Hygroscopic Properties of Pasadena,California Aerosol

The hygroscopic behavior of Pasadena, CA aerosol was continuously measured from August 15 to September 15, 1999 using a tandem differential mobility analyzer. Two dry particle sizes were sampled, 50 nm and 150 nm in diameter; humidification of the dry aerosol was carried out at 89% relative humidity. Complex growth patterns were observed for both size modes, with aerosol distributions splitting from a single mode at times to more than 6 modes. Diurnal profiles for the observed multiple peaks were noted, with the greatest number of measurable growth modes being found during the late night and predawn hours for 50 nm particles. For 150 nm particles, more modes were present during the afternoon hours, with the humidified aerosol becoming bimodal during the late night/early morning hours. Growth factors, defined as the ratio of humidified particle diameter (at 89%) to dry diameter, were determined for modes with significant number concentrations. Average growth factors over the sampling period for the 2 particle sizes ranged from 1.0 to 1.6. Hygroscopic growth increased in the latter half of the sampling period when forest fires were present. In short, treating this complex urban aerosol as a combination of "less" and "more" hygroscopic fractions is an oversimplification.     

Hygroscopic Behavior of Aerosol Particles Emitted from Biomass Fired Grate Boilers

This study focuses on the hygroscopic properties of submicrometer aerosol particles emitted from two small-scale district heating combustion plants (1 and 1.5 MW) burning two types of biomass fuels (moist forest residue and pellets). The hygroscopic particle diameter growth factor (Gf) was measured when taken from a dehydrated to a humidified state for particle diameters between 30–350 nm (dry size) using a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA). Particles of a certain dry size all showed similar diameter growth and the Gf at RH = 90% for 110/100 nm particles was 1.68 in the 1 MW boiler, and 1.5 in the 1.5 MW boiler. These growth factors are considerably higher in comparison to other combustion aerosol particles such as diesel exhaust, and are the result of the efficient combustion and the high concentration of alkali species in the fuel. The observed water uptake could be explained using the Zdanovski-Stokes-Robinson (ZSR) mixing rule and a chemical composition of potassium salts only, taken from ion chromatography analysis of filter and impactor samples (KCl, K₂SO₄, and K₂CO₃). Agglomerated particles collapsed and became more spherical when initially exposed to a moderately high relative humidity. When diluted with hot particle-free air, the fractal-like structures remained intact until humidified in the H-TDMA. A method to estimate the fractal dimension of the agglomerated combustion aerosol and to convert the measured mobility diameter hygroscopic growth to the more useful property volume diameter growth is presented. The fractal dimension was estimated to be ～ 2.5.     

Miniature Pipe Bundle Heat Exchanger for Thermophoretic Deposition of Ultrafine Soot Aerosol Particles at High Flow Velocities

The deposition of submicrometer soot aerosol particles in a miniature pipe bundle heat exchanger system has been investigated under conditions characteristic for combustion exhaust from diesel engines and oil or biomass burning processes. The system has been characterized for a wide range of aerosol inlet temperatures (390–510 K) and flow velocities (1–4 m s^?1), and particle deposition efficiencies up to 45% have been achieved over an effective deposition length of 27 cm. Thermophoresis was the dominant deposition mechanism, and its decoupling from isothermal deposition was consistent with the assumption of independently acting processes. The measured deposition efficiencies can be described by simple linear parameterizations based on an approximation formula for thermophoretic plate precipitators. The results of this study support the development of modified heat exchanger systems with enhanced capability for filterless removal of combustion aerosol particles.     

Development of a Portable Aerosol Collector and Spectrometer (PACS)

This article presents the development of a Portable Aerosol Collector and Spectrometer (PACS), an instrument designed to measure particle number, surface area, and mass concentrations continuously and time-weighted mass concentration by composition from 10?nm to 10?µm. The PACS consists of a six-stage particle size selector, a valve system, a water condensation particle counter to detect number concentrations, and a photometer to detect mass concentrations. The stages of the selector include three impactor and two diffusion stages, which resolve particles by size and collect particles for later chemical analysis. Particle penetration by size was measured through each stage to determine actual collection performance and account for particle losses. The data inversion algorithm uses an adaptive grid-search process with a constrained linear least-square solver to fit a tri-modal (ultrafine, fine, and coarse), log-normal distribution to the input data (number and mass concentration exiting each stage). The measured 50% cutoff diameter of each stage was similar to the design. The pressure drop of each stage was sufficiently low to permit its operation with portable air pumps. Sensitivity studies were conducted to explore the influence of unknown particle density (range from 500 to 3,000?kg/m³) and shape factor (range from 1.0 to 3.0) on algorithm output. Assuming standard density spheres, the aerosol size distributions fit well with a normalized mean bias of ?4.9% to 3.5%, normalized mean error of 3.3% to 27.6%, and R² values of 0.90 to 1.00. The fitted number and mass concentration biases were within ±10% regardless of uncertainties in density and shape. However, fitted surface area concentrations were more likely to be underestimated/overestimated due to the variation in particle density and shape. The PACS represents a novel way to simultaneously assess airborne aerosol composition and concentration by number, surface area, and mass over a wide size range.

