An Experimental Investigation for Agglomeration of Aerosols in Alternating Electric Fields

Electrostatic precipitators have a relatively low efficiency for the collection of submicron particles. One way to increase their efficiency is to enforce the agglomeration and thereby form larger particles. In this work, a study has been initiated for enhancing the agglomeration between oppositely charged particles by using an alternating electric field to increase the relative motion between such particles. A simple first order computer model was developed for establishing the magnitude of agglomeration and for characterizing the interdependence of different parameters of importance. A laboratory experimental unit was established for studying various practical configurations for agglomeration in alternating electric field, using finely dispersed limestone powder. For the measurements of particle size distributions, a Berner Low Pressure Impactor was used. The preliminary results have shown a 50% reduction in the mass concentration of submicron particles, for oppositely charged particles in an alternating electric field. On the other hand, no agglomeration was measured by the use of a quadrupole field.     

Unipolar Charging of Fine and Ultra-Fine Particles Using Carbon Fiber Ionizers

A simple and novel unipolar charger using carbon fiber ionizers was developed to effectively charge fine and ultra-fine aerosol particles without the generation of ozone. The particle penetration in the charger was investigated for non-charged, neutralized, and singly charged particles in the size range of 20–200 nm. Particle loss and the intrinsic, exit and extrinsic charging efficiencies of fine and ultra-fine particles were also investigated for non-charged particles at different applied voltages to the charger. Particle penetrations in the charger were nearly 100% for particles larger than 20 nm, irrespective of the initial particle charging state. Particle losses in the charger could be decreased by decreasing the applied voltage to the charger from 4.0 kV to 2.3 kV. The intrinsic charging efficiencies were proportionally increased with the applied voltage, whereas the exit charging efficiencies were almost independent of the applied voltage. Therefore, the extrinsic charging efficiency of the charger becomes higher for the lower applied voltage (2.3 kV), at which about 60% of 20 nm particles were charged. Little (less than 4 ppb) to no ozone was generated under all operation conditions. It can be concluded that the newly developed unipolar charger using carbon fiber ionizers can charge fine and ultra-fine particles at least as effectively as currently available unipolar chargers, but with the major advantage of negligible ozone generation, a highly desirable feature if the charged particles are to be used for chemical or biological analysis.     

声波与喷雾对于飞灰颗粒团聚的影响

为了研究声波团聚的影响因素,以燃煤飞灰细颗粒作为声波团聚的实验对象,使用光学颗粒物粒径谱仪测量颗粒的粒径分布与浓度,主要研究了声波频率与喷雾对声波团聚的影响。结果表明：在声波的作用下,细颗粒浓度显著减少,且声波团聚效果对频率较为敏感;无论在高声压还是低声压级下,1400Hz的频率下能获得最佳的团聚效果;在加入喷雾后,颗粒物浓度显著减小,且随着喷雾量增大,颗粒物浓度越小;分析了喷雾增强团聚效果的机理：在加入喷雾后,细颗粒间的相对运动增强;同时喷雾颗粒增大了颗粒浓度,增大了细颗粒碰撞概率;此外,喷雾改变燃煤飞灰细颗粒的表面特性,使颗粒的表面黏性增大,有助于团聚体形成。     

Fundamental Study of a Miniaturized Disk-Type Electrostatic Aerosol Precipitator for a Personal Nanoparticle Sizer

We have developed a low-cost, miniaturized disk-type electrostatic aerosol precipitator for a personal nanoparticle sizer, often needed in applications requiring spatially distributed measurement or personal exposure monitoring. The performance of prototype mini-disk precipitator was evaluated in this study. Measurement of particle transmission through the precipitator for both neutral and singly charged particles shows that the compact size of the disk precipitator does not lead to serious particle loss resulting from particle diffusion and/or electrical image force. The transmission of singly charged particles of 10 nm is 64% at an aerosol flowrate of 0.3 lpm. The device consists of two precipitation chambers, separated by a metal disk. The design further allows the device to be configured to precipitate charged particles by establishing electrical fields in one or both precipitation chambers. Both operations work well to precipitate particles by electrical mobility. The operation of dual-chamber precipitation, with electrical field established on both sides of the middle disk, is preferred since it lowers the maximum requirement of applied voltage to precipitate particles with a specific electrical mobility for a given flowrate. Semi-empirical models were also developed to describe the dependence of the particle penetration curves on particle electrical mobility.     

