehensively characterized in terms of its physical and chemical properties. The wet scrubber led up to a 40% reduction in particle number, whereas a reduction in particle mass emissions was not generally determined. We observed a shift in the size distribution by the scrubber to larger particle diameters when the engine was operated on conventional HFOs.The reduction in particle number concentrations and shift in particle size were caused by the coagulation of soot particles and formation/growing of sulfur-containing particles. Combining the scrubber with a wet electrostatic precipitator as an additional abatement system showed a reduction in particle number and mass emission factors by >98%.Therefore, the application of a wet scrubber for the after-treatment of marine fuel oil combustion will reduce SO2 emissions, but it does not substantially affect the number and mass concentration of respirable particulate matters. To reduce particle emission, the scrubber should be combined with additional abatement systems.

Theoretical Study of the Coagulation of Ash Particles in Waste Gases during the Vibrating Combustion of Solid Biofuel

Consideration has been given to the influence of acoustic oscillations on the process of coagulation of soot particles suspended in the flow of combustion products of wood and peat. It has been shown that at the characteristic parameters of vibrating combustion of these materials, the main factor of growth of the soot particles in the process of their coagulation is the deposition of small particles well entrained by the gas on large ones that are streamlined by the gas but are poorly entrained by it.

1D Modeling of the Microwave Discharge in Liquid n-Heptane Including Production of Carbonaceous Particles

Microwave plasma in the liquid is initiated inside a gas bubble formed at the end of the electrode-antenna, through which microwave energy is introduced into the liquid. A 1D model a set of gas phase kinetic reactions describes the evolution processes of ionization, heat transfer and formation of gas and solid products inside the plasma bubble. The code is based on joint solution of the Boltzmann equation for free electrons of the plasma, a simplified equation for the microwave field, the heat conduction equation, the balance equation for the electron density and the balance equations for the weight fraction for all gas and solid products of n-heptane pyrolysis. The Joule heat released in the plasma is expended on the evaporation of liquid n-heptane into the bubble and the decomposition of the n-heptane molecules. The model includes both the description of gas phase processes and formation of solid carbon-containing particles. The growth mechanism for generation of solid particles describes simultaneous processes of the initial nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results.

A 0D kinetic model for the microwave discharge in liquid n-heptane including carbonaceous particles production

A 0D model of time dependent processes, occurring in a gas bubble created by a microwave discharge in liquid n-heptane, is presented. The model includes both the description of gas-phase processes and formation of solid carbon-containing particles at atmospheric pressure. Two mechanisms of generation of solid particles are considered. The first one relates to the ‘linear PAH growth model’ and the second to ‘planar PAH growth model’. Both of them describe simultaneous processes of the initial n-heptane pyrolysis and nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results. It is concluded that the second model describes the results of the experiments better.

Numerical simulations of microgravity ethylene/air laminar boundary layer diffusion flames

Microgravity ethylene/air laminar boundary layer diffusion flames were studied numerically. Two oxidizer velocities of 250 and 300 mm/s and three fuel injection velocities of 3, 4, and 5 mm/s were considered. A detailed gas-phase reaction mechanism, which includes aromatic chemistry up to four rings, was used. Soot kinetics was modeled by using a pyrene-based model including the mechanisms of nucleation, heterogeneous surface growth and oxidation following the hydrogen-abstraction acetylene-addition (HACA) mechanism, polycyclic aromatic hydrocarbon (PAH) surface condensation and soot particle coagulation. Radiative heat transfer from CO, CO2, H2O and soot was calculated using the discrete ordinate method (DOM) coupled to a wide-band correlated-k model. Model predictions are in quantitative agreement with the available experimental data. Model results show that H and OH radicals, responsible for the dehydrogenation of sites in the HACA process, and pyrene, responsible for soot nucleation and PAH condensation, are located in a thin region that follows the stand-off distance. Soot is produced in this region and, then, is transported inside the boundary layer by convection and thermophoresis. The combustion efficiency is significantly lower than 1 and is reduced as the flow residence time increasing, confirming that these sooting micro-gravity diffusion flames are characterized by radiative quenching at the flame trailing edge. In particular, this quenching phenomenon explains the increase in flame length with the oxidizer velocity observed in previous experimental studies. The effects of using approximate radiative-property models, namely the optically-thin approximation and gray approximations for soot and combustion gases, were assessed. It was found that the re-absorption and the spectral dependence of combustion gases and soot must be taken into account to predict accurately temperature, soot volume fraction, flame geometry and flame quenching.

