Characteristics of diesel fuel combustion in a burner with injection of a superheated steam jet

Basic characteristics of combustion of the diesel fuel in a novel autonomous burner with injection of superheated steam into the combustion region are studied. The temperature distribution in the flame is obtained. Calorimetric measurements of heat release and gas analysis of combustion products are performed. The environmental effects of fuel combustion are compared for regimes with injection of a steam jet and an air jet. It is demonstrated that the combustion regime with steam gasification ensures high combustion intensity and combustion efficiency; moreover, the combustion process becomes more environmentally friendly.     

A novel miniature inverted-flame burner for the generation of soot nanoparticles

Lab-scale soot nanoparticle generators are used by the aerosol research community to study the properties of soot over a broad range of particle size distributions, and number and mass concentrations. In this study, a novel miniature inverted-flame burner is presented and its emitted soot particles were characterized. The burner consisted of two co-annular tubes for fuel and co-flow air and the flame was enclosed by the latter. The fuel used was ethylene. A scanning mobility particle sizer (SMPS) and an aerodynamic aerosol classifier (AAC) were used to measure mobility and aerodynamic size distribution of soot particles, respectively. Particle morphology was studied using transmission electron microscopy (TEM). The elemental carbon (EC) and organic carbon (OC) content of the soot were measured using thermal-optical analysis (TOA). The burner produced soot particles with mobility diameter range of 66–270?nm, aerodynamic diameter range of 56–140?nm, and total concentration range of 2?×?10⁵–1?×?10⁷?cm^?3. TEM images showed that most soot particles were sub-micron soot aggregates. Some soot superaggregates, typically larger than 2?µm in length, were observed and their abundance increased with ethylene flow rate. TOA showed that the concentration of EC in the generated soot increased with ethylene flow rate, and the soot was observed to have high EC fraction at high ethylene flow rates. The miniature inverted-flame burner was demonstrated to produce soot nanoparticles over a range of concentrations and sizes with high EC content, making it a practical device to study soot nanoparticle properties in different applications.

Copyright © 2019 American Association for Aerosol Research     

Observations of a Correlation Between Primary Particle and Aggregate Size for Soot Particles

For decades, soot has been modeled as fractal-like aggregates of nearly equiaxed spherules. Cluster–cluster aggregation simulations, starting from a population of primary particles, give rise to structures that closely match real aerosols of solid particles produced in flames. In such simulations, primary particle size is uncorrelated with aggregate size, as all aggregates contain primary particles drawn from the same population. Aerosol measurements have been interpreted with this geometric model. Examination of transmission electron micrographs of soot samples from various sources shows that primary particle sizes are not well mixed within an aerosol population. Larger aggregates tend to contain larger primary particles and the variation in size is much larger between aggregates than within aggregates. The soot sources considered here are all substantially not well-mixed (aircraft jet engine, inverted diffusion flame, gasoline direct injection engine, heavy-duty compression ignition engine). The observed variations in primary particle size can be explained if soot aggregates are formed and grew by coagulation in small zones of the combustion chamber, prior to dilution and transport (with minimal coagulation) to the sampling system.

Copyright 2014 American Association for Aerosol Research     

Mapping the Operation of the Miniature Combustion Aerosol Standard (Mini-CAST) Soot Generator

The Jing Ltd. miniature combustion aerosol standard (Mini-CAST) soot generator is a portable, commercially available burner that is widely used for laboratory measurements of soot processes. While many studies have used the Mini-CAST to generate soot with known size, concentration, and organic carbon fraction under a single or few conditions, there has been no systematic study of the burner operation over a wide range of operating conditions. Here, we present a comprehensive characterization of the microphysical, chemical, morphological, and hygroscopic properties of Mini-CAST soot over the full range of oxidation air and mixing N₂ flow rates. Very fuel-rich and fuel-lean flame conditions are found to produce organic-dominated soot with mode diameters of 10–60 nm, and the highest particle number concentrations are produced under fuel-rich conditions. The lowest organic fraction and largest diameter soot (70–130 nm) occur under slightly fuel-lean conditions. Moving from fuel-rich to fuel-lean conditions also increases the O:C ratio of the soot coatings from ～0.05 to ～0.25, which causes a small fraction of the particles to act as cloud condensation nuclei near the Kelvin limit (κ ～ 0–10^?3). Comparison of these property ranges to those reported in the literature for aircraft and diesel engine soots indicates that the Mini-CAST soot is similar to real-world primary soot particles, which lends itself to a variety of process-based soot studies. The trends in soot properties uncovered here will guide selection of burner operating conditions to achieve optimum soot properties that are most relevant to such studies.