燃煤飞灰低频下声波团聚的实验研究

通过实验研究了低频下燃煤飞灰的声波团聚。燃煤飞灰气溶胶在声波作用下粒径分布发生明显变化,大量细颗粒团聚产生粗颗粒,气溶胶浓度降低。在147 dB、1400 Hz时,气溶胶总浓度和PM_2.5浓度分别减少了68.4%和75.6%,达到很好的团聚效果。从扫描电镜照片中可以观察到声波团聚过程中产生了粒径大于10 μm的链状团聚体。在声波作用下,气溶胶总浓度随团聚时间呈指数衰减规律降低。实验证明了声波团聚中存在最佳频率,在此频率下团聚效果最好,偏离该值团聚效果急剧降低。另外,实验发现最佳频率随声压级的增大而轻微降低。同时,研究了声压级、停留时间和气溶胶初始浓度等参数对声波团聚的影响。     

Aerosol Charge Neutralization by a Mixing-Type Bipolar Charger using Corona Discharge at High Pressure

An aerosol neutralizer called the Mixing-type Bipolar Charger using Corona-Discharge at High Pressure (MBCCHP) was developed. In the MBCCHP, a corona discharge (High-Pressure Corona Ionizer; HPC Ionizer) induced by high frequency voltage (>100 Hz) at high pressure (>0.2 MPa) is used to generate bipolar ions at high concentration (1–3 × 10⁹ ions/cm³) that are then mixed with aerosol particles flowing in a charging chamber where no external electric field is present. The charging performance of the MBCCHP was evaluated by comparing the measured and theoretical number ratios of positively and negatively charged particles to the total number of particles, and by comparing those of negatively charged to positively charged particles for an equilibrium charge distribution. The theoretical and measured results agreed well in the particle size range of 5–80 nm. Particle loss in the MBCCHP for the size range of 5–100 nm was less than 15%, and particle generation from the electrode due to spattering or from the carrier gas containing SO_x due to chemical reaction was either negligible or not observed. The MBCCHP can effectively provide aerosol particles in the equilibrium charge state. Advantages include (1) no selective deposition of charged particles by an electric field, (2) no generation of new particles by reactive molecules, such as atmospheric pollution gases contained in a sample aerosol by chemical reactions with active species, such as OH radicals, produced by discharge, and (3) no effect of carrier gases of the sample aerosol on the ion properties.     

细颗粒物凝并技术机理的研究进展

凝并技术是提高烟气中细颗粒物(PM2.5)去除效率的关键技术之一。凝并机理的研究有利于加深对细颗粒物凝并过程的理解,最大限度地提高PM2.5的凝聚速度,使PM2.5在较短的时间内团聚成大颗粒。本工作对电凝并、化学凝并和声凝并3种凝并效果显著的凝并技术机理进行概述,分别介绍了电凝并机理的核心电凝并系数方程,不同化学添加剂对颗粒的作用机制,同向运动、流体力学和声致湍流作用下的声凝并机理的发展现状。阐述了现有研究的不足,并提出在后续凝并机理的研究中,可利用高速显微摄像技术实时观测颗粒的凝并过程,对已有凝并机理进行验证及修正。同时还需考虑实际烟气成分对颗粒凝并的影响,进一步完善颗粒的凝并机理。     

Population balance modelling for a flow induced phase inversion based granulation in a two-dimensional rotating agglomerator

A novel two-dimensional rotating agglomerator was developed to carry out the flow induced phase inversion (FIPI) based granulation. The process in this agglomerator shows that a continuous paste flow (mixed with liquid binder and primary particles) is extruded into the interstice of two relatively rotating disks, as the paste becomes solidified due to the loss of heat to the disks, it is then broken into granules by the shearing force imposed by the rotating disk. Experimental measurements have shown that the size of these granules is enlarged along the positive radial direction of the disks. It is also found that these granules contain approximately the same quantity of binder in terms of its volume fraction. The paper thus proposes a population balance (PB) model to describe the growth of the granules by considering a size independent agglomeration kernel. The PB simulated results are found to be well capable of describing the change of the particle size distribution (PSD) of the granules in the radial direction. This study also proposes a velocity profile for the paste flow and attempts to establish a quantitative relationship between the granulation rate and the deformation rate as this would help us understand the mechanism of the agglomeration. It is hoped that this study would be used to improve the design of the agglomerator and to assure the control of the process and the granular product quality.     