Numerical study of soot formation in laminar coflow diffusion flames of methane doped with primary reference fuels

Laminar methane diffusion flames doped by vapors of different blends of n-heptane and iso-octane burning in a coflow of air diluted with CO2 were simulated numerically in this study. These laminar coflow diffusion flames have been experimentally investigated recently by Kashif et al. (2014). Numerical simulations were performed using a detailed reaction model containing 175 species and 1189 reactions together with a sectional soot model. The soot model consists of inception as a result of the collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition (HACA) mechanism, soot particle coagulation, and PAH surface condensation. The numerical model provides prediction in fairly good quantitative agreement with the experimental data and reproduces successfully the influence of the fuel blend composition and CO2 dilution of air on the flame structure and soot production. The soot production increases and decreases linearly with the molar concentration of iso-octane in the fuel stream and CO2 content of the air stream, respectively. In addition, the efficiency of CO2 dilution to suppress soot is reduced as the concentration of iso-octane increases. This is likely due to the greater contributions of PAH condensation with increasing iso-octane concentration relative to soot surface growth through HACA. The increase in soot production with the molar concentration of iso-octane results primarily from an increase in the concentration of propargyl radical. The self-reaction of propargyl then produces higher concentrations of benzene and pyrene, leading to higher nucleation rates and consequently an increase the soot surface area. The higher surface area then results in higher surface growth and condensation rates. The enhanced pyrene concentration is another factor to prompt soot surface condensation. An analysis of the reaction rates shows that propargyl radicals are mainly produced from propylene. The decomposition and oxidation of iso-octane was found much more efficient to produce propylene than that of n-heptane.

Numerical study of the effects of pressure on soot formation in laminar coflow n-heptane/air diffusion flames between 1 and 10 atm

Laminar nitrogen-diluted n-heptane diffusion flames burning in coflow air were numerically simulated at pressures from 1 to 10atm. Numerical simulations were performed by using a detailed reaction mechanism containing 175 species and 1086 reactions with PAH formation up to pyrene and a sectional soot model. The soot model consists of inception as a result of the collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition (HACA) mechanism, and soot particle coagulation and PAH surface condensation. Comparisons with available experimental data indicate that the numerical model reproduces successfully the influence of pressure on the flame structure and soot production. All the soot formation processes are enhanced with increasing pressure and HACA dominates the soot mass growth over the pressure range considered. PAH condensation is the most sensitive process to pressure whereas the inception and HACA processes exhibit similar pressure dependency. These trends are directly related to the pressure dependence of the molar concentrations of the species involved in soot formation. The increase in the molar concentrations of benzene and pyrene with pressure results from both physical and chemical effects. Propargyl recombination and interconversion of phenyl to benzene dominate the formation of benzene low in the flame over the pressure range considered, though the contributions of the addition of acetylene to n-butadienyl radical increases significantly with pressure in the flame centerline region.

A Modeling Study on the Nucleation, Growth and Coagulation of Soot Particles in Combustion Flame

Soot, a crucial component of PM2.5 in the ambient air, is a prime product of incomplete combustion of fule. The present paper uses a series of models, including nucleation of soot precursors, surface growth of soot particles as well as particles coagulation, to do numerical simulation of soot particles formation mechanism over time in combustion flame, with the aid of MATLAB platform by C language. Simulation results indicate that the concentration distribution of soot precursors like polycyclic aromatic hydrocarbons (PAHs) have direct effect on the concentration distribution of soot nuclei. Mass addition rate on soot particles surface rises over time since the enlargement of the surface area. In coagulation process, the number of small diameter particles decreases, while that of large diameter particles increases.