Copyright © 2018 American Association for Aerosol Research     

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     

Inkjet Aerosol Generator as Monodisperse Particle Number Standard

The AIST-inkjet aerosol generator (IAG) can generate highly monodisperse solid or liquid aerosol particles in the particle diameter range from 0.3 to 20 μm at precisely known particle generation rates. The device has been developed for evaluating the counting efficiencies of optical and condensation particle counters. Particle generation efficiency of the IAG is defined as the number of aerosol particles generated by one voltage pulse sent to an inkjet head. The 95% confidence interval of the efficiency were 0.998 ± 0.006 within the 0.4 to 10 μm particle diameter range. The efficiencies remained close to unity when the droplet generation rates were within 20–500 s^?1 and 100–900 s^?1 using ultrapure-water and isopropyl alcohol (IPA) as the solvent of the inkjet solution, respectively. The operating aerosol flowrate range of the IAG is currently 0.5 and 1.0 L/min. The coefficients of variations (C.V.) of the size distributions were 2 to 3% indicating the generated particles were highly monodisperse. The generated particle sizes were defined as the volume equivalent diameter, D_ve. The uncertainty analysis on the factors affecting D_ve indicated that 95% confidence interval of the D_ve is expected to be ±5%. The uncertainty of D_ve was entirely caused by the uncertainty of the average mass of a droplet. The reproducibility of particle sizes within 0.5 to 10 μm was evaluated using an aerodynamic particle sizer. The C.V. of the measured particle sizes were less than 6% and 4% when NaCl particles and ionic liquid droplets were generated, respectively.

Copyright 2014 American Association for Aerosol Research     

Production of Well-Controlled Laminar Aerosol Jets and Their Application for Studying Aerosol Combustion Processes

Generation of steady-state solid aerosol jets with controllable parameters is often necessary in experimental studies and industrial processes. Most of the current approaches use a fluidized bed to produce an aerosol flow and always introduce initial turbulence into the jet. Toproduce a laminar aerosol jet, flow straighteners and long tubes are used that make the design cumbersome and inflexible. In addition, in a fluidized bed-type system, the aerosol number density and gas flow rate are inherently interdependent. In a new apparatus described in this paper, metal aerosol is produced using an electrostatic recharging of particles in a DC electric field of a parallel plate capacitor, a so-called electrostatic particulate method. The powder is aerosolized within the capacitor without using any gas flows and only a small velocity, a laminar gas jet is used to carry the aerosol away from the chamber through a small nozzle made in the top plate of the capacitor. It is shown that the aerosol number density is controlled by an electric field, independently of the gas flow rate. The usefulness and flexibility of the new technique for the aerosol combustion studies is demonstrated. Preliminary results on characterization of the produced small-scale, laminar, premixed, lifted aluminum-air flames are reported. The flame propagation velocities are measured and compared to the earlier results; overall flame dimensions and radiation profiles are determined. Individual particle flame zones are visualized in the aluminum-air aerosol flame for the first time.     

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     

Ash Vaporization in Circulating Fluidized Bed Coal Combustion

ABSTRACT

In this work, the vaporization of the ash forming constituents in circulating fluidized bed combustion (CFBC) in a full-scale 80 MW_th unit was studied. Ash vaporization in CFBC was studied by measuring the fly ash aerosols in a full-scale boiler upstream of the electrostatic precipitator (ESP) at the flue gas temperature of 125°C. The fuel was a Venezuelan bituminous coal, and a limestone sorbent was used during the measurements. The fly ash number size distributions showed two distinct modes in the submicrometer size range, at particle diameters 0.02 and 0.3 μm. The concentration of the ultrafine 0.02-μm mode showed a large variation with time and it decreased as the measurements advanced. The concentration of the 0.02-μm mode was two orders of magnitude lower than in the submicrometer mode observed earlier in the bubbling FBC and up to three orders of magnitude lower than in the pulverized coal combustion. Scanning electron micrographs showed few ultrafine particles. The intermediate mode at 0.3 μm consisted of particles irregular in shape, and hence in this mode the particles had not been formed via a gas to particle route. We propose that the 0.3-μm mode had been formed from the partial melting of the very fine mineral particles in the coal. The mass size distribution in the size range 0.01–70 μm was unimodal with maximum at 20 μm. Less than 1% of the fly ash particles was found in the submicrometer size range. Ninety percent of Mg in coal was organically bound, and it was found to react with quartz and aluminosilicate minerals inside the coal particle. No Mg was found to be released to the gas phase and Mg mass fraction size distribution was size independent. A fraction of halogens CI, Br and I were found to be in the gas phase after the combustion.     