Coincidence in Time-of-Flight Aerosol Spectrometers: Phantom Particle Creation

When using time-of-flight aerosol spectrometers, particle size measurement is based upon a particle's transit time between two laser beams. The particle's transit time is assumed to be the time difference between the two pulses of light that are produced as the particle passes through the two laser beams. Particle coincidence, which occurs when a second particle crosses the first laser beam before the first particle crosses the second laser beam, has a complex effect upon the measured size distribution. As a result of coincidence, time-of-flight aerosol spectrometers can replace real particles of one size with spurious, or phantom, particles of a different size in the measured distribution. When partial detection of a particle occurs, i.e., only one pulse from a particle is detected, another particle producing a pulse that occurs while the timer is open can cause the recording of a randomly sized phantom particle. The creation of these phantom particles, which we termed “open-timer” phantom particles, has been investigated theoretically and experimentally in a commercially available time-of-flight aerosol spectrometer. The number of these open-timer phantom particles was found to increase with particle size and aerosol concentration. In addition, the instrument's detection logic affects the number and size of the phantom particles. These are most apparent in the tails and minima of the measured distribution. In order to minimize phantom particle creation, the concentration of partially detected particles must be minimized. Strategies to reduce phantom particle concentration involve reducing the concentration of small particles near the optical detection threshold of the spectrometer.     

Towards traceable particle number concentration standard: Single charged aerosol reference (SCAR)

A concept of realizing a standard for aerosol particle number concentration was tested, based on generating singly charged aerosol particles in the size range from 10 up to 500 nm. To this end, a device named single-charged aerosol reference (SCAR) was designed, built, and tested. The device is based on electrical charging of nanoparticles and subsequent growth of the particles. With an accurate measurement of volume flow and electrical current from the singly charged particles, the number concentration can be accurately, and in the end, traceably determined. Laboratory tests have shown that the device can be used to generate a narrow (GSD<1.3) particle size distribution of singly charged particles. The device can be used for traceable calibration of instruments measuring the number concentration of the particles.     

Direct Transfer of Gas-Borne Nanoparticles into Liquid Suspensions by Means of a Wet Electrostatic Precipitator

The direct transfer of flame-synthesized aerosols of silica nanoparticles into aqueous suspensions is investigated. Silica nanoparticle aerosols with production rates of 0.5 g/h and different mean diameters and degrees of agglomeration are transferred into liquid suspensions by means of a novel wet electrostatic precipitator. Particle collection efficiencies above 99.999% were measured. The influence of the transfer on the particle size distribution was investigated by comparison of aerosol and suspensions size measurements. Aerosol sizes were measured with the scanning mobility particle sizer (SMPS), and suspension size measurements were conducted by dynamic light scattering (DLS) and by SMPS measurements of the aerosolized suspension employing a novel nebulizer. Depending on the aerosol and stabilization conditions, particle transfer with nearly no influence on the particle size distribution is possible. Suspensions generated from the same particle aerosol by direct transfer and by sonication of the respective powder were compared. In contrast to the direct transfer, the aerosol particle size distribution could not be restored by ultrasonication.

Copyright © 2015 American Association for Aerosol Research     

Ozone-free post-DBD aerosol bipolar diffusion charger: Evaluation as neutralizer for SMPS size distribution measurements

A postplasma neutralizer for submicron particles size measurements by mobility analysis has been evaluated. Bipolar ion currents have been measured downstream a dielectric barrier discharge (DBD) to estimate the ion fluxes at the inlet of charging volume and the n_i·τ product that define the theoretical maximal concentration that can be neutralized. Charge distributions were measured versus DBD voltage, aerosol diameter and concentration for monodisperse aerosols. It is confirmed that the charge distribution of particles depends on the ratio of initial positive and negative ion currents controlled by the DBD voltage leading to a tuneable mean charge of aerosol in this post-DBD bipolar charger. As expected from Gunn's law, the mean charge and the variance are proportional to particle diameter above 50 nm and independent of the aerosol concentration. The size distributions measured with ⁸⁵Kr and post-DBD neutralizer present the same modal diameters and a maximal overestimation of the total concentration of 10%, for aerosol from 15 to 730 nm with concentrations up to 6 × 10¹² m^?3. This post-DBD bipolar charger can be used for submicron aerosol neutralization and thus for scanning mobility particle sizer size distribution measurements in air as well as in nitrogen to suppress ozone downstream DBD.

