Hybrid electrostatic filtration systems for exhaust gas cleaning

Emission of submicron and nanoparticles by industrial processes, like coal or biomass combustion, solid waste incineration, cement kiln heating, and glass production is a serious environmental problem. The application of most effective bag filters is limited due to very high temperature of the exhaust gases from most of these processes, sometimes >1000 K, which the filter material cannot resists. Cyclones, which could be resistant to these temperatures, are not effective for PM2.5 particles removal, which are the most abundant in these processes. Electrostatic precipitators remove dust from flue gases with high mass collection efficiency, but in submicron range (~100 nm to 1 μm) the so called “penetration window” occurs, in which the fractional collection efficiency decreases even below 50% at minimum. Hybrid electrostatic filtration systems, which combines various conventional gas-cleaning devices were therefore proposed and tested in several research papers, in order to improve the overall collection efficiency of the precipitation processes. The heart in all of these devices is an electrostatic particle charger, which can be a conventional electrostatic precipitator, a corona-discharge particle precharger, or electrostatic particle agglomerator. After charging and/or agglomeration, the particles can be further removed by other devices like cyclones, electrostatic precipitators, or bag filters (after gas cooling to an allowable temperature). The paper reviews specific problems encountered in exhaust gas cleaning from various industrial processes, and the most interesting constructions designed for this goal. In particular, an example of a system comprising a unipolar electrostatic agglomerator, which simultaneously charges the particles and agglomerates them in alternating electric field, for the formation of aggregates of finer particles (PM1) with larger species, which could be removed by any conventional gas cleaning device, with sufficiently high collection efficiency, has been discussed in details.


Introduction
Hybrid electrostatic filtration systems are the gas cleaning installations arranged from one or more electrostatic devices, such as, electrostatic precipitator, electrostatic pre-charger and/or electrostatic agglomerator, and a conventional mechanical device, for example, fibrous filter, cyclone or granularbed filter.Hybrid electrostatic filtration systems are used in order to increase the overall collection efficiency or fractional collection efficiency for submicron dust particles, which cannot be removed using conventional mechanical gas cleaning devices or electrostatic precipitators operating alone.The collection efficiency of these devices gradually decreases with decreasing particle size below 10 µm, obtaining the minimum in the range of sizes between 100 nm and 1 µm, called the "penetration window".Depending on the filter material, fibrous filters can provide higher filtration efficiency than other devices for submicron particles, but the pressure drop across fibrous filter is much higher, to 2 kPa [1], and increases with the growing dust cake.This requires additional energy for the exhaust gas fan to sustain the gas flow rate through the filter that increases the operating costs of the system.Nanoparticles are also difficult to remove because they can seep between the fibers of filter and are discharged to the atmosphere.Additional disadvantage is the lack of filter resistance to high temperatures of exhaust gases.Smaller particles (nanoparticles, <100 nm) would also be difficult to remove using the mechanical or electrostatic forces, but due to other mechanisms such as Brownian diffusion, thermophoresis and diffusiophoresis they are deposited onto larger particles (>5 m) in the collisions with them.The small particles are kept on the larger ones due to Van der Waals, electrostatic or liquid bridges forces, and are removed from the gas flow with these large particles.Another mechanism is the coagulation of these nanoparticles to larger species built in the form of dendrites or tight collapsed aggregates.
Electrostatic precipitators provide high overall mass collection efficiency and low pressure drop, but their collection efficiency decreases even below 50% at the minimum in the submicron size range, usually between 200 and 500 nm.This low collection efficiency is an effect of low electric charge (several elementary charges) the particles from this size range can be charged to in the corona discharge, and particles <100 nm can be uncharged [2].The low electric charge implies weak electric force, which drives the particles to the collection electrode, and the particles <1 m can be easily conveyed to the outlet of electrostatic precipitator by the flowing gas [3].
Hybrid electrostatic filtration systems augmenting the removal efficiency of the devices using various filtration mechanisms, by an addition of electrostatic forces, became a promising alternative to conventional devices, achieving highly efficient fly ash emission control in submicron and nano-range.
This paper provides a brief outlook on various constructions of hybrid electrostatic filtration systems and compares their performances reported in the literature, with particular attention to the last 10 years experience.The report will consider mainly the systems with electrostatic precipitators, electrostatic agglomerators or electrostatic prechargers and fibrous filters of rectangular cross section of the channel used in industrial sector.Special attention will be devoted to those devices, which were designed and tested for the removal of PM2.5 fly ash particles.