Wall-to-bed heat transfer in liquid—solid and gas—liquid—solid fluidized beds part I: Liquid—solid fluidized beds

Experimental work was conducted to investigate the effect of particle size and particle density upon the wall-to-bed heat transfer characteristics in liquid—solid fluidized beds with a 95.6 mm column diameter over a wide range of operating conditions. The radial temperature profile was found to be parabolic, indicating the presence of a considerable bed resistance. The effective radial thermal conductivity and the apparent wall film coefficient were obtained on the basis of a series thermal resistance model. The modified Peclet number of the radial thermal conductivity decreases upon the onset of fluidization, has a minimum at a bed porosity of 0.6 to 0.7 and increases with further increase of bed porosity. The modified Peclet number decreases considerably with decreasing particle size or increasing particle density. The apparent wall heat transfer coefficient can be represented well by a Colburn j-factor correlation over a wide range of data as follows: j′_H = 0.137 Re′^?0.271 A close analogy is found to exist between the modified j-factor for wall heat transfer coefficient and that for wall mass transfer coefficient, in liquid—solid fluidized beds.     

惰性粒子流化床干燥钻井废泥浆实验研究

实验考察了惰性粒子流化床干燥钻井废泥浆,关联了干燥器的体积传热系数表达式,可定量计算进风速度、进风温度、废泥浆体积流量和含水量等参数的影响。结果表明,惰性粒子流化床干燥热效率达55％,体积传热系数可达6kW·m^-3·K^-1,可用于钻井现场柴油机废气干燥钻井废泥浆。激光粒度分析仪测试干燥产物粒度平均值为12／μm,且分布较为集中,其密度为3．2g·cm^-3,可以回用于钻井泥浆的加重材料。浸毒试验表明,干燥产物CODCr值大幅度下降。     

Physical Characterization of Cigarette Smoke Aerosol Generated from a Walton Smoke Machine

Mainstream cigarette smoke generated using a Walton smoking machine and Kentucky 2R1 research cigarettes was studied. Results showed that puff volume and total particulate matter were consistent after the first puff, with average values of 35.6 cm³ and 3.37 mg, respectively. The particle size distribution, measured with a multijet cascade impactor, was not related to butt length or relative humidity (≤95%), but was strongly dependent on the aging time. Based on simple monodisperse coagulation, the mass median aerodynamic diameter was calculated to be 0.45 μm at a dilution ratio of 21.7. Using a technique based on the dimensional change of collected droplet particles at various viewing angles of a scanning electron microscope, the count median diameter was estimated to be 0.22 μm. These values were in good agreement with those reported by others. The results suggest that there is a dilution value critical to the rapid evaporation and final particle size of the cigarette smoke aerosol. Once reaching this value, further dilution has little effect on the final particle size. By using the derived mass concentration and size distribution, the particle density, number concentration, and coagulation coefficient of the cigarette smoke aerosol were estimated to be 1.12 g/cm³, 7.20 × 109 particles/cm³, and 6.64 × 10 ^?10 cm³/s respectively. Solid particles > 1 μm were found in the first few puffs. These were considered to consist of tobacco debris.     

Calibration and Testing of Samplers with Dry,Polydisperse Latex

A new method for measuring the collection efficiency of an aerosol sampler as a function of particle size has been developed, featuring the use of dry, polydisperse latex particles. Test aerosol is generated by placing a polydisperse latex powder sample into a fluidized bed of glass beads. An Aerodynamic Particle Sizer (APS) measures the particle size distribution entering and leaving the sampler's size-selector, yielding the penetration efficiency. The use of dry latex minimizes the ''phantom'' particle problem inherent with the APS by avoiding the generation of high concentrations of small particles such as those produced by nebulizers. In addition to having useful properties for determining particle size cutoff characteristics, including spherical shape, near-unit density, and white color, latex particles afford a test for the presence of particle bounce and reen trainment. A complete efficiency measurement can be made in a little over three minutes, facilitating experimentation with parameters such as sampler flow rate, which require repeated measurements. The method has been used extensively for the development and calibration of respirable and PM-2.5 samplers.     

Particle characteristics of a stable fluidized bed aerosol generator

An aerosol generator consisting of a vibrating system for feeding dust into a fluidized bed was developed and tested to determine its dust output characteristics. The dust feed unit can produce 0–40 g min⁻¹ of coal dust and shows constant output up to 3 h operation durations. These correspond to mass concentrations of 0–101 g m⁻³ of coal particles for an air flowrate of 395 l min⁻¹ through the aerosol generator. The aerosolized coal particles show constant particle size distribution with time for up to h of testing under varied operation parameters. The normalized particle size distribution remains almost identical for a given feed material for a range of dust loadings. The time required to reach steady state aerosol generation is negligible for the sizes of coal particles used in this investigation.     

