Electrostatic behavior of different fines added to a Faraday cup fluidized bed

Experiments were performed in a Faraday cup fluidization column to determine the changes in the electrostatic charges on various fine particles after their addition to gas–solid fluidized beds to better understand their role in influencing electrostatic charge generation/dissipation. The charges transported by different fines (Larostat 519 antistatic agent, glass beads (GB), silver-coated GB, catalyst and silica particles) were determined after their injection into an initially charged bed of much larger mono-sized particles (GB or polyethylene) for a range of relative humidities. Entrained fines carry significant charges out of the column, therefore leaving a net charge behind, with the polarity and quantity of charge depending on the size, physical properties and chemical structure of the particles, and on the moisture content of the fluidizing gas.     

Electrostatic potentials in gas–solid fluidized beds influenced by the injection of charge inducing agents

Accumulation of electrostatic charges in gas–solid fluidized beds can lead to particle adhesions, electrostatic discharges, wall sheeting and even explosion. Therefore, it is of vital importance to reduce the electrostatic level in industrial fluidized beds. Adding chemical static agents was reported as one of the most promising ways to control fluidized bed electrostatic potentials. In this study, the electrostatic potential distributions were measured in a three-dimensional column gas–solid fluidized bed with the LLDPE particles for fluidization and the introduction of chemical static agents. Seven charge inducing agents including TiO₂, Al₂O₃, MgO, H₂O, C₂H₅OH, ZnO, and Fe₂O₃ were employed to investigate their influence on electrostatic potentials distribution respectively. Additionally, Al(Et)₃, which is normally used as the cocatalyst for olefin polymerization, was also studied for comparison. It was found that the adding of negative charge inducing agent such as TiO₂ made the positive electrostatic potentials shift to higher values and the negative electrostatic potentials shift to lower values. The injection of positive charge inducing agent such as Al₂O₃ and MgO caused the positive electrostatic potentials dropping to lower values and the negative electrostatic potentials shifting to the values near zero at low injection amount. Affecting mechanism of charge inducing agents was discussed based on the experimental results and the electronegativity of corresponding compounds. It was found that the electronegativity (χ_i) of the metal ion played an important role in determining the charge polarity on the LLDPE particles and the change in the electrostatic potentials after contact and separation with the charge inducing agent particles.     

Novel wet electrostatic precipitator for collection of fine aerosol

A novel wet electrostatic precipitator (WESP) is designed for effective control of fine aerosol from humid gases. It operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge. The new precipitator is characterized by high gas velocity in the ionizing stage. Tests were carried out for gas with (NH₄)₂SO₄, HCl and (NH₄)Cl aerosol at particle number concentration up to 5·10⁷#/cm³ and mass concentration 10–1000 mg/Nm³. For test conditions one-field WESP ensures mass collection efficiency 90–97% and two-field electrostatic precipitator up to 99%.     

Investigations into sensing characteristics of electrostatic sensor arrays through computational modelling and practical experimentation

Recently, great advance has been made on electrostatic sensing techniques for gas–solid flow measurement. However, reports on research studies of the sensing mechanism and characteristics of the electrostatic sensor arrays (ESA) are scarce. The ESA is a key element of the Electrostatic Tomography system (EST) and multi-channel electrostatic sensors for the local characteristic measurement of gas–solid flow. The geometric sizes of the ESA and the velocity and distribution of particles in the pipeline have important effects on the spatial sensitivity, spatial filtering effect and temporal frequency response of the ESA. In this paper, the charge induced on the electrodes of an ESA with different geometric sizes from a single particle having a unity charge is modelled mathematically, and the 3-dimensional electrostatic field due to the charged particle in the sensing zone of the ESA is solved by using a Finite Element Method. The effects of geometric and material parameters of the ESA, including the width and angel of the electrode, the thickness, length and permittivity of the dielectric pipe, the radius of metal screening on the sensing field of the electrodes are investigated numerically. Furthermore a computational model of the sensitivity of the ESA is proposed based on a fitted Gaussian function to the finite element results and its spatial filtering characteristics are also theoretically analyzed. The temporal frequency response of the ESA is also derived. Experimental work is performed on a purpose-built particle flow test rig to verify the modelling results. The theoretical and experimental results obtained demonstrate that the ESA acts as a low-pass filter in the spatial frequency domain. The measurement system, including the ESA and an interface circuit, acts as a band-pass filter. And the space position of charged particle, the electrode width and particle velocity affect the temporal frequency responses of the ESA. These results provide the basis for the performance improvement and optimized design of the ESA.     