Hybrid filtration background
In order to increase the collection efficiency of hybrid systems, the processes of particle charging filtration by a bag filter were combined [5].Three types of hybrid electrostatic filtration systems, using electrostatic phenomena to support filtration processes, have been developed and reviewed in [6][7][8][9].Single-line diagrams of these systems are drawn in Figure 1.
1. Electrically energized filters (Figure 2).The filtration fabrics is equipped with electrodes embodied to the filter tissue and/or outside it to produce electric field throughout the fabric [10].The filtration process of electrically energized filters depends on the magnitude of external electric field and the surface charge density induced on the filter fibers [8].These parameters and the charge on particles determine the deposition efficiency of the incident particles onto the fibers.The electric fields parallel and perpendicular to the gas flow were tested [10].It was noticed by several authors that the penetration of particles into the filter tissue was decreased regardless of the electric field vector, and the pressure drop was lower than that for the same filters without the electric field [11][12][13].It was determined that the particle penetration through the filter decreases with increasing electric field magnitude [14,15].
2. Hybrid electrostatic filters (Figure 3).This is a two-stage device, in which electrostatic particle precharger precedes a fibrous/bag filter [16,17].It is desired that charged particles penetration through the electrostatic precharger should be high, and the filtration process have to occur only at the filter by building loose dendrites on the fibers [18][19][20][21][22].In another version, the particle precharger is integrated with bag filters by interlacing a row of bag with a row of discharge electrodes separated by a grounded grid or arranged in a "matrix" [4,9,[23][24][25][26].In such a device, the particles are charged and partly precipitated by an electrostatic precharger or precipitator with perforated collection electrodes.Small charged particles, which are not precipitated, pass the openings in the collection electrodes and flow directly to the bag filters placed between adjacent parallel collection electrodes.
3. Hybrid electrostatic agglomeration filters (Figure 4).This is a two-or three-stage device.Electrostatic precipitator is followed by electrostatic agglomerator and fibrous/bag filter.In this type of device, the electrostatic precipitator removes the coarse particles, electrostatic agglomerator produces larger agglomerates from primary particles, depositing the smallest particles onto the medium size ones (5-20 µm), and then these agglomerates are captured by bag filter [27][28][29][30].Hybrid electrostatic agglomeration filter differs from a hybrid electrostatic filter in that in the former, larger particles are removed by electrostatic precipitator and only fine particles and their agglomerates produced by the agglomerator are captured by the bag filter that reduces the filter loading and frequency of its regeneration, while in the later the particles are only electrically charged in electrostatic precharger with only small amount of their precipitation within this device, and the filtration process occurs at the fibrous filter with higher fractional collection efficiency than for uncharged particles.
Hybrid electrostatic filtration systems [5] differ from two-stage electrostatic precipitators [31] in that in the former the concentration of particles is partly reduced in a precharger, precipitator and/or agglomerator and then charged particles are removed by fibrous/bag filter, with or without electric field supporting the filtration process, while in the later the particles are charged in the first stage by the corona discharge, and then are precipitated in the second electrostatic stage by electric field, without particle charging in this stage, or by a conventional electrostatic precipitator.Most of the hybrid electrostatic filtration systems were constructed in order to remove fly ash particles from exhaust gases from power plants burning coal or biomass, or from solid waste incinerators.Charging the particles increases several times the filtration efficiency of fibrous filter and reduces the pressure drop across it.It was also observed that the filtration efficiency is higher for filtration materials of higher dielectric constant and larger tortuosity [32].

