Wet electroscrubbers for state of the art gas cleaning

Effects of electrode materials and dimensions of an electrostatic spray scrubber on water droplet charging for dust removal

Droplet charging is an effective method to enhance dust removal efficiency using an electrostatic spray scrubber (ESS). However, effects of the materials and dimensions of different electrodes on droplet charging efficiency have not been studied systematically. In this study, ring-shaped electrodes were selected to test effects of three types of electrode materials (copper, stainless steel, and graphite) with various dimensions on droplet charging efficiency. A Faraday pail charge measurement device was used to measure the droplet charge generated by the electrodes. A reduced factorial design with four factors was used in this study to investigate the charging efficiency affected by the factors. The four factors tested were electrode material, electrode diameter, nozzle height, and applied voltage. A 304L stainless steel electrode was found to achieve higher droplet charge-to-mass ratio (CMR) values than identically shaped electrodes made from either copper or graphite. The optimal stainless steel electrode inner diameter was 100 mm. The 304L stainless-steel electrode with this optimal diameter and a height of 45 mm achieved the highest mean CMR of 0.435 ± 0.002 mC kg^-1 at an applied voltage of -11 kV with the bottom of the nozzle tip positioned 15 mm below the top of the electrode.Implications Currently, the low removal efficiency of submicron particles cannot meet the environmental control requirements of industrial applications that may have significant concentrations of particulate matter (PM) in this size range. As stricter environmental regulations are increasingly enforced, there is an urgent need to efficiently remove submicron dust particles from the air. Experiments show that the dust capture process is improved by charging droplets in the spray separator and reducing the secondary escape of dust particles in the electrostatic precipitatorAlthough high-voltage charging of droplets has been investigated in many studies, the effect of electrode material and size on droplet CMR is still not fully understood. The purpose of this study was to (1) quantify the performance of 304L stainless steel (304L SS), copper, and high-purity graphite electrodes for spray droplet CMR, (2) evaluate effects of the applied voltage, nozzle height, electrode material and dimension on CMR, and (3) optimize the specific values of key design factors and operating parameters that lead to the highest CMR. The results of this study aim to optimize electrostatic spray scrubbers to achieve high droplet CMR values.

Multiplexed electrospraying of water in cone-jet mode using a UV-embossed pyramidal micronozzle film

The electrospraying of water in the cone-jet mode is difficult in practical applications owing to its low throughput and the electrical discharge caused by the high surface tension of water. A film with multiple dielectric micronozzles is essential for multiplexed electrospraying of water in cone-jet mode without electrical discharge. Thus, a pyramidal micronozzle film with five nozzles was fabricated using the UV-embossing process. The pyramidal micronozzle film consisted of pyramidal micronozzles, a micropillar array, and an in-plane extractor, which were proposed to minimize wetting and concentrate the electric field to the water meniscus at the tip of the pyramidal micronozzle. The electrospraying of water using a single pyramidal micronozzle was visualized by a high-speed camera at a flow rate of 0.15-0.50 ml/h with voltages of 0.0-2.3 kV, -1.6 kV, and -4.0 kV at the water, guide ring, and collector, respectively. Three distinct modes, the dripping, spindle, and cone-jet modes, were observed and distinguished according to the motion of the water meniscus at the nozzle tip. The steady Taylor cone and jet were observed in a voltage range of 1.3-2.0 kV in water, particularly in cone-jet mode. Multiplexed electrospraying of water in cone-jet mode at a flow rate of 1.5 ml/h was performed using a pyramidal micronozzle film, demonstrating the potential for a high-throughput electrospraying system.

Potential Use of Wet Scrubbers for the Removal of Tobacco Dust Particles in the Tobacco Industry

Tobacco dust represents a critical threat to the operators’ respiratory health. Even when wet scrubbers display high purification efficiencies and strong adaptability to dust, their potential applications in the tobacco industry have not been studied. In the present research, removal efficiency of wet scrubber for tobacco dust and fine dust were examined at laboratory and industrial level. Results showed that the dust removal efficiency of the wet scrubber under laboratory conditions reached a maximum value of 99.17%. In addition, the dust discharge concentration was reduced to a minimum value of 1.1 mg/m3. Our data indicated that after using the wet scrubber, large dust particles (≥10 μm) were more effectively removed than fine dust particles (≤5 μm). After the use of the wet scrubber at the industrial facility, the overall dust removal efficiency reached a value of 98.9%, and dust concentration at the discharge point was as low as 1 mg/m3. This value complies with the ultra-low discharge standard and industrial production requirements. Our study demonstrated the feasibility of using wet scrubbers in the removal of tobacco dust and provides new insights for dust suppression in the tobacco processing industry.