Quantitative Photoelectron Spectroscopy (ESCA) of Sulfate Aerosol on Filtration Media

Photoelectron spectroscopy (ESCA) was applied to sub-micron sodium sulfate particles deposited on different filter surfaces. Only flat membrane filters were found to be suitable for aerosol analysis by ESCA. A linear dependence of ESCA signal on aerosol mass was observed for different polydisperse mass loadings on Nuclepore filters. We have also performed a systematic study of the relation between ESCA signal and particle diameter (30–220 nm). Lower detection limits range between 25 and 100 ng SO² ₄ per 0.25-cm² analyzed filter area, depending on particle radius and/or size distribution. We discuss future applications of ESCA in aerosol analysis.     

Factors Affecting the Collision of Aerosol Particles with Small Water Drops

The factors influencing the collision of aerosol particles with small water drops at low collision efficiencies are examined. The gravitational force and velocity slip of air on the drop surface are found to affect the collision efficiency in the range of values of 10^?4–^?2. The efficacies of the different computational models are compared for ratios of particle radius to drop radius of less than 0.1. The accuracy of the numerical scheme in the trajectory model can be verified by comparing the efficiencies obtained for submicrometer particles with the convective-diffusion model.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

Fluidization Characteristics of a Fine Magnetite Powder Fluidized Bed for Density-Based Dry Separation of Coal

Gas-solid fluidized bed separation technique is very beneficial for saving water resources and for the clean utilization of coal resource. The hydrodynamics of 0.15–0.06 mm fine Geldart B magnetite powder were experimentally and numerically studied to decrease the lower size limit. The results show that the static bed height should be controlled near 300 mm (e.g., 300–350 mm). The bubble size, amount, and frequency of the fine particle bed are smaller than those of the bed containing 0.3–0.15 mm large Geldart B particles, thus leading to a higher bed activity. The pressure drop and density of the fine particle bed are uniform and stable, which indicates a good fluidization quality. Furthermore, simulated results are consistent with experimental data, which indicates the correctness and effectiveness of the simulations. The superficial gas velocity should be adjusted to not more than 1.8U _mf for the fine particle bed. Additionally, wide size range magnetite powder, which contains 94.23 wt% < 0.3 mm particles with a 0.3–0.06 mm particles content of 91.38 wt%, was used in an industrial scale modularized demonstration system for 50-6 mm coal density separation. The ash content of feed coal was reduced from 55.35% to 14.67% with a probable error, E, value of 0.06 g/cm^3.     

Monodisperse fine aerosol generation using fluidized bed

Monodisperse, fine aerosols are needed in many applications: filter testing, experiments for testing models, and aerosol instrument calibration, among others. Usually, monodisperse fine aerosols are generated in very low concentrations, or mass flow rates, in the laboratory scale. In this work, we needed to generate aerosols with higher mass flow rate than typically available by the laboratory-scale methods, such as atomizers, nebulizers, ultrasonic generators, vibrating orifice generators, and condensation generators. Therefore, we constructed a fluidized bed aerosol generator to achieve particle mass flow rates in the range of 15-100 g/h. Monodisperse, spherical SiO₂ particles of two sizes with geometrical diameters of 1.0 and 2.6 µm were used in the aerosol generator. The aerosol generator was used at both atmospheric pressure, and at high pressures up to 5 bar (abs).The particle size, mass concentration and the net average particle charge were measured after mixing the aerosol with nitrogen. The particle size distributions with both particle sizes were monodisperse, and no particle agglomerates were entrained from the fluidized bed. The behavior of the fluidized bed generator was found to be markedly different with the two particle sizes in regard to particle concentration, presumably due to different particle charging inside the generator. After determining the net average charge of the particles, an ion source Kr-85 was used to reduce the charge of the particles. This was found to be effective in neutralizing the particles.     

Influence of high density particles in reduction of axial dispersion in liquid fluidized bed with residence time distribution curve studies

iquid phase RTD curves were investigated in classical fixed and fluidized bed regimes with high density particles. The effect of liquid velocity was studied on bed hydrodynamics. Using an impulse tracer injection technique in a column of 5 cm inner diameter and 1.2 m height, liquid RTD, mean residence time (MRT), axial dispersion coefficient (ADC) and vessel dispersion number (N _D ) were determined. ADC increases with liquid superficial velocity. It varied from 4.63 to 20.7 cm²/s for the particle Reynolds number of 43 to 279, respectively. The experimental results show that the hight density particles cause less ADC than the low density particles at an identical Reynolds number.