Electrostatic prechargers
Electrostatic precharger is used for imposing an electric charge on particles, which are removed in the next stage with higher filtration efficiency by fibrous filter.A schematic of hybrid electrostatic filter with corona precharger as the first stage and conventional bag filter as the second stage is shown in Figure 3.The particles are charged by corona-generated ions from sharp electrodes, such as spiked-rod or spiked-plate, or similar, connected to a high voltage source.The particles flowing with the exhausts are charged by gaseous ions by two well-known mechanisms, depending on the particle size: field charging and diffusion charging [33].Electrostatic particle precharger plays a key role in the hybrid electrostatic filters.It is desired that a precharger only imparts a maximal charge to the particles without precipitating them on the precharger electrodes, i.e., the particle penetration have to be maximized.
Various particle prechargers, supplied by AC or DC voltage, as a separate stage prior to bag filter have been investigated by many authors [31].From various types of these prechargers, the following were used or potentially can be used in hybrid electrostatic filters of rectangular cross section of the channel: 1.
Masuda boxer charger [34][35][36].In this type of precharger, strip electrodes embedded in ceramic walls on both sides of the channel, excited by a high frequency signal cause the gas ionization by a surface discharge.The gaseous ions present in the generated plasma are charging by negative ions the particles conveyed by the flowing gas.Additional pair of electrodes supplied with high voltage produce low frequency electric field, which forces the flowing particles to oscillate.

2.
HF Dielectric barrier discharge precharger [37][38][39].In dielectric barrier discharge (DBD) precharger two parallel plate electrodes, energised by a high-voltage, high-frequency (HF) signal, are covered with a dielectric layer.In the generated plasma, the particles are charged to both signs.Due to differences in the electric mobility of positive and negative gaseous ions, the concentration of positively charged particles was slightly higher than those charged negatively [38].DBD reactors proposed by Yao et al. [40], Fushimi et al. [41] and Song et al. [42] were used for simultaneous removal of particles and noxious gases: NOx and hydrocarbons.Due to the dielectric barrier, the gaseous ions cannot be drawn off from the plasma because of surface charge density built on the dielectric, which produces a repelling electric field.

3.
Cross-flow DC corona precharger [43][44][45].Rod or plate electrodes with sharp spikes mounted on their surface, or smooth wires, produce the gaseous ions in a corona discharge.The discharge current flows perpendicularly to the gas flow, similarly to electrostatic precipitator.The discharge electrodes are placed mid-plane between two grounded parallel plates or smooth rod electrodes.The discharge electrodes are supplied with DC high voltage.Spikes are more advantageous because they generate more stable corona discharge from the fixed spikes than thin-wire electrodes.In order to reduce the precipitation of particles in the precharger, short-length ground electrodes downstream the precharger and high gas velocities are used in such prechargers.

4.
Co-flow DC corona precharger [46][47][48][49].Perforated plates, rings, rod electrodes or smooth wires with sharp spikes mounted on their surface, connected to a DC high voltage power supply, produce the gaseous ions in corona discharge parallel to the gas flow.Grounded mesh or rod-palisade is facing these sharp points or wires to produce the electric field.In the device constructed by Chang et al. [26,50], known as quadrupole particle precharger, the discharge is generated between thin wires stretched perpendicularly to the gas flow and parallel rods of larger diameter.The particles are charged by the ions generated by corona discharge and co-flowing with the gas.Similar solutions but using bipolar prechargers to promote simultaneous particles agglomeration were used by Chang et al. [51] and Jia and Lin [52].A palisade of spiked rods of tips facing a parallel palisade of smooth rods and generating gaseous ions downstream the gas flow was tested by Jaworek et al. [49].Such electrode pairs can be arranged in series to increase the electric charge of particles and increase the collection efficiency.

5.
AC field corona precharger [53,54].In this type of precharger, two spiked-plates or spikedrod discharge electrodes are placed by sides of both walls of the channel and they are facing each other.These discharge electrodes are separated by two parallel grids made form a smooth rods.The discharge electrodes generate the ion current perpendicularly to the gas flow and the particles to be charged are flowing between these grids.The electrode system is supplied by AC voltage.The negative potential is alternatingly provided to the discharge electrodes.Each grid, which is close to the negatively polarized discharge electrode is grounded and the opposite grid is at positive potential to attract the ions flowing throughout the charging zone.The charged particles flowing between the grids perform oscillatory motion due to alternating electric field produced between these grids.These oscillations prevent the particles from their precipitation on the electrodes and high gas velocities are not necessary.
The particle precharger can also be combined with bag filter in the same bag house.The discharge electrodes are facing the conducting surface of fibrous filter or a grid (perforated metal sheet) in front of bags, and the particle charging and filtration proceeds in the same device [15,18].