Copyright © 2017 American Association for Aerosol Research     

A Potential Soot Mass Determination Method from Resistivity Measurement of Thermophoretically Deposited Soot

Miniaturized detection systems for nanometer-sized airborne particles are in demand, for example in applications for onboard diagnostics downstream particulate filters in modern diesel engines. A soot sensor based on resistivity measurements was developed and characterized. This involved generation of soot particles using a quenched co-flow diffusion flame; depositing the particles onto a sensor substrate using thermophoresis and particle detection using a finger electrode structure, patterned on thermally oxidized silicon substrate. The generated soot particles were characterized using techniques including Scanning Mobility Particle Sizer for mobility size distributions, Differential Mobility Analyzer—Aerosol Particle Mass analyzer for the mass–mobility relationship, and Transmission Electron Microscopy for morphology. The generated particles were similar to particles from diesel engines in concentration, mobility size distribution, and mass fractal dimension. The primary particle size, effective density and organic mass fraction were slightly lower than values reported for diesel engines. The response measured with the sensors was largely dependent on particle mass concentration, but increased with increasing soot aggregate mobility size. Detection down to cumulative mass as small as 20–30 μg has been demonstrated. The detection limit can be improved by using a more sensitive resistance meter, modified deposition cell, larger flow rates of soot aerosol and modifying the sensor surface.     

Measurement of the Aerodynamic Drag Force on Single Aerosol Particles

The “picobalance” (quadrupole) was used to measure the aerodynamic drag force on individual solid particles and droplets by suspending the object in a laminar jet of gas introduced through the bottom electrode. Particles ranging in diameters from about 1 to 150 μm can be studied in this manner. The DC voltage required to maintain the particle position against the opposing forces of aerodynamic drag and gravity was measured to determine the drag force. The flow velocity at which the aerodynamic drag force balances the gravitational force yields information on the aerodynamic size, and the DC voltage required to suspend the particle against gravity with no flow provides a measure of the particle mass. Particle mobilities for spherical and irregularly shaped solids are presented. Light-scattering measurements for spherical particles provide an independent determination of size; the results are generally in good agreement with the aerodynamic size. It is shown that the electrodynamic balance can be used to measure drag forces much larger than the particle weight.     

Absolute Mass and Size Measurement of Monodisperse Particles Using a Modified Millikan's Method: Part I—Theoretical Framework of the Electro-Gravitational Aerosol Balance

A new method for accurate mass and size measurement of monodisperse particles is proposed. In this method, charged aerosol particles are introduced into parallel plate electrodes similar to the Millikan cell, and the number of particles left suspended after a certainty holding time has elapsed is measured. The particle survival rate as a function of the voltage applied to the electrodes is used to determine the particle mass. The particle size is deduced by using the particle density which is determined in a separate experiment. The expression of the particle survival function, which is defined as the survival rate as a function of the mass, for particles with and without Brownian diffusion is derived. The sensitivity of this method to the number average diameter, as well as other size distribution parameters, is analyzed on the basis of the survival function.     

COUNTING EFFICIENCY OF THE API AEROSIZER

The Aerosizer (Amherst Process Instruments, Inc. Hadley MA) is a time-of-flight instrument frequently used to measure the size distribution of an aerosol. However, if the Aerosizer’s counting efficiency, defined as the number of particles counted divided by the total number entering the instrument, is not 100% or varies with particle size, the resulting size distribution will be inaccurate.Experiments were conducted to determine the effect of particle diameter, particle concentration, photomultiplier tube (PMT) voltage, and model type on the Aerosizer’s counting efficiency. To calculate counting efficiency, the number of particles between 0.3 and 10 μm recorded by the Aerosizer was divided by the number of particles of the same size collected on each stage of a cascade impactor.Particle diameter, aerosol concentration, Aerosizer model, PMT voltage, and the diameter interaction terms influenced counting efficiency. Counting efficiencies were less than 1% for particles smaller than 0.45 μm, and more than 100% for particles larger than 7 μm. Increasing the PMT voltage increased the counting efficiency for the smaller particles, but also created false, larger particles. Counting efficiency decreased as count rate increased for count rates greater than 20,000 particles per second. The Aerosizer LD counted particles more efficiently than the Aerosizer Mach 2 because of improved laser and optics systems. Four regression models that relate counting efficiency to the salient operating parameters were developed, one for each combination of Aerosizer model and photomultiplier tube voltage studied.     