Two-stage electrostatic precipitator with co- and counter-flow particle prechargers

Lab-scale, two-stage electrostatic precipitation system comprising of precharging stage, in which PM2.5 particles are electrically charged, and collection stage, in which the charged particles are removed from the flowing gas by electric field, was investigated in this paper. Two types of electrostatic particle prechargers were compared with respect to the collection efficiency of the system: (1) co-flow precharger, in which ionic current was generated co-currently with the gas conveying the particles, and (2) counter-flow precharger, in which ionic current was generated oppositely to the flowing gas. In each case, the electrodes of precharger were supplied with DC or AC high-voltage in order to compare the effect of discharge mode on the collection efficiency of two-stage electrostatic precipitator. The collection stage was formed by two parallel-plate electrodes connected to DC high voltage source. Plate electrodes without discharge points (spikes) are corona-free electrodes, which prevent the collection stage from electrical discharges, and reduce the probability of back discharge ignition. The back discharge decreases collection efficiency of conventional electrostatic precipitators.It was concluded that the co-flow electrode configuration of the precharger, supplied with DC high voltage, has the highest total number collection efficiency for PM2.5 particles, higher than 95% and the mass collection efficiency larger than 99%. The counter-flow precharger provided only about 90% number collection efficiency of two-stage electrostatic precipitator. It was also shown that by AC electrode excitation, the collection efficiency of the system is lower than for DC supply. The two-stage electrostatic precipitators allowed obtaining higher fractional collection efficiency for PM2.5 particles than other conventional systems and can be recommended as highly effective devices for gas cleaning in power plants or cement industry.     

Corona discharge ion sources for fine particle charging

Charging of aerosol droplets and solid particles is applied in many industrial processes such as electrostatic painting, particle separation and electrostatic precipitation. In most of charging devices, electrical discharges are used as a source of ions, which are deposited onto the particles. In the present paper, the charging process by ionic current in alternating electric field was optimized experimentally. Alternating electric field charger was used as a charging device in these experiments. The current voltage characteristics of electrical discharge in this device, and the charge imparted to the particles were determined. The level of charge was measured at the outlet of the charger and was compared to the Pauthenier limit for different supply voltages, and frequencies. MgO powder was used as a source of particles in these experiments. It was noticed that higher supply voltage of the charger gives higher level of particle charge, but at the same time, the particle deposition on the charger elements was increased, decreasing the particle penetration. A compromise between these two tendencies is therefore necessary. As a result we have proposed a criterion maximizing the total charge born by the particles which is a product of relative particle charge and particle penetration.     

Experimental study on electrostatic removal of high-carbon particle in high temperature coal pyrolysis gas

A high-temperature electrostatic precipitator (ESP) presents a good solution for hot gas cleaning, which can remove fly ash from pyrolysis gas at temperatures higher than the tar dew point. In this paper, the characteristics of negative DC corona discharge in air and simulated coal pyrolysis gas were studied. The removal of coal pyrolysis furnace fly ash (ash A) was investigated and compared with that of coal-fired power plant fly ash (ash B) in ESP with a temperature ranging from 300?K to 900?K. The current density of simulated gas was higher than that of air under the same discharge voltage and at different temperatures. The simulated gas also had a higher spark voltage and a lower onset voltage compared with air. The fractional collection efficiency of ash A was lower for particles with diameters of larger than 0.1?µm at high temperature, compared with ash B. A lower collection efficiency in simulated gas was obtained for particles with diameters of less than 0.1?µm compared with air. The collection efficiency of submicron particles in simulated gas was usually higher than it in air, especially for particles with diameters of less than 0.04?µm. In simulated gas, the overall collection efficiency of ash A was obviously lower than that of ash B, especially at high temperature. From 300?K to 700?K, the collection efficiencies of both ash samples were as high as above 93%, but the collection efficiency of ash A in simulated gas decreased to 78.7% at 900?K.     

Particle Dynamics in an Electrohydrodynamic Flow Field investigated with a two-component laser-doppler velocimeter

A laser-Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40-μm particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady-state velocity for a 1.40-μm particle, provided there is a saturation charge of at least 90f%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (superposition of moving gas ions and neutral gas molecules) on the particle transport, characterized by the dimensionless electrohydrodynamic number N_EHD. A comparison between six different electrohydrodynamic states revealed that N_EHD ≈? 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for N_EHD values > 1 the stochastic particle motion is chiefly determined by the nonsteady-state character of the negative corona, for N_EHD values < 1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These finding might be helpful in adjusting the operating conditions in electrical precipitators for and optimized particle separation.     