Electrostatic particle agglomerators
The location of electrostatic agglomerator in a hybrid electrostatic agglomeration filter with a bag filter as the last stage is schematically shown in Figure 4. Electrostatic precipitator removes coarse particles from the gas flow after their charging.Electrostatic agglomerator scavenges small (PM2.5)particles by larger (>5 µm), highly charged ones to form agglomerates, while the bag filter removes all the particles, which penetrate the electrostatic agglomerator.The concentration of submicron particles is lower at the outlet because of their agglomeration, and simultaneously the size distribution shifts towards larger particles.An electrostatic agglomerator is charging the flowing particles in corona discharge to the same or opposite polarities, and then, using electrostatic or aerodynamic forces, promotes collisions between these particles in an alternating electric field.The stability of produced agglomerates depends on the van der Waals and liquid-bridge forces between coagulated particles, and in the case of particles of low conductivity, on the image forces, which must be larger than the Coulomb repulsion force on particles.
Depending on the number of signs the particles are charged, to one sign or both signs, the electrostatic agglomerators are called Unipolar agglomerators or Bipolar agglomerators: In Unipolar agglomerators all particles are charged to the same polarity [55][56][57][58].The particles of the same sign of charge are agglomerated due to their collisions in an AC electric field of frequency usually between 50 and 300 Hz, causing oscillatory motion of these particles.Larger particles of high electric charge and high electric mobility oscillate with large amplitude and collide with smaller particles of low charge and low mobility.In this process, inertia of small particles, drag forces and electrostatic image forces act against the Coulomb repulsion forces.The unipolar agglomeration can be a one-stage process, in which charging and agglomeration proceed in the same device [29,49].Particles leaving unipolar agglomerator remain electrically charged that is advantageous in building more porous cake at the surface of bag filter.
In Bipolar agglomerators the process is a two-step one.In the first step the particles are divided up into two streams, which are electrically charged to opposite signs, in two separate prechargers [59].In the second step they are agglomerated, in the AC [60][61][62] or DC [63,64] electric field, utilizing Coulomb attraction between the particles of opposite charges [65,66].In another version, the particles are electrically charged in an AC corona discharge.The two particle streams of opposite signs flowing one after another are then agglomerated in DC or AC electric field [67,68].In order to increase the probability of collision between oppositely charged particles, in some of the constructions a turbulent flow is induced by the obstacles crossing the gas flow by a high gas velocity, about 10 m/s [69][70][71][72].The particles collide due to the Coulomb attraction forces between them, and are agglomerated.
When the particles flow through the AC electric field of frequency of 50 Hz the amplitude of their oscillations in the air at NTP is of the order of magnitude of 10-100 µm, for particles of a size between 0.1 µm and 1 µm (neglecting their Brownian motion), but it is larger for 10 µm particles and is about 1 mm [60].Because of increasing mass of larger fly ash particles, the amplitude of their oscillations decreases for particles larger than 20 µm [60].In unipolar parallel-plate electrostatic agglomerator, the size of agglomerated particles increased for higher frequencies of oscillations [56].For example, the agglomerates of a size from 4 to 8 µm were obtained in electric field of frequency of 50 Hz, and their size increased to 30-50 µm for 300 Hz [56].However, an increase in supply voltage frequency results also in decreasing amplitude of oscillations [29].
The number size distribution of particles is usually changed at the outlet of an agglomerator, compared to that at its inlet, which depends on the type of agglomerator and the kind of agglomerated particles.For example, in a bipolar parallel-plate agglomerator investigated by Kanazawa et al. [63], the number concentration of submicron particles decreased from 75% to 18%.In the system designed by Laitinen et al. [61], five-time reduction in the concentration of submicron particles was obtained.The same authors concluded that nanoparticles (<100 nm) can be easier agglomerated in unipolar parallelplate agglomerator than those larger than 1 µm.