An experimental study on the removal of submicron fly ash and black carbon in a gravitational wet scrubber with electrostatic enhancement

It is of great significance to adopt a cost-effective and highly efficient method to capture submicron particles produced by small-scale industrial boilers. In this study, a middle-scale wet electrostatic scrubbing (WES) setup was built to investigate its performance in the removal of both fly ash particles and black carbon, with special attention to the submicron size range. Major factors including the particle properties and charging conditions were expatiated in detail to popularize this method. The results showed that the efficiency increase in black carbon is significantly higher than that of fly ash particles at the charging condition. For the case of droplet charging, the highest efficiency increase in black carbon in the submicron size range is up to 60%, while that of fly ash is only 40% under same conditions. In comparison with particle charging, droplet charging plays a more significant role in removing both fly ash particles and black carbon, which is beneficial for reforming conventional wet scrubbers. Moreover, more small particles could adhere to the surface of large fly ash particles after scavenging while this phenomenon was not found for black carbon due to the characteristic of fractal agglomerates.

The efficiency increases of black carbon and fly ash upon the addition of an electric field to a wet scrubber were expatiated.

Impact of hydrophobic micron ellipsoids on liquid surfaces

HYPOTHESIS

Hydrophobic spheres may exhibit three impact modes after impacting the liquid surface. For the impact of common non-spherical particles in practical processes, particle shape affects the flow of surrounding fluid and forces acting on them, thus the impact behaviors of non-spherical particles may differ from those of spherical particles and remain to be revealed.

SIMULATION

The impact of hydrophobic micron ellipsoidal particles is numerically studied by solving the coupled equations of particle motion and surrounding fluid flow combining VOF method and dynamic meshing technique. The motion characteristics of the ellipsoids and fluids, and the main forces acting on the ellipsoids during impact are analyzed.

FINDINGS

The increase in the axis ratio (AR) of ellipsoid decreases the surface tension and fluid force, resulting in a smaller total force acting on the ellipsoid. Correspondingly, the ellipsoid's impact behavior changes. An impact-mode phase diagram is presented for the studied ellipsoids. Three impact modes, namely, submergence, rebound, and oscillation, exist when AR > 0.8, while only submergence and oscillation exist when AR ≤ 0.8. The critical velocities decrease with the increase in AR, which is well illustrated by the analysis of energy conversion under critical conditions.

Experimental and Modeling Studies on the Filtration of SiO2 Nanoparticles Aerosolized from Different Solvents

The filtration performance of a fibrous filter in removing nano-SiO₂ aerosols atomized using different solvents including methanol, ethanol, 1-propanol, water, and the ethanol/water mixture has been investigated. Through discrete element method (DEM) simulation and filtration experiments, the efficiency variation caused by the combinative interaction of the particle-filter adhesion and interparticle attraction has been analyzed and verified. The adhesion force between the solvent-coated nanoparticles and the filter is considered as the key factor to influence their initial filtration efficiency and can be balanced by their interparticle interaction. The stronger the adhesion, the higher the initial filtration efficiency. Primary aggregate is formed through the particle-fiber interaction, and further agglomerate is caused by particle migration on the fibers, i.e. secondary aggregate. Hydrogen bonding interaction is considered as the main factor causing interparticle secondary agglomeration, and plenty of OH groups existing in the nano-SiO₂ aerosols yielded from alcohol promotes the particle secondary aggregation. As a result, the Brown diffusion capture of the filter is significantly abated, and the as-formed agglomerate is scraped off the filter surface by the alcohol molecules, causing the filtration efficiency decreases. This study highlights the surface affinity properties of nanoaerosols and their balance between particle-particle and particle-fiber interactions in the filtration process.

Laboratory evaluation of electrostatic spray wet scrubber to control particulate matter emissions from poultry facilities

Particulate matter (PM) is a major air pollutant from animal production with significant impacts on human health and the environment. Abatement of PM emissions is imperative and effective PM control technologies are strongly needed. In this work, an electrostatic spray wet scrubber (ESWS) technology designed for removal of PM (PM₁₀ and PM_2.5) emissions from poultry facilities was evaluated under simulated laboratory conditions. Effects of primary operating parameters - including charging voltage (0-14 kV), air speed (0.5-3.5 ms^-1), droplet diameter (100-300 µm), and PM concentration (1-5 mg m^-3) on the PM removal performance of the ESWS - were investigated. Predictive empirical models for PM₁₀ and PM_2.5 removal efficiencies were also developed. The preliminary results showed that the ESWS technology reduced emissions of PM₁₀ by 85-94% and PM_2.5 by 85-88% with a charging voltage of 7 kV, air speed of 0.5-0.75 m s^-1, and droplet diameter of 100-150 µm. The water consumption rate was approximately 2.35 L min^-1, while the total power consumed was 270 W. This study demonstrates that the ESWS could be a potentially effective and feasible tool in controlling PM emissions for commercial poultry facilities.

