Electrostatic Precipitation of Ultrafine Particles Enhanced by Simultaneous Diffusional Deposition on Wire Screens

Abstract

A laboratory-scale electrostatic precipitator has been designed and constructed in which the grounded collector plate has been substituted by a set of wire screens placed perpendicularly to the gas flow. Particles are deposited onto the screens by two mechanisms—electrostatic deposition and diffusional deposition—which act simultaneously. On the one hand, electrostatic deposition is effective for relatively large particles, but it is quite ineffective for the smallest ones because their charging probability in the corona field is too low. On the other hand, the diffusional collection efficiency of particles on fibers is high for small particles but low for the larger ones. Therefore, the simultaneous diffusional-electrostatic precipitation may become a useful technique for efficient filtration of particles below 0.1 μm. A preliminary experimental evaluation of this filtering device has shown that submicrometer particles with diameters down to a few nanometers can be collected with number efficiencies greater than 99%.     

Design and Testing of a New Size Classifying Isokinetic Sequential Aerosol Sampler

Two versions of a size-classifying isokinetic sequential aerosol sampler (SCISAS) have been designed, built, tested, and deployed in a field program in the southwestern United States. The SCISAS units can operate at unattended sites, exposing four or more filter types simultaneously, in two size ranges, for six sampling time Intervals. Design considerations included theoretical estimates of aerosol particle losses in the 0-15 μm size range.

SCISAS prototypes have been tested to evaluate their sampling efficiency as a function of flow rate, the sensitivity of the sampling efficiency to isokinetic matching within the SCISAS sampling stack, the equivalency of their sampling ports, and their passive deposition characteristics. The prototypes were also compared to several other types of aerosol filtration samplers already in common use. These tests show that particle loss mechanisms within the SCISAS usually cause no more than 5 percent losses, and that the SCISAS units agree, within one to two measurement uncertainty intervals, with other types of aerosol samplers.     

Electrostatic precipitation of ultrafine particles enhanced by simultaneous diffusional deposition on wire screens

A laboratory-scale electrostatic precipitator has been designed and constructed in which the grounded collector plate has been substituted by a set of wire screens placed perpendicularly to the gas flow. Particles are deposited onto the screens by two mechanisms--electrostatic deposition and diffusional deposition--which act simultaneously. On the one hand, electrostatic deposition is effective for relatively large particles, but it is quite ineffective for the smallest ones because their charging probability in the corona field is too low. On the other hand, the diffusional collection efficiency of particles on fibers is high for small particles but low for the larger ones. Therefore, the simultaneous diffusional-electrostatic precipitation may become a useful technique for efficient filtration of particles below 0.1 microm. A preliminary experimental evaluation of this filtering device has shown that submicrometer particles with diameters down to a few nanometers can be collected with number efficiencies greater than 99%.     

Feasibility of granular bed filtration of an aerosol of ultrafine metallic particles including a pressure drop regeneration system

A process for filtering an aerosol of ultrafine metallic particles (UFP) has been designed and tested, based on the principle of a multistage granular bed. The filtration system comprised a succession of granular beds of varying thickness composed of glass beads of different diameters. This system allows the pressure drop to be regenerated during filtration (“on-line” mode) using a vibrating probe. Tests monitoring the pressure drop were conducted on a “10-L/min” low airflow rate device and on a “100-m³/hr” prototype. Granular bed unclogging is automated on the latter. The cyclic operation and filtration performances are similar to that of filter medium-based industrial dust collectors.

Implications:?Filtration of ultrafine metallic particles generated by different industrial processes such as arc welding, metal cutting, or spraying constitutes a difficult problem due to the high filter clogging properties of these particles and to the high temperatures generally encountered. Granular beds represent an advantageous means of filtering these aerosols with difficult properties.     