Characteristics of hybrid electrostatic filtration systems
The electrostatic stages of hybrid electrostatic filtration systems are characterized by current -voltage characteristics, which are the dependence of the discharge current on supply voltage of the electrodes for various operating conditions (gas temperature, gas pressure, gas velocity, dust loading, pressure drop), and the collection efficiency of these stages.The bag filters are characterized by the pressure drop across the filter with increasing filter loading, and the filtration efficiency.
The performance of each stage of hybrid filtration system is characterized by the collection efficiency, which is the ratio of mass concentration of particles removed from the gas flow m=m in -m out to the mass concentration m in at the inlet of each stage: The percentage of dust leaving each stage is characterized by the penetration, which is defined as the ratio of mass concentration of particles at the outlet m out to the mass concentration at the inlet m in of this stage: The total collection efficiency of a hybrid filter can be determined from the product of penetrations of all stages in this system: This theoretical equation is only an approximation of actual filtration process, but it does not reflect other phenomena that can occur in actual system, for example, the interaction of the consecutive stages of a hybrid filtration system, or synergistic effects due to charging of particles.Depending on the process parameters, the total collection efficiency can be larger than that resulting from equation ( 3), but fractional collection efficiency can also be lower or negative in certain particle size ranges [73].This effect can be caused by Brownian coagulation of nanoparticles [74] and/or space electrostatic agglomeration to larger species, and was observed by many authors [48,75].The negative fractional collection efficiency occurs mainly within the penetration window of electrostatic gas cleaning devices (100 nm -1 µm) because the agglomerates of this size cannot be precipitated by the agglomerator, similarly to primary particles.The negative collection efficiency for submicron smoke particles was also considered by Atten [76].The author explained this phenomenon as an effect of in-flow deagglomeration of electrically charged agglomerates.
Another phenomenon also causing negative collection efficiency, but in the range of larger particles (usually >1 µm is the re-entrainment (detachment/re-suspension) of clusters of deposited particles from the electrodes of electrostatic precipitator or electrostatic agglomerator, or as an effect of dust eruption during back-corona discharge [77].Although this negative collection efficiency was obtained mainly at the outlet of electrostatic agglomerators, these agglomerates can be removed by the bag filter [20].

Example: Hybrid electrostatic agglomeration filter HYBRYDA+
The experimental, industrial scale electrostatic hybrid filtration system HYBRYDA+, discussed in this section, is of the construction shown schematically in Figure 4.This system combines a barbed-rods vs. plate electrodes electrostatic precipitator, followed by kinematic unipolar electrostatic agglomerator, and bag filter.The exhausts to this hybrid system were injected upstream of electrostatic precipitator via a by-pass from the main flue gas duct downstream of a coal-fired boiler.The gas flow rate through this hybrid system was 3000-7500 Nm 3 /h, and the dust loading 10000-30000 mg/Nm 3 .The exhaust gas temperature varied between 130 and 170 o C. The cross section of the electrostatic precipitator and electrostatic agglomerator chamber was 1700 mm width and 2500 mm height.In this electrostatic agglomerator with alternating electric field, supplied with trapezoidal high voltage of 50 Hz, the small PM2.5 particles were deposited onto medium size particles (5-20 µm).These agglomerates were finally removed by the bag filter.The details of this type filtration system were presented in papers [28,30].A comparison SEM micrographs of fly ash particles collected at the outlet of electrostatic agglomerator in such hybrid electrostatic agglomeration filter for two cases: electrostatic agglomerator off and on are shown in Figure 5.The number of PM2.5 particles deposited on a large particle is higher than 100 by the electrostatic agglomerator on.The time-changes of pressure drop across the bag filter in hybrid electrostatic agglomeration filter, after filter regeneration is shown in Figure 6, for various operation conditions: bag filter only (electrostatic precipitator and electrostatic agglomerator off -, labelled BF), electrostatic agglomerator on (AGGL (20 kV) + BF), electrostatic precipitator on (ESP (57 kV) + BF), and electrostatic precipitator and electrostatic agglomerator on (ESP (57 kV) + AGGL (32 kV) + BF).The increase of pressure drop across the bag filter is about 3 time slower for the electrostatic agglomerator on than in the case with both these electrostatic devices off.Table 1 presents the pressure drop across the bag filter and regeneration period of the filter for various operation conditions, and the mass collection efficiencies for PM2.5 particles for these cases.The regeneration period increases more than 20 times when electrostatic precipitator and electrostatic agglomerator are on.
The considered hybrid electrostatic agglomeration filter HYBRYDA+ allows for a significant reduction in dust emissions.Figure 7 shows a comparison of the dust collection efficiency and dust concentration at the outlet of three types of installations: three-field electrostatic precipitator, bag filter and the system HYBRYDA+.The measurements were made downstream of the three-stage electrostatic precipitator and the hybrid filter for the same precipitator chamber cross section and comparable exhaust gas and fuel parameters.