Experimental characterization of a short electrical mobility spectrometer for aerosol size classification

A prototype of a short column electrical mobility spectrometer (EMS) for size measurement of aerosol particle was design, constructed, and experimentally characterized. The short EMS consists of a particle charger, a size classifier column, and a multi-channel electrometer. Its particle size resolution is derived from a 10 channel electrometer detector. The short EMS is capable of size measurements in the range between 10 nm to 1,000 nm with a time response of about 50 s for full up and down scan. Particle number concentration in which the short EMS can measure ranges from 10¹¹ to 10¹³ particles/m³. The operating flow rate of the short EMS is set for the aerosol flow rate of 1.0–2.0 l/min and the sheath air flow rate fixed at 10.0 l/min. The inner electrode voltage of the classifier can be varied between 500–3,000 VDC. The short EMS operates at sub-atmospheric pressure, typically at 526 mbar. Validation of the short EMS performance was performed against a scanning electron microscope (SEM). Good agreements were obtained from comparison between sizes determined from the short EMS classifier and the SEM analysis. Signal current from the detector was also analyzed to give rise to number concentration of particles. Experimental results obtained appeared to agree well with the theoretical predictions.     

Effect of selective catalytic reduction on exhaust nonvolatile particle emissions of Euro VI heavy-duty compression ignition vehicles

The nonvolatile particle number (PN) emissions of late technology diesel heavy-duty vehicles (HDV) are very low due to the introduction of Diesel Particulate Filters (DPF). Nevertheless, a large fraction (50%) of particles below the current lower regulated size (23?nm) was recently reported. Moreover, large differences between laboratory and PN Portable Emission Measurement Systems (PN-PEMS) have been observed. In order to better understand such differences, the physical properties of the exhaust aerosol from two Euro VI technology diesel heavy-duty engines were studied. It was found that urea injection leads to formation of nonvolatile particles. The produced particles covered a wide size range spanning from below 10?nm to above 100?nm. As such, they contribute to the regulated PN emissions, with measured concentrations corresponding to as high as 2?×?10¹¹ #/kWh over a World Harmonized Transient Cycle (WHTC). However, a large fraction of them was undetected owing to their small particle size. Low-cutoff size (10?nm) Condensation Particle Counters (CPCs) (which are under discussion to be included in the regulations) measured up to twice as high concentrations. Considering the large particle losses in the sampling systems at this size range, the true concentrations can be two times higher from what the low-cutoff CPCs reported. When the temperature of the SCR system exceeded a threshold of 300?°C, the produced particles were found to be positively charged, increasing the average exhaust aerosol charge up to +3 elementary charges per particle. Scanning Mobility Particle Sizer (SMPS) measurements of non-neutralized samples revealed that even the smallest of them can carry more than one positive charge. The findings of this study can explain the differences reported between PEMS and laboratory systems and especially those based on diffusion charging. They also provide insight for a refinement of technical requirements prescribed in the European PEMS regulation to more accurately quantify the PN emissions from such technologies.     

Particle size analysis of dioctyl phthalate aerosols using the stöber aerosol spectrometer

Particle size distributions of nearly monodisperse dioctyl phthalate aerosols (dia. between 0–5 and 1–4 μm) have been determined using the Stöber aerosol spectrometer. The particle size distributions can be approximated very well by bimodal distribution functions. From a statistical analysis it turned out that the accuracy of the approximation is limited in case of small particles (dia. ～ 0·5 μm). This is due to evaporation of the particles during the analysis.The mean of the particle size distribution determined with the Stöber aerosol spectrometer was in fair agreement with the particle diameter determined with the higher order Tyndall spectrometer.     

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.