Preparation of electrospun polyurethane filter media and their collection mechanisms for ultrafine particles

Electrospinning is a simple and versatile process to produce polymer nanofibers, which are useful for ultrafine particle filtration. In this study, a polyurethane filter with an average fiber diameter of 150–250 nm was prepared through the electrospinning process and its filtration characteristics were investigated. We found that the electrospun fiber diameter was highly dependent on the polyurethane concentration, electric field, and tip-to-collector distance. As the polyurethane concentration, electric field, and tip-to-collector distance under the same electric field increased, the fiber diameter increased. We also found that the produced filter media had a minimum collection efficiency at particles sizes from 80 to 100 nm, which implies an electrostatic attraction between the filter and the test particles. Furthermore, we observed that interception was a predominant collection mechanism at Peclet numbers higher than 10 in nanofiber filtration for ultrafine particles.

Implications:

A polyurethane nanofiber filter with excellent mechanical properties was prepared, and the effect of operating conditions on fiber morphology was examined. The filter fabricated by an electrospinning process is charged and has high filtration efficiency due to electrostatic force. Therefore, it can be a good alternative to control hazardous ultrafine particles.     

Low-resistivity Related ESP Performance Problems in Dry Scrubbing Applications

This paper describes the evaluation of the performance of ESPs operating downstream of spray dryers in high- and medium-sulfur coal flue gas streams. Tests were conducted at the TV A10 MW Spray Dryer/ESP Pilot Plant and the EPRI High Sulfur Test Center. The results of the analysis of particle characteristics, spray dryer operating parameters, and ESP operating variables identify the occurrence of severe particle reentrainment due to the low resistivity (10⁸ ohm-cm and lower) of the sorbent/flyash mixtures at low approach-tosaturation temperatures. The reentrainment has a significant impact on the collection efficiency of ESPs which could represent a fundamental limitation on their ability to adequately perform in this environment. Although this program has been focused on spray dryer applications, because of the similarities of the gas and particle characteristics produced from spray drying and other dry scrubbing processes, the results also have implications to duct slurry injection, dry sorbent injection with humidification, and processes involving furnace sorbent injection with humidification.

The performance characteristics of the ESPs are presented under both baseline and spray dryer conditions. The results are analyzed and the Southern Research Institute ESP Computer Model was used to evaluate the data. Special techniques for measuring particle resistivity at these conditions are described. A theoretical examination of particle reentrainment was undertaken which indicated that at low-resistivity levels the electrostatic forces reverse and tend to pull the particles off the plates with a force proportional to the square of the electric field. This repulsion of particles from the plates at spray dryer conditions was confirmed by laboratory experiments. Chloride content of the coal was found to be an important parameter effecting the performance of the ESP. Implications of the results of this evaluation relative to ESP upgrades are presented.     

Modeling PAH uptake by vegetation from the air using field measurements

We examined PAH uptake by Norway spruce needles following the emergence of new buds in spring 2004–June 2005. Atmospheric PAH concentrations (gaseous phase and particle-bound) were monitored during this period, and PAH concentrations from these three environmental media were then used to calculate deposition and transfer velocities. Benzo(a)pyrene was found almost exclusively associated to particles and thus was used to determine a particle-bound deposition velocity of 10.8 m h^?1. PAHs present in both compartments had net gaseous transfer velocities ranging from negligible values to 75.6 m h^?1 and correlated significantly with log K_OA. The loss velocities thereafter calculated were found to be higher for more volatile PAHs. Using the calculated average atmospheric PAH concentrations and deposition velocities, it was thus possible to model PAH uptake by vegetation through time. We demonstrate that this approach can be used to determine deposition velocities without the use of a surrogate surface. In considering both particulate-bound and gaseous deposition processes this model can be used not only to study air–foliage exchange of semi-volatile organic compounds, but also to illustrate the relative contribution of gaseous deposition and particulate-bound deposition in the overall atmospheric vegetation uptake of semi-volatile organic compounds.     