Conclusions
It was shown in this paper that the activation of electrostatic processes in the removal of aerosol particles by conventional bag filters can significantly improve the filtration processes for particles of all sizes.This type of hybrid electrostatic filtration systems are particularly suitable for the removal of fly ash particles produced by coal or biomass fired boilers.The hybrid electrostatic filtration systems, were classified into three types, depending of the function of each stage: (1) electrically energized filters, conventional filters operating with support from electric field, (2) hybrid electrostatic filters, with particles prechargers, and (3) hybrid electrostatic agglomeration filter, with or without electrostatic agglomerator preceding the bag filter.The increased collection efficiency for PM2.5 particles is characteristic of all considered hybrid systems, by simultaneous reduced pressure drop across the filter.Because of the lower pressure drop, the gas face velocity in the bag filter can be increased without decreasing the collection efficiency, or the number of bags used for the filtration can be lower.Because the charged dust cake is loose and more fragile, the filter bag regeneration is easier by dust dislodging.The pressure drop across bag filter increases slower than for conventional filters and the regeneration period of those filters can be much longer and the wear of bags is slower.
The advantages of hybrid electrostatic filtration systems compared to electrostatic precipitators or bag filters result from: (1) the particles leaving the electrostatic precipitator and/or electrostatic agglomerator or clusters reentrained from the electrodes are removed by the bag filter; (2) the charged particles leaving the electrostatic precipitator and/or electrostatic agglomerator are building the porous dendrites on the surface of bag filter that prevents the bag penetration; (3) the charged particles approaching the bag filter loaded with formerly deposited charged particles are decelerated by Coulomb repulsion force that decreases their apparent face velocity; (4) the smallest particles flowing through the agglomeration stage are agglomerated with larger particles that prevents building compact dust cake on the bag filter, which is difficult to dislodge; (5) the particles are removed by electrostatic precipitator and electrostatic agglomerator in a large amount that decelerates building the dust cake at filter bag and increases the dislodging period; (6) the particles <2.5 µm (PM2.5) are removed with high mass collection efficiency >99%, even those which penetrate the conventional bag filters.
The optimal solution is that comprising of electrostatic precipitator for the removal of coarse particles and electrostatic agglomerator, which agglomerates the particles.The bag filter is therefore prevented from fouling the pores between the filter's fibres.
Many of the presented hybrid electrostatic filtration systems are only at the lab or semi-industrial scale readiness level and their optimal construction and operation at the industrial conditions require further research for their full maturity to their practical implementation.

Figure 1 .
Figure 1.Single-line diagram of hybrid electrostatic filtration systems: Electrically energized filter (a) Hybrid electrostatic filter (b) Hybrid electrostatic agglomeration filter (c).

Figure 5 .
Figure 5. SEM micrographs of fly ash particles collected at the outlet of electrostatic agglomerator of hybrid electrostatic agglomeration filter by electrostatic precipitator off.Electrostatic agglomerator off (a) and on (b).

Figure 6 .
Figure 6.Pressure drops across the bag filter during the first 2000 s after pulse-jet regeneration for various operation conditions.

Table 1 .
Comparison of regeneration period, pressure drop across bag filter and collection efficiency for PM2.5 particles measured at the outlet for various operation conditions of the system HYBRYDA+.