Thermophoretic motion behavior of submicron particles in boundary-layer-separation flow around a droplet

As a key mechanism of submicron particle capture in wet deposition and wet scrubbing processes, thermophoresis is influenced by the flow and temperature fields. Three-dimensional direct numerical simulations were conducted to quantify the characteristics of the flow and temperature fields around a droplet at three droplet Reynolds numbers (Re) that correspond to three typical boundary-layer-separation flows (steady axisymmetric, steady plane-symmetric, and unsteady plane-symmetric flows). The thermophoretic motion of submicron particles was simulated in these cases. Numerical results show that the motion of submicron particles around the droplet and the deposition distribution exhibit different characteristics under three typical flow forms. The motion patterns of particles are dependent on their initial positions in the upstream and flow forms. The patterns of particle motion and deposition are diversified as Re increases. The particle motion pattern, initial position of captured particles, and capture efficiency change periodically, especially during periodic vortex shedding. The key effects of flow forms on particle motion are the shape and stability of the wake behind the droplet. The drag force of fluid and the thermophoretic force in the wake contribute jointly to the deposition of submicron particles after the boundary-layer separation around a droplet.

Surface-charge distribution on a dielectric sphere due to an external point charge: examples of C60 and C240 fullerenes

An analytical solution for the distribution of surface charge on a dielectric sphere due to the presence of an external point charge is presented. This solution describes how charge on the surface of the sphere is polarised in the electric field into regions of negative and positive charge. The polarisation effect (distribution of surface charge) generally varies with the separation between the sphere and the charge, and it is particularly significant at very short separations. Results obtained from the classical electrostatic model are in qualitative agreement with density functional theory calculations of charge separation in C60 and C240 fullerenes in the presence of an external point charge. This suggests that, from an electrostatic point of view, in the static electric field of external charges these molecules exhibit dielectric behaviour.

Integration of a nonmetallic electrostatic precipitator and a wet scrubber for improved removal of particles and corrosive gas cleaning in semiconductor manufacturing industries

To remove particles in corrosive gases generated by semiconductor industries, we have developed a novel non-metallic, two-stage electrostatic precipitator (ESP). Carbon brush electrodes and grounded carbon fiber-reinforced polymer (CFRP) form the ionization stage, and polyvinyl chloride collection plates are used in the collection stage of the ESP The collection performance of the ESP downstream of a wet scrubber was evaluated with KC1, silica, and mist particles (0.01-10 pm), changing design and operation parameters such as the ESP length, voltage, and flow rate. A long-term and regeneration performance (12-hr) test was conducted at the maximum operation conditions of the scrubber and ESP and the performance was then demonstrated for 1 month with exhaust gases from wet scrubbers at the rooftop of a semiconductor manufacturing plant in Korea. The results showed that the electrical and collection performance of the ESP (16 channels, 400x400 mm2) was maintained with different grounded plate materials (stainless steel and CFRP) and different lengths of the ionization stage. The collection efficiency of the ESP at high air velocity was enhanced with increases in applied voltages and collection plate lengths. The ESP (16 channels with 100 mm length, 400x400 mm2x540 mm with a 10-mm gap) removed more than 90% of silica and mistparticles with 10 and 12 kV applied to the ESPat the air velocity of 2 m/s and liquid-to-gas ratio of 3.6 L/m3. Decreased performance after 13 hours ofcontinuous operation was recovered to the initial performance level by 5 min of water washing. Moreover during the 1-month operation at the demonstration site, the ESP showed average collection efficiencies of 97% based on particle number and 92% based on total particle mass, which were achieved with a much smaller specific corona power of 0.28 W/m3/hr compared with conventional ESPs.

Evidence-based integrated environmental solutions for secondary lead smelters: pollution prevention and waste minimization technologies and practices

An evidence-based methodology was adopted in this research to establish strategies to increase lead recovery and recycling via a systematic review and critical appraisal of the published literature. In particular, the research examines pollution prevention and waste minimization practices and technologies that meet the following criteria: (a) reduce/recover/recycle the largest quantities of lead currently being disposed of as waste, (b) technically and economically viable, that is, ready to be diffused and easily transferable, and (c) strong industry interest (i.e., industry would consider implementing projects with higher payback periods). The following specific aims are designed to achieve the study objectives: Aim 1 - To describe the recycling process of recovering refined lead from scrap; Aim 2 - To document pollution prevention and waste management technologies and practices adopted by US stakeholders along the trajectory of LAB and lead product life cycle; Aim 3 - To explore improved practices and technologies which are employed by other organizations with an emphasis on the aforementioned criteria; Aim 4 - To demonstrate the economic and environmental costs and benefits of applying improved technologies and practices to existing US smelting operations; and Aim 5 - To evaluate improved environmental technologies and practices using an algorithm that integrates quantitative and qualitative criteria. The process of identifying relevant articles and reports was documented. The description of evidence was presented for current practices and technologies used by US smelters as well as improved practices and technologies. Options for integrated environmental solutions for secondary smelters were introduced and rank ordered on the basis of costs (i.e., capital investment) and benefits (i.e., production increases, energy and flux savings, and reduction of SO(2) and slag). An example was provided to demonstrate the utility of the algorithm by detailing the costs and benefits associated with different combinations of practices and technologies. The evidence-based methodology documented in this research reveals that it is technically and economically feasible to implement integrated environmental solutions to increase lead recovery and recycling among US smelters. The working example presented in this research can be confirmed with US stakeholders and form the basis for implementable solutions in the lead smelter and product industries to help reverse the overall trend of declining life-cycle recycling rates.