Collection of Aerosol Particles by Electrostatic Droplet Spray Scrubbers

Theoretical calculations and experimental measurements show that the collection of small aerosol particles (0.05 to 5 micron diameter range) by water droplets in spray scrubbers can be substantially increased by electrostatically charging the droplets and particles to opposite polarity. Measurements with a 140 acfm two chamber spray scrubber (7 seconds gas residence time) showed an increase in the overall particle collection efficiency from 68.8% tit uncharged conditions to 93.6% at charged conditions, with a dioctyl phthalate aerosol (1.05 μm particle mass mean diameter and 2.59 geometric standard deviation). The collection efficiency for 0.3 μm particles increased from 35 to 87% when charged. During 1973–1974 a 1000 acfm pilot plant electrostatic scrubber was constructed inside a 40 ft trailer for evaluation on controlling particu-late emissions from pulp mill operations (funded by Northwest Pulp and Paper Association). Field tests performed on the particle emissions exhausting from SO₂ absorption towers treating the gases from a magnesium based sulfite recovery boiler have shown particle collection efficiencies ranging from about 60 to 99% by weight, depending on the electrostatic scrubber operating conditions. Energy requirements for the University of Washington electrostatic scrubber are about 0.5 hp/1000 acfm (350 Watts/1000 acfm) including gas pressure drop, water pressure drop, and electrostatic charging of the water spray droplets and the particles.     

接触絮凝研究进展

接触絮凝的概念源于直接过滤 ,是指介质存在时的絮凝作用。由于简便易行、处理效果良好 ,接触絮凝方法处于迅速发展之中 ,并且具有广泛的应用前景。其研究范围包括物理、化学、胶体界面过程等相互作用。本文以溶液中的颗粒物相互作用为基础 ,对接触絮凝理论的发展加以简要概述     

Empirical correlations to estimate agglomerate size and deposition during injection of a polyelectrolyte-modified Fe0 nanoparticle at high particle concentration in saturated sand

Controlled emplacement of polyelectrolyte-modified nanoscale zerovalent iron (NZVI) particles at high particle concentration (1-10 g/L) is needed for effective in situ subsurface remediation using NZVI. Deep bed filtration theory cannot be used to estimate the transport and deposition of concentrated polyelectrolyte-modified NZVI dispersions (>0.03 g/L) because particles agglomerate during transport which violates a fundamental assumption of the theory. Here we develop two empirical correlations for estimating the deposition and transport of concentrated polyelectrolyte-modified NZVI dispersions in saturated porous media when NZVI agglomeration in porous media is assumed to reach steady state quickly. The first correlation determines the apparent stable agglomerate size formed during NZVI transport in porous media for a fixed hydrogeochemical condition. The second correlation estimates the attachment efficiency (sticking coefficient) of the stable agglomerates. Both correlations are described using dimensionless numbers derived from parameters affecting deposition and agglomeration in porous media. The exponents for the dimensionless numbers are determined from statistical analysis of breakthrough data for polyelectrolyte-modified NZVI dispersions collected in laboratory scale column experiments for a range of ionic strength (1, 10, and 50mM Na(+) and 0.25, 1, and 1.25 mM Ca(2+)), approach velocity (0.8 to 55 × 10(-4)m/s), average collector sizes (d(50)=99 μm, 300 μm, and 880 μm), and polyelectrolyte surface modifier properties. Attachment efficiency depended on approach velocity and was inversely related to collector size, which is contrary to that predicted from classic filtration models. High ionic strength, the presence of divalent cations, lower extended adsorbed polyelectrolyte layer thickness, decreased approach velocity, and a larger collector size promoted NZVI agglomeration and deposition and thus limited its mobility in porous media. These effects are captured quantitatively in the two correlations developed. The application and limitations of using the correlations for preliminary design of in situ NZVI emplacement strategies is discussed.     