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C 6H 6 and n-C 6H 7 and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.

Structure and optical properties of soot aerosol in a moist atmosphere: 1. Structural changes of soot particles in the process of condensation

A laboratory setup and a procedure for measuring the volume coefficients and mean cross sections of extinction, scattering, and absorption of soot particles in the medium of saturated water vapor are described. A method for hydrophilization of the surface of soot particles, which makes it possible to obtain model objects with specified hygroscopic properties, is presented. The processes of transformation of soot particles are analyzed with the use of data of electron-microscopic investigations on the basis of the fractal approach. The structural parameters of hydrophobic soot are shown to depend on the conditions of moistening, whereas hydrophilic particles are subject to substantial structural changes indicative of their watering. Investigation of the coagulation of soot particles with drops shows that hydrophobic particles form a weakly bound system; they coagulate on the surface of drops and cause no changes in the structure of soot aggregates. During the coagulation, hydrophilic soot particles penetrate inside a drop and irreversibly form a mixed system.

Monte-Carlo simulation of soot particle coagulation and aggregation: the effect of a realistic size distribution

The evolution of the morphology of primary soot particles was investigated in a laminar premixed flame with the emphasis on the effect of a realistic particle size distribution. A time-dependent Monte-Carlo method was used to calculate flame trajectories of single particles. In the nucleation zone of the flame, particles were grown by coagulation and simultaneous surface growth. The particles for coagulation were selected according to a pre-calculated size distribution obtained by solving the dynamics of the entire soot particle ensemble. In the post-nucleation zone, defined by the point of transition from coalescent to aggregate growth, only surface growth was applied to the particles. The simulation results provide further support to the notion that particle nucleation and the presence of small particles influence the morphology of primary particles and the location of transition. It is demonstrated that the mean size of subparticles comprising the primary particles is substantially smaller when using a realistic size distribution in the nucleation zone of the investigated flame. This, in turn, reduces the degree of aggregation of the primary particles at the time of transition to an extent that surface growth is able to recover their spherical shape in the post-nucleation zone.

Modelling coagulation of soot particles

Modelling coagulation of soot particles

Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C 2 hydrocarbons

The present study reports an updated detailed chemical kinetic model for soot formation. The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth. The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene. The numerical agreement between the model predictions and experimental data is generally within a factor of 3. This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry. The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model.

Measurements of the growth and coagulation of soot particles in a high-pressure shock tube

The advantage of well-defined experimental conditions in shock tubes has been used to investigate the growth and coagulation of soot particles at high pressures. The measurements have been made for fuel-rich oxidation and pyrolysis of different hydrocarbons behind the reflected shock at pressures between 10 and 60 bar and temperatures between 1500 and 2300 K. In addition to soot volume fraction, time-resolved scattering measurements yielded particle diameters and number densities; all these give insight into both surface growth and coagulation at enhanced pressures. The temperature behind the reflected shock was monitored by two-color pyrometry. Soot growth was characterized by induction periods and soot growth rates. At low final soot yields, the growth rate of soot depends on the square of the carbon concentration. At high soot yields, reduced growth rates of soot volume fraction were observed and can be attributed to a lack of growth species. At constant carbon concentration no pressure dependence of soot volume fraction could be found. Particle diameters between 15 and 40 nm were measured. The number density of particles was found to increase strongly with soot volume fraction. Calculated and measured particle number densities agree well during early soot growth. However, at longer times the experiments reveal coagulation rates which are significantly smaller than predicted. This behavior indicates that collisions of deactivated soot particles are characterized by sticking probabilities lower than unity. A correlation for the sticking probability has been established to match both the experimental results and calculations.