Copyright 2014 American Association for Aerosol Research     

Generation of Silica Nanoparticles from Tetraethylorthosilicate (TEOS) Vapor in a Diffusion Flame

Silica (SiO2) nanoparticles were synthesized by the gas phase thermal oxidation of tetraethylorthosilicate (TEOS) in a laminar diffusion flame reactor. Characteristics of the formation of silica nanoparticles along the axial distance above the burner outlet were investigated. Effects of maximum flame temperature, TEOS concentration, residence time, and water vapor on the particle size were also investigated. Silica nanoparticles less than 20 nm in average particle diameter were synthesized in all of the experiments. Morphological changes of particles were found along axial distance above the burner outlet; many small aggregates of particles (dp = ～ 7 nm) were found up to 2 cm, isolated smaller particles ( dp = ～ 5 nm) at 4 cm, and aggregates of bigger particles (dp = ～ 10 nm) at 10 cm. Larger particles at higher TEOS concentrations are generated in the flame synthesis. As the maximum flame temperature increased, the average particle size of silica also increased. Smaller particles were produced with decrease of the residence time of TEOS vapor in the flame. The average particle size decreased with the injection of water vapor to the flame.     

Design and Testing of the NCTU Micro-Orifice Cascade Impactor (NMCI) for the Measurement of Nanoparticle Size Distributions

Soot processes within a turbulent nonpremixed flame burning acetylene/air were investigated by conducting thermophoretic sampling experiments at various axial and radial locations. Analyses of transmission electron microscope images yielded the mean soot spherule diameter, number of spherules per aggregate, and fractal morphology within this highly luminous turbulent flame. Specifically, translucent particles were observed at low-to-intermediate heights above the flame with the formation and evolution of young soot precursors. The soot spherule diameter peaked at 34 nm halfway along the centerline, identifying the flame regions of surface growth and oxidation processes. In the meantime, the aggregation was continuous along the flame axis with the mean number of spherules per aggregate reaching 150 at the highest sampling location. Size ranges of spherules and aggregates were narrow and broad, respectively, while the relative widths of both size distributions remained similar throughout the flame. In contrast to the observed axial variations, the radial changes of the mean spherule and aggregate sizes appeared to be small. Aggregate morphologies were universally characterized by a fractal dimension of 1.82 and a fractal prefactor of 1.9 for all the flame positions. In comparison to a lightly sooting ethylene flame, these measurements in the acetylene turbulent flame revealed that the fuel type mainly affected the axial evolution of spherule diameters but not their range, and enhanced the aggregate sizes but not their morphology. The effective decoupling of spherule and aggregate sizes permitted the separation of soot surface growth and oxidation from aggregation. This key aspect provided a stringent test for the existing particulate diagnostics and predictive models in their ability to quantify the actual particle surface area, particularly in optically thick conditions that are encountered in many practical combustion environments.     

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII,HIM, and 1 nm-SMPS

Laser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermophoretically and (2) online size distribution analysis of microprobe-sampled particles using a 1 nm-SMPS. The size distributions of the incipient soot particles, found in the nucleation flame and in the early soot region of the Φ = 1.95 flame, derived from time-resolved LII signals are in good agreement with HIM and 1 nm-SMPS measurements and are in the range of 2–4 nm. The thermal and optical properties of incipient soot were found to be not radically different from those of mature soot commonly used in LII modeling. This explains the ability of incipient soot particles to produce continuous thermal emissions in the visible spectrum. This study demonstrates that LII is a promising in situ optical particle sizing technique that is capable of detecting incipient soot as small as about 2.5 nm and potentially 2 nm and resolving small changes in soot sizes below 10 nm.

© 2017 American Association for Aerosol Research     

Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H_p, to visible flame length, H_f, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINO_X happens to occur closer to stoichiometric overall equivalence ratio.     