Effects of Sorbent Injection on Participate Properties: Part I. Low-Temperature Sorbent Injection

Abstract

This article is the first of a two-part series dealing with the effects of sorbent injection processes on particulate properties. Part I reviews the effects on particulate properties of low-temperature sorbent injection processes (those processes that treat flue gas at temperatures near 300 °F). Part II reviews the effects on particulate properties of high-temperature sorbent injection processes (those processes that involve sorbent injection into the combustion or economizer sections of a boiler). In this article, we review what is currently known about the effects of the low-temperature sorbent injection processes on electrical resistivity, particulate mass loading, particulate size distribution, particulate morphology and cohesivity.

Mixtures of ash and sorbent produced by low-temperature sorbent injection processes are typically less cohesive than most types of fly ash. At temperatures within 30 °F of the water dew point, the combination of low cohesivity and low electrical resistivity of the ash and sorbent mixtures can cause electrical reentrainment in electrostatic precipitators. Deliquescent additives such as calcium chloride cause the water to be retained on the particle surface, thereby increasing cohesivity.

Sorbent injection has been reported to increase the particulate mass loading by a factor of 1.8 to 10, depending upon the reagent ratio and the coal sulfur content. Conventional and in-duct spray drying processes tend to shift the particle size distribution toward larger particles, while dry injection processes tend to shift the particle size distribution toward smaller particles.     

Dynamic of submicrometer particles in urban environment

Many studies show that particle toxicity increases with decreasing their size, emphasizing the role of submicrometric particles, in particular of ultrafine particles (<100 nm). In fact, particles greater than 2.5 μm are quickly removed through dry and wet deposition on the timescale of hours whereas submicrometer particles may reside in atmosphere for weeks, penetrate in indoor environment, and be long-range transported. High aerosol size resolution measurements are important for a correct assessment of the deposition efficiency in the human respiratory system, and time resolution is another important requisite. Starting from such considerations, time-resolved aerosol particle number size distributions have been measured in downtown Rome. Fast Mobility Particle Sizer (FMPS) and Scanning Mobility Particle Sizer (SMPS) measurements have been carried out at the INAIL’s Pilot Station, located in downtown Rome, in an area characterized by high density of autovehicular traffic. The two instruments have allowed to investigate deeply the urban aerosol in the range of 5.6–560 and 3.5–117 nm, respectively. In particular, the FMPS measurements have confirmed the interpretation about the transition phenomena in the time interval of few seconds, timescale typically associated with the emission of gasoline and diesel engines. In downtown Rome, the hourly average size distribution is bimodal or trimodal with maxima at about 5–15, 20–30, and 70–100 nm. Particle formation in the nucleation mode was associated to freshly emitted autovehicular exhaust.     

Performance Characteristics of PM10 Samplers under Calm Air Conditions

ABSTRACT

The size range of airborne particles that is closely related to specific deposition regions in the human respiratory tract and excess lung burden of these deposited particles is associated with disease. Size-selective sampling, therefore, needs to be performed to assess the related health risks. Performance criteria applied to these samplers must be well characterized in order to provide accurate and reliable results. The PM₁₀ samplers that have been used in place of the total suspended particulate samplers for the collection of ambient air particles are more relevant to potential inhalation hazards. In order to be certified, a PM₁₀ sampler must meet reliable performance specifications, primarily the aerosol penetration test with liquid and solid particles in a wind tunnel (wind speeds of 2, 8, and 24 km/hr). This testing is intended to assure reasonable accuracy in aerosol measurements. However, the sampler performance under calm air conditions has not been well studied.

In the present study, the sampling heads of three devices—the Harvard impactor, the Personal Environmental Monitor (PEM), and the Sierra Andersen model 241 dichotomous sampler PM₁₀ inlet head—were tested for aerosol separation efficiency. With the consideration of bias and imprecision of the measurements, five specimens of each type of sampler were chosen for performance testing, repeating the tests 5 times for each specimen. An ultrasonic atomizing nozzle was used to nebulize potassium sodium tartrate tetrahydrate and dioctyl phthalate particles as the solid and liquid challenge aerosols, respectively. The aerosol number concentrations and size distributions upstream and downstream of the samplers were measured by using an aerosizer calibrated against a settling velocity chamber. The results showed that among the samplers tested, the dichotomous sampler PM₁₀ inlet head had the best fit to the PM₁₀ convention, while the other two samplers not only appeared to have a steeper separation-curve slope but also had significant particle bounce when challenged with solid particles. Analysis of variance also confirmed the superiority of the dichotomous samplers. Surface-coating with oil or grease greatly reduced the problem of particle bounce.     