Modern electrostatic devices and methods for exhaust gas cleaning: A brief review

Conventional electrostatic precipitators (ESPs) have been modernized over the last few decades. In recent years, many new methods of construction have been proposed with the goal of increasing cleaning efficiency, particularly for particles in the submicrometer size range. Adding electrical forces to traditional filters has also resulted in an increase in their collection efficiency for removing dust particles. This paper reviews modifications to ESPs aimed at increasing overall collection efficiency, as well as electrostatically assisted non-electric gas cleaning devices such as cyclones, fibrous filters, and granular-bed filters assisted by electrostatic field or ionization current.     

Charged spray generation for gas cleaning applications

The paper presents results of experimental investigation of properties of charged sprays generated by two types of pressure atomizers with charging by induction. Among other possible methods of charged spray generation, the induction charging has been considered due to its most practical importance. The goal of this research is to optimise the charging process with respect to obtain droplets of required size and charge for their application for exhaust gas cleaning from submicron particles in electrostatic scrubber used for the removal of PM from Diesel engine exhausts. Electrostatic scrubbers use electrostatic forces in order to deposit fine charged particles onto oppositely charged droplets.     

Optical In Situ Size and Concentration Measurement of Particles Dispersed in Gases at Temperatures up to 1000°C

Based on previous experience, an optical particle counter has been designed which allows the direct measurement of the size and concentration of dust particles in gas flows at high temperatures. The instrument features a new optical measuring volume definition together with a signal control to avoid border‐zone error effects. The instrument has a free working distance of 200 mm. Hence it is possible to make true in situ measurements in pipe flows with a cross‐section of ca. 60 cm² and additionally to protect the device against heat and dust precipitation on the optical windows. The instrument was initially used to examine the separation behaviour of cake‐forming rigid ceramic barrier filters at temperatures up to 1000 °C. In particular, the fractional efficiencies and the time‐related concentration changes on the clean gas side caused by a pulse jet cleaning event could be determined. The results obtained so far demonstrate that ceramic barrier filters show basically the same behaviour as conventional filters, but possess a substantially higher separation efficiency at a corresponding higher pressure drop (fractional penetration values between 10⁻⁵ and 10⁻⁹ depending on the type of filter material and the cake formation).     

New technique for electrostatic charge measurement in gas–solid fluidized beds

Electrostatic charge generation within gas–solid fluidized bed reactors has been a concern to industry for many years. Over the years, numerous methods for measuring this phenomenon within a fluidization column have been proposed. This paper focuses on the design of a new method that minimizes effects such as extra charging due to particle handling and bed hydrodynamic disruption due to the location of the measurement device. In addition, the new method provides the bulk charge of the bed particles rather than a local measurement. The device is flexible and can be adapted to a range of fluidization columns. The new method developed consisted of a Faraday cup placed within the windbox of a fluidization column. The distributor plate was designed in such a way that it can be automatically opened to drop the charged fluidizing particles into the Faraday cup below. The new measurement technique was validated by conducting fluidization experiments in a system consisting of a 0.10 m in diameter carbon steel column with glass beads as the fluidizing particles. The technique was proven to be suitable for measuring the total net electrostatic charges in gas–solid fluidized beds.     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

In‐Situ Determination of the Charging of Nanometer and Submicron Particles at High Temperatures

Thermal charging of submicron and nanometer particles has been studied for model aerosols of TiO₂ and SiO₂ as well as Al‐Si (aluminosilicate) at 1 000 °C with a new quasi in‐situ technique. The size dependence of the particle separation efficiency for electrostatic precipitation was determined. The charging state of the particles was obtained from evaluating the global Deutsch number for precipitation in an electric field applied to a laminar flow based on particle trajectory considerations.     

强场和弱场侧超声分子束注入对加料的影响

报道了HL-2A装置最新的实验结果,讨论并研究了超声分子束的注入位置对分子束在等离子体中的消融和穿透的影响,其中包括电离后的分子束粒子在磁场梯度作用和 E × B 漂移下的加速或减速及由此形成的冷通道效应.研究结果表明,磁场梯度和 E × B 漂移对于超声分子束的加料效果、消融和穿透有着重要的作用.强场侧注入可使电离后的电子和离子更深地进入等离子体芯部.这些研究对于更好地理解超声分子束与等离子体的相互作用和优化设计加料系统有一定作用.     