A Study of Optical Properties of Soot Aggregates Composed of Poly-Disperse Monomers Using the Superposition T-Matrix Method

The diameters of soot monomers may not be constant in the single fractal aggregated soot particle. The optical properties of light absorbing soot particles aggregated with poly-disperse monomers were studied using the superposition T-matrix method. Soot aggregates were generated with different log-normal probability distribution functions (PDF) of soot monomer diameter, according to the same soot volumes and monomer numbers. The single scattering properties of soot particles were calculated at a wavelength of 550 nm, assuming a soot refractive index of 1.95 + 0.79i and a mass density of 1.8 g/cm³. The random-orientation averaging results indicated that the optical properties of soot aggregates were fairly varied for the different distributions of the monomer diameters. In these simulations, the extinction and absorption of soot aggregates were slightly (<10%) affected by the monomer poly-dispersity. The simulated mass absorption cross-sections (MAC) of fresh dry soot particles aggregated with poly-disperse monomers reached up to 6.62 ± 0.07 m²/g, which was closer to the measurement (7.5 ± 1.2 m²/g) than the assumption of volume-equivalent mono-disperse monomer (6.36 ± 0.06 m²/g). Moreover, the optical properties of soot coated with an organic shell were calculated, and the optical results showed that the absorption cross-sections of the internally mixed soot particles were also slightly (<8%) influenced by the monomer poly-dispersity. We found that the effect of the monomer poly-dispersity on the light scattering and the single scattering albedo may be considerably large (up to ?50% in extreme cases) for fresh dry soot aggregates. This effect on light scattering should be taken into account for those aggregates composed of monomers with widely distributed diameters.

Copyright 2015 American Association for Aerosol Research     

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.     

Characterization of Soot Aerosol Produced from Combustion of Propane in a Shock Tube

The knowledge of yields and properties of soot from combustion of hydrocarbon fuels is crucial for accurate evaluation of the impacts of primary aerosols on air quality and climate. This study presents measurements of soot generated from combustion of propane in a shock tube, using independently adjustable fuel equivalence ratio (φ), temperature, and pressure. The characterization of soot yields inside the shock tube by in situ laser extinction is complemented with a set of comprehensive measurements of soot transferred into a fluoropolymer chamber, including particle size distributions, elemental carbon (EC) mass fraction, effective density, mass fractal dimension (Dfm), dynamic shape factor (χ), and optical properties. The properties of soot particles and the soot yield are sensitive to combustion conditions and the duration of the combustion experiment. High-temperature combustion with φ = 2.5 produces small fractal (Dfm = 2) soot particles composed mainly of EC (up to 90%), at a low mass yield. Particles from lower temperature combustion contain a significant fraction of organic material (～50%). Using rich fuel mixtures (φ = 4.0 and 8.0) significantly increases particle size and soot mass yield. At lower temperatures, compact (Dfm = 3) and nearly spherical (χ = 1.1) aggregates with high organic content are formed, whereas at higher temperatures, the particles are fractal and closely resemble those obtained using φ = 2.5. Single scattering albedo (SSA) varies from 0.15 for fractal particles to 0.75 for compact particles. For soot generated at high equivalence ratios, SSA can be used as a proxy for particle morphology and EC content.

Copyright 2012 American Association for Aerosol Research     

Morphological and Elemental Classification of Freshly Emitted Soot Particles and Atmospheric Ultrafine Particles using the TEM/EDS

The Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) were used to determine morphology and elemental composition of a variety of freshly emitted soot particles (acetylene flame, candle flame, kerosene flame, diesel exhaust, electric arc, plastic burning, styrofoam burning, wood burning [white oak and pine bark], and rice straw burning), which can be possible candidate soot in the ambient atmosphere, and ultrafine particles sampled in urban, industrial, and coastal sites during ultrafine particle formation events (combustion and photochemical events). By using mobility-classified non-refractory ((NH ₄ ) ₂ SO ₄ ) and refractory (Polystyrene latex (PSL) and salt (NaCl)) particles, limitation of the TEM was tested. Data showed that the TEM method can be used to examine shapes of both volatile particles such as (NH ₄ ) ₂ SO ₄ (100 nm) at low, but not high magnification (refer to low and high beam intensity, respectively), and non-volatile particles like NaCl (100 nm) and PSL (84 nm) at either low or high magnification. Distinct differences in morphological properties such as primary particle diameter, fractal dimension, and microstructure were observed among the different types of fresh soot particles. The atmospheric ultrafine particles were classified as agglomerates, sulfate mixtures (spherical), metallic oxides (spherical and polygonal), C-rich refractory (not agglomerated), C-rich non-refractory (not agglomerated), Si-rich (spherical), Na-rich (porous), or P-containing (non-spherical) particles. At the urban Gwangju site, a higher fraction of fresh and aged agglomerates was observed than at other sites. The C-rich non-refractory and sulfate mixtures were often observed in the photochemical event. The C-rich refractory particles were abundant at the Gwangju and Yeosu sites. The coastal Taean site had few agglomerates due to limited anthropogenic combustion source.     