Size distribution of polycyclic aromatic hydrocarbons in a roadway tunnel in Lisbon, Portugal

Atmospheric aerosols of four aerodynamic size ranges were collected using high volume cascade impactors in an extremely busy roadway tunnel in Lisbon (Portugal). Dust deposited on the tunnel walls and guardrails was also collected. Average particle mass concentrations in the tunnel atmosphere were more than 30 times higher than in the outside urban background air, revealing its origins almost exclusively from fresh vehicle emissions. Most of the aerosol mass was concentrated in submicrometer fractions (65%), and polycyclic aromatic hydrocarbons (PAH) were even more concentrated in the finer particles with an average of 84% of total PAH present in sizes smaller than 0.49 μm. The most abundant PAH were methylated phenanthrenes, fluoranthene and pyrene. About 46% of the total PAH mass was attributed to lower molecular weight compounds (two and three rings), suggesting a strong influence of diesel vehicle emissions on the production of local particulate PAH. The application of diagnostic ratios confirmed the relevance of this source of PAH in the tunnel ambient air. Deposited dust presented PAH profiles similar to the coarser aerosol size range, in agreement with the predominant origin of coarser aerosol particles from soil dust resuspension and vehicle wear products.     

Public Relations Handbook

A major difficulty encountered in laboratory research on the atmospheric interaction of an aerosol-gas system is the unstable nature of the aerosol phase. Previously reported aerosol stabilizing techniques often severely alter the aerosol so that laboratory results cannot be validly extrapolated to the atmospheric environment. A new technique which does not alter the nature of the aerosol is described in this paper.

Aerosol particles are deposited on an inert substrate such as Teflon beads. The deposition is carried out in a fluidized-bed to ensure discrete aerosol deposition and to achieve a uniform distribution of aerosol concentration on the supporting beads. Aerosol-gas interactions can be investigated conveniently by exposing these stabilized aerosols to the reacting gases in dynamic or static systems. Laboratory results obtained by using stabilized aerosols may be extrapolated to the atmospheric environment.

This aerosol stabilizing technique was incorporated into an investigation of the particulate-catalyzed atmospheric oxidation of sulfur dioxide. Teflon beads with deposited aerosol particles of CuCl₂, MnCl₂, and NaCI were exposed to 4–42 ppm of sulfur dioxide in a plug flow reactor. The rate of oxidation of sulfur dioxide was found to be influenced by type of catalyst, concentration of catalyst, relative humidity and concentration of sulfur dioxide. The rate of oxidation by sodium chloride particulate was measurable at low to moderate relative humidities (45–60%), but the rate was several times higher when the sodium chloride catalyst particles change from solid form into droplet form at high relative humidities (>80%).     

超高效气溶胶净化系统有效性现场检验研究

各类超高效气溶胶净化系统普遍存在的较为突出的问题是:缺乏系统工作有效性现场检测手段,难以判断系统防护是否失效。根据超高效滤料过滤特性和测试原理,并在分析了大气气溶胶对过滤效率测试影响的基础上,提出大气气溶胶背景下,基于粒数浓度测量和使用发生的高浓度单分散气溶胶作为实验气溶胶的净化系统防护有效性现场检验测试技术及系统组成,并进行了验证。     

Inhibition and promotion of trace pollutant adsorption within electrostatic precipitators