The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator

An analysis of the electrostatic gas cleaning fundamental phenomenon shows an essential influence of discharge electrode construction on the gas cleaning process efficiency.In the physical model tests there were used rigid discharge electrodes with corona emitting elements of various geometries. Different constructions of discharge electrode were tested in the aspect of discharge current uniform distribution on collecting electrode surfaces. Measurements of discharge current distribution has been carried out for discharge electrodes with different spike shapes and in different electric field geometry. The research aim was to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Collection efficiency measurements of selected fly ash samples (from coal fired boilers) were carried out on a laboratory testing bench in a horizontal electrostatic precipitator model.     

Electrostatic charging behaviour of dielectric particles in a pressurized gas–solid fluidized bed

The charging behaviour of insulating particles in pressurized fluidized beds was investigated by fluidizing five polyethylene resins in a column of 150 mm inner diameter and 2.0 m height. Seven collision ball probes at different levels and radial positions monitored the electrostatic charge generation in the bed. The influences of operating pressure and superficial gas velocity on the degree of electrification were studied. For each polyethylene resin, the electrostatic charges of the particles in the upper part of the bed gained a polarity which differed from the particles in the lower part of the bed due to bipolar charging and particle segregation. The hydrodynamics in the fluidized bed significantly influenced the particle electrification. Due to increased bubble size and rise velocity, electrostatic charge generation was enhanced as the superficial gas velocity increased. However, it was difficult to predict the influence of elevated pressure on the charging behaviour of each resin as a result of the complex impacts of pressurization on the hydrodynamics and electrification.     

A versatile sonication-assisted deposition–reduction method for preparing supported metal catalysts for catalytic applications

This work aims to develop a rapid and efficient strategy for preparing supported metal catalysts for catalytic applications. The sonication-assisted reduction–precipitation method was employed to prepare the heterogeneous mono- and bi-metallic catalysts for photocatalytic degradation of methyl orange (MO) and preferential oxidation (PROX) of CO in H₂-rich gas. In general, there are three advantages for the sonication-assisted method as compared with the conventional methods, including high dispersion of metal nanoparticles on the catalyst support, the much higher deposition efficiency (DE) than those of the deposition–precipitation (DP) and co-precipitation (CP) methods, and the very fast preparation, which only lasts 10–20 s for the deposition. In the AuPd/TiO₂ catalysts series, the AuPd(3:1)/TiO₂ catalyst is the most active for MO photocatalytic degradation; while for PROX reaction, Ru/TiO₂, Au–Cu/SBA-15 and Pt/γ-Al₂O₃ catalysts are very active, and the last one showed high stability in the lifetime test. The structural characterization revealed that in the AuPd(3:1)/TiO₂ catalyst, Au–Pd alloy particles were formed and a high percentage of Au atoms was located at the surface. Therefore, this sonication-assisted method is efficient and rapid in the preparation of supported metal catalysts with obvious structural characteristics for various catalytic applications.     

Multi-nozzle electrospray system for gas cleaning processes

A multi-nozzle electrospray system was developed as a charged droplet source for cleaning a gas contaminated with fine particles. The efficiency of removal of fine particles from the gas can be significantly increased, as compared to uncharged sprays, when the droplets are electrically charged. In the presented experiments, the spray of the droplets of size lower than 100 μm was charged either positively or negatively. Cigarette smoke was used as a source of submicrometer particles. The suppression of the particle concentration was determined after different time intervals of spraying of water. Further improvement in gas cleaning was obtained after charging the smoke particles using a specially designed corona charger. The efficiency of the cleaning process was similar to that obtained for droplets generated by mechanical atomisers with induction charging, but the electrospraying allowed decreasing the water consumption up to about three times.     

Thermal diffusion in disperse systems

This paper discusses a theory for a new effect, the migration of solid dispersed particles initiated by a nonuniform temperature field. The reason for the motion is the inhomogeneity of the properties of a thin protective layer around a particle. The example of ionic dispersion shows that the sign of the coefficient of thermodiffusion depends on the magnitude of the electrostatic potential at the particle surface and the thickness of the Debye layer and that the coefficientis larger than the values known for molecular systems by a factor of 100 to 10000. In contrast to molecular systems, in disperse systems thermodiffusion should play a much more important role. Zh. éksp. Teor. Fiz. 115, 1721–1726 (May 1999)