Combustion of Mechanically Alloyed Aluminum‐Magnesium Powders in Steam

Mechanically alloyed Al ⋅ Mg powders with the mole fraction of Al varied from 0.47 to 0.9 were burned at atmospheric pressure in water vapor. The powders were carried by nitrogen through the center of a hydrogen‐oxygen diffusion flame. The particles ignited in the steam at approximately 2500 K, generated as the hydrogen‐oxygen flame product. Filtered photomultiplier tubes were used to capture the optical emission traces of individual particles as they burned. It was assumed that the measured durations of individual emission pulses are representative of individual particle burn times. Distributions of the burn times were obtained for each powder and correlated with respective particle size distributions to relate particle burn times with their sizes. Color temperatures corresponding to the particle emission signals were also obtained. It was observed that the burn times measured for alloys were more close to those of pure Al than Mg; for particles smaller than 2–3 μm, burn times for the alloys were shorter than for pure metal particles. The effect was strongest for the alloy with 50 wt‐% of Mg (Al_0.47Mg_0.54). Approximately, burn times, τ, as a function of particle size, d, could be estimated using a τ∼dⁿ law, where n increased from 0.72 to 1.05 as the mole fraction of Mg increased from 0.1 to 0.53. The particle flame temperatures varied between 2500 and 3100 K for all alloys except for Al_0.7Mg_0.3, for which the temperatures were somewhat lower. The measured flame temperatures were reasonably close to the adiabatic flame temperatures calculated for combustion of mixed elemental Al and Mg in steam.     

Quality Changes of Superheated Steam–Dried Fibers from Oil Palm Empty Fruit Bunches

Conventional drying of the fibers from oil palm empty fruit bunches (EFB) using flue gas from diesel burners frequently causes browning and dust explosion. Replacing the drying medium with superheated steam is expected to improve the quality of EFB fibers as well as improve the safety of the dryer operation. In this study, the effects of steam temperature and steam velocity on the quality of steam–dried EFB fibers was investigated. The drying experiment was carried out at atmospheric pressure with steam superficial velocity in the range of 0.3 to 0.49 m s^?1 and temperature in the range of 135 to 200°C. Three quality parameters of the EFB fibers, the color, strength, and microstructure, were used to assess the changes in EFB fiber quality as a result of superheated steam drying. The color of the EFB fiber was either improved or not significantly degraded. The strength of the superheated steam–dried EFB fibers was higher than that of undried and hot air–dried EFB fibers. The microstructure of fresh undried EFB fibers as seen by scanning electron microscopy (SEM) showed the presence of round silica particles of 10–20 µm in diameter all over the EFB fiber strand, which complicates pulping and bleaching. Superheated steam drying successfully removed the silica particles from the EFB fibers at temperatures of at least 200°C and a velocity of steam of at most 0.49 m s^?1, which is better than hammering, which can only remove 88% of the silica particles. The high temperature of the superheated steam loosened the silica particles from their craters. The EFB fibers cracked and split at steam velocities at or above 0.49 m s^?1 and high superheated steam temperatures at or above 200°C and as a consequence became weaker at these conditions. The removal of silica particles by superheated steam drying makes the EFB fiber amenable to pulping and bleaching. Superheated steam drying is therefore found to improve the overall quality of EFB fibers compared to hot air drying.     

Effect of addition of potassium carbonate to aluminum powder on the grain size of Al2O3 nanoparticles formed in the laminar dusty flame

Results of studying the effect of K₂CO₃ additives on the grain size of the products of combustion of a gas suspension of Al particles (with the mean particle diameter of 4.8 µm) in a laminar diffusion flame are reported. An extreme character of the dependence of the mean size of Al₂O₃ particles on the additive concentration is experimentally observed. For the concentration of the K₂CO₃ additive equal to 0.5%, the mean diameter of Al₂O₃ particles is 30 nm; for the additive concentration of 5%, the mean particle size increases to 67 nm. It is demonstrated that the change in the mean size of Al₂O₃ particles as a function of the concentration of the readily ionized additive is caused by interaction of the dusty and ionic subsystems of the plasma of the combustion products in the reaction zone in the flame. At a high concentration of ions (above 10²⁰ m^?3), this interaction increases the rate of coagulation of Al₂O₃ particles.     