Among the technologies available for reducing mercury emissions from coal-fired electric utilities is the injection of a powdered sorbent, often some form of activated carbon, into the flue gas upstream of the particulate control device, most commonly an electrostatic precipitator (ESP). Detailed measurements of mercury removal within ESPs are lacking due to the hazardous environment they pose, increasing the importance of analysis and numerical simulation in understanding the mechanisms involved. Our previous analyses revealed that mercury adsorption by particles suspended in the gas and mercury adsorption by particles collected on internal ESP surfaces are not additive removal mechanisms but rather are competitive. The present study expands on this counterintuitive finding. Presented are results from numerical simulations reflecting the complete range of possible mass transfer boundary conditions representing mercury adsorption by the accumulated dust cake covering internal ESP collection electrodes. Using the two mercury removal mechanisms operating concurrently and interdependently always underperforms the sum of the two mechanisms’ individual contributions.

Implications: The dual use of electrostatic precipitators (ESPs) for particulate removal and adsorption of trace gaseous pollutants such as mercury is increasing as mercury regulations become more widespread. Under such circumstances, mercury adsorption by particles suspended in the gas and mercury adsorption by particles collected on internal ESP surfaces are competitive. Together, the two mercury removal mechanisms always underperform the sum of their two independent contributions. These findings can inform strategies sought by electric utilities for reducing the usage costs of mercury sorbents.     

Sampling and analysis of particulate and gaseous polycyclic aromatic hydrocarbons from coal tar sources in the working environment

A sampling method for polycyclic aromatic hydrocarbons (PAH) in workplace atmospheres using a standard glass fiber filter with a back-up section of Amberlite XAD-2 is described. Filters and XAD are solvent-desorbed and, without further clean-up, the solutions are submitted to high performance liquid chromatographic analysis using fluorimetric detection. Recovery of PAH from XAD-2 was studied, and was found to be in the range 80–100%. The sampling method was evaluated in a coke plant, an aluminium plant and in a creosote impregnating plant. The method described is rapid and sensitive for the determination of the most important workplace PAH, and accounts for both particulate and gaseous PAH.     

Role of surface functionality in the adsorption of anionic dyes on modified polymeric sorbents

While synthetic polymeric sorbents effectively treat dye wastewaters by adsorption, the underlying mechanisms remain to be understood. This work determined the adsorption of an anionic dye by three polymers differing significantly in surface functionality. Surface functional groups of polymers were indicated in FTIR spectra and quantified by the Boehm titration. In reference to the commercial sorbent XAD-4 with a low degree of functionality, the laboratory synthesized NG-8 had primarily acidic functional groups, whereas its aminated product MN-8 had mainly basic amino groups. Electrophoresis determined the points of zero charge of 4.18, 3.23, and 4.51 for XAD-4, NG-8, and MN-8, respectively. The adsorption of Reactive Black 5 dye (RB5) by all the sorbents on a unit surface area basis increased with decreasing pH. At the same low pH (4.40), the adsorption by NG-8 was similar to that by XAD-4, indicating little influence of protonated (neutral) surface functional groups. In contrast, the adsorption by NG-8 at pH 6.05 was 75% lower than that by XAD-4, resulting apparently from the strong electrostatic repulsion between RB5 and deprotonated (negative) groups. Amination substantially enhanced RB5 adsorption by eliminating acidic groups and creating a positive charge on the surface of MN-8. The adsorptive enhancement was also achieved in the presence of CaCl2, due presumably to the neutralization of negative surface charge by Ca2+ and the RB5-Ca2+ pairing. These results manifest the important role of surface functionality in the adsorption of dyes by synthetic polymers.     

Dust Deposition in Granular Bed Filters: Theories and Experiments

The paper reviews the status of granular bed dust filtration theory and its experimental verification. First, works on fluid flow around a single sphere and their relevance to filtration theory are reviewed and solutions of flows in packed and fludized beds of granules are presented. Solutions for creeping flows, potential flows, intermediate and high Reynolds number flows are discussed in relation to filtration theory. Then the various dust deposition mechanisms and their development are presented. Theories based on inertia, diffusion, and other mechanisms are reviewed. Some of the latest theories combining the different effects are described. Experimental data from the literature are presented and compared with various theoretical models.