Coagulation of soot particles and fractal dimension

Coagulation of soot particles and fractal dimension

Dynamic Modeling of Soot Particle Coagulation and Aggregation: Implementation With the Method of Moments and Application to High-Pressure Laminar Premixed Flames

The method of moments was extended to include a more complete description of particle coagulation, namely, the transition and continuum regimes as well as the formation and growth of fractal aggregates. The formalism preserves the numerical efficiency and the physical rigor of the method of moments by inclusion of just two additional moment equations and without prescribing a mathematical form to the particle size distribution function. The extended model was used to simulate soot formation in several 10 bar laminar premixed ethylene-air flames. The results demonstrate that the more complete formulation improves significantly model prediction of experimental data, thereby explaining the appearance of a catastrophic decrease in coagulation rates, which could not be rationalized within the classical theory of Brownian coagulation.

Soot Particle Coagulation in Premixed Ethylene/Air Flames at 10 bar

Soot Particle Coagulation in Premixed Ethylene/Air Flames at 10 bar

Soot mass growth and coagulation of soot particles in C2H4/air-flames at 15 bar

The growth of soot particles has been studied in flat C 2 H 4 /air flames burning at 15 bar. Laser extinctionand scattering were measured simultaneously as a function of distance from the burner, together with temperature and concentration profiles of aliphatic and polycyclic hydrocarbons. In 15 bar C 2 H 4 /air flames with surplus carbon densities ϱ c >10 18 C atoms/cm 3 , final particle numbers N ∝ are found that are higher than at lower pressures. The growth of f ν∝ , the final soot mass, proceeds via increasing particle number density and not as at normal pressure via larger particles. The particle coagulation rate decays quite rapidly with time leading to larger final particle number density. The apparent active lifetime of soot particles ( k f −1 ) at 15 bar depends on temperature and, in addition, on the surplus carbon density ϱ c . The quantity k f /ϱ c is a function of temperature. The polycyclic aromatic hydrocarbons (PAH) appear very early in the C 2 H 4 /air flame, but they are also consumed very rapidly to very low final mole fraction x PAH ≈10 −9 . The carbon density of C 2 H 2 decreases strongly in the phase of soot growth. The final value [C 2 H 2 ] ∞ of about 10 −6 g/cm 3 is high enough for further soot growth, so that the ceasing of coagulation and soot mass growth at 15 bar cannot be explained by a lack of C 2 H 2 . It rather seems to be the decreasing activity of soot particles.

Mass growth and coagulation of soot particles in low pressure flames

The mass growth and coagulation of soot particles was studied in low pressure flat flames of acetylene and oxygen (+nitrogen) burning with C/O ratios ranging between 0.95 and 1.2. A particle mass spectrometer (PMS) was used to determine the size and size distribution of charged particles. From 10 different flames the growth behaviour of singly charged soot particles in the mass range 0.5×10 −20 g≤ m p ≤50×10 −20 g was measured without interference by particle coagulation. The data obtained could be interpreted in terms of a specific mass growth rate. For one flame burning at C/O=1.1 the coagulation of primary particles was also determined based on PMS measurements. The results obtained agree quite well with computer simulations based on a coagulation model including van der Waals, electrostatic, and electric image potentials during particle collision.

Computer simulation of soot particle coagulation in low pressure flames

A computer model for soot particle coagulation in the combustion zone of a low pressure acetylene/oxygen flame has been developed. It is assumed that coagulation is the dominant particle growing process. The computations include the one-dimensional numerical solution of the General Dynamic Equation (GDE) for a stationary flame without considering particle nucleation, diffusion, and external forces. The coagulation coefficients were determined from the kinetic theory of gases and include particle interactions caused by van der Waals, coulombic and image forces. The developed computer model is able to predict the experimentally observed particle behaviour. The agreement between calculated and measured ratios of singly charged agglomerates to singly charged primary particles is nearly exact.