Effective Density Characterization of Soot Agglomerates from Various Sources and Comparison to Aggregation Theory

Soot particle (black carbon) morphology is of dual interest, both from a health perspective and due to the influence of soot on the global climate. In this study, the mass-mobility relationships, and thus effective densities, of soot agglomerates from three types of soot emitting sources were determined in situ by combining a differential mobility analyzer (DMA) and an aerosol particle mass analyzer (APM). High-resolution transmission electron microscopy was also used. The soot sources were diesel engines, diffusion flame soot generators, and tapered candles, operated under varying conditions. The soot microstructure was found to be similar for all sources and settings tested, with a distance between the graphene layers of 3.7–3.8 Å. The particle specific surface area was found to vary from 100 to 260 m²/g. The particle mass-mobility relationship could be described by a power law function with an average exponent of 2.3 (±0.1) for sources with a volatile mass fraction <10% and primary particle sizes of 11–29 nm. The diesel exhaust from a heavy duty engine at idling had a substantially higher volatile mass fraction and a higher mass-mobility exponent of 2.6. The mass-mobility exponent was essentially independent of the number of primary particles in the range covered (N_pp = 10–1000). Despite the similar exponents, the effective density varied substantially from source to source. Two parameters were found to alter the effective density: primary particle size and coating mass fraction. A correlation was found between primary particle size and mass-mobility relationship/effective density and an empirical expression relating these parameters is presented. The effects on the DMA-APM results of doubly charged particles and DMA agglomerate alignment were investigated and quantified. Finally, the dataset was compared to three theoretical approaches describing agglomerate particles’ mass-mobility relationship.

Copyright 2013 American Association for Aerosol Research     

Detached eddy simulation of particle dispersion in a gas-solid two-phase fuel rich/lean burner flow

To predict the particle dispersion in the burner jet is of great importance in industrial application and in the designing of coal burner with good performance. The objective of this study was to numerically investigate the particle dispersion mechanisms in the gas-solid two-phase jet from a fuel rich/lean burner. The detached-eddy-simulation (DES) approach was employed to study the turbulent flow in the fuel rich/lean separator and the gas-solid multiphase jet from the exit of a fuel rich/lean burner. The vortex shedding process was simulated and its effect on the fuel rich/lean separating performance was evaluated. Combined with the Lagrangian tracking procedure for the particle phase, the coal particles with various Stokes numbers equal to 0.000434, 0.043, 1.08, 4.34 (corresponding to particle diameter 1, 10, 50, 100 μm, respectively) in the gas-solid fuel rich/lean jet were studied, which shows that although there are coherent vortex structures behind the central partition plate, these vorticities are small, the fuel-rich stream and the fuel lean stream will not mix quickly downstream of the exit of the nozzle. The large vortex structures at the jet outer boundary are the main factors that make the small particles to mix together. The coal particles with large Stokes number (St>1) disperse very slowly in the jet flow, which realizes the fuel rich/lean combustion in a rather long distance downstream the exit of the nozzle, resulting in low NO_x emissions.     

The Morphology of Macroscopic Soot

ABSTRACT

The morphology of soot collected from a laminar acetylene/air diffusion flame was studied. Collection methods included both thermophoretic and impaction sampling from both the luminous and nonluminous portions of the flow. The soot was viewed with both electron and optical microscopy. Cluster sizes ranged over four orders of magnitude from 50 nm to 400 yam to include some clusters visible to the naked eye. A new method of micrograph analysis, necessary when the clusters were large, was developed to account for the unresolved primary particles. Over this entire size range, the same fractal morphology was found with a fractal dimension of D = 1.8 and, within a rather large uncertainty, the same prefactor k₀=1.7. Thus, the fractal morphology of soot remains constant from clusters of about 10 primary particles per aggregate to macroscopic clusters of over 10⁸ primary particles.     

Experimental Study of Combustion of a Cloud of Boron Particles in Air

Stabilization of a premixed boron—air flame at the Bunsen burner nozzle exit is studied. The normal burning velocity of the gaseous suspension of boron particles is determined by spectral zonal camera recording. Qualitative spectral analysis of the particle-gas flame showed the presence of fluctuation bands in the range of 508–600 nm due to the radiation from BO₂ molecules, which confirms gas-phase combustion of the boron particles in the gaseous suspension. Chemical, particle-size, and morphological analyses of the condensed combustion products of powdered boron in air showed the presence of submicron and micron particles with number mean diameters of 100 nm and 14 